Calcium carbonate (CaCO3 or CC) is the most used filler in polyolefins. CC’s importance comes from its role as a bulk extender used for displacing the relative amount of resin needed in an application. Producers of commodity HDPE (high-density polyethylene) containers and packaging, for example, rely on these fillers for minimizing raw resin costs. However, fine particles of CaCO3 loaded at 10-60% and using the proper surface treatment or coupling agent to bond them to the polymer also tend to increase stiffness, hardness, and dimensional stability (while normally lowering tensile and impact strength properties, depending on the grade of CaCO3 and its coating).
Commercial calcium carbonate products typically contain at least 95-98% CC, with the remainder being various metal oxides. CC for plastics may be ground from limestone (such as dolomite) or another natural raw source, or chemically precipitated into fine, uniform particles with predictable properties. Recently, a kind of naturally precipitated oolitic aragonite mineral form of CC is being harvested from the sea floor; it has been commercialized (under the trade name Oshenite) for use in applications like PE milk containers for displacing up to 25% of the resin content. CC’s specific gravity is 2.7, and particle sizes may vary from 1 to 100 microns, with the median size usually being from 3 to 20 microns. Unfortunately for processors, CC filler has relatively high moisture take-up.
Surface-treating calcium carbonate
particles allow maximum dispersion of the filler, easier processing of highly filled compounds, and higher impact strengths than in polyolefins filled with untreated CC. Typical materials for coating CC particles include fatty acid-based stearic acids, which are molecules having carboxylate functional groups that anchor them to the filler surface. Stearic acid aids CC dispersion but has limited ability to bond or entangle with the polymer, limiting its effectiveness as a true coupling agent. By contrast, maleic anhydride-grafted polymers do bond to the filler particle and the polymer for better coupling. But such formulations may consume 5-10% of the polymer, increasing viscosity and affecting mechanical properties. Alternatively, silane-based agents bond to the polymer and at least one product (Solplus C800 from Lubrizol Advanced Materials) is said also to bond effectively to the relatively inactive surface of CaCO3. This product, loaded at about 1% in highly loaded CC/PP systems, reportedly results in higher strength and modulus properties, while better maintaining impact strength at levels above those of stearatetreated CaCO3 grades and closer to the values of the unfilled polymer.
Advantages of Calcium Carbonate Masterbatch
- Cost reduction
- Use of high-quality calcium carbonate with fine and uniform grain distribution in the polymer matrix
- White coloring without the need for white masterbatch
- Increased fiber strength against tears
- Decreased wearing and tearing in bags
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