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An Investigation of the Effect of Ageing on the Physical Properties of Natural Rubber

The effects of ageing on the strength properties of natural rubber (NR) were examined in this thesis. The work involved the use of experimental techniques to investigate possible mechanisms for the change in the strength properties of rubbers after ageing. The mechanisms were an increase in the crosslink density, scission of the molecular chains, changes to the crosslink chemistry and an inhibition of the strain-crystallisation process.

A series of experimental tests were carried out on NR compounds with different curing systems. The tests were conducted at room temperature after ageing at 100C in air (aerobic) or in a vacuum (anaerobic).

Through thickness measurements provided evidence that ageing was not uniform throughout the thickness of the sample. Due to the diffusion limited oxidative reactions, the outer layer degrades more rapidly than the bulk. This leads to essentially anaerobic ageing in the interior of the materials. From an oxygen uptake test, the reaction rate constant can be calculated. This makes it possible to estimate the rate and thickness of oxidised layer formation during ageing.

Cyclic crack growth and trouser tear tests were carried out to measure changes in the strength properties utilising a fracture mechanics approach. The results showed that after aerobic or anaerobic ageing the strength properties were significantly reduced, especially for conventionally cured NR. The effects were more pronounced than for aerobic ageing.

Results from stress-strain measurements showed that different curing systems exhibited different effects after ageing aerobically at 100C. Peroxide cured rubbers did not show much change in their mechanical behaviour when compared to sulphur cured rubbers, which softened significantly.

A two-network theory is used to establish the magnitude of main chain and/ or crosslink scission and crosslink formation during ageing. The extent of permanent set is dependent upon the extent of these processes. The higher permanent set value suggests more new crosslinks have been reformed. This effect was more pronounced for conventionally cured NR compared to efficiently cured NR.

Calorimetry showed that ageing inhibited strain induced crystallisation. Low temperature crystallisation was also inhibited. After ageing for 3 days at 100C, a significant slowing of the rate of crystallisation was observed in stress relaxation test conducted at low temperature (-26C).