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Polyetheretherketone (PEEK)
How to Choose the Right Grade?
Polyetheretherketone is commonly abbreviated as PEEK. This high-temperature resistant polymer is renowned for its exceptional mechanical, thermal, and chemical properties. Precisely because of this, you will find it in aerospace, automotive, medical, and electronics industries.
But the real question is, how does PEEK differ from other high-temperature plastics? Why do engineers specifically choose PEEK over other alternative materials?
In this guide, we will walk you through how PEEK is manufactured and what happens at the molecular level. Then, based on your specific requirements and application, we will help you select the ideal PEEK grade on our platform.
PEEK
Definition, Chemical Structure, and Synthesis
What is PEEK Plastic?
Polyetheretherketone (PEEK) belongs to the polyketone family of polymers (PEK, PEEK, PEEKK, PEKK, PEKEKK).
Among them, PEEK is the most widely used and mass-produced type.
The abbreviation PEEK reveals all the information about its chemical backbone:
Poly: Many repeating units (it is a polymer)
Ether: Oxygen linkages in the chain
Ether: Another set of oxygen linkages
Ketone: Carbonyl (C=O) linkages
This specific arrangement of chemical bonds is precisely what gives PEEK its outstanding properties. Commercialized since the early 1980s by Imperial Chemical Industries (ICI, later becoming Victrex PLC), PEEK is a fully recyclable plastic.
PEEK's Chemical Structure
Molecular Composition
At the molecular level, PEEK consists of aromatic rings (benzene ring structures) connected by ether and ketone linkages. Its structure is as follows:
CAS Number: 29658-26-2
Chemical Name: Poly(oxy-1,4-phenylen-oxy-1,4-phenylen-carbonyl-1,4-phenylene)
Chemical Formula: (C19H14O3)n
The repeating unit contains:
Three aromatic rings (providing rigidity and thermal stability)
Two ether linkages (–O–) (providing flexibility and toughness)
One ketone group (C=O) (enhancing chemical resistance and strength)
The aromatic rings form the rigid, heat-resistant backbone. The ether linkages provide sufficient flexibility to prevent brittleness. The ketone group contributes to chemical resistance and high-temperature performance. It is this precise combination that sets PEEK apart from other polymers.
Semi-Crystalline Nature
PEEK is a semi-crystalline polymer. Its internal structure has two regions:
Synthesis of PEEK
The PEEK polymer is produced via step-growth polymerization (nucleophilic substitution polycondensation) of bisphenol salts.
Monomer Preparation
The two main monomers involved are:
4,4'-Difluorobenzophenone or 1,4-bis(4-fluorobenzoyl)benzene
Hydroquinone
The reaction is carried out in the presence of alkali metal carbonates. Hydroquinone is treated with a strong base (e.g., sodium carbonate) to form its sodium salt.
Polymerization
The sodium salt of hydroquinone reacts with 4,4'-difluorobenzophenone in a polar aprotic solvent (e.g., diphenyl sulfone) at high temperatures (around 300°C).
This reaction is a nucleophilic aromatic substitution where the phenolate salt attacks the fluorine atoms in difluorobenzophenone, forming ether linkages.
The polymerization process continues, forming increasingly longer polymer chains.
Separation and Purification
The resulting polymer solution is cooled and precipitated into a non-solvent (like water) to separate the PEEK polymer.
The polymer is then washed and dried to remove impurities.
Due to its rigid aromatic polymer backbone, this polymer exhibits the highest thermal transition temperatures among all commercialized polymers. Consequently, they can be used at temperatures up to 240°C.
Performance Comparison with Other High-Temperature Polymers
PEEK offers a unique combination of mechanical, chemical, electrical, and thermal properties. Understanding PEEK's unique characteristics is crucial to help you select the right grade for your application. Let's look at the distinctive properties of PEEK polymers.
Physical Properties
PEEK is a semi-crystalline, high-performance engineering thermoplastic.
Crystallinity gives it excellent tolerance to a wide range of liquids. It also provides excellent fatigue resistance.
PEEK is insoluble in all common solvents and does not undergo hydrolysis.
Can be used in steam or pressurized water for 1000 hours without significant performance degradation. Thus, PEEK has excellent resistance to sterilization at high temperatures.
PEEK has good dimensional stability, inherent purity, and biocompatibility.
Mechanical Properties
PEEK polymers exhibit outstanding mechanical properties.
PEEK has exceptional tensile properties, high strength, and toughness.
The polymer also exhibits excellent creep resistance. Combined with its flexural and tensile properties, it offers an excellent balance of performance. For example, in applications where materials need to withstand high loads at elevated temperatures for long periods without permanent deformation.
PEEK maintains good flexural modulus at very high temperatures.
Parts made from PEEK are lightweight. They can work longer in harsh environments.
Thermal Properties
PEEK and its composites are suitable for high-temperature applications, including aerospace, automotive, structural, electrical, and biomedical applications. Thermal properties exhibited by PEEK polymers include:
High Melting Point (Tm): 343°C
High Glass Transition Temperature (Tg): 143°C
High Continuous Use Temperature: Up to 260°C
Electrical Properties
PEEK polymers exhibit high volume and surface resistivity.
PEEK maintains good insulating properties over a wide temperature range and is less affected by environmental changes.
Flammability
PEEK has a V0 flame retardant rating at 1.45 mm thickness.
PEEK has a Limiting Oxygen Index (LOI) of 35%.
Its smoke emission and toxic gas generation are extremely low.
Reinforcements
Adding reinforcements to PEEK polymers can improve their creep and fatigue resistance and further enhance thermal conductivity and heat deflection temperature. For example:
The flexural modulus of the polymer can be increased by adding glass and carbon fibers.
When reinforced with carbon fiber, tensile strengths as high as 29,000 psi (~200 MPa) can be achieved while maintaining performance up to 299°C.
The table below compares the properties of unfilled vs. filled PEEK grades:
PEEK vs. Other High-Temperature Polymers
Polyetheretherketone exhibits significantly superior properties compared to other high-temperature polymers.
Compared to fluoropolymers, PEEK is superior in tensile strength, heat deflection temperature, operating temperature, bondability, processability, and toxic gas emission. However, it is slightly inferior in chemical resistance, cost, toughness, and UV aging resistance.
Compared to Polyphenylene Sulfide (PPS), PEEK is significantly superior in heat resistance, toughness, and flash control.
Compared to Polyethersulfone (PES), PEEK has higher temperature performance, wear resistance, chemical resistance, and fatigue resistance.
PEEK is generally not blended with other polymers. However, it can form miscible blends with a range of other polyketones and polyetherimides (PEI). PEEK/PEI blends have a higher Tg. Its blends with PES and PPS are highly compatible.
Addressing Limitations of PEEK Plastic
When selecting a PEEK grade for your application, the following limitations must be considered:
Expensive, suitable for the most demanding applications.
Requires processing at high temperatures.
Can be attacked by concentrated sulfuric acid, nitric acid, and chromic acid.
Can be attacked by halogens and sodium.
Low resistance to UV light.
Physical Forms and Processing Techniques
Polyetheretherketone can be processed by various conventional methods described below. The processing conditions used to shape PEEK affect its crystallinity and mechanical properties. As a linear thermoplastic, PEEK can be melt-processed in the temperature range (370 to 420°C). No corrosive gases are released when processing PEEK.
Morphology of PEEK
Different morphologies of PEEK polymer affect the efficiency of the final plastic part. Selecting a PEEK grade based on this can avoid production interruptions and unnecessary capital investment. Commercially available physical forms of PEEK include:
Pellets: Small, uniform pellets for injection molding and extrusion processes.
Powder: Fine particles suitable for compression molding, powder coating, and additive manufacturing.
Rods: Blank shapes for machining into custom parts.
Granules: Similar to pellets but typically larger, used for various molding processes.
Before processing PEEK, it is recommended to dry at 150°C for 3 hours or at 180°C for 2 hours to avoid any molding defects.
Processing Methods for PEEK
PEEK is processed using the following conventional methods. Processing guidelines for each process are described below.
Injection Molding
A mold temperature of 160-190°C is recommended for good crystallinity and to minimize warpage.
Post-crystallization can be done at 200°C, but is not recommended for applications requiring high dimensional stability.
PEEK is suitable for injection molding of very small parts with tight dimensional tolerances.
Injection Pressure: 70-140 MPa
Mold Shrinkage: Unfilled – 1.2-2.4%, Filled – 0.1-1.1%
Extrusion
Cooling temperature strongly affects crystallinity, which in turn affects properties.
For film and sheet extrusion, cooling rolls at 50°C yield transparent amorphous material.
Cooling rolls at 170°C yield opaque and highly crystalline material.
Oriented or biaxially oriented films can also be produced via extrusion.
3D Printing
Thanks to PEEK's unique properties, 3D printing allows building almost any complex design geometry that cannot be manufactured by other techniques. 3D printing with PEEK filament typically uses Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF) methods.
Nozzle Temperature: 360-400°C
Heated Bed Temperature: 120°C
Due to its low moisture absorption (compared to other common FFF materials like ABS), PEEK is an excellent candidate for filament fabrication.
Sustainability
Here are key aspects to consider when choosing sustainable PEEK grades.
Recyclability: PEEK is a fully recyclable plastic. However, this is a complex process due to the material's high-performance characteristics and cost. Common recycling methods include mechanical and chemical recycling.
VOC Emissions: PEEK's manufacturing process uses high-temperature melt polymerization, reducing the need for solvents that could generate low VOC emissions. The raw materials used in PEEK production are also relatively stable, making them less prone to gas release during processing.
Bio-based Versions: 100% bio-based PEEK is not yet available on a large scale. However, significant progress has been made in this area. Some companies are working towards partially bio-based versions of PEEK, which often exhibit properties similar to petroleum-based alternatives.
Renewable Feedstocks: Some innovators are producing sustainable renewable or bio-based MDA (4,4'-methylenedianiline), a precursor in the PEEK manufacturing process.
Regulatory Compliance: PEEK is considered safe for food contact applications per FDA regulations. Many PEEK grades also comply with REACH and RoHS directives.
PFAS-Free: PEEK's chemical structure contains no fluorinated compounds, making it an attractive choice for applications requiring materials free from per-/polyfluoroalkyl substances (PFAS).
Applications of Polyetheretherketone (PEEK)
PEEK is used in aerospace, automotive, electrical, and medical industries. Due to its robustness and durability, it is also used in equipment manufacturing for chemical and engineering industries. Common examples include bearings, piston parts, pumps, HPLC columns, compressor plate valves, and cable insulation. Let's explore some of its applications in detail.
Automotive Industry
With the trend towards engine compartment miniaturization, PEEK offers solutions to replace metals, enabling weight reduction, noise reduction, and functional integration.
The most important performance benefits of polyetheretherketone lie in enhanced dry and lubricated surface interactions, excellent mechanical properties over a wide temperature range, ease of processing, and superior fatigue performance.
Key automotive applications of PEEK include:
Piston units in the engine compartment
Seals
Gaskets
Bearings
Various moving parts for transmission, braking, and air conditioning systems
Aerospace
PEEK polymers and their composites are replacing aluminum and other metals for numerous aircraft components.
Large volumes of large-volume parts with fine tolerances can be shaped cost-effectively and used directly without assembly or modification.
Applications of PEEK in the aerospace market include:
Critical engine components: Because the polymer can withstand high temperatures as well as tribological interactions of dry and lubricated material contact.
Aircraft exterior parts: PEEK provides excellent rain erosion resistance.
Interior parts: Its inherent flame retardancy combined with low smoke and toxic gas emission reduces hazards in case of fire.
Electrical systems: For manufacturing bellows that protect wires and optical fibers.
Medical & Healthcare
Polyetheretherketone offers cost-effective innovative components with excellent wear resistance, heat resistance, electrical insulation, and chemical resistance. Its applications in healthcare primarily include dental instruments, endoscopes, and dialyzers.
PEEK is replacing aluminum for dental syringe handles and sterile boxes storing root canal files.
The polymer can withstand up to 3000 autoclave cycles, where temperatures typically reach 134°C.
It maintains excellent mechanical strength, superior stress crack resistance, and hydrolytic stability in the presence of hot water, steam, solvents, and chemicals.
It offers better biocompatibility for load-bearing implants.
Electrical/Electronics
Polyetheretherketone has excellent electrical properties, making it an ideal electrical insulator. It is used in electrical markets and electronic applications.
PEEK provides parts with long-term operational reliability over wide fluctuations in temperature, pressure, and frequency.
Its inherent purity, combined with excellent mechanical and chemical stability, minimizes contamination and maximizes safety during silicon wafer handling.
Excellent thermal properties allow PEEK polymer parts to withstand high temperatures during soldering.
Some interesting examples of current applications include: coaxial connector jacks for hands-free phone kits; surface-mount trimming potentiometers (an electromechanical device, called SMD, designed to correct voltage or resistance errors on printed circuit boards); and insulators used as connector pins for subsea environmental control equipment.
Other Key Markets
PEEK polymers outperform metals and other materials on many parts used in industrial, chemical, and processing industries.
Following FDA approval, PEEK polymers are now used in food contact applications.
PEEK is replacing stainless steel for the impellers of regenerative pumps. It significantly reduces wear, lowers noise levels, and provides more consistent operating characteristics.
In modern connector technology, polyetheretherketone also offers greater application potential for pipe and hose connections. They can withstand pressures up to 25,000 psi and temperatures up to 260°C.
