How Plastics Are Made:

The Basics

The term "plastics" encompasses organic materials, such as the elements carbon (C), hydrogen (H), nitrogen (N), chlorine (Cl) and sulfur (S), which have properties similar to those naturally grown in organic materials such as wood, horn and rosin. Organic materials are based on polymers, which are produced by the conversion of natural products or by syththesis from primary chemicals coming from oil, natural gas or coal.

The plastic production process begins by heating the hydrocarbons in a "cracking process." Here, in the presence of a catalyst, larger molecules are broken down into smaller ones such as ethylene (ethene) C2H4, propylene (propene) C3H6, and butene C4H8 and other hydrocarbons. The yield of ethylene is controlled by the cracking temperature and is more than 30% at 850°C and such products as styrene and vinylchloride can be produced in subsequent reactions. These are then the starting materials for several other types of plastics. Therefore, this process results in the conversion of the natural gas or crude oil components into monomers such as ethylene, propylene, butene and styrene.

These monomers are then chemically bonded into chains called polymers. Different combinations of monomers yield plastic resins with different properties and characteristics. Each monomer yields a plastic resin with different properties and characteristics. Combinations of monomers produce copolymers with further property variations.

The resulting resins may be molded or formed to produce several different kinds of plastic products with application in many major markets. The variability of resin permits a compound to be tailored to a specific design or performance requirement. This is why certain plastics are best suited for some applications while others are best suited for entirely different applications. For instance, impact strength measures the ability of a material to withstand shock loading. Heat resistance protects the resin from exposure to excessive temperatures. Chemical resistance protects the resin from breakdown due to exposure to environmental chemicals.

Some examples of material properties in plastic product applications are:

     -Hot-filled packaging used for products such as ketchup

     -Chemical-resistant packaging used for products such as bleach

     -Impact strength of car bumpers

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The Structure of Polymers

Polymers are created by the chemical bonding of many identical or related basic units and those produced from a single monomer type are called homopolymers. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the carbon backbone.

Polymers that contain primarily carbon and hydrogen are classified as organic polymers. Polypropylene, polybutylene, polystyrene, and polymethylpentene are examples of these. Below is a diagram of polyethylene, the simplest polymer structure.

Even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved. Oxygen, chlorine, fluorine, nitrogen, silicon, phosphorous, and sulfur are other elements that are found in the molecular makeup of polymers. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen. There are also some polymers that, instead of having a carbon backbone, have a silicon or phosphorous backbone and these are considered inorganic polymers.

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The Additives

When plastics emerge from reactors, they do not have the desired properties that make it a material of choice, that is, it is considered a raw material. In order to achieve a commercial product, the plastic is subject to further treatment and the inclusion of additives which are selected to give it specified properties. Most polymers are blended with additives during raw material processing into their finished parts. Additives are incorporated into polymers to alter and improve their basic mechanical, physical or chemical properties. Additives are also used to protect the polymer from the degrading effects of light, heat, or bacteria; to change such polymer properties as flow; to provide product color; and to provide special characteristics such as improved surface appearance or reduced friction.

Types of Additives:

     -antioxidants: for outside application

     -colorants: for colored plastic parts

     -foaming agents: for Styrofoam cups

     -plasticizers: used in toys and food processing equipment

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Two Characterizations Of Plastic

A Thermoset is a polymer that solidifies or "sets" irreversibly when heated. Similar to the relationship between a raw and a cooked egg, once heated, a thermoset polymer can't be softened again and once cooked, the egg cannot revert back to its original form. Thermosets are valued for their durability and strength and are used primarily in automobiles and construction, although applications such as adhesives, inks, and coatings are also significant. Other examples of thermoset plastics and their product applications are:

Polyurethanes:	Unsaturated Polyesters:	Epoxies:
-mattresses -cushions -insulation -ski boots -toys	-lacquers -varnishes -boat hulls -furniture	-glues -coating electrical circuits -helicopter blades

A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding or pressing. Ice cubes are a common household item which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled. Like a polymer, this process may be repeated numerous times. Thermoplastics offer versatility and a wide range of applications. They make up the greatest share of plastics used in food packaging because they can be rapidly and economically formed into any shape needed to fulfill the packaging function. Examples include milk jugs and soda bottles. Other examples of thermoplastics are:

Polyethylene:

     -packaging

     -electrical insulation

     -milk and water bottles

     -packaging film

     -house wrap

     -agricultural film

Polypropylene:

     -carpet fibers

     -automotive bumpers

     -microwave containers

     -external prostheses

Polyvinyl chloride (PVC):

     -sheathing for electrical cables

     -floor and wall coverings

     -siding

     -credit cards

     -automobile instrument panels

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