Granular PTFE

Granular material Granular PTFE Granulated sugar Granulating agents Granulation... 

As manufactured, PTFE is of two principal types dispersion polymer, made by suspension polymerization followed by coagulation, and granular PTFE, polymerized and generally comminuted to a desirable particle size. Some details are given by Sperati. We have observed cast films of an aqueous colloidal dispersion and see that it consists of peanut-shaped particles, approximately 0.25 pm in size, which are composed of even finer particles. Electron micrographs of as-polymerized granular particles show three structures bands arranged in parallel, striated humps, and fibrils, some of which have the shish-kebab structure."... 

We have done most of our studies on granular PTFE by compacting it into a preform and sintering to make a shape matching ASTM D-1708-66 for tensile testing. Orientation measurements by IR dichroism require thinner specimens, and for this purpose we used dispersion-grade material, commercially cast and processed into film (Dilectrix Corp., Farmingdale, NY). These films were used as received without further heat treatment. ... 

The morphology and microstructure of as-polymerized polytetrafluoro-ethylenes is a study in itself. We observe that fibrils are common in some lots of granular PTFE while other specimens consist of beadlike particles, the surfaces of which bear markings suggesting lamellar crystals. Of special note is the (rare) occurrence of shish-kebab structures in as-polymerized PTFE (Figure 1.3). 

Broad-line NMR derivative spectra were obtained using a Brucker HFX-90 spectrometer to record the resonance at 84.67 MHz. The specimens, made by compacting granular PTFE into preforms, sintering at 380°C, and cooling slowly at a rate of 0.02 deg/min had a specific gravity of 2.205. The second moment of tire NMR line shape is of interest because the fourth moment of the orientation distribution function is proportional to it. 

Granular PTFE resins are most frequently processed by compression molding using a technique similar to that common in powder metallurgy and by ram extrusion. Each of these processes requires a specific type of granular resins. 

Granular PTFE resins are most frequently extruded as rods or tubes, but it is possible to produce extrudates of noncircular cross sections. 

Granular PTFE resins are produced by polymerizing tetrafluoroethylene alone or with a trace of comonomers [19,20] with initiator and sometimes in the presence of an alkaline buffer in aqueous suspension medium. The product from the autoclave can consist of a mixture of water with particles of polymer of variable size and irregular shape. After the water is removed from the mixture, the polymer is dried. 

PTFE is extremely inert and stable up to temperatures of 250°C (482°F). When heated above that temperature, it begins to decompose very slowly, giving rise to small quantities of gaseous products. For example, the initial rate of weight loss for granular PTFE has been reported as 0.0001%/h at 260°C (500°F) and 0.004%/h at 371°C (700°F) [3]. Thermal degradation of PTFE at different temperatnres is illns-trated in Table 8.1. 

Granular PTFE -CF2-CF2- 327 10lO iol2(3) Compression molding... 

Polytetrafluoroethylene selection. Polymer selection for compounding granular PTFE is relatively straightforward. Fine cut resins are used as a starting point to produce filled compounds. These powders have relatively small particle size and form the most uniform compounds. Typically, smaller particle size resins produce compounds with higher physical properties. 

Biaxially Oriented Unfilled Resin Unfilled Paste- Extruded PTFE Biaxially Oriented 40% Graphite-Filled Fine Powder PTFE 35% Graphite- Filled Granular PTFE Biaxially Oriented 25% Glass Fiber-Filled Fine Powder PTFE 35% Glass Fiber-Filled Granular PTFE... 

Properties of granular PTFE compounds are presented in this section. A few points should be noted about filled PTFE compounds. Shape and size of the filler and resin particles, type of molding process, and processing conditions all influence the properties of parts made from compounds. Properties of compounds made with non-spherical particles are... 

The only continuous process for manufacturing parts from granular PTFE is called ram extrusion. 

This section describes the three processes for producing a billet from granular PTFE preforming, sintering, and cooling. 

Ram extrusion is the only continuous process for fabrication of parts from granular PTFE powders. All the required steps of granular processing are performed in one machine called a ram extruder. The most common shapes are solid round rods and tubes. Rectangular rods, L-shaped cross sections, and other ram extrudable profiles are occasionally fabricated. 

Examples of parts made from granular PTFE resins are shown in Figs. 5.19-5.23. 

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