Document jgY56JzrjJaXMYGe5Ddne66N

v 30 INDUSTRIAL AND ENGINEERING CHEMISTRY___________________ Vol, 18, No. 1 / Effect of Zinc Oxide Pigments upon Rate of Oxidation of Linseed Oil1 By F. H. Rhodes and R. A. Mathes Co r n e l l Un iv e r s it y , It h a c a , N Y. N THEIR preliminary work on the effect of various I pigments on the oxidation of linseed oil, Rhodes and Van Wirt2 found that under certain conditions zinc oxide shown to represent the average results of the two or more check determinations made with each paint. U. S. P. Zin c Ox id e (Fig u r e 1)--This pure zinc oxide has causes an oxidizing film of linseed oil to set or harden at anvery little effect on the rate of oxidation of the oil. The early stage in the oxidation, They did not, however, de slight retardation of the initial rate of oxidation may be termine the cause of this hardening or the extent to which due to a slight adsorption of the lead drier or simply to the it depends upon the exact composition of the pigment. The mechanical effect of the pigment in retarding the diffusion work described in the present article was undertaken for of oxygen into the interior of the film. the purpose of investigating more closely the relation existing Ka d o x Bl a c k La b e l Zin c Ox id e (Fig u r e 2)--Paints between the exact composition of a zinc oxide pigment and made with this pigment harden somewhat more rapidly than the effect of that pigment do those made with the upon the drying of linseed purer U. S. P. zinc oxide, so oil. Materials The linseed oil used in this work was pure refined oil from North American seed. It showed the fol lowing analysis: Specific gravity at 15.5 C. 0.938 Refractive index at 25 C. 1.4801 Acid number 0.496 Saponification number 193.5 Iodine number 173.7 Pure zinc oxide has no effect on the rate of oxidation of linseed oil. Zinc oxide containing lead as an im purity may increase the rate of oxidation because of the formation of lead driers by interaction between the oil and the pigment. The presence of soluble com pounds of zinc causes the drying oil to coagulate at an earlier stage in the oxidation and to form a harder and less permeable film. The action of soluble compounds of zinc as coagulants or hardeners for Iinoxyn explains many of the observed phenomena in connection with the use of zinc oxide pigments and zinc driers. that the diffusion of oxygen into the film of partly oxi dized oil takes place more slowly and the final oxidation of the oil is retarded. Red Seal and White Seal zinc whites gave results which were so nearly iden tical with those obtained with the Kadox Black Label oxide that a single curve may be drawn to represent all three sets of results. - The analyses of the pig The most significant differ ments used are. shown by Table I. ences in composition between these three pigments and the U, S. P. material are in the amounts of lead and of zinc sulfate Table I--Analyses of Pigments (Figures in per cent) Pig me n t HaO SiOa SOa U. S. P. Zinc Oxide Kadox Black Label Red Seal White Seal Standard American 0.10 0.10 0.03 0.07 0.07 0.37 0.50 0.50 0.53 0.24 0.00 0.00 0.01 0.00 0.03 Pb 0.02 0.23 0.22 0.30 1.95 ZnSO* 0.05 0.15 0.11 0.14 0.41 which are present. Since the larger amount of lead in the Kadox oxide should tend to increase rather than to decrease the final rate of drying, it seems reasonable to assumethat the solu ble salts of zinc are responsible for the more rapid hardening of the oil and the decrease in the final rate of oxidation. St a n d a r d Ame r ic a n Zin c Wh it e (Fig u r e 3)--This pig Procedure ment at first slightly retards the oxidation but later acceler ates it. The initial retardation may be due to the partial The procedure used was essentially similar to that de adsorption of the lead drier by the pigment. The final scribed by Rhodes and Van Wirt.4 Paints prepared from acceleration is probably caused by the interaction of the zinc oxide and linseed oil were exposed to an atmosphere of compounds of lead in the pigment with the acidic products pure oxygen at 30 C., and the rate of absorption of oxygen of the oxidation of the oil, with the resulting formation of and the rate of evolution of volatile matter were measured. lead driers. It will be observed that with the U. S. P. zinc Each paint was made up from two parts by weight of pigment oxide, which is practically free from lead, no such acceleration and three parts of linseed oil in which had been dissolved sufficient lead linoleate to contain an amount of lead equiv alent to 0.2 per cent by weight of the oil. All paints were aged by being allowed to stand for 2 weeks in a sealed con of the final drying takes place. Ef f e c t of So l u b l e Co mp o u n d s o f Zin c (Fig u r e 3)-- If the presence of very small amounts of soluble salts of zinc does cause the more rapid and more marked hardening of i tainer before being tested. With each paint there were made two or more parallel determinations, giving results which agreed to within the limits of experimental error. the drying oil, it should be possible to cause this hardening to take place to a greater extent and at an earlier stage in the oxidation by adding soluble compounds of zinc to the paint. Paints were prepared from Standard American zinc Results white to which had been added 5 per cent by weight of de hydrated zinc sulfate, and from Standard American zinc The results are shown graphically by the accompanying white to which had been added 5 per cent by weight of zinc curves, in which the amounts of oxygen absorbed and the linoleate (obtained by adding a solution of zinc sulfate to a amounts of volatile matter evolved, each expressed in terms neutral solution of sodium linoleate and drying the resulting of percentage by weight of the oil in the paint, are plotted precipitate). The paint containing the zinc sulfate showed against the lengths of time of exposure. A single curve is sudden and marked hardening after about 50 hours' exposure. 1 Received June 16, 1925. 'Th is Jo u r n a l , 15, 1135 (1923). The hardened film continued to oxidize very slowly, so that the total amount of oxygen absorbed after 400 hours' ex- % ih January, 1926 INDUSTRIAL AND ENGINEERING CHEMISTRY 31 posure was only 32 per cent instead of 43 per cent, as in the paints are harder and less elastic than white lead paints,"* case of the paint made with the pigment to which no zinc and that "the addition of zinc oxide to a white lead paint sulfate had been added. The paint containing zinc linoleate renders it sufficiently hard to resist some of the peculiarly hardenedjiter about 25 hours' exposure, but the hardened inherent forms of decay and discoloration.4 Since other very film was somewhat more permeable to oxygen and continued fine pigments do not show the same effect, this hardening cannot be due to the merely physical action of the finely di vided zinc white in decreasing the voids in the film, but must be caused by some change in the hardness of the oxidized vehicle. The zinc oxide causes the linoxyn film to harden and thus gives a film which is not only firmer but less per meable to oxygen and therefore more resistant to the slow "after oxidation" which is responsible for the gradual dis integration and ultimate destruction of the paint. If the paint contains too much zinc in soluble form, however, the hardening effect may become so pronounced that the dry film fails because of brittleness. Excessive amounts of soluble zinc may also tend to gelatinize or coagulate the liquid paint and may therefore cause "stiffening," "livering," or "granula tion." Such excessive amounts of soluble zinc may be intro duced by the use of a zinc white, which contains zinc sulfate,* to oxidize more rapidly than did the film from the paint con or by grinding zinc oxide in an oil of high acid value'.3*6 taining zinc sulfate, so that the total amount of oxygen taken up in 400 hours was about the same in the two cases. A number of investigators have found that the addition of soluble compounds of zinc to linseed oil decreases the time required for the film of oil to become dry to the touch, and Discussion have therefore classified zinc, along with lead, manganese, cobalt, and iron, as a drier for linseed oil. Many of the" Pure zinc oxide, free from lead and from soluble compounds of zinc, has no effect upon the rate of oxidation of well-refined linseed oil and does not affect the hardness or other character istics of the linoxyn film formed during at least the first 400 hours of drying. The presence of compounds of lead in the zinc oxide tends to increase the rate of oxidation of the oil. This effect is probably due to the formation of lead driers by the interaction of the lead compounds in the pigment and the acidic products of the interaction of the oil. Very probably the extent to which this drying effect of the lead compounds appears de pends, not only on the amount of the lead in the pigment, but also upon the form in which this lead is present. The presence of soluble compounds of zinc tends to cause the oil to gelatinize or solidify at an early stage in the oxida tion and to form a harder and more dense film. Among the soluble compounds of zinc that may find their way into a paint are zinc sulfate, which may be introduced as an impurity in the zinc oxide pigment, and zinc linoleate, which may be formed by the interaction of the zinc oxide with a linseed oil of high acidity. It is interesting to note that lithopone, which may also contain small amounts of soluble compounds of zinc, also acts as a hardener for linoxyn and tends to in hibit the final oxidation of the film.2 special patent driers contain zinc sulfate - as an essential ingredient. The writers' experimental work has shown that zinc is not, properly speaking, a drier, but is a hardener for linseed oil. Compounds of zinc do not catalyze the oxidation of the oil, but they do cause the oil to solidify at an earlier stage in the oxidation. Since, as is well known, paints made with lampblack or carbon black sometimes tend to remain sticky for a long time, it would appear that the addition of zinc sulfate or zinc linoleate to such paints would be of con siderable advantage.7 * Sabin, "Technology of Faint and Varnish/* 1917, p. 184. * Gardner, "Faint Researches and Their Practical Applications/* 1917, p. 27. * Holley. "Lead and Zinc Pigments/* 1909, p. 178. * Morrell and de Waele, "Rubber, Resins, Paints, and Varnishes/* 1920, p. 118. / * Toch, "Chemistry and Technology of Paints,** 1910, p. 45. O 50 100 150 ZOO 250 300 350 "tOO Figure 2 This action of soluble compounds of zinc as coagulants or hardeners for linseed oil explains many of the phenomena that have been observed in connection with the use of zinc oxide pigments. It is commonly recognized that "zinc oxide Austria Exempts Duty on Certain Chemicals--By a decree of the Austrian Minister of Finance, the following substances, intended exclusively for the preparation of medicines have been exempted from import duty until further notice, provided they are loosely packed and imported as chemicals. Salvarsan, neosalvarsan, and salvarsan compounds are duty-free in any form. 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