Welding Fluoropolymer

The anode and cathode chambers are separated by a cation-permeable fluoropolymer-based membrane (see Membrane technology). Platinum-electroplated high surface area electrodes sold under the trade name of TySAR (Olin) (85,86) were used as the anode the cathode was formed from a two-layer HasteUoy (Cabot Corp.) C-22-mesh stmcture having a fine outer 60-mesh stmcture supported on a coarse inner mesh layer welded to a backplate. The cell voltage was 3.3 V at 8 kA/m, resulting ia a 40% current efficiency. The steady-state perchloric acid concentration was about 21% by weight. 

Temperatures required to adequately produce a molten layer of the surfaces vary from 210°C-290°C. Welds may be particularly susceptible to stress cracking in strongly alkali solutions where discoloration develops as a result of dehydrofluorination. VF2/ HFP fluoropolymers exhibit improved resistance to a high pH environment, where PVDF homopolymers have been known to become brittle over time. 

Dual Laminate (Fluoropolymer-lined FRP) Same as Adhesive-bonded Fabricate liner first on a mandrel (hand and machine welding) and build FRP laminate over the liner. Use carbon cloth for spark testing Shop and Field Pressure OK (RTP-1 Dual lam) Vacuum OK for FRP/fluoropolymer bond. Design FRP for vacuum 33 dia max Visual Spark AE CRBBD Possible Testing recommended... 

Melt-processible fluoropolymers can be directly welded while PTFE requires an adhesive ring. PFA film (13-50 pm thick) is the most effective adhesive material for welding PTFE. A PFA ring is inserted on each end of the tube and the ends are brought into contact be means of the welding device (Fig. 

Figure 8.22 Machine butt welding of fluoropolymer prior to dual laminate construction. (Courtesy CPF Dualam, Montreal, Canada.)...

Repairability is determined by welding a small bead on the lining at a location such as a manway cover or a blind flange. Adhesion of the weld bead is determined and any signs of foaming or other reaction between the heat input and the chemicals trapped in the fluoropolymer are noted. Once satisfied, weld repair can be carried out successfully. The repair area is then inspected by spark test. 

Welding and Flame-Cutting Fluoropolymer-Lined Metals... 

Welding arcs and torches are capable of massive destmction of fluoropolymers due to the exeessive heat that they generate. The plastic part should be removed before metal is cut or welded. Local exhaust ventilation should be provided whenever it is not possible to remove the polymer from the area to be welded or cut. 

For process applications which require a lining that will resist harsh chemicals usually one of the fluoropolymers is selected. Since these linings are expensive, representing approximately 80% of the cost of the vessel, a fabricator must be selected that is experienced and knowledgeable in the handling, welding, and forming of the specific fluoropolymer sheet. 

THV (tetrafluoroethylene-hexafluoropropylene-vinylidenefluoride copolymer) foil has very high transparency and elasticity, but low strength, so it is mostly used as a covering for load-carrying nets. As a fluoropolymer, THV foil may be welded and is non-combustible and not susceptible to contamination, nor is it affected by UV radiation or chemicals. 

PTFE (and other fluoropolymers) tubes and lined pipes are welded by the butt fusion technique (Fig. 14.45). A uniform bead is required for the formation of a strong butt weld. The two ends of the plastic tubing are cut back to obtain square ends. The pipe ends are faced together by a device such as the one shown in Fig. 14.46 which allows the application of a measured axial force to the tubes. A heated plate or a clam shell heater can be used to heat the ends of the tube. An important step is to thoroughly clean and wipe the ends of the tube with acetone or another solvent. 

Thermoplastic fluoropolymers are also used in this way to line vessels. Compared with PTFE, they are easier to weld, and unlike PTFE, they can be readily... 

Improvements in processibility and technology are making it easier to use fluoropolymers to protect chemical equipment. Relatively new modified PTFE resins, for example, allow welding and thermoforming to reduce manufacturing costs. 

For the thermoplastic fluoropolymers, rotolining is proving to be a reliable and versatile method for lining complex parts. The need to eliminate flanged connections wherever possible is increasing interest in dual laminate constructions that permit welding of joints. 

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