Shrinkage reduction: fillers

Reinforcement reduces shrinkage far more than fillers do. Fillers help balance shrinkage, however, because they replace shrinking polymer. The sharp shrinkage reduction in reinforced crystalline resins can often lead to warpage. The best mold-to-size composites are reinforced amorphous composites. 

One of the principal reasons for the use of mineral fillers in these polymers is to reduce this shrinkage problem. Platy fillers have generally been found to be the most effective in this respect, giving the highest levels of shrinkage reduction consistent with maintaining adequate properties in other areas. 

Fillers offer a variety of benefits increased strength and stiffness, reduced cost, shrinkage reduction, exothermic heat reduction, thermal expansion coefficient reduction, improved heat resistance, slightly improved heat conductivity, improved surface appearance, reduced porosity, improved wet strength, reduced crazing, improved fabrication mobility, increased viscosity, improved abrasion resistance, and/or impact strength. Fillers also can have disadvantages. They may limit the method of fabrication, inhibit cure of certain resins, and shorten pot life of the resin. 

Fillers and reinforcements are widely used in compression-molded systems. These materials enhance the molded object in a variety of ways, such as shrinkage reduction, lessened crazing, improved color, and finish. Also, very importantly, the costs of materials is reduced. > dely used fillers include clays and minerals of various types. 

ASTM D883 defines a filler as "...a relatively inert material added to a plastic to modify its strength, permanence, working properties, or other quaHties or to lower costs." EiHers (qv) that modify the properties and characteristics of epoxies are employed in epoxy resins for a variety of reasons. Then principal functions are to control viscosity, reduce shrinkage and the coefficient of thermal expansion, effect a cost reduction, and color the epoxy resins. 

There are also hundreds of additives used as process aids to improve such things as air release, cure rate, thickening, viscosity reduction, mold release, wetting and dispersion of fillers, thixotropy, shrinkage and static reduction. 

Very often particles are blended into polymers, in thermoplasts as well as in thermosets and in synthetic rubbers. This is done for various reasons the aim may be stiffness, strength, hardness, softening temperature, a reduction of shrinkage in processing, reduction of thermal expansion or electric resistance, or, simply, to reduce the price of the material. The fillers used are wood flour, carbon black, glass powder, chalk, quartz powder, mica, molybdene sulphide, various metal oxides, etc. etc. 

The improvements in adhesive strength of cured epoxy joints that are attributable to fillers are not as much related to the improved cohesive characteristics of the adhesive as to the reduction in internal stress due to modification of the coefficient of thermal expansion, shrinkage, etc. 

Sometimes filled adhesives will show better resistance to moisture resistance than unfilled adhesives simply because incorporating inert fillers into the adhesive lowers the organic volume that can be affected by moisture. Aluminum powder seems to be particularly effective, especially on aluminum substrates. The filler can provide a reduction of shrinkage on cure, a reduction of the thermal expansion coefficient, and a reduction of the permeability to water and other penetrants. However, fillers do not always produce more durable bonds. 

Shrinkage is the reduction in volume during cure. Epoxies have a low curing shrinkage. Shrinkage may be reduced through the use of fillers. 

There are several reports on the influence of fillers on shrinkage. " Figure 8.56 shows mold shrinkage vs. concentration of filler. Mold shrinkage can be reduced to half of the value for unfilled resin by the incorporation of mica. Additional reduction of shrinkage is possible if the interaction between filler and the matrix can be increased. This can be achieved by reacting polypropylene with maleic anhydride. ... 

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