Theodore F. Bradley and Roy W. Tess Shell Development Company, Enzeryidle, Calif.
T h e viscosity and gelation characteristics of esters of drying oil acids as observed during the course of their thermal polymerization (as well as during air drying) are influenced and determined by the functionality of the reactants. For this reason the number of hydroxyl groups of the alcohols and the number of carboxyl groups of the acids used in the synthesis of the ester become just as important as do the number of unsaturated bonds as determinants of the nature of the polymer and of the rates of viscosity increase and of gelation. The difficulties which
RECENT author (46) has stat,ed that the ability of an oil to polymerize or to dry in films is related to its unsaturat,ion, since it is the double bonds which are the reactive centers about which polymerization occurs. He neglected to emphasize that double bonds are not the only functional groups concerned and that in drying oil polymerization, as in all other polymerizations, all of the functional groups should be considered. Failure to do so can easily overcmphasize the importance of a particular kind of functional group. The double bond has attained much of its prominence because it is one of the few kinds of functional groups which will readily undergo reaction a t ordinary or a t slightly elevated temperat,urcs. Frequently the reaction occurs by simple addition wit,hout the necessity of eliminating any volatile by-products, and this fact has many practical benefits. This kind of funct'ional group is abundantly present in so-called drying oils. Their intelligent and future use will be governcd by ability to comprehend more of their chemistry and to apply whatever is learned to constructive ends. Overemphasis of the role of unsaturation in the polymerization and drying of the varnish and paint oils is believed to have retarded progress in this industry. Seemingly it is only because of the knowledge gained in t,he synthesis of alkyd resins and other polymers and the commercial inroads made by these synthetics that the tide has recently turned. This new chemistry has taught that polymers may be formed by condensation as well as by addition mechanisms and frequently by combinations of both. A given viscosity or the gel point of a polymerizable oil therefore may be attained by either or both mechanisms according to the nature and the number of functional groups involved and the reaction conditions imposed. Hydroxyl and carboxyl groups therefore become equally as important as the double bonds in the synthosis and in the polymerization and drying of drying oils. Moreover, natural drying oils must be considered from the standpoint of their component parts (glycerol and monobasic carboxylic acids) since the first step of their polymerization (molecular growth) occurred during their synthesis. That this is not a mere academic abstraction is best illustrated by the fact that improved drying oils or resins are being manufactured currently by replacing glycerol with alcohols having more hydroxyl groups, as in the case of the pentaerythritols, sorbitol, and mannitol and/or by replacing a portion of the drying oil fatty acids with polybasic acids, as in the formation of oil-modified alkyd resins. In both of these cases the manufacturer is designing a new form of drying oil in which a larger and more polyfunctional molecule
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arise during the attempted isolation, purification, and characterization of the polymers of the esters of drying oil acids are reviewed, the shortcomings emphasized, and constructive improvements suggested. The shortcomings of statistical methods as applied to the polymerized mixtures, including their neglect of intramolecular reaction, are emphasized. Two classes of copolymers which are capable of being thermally copolymerized with other compounds to modify them usefully for industrial applications are defined and illustrated.
is created through a condelisation mechanism which in n o way involves the double bonds. Improved drying and film characteristics result and the products heat body or polynierize faster in the varnish ket,tlc; yet the iodine values are generally lower than are found in the natural drying oils. The degree of unsaturation per unit of weight as expressed by the iodine value may decrease, yet because a larger molecule has been synthesized as described, the degree of unsaturation per molecule has increased thereby; this can be recognized to impart much of the significant, improvement. According to modern findings, the thermal treatment of natural, drying oils to effect their partial polymerization and to providc improved drying and film characteristics involves the creation or polybasic acid esters by addition polymerization of the unsaturated fatty acid radicals, thereby creating an alkyd resin nrhioh except for the specific nature of its polybasic acid componc:nt,s does not differ materially from ordinary alkyd resins. Although conceptions of the influence of colloidal forces arid aggregates in the polymerization and drying of oils have becri relegated t,o a secondary role following the impact of polymw chemistry, disagreement still exists with respect to the mcchariisms of double bond rcactions in the polymerization of drying oils. Much of this now seems to have been due to the lack of prcparetive and analytical tools and techniques suitable for appiicatiora to compounds of such relatively high molecular weight, arid complexity as have been dealt n.it,h. Even when the availahle tools are applied to the polymerized methyl esters of drying oil fatty acids or to the polymerized fatt,y acids themselves great, diffioul-. ties are encountered in their fractionation and purification. Such distillations as have been reported, with few exceptions, Icavc: much improvement to be desired, as will he s h o r n subsequcntly . The situation rapidly becomes worse as one attempts to dc%l vvith still higher molecular weight mixtures and compounds of' greater functionality. This has occasioned various hypothetical and statistical approaches to the problems at hand. Pending the development and application of improved rost:arcli tools, attention may be centered profitably on the polymcrizai ion of lower molecular weight cornpounds bcaring st,rutrtuml similarity to the drying oils and including some unsaturated hydrocarbons and simple esters. After due considerat,iorl the resul tiiig findings then may be compared with much that has boen 1cur11c~l about the drying oils and their simpler esters and conclusiom !h(.ta reached which may help to justify t,hii approach and to dt~i.il'y the situation.
Pebruary 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
Polymerization sf Hydrocarbons Although all addition polymerizations involving double bonds have been held by some investigators to involve the formation and subsequent reaction of free radicals, this generalization seems to hold little benefit as such. Free radical polymerization in its best known examples such as vinyl polymer and synthet
