Radiation processing of polymers was introduced after world war II with the development of the nuclear reactor. Basic research, started in the late 1940’s are continuing with the commercialization of the technology.
In recent years various radiation sources e.g., X-rays (soft and hard), gamma (y) and ultraviolet (UV) rays and electron beam (EB) are being used in the curing of various polymers, paints, ink, coatings, adhesives etc and in graft co-polymerization.
Radiation processing particularly with electron beam offers some distinct advantages, some of which are:
Advantages of Electron Beam Processing For Polymers
- The process is very fast, clean and can be controlled precisely.
- There is no permanent radioactivity since the machine can be switched off.
- The electron beam can be steered very easily to meet the requirements of various geometrical shapes of products to be irradiated. This cannot be achieved by X-rays and y-rays.
- The high penetrating power of the electron beam allows the efficient curing of thick polymeric articles, highly pigmented inks and coatings. Pigmented inks and coatings cannot be otherwise cured by UV radiation due to low penetrating power.
- The EB radiation process is practically free of waste products and hence there is no serious environmental hazard.
- It gives large through-puts compared to other radiation processes.
- It can be easily implemented at any stage of production.
Now-a-days electron beam radiation processing has wide applications particularly in the wire, cable, coating and the tire industries. This chapter deals with the processing of polymers by electron beam radiation and the properties as well as the applications of modified polymers.
Like infrared and microwave, electron beam is an electromagnetic radiation except that its wave length is very short and the frequency is very high. Table 1 gives comparative chart of various electromagnetic radiations, EB has greater penetrating power as compared to other radiation processes and is associated with very high energy. However, EB is classified as ionizing radiation. This is different from the microwave and the infrared radiation which are non ionizing.
In the electron beam process, the electrons are generated by the electron gun or injector by thermo-ionic emission and accelerated through a potential of 150 to 250 thousand volts under vacuum of 10m6 torr. These are then passed through the window into the target. The machine in which the electrons are generated and accelerated is conventionally termed an electron accelerator or EB generator. Figure 1 shows the schematic diagram of a typical electron beam accelerator.
The main part of an accelerator is obviously the electron injector system or electron gun placed on the upper portion of a cavity called the resonator cavity. The electron injector produces a triode comprising the cathode (generally made of LaB,), anode and a control grid. An equivalent electron gun circuit is shown in Figure 1. A conical spiral heater made of tungsten wire of about 0.6 mm in diameter is attached with the cathode for heating. The electrons produced in the cathode by thermo-ionic emission, are accelerated through the accelerating gap by a high voltage. The accelerated beam current is controlled by changing the value of the positive bias voltage on the cathode with respect to the grid.
The energy of accelerated electrons is around 0.5 to 2.5 MeV and the beam power is about 20 kW over the entire energy range. The unit of radiation dose is the Gray (Gy) and is defined as the dose required by 1 kilogram of material to absorb I Joule of energy. 1 Gy = 1 J in 1 kg = 1 w-set in 1 kg = 100 Rad = 10m4 Mrad The dose rate (DR) for an electron accelerator can be written in terms of beam current (I) and irradiation field area (R)
where K is a stopping power of electrons, which depends on the energy of electrons and density of materials being irradiated . The current can be varied between extremely low to high values which provide good flexibility for the utilization of the accelerator for processing applications.
An important point should be noted here that some ozone is formed during irradiation which is harmful to polymers and should be removed. Typical ozone output within an air gap between an irradiated subject and an extraction foil of 0.05 mm thickness is 60 mg/sec.