In most cases, the actual chemical composition of the filler is not of direct importance, as long as it is inert, insoluble, nontoxic, and sufficiently thermally stable. The presence of deleterious impurities is often of more concern. These vary with the application but are usually those that can affect color (e.g., humates, iron compounds), equipment wear (e.g., quartz), polymer stability (many metal compounds), electrical properties (soluble materials), and toxicity (crystalline silicas, asbestos). In some cases, even a few parts per million (ppm) of these impurities can be detrimental.
While the bulk chemical composition is of little direct importance, surface chemical composition is very relevant for a number of reasons.
First and foremost is its effect on the strength of interaction between the particles and the polymer matrix. This interaction is critical to many composite properties and can vary from strong to weak. While strong interaction is often best, some applications, such as microporous films, require weak interaction. Chemical treatments such as coupling agents and fatty acids are often used to alter the amount of interaction.
Carbon black is the main example of a filler whose own surface is able to interact strongly with most polymer types and where little use is made of surface treatments. Fumed and precipitated silicas are examples of inorganics where the surface is able to interact strongly with some polymers, notably silicone elastomers.
The surface of most mineral fillers leads to weak interaction with nonpolar polymers, especially when polar additives, such as many processing aids and dispersants, are present. These are often more strongly attracted to the filler than the polymer is and collect at and weaken the interface. In these cases, the interaction can often be boosted by the use of coupling agents (chemicals which can react with both the filler surface and the polymer). In some cases, noncoupling treatments are used, such as fatty acids, and these reduce the already weak interaction. Polar polymers such as polyamides and polyesters are able to interact strongly with many filler surfaces, especially carbonates. Even so, coupling agents still help in many cases, especially with siliceous fillers. As a very rough rule of thumb, where strong interaction is required, one usually finds siliceous fillers with silane coupling agents. Where low interaction is required, one usually finds carbonate fillers with fatty acids.
Deleterious impurities are often concentrated at the filler surface and can have a disproportionate effect compared to their bulk concentration on composite properties such as water absorption and polymer stability. This is particularly true for high surface area fillers.
Untreated filler surfaces can also adsorb important additives such as processing aids, stabilizers, and curatives. Such interactions result in removal of the additive from the bulk matrix and deactivation of their desired effects. Additive adsorption can be countered using simple filler surface treatments such as fatty acids. When adsorbed, the latter occupies the sites on the filler surface that would otherwise have attracted and deactivated polar additives in the formulation.