Metal Pigments
Certainly there is no resemblance or connection between metal
pigments and metallic stearates. It was merely a matter of convenience
to group these two short subjects together in one chapter.
The principal metal pigments are those made from aluminium,
zinc and mixtures of copper and zinc which range from 100% copper
to about 70% copper and 30% zinc. Pigments from the copper-zinc
alloys are known as “bronze powders.” Metal lead pigments are
available and metal silver pigments have specialized uses such as
conductive coatings for printed circuits. Similarly, nickel and stainless
steel pigments find use where a combination of metallic appearance
and alkali resistance is required.
The metal industry produces fine particle size metals in both
powder and flake forms which are used in a variety of industries. The
paint industry uses chiefly the flake form of aluminium and bronze
and zinc in the powder form. Since metal, even as very thin foil, is
completely opaque to visible and ultraviolet light, it could be anticipated
that metal pigments would have exceptionally good hiding power.
However, they have relatively low tinting strength and for some finishes
they are coloured with various amounts of colour pigments.
Finely divided metals react with moisture, and hydrogen gas is
one of the products. If this reaction occurs in tightly closed containers
it may develop sufficient pressure to bulge or rupture the container.
Therefore, every effort must be made to store metal pigments under dry
conditions and to use paint vehicles which are as nearly anhydrous
as possible. In many cases two containers are used to ship metallic
paints. One container has the metal pigment and the other contains
the remainder of the paint. Shortly before application the metal pigment
is mixed into contents of the second container.
Metal pigments fulfill several important functions in coatings.
Bronze powders are widely used for decorative coatings. Aluminium
pigments also are used for decorative purposes; in addition, they impart
heat reflective properties, reduced permeability to moisture, and good
durability to coatings. Zinc dust contributes corrosion- inhibitive
properties in primers for iron and steel and excellent adhesion in primers
for galvanized iron. The many applications for metal pigments and
metallic stearates are discussed in later chapters. This chapter outlines
the types, method of manufacture, and general properties of the principal
metal pigments and metallic stearates available at present.
The lubricant is added to prevent the aluminium particles from
being mechanically welded together under impact and also to develop
a bright metallic lustre on the flakes. Usually, the lubricant is stearic
acid but other fats and oils may be used, such as tallow and olive and
rapeseed oils. It is believed that chemical reaction occurs between
stearic acid and aluminium during the milling operation to form a
strongly adhering coating of aluminium stearate on the finished flake.
This coating makes possible the leafing action of the flakes in a paint
film with the resulting brilliant metallic finish. The coating is quite
stable under conditions of normal use, but it may be loosened or
removed at temperatures above 180oF, also by certain solvents, and by
chemical reaction with materials such as lead driers and free acids in
vehicles. Further discussion of conditions for proper leafing is given
later under physical and chemical properties.
The liquid medium for wet milling usually is mineral spirits, but
liquids such as solvent naphtha or certain plasticisers are used for
specific applications. The mill is run until test shows the required
degree of fineness has been attained; then the paste is washed from the
mill with mineral spirits. The batch is filtered and adjusted to the
specified solid content either by drying or by addition of liquid medium.
The solid content of commercial aluminium pastes usually is 65%, but
special grades may be as high as 73.5%. Flake aluminium powders are
produces by complete drying of the paste through evaporation of the
mineral spirits under vacuum. The foregoing process produces the
leafing type of aluminium pigments. These may be treated to convert
them into the non-leafing type, or a special lubricant may be used
which produces the non-leafing type directly.
An indirect indication of the particle size is obtained from the
water-covering test. A weighed amount of powder is dusted on to the
surface of a rectangular pan of water as uniformly as possible between
two movable baffles. The layer of powder then is manipulated by means
of the baffles to produce as extensive an area as possible and still
maintain a continuous layer. The area of this layer is measured, and
the results are expressed as square centimenters covered per gram of
powder. The coverage ranges from about 8000-10,000 sq cm/gm for
the coarse grades, 14,000-18,000 sq cm/gm for the lining grades, and
25,000-30,000 sq cm/gm for the extra fine lining grades.
Leafing grades may be distinguished from the non-leafing grades
by mixing a small amount of paste or powder with mineral spirits of
xylene. The leafing grades produce the familiar metallic surface on the
liquid, whereas the non-leafing graders yield a gray suspension. This
test does not give satisfactory results with liquids having surface tension
values of less than 25. For example, VM&P naphtha does not permit
satisfactory leafing because of its low surface tension values. The extent
of leafing of aluminium pigments is measured by the spatula immersion
test. A polished spatula of standard dimensions is dipped in a specific
mixture of pigment and coumarone resin solution, withdrawn rapidly,
and allowed to drain in a cylinder for three minutes. The coating then
is examined for extent of leafing over the total depth of immersion. The
ratio of the depth of the fully leafed portion to the total depth is
calculated and expressed as percent leafing. This test may be used to
determine possible loss of leafing of an aluminium paint on aging.
Handling and Storage. Aluminium pastes and powders should
be stored in closed containers at normal temperatures. Open containers
permit loss of volatile liquid from pastes with consequent drying and
uncertainty of composition. Free access of pigments to air and moisture
should be avoided. Long exposure to air produces oxidation of the
metal with a consequent loss of adhesion of the stearate layer and
reduction of leafing property. Moisture reacts chemically with finely
divided aluminium with a resulting production of hydrogen gas and
loss of leafing property. The use of paste eliminate the dust hazard
connected with the powder type. In addition, paste has a higher
apparent density, therefore it occupies less space and mixes with paint
vehicles more easily than powder.
Black Pigments
Charred bones and soot from smoky fires were used as black
colouring materials by pre-historic man. Modern man also uses bone
black and soot as black pigments for paints and printing inks, but he
has wide range of types and grades of black pigments for specific
applications. In addition to the blacks composed chiefly of carbon, he
used certain inorganic blacks in which good filling properties are more
important than depth of colour. The general types of black pigments
are listed in Table 1 together with their raw materials and approximate
range of composition.
Bone black is obtained by pulverizing carbonized bones. It has
good blackness and also low oil absorption and good filling properties.
Vegetable black is obtained by carbonizing wood and other plant
products. At present it finds-only limited use in coatings, and it is
being replaced by blacks having more uniform composition varies with
source and type of raw material, and they may be replaced with greater
uniformity with mixtures of extender pigments and carbon black.
Mineral black is used in coatings such as freight car paints and metal
primers and surfacers. The natural and synthetic black oxides of iron
are described. They are used where good filling properties are more
imortant than blackness of colour. Antimony sulfide is used almost
excliusively in camouflage paints, since it reflects the same as green
foliage when photographed with infrared film. Black toners are organic
compounds used in conjunction with carbon black to increase the
blackness of specialty finishes. An apprarent increase in blackness of
finishes made with certain standard carbon blacks may be obtained by
addition of up to 25% of in iron blue. The type of iron blue generally
used is the toning blue.
In addition to the black pigments listed in Table 1 the paint
industry uses a variety of bituminous materials such as pitches,
asphalts, and gilsonite in black and dark coloured coatings.
Furnace Black. The furnace process not only give higher yields of
carbon, but also the plant occupies much less space and is free from
the smoke nuisance associated with burner houses. Also, the furnace
process may be adjusted to use either gas or oil as the raw material.
Continued improvements since the furnace process was inaugurated
have produced smaller size particles with better blackness, but it
remains to be seen whether this trend can be continued to produce
colour equal to the better grades of channel black.
In the furnace process the same operations of partial combustion
to produce the necessary temperature and thermal decomposition of
the remainder take place, but a single large flame replaces the large
number of small flames of the channel process. Gas and air are admitted
separately to a firebrick-lined furnace which operates at a temperature
of about 2400oF. Through controlled air supply and the particular
design of the inlet ports, a large portion of the gas is decomposed
instead of being burned completely. The hot gases from the furnace,
carrying the black in suspension, are cooled by water sprays. Then the
products pass through an electrostatic field which agglomerates the
carbon particles so that they may be collected in cyclone collectors,
and the exhaust gases are vented to the atmosphere. From the collectors
the black may be bagged or routed through pelletizing equipment. The
variables in the furnace process are:
Thermal Black. Thermal black represents only a small percentage
of carbon black production. Two general types are produced; one is
based on natural gas as the raw material and the other on acetylene.
The black from natural gas has the largest particle size and greyest
colour of the carbon blacks, and the black from acetylene is intermediate
in size and colour between furnace and channel black. Thermal black
is produced by thermal decomposition without simultaneous
combustion.
In the natural gas process the thermal decomposition takes place
in an insulated chamber containing a network of firebrick. First, the
chamber is heated to 1800-2500oF direct combustion of an air-gas
mixture. Then the combustion is stopped, and a charge of “make-gas”
is passed through the heated chamber. The resulting carbon and spent
gases are cooled by water sprays, and the carbon collected in bag
filters. This process gives a high yield of carbon, but the particle size is
large and the colour is quite grey. Somewhat smaller particle size may
be obtained by diluting the make-gas with spent flue gases.
The carbon content of bone black is about 20% that of carbon
black. Since the hiding power and tinting strength depend on the carbon
content, it will be apparent that bone black is much weaker than carbon
black in these respects. However, bone black is much lower in oil
absorption, therefore a greater percentage can be incorporated in a
formulation without developing excessive consistency. This feature is
desirable in the production of low-sheen black finishes or when good
filling properties are required, such as in leather finishing. Its particle
size, relatively large, is expressed as percent retained on a 325-mesh
screen. When used with other colours it shows less tendency to float
than carbon black because of the larger particle size. Bone black is
fairly easy to disperse in coating vehicles and is wetted by water much
easier than regular carbon black. For this reason it finds use in tinting
calcimine, casein and latex paints, and water base inks. Generally,
bone black is considered too abrasive for lithographic inks, but is used
in artists colours and many standard coatings.
Vegetable Black. Vine black was produced originally from stems
and twigs of grape and hop vines and from organic wastes of the wine
industry. It is the most important member of the family of “vegetable”
blacks which are produced from many kinds of cellulosic materials
obtained from plants and trees. Vegetable blacks are made by dry
distillation and carbonization of the vegetable material in the absence
of air. They range from 30-70% in carbon content, the remainder being
a mixture of calcium and potassium carbonates. As may be expected,
vegetable blacks show an alkaline reaction. They are somewhat low in
colour value and in oil absorption. At present they find very limited
application in coatings and have been replaced to a great extent by
mixtures of furnace black and black iron oxides.
Mineral blacks are coarse pigments with low oil absorption and
good filling properties. They may be used in coatings such as freight
car paints and sanding surfacers. In view of the availability at low
cost of carefully classified extended pigments described, and of the
newer furnace blacks, it would appear that combinations of these
materials could be made by paint manufacturers to meet the
requirements of mineral blacks with possible improvement in
performance.
Antimony Trisulfide. Antimony sulfide occurs in nature as the
grayblack mineral known as stibnite. It may be pulverized to the
required particle size and used as a pigment in camouflage paints.
When such paints are photographed with infrared film, the antimony
sulfide gives the same reflection characteristics as green foliage.
Antimony sulfide may be made by reaction between antimony
and sulfur or by precipitation from solution of antimony trichloride
with hydrogen sulfide. The natural products range from 64-66% Sb2S3,
and the technical grade of precipitated material contains about 94%
Sb2S3. Owing to its low colour value, its use in paints is limited almost
entirely to the camouflags type.
Extender Pigments
Introduction
Extender pigments are much lower in price than prime pigments
and are used in paints primarily to reduce cost. However, by careful
selection of the partiuclar type of extender for specific paints, it is often
possible to improve certain properties of the paint or the dry coating.
Proper choice of extender may improve properties such as consistency,
leveling, and pigment settling in the paint. Certain extenders reinforce
the structure of the dry coating mechanically, while others increase its
resistance to the transmission of moisture.
Flat finish and semi-gloss paints are produced by using pigment
concentrations high enough to prevent the formation of a layer of clear
oil or resin in the surface of the coating which would give it gloss.
When pigment particles are in the surface they diffuse the light and
prevent specular or glossy reflection. In paints with high pigment
concentration sufficient hiding may be obtained by replacing some of
the opaque pigment with extender pigment. Since extenders are lower
in price than opaque pigments, a reduction in material cost will result.
The refractive indexes of extender pigments range from 1.55 to
1.65, and since these values are only slightly different from those for
oils and resins, extenders generally do not contribute to the hiding
power of paints. In special cases, such as their use in wood fillers and
flat varnishes, lack of opacity is desirable to prevent a “muddy” effect
in clear furniture finishes. Certain chemically prepared extenders such
as Micro-Cel pigments have refractive indexes in the range of those for
oils and resins, but in combination with while opaque pigments they
contribute slightly to hiding power. The reson for this phenomenon is
not clearly understood at characteristics, and the effect of these factors
on the scattering of light in the coating. Detailed knowledge of the
various types and grades of extenders will enable the paint formulator
to produce paints having maximum properties at minimum costs.
Type of Extenders
Extender pigment are obtained from two general sources: (1) by
pulverization of certain rocks and sedimentary deposits; (2) by chemical
precipitation. The two types are referred to as natural and precipitated
extenders respectively. The natural deposits such as limestone, quartz,
and clay are found in various parts of the country, and the pigments
usually are processed at the deposits. The markets are supplied from
local deposits whenever possible, because the low price of extender
pigments will not permit high freight charges. There may be
considerable variation in composition and properties of natural
deposits with corresponding differences in extender pigments obtained
from them. Therefore paint manufacturers having plants in eastern,
central, and western states may have to adjust their formulations
containing extenders to accommodate variations in extenders supplied
to the different plants.
Extender pigments also may be used to advantage to increase the
consistency of paints. Some extenders that have much higher oil
absorption values than white pigments may be employed to increase
the consistency of white paints without raising the cost. The oil
absorption of a particular pigment is directly proportional to its
available surface; therefore the grade having finer particle size usually
has higher oill absorption. Since oil absorption also is affected by the
nature of the pigment surface, the oil absorption values vary among
the different type of extender pigments because their different surface
characteristics as discussed before.
Extender pigments are marketed as white powders, but because
of their low refractive indexes they do not contribute whiteness or high
reflectance to oleoresinous paints. However, some grades of natural
extenders contain traces of metallic oxides, such as iron oxide, which
cause slight discolouration or reduction in reflectance of white paints.
Such grades should not be used for flat or semi-gloss white finishes
but would be entirely satisfactory for coloured finishes or in metal
primers and surfacers.
Extender pigments may vary considerably in characteristics such
as reactivity with components of paint vehicles, sensitivity to and
solubility in water, and in the pH of water slurries. Their reactivity is
due in part to variations in origin and methods of processing, but
some of the reactivity is inherent in the particular extenders. For
example, barytes is very inert chemically and not sensitive to moisture
adsorption, whereas calcium carbonate is more readily affected by acidic
conditions. Calcium sulfate may adsorb sufficient moisture in humid
weather to change the consistency characteristics of a paint. The pH
and specific resistance of water slurries of extenders are important if
these pigments are to be used in water-dispersed paints; they should
be checked carefully. Some extenders have sufficient solubility in water
to liberate enough cations to affect the stability of latex or other types
of emulsion paints.
Calcium Carbonate Extenders. Calcium carbonate extender
pigments are very widely used and are frequently referred to as whiting.
They are available as both natural and precipitated types and in a
wide range of particle size. The very coarse grades are preferred for
putty and glazing compounds, the intermediate sizes for oleoresinous
flat and semi-gloss finishes and caulking compounds, and ultrafine
precipitated grades are available for gloss finished and printing inks.
Natural whiting is obtained from two main sources: limestone
and chalk. Limestone is widely distributed throughout the world and
is found in most of the states of the United States. Limestone and the
related rocks, marble and calcite, are crystalline and were formed in
the earth’s crust by reactions between calcium salts and water
containing carbon dioxide. When magnesium salts were present, a
mixture of magnesium and calcium carbonates were precipitated which
is known as dolomite limestone. Chalk is an oolitic variety of calcium
carbonate and was formed by deposition of shells and marine animals.
Natural whitings are made by quarrying the rock, crushing and
grinding it, and then classifying the powdered material for particle
size requirement. The grinding process may be either dry or wet. In dry
grinding the screened crushed rock is powdered with a hammer or
roller mill and classfied for particle size by the air-flatation process. A
stream of compressed air passes through the mill and carries out the
particles which are small enough to “float” on it; the larger particles
drop back into the mill. The rate of air flow is a factor in the particle
size obtained in the product.
High Temperature Stable Inorganic Pigments
The pottery industry has, for several thousand years, been using
high temperature stable inorganic pigments, which do not fade or
discolour even at the high temperatures (between 400o and 1400oC)
required for the maturing of enamels and glazes. Moreover, there are
many examples of beautiful colour and decoration work from as early
as the Shang dynasty in China (1500 B.C.), which have come down to
our period without much change in the shade or brightness of the
colours. While it is true, that the glaze substrate in which these pigments
are embodied, provides substantial protection from contact with air,
water, chemicals and other destructive elements, it is nevertheless also
true that the light-fastness of these pigments must be something fantastic
for them to have remained unchanged for periods of 3000 years and
more.
While the tinting power of many inorganic colours tends to the
rather less than that of organic pigments, inorganic pigments tend to
have greater opacity, hiding power, bleed resistance, and of course
light-fastness. They are more resistant to heat, and, being mostly ionic
bonded, are usually less reactive with the organic vehicles, which are
usually covalent bonded.
High temperature stable inorganic pigments can also be used for
normal temperature applications such as the pigmentation of rubbers,
plastics, paints, cements, etc. Almost any colour and shade, including
those shown in IS:5-1978 and many more not shown in the I.S. charts,
are obtainable with high temperature stable pigments.
High temperature stable pigments are mostly exides, sulphides
or silicates of various metals
Shades of red, yellow, pink and violet are also obtainable from
compounds of tin, copper and gold. Such compounds are, naturally,
quite expensive, but produce very attractive, long lasting shades. Blues
and violets are produced from compounds of vanadium, titanium,
uranium, copper and cobalt. Cobalt colours are, of course, fairly
expensive, but have excellent light-fastness and binding power. They
come mostly in the spinel crystalline form, which are stable upto
1600oC., and inert to acids and alkalies.
Because of chemical stability, light-fastness, resistance to dilute
acids and alkalies, dispersability and compatibility with other organic
and inorganic pigments, high temperature stable pigments make
excellent colours for.
Preparation of Iron Oxide Pigment from Industrial Waste
Paint is used for decoration, protection of metals and functional
applications. The constituents of a paint are vehicle, pigment, solvent
and additivies. Pigments are mainly used for giving body to the paint,
protection and for specialised functions. These are finely divided solids
of different shades used in the paint to give colour, hiding, consistency,
durability, build, etc. These particles are substantially insoluble in water.
Pigments may be classified as natural and synthetic, depending on
their origin.
Iron oxides are extensively used as inert pigments in the paint
industy. A dye intermediate manufacturing industry in the country
was faced with the problem of accumulation of huge volumes of slude
in their factory during the iron and acid reduction process. Efforts
have been made elsewhere to convert the sludge into useful pigment.
These results of the preliminary studies made in this direction are
reported in this paper.
Preparation of the pigment
The sample supplied by the firm contained total iron as Fe2O3-
90.5% and ferrous iron as FeO 3%. The colour of the product was
black. The sludge was washed first with water to remove the soluble
impurities such as chlorides, etc. This washed product was
subsequently subjected to the treatments as shown in Table 1 and visual
observations were made.
A water wash was given to remove the chloride present in the
sludge. It may be seen from the above table that the sludge heated to
500oC at the end of one hour only showed some slight change in the
colour. The temperature was gradually increased from 150oC and the
colour change was observed at 500oC only. The time of heating also
determined by increasing the duration from 30 minutes onwards and
changes could be seen only at the end of 60 minutes. Further heating
did not show any change. No change could be observed when the
slude was treated with different treatments as indicated in Sl. Nos. 2 to
6. When the chloride-free sludge was treated with 10% iron oxide or
pigment grade synthetic iron oxide, the colour of the sludge changed
to that of the pigment grade red oxide at 500oC itself. But the product
was heated to 700oC and maintained there for three hours for the
transformation to be completed. Thus the waste product was converted
into a pigment grade iron oxide.
The water extract of the waste as well as the treated products
were analysed for pH and chloride. It was found that the pH was
neutral i.e. 6.5-7.0 in all the cases except the sludge which was slightly
acidic (<6). The chloride content in the water extract was determined.
It was found that the sludge contained 30 mg of chloride/100 ml of the
extract, whereas it was negligible for others.
The oil uptake value for the pigment grade iron oxide with the
commercial grade LSO was 20 and the oil uptake value of the converted
product by both the methods viz. Sl. Nos. 7 and 8 of Table 1 was 18-22.
It shows that the oil uptake of the converted pigment from waste is on
par with the pigment grade iron oxide. This again shows that the
fineness of the pigment got from the waste is more or less similar to
that of the pigment grade iron oxide.
An Overview of Aluminium Pigment Technologies
Through the use of aluminum pigments in coatings and inks, a
wide and varied set of aesthetics is achievable. With the ever-changing
aluminum pigment technologies, the opportunities for applications in
these areas in the nineties are indeed exciting.
An overview is provided, covering a general introduction to
aluminium pigments, along with those physical properties which make
them particularly appearling in the automotive; general industrial; and
ink markets. Along with the physical properties of aluminium
pigments, formulation parameters including-pigment grades; resins;
solvents; and additives, typically used by the coatings formulator, are
detailed. Water-based and high solids systems suggesting starting
points for the nineties are exhibited. Application procedures along
with key clay; handling; and storage procedures are also presented.
At the close of the last decade, the Automotive Industry, worldwide,
produced over forty-four million automobiles and trucks, totaling
approximately five hundred and sixty billion dollars in sales. An
analysis from a geographical standpoing shows that the United States
produced 37.7% of these units, followed closely by Japan, with 29.6%.
The European community was led by West Germany with 9.5%,
followed closely by France, with 8.8%; then Italy, with 5.4%, Sweden,
with 1.3%; Britain, with 1.1%, and finally, the Soviet Union, with 3%.
The Far East has Korea as a rapidly growing producer of automobiles
with 3% market share.
One of the fastest growing markets is in Asia. It is estimated that
automobile sales in Aisa, excluding Japan, Australia and New Zealand,
grew from 3.2% of global unit sales in 1980 to 4.7% in 1987 and had
increase to 8.7% in 1995. While the number of units on a global basis
are not large in this area, the growth potential is enormous.
Aluminium pigments, used in metallic automotive top coats in
North America, accounted for nearly 50% of the cars produced. In
Europe, the figure is slightly higher, while in Japan the figure is lower,
but rapidly catching up.
The use of aluminium pigments in automotive coatings is a
relatively recent invention. Up until the 1930‘s all automotive top coat
finishes were solid colours, with black being the most popular. The
introduction of aluminium pigments in automotive finishes, by Chrysler
in 1934, signaled their rise to today’s level of popularity. By the close
of the 1940’s, approximately 20% of American automobiles utilized
metallic finishes. By the close of the 1950’s, brighter metallic finishes
were seen in the market place, utilizing coarser controlled grades. Prior
to the 60’s, the grade typically used were the non-leafing, non-acid
resistant types. With the advent of the ’60’s, newer, innovative, acid
resistant grades of aluminium pigment were introduced to the
marketplace, which in addition, offered greater control over particle
size distribution. The culmination of these improvements was the
introduction of Sparkle Silver type aluminium flake pigments in the
1970’s. The Sparkle Silver grades offer exceptional brilliance, sparkle,
and whiteness in a wide range of grades from very coarse to very fine
particle size. During the ’80’s, the Coating Industry experienced
environmental limitations being placed on coatings, in terms of VOC
(volatile organic compound) emissions allowed into the atmosphere.
The legislation forced the Industry into developing higher solids
coatings, which presented the formulator with many problems. The
most significant one is the development of aesthetically pleasing
metallic finishes. Along with the advent of high solids, research and
development work in the area of waterborne unicoat and base coat/
clear coat systems was ongoing. During this time, the need also arose
for more degradation resistant pigments, as well as more aesthetically
pleasing pigments to maximize the styling changes going on in the
Industry. With the automobile stylists developing more rounded, softer
looking body styles to lower air drag and increase fuel efficiency, the
stylist was faced with the responsibility of accentuating these body
designs. New developments in the base coat/clear coat grades were
offered, by providing whiter, finer Sparkle Silver grades with deeper
flop than previously available in the marketplace. Whiter grades were
developed for higher solids finishes, while Tufflake grades were
introduced to solve the problems of colour change, during coating
application in the automotive plants. Along with these challenges,
aluminium pigment grades for use in waterborne systems were
introduced.
Generally, to overcome this type of phenomenon, a coarser grade
is required. Coarser grades being brighter enables one to achieve
approximately the same colour with the high solids system as you
would have had in the conventional system, with a finer particle size
aluminium flake.
In automotive OEM coatings, it is interesting to note that normally
high solids solvent borne base coat/clear coat systems tend to have
difficulty in achieving high gloss and smoothness on vertical surfaces,
when compared to horizontal surfaces. The VOC restrictions for the
high solids systems does hamper the flow and leveling on vertical
surfaces. The use of waterborne base coats, coupled with higher solids
clear coats, marries the two types of technology, yielding improved
aesthetics in the Automobile Industry today.
Within the Ink Industry, similar environmental restrictions are
being faced. The ink formulator faces ever-increasing pressure to develp
waterborne inks, which meet the performance and application
characteristics of their solvent-based counterparts. For acrylic emulsion
ink, finding the best resin is just the start of the quest to develop a
suitable ink. The formulator also must select and evaluate defoamers,
dispersion agents, surface tension modifiers, flow control agents,
coalescents, and co-solvents before a final formulation is developed
and fully tested on line.
In meeting the needs of this Industry, Siliberline offers a wide
variety of products: standard aluminum pastes (mineral spirits); Silvex;
Silvet granules; Isopropyl Alcohol (IPA) based. Summarized in Figure
6 is a matrix illustrating which class of products is applicable to
Letterpress and Litho-Offset; flexo and gravure, screen and UV ink
systems.
Recent laboratory efforts have been focused in developing new
grades, with (IPA) as the solvent carrier, which find application in
aqueous, liquid, flexo and/or gravure inks.
In summary, Silberline continues to support research activity for
the present and future needs of their industries, worldwide.
Reactive Dyes
A reactive dye, according to a useful definition by Rys and
Zollinger, is a coloured compound which has a suitable group enable
of forming a covalent bond between a carbon atom of a hydroxy, an
amino or a mercapto group respectively of the substrate. They point
out that this definition excludes mordant dyes and 1:1 chromium azo
dye complexes, which are used in dyeing protein fibres, may form
covalent bonds between metal ion and nucleophilic groups of the
fibre.
The idea that the establishment of a covalent bond between dye
and substrate would result in improved wash fastness compared with
that of ordinary dye-substrate systems where weaker forces were
operative is an old one. The invention consisted in the synthesis of
dyes containing a reactive group, the 2,4,6-dichlorotriazinylamino
group which has two labile chlorine atoms activated by the electronwithdrawing
action of the three N atoms, and the Devising of dyebath
conditions, which, while bringing about the formation of a covalent
bond, were mild enough to avoid serious damage to the fibre.
The chlorotriazinyl reactive dyes are by far the most important
class and have proved a serious rival to the vat dyes as regards washfastness
and in other ways. The main chromogens employed are azo,
metal-azo, anthraquinone and phthallocyanine systems. The question
of cotton substantivity is an important one. It should be high enough
to ensure a high ‘fixation-yield’ but at the same time a substantivity of
the unfixed, hydrolysed dye should be low enough to permit easy
removal by soaping and rinsing to ensure maximum fastness to wet
treatments in the finished dyeing. Structural modifications to the
molecule, which (a) inhibit coplanarity or (b) increase the watersolubility,
tend to reduce substantivity.
Since their introduction reactive dyes have been the subject of a
very large number of patents comparable only with the numbers
granted for inventions in the disperse dye field and in that of synthetic
organic pigments. Most dye manufacturers have invested heavily in
research programmes concerning new reactive systems and variations
of molecular structure to achieve optimum fastness and other properties.
Attention has naturally turned to reactive dyes for substrates other
than cellulose and dyes have been developed which are suitable for
wool and polyamides. Water-insoluble disperse dyes having reactive
groups (Procynyl dyes, ICI) have been introduced principally for the
dyeing of polyamide fibres on which they show improved washing
and heat fastness. Reactive systems may be divided into two main
types:
• Those involving nucleophilic substitution
• Those involving nucleophilic addition
Nucleophilic Substitution Systems
The monochloro and dichlorotriazinyl dyes, of which early
examples have already been given, account for 50% of all reactive dyes
used in commerce.
Evidence for Chemical Combination Cellulose
Stamm, Zollinger and co-workers have endeavoured to obtain
experimental evidence of the formation of a covalent link and to
demonstrate its position in the D-glucose unit of cellulose. Cotton
dyed with a Remazol dye was subjected to microbiological hydrolysis,
a mixture of oligomers being formed. Further degradation, with dilute
sulphuric acid, gave a glucose derivative in which one hydroxyl group
was blocked by a dye molecule. Methylation of this under very mild
conditions, followed by alkaline treatment to remove the dye molecule,
and then acid hydrolysis to remove the glucosidic methyl group gave
finally a known trimethylglucose. Stamm later showed that a glucoside
is normally formed by Remazol dyes acting on cellulose and concluded
that the earlier findings were ambiguous.
Cellulose dyed with a chlorotriazinyl reactive dye however will
not dissolve in cuprammonium solution, whereas cellulose dyes with
direct dyes will dissolve.
Direct Dyes
The direct dyes, also known as the substantive colours, differ
from the basic and acid dyes because cellulosic fibres have a strong
affinity for them. Many of them will also dye the protein fibres and, as
was explained in the previous chapter, the majority is sulphonated azo
compounds very similar to the acid dyes in constitution, there being
no clear demarcation between the two classes. Selected substantive
dyes can be used to give solid shades on wool and cotton mixtures.
This was the first direct dye, and its discovery was quickly
followed by the preparation of many similar colours, opening a new
era in cotton dyeing. Before 1884 cellulosic fibres could only be dyed
on a mordant or by means of indigo and a limited number of other
naturally occurring vat dyes. Both of these methods were troublesome
and expensive. Cotton was made in large quantities in the last century
for markets where cheapness was a most important consideration. The
direct dyes were inexpensive and easy to apply and, although of
indifferent wet-fastness, their use spread with great rapidity because
they fulfilled an outstanding demand. New members with improved
fastness are still being added to this class.
CLASSIFICATION ACCORDING TO DYEING BEHAVIOUR
It was appreciated by earlier workers that the behaviour of
individual direct dyes varied considerably. This necessitated special
care in selection, particularly in mixture, in order to achieve optimum
results and to prevent the occurrence of faults, such as uneven or
insufficiently penetrated dyeings on all types of materials and listing
or ending with jig-dyed fabrics. As a result attention was given to
devising suitable laboratory test methods to characterise the dyeing
behaviour of individual direct dyes and thereby enable the best selection
to be made for a particular dyeing method, highlighting the parameters
to be observed in controlling the dyeing cycle.
In the UK pioneer work in this area by C M Whittaker, John
Boulton and their colleagues at Courtaulds in the 1940s was concerned
with the dhyeing of viscose. A characteristic of individual direct dyes,
described as the time of half dyeing (i.e. the time taken to reach 50%
of the equilibrium absorption under specified conditions), is an
indication of the rate at which a direct dye is absorbed by the fibre. In
the direct dye range it varies from 0.72 to 280 min. Arising from this
work, it was suggested that dyes exhibiting a similar time of half
dyeing would be the preferred choice in mixtures. It was found later,
however, that measurements of the so-called rate of dyeing, related to
time of half dyeing, were inadequate to obtain a full understanding of
the compatibility of direct dyes. Subsequently it was confirmed that
rate of dyeing alone is insufficient to predict compatibility and that
rate of migration and salt controllability are of greater importance.
As a result of a detailed study of the subject by the Society of
Dyers and Colourists’ Committee on the Dyeing Properties of Direct
Cotton Dyes it was concluded that determination of four parameters
was necessary, i.e., migration (or leveling power), salt controllability
and the influence of temperature and of liquor ratio on exhaustion.
Tests are prescribed for migration and salt controllability whilst a
statement covers the influence of temperature and liquor ratio, no tests
being prescribed. The aforementioned SDC committee recommended
that direct dyes be classified as follows.
Temperature-ranges tests are useful for determining the behaviour
of individual dyes at various temperatures of dyeing and are of
particular value in the selection of compatible dyes for mixtures. The
percentage absorption of dye under standard conditions of electrolyte
concentration, liquor ratio and time of dyeing at a variety of
temperatures is estimated visually or colorimetrically and the results
are given in the form of graphs.
The selection of compatible dyes for padding and jig dyeing
processes is not whooly covered by the SDC ABC classification and
related tests. This can be done, however, by carrying out simple dip or
strike tests in which fabric or yarn samples are dyed for short periods,
e.g. for 1-2 min, removed from the dyebaths, replaced by fresh samples
and the procedure repeated several times; the patterns are mounted in
series and assessed visually for change of hue and depth. Marked
changes of hue indicate incompatibility.
The various tests described are simple to perform, required the
minimum of apparatus and skill, and the results obtained are easy to
interpret. They provide valuable information on the performance of
individual direct dyes, either alone or mixtures.
Sulphur Dyes
Introduction
These constitute a group of dyes of unknown constitution which
can be applied to fibres when reduced with sodium sulphide. Most of
them are insoluble in water before reduction. After reduction they are
soluble and can be absorbed by fibres by fibres and than oxidised to an
insoluble form with air. These dyes are popular because of their heavy
shades, such as blue, green, black, brown, etc. of reasonable fastness to
light and ordinary washing at a low cost. These dyes are second to the
azo dyes in quantity produced.
Although structures cannot be written for the sulphur dyes, the
methods for reproducing individual types are well established. These
are manufactured by treating aromatic amines, phenols, ammo-phenols,
with sulphur and or sodium polysulphide at 150-200°C. Some important
sulphur dyes are described as follows :
(i) Sulphur black I is manufactured by heating //i-dinitrophenol
with sodium polysulphide. The fused mass is dissolved in
water and blown with air until all the dye has separated. It
is then filtered, washed and dried.
(ii) Brown sulphur dyes are obtained by fusing m-diamines (e.g.
m-toluenediamine) with sulphur. During this preparation,
hydrogen sulphide gas is evolved.
(iii) Red shades are obtained by fusing sulphur with derivatives
of azine, such as the compound below which produces
sulphur red 6.
Properties of Sulphur Dyes
From the name it is clear that these dyes contain little amount of
sulphuric acid. The fibers those can be dyed by these dyes are Viscous,
Staple fibers. Yarn, any materials which give a resin finish, silk etc.
• These dyes have an excellent light fastness properties.
• Dyeing temperature: 80-95 degree C (Optimum) but
sometimes at cold temperature also.
• It is a good soluble in Na2S.
• It has a good exhaustion.
• Its dyeing rate is moderate.
• It is a soluble in water.
• Make rapid black on cellulose materials.
• Sometimes create direct prints on cellulose.
Since so little is known of their structures, sulphur dyes are
usually classified according to the chemistry of their starting materials.
The manufacturing processes are chiefly of three types:
1. A dry mixture of the organic starting material (or material)
with sulphur is heated (the temperature usually exceeding
200°C).
2. As 1, but using sodium polysulphide instead, sulphur. The
baking temperature varies widely.
3. The starting material is heated with aqueous sodium
polysulphide, either under reflux or in a closed vessel under
pressure. Some or all of the water may be replaced by butanol.
The shade and properties of the resulting dyes may vary
considerably with the reaction temperature and duration of heating. In
all cases hydrogen sulphide is evolved during reaction and it is absorbed
in aqueous caustic soda. The dyes are usually isolated from alkaline
solution by air oxidation. Many of them are subject to deterioration
during prolonged storage.
The properties of sulphur dyes are intermediate between those of
direct dyes and vat dyes. As already stated, reds are poorly represented,
only dull Bordeaux shades being available. Other hues are plentiful,
but almost all sulphur dyes are somewhat dull. Wet fastness properties
are usually good, but resistance to bleaching is poor. With some notable
exceptions, as in sulphur black T and its equivalents, light-fastness is
only fair or moderate. The great demand for sulphur dyes is due to
their moderately good properties and low cost.
They are applied almost exclusively to cellulosic fibres, the alkaline
batch required being unsuitable for wool and silk. The process consists
in dissolving the dye in a solution of sodium sulphide, whereby it is
reduced to a leuco compound with affinity for the fibre, carrying out
dyeing just below the boil, then exposing the dyed material to air so
that oxidation and development of the shade take place. Sometimes
the dyeings are aftertreated with a mixture of a dichromate and copper
sulphate for improvement in fastness to light and wet treatments, but
this is liable to result in tendering of the fibre by slow liberating of
sulphuric acid. Cotton dyed with sulphur colours acquires affinity for
basic dyes, and there are sometimes applied as ‘topping’ colours in
order to brighten the shades. Sulphur blacks can also be topped with
aniline Black to give very deep black shades with increased fastness to
milling.
