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General Principles

Virtually all surface modifiers work by chemically bonding an organic species to the filler surface with the nature of the organic molecule being chosen to have good compatibility with the polymer and in many instances to also be able to chemically interact with it, often during the curing process if there is one.

The general structure of a surface modifier can be thought of as an anchoring group, optionally a spacer and a polymer-compatible or reactive part. The role of anchoring groups is to react with the filler surface and bind the additive to it. The most common anchor groups are acids or acid precursors, such as anhydrides (for basic or amphoteric fillers) and alkoxysilanes (for fillers with metal hydroxyls present, especially siliceous fillers). The alkoxysilanes are frequently referred to merely as silanes. Other organometallics (titanates, aluminates, etc.) can also be used. The organic part of the molecule provides the polymer compatibility and reactivity. Spacer groups are not an essential part of the structure, but can be used to remove reactive functionalities from the vicinity of the anchor group and lift them proud of the filler surface.

Two distinct types of modifier can be recognized, depending on the type of organic group: noncoupling and coupling (see Fig. 1). Both have strong anchorage to the filler surface, but only the coupling type has strong interaction with the polymer. This interaction can be chemical or physical (e.g., entanglement). Fatty acids are the most common noncoupling treatment, while the silanes (see later for definition) are most often used as coupling agents (although noncoupling versions are available and are sometimes used).

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