PREFACE "Cor the past 20 years, radiation chemistry has received considerable attention in polymer research. High energy radiation, as obtained from electron beams and x- or beta rays, initiates ionization and radical formation. It may cause either crosslinking or chain scission, depending on the chemical structure of the polymer and dose rate. In the presence of oxygen, peroxides may be formed. When they are trapped, ions, radi cals, or peroxides can produce postirradiation reactions. High energy radiation may also be used to initiate polymerization or to engraft monoor multifunctional monomers upon polymeric chains. Low energy radia tion, such as ultraviolet light, is less penetrating and has been restricted to surface treatment. For several years, crosslinking of polyethylene had been the only commercial process used in the polymer industry. Recently, other proc esses have gained importance, especially in the area of grafting. The Division of Industrial and Engineering Chemistry, recognizing the poten tial of polymer irradiation for industrial applications, held a two-day symposium on this subject during the 1966 spring meeting of the Ameri can Chemical Society in Pittsburgh. The participants were selected among the world experts in this field, well known through their books, publications, and patents. This volume contains the papers presented at that symposium. When considering industrial use of polymer irradiation, the following four questions arise: 1. What are the advantages of irradiation? 2. Which properties can be improved by irradiation? 3. What kind of radiation source is preferred? 4. What does irradiation cost? Most of these pertinent questions are answered in the chapters of the various authors. As a guideline, I shall attempt to discuss these questions briefly by referring to the different chapters and by filling in a few gaps, not covered by the authors.
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A
What
Are
the Advantages
of
Irradiation?
The major advantages are: (a) that the desired reactions can be carried out generally at lower temperatures than in operations employing chemical means, (b) that monomers can be polymerized free of catalyst contamination, (c) that crosslinking and grafting can be performed in vii Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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situ on fabricated articles, and (d) that coatings can be applied in monomeric form, eliminating solvents. New theories and findings on the formation of ionic species and radicals, which become trapped in the polymers, are discussed in the first two chapters, written by the two European authorities, Chapiro and Charlesby. The kinetics of crosslinking polyethylene is the subject of the American authority, Dole. A higher yield of crosslinking polyolefins was observed in the presence of nitrous oxide by the Japanese scientist, Okada. Certain high polymers crosslink when irradiated whereas others degrade. This characteristic is related to the hydrogen which is bonded to the polymeric chain. Generally, polymers crosslink if all the carbon atoms of the main chain carry at least one hydrogen atom, but they degrade if tetrasubstituted carbons are present in the chain. This is probably caused by a larger steric hindrance which can be observed through a lower heat of polymerization than theoretically calculated and through a higher yield of recoverable monomer on pyrolysis. Crosslink ing of ethylene-acrylate copolymers is the topic of Potts, and of poly ethylene oxide is that of King, both with the Union Carbide Corp. Crosslinking of ionomers (Surlyn A ) has been investigated by Lyons and Brenner, and of polyethylene terephthalate (Mylar) by Turner. Gamma radiation-induced mass and emulsion polymerization of vinyl monomers has been studied by the American, Metz, and the Ger man, Hummel. Irradiating a polymer and adding a monomer may lead to the for mation of a graft copolymer when irradiated. It causes the formation of free radicals in both the polymer and the monomer. Graft copolymerization is favored over homopolymerization of the added monomer if the G value (free radicals per 100 e.v.) of the polymer is significantly higher than the G value of the monomer. Homopolymerization predominates if the G value of the monomer is larger than that of the polymer. Poly (vinyl chloride), for example, has a high G value and is well suited as a backbone for grafting, whereas polystyrene has a low G value. Grafting 10 to 20% styrene upon poly (vinyl chloride) and the resulting increase in density, tensile strength, and flexural strength are discussed by Harmer of the Dow Chemical Co. Polystyrene, which is unsuitable as a backbone, can be made susceptible to grafting by incorporating rubber. Allyl methacrylate and allyl acrylate are difunctional monomers, triallyl phosphate is a trifunctional monomer, and polyethylene glycol dimethacrylate is a polyfunctional monomer. A l l these lead to crosslinked graft copolymers. Grafting vinyl monomers to flexible polyurethane foam is the subject of Skrabas chapter, and to wool is that of Stannett. R
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viii Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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Which
Properties
Can
Be Improved
by
Irradiation?
Increasing heat resistance and strength, reducing stress cracking, and lowering permeability are a few of the advantages obtainable through gamma or electron irradiation. Surface improvements, such as weather resistance, hardening, dyeability, or destaticizing, are obtainable through ultraviolet light treatment. Prestressed irradiated polyethylene film and tubing shrink when heated above the crystalline melting temperature and form tight covers over the enclosed object. Irradiated polyethylene film is used for food wrapping and for protecting metal parts and equipment during pro longed storage. Its sale is expected to rise from 1965's $8 million to $20 million by 1970. Heat-shrinkable tubing and tape are used for electrical insulation and moisture protection. Irradiated polyethylene bottles may be sterilized at 135 °C. Swelling of polymers by gas generation during irradiation has been observed by Hoffman and Bell, who report on this phenomenon. Impregnating wood with monomers, such as methyl methacrylate, hydroxyethyl methacrylate, or vinyl acetate, and polymerizing it by irra diation results in increased hardness, higher strength, and better form stability. For instance, the hardness of pine can increase by 700% and oak made as hard as teak by irradiation. Considerable research is being devoted to the graft- and homopolymerization of various monomers on sheet paper and on paper-making fibers. This enhances bulkiness, resili ence, acid resistance, and tensile strength of paper—all properties in demand for applications such as grocery bags. The properties of impact polystyrene can be improved by simulta neous grafting and crosslinking upon rubber. Changing cis-polybutadiene into frans-polybutadiene and eliminating the 1,2-vinyl configuration through radiation are discussed by Parkinson. Chloromethylstyrene has been grafted to polypropylene fiber to facilitate dyeing. Since moisture pickup is improved at the same time, ironing the textile product is also easier. Preirradiation of polyamide and polyester fibers at a dose of 0.5 to 1.5 megarads, followed by grafting of methacrylic acid to the surface, also increases wetting and dye acceptance and reduces static. Styrene monomer is grafted in commercial operation to the surface of Teflon and Kel-F sheets and films to make them bondable, dyeable, and printable. There, a dose of 1.75 megarads is required. The surfaces of vinyl tiles are hardened by grafting acrylonitrile and glycol mono- and diacrylates at a dose rate of 0.4 to 1.5 megarads. What
Kind
of Irradiation
Source
Is
Preferred?
Radioactive isotopes appear practical when considerable depth of treatment is required. For upgrading the physical properties of the ix Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
polymer, an electron accelerator may be used. For surface treatment, an electron accelerator, x-ray machine, or ultraviolet light can be em ployed, depending on the depth of treatment desired. Radioactive sources of commercial interest are γ- and β-ray emitters. The choice of sources has narrowed down to roughly three elements— cobalt-60, strontium-90, and cesium-137—as a result of their useful char acteristics, reasonably long half-life, availability, and cost.
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Radioactive Isotopes Element Cobalt 60 1.17,1.33 0.306 14,400 5 5.27
Atomic weight γ-rays, m.e.v. β-rays, m.e.v. Mr/hr. output/curie at 1 ft. Curies in % χ Vs inch source Half life, years • 0.66 m.e.v. gamma from B a * 2.18 m.e.v. beta from Y»o.
i:î7
Strontium 90 0.61, 2.18 28
b
Cesium 137 0.66° 0.51(95%), 1.17(5%) 3750 1.45 33
.
Cobalt-60 is made by exposing ordinary inexpensive cobalt in an atomic reactor. Strontium-90 is a fission product in nuclear power plants and has a higher beta radiation than cobalt-60. Cesium-137 is a fission product found in all nuclear reactors and must be removed from time to time to maintain efficiency. Evidently large quantities of strontium-90 and cesium-137 will be available in the year to come. Electron accelerators are available for producing electron beams with energies ranging from 10 k.e.v. to 10 m.e.v. and overlapping into the area of x-rays and γ-rays. There are two general types of electron beam machines: the direct and the indirect. In the direct, such as the Van de Graaff, the resonant transformer, and the Dynamitron, the high voltage is applied directly between the anode and the cathode of the accelerator tube. The accelerator tube is essentially the same in all three, and they differ only in the manner in which the high voltage is generated. In the Van de Graaff accelerator, the high voltage is gener ated by electrons carried on a rapidly moving insulated belt. In the resonant transformer, the high voltage is generated by a parallel circuit in its secondary which resonates at 180-cycle frequency fed to the pri mary. In the Dynamitron, the high voltage is generated by a set of cascaded rectifiers coupled to a 300-ke. RF-oscillator. The insulated core transformer is another direct electron beam machine. It is similar to a three-phase power transformer in that a core and coils are used. One insulated core transformer will drive three accelerator tubes, which allows great flexibility and good power utiliza tion. The Linac (linear accelerator) is an indirect electron beam max Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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chine. There, power is generated externally in a microwave oscillator at 3 kilomegacycles, and the accelerator tube takes the form of a circular copper wave guide. The advantage of this machine is that high voltage is not required and therefore insulation problems are not as severe as in the direct machines, and, in addition, rather higher powers can be generated. A radiation dose of 1 kw. could irradiate about 795 pounds per hour of any material to a dose level of 1 Mrad at 100% efficiency. The power efficiency of the different machines varies from a low of 15% for a Van de Graaff accelerator or resonant transformer to 90% for an insulating core transformer. On the other hand, the Van de Graaff accelerator, being a d.c. machine, can be precisely controlled and adjusted, which is important for research and development. The resonant transformer, Dynamitron, and ICT are less precise, but they have lower operating costs and higher output power, making them more suitable for production purposes. The use to which the machine will be put has a bearing on its choice. If it is used for sheets, films, or surface treatment, where only modest penetration, is called for, a low voltage machine, such as the ICT, is suit able. If greater penetration is necessary, a higher voltage machine is needed, and the choice is between the resonant transformer and the Dy namitron. For surface treatment, an ultraviolet light might be sufficient. This is created when an electric arc passes between electrodes separated by gas or vapor. There are two main classes of arcs: open, such as the carbon arc, and closed as the various vapor lamps. Types of Ultraviolet Radiation Sources UV Source Plasma arc Tungsten filament Mercury xenon tube Carbon arc α 6
Max. Power, Efficiency Spectrum Range, Kw. (UV and Visible), % A. 250 10 5 25
30 15 40 10
1500- 3500" 4000- 7000 4000- 7000 5000-15,000 6
6
6
Controlled within spectrum range by gas or gas mixture. Not controllable within spectrum range.
The plasma arc is a recent development, with a potential of high intensity and efficiency. By using photosensitizers, visible light or laser beams may also be employed. The effect of ultraviolet light on polymers is discussed in two chapters by Chen and Rânby. What
Does Irradiation
Cost?
Cobalt-60 now costs $1 per curie or roughly $70,000 per kilowatt produced while electron accelerators are now between $5000 and $10,000 xi Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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per kilowatt output. The price of a plasma arc is $1000 to $1500 per kilo watt. It is expected that, in the future, cobalt-60 may be reduced to as low as 20 cents per curie. This is still about $13,000 per kilowatt, and the cost of electron accelerators and ultraviolet sources may also be reduced. Shielding for isotope sources, which are gamma emitters, is considerably more expensive than for electron accelerators and ultra violet sources. The cost of polymer irradiation depends on the type of polymer, formulation, and shape of the fabricated polymer. Upgrading polymers by electron beam radiation costs about 1 to 4 cents per pound. Some of this cost can be equalized through material saving in view of property improvement. Surface treatment might be as low as 0.1 to 0.2 cents per pound. For polymerizing monomers into commodity resins, cost of irradia tion would have to be reduced to the cost of catalyst in order to compete economically with the present commercial polymerization processes. For engrafting monomers to finished articles, the cost can be higher, especially when solvents are eliminated. This book is not intended to give a complete survey of polymer irradiation. For this, we have to refer to such excellent texts as "Atomic Radiation of Polymers" by Charlesby and "Radiation Chemistry of Polymeric Systems" by Chapiro. Since these books were issued in 1961 and 1962, more experimental research has been done and more theories have been developed by these authors, who are also the authors of the first two chapters of this volume. New trends have been discovered, and more light has been shed on polymer irradiation by the authors of the following 18 chapters. Thanks to their work and efforts, polymer irradiation is making inroads into the plastic and related industries. It is a privilege to express the divisions and my deepest appreciation to all of those who made the symposium and this book possible. NORBERT A. J . PLATZER
Springfield, Mass. April 1966.
xii Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
