The Theory of Permanent Set at Elevated Temperatures in Natural and Synthetic Rubber Vulcanizates

Abstract

Certain materials, including natural and synthetic rubbers, after being deformed for finite periods of time and then released, do not return completely to their original dimensions. This phenomenon is called “permanent set”. There are many different types of measurements, made under a great variety of conditions, all of which are described as measurements of permanent set. The molecular causes of permanent set are not the same under all of these conditions. For this reason we shall restrict ourselves in the present article to a discussion of the permanent set taking place in thin rubber samples which are maintained at constant extension at elevated temperatures. We shall show that the permanent set which occurs under these conditions is due to chemical changes which affect the structure of the rubber network, and that a quantitative molecular description of permanent set can be accomplished by the use of concepts already developed concerning the nature of these deteriorative chemical processes.

Chemical origins of permanent set in a peroxide cured filled silicone elastomer – tensile and 1H NMR analysis

“…In addition, one can predict the change in crosslink density while irradiated under stress from the mathematical framework developed by Tobolsky. In this case, using the assumptions that a) the network can be described as near Gaussian and b) no net change occurs in the number of elastically ineffective chains, the permanent set is a function of the ratio of the crosslink density at the start of the experiment to the crosslink density at the end of the experiment [4,5]:…”

“…For example, elastomeric materials subject to physical stress while undergoing crosslinking reactions are known to be subject to permanent set [4,5]. This degradation can limit component lifetime or simply have important consequences on the assessment of physical properties for lifetime assessments of field aged materials.…”

Report on ASC project degradation of organic materials.

“…The independent network hypothesis (originally proposed by Tobolsky and coworkers 6 ) states that the overall stress in the sample can be considered as the sum of the stresses in each network individually. Since networks in their state of ease should have zero stress contribution, simply adding more crosslinks at a given deformation should have no effect on the stress.…”

“…Crosslinking history can be inferred using normalized stress relaxation results by making a few assumptions and that procedure has been employed here on two experimental studies. The classic work on permanent set and the independent network hypothesis was done by Tobolsky and coworkers 6 . Consequently, their results on butyl rubber are used as one of the experimental data sets to validate the new model.…”

Entanglement networks of 1,2‐polybutadiene crosslinked in states of equibiaxial extension

“…The dimensions in the state of ease are related to u, and UN by U , / U N = A02(1 -As6)/(A,6 -A 0 9 (2) where AO is the extension ratio at which crosslinks are formed and A, is the extension ratio in the state of ease. Young's modulus relative to the state of ease, E, is given by…”