The Preparation of Bakelite
Chemical Concept Demonstrated
The polymer expands out of the beaker, along with a prodigious amount of heat.
This reaction is highly exothermic, but that is not why the plastic produced is referred to as "thermoset".
Bakelite is a space-network polymer. Unlike linear and branched polymers, which are composed of long molecules that make them more or less crystalline, space-network polymers are highly and irregularly cross-linked throughout the structure. The sheer extent of the cross-linking means that a sample of the material is essentially one gigantic molecule.
Although heat softens and melts linear and branched polymers, heating doesn't soften space-network polymers because such a softening would require the breaking of covalent bonds. In fact, heating usually produces additional cross-linking in these polymers, making them harder. It is for this reason that space-network polymers, such as bakelite, are called thermoset plastics.