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Effects of Fatty Acid Coating in Composites
The saturated fatty acid chains are too short to entangle with the polymer matrix and have no means of chemically reacting, so they are noncoupling additives and result in low bond strengths between the filler and matrix and in filler debonding at relatively low stress levels. This leads to stress whitening and a decrease in ultimate tensile strength. On the other hand, there can also be useful improvements in other properties such as elongation and toughness.
Typical effects obtained as a function of coating level for a magnesium hydroxide-filled EVA compound are illustrated in Fig. 5.
The improvement in impact resistance obtained by using a fatty acid coating on calcium carbonate in a polypropylene homopolymer compound is illustrated in Table 3.
Fig. 5 Effect of fatty acid coating level on the properties of a magnesium hydroxide-filled EVA compound (adapted from Rothon et al. 2002a)
Table 3 Effect of fatty acid treatment level of calcium carbonate on the impact resistance of filled polypropylene homopolymer (50% w/w filler) (based on Hancock et al. (1980))
Fatty acid coatings are predominately used in thermoplastic composites, where their combination of effects is of the most used. The general belief for these polymers is 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.
In elastomers, the main fatty acid effects observed are reduced viscosity but also reduced filler polymer interaction (indicated by less bound rubber) and reduced reinforcement.
The effects of fatty acid surface modifiers also vary markedly according to the elastomer type. The effects are much more noticeable in solution-polymerized elastomers than in emulsion-polymerized ones. As shown by Rothon, this is due to the presence of large amounts of residual surfactants in the emulsion-polymerized case (1984). These can duplicate the effects of fatty acids.
Fatty acid treatments do not have much use in thermoset applications. Many of these (e.g., unsaturated polyesters) contain abundant acidic groups in any case.
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