A brief introduction to the properties of commonly used plastic materials

Engineering plastics refer to a class of resins with good mechanical and thermal properties, can withstand large mechanical stress, can be used in more harsh chemical and physical environments, and are used to replace some metals as structural materials.
The tensile strength of engineering plastics is generally more than 50MPa, the flexural modulus is more than 2GPa, and the impact strength is more than 60J/m. Generally, engineering plastics that are used in large quantities and can be used at temperatures above 100 °C for a long time are called general engineering plastics, while a class of engineering plastics that are used in small quantities and use temperatures above 150 °C, some at 150~250 °C or even up to 300 °C are called special engineering plastics. Here are some of the characteristics of general-purpose engineering plastics.
1. Polyamide plastics
Polyamide is commonly known as "nylon", abbreviated as PA in English, and it is currently the largest variety of general-purpose engineering plastics. PA includes kinds
There are many, specifically named PAxy, where x represents the number of carbon atoms of the dibasic amine and y represents the number of carbon atoms of the dibasic acid. The PAs suitable for plastics are mainly aliphatic PA and a small amount of aromatic PA. The specific varieties are PA6, PA66, PA610, PA1010, PA11, PA12, PA9, PA612, PA46, PA1212, cast nylon (MC5) and aromatic nylon. Of these, PA6 accounted for 47% and PA66 accounted for 45%.
At present, the largest application of PA is auto parts, accounting for 30.7% in the United States, 31.6% in Western Europe, and 34.5% in Japan. Other uses include packaging films, electronic appliances, mechanical parts, daily necessities, etc.
1. The performance of amide plastics
(1) General performance The appearance of PA is transparent or opaque milky white or light yellow granules, the appearance is, hard, and the surface of the product is shiny. The larger the proportion of amide groups, the higher the water absorption, specifically:
PA6＞PA66＞PA610＞PA1010＞PA11＞PA12＞PA1212. PA is a self-extinguishing plastic that burns with a burnt wool or nail smell. PA is a medium barrier plastic, and the barrier property increases with the increase of the ratio of amide group/methylene, and PA6 has the best barrier effect. The O2 transmittance coefficient of PA6 is 25~40cm3•mm/㎡•d•Mpa
The CO2 permeability coefficient is 150~200cm3•mm/㎡•d•Mpa
The transmission coefficient of H2O is 150g•mm/㎡•d•Mpa
(2) Mechanical properties PA has higher tensile strength and impact strength at room temperature, but the impact strength is not as high as PC and POM. As the temperature and humidity increase, the tensile strength decreases sharply, while the impact strength increases significantly. The strength of glass fiber reinforced PA is less affected by temperature and humidity.
The fatigue resistance of PA is better, second only to POM, and can be increased by about 50% after glass fiber reinforcement treatment.
The creep resistance of PA is poor, and it is not suitable for the manufacture of precision stressed products, but it can be improved after glass fiber reinforcement.
PA has excellent friction resistance and wear resistance, and is a commonly used wear-resistant plastic variety. Among them, the friction factor of different varieties is not much different, and the friction factor of oil-free lubrication is only 0.1~0.3. PA1010 has the best abrasion resistance. The addition of molybdenum disulfide, graphite, F4 and PE to PA can further improve friction and wear resistance.
(3) Thermal performance The heat deflection temperature of PA is not high, generally at 50~75 °C. After being reinforced with glass fiber, it can be increased by more than 4 times, and the thermal conductivity coefficient of CPA up to 200°C is very small, only 0.16~0.4W/M•K. The linear expansion coefficient of PA is large and decreases with the increase of crystallinity.
(4) Electrical properties PA is an excellent insulating material under low temperature and low humidity conditions, but the insulation performance deteriorates dramatically with the increase of temperature and humidity, and the one with a large proportion of amide groups in the molecule is the most sensitive, for example, PA6 is the largest and PA12 is the smallest.
(5) Environmental performance PA has excellent chemical resistance and stability, and can resist most organic solvents such as alcohols, aromatics, esters and ketones, especially with outstanding oil resistance. However, the acid, alkali and salt resistance of PA is not good, which can lead to swelling, and the most harmful inorganic salt is zinc chloride. PA is soluble in formic acid and phenolic compounds.
PA has poor lightfastness, and its strength quickly decreases and becomes brittle in sunlight, so it cannot be used outdoors.
2. Molding and processing of amide plastics
(1) Processing characteristics
PA has an obvious melting point, and the melting point is high, the melting range is narrow, so the processing temperature is high, PA6 is 220~300 °C, PA66 is 260~320 °C.
PA has a low melt viscosity and good fluidity, and the melt viscosity is sensitive to both temperature and shear rate. However, its fluid characteristics are close to Newtonian fluids, that is, it is more sensitive to temperature.
The thermal stability of PA is poor, and the tendency of thermal degradation is serious, so diphenylamine should be added to improve it, and the temperature should be strictly controlled.
Crystallization occurs during PA molding, and the molding shrinkage is large, and the degree of crystallinity is greatly affected by processing conditions.
The water absorption rate of PA is relatively large, and it must be dried before processing to make the water content less than 0.1%. The drying condition is 100~110 °C, and the time is 10~12 hours.
After molding, PA products need to be humidified to reduce the impact of water absorption on performance and improve dimensional stability.
PA is prone to internal stress during processing and should be annealed.
(2) Processing method
PA can be molded by injection molding, extrusion, and blow molding.
3. Modified varieties of polyamide plastics
It mainly includes two types of reinforced PA and PA alloys.
(1) Reinforced PA is mainly made of glass fiber as the reinforcing material. The mechanical properties, hardness, creep, dimensional stability and heat resistance of glass fiber content greater than 30% have been significantly improved.
(2) There are many types of PA alloys, and the technology is mature, and the common ones are as follows:
① PA/PO. This alloy can increase the impact strength of PA in dry and low temperature conditions by 1.5-3 times, and reduce water absorption
300%. Unsaturated acid graft of PO for compatibilizer.
② PA/ABS. This alloy can improve the toughness, rigidity, hardness and arc resistance of products. The impact strength increases most when the ABS content is in the range of 15%-20%.
(3) PA/styrene-N-phenylmaleimide. This alloy mainly improves the heat-resistant temperature of PA, which can generally be increased to
110℃. In addition, it can improve impact strength and chemical resistance.
4. Application range of polyamide plastics
(1) Automobile industry The automobile industry is the largest market for PA, mainly used for engine parts, electrical parts, car body parts and oil conveying parts. The specific products are: oil pipes, air conditioning pipes, fuel injectors, fuel tanks, fuel filters, oil storage tanks, tanks, gears, wheel covers and automobile exterior trim panels.
(2) Machinery industry can widely manufacture gears, worm gears, gaskets, bolts, nuts, bearings, etc.
(3) Electronics/electrical appliances are mainly used for switches, wiring boards, resistors, etc. of civil electrical appliances such as rice cookers, vacuum cleaners, microwave ovens, etc.
(4) Packaging material PA has good gas barrier, often blended, co-extruded and compounded with HDPE, and is used for freezing and vacuum packaging of cooked meat, ham and other foods.
(5) Daily necessities PA is the first generation of zipper material, which is still widely used. In addition, PA can also be used in disposable lighter housings, alkaline dry cell liners, helmets, office equipment housings, etc.
(6) Sporting goods mainly include skis, racket strings, racket frames, surfboards, ice skates, fishing rods and fishing lines, etc.
(7) Medical devices can be used for blood vessels, hemostats, infusion sets, surgical sutures, wigs, etc.
5. Other varieties
(1)PA11
PA11 is polyundecamide, which is a long-chain soft nylon prepared from castor oil. PA11 has excellent mechanical properties, dimensional stability and oil resistance, and is a high-grade engineering plastic, mainly used for automobile oil pipes and brake pipes. PA11 has good flexibility, good stress crack resistance and dynamic fatigue resistance, the friction coefficient of copper in the non-lubricated state is 0.18, good wear resistance, and self-extinguishing. It is resistant to alkalis, alcohols, ketones, aromatics and oils, but has poor acid resistance.
PA11 is widely used in the manufacture of fuel pipes, brake pipes, pressure pipes and threaded pipes in the automobile industry, and is also used in cable sheath pipes in various vehicles such as aircraft, ships and automobiles, and military equipment such as gun stocks, gun handles, bullet casings, training bullets, etc.
(2)PA12
In addition to the similar characteristics of PA11, PA12 also has low water absorption, good flexibility, better electrical properties than other PAs, and good dimensional stability.
(3)MCPA
MCPA is monomer cast nylon, also known as MC nylon or molded nylon. MCPA uses caprolactam or valerolactam as monomer and is formed by alkali polymerization in a mold by alkali polymerization.
The molecular weight of MCPA is twice as large as that of PA6, so its mechanical properties, dimensional stability, fatigue resistance, wear resistance, impact strength, heat resistance, water absorption and electrical properties are more than twice that of PA6.
MCPA is mainly used for large-scale products, especially for products with low demand, complex structure, and high cost of opening injection molds. Specific products such as large gears, sliding bearings, rollers, valve seats and guide rails, etc., can be manufactured by casting first and then machining.
(4)RIMPA
RIMPA is a liquid injection molding method that is developed on the basis of MCPA and is a liquid injection molding method that completes the reaction and molds in a closed mold. Compared with PA6, it has higher crystallinity and rigidity, and lower water absorption.
(5) Transparent PA
Ordinary PA is a crystalline polymer, while transparent PA is an amorphous polymer, and the specific varieties are:
(1) Terephthalatetrimethylhexanediamine
Its light transmittance can reach more than 90%, the water absorption rate is 0.41%, it is not easy to scratch, it has good thermal stability, outstanding impact resistance, good dimensional stability, and good chemical corrosion resistance.
② PACP9/6
The light transmittance is 92%, which is close to that of optical glass. In addition, its tensile strength, rigidity, wear resistance, chemical corrosion resistance, surface hardness, etc. are better than other engineering plastics.
(6) Aromatic PA
Aromatic PA is a new type of PA variety developed in the 60s of the 20th century with high temperature resistance, radiation resistance and corrosion resistance. At present, the main application varieties are:
(1) Polyisophthalyl m-phenylenediamine
The English abbreviation is mPIPA, commonly known as aramid 1313. It has a melting point of 410 °C, a decomposition temperature of 450 °C, a embrittlement temperature of -70 °C, and can be used continuously at 200 °C without deformation. It has excellent mechanical and electrical properties, its tensile strength is 80-120Mpa, compressive strength is 320Mpa, the insulation performance is less affected by temperature and humidity, and the insulation is better than mica under humid conditions, and it can resist the radiation of γ rays of 5×109mGy.
(2) Poly-terenamide
The English abbreviation is mPTPA, commonly known as aramid 1414. It is the fastest-growing high-strength, high-modulus, and high-temperature-resistant fiber in recent years, and can also be made into film or laminated materials. MPTPA has a pretensile time of up to 200Mpa, a softening temperature of 280°C, and has the properties of fatigue resistance, good dimensional stability, low linear expansion coefficient and good environmental resistance.