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What Is Plastic?

A plastic is a type of synthetic or man-made polymer; similar in many ways to natural resins found in trees and other plants. Webster's Dictionary defines polymers as: any of various complex organic compounds produced by polymerization, capable of being molded, extruded, cast into various shapes and films, or drawn into filaments and then used as textile fibers.

 Raw Materials

Oil and natural gas are the major raw materials used to manufacture plastics. The plastics production process begins by heating components of crude oil or natural gas in a "cracking process." This process results in the conversion of these components into hydrocarbon monomers such as ethylene and propylene . Further processing leads to a wider range of monomers such as styrene, vinyl chloride, ethylene glycol, terephthalic acid and many others. These monomers are then chemically bonded into chains called polymers. The different combinations of monomers yield plastics with a wide range of properties and characteristics.

Plastics

Many common plastics are made from hydrocarbon monomers. These plastics are made by linking many monomers together into long chains with a carbon backbone. Polyethylene, polypropylene and polystyrene are the most common examples of these. Below is a diagram of polyethylene, the simplest plastic structure.

Even though the basic makeup of many plastics is carbon and hydrogen, other elements can also be involved. Oxygen, chlorine, fluorine and nitrogen are also found in the molecular makeup of many plastics. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen.

  Additives

Most plastics are blended with additives as they are processed into finished products. The additives are incorporated into plastics to alter and improve their basic mechanical, physical or chemical properties. Additives are used to protect plastics from the degrading effects of light, heat, or bacteria; to change such plastic properties as flow; to provide color; and to provide special characteristics such as improved surface appearance or reduced tack/friction.

Plasticizers are materials incorporated into certain plastics to increase flexibility and workability. Plasticizers are found in many plastic film wraps and in flexible plastic tubing, both of which are commonly used in food packaging or processing. All plastics used in food contact, including the additives and plasticizers, are regulated by the U.S. Food and Drug Administration (FDA) to ensure that these materials are safe.

Processing Methods

There are several different processing methods used to make plastic products. Below are the four main methods in which plastics are processed to form the products that consumers use, such as plastic film, bottles, bags and other containers.

Extrusion - Plastic pellets or granules are first loaded into a hopper, then fed into an extruder, which is a long heated chamber, through which it is moved by the action of a continuously revolving screw. At the end of the extruder, the molten plastic is forced out through a small opening or die to shape the finished product. As the plastic product extrudes from the die, it is cooled by air or water. Plastic films and bags are made by extrusion processing.

Injection molding - In injection molding, plastic pellets or granules are fed from a hopper into a heating chamber. A plunger pushes the plastic through the heating chamber, where the material is softened into a fluid state. At the end of this chamber, the resin is forced into a cooled, closed mold. Once the plastic cools to a solid state, the mold opens and the finished part is ejected. This process is used to make products such as butter tubs, yogurt containers, closures and fittings.

Blow molding - Blow molding is a process in which a molten tube of thermoplastic material is formed. Using compressed air, the tube is blown to conform to the interior of a chilled mold that clamps around the tube. This method is used to make plastic bottles such as milk jugs.

Rotational Molding - Rotational molding consists of a closed mold mounted on a machine capable of rotation on two axes simultaneously. Plastic granules are placed in the mold, which is then heated in an oven to melt the plastic Rotation around both axes distributes the molten plastic into a uniform coating on the inside of the mold until the part is set by cooling. This process is used to make hollow products, for example large toys or kayaks.

Durables Vs. Non-Durables

All types of plastic products are classified within the plastic industry as being either a durable or non-durable plastic good. These classifications are used to refer to a product's expected life.

Products with a useful life of three years or more are referred to as durables. They include appliances, furniture, consumer electronics, automobiles, and building and construction materials.

Products with a useful life of less than three years are generally referred to as non-durables. Common applications include packaging, trash bags, cups, eating utensils, sporting and recreational equipment, toys, medical devices and disposable diapers.

 PETE

Polyethylene Terephthalate (PET or PETE) is clear, tough and has good gas and moisture barrier properties making it ideal for carbonated beverage applications and other food containers. The fact that it has high use temperature allows it to be used in applications such as heatable pre-prepared food trays. Its heat resistance and microwave transparency make it an ideal heatable film. It also finds applications in such diverse end uses as fibers for clothing and carpets, bottles, food containers, strapping, and engineering plastics for precision-molded parts.

HDPE

High Density Polyethylene (HDPE) is used for many packaging applications because it provides excellent moisture barrier properties and chemical resistance. However, HDPE, like all types of polyethylene, is limited to those food packaging applications that do not require an oxygen or CO2 barrier. In film form, HDPE is used in snack food packages and cereal box liners; in blow-molded bottle form, for milk and non-carbonated beverage bottles; and in injection-molded tub form, for packaging margarine, whipped toppings and deli foods. Because HDPE has good chemical resistance, it is used for packaging many household as well as industrial chemicals such as detergents, bleach and acids. General uses of HDPE include injection-molded beverage cases, bread trays as well as films for grocery sacks and bottles for beverages and household chemicals.

PVC

Polyvinyl Chloride (PVC) has excellent transparency, chemical resistance, long term stability, good weatherability and stable electrical properties. Vinyl products can be broadly divided into rigid and flexible materials. Rigid applications are concentrated in construction markets, which includes pipe and fittings, siding, carpet backing and windows. PVC's success in pipe and fittings can be attributed to its resistance to most chemicals, imperviousness to attack by bacteria or micro-organisms, corrosion resistance and strength. Flexible vinyl is used in wire and cable sheathing, insulation, film and sheet, floor coverings, synthetic leather products, coatings, blood bags and medical tubing.

LDPE

Low Density Polyethylene (LDPE) is predominantly used in film applications due to its toughness, flexibility and transparency. LDPE has a low melting point making it popular for use in applications where heat sealing is necessary. Typically, LDPE is used to manufacture flexible films such as those used for dry cleaned garment bags and produce bags. LDPE is also used to manufacture some flexible lids and bottles, and it is widely used in wire and cable applications for its stable electrical properties and processing characteristics.

PP

Polypropylene (PP) has excellent chemical resistance and is commonly used in packaging. It has a high melting point, making it ideal for hot fill liquids. Polypropylene is found in everything from flexible and rigid packaging to fibers for fabrics and carpets and large molded parts for automotive and consumer products. Like other plastics, polypropylene has excellent resistance to water and to salt and acid solutions that are destructive to metals. Typical applications include ketchup bottles, yogurt containers, medicine bottles, pancake syrup bottles and automobile battery casings.

PS

Polystyrene (PS) is a versatile plastic that can be rigid or foamed. General purpose polystyrene is clear, hard and brittle. Its clarity allows it to be used when see-throughability is important, as in medical and food packaging, in laboratory ware, and in certain electronic uses. Expandable Polystyrene (EPS) is commonly extruded into sheet for thermoforming into trays for meats, fish and cheeses and into containers such as egg crates. EPS is also directly formed into cups and tubs for dry foods such as dehydrated soups. Both foamed sheet and molded tubs are used extensively in take-out restaurants for their lightweight, stiffness and excellent thermal insulation.

Other Plastics

There are many other plastics beyond the most common ones described above, for example Nylon, ABS copolymers and Polymethyl Methacrylate.

 Whether you are aware of it or not, plastics play an important part in your life. Plastics' versatility allow it to be used in everything from car parts to doll parts, from soft drink bottles to the refrigerators they get stored in. From the car you drive to work in to the television you watch when you get home, plastics help make your life easier and better. So how is it that plastics have become so widely used? How did plastics become the material of choice for so many varied applications?

The simple answer is that plastics can provide the things consumers want and need. Plastics have the unique capability to be manufactured to meet very specific functional needs for consumers. So maybe there's another question that's relevant: What do I want? Regardless of how you answer this question, plastics can probably satisfy your needs.