thermoplastic polyester

Thermoplastic polyesters include polybutylene terephthalate and polyethylene terephthalate.
(1) Polybutylene terephthalate (PBT)
Polybutylene terephthalate is a polymer made by the transesterification method of polyethylene terephthalate and butylene glycol or the direct esterification method of polyethylene terephthalic acid and butylene glycol, which is referred to as PBT in English.
The performance of pure PBT is average, but its mechanical and thermal properties are greatly improved after being reinforced and modified with glass fiber. Its specific performance characteristics are: mechanical properties are similar to PA and POM, small friction coefficient, good self-lubrication, good thermal and chemical stability, excellent electrical insulation, high rigidity and hardness, good impact resistance, high dimensional stability, good processability, good heat resistance, self-extinguishing.
PBT is mainly used in the fields of electronic appliances, automobiles, machinery and instrumentation. 65% is used for electronic appliances and 10% for automobiles in China, 44.6% for automobiles, 19.1% for electronic appliances and 8.9% for instrumentation in the United States, and 44.6% for electronic appliances, 29.7% for automobiles and 25.7% for machinery and instrumentation in Japan.
1. Performance of PBT
The mechanical properties and heat resistance of pure PBT are not good, so it is rarely used alone, and more than 80% of PBT used for engineering plastics are modified varieties.
(1) General performance
PBT is a crystalline thermoplastic resin, non-toxic, tasteless and odorless, with a relative density of 1.31, a water absorption rate of 0.07%, and a shiny surface.
(2) Mechanical properties The mechanical properties of PBT after being reinforced and modified by glass fiber are greatly improved, such as tensile strength can be increased by as much as 150%. PBT has excellent creep resistance. The mechanical properties are less affected by temperature, and the dimensional stability is very good at high temperature, which can be used for high-temperature stressed structural parts.
(3) Thermal performance The heat deflection temperature of PBT can be increased by as much as 280% after enhanced modification, which is the largest increase in plastics. Among engineering thermoplastics, reinforced PBT has the smallest coefficient of linear expansion.
(4) Electrical properties PBT has excellent electrical insulation properties, and the dielectric strength after enhanced modification is as high as 23Kv/mm. Electrical insulation is less affected by temperature and humidity, but more by frequency.
(5) Chemical properties PBT is an ester polymer, not resistant to strong acids, strong alkalis and phenol chemical reagents, good oil resistance, stable to alcohols, ethers, aliphatic hydrocarbons, high molecular weight esters, etc., and strong stress cracking resistance to organic solvents.
2. Modified varieties of PBT
The most important modified varieties of PBT are enhanced PBT, and there are also modified varieties such as blending.
(1) Reinforced PBT More than 90% of reinforced PBT is reinforced with glass fiber, and the modification effect of reinforced PBT in terms of mechanical properties and thermal properties is very prominent, as mentioned above.
(2) PBT alloy
PBT/PET alloy Improve the warpage of PBT and the surface gloss of the product.
PBT/PC alloy Improves heat resistance and impact strength.
When the blending ratio of PBT/PU/PC alloy is 50/25/25, the notched impact strength is 850~1400J/m.
Low-warpage PBT is filled with inorganic fillers such as talc, mica and glass beads to improve warpage.
3. Molding and processing of PBT
(1) Processing characteristics of PBT
Both PBT and reinforced PBT have good molding fluidity and can be made into thin-walled products. However, the rheological curve of PBT is relatively steep, the molding temperature range is relatively narrow, and the temperature requirements in the molding process are relatively strict, the temperature is too high will overflow, and the temperature is too low will block the nozzle.
PBT has a fast crystallization speed and a short molding cycle. The difference between 0.25% shrinkage in the flow direction and 1% shrinkage in the vertical direction is significant. PBT is one of the varieties with the lowest water absorption rate among engineering plastics, and generally does not need to be dried. If it is to be dried, the specific conditions are: 120~130 °C, 3~5h, so that the water content is below 0.05%.
PBT processing waste is generally not used alone, and is often added to new materials in a proportion of 20%~30%.
(2) PBT is easy to process and can be molded by injection molding and extrusion methods, but injection molding is the main method.
4. Application scope of PBT
(1) Electronic appliances such as connectors, switches, socket covers, circuit breaker covers, picture tube sockets, digital tube sockets, integrated circuit bases, coil winding tubes, transformer skeletons and small motor covers, etc.
(2) At present, the amount of automobiles is not as much as that of PA, PC and POM, and is mainly used for bumpers, fenders, steering wheels, automobile ignitions, rear corner grilles, engine heat vent covers and brush brackets.
(3) Machinery manufacturing gears, cams, transmission shafts and buttons, etc.
(4) Instrumentation Office equipment shells, camera parts and meter accessories, etc.
(2) Polyethylene terephthalate (PET)
Polyethylene terephthalate is a polymer made by direct esterification of polyethylene terephthalic acid and ethylene glycol or polyethylene terephthalate and ethylene glycol esterification method, commonly known as "polyester". The English abbreviation is PET.
At present, PET is used in fiber and plastic products, and plastic products are mainly used in transparent bottles, films and sheets;
The outstanding advantages of PET film are good barrier, mechanical properties and toughness, and the characteristics of PET glass fiber reinforced and modified are high mechanical properties and little influence by temperature, high heat resistance, high impact strength, good friction resistance, creep resistance, high rigidity, high hardness and good dimensional stability. Reinforced PET surpasses reinforced PBT in terms of mechanical properties, rigidity, and heat resistance, but its processability is not as good as that of PBT.
1. Performance of PET
(1) General performance PET resin is milky white translucent or colorless transparent body, with a relative density of 1.38 and a light transmittance of 90%. PET is a medium barrier material, with a transmittance coefficient of 50~90cm3•mm/(m2•d•MPa) for O2 and 180cm3•mm/(m2•d•MPa) for CO2. The water absorption rate of PET is 0.6%, and the water absorption is large.
(2) Mechanical properties The tensile strength of PET film is very high, comparable to aluminum foil, 9 times that of HDPE film, and 3 times that of PC and PA film. Reinforced PET has low creep, excellent fatigue resistance (better than reinforced PC and PA), and good abrasion and friction resistance. The mechanical properties of PET are less affected by temperature.
(3) Thermal performance The heat resistance of pure PET is not high, but it is greatly improved after enhanced treatment, and the mechanical properties are better than PF laminates at 180 °C, and it is a variety with better heat resistance among reinforced thermoplastic engineering plastics. PET has good heat aging resistance, embrittlement temperature is -70 °C, and it still has a certain toughness at -30 °C. PET is not flammable, the flame is yellow and dripping.
(4) Electrical properties Although PET is a polar polymer, it has excellent electrical insulation and can still be well maintained at high frequencies. PET has poor corona resistance and cannot be used for high-voltage insulation, and electrical insulation is affected by temperature and humidity, and is greatly affected by humidity.
(5) Environmental performance PET contains ester bonds, which are not resistant to water, acid, and alkali under the conditions of high temperature and water vapor. PET is stable to organic solvents such as acetone, benzene, toluene, trichloroethane, carbon tetrachloride and oils, and is also highly resistant to some oxidants such as hydrogen peroxide, sodium hypochlorite and potassium dichromate. PET has excellent weather resistance and can be used outdoors for a long time.
2. Modified varieties of PET
(1) Reinforced modified PET is mainly used for glass fiber, in addition to carbon fiber, silicon fiber, boron fiber, etc. Enhanced modification mainly improves the heat resistance, mechanical properties and dimensional stability of PET under high loads.
(2) Blended modified PET
PET/PBT PET is blended with PBT and 0.5% talc is added as a nucleating agent, and the blend has the properties of low shrinkage, heat resistance and excellent impact resistance.
PET/PC to improve the impact strength of the product, specifically adding a small amount of maleic anhydride grafted PE to PET/PC, or PET/PC/ABS ternary blending and adding talcum powder as a nucleating agent.
PET/PA improves the impact strength and dimensional stability of products, and PP-MAH compatibilizers are often added to PET/PA blends.
In addition, PET/PE, PET/EPDM and PET/SBS are available to improve impact performance.
(3) Crystallized modified PET
Crystallization modification is to speed up the crystallization speed, often add sodium salt of ethylene-methacrylic acid polymer, polyethylene glycol diglycidyl ether, polyethylene oxide, sodium salt of ethylene-maleic anhydride copolymer, glycidyl methacrylate, sodium acetyl acetate and polybutyl adipate.
3. PET molding and processing
(1) Processing characteristics of PET
PET is a polar polymer with large melting temperature and melt viscosity, while PET is a non-Newtonian fluid with low viscosity sensitivity to temperature and high sensitivity to shear rate.
PET has high water absorption, and must be dried before processing, and the drying conditions are: 130~150 °C, 3~4h.
The processing temperature range of PET is narrow, generally 270~290 °C, close to the decomposition temperature of 300 °C, therefore, the temperature should not be too high during processing. The crystallization speed of PET is slow, in order to promote crystallization, high temperature mold is often used, and the mold temperature is 100~130 °C.
The shrinkage rate of PET molding is large, which can be greatly reduced after enhanced modification, but post-processing is required when producing high-precision products. The conditions of post-treatment are: 130~140°C, 1~2h.
(2) PET processing method
Injection molding transparent products often use hot runners, and the screw length to diameter ratio is larger.
Extrusion is used in the production of film and sheet products. In order to improve the mechanical properties and optical properties of the product, bidirectional stretching is often carried out.
Blow molding is used to produce PET bottles, which are commonly formed by injection-pull-blow molding to ensure the tensile modification effect.
4. The application range of PET
In addition to fibers, PET is mainly used in three categories: films and sheets, bottles and engineering plastics.
(1) Films and sheets are mainly used for packaging materials, such as food, medicine and non-toxic and sterile sanitary packaging, high-grade packaging of textiles, precision instruments and electronic components, audio tapes, video tapes, photographic negatives, motion picture films, disks, optical discs, magnetic cards and other substrates, capacitor films, flexible printed circuit boards and membrane switches.
(2) Bottles PET bottles have high transparency and good barrier properties, and can be used for fresh-keeping packaging materials. Such as beer, liquor, carbonated drinks, edible oil, food, condiments, medicines, cosmetics and health food.
(3) The reinforced modified varieties of engineering plastics PET are mainly used in the following aspects:
Electronic and electrical connectors, coil coil coils, integrated circuit shells, capacitor shells, transformer shells, TV accessories, tuners, switches, timer shells and relays, etc.
Auto Parts Switchboard covers, valves, exhaust parts, distributor covers and small motor housings, etc.
Mechanical parts Gears, cams, pump housings, pulleys, motor frame frames, clock parts, etc.
Zipper material is the third generation of zipper material after PA and POM, and can be used in both wide and narrow specifications.