Polytetrafluoroethylene fillers

Blends of the polysulfone tesia have been made with ABS, poly(ethylene terephthalate), polytetrafluoroethylene (PTFE), and polycarbonate. These ate sold by Amoco under the Miadel trademark. Additional materials ate compounded with mineral filler, glass, or carbon fiber to improve properties and lower price. 

Polytetrafluoroethylene, molybdenum disulfide, graphite, and aramid fibers reduce the frictional coefficient. These may be used as single friction additive, in combination with other fillers, and in combination with silicone oil. Table 5.17 illustrates effect of PTFE on the frictional properties of different polymers. 

Tapes. A great variety of tapes find application in electrical equipment. Some tapes contain filler materials in macroscopic form such as glass fibers, mica flakes, and cloth others have finely divided filler particles or no fillers at all. In the heavily filled materials the polymeric binders are present in small fractions, and the major emphasis may be on their adhesive capabilities rather than on their properties as dielectric materials. Most of the polymers used in tapes have already been mentioned in connection with other insulation applications, for example, polyesters, aromatic polyamides, polyimides, and polypropylene. Other polymers frequently used for electrical tapes are vinyls, including poly(vinyl fluoride) these are particularly well suited as conformable tapes. Polytetrafluoroethylene (Teflon TFE) has also been fabricated into tape constructions, frequently in combination with adhesives to provide a bondable material. 

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. 

Filled PTFE—production techniques. Granular polytetrafluoroethylene compounds containing fillers are converted into parts by the same molding techniques as those used for neat resin. The compounding techniques aim at producing uniform blends of PTFE with fillers that can be processed in the same molding equipment. The rest of this section describes methods by which compounds can be made. 

Co-coagulation is the method by which large quantities of fillers can be incorporated in dispersion polymerized polytetrafluoroethylene. The addition of fillers takes place prior to coagulation of the resin from its dispersion state. In the process of co-coagu-lation, the additives are added to the polytetrafluoroethylene dispersion and mixed. This dispersion is coagulated and the compound is recovered. The smaller the filler particles, the smaller the points of stress rise in the compound will be. Significantly larger quantities of filler can be compounded in PTFE by this technique. 

Deformation under load of all filled polytetrafluoroethylene compounds decreases in comparison to unfilled resin, as seen in Table 3.13. Combinations of carbon and graphite reduce deformation the most at room and at elevated temperatures. The next effective filler in reducing deformation under load is bronze at 60% by weight. Hardness is increased by the addition of additives, particularly bronze, carbon, and graphite (Table 3.14). 

Electrical properties. Fillers and additives significantly increase the porosity of polytetrafluoroethylene compounds. Electrical properties are affected by the void content as well as the filler characteristics. Dielectric strength drops while dielectric constant and dissipation factor rise. Metals, carbon, and graphite increase the thermal conductivity of PTFE compounds. Tables 3.19 and 3.20 present electrical properties of a few common compounds. 

Polytetrafluoroethylene has excellent chemical resistance properties. The effect of incorporation of additives on chemical properties depends on the t) e of the filler and the specific chemicals. In general, chemical properties of filled PTFE compounds are not as good as those of the unfilled resin. Table 3.21 shows the effect of a number of chemicals on car-bon/graphite, glass, and bronze compounds. 

Polytetrafluoroethylene has a somewhat higher coefficient of expansion than other plastics. This differential expansion can result in leaking of joints when PTFE is combined with other materials. Addition of fillers such as glass, fiber, graphite, bronze, and molybdenum disulfide alters the coefficient of expansion of polytetrafluoroethylene compounds (Table 3.36). A compound containing 25% filler has a coefficient of expansion about half that of the unmodified resin. 

Fillers usually increase the thermal conductivity of polytetrafluoroethylene as with other plastics (Table 3.38). Specific heat of PTFE at various temperatures is given in Table 3.39. Enthalpy of molded PTFE is given in Fig. 3.33. 

Finishes - Highly formulated dispersions of polytetrafluoroethylene containing a variety of fillers such as pigments, resins, extenders, and others. Finishes are used to coat different surfaces such as cookware, houseware, and industrial equipment. 

Polytetrafluoroethylene Compounds - Material obtained by intimate mixing of fillers (metallic and nonmetallic) with polytetrafluoroethylene. One or more of polymer properties sueh as eold flow, wear, and surfaee hardness are altered by the addition of fillers. 

Ryton Polyphenylene Sulfide is a new commercial plastic which is characterized by good thermal stability, retention of mechanical properties at elevated temperatures, excellent chemical resistance, a high level of mechanical properties, and an affinity for a variety of fillers. It is produced from sodium sulfide and dichlorobenzene. Its unusual combination of properties suggests applications in a variety of molded parts such as non-lubricated bearings, seals, pistons, impellers, pump vanes, and electronic components. Tough coatings of polyphenylene sulfide can be applied to metals or ceramics by a variety of techniques and are used as protective, corrosion-resistant coatings in the chemical and petroleum industries. Incorporation of small amounts of polytetrafluoroethylene provides excellent non-stick properties in both cookware and industrial applications. 

Fillers/reinforcements can be used to increase the thermal conductivity of the material such as glass and metal fibers or spheres. The filter can be a material like PTFE (polytetrafluoroethylene) plastic that has a much lower coefficient of fnction and the surface exposed material will reduce the fiiction. 

Bentonite Kaolin Strontium sulfate Wollastonite filler, reinforcing Aluminum (III) silicate (2 1) Polytetrafluoroethylene Sisal filler, reinforcing cleaners Silica, hydrated... 

The part may need to be in contact with service fluids such as mineral and vegetable based oils. The selection of the correct polymer depends on the exact nature of the fluid and the service temperature. For mineral oils a polychloroprene or acrylonitrile -butadiene copolymer based compound may be appropriate but small variations in lubricant constituents make it worthwhile to measure the changes that can occur at operating temperatures to properties such as modulus and tear resistance. For solvents it may be more viable to use a physical sheath of an impervious material such as polytetrafluoroethylene. Swelling or shrinkage is strongly influenced by the nature of fillers and oils used to compound the rubber. 

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