The Reinforcement of Rubber by Resins
Abstract According to the partial conclusions drawn from the results given in each of the three chapters of this review, one can say that the question of reinforcement by resins, if it has not yet attained the aims sought for, has accumulated a great deal of useful information and now seems in a position to make rapid progress. The most significant point that seems to emerge is the necessity for the establishment of strong chemical bonds between the resin particle and the elastomer. This is undoubtedly not sufficient, and other characteristics of the particle must also intervene; but we have said above that one could now imagine a systematic study of the influence of these characteristics, and we need not return to it here. Many questions are still posed and the investigator will have the task of answering them. We consider it important to insist, however, in this conclusion, on some very recent results which it seemed preferable to us to mention here rather than to incorporate them in the text. In fact, they will either bring a confirmation on the influence of chemical bonds or show the possibility of preparing rubbers which should lend themselves remarkably well to this kind of reinforcement. In studying the crosslinking phenomena that involve the well known hardening of dry natural rubber on storage, Sekhar has shown that reactive groups are present as in integral part of the polyisoprene chain when it leaves the tree. These reactive groups have the characteristic property of carbonyl groups or, more specifically, aldehyde groups. They are responsible for crosslinking of the rubber molecules, and reactive monofunctional amines or other carbonyl reagents are capable of inhibiting this crosslinking effectively. From the critical concentration of reagent required to inhibit hardening, one has to postulate the presence of 9 to 29 aldehyde groups per polyisoprene molecule, assuming a molecular weight of 1,000,000). Because of the presence of these aldehyde groups on the chain of the rubber hydrocarbon, one was led logically to suppose that molecules of aminoplast resins formed in the latex might be fixed chemically on the rubber through the aid of these groups. This is what Sekhar and Angove realized with hydrazine-formalde-hyde resins. At a concentration of 5% and less of resin based on rubber, the latex remained fluid with no tendency to gel. Films and foam rubbers prepared from such resin latex showed considerable reinforcement: thus for a 3% resin content, for example, the tensile strength of a latex film passes from 285 kg/cm2 for the control to 362 kg/cm2 (the elongation in both cases being 925%), the 600% modulus from 32 to 75 kg/cm2, the tear resistance from 64 to 100 kg/cm. It is therefore with the greatest interest that one should consider such reinforcement results obtained with such a small proportion of resin; they emphasize the very important part that must be played by a strong chemical bond between the filler particle and the elastomer. It is evident, however, that the small proportion of aldehyde groups present on the rubber molecule limits the possibilities of such a fixation and must not permit to obtain the maximal reinforcing effects. That is why it seems necessary to pay a great attention to the reaction of glyoxal on rubber, which is endowing this latter with aldehyde-α alcohol side groups and gives it the reactivity toward the resins that seems to be desirable. We may therefore think that the years to come will bring into this field new and useful results, with a view toward the improvement of the characteristics of vulcanized rubber and ever-widening development of its application.