A Century of Progress in Driers A. C. ELM,The N e w Jersey Zinc Company, Palmerton, Pa.
T
H E use of litharge or red lead and possibly umber for the purpose of accelerating the drying of oil paints or glazes is almost as old as the use of linseed oil itself. Tarnish formulas described by Jacobus de Tholeto (1440), Joseph Petitot (1644), and Alberti of Magdeburg (1750) are among the first written records to contain references to the use of metallic oxides for this purpose. It is highly probable, however, that these formulas were several hundred years old when they were published and that the accelerating effect of litharge, a t least, was known to the Egyptians who made use of it in the preparation of the oil employed in the mummification of their dead. These formulas were handed down through the ages as carefully guarded trade secrets, and thus litharge and umber remained the only driers for centuries.
Stas laid the foundation for the modern theories of the effect of driers on drying oils and paved the way for the next important development in the history of driers-the soluble drier.
THEORY OF DRIERS The develpment of a theory for the action of driers on drying oils followed closely the knowledge of these oils and their chemical and physical changes when exposed to air in thin films. Until a few years ago the drying of oils was thought to be an oxidation of the liquid unsaturated glycerides to solid oxyglycerides, and the action of the driers was assumed to be merely an acceleration of the rate of oxidation. The original picture in which the metallic oxide furnished the oxygen for the oxidation of the glycerides was changed t o an improved picture in which the dissolved metal atoms, while oscillating between two stages of oxidation, activated the oxygen of the air and passed it on to the oil molecules. This “bucket elevator theory,” as Long called it, was quite satisfactory as long as the colloid-chemical aspect of the drying of oils was not fully recognized. More recent investigations made i t evident that the presence of a drier not only accelerates the rate but also modifies the course of the oxidation, because the total amount of oxygen absorbed up to the time the film solidifies is lower than that absorbed when no drier is present. Furthermore, in the presence of a drier the amount of carbon dioxide evolved during the oxidation is approximately the same as for the oil alone, whereas the evolution of hydrogen peroxide is suppressed entirely. In the light of this evidence it is obvious that the drier does not act as a catalyst in the strictest sense of the term, because there is no doubt that it changes the course of the reaction. With the development of the modern combined chemical and colloidal theory of oil drying came a new and improved theory of the action of driers. The principal points of the modern theory of the drying of oils are as follows: During the transition of the liquid into the solid oil film, there are taking place three primary reactions: DEVELOPMEKT OF
OF DRIER RESEARCH BEGIKNING
It is doubtful that anything was knoun about the nature of the action of the driers until 1835 to 1850 when the efforts of the French zinc oxide manufacturers to substitute zinc white for white lead in paints started a search for compounds which would improve the drying of zinc oxide paints. This mas the beginning of the science and the industry of driers. The results of the first few years of drier research were critically reviewed and briefly summarized by J. S. Stas in a report presented a t the International Exposition a t Paris in 1855 on the value of zinc white as a substitute for white lead. Although there is no definite statement to this effect in the literature, it may be assumed that litharge was the first metallic oxide to be incorporated into oil by heating. Stas says: “It is also well known that lead oxide in very large quantities can dissolve in linseed oil . . . The famous Liebig was the first to show this. Linseed oil thus acquires high drying power. The ancient use of lead oxide to render this oil more siccative is abundant proof of this fact.” Leclair, in his studies to substitute zinc oxide for white lead (1835-1850), found that heating linseed oil for a long time with manganese dioxide makes it drying in the presence of zinc oxide. These observations led to the theory that metallic oxides high in oxygen yielded their oxygen to the oil, making it more siccative. The observation that, when heating oil with litharge or red lead, the solid residue sometimes contains metallic lead seemed to substantiate this theory. Sorel’s observation, about 1840, that anhydrous manganous chloride, and M. E. Barruel’s discovery (about 1845) that manganese borate are very effective driers could not be readily reconciled with this theory. Stas, then, pointed out that on this basis zinc oxide should be an excellent drier. Based on these facts and the results of his own investigation conducted with M. W. de la Rue, Stas concludes that “every metal, of which the oxide, linoleate, and margarate are soluble in a siccative oil,” possesses drying properties. TO explain the nondrying properties of zinc oxide, he states: ‘‘although zinc oxide in the presence of moisture may react with linseed oil to form soaps, they never remain in solution upon cooling, the cooled and separated clear oil scarcely retaining more than 0.2 per cent zinc oxide. Thus zinc linoleate may be said to be insoluble in oil and therefore have no drying action.” Thus, by emphasizing the necessity of solubility of a drier,
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1. Oxidation of the unsaturated compounds to oxyns. 2. Association or polymerization of the oxidized oil molecules
t o colloidal dimensions. 3. Gelation or coagulation of the colloidal system.
The rate of oxidation of the oil is very low a t the beginning of the exposure. This period, which passes before the rate of oxidation becomes perceptible, is called the “induction period” and may be caused by the presence in the oil of natural antioxidants. Oxidation results in the formation of highly polar compounds which, in their desire to satisfy as far as possible their free energy, associate or polymerize, yielding colloida1 systems which eventually gel and solidify. As the concentration of oxidized oil molecules increases, the rate of association increases; and with increasing concentration of particles of colloidal dimensions the rate of gelation increases. Thus, these three primary reactions overlap, each in turn being the predominant reaction. The modern theory of the action of driers attempts t o answer the question: Which one of these three basic reactions is influenced b y the drying metal and what is the mechanism of its action? I n 1932 the Philadelphia Paint
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April, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
Superintendents’ Club reviewed and analyzed this subject and came to the following conclusions: 1. The driers are primarily accelerators of the oxidation reaction, neutralizing antioxidants and increasing the rate of oxidation. 2. To a lesser degree the driers accelerate association or polymerization of the oxidized oil molecules by increasing the concentration of the polar compounds and by exerting an orienting influence upon them. 3. It is undecided but it is quite conceivable that the driers accelerate gelation by serving as coagulation centers or nuclei. 4. I n order t o be able to act in this threefold manner, the drying metal must be in an “available” form and occur in two stages of oxidation of which the higher one is stable in air but unstable in a drying oil, and the lower one is stable in oil but unstable in air.
The Philadelphia Club, using tetraethyllead as an example, points out that solubility of the metallic compound is of no avail unless the metal is “available.” It is difficult to define clearly this term “availability,” but an idea of what it seeks to describe might be obtained from its analogy to ionization in aqueous solutions. Although the theories on the action of the driers on the solidification of drying oils have not yet been developed to an entirely satisfactory end, they should, in their present form, prove of considerable help in future developments in the field of metallic driers. Modern drier research which dates from the introduction of the so-called soluble driers has slowly recognized the two main requirements-solubility and availability of the metal-and has been guided by them. THE SOLUBLEDRIERS(ABOCT1885) Strictly adhering to every detail of his secret formulas, the varnish maker of the nineteenth century incorporated lead and manganese oxides, acetates, and borates into his boiled oils and varnishes much in the same n a y as it is still practiced today. The secrecy surrounding all operations in the varnish industry prevented an exchange of ideas which, together with the antipathy of the old-timt: varnish maker to deviate even the slightest degree from his secret formulas, retarded progress in driers as much as in the other branches of the paint and varnish industry. It is not a t all surprising therefore that it took about thirty years before Stas’ ideas on the relation between solubility of a metallic compound and its effectiveness as a drier were applied. Stas, in 1855, had pointed out that a metal, in order to act as a drier, must be soluble in oil in the form of its linoleate or margarate. The varnish maker was slow in realizing that, when boiling oils with litharge or manganese dioxide, he was converting the metallic oxides into the soaps by a slow and wasteful process yielding dark colored products. Nevertheless, when in 1885 the so-called soluble driers (the fused and precipitated linoleates and resinates of lead and manganese) made their appearance, the industry was very slow in accepting them in spite of their easily recognizable advantages. This may have been caused partly by the difficulty of manufacturing entirely satisfactory soluble driers of this type. Although in the course of the years the basic principles involved in these manufacturing processes did not change appreciably, the introduction of more scientific methods and equipment and a thorough control of the individual manufacturing stages resulted in products improved as t o purity, uniformity, and solubility. Kevertheless, they never entirely replaced the oxides, acetates, and borates.
INTRODUCTION OF COBALT For a long time lead and manganese remained the only driers. The writer was unable to locate in the literature a
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definite reference as to the discovery of the drying action of cobalt, but the indications are that it took pIace about 1901. The Russian chemist Fokin, who in 1907 published the first series of metals in the order of their ability of accelerating the drying of oils, treats cobalt in a way which leads one to believe that it was an old acquaintance and not a new discovery of his own. Earlier publications on the drying of oils-for instance, Livache’s articles in 1901-contain references to lead and manganese only. A report by H. T. T‘ulte and H. W. Gibson on their studies of the solubilities of linseed oil soaps in certain hydrocarbons, published in 1902,l contains the first reference to cobalt linoleate the writer was able to find. It is therefore permissible to assume that the discovery of cobalt as one of the most efficient metallic driers was made in 1901. I n spite of their advantages over lead and manganese in color and effectiveness, cobalt driers, on account of their high price, were not used commercially until about 1911.
IRONAS
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DRIER
The only other metallic drier of importance is iron. Tliii was included in Fokin’s series (1907), but it is difficult t o establish the date of its first commercial application. Because of its comparatively low acceleration effect and the extremely dark color it imparts to oils and varnishes, it has found only limited application, especially in dark colored products. In the form of Prussian blue it is incorporated into asphalt-base black baking enamels and into dark colored oilcloth and patent leather oils, and it is supposed to be largely responsible for the beautiful gloss and luster of these products. OTHERMETALS Cerium, vanadium, thorium, and nickel conipounds have been suggested and offered as driers during the last two decades. Since, however, they do not offer any advantages over lead, cobalt, and manganese, and did not induce properties which would not be obtained with the three established drying metals, they found only limited use in a few specialties. Lead, cobalt, and manganese still are the common driers and the indications are that they will retain this position for some time to come.
MOBTRECENTDEVELOPMENTS The last ten years have seen an unusual activity in drier research. The recognition of the importance of solubility and of the fact that fused and precipitated linoleates, tungates, and resinates are a t best only colloidally dissolved in the vehicles and thinners and are, therefore, subject to flocculation and coagulation caused some drier chemists t o resort to the use of solubilizing agents, mostly of an acidic nature. When it was realized that the tendency of the linoleate and resinate driers to oxidize spontaneously on exposure to air during storage is, to a large degree, responsible for the unsatisfactory solubility characteristics, a search was made for bases which would yield more soluble and stable metallic salts. Drying metals were combined with more saturated organic acid radicals to yield salts of solubilities approaching molecular solution in the usual paint oils and thinners. Thus lead, cobalt, and manganese were combined with naphthenic acids, acidic oxidation products of crude and refined waxes and petroleum fractions, sulfonic acids, and other organic acids especially synthesized for this purpose. Thus a radical departure from ancient practices and the adoption of sound scientific reasoning resulted in the development of drying compounds of superior solubility and stability, improved color, and a higher concentration of 1
J . Am. Chem. Soc., 24, 215 (1902):
INDUSTRIAL AND EXGINEERING CHEMISTRY
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active metal which permits an accurate control of the metal content and a reduction of the total amount of drying compound necessary to obtain the desired drying properties. The many advantages offered by the soluble driers are so striking that it is not difficult to visualize the time when varnishes and paints free from driers will be tanked, stored, and even sold in order to prevent such common troubles as skinning. The proper type and correct amount of drier will then be added just before the paint or varnish is used and will make it possible to adjust the amount of drying metal to the individual requirements. Excess and misuse of drier can thus be avoided, and paints and varnishes will be greatly improved. During the past century the drier industry has progressed from the empirical use of litharge and umber by a few craftsmen or artists, who had neither knowledge nor theory to ex-
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plain the results, to a great industry well founded on scientific knowledge and research. The scientific approach to the drier problems is, to a large degree, responsible for the advances made during the last twenty-five years. It is to be expected that a careful study and intelligent application of the modern theory of the action of driers and especially the laws governing the solubility of organic compounds will result in still further improvements.
ACKNOWLEDGMENT The author wishes to acknowledge the suggestions and criticisms offered by h-elson and Werthan of the Research Division of The Sew Jersey Zinc Company. RECEIVED October 4, 1933. Presented 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.
Aerogel Catalysts
Thoria : Preparation of Catalyst and Conversions of Organic Acids to Ketones S. S. KISTLER,SHERLOCK SWANN,JR., AND E. G. APPEL,University of Illinois, Urbana, Ill.
N V I E W of t h e u n i q u e physical properties of the
The conversion of aliphatic acids to ketones
has been studied over thoria aerogel. It has been
more uniform subdivision in the c l e a r jellies. No satisfactory method was found in the literature. Eventually two methods
aeroge*s, it has seemed defound that the aerogel catalyst is distinctly sirable to study them as catasuperior to thoria thoria prepared were devised that gave products lysts. The extent of their surface per unit mass is probably from the oxalate, and thoria on pumice for this of excellent catalytic activity. With the present data it is imgreater than that of any other purpose. The yields of ketones compare favorthe best reported in the chemical literapossible to discriminate between form of solid available for catalyture. the products. sis. I n addition, the structure 1; the first method 50 grams is such that the accessibility of of c. P. thorium nitrate in soluthe surface is a maximum. The ratio of free space to volume of the solid is so great that diffusion tion were treated in the cold with an excess of ammonia, through the structure occurs rapidly, while the diameter of the and the precipitate was washed repeatedly by decantation capillaries is so large that capillary condensation is negligible, with distilled water until free of ammonia. It was then made a factor that seriously reduces the efficiency of the usual gel up to a volume of 600 cc. with distilled water, heated to type of catalyst under certain circumstances. On the other 90" C., and peptized by the addition of 5 grams of thorium hand, unpublished measurements made in this laboratory nitrate with rapid stirring. Stirring was continued a t 90" C. show that the average distance between surfaces in the aero- for one hour, after which the material was cooled to 60-70" C. gels is smaller than the mean free path of a gas at atmospheric and the peptization continued with stirring until complete pressure, so that under normal operating conditions a mole- (about 1 hour) The sol was yellow-orange and transparent. cule will bound from surface to surface and experience a It was then dialyzed for 24 hours in a Cellophane bag, concentrated to 33-35 per cent thorium dioxide (by ignition of a maximum number of collisions. The methods of preparation of the aerogels are so versatile sample) by evaporation under vacuum on a steam bath, and that it may be possible to prepare gels of a given material cooled. A 10 per cent solution of citric acid in alcohol was which will show marked differences in activity. It is possible added dropwise with stirring until the viscosity had risen to a to support films of substances that do not form gels, such as point where by experience it was known that, on standing, the sol would set to a gel. An alcoholic solution was used to the metals, on aerogels and thus obtain added versatility. The selection of the first catalyst for investigation was more prevent precipitation before the drop was dispersed. The or less fortuitous. One of the authors needed certain ke- amount of citric acid used will depend largely upon the extent tones, and the catalytic decomposition of aliphatic acids was of dialysis. The gel was cast in a beaker and, when thoroughly set, was therefore investigated. Thorium dioxide is one of the precovered with acetone in which it was allowed to stand for 4 ferred catalysts for this reaction, according to Sabatier. days. Care was taken not to disturb the gel. The acetone PREPARATION OF CATALYST was changed daily. The gel was then loosened from the The general method for the preparation of aerogels has beaker by a wire inserted a t the edges. After about 5 days been given in detail in a former publication (8). I n the the gel was transferred to another beaker in as large lumps a s preparation of the catalyst a transparent jelly of thorium di- possible. It was again allowed to stand in acetone for 24 oxide was desired, since the gelatinous precipitate usually used hours. The acetone was then exchanged for methanol b y in the preparation of gel catalysts would not have the desired allowing the gel to stand in the latter for I 2 hours. The gel mechanical strength, and since there is possibly better and was carefully removed to an autoclave. The autoclave was I
