Electron beam processing of elastomers.

Electron beam radiation is an ionizing type of radiation that is capable of introducing profound changes of organic matter. It affects the electron shells of atoms without having any significant effects on their nuclei. Radiation crosslinking of polymers by electron beam is a continuous process, which accomplishes a variety of reactions in a fraction of a second. It has been used in the crosslinking and modification of polymers for several decades successfully. Its major advantages are a high speed of conversion, cleanliness and very accurate process control. The objective of this article is to review the chemistry, equipment, processes, applications and the newest developments in the electron beam technology as used mainly in the manufacture of rubber products.

Radiant energy is one of the most abundant forms of energy available to mankind. Nature provides sunlight, the type of radiation essential for many forms of life and growth. Some natural substances, such as radioactive elements (uranium, radium and radioactive isotopes of other elements, etc.) generate the kind of radiation, which can be destructive to life, but when harnessed, it can be useful for medical or industrial applications.

Human genius has created devices for the generation of radiant energy useful in a great variety of scientific, industrial and medical applications. Cathode rays emit impulses that activate screens of televisions and computer monitors. X-rays are used not only as diagnostic tools in medicine, but also as an analytical tool in the inspection of manufactured products, such as tires and other composite structures. Microwaves are used not only in cooking, but also in heating of certain materials and a variety of electronic applications including UV curing lamps. Infrared (IR) radiation is used in heating, analytical chemistry and electronics.

Ultraviolet (UV) and electron beam (EB) radiations are classified as electromagnetic radiation, along with infrared and microwave radiations. The differences between them are shown it table 1.

Industrial processes involving man-made electromagnetic radiations, specifically UV and EB types, depend essentially on two electrically generated sources of radiation: Photons from high-intensity ultraviolet

lamps and accelerated electrons, respectively. The difference between those two is that accelerated electrons can penetrate matter and are stopped only by mass, whereas high-intensity UV light affects only the surface. Both EB and UV radiations represent a clean and efficient use of electric energy.

In this article, we will limit our attention to ionizing radiation, which includes electron beam (EB) radiation and gamma-radiation, with particular stress on the former. The units of measurement for ionizing radiation relevant to its applications are:

* Absorbed dose: It is a mean value of energy of the ionizing radiation absorbed by the unit of mass of the exposed material. The unit is 1 Gray (1Gy) = 1 J/kg. The older unit, used officially until 1986, was 1 Megarad (Mrad), equal to 10 kGy.

* Dose rate: Dose absorbed per unit of time, expressed in Gyx[s.sup.-1] or Jx[kg.sup.-1] x [s.sup.-1]. Ionizing radiation has a profound effect on materials, especially on organic matter. The effects resulting from the exposure to different dose levels are in table 2. This table also shows some of the common applications of ionizing radiation.

Industrial sources of ionizing radiation

Gamma-radiation

The most widely used source of gamma-radiation (often denoted as [gamma]-radiation) in industrial and medical applications is the cobalt isotope 60 ([sup.60]Co). The [gamma]-radiation has a very high penetration, but exhibits a high dose rate when compared to electron beam radiation (table 3). Its major drawback is that it is radioactive and thus presents a major health hazard. It is still widely used in medical procedures and in device sterilization.

Electron beam radiation

Electron beam radiation is generated in a high vacuum (typically [10.sup.-6] torr) by a heated cathode. The electrons emitted from the cathode are then accelerated in an electrostatic field applied between cathode and anode. The acceleration takes place from the cathode that is on negative high voltage potential to the grounded vessel as anode. The accelerated electrons are often focused by an optical system to the window plane of the accelerator (figure 1).

[FIGURE 1 OMITTED]

The energy gain of the electron beam is proportional to the acceleration voltage and is expressed in electron volts (eV) that represent the energy gained by a particle of unit charge by passing the potential difference of 1 V. The electrons leave the vacuum chamber only if their energy is high enough to penetrate the 5-20 [micro]m thick titanium window of the accelerator.

When an electron beam enters a material (this includes the accelerator exit window, the air gap and the material being irradiated), the energy of the accelerated electron is greatly altered. They lose their energy and slow down almost continuously as a result of a large number of interactions, each with only a small energy loss. As any other charged particles, electrons transfer their energy to the material through which they pass in two types of interactions:

* In collision with electrons of an atom resulting in material ionization and excitation; and

* in interaction with atomic nuclei leading to the emission of x-ray photons. This is referred to as Bremsstrahlung (from German, meaning "breaking radiation"), an electromagnetic radiation emitted when a charged particle changes its velocity due to such an interaction.

Somewhat simplified, the process of interaction of high-energy electrons with organic matter can be divided into three primary events:

* Ionization: Ionization takes place only when the transferred energy during the interaction is higher than the bonding energy of the bonding electron:

AB [right arrow] A[B.sup.+] + [e.sup.-]

At almost the same time, the ionized molecule dissociates into a free radical and a radical ion:

A[B.sup.+] [right arrow] ABx + x[B.sup.+]

* Excitation: Excitation moves the molecule from the ground state to the excited state:

AB [right arrow] A[B.sup.*]

The excited molecule eventually dissociates into free radicals: A[B.sup.*] [right arrow] Ax + Bx

* Capture of electron: This process is also ionization. Electrons with still lower energy can be captured by molecules. The resulting ion can dissociate into a free radical and a radical ion:

AB +[e.sup.-] [right arrow] A[B.sup.-]

A[B.sup.-] [right arrow] Ax + x[B.sup.-]

Besides these primary reactions, there are various secondary reactions in which ions or excited molecules take part. The final result of these three events is that, through the diverse primary and secondary fragmentations, radicals are formed. The complete cascade of reactions triggered by the primary excitation of molecules may take up to several seconds. The energy deposited does not always cause changes in the precise position where it was originally deposited, and it can migrate and affect the product yield considerably.

Radiation processing of polymers

As pointed out earlier, free radicals are formed that can initiate a free-radical process, which in monomers and polymers can lead to polymerization, crosslinking, backbone or side-chain scissions, structural rearrangements, etc. The final outcome of the reaction depends on the structure of the polymer, the dose absorbed and the presence of other compounds in the material. Essentially, some polymers are crosslinked, some are degraded.

Radiation crosslinking of polymers, the most desirable process, is enhanced by the addition of relatively small amounts (typically 1-10% by weight) of certain chemicals, called radiation crosslink promoters (prorads). Examples include maleimides, thiols, acrylic and allylic compounds. These are, in general, essentially similar to coagents used for peroxidic crosslinking. Chemicals having an opposite effect, namely protecting polymers against the effects of radiation, are antirads. These compounds are certain aromatic amines, quinones and hydroquinolines.

As pointed out earlier, the depth of penetration of high-energy electrons depends on the acceleration voltage, and the dose deposited in the material depends on the accelerator current. The electron penetration range is related to the path length, which the electron travels during the energy degradation process. It can be estimated from the depth-dose distribution. In applications involving chemical changes, electrons have penetrated reactive solids and liquids with typical masses per unit area of one to several hundred g/[m.sup.2] (1 g/[m.sup.2] = 1 [micro]m at unit density). An example of a depth-dose profile curve is in figure 2.

[FIGURE 2 OMITTED]

The dose and dose distribution are measured conveniently by dosimeters. The most widely used are thin film dosimeters that are essentially dyed or clear plastic films. These films (also referred to as radiachromic films) change their optical absorbance in proportion to the absorbed dose. Thin film dosimeters are calibrated by a special calorimeter, designed to measure the energy deposited from electron accelerators.

Electron beam processing equipment

The basic electrical parameters of electron beam processing equipment are its acceleration voltage, the electron beam current and the electron beam power. The process parameters are line speed, penetration range and dose rate. When the line speed and dose rate are combined, the total delivered dose can be calculated.

The acceleration voltage is the potential difference between cathode and anode of the accelerator expressed usually in kV or MV, and determines, as mentioned already, the penetration depth of the beam.

The electron beam current is a number of electrons emitted per second from the cathode, measured in mA at the high-voltage unit. At a constant accelerating voltage, the beam current determines the dose rate.

The equipment manufacturers currently offer a wide variety of processing equipment with a range of accelerating voltages from 80 kV to 10 MV and with different designs, such as direct and indirect accelerators. The beam can be scanned or delivered as a "curtain" or a broad beam. The three designs that have been used over the years are in figure 3. Since the accelerator operates with a high vacuum, the vacuum chamber is sealed by a 5-25 [micro]m thick titanium window. A schematic showing the design of a direct high voltage accelerator is in figure 4.

[FIGURES 3-4 OMITTED]

Because of the generation of x-rays during the interaction of the high energy electrons with the material and parts of the reactor, the reaction chamber of the processor must be shielded to protect the operating personnel and anyone working nearby. Some reactors with very high accelerating voltages, typically more than 500 kV, are shielded by a concrete or steel vault built around it (figure 5). Equipment operating at lower accelerating voltage is of self-shielded design using one or more inches thick lead cladding. An example of a modern self-shielded reactor is in figure 6.

[FIGURES 5-6 OMITTED]

Processing of elastomers by electron beam irradiation

As with any other polymeric substances, elastomers respond generally to the irradiation by electron beam by either forming crosslinks or degrading. Crosslinking is the more desirable process. The advantages are a fast reaction and the possibility of desired degree of crosslinking. Moreover, the depth of penetration can be set by the choice of accelerating voltage. Thus, it is possible to have a product that is crosslinked only in part of its thickness, and the other part is still unchanged.

The electron beam process is widely used in pre-curing components of tires. The result is the improvement of green strength of the robber compounds, especially of the inner liner, and stabilization of body plies (refs. 1 and 2). This represents potential savings on material up to 20%. Most current arrangements include the EB equipment in-line. An example of a partially pre-cured tire body ply is shown in figure 7.

[FIGURE 7 OMITTED]

Another major application is in wire and cable technology, where elastomeric insulations are cured in a continuous process. Depending on the diameter of the wire or cable, special devices are used to assure a sufficient exposure over the entire surface. An example of the arrangement is shown in figure 8. Other applications are continuous crosslinking of elastic films and sheet and other parts, laminations with elastomeric adhesives, crosslinking of latex articles and elastomeric coatings.

[FIGURE 8 OMITTED]

The changes of polymers during the irradiation are rather complex. In general, there are always competing reactions occurring simultaneously. For example, crosslinking and main chain scission compete, and the result depends on which of the two prevails. The response of different elastomers to ionizing radiation is shown in table 4.

Recent developments

Over the period of the past several years, an appreciable amount of work has been done on surface modification of polymers, such as radiation induced grafting. This introduces new functional groups that can be used for a variety of reactions (refs. 3 and 4). Although there is a large number of possibilities, only a few so far have been used commercially. Examples are battery separators, separation membranes, and applications in the textile industry and in medicine (ref. 5). Radiation rapid curing (RRC) is another promising application of radiation grafting with a potential application in packaging and coatings (ref. 6).

Other recent work, more specific to elastomeric materials, includes:

* Preparation of compatible blends of natural rubber and linear low density polyethylene (LLDPE) (ref. 7);

* vulcanization of mixtures of natural rubber and polybutadiene rubber by simultaneous irradiation by electron beam and microwaves (ref. 8);

* enhancement of reinforcing properties of clay fillers by coating them with acrylate monomer and silane coupling agent followed by exposure to electron beam (ref. 9);

* radiation cure of hydrogenated acrylonitrile-butadiene rubber (HNBR) (ref. 10);

* recycling of butyl rubber waste using EB (ref. 11);

* modification of acrylic elastomer in the presence of polyfunctional monomers (ref. 12);

* radiation induced vulcanization of natural rubber latex in the presence of SBR latex (ref. 13); and

* radiation-induced grafting of methyl methacrylate onto polybutadiene rubber latex (ref. 14).

In equipment, the trend is to develop low cost low energy accelerators to compete with other methods, such as UV cure in coatings, adhesives, crosslinking of thin films and surface modifications.

In dosimetry, the tendency is to provide real-time monitoring and process control using a collimator (cell) (ref. 15) or closed loop microcomputer control system (ref. 16).

Summary

Radiation processing of polymers with an electron beam offers several distinct advantages when compared with other radiation sources, particularly y-rays and x-rays and with conventional methods:

* The process is very fast, clean and can be controlled with much precision;

* there is no permanent radioactivity since the machine can be switched off;

* in contrast to y-rays and x-rays, the electron beam can be steered relatively easily, thus allowing irradiation of a variety of physical shapes; and

* the electron beam radiation process is practically free of waste products and therefore is not a serious environmental hazard.

Table 4--classification of elastomers according to
their response to ionizing radiation

Polymers predominantly crosslinking
Copolymer of styrene and butadiene (SBR)
Chlorinated polyethylene (CM)
Chlorosulfonated polyethylene (CSM)
Polybutadiene (BR or PB)
Natural rubber (NR)
Polychloroprene (CR)
Copolymer of acrylonitrile and butadiene (NBR)
Hydogenated NBR (HNBR)
Ethylene-propylene rubber (EPM)
Ethylene-propylene-diene rubber (EPDM)
Polyurethanes (PUR)
Polydimethyl silicone (MQ)
Polydimethylphenylsilicone (PMQ)
Fluorocarbon elastomers, based on vinylidene fluoride (FKM)

Polymers predominantly degrading
Isobutylene-isoprene rubber or butyl rubber (IIR)
Chlorobutyl rubber (CIIR)
Bromobutyl rubber (BIIR)
Isobutylene rubber (IM)

References

(1.) Thorburn, B., Rubber World, 228, No. 3, June 2003, p. 24.

(2.) Thorburn, B., Rubber & Plastics News, April 5, 2004, p. 14.

(3.) Dworjanyn, P and Garnett, J.L. in Radiation Processing of Polymers, Chapter 6 (Singh, A. and Silverman, J., Eds.), Hanser Publishers, Munich, 1992.

(4.) Wendrinsky, J., RadTech Europe 2001 Conference, October 8-10, 2001, Basel, Switzerland.

(5.) Drobny, J.G., Radiation Technology for Polymers, CRC Press, Boca Raton, FL, 2003, p. 96.

(6.) Stanett, V.T., Radiat. Phys. Chem. 35, p. 82 (1990).

(7.) Dahlan, H.M. et al., Radiat. Phys. Chem. 64, p. 429 (2002).

(8.) Martin, D. et al., Radiat. Phys. Chem. 65, p. 63 (2002).

(9.) Ray, S., et al., Radiat. Phys. Chem. 65, p. 627 (2002).

(10.) Bik, J. et al., Radiat. Phys. Chem. 67, p. 421 (2003).

(11.) Telnov, A.V., Radiat. Phys. Chem. 63, p. 248 (2002).

(12.) Vijaybaskar, V. et al., Radiat. Phys. Chem. 71, p.1,045 (2004).

(13.) Chaudhari, C.V. et al., Radiat. Phys. Chem. 72, p. 613 (2005).

(14.) Peng Jing et al., Radiat. Phys. Chem. 72, p. 739 (2005).

(15.) Korenev, S. et al., Radiat. Phys. Chem. 71, p. 315 (2004).

(16.) Zhou, Xinzhi, et al., Radiat. Phys. Chem. 63, p. 267 (2002).

Jiri George Drobny, Drobny Polymer Associates

Table 1--frequency and wavelength of various
types of electromagnetic radiation

Radiation              Wavelength,                Frequency,
                        [micro]m                      Hz

Infrared              1-[10.sup.2]          [10.sup.15]-[10.sup.12]
Ultraviolet           [10.sup.-2]-1         [10.sup.17]-[10.sup.15]
Microwave         [10.sup.3]-[10.sup.5]     [10.sup.15]-[10.sup.10]
Electron beam    [10.sup.-7]-[10.sup.-4]    [10.sup.21]-[10.sup.18]

Source: Datta, S.K. et al. in Advanced Processing Operations,
Noyes Publications, Westwood, NJ, 1998.

Table 2--dose requirements for various radiation
effects

Radiation effect                                     Dose requirement

Radiography (film)                      [10.sup.-3] to [10.sup.-2] Gy
Human lethal dose (LD 50)                                   4 to 5 Gy
Sprout inhibition (potatoes, onions)                    100 to 200 Gy
Potable water cleanup                                   250 to 500 Gy
Insect control (grains and fruits)                      250 to 500 Gy
Waste water disinfection                                 0.5 to 1 kGy
Fungi and mold control                                     1 to 3 kGy
Food spoilage bacteria                                     1 to 3 kGy
Municipal sludge disinfection                             3 to 10 kGy
Bacterial spore sterilization                            10 to 30 kGy
Virus particle sterilization                             10 to 30 kGy
Smoke scrubbing (S[O.sub.2] and
  N[O.sub.x])                                            10 to 30 kGy
Polymerization of monomers                               10 to 30 kGy
Modification of polymers                                 10 to 50 kGy
Degradation of cellulosic materials                     50 to 250 kGy
Degradation of scrap PTFE ("Teflon")                 500 to 1,500 kGy

Table 3--comparison of electron and [gamma]-Ray sources

Characteristics               Electrons      [gamma]-rays

Power, kW                        150              15
Throughput, kGy-ton/hr.           30             1.5
Energy, MeV                      4.5             1.3
Maximum penetration, cm *        4.0              20
Dose rate                    100 kGy/sec.     10 kGy/h.

* Unit density

Source: Clelland, M.R., "Basic Concepts in Radiation Processing,"
RDI Technical Information Series, TIS 81-4, Radiation
Dynamics.