Introduction to Fluoropolymers - …

1 4 Introduction to FluoropolymersSina Ebnesajjad, PhDFluoroConsultants Group, IntroductionFluoropolymer is usually olefinic polymer which consistsof partially or fully fluorinated olefinic monomers such asvinylidene fluoride (CH2 CF2) and tetrafluoroethylene(TFE) (CF2 CF2). These polymers have been covered ingreat detail in a number of references (see General Refer-ences). More specialty fluorinated polymers include per-fluoroethers, fluoroacrylates, and fluorosilicones which areused in significantly smaller volume than olefinic fluo-ropolymers. These Fluoropolymers and others are covered indetail elsewhere[1,2].Commercial Fluoropolymers include homopolymers andcopolymers. Homopolymers contain 99% or more by weightone monomer and 1% or less by weight of another monomeraccording to the convention by American Society for TestingMaterials (ASTM). Copolymers contain more than 1% ormore by weight of one or more comonomers. The majorcommercial Fluoropolymers are based on three monomers:TFE, vinylidene fluoride (VF2), and to a lesser extentchlorotrifluoroethylene (CTFE).

2 Examples of comonomersinclude perfluoromethyl vinyl ether (PMVE), perfluoroethylvinyl ether (PEVE), perfluoropropyl vinyl ether (PPVE),hexafluoropropylene (HFP), CTFE, perfluorobutyl ethylene(PFBE), and exotic monomers such as 2,2-bistri-fluoromethyl-4,5-difluoro-1, Fluoropolymers reviewed in this chapter includepolytetrafluoroethylene (PTFE), perfluoroalkoxy polymer(PFA), fluorinated ethyleneepropylene polymer (FEP), eth-yleneetetrafluoroethylene copolymer (ETFE), ethyleneechlorotrifluoroethylene copolymer (ECTFE), polychlorotri-fluoroethylene (PCTFE), polyvinylidene fluoride (PVDF),and polyvinyl fluoride (PVF). Classification, preparation,properties, fabrication, safety considerations, and economicsof Fluoropolymers are reviewed. Monomer synthesis andproperties have also been good rule of thumb to remember is that increasing thefluorine content of a polymer molecule increases its chemicaland solvent resistance, flame resistance, and photostability;improves its electrical properties such as lower dielectricconstant; lowers coefficient of friction; raises melting point;increases its thermal stability; and weakens its mechanicalproperties.

3 Solubility of polymers in solvents usuallydecreases by increasing the fluorine content of the fluoropolymer ClassificationThe serendipitous discovery of PTFE in 1938 by RoyPlunkett of DuPont Company[3] began the era of fluo-ropolymers. PTFE has found thousands of applicationsbecause of its unique properties. Various fluoroplastics(Figure ) have been developed since the discovery ofPTFE. A number of companies produce these plastics aroundthe world (Tables and ). Fluoropolymers are dividedinto two classes of perfluorinated and partially fluorinatedpolymers. Perfluorinated Fluoropolymers are homopolymersand copolymers of TFE. Some of the comonomers maycontain a small amount of elements other than C or example, PFA is a copolymer of TFE and per-fluoroalkyl vinyl ether that contains oxygen. Rfis a per-fluoroalkyl group of (1937)ETFE(1972)FEP(1960)PFA (1973)PVF (1949)PVDF (1948)AF(1985)ECTFE(1970)PerfluorinatedP artially FluorinatedFirst melt-processible perfluoropolymerProperties similar to PTFEPTFE(1938)Figure of fluoropolymer development over time.

4 (PCTFE polychlorotrifluoroethylene, PVDF polyvinylidenefluoride, PVF polyvinyl fluoride, FEP fluorinated ethyle-neepropylene polymer, ECTFE ethylene chlorotrifluoro-ethylene polymer, ETFE ethyleneetetrafluoroethylenepolymer, PFA perfluoroalkoxy polymer, AF amorphousfluoropolymers).Applied Plastics Engineering HandbookCopyright 2011 Elsevier Inc. All rights fluorinated Fluoropolymers contain hydrogen (H)or other atoms such as chlorine, in addition to fluorine andcarbon. The most significant are homopolymers and copoly-mers of PVDF. There are also copolymers and homopoly-mers of chlorotrifluoroethylene (PCTFE), although somehave elastomeric properties. Other significant fluoroplasticsinclude ETFE, ECTFE, and Polymer Development HistoryPTFE cannot be fabricated by melt-processing techniquesbecause of its high viscosity (1010e1012poise at 380 C).Melt-processible Fluoropolymers have been developed bycopolymerization of TFE. FEP, a copolymer of TFE and HFP,has a lower maximum continuous use temperature than PTFE(200 C vs.)

5 260 C) because of the deterioration ofmechanical properties. PFA, a copolymer of TFE with PPVEor PEVE, offers thermal stability, melt-processibility andmaximum continuous use temperatured260 C. Both FEPand PFA are considered of ethylene with tetrafluoroethylene (ETFE)and chlorotrifluoroethylene (ECTFE) are mechanicallystronger than perfluoropolymers, accompanied with trade-offs of reduction in their chemical resistance and continuoususe temperature and an increase in the coefficient of copolymers of TFE are soluble in specialhalogenated solvents and can be applied to surfaces asa polymer solution to form thin coatings. The dried coating isas resistant to almost as many chemicals as PTFE.[4] Monomer Synthesis of TetrafluoroethyleneThe first reliable and complete description of TFEsynthesis was published in 1933 by Ruff and Bretschneider[5] in which they prepared TFE (CF2 CF2, CAS number116-14-3) from decomposition of tetrafluoromethane in anelectric arc.

6 TFE was obtained by bromination and separationof the dibromide (CF2 BreCF2Br) from the other reactionproducts. Dehalogenation with zinc was the next step forobtaining pure TFE. Commercially significant techniques forTFE preparation list fluorspar (CaF2), hydrofluoric acid, andchloroform as the starting ingredients[6e13], as shown inthe reaction sequence inFigure other compounds produced are HFP and a smallamount of highly toxic [14]and others[15]have provided completedescriptions of the preparation of TFE. The overall yield ofTFE production depends on the pyrolysis reaction. Theproducts of pyrolysis are cooled, scrubbed with a dilute basicsolution to remove HCl, and dried. The resulting gas isHF preparation:Chloroform preparation:Chlorodifluoromethane preparation:TFE synthesis:CaF2 + H2SO4CH4 + 3Cl2 CHCl3 + 2HF2 CHClF2 (pyrolysis)CF2=CF2 + 2 HCl (SbF3 catalyst)CHClF2 + 2 HClCHCl3 + 3 HCl2HF + CaSO4 Figure reactions of producers of fluoropolymersCompanyProductDuPontPTFE, FEP, PFA, ETFE,PVFD aikinPTFE, FEP, PFA, ETFE,PVDF, PCTFE, EeFEPA rkemaPVDF, PCTFEA sahi GlassPTFE, PFA, ETFES olvay SolexisPTFE, PVDF, ECTFE,PFA/MFAD yneonPTFE, FEP, PFA, THV,PVDFC onstructive BureauRussiaPTFE, small amounts ofother PolandPTFEK urehaPVDFH oneywellPCTFET able producers of fluoropolymersCompanyProductGujarat Fluorochemicals FluoropolymerPTFES hanghai 3 FPTFE, PVDF, FEPF uxin HengtongPTFEJ iangsu MeilanPTFEJ inan 3 FPTFE, FEPZ hejiang JushengPTFEZ honghao Chenguang Res , PVDFS handong DongyuePTFE50 APPLIEDPLASTICSENGINEERINGHANDBOOK compressed and distilled to recover the unreacted CHClF2andhigh purity TFE.

7 Polymerization of TFE to high molecularweight requires extreme purity, thus ensuring the removal ofall traces of telogenic hydrogen or chlorine-bearing impuri-ties. TFE can autopolymerize if it is not inhibited by terpenes,such asa-pinene, terpene B, and d-limonene[16]. Synthesis ofHexafluoropropyleneHFP (CF3CF CF2, CAS number 116-15-4) was firstprepared by Benning et al.[17]by pyrolysis. The fullsynthesis and identification of HFP was conducted byHenne[18]. A six step reaction scheme starting with thefluorination of 1,2,3-trichloropropane led to 1,2-dichloro-hexafluoropropane. The latter was dehalogenated with zincin boiling ethanol to yield HFP. HFP is a co-product in thesynthesis of TFE. HFP yield can be increased, in lieu of TFEproduction, by altering the reaction conditions such asreduction in the pyrolysis temperature and use of steam asdiluent of the reactants[19,20]. Synthesis of PerfluoroalkylVinyl EthersPerfluoroalkyl vinyl ethers such as perfluoropropyl vinylether (CF2 CFeOeC3F7, CAS number 1623-05-8) aresynthesized according to the steps shown inFigure as alternative techniques[21].

8 There are other processesincluding electrochemical methods for the production ofperfluoro-2-alkoxy-propionyl fluoride[22]. Synthesis ofChlorotrifluoroethyleneThis monomer is fairly simple to manufacture comparedto the perfluorinated monomers[25e28]. The commercialprocess for the synthesis of CTFE (CF2 CClF, CAS number79-38-9) begins with 1,1,2-trichloro-1,2,2-trifluoroethane(TC TFE). It is dechlorinated by pyrolysis at 500e600 Cinvapor phase. An alternative method for preparation ofTCTFE is catalytic dechlorination:CCl3eCCl3 HF/CCl2 FeCClF2 2 HCl catalystSbClxFy CCl2 FeCClF2 Zn/CFCl CF2 ZnCl2 at50e100 C in methanol The reaction stream is put through a number of purifica-tion and distillation steps to remove the gaseous and liquidcontaminants. CTFE is further purified by the removal ofmethyl chloride, dimethyl ether, and water by passing the gasstream through sulfuric acid. Water and hydrochloric acid areremoved by passing the CTFE through an alumina columnbefore condensing it into a Synthesis of Vinylidene FluorideThere are numerous ways to prepare vinylidene fluoride(VDF) (CF2 CH2, CAS number 75-38-7)[29,30].

9 TwoFigure of perfluoroalkyl vinyl ethers[23e24].S. EBNESAJJAD51methods, including the popular commercial technique forVDF production, are described. Conversion of 1,1,1-tri-fluoroethane[31]begins by passing this gas througha platinum-lined Inconel tube, which is heated to 1200 time is about seconds. The exit gases arepassed through a sodium fluoride bed to remove thehydrofluoric acid and are then collected in a liquidnitrogen trap. VDF is separated by low temperaturedistillation. Unreacted trifluoroethane is removed at Cand is commercial method begins with hydrofluorination ofacetylene followed by chlorination[32], by hydrofluorinationof trichloroethane[33], or by hydrofluorination of vinylidenechloride[34]. In each case the final product, 1-chloro-1,1-difluoroethane, is stripped of a molecule of hydrochloricacid to yield Synthesis of Vinyl FluorideVinyl fluoride (VF)(CHF CH2, CAS number 75-02-5)[35e39] was first prepared by the reaction of 1,1-difluoro-2-bromoethane (CAS number 359-07-9) with zinc.

10 Mostapproaches to VF synthesis have employed reactions ofacetylene (CAS number 74-86-2) with hydrogen fluoride(HF) either directly or utilizing catalysts. Other routes haveinvolved ethylene (CAS number 74-85-1) and HF, pyrolysisof 1,1-difluoroethane (CAS number 624-72-6) and fluoro-chloroethanes, reaction of 1,1-difluoroethane with acetylene,and halogen exchange of vinyl chloride (CAS number 75-01-4)with HF[40e42]. Monomer Properties of TetrafluoroethyleneTetrafluoroethylene15 , 44is a colorless, odorless, taste-less, nontoxic gas which boils at C and melts at C. The critical temperature and pressure of TFE C and MPa. TFE is stored as a liquid; vaporpressure at 20 C is 1 MPa. Its heat of formation is reportedto be kcal/mole. Polymerization of TFE is highlyexothermic and generates kcal/mole heatdthe extentof which can be compared with the heats of polymerizationof vinyl chloride and styrene, at 23e26 kcal/mole kcal/mole, storage of TFE requires its oxygen content to be lessthan 20 ppm.