Surface Treatments - Dispersants and Coupling Agents

Surface treatments for fillers have been extensively reviewed [124]. Ernstsson and Larsson studied a range of mineral fillers in terms of the Lewis acid-base character [125, 126]. It was found that each one had a different surface chemistry and they were all amphoteric (possessing both Lewis acid and basic sites). Another point was that trace impurities of iron on the filler surface, presumably picked up during processing, significantly changed the surface chemistry of silica. 

In unpublished work, DeArmitt and Breese used a novel rheological method to characterise a wide variety of mineral fillers. The method was especially attractive in that it allowed investigation of the surface chemistry of the fillers, while at the same time showing which types of organic probe molecules were most effective as dispersants. This work confirmed the findings of Ernstsson and Larsson [125,126]. Each mineral displayed a unique affinity for the probe molecules and each was appreciably amphoteric, adsorbing Lewis acid and 

Lewis basic probes. This shows that the dispersant should be chosen to optimise its affinity for the filler or pigment. 

It is important to ensure that the chosen surface treatment agent does in fact bind to the filler. If the additive does not bond to the surface then it cannot fulfil its function. Excess additive is, at best, a waste of money, but in some cases it may have worse consequences such as destabilising the polymer. For example, it has been shown that calcium stearate, a common dispersant, can destabilise polyolefins and cause yellowing, by interacting with the antioxidant [127]. 

As mentioned, dispersant design and mode of action are well understood by surface and colloid scientists [43, 112-115]. In that field, the term dispersant refers to any additive that reduces the interparticle interactions, thereby encouraging dispersion of the particles. This is achievable via a number of mechanisms using low molecular weight, oligomeric, or polymeric additives [128]. Steric stabilisation is most relevant to mineral fillers in polymers because it is the main way to achieve colloidal stability in low polarity solvents. This stabilisation mechanism operates by strong adsorption of a layer of organic additive that physically prevents close interparticle approach. 

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