13

Fluorine-containing Polymers

13.1 INTRODUCTION

The high thermal stability of the carbon-fluorine bond has led to considerable interest in fluorine-containing polymers as heat-resistant plastics and rubbers. The first patents, taken out by IG Farben in 1934, related to polychlorotri- fluoroethylene' (PCTFE) (Figure 13.1 (a)), these materials being subsequently manufactured in Germany and the United States. PCTFE has been of limited application and it was the discovery of polytetrafluoroethylene (PTFE) (Figure 13.1 (b)) by Plunkett2 in 1938 which gave an impetus to the study of fluorine- containing polymers.

The inability to process PTFE by conventional thermoplastics techniques has nevertheless led to an extensive search for a melt-processable polymer but with similar chemical, electrical, non-stick and low-friction properties. This has resulted in several useful materials being marketed, including tetrafluoro- ethylene-hexafluoropropylene copolymer, poly(viny1idene fluoride) (Figure 13.1 (d)), and, most promisingly, the copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether. Other fluorine-containing plastics include poly(viny1 fluoride) and polymers and copolymers based on CTFE.

The fluororubbers also form an important class of speciality elastomers and although the market is dominated by the vinylidene fluoride-hexafluoro- propylene copolymers a wide range of materials has been produced over the past 40 years.

(a) (h) ((.) (4 Figure 13.1. (a) Polychlorotrifluoroethylene (PCTFE). (b) Polytetrafluoreoethylene (PTFE).

(c) Poly(viny1 fluoride), (d) Poly(viny1idene fluoride)

363

364 Fluorine-containing Polymers

World-wide capacity for fluoropolymers in the late 1990s has been estimated at 90 000 t.p.a. divided roughly equally between Western Europe, North America and the rest of the world. This total is dominated by PTFE although this has decreased from about 80% of the total in 1980 to about 60% in the late 1990s.

13.2 POLYTETRAFLUOROETHYLENE

In addition to the presence of stable C-F bonds, the PTFE molecule possesses other features which lead to materials of outstanding heat resistance, chemical resistance and electrical insulation characteristics and with a low coefficient of friction. It is today produced by a number of chemical manufacturers such as Du Pont (Teflon), IC1 (Fluon), Hoechst (Hostaflon TF), Rh6ne-Poulenc (Soreflon), Montecatini (Algoflan), Nitto Chemical-Japan (Tetraflon) and Daikin Kogyo- Japan (Polyflon).

13.2.1. Preparation of Monomer

Tetrafluoroethylene was first prepared in 1933. The current commercial syntheses are based on fluorspar, sulphuric acid and chloroform.

The reaction of fluorspar (CaF2) and sulphuric acid yields hydrofluoric acid

CaF2 + H2S04 __j CaS04 + 2HF

Treatment of chloroform, obtained by reacting methanol and chlorine, with the hydrofluoric acid yields monochlorodifluoromethane, also used as a refrigerant, which is a gas boiling at -4023°C.

CHC1, + 2HF - CHClF2 + 2HC1

The monochlorodifluoromethane may be converted to tetrafluoroethylene by pyrolysis, for example by passing through a platinum tube at 700°C.

2CHClF2 + CF2 CF2 + 2HC1

Other fluorine compounds are produced during pyrolysis, including some highly toxic ring structures. Since very pure monomer is required for polymerisation, the gas is first scrubbed to remove any hydrochloric acid and then distilled to separate other impurities. Tetrafluoroethylene has a boiling point of -76.3"C. For safe storage under pressure the oxygen content should be below 20 ppm. Traces of compounds which react preferentially with oxygen such as 0.5% dipentene, benzaldehyde or methyl methacrylate may be added as stabilisers.

13.2.2 Polymerisation Pure uninhibited tetrafluoroethylene can polymerise with violence, even at temperatures initially below that of room temperature. There is little published information concerning details of commercial polymerisation. In one patent3 example a silver-plated reactor was quarter-filled with a solution consisting of 0.2 parts ammonium persulphate, 1.5 parts borax and 100 parts water, and with a pH of 9.2. The reactor was closed and evacuated, and 30 parts of monomer

Polytetrafuoroethylene 365

were let in. The reactor was agitated for one hour at 80°C and after cooling gave an 86% yield of polymer.

PTFE is made commercially by two major processes, one leading to the so called ‘granular’ polymer and the second leading to a dispersion of polymer of much finer particle size and lower molecular weight. One method of producing the latter4 involved the use of a 0.1 % aqueous disuccinic acid peroxide solution. The reactions were camed out at temperatures up to 90°C. It is understood that the Du Pont dispersion polymers, at least, are produced by methods based on the patent containing the above example.

13.2.3 Structure and Properties

Polytetrafluoroethylene is a linear polymer free from any significant amount of branching (Figure 13.2).

Figure 13.2

Whereas the molecule of polyethylene is in the form of planar zigzag in the crystalline zone this is sterically impossible with that of PTFE due to the fluorine atoms being larger than those of hydrogen. As a consequence the molecule takes up a twisted zigzag, with the fluorine atoms packing tightly in a spiral around the carbon-carbon skeleton. A complete turn of the spiral will involve over 26 carbon atoms below 19°C and 30 above it, there being a transition point involving a 1% volume change at this temperature. The compact interlocking of the fluorine atoms leads to a molecule of great stiffness and it is this feature which leads to the high crystalline melting point and thermal form stability of the polymer.

The intermolecular attraction between PTFE molecules is very small, the computed solubility parameter being 12.6 (MJ/m3)’’2. The polymer in bulk does not thus have the high rigidity and tensile strength which is often associated with polymers with a high softening point.

The carbon-fluorine bond is very stable. Further, where two fluorine atoms are attached to a single carbon atom there is a reduction in the C-F bond distance from 1.42 A to 1.35 A. As a result bond strengths may be as high as 504 kJ/mole. Since the only other bond present is the stable C-C bond, PTFE has a very high heat stability, even when heated above its crystalline melting point of 327°C.

Because of its high crystallinity and incapability of specific interaction, there are no solvents at room temperature. At temperatures approaching the melting point certain fluorinated liquids such as perfluorinated kerosenes will dissolve the polymer.

The properties of PTFE are dependent on the type of polymer and the method of processing. The polymer may differ in particle size and/or molecular weight. The particle size will influence ease of processing and the quantity of voids in the finished product whilst the molecular weight will influence crystallinity and

366 Fluorine-containing Polymers

hence many physical properties. The processing techniques will also affect both crystallinity and void content.

The weight average molecular weights of commercial polymers appear to be very high and are in the range 400 000 to 9 000 000. IC1 report that their materials have a molecular weight in the range 500 000 to 5 000 000 and a percentage crystallinity greater than 94°C as manufactured. Fabricated parts are less crystalline. The degree of crystallinity of the finished product will depend on the rate of cooling from the processing temperatures. Slow cooling will lead to high crystallinity, with fast cooling giving the opposite effect. Low molecular weight materials will also be more crystalline.

Figure

240

2.50 * c z * 220 = 0 5 0 w L v)

240

0 20 40 60 80 loo 2.00

CRYSTALLINITY IN '/a

?.3. Density as a function of crystallinity in PTFE (After Thomas et i

Figure 13.3 shows the relationship between percentage crystallinity and specific gravity at 23°C. By measuring the specific gravity of mouldings prepared under rigorously controlled conditions Thomas' and co-workers were able to obtain a measure of molecular weight which they were able to calibrate with results obtained by end-group and infrared analysis (Figure 13.4).

Figure 13.4. Standard specific gravity of FTFE as a function of molecular weight. (After Thomas et a i .3

It is observed that the dispersion polymer, which is of finer particle size and lower molecular weight, gives products with a vastly improved resistance to flexing and also distinctly higher tensile strengths. These improvements appear to arise through the formation of fibre-like structures in the mass of polymer during processing.

Polytetrafluoroethylene 361

There has been some recent interest in polymers containing very small proportions (<2000 ppm) of a second comonomer. These can interfere with crystallisation and the resulting products are claimed to have improved compression strength, electrical insulation properties, weldability and transpar- ency compared with the unmodified homopolymers.

13.2.4 General Properties

PTFE is a tough, flexible, non-resilient material of moderate tensile strength but with excellent resistance to heat, chemicals and to the passage of an electric current. It remains ductile in compression at temperatures as low as 4K (-269°C).

Table 13.1 lists some typical values of FTFE mouldings compared with other fluorine-containing thermoplastics.

As with other plastics materials, temperature has a considerable effect on mechanical properties. This is clearly illustrated in Figure 13.5 in the case of stress to break and elongation at break. Even at 20°C unfilled FTFE has a measurable creep with compression loads as low as 3001bf/in2 (2.1 MPa).

I I I I I 1600

I I 0 40

TLHPERATURE IN 'C

Figure 13.5. Effect of temperature on the stress at break and elongation at break of PTEF.' (Reproduced by permission of IC1 Plastics Division)

The coefficient of friction is unusually low and stated to be lower than that of any other solid. A number of different values have been quoted in the literature but are usually in the range 0.02-0.10 for polymer to polymer.

PTFE is an outstanding insulator over a wide range of temperature and frequency. The volume resistivity (100 s value) exceeds lo2' 0 m and it appears that any current measured is a polarisation current rather than a conduction current. The power factor is negligible in the temperature range -60°C to +250"C at frequencies up to 10"Hz. The polymer has a low dielectric constant similarly unaffected by frequency. The only effect of temperature is to alter the density which has been found to influence the dielectric constant according to the relationship

1 + 0.238D Dielectric constant =

1 - 0.1190 where D = specific gravity

8 4

3 4

Polytetrafluoroethylene 369

Figure 13.6 shows the influence of temperature on specific volume (reciprocal specific gravity). The exact form of the curve is somewhat dependent on the crystallinity and the rate of temperature change. A small transition is observed at about 19°C and a first order transition (melting) at about 327°C. Above this temperature the material does not exhibit true flow but is rubbery. A melt viscosity of 10'o-lO" poises has been measured at about 350°C. A slow rate of decomposition may be detected at the melting point and this increases with a further increase in temperature. Processing temperatures, except possibly in the case of extrusion, are, however, rarely above 380°C.

Figure 13.6. Variation of specific volume of PTFE with (Reproduced by permission of IC1 Plastics Division)

The chemical resistance of PTFE is exceptional. There are no solvents and it is attacked at room temperature only by molten alkali metals and in some cases by fluorine. Treatment with a solution of sodium metal in liquid ammonia will sufficiently alter the surface of a PTFE sample to enable it to be cemented to other materials using epoxide resin adhesives.

Although it has good weathering resistance, PTFE is degraded by high-energy radiation. Exposure to a dosage of 70Mrad will halve the tensile strength of a given sample. The polymer is not wetted by water and does not measurably absorb it. The permeability to gases is low, the water vapour transmission rate being approximately half that of low-density polyethylene and poly(ethy1ene terephthalate).

13.2.5 Processing

PTFE is normally available in three forms:

(1) Granular polymers with median particle size of 300 and 600 pm. ( 2 ) Dispersion polymer obtained by coagulation of a dispersion. It consists of

agglomerates with an average diameter of 450pm made up of primary particles 0.1 pm in diameter.

(3) Dispersions (latices) containing about 60% polymer in particles with an average diameter of about 0.16 pm.

The exceptionally high melt viscosity above the melting point (about 10"- 10" poises at 350°C) prevents the use of the usual techniques for

370 Fluorine-containing Polymers

processing thermoplastics. In the case of granular polymers, methods allied to those used with ceramics and in powder metallurgy are employed instead. In principle this involves preforming the powder, usually at room temperature, sintering at a temperature above the melting point, typically at about 370"C, and then cooling.

Preforming is carried out by compressing sieved powder that has been evenly loaded into a mould. It has been shown' that if the pressure is too low there is an excessively large void content whereas if the pressure is too high cleavage planes are produced by one portion of polymer sliding over another. Best results are obtained using a pressure of about 1 tonf/in2 (16 MPa). If the powder is preheated at 100°C Immediately before preforming, optimum results are obtained at performing pressures of 3.5 tonf/in2 (54MPa). Mouldings made from reforms prepared in this way, i.e. using preheated powder, are found to have tensile strengths appreciably higher than those using cold powder (e.g. 2800 lbf/in2 (1 9 MPa) compared with about 2000 lbf/in2 (14 MPa)).

For many applications it is found that the technique of free sintering is quite satisfactory. This simply involves heating the preform in an oven at about 380°C for a time of 90 minutes plus a further 60 minutes for every 0.25 in (0.65cm) thickness. For example a sample 0.5 in (1.25 cm) thick will require sintering for 3.5 hours. The ovens should be ventilated to the open air to prevent toxic decomposition products accumulating in the working area.

After sintering, the moulding is cooled. Thin sections, i.e. less than &in (0.5 cm) thick, may be removed from the oven and allowed to cool naturally or may be quenched by placing between cold flat metal plates and light pressure applied. Sections up to 1 in (2.5 cm) thick are preferably cooled in an oven, cooling at a rate of 20°C/h when maximum dimensional stability is required. Thicker sections are usually cooled under pressure. In this case the preform can be sintered in the preform mould and the mould and moulding then transferred to a press in which they are cooled under a gradually increased pressure. It is necessary that this should reach the preforming pressure as the sample goes through the transition temperature (327°C) and it should be maintained until the sample reaches room temperature.

Shrinkage of about 5-10% occurs at right angles to the direction of the preforming force. The amount of shrinkage is mainly dependent on the rate of cooling, but also to a minor extent, on the preforming pressure.

The above process is limited to simple shapes whose principal dimension is not more than four times, and preferably less than twice, that of the next largest dimension. More intricate shapes must be made by machining or in some instances by a coining operation which involves stamping a sintered moulding of the same weight and approximate dimensions as the finished part at 320°C.

Granular polymer may also be extruded, albeit at very low rates (1-6 in/min, 2.5-16 cm/min), by means of both screw and ram extruders. In both machines the extruder serves to feed cold powder into a long, heated sintering die whose overall length is about 90 times its internal diameter. The polymer, preferably a presintered grade of 600 pm particle size, is compacted, sintered and partially cooled before leaving the die. Since compacting is still taking place as the polymer enters the sintering zone it is possible to obtain extrudates reasonable free from voids, a factor which is reflected in their high tensile strength and elongation.

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.

Polytetrafluoroethylene 31 1

Dispersion polymer, which leads to products with improved tensile strength and flex life, is not easily fabricated by the above techniques. It has, however, been found possible to produce preforms by mixing with 15-25% of a lubricant, extruding and then removing the lubricant and sintering. Because of the need to remove the lubricant it is possible to produce only thin-section extrudates by this method.

In a typical process a preform billet is produced by compacting a mixture of 83 parts PTFE dispersion polymer and 17 parts of petroleum ether (100-120°C fraction). This is then extruded using a vertical ram extruder. The extrudate is subsequently heated in an oven at about 105°C to remove the lubricant, this being followed by sintering at about 380°C. By this process it is possible to produce thin-walled tube with excellent flexing fatigue resistance and to coat wire with very thin coatings or polymer.

Tape may be made by a similar process. In this case the lubricant selected is a non-volatile oil. The preform is placed in the extruder and a rod extruded. The rod is then passed between a pair of calender rolls at about 60-80°C. The unsintered tape is often used for lapping wire and for making lapped tube. Sintering is carried out after fabrication. The current most important application of unsintered tape is in pipe-thread sealing.

If sintered tape is required, the product from the calender is first degreased by passing through boiling trichlorethylene and then sintered by passing through a salt bath. This tape is superior to that made by machining from granular polymer mouldings.

PTFE dispersions' may be used in a variety of ways. Filled PTFE moulding material may be made by stirring fillers into the dispersion, coagulating with acetone, drying at 280-290°C and disintegrating the resulting cake of material. Asbestos and glass cloth fabrics may be impregnated with PTFE by passing through the dispersion, drying and sintering the polymer. Glass-cloth PTFE laminates may be produced by plying-up layers of impregnated cloth and pressing at about 330°C. The dispersions are also used for coating PTFE on to metal to produce surfaces which are non-adhesive and which have a very low coefficient of friction.

Whenever PTFE is used in a sintered form there are two points that should always be borne in mind. Firstly, at sintering temperatures toxic cyclic fluorinated compounds are formed and it is thus necessary to ventilate ovens and to use fume hoods whenever fumes of such toxic compounds are produced. It is particularly important that dust should not contaminate cigarettes or tobacco since the smoker will inhale the decomposition products. Secondly, scrupulous standards of cleanliness are necessary to prevent dust, which is frequently organic in nature, contaminating PTFE products before sintering. If this did happen organic dust would carbonise on sintering leaving a product both unsightly and with inferior electrical properties.

13.2.6 Additives Because of the high processing temperatures there are few pigments suitable for use with PTFE. A number of inorganic pigments, particularly the cadmium compounds, iron oxides and ultramarines, may, however, be used.

The resistance of FTFE to creep can be improved by blending in up to 25% of glass or asbestos fibre using PTFE dispersions as mentioned in the previous section. By the same technique alumina, silica and lithia may be incorporated to

312 Fluorine-containing Polymers

give compounds of improved dimensional stability coupled with good electrical insulation properties. Molybdenum disulphide and graphite improve dimensional stability without losing the low coefficient of friction whilst the use of barium ferrite will produce a material that can be magnetised. The incorporation of titanium dioxide serves to increase the dielectric constant whilst certain compounds of boron increase the resistance to neutron bombardment.

%

35 25 15

25

13.2.7 Applications

The use of PTFE in a great diversity of applications may be ascribed to the following properties:

Consumption (t.p.a.) %

1500 42

830 22 900 25 370 11

3600

(1) Chemical inertness. ( 2 ) Exceptional weathering resistance. Samples exposed in Florida for 10 years

showed little change in physical properties. ( 3 ) The excellent electrical insulation characteristics. (4) The excellent heat resistance. (5) The non-adhesive properties. (6) The very low coefficient of friction.

However, world production is only about 55 000 tonnes per annum and this is a reflection of the high volume cost, the rather specialised techniques involving lengthy processing times and to a smaller extent the high creep rate under load.

Because of its chemical inertness over a wide temperature range it is used in a variety of seals, gaskets, packings, valve and pump parts and in laboratory equipment.

Its excellent electrical insulation properties lead to its use in wire insulation, in valve holders, in insulated transformers, in hermetic seals for condensers, in laminates for printed ciruitry and for many other miscellaneous electrical applications.

PTFE is used for lining chutes and coating other metal objects where low coefficients of friction or non-adhesive characteristics are required. Because of its excellent flexing resistance, inner linings made from dispersion polymer are used in flexible steam hose. A variety of mouldings are used in aircraft and missiles and also in other applications where use at elevated temperatures is required.

Table 13.2 Consumption of fluorine-containing plastics for Western Europe 1991

I PTFE I Thermoplastic fluor0 materials

Consumption (t.p.a.)

I

Chemical industry Motor construction Electrical engineering Wire/cable insulation Coating Other Total

3 900 2 700 1700

2 700 11 000

Tetrafluoroethylene-Hexafluoropropylene Copolymers 373

Because of its high volume cost PTFE is not generally used to produce large objects. In many cases, however, it is possible to coat a metal object with a layer of PTFE and hence meet the particular requirement.

One significant development in recent years has been the widespread treatment of clothing fabrics to give a measure of water and stain resistance. Mention may also be made of the expression 'Teflon-coated' (Teflon is the DuPont trade name for PTFE) to describe a person, usually a politician, to whom no dirt (i.e. scandal) sticks, a reflection of the non-stick characteristics of the polymer!

Some indication of the relative importance of the various applications is given by the consumption breakdown for fluorine-containing plastics in Western Europe in 1991 is (Table 13.2).

13.3 TETRAFLUOROETHYLENE-HEXAFLUOROPROPYLENE COPOLYMERS

These materials were first introduced by Du Pont in 1956 and are now known as Teflon FEP resins. (FBP = fluorinated ethylene-propylene.) Subsequently other commercial grades have become available (Neoflon by Daikin Kogyo and Teflex by Niitechim, USSR). These copolymers may be regarded as the first commercial attempt to provide a material with the general properties of PTFE and the melt processability of the more conventional thermoplastics.

The commercial polymers are mechanically similar to PTFE but with a somewhat greater impact strength. They also have the same excellent electrical insulation properties and chemical inertness. Weathering tests in Florida showed no change in properties after four years. The material also shows exceptional non-adhesiveness. The coefficient of friction of the resin is low but somewhat higher than that of PTFE. Films up to 0.010 in thick show good transparency.

The maximum service temperature is about 60°C lower than that of PTFE for use under equivalent conditions. Continuous service at 200°C is possible for a number of applications. The polymer melts at about 290°C.

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 102s-') than with low-density polyethylene (about IO3 s-l) or nylon 66 (about lo5 s-l). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds.

The advantage of being able to injection mould and extrude these copolymers has perhaps had a less marked effect than might have been expected. This is because the fabrication of PTFE has been developed by firms closely related to the engineering industries rather than by the conventional plastics fabricators. The PTFE fabricators, because they do not normally possess conventional injection moulding and extrusion machines, would see no obvious advantage in melt processability. At the same time the conventional plastics fabricators, if they wished to enter the field of fabricated fluorine-containing thermoplastics, would have to modify their existing machinery in order to cope with the processing temperatures and high melt viscosity. In spite of these retarding influences the use of FEP copolymers has grown steadily.

At the present time they are used for a variety of electrical and chemically resistant mouldings, for corrosion-resistant linings, for coatings, for flexible printed circuits and for wire insulation. One particular growth area arising from

374 Fluorine-containing Polymers

the inherent flame retardancy has been for wire and cable insulation, particularly for data networks and for optical fibre insulation.

In the mid-1980s Hoechst introduced a related material, Hostaflon TFB, a terpolymer of tetrafluoroethylene, hexafluoropropene and vinylidene fluoride.

1 3.4 TETRAFLUOROETHYLENE-ETHYLENE COPOLYMERS (ETFE)

A second melt-processable copolymer containing tetrafluoroethylene residues was introduced by Du Pont in 1972 as Tefzel. This material is similar in many properties to the TFE-HFP copolymers but claimed to have exceptional abrasion resistance for a fluorine-containing plastics material. It also has very high impact strength and does not fracture in a notched Izod test at room temperature when subjected to impact stresses as high as 20 ft lbf in-' (10.9 kgf m cm-I). Unlike PTFE it cross-links during irradiation. It also differs from PTFE in that glass fibre actually reinforces the polymer, giving tensile strengths as high as 12 0001b/in2 (85/MPa).

This copolymer has proved particularly suitable for wire and cable insulation, with many grades being rated at 155°C for 20000h continuous exposure. It is extensively used in electrical systems for aircraft, underground railways, computers, telecommunications installations and heating circuits. Because of its toughness combined with its heat and chemical resistance it also finds use for lining pumps and valves and other equipment for the chemical industry and for laboratory ware.

Typical physical properties are listed in Table 13.1. Whereas Tefzel is said to be an internally stablised copolymer of TFE and

ethylene, other copolymers that are compounds of similar copolymers with stabilisers of antioxidants are now also available (Hostaflon ET by Hoechst and Aflon by Asahi Glass Co.). Glass-fibre-filled grades are also available.

1 3.5 POLYCHLOROTRIFLUOROETHYLENE POLYMERS (PCTFE) AND COPOLYMERS WITH ETHYLENE (ECTFE)

Polychlorotrifluoroethylene was the first fluorinated polymer to be produced on an experimental scale and polymers were used in Germany and in the United States early in World War 11. PCTFE was used, in particular, in connection with the atomic bomb project in the handling of corrosive materials such as uranium hexafluoride.

The monomer may conveniently be produced from hexachloroethane via trichlorotrifluoroethane

HF Z" CC13*CC13 -----+ CClFZ.CC12F + CF2=CFCl

Polymerisation may be carried out by techniques akin to those used in the manufacture of PTFE. The preparation of polymers in yields of up to 88% are described in one patent.' Water was used as a diluent in concentrations of from one to five times the weight of the monomer, a gas with boiling point of -27.9"C. Solid polymers were formed with reaction temperatures of W 0 " C ; at higher reaction temperatures liquid polymers are formed.

Polychlorotr@oroethylene Polymers (PCTFE) 375

Pressures varied from 20 to 1500 lbf/in2 (0.14 to 10.5 MPa) and reaction times were of the order of 5-35 hours. Reaction promoters included peroxides and salts of persulphuric and perphosphoric acids. ‘Activators’, ‘accelerators’ and buffering agents were also discussed in the patent. The process of manufacture of Kel-F is understood to be based on this patent.

The major differences in properties between PTFE and PCTFE can be related to chemical structure. The introduction of a chlorine atom, which is larger than the fluorine atom, breaks up the very neat symmetry which is shown by PTFE and thus reduces the close chain packing. It is still, however, possible for the molecules to crystallise, albeit to a lower extent than PTFE. The introduction of the chlorine atom in breaking up the molecular symmetry appears to increase the chain flexibility and this leads to a lower softening point. On the other hand the higher interchain attraction results in a harder polymer with a higher tensile strength. The unbalanced electrical structure adversely affects the electrical insulation properties of the material and limits its use in high-frequency applications.

Because of the lower tendency to crystallisation it is possible to produce thin transparent films.

The chemical resistance of PCTFE is good but not as good as that of PTFE. Under certain circumstances substances such as chlorosulphonic acid, molten caustic alkalis and molten alkali metal will adversely affect the material. Alcohols, acids, phenols and aliphatic hydrocarbons have little effect but certain aromatic hydrocarbons, esters, halogenated hydrocarbons and ethers may cause swelling at elevated temperatures.

The polymer melts at 216°C and above this temperature shows better cohesion of the melt than PTFE. It may be processed by conventional thermoplastics processing methods at temperatures in the range 230-290°C. Because of the high melt viscosity high injection moulding pressures are required.

PCTFE is more expensive than PTFE and its use is comparatively limited. With the advent of FEP copolymers, TFE-ethylene copolymers and the peffluoroalkoxy polymers the advantage of melt processability is no longer, alone, a sufficient justification for its use. The particular advantages of the material are its transparency in thin films and its greater hardness and tensile strength as compared to PTFE and FEP copolymers. Examples of its use include gas-tight packaging film for medical and military applications (the main use), transparent windows for chemical and other apparatus where glass or other materials cannot be used, seals, gaskets and O-rings and some electrical applications such as hook-up wire and terminal insulators. Consump- tion, estimated at 350-400 tonnes per annum, is only about 1% that of PTFE.

PCTFE is marketed by Hoechst as Hostaflon C2 and in the United States by Minnesota Mining and Manufacturing (Kel-F) and Allied Chemical (Halon). Typical values for various physical properties are given in Table 13.1.

Copolymers of chlorotrifluoroethylene and ethylene were introduced by Allied Chemicals under the trade name Halar in the early 1970s. This is essentially a 1 : 1 alternating copolymer compounded with stabilising additives. The polymer has mechanical properties more like those of nylon than of typical fluoroplastic, with low creep and very good impact strength. Fur- thermore the polymers have very good chemical resistance and electrical insulation properties and are resistant to burning. They may be injection moulded or formed into fibres.

376 Fluorine-containing Polymers

13.6 POLY(V1NYL FLUORIDE) (PVF)

Poly(viny1 fluoride) was first introduced in the early 1960s, in film form, by Du Pont under the trade name Tedlar. Details of the commercial method of preparing the monomer have not been disclosed but it may be prepared by addition of hydrogen fluoride to acetylene at about 40°C.

HgClz

on Charcoal CHGCH + HF - CH2=CHF

It may also be prepared by pyrolysis of 1,l-difluoroethane at 725°C over a chromium fluoride catalyst in a platinum tube or by the action of zinc dust on bromodifluoroethane at 50°C.

The polymers were first described by Newkirk." Polymerisation may be brought about by subjecting acetylene-free vinyl fluoride to pressures to up to 1000 atm at 80°C in the presence of water and a trace of benzoyl peroxide.

Although poly(viny1 fluoride) resembles PVC in its low water absorption, resistance to hydrolysis, insolubility in common solvents at room temperature and a tendency to split off hydrogen halides at elevated temperatures, it has a much greater tendency to crystallise. This is because the fluorine atom (c.f. the chlorine atom) is sufficiently small to allow molecules to pack in the same way as polythene.

PVF has better heat resistance than PVC and exceptionally good weather resistance. It will burn slowly. Instability at processing temperatures makes handling difficult but this problem has been sufficiently overcome for Du Pont to be able to market their Tedlar film.

PVF film is now being used in the manufacture of weather-resisting laminates, for agricultural glazing and in electrical applications.

13.7 POLY(VINYL1DENE FLUORIDE)

This melt-processable homopolymer was first introduced in 1961 as Kynar by the Pennsalt Chemical Corporation (the company name being subsequently changed to Pennwalt). Other companies now manufacturing similar polymers are Dynamit Nobel (Dyflor), Kureha (KF), Solvay (Solef) and Atochem (Foraflon).

The monomer is a gas boiling at -84°C which may be made by dehydrochlorination of 1 -chloro- 1,l -difluoroethane:

CF2Cl.CH3 + CF2=CH2

or by dechlorination of 1,2-dichloro- 1,l -difluoroethane:

CF2 C1 CH2 CY----+ CF, = CH2

Poly(viny1idene fluoride) is a crystalline polymer melting at 171 "C. Amongst the melt-processable fluoroplastics the polymer is of interest because of its good mechanical properties and relatively low price. Tensile and impact strengths are good and the material is flexible in thin sections. Although it has generally good chemical resistance, strongly polar solvents such as dimethyl acetamide tend to

Other Plastics Materials Containing Tetrafluoroethylene 377

dissolve the polymer whilst some strongly basic primary amines such as n-butylamine tend to cause embrittlement and discolouration. The polymer is also attacked by some concentrated acids. A further disadvantage of the material is that its dielectric properties are frequency dependent and this limits its use as an electrical insulator. The high dielectric constant is a particular feature.

Of greater interest in recent years have been the peculiar piezolectric propertie~"-'~ of poly(viny1idene fluoride). In 1969 it was observed" that stretched film of the polymer heated to 90°C and subsequently cooled to room temperature in a direct current electric field was 3-5 times more piezoelectric than crystalline quartz. It was observed that the piezolectric strain coefficients were higher in the drawn film and in the normal directions than in the direction transverse to the film drawing.

The piezoelectric phenomena have been used to generate ultrasonic waves up to microwave frequencies using thin poly(viny1idene fluoride) transducers. In the audio range a new type of loudspeaker has been introduced using the transverse piezolectric effect on a mechanically biased membrane. This development has been of considerable interest to telephone engineers and scientists.

Poly(viny1idene fluoride) also has interesting pyroelectric properties showing a stable and reversible polarisation which persists after several heating cycles. In consequence the film is used in pyroelectric detectors. PVDF has a wide processing window in that there is a big difference between the melt temperature and the decomposition temperature. Thermal stability may, however, be drastically affected by contaminants, and scrupulous cleanliness is important when processing. The generation of HF should decomposition occur during processing is an obvious hazard. Typical melt temperatures are in the range 24O-26O0C, with mould temperatures being anything from 30 to 120°C.

The polymer, like many fluorine-containing polymers has very good weathering resistance and may also be used continuously up to 150°C. Outside of the electrical field it finds use in fluid handling, in hot water piping systems, in packaging and in chemical plant. A widely used specific application for PVDF is in ultra-pure water systems for the semiconductor industry.

13.8 OTHER PLASTICS MATERIALS CONTAINING TETRAFLUOROETHYLENE

In 1972 Du Pont introduced Teflon PFA, a copolymer of tetrafluoroethylene and perfluoro(propy1 vinyl ether) (CF2 = CFOCF2CF,CF3). Similar materials are now also produced by Asahi Glass, Daikin, Hoechst and Monteflos and are commonly referred to as PFA fluoropolymers. In 1994 Hoechst introduced Hostaflon PFA-N, claimed to have significantly lower melt viscosities than earlier grades of material.

Properties are similar to those of PTFE, and PFA fluoropolymers are generally considered to be the best melt-processable alternative to PTFE yet available. They are, however, more expensive than PTFE. Compared with the TFE-FEP copolymers such as Teflon FEP the PFA fluoropolymers:

(1) Have a higher melting point (300-310°C). (2) Have better processability. (3) Retain a higher proportion of their room temperature stiffness and strength at

elevated temperatures.

318 Fluorine-containing Polymers

In addition, the polymers are noted for their outstanding flex life, toughness and stress cracking resistance.

PFA fluoropolymers may be processed by injection moulding, extrusion, extrusion blow moulding and transfer moulding. All machine parts coming into contact with the melt should be made from highly corrosion-resistant high nickel content alloys. Processing melt temperatures can be as high as 420°C and mould temperatures may be in the range 50-250°C.

Applications include high-performance insulation for wire and cables (particularly heater cables), and corrosion-resistant linings for pumps, valves, pipes and other chemical equipment. Its availability in the form of film and tubing has led to its demand for both corrosion protection and antistick applications.

Somewhat between PTFE and PFA materials is the product Hostaflon TFM, which is a copolymer of TFE and a small amount of the perfluoro(propy1 vinyl ether). It has improved impact strength and weldability and has been promoted as a suitable material for forming into bottles. Yet another TFE-perfluoroalkoxy copolymer was introduced by Du Pont in 1979 as Teflon EPE. This material had a somewhat lower melting point (295°C) than the more common PFA fluoropolymers but it is no longer marketed.

In 1989 Du Pont introduced Teflon AF, said to be a copolymer of tetrafluoroethylene and trifluoromethyldifluorodioxol. This amorphous fluoro- polymer has a similar heat and chemical resistance to PTFE but possesses several notable properties, including:

High optical clarity (>95% in the visible range extending into the near infrared together with a good level of transparency to ultraviolet light). A very low refractive index (1.29-1.31). The lowest dielectric constant (1.83-1.93) of any known plastics material. (It is to be noted that this is in spite of the fact that the dielectric constant is more than the square of the refractive index, indicating that polarisations other than electronic polarisations are present-see Section 6.3). Limited solubility in selected perfluorinated solvents (unique amongst commercial fluoropolymers), enabling solution-cast ultra-thin coatings in the submicrometre thickness range. A high coefficient of friction. At about &1500/lb it is one of the most expensive plastics materials commercially available.

At the time of writing two grades of the material were available with different comonomer ratios. Typical properties are given in Table 23.3.

Table 13.3 Typical properties of Teflon AF amorphous fluoropolymers

Grade I ASTMMethod I

Tg ("C) Tensile strength (MPa) Ultimate elongation (%) Tensile modulus (GPa) Specific gravity Melt viscosity (Pa.s) (at loo-')

D34 18 D1708 D1708 D1708 D792 D3835

AF1600

160 27

20.5 1.55 1.78

2657 at 250°C

AF2400

240 24.6 6.1 1.54 1.67

540 at 350°C

Fluorine-containing Rubbers 379

The AF polymers are of potential interest in a number of high-technology applications, including the following:

(1) For coating optical devices for use in chemically aggressive environments. (2 ) Fibre optics applications. (3) Semiconductor and dielectric applications. (4) Release film coatings of very low thickness. ( 5 ) Corrosion-resistant coatings and high-permeability separation membranes.

1 3.9 HEXAFLUOROISOBUTYLENE-VINYLIDENE FLUORIDE COPOLYMERS

A 5050 mol/mol copolymer of hexafluoroisobutylene (CH2 = C(CF3)J and vinylidene fluoride was made available by Allied Chemical in the mid-1970s as CM-1 Fluoropolymer. The polymer has the same crystalline melting point as PTFE (327°C) but a much lower density (1.88 g/cm3). It has excellent chemical resistance, electrical insulation properties and non-stick characteristics and, unlike PTFE, may be injection moulded (at -380°C). It is less tough than PTFE.

13.10 FLUORINE-CONTAINING RUBBERS

Fluorine-containing rubbers were originally developed during the search for fluid- resisting elastomers which could be used over a wide temperature range. Much of the initial developmental work was a result of contracts placed by the US Army and Air Force. Whilst the current commercial materials are very expensive compared with general purpose rubbers they find a number of both military and non-military applications, particularly in the area of seals and O-rings.

In order to produce a rubbery material the polymer must have a flexible backbone, be sufficiently irregular in structure to be non-crystalline and also contain a site for cross-linking. These are of course requirements applicable equally to any potential elastomer whether or not it contains fluorine.

The first material to be marked, Fluoroprene, was introduced by Du Pont in 1948. A polymer of 2-fluorobuta-1,3-diene it was the fluoro analogue of polychloroprene. However, its properties were far from outstanding and production was soon discontinued.

In the early 1950s the fluoroacrylate polymers Poly-IF4 and Poly-2F4 (known initially as PolyFBA) and PolyFMFPA) were introduced. These materials had the structures given in Figure 13.7. These materials are also no longer of commercial significance.

Much greater success has been achieved with fluororubbers based on vinylidene fluoride (see Table 13.4). The copolymer of VDF with hexa- fluoropropylene (HFP) (typified by Viton A) and the terpolymer of VDF, HFP and TFE (typified by Viton B) are of similar importance and between them probably hold about 95% of the fluororubber market. The terpolymers have better long-term heat resistance, better resistance to swelling in oils and better resistance to chemical degradation, particularly from oil additives. On the other hand, the copolymers have a good balance of properties with a better retention of tensile strength after high-temperature aging, and some copolymer grades have outstanding compression set resistance. Polymers containing hydropenta-

380 Fluorine-containing Polymers

(CH2- CH),m (CH, - CH),m I I I I I

CH, CH, I I

c=o 0

c=o 0

CF, - CF, - 0 - CF, (b)

Figure 13.7. (a) Poly 1F4, (b) Poly 2F4

fluoropropylene (early grades of Tecnoflon) appear to have been introduced primarily to circumvent patents but are no longer of importance. On the other hand DuPont have introduced Viton GLT, a terpolymer of tetrafluorethylene, vinylidene fluoride and perfluoromethyl ether. All of the materials referred to in this paragraph are collectively classified by ASTM as FKM rubbers.

Since their appearance in the 1950s the main developments with these materials have been in their method of vulcanisation. Being saturated rubbers they cannot be vulcanised with sulphur but they could be cross-linked by irradiation or the use of peroxides. Until the 1970s, however, the only agents of commercial importance were diamines and certain of their derivatives. Typical of these materials were ethylenediamine carbamate, hexamethylenediamine carba- mate and N,N'-dicinnamylidenehexane- 1,6- diamine. Amongst the disadvantages of these systems were the high level of compression set shown by the vulcanisates, the generation of double bonds during vulcanisation providing a possible site of degradation, and the generation of up to 2% of water during cure which can cause both porosity and some de-vulcanisation.

Reduction in compression set began to be achieved in the late 1960s when it was found that tropolene and phenanthroline not only accelerated amine cures but were also effective with certain bisnucleophiles such as resorcinol, hydroquinone and bis-phenol AF. In due course even better results were obtained with quaternary ammonium or phosphonium salts being used in conjunction with aromatic dihydroxy compounds.

As with the amine systems such systems still suffered the disadvantage that water was split out during cure. This led to the availability in the late 1970s of peroxide-curable materials containing a cure site of enhanced receptivity to attack by aliphatic radicals. These peroxide-cured elastomers are claimed to have superior resistance to steam, hot water and mineral acids than the earlier systems.

It has been estimated that about 75% of FKM consumption is for O-rings, packings and gaskets for the aerospace industry, whilst automotive and other mechanical goods accounts for about 12%. Although the parts are expensive, motor manufacturers, are nowadays more appreciative of the demand by customers for reliability and increased service intervals. For this reason FKM is now used in valve stem seals, heavy duty automatic and pinion seals, crankshaft seals and cylinder liner O-rings for diesel engines. Other uses include seals for diesel engine glow plugs, seals for pilot-operated slide valves, protective suiting and flue duct expansion joints.

As will be seen from Table 13.4 elastomers are also available which are copolymers of vinylidene fluoride and chlorotrifluoroethylene. These materials

382 Fluorine-containing Polyniel-s

are notable for their superior resistance to oxidising acids such as fuming nitric acid. Elastomeric copolymers of vinylidene fluoride and hydropentafluoropylene have also been marketed (Tecnoflon by Montedison).

In attempts to further improve the stability of fluorine-containing elastomers Du Pont developed a polymer with no C-H groups. This material is a terpolymer of tetrafluoroethylene, perfluoro(methy1 vinyl ether) and, in small amounts, a cure site monomer of undisclosed composition. Marketed as Kalrez in 1975 the polymer withstands air oxidation up to 290-315°C and has an extremely low volume swell in a wide range of solvents, properties unmatched by any other commercial fluoroelastomer. This rubber is, however, very expensive, about 20 times the cost of the FKM rubbers and quoted at $1500/kg in 1990, and production is only of the order of 1 t.p.a. In 1992 Du Pont offered a material costing about 75% as much as Kalrez and marketed as Zalak. Structurally, it differs mainly from Kalrez in the choice of cure-site monomer.

A terpolymer of tetrafluoroethylene, propylene and a cure site monomer (suggested as triallyl cyanurate by one commentator) has now been marketed by Asahi Glass as Aflas. This rubber may be cross-linked by peroxides to give vulcanisates that swell only slightly in inorganic acids and bases but strongly in chloroform, acetone and hydrocarbons. Compared with the Du Pont material Kalrez, this rubber has a higher Tg(-2"C, c.f. -12°C) and a lower long time heat distortion temperature (less than 200°C) and thus has a narrower temperature range of application. It is stated to be significantly cheaper.

The excellent chemical resistance of Aflas has led to important applications in oilfields and, more recently, in the car industry in place of FKM rubbers because of the better resistance to new types of engine oils, transmission fluids, gear lubricants and engine coolants.

In 1991 MMM announced Fluorel 11, a terpolymer of tetrafluoroethylene, vinylidene fluoride and propylene. As might be expected from the structure, this is intermediate between FKM and Aflas, having better resistance to many newer automotive oils, lubricants and transmission fluids than the former but better heat resistance than the latter.

In 1955 Barr and Haszeldine, working in Manchester, prepared nitroso- fluoroelastomers of the general type:

Interest has continued with these materials because of their non-inflammable nature (they will not bum, even in pure oxygen), their excellent chemical resistance, including that of nitrogen tetroxide and chlorine trifluoride, a low Tg of -5 1 "C and an extremely low solubility parameter of 10.6 MPa'''.

The earliest materials were copolymers of tetrafluoroethylene and tri- fluoronitrosomethane but they were cross-linked with difficulty and the vulcanisates had little strength. Somewhat better results were obtained using carboxynitrosopolymers of the type

-NO- CF, - CF, -NO- I

CF, (CF,), COOH I

CF,

in which perfluoro(nitrosobutyric acid) was used as the cure site monomer.

Thermoplastic Fluoroelastomers 383

In general the nitroso rubbers also suffer from a poor resistance to ionising radiation, sensitivity to degradation by organic bases, highly toxic degradation products and an exceptionally high cost. The advent of the rubbers based on perfluoro(methy1 vinyl ether) considered above and of the phosphonitrilic elastomers considered below would appear to put the commercial future of these materials in extreme doubt.

These last named materials may be considered as derivatives of the inorganic rubber, polyphosphonitrilic chloride, discovered by Stokes in 1895. This was prepared by the reaction of phosphorus pentachloride with ammonium chloride as follows:

n PC1, + nNH, CI __+ (NPCl,), + 4nHC1 120T 3

This material had poor hydrolytic stability and was no more than a laboratory curiosity. Treatment with sodium trifluoroethoxide and heptafluorobutoxide has recently been found to yield a useful fluorophosphazene polymer:

- n NaOCH,CF, OCH,CF,

+ 2nNaC1

n NaOCH, (CF,), CF,H OCH,(CF,),CF,H-,

The rubber has a very low Tg of -68"C, excellent hydrolytic stability and excellent resistance to ozone, solvents and acids. In addition the rubber does not burn even in an oxidising atmosphere. Although its properties are virtually unchanged in the range -75 to + 120°C it does not possess the heat resistance of other fluoroelastomers. This polymer was marketed by Firestone in the mid- 1970s as PNF rubber, but in 1983 the Ethyl Corporation obtained exclusive rights to the Firestone patents and the polymer is now marketed as Eypel F.

In addition to the elastomers already described, others, have been produced on an experimental scale. These include the perfluoroalkylenetriazines with their unsurpassed thermal oxidative stability for an elastomer but with many offsetting disadvantages, and poly(thiocarbony1 fluoride). It is probably true to say that material does not have any outstanding desirable property that cannot now be matched by an alternative and commercially available material.

13.11 THERMOPLASTIC FLUOROELASTOMERS

Over the past 40 years there have been a number of developments that have resulted in the availability of rubbery materials that are thermoplastic in nature and which do not need chemical cross-linking (vulcanisation or setting) to generate elastomeric properties (see also Section 11.8 and 31.2). This approach has been extended to the fluoroelastomers.

384 Fluorine-containing Polymers

The Japanese company Daikin Industries has marketed block copolymers of the ABA type where B is a soft segment that is a terpolymer of vinylidene fluoride, hexafluoropropene and tetrafluoroethylene and A is a hard segment which is either a polyvinylidene fluoride segment or an ethylene, tetrafluoro- ethylene, hexafluoropropene copolymer. If desired, in order to enhance the properties, the soft segment may be thermoset either by radiation or chemical curing mechanisms. These polymers are made by free radical polymerisation of the B monomers in the presence of organic iodides. At the end of this reaction monomer(s) for the hard segment are charged into the reactor and the terminal iodines cleaved by radicals leaving free radical ends which can initiate chain extension polymerisation of the A segment monomers.

The polymers are marketed under the name Dai-el. Dai-el T530 has a hard segment based on ethylene, tetrafluoroethylene and hexafluoropropene which has a melting point of 220"C, tensile strength of 12 MPa, a resilience of 10% and a 24 h compression set @50°C of 1 1 %. Dai-el T630, with the hard segment based on vinylidene fluoride has a lower melting point of 160"C, a tensile strength of only 2 MPa and a compression set (24 h @ 50°C) of 80%.

13.12 MISCELLANEOUS FLUOROPOLYMERS

In addition to the fluoroplastics and fluororubbers already described other fluoropolymers have been marketed. Polymers of hexafluoropropylene oxide are marketed by Du Pont (Krytox). These materials have a low molecular weight (2000-7000) and are either oils or greases. The oils are uses as lubricants, heat transfer fluids and non-flammable oils for diffusion pumps. The greases are also used as lubricants. They have good heat and oil resistance but it is said that explosions may result from contact with the surfaces aluminium or magnesium cuttings.

Another Du Pont material (XR-resin) is prepared by copolymerisation of tetrafluoroethylene and the following sulphonyl fluoride vinyl ether:

Saponification to the sulphonic acid yields the product marketed as Nafion. This material is said to be permselective in that it passes cations but not anions. It is used as a membrane material in electrochemical processes, in for example the manufacture of sodium hypochlorite.

Very similar materials have been produced by Asahi Glass which are copolymers of tetrafluoroethylene and o-carbalkoxy-perfluoroalkoxy vinyl ethers of the general structure

CF2 = CF-(O-CF~-CF(CF~))-O-(CF~),-CO-OR

Films of the copolymers are, as with Nafion, saponified and used for permselective membranes. They have a much higher tensile strength than the Du Pont material and are also claimed to have a higher ion exchange capacity.

An interesting aromatic fluoro compound is polytrifluoromethylstyrene, which is claimed to have excellent optical properties (ref. 14).

Reviews 385

References 1 German Patent, 677,071; French Patent, 796, 026; British Patent, 465, 520 (IG Farben) 2. U.S. Patent, 2,230,654 (Kinetic Chemicals Inc.) 3. U S . Patent, 2,393,967 (Du Pont) 4. U.S. Patent, 2,534,058 (Du Pont) 5. THOMAS, P. E., LONTZ, I. F., SPERATIC, c. A,, and MCPHERSON, I. L., SOC. Plastics Engrs J . , 12,(5) 89

(1956) 6. Technical Trade Literature, IC1 Ltd. (Plastics Division), Welwyn Garden City 7. BOWLEY, G. w., Plastics Progress 1957 (Ed. P. Morgan), Iliffe, London (1958) 8. WHITCUT, H. M., Plastics Progress I955 (Ed. P. Morgan) Iliffe, London, p. 103 (1956) 9. U S . Patent, 2,689,241 (M. W. Kellogg)

10. NEWKIRK, A. E., J . Am. Chem. SOC., 68, 2467 (1946) 11. KAWNI, H., Japan J . Appl. Phys., 8, 975 (1969) 12. ZIMMERMAN, R. L., SUCHICITAL, c., and FUKADA, E., J . Appl. Polymer Sci., 19, 1373 (1975) 13. SUSSNER, H., and DRANSFELD K., J. Polymer Sci. (Phys.), 16 529 (1978) 14. B ~ M E R , B. and HAGEMANN, H ., Angew. Makromol. Chem., 109-110, 285 (1982)

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RUDNER, M.A., Fluorocarbons, Reinhold, New York (1958) SCHILDKNECHT, c.E., Vinyl and Related Polymers, John Wiley, New York (1952) SHERRATT, s., Contribution to Encyclopedia of Chemical Technology, Vol. 9, Interscience, New York,

WALL, L.A. (Ed.), Fluoropofymers (High Polymer Series Vol. 25), Wiley-Interscience, New York

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