2.1 Major Families of Fluorocarbon Elastomers

Fluorocarbon elastomers are copolymers of various combinations of monomers. Composition can be chosen to get a desired combination of properties. The main characteristics affected by composition are fluid resistance, stability at high temperatures, and flexibility at low temperatures. Ease of processing and curing also vary with composition. The situation with fluoroelastomers is analogous to that of several hydrocarbon-based elastomers in which composition determines the trade-off between oil resistance and low-temperature characteristics. For example, in the nitrile rubber family (NBR, copolymers of butadiene and acrylonitrile), higher acrylonitrile content enhances oil resistance, but gives poorer low-temperature flexibility. The same trade-off exists for acrylate content of elastomers based on copolymers of ethylene and ethyl acrylate. Chlorine content exerts similar effects in chlorinated polyethylene elastomers. The range of property variation that can be attained by varying composition is much greater for fluoroelastomers than for the hydrocarbon-based elastomer families.

Effects of various major monomers on important fluoroelastomer characteristics are indicated in Table 2.1. Three of the monomers (vinylidene fluoride, tetrafluoroethylene, and ethylene) would contribute to crystallinity if incorporated in sufficiently long sequences. The other three monomers [hexafluoropropylene, perfluoro(methyl vinyl ether), and propylene] have bulky side groups that hinder crystallization and allow synthesis of amorphous elastomers. VDF and PMVE contribute to low glass-transition temperature (Tg) and thus to good low-temperature flexibility. All the fluoromonomers impart good resistance to hydrocarbons. VDF is a polar moiety, especially when incorporated adjacent to perfluorinated monomer units, so it contributes to swelling in contact with low molecular weight polar solvents and is susceptible to attack by base. Ethylene and propylene units contribute to swelling in contact with hydrocarbons, but are resistant to polar solvents and base. Several families of commercial fluoroelastomers have been designed with various combinations of these major monomers to get characteristics necessary for successful performance in wide ranges of applications and environments.

Dipolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) make up the largest volume of fluoroelastomers sales. Only one composition (VDF/HFP about 60/40 weight percent or 78/22 mole %, 66% fluorine) is of commercial importance, but it is offered in a wide range of viscosities and in numerous formulations tailored for specific applications. Other dipolymer compositions can be made, but higher VDF content leads to significant crystallinity, while lower VDF levels give much higher glass-transition temperatures, both detrimental to low-temperature flexibility. Terpolymers of VDF and HFP with tetrafluoroethylene (TFE) afford a better way to get enhanced fluid resistance without such severe effects on low-temperature characteristics. Useful terpolymers can be made with VDF content as low as about 30% to get fluoroelastomers with higher fluorine content (up to about 71%) and better fluid resistance. Most curing of these elastomers is based on the versatile bisphenol cure system, but some TFE-containing polymers are designed for free radical (peroxide) curing.

A family of fluoroelastomers growing in importance because of better low-temperature characteristics is based on use of perfluoro(methyl vinyl ether) [PMVE] in place of HFP in copolymers with VDF and TFE. Incorporation of a small amount of cure site is necessary to facilitate curing with peroxide systems. These PMVE-containing elastomers are useful at temperatures 10°C to 20°C lower than possible with HFP-containing polymers with comparable VDF content. Figure 2.1 shows trends in fluid resistance and low-temperature flexibility for vulcanizates of VDF/HFP/TFE and VDF/PMVE/

TFE elastomers with varying fluorine (VDF) content. All these elastomers are resistant to a wide range of fluids. However, for this comparison, a fuel mixture (M15 Fuel) containing 15% methanol and 85% standard hydrocarbon Fuel C was chosen to show the relatively large increase in volume swell with higher VDF content (lower fluorine content). The measure of low-temperature characteristics shown, TR-10, is a test carried out on cured strips of elastomer. The specimen is stretched, locked in the elongated condition, and cooled to very low temperature; then the specimen is released, and allowed to retract freely while raising the temperature at a uniform rate. TR-10 is the temperature at which the specimen has retracted 10%. For a vulcanizate of medium hardness, TR-10 is close to the glass transition temperature of the base polymer. TR-10 decreases with increasing VDF content, and is much lower for PMVE-based fluoroelastomers.

Perfluoroelastomers, copolymers of TFE with PMVE or a perfluoro(alkoxyalkyl vinyl ether), have excellent resistance to most fluids. With properly designed cure systems, TFE/PMVE elastomer vulcanizates have long service life at temperatures up to 300°C. Perfluoroelastomer parts can be designed for use in extreme environments that would destroy other elastomers.

Two families of fluoroelastomers are based on copolymerization of fluoromonomers with ethylene or propylene. Copolymers of TFE and propylene are resistant to polar fluids and base, but susceptible to high swell in hydrocarbons. Incorporation of VDF improves oil resistance at the expense of some base resistance. Ethylene may be used in place of VDF in copolymers with TFE and PMVE to get excellent resistance to most solvents and polar fluids, including base and amines.

Determination of fluoroelastomer composition is rather difficult. VDF/HFP dipolymer composition and monomer sequencing were determined by 19F nuclear magnetic resonance (NMR).[1] A typical spectrum is shown in Fig. 2.2. In studies related to curing, W. W. Schmiegel has interpreted the more complicated VDF/HFP/TFE and VDF/PMVE/TFE terpolymer spectra.[2] For quantitative analysis of terpolymers, VDF may be determined by 1H NMR; then TFE and HFP or PMVE can be calculated from 19F NMR on the same polymer. In practice, NMR analysis is not sufficiently rapid or precise for routine use in polymerization plant control. Elemental analyses for C, H, and F are of limited utility, with fluorine determination being particularly susceptible to bias errors because of interaction of fluorine with laboratory glassware used in the analysis. (Values of fluorine content of fluoroelastomers reported by suppliers are based on calculations from overall monomer composition, rather than direct analysis.) Usually a number of well characterized copolymers of varying composition are used as standards for calibration of suitable Fourier Transform Infrared (FTIR) methods for the various fluoroelastomer families. Careful monomer mass balances around well-controlled laboratory polymerization reactors allow preparation of precise composition standards. Even so, some inconsistencies probably exist in reported values of fluorine content by different fluoroelastomer suppliers.

2.2 VDF/HFP/(TFE) Elastomers

A ternary plot[3] of all the possible polymer compositions from VDF, HFP, and TFE monomers is shown as Fig. 2.3, based on polymer synthesis and thermal characterization studies by the author. A number of VDF/HFP and VDF/TFE dipolymers and VDF/HFP/TFE terpolymers were made by emulsion polymerization in a continuous reactor, with compositions determined by monomer mass balances. Glass transition temperatures, melting points, and heats of fusion were determined by differential scanning calorimetry (DSC). Polymers were designated as elastomers if they had glass transition temperatures less than 20°C, crystalline melting points below 60°C, and heats of fusion below 5 joules per gram. The Tg limit set the high-HFP, low-VDF boundary, and the limits on crystallinity set the low-HFP, high-VDF or high-TFE boundary of the elastomeric range. The large region of high-VDF or high-TFE plastics was characterized by high crystallinity (heats of fusion above 10 J/g) with melting points above 120°C. These plastics have been described in the first two volumes of this handbook series.[4] The intermediate region labeled “elastoplastics” comprises polymers with considerable crystallinity melting at 60°C-120°C. These are rather stiff polymers with higher modulus than elastomers. They do not generally have adequate mechanical properties for commercial usefulness. The unlabeled region of high-HFP compositions would have high Tg and low crystallinity; these are impractical to make because of poor polymerizability of high-HFP mixtures.

Approximate compositions of commercial elastomeric products are shown on the ternary diagram, with letters denoting the various composition families using the...