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SYNTHETIC PLASTIC INSULATION P. C. Woodland The Dow Chemical Company Midland, Michigan Tntroduc~Gion ~_ The end of the war and the ~ if tiny of travel restrictions hes made possible a larger number of scientific meetings. The Committee on Chemistry of the Ne-~onal Research Council had its first meeting since the start of the war, at enrich a great many balers were Presented on diel ectrics. The Ameri can Chemical Society els o had a sy~r'csium on electrical insulating materials nt At? antic Otto. "New Plastic Insulators'' 'yea the tonic of discussion et the 19th kneeling of the Electrochemical Society. Some of the ~na30r clevelo~.ents in plastics insulation are re~rie,-,red und.er the Boll owing subtopics: general Articles The ory S4Gendard Plastic Materials Silic ones lfire and Cable Insulation Di electri c Heeting Genera] Ar fiches .~ Plastics are useful in electrical applications because o' they r unique combs nation of Properties, the most important of these being good dielectric properties, mechanical strength, end ease of fabrication. Whit e no substantial change in di- elec~cric ~ro~er1:ies of existing plastics is anticipated, constant Progress is being made in improving their mechanical properties and their heat resistance. Several Caters have been smitten re- vie,~ing the el ectric~] r)ronerties of ~1 estics and. their use as _nsulating materials)--~. A book has been Published Tiering ~ sure ear of the r,1~stics and their n~1 Scion in the electrical end radio irilustry~e Light Eight, high dielectric strength, lo,Ar ,.r~ater absorption, end resistance to attack by chemicals are ~ one of the ad v~ nt ~ ge .n c ~ aim ed f or Ala ~ t i c Mimi net e ~ )0 . Kit i ne has Win reviewed ~enera.I developments in the entire Stick lie Td e Met;~.l s~re.yin~, c~t;hode sputtering, metal erosion, - 90 -

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meccas hoarder ~ acauer, and chemical reduction fire methods outlined ~ ~ render ~ Gastric concluctive72. Prectical procedures for com- merci~l production of silver deposits on ~1 emetics are given. The cry A report of discussions at meetings of the Royal Institute of Chemistry with the Institute of Physics and of the Faraday Society has be en pubs ished 3. The former surveyed the Fiji el cts of physical theory, chemical preparations end industrie ? aged icons; the Aver debug ';rith surveys and origine'.l darers on the Resent state and irnunediate trends of ~hysice.1, che-~.ica3, a.~d physic- recearch. Two Unmoor ~.n&c genera.] tonics were considered; one c one erned the theory of the internal fi e] ~ of die ~ e ctrics and the others discussed advances in high frequency dielectric measure- ment.s . \~en a. diele~ct~ic it placed in an atmosphere of 100% relative humidity, an ionized film of water forms' hose conducti- ~rity at the end of one minute is within a factor of ten of its en; ~ilibriu~ values This equilibrium conductivity ranges from essenti~.~ y zero for certain hydrocarbon praxes, silicone resins end silicone treated glass to 100 micro-ohms for ordinary glass and Quartz. The ionized wincer film els o produces interracial got ~ri~.~tion which effects ~ marked increase in both capacitance only dis~i~.tion factor at audio frequencies)4,15. Two haters have been written that are concerned Erich the ~Gheor,y of die?lectric.s . One Jisucs ses the behavior of electrons in crystalline solids with particular reference to the properties or dielectrics. In insulators the occupied energy bands in the atoms are compel Ethel jr filled, :~.~i' e conduction in mete.1 ~ i ~ e.s ~ oc fated ~ th inc obsolete I,y ~ ill ed banes . Di ele c tric breakdown occurs ashen electrons are continuous] y raised to higher energy 1 evels until internal ionizati on occurs . Dies ectric strength increases moth t,em~erature up to a critical tem~erat~e t which colic signs between electrons become important; dielectric strength d.ecree.ses with further temperature increase)6. The sigher Hater delis trith ~ auantita4Give theory of . , . the dielectric properties of crystalline solids consisting of long chain discolor molecule es ~ In these solids -the dipoles are ~ . . . c oncentrated in dipole plane s . In the ground state the di- r~olar planes have ~ Permanent polarization, but usually the boo arizetion of successi~re Ones has opposite direct' one. The static dielectric constant rises with increasing temperature up to ~ critical temperature end then decreases. At T the eub- stance has a Chase transition of the second kind. For chains 'with an even number of carbon atoms, metestab~ e states with r~e'~nenent boot e.riz~..tion are predicted, and a method to reach there states is discussed. The interaction between. dipoles ,?1 e.ys ~ Bred omine.~Ge role at ~Gemceraturee belong To ~ It is chosen thet Lorentz' ~ or Onsager's methods are invalid in this tem- ~erP~tu-^e ranged ~ _ 83

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Another paper gives the properties of plastics as a function of their molecular structured '. The absorption of microwaves by various polar gases has been studiedly. Investi- gations have been made of polarity in hydrocarbon possessing conjugated systema20. ~ article has been published giving the effects of high humidity and fungi on the insulation resistance of various plastice21 . Relations hairs been given between the power factor and the temperature coefficient of the dielectric constant of solid dielectrics. rnis ratio at a given temperature and frequency is nearly the same for most solid dielectrics22. Standard Plastic Mat eriale ~D~ The dlelectrlc constants and power factors of pure and modified phenol _ formaldehyde resins have been determined over a wide range of frequenci es23. The dielectric constants of both cured and uncured resins decrease gradually with increasing frequency. Ouring the resins reduces the dielectric constants ~0Ut 9,` over the entire range. The power factors are increased at low frequencies and decreased at high freauencles wl th hexamethylenetetramine. Lee power factor and dielectric con- stant are reduced markedly by substituting alkyl or aryl group e in the methylene bridge or in the pare position of the benzene ring ~ 5~ e. Polytetrafluoroeth~rlene or Teflon is a comparatively new plastic being manufactured on an experi- ~nental plant scale. Arc resistance and hoer electrical losses over a frequency range of 60 cycl es to 3000 megacycles, combined with high heat resistance, suggest the use of this plastic as a dielectric in coaxial cable for television and radar. Because of its heat resistance and unusual chemical resistance, Teflon is also highly suitable as a packing and Casketing material~4~25~26,27. Poll. While polyethylene does not have the unusual chemical resistance or high heat stability of Teflon, it does have comparable dielectric properties It also has an adder en_ tage of ease of fabrication. On^e article2 states that the power factor decreases for wavelengths below 10 centimeters. Poly- ethylene can be used with confidence as a low 109s dielectric at one centimeter wavelength. A graph shows measurements of power factor versus frequency over ~ range of 105 to 2 x 1910 cyc~ies' Many papers have been written reviewing polyethylene 9~30~ 3 Pol st ren . One article describes in detail (with 90 referen~anufacturing and properties of styrene and olystyrene33. An extensive study of the effect of different _ 82 ~

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impurities on the electrical properties of styrene monomer has been made. A moderately high vacuum and the presence of an inert atmosphere during polymerization is said to reduce the el ectrical losses34. The advantages of polystyrene capacitors over Chose made from mica End painter have been pointed out35. Styrene may be conolymerizeS with fatty acids and oils Go prom duce a variety of coating resine36. They have good electrical properties, particularly with regard to anti-tracking. A privately printed monograph on styrene in the form of six individually bounty chapters has appeared. The first flee concern styrene monomer Physical Properties, Chemistry, Analysis, Managing and Polymerization`. The last chapter covers analytical methods for Polystyrene36a. The present interest in ~olydichiorostyrenes stems from the fact that this new plastic combines high Heal distortion with good electrical and physical properties5 ,3 . ~QQ~ Although silicone products are relatively new, the plastics industry is already familiar with the general nature of their compositions as a result of the wide publicity given these materiala. The dielectric onetants of dimethyl si~oxane polymers have been measured as a function of temperatures ~ The results, together with density and optical data, have been used to calcu- late the dipole, atomic and electronic pola:~zatione by means of the Onsager_Rirkwood theory. Dielectric constants have also been reported for a series of liquid linear and cyclic poly- methy~polysiloxanes40. When industry needs a met Eric that will withstand a wide range of operating temperatures, there is a good possibility that silicones will be accepted as the final choice. Specially notable has been the growing acceptance of silicone rubber, the youngest of the silicone family4~42~43. A Russian report emphasizes the importance of organo- silicon compounds for inaUstria1 uses in making polymers and lubricating oils44. Ike advantages of these compounds lie in their high boiling Point end low freezing point, and the small effect of temperature on their viscosity end electrical properties. _ 83 _

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Silicone coatings for '~ire- wound resistors are re , boated to halve for thstood repeated 275oC. the real shocks and immersions in hot Id col ~ sat ~ crater Hirsch out cracking, crazing, or r~eeling45e [,iould di~nethyIsilicones have been used to oro- dolce water-re~ellent surfaces on g\RSS insulator bodies. The resu1 ts of tests on heated and unfree. ted glass insul ators are discussed in detail460 Silicone resins have been developed and Produced for . use es a heat s~Gable bonding material for inorganic fabrics and ~i] ~ ere ~ The properties or these ~ aminetes suggest numerous explications. The construction of motors for service uncler ad_ verse conditi ons cat ls for heat-resistant insulation. S1 ot wedges a: si]4cone_gle.~s laminate together With Icing strips, slot liners, arid tenoning] boards provide insulation superior to that HOYA! used in C1ass ~ insulation. The flow dielectric loss factor, high dielectric c trength,, high incus atiort re Distance, and good arc resistance indicate uses in the insulation of electronic circuits Where the above requirements along Keith high heat resistance are reC:uired47~4S. Silicone inflation for electrical machinery has the s~'rant~ges of less weight, operation at high ~ce~eratures, chemical resistance,, and longer service life . Applying silicone resins to insu]nting materials makes Possible a ~re~gh~G reduction r)' Bt3 much e.s 5040 in certain General Purpose ~notors49,50,51. Be~reral parers have been glibly shed giving the history, structure, synthesis, and Shorties of silicone resins, oils, greases, films and rubbers ~ Wire and Cable Incus ati on . . . With the end of the war) the demand for high frequency coaxial cabs es has been grew. tly reduced, although the constantly increasing use of the higher frequencies for frequency modu- ~ation,, television, ratter, aircraft communication and general research requires continuing interest in low loss coaxial cables. The Russians have de~re1~ced a near material for coaxial cables called ''I:sc?rc~n" '~hose died ec`,~ric ~ro~e-~tie.s are said to be ~ over i or to tho s e of ~ of ys ~ ;' er; ~ ~ ~ t is me lie by tr e at i ng stadium Polymerized buta~ie6e for several hours at 200C, and fifty atmospheres fires sure 2. A ~ ar' er ha s be err pubs i shed go viny the r e~Lati ve eri of ~ number of plastics Band other maters ~ ~ for use as ~ro- tecti~re ,rokets or covering. for Rich frequency transmis lion lines. The lest Procedures for the evaluation of these mat;e-ris' is ~] so nu,JI1ne<3L63. Vinyl resin production rose from 1S,000~000 o~nd~ in 1940 to 125, 000, 000 wounds in 194S. P] ~ aticized dot y_ Cindy ch? oride is of interest to the spire a~d coating industry. The inmost c~,mm.on ~1 rsticizers used are tricresy~ r~hos~hate end _ 84 -

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dioctyl nthelate. Polyvinyl ch~orld.= is a superior Ire coating for use in to onical climates. Te.bles are given shooting the hyslce1 Cedric and electrical ~ro~e-~ties of polyvinyl chloridet4, 65 ~ Polyethylene is the most Tritely used synthetic di_ electric in low 108B, high frequency coaxial cables66~67~68. The properties of butyl insulation for refire and cables has been diCcuesed69. ~ namer has been written70 Which revievrs the available insulating materials for wires and cables in the com- municatione field.. Tt tabulates their physical and electrical pro-.. ~ ~ f or a Aide range of a.~1 ications. The principles used in calcul ating the temperature rise in cables which are required to transit radio frequency power have been considered7~. The theoretical attenuation characteris_ tics end Coverer ratings of a number of standard radio frequency cables hare been set down in graphical form for a Ire range of. operating frequencies. The ratings are based on ~ maximum tem_ literature rise of 3000. in an ambient of 55C., established in a matched line for a steeply state condition. Methods of measuring cabl e attentuation by insertion loss have been briefly described72. Measurements at frequencies from. 120 to 420 megacycles Carl be made directly and can be ex_ tended to 3000 megacycles by a heterodyne system. Characteristic impedance is obtained at 100 megacycles by a resonance method using a Q meter and standard calibrating resistors. Several other test procedures for t7he e~re.luation of syn- thetic insulated cables have been published 3~74,75. The design dale a. f or be a de ~ c oaxi al ~ i ne s has al ~ o be en gi ire n76 . sass A Dater presented at the National Electronics Conference . i. gave a discussion of microwaves and their possible use in high f-reouency heating. Limitations anct advantages Crepe discussed and nossib~ e applications given for the use of ~nicrowaves e.ncI their associated circuit elements for high frequency heating purposes. ~77 The recent proposed frequency allocations for medical .~ and industrial generat,ors by the Federal Communications Commission emphasizes the problem of c onstent frequency generator, . The changing loach during a heat circle makes some sort of frequency control essential to avoid interference Keith communication ser~rices78. For small generators an economical solution con be found by the design of e. self_exc4ted oscillator Keith high capacity tank circuit or a crystal controlled oscil lator 'filch a power e.m~1 if ier. In the case of high power industrial generators other solutions must be found. _ 85 -

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The constant frequency reGuirement bores intro problems namely, to maintain a. constant frequency of operP,.tion Id to force the generator to this frequency. Systems are described 9 in which the acceleration of the correcting means is ~ro~ort'ona~ to the nagnitude of the frequency deviation, thus providing close fre_ quency regulation with out o~rer_regu] ation. The problems intro- ~uced by the necessity to maintain constant power output of the generatc~r are discussed. Suf f i c i ent at t enua.ti on of th e r adi e-tion fr om h igh pourer I, frequency generators is obtained by the use of ~ double shi.eJ5ed room constr,~cted of copper or steel nether A method I controlling the moorer of high poster radio frequency units hate been descry teed. Control of the output voltage is obtained by varying the Chase of an auxiliary at terneting current sUpT,ly on a tnyre.tron grld81. An article has a.Iso appeared stating some o' the ~roble~P.s in the design of high frequency heating equipment82. ,im~le analysis of a tre.nsformer coupled circuit is given and the advantages of close and loose coupling compared. A generator can be made more versatile if lotus impedance tank circuits and cose count are used. The old method of heat see~irtg plastics or Alas tic coatea materials involved the application of heart from the out- side by means of hot dies, bars or rollers85,84,85. The heat meal ing time thus depends on the heat conduction rate and the C O`31i~g period New equipment has be en developed that no longer rely es on heat conduction but on the Cation of high frequency electrical energy which au' ckly raises the temperature of the plastic to the bonding point. Cold metal electrodes simultaneously early pressure and cool the outside surface of the plastic. Radi ~ frequency has been its ecu quit e e:%:tensi ve 1 y for drying. The textile industry has applied it to the drying and setting the twist in cottons rayon,, nylon and enroot fibers. The processing of foods and the deh,ydrati on of Penici~ lin are other a~liCetions86-~. Dielectric heating has al so been used in the . . . rubber incI,~stry for the heating, drying; and vulcanizing of rubber and simlls~ reslns~9~~3. A patent has been issued for the modi- ficetion of plastics and adhesives for induction heating. It covers the interposition or mixing of the rustic mingler Keith Particles of metals of relatively low curie joint such as alloys of .tTi-0r, Mn-Al-Cu, .'Ji-Cu, Mi-Mn, and ~Ji-Al9-. The compression molding of therrnosetting materials has been Simon if i ed and the rate of ~roduc tl on increased through the use of high frequency preheating of the ~3 a.~.ic. The fold ow_ ing e.re some of the advantages gained thereby95-99: (~) Increase introduction due to a decrease of up to 80% in cure time; (2) Moorer mo:Ld repair; (3) bees me. serial ~ s required in the preform to make a dense molded ~art; (43 Favorer defective pieces are ~ro- duced, (5 ) Finishing costs are decreased; (6) In some cases high ~ 86 _

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frequency preheating permits the use of general purpose material and thus eliminates the need for higher priced special materials; (7) April ications that require the strength of rag filled types but are somewhat intricate in design can now be successfully molded; (S ~ There is an improvement in appearance of the molded product. ~ ~~ ~ ~ Suggestions have been given for systematic analysis and tm~ro~rements of molding cycles in the press room, together with beneficial changes in factory operation. There have been a multitude of general and review papers written during the lint year concerning radio frequency heating in the plastic field OC)-~lO. Modern Plastics Encyclopedia The Modern Plastics Encycl opedialll for 1947 has appeared in three ~rol,;~mes. Volume I covers materials and technical data; Volume TI discusses engineering and fabrication; while Volume TIT is a folder containing various charts on plastics, plasti_ cizera, solvents, adhesives, fibers, synthetic rubbers, etc. Of the charts present a comprehensive picture of low pressure laminating resins, giving Sources, chemical types, modifying agents, curt rot conditions, arid uses. ~ 87

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BIBLIOGRAPlIY 3. Ral1s E. E Feb. t1946) 2* Del!nonte, ~r., '*ra~chine Design, lS, ll4_1S, July (1946) `~. Engineer (Lor~don), 181, 519_20, June 7 (1946) a. ~Og-~n, T'!. 5. ~nd Boyer' R. F., Ind. and Eng. Chem., 3S, 1090_] C`96, ~:ov. ~ 1946 ~ Gibson, P. R. S., Elect-rician, 137, 443_5, 517_2O3 582-5 ~ na 649-652 ~ Allg. 16 and Se~G . ~1946 ~ 6. ~.ckson, ~'r. , Engineering (London), 162, 427_S, Nov. (1946) 7. Fie~din-, m. T. ~ Elect;rician, 137, 1370_4, Nov. 15 (1946) B. ?enn, '!~e C,, , Electronic EngO, 18, 280-1, Sent. (1946) a. Tucker, ,r. J. P~ Poberts, R. S., Technlce.1 Press (London) ttp7 ~st'c~ fo~ 1;he Electrical and Radio Engineertt o Rose, K. , ilaterials P.!ld Methods, 24, 653_664, Ser~t. (1946) 11. K1 ine, G. M., Modern P1~stics, 24, 153, Jan. (1947) N~rcus ~ R. ~ Mete~ Finishing, 44, 240_2, (1946) M~.tu-e ~ 153, 121_4 ) Ju] y 27 ( 1946 ) ~ield' R. F. 7 Jour. A~lied Phys., 17, 318_25 (1946) 1.5. Field, p. F., Phys. Rev. fi9, 688, June (1946) 1. 6 . Frohli ch, N~.-ur e , ] 58 , 332-4 , Se~t . 7 ( 1 946 ) 7. Froh] ich, H., Proc . Roy. Soc ., 185, 39_414, A~ril 5 ( 1946 ) 18. ~enn, w. S., Pl?^PtiCS (London), 10, 33_42, Jan. (1946) 15. Xershterger, W. n., J. Anclied Phys., l7, 495-500, June (1946) H~nney, M. B. ~nd Smyth' C. P., A-`e-. Chem. Soc. J1., 68, 244-7 , Febru~r~- ( 3~946 ) ?1 . A . ~ . ~ . ', ., !3ul . .`' o . 138 , 25_32 , J~.n . ( 1946 ) Ge~rers, I~., Ph4lir~s Res. R.e~., l, lQ7-22.4, Al)ril (1C~46) ebing, L. 1l~r., T-~ns. Electrochern. Soc., 90, 2a., (19~-6) . ~Ple.~tice (London) 10, 25_32, 59-73,, Jan*,

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24. Modern Plastics, 23, 134-136, June (1946) Hanford, W.- Ee ~ and Joyce, R. M., J. An. Chem. Soc., 6S, 2082_5, October (1946) Yelton, E. B., Plastics and Resins, 5, ~No. 5, 14_16 (1946) Rerlfre~r, [I. M. end L,e~'is, E. E., Ind. Erlg. Chem. ~ 3S, 870_7, Sent e ( 1946 ) Po,~r] es , ]. G. , and Cakes, W. G. , Nature {London) , 157 9 840-1, June 22 ~ 1~46 ) 29. Honwink R.9 Brit. Plastics, 18, 192-8 tl946) 30. Reendow f, A. F., Colloid Chem., 6, 976_83 (1946) Reinhold Publo CO. - 31. meknish T13Skrift, 76, 139_46, Feb. 9, tl946) 52-. Freeth, F. A., Brit. Plastics9 IS, 44~-6, Oct. (1946) 33. ATnO8J L. A., Coll.oid Chem., 6, 992-1009 (1946)Reinhold Publ. Co. 34. Ron Hillel, A. and Season, L. G. ~ Inde Eng. Chem. AS, 1 121_9 ~ 1946 ) 35. Weeks, J. F1., Plastics and Resins, 5, No. 4, 5_8 (1946) 36. lIeT.ritt,, D. R., J. 011 and Colour Chem. Assoc., 29, 109_28, June ~ 1946 ) 36ae The Dow Chemical Company' Mi]landl, Michigan. 57. Michalek, J. C. , and Cl ark C. C . , Colloid Chem. 6, 1010_13 (1946) Reinhold Publ. Co. 38. Fussell, L. E., Mectlahon, J. D., and Vincent, G. P., Plastics and Resins,, 5, 5~12, Nov. Dec. (1946) 39. maker, F. 13., Barry, A. J. , Runter, M. J., Ind. and Eng Chem., 38, 1117_1120, Nov. (1946) Se.uer,, R. O., and Read, D. J., J. Ala. Chem. Soc., 68, 1794_7, ~ eat . ( lQ46 ) 43. Ser~rn.is, P. C., India Pier World, 114, 657_9, Aug. (1946) 42. Modern P]rstics, 24, to. 4, 102_104, Dec. (1946) 43. Moffitt,, J. A., and Pane.grossi, A., Machine Design, IS, 109_13, Bent. ~ 1946 ~ Andre anov, E. A., Khimicheskaya Prom., 2, 1-5 (1945)

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K41~0Ur~e 52. MCND.~,, D. ~3 54 55 45. Marbaker, E. E. ~ Rabin, 30, 10, F6b. (1946) 46e JOh9.nnSOn O" K. , en] TOrOk., J. ]., PrOo. I.R.E., 296, My ~Ig46~ 47. Larsen, IS. NT., T.~1ttOn, J. J., PY1e, J. J., MOD. PI~StICS, 23., NO ~ 7 ~ 160 ~ T(?.rCh ~ 1946 ) 48. Mod. P''~5 cs ~ 24) tJo. 43 112, DeC. (1946) 71a. 1:Iorrell, F.. F., P1?~11CB (Chicago3, 4, NO. 6, 50_55 (1946) 53. Fuller, X, L., E1eC. Mfg., 37, 125_7, April (1946) 5~. KI1DOUrne, C. E., epoch. Design, 1~3, 109_13, Hug. (1946) ~ , C8n. Cheep Process IT15., 30, 30-2, April (1946) Stee18 118, 78-:S5, April (1946) Callings, W. R. ~ Thorns. Are. Inst. Chef. EngrS. 42, 455_71 (1946) lIalle, Ee E., P1.~stics (I~ondon), lO, 187-200, 248-55, 303_9, 37 6-81, 406_15, Ar~r i ] , Me y, June, July, Aug . ( 194 6 ) 56 ~ Youngs ,, D ~ C ~ and Kin, M ., Plastlc ~ and Re sine, 5, No . 12 13_18, No~r. ~ Dec. (1946) cr7. Bass, S. L., Hunter, M. J., Eaup~i 5 Te A., Tr~ns. Electro Chem4 Soc., 90, (1946) ~renrint. 58. A~e7le, F-, Rev. Gen. Electricite9 55, 99-103 (1946) 59 ~ Electricievn, ~L37, '5~35-6, Aug. ( 1946 ) 60. Scn=Iott, C. A., Pl?~stics e~nd l?:esins, 5, No. 2, 7-12 (1946) 61. Pochow, E. G., Par~er ~res~nted at lOth meeting of Electro. Che~n. 130ce 6Z. LP.ngmuir, Chem. Eng. News, 24, 764, March 25, 1946. 63. Warner, A. J., Elec. Commun., 23, 63_69, Ma.rch, 1946. 64. Fo'~Jels> G. ~Rubber Age, 5S, 703, 11arch, 1946. 65. 66. 67 68. I s enburg, () . E ., Pla. s t ic s ( Chicago ), 5, Mo . 3, 63_4 ( 1946 ) Krueger> R. M. , Q3T, 30, 51_3' A~ri? (1946) Modern Plastics, 24, No. 2,, 196, October (1946) Grahe~m, R. C ., Radl ~ News , 36 , 46-8 , oct . ( 1946 ) _ 90 -

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- . 69 . McKinley, R . B . ~ Dis tributl on, 8, 34, July ( 1946 ) 70. Lunt, Ae Po ~ (;omnlun. , 26, 30, 53 ~ June ~ 1946 ) 71. pilsner, P. L. , Jour. In8tO Elec. Eng. (London) ~ 93, 29~304, July ( Ig46 ) ~2~ ~.~.~14_~g9 G. tin In- L aos. Rec., 2~, Id, Jan. (1946) '~"23 i~ ~ i~ ~1 0~$ ~ ~ ~ ig S 128~9 ~ Ju~ y ~ 1940 ) 74o Eddy' We' `!o and Fenn, W. D, 9 (;5 ~ 5 76 ) 1Lug 0 Se~ t e ~ 194~; ) Rosen, A. 9 Joilre Ins+. Elec. Eng. (London), 93, <~83-386, Augus t ( lg46 ) 75~ Cox, C. R. , Electronics, 113, 130_5, ~lay (1946) 77. Kinr~, T. P. end Mercum, Je ) Pa~er presented at 2nd National Elec~Gronice Conference. Trans. Amer. Inst. Elec. Eng. ~ 780 Mittlemann, ~.,, ~ ~ o~erence ~ ~embo, S. I. 9 E1ectronics, l9,, 120~122, Anri1 (1946) 8C'. Swan, Ao G. ~ Electronics, 19, 162' 170' May (1946) 81. Boyd,, B. , Electronics, 19, 125_127, O-~. (1946) S2, Roberds, W. '~,, Proc. I.R.E., 34$ 4Ps9_500, 5121y (194~3 83. Wenger, W. D*, vodern Plastlc$, 84. Kohlor, F., Plastics, 5, 50-52, Dec. (1946) 85. Moderm Packe.gingg 19, 120~' May (1946) 86. Batsel, CO N., Radlo Ne~'s ,, 56, =r`~ - ~nt:~?~ ~; Pnd ~ti ~na1 Electronice 23 510 1i 9 135 ~ r~l ~ ( ~ 946 7 ~ O , Oc t O ( 1946 ) 87. Wintereg S. R. ,, Radio News, 35' 3~9 123, June (~46) 88. Bro~m, G. H. and others, I:-oc. I. R. E. ~ 34, Feb. (1946) 89. Westinghouse ISlec. Corral, Ind. Ecui~. News, 14, Feb. (1946) 90. Purcha.sing, 20, 180-184, Mar. (1946) 91. Mittelmann, E. and 80.somo~.,rrth, G. P. , Electronics, 19, 128-~.~0, March ~ 1946 ~ 92. S~nither.s, V. L., India Rubber t.'!o~ld, :~15, 505, 566 (19463 _ 9], _

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