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Polymeric Acids and Anhydrides
Using suitable polymeric backbones to carry the carboxylic acid functionality is a more promising approach for obtaining coupling in many cases. Two main classes of product are commercially available: one with a saturated hydrocarbon backbone and the other with an unsaturated one.
Acid Functional Saturated Polymers
The main modifiers of this type are based on polyolefin homo- and copolymers. The acid functionality is usually introduced by the use of acrylic acid or maleic anhydride, and this can be done by grafting or by copolymerization. For various reasons, the level of acid functionality is relatively low compared to fatty acids.
These additives are for use in filled polyolefins, and coupling to the polymer matrix is thought to be due to processes such as chain entanglement and cocrystallization (Godlewski and Heggs 1989). They are usually used as additives to the formulation, rather than being precoated.
Acid Functional Unsaturated Polymers
Unsaturated polymers offer the possibility of chemical bonding in addition to the physical methods described for the saturated versions. Polybutadienes (PBDs) have been found to be particularly suitable for this and can be made into excellent coupling agents for basic and amphoteric fillers. Just as for their nonpolymer equivalents, they are best in systems where some curing or cross-linking is taking place, but unlike the simple unsaturated acids, they are effective in both peroxide- and sulfur-cured materials.
The structure of a typical unsaturated polymeric acid anhydride surface modifier based on maleinized polybutadiene (MPBD) is shown in Fig. 6. The anhydride groups are believed to lead to the reaction with the filler surface, probably by salt formation, while the residual unsaturation is available for participation in various curing or cross-linking processes.
Much higher acid levels are possible compared to the functionalized saturated polymers and these give the best results. Moreover, high levels allow water- dispersible or even water-soluble forms to be made and used for precoating.
The effectiveness of an MPBD-type additive when precoated onto precipitated calcium carbonate is demonstrated in Table 4. This compares uncoated, fatty acid- coated, and MPBD-coated fillers in a cross-linked EPDM elastomer and clearly shows the benefits arising from the use of the MPBD.
It has been found that performance of the MPBD increases with both anhydride content and molecular weight but begins to plateau at about 10,000 molecular weight and 20-25% by weight of anhydride (Rothon 1990).
Fig. 6 Structural elements of maleinized polybutadiene
Table 4 A comparison of fatty acid and maleinized polybutadiene coating of precipitated calcium carbonate on performance in a sulfur-cured EPDM elastomer (100 phr filler, both coatings at 2.6% w/w on filler)
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