Thermoplastic vs Thermoset
Thermoplastic and Thermoset are terms used when characterizing polymers depending on their behaviour when subjected to heat, hence the prefix, ‘thermo’. Polymers are large molecules made of repeating subunits, and these subunits are called monomers. The main difference between the two is that a thermoset polymer doesn’t melt upon heating and withstands high temperatures, whereas a thermoplastic polymer melts beyond a certain temperature and thus acquiring mouldable properties and solidifies upon cooling.
More on Thermoplastics
These polymers are also called ‘Thermo-softening Plastics’, and as mentioned above can be melted down at high temperatures and cooled to gain back solid form. Thermoplastics are generally of high molecular weight where the polymer chains are associated together via intermolecular forces. These intermolecular forces can be easily broken when energy is supplied. This explains why the polymer is mouldable and will melt upon heating. When enough energy is supplied to get rid of the intermolecular forces that hold the polymer as a solid, we see the solid melting. When it is being cooled, the polymer gives off heat and re-forms the intermolecular forces making it a solid. Therefore, the process is reversible.
Once the polymer is melted, it can be moulded into different shapes and upon re-cooling different products can be obtained. Thermoplastics also do possess a characteristic feature by showing different physical properties between the melting point and the temperature where solid crystals are formed. It is observed that they possess a rubbery nature between those temperatures. Some commonly known thermoplastics include; Nylon, Teflon, Polyethylene, Polystyrene etc.
More on Thermosets
These polymers are also called ‘Thermosetting Plastics’ and are able to withstand high temperatures without melting. This property is brought about by toughening or hardening the soft and viscous pre-polymer through the introduction of cross-links between polymer chains. These links are introduced at chemically active sites (unsaturation etc.) with the aid of a chemical reaction. This process is commonly referred to as ‘curing’ and can be initiated by heat above 200˚C, UV radiation, high energy electron beams and additives. The cross links are chemical in nature, more correctly, they are stable chemical bonds. Once the polymer is cross-liked, it gets a 3D structure which is very rigid and strong, that refuses to melt upon heating. Therefore, this process is irreversible converting the soft starting material into a thermally stable polymer network.
During the process of cross-linking, the molecular weight of the polymer is increased and hence the increase of the melting point. Once the melting point is elevated above the ambient temperature it remains solid. When thermosets are heated up to uncontrollably high temperatures, they decompose instead of melting, due to reaching the decomposition point before the melting point. Some common examples for thermosets include; Polyester Fibreglass, Polyurethanes, Vulcanized Rubber, Bakelite, Melamine etc.
What is the difference between Thermoplastic and Thermoset?
• Thermosets are usually stronger than thermoplastics due to the presence of the 3D network of cross-linking bonds.
• Thermoplastics melt upon heating whereas thermosets are able to withstand high temperatures; hence thermosets are more brittle in nature.
• Thermosets have a permanent shape and cannot be recyclable into new forms of plastic, whereas thermoplastic can be melted into any shape and re-used.