Drift and Relaxation of Rubber

Abstract

Drift tests lasting 8 years have been carried out on rubber blocks in compression at 35°C. Rate of drift, initially high, attains a low constant value in 200 days or less. The initial, rapid drift is termed transient drift; and the slower, constant drift is termed steady drift. Drift varies considerably with the type of accelerator in the compound, Ureka giving the lowest drift of the accelerators tested. A new method has been developed for measuring stress relaxation in tension. The stress is measured to 0.1 percent by the resonance frequency of lateral vibrations of the stretched sample. The vibrations are impressed on the sample by a mechanical oscillator in which the source of vibrational energy is a steel wire which is under adjustable tension and is kept in circular vibration by a pair of air jets. Relaxation measurements extending over many months are reported on a soft vulcanized rubber at elongations from 10 to 400 percent at 35° and 70°C. The experimental data can be fitted by a two-term stress equation of the Tobolsky-Eyring form representing two slip mechanisms, or transient and steady relaxation. Steady relaxation follows an exponential decay law; and this means that the residual stress goes to zero at infinite time. Other stress relaxation data are reported for rubber tested at 150 percent elongation, at temperatures from 35° to 113°C, in air and in vacuum. Air, as compared with vacuum, produces little effect at 35°; but at 70° and higher, the rate of steady relaxation is greatly increased. Thus oxidation appears to be the major factor in steady relaxation at elevated temperatures. Total transient relaxation is unaccountably increased by vacuum and by heat. A modification of the Tobolsky-Eyring equation is developed for steady relaxation. For transient relaxation a new theory is developed which leads to the Tobolsky-Eyring equation, but involves different interpretation of the parameters. In this theory, transient relaxation is attributed to rupture of secondary chemical bonds or crystal forces, followed by longitudinal slippage of chain molecules, with partial or local equalization of tension along the chains. The crystallites then reform, and the rupture and slippage process is repeated. The energy dissipation is attributed primarily to release of local elastic stresses following bond rupture. Transient relaxation is complete when the tension is completely equalized over the total length of each chain between cross links. Neither of the new equations can be distinguished from the Tobolsky-Eyring equation on the basis of present data.