Using Fillers in Composites

When used in composite laminates, inorganic fillers can account for 40 to 65% by weight. They perform a function similar to silica fume in concrete. In comparison to resins and reinforcements, fillers are the least expensive of the major ingredients. These materials are nevertheless very important in establishing the performance of the composite laminate for the following reasons:

	Fillers reduce the shrinkage of the composites part.
	Fillers influences the fire resistance of  laminates.
	Fillers lower compound cost by diluting more expensive resin and may reduce the amount of reinforcement required.
	Fillers can influence the mechanical strengths of composites.
	Fillers serve to transfer stresses between the primary structural components of the laminate (i.e., resin and reinforcement), thereby improving mechanical and physical performance.
	Uniformity of the laminate can be enhanced by the effective use of fillers. Fillers help maintain fiber-loading uniformity by carrying reinforcing fibers along with the flow as resin is moved on the mold during compression molding.
	Crack resistance and crack prevention properties are improved with filled resin systems. This is particularly true in sharp corners and resin-rich areas where smaller particles in the filler help to reinforce the resin in these regions.
	The combination of small and medium filler particles helps control compound rheology at elevated temperatures and pressures, thereby helping to ensure that compression molded parts are uniform.
	Low-density fillers are used extensively in marine putty and the transportation industry. They offer the lowest cost of filled systems, without the increases of weight that affect the performance of the final product.

Surface Treatments Improve Some Fillers

Some fillers are chemically modified by treating the surface area of the particles with a coupling agent. These coupling agents help to improve the chemical bond between the resin and filler and can reduce resin demand.

Summary of Fillers

Effective use of fillers in composites can improve performance and reduce cost. In today’s market, many of the filler systems being sold are providing several different properties for the composite in one filler system. Flame/smoke, shrink control, weight management and physical properties are often modified by using a designed filler package that has a blend of specialty and commodity fillers. Product and technical information exists and is readily available to those interested in this very important composite ingredient that affects material, process and design and cost.