Sintered fluorocarbons

A similar set of materials has been produced by Springsteen and coworkers at Labsphere. Known as Spectralon WCS standards, the patented materials are each of the three rare earth oxides used in NIST SRM-1920 sintered into a fluorocarbon matrix. As with the Verrill ceramic standard, the positions of some of the peaks are slightly shifted from that of the unprocessed powders. Although the cause of these peak shifts is unknown, it is likely due to some change in the lattice of the oxides when heated to the temperatures needed to process the ceramics or sintered fluorocarbons. 

Because PTFE melts at such a high temperature and the melt is very viscous, it is difficult to work by conventional plastics techniques such as injection molding or extrusion. It is usually formed into useful shapes by sintering at about 380 °C sometimes liquid alkanes are used as a carrier, but the product then tends to be porous (this emerges as a serious problem when thin-walled objects are machined out of PTFE stock). Accordingly, several fluorocarbon thermoplastics have been developed that have lower melt viscosities, at the expense of somewhat poorer thermal and mechanical properties than PTFE. For example, the thermoplastic FEP (fluori-nated ethylene propylene), made by copolymerization of F2C=CF2 and... 

Inks having a fluorocarbon base can be used to print stripe patterns on wire for identification. In practice a wheel coated with the desired color runs along the wire prior to the sintering step of the manufacturing process. For more than one stripe, additional wheels are needed. The ink is sintered at the same time as the insulation. PTFE and FEP dispersion have been used as the base to produce ink. Inorganic pigment that is stable under the sintering conditions of polytetrafluoroethylene must be used. 

Fig. 5. Platinum hydrofluorination apparatus (88). A, platinum spheres with filter disks of sintered platinum B, brass needle valves C, stainless steel block clamps D, platinum reactor E, platinum vessel F, to manifold O, furnace H, plastic bubbler with fluorocarbon oil.

There are of course many plastics materials which have better high temperature performances than the polysulphones, for example the polyimides and fluorocarbon polymers, but most of these materials are thermosetting resins and all require somewhat specialised techniques for handling eg sintering or thin layer coating techniques (12). 

Dispersion spraying. The plastics to be applied is dispersed in an aqueous or solvent base and is sprayed in a manner similar to the application of paint. After sintering one or two further coatings are usually applied with intermediate sintering in order to build up a reasonable coating thickness. The significance of this method can be appreciated when fluorocarbon (PTFE, FEP, etc) coatings are required since they can only be applied from dispersions. 

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