Monday, April 20, 2009

Fluorocarbon rubber FKM (FPM)


In the United States fluorocarbon rubber is well known by its trade name of Viton (Based on vinylidene fluoride and hexafluoro-propylene the grades available differ in the chemical building blocks which were used to construct the polymer. Like silicone rubber, FKM has excellent high temperature resistance with an upper continuous heat aging temperature limit of 205°C. DuPont literature quotes continuous dry heat service to be >3,000 hours at 232°C decreasing to >48 hours at 316 0C. At the opposite end of the scale Nagdi points out that conventional FKM is usually serviceable at temperatures down to -20°C in dynamic applications, while for static use the temperature can be lower, although this will depend on the grade chosen. A primary variable in FKM grades is the level of fluorine in the elastomer molecule, FKMs being fluorohydrocarbons. Terpolymers tend to have higher fluorine content than copolymers and therefore have better resistance to various media. In general, fluoroelastomers have excellent resistance to oxidation, ozone, fuels and petroleum oils and are resistant to most mineral acids at high concentrations. Although FKM has good resistance to many chemicals, excessive swelling occurs in some polar solvents such as low molecular weight ethers, esters and ketones. Chemicals such as alkalis and amines should be used with caution, with standard fluorocarbon grades, especially at higher temperatures because alkalis harden the general purpose FKM, which will eventually embitter and then crack. FKM has a tendency to self extinguish when a flame is removed. This is of benefit in situations where the results of a fire would be catastrophic, for example in a coal mine. Other elastomers might burn out of control, when the source of the originating flame (such as methane gas explosion) is removed. Applications for FKM include automotive fuel hose liners and seals and flue duct expansion joints, where high temperatures and acidic products from gas desulfurization are involved. The relative cost of FKM is high, also a high specific gravity (around 1.8) means less cured product (volume) per unit weight A recent addition to the FKM family is an 'alloy' of a polar ethylene copolymer with a fluoroelastomer which optimizes cost, oil and heat resistance.

Reference: An Introduction to Rubber Technology by Andrew Ciesielski 1999

No comments: