Important Types Of Plastics
A wide variety of both thermoplastics and thermosetting plastics are manufactured. These plastics have a spectrum of properties that are derived from their chemical compositions. As a result, manufactured plastics can be used in applications ranging from contact lenses to jet body components.
A Thermoplastics
Thermoplastic materials are in high demand because they can be repeatedly softened and remolded. The most commonly manufactured thermoplastics are presented in this section in order of decreasing volume of production.
A1 Polyethylene
(PE) resins are milky white, translucent substances derived from CH2). Polyethylene, with the chemical formula9ethylene (CH2 ]n (where n denotes that the chemical formula inside the8CH28CH28[ brackets repeats itself to form the plastic molecule) is made in low- and high-density forms. Low-density polyethylene (LDPE) has a density ranging from 0.91 to 0.93 g/cm3 (0.60 to 0.61 oz/cu in). The molecules of LDPE have a carbon backbone with side groups of four to six carbon atoms attached randomly along the main backbone. LDPE is the most widely used of all plastics, because it is inexpensive, flexible, extremely tough, and chemical-resistant. LDPE is molded into bottles, garment bags, frozen food packages, and plastic toys.
High-density polyethylene (HDPE) has a density that ranges from 0.94 to 0.97 g/cm3 (0.62 to 0.64 oz/cu in). Its molecules have an extremely long carbon backbone with no side groups. As a result, these molecules align into more compact arrangements, accounting for the higher density of HDPE. HDPE is stiffer, stronger, and less translucent than low-density polyethylene. HDPE is formed into grocery bags, car fuel tanks, packaging, and piping.
A2 Polyvinyl Chloride
Polyvinyl chloride (PVC) is prepared from the organic compound vinyl CHCl). PVC is the most widely used of the amorphous9chloride (CH2 plastics. PVC is lightweight, durable, and waterproof. Chlorine atoms bonded to the carbon backbone of its molecules give PVC its hard and flame-resistant properties.
In its rigid form, PVC is weather-resistant and is extruded into pipe, house siding, and gutters. Rigid PVC is also blow molded into clear bottles and is used to form other consumer products, including compact discs and computer casings.
PVC can be softened with certain chemicals. This softened form of PVC is used to make shrink-wrap, food packaging, rainwear, shoe soles, shampoo containers, floor tile, gloves, upholstery, and other products. Most softened PVC plastic products are manufactured by extrusion, injection molding, or casting.
A3 Polypropylene
CH2) and has9CH8is polymerized from the organic compound propylene (CH3 CH3) branching off of every other carbon along the8a methyl group ( molecular backbone. Because the most common form of polypropylene has the methyl groups all on one side of the carbon backbone, polypropylene molecules tend to be highly aligned and compact, giving this thermoplastic the properties of durability and chemical resistance. Many polypropylene products, such as rope, fiber, luggage, carpet, and packaging film, are formed by injection molding.
A4 Polystyrene
CH2), has phenyl groups (six-member9, produced from styrene (C6H5CH carbon ring) attached in random locations along the carbon backbone of the molecule. The random attachment of benzene prevents the molecules from becoming highly aligned. As a result, polystyrene is an amorphous, transparent, and somewhat brittle plastic. Polystyrene is widely used because of its rigidity and superior insulation properties. Polystyrene can undergo all thermoplastic processes to form products such as toys, utensils, display boxes, model aircraft kits, and ballpoint pen barrels. Polystyrene is also expanded into foam plastics such as packaging materials, egg cartons, flotation devices, and styrofoam. (For more information, see the Expansion Processes section of this article.)
A5 Polyethylene Terephthalate
Polyethylene terephthalate (PET) is formed from the reaction of CH2OH),8COOH) and ethylene glycol (HOCH28C6H48terephthalic acid (HOOC ]n. PET molecules8CH2CH28COO8C6H48OOC8which produces the PET monomer [ are highly aligned, creating a strong and abrasion-resistant material that is used to produce films and polyester fibers. PET is injection molded into windshield wiper arms, sunroof frames, gears, pulleys, and food trays. This plastic is used to make the trademarked textiles Dacron, Fibre V, Fortrel, and Kodel. Tough, transparent PET films (marketed under the brand name Mylar) are magnetically coated to make both audio and video recording tape.
A6 Acrylonitrile Butadiene Styrene
Acrylonitrile butadiene styrene (ABS) is made by copolymerizing (combining two or more monomers) the monomers acrylonitrile (CH2CHCN) CH2). Acrylonitrile and styrene are dissolved in9and styrene (C6H5CH ] n, which allows these monomers to8CH9CH8CH9CH8polybutadiene rubber [ form chains by attaching to the rubber molecules.
The advantage of ABS is that this material combines the strength and rigidity of the acrylonitrile and styrene polymers with the toughness of the polybutadiene rubber. Although the cost of producing ABS is roughly twice the cost of producing polystyrene, ABS is considered superior for its hardness, gloss, toughness, and electrical insulation properties. ABS plastic is injection molded to make telephones, helmets, washing machine agitators, and pipe joints. This plastic is thermoformed to make luggage, golf carts, toys, and car grills. ABS is also extruded to make piping, to which pipe joints are easily solvent-cemented.
A7 Polymethyl Methacrylate
Polymethyl methacrylate (PMMA), more commonly known by the generic name acrylic, is polymerized from the hydrocarbon compound methyl methacrylate (C4O2H8). PMMA is a hard material and is extremely clear because of the amorphous arrangement of its molecules. As a result, this thermoplastic is used to make optical lenses, watch crystals, aircraft windshields, skylights, and outdoor signs. These PMMA products are marketed under familiar trade names, including Plexiglas, Lucite, and Acrylite. Because PMMA can be cast to resemble marble, it is also used to make sinks, countertops, and other fixtures.
A8 Polyamide
Polyamides (PA), known by the trade name Nylon, consist of highly ordered molecules, which give polyamides high tensile strength. Some polyamides are made by reacting dicarboxylic acid with diamines (carbon molecules with the ion –NH2 on each end), as in nylon-6,6 and nylon-6,10. (The two numbers in each type of nylon represent the number of carbon atoms in the diamine and the dicarboxylic acid, respectively.) Other types of nylon are synthesized by the condensation of amino acids.
Polyamides have mechanical properties such as high abrasion resistance, low coefficients of friction (meaning they are slippery), and tensile strengths comparable to the softer of the aluminum alloys. Therefore, nylons are commonly used for mechanical applications, such as gears, bearings, and bushings (see Engineering: Mechanical Engineering). Nylons are also extruded into millions of tons of synthetic fibers every year. The most commonly used nylon fibers, nylon-6,6 and nylon-6 (single number because this nylon forms by the self-condensation of an amino acid) are made into textiles, ropes, fishing lines, brushes, and other items.
B Thermosetting Materials
Because thermosetting plastics cure, or cross-link, after being heated, these plastics can be made into durable and heat-resistant materials. The most commonly manufactured thermosetting plastics are presented below in order of decreasing volume of production.
B1 Polyurethane
Polyurethane is a polymer consisting of the repeating unit ]n, where R may represent a different alkyl group than R’.8R’8OOCNH8R8[ Alkyl groups are chemical groups obtained by removing a hydrogen atom from an alkane—a hydrocarbon containing all carbon-carbon single bonds. Most types of polyurethane resin cross-link and become thermosetting plastics. However, some polyurethane resins have a linear molecular arrangement that does not cross-link, resulting in thermoplastics.
Thermosetting polyurethane molecules cross-link into a single giant molecule. Thermosetting polyurethane is widely used in various forms, including soft and hard foams. Soft, open-celled polyurethane foams are used to make seat cushions, mattresses, and packaging. Hard polyurethane foams are used as insulation in refrigerators, freezers, and homes.
Thermoplastic polyurethane molecules have linear, highly crystalline molecular structures that form an abrasion-resistant material. Thermoplastic polyurethanes are molded into shoe soles, car fenders, door panels, and other products.
B2 Phenolics
Phenolic (phenol-formaldehyde) resins, first commercially available in 1910, were some of the first polymers made. Today phenolics are some of the most widely produced thermosetting plastics. They are produced by reacting phenol (C6H5OH) with formaldehyde (HCOH). Phenolic plastics are hard, strong, inexpensive to produce, and they possess excellent electrical resistance. Phenolic resins cure (cross-link) when heat and pressure are applied during the molding process. Phenolic resin-impregnated paper or cloth can be laminated into numerous products, such as electrical circuit boards. Phenolic resins are also compression molded into electrical switches, pan and iron handles, radio and television casings, and toaster knobs and bases.
B3 Melamine-Formaldehyde and Urea-Formaldehyde
Urea-formaldehyde (UF) and melamine-formaldehyde (MF) resins are composed of molecules that cross-link into clear, hard plastics. Properties of UF and MF resins are similar to the properties of phenolic resins. As their names imply, these resins are formed by condensation reactions between urea (H2NCONH2) or melamine (C3H6N6) and formaldehyde (CH2O).
Melamine-formaldehyde resins are easily molded in compression and special injection molding machines. MF plastics are more heat-resistant, scratch-proof, and stain-resistant than urea-formaldehyde plastics are. MF resins are used to manufacture dishware, electrical components, laminated furniture veneers, and to bond wood layers into plywood.
Urea-formaldehyde resins form products such as appliance knobs, knife handles, and plates. UF resins are used to give drip-dry properties to wash-and-wear clothes as well as to bond wood chips and wood sheets into chip board and plywood.
B4 Unsaturated Polyesters
Unsaturated polyesters (UP) belong to the polyester group of plastics. Polyesters are composed of long carbon chains containing CH2]n. Unsaturated polyesters (an unsaturated8CH28COO8C6H48OOC8[ compound contains multiple bonds) cross-link when the long molecules are joined (copolymerized) by the aromatic organic compound styrene (see Aromatic Compounds).
Unsaturated polyester resins are often premixed with glass fibers for additional strength. Two types of premixed resins are bulk molding compounds (BMC) and sheet molding compounds (SMC). Both types of compounds are doughlike in consistency and may contain short fiber reinforcements and other additives. Sheet molding compounds are preformed into large sheets or rolls that can be molded into products such as shower floors, small boat hulls, and roofing materials. Bulk molding compounds are also preformed to be compression molded into car body panels and other automobile components.
B5 Epoxy
Epoxy (EP) resins are named for the epoxide groups (cycl-CH2OCH; cycl or cyclic refers to the triangle formed by this group) that terminate the molecules. The oxygen along epoxy’s carbon chain and the epoxide groups at the ends of the carbon chain give epoxy resins some useful properties. Epoxies are tough, extremely weather-resistant, and do not shrink as they cure (dry).
Epoxies cross-link when a catalyzing agent (hardener) is added, forming a three-dimensional molecular network. Because of their outstanding bonding strength, epoxy resins are used to make coatings, adhesives, and composite laminates. Epoxy has important applications in the aerospace industry. All composite aircraft are made of epoxy. Epoxy is used to make the wing skins for the F-18 and F-22 fighters, as well as the horizontal stabilizer for the F-16 fighter and the B-1 bomber. In addition, almost 20 percent of the Harrier jet’s total weight is composed of reinforcements bound with an epoxy matrix (see Airplane). Because of epoxy’s chemical resistance and excellent electrical insulation properties, electrical parts such as relays, coils, and transformers are insulated with epoxy.
B6 Reinforced Plastics
Reinforced plastics, called composites, are plastics strengthened with fibers, strands, cloth, or other materials. Thermosetting epoxy and polyester resins are commonly used as the polymer matrix (binding material) in reinforced plastics. Due to a combination of strength and affordability, glass fibers, which are woven into the product, are the most common reinforcing material. Organic synthetic fibers such as aramid (an aromatic polyamide with the commercial name Kevlar) offer greater strength and stiffness than glass fibers, but these synthetic fibers are considerably more expensive.
The Boeing 777 aircraft makes extensive use of lightweight reinforced plastics. Other products made from reinforced plastics include boat hulls and automobile body panels, as well as recreation equipment, such as tennis rackets, golf clubs, and jet skis.
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