Plastics can be classified into general-purpose plastics, engineering plastics and high-temperature engineering plastics according to their long-term use temperature. Among them, high-temperature plastics are also called heat-resistant plastics, high-performance plastics, special engineering plastics, etc.

 

Long-term use temperature of general-purpose plastics is below 100℃. Usually including PE, PP, PS, PVC, ABS. General-purpose plastics are the largest type of plastics used in our daily life, usually used for packaging, daily necessities, toys, etc.
Long-term use temperature of engineering plastics is around 100℃ to 150℃. Five major engineering plastics include PA, POM, PBT, PC and PPO. They are usually used in mechanical parts, automobiles, electrical and electronic fields.
Use temperature of high-temperature engineering plastics is above 150℃. In addition to high heat resistance and flame retardancy, these materials usually have excellent mechanical processing performance, aging resistance, dimensional stability and excellent electrical properties. It can be used to replace metal materials and is widely used in electronic appliances, aerospace, medical equipment, automobiles, and military fields. It is material at the top of plastic pyramid.
Common high-temperature engineering plastics are divided into following 8 categories:

Fluoroplastics (Fluoroplastie) refers to plastics made of fluorine resin. Common main varieties include polytetrafluoroethylene (PTFE), tetrafluoroethylene-ethylene copolymer (ETFE), polyperfluoroethylene propylene (FEP), polyvinylidene fluoride (PVDF), etc. Operating temperature is between 150℃-260℃.

 

For example, so-called king of plastics - polytetrafluoroethylene (Teflon) has following characteristics:
(1) High chemical stability: It has good chemical inertness, and strong acids, strong alkalis or strong oxidants and organic solvents have no effect on it.
(2) Good thermal stability: Cracking temperature is above 400℃, so it can work normally in temperature range of -200℃ to 300℃.
(3) Good anti-friction property: Friction coefficient between PTFE materials is 0.1-0.2, dry friction coefficient between PTFE materials and metal is 0.2-0.3, and dry friction coefficient with liquid lubrication is 0.02-0.04. Its dynamic and static friction coefficients are relatively small, and its initial friction coefficient is almost same as dynamic friction coefficient.
(4) Excellent self-lubrication: Surface of PTFE materials has outstanding surface self-lubrication, and almost all sticky substances cannot adhere to its surface.

Polyaryletherketone (PAEK) is a polymer whose main chain is composed of phenylene rings connected by ether bonds and ketone bonds. According to number and order of ether groups and ketone groups, it can be divided into polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherketone (PEK), polyetherketoneetherketoneketone (PEKEKK), etc.

PEEK is a special engineering plastic with excellent comprehensive performance. It has excellent heat resistance, water resistance, solvent resistance, and electrical insulation; high fatigue strength; the best radioactivity resistance among all plastics; high oxygen index, less smoke and non-toxicity when burning.
PEEK, top product in engineering plastics industry
Polyetheretherketone (PEEK) is an ultra-high performance special engineering plastic developed by British Imperial Chemical Industries (ICI) in 1978. Since PEEK is an aromatic crystalline thermoplastic polymer material with a melting point of 334℃, it has high mechanical strength, high temperature resistance, impact resistance, flame retardancy, acid and alkali resistance, hydrolysis resistance, wear resistance, fatigue resistance, radiation resistance and excellent electrical properties. In most cases, it can replace metals, alloys, ceramics and other materials. Although it is a compound material, it does not produce non-toxic gases when burning, so today, when green and environmentally friendly design is advocated, its application field has been rapidly expanded.
01 Molecular structure of PEEK

 

02 Characteristics of PEEK
(1) High temperature resistance: PEEK has a high glass transition temperature (Tg=143℃) and melting point (Tm=343℃). Its load heat deformation temperature is as high as 316℃, long-term use temperature is 260℃, and instantaneous use temperature can reach 300℃.
(2) Self-lubricating property: PEEK has good sliding properties and is suitable for occasions with strict requirements for low friction coefficient and wear resistance. In particular, PEEK modified with carbon fiber, graphite, and PTFE has excellent wear resistance.
(3) Corrosion resistance: PEEK is insoluble in any solvent except concentrated sulfuric acid and has high chemical stability.
(4) Flame retardancy: PEEK is self-extinguishing and can reach UL standard 94V-0 level even without any flame retardant.
(5) Processability: Due to its good high-temperature fluidity and high thermal decomposition temperature, PEEK can be processed in a variety of ways, such as injection molding, extrusion molding, compression molding and melt spinning.
(6) Mechanical properties: PEEK has good toughness and rigidity, and it has excellent fatigue resistance comparable to alloy materials.
(7) Electrical properties: PEEK has high volume resistivity and surface resistivity, and can withstand AC potential field strengths of various frequencies under a wide temperature range and environmental changes, maintaining good insulation properties.
(8) Hydrolysis resistance: Resins and their composite materials are not chemically affected by water and high-pressure steam, can be used continuously in high temperature and high-pressure steam or water environments while maintaining good mechanical properties.
(9) Weather resistance: With excellent weather resistance, polymers can be used to manufacture components that have strict working environment requirements or require frequent sterilization treatment.

Comparison of PEEK and several typical high-temperature resistant plastics
Application of PEEK: PEEK materials have been developed abroad for decades and are widely used in the automotive, military, medical, aviation, electrical and other industries. However, relatively high price has long limited its penetration into general civilian market. At present, it is mainly used in aerospace, high-end industry and medical fields.
Aerospace: PEEK can replace aluminum and other metal materials to manufacture aircraft parts, reducing weight by up to 70% compared with metal and improving fuel efficiency. At present, Boeing, Airbus and other companies have adopted PEEK materials in large quantities.
Medical: PEEK has advantages of non-toxicity, light weight, corrosion resistance, excellent biocompatibility, and is a very promising bioprosthetic material. Used for intervertebral fusion devices, medical catheters, bones, etc.
Energy: PEEK is specially used in high temperature, high pressure and chemically corrosive environments, which can expand scope of oil and gas exploration. It can be used to manufacture annular and mesh valves for hydrogen compressors and petroleum gas compressors on large petrochemical production lines. In addition, PEEK can also be used for nuclear power components.
Automobile: PEEK has excellent mechanical properties and excellent chemical corrosion resistance to automotive working fluids, good wear resistance, etc. It is widely used in automotive gears, vacuum pumps, clutches, valve cones, etc. to improve fuel efficiency and increase durability.
Food and Beverage Industry: PEEK can be used for food processing, storage, cooking equipment parts and water and beverage transmission and storage equipment, such as pipes, valves, containers, etc.
Electronics and Electrical: PEEK has excellent electrical properties and is an ideal electrical insulator. It can still maintain good electrical insulation under harsh environments such as high temperature, high pressure and high humidity. It is used to manufacture chip carriers, insulating diaphragms, printed circuit boards, high-temperature connectors and various electrical connectors. PEEK film can be used in micro speaker diaphragms, 5G RF antenna substrates, etc.

Because the proportion of ether bonds and ketone groups in its molecular structure is lower than that of PEEK, its melting point and glass transition temperature are higher than PEEK, and its heat resistance is better than PEEK. Continuous use temperature is 250℃.

PEKK, whose Chinese name is polyetherketoneketone, is a high-performance material. This material has a high melting point, about 300℃ to 600℃, has strong chemical resistance and wear resistance. In recent years, application of PEKK in the field of 3D printing has made rapid progress, and it has better performance than traditional 3D printing materials.

 

PEKK used in 3D printing

High-temperature nylon is divided into three categories:

PA46 is an aliphatic polyamide formed by condensation of butanediamine and adipic acid. Compared with PA6 and PA66, PA46 has more amides on each given length of chain and a more symmetrical chain structure, which makes its crystallinity up to 70% and gives it a very fast crystallization speed.
Melting point of PA46 is 295℃. HDT (heat deformation temperature) of unreinforced PA46 is 160℃. After being reinforced with glass fiber, its HDT can be as high as 290℃, and long-term use temperature is also 163℃. Unique structure of PA46 gives it unique properties that other materials cannot achieve.
PA46 is mainly used in electronics, aerospace, and automobiles.

 

PPA is formed by the polycondensation of dibasic acids containing benzene rings and aliphatic diamines. Its melting point is between 310-325℃ and its heat deformation temperature is between 280-290℃. Main varieties are PA4T, PA6T, PA9T, PA10T, etc.
Compared with ordinary PA66, PPA has a very low water absorption rate. Even if it is soaked in cold water for several years, its tensile strength can be maintained at more than 80%. PPA has good oil resistance and is extremely resistant to lubricating oil and fuel even at high temperatures. PPA also has excellent dimensional stability and weather resistance.
Commonly used in automobiles, electrical and electronic equipment, mechanical industry, and daily necessities.

 

Connector

PARA was invented by DuPont, the most famous of which are Nomex (polyisophthalamide, also known as aramid 1313) and Kevlar (full para-polyaramid, also known as aramid 1414).
This type of material is mainly used to prepare high-performance fibers and sheets. Fibers produced have characteristics of high strength, high rigidity, high modulus, high heat resistance, and high dielectric strength.
It can be applied to super-strong fibers and reinforcement materials for structural components such as military, aviation, and aerospace. Aramid 1414 can be used in bulletproof vests, and Solvay PARA is used in disposable surgical instruments.

Polyphenylene sulfide (PPS) is the fastest growing and most widely used special engineering plastic in recent years. It has excellent high temperature resistance, chemical resistance, weather resistance, flame retardancy, electrical properties, good dimensional stability and other advantages. It is widely used in automobiles, electronics and electrical, machinery, petrochemicals, pharmaceuticals, light industry, military industry, aerospace, 5G communications and other fields. It is the most widely used special engineering plastic.

 

PPS is also special engineering plastic with the highest degree of localization in China, with a self-sufficiency rate of more than 80%.
According to incomplete statistics, production capacity of major global polyphenylene sulfide (PPS) companies exceeds 200,000 tons. From perspective of capacity distribution, current global polyphenylene sulfide (PPS) industry capacity is mainly concentrated in Japan and China.
Summary of capacity of major PPS companies as of 2021

Traditional PPS giants such as DIC, Toray, Solvay, and Kureha are still relatively advanced in PPS production and technology. Production capacity of four companies accounts for more than 56% of global capacity. However, Chinese enterprises are showing a rapid rise. Domestic enterprises such as Zhejiang Xinhecheng and Chongqing Jushi have formed large-scale production, and many other enterprises have already mass-produced or are planning to layout.

Polyimide (PI) is an aromatic heterocyclic polymer compound with an imide chain link in its molecular structure. It is one of the most heat-resistant varieties of engineering plastics at present. It can withstand extreme temperatures, with a thermal decomposition temperature of up to 600℃, and will not brittle in liquid helium at -269℃. In addition, it has excellent mechanical properties, acid and alkali resistance, biocompatibility and electrical properties.

 

Polyimide for high-performance parts
Polyimide engineering plastics can be divided into both thermosetting and thermoplastic, and can be divided into polymellitic imide (PMMI), polyetherimide (PEI), polyamide-imide (PAI), etc., which have their own uses in different fields.
PMMI has a heat deformation temperature of 360℃ under a load of 1.8MPa and excellent electrical properties. It can be used for precision parts under special conditions, high-temperature resistant self-lubricating bearings, seals, blower impellers, etc. It can also be used for valve parts in contact with liquid ammonia and jet engine fuel supply system parts.
PEI has excellent mechanical properties, electrical insulation properties, radiation resistance, high temperature resistance and wear resistance, good melt fluidity, and a molding shrinkage rate of 0.5% to 0.7%. It can be molded by injection and extrusion, and post-processing is relatively easy. It can also be combined with other materials by welding. It is widely used in electronics, aviation, automobiles, medical equipment and other industries.
PAI has the highest strength among current non-reinforced plastics, with a tensile strength of 190MPa and a flexural strength of 250MPa. Heat deformation temperature is as high as 274℃ under a load of 1.8MPa. PAI has good ablation resistance and electromagnetic properties under high temperature and high frequency, has good bonding properties to metals and other materials. It is mainly used for gears, bearings and copier separation claws, etc. It can also be used for ablative materials, magnetic permeable materials and structural materials of aircraft.

PSU is a slightly amber, amorphous transparent or translucent polymer with excellent mechanical properties, high rigidity, wear resistance, high strength, and excellent mechanical properties even at high temperatures. Its outstanding advantages are that it can maintain excellent mechanical properties; its range is -100~150℃, long-term use temperature is 160℃, and the short-term use temperature is 190℃.
Polysulfone includes ordinary bisphenol A type PSU (commonly known as PSU), polyphenylsulfone and polyethersulfone.
1. Molecular formula of ordinary bisphenol A type PSU:

 

Conjugated effect of the sulfone group provides antioxidant and thermal stability; ether chain improves toughness, benzene ring ensures its mechanical strength and modulus.
2. Molecular formula of polyphenylsulfone PPSU:
Benzene ring on main chain of polyphenylsulfone provides high heat resistance and mechanical properties; ether bond provides excellent fluidity and processing performance.
3. Molecular formula of polyethersulfone PESU:
Ether group provides flexibility and high fluidity, sulfone group provides heat resistance, and phenylene group provides rigidity.
Performance comparison of bisphenol A type polysulfone, polyarylsulfone, and polyethersulfone

Polyarylate (PAR) is a thermoplastic resin containing benzene rings and ester bonds on main chain. PAR has good light transmittance (nearly 90%), toughness, heat resistance, elastic recovery, weather resistance and flame retardancy, and continuous use temperature can reach up to 170℃. It is mainly used in precision devices, automobiles, medical treatment, food and daily necessities.
PAR is used for car lamp parts
PAR has a similar structure to PC, and its performance is roughly the same. It can be molded in same mold, but high density of aromatic rings on main chain of PAR makes PAR better in heat resistance than PC. Heat deformation temperature is 20~40℃ higher than PC, it has excellent UV resistance and creep resistance. However, its elongation at break and impact resistance are not as good as polycarbonate.

 

Performance comparison of PAR and PC

 

LCP is called liquid crystal compound in Chinese. So-called "liquid crystal" is a substance that has both fluidity of liquid and orderly arrangement of crystal molecules in molten state.
LCP has excellent mechanical properties, and its biggest feature is that as wall thickness becomes thinner, relative strength increases. LCP has good thermal properties, and continuous use temperature can reach 200℃-300℃.

 

LDS-LCP material used for mobile phone antennas
LCP has very small dielectric constant and dielectric loss, so it is used in electronic devices such as connectors, slots, switches, brackets and sensors. The most widely studied application is the application of mobile phone 5G antennas.