Processing of PTFE Dispersions

The Processing of PTFE Coagulated Dispersion Powders, Technical Service Note F3/4/5, 4th ed., ICI Fluoropolymers, Imperial Chemical Industries PLC, Blackpool, Lancs, U.K., p. 18 (1992). 

The major utility of PTFE dispersions is that they allow processing of PTFE resin, which cannot be processed as ordinary polymeric melt, because of its extraordinarily high melt viscosity, or as solution, because it is insoluble. Thus, PTFE dispersions can be used to coat fabrics and yams, impregnate fibers, nonwoven fabrics, and other porous structures to produce antistick and low-friction coatings on metals and other substrates and to produce cast films. 

Another application of PTFE dispersions is the preparation of a variety of compositions with other materials, such as mineral fillers, other polymers in powdered form by co-coagulation. The dispersion of the other component is blended with the PTFE dispersion and the blend is then coagulated. The resulting composition can be processed by extrusion with lubricants (see processing of fine powders) or by compression molding.16... 

The Processing of PTFE Coagulated Dispersion Powder, Fluon PTFE Resins, Imperial Chemical Industries, Ltd.(1986)... 

A PTFE dispersion polymer leads to products with improved tensile strength and flex life. Preforms are made by mixing the polymer with 15-25% of a lubricant and extruded. This step is followed by lubricant removal and sintering. In a typical process a mixture of PTFE dispersion polymer (83 parts) and petroleum ether (17 parts) with a 100-120°C boiling range is compacted into a perform billet which is then extruded by a vertical ram extruder. The extrudate is heated in an oven at about 105°C to remove the lubricant and is then sintered at about 380°C. Because of the need to remove the lubricant, only thin sections can be produced by this process. Thin-walled tubes with excellent fatigue resistance can be produced, or wire can be coated with very thin coatings of PTFE. 

In a typical process a preform billet is produced by compacting a mixture of 83 parts PTFE dispersion polymer and 17 parts of petroleum ether (100-120°C fraction). This is then extmded using a vertical ram extruder. The extrudate is subsequently heated in an oven at about 105°C to remove the lubricant, this being followed by sintering at about 380°C. By this process it is possible to produce thin-walled tube with excellent flexing fatigue resistance and to coat wire with very thin coatings or polymer. 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

PTFE aqueous dispersions are made by the polymerization process used to make fine powders. Raw dispersions are polymerized to different particle sizes.24 The optimum particle size for most applications is about 0.2 pm. The dispersion from the autoclave is stabilized by the addition of nonionic or anionic surfactants, followed by concentration to a solids content of 60 to 65% by electrodecantation, evaporation, or thermal concentration.25 After further modification with chemical additives, the commercial product is sold with a polymer content of about 60% by weight, viscosity of several centipoise, and specific gravity around 1.5. The processing characteristics of the dispersion depends on the conditions for the polymerization and the type and amounts of the chemical additives contained in it. 

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. 

The ability to dr aw approximately 100 - A diameter fibrils from PTFE dispersion particles, either by fracture after cold compaction [34] or by uniaxial or biaxial expansion of sheets or tubes after paste extrusion (extrusion of mixtures of the dispersion particles and lubricants such as mineral spirits) and lubricant removal [35], was demonstrated a number of years ago. The former process results in the development of a myriad of fibrils spanning the gap between the fracture faces these were utilized for ED characterization of the PTFE conformation and crystal packing. 

To improve the properties of the raw polymer (wear resistance, creep resistance, thermal and electrical conductivity), various fillers, such as glass fibers, powdered metals, and graphite, are combined with all three types of PTFE polymers, mostly by intimate mixing. Filled fine powders are produced mostly by adding fillers into a dispersion and then coagulating the mixture. Aqueous dispersions can also be modified by the addition of certain fillers, pigments, heat resistant dyes, carbon blacks, and powdered metals, especially when processed into films (see Chapter 6). 

---