An important decision is the choice of materials, which can react quite differently to the radiation. Metals and most inorganic materials will not be changed by radiation, although colour changes can occur in transparent materials. Polymers undergo two principally different reactions, which occur in parallel and whose process depends on the chemical properties. The final result depends on the dominant reaction:
Radiation creates free radicals in the polymer matrix that are generated through the breaking of chemical bonds.These radicals can re-combine and form new chemical bonds, leading to a three-dimensional network, with improved properties regarding heat stability as well as chemical and mechanical resistance. Some polymers react so slowly that special crosslinking accelerators are required to achieve a sufficient crosslinking level. They have to be added to the polymer compound in small quantities. The major advantage of the radiation crosslinking process is that it takes place at low temperatures (room temperature) and generally no chemical additives are required; accordingly no contamination with residues occurs.
Some materials react in the form of chain scission (degradation). In this case, the molecular weight is reduced in proportion to the radiation dose applied. As a result, the material loses its strength and becomes brittle. For some applications, this is an unwanted effect as it negatively affects the function. With other materials, it can be a desired effect as the molecular weight can be adjusted very precisely, so that flow and processing properties (rheology) can be modified precisely. The modification of starch, cellulose or polypropylene are important examples for utilising this effect.
Radiation crosslinking improves the properties of inexpensive commodity plastics and technical plastics and thus enables them to be used in conditions which they would not be able to withstand otherwise. The advantages at a glance:
Thermal property improvements:
Tribological property improvements:
Chemical property improvements:
Mechanical property improvements: