Fusible polytetrafluoroethylene (PFA) is a very important new type of fluororesin. Its chemical stability, physical and mechanical properties, electrical insulation properties, lubricity, non-stickiness, aging resistance, non-combustibility and thermal stability The properties are very good, similar to PTFE, the high temperature mechanical strength is about 2 times higher than that of PTFE; and, it can be processed by extrusion, blow molding, injection molding and other general thermoplastic molding processes. This is because the composition of PFA contains 1wt%-10wt% of perfluoropropyl vinyl ether (PPVE), which significantly improves the flexibility of the PFA polymer chain, reduces the crystallinity of PFA, and makes PFA have good thermoplasticity. The melt viscosity of 109 poise is reduced to about 104-106 poise, which overcomes the shortcomings of difficult processing of polytetrafluoroethylene.

     Currently, there are four major manufacturers in the world: Du Pont, Daikin, AsahiGlass, and Dyneon. The global consumption in 2002 was about 5 kt/a, of which 80% were used in the semiconductor manufacturing industry.

     PFA polymerization process:

     In a 1L stainless steel autoclave, after vacuuming to remove O2, start stirring. After adding 800ml of perfluoro-N-methylmorpholine (PFNMM), 0.38g of ethane chain transfer agent, and 20ml of PPVE, the temperature is raised to 60℃, the stirring speed is 1000 r/min, and TFE is passed until 0.5 MPa, and then 10ml is injected with a pump The FC-ll3 solution of bisperfluoropropionyl is oxidized to start the polymerization reaction. During the polymerization, continuously add TFE to keep the kettle pressure between 0.5 and 0.6 MPa; continuously inject the initiator solution at a rate of 0.5 ml/min. After the polymerization, the polymer slurry was cooled, vented, and discharged. After most of the solvent was evaporated at room temperature, the mixture was heated to 150° C., and air was used to drive out the remaining solvent to obtain 14.3 g of resin. After analysis, the melt viscosity of PFA was 2.55×10 poise, and the content of PPVE was 4.44 wt%.

    After the PFA resin is synthesized, a post-treatment process of unstable end groups is required to convert the unstable end groups into stable end groups to prepare high-purity PFA. Unstable end groups can be converted into two types of stable end groups: -CONH2, -CF3. Accordingly, there are two treatment methods for unstable end groups: (1) NH3 and ammonium salt treatment of PFA; (2) fluorine gas Deal with PFA.

In the 21st century, the semiconductor manufacturing industry has put forward higher requirements for the surface finish of PFA products. The inner surfaces of containers, pipes, valves, and pumps made of PFA are required to be smooth, to prevent particles and other impurities from adhering and staying on the walls. It will contaminate the storage and transportation of high-purity chemicals, ultra-pure water, and facilitate cleaning.

The surface roughness of the PFA product is related to the size of the PFA crystal nucleus. When the spherulite size is less than or equal to 15u, the surface of the PFA product is relatively smooth.

     DuPond company uses PTFE and modified PTFE as the nucleus-forming agent of PFA, which is added during PFA processing to increase the number of spherulites to reduce the size of the crystal nucleus, which can control the size of the crystal nucleus in the PFA component ≤ 11u. Daikin uses amorphous fluoropolymers or copolymers containing amorphous blocks as the spherulite-micronizing agent. Blending with PFA can slow down the growth rate of spherulite in PFA. In the case of slow cooling, the spherulites do not even grow, so that the size of the spherulites ≤5u.

可熔性聚四氟乙烯PFA的聚合工艺

可熔性聚四氟乙烯(PFA)是一类很重要的含氟树脂新品，它的化学稳定性能、物理机械性能、电绝缘性能、润滑性、不粘性、耐老化性、不燃性和热稳定性都非常优良，与PTFE相似，高温机械强度比PTFE高2倍左右；并且，能用挤塑、吹塑、注塑等一般热塑性塑料的成型加工工艺进行加工。这是因为PFA组成中含有1wt％-10wt％的全氟丙基乙烯基醚(PPVE)，显著改善了PFA高分子链的柔顺性，降低了PFA的结晶度，使PFA具有良好的热塑性，从109泊的熔融粘度降至104～106泊左右，克服了聚四氟乙烯难加工的缺点。

     现世界上生产厂家主要有Du Pont、Daikin、AsahiGlass、Dyneon四家公司，2002年的全球消费量为5 kt/a左右，其中80％应用于半导体制造工业。

     PFA聚合过程：

     在一个1L的不锈钢高压釜中，抽真空除O2后，开启搅拌．加入800ml全氟-N-甲基吗啉(PFNMM)、0.38g乙烷链转移剂、20mlPPVE后，升温至60℃，搅拌转速1000 r/min，通TFE直至0.5 MPa，再用泵注入1Oml过氧化双全氟丙酰的FC-ll3溶液，开始聚合反应。在聚合期间，连续添加TFE、使釜压保持在0.5~0.6 MPa之间；连续以0.5ml/min的速度注入引发剂溶液。聚合结束后，冷却、放空、放出聚合物浆料，室温下挥发掉大部分溶剂后，加热至150℃，用空气驱赶残留的溶剂，得14.3g树脂。经分析，PFA的熔融粘度为2．55x10泊，PPVE含量为4.44wt％。

    PFA树脂合成出来后，需进行不稳定端基的后处理工艺，把不稳定端基转化为稳定端基，就制备出高纯度的PFA。不稳定端基可转化为两种类型的稳定端基：-CONH2、-CF3，相应地，有两种不稳定端基的处理方法：(1)NH3和铵盐处理PFA；(2)氟气处理PFA。  

进入21世纪，半导体制造工业又对PFA制品的表面光洁度提出了更高的要求，要求用PFA制造的容器、管道、阀门、泵的内表面光滑，抑制粒子等杂质在壁上附着、滞留，不会污染所贮存、输送高纯化学品、超纯水，且利于清洗。

PFA制品表面的粗糙度与PFA的晶核尺寸大小有关，当球晶尺寸≤15u时，PFA制品表面就比较光滑。

     DuPond公司采用PTFE和改性PTFE作为PFA的成核剂(nucleus-formingagent)，在PFA加工过程中加入，增加球晶的数量以减少晶核的尺寸，可控制PFA组份中的晶核尺寸≤11u。Daikin公司则采用无定型含氟聚合物或含有无定型嵌段的共聚物作为球晶*小化剂(spherulite-micronizingagent)，与PFA共混，可减慢PFA中球晶生长速度，在PFA制品缓慢冷却的情况下，球晶甚至不生长，从而使球晶尺寸≤5u。