Calcium carbonate thermal conductivity

Slime masses or any biofilm may substantially reduce heat transfer and increase flow resistance. The thermal conductivity of a biofilm and water are identical (Table 6.1). For a 0.004-in. (lOO-pm)-thick biofilm, the thermal conductivity is only about one-fourth as great as for calcium carbonate and only about half that of analcite. In critical cooling applications such as continuous caster molds and blast furnace tuyeres, decreased thermal conductivity may lead to large transient thermal stresses. Such stresses can produce corrosion-fatigue cracking. Increased scaling and disastrous process failures may also occur if heat transfer is materially reduced. 

The mechanisms described above tell us how heat travels in systems, but we are also interested in its rate of transfer. The most common way to describe the heat transfer rate is through the use of thermal conductivity coefficients, which define how quickly heat will travel per unit length (or area for convection processes). Every material has a characteristic thermal conductivity coefficient. Metals have high thermal conductivities, while polymers generally exhibit low thermal conductivities. One interesting application of thermal conductivity is the utilization of calcium carbonate in blown film processing. Calcium carbonate is added to a polyethylene resin to increase the heat transfer rate from the melt to the air surrounding the bubble. Without the calcium carbonate, the resin cools much more slowly and production rates are decreased. 

Calcium Carbonate, Calcium Silicate, Powdered Aluminium, Copper Alumina, Flint Powder, Carborundum, Silica, Molybdenum Disulphide Chopped Glass Mica, Silica, Powdered or flaked Glass Metallic Filler or Alumina Colloidal Silica, Bentonite Clay Improved Thermal Conductivity Improved Machinability Improved Abrasion Resistance Improved Impact Strength Improved Electrical Conductivity Improved Thixotropic Response... 

Thermal expansion-contraction of inorganic fillers is much lower compared with that of plastics. Therefore, the higher the filler content, the lower the coefficient of expansion-contraction of the composite material (see Chapter 10). Many inorganic nonmetallic fillers decrease thermal conductivity of the composite material. For example, compared with thermal conductivity of aluminum (204 W/deg Km) to that of talc is of 0.02, titanium dioxide of 0.065, glass fiber of 1, and calcium carbonate of 2-3. Therefore, nonmetallic mineral fillers are rather thermal insulators than thermal conductors. This property of the fillers effects flowability of filled plastics and plastic-based composite materials in the extruder. 

Reasons for use abrasion resistance, cost reduction, electric conductivity (metal fibers, carbon fibers, carbon black), EMI shielding (metal and carbon fibers), electric resistivity (mica), flame retarding properties (aluminum hydroxide, antimony trioxide, magnesium hydroxide), impact resistance improvement (small particle size calcium carbonate), improvement of radiation stability (zeolite), increase of density, increase of flexural modulus, impact strength, and stiffness (talc), nucleating agent for bubble formation, permeability (mica), smoke suppression (magnesium hydroxide), thermal stabilization (calcium carbonate), wear resistance (aluminum oxide, silica carbide, wollastonite)... 

More recently nanoscale fillers such as clay platelets, silica, nano-calcium carbonate, titanium dioxide, and carbon nanotube nanoparticles have been used extensively to achieve reinforcement, improve barrier properties, flame retardancy and thermal stability, as well as synthesize electrically conductive composites. In contrast to micron-size fillers, the desired effects can be usually achieved through addihon of very small amounts (a few weight percent) of nanofillers [4]. For example, it has been reported that the addition of 5 wt% of nanoclays to a thermoplastic matrix provides the same degree of reinforcement as 20 wt% of talc [5]. The dispersion and/or exfoliahon of nanofillers have been identified as a critical factor in order to reach optimum performance. Techniques such as filler modification and matrix functionalization have been employed to facilitate the breakup of filler agglomerates and to improve their interactions with the polymeric matrix. 

The other, almost universaL additive is inorganic powdered fillers, used to increase viscosity, hardness, modulus, thermal conductivity, heat deflection temperature, opacity, and UV resistance, and to decrease exotherm, cure shrinkage, coefficient of thermal expansion, and cost. Calcium carbonate is the least expensive and most widely used. Clay gives higher electrical and chemical resistance. Talc gives high viscosity for gel coats and auto body repair. Alumina trihydrate gives flame retardance. 

Typical mineral fillers like calcium carbonate, talc, kaolin, mica, silica, and wollastonite all have thermal conductivities an order of magnitude higher than that of polymers. However, specialty fillers are used to achieve better thermal conductivities. Examples include alumina, beryllium oxide, boron nitride (cubic and hexagonal), graphite, carbon nanotubes, metals, and the best of all, diamond. 

DIATOMACEOUS EARTH. (Diatoma ceous silica, diatomite, Kieselguhr.) A highly siliceous mineral derived from skeletons of diatoms (microscopic organisms). Density 0.8-1.2 g/cm fusion point 1715°C. The material s closed cells and high porosity provide a low density and low thermal conductivity. Diatomaceous earth is mined in California, Nevada and Arizona. A typical analysis 85.3% silica, 5.4% alumina, 1.1% FejOj, 1.1% calcium carbonate, 5.6% moisture. 

Although in refractory practice there are hundreds of heat insulation materials, the list of heat insulation materials for the lining of reduction cells is rather limited. For one thing, economic considerations add some limitations, but for another, the heat insulation materials in reduction cells should withstand mechanical compression loads without deformation at temperatures up to 900 °C for a long time, and numerous inexpensive fiber heat insulation materials don t correspond to this requirement. In the Hall-Heroult reduction cell, the heat insulation materials should withstand the pressure of the layer of the electrolyte, the layer of molten aluminium, cathode carbon blocks (taking into account collector bars), and the refractory layer. Currently, only four or five heat insulation materials are used in the lining of reduction cells diatomaceous (moler) and perlite bricks, vermiculite and calcium silicate blocks (slabs), and sometimes lightweight fireclay bricks (but their thermal conductivity is relatively big, while the cost is not small) and fiber fireclay bricks. 

At significant levels of flammable plasticizer, highly FR vinyl compositions require not only additives such as antimony trioxide, zinc borate, or proprietary combinations, but also replacement of calcium carbonate or calcined clay with an FR filler such as ATH or BMC. Results in flame tests are generally enhanced by coating the FR filler to improve dispersion and surface contact with the vinyl matrix and by fine particle size. These factors favor development of char with poor thermal conductivity, and probably promote release of water of hydration to the flame. 

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