Mica flakes, expansion

Vermiculite resembles mica in appearance. In industrial process, vermiculite flakes are rapidly heated at flame temperature approaching 1000°C. Some of the water of hydration is removed and the pressure generated by the water vapor expands (or exfoliates) vermiculite particles which increases in volume by 15 to 20 times. This expansion process must be precisely controlled to achieve the required expansion and to retain its water absorption properties. If the time of heating is extended, vermiculite will no longer absorb water. Thus, different grades may be produced by varying the heating time. 

Silica (silicon dioxide) particles, synthetic or natural based, have commonly been used in various polymer systems. Silica has a low coefficient of thermal expansion and high stiffness, translating into increased modulus of the compounded polymer. However, silica filler particles are not flake or plate like as talc or mica particles are, and typically have low aspect ratios. This means that unless the particles are very small, a silica filler addition provides a relatively low surface area for contacting the polymer, and thus it reinforces the resin system less than platy fillers. Researchers have also noted that mica-filled PP, for instance, contains fewer voids than silica-filled PP, which helps explain the higher strengths of mica compounds at 20% filler loadings. This tendency to form voids or cavities increases as silica filler content increases [7-20, 7-21). 

The incorporation of mica generally reduces thermal expansion and helps eliminating the nonuniform thermal shrinkage in injection molding both effects are obviously associated with the natural flake-like form of ground mica. Like talc, mica has also excellent barrier properties, all the more if the particles are properly oriented with respect to the trajectory of the permeant. 

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