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| Plastics diagram. |
Plastics, some ubiquitous 'mers' are ethylene, styrene and acrylamide. Each of these may be polymerized to make, respectively, polyethylene (the soft clear plastic that plastic bags are made of), polystyrene (the stiffer, usually white plastic that the covers for soft-drink cups are made of), and polyacrylamide (the very tough, clear plastic that compact discs are made from). Look on the bottom of a recyclable plastic bottle - chances are you will see a PE or PS which means polyethylene or polystyrene. These materials are examples of what happens to polymers when they solidify: the chains are entangled and packed together to make light, tough, flexible materials. A way to think about some of these materials is to think of what a big glob of cooked spaghetti is like. If you stretch it a bit, it is kind of elastic, but if you really pull hard, the noodles start to slide past one another and the whole glob starts to permanently deform. At least that is the idea! Does this remind you of what happens to a PE plastic bag when you stretch it? Think about what must be happening to the microscopic spaghetti that the bag is made up of! This way of thinking doesn't work very well to describe stiffer plastics like PS: the chemical units in stiffer plastics are actually packed together in a orderly way, into pseudo-crystals. If you heat up PE or PS to moderate temperatures, if the chains have not been chemically stuck together (`cross-linked') they will melt, and turn into goopy liquids, which are called polymer melts. Some polymers are melts even at room temperature, like polydimethylsiloxane (PDMS), or poly(ethylene-propylene) (PEP). Remembering that paper is made of cellulose, which is a polymer of biological origin, if you look around the room that you are in, you will see that a good fraction of the stuff in it is made of polymers. The plastics are a very large group of synthetic materials whose structures are based on the chemistry of carbon (see organic chemistry). Plastics are also called polymers because they are made of extremely long chains of carbon atoms. An important characteristic of plastics is that they can be readily molded into finished products by the application of heat. The group has now become so diverse that some polymers do not conform comfortably to this definition (see polysilane; silicone). Since the 1940s many outstanding and indispensable plastics have been developed, and these have been used in a wide range of critical applications, including machine gears, artificial hearts, and bonding cements for such things as aircraft structures.
HISTORY
The first synthetic plastic was celluloid, a mixture of cellulose nitrate and camphor. Invented in 1856 by Alexander Parkes, it was used initially as a substitute for ivory in billiard balls, combs, and piano keys. The high flammability of celluloid has restricted its use to products that are small in size. For years celluloid was widely used in photographic and motion picture film stock, until it was superseded by the less dangerous polymer cellulose acetate. In 1909 the second synthetic plastic, phenol-formaldehyde (also called Bakelite), was invented by Leo Baekeland when he simply heated a mixture of phenol and formaldehyde. Shortly before World War II a number of synthetic polymers were developed, including casein, nylon, polyesters, polyvinyl chloride (see vinyl), polystyrene, and polyethylene. Since then the number as well as the types and qualities of plastics have greatly increased, producing superior materials such as epoxies, polycarbonate, Teflon, silicones, and polysulfones. Two modern trends found in the development of plastic materials are of interest. One is the increased number of foamed plastics--plastics that are imbedded with gas--and the other is the specific designing of plastics to satisfy particular service requirements. The ability of chemists to tailor the properties of plastics has become powerful and dramatic. This may be illustrated by polyethylene, which is soft and waxy when used as a film, but hard and abrasion-resistant when used as a socket for an artificial hip joint.
PROPERTIES OF PLASTICS
The bonding properties and chemical versatility of carbon account for the great number of plastics. Although carbon is the backbone of polymer chains, other elements are included, to varying degrees, in the chemical structures of plastics. These include hydrogen, oxygen, nitrogen, chlorine, fluorine, and occasionally other elements, such as sulfur and silicon. While progress in polymer technology makes it increasingly difficult to make general statements about these materials, the following properties are characteristic of most plastics:
1. Low strengthfor the familiar plastics, about one-sixth the strength of structural steel.
2. Low stiffness (technically, modulus of elasticity) less then one-tenth that of metals, except for reinforced plastics.
3. A tendency to creep, that is, to increase in length under a tensile stress.
4. Low hardness (except formaldehyde plastics).
5. Low density, usually an advantage, the density of most plastics being close to that of water.
6. Brittleness at low temperatures and loss of strength and hardness at moderately elevated temperatures (thermal expansion of plastics is about ten times that of metals).
7. Flammability, although many plastics do not burn.
8. Outstanding electrical characteristics, such as electrical resistance.
9. Degradation of some plastics by environmental agencies such as ultraviolet radiation, although most plastics are highly resistant to chemical attack.
Almost all of the characteristics mentioned above can be modified to some degree by the addition to a given plastic of suitable fillers or reinforcing fibers (see composite materials). For example, a number of plastics have been developed that can sustain elevated temperatures, including Teflon and the silicones. Addition of other materials to plastics generally reduces their property of electrical resistance. On the other hand, a number of plastics have more recently been developed for the specific purpose of making them electrically conductive. The aim of such research is to produce cheap and lightweight components for use in the electronics industry.
CHEMICAL STRUCTURE OF PLASTICS
High molecular-weight polymer molecules are built up by joining together into chains repeating chemical units called monomers. Monomer molecules may be either gases or liquids. In the case of polyethylene the monomer unit is ethylene, C2H4, which is obtained from the dehydrogenation of ethane, C2H6. With the aid of a catalyst, ethylene molecules attach to each other in a process known as polymerization. The lengths of the resulting polymer chains, that is, the average number of monomer units, and the average molecular weight, can be controlled. This is important, because large variations in chain lengths can result in variations in properties. In the case of polyethylene, longer chains create plastics that are harder and stronger, but more difficult to shape. Although polymer chains are, for the most part, linear, they may include side branches. In addition, although chains are often shown as straight lines in diagrams, they actually tend to twist around each other in a random manner. Linear polymers such as polyethylene that can be repeatedly softened or melted by heating, are called thermoplastics. Plastics that cannot be softened by heating are called thermosetting plastics or thermosets. For structural reasons, Teflon and a few more complex thermoplastics cannot be softened by heat, and in this respect they resemble thermosets. Thermosets are set or hardened by heat during the molding operation and thereafter cannot be reshaped. Wood is an example of a natural thermoset. In a thermosetting plastic a cross-linking agent joins one linear chain to another, thereby creating a three-dimensional network. Oxygen atoms are the most commonly used cross-linking agents. Because cross-linking reduces the mobility of polymer chains, the thermosets are more brittle than the thermoplastics. It is not necessary for all the monomer units in a polymer to be identical. Two kinds of monomers may be blended into a polymer chain, as in the case of styrene and butadiene. Such plastics are called copolymers. ABS plastic is a copolymer of the monomers acrylonitrile, butadiene, and styrene. Copolymerization of two or more monomers is analogous to the alloying of two or more metals. Pure polystyrene is brittle, but if a percentage of butadiene monomers is incorporated into the chain of styrene monomers, a high-impact grade of polystyrene results. Polybutadiene is too soft to be used in the tires of vehicles but can be modified with styrene monomers to create a suitable synthetic rubber.
THERMOPLASTICS
Ethylene-Based Plastics
The simplest structure among the many thermoplastics is that of polyethylene. Addition polymerization is the name given to the process in which each ethylene monomer opens up at a double bond and joins to the end of the lengthening chain. The earliest thermoplastics to be developed had the basic structure of polyethylene and were made by addition polymerization. These polymers could be created simply by substituting other atoms or groups of atoms for one or more of the four hydrogen atoms in the ethylene monomer. Polyvinyl chloride is made from an ethylene monomer in which one chlorine atom has replaced one hydrogen atom. The result is a polymer that is nonflammable. Polyvinyl fluoride is made from an ethylene monomer in which a fluorine atom has replaced a hydrogen atom. The result is another polymer with improved heat resistance. Polyvinyl alcohol involves the substitution of an OH group, which causes the polymer to be water soluble. Polytetrafluoroethylene (Teflon) contains fluorine atoms in place of all hydrogen atoms. The well-known properties of this plastic include remarkable heat resistance as well as the inability to be softened by heat. In polypropylene a methyl group (CH(3)) replaces one hydrogen atom. In the monomer of polystyrene a phenyl ring of six carbon atoms is attached to the ethylene unit in place of one hydrogen atom. This bulky side group results in a brittle plastic. Except for the fluorinated polymers and the acrylic polymers, thermoplastics must be protected from destruction caused by ultraviolet radiation. Carbon black provides such protection in polyethylene pipe, but other additives must be used if the product must be white or pigmented. The consumption of polyethylene exceeds that of any other plastic. This soft, flexible, waxy material is produced in five grades: low density, medium density, high density, ultrahigh molecular weight (UHMW), and irradiated (cross-linked by radiation). It is also made into a flexible foam. The differences in density result from differences in the degree of crystallinity. When the long polymer chains are ordered in a parallel arrangement like the atoms in a metal crystal, the result is a higher density than would be possible in a random or disordered distribution. The branching of polymer chains also leads to lower densities. Although low-density polyethylene has the highest vapor transmission rate, it is the least expensive of the five grades and is used as a vapor barrier in buildings. High-density polyethylene is used in blown bottles and pipes. The UHMW grade is a harder, stronger material. Polypropylene is hard and strong, and has a higher useful temperature range than polyethylene, polyvinyl chloride, and polystyrene. It is highly crystalline. At low temperatures it becomes brittle, but this is overcome by copolymerization with ethylene or other monomers. Polymethyl methacrylate (PMMA), also called acrylic, is known by its trade names Lucite and Plexiglas. Its monomer contains a complex side group, which prevents crystallization. PMMA has outstanding resistance to outdoor environments, including ultraviolet radiation. It has excellent optical properties and unlimited coloring possibilities. It is also harder and stronger than the plastics previously mentioned, although it is brittle. PMMA is familiar in lighting fixtures, outdoor signs, aircraft windows, and automobile taillights. The fluorocarbon group consists of several polymers, all containing fluorine. The presence of fluorine makes these polymers nonflammable. The carbon-fluorine bond is extremely stable and provides chemical and heat stability and low surface tension, thus leading to low friction and nonwetting, nonstaining, nonsticking properties. New resins called Teflon AFs are also amorphous, enhancing their physical properties and making them of potential great usefulness in optical and electronic circuits for computers and instruments. Polyvinylidene dichloride (PVDC) is a tough, protective plastic that can be processed to exhibit piezoelectricity, making it valuable for many applications in electronics. Polyvinyl chloride (PVC) is a stiff plastic made soft and flexible by adding plasticizers. It is used as shower curtains, hoses, and electrical insulation. Polystyrene is a clear, hard, brittle plastic that is attacked by many solvents.
Engineering Plastics
The so-called engineering plastics, those with superior properties that make them suited to such applications as machine parts, do not have the straight carbon chain. The first of these to be developed were the nylons, a group of polymers that incorporate nitrogen into the chain along with carbon. The nylons are crystalline, strong, abrasion resistant, and white in color. Their property of low friction accounts for their employment in such machine parts as noiseless small gears, bearings, slides, rollers, and aerosol valves. Acetal, which goes by the name Delrin, is made from a monomer that alternates carbon and oxygen atoms. It resembles nylon in its appearance, properties, and uses. Like nylon, acetal is crystalline, and so has a sharp melting point. Although the incorporation of the phenyl ring as a side attachment to the polymer chain in polystyrene results in no favorable properties, the incorporation of a phenyl ring into the chain itself produces dramatic results. In polycarbonate the incorporation of the phenyl ring in the chain leads to properties of transparency, heat resistance, flame resistance, dimensional stability, and remarkable toughness. Polycarbonate has been used as vandal-proof glazing and in hard hats, nails, screws, and power-tool housings. The sulfone plastics are similar to polycarbonate, with phenyl rings as well as sulfur atoms in the chain. They too are tough, and resistant to heat and flame. They are commonly used in the housings of smoke detectors attached to ceilings. Poor resistance to sunlight confines them to indoor applications.
THERMOSETS
The thermosets, such as wood, wool, Bakelite, epoxy, polyurethane, and paints, cannot be softened following polymerization and cross-linking. Because the thermosets do not offer the wide range of properties found in the thermoplastics, fewer thermosets are in use. In general, they are harder and more brittle than thermoplastics. The polymerization of a thermoset is a more complex chemical process than the addition polymerization of a thermoplastic. It frequently proceeds by the process called condensation polymerization, in which a compound reacts with itself or another compound and in the reaction releases, or "condenses" some small molecule such as water. In the case of phenol-formaldehyde (Bakelite), phenol and formaldehyde molecules attach to each other in an alternating-chain fashion, releasing water molecules in the process. It was noted above that cross-linking reduces the freedom of movement of polymer chains under stress, resulting in brittleness. Rubbers, however, though cross-linked, are not brittle even though they are thermosets. Common cross-linking agents include sulfur for rubbers, styrene for polyesters, and oxygen for linseed oil and many paints and varnishes. Polymer paints are obtained in the thermoplastic condition. After paint is brushed onto a surface, it cross-links by means of oxygen in the air or other agents, becoming thermosetting and brittle. To counter shrinkage that occurs during molding and to improve properties such as impact resistance and tensile strength, thermosets are usually compounded with fillers such as wood flour, minerals, or glass fiber. The epoxies undergo very little shrinkage, however, and are rarely compounded with fillers. Phenol-formaldehyde is commonly used in pot handles, bottle caps, wall switches, and other electrical hardware and as a plywood adhesive. It is available only in black and brown colors. When a differently colored formaldehyde is needed, as in countertops and tabletops, urea-formaldehyde or melamine-formaldehyde are commonly chosen. Urea-formaldehyde is not suited to outdoor exposure, however. The superior properties of the epoxy thermosets are in part accounted for by oxygen atoms and carbon rings in the polymer chains. Epoxies are usually supplied as two components to be mixed and set. These are strong, corrosion resistant materials that adhere well to most materials, including metals. Their low shrinkage and high strength make them the preferred filler-adhesive in demanding applications such as aircraft structures. Polyesters are thermosetting plastics familiar as fiberglass-reinforced materials in boats, fishing rods, and furniture. There are also thermoplastic polyesters. Polyesters are synthesized in a wide range of reactions involving complex organic acids and alcohols. By suitable selection of the acid and alcohol, specific properties such as flexibility and heat resistance can be obtained. Styrene is commonly used as a cross-linking agent for polyesters; methyl methacrylate is used when improved color and weatherability are needed. Because neither heat nor pressure is required for the production of polyesters, there is almost no limit to the size of the part that may be produced. With glass as a reinforcement, polyesters can be created with strengths equal to those of metals.
SUBCLASSIFICATIONS
It has been stated that the first plastics were based on carbon chains and that improved properties resulted when other atoms such as oxygen were included in the chain. Incorporation of the phenyl ring within the chain, first achieved with polycarbonate and polyesters, also increased the possibilities. The sequence of developments of polymer chains--past, present, and future--may be approximated by the following scheme, which may also serve as a kind of classification of linear polymers. The term linear is used here rather than thermoplastic, because some of the more recent linear polymers are not meltable. The ladder type of polymer has two bonds between units in the chain, providing remarkable resistance to heat. The breaking of one bond does not result in depolymerization. Rings in the chain also yield thermal stability at higher temperatures. As the number of cyclic units in the polymer chain increases, however, difficulties in polymerization and in the molding of these materials also increase. Many of these linear polymers of more complex structure must be condensation-polymerized.
Composites
Most plastics are manufactured as composite materials: fiberglass-reinforced polyester; phenol-formaldehyde compounded with wood flour to reduce mold shrinkage; polyvinyl chloride floor tile with clay filler to reduce moisture absorption and improve surface gloss; polyethylene compounded with carbon black for ultraviolet protection; epoxies filled with aluminum for ease of machining; plastic foams, which are composites of plastic and gas cells. These examples indicate the wide range of advantages that filler materials give to plastics. To obtain strength in a plastic comparable to that of metals, reinforcing fibers must be used. The strength of the reinforced composite is proportional to the weight percentage of the reinforcing fiber. Strength is also influenced by the orientation of the fibers, and three orientations are possible: (1) a unidirectional reinforcement with fibers parallel to one another as well as to the stress direction, as in a fishing rod; (2) a bidirectional reinforcement, with half of the fibers at right angles to the other half, as in boat shells and swimming pools; and (3) an equal reinforcement in all directions by random orientation of fibers, as in safety helmets and office-machine housings.
Foams
The first of the foamed plastics to be developed was polystyrene (Styrofoam). It is commonly used as building insulation and in flotation devices. Polystyrene foams are either extruded with a blowing agent or created in a mold by using expandable beads. The latter method is used to make the familiar white coffee cup. Like solid polystyrene, the foam version is low in cost, brittle, and attacked by solvents and ultraviolet radiation. Although most plastics have been foamed, only polystyrene, ABS, polyethylene, polyvinyl chloride, urea-formaldehyde, and polyurethane have found extensive applications. These foams have three principal uses: thermal insulation, cushioning materials, and structural materials. Insulating foams must be of low density, whereas structural foams must have high density to obtain strength and hardness. Plastic foams may be open celled or closed celled. An open-celled foam has interconnected gas cells and hence can absorb water and other liquids. The gas cells in a closed-cell foam are completely isolated from one another by thin walls of plastic. Closed-cell foams are required for flotation devices and are also preferred for building insulation. Polystyrene, polyurethane (for insulation), and ABS foams are closed celled, while polyethylene, urea-formaldehyde, and other polyurethane foams are open celled. Low-density urethane insulating foams provide the best but also the most expensive insulation (see insulating materials). These foams are poured or sprayed from a gun, and they bond well to clean, dry surfaces. In higher densities, urethane foam is used for cupboard doors, artificial limbs, and furniture and will hold nails and screws. Strength and hardness increase with density. By using a chilled mold, an integral-skin foam can be produced--that is, a foam with a dense and hard surface. Polyvinyl chloride is commonly used as either a flexible foam or as a high-density rigid foam. The latter is used as a substitute for moldings and other wood products and has the same weight as softwoods.
Recycling plastic bottles
Characteristics
The term 'plastics' is used to describe a wide variety of resins or polymers with different characteristics and uses. Polymers are long chains of molecules, a group of many units, taking its name from the greek 'poly' (meaning 'many') and 'meros' (meaning 'parts' or 'units'). While all plastics are polymers, not all polymers are plastic. For the discussion of recycling, an understanding of two basic types of polymers is helpful:
-Thermoplastic polymers can be heated and formed, then heated and formed again and again. the shape of the polymer molecules are generally linear or slightly branched. this means that the molecules can flow under pressure when heated above their melting point.
-Thermoset polymers undergo a chemical change when they are heated, creating a three-dimensional network. after they are heated and formed, these molecules cannot be re-heated and re-formed. Comparing these types, thermoplastics are much easier to adapt to recycling. Plastic from a 'blow mold' (the neck of the bottle is narrower than the body) has a slightly different structure from the exact same plastic used in an ‘injection mold’ (where the opening is the widest part of the product).
Plastic identification / recycling code
When working with plastics there is often a need to identify which particular plastic material has been used for a given product. Most consumers recognize the types of plastics by the numerical coding system created by the society of the plastics industry in the late 1980s. There are seven different types of plastic resins that are commonly used to package household products. the identification codes listed below can be found on the bottom of most plastic packaging.
1- Polyethylene terephthalate (PET), soda & water containers, some waterproof packaging, tennis balls.
2 - High-density polyethylene (PE), milk, detergent & oil bottles. toys and plastic bags.
3 - Vinyl / polyvinyl chloride (PVC), food wrap, vegetable oil bottles, blister packages.
4 - Low-density polyethylene, many plastic bags. shrink wrap, garment bags.
5 - Polypropylene, refrigerated containers, some bags, most bottle tops, some carpets, some food wrap, chairs (back/seats).
6 – Polystyrene, throwaway utensils, meat packing, protective packing.
7 - Other. usually layered or mixed plastic. No recycling potential - must be landfilled.
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