LOW GLASS TRANSITION TEMPERATURE FLUOROELASTOMERS: THE EFFECT OF SPECIFIC MONOMERS

摘要

Fluoroelastomers are well known for their excellent thermo-chemical resistance; however, the need of improving their low temperature behavior has been always recognized. The glass transition temperature of classical copolymers and terpolymers is in the range of-25 to -10 °C (as low as -35 to -30°C when HFP is substituted by perfluoro-methylvinylether), while that of perfluoroelastomers (copolymers of TFE and perfluoro-methylvinylether) is between -5 and 0°C. The presence of more than one ether oxygen in the pendant group of the perfluoro-vinylether utilized substantially reduces T_g, but this work shows that the amount of oxygen in the pendant group is not sufficient to characterize the ability of the perfluoro-vinylether to lower the T_g of fluoroelastomers. Specific vinylethers of the type CF_2=CF-OCF_2O-R_f are particularly effective. The T_g of fluoroelastomers that incorporate such monomers are well below the values expected from applying either the Fox or the Gordon-Taylor equation. Models that take into account the effect of sequences are necessary to explain the data. The characterization of these fluoroelastomers shows that sequences of these vinylethers are rare and head-to-head inversions are absent. Three of these vinylethers have been developed for the production of technically valuable Tecnoflon~R fluoroelastomers with T_g around -40°C.