Fluoroplastics limitation

The description of the physical properties of fluoroelastomers is necessarily less precise than that of fluoroplastics because of the major effect of adding curatives and fillers to achieve useful cross-linked materials of a given hardness and specific mechanical properties Generally, two parameters are varied increasing cross-link density increases modulus and decreases elongation, and raising filler levels increases hardness and decreases solvent swell because of the decreased volume fraction of the elastomer In addition to these two major vanables, the major determinants of vulcanizate behavior are the chemical and thermal stabilities of its cross-links The selection of elastomer, of course, places limits on the overall resistance to fluids and chemicals and on its service temperature range... 

Coextrusion is the process of forming an extru-date composed of more than one thermoplastic melt stream. The process came about because some service demands, particularly from the packaging industry, could not be satisfied by a single polymer although they could be met by a combination of polymers. Coextrusion was first practiced in the production of cast film and is now also used in blown film and sheet extrusion. The intention is normally to produce a laminar structure in which each layer contributes a key property to the overall product performance. Coextruded films may be very complex structures composed of many different functional layers, including tie layers whose purpose is to bond neighboring layers of limited compatibility. Five layers are not uncommon. However, side-by-side coextrusion is also possible. Fluoroplastics can be coextmded with other polymers such as ETFE and nylon. 

Failure of parts, irrespective of plastic t5 e, is an inevitable fact of the operation of chemical plants. Fluoropolymers are no exception in spite of their excellent chemical, thermal, and mechanical properties. These plastics form the processing surfaces of equipment where they are exposed to the most aggressive and corrosive chemicals. The repeated exposure of fluoroplastics to these chemicals, in addition to other factors, can affect the integrity and surface quality of the parts. The chapters dealing with properties and part fabrication techniques of fluoropolymers should be consulted extensively. An understanding of the limitations of fluoropolymers and flaws created by fabrication methods is required for successful failure analysis of parts. 

HCFC-22 and HCRI -142b are used in the production of tetrafluoroethylene and vinylidene fluoride, respectively. They are used for producing fluoroplastics and fluoroelastomers and the use of chlorine in this market segment will continue to increase. Chlorinated Methanes. While methyl chloride, used mainly to make silicones, will grow marginally, the market for methylene chloride will decline because of environmental and occupational concerns calling for restricted emissions and limited use, and worker exposure. 

Every plastic manifests a limited shear resistance, determined as the permissible shear rate exceeding this value causes mechanical destruction and tearing of molecules of the plastic as a result of excessive internal friction, which has a bearing on the mechanical, electrical or thermal properties of the moulded part. The plastics with the greatest shear resistance are those of low viscosity, e.g., readily fluid versions of PP and PE, PA, etc. Low shear resistance is a feature of plastics like PP (of a high viscosity), PC, PSU and PPS. PVC, CA, CAB, EVA, POM and fluoroplastics have a particularly low shear resistance. 

In the remainder of this seetion, examples of impurity data for different fluoropolymers are listed. An important aspect of these measurements is the detection limit of each technique when measured by ICP-MS, of whieh examples are shown in Tables 15.10 and 15.11. Deteetion limit to less than 100 ppt (part per trillion, or 1 in 10 ) and part per quadrillion (1 in 10 ) is being diseussed as a near future need. Current detection limits for leaching metals from fluoroplastics resins, in a mixture of 80% by volume hydrofluoric acid (49% by weight solution in water) and 20% by volume /-propanol, are summarized in Table 15.11. 

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