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. Polyethylene, with the chemical formula [-CH2-CH2-]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.
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.
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. 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. 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.
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.
Additionally, the metallic, reflective outer covering of the insulation reflects electromagnetic radiation, further isolating the interior of the building from the outside. 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. Tough, transparent PET films marketed under the brand name Mylar are magnetically coated to make both audio and video recording tape. 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.
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. 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, 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.
Velcro Loops and Hooks This color-enhanced electron microscope image shows the tiny nylon hooks and loops that give Velcro its unique fastening ability.
Due to the strength of nylon, Velcro can be fastened and unfastened thousands of times. Velcro fastens when dry or wet, and is used in sports shoes, jackets, tents, sleeping bags, and countless other products—even spacesuits.
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. 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. 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 , were some of the first polymers made.
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. 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 an unsaturated 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. 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.
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 and F fighters, as well as the horizontal stabilizer for the F fighter and the B-1 bomber. 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. 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 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. For example, the early Egyptians soaked burial wrappings in natural resins to help preserve their dead.
People have been using animal horns and turtle shells which contain natural resins for centuries to make items such as spoons, combs, and buttons.
During the midth century, shellac resinous substance secreted by the lac insect was gathered in southern Asia and transported to the United States to be molded into buttons, small cases, knobs, phonograph records, and hand-mirror frames. During that time period, gutta-percha rubberlike sap taken from certain trees in Malaya was used as the first insulating coating for electrical wires. In order to find more efficient ways to produce plastics and rubbers, scientists began trying to produce these materials in the laboratory.
In American inventor Charles Goodyear vulcanized rubber by accidentally dropping a piece of sulfur-treated rubber onto a hot stove. Goodyear discovered that heating sulfur and rubber together improved the properties of natural rubber so that it would no longer become brittle when cold and soft when hot.
In British chemist Alexander Parkes synthesized a plastic known as pyroxylin, which was used as a coating film on photographic plates. The following year, American inventor John W. Hyatt began working on a substitute for ivory billiard balls. Hyatt added camphor to nitrated cellulose and formed a modified natural plastic called celluloid, which became the basis of the early plastics industry.
These early plastics based on natural products shared numerous drawbacks. For example, many of the necessary natural materials were in short supply, and all proved difficult to mold. Finished products were inconsistent from batch to batch, and most products darkened and cracked with age. Furthermore, celluloid proved to be a very flammable material. Due to these shortcomings, scientists attempted to find more reliable plastic source materials.
In American chemist Leo Hendrik Baekeland made a breakthrough when he created the first commercially successful thermosetting synthetic resin, which was called Bakelite known today as phenolic resin. Use of Bakelite quickly grew. It has been used to make products such as telephones and pot handles.
The chemistry of joining small molecules into macromolecules became the foundation of an emerging plastics industry. Between and , the I. Farben Company of Germany synthesized polystyrene and polyvinyl chloride, as well as a synthetic rubber called Buna-S.
In Du Pont made a breakthrough when it introduced nylon—a material finer, stronger, and more elastic than silk. By acrylics were being produced by German, British, and U. That same year, the British company Imperial Chemical Industries developed polyethylene.
In the German company I. Farbenindustrie filed a patent for polyepoxide epoxy , which was not sold commercially until a U. After World War II , the pace of new polymer discoveries accelerated. In a small English company developed polyethylene terephthalate PET.
In the postwar era, research by Bayer and by General Electric resulted in production of plastics such as polycarbonates, which are used to make small appliances, aircraft parts, and safety helmets. In Union Carbide Corporation introduced a linear, heat-resistant thermoplastic known as polysulfone, which is used to make face shields for astronauts and hospital equipment that can be sterilized in an autoclave a device that uses high pressure steam for sterilization.
Today, scientists can tailor the properties of plastics to numerous design specifications. Modern plastics are used to make products such as artificial joints, contact lenses, space suits, and other specialized materials. As plastics have become more versatile, use of plastics has grown as well.
By the year , annual global demand for plastics is projected to exceed million metric tons billion lb. As municipal landfills reach capacity and additional landfill space diminishes across the United States, alternative methods for reducing and disposing of wastes— including plastics—are being explored.
Some of these options include reducing consumption of plastics, using biodegradable plastics, and incinerating or recycling plastic waste. A Source Reduction Source reduction is the practice of using less material to manufacture a product.
For example, the wall thickness of many plastic and metal containers has been reduced in recent years, and some European countries have proposed to eliminate packaging that cannot be easily recycled. Plastics are therefore not considered biodegradable. However, researchers are working to develop biodegradable plastics that will disintegrate due to bacterial action or exposure to sunlight. For example, scientists are incorporating starch molecules into some plastic resins during the manufacturing process.
When these plastics are discarded, bacteria eat the starch molecules. This causes the polymer molecules to break apart, allowing the plastic to decompose. Researchers are also investigating ways to make plastics more biodegradable from exposure to sunlight. Prolonged exposure to ultraviolet radiation from the sun causes many plastics molecules to become brittle and slowly break apart.
Researchers are working to create plastics that will degrade faster in sunlight, but not so fast that the plastic begins to degrade while still in use.
C Incineration Some wastes, such as paper, plastics, wood, and other flammable materials can be burned in incinerators. The resulting ash requires much less space for disposal than the original waste would. Because incineration of plastics can produce hazardous air emissions and other pollutants, this process is strictly regulated. D Recycling Plastics All plastics can be recycled. Thermoplastics can be remelted and made into new products. Thermosetting plastics can be ground, commingled mixed , and then used as filler in moldable thermoplastic materials.
Highly filled and reinforced thermosetting plastics can be pulverized and used in new composite formulations. Collecting, sorting, and recycling plastics is an expensive process. Although automated plastic sorting machines are being developed, many recycling operations sort plastic by hand, as shown here.
Only about 5 percent of plastic products in the United States are reused. Chemical recycling is a depolymerization process that uses heat and chemicals to break plastic molecules down into more basic components, which can then be reused.
Another process, called pyrolysis, vaporizes and condenses both thermoplastics and thermosetting plastics into hydrocarbon liquids. Collecting and sorting used plastics is an expensive and time-consuming process. While about 35 percent of aluminum products, 40 percent of paper products, and 25 percent of glass products are recycled in the United States, only about 5 percent of plastics are currently recovered and recycled.
Once plastic products are thrown away, they must be collected and then separated by plastic type. Most modern automated plastic sorting systems are not capable of differentiating between many different types of plastics. However, some advances are being made in these sorting systems to separate plastics by color, density, and chemical composition.
For example, x-ray sensors can distinguish PET from PVC by sensing the presence of chlorine atoms in the polyvinyl chloride material. Other factors can adversely affect the quality of recycled plastics. These factors include the possible degradation of the plastic during its original life cycle and the possible addition of foreign materials to the scrap recycled plastic during the recycling process.
For health reasons, recycled plastics are rarely made into food containers. Instead, most recycled plastics are typically made into items such as carpet fibers, motor oil bottles, trash carts, soap packages, and textile fibers.
To promote the conservation and recycling of materials, the U. In the Plastic Bottle Institute of the Society of the Plastics Industry established a system for identifying plastic containers by plastic type. The purpose of the "chasing arrows" symbol that appears on the bottom of many plastic containers is to promote plastics recycling. By , 40 states had legislative mandates for litter control and recycling. Today, a growing number of communities have collection centers for recyclable materials, and some larger municipalities have implemented curbside pickup for recyclable materials, including plastics, paper, metal, and glass.
Recycling, collection, processing, and reuse of materials that would otherwise be thrown away. Materials ranging from precious metals to broken glass, from old newspapers to plastic spoons, can be recycled.
The recycling process reclaims the original material and uses it in new products. Overflowing Landfill An average city dweller may produce a ton of refuse in a year, a volume that rapidly overflows local dumps. Cities running out of space for landfill often turn to incinerating their waste or transporting it to other areas, although up to 90 percent of the material might have been recycled.
In general, using recycled materials to make new products costs less and requires less energy than using new materials. Recycling can also reduce pollution, either by reducing the demand for high-pollution alternatives or by minimizing the amount of pollution produced during the manufacturing process. Recycling decreases the amount of land needed for trash dumps by reducing the volume of discarded waste Recycling can be done internally within a company or externally after a product is sold and used.
In the paper industry, for example, internal recycling occurs when leftover stock and trimmings are salvaged to help make more new product. Since the recovered material never left the manufacturing plant, the final product is said to contain preconsumer waste. External recycling occurs when materials used by the customer are returned for processing into new products. Materials ready to be recycled in this manner, such as empty beverage containers, are called postconsumer waste Plastics are more difficult to recycle than metal, paper, or glass.
One problem is that any of seven categories of plastics can be used for containers alone. For effective recycling, the different types cannot be mixed. The code assigns a particular number to each of the seven plastics used in packaging.
PET can be made into carpet, or fiberfill for ski jackets and clothing. HDPE can be recycled into construction fencing, landfill liners, and a variety of other products. The pellets are melted into a final product. So a hard material that we cannot easily scratch, is equally hard. Material that if we bend or twist, how much energy can absorb before it breaks is called Toughness.
The toughness of the material has been decreased when it is heated. So Toughness is properties that provide information about the capacity to absorb maximum energy. In this, we suddenly impact and check how much energy is absorbed at that time. It has a pendulum that suddenly attacks the material, and connects its maximum energy absorbing capacity. Suppose we have a material and we impact it and it should be broken, without deform is called Brittleness.
Or If we pull such a material, it breaks instead of pulling it, we call it Brittleness. Cast iron is a brittle material.
Such material on which we apply pressure, bend, and pull, but do not break it in that condition is called Tenacity. When a material loads more than a specific load, then there is a chance of failure But in fatigue, Any material fails even at low load if we apply a repetitive load. This failure is known as fatigue. Fatigue value is many times less than that stress, in which a material has to fail in actual.
The factor of fatigue on materials: Less strength, life, and Durability. Fatigue property is used for observing In designing shafts, connecting rod, springs, gears, etc. Causes of fatigue in the material : 1. Dynamic forces 2. In spite of repetitive loads on a material, it is not broken then it is its fatigue property. Such a material that is easy to work on, such as cutting, using a tool, and machining, we call it machinability.
Brass can be easily machined than steel. If we put a load on metal, it is without changing its shape or if it is able to bear it without breaking it then it is called its strength. So the ability or capacity of a material to withstand or support a load without fracture is called its strength.
If we put a load in a body or material, then the body is elastic up to a particular limit in the stress-strain curve, so the energy that the body stores up to that elastic limit is called Strain energy. Such material in which the strain energy is stored in the body till the elastic limit only, is called as the resilience.
This property is essential for spring materials. How much maximum strain energy stored in material up to the elastic limit is called proof resilience. If we divide the proof resilience from the volume of body, then it will come out with the Modulus of Resilience. When we put the material under constant load, for a long time, at high temperature, then the deformation that happens inside it, is called Creep.
It is also known as cold flow. Creep is used for examine in designing internal combustion engines, boilers, and turbines. Rupture meaning is a break or burst suddenly. Stress rupture testing is similar to creep testing except that the stresses are higher than those used in creep testing.
0コメント