Additives, fillers, and reinforcements

Why they're important

Additives, fillers, and reinforcements are used to change and improve the physical and mechanical properties of plastics. In general, reinforcing fibers increase the mechanical properties of polymer composites while particular fillers of various types increase the modulus.
TABLE 1. Effects of additives, fillers, and reinforcements on polymer properties
Additive / Filler /
Common materials  Effects on polymer properties 
Reinforcing fibers  Baron, carbon, fibrous minerals, glass, Kevlar   Increases tensile strength.
 Increases flexural modulus.
 Increases heat-deflection temperature (HDT).
 Resists shrinkage and warpage.
Conductive fillers  Aluminum powders, carbon fiber, graphite   Improves electrical and thermal conductivity.
Coupling agents  Silanes, titanates   Improves interface bonding between polymer matrix and the fibers.
Flame retardants  Chlorine, bromine, phosphorous, metallic salts   Reduces the occurrence and spread of combustion.
Extender fillers  Calcium carbonate, silica, clay   Reduces material cost.
Plasticizers  Monomeric liquids, low-molecular-weight materials   Improves melt flow properties.
 Enhances flexibility.
Colorants (pigments and dyes)  Metal oxides, chromates, carbon blacks    Provides colorfastness.
 Protects from thermal and UV degradation (with carbon blacks).
Blowing agents  Gas, azo compounds, hydrazine derivatives   Generates a cellular form to obtain a low-density material.

Modifying polymer properties
Electrical properties can be affected by many fillers. For example, by adding conductive fillers, an electromagnetic shielding property can be built into plastics, which are normally poor electrical conductors. Anti-static agents can be used to attract moisture, reducing the build-up of static charge.

Coupling agents are added to improve the bonding of the plastic matrix and the reinforcing fibers. Different fillers are used to lower the cost of materials. Other additives include flame retardants to reduce the likelihood of combustion, lubricants to reduce the viscosity of the molten plastic, plasticizers to increase the flexibility of the materials, and colorants to provide colorfastness.

Low-aspect fillers
Fillers modify the properties and molding of the compound to which they are added. If the fillers are characterized with a low aspect ratio between the longest and the shortest dimensions, the basic properties will be less changed from those of the unfilled polymer. Fillers benefit plastics parts in the following ways:

High-aspect fillers: fibers
When the aspect ratio between the longest and the shortest dimension of the filler is large, for example, greater than 25, the filler can be characterized as a fiber. Fiber reinforcements will significantly affect the properties of the compounds to which they are added.

Fibers impact strength  
Assuming good bonding between the fiber and the polymer matrix, the strength in the fiber direction will be significantly increased. If many fibers are oriented in the same direction, large differences will be noted between the modulus in the orientation direction and in the direction perpendicular to the orientation. The latter will be very close to that for the unfilled polymer.

Fibers affect shrinkage  
The fibers will also have a significant effect on the shrinkage properties of the compound: shrinkage in the orientation direction will be much less than the shrinkage in the cross direction.

Importance of predicting fiber orientation  
Because the fiber orientation varies with the flow direction, in the thickness direction, and at weld line locations, it is important to be able to predict these orientations, in order to predict the properties of the molded article.