Polyether ether ketone (PEEK Plastic): Properties & Material Guide

What is PEEK – Polyetheretherketone?

Polyetheretherketone (PEEK) belongs to the polyketone family of polymers (PEK, PEEK, PEEKK, PEKK, PEKEKK). Amongst them, PEEK is the most widely used and manufactured on a large scale. It has been commercially available from ICI since the early 1980s. PEEK is fully recyclable plastic.

CAS Number: 29658-26-2
Chemical name: poly(oxy-1,4-phenyene-oxy-1, 4-phenyene)]
Chemical formula: C₁₉H₁₄O₃

Synthesis of PEEK

PEEK polymers are obtained by step-growth polymerization by the dialkylation of bisphenolate salts. It involves the reaction of 4,4’-difluorobenzophenone or 1,4-bis(4-fluorobenzoyl) benzene with hydroquinone. This reaction takes place in the presence of alkali carbonates. The polymer is then isolated by removal of alkali metal fluoride and the polymerization solvent.

Molecular Structure of PEEK

The polymer exhibits the highest thermal transitions among other commercial polymers. This is because of its stiff aromatic polymer backbone. Hence, they can be used at temperatures as high as 240°C.

Typical performance profile

PEEK is a semi-crystalline, high-performance engineering thermoplastic. This rigid opaque (grey) material offers a unique combination of:

mechanical properties,
resistance to chemicals and wear,
exceptionally high-temperature resistance

The polymer also exhibits excellent creep resistance when combined with flexural & tensile characteristics. The material withstands high loadings for long periods at high temperatures without permanent deformations.

Along with these features, PEEK has:

High Melting Point (Tm), 343°C
High Glass Transition (Tg) temperature, 143°C
High continuous service temperature, up to 260°C

These properties make PEEK and its composites suitable for high-temperature applications. These include aerospace, automotive, structural, electrical, and biomedical applications.

Despite the high price of the polymer, the added value that PEEK materials bring to manufacturing parts include:

lightweight,
strength or toughness, and
ability to survive longer in harsh environments.

Polyetheretherketone (PEEK): A Complete Guide on High-Heat Engineering Plastic

The polymer exhibits high volume resistivity and surface resistivity. It can maintain good insulating properties in a broad temperature range. It can also undergo environmental changes.

PEEK has a V0 flammability rating down to 1.45 mm and an LOI of 35%. It's smoke and toxic gas generation is extremely low. Crystallinity imparts excellent resistance to a wide range of liquids and superb fatigue performance.

PEEK is insoluble in all common solvents. It does not undergo hydrolysis. Used for 1000s of hours in steam or high-pressure water without significant property degradation.

More properties of PEEK include:

low friction
good dimensional stability
excellent sterilization resistance at high temperature
biocompatible
long life
inherent purity

Property comparison: PEEK versions and other high heat polymers

Neat vs. Reinforced PEEK version

Reinforced carbon fiber PEEK grades have exceptional tensile properties. A tensile strength of 29000psi can be achieved with properties retained at 299°C.

Reinforced grades have improved flexural modulus at very high temperatures. Glass and carbon reinforcements can also increase:

creep and fatigue resistance,
thermal conductivity, as well as
heat distortion temperature of the polymer

The table below lists the properties of unfilled vs. filled PEEK grades.

Property	Unfilled	30% carbon fiber	30% glass fiber
Dielectric Constant	3.2	3.2 - 3.4	3.3 - 4.2
Dielectric Strength (kV/mm)	20	18.5 - 19	15 - 24
Volume Resistivity (x 10¹⁵ Ohm.cm)	16 - 17	1 - 8	15 - 16
Elongation at Break (%)	30 - 150	1 - 3	2 - 3
Flexibility or Flexural Modulus (GPa)	3.7 - 4	13 - 19	9 - 10
Young Modulus (GPa)	3.5 - 3.9	13 - 22.3	9 - 11.4
Toughness or Notched Izod Impact at Room Temperature (J/m)	80 - 94	85 - 120	95 - 130
Density (g/cm³)	1.26 - 1.32	1.4 - 1.44	1.49 - 1.54
Thermal Insulation or Thermal Conductivity (W/m.K)	0.25	0.9 - 0.95	0.43
Commercial Grades	Neat PEEK Grades	Carbon Fiber Filled PEEK Grades	Glass Fiber Filled PEEK Grades

PEEK vs. other high heat plastics

In comparison to fluoropolymers, PEEK shows superior tensile strength, HDT, operating temperature, bonding, processing, and toxic gas emission. However, they are inferior in terms of chemical resistance, cost, toughness, and UV weathering.

As compared to PPS, PEEK has markedly superior thermal resistance, toughness, and flashing
PEEK has much higher temperature performance, wear resistance, chemical resistance, and fatigue performance when compared to PES.
PEEK is not normally blended with other polymers. However, it forms miscible blends with a range of other polyketones and PEI. PEEK/PEI blends have higher Tg. Its blends with PES and PPS are highly compatible.

PEEK Limitations

Expensive, applicable for highly demanding application
Processing at high temperatures
Attacked by some acids, such as conc. Sulphuric, Nitric, Chromic
Attacked by halogens and sodium
Low resistance to UV light

How are PEEK polymers processed?

Polyetheretherketone is processed by conventional methods such as:

injection molding,
extrusion,
compression molding, etc.

However, processing conditions used to mold it can influence crystallinity and mechanical properties.

As a linear thermoplastic, PEEK can be melt processed in the temperature range (370 to 420°C). While processing Polyetheretherketone, no corrosive gases are evolved. Effectively monitor melt temperature of your high heat plastics for improved quality product.

Before processing Polyetheretherketone, drying is recommended for 3 hours at 150°C or 2 hours at 180°C. This is done to avoid any molding defects.

The processing guidelines of PEEK polymers are described below:

Injection Molding

A mold temperature of 160-190°C is recommended to obtain good crystallization and minimize warping.
Post crystallization at 200°C is possible but it is not recommended for applications requiring high dimensional stability.
PEEK is suitable for injection 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 have a strong influence on crystallinity and then on performances.
For film and sheet extrusion, cooling cylinders at 50°C lead to transparent amorphous material.
Cooling cylinders at 170°C lead to opaque and highly crystalline material.
It is also possible to produce oriented or bioriented films by the process of extrusion.

3D printing

Thanks to its unique properties, 3D printing of PEEK allows construction of almost any complex design geometry, which cannot be manufactured using other technologies. Fused deposition modeling – FDM (or fused filament fabrication – FFF) methods are used for 3D Printing with PEEK filaments.

Nozzle temperature: 360-400°C
Heated bed: 120°C

It is an excellent candidate for FFF due to its low moisture absorption as opposed to other common FFF materials, such as Acrylonitrile Butadiene Styrene (ABS).