Incorporating Compressive Stress Relaxation Into the Compound Development Process

Compressive stress relaxation (CSR) is gaining momentum as a test of compound suitability particularly in the automotive and off-road equipment sectors. Although sometimes viewed as a glorified compression set test, CSR does not measure a single physical property of a compound, but addresses time-dependent, complex, and sometimes competing effects present in sealing applications. Since the call for this long-term test is not going to go away, the factors that impact the results need to be included in the compound development process to avoid extending the material development time frame with unanticipated failures late in the testing phase. It is common knowledge that different compounds, even those meeting the same specification, do not always have the same ability to seal. CSR provides a method to generate the data to quantify that ability. A study was undertaken to look at existing compounds for which CSR requirements were being added to the specification. Testing began with the hypothesis that compounds with higher durometer and lower compression set would show better sealing force retention, but some of the baseline tests produced unexpected results and further studies confirmed that conventional wisdom did not necessarily apply.