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What is Plastic?

Plastic is the general term for a wide range of synthetic or semisynthetic polymerization products. They are composed of organic condensation or addition polymers and may contain other substances to improve performance or economics. There are few natural polymers generally considered to be "plastics". Plastics can be formed into objects or films or fibers. Their name is derived from the fact that many are malleable, having the property of plasticity.

 

Overview

 

Plastic can be classified in many ways, but most commonly by their polymer backbone (polyvinyl chloride, polyethylene, polymethyl methacrylate and other acrylics, silicones, polyurethanes, etc.). Other classifications include thermoplastic, thermoset, elastomer, engineering plastic, addition or condensation or polyaddition (depending on polymerization method used), and glass transition temperature.

 

Some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point (the temperature at which the attractive intermolecular forces are overcome) and one or more glass transitions (temperatures above which the extent of localized molecular is substantially increased). So-called semi-crystalline plastics include polyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some polyurethanes. Many plastics are completely amorphous, such as polystyrene and its copolymers, poly (methyl methacrylate), and all thermosets.

 

Plastics are polymers: long chains of atoms bonded to one another. Common thermoplastics range from 20,000 to 500,000 in molecular weight, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as "repeat units", derived from "monomers"; each polymer chain will have several 1000's of repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. (Some of commercial interest are silicon based.) The backbone is that part of the chain on the main "path" linking a large number of repeat units together. To vary the properties of plastics, both the repeat unit with different molecular groups "hanging" or "pendant" from the backbone, (usually they are "hung" as part of the monomers before linking monomers together to form the polymer chain). This customization by repeat unit's molecular structure has allowed plastics to become such an indispensable part of twenty first-century life by fine tuning the properties of the polymer.

 

The development of plastics has come from the use of natural materials (e.g., chewing gum, shellac) to the use of chemically modified natural materials (e.g., natural rubber, nitrocellulose, collagen) and finally to completely synthetic molecules (e.g., epoxy, polyvinyl chloride, polyethylene).

 

The environment

 

Plastics are durable and degrade very slowly. By the 1990s, plastic recycling programs were common in the United States and elsewhere. Thermoplastics can be remelted and reused, and thermoset plastics can be ground up and used as filler, though the purity of the material tends to degrade with each reuse cycle. There are methods by which plastics can be broken back down to a feedstock state. To assist recycling of disposable items, the Plastic Bottle Institute of the Society of the Plastics Industry devised a now-familiar scheme to mark plastic bottles by plastic type. A recyclable plastic container using this scheme is marked with a triangle of three "chasing arrows", which enclose a number giving the plastic type:

 

1-PETE 2-HDPE 3-PVC 4-LDPE 5-PP 6-PS 7-Other

 

Plastics type marks: the Resin identification code

 

1, PET (PETE): Polyethylene Terephthalate - Commonly found on: 2-liter soft drink bottles, cooking oil bottles, peanut butter jars.

2, HDPE: High Density Polyethylene - Commonly found on: detergent bottles, milk jugs.

3, PVC: Polyvinyl Chloride - Commonly found on: plastic pipes, outdoor furniture, shrink-wrap, water bottles, salad dressing and liquid detergent containers.

4, LDPE: Low Density Polyethylene - Commonly found on: dry-cleaning bags, produce bags, trash can liners, food storage containers.

5, PP: Polypropylene - Commonly found on: bottle caps, drinking straws

6, PS: Polystyrene - Commonly found on: "Styrofoam peanuts," cups, plastic tableware, meat trays, take-away food clamshell containers

7, OTHER: Other - This plastic category, as its name of "other" implies, is any plastic other than the named #1 - #6, Commonly found on: certain kinds of food containers, Tupperware, and Nalgene bottles.

Date:Aug 07, 2010 Click:/
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