Thermoplastics are a type of plastic that is plastic at a certain temperature, solidifies after cooling, and can repeat this process. The molecular structure is characterized by linear polymer compounds. Generally, they do not have active groups and do not undergo linear molecular cross-linking when heated. Waste products can be recycled and reprocessed into new products. The main varieties include polyolefins (vinyls, olefins, styrenes, acrylates, fluorinated olefins, etc.), celluloses, polyether polyesters, and aromatic heterocyclic polymers.
Definition
Thermoplastics are the most widely used type of plastics. They are made of thermoplastic resins as the main component and various additives added to make plastics. Under certain temperature conditions, plastics can soften or melt into any shape, and the shape remains unchanged after cooling; this state can be repeated many times and always has plasticity, and this repetition is just a physical change. This type of plastic is called thermoplastic.
These include polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyoxymethylene, polycarbonate, polyamide, acrylic plastics, other polyolefins and their copolymers, polysulfones, and polyphenylene ether.
Structural classification
Thermoplastics can be divided into general-purpose plastics, engineering plastics, special plastics, etc. according to their performance characteristics, wide range of uses and universal molding technology. The main characteristics of general-purpose plastics are: wide range of uses, easy processing and good comprehensive performance. For example, polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), acrylonitrile -butadiene-styrene (ABS) are also known as the “five major general-purpose plastics”. The characteristics of engineering plastics and special plastics are: some structures and properties of polymers are particularly prominent, or the molding and processing technology is difficult, etc., and they are often used in professional engineering or special fields and occasions. The main engineering plastics are: nylon (Nylon), polycarbonate (PC), polyurethane (PU), polytetrafluoroethylene (Teflon, PTFE), polyethylene terephthalate (PET), etc., and special plastics such as “synthetic heart valves” and “artificial joints” of the ” medical polymer ” category.
According to the aggregate structure and performance characteristics of copolymers, they can be divided into two categories: crystalline plastics and non-crystalline plastics. Non-crystalline plastics are also called amorphous plastics.
TPV
Thermoplastic vulcanizate (TPV) is abbreviated as TPV. The Chinese abbreviation of thermoplastic vulcanizate is thermoplastic rubber (TPR), but this name is easily confused with other types of thermoplastic elastomers (TPR) because thermoplastic elastomers are usually also called thermoplastic rubbers, especially styrene elastomers. At least in China, “TPR” seems to have become its proprietary name. When TPR is mentioned, it refers to thermoplastic elastomers based on styrene elastomers such as SBS and SEBS. This is inseparable from the large consumption of styrene elastomers in the fields of civil and terminal consumer products.
If the name of thermoplastic vulcanizate is described in more detail, it should be thermoplastic dynamic vulcanizate (English: Thermoplastic Dynamic Vulcanizate). The word “dynamic” is added to more specifically describe the process of producing this thermoplastic vulcanizate – dynamic vulcanization. This process refers to the vulcanization of rubber during the melt blending of rubber and thermoplastics. Of course, while the rubber is being vulcanized, it is also continuously mixed with the thermoplastics. Therefore, the vulcanized rubber is distributed as a dispersed phase in the continuous phase of the thermoplastics. In contrast, thermoplastic static vulcanizate refers to the rubber that is first vulcanized by the traditional method, then ground into powder by a grinding device, and finally blended with the molten thermoplastic. In theory, this method can also produce TPV with excellent performance, but it is only in the laboratory stage.
Composition
TPV is mainly composed of two parts, one is plastic as the continuous phase, and the other is rubber as the dispersed phase. Usually rubber needs to be combined with softening oil or plasticizer. Vulcanizer and some auxiliary additives are also indispensable. In addition, in order to reduce costs or improve certain performance, some inorganic fillers will be added.
Many plastics and rubbers can form TPV, but only some blends have practical value after dynamic vulcanization, including PP/PE/EPDM, PP/NBR, PP/ACM, and PS/SEBS. In the book “Thermoplastic Elastomers” published by Chemical Industry Press, 99 rubber/plastic blends prepared from 11 rubbers and 9 plastics were reviewed. The study found that in order to obtain the best performance of rubber/thermoplastic dynamic vulcanization blends, the following conditions must be met:
- The surface energies of the two polymers, plastic and rubber, match;
- The length of rubber entanglement molecular chain is relatively low;
- The crystallinity of the plastic is greater than 15%. When the polarity or surface energy difference between the (PA66) plastic and the rubber is relatively large, adding a suitable compatibilizer and then performing dynamic vulcanization can also produce a blend with excellent performance.
Specific performance
1. Good elasticity and resistance to compression deformation, environmental resistance and aging resistance are equivalent to EPDM rubber, while its oil and solvent resistance is comparable to that of general-purpose chloroprene rubber.
2. It has a wide application temperature range (-60-150℃), a wide range of soft and hard applications (25A-54D), and the advantage of easy dyeing greatly increases the freedom of product design.
3. Excellent processing performance: It can be processed by thermoplastic processing methods such as injection and extrusion, which is efficient, simple and easy, without adding equipment, with high fluidity and small shrinkage.
4. Green and environmentally friendly, recyclable, and no significant performance degradation after six repeated uses, in line with EU environmental protection requirements.
5. Light specific gravity (0.90-0.97), uniform appearance quality, high surface grade and good hand feel.
Based on the above performance characteristics, TPV has certain substitution advantages in comprehensive performance and comprehensive cost compared with traditional rubber materials, other TPE elastomers (including TPR\SBS, SEBS, TPU, etc.) or plastic materials such as PVC in a wide range of application fields, thus providing new options for product companies in product innovation, increasing product added value and improving competitiveness.
TPV is the abbreviation of Thermoplastic Vulcanizate. Its Chinese name is thermoplastic EPDM dynamically vulcanized elastomer or thermoplastic EPDM dynamically vulcanized rubber. It is a polymer elastomer material composed of highly vulcanized EPDM particles dispersed in a continuous polypropylene PP phase. The physical properties and functions of TPV at room temperature are similar to those of thermosetting rubber. At high temperatures, it exhibits the characteristics of thermoplastic plastics and can be processed and formed quickly, economically and conveniently. TPV thermoplastic EPDM dynamically vulcanized elastomer/rubber disperses EPDM particles with a size of less than 2 microns in a polypropylene PP plastic matrix through dynamic vulcanization of vulcanized rubber materials, combining the characteristics of rubber and plastic well to obtain a high-performance elastomer material with excellent comprehensive performance.
Main properties of TPV thermoplastic EPDM dynamically vulcanized elastomer/rubber:
1. TPV can be used in the temperature range of -60℃～135℃, with a wide range of application temperature;
2. TPV hardness range is between 25A and 65D, which can meet a wide range of hardness requirements;
3. TPV has good weather resistance and excellent anti-aging, ozone resistance and UV resistance;
4. TPV does not require vulcanization when used and can be directly processed by injection, extrusion, calendering, blow molding, etc., which can shorten the processing process and reduce processing costs;
5. TPV’s environmental resistance is similar to that of EPDM, and its oil and solvent resistance is similar to that of chloroprene rubber;
6. TPV is easy to weld, reusable, environmentally friendly and non-toxic.
Main features of TPV thermoplastic EPDM dynamically vulcanized elastomer/rubber:
1. Excellent anti-aging performance and good weather resistance and heat resistance;
2. Excellent resistance to permanent deformation;
3. Excellent tensile strength, high toughness and high resilience;
4. Excellent environmental performance and reusable;
5. Excellent electrical insulation performance;
6. Wide range of hardness;
7. Wide operating temperature range;
8. Diversified colors, including fully transparent, semi-transparent and light-colored series, easy to color and process;
9. It can be co-injected or extruded with various materials such as PP, PA, PC, ABS, PS, PBT, PET, etc.
Application areas of TPV thermoplastic EPDM dynamically vulcanized elastomer/rubber:
Automotive industry
1. Automobile sealing strips and seals series;
2. Automobile dust cover, fender, ventilation pipe, buffer, bellows, air intake pipe, etc.;
3. Automobile high voltage ignition wire. It can withstand 30-40KV voltage and meet UL94 V0 flame retardant requirements.
Consumer goods
1. Parts of hand tools, power tools, lawn mowers and other gardening equipment;
2. Gaskets and parts used in household appliances;
3. Handles of scissors, toothbrushes, fishing rods, sports equipment, kitchen supplies, etc.
4. Various packaging for cosmetics, beverages, food, sanitary products, medical appliances, etc.
5. Soft parts of various wheels, buzzers, pipes, belts and other joints.
6. Needle stoppers, bottle stoppers, straws, cannulas and other soft plastic parts;
7. Flashlight housings, children’s toys, toy tires, golf bags, various grips, etc.
Electronic appliances
1. Various headphone cable covers and headphone cable connectors;
2. Mining cables, CNC coaxial cables, common and high-grade wire and cable insulation layers and sheaths;
3. Power sockets, plugs and sheaths, etc.;
4. Batteries, wireless telephone housings and electronic transformer housing jackets;
5. Insulation layer and sheath of power cables in ships, mines, drilling platforms, nuclear power plants and other facilities.
Transportation equipment
1. Expansion joints of roads and bridges;
2. Road safety facilities, buffer and anti-collision components;
3. Container sealing strip.
Construction materials
1. Power component sealing strip
2. Building expansion joints and sealing strips
3. Seals for water supply and drainage pipes, water irrigation system control valves, etc.
TPR
Thermoplastic Elastomer (TPE), also known as Thermoplastic Rubber (TPR), is a material that has the characteristics of both rubber and thermoplastic plastics. Thermoplastic elastomers have a variety of possible structures. The most fundamental one is that there must be at least two mutually dispersed polymer phases. At normal use temperature, one phase is a fluid (the temperature is higher than its Tg-glass transition temperature), and the other phase is a solid (the temperature is lower than its Tg or equal to Tg), and there is interaction between the two phases. That is, a polymer material that shows rubber elasticity at room temperature and can be plasticized and formed at high temperature has mechanical properties and performance similar to rubber, and can be processed and recycled as thermoplastic plastics. It builds a bridge between plastic and rubber. Therefore, thermoplastic elastomers can process rubber products as quickly, effectively and economically as thermoplastic plastics. In terms of processing, it is a plastic; in terms of properties, it is a rubber. Thermoplastic elastomers have many advantages over thermosetting rubbers. There is no unified naming for thermoplastic elastomers. It is customary to use the English abbreviations TPR for thermoplastic rubber and TPE for thermoplastic elastomer. Both are used in relevant materials and books. For the sake of uniformity, they are all referred to as TPE or thermoplastic elastomers. In China, thermoplastic styrene-butadiene block copolymers are called SBS (styrene-butadiene-styren block copolymers), thermoplastic isoprene-styrene block copolymers are called SIS (styrene-isoprene block copolymers), and saturated SBS is called SEBS, which is the abbreviation of Styrene-ethylene-butylene-styrene block copolymer, that is, styrene-ethylene-butylene-styrene block copolymers. Other types of thermoplastic elastomers are called by the manufacturer’s product name. China also uses the code SBS to represent thermoplastic styrene-butadiene-styrene block copolymers, which are customarily called thermoplastic styrene- butadiene rubberElastomer is a man-made thermoplastic elastomer with unique properties and has a very wide range of uses. Its excellent product applicability comes from the adjustability and controllability of its special molecular structure, thus showing the following excellent properties:
Excellent physical properties: good appearance texture, mild touch, easy to color, uniform and stable color tone; adjustable physical properties, providing a wide range of product design space; mechanical properties comparable to vulcanized rubber, but without vulcanization and cross-linking; wide hardness range, adjustable from SHORE-A 0 degrees to SHORE-D 70 degrees; excellent tensile resistance, tensile strength up to more than ten MPa, elongation at break up to more than ten times; long-term temperature resistance can exceed 70℃, good low temperature environment performance, still maintain good bending performance at -60℃; good electrical insulation and voltage resistance characteristics. It has outstanding anti-slip performance, wear resistance and weather resistance.
Excellent chemical properties: resistant to general chemicals (water, acid, alkali, alcohol solvents); can be processed in solvents and can be immersed in solvents or oils for a short period of time; non-toxic; good resistance to ultraviolet radiation and oxidation, can be used in outdoor environments; good bonding properties, using appropriate adhesive technology can directly and firmly bond to the surface of genuine leather, synthetic leather or artificial leather.
Production and processing advantages: It has the characteristics of traditional vulcanized rubber without vulcanization, saving auxiliary raw materials such as vulcanizers and accelerators; It is suitable for various processes such as injection molding, die-casting, hot melt and dissolving coating; Edge materials, residual materials and waste materials can be completely recycled and reused without changing the performance, reducing waste; Simplify the processing technology, save processing energy consumption and equipment resources, shorten the processing cycle, reduce production costs and improve work efficiency; The processing equipment and process are simple, saving production space and reducing the rate of defective products; The product is non-toxic, has no irritating odor, and does not harm the environment, equipment and personnel; The material can be reused repeatedly, and the edge waste can be recycled. It can be said that there is no waste in the production; There are fewer processing aids and compounding agents, which can save the cost of product quality control and testing; The product has high dimensional accuracy and easier quality control; The material has a low specific gravity and is adjustable; It can be directly mixed with PP, ABS and other plastics to make special plastic alloys.
Elastomers have excellent physical and chemical properties and are easy to process. At the same time, the product is non-toxic, pollution-free and can be recycled for secondary processing. Therefore, it is widely used in many industrial fields, such as toys, sports equipment, footwear, stationery, hardware, power tools, communications, electronic products, food and beverage packaging, household appliances, kitchen supplies, medical equipment, automobiles, construction projects, wires and cables, etc. More importantly, it is a high-quality material that leads new product design and market orientation. Its soft texture, adjustable physical properties, hardness, suitability for a variety of processing techniques and environmental advantages provide a huge space for product designers to play. This will undoubtedly provide great help for you to innovate products, increase value and lead market trends.
Performance characteristics
The molecular weight of polymers in general thermoplastics can reach hundreds of thousands to millions, and the length of macromolecular chains can reach 10 -3 mm. These macromolecules can be linear, such as LLDPE and HDPE, or branched, such as LDPE. The macromolecules are entangled with each other, arranged in a disordered or relatively orderly manner, forming an ” aggregate structure “.
When the macromolecules are completely disordered, we call it amorphous thermoplastics, such as PVC, PC, PMMA, etc. Its performance characteristics are: good transparency, low mechanical strength, and good flexibility.
The ones with some macromolecules or macromolecules evenly arranged are called crystalline thermoplastics, such as LLDPE, POM, nylon, etc., and their performance characteristics are: poor transparency, high mechanical strength, and low flexibility.
——Since polymer molecular chains are very long, unlike low molecular weight substances, the entire molecule cannot enter the “crystallization zone” and achieve complete crystallization. Therefore, ” crystallinity ” is often used to describe the degree of crystallization (or the size of the crystallization zone) of crystalline thermoplastics.
The characteristic temperature of amorphous thermoplastics is the glass transition temperature (Tg). When it is lower than Tg, the polymer exhibits “glass” characteristic properties, which is professionally called ” glass state “. At this time, the polymer has the function of use, but cannot be “plastic”; when it is higher than Tg, the polymer has high elasticity and certain plasticity characteristics, and loses the function of use, which is professionally called ” high elastic state “. After further heating, its elasticity is lost and it becomes completely plastic. Therefore, its maximum use temperature should be below Tg, and its minimum processing temperature should be above Tg.
The characteristic temperature of crystalline thermoplastics is the crystallization temperature (Tc). When it is lower than Tc, the polymer is hard and functional, and cannot be “plasticized”; when it is higher than Tc, the polymer is melted and plasticized, loses its functional use, but can be plasticized.
Compared with crystalline thermoplastics, amorphous thermoplastics have three physical property states, while crystalline thermoplastics have only two physical property states without “high elastic state”. This is reflected in the processing technology, where the former often uses a “gradual screw” while the latter uses a ” mutated screw”.
For crystalline thermoplastics, the area containing regularly arranged molecular chains is usually called the crystalline region. The crystallinity of many crystalline thermoplastics can be adjusted by controlling the cooling rate of the molding temperature. When the cooling rate is fast, the crystallization process is inhibited, and the final product with better transparency is obtained, such as PET bottles, transparent PET sheets and transparent polypropylene sheets.
Distinguish
Plastics can be divided into two categories: thermosetting plastics, which cannot be reshaped, and thermoplastic plastics, which can be produced over and over again.
Thermoplastics
When heated, it softens and flows, and when cooled, it hardens. This process is reversible and can be repeated. Polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyoxymethylene, polycarbonate, polyamide, acrylic plastics, other polyolefins and their copolymers, polysulfones, polyphenylene ether, chlorinated polyether, etc. are all thermoplastics. The resin molecular chains in thermoplastics are linear or branched structures, and there are no chemical bonds between the molecular chains. When heated, it softens and flows. The process of cooling and hardening is a physical change.
Thermosetting plastics
It can soften and flow when heated for the first time. When heated to a certain temperature, a chemical reaction occurs, cross-linking and solidification occurs, and it becomes hard. This change is irreversible. After that, it can no longer soften and flow when heated again. It is precisely with the help of this characteristic that molding is carried out. The plasticization flow during the first heating is used to fill the mold cavity under pressure, and then solidify into a product of a certain shape and size. This material is called a thermosetting plastic.
The resin of thermosetting plastics is linear or branched before curing. After curing, chemical bonds are formed between the molecular chains to form a three-dimensional network structure. It is not only insoluble in touch, but also insoluble in solvents. Plastics such as phenolic, aldehyde, melamine formaldehyde, epoxy, unsaturated polyester, and silicone are all thermosetting plastics.
Plastics used in harsh environments such as heat insulation, wear resistance, insulation, and high voltage resistance are mostly thermosetting plastics. The most commonly used plastics are probably frying pan handles and high and low voltage electrical appliances.
Note
Factors that affect the shrinkage of thermoplastic molding are:
1. Plastic types: During the molding process of thermoplastics, there are volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors. Therefore, compared with thermosetting plastics, their shrinkage rate is larger, the shrinkage rate range is wider, and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity adjustment treatment is generally larger than that of thermosetting plastics.
2. Plastic part characteristics When the molten material contacts the cavity surface during molding, the outer layer immediately cools to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of the plastic part cools slowly to form a high-density solid layer with large shrinkage. Therefore, the thicker the wall, the slower the cooling, and the thicker the high-density layer, the greater the shrinkage. In addition, the presence or absence of inserts and the layout and number of inserts directly affect the material flow direction, density distribution, shrinkage resistance, etc., so the characteristics of the plastic part have a greater impact on the shrinkage size and directionality.
3. The form, size and distribution of the feed port directly affect the material flow direction, density distribution, pressure holding and shrinkage compensation and molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have small shrinkage but large directionality, while feed ports with wide widths and short lengths have small directionality. Feed ports that are close to the feed port or parallel to the material flow direction have large shrinkage.
4. Molding conditions: When the mold temperature is high, the molten material cools slowly, has high density, and shrinks greatly. Especially for crystalline materials, the shrinkage is greater due to the high crystallinity and large volume change. The mold temperature distribution is also related to the cooling inside and outside of the plastic part and the uniformity of density, which directly affects the shrinkage amount and directionality of each part. In addition, the holding pressure and time also have a great influence on the shrinkage. The shrinkage is small but the directionality is large when the pressure is high and the time is long. The injection pressure is high, the viscosity difference of the molten material is small, the interlayer shear stress is small, and the elastic rebound after demolding is large, so the shrinkage can also be reduced appropriately. The material temperature is high and the shrinkage is large, but the directionality is small. Therefore, adjusting the mold temperature, pressure, injection speed, cooling time and other factors during molding can also appropriately change the shrinkage of the plastic part.
When designing a mold, the shrinkage rate of each part of the plastic part is determined empirically based on the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the feed port, and the cavity size is then calculated.
Commonly used thermoplastics
PP is the abbreviation of Polypropylene in English, and its Chinese name is polypropylene. Advantages of polypropylene (PP):
1. It has excellent mechanical properties. Its strength and elasticity are higher than HDPE, and its resistance to bending fatigue is good.
2. It has good heat resistance, with a melting point of 164-170°C. The products can be sterilized at temperatures above 100°C. The heat deformation temperature can usually reach 110°C, and the brittle temperature is -35°C.
3. It has good chemical stability. It is relatively stable to other chemical reagents except being corroded by concentrated sulfuric acid and concentrated nitric acid. 4. Polypropylene has excellent high-frequency insulation performance. Since it hardly absorbs water, its insulation performance is not affected by temperature.
Disadvantages of polypropylene (PP):
1. The shrinkage rate is large, and thick-walled products are prone to dents.
2. At low temperatures, the impact strength is poor.
3. High static electricity, easy to age when in contact with copper.
4. Very sensitive to ultraviolet rays.
Compared with pure PP, the advantages of impact-modified PP are:
1. The impact strength, toughness and mechanical modulus are significantly improved. It can be seen from the performance table that the tensile strength, bending strength and hardness of the modified PP, which represent rigidity, are higher than those of pure PP, and the impact strength representing toughness is also improved, especially the low-temperature brittleness of PP.
2. Reduce the shrinkage rate and effectively improve the warping deformation and surface shrinkage of the product.
3. Improve the aging resistance of PP and greatly increase the service life of the product.
HDPE is the abbreviation of High Density Polyethylene, and its Chinese name is high-density polyethylene. Advantages of high-density polyethylene (HDPE):
1. Good impact resistance and cold resistance, and resistance to environmental stress cracking.
2. Excellent chemical stability and good oil resistance.
3. It absorbs very little water, has low water permeability, and has a high permeability of organic vapor.
4. Good electrical insulation, and the dielectric properties are extremely excellent in all frequency ranges.
Disadvantages of high-density polyethylene (HDPE):
1. The operating temperature of HDPE is not high, generally below 110°C.
2. HDPE has poor aging resistance. Under the influence of atmosphere, sunlight and oxygen, it gradually becomes brittle, and its mechanical strength and electrical properties decrease.
3. At the molding temperature, oxidation will cause the viscosity to decrease, discoloration and streaks to appear.
ABS is the abbreviation of Acrylonitrile Butadiene Styrene, and its Chinese name is acrylonitrile-butadiene-styrene copolymer. Advantages of acrylonitrile-butadiene-styrene copolymer (ABS):
1. Good rigidity, high impact strength, and will not drop rapidly at low temperatures.
2. Good heat resistance and low temperature resistance, high wear resistance, chemical resistance, and excellent electrical performance.
3. Easy to process and stable processing dimensions.
4. The surface has good gloss and is easy to paint and color. It can also be subjected to secondary processing such as metal spraying, electroplating, welding and bonding.
Disadvantages of acrylonitrile-butadiene-styrene copolymer (ABS):
1. ABS has strong hygroscopicity in the air and must be dried before injection molding. The resin needs to be pre-dried at 70-80°C for more than 4 hours.
2. Poor weather resistance.
