25.6 The Preparation of Bakelite

Chemical Concept Demonstrated: Thermoset plastics

Demonstration:

Formaldehyde is added to the beaker inside a fume hood. Aniline hydrochloride (C6H5NH3Cl) is rapidly added.

Observations:

    The polymer expands out of the beaker, along with a prodigious amount of heat.

Explanation:

    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 (see demonstration 25.2), 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.