Patented Apr. 16, 1946 2,398,350 UNITED STATES PATENT OFFICE METHOD of PRODUCING ELASTIC POLYMER, Francis Clarke Atwood, Newton, and Henry A. Hill, Cambridge, Mass, assignors, by mesne as signments, to National Dairy Products Corpo ration, New York, N.Y., a corporation of Dela ... We No Drawing. Application August 14, 1942, Serial No. 454,864 2 Claims. (Cl. 260-83) ' This invention relates to polymers having elas these well known properties of natural rubber and tic properties similar in some respects to those its simple reaction with sulfur. In producing an of natural rubber. More particularly, the inven elastic composition, a so-called 'synthetic rub tion relates to polymers in which the essential ber,' which may not be the same as natural rub polymerizing monomer is an alkyl acrylate, and 5 ber chemically, it is very desirable that such a which are formed so that at an intermediate product be capable of passing through this two stage of their manufacture, they have plastic . stage process so as to permit molding while the flow properties suiting them for molding, follow product has plastic flow characteristics, follow ing which the plastic flow characteristic may be ing which the plastic flow is eliminated. It is eliminated and the elastic characteristics en 10 particularly desirable that the plastic flow may hanced. be eliminated by a simple method after molding, Rubber in its natural form is elastic but pos and preferably by the use of heat and mechani eSSes a certain amount of plastic flow properties cal operations similar to vulcanization so that as well as elasticity. These plastic flow proper the composition may be processed by the well ties can be enhanced by milling the crude rub 5 established rubber technique and by workmen ber. In order to better appreciate and under already skilled in that technique. stand the invention, it is desirable to restate It is an object of the invention to produce the simple rubber chemistry. Natural rubber , materials which, while quite different chemically is thought to consist of long chain molecules from natural and vulcanized rubber, are adapted which contain unsaturated or double bonds, and 20 to be processed in accordance with established the ability of rubber in this condition to flow rubber processing procedures, and which upon has been explained on the basis of the long chain completion of processing, have, among other molecules slipping past One another. The molec things, a number of properties similar to those ular structure of natural or milled rubber is not of Vulcanized natural rubber. Such as to cause the chains to return to their 25. It is a further object of the invention to elimi original positions relative to each other. This nate the plastic flow from alkyl acrylate polymers makes it possible to mold the rubber and make by trans-esterification, ester interchange, saponi it take any desired form. fication or other reaction using the alkyl group In order to eliminate this plastic flow, it is the of the polymer, and without the use of vulcaniza practice to mill Sulfur into the rubber during the 30 tion. milling Operation. After or during molding of It has long been known heretofore that con the rubber, the rubber-sulfur mixture is heated pounds containing ethylene linkages, such as and the plastic flow is eliminated. This is termed ethylene, acrylic acid, and the alkyl acrylates, "vulcanization' and it is thought that sulfur re may be polymerized into long chain polymers. aets with the unsaturated bonds in the long 35 These polymers possess some elasticity, but all chain molecules of the rubber to effect cross of them possess plastic flow properties. The linkages between the chains. These cross link elasticity, particularly in the case of the alkyl ages are assumed to prevent slippage of the acrylates possibly may be attributed to the alkyl chains past each other and eliminate plastic radical extending as side groups from the long flow. At the same time the elasticity is preserved 40 chain polymers. However, these side groups do or enhanced. The latter may be regarded as not prevent plastic flow. due to the lastic characteristic of the chains It is important to note that these polymers themselves ahd to the cross links. which contain plastic flow properties, and which It is the standard practice in the rubber indus properties may be enhanced by a milling opera try to process the rubber through an intermedi 45 tion similar to that performed on natural rubber, ate stage in which it possesses plastic flow to are all Saturated. Thus the long chain mole permit molding, following which the plastic flow cules of the polymers do not contain any unsat properties are eliminated by reaction with Sul urated or double bonds, and it is not possible to fur in the process known as vulcanization. It cross link the chains of the polymers by a reac is absolutely necessary that the final product be 50 tion with sulfur, as is common in the rubber free from plastic flow or have a minimum of it. industry. On the other hand it is desirable at some stage It may be possible to co-polymerize the alkyl to have this plastic flow to permit molding. acrylates with other unsaturated compounds The techniques in the rubber industry have which offer a possibility of cross linking the co been Well developed and standardized because of 55 polymers. But the establishment of cross link 2 2,398,850 ages between homogeneous saturated polymers, ene:glycol is used, the milled polymer and the or by the use of the radicals contained in these ethylene glycol exist as a mere mixture and are homogeneous polymers, whereby plastic flow may capable of being molded in accordance with the be eliminated, has not been perceived as a possi conventional rubber technique. , bility by the prior art, nor has it been accom 5 If desired a small amount, i. e., 0.01% to 5% plished by any method as far as is known. and preferably 0.1% to 2% of a material may be In accordance with this invention, rather than added to facilitate the reaction. The amount attempt the difficult procedure of introducing will depend on the thoroughness of mixing, the double bonds into or adding double bonds onto rate of the particular reaction involved, and the the long chain polymers to permit conventional curing temperature used. A mineral acid, alkali, Vulcanization. With sulfur, or to form co-polymers Zinc chloride, aluminum chloride, sodamid, a low having groups through which cross linkage may er Organic acid chloride or any compound known be effected, it has been discovered that it is pos to promote esterification or trans-esterification sible to cross link the chains of the polymer by are desirable ingredients for this purpose. using the alkyl groups that are available in a 5 Following or during the molding operation the homogeneous alkyl acrylate polymer, such as by mass is heated, whereupon trans-esterification or trans-esterification, ester interchange, or saponi ester interchange occurs, and the plastic flow is fication. In general, any di-type compound may eliminated. The chemistry involved is very com be used which contains two groups reactive with plicated but it is speculated that the chains prob the alkyl groups, such as polyhydroxy compounds, 20 ably may be cross linked in the following manner: for example, the dihydric alcohols (HO(CH2):OH) E. E. E. H. H. where ac is an integer as typified by ethylene gly. col; the trihydric alcohols as typified by glycerol; esters of hydroxy acids such as poly-ricinoleate and lactate esters of polybasic alcohols, citrates; 25 simple di-esters Such as glycol di-chloride or di acetate; and hydroxides of bivalent metals such as hydrated lime. In accordance with a preferred embodiment of my invention, which is described merely as 30 illustrative, an alkyl acrylate, such as the methyl, ethyl, propyl, butyl or lauryl acrylates, or a mix ture thereof, and preferably one of the lower alkyl acrylates, is polymerized in accordance with tech niques well understood in the art and which need not be described in detail here. If it is desired, however, a preferred form of the polymerization using an excess of the monomers may be utilized. The alkyl acrylate monomers, when so poly The relative proportions of the polymer and merized, result in simple polymers having long 40 the coupling compound may be varied over wide chains, a section of which may be illustrated by ranges, depending upon the conditions of elastic the following formula: ity, strength, etc., desired in the final product. In general, it has been found that 1 to 10% of the coupling compound by weight may be used with 45 a complementary percentage of the acrylate poly le. Ethylene glycol in the amount of 5% pro duces a particularly satisfactory product. The properties of the final product may be in which X is the alkyl group. It is readily ap controlled to Some extent by the amount of the parent that Such polymers are Saturated. 50 coupling compound. Thus if a large amount of When the monomer is solely methyl acrylate the coupling compound is milled in, and such an a section of the long chain has the following for amount is 'completely cross linked, a harder prod mula: - uct would result approaching the properties of hard rubber. Even though a large amount of the 55 coupling compound is present in admixture with the polymer, it may be possible to produce a softer and more elastic product by terminating the reaction short of complete coupling. In gen eral, however, the amount of the cross linking The polymers of the alkyl acrylates are elastic. 60 compound should not greatly exceed that re This may be attributed to the side groups ex quired for a product of the desired elasticity when tending from the long chain molecules. The completely cured. polymer also has plastic flow and may be milled After the product has been milled, it may be on Conventional rubber milling rolls to sheet form and this condition has plastic flow characteristics molded by any of the conventional molding tech suiting it to be molded in accordance with the niques now applicable to the rubber industry." conventional rubber technique. During the mill During or following the molding the composition ing pigments, modifiers, catalysts or other mate is heated, whereupon the polymer and Cross link rial may be included. ing compound react to accomplish the trans-es In accordance With the invention a cross link 70 terification. The temperature for this reaction ing compound is mixed with the polymer. This may be anywhere from 100 to 200° C. but must may be done at any time or in any way but for be selected with reference to the boiling point of convenience it is preferred to mix it during the the cross linking compound and the boiling point milling operation in a manner similar to that of the compound formed during the reaction and now practiced in the rubber industry. If ethyl 75 whether the latter is to remain or be eliminated, 2,898,850 3 A temperature in the range of 130' to 150 C. has a selecting the compounds for the reaction been found particularly satisfactory. , consideration should be given to the boiling point Referring more particularly to a specific exam of the product formed in the reaction and the ple as illustrative of the method comprising my relation of the curing temperature. If it is de invention, 200 pounds of poly methyl acrylate sired that the product remain, its boiling point polymer, 100 pounds of carbon black and 10 should be above the curing temperature. In fact pounds of Zinc oxide are milled on a conven it ic possible to form products during tile cur tional rubber mill. Into this mixture is milled 10 ing which have a pasticizing or elasticizing potlinds of diethylene glycol and 10 pounds of action. sodamid. The milling is continued and the mill 0 While the invention has been described partic ing roils are cooled so that any heat generated ularly with reference to the use of certain com during the milling is dissipated and does not pounds, it will bf understood that any other di cause a reaction. When the materials are thor type compound may be used, such as those men oughly millied, they are sheeted of the rolls in a tioned heretofore. In selecting the compound conventional manner and may be molded to any consideration should be given of course, to the desired shape. During or following the molding reactivity thereof with the polymer and the tem the product is heated to a temperature of about peratures at which the various operations are 140 C. for from 5 to 40 minutes. The material, preferred to be carried out. after this treatment, is subjected to test with a It will be obvious that many materials and durometer in much the same way that rubber 20 procedures may be possible in accordance with tube stock is tested, and this showed no plastic the invention described herein other than the flow. . . specific illustrative examples and all of the same The amounts of the pigments may be varied that fall within the following claims are intended depending upon the uses to which the product is to beincluded. to be put and the amount of the cross linking 25 We claim: compound may be varied within the limits men 1. The method of forming an alkyl acrylate tioned above. Instead of sodamid, other reac tion promoting materials may be used as men polymer into an elastic substance having proper tioned heretofore. The amount of this reaction ties similar to those of vulcanized natural rub promoting material may be varied as indicated ber, which comprises milling hydrated line into heretofore, 30 an alkyl acrylate polymer while it is in a plastic In a further example, a methyl acrylate poly. condition, molding the milled plastic mixture, mer was milled with castor oil, following which and then heating the molded mixture, the Zinc oxide and sodamid were milled in. This ma-. amount of said hydrated line milled into the terial was tested with a diurometer and showed pdlymer and the extent of heating being such as . a typical uncured action both as to hardness and to eliminate plastic flow characteristics from the plastic flow. This material was cured for about composition and to provide the above mentioned ten minutes at about 140 C. When this was elastic properties. - tested with a durOmeter it showed no plastic flow 2. The method of forming a methyl acrylate and was fully cured. In this example it is likely polymer into an elastic substance having proper that the hydroxy groups of the ricinoleate radi ties similar to those of vulcanized natural rub cal act in the trans-esterifying action. ber, which comprises milling hydrated lime into In a further example hydrated lime is milled a methyl acrylate polymer while it is in a plastic with the alkyl acrylate polymer and the cross condition, molding the milled plastic mixture, linkage is thought to be accomplished during 45 and then heating the molded mixture, the amount curing by saponification and the formation of the of said hydrated lime milled into the polymer and alkyl alcohol. the extent of heating being such as to eliminate In still a further example a di-ester such plastic flow characteristics from the composition as glycol dichloride or glycol diacetate is milled andproperties, to provide the above mentioned elastic with the alkyl acrylate polymer and cured; the 50 alkyl chloride and alkyl acetate are formed re FRANCS CAR3E ATWOOD. spectively. HENRY A. H.I. . .