An important consideration is the choice of material, which can react quite differently to the radiation. Metals and most inorganic material will not be changed by radiation, although colour changes can occur by transparent materials.
Polymers undergo two principally different reactions, which occur in parallel and depend on the chemical nature of the polymer. Which reaction dominates, decides on the final result:
Radiation creates free radicals in the polymer matrix, which are generated from chemical bonds, which have been broken up. These radicals can re-combine and form new chemical bonds, leading to a three-dimensional network, having much improved properties regarding heat stability, chemical and mechanical resistance. Some of the polymers react too slowly, so if crosslinking is the desired result, a special crosslinking promoter is required as an added chemical to the polymer compound.
The major advantage of the radiation crosslinking process is, that it takes place at low temperatures (room temperature) and that in many cases no chemicals are required, accordingly no contamination with residues is possible.
Chain scission (degradation)
In some materials, chain scission (degradation) is the predominant reaction. In this case, the molecular weight of the polymer is reduced in proportion to the radiation dose applied. As a result, the material will lose their strength and become brittle. For some applications this is an unwanted effect as it negatively affects the function. With other materials, it can be a wanted effect, as the molecular weight can be adjusted very precisely, so that flow and processing properties (rheology) can be tailored accordingly. Modification of starch, cellulose or polypropylene are important examples for utilizing this effect.