May, 1934
I N D U S T R I .4 L A N D E N G I N E E R I J’T G C H E M I S T R Y
of 2.5 F) does not meet the others a t this common point. However, this line like all the others is a straight line. I n the case of nitric acid, the isoviscoidal lines for concentrations of 0.1, 0.25, and 0.5 F meet a t (11.0, l l . O ) , while the line for a fourth concentration-1.0 F-fails to do so. Sodium iodide, ammonium bromide, ammonium dichromate, and sodium bromide are the other salts whose solutions do not give isoviscoidal lines that meet in a point. However, for any one concentration of any of these four salts the isoviscoidal line is straight. The viscosity of liquids is affected by association and dissociation, and Porter’s rule applies only when there is no increase or decrease in the degree of association of the molecules in a solution. The degree of association is markedly different in solutions of extremely high or low concentrations. Consequently, solutions whose molecules change their degree of association with variation in concentration will not follow the isoviscoidal rule when quite dilute or highly concentrated. These considerations may explain the exceptions to the general rule that were noted. Figure 2 shows the different groupings taken by the isoviscoidal lines. In Figure 2, F is the concentration in formula weights per 1000 grams of water. For sodium chloride the isoviscoidal point is on the diagonal Y = X, a t the same time being below zero. The lines for sulfuric acid show the isoviscoidal point also to be below zero but not on the diagonal Y = X . Data for sodium iodide show a case where the isoviscoidal lines do not meet in a point. The point for cesium
551
nitrate is above zero and lies very close to t.he diagonal Y = X. Cesium nitrate is one of the salts which decreases the viscosity of water, as does potassium chloride. Below are listed the Y and X coordinates of the isoviscoidal points for the aqueous solutions studied: None None ( - 2 3 , -23) ( + 6 7 , f67.5) (-2.75, -2.75) None None (-15, -15)
RbzSOi NaBr NaCl NaI NaNOa SrClz
( - 7 . -7) None ( - 3 3 , -33)
None
( - 5 1 , -56) (-43, -45) ( - 4 5 , -48)
Porter’s lines (isoviscoidal lines) for various concentrations of solutions of a given electrolyte have been shown to be
straight lines and, in general, when extended, to meet in a point. I n many cases this point lies on, or near, the diagonal Y = X. A method determining the viscosity us. temperature relationship of a particular concentration of an electrolyte has been developed. LITERaTURE CITED
(1) Duhriny, “Neue Grundgesetre rur nationelle
Physik und Chemie,” Erste Folge, Leiprig, 1878. (2) International Critical Tables, Vol. V pp. 13-18. McGraw-Hill, New York, 1929. (3) Porter, A. W., Phil. Mag., 23, 458 (1912). (4) Walker, Lewis, and McAdams, “Principles of Chemical Engineering,” p. 82, McGraw-Hill, New York, 1927.
RECEIVED August 15, 1933.
Lacquer Development during 100 Years D. R. WIGGAM ANI) W. E. GLOOR, Hercules Powder Co., Wilmington, Del. HE year 1933 marked riot only the Chicago Century of Progress, but also a century of progress in the development of cellulose lacquers. Just a hundred years ago Braconnot (4) published his account of the preparation of xyloidin by treating starch, sawdust, cotton, etc., mith nitric acid and washing. This material proved soluble in acetic acid or pyroligneous acid, from which “a hard varnish-like film” could be obtained on evaporation. Braconnot also found this film to be water-resistant and actually tried to make small microscope lenses from it, as it was “colorless as white glass, and keeps its clarity under water.” Here we have the genesis of both the plastic and lacquer industries. Braconnot’s work did not receive wide attention a t the time, but Liebig recognized its fundamental importance and in a footnote to a translation of Braconnot’s article (5) he described certain of his own experiments showing the instability of the material and recognized the need of further analytical data before xyloidin could be callecl a compound of nitric acid. Boettger (3)reported negative results in attempting to make the material, but Pelouze, one of Braconnot’s assistants, provided further experimental data (30). He nitrated starch and paper with nitric acid of 1.5 specific gravity and was the first to affirm that these materials actually reacted as “bases toward the acid.” While he suggested the possibility of use of this material as an explosive, it was some time later before this observation was made use of. Schonbein (33) was the first to nitrate cotton and starch with mixed sulfuric and nitric acids, a step which first rendered the commercial production of nitrocellulose practical. He was primarily interested in the use of these materials in explosives, and between Schonbein’s work, the announcements of Boettger and Otto that they too had made guncotton, and the controversy with Pelouze over priority of discovery,
T
Braconnot’s description of varnish-like films was apparently overlooked. However, Schonbein had used solutions of his guncotton in ether-alcohol as an “ether glue” (4). Maynard (25) proposed the use of ether-alcohol solutions of nitrocellulose in a surgical dressing. This was the first solvent combination available, and for nearly 40 years inventors tried to overcome its volatility and hygroscopicity. With the contribution of Abel (1) who stressed the importance of using pure raw materials as well as the careful purification of the nitrated product and who developed a satisfactory test for stability, the art of nitrating cellulose became firmly established. The plasticizing value of such materials as castor oil (29)and camphor (18)was discovered. A clear recognition of the desirability of mixing gunis and plasticizers with nitrocellulose to produce varnish-like materials for protective coatings seems to be due to Parkes ($8) ; however the practical application of this idea had to await Stevens’ disclosure (39) of amyl acetate as a nitrocellulose solvent in 1882 almost 50 years after Braconnot. With this slow-drying nonhygroscopic solvent, the way toward the working out of the technical details of Braconnot’s ideal was cleared. Amyl acetate lacquers, first developed by Hale and Crane (43),proved to be good metal finishes and vehicles for bronze. This commercial success undoubtedly stimulated the research which led to the discovery of the solvent properties of ketones, esters, and a host of new plasticizers. Important in this period, Crane’s use of acetone oil as a solvent (9), the disclosure of some two hundred esters as solvents by Nobel, the use of benzene and petroleum hydrocarbons as cheapening diluents, and the discovery of the plasticizing value of phthalates (20) and phosphates (46) are all essential steps in the development of lacquers.
INDUSTRIAL AND ENGINEERING CHEMISTRY
552
Vol. 26, No. 5
The combination of a large demand for a quick-drying, durable finish and the availability of facilities for manuThe field of use for these early lacquers was limited chiefly facturing ingredients in quantity resulted in a rapid deby lack of suitable solvent. Fusel oil, the source of amyl velopment of the lacquer industry in this country, as the acetate, the only nonhygroscopic solvent available, was preceding table demonstrates (41). expensive, and the uncertainty of price and supply was Much research has been necessary to bring this field of another factor hindering progress in the development of endeavor to the highly developed stage that we find today, lacquers. The following table gives the price ranges of this both of a fundamental character to elucidate the basic charessential solvent over a period of years (41): acteristics of the materials that we are dealing with, and of a technological nature to improve the quality and to cheapen PRICE PRICE PRICE the essential ingredients entering the lacquer. RANQE RANQE RANQE YEAR PERGAL. YEAR PERGAL. YEAR PERGAL. Sponsler and Dore (36) were the first to apply x-ray $3.00-3.25 1924 $0.10-0.30 1911 $2.744.75 1885 methods to a study of the structure of cellulose fibers. Shortly 1.35-1.50 1925 3.10-3.75 0.55-0.60 1913 1898 5.75 1932 1.87 1918 1901 0.60-0.75 afterwards Meyer and Mark (26) extended this work and deduced that cellulose is made up of long chains of hexose In 1915 Todd (40) called attention to the wide market groups linked together by oxygen bridges, the chain being that would be opened to lacquer finishes with the discovery held in bundles by secondary valence forces. The brilliant of a new solvent of the right characteristics. This need was investigations by Haworth (16), using the methods of organic not met until 1920-23 when butyl alcohol produced by the chemistry, supported fully the impressions gained by the Weizman process first began to be used in making butyl physical methods. From energy relations it has been calcuacetate (11). About this time also, anhydrous ethyl acetate lated (24) that the strength of a cellulose chain is equal to was offered as a solvent (42). that of a piece of mild steel of equal cross section. This The high cost of nitrocellulose ($1.00 to $1.25 a pound accounts for the great toughness and strength of films of before the war), and the thin coatings given by these lacquers cellulose compounds. also lessened their utility. In 1852, BBchamp had mentioned Working from this basis, Fikentscher and Mark (23) and the fact that ammonia present in a solution of nitrocellulose Staudinger (38) have tried to explain the behavior of cellulose in ether-alcohol stored at 86' C. would lower the consistency compounds in solution. The former think that a hull of of the solution (2). W. F. Doerflinger was one of the first solvent surrounds each micelle (made up of a bundle of chains) (1911) to make commercial use of this process with the pur- in solution, while the latter believes that each chain is free pose in mind of raising the solids content of a lacquer. He to move in the solvent and acts as though it swings through dissolved high-viscosity medium-nitrogen nitrocellulose in a a flat disk surface of diameter equal to its length; the controsolvent mixture consisting mainly of wood alcohol and higher versy is by no means settled. The osmotic pressure methods ketones, and added a small amount of ammonia. After used by Buchner and Samwell ( 6 ) and the supercentrifugal standing a few days a t 40" C. the viscosity of the solution methods used by Stamm (37) point to a molecular weight for was greatly reduced, the viscosity of the nitrocellulose approxi- cellulose of 30 to 40,000, a value in conformity with the mating that of the grade now known as R. S. l / 2 second. views of Haworth and Staudinger. The viscosity of nitroAfter neutralizing the excess ammonia with acetic acid, gums cellulose is a direct function of the average molecular weight, and plasticizers were added to make a finished lacquer. and in turn the tensile strength of films increases as the Carlson and Thall ( 7 ) merely heated high-viscosity nitro- viscosity goes up. cellulose dissolved in amyl acetate to reduce the viscosity. The work of Highfield ( l 7 ) , Mardles (22), and more reGroves and Ward (15) used zinc chloride and other metallic cently that of Hessand Trogus (4OA)indicates that complex chlorides in a solvent medium as the agent for viscosity re- formation between cellulose ester and solvent plays a part in duction, while Pitman (32) preferred sodium acetate. None both solution and plasticization processes. Sheppard (35) of these processes attained the wide significance of simple has shown that the structure of a cellulose compound film heating under pressure in water, the time for reduction cast on various surfaces varies with the ester used, the solvent, depending on the viscosity of the original nitrocellulose and and the surface on which it is cast. Most lacquer films have the temperature of heating. Milliken (27) devised a method the long cellulose molecules randomly oriented in the plane for carrying out this process continuously. of the surface on which they are applied, the resin and plasticizer probably filling in the interstices. INCREASED DEVELOPMENT AFTER THE WAR LIGHT, HEAT,AND MOISTURE PROBLEMS After the war four factors combined to accelerate the
DISADVANTAQES OF EARLY LACQUERS
*
development of nitrocellulose lacquer in this country: (1) an assured supply of a desirable solvent; (2) an excess capacity for producing nitrocellulose, as well as huge surplus stocks of both this material and butyl alcohol; (3) dependable methods of viscosity reduction; (4) a demand for a hard, quick-drying finish by the automobile industry that would approximate the hardness of baked enamel without requiring the elaborate equipment and long finishing periods then customary. NITRO-
NITRO-
~~
CELLULOSE
CELLULOSE
-
ITBED .~
YEAR 1920 1922 1924 1926 1927
IN
~
LACQUERS 1000 lb. ca. 640 ca. 1,070 ca. 2,400 9,200 12,700
YEAR 1928 1929 1930 1931 1932
USED IN
LACQUERS I000 lb. 17,300 20,000 14,600 13,400 9,500
In addition to these problems of a basic character, the cellulose lacquer technologist is faced with a great many questions of an intensely practical nature, the solution for which must be found without awaiting the slower results of the fundamental studies. Outdoor exposure tests are of greatest importance in evaluating a protective coating but are very time-consuming. More rapid but not such accurate results are given by the various accelerated tests. Light, heat, and moisture have the most destructive action on protective coatings, resulting in embrittlement of the film with a loss in adhesion to the base. Of these, light seems to be most vigorous in its action, effecting discoloration of the gums and plasticizers (10) and degradation of the nitrocellulose (14) which induces a loss in tensile strength of the film and eventual cracking. Wave lengths below 3100 A. which make up but a minor proportion of the solar spectrum are the most active. The usual gums and plasticizers have a higher light absorp-
May, 1934
INDUSTRIAL AND ENGIKEERING CHEMISTRY
tion in the near ultra-violet region than do the cellulose compounds. The absorption of light by nitrocellulose does not seem to be affected by the nitrogen content (11.0 to 12.5 per cent nitrogen) or by the viscosity. While heat has not been found to exert an effect on this photochemical process, as would be expected, it does have a degrading and denitrating action on nitrocellulose. In lacquer formulas, heat discoloration may be very important a t temperatures above 100” C. Extreme cold induces brittleness apparently by changing compatibility in films as well as by differing thermal expansions of coating and base. Moisture has little effect on freshly exposed lacquer surfaces, but on very long exposure where there is some denitration the film may become water-sensitive. I n addition to having good durability, a lacquer film must be satisfactorily hard and resistant to abrasion. The Pfund meter (31) is useful for measuring hardness, and an instrument as described by Koch (19) and Schuh (34) may be used for measuring abrasion. Before testing the durability of lacquers on outdoor exposure, much time will be saved if only those formulas are examined which have satisfactory hardness and resistance to abrasion. Further useful tests are determination of discoloration by various wave bands of light, resistance to sudden changes of temperature, and rate of chalking. By means of these tests many gums, plasticizers, and pigments have been evaluated for lacquer use, and new types of nitrocellulose suitable for restricted fields have been developed. Practical formulation is facilitated not only by the tests just described, but also by the steady development of new raw materials. After the advent of butyl acetate, continuous processes were devised for making ethyl acetate (4.2); from ethylene, the cellosolves were synthesized (21) ; isopropyl, isobutyl, and the amyl alcohols were produced from the wartes of cracking stills (8); the chlorinated solvents (1.2) and higher ketones from the same raw material have been added to the list. Diluents prepared by the hydrogenation of petroleum fractions have appeared recently. MATERIALS USED IX LACQUERS Scores of plasticizers have been suggested and used; however, the phthalates, phosphates, and castor oil remain of chief importance in the industry. A field having attractive possibilities as a source of cheap, useful plasticizers, as yet imperfectly explored, is offered by oxidation of the petroleum hydrocarbons (23). No recent additions have been made to the natural resins, the present tendency being entirely toward synthetic substances such as the glycerol phthalates, vinyl compounds, chlorinated diphenyls, modified phenol-formaldehyde, and the polystyrenes. Because of the possibility of controlling molecular weight and other physical properties in preparing these resins, opportunities are opened up for further improvements of cellulose compound lacquers. Other cellulose compounds than the nitrate for use in protective coatings are appearing on the market. Small amounts of cellulose acetate have long been used, particularly for coating airplane wings, but the broad expansion of this cellulose compound into protective coatings suffers from the drawback that an entirely satisfactory plasticizer has not yet been developed for it nor is it readily compatible with gums. With the attention now being given to these points improvements may be expected. Cellulose formate is too unstable to isolate from the esterification medium; cellulose propionate behaves similarly to acetate, while cellulose butyrate is too soft and low-melting to be useful in protective coatings. The higher fatty acid esters require more drastic methods for preparation and are too degraded in viscosity
553
to produce strong coatings. The cellulose ethers have attracted considerable interest both here and abroad because of their noninflammability, stability to light and heat, resistance to alkalies, ready solubility in aromatic hydrocarbon solvents, and wide compatibility with gums and plasticizers (46). These properties appear to be of sufficient importance to warrant detailed study by lacquer technologists. I n working with these compounds, it should be recognized that they differ from nitrocellulose in physical properties, as a consequence of which different principles of formulation must be developed. These cellulose compounds have been too expensive to permit their use in any but highly specialized fields; however, with a demand for them the price could, no doubt, be greatly reduced. LITERATURE CITED (1) Abel, Trans. Roy. SOC.(London), 1867, 181-253; MacDonald, “Historical Papers on Modern Explosives,” p. 97, Whittaker & Co.. London, 1912. 12) BBchamD. Zentralblatt. 23. 8 2 3 4 (18521. i3j Boettge;,’Ibid., 9, 128’ (1838). (4) Braconnot, Ibid., 4, 537 (1833); “Historical Papers on Modern Exdosives.” DD. 5-9. Whittaker & Go.. London. 1912. ( 5 ) Braconnot, Ann.;7, 244 (1833). (6) Buchner and Samwell, Trans. Faraday SOC.,29,32 (1933). (7) Carlson and Thall, British Patent 136,141 (1919). (8) Clough and Johns, IND. ENG.CHEM.,15, 1030-2 (1923). (9) Crane. British Patent 6543 (1892). (10) Davey, Duncan, and Wiggam, I ~ D ENG. . CHEM.,23, 904 (1931). (11) Davis, Ibid., 15, 631 (1923). (12) Fife and Reid, Ibid., 22, 513 (1930). (13) Fikentscher and Mark, KoZZoi&Z., 49, 135-48 (1929). (14) Gloor, IND. ENG.CHEM.,23, 980 (1931). (15) Groves and Ward, British Patent 128,659 (1917). (16) Haworth, Trans. Faraday SOC.,29, 14 (1933). (17) Highfield, Ibid., 22, 57 (1926). (18) Hyatt, U. S. Patent 105,338 (1870). (19) Koch, IND. ENG.CHEM.,Anal. Ed., 2, 407-9 (1930). (20) Ldienfeld, British Patent 592 (1907). (21) Lowry, IND.ENQ.CHEX.,News Ed., 10,6 (1932). (22) Mardles, Kolloid-Z., 49, 4-11 (1929). (23) Marek and Hahn, “Catalytic Oxidation of Organic Compounds in the Vapor Phase,” p. 255, Chemical Catalog, 1932. (24) Mark, “Der Aufbau der Hochpolymeren Organischen Naturstoffe,” p. 154, Akademische Verlagsgesellschaft, Leipzig, 1930. (25) Maynard, in Worden’s “Technology of Cellulose Esters,” Vol. I, Pt. IV, p. 2789, Van Nostrand, 1921. (26) Meyer and Mark, Ber., 61B, 593-614 (1928). (27) Milliken, IND.ENG.CHEM.,22, 326 (1930). (28) Parkes, British Patent 2359 (1855). (29) Pellen, British Patent 2256 (1856). (30) Pelouze, ZentraZbZutt, 10, 122 (1839) : MacDonald, “Historical Papers on Modern Explosives,” p. 7, Whittaker & Co London, 1912. (31) Pfund, Proc. Am. SOC.Testing Materials, 25, Pt. 11,392 (1925). (32) Pitman, U. S. Patent 1,535,438 (1925). (33) Schonbein, British Patent 11,407 (1846). (34) Schuh, IND.ESQ. CHEM.,Anal. Ed., 3,72-6 (1931). (35) Sheppard, Trans. Faraday SOC.,29, 77 (1933). (36) Sponsler and Dore, 4th CoZloid Symposium Monograph, 174202 (1926). (37) Stamm, J . A m . Chem. SOC.,52, 3047 (1930). (38) Staudinger, Ber., 63, 2308, 3242 (1930). (39) Stevens, U. S. Patent 269,340 (1882). (40) Todd, Brass W o r l d , 11, 170-1 (1915). (40A) Trogus, Tomonari, and Hess, 2. physik. Chem., B16, 351-81 (1932). (41) Wiesel, J. B., private communication. (42) Willkie, Chem. & Met. Eng., 25, 631 (1923). (43) Worden, “h’itrocellulose Industry,” Vol. I , p. 301, Van Nostrand, 1911. (44) Worden, “Technology of Cellulose Esters,” Vol. I, Pt. IV, pp. 2412, 2788, Van Nostrand, 1921. (45) Worden, “Technology of Cellulose Ethers,” Chemical Catalog, 1933. (46) Zuhl, U. S. Patent 700,885 (1902).
.
,
RECEIVEDNovember 2, 1933. Preaented before the Division of Paint and Varnish Chemistry a t the 86th Meeting of the American Chemical Society, Chicago, Ill., September 10 t o 15, 1933.
