Polytetrafluoroethylene extrusion

Commercially, suspension polymerizations have been limited to the free radical polymerization of water-insoluble liquid monomers to prepare a number of granular polymers, including polystyrene, poly(vinyl acetate), poly(methyl methacrylate), polytetrafluoroethylene, extrusion and injection-molding grades of poly(vinyl chloride), poly(styrene-co-acrylonitrile) (SAN), and extrusion-grade poly(vinylidene chloride-covinyl chloride). It is possible, however, to perform inverse suspension polymerizations, where water-soluble monomer (e.g., acrylamide) is dispersed in a continuous hydrophobic organic solvent. 

AMS 3656D Polytetrafluoroethylene Extrusions, Normal Strength, as Sintered, Radiographically... 

Compositions whose products of combustion produce energy in the infrared wave band are generally composed of magnesium powder, polytetrafluoroethylene (PTFE) and a binder. For efficient tactical utilization of the energy developed by the combustion process the composition is normally formed into pellets either by press consolidation or by press extrusion. The process being used at Longhorn at the time the electrostatic problem was encountered was press consolidation. The composition was being consolidated into a pellet... 

Above its melting point of 327° C, polytetrafluoroethylene has some properties more like a rubber than a liquid. The instantaneous Young s modulus is 2—3 X 107 dynes/cm2, and the melt viscosity is about 10u poises at 380° C (Nishioka and Watanabe). Because of this very high melt viscosity, it is not feasible to process the polymer by conventional extrusion or injection molding. Instead, techniques similar to those of powder metallurgy are employed. These involve three basic steps. 

The extrusion of lubricated powders, which has been described by Lontz, Jaffe, Robb, and Happoldt, is a unique method for fabricating polytetrafluoroethylene. An aqueous dispersion is coagulated by mechanical agitation. The powder is then dried and blended with 18—20% of a... 

The rheology of lubricated polytetrafluoroethylene compositions was studied by Lewis and Winchester. The mechanism appeared to be a combination of permanent and elastic deformations in the region just before the orifice of the die in the extruder. As a result of permanent deformation, the polymer particles are partially transformed into long fibers. The relative amounts of permanent and recoverable deformation were related to the rate and temperature of extrusion and the geometry of the extruder. Plastic deformation is favored by extruding at temperatures above the 19 and 30° transitions (Snelling and Lontz). 

Extrusion-Applied Insulations. The polymers used in extrusion applications can be divided into two classes low-temperature applications and high-temperature applications. Polymers in the first category are poly(vinyl chloride), polyethylene, polypropylene, and their copolymers along with other elastomers. Polymers in the second category are mainly halocarbons such as Teflon polytetrafluoroethylene (which requires special extrusion or application conditions), fluoroethylene-propylene copolymer (FEP), perf luoroalkoxy-modified polytetrafluoroethylene (PFA), poly(ethylene-tetrafluoroethylene) (ETFE), poly(vinylidene fluoride) (PVF2) (borderline temperature of 135 °C), and poly(ethylene-chlorotrifluoroethylene). Extrusion conditions for wire and cable insulations have to be tailored to resin composition, conductor size, and need for cross-linking of the insulating layer. 

Teflon 62 Polytetrafluoroethylene (PTFE) fine powder, paste extrusion resin. 

Paste Extrusion - Process by which dispersion po-l5mierized polytetrafluoroethylene (PTFE) is extruded with the aid of a hydrocarbon lubricant. A paste of the PTFE and hydrocarbon is made... 

Ram Extrusion - Ram extrusion is the only continuous process for fabrication of parts from suspension polymerized (granular) polytetrafluoroethylene. All three required steps of processing are performed in one machine called ram extruder. The most common shapes are solid rods... 

Reduction Ratio - This factor is the ratio of cross-sectional surface areas of the preform and the extrudate in paste extrusion. Different dispersion polymerized polytetrafluoroethylene powders accommodate different reduction ratio ranges. In general, reduction ratio of the resin decreases as molecular weight increases. Melt fracture and defects appear in the extrudate if it is processed at above its maximum reduction ratio. 

Stretched Membranes. Another relatively simple procedure for preparing microporous membranes is the stretching of a homogeneous polymer film of partial crystallinity. This technique is mainly employed with films of polyethylene or polytetrafluoroethylene which have been extruded from a polymer powder and then stretched perpendicular to the direction of extrusion.10 11 This leads to a partial fracture of the film and relatively uniform pores with diameters of 1 to 20jum. A typical stretched membrane prepared from tetrafluoroethylene is shown in the scanning electron micrograph of Figure 1.2. 

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]. 

Polytetrafluoroethylene incorporation in polymers to yield multiple traversal wear resistant blends is such an example. Microfiber formation via extrusion of highly immiscible blends is another example [Robeson and Axelrod, 1992 Robeson et al., 1994] (see Figure 17.2). 

ASTM D1457, Polytetrafluoroethylene (PTFE) molding and extrusion materials, 1992. 

ASTM D4894-98a Standard Specification for Polytetrafluoroethylene (PI PE) Granular Molding and Ram Extrusion Materials... 

Various micronized polytetrafluoroethylene powders were compounded with silicone rubber (MQ) and the mechanical properties of the composites were evaluated. At a PTFE level of only 5 wt%, the fractured surface of the composites showed layered structure morphology. This stracture effectively improved the tear strength of the MQ but it also lowered the tensile properties of the composites. The addition of fluorosUicone rubber (FMQ) as a compatibilizer, improved considerably the tensile and tear strength of the composites. Extrusion of the MQ/PTFE/FMQ composites on an electric wire indicated that the spherical PTFE powder was suitable for the extrusion process [55]. 

The unique invention that led to PTFE polymer products expanding and becoming desirably porous made the material suitable for use as implants. The modified structure consists of nodes interconnected by very small fibrils (Fig. 1.11). Briefly, the process is as follows. A fine powder of un-sintered polytetrafluoroethylene is uniformly mixed with a Uquid lubricant and then shaped into a product (sheet, rod or tube) by extrusion. After removal of the... 

For practical purposes there are eight types of fluoropolymers, as summarized in Table F.7. Included in this family of plastics are polytetrafluoroethylene (FIFE), polychlorotrifluoroethylene (PCTFE), polyvinyl fluoride (PVF), fluorinated ethylene propylene (FEP), and others. Depending on which of the fluoropolymers are used, they can be produced as molding materials, extrusion materials, dispersion, film, or tape. Processing of fluoropolymers requires adequate ventilation for the toxic gases (HF) that may be produced. 

In this method an extruded film or foil made from a partially crystalline polymeric material (polytetrafluoroethylene, polypropylene, polyethylene) is stretched perpendicular to-the direction of the extrusion, so that the crystalline regions are located parallel to the extrusion direction. When a mechanical stress is applied small ruptures occur and a porous structure is obtained with pore sizes of about 0.1 pm minimum to a maximum of about 3 pm maximum. Only (semi) crystalline polymeric materials can be used for this technique. The porosity of these membranes is much higher than that of the membranes obtained by sintering, and values up to 90% can be obtained. 

D 1457 Specification for PTFE Molding and Extrusion Materials D1675 Method of Testing Polytetrafluoroethylene Tubing D 1710 Specification for TFE Fluorocarbon Rod... 

The biggest application for fiuorinated ionomers currently is as unreinforced membranes for fuel cells, or polytetrafluoroethylene (PTFE) fiber-reinforced composite membranes for electrolytic baths. The membranes can be fabricated by the extrusion method or the solution-cast method. 

Lubricants are added to most polymers at low levels to increase the overall rate of processing or to improve surface properties [1-4], They have been used in the past to facilitate extrusion, injection, compression, etc., of many polymers, mainly PVC, acrylonitrile-butadiene-styrene terpolymer (ABS), PS, PMMA, cellulose acetate (CA), polytetrafluoroethylene (PTFE), and so on. Figure 2.1 shows the relative importance of these polymers as far as the use of lubricants is concerned [5]. The most used lubricants and their relative importance in the United States in 1978 were [6] ... 

Extruder, Dual-Ram n A modification of the original ram extruder employing two identical units, stroking alternately, and delivering to the same die. Aided by values, the result is the conversion of a batch operation into one that makes continuous extrusions from sinterable resins such as polytetrafluoroethylene and ultra-high-molecular-weight polyethylene. 

This book is the second of two volumes about fluoropolymers. The division of the volumes is based on the processing techniques of commercial fluoropolymers. Volume One covers the homopolymers of tetrafluoroethylene (TFE) or polytetrafluoroethylene plastics, which are processed by nontraditional techniques. The extremely high melt viscosity of TFE homopolymers precludes its processing by conventional melt processing methods such as injection molding and melt extrusion. The copolymers of TFE and other fluorocarbon polymers, which are processed by melt-processing methods, have been covered in Volume Two. This book is devoted to exploring the various perflu-orinated and partially fluorinated copolymers of tetrafluorethylene and chlorotrifluoroethylene. Polymers of vinyl fluoride and vinylidene fluoride that are, for the most part, melt-processible have been discussed in the second volume. 

---