Thermosetting polymer
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Thermosetting plastics (thermosets) are polymer materials that irreversibly cure. The cure may be done through heat (generally above 200 degrees Celsius), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing.
Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC's).
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[edit] Process
The curing process transforms the resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. During the reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.
Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material.[1]
[edit] Statistics
Thermoset materials are generally stronger than thermoplastic materials due to this 3-D network of bonds, and are also better suited to high-temperature applications up to the decomposition temperature.
[edit] Examples
Some examples of thermosets are:
- Polyester fiberglass systems: (SMC Sheet molding compounds and BMC Bulk molding compounds)
- Vulcanized rubber
- Bakelite, a phenol-formaldehyde resin (used in electrical insulators and plasticware)
- Duroplast, similar to Bakelite
- Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard)
- Melamine resin (used on worktop surfaces)
- Epoxy resin (used as an adhesive and in fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic)
- Polyimides (used in printed circuit boards and in body parts of modern airplanes)
- Mold or Mold Runners (the black plastic part in Integrated Circuits (IC) or semiconductors)
Some methods of molding thermosets are:
- Reactive injection molding (used for objects like milk bottle crates)
- Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables)
- Compression molding (used to shape most thermosetting plastics)
- Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)
[edit] See also
[edit] References
- ^ The Open University (UK), 2000. T838 Design and Manufacture with Polymers: Introduction to Polymers, page 9. Milton Keynes: The Open University
