Thermosetting polymers (or thermosetting resins) belong to the group of plastic materials; they are particular polymers that, after an initial softening phase due to heating, harden due to the effect of three-dimensional cross-linking; during the softening phase due to the combined effect of heat and pressure they are deformable and once hardened they are very resistant.
Thermosetting polymers have a much higher degree of cross-linking of the polymer chain than elastomers, thus giving greater stiffness and consequently greater fragility. If thermosetting polymers are subjected to high temperatures close to or equal to the melting temperature, they undergo chemical degradation (carbonization).
Thermosetting polymers are those polymers that, in appropriate temperature conditions and/or in the presence of particular substances, are transformed into rigid, insoluble and infusible materials. This transformation occurs as a result of cross-linking reactions (process through which the polymeric chains undergo a reaction that creates bonds between different chains at the level of reactive functional groups) called curing that occur between the polymeric chains with the formation of strong bonds (covalent or ionic).
Some thermosetting polymers are cross-linked by heat alone or through combinations of pressure and heat, while others can be cross-linked through chemical reactions at room temperature (cold cross-linking). These polymers are difficult to recycle because the new bonds formed as a result of crosslinking operations are permanent. Examining the trend of viscosity with temperature, when the softening point is exceeded there is an initial decrease in viscosity: it is in the presence of a plastic state that allows the processing of the material. At a certain point, however, cross-linking takes over and there is a progressive increase in viscosity that leads to hardening of the material.
It follows that thermosetting materials can be processed with the same technologies as thermoplastic materials, provided that the processing is carried out under conditions in which the polymers retain their plasticity and care is taken that the cross-linking occurs at a later stage when the material has been given its final shape. Thermosetting polymers are used as molding materials, in adhesives, in paints and enamels and as insulators for aircraft. Some of the best known thermosetting polymers are epoxy resins, most polyurethanes (PU), Bakelite, melamine resins, etc.
Thermosetting plastics are not weldable, if cured by aging in organic solvents they are not soluble, sometimes expansible. Preliminary products of thermosetting plastics are available on the one hand as “molding material” for processing as a melt and subsequent thermal aging; on the other hand as “reaction casting resins” that can be processed at room temperature and catalytically aged.
Types of thermosetting polymers (resins)
Thermoplastic materials are weldable. Semi-finished products of hard thermoplastic materials can be extensively heat deformed. The main types of thermosetting resins are:
- Epoxy resins
- Phenolic resins
- Melamine resins
- Alkyl Resins
- Furan Resins
- Allylic Resins