Polytetrafluoroethylene Permeability

Oxidation is also dependent on the permeability of the polymer to oxygen. Table 10.1 lists the permeabilities of selected polymers to oxygen. Because bulk oxidations are dependent on the permeability to oxygen, crystalline polymer forms are more resistant to oxidation than amorphous forms. Also, the very nature of the molecules present in the chains affects the tendencies toward oxidation. Thus the fluorine atom in polytetrafluoroethylene (ptfe)... 

As discussed above for simple adsorption, polymer sorption can be treated in both thermodynamic and kinetic contexts. The quantity of an analyte that is sorbed by a polymer at equilibrium is referred to as the solubility of the analyte, while the rate at which the analyte is transported through the polymer is referred to as permeability. Although high solubility is generally a prerequisite for high permeability (on any reasonable time scale), there are some notable exceptions. Poly-siloxanes and polytetrafluoroethylene (Teflon ), for example, are quite permeable to water, but the solubility of water is not particularly large in either material. 

Large differences in permeabilities of membranes can be attributed to differences in interchain displacement and flexibility related to polar and steric effects. The polar molecules such as polytetrafluoroethylene have a stronger tendency to form rigid associations leading to crystal formation than nonpolar molecules. Polytetrafluoroethylene polymers are highly... 

Hyflon AD amorphous fluoropolymer is used in optical devices, pellicles in semiconductor manufacture, as a dielectric and as a separation membrane. Small amounts of TDD have been used as a modifier in ethylene-chlorotrifluoroethylene polymers to increase stress crack resistance. Minute amounts of TDD are used also as a modifier in polytetrafluoroethylene to improve elastic modulus, reduce creep and permeability and increase transparency. It has been suggested that the much higher reactivity of TDD and other fluorinated dioxoles relative to other modifiers gives a more uniform distribution of the modifier in the polymer chain that results in a greater increase in the desired properties at lower concentration of modifier in the polymer. 

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. 

The most suitable material for making reactor tubes is polytetrafluoroethylene (PTFE), which, in addition to being chemically resistant, absorbs less solutes on its surface. Polyethylene or polypropylene is inexpensive and easy to flange. As these three materials are highly permeable to oxygen, they pose serious problems in dealing with strong reductants or... 

MF and UF membranes can be polymeric or inorganic. Membrane materials must be chemical resistant to both feed and cleaning solutions, mechanically and thermally stable, and characterized by high selectivity and permeability. Polysulfone (PS), polyethersulfone (PES), polyamide (PA), cellulose acetate (CA), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), poly(vinylidene F ) (PVDF) and polypropylene (PP) are typical materials commonly used to cast the membrane. Alumina, zirconia and ceramic materials are usually used as inorganic materials. 

Microporous polyethylene (PE) membranes with various pore diameters and porosities and microporous polytetrafluoroethylene (PTFE) membrane were used as substrates for the plasma graft polymerization (Table II). Besides these porous substrates, homogeneous poly[ l-(trimethyl si lyl)-l-propyne] (PTMSP), which has the highest gas permeability among polymeric materials, was used as the substrate. The poly[l-(trimethylsilyl)-l-propyne] was synthesized from l-(trimethylsilyl)-l-propyne according to the literature procedure (19). Films were prepared by casting polymers from toluene solutions. Hereafter, the respective substrate membranes will be abbreviated as shown in Table II. 

Commercial interpenetrating polymer networks (not including thermoplastic compositions) include artificial teeth (Dentsply) made Ifom crosslinked PMMA mixed with MMA monomer and polymerized (sequential IPN), sound and vibration damping compositions (e.g., vinyl-phenolic Hitachi) and sheet molding compositions (acryhc/urethane/polystyrene Ferro Chemical) [164]. Water-borne acrylic methane semi-IPNs are commercial Ifom several sources where acrylate monomers are polymerized in the presence of a polymethane water dispersion. A silicone/polytetrafluoroethylene composition described as an IPN is offered by Biomed Sciences under the Silon tradename. The fluoropolymer provides the mechanical strength and the silicone rubber offers the softness and oxygen and moisture permeability for applications in the wound care area. 

---