This is one of the best books on Textile Dyes and Pigments covering Formulae, Manufacturing Processes of Various Textile Dyes and Pigments. This book will be very helpful to new entrepreneurs, professionals, libraries or those who wants to diversify in this field. This book will also serve as a reference book for the people who are in this field.
Organic Colour Pigments
Introduction
Organic colouring materials of natural origin have been used for centuries. They are obtained from plants and animals and include blues from indigo and the woad plant, green from chlorophyl, reds from madder root, lac insects, and the female cochineal bug, and many other colours and sources. The synthetic organic colouring materials are generally considered to stem from Perkin’s discovery of violet in 1856. It is also recognized that Woulfe in 1771 prepared picric acid which is a yellow dye for wool, and Runge in 1834 produced the red dyestuff known as rosolic acid. The very useful and convenient chart showing the significant developments in organic colouring materials since Perkin’s violet is reproduced by courtesy of the Interchemical Corporation. Vesce gives the visual and infra red spectral characteristics and the structural formulas of a number of organic and inorganic pigments.
Pratt gives a detailed account of the discovery and development of natural and synthetic organic colouring materials. Pratt also gives a very comprehensive discussion of the chemistry and physics of organic pigments which includes the raw materials and the intermediates used in the preparation of organic pigments. He illustrates profusely the chemical reactions and structural formulas and points out the extreme importance of the physical structure of the pigment particles to the development of maximum colour and pigment properties. Pratt’s book also contains an account of Vesce’s microcrystallization technique for the identification of organic dyestuffs and photomicrographs of the crystals obtained from various organic pigments. Another section gives the spectrophotommetric reflectance curves for a variety of organic colours which also serve to identify these pigments. A qualitative spot test for the identification of organic colours is given later in this chapter.
Pratt’s book should be consulted by readers who wish more nearlv complete details than can be given in the present survey of organic pigments. The present chapter simply outlines the structural theory of colour in organic pigments and illustrates it with the more important types of pigments. In the present chapter, the physical and chemical properties of specific commercial pigments will be given to illustrate the types currently available. It should be kept in mind that comparable pigments usually are made by other manufacturers most of whom are listed in the Pigment Index issued by chemical weekly. Also, the reader should consult the various pigment manufacturers regarding specialties which they may produce and for the latest developments in the field.
Toners and Lakes
The difference between dyes and pigments is their relative solubility; dyes are soluble and pigments are essentially insoluble in the liquid media in which they are dispersed. In the manufacture of organic pigments certain colouring materials become insoluble in the pure form, whereas others require a metal or an inorganic base to precipitate them. Usually the base is either alumina hydrate or gloss white. The colouring materials which are insoluble in the pure form are known as “toner pigments” or “toners,” and those which require a base are referred to as “lake pigments” or simply “lakes.” This definition is quite clear cut, but there is a difference of opinion in the trade regarding organic pigments which require metals such as barium, calcium, and sodium to insolubilize the colour and also regarding those pigments which are rosinated. For example, the important lithol pigments are available as barium lithols, sodium lithols, etc., and also in rosinated and non-rosinated types. These “metal toners” are not completely organic, but it is general practice to call them rosinated toners. The rosinated types may contain 15-30% of metallic rosinate. Rosination is accomplished during manufacture of the pigment by insolubilizing alkali rosinate simultaneously with the dyestuff. The rosinated type is more readily dispersed in coating and ink vehicles and produces greater brilliance and gloss in printing inks.
It may be surmised from the foregoing discussion of insolubilizing organic pigments that there might be differences in the degree of insolubility among them. Since this surmise is true, the “bleed” test is very important in the evaluation of organic pigments. The bleed test is referred to in earlier chapter, but the only inorganic pigment which was rated for bleeding was zinc yellow. It bleeds in water because of its slight solubility in this medium. When organic pigments are used in coatings, usually it is sufficient to rate them for bleed in mineral spirits, aromatic hydrocarbons, lacquer solvents, oils, and water. However, for use in printing inks it may be necessary to rate them for bleed in solvents such as alcohol and glycol, in mineral oil and fats such as lard or butter, and in hot paraffin wax. Some pigments bleed in contact with soap because of the fats in the soap; yet these pigments may be quite resistant to alkali.
General Characteristics
Before discussing the theory and production of colour in organic pigments it would be well to consider briefly their general characteristics and to make some comparison with the inorganic pigments. In general, organic pigments are higher in price than the inorganics with a few exceptions such as the cadmium colours, hydrated chromium oxide, and inorganic maroon. The most distinguishing characteristic of the organic pigments is their brilliant colour and high tinting strength. Unfortunately, some of the most brilliant colours fade badly when exposed to strong light, and others which are lightfast in mass tone fade when used in tints. However, many organic pigments have good permanence to light and are satisfactory for automobile finishes and other exterior applications. The less permanent colours are used for interior organic coatings and printing inks in which brilliance of colour is more important than permanence to light.
In general, organic pigments have lower hiding power but greater tinting strength than inorganic pigments. Some of the yellow and orange lakes are so transparent that the are used in coatings for tin plate or polished aluminium to produce brass and gold effects. Many organic toners may be used in low concentration for transparent coloured coatings on metals. In some cases the low hiding power and inability to protect the binder from ultraviolet radiation results in poor durability of exterior finishes. There is considerable variation among organic pigments in their resistance to acids and alkalies; therefore this feature should be checked carefully when chemical resistance of the coating is important. It was mentioned previously that it is necessary to know the resistance to bleeding of organic pigments because some show partial solubility in the oils, plasticizers, and solvents used in coatings.
As may be expected, organic pigments are lower in specific gravity; consequently they are higher in bulking value than inorganic pigments. In general, organic pigments have small particle size which,combined with low specific gravity, produces high oil absorption. Also they are more difficult to disperse in coating vehicles than most inorganic pigments.
Although there are a great many organic pigments available they may be classified in about six groups based on some characteristic of their chemical composition. The major groups are shown in Table 1, but the “basic PMA, PTA” group and some of the pigments in the “acid azo” group are used almost exclusively in printing inks. The significance of the method of classification will be apparent after the discussion of the chemical structure of the pigments. Some of the well-known types of pigments of each group are shown in following Table 1.
Typical Pigment in Groups of Table 1
Azo, insoluble: Toluidine, para, chlorinated nitranilines; naphthol reds, Hansa, benzidine, dinitraniline orange
Acid azo: Lithol, lithol rubine, BON colours, red lake C, Persian orange, tartrazine
Anthraquinone: Alizarine, madder lake, indanthrene, vat colours
Indigold: Indigo blue and maroons
Phthalocyanine: Phthalocyanine blues and greens
Basic, PMA, PTA: PMA and PTA tones and lakes, rhodamine malachite green, methyl violet, Victoria blue
*Rating: Ex = excellent; G = good; F = fair; P = poor.
Colour in Organic Materials
Certain organic materials are coloured when viewed in normal light for the basic reason, that they selectively absorb and reflect specific wavelengths of the visible spectrum. The colours these materials exhibit correspond to the wavelengths which they reflect. The materials absorb certain wavelengths because the frequencies of their electrons are essentially the same as those of the specific waves of light which they absorb.
Witt is generally considered the first to relate colouring organic compounds to certain groups of atoms in their chemical structure. The groups which are essential for the production of colour is called chromophores by him. He noted that other groups improved the colour; he called these groups auxochromes. Typical chromophore and auxochrome groups are:
An-organic molecule which contains a chromphoric group is known as a chromogen. However, the chromophores vary in their ability to contribute colour and in the type of colour which they impart. For example, each of the structures shown below contains a chromophoric group, but stilbene is colourless, azobenzene is orange, and thiobenzenophenone is blue.
Each of the foregoing three structures contains benzene rings which are common to a large majority of dyes and organic pigments. However, benzene rings are not essential to colour in organic compounds as indicated by the following structures:
It will be noted in the three structures above that colour is not produced until conjugated unsaturation is present. In addition, the colour is more intense as conjugation is increased from two conjugated groups in diacetyl to three conjugated groups in triketopentane. However, conjugation cannot be accepted as a definite criterion of colour, because it is present in each of the three structures in the first group, and one of these is colourless. The addition of auxochromes to a chromogen usually intensifies the colour as shown by the folowing structures.
An excellent discussion of colour and the chemical constitution of dyes is given in earlier chapter. Lewis developed the relationship between the characteristic frequencies of electrons in chemical structures and the ability of such structures to produce colour by absorbing corresponding frequencies in the visible spectrum. From the discussions of resonance in organic structures it will be apparent that the coloured structures given previously are capable of resonating. It appears to be a general rule that as the possibility of resonance increases in a structure there is greater possibility for colour and intensity of colour. From this it will also be apparent that the introduction of one or more auxochromes into a chromogen must increase its possibility of resonance since the colour is intensified. Schneid applies the theory of resonance to a wide range of coloured organic structures and shows that as resonance is increased the colour is intensified, and as resonance is inhibited a loss of colour results.
An examination of the structures of the various organic pigments given later in this chapter will show that they contain centers of unsaturation which are usually conjugated systems. The electrons in such systems are able to shift their positions within the system, which is the phenomenon or resonance. Under such conditions the characteristic frequencies of the electrons are lowered to such an extent that they correspond to those of the visible spectrum. Therefore, when they are exposed to visible light they absorb selected wavelengths, reflect the others and thereby appear as the colour of the reflected wavelengths.
Raw Materials and Intermediates. For the manufacture of organic pigments the basic raw materials first are converted into intermediates. These are reacted in various combinations to produce a very wide variety of colours. The basic raw materials are the aromatic hydrocarbons.
Intermediates are produced from the hydrocarbons by reactions such as nitration, sulfonation, halogenation, hydroxylation, and carboxylation. The nitrated products may be reduced to produce intermediates such as aniline. Pratt, who gives a detailed account of these reactions, lists the wide variety of products which may be obtained from the various hydrocarbons. A few typical intermediates are shown below:
Another important reaction in the production of organic colours occurs between an amine and nitrous acid in the presence of a mineral acid to produce a diazonium compound. It is believed that the diazonium compound exists in equilibrium with the diazo form. The reaction between aniline and nitrous acid in the presence of hydrochloric is illustrated below:
A wide variety of diazonium compounds may be made by varying the substituents on the amine. These compounds are used in the manufacture of the important azo pigments referred to in Table 1. When the reaction occurs with a diamine instead of a monoamine, a tetrazo compound is produced. This is illustrated by the reaction with benzidine to produce a tetrazo compound.
Again a variety of tetrazo compounds may be obtained by using different diamines. For example, the o,o’-dichlorobenzidine produces a tetrazo compound used in the manufacture of the important benzidine yellow toner.
Coupling Reaction. As stated previously, various intermediates are reacted to produce dyestuffs, some of which are insoluble in water and are filtered and dried and used as toners. Other dyestuffs require metals or materials such as alumina hydrate to insolubilize them. A very important reaction between two intermediates for the production of the azo pigments is known as the coupling reaction. This reaction occurs between a diazonium compound and a phenolic or an amino compound to form an azo compound. This reaction is used for the production of such important pigments at toluidine, para, and lithol reds,and the Hansa and benzidine yellows. A very wide range of azo pigments may be formed by varying the diazonium compound, two of which have shown, and also by changing the coupling compound. The coupling reaction in a simple form is shown here:
Beta naphthol, just shown, is a very important intermediate for the coupling reaction. Many pigments are made from it by varying the diazonium salt. For example, when b-naphthol is coupled with the diazonium salt made from m-nitro-p-toluidine, the product is toluidine red. Other combinations are illustrated in the section on azo pigments.
Another important intermediate used in the coupling reaction is b-oxynaphthoic acid, illustrated previously. It is coupled with a series of diazonium salts to produce the line of pigments known as BON colours. These colours are azo pigments because they contain the azo group, —N=N—, but their properties are different from the azo pigments made with b-naphthol instead of b-oxynaphthoic acid.
Another important intermediate used in azo pigments is acetoacetanilide. It is made by reaction between an acetoacetic acid ester and aniline. It exists in both keto and end forms, but the enol form is used to illustrate its reaction with a diazonium salt to produce the azo pigment. The coupling reaction between the diazonium salt of m-nitro-p—toluidine and acetoacetanilide to produce the type of pigment known as Hansa yellow is shown below:
Again, different azo pigments may be produced by varying either or both the diazonium salt or the acetoacetic arylide. For example, benzidine yellow is made by coupling acetoacetanilide with the diazonium salt of dichlorobenzidine. The structure of benzidine yellow is shown in a later section.
Another group of azo pigments contains the pyrazolone ring structure with various substituents. The pyrazolone compounds are coupled with diazonium salts to produce such pigments as the pyrazolone reds discussed in a later section, and also a type of Hansa yellow which is much redder than the regular type shown previously. A comparison of the structures of the two types of Hansa indicates the wide range of possibilities for developing organic colours. Of course, the relative cost of production and performance of the colours determines the extent of their commercial applications. The pyrazolone ring structure with five positions for substituents is shown below together with the red type of Hansa yellow and a tartrazine yellow.
Antnraquinone Group. An important group of pigments listed in Table 1 is the anthraquinone group. The basic raw material for this group is antracene which is oxidized to the chromogen, anthraquinone. The possibility for resonance and the development of colour may be seen from its conjugated structure. Various intermediates and dyestuffs are produced by addition of auxochromic groups to the anthraquinone chromogen as indicated on former page.
Lakes and toners are produced from the foregoing materials by complicated procedures. Typical examples of these pigments are the alizarine reds, maroons and purples, the red madder lake, and indanthrene blues and violets. In general, the indanthrone pigments have very good lightfastness both in mass tone and tints and are statisfactory for exterior paints and automotive finishes.
The natural anthraquinone type pigment known for centuries as madder red is obtained from the roots of a plant grown in Europe and Asia Minor. Synthetic madder is known as alizarine red and is available as lakes. The alizarine pigments have a considerable range in colour from red to violent depending on modification of their structures and variations in processing. As may be expected such variations also may produce differences in their other physical and chemical properties. The characteristics of the two commercial alizarine lakes shown in Table 2 may be taken to illustrate the good properties of these pigments and also the variations which may be expected among different members of the group.
The madder lake in Table 2 is a deep red in mass tone and produces tints with white which have a yellowish pink colour. The alizarine lake B is a deep violet maroon in mass tone, but its tints with white range from deep to light orchid in colour. Madder lake is used in combinations with red iron oxide to brignten the colour and produce the Tuscan reds. Tuscan reds made with madder lake have good permanence and do not bleed into subsequent coats of oil paint.
Many extensions of the anthraquinone structure are described by Pratt which are used to produce various colours. A typical example is the complex structure of indanthrene blue given previously. Another illustration is the Platinum Violet pigment manufactured by du Pont. This pigment has the following dibenzanthrone structure:
This pigment is available in two forms, a rosinated high-strength type and a lake pigment. Both have excellent lightfastness and resistance to acid and alkali. They also have good resistance to darkening at normal baking schedules. It will be seen that the anthraquinone-type pigments have good properties and a wide range of colours. There is no doubt that they would be used more extensively if their cost were not so high.
Indigoid Group. A few red, maroon, and violet pigments are available known as thioindigo colours. They are related to the natural blue dyestuff, indigo. They all contain the chromophoric group
but the chromogens vary as indicated on next page.
Synthetic indigo has not been widely used as a pigment. However, the thioindigo pigments have rich colours and good lightfastness and are satisfactory for such exterior applications as automotive finishes.
The Basic Group. The group of pigments based on the so-called “basic dyes” usually contain diphenyl or triphenyl methane as the chromogen. The chromophores attached to these chromogens are amino or substituted amino groups which impart the basic characteristics to these compounds.
The amino groups are neutralized with acids or acid salts to produce the “basic pigments.” The lightfastness of these pigments is changed markedly by the type of acid used to precipitate them. Formerly, acids such as tannic, oxalic, and tartaric were used but the pigments were quite fugitive to light. The use of phosphotungstic acid (PTA) or phosphomolybdic acid (PMA) or mixtures of these acids has resulted in much greater lightfastness. However, these pigments are not sufficiently permanent for exterior exposures. A smaller group of the basic pigments are based on xanthene, thiazine, and thiazole as chromogens. Pratt gives a comprehensive discussion of the chemistry of the PTA and PMA pigments, and Linz and Coffer of the Climax Molybdenum Company describe their manufacture and use in printing inks.
The PTA and PMA pigments constitute a group of very brilliant colours which include red, yellow, green, blue, and violet. Typical structures of the dyestuffs from which these colours are made are shown in Fig. 1. Differences in colour and other properties result from use of either PTA or PMA or a mixture of these acids. In general, tungsten is more expensive than molybdenum. Theoretically, the PMA pigments can be made stronger than PTA pigments because less PMA is required to neutralize the basic dyestuffs. These pigments are used almost exclusively in printing inks and metal decorating coatings and to only a minor extent in paints and enamels.
Rhodamine red toners are available in both yellow and blue tones. It will be noticed in Fig. 1 that the rhodamines have the xanthene chromogen in contrast to the triphenyl methane for the others. Typical of these pigments the rhodamines have only fair lightfastness in either mass tone or tints. Also, they bleed considerably in lacquers and slightly in enamels, and some grades have sufficient heat resistance for regular baking schedules. They have quite poor resistance to alkali.
The group of green toners and lakes based on Malachite green and Brilliant green range in colour from yellow to blue greens. They are similar to the rhodamines in general properties but have somewhat better heat resistance. The Victoria blue toners generally are the best in this group of pigments for light fastness. They also have better chemical resistance and will resist baking temperatures up to 270°F. Some grades are reduced with alumina hydrate or gloss white to improve dispersion in organic media. The violet toners based on methyl violet have considerable bronze and have an adverse effect on the drying of oxidizing vehicles.
It will be realized from the foregoing brief descriptions that these pigments are not used extensively in regular coatings but are restricted to special applications. Their very brilliant colours are desirable for decorative inks where permanence to light is not as important as brilliant colour. For some colours the phthalocyanine blues and green are almost as brilliant with added advantage of permanence to light, heat, and chemicals.
The Acid Dye Group. The so-called acid dye pigments are somewhat related to the basic group. Many of them are based on triphenylmethane with amino groups on this structure, but in addition they have sulfonic acid groups. This increases their solubility in water and necessitates a base such as alumina hydrate for effective precipitation of their metal salts. Like the basic group these pigments have brilliant colours but poor lightfastenss and alkali resistance. They are used principally in printing inks and some metal decorating coatings. The best known pigments in this group are peacock blue lake, alkali blue, and acid green. The general relationship of these pigments to the basic group will be seen from their structures in Fig. 1.
Fig. 1.
Manufacture of Organic Pigments
Before describing the various types of organic pigments which are available commercially, a brief outline the method used for their manufacture will be given. It was pointed out paurously that the particle size and crystal form of organic pigments are extremely important factors in their colour, hiding power, and other performance characteristics. Since particle size and crystal form frequently can be varied by conditions of manufacture, it will be realized that very rigid control of the various operations must be maintained. Slight changes in pH, temperature of the reaction, rate of agitation, or even the size of the batch may change the characteristics of the pigment.
The colours are made or “struck” in large wooden vats, equipped with slow-speed agitators and steam and cold water connections. The intermediates are placed in smaller tanks located above the striking vat. The intermediates are run into the vat at a predetermined rate with proper agitation and temperature conditions. Many colours are struck at low temperature which necessitates placing ice in the vat. Above normal temperatures are obtained from addition of steam. In the manufacture of lake pigments, the base, such as alumina hydrate, is placed in the vat before striking the colour.
After precipitation or insolubilization is complete the pigment is filtered, dried, and pulverized. Care must be taken in the drying process, because the colour of many organic pigments is sensitive to heat, and some have relatively low melting points and their very fine particles tend to sinter together. Excessive pulverization may damage the crystals and change the colour properties. Each batch of pigment is tested for such properties as colour, tinting strength, texture, permanence to light, bleed, and chemical resistance.
Pratt gives many details regarding the manufacture of specific types of organic pigments. Locke describes a process in which diazotization and coupling of azo pigments may be carried on in the same reaction medium. By suitable adjustments, the process can be made a continuous one. Locke also describes a continuous process which includes the diazotization, coupling, and laking reactions. Schmidt-Nickels describes an improved process for production of vat dyestuffs of the dibenzanthrone series and their leuco sulfuric add esters. The large number of articles in the trade and patent literature indicate continuing developments in the organic pigment field.
Spot Tests for Colour Pigments
Spot tests for identifying colour pigments are rapid and relatively easy to make and are very useful if the limitations of the method are recognized. The test may be applied to an unknown pigment or to a coating containing an unknown pigment. In some cases the test may be used on mixtures of pigments. The spot test is based on the specific colours obtained from solutions of a pigment or coating in the cups of a white porcelain spot plate.
The insoluble azo pigments may be distinguished from the acid azo pigments by a chloroform solubility test. The insoluble azo pigments are insoluble in water but are soluble in chloroform, whereas the acid azo pigments do not dissolve in chloroform. If a very small quantity of pigment is dissolved completely when placed in a test tube about one third full of chlorofom, it may be classed as an insoluble azo, and in subsequent spot tests the presence of other pigments need not be considered. If part of the pigment is insoluble it may be considered to be a mixture. The insoluble portion may be filtered off and tested by the spot method.
The spot test is made by dissolving a little of the unknown in concentrated sufluric acid in one of the cups of the spot plate and then placing some of the solution in two other cups containing distilled water. One of these two cups is then made alkaline with ammonium hydroxide. The various colours and/or precipitates obtained in the three cups serve to identify the unknown. Tables 3 to 6 inclusive give the colours and reactions in the three cups obtained with various pigments. The colour of the acid solution may be observed by puddling a little of it over the side of the cup. A drop of the acid solution should be transferred to cups containing distilled water with a stirring rod. Sufficient concentrated ammonium hydroxide should be added to one of these cups until its contents are strongly alkaline. The colour and reactions in the three cups together with confirmatory tests will serve to identify the unknown.
The methods for determining specific gravity, bulking value, and oil absorption of pigments are given in the next chapter. The method for determining fineness by the amount retained on a 325-mesh screen also is described. However, as pointed out previously, this method determines only the large aggregates because the screen openings are 44 m across. Since actual particle size distribution tests are time-consuming, the more practical “ease of grinding” test is made to indicate particle size and “texture.” The term “texture” sometimes is used to mean the relative hardness or softness of pigments, but the more general meaning is the relative ease of dispersion in paint vehicles.
Ease of Dispersion. This test is made by dispersing the pigment in a vehicle and noting the number of passes over a roller mill that are required to obtain a specified fineness of dispersion as indicated on a Hegman-type gage. The rating also may be based on the Hegman gage reading obtained from one or more passes over the roller mill, but its rating is based on the minimum thickness of paint film required to cover the coarsest particles in the dispersion. The gage produces a wedge-shaped film, and the point at which coarse particles appear above the film can be read numerically. Pigment manufacturers may use a standard vehicle such as an oil, varnish, or alkyd resin in which to desperse the pigment, but paint manufacturers will obtain best results by using the actual vehicle in which the pigment is used.
A number of “additives” are discribed in the next chapter which may be added to pigment-vehicle grinding pastes to facilitate the mixing and grinding operation. Pigment manufactures have produced “easy-grinding” types of pigments by surface treating the pigment particles to reduce the interfacial tension between the pigment and the vehicle. A special case of surface treatment for certain lithol pigments is “rosination” referred to previously. These pigments are alkaline earth salts of the dyestuffs and, if alkali rosinate is present when they are precipitated, the rosinate is incorporated in the finished pigment. Rosinated lithols are more brilliant in colour and produce greater gloss in printing inks. They have higher oil absorption than the non-rosinated type and they tend to produce a thixotropic consistency. They contain 15-30% of rosinate and are more transparent than the non-rosinated type.
Bleed Test. The best method of test for bleed is to incorporate the pigment in the actual coating in which it is to be used, apply it to a panel, and after drying, apply over it the coating or other material into which it must not bleed. For instance, if a red baking enamel must resist bleeding into a white lacquer which is applied as a stripe over it, the pigment should be tested under such conditions. This type of test may be used by paint manufacturers because they know the particular formulation and resistance to bleed to which the pigment will be subjected. Since this information is not always available to pigment manufacturers they are more apt to use a “solubility” test. For example, about 0.5 gm of pigment may be shaken with 15 to 20 cc of water or various organic solvents and then filtered through several filter papers.
The clear filtrate then is compared with the original solvent for colour. For the hot paraffin wax bleed test the pigment may be stirred into melted wax and then filtered and rated for colour change. For oils which are too viscous for proper filtering, the pigment-oil paste may be applied as a spot to a pile of filter papers, and the colour of the oil which penetrates into the underlying sheets is compared with that of the clear oil on the same paper. In this case, care should be taken to distinguish between finely dispersed pigment which may migrate with the oil as it is absorbed by the paper and a true bleed due to solubility of the pigment in the oil. For bleed into such materials as animal and vegetable fats, bacon, etc., these materials may be placed in contact with a coated panel or paper, removed at stated time intervals, and rated for degree of staning. The temperature of this test is important because it may vary over a wide range under conditions of actual use.
Permanence to Light and Heat. Permanence to light should be tested as indicated by incorporating the pigment into the actual coating or printing ink and exposing the dried coating to subdued light, direct sunlight, and in the Fadeometer or Weatherometer. In each case, a part of the coating is protected from the light source and comparison between exposed and protected areas is made at stated time intervals. The extent of colour change may be rated visually or measured photometrically. Tests should be made on both mass tone and tints because some colours are fairly permanent in mass tone but fade badly when used in tints.
Heat resistance of organic pigments also should be tested on actual coatings because the results are affected by type of vehicle and concentration of pigment.
Chemical Resistance. A general rating for resistance to acids or alkalies is only an approximate rating for specific end uses. Several factors affect this test such as length of time of exposure, pH, and amount of chemical. Some pigments may have satisfactory resistance to inorganic acids but not to organic acids or vice versa. Other pigments may be resistant to alkali but bleed in soap or vice versa. Other pigments may be resistant to alkali but bleed in soap or detergent solutions. In view of this situation, the chemical resistance tests should be designed to approximate the actual end use conditions as closely as possible. However, such a procedure is not practical for pigment manufacturers as indicated by the description of tests as follows:
Resistance to Alkali. The testing and rating of pigments for resistance to alkali have been made in the past by incorporating the pigment into a typical alkali-resistant vehicle and immersing the film in an aqueous alkaline solution until failure has occurred or for a specified prolonged period of immersion. In this method of test no pigment failure can occur until the alkali has penetrated through the vehicle and contacted the pigment particle. Since pigments occasionally alter the rate of cure and hence the alkali resistance of the vehicle, it has become increasingly apparent that such a test, although probably practical from the standpoint of the paint formulator using a specific type of vehicle, is not satisfactory for the general grading and classification of pigments.
Since both sodium and calcium are very important and typical alkaline reagents, resistance of pigments to these two types seems to be particularly important. Certain pigments are highly sensitive and others highly resistant to both types. Others are quite sensitive to one type but not to the other. Where the pigment is test in an oil, varnish, or resin vehicle, the vehicle itself will show quite while different resistance to each of these alkalies and therefore show markedly different protective effects in a test. For instance, with a saponifiable oil or varnish vehicle, lime will saponify the vehicle but may not break through and contact the pigment surface because of the water-repellent nature of calcium soaps. On the other hand sodium or potassium alkalinity will destroy the vehicle rapidly and attack the pigment.
To eliminate these variables and test the pigment itself, a simple test has been set up wherein the pigment is dispersed in glycerine on a Hoover Muller by a conventional mass tone rub-up procedure. A rub-up of 2 × 100 rotations at full Hoover Muller loading (150 psi weighting) is used. Tight draw-downs on bond paper are prepared and allowed to stand for two hours for film setting, which is mainly by penetration. The film is then partially immersed in a beaker containing the solution of alkali. There is some tendency for the pigment paste to wash off, but it is quite easy to detect any significant degradation or colour change of the pigment at once without influence of the glycerine which for purpose of test can be considered entirely inert to the alkaline reagent. Immersion periods ranging up to one hour have been used depending on the sensitivity of the pigment. For instance, Chrome Greens may be immersed and immediately withdrawn and show substantial failure, whereas highly alkali-resistant pigments stand one hour or longer without noticeable degradation.
In carrying out this test, three alkalies have been used as follows:
- Two percent sodium hydroxide (pH 11.5).
- Saturated solution of calcium hydroxide (pH 12.5).
- Five percent solution of sodium carbonate (pH 11.1)
The adherent alkali is washed from the test strip by a gentle stream of water upon removal from the reagent. These strips are dried and retained for record purposes.
Ratings for alkali resistance have the following significance in the descriptions of commercial pigments given later.
Excellent—Almost no change after 60 min immersion.
Good—Significant change, but not complete failure after 60 min immersion.
Fair—No significant failure after 5 min immersion, but complete failure at 60 min.
Poor—Failure after immersion for 5 min or less.
In grading resistance to alkalinity, the alkali which causes most rapid or most marked failure has been used. This means that certain pigments graded relatively, poor in alkali resistance may show quite good resistance to one alkaline medium. For instance, sodium lithol or sodium red lake C shows quite good resistance to colour change or degradation in sodium alkalinity but will show marked colour change in calcium alkalinity. In the same manner, calcium lithol is quite resistant to colour change in contact with lime, but it is sensitive to sodium or potassium alkalinity. Other classes of pigments such as iron blues, ultramarine blues, and many lakes show different degrees of sensitivity depending upon the alkali involved. Ultramarine blue shows good resistance to all alkalies in this test which is limited to 1-hr immersion. It is well known, however, that on very long exposure to calcium alkalinity, ultramarine blue does react and degrade in colour although it is, as a type, considered to be quite alkali resistant.
It should also be noted that very mildly alkaline soap may cause a bleed with certain pigments, primarily insoluble azo toners, that show a high degree of resistance to alkali. Apparently this is a result of bleed into the organic constituents of the soap.
Resistance To Acid. In evaluating the resistance of pigments to acid, ratings have been made by immersing thoroughly dried panels in a 5% sulfuric acid solution at room temperature for period of 18 hr and noting the colour change or pigment degradation. The panels were prepared by dispersing the pigment in typical chemical resistant rung oil varnish at practical enamel pigment concentration.
Acid conditions to which a paint film might be subjected in actual practice could arise from fruit acids, spillage of battery acids, atmospheric acid conditions in the neighbourhood of industrial plants, or from special conditions in the plants themselves. Under such varying circumstances, of course, the acid conditional might be much more severe than those represented by these tests but in most cases, probably would be less severe. It is recommended that the pigments be tested under the actual conditions of formulation and service to which they will be subjected in use. The evaluations have the following significance:
Excellent—Almost no change
Good—Slight change
Fair—Considerable change
Poor—Almost complete failure
The alkalinity or acidity of the pigment itself may be determined by ASTM Designation: D278-31 (13). In this method a 20-gm sample of the dry pigment is placed in 200 ml of distilled water and shaken for 5 min. Allow the pigment to settle then filter through a dry filter paper, but discard the first 10 ml of filtrate. Transfer 100 ml of filtrate to a flask, add 3 drops of methyl orange solution and titrate with either 0.02 NH2SO4 or 0.02 N NaOH or Na2 CO3 solutions. The alkalinity is calculated to milligrams of NaOH in 1 gm of sample, and the acidity to milligrams of NaOH required to neutralize 1 gm of sample.
Commercial Pigments
A group of commercial pigments from some of the major types will be used to illustrate the properties and characteristics of organic pigments which are available at the present time. The reader should check with other manufacturers than those cited for comparable pigments or for specialties. The brief outline of the chemistry of organic pigments given in this chapter indicates the vast number of modifications which are possible in this field. Since the physical characteristics of these pigments are almost as important as their chemical composition it will be realized that new developments in methods of processing also may produce improved types.
The various classes of organic pigments will be discussed in approximately the same order as that given in Table 1. Wherever possible the basic chemical formula for the various pigments will be given. In many cases the pigment manufacturer has a range of shades of a particular type of colour. These result from slight modifications of the basic formula or from variations in method of manufacture. The reader should compare the basic formulas for the different types of pigments and note the variations used to produce the particular types.
Insoluble Azo Pigments. The insoluble azo pigments are the azo (—N=N—) dyestuffs which are insoluble in water and therefore do not require a metal precipitant. However, this does not mean that they are insoluble in organic liquids. In fact, the spot test used for identification of organic pigments shows that all of the insoluble azos are soluble in chloroform. We shall also see that some of them bleed badly in oils and other organic liquids. The azo insoluble pigments constitute a very important group because they have excellent resistance to acids and alkalies and are quite permanent to light in mass tone. However, they tend to fade when exposed in tint tones, and are sensitive to bleed in organic liquids. The insoluble azo group includes such well-known pigments as toluidine reds and maroons, para reds, chlorinated para nitraniline reds, dinitraniline oranges, naphthol reds, and the Hansa and benzidine yellows.
Toluidine Reds. The five toluidine reds shown in Table 7 are bright rich reds with a limited range in colour. They are produced from the diazonium salt of m-nitro-p-toluidine coupled with b-naphthol. The general reaction may be illustrated as follows:
The range in colour of the toluidine reds in Table 7 is indicated by the letters XL for extra light to XD for extra dark. The differences in colour are relatively slight and are produced by variation in processing rather than by changes in composition. The toluidine maroons are much deeper in colour and have a different chemical composition as indicated later. As the mass tone of the toluidine reds changes from light to dark, the colour of the tints made with white are more bluish. Also, the tinting strength decreases somewhat as the mass tone deepens in shade. The general characteristics of a typical group of toluidine reds are given in Table 7.
Table 7 shows the low specific gravity and high bulking value typical of the organic pigments and also their medium to high oil absorption. Toluidine reds are easier to grind than para reds and some of the lithols, but, like all pigments, they must be well ground to develop maximum colour strength and gloss. They have fairly good hiding power and are less transparent than the lake colours.
Toluidine reds are widely used in exterior finishes because of their rich colour and permanence to light, but they should not be used in exterior tints. Their heat stability makes them satisfactory for industrial baking finishes which are baked at temperatures of 250-300°F. Toluidine reds have good acid and alkali resistance and may be used in chemical-resistant finishes. They have satisfactory alkali resistance for use in latex paints.
The data in Table 7 show toluidine reds to have somewhat poor resistance to bleed in strong solvents. They are usually satisfactory in oxidizing finishes for multicoat work, but the particular finishing system should be checked for bleed if white lacquer stripes are applied over them.
Gloss enamels made with toluidine reds tend to develop a slight haze on their surface after drying. This applies to air-drying oleoresinous enamels and also to alkyd-amino resin baking enamels. The cause of the haze has not been determined precisely, but it may be related to the slight solubility of toluidine reds in the solvents used in these enamels. The residual solvent on leaving the partially set coating may carry a very small amount of pigment to the surface which is deposited as micro crystals when the solvent finally evaporates. There is some variation among commerical toluidine reds in their tendency to haze, and types have been developed recently which are relatively free from this objectionable characteristic. The new types are not quite as bright in mass tone but are stronger in tinting strength.
Toluidine Maroons. Several variations of toluidine maroon pigments are available commercially. Since they differ considerably in their properties the manufacturer should be consulted before selecting one for a particular application. For example, some toluidine maroons are suitable for nitrocellulose lacquer finishes but are not satisfactory for alkyd finishes, and others are designed for alkyd finishes.
The toluidine maroons are similar to toluidine red in that they are prepared from diazotized m-nitro-p-toluidine but the (3-naphthol is replaced with other compounds. Vesce describes several of these compounds among which is naphthol AS (see “Naphthol Reds”). A modification of naphthol AS, known as m-nitroanilide of p-oxynaphthoic acid, was used in a toluidine maroon suitable for use in nitrocellulose lacquers as indicated below next page.
Comparison of the structures of toluidine red and maroon shows both are azo (—N=N—) colours. They are the insoluble azo type because neither a metal nor an inorganic base is required to insolubilize them. The maroon has a larger molecular structure than the red which is frequently though not always a criterion of deeper colours.
A typical range of toluidine maroon pigments is made by Allied Chemical Corporation, Harmon Colours. Sometimes they may be referred to as naphthol or arylamide maroons because of the coupling component as indicated previously. Their range in physical properties is shown here:
Specific gravity 1.28-1.42
Pounds/gallon 10.66-11.83
Gallons/100 lb 9.38-8.46
The toluidine maroons are sensitive to bleed in various solvents; the type of solvent and extent of bleed varies with the individual pigments. Toluidine maroons have good resistance to both acids and alkalies. They are quite permanent to light in mass tone, but they should not be used in tints. They are durable in exterior finishes but the particular pigment-vehicle combination should be selected carefully as indicated previously. In general, the toluidine maroons are not as durable as the anthraquinone and indigoid maroons described later, but they are somewhat cleaner in colour and lower in cost.
Para Reds. Para reds are available in a wider range of colour than the toluidine reds. The para Y type is lighter and yellower than the deep para B type which has a blue tone. The variation in colour is produced by replacing part of the basic ingredients with other compounds.
The basic composition is obtained from coupling the diazonium salt of paranitraniline with (3-naphthol as indicated below:
Pratt describes the preparation of the deep blue-tone para reds by replacing part of the b-naphthol with “monoacid F” or 2-naphthol-7-sulfonic acid. The product obtained from the coupling reaction with diazotized paranitraniline may be converted into a metallic derivative desired.
Table 8 shows some variations in the properties of the para reds with change in colour from the lighter and yellower type to the deeper and bluer b type. In general, the b type is more difficult to grind than the type, and it is more sensitive to acid and water. The excellent permanence in mass tone makes para reds suitable for exterior applications, but they are not as durable nor is their colour as brilliant as the toluidine pigments.
The data indicate clearly that para reds should not be used in tints or in baking enamels. It will also be noted that they bleed badly in oils and solvents. Para reds have excellent alkali resistance, but they may tent to bleed in contact with soap because of the organic consituents of the soap.
Some manufacturers produce a line of para toners reduced with materials such as whiting or blanc fixe. The reduced para reds are useful for interior applications, but they are not permanent for exterior use.
Chlorinated Paranitraniline Reds. There are two varieties of paranitraniline reds, namely the o-chlor-p-nitraniline and the p-chlor-o-nitraniline. These reds are lighter or yellower in mass tone colour than toluidine reds, but much redder than nitraniline oranges The basic reaction for their manufacture is illustrated by the coupling of diazotized o-chlor-p-nitraniline with b-naphthol as indicated below:
A variety of commercial chlorinated paranitraniline toners is available from pigment manufacturers. The range in their physical properties is indicated in Table 9.
The chlorinated paranitraniline reds are relatively easy to grind and have good hiding power. They have very good permanence to light in mass tone, although they are not quite as good as toluidine reds in this respect. The parachlor type is reducer and more transparent than the orthochlor, and it has better lightfastness to tints. These reds show considerable bleed in coal tar solvents and lacquer thinners and slight bleed in mineral spirits and linseed oil, but they do not bleed in water. They have good resistance to acids and alkalis, but they may be sensitive to soap because of their tendency to bleed in fats and oils.
Chlorinated paranitraniline reds have sufficient permanence in mass tone to be used in such applications as exterior enamels for pumps, signs, and awning stripes. They may be blended with benzidine yellow to produce orange colours similar to molybdate orange, and the blends have better alkali resistance. The blends, which also are lead free, may be used for such applications as toy enamels and fume-resistant coatings. There is a tendency for the parachlor type to discolour when used with cobalt driers.
Naphthol Reds -A group of azo insoluble colours may be made by replacing b-naphthol in the coupling reaction with various derivatives of b-naphthol. These colours include the toluidine maroons which are described in other sections. More recently a series of reds have been produced which are referred to genericaly as “naphthol reds.” They are bright reds, more expensive than toluidine and para reds, but they are quite resistant to alkali and to soap.
A series of so-called “naphthol AS” derivatives may be made for these naphthol reds by condensing amino compounds with p-oxynaphthoic acid. A typical example is the condensation of aniline with b-oxynaphthoic acid as indicated next page.
The particular naphthol AS derivatives used in commercial naphthol reds generally are not disclosed, since at least thirty derivatives are possible and compounds other than derivatives of P-oxynaphthoic are used. The difference in properties of a typical group of commercial naphthol reds is shown in Table 10.
The naphthol reds are durable colours, but they are not as permanent to light as the para reds or the toluidines. Their excellent resistance to alkali and soap makes them suitable for such applications as latex paints and soap wrapper inks.
Nitraniline Oranges. A series of orange toners is available produced by coupling either diazotized o-nitraniline or 2, 4-dinitraniline with b-naphthol. Those made from o-nitraniline are isomers of para reds since these are made from p-nitraniline. The o-nitraniline oranges are somewhat paler in shade than the dinitraniline pigments and they are more sensitive to bleed in solvents and oils. The structures of the ortho- and di-nitra-niline oranges may be represented as follows:
The nitraniline oranges listed show the following range in physical properties:
There is some variation in chemical and light resistance properties of the orange toners depending on variations in composition and manufacture. In general, they have good lightestness in mass tone but are poor in tints. They have good resistance to acids and alkalies, fair resistance to bleed in mild solvents and oils, but they bleed in strong solvents. Since the nitraniline oranges are lead-free they may be used in such applications as toy enamels. They are much brighter in shade than the chrome or molybdate oranges. However, they are sensitive to heat, therefore enamels should not be baked at temperatures higher than 250°F. The o-nitraniline orange has somewhat better heat resistance than the dinitraniline orange and is frequently used in baking enamels where bleeding is of no consequence.
Hansa and Benzidine Yellows. Hansa and benzidine yellows are used instead of chrome yellow in coatings which are required to have alkali resistance, fume resistance, and to be lead-free. Cadmium yellow also is alkali and fume resistant, but it is generally considered to be toxic. Cadmium yellow does not have as high tinting strength or as good working properties in printing inks as the organic yellows. Hansa and benzidine yellows have considerably less hiding power than chrome yellow, but they have much greater tinting strength. In this respect Hansa has 4 to 6 times the tinting strength of chrome yellow and benzidine has 10 to 12 times.
There is a range of both Hansa and benzidine yellows available which vary from yellows which have a green shade to those having a red tone. The differences in shade are produced by variation in composition. A very popular shade of Hansa is known as Hansa yellow G, and others are referred to as 3G, 5G, 10G, etc. Hansa yellow G is made by coupling diazotized m-nitro-p-toluidine with acetoacetanilide. The chemical formula for this reaction was given previously.
By comparison of the formulas for toluidine red and Hansa yellow G, it will be seen that both start with m-nitro-p-toluidine but b-naphthol in the red is replaced with acetoacetanilide for the yellow. Pratt shows that the m-nitro-p-toludine in Hansa yellow G is replaced with 4-chloro-2-niraniline to produce Hansa 3G, and with o-nitraniline for Hansa 5G. The 10G uses 4-chloro-2-nitraniline as in Hansa 3G, but it is coupled with acetoacet-2-chloroanilide. This illustrates a few of the many variations which are used in the manufacture of Hansa yellow pigments.
Benzidine yellows also are available in a range of shades depending on variations in their composition. They are made by coupling the tetrazo compounds of benzidine derivatives with acetoacetanilide to its derivatives. Pratt gives details of the reaction and describes some of the derivatives. One of the widely used benzidine yellows is made by coupling the tetrazo compound of 0, 0-dichlorobenzidine with acetoacetanilide as indicated in the reaction shown on the next page.
It was mentioned previously that benzidine yellows are quite transparent compared with chrome yellow, and they have greater tinting strength than Hansa yellow. Benzidine yellows vary somewhat in transparency and permanence depending on variations in composition. For example, the o-toluidine type is quite opaque by comparison with the transparent anilidid and o-anisidide types. Also, the anilidide type is not as good as the others for mass tone permanency. The tint permanency is even more sensitive with the result that benzidine yellows are not generally considered lightfast in tints of interior latex paints. Both Hansa and benzidine yellows have good resistance to acid and alkali, and they do not bleed in oil or water. Benzidine yellow does not bleed in mineral spirits and has fair resistance to strong solvents. Hansa bleeds slightly in mineral spirits and considerably in strong solvents.
Acid Azo Pigments. As indicated previously the acid azo pigments are obtained from dyestuffs which have acids groups (— SO3H, —COOH) in their structures. These dyestuffs are relatively soluble in water but are insolubilized by reaction with salts of sodium, barium, calcium, or strontium, so that they may be used as pigments. The acid azo pigments include such well-known types as the lithols, rubines, lithol rubines, red lake C, red lake D, and certain maroons.
Lithol Reds and Maroons. At present the lithol reds and maroons constitute one of the most widely used groups of organic colours. This is due to their relatively low cost, wide range of colour and good colour strength. Lithols are prepared by diazotizing tobias acid and coupling it with b-naphthol in the presence of sodium hydroxide. This gives the relatively insoluble sodium lithol. The barium, calcium, or strontium lithols are obtained by reacting the sodium lithol with suitable salts of the other metals. The basic reactions for the preparation of sodium lithol are indicated below:
The differences in colour of the lithols depends chiefly on the metal used for precipitation. The sodium lithols are light yellow reds, barium lithols are medium reds, and the calcium lithols are deep reds to maroons.The strontium lithols range in colour between the barium and calcium lithols. Lithols are available in both resinated and non-resinated forms. Resination makes the colour brighter and the pigment more transparent. Resinated lithols produce more thixotropy in enamels and printing inks than the non-resinated type.
The characteristic physical and chemical properties of a group of commercial lithol pigments are given in Table 11.
The mass tone colours in Table 11 range from a light red sodium lithol, 20-4018, to a deep red calcium lithol, 20-4620, and include two maroons, 20-4910 and 20-4995. Their tints with white range from a yellow pink for 20-4018 to blue tone pinks for the deeper reds and lavender or violet colours for the two maroons. The data show that these pigments are not satisfactory for exterior exposure either in mass tone or tints. They have fair baking stability which indicates that they may be used in finishes which are baked for 20 min at temperatures 200-250°F.
The sodium lithol has relatively poor chemical resistance, whereas the others have fair to good resistance. However, the barium and calcium lithols are more sensitive to attack by sodium alkalies than the sodium lithol. Also, the barium lithol is more sensitive to calcium alkalinity than the calcium lithol. Bleed-resistance data show that lithols have better resistance to mineral spirits and coal tar solvents than toluidine reds. They have good resistance to oils but are sensitive to strong solvents. It should be noted that the sodium lithol tends to bleed in water. Some of the many applications for lithol reds and maroons are described in later chapters.
Lithol Rubine Toner. Lithol rubine toner is a very dark bluish red frequently used in combination with a red shade molybdate orange to produce a range of low-cost bright reds for printing inks, tools, toys, and other industrial coatings. They are not sufficiently permanent for the highest grade of exterior coatings but are satisfactory for many less critical exterior uses. Lithol rubine is made by diazotizing p-toluidne-m-sulfonic acid, coupling the product with b-oxynaphthoic acid in the presence of sodium hydroxide to obtain the sodium salt, then precipitating with calcium chloride to obtain the calcium salt:
A typical commmercial example of a lithol rubine is Selkirk Red. Although it has fairly good light resistance it is not satisfactory for exterior finishes. It bleeds slightly through a white lacquer coating but does not bleed through white air-drying or baking oleoresinous enamels which do not contain coal tar solvents. It shows almost no colour change in coatings baked for 30 min at 250°F. The pigment has only fair resistance to alkali.
Rubine Toners. Another group of bright red pigments, known as rubine toners, is produced from diazotized p-chloroaniline-m-sulfonic acid and b-oxynaphthoic acid. The sodium salt is precipitated with one of the following metals; barium, calcium, strontium, or manganese. The chemical structures for these pigments are similar to that shown previously for lithol rubine, except for the various metal precipitations and the replacement of the methyl group in the para position with a chlorine group.
The series of rubine toners produced by Imperial Paper and Colour Corporation includes pigments obtained by precipitating with each of the four metals and also resinated and non-resinated types. These pigments have slightly better resistance to light, bleed, heat, and alkali than the lithol rubine given in the preceding section. They range in physical properties as follows:
Specific gravity 1.49-2.09
Pounds/gallon 12.41-17.41
Gallons/100 lb 8.06-5.74
Grinding ratio 39-45
Red Lake C. Red lake C pigments are light to medium reds with brilliant colour and good lightfastness but they are not sufficiently permanent for exterior use. The barium toners are more widely used than the sodium toners because they are superior in light and heat resistance. The red lake C pigments find wide application in printing inks but only limited use in coatings.
These pigments are produced from the diazonium salt of o-chloro-m-toluidine-p-sulfonic acid by coupling with b-naphthol in the presence of sodium hydroxide. The resulting sodium toner is reacted with barium salts to produce the barium toner.
Typical commercial red lake C pigments are the toners L20-5200 and M20-5650 of the American Cyanamid Company which have the following range in physical properties:
Specific gravity 1.70-1.75
Pounds/gallon 14.2-14.6
Gallons/100 lb 7.12-6.88
Grinding ratio 45
These pigments have good resistance to bleed in hydrocarbon solvents and oils but are sensitive to lacquer solvents. They are similar to lithols in that they are not highly resistant to alkali. Although they have better light resistance than lithols they are not satisfactory in this respect for use in exterior coatings.
Persian Orange. Persian orange and tartrazine yellow are typical examples of the acid azo type of organic lake pigments. They are made by precipitating the metallic salts of acidic dyes on alumina hydrate or a combination of alumina hydrate and blanc fixe. These lake pigments tend to be reactive with bodied oils and varnishes owing to the alumina hydrate content. Some types also retard the drying of oxidizing vehicles. Nace and Walker (20) showed that the loss of drying increases with increasing acidity and moisture content, and the loss is severe in lakes containing excessive dye and in the presence of Turkey-red oil. The loss was shown to be caused by adsorption of cobalt drier onto the pigments. A cobalt adsorption test was developed which was used to indicate differences among alumina hydrate lakes in their effect on loss of drying on aging.
Persian orange is made by diazotizing sulfanilic acid and coupling the diazo compound with b-naphthol to obtain the dyestuff Orange II (C.I.151). The dyestuff is run into a suspension of aluminium hydroxide and precipitated by addition of barium chloride. Considerable variation in properties may be obtained by changes in processing. The structure of the barium salt may be represented as follows:
Persian orange is a very bright transparent orange used in printing inks and baking coatings for metal decorating. It has satisfactory heat resistance for the usual baking schedules of 20-30 min at 250-270°F, but it has poor light resistance. It has poor chemical resistance because it is sensitive to both acids and alkalies. It does not bleed in oil or water but bleeds in alcohol and lacquer solvents.
Tartrazine Lakes. Tartrazine lake pigments are available ranging from bright yellow to orange for use in printing inks and transparent metal decorating coatings. They are made by precipitating the acid hydrazine dyestuff on alumina hydrate or gloss white by barium chloride. The structure of tartrazine yellow, containing a pyrazolone ring, was given previously.
The tartrazine lakes produce brilliant gold, brass, and copper coatings on bright tin or aluminium. They are resistant to baking temperatures of 250-270°F but have only fair light resistance. They have good bleed resistant but are quite sensitive to acids and alkalies.
Green-Gold Pigment. It will be apparent from the foregoing discussion of azo pigments that a very wide variety of types and colors may be produced. Only a limited number of the basic types of commercial azo pigments have been described; therefore the reader should consult with the manufacturers for more details or later products.
An interesting example of the new types of azo pigments which may be produced is the Green-Gold pigment. This pigment is described as the nickel complex of an azo compound and is covered by U.S. Patent 2.396.327. The Green-Gold pigment is a green colour in mass tone but is a greenish yellow in tint tone. It has very good lightfastness in both mass tone and tints. It is somewhat transparent, therefore it has low-to-medium hiding power, but it is strong in tints. In addition to the regular pigment uses it has been found very satisfactory in the so-called metalized finishes for automobilies and novelty products. It produces brilliant and permanent greens when blended with phthalocyanine blue. The Green-Gold pigment is satisfactory at baking schedules ranging from 60 min at 225°F to 10 min at 375°F and is resistant t6 an overbake of 45 min at 250°F or 15 min at 300°F. It has excellent chemical resistance, but it bleeds slightly in linseed oil, alkyd resins, and lacquers.
Pigment Green B. Pigment green B is an example of a pigment produced almost exclusively for specific applications such as wall paper, cold water paints, and latex paints. Its value for these applications is its good alkali resistance and economy in cost. It has brighter colour than chromium oxide and lower cost than either chromium oxide or the phthalo-cyanine pigments. Pigment green B is not used in mass tone because it is almost black in colour. It has relatively good lightfastness in mass tone, but it is poor in tints and is recommended for interior finishes only. The pigment is available for dispersion in organic media and also as water dispersible grades and dispersed water pastes.
Pigment green B is the iron salt of nitroso-b-naphthol; therefore it is a member of the nitroso group of pigments discussed by Pratt. They all contain the nitroso group,—N=O, and are prepared by reaction between phenols and nitrous acid. When b-naphthol is used as the phenol the product is nitroso-b-naphthol. This compound forms complexes with metals such as iron and cobalt which may be used as pigments. Pratt shows that the complexes may exist in tautomeric forms as indicated below:
Phthalocyanine Pigments. Phthalocyanine blue and green are very dark in mass tone and somewhat transparent, and although not generally used in mass tone, they are satisfactory at low concentrations in decorative transparent finishes and the metalized finishes. The phthalocyanine pigments prodcue very brilliant tint colours when reduced with white or bright yellows, and these have very good hiding. The tints have excellent lightfastness and chemical resistance, and they do not bleed in oils or solvents. The phthalocyanine blue appeared on the market in the United States in 1936. The early blues were coarse in texture and difficult to disperse in oleoresinous vehicles, and they tended to flocculate in paints and lacquers. Also, they increased in crystal size when used in formulations containing organic solvents such as the aromatic hydrocarbons and certain esters. Subsequent research by pigment manufacturers has eliminated these early difficulties, and today several grades are available for specific applications.
Phthalocyanine blue may be prepared by reacting phthalic anhydride with a copper salt in the presence of urea or by reacting phthalonitrile with a copper salt with or without ammonia. British patent 464, 673 gives the following example: react a mixture of urea and boric acid with phthalic anhydride and cupric chloride and heat the mixture until reaction is complete. Cool the melt and grind it, then wash it free of reaction by products first with sodium hydroxide solution, then with dilute hydrochloric acid. The crude pigment from such reactions is conditioned by dissolving or “pasting” it in strong sulfuric acid and then drowning it in a large volume of water to precipitate the pigment in the desired fine particle size.
Pigments prepared in the foregoing manner have a different crystal form from those studied by Robertson and prepared by subliming phthalocyanine blue. Since Robertson’s crystal form was described first, it has been called the alpha form and the crystal form obtained from the acid pasting process is referred to as the beta form. Von Suich also studied the two crystal forms, but he used the reverse order of alpha and beta for their naming. Von Suich’s nomenclature will be used in this discussion because it is the one generally used by industry. The structural formula for phthalocyanine blue may be represented as follows:
Diesbach and Weid in 1927 described insoluble blue compounds which they believed to be metal salts of aromatic o-dinitriles with copper and pyridine but which were undoubtedly copper phthalocyanine. Dandridge, Drescher, and Thomas in 1929 described coloured organic compounds produced by reaction of ammonia with phthalic anhydride, phthalamide, or phthalimide in the presence of metals. Iron was the metal used in their first products, but products obtained with copper and nickel were described shortly afterward. The structure of these compounds was determined by R.P Linstead and his students at the Imperial Institute of Science and Technology in London. Dahlen reviewed the history and chemistry of the phthalocyanines and described the properties obtained by variation in metal and also the metal-free phthalo-cyanine. Copper phthalocyanine blue pigment was marketed first in England in 1935 under the trade name of “Monastral” blue, and in 1936 it was placed on the market in the United States.
Siegel in 1939 described a method for resinating the wet phthalocyanine pigment to improve its wettability by organic vehicles. Vesce in 1941 added an emulsion of a vlatile liquid such as petroleum naphtha to the phthalocyanine pigment slurry before drying. This addition reduced the tendency to form hard aggregates in the drying process and facilitated dispersion of the pigment in coating and ink vehicles. Vesce and Staltzer in 1943 describe the production of aluminium benzoate lakes of the phthalocyanine pigments to prevent flocculation and improve colour stability. This reduced the hiding power somewhat because about 30% of aluminum benzoate was required for satisfactory results. Loukomsky and Lacey in 1945 showed that the addition of a surface active agent such as xylene-sulfonic acid to the acid solution of the copper phthalocyanine improved dispersibility of the resulting pigment to a marked degree.
None of the foregoing procedures eliminated the crystal growth of the blue pigment in formulations containing solvents such as aromatic hydrocarbons and esters. When the pigment particles increased in size, parti-beyond 2 m, the hiding of the coating or ink was reduced, and also there was a change in colour. Wiswall in 1949 described the first successful method for producing phthalocyanine blue pigment which was stable in contact with organic solvents. He found that the alpha form, obtained by conditioning the acid pasted pigment, increased in particle size considerably beyond 2 m and changed to the beta form when it was exposed to aromatic hydrocarbons. He could not reduce these particles to pigment size by ordinary grinding methods. However, the particles could be ground satisfactorily in a ball mill in the presence of an inert solid grinding aid. If sodium chloride were used as the grinding aid, it could be washed out of the product with water, and calcium carbonate could be removed by acid leaching. Materials such as blanc fixe or powdered silica could remain in the finished product if an extended pigment were required. Wiswal found that the finely ground product obtained in this manner changed from the beta back to the alpha form in the grinding process. However, when these finely ground alpha particles were subjected to the action of organic solvents, only a very slight increase in crystal size resulted, and the particles were then stable to further crystal growth. The stable blue pigments change to a somewhat greener shade in the processing. Loukomsky showed that the Wiswal three-step process could be accomplished in a single operation. He subjected a mixture of acid pasted phthalocyanine, grinding aid, and organic liquid to intensive mixing and obtained stable pigment particles of the required size. Factors other than the change from the alpha to the beta form have been found to affect the stability of phthalocyanine blue. For example, the red shades of alpha blue are stable to active solvents, and the greenish beta type is stable to both solvents and temperature. The introduction of halogens into the molecule increases stability and the solvent stable alpha form usually contains 2-5% of chlorine.
As a result of the foregoing research and development, commercial phthalocyanine blue pigments now are available which are highly resistant to crystallization in organic solvents. Improvements also have been made in ease of dispersion in organic media, but these pigments have a tendency to flocculate in polar vehicles such as nitrocellulose lacquers. The tendency to flocculate is related to the non-polar character of the phthalocyanine blue. It may be overcome by laking, as mentioned previously, but this reduces the hiding power because a fairly high percentage of base is necessary. More recent developments have produced phthalocyanine blues whkh are essentially free from flocculation in normal coatings and which retain their toner strength.
Phthalocyanine blue is too dark and transparent in mass tone for general use, but it reduces with white or yellow to brillliant tints. Some blues have a greenish tint tone and others a reddish tint. The tints are very lightfast and have excellent durability in exterior finishes, but in mass tone the blue tends to bronze considerably. The blue pigments are not affected by normal baking conditions and are highly resistant to acids and alkalies. They are not subject to reduction with consequent colour change as are the iron blues. They are about twice as strong in tinting strength as iron blue and many times stronger than ultramarine blue.
The non-polar character of regular phthalocyanine blue makes it difficult to disperse in aqueous media. However, water-dispersible grades are available as a result of surface treatments or addition of hydrophilic materials. The pigment also may be obtained in pulp form for use in latex paints.
Phthalocyanine Green. Phthalocyanine green may be prepared by chlorination of copper phthalocyanine blue. At least 12 of the available 16 hydrogens on the benzene nuclei must be replaced with cholerine before a definite change from blue to green occurs. The best green is obtaiend when 14 to 16 hydrogens have been replaced with chlorine. Direct chlorination requires relatively high temperature and anhydrous conditions. The resulting crude product does not have good pigment properties; therefore it is acid pasted as described for the phthalocyanine blue. The final green pigment does not crystallize in the presence of organic solvents and is relatively free from flocculation in polar vehicles.
Phthalocyanine green is very dark in mass tone and is rarely used as such. It has high tinting strength, and it produces very brilliant medium and light tints with white or bright yellows which are extremely lightfast. The green has excellent resistance to soap, alkalies, and acids, and is satisfactory for soap wrapper inks and latex paints. Water dispersible grades and pulp colours are available for the latter application. It has very good exterior durability and may be used in bright green automotive finishes or metalized finishes. Its heat resistance is satisfactory for all normal baking schedules.
Lightfastness in Tints
As a result of extensive research on organic pigments, Vesce gave detailed information on the chemical structure (where known), generic chemical classification, spectrophotometric reflectance curves, Munsell values, and actual colour chips of a range of organic pigments which have outstanding lightfastness in light tints. Vesce points out the necessity for evaluating these pigments in the pigmented system employed for actual end-use rather than the pigment itself. Some of the pigments were found to behave differently in systems such as lacquer, enamel, printing ink, or plastic. The types of pigments developed may be visualized from the classification in Table 12.
Coatings were made with the pigments in several tints with titanium dioxide and exposed at Miami, Florida, for one year. Colour change was measured with a spectrophotometer and Munsell values calculated. An indication of the results from this important research may be obtained from the following outline.
The colour retention of a violet pigment based on carbazole dioxazine in an alkyd enamel was very satisfactory. This pigment shows promise because of its clean, strong tone and non-bleeding property.
Red and green shades of indanthrene blue had good durability and light-fastness even in very light tints,they are non-bleeding, and they have good baking characteristics.
Although phthalocyanines have been used by paint industry since 1934, continued research has raised the standards of performance of these colours. The following three types of phthalocyanines were investigated for colour stability; the green shade of blue (unchlorinated, betamodification), the red shade of blue (monochloro copper phthalocyanine), and phthalocyanine green. These pigments showed excellent colour stability on exposure with a slight increase in chromaticity.
A dibenzanthrone green exhibited only slight difference in Munsell notation between control and exposed panels indicating good light stability. Although this colour bleeds, it is used with aluminium to produce unusual colour effects in metallic finishes.
A flavanthrone yellow showed satisfactory durability in lighter shades but darkened somewhat in fuller tones. It is non-bleeding and possesses excellent baking properties.
The extreme transparency and high tinctorial value of a brown anthrimide pigment make it useful for special metallic formulations despite a slight tendency to bleed.
The high chroma of a tribrom pyranthrone scarlet is approached by very few colours of comparable lightfastness. By comparison, the dibrom pyranthrone is much yellower and more fugitive.lt is not satisfactory in the lighter tints, but is acceptable in the deeper tones.
A halogenated anthanthrone orange-red gave good durability even in light shades. However, this pigment lost almost all its colour in less than 100 hr in a carbon-arc, accelerated fading apparatus which shows the importance of testing colours under conditions approximating those of actual end use.
An imidizole pigment, vat brilliant orange GR, was found to be unsatisfactory at lower concentrations, but it could be used at higher pigment content. An unhalogenated pyranthrone orange, vat golden orange GA, was shown to be unsuitable for very light pastels.
An acridone red was tested in three dilutions in a white alkyd enamel. It has satisfactory lightfastness as a medium red and, although it bled slightly, it has found acceptance for automotive lacquer finishes. A perylene colour, which is a brighter shade of red, had good colour retention and baking characteristics and was nonbleeding for practical purposes.
An oxazole type of the anthraquinone vat group of colours showed some fading at high dilutions in an alkyd enamel but was satisactory in a nitrocellulose lacquer. This fact shows the importance of considering the coating system as a whole rather than the pigment alone.
Two new thioindigo maroons were compared with one of the older type. The new colours are heatproof and do not sublime at temperatures up to 350°F. These colours also are non-bleeding and have excellent lightfastness.
An interesting comparison was made between three different chemical types of pigment of produce a magenta hue when reduced 25% color with 75% rutile TiO2. The three pigments were (1) a manganese toner of permanent red 2B, (2) a chlorinated thioindigo red and (3) the azo arylamide known as permanent carmine. The first showed a severe change in colour on exposure. The second showed some change, but this pigment is considered suitable for exterior use at this concentration. The third is considered among the most lightfast of the azo reds, but the change in colour observed in this test makes it unsuitable for exterior use at this concentration.
Experience in the automotive industry has shown the pastel shades of the halogenated thioindigos as a group are better in lightfastness than any azo color of similar hue. However, some of the individual members of this thioindigo group are much better than others; some are suitable for the most rigid fastness requirement, whereas others must be limited to moderate dilutions.
The following blue and yellow shades of BON red were found to be not lightfast enough for use in permanent pastel shades: manganese toner of permanent red 2B and manganese toner of chlor amino benzoic acid plus BON. The same is true of Hansa yellow G which showed a perceptible fade at 10:90 dilution and serverfade at lower colour concentrations. The azo yellow known as permanent yellow NCG showed little change at 50:50 dilution but had very poor colour retention at 5:95 dilution. By comparison the metalized azo yellow, Virescent Gold, had excellent colour retention even at 1:99 dilution.
Vesce emphasized that the proper assessment of what constitutes “satisfactory durability or lightfastness” depends upon the end use. For example, colours considered unsuitable for the most stringent requirements, such as automotive work, could be completely satisfactory for other uses, such as metal decorating or interior finishes. An outstanding contribution to our knowledge of the behavior of organic colour pigments in exterior exposures was made by Vincent C. Vesce in the 1959 Mattiello Memorial Lecture.
Doctor Vesce exposed a very wide range of colours in the following five vehicles: alkyd air-drying, alkyd-melamine baking, nitrocellulose lacquer, acrylic lacquer, and acrylic emulsion. The colours were used in full tone and also let down with white. Coated panels were exposed in Florida three to twelve months and the change in colour was given in NBS units and also photographically. It was observed that lightfastness of some of the pigments varied with the vehicle in which they were dispersed as noted previously. The data in this lecture provide a wealth of information particularly for the formulator of colored industrial finishes.
Smith and Stead also emphasize the importance of determining the lightfastness of actual pigmentations rather than specific pigments. They illustrate the effect of combinations with other pigments and variations in resinous binders on lightfastness. These authors discuss the problem of measuring lightfastness in the laboratory so that the results will correlate with actual exposure. They propose the adoption of the British Standards Blue Scale to indicate the relative lightfastness of pigmentations. This scale is used by the textile industry and consists of eight standard blue dyeings on wool. Standard 8 is extremely fast to light and standard 1 is very fugitive, and the standards between these limits are arranged to fade geometrically with respect to time. Direct comparison of coatings with the Blue Scale standards could be used to indicate relative lightfastness.
Carr and Musgrave discuss the behaviour of organic pigments at elevated temperatures in plastics and coatings. They also point the importance of the binding medium on the lightfastness, heat stability, and the hazing and bleeding characteristics of specific pigments. These authors conclude that for baking enamels at schedules of 180°C (356°F) for 30 min there are few if any satisfactory organic yellows or violets, but benzidine orange, BON maroon toners, phthalocyanine blue and green, and Pigment Green B may be used. If the baking temperature is lowered to 150-120°C (302-248°F), the number of satisfactory organic pigment is not increased appreciably because of bleeding or other disadvantages.
Monastral Red and Violet Pigment. Reeve and Botti describe an important development in organic pigment that are marketed by Du Pont as Monastral red and violet. The basic chromphore for these pigments is the linear quinacridone structure shown below:
Struve describes the process for making such linear quinacridones and shows how various derivatives can be prepared to yield pigments ranging in hue from bluish red to violt. These pigments show a very high resistance to fading on exposure to the elements and exhibit insolubility in solvents for organic coatings. They are also insoluble in ordinary dilute acids and alkalies. Additional developments on these pigments are described by Struve in a series of patents.
At prsent, a family of these distinctly different pigment colours is commercially available, namely:
Monastral Red B: a dark, transparent; blue-shade red toner
Monastral Red Y: a yellow-shade red toner, lighter and more opaque in mass tone than red B.
Monastral Violet R: a dark, transparent, red-shade iolet toner.
The outstanding combination of properties of these pigments in paint systems is described in detail by Reeve and Botti. These pigments are bright and strong in tints and demonsrate outstanding lightastness, even at high dilution. They are somewhat transparent in mass tone which, of course, is an advantage in polychromatic finishes containing aluminum pigments, and in coloured transparent finishes for aluminium foil.
The Monastral red and violet pigments, when tested for exterior durability in air-dried and baked alkyd enamels, nitrocellulose lacquers and emulsion paints, exhibit excellent results. No colour change attributable to the pigments is observed in baked enamel and silicone coatings, either in mass tone or light tints, when heated at 500°F for 2 hr. Baked enamel and lacquer coatings are not affected by a 1-hr test in the following acids and 1-10% concentration: sulfuric, hydrochloric, acetic, citric and phosphoric. Tests on these coatings with 1% caustic soda for 1 hr, and 4% soap solution at 100°F for 4 hr show no detrimental effect. The Monastral reds and violet do not bleed in baked enamel or lacquer coatings when overstriped with either white enamel or white lacquer.
The intense colour and high tinting strength of these pigments can be used to advantage in blends with other colours such as vat oranges, red shade molybdate orange, red iron oxide, and in metallics or tints with phthalocyanine blue. An interesting application for Monstral Violet R is the toning of white finishes when a high degree of heat and chemical resistance is required. Additional applications for these pigments are discussed in the chapter on industrial finishes.
High Grade Organic Pigments
In the field of organic pigments the development is most extensive. There is growing demand for pigment of high degree of fastness in all the sector of pigment consuming industry such as paint, printing ink, plastic, etc.
The fastness of coloured pigment, refers to its inherent ability to withstand the chemical and physical factors to which it is or will be exposed both during incorporation into the pigmented system and in its end-use application. Fastness refers to the behaviour of a pigment in terms of retaining its initial colour value, either alone or as a component of a pigment system or exposure to light, weather, heat, solvent or chemicals, ideally a pigment should be chemically inert.
High grade pigments can be classified according to their chemical classification as follows:
- Azo condensation.
- Vat pigments and related compounds.
- Dioxazine.
- Quinacridone.
- Isoindolinone.
- Phthalocyanine.
Azo Condensation
Azo pigments have poor fastness properties, and also shows a marked degree of solvent bleed and migration. However these new azo condensation pigments have overcome these fastness problem. They are high molecular weight pigments. The molecular weight of azo condensation can reach 1500, where as the coupling method only yields molecular wts. of about 600. By applying the condensation method to the most varied diazo components, coupling, components, and arylide components, a vast diversity of high molecular azo pigments can be obtained which not be accessible by the classical coupling method.
Azo condensation pigments differ from the azo coupling pigments in their fastness properties are well balanced and of a higher general standard. Apart from possess of good to excellent fastness to light and outdoor exposure, they exhibit a high degree of resistance to bleeding, migration and dry cleaning as well to alkalies, acids and other chemicals and to heat.
Vat Pigments and Related Compounds
The vats is general had the reputation of being deficient in both brilliance and tinctorial strength when used as pigments. The major problem with vat dyes, is that they are not easily dispersible in the substrate and hence no tinctorial value. It was mainly Harmon Colours in the United States, who did some very valuable pioneering work in this direction. The group of pigments can be further divided into:
- Anthraquinone
- Thioindigo
- Perylene-perionone.
Anthraquinone pigments
Both the anthrapyrimidine yellow and flavanthrone yellow exhibit maximum lightfastness, even in very light tints. The relatively clean, greenish shade of anthrapyrimidine yellow favours its use in automotive finishes, inspite of a trace of solvent bleed. The flavanthrone yellow which is absolutely insoluble, has also proved useful for plastics. The anthranthrone orange and the isoviolanthrone violet show good fastness to light in pale tints and give attractive clean shades. The anthranthrone pigment Hostaperm scarlet G (manufactured by Hoechst) is high quality pigment. Its fastness to light and weathering in full shades and strong reduction is excellent. It is widely used for automotive paints despite the fact that it is not entirely fast to re-coating with stoving enamels. The chlorinated Indanthrone blue have reddish shade, which is widely used. Indanthrone Blue in a modification is a good pigment and it has excellent fastness properties in all fields of application.
Generally anthraquinone pigments are relatively high in price.
Thioindigo Pigments
Many derivatives of thioindigo prepared but only those, that contain methyl or chloro groups (or both) have been found to be useful as organic pigments. These pigments are characterised by their clear red and orange colours.
The thioindigo pink, the red, the magenta and the red violet, a pigment of long standing are only moderately fast to light and solvents.
Thioindigo can be used for colouring high quality organic coatings and plastics in both masstones and pastel shades.
Perylene-Perinone pigments
These pigments which range in colour from red to blue are characterised by their clear red and orange colours.
The perylene pigments are all di-imides of perylene-3, 4, 9, 10 tetracarboxylic acid and perinone are di-imides of naphthalene 1, 4, 5, 8 tetracarboxylic acid.
It was Hoechst who did the main development work on this important class of compounds. Scarlet and vermillion have found their main outlet in plastic industry although their light-fatness does not meet the highest demands. In comparison the red, the maroon and the bordeaux particularly attain the phthalocyanine standard also as to their fastness to light. A notable feature of the perylene pigments is their high degree of resistance to heat and reducing agencies, perinone orange is used of shading automotive finishesd. The use of perinone red is limited by its relatively high solvent bleed.
Permanent Red TG 01 manufactured by Hoechst is a Naphthalene tetracarboxylic acid perinone pigment with very good general fastness properties in pastel shades. Fastness to re-coating with stoving enamels is not always satisfactory, but fastness to re-coating with white nitrocellulose paints is very good.
Dioxazine
Dioxanines or more exactly triphene-dioxazines have the following formula.
Its many conjugated double bonds, united with a quinone form in a planar structure make this subtance a chromogen of choice. Dioxazines are generally prepared from dianiles, which are obtained by condensation of primary aromatic amines and quinones. Polymorphism is found in different groups of dioxazine pigments. It has been also found that certain substituents promote the formulating of distinct crystalline forms to which specific colours correspond.
Carbozole dioxazine violet is the only type which has yet appeared on the market. As far back as the 1930’s this compound was sulphonated to form a very fast to light substantive cotton dye sinus light blue FFGL. Carbazole dioxazine is a clean violet of extremely high tinctorial strength and good light fastness, even in light tints.
It is widely used as a reddening component for greenish phthalocyanine pigments. Another important use of Carbazole Dioxazine violet is the bluing of white paints, it is added in a ratio of 0.004 to 0.012 parts per 100 parts of titanium dioxide rutile grade.
Quinacridone
Quinacridone was first described by Liebermann in 1935, but commercially as a pigment it was put forward by Dupont in 1958.
Quinacridone can be prepared in linear transform by following methods:
- Tetraphthalic acid and 2.5 di-bromotetraphthalic acid.
- Heated in the presence of suitable amine and converted into, 2.5, dianilinotetrephthalic acid.
- 2, 5 dianilinotetrephthalic acid cyclised in presence of condensing agent and solvent such as nitrobenzene or high boiling halogenated liquid. Polyphosphoric acid, aluminium chloride, sodium chloride melt. Oleum, phosgene and anhydrous hydrofluoric acid can be used as condensing agents, which improve the yield and quality of quinacridone.
In the second method, diethyl succinyl succinate (IV) is refluxed and aromatic amine (V) in non-oxidising atmosphere and give 2.5,-diarylamine 3.6, -dihydrotere phthalate (VI), cyclisation effected by boiling VI in a mixture of diphenyl and diphenyl oxide dihydro derivative. VII on oxidation with sodium m-nitrobenzene-sulphonate give quinacridone.
A smooth reaction and good yield is obtained if the oxidation is carried out with chloroanil or an aqueous sulphonic acid.
Thirdly, quinacridones can be prepared by selective reduction of two carbonyl groups in quinacridone quinone (VIII), as shown below. The reduction is carried out in molten zinc and aluminium chloride at 130°C, using o-dichlorobenzene as solvent.
Special conditioning is necessary to convert the crude product into a pigmentary form. Quinacridones range give colour from gold through red to violet.
Quinacridones have made valuable contribution to the high grade pigments. Quinacridone exists in four different crystalline forms which differ in appearance, colour, and X-ray diffraction pattern. It is possible to convert one form into another by special processess. Intensive work is being carried out everywhere on the extension of this promising class of compounds the main aim being to produce shades other than the bluish, red. Unfortunately quinacridone red is more expensive than copper phthalocyanine, and very much more. So, than many of the azo compound at present in use.
All quinacridone pigments are completely fast to recoating and fast to stoving at any concentration. Their excellent light fastness permits them to be used for very pale reductions with white pigments and in admixture with high concentrations of aluminium powder for special metallic finishes. Of the greatest importance are combinations with the light stabilised molybdate orange grades. The mixture ratio of organic to inorganic pigment can be varied according to the shade required and generally lies between 1:2 and 1:9.
Such blends show:
- Good lightfastness; fading which is associated with conventional reds, is absent, Any darkening that occurs is related to the molybdate orange in the blends.
- Good to excellent gloss retention in most instances.
Toning White Enamels
Quinacridone violet is an excellent toning pigment for white enamels and shows superior performance (better alkali resistance and lightfastness)
Interblending of quinacridones, as well as interblending with other light fast colours, such as the phthalocyanines, can lead to a wide variety of colours for automotive industry.
Quinacridones are infusible or high-melting solids, insoluble in common solvents. Consequently, as pigments they are non bleeding and quite heat resistant. Furthermore, they are resistant to many chemical agents. Because of their over-all excellence, they are widely used in high-quality finishes of all types.
Isoindolinone
Introduction of chlorine into an isoindolinone molecule is another factor that often improved the pigmentary properties, particularly with respect to light fastness and solvent resistance. This is well illustrated by the group of isoindolinone derivatives, recently introduced by Geigy.
Isoindolinone vary in colour from yellow to orange, red and violet. Claims are made for relative insolubility in most organic solvents, excellent fastness to migration in plastics, and excellent fastness to light and weathering, particularly in light tints. Presence of four chlorine atoms in the 4-, 5-, 6- and 7-positions of the isoindolinone nucleus is essential for improving light, weathering and solvent fastness properties to the pigments.
Applications
Isoindolinone pigments have found major applicability in various segments of the paint industry, including:
automotive exterior finishes (thermoset and thermoplastic acrylics);
automotive refinish and interior enamels;
trade sales paints (latex and oil based systems);
industrial fmishes, e.g., coil coating , toy finishes and appliance finishes;
airplane and marine finishes.
Phthalocyanine
Phtyalocyanine discovered by Diesbeach and Vonderwoid in 1927 and quite independently by Chemists of Scottish Dyes ltd., in 1928. In phthalocyanine, copper-phthalocyanine is a very popular product. Copper phthalocyanine provides almost all the blue organic pigments used today.
The reason for this is that of all known colouring matters, it approximates most closely to an ideal pigment. It has strength, brightness and outstanding all round fastness properties and can be made of a relatively simple process from cheap intermediates. Unlike the azo-pigments, whose development dates back to the century, phthalocyanines are of recent origin. The most outstanding green pigments, however, are the polychloro-copper phthalocyanines and possess the outstanding all round fastness properties.
It should be emphasised that whilst insolubility and heat stability are essential properties of high grade pigments, other factors such as shades, strength and lightfastness are equally important. Pigment research during the past twenty-five years has resulted in substantial advances not only in the development of new pigments but in the improvement of existing ones.
Pigments for Textiles
Lately the use of pigments have increased in textile printing. Pigment dyeing is of much less significance than pigment printing. Pigment are normally supplied as powders, which cannot be directly utilised in textile printing. Pigments are generally supplied as pigment-emulsion prepared by processing raw pigment with water and auxiliaries. The dispersing agents contained in the emulsion stabilise the particle form and size during grinding and protect them from subsequent agglomeration. Pigment-emulsion have no substantivity for the fibre in pigment printing and their fixation is achieved by use of a binder which encloses them and provides a bond between them and the fire. Emulsions are inherently unstable systems but with proper pigment grinding and formulation it can be preserved for 4 to 6 months. Pigment printing is preferred than dye printing due to ease of application and the reduced cost of pigment printing. Pigment colours do not change during processing, whereas final colour of a dye is determined only after fully washed and dried. Pigments of all chemical classes can be utilised in pigment printing including inorganic pigment such as carbon black, and iron oxide. The selection of particular pigment emulsion from the extensive range depends on following criteria.
- hue and brilliance.
- fastness.
- cost.
The brilliance of a pigment depends on the form of the pigment crystals as well as on the pigment finish.
During pigment emulsion preparation the particle size of pigment is reduced to optimum performance, and it is prepared by processing the raw pigment with water and auxiliaries. The surface active agent contained in the pigment-emulsion stabilise the particle form and size during grinding and protect them from subsequent agglomeration. The pigment emulsion is prepared either by soaking pigment in kneader or mixer and then pass through cone mills, or colloid mill or two-roll mill. Another simple method which is widely followed is ball milling of pigments with different auxiliary and then passing through unirol. Ball mill method is extremely simple, and to get good results, following factors are to be looked in :-
- porcelain ball mill should be used to get clean tone of pigment emulsion.
- Grinding media.
Grinding media
High density balls 1" diameter.
Sp. gravity 3.6 to 3.8
These provides faster grinding than is usually obtained with porcelain balls of low sp. gravity.
Speed of Rotation
The action of the grinding media inside a mill is determined by the speed at which the mill cylinder is turned. It is generally accepted that a speed which induces a cascading action of the media is more desirable. Cascading usually occurs when the mill speed is such that the pebble charge breaks away from the mill wall at an angle of 45°C to 60°C from the horizontal.
Cascading actions lead to intensive disintegration, better dispersion and in wet milling, more complete particle wetting due to the high rate of shear from the spinning action of the grinding media.
When mills are rotated at too fast a rate, centrifuging
