Mechanism of Drier Actio A Proposal and Panel Discussion presented before the Division of Paint, Plastics, and Printing Ink Chemistry a t tlie 124th Meeting of the ACS, Chicago, Ill., J o h n K . Wise, presiding
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discussion departs from the usual presentation of cxperimental evidence, the results of original research, and subordinates experimental evidence to theory and perhaps to speeulation. The subject is a very controversial one in the field of paint and varnish chemistry-how do oil-soluble metal salts accelerate the oxidation and polymerization of unsat’urated triglycerides? I n the last 10 years, much good Twrk has been done on tlie chemical reactions involved in the drying of oils. Today, the geri-
era1 mechanism is fairly well accepted. The prinripal article in this discussion considers those chemical reactions iiivolwtl in the drying of oils and proposes a mechanism to explain ho~v oil-soluble metal compounds affect) these reactions. I2our ~ x perts in the field comment on and evaluate this proposal. Thi.: discussion is not expected to eliminate all cont,ro cerning drier action, but, it is designed t o spotlight arm.. of UTIcertainty a,nd to point the way for future research. JOHN K. WISE
E. R . 3IUELLER Organic Coatings Division, Battelle Memorial I n s t i t u t e , Columbus, Ohio
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given by Elm ( 1 9 ; . This work was started almost a centu However, little real progress Tq-as made until after the first \Yorltl Il’ar. Most useful information has appeared since about 1930. Since then, a large number of investigators have added greatly t o our knowledge of the thickening, gelling, and set,ting of oils, Colloid chemists made valuable contributions which helped t,o explain t h e process of film format’ion. It remained for the high polymer chemists, however, t o shed t h e most useful light on this highly intricat,e and complex subject during the last 15 to 20 years. P O I . Y l f E R I Z A T I O S BY HEAT. I n order t o understand more fully the oxidative polynierizat,ion process and t h e role of t,lie driers in it, the heat-induced polymerization of oils should be considered. Recently, there have appeared several excellent rcviem on this subject, (IO, 12, 67, 68). It has been rather clearly demonstrated by these and other investigators ( 1 5 ) that, threedimensional polymers must be formed before gelation can occur. Therefore, it must be assumed t h a t this is what happens when films are baked or force dried. It has been shown by many Lvorkers t h a t such a three-dimensional structure may be derived in several a-ays. hIost applicable in this discussion arc m t c r interchange of higher and lower molecular weight polymers arid cross linking a t the unsaturat,ed centers. Figure 1 depicts two hypothetical synthetic oil molecules which simply show the structure of several fatty acids attached to polyols. By cross linking of two fat’ty acids, one from each of tn-o such niolecules or similar molecules, one dimer fatty acid already forms a double-eized molecule. Ester interchange occurring in h o such or similar molecules would form a four-unit structure. By cross linking a t t h e unsaturated centers, it is generally believed that more or less cyclic polymers are formed, depending on the degree of unsaturation of the fatty acids involved.
NY investigators have studied t h e drying of oils and air-drying finishes. T h e effect, of factors--e.g., heat, oxygen, light, a.nd catalysts-on the conversion of an oil film t o R “set” condition, has been ext,ensively investigated. Most of t,hk work has dealt with t h e effects of heating t h e oils or est,ers of fatty acids t o relatively high temperatures (260’ t o 320’ C.) or osidizing them a t temperatures ranging from room tempcraturc or slightly above to 130’ C. Investigations concerning the efYect of catalysts, for the most part, have been almost entirely empirical. JVhile a lot of illformation is now available on t h e effect of these “accelerators,” m c h as the common drier metals, on the air-drying process, t h e precise manner in Thich these materials function is still a subject of considerable controversy. I n the organic coatings industry, great strides have been made during t h e last 25 years in modifying the common vegetable oils to improve their over-all properties, including drying rate. Of particular interest are t h e quickdrying varnishes of various types made from these oils. Jl‘hile most, varnishes dry much faster than the oils, the process of air drying is generally considered t o be affected very little, if a t all, by the resin component, regardless of t h e type of resin (4bj. For the sake of simplicity, this paper, therefore, nil1 deal only with the role of the drier metals in the oxidative-polymerization of oil films. I n order t o better understand how catalysts or accelerators aid in the drying process, it is essential t o consider first t h e processes of polymerization and drying in the absence of these accelerators. PROCESSES IIVVOLVED DURING DRYING
T h e physical and chemical processes operative in film formation have been extensively investigated b y many workers, both in this country and abroad. A good historical review has been
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