Introduction 

The polymer chains can be free to slide past one another (thermoplastic) or they can be connected to each other with crosslinks (thermoset). Thermoplastics (including thermoplastic elastomers) can be reformed and recycled. This reflects the fact that above T_g they may be shaped or pressed into molds, spun or cast from melts or dissolved in suitable solvents for later fashioning. On the other hand, Thermosets which also includes cross linked elastomers aren't reworkable. They are distinguished via a high degree of cross-linking, resist deformation and solution once their final morphology is achieved. Such polymers are usually prepared in molds that yield the desired object since once formed, can't be reshaped via heating.

Thermoplastics

Polymers that flow when heated; therefore, simply reshaped and recycled. This property is due to presence of long chains through limited or no crosslinks. In a thermoplastic material the very long chain-like molecules are held together via comparatively weak Vander Waals forces. When the material is heated the intermolecular forces are weakened so that it becomes soft and flexible and eventually, at high temperatures, it is a viscous melt (it flows). When the substance is permitted to cool it solidifies once more. For example, polyethylene (PE), polypropylene (PP), poly (vinyl chloride) (PVC), polystyrene (PS), poly (ethylene terephthalate) (PET), nylon (polyamide), unvulcanized natural rubber (polyisoprene)

Thermosets

Decompose when heated; therefore, can't be reformed or reprocessed. Presence of extensive crosslinks between long chains induces decomposition upon heating and renders thermosetting polymers brittle. A thermosetting polymer is created through a chemical reaction that has 2 stages. The 1^st stage consequences in the formation of long chain-like molecules similar to those present in thermoplastics, but still able of additional reaction. The second stage of the reaction (crosslinking of chains) takes place during moulding, generally under the application of heat and pressure. During the 2^nd stage, the long molecular chains have been interlinked via strong covalent bonds so that the substance can't be softened again through the application of heat. If excess heat is applied to such materials they will char and degrade.

For example, epoxy, unsaturated polyesters, phenol-formaldehyde resins, vulcanized rubber. 

    Table: Characteristics of Thermoplastic and Thermosetting Polymers

Industrially important thermoplastic and thermosetting polymers

A number of thermoplastics with very good mechanical, electrical, thermal and chemical characteristics are usually termed to as engineering plastics. They are polycarbonates, polyacetals, polyamides, polyethylene, polystyrene, polypropylene, poly(vinyl chloride) and polytetrafluoroethylene. It doesn't mean that only these are helpful within the engineering context. Therefore there are epoxides, formaldehyde-based resins (for instance, phenol-formaldehyde, urea-formaldehyde, and melamine-formaldehyde) and unsaturated polyesters (reinforced as appropriate).

There are 5 main areas of application for polymers, namely, plastics, elastomers, fibers, surface finishes, protective coatings and adhesives. The equivalent polymer might be utilized in two or more applications. For instance, nylon, a well known textile fiber is as well utilized as a plastic molding material and as a film. Similarly epoxides are used in paints and adhesives as well as composites. The main factors determining the application of a given polymer is its mechanical behavior. If this can be appropriately altered via physical and chemical means then the polymer can be utilized in more than one application. 

Table: Industrially Important Thermoplastics Polymers

HDPE (High Density Polyethylene): Linear structure, better mechanical properties but more difficult to process than LDPE.

 LDPE (Low Density Polyethylene): Branched structure, easier to process than HDPE.

Table: Industrially Important Thermosetting Polymers

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