Polypropylene is a polymer formed by the polymerization of propylene monomer by free radicals, abbreviated as PP in English. According to different structures, PP can be divided into three categories: isoregular, inter-gauge (also known as metallocene PP), and random. At present, the main application is isometric PP, and the dosage can account for more than 90%. Random PP cannot be used for plastics, and is often used for modified carriers. Intermodal PP is a highly elastic thermoplastic material produced by metallocene catalysts for low-crystalline polymers. Due to the high price, the current application of inter-regulated PP is not wide, but it is very promising, and it is a new growth point for PP resin.
The advantages of PP are excellent electrical insulation and chemical resistance, the highest mechanical properties and heat resistance among general-purpose thermoplastics, good fatigue resistance, and the lowest price among all resins. PP reinforced with glass fiber has high strength and performance close to engineering plastics, and is often used as engineering plastics. The disadvantage of PP is that it is brittle at low temperature and has poor aging resistance.
1. Structural properties of polypropylene plastics
(1) Structure of PP
PP is a linear structure, and the arrangement of methyl groups on its macromolecular chain is different, and the properties are also different: the structure of isometric PP is regular, with a high degree of crystallinity, high melting point, large hardness and rigidity, and good mechanical properties; random PP is amorphous, with very low strength, and is difficult to be used as a plastic; the performance of interstructured PP is between the two, with small hardness and rigidity, but good impact performance. The molecular structure characteristics of PP lead to poor oxidation resistance and radiation resistance of PP, which is difficult to use outdoors.
(2) Performance of PP
(1) General performance PP resin is a white waxy solid, similar in appearance to PE, but more transparent and lighter than PE, and is the lightest variety after TPX. PP is flammable, it continues to burn away from the flame, the flame is yellow on the top and blue on the bottom, there is a small amount of black smoke, molten dripping, and there is an oil smell. PP has low water absorption and low gas permeability. PP has good fibrillation and can be used in the production of propylene lun.
(2) Mechanical properties PP has good mechanical properties, its tensile yield strength and tensile strength are more than PE, tensile strength is also more than PS and ABS, and can be greatly improved after reinforcement and tensile treatment. The mechanical properties of PP are less affected by temperature, and the tensile strength can still be maintained at half of the temperature of 100°C.
The impact strength of PP is greatly affected by temperature, and the impact performance of PP is better above room temperature, but its impact performance deteriorates rapidly at low temperature. The impact strength of PP is also related to factors such as molecular weight, crystallinity, and crystal size.
The surface hardness and rigidity of PP products are higher, and have a good surface gloss, but they are not as high as PS and ABS.
The dry friction coefficient of PP is 0.12, which is close to that of PA, but does not decrease significantly in the lubricated state, and is only suitable for gears and bearings with low PV value and no impact. The abrasion resistance of PP is average, less than that of rigid PVC and PMMA, and slightly higher than that of HDPE.
PP has outstanding bending fatigue resistance, and the hinges made from it are not damaged after 70 million folds and bending. PP has better creep resistance than NDPE. Therefore, it can be used as engineering plastics after appropriate reinforcement modification treatment.
(3) Thermal performance PP has good heat resistance, the product can withstand 100 °C hot water boiling, and can be used for a long time at 100-120 °C. It is used in hot water pipes, and can be used at 150°C without deformation when it is not affected by external forces. But the low temperature resistance of PP is not good, in
-5~-20°C is embrittlement, and the product cannot be used in a low-temperature environment. The linear expansion coefficient of PP is large, and the thermal conductivity is medium.
(4) Electrical properties PP is a non-polar polymer with excellent electrical insulation properties, less affected by humidity, temperature and frequency, and good arc resistance, but not corona resistance. Due to the influence of low temperature brittleness, the application of PP in the field of electrical insulation is far less extensive than that of PE and PVC.
(5) Environmental performance PP has high chemical resistance, can resist acids, alkalis, salts and most organic solvents (such as alcohols, phenols, aldehydes, ketones and most carboxylic acids, etc.) in addition to strong oxidants, concentrated sulfuric acid and concentrated nitric acid, but low molecular weight aliphatic hydrocarbons, halogenated hydrocarbons and aromatics can make them swell, and can be soluble in aromatics and halogenated hydrocarbons such as decachlorinated naphthalene, tetrahydronaphthalene and 1,2,4-trichlorobenzene at high temperatures. PP has poor weather resistance and is sensitive to ultraviolet rays, so antioxidants and light stabilizers need to be added before it can be used outdoors. PP has better stress crack resistance than HDPE and PS, except for corrosive media such as concentrated sulfuric acid, concentrated chromic acid and aqua regia.
1. Molding and processing of polypropylene plastics
(1) Processing characteristics of PP
The water absorption rate of PP is low, and the water absorption rate is less than 0.01% when soaked in water for 24h, so it does not have to be dried before processing.
The melt of PP is close to that of a non-Newtonian fluid, and the viscosity is less sensitive to temperature, depending on the magnitude of the shear rate.
The shrinkage rate of PP molding is large, generally up to 1.6%-2%, which has a great impact on the accuracy of the product, and it is easy to produce orientation in the processing, resulting in the performance difference of the product in different directions, which should be paid attention to in the production.
PP is sensitive to notches, and sharp corners and notches should be avoided in the product to avoid stress concentration.
PP is particularly sensitive to oxygen at high temperatures, and antioxidants are generally added to resin synthesis to prevent thermal degradation during processing. PP melt contact with copper will cause degradation, and contact with copper or copper anti-agent should be avoided during processing. (Why is there PP material used in the existing wire rod to make the core jacket, and the core wire is made of copper, which will not be degraded?)
After annealing of PP products, the residual internal stress can be eliminated and the impact strength can be improved.
(2) Processing method of PP
PP can be processed and molded by injection molding, extrusion and blow molding.
A general-purpose injection molding machine can be used for injection molding, and the melt index of the raw material is medium (MI is 1-4.5).
The length of the screw feeding section of the extruder is longer than that of the PE screw to overcome the effects of low thermal conductivity.
The cooling conditions have a great influence on the transparency and impact performance of the product.
PP extruded products can be stretched. It can be stretched in both one and both directions, and the stretching ratio can reach more than three times. The strength, impact, transparency, heat resistance, surface gloss and barrier properties of PP products after stretching have been significantly improved.
3. Modified varieties of polypropylene plastics
(1) Metallocene PP (m-PP)
Compared with ordinary PP, m-PP has good fluidity, high strength, high hardness, better heat resistance and transparency, low melting point, excellent gloss and toughness. m-PP is mainly used for packaging films, automotive bumper injection molded parts, sheets and bottles.
m-PP can be processed by ordinary PP, but the barrel temperature is 30-40°C lower than that of ordinary PP.
(2) Copolymer PP (PP-C)
PP-C is a copolymer of propylene and ethylene, which can be divided into two types: random copolymer (PP-R) and block copolymer (PP-B).
The ethylene content of PP-R is 1%-7%, and compared with ordinary PP, its crystallinity and melting point are low, soft and transparent, and the temperature is lower
It still has good impact strength at 0°C, and the application limit is reached at -20°C. However, its hardness, rigidity, creep resistance, etc. are 10%-15% lower than ordinary PP, and are mainly used for water supply pipes and heating pipes.
The ethylene content of PP-B is 5%-20%, which has both good rigidity and good low-temperature toughness. It is mainly used for large containers, turnover boxes, hollow blow molding containers, mechanical parts, wire and cable coating products, etc.
(3) Reinforced PP
Glass fiber is commonly used as a reinforcing material. The reinforcement not only retains the original excellent properties of PP, but also significantly improves the tensile strength, heat resistance, rigidity, hardness, creep resistance, linear expansion coefficient and molding shrinkage.
(4) Fill PP
Filler modification of PP is the most commonly used. The filling materials include calcium carbonate, talc, mica and wood powder. Coupling agent activation is required prior to filling to improve compatibility. The density, stiffness, hardness, heat deflection temperature, creep resistance, forming shrinkage and linear expansion coefficient of filled PP were improved, but the tensile strength, impact strength and elongation at break decreased.
(5) Blended PP
The main purpose of blending PP is to improve the low-temperature impact performance.
PP/HDPE blend HDPE content is 10%-40%, and the impact strength can be increased by as much as 8 times.
PP/EPDM (or EPR) blending improves impact performance and has obvious modification effect, which can be used for automotive bumpers and safety helmets.
PP/butadiene rubber blend The impact strength of the cantilever beam can be increased by more than 6 times, and the embrittlement temperature can be reduced by 8°C.
PP/PA blending Improves impact, abrasion and heat resistance.
(6) PP chloride
The flame retardancy, hardness, wear resistance, acid resistance, heat resistance, light resistance, aging resistance and adhesion of chlorinated PP are better than ordinary PP, and chlorinated PP is mainly used for raw materials such as coatings and films.
4. Application range of polypropylene plastics
(1) Injection molded products
PP resin injection molded products can account for about half, as follows:
Auto Parts Modified toughened PP for bumpers and wheel housings, etc., reinforced PP is used for instrument panels, steering wheels, fan blades, handles, and battery housings.
Daily necessities hangers, chairs, stools, buckets, basins, bookshelves, bathtubs, toys, stationery, office supplies, furniture, turnover boxes and cargo boxes, etc.
Electrical appliances Washing machine barrels, TV cases, electric fan blades, refrigerator linings, telephone cases, etc.
(2) Film products
PP film can account for about 10% of the amount of PP, which is characterized by transparency and surface gloss close to cellophane, but the softness is not good, and there is a strong sound when kneading by hand; high strength, can be used for heavy packaging materials; oxygen permeability is only 30% of HDPE film, suitable for moisture-proof packaging materials, such as high-end clothing, medicine and cigarettes.
PP film has good heat resistance, can be boiled and sterilized, used for refrigeration and fresh food packaging; PP film has good electrical insulation performance, and the heat-setting oriented film can be used as an insulating material for capacitors, motors and transformers, which is better than PET film; PP biaxially oriented film has good strength, transparency and gloss, and can be used for typewriter tape, adhesive tape base film and cigarette packaging film.
(3) Fiber products
It mainly includes three categories: monofilament, flat filament and fiber. PP monofilament has low density, good toughness and wear resistance, and is suitable for the production of ropes and fishing nets. PP flat wire has high tensile strength and is suitable for the production of woven bags and woven fabrics. PP fiber is widely used in carpets, blankets, clothing, mosquito nets, artificial turf, artificial wool, filter cloth, non-woven fabrics, etc.
(4) Extruded products
Pipes and fittings are new application fields of plastic products, mainly using PP-C as raw materials, used for water, drainage, heating and chemical corrosive media pipeline systems, and pipes and fittings are connected by hot melt method.
Sheet, PP sheet is made of PP/PE blend as raw material, mainly used for blister products. In addition, PP can also be used to make plates and bars, which can be used to produce automobile fenders, car seats, motor and pump housings, liquid storage tanks, etc.
(5) Hollow products
PP hollow products have good transparency and mechanical properties, and single-layer bottles are mainly used for packaging detergents, cosmetics and medicines, etc., and bottles compounded with barrier materials can be used for the packaging of food such as soy sauce, liquid fuels and chemical agents.
