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PEEK (Polyetheretherketone)

Polyetheretherketone (PEEK) is a polymer that is used in high-performance applications.

Its homopolymer can be seen in Figure 1. It was invented by Imperial Chemical Industries in

1982 and is an aromatic polyketone. Some of its various trade names are Victrex, Ultrapak,

Hostatec and Kadel. Since its discovery it has been used in various applications ranging from

electrical insulation, high-performance mechanical parts, and films. PEEKs biggest

consumption is the automobile industry. Around the world, Western Europe uses more than 50%

of this polymer in automotive applications.

Figure 1: The PEEK molecule consists of a repeating unit of three stiff benzene rings and one ester functional

group.

The benzene rings in the molecule have strong double bonds between carbon atoms.

These bonds contribute greatly to the high strength and elastic modulus of the plastic. Its stiff

benzene rings give its structure a large degree of crystallinity.

The polymerization process used to produce PEEK is condensation polymerization, also

known as step growth. The polymers monomer is created when phenoxides are combined with

difluorobenzophenone in the presence of potassium carbonate to yield PEEK and the excess

products of carbon dioxide, water, and potassium fluoride. This polymerization process results in

low smoke and toxic gas emissions since PEEKs off gasing values are among the lowest of

thermoplastic materials.

PEEK is an excellent replacement for steel because it is lighter with a relatively high

strength and modulus of elasticity.These properties can be significantly improved when

combined with carbon or glass fibers to form a composite material. Composites can be useful in

applications ranging from aerospace to automotive. The combination of the two materials creates

properties that cannot be achieved by either material on its own. The fibers in the composite bear

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the load while the PEEK matrix holds the fibers in place, protects the fibers and transfers the

load to the fibers. The resulting material has an extremely high strength to weight ratio and

improved thermal properties.

PEEK is also used extensively in wire coatings and electrical applications. The deciding

factor for these applications is the high electrical resistivity of 3.3x1021 to 3x1022 ohm-m,

allowing current to flow through the metallic wire and be protected by the polymer coating. A

low thermal conductivity of 0.14 to 0.15 Btu/ hr-ft-F also keeps the wire from becoming hot

after long periods of use. PEEK coatings also improve wear resistance and durability, improving

the life of the electrical components and wires.

PEEK is a useful biomaterial since it is inert to all common organic and inorganic liquids

and common solvents. This characteristic is key in allowing PEEK to have a lack of interaction

with biological systems. For this reason and its extraordinary mechanical and thermal properties,

it is a good choice for in vivo implants and devices. Its high surface lubricity makes PEEK a

good selection for artificial joints. Additionally it has good radiation resistance with the ability to

absorb more than 10000 M rads of infrared radiation without a significant reduction in

mechanical characteristics, and PEEK has exceptional resistance to high exposure of gamma

radiation.

PEEKs exceptional mechanical properties among thermoplastics make it useful for high-

performance applications. Its high stiffness, strength, lubricity and resistance to heat make it

ideal for applications such as bearings, valves, and bushings. Its high hardness values, compared

to other plastics, give it exceptional wear and abrasion resistance. PEEK can be easily injection

molded into intricate shapes for nearly any mechanical application and can be easily machined if

necessary. It has a maximum service temperature of approximately 250C which makes it a good

choice for numerous applications.

PEEKs outstanding mechanical, chemical, thermal and electrical properties come at a

price. It is among the most expensive thermoplastic materials. The result is a material that is 50

times more expensive than PP, and 10-20 times more than nylon. For this reason, PEEK is

typically limited to applications in which high performance is critical. A comparison of PEEK

with a plastic of similar structure, polycarbonate, can be seen in Table I. Due to PEEKs high

melt and processing temperatures, it also makes it an expensive material to process, since its

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shaping processes require high temperatures and pressures. Additionally PEEK has poor

chemical stability in the presence of strong acids.

Table I: Comparison of PEEK and PC

PEEK PC

Cost $47.25/lb $1.82/lb

Density 81.8 lb/ft^3 73.35 lb/ft^3

Elastic Modulus 559,000 psi 322,000 psi

Yield Strength 13,200 psi 9,380 psi

Melting Temperature 634F or 334C 581F or 305C

Glass Temperature 302F or 150C 361F or 183C

Maximum Service Temperature 481F or 249C 277F or 136C

Transparency Opaque Optical Quality

Processes for PEEK include extrusion, injection molding, extrusion molding,

compression molding, and thermoforming. Rotational molding is not typically used to

manufacture high quality parts and is therefore not an ideal shaping process for PEEK. More

expensive processes such as injection blow molding do not typically use PEEK since the

materials cost is too high, a cheaper alternative is often chosen.

In summary, polyetheretherketone has a variety of useful properties including high

strength and stiffness, good thermal stability, and a high Tg and Tm. These properties make

PEEK one of the most expensive thermoplastic materials. Its properties permit it to be used in an

endless number of useful applications.

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Sources:

1. Seferis, J. C. (1986), Polyetheretherketone (PEEK): Processing-structure and propertiesstudies for a matrix in high performance composites. Polymer Composites, 7: 158169.

2. Cambridge Education Selector Edupack 2011. Vers. 7.0. Cambridge: Granta DesignLimited, 2011. Computer software.

3. Platt, David. "Engineering and High Performance ... - David Platt." Google Books. Web.30 Nov. 2011. .