A thermoplastic is a class of polymer that can be softened through heating and then processed using methods such as extrusion, injection moulding, thermoforming and blow moulding.
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Thermoplastics harden once cooled and do not show any changes in chemical property after being heated and cooled multiple times, making them easily recyclable.
Thermoplastics are made by joining small molecules, called monomers, together to form long chains using a process called polymerisation. A single polymer chain can be made from many thousands of monomers. The atoms in a polymer chain are joined by strong covalent bonds, whereas the forces between chains are weak.
Depending on the type of monomer, polymer chains may have side branches. If a polymer chain has only a few, short side branches then the chains can form ordered, crystalline regions, called spherulites. However, if the chain has many large side branches, then it is not possible for ordered regions to be formed and the polymer is amorphous. Examples of amorphous polymers are polystyrene (PS), polyvinyl chloride (PVC) and acrylonitrile-butadiene-styrene (ABS). Even for polymers with crystalline regions, there are always some amorphous regions between the crystallites, so these polymers are called semi-crystalline. Examples of semi-crystalline polymers are polyethylene (PE), polyamide (PA) and polypropylene (PP). For semi-crystalline polymers, as the temperature increases, the bonds between the polymer chains weaken to create a pliable solid and then a viscous liquid, which allows the plastic material to be shaped to produce parts.
Amorphous plastics are used for applications where optical clarity is required since light is scattered by crystallites. These amorphous plastics are, however, less resistant to chemical attack and environmental stress cracking due to the lack of crystalline structure.
Before a thermoplastic polymer can be used it is normally mixed with additives, such as stabilisers, plasticisers, lubricants, flame retardants and colourants, to improve the polymer’s functionality, stability or appearance. For example, stabilisers are added to reduce degradation due to sunlight or heat and plasticisers can be added to increase the mobility of amorphous chain segments, lowering the glass transition temperature and decreasing brittleness.
The advantages of thermoplastics include:
Despite the many advantages, there are also some limitations associated with thermoplastics. Due to their low melting point compared to metals, thermoplastics are inappropriate for use on some high temperature applications. In addition, some thermoplastics are susceptible to creep when exposed to long-term stress loads.
Thermoplastics come in a range of types with their own unique applications. Examples of thermoplastic polymers include:
Polyethylene is the most commonly used plastic in the world. It is in fact a family of materials that come with a range of densities and molecular structures, each with their own applications. Trade names include Alathon, Borstar, Dowlex, Eltex, Finathene, Hostalen, Lacqtene, Lupolen, Rigidex and Vestolen A. Strong and resistant to most chemicals, ultra high molecular weight polyethylene (UHMWPE) is used to manufacture moving machine parts, bearings, gears, artificial joints and bulletproof vests. High density polyethylene (HDPE) is used to make items such as chemical tanks, gas and water pipes, toys, shampoo bottles and margarine tubs. Medium density polyethylene (MDPE) is used for gas and water pipes, packaging film and pond liners. Linear low density polyethylene (LLDPE) is used for plastic bags, shrink/stretch films and food packaging. Being the softest and most flexible of these materials, low density polyethylene (LDPE) is used for the manufacture of squeeze bottles, sacks and sheets.
Polypropylene is the second most widely used commodity polymer in the world. It is used across a wide range of industries to create items including reusable food containers, sanitary products, heat resistant medical equipment, ropes, carpets, car batteries, cable insulation, storage bins, and even banknotes! Trade names include Adstif, Clyrell, Hifax, Hostalen, Inspire, Isoplen, Moplen, Novolen, and Vestolen.
Being tough, lightweight, and resistant to acids and bases, polyvinyl chloride (PVC), also known as vinyl, is used by the construction industry for items including water pipes, drainpipes, gutters and roofing sheets. Trade names include Astraglas, Benvic, Vestolit and Vinnolit.
PVC can also be made more flexible with the addition of plasticisers, where it is used for hoses, tubes, electrical insulation, clothing, upholstery and inflatable products such as waterbeds and pool toys. Trade names include, Acvitron and Lifolit.
Poly(ethylene terephthalate) (PET) or polyester has a good combination of mechanical and thermal properties, chemical resistance and dimensional stability. It is used for liquid containers, especially carbonated soft drinks, food containers and, in fibre form, for clothing. It is the most recycled polymer worldwide. Trade names include Dacron, Eastapak, Rynite and Terylene.
Polyamide (PA) is also known by the trade names Nylon, Akromid, Akulon, Grilamid, Grilon, Rislan and Ultramid. It was originally used as a replacement for silk when making items such as flak vests, parachutes and stockings. Nylon fibres are also used for fabric, carpets, rope and strings for musical instruments. It is also used for machine screws, gear wheels and power tool casings.
Polystyrene (PS), also known by the trade names Styron and Vampstyr, is manufactured in different forms that are suitable for different applications. It is used to make items such as disposable cutlery, cases for CDs and DVDs, and smoke detector housings. Expanded polystyrene (EPS) foam, also called by the trade name Styropor, is used for insulation and packaging materials and extruded polystyrene foam (XPS), also called by the trade name Styrofoam, is used for architectural models and drinking cups. Elsewhere, polystyrene copolymers are used for the manufacture of toys and product casings.
ABS, also known by trade names Cycolac and Ensidur, is a lightweight polymer that shows high impact resistance and mechanical toughness compared to most thermoplastics and is widely used in everyday consumer products like toys and telephones.
Polycarbonate (PC) is also known by trade names including arcoPlus, Lexan, Makroclear and Makrolon. Easy to mould and thermoform, it is used in a range of applications in the medical, construction, electronics, automotive and aerospace industries, including safety glasses, bullet-resistant glass, CDs and DVDs, car headlamp lenses and safety helmets.
Poly(methyl methacrylate) (PMMA) or acrylic, is also known by the trade names Acrylite, Altuglas, Lucite, Oroglas, Perspex and Plexiglas. It is widely used as a substitute for glass in aquariums, aircraft windows, motorcycle helmet visors and for the lenses on exterior automobile lights. Acrylic is also used for signage, for eye lenses and in bone cement for medical use, and also in paint, where PMMA particles are suspended in water.
Demonstrating a high stiffness, good dimensional stability and low friction, polyoxymethylene (POM), also known as acetal, polyacetal and polyformaldehyde, is used for parts that require high precision, such as bearings, valve parts, gears and electrical components, and is also known by the trade names Celcon, Delrin, Duracon, Hostaform, Kepital, and Ramtal.
Derived from renewable resources like sugar beet pulp, corn starch, chips, sugarcane and tapioca roots, poly(lactic acid) (PLA) is a compostable thermoplastic. It is used in tableware, food packaging and additive manufacturing (3D printing). Trade names include Bio-Flex, Fozeas and Ingeo.
Poly(phenylene oxide (PPO) offers a range of attractive properties, including high impact strength, heat distortion, and chemical stability against mineral and organic acids. It also offers low water absorption, but can be difficult to use due to the high processing temperature. Commercial resins, such as Noryl, blend PPO with high impact polystyrene (HIPS) to lower the processing temperature, making it easier to process. Applications include electrical components and washing machine parts.
Polytetrafluoroethylene (PTFE) belongs to a class of thermoplastics known as fluoropolymers, and is also known by the trade names Teflon, Dyneon, Fluon and Hostaflon. It has one of the lowest friction coefficients of any known solid and is well-known for its use on non-stick cookware. It is also used as a lubricant to reduce frictional wear between sliding parts like gears, bearings and bushings. Because it is chemically inert, it is also used for pipes and containers that come into contact with reactive chemicals.
Poly(vinylidene fluoride) (PVDF) is another member of the fluoropolymer family. It is also known by the trade names Kynar, Hylar and Solef, and is known for its chemical inertness and resistance, used for engineering sheets and pipes as well as to make powders and coatings. PVDF is also widely used in the chemical industry for piping to transport aggressive chemicals and high purity liquids.
Polyetheretherketone (PEEK) is a high-performance thermoplastic used for a range of engineering applications, including bearings, pumps, valves and medical implants, due to its good abrasion resistance and low flammability as well as low emission of smoke or toxic gases. Trade names include Victrex and Vestakeep.
Poly(phenylene sulphide) (PPS) delivers superb chemical resistance, electrical properties, flame retardance, and transparency to microwave radiation as well as a low coefficient of friction. These properties mean that, when injection or compression moulded at temperatures high enough to create crosslinks, PPS can also be used to make cookware, bearings and pump components suitable for corrosive environments. Trade names include Torelina and Ryton.
With a high heat distortion temperature, modulus and tensile strength, polyetherimide (PEI) is used in high performance electronic and electrical parts, including for the automotive industry, as well as in consumer items like microwave cookware. Trade names include Ultem.
Polyethersulfone (PESU, PES) has high hydrolytic, oxidative, and thermal stability as well as a good resistance to alkalis, salt solutions, acids from aqueous minerals, oils and greases. Application include medical components, gas separation membranes and freezer-to-microwave food containers. Trade names include Ultrason and Veradel.
Polybenzimidazole (PBI), also known by trade names including Celazole and Duratron, has a very high melting point compared with other thermoplastics and shows excellent chemical and thermal stability. PBI’s superb stability, retention, stiffness and toughness at high temperatures has lent it to being used for firefighting clothing, space suits for astronauts, protective gloves, welding apparel, wall fabrics for aircraft and for membranes in fuel cells.
Thermoplastics are easily recyclable as the polymer chain does not degrade when heated. Because the chemical bonds within the chain remain intact while the weaker bonds between polymer chains break down, thermoplastics can be melted and re-used repeatedly.
Most types of thermoplastic are safe to use as intended. However, there have been concerns raised over PVC because of the vinyl chloride monomer (VCM) that is used in production. However, modern manufacturing methods mean that the release of VCM is very low while the residual VCM left in the polymer is so low that it can’t be detected.
Most thermoplastics are not biodegradable. However, some thermoplastics, such as poly(lactic acid) (PLA), poly (vinyl alcohol) (PVAL, PVOH) and polyhydroxyalkanoates (PHAs) are.
Below their glass transition temperature (Tg), thermoplastics are brittle and deform by elastic deformation. However, when above their Tg, thermoplastics are ductile and deform mainly through plastic deformation. So, in short, thermoplastics go from brittle to ductile as they are heated through their Tg.
Thermoplastics can be remoulded repeatedly by heating and then reforming them into new shapes.
Semi-crystalline thermoplastics melt at a particular temperature when their crystalline regions transition to a random arrangement. This melting point is different for different thermoplastics. Amorphous thermoplastics do not have an ordered structure and therefore do not melt; they have a glass transition temperature, below which the material is brittle and, as the temperature increases, the material softens and becomes more rubbery.
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Thermoplastics can be painted to provide a different surface finish. However, you will need to use the correct type of paint so that it doesn’t react with any polymer coating and cause discolouration and lowering weather resistance. Acrylic based paints, including spray paints, are a good option for painting thermoplastics.
Thermoplastics can be welded using a variety of different techniques. You can find out more about welding thermoplastics here.
Thermoplastics are polymers that can be softened through heating before being processed and then left to cool and harden. Once cooled, they show no changes in chemical properties, meaning they can be re-melted and re-used several times.
There are many types of thermoplastic, each with their own distinct applications and properties, including being non-stick, tough, flexible, and so forth.
Thermoplastics are synthesised from a range of different materials, including renewable and biodegradable resources such as sugar beet, and have uses in industries including construction, aerospace, automotive, electronics, rail, oil and gas, and power, as well as for a huge range of domestic and consumer products.
TWI provides our Industrial Members with support in using a wide range materials, including thermoplastics. Our expertise includes testing different plastics and composites as well as materials selection and joining methods for polymeric materials used in different applications.
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A thermoplastic, or thermosoftening plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.[1][2]
Most thermoplastics have a high molecular weight. The polymer chains associate by intermolecular forces, which weaken rapidly with increased temperature, yielding a viscous liquid. In this state, thermoplastics may be reshaped, and are typically used to produce parts by various polymer processing techniques such as injection molding, compression molding, calendering, and extrusion.[3][4] Thermoplastics differ from thermosetting polymers (or "thermosets"), which form irreversible chemical bonds during the curing process. Thermosets do not melt when heated, but typically decompose and do not reform upon cooling.
Above its glass transition temperature and below its melting point, the physical properties of a thermoplastic change drastically without an associated phase change. Some thermoplastics do not fully crystallize below the glass transition temperature, retaining some or all of their amorphous characteristics. Amorphous and semi-amorphous plastics are used when high optical clarity is necessary, as light is scattered strongly by crystallites larger than its wavelength. Amorphous and semi-amorphous plastics are less resistant to chemical attack and environmental stress cracking because they lack a crystalline structure.
Brittleness can be decreased with the addition of plasticizers, which increases the mobility of amorphous chain segments to effectively lower the glass transition temperature. Modification of the polymer through copolymerization or through the addition of non-reactive side chains to monomers before polymerization can also lower it. Before these techniques were employed, plastic automobile parts would often crack when exposed to cold temperatures. These are linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling.
Today's acrylics industry can be divided into two distinct multibillion-dollar markets: the polyacrylic acids (PAA) and its ester derivatives (PAc) market, and the poly(methyl methacrylate) (PMMA) market.[5] PMMA is also known by trade names such as Lucite, Perspex and Plexiglas. It serves as a sturdy substitute for glass for items such as aquariums, buttons, motorcycle helmet visors, aircraft windows, viewing ports of submersibles, and lenses of exterior lights of automobiles. It is extensively used to make signs, including lettering and logos. In medicine, it is used in bone cement and to replace eye lenses. Acrylic paint consists of PMMA particles suspended in water.
For many decades, PMMA has been the predominant methacrylic ester produced worldwide. Major players in the PMMA market are Mitsubishi Rayon (Japan), Arkema SA (France), LG MMA (South Korea), Chi Mei Corp. (Taiwan), Sumimoto Chemical Company Ltd (Japan), Evonik Industries (Germany), BASF (Germany), Dow Chemical Company (U.S.), AkzoNobel (The Netherlands), Quinn Plastics (UK) and Cytec Industries (U.S.). Regarding the PAA and PAc market, key manufacturers are Nippon Shokubai Company Ltd. (Japan), Arkema SA (France) and Dow Chemical Company (U.S.).
Acrylonitrile butadiene styrene (ABS) is a terpolymer synthesized from styrene and acrylonitrile in the presence of polybutadiene, resulting in a long chain of polybutadiene crisscrossed with shorter chains of poly(styrene-co-acrylonitrile). ABS is a light-weight material that exhibits high impact resistance and mechanical toughness. It poses few risks to human health under normal handling. It is used in many consumer products, such as toys, appliances, and telephones.
Nylon belongs to a class of polymers called polyamides. It has served as a substitute mainly for hemp, cotton and silk, in products such as parachutes, cords, sails, flak vests and clothing. Nylon fibres are useful in making fabrics, rope, carpets and musical strings. In bulk form, nylon is used for mechanical parts including machine screws, gears and power tool casings. In addition, it is used in the manufacture of heat-resistant composite materials.
Polylactic acid (polylactide) is a compostable thermoplastic aliphatic polyester derived from renewable resources, such as corn starch (in the United States), sugar beet pulp (in Europe), tapioca roots, chips or starch (mostly in Asia), or sugarcane. It is the most common material used for 3D printing with fused deposition modeling techniques.
Polybenzimidazole (Poly-[2,2’-(m-phenylen)-5,5’-bisbenzimidazole], PBI) fiber is a synthetic fiber with a very high melting point. It has exceptional thermal and chemical stability and does not readily ignite. It was first discovered by American polymer chemist Carl Shipp Marvel in the pursuit of new materials with superior stability, retention of stiffness, toughness at elevated temperature. Due to its high stability, polybenzimidazole is used to fabricate high-performance protective apparel such as firefighter's gear, astronaut space suits, high temperature protective gloves, welders' apparel and aircraft wall fabrics. In recent years, polybenzimidazole found its application as membrane in fuel cells.
Polycarbonate (PC) thermoplastics are known under trademarks such as Lexan, Makrolon, Makroclear, and arcoPlus. They are easily worked, molded, and thermoformed for many applications, such as electronic components, construction materials, data storage devices, automotive and aircraft parts, check sockets in prosthetics, and security glazing. Polycarbonates do not have a unique resin identification code. Items made from polycarbonate can contain the precursor monomer bisphenol A (BPA). Susceptible to UV light, exposure results in yellowing. Degradation is especially visible in headlamps that lost or didn't have proper protective coating.
Polyether sulfone (PES) or polysulfone is a class of specially engineered thermoplastics[6] with high thermal, oxidative, and hydrolytic stability, and good resistance to aqueous mineral acids, alkalis, salt solutions, oils and greases.
Polyoxymethylene (POM), also known as acetal, polyacetal and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic polymers, it is produced by different chemical firms with slightly different formulas and sold variously by such names as Delrin, Celcon, Ramtal, Duracon, Kepital and Hostaform.
Polyether ether ketone (PEEK) is a colourless organic thermoplastic polymer in the polyaryletherketone (PEAK) family, used in engineering applications. It was originally introduced by Victrex PLC, then Imperial Chemical Industries in the early s. It has attractive properties such as good abrasion resistance, low flammability, and emission of smoke and toxic gases.
Polyetherimide (PEI), produced by a novel nitro displacement reaction involving bisphenol A, 4, 4’-methylenedianiline and 3-nitrophthalic anhydride, has high heat distortion temperature, tensile strength and modulus. They are generally used in high performance electrical and electronic parts, microwave appliances, and under-the-hood automotive parts.
Polyethylene (polyethene, polythene, poly, PE) is a family of similar materials categorized according to their density and molecular structure. It is obtained by polymerization of ethylene. It may be of low density or high density depending on the process used in its manufacturing. It is resistant to moisture and certain chemical agents. It is flexible at room temperature and low temperature and can be heat-sealed. As an inexpensive plastic, it is made in large amounts for varying uses. For example:
Polyphenylene oxide (PPO), which is obtained from the free-radical, step-growth oxidative coupling polymerization of 2,6-xylenol, has many attractive properties such as high heat distortion and impact strength, chemical stability to mineral and organic acids, and low water absorption. PPO is difficult to process, and hence the commercial resin (Noryl) is made by blending PPO with high-impact polystyrene (HIPS), which serves to reduce the processing temperature.
Polyphenylene sulfide (PPS) is obtained by the condensation polymerization of p-dichlorobenzene and sodium sulfide. PPS has desirable chemical resistance, electrical properties, flame retardance, low coefficient of friction and high transparency to microwave radiation. PPS is principally used in coating applications. This is done by spraying an aqueous slurry of PPS particles and heating to temperatures above 370 °C. Particular grades of PPS can be used in injection and compression molding at temperatures of 300 to 370 °C at which PPS particles soften and undergo apparent crosslinking. Principal applications of injection and compression molded PPS include cookware, bearings, and pump parts for service in various corrosive environments.
Polypropylene (PP) is useful for such diverse products as reusable plastic food containers, microwave- and dishwasher-safe plastic containers, diaper lining, sanitary pad lining and casing, ropes, carpets, plastic moldings, piping systems, car batteries, insulation for electrical cables and filters for gases and liquids. In medicine, it is used in hernia treatment and to make heat-resistant medical equipment. Polypropylene sheets are used for stationery folders and packaging and clear storage bins. Polypropylene is defined by the recyclable plastic number 5. Although relatively inert, it is vulnerable to ultraviolet radiation and can degrade considerably in direct sunlight. Polypropylene is not as impact-resistant as the polyethylenes (HDPE, LDPE). It is also somewhat permeable to highly volatile gases and liquids.
Polystyrene is manufactured in various forms that have different applications and can have medium to very low density. Extruded polystyrene (PS or xPS, sometimes colored pink/blue) is used in the manufacture of disposable cutlery, rigid ground contact rated insulating foam board, CD and DVD cases, plastic models of cars and boats, and smoke detector housings. Expanded polystyrene foam (EPS or "styrofoam", white) is used in making insulation and packaging materials, such as foam "peanuts" and molded foam used to cushion fragile products. Polystyrene copolymers are used in the manufacture of toys and product casings.
Polyvinyl chloride (PVC) is a tough, lightweight material that is durable, fairly rigid and versatile, and is resistant to acids and bases. PVC is used by the construction industry, such as for vinyl siding, drainpipes, gutters and roofing sheets. It is also converted to flexible forms with the addition of plasticizers, thereby making it useful for items such as hoses, tubing, electrical insulation, coats, jackets and upholstery. Flexible PVC is also used in inflatable products, such as water beds and pool toys. PVC is also a common material in vinyl action figures, especially in countries such as Japan, where the material is used extensively in so-called Sofubi figures (Soft vinyl toys[7]). As PVC bends easily and has a tendency to be bent during transit, a method to mitigate this deformation is to heat the plastic until it becomes mobile, then reform the material into the desired shape.
PVC is produced in many specific modifications to affect its chemical and physical properties. In plasticized polyvinyl chloride (pPVC), plasticizers are added to the raw material before molding to make it more flexible or pliable. Early on, the health and environmental aspects of this were poorly understood and replacements and product bans resulted after studies. The original form is often referred to as unplasticized polyvinyl chloride (uPVC), which is the more commonly used type for installations such as water, waste, and sewer conveyance plumbing.
Chemical modification often produces more drastic changes in properties. Chlorinated polyvinyl chloride (CPVC) is produced through exposing PVC to the continued free-radical chlorination reaction that originally formulates the PVC polymer. The chlorination reaction continues to add chlorine atoms to the polymer hydrocarbon backbone until most commercial applications reach a percent range between 56 and 74% total chlorine.[8] This increase in elemental chlorine content contributes to CPVC's increased expression of chlorine-based characteristics, such as chemical durability, resistance to acids, bases, and salts; susceptibility to ammonia-based compounds, aromatics, esters, ketones;[9] chemical stability; heat energy transfer resistance. CPVC is commonly used in water, chemical, hot and cold, delivery systems for residential, commercial, and industrial applications.
Polyvinylidene fluoride, PVDF, belongs to the fluoropolymer class of thermoplastics and is known for its high chemical inertness and resistance. PVDF is obtained through the polymerization of the vinylidene fluoride monomer. PVDF thermoplastic is fabricated into sheets and pipes for engineering uses as well as powders and coatings that can be dissolved in solvents and applied across a product surface. PVDF is widely used in the chemical industry as piping for aggressive chemicals and high purity liquids. The PVDF material is used in construction, transportation, chemical processes, electricity, batteries, waste water and treatment.[10]
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene is commonly recognized under the brand name Teflon. PTFE is hydrophobic: aqueous liquids do not wet the material, as fluorocarbons demonstrate mitigated London dispersion forces due to the high electronegativity of fluorine. This also supports its use in coatings of cooking ware. The polymer has one of the lowest coefficients of friction of any solid and is therefore commonly used for bearings and support of moving mechanical parts.
Thermoplastic composites (TPCs) are a class of composite materials that consist of a thermoplastic polymer matrix reinforced with fibers of carbon, glass, aramid, and others. Unlike thermoset composites, which undergo irreversible curing, thermoplastics can be melted and reshaped multiple times, making them more recyclable and repairable. Due to their high strength-to-weight ratio, impact resistance, and processability, thermoplastic composites are widely used in aerospace, automotive, and industrial applications.[11]
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