CHEMISTRY OF PLASTICS

Plastics consist of very long molecules each composed of carbon atoms linked into chains. One type of plastic, known as polyethylene, is composed of extremely long molecules that each contain over 200,000 carbon atoms. These long, chainlike molecules give plastics unique properties and distinguish plastics from materials, such as metals, that have short, crystalline molecular structures.

Although some plastics are made from plant oils, the majority are made from fossil fuels. Fossil fuels contain hydrocarbons (compounds containing hydrogen and carbon), which provide the building blocks for long polymer molecules. These small building blocks, called monomers, link together to form long carbon chains called polymers. The process of forming these long molecules from hydrocarbons is known as polymerization. The molecules typically form viscous, sticky substances known as resins, which are used to make plastic products.

Ethylene, for example, is a gaseous hydrocarbon. When it is subjected to heat, pressure, and certain catalysts (substances used to enable faster chemical reactions), the ethylene molecules join together into long, repeating carbon chains. These joined molecules form a plastic resin known as polyethylene.

Joining identical monomers to make carbon chains is called addition polymerization, because the process is similar to stringing many identical beads on a string. Plastics made by addition polymerization include polyethylene, polypropylene, polyvinyl chloride, and polystyrene. Joining two or more different monomers of varying lengths is known as condensation polymerization, because water or other by-products are eliminated as the polymer forms. Condensation polymers include nylon (polyamide), polyester, and polyurethane.

The properties of a plastic are determined by the length of the plastic’s molecules and the specific monomer present. For example, elastomers are plastics composed of long, tightly twisted molecules. These coiled molecules allow the plastic to stretch and recoil like a spring. Rubber bands and flexible silicone caulking are examples of elastomers.

The carbon backbone of polymer molecules often bonds with smaller side chains consisting of other elements, including chlorine, fluorine, nitrogen, and silicon. These side chains give plastics some distinguishing characteristics. For example, when chlorine atoms substitute for hydrogen atoms along the carbon chain, the result is polyvinyl chloride, one of the most versatile and widely used plastics in the world. The addition of chlorine makes this plastic harder and more heat resistant.

Different plastics have advantages and disadvantages associated with the unique chemistry of each plastic. For example, longer polymer molecules become more entangled (like spaghetti noodles), which gives plastics containing these longer polymers high tensile strength and high impact resistance. However, plastics made from longer molecules are more difficult to mold.

THERMOPLASTICS AND THERMOSETTING PLASTICS

All plastics, whether made by addition or condensation polymerization, can be divided into two groups: thermoplastics and thermosetting plastics. These terms refer to the different ways these types of plastics respond to heat. Thermoplastics can be repeatedly softened by heating and hardened by cooling. Thermosetting plastics, on the other hand, harden permanently after being heated once.

The reason for the difference in response to heat between thermoplastics and thermosetting plastics lies in the chemical structures of the plastics. Thermoplastic molecules, which are linear or slightly branched, do not chemically bond with each other when heated. Instead, thermoplastic chains are held together by weak van der Waal forces (weak attractions between the molecules) that cause the long molecular chains to clump together like piles of entangled spaghetti. Thermoplastics can be heated and cooled, and consequently softened and hardened, repeatedly, like candle wax. For this reason, thermoplastics can be remolded and reused almost indefinitely.

Thermosetting plastics consist of chain molecules that chemically bond, or cross-link, with each other when heated. When thermosetting plastics cross-link, the molecules create a permanent, three-dimensional network that can be considered one giant molecule. Once cured, thermosetting plastics cannot be remelted, in the same way that cured concrete cannot be reset. Consequently, thermosetting plastics are often used to make heat-resistant products, because these plastics can be heated to temperatures of 260° C (500° F) without melting.

The different molecular structures of thermoplastics and thermosetting plastics allow manufacturers to customize the properties of commercial plastics for specific applications. Because thermoplastic materials consist of individual molecules, properties of thermoplastics are largely influenced by molecular weight. For instance, increasing the molecular weight of a thermoplastic material increases its tensile strength, impact strength, and fatigue strength (ability of a material to withstand constant stress). Conversely, because thermosetting plastics consist of a single molecular network, molecular weight does not significantly influence the properties of these plastics. Instead, many properties of thermosetting plastics are determined by adding different types and amounts of fillers and reinforcements, such as glass fibers.

Thermoplastics may be grouped according to the arrangement of their molecules. Highly aligned molecules arrange themselves more compactly, resulting in a stronger plastic. For example, molecules in nylon are highly aligned, making this thermoplastic extremely strong. The degree of alignment of the molecules also determines how transparent a plastic is. Thermoplastics with highly aligned molecules scatter light, which makes these plastics appear opaque. Thermoplastics with semialigned molecules scatter some light, which makes most of these plastics appear translucent. Thermoplastics with random (amorphous) molecular arrangement do not scatter light and are clear. Amorphous thermoplastics are used to make optical lenses, windshields, and other clear products.

MANUFACTURING PLASTIC PRODUCTS

The process of forming plastic resins into plastic products is the basis of the plastics industry. Many different processes are used to make plastic products, and in each process, the plastic resin must be softened or sufficiently liquefied to be shaped.

Forming Thermoplastics

Although some processes are used to manufacture both thermoplastics and thermosetting plastics, certain processes are specific to forming thermoplastics.

Injection Molding

Injection molding uses a piston or screw to force plastic resin through a heated tube into a mold, where the plastic cools and hardens to the shape of the mold. The mold is then opened and the plastic cast removed. Thermoplastic items made by injection molding include toys, combs, car grills, and various containers.

Extrusion

Extrusion is a continuous process, as opposed to all other plastic production processes, which start over at the beginning of the process after each new part is removed from the mold. In the extrusion process, plastic pellets are first heated in a long barrel. In a manner similar to that of a pasta-making or sausage-stuffing machine, a rotating screw then forces the heated plastic through a die (device used for forming material) opening of the desired shape.

As the continuous plastic form emerges from the die opening, it is cooled and solidified, and the continuous plastic form is then cut to the desired length. Plastic products made by extrusion include garden hoses, drinking straws, pipes, and ropes. Melted thermoplastic forced through extremely fine die holes can be cooled and woven into fabrics for clothes, curtains, and carpets.

Blow Molding

Blow molding is used to form bottles and other containers from soft, hollow thermoplastic tubes. First a mold is fitted around the outside of the softened thermoplastic tube, and then the tube is heated. Next, air is blown into the softened tube (similar to inflating a balloon), which forces the outside of the softened tube to conform to the inside walls of the mold. Once the plastic cools, the mold is opened and the newly molded container is removed. Blow molding is used to make many plastic containers, including soft-drink bottles, jars, detergent bottles, and storage drums.

Blow Film Extrusion

Blow film extrusion is the process used to make plastic garbage bags and continuous sheets. This process works by extruding a hollow, sealed-end thermoplastic tube through a die opening. As the flattened plastic tube emerges from the die opening, air is blown inside the hollow tube to stretch and thin the tube (like a balloon being inflated) to the desired size and wall thickness.

The plastic is then air-cooled and pulled away on take-up rollers to a heat-sealing operation. The heat-sealer cuts and seals one end of the thinned, flattened thermoplastic tube, creating various bag lengths for products such as plastic grocery and garbage bags. For sheeting (flat film), the thinned plastic tube is slit along one side and opened to form a continuous sheet.

Calendering

The calendering process forms continuous plastic sheets that are used to make flooring, wall siding, tape, and other products. These plastic sheets are made by forcing hot thermoplastic resin between heated rollers called calenders. A series of secondary calenders further thins the plastic sheets. Paper, cloth, and other plastics may be pressed between layers of calendered plastic to make items such as credit cards, playing cards, and wallpaper.

Thermoforming

Thermoforming is a term used to describe several techniques for making products from plastic sheets. Products made from thermoformed sheets include trays, signs, briefcase shells, refrigerator door liners, and packages. In a vacuum-forming process, hot thermoplastic sheets are draped over a mold. Air is removed from between the mold and the hot plastic, which creates a vacuum that draws the plastic into the cavities of the mold. When the plastic cools, the molded product is removed. In the pressure-forming process, compressed air is used to drive a hot plastic sheet into the cavities and depressions of a concave, or female, mold. Vent holes in the bottom of the mold allow trapped air to escape.

Forming Thermosetting Plastics

Thermosetting plastics are manufactured by several methods that use heat or pressure to induce polymer molecules to bond, or cross-link, into typically hard and durable products.

Compression Molding

Compression molding forms plastics through a technique that is similar to the way a waffle iron forms waffles from batter. First, thermosetting resin is placed into a steel mold. The application of heat and pressure, which accelerate cross-linking of the resin, softens the material and squeezes it into all parts of the mold to form the desired shape. Once the material has cooled and hardened, the newly formed object is removed from the mold. This process creates hard, heat-resistant plastic products, including dinnerware, telephones, television set frames, and electrical parts.

Laminating

The laminating process binds layers of materials, such as textiles and paper, together in a plastic matrix. This process is similar to the process of joining sheets of wood to make plywood. Resin-impregnated layers of textiles or paper are stacked on hot plates, then squeezed and fused together by heat and pressure, which causes the polymer molecules to cross-link. The best-known laminate trade name is Formica, which is a product consisting of resin-impregnated layers of paper with decorative patterns such as wood grain, marble, and colored designs. Formica is often used as a surface finish for furniture, and kitchen and bathroom countertops. Thermosetting resins known as melamine and phenolic resins form the plastic matrix for Formica and other laminates. Electric circuit boards are also laminated from resin-impregnated paper, fabric, and glass fibers.

Reaction Injection Molding (RIM)

Strong, sizable, and durable plastic products such as automobile body panels, skis, and business machine housings are formed by reaction injection molding. In this process, liquid thermosetting resin is combined with a curing agent (a chemical that causes the polymer molecules to cross-link) and injected into a mold. Most products made by reaction injection molding are made from polyurethane.

Forming Both Types of Plastics

Certain plastic fabrication processes can be used to form either thermoplastics or thermosetting plastics.

Casting

The casting process is similar to that of molding plaster or cement. Fluid thermosetting or thermoplastic resin is poured into a mold, and additives cause the resin to solidify. Photographic film is made by pouring a fluid solution of resin onto a highly polished metal belt. A thin plastic film remains as the solution evaporates. The casting process is also used to make furniture parts, tabletops, sinks, and acrylic window sheets.

Expansion Processes

Thermosetting and thermoplastic resins can be expanded by injecting gases (often nitrogen or methyl chloride) into the plastic melt. As the resin cools, tiny bubbles of gas are trapped inside, forming a cellular plastic structure. This process is used to make foam products such as cushions, pillows, sponges, egg cartons, and polystyrene cups.

Foam plastics can be classified according to their bubble, or cell, structure. Sponges and carpet pads are examples of open-celled foam plastics, in which the bubbles are interconnected. Flotation devices are examples of closed-celled foam plastics, in which the bubbles are sealed like tiny balloons. Foam plastics can also be classified by density (ratio of plastic to cells), by the type of plastic resin used, and by flexibility (rigid or flexible foam). For example, rigid, closed-celled polyurethane plastics make excellent insulation for refrigerators and freezers.

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.

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.

Polyethylene

Polyethylene (PE) resins are milky white, translucent substances derived from ethylene (CH₂9CH₂). Polyethylene, with the chemical formula [8CH₂8CH₂8]_n (where n denotes that the chemical formula inside the 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.

Polyvinyl Chloride

Polyvinyl chloride (PVC) is prepared from the organic compound vinyl chloride (CH₂9CHCl). PVC is the most widely used of the amorphous 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.

Polypropylene

Polypropylene is polymerized from the organic compound propylene (CH₃8CH9CH₂) and has a methyl group (8CH₃) branching off of every other carbon along the 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.

Polystyrene

Polystyrene, produced from styrene (C₆H₅CH9CH₂), has phenyl groups (six-member 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.

Polyethylene Terephthalate

Polyethylene terephthalate (PET) is formed from the reaction of terephthalic acid (HOOC8C₆H₄8COOH) and ethylene glycol (HOCH₂8CH₂OH), which produces the PET monomer [8OOC8C₆H₄8COO8CH₂CH₂8]_n. PET molecules 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.

Acrylonitrile Butadiene Styrene

Acrylonitrile butadiene styrene (ABS) is made by copolymerizing (combining two or more monomers) the monomers acrylonitrile (CH₂CHCN) and styrene (C₆H₅CH9CH₂). Acrylonitrile and styrene are dissolved in polybutadiene rubber [8CH9CH8CH9CH8] _n, which allows these monomers to 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.

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.

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. 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.

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.

Polyurethane

Polyurethane is a polymer consisting of the repeating unit [8R8OOCNH8R’8]_n, where R may represent a different alkyl group than R’. 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.

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.

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.

Unsaturated Polyesters

Unsaturated polyesters (UP) belong to the polyester group of plastics. Polyesters are composed of long carbon chains containing [8OOC8C₆H₄8COO8CH₂8CH₂]_n. Unsaturated polyesters (an unsaturated compound contains multiple bonds) cross-link when the long molecules are joined (copolymerized) by the aromatic organic compound styrene.

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.

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.

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.

Date: 2015-01-02; view: 1134