Calcium carbonate fatty acid coating

In the thermoplastics area, precipitated calcium carbonate is principally used in PVC applications, a market with which it has been associated since the early days of the polymer. Despite some erosion by coated natural products, the combination of small particle size and fatty acid coating continues to give a unique blend of properties in both unplasticised and plasticised PVC formulations. The advantages include easier processing, better surface finish, good low temperature properties and resistance to crease whitening and to scratching. 

Properly applied fatty acids provide the filler with a hydrocarbon-like surface, which is much less polar than the filler itself For example, the treatment of a precipitated calcium carbonate with a fatty acid coating was found to reduce the dispersive component of surface energy from 54 to 23 mj/m 3]. As a result, the filler is made more compatible with many polymer types, resulting in benefits such as faster incorporation and mixing, better dispersion, less energy consumption, lower viscosity, and easier extrusion. The filler generally also has lower adsorbed water content. 

The effectiveness of an MPBD type additive when pre-coated onto precipitated calcium carbonate is demonstrated in Table 4.5. This compares uncoated, fatty acid coated, and MPBD coated fillers in a crosslinked ethylene-propylene-diene terpolymer (EPDM) elastomer and clearly shows the benefits arising from the use of the latter coating. 

The ultrafine materials are all semi-reinforcing in character, as a consequence of their fine particle size. The uncoated and fatty-acid-coated grades exhibit very poor interaction with polymer, so the modulus (a function of polymer-filler interaction) is very low, thus soft compounds are readily obtained. The high purity ensures that these fillers are very white and heat-ageing performance is superior to naturally derived calcium carbonates. One of the outstanding features of these fillers is their good high-temperature tear resistance [60]. 

Dry coating is extensively used with fatty acid treatment of natural calcium carbonates. The challenge is to convert as much as possible of the coating to a bound surface layer, with as little unbound salt and remaining free acid as possible. There is little scientific literature on this procedure but some useful studies have been made[51,64]. A number of different methods are employed. In most cases, unless a small amount of solvent is used, it is necessary for the procedure to be carried out at a temperature where the fatty acid blend is molten. With stearate mixtures this is about 80 °C. Some fatty acids such as iso-stearic acid have the advantage of being liquid at room temperature, but are not widely used as they are more expensive. 

Much of the natural calcium carbonate used in thermoplastics is fatty acid treated. Manufacturers give little detail about their coating processes but it is likely that both wet and dry coating procedures are utilised. 

The fatty acid derivatives give a very good performance on calcium carbonate but are inferior on kaolin. The results of mechanical testing show that the ease of dispersion and mechanical properties of fillers are governed by interactions with the matrix polymer. Thus, mechanical testing of the filled material must be carried out before the best coating can be selected for a given polymer. 

Some grades of PCC are surface coated to improve handling characteristics and dispersability in, for example, plastics. Additives include fatty acids, resins and wetting agents. They help to reduce the surface energy of the calcium carbonate and improve dispersion in organic materials. 

Fatty adds are predominantly used as intermediates. Main applieations are water soluble soaps for household eleaning, personal care, industrial and institutional (I I) cleaning and synthetic rubber manufacturing by emulsion polymerization. Soaps are made by reaction of fatty acids with caustic alkalis, alkali carbonate or ammonia or (>90%) by direct saponification of the triglyceride oil. Another important group of fatty add soaps are dry, water-insoluble metal soaps used as lubricants or stabilizers for PVC and other plastics and aqueous calcium stearate dispersions applied as paper coating... 

Although it seems that the calcite surface in these fillers is often covered by either organic materials or silicate minerals, their chemistry is determined by the basic nature of calcium carbonate and its reactivity towards acids. This is of considerable importance. In particular, their reaction with fatty or other organic acids, but especially stearic acid, has been used for many years [7, 8] to improve compatibility with, and dispersion in, polymers. The coated fillers are much more hydrophobic than uncoated ones, reducing water pick-up, and they have also been shown to have an effect on polymer morphology, and hence modifying properties (see Chapter 1). 

In the wet coating process, a hot concentrated aqueous solution of a salt of the fatty acid is added to an aqueous slurry (usually also hot) of the filler. Under these conditions, rapid reaction with the surface of fillers such as calcium carbonate occurs. (Note, there is some evidence that precipitation may occur first). As mentioned earlieg this procedure can aid product isolation and handling. While sodium salts are often used for convenience, this can lead to undesirable levels of sodium in the final product, unless extensive washing is carried out. Ammonium salts are used to overcome this problem. 

Precipitated calcium carbonates are produced by controlled precipitation from calcium hydroxide solution by carbonation. The products of these processes are fine particles (0.05-0.2 pm) and are often coated with a fatty acid (stearic) or a reactive resin (carboxylated polybutadiene). Uncoated grades are also available. 

Surface treatment of CaCOj can change its nucleating activity in iPP. In Reference [134] iPP composites with nano-CaCOs were modified with iPP grafted with acrylic acid (PP-g-AA). of iPP in the composites increased with increasing nano-CaCOs content, and it was further increased by the addition of PP-g-AA. Recently, composites of iPP with 1 wt% and 3 wt% of nanosized calcium carbonate, both calcite and aragonite, coated either with PP-g-MA or fatty acids were... 

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