Fluoroplastics mechanical properties

Copolymers of chlorotrifluoroethylene and ethylene were introduced by Allied Chemicals under the trade name Halar in the early 1970s. This is essentially a 1 1 alternating copolymer compounded with stabilising additives. The polymer has mechanical properties more like those of nylon than of typical fluoroplastic, with low creep and very good impact strength. Furthermore the polymers have very good chemical resistance and electrical insulation properties and are resistant to burning. They may be injection moulded or formed into fibres. 

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... 

The moduli of fluoropolymers are a function of a number of variables, most important of which is the composition of the polymer and, more specifically, the presence or absence of hydrogen in the polymer. Other variables are those affecting the other mechanical properties of these plastics. They include testing temperature, molecular weight, and crystallinity, which affect the modulus of these plastics independently of the regime/tyqie of measurements. Perfluo-ropolymiers have lower moduli than partially fluori-nated fiuoropiastics. An increase in the fluorine content of the fluoroplastic increases the modulus. 

Fluoroplastics are used in a large number of applications that involve operations at temperature extremes because of the ability of these plastics to withstand very high or low temperatures. A popular method of testing a part is based on monitoring the physical or mechanical properties, such as tensile strength and break elongation, as a result of thermal exposure over time. To check the impact of process-... 

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. 

Chemical changes occur when a fluoroplastic part is exposed to the few compounds and elements that can degrade them. Chemical attack over time can result in changes in surface or bulk composition, and a loss of mechanical properties such as tensile or burst strength. The loss of properties is because of a reduction in molecular weight or mass loss of the part. Degradation can introduce impurities into the process stream in the extreme case when massive chemical degradation takes place. 

ETFE is a fluoroplastic with excellent electrical and chemical properties. It also has excellent mechanical properties. ETEE is especially suited for uses requiring high mechanical strength, chemical, thermal, and/or electrical properties. The mechanical properties of ETEE are superior to those of PTEE and EEP. ETFE has ... 

With this correlation of the hardness test established, efforts were then directed toward the determination of the effect of crystallinity on the mechanical properties of the fluoroplastics at cryogenic temperatures. 

A series of mechanical property determinations was made to determine the reproducibility and spread of the tests results when materials of various levels of crystallinity were tested at cryogenic temperatures. It was found that there was a definite correlation between the crystallinity (as determined by the hardness test) of the samples and their low-temperature mechanical properties. As an example, samples of relatively low crystallinity were much stronger and more ductile at cryogenic temperatures than the highly crystalline samples. As a result of these determinations, it was found that a reasonably accurate set of low-temperature mechanical properties could be assigned to any fluoroplastic by performing this simple non-destructive room temperature hardness test. 

Partially fluorinated fluoropolymers (PVDF, ETFE, ECTFE) are much more resistant to electron beam and y-radiation than perfluorinated fluoroplastics. Table 13.47 provides mechanical property data for ECTFE as a fiinction of Co radiation (y-rays). Figure 13.108 shows the effect ofy-rays on the break elongation and yield strength of PVDF. Exposure of... 

Partially fluorinated fluoroplastics can be cross-linked to improve the mechanical properties of shaped articles such as wire insulation. For example, the aerospace industry requires wire and cable that can withstand abrasion and cutting in addition to flammability. Cross-linking a fluoropolymer like ETFE, ECTFE, or PVDF enhances its mechanical properties. For example, actual cross-linking is accomplished by incorporating a cross-linking agent in ETFE after the extrusion of the insulation, followed by irradiation.fi l... 

More recently, modified fluoroplastics such as fluorinated ethylene/propylene copolymer, polychlorotrifluoroethylene, and polyvinylidene fluoride have been offered by DuPont, Allied Chemical, 3M, and Pennwalt respectively, to provide improved processability and mechanical strength at some sacrifice in heat-resistance, electrical properties, and chemical resistance and at prices of 3.70-7.15 these have also been finding appropriate if smaller markets. 

Ethylene chlorotrifluoroethylene copolymer (ECTFE, E/CTFE) n. A fluoroplastic with good mechanical, thermal, electrical, processing, and resistance properties. 

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. 

This chapter begins with a brief discussion of free radical polymerization, the mechanism by which fluoroplastics are polymerized. Next, preparation of different polymers is described. Each polymerization technique and the corresponding finishing steps are discussed. Characterization methods and the defining properties for each polymer group are also provided. 

Carbon, fluorine, and hydrogen are the major elements that form the perfluorinated and partially fluori-nated fluoropolymers. The presence of fluorine is the main reason that these plastics have many special properties, which surpass those of most polymers. The desirable properties span across mechanical, tribological, electrical, and thermal characteristics of these polymers in addition to chemical resistance. Increased fluorine content of the fluoropolymers enhances these properties. Consequently, perfluoropolymers should be sought out when ultimate chemical resistance, electrical properties, etc., are required. This chapter concentrates on presenting the key properties of fluoropolymers. Properties of fluoroplastics films can be found in Ch. 6 and Appendix VI. 

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