High-Green-Strength Silicone Rubber

A high-green-strength silicone rubber has been developed which can be processed on equipment designed to handle organic rubber in the uncured state. This paper describes the processing of silicone rubber for curved radiator hose where green strength is needed. A correlation between green strength, room temperature Mooney viscosity, and plasticity has been established and found to be strict enough to allow green strength to be estimated from room temperature Mooney viscosity and plasticity data. Good shelf life along with excellent processing properties on a 2-roll mill are characteristic of this silicone rubber stock. Data will be presented covering handling properties, immersion tests, typical physical and thermal properties. Currently, this silicone rubber is being used in hose applications and could be considered for others where green strength is a requirement.

Studies of Heat Generation in Polyester Cord Tires

Measurements of loss factor in polyester cord and cured rubber stock were made. From the results, the equilibrium cord temperatures in a rotating tire were estimated. These calculated values agree with measured values reported in the literature. It is shown that, for truck tires, cord temperatures can easily rise to levels where cord degradation in the presence of sulfenamide accelerators is known to occur. Measurements on cords which had been degraded by aminolysis indicate that loss factors increase as a result of the degradation and cause significant increases in local steady-state temperatures. This effect will tend to accelerate the rate of degradation at those locations. The increases in loss factor agree with observations on loss factors at increasing levels of crystallinity in polyester fibers in the literature.

Effects of Rubber-Curing Chemicals on Tensile Strength of Polyester Yarns

The degradative effects of accelerators of the sulfenamide and thiuram types on polyester tire yarns depends on the stability of the accelerator and the reactivity of the amines generated. Thermal decomposition of the accelerator alone is not sufficient to explain the degradative effects reported by other workers. The complexes formed by the accelerators in the presence of activators, such as zinc oxide and stearic acid, have lower thermal stability than the accelerators alone and release amines at lower temperatures. The formation of the complexes can be shown by differential scanning calorimetry. The findings of this investigation may have important implications with regard to the release of amines during the life of a tire and to the efficient compounding of rubber stock. If, as it appears, the decomposition of TMTM and TBBS occurs more efficiently if they are previously allowed to form a complex, there may be advantages in forming such complexes before addition of the ingredients to the rubber stock. If the ingredients are added separately, the formation of the complex would be controlled by diffusion of the ingredients in the rubber, and incomplete complex formation may result, leaving residues of unreacted components. These would, over a period of time, release more amine, causing degradation of the polyester cord. Advantages of prior complex formation, beside reduction of the above effect, might be lower concentration requirements for the accelerator and possibly better long-term physical properties of the elastomer. As pointed out in the introduction, effects of moisture, which were discussed by Iyengar, were not taken into consideration in this investigation. The conclusions drawn here are therefore strictly applicable only to the thermal decomposition of the accelerators and accelerator-activator complexes. This mode of decomposition is, however, felt to be of major significance for the degradation of polyester tire cords in truck tires during actual use.

The Tensile Strength of Glass Yarn in Rubber and the Effects of Moisture. I

The strength of glass tire yarn depends on its environment so that testing in rubber is necessary to determine the strength of the yarn as used. The techniques described for building and testing glass yarn-rubber composites lead to tensile values of good precision. The techniques allow specification of a standard tensile in rubber for a glass yarn. Tensiles which would exist under non-standard conditions experienced in manufacturing or use can also be determined. Tire operating temperature is one use condition covered. The techniques discriminate sufficiently to show the effects of small differences in moisture level on hot tensile strength. The rubber stock used here did not harm the tensile strength of the glass yarn. The observed tensile strength increase of the glass yarn encased in rubber as compared to air can be attributed to the stress transfer ability of the rubber and to the test piece configuration. The strength of a glass yarn-rubber composite is inversely related to the level of moisture in the composite. High moisture levels in the glass yarn and rubber stock at building and cure can hurt tensile since moisture loss from the composite can be slow. Also, high moisture level at cure may degrade the resin-adhesive system of some yarns. For this particular glass yarn under high humidity, storage at advanced temperatures and/or curing into rubber permanently damages the yarn. These observations were made under absolute humidity levels not usually encountered. The question of the extent of the humidity degradation of this type of glass yarn under practical humidities will be examined. Since the tensile loss is probably due to some degradative action on the sizing or resin, it cannot be assumed that all glass yarns will behave similarly. It will be necessary to determine the susceptibility of each glass yarn to moisture.

A Study on Heat History of Organic Accelerators in Rubber Stock (BR Base)

Rubber with some exceptions must generally undergo such processes as mastication, mixing, warming-up, extrusion, spreading, calendering, etc. prior to vulcanization under heat to obtain cured articles. Consequently the rubber matrix receives a heat history caused by mechanical frictional heat or the heat which cannot be avoided during these processes. On the other hand, when an uncured rubber compound, ready for vulcanization, containing such curing agents as sulfur, such activators as zinc oxide, and organic accelerators is heated during the processes or during storage between individual processes, each incremental effect of heat is accumulated with time. It is a well-known fact that this accumulation of heat can lead to the trouble of scorching, etc. As a cause for the trouble, organic accelerators seem to play the most important role. A few reports have been published on the action of accelerators under heat, but, to my knowledge, no report is available on the behavior of accelerators in rubber stocks, namely, on the change of the properties of uncured rubber compounds and on its influence on the properties of vulcanizates. This paper shall report these problems, though it describes only the results of the tests carried out under specific conditions.

The Chemistry of Thiazoles and Dithiocarbamates as Antioxidants

Using a strictly qualitative approach, the present work suggests a possible explanation for the observed antioxidant action of dithiocarbamates. It has been shown that dithiocarbamates react with hydroperoxides to form a new oxidized intermediate which in most cases is stable. These have been identified as semisulfinates: (see PDF for diagram) With continued oxidation, a normal sulfinate is formed, the structure of which is probably best represented as follows where M is a divalent metal and R is, e.g., a simple alkyl group: (see PDF for diagram) The stability of the above compound will depend upon the metal M and the nature or size of the alkyl group. These compounds further decompose into a thiuram monosulfide and the corresponding metal sulfate. Particularly active or sensitive dithiocarbamates are oxidized directly to the metal sulfate and thiuram monosulfide. In these cases, the ephemeral formation of the semi-sulfinate and normal sulfinate as intermediates is probable. In the presence of excess hydroperoxide, the thiuram monosulfide undergoes further oxidation to form a carbamyl thiocarbamyl disulfide: (see PDF for diagram) Compounds of this structure are moisture sensitive and readily undergo hydrolysis. In the presence of zinc oxide and water such as would be found on the surface of a conventional rubber stock, it is suggested that a further reaction occurs to form a zinc dithiocarbamate which will then undergo the sequence of reactions described above until the dithiocarbamate is completely consumed. It is suggested that these reactions account for the antioxidant behavior of dithiocarbamates in a hydrocarbon system such as rubbers and petroleum products where the oxidizing agent is a hydroperoxide or some peroxygen compound which, if not destroyed, would in turn degrade the product to one of no utility.

Influence of the Temperature upon the Durability of Rubber in Corrosion Cracking

1. The rate of reaction of an undeformed rubber stock with a reactive medium is determined by the diffusion, while the rate of destruction of a deformed stock is determined by its chemical interaction with the medium. 2. The magnitude of the apparent activation energy (u) of the rupture of a stock in the presence of a reactive medium does not undergo perceptible alteration in the range of deformations 30 to 80%, while on transition to deformations of 500 to 700% the values of the apparent activation energy increase. 3. The temperature coefficient of rupture depends upon the nature of the bonds being destroyed and upon the capacity of the reactive medium for adsorption on the stock. On rupture in the gaseous phase the apparent activation energy has a lower value than on rupture in a solution of the same agent. 4. In a wide range of deformations the time to rupture of stocks obeys a complex system of laws, passing through a minimum value in the region of the critical deformation, εcrit. The position of εcrit depends upon the temperature, type of ‘aggressor’ (i.e. reactive agent) and physical state of the medium (gas, solution). 5. As a result of the shift of εcrit with alteration in temperature there is possible the phenomenon of anomalous dependences for certain deformations, i.e., the time to rupture at low temperatures are less than for higher temperatures with identical deformations and concentrations of the medium.