Figure 1. The chemical structure of FFKM. Source: Precision Polymer EngineeringFigure 1. The chemical structure of FFKM. Source: Precision Polymer EngineeringElastomers (rubber variants of polymers) have been used as sealing materials in life science applications for many years. Typically, the most common elastomers include EPDM, nitrile (NBR), hydrogenated nitrile (HNBR) and fluoroelastomer (FKM). This article looks at the pros and cons of these sealing materials and makes the case for perfluoroelastomers (FFKM) and why they are now considered a compelling alternative.

EPDM elastomers provide outstanding resistance to water and steam, but are incompatible with oils. Nitrile elastomers have good resistance to oils but are unsuitable for water or steam applications. Silicone rubber, a silica-based elastomer, has good low-temperature resistance but its high gas-permeability makes it unsuitable for some sealing applications.

Fluoropolymers — fluorocarbon-based elastomers containing carbon-fluorine bonds — are characterized by a high resistance to solvents, acids and bases. The best known are FKM and FFKM.

FKMs are widely used in sealing solutions for critical industries, like aerospace, because of their superior thermal and chemical resistance. Their popularity for sealing has surged in life science applications, including food and pharmaceutical production, due to advances in filler and additive technologies that allow them to pass the most stringent industry standards and regulatory requirements.

Fluorocarbons: Strengths and weaknesses

Depending on the polymer composition, FKM rubber is available as a dipolymer Type 1 or terpolymer Type 2 and Type 3. The type determines the fluorine content and thus the chemical resistance; Type 3 terpolymers are designed to provide improved low temperature flexibility. FKM polymers can also be classified according to their vulcanization chemistry, namely bisphenol AF curing for better high temperature service and peroxide curing for improved resistance to aqueous environments, steam and additive containing lubricants.

Typical properties for FKM include resistance to heat, aliphatic and aromatic hydrocarbons, chlorinated solvents and petroleum fluids. FKMs are superior to other oil-heat resistant rubbers in O-ring sealing force retention, except for FFKM. FKMs show poor resistance to ethers, ketones, esters, amines and hydraulic fluid-based phosphate esters. Special compounds are required to provide suitable resistance to hot water, steam and wet chlorine. FKMs are typically not recommended for alkaline fluids.

Base resistance of FKM

The FKM polymer consists of long chains of carbon atoms with a combination of fluorine and hydrogen atoms attached. The carbon fluorine bond is very strong but in comparison the carbon hydrogen bond is weaker. The fluorine atoms are electronegative and want to attract all the electrons to themselves, making the electropositive hydrogen atom vulnerable to attack by any alkaline species. The hydrogen atom and fluorine atom are lost via dehydrofluorination reaction, forming a double bond that is now a weak point in the polymer chain that is more susceptible to subsequent chemical attack and cross linking. Eventually, the material becomes hard and loses elasticity.

FFKM grades are the best base-resistant materials for use in these situations due to the absence of the acidic C-H bond. An alternative polymer type for alkaline conditions is EPDM, but EPDM is limited by temperature and the presence of oils and plasticizers, which may severely inhibit seal performance.

Typical applications for FKMs

Typical applications of FKM seals are valve stems, crankshafts and diesel engine cylinder O-rings. In life science applications, special compounds based on peroxide curable FKM would be the most suitable option, due to the steam and chemical resistance required for both process media and aggressive cleaning regimes.

Careful consideration is needed when specifying elastomer sealing solutions in life science, as the repercussions of ‘getting it wrong’ can be severe.

How does FFKM produce a more reliable sealing solution than FKM?

FFKMs have a carbon-carbon polymer backbone surrounded by relatively large fluorine atoms, creating a shielding effect against virtually all types of chemical attack. Also evident is the high bond dissociation energy C-F bond in the FFKM, in comparison to the weaker C-H bonds found in the FKM material (Figure 2). This more robust chemical makeup gives FFKM materials their high-temperature sealing capability and significant chemical-resistance performance. This is especially important for consumer health to ensure an end product free of contamination.

Benefits of FFKM in life science

Figure 2. Structures of FKM and FFKM materials. Source: Precision Polymer EngineeringFigure 2. Structures of FKM and FFKM materials. Source: Precision Polymer EngineeringThe human health component of life science applications means absolutely nothing can be left to chance because any seal failure has a potential cost beyond the monetary. Elastomers used in life science process equipment must withstand a wide range of process chemical media, potent active pharmaceutical ingredients (APIs) and aggressive sterilization-in-place and clean-in-place cleaning regimes. FFKM material has no thermal or chemical weakness in this respect and provides peace of mind.

The excellent gas and liquid permeation resistance, and good mechanical properties, are additional factors in the suitability of FFKM grades for sealing solutions in life science applications.

Food contact compliance

Food and pharmaceutical production is regulated by a robust legislature governing both manufacturing regulations and hygiene standards.

FFKM materials that are complaint with FDA, USP Class VI, 3A Sanitary Standard and many other industry standards are available to suit every life science application.

Why is FFKM so expensive?

FKKMs are the premium choice for applications demanding high-performance sealing with long-term reliability, particularly in life science applications. However, the higher amounts of fluorine that give FFKM near universal chemical compatibility also make the base material considerably more expensive than alternatives.

Figure 3. FFKMs are the premium choice for applications demanding high-performance sealing with long-term reliability. Source: Precision Polymer EngineeringFigure 3. FFKMs are the premium choice for applications demanding high-performance sealing with long-term reliability. Source: Precision Polymer Engineering

The purity of FFKM material relative to other elastomers is another reason for its elevated cost.

Many sealing materials have low molecular weight species additives in the compounds as part of the cure package or to aid processing. At elevated temperatures, these additives can be released from a seal, causing hardening and a subsequent loss of elasticity and sealing. Water vapor and hydrocarbons can leach at ambient conditions, with an increased rate of out-gassing or leaching at higher temperatures. This in turn increases the permeation rate and can even initiate chemical reactions within the elastomer, which release other gases. These substances can condense onto surfaces in the process equipment, or react with process chemicals and process media — particularly prohibitive when the purity of the end product is paramount.

FFKM materials have very few, if any, low molecular weight species added during the compounding step of material production, this level of purity results in a much lower outgassing rate.

FFKM — A cost-raiser or a cost-saver?

The short-term cost savings of choosing FKM neglects the long-term savings on equipment downtime, unscheduled maintenance and increased contamination risk that comes with an incorrectly specified material grade.

FKM material grades, when specified correctly by an experienced sealing specialist, can provide many years of incident-free sealing performance. However, potential difficulty can arise when an FKM has been recommended based on one metric (temperature, pressure or motion) without a thorough understanding of how all factors might impact that value. FFKMs provide a considerable safety buffer. In life science especially, the reputational cost to a business of a failed seal cannot be understated.

In a life science sealing system, higher-rated FFKM comes with the peace of mind of operating a safer process line as well as the sizable cost savings of increased time between maintenance, replacement and overhaul windows.

Having a standardized sealing material on hand for deployment in any temperature or chemical environment also reduces the complexity of stock management.

Conclusion

FFKM material grades are fast becoming a mainstream choice when sealing in life science applications due to their outstanding chemical resistance and temperature range. In terms of sealing performance, in virtually any industrial operating environment, FFKM materials perform better and more reliably than FKM materials.

While FFKM materials are initially more expensive than FKM materials, this expense is largely offset by medium- to long-term cost savings, including relative reliability and longevity, less frequent maintenance, shorter repair and overhaul windows and, most notably, the potential to rationalize onto a single material grade across all applications.

Additionally, lower risk of process contamination reduces the large financial and reputational cost of batch loss and product recall, thus offering greater consumer protection.

Precision Polymer Engineering has developed FFKM Perlast® G74S specifically for the unique challenges of life science applications. Perlast® G74S is compliant with FDA, USP Class VI and 3-A, proven in both static and dynamic applications, and available in O-rings, hygienic gaskets and other custom profiles.

For more information, visit www.prepol.com/perlast.