1NDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 23. No. 11
Gelation Phenomenon o f Synthetic Resins and Other Organic Polymers' R. H. Kienle GENERAL ELECTRIC COXPAXY, SCHENECTADY, hT. Y.
T
HE industrial usefulness of synthetic resins and other organic polymers depends largely on their ability to convert or non-convert to the so-called insoluble, infusible state. This conversion process is generally referred to as polymerization, although strictly speaking it is a gelation phenomenon, that is a sol -,gel transition. It is to be differentiated from the reversible gelation which is encountered when an organophillic solution of a synthetic resin or polymer is evaporated either en masso or to form a film. Only the gelation which the synthetic resin or polymer itself undergoes, irrespective of the presence of any solvent or dispersing medium is involved. A previous survey has shown that three classes of organic polymers are obtainable, depending upon their ability to gel or upon their mechanism of gelation-namely, heat convertible, heat non-convertible, and element convertible (6). Synthetic polymers of all these three classes have been prepared, the fundamental chemical reaction of preparation being either condensation, addition, or both. The fundamental question a t the present time in the formation of the final useful state of synthetic resins and other organic polymers is whether this so1 -c gel transition is chemical, physical, colloidal, or combinations thereof. It is the relative importance of these several processes that differentiates the ideas of the various workers in this field of organic polymers-e. g., Staudinger (9), Mark and Meyer (7), Long (4, Carothers ( I ) , and Eibner (9). Thus, Staudinger stresses the chemical side, laying emphasis on primary valence forces by assuming the formation of macromolecules. This is probably because he and his co-workers have studied organic polymers from a synthetic viewpoint, and particularly as they have worked with addition polymers where the reaction reactivity was 2,2 (.-)-e. g., styrene, polyoxymethylene, etc. Mark and Meyer, on the other hand, stress the importance of secondary valence forces, such as association. They assume that such forces act between chain molecules, forming micelles, and that similar forces subsequently act between the micelles. By following the fundamental chemical reactions in a number of different types of synthetic polymers up to, and to some extent through, gelation, it has been shown, a t least in the case of polymers studied so far, that the primary valence union of the interacting molecules occurred continually. Both convertible and non-convertible polymers have been prepared, even though the same basic chemical reaction took place. Thus non-convertible polymers have been formed: by esterification-glycol and phthalic anhydride; by condensation-phenol and formaldehyde in the presence of acid catalyzers; and by addition polymerization-cyclopentadiene. On the other hand, convertible polymers have been formed: by esterification-glycerol and phthalic anhydride; by condensation-phenol and formaldehyde in the presence of alkaline catalyzers; and by addition polymerization-cyclopentadiene under pressure. The alkyd resins, because they could be prepared from a wide variety of molecules of various sizes, shapes, and degrees I Received April 9, 1931. Part of Symposium on "Polymerization as Related to Paint and Varnish" presented before the Division of Paint and Varnish Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind.. March 30 to A p d 3. 1931.
of reactivity, afforded a particularly good opportunity to study the effect of such factors on the gelation phenomenon. All types of alkyd resins have been prepared, together with amorphous powders and even crystalline compounds, although the same basic chemical reaction-namely, the condensation of a hydroxyl and carboxyl g r o u p w a s common throughout. These studies lead to the formulation of three postulates as being fundamental to resin formation. These postulates have been given previously (3, 5 ) . However, before the cause of the sol -+ gel transition of synthetic resins is definitely understood, the properties and reactions after gelation as well as up to gelation must be taken into account. Depolymerization
The reason that rubber, metastyrene, heat-converted alkyd resins, and heat-converted drying oils can be depolymerized must be explained. Depolymerization is accomplished by simple heating in a confined space, as, for example, in an autoclave or partially closed container. The process is not thoroughly understood, but seems to be tied up with the pyrogenic decomposition of the gelled polymer t o form highboiling nonvolatile constituents that solvate and eventually liquefy the gel. Depolymerization can be illustrated with China wood oil gel which is normally regarded as quite stable. If China wood oil gel, formed by heating China wood oil to 290" C., is kept at 300-325' C. in a loosely covered vessel without agitation, a dark brown fluid is slowly formed a t the bottom and sides of the vessel which eventually permeates the gel until the entire mass is converted to a dark brown liquid-the depolymerized gel. At room temperatures, depolymerized China wood oil is a viscous, thick, tarlike mass. It is non drying, although in thin films it can be baked. E n masse it stays liquid even at elevated temperatures, slowly evaporating. It can be regelled by treating with 510 per cent of glycerol. The new gel thus formed can be depolymerized as before. Other polymers similarly depolymerize, but only certain of them have been regelled. Furthermore there is the fact that it has been possible to prepare sols from cured alkyd resins, heat-converted drying oils, and cured phenol-formaldehyde resin by simple, prolonged, and restricted solvation. To prepare such sols, the principal solvents should be those in which the original reactants are very soluble, and the conditions so adjusted that prolonged solvation could occur without loss of solvent. In that may clear transparent sols of cured glyceryl phthalate in glycol diacetate and in acetone, of linseed oil gel in benzyl benzoate and in petroleum ether, and of cured phenol formaldehyde in o-cresyl benzoate were prepared. There also is the fact that gelled alkyd resins on curing no longer appear to evolve water of condensation but rather glycerol and phthalic anhydride, as certain experiments of Kienle and Wright (unpublished) indicate. I n these experiments the volatile products evolved during the curing of a gelled glyceryl phthalate resin, especially prepared from pure ingredients, were carefully collected and quantitatively determined. The only products found were carbon dioxide, water, and phthalic anhydride. The carbon dioxide and water were present in the proportions to calculate as glycerol. The glycerol, so calculated, and the phthalic anhydride were
I-VDUSTRIAII A S D ENGINEERING CHEMISTRY
November, 1931
found to be present in the stoichiometric proportions present in the original uncured resin. In the attempt to better correlate the properties of gelled resins with resins prior to gelation, a quantitative study of the imbibition of gelled synthetic resin has been started by Winslow and Kienle ( I O ) . This study is by no means concluded. But as far as it has gone, there seems to be but one criterion of swelling. This is the product of tjwo factors: the maximum amount of solvation for which a particular solvent is capable, as measured by the swelling maximum M ; and the rate a t which this solvation proceeds, as rneasured by the velocity constant of swelling K . This is illustrated in the data obtained for glyceryl phthalate given in Table I. Table I-Swelling
SOLVENT
Data f o r Glyceryl P h t h a l a t e SWELLING DIELECTRIC ELECTRIC MAXIMUMVELOCITY CONSTANTMOMENT M (VOL. CONSTANT AT 20" C. ( p X 1018) INCREASE) ( K X 100) (M X K ) % , ,"
Dioxan Benzyl alcohol (1:l) (13) Acetone Ethyl acetate Ethylene glycol Benzene Water
3.0
2i.'3 6.4 41 2.3 81
0.5 2:io 1.74 2.5
0.06
1.7
48 53 150 60 5.8 8.0 Low
26.7
12.80
21.6 7 .3 7.1 19.8 3.0
11.45 11.25 4.28 1.15
..
0.24
Low
I n this table the solvents are arranged in the order of decreasing values of the product M X K . This order is in agreement with past experience as to the effectiveness of these liquids as glyceryl phthalate solvents. The dielectric constants and electric moments of the solvents are included in the table t o illustrate the minor importance which these two factors seem to play in the case of organic polymers for determining the effectiveness of liquids as solvents and swelling agents. The importance of solvation, together with the rate with which solvation proceeds in the swelling process is, however, evident. Properties of Gelled Organic Polymers The properties of gelled organic polymers, some of which have just been discussed, are of equal importance to an understanding of the gelation phenomenon as are the studies on the formation of the original high-molecular weight, i. e., sol state. To understand both the sol and gel structure is to understand the sol + gel transition. Add thereto a knowledge of the reasons for the formation of non-convertible polymers under certain conditions in preference to convertible polymers, although the same basic chemical reactions are involved in both cases, and the mysteries of organic polymers begin to unravel. In organic polymers the fact that chemical reaction proceeds up to the gelation point is consistent with a strictly chemical theory of gel formation, that is, macromolecules being formed and chemically bonded together to give the gel state. Further evidence in support of this contention is that either convertible or non-convertible polymers can be obtained in a perfectly definite manner by simply changing the number or position of the reactive points in the interacting molecules. Howeyer, the ability of the gelled polymers to reconvert, with the proper swelling agents and under the proper conditions, to a sol state, together with the imbibition studies on glyceryl phthalate point to a colloidal structure. Taking all these facts into consideration, about the best conception of organic polymers a t the present time seems to be that they are formed from simple molecules which have interacted, according to chance contact in space, in a strictly chemical manner, producing a mixture of various-sized molecules. According as the molecules, through their primary valence bonding, are built up by intertwining, convertible polymers result. Whenever straight-chain unions occur, non-convertible polymers are formed. A mixture of polymers may therefore be obtained, in which small molecules of
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low degrees of condensation are associated with large polymeric molecules. This leads to an organo-colloidal system whenever the size attained by the larger molecules is within the colloidal domain. With this concept nearly all the known formation reactions and sol + gel phenomena are understandable, except possibly in the case of those polymers prepared from highly polar ingredients, such as the phenol-formaldehyde, aniline, and urea resins, although even here the concept holds in general. With these resins, association forces seem to play a more important role during the sol .-,gel transition than otherwise. This is probably due t o the fact that these resins are in reality hydro-colloidal systems, a fact which is not surprising when it is considered that their ingredients are polar and that the condensation product is water, thus giving the conditions requisite for marked hydration. The importance of dehydration and of its bearing on the gelation of these resins has long been recognized by those interested in their industrial preparation and use. In particular the work of Pollak and Ripper (8) bears mention in this connection, as they have definitely shown the importance of proper dehydration in the case of urea resins. Pollak and Ripper further showed that considering urea resins as hydro-colloids and applying known colloidal treatment thereto a variety of products results. Although not suspected and certainly not recognized a t first, recent work has shown that somewhat analogous is the case with the aniline and phenolic resins. Without doubt the other factors mentioned, such as continued intertwining due to purely chemical interaction, are also important and present. It is probable, therefore, that bhe same gelation mechanism may not hold for all synthetic resins and organic polymers. It is suggested that two general mechanisms occur. In both cases, however, the formation of the polymeric molecules is fundamentally a primary valence linking of molecules. But in one case during gelation, the primary valence linkage is of fundamental importance with association forces, if acting a t all, of secondary importance; while in the other case, association is paramount'. Summary
As the experimental facts exist today, the explanation of the gelation phenomenon of synthetic resins is uncertain. The importance of primary valence union in the formation of high polymers is recognized. Furthermore, the importance of the reactivity of the interact'ing molecules, their size and shape, and their chance union in space in determining the gelation properties, has been shown. But whether gelation is due to further chemical union of polymers into macromolecules, to association or secondary valence forces binding polymers into aggregates, to the building up of a certain concentration of polymers followed by a sort of unoriented crystallization process, or to a process still to be uncovered, is by no means proven. Above all, further quantitative experimental work is necessary before this industrially important gelation phenomenon can be fully explained. Literature Cited (1) Carothers, W. H . , J . A n . Chem. SOL., 51, 2548 (1929). (2) Eibner, .4., "Das Oeltrocknen," Allgemeiner Industrie-Verlag, Berlin 1930. (3) Hoenel, H . , Paint, Oil Chem. Rev., 91, 19 (1931). (4) Long, J. S., Ibid., 89, 8 (1930). (5) Kienle, R . H., IND. ENG. CHEM.,22, 590 (1930). (6) Kienle, R. H . , and Ferguson, C. S., Ibid., 21, 349 (1929). (7) Mark, H . , and Meyer, K . H . , "Der Aufbau der hochpolymeren Organischen Naturstoffe," Akademische Verlagsgesellschaft, M. B. H . Leipzis, 1930. (8) Pollak, F., and Ripper, K., Chem.-Ztg., 48, 569 (1924). (9) Staudinger, H . , Ber., 62, 2893 (1929). (10) Winslow, E. H., and Kienle, R . H . (To be published.)
