While there are hundreds of thousands of polymer grades in existence today they can all be divided into two groups; Thermosets and Thermoplastics. The difference relates to the number of times that the material can be heated and processed.
Thermosets can be molded and set only once - rather like an egg. They cannot be melted and reformed.
Thermoplastics are molded, usually under heat and pressure, and are then set by cooling. Like chocolate they can be reheated and remolded into different shapes. In theory this can happen indefinitely - although the properites and quality of the plastics may change after a number of reprocessing cycles.
There are advantages and disadvantages to each category. Because they don't soften reaily with heat Thermset plastics are good for electrical fittings that get hot. Thermoplastics are suitable for a wider number of processing techniques because of the ease with which they can be melted.
Those which soften on heating and then harden again on cooling.
These are called thermoplastic polymers because they keep their plastic properties.
These polymer molecules consist of long chains which have only weak bonds between the chains.
The bonds between the chains are so weak that they can be broken when the plastic is heated.
The chains can then move to form a different shape.
The weak bonds reform when it is cooled and the thermoplastic material keeps its new shape.
Those which never soften once they have been moulded.
These are called thermosetting polymers because once set in shape, that shape cannot be altered.
These polymer molecules consist of long chains which have many strong chemical bonds between the chains.
The bonds between the chains are so strong that they cannot be broken when the plastic is heated.
This means that the thermosetting material always keeps its shape.
The bonding process: When thermoplastic polymers are heated they become flexible. There are no cross-links between chains and the molecules can slide over each other. Thermoset polymers do not soften when heated because the molecules are cross-linked together and remain rigid. The chemical bonding formed within a polymer, and the shape of the resulting polymer, affect its properties.