Fusible polytetrafluoroethylene (PFA) is a very important new fluorine-containing resin. Its chemical stability, physical and mechanical properties, electrical insulation, lubricity, non viscosity, aging resistance, non flammability and thermal stability are very good. Similar to PTFE, its high temperature mechanical strength is about 2 times higher than that of PTFE. Moreover, it can be molded by extrusion, blow molding, injection molding and other general thermoplastic plastics Processing technology. This is because PFA contains 1wt% - 10wt% of perfluoropropyl vinyl ether (ppve), which significantly improves the flexibility of PFA polymer chain, reduces the crystallinity of PFA, and makes PFA have good thermoplastic properties. The melt viscosity of PFA is reduced from 109 to 104 ~ 106 poise, which overcomes the disadvantage of difficult processing of polytetrafluoroethylene.
At present, the world's main manufacturers are Du Pont, Daikin, Asahi Glass and dyneon. The global consumption in 2002 was about 5 kt / A, 80% of which was used in the semiconductor manufacturing industry.
PFA polymerization process:
In a 1L stainless steel autoclave, after vacuum removing O2, start stirring. After adding 800ml perfluoro-n-methylmorpholine (pfnmm), 0.38g ethane chain transfer agent and 20ml ppve, the temperature was raised to 60 ℃, the stirring speed was 1000 R / min, TFE was turned on until 0.5 MPa. Then, 1 ml of fc-ll3 solution was injected into the pump to start polymerization. During the polymerization, TFE was added continuously to keep the reactor pressure between 0.5 MPa and 0.6 MPa, and the initiator solution was continuously injected at the rate of 0.5 ml / min. At the end of polymerization, the polymer slurry was cooled, vented and discharged. After most of the solvent was volatilized at room temperature, it was heated to 150 ℃ to drive away the residual solvent with air to obtain 14.3g resin. The melting viscosity of PFA is 2.55x10 poise and the content of ppve is 4.44wt%.
After the synthesis of PFA resin, the unstable end group needs post-treatment process to convert the unstable end group into the stable end group, then the high-purity PFA is prepared. The unstable end groups can be transformed into two types of stable end groups: - CONH2, - CF3. Correspondingly, there are two treatment methods of unstable end groups: (1) PFA treated with NH3 and ammonium salt; (2) PFA treated with fluorine gas.
In the 21st century, the semiconductor manufacturing industry has put forward higher requirements for the surface finish of PFA products. It requires that the inner surfaces of containers, pipes, valves and pumps made with PFA should be smooth, and the impurities such as particles should be prevented from adhering and retaining on the walls, and the products would not pollute the stored and transported high-purity chemicals and ultra-pure water, and be convenient for cleaning.
The surface roughness of PFA products is related to the size of PFA crystal nucleus. When the spherulite size is less than or equal to 15u, the surface of PFA products is relatively smooth.
Dupond company uses PTFE and modified PTFE as nucleating agents for PFA, adding them in the process of PFA processing, increasing the number of spherulites to reduce the size of crystal nucleus, and controlling the size of crystal nucleus in PFA component ≤ 11u. Daikin company uses amorphous fluoropolymer or copolymer containing amorphous block as spherulite micromanizing agent, which can slow down the growth rate of spherulite in PFA. When PFA products are cooled slowly, spherulite does not grow, so that the spherulite size is less than 5u.