Chemical Stress Relaxation of Polyacrylate Elastomers

Abstract

The continually widening range of thermal stresses imposed on elastomeric parts used in the automotive and aeronautical fields intensifies both the need for more thermally versatile elastomers and the need for effective laboratory methods for evaluating elastomer performance. Because stress relaxation provides a means of gauging product performance under both high temperature and external stress that approximate conditions of use, this technique gives a good estimation of an elastomer's commercial potential and its adaptability to a particular application. To demonstrate the value of stress relaxation, the thermal stability of a polyacrylate elastomer crosslinked with various curatives and the effect of certain additives on thermal stability were studied. Polyacrylates are of major interest for applications that require a high degree of thermal stability because of their ability to perform well over wide temperature ranges (−25° to 400° F). These elastomers are mainly copolymers of ethyl acrylate and a chlorine containing monomer such as vinyl chloroethyl ether. Some newer types are based on ethyl and butyl acrylates and chlorine free monomers such as epoxides and substituted amines. Generally, polyacrylates are compounded and prepared for curing according to standard techniques and they can be crosslinked with a variety of curative systems. Curing is accomplished at between 300° and 400° F in times ranging from 45 seconds to 45 minutes, depending on the elastomer and curing recipe used. The thermal stability of these elastomers is frequently determined by measuring the percentage changes in physical properties, e.g., tensile strength, elongation, modulus, after oven or oil aging or both at elevated temperatures. However, heat aging is carried out with no external stresses applied to the test specimens and the physical measurements before or after aging are made at room temperature. Although this type of test has some value, the results do not necessarily reflect the behavior of the elastomers in practice, as is the case with stress relaxation.

Cast polyurethane elastomers with improved dynamic fatigue resistance

“…The wheel temperature has an evident effect on the wheel performance characteristics, such as the durability, wear resistance, handling, and traction, and on the rolling resistance. An increase in the wheel temperature is the principal cause of urethane thermal degradation; this leads to fatigue cracking and bond failure over time 6. Sometimes, a dramatic heat buildup in the wheels results in the melting of urethane elastomers, commonly to referred as blowout .…”

“…Studies of the heat buildup in urethane elastomers can be dated back to the mid 1960s. Using a flexometer that was capable of compression and lateral shear, Singh et al6 measured the heat buildup in both polyester‐ and polyether‐based elastomers. The authors emphasized that urethane parts subjected to dynamic stresses or strains generally showed great heat buildup.…”