- Email: [email protected]
Industrial application of radiation processing in Japan
Radiat. Phys. Chem. Vol. 14, pp, 155-170. Pergamon Press Ltd.
1979. Printed in Great Britain.
INDUSTRIAL APPLICATION OF RADIATION PROCESSING IN JAPAN
Sueo Machi Japan Atomic Energy Research Institute Takasaki Radiation Chemistry Research Establishment Takasaki, Gunma 370-12, Japan
ABSTRACT A review is presented of the status of industrial application and advanced research projects of radiation processing in Japan. More than 20 industrial radiation facilities are now in operation. About 35 electron beam accelerators with the total capacity of ca. 1,200 kW are used for industrial applications, such as c r o s s l i n k i n g ~ f wire and cable insulation, heat shrinkable tubing and sheet, curing of surface coating and polyethylene foam. Three Co-60 facilities with i,i00 kCi in total are installed for the sterilization of medical supplies, plastics irradiation and the s p r o u t inhibition of potato. The radiation research projects promissing for industrial application are radiation treatment of exhaust gas, modification of emulsion-film, permselective membrane, immobilization of enzyme, fluoroelastomer, organic glass by cast polymerization, paint synthesis by electron beams and precoating of steel sheet.
INTRODUCTION Applied research of radiation chemistry has been eagerly and extensively performed in Japan for about 20 years. The Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment which has about 160 workers was organized in 1968 for the promotion of research, development and industrial application of radiation processing. The first industrial radiation application in Japan was started in 1964 by crosslinking of wire and cable insulation at Sumitomo Electric Industry, and steady growth has been continued since 1970. Japan is now one of the most advanced countries in this field next to the United States. More than 20 industrial plants of radiation processing are now in operation, and the first and world's largest commercial irradiation plant of potato for the sprout inhibition was constructed in 1973 and is being operated. The purpose of this paper is to present the status of industrial application of radiation processing in Japan as well as advanced development which is promissing for the industrial application.
155 RPC 14:1/2 I<
156
S. Machi GROWTH OF INDUSTRIAL RADIATION APPLICATION
IN JAPAN
Potential advantages of the radiation processing are
(i):
Temperature independence Low temperature capability Ease of control Free of catalyst - high purity product Capability in solid substrates External to the system High speed reaction In spite of these advantages, the radiation processing is always forced to compete with alternative conventional technology, and general lack of understanding and in some cases ungrounded fear of radiation have been reasons for the somewhat slower adaption in industry than we expected. Table 1 contains the radiation processes which have reached the stage of commercial production in Japan. It should be noted that a number of plants have been installed since 1969 for crosslinking of polymers, paint curing and steri!i. zation of medical supplies. Another characteristic of Table 1 is that the majority of processes use accelerator as radiation source. TABLE 1
Cca~mercial Radiation Processes in Japan
Process
Crosslinking of Wire and Cable Insulation
Company
Sumitomo Electric Industry and ii companies
Year of Initial Corauercial Use 1964
Radiation Source Accelerator
1972 - 75
Accelerator
Toray Company
1969
Accelerator
Sekisui Chemical Company
1969
Accelerator
Sumitomo Electric Industry
1974
Accelerator
Curing of Surface Coating
Suzuki Motor Company
1973
Accelerator
Heat Shrinkable Tubing and Sheet
Nitto Electric Industrical Company
1976
Accelerator
Radiation Services
~apan Radioisotope Radiation Service Corporation
1969
Cobalt-60
Radia Industry
1974
Cobalt-60
Shihoro Agricultural 1973 Co-operative Association
Cobalt-60
Polyethylene Foam
Food Irradiation
The growth of radiation processing in Japan through 1978 are more quantitatively shown in Figs. 1 and 2. Figure 1 demonstrates a plot of number and capacity of electron accelerators installed for industrial application, and Fig. 2 for Co-60 source capacity.
Industrial Application of Radiation Processing in Japan
157
1200 eROW'rH IN RADIATION CAPACITY FOR COMNERCIAL P R O C E S S / KX)O
J..,
ELECTRON BEAN ACCELERATOR
30
60G
2o ~ 400
,e./
[
F.'---> 200
,o
""
Z 1964
66
68
70
72
74
76
YEAR
Fig. i.
Growth in electron beam accelerator capacity for industrial processes in Japan
78
]58
S. Machi
1200
GROWTH IN RADIATION CAPACITY FOR COMMERCIAL PROCESS COBALT -BO
"~
SOURCE
I000
bJ 800
o I0
0 600
8 8 !
g, 400
5 J Z
1965
I
I
I
I
I
I
67
69
71
T3
7'5
77
YEAR Fig. 2.
Growth in Co-60 source capacity for industrial processes in Japan
79
Industrial Application of Radiation Processing in Japan
159
Remarkable growth more than 50 % is achieved during the period from 1970 to 1974 in both electron accelerator and Co-60 capacity. This growth in electron accelerator is, to great extent, due to the expansion of the radiation crosslinked wire and cable. The energy crisis in 1973 is apparently responsible for the s l o w i n g down during the 1974-1976. It is to be recognized that the commercial application are limited to the crosslinking of polymers, and radiation-induced polymerization including graft polymerization, except the paint curing, has not reached to the commercial application though there has been considerable hope. The steady growth with high rate is expected to be maintained over the following years. It is also notable in comparing Fig. 1 and Fig. 2 that the radiation capacity of electron accelerator is far beyond that of Co-60 source. The use of Co-60 facilities is almost limited to the sterilization of medical supplies and the potato irradiation.
INDUSTRIAL RADIATION PROCESSES Crosslinking of Wire and Cable Insulation The crosslinking of wire and cable is the largest industrial application of radiation processing in Japan and has rapidly grown during the past several years. In Table 2 are listed the companies which have commercial line, and their estimated radiation capacities. The Sumitomo Electric Industry first commercialized the radiation crosslinking of wire and cable in 1964, and is presently the largest in this business. In Fig. 3 is shown a photograph of a view of the radiation facility of the Sumitomo Electric Company for the crosslinking. There are now 12 suppliers of radiation crosslinked wire and cable, most of which entered this area after 1972. It is estimated that 20 electron beam accelerators have been installed with total output power of about 800 kW. The annual sales of crosslinked wire and cable is estimated to be about 12 billion yen, 60 million dollars, and its increase in the past few years is about 30 % per year which is much higher than the average national economy growth.
Fig. 3.
Radiation facility of Sumitomo Electric Industry for the crosslinking of wire and cable
160
S. Machi
TABLE 2
Radiation Crosslinkin~ F1an~s of Wire and Cable in Japan
Company
Capacity of Electron Beam Accelerator (Assumed) kW
Sumitomo Electric Industry
152
Furukawa Electric Company
133
Hitachi Cable Company
87
Yazaki Cable Works
78
Dainichi Nihon Cable Campany
50
Taisho Electrical Industry
5O
Showa Cable Works
33
Tokai Electric Wire Cempany
33
Tokyo Electric Company
60
Hirakawa Cable Works
23
Fujikura Cable Works
ii
Oki Electric Cable Company
52
The insulation materials used for the radiation crosslinked wire and cable are mainly polyethylene and polyvinylchloride. The application of radiation crosslinking is limited to the wire and cable with small diameter and thin insulation less than 1.2 m m which are mainly used for low voltage applications, such as home electrical appliances, electronics industry, communication apparatus, automobile and airospace. In these applications, the electron beam process is evaluated as an excellent technology, which has already established. A real efforts are being expanded to apply the process to large diameter cable with thicker insulation, which is crosslinked by conventional thermal catalytic method using i00 meter tall tower oven with large energy consumption. In case of the crosslinking Of thick insulation materials, a serious problem is the void formation due to the evolution of hydrogen gas at elevated temperature during irradiation (ca. 5°C increase per Mrad). The irradiation heat is not easily removed in the thick insulation material because of the low thermal conductivity. Reducing the required dose for crosslinking by the addition of polyfunctional monomers is, therefore, studied to minimize the temperature increase and void formation. The discharge breakdown Of polyethylene insulation by charge accumulation during irradiation is another problem. Workers of the Takasaki Radiation Chemistry Research Establishment and the Furukawa Electric Company have found that dipropargyl succinate is most effective to reduce reqired dose, void formation and discharge breakdown. T h e details will be presented in this international meeting (2).
Industrial Application of Radiation Processing in Japan
I6]
Crosslinked Polyethylene Foam This is the technology which was originally developed in Japan and transferred to the United States and Euorope. The Toray Industies Inc., Sekisui Chemical Company, Hitachi Chemical Company and Sumitomo Electric Industry began in about 1963 research and development toward the production of crosslinked polyethylene foam. By 1966, the Toray Industries and Sekisui Chemical had reached a point where a decision was made to commercialize the radiation crosslinked polyethylene foam. In 1969, the both companies started the commercial production, while the Hitachi Chemical Company began the production by means of chemical crosslinking with peroxide. The Sumitomo Electric Industry started the production in 1973 focusing on polyethylene foam tubings by radiation. The Sekisui Chemical Company and Toray Industries Inc. exported their technology to Voltek Inc. and Dow Chemical Company in the United States, and Alveo in Europe. The production rate of radiation crosslinked polyethylene foam in Japan is now about 7,000 ton per year, approximately one third of total polyethylene foam. The major application are in insulation, packing, backing, mat and padding for sports clothing. A block diagram of the production line of crosslinked polyethylene foam is shown in Fig. 4. Azodicarbonamide (H2N-OC-N=N-CO-NH2) is used as a foaming agent which decompose at 190-200°C to form 200-240 ml gases of N 2, CO 2, CO and NH 3. The degree of expansion is in the range of i0 to 40 corresponding to 0.i to 0.025 g/ml density of the product. Polyethylene blended with the foaming agent is molded to sheet, which is then irradiated by electron beams at high speed to be crosslinked. The sheet is then heated in foaming step. Polypropylene foam is also being produced by the same technology. Production cost of the radiation crosslinked foam was estimated to be lower than that of chemically crosslinked one in case where the plant capacity was larger than 100-200 ton per month in spite of the extra crosslinking step (3).
~POLYETHYLENE~BLENDING ~_.~SHEET I ~EXTRUSION
~
IRRADIATION. ~---~FOAMING ~
PRODUCTI
IAGENTS Fig. 4.
Block diagram of production line of radiation crosslinked polyethylene foam
Curin@ of Surface Coatin@ The Ford Motor Company completed the development of an electron beam curing process for paints and laquers, and introduced it into the production line in 1970. This technology is known to have several advantages, such as solvent free or I00 % solid, high speed curing, better bonding to plastic substrate, no heat damage of plastic substrate, space saving and energy Saving. The Ford system has been transferred to Nippon Electrocure Company, which is a joint venture between Kansai Paint Company and Itoh-chu Trading Company. In 1973, the Suzuki Motor Company which is the second largest supplier of motorcycle and has production capacity of 1.2 million motorcycles per year, introduced this
162
S. Machi
system into the production line. Two electron beam accelerators of 300 keY and I00 m A are being used to cure 170,000 parts per month. The line speed is 3 m per min, and curing dose is from 7 to 15 Mrad. The inert gas, whose oxygen content is less than i00 ppm, for curing atmosphere is produced from ccm~bustion of butane. The system has been satisfactorily operated without any serious problems during the past 5 years for 23,000 hr of total operation time. The components to be radiation cured are oil tank of metal and flame cover of ABS (poly[acrylonitrile-butadiene-styrene]) resin. The major advantages of this system which the company really appreciates are: Space saving Excellent surface hardness and durability Glossy surface Simultaneous curing capability on metal and plastic cemponents Mitsubishi Rayon Company has also done considerable work on electron beam curing of sheet and coil with the use of 500 keY 65 m A machine. Eidai Industry has been developing surface coating of plywood for housing construction material in pilot scale with 600 keY and i00 mA accelerator. These technologies are not yet adopted in commercial line. The system of the Energy Science using linear filament type accelerator is interesting, but not yet imported in Japan. From the advantages of energy saving and free from air pollution, which are now strong social demands, the general area of the electron beam curing is expected to expand in industry in the year ahead. Heat Shrinkable Tubing and Tape commercial production of heat shrinkable tubing, sheet and tape by radiation crosslinking was far behind to the United States. In 1976, the Nitto Electric Industrial Company, which is one of the largest supplier of insulating tapes, first introduced the radiation crosslinking to produce the heat shrinkable tubing and tape. The major applications are insulation of electrical wires and protection of pipe line from corrosion. The shrinkable film for~food packaging, which is one of the largest application in the United States, is not yet produced by radiation process in Japan. Sterilization of Medical Supply and Radiation Services There are two companies for radiation sterilization of medical supplies and for radiation services. One is the Japan Radioisotope Radiation Service Corporation which was established in 1969 in Tochigi. They started with ii0 kCi of Co-60 and has reached 360 kCi capacity. The radiation facility is characterized by 4 straight conveyors for one passage system. Mayor items irradiated in this company are medical supply (70 %), animal food (15 %), and plastics (15 %). Another company is the Radia Industry which started the business in 1974 in Takasaki. The initial Co-60 capacity was 200 kCi. The details of the plant design and characteristics are reported in the ist International Meeting on Radiation Processing (4). Very recently, they increased the plant capacity by adding i00 kCi Co-60 in accordance with continuing market expansion. The present capacity is 460 kCi. Medical supplies are major items to be irradiated. A new radiation facility is being planned by the Japan Radioisotope Association. It is reported that 550 kCi Co-60 will be installed and major items to be irradiated will be medical supplies. In Japan, majority of medical supplies is still sterilized by ethylene oxide
Industrial Application of Radiation Processing in Japan
163
gas, therefore an appreciable growth is anticipated in this area in the near future because of the perfect sterilization and no toxic residue. Potato Irradiation for Sprout Inhibition The radiation preservation of food has been studies for 20 years in Japan. In 1969 a national project was started in the cooperation among various organizations, such as the National Food Research Institute for the study of radiation effect, the National Institute of Hygienic Science for wholesomeness, Takasaki Radiation Radiation Chemistry Research Establishment for radiation engineering, and Institute of Physical and Chemical Research for basic research (5). This project resulted the authorization of the application of gamma radiation for sprout inhibition of potatoes in 1972. The world's largest commercial irradiation plant of the Shihoro Agricultural Co-operative Association was started in 1973 in Hokkaido. The plant has a capacity of irradiating i0,000 ton potatoes per month, and actually 30,000 ton is being irradiated in a year for a period of three months. The plant will he also used for the onion sprout inhibition in future.
Fig. 5.
A view of irradiation cell of Shihoro Agricultural Co-operative Association plant
164
S. Machi
Figure 5 is a photograph of a view of the radiation cell. The 300 kCi Co-60 source is arranged in a circular frame of 1 m diameter. The potato container, whose dimention is i00 cm in width, 160 om length and 130 cm height moves around the source with circular conveyor. In order to obtain the maximum and minimum absorbed dose of less than 15 krad and more than 6 krad, the container is placed 4 m away from the source (6). The container is 1.7 ton in weight and can contain 1.5 ton potatoes. The items studied in the national project of food irradiation research are onion, rice, wheat, sausage, kamoboko and satsu~a orange.
PROMISSING RADIATION RESEARCH PROJECTS As listed in Table 3, several radiation research projects aiming the industri. al application are being performed by the Takasaki Radiation Chemistry Research Establishment and by private companies. The results of some projects have not been fully disclosed because of the company's policy. Radiation Treatment of Comustion Gases One of the most significant project is the radiation treatment of combustion gases to remove sulfer dioxide and nitrogen oxide. This work was started in 1972 with cooperation of the Takasaki Radiation Chemistry Research Establishment and the Ebara Manufacturing Company. They demonstrated by using a flow irradiation apparatus that if combustion gas from heavy oil containing about i00 p p m N O x and 800 ppm SO 2 was irradiated, mosts of SO 2 and NO x were removed simultaneously (7). Further development of this project for the treatment of exhaust gas from steel sintering plant is being done by the Research Association for Abatement and Removal of NO x in the Steel Industry in colabora£ion of the Ebara Mfg. with the goverment support. A pilot scale plant was constructed at the Yahata Works of the Nippon Steel Corporation which is a leading company of the Research Association. The plant has two electron accelerators of 0.75 MeV and 60 m A and the treatment capacity is 3000 Nm3/hr. The photograph in Fig. 6 is a view of the plant. The exhaust gas is introduced into a cylindrical vessel at 100°C to be irradiated with two accelerators from both sides. Small amount of ammonia is added before irradiation. Under irradiation, SO 2 and N O x are converted to complex salt of a/m~onia sulfate and nitrate, which is collected in an electron precipitator. The operation has been satisfactorily continued since August 1977. Continuing basic researches supporting the project are being done at the Takasaki Radiation Chemistry Research Establishment, the University of Tokyo and the Tokyo Institute of Technology. It was recently reported that asm~nia is effective to increase removal rate of NO and to decrease the formation of N O 2 as shown in Fig. 7 (8). The significant reactions in this system are the radiation-induced oxidation of NO x and SO 2 by the oxidative species from radiolysis of 02 and H20 , and subsequent formation of ammonia salts Of nitric acid and sulferic acid. Major advantages of this process are: Capability of simultaneous removal of SO 2 and NO x Capability in exhaust gas containing dust Dry process - no waste water Development of economical accelerator with large capacity will be one of the crucial facotor for the industrial application of this process.
Industrial Application of Radiation Processing in Japan
165
r-4
H •
•
~o
O O
0
• ,.4 u,..i ,.~ 0 -,4 0
-~1 o
o 0 ,--t
•
,d ° g -,-4
166
S. Machi
I
'I0
I
I
I
0
0
o
4OO
=o 3oo
Q. 4
.,~ ,~ I00 0 0
I
I
500
I000
!
1500
I
2000
[NH3], ppm Fig. 7. Effect of ammonia on the rates of NO removal and NO 2 formation. (O) initial rate of NO removal, ~ ) initial rate of NO 2 formation; NO 250 ppm, 02 11.6 %, N 2 80.2 % : irradiation temperature, 120°C
Synthesis and Modification of Polymer Emulsion There is a continuing and strong interest in radiation application for polymeric system, such as polymerization, graft polymerization and crosslinking. Dr. Makuuchi and his coworkers of the Takasaki Radiation Chemistry Research Establishment demonstrated that water resistance of emulsion-fill of polyethylacrylate was effectively improved by radiation crosslinking (9). It is notable that smaller gel dose is required than the crosslinking of bulk polymer because radicals formed from water radiolysis play an important role for the crosslinking. Figure 8 shows that emulsion-fill with higher gel fraction or crosslinking has smaller water absorption. They also reported that the fill of polyethylacrylate emulsion prepared by the radiation-induced polymerization had excellent water resistance because of the absence of conventional redox type initiator (for instance, ammonium persulfate). Apilot plant with Co-60 source has been constructed. Another group of Takasaki found that the emulsifier and catalyst free polymer emulsion could be prepared by the radiation-induced polymerization of ethylene and tetrafluoroethylene (i0, ii). Permselective Membranes Preparations of permselective membrane by radiation-induced grafting of hydrophylic monomers to polyethylene and fluoro-containing polymers are being
Industrial Application of Radiation Processing in Japan
°
167
c,
¢-2
R-2 iO
i
0
0.5 GEL
--
1.0
FRACTION
Fig. 8.
Relation between the gel fraction and the water absorption of the emulsion-film Samples C-2 and C-3, catalyst polym.; R-2, radiation polym. studied by Dr. Machi and his coworkers of Takasaki Radiation Chemistry Research Establishemnt with the strong industrial interest and cooperation. This program is extensive and involves study of various monomer and polymer systems, microstructure of grafted membrane, electro-chemical properties in connection with the application and engineering study of grafting. Majority of the monomer and polymer used are para-vinylphenyl acetate, acrylic acid, polyethylene and polytetrafluoroethylene. Preirradiation technique is mainly used. Some of the results will be reported in this meeting (12). A pilot scale plant is-going to be installed this year for the engineering study and sample preparation, by which membrane with 30 cm width and i00 m length is produced in one operation. Immobilization of Enzyme One of the important characteristics of radiation initiation is low temperature capability. This advantage is used for the preparation of immobilized enzyme by radiation-induced polymerization at the Takasaki Radiation Chemistry Research Establishment and the National Food Research Institute. Dr. Kaetsu and his coworkers of the Takasaki Establishment demonstrated that if enzyme was immobilized by the polymerization of mixture of glass-forming monomer (e.g., hydrooxyethyl methacrylate) dissolved in buffer solution, 30 to 50 % activity yield was obtained. Low temperature polymerization at -i00 to O°C is essential to maintain high activity yield. The details will be reported in this meeting (13). Workers of the National Food Research Institute are developing enzyme immobilization by radiation-induced polymerization of salt of acrylic acid aqueous solution containing enzyme at low temperature in solid state. It is reported that the project will be developed for the co~m~ercial production by a private company with the support of the goverment organization , the Research Development Corporation of Japan.
168
S. Machi
Synthesis of Paint by Electron Beam Aishin Chemical Company has been developing the synthesis of paint of acrylates by the polymerization induced by electron beams irradiation. T h e monomer used in this system are acrylate, methacrylate and styrene and their mixtures. The pilot plant equipped with an accelerator of 2 MeV and 20 mA and the reaction vessel of 300 1 is in operation. The polymerization is carried out in batch system, where 280 kg production is obtained for 1 hr at 99 % conversion. The characteristics of the product are low viscosity in high solid content (70 %) and capability of thick coating. Fluoroelastomer Synthesis of a new fluoroelastomer by radiation-induced copolymerization of tetrafluoroethylene and propylene has been extensively studied at the Takasaki Radiation Chemistry Research Establishment including engineering studies with a pilot scale plant (14). The copolymer with alternative sequence of the monomers has excellent thermal stability and chemical resistance for polar solvent, sulferic acid, nitric acid, hydrofluoride and so on even at high temperature. Recently, the Asahi Glass Company started commercial production of the elastomer at 40 ton per year rate by using catalyst not radiation. The comparative study of the production cost indicates that radiation polymerization is more economical than catalytic method if the production rate is larger than 300 ton per year.
TABLE 3
Radiation Processes under Advanced Development Company or Organization
Process
Radiation Source
SO 2 and NO x Removal from exhaust gases
Res. Assoc. for Abatement and Removal of NO x in Steel Industry
Accelerator
Synthesis and Modification of Polymer Emulsion
Japan Atomic Energy Research Institute (JAERI)
Co-60
Permselective Membrane
JAERI
Accelerator
Immobilized Enzyme
JAERI, National Food Res. Inst.
Co-60
Fluoroelastomer
JAERI
Co-60
Paint Synthesis
Aishin Chemical Company
Accelerator
Precoating of Steel Sheets
Toray Industries, Inc. and Nippon Steel Corporation
Accelerator
Organic Glass by Cast Polymerization
JAERI
Co-60
Industrial Application of Radiation Processing Cast Polymerization
in Japan
169
for Organic Glass
The radiation-induced cast polymerization of glass-forming monomers has been extensively studied at the Takasaki Radiation Research Establishment to produce organic glass. Various monomers, such as acrylate, methacrylate and vinylether are called glass-forming monomer since their viscosity is extremely high at low temperature like as amorphous solid. Workers at the Takasaki Establishment found that if these monomers are polymerized in glassy state, the transparent organic glasses are obtained at high polymerization rate. The characteristic of the product is free from optical stress and strain. Details will be reported in this meeting (15). Precoating of Steel Sheets Toray Industries Inc. and Nippon Steel Corporation have been developing the electron beam curing process for the precoating of steel sheets. They announced that they opened bright prospects for the production of high quality precoated steel products with excellent hardness, gloss and thick embossed surfaces which conventional thermal cure could not atained.
REFERENCES
(i)
D.E. Harmer and D. S. Ballantine,
(2)
T. Sasaki, F. Hosoi, M. Hagiwara, K. Araki, E. Saito, H. Ishitani, K. Uesugi, Development of Radiation Crosslinking Process for High Voltage Power Cable, 2nd International Meeting on Radiation Processing, Miami, 1978
(3)
I. Tamai, (1976)
(4)
K. Temita and S. Sugimoto,
(5)
K. Umeda, "Requirement for the Irradiation of Food on a Commercial Scale", 113, IAEA, Vienna 1975
(6)
T. Kume, T. Tachibana, (1976)
(7)
S. Machi, O. Tokunaga, K. Nishimura, S. Hashimoto, W. Kawakami, M. Washino, K. Kawamura, S. Aoki, K. Adachi, Radiat. Phys. Chem., 2, 371 (1977)
(8)
O. Tokunaga, K. Nishimura, Chem. in press.
(9)
K. Makuuchi,
Chem. Eng. Apr. 19, 98 (1971)
"Data Book of Radiation Processing",
231, Brain Service, Tokyo
Radiat. Phys. Chem. 2, 567 (1977)
H. Aoki, T. Sato, J. Food Sci. Tech. Japan, 20, 492
N. Suzuki, S. Machi, M. Washino,
Radiat. Phys.
T. Takagi, K. Araki, Shikizai Kyokai Shi, 51, 214 (1978)
(10) T. Suwa, H. Nakajima, M. Takehisa, S. Machi, J. Polymer Sci., Polym. Letter, Ed. 13, 369 (1975)
(11) T. Suwa, T. Watanabe, S. Machi, J. Polymer Sci., Chem. Ed., in press. (12) S. Machi, T. Sugo, A. Sugishita, K. Senou, H. Fujiwara, I. Ishigaki, Inon Exchange Membrane by Radiation Grafting, Radiation Processing, Miami, 1978
2nd International Meeting on
!70
S. Machi
(13) I. Kaetsu, M. Kumakura, M. Asano, M. Himei, M. Tamura, K. Hayashi, Immobilization of Enzyme for Medical Use. The 2nd International Meeting on Radiation Processing, Miami, 1978 (14) S. Machi, O. Matsuda, 403 (1977)
Y. Tabata, J. Okamoto, Radiat. Phys. Chem. 9,
(15) I. Kae~su, K. Yoshida, H. Okubo, K. Hayashi, Cast Polymerization by Irradia. tion, The 2nd International Meeting on Radiation Processing, Miami, 1978
1979. Printed in Great Britain.
INDUSTRIAL APPLICATION OF RADIATION PROCESSING IN JAPAN
Sueo Machi Japan Atomic Energy Research Institute Takasaki Radiation Chemistry Research Establishment Takasaki, Gunma 370-12, Japan
ABSTRACT A review is presented of the status of industrial application and advanced research projects of radiation processing in Japan. More than 20 industrial radiation facilities are now in operation. About 35 electron beam accelerators with the total capacity of ca. 1,200 kW are used for industrial applications, such as c r o s s l i n k i n g ~ f wire and cable insulation, heat shrinkable tubing and sheet, curing of surface coating and polyethylene foam. Three Co-60 facilities with i,i00 kCi in total are installed for the sterilization of medical supplies, plastics irradiation and the s p r o u t inhibition of potato. The radiation research projects promissing for industrial application are radiation treatment of exhaust gas, modification of emulsion-film, permselective membrane, immobilization of enzyme, fluoroelastomer, organic glass by cast polymerization, paint synthesis by electron beams and precoating of steel sheet.
INTRODUCTION Applied research of radiation chemistry has been eagerly and extensively performed in Japan for about 20 years. The Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment which has about 160 workers was organized in 1968 for the promotion of research, development and industrial application of radiation processing. The first industrial radiation application in Japan was started in 1964 by crosslinking of wire and cable insulation at Sumitomo Electric Industry, and steady growth has been continued since 1970. Japan is now one of the most advanced countries in this field next to the United States. More than 20 industrial plants of radiation processing are now in operation, and the first and world's largest commercial irradiation plant of potato for the sprout inhibition was constructed in 1973 and is being operated. The purpose of this paper is to present the status of industrial application of radiation processing in Japan as well as advanced development which is promissing for the industrial application.
155 RPC 14:1/2 I<
156
S. Machi GROWTH OF INDUSTRIAL RADIATION APPLICATION
IN JAPAN
Potential advantages of the radiation processing are
(i):
Temperature independence Low temperature capability Ease of control Free of catalyst - high purity product Capability in solid substrates External to the system High speed reaction In spite of these advantages, the radiation processing is always forced to compete with alternative conventional technology, and general lack of understanding and in some cases ungrounded fear of radiation have been reasons for the somewhat slower adaption in industry than we expected. Table 1 contains the radiation processes which have reached the stage of commercial production in Japan. It should be noted that a number of plants have been installed since 1969 for crosslinking of polymers, paint curing and steri!i. zation of medical supplies. Another characteristic of Table 1 is that the majority of processes use accelerator as radiation source. TABLE 1
Cca~mercial Radiation Processes in Japan
Process
Crosslinking of Wire and Cable Insulation
Company
Sumitomo Electric Industry and ii companies
Year of Initial Corauercial Use 1964
Radiation Source Accelerator
1972 - 75
Accelerator
Toray Company
1969
Accelerator
Sekisui Chemical Company
1969
Accelerator
Sumitomo Electric Industry
1974
Accelerator
Curing of Surface Coating
Suzuki Motor Company
1973
Accelerator
Heat Shrinkable Tubing and Sheet
Nitto Electric Industrical Company
1976
Accelerator
Radiation Services
~apan Radioisotope Radiation Service Corporation
1969
Cobalt-60
Radia Industry
1974
Cobalt-60
Shihoro Agricultural 1973 Co-operative Association
Cobalt-60
Polyethylene Foam
Food Irradiation
The growth of radiation processing in Japan through 1978 are more quantitatively shown in Figs. 1 and 2. Figure 1 demonstrates a plot of number and capacity of electron accelerators installed for industrial application, and Fig. 2 for Co-60 source capacity.
Industrial Application of Radiation Processing in Japan
157
1200 eROW'rH IN RADIATION CAPACITY FOR COMNERCIAL P R O C E S S / KX)O
J..,
ELECTRON BEAN ACCELERATOR
30
60G
2o ~ 400
,e./
[
F.'---> 200
,o
""
Z 1964
66
68
70
72
74
76
YEAR
Fig. i.
Growth in electron beam accelerator capacity for industrial processes in Japan
78
]58
S. Machi
1200
GROWTH IN RADIATION CAPACITY FOR COMMERCIAL PROCESS COBALT -BO
"~
SOURCE
I000
bJ 800
o I0
0 600
8 8 !
g, 400
5 J Z
1965
I
I
I
I
I
I
67
69
71
T3
7'5
77
YEAR Fig. 2.
Growth in Co-60 source capacity for industrial processes in Japan
79
Industrial Application of Radiation Processing in Japan
159
Remarkable growth more than 50 % is achieved during the period from 1970 to 1974 in both electron accelerator and Co-60 capacity. This growth in electron accelerator is, to great extent, due to the expansion of the radiation crosslinked wire and cable. The energy crisis in 1973 is apparently responsible for the s l o w i n g down during the 1974-1976. It is to be recognized that the commercial application are limited to the crosslinking of polymers, and radiation-induced polymerization including graft polymerization, except the paint curing, has not reached to the commercial application though there has been considerable hope. The steady growth with high rate is expected to be maintained over the following years. It is also notable in comparing Fig. 1 and Fig. 2 that the radiation capacity of electron accelerator is far beyond that of Co-60 source. The use of Co-60 facilities is almost limited to the sterilization of medical supplies and the potato irradiation.
INDUSTRIAL RADIATION PROCESSES Crosslinking of Wire and Cable Insulation The crosslinking of wire and cable is the largest industrial application of radiation processing in Japan and has rapidly grown during the past several years. In Table 2 are listed the companies which have commercial line, and their estimated radiation capacities. The Sumitomo Electric Industry first commercialized the radiation crosslinking of wire and cable in 1964, and is presently the largest in this business. In Fig. 3 is shown a photograph of a view of the radiation facility of the Sumitomo Electric Company for the crosslinking. There are now 12 suppliers of radiation crosslinked wire and cable, most of which entered this area after 1972. It is estimated that 20 electron beam accelerators have been installed with total output power of about 800 kW. The annual sales of crosslinked wire and cable is estimated to be about 12 billion yen, 60 million dollars, and its increase in the past few years is about 30 % per year which is much higher than the average national economy growth.
Fig. 3.
Radiation facility of Sumitomo Electric Industry for the crosslinking of wire and cable
160
S. Machi
TABLE 2
Radiation Crosslinkin~ F1an~s of Wire and Cable in Japan
Company
Capacity of Electron Beam Accelerator (Assumed) kW
Sumitomo Electric Industry
152
Furukawa Electric Company
133
Hitachi Cable Company
87
Yazaki Cable Works
78
Dainichi Nihon Cable Campany
50
Taisho Electrical Industry
5O
Showa Cable Works
33
Tokai Electric Wire Cempany
33
Tokyo Electric Company
60
Hirakawa Cable Works
23
Fujikura Cable Works
ii
Oki Electric Cable Company
52
The insulation materials used for the radiation crosslinked wire and cable are mainly polyethylene and polyvinylchloride. The application of radiation crosslinking is limited to the wire and cable with small diameter and thin insulation less than 1.2 m m which are mainly used for low voltage applications, such as home electrical appliances, electronics industry, communication apparatus, automobile and airospace. In these applications, the electron beam process is evaluated as an excellent technology, which has already established. A real efforts are being expanded to apply the process to large diameter cable with thicker insulation, which is crosslinked by conventional thermal catalytic method using i00 meter tall tower oven with large energy consumption. In case of the crosslinking Of thick insulation materials, a serious problem is the void formation due to the evolution of hydrogen gas at elevated temperature during irradiation (ca. 5°C increase per Mrad). The irradiation heat is not easily removed in the thick insulation material because of the low thermal conductivity. Reducing the required dose for crosslinking by the addition of polyfunctional monomers is, therefore, studied to minimize the temperature increase and void formation. The discharge breakdown Of polyethylene insulation by charge accumulation during irradiation is another problem. Workers of the Takasaki Radiation Chemistry Research Establishment and the Furukawa Electric Company have found that dipropargyl succinate is most effective to reduce reqired dose, void formation and discharge breakdown. T h e details will be presented in this international meeting (2).
Industrial Application of Radiation Processing in Japan
I6]
Crosslinked Polyethylene Foam This is the technology which was originally developed in Japan and transferred to the United States and Euorope. The Toray Industies Inc., Sekisui Chemical Company, Hitachi Chemical Company and Sumitomo Electric Industry began in about 1963 research and development toward the production of crosslinked polyethylene foam. By 1966, the Toray Industries and Sekisui Chemical had reached a point where a decision was made to commercialize the radiation crosslinked polyethylene foam. In 1969, the both companies started the commercial production, while the Hitachi Chemical Company began the production by means of chemical crosslinking with peroxide. The Sumitomo Electric Industry started the production in 1973 focusing on polyethylene foam tubings by radiation. The Sekisui Chemical Company and Toray Industries Inc. exported their technology to Voltek Inc. and Dow Chemical Company in the United States, and Alveo in Europe. The production rate of radiation crosslinked polyethylene foam in Japan is now about 7,000 ton per year, approximately one third of total polyethylene foam. The major application are in insulation, packing, backing, mat and padding for sports clothing. A block diagram of the production line of crosslinked polyethylene foam is shown in Fig. 4. Azodicarbonamide (H2N-OC-N=N-CO-NH2) is used as a foaming agent which decompose at 190-200°C to form 200-240 ml gases of N 2, CO 2, CO and NH 3. The degree of expansion is in the range of i0 to 40 corresponding to 0.i to 0.025 g/ml density of the product. Polyethylene blended with the foaming agent is molded to sheet, which is then irradiated by electron beams at high speed to be crosslinked. The sheet is then heated in foaming step. Polypropylene foam is also being produced by the same technology. Production cost of the radiation crosslinked foam was estimated to be lower than that of chemically crosslinked one in case where the plant capacity was larger than 100-200 ton per month in spite of the extra crosslinking step (3).
~POLYETHYLENE~BLENDING ~_.~SHEET I ~EXTRUSION
~
IRRADIATION. ~---~FOAMING ~
PRODUCTI
IAGENTS Fig. 4.
Block diagram of production line of radiation crosslinked polyethylene foam
Curin@ of Surface Coatin@ The Ford Motor Company completed the development of an electron beam curing process for paints and laquers, and introduced it into the production line in 1970. This technology is known to have several advantages, such as solvent free or I00 % solid, high speed curing, better bonding to plastic substrate, no heat damage of plastic substrate, space saving and energy Saving. The Ford system has been transferred to Nippon Electrocure Company, which is a joint venture between Kansai Paint Company and Itoh-chu Trading Company. In 1973, the Suzuki Motor Company which is the second largest supplier of motorcycle and has production capacity of 1.2 million motorcycles per year, introduced this
162
S. Machi
system into the production line. Two electron beam accelerators of 300 keY and I00 m A are being used to cure 170,000 parts per month. The line speed is 3 m per min, and curing dose is from 7 to 15 Mrad. The inert gas, whose oxygen content is less than i00 ppm, for curing atmosphere is produced from ccm~bustion of butane. The system has been satisfactorily operated without any serious problems during the past 5 years for 23,000 hr of total operation time. The components to be radiation cured are oil tank of metal and flame cover of ABS (poly[acrylonitrile-butadiene-styrene]) resin. The major advantages of this system which the company really appreciates are: Space saving Excellent surface hardness and durability Glossy surface Simultaneous curing capability on metal and plastic cemponents Mitsubishi Rayon Company has also done considerable work on electron beam curing of sheet and coil with the use of 500 keY 65 m A machine. Eidai Industry has been developing surface coating of plywood for housing construction material in pilot scale with 600 keY and i00 mA accelerator. These technologies are not yet adopted in commercial line. The system of the Energy Science using linear filament type accelerator is interesting, but not yet imported in Japan. From the advantages of energy saving and free from air pollution, which are now strong social demands, the general area of the electron beam curing is expected to expand in industry in the year ahead. Heat Shrinkable Tubing and Tape commercial production of heat shrinkable tubing, sheet and tape by radiation crosslinking was far behind to the United States. In 1976, the Nitto Electric Industrial Company, which is one of the largest supplier of insulating tapes, first introduced the radiation crosslinking to produce the heat shrinkable tubing and tape. The major applications are insulation of electrical wires and protection of pipe line from corrosion. The shrinkable film for~food packaging, which is one of the largest application in the United States, is not yet produced by radiation process in Japan. Sterilization of Medical Supply and Radiation Services There are two companies for radiation sterilization of medical supplies and for radiation services. One is the Japan Radioisotope Radiation Service Corporation which was established in 1969 in Tochigi. They started with ii0 kCi of Co-60 and has reached 360 kCi capacity. The radiation facility is characterized by 4 straight conveyors for one passage system. Mayor items irradiated in this company are medical supply (70 %), animal food (15 %), and plastics (15 %). Another company is the Radia Industry which started the business in 1974 in Takasaki. The initial Co-60 capacity was 200 kCi. The details of the plant design and characteristics are reported in the ist International Meeting on Radiation Processing (4). Very recently, they increased the plant capacity by adding i00 kCi Co-60 in accordance with continuing market expansion. The present capacity is 460 kCi. Medical supplies are major items to be irradiated. A new radiation facility is being planned by the Japan Radioisotope Association. It is reported that 550 kCi Co-60 will be installed and major items to be irradiated will be medical supplies. In Japan, majority of medical supplies is still sterilized by ethylene oxide
Industrial Application of Radiation Processing in Japan
163
gas, therefore an appreciable growth is anticipated in this area in the near future because of the perfect sterilization and no toxic residue. Potato Irradiation for Sprout Inhibition The radiation preservation of food has been studies for 20 years in Japan. In 1969 a national project was started in the cooperation among various organizations, such as the National Food Research Institute for the study of radiation effect, the National Institute of Hygienic Science for wholesomeness, Takasaki Radiation Radiation Chemistry Research Establishment for radiation engineering, and Institute of Physical and Chemical Research for basic research (5). This project resulted the authorization of the application of gamma radiation for sprout inhibition of potatoes in 1972. The world's largest commercial irradiation plant of the Shihoro Agricultural Co-operative Association was started in 1973 in Hokkaido. The plant has a capacity of irradiating i0,000 ton potatoes per month, and actually 30,000 ton is being irradiated in a year for a period of three months. The plant will he also used for the onion sprout inhibition in future.
Fig. 5.
A view of irradiation cell of Shihoro Agricultural Co-operative Association plant
164
S. Machi
Figure 5 is a photograph of a view of the radiation cell. The 300 kCi Co-60 source is arranged in a circular frame of 1 m diameter. The potato container, whose dimention is i00 cm in width, 160 om length and 130 cm height moves around the source with circular conveyor. In order to obtain the maximum and minimum absorbed dose of less than 15 krad and more than 6 krad, the container is placed 4 m away from the source (6). The container is 1.7 ton in weight and can contain 1.5 ton potatoes. The items studied in the national project of food irradiation research are onion, rice, wheat, sausage, kamoboko and satsu~a orange.
PROMISSING RADIATION RESEARCH PROJECTS As listed in Table 3, several radiation research projects aiming the industri. al application are being performed by the Takasaki Radiation Chemistry Research Establishment and by private companies. The results of some projects have not been fully disclosed because of the company's policy. Radiation Treatment of Comustion Gases One of the most significant project is the radiation treatment of combustion gases to remove sulfer dioxide and nitrogen oxide. This work was started in 1972 with cooperation of the Takasaki Radiation Chemistry Research Establishment and the Ebara Manufacturing Company. They demonstrated by using a flow irradiation apparatus that if combustion gas from heavy oil containing about i00 p p m N O x and 800 ppm SO 2 was irradiated, mosts of SO 2 and NO x were removed simultaneously (7). Further development of this project for the treatment of exhaust gas from steel sintering plant is being done by the Research Association for Abatement and Removal of NO x in the Steel Industry in colabora£ion of the Ebara Mfg. with the goverment support. A pilot scale plant was constructed at the Yahata Works of the Nippon Steel Corporation which is a leading company of the Research Association. The plant has two electron accelerators of 0.75 MeV and 60 m A and the treatment capacity is 3000 Nm3/hr. The photograph in Fig. 6 is a view of the plant. The exhaust gas is introduced into a cylindrical vessel at 100°C to be irradiated with two accelerators from both sides. Small amount of ammonia is added before irradiation. Under irradiation, SO 2 and N O x are converted to complex salt of a/m~onia sulfate and nitrate, which is collected in an electron precipitator. The operation has been satisfactorily continued since August 1977. Continuing basic researches supporting the project are being done at the Takasaki Radiation Chemistry Research Establishment, the University of Tokyo and the Tokyo Institute of Technology. It was recently reported that asm~nia is effective to increase removal rate of NO and to decrease the formation of N O 2 as shown in Fig. 7 (8). The significant reactions in this system are the radiation-induced oxidation of NO x and SO 2 by the oxidative species from radiolysis of 02 and H20 , and subsequent formation of ammonia salts Of nitric acid and sulferic acid. Major advantages of this process are: Capability of simultaneous removal of SO 2 and NO x Capability in exhaust gas containing dust Dry process - no waste water Development of economical accelerator with large capacity will be one of the crucial facotor for the industrial application of this process.
Industrial Application of Radiation Processing in Japan
165
r-4
H •
•
~o
O O
0
• ,.4 u,..i ,.~ 0 -,4 0
-~1 o
o 0 ,--t
•
,d ° g -,-4
166
S. Machi
I
'I0
I
I
I
0
0
o
4OO
=o 3oo
Q. 4
.,~ ,~ I00 0 0
I
I
500
I000
!
1500
I
2000
[NH3], ppm Fig. 7. Effect of ammonia on the rates of NO removal and NO 2 formation. (O) initial rate of NO removal, ~ ) initial rate of NO 2 formation; NO 250 ppm, 02 11.6 %, N 2 80.2 % : irradiation temperature, 120°C
Synthesis and Modification of Polymer Emulsion There is a continuing and strong interest in radiation application for polymeric system, such as polymerization, graft polymerization and crosslinking. Dr. Makuuchi and his coworkers of the Takasaki Radiation Chemistry Research Establishment demonstrated that water resistance of emulsion-fill of polyethylacrylate was effectively improved by radiation crosslinking (9). It is notable that smaller gel dose is required than the crosslinking of bulk polymer because radicals formed from water radiolysis play an important role for the crosslinking. Figure 8 shows that emulsion-fill with higher gel fraction or crosslinking has smaller water absorption. They also reported that the fill of polyethylacrylate emulsion prepared by the radiation-induced polymerization had excellent water resistance because of the absence of conventional redox type initiator (for instance, ammonium persulfate). Apilot plant with Co-60 source has been constructed. Another group of Takasaki found that the emulsifier and catalyst free polymer emulsion could be prepared by the radiation-induced polymerization of ethylene and tetrafluoroethylene (i0, ii). Permselective Membranes Preparations of permselective membrane by radiation-induced grafting of hydrophylic monomers to polyethylene and fluoro-containing polymers are being
Industrial Application of Radiation Processing in Japan
°
167
c,
¢-2
R-2 iO
i
0
0.5 GEL
--
1.0
FRACTION
Fig. 8.
Relation between the gel fraction and the water absorption of the emulsion-film Samples C-2 and C-3, catalyst polym.; R-2, radiation polym. studied by Dr. Machi and his coworkers of Takasaki Radiation Chemistry Research Establishemnt with the strong industrial interest and cooperation. This program is extensive and involves study of various monomer and polymer systems, microstructure of grafted membrane, electro-chemical properties in connection with the application and engineering study of grafting. Majority of the monomer and polymer used are para-vinylphenyl acetate, acrylic acid, polyethylene and polytetrafluoroethylene. Preirradiation technique is mainly used. Some of the results will be reported in this meeting (12). A pilot scale plant is-going to be installed this year for the engineering study and sample preparation, by which membrane with 30 cm width and i00 m length is produced in one operation. Immobilization of Enzyme One of the important characteristics of radiation initiation is low temperature capability. This advantage is used for the preparation of immobilized enzyme by radiation-induced polymerization at the Takasaki Radiation Chemistry Research Establishment and the National Food Research Institute. Dr. Kaetsu and his coworkers of the Takasaki Establishment demonstrated that if enzyme was immobilized by the polymerization of mixture of glass-forming monomer (e.g., hydrooxyethyl methacrylate) dissolved in buffer solution, 30 to 50 % activity yield was obtained. Low temperature polymerization at -i00 to O°C is essential to maintain high activity yield. The details will be reported in this meeting (13). Workers of the National Food Research Institute are developing enzyme immobilization by radiation-induced polymerization of salt of acrylic acid aqueous solution containing enzyme at low temperature in solid state. It is reported that the project will be developed for the co~m~ercial production by a private company with the support of the goverment organization , the Research Development Corporation of Japan.
168
S. Machi
Synthesis of Paint by Electron Beam Aishin Chemical Company has been developing the synthesis of paint of acrylates by the polymerization induced by electron beams irradiation. T h e monomer used in this system are acrylate, methacrylate and styrene and their mixtures. The pilot plant equipped with an accelerator of 2 MeV and 20 mA and the reaction vessel of 300 1 is in operation. The polymerization is carried out in batch system, where 280 kg production is obtained for 1 hr at 99 % conversion. The characteristics of the product are low viscosity in high solid content (70 %) and capability of thick coating. Fluoroelastomer Synthesis of a new fluoroelastomer by radiation-induced copolymerization of tetrafluoroethylene and propylene has been extensively studied at the Takasaki Radiation Chemistry Research Establishment including engineering studies with a pilot scale plant (14). The copolymer with alternative sequence of the monomers has excellent thermal stability and chemical resistance for polar solvent, sulferic acid, nitric acid, hydrofluoride and so on even at high temperature. Recently, the Asahi Glass Company started commercial production of the elastomer at 40 ton per year rate by using catalyst not radiation. The comparative study of the production cost indicates that radiation polymerization is more economical than catalytic method if the production rate is larger than 300 ton per year.
TABLE 3
Radiation Processes under Advanced Development Company or Organization
Process
Radiation Source
SO 2 and NO x Removal from exhaust gases
Res. Assoc. for Abatement and Removal of NO x in Steel Industry
Accelerator
Synthesis and Modification of Polymer Emulsion
Japan Atomic Energy Research Institute (JAERI)
Co-60
Permselective Membrane
JAERI
Accelerator
Immobilized Enzyme
JAERI, National Food Res. Inst.
Co-60
Fluoroelastomer
JAERI
Co-60
Paint Synthesis
Aishin Chemical Company
Accelerator
Precoating of Steel Sheets
Toray Industries, Inc. and Nippon Steel Corporation
Accelerator
Organic Glass by Cast Polymerization
JAERI
Co-60
Industrial Application of Radiation Processing Cast Polymerization
in Japan
169
for Organic Glass
The radiation-induced cast polymerization of glass-forming monomers has been extensively studied at the Takasaki Radiation Research Establishment to produce organic glass. Various monomers, such as acrylate, methacrylate and vinylether are called glass-forming monomer since their viscosity is extremely high at low temperature like as amorphous solid. Workers at the Takasaki Establishment found that if these monomers are polymerized in glassy state, the transparent organic glasses are obtained at high polymerization rate. The characteristic of the product is free from optical stress and strain. Details will be reported in this meeting (15). Precoating of Steel Sheets Toray Industries Inc. and Nippon Steel Corporation have been developing the electron beam curing process for the precoating of steel sheets. They announced that they opened bright prospects for the production of high quality precoated steel products with excellent hardness, gloss and thick embossed surfaces which conventional thermal cure could not atained.
REFERENCES
(i)
D.E. Harmer and D. S. Ballantine,
(2)
T. Sasaki, F. Hosoi, M. Hagiwara, K. Araki, E. Saito, H. Ishitani, K. Uesugi, Development of Radiation Crosslinking Process for High Voltage Power Cable, 2nd International Meeting on Radiation Processing, Miami, 1978
(3)
I. Tamai, (1976)
(4)
K. Temita and S. Sugimoto,
(5)
K. Umeda, "Requirement for the Irradiation of Food on a Commercial Scale", 113, IAEA, Vienna 1975
(6)
T. Kume, T. Tachibana, (1976)
(7)
S. Machi, O. Tokunaga, K. Nishimura, S. Hashimoto, W. Kawakami, M. Washino, K. Kawamura, S. Aoki, K. Adachi, Radiat. Phys. Chem., 2, 371 (1977)
(8)
O. Tokunaga, K. Nishimura, Chem. in press.
(9)
K. Makuuchi,
Chem. Eng. Apr. 19, 98 (1971)
"Data Book of Radiation Processing",
231, Brain Service, Tokyo
Radiat. Phys. Chem. 2, 567 (1977)
H. Aoki, T. Sato, J. Food Sci. Tech. Japan, 20, 492
N. Suzuki, S. Machi, M. Washino,
Radiat. Phys.
T. Takagi, K. Araki, Shikizai Kyokai Shi, 51, 214 (1978)
(10) T. Suwa, H. Nakajima, M. Takehisa, S. Machi, J. Polymer Sci., Polym. Letter, Ed. 13, 369 (1975)
(11) T. Suwa, T. Watanabe, S. Machi, J. Polymer Sci., Chem. Ed., in press. (12) S. Machi, T. Sugo, A. Sugishita, K. Senou, H. Fujiwara, I. Ishigaki, Inon Exchange Membrane by Radiation Grafting, Radiation Processing, Miami, 1978
2nd International Meeting on
!70
S. Machi
(13) I. Kaetsu, M. Kumakura, M. Asano, M. Himei, M. Tamura, K. Hayashi, Immobilization of Enzyme for Medical Use. The 2nd International Meeting on Radiation Processing, Miami, 1978 (14) S. Machi, O. Matsuda, 403 (1977)
Y. Tabata, J. Okamoto, Radiat. Phys. Chem. 9,
(15) I. Kae~su, K. Yoshida, H. Okubo, K. Hayashi, Cast Polymerization by Irradia. tion, The 2nd International Meeting on Radiation Processing, Miami, 1978