WAXES
Waxes may be either natural or synthetic and of petroleum mineral vegetable or animal origin. They are generally smooth glossy lustrous and relatively firm solids at room temperature and are fusible when warmed. Originally the term wax referred to beeswax but now has the broader meaning of all materials that have wax like properties. Waxes include various types of chemical composition such as paraffin hydrocarbons fatty esters acids alcohols and ketones. The utilization of waxes in polymers and of polymer additives in wax is based on the improvement in performance or properties conferred by the components in the blend.
Waxes are discussed in this article in the following four classes Petroleum waxes covering paraffin and microcrystalline wax neutral waxes covering paraffin and microcrystalline wax neutral waxes including plant insect and animal wax mineral waxes such as montan and ozokerite and synthetic waxes including polyethylene and Fischer Tropsch wax made from nonwax raw materials. The subject of waxes is covered in texts by Warth (1) by Bennett (2) and by Guthrie (3) which are quite thorough although not recent. See also HYDROCARBON RESINS.
Petroleum Waxes
Petroleum waxes paraffin and microcrystalline waxes essentially are saturated hydrocarbon mixtures obtained by the refining of crude waxes from petroleum. These petroleum waxes comprise by far the largest amount of all the different kinds of wax used in the United States and in the world (4). Paraffin waxes and microcrystalline waxes differ sufficiently from each other in hydrocarbon compositions physical properties and the crystal form of the solid so that there are marked differences in their functional properties and uses in polymers and other industrial formulations.
Paraffin Waxes
Paraffin waxes are solid firm materials that are basically mixtures of saturated straight chain hydrocarbons obtained from refining waxy distillates derived from paraffinic crude oils. Fully refined paraffin wax is usually obtained by deoiling crude scale wax which is a soft paraffin wax intermediate containing up to 50% oil. Scale wax is principally made from slack wax obtained from wax bearing crude (5). The old pressing and sweating processes and the newer dew axing and deoiling technology have been described by Nelson (6) and by Tuttle (7). Typical petroleum test properties of most commercial paraffin wax scale wax and slack wax fall in the range given in Table 1.
Paraffin waxes show the same general lack of chemical reactivity as the nalkanes that are their principal components. However paraffin waxes do undergo a number of chemical reactions including formation of adducts cracking reactions and freeradical substitution reactions. The nalkane components in paraffin wax can be reacted with urea to give crystalline clathrated adducts. Also normal paraffins can be separated from hydrocarbon by use of molecular sieves such as Lindes type 5A Linde Molecular Sieves Union Carbide Corp. The thermal cracking of paraffin wax is a commercial process for making olefins.
Paraffin wax can be chlorinated to introduce various percentages of chlorine up to about 70% which corresponds to an average of twenty two chlorine atoms per molecule of hydrogen chloride at 150°C. Fluorinated paraffin wax has been made by fluorination of paraffin wax particles coated with sodium fluoride.
The liquid phase oxidation of paraffin wax as well as the chlorination reaction has been known and investigated for more than a century. It has been used in Germany as a commercial source of fatty acids. The crude oxidized product also contains esters alcohols aldehydes ketones lactones hydroxy acids and unioxidized alkanes.
Natural Waxes
Vegetables Waxes
Vegetable waxes are obtained from the coating on leaves stems grasses fruits and barks of various plants and trees. These waxes are mixtures of esters of fatty acids and high molecular weight alcohols and unsaponifiable materials. Properties of a number of the number of the more commercial vegetable waxes are listed in Table 6.
The most important commercial vegetable waxes are carnauba candelilla ouricuri and Japan. These waxes are used to a great extent either by themselves or in conjunction with the others in the formulation of resin wax polishes (eg liquid polishes for wood floor waxes shoe pastes). The value of these waxes in polishes lies in the fact that they produce polishes with very durable luster and hardness. Carnauba is most preferred but candelilla ouricuri and Japan wax have been used as substitutes. These waxes are generally used in conjunction with resins such as acrylics polystyrene and poly (vinyl chloride).
AMINES
Amines are named in a number of different ways. The different nomenclature systems in use are outlined below.
Trivial names such as aniline mtoluidine can be used.
Radical or common names can also be used in this system the suffix amine is used together with the appropriate hydrocabon radical or radicals. Aliphatic amines possessing only one amino group are usually named in this manner for example butylamine diethylamine. Mixed amines are named by choosing the larges radical present for the parent compound. Thus CH3CH2CH2N(CH3)C2H5 is named as ethylmethylpropylamine or more accurately NethylNmethylpropylamine.
The IUPAC system which follows the rules laid down by the International Union of Pure and Applied Chemistry (formerly the International Union of Chemistry) is particularly used for polyamines. The name of the diamine triamine etc is derived from the name of the hydrocarbon having the longest straight (unbranched) chain attached to the amino groups by adding the suffix diamine triamine etc. The positions of the NH2 groups are indicated by the lowest possible numbers which are normally placed before the name as in 1 2butanediamine CH3CH2CH(NH2) CH2NH2. Although the practice is generally discouraged diamines are frequently named as for example propylenediamine. Linear polyamines containing recurring amino groups in the chain may be named like triethylenetetramine H2N(CH2)2NH(CH2)2NH(CH2)2NH2.
Physical Properties
The simple amines are derivatives of ammonia in which one or more of the hydrogen is replaced by an alkyl or an aryl group. The trivalent nitrogen might also be part of a saturated or aromatic heterocyclic system. Since the carbon containing portion generally constitutes the largest segment of the molecule in most of the amines the physical properties of these materials generally vary as they do within any homologous series of compounds that is the melting points and boiling points show a generally increasing progession with higher moleculare weights. For equivalent molecular weight compounds however other structural features such as whether the compounds are primary secondary or tertiary amines or whether other functional groups are also present have a decided influence on the physical properties. Primary amines generally have higher boiling points than secondary and secondary higher than tertiary in a series of equivalent molecular weights aliphatic amines. The relationship between melting points is not quite. As clear cut the branched chain amines have greater volatility than the corresponding straight chain compounds. The densities of the aliphatic unsubstituted amines are less than one. The odors of the lower primary amines (C1C3) resemble that of ammonia they become fishy in the C4C7 range and then decrease intensity with increasing molecular weight. The water miscibility of the aliphatic amines is related to their hydrocarbon composition. The lower primary amines (C1C5) are miscible with water as well as with alcohol and ether but as the formula weight increases (above C6) the solubility in water decreases. The secondary and tertiary aliphatic amines are soluble in alcohol and ether but have limited water solubility (only R2NH in which R is C1C4 and (CH3)3N are miscible with water). The lower amines form stable hydrates making purification difficult whereas the hydrates of the higher amines (C10 and higher) are unstable. The introduction of additional amino groups raises the boiling points of the aliphatic amines the corresponding alkanolamines have still higher boiling points. The alicyclic amines have physical properties that are not too different from the corresponding secondary or tertiary aliphatic amines but with slightly higher boiling points compared to the equivalent molecular weight compounds. The aromatic amines are high boiling oily liquids or crystalline solids the liquids are unstable on exposure to light and air and become dark colored. The unsubstituted monoamines (except aniline) are only slightly soluble in water but the diamines are more soluble. All the aromatic amines are soluble in organic solvents. The simple aromatic diamines such as the phenylene diamines are colorless crystalline solids that turn brown in air. Some of the physical properties of representative amines and amine derivatives of various classes are shown in Table 14.
Chemical Properties
The amines and amine derivatives under consideration in this article are alkyl aryl acyl and sulfonyl derivatives of ammonia as well as heterocyclic compounds such as piperazines pyridines and pyrroles. These compounds all contain the trivalent nitrogen atom and their chemical reactions generally involve their behavior as nucleophilic reagents. The degree of reactivity they possess is dependent upon how the various substituents attached to the nitrogen influence the electron density associated with the nonbonded pair of electrons at this site . This electron density which may be referred to as basicity is related to the electron donating or withdrawing influence of the attached groups. As might be expected alkylamines are stronger bases than ammonia and secondary amines more basic than primary the aryl acyl and sulfonyl compounds are more weakly basic than primary the aryl acyl and sulfonyls compounds are more weakly basic in that general order. A qualitative order of basicity dependent on the groups attached to the nitrogen is indicated by the following sequence
In the case of arylamines the socalled metadirecting substituents on the aryl group decrease the basicity whereas orthoparadirecting substituents increase the base strength. These generalities are actually an over simplification of the facts since steric factors as well as the type of group have a decided effect on the basicity or reactivity of the amines as nucleophilic reagents.
The basic character of the amines is responsible for their ability to react with acids to form salts. Salt formation is a highly distinctive property of the amines. Amine salts derived from mineral acids are analogous to ammonium salts and can be formed both in aqueous solution and under anhydrous conditions. For example passing anhydrous hydrochloric acid gas into an ether solution of the amine results in a white precipitate of the insoluble amine salt.
Amine salts with halogen acids often possess organic acids form salts of low stability. Stronger carboxylic acids such as oxalic acid and other strongly acidic organic structures such as picric acid give stable salts that are less water soluble than the mineral acid salts and have characteristic melting points. The stable organic acid salts are useful for identification purposes. The complexes are generally less stable than the salts of protonic acids and they can usually be decomposed into their respective components with heat. The stability is dependent upon the Lewis acid and base strengths of the materials and is also highly dependent upon steric factors.
The substituents that affect the basicity of the nitrogen atoms of the amines also influence the acidity of the hydrogen atoms attached directly to the nitrogen the groups that decrease basicity also increase acidity as shown by the higher reactivity of the hydrogen atoms with metals. All NH compounds react with Grignard reagents. Active metals such as sodium also displace the hydrogen atoms of amines. For example aniline forms sodium anilide although the reaction must be catalzed copper being most effective (eq. 1). Phthalimide reacts with potassium hydroxide in ethanolic solution to give potassium phthalimide (eq.2).
As active hydrogen compounds amines (primary and secondary) react with activated double bonds in the Michael addition reaction. The cyanoethylation reaction with acrylonitrile represents a special case of this. Both hydrogens of a primary amine may react (eq.3).
Amines generally react as active hydrogen compounds with carbonyl compounds adding to the carbonyl group (eq.4).
This reaction between a secondary amine and formaldehyde in the presence of another active hydrogen compound constitutes the Mannich reaction (eq.5).
Primary and secondary amines as well as ammonia react with alkylene oxides to provide hydroxyalkylated derivatives. Ethylene oxide reacts with ammonia to give mono di or triethanolamine depending on the ratio of reactants (eq.8). The reaction between epoxides and amines forms the basis for the hardening of epoxy resins.
Acylation of primary and secondary amines to form amides is commonly accomplished by the reactions of the amine with an acid halide or with a carboxylic acid anhydride.
Ammonia reacts readily with esters to provide the corresponding amides. A method for preparing amides that is significant in polymer chemistry particularly in preparing polyamides is heating the ammonium salts of the carboxylic acid (eq.9).
Aqueous ammonia is corrosive to copper alloys and galvanized surfaces. Aluminium alloys can be used for strong ammonia liquor solutions but their use for weak solutions is not advisable. Mercury should never be used in contact with ammonia as explosive chemical compounds can result.
Ethanolamines. The ethanolamines due to their bifunctional nature are highly reactive compounds. Most reagents attack the amine group preferentially but reaction with the hydroxyl group can be accomplished in many instances. Some of the characteristic reactions of the ethanolamines are illustrated by the following examples.
SYNTHETIC ORGANIC CHEMICALS
Synthetic organic chemicals can be defined as derivative products of naturally occurring materials (petroleum natural gas and coal) which have undergone at least one chemical reaction such as oxidation hydrogenation halogenation sulfonation and alkylation.
The volume of synthetic organic chemicals increased form 17 billion pounds in 1949 to more than 130 billion pounds in 1969. The production for the past two decades is shown in Fig. 1. Much of this phenomenal growth has been due to the replacement of natural organic chemicals. Since this replacement is now essentially complete future growth for synthetic materials will be dictated by the expansion of present markets and development of new organic chemical end uses.
More than 2500 organic chemical products are derived principally from petrochemical sources. These are commercially produced form five logical starting points. Consequently this chapter has been subdivided into five major raw material classifications methane ethylene propylene C4 and higher aliphatics and aromatics.
CHEMICALS DERIVED FROM METHANE
It has been stated that every synthetic organic chemical listed in Beilstein can be made in some way or other starting with methane. This section however deals only with the relatively small number which can be made economically and which are useful enough to warrant large volume production. A diagram of the principal materials covered is shown in Fig. 1.
Synthesis Gas
The most important route for the conversion of methane to petrochemicals is via either hydrogen or a mixture of hydrogen and carbon monoxide. This latter material is known as synthetic gas.
Two important methods are presently used to produce the gas mixture from methane. The first is the methane steam reaction where methane and steam at about 900°C are passed through a tubular reactor packed with a promoted iron oxide catalyst.
The second commercial method involves the partial combustion of methane to provide the heat and steam needed for the conversion. Thus the reaction can be considered to take place in at least two steps.
The process is usually run with nickel catalysts in the temperature range 8001000°C.
The main outlets in the chemical industry for the gas mixtures obtained by the reforming of methane are in the manufacture of ammonia the methyl alcohols synthesis and in the Fischer Tropsch and Oxo reactions.
Ammonia. Ammonia derived from petroleum and natural gas sources accounts for more than 97 percent of the almost 26 billion pounds produced annually. Consequently if can be termed the number one petrochemical in volume. About three fourths of the ammonia produced goes directly into fertilizer uses and the rest is used to produce such chemicals as ammonium nitrate ammonium sulfate caprolactam nitric acid urea acrylonitrile and organic amines.
Methanol. Before 1926 all American methanol was obtained commercially as a byproduct of the wood distillation process (wood alcohol). That year however marked the first appearance of German synthetic methanol. Presently only a negligible amount of the four billion pounds produced comes from wood.
The methyl alcohol synthesis is well known. It resembles the synthesis of ammonia in that the catalysts operate only at high temperature levels and conversion and equilibrium are greatly assisted by high-pressure operation. The industrial reaction conditions are pressures of 250350 atmospheres and temperatures in the range of 300400°C. The catalysts employed are based on zinc oxide which is mixed with other oxides to provide temperature resistance. Variations between synthetic methanol plants are quite similar to those between synthetic ammonia plants. In fact many ammonia operations are designed so that methanol could also be produced in them.
Methanol production has generally paralleled that of its largest end-use formaldehyde. However this is no longer completely true because fairly large quantities of formaldehyde now come from the hydrocarbon oxidation process. Other major uses of methanol are as solvents inhibitors and in the synthesis of methyl amines methyl chloride and methyl methacrylate.
Formaldehyde. Formaldehyde may be made from methanol either by catalytic vapor phase oxidation
It can also be produced directly from natural gas methane and other aliphatic hydrocarbons but this process yields mixture of various oxygenated materials.
Since both gaseous and liquid formaldehyde readily polymerize at room temperature it is not available in the pure form. It is sold instead as a 37 percent solution in water or in the polymeric form as paraformaldehyde (HO(CH2O)nH where n is between 8 and 50] or as trioxane [CH2O3]. The largest end use of formaldehyde is in the field of synthetic resins either as a homopolymer or as a copolymer with phenol urea or melamine. It is also reacted with acetaldehyde to produce pentaerythritol [C(CH2OH)4] which finds use in polyester resins. Two smaller volume uses are in urea formaldehyde fertilizers and hexamethylenetetramine the latter being formed by condensation with ammonia.
Oxo Chemicals. The oxo chemicals are compoundsprimarily C4 and higher alcohols made by the so called ox process. This process is a method of reacting olefins with carbon monoxide and hydrogen to produce aldehydes containing one more carbon atom than the olefin these in turn are converted into alcohols. The earliest reaction studied used ethylene to produce both an aldehyde and a ketone. Thus the name oxo which was adapted from the German oxierung meaning ketonization. However even though other names such as hydroformylation would much more accurately describe the process the term oxo appears too deeply entrenched to be replaced.
A flow sheet of a typical process is shown in Figure 3. The steps involved in the reaction are
The cobalt catalyst used under these conditions is in the form of dicobalt octacarbonyl and cobalthydrocarbonyl.
At the present time the plant capacity in the United States amounts to more than 700 million pounds per year of oxo chemicals. These include such products asnbutanol isobutanol propionaldehyde butyraldehydes butyronitriles isooctyl alcohol decyl alcohol and tridecyl alcohol.
Chlorinated Methanes
The chlorination of methane can be carried out either thermally or photochemically to produce methyl chloride (CH3Cl) methylene chloride (CH2Cl2) chloroform (CHCl3) and carbon tetrachloride (CCl4). If only a particular chlorinated material is desired other methods such as the chlorination
of CS2 or the reaction of methanol with HCl are generally used. These are fairly large volume chemicals with the production of carbon tetrachloride alone estimated at almost a billion pounds.
Although largely replaced by other solvents in the drycleaning field carbon tetrachloride has shown considerable growth as a raw material in the manufacture of chlorofluorohydrobons. Next in volume is methyl chloride which is used to make silicones and tetramethyl lead. Methylene chloride finds use in paint removes solvents and aerosols. Chloroform is a raw material for fluorohydrocarbons.
Acetylene
Acetylene is made commercially in two ways from calcium carbide or from hydrocarbons. The choice of method is determined mainly by the fact that acetylene cannot be shipped easily so large users must be at or near the point of origin. The carbide plant in turn must be near a cheap source of electric power since each pound of carbide requires about 1.5 kwh of electricity.
Acetylene has long been a valuable building block in the chemical industry. Major consumers of the more than a billion pounds produced are the manufacture of vinyl chloride neoprene vinyl acetate acrylic acid and esters and chlorinated ethylene. Growth in the demand for acetylene has been small in recent years however due to competition from cheaper raw materials. Ethylene is now preferred over acetylene as the starting material for vinyl chloride and vinyl acetate propylene has completely supplanted it for acrylonitrile and is making inroads into the acrylates and neoprene can now be made from butadiene.
Vinyl Chloride. Less than 20 percent of the vinyl chloride (or about 700 million pounds) now comes from the addition of hydrogen chloride to acetylene. The process involves a mercuric chloride catalysts and temperatures around 200°C. All vinyl chloride is used to make plastics the most important of which are the homopolymer (PVC) and copolymers with vinylidene chloride or vinyl acetate.
Vinyl Acetate. Vinyl acetate can be produced by combining acetylene and glacial acetic acid. This is a catalytic reaction (zinc or mercury compounds) and it may be carried our either in the liquid or vapor phase.
Approximately 800 million pounds are produced annually all of which is utilized in the polymeric form. Polyvinyl acetate (PVA) can be found in films and latex paints. It also can be used to produce polyvinyl alcohol (a water soluble polymer) polyvinyl butyral (for safety glass) polyvinyl formal and various copolymers.
Acrylates and Methacrylates. The acrylates are esters of acrylic acid (CH2 = CHCOOR) with the R generally ranging from methyl to ethylhexyl. The main method of preparation involves reacting a mixture of acetylene hydrogen chloride nickel carbonyl carbon monoxide and the appropriate alcohol. About 80 percent of the carbonyl group in the product ester is derived from the carbon monoxide and the remainder from the nickel compound. Other methods involve ethylene cyanohydrin ketene the esterification of acrylic acid or the oxidation of propylene to acrolein.
The most important products are ethyl acrylate and butyl acrylate. They are used in making emulsion polymers for latex paints and textiles.
Methyl Methacrylate. Methyl methacrylate is formed from acetone cyanohydrin in a two step process.
While this is the major process in operation there have been reports of a route involving the oxidation of isobutylene to methacrylic acid.
Production of methyl methacrylate now totals more than 450 million pounds. The largest part of this goes into cast sheet where the clarity and resistance of poly (methyl methacrylate) are desirable. Other uses are in surface coating resins and molding powders.
Hydrogen Cyamide. Hydrogen cyanide is prepared as shown in Fig. 3 by passing a mixture of air ammonia and natural gas over a platinum catalyst. The converter is operated at a temperature of about 1800°F and care must be taken to minimize the decomposition of the ammonia and methane as well as the oxidation of methane to carbon monoxide and hydrogen. The effluent gases are called washed with dilute sulfuric acid and then passed through a column where the hydrogen cyanide is absorbed in water. This is concentrated by distillation and an inhibitor is added to prevent polymerization. Although all new plants follow the methane ammonia route HCN can also be produced from cokeoven gas from sodium and calcium cyanides and by the decomposition of form amide.
Because of the safety problems most production is captive to avoid the need for shipment. About one third of the HCN goes into the production of acetone cyanohydrin while almost another third is used to make adiponitrile. Other uses are for chelating agents and sodium cyanide.
The advent of the propylene ammonia process for acrylonitrile has had an interesting effect on this material. Ten years ago acrylonitrile manufacture was a major consumer of HCN. Now almost 28 percent of our HCN is produced as a byproduct in acrylonitrile manufacture.
Carbon Disulfide
Carbon disulfide is made by the catalytic reaction of methane and sulfur vapor. Production is about 800 million pounds with the largest portion going to the manufacture of rayon and cellophane. The other major use is production of carbon tetrachloride.
CHEMICALS DERIVED FROM ETHYLENE
Ethylene far surpasses all other hydrocarbons both in volume and in diversity of commercial use. In the whole field of petrochemicals it is exceeded in tonnage only by synthetic ammonia. Consumption of ethylene has grown remarkably in just the last 30 years. In 1940 300 million pounds were produced mostly for ethanol and ethylene oxide. The wartime demand for styrene and the postwar impact of polyethylene aided in causing this figure to swell to almost 5 billion pounds in 1960. a boom in polyethylene use and strong growth in ethylene dichloride and ethylene oxide expanded ethylene production to over 16 billion pounds in 1968 and over 18 billion pounds in 1970. The major consumers of ethylene in 1969 are shown in Table 1.
Polyethylene
Polyethylene has shown a spectacular growth accounting for only 4 percent of total ethylene consumption in 1950 and almost 25 percent ten years later. In 1961 polyethylene surpassed ethylene oxide as the principle ethylene consumer. In 1969 polyethylene and ethylene copolymer manufacture consumed nearly 40 per cent of all ethylene produced that year.
Three types of processes are used to produce polyethylene. The high pressure process yields a product of low pressure process yields either high density polymer via the Ziegler process or low or medium density polymers if the recent Phillips low pressure process is used. Low density polyethylene and ethylene copolymers accounted for about 70 percent of polyethylene produced in 1969.
Ethylene Oxide
Ethylene oxide was discovered in 1859 by Wurtz who named it because of certain analogies with inorganic oxides. The method which he used was what is today known as the chlorohydrin process. He considered that direct oxidation was an impossibility and stated flatly that ethylene oxide cannot be made by the direct combination of ethylene and oxygen. It took almost eighty years to disprove this statement.
A flow sheet of this process is shown in the first part of Fig. 4. Ethylene chlorine and water are fed into the bottom of a large acidproof brick lined tower at somewhere below 50°C. The water and chlorine form hypochlorous acid which then reacts rapidly with ethylene. The dilute solution emerging from the tower contains about 5 percent chlorohydrin. The major side reaction is the formation of ethylene dichloride. The solution next passes to a hydrolyzer where the chlorohydrin is treated with either slaked lime or caustic soda to produce the oxide. The crude ethylene oxide contains about 10 percent ethylene dichloride which is removed by distillation. This process accounted for about 10 percent of the ethylene oxide capacity in 1969 but many former ethylene oxide chlorohydrin plants are now used for the production of propylene oxide.
The most important process involves the direct oxidation of ethylene with air in the presence of a silver catalyst.
A number of processes are available and a typical one is shown in Figure 5. Ethylene compressed air and recycle gases are fed to a tubular reactor containing a silver catalyst. The oxygen and ethylene concentrations are maintained at a low level to avoid explosion hazards. The reaction temperature is 250 300°C with a pressure of 120300 psi. Two competing side reactions which must be minimized are the total combustion of ethylene to carbon dioxide and the isomerization of ethylene oxide to acetaldehyde. Some ethylene oxide direct oxidation plants use purified oxygen instead of air as the oxidizing agent.
Ethylene oxide is the most important of the olefin oxides. While it can be used directly as a fumigant for foodstuffs (usually mixed with carbon dioxide) it finds its chief outlet as a chemical intermediate. It owes its value to a combination of two types of reactivity it can combine with chemicals containing replaceable hydrogen and it can polymerize to give a polyethenoxy chain. The first is typified in the formation of ethanol amines while the second occurs in the synthesis of the polyglycols and higher glycol ethers.
The estimated production for 1969 of ethylene oxide derived materials and the amount of ethylene oxide they consume is shown in Table 2.
Ethylene oxide is expected to maintain its present growth trend for at least the next few years. Ethylene glycol will remain the largest end use.
Ethylene Glycols. Ethylene glycol can be prepared directly by the hydrolysis of chlorohydrin but the indirect hydrolysis via ethylene oxide is the preferred method. This is shown in the second part of Figure 6. The feed stream consists of ethylene oxide (either from the chlorohydrin or direct oxidation process) and water. This mixture is fed under pressure into the reactor vessel at about 100°C.
Polyethylene glycols are produced by passing ethylene oxide into a small amount of a low molecular weight glycol using a sodium or caustic soda catalyst. The molecular weight of liquid polyglycol products ranges from 200 to 1000. They are used as plasticizers dispersants lubricants and humectants. Above a molecular weight of about 1000 the polyglycols are waxy solids suitable for use as softening agents in ointments and cosmetics and as lubricants.
Surfactants. In 1969 nonionics made up approximately 25 percent of all synthetic detergents produced in the U.S. or nearly one billion pounds. In the years between 1960 and 1969 production of ethoxylated nonionics doubled. Behind this rise are several characteristics most nonionics are liquid they are low sudsing and they can built into readily biodegradable surfactants.
There are many nonionic surfactants but four classes account form more than 80 percent of the production. These are (1) the alkylphenolethylene oxide derivatives (2) fatty acid alkanolamine condensates (3) tall oilethylene oxide adducts and (4) the fatty acid ethylene oxide adducts. The alkanolamine condensates are foam stabilizers in various detergent formulations. Tall oil adducts find use in household detergents chiefly automatic washer products because of their lowsudsing properties. The fatty alcohol adducts are used mainly as light duty detergents though some tridecyl adducts are utilized as foam stabilizers. The trend has been away from the benzenoid ethers because of their biodegradability limitations.
Ethanolamines. Ethanolamines are manufactured by reacting ethylene oxide and ammonia. The relative amounts of the three amines will depend primarily on the ammonia to oxide feed ratio.
The products from the reaction are separated by distillation. During the last few years each of the amines has in turn been in the greatest demand so processing flexibility must be maintained.
Monoethanolamine is used primarily in detergents and as an absorbent for acidgas (H2S CO2) removal it is to a lesser extent a chemical intermediate for compounds such as ethylene imine. Diethanolamines major end use is in detergents but it is also utilized in textiles and as a gaspurification agent. Most of the triethanolamine goes into the production of cosmetics and textile specialities.
Isopropanolamines derived from propylene oxide and ammonia are competitive with the ethanolamines and both are unique in that they are organic compounds and yet strongly alkaline.
Glycol Ethers. In the same way that water reacts with one or more molecules of ethylene oxide alcohols react to give monoethers of ethylene glycol producing monoethers of diethylene glycol triethylene glycol etc as byproducts.
Since their commercial introduction in 1926 glycol ethers have become valuable as industrial solvents and chemical intermediates. Because glycol monoethers contain a OCH2CH2OH group they resemble a combination of ether and ethyl alcohol in solvent properties. The most common alcohols used are menthanol ethanol and butanol. Principal uses for the glycol ethers are as solvents for paints and lacquers as intermediates in the production of plasticizers and as ingredients in brake fluid formulations. The most common trade names are Dowanol Cellosolve and Polysolve Condensation of the monoethers produces glycol diethers which are also useful as solvents.
SILICONE RESINS
Silicone resin polymers differ from fluids and gums in containing a significant proportion of silicon atoms with only one or with no organic substituent groups. The high degree of latent cross linking in these polymers causes the formation of harder less elastic matrixes when the resins are completely cured and necessitates handling many resins in solution to permit easy application and to prevent premature cure. The glass transition temperature of cured commercial silicone resins range up to 200°C in contrast to typical silicone rubber glass transition temperatures near 60°C.
There are two broad groups of silicone resins those probably having a continuous cross linked network produced by hydrolysis of appropriated mixtures of primarily di and trichlorosilanes (DT resins) and those presumably having a knotandchain structure produced by combining a cohydrolyzate of mono and tetrachlorosilanes with a hydrolyzate of dichlorosilanes (MQD resins). Since a great many specialized variations of composition processing technology application and curing techniques exist only general outline of the major methods and applications is given here.
Manufacture
The first step in preparing any silicone resin consists of formulating an appropriate blend of organochlorosilanes. Monomethyl dimethyl monophenyl diphenyl methylphenyl monovinyl and methylvinylchlorosilanes together with silicon tetrachloride have been most widely used. Prediction of properties of the finished resin as a function of composition frequently fails since processing and cure have considerable influence on the final molecular configuration and related characteristics. However some generalizations can be made (1) Trifunctional siloxy units generally produce harder less flexible resins which are frequently immiscible with organic polymers. (2) Difunctional siloxy units increase softness and flexibility. (3) Phenylsiloxanes are generally more miscible with organic polymers than are methylsiloxanes and produce resins which are less brittle and have superior thermal resistance. The chlorsilane blend may be mixed with inert solvents which serve both to modify the rate of hydrolysis and to provide a diluent for the hydrolyzed resin. The most commonly used solvents are mineral spirits esters such as butyl acetate chlorinated hydrocarbons toluene and xylene.
Resin hydrolysis is complicated by a number of factors not encountered in processing silicone fluid. The first is the tendency of the hydrolyzate to gel and become insoluble if the proportion of trifunctional or tetrafunctional chlorosilanes is too high if the solvent level is too low or if conditions are not carefully controlled to prevent excessive silanol condensation. One means of minimizing gel formation is the addition of modifiers usually low molecular weight alcohols which can react with chlorosilanes to yield less easily condensed alkoxysilanes rather than silanols. The final conversion of these alkoxysilanes to siloxanes can then be accomplished at a regulated rate during subsequent processing.
A second complicating factor is the difference in the hydrolysis rates of various chlorosilanes. In general the rate of hydrolysis under given conditions increases with increasing functionality and decreases with increasing molecular weight of the organic substituent groups. Conditions must be balanced to promote the incorporation of all the hydrolyzate products in the average resin molecule if the desired final properties are to be obtained. This can be achieved through proper selection of solvent through intensive agitation and in some cases through sequential addition of the chlorosilanes to be hydrolyzed.
The hydrolysis process for resins is not understood in detail and the products generally represent a statistical distribution of constituent groups and of molecular weights. It is probable that tetrameric ring structures as in the case of fluid hydrolyzates are the most common intermediate configurations. A number of these structures have been characterized. In one well defined ease a resin product of known structure is obtained. When phenyltrichlorosilane is hydrolyzed and the hydrolyzate is condensed with base in bulk or in selected solvents a regularly cross linked bifilar molecule or ladder polymer results. These ladder polymers are thermoplastics of high intrinsic viscosity and of sufficiently high softening point and physical toughness to have possible use as self supporting films and as high temperature coatings. The chemistry of resin hydrolysis and polymerization has been discussed in an earlier section where typical intermediate and final hydrolyzate structures may be found.
In practice hydrolysis is commonly accomplished by one of two techniques. In the first a considerable excess of water is used for hydrolysis and the chlorosilane solvent mixture is fed in at a controlled rate. Evolved hydrogen chloride dissolves in the aqueous phase and is later separated. In the second a near stoichiometric quantity of water is used and the order of addition may vary. The hydrogen chloride escapes as a gas from the hydrolysis system and may be recovered in an absorption tower. In both cases the resin hydrolyzate enters the solvent phase but its average molecular weight and structure and the proportion of silanol groups generated for further condensation differ considerably with the hydrolysis conditions used. Water input rate of feed hydrolysis temperature proportion and type of solvent and intensity of agitation can affect molecular structure of the hydrolyzate and properties of the finished resin. After hydrolysis the aqueous layer if any is withdrawn and the resin is washed. When chlorinated solvents are used the resin layer has a higher specific gravity than the aqueous layer and in batch equipment must be withdrawn and recycled to the hydrolysis vessel. When other solvents are used it is frequently necessary to use a salt solution for washing to permit ready separation of the phases.
Resin hydrolysis may be carried out either batch wise or continuously. Continuously processed resin have in theory at least the advantage of greater uniformity since each increment of hydrolyzate experiences a uniform processing history the molecular weight distribution should therefore be narrower for the continuous product. As in the case of batch hydrolysis of silicon fluids the standard equipment for batch hydrolysis of resins is an agitated jacketed kettle. Auxiliary equipment includes a feed tank and metering system. Continuous processing is generally done in an agitated multistage contactor with provision for escape of gaseous hydrogen chloride if necessary. For both types of hydrolyzers glass lined or other acid resistant construction is required.
Following hydrolysis any residual acid may be removed by stripping out a small amount of solvent or the acid may be left in to serve as a catalyst for further condensation of silanol groups if this step is required. The purpose of the partial condensation or bodying is to generate larger molecules with lower residual silanol concentration so that a rapid cure to a completely cross linked resin can be obtained after the solvent has been removed. If the resin hydrolyzate has been washed and neutralized special condensation catalysts such as metal soaps or acid treated clays may be added to promote bodying. Some solvent may be stripped off to permit attaining the higher temperature necessary for rapid condensation or to promote intermolecular silanol reactions. The end point is determined by viscosity measurement and is influenced by the concentration of resin solids in the solution. At the completion of the bodying step the temperature is reduced by quenching with additional solvent. After addition of curing catalyst filtration and blending to the desired final solvent content the resin solutions are drummed. For most bodying and finishing operations stainless steel or even carbon steel equipment may be used provided that the system in kept dry.
Processing of MQD resins is generally similar to that outlined above for DT resins except that the separate hydrolyzates are blended and equilibrated in a process similar to that described in manufacture of silicone fluids. As an alternative to the use of siliconterachloride as a starting tetrafunctional material processes have been designed based on the conversion of sodium silicate to a silicic acid solution which then reacts with chlorosilanes.
In addition to the usual types of resins used in solution solventless resins in solid form have been offered for sale. These are made by careful removal of solvent at relatively low temperatures using equipment such as a vacuum drum dryer. Such resins can be compounded with glass flock or other filters to produce silicone molding compounds.
Cure
Two general types of cure are applied to silicone resins. The more common involves the removal of solvent from the resin solution with concurrent condensation of some silanols this is followed by catalyzed condensation of the relatively small proportion of silanol groups remaining at temperatures above 100°C. The catalyst may be a metal soap such as tin octoate either left from the bodying operation or added specifically as a cure promotor or it may be a member of one of several classes of amines which are effective. The second type of cross linking reaction is that used in heat curing silicone rubber involving a free radical reaction to form intermolecular ethylene bridges. This type of cure is generally more difficult to control.
Typically application of a silicone resin involves dipping a part or a sheet like material in the resin solution draining or scraping off excess solution allowing the solvent to evaporate and then curing in batch or continuous ovens such as cloth coating towers. The finished coating is hard and relatively insoluble in the solvents used for the uncured polymer. Further cross linking and hardening of the resin may occur on continued exposure to elevated operating temperature during normal use of the substrate or part.
Properties and Uses
The earliest uses of silicone resins were in electrical equipment operating at higher temperatures than were previously permissible. Such motors generators and transformers still constitute a major market for these coatings which have made possible equipment rated for continuous service at 220°C. In hermetically sealed systems although the high thermal stability of silicones is particularly valuable gradual loss of volatiles from the resins may lead to deposition of a nonconducting silicone layer on contacts and on commutator brushes. The rapid brush wear encountered under these conditions has been partially overcome through a special brush design for this class of electrical equipment.
Silicone resins are also used to coat or impregnate glass cloth mica paper asbestos paper and similar materials to form high temperature electrical insulating constructions. For these uses relatively soft flexible resins are preferable. Self supporting substrates such as glass cloth can be coated by dipping. When the resin is to serve as a binder it can be sprayed onto a layer of flakes or fibers the solvent allowed to evaporate and the mass compacted with rollers before being cured at high temperature. The composite strips so formed may be split to produce tapes for winding around irregularly shaped components of electrical machinery or cut into shapes to serve as inserts and spacers.
An extension of impregnation techniques involves stacking a number of glass cloth layers impregnated with partially cured resin and curing the assembly to form a laminated structure. Cure of the laminate may be carried out in a heated hydraulic press at pressures up to 10000lb in.2 to form dense rigid boards having thicknesses up to several inches. These laminates are used principally as electronic circuit mounting boards where freedom from distortion low moisture absorption dimensional stability and constancy of electrical properties are essential. Alternatively the glass cloth plies may be placed in layers over the surface of a model or template. The assembly is then enclosed in a flexible heat resistant bag and cured in a steam chamber. Relatively easily cured resins are necessary for this kind of fabrication.
Most silicone resins like unfilled silicone elastomers are relatively weak and hence are not used where physical strength is the main criterion of performance. There are however applications in electrical machinery and in electronic encapsulation where the need for the thermal and dimensional stability of silicones dictates the use of silicone moldings. When used as molding resins silicones are generally reinforced with fibrous or particulate fillers to improve toughness. Silicone resin powders may also serve as binders for ceramic frits or oxide powders in the molding and firing of ceramic parts requiring precise control of dimensions.
Silicone resins have long been used as paint vehicles for extreme high temperature service. When pigmented with aluminum flake they provide durable protection for metal smokestacks and similar equipment operating at temperatures as high as about 550°C. More recently silicone resins have been used to improve the weather durability and gloss retention of maintenance paints by blending or reacting with alkyd acrylic and other conventional paint bases. If a copolymer is to be formed with for example an alkyd resin the silicone must have a relatively high proportion of reactive silanol or alkoxy groups which can combine with alkoxy or carboxy groups on the alkyd under conditions attainable in normal alkyd processing. Whether reacted or blended the silicone resin must have a sufficient content of phenyl or moderate length alkyl groups to confer ready compatibility with organic polymer. Silicone containing paints are accepted in manufacture of metal siding for buildings where the durability against weathering that can be achieved is matched only by fluorocarbon polymers. Application of the paint to the metal takes place while the metal is still in the form of a flat strip. Curing conditions can be controlled in a continuous coating line to provide optimum adhesion and flexibility of the paint layer so that subsequent coiling slitting and bending to form the siding strips can be accomplished without chipping or cracking the coatings.
Applications of resins as release agents include those in which a degree of permanence despite temperature cycling is desired as in bake ware and those involving extremely high temperatures such as glass molding and metal casting. Silicone resin coated pans for baking bread can be used without greasing and permit the loaves to be discharged merely by inverting the pan. After several hundred baking cycles the pans must be cleaned and recoated. Recently silicone coated bake ware for home use has been introduced.
Like other types of silicone polymers the resins wet most substrates well before cross linking and can be used as bonding agents for high temperature assemblies. The MQD polymers are outstanding in their ability to adhere to almost any solid including polymers with poor wettability such as poly tetra fluoro ethylene. They will also adhere to many wet substrates. For these reasons they have been used as pressure sensitive adhesives for high temperature tapes with glass cloth or silicone rubber backing. At ordinary temperatures their resistance to environmental factors makes them useful in tapes for semi permanent positioning and assembly of industrial components. And since they retain tackiness at temperatures as low as 40°C they can be used in low temperature tape application such as field repairs of electrical systems in winter. For some purposes the pressure sensitive adhesives can be cured with peroxides at temperatures over 125°C to provide the properties generally associated with cross linked silicones.
Surfactants and Specialtics
Emulsions
Emulsions of silicone fluids and a few resins are available commercially for use primarily as antifoaming agents and as release agents. The features which make the use of emulsions desirable include nonflammability compatibility with aqueous systems case of dilution and effectiveness of these highly dispersed forms of silicones is applications where surface properties are important.
Silicone emulsions are generally made from standard fluids emulsifying agents water and in some cases finely divided solids which apparently act as carriers for the silicone increasing the exposed silicone interfacial area and consequently the effectiveness of the emulsion as a surfactant. All classes of emulsifiers can be used anionic cationic and nonionic. In most cases a waterinoil dispersion is prepared first by passing a mixture of silicone fluid emulsifier some water and solid dispersant (if used) through a high shear blending device such as a colloid mill or homogenizer. The resulting paste is then dispersed in a larger amount of water with vigorous agitation. The final product is a silicone in water emulsion even though the silicone fluid may constitute up to 70% of the product composition. Most silicone emulsions as solid contain 1070% silicone but are usually diluted too much lower concentrations before use.
Commercial silicone emulsions are pourable systems of low to moderate viscosity with good shelf stability and resistance to phase separation. They are susceptible in most cases to breaking at high (80°C) or low temperatures and must be protected from freezing. Extremes of hydrogen ion concentration will also cause deemulsification. Certain emulsions for use in polishes are intentionally designed to break down under shear so that good gloss is obtained without excessive polishing effort. Other applications of emulsions antifoaming agents and release agents have already been described in the section on Silicone Fluids.
Greases and Compounds
Compounds of silicone fluids with particulate solids and other thickening agents to achieve a grease like consistency are used in many cases where noncuring flowstable silicone containing compositions are required. When used primarily as lubricants these compositions are called greases otherwise the more general term compound applies.
The most commonly used thickening agents in silicone compounds are various forms of silica although clays and other finely divided solids may be used. In greases the thickening agents are generally soaps. Other ingredients include organic thickeners diluents and additives to improve properties ranging from oxidative stability to radiation resistance. Compounding is usually carried out in a dough mixer or kettle with high shear agitation. Thorough wetting of the thickening agent may be promoted by selective addition of the components to the mixer or by mixing at elevated temperature. In some cases the compound is passed through a paint mill or homogenizer after mixing to insure that filler agglomerates are broken down. The finished product is packaged in jars cans or collapsible tubes.
Silicone compounds are used principally in electrical applications as potting materials or insulating coatings with excellent corona resistance. Greases are used in a number of non load bearing applications such as laboratory glassware joints and electric clock motors where chemical inertness or resistance to aging are important. High temperature greases are used in oven conveyors and similar systems. Although the relatively poor lubricating characteristics of dimethyl silicones have kept them from wide use in heavy duty greases applications are gradually increasing in the automotive and aerospace industries for products based on chlorophenyl fluoro alkyl and long chain alkyl silicones. Thermal and oxidative resistance relative invariance of properties over a range of temperature resistance to aging and weathering and resistance to hydrocarbon solvents are among the more important reasons for selection of one of these products.
Surfactants
One of the most successful specialized applications of silicone materials has been the use of silicone organic block copolymers to control the structure of organic polymer foams primarily polyurethans. These copolymers added at about 1% to the foam formulation serve to control cell size and uniformity to produce even textured foams with reproducible physical properties. As the proportion of surfactant is decrease wild cells of unusually large size appear followed at lower silicone levels by general coarseness and a tendency of the foam to split mechanically. With no surfactant many commercial foam formulations collapse completely. Polyester vinyl synthetic rubber and other polymeric foams have also been improved through the use of silicone surfactants.
Among the various chemical types of silicone surfactant one of the most widely used in based on an alkoxyterminated branched silicone containing an average of one tri functional siloxy group and three alkoxy groups per molecule. This is combined with a hydroxyl terminated polyether in an ester exchange reaction using a suitable catalyst. The displaced low molecular weight alcohol is continuously distilled away yielding a molecule with a silicone segment attached on the average to three long polyether chains. Much other linkage can be employed to join the silicone and organic portion of the molecule carboxylic acid ester carbamate thioester direct silicon carbon linkages and others have been patented or disclosed in the technical literature.
The function of these additives is complex and has been the subject of a number of articles. Whether the silicone copolymers promote nucleation of gas bubbles or serve merely to stabilize bubbles introduced mechanically during the mixing of the polyurethane reactants the surfactants are essential to maintaining the integrity of the cell walls during the expansion of the foam. In the case of flexible foams intended for use in mattresses and seating a relatively low foam density and high resiliency are of paramount importance. To achieve high resiliency the foam must have a significant proportion of open cells to permit ready deformability while maintaining sufficient mechanical strength. This requires a fine balance between the rate of gas generation and the rate of polymerization of the urethan so that a controlled proportion of cell walls rupture just before the foam becomes set. Thus the type and proportion of amine catalyst promoting the foaming reaction the tin catalyst promoting polymer formation and the surfactant can all have an effect on the ultimate density and resiliency of the foam.
The sensitivity of the foam forming process to the surfactant level is attributable to the tendency of the surfactant to concentrate at the polymer gas interfaces with the polyether oriented inward toward the film and the silicone portion extending into the gas phase. The low surface tension of the surfactant tends to stabilize the cell walls and to induce flow of the still plastic polyurethan to maintain uniform surface energy. This ability is a function of the solubility of the polyether portion of the surfactant copolymer in the urethan phase and of the plasticity of the bulk material. If prepolymer is used as a starting ingredient for the foam polydimethylsiloxane surfactants perform effectively as foam promoters. With monomeric starting materials silicone homopolymer (with no polyether content) act as defoaming agents.
In the case of rigid foams for thermal insultion low thermal conductivity must be obtained by forming a high proportion of closed cells in a low density foam. In this case the cell walls are strong enough to prevent shrinkage of the foam when the cured structure cools. In general somewhat different types of silicone copolymers are required for rigid foams than those found most effective in flexible foams. In furniture molding higher foam densities approximating those of hard woods are desired. Here too although the density of the finished foam is primarily a function of the formulation used the type of concentration of silicone surfactant must be carefully selected to achieve the desired result.
Silicones have also been incorporated in organic polymers intended for molding of solid plastic parts. Improved surface texture and easy release from the mold are among the benefits obtained.
Primers and Adhesion Promoters
Although several types of silicones notably MQD resins and one part RTV compounds exhibit good adhesion to a variety of substrates without priming best results in achieving adherence to other materials are frequently obtained by using specialized primers. Few generalizations can be made concerning the utility of various kinds of primers in specific applications and an empirical approach to adhesion problems is frequently necessary. Primers are often based on silicone resins with appropriate additives to improve wetting of the substrate or to achieve effective bonding to reactive surface groups. The primers may serve to shield sensitive curing catalysts in the silicone polymer systems from inhibitors in the substrate materials or simply provide a well anchored compatible base to which the bulk silicone resin or rubber can become attached.
One class of primer which has attained wide use in glass reinforced plastics as adhesion promoters between resin and glass fibers is the family of carbon functional trialkoxysilanes. These are derivatives of trichlorosilane with the general formula RSi(OR)3. Carbonfunctional groups which have been successfully utilized include vinyl aminoalkyl acrylatoalkyl glycidoxyalkyl and variations of these. The alkoxy substituents are generally methoxy ethoxy or methoxyethoxy used to impart water solubility. The trialkoxysilanes are usually applied to the glass fibers from dilute aqueous solution directly after the fibers are blown and collected. Partial or complete hydrolysis of the alkoxy groups takes place in the bath forming silanols which can interact with similar reactive sites on the glass surface bonding the silane chemically to the glass through siloxy linkages. The carbon functional group is not attached to the glass surface and is available to provide bonding to the resin. The most likely mechanism of bonding involves solubilization of the organic group in thermoplastic matrixes whereas actual copolymerization of the carbon functional group with the bulk polymer appears to take place in thermoset polymers. Other factors including polarity of the carbon functional group critical surface tension of the treated fiber surface and ability to form hydrogenbonds may also be significant in the performance of these bonding agents. The coating is generally much thicker than a monolayer and additional silanes or their hydrolysis products are held by capillary action between the individual fibers. The benefits of the silane treatment are seen in improved reinforcement of the glass fiber plastic matrix as measured by flexural strength of the composite and are especially apparent in retention of physical properties after exposure to water. The treatment prevents absorption of water by capillary action along the fiber polymer interfaces and the consequent loss of reinforcing action of the fibers. A notable current application is in the treatment of glass cord used in automobile tire construction.
STABILIZERS
Stabilizations as discussed in this article denotes the treatments or manipulations to which polymers are subjected in an effort to control or adjust effectively the deteriorative physicochemical reactions at work during the manufacture compounding processing and subsequent life of the polymer.
Since the goal of stabilization is to maintain insofar as is possible the original characteristics of the polymer it follows that the subject is of interest to all concerned with the various manners in which polymers are employed e.g. as in plastics elastomers foams textile fibers coatings and adhesives.
Effective polymer stabilization involves the simultaneous control of numerous degradative forces and mechanisms which may be at work on the polymer system at any given time. Maintenance and control of color rheological characteristics mechanical properties electrical properties chemical resistance biological resistance thermal and optical properties and resistance to long term aging and weathering are all aspects of polymer stabilization. This article is concerned mainly with the effective use of certain additive to achieve a close control and regulation of properties of a composite polymer matrix or system. Specifically the stabilization of halogenated polymers in general and poly (vinyl chloride) in particular will be discussed.
The section on stabilization of polyolefins discusses how the antioxidants and other special organic inhibitors are applied to the total stabilization of a hydrocarbon polymer. By way of illustration the polyolefins were chosen to be generally representative of the type of polymer deterioration one might expect to encounter with hydrocarbon resins and the discussion in this section illustrates the means one might take in combating these phenomena. Additional information can also be found in articles on the specific polymer see for example ETHYLENE POLYMERS PROPYLENE POLYMERS STYRENE POLYMERS etc. for information relation to the degradation of other nonhydrocarbon polymer systems.
Methods
Polymer stability can be achieved effectively by pursuing either of two possible lines of attack preventive stabilization or arrestive stabilization. Better results however are usually attained by a combination of the two approaches. In preventive stabilization methods such as closer control and regulation of the polymerization reaction to reduce or eliminate byproduct formation are effective since the presence of residual byproducts (including structural aberrations of the desired polymer) may exert a deteriorative influence on the properties of the newly produced polymer. Better regulation may be achieved by (a) closer control of monomer purity and the purity of all of the other materials employed in the polymerization reaction - water organic solvents buffers suspending aids emulsifiers catalysts and initiators chain transfer agents etc (b) cleaner polymerizations such as polymerization in bulk rather than in emulsion solution or suspension initiation of polymerization by nonochemical means eg radiation initiation more painstaking workup of the polymers after reaction termination to remove or inactivate potentially troublesome catalyst residues and other polymerization reaction additives and combinations of any or all of the preceding. The realization of these objectives is slowly being attained as polymer manufacturers realize that the manner in which they adjust conditions in the polymerization kettle will ultimately determine the inherent stability of the polymer and that process changes which may be slightly more costly to incorporate at this point may ultimately be the less expensive.
Another form of preventive stabilization occasionally practiced is through copolymerization with a minor amount of a second monomer in an effort to build in blocks to the progressive chain disruptive forces that are encountered eg in the unzippering of hydrogen chloride from a poly(vinyl chloride) backbone. The inclusion of very small quantities of ethylene or propylene on the order of 13% in a vinyl chloride polymerization has resulted in copolymer of greatly improved that stability as compared to a vinyl chloride homopolymer of equivalent intrinside viscosity yet with very little change in all of the other properties characteristic of poly (vinyl chloride). In a similar manner the partial replacement of styrene monomer with methyl methacrylate in the cross linking of unsaturated linear polyester resins such as those used to impregnate glass fibers has resulted in glass fiber rein forced plastics of greatly improved light stability as compared to the similar system compounded with styrene monomer used as the sole cross linking agent.
Cases such as the above two examples in which copolymerization effects improvements in stability are admittedly the exception rather than the rule. They are found to occur in those relatively rare instances where the resultant copolymer is a true block copolymer rather than a random or graft copolymer.
The concept of arrestive stabilization denotes the remedial treatment of polymer systems within which some albeit perhaps only incipient degradation has already been initiated. Arrestive stabilization can be carried out effectively by removal or inactivation of the degradative source to prevent further more serious deterioration from occurring or else it can be accomplished by introducing reactive sites or structures which will combine with a repair the polymer by altering the degradation to a less objectionable form. A third means of arrestive stabilization is through the introduction of reactive species which will compete with the normal series of degradation reactions for the active susceptible sites on the polymer and will replace those groups or ligands with others of greater inherent stability. All of these remedial stabilization mechanisms will be dealt with a greater length in the discussion of poly(vinyl chloride) stabilization.
At present two techniques have been developed for achieving stability by arrestive means. One of these is achieving stability by arrestive means. One of these is through copolymerization with monomeric moieties which are themselves capable of functioning as stabilizers through any one of the three modes described above. In effectiveness this approach has been only partially successful and is still open to considerable improvement. It does however have the singular characteristic of permanence because the stabilizing group is made in integral part of the polymer backbone. Whether or not this characteristic is an advantageous one is a question still open to much debate. Much of the answer resides in the nature of the particular stabilization mechanism being introduced into the system through copolymerization and in the end use for which the polymer composition was developed.
By far the most effective and widely employed means of arresting polymer degradation is through the incorporation of chemical additives. For purposes of this discussion an additive is defined as a substance which is mechanically dispersed or dissolved (usually with the aid of heat) in the polymer system requiring stabilization. It almost always is a minor constituent of the total system usually representing 10% or less by weight and most often constituting an even smaller amount eg on the order of 0.5% 3.0% of the system by weight. The additive(s) maybe low molecular weight simple chemical substances or may themselves be highly polymeric and polyfunctional in nature.
Traditionally the additive approach is among the oldest of the various stabilization techniques and is still the route most preferred by resin manufacturers compounders processors and captive users who have the power of choice. The reason for this can be summed up as simplicity versatility and economy. The literature is replete with examples that show the favourable results obtained with a stabilizing additive in comparison with the same stabilizing group introduced by copolymerization. Although the copolymerization method will certainly continue to be pursued and may eventually be developed to the point where it will surpass the additive compounding technique for general applications the abandonment of stabilization through the comparatively simpler stirin method is not yet in sight.
An examination of Table 1 will serve to illustrate the current commercial importance within the United States of just three classes of additive stabilizers antioxidants ultraviolet inhibitors and proprietary poly component mixed stabilizer systems primarily designed for use in poly (vinyl chloride).
These statistics pertain to the use of the additives in plastics applications only applications involving rubbers synthetic fibers paints and organic coatings are excluded although these markets also account for the consumption of considerable volumes of stabilizers. The rubber industry alone for example in 1963 consumed over 96 million lb of antioxidants in the United States without including antiozonants and other inhibitors. With respect to all stabilizers it is further estimated that the total worldwide consumption (excluding U.S.S.R. and other Eastern European countries for which figures are not available) is roughly 2.2 times the market for the U.S. alone. Although some of the U.S. production is used for export purposes the amount is not considered significant.
Stabilization of Polyolefin Resins
Within each of these five classes of material the number and variety of commercially acceptable materials have been somewhat limited due to additional considerations chief among which is the factor of compatibility. Olefin polymers are inherently similar in structure to aliphatic hydrocarbons and therefore similar also in solubility characteristics i.e. they are hydrophobic and oleophilic. Certain otherwise extremely effective additives are often ruled out for use with olefin polymers because they are too polar for optimum solubility in and retention by the polymer. Although the situation is not so restrictive in the same of olefin copolymers and in polymers containing long and/or branched pendant groups eg poly (4methyl1pentene) materials such as polyethylene and polypropylene still present problems in certain demanding applications.
The toxicological characteristics of the additive may limit many otherwise useful material from applications in which the health safety factor is important. Although this situation applies to all stabilizers regardless of polymer application it is especially important with olefin plastics since these materials have come to dominate the packaging industry.
The effect of the additive color on the formulation is another consideration which tends to rule out many otherwise highly desirable materials. Alkyl or aryl derivatives of pphenylenediamine for example are extremely effective antioxidants for polyethylene (use as they also are in rubber and other hydrocarbons) but they are scarcely used today other than in dark colored stocks because of their staining nature.
Ultra violet radiation absorbers have special requirements of their own which apply not only to polyolefins but wherever these materials are used (1) high absorption in the ultraviolet region of 29003900A with sharp cutoff to near zero absorption above 4000A (absence of visible color) (2) ability to transform absorbed energy into inactive energy without the formation of color (3) stability against eventual self destruction due to absorption of energy (4) compatibility (solubility) with the substrate (5) nonmigration.
Table 2 lists the materials most frequently employed in the stabilization of polyolefin resins. For the most part one selected member from each class is used in admixture with a material from each of the other groups excepting that metal deactivators and ultraviolet absorbers are generally used only when special circumstances warrant their inclusion eg. polyethylene insulation over copper wire or polypropylene filaments for indoor outdoor carpeting.
The degree to which olefin polymers are capable of resisting oxidative degradation in any given environment is determined by such obvious factors as the chemical composition of the particular polyolefin the polymerization reaction conditions and the degree of effectiveness and the concentration of the protective agent(s) included in the resinous compound. In addition there are certain less obvious factors which can affect the degree of stability of the system. Chief among these are the influences contributed by the other formulation constituents including colorants lubricants mineral fillers etc. With respect to stability their inclusion or omission can be good or bad depending upon their own chemistry. From a practical point of view achieving optimum stability in a formulation entails a study and recognition of the effect contributed by other formulation constituents.
Synergism and Antagonism. Some stabilizers behave synergistically that is their combined effect is greater than the sum of their effect when each is used alone. For example a particular antioxidant tested with three different ultraviolet radiation absorbers in a polyolefin may in the first case yield an outdoor durability improved several fold over that of the antioxidant used alone substantially unaffected in second and poorer in the third. If in the first case the performance of the antioxidant is also improved the materials perform synergistically. This unanticipated benefit is clearly attributable to some complementary relationship existing between the two materials. Examples of true synergistic activity among stabilizer components are comparatively rare whereas reverse synergism or antagonism occurs rather frequently. Two examples of synergism which have been studied extensively are the synergism between hindered phenolic antioxidants on the one hand and various sulfur bearing organic materials particularly sulfides (thioethers) on the other and the apparent synergism between certain grades of carbon black and sulfer containing antioxidants. The impetus behind the investigation into the carbon black/sulfer synergism originally resulted from an exploration for novel antioxidants which might effectively replace amines and/or phenols owing to the fact the latter two groups or primary antioxidants were known to lose most or all of their effectiveness in the presence of carbon black. The latter illustrates a typical example of antagonism.
Effects of Formulation Constituents. Compared to the volume of published literature concerning the elucidation of structure and mechanisms of degradation of olefin polymers relatively little has been said concerning the nature of formulation additives and constituents and their effects on the stability characteristics of the formulation. It is common practice not only in the United States but elsewhere for the producers of polyolefins to compound their polymers into forms suitable for direct processing into final compositions and shapes eg extrusion and molding compounds rotational molding powders. In the compounded form the polymer blend contains the colorants lubricants various stabilizing agents and whatever other special purpose additive(s) may be required to modify the properties of the composition for the particular end application. The accumulated body of knowledge concerning the evaluation and selection of these agents is regarded by the polymer producers as proprietary information and this accounts for their reluctance to publish information of this type. However some findings have recently come to light mainly through the laboratory evaluations of additive producers and large volume users of olefin polymers who are in a position to specify formulations to the resin producers.
The effects of carbon black have been alluded to previously. Effects reported on various pigments as well as carbon black in polyethylene wire and cable formulations were also reported in 1957 carbon black at 2% concentration extended the outdoor life from one to twenty years. Similar studies have been conducted more recently.
Studies of the role of stearate processing aids upon the melt flow stability of polypropylene particularly in the presence of copper show that the stearates of calcium aluminum zinc and cadmium as well as free stearic acid adversely affect the rheology of the compositions at elevated temperatures and concluded that the use of stearate processing aids should thus be a voided if maintenance of properties and stability are of prime consideration in the fabricated article.
The specific effects of various stabilizing agents have been described with respect to a host of different properties in various olefin systems. The performance of 112tris (2methyl4hydroxy5textbutylphenyl)butane which refers to a condensate of 3methyl6tertbutylphenol and crotonaldehyde has been compared with a variety of other hindered phenols alone and in the presence of dilauryl thiodipropionate in polypropylens polyethylene and a copolymer of ethylene and vinyl acetate. The effects of antioxidants and ultraviolet absorbers in polypropylene outdoor weathering and extraction resistance of the additives heat aging stability and light stability of polyethylene extraction resistance and additive loss through volatilization have been discussed.
Stabilization of Halogenated Polymers
From an industrial point of view the only halogenated polymers which require some sort of stabilization to be commercially useful are those containing chlorine. Fluorinated polymers such as poly tetra fluoro ethylene or poly (vinyl fluoride) ordinarily require no remedial or corrective treatments.
The following discussion deals for the most part with the degradation and stabilization of poly (vinyl chloride) since this polymer has been studied in greater depth than any of the other chlorine containing polymers. The knowledge which has been gained from studies related to poly (vinyl chloride) has also been applied with some success to other material similar to structure to poly (vinyl chloride) and which generally degrade in a like manner. Thus the polymer industries have been able to develop and use successfully such materials as poly (vinylidene chloride) chlorinated polyethylene chlorinated polypropylene copolymers of vinyl chloride with such other monomers as vinyl acetate vinyl alkyl ethers maleic anhydride and its esters acrylonitrile ethylene and propylene. A post chlorinated poly(vinyl chloride) series of compounds designed and used primarily in applications at temperatures above which poly(vinyl chloride) is not usable (eg in hot water pipes) has been successfully commercialized. The stabilization of each of these materials follows the general precepts of poly (vinyl chloride) stabilization and also takes into account the different types of formulations and processing conditions to which these diverse chlorinated polymers are exposed.
Structure and Degradation of Poly (vinyl Chloride). The polymerization of vinyl chloride monomer in common with other vinyl monomers proceeds by a free radical mechanism involving the usual steps of initiation propagation and termination. Poly(vinyl chloride) is formed in a regular head to tail manner(I) although it had been previously suspected that the structure was random containing both head to head and tail to tail units in addition to head to tail segments.
Some chain branching (radical transfer to polymer) occurs during the propagation steps leading to two possible structures (3 and 4) at the branch points both of which are believed to be present in commercially prepared poly(vinyl chloride) whether by suspension (as is most common) emulsion mass or solution polymerization.
Termination is responsible for a number of different end group structures as well as certain irregular groups found within poly(vinyl chloride) molecules which have been terminated by coupling. When polymerization of two growing radical chains is terminated by direct coupling the resultant polymer molecule has an initiator fragment at each end and a dichloro structure somewhere within the chain at the point of coupling (eq.1).
In addition to coupling termination also occurs by disproportionation and by chain transfer to monomer polymer initiator solvent or to any of the many other ingredients usually present in a polymerization reaction. Termination by disproportionation is illustrated by equation 2.
SULFURIC ACID
Sulfuric acid a strong acid is oily viscous water white nonvolatile liquid. It absorbs water from the atmosphere. A drop of it on the skin causes a severe burn. It is made in large volume by the chemical industry. It is used as a solvent a dehydrating agent a reagent in chemical reactions or processes an acid a catalyst an absorbent etc. The concentrated acid is usually stored in steel tanks. The dilute acid may be stored in lead lined or plastic tanks. It is used in very dilute concentrations and as strong fuming acid. It is often recovered and reused. After use in some phases of the explosives petroleum and dye industry it is often recovered in a form unsuitable for reuse in that industry but suitable for use in another industry. It is a versatile useful acid and has been called the work horse of the chemical industry.
USES OF SULFURIC ACID
Sulfuric acid is one of the most widely used of all manufactured chemicals and its rate of production has long been a reliable index of the total chemical production and the Industrial activity of a nation. For 1969 in the United States the average per capita consumption of sulfur was 103.8 pounds of which most went into the manufacture of sulfuric acid. In the world the per capita consumption in 1969 was about 26 pounds.
The sulfuric acid consuming industries in the United States are listed in Table 1. Of these the largest consumer is the fertilizer industry which treats phosphate rock with sulfuric acid to produce super phosphate (a mixture of mono calcium phosphate and calcium sulfate) or crude (wet process) phosphoric acid. There is hardly an article of commerce which has not come into contact with sulfuric acid at one time or another during its manufacture or in the manufacture of its components. The consumption of acid in the various industries is undergoing constant change. Progress demands that manufacturers strive to decrease the consumption of acid per unit product manufactured. Progress also is continually turning up new uses.
Kinds of Acid
Sulfuric acid is marketed in the United States as a large tonnage product. It is made in numerous grades and strengths and shipments are made in both packaged containers and in bulk. It is produced in grades of exacting purity for one in storage batteries and for the rayon dye and pharmaceutical industries. It is produced to less exacting purity specifications for use in the steel heavy chemicals and fertilizer industries. Originally sulfuric acid was marketed in four grades known as chamber acid 50° Beaume (Bé) tower acid 60° Bé oil of vitriol 66° Bé and fuming acid. At present it is marketed in the strengths listed in Tables 2 and 3.
The Manufacture of Sulfuric Acid
History. Sulfuric acid is formed in nature by the oxidation and chemical decomposition of naturally occurring sulfur and sulfur containing compounds. It is formed by the weathering of coal brasses or iron disulfide discarded on refuse dumps at coal mines. It is formed by bacteria in hot sulfur springs. It is formed in the atmosphere by the oxidation of sulfur dioxide emitted form the combustion of coal oil and other substances. It is formed by chemical decomposition resulting from geological changes.
In ancient times sulfuric acid was probably made by distilling niter (potassium nitrate) and green vitriol (ferrous sulfate heptahydrate). Weathered iron pyrites were usually the source of the green vitriol. About 1740 the acid was made in England by burning sulfur in the presence of saltpeter (potassium nitrate) in a glass balloon flask. The vapors united with water to form acid which condensed on the walls of the flask. In 1746 the glass balloon flask was replaced by a large lead lined box or chamber giving rise to the name chamber process. In 1827 GayLussac and in 1859 Glover changed the circulation of gases in the plant by adding towers which are now known as GayLussac and Glover towers. These permit the recovery from the exit gases of nitrogen oxides which are essential to the economic production of chamber acid. Today most sulfuric acid made in the United States is produced by the contact process based on scientific technology developed about 1900 and thereafter.
BASF built a successfully operating contact plant in the United States in 1898. General Chemical erected a pyriteburning contact plant using the Herre shoff furnace in the United States in 1900.
Development of the Sulfuric Acid Industry in the United States
The manufacture of sulfuric acid has been a basic industry in the United States for many years. It has been made by two well established methods the chamber process and the contact process. Initially the production of acid was concentrated on the Eastern seaboard. After the Civil War the industry spread to the west and into the South. Between 1899 and 1904 the number of acid manufacturers increased rapidly in Ohio and Illinois and just before the turn of the century there was great activity in the South as a result of the discovery of phosphate deposits in South Carolina and Florida and the development of the phosphate fertilizer industry.
Over the years the number of contact plants has increased while the number of chamber plants has decreased. During the mid 1940s the number of chamber plants was approximately equal to the number of contact plants. Today the contact plants are in the majority and the trend is toward contact plants of larger and larger capacity. Contact plants producing 2000 tons of acid a day in a single train are not unusual and they may become larger in the future.
Consumers confronted with the task of disposing of waste or spent acid find it advantageous to arrange with an independent producer to exchange the waste acid for fresh acid. Methods have been developed which permit such producers to reprocess the waste acid and obtain a product of virgin quality. Also they can operate centrally located plants of large size which can produce acid at a much lower cost than can be realized in the small plant needed by the average user. The end use of sulfuric acid more than any other factor determines the location of sulfuric acid plants. Data on the production of acid in the United States are listed in Table 4.
The Chamber Process for Making Sulfuric Acid
The chamber process for making sulfuric acid at first glance appears to be a rather simple process requiring simple equipment. The reactions involved however are not simple and even today there is disagreement among experts as to just what does take place in the chambers. All agree however that the oxidation of sulfur dioxide to sulfuric acid in the chambers is not directly effected by oxygen but that intermediate compounds involving nitrogen oxides are formed and that the reaction is really a cyclic process involving the alternate formation and decomposition of the intermediate compounds. Many operators say that operation of a chamber plant is more an art than a science.
In the chamber process chemical reactions take place between sulfur dioxide oxygen nitrogen oxides and water vapor. A series of intermediate compounds are formed which decompose to yield sulfuric acid and nitrogen oxides. The overall effect is that the sulfur dioxide is oxidized to sulfur trioxide which combines with water vapor to form sulfuric acid. The nitrogen dioxide acts as the oxidant and is reduced to nitric oxide which must be continually reoxidized by oxygen in the air. When all the sulfur dioxide has been consumed the nitrogen oxides appear as equal moles of nitric oxide and nitrogen dioxide in which ratio they are absorbed in sulfuric acid in the GayLussac tower as nitrosylsulfuric acid. The solution of nitrosylsulfuric acid (nitrose) from the GayLussac tower is pumped to the denitration (Glover) tower where heat releases the nitrogen oxides for reuse in the cycle. In the Glover tower the denitrated sulfuric acid is concentrated to 60°Be. Part of this acid is returned to the GayLussac tower for recovery of the nitrogen oxides from the exit gases. The balance is available for use or sale.
If elemental sulfur is burned a substantially clean gas containing 8 to 11 per cent sulfur dioxide by volume is formed. If sulfide ores or other sulfur bearing materials are burned a gas containing about 7 per cent sulfur dioxide is produced. This gas is usually contaminated with varying amounts of dust metallic fumes water and other gaseous impurities which must be removed at least in part. The sulfur dioxide gas of suitable purity is then conducted to the Glover tower of the chamber plant where it meets a countercurrent flow of sulfuric acid 50° to 54° Be containing nitrosylsulfuric acid. The hot gas concentrates the acid to 60° Be and decomposes the nitrosylsulfuric acid releasing the oxides of nitrogen. The gas leaving the Glover tower and containing sulfur dioxide nitrogen oxides of nitrogen water and an excess of oxygen then enters the lead chambers.
The concentration of sulfur dioxide gas in the system must be controlled to allow an excess of oxygen throughout the system. Nitrogen oxides obtained by adding nitric acid by the decomposition of sodium nitrate reacted with sulfuric acid or in more recent plants by burning ammonia are added to the chambers. A fine spray of water is also added. The sulfuric acid produced condenses on the walls of the lead chambers. The unreacted gas flow to the GayLussac towers. Here the nitrogen oxides in the gas are recovered by absorption in a countercurrent flow of 60° Be acid to form nitrosylsulfuric acid. This acid containing the nitrosylsulfuric acid is then pumped to the Glover tower and the cycle repeated. Up to 50 per cent of all the acid produced in the plant is formed in the Glover tower.
The acid produced is drawn from the pans in the bottom of the chambers or from the Glover tower. That produced in the first and intermediate chambers usually contains from 63.66 to 68.13 per cent sulfuric acid (52° to 54° Be). That which is produced in the last chamber is weaker and contains about 59.32 per cent sulfuric acid (48° Be). That which flows from the Glover Tower contains from 75 to 85 per cent sulfuric acid.
Chamber acid may contain small amounts of impurities such as oxides of nitrogen arsenic and selenium and sulfates of iron copper zinc mercury lead and antimony depending in part on the kind of sulfur bearing material used in making the sulfur dioxide gas charged to the plant. In some applications as in the manufacture of fertilizers these impurities are not harmful. In other they are harmful and must be removed.
The Contact Process
The basic features of the contact process for making sulfuric acid as practiced today were described in a patent issued in England in 1831. It disclosed that if sulfur dioxide mixed with oxygen or air is passed over heated platinum the sulfur dioxide is rapidly converted to sulfur trioxide which can be dissolved in water to make sulfuric acid. The practical application of this disclosure however was delayed. An understanding of the complex reactions occurring in the gas phase over the catalyst required the development of that branch of physical chemistry known as chemical kinetics and also the development of that branch of engineering known as chemical engineering. A demand for acid stronger than that which could be produced readily by the chamber process stimulated this development. The success of this process for making sulfuric acid led to the development of other catalytic processes for making many of the synthetic chemicals known today.
The heart of the contact sulfuric acid plant is the converter in which sulfur dioxide is converted catalytically to sulfur trioxide. Over the course of the years a variety of catalysts have been used including platinum and the oxides of iron chromium copper manganese titanium vanadium and other metals. The first catalyst used was platinum. It proved to be extremely sensitive to poisons such as arsenic compounds present in small amounts in some sources of sulfur dioxide. The successful development of the contact process depended in part on the recognition of the existence of catalytic poisons and in devising methods for their removal. Platinum and iron catalysts were the main catalysts used prior to World War I. At present vanadium catalysts in various forms combined with promoters are generally used.
A number of different plant designs have been developed for the efficient production of sulfuric acid by the contact process. These in the United States are often referred to by the name of the builder or designer e.g. a Chemical Construction (Chemico) a LeonardMonsanto or a WellmanLord plant. In Europe and other parts of the world Lurgi Gesellschaft fur Chemie and Huttenwesen mbH Chemiebau Dr. A. Zieren GmbH & Co. KG and SimonCarves Chemical Engineering Ltd. Are noted for designing and building contact sulfuric acid plants. Contact plants are also classified according to the material used in the production of the sulfur dioxide charged to the plant e.g. sulfur hydrogen sulfide gypsum iron pyrites smelter gas or spent and sludge acids.
Dithiocarbamates
Dithiocarbamates are commercially important class of substances derived from amines and carbon disulfide. They possess the common functionality and are formally derivatives of dithiocarbamic acid. Dithiocarbamates of commercial importance may be divided into three categories dithiocarbamate salts neutral dithiocarbamate esters and thiuram disulfides.
These compounds are widely used in analytical chemistry as agricultural fungicides and in rubber technology. Despite a certain commonality of application the chemistry both preparative and analytical of each class is best considered individually.
DIOTHIOCARBAMIC ACID SALTS
Metal salts and, or chelates of dithiocarbamates are perhaps the most important dithiocarbamates in a commercial sense. They are utilized mainly as pesticides and as accelerators in rubber vulcanization. Some use has been made of the strong metal binding properties of dithiocarbamates to effect separation identification and determination of various metal ions. Dithiocarbamate salts are also used in the preparation of thiuram disulfides and dithiocarbamate esters.
If carbon disulfide is added to an ethanolic solution of ammonia colorless crystals of ammonium dithiocarbamate separate after a time. Careful acidification of the salt permits isolation of the free thiocarbamic acid. The latter is however extremely unstable and upon heating reverts back to ammonia and carbon disulfide
These reactions hold also for primary and secondary amines and this is the method by which almost all dithiocarbamate salts are prepared
One mole of alkali may be substituted for part of the amine in which case the metal dithiocarbamate salt is obtained
Acidification also liberates the unstable free acids which easily disintegrate into carbon disulfide and amine. Ethylenediamine or similar aliphatic diamines yield mixtures of monomeric and polymeric salts or bisdithiocarbamate salts depending on the amount of carbon disulfide available at the instant of reaction
A great many heavy metal dithiocarbamates are known. They are often highly colored and monionic in character being in fact strongly chelated. Preparation is effected simply by the addition of a solution of the heavy metal as the chloride or sulfate to a solution of an ammonium or alkali metal salt of the dithiocarbamine acid preferably with exclusion of oxygen. The heavy metal salts are sparingly soluble in water being more soluble in organic solvents such as chloroform carbon tetrachloride or ethyl ether. They can be crystallized from solvents such as benzene and petroleum ether.
A great number of metal dithiocarbamate salts have been prepared and references to a substantial number are listed by Thorn. The salts vary widely from completely ionic salts to covalent chelates. Various physicochemical properties of these materials have been summarized. Some physical property data for metal dithiocarbamates derived from simple dialkylamines are shown in Table 1. Some of commercial dithiocarbamates along with their more frequently encountered trade designations are listed in Table 2.
Analysis of Dithiocarbamate Salts
A variety of methods have been devised for the detection and determination of dithiocarbamate salts the approach found most useful involves decomposing the dithiocarbamates to the amine and carbon disulfide with hot mineral acids.
Acid Hydrolysis. In general dithiocarbamates are smoothly decomposed by hot dilute mineral acid to yield carbon disulfide and the corresponding amine either of which may be determined.
Decomposition by route A is said to be quantitative when boiling dilute sulfuric acid is used and yields the anticipated two moles of carbon disulfide per mole of ethylene bisdithiocarbamate. Route B is a slower reaction favored by the use of cooler acid. If this reaction occurs 50% of the available sulfur which would otherwise have been evolved as carbon disulfide is fixed as ethylenethiourea plus hydrogen sulfide and a low result is obtained.
The conditions for the acid digestion have been investigated by a number of workers mostly with a particular compound or type of compound in mind and with attention being paid to acid concentration. Generally water soluble dithiocarbamates decompose rapidly in sulfuric acid and only a short digestion time is required. Digestion problems and consequent erratic results are most often encountered in the analysis of water insoluble heavy metal dithiocarbamates or in the analysis of pesticide formulations in which copper salts (copper oxychloride) are present.
Lowen first suggested that the analysis of manganese ethylenebisdithiocarbamate could be markedly improved by the addition of ethanol to the digestion mixture. Subsequently Levitsky and Lowen used a 34% tetrasodium ethylenediaminetetraacetate solution. This complexing agent has also been recommended by Pease and its use has subsequently been quite generally accepted. Rosenthal et al. have used 85% phosphoric acid to disperse and digest dithiocarbamates. Ethylenethiourea was found to interfere but it could be removed from the sample zinc ethylenebisdithiocarbamate by a preliminary washing with 13 acetic acid. Hilton used an ethanolphosphoric acid mixture. More recently Roth used a pyridinephosphoric acid mixture for the digestion. Although this mixture is claimed to be generally applicable to dithiocarbamate and thiuram analysis collaborative studies indicate that with respect to dithiocarbamates at least it is less reliable than the socalled Clarke modified by the addition of tetrasodium EDTA.
Gardner employ zinc sulfide to tie up copper. If the zinc sulfide is finely divided results obtained when phosphoric acid is used for decomposition are similar to those obtained in the absence of copper. Bonnet has attempted to remove interfering copper by dissolving it in ammonium hydroxide. The method is not entirely satisfactory and correction factors must be used. Del Re and Fontana decompose formulations containing copper oxychloride by digestion with anhydrous copper sulfate in an aprotic medium of dimethyl sulfoxide and carbon tetrachloride. Other methods such as decomposition with ferricyanic acid or sulfuric acid plus potassium ferricyanide in aqueous solution have also been suggested as a means of overcoming the copper problem.
Xanthate Method. This is the most common technique of measuring the carbon disulfide formed in the acid decomposition of dithiocarbamates. The carbon disulfide is collected in alcoholic potassium hydroxide forming the xanthate and then determined iodimetrically.
Hydrogen sulfide formed by acid digestion of other sulfurcontaining materials interferes. To eliminate errors due to sulfide a scrubber solution of lead acetate or a cadmium salt is placed between the decomposition vessel and the carbon disulfide absorber. Hydrogen cyanide if formed in the hydrolysis mixture also produces high results. Carloni has recommended the use of a silver nitrate absorber in addition to the lead acetate scrubber to eliminate this interference. Sulfide ion produced by hydrolysis of the xanthate is also a source of error
Any sulfide formed will consume two equivalents of iodine per mole as opposed to the one equivalent required by the xanthate.
Matuszak has pointed out that for best results the methanolic potassium hydroxide solution must be fresh and that the xanthate solution should be cooled to 0°C and neutralized to the phenolphthalein end point with acetic acid before titrating with aqueous iodine in order to increase the permanency of the starchiodine titration. Cooling has an additional advantage in that it prevents oxidation of xanthate to dixanthogen. The latter reaction can also be avoided by using nitrogen or helium instead of air to sweep the carbon disulfide forward. Roth titrate the methyl xanthate solution with methanolic iodine.
The apparatus assembly is shown in Figure 1. It consists of a digestion flask and a reflux condenser to the top of which are attached one or two hydrogen sulfide scavenge traps followed by the methanolic potassium hydroxide scrubber for collecting carbon disulfide. Air is aspirated through the system to entrain the gaseous products through the scrubber train. The following generalized procedure taken from methods of Bontoyn is suitable for most commercial dithiocarbamate salts including manganese and zinc ethylenebisdithiocarbamates.
Procedure
Weigh not more than 5 g of the sample containing 0.10.3 g of dithiocarbamate and place in a dry reaction flask. Assemble the apparatus as shown in Figure 1. Charge each lead acetate trap with about 20 ml of 10% lead acetate solution. Charge the alkaline scrubber with about 50 ml of 2 N methanolic potassium hydroxide solutions and cool with an icewater bath. Equip the reaction flask with a 50 ml additional funnel and a small magnetic stirring bar. Charge approximately 10 ml of 34% tetrasodium ethylenediaminetetraacetate solution through the addition funnel. (For dithiocarbamates other than manganese or zinc ethylenebisdithiocarbamate EDTA need not be used. With compounds such as iron (III) dimethyldithiocarbamate having some water repellency addition of a trace of wetting agent is permissible). Stir the sample with the EDTA solution for 1 min. Commence careful aspiration of air through the reaction flask and then add 50 ml of hot 95100ºC 4N sulfuric acid rapidly through the addition funnel. Begin heating the flask to initiate boiling. As the reaction proceeds adjust the system so that rates of boiling and aspiration are almost in equilibrium producing only a very slow rate of bubbling through the methanolic potassium hydroxide solution (approximately 50 ml air/min). Continue a brisk efflux for 1.5 hr (a lesser time is required for water soluble or more easily digested dithiocarbamates).
Disconnect the cold potassium hydroxide absorber and quantitatively transfer the contents to a 500ml flask. Add one or two drops of phenolphthalein indicator just neutralize with acetic acid added from a buret and then add three drops in excess. Next with continuous stirring titrate the solution immediately with standardized 0.1 N iodine solution to the starch end point. Determine the solution blank by titrating a corresponding mixture of methanolic potassium hydroxide solution water phenolphthalein and acetic acid.
Spectrophotometric Methods. Hilton has determined carbon disulfide in dimethylamine using the thiocarbamic acid ultraviolet maximum at 287 nm. Xanthate salts also absorb strongly in the ultraviolet and hence can be determined by ultraviolet spectroscopy rather than by iodimetry. Nevertheless the most popular spectrophotometric method is based on absorption of the carbon disulfide in an ethanolic solution of copper acetate containing triethanolamine and diethylamine. This solution is known as the Viles or the DickinsonViles reagent. This method is best suited for determining small amounts of carbon disulfide such as obtained from dithiocarbamate residues in food crops. Its application for this purpose was first published by Clarke et al. and by Lowen. Subsequent refinements have been made by Pease.
The wavelength of maximum absorption of the copper (II) diethyldithiocarbamate chelate is 425 nm. Measurement at 440 nm 435 nm 430 nm 400 nm and 380 nm has also been recommended. The 11 complex is favored by high copper concentration which results in a high base line due to excess blue Cu2. Cullen therefore recommends limiting the copper iron concentration to promote 12 complex formation and use of the 435 nm maximum. He observed that a Cu to CS2 ratio of 0.5 was optimum but within the limits 0.3 and 2.5 at least 85% of the maximum absorption could still be attained.
The following procedure is applicable to residues of dithiocarbamate fungicides such as iron (III) and zinc (II) dimethyldithiocarbamate and disodium manganese (II) and zinc(II) ethylenebisdithiocarbamate. It is advisable to standardize the method with untreated crop samples to which known amounts of fungicides have been added. Recoveries ranging from 0.1 to 7.0 ppm have been demonstrated. The apparatus shown in Figure 2 is similar to that used in the xanthate method but modified for the larger sample size required. A lead acetate trap is used to remove hydrogen sulfide.
Procedure
Prepare Viles reagent by dissolving 0.05 g of copper (II) acette in 25 ml of water and then adding 975 ml of ethanol 1 ml of diethylamine and 20 ml of triethanolamine. Dissolve 30 g of neutral lead acetate Pb(OAc)2.3 H2O in water and dilute to 100 ml.
Dice the crop sample or otherwise subdivide it into ¼ in. or smaller cubes and weigh a representative amount containing 20160g of dihiocarbamate. Transfer the sample to the digestion flask. Add 200 ml of water or for manganese or zinc ethylenebisdithiocarbamate 200 ml of N disodium EDTA solution. Place 10 ml of lead acetate solution in the first absorption tower and 12.5 ml of Viles reagent in the second tower and assemble the apparatus as shown in Figure 2 leaving the vacuum source disconnected. Heat the contents of the reaction flask to just short of boiling (8590ºC). Apply gentle vacuum then cautiously add 40 ml of boiling 10 N sulfuric acid through the dropping funnel. For manganese or zinc ethylenebisdithiocarbamate use 60 ml of hot acid. Reflux the mixture for 45 min.
After digestion disconnect the apparatus and drain the contents of the Viles reagent trap into a 25 ml volumetric flask. Wash the column with several 34 ml portion of ethanol and add the washings to the volumetric flask. Dilute to volume with ethanol and mix thoroughly. Determine the absorbance at 380 nm vs a reference solution prepared by diluting 12.5 ml of Viles reagent to 25 ml with ethanol.
Prepare standard solutions as follows. Dissolve 0.04 g of iron or zinc dimethyldithiocarbamate in 100 ml of chloroform and dilute a 5 ml aliquot to 100 ml chloroform. For disodium ethylenebisidithiocarbamate use water as the solvent and for manganese or zinc ethylenebisdithiocarbamate use N tertasodium EDTA solution. Use 1.0 2.0 3.0 5.0 and 8.0ml a liquots of the appropriate standard solution for preparation of the calibratrion curve and carry through the method as given for the samples. Remove chloroform solvent by evaporation at room temperature under a stream of nitrogen before acid decomposition.
Gas Chromatography. Bighi have studied the acid decomposition of pure and commercial dithiocarbamates by gas chromatography. Gaseous products were swept from the digestion flask by a slow stream of helium passed through two drying traps containing concentrated sulfuric acid and hydrogen sulfide and carbon disulfide were condensed in a liquid air trap of special design. Carbon disulfide was determined using the following operating conditions with a retention time of about 4 min.
Hydrogen sulfide plus carbon disulfide are determined on a column packed with 25% triceresyl phosphate on Celite C 30/60 mesh 22 at 20ºC. The retention times are about 5 min for hydrogen sulfide and 45 min for carbon disulfide. Less than 5g of carbon disulfide can be determined.
Determination of Amine Acid decomposition of a dithiocarbamate salt in addition to producing carbon disulfide liberates the amine portion of the molecule which remains in the aqueous phase as nonvolatile amine salt. As a practical matter only the steam volatile lower aliphatic amines are conveniently measured quantitatively. The aqueous residue is made basic amines are distilled and determined by titration with acid. Alternatively the amine may be treated with carbon disulfide and Cu (II) ion to produce the copper dithiocarbamate which can then be measured colorimetrically.
Brock and Louth have identified accelerators and antioxidants in compounded rubber products by decomposing the dithiocarbamate with N hydrochloric acid making the solution alkaline and identifying the distilled amine as its hydrochloride by xray diffraction methods. Zijp determined amine hydrochlorides obtained in a similar manner by paper chromatography.
Ethylenediamine and other diamines not sufficiently volatile to be removed by steam distillation can be isolated by vacuum distillation. Bighi nd Penzo have determined ethylenediamine in a dithiocarbamate residue by a chlorimetric measurement of its copper compled at 550 nm.
In certain cases particularly with the watersoluble dithiocarbamates decomposition in the presence of a known excess of acid followed by back titration with base constitutes a simple method of assay. When the acid decomposed residue is titrated with alkali the neutralization proceeds stepwise. At first the excess of strong acid is titrated a second step involves neutralization of the protonized amine. The last pH change corresponds to addition of excess base. When dithiocarbamate salts of weak bases are titrated an additional neutralization takes place.
Critchfield has devised an ingenious method for determining primary and secondary amines in the presence of tertiary amines. In this method an excess of carbon disulfide is caused to react with the primary or secondary amine in an essentially nonaqueous medium such as isopropyl alcohol or pyridineisopropyl alcohol mixture. The dithiocarbamine acid formed in the reaction is then titrated with sodium hydroxide solution using phenolphthalein potentiometric indiction.
Spectroscopy. Data on the ultraviolet absorption of various simple dithiocarbamate salts are summarized in Table 3. For water soluble dithiocarbamates the ultraviolet absorption at or about 285 nm can be measured as was done by Bode in his study on the stability of sodium diethyldithiocarbamate at various pH values. Kress determined zinc diethyldithiocarbmate by its absorption at 262 nm in ethyl ether solution.
Many of the heavy metal salts of dithiocarbamic acids absorb in the visible region. Copper (II) dimethyldithiocarbamate for example has an absorption band at 425 nm. Ultraviolet spectra and simultaneous determination of the copper nickel and cobalt salts of diethyldithiocarbamic acid in carbon tetrachloride have been recorded by Chilton. Morrison detected the presence of dithiocarbamates in rubber by a color reaction with copper chelate. A colorimetric method for dithiocarbamate residues which obviates degradation of the sample and distillation has been proposed by Kerssen. The residue on glass plates or leaves is removed with a detergent and phosphate buffer and copper sulfate are added to the washings. The copper salt is extracted into an organic solvent and estimated colorimetrically.
The infrared spectra of many dithiocarbamates have been investigated by Chatt. Nevertheless little use of infrared spectroscopy has been made in the analysis of dithiocarbamates although the method has been of considerable use in determining the structure of the various metal coordination compounds. Fisher has devised infrared methods for determining iron dimethyldithiocarbamate zinc ethylene bisdithiocarbamate and tetramethylthiuram disulfide in mixtures of various nonthiocarbamate pesticides.
Liquid Chromatography. Various chromatographic techniques have proved useful in separating and in some cases identifying dithiocarbamate salts. Paper chromatography has been most frequently employed. Salts of the dithiocarbamic acids derived from amino acids ie (dithiocarboxyamino) carboxylic acids have been separated satisfactorily by paper chromatography by Jensovsky and by Zahradnik. The most suitable developing solvent found by Jensovsky was 37 2 N ammonium hydroxidepropanol using Whatman No. 4 paper. The spots were located by spraying with ammoniacal silver nitrate solution. Zahradnik and Kobrle used boraximpregnated Whatman No. 1 paper the developing solvent was 70525 propanol25% ammonium hydroxide 0.05 M sodium tetraborate.
McKinley and Magarvey resolved iron and zinc dimethyldithiocarbamate disodium manganese and zinc ethylenebisdithiocarbamate and tetramethylthuiram disulfide into two groups on fiber glass paper impregnated with formamide. The fungcides derived from dimethylamine moved readily with the solvent systems used whereas the dithiocarbamates derived from ethylenediamine remained relatively immobile. Chloroform petroleum ether and a mixture of nhexane and chloroform were used as mobile phases. The compounds were detected by spraying with 3% aqueous sodium azide solution subjecting the papers to iodine vapor and immediately spraying with 1% soluble starch solution.
Disodium ethylenebisdithiocarbamate and its air oxidation products ethylenethiuram monosulfide and ethylenethiourea have been separated by Thorn and Ludwig on Whatman No. 1 paper using 1203357 butanolethanolwater as the developing solvent. Again the spots were detected using the iodineazide reagent or Grotes reagent a solution of sodium nitroprusside reduced with hydroxylamine and oxidized with bromine. Some simple dithiocarbamates and thiuram sulfides have been separated by Lu using Whatman No. 2 paper impregnated with 2% acrylonitrilebutadiene copolymer in 21 benzeneacetone as stationary phase. Various organic solvents and water were used as the mobile phase in the ascending method and dilute acidified sulfate was used to detect the spots. Paper chromatographic techniques have been employed by Zijp to identify dithiocarbamate rubber accelerators and antioxidants. Here however the dithiocarbamic acid derivatives themselves were not subjected to chromatographic separation but rather the amine hydrochlorides obtained by acid decomposition of these compounds.
Manganese and zinc ethylene bisdithiocarbamates and their degradation productsethylenethiuram monosulfide ethylenethiourea and sulfur-have been separated by thinlayer chromatography on silica gel DF5 and silica gel G plates. Three solvent systems were utilized (a) immobile 5% formamide in acetone mobile chloroform (b) 1203357 butanolmethanolwater (c) immobile 5% paraffin oil in ethyl ether mobile dimethylformamide. Iodineazide zincon (2carboxy2hydroxy5sulfoformazyl) dithizone and potassium ferricyanideferric chloride reagents were employed as detecting reagents.
Organic Acids
The microbiological production of organic acids represents one of the earlier areas of fermentation necessary for the accumulation of information which made possible the large scale production of antibiotics and other microbial products of more recent date.
Citric Acid
Citric acid is one of the most important organic acids used in foods beverages and pharmaceuticals. During the past few years it has also become important as an organic intermediate.
Citric Acid
Citric acid was first isolated from lemon juice in 1784 by Scheele. In 1917 Currie of the U.S. Department of Agriculture found citric acid could be produced microbiologically by using Aspergillus niger grown on a sugermineral salts solution. Since then many other microorganisms have been shown to produce the citric acid however A. Niger has always given the best result in industrial production. Citric acid may be fermented either by using shallow pans or by employing a submerged or deep fermentation process with aeration.
Sucrose in the form of cane or beet molasses is the principal source of sugar. A 1220 per cent sugar solution is normally used along with mineral supplements. The duration of shallow pan fermentation is 710 days at 2628ºC. Submerged fermentation periods are shorter but yields are less. On shallow pans yields on the sugar used may be 9095 per cent while the submerged process normally runs 7578 per cent. The citric acid is recovered as the calcium salt and treated with sulfuric acid to precipitate calcium sulfate which is removed. Citric acid crystallizes upon concentrating the resulting solution. Some oxalic acid is recovered as a byproduct of the citric fermentation process.
Recently it has been shown that certain strains of Candida (a yeast) can produce citric and isocitric acid from nparaffins or carbohydrates. The impact of this research development may affect the future production method.
Current production of citric acid in the U.S. is approximately 130000000 lbs as it continues to be the leading food acidulant.
Gluconic Acid
Gluconic Acid is produced by the oxidation of the aldehyde grouping of glucose.
Gluconic acid may be prepared from glucose by oxidation with a hypochlorite solution by electrolysis of a solution of sugar containing a measured amount of bromide or by fermentation of glucose by molds or bacteria. The latter method is not preferred form an economic standpoint. The most important microorganisms used are Aspergillus niger and Acetobacter suboxydans grown on a glucosesalt solution in deep tank fermentation. Yields as high as 90 per cent on the sugar consumed have been reported. Gluconic acid is marketed in the form of several crystalline metal salts 50 per cent aqueous acid and the deltalactone. Calcium gluconate is frequently used as a nutritional calcium source because of its solubility. The sequestering properties of sodium gluconate particularly for Ca and heavy metal ions in strong caustic solution make it useful in cleaning operations.
Acetic Acid
Acetic acid in the form of vinegar (by law 5 per cent acetic acid) is a widely used food adjunct. Vinegar is produced by the oxidation of ethanol by bacteria of theAcetobacter genus. In the food industry many vinegar types are classified on the basis of the source of alcohol. Most vinegar is made from apple cider a yeast converts the sugar to ethanol and the acetification is accomplished by Acetobacter aceti strains.
Ethanol may also be converted into acetic acid by catalytic oxidation at high temperatures but synthetic acid cannot be used in foods.
Taconic Acid
Itaconic acid is an unsaturated dibasic acid which may be used for the preparation of resins or surfaceactive agents or in the manufacture of synthetic organic chemical compounds. Its esters may be polymerized.
Itaconic acid may be produced by either a shallowpan or a deeptank fermentation process by growing Aspergillus terreus or A. itaconicus on lactose glucose or molassessalt media. Fermentation of solutions of 2025 per cent glucose gives yields equivalent to 5070 per cent based on the sugar consumed.
Kojic Acid
Kojic acid was first discovered in Japan in 1907 by Saito it was a byproduct of the fermentation of steamed rice by Aspergillus oryzae. Various other investigators have found that numerous species of Aspergillus and some Acetobacter bacterial strains produce kojic acid. In 1955 it was first produced on a commercial scale by a fermentation process. Kojic acid is an acid weaker than carbonic. It is reactive at every position and forms a number of products.
One of its major uses has been for the manufacture of maltol and ethyl maltol widely used in foods as flavorenhancing agents. Chemically the CH2OH group is oxidized to COOH (comenic acid) which is removed by pyrolysis (pyromeconic acid). The 14pyronenucleus is reactive at the 5position with formaldehyde or acetaldehyde and the reduction of the respective aldehydes give maltol or ethyl maltol.
OTHER KETOGENIC FERMENTATIONS
Sorbose
Sorbose fermentation the first bacterial ketogenic fermentation discovered is one of the simplest. LSorbose is produced from the polyhydric alcohol sorbitol by the action of several species of bacteria of the genus Acetobacter. Sorbitol is made by the catalytic hydrogenation of glucose. The most commonly used microorganism is Acetobacter suboxydans. Since this organism is very sensitive to nickel ions it is important that the medium and fermentor be free of nickel. The medium normally consists of 100200 grams per liter sorbitol 2.5 grams per liter corn steep liquor and antifoam such as soybean oil. The medium is sterilized and cooled to 3035ºC where about 2.5 per cent inoculum is added. The tank is aerated and sometimes stirred. Yields of 8090 per cent of the sugar used are commonly obtained in 2030 hours.
The only commercial use of Lsorbose is in the manufacture of ascorbic acid (vitamin C). The chemical steps in the conversion of sorbose to ascorbic acid involve the preparation of the diacetone derivative which is then oxidized the acetone groups are removed and the resultant 2ketoLgluconic acid is isomerized to the enediol with ring closure. This concept of using combined microbiological and chemical conversions has recently been applied with commercial success to the preparation of new steroid drugs.
2Ketogluconic Acid
2Ketogluconic acid may be produced by a bacterial fermentation involving various strains of Acetobacter and Pseudomonas. Selcted strains of Pesudomonas Fluorescens have been reported as giving the highest yields (up to 70 per cent) when glucose or gluconate is used in the medium is highly aerated processes. DGluconic acid is an intermediate in the oxidation of glucose to 2ketogluconic acid. 2ketogluconic acid is structurally related to both gluconic acid and glucosone and may be derived from both by oxidation. The 2ketogluconic acid is recovered as the calcium salt. The principle use of 2ketogluconic acid is as an intermediate in the preparation of Darabo greater than four or five units fails to enhance activity. An extensive treatement of anionic surfactants based upon nonionic materials can be found elsewhere.
Taurate Surfactants. In 1930 both H.T. Bohme A.G. and I.G. Farbenindustrie recognized that the weakness of soap was centered at the carboxyl linkage the former chose the alkyl sulfate route the latter esters of fatty acids. Subsequently because the fatty esters were too unstable for many purposes fatty amides which were taurine derivatives were developed. Igepon T (oleyl methyl taurate) was one of the first widely used surfactants which still have many applications.
It is of considerable interest that in the patent covering compounds of this type in which R1 R2 and R3 are branched or straight chain aliphatic cycloaliphatic or aromatic hydrocarbon groups or heterocyclic rings and in which R3 may be a sulfornic or sulfuric ester (these groups may have many forms) that over 1620000 variations are possible. Obviously only relatively few of this number have been synthesized and of these fewer still have been merchandized.
Nonionic Surfactants
As the name indicates these products are not ionic in nature and in contrast to sulfates sulfonates or phosphates their solubility generally depends on hydrogen bonding through a multiplicity of oxygen groups in the molecule. The most widely manufactured products are ethylene oxide adducts although mixed ethylene and propylene or butylenes oxide compounds are or can be produced.
Ethylene Oxide Adducts. One main requirement of the nonionic hydrophobe used for ethylene oxide addition is that it contains reactive hydrogen the most important hydrophobes are given in Table 1. As discussed under hydrophilhydrophobe balance these adduct are not single compounds but represent mixtures approximating Poisson distribution. Production of ethylene oxide adducts is not difficult but because of corrosion and explosion hazards the equipment is necessarily expensive and handling and storing raw materials and products requires considerable capital. Operating costs are largely dependent on the volume produced short chain ethylene oxide adducts can be produced in much shorter cycles than the more generally used longer chain products.
Shick has edited a very thorough volume concerning nonionic surfactants. Tall oil adducts probably comprised the largest single group of nonionics largely because of their relatively low cost. However they have been replaced by other nonionics for controlled lowsudsing detergents. The currently most used adducts are those derived from primary or secondary alcohols and from alkylphenols. Use of the fatty alcohol adducts as sulfates is discussed under Anionic Surfactants.
Polymeric Nonionics. Optimum polypropylene glycol molecular weights appear to lie between 800 and 2500 g. It is obvious that a large number of compounds varying marketdly in characteristics can be produced many of these have already been investigated. If instead of polypropylene glycol the central portion of the molecule becomes ethylenediamine block polymers having four hydrophilic tails can be made by reaction with ethylene oxide. Where the molecular weights of the polyoxiypropylenepolyoxyethylene compounds can approximate 10000 molecular weights of the ethylenediamine products can approach 27000.
New 11 types have been developed having over 90 per cent crude amide content achieved by an ester interchange of 1 mole DEA with 1 mole of fatty acid methyl ester under special synthesis conditions. The importance of these compounds lies in their detergent and foaming ability and in the fact that they act as foam boosters and stabilizers for dodecylbenzene sulfonates. In addition they are compatible with both anoinic and cationic surfactants are emollients can affect the viscosity of liquid detergents and are corrosion inhibitors.
Sugar Surfactants. Sugar is a desirably priced raw material on which a process of surfactant preparation has been based. The product is a sucrose fatty acid monoester (the monosterate for example) the eleven oxygen atoms of sucrose contributing about the same hydrophilic effect as a polyoxyethylene with the same number of oxygen atoms.
The process is typified by the following run
Three moles of sucrose one mole of methyl stearate and 0.1 mole of potassium carbonate (catalyst) are dissolved in dimethylformamide (or dimethyl sulfoxide). Potassium carbonate is a preferred catalyst because unlike a more alkaline catalyst (e.g. sodium methoxide) it will not take part in undesirable side reactions at high temperatures.
The reaction mixture is agitated heated at 9095ºC at 80100 mm of mercury for 912 hours. The methyl stearate reacts with the sucrose to give a sucrose monostearate and methanol. The later is stripped off.
After the solvent is distilled off and the product dried it contains about 45 per cent monostearate 12 per cent potassium carbonate and about 54 per cent unconverted sugar (because of the large excess used). The product can be used for many jobs as is. More likely however the economics of the situation will dictate that the sugar be recovered by further purification of the product.
The sugar can be removed by adding toluene as a solvent.
Conversions of over 90 per cent are claimed. An excess of sucrose produces the monoester best for detergent products while an excess of nonsugar ester yields the diester product which is superior for food applications. Raw material costs are a controlling factor due to variation in world supplies. Purification is also difficult. The flow sheet is shown in Fig. 2.
In the equipment shown in Figure 3 the reactants the solvent and the catalyst are placed in the reaction vessel (5) in which the conversion from nonsugar ester to sucrose monoester is carried out. The product alcohol and part of the solvent are striped from the system in a turbulentfilm evaporator (1). The product alcohol and solvent are fractionated in the packed reflux tower (2). The solvent is returned to the system through (4) while the alcohol is condensed in (3) and collected in vessels (7).
Recovery of the sucrose ester from the slurry containing unreacted sugar can be accomplished using xylene as the ester solvent since filtration is unsuccessful due to the sugar particle size. The xylene is then recovered by steam distillation and the sugar ester remains. Important to economic operation is minimization of diester formation (which is effected by controlling the water content during alcoholysis) recycling of unreacted sugar and recovery of DMF and xylene. Several licenses have been granted for this process.
Sorbitol Compounds. Sorbital may be produced by the hydrogenation of sugars such as glucose. Then this hexahydric alcohol may be reacted with ethylene glycol and the reaction product esterified to varying degrees with lipohilic fatty acids. Or sorbitol may be partially esterified with fatty acids and then these innerester sorbitol anhydrides may be further reacted with ethylene glycol.
