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go to: About PlasticsPlastics Science: Polymers

There are thousands of kinds of plastic; each has been developed to have a unique set of characteristics tailored for its intended use.

These characteristics help determine the recyclability of each plastic product.

intro to plastics
thermoplastics, thermosetting, elastomers
polymer and plastic properties

plastics molding processes
plastics recyclability
helpful links to toxic chemicals in plastic
helpful links to manufacturing plastics

Intro to Plastics

Plastics are a large family of related materials with a broad range of physical and chemical properties.

All plastics are easily moldable into different shapes. Plastics can, for example, be stretched out into long thin strings or filaments, blown out into hollow containers of varying shape, or pressed flat into sheets.

The majority of plastic products in use today are "synthetic" plastics; that is, they are derived from petrochemical substances (oil, natural gas, or coal).

A growing class of plastics are bioplastics, which are derived from plant matter such as vegetable oil, corn starch, sugar cane stalks, and other starchy agricultural by-products.

Three main factors establish the characteristics and behavior of each plastic product:

  • kind of plastic: the polymer, or molecular make-up, of the plastic material
  • plastic additives: additional chemicals added to the plastic materials
  • molding processes: how the plastic materials are shaped into new products

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Plastics are identified by their chemical properties. The basic building blocks of plastics are molecules called monomers that consist of some combination of carbon, hydrogen, and other elements, such as oxygen, chlorine, and nitrogen. Monomer molecules link together to form giant molecules called polymers.

You can think of a polymer as a chain and each link is a monomer. In Greek, poly means many, mono means one, and mer means part. A monomer is "one part"; a polymer has "many parts".

The simplest polymer structure is a chain of a single repeating monomer, a homopolymer.  illustration: homopolymer  placeholder

Other polymer structures are more complicated:

More than one kind of monomer can link together to form a copolymer.  illustration: copolymer  illustration: copolymer
A polymer chain of one or more monomers can branch off in different directions to form a branched polymer.  illustration: branched polymer  illustration: branched polymer
Two or more polymer chains can be joined by side chains to form a cross-linked polymer.  illustration: cross-linked polymer  placeholder

To find out more about polymer science, see helpful links about plastics manufacturing.

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Chemicals, heat, and/or pressure are needed to get monomers to join together into a polymer. The polymer will take on its distinct structure (a long, single chain, a branching chain, or some other structure) depending on the chemicals used and the amount of heat/pressure applied.

This means the same monomer can be used to create polymers with different structures.

Polymers which have been dried and shaped into pellets are called plastic resins. These are the basic building blocks of plastic products.

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Most plastic products contain more than just polymer resins. Polymers are typically combined with additional chemical compounds called additives to further alter and refine their properties, and to assist in processing the material as it is made into a product.

Plastics are so diverse in form and use because virtually any desired combination of properties can be obtained depending on the combination of polymers and additives.

Common additives include:

  • Stabilizers that prevent degradation due to moisture or UV light.
  • Lubricants that make the polymer easier to form and self-lubricating when in use.
  • Pigments to add color.
  • Plasticizers to make the plastic less hard and brittle, which increases flexibility and toughness.
  • Fillers to enhance properties such as wear, strength, toughness, or durability (shock resistance, for example.)

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Thermoplastics, Thermosetting, and Elastomers

There are three main categories of materials made from polymers:

Thermoplastics can be resoftened by heat or pressure and remolded over and over again. They are made up of single chain or branched polymers.

The most common plastics used in containers and packaging are thermoplastics, such as polypropylene and polyethylene. While thermoplastics can be remolded and recycled, the purity of the material tends to degrade with each reuse cycle.

Thermosetting plastics, once molded, cannot be re-softened and molded again. They are made up of cross-linked polymers.

Examples of thermosetting plastics include Kevlar, fiber glass, electrical fittings, and adhesives (epoxy), which are commonly used in durable products such as in automobile parts and construction.

Elastomers (also known as elastic polymers and rubber) can be stretched and will return to their original shape again and again, like a rubber band.

Primary uses for elastomers include seals, adhesives, and molded flexible parts.

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Polymer and Plastics Properties

Examples of mechanical, thermal, and chemical properties of plastic polymers and plastic include:

  • Rigidity/Flexibility: how much a plastic bends or flexes when you push on it.
  • Strength: how much force can you put on a plastic before it breaks.
  • Toughness: how much force and flexing you can subject a plastic to before it breaks. A plastic that is strong, but rigid, is considered "brittle".
  • Density: mass per unit volume, that is, how closely packed together the molecules are in the plastic.
  • Melting Point: the temperature above which a plastic returns to a "liquid" state in which it can be remolded.
  • Glass Transition Temperature: the temperature range below which a plastic is hard and brittle (like "glass") and above which it becomes soft and pliable.
  • Thermal Resistance: how resistant a plastic is to being deteriorated from heat or cold.
  • Chemical Resistance: how resistant a plastic is to being deteriorated by different chemicals.
  • UV Resistance: how resistant a plastic is to being deteriorated by UV radiation.

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To build products from the plastic polymers and additives, these materials undergo one or more molding processes. Molding further exposes the materials to heat, pressure, and chemicals, which further defines the plastic's structure and behavior.

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