synopsisSynergism in poly(viny1 chloride) stabilization has been studied by measuring rates of hydrogen chloride evolution from samples of polymer in the presence of stabilizers in di-(a-ethylhexyl) phthalate solution in an inert atmosphere. Barium, cadmium, calcium, and zinc laurates, when used alone, allow escape of hydrogen chloride well before stoichiometric uptake is achieved, whereas synergistic mixtures of calcium-zinc and barium-cadmium laurates absorb almost the theoretical quantity of hydrogen chloride. Cadmium and zinc laurates replace labile chlorine atoms in the polymer backbone by ester groups, reducing formation of long polyene sequences: barium and calcium laurates delay the formation of cadmium and zinc chlorides, apparently by reconverting them into their respective laurates. Polyols function by forming complexes with the prodegradant cadmium and zinc chlorides, but contrary to popular belief phosphites possess little activity in this respect. Instead, they slow down the rate of polymer degradation by removal of labile chlorine atoms, by reaction with hydrogen chloride, and by peroxide decomposition.

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.

The present study and that published previously show that crosslinks based on quaternary ammonium halide salts introduced into emulsion SBR-type polymers confer green strength on blends with other compatible rubbers. The crosslink density employed did not exceed 1 link per 3000 combined monomer groups, equivalent to two links per weight-average polymer chain. Judging by the positive slopes of stress-strain curves, green strength obtained in this manner will persist up to at least 50°C. The crosslinks can be broken by mechanical shear and will re-form under resting conditions during times ranging from days at room temperature to minutes at 150°C. Prolonged mechanical shear in laboratory mixing equipment causes progressive loss of modulus and reduction in the slope of the stress-strain curves of compounds containing these labile crosslinks. Similar effects are observed in corresponding control compounds containing no labile crosslinks, but the initial advantage of the crosslinked compounds is progressively reduced and may be lost if shearing is sufficiently prolonged. Available evidence suggests that the principal cause of loss of modulus through shear is selective breakdown of the high molecular weight fraction of the base polymer. The breakdown of the longer chains, which are those most likely initially to carry three or more crosslinks, eliminates most of the network structure responsible for enhanced modulus. Whether chemical effects contribute to loss of modulus remains to be determined, but the evidence suggests that any such effects will be of lesser importance. Because the shear encountered in small laboratory mixing devices differs from that in full-scale factory equipment and stress-strain curves may not reliably predict green-stock behavior, the significance of the present findings can only be determined by factory trials. It is evident, however, that for the most efficient utilization of high green-strength SBR, exposure of the crosslinked SBR component to mechanical shear during mixing should be held to a minimum. On the other hand, to achieve smooth sheets or profiles at the forming stage, the final compound should be subjected briefly to a high degree of shear at as low a temperature as practicable, to break the labile crosslinks in the stock. Green strength will be recovered during subsequent rest periods. The present study suggests that these conditions should be readily achieved in a calendering operation, such as normally used for preparing carcass plies. However, to a greater extent than in most polymer developments, the final proving ground for applications for this new modification of SBR must be full-scale factory trials.

The use of simple mechanical tests, conducted in an environment of rapidly changing temperature, proves to be a valuable approach to the preliminary study of rubber compositions. The differential strain test in particular is a most useful and convenient scanning procedure, allowing the estimation of basic quantities and the assessment of the general physical character of experimental vulcanizates. The test serves to identify types and regions of unusual behavior, which can then be pursued if desired by methods appropriate to the case. Where the careful study of particular questions requires the application of more refined and specialized techniques, these simple mechanical procedures, requiring little time and material, can contribute greatly to intelligent experimental design.