Platy fillers, oriented

In addition, the presence of impermeable filler in a polymer forces the diffusant molecule to travel further around the filler particles. This physical blocking effect is known as tortuosity, because the filler forces the diffusant to take a more indirect, or tortuous, path through the material. The degree of tortuosity imposed is dependent upon the anisotropy and orientation of the filler particles with respect to the direction of diffusion. For example, platy particles oriented perpendicularly to the diffusion vector will be particularly effective in retarding diffusion. The permeability of a composite can be calculated using an equation that allows for the reduction in permeant solubility and for the tortuosity (Equation 8.3). Where P and Pp are the permeability of the composite and the unfilled polymer, respectively. The terms w and t refer to the width and thickness of the filler and (pp and (pf represent the volume fraction of polymer and filler. 

R. J. Eldred. Effect of oriented platy filler on the fracture mechanism of elastomers. Rubb. Chem. TechnoL, 61,620-629,1988. 

Fibrous reinforcements, such as glass and asbestos, and acicular reinforcements, such as wollastonite, also tend to orient parallel to the flow direction. Since they all have lower thermal expansions than plastics, use of these minerals tend to promote warping, especially in crystalline polymers. Fillers such as mica, talc and kaolin, also orient in the flow direction, but their platy, more two-dimensional shape does not contribute to warping. High aspect ratio mica, because of its extreme platiness, is used to counteract the natural warping tendency of highly crystalline thermoplastic polyesters. 

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