Weathering Studies on Polyethylene J
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WIRE AND CABLE APPLICATIONS V. T. WALLDER, W. J. CLARKE, J. B. DECOSTE, AND J. B. HOWARD Bell Telephone Laboratories, Inc., Micrray Hill,N . J .
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INCE its development First attempts to devise uti Polyethylene has been used for a rinmber of years as by the Imperial Chemia dielectric material but only recently has i t been conaccelerated test involved the cal Industries in 1933 (6) use of a National iModr1 sidered as a mechanical protection for wirea and cables inpolyethylene has seen a rapid X-1-Aaccelerated weathering tended for direct exposure to the weather. Data are pregrowth, particularly during unit (IO) using National Sunsented on the results of a 10-year program on the effects World War 11, as the prime of weather on polyethylene. An accelerated test, which shine carbons as the elecfor the materials tested shows good correlation with wire and cable insulating trodes with the Corex D glanr material for high frequency natural aging, is described and used to evaluate the agfilters removed. This apuw. Its use as H dielectric ing characteristics of compounds of polyethylene conparatus involves a range of has been well dewribed in taining carbon black. Data are given showing effects on wave length8 shorter than the literature (6, 9, II, IS), aging of different types of carbon blacks such as furnace those found in natural sunbut little information has apand channel blacks, effects of carbon black concentration light. However, since the peared with regard to the on aging, the necessity for efficient dispersion of the carspertral transmission of polyeffect of weathw onpolyethylbon black in the polyethylene, and the relation between e t h y l e n e is essentially a ene. Crafton and SIade (9) aging and carbon-black particle size. Age resistance of smooth curve down t o a wave have claimed that sunlight length of about 225 mp (8), polyethylene is shown to increase as the average molecular has no effect on polyethylene, weight of the polymer is increased. These data indicate the authors assumed that but Maibauer and Myers (8) that channel grades of carbon black which have a particle inclusion of these shorter diameter of about 25 m p or less when well dispersed in have pointed out that unprowave lengths would not introan appmpriate polyethylene at concentrations of 1 to 2% tected polyethylene or a polyduce any effects which would can produce compositions having a natural outdoor life not occur on exposure to ethylene containing antioxiexpectancy sufficiently long to be considered for most outnormal sunlight for longer dants shows evidence of surdoor applications in the wire and cable field. periods of time. Correlation face degradation after only a few months of outdoor eupoof this type of exposure with natural outdoor aaina sure. Their work also shows - - has been that black and brown pigments produce a greater protective effect satisfactory. Initial tests with this equipment showed that degrathan either lightcolored pigments or antioxidants. The data predation proceeded rapidly but that on dark-colored samples the radiated energy of the bare arc was sufficiently high to produce a sented here may clarify some of the confusion which exists on the weather resistance of polyethylene and to point out the principal sample temperature above the melting point of polyethylene factors which are involved in the formulation of polyethylene (110" C . ) so that flow occurred. The machine was then modified to include a curved aluminum baMe to protect the samples compounds intended for exposure to the weather. from the direct rays of the car_bon arc for an angular distance of The Bell System first became interested in the use of polyethylene as a dielectric (7) in 1939 shortly after the material beabout 90". This baffle was placed on an arc approximately 5 inches shorter in diameter than the arc containing the samples came commercially available and since that time has become and was so oriented that the samples would emerge from behind increasingly concerned with its advantages as a mechanical prothe baffle directly into the water spray. Figures 1and 2 show the tection for wires and cables in the form of outer protective test equipment. Additional specimen-supporting rings have been sheaths and jackets. Certain samples of polyethylene exposed in 1941 in the form of flat sheets and as wire jackets, although added to increase the capacity of the unit, but in no case are showing some change, are stili continuing to weather satisfactorily. specimens exposed more than 8 inches above or below the horizontal center line of the arc light. The specimens are exposed in Based on this early experience together with the work described the form of molded sheets 1 X 3 X 0.075 inch at a mean distance herein, the Western Electric Company, the manufacturing unit of 20 inches from the arc. Except where indicated, polyethylene of the Bell System, has over the last 2 years made more than 1500 having a Williams plasticity (I) of approximately 52 was used. miles of cable sheathed with polyethylene for aerial use exposed to the weather. CRITERION OF DETERIORATION ACCELERATED AGING
Although the exposure of polyethylene compounds to the weather was begun early in the development of this material as a wire and cable jacket, an accelerated laboratory teat that could be related to outdoor performance was needed. The sensitivity of polyethylene to the weather is primarily an oxidation reaction catalyzed by the action of the ultraviolet energy present in normal sunlight combined v ith ordinary thermal oxidation.
As polyethylene ages either outdoors or in the accelerated aging unit it gradually loses flexibility until a point is reached where it becomes extremely brittle and fractures into many pieces under the application of slight stress. Figure 3 illustrates the appearance of a clear-unpigmented polyethylene-insulated wire after 29 months' exposure in Xew Jersey. The condition shown actually became apparent after only 0 months. This type brittlenevs is arcompanied by a chemical change in the polymer
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change from B msterial originnlly flexible st -76" C. to one completely brittle at this temperature in less than 16 hours' aging. In
the anme period of time the stres-strain and the dielectric data show substentially m a l l e r change% In the ease of the StreSSet,sltin properties approrimzitely twice the agirrg time is required to muse a reill d w r m in elongation, the only str-train vdne which is markedly chssged l>ya&g. A similar period of aging is required to produce a compasable ahangc i n the power fwtm or the surfare resiativity. The dio1sr:tric. Cowtarit is litt,lc affmted by aging histosy. On the hmk of t h e w datu, in d l mltmquent uvaluutioii of t,he rffoct? of aging on polyethylene, low-temperaturn brittleness hlIs beeen us'& as the witeriozi of change. Jmwkmperature irrittlencnv itlso has practical significanre nitice cables nheathed with polyethylene, or othw materials, must be c,apsble of being bent and handled :tt, low tempomtures. h r *,me of the work actus1 brit,tie poink were determind wimp: the prmedures of A.S.T.M. D 746, which involw Ihr detwminnt.ion of the l o r e s t temperaturn of nonfailun, f o r five
