EPOXY
plasticizers-stabilizers were introduced in 1950, and today represent the third largest class of plasticizers. They encompass all structures containing an epoxy group which imparts plasticizing and/or stabilizing properties to polymer systems. Many such structures have been described in the literature, but only two classes, epoxy fatty acid esters and glycidyl ethers, are used commercially. Of these two groups, epoxy fatty acid esters represent by far the largest tonnage-an estimated 30,000,000 pounds will be produced in 1957, of which approximately 77.5% will be epoxy-triglycerides and 22.5% alkyl epoxystearates. Usage of epoxy plasticizers-stabilizers has grown because of the unique ability of the epoxy group to stabilize poly(vinyl chloride) resins, which like other highly chlorinated materials tends to undergo dehydrochlorination, resulting in degradation and discoloration, when exposed to heat and light. The exact mechanism of stabilization is not known, but one theory ties this in with ability of the epoxy group to scavenge acidic materials. These epoxy plasticizers have been used mostly with poly(viny1 chloride) resins-e.g., in gasketing, floor tile, garden hose, vinyl foam, wall coverings, food packaging, vinyl toys, coated fabrics, film, organosols, and plastisols. But also they are used to stabilize chlorinated derivatives of such materials as rubbers, paraffins, hydrocarbons, insecticides, and to plasticize other polymer systems such as poly(viny1 acetate), ethylcellulose, Neoprene, synthetic rubbers, and epoxy resins. The degree of plasticization and stabilization is directly related to structure and may be achieved sjngly or in combination-e.g., diglycidyl ethers are good stabilizers but poor plasticizers. O n the other hand, epoxy fatty acid esters are generally good both as plasticizers and stabilizers; thus, they can be described as plasticizers with stabilizing action or stabilizers with plasticizing action. In contrast to this, some epoxy structures are good plasticizers but poor stabilizers. Copolymerization of vinyl epoxystearate with vinyl chloride gives a linear polymer with epoxy fatty acid ester groups spaced along the chain. This structure is well plasticized but poorly stabilized. Paradoxically, vinyl epoxystearate milled into poly(viny1 chloride) resin as an external plasticizer gives excellent stabilization along with plasticization. Also, a given epoxy structure may stabilize in one instance but not in another. Epoxy fatty acid esters as a class are good stabilizers as well as plasticizers for poly(vinyl chloride), but in poly(viny1 acetate) they only plasticize. Epoxy plasticizers are generally used in poly(viny1 chloride) formulations a t the secondary level-i.e., as a partial replacement (IO to 20%) of the primary plasticizers. In recent years there has been an increased tendency to incorporate a few parts of epoxy plasticizer into all poly(viny1 chloride) formulations and thus obtain a “free ride” on stabilization. When combined with standard metallic soap stabilizers, increased stabilization is achieved at low cost-usually about one half of the metallic soap is needed for equal results. Although heat and light stabilization has received most attention, epoxy triglycerides are polymeric-type plasticizers having extremely low volatility, excellent soapy water resistance, and fine pigment-wetting properties. The ex-
EPOXY
Reprints of this group of articles may be purchased at $1.00 for single copies or for $0.75 each, in lots of ten or more. Address Special Issue Sales Department, American Chemical Society, 1 155 16th Street N.W., Washington 6, D. C.
VOL. 50, NO. 6
JUNE 1958
861
cellent low temperature properties of alkyl epoxystearates, which are usable at primary plasticizer levels, are comparable to the adipates. Glycidyl ester stabilizers are made by condensation of epichlorohydrin with a polyhydroxy material, generally a polyphenol such as bisphenol A. The reaction is carried out with excess epichlorohydrin to give a low molecularweight liquid product. These are identical with liquid epoxy casting resins. Epoxy fatty acid esters are prepared by epoxidizing the corresponding unsaturated fatty acid ester with a n aliphatic peracid, generally through the medium of hydrogen peroxide. A number of efficient reaction procedures use hydrogen peroxide in the presence of acetic or formic acid. Numerous epoxy plasticizer-stabilizer structures have been described, largely fatty chemical based. But numerous other compounds will undoubtedly be found, which are not dependent on a fatty chemical structure. F. P. GREENSPAN Chairman FMC Organic Chemicals Division New York, N. Y.
As Plasticizers
COMPATIBILITY
BETWEEN polar polymers and plasticizers usually results from the right kind of polar groups being properly distributed on the plasticizer molecule. The requirements for such polar groups are so exacting that only a few, most recently the epoxy group, have attained widespread commercial acceptance. Some believe that the future importance of epoxy groups will exceed that of all other compatibilizing groups. There is much to justify this optimism. In most commercially important systems, the compatibilizing potency of the epoxy group is greater than that of the ubiquitous carboxylic ester group. Also, these epoxy groups may readily and economically be incorporated into a wide variety of materials which contain ethylenic unsaturation. As a result, a large number of relatively low cost materials have suddenly become important, or potentially important, plasticizer raw materials. Much of this optimism has resulted from theoretical considerations and laboratory observations supplemented by pure speculation. But how well have these epoxides survived exposure in the marketplace? Because their molecular weights are a t least double those of common monomeric plasticizers, vegetable oil epoxides permit formulation of compounds which in service show little loss of plasticizer from volatilization, extraction, and migration. The plasticizing characteristics of alkyl epoxystearates are similar to those of the common dialkyl esters of aliphatic dibasic acids. They are efficient and contribute good flexibility a t low temperatures. Because of their low viscosities, they are useful in formulating plastisols and organosols. An important asset of both these classes of epoxide plasticizers is their ability to function concurrently as a plasticizer and as a stabilizer. Although the stabilizing characteristics of the epoxides have been discussed separately, this dual functionality offers so many technical and economic advantages to the vinyl formulator that its practical importance cannot be overemphasized. When used a t high concentrations in poly(viny1 chloride), all commercially available epoxy plasticizers are subject, in varying degrees, to loss in compatibility on exposure to light. This tendency is the major limitation in the way of still larger volume usage of epoxy plasticizers; and the extent to which it may occur depends on composition of the plasticizer and of the formulation in which it is used, and the conditions under which the exposure to light takes place. However, by proper control of the epoxidation
862
INDUSTRIAL A N D ENGINEERING CHEMISTRY
process, compounds may be prepared which give flexible vinyl plastics showing no appreciable loss of compatibility under the most rigorous exposure conditions. The merits of epoxy plasticization have been demonstrated in all major fields of vinyl compounding-more durable automotive upholstery, more permanently flexible, high temperature electrical insulation, wash-proof wearing apparel, nontoxic food-packaging film, and more colorful and more durable flooring compositions. R. F. CONYNE Rohm & Haas Co., Philadelphia, Pa.
As Stabilizers
THE general effect of all stabilizing constituents added to
a poly(viny1 chloride) formulation is to direct and modify
the decomposition reactions to an unobjectionable form.
TOachieve this, a modern stabilizer suitable for calendering might consist of organic salts of barium, cadmium, and zinc, a phosphite, and an epoxy plasticizer or resin. Most stabilizer systems include hydrogen chloride acceptor as a t least one of the components. Although ability to combine with hydrogen chloride is certainly a desirable stabilizer characteristic, this apparently is not the controlling influence. Early thought on degradation mechanisms held that poly(viny1 chloride) degradation was autocatalyzed by hydrogen chloride and, that to prevent further damage, a stabilizer should immediately combine with each molecule of hydrogen chloride as it is produced. Arlman has shown that hydrogen chloride does not have such an autocatalytic effect [J.Polymer Sci. 12, 543 (1954)]. Epoxy compounds extend over-all heat stability of a vinyl formulation when used with stabilizers containing salts of cadmium or zinc, but little beneficial effect is observed with tin or lead stabilizers. Although epoxy materials are capable of combining with hydrogen chloride to form chlorohydrin, this does not explain their stabilizing action. In a formulation stabilized with a barium-cadmium system, any chlorohydrin formation is apparently only transitory. This has been demonstrated by extraction and analysis of plasticizer from a heat-aged vinyl compound stabilized with a barium-cadmium system and a n epoxy plasticizer. The epoxy-oxygen content of the extracted plasticizer is the same as that of the plasticizer blend used in preparing the vinyl compound. From this, it may be concluded that metallic stabilizers used in such a compound operate preferentially to the epoxies as hydrogen chloride acceptors. Any chlorohydrin formed by heat-aging of the vinyl compound is apparently destroyed by the metallic stabilizers with subsequent reformation of the epoxide group. Commercially available epoxy compounds and bariumcadmium stabilizers exhibit a synergistic action in improving the light stability of a vinyl formulation. Use of both components provides three to four times the weathering resistance that could be obtained from either epoxy or barium-cadmium alone. During the early days of epoxy use, difficulties were encountered because of exudates which developed on surfaces of vinyl when exposed to normal light for several months. This phenomenon, called aging incompatibility, was traced to epoxy plasticizers having a high degree of residual unsaturation. More complete epoxidation, with consequent reduction of residual unsaturation, eliminated this exudation problem. Unsaturated additives must be excluded from any vinyl formulation containing a n epoxy plasticizer if aging incompatibility is to be avoided. If unsaturated additives are not employed, a low iodine value epoxy plasticizer may be used a t fairly high levels without danger of aging incompatibility. N. L. PERRY Argus Chemical Corp., Brooklyn, N. Y.
