Melt processible PTFE

PTFE is highly crystalline (92 to 98 percent) and degrades near its melting temperature of 327 C therefore, it cannot be melt processed even though it is a thermoplastic. Due to its inability to be melt processed, PTFE is formed by pressing PTFE powder, followed by heating to sinter the powder, or it is heated and pressed simultaneously (pressure sintered). ... 

Lehmaim (2011) prepared compatibilized blends of PPS and PTFE (component ratios 0-100 to 100-0) using melt-processable PTFE treated by radiation to introduce -COF and -COOH functional groups by chain scission. It was proposed that functional groups on PTFE may react with thiol end-groups on PPS. Blend characterization techniques included mechanical and tribological properties. 

PEEK and polytetrafluoroethylene (PTFE) are highly incompatible. However, fine PTFE powder is commonly added to PAEK to act as an internal lubricant in tribiological applications. The PTFE smears across the wear surface and reduces interfacial friction. This reduces interfacial forces and the heat build-up that can lead to failure by melting. PTFE is particularly suitable in applications where there is no external lubricant and the compounds are often reinforced with carbon fibre. PEEK can also be added to PTFE to improve the wear properties of PTFE - although other less expensive polymers can have similar effects. More recently PAEK and PTFE have been blended so as to produce melt-processable PTFE which has a number of interesting properties [24]. This is perhaps the most luilikely example of the use of PAEK to improve the melt-processability of an otherwise hard-to-process material. 

However, reeently Tervoort et al. reported on melt-processable PTFE blends of various eommereially available fluoropolymers [636,637]. They identified a window of viseosities that permits standard melt-processing of PTFE. 

Peifluorinated ethylene—piopjiene (FEP) lesin [25067-11-2] is a copolymer of tetiafluoioethylene [116-14-3] (TFE) and hexafluoiopiopylene [116-15-4] (HEP) thus its blanched stmctuie contains units of —CF2—CF2— and units of —CF2—CF(CF2)—. It retains most of the desirable characteristics of polytetrafluoroethylene (PTFE) but with a melt viscosity low enough for conventional melt processing. The introduction of hexafluoropropylene lowers the melting point of PTFE from 325°C to about 260°C. 

There are a number of polymers which in fact cannot be melt processed because of their high molecular weights. These include PTFE, very high molecular weight polyethylene and most grades of cast poly(methyl methacrylate). In such cases shaping in the rubbery phase is usually the best alternative. 

The inability to process PTFE by conventional thermoplastics techniques has nevertheless led to an extensive search for a melt-processable polymer but with similar chemical, electrical, non-stick and low-friction properties. This has resulted in several useful materials being marketed, including tetrafluoro-ethylene-hexafluoropropylene copolymer, poly(vinylidene fluoride) (Figure 13.1(d)), and, most promisingly, the copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether. Other fluorine-containing plastics include poly(vinyl fluoride) and polymers and copolymers based on CTFE. 

These materials were first introduced by Du Pont in 1956 and are now known as Teflon FEP resins. (FEP = fluorinated ethylene-propylene.) Subsequently other commercial grades have become available (Neoflon by Daikin Kogyo and Teflex by Niitechim, USSR). These copolymers may be regarded as the first commercial attempt to provide a material with the general properties of PTFE and the melt processability of the more conventional thermoplastics. 

The polymer melts at 216°C and above this temperature shows better cohesion of the melt than PTFE. It may be processed by conventional thermoplastics processing methods at temperatures in the range 230-290°C. Because of the high melt viscosity high injection moulding pressures are required. 

Properties are similar to those of PTFE, and PFA fluoropolymers are generally considered to be the best melt-processable alternative to PTFE yet available. They are, however, more expensive than PTFE. Compared with the TFE-FEP copolymers such as Teflon I P the PFA fluoropolymers ... 

Fluorinated ethylene propylene (Teflon) (FEP) is a fully fluori-nated plastic. This polymer was developed to have a combination of unique properties. It combines the desirable properties of PTFE with advantageous melt processing properties. 

Perfluoroalkoxy (PFA) is very like PTFE with very similar fluorine content and properties, but it is melt processable and more expensive. 

PEP, copolymer of tetrafluoroethylene (TFE) and hexafluoropropylene (HEP), has physical and chemical properties similar to those of PTFE, but it differs from it in that it can be processed by standard melt processing techniques. 

Fluorinated ethylene-propylene (FEP) is a copolymer of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP). It has a branched structure containing units of -CF2-CF2- and -CF2-CF (CF3)-. It retains most of the favorable properties of PTFE but its melt viscosity is low enough for conventional melt-processing. The introduction of HFP reduces the melting point of polytetrafluoroethylene from 325°C (617°F) to about 260°C (500°F).26... 

Gases and vapors permeate FEP at a rate that is lower than for most plastics. It occurs only by molecular diffusion, because the polymer was melt-processed. Because of the low permeability and chemical inertness, FEP is widely used in the chemical industry. Its permeation characteristics are similar to those of PTFE with some advantage because of the absence of microporosity often present in PTFE. For the permeation through FEP films an inverse relationship between permeability and film thickness applies 49... 

In contrast to PTFE with measurable void content, the melt-processed PFA is intrinsically void free. As a result, lower permeation coefficients should result because permeation occurs by molecular diffusion. This is indeed the case, but the effect levels off at higher temperatures.53... 

The need for highly fluorinated thermoplastic polymers that, unlike PTFE, could be fabricated by conventional melt-processing methods led to the development of a group of resins that are copolymers of tetrafluoroethylene (TFE) with other perflu-orinated monomers. Commercially, the copolymer of TFE and hexafluoropropylene (HFP) is commonly known as fluorinated ethylene propylene (FEP). Copolymerization of TFE with perfluoropropylvinyl ether (PPVE) leads to PFA resins, and copolymerization of TFE with perfluoromethylvinyl ether (PMVE) produces MFA resins. 

Aqueous dispersions of these two melt-processible perfluoropolymers are processed in a way similar to PTFE dispersion. FEP dispersions can be used for coating fabrics, metals, and polyimide films. They are very well suited for bonding seals and bearings from PTFE to metallic and nonmetallic components and as nonstick and low-friction coatings for metals.16 FEP can be fused completely into a continuous film in approximately 1 min at 400°C (752°F) or 40 min at 290°C (554°F).17 PFA is used to coat various surfaces, including glass fabric, glass, and metals. 

PFA — Copolymer of TFE with perfluoro(propylvinyl ether), an engineering thermoplastic characterized by excellent thermal stability, release properties, low friction, and toughness. Its performance is comparable to PTFE except it is melt-processible. 

Figure 3. Crystallinities of several melt-processed and annealed-virgin PTFE samples as determined from decomposition of chemical shift lineshapes vj. crystallinities determined from density measurements...

The chemical shift lineshape changes observed for the crystalline fraction of melt-recrystallized PTFE from /L14°C to w28°C (Figure 6) are due to reorientation about an axis essentially parallel to Ozz (which is tilted VL2° from the molecular chain axis). The temperature dependence of the line-shape changes can be simulated as an increase in the rate of rotational diffusion about this axis (Figure 6) and this process has an activation energy of 48+11 kcal/mole. In addition, the... 

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