THERMOSET VS. THERMOPLASTIC MATERIALS
Polymer properties are linked to heat-induced changes.
Thermoset polymers feature chemical cross-links
(such as those shown here in yellow) between the
Though materials may be categorized in many ways (natural versus synthetic, oil-resistant versus non-oil-resistant, etc.), they can also be classified based on the ways in which they respond to heat. Some polymers (large, chainlike molecules) undergo a permanent chemical reaction when heated. Others only undergo a temporary physical change. A clear understanding of why various polymers react the way they do will help you make good decisions when choosing materials. Here’s a closer look:
As the name suggests, a thermoset is a material (such as rubber) that cures or hardens (sets) into a given shape, generally through the application of heat (a thermal increase). Curing (also referred to as vulcanizing) is an irreversible chemical reaction in which permanent connections (known as cross-links) are made between the material’s molecular chains. These cross-links (you can think of them as chemical bridges) give the cured polymer a three-dimensional structure, as well as a higher degree of rigidity than it possessed prior to curing.
Keep in mind that a cured, thermoset material will not remelt or otherwise regain the processibility it had before being cured. Curing changes the material forever. Thermoset polymers outperform other materials (such as thermoplastics, see below) in a number of areas, including mechanical properties, chemical resistance, thermal stability, and overall durability. For these reasons, thermoset parts tend to make more effective seals.
A thermoplastic material softens (becomes pliable and plastic) when heated, but it does not cure or set. A thermoplastic often begins in pellet form, then becomes softer and more fluid as heat increases. This fluidity allows it to be injected under pressure from a heated cavity into a cool mold. As it cools, the thermoplastic will harden in the shape of the mold, but there is no chemical curing at work. No cross-links are formed as with a thermoset material. The changes seen in the thermoplastic are purely physical and, with the reapplication of heat, wholly reversible. A thermoplastic material can therefore be reprocessed many times, though continual recycling will eventually degrade the polymer.
A thermoplastic elastomer (TPE) combines the rubber-like properties of a thermoset elastomer and the processing characteristics of a thermoplastic. The TPE achieves this blend because it is composed of two regions (or phases): soft phases (cured thermoset rubber particles) dispersed within hard phases (the thermoplastic portion). Be aware that the physical, chemical, and thermal limits of both phases will determine the overall limits for the TPE. Because it is a blended material, a TPE is also considerably more expensive than a simpler thermoset material.