Welding PTFE

Polytetrafluoroethylene for these applications is known as modified, which refers to the presence of a small amount of a second perfluorinated monomer, known as a modifier, in its structure. The modifier molecule always contains a pendent group. 

The quality of a welded area is defined by the strength of the bond. One of the ways to measure 

Heated Metal Welding Jaws With Smooth Welding Surface 

For Example, Pneumatic Closing System (Controlled Contact Pressure) 

Another method used to weld the liner of large tanks is hot gas welding using a PFA (melt-proces-sible) rod. Sheets of skived polytetrafluoroethylene are joined together by welding to lined chemical storage tanks. PTFE is heated at the seam by hot air until it is in a gel state. Simultaneously, a PFA rod is molten and used to fill the seam. 

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Other Techniques. The FEP resin is bonded to metal surfaces by the appHcation of heat and pressure it can be heat sealed or hot-gas welded. Heating FEP at 260°C and allowing it to cool slowly results in stress relieving, or annealing. The FEP film is used to weld PTFE-coated surfaces. 

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. 

Polytetrafluoroethylene (PTFE) is available in highly concentrated dispersions which have to be sintered at temperatures up to 400°C.This means that only glass fibre substrates are suitable for PTFE coatings. Modified PTFE types with thermoplastic properties can be welded. PTFE displays very good chemical stability but reduced mechanical stability. It is transpar-... 

Melt processible fluoroplastics can be directly welded while PTFE requires an adhesive ring. A PFA (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. 14.46). Next, a cylindrical heater that has been heated to 410-425°C is inserted over the tube ends and held in position for 4-5 minutes while a force of 8-12 kg is applied to the tube ends. After removal of the heater at the end of the cycle, the application of force on the tube ends is continued until the weld has cooled down.l l... 

PTFE mouldings and extrudates may be machined without difficulty. Film may be obtained by peeling from a pressure sintered ring and this may be welded to similar film by heat sealing under light pressure at about 350°C. 

Welding is very difficult for PTFE and definitely alters mechanical properties. 

Modified PTFE represents a relatively new technology that is designed to overcome the limitations of conventional PTFE, namely, poor creep resistance (i.e., tendency to cold flow), difficult welding, and high level of microvoids [35], These changes of properties are accomplished by addition of small amount comonomer in amounts less than 0.1%. Such modifier is most frequently perfluoropropylvinyl ether (PPVE). The copolymerization is carried out in aqueous suspension under practically the same conditions as the homopolymerization of tetrafluoroethylene, that is, at TEE pressures in the range 5 to 20 bar (72 to 290 psi) and temperature range 35°C to 90°C (95°E to 194°E) [36]. There is more on modified PTFE in Section 7.7. 

Perfluoro alkoxy oxane PFA 260°C Similar to PTFE, but can be melted, molded and welded... 

Open the jaws of the welder with one hand once it is fully heated. Hold the PTFE strip with the other hand (a safe distance along the strip), and place a section of the strip between the welder jaws, feeding the strip in width-ways (with the 9 cm edge leading), and taking care that the double strip remains flat. Allow the jaws to close briefly. Open the welder and remove the end of the PTFE strip, which now has the bag outline welded into it. 

Take the rectangular piece of doubled PTFE film now imprinted with the bag outline and carefully cut around the edge of the bag. Cut as close as possible to the edge, but do not cut through the welded outline. 

Adhesive- bonded Synthetic and glass backed PVDF(60, 90, 118 mils) ECTFE (60, 90) Glass backed ETFE (60, 90 mils) PTFE (80, 120 mils) FEP ( 60, 90 mils) PFA (90, 110 mils) MFA (60, 90 mils) Contact or thermosetting adhesive Weld rod and Cap strip welds Shop or field Pressure OK Full vacuum up to 120°F (49°C) Max Temp 275°F (135°C) None Visual Spark Possible Testing recommended... 

Loose lining FEP PFA Modified PTFE All 60, 90, 125, 187 mils Liner fabricated outside the housing and slipped inside Liner fabricated by hand and machine welding Shop only No vacuum. Pressure OK Determined by body flange (12) and section height (12) Visual Spark Difficult... 

Figure 7.14 shows a comparison of the stress-strain curves of a conventional and a modified PTFE for the original and welded material. The weld line in conventional PTFE when welded to itself, at best, fails at very low strains. In the case of a modified resin welded to itself, the weld factor attains value of 0.80-0.85. The weld factors for welding of conventional to modified PTFE have been reported in the range of 0.66-0.87.F 1... 

PTFE (and other fluoropol5miers) tubes and lined pipes are welded by the butt fusion technique (Fig. 

PFA tubes and pipe liners can be butt-welded in a similar procedure to that used for PTFE (Sec. 7.4.2). PFA does not require adhesive to form a bond. PFA is easily welded by a heated platen technique in which the liner ends contact the heated plate until a bead is formed. A heater plate temperature of 399°C, a contact time of 15-30 secs, and a soak time of 90-105 secs are required for liners with a diameter of 2.5-... 

With extrusion welding, the support is prepared (beveled or preferably double beveled) and preheated using a hot air gun to reach a surface temperature of 250°C-260°C. The molten extrusion (maximum length— 20 cm) is pressed into the bevel using stainless steel or PTFE coated tools. 

In the extrusion welding technique, the mating parts are heated by pressing onto a metal heater which is held at 250°C-270°C and which has been surface-treated (usually with PTFE) to minimize adhesion. For butt welding, the heater is a flat plate for spigot and socket welding, there are male and female surfaces. The ideal pressure on the hot surface is 50-60 kPa, and the time should be sufficient for the fusion of the material to a depth of 4-5 mm at the contact surface. The heating unit is removed and contact is made under a pressure of ideally 0.06 to... 

Polytetrafluoroethylene (PTFE, or Teflon) is reported to resist friction welding to other polymers even at high speeds and pressures [24]. PTFE has a much lower coefficient of friction and relatively higher softening point among the polymers. To reach the necessary conditions to weld it, sliding speed and pressure should be increased by a factor of 50 and 20 in comparison with high-density polyethylene [19]. 

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