GLUES OF ANIMAL ORIGIN
Animal glues are essentially natural high-polymer proteins.
These organic colloids are derived from collagen which is the
protein constituent of animal hides, connective tissues, and
bones. There are two principal types of animal glues, hide and
bone, differing in the type of raw materials used. In both cases,
animal glue is obtained by hydrolysis of the collagen in the raw
material.
Animal glues find application in a wide range of industrial
uses. They are used in woodworking for such applications as
assembly, edge gluing, and laminating. In the paper industry,
they are used as sizing materials and as binders in paper coating,
and also for paper creping. Animal glues find wide use during
paper manufacture for the retention and recovery of paper fibers
and pigments.
The coated abrasive industry uses animal glues in the
manufacture of abrasive paper and cloth. Closely allied with the
coated abrasives is the use of animal glue in preparation of
compounds for coating wheels, discs, belts, etc.
Animal glues are widely used in the manufacture of gummed
papers and tapes and in paper and paperboard converting.
Animal glues and glue-based compounded products are used
in paper containers—set up and folding boxes, spiral and
convolute tube winding, and laminating. Applications in
bookbinding, magazine and catalogue production, and allied fields
include binding, casemaking, padding, looseleaf binders, and
various luggage and case covering applications.
Animal glues are employed as warp sizing, throwing, and dyeleveling
agents in the textile industry. They are used in the
match industry for match-head compositions. Other uses include
paper gaskets, cork compositions, rubber compounding,
compositions for printing, coating and graining rollers, mining,
ore refining, and metal plating.
Glue molecules consist of amino acids connected through
polypeptide linkages to form long-chain polymers of varying
molecular weights. In hot aqueous solution the glue molecules
take up random configurations of essentially linear form. A wide
range of molecular weights, varying from 20,000 to 250,000 have
been reported. Acidic and basic sites on the amino acid side
chains and terminal groups affect the interactions among the
protein molecules and water, and are believed to be responsible
for the gelation and rheological properties of animal glues.
Because of the presence of both acidic and basic functional
groups in the protein molecule, the molecules are amphoteric
and can bear either a positive or negative charge. Animal glues
can act either as acids or bases depending upon the pH in water
solution. In acidic solution, the protein molecules have an
overall positive charge and function as cations, in alkaline
solution the molecules are negatively charged and behave as
anions. The point where the net charge on the protein is zero
is known as the isoelectric point (IEP). The isoelectric point of
animal glues usually lies in the pH range of 4.5-5.6. Glues in
solution at pH values lower than their IEP have cationic
characteristics while they have anionic characteristics at pH
values above their IEP. Many properties of glue solutions, such
as viscosity, solubility, gel strength, and optical clarity, pass
through a maximum or minimum at this point.
Commercial animal glues are dry, hard, odorless materials
available in granular or pulverized form which vary in color from
light amber to brown. They may be stored indefinitely in the dry
form.
The density of dry animal glue is approximately 1.27 g/ml.
A moisture content range of 10-14% is considered normal for
the commercially dried product. Inorganic ash content, consisting
mainly of calcium salts, may vary from 2% to 6%. Hide glues
are generally neutral in water solution with a usual pH range of
6.5-7.5, and bone glues are slightly acidic with values in the pH
range of 5.5-6.5.
Animal glues are soluble only in water. They are insoluble
in oils, greases, alcohols, and other organic solvents. When
placed in cold water, the glue particles absorb water and swell
to form a spongy gel. When heated the particles dissolve to form
a solution. When the solution is cooled the glue forms an elastic
gel. This property is thermally reversible, and upon application
of heat the gel liquifies. The gelling or melting point of an animal
glue solution will vary from below room temperature to over 120ºF,
depending upon glue grade, concentration, and the presence of
modifiers.
Viscosity in solution and the gel-forming characteristic when
cooled are important properties of animal glues, especially in
adhesive and sizing or coating applications. These properties vary
with the degree of hydrolysis of the collagen precursor and have
a marked bearing on working properties. Animal glues are graded
as to viscosity (fluidity) and gel strength (stiffness of gel
formation) under standard conditions and are available in a wide
range of viscosities and gel strengths.
Animal glues are compatible with and may be modified by
such water soluble materials as glycerin, sorbitol, glycols, sugars,
syrups, and sulfonated oils to act as plasticizers and modify the
working properties of the glue. A degree of moisture resistance
and increase in the solution melting point of animal glues may
be imparted by the proper use of such materials as aldehyde
donors and metal salts.
Since they possess amphoteric properties, animal glues are
highly effective with suitable modification as colloidal flocculants
or suspending agents.
Methods of Manufacture
Both major types of animal glues are prepared by the
hydrolysis of collagen and differ mainly in the type of raw material
used and the manufacturing processes employed.
Hide glues are prepared by initially washing the raw material
with water, followed by curing in a calcium hydroxide (lime)
solution which conditions the collagen for subsequent glue
extraction by hydrolysis. The cured stock is then washed, treated
with dilute mineral acid, such as sulfuric, sulfurous, or
hydrochloric, for pH adjustment, followed by a water rinse. The
stock is then transferred to extraction kettles or tanks and is
heated with water to extract the glue. Several hot water
extractions are made until the glue is completely removed from
the stock.
Dilute glue solutions are filtered, concentrated by vacuum
evaporation and dried. The dry product is ground to the desired
particle size.
Bone glues are made from the collagen occurring in animal
bones. Green bone glues are prepared from fresh bones and
extracted bone glues from bones which have been degreased
prior to processing for glue.
Both types of bone glues are initially conditioned by cleansing
with water and/or dilute acid solutions. The glue is extracted
in pressure tanks with a series of steam and hot water
applications. The dilute glue solutions are filtered or centrifuged
to remove suspended particles and free grease, followed by
vacuum evaporation, drying, and grinding.
Animal glues contain preservatives added during
manufacture to provide adequate protection under conditions of
normal usage and may contain foam control agents, depending
upon the end use.
Commercial Grades and Specifications
Animal glues are graded according to standard methods
developed and adopted by the National Association of Glue
Manufacturers (NAGM). Grades are based on gel strength and
viscosity values.
It is common to market animal glue under brand names or
grade designations identified by the midpoint gram values shown
in Table 1 or by National Association of Glue Manufacturers’
grade number.
Table 2 lists the typical properties of hide and bone types
of glues.
Viscosity of animal glue solutions vary over a wide range,
depending upon grade, concentration, and temperature. Table 3
lists typical viscosity values at 140ºF for a range of dry glue grades
at various concentrations.
CASEIN GLUES AND ADHESIVES
Introduction
Casein is milk protein, obtained from skimmed milk by
precipitation with sulphuric, hydrochloric or lactic acid, to a pH
of about 4.5.100 kgs. of milk usually yields about 3 kgs. of casein.
The precipitated casein is filtered, washed thoroughly, ground
and screened to get 20 mesh or finer product for glue
manufacture. Commercial casein contains about 80-90 per cent
of protein, 1-4 per cent ash, 0-1-3 per cent of butter fat, 7-10
per cent moisture, 0-4 per cent lactose and 0-3 per cent acids
expressed as lactic acid. The composition and amounts of
impurities depend on, among other factors, the method of
manufacture. Reunet casein is not, as it is, suitable for glue
manufacture due to high ash content.
Properties
Casein is an off white powder. Its molecular weight is about
13000-19,000. It is insoluble in water at its isolectric point pH
4.6; the solubility increasing acidity or alkalinity, in the latter
it is more readily soluble. Fixed alkalies like sodium hydroxide,
remain in the glue line as sodium caseinate, a soluble salt while
calcium hydroxide on which water-resistant wood glues are
based, forms insoluble calcium caseinate in the glue line.
Similar insoluble caseinates are formed by zinc, chromium and
aluminium salts etc. Casein powder has a shelf life of above 1
year at 20ºC.
Casein adhesives are unsuitable for outdoor use although
they are more resistant to temperature changes and moisture than
other water-based adhesives. Resistance to dry heat up to 70ºC
is good, but under damp conditions the adhesives lose strength
and are subject to biodeterioration. Their resistance to organic
solvents is generally good. Casein adhesives are often compounded
with materials such as latex and dialdehyde starch to improve
durability. Strong alkaline nature of mixed casein adhesives often
affects the bonding of timber with high resin or oil content by virtue
of a saponification action on poorly wetting surface contamination.
Resultant bonds may be stronger than those obtained from
synthetic resins. Hard woods are subject to staining. Gap-filling
properties are good. Alkaline nature of casein glues precludes the
use of copper or aluminium mixing vessels.
Like animal glues, casein glues have fairly good bond strength
and those containing sodium hydroxide have even better
resistance to water than animal glues; also they recover their
original strength on redyeing. The fact that casein glues do not
gelatinise makes them much easier to handle. The most serious
drawback of casein for use in adhesive is the presence of fatty
matter which has a very adverse effect on the tensile strength
of joint made.
Classification of casein glues and adhesives:
Water-resistant Casein-lime Glue
Water-resistant casein glue sets to a gel as the result of a
slow chemical reaction, sodium caseinate gradually converted to
calcium caseinate. Some of the calcium hydroxide in the formula
has produced sodium hydroxide from a sodium salt also in the
formula, dissolving the casein. The chemicals are dry mixed with
casein as a ready mix powder and shipped to the user as a
complete prepared glue for dispersing in water.
Calcium hydroxide when present in excess, shortens the
working life but increases the water resistance of the glue line.
Addition of sodium silicate (silica: soda ratio 3:1) increases the
working life of the glue, at all levels of alkalinity. A simple
formula using lime and alkali is given below. It has good water
resistance, good working life about 7 hours.
Casein Blend Glues
Casein blends with blood are dry powder glues for cold
pressing plywood. The blood constituent in casein glue
contributes quick setting, thus reducing clamp time, and both
dry and wet strengths are improved. These glues are used in
the construction of flush doors, boxes, furniture and other wood
assembly work where dark colour imparted by the blood is
accepted. Blend glues may compromise mixtures of casein with
soyabean meal. This type of blend is a way to the reduction of
the cost.
Lime free Casein Adhesives
Casein solution not involving lime may be used as adhesives
for adherends other than wood. These are prepared by dissolving
casein with sodium salts, which provide a sufficiently medium
alkali to dissolve the casein. Commonly used sodium salts are
borax soda ash, trisodium phosphate, and others. Casein in
solution in strong alkalies, such as sodium hydroxide, and
ammonium hydroxide, also have the adhesive value. Organic
amines dissolve casein and there is some small use of alkyl
amines, ethanolamines and morpholine as the solvent for casein
adhesives. To give limefree casein adhesives some measure of
water resistance, formaldehyde or formaldehyde donor in the
form of resin or hexamethylene tetramine may be added. More
commonly, an oxide or salt of zinc, aluminium or chromium are
added to improve water resistance. A formulation is given below
used for plywood.
AMINO RESIN ADHESIVES
Introduction
Amino resins are the condensation products of amino
compound with aldehydes. The most common and widely used
amino compounds are urea and melamine where as formaldehyde
is almost always the aldehyde.
In a poly condensation reaction, a reactant of functionality
greater than two leads to branching and crosslinking. The
resultant three dimensional network can attain greater size
indefinitely, becoming insoluble and infusible. The over all
reaction of amino resin can be described in three stages. The
first stage is the reaction of amino compound and formaldehyde
to a form a methylol derivatives
RNH2+HCHO RNHCH2OH
Urea is tetrafunctional and melamine is hexafunctional.
Theoritically therefore, the initial reaction can lead to the
formation of a tetramethylol derivatives of urea or a hexamethylol
derivatives of melamine, of the ratio of formaldehyde to ammonia
is high enough for urea, formation of a methyl group slows
formation of another. These methylol derivatives are condensed
with the evolution of water of formaldehyde.
The properties of the adhesive intermediates are very much
dependent on the reaction condition. Molecular weight may vary
from a few hundred to a few thousand, with a wide distribution
of molecular size. Characteristic of commercial products are
solubility, viscosity, pH and concentration. The products are
available either in dry form or in aqueous solutions. Urea resin
adhesives are usually marketed in aqueous solution whereas
melamine resin adhesives are available in powder form.
Manufacturing Technology
Fig. 25.1 is a flow chart for the manufacture of amino resin
adhesives. All the commercial processes are batch type. The unit
operations are reflux and condensation, filtration and spray
drying. Because of the corrosiveness of formaldehyde and its
formic acid content, the reaction is usually carried out in a
stainless steel vessel. The reaction vessel is equipped with a
turbine agitator and reflux condenser and jacketed for heating
or cooling.
The order of addition to the reactor is formaldehyde, boric
acid and urea. When all the components are added to the reactor.
pH is adjusted to 7.0-7.8 and the charge is heated to 120ºC.
Disappearance of urea causes the pH to drop to about 4.0. The
reaction mixture is refluxed at atmospheric pressure for 2 hr.
Vacuum is applied and distillation is carried out under vacuum
of 28-29 in. Of mercury, until approximately 33 parts by weight
of water is removed. Then the system is shifted to total reflux
and cooled to about 30ºC. The pH is adjusted to 7.2-7.4 with
sodium hydroxide. The molar ratio: formaldehyde, urea, is
usually 1.75:1 to 2:1 for plywood adhesives.
board applications, to avoid the smell of formaldehyde in the final
product. General practice is about 1.3-1.5 moles of formaldehyde
per mole of urea. The pH is adjusted to 8.5 to 9.0 and the mixture
is heated to the boiling point under agitation. The solution is
refluxed for 40 min and cooled. The pH is then adjusted to
7.0-8.0 with a saturated solution of trisodium phosphate.
Adhesives for Hardwood Plywood
Plywood is an assembly of an odd number of layers of wood
(Veneer) joined together by means of an adhesive. The difference
between hard wood, plywood and soft wood plywood is that the
former has a ply of wood from the broad-leaf tree, e.g. oak,
walnut, maple etc. where as the veneers for softwood plywood
comes from coniferous trees. Hardwood plywood is generally used
for decorative purposes, softwood plywood for structural purposes.
The adhesive is applied by means of rubber covered rollers in
the glue spreaders. The coated veneers are alternated with the
uncoated veneer in the final assembly. Then the assembled
veneers are pressed in a hot press at approximately 90ºC and
150-300 psi pressure. Press time is about 5-7 min.
Sand Core Binder
Cores are projections of sand in the mould cavity for the
purpose of marking holes in the casting. After casting, the cores
are surrounded by metal and should be removed without
damaging the casting. Urea resins are capable for forming
mechanically strong cores.
PHENOLIC RESIN ADHESIVES
Introduction
Phenolic resins are the reaction products of phenol or
substituted phenols with formaldehyde. An unlimited variety of
resins are possible depending on (1) the choice of phenol (2) the
phenol: formaldehyde molar ratio (3) the type and amount of
catalyst used (4) the time and temperature of the reaction.
Resole resin
The active positions on the phenol molecule are the two
ortho and one para positions. When there is more than one mole
of phenol in the presence of an alkaline catalyst, resole is
formed. The amount of heat determines the final form of product,
e.g., whether the resin is of low viscosity, water soluble liquid
or a grindable solid. If the reaction is carried too far, the resole
can gel. Therefore the reaction is always conducted under
carefully controlled conditions of time, temperature, pH and mole
ratio of formaldehyde to phenol.
Novolac Resins
The reaction of one mole of phenol with less than one mole
of formaldehyde, under acid conditions, results in a novolac
resin. Novolac resin contains methylene links and are phenol
terminated. Methylol and methylene ester groups that are
present in resole resins are absent in novolac. Therefore, this
type of resin is incapable of further reaction without the addition
of more formaldehyde. This is accomplished by the addition of
hexamethylene tetramine, which is known as “hexa”. Hexa
makes the non-heat-reactive thermoplastic novolac capable of
reacting under heat to a cross linked advantage that novolac
resins have over resoles is that no water of reaction is evolved
during cure with hexa. Molecular weight of phenolic novolacs are
in the 500-900 range.
Manufacture
A typical phenolic resin is made by a batch process in a
jacketed stainless steel reaction kettle, equipped with anchor
type agitator and condenser. Molten phenol and formaldehyde
(37-40%) are charged into the kettle and agitation begin. For a
novolac, an acid catalyst is added and steam is introduced into
the jacket to heat the batch with atmospheric reflux. The reaction
is continued for 3-6 hrs at 100ºC. The reaction time is dependent
upon pH and phenol: formaldehyde mole ratio. Following the
reaction period, the batch is dehydrated under atmospheric
pressure and than vacuum. If the resin is to be solid in solution,
the solvent is slowly added to the molten resin in the still,
cooled by refluxing and discharged into drums. Most of the solid
resins discharged into pans are pulverized and blended with
hexa before packaging.
To make resole resin, an alkaline catalyst such as sodium
hydroxide is added to the phenol and formaldehyde before
heating the batch to 80-100ºC. Reaction times are generally 1-
3 hr. Since resole resin is capable of gelling in the still,
dehydration temperature is kept below 105ºC. By the application
of vacuum solid resoles are discharged into resin coolers. The
low molecular weight, water soluble resins are finished at as
low a temperature as possible, usually about 50ºC, whereas the
less reactive para-substituted resoles can be finished at
temperature as high as 120ºC.
Adhesive Compounding
There are two methods in general used for compounding
polychloroprene/phenolic adhesives. The first method involves
masticating the rubber on a two roll mill to reduce crystallinity
and improve solubility. The time of milling and degree of shear
are frequently used to control adhesive viscosity. The magnesium
oxide and zinc oxide are compounded into the rubber on an
unheated mill. The magnesium oxide is always added before zinc
oxide to preclude premature curing of the rubber. The antioxidant
is also added. The compounded rubber is then dissolved with
the resin in the solvent blend in a cement tub.
Compounders who do not have milling equipment use the
slurry method for adhesive preparation. This method consists
of simply adding of resin, pigments and antioxidants together
with the unmilled rubber to the solvent blend in the cement tub.
Adhesives made from unmilled rubber will be more viscous and
therefore, are usually produced at lower solid content. They will
also have higher initial cohesive strength. Adhesives from milled
polymer are however uniform and retain their uniformity upon
ageing.
Vinyl/Phenolic
Vinyl formal, vinyl acetal, and vinyl butyral may be combined
with phenolic resin to produce tough structural adhesives with
good impact strength, resistance to oil and aromatic fuels and
good salt spray and weathering resistance. The presence of
hydroxyl groups on the vinyl chain makes it likely that
crosslinking occurs between the phenolic resole resin and
hydroxyl groups during elevated temperature cure.
ACRYLIC ADHESIVES AND SEALANTS
POLYMERIZATION
All industrial polymerization processes are carried out at an
elevated temperature in the presence of an initiator.
Polymerization can be carried out in bulk, solution, suspension,
or emulsion. The most important processes for producing acrylics
for adhesives-solution and emulsion polymerization are dealt
with here.
SOLUTION POLYMERIZATION
In solution polymerization, the monomer or monomer
mixture is dissolved in a solvent which is relatively inert to free
radicals, e.g., ethyl or butyl acetate, benzene, toluene, petroleum
solvent of ketones; then polymerization is effected at elevated
temperatures in the presence of an initiator such as an organic
peroxide or an azo compound which is soluble in the solvent.
Properties of the product
The type of solvent used has a great influence on the reaction
speed and the molecular weight of the resulting polymer in
solution, because of the different chain transfer activities of the
various solvents. Thus, the viscosity of a polymethyl acrylate
solution, and the molecular weight of the polymer, decreases in
the following order: benzene, ethyl acetate, ethylene dicholoride,
butyl acetate, methyl isobutyl ketone, and toluene. The solvent
with the highest chain transfer activity gives polymers of lowest
molecular weight. The molecular weights of solution polymers
are normally lower than those of emulsion polymers. In selecting
the solvent to be used, consideration must also be given to the
economic and safety aspects.
Emulsion polymerization
The emulsion polymerization, process for homo-and
copolymerization of acrylic compounds is of greater significant
than the solution polymerization process. It is effected in the
presence of emulsifiers and initiators (e.g. alkali persulfaces),
normally in water as the external phase. Suitable emulsifiers
are, for example, alkali salts of longchain aliphatic carboxylic or
sulfonic acids, of sulfated ethylene oxide adducts.
PROPERTIES
At room temperature, the homopolymers of methacrylic and
acrylic acid as well as those of the lower methacrylic acid esters
are hard, nontacky products which are suitable only for special
applications in the adhesives field. Homopolymers of acrylic acid
esters from alcohols with at least 2 carbon atoms, which are
elastic, soft, and partially highly tacky products, are used for a
much larger range of adhesive applications.
Further physical properties of the most important acrylic
homopolymers in comparison to those of polyvinyl acetate are
given in Table 1 (properties increasing in direction of arrow).
FORMULATIONS AND APPLICATIONS
Adhesives for paper converting
The requirements imposed on adhesives for bonding paperto
paper are normally not too severe. Because the absorptivity
and surface condition of paper, animal and vegetable adhesives
can attain satisfactory wetting and encourage and thus yield
adequate bonding strength.
However, since the requirements imposed on the bonding
speed-have increased steadily, they can no longer be met with
adhesives based on natural products. Thus, it was not possible
to design and use modern automatic paper converting machines
until it was discovered that polymer dispersions based on
polyvinyl acetate are suitable for this application. By modifying
these homopolymeric polyvinyl acetate dispersions by adding, for
instance, plasticizers, solvents and resins, it was even possible
to render these adhesives suitable for bonding coated and
lacquered paper and to a certain extent also for bonding paper
to polymer films. The range of application for dispersions
modified in this way is, however, limited by the fact that
plasticizer migration may adversely affect the adhesion and/or
bonded materials. The demand for raw materials and adhesives
with improved specific adhesion therefore increased with the
improvements of the materials used in the paper converting
industry, such as printing, lacquering, application of water vapor
impermeable coatings on paper and board, and the use of polymer
films.
Because of their high specific adhesion to a great variety of
surfaces, polyacrylic acid esters in the form of aqueous
dispersions and organic solutions were found suitable for the
production of adhesives for surfaces which are difficult to bond.
The products in question are either copolymers of acrylic acid
esters with one another, or, especially in the case of packaging
adhesives, copolymers of acrylic acid esters with vinyl propionate
or vinyl acetate i.e. copolymers of vinyl acetate with acrylic acid
esters. Terpolymers produced from acrylic acid, acrylic acid ester,
and vinyl acetate have increased adhesion to metal foils and
various plastics and are therefore used for producing adhesives
for this field of application. Terpolymers of this type are also
very resistant to plasticizer migration, e.g., from plasticized PVC
film.
Flame Resistant & Pressure Sensitive Adhesive
For some applications it may be necessary to use a flame
resistant pressure-sensitive adhesive. Acrylics can be rendered
flame resistant.
Pressure-sensitive acrylic adhesives can normally be applied
by the conventional methods, e.g., direct or reverse roll coating
or by air-knife coating. The adhesive compound is applied either
directly to the final substrate or a release paper. In the latter
case the adhesive on the carrier is dried or crosslinked before
it is transferred to the final substrate. This transfer method is
necessary in those case where the backing material would
deteriorate during the drying or crosslinking process. The transfer
method is, for instance, commonly used for producing decorative
films.
For protecting the adhesive coat of pressure sensitive
adhesive materials during transport and storage, the adhesive
coat except in the case of adhesive tapes in rolls, is usually
covered by a release paper or a release foil, Silicone treated
paper, polyethylene or PVC films are, for instance, suitable for
this purpose. Long-chained acrylates exhibit good release effects.
The release material must not have any adverse effect on the
pressure-sensitive adhesive coating. Undesirable effects can be
obtained when unsuitable silicones are used or when the
silicones are not properly processed.
Acrylic Sealants
Linseed oil and bitumen were for a long time the commonest
base materials for building sealants. Further developments in
building construction and the steadily increasing demands on
quality resulted in the development of a number of synthetic
polymers for sealants. The first work on acrylics for building
sealants was carried out in response to the technical and
economic success which was achieved by this product class in
the last 10-15 years in the field of surface coatings, such as
paints. The first serviceable acrylic compounds were placed on
the market in about 1960. In the meantime the acrylics,
particularly the aqueous acrylic dispersions, gained considerable
significance in the production of sealants because of their
outstanding aging resistance and adhesion properties as well
as their favourable price.
It is expected that the increase of the consumption of
aqueous acrylic sealants will be above average in the next few
years. The total consumption of sealants will increase only be
approximately 20% during the same period.
The acrylics used nowadays for sealants are tailor-made
copolymers of acrylic and/or methacrylic acid esters and other
monomers.
Usually several monomers are present in order to achieve
the desired properties, such as elasticity, adhesion, resistance
to UV radiation, resistance to chemicals, and hardness, suitable
polymers are linear polymers, most of which are thermoplastic,
as well as polymers which can be rendered adequately elastic
by cold vulcanization, oxidation, or by the effect of alkaline
substance, such as caustic soda solution, cement, or lime.
Acrylics which can be crosslinked with the aid of oxidative
catalysts or epoxy resins are also well known. The dominating
raw materials for acrylic sealants are based on aqueous acrylic
dispersions.
Solvent-containing products and solvent-free products are
also available on the market. The solvent-free products have
been on the market only for a short time and the experience
gained with them is still inadequate. The solvent-containing
products have been on the market for a long time but they gained
considerable less significance than the acrylic dispersions.
Solvent-containing products are usually 80-90% solutions in
xylene. In the initial stage aqueous dispersions were available
with a solids content of only 50-55%. Dispersions with a higher
solids content, were obtained by improving the polymerization
technique. All acrylics must be modified with fillers and other
aids in order to achieve optimum properties.
Aqueous Acrylic Sealants
Aqueous acrylic sealants are employed mainly for those
applications for which compounds bases on linseed oil, butyl
rubber, or polyisobutylene have been used hitherto, i.e., the
sealing of joints which are subject to little elongation. Viz joints
between curtain walls and door and window frame joints.
In view of the experience gained hitherto, it appears that
the aqueous acrylic sealants are also suitable for joints between
prefabricated concrete building components with a practical
elongation of approximately 10-15%.
Soft acrylic sealants with a high degree of elongation have
already been successfully used for many years for joints between
small building components and special applications, e.g., aerated
concrete.
Even the results obtained hitherto in the trials for expansion
joints, which were commenced some time ago, have been positive
until now. Practice will show whether aqueous acrylic sealants
are in fact suitable for this application.
Harder compounds are mainly used for do-it-yourself
application and for sanitary equipment, e.g., bathtub and
washbasins.
AMINO RESINS
Introduction
Amino resin are manufactured throughout the industrialised
world to provide a wide variety of useful products. Adhesives (qv),
representing the largest single market, are used to make plywood,
chipboard, and sawdust board. Other types are used to make
laminated wood beams, parquet flooring, and for furniture
assembly. Some amino resins are used as additives to modify
the properties of other materials. For example, a small amount
of amino resin added to textile fabric imparts the familiar washand-
wear qualities to shirts and dresses. Automobile tires are
strengthened by amino resins which improve the adhesion of
rubber to tire cord. A racing sailboat may have a better change
to win because the sails of polyester have been treated with an
amino resin. Amino resins can improve the strength of paper
even when it is wet. Molding compounds based on amino resins
are used for parts of electrical devices, bottle and jar caps,
molded plastic dinnerware, and buttons.
Amino resins are also often used for the cure of other
resins such as alkyds and reactive acrylic polymers. These
polymer systems may contain 5-50% of the amino resin and are
commonly used in the flexible backings found on carpets and
draperies, as well as in protective surface coatings, particularly
the durable baked enamels of appliances, automobiles, etc. The
term amino resin is usually applied to the board class of
materials regardless of application, whereas the term aminoplast
or sometimes amino plastic is more commonly applied to
thermosetting molding compounds based on amino resins. Amino
plastics and resins have been in use for the past fifty years.
Compared to other segments of the plastics industry, they are
mature products, and their growth rate is now only about half
of that of the plastics industry as a whole.
Most amino resins are based on the reaction of formaldehyde
with urea or melamine. Although formadehyde combines with
many other amines, amides, or amino triazines to give useful
products, only a few have found commercial utility, and they are
of minor importance compared to the major products based on
urea and melamine. Benzoyuanamine, e.g., is used is amino
resins for coatings because it provides excellent resistance to
laundry detergent, a definite advantage in coatings for automatic
washing machines, dihydroxyethyleneurea is used for making
amino resins that provide wash-and-wear properties to clothing.
Aniline-formaldehyde resins were formerly important because of
their excellent electrical properties, but have been supplanted
by newer thermoplastics. Nevertheless, some aniline resins are
still used as modifiers for other resins. Acrylamide occupies a
unique position in the amino resin field since it not only contains
a formaldehyde-reactive site but also a polymerisable double
bond. Thus it forms a bridge between the formaldehyde
condensation polymers and the versatile vinyl polymers and
copolymers.
Formaldehyde links two molecules together and is hence
diffunctional. Each amino group has two replaceable hydrogens
that can react with formaldehyde and thus is also difunctional.
Since urea and melamine, the amino compounds commonly used
for making amino resins, contain two and three amino groups,
they react polyfunctionally with formaldehyde to form threedimensional,
cross-linked polymers. Compounds with a single
amino group, such as aniline or toluenesulfonamide, can react
with formaldehyde to form only linear polymer chains.
This is true under mild conditions, but in the presence of
an acid catalyst a higher temperatures, the aromatic ring of
aniline, e.g., may react with formaldehyde to produce a crosslinked
polymer. The use of thiourea improved gloss and water
resistance, but stained the steel molds. As amino resins
technology progressed the amount of thiourea the formulation
could be reduced and finally eliminated altogether.
Melamine resins were introduced about ten years after
molding compound. They were very similar to those based on
urea but had superior qualities. Melamine resins rapidly
supplanted urea resins and were soon used in molding,
laminating, and bonding formulations, as well as for textile and
paper treatments. The remarkable stability of the symmetrical
triazine ring made these products resistant to chemical change
once the resin had been cured to the insoluble, crosslinked
state. Future markets for amino resins and plastics appear to
be secure because they provide unusual qualities. New
developments will probably occur in the areas of more highly
specialine materials for treating textiles, paper, etc, and for use
with other resins in the formulation of surface coatings where
a small amount of an amino resin can significantly increase the
value of the basic material. Looking further into the future, the
fact that amino resins are largely based on nitrogen may put
them into a position to compete with other plastics as raw
materials based on fossil fuels become more costly.
Raw materials
Urea
Urea (carbamide) is the most important building block for
amino resins because urea-formaldehyde is the largest selling
amino resins, and urea is the raw material for melamine, the
amino compound used in the next largest selling type of amino
resin. Urea is also used to make a variety of other amino
compounds, such as ethyleneureas, and other cyclic derivatives
used for amino resins for treating textiles. They are discussed
below:
Urea is soluble in water, and the crystalline solid is some
what hygroscopic, tending to cake when exposed to a humid
atmosphere. For this reason , urea is frequently pelletised or
prilled (formed into little beads) to avoid caking and making it
easy to handle. Only about 10% of the total ureas production
is used for amino resins, which thus appear to have a secure
source of low-cost raw materials. Urea is made by the reaction
of carbon dioxide and ammonia at high temperature and
pressure to yield a mixture of urea and ammonium carbamate;
the latter is recycled.
CO2 + 2HN3 → NH2CONH2 + H2O = H2NCOONH4
Melamine
Melamine (cyanurrotriamide, 2,4,6-triamino-s-triazine) is a
white crystalline solid, melting at approximately 350ºC with
vaporisation, only slightly soluble in water, commercial product,
recrystallised grade, is at least 99% pure. Melamine was
systhesised early in the development of organic chemistry, but
it remained of theoretical interest until it was found to be a
useful constituent of amino resins. Melamine was first made
commercially from dicyandiamine but is now made from urea, a
much cheaper starting material. The urea is dehydrated to
cyanamide which trimerises to melamine in an atmosphere of
ammonia to suppress the formation of deamination products.
The ammonium carbamate also formed in recycled and converted
urea. For this reason the manufacture of melamine is usually
integrated with much larger facilities with much larger facilities
making ammonia and urea. Since melamine resins are derived
from urea, they are more costly and are therefore restricted to
applications requiring superior performance. Essentially all of
the melamine produced is used for making amino resins and
plastics.
Formaldehyde
Pure formaldehyde is a colorless, pungent smelling reactive
gas. The commercial product is handled either as solid polymer
paraformaldehyde, or in aqueous or alcoholic solutions. Marketed
under the trade name Formcel, solution is methanol, n-butanol,
and isobutanol, are widely used for making alcohol-modified urea
and melamine resins for surface coatings and treating textiles.
Aqueous formaldehyde, known as formalin, is usually 37 wt %
formaldehyde, though more concentrated solutions are available.
Formalin is the general-purpose formaldehyde of commerce
supplied unstabilised or methonol-stabilised. The latter may be
stored at room temperature without precipitation of solid
formaldehyde polymers because it contains 5-10% of methyl
alcohol. The uninhibited type must be maintained at a
temperature of at least 32ºC to prevent the separation of solid
formaldehyde polymers. Large quantities are often supplied in
more concentrated solutions. Formalin at 44, 50, or even 56%
may be used to reduce shipping costs and improve manufacturing
efficiency. Heated storage tanks must be used. For example,
formalin containing 50% formaldehyde must be kept at a
temperature of 55ºC to avoid precipitaton. Formaldehyde
solutions stabilised with urea are used and various other
stabilisers have been proposed. With urea-stabilised
formaldehyde the user only adjust the U/F (urea/formaldehyde)
ratio by adding more urea to produce a urea resin solution ready
for use.
Paraformaldehyde is a mixture of polyoxymethylene glycols,
HO (CH2O)n H, with n from 8 to as much as 100. It is
commercially available as a powder (95%) and a flake (91%). The
remainder is a mixture of water and methanol. Paraformaldehyde
is an unstable polymer that easily regenerates form-aldehyde
in solution. Under alkaline conditions, the chains depolymerize
from the ends, whereas in acid solution the chains are randomly
cleaved. Paraformaldehyde is often used when the presence of
a large amount of water should be avoided as in the preparation
of alkylated amino resins for coatings. Formaldehyde may also
exist in the form of the cyclic trimer trioxane. This is a fairly
stable compound that does not easily release formaldehyde,
hence it is not used as a source of formaldehyde for making
amino resins. Approximately 25% of the formaldehyde produced
in India is used in the manufacture of amino resins and plastics.
Other materials
Benzoguanamine and acetoguanamine may be used in place
of melamine to achieve greater solubility inorganic solvents and
greater chemical resistance. Aniline and toluenesulfonamide
react with formaldehyde to form thermoplastic resins. They are
not used alone, but rather as plasticizers for other resins
including melamine and urea-formaldehyde. The plasticizer may
be made separately or formed in situ during preparation of the
primary resins.
Water borne epoxy resins and derivatives
Electrodeposition is an important new technique for coating
metals, and water-borne epoxy ester-based vehicles are among
the leading coating systems thus applied. The coatings are used
as corrosion-resistant primers for automobiles, appliances, and
electrical parts. An early approach involved maleinising epoxylinsed
fatty acid ester (effecting a Diels-Alder condensation,
between maleic anhydride and a fourcarbon conjugated
unsaturated segment of the fatty acid), then dissolving in butyl
cellosolve and subsequently neutralising 80% of the composition
by ammonia solution or tertiary amine.
R-COOH + NH3 → R-COO- + NH4
+
Maleated resin Resin anion
The resin anion is deposited on an anode under an applied
dc voltage of 100 volts or more. The coating is then cured by
baking at elevated temperatures. The epoxy ester can be made
water-soluble by esterifying unreacted hydroxyl groups with
phthalic acid and thus preparing a phthalic acid half ester of
specific acid number. Other approaches include the use of dimer
acid and versatic acid. This is still an expanding field, and further
advances are expected.
Emulsions of epoxy resins themselves are generating
interest because of their ecological advantages. Coatings applied
from water-based epoxies reduce hazards from fire, toxicity,
pollution etc. A bisphenol resin modified with a reactive diluent,
and containing emulsifier, is easily emulsified just before use
in a high-speed agitator with gradual addition of water. The
coatings based on this resin are cured with polyamide emulsions.
Among suitable emulsifying agents for a bis-epi resins are the
derivatives of nonylphenol and ethylene oxide.
Diluents and modifiers
Many applications have requirements for viscosity, flexibility,
impact resistance, adhesion, pot-life, cost etc., which can be met
by the use of diluents and various modifiers.
Diluents
These liquids are used primarily to reduce the viscosity of
the epoxy resin system. They may be nonreactive. The
nonreactive diluents may be volatile organic solvents or
nonvolatile plasticizers. Solvents are used to obtain deep
penetration in such applications as prepreg laminating and
filament winding. Ketones, esters, and glycol ethers are true
solvents for epoxy resins; but aromatic hydrocarbons and
alcohols are sufficiently compatible to function as diluents. Some
of the commercial medium-viscosity (2000-4000 cps) resins
contain dibutyl phthalates as a nonreactive diluent. Viscosity
reduction of 70-80% is obtained by the use of about 15 phr of
dibutyl phthalate. The products obtained from such resins are
generally softer, less brittle and have less solvent resistant than
products based on unmodified or 100% reactive resins. Pine oil
was suggested as a nonreactive diluent.
Reactive diluents are those that may take part in the curing
reaction and become an integral part of the crosslinked system.
The reactive sites on these may be either epoxides or other
functional groups. Monoepoxide diluents include butyl glycidyl
ether, cresyl glycidyl ether, phenyl glycidyl ether, styrene oxide.
The first two are highly efficient diluents providing very great
reduction of viscosity in small amounts. About 12 p butyl glycidyl
ether per 100 standard liquid resin brings the viscosity down
from more than 10,000 cps to about 500-700 cps at 25°C. Since
monoepoxides reduce crosslinking, some of the properties of the
cured resin, such as water resistance, flexural strength, and
heat distortion temperature, are somewhat lowered. To overcome
this disadvantage, diepoxides may be used as diluents, e.g., 1,4-
butanediol diglycidyl ether, bis (2,3-epoxy cyclopentyl) ether. The
nonepoxy type reactive diluents include triphenyl phosphite and
butyrolactone. Details of reactive diluents are shown in Table
6.
Flexibilisers
The bis-epi type resins, epoxy nonvolacs, and other epoxy
resins containing aromatic ring structures, when cured with the
usual amines, or anhydrides, give products that are hard and
brittle, with rather low impact resistance and poor elongation.
Flexibilisers are employed to improve the impact resistance and
increase the elongation of the cured products. Some improvement
in these properties may be achieved with plasticizers such as
dibutyl phthalate, but only at the expense of gross reduction in
other properties such as solvent resistance. In epoxy technology
the term flexibiliser generally refers to those compounds that
undergo reaction and impart flexibility to the system by
increasing the distance between the crosslinks, interposing
segments with greater free rotation. Among favoured categories
of reactive flexibilisers are the aliphatic diepoxides, the
polysulfide telomers, and the amido-amine crosslinking agents
discussed later. The flexible aliphatic epoxy resins, when used
alone give soft-cured compositions having low physical strength.
They are best utilised in blends with the bisphenol-A based
epoxy resins. Incorporation of about 10-30% of flexible resins
retains most of the desirable properties of unmodified system
while improving the impact resistance and elongation.
Improvement in the flexibility of the modified system will also
depend on the type and chain length of the flexibilising resin,
the ether type and long-chain resins being more effective than
the ester type and shorter chain resins. Some epoxy are based
on dimerised C18 fatty acid, polyurethane diglycidyl ethers and
cycloaliphatic diglycidyl ethers. The diglycidyl ethers with
urethane linkages provide tough products with excellent impact
resistance at temperatures as low as 6-55°C. The glycidyl groups
of the cycloaliphatic resins react readily with polyamines at
ambient temperatures.
The polysulfides of commercial significance in epoxy resin
technology are Thiokol’s LP liquid polymers, which are essentially
mercaptan terminated poly (ethyl formaldisulfide):
The various grades of polysulfide polymers differ in amount
of branching and in molecular weight, which may range from 600
to 7500. The liquid polymers most often used with epoxy resins
are low molecular weight polymers with approximate molecular
weight of 1000. These polymers flexibilise epoxy resins by
extending chain length.
This reaction proceeds very slowly at room temperature, and
no useful products are obtained unless curing agents such as
amines or anhydrides are used. The polysulfide-epoxy resin
compounds are usually formulated as two component systems
to give good shelf life and permit easy handling. In most
applications, for every 100 parts of epoxy resin, about 75 parts
of polysulfide liquid polymer is used with about 10 parts of
amine curing agent, commonly a tertiary amine such as 2,4,6-
tris (dimethylaminomethyl) phenol. Aliphatic amines are not
compatible with polysulfides and tend to settle out.
Bituminous modifiers
Coal tar-modified epoxy coatings are used for pipes, tanks,
machinery foundations, and boats, because of their outstanding
resistance to acids, alkalies, and brine. A 50/50 mixture of a
low molecular weight epoxy resin and coal tar pitch, incorporating
7.5 parts of diethylene triamine per hundred parts of blends,
cured to a corrosion-resistant, rubbery product within 24 hours.
Such mixtures also can be cured with amine adducts and
polyamides. When added to flexible epoxy resins, coal tar
provides better elongation without reduction of tensile strength.
These resins can be compounded with asphalt; addition of 30
phr aromatic distillate results in elongation in excess of 300%
even at -18°C.
Synthetic polymers as modifiers
Various thermoplastic and thermosetting polymers, including
elastomers, have been incorporated to modify the properties of
the cured epoxy resin products. A nylon soluble in ethanol-water
mixture, is used in epoxy-nylon film adhesives to obtain high
peel strength as well as good heat resistance. The nylon can be
a major or minor component in the blend. Room temperature
peel strength usually increases with increasing amount of
polyamide, but with the sacrifice of high temperature resistance.
Excessive deformation under high temperature curing can be
reduced by blending with high-temperature-melting nylon
particles of uniform fine size. A thermoplastic polyurethanemodified
epoxy resin has been developed which is reported to
give better peel strength at cryogenic temperatures than that
obtained with epoxy-nylon.
Polyvinyl formal and polyvinyl acetals show good compatibility
with epoxy resins and improve peel strength of adhesives
Polyvinyl formal improves impact resistance of powder coatings
remarkably when used at 35-100 phr level with silica and BF3-
amine complex. Tough powder coatings are claimed by blending
with irradiated polyethylene. Among the thermosetting resins,
phenolics have long been blended to obtain heat resistance in
adhesives and chemical resistance in coatings. Xyleneformaldehyde
resins are useful in formulating epoxy casting
systems. Butylated amino resins are used to crosslink high
molecular weight epoxy resins or epoxy esters to obtain light
colored, chemical-resistant coatings. Solid epoxy resins modified
with hydroxy-functional silicone intermediates yield reaction
products with free terminal epoxy groups available for further
reaction with fatty acids or common curing agents. The siliconeepoxy-
based products have good heat stability, chemical and
moisture resistance as well as good electrical properties making
them suitable for protective coatings, laminates, and molding
materials.
Elastomers provide greater elongation and impact strength.
Polysulfides, the most commonly used elastomer to flexibilise
epoxy resins, have been discussed already. Epoxy-chloroprene
(neoprene) rubber blends have been cured with polyphenols or
aromatic amines to give tough, chemically resistant products.
Epoxy-nitrile rubber blends yield high-peel-strength adhesives.
Carboxyl-terminated nitrile rubbers, introduced by Goodrich,
were shown to toughen the cured epoxy resin at only 5 phr
loading. Improvements in impact have been obtained by the
addition of 5-35% by weight of carboxyl terminated nitrile rubber
to cycloaliphatic resins.
Fillers, reinforcements, and other additives
Incorporation of fillers and reinforcements into epoxy
formulations can result in higher viscosity, longer pot life, lower
exotherm and lower shrinkage. Properties of the cured polymers
may be improved. Above all, use of fibrous fillers may lower the
cost of the formulations. For mechanical strength, asbestos,
glass, graphite and boron fibers are used. Glass fibers the most
common reinforcement, not only increases tensile, flexural, and
impact strength, but also raised heat resistance.and reduces
shrinkage and thermal expansion. Graphite and boron fibers, very
high in modulus and thermal tensile strength, are used for high
performance aerospace application where strength-weight
characteristics are critical. Coated graphitised carbon fibers of
391,000 psi have been incorporated in 60 volume % in epoxy
resins.
