Elastomer seals in pharmaceutical equipment need to be resistant to many chemicals and solvents. With the trend towards more aggressive equipment cleaning regimes, Darryl Turland, of Precision Polymer Engineering, reports on new developments in fluoroelastomer chemistry that are improving the elastomer's water and steam resistance
Pharmaceutical equipment features a variety of different types of FDA and USP Class VI compliant sealing materials. For example, water-resistant ethylene propylene diene M-class rubber (EPDM) is widely used in equipment that processes water-based materials. Where chemicals are used a hydrogenated nitrile butadiene rubber (HNBR) is typically adopted. But both materials have their weaknesses. EPDM lacks chemical resistance and HNBR will swell in steam; and both lack high temperature resistance above 150ËšC.
Fluoroelastomer (FKM) seals, on the other hand, provide high temperature and chemical resistance to a broad range of chemicals and solvents. However, this type of elastomer typically shows poor resistance to hot water and steam. At present, process engineers have to replace FKM seals more frequently than perhaps they would like in order to prevent seal failure during production.
The alternative is to avoid using FKM seals completely and instead use perfluoroelastomer (FFKM) seals from the outset to cope with pharmaceutical compounds and steam cleaning operations. However, in most cases the productivity benefits of using FFKM seals are outweighed by the cost of the elastomer.
Developments to increase the water and steam resistance of FKM have focused on changing the way in which the elastomer is manufactured. Within the FKM family of materials, there are differing "cure systems" - the chemical cross-linking reaction (vulcanisation) that occurs to join the polymer chains together. Common FKM sealing materials are of two types: co-polymer and terpolymer. The widely-used traditional, bisphenol-cured, co-polymer fluoroelastomers are cured by utilising a "condensation" reaction whereby water is generated during the cure process. When the fluoroelastomer is exposed to high temperature water and steam environments, the cure is reversed, breaking down the cross-links of the material, leading to premature failure of the seal.
Peroxide-cured, terpolymer fluoroelastomers do not suffer the reverse condensation reaction. Instead, the peroxide cure system is a "free-radical" reaction, and as such provides superior water and steam resistance. This and other new compounding techniques now enable FKM elastomers to withstand steam-cleaning regimes up to 200ËšC.
Recently a new, peroxide-cured fluoro-elastomer achieved FDA compliance, enabling it to be used in pharmaceutical processing equipment. The new steam-resistant fluoro-elastomer has been used in applications such as sanitary pipe couplings and ball segment valve seals. In both cases the FKM seals withstood many more steam cleaning cycles before replacement than traditional FKM seals.
The development of steam-resistant, peroxide-cured fluoroelastomers means that process engineers are no longer faced with having to make a trade-off between the poor performance of traditional FKM seals in water and steam, and EPDM materials that have good water/steam resistance but lack FKM's chemical and high temperature resistance. Moreover, the new fluoroelastomer material offers the prospect of significant productivity benefits arising from using the same FKM seals for longer and through more steam cleaning cycles before requiring replacement.