Calcium type fillers

Calcium carbonate fillers are either of ground limestone type or precipitated chalk type. They are inexpensive and have low oil-absorption, which allows considerable freedom in filler levels while keeping the viscosity under control. Relatively coarse particle size fillers ranging from 30 to 100 mesh are used to incorporate high loadings. 

Figure 8.32 shows the effect of filler type and concentration on the mode of fracture. Several factors are responsible for the behavior. CaT (calcium terephthalate) fillers have good adhesion to the matrix and have an elongated shape. CaCOs (1) and (2) are both untreated fillers of smaller particle sizes (2.2 and 4.1 (dm, respectively). CaCO (3) is a stearate coated filler (better dispersion, but poor adhesion to matrix) with a particle size of 6.1 im. The mode of fracture depends on filler concentration, the degree of adhesion to the matrix, and particle size. 

Clays - Most commercial clays are classified as hydrous. They are undesirable for use in liquid polysulfide base compounds because of their low pH. However, calcined clays have been found to be satisfactory for use in polysulfide compounds because they are less acidic. The best use of calcined clay fillers is in conjunction with precipitated calcium carbonate fillers to produce a compound with a relatively neutral pH value. For example, calcined clay of the type similar to Icecap K has given good results. 

Selected unsaturated carboxylic acids are particularly useful as coupling agents for calcium carbonate-type fillers. They interact strongly with the basic surface of the carbonate, unlike silanes that only show a weak interaction. In addition, unlike simple saturated carboxylic acids, which just have a beneficial effect on impact strength, unsaturated carboxylic acids may increase tensile strength. Table 6.5 illustrates this effect in a 60% filled CaCOs/PP homopolymer system. This system is heavily stabilized with both hindered phenol and phosphite-type antioxidants, and a small amount of peroxide was therefore added. 

The most common form of the calcium carbonate filler used in rubber is from the simple grinding of limestone into fine-particle-size filler, usually with an average particle size down to about 2 micrometers in diameter. This type of filler is very inexpensive but can be a degrading filler, deteriorating rubber compound properties. These types of calcium carbonate fillers are generally not used in tire technology or in rubber compounds used to make parts where dynamic properties are important. 

High alpha-ceUulose chemical woodpulp paper, machine-made primarily from fast-growiag softwoods, sized usiag alkaline calcium compounds, and loaded with fillers and other additives, constitutes a presumably more stable material. Different types of paper are used for art, manuscripts, documents, books, etc, each having its own properties of color, texture, feel, etc. 

Fillers. These are used to reduce cost in flexible PVC compounds. It is also possible to improve specific properties such as insulation resistance, yellowing in sunlight, scuff resistance, and heat deformation with the use of fillers (qv). Typical filler types used in PVC are calcium carbonate, clays, siHca, titanium dioxide, and carbon black. 

Plastics. The fastest-growing use of whiting (microcarbonate fillers) is in the plastics industry where dry, pulverized limestone is used intensively for most types of plastics. Other carbonate fillers, precipitated calcium carbonate, oyster sheU, marble, and wet-ground limestone, are also used. 

Sihcone polymers when cured into elastomers by themselves ate weak, gel-like materials. For this reason, fillers must be used to provide reinforcement. The type of fillers (qv) used in sihcone sealants varies widely two of the most common fillers are fumed sihca and calcium carbonate. 

Calcium carbonate is one of the most common filler/extenders used in the paint and coatings industry. Consumer and contractor paint formulas can include products from submicrometer size to coarse mesh sizes. The main function of calcium carbonate in paint is as a low cost extender. It is also used to improve brightness, appHcation properties, stabiHty, and exposure resistance. Coarse products help to lower gloss and sheen or even provide textured finishes. The selection of product type and particle size is deterrnined by the desired performance and cost of the coating. 

Carbon blacks are usually used as fillers. The semi-reinforcing types, such as PEP (Past Extmsion Pumace) and SRE (Semi-Reinforcing Pumace) give the best performance (see Carbon, carbonblack). To lower the cost and improve the processibiUty of light compounds, or to lower the cost of black compounds, calcined clay or fine-particle calcium carbonate are used. 

Particulate fillers are divided into two types, inert fillers and reinforcing fillers. The term inert filler is something of a misnomer as many properties may be affected by incorporation of such a filler. For example, in a plasticised PVC compound the addition of an inert filler will reduce die swell on extrusion, increase modulus and hardness, may provide a white base for colouring, improve electrical insulation properties and reduce tackiness. Inert fillers will also usually substantially reduce the cost of the compound. Amongst the fillers used are calcium carbonates, china clay, talc, and barium sulphate. For normal uses such fillers should be quite insoluble in any liquids with which the polymer compound is liable to come into contact. 

In other parts of the world, plywood adhesive fillers are obtained from local sources and may be quite different than those used in North America. In Southeast Asia, banana flour is quite important. In Europe, calcium carbonate (chalk) is often used. Nearly any fibrous material or fine particulate material capable of forming a functionally stable suspension can be made to work if the formulator is sufficiently skillful. However, the mix formulator will be very specific about the type and grade of filler to be used in a particular mix. Substitutions may lead to serious gluing problems. 

On the other hand, the alkoxide system presented several problems in formulation. The system first chosen as a model consisted of a trimethoxymethyl silane crosslinker, 8000 centistoke HEB siloxane, and a catalyst. A number of catalysts were used and each exhibited different cure rates and electrical properties. DuPont tetraalkoxytitante-Tyzor appears to he one of the better catalysts used in this type of curing system. Fillers are usually incorporated into the silicone formulation to improve mechanical properties, promote adhesion, and to serve as light screening and pigment agents. Cab-o-sil, a form of fumed silica, carbon-black, titanium dioxide and calcium carbonate are then used as RTV fillers. 

It is estimated that over one million tons of mineral fillers were used in thermoplastic applications in western Europe in 1986 [2], and the figure is doubtless much greater today. Mineral fillers are used to some extent in virtually all the commercially important thermoplastic polymers but, in volume terms, the principal markets are in PVC and polyolefins, where calcium carbonate dominates the filler types with over 80% of the volume consumption [2]. 

In most, but not all, cases the incorporation of mineral fillers at the levels needed to produce useful effects causes a significant decrease in the toughness or impact strength of the composite. The major exception is in polypropylene, where certain types of calcium carbonate can give a very significant increase in toughness at loadings of 30-40 wt% [33]. 

Aluminium and magnesium hydroxides are difficult to produce directly in any useable form from their natural ores. Filler grade calcium carbonate is widely produced from natural sources, but grinding costs appear to become prohibitive when ultra-fine particles are required and precipitation procedures then become competitive. Further details of precipitation procedures will be found under the specific filler types. 

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