Granular-based Compounds

Compounds, known as granular powders, are made with granular polytetrafluoroethylene resin. The choice and concentration of the filler depends on the desired properties of the final part. Glass fiber, bronze, steel, carbon, carbon fiber, and graphite are among the common filler materials. Up to 40% by volume of filler can be added to the resin without complete loss of physical properties. The impact of additives below 5% by volume of filler on the properties of compounds is insignificant. Above 40%, most physical properties of the compounds drop sharply. 

Fillers. Polytetrafluoroethylene is one of the more difficult polymers to compound. This is due to the extreme neutrality of PTFE chains, which pre- 

The only requirement for an additive to qualify as filler for PTFE is that it should be able to withstand the sintering temperatures of polytetrafluoroethylene. Sintering involves exposure to temperatures close to 400°C for several hours, which excludes a great many materials. Characteristics of the filler such as particle size and shape and the chemical composition of the filler affect the properties of compound. A list of most common fillers and descriptions of their important characteristics can be found in Table 3.6. 

T q)ical properties of two compounds are summarized in Table 3.7. Increasing the glass fiber content of the compound reduces standard specific gravity (SSG) and shrinkage moderately, while physical properties decline more drastically. 

Carbon reduces creep, increases hardness and elevates thermal conductivity of polytetrafluoroethylene. Wear resistance of carbon filled compounds improves, particularly when combined with graphite. Carbon-graphite compounds perform well in non-lubricated applications such as piston rings in compressor cylinders. Carbon-filled PTFE has some electrical conductivity. Close tolerances can be achieved 

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Kuwabara, M., Hayamizu, S., and Hatakeyama, A., Trends in Urea-Based Compound Fertilizer Technology, in Granular Fertilizers and Their Production, pp. 125-147, British Sulphur Corporation, London, England, 1977. 

The resole-based compounds are generally only available as granular powder or small pellets. Both resin system compounds are easily molded in all thermosetting molding procedures. 

ChlorinatedIsocya.nura.tes. The cyanuric acid-based sanitizers, introduced for pool use in 1958, are stable crystalline compounds with moderate-to-high available CI2. Sodium dichloroisocyanurate (Dichlor), sold in granular form, dissolves rapidly, whereas trichloroisocyanuric acid (Trichlor) dissolves very slowly and is widely used in the form of tablets or sticks in feeders, floating devices, or in the pool skimmer. 

Fig. 6. Breakthrough curves for aqueous acetone (10 mg 1" in feed) flowing through exnutshell granular active carbon, GAC, and PAN-based active carbon fibers, ACF, in a continuous flow reactor (see Fig. 5) at 10 ml min" and 293 K [64]. C/Cq is the outlet concentration relative to the feed concentration. Reprinted from Ind. Eng. Chem. Res., Volume 34, Lin, S. H. and Hsu, F. M., Liquid phase adsorption of organic compounds by granular activated carbon and activated carbon fibers, pp. 2110-2116, Copyright 1995, with permission from the American Chemical Society.

Advanced treatment plants employ either granular filters or membrane filters. The former is exemplified by activated carbon, whereas membrane filtration has been developed only in recent memory. Besides the general principle of excluding contaminants based on size, these advanced filtration systems also have a charge that enables them to exclude particles, with the removal of anionic compounds being higher than that of nonionic ones. Both systems come at a premium. 

A colorless or slightly yellow solid occurring in filamentous, granular, and powdered form. Forms a viscous cnlloidal solution with water, insoluble in alcohol, ether, and chloroform. It is extracted from brown seaweeds. The gum is used as a thickener, stabilizer, and emulsifier in foods, especially ice cream. Also used in boiler compounds, pharmaceuticals, textile printing, cement compositions, paper coatings, and in some water-base paints. 

Bansode, R.R., Losso, J.N., MasrshaU, W.E., et al. (2003). Adsorption of volatile inorganic compounds by pecan sheU- and almond shell-based granular activated carbons. Bioresource TechnoL, 90, 175—84. 

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