What is Radiation Processing?

　　Radiation processing is a technology that utilizes ionizing radiation to induce useful changes in the physical properties of materials or their components. This type of radiation possesses sufficiently high energy to cause ionization of atoms or molecules. This ionization breaks chemical bonds, generating free radicals. These free radicals are highly reactive and are the root cause of many desired changes induced by radiation.

　　Several factors have contributed to the expansion of radiation applications in industry and the proliferation of radiation facilities. These include the availability of high-power electron accelerators with low unit radiation costs, the accessibility of strong gamma radiation sources, a deeper understanding of the underlying chemical principles, and the rising energy costs of competing processing methods.

Process

　　In various plastics, elastomers, or other macromolecular materials, crosslinking and degradation can occur simultaneously or competitively. Crosslinking involves the bonding of molecules, reducing their usual mobility, which means they no longer lose their shape and do not have a true melting point. Degradation, on the other hand, involves the breaking of molecular chains. Both processes cause significant changes in the material, with one often dominating (though this is not always the case). Depending on the relative kinetics of these processes, materials can be classified as "crosslinking" or "degrading," based on which process is predominant. Additionally, materials with high aromatic (benzene ring) content are often referred to as "resistant" materials. Crosslinking or degradation can be induced by several methods, typically through ionizing radiation or chemical means. Chemical methods have the advantage of requiring only small amounts of substances, while radiation offers the benefits of speed and cost-effectiveness.

Radiation Sources

　　Industrial ionizing radiation generally comes in two forms: gamma rays from radioactive substances like cobalt-60 and high-energy electron beams from accelerators.

- Gamma rays have strong penetrating power and can process very thick objects, but the processing rate is usually slow.
- Electron beams have much lower penetration power, but electron generators can achieve very high power outputs, up to several hundred kilowatts, allowing production rates that can match or exceed those of other industrial processes involving radiation equipment.

　　Currently, industrial electron generators are available with higher electron voltages (up to 5 million volts), relaxing the limitations imposed by penetration depth. However, small, low-voltage systems (200–500 kV) are also available, occupying as little as 819 cm² while delivering tens of kilowatts of electron power. The versatility of existing equipment has made electron accelerators the dominant choice for industrial radiation processing due to their cost-effectiveness, efficiency, high production rates, and the high quality of processed products.

Radiation Processing Parameters

　　The radiation produced by electron accelerators is characterized by the accelerator's electron voltage (which determines penetration capability) and the electron beam current or the number of electrons emitted per second. If the electron voltage is sufficient to penetrate the product, the electron beam current determines the production rate or processing speed, provided the energy required for the desired process is available. The energy deposited by the electron beam is called the dose, typically expressed in megarads (Mrad) or kilograys (kGy). One megarad is one million rads; one rad dose represents 10⁻⁵ joules of energy absorbed per gram of product. Ten kilograys are equivalent to one megarad.

　　The output rating of an accelerator is expressed in kilowatts (electron volts × current). As a rule of thumb, 1 kW of electron power can theoretically deliver a dose of 1 Mrad to 362.87 kg of product. The actual processing rate can be calculated by multiplying the theoretical processing rate by the efficiency of the actual process.

　　In the competition for overall economic efficiency, radiation processing methods can succeed based solely on lower unit costs. The key here is that the required production capacity must be large enough to fully utilize the capabilities of the radiation system. Newer, more powerful electron beam systems can thus offer very low unit processing costs. While other systems may have much lower investment costs, radiation systems have the advantage of significantly higher processing rates, making them more cost-effective in terms of unit processing costs.

Applications

　　Radiation processing applications include the following areas:

- Crosslinking of insulation jackets for wires and cables: This is perhaps the most well-known and widely used application of radiation processing. Crosslinking improves resistance to stress cracking, abrasion, and deformation, while also enhancing temperature tolerance and resistance to fluid effects.
- Improving the shrinkage memory of heat-shrinkable plastics through crosslinking: Similar to its use in wires and cables, crosslinking is applied to plastic pipes and components to enhance performance.
- Cold sterilization of surgical instruments and other disposable medical devices.
- Graft polymerization of monomers onto fabric substrates to improve stain resistance and dyeability.
- Eliminating or significantly reducing residual monomers in many plastic packaging materials and food/beverage packaging materials (Lomo method).
- Curing of solvent-free or low-solvent, emission-free, room-temperature rapid-polymerization organic coatings.
- Crosslinking of polyethylene foam to control cell structure, foaming degree, improve surface conditions, or restore toughness.
- Vulcanization of rubber or other elastomers (though generally not including butyl rubber).
- Controlled degradation of plastics such as polypropylene and cellulose plastics.
- Controlled modification of the melt index of homopolymers.