Resumen de Main content area Looking for the right mineral filler for plastics
Limestone is the main raw material for producing calcium carbonate. It is also found in nature in crystalline form, which is known as calcite. Naturally ground calcium carbonate (GCC) is available as dry ground (size 200- 325 mesh) or wet ground (Size d 50 - 0.7 to 12 [mu]m and d 98 - 10 to 44 [mu]m). Its particles are generally rhombohedral or prismatic in shape. While there are more than 300 crystal shapes for calcite alone, it is generally an irregular particle with a low surface area and is non-toxic, non-irritating and odourless. It lacks water of crystallisation and its PSD is controllable for optimal packing in each polymeric system. It can be easily coated in dry form in a high-intensity mixture to improve the plastic's melt rheology. These fillers can be smoothly mixed into formulations, sometimes aiding the mixing of other ingredients. It also reduces shrinkage during moulding and curing, for example in no-shrink reinforced polyester sheet moulding compounds. It has a relatively low stiffness, even with high loading, and is usually stable over a wide temperature range.
The disadvantage in using GCC is that when it is attacked by acids, CO 2 is evolved and soluble salts are formed. Many plastics, such as epoxy and polyester, successfully and thoroughly wet and bind CaCO 3 so that the compound, even though highly loaded, resists acid attack. On heating at 800 - 900 o C, CO 2 is evolved and CaO is formed. In polyethylene and polystyrene CaCo 3 loading tends to make the product more brittle. An additional problem is that the triangular crystal shape of CaCO 3 provides little reinforcing action compared with strongly reinforcing materials. In PP, talc and asbestos fillers have higher stiffness, flexural modulus and deflection temperature than CaCO 3. However, CaCo 3 in PP has better impact resistance, possibly due to better bonding between the polymer and calcium carbonate.
Wollastonite particles (size 100 mesh to 8 [mu]m) are needle-shaped and are known for their high-aspect ratio (15:1 to 20:1). Producers have to be careful not to destroy this structure while grinding during dry processing. Powder grades are milled to a low-aspect ratio (3:1, 5:1), either from naturally low-aspect ratio ores or from high-aspect ratio ores that have been ground in a way that breaks the needles width-wise. Despite their low average-aspect ratios, powder grades can retain a significant portion of acicular particles. Both powder and acicular forms of wollastonite are available with surface coatings, usually silane. The silane treatment is a chemical function of the minerals. Performances of properties, particularly nylon 6/66 and PVC, are significantly enhanced by using suitable grades of wollastonite.
