Fillers for Resin Systems

Other than the resin and catalyst, fillers are probably the most common ingredient in resin systems for potting and casting applications. They are used in formulations for a variety of reasons, the most important of which is cost reduction. Frequently, they account for the largest percentages, both weight and volume, of the casting and potting system. The normal filler loading level is 20-30%, but levels as high as 70% have been used in certain applications.

In selecting a proper filler, a number of parameters and properties must be taken into consideration. The absorbent qualities and particle size must be considered since they both affect the properties of the cured resin. Fillers of large particle size, if not dispersed/suspended properly, may tend to settle, resulting in an uneven distribution of the filler during cure. Fillers having high absorption, on the other hand, can lead to very high viscosity and cause poor flow properties. In general, the effects provided by fillers are more dependent on the amount incorporated rather than the type. The more common fillers in use today are silica, talc, alumina and clays.

Specific fillers may be used to provide specific performance characteristics such as:

• Lower peak exotherm
• Lower shrinkage during cure
• Reduce Coefficient of thermal expansion, thus increasing thermal stability and enhancing Thermal shock properties
• Improved machinability: CaCO₃, Ca-silicate, powered aluminum
• Improved abrasion resistance: alumina, silica
• Improved impact strength: chopped (fibre) glass
• Improved electrical properties: mica, silica, powered or flaked glass
• Improved thermal conductivity: metallic fillers, alumina, beryllium oxide
• Improved thixotropic properties: silica