PEEK

What PEEK Is and the PAEK Family

PEEK is a linear aromatic semi-crystalline thermoplastic whose repeat unit alternates ether, ether, and ketone linkages between para-substituted benzene rings. The repeat unit has the formula (C19H12O3)n, and the polymer backbone of rigid aromatic rings is what gives PEEK its combination of high melting point, stiffness retention at temperature, and resistance to chemical attack. Because the chain crystallizes, PEEK keeps a usable fraction of its mechanical strength above its glass transition, which is the property that separates it from amorphous high-temperature plastics that go soft once the glass transition is crossed.

PEEK belongs to the broader PAEK (polyaryletherketone) family. The family members differ by the ratio of ether to ketone groups along the backbone. More ketone groups make the chain stiffer and raise both the glass transition and the melt temperature. PEEK, with two ethers and one ketone per repeat unit, sits at the accessible end of the family: a glass transition near 143 degrees Celsius and a melt near 343 degrees Celsius. PEK (polyetherketone) and PEKK (polyetherketoneketone) carry a higher ketone fraction, which pushes the continuous-use ceiling roughly 20 to 30 degrees Celsius higher but makes them harder and more expensive to process. For the vast majority of industrial parts, PEEK is the right balance of performance, processability, and cost.

The industrial history is short by polymer standards. ICI first synthesized PEEK in 1978, and the material reached the market in the early 1980s under the Victrex name, which became an independent company after a management buyout from ICI. Solvay (KetaSpire), Evonik (VESTAKEEP), and several Chinese producers (Jilin Joinature/JUSEP, Zhongyan) later entered the resin market, while compounders and stock-shape makers such as Ensinger (TECAPEEK), Mitsubishi Chemical Group (Ketron), and Quadrant supply machinable rod, plate, and tube. Implant resin is supplied by Invibio (PEEK-OPTIMA) and by medical grades from the resin houses.

In application terms, PEEK occupies the apex of the engineering-plastic pyramid: above commodity engineering plastics like nylon and acetal, alongside polyimide and PAI, and competing with the lower polyaryletherketones and with PTFE in chemically aggressive duty. It is the default when a single material must survive heat, chemical attack, and mechanical load at once. Typical end uses include oil and gas downhole seals and connectors, semiconductor wafer-handling and fluid components, aerospace brackets and clamps, pump and valve internals, electrical insulators and connector bodies, medical and analytical instrumentation (including HPLC tubing), and unlubricated bearings and bushings.

Four engineering attributes govern where PEEK earns its premium: continuous-use temperature, chemical and hydrolysis resistance, mechanical strength and creep behavior at temperature, and tribological (friction and wear) performance. The chapters that follow address each, then close with a structured selection sequence so the right grade is matched to the dominant duty rather than over-specified.

Grades and Filler Systems

PEEK is sold not as one material but as a family of compounds. The base resin is blended with fillers and reinforcements that trade ductility for stiffness, strength, wear resistance, or conductivity. Selecting the wrong filler is one of the most common and most expensive PEEK mistakes, because a glass-filled grade in a sliding-bearing duty will gall the mating metal, while a bearing grade in a structural duty lacks the stiffness the part needs. The table below summarizes the four mainstream grade families, using Victrex 450-series designations as the reference; equivalent grades exist from Solvay, Evonik, and Ensinger under their own naming.

Unfilled natural PEEK (Victrex 450G, Ensinger TECAPEEK natural) is the baseline. It carries the highest elongation at break, around 30 to 45 percent depending on test method, the best impact strength, and the highest electrical resistivity of the family. It is the grade for ductile parts, electrical insulators, and applications where purity and the absence of conductive or abrasive fillers matter, such as semiconductor fluid handling and medical components. Its tensile strength sits near 90 to 100 MPa.

Glass-fiber grades (450GL30, TECAPEEK GF30) add roughly 30 percent short glass fiber. This raises the tensile modulus to around 11.5 GPa, lifts the heat deflection temperature, improves dimensional stability, and reduces the coefficient of thermal expansion, at the cost of dropping elongation to a few percent and making the part brittle. Glass-filled PEEK stays electrically insulating, which is why it is preferred over carbon-filled grades where stiffness and insulation must coexist.

Carbon-fiber grades (450CA30, TECAPEEK CF30) push stiffness and strength higher still: tensile strength near 265 MPa, tensile modulus near 28 GPa and flexural modulus near 24 GPa, plus lower density and useful thermal conductivity that helps dissipate frictional heat. The trade-off is that carbon fiber makes the part electrically conductive and abrasive to soft mating metals, so it is unsuitable where electrical insulation is required.

Bearing and wear grades (450FC30 and similar carbon-graphite-PTFE blends, plus PTFE-filled TF10 and ceramic-filled CMF variants) are tribological compounds. The carbon fiber carries load and conducts heat, graphite and PTFE lower the friction coefficient to around 0.11 and reduce wear, making these grades the choice for unlubricated bushings, thrust washers, wear rings, and dynamic seals. Beyond these four families, specialty grades exist: implant grades to ASTM F2026, antistatic and ESD-dissipative grades, and high-flow grades tuned for thin-wall molding.

Thermal and Mechanical Properties

The defining numbers of PEEK are thermal. Its glass transition temperature sits near 143 degrees Celsius and its crystalline melt near 343 degrees Celsius. Continuous-use temperature for unfilled grades is 250 to 260 degrees Celsius, with the 260 degree figure validated to the UL 746B Relative Thermal Index, and short-term excursions reach about 300 degrees Celsius. Because the polymer is semi-crystalline, it retains load-bearing strength above the glass transition, unlike amorphous high-temperature plastics. The table below collects the key specifications for the four grade families. Values are typical datasheet figures for injection-molded specimens (Victrex 450 series and Ensinger TECAPEEK); always confirm against the specific lot datasheet before design.

Glass transition and melt. The glass transition near 143 degrees Celsius marks where the amorphous fraction softens; stiffness drops there, but crystallinity holds the part together up to the melt near 343 degrees Celsius. For load-bearing parts above about 150 degrees Celsius, design with the elevated-temperature modulus and account for creep, not the room-temperature stiffness, which overstates capability.

Tensile strength and modulus. Unfilled PEEK reaches roughly 90 to 100 MPa tensile strength with a modulus of 3.6 to 4.2 GPa and elongation around 30 to 45 percent, a ductile combination. Glass and carbon fibers trade that ductility for stiffness and strength: glass lifts modulus to about 11.5 GPa, carbon lifts strength to about 265 MPa and stiffness to roughly 28 GPa, while elongation falls to a few percent in both. The lower elongation means filled grades are notch-sensitive and should be designed with generous radii.

Wear and friction. Unfilled PEEK already runs reasonably dry, but bearing grades blend carbon fiber, graphite, and PTFE to lower the friction coefficient to around 0.11 and extend wear life. Polymeric bearing performance is bounded by the limiting PV (pressure times velocity): the combination of contact pressure and sliding speed at which friction or surface temperature stops stabilizing and the bearing fails or wears excessively. Specify against the supplier limiting-PV curve for the actual grade, and derate for elevated ambient temperature.

Thermal expansion and conductivity. Unfilled PEEK has a relatively high coefficient of thermal expansion for a structural plastic, near 45 to 50 micrometers per meter per Kelvin below the glass transition, which fiber fillers reduce substantially; carbon-filled grades are also thermally conductive, which helps dissipate frictional heat in bearings. Account for the higher expansion of unfilled grades when designing close-tolerance fits against metal.

Flammability and electrical. PEEK is inherently flame-retardant without additives: it carries a UL 94 V-0 rating at thin sections and a limiting oxygen index around 35 percent, with low smoke and low toxic-gas emission, which is why it is used in aircraft interiors and rail. Unfilled grades are good electrical insulators with a dielectric constant near 3.1 at room temperature that is stable across frequency; carbon-filled grades are conductive and must not be used as insulators.

Chemical Resistance and Media Compatibility

Chemical resistance is one of the two reasons engineers pay for PEEK, the other being heat. PEEK resists an unusually broad range of media even at elevated temperature: water and steam, most organic solvents, oils and fuels, hydraulic fluids, weak acids and alkalis, and seawater. It also resists hydrolysis, meaning it survives prolonged hot-water and steam exposure that degrades many other plastics, which is why it appears in autoclavable medical parts, hot-water and CIP (clean-in-place) fittings, and downhole oil and gas hardware. Crucially, this resistance is retained at temperatures where commodity plastics would have softened.

The principal weakness of PEEK is concentrated sulfuric acid, which dissolves it at room temperature, and other strongly oxidizing or halogenating acids such as concentrated nitric acid, fuming acids, and concentrated hydrofluoric acid. Some halogens and a few halogenated reagents at high temperature also attack it. For these media, fluoropolymers such as PTFE or PVDF, or fully fluorinated linings, are the correct choice. The table below is a quick-reference lookup for initial screening; before engineering implementation, always obtain the supplier corrosion chart and verify the actual concentration, temperature, flow, and applied stress, because a stressed part can fail at exposures that an unstressed coupon survives.

Food, water, and medical contact. Standard FDA-compliant and EU 10/2011 listed grades exist for food and potable-water contact, and many Victrex and Ensinger grades carry that compliance. For implants and intracorporeal devices, the material must be an implant grade specified to ASTM F2026 (the standard specification for PEEK polymers for surgical implant applications) and biologically evaluated to the ISO 10993 series. PEEK is radiolucent and has an elastic modulus closer to cortical bone than titanium, which is why it dominates spinal interbody cages and is used in trauma fixation. Implant resin such as Invibio PEEK-OPTIMA and Ensinger TECAPEEK MT is supplied as traceable lots; do not substitute a general industrial grade.

Radiation and outgassing. PEEK has good resistance to gamma and electron-beam sterilization radiation, supporting repeated sterilization cycles, and its low outgassing and low moisture uptake (water absorption near 0.45 percent at saturation, and only about 0.1 percent at 24 hours) make it suitable for high-vacuum and semiconductor service. Low moisture uptake also means parts hold dimension in humid environments better than nylon, which can swell measurably with absorbed water.

Weathering. Like most unstabilized engineering thermoplastics, natural PEEK is not optimized for prolonged outdoor ultraviolet exposure; carbon-filled black grades resist UV far better and are the choice for outdoor or sunlit service. For chemical service the governing reference is always the supplier compatibility chart at the real operating conditions, not a generic resistance class.

Decoding the Datasheet

A PEEK datasheet can list 20 or more parameters, but only a handful drive a sound selection. Reading them correctly, and knowing which test method produced each number, is a core skill for the purchasing engineer. The points below decode the parameters that matter most and the traps that hide in them.

Grade designation and filler. Read the grade code first. Victrex uses a base flow number (for example 450) plus a filler suffix: G for unfilled, GL30 for 30 percent glass, CA30 for 30 percent carbon, FC30 for the carbon-graphite-PTFE bearing blend. Ensinger uses TECAPEEK with GF30, CF30, and bearing or ceramic suffixes. The first 450 in Victrex naming indicates a standard melt viscosity; lower numbers are higher-flow grades for thin walls. The filler suffix changes strength, expansion, friction, and conductivity together, so never compare a bare tensile number between two grades without checking the filler.

Property values are method-dependent. Mechanical numbers are reported to ISO 527 or ASTM D638 (tensile), ISO 178 or ASTM D790 (flexural), and ISO 75 or ASTM D648 (heat deflection). ISO and ASTM specimen geometries and rates differ, so a small numeric gap between two datasheets can be a method difference rather than a real material difference. Confirm both datasheets cite the same method before treating the numbers as comparable.

Continuous use versus short-term temperature. The continuous-use temperature (250 to 260 degrees Celsius, ideally a UL 746B Relative Thermal Index) is the figure that governs long-term service. The short-term or peak figure near 300 degrees Celsius and the melt near 343 degrees Celsius are not service ratings; designing to them risks creep failure and dimensional loss.

Crystallinity and processing state. PEEK properties depend on how fully the part crystallized. Injection-molded parts need a mold held near 180 to 200 degrees Celsius to crystallize fully; a cold mold yields an amorphous, weaker, less chemically resistant part. Machined stock should be stress-relieved (annealed) to avoid distortion when material is removed. A datasheet number assumes the properly crystallized state, so ask the molder or stock supplier to confirm processing.

Mechanical, thermal, and tribological set. For a structural part, weigh tensile and flexural strength, modulus, heat deflection temperature, and coefficient of thermal expansion. For a bearing or seal, weigh the friction coefficient, the limiting-PV curve, and the wear factor. For an electrical part, weigh dielectric strength, dielectric constant, and volume resistivity, and remember carbon-filled grades are conductive. Do not collapse these into a single quality score; they trade against each other.

Compliance and traceability. Confirm the specific compliances the application requires: FDA or EU 10/2011 for food and water, ASTM F2026 and ISO 10993 for implants, aerospace flame and smoke standards such as FAR 25.853 for aircraft interiors, and lot traceability for regulated industries. A general industrial grade with the right mechanical numbers can still be the wrong material if it lacks the compliance the application demands.

Selection Decision Factors

To turn the preceding five chapters into a specific grade and supplier, follow the decision sequence below. Most PEEK selection errors come not from a single wrong number but from deciding the grade before the duty is fully defined, or from over-specifying a premium filled grade where a simpler one would serve. These steps double as an RFQ template.

1. Confirm PEEK is the right tier. PEEK earns its premium only when heat, chemical attack, and mechanical load coexist. If the duty is mild, a cheaper engineering plastic such as nylon, acetal, or PTFE may suffice. If the duty exceeds PEEK (continuous service above about 260 degrees Celsius, or concentrated oxidizing acids), step up to PEK, PEKK, or a polyimide, or to a fluoropolymer for the chemistry.
2. Define the dominant duty and pick the filler. Structural and stiffness-driven: glass-filled (GL30) or carbon-filled (CA30). Bearing, bushing, or dynamic seal: a bearing grade (FC30 carbon-graphite-PTFE). Ductile, insulating, or high-purity: unfilled natural. Conductive or ESD requirement: a carbon or antistatic grade. Match one filler to the one duty that dominates.
3. Set the thermal envelope. Specify both the maximum continuous service temperature against the continuous-use rating (ideally UL 746B RTI), and any short-term peak against the 300 degree excursion limit. Design load-bearing parts above 150 degrees Celsius with the elevated-temperature modulus and creep data, not room-temperature values.
4. Verify chemical and media compatibility. Check every contacting medium against the supplier corrosion chart at the actual concentration, temperature, flow, and applied stress. Rule out concentrated sulfuric, nitric, and other oxidizing or halogenating acids, for which a fluoropolymer is correct.
5. Choose the form and process. High volume: injection-molding granule grade, with a high-temperature mold near 180 to 200 degrees Celsius for full crystallinity. Low volume or large precision parts: machined stock rod, plate, or tube, stress-relieved. Prototypes or lightweight aerospace brackets: high-temperature fused-filament printing with a heated chamber and post-print annealing.
6. Fix tolerances around thermal expansion and crystallinity. Account for PEEK thermal expansion (higher for unfilled, much lower for fiber-filled grades) in close-tolerance fits against metal, and confirm the part will be fully crystallized so the datasheet properties actually apply.
7. Specify compliance and certification. Name the exact requirements: FDA or EU 10/2011 for food and water contact, ASTM F2026 plus ISO 10993 for implants, FAR 25.853 for aircraft interiors, and lot traceability for regulated use. A grade without the required certificate is disqualified regardless of its mechanical numbers.
8. Total cost of ownership. Compare resin or stock price plus processing or machining cost plus the cost of failure and downtime that PEEK avoids over the service life. PEEK rarely wins on initial price; it wins when a cheaper plastic would fail repeatedly in hot, chemically aggressive, or wear duty.

One last commonly overlooked dimension is supplier serviceability and supply security: lot-to-lot consistency, traceable certificates of conformity, available stock-shape inventory and machining partners, and dual sourcing. The PEEK resin base is concentrated (Victrex, Solvay, Evonik, and a growing set of Chinese producers such as Jilin Joinature), so confirm the resin pedigree behind a compounded or stock grade and qualify a second source for production-critical parts. Established stock-shape and compound suppliers including Ensinger, Mitsubishi Chemical Group (Ketron), and Quadrant maintain inventory and technical support that shorten lead time and reduce qualification risk for large programs.