RESEARCH
H o w Does α Polymer Cross Link? Effect of oxygen may be one complicating factor in cross linking and degradation of polymers by irradiation ANN ARBOR, MICH.-Since the dis covery a few years ago that plastics irradiated in a nuclear reactor undergo significant changes, a great deal of both applied and basic research has been devoted to irradiation-induced reac tions of polymers. The most practical accomplishment so far has been com mercial electron beam irradiation of polyethylene to bring about cross link ing of polymer chains with consequent improvement of heat resistance and rigidity. The basic mechanism of re actions of this type, as well as the degration reactions which many other plastics undergo on irradiation, have been the subject of some controversy. Among the factors which may give rise to conflicting experimental results is the effect of oxygen. At the Ann Arbor High Polymer Research Confer ence, L. A. Wall, National Bureau of Standards, explained that in the pres ence of oxygen a degrading polymer degrades less than it does in a vacuum; a cross linking polymer cross links less, or may actually be di ,Taded instead. Polyethylene affords a good example for pointing out fundamental reactions polymers undergo on irradiation. There is much disagreement in regard to de tails, but a widely accepted theory as sumes that the first step is the breakage of a carbon—hydrogen or carbon—car bon bond by radiation. A C—C bond break ruptures the chain, while a C—H bond break merely splits off an atom of hydrogen and leaves a free bond on the chain. The active hydrogen then "extracts" another hydrogen atom from an adjacent chain and forms a second free radical. The two free radicals then combine to form a cross link. Since there are twice as many C—H bonds as there are C—C bonds in poly ethylene, it would be expected that the ratio of cross links to chain scissions would be 1 to 0.5. Experimental re sults give a 1 to 0.35 ratio, however. This is probably due to recombination of some of the broken chains. From experimental information on the degradation of methyl methacrylate and the cross linking of polystyrene polymers in high vacuum compared with results obtained in air, Wall con cludes that the small free radicals (or hydrogen atoms) react with oxygen in preference to the polymer. Other free radicals may combine with oxygen to form hydroperoxides, which break the 3390
polymer chain when they decompose. Differences Explained. Wall finds an energy requirement of 50O electron volts to obtain each cross link in poly styrene. This compares with a figure of 3000 to 5000 reported b y A. Charlesby, of the British Atomic Energy Research Establishment, pio neer in polymer irradiation research. This discrepancy could probably be explained by traces of oxygen in Charlesby's samples. Wall finds that in the degradation of methyl methacrylate (which does not cross link), 60 electron volts are.needed per scission in a vacuum, but 120 are required in air. Studies are not yet complete, but it appears that the aromatic rings of poly styrene are more involved in irradiation reactions than the main "backbone" of the chains, according to AVall. Deu terium tracer techniques are being used in this research. PyroSysis Depolymerizotion. Most present commercial polyethylene con sists of branched chain polymer mole cules. Newer material made b y lowpressure processes i s predominandy straight chained, so-called "polymethylene." Polymer chains might be ex pected to decompose b y a random proc ess when heated, according to Robert Simha, New York "University. If this is the case, one can calculate that the rate of conversion to volatile material should reach a maximum when 25% of the polymer has been decomposed. Ex periments show this is true w i t h polymethylene, but polyethylene does not show a maximum. Branches in polyethylene chains re sult in some tertiary C—H linkages. These react about 10 times as rapidly as the more common secondary link ages, which make up most of the C—H bonds in the molecule. The amount of branching in commercial polyethylene does give rise to a different reaction rate because of this, but calculation of the point of maximum rate of volatization shows that it should not b e very different from the point expected for linear material and does not explain the experimental results. Therefore, the existence of branches, in itself, does not account for the lack of a maximum. Mathematically it can be shown that a maximum would not appear if the difference between ter tiary and secondary linkages is larger
than assumed. This would be the equivalent to more than one bond be ing weakened for each branch present.
• Cockroaches a r e being used in a
cholesterol research project of the Vet erans Administration in Wood, Wis. Cockroaches must have cholesterol in their diet for normal growth, but they cannot make it from simple materials as humans do. In th« tests, Jerre Noland, head of the project, is giving the insects compounds similar to choles terol, which substitute in part for chol esterol to furnish information on the steps occurring in natural synthesis. If these compounds act as antagonists to cholesterol, they could be used in humans to prevent hardening of the arteries. • Prednisolone is being sold by Sharp & Dohme under the name of Hydeltra. It makes prednisone under the name Deltra. Both are being widely used to treat arthritis. • A polio research grant of $85,198 goes to Wendell M. Stanley, Univer sity of California. The grant, made by National Foundation of Infantile Paral ysis allows continuation of purifica tion studies on the polio virus to deter mine its chemical composition. ÎAEC awards 5 3 unclassified contracts for research in the physical sciences. Eight are new contracts; among them are: Milton Kerker, Clarkson College of Technology, $8018 for A Study of Size and Shape of Colloidal Particles by Light Scattering and Electron Microscopy; R. H. Johnsen, Florida State University, $4539 for Radiation Induced Effects in Heterogeneous Organic Systems; and Ernest Griswold and Jacob Kleinberg, University of Kansas, $8500 for Some Problems in Chemistry of Low Oxidation States of Metals. • Damon Runyon Fund for Cancer Research allocates $130,890 in July. To date $9.2 million has been allocated. • Sharp & Dohme's research division
made 22 grants totaling $54,000 since Aug. 1, 1954; in the same period, the medical division granted $38,550 for clinical studies. • A
$2500
graduate
fellowship
in
chemical engineering has been established at University of Oklahoma by Autoclave Engineers of Erie, Pa. Leon Vernon receives the first grant for a project involving heat and mass transfer in a newly designed reactor.
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PICCOLASTIG ft E S I Ν S
24 standard grades—polymers of styrene and its homologues Piccolastic Resins are made in five series and twenty-four standard grades. They are polymers, in a wide range of aver age molecular weights, of styrene and its homologues. They possess sufficient pale color so as to be suitable for the majority of uses. The entire line is soluble in aromatic hydrocarbons. The Piccolastic Resins, with the exception of one type, are wholly hydrocarbon in structure, and therefore are alkali and acid resistant to a high degree, do not support mold or other fungus growth, and are not subject to enzyme reaction. The Piccolastic Resins are permanently thermoplastic, and because of their heat stability at temperatures up to at least 175 C, make excellent stable, hot melt compounds. Piccolastic Resins vary from viscous liquids through tacky solids, brittle solids to resins of hard horny toughness. Inter mixtures of the various items permit an unlimited range of properties.
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What Chemical Research Can Do for the Paper Industry
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FOR T H E CHEMICAL INDUSTRY
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It has been said that in 25 years over half of the chemical industry's income will come from products it doesn't κηονυ about now, &r doesn't know how to make commercially. Some of these unknowns undoubtedly will add new items to the roster of paper products, according to Crown ZeUerbach's Walter F. Holzer. As an example of the new vistas chem istry can give to paper, Holzer cites the enormous growth in use of syn thetic resins in paper. Before the Western· Chemical Market Research Group, he listed a few things the paper industry can use if the chemical industry can make them. A chemical solvent for lignin which would allow us t o recover more fibrous material, and at the same time obtain lignin in more usable form. This could be the basis of a n e w chemical industry. Reagents to V centrations.
react with cellulose hydroxyls i n water at dilute con Present reactions are in strong caustic or anhydrous media. With such a reagent we could modify cellulose properties economically.
ν ν ν Laboratory wares of a l l description. Sheet, W i r e , Tubing, G a u z e and Fine Foils. Stills, Retorts, Electrodes and other Special Process Equipment to order. Salts a n d Solutions. Platinum M e t a l Catalysts—Concentrated forms and on carriers. Palladium, Iridium, Osmium, Rhodium and Ruthenium.
We pay highest prices for scrap platinum and also have facilities for prompt recovery of spent platinum and palladium catalysts. W E I N V I T E YOUR I N Q U I R I E S AND W I L L BE GLAD T O SEND ON REQUEST OUR FOLDER N-20, " P L A T I N U M , G O L D A N D S I L V E R FDR S C I E N C E , I N D U S T R Y AND T H E A R T S "
^THf AMERIGAN PLATINUM V/0FÎKS 2 3 1 NEW JERSEY R. R . : ^ E ; ^ • · 1 NEWARK; 5, N. J. r
3392
M'EIALS SINC£
Additives for greater strength. Cellulose fibers have tensile strengths approaching that of steel. The strength of paper is limited b y the fiber-fiber bonds. Inhibitors to prevent oxidative degradation of cellulose in bleaching. A dimensional stabilizer for cellulose. Tlie expansion and contraction of cellulose with moisture is well known, a n d is a major headache with paper manufacturers and users.
mill systems are excellent environments for slime growths. W e V Paper need more really effective slimicides to go with the very few w e now have, in order to permit change of treatment. Of course, they should be odorless, tasteless, and nontoxic to humans. can be strong yet not resistant to shock. W e need additives V Paper to improve impact and fatigue resistance t o paper at a reasonable cost. are notorious for lowering paper V Plasticizers would not, would be a real contribution.
strength,
One which
priced flameproofing V Athereasonably beater, would be noncorrosive,
agent which could be added in and would not deteriorate paper strength could make an enormous market for this type of paper.
rodent repellent that would protect foodstuffs stored in paper bags V Awould save millions of dollars a year, and greatly expand t h e use of multiwall sacks. Transparent films are expensive, and the only way to make paper transparent is an almost equally expensive physical treatment. A chemical to do this in simple fashion would b e welcome.
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W e add fillers to papers for various reasons, but can retain only 30 to 60% in the sheet. This is a serious economic loss. Can you give us a chemical retention agent? In coated papers w e need better adhesives which are compatible with printing, and resistant to moisture.
in cost of chemicals would b e welcome. V Athatreduction any ^appreciable saving could be made in th**
robable „ ones such as chlorine, caustic, salt cake, and the like. O n die onier hand, more expensive ones like chlorine dioxide, peroxides, resins, latices, and many others would find much wider «sage pretty much in direct proportion to a decrease in price. CHEMICAL
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