Polytetrafluoroethylene content

Dai W, Wang H, Yuan X Z, Martin J, Shen letal. (2009a), Measurement of water transport rates across the gas diffusion layer in a proton exchange membrane fuel cell, and the influence of polytetrafluoroethylene content and micro-porous layer . Journal of Power Sources, 188, 1, 122-126. 

Ultraviolet-visible (UV-vis) diffuse reflectance spectra of supported WOx samples and standard W compounds were obtained with a Varian (Cary 5E) spectrophotometer using polytetrafluoroethylene as a reference. The Kubelka-Munk function was used to convert reflectance measurements into equivalent absorption spectra [12]. Spectral features of surface WOx species were isolated by subtracting from the W0x-Zr02 spectra that of pure Z1O2 with equivalent tetragonal content. All samples were equilibrated with atmospheric humidity before UV-vis measurements. 

Being microwave-transparent, the reaction vessels will be no hotter than their contents. As mentioned above, they usually are made from insulating polymeric materials like polytetrafluoroethylene (PTFE), which have inherent advantages for cleaner processing. In contrast with other materials, PTFE is resistant to attack by strong bases or HF and is not corroded by halide ions. 

Fig. 5. Calculation of the amorphous content of polytetrafluoroethylene from the x-ray diffraction pattern. (After Ryland)...

A routine method for determining relative crystallinity based on the amorphous bands in the spectrum has proved more rapid and precise than the x-ray method. In practice, the ratio of the 778 cm-1 (12.85 ft) and 2367 cm-1 (4.22 ft) band intensities is measured. Use of a ratio eliminates the thickness measurement and increases precision to about 1% at 50% crystallinity and considerably better at higher levels. A density measurement and an infrared crystallinity determination when combined give an estimate of the fraction of microvoids which can occur in molded specimens of polytetrafluoroethylene. The density of a sample is predicted on the basis of its crystallinity as measured by the infrared method and the difference between this density and the actual density measured by displacement in water is a measure of the microvoid content. This determination is precise to about 0,2% voids by volume. By the use of confirmatory infrared measurements, it is possible to check the possibility that the presence of a substantial percentage of voids may have led to erroneous indications of the molecular weight in the standard specific gravity test discussed earlier. 

The stability of methylmercury contained in samples of fish, fish muscle and dried shellfish was investigated by storing samples in a deep-freeze and subjecting them to thermal cycling over a period of 32 months (Horvat and Byrne, 1992). Losses of up to 30% of methylmercury contents of shellfish were recorded, but in the other samples contents were stable. Aqueous samples containing methylmercury stored in polytetrafluoroethylene containers were stable for up to 20 days, and acidified solutions (nitric acid) were stable in glass containers stored in a refrigerator. Seawater samples were acidified with 2% hydrochloric acid for better stability (Leermakers et al., 1990). 

DSC curves can also be used to identify individual polymers in a polymer mixture. This is a rather unique capability of DSC or DTA. Figure 10.17 shows DSC curves of plastic waste. The individual melting peaks reveal the waste s polymer content. By comparing these curves with DSC curves of pure polymers, we can assign the melting peaks as those of low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), Nylon-6, Nylon-66 and polytetrafluoroethylene (PIPE). 

In Chapter 18, we described solvent extraction and solid-phase extraction sample preparation methods, which are applicable to GC analyses as well as others. A convenient way of sampling volatile samples for GC analysis is the technique of head-space analysis. A sample in a sealed vial is equilibrated at a fixed temperature, for example, for 10 min, and the vapor in equilibrium above the sample is sampled and injected into the gas chromatograph. A typical 20-mL glass vial is capped with a silicone rubber septum lined with polytetrafluoroethylene (PTFE). A syringe needle can be inserted to withdraw a 1-mL portion. Or the pressurized vapor is allowed to expand into a 1-mL sample loop at atmospheric pressure, and then an auxiliary carrier gas carries the loop contents to the GC loop injector. Volatile compounds in solid or liquid samples can be determined at parts per million or less. Pharmaceutical tablets can be dissolved in a water-sodium sulfate solution... 

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. 

Another, more common commercial use of the phenomenon is the addition of fluoropoly-mers to polyolehns. In this case, a small amount of fluoropolymer progressively migrates to the die surface, reducing the die pressure drop and making it possible to extrude the resin at high throughput without the melt fracture. It has been shown that this approach also works for other polymers, viz. PEEK. Thus, blends of PEEK with polytetrafluoroethylene, 1-5 wt% PTFE, were extruded. The pressure drop across the die was reported to decrease with time to an equilibrium value, R.. The value of P,. depended on PTFE content, whereas the time to reach it depended on the rate of extrusion — the higher was the rate, the shorter was the saturation time [Chan et al., 1992]. 

The presence of moisture in the gas stream, which above 100 C will be present in the form of superheated steam, will also cause a rapid degradation of many fibres through hydrolysis, the rate of which is dependent on the actual gas temperature and its moisture content. Similarly, traces of acids in the gas stream can pose very serious risks to the filter fabric. Perhaps the most topical example is found in the combustion of fossil fuels. The sulphur that is present in the fuel oxidises in the combustion process to form SO, and in some cases, SO3 may also be liberated. The latter presents particular difficulties because, in the presence of moisture, sulphuric add will be formed. Hence, if the temperature in the collector were to be allowed to faU below the acid dew point, which could be in excess of 150°C, rapid degradation of the fibre could ensue. Polyaramid fibres are particularly sensitive to acid hydrolysis and, in situations where such an attack may occnr, more hydrolysis-resistant fibres, such as those produced from polyphenylene sulphide (PPS), would be preferred. On the debit side, PPS fibres cannot snstain continuous exposure to temperatures greater than 190 °C (or atmospheres with more than 15% oxygen), and where this is a major constraint, consideration would have to be given to more costly materials, such as polytetrafluoroethylene (PTFE). 

Moisture resistance. The adhesive should be formulated to minimise moisture transport through the adhesive itself. The equilibrium water content (A/ ) should not exceed 3% by weight after immersion in distilled water at 20 °C. The permeability, obtained from the product of the coefficient of diffusion (D) and My, should not exceed 5 x 10 " m-/s at 20 °C (see Fig. A.l). A film of adhesive, approximately 1 mm thick cast in polytetrafluoroethylene-lined moulds and weighing at least 3 g, is suggested for this test(3). This requirement is to enhance the potential for a durable adhesive/adherend interface, even if moisture uptake is not deleterious to the adhesive itself. 

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