Fine Powder Resin Processing

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

The first step in the manufacture of fine powder resins is to prepare an aqueous colloidal dispersion by polymerization with initiator and emulsifier present.21 Although the polymerization mechanism is not a typical emulsion type, some of the principles of emulsion polymerization apply here. Both the process and the ingredients have significant effects on the product.22 The solids contents of such disper-... 

Fine powder resins are extremely sensitive to shear and the sheared polymer cannot be processed. Because of that they have to be handled with a great care during transport and processing. 

Most commonly, fine powder resins are processed in the form of a paste. Such a paste is prepared by mixing the powder with 15 to 25% hydrocarbon lubricant, such as kerosene, white oil, or naphtha, with the resultant blend appearing much like the powder alone.7... 

Source Adapted from Processing Guide for Fine Powder Resins, Publication H-21211-2, E. I. Du Pont de Nemours Co., Inc., Wilmington, DE, p. 4. (1994) (With permission). 

Cascade coating n. A process for applying epoxy and other thermosets to objects such as electrical resistors and capacitors, in which finely powdered resin is poured over the preheated object. The article is usually rotated as the powder is applied. 

Except for a small introduction in the section on aqueous dispersions, high MW PTFE is one perfluoroplastic which is not covered in this chapter. This is because, in the strictest sense, resins of high MW PTFE are not melt processible. Granular PTFE resins are processed by sintered metal technology techniques. PTFE fine powders are processed by paste extrasion techniques. The structures created with the use of these resins can be quite similar to melt processible perfluoroplastic parts or can be very... 

Commercially PTFE is available in granular and fine powder resin forms and aqueous dispersions. The granular form of PTFE resin is prepared by suspension polymerization process in an aqueous mediiun with little or no dispersing agent. These forms of PTFE resins are mainly used for compression molding and ram extrusion. The fine PTFE powder is prepared by controlled emulsion polymerization and it is useful for paste extrusion into tapes, tubes, pipe liners, insulation layer of wires, gas-liquid separation membranes and fibers. 

Different resins have been developed for use in different reduction—ratio appHcation ranges (111,112). The powders suitable for high reduction—ratio appHcations, such as wire coatings, are not necessarily suitable for the medium reduction—ratio appHcations, such as tubings, or the low reduction—ratio appHcations, such as thread-sealant tapes or pipe liners. AppHcations and processing techniques are being used, which utilize the unique combination of properties offered by PTFE in fine powder form (113—115). 

Chemical Applications. The chemical processing industry uses large amounts of granular and fine powder PTFE. Soft packing appHcations are manufactured from dispersions, and hard packings are molded or machined from stocks and shapes made from granular resin. 

For rayon fiber based composites (Sections 3 and 4) the fiber and powdered resins were mixed in a water slurry in approximately equal parts by mass. The isotropic pitch carbon fiber composites (Section 5) were manufactured with less binder, typically a 4 1 mass ratio of fiber to binder being utilized. The slurry was transferred to a molding tank and the water drawn through a porous screen under vacuum. In previous studies [2] it was established that a head of water must be maintained over the mold screen in order to prevent the formation of large voids, and thus to assure uniform properties. The fabrication process allows the manufacture of slab or tubular forms. In the latter case, the cylinders were molded over a perforated tubular mandrel covered with a fine mesh or screen. Moreover, it is possible to mold contoured plates, and tubes, to near net shape via this synthesis route. 

Since the binder systems are solid at room temperature, they can be produced by the existing methods used for powder coafingsd Solid resins, pigments, photoinitiators, and other additives are premixed, then melted and dispersed in an extruder at 100 to 130°C (212 to 266°F). The molten blend is then squeezed into a thin ribbon between chilled rolls. This ribbon is further cooled to near room temperature on a water-cooled cooling belt. The cooled ribbon is broken first into flake and then ground into a fine powder ready for use. The process is illustrated in Figure 7.15. 

In a nonaqueous copolymerization, fluorinated acyl peroxides are added that are soluble in the medium.43 A chain transfer agent may be added to control the molecular weight of the resin. The polymer is separated from the medium and converted into useful forms such as melt-extruded cubes for processes working with melt (e.g., extrusion, injection molding). The resins are also available as aqueous dispersions, molding powders, and fine powders for powder coating.44 45... 

Pelletized granular resins can be obtained by agglomeration of fine-cut resins. The agglomeration process increases the powder flow and apparent density. The goal of this process is to make the small PTFE particles adhere together. Essentially, there are two processes of agglomeration namely, dry and wet techniques [24]. 

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