Anestis Leonidas Logothetis

CHEMISTRY OF FLUOROCARBON ELASTOMERS

Abstract Fluorocarbon elastomers represent a broad class of high performance elastomers which combine excellent thermal, oxidative and fluid resistance with very good tensile and compression-set resistance properties. These polymers are characterized by having carbon-carbon bonds in the backbone and a considerable amount of fluorine (50–73 wt%). They are made by using different combinations of some of the same fluoroolefin monomers. The commercially important types are classified into three categories: 1. (a) Random copolymers of vinylidene fluoride with perfluorooelfins such as hexafluoropropylene, tetrafluoroethylene, and perfluoroalkyl vinyl ethers. Cure site monomers may also be incorporated. 2. (b) Copolymers of tetrafluoroethylene and propylene (in about equal molar amounts) which have mostly an alternating structure. Cure site monomers may also be incorporated. 3. (c) Copolymers of tetrafluoroethylene with perfluoroalkyl vinyl ethers. Cure sites are imperative for curing these highly inert materials. Curing chemistry suitable for each composition has been developed. The polymer structure is very important in determining the physical properties of an elastomer. However, the crosslinking chemistry used in curing determines to a great extent the properties of the final rubber part. Because of their excellent properties these polymers are being increasingly used in applications where no other material is suitable. In spite of their higher costs, fluoroelastomer uses are expected to keep growing in volume at an estimated rate of 10% per year. Most of the current research effort is aimed towards improving the processability of the polymers and the properties of the resulting rubber parts. It is expected that new polymeric compositions and new curing chemistry will appear in the future.

The radiation chemistry of the copolymer of tetrafluoroethylene with 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole

The radiation chemistry of the copolymer of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (Teflon AF(R)) was investigated using gamma-irradiation under vacuum. Two types of resin were studied which differed in dioxole content; AF1600 65 mol% dioxole and AF2400 87 mol% dioxole. The cyclic fluoroplastic was found to undergo predominant main chain scission upon radiolysis, both above and below the glass transition temperature, which was characterised by a decrease in the glass transition temperature. FTIR analysis showed the formation of new carboxylate end groups as well as terminal unsaturation. Both CF . and CF2., radicals were identified using Electron Spin Resonance upon gamma-radiolysis and subsequent thermal annealing. The G-value for radical production at 77 K, G(R), was 1.6 for both resins

Effect of temperature on the γ-radiolysis of poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The effect of irradiation temperature on the polymer properties was investigated for the fluoroelastomer poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (TFE/PMVE). TFE/PMVE samples were gamma-irradiated to 150 kGy at temperatures ranging from 77 K to 373 K. Analysis of the sol/gel behaviour, tensile properties, and glass transition temperatures indicated that crosslinking commenced in the temperature range 195 to 263 K, for a dose of 150 kGy. The latter temperature was 13 K below the glass transition temperature. Crosslinking remained relatively constant to higher temperatures. Chain scission reactions were found to occur well below the glass transition temperature and increased at higher temperatures. The optimum temperature for the radiation crosslinking of TFE/PMVE, for the temperatures investigated, was 263 K

Thermal and mechanical properties of radiation crosslinked poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The γ-radiolysis of poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether) (TFE/PMVE) was investigated using chemical and mechanical analyses. The polymer was found to form an insoluble network with a dose of gelation of 15.8 kGy. Tensile and glass transition temperature measurements indicated the predominance of crosslinking, with optimal elastomeric properties reached in the dose range of 120 to 200 kGy. Photoacoustic FTIR spectroscopy (PAS) showed the formation of new carboxylic acid end groups on irradiation. These new end groups were shown to decrease the thermal oxidative stability of the crosslinked network as determined by thermal gravimetric analysis. Electron spin resonance (ESR) studies of the polymer at 77 K indicated the presence of radical precursors. A G-value of 1.1 was determined for radical production at 77 K. Comparison of radical concentrations for a copolymer with a different mole ratio of PMVE, indicated that the PMVE units contribute to scission reactions.

The use of crosslinking promoters in the γ-radiolysis of poly(tetrafluoroethylene-co-perfluoromethylvinyl ether). II.

Incorporation of 1 wt % of triallyl isocyanurate (TAIC) significantly enhanced the radiation crosslinking of the perfluoroelastomer, poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (TFE/PMVE). The dose for gelation was lowered by 70% with the presence of TAIC. The additive also improved the tensile properties of TFE/ PMVE both before and after crosslinking by irradiation. Higher radical yields were obtained with the presence of TAIC at 77 K, indicating the crosslinking promoter was acting as a radical trap. ESR studies showed that radiolysis of TAIC and subsequent photobleaching cleaved an allyl branch from the ring structure. Upon thermal annealing, an allyl radical on the TAIC molecule was observed

Effect of temperature and a crosslinking promoter on the γ-radiolysis of a perfluoro-elastomer

Methods of promoting the radiation-induced cross linking of poly(tetrafluoro-ethylene-co-perfluoromethyl vinyl ether) (TFE/PMVE) have been investigated. Greater control of the crosslinking and chain-scission reactions was achieved by varying the radiolysis temperature. This was attributed to temperature affecting the mobilities of reactive species such as polymeric free radicals. These reactive species are precursors to radiation-induced cross links and chain-ends. Analysis of the sol/gel behaviour, tensile properties and FTIR indicated that the optimum temperature for the radiation crosslinking of TFE/PMVE, at a dose of 150 kGy, was 263 K. This temperature was 10 K below the glass transition temperature. Incorporation of 1 wt% triallyl isocyanurate (TAIC) greatly amplified the radiation crosslinking of TFE/PMVE, The dose for gelation was decreased by 70%, and the additive imparted superior mechanical properties compared to the neat irradiated TFE/PMVE. Electron spin resonance (ESR) measurements showed higher radical yields at 77 K with the 1 wt% TAIC, indicating that the crosslinking promoter was acting as a radical trap