Calcium Carbonate silane treatment

While organo-silane treatments are extensively used in both thermoset and elastomer applications, their use in thermoplastics has so far been somewhat restricted. This is because they do not react with the surface of calcium carbonate, one of the principal fillers used in this type of polymer and because of the lack of a suitable reactive functionality for most of the thermoplastic polymers. Today they are principally used in conjunction with glass fibres, calcined clays, aluminium and magnesium hydroxides, micas and wollastonite. The main thermo-... 

Nevertheless, with regard to filler treatment and limestone impregnation, it is still considered difficult to coat calcium carbonate with silicone. As shown in Table 1, the commercial monomeric silanes that are used for coating the surfaces of fillers nowadays are not effective for coating calcite [19, 20, 21]. The idea that the production of a silicone-coated surface is difficult may result from the misconception that attachment of high silicone polymers to calcium carbonate is equally problematic. The issue of attachment of silicone to the mineral surfaces has still not been finally resolved [62]. 

Both the ground and precipitated calcium carbonates can by treated with stearic acid to control water absorption, improve dispersabUity, and promote better wetting of the flUer by rubber. Silane treatment of these fillers is not effective. However, there is an ultra-fine grade coated with carboxylated polybutadiene, which reactively links to the particle surfaces. Such treated ultra-fine products can give reinforcement of about the same level of the semireinforcing thermal carbon blacks. 

Non-reinforcing fillers such as clay and calcium carbonate may be enhanced with surface treatments such as carboxylated polybutadiene, calcium stearate, or an organo-silane coupling agent. 

Water molecules are eliminated during silane bonding reactions, and oxane linkages (M-O-Si) are formed. Since calcium carbonate is not responsive to chemically reactive silanes, stearic acid treatment has become an important technique, widely used in calcium carbonate filled PVC, including rigid PVC products, flexible PVC and plastisols. 

Organotitanates, aluminates, zirconates and zircoaluminates can also act like silanes as adhesion promoters. They perform similar functions, but unlike silanes there is no need for water molecules to be eliminated. These other treatments bond the polymer to the filler surface by a chemical bond involving proton co-ordination, and they can also be used with fillers that are not receptive to silanes, such as calcium carbonate, carbon black and barium sulfate, as well as barium ferrite, magnesium hydroxide, aluminium trihydroxide, titanium dioxide, talc and the nanoclays. 

The use of such materials was prompted by the need to find coupling agents for calcium carbonate, as silanes were largely ineffective with this filler. Simple unsaturated acids also appeared to have little effect, but Hutchinson and Birchall found unsaturated polymeric acids to be excellent treatments for precipitated calcium carbonate [39]. The concept was developed and extended by Evans and co-workers [40, 41]. 

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