Stearic acid coated calcium carbonate

Stearic acid coated calcium carbonate (Multiflex SC) 17.5... 

Some studies with filled and unfilled rigid PVC were made in our laboratory to describe the effect of chalk as additional stabilizer. A suspension PVC (K-value 70) with an organotin stabilizer (2 phr) and lubricants (1.8 phr) was used as the experimental material. A part of this compound was filled with a stearic acid coated calcium carbonate (10 phr) as an additional component. The PVC powder and the additives were mixed at a high speed in an intensive mixer. The received dry blends were pelletized by extrusion to get a better dispersion of the additives in the polymer material. Finally, the granulates were processed by injection molding to test specimens for measurements of mechanical properties. 

Papirer and co-workers have also used IGC to study stearic acid coated calcium carbonates [17]. In their work, a high surface area precipitated filler was used, and coating was from toluene solution. They also prepared fractional coating levels based on solution adsorption isotherms. Stearic acid treatment was again found to decrease both the dispersive and polar contributions of surface energy to values typical of a hydrocarbon. Both acidic and basic probes were used in this work and interestingly, the uncoated filler was found to contain sites capable of interaction with both. 

Manufacturers literature offers recommendations on estimating optimum coating levels, and in practice it is usually best to test several coating levels so that the required properties can be optimized. Even the concept of optimum properties is not straightforward, as illustrated in Figure 4 for a compound containing stearic acid coated calcium carbonate in polyethylene. While yield strength is at a maximum... 

Figure 4 Mechanical properties of compounds containing stearic acid coated calcium carbonate in polyethylene (a) tensile yield stress (b) impact energy using instrumented falling weight impact tester.

Figure 22.8 Stearic acid-coated calcium carbonate dispersed in thermoplastic via twin screw extrusion mild agglomeration.

Normally, addition of filler reduces impact resistance, sometimes dramatically so. Even low levels of well-dispersed, stearic acid-coated calcium carbonate can lead to drastic reductions in impact resistance of ABS and to a lesser extent in HIPS. ABS and HIPS are impact modified using rubber particles whereby... 

Semicompatible plasticizing polymers such as chlorinated polyethylene (CPE), ethylene-vinyl acetate (EVA), vinylchloride graft polymers and inorganics such as stearic acid-coated calcium carbonate are also used for impact modification of PVC. 

The study of the adsorption of stearic acid onto calcium carbonate from alcohol solution by Ivanishchenko and Gladkikh [6] has already been mentioned. They also determined adsorption isotherms and the variation of ohmic resistance and water adsorption with coating level for shorter chain acids. Acids with ten and twelve carbon atoms appeared to form mono-layers on chalk with 1.5 x 10 moles g of filler while longer chain acids only required 0.9 x 10 moles g (unfortunately the surface area of the filler was not specified). With the shortest chain acid (Cjo), about three molecular layers were required to maximise hydrophobicity, but when more than 14 carbon atoms were present then only one mono-layer was needed. Their results also indicated that, for the shorter acids at least, the first molecular layer was packed differently from subsequent layers. 

Calcium carbonate used as an additive in polymeric systems, is a hydrophilic, finely divided powder. When used as a component in polyolefins, the calcium carbonate s surface is modified by coating it with a thin layer of stearic acid. Why would compounders prefer to use the stearic acid coated material over the uncoated mineral ... 

Fillers. Fillers are used to improve strength and stiffness, to lower cost, and to control gloss. The most common filler is calcium carbonate, which ranges in size from 0.07 to well over 50 m. Some forms are treated with a stearic acid coating. Clay fillers, such as calcined clay, improve electrical properties. Glass fibers, talc, and mica improve tensile strength and stiffness, but at a loss in ductility. 

Particle size is important and, for some applications requiring good weathering and impact performance (window profile), the ultrafine milled, high whiteness, natural version is normally used. To ease dispersion, the filler is usually coated with stearic acid. Coated ultrafine and precipitated calcium carbonates are claimed also to have a positive effect on impact properties in impact modified formulations (52, 294, 462). The abrasive wear of calcium carbonate, on melt processing equipment, is not significant but increases with increasing levels (177). 

Precipitated calcium carbonate with stearic acid coating 10-40... 

Stearic acid coated, precipitated calcium carbonate 0.1... 

One way of improving the adhesion between polymer and filler is to improve the level of wetting of the filler by the polymer. One approach, which has been used for many years, is to coat the filler with an additive that may be considered to have two active parts. One part is compatible with the filler, the other with the polymer. Probably the best known example is the coating of calcium carbonate with stearic acid. Such coated or activated whitings have been used particularly with hydrocarbon rubbers. It is generally believed that the polar end attaches itself to the filler particle whilst the aliphatic hydrocarbon end is compatible with the rubbery matrix. In a similar manner clays have been treated with amines. 

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. 

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