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Recent developments in the application of electron accelerators for polymer processing
Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Recent developments in the application of electron accelerators for polymer processing A.G. Chmielewski a, M. Al-Sheikhly b, A.J. Berejka c,n, M.R. Cleland d, M. Antoniak a a
Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland University of Maryland, Department of Materials Science and Engineering, College Park, MD 20742-2115, USA c Ionicorp +, Huntington, NY, USA d IBA Industrial, Inc., Edgewood, NY, USA b
H I G H L I G H T S
The development of high current electron accelerators starting in the 1950s and their early commercial use is summarized. The down-sizing of mid and low-energy self-shielded electron beams facilitates process integration for proven and new uses. Surface decontamination of packing materials used in aseptic filling and curing of metal coil coatings are new applications. Powerful new accelerators (700 kW) make X-ray conversion an industrially viable alternative to isotope use for sterilization. Promising developments with natural polymers, nano-technology, fiber composites, nano-gels and grafting are noted.
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2012 Accepted 24 June 2013
There are now over 1700 high current, electron beam (EB) accelerators being used world-wide in industrial applications, most of which involve polymer processing. In contrast to the use of heat, which transfers only about 5–10% of input energy into energy useful for materials modification, radiation processing is very energy efficient, with 60% or more of the input energy to an accelerator being available for affecting materials. Historic markets, such as the crosslinking of wire and cable jacketing, of heat shrinkable tubings and films, of partial crosslinking of tire components and of low-energy EB to cure or dry inks and coatings remain strong. Accelerator manufacturers have made equipment more affordable by down-sizing units while maintaining high beam currents. Very powerful accelerators with 700 kW output have made X-ray conversion a practical alternative to the historic use of radioisotopes, mainly cobalt-60, for applications as medical device sterilization. New EB end-uses are emerging, such as the development of nano-composites and nano-gels and the use of EB processing to facilitate biofuel production. These present opportunities for future research and development. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Electron beam processing X-ray processing Accelerator industrial uses
1. Industrial electron beam accelerators The industrial use of electron beam accelerators (EB) started in the late 1950's with the crosslinking of polyethylene wire insulation. In the 1940's and 50's, high current accelerators were developed by the General Electric Corporation (the GE Resonant Transformer) (Westendorp, 1940), by the High Voltage Engineering Corporation (the Insulating Core Transformer, ICT system) (Van de Graaff, 1965), and by Radiation Dynamics, Inc., (the parallel capacitive-coupled, cascaded rectifier, direct-current Dynamitron™) (Cleland, 1959). These coincided with the discovery by Arthur Charlesby and Malcolm Dole that ionizing radiation would crosslink the emerging
n
Corresponding author. Tel.: +1 631 549 8517. E-mail address: [email protected] (A.J. Berejka).
commodity plastic, polyethylene (PE) (Dole, 1950; Charlesby, 1952, 1960). By the end of the 1950's, the Raytherm corporation was formed to bring together the innovation of using ionizing radiation to crosslink PE with the then available industrial accelerators to provide EB crosslinked aircraft wire, which was lighter in weight and better in performance than insulations made using historic thermal processes (Cook and Muchmore, 1963; Ross, 1997). In the 1970's, self-shielded, lead encased low-energy accelerators (300 keV and below) were developed. These had elongated filaments, as units made by Energy Sciences Incorporated, or segmented filaments, as in the Broadbeam™ technology (Quintal, 1972; Nablo, 1993). Also, in the 1970's, the Budker Institute of Nuclear Physics in Novosibirsk and the Efremov Institute of Electrophysical Apparatus in Saint Petersburg, both then in the USSR, began producing high current accelerators for industrial market applications in Eastern Europe (Budker, 1977).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.06.024
Please cite this article as: Chmielewski, A.G., et al., Recent developments in the application of electron accelerators for polymer processing. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2013.06.024i
A.G. Chmielewski et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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A common characteristic of industrial accelerators is their high beam current. For high energies, beam output powers of 30 kW at 10 MeV and 700 kW at 7 MeV are now available. At low and midenergy ranges, beam currents range from a few tens of milliamps (mA) up to 100 mA. Such power is required so that economic and profitable process through-put rates can be attained. While there are many more numerous and diverse research accelerators, often operating in the micro-amp range, such as Van de Graaff generators and many linear accelerators, such accelerators lack the output power needed for industrial applications (IAEA, 2011). Over the decades, different energy accelerators have been found to be more suitable for different industrial polymer applications: 1. Low-energy (from 120 keV to 300 keV) — printing inks; coatings for paper, wood, metals and plastics; adhesives; multilayer packaging; and surface grafting for membranes. 2. Mid-energy (from 300 keV to 5 MeV) — wire and cable insulation; heat shrinkable tubing and films; fiber modification; polymer blends with nano-particles and with natural polymers. 3. High-energy (from 5 MeV to 10 MeV) — sterilization of medical devices and some pharmaceutical and biological products; treatment of industrial and domestic effluents and sludge; treatment of lignocellulosic materials to produce ethanol biofuels (IAEA, 2008). Two equipment trends have emerged: (1) the down-sizing of mid and low-energy EB equipment and (2) the development of industrial processing based on X-rays derived from high powered (up to 700 kW output) accelerators. Accelerated electrons are converted to X-rays by interposing a target material, such as a water cooled tantalum construction, between the electron source and the object to be irradiated (Berejka, 2009). As noted, the development of high-energy electron accelerators, as the 700 kW Rhodotron™ developed by IBA, makes X-ray processing a viable alternative to gamma processing for applications. The sterilization of packaged medical devices and the preservation of food often require greater penetration than can be provided by industrial electron beams, even at 10 MeV (IAEA, 2004). The down-sizing of mid to low-energy EB units (800 keV down to 100 keV) in selfshielded systems has made such accelerators more amenable to installation of conventional process equipment, such as printing presses and coatings lines, and doing so while also providing more cost-effective equipment, thereby reducing some of the burdens of capital investment (Berejka, 1995).
2. Emerging industrial applications: A report commissioned by the International Atomic Energy Agency (IAEA) on the “Industrial Radiation Processing of Polymers Status and Prospects — August 2005” concluded that: “Over the past several decades, no new major end-use markets for radiation processing have developed.” This same report also concluded that barriers to the public acceptance of such major end-use applications as food irradiation might be overcome: “Public understanding of radiation processing could be fostered by forcing downward to at least the secondary school level basic concepts underlying all industrial radiation processing” (Berejka, 2005). Even at the college level, engineers are unaware of the benefits of radiation processing as an alternative to the use of heat and its energy inefficient thermal systems. Since then major new end-use markets for industrial accelerators have emerged: as the use of low-energy accelerators to decontaminate the surfaces of packaging materials for food products and medicinals that are to be filled in aseptic systems, as the use of low-energy EB to cure the pigmented coatings onto
metal coil, and as the use of X-rays to sterilize medical devices in lieu of the use of radioactive isotope sources. 2.1. Surface decontamination of packaging materials for aseptic filling systems At the May, 2009 International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators (AccApp ’09), which was cosponsored by the IAEA and the American Nuclear Society, it was reported that 19 low-energy EB systems, with three triangulated 200 keV beams each, had by then been installed for the surface decontamination of packaging materials to be used in the aseptic filling of medicinals. Another eight systems with two 125 keV electron emitters each had been installed for decontamination of the surfaces of packaging materials to be used in aseptic filling of products such as fruit juices. This area of low-energy EB use represents the single most significant market development of industrial electron beam processing to have emerged in recent years. By treating materials as they enter an aseptic filling system, this use of EB eliminates the need for terminal product sterilization (Morisseau, 2009). 2.2. EB curing of coatings on metal coil In 2011, a low-energy EB line was launched in France to cure coatings that were applied to coiled flat metal sheet. The concept of using low-energy EB for coil coatings had been envisioned for years. This installation marks a break-through of EB curing into a market other than the traditional curing of inks, coatings (as applied to wood, paper, and plastics for example) and adhesives. At a US Department of Energy symposium on “Accelerators for America's Future,” held in Washington, DC, in October 2009, an industry working group estimated that if just this one segment, coil coating, of the entire coatings industry were to convert to EB, there would be energy savings comparable to the output of a midsized electrical power plant. This is one market area in which data on the use coatings to be used on metal coil was available through analyses performed by the US Environmental Protection Agency. In general, with EB it takes only about 5% of the input energy needed to dry or cure a coating than with solvent based systems. In addition, EB coatings, having near-zero volatile organic compounds (VOCs), eliminate air pollutants. When compared to waterborne systems, which may have very low VOCs, the energy savings with EB is even more dramatic, with EB requiring less than 1% the input energy demand than that needed to dry water-based systems, and does so at much higher product through-put rates. As noted, EB processes are based on high beam current accelerators with their use resulting in high productivity and throughput rates (Cleland and Amm, 2009). 2.3. X-ray sterilization of medical devices Since Wilhelm Roentgen's discovery of X-radiation in 1895, it has been known that such rays can penetrate matter. X-rays have been used for diagnoses in the medical area, relying upon doses of only about 0.01 Gy or 10 milli-grays. Industrial applications require doses in the kilo-gray range, six orders of magnitude greater. The conversion of high current electron beams into X-rays requires the use of a metallic target, preferably a water-cooled system made from tantalum which lends itself to such fabrication. The tantalum target converts EB output into X-rays with relative efficiency, in the 8–12% range, depending on the accelerator voltage. The depth-dose profile of X-rays is comparable to that of gamma emissions from radioactive sources, such as cobalt-60, and significantly greater than the highest voltage industrial accelerators at 10 MeV. The table above summarizes some key features of
Please cite this article as: Chmielewski, A.G., et al., Recent developments in the application of electron accelerators for polymer processing. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2013.06.024i
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Table 1
Power source: Power activity: Penetration: Dose-rate:
Electron beams
X-rays
Gamma rays
Electricity Electrical on-off 10 MeV¼ 38 mm 360,000 kilogray/h 100 kGy/sec
Electricity Electrical on-off 7.5 MeV ¼450 mm 100 kilogray/h 2.7 10−2 kGy/sec
Radioactive isotope (mainly cobalt-60) 5.27 year half-life 450 mm 10 kilogray/h 2.8 10−3 kGy/sec
these three sources of ionizing radiation, EB, X-ray and isotope based gamma (IAEA, 2011) . EB and X-ray differ by nearly four orders of magnitude in doserate. The lower dose-rate of X-rays has been found to be advantageous in the polymerization of monomers, such as those used in wood impregnation. Although X-rays and gamma rays differ by but one order of magnitude in dose-rate, X-rays being higher, studies on medical device grades of polypropylene, stabilized to deter chain scissioning, have shown X-rays to be less deleterious than gamma rays (Portnoy et al., 2007). The higher through-put potential of X-ray processing versus gamma due simply to doserate differences and the less deleterious effects on polymers commonly used in medical devices is being assessed in a fullscale X-ray sterilization facility in Switzerland that is based on a 7.0 MeV, 700 kW output Rhodotron. Other benefits, such as better dose uniformity with X-rays, are also being observed. With X-rays, there is no need to be concerned over source replenishment, shipping of isotopes, especially across borders, and the reliability of source suppliers.
3. Promising developmental programs: 3.1. Natural polymers Natural polymers are being evaluated since some are biocompatible and they provide a renewable resource in polymer compositions. For example, polymer-composites are being developed which use natural fiber reinforcement in lieu of glass or carbon fibers. An agro-fiber, such as from the Kenaf plant, has been evaluated in composites for non-structural automotive components, as interior body trim, in Malaysia. Wood-polymer composites based on the impregnation of fast growing, soft woods with a specific acrylic monomer and EB or X-ray cured have shown outstanding dimensional stability under humidity cycling. This technology affords more durable wood based materials (Cleland et al., 2009a, 2009b). Natural polymers are also being used with radiation processing for enhanced biofuel manufacture. EB processing is being used to degrade cellulose so that it is more amenable to enzymatic digestion and the production of alcohols (Driscoll et al., 2009). In Brazil and elsewhere, the use of EB with sugar plant based sources has shown EB to facilitate the breakdown of the natural fibers and thus also expedite alcohol manufacture for use as a fuel. Another emerging interest is in the production of methane for use in combustion processes based upon anaerobic digestion of radiation treated biomass. 3.2. Nanotechnology Nanotechnology is one of the fastest growing areas in science and engineering. The subject arises from the convergence of electronics, physics, chemistry, biology and materials science to create new functional systems of nano-scale dimensions. It has been shown that EB treatment of metal-ion based solutions can themselves result in a precipitate of nano-scale particles. Thus,
radiation processing can be used in the production of nanoparticulates themselves (Chmielewski et al., 2007). 3.3. Fiber composites X-rays derived from high-energy electron accelerators have been used to cure the matrices of fiber reinforced composites, even when the composite itself is constrained within a mold. The X-rays penetrate through the mold walls. This method eliminates the need for chemical catalysis and high temperature thermal processing (Herer et al., 2009). Using carbon-fiber, structural automotive components, including the chassis, can be made for diverse end-uses. Substantial fuel savings (energy efficiency) can be attained when building an auto entirely from carbon-fiber composites. Large autoclaves can be eliminated and cure cycles reduced from hours to a matter of minutes (Cleland et al., 2009a, 2009b). 3.4. Nano-gels A two-step synthesis procedure, consisting of low-dose-rate irradiation of a semi-concentrated polymer solution followed by pulsed irradiation of this dilute solution, yields polymeric nanogels with controlled properties (molecular weight and the size of the gels). This method produces nano-gels in a pure polymerwater system. The use of some auxiliary materials, as some monomers and crosslinking agents, is eliminated (Kadłubowski et al., 2012). Water-based nano-gels are soft polymeric materials that exhibit mutability and responsiveness to mechanical stimuli. These nano-gels can be used for drug or vaccine delivery and are especially suited for the delivery of anti-cancer agents into tumor tissues. 3.5. Grafting Reversible Addition/Fragmentation Chain Transfer (RAFT) technology is a way to readily synthesize polymers which have a predetermined molecular weight and narrow molecular weight distribution based on a wide range of monomers and reaction conditions and which have reactive terminal groups that can be used for chain addition polymerization (Chiefari et al., 1998). Unlike the historic uses of radiation grafting, which bound a reactive monomer to a substrate, RAFT controls the conditions to which precursors are exposed so as to permit polymer chain extension to proceed via a residual reactive moiety.
4. Conclusion The penetration of electrons is dependent upon the electron energy and the density of the irradiated material. The energy transfer efficiency of electron accelerators depends on their specific designs. Radiation processing based on accelerated electrons, X-rays or both are used in the processing of polymeric materials to produce value-added products. Some new commercial applications have emerged in recent years: (a) the use of lowenergy EB to decontaminate surfaces of materials to be used in
Please cite this article as: Chmielewski, A.G., et al., Recent developments in the application of electron accelerators for polymer processing. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2013.06.024i
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aseptic packaging; (b) the use of low-energy EB to cure the pigmented coatings on metal coil stock; and (c) the use of X-rays in lieu of radioactive isotopes to sterilize medical devices. In the research and commercial development stage are additional new fields involving natural polymers, nanotechnology as nanocomposites and nano-gels and the greater use of grafting. References Berejka, A.J., 1995. Irradiation Processing in the 90’s: Energy Savings and Environmental Benefits. Radiat. Phys. Chem. 46 (Nos. 4–6), 429–437. Berejka, A.J., June 2005. Industrial Irradiation Processing of Polymers – Status and Prospects” Report for the IAEA. Berejka, A.J., 4–8 May 2009. Prospects and Challenges for the industrial use of electron beam accelerators, IAEA. International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators, Vienna, SM/EB-01. Budker, G.I., April 5 1977. Charged particle accelerator. US Patent 4,016,499. Charlesby, A., 1952. In: Proceedings of the Royal Society, A215, London, p. 187. Charlesby, A., 1960. Atomic Radiation and Polymers. Pergamon Press, London. Chiefari, J., Chong, Y.K., Ercole, F., Krstina, J., Jeffery, J., Le, T.P.T., Mayadunne, R.T.A., Meijs, G.F, Moad, C.L., Moad, G., Rizzardo, E., Thang, S.H., 1998. Living freeradical polymerization by reversible addition-fragmentation chain transfer: the RAFT process. Macromolecules 31, 5559–5562. Chmielewski, A.G., Chmielewska, D.K., Michalik, J., Sampa, M.H., 2007. Prospects and challenges in application of gamma, electron and ion beams in processing of nanomaterials. Nucl. Instrum. Methods Phys. Res. 265 B. Cleland, M.R., February 24 1959. Voltage multiplication apparatus. US Patent 2,875,394. Cleland, M.R., Galloway, R.A., Montoney, D., Dispenza, D., Berejka A.J., 4–8 May 2009a. Radiation curing of composites for vehicle components and vehicle manufacture. International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators, Vienna, AP/IA-04. Cleland, M.R., Galloway, R.A., Berejka, A.J., Montoney, D., Driscoll, M., Smith, L., Larsen, L.S., 2009b. X-ray initiated polymerization of wood impregnants. Radiat. Phys. Chem. 78, 535–538. Cleland, M.R., Amm, K.A., December 2009. The Report of the Industry Working Group. US Department of Energy workshop Accelerators for America’s Future.
Cook, P.M., Muchmore, R.W., April 23 1963. Process and Apparatus for Producing Materials having Plastic Memory. US Patent 3,086,242. Dole, M., 1950. Report of Symposium IX “Chemistry and Physics of Radiation Dosimetry” Army Chemical Center, Maryland, p. 120. Driscoll, M., Stipanovic, A., Winter, W., Cheng, K., Manning, M., Spiese, J., Galloway, R.A., Cleland, M.R., 2009. Electron beam irradiation of cellulose. Radiat. Phys. Chem. 78, 539–542. Herer, A., Galloway, R.A., Cleland, M.R., Berejka, A.J., Montoney, D., Dispenza, D., Driscoll, M., 2009. X-rays -cured carbon-fiber composites for vehicle use. Radiat. Phys. Chem. 78 (7–8), 531–534, July–August. IAEA, 2004. Emerging applications of radiation processing. In: Proceedings of a Technical Meetingheld in Vienna, 28–30 April 2003. IAEA-TECDOC-1386. IAEA, 2008. Industrial electron beam processing. In: Proceedings of the Consultants’ Meeting on the Preparation of the status report on low energy, self-shielded electron accelerators and of industrial scale electron/X-ray irradiators, Vienna, Austria, July 21–25, 2008. IAEA, 1 May 2011. Industrial Radiation Processing with Electron Beams and X-rays. Revision 6. Kadłubowski, S., Ulański, P., Rosiak, J.M., 25 April 2012. Synthesis of tailored nanogels by means of two-stage irradiation, vol. 53, no. 10, pp. 1985–1991. Morisseau, D., 4–8 May 2009. New Applications for Accelerators in Pharmaceutical Processes. International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators, Vienna. AP/IA-04. Nablo, S.V., 1993. Electron-Beam Processing Machinery. Radiation Curing in Polymer Science and Technology. Fundamentals and Methods, Volume 1. Elsevier Applied Science, London, pp. 504–551. Portnoy, R.C., Berejka, A.J., Cleland, M.R., Galloway, R.A., May 2007. A Comparison of the Effects on Polypropylene using Sterilizing Doses from Three Different Sources of Ionizing Radiation. Society of Plastics Engineers. International Polyolefins Conference. Quintal, B.S., November 7 1972. Apparatus For and Method of Producing an Energetic Electron Curtain. US Patent 3,702,412. Ross, J.R., 1997. The Cryovac Story. Cryovac Division. W. R. Grace & Co., Connecticut p. 37. Van de Graaff, R.J., June 1 1965. High Voltage Electromagnetic Apparatus Having an Insulating Magnetic Core. US Patent 3,187,208. Westendorp, W.F., September 17 1940. Voltage Generating Apparatus. US Patent 2,214,871.
Please cite this article as: Chmielewski, A.G., et al., Recent developments in the application of electron accelerators for polymer processing. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2013.06.024i
Contents lists available at SciVerse ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Recent developments in the application of electron accelerators for polymer processing A.G. Chmielewski a, M. Al-Sheikhly b, A.J. Berejka c,n, M.R. Cleland d, M. Antoniak a a
Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland University of Maryland, Department of Materials Science and Engineering, College Park, MD 20742-2115, USA c Ionicorp +, Huntington, NY, USA d IBA Industrial, Inc., Edgewood, NY, USA b
H I G H L I G H T S
The development of high current electron accelerators starting in the 1950s and their early commercial use is summarized. The down-sizing of mid and low-energy self-shielded electron beams facilitates process integration for proven and new uses. Surface decontamination of packing materials used in aseptic filling and curing of metal coil coatings are new applications. Powerful new accelerators (700 kW) make X-ray conversion an industrially viable alternative to isotope use for sterilization. Promising developments with natural polymers, nano-technology, fiber composites, nano-gels and grafting are noted.
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2012 Accepted 24 June 2013
There are now over 1700 high current, electron beam (EB) accelerators being used world-wide in industrial applications, most of which involve polymer processing. In contrast to the use of heat, which transfers only about 5–10% of input energy into energy useful for materials modification, radiation processing is very energy efficient, with 60% or more of the input energy to an accelerator being available for affecting materials. Historic markets, such as the crosslinking of wire and cable jacketing, of heat shrinkable tubings and films, of partial crosslinking of tire components and of low-energy EB to cure or dry inks and coatings remain strong. Accelerator manufacturers have made equipment more affordable by down-sizing units while maintaining high beam currents. Very powerful accelerators with 700 kW output have made X-ray conversion a practical alternative to the historic use of radioisotopes, mainly cobalt-60, for applications as medical device sterilization. New EB end-uses are emerging, such as the development of nano-composites and nano-gels and the use of EB processing to facilitate biofuel production. These present opportunities for future research and development. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Electron beam processing X-ray processing Accelerator industrial uses
1. Industrial electron beam accelerators The industrial use of electron beam accelerators (EB) started in the late 1950's with the crosslinking of polyethylene wire insulation. In the 1940's and 50's, high current accelerators were developed by the General Electric Corporation (the GE Resonant Transformer) (Westendorp, 1940), by the High Voltage Engineering Corporation (the Insulating Core Transformer, ICT system) (Van de Graaff, 1965), and by Radiation Dynamics, Inc., (the parallel capacitive-coupled, cascaded rectifier, direct-current Dynamitron™) (Cleland, 1959). These coincided with the discovery by Arthur Charlesby and Malcolm Dole that ionizing radiation would crosslink the emerging
n
Corresponding author. Tel.: +1 631 549 8517. E-mail address: [email protected] (A.J. Berejka).
commodity plastic, polyethylene (PE) (Dole, 1950; Charlesby, 1952, 1960). By the end of the 1950's, the Raytherm corporation was formed to bring together the innovation of using ionizing radiation to crosslink PE with the then available industrial accelerators to provide EB crosslinked aircraft wire, which was lighter in weight and better in performance than insulations made using historic thermal processes (Cook and Muchmore, 1963; Ross, 1997). In the 1970's, self-shielded, lead encased low-energy accelerators (300 keV and below) were developed. These had elongated filaments, as units made by Energy Sciences Incorporated, or segmented filaments, as in the Broadbeam™ technology (Quintal, 1972; Nablo, 1993). Also, in the 1970's, the Budker Institute of Nuclear Physics in Novosibirsk and the Efremov Institute of Electrophysical Apparatus in Saint Petersburg, both then in the USSR, began producing high current accelerators for industrial market applications in Eastern Europe (Budker, 1977).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.06.024
Please cite this article as: Chmielewski, A.G., et al., Recent developments in the application of electron accelerators for polymer processing. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2013.06.024i
A.G. Chmielewski et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
A common characteristic of industrial accelerators is their high beam current. For high energies, beam output powers of 30 kW at 10 MeV and 700 kW at 7 MeV are now available. At low and midenergy ranges, beam currents range from a few tens of milliamps (mA) up to 100 mA. Such power is required so that economic and profitable process through-put rates can be attained. While there are many more numerous and diverse research accelerators, often operating in the micro-amp range, such as Van de Graaff generators and many linear accelerators, such accelerators lack the output power needed for industrial applications (IAEA, 2011). Over the decades, different energy accelerators have been found to be more suitable for different industrial polymer applications: 1. Low-energy (from 120 keV to 300 keV) — printing inks; coatings for paper, wood, metals and plastics; adhesives; multilayer packaging; and surface grafting for membranes. 2. Mid-energy (from 300 keV to 5 MeV) — wire and cable insulation; heat shrinkable tubing and films; fiber modification; polymer blends with nano-particles and with natural polymers. 3. High-energy (from 5 MeV to 10 MeV) — sterilization of medical devices and some pharmaceutical and biological products; treatment of industrial and domestic effluents and sludge; treatment of lignocellulosic materials to produce ethanol biofuels (IAEA, 2008). Two equipment trends have emerged: (1) the down-sizing of mid and low-energy EB equipment and (2) the development of industrial processing based on X-rays derived from high powered (up to 700 kW output) accelerators. Accelerated electrons are converted to X-rays by interposing a target material, such as a water cooled tantalum construction, between the electron source and the object to be irradiated (Berejka, 2009). As noted, the development of high-energy electron accelerators, as the 700 kW Rhodotron™ developed by IBA, makes X-ray processing a viable alternative to gamma processing for applications. The sterilization of packaged medical devices and the preservation of food often require greater penetration than can be provided by industrial electron beams, even at 10 MeV (IAEA, 2004). The down-sizing of mid to low-energy EB units (800 keV down to 100 keV) in selfshielded systems has made such accelerators more amenable to installation of conventional process equipment, such as printing presses and coatings lines, and doing so while also providing more cost-effective equipment, thereby reducing some of the burdens of capital investment (Berejka, 1995).
2. Emerging industrial applications: A report commissioned by the International Atomic Energy Agency (IAEA) on the “Industrial Radiation Processing of Polymers Status and Prospects — August 2005” concluded that: “Over the past several decades, no new major end-use markets for radiation processing have developed.” This same report also concluded that barriers to the public acceptance of such major end-use applications as food irradiation might be overcome: “Public understanding of radiation processing could be fostered by forcing downward to at least the secondary school level basic concepts underlying all industrial radiation processing” (Berejka, 2005). Even at the college level, engineers are unaware of the benefits of radiation processing as an alternative to the use of heat and its energy inefficient thermal systems. Since then major new end-use markets for industrial accelerators have emerged: as the use of low-energy accelerators to decontaminate the surfaces of packaging materials for food products and medicinals that are to be filled in aseptic systems, as the use of low-energy EB to cure the pigmented coatings onto
metal coil, and as the use of X-rays to sterilize medical devices in lieu of the use of radioactive isotope sources. 2.1. Surface decontamination of packaging materials for aseptic filling systems At the May, 2009 International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators (AccApp ’09), which was cosponsored by the IAEA and the American Nuclear Society, it was reported that 19 low-energy EB systems, with three triangulated 200 keV beams each, had by then been installed for the surface decontamination of packaging materials to be used in the aseptic filling of medicinals. Another eight systems with two 125 keV electron emitters each had been installed for decontamination of the surfaces of packaging materials to be used in aseptic filling of products such as fruit juices. This area of low-energy EB use represents the single most significant market development of industrial electron beam processing to have emerged in recent years. By treating materials as they enter an aseptic filling system, this use of EB eliminates the need for terminal product sterilization (Morisseau, 2009). 2.2. EB curing of coatings on metal coil In 2011, a low-energy EB line was launched in France to cure coatings that were applied to coiled flat metal sheet. The concept of using low-energy EB for coil coatings had been envisioned for years. This installation marks a break-through of EB curing into a market other than the traditional curing of inks, coatings (as applied to wood, paper, and plastics for example) and adhesives. At a US Department of Energy symposium on “Accelerators for America's Future,” held in Washington, DC, in October 2009, an industry working group estimated that if just this one segment, coil coating, of the entire coatings industry were to convert to EB, there would be energy savings comparable to the output of a midsized electrical power plant. This is one market area in which data on the use coatings to be used on metal coil was available through analyses performed by the US Environmental Protection Agency. In general, with EB it takes only about 5% of the input energy needed to dry or cure a coating than with solvent based systems. In addition, EB coatings, having near-zero volatile organic compounds (VOCs), eliminate air pollutants. When compared to waterborne systems, which may have very low VOCs, the energy savings with EB is even more dramatic, with EB requiring less than 1% the input energy demand than that needed to dry water-based systems, and does so at much higher product through-put rates. As noted, EB processes are based on high beam current accelerators with their use resulting in high productivity and throughput rates (Cleland and Amm, 2009). 2.3. X-ray sterilization of medical devices Since Wilhelm Roentgen's discovery of X-radiation in 1895, it has been known that such rays can penetrate matter. X-rays have been used for diagnoses in the medical area, relying upon doses of only about 0.01 Gy or 10 milli-grays. Industrial applications require doses in the kilo-gray range, six orders of magnitude greater. The conversion of high current electron beams into X-rays requires the use of a metallic target, preferably a water-cooled system made from tantalum which lends itself to such fabrication. The tantalum target converts EB output into X-rays with relative efficiency, in the 8–12% range, depending on the accelerator voltage. The depth-dose profile of X-rays is comparable to that of gamma emissions from radioactive sources, such as cobalt-60, and significantly greater than the highest voltage industrial accelerators at 10 MeV. The table above summarizes some key features of
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Table 1
Power source: Power activity: Penetration: Dose-rate:
Electron beams
X-rays
Gamma rays
Electricity Electrical on-off 10 MeV¼ 38 mm 360,000 kilogray/h 100 kGy/sec
Electricity Electrical on-off 7.5 MeV ¼450 mm 100 kilogray/h 2.7 10−2 kGy/sec
Radioactive isotope (mainly cobalt-60) 5.27 year half-life 450 mm 10 kilogray/h 2.8 10−3 kGy/sec
these three sources of ionizing radiation, EB, X-ray and isotope based gamma (IAEA, 2011) . EB and X-ray differ by nearly four orders of magnitude in doserate. The lower dose-rate of X-rays has been found to be advantageous in the polymerization of monomers, such as those used in wood impregnation. Although X-rays and gamma rays differ by but one order of magnitude in dose-rate, X-rays being higher, studies on medical device grades of polypropylene, stabilized to deter chain scissioning, have shown X-rays to be less deleterious than gamma rays (Portnoy et al., 2007). The higher through-put potential of X-ray processing versus gamma due simply to doserate differences and the less deleterious effects on polymers commonly used in medical devices is being assessed in a fullscale X-ray sterilization facility in Switzerland that is based on a 7.0 MeV, 700 kW output Rhodotron. Other benefits, such as better dose uniformity with X-rays, are also being observed. With X-rays, there is no need to be concerned over source replenishment, shipping of isotopes, especially across borders, and the reliability of source suppliers.
3. Promising developmental programs: 3.1. Natural polymers Natural polymers are being evaluated since some are biocompatible and they provide a renewable resource in polymer compositions. For example, polymer-composites are being developed which use natural fiber reinforcement in lieu of glass or carbon fibers. An agro-fiber, such as from the Kenaf plant, has been evaluated in composites for non-structural automotive components, as interior body trim, in Malaysia. Wood-polymer composites based on the impregnation of fast growing, soft woods with a specific acrylic monomer and EB or X-ray cured have shown outstanding dimensional stability under humidity cycling. This technology affords more durable wood based materials (Cleland et al., 2009a, 2009b). Natural polymers are also being used with radiation processing for enhanced biofuel manufacture. EB processing is being used to degrade cellulose so that it is more amenable to enzymatic digestion and the production of alcohols (Driscoll et al., 2009). In Brazil and elsewhere, the use of EB with sugar plant based sources has shown EB to facilitate the breakdown of the natural fibers and thus also expedite alcohol manufacture for use as a fuel. Another emerging interest is in the production of methane for use in combustion processes based upon anaerobic digestion of radiation treated biomass. 3.2. Nanotechnology Nanotechnology is one of the fastest growing areas in science and engineering. The subject arises from the convergence of electronics, physics, chemistry, biology and materials science to create new functional systems of nano-scale dimensions. It has been shown that EB treatment of metal-ion based solutions can themselves result in a precipitate of nano-scale particles. Thus,
radiation processing can be used in the production of nanoparticulates themselves (Chmielewski et al., 2007). 3.3. Fiber composites X-rays derived from high-energy electron accelerators have been used to cure the matrices of fiber reinforced composites, even when the composite itself is constrained within a mold. The X-rays penetrate through the mold walls. This method eliminates the need for chemical catalysis and high temperature thermal processing (Herer et al., 2009). Using carbon-fiber, structural automotive components, including the chassis, can be made for diverse end-uses. Substantial fuel savings (energy efficiency) can be attained when building an auto entirely from carbon-fiber composites. Large autoclaves can be eliminated and cure cycles reduced from hours to a matter of minutes (Cleland et al., 2009a, 2009b). 3.4. Nano-gels A two-step synthesis procedure, consisting of low-dose-rate irradiation of a semi-concentrated polymer solution followed by pulsed irradiation of this dilute solution, yields polymeric nanogels with controlled properties (molecular weight and the size of the gels). This method produces nano-gels in a pure polymerwater system. The use of some auxiliary materials, as some monomers and crosslinking agents, is eliminated (Kadłubowski et al., 2012). Water-based nano-gels are soft polymeric materials that exhibit mutability and responsiveness to mechanical stimuli. These nano-gels can be used for drug or vaccine delivery and are especially suited for the delivery of anti-cancer agents into tumor tissues. 3.5. Grafting Reversible Addition/Fragmentation Chain Transfer (RAFT) technology is a way to readily synthesize polymers which have a predetermined molecular weight and narrow molecular weight distribution based on a wide range of monomers and reaction conditions and which have reactive terminal groups that can be used for chain addition polymerization (Chiefari et al., 1998). Unlike the historic uses of radiation grafting, which bound a reactive monomer to a substrate, RAFT controls the conditions to which precursors are exposed so as to permit polymer chain extension to proceed via a residual reactive moiety.
4. Conclusion The penetration of electrons is dependent upon the electron energy and the density of the irradiated material. The energy transfer efficiency of electron accelerators depends on their specific designs. Radiation processing based on accelerated electrons, X-rays or both are used in the processing of polymeric materials to produce value-added products. Some new commercial applications have emerged in recent years: (a) the use of lowenergy EB to decontaminate surfaces of materials to be used in
Please cite this article as: Chmielewski, A.G., et al., Recent developments in the application of electron accelerators for polymer processing. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2013.06.024i
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aseptic packaging; (b) the use of low-energy EB to cure the pigmented coatings on metal coil stock; and (c) the use of X-rays in lieu of radioactive isotopes to sterilize medical devices. In the research and commercial development stage are additional new fields involving natural polymers, nanotechnology as nanocomposites and nano-gels and the greater use of grafting. References Berejka, A.J., 1995. Irradiation Processing in the 90’s: Energy Savings and Environmental Benefits. Radiat. Phys. Chem. 46 (Nos. 4–6), 429–437. Berejka, A.J., June 2005. Industrial Irradiation Processing of Polymers – Status and Prospects” Report for the IAEA. Berejka, A.J., 4–8 May 2009. Prospects and Challenges for the industrial use of electron beam accelerators, IAEA. International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators, Vienna, SM/EB-01. Budker, G.I., April 5 1977. Charged particle accelerator. US Patent 4,016,499. Charlesby, A., 1952. In: Proceedings of the Royal Society, A215, London, p. 187. Charlesby, A., 1960. Atomic Radiation and Polymers. Pergamon Press, London. Chiefari, J., Chong, Y.K., Ercole, F., Krstina, J., Jeffery, J., Le, T.P.T., Mayadunne, R.T.A., Meijs, G.F, Moad, C.L., Moad, G., Rizzardo, E., Thang, S.H., 1998. Living freeradical polymerization by reversible addition-fragmentation chain transfer: the RAFT process. Macromolecules 31, 5559–5562. Chmielewski, A.G., Chmielewska, D.K., Michalik, J., Sampa, M.H., 2007. Prospects and challenges in application of gamma, electron and ion beams in processing of nanomaterials. Nucl. Instrum. Methods Phys. Res. 265 B. Cleland, M.R., February 24 1959. Voltage multiplication apparatus. US Patent 2,875,394. Cleland, M.R., Galloway, R.A., Montoney, D., Dispenza, D., Berejka A.J., 4–8 May 2009a. Radiation curing of composites for vehicle components and vehicle manufacture. International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators, Vienna, AP/IA-04. Cleland, M.R., Galloway, R.A., Berejka, A.J., Montoney, D., Driscoll, M., Smith, L., Larsen, L.S., 2009b. X-ray initiated polymerization of wood impregnants. Radiat. Phys. Chem. 78, 535–538. Cleland, M.R., Amm, K.A., December 2009. The Report of the Industry Working Group. US Department of Energy workshop Accelerators for America’s Future.
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Please cite this article as: Chmielewski, A.G., et al., Recent developments in the application of electron accelerators for polymer processing. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2013.06.024i