This study considers high temperature (160° C) water absorption in the absence of oxygen. Examination of the freezing behavior of water-swollen butyl gum vulcanizates shows the water to be disposed as discrete droplets of the order of 3µ diameter, corresponding in this case with soluble residues based on zinc. Data are shown for peroxide gum vulcanizates of natural rubber, SBR, NBR, ethylene-propylene rubber, and cis-1,4 poly butadiene. Equilibrium absorption occurs, the degree being dependent on polymer impurities and state of cure. Model systems based on cis-polybutadiene show the effects of typical emulsion polymerization residues and the nature of the equilibrium balance between osmotic pressure and the retractive pressure exerted by the rubber. Classes of absorption behavior, whether low equilibrium, moderate to high equilibrium, or nonequilibrium, are discussed in terms of osmotic and rubber pressures. The temperature dependence of absorption rate is shown for the various polymers for the range 25° to 92° C. Five decades are required to accomodate the data, ranging from low rates for butyl rubber to high rates for NBR and natural rubber. Curing systems are compared, with cis-polybutadiene and butyl rubber as base polymers. Common classes of fillers and reinforcing agents are compared, particular consideration being given to the hydrophylic calcium silicate and silica fillers, which appear to facilitate leaching of electrolytes when present in sufficient loading. From the combined experience, illustrative water-resistant formulations are derived.

One hesitates to close this review by adding a paragraph of conclusions. The term “oil-extension principle” introduced a new concept to the manufacturer of synthetic rubbers. The principle (if it may be identified by such a word) and the application of the products it generated do not involve any novel or unfamiliar approaches to the art of rubber compounding. Indeed, it would be rendering a disservice to the progress of their adoption by industry to suggest that it required a revision in the viewpoint of the rubber compounder. The use of softeners in compounding was almost coincidental with the discovery of rubberlike substances by the explorers of the 16th century. The development of carbon blacks having more useful characteristics in rubber came after the compounder had become familiar with the application of innumerable inorganic fillers in rubber and, indeed, soots and lampblacks. The tailor-made synthetic rubbers were developed by the chemical industry in the second-quarter of the 20th century and it is scarcely likely that higher molecular weight types would not be produced nor that any inherent processing problems would not be solved when there existed an economic and raw material incentive to do so. These are the contributions of the polymerization chemists and the synthetic rubber industry. Where it is more economically-attractive and technically-desirable to add softener and filler to the synthetic rubber in the manufacturing process, oil-extended rubbers and filler masterbatches will be provided as raw materials for the rubber industry. If on the other hand, these ingredients may be added quite readily during factory mixing operations, without any detrimental effects on the polymer quality, the synthetic rubber producer would be quite unwise to attempt to usurp the functions of the rubber manufacturer. The income of the rubber manufacturer, depends upon the skill and economy that he applies to the operation of mixing rubber with liquid and solid materials and this is the prime occupation of a rubber compounder. It would avail the synthetic rubber producer nothing to try and convince the compounder that he is doing something novel and unusual by preparing softener-filler masterbatches. However, the compounder will be receptive to a pre-blended product that enables him to produce rubber mixes of better or different quality at an equal, or lower, cost. This is the aim and accomplishment of oil-extended rubbers.

No abstract

A method has been developed whereby the separate indices may be obtained for the flex life and heat build-up properties of elastomers. These indices are independent of the state of cure, but depend on the test recipe and the dispersion of the compounding ingredients. The method may be used to examine the effect of polymerization variables, compounding ingredients and procedures, or ambient temperature, on these two properties.

A simple procedure has been described to provide an estimate of the wear characteristics of a vulcanized polymer by means of a towing device. The present paper describes modifications that have been adopted to increase the efficiency of this procedure and reports the effect of various test conditions that have been explored by statistical methods. Estimates of the wear resistance of various types of rubber in suitable tread recipes are given together with an observed order of rating based upon their response to the severity of the imposed conditions.