Note: The following index list will take you to separate articles pertaining to the specific subject.
The word Polymer comes from the Greek "poly" meaning many, and "meros", parts or units. A polymer is a group of many units. You combine many monomers (one unit) to create a polymer.
Polymer is often used as a synonym for "plastic", but many biological and inorganic molecules are also polymeric. All plastics are polymers, but not all polymers are plastics. Plastic actually refers to the way a material melts and flows.
Commercial polymers are formed through chemical reactions in large vessels under heat and pressure. Ingredients are added to control how the polymer is formed and to produce the proper molecular length and properties. This chemical process is called "polymerization".
The first injection molding machining was patented in the 1870's and together with profile extrusion, came into common commercial usage during the 1930's. Compression molding was developed in the 1920's. Blow molding as we know it today did not develop until the 1940's. Rotational molding in the 1960's and the process of recycled plastics in the 1980's.
Plastic raw materials are roughly divided into 2 kinds: commodity grade and engineering grade. Commodity grade resins are more widely used and include polyethylene, polypropylene, polystyrene and PVC (polyvinyl chloride) Engineering grade resins are generally more difficult to process but have characteristics which make them desirable for specialized use.
Plastic resins are also broadly classified as either thermoplastic or thermoset.
A material that softens when heated and hardens upon cooling. Resins can be remelted after processing. Flash and rejected parts can be reground and added to virgin resin for reprocessing. They can be subdivided into the following types:
Thermoplastic polyurethane (TPU) are the most durable when it comes to flexible materials. TPUs don't use plasticizers which tend to leach to the surface of parts and weaken the material. A drawback to choosing TPUs, however is cost. While TPUs offer higher performance, they also carry a preminum price compared to other flexible materials. The medical and healthcare industries are among the fastest-growing consumers of TPUs.
A material that once having been hardened by a chemical reaction, will not soften or melt when subsequently heated. Once processed, if they are reheated, they simply burn up and disintegrate. Thus, rejected parts and flash cannot be reused. They can be subdivided into the following types:
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Polymers are characterized in many ways - by chemical or physical structure, by strength or thermal performance, by optical or electrical properties, etc.
Most textbooks will give qualitative and some quantitative data on polymer properties. Properties can vary widely however, between manufacturers, for different performance grades, due to additives and reinforcements, or other reasons. For more precise data, contact a representative from a polymer producer, compounder, or distributor for a spec sheet on a particular material and grade. Often grades are offered to suit the needs of specific types of applications.
Properties of interest typically include:
Physical Properties
Mechanical Properties
Thermal Properties
Processing Characteristics
Optical Properties
Electrical Properties
Environmental Properties
Morphology
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There are many ways in which polymer properties or behavior are classified to make general descriptions and understanding easier. Some common classifications are:
Thermoplastic vs. Thermoset:
Thermoplastics can be heated and formed, then re-heated and re-formed repeatedly. The shape of the polymer molecules is generally linear, or slightly branched, allowing them to flow under pressure when heated above the effective melting point.
Thermosets undergo a chemical as well as a phase change when they are heated. Their molecules form a three-dimensional cross-linked network. Once they are heated and formed they can not be reprocessed - the three- dimensional molecules can not be made to flow under pressure when heated.
Amorphous vs Crystalline:
Polymers with nearly linear structure, which have simple backbones, tend to be flexible and fold up to form very tightly packed and ordered areas called crystals. Levels of Crystallinity can vary from zero to near 100%. Time and temperature during processing influence the degree of Crystallinity. Crystalline polymers include: polyethylene, polypropylene, acetals, nylons, most thermoplastic polyesters, and in some cases polyvinyl chloride. Crystalline polymers have higher shrinkage, are generally opaque or translucent, good to excellent chemical resistance, low friction, good to excellent wear resistance. Polymers with bulkier molecular chains or large branches or functional groups tend to be stiffer and will not fold up tight enough to form crystals. These polymers are referred to as "amorphous" and include: polystyrene, polycarbonate, acrylic, ABS, SAN, and polysulfone. Amorphous polymers have low shrinkage, good transparency, gradual softening when heated (no melting point), average to poor chemical resistance, high friction, and average to low wear resistance.
Addition vs. Condensation:
Polymers such as nylons, acetals, and polyesters are made by condensation or step-reaction polymerization where small molecules (monomers) of two different chemicals combine to form chains of alternating chemical groups. The length of molecules is determined by the number of active chain ends available to react with more monomer or the active ends of other molecules.
Polymers such as polyethylene, polystyrene, acrylic, and polyvinyl chloride are made by addition or chain-reaction polymerization where only one monomer species is used. The reaction is begun by an initiator which activates monomer molecules by the breaking a double bond between atoms and creating two bonding sites. These sites quickly react with sites on two other monomer molecules and so on. This continues until the initiator is used up and the reaction stops. The length of molecules is determined by the number of monomer molecules which can attach to a chain before the initiator is consumed and all molecules with initiated bonding sites have reacted.
Commodity, Engineering, High Performance:
Another common classification of plastics is "commodity" versus "engineering". Commodity polymers have relatively low physical properties. They are used for consumer products which require low cost, disposability, packaging or container related, low stress and low temperature resistance, limited product life, and high volume production. (e.g. PE, PS, PP)
Engineering polymers have properties towards the high end of the spectrum. Strength and thermal resistance are the most significant. Their price may range from two to ten times as much as a commodity polymer. They are used in: housings, brackets, load bearing members, machine enclosures, and applications requiring wear resistance, long life expectancy, flame resistance, and the ability to endure cyclic stress loading. (e.g. PC, POM, PBT)
The properties of high performance polymers are at the highest end of the spectrum, generally with very high strength and thermal resistance. They tend to be very expensive, priced above most engineering polymers. They are used in high temperature, high stress applications, in harsh environments, and low to medium volume production. (e.g. PEEK, PEI, LCP)
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Most thermoplastic polymers can be recycled - that is converted from their initial use as a consumer, business, or industrial product, back into a raw material from which some other product can be manufactured. Recycled materials are often classified as Post-Industrial and Post-Consumer. Post-Industrial includes such things as manufacturing scrap, containers and packaging. Post-Consumer is basically any product, container, packaging, etc. that has passed through the hands of a consumer, e.g. plastics bags, beverage containers, carpeting, home appliances, toys, etc.
Thermoset polymers can only be recycled for use as an inert filler (something to take up space) in another material.
The keys to effective recycling are:
There are many arguments whether there is not enough of a market for recycled materials to create the proper recycling infrastructure, or not a consistent supply of recycled material to encourage the growth of a market. In the case of the US paper industry, decreasing availability of virgin wood pulp rapidly created a profitable market for recycled paper.
The contamination issue is very important for plastics. While oil, grease, paper labels, glue, etc. will burn off when glass or metals are recycled, they become contaminants and degrade thermoplastics during reprocessing.
There are three versions of the recycling logo. The original one was three arrows chasing each other in the shape of a triangle, the second was just a triangle, and the current one is a pair of angle brackets <>. The number inside the triangle or brackets indicates the material used in the part.
There are six specific categories, and a generic seventh for "other". In the case of "other" it is good form to put the material name under the recycling logo.
SYMBOL/MATERIAL
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SPE
Society of Plastic Engineers
14 Fairfield Drive
Brookfield CT 06804
Phone: 203-775-0471 Fax: 203-775-8490
http://www.4spc.org
SPI
Society of the Plastics Industry
1275 K Street NW, Suite 400
Washington, DC 20006
Phone: 202-974-5200 Fax: 202-296-7005
http://www.socplas.org/
American Plastic Council
1275K Street N.W. #400
Washington DC USA 20005
Phone: 1-800-243-5790 Fax: 202-371-5679
http://www.plasticresource.com/
PINZ
Plastics Institute of New Zealand
P.O. Box 76378 Manakau City
Auckland, New Zealand
Phone: +64-9-262-3773
CANZ
Composites Association of New Zealand
5 Balmacewen Road
Dunedin, New Zealand
Phone: +64-3-476-2514
PIA
Plastics Industry Association (Australia)
41-43 Exhibition Street
Malbourne Vic 3000 AUSTRALIA
Phone: +61-3-654-2199 Fax: +61-3-654-2384
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