Comparison of methods for determining the gel content in natural rubber and the effect of gel content on rheological behavior

Natural rubber, given its natural origin, is not composed exclusively of polyisoprene chains, but also of so-called non-rubber components such as proteins or phospholipids. Such compounds are responsible for gel formation - insoluble phase if natural rubber (NR) is in a proper solvent. Gel is one of the causes why the behavior of NR is non-standard during processing. The gel content is determined by various methods, while it is not examined in detail which method has a stronger relation to the rheological behavior of rubber. Three different methods (filtration, dissolution in toluene for three different times and Soxhlet extraction) used for determining the gel content of NR are compared in this paper. Analytical (ash content, molar mass measurement, …) and rheological (Mooney viscosity, rubber process analyzer measurements, …) measurements are performed and their results are correlated with the gel content values.