1104
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
ranges of reference fuels is shown in Figure 12. The curves are not linear and large octane gains have to be obtained by disproportionate amounts of injection. The effect of gasoline composition is demonstrated in Figures 13 and 14 in which the increase of performance number is plotted against injection ratio a t 1000 and 2500 r.p.m., respectively. As stated previously, present commercial gasolines are less responsive to alcohol-water injection than standard and the so-called sensitive reference fuels.
Vol. 44, No.
5;
LITERATURE C I T E D
(1) Bogen, J. S., and Nichols, R. RI., IXD.ENG.CHEM.,41, 2629
(1949). (2) Colwell, A. T., paper presented at 1948 Soc. Automotive Engrs.
National Tractor and Diesel Engine Meeting, Nilaaukoc, Wis., Sept. 9, 1948. 13) “CRC Handbook.” compiled by the Coordinating - Research Council, Inc., New York, 1946. (4) Heron, S. D., and Beatty, H. A., J . Aeronaut. Sciences, 5 , 463
.
11938). ---,
For performance number gains of 5 to 10 with present-day engines and where the base fuel used with injection is in the octane range below 85, a 50-50 alcohol-water mixture appears to be as good as an 85 to 100% ethyl alcohol mixture and more economical. For very small octane gains 100% water may be used. For performance number gains of 10 to 30 with present-day engines, the alcohol content of the mixture should be between 50 to 100% t o prevent an excessive injection requirement. In many case6 a compromise may be made. For the General Motors Research high compression test engine a t 10 to 1, having octane requirements of 100 or more at low speeds and of about 95 at high speeds and using a base fuel of 90 to 95 (Research octane number), a 50-50 alcohol-water mixture appears to be a good compromise. Under these conditions the performance number gain is 20 to 30 at low speeds and 5 to 10 a t high speeds. I n previous work (7, I S ) it was shown that methanol is for all practical purposes equivalent to ethyl alcohol in antidetonant mixtures, but that isopropyl alcohol alone is somexThat less efficient.
(5) Hershey, D. S., Eberhardt, J. E., and Hottel, H. C., S.A.E. Journal, 39, 409T (1936). (6) Hottel, H. C., Williams, G. C., and Satterfield, C. N., “Thermodynamic Charts for Combustion Processes,” Pt. I and 11. New York. John Wiley & Sons, Inc., 1949. ( 7 ) Porter, J . C., Gilbert, M. M., Lykins, 1%. A., and Wiebe, Richard, Agri. Eng., 31, 71 (1950). (8) Porter, J. C., Roth, W. B., and ?Tiebe, Richard, Aztlomotite I n d s . , 98, No.8, 34 (1948). (9) Potter, R. I., SAE preprint, Soc. Automotive Engrs. bleeting, June 1948. (10) Rowe. M. R.. and Ladd. G. T., S.A.E. Journal, 54, 26T (1946). (11) Smith, J. T., Shaw, G. N., Van Hartesveldt, C. H., and Kilgore, W.E., SAE preprint, SOC.Automotive Engrs. meeting, Dec. 13, 1948. (12) Taub, Alex., Automotzve I n d s , 101, No. 1, 28; No. 2, 36; S o . 3. 34 (1949). (13) Van Hartesveldt, C. H., S.A.E. Quart. Trans., 3,277 (1949). (14) Veal, C. B.. S.A.E. Journal, 35, 131‘ (1934). (15) Veal, C. B., Best, H. W., Campbell, J. AT., and Holaday, 11’. AI., Ibid.. 32. 105T (1933). (16) Wiebe,’ Richard, and Nowakowska. Janina, U. 8. Dept. Agr., Bibliography Bull. 10 (1949). (17) Wiebe, Richard, and Porter, J. C., U. 8.Dept. Agr., AZC 240 (1949). (18) Wiebe, Richard, Schults, J. F., and Porter, J. C., IND.ENG. CHEM.,34, 575 (1942); 36, 672 (1944).
The assistance of C. F. Elder, M. RI. Gilbert, H. A. Lykins, A. P. McCloud, and C. R . Martin in obtaining the data is acknowledged.
RECEIVED for review April 2 6 , 1951. ACCEPTED December 4,1951. Presented before t h e Division of Petroleum Chemistry at t h e 119th Meeting of t h e Axmuc.m CHEMICAL SOCIETY, Cleveland, Ohio. Report of a study made under t h e Research a n d Marketing Act of 1946
SUMMARY
ACKNOWLEDGMEYT
Pigment Colors for Vinvl C d
GEORGE WORRZALD AND W. F. SPENGEMAN E. I. d u Pont de Nemours & Co., Inc., Newark, N. J .
ITH the commercial introduction of plasticized vinyl chloride polymers about a decade ago, problemsin coloring arose which were somewhat different from those faced by formulators of paint, printing ink, and related polymeric systems wherein pigment colors are widely used. The choice of color for the polymeric systems was determined largely by considerations of end use-Le., lightfastness, chemical resistance, etc. Rigid, i.e., unplasticized vinyl chloride polymers, are also included in this category since the coloring problems are relatively simple, both dyes and pigments being used depending on the ultimate use. I n vinyl systems, however, factors in addition to end use exert effects considerably greater and different from those observed in the other common polymers. Specifically these factors were shown to be related to the chemical and physical reactions between the colorant, plasticizers, and stabilizers used, and to the chemical effect of the colorant on the stability of the vinyl chloride. When these factors were thoroughly recognized, i t became apparent that the chemical and physical nature of the colors used would have t o be scrutinized more closely than they were for most
uses. Oil-soluble dyes were shown to be much too soluble and t o cause excessive crocking and migration. Accordingly, essentially nothing but pigment colors are used today. Since chemical and physical factors are so important in pigmenting vinyl polymers, i t is the purpose of this discussion to present a simplified classification of pigment colors from the cfiemical viewpoint and to comment on the suitability of the various types of colors for pigmenting vinyl polymers. The term “vinyl polymer” or “vinyl plastic” refers in this discussion to plasticized vinyl chloride polymers and copolymers as represented by a typical formula containing 100 parts of polyvinyl chloride or copolymer resin, 30 to 60 parts of plasticizer (primary and secondary plus a diluent), 1 to 5 parts of stabilizer (light and heat), and 1 to 5 parts of lubricant. For purposes of review, Table I shows a simplified classification of synthetic pigment colors broken down into two major groups, the organic and inorganic. Since naturally occurring pigments, such as the ochres and umbers, are seldom used in this type of plastic, they are not considered in the classification.
INDUSTRIAL AND ENGINEERING CHEMISTRY
M a y 1952 TABLEI.
CLASSIFICATION OB
SYNTHETIC PIGMENT
COLORS
INORQANIC Lead chromate jCI-1270)a, Molybdate Orange, strontium chromate Ferrocyanides iron blue (CI-1288) Iron oxides (61-1276). chromium oxide (CI-1291) Cadmiums ((31-1272) Ultramarine blue (CI-1290)
ORQANIC
Azo Azo Pigment Dye Precipitated Azo Pigment Toluidine Red (GI-6s) Lithol Red (CI-189) Para Red ('21-44) Alkali-resistant reds Hansa Yellow Benzidine Yellow Pyrazolone Red
Lake Red C ((31-165) 2-oxynaphthoic acid reds and maroons Lithol Rubine Permanent Red 2B Lake Bordeaux B
Nonazo Acidd elrtkes Basic drye lakes Phthalocyanines Vat dyes
.
Pigment Scarlet (GI-216) Nickel azo complex yellowt a Figures in parentheses refer to the color index. b A metallized azo pigment, classified here as precipitated azo pigment because of similarity in properties.
INORGANIC PIGMENTS
In the field of inorganic pigments, several members find particular application as vinyl resin colorants, One such member is the family of lead chromate pigments shown in Table 11. This family can be subdivided into five groups-namely, the medium yellows, the light yellows, the primrose and lemon yellows, the chrome oranges, and the molybdate oranges. Relative to other inorganic pigments, they are recognized for their ease of dispersion, brightness, high tinting strength and opacity, low cost, and their good coverage of shades of yellow and orange. Their lead content contributes to the heat stability of the vinyl resin; however, the pigments tend to darken a t temperatures in excess of 400" F. Further, their use is limited in those cases where maximum lightfastness (particularly in deep or masstone shades), maximum resistance to alkali, and resistance to discoloration in the presence of sulfur are required.
TABLE 11. LEADCHROMATE PIGMENTS Type Medium yellow Light or Excelsior yellow
Composition Normal lead chromate Coprecipitated lead chromate lead sulfate
Crystal Form Monoclinic Monoclinic
The tinctorial properties of the chromate pigments are a function not only of their chemical composition but of their crystal form. The medium yellows are essentially normal lead chromates existing in the monoclinic form. They are the reddest, strongest, and most lightfast of the chrome yellows. The light yellows are also monoclinic but differ from the medium yellows in being coprecipitated mixtures of lead chromate and lead sulfate; they are lighter, weaker, and greener in color compared with the medium yellows. Precipitation of lead chromate-lead sulfate mixtures in the orthorhombic form rather than the monoclinic, yields pigments considerably greener in hue, namely, primrose and lemon yellows. These yellows, being generally poorer in masstone lightfastness and appreciably weaker in tinting strength than the light yellows, are not widely used in vinyls. Instead, primrose yellow patterns with good lightfastness can be made with strontium chromate. This pigment, although quite weak, is superior in lightfastness to lead chromate pigments of comparable shade and does not blacken in the presence of sulfides. The chrome oranges differ chemically from the chrome yellows in that they contain substantial amounts of basic lead chromate. As the depth of shade increases, the basic lead chromate content
1105
increases, the oranges become weaker and redder in tint and poorer in lightfastness. All are precipitated in the tetragonal crystal configuration. Because of their weakness they are not widely used in vinyl compounds; the stronger and brighter molybdate orange is more widely used. Molybdate orange is also a tetragonal form of lead chromate consisting of coprecipitated crystals of normal lead chromate, lead sulfate, and lead molybdate. I t s low cost, high tinting strength and opacity, and outstanding color intensity render it ideally suited for coloring many vinyl compounds as a single pigment as well as in blends with reds. The alkali resistance and lightfastness of molybdate orange are similar t o those of medium yellow. This group of inorganic pigments, indicated as the ferrocyanides, is represented mainly by a type of pigment commonly referred to as iron or Prussian blue, the composition of which is essentially ferric ammonium ferrocyanide. Although some evidence has been presented t o the contrary (3), iron blue pigments are generally considered to be unsatisfactory for use in vinyl chloride polymer systems. It has been well established that ferric chloride, produced through the reaction of iron compounds with hydrogen chloride liberated from the vinyl polymer, will serve as a catalyst for the degradation of the resin. For the same reason chrome greens, which are intimate mixtures of lead chromate yellow and iron blue (not to be confused with chromium oxide green), are also not generally recommended for use in vinyl chloride systems. This is not to say that vinyl chloride resins cannot be pigmented with iron blue or chrome green; if properly stabilized and processed, vinyl coatings satisfactory for some purposes can be made. Included in the ferrocyanide group is a copper ferrocyanide known as Copper Maroon. Like iron blue, it is not used t o any great extent in vinyls. In contrast to the ferrocyanides, iron oxides of good quality are generally considered as satisfactory vinyl colorants requiring only moderate precautions with regard t o proper stabilization. They are rather dull weak pigments ranging in shade from light yellow to dark red. By virtue of their low cost and excellent lightfastness, the iron oxides are frequently used to produce opaque olive green and brown vinyl films and sheeting intended for outdoor or window use. The chromium oxides are olive green pigments of moderate color intensity, having excellent lightfastness, heat stability, and chemical resistance. Their use is limited, however, by their relatively narrow color range and low tinting strength. The cadmium pigments are important vinyl colorants ranging in shade from very light greenish yellows to relatively dark maroons. As a group, they are resistant to light, heat, and alkali, and they are insoluble in most solvents and vehicles. They have only moderate color intensity and comparatively low tinting strength. Gadmium yellows are essentially cadmium sulfide; the very pale yellows may contain some zinc sulfide ( 6 ) . The poor resistance of cadmium sulfide to dilute acid requires special consideration for proper stabilization of the film if sulfide odors are to be avoided. The problem of stabilization will be all the more difficult if zinc sulfide is present. Experience indicates that zinc, like iron, serves as a catalyst for heat and light degradation of the vinyl chloride polymer; Lally ( 6 ) , however, suggests that if properly handled, zinc may be used as a vinyl stabilizer, T o avoid sulfide darkening, compounds of lead-i.e., lead stabilizers, lead chromate pigments, etc.-should not be used in combination with cadmium sulfide. Cadmium reds are calcined coprecipitated mixtures of cadmium sulfide, cadmium selenide, and varying amounts of barium sulfate. Ultramarine blue, prepared by calcining a mixture of either soda ash or sodium sulfate together with clay, sulfur, and some form of carbon (Y),is characterized by its distinctive brilliant red shade of blue not readily matched with any other pigment or pigment combination. It has good resistance to light, heat, and alkali, is insoluble in most solvents and vehicles, but is com-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 44, No. 5
migration. There are known exceptions to this broad generalizntion, such as some of the pyrazolone pigments. The most satisfactory azo pigments for vinyl are the precipitated type. The precipitated azo pigments are distinguished by the presence of one or more acid groups, such as sulfonic or carORGANIC PIGMENTS boxylic, in the molecule. Interaction of these acid groups with The second major group of synthetic colored pigments to be metallic ions, such as barium, calcium, strontium, and manganese, considered is the organic group. For the purpose of simplificaresults in the precipitation of the azo dye as a precipitated azo tion this very large and extensive group can be subdivided into pigment. the azo and the nonazo types. I n the azo group, those that are The structural formulas of several important members of the naturally insoluble in water are termed the azo pigment dyes as precipitated azo pigment group are shown below. Although both shon-n in Table I; they contain no metals in their chemical Lithol Red and Lake Red C pigments are important printing ink make-up. Those that must be insolubilized by precipitation pigments, they are seldom, if ever, used in vinyl because of their with suitable metallic salts to form metallic derivatives are relatively poor lightfastness. (See column 1.) termed precipitated azo pigments. Generally the first type are Of all the organic red and maroon pigments, those derived somewhat soluble-Le., they bleed in oils and organic solvents, from 2-0x1naphthoic acid are probably the most widely used in whereas the nonazo pigments do not. vinyl today. Among the more familiar members of this family of The bleeding characteristics of the azo pigAzo PIGMENTS. pigments are Lithol Rubine, Permanent Red 2B, and Lake Borment dyes, e.g , Para Red, Toluidine Red, Hansa Yellow, etc., deaux The Permanent Red 2B pigments, precipitated as barium, generally render these pigments unsuitable for plasticized vinyl calcium, or manganese salts, provide a series of reds which range systems. The use of such colorants, even a t concentrations as in masstone depth from light to dark, and vary in hue from the low as 0.005 of 1% usually results in crocking, bronzing, and near-orange to light maroon, The Lake Bordeaux B pigments are deep blue-shade maroons usually precipitated v ith manganese. It is generally found that as the masstone depth of the pigments in the latter two groups increases, the ease of dispersion and lightfastness in pastel shades decreases. In order to provide dark red and maroon pigments of these types which can be disL persed in vinyl systems, lakes on special substrata, such as betaoxynaphthoic acid ( R O S ) 4-t oluidine-&sulfonic acid alumina hydrate, have been prepared. Akniong organic pigLithol Rubine ments, the 2-oxynaphthoic acids are recognized for their bleed resistance in most organic vehicles and for their good lightfastness in deep shades. In general, this group iq sensitive to the action of the alkalies. Another distinct type of precipitated azo pigment is the adsorbed lake type of 17 hich Pigment Scarlet is a well knonn example. In this case, in order to obtain satisfactory pigment 2-naphthylamine-1-sulfonic betaoxynaphthoic acid (BON) acid (Tobias acid) properties, it is necessary to precipitate the dye in the presence Lake Bordeaux B of a substrate such as alumina hydrate. This pigment is also nonbleeding in organic vehicles and it is sensitive to the action of alkali. Zinc oxide is often used in the preparation of Pigment Scarlets; because of this it is necessary to provide good heat stabilization of the vinyl compound in which they may be used. Finally, in the precipitated azo pigment class there is a relatively new pigment (4)x hich may be identified as a nickel azo 2-chloro-4-toluidine-5-sulfonic betaoxynaphthoic complex yelloiv. This pigment is more properly classified as a acid acid (BOX) Permanent Red 2B metallized azo pigment. For simplicity, and because of similarity in properties, it is included in the precipitated azo pigment group. The nickel is believed to be coordinately bonded with the azo linkage and other polar groups present in the molecule. This pigment is characterized by its green masstone and yellow undertone or tint, and by its excellent heat, light, bleed, and chemical resistance. The precipitated azo pigments are resistant t o bleed in water 2-aminobenzoic acid 2-naphthol-3,6-disulfonic and organic solvents but are, hoTvever, usually sensitive to the ac(anthranilic acid) acid (R salt) Pigment Scarlet tion of alkalies. Most vinyl stabilizers, being acid acceptors, are basic materials and, as such, are sometimes capable of stripping the precipitating metal H O H I from the oiement molecule. This altered DigII I -N=N-C-C-b-O-CaHs H6C2--O-C-c-c--N=N ment would be more water soluble, and might II 1 I I 1 be more soluble in the plasticizers and/or luN C=O O=C N bricantpresent in thevinyl than was the original \/ pigment. Under such conditions a particular pigment might crock 4 \ N I 1 and migrate, whereas if a less alkaline stabilizer had been used the pigment might not crock. Because of the increased water solubility, i t would be expected that the wet crock would be more severe than the dry crock. On the other hand, in an unstabilized or insufficiently stabilized 3,3’-dichlorobenzidine (l-phenyl-3-carbethoxy-5-pyrazolone)~ Pyrazolone Red system, it is likewise possible, in the presence of hydrogen chloride
paratively low in tinting strength, The resistance of ultramarine blue to acids being poor, as with cadmium yellows, special attention to stabilization is necessary to prevent sulfide odor development and lead sulfide discoloration in the pigmented vinyl film.
?
0
3
_
Y
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
May 1952
liberated from the vinyl, to strip the precipitating metal from the pigment, leaving the dye in the acid form. As before, this could result in severe migration and crocking, as was revealed by Clark (5) in his work with a 2-oxynaphthoic acid maroon and madder lake; both pigmented films showed pronounced migration from unatabilized films but no migration when the films were properly stabilized.
I
1 ‘
--r.J
/
i
I
It
In such cases the crystal-stable types are necessary. Crystalstable phthalocyanine blue represents a comparatively recent product of phthalocyanine pigment research. The question of crystal stability is not encountered with copper phthalocyanine green. The vat dye group includes indanthrone blues and thioindigo reds and maroons. Some pigments in this group are very resistant t o migration and possess excellent lightfastness even a t very low pigment concentrations; some have poor migration resistance. Many vat dye pigments, like various other pure organic types, such as Benzidine Yellow, exhibit a peculiar tendency to crock more when used at low concentrations (less than 1% on the resin basis) than a t high concentrations ( 2 ) . I n addition to the chemical and physical nature of colorants used in vinyl compounds, particularly solubility and the metal constituents, it is important to recognize that compounding ingredients other than the pigments may exert a pronounced effect on the tinctorial properties of the vinyl film. For example, in the selection of titanium dioxide for tinting, rutile grades (zinc oxide free) afford considerably greater ultraviolet resistance than do anatase grades. I n some cases ’u hat may appear to be fading in pastel films may be caused by the hiding power of various white opaque metal chlorides-Le., lead chloride produced in acid-accepting process of the lead containing stabilizer. The splitting out of hydrogen chloride during the degradation of the vinyl chloride polymer is thought to result in a linear conjugated double bond of polyene structure; such a structure, being chromophoric, is quite capable of causing a color change that a t first glance might be attributed to the pigment.
---O
-[,/
‘
1107
i
Figure 1. Chemical S t r u c t u r e of Phthalocyanine Blue Still further evidence suggests that a precipitating metal of an azo pigment is capable of reacting with the other vinyl film ingredients. Rancid odor development and plasticizer exudation may be caused by the presence of an oxidation catalyst, such as manganese, used as the precipitant for the aBo dye. Clark (2) observes that Cellosolve phthalate and plasticizers derived from castoi oil are particularly susceptible to thiR reaction. The effect can be minimized by the use of suitable antioxidants, or by changing the plasticizer and/or stabilizer. Sosazo. The nonazo pigments constitute the last major group of organic pigments shown in Table I. Included in this group are such pigments as the acid dye lakes (Eosine Red and Peacock Blue), basic dye lakes (frequently referred to as PTA’s and PMA’s because the precipitants used are phosphotungstic and/or phosphomolybdic acids), copper phthalocyanine blue and green, and the vat dyes. Of these, the first two are not recommended for plasticized vinyl systems because of their poor bleeding characteristics and poor lightfastness. The phthalocyanine pigments ( 1 ) are regarded by most consumers as representing the highest standards for lightfastness, heat stability, and chemical resistance among all organic pigments. Figure 1 represents the copper phthalocyanine blue molecule. The green is derived from the blue by chlorination Le., replacement of 14 or 15 of the 16 hydrogens in the molecule, with chlorine. Although the phthalocyanine pigments are relatively expensive on a weight basis, their high tinting strength frequently makes them more economical to use than other pigments of lower selling price. Phthalocyanine green is considerably more expensive than the blue; thus, when maximum brilliance is not required, greens are usually formulated with phthalocyanine blue and a suitable yellow pigment. Certain commercial varieties of copper phthalocyanine blue exist in a crystal-unstable modification. Under high temperature processing or in the presence of aromatic solvents, these unstable types tend to lose strength and drift greener in shade.
CONCLUSION
Lead-containing pigments frequently contribute to the heat stability of vinyl compositions, Their use with sulfide-containing pigments is to be avoided in order to prevent darkening. Acid-sensitive sulfide containing pigments require special and careful stabilization to avoid sulfide odors. Since iron and zinc are known to catalyze the degradation of vinyl polymers, pigments containing these elements generally require special consideration, particularly with regard to the problems of stabilization. The presence of oxidation catalysts, such as manganese, in some vinyl formulations may promote rancid odor development and plasticizer exudation. The slight solubility in organic media of certain organic pigments makes careful consideration of bleeding and crocking characteristics very important. Precipitated azo pigments are usually sensitive to the action of alkalies; therefore, special selection of stabilizers may be necessary in order to avoid stri ping of the precipitating metal from the pigment molecule, thus cianging its color as well as its crocking and migration characteristics. Crystal-unstable forms of phthalocyanine blue are sometimes subject to color and strength change when processed a t elevated temperatures. For such special uses, crystal-stable forms of‘ phthalocyanine blue should be used. LITERATURE CITED
(1) Allen, E. R., and J. Mattiello, “Protective and Decorative Coatings,” 2nd ed., Vol. 2, chapt. 8, New York, John Wiley & Sons, 1944. (2) Clark, F. G., India Rubber World, 123, 571-3 (1951). (3) Clark, W. X., Modern Plastics, 26, No. 11, 97 (1949). (4) Kvalnes, D. E., and Woodward, H. E. (to E. I. du Pont de Nemours & Co., Inc.), U. S. Patent 2,396,327 (March 12. 1946). (5) Lally, R. E.,“Recent Developments in Stabilizers,” Proc. Plastic Industry Meeting, New York, December 1950. (6) Wolfe, H.G.,“Printing and Litho Inks,” 4th ed., pp. 96, 97,
New York, Mac-Nair-Dorland Co. (7) Ibid., pp. 114-15. RECEIVED for review August 28, 1951. ACCEPTED November 14, 1951. Presented as part of the Symposium on Vinyl Resins before the Division of Paint, Varnish, and Plastios Chemistry at the 120th Meeting of the AMERICAN New York, N. Y. CAEMICAL SOCIETY,
