Technology advances are pushing many industries to operate at higher temperatures and pressures than ever before . Hydrogen fuel cells operate more efficiently at high temperatures , semiconductor manufacturers are pushing the limits of process conditions to make chips smaller and more precise , and oil & gas wells are deeper and hotter than ever before .

By Brian Callahan , Product Manager – Thermoplastics Sealing Systems , Greene Tweed

As operating conditions become more severe , polymeric components such as valve seats , backup rings , seals , connectors and structural components are pushed to their limits because their strength and durability decrease with increasing temperatures and more aggressive chemistries . However , advances in polymer chemistry are enabling new materials that can solve these challenges and usher in new options for material selection in valve seats , seal stacks , backup rings , bushings and bearings .

Limitations of Traditional Polymers

Many valve seats and valve stem seals are made of PTFE for its excellent friction properties and nearly universal chemical resistance , or from PEEK for its high durability , high strength , outstanding creep / extrusion resistance and outstanding chemical resistance . These materials are excellent up to their limits , but PEEK physical properties begin to decline above 300 ° F ( 149 ° C ), the glass transition temperature ( Tg ) of PEEK , where the strength and stiffness drop-off , with a maximum continuous service temperature around 500 ° F ( 260 ° C ) in severe applications , and maximum excursions to 600 ° F ( 316 ° C ). PTFE ’ s mechanical performance limit starts significantly lower than PEEK and tends to decrease with increased temperature , with a maximum service temperature around 500 ° F ( 260 ° C ). As temperatures increase , the addition of reinforcing fillers such as glass and carbon fibers can extend this range by increasing stiffness , which has a particularly big impact over the glass transition temperature , taking the continuous service temperature of glass or carbon filled PEEK up to roughly the heat deflection temperature of 608 ° F ( 320 ° C ). The limitations of these materials drive the switch to metal seals and valve seats at extremely high temperatures , but this sacrifices the improved sealing , reduced friction , weight reduction and improved chemical resistance properties of the polymers .

Advancements with Crosslinked Polymers

A recent advancement from a key manufacturer in the industry clearly displays crosslinking polymer as a solution to volatile environments . This was developed to withstand more extreme conditions , retaining the mechanical properties associated with PEEK up to 650 ° F ( 343 ° C ). Crosslinking the polymer helps to retain the physical properties of the polymer well above the glass transition and heat deflection temperatures of conventional PEEK materials , delivering enhanced mechanical performance particularly well suited for high-pressure ( HP ) and high-temperature ( HT ) conditions , along with improved chemical resistance compared to traditional PEEK materials . The depicted dynamic mechanical analysis ( DMA ) curves graph the complex modulus to show the relative stiffness from around 79 ° F ( 26 ° C ) to 500 ° F

( 400 ° C ) of unfilled PEEK , 30 % glass filled PEEK , a leading wear grade of PEEK and their corresponding crosslinked PEEK versions .

Note the upward Tg Shift of roughly 10 to 15 ° C , significantly increased modulus and no observed melting for the crosslinked PEEK materials . Through increased capability , reliability and extended service life , crosslinked polymers expand design headroom in extreme environments , expanding the capabilities of high-performance polymers while avoiding many of the compromises of imidized plastics which have been used with PEEK in demanding applications , such as the greatly reduced steam resistance , reduced ductility and lower toughness of imidized plastics compared to PEEK .

Crosslinked PEEK materials can improve valve seat performance at higher temperatures with substantially improved creep , extrusion and wear resistance at temperatures up to 650 ° F ( 343 ° C ), as the crosslinks retain material strength and stiffness well above the glass transition and melt temperature ( Tm ) of the crystallites in conventional PEEK , a semicrystalline amorphous thermoplastic . Creep is the second leading cause of failure in plastics , behind chemical resistance , which can be substantially improved by utilizing crosslinked polymers compared to conventional PEEK . The Pareto chart shows the relative percentage of failures of plastic materials caused by various common failure modes .

Comparing Testing Data

Higher temperatures and higher pressures ( corresponding to higher loads ) increase the degree of creep ( extrusion ) in applications , so this difference is compounded in severe service valves . This is especially true

in pressure relief valve seats that remain under high pressure for long durations . The comparison measuring the extrusion tail of discs subjected to 35,000 psi ( 241 MPa ) at 550 ° F ( 288 ° C ) shows a substantial advantage in the performance of the crosslinked PEEK , while the backup ring test shows a significant advantage even at 350 ° F ( 177 ° C ) and 25,000 psi ( 172 MPa ) -- and both cases show that over time the advancements such as the Arlon ® 3000XT unfilled crosslinked PEEK not only substantially outperforms conventional PEEK , but also 30 % carbon fiber filled PEEK .

This data can be further compared against application needs to predict service lifetime of the valve seat , backup ring or sealing component using time-temperature equivalency calculations by comparing compressive modulus retention , creep / stress relaxation . The included lifetime prediction graph compares the relative performance of unfilled crosslinked PEEK to conventional unfilled PEEK .

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Common Failure Modes of Plastics

Chemical Resistance

Failure Mode

Cumulative Total

Literature shows chemical resistance and creep account for close to 60 % of field failures of plastics , across multiple industries . 1

This clearly shows that crosslinking significantly improves high temperature creep resistance of PEEK . In DMA ( Dynamic Mechanical Analysis ) testing , the manufacturer ’ s innovationhad a glass transition temperature ( Tg ) 35 ° F ( 19.5 ° C ) higher than PEEK , and provided superior mechanical property retention from 300 ° F ( 149 ° C ) – 650 ° F ( 343 ° C ). In extrusion testing at 35 ksi and 550 ° F ( 288 ° C ), it outperformed both virgin and filled grades of PEEK and PEKEKK . Crosslinking also offers improvement in tensile strength and modulus at higher temperatures .

It is common to modify PEEK to optimize the desired physical properties . For example , PTFE or graphite could be added to improve friction properties or glass or carbon fiber can be added to improve creep resistance , increase tensile modulus , flexural modulus , heat deflection temperature ( HDT ), tensile strength , flexural strength and compressive strength . The same fillers can be added to the crosslinked PEEK to design a material that offers the sealing

Chemical Resistance

• Limits the polymers we can use in extreme service

• Polyimides , BMI , and epoxies show poor hydrolysis resistance 2 , 3 , 4

• Some polyketones have issues as well 3

Creep

• Failures of thermoplastic components are seen as a major failure mode of seals in simulations and in the field

• Higher temperatures and higher pressures ( loads ) increase the degree of creep in applications

14 Valve World Americas | November 2024 • www . valve-world-americas . net