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Except for carbon blacks, the surface of commonly used particulate fillers is suboptimal for maximizing processing and properties of the polymer composites in which they are used. This is often remedied by treating the surface to enhance wetting and reaction with the polymer matrix. Surface modifiers are molecules which are chemically attached to the particle surface to improve compatibility. This differentiates them from weakly attached molecules that are physically adsorbed.


The surface of particulate fillers plays a vital role in determining both the processing behavior and properties of composites. Two factors are involved in the interaction between the filler and polymer: wetting and adhesion. The ease with which their surface is wetted by the polymer governs how easily they can be incorporated and how well they disperse. The strength of the interaction between the surface and the polymer determines the magnitude of many of the final composite properties.

The natural surface of most fillers is not ideal for adhesion to the polymer, and hence, surface modifiers are frequently used to improve performance.

While improved composite processing and end properties are the usual reasons for using modifiers, they can also produce other useful effects such as a reduction in the adsorption of curatives and stabilizers. The presence of surface modifiers can also affect filler handling properties, such as bulk density, powder flow, and dustiness.

In addition to their deliberate addition to improve wetting and adhesion, chemicals that can act as surface modifiers may also be present for other purposes. Typical examples are:

They may have been used as milling aids during grinding processes.

They may have been added to improve filtration and drying of precipitated products. They may be used to protect the filler during storage. For instance, to reduce water pickup.

The presence of these additives is often not disclosed by the producer, with the use of milling aids being a prime example.

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