Thermoplastics are a class of polymer that can be softened through heating and then harden when cooled. Thermoplastics can be processed using methods such as injection molding, extrusion, thermoforming and blow molding. They can be cooled and heated several times without any change in their chemical or mechanical properties and differ from thermosets which form irreversible chemical bonds during the curing process and do not melt.
The benefits of thermoplastics for foot orthotic use are they are lightweight, durable, easy to work with and fabricate, resistant to water and chemicals, easily adjustable and can be recycled. A disadvantage of some thermoplastics are that they can be susceptible to creep when exposed to longer term stress loads (eg foot orthotics).
Thermoplastics come in sheets with many different thicknesses, strengths and colors.
There are two types of thermoplastics depending on the nature of the chemical structure: amorphous and semi-crystalline.
Semi-crystalline:
If a polymer chain has only a few, short side branches then these chains can form a more ordered structure with crystalline regions.
Examples: polyethylene (PE), polyamide (PA) and polypropylene (PP)
Amorphous:
If the polymor chain has a lot more large side branches, then it is not possible for these ordered regions to be formed and the polymer is called an amorphous.
Examples: polystyrene, polyvinyl chloride, acrylonitrile butadiene styrene (ABS)
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