Particulate fillers are powdered substances, with particles usually less than 100 pm in size, which are added to polymers to reduce cost, to improve processing, and/or to modify one or more properties. Nanoparticle fillers are those where at least one dimension is in the 1-10 nm range, but fibers and nanoparticle fillers are outside of the scope of this entry. A particulate-filled polymer composite is an admixture of a polymer matrix with particulate fillers.
Fillers have been incorporated into all polymer types, thermoplastics, elastomers, and thermosets, from the very beginning. Indeed, it is true to say that the development of many of today’s polymers would not have been possible without them. One of the original purposes was simply to reduce cost, especially in the early days when polymers were relatively expensive. Today, as polymers have become more of a commodity and less expensive, addition of fillers to reduce cost is less important and other reasons have come to the fore. Typically, the filler and the polymer have very different properties, and by judicious blending of the two, one can achieve a spectrum of materials with properties intermediate between those of the two ingredients. If one looks at the commercial polymers on the market, there are performance gaps between them, incorporation of fillers can bridge such gaps and extend performance capabilities to address a much broader market. In some cases, such as elastomers, particulate fillers are essential to develop acceptable properties for most applications.
In selecting particulate fillers, one must first understand the performance criteria for the material in use and then select or create a material that matches those criteria at a competitive cost. There is no ideal polymer or filler; so the skill is to blend ingredients to maximize certain properties and mitigate against loss of others. In essence, the winner is the one able to make the best compromises by understanding materials better than their competitor. There are at least 30 properties that can be altered by filler addition so you may be adding a filler to increase modulus but discover that most of the other properties are changed, possibly detrimentally, even though that was not the intent. Burditt (1991) listed 21 reasons that fillers are added to polymers. His list, with some modifications and updating, is:
Reduce cost Increase heat resistance Increase stiffness Reduce creep
Reduce shrinkage during polymerization or molding
Reduce cure exotherm
Change electrical properties
Modify specific gravity (density)
Increase compressive strength Increase lubricity Improve abrasion resistance
Decrease permeability (today we could add also increase permeability)
Improve tensile and flex strengths Improve impact strength Improve dimensional stability Improve thermal conductivity Improve processability Improve moisture resistance
Increase degradability (e.g., starch filler added to produce biodegradable film) Modify adhesion, either to itself (e.g., antiblocking) or to other substrates Change appearance, opacity, and gloss
It is important to remember that not all fillers improve all of the above properties and that some can actually be decreased by certain fillers.