Polytetrafluoroethylene measurement methods

If the dirt or contaminant spots can be dissolved in a low boiling-point solvent, solvent extraction (removal) may be an effective method for collecting the sample. If the base plate is a plastic material, the solvent must be carefully selected to ensure that none of the polymer support is dissolved. The solution obtained may then be slowly dripped from a syringe onto a thin film, for example, of polyethylene or polytetrafluoroethylene, and after the solvent has evaporated completely, the remaining powder can then be made into a KBr disk. If the solution is dropped directly onto an infrared-transparent window or a metal plate, the spot often tends to spread to form a circle of the sample after evaporation of the solvent, making the sample then not suitable for either the transmission or transmission-reflection measurement method. If the solution is dropped into a small hole (for example, with a diameter of 1 mm and a depth of 3 mm drilled into a metal support) filled with KBr powder, a diffuse-reflection spectrum may be measured from this after complete evaporation of the solvent. 

There are various kinds of polytetrafluoroethylene. One is granular polymer consisting of spongy, white particles having a median size of about 600/l The specific surface of this polymer is on the order of 2 m2/g (determined by nitrogen adsorption and calculations by the method of Brunauer, Emmett, and Teller). Since this specific surface area is about 1700 times the observed outer surface of the particles, these measurements confirm the porous, spongelike structure that can be seen in the photomicrograph of a cross section of several particles in Fig. la. 

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. 

Polytetrafluoroethylene parts have good wear properties, as seen from the data in Table 3.27. The resistance of unfilled PTFE to wear is less than that of filled compositions. Data from tests measuring wear rate are presented in Tables 3.28-3.30. They should be viewed with an understanding that none of the techniques represent an actual wear situation. In all three methods, a new surface is exposed to abrasion during the repeated motion of the abrading surface. 

Torsional Damping - One of the methods to measure the crystallinity of polytetrafluoroethylene is torsional damping. Other methods include infrared spectroscopy, x-ray diffraction, and ultrasonics. 

To examine this possibility, the degree of complement activation that results when rabbit plasma is incubated with polytetrafluoroethylene (PTFE) or silicone rubber or cellophane has been measured (26). Each of these materials was primed in two ways before it was exposed to the plasma. One method of priming removed the air nuclei from the surface roughness of the material and the other was simply the normal priming technique in which the material was immersed in the physiological saline before it was exposed to the plasma... 

Many experimental techniques have been used to examine the detailed structure of perfluorinated polymeric membranes. These include transmission electron microscopy [23], small angle X-ray scattering [24], Infra Red spectroscopy [25,26], neutron diffraction [27], Nuclear Magnetic Resonance [26,28], mechanical and dielectric relaxation [25,29], X-ray diffraction, and transport measurements. All these methods show convincing evidence for the existence of two phases in the perfluorosulfonate and perfluorocarboxylate polymers. One phase has crystallinity and a structure close to that of polytetrafluoroethylene (PTFE), and the other is an aqueous phase containing ionic groups. 

Figure 12. Population density (A) and adhesion area (B) of osteoblast-like MG 63 cells on day 2 after seeding on tissue culture polystyrene dish (TCPS). carbon fibrereinforced carbon composites (CFRC) and CFRC coated with a fullerene layer (CFRC+full). C Growth curves of MG 63 cells on a terpolymer of polytetrafluoroethylene. poljcvinyldifluoride and polypropylene (Ter), terpolymer mixed with 4 wt. % of single-wall carbon nanohorns (SWNH) or 4 wt.% of high crystalline electric arc multi-wall nanotubes (MWNT-A). D Growth curves of MG 63 cells on TCPS. a nanostructured diamond layer (Nano) and a layer with hierarchically organized micro-and nanostructure (Micro-Nano). Mean S.E.M. from 4-12 measurements. ANOVA. Student-Newman-Keuls method. Statistical significance TCPS. CFRC. Ter p<0.05 compared to the values on tissue culture polystyrene, pure CFRC and pure terpolymer [23].

Resistance in the samples was measured by means of 34401A (Hewlett Packard) device connected with the samples by means of a measurement system in Kelvin (four-point) configuration. The samples were in the form of the stack composed of two layers of carbon composite (carbon paper) which performs the role of a diffusion layer (GDL) in the cell. A plate made of composite material was placed between the carbon paper in order to ensure even distribution of reactants to the electrodes. The set of studied layers were connected with the resistance meter by means of the electrodes made of polished cuprum. The sample was electrically isolated from the press components by means of the plates made of nonconducting PTFE (polytetrafluoroethylene. Teflon). The diagram which illustrates the method of measurement is presented in Fig. 7. 

PFA polymers are fully fluorinated and melt processible. They have chemical resistance and thermal stability comparable to polytetrafluoroethylene (PTFE). Melt viscosity of PFA is over one million times lower than PTFE. Perfluoroalkoxy resins are in general copolymers of tetrafiuoroethylene with one or more of perfluoroalkyl vinyl ether comonomers. Commercial examples of the latter include perfluoromethyl vinyl ether (PMVE), perfluoroethyl vinyl ether (PEVE), and perfluoropropyl vinyl ether (PPVE). PFA resins are specified by ASTM Method D3307, which also provides procedures or references to other ASTM methods for the measurement of resin properties. Commercial PFA resins offered by major manufacturers have been listed in Tables 6.1 through 6.7. 

Table 14.3 contains data that compares the results of plasma treatment and sodium etching for four fluo-ropolymers. Peel strengths of untreated and treated samples were measured by bonding them into T-peel specimen using the flexibilized epoxy adhesive Scotch-Weld 3553 (available from 3M Corp.). The laminates were cured for several hours at 70°C and peel tested at 12.5 cm/min pull rate. Polytetrafluoroethylene does not accept plasma treatment as well as PEA and FEP, as indicated by its relatively low peel strength. Sodium etching is the only effective method of modifying the surface of PTFE. 

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