Does free radical halogenation only happen under application of UV?
The initiation step in a free radical halogenation involves the homolytic cleavage of a halogen molecule.
Breaking a bond requires an input of energy to the molecule
In the lab, the most convenient energy source is UV light.
Commercially, however, it is cheaper to use heat as the energy source.
300 °C is required to dissociate chlorine molecules fast enough to effectively initiate the chain reaction.
This temperature range is easily reached in an industrial process.