Polyether ether ketone (PEEK) is a high temperature semi-crystalline thermoplastic. PEEK was developed by ICI in 1982. This polymer has the ability to be used continuously to a temperature of 250°C (480°F) and in hot water or steam without the material permanently losing any physical properties. PEEK has been given a V-O flammability rating as it produces very less smoke and toxic gas emissions when exposed to flame. It can be processed by the traditional methods such as injection molding, extrusion, compression molding and powder coating processes.
PEEK is a high strength alternative to fluoropolymers especially in tough environments. Unreinforced PEEK offers the highest elongation and endurance of all PEEK grades. PEEK is also available as a composite or with carbon fiber reinforcement /fiberglass. However, PEEK is very expensive to process, hence it is widely used in major industries such as nuclear and aerospace
Molecular formula - (CH2O)n
Density - 1.31 g/cm³
Melting point – 340°C (644°F)
Chemical and Physical Properties of PEEK
The key properties of PEEK are given below:
- Excellent dielectric properties with low loss
- Good radiation resistance, solvent, wear and abrasion resistant
- Low coefficient friction, low smoke and toxic gas emissions
- Very low moisture absorption
- Unaffected by continuous exposure to hot water or steam
- Excellent fatigue, stress-crack, oxidation and acid resistance
- Thermal stability, very good mechanical properties
- Excellent UV resistance, high purity and light weight
Manufacturing Process of PEEK
There are several production processes to form PEEK. The conventional method is to form PEEK by allowing bisphenolate to react with difluorobenzophenone.
Synthesis of PEEK requires accurate reaction conditions because of its insolubility. PEEK can been synthesized by using nucleophilic substitution of 4,4' - difluorobenzophenone with hydroquinone in the presence of anhydrous potassium carbonate under microwave irradiation to produce a good yield. A three-necked flask fitted with a Dean-Stark trap, condenser, nitrogen inlet, and mechanical stirrer is used for this process. 1.31 g of 4,4'- difluorobenzophenone, 0.66 g of hydroquinone, and 1.24 g of K2CO3 has to be dissolved in a mixture containing 15ml of solvent and 35ml of toluene. Toluene helps to remove water. The flask is then placed inside a microwave apparatus with maximum power of 700W and 2455 MHz. This operation has to be performed in a nitrogen atmosphere with constant stirring.
During the initial stage of the polymerization, the temperature should be at 80-110°C (176-230°F), and the reaction mixture should be heated under reflux for 20 min. The water generated during the formation of the phenate is removed by azeotropic distillation. After the water was removed, the temperature has to be maintained at 110-130°C (230-266°F) for 15 min. At this time, toluene is also removed by distillation process. Then the reaction is maintained at 180- 200°C (356-392°F) for varying lengths of time. The reaction is cooled to room temperature and the polymer was obtained by precipitating from water. The material is refluxed with water and subjected to Soxhlet extraction using methanol. The clean-up of the polymer is to be performed with care to ensure that the solvent and the inorganic salts are completely removed. The polymer is then dried under vacuum at 100°C (212°F) for 24h.
Applications of Virgin Material PEEK
PEEK has widespread applications in a variety of fields such as oil and petroleum environments, electronics, nuclear, automotive, marine, medical, general industrial, and aerospace.
- Thermal plastic applications involving combustion, thermal, and chemical operations
- Chemical and hydrolysis resistant valves replacing glass
- Internal combustion engines replacing thermosets
- Cooker components replacing enamel
- Automotive components replacing metal
- High temperature and chemical resistant filters from fiber
- Low friction bearings
- Pressure sensor membranes, flexible surface heaters
- It is used as a biomaterial for spinal applications
- Flexible printed circuit boards, injection molded engineering components
- Products used in radiation environments
Environmental Impacts of PEEK
On overheating and combustion of PEEK, carbon monoxide and carbon dioxide are formed. Formation of other hazardous decomposition depends upon the fire conditions. In many cases, recycling is not feasible, hence waste disposal of PEEK products should be by incineration or landfill. However, local governmental regulations should be followed in disposal. The PEEK waste is not biodegradable.
Toxicity of PEEK is expected to be low based on insolubility of polymer in water. Natural polyetheretherketone (PEEK) such as UNITREX PEEK does not contain any known toxic chemicals listed under Section 313 of Title III of the Superfund Amendments and Reauthorization Act (SARA) of 1986 and 40CFR part 372.
PEEK is basically manufactured and used in low tonnages. It is also very expensive, thus making recycling a necessity. The use of thermosetting polymers is only increasing and it is now important to find ways to dispose or recycle them without it becoming a major global concern.
There are different recycling treatments available. Incineration process is a treatment with considerable non-combustible residues. Another treatment is thermolysis process which causes decomposition of products and mechanical weakening. Mechanical recycling techniques are based on granulation and comminution, which cause specific size fractions that can be incorporated into new sheet molding compounds (SMC) parts in a thermoplastic matrix, or in concrete. Solvolysis is a recovery method of composite wastes. Chemical recycling process is a proven and effective recycling method for plastics. Further research is ongoing and PEEK is not being recycled extensively at present.
Applications of Recycled PEEK
Recycled PEEK will be more economical than PEEK; however, since PEEK is used for high value applications, research is still being done whether recycled PEEK can be used for the same applications as virgin PEEK.
Sources and Further Reading