Gels, Gel Content, and Gelation

The term gel has different meanings in the polymer extrusion industry. The term gel is used as gel content in crosslinked polymers. Determination of gel content is described in ASTM D2765, standard test methods for determination of gel content and swell ratio of crosslinked polyethylene plastics [163]. The gel content (insoluble fraction) produced in ethylene plastics by crosslinking is determined by extracting with solvents such as decahydronaphthalene or xylenes. 

The term gelation is used in extruded products made of rigid polyvinylchloride (PVC). The term gelation is the fusion of the primary particles of the PVC [164]. In 

Gels are generally defined as small, more or less round defects in extruded products, especially film or thin walled tubing. Some people define gels as any particle in an extruder plastic product that has visual properties different from the rest of the product. This includes discolored specks, contamination, crosslinked polymer 

We will define gels as small spherical droplets or specks with a distinct boundary that can be observed by simple visual inspection. The material making up the gel particle is basically the same as the polymer of the surrounding film. Therefore, a gel particle is different from contamination. In many cases the gel particle has no discoloration see for instance Fig. 11.38. Small droplets with strong discoloration are generally referred to as discolored specks this topic is discussed in Section 11.3.7.4. 

Section 11.3.7.4 also shows expressions that can be used to relate the frequency of discolored specks in the raw material to the frequency of discolored specks in the extruded product. These expressions can also be used to relate the frequency of gels in the raw material to the frequency of gels in the extruded product. 

---