Use of Crosslink Density Measurements to Assess Heat History in Molded Rubber Parts

There is a question asked repeatedly in the analysis of rubber, especially of a part returned from the field: “has the rubber decomposed?” or “is the rubber viable?”.   If the rubber piece is from a known compound and is an ASTM slab then the matter is relatively simple.  Tensile measurements may be taken and compared to original physical properties.  Typically this cannot be done with molded rubber articles; the dimensional requirements of a tensile test can almost never be met for such parts. 

In this paper we will show that in some instances the question (viable/not viable/severity of heat history) may be answered with a high degree of confidence.  This is done by preparing a series of air aged ASTM slabs which  represent a range of samples from slightly aged to severely aged.  The latter target is set objectively, with reference to SAE J2236 which defines the temperature ceiling for a compound as that temperature at which there is at least 50%  for both tensile and %elongation. 

Once prepared, the reference samples are measured for both tensile properties and crosslink density.  With the resulting curve we are able to use crosslink density to predict tensile properties and thus answer the question: is the rubber viable?  A case study will be provided in which such a study answered a critical question concerning a customer returned sample.