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79 SYNTHETIC PLASTIC INSULATICN R. P0 Boyer and P. 5. ~oodiand The Don Chemical Company Midiandg Michigan Introduction . ~ , The 1946 Tariff Commission report shoes a total production of synthetic resins for that year of approximately one billion pO]~3S9 exclusive of cellulosics and synthetic rubber. There was a time when the electrical industr~t constituted a major outlet for synthetic plastics and provided strong stimulus for the development ·of new plastics In fact' much of the drive for the development of polystyrene in this country came from the fact that this material was an outstanding dielectric substance which.should demand considerable use in the electrical industry. It is aifficult,to tell...~hat fraction of current polystyrene production finds its ultimate use in the,-electrica1 ~.ndustry9 but the fraction must be small. A similar case preva~.s With polythen£,.~hose 'arge scale development arose from an urgent war-time requirement for a dielectric materia.]O Fo~.~ever9 it is.safe to predict that the non- electrica]. peace-.t~me uses for pol~thene are already or will dwarf any d.ie~ectr~c applications. ~hi~e'.~ese't~o important plastics, pol,ythene and pol-~rstyrene9 mere both sti.mulaued'primarily on tile basis of e]..ect.,r~cal properties, yet the character of the plastics industry leas so changed in magnitude and outlook that it mill prob- ably be d~.fficu~t in tine future to encourage plastics developments solely on the. basis of their possible use in the electrical field. Protective coatings' synthetic fibers 9 molding ponders and structural materials are suck. large volume ventures as to attract the major research effort. If the new material' developed for these fields happen also to possess good electrical properties, no much the better. A second important consequence of the growth in the plastics industry is that its.rav! materials requirements aloe so tremendous as to disturb the equil~.br~um of 'Ghe entire c'nemical industry. Phenol, G. R. S. Rubber and polystyrene combined can cons.~me over half of the benzene ~rod.uced, ~h~..le ~hthalic anh.-ydrid.e for aDkyd- 'resins and,for ~htLalate plasticizers requires a.~.arge notion of the available rlaphthalen~e. Various peop]..e are express ng concern over this vital. dependence. of; plastics on coal. tar chemistry, and also on the ::etro].eum industry It is signifi- can+ to this situation that Nylon production has alr~es.d;y turned for one of its major raw materials to furf-ara' g an agricultural cl~e?nica~ O Genera] Articles Kline3 has prepared his cu.s~vomary anml.a] review articles which attempts to give a broad picture of develo-.~ments in the ent re plrsti.cs field during 1947. Kline4 has al so presented a gener`ai revile article concernln~ recent developments of plastics for use on corrosion resistance equipment. Mar of the nester plastic materials, as ;;'el~ as the snore standard ones, fire countered as to properties and acrlic,~-~tions. ' ' r
so An excel:l ent, reviews ent' tle.d "pylon 2nd Other fJonce.2 lu? os7 c Synthetic Fibers'' by Ruth Peterson, is to be recommended because f hers ser-'e not only as Ire covering and on ~ aminat~es for Eke e~ ectr;.~3 industr.,r9 but gene-ra~3y those sy~thei,ics which hare desirable properties for fibers may be of direct interest as molding and extruding materials for elec-~rj.c~al ~nsu3=t~ors. T-n:~s survey covers both the mote generally ~ ecognized synthetic fibers (ny].on:., hi n"`T;on9 Area on ~ g3.ass, etc ~ ) bill al so gives the available background on the never fibers. (acryIoni tr~.le, Ter~y].ene . aig~nates, etch An excellent book g "The Chili strv o:E Coi~nerci.21 Elastic s t, by ~lakerr.a~ i is scholarly and well writtan9 and gives a broad. pi.ct~we of the entire pias~1cs iridustry include ng the .nerier plastic mater~ais. ,.. . . .. . A report from the IT, ,, Pureau of Minxish on ti~:oxici.~: and inflame Resistance of~rTher~iosetting Plastics' is obvic~usl~y of consid.erable i~Aportar~e to the entire insulation field. . Reports from UO SO Fleet un~.~s concerning battle damage :suggestied that., toxic fumes are generated! then tt~er~iosett~ng ple.stic ~ns<~]at~cn ?s.burned or . decomposed in enclosed spaces. La-~orat,ory .f] ame-~esistar3ce tests modified to study -these toxic f~lmes9 re~ea'ea t,k~e fo310t~in~ factse Ph=~1~,c-~t,~?~-ics-fa-~c fi3~1ed. materials end phenolic~fabric-cotton fil~edmc~.ded~nauer~a.ls give off rollgh].y equal volumes of NED,` CO, and c,Fan,.de~. Iibe quantity, of &11 three- toxic gases Was greater for me].am~ne-m~neral filled molding materials .-7hich d;.ri not ignite, than for the :?heno1ic-n'.inera1 f~.1led materials. For the same torte of filler, me1an~inc;.s veered more flange resistant Chars -~ohenolics,` for the same -e of desire' inQrgar~ic f~13.ers pros vided more flame resistance than organic fillers, A methq] was described fcr`,det=rr:~ning the surface ten:~e~rat.~.re.at the moment of ignition of rig~.plastics;~'. ~lamm.abi.lity data are r~resented..for over twenty~five different materials uncle' a variety obtest condition.s. A n-~okabJe mechanism of ignition is included in the discussion. A Literature Survey on the Properties of Plastics Infl~encl.n~ Their Stabijity in Storage" Zla.s been prepared tv members of the Battelle Mc~o;-ial Insti- tut~o C~timum storage conditions, as extra~.ted from publ.shed.~.~"y.$~l nroper- ti.es, are. an oxygen-free atmosphere of 50-70 per cent relative..hum~.oi~,~yg and tem- perature between Roland. SO° F.' the absence of '.ight9 and a minions of cyc.~.-icai variations. Fore rea1~sti.c cond.it~ons are given as ,~:ir9 50-80 per cent. re1ati`.,e hum~dity9 30-100° F., absence of light and a r.r~inimum of cyclical ~rariciticn. Delmonte1O has presented further results based on his ph.ot~dielect.ric arat.~;sis of plast~icso In this method ~ transparent plastic its v-.;e~,Ted by colariz,ed light as increasing voltage ~ to tile dielectric strength or the material is arrI.ied. Strains resulting in the material, from the high voltages and. the effects of mechan ~cally induced strains con the dielectric breakdown can Act? fo]1ored. Eventually sac'': studies should lead to a belter un.derstandirig of insu]a.tor design. Standard P3 ass'' c ~ -~qateria] s ~.,~_,~ ~^ ~ _ ~ ~ _. Pherlol:Lcs. The stiriLnka(~;e Cal various pLeno1ic Mastics d~.ririg prolonged ~ . ~ heating It 1~° C. D8.S been investigated.- 1 A 30 minute preheating of the mod cling panders at 90° C. gives more beat-s+..a'n].e moldings than parts mol.der5. from the regular pander as received from the manu~actu.rerO
gl A study of the dielectric properties of pure and modified phenol-formal- dek~rde resins in the frequency range 10: circles to 2,5 megacycles has been reportedi2 The power factor and dielectric constant are lowered substantially by the benzy~a~ tion of the hyc~rox] group or by substituting an alkyl or ary] group either in the methylene bridge or in the pare position of the benzene ring. A study of the rate and event of cross-li.=ing pheno1.-Porme.~deh~J4e resins has been made by the joint use of electrical resistance and density measurement The former is sensitive to internal molecular arrangements while the latter appears to fcl1 0757 the gross properties of the resin. Comprehensive articles on fillers for phenolicsl4 and for ureas and me]a _ r mi nes- -7 have appeared. Mood flours, cotton, asbestos, sisal, di~atomaceo~.'s earth, and sheJ1 fillers are among the types discussed. . ~ . Cellulose Derivatives. Th.ermo~lastic laminates have been prepared by _~._, .. . ~.~ ., . adhering molten cellulose acetate or ethy~cellulose to fabric sheets and then mold- ~ng several. of these sheets together into the finished laminate. The toughness and 'neat resistance of these thermoplastic laminates has found use in electr~.ca].. hous- irgs9 solenoid covers, etc.1 Ce]lulose acetate propionate has recently been offered for sale under the trade name of Penile III.17; This material is said to have better impact strength and dimensional stability than.cel]^ulose acetate and less odor than cellulose - acetate-but~yrate. ~ general article -reviewing the history of ethyl cellulose stresses the part which this material played in the VT-fuze program^3 and how this program lead to a bette~gand more general knowledge of the molding cha.rac;tel~istics of ethyl cellu10se. Acr:;lates. Plexigas A is an acrylic injection molding powder With a heat A__ ~ . t. distortion temperature of 90-94`' C. It possesses improved ~oldab~li+vy by virtue ~5 low f1c,~r temperature and. yet,, it shows little tendency for shr..nkage up to lOOiC. PolYstYren.e. A mod.if~.ed po~yst~rene9 knowers as Lustrex shores a measured _ . ~ .. ~ heat distortion temperature of S7-~° C., but is actually able to withstand bolting Hater for short periods. Some lmproveme~G,-'n toughness and a 30%.reduction in molding cycle are additional a.~vante~esO A copolymer of styrene and fumarodinitr~le is claimed to have higher heat resistance and better solvent resistance than polystyrene This patent is possibly the basis of Cerex, a high heat distortion s Serene base material Chick has been mentioned in previous issues of this Digest. ... ~ ! ~i The perennial 'out never too promising subject GO plasticizers for polysty- rene finds expression in a patent Filch mentions alk,yi napkthalenes where the alk,vl chain is five carbon atoms or le.ssO
~2 Pclyvin~1 CarkazGle Resin. The general. properties of polyvi.nylcarba7Jole have been known for some time - particularly its high heat dis~vor~;on temperature and excellent electrice.1 character.-,.sti.cs. Its v~ar-time development under the name of Polectron as a mica replacement, leas been mentior~ed in nrevicus reviews. A recent arti21e describes monomer properties end nietliods of using 3.t to impregnate condensers. 3 The polymer itself suffers from bri1;tl:eness' although compression moldings of pl.astici.zed pol--yvinyl carbazole containing or~ent,ed fibers of flare polyvin''1 carbazole shoe.' improved t~ougI~ness. Just as T?ll~h polystyrene, the presence of a plasticizer lowers the flow tempera+ure9 but does not: increase toughness. Silicones. A plastic dielectric comprising a major Portion of a liquid polymer of an organosilicone, eg9 dimethy' silicone mixed Ruth p~ inorganic a~rogel, is used in aircraft ignition-cable to prevent corona discharge.~~ This its a 'i~is-~ori.c patent whose importance cannot be gauged by the simple description above. not only did it solve an important aircraft ignition problem for the armed forces' but9 being perhaps one of the first commercial appli.cat~on for silicones' it gave con- siderable impetus to the development of this indust.,ry~. Recent developments in the use of silicone resin DC2103 as a heat stable bonding agent for fibers and fined divided ponders such as Ordered metals, micas SiO2 have been listed. A generate review of the silicone .¢ield has been given. The Navy27 in its search for more ordeal electrical. insul.at~.on is nor encore aging development of silicone-glass laminates as high strength' non-toxic, heat and arc resistant materials. Thile some mechanical properties of the present silicone laminates are inferior to those ob+,ainabJe from melamine-~ass cloth laminates9 it Was felt that most of tI:e outstanding problems could yield to further research. ~ , .. -Polvtef,raflurorcethylene (Teflon). Inoreased Production, bore experience in fabrication, and greater appreciation of its chemical inertness' held resistane,e Arid dielectric strength' charact,er~ze recense developments with Teflon. Shale it,s electrical properties are ideals yet its remark&tile chemical resistance has perhaps Excited even more interest for use in gaskets. New Plastics Te~Q_ene is ~ crystalline polyester prepared from terephthalic aced And ethylene glycol (or their equiva]~ents.) ~hil..e it is being protected primarily As textile fiber material, yet like nylons it may find use for wire covering and as a thermoplastic molding m&serial because of its heat and ligh' stability chemical resistance, and low moisture absorption. 9, 30, Al . . . . . . Dually] Phenol Phoschonate. This interesting monomer can be co~ol.~ymernzed with other monomers such as methyl methac~ylate or styrene to give both cross-li.~:ing action and flame resistance. Polymerized by itself' it forms an insoluble resin With a refractive index of l.57O It is-said to form transparent laminates with glass cloth.32 A general discussion of the monomeric and polymeric bent aky1 ethers has been presented. These polymers are finding use as plasticizers and modifiers for other polymers, and as aid. in latex dip coating applicat~.ons.
83 Stable polymer oils, greases and waxes have been prepared by the poly- merization of chlorotrifluoro~e~G_~lene.34' 35 Electrical properties were not. reported for these materials. High polymers of this type were not discussed although one would suspect developments along this line to be in progress. Such a polymer would be similar in general structure to ~oly~in.~l chloride without the nossibi~i.ty of PCI evolutions and it would constitute a modification of Teflon, probably in the direction of greater ease in molding and inferior dielectric properties. A patent, claims that in in.teroo3~ymer of ethylene and chlorotr~fluore-~- lene in 1:1 mol. ratio this a higher softening point than that of either component when pol',r~nerized.alorle. A copolymer of tetrafluoreth..,vlene with SO2 is said to have a lacer melting point and improved workability as compared With Boll etrafluorethylene.~' The dielectric proper"' s of ch].ori.nated po],ythenes in the range from 104 .3.j ~ _~ __ a. . ~ ~ :~ . . ~ . ,,~. _ - to 10~ cycles has been reportedly The increased rigidity of the material resulting from the addition of chlorine is reflected in an increased dipole relaxation time. It should prove extremely interesting to follow developments.in the family of compounds: Polyethylene, chlorinated ~olyethyIene9~ pol~,rtetraf~uoroethylene, Polych2~orotrifluorethylene' tetrafluorethylene-S,Q2 ,copol~,mers, . and the trifluor- ettl~rlene-ettlylene copol,ymers. 'Cr,~r,stallinit,T, solvent resistances high meltir~g points and dielectric properties can ar)~arently,be.,,controlled in a region of' im- portance to the elec~tric'd;l;'industr~r. ' ' ' " A brief note on a new method of synthesizing extremely high molecular Height proteins by an ionic chain polymerization method has. occasioned considerable lo. _.. ma., .~ A_ . . . . . . . . .. interest among pl.ast,ic chemists. There is annarerlt~v 'no cQmmerci~;3 activity in this field as yet, but such developments may '~11' be anticipe.*ted. Wire and Cable Nation ~· ~, , ~ discuss:.on4° of tile voltage characteristics of.pol~ell~tlene cables has been given2by Davis, Austin and Jackson. They have made an experimental ~nveStiga- ti.on of breakdown voltage, Dower factor, life with pulse voltages, and discharge characteristics at power frequencies and performance Edith 6~)0 me radio frequency pulse operation. Tentative voltage ratings based on this work.are proposed, A .patent4) has been issued for a coating of high dielectric strength Bale from ~ solution of l-5 carts of polyLutene in 30-2C parts of benzene and .2-5~ of dodecy1 arr.ine acetate. . . . . A tab]:p has been publ-'shc36.42 showing the comparative properties of natural. arid synthetic polymer used in ~nsul~-..ted. afire. Polymeric Plasticizers Po1:Trneric plasticizers have been mentioned in previous issues of this review but the past year has witnessed ~ considerable amount of scientific and
AL industrial acti~rit,7 in this field. Perhaps the first co~neric.~1 use of a polymeric plasticizer novas the ~nccr~cratior~ of ~nol~vrisobutylL!er~e with polystyrene to form a aemi-flexible insulation for coaxial cables during the early days of the car before clythene became avai.laLle. Polvisob'~t,y].en.e was also used as a r:L`ast:.cizer or softening agent for polyethylene. Toward tile end of the car, polyester resins were used ~n the vinylite jacket on coaxial cabbies to~replace conversional pasters Which migrated into and ruined the dielectric cro~er+,ies of tee po~yi;lene cable core. The Federal significance of this deve'onr~ient to aid. types of `~-asticizec problems was soon realized with the result Chaff an i.1nt-Jortant new trench in p3.asti- c izers i s row eve .; ~ An exact definition of a polymeric plasticizer has not been formulated. The following somewhat arbitrary def~niticn V'7il] describe the essential features: an inherently flexible polymer or copolynner with ~ molecular wei~g.~t in excess of 5,000-IO9OQO such that both the vapor pressure and the diffusion rate are negI-~ble. In general, a cot~b~natlon of flexible elements along the chair (met,h~ylLene or ester DLink~ages) to ensure 1 sow temperature f ~ exibi~it~, and of polar groups f -G = C, -C N) to promote compatibility with the polymer: appear to be necessary at leas' with the vinyls. The three types of polymeric pl$st~'cizers*which haste received most attention; thus. far for use With polyvinyl chloride are . ~ . a) Polyesters43 b) ~Copoly~ers of but&diene and acrylonitr:~e44s 45 c ~ Copolymers of styrene and i sobutylene46 The major di~ficu:~ties inherent in the use of such pl.asticizere are: al Difficulty of i.ncorporation:~:n the r301y-.er. b) Not as efficient, on a Weight basis, in Ic~b;7erir~g the brit~3.e temperature as are con~rentiona] ].o~ ~no.lecul.ar weight plasti.- cizers or the same chemical s tructu-re. c) Tendency for phase separation Beading to loss of flexibility of the plasticized polymer. d) Sensitivity to light and oxygen of the o~efin linkages in butadiene contai.nin..g polymeric plast,.ci7.ers. e) The apparent necessity of using small amounts of I molecular Weight plasticizers to supplement the action of the polymeric Plasticizer. Some of these difficulties will undoubtedly-disappear as more experience is gained in handling such Plasticizers. They should not detract from the ~m~cbr- tance of this new development. For examples 1 '0 5,ci of tet;raalkyl thiura~ d~s,'~fid.e is recommended as a stabilizer against the effects of li-ght~ and air for v:: nay resins compounded ~,~.th butadiene acrylonitrile polymers . 47 ` , ~
~5 what is prot;ably one of the basic po3 Anemic p3 asticizer patents clad me a linear polyester of 1,2-prcrylene glycol and sebaccic acid havlug a molecular weight between 1700 and 20,000 as a non-m~gratlug plasticizer for pol~-in:~]ckloride, polyvi.ryl acetate g or copolymers thereof a ~ A spectra' var:i~t~on on the po'ymoric plas+,nc~zers consists in using a ~Tin,yl chloricn~e--20 per cent vinyl acetate copo~yr`'er of molecular weight S,OOO - 159G(") as a processing aid for v:inylite co'co:Lymers containing up to 7 per cent vinyl acetate and haling molecular Eights in excess of lS,O00.5° Internal po]~neric plast~ic~-zers have been used to toughen phenol forma]- deL~Tde resins.51 Polyolefins9 or 3or molecular weight, vinyl or butadiene pol',~ners, or fatf,y acids are condensed with phenol in tile presence of BF3 to form compounds having trTo or more phenol groups per mo~ecu]~. these in learn are condensed with fo,~ma]del~yde. Con~renticnal inert r?lastic~ze~rs added to phenolic resins do not cite elastic products v'Pe'-eas the interna:l~ly lineal. p~l.ast-.c3zers appear able to take d=-~:Corn~ation forces elasc~cally and reversibly. A symposium on conventional type plasticizers has served to review this fiel¢2and bring out some special work performed for the armed services during the war.' ~ dielectric idc?~..tit; test has began developed for plasticiscrs of the three used in polyvinyl chloride, 5 Reprod.uclble loss factor-ten~perature, and dielectric constant~t~mpera~ure curares Were obtained at 70 megacycles on a number of typical. plasticizers. D; c- ~ ec~tr~ c Tahiti nil The FCC on Dece~ncer 3, 3,947, altered the frequencies alloca.t,ed for in- dustrial dielectric heating use. It non has provided the frequencies of i~.569 27.12 encl. 40.68 r^,ogacyc~es for this use. On comparing the reed allocations r?i.th the old, it will be observed that the new ~requer~cies differ only slightly from the pre~r:~ous allocations. ~n addition bc these three Preq~enc~- banish the commission has also allocated a nu~nb=~ or' frequencies in tier? ~ltra-hiph frequency region. Re.diation Chicle may be produced by electronic heating equipment can inters fere seriously r~it,h radio con~rnun~cations' thus posing a problem for both industry and the Federal Communications Com~r;iss.~on. This; along raith other !?rokJ ems, is discussed by the ~{,F,,~. subco~i~i.t tee on e]octronic 'neati.ng and. reported by'--. CO Rude. 54 The Pritish55 have published a repour on the in~erferer~ce from :industrial radio frequency heating equipment. T'ney "Lnvest.ige.t~?d Foul different p:.ecc;s of appa- r~J~us ranging in pouch -Strom 2.5 lo 45 T`~.r and operating corer a frequency range from 600 KC to 20 MCo For the study of the application of dielectric heating in various indus- tri.al processes ~hi+,ehead5 suggests t:Lle use of a calorimetric substitution method for the ~Teasuroment of losses in dielectrics.
With the ever increasing demand for greater production and speed, the use of hi.giler end. higher frequencies is being pursued. The ''nor~'ledge and teehni.ques gained in racer Fork during the solar are now being applied to radio frequency, heating as a means of increasing heating rates. In a recent article57 by Farce and R:nn, techniques are discussed for generating and handling microwave energy! Methods are suggested for utilizing wave guides for applying microwaves to moving or sta+,io~lary hires and threads, sheets or irregularly shaped objects to achieve uniform diel.ectric.heat,~ng. A survey of tubes offering possibilities for continuous ope^~.~ion at n,.icro~re frequencies is also givers. Development of a magnetron and horn antenna technique :for Cookir.g has resulted in a practical. design of a mode:! having an oven 13 inches wide, 14 inches deep, and 15 inches high.5 A number of these.units are non in commercial use in restaurants throughout the country. Perhaps '=~its of this general type could be used for high speed heating of p]-astic'pre~orms. Force and Bevercomb have described59 a magnetron of the continuous Have type capable of providing five kilowatts of output poller at ],000 megacycles. Brown and Baylor have published a kook6° on the theory and application of radio :frequenc~y heating. The authors have placed .~.n ore volume the basic princl- ,ples' design equations, and practical applications data for both Induction and di- e].ectric heatnugO Eight chapters are devoted to tile 'nearing of poor conductorsO 'l'hese include wood gluing pressed radio ~requenc~',~ se~'i.ng mach.;nes.for sheet , .- ~iastacs, ref. dehydration' pasteurization, ster~.~ation; ccok~ng of'food 'end the '' heating of drugs and other materials. . . . . · . A. P. Pock exclains61 the.factors involved l.?hen.heatirlg a substance by induction or die2.ect,ric heating. Examples are given for calculating the power re- quiremer~ts for the heating of plastic preforms. Si~ilar'Galculations Here also made by Courval. ~ ' Finder outlines63,the fundamental principles of induction and dielectric heating and points out ~rario~ls.gerleral types of operaticns.~rrhere Induction or d electric heating can be Used to save time, crest? equipment and materials . ~. A British patented has been issued claiming Improved dielectric scaling of.' superficial ch].orinated po].yeth~r].ene films. ~ , ~ . ' J. F. Capner65 discusses methods and advantages c!f automatic loading control in generators used in the dielectric beating.~.eld. ~ .'..',, .. ' . · . . . . Surface Effects An OSRD,reporu (.-i'To. 5324, PB-11955) from Johns i~'opki.ns Un;.versi+.y has attempted to collect from! various government and indvstria]. research.laboratories data pertaining to the effect of moisture and fungus on the electrical and mechani- ca3. properties of plastic ~ns~.Jat=ng materials. As the author recognizes, the data avai3.able to him at the time of the -deport (Cctober, 1945) was too incomplete to
~7 a]]or: many general conclusions. Most of the data presented refers to filled pk~enol- "ics, but, there are some data on r.~ethy~ methacryxate and glass bonded mica. ^ti stud.-r66 has appeared on tile surface conductivity of mica, glassy methyl met7~clc~vlategpolystyrene and pol~risc~butylene, in vacua and in the presence of acid9 alkaD!i and. range of water vapor concentrations. No surface conductivity is observed in vacuum even With aced or alkali or the surface. Mica9 methyl. methacr~rlate and polystyrene shoe: no surface conductivity up to saturation Linen distillated Water valor is used, but minute traces of acids in the hater vapor do produce conductivity at high rater vapor pressures. A nerT surface coating material for polystyrene, known as Logoquantg its reported. to give an l8% increase ire scratch resistance, 14% greater transmission of incident..light,g ~ reve-rsec] electrostatic charge' and better resistance to to~uene9 carbon tet~acI]oride, and gasoline. This coating, which can be applied by spraying Or dippings becomes integra'3iy borld.od~'to the base polystyrene. The coating material is a}?7~are--ltly a cede derivatively A study of the mechanism of current creep and tracking conclud.es Thai; an irregular dis~;ibu.tion of surface condtlcti.v~ty is necessary for the occurrence of current creep. a A stud, has been made of contribution of moisture to the high frequency e].ectr~cal characteristics of, a synthetic resin. About half of the Icss angle at room temperature is ascribed to l~ygrosco~icity.69 Pott~.n~ Conmounds fifth the increasing produc+~..on of home television receivers, With their I~i.gh voltage components, there should be increasing activity in potting resin A. r.t'he JAN T-27 specs. covers the testing of potted transformers. The follo~eir~g has been given70 as the best National. Bureau of Standards casting mixture. 33 . ON 2- 5 di c h] oro s Anyone 2] ~ 5~ poll ~rdichiorostyrene 21. 0% st!',~rene monomer ~ l. ~polystyrene hydroge:rlated terphenyl di viny]: of 60~ strength
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