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3Pi t h e r e o r e some 3 6 0 0 Eastman O r g a n i c Chemicals for science and industry
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58
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FEB. 24,
I9S8
Bell's n e w thermal antioxidants for polyethylene increase in effectiveness when used with carbon black. Here, Belfs W . L. Hawkins makes accelerated oxida tion tests to determine comparative effectiveness of the various additives
Additives Check Oxidation Bell Laboratories turns up some thermal antiox idants for use, with carbon black, in polyethylene / I L NUMBER OF NEW ADDITIVE COMBI NATION'S that inhibit oxidation of poly ethylene have been discovered b y Bell Telephone Laboratories. And these antioxidants, says Bell, are up t o 10 times a s effective as those now in use. Much polyethylene has carbon black added to it to cut down on photo-oxi dation. But conventional thermal anti oxidants tend to lose their effectiveness in the presence of carbon black. Bell finds that its new additives, when used in conjunction with carbon black, ac tually have more antioxidant activity than they do when used in polyethylene without carbon black. The new antioxidants are certain substituted thioethers t h a t Bell forms by the condensation of conventional thermal antioxidants, such as aromatic amines or phenols, with sulfur dichlo ride. Bell finds that alkyl, aryl, and
heterocyclic thiols and disulfides with out amine or phenolic substituents are also effective antioxidants when used with polyethylene that contains carbon black. About 0 . 1 % of the sulfur com pound with 3 % carbon black is a highly effective combination for reducing both thermal and photo-oxidation, t h e com pany says. • Oxidation "Runs A w a y . " Bell has carried out tests of the n e w com pounds under accelerated oxidation conditions of 140° C. in an atmosphere of oxygen. T h e company finds t h a t the best antioxidants currently used will protect carbon black-polyethylene for mulations for less than 200 hours under these test conditions. Furthermore, the oxidation process becomes autocatalytic—it reaches a point at w h i c h it "runs away.'* However, with several of t h e new
protectants, Bell says, oxidation proceeds at a slow, even rate and does not become autoeatalytic even after 2000 hours at the accelerated conditions. This, adds *he company, is the equivalent of over 20 years of normal service life. In addition, Bell finds that some of the new antioxidants possibly offer other advantages. Into this group fall such additives as polymeric organosulfur compounds including thiols and the polymer from 1,10-decamethylenedithiol. Control of the molecular weight of the antioxidant will vary its diffusion rate (rate of migration to the surface) in polyethylene, says the company. Also, by varying the size of the repeating unit in the antioxidant, the concentration of the active groups can be changed without necessarily affecting their diffusion rates. Thus, you can get a high concentration of functional groups in comparatively high—molecular weight additives. This gives maximum protection over longer periods of time, adds Bell Finally, the company says, highmolecular weight antioxidants can possibly be added in large enough amounts to act as * ^sticizers for the polyethvlene.
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