I
A.
J.
RESTAINO
Atlas Powder Co., Industrial Reactor Laboratories, Plainsboro, N. J.
Radiation Technology for Nonbiological Materials Present outlook for nonbiological radiation processing is not encouraging except in a few areas. If chemical process costs increase, certain radiation processes m a y capture a noticeable portion of the industrial market during the interval 1965-1970
A of review articles have been written concerning the effects LARGE NUMBER
of radiation on chemical systems to induce reactions, or to report studies on the properties of materials subsequent to high energy radiation exposure. Most of these papers have as their objective the reporting or correlation of information or the elucidation of a mechanism for a radiation-induced process. The purpose of the present review is to select from the myriad of publications in this field the relatively small number of findings which contribute to radiation technology and imply commercial potential. Commercial potential depends on many factors of which the nature and use of the product, the yield, and the economics are some of the more important aspects. The present review, for the most part, will be concerned with those radiation processes which yield end products for which a use, need, or market exists, and the costs involved are in the "ball park" for making the same or similar products by nonradiation techniques.
.a
Chain, Noncha in RadiationInduced Reactions A chain reaction is one in which many reactant molecules are converted into product for every active species formed. In radiation processes the active center is usually a free radical, although it may be an ion. The very nature of chain reactions, assuming the propagating species remains unchanged, appears to be much more highly dependent on the chemical system than on the mode of initiation. Hence, although certain vinyl polymerizations are highly efficient chain reactions when high energy radiation is the initiating source, the same reaction can be duplicated in chain length by using heat, chemical catalysts, or ultraviolet radiation as the initiator. Likewise, many reactions exist of which the halogenation and sulfochlorination of certain hydrocarbons are typical, which represent highly efficient chain reactions whether high energy radiation or ultraviolet is used to initiate the chain. Additional examples could be given to show
that while high G-values (number of molecules of product formed per 100 ev. of energy absorbed) and chain-type reactions are desirable in a process, they are not a sufficient condition to justify the use of high energy radiation, but rather serve to demonstrate the nearly overwhelming economic competition from already existing processes of accomplishing the same or similar phenomena. A number of investigators report some advantqges in organic syntheses of hydrocarbon derivatives by means of high energy radiation. Some examples, including the sulfochlorination of cyclohexane, the sulfoxidation of aliphatic hydrocarbons, and the chlorination of benzene and toluene, will be discussed subsequently. A good deal of the research in radiation chemistry, particularly of an industrial nature, has been devoted to studies of chain-type reactions. Hence, vinyl polymerizations, including graft copolymerization, have received some of the heavier research expenditures from industry. However, the vulcanization of rubber, the cross linking of plastics, the cracking of hydrocarbons, the production of phenol and of oxides of nitrogen have also aroused considerable interest although these reactions induced by radiation are not of the chain type. I n the radiation synthesis of phenols and of oxides of nitrogen, although the G-values are very low and the economics discouraging, both radiation processes have the advantage of starting with cheaper raw materials than their commercial counterparts. This means that much lower Gvalues could be tolerated when compared
to chain reactions in which the starting materials are the same for both processes. However, the limit in the value of G which is economically tolerable in a radiation process should be related to the difference in materials and processing costs from the chemical process. The chlorination of benzene to the hexachloro derivative, the sulfochlorination of cyclohexane, and the sulfoxidation of certain aliphatic hydrocarbons by high energy radiation offer some distinct advantages over the ultraviolet-initiated reaction. Some of the differences are related to yields and to the extended applicability of the high energy process. The details of these distinguishing features are discussed below.
Chain Reactions One of the fundamental differences between peroxide initiated polymerizations and those initiated by high energy radiation is the initiation step. As in homogeneous vinyl polymerization, propagation, termination, and chain transfer are identical in both processes, variations in the over-all mechanism, the physical properties of the polymer, and the economics are due primarily to the difference in the initiating system. Most commercial vinyl polymerizations of the free radical type are initiated by peroxide catalysts and in the absence of additives, the over-all activation energy is most seriously affected by the activation energy for the decomposition of the peroxide. As the activation energy for most of the commercially used
Here Are Some Companies Known To Be Active in Radiation Processing Dow Chemical Co. General Electric Co. W. R. Grace & Co. Hercules Powder Co.
High Voltage Engineering Co. Irradiated Insulations Radiation Applications Inc. Raytherm
VOL. 52, NO. 8
AUGUST 1960
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peroxides is approximately 30 kcal. per mole, a value which is large compared to the activation energy for propagation and termination, temperatures from 50 O to 100’ C. are usually required to obtain reasonable rates of conversion. O n the other hand, the over-all activation energy for the radiation process is small because the temperature dependence for molecular shattering and free radical formation by high energy radiation is zero. The consequence is that a temperature increase in the radiation process increases both the polymerization rate and the molecular weight to a moderate extent, whereas in the peroxide process, temperature increase causes a more significant increase in rate, but a decrease in molecular weight. The physical properties of polymers are a serious function of molecular weight; hence reaction temperature and the problem of temperature control for the exothermic polymerization reaction are a less rigid requirement in the radiation process. The economics of the two processes generally reflect the difference in costs due to the initiation step, because a number of studies demonstrate that it is possible to design a radiation process which is quite similar to conventional methods used for commercial production. Thus, in a proposed 20,000,000 pounds per year poly(viny1 chloride) production facility (72), the production costs attributable to monomer, utilities, direct labor, overhead, depreciation, insurance, and taxes (less radiation source in the last three items) are identical in both processes. The cost difference is attributed to the maintenance and operational costs connected with the radiation facility. On this basis, the cost difference between the two processes is about 0.5 cent per pound. Similar studies on other free radical vinyl polymerizations demonstrate that except for the radiation costs, processing costs differ very little, if at all, from conventional commercial production. The comparative costs of the radiation and peroxide processes, and selling prices of poly(viny1 chloride) and polyethylene ( 3 ) are :
Polymer
Polymerization, selling Cost per Pound, PS price Radia- Convenper tion tional Pound, B
Poly( vinyl
chloride) Polyethylene
0.1666= 0 . 161Sb 0.275d
0.2~5~
0.235c 0.35
