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91 RABBI AT ~ ~ 3. T. Blake and R. I.yle Simplex dire & (able Comply Cambridge, Mass. The question of the supp] y of natural and synthetic rubber and their uses in electrical insulation continues to be an important One. In the country as a whole, the proportion of synthetic rubber cased fell to about 42 per cent in 19470 About 80 per cerLt of the rubber used in electrical insulation was synthetic, since this material :Ls preeminently aciapted to this use. :Et wouic] have been shill greater had not a shortage of GR~S developed. This shortage was caused by the premature closing by the Government of some synt;het,ic rubber p' ants. At the present time, GR-S :Ls in adequate supply due to the curtailment of tire production schedules, and to the proposed reactivation of several producing plants . There was a relatively high rate of usage of GR-S in spite of. the fact that9 for a portion of the years the price of natural rubber was as much as 5 cants less than that of GR-S. At the present time, stockPi].irlg policies of the Govern- ment are holding the price of natural rubber slightly higher than that of OR-So It is protiable that on a supply and demand basis' the price of natural rubber drill drop In the next fey? years. Even at the present price, the rubber plantations are not happy, since their costs have increased aT,precia'~ly since before the -zero In the past, plantation costs leave had little to do pith selling prices. There are indications, however, that~the price of GR-S could be lowered several cereals and still be profit&tile. A1 though manufacturorp of rubber insulation ares in general' v~>e3.~-satis- fied with GP`-S' consumers are not entirely educated to its use. They still remember the higher tensile strength insulations obtainable With natural rubber, although this is, of course, an unimportant property for insulation. Prob2.bl,nr-more attention should be paid by the consumer to the better aging properties of GR-S9 and tile better resistance to sunlight of such ~nsulation..,In ad.,dition9 the greater c~ean- liness and uniformity of GR-S are.arg~ments for 'its use'. ' . . . . . . . . . . The recent announcement of types of GR-S ~ol'~mer~zed at loper temperatures should make it much more desirable for insulation. These 'never polymers, a.libough . . . . not yet available in quantity and not yet made in electrical grades, should give vu~canizates With greater touchiness, and the tensile strength will, incidentl,y) be higher. They 'v'~73)1 have higher compression resistance at elevated temperatures, and the good aging qualities and resistance to sunlight of regular GR-S should be main- tained. In addition to the materials already -in sights it its certain that better polymers mill be produced in the near future and remove finally any possible advan- ta~Fe that natural rubber might posses.
c>? This year ~ as in the past se`Jere1 Steals ~ the most significant improve - ments in rubber insulations were in the field of synthetic rubbers. The properties of sem~-conductir~g rubbers have continued to provide considerable interest. LThe marketing of b;lty1 rubL-er insulated. cables for high Frontage uses Was announced . The properties of such an insulation Were discussed bay Eaton sold others in a paper ~.'h~c~h was abstracted at some length last years Such cable. are recommended for operation s.t a copter temperature of 80°CO Bo-;~ev=-r9 the most stringent test reported requires orally 7 days aging in an ox,rger~ bomb at 70°C. No data are given for aging at 80°C or higher. The use of poi.yeth:ylene as a filler in busy]. rubber vitas annolmced3O Polar_ ethylene filled butyl rubber insulation is claimed to have the sane excellent corona resistance and aging characteristics as betel rubber. The physical properties are cc~llsi-derab~y improved, particularly hardness and modu:Lus9 a~r~roximating9 it is claimed, heat-resistant natural rubber compounds. These compounds are recor ted as retaining 60 per cent of their tensile strength after 20 days at 100°C in an air ovens and 30 per cent after 2Q days at 121 °CO The air pressure aging at 127°O shored ~ retention of 70 per chant of original tensile strength after 20 I1OUrS9 SD] 30 per cent after 80 hours O After 28 days in an o~-~gyg~n bomb at BORIC, the compound retained 65 per cent of its original tens)]= strength. The dielectric strength of a polyeth:y'ene-fi11~d butyl rubber its not high, and is reported to be 289 volts per mi3! --- about tile same as for buty]~ insu:~&tion. Advances were made in the preparation of GR-S For electrical r~ur~oses~ TVilson4 and co-workers report that the addition of about 1 per cent of Q unprotective agent's such as Dixie C3-,ay during the coagul`~-tion of GR-S latex results in a much lower electrolyte canteens in the coagulant The amount of electrolyte gal, estimated from resistivity measurements of a slurry of hater and;the ash obtained from for temperature incineration of the rubterO The values thus ob+,~ined are compared Lithe a set of s+..anclard v.?.lues obtained from sar.^ples Faith known a~,ou.~ts of e~ectroly'c. kali extraction of the rubber is recommended to;~emove.fa~ty acid. The clays it. is claimed' aids in the formation of' a smaller, more uniform' and more paroles crumb than usual permitting better extraction of electrolyte. The deposition of epic- - tro~yto ir1 the crumb through syneresis is retarded. The rubber is said bo be su- ~erior to CTR-S 65. NTm~canizates of salr.plcs processed in this manner showed lower mater absorptions (A.S,T.M. method), and greater electrical~stabilit~- in water, Ten Broeck and June' have reported the development or ~ net-? boll moistures absorptive GR-S (X-361) using alum coagulation from dispute solution (4 per cent latex solids). This technique reduces the soap and electrolyte content of- the coagulum. Water absorptions are reported from 0.51 to I.4 ing.,~sq. cm. in 2-0 homers at. 700C. The change in S.:OC. from soaking in Hater at 70°C and 90°5 is reduced considerably. GR-S compounds prepared from this rubber are said to be superior to those prepared from GROSS 650 A scuds of the dielectric properties of mixtures of polystyrene and poly- butadiene was reported by Boyor and co-workers. The Over factor vs. frequency curves Were found to pass through a maximum at 100 megacycles' and the Sam.. maximum was found to occur in natural rubber. The theory is-;presented that a long chain of conjugated double bonds is a perfect conductor, and a regular distrib-~ltion of double · . .
93 bonds as in a long po2yisoprene chain causes a Metric di.s~ors~on of electrons wher' their transit time along the po'~ner is equal to the freqliex~ey of the applied field. r1 Bonner' has developed a method of testing snail pressed sheet samples of rubber co~.pounds for they r electr~.ca]~ property. es. lie recommends using, l,infoi] electrodes which are assured Or good contact ~.r~th the sample by wetting with pet,:o- le~'m jel].y. Reproducible results Were obtained for lTollune res~st,'~ity7 poker factor arid S O I . I, . No correlate ons between resllits obtained on pressed sheets and respites obtained on ex+~ruded wire revere given. In the i'ield of cond~lct:.ng rubbers Rack and othereg made measurements of the DC cond~lcti.~rity of rubbers containing ShaT`~;n~gan;acetylene back arid Ccntinen- tal. R-40. The resistive ties Were st,ud~ed as functions of time, t~em~erature9 cons certration of black and elongation of the s+,ock. The res~st~vit~r at first decreased very rawer With ages and then Revealed off at a veer sIoyT rate of change. Ttle importance of annealing samples to remove strains wars stressed. Te~nperature co- efficients were positive for Shaw~.nigan and negative for R-40. Temperature CG- e~ficients (defined as ~X: 50°C = ~ C) tended to increase With increasing concentration of black arid with in -~;asf~g elongation The resistivit,.- increased with elongation for R-40, but decreased with Shawinigan at low concent~at,ons, but after 20 per cent elongation increased with increasing extension. Hanson9 has developed a method for measuring the resistivi.~.y c. conduct- ing blacks which eliminates the Usual high electrode contact resistances It in- volves a double elec+,rode system With two circuits, (13 a current ci.rcuiL, and (2) a potential measuring circuit. The resisti.~rity Was determined f' om the potential drop "long a portion of the sample (circuit 2) Chill ~ current was being passed through the sample (circuit l)0 ITanson found the fol.lowi?~g factors affected the resistibility: ]. comporting ingred~en~s~ 2. terse of black' 3. amok Of flexing of the sample g 4. red ative humidity 5 0 temperature ~ 6. age of sample. He foment it necessary to anneal the same to eliminate the effect of fic`~.~, and to store samples in a dessicator before testing. Testing at a starboard tempera4;;u?:e is recommended for reproducible res its. s
94 BIBLIOGRAPHY 1. S=uper 5oronol. Publicity release of General Electric Company' Apparatus Department g Schenectady' AT, Y. 2c The Electrical Properties of BU{Y: Rubber. I. C. Eaton, A. lT. Roberts and 3. E. Sheldon, India Rubber World' 1149 No. 29 227 (1946~9 (Abstract).May, 19469 AoCeSe seeing High Poisoner Foruan. See also ~Jolur.qe X, Digest of I!iterature on Dielectrics, c. 3. be ABP High Voltage Insulation. L. F. H~ckernell, Publication toe C-639 Anaconda fire 8 Cable Company 4. Develo~b~on Process for the Reduction of- Electrolyte in GR-S. E. A. bisons [. ~. Riggs' G. [a ~heelock and B. M. Go bicker' Rubber Age - Vo1. 623 No. 29 pp. 183-:LS69 November (1947 J S. Cow Moisture Absorptive GR-S. To T ~ Ten 3xoeck and R. D. Juve9 India _ ~ _ O ~ - Rubber World - VolO 116, No. 6, pp. 78~-7229 Septe~her (1'P47) . . . 6, Some Dielectric Properties of Bu diene-Conta~n~n.~ P~r_and Cocolsr,ers. R. F. Boyer' :~. BO Baker and PO C4 Woodland. Paper presented before the Division of IIigh-Polymer Physics of The American Physical Society' January? 1947. Rubber Age - -Iol. 60, Into. 49 p. 455 (Abstract one 7. Electrical Tuber. Ro BonnerO Transactions of the :Tnstitutior. . _..,. of the Rubber Industry - Vow.. 23' p. 155-16] 9'' (October 1947) . . . . . . 80 Electrical Conductiv~.t~r of GR-S and M_a1 Rubber Stocks Loaded w it Shaw~L~|acksO P. 13. Sac R. Tto Ar~t''~ony and E. Guth, Journal of Applied Pl~y.sics, box. 1.89 pie' 45i6-~69 (May ].S47) 9. Measurement of Resist~onductive Rubber. A. C. lIansonO Paper _.. ~ it, presented before the Division of Rubber Chemistry, A~r.erican Cheek Cal Society9 September, ~Q,./~70 Rubber Age g VolO 63. NoO 5, p. 576 (abstract only) (August 1947)
