Physico-chemical modification of polymers by ionization

Ionizing radiation (accelerated electrons and gamma γ rays – emitted by a source of ⁶⁰ Co or ¹³⁷ Cs) has been industrially developed since the 1960s for the sterilization of medical-surgical equipment and the preservation of agri-food products.

At the same time, they have led to the development of radical radiation chemistry, applied mainly to polymers. By triggering a chemical process involving the formation of free radicals, ionizing radiation can then initiate a number of chemical reactions such as chain splitting, polyaddition, polymerization, etc., which can give rise to a variety of applications, the main ones being :

• degradation of plastics (in particular to improve their recyclability ;

• cross-linking of plastics and resin-impregnated wood;

• grafting onto polymers.

Until now, radiation treatment of plastics has been limited to a few applications: the manufacture of rubber, cross-linked cable and tubing (tubing for underfloor heating), and heat-shrink tubing. It was mainly implemented with low-energy electron gas pedals (0.1 MeV to 3 MeV), enabling surface treatments only (limited to a few millimeters in depth).

High-energy electron gas pedals (10 MeV) and Gamma accelerators (high radiation penetration power) have since been developed, and are suitable for processing much greater thicknesses, from the size of a cardboard box (several centimetres) to that of a pallet (1 m), enabling products to be processed directly in their packaging.

The industrial treatments carried out on plastics using such equipment are described in this dossier, from the point of view of their effects, their industrial implementation and their applications.

Plastics crosslinking and grafting using ionizing radiation (also known as radiocrosslinking and radiografting respectively) are the focus of this article. Radiation crosslinking is currently the most widely applied, while radiografting, which is attracting increasing interest due to its wide range of applications, is giving rise to a growing number of developments.