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INDIANA Rubber & Plastics Company, Inc.
6620 East 266th Street
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Custom molded plastics. Custom molded rubber. Molding alternatives.
The basics of thermoplastic versus thermosetting

The difference between thermoplastic
and thermoset materials


Most rubber compounds and some plastics are themosetting materials. Most plastics are thermoplastics. What's the difference?

Thermoplastics can repeatedly be re-melted when heated. Themosets cannot. Thermoplastics are commonly turned into finished molded parts by injection molding. Thermoplastics include the following common plastics materials, referred to as "resins":

ABS
acetal
nylon
polycarbonate
polyethylene
polypropylene
styrene
vinyl and polyvinyl chloride (PVC)

Now, in contrast, think of a thermoset as a cake mix. Once baked, a cake cannot be reheated. It cannot be "un" baked. Once the thermoset material is set up, further heating will only cause the material to deteriorate and lose physical and chemical properties. Think in terms of over-baking a cake; there are similarities. Most rubber compounds are thermosets, and they are turned into finished molded parts by compression, transfer or injection molding.

Notice that there are three methods available for molding thermosets versus one for thermoplastics. The explanation is slightly involved but much has to do with the form of the raw material. Thermoplastics begin as beads poured into a hopper, then fed into a screw and ultimately into the injection mold in molten form. Thermoset rubber begins as a sheet or strip, somewhat resembling unchewed chewing gum, which can be preloaded into a compression or transfer mold; or which is fed into an injection press in continuous strip form approximately 2 inches wide and 1/2 inch thick.

Examples of common thermosetting rubber "compounds" include:

butyl
EPDM
hypalon
natural rubber
nitrile
neoprene
SBR
silicone
synthetic polyisoprene

Without muddying the waters too much, there are thermoplastic forms of rubber and there are also thermosetting plastics. Some materials, such as polyurethane, are available in both thermoplastic and thermosetting forms. Competent technical support will prove valuable in helping you decide which is best for your particular design and application.

While most plastics we encounter today are thermoplastics, some of the oldest plastics resins are actually thermosets and while themoset plastics consumption has declined over time, for some applications, particularly high temperature and high voltage electrical applications, there is not a thermoplastic substitute.

Rubber compounds remain overwhelmingly thermosetting largely because of the nature of rubber chemistry in which the application of heat and pressure causes the compouind to "vulcanize" or "cure" which means that crosslinks form between the long chain molecules (macromolecules) common to most rubber compounds and which give rubber its stretch, its compressibility and its optimal strength and chemical properties.

There are, however, some materials with rubberlike properties which are actually thermoplastic and which are known as thermoplastic rubber(tpr), thermoplastic elastomer (tpe) or even thermoplastic vulcanizate (tpv), the latter being a material which partially crosslinks, imparting improved properties versus non-crosslinking materials. Examples include Alcryn®, Santoprene®, Kraton®, and thermoplastic urethanes such as Elastollan® and Pellathane®.

Alcryn is a tradename of A. Schulman, Inc.                                    Santoprene is a tradename of Exxon-Mobil Chemical
Kraton is a tradename of Kraton Performance Polymers, Inc.              Elastollan is a tradename of BASF Corporation
Pellethane is a tradename of Lubrizol Advanced Materials, Inc.

10-2014 NO REV

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