Radiation Processing: The Industrial Applications of Radiation Chemistry Joseph Silverman Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742 Although unknown to most of the scientific community, and even more unknown to the lay public, the industrial applications of radiation chemistrv have alreadv had a urofound and widespread impact on the way we live. At present the value of irradiated pruducts pr~rlucedannually is several hillion\ of dollars and growing at a wmpound rate approaching 2S'c oer annum. Thv coml~ined111sta11ed Dower outout of the 250 electron machines and 100 large gamma sources engaged in industrial urodnction is 15 megawatts. (As demonstrated below this is a very large power level.) The installed power is also showing a 25% growth rate. Those of us who drive to work are almost certain to encounter irradiated components. The paint on the dashboard is often cured by electron beams. Much of the foamed plastics used to insulate us from noise, shock, and extreme temperatures are irradiated ulastics. Insulated comuonents of auto ignition system> w n - ~ uf i ~r?diatiun,crosslinked insuln~i 2, M , decreases with increasing dose and the ~ o l v m eremains r soluble in its usual solvents. If .~ "n /." oo < 2, ~ i i n c r e a s e with s dose. At the gel dose,
D,,
=
1 J$ Mw ( 9 0 - %PO)
(6)
a three-dimensional network begins to appear. The gel is insoluble and infusible; increasing doses add a greater fraction of the polymer to the gel. Unless po = 0, there will always he a soluble part of the polymer even a t very high doses. Beyond the gel point, the Charleshy-Pinner equation can he used to describe the sol fraction, s , as a function of dose: Volume 58 Number 2
February 1981
171
Figure 6. Chariesby-Pinnw plot of of
sol fractionas atunction of dose in sampler
inadiated polyelhyiene.
Figure 5. Mmbined elfects of scissim and crasslir&ingon lhe mlacular wemt 01 irradiated polymers.
s
+
s
~
40
~
2
=
~
+
m
(7)
L70MluD
Figure 6 shows the Charleshy-Pinner plot for irradiated polyethylene. It should he recognized that these equations describe idealized conditions. They assume that (1) the initial molecular weieht distribution is random. (21 the radiation vields are independent ofdow, and r 3 , rlwre are no cornplicatini: i w ondary reactions. TIw lirst is rarelv true, hut rhe equatloni can be adjusted for any disfril)ution. The second assumption is reasonable for low d o i i only. ~ The last assumption is often untrue because of impurities which are common components of industrial . polymers. Als~,.asrwted above, the m o r ~ h c h-r.r , . of polymers can be a complicating feature. Most of the applications of practical interest involve doses of 5-20 Mrad. Depending upon'efficiency of radiation ahsorption and other process details, this would lead to radiation costs of $0.10-0.50lke. In hieh volume Drocesses. such as the crosslinking of cent& station hookup'wire by the Western Electric Company, this radiation cost is significantly reduced by the economy of scale and the use of added crosslink "sensitizers." The latter reduce the dose requirements hut a t the cost of the chemical agent. The beneficial effect ohtained from the irradiation of wire insulated by polyethylene or polyvinyl chloride is the improved dimensional stability of the insulation of elevated temperatures. The doses required to give improvements are relatively high since a gel content of 70% is usually needed. For packaging applicati& the dose requireme& are more modest because the required gel content is about 30%. Radiation crosslinking of rubber sheet with submegarad doses imparts added green strength prior to further handling. An i m ~ o r t a n feature t of radiation-crosslinked sem&rvstalline p61smers ia i t ? viswr13tw memory. If the radiat&i crosslinked u r d u t is fir>! hmred to the cr\.stalline rncltinr temperaturd, then stretched and quickl; cooled in th; stretched confieuration, i t will retain the distorted shaoe a t room ternpernt.trt.. On iulwqurnt heating of the pulymtv above the meltinr pmnt, the ( m s l i n k s cause a rubber-like contraction of the film to its original form. This is the basis of the heat shrinkable packaging of the W. R. Grace Company and the heat st~rinkahleelectrical connectors pioneered by Raychem Corporation. ~~~
~~~~~
~
~
Graftlng . Radiation-induced grafting of a monomer to a formed polymer, already a minor success, is potentially one of the 172
Journal of Chemical Education
most important industrial applications of ionizing radiation. In considering such applications, one may ask why are graft copolsmers useful, and why should radiation be used to orodice them? At present the materials engineer trying to meet a particular need i s faced with the choice of severaithousand polymers ranging in price from $0.50 to $20/kg. Frequently, he chooses an expensive polymer when a slightly modified inexpensive one could meet his requirements. As a consequence, it has been a coal of manv..~ o l.v m e eneineers r to develoo. a .oolvmer . tecltnology based on a few cheap basic ~ r ~ l y i n eand w to obtain slwvinl surface or bulk orooerties hv- s i m- ~ l mdifications. e One sich attractive possibilit;, is grafting. A graft co~olvmer is composed of a main chain consistinn. . ofonr set of repeating units and side chains consisting of an
