Effects of Stearic Acid Coating in Composites

As already alluded to, stearic acid is often added to aid filler handling during production. It is especially useful with very fine precipitated materials, such as calcium carbonate, where it reduces shrinkage on drying and thus improves dispersability. It also makes the filler hydrophobic, and reduces water adsorption, but can make some fillers considerably dustier. 

In thermoplastics it is generally held that fatty acid treatments reduce melt viscosity, improve filler dispersion, decrease modulus, reduce tensile strength but improve elongation and impact resistance. While generally true, there are a significant number of exceptions reported. 

Fulmer and co-workers [22] have shown clear evidence of faster incorporation and lower melt viscosity in calcium carbonate filled polypropylene homo-polymer. However, despite the widespread use of fatty acids for improving processability, melt viscosity reductions are not always obtained. Bohlin and co-workers have published an interesting paper on the effect of stearic acid treatment of a dolomite filler in polypropylene [23]. They found 

In the author s experience, correctly chosen fatty acid treatments can improve the colour of some filled polymers. Fulmer and co-workers have also reported this effect [22]. 

In elastomers the effects of stearic acid coating of fillers appear mnch more dramatic in solution polymerised polymers than in emnlsion ones. As shown by Rothon, this is dne to the presence of large amounts of residnal snrfactants in the emulsion polymerised polymer [29]. These probably adsorb on the filler snrface, prodncing effects similar to stearic acid. In the absence of these competing effects, stearic acid reduces viscosity but also reduces the filler polymer interaction, resnlting in decreased bound rubber and 

---