|University Federico II
of Naples, Italy
Thermosetting polymers are plastics that change their chemical constitution from the time they are synthetized to the time they are put in use.
In a thermosetting plastic, the polymer is made of atomic chains, but in addition there are many bonds between different chains. A linear wirelike chain is the normal structure for all polymers; bonds between chains are called cross-links and they stabilize the plastic shape. The molecules cannot shift relative to their neighbors, and the whole object is solid and non-deformable.
Such a material cannot be worked, right because of its rigidity. Thus plastics objects are not prepared by using the polymer directly, but from a pre-polymer, constituded of relatively short chains, with no interchain bonds. This pre-polymer is viscous liquid, and can easily be formed in the desired shape.
The process leading to formation of a thermosetting polymer. left: Molecules of the thermoplastic pre-polymer associate, forming a "Spaghetti-like" structure. right: After curing, the macromolecules are chemically connected through cross-link; the material assumes a three-dimensional network structure.
Now it is necessary to make this shape stable. The polymer must undergo a chemical transformation, called hardening, that forms the cross-links. This reaction is often carried out by heating, hence the name thermosetting polymers. Sometimes hardening can be achieved without heating, by adding a suitable reactant that induces formation of bonds between chains.
Thermosetting polymers are somewhat awkward for the final user. In order to prepare a product, it is not sufficient buying the raw material and shaping it, but one has to carry out a chemical process. In order to make things easier, producers try to keep the hardening process as simple as possible.
These materials, often called thermosetting resins, are common components of everyday artifacts, such as tables and desks, plastic laminates, electric plugs and switches, rigid boxes. There are many types of thermosetting resings, including phenol-formaldehyde, melamine, epoxy, acrylic, unsaturated polyesters, polyurethans.