POLYS1LOXANE ELASTQMERS -
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C. M . D O E D E A N D A H M E D P A N A G R O S S I T H E CONNECTICUT H A R D RUBBER C O M P A N Y . NEW HAVEN, C O N N .
THE silicone rubbers are pol?niers made I)? the concleii-ation and oxidation of especiall? prepared niethrlsiloxane. The? are flexible a t -65' and remain stable a t 200" C. These characteristics nial\e them useful a t temperatures helot+ and aboie those a t \+hich t h e ordinar? rubbers can function. Because these rubbers adhere reaclil? t o glass, it is possible t o tIe\ise material constructions w i t h ~ O I J I Cof t h e desirable properties of both substances. Their rheniical resistance nial\es them desirable for gabhet applir ations. The ease with which the? adhere to steel makes possible t h e fabrication of high temperature rubber belting for the process industries. Other tjpes of coated faljrics ma? s e n e as diaphragms or hot pas ducts. Still others, becauqe of their excellent ozone resistance and abilit? to withstand corona cutting, h a l e become desirable elec trical insulating tapea. The fact t h a t the rubbers are unplasticiaed and do not contain sollent-extractable materials, proniotes their use in sJstems where contaniinantq of the ga5keting material should not exist. This report OJI the characteristic9 of these rubbers and a dewription of 'I fe\+ engineering applications w ill probably suggest soliitions to other similar problems in t h e process industries.
silic.oi~i~ pol,~.niei~s.Thi. polymers thus formed have bec~ii reiiortcd t o he both chairililce (9) and cyclic (4)in nature. Prol)ably the find structui,c is unique for each type of polymer involvcd, :iniI is a conibiiieti Iiiodification of the t w o types, straight and eyc.lic. the silicon-os>-gen bond is apparently one of those upon \vhioli thc structure of the silicoiie po1ynii.r is based, the f':irt tlint its Iiond energy ( 2 1 ) is about half again an much as that for carbon-carbon or carbon-sulfur linkage would indicate a tliei~niallyniorc stable type of structurtx. The bond energies, in I;g.-cd. pl'r moll^, folio\\.: c'-('
c'-0 c'-S
C--e
58 0 200 48
('-PI C-H
(i
54.0 66.3 87.3
Si-C
Si-0
57.0 89.8
Tlir 1.ulilier (8)lias I)c.c.ii rcyo1,tcd to l x a specially prepared pcilynii~~ of :i purified tlimctliylailosarie in which the condensation aud aggregntc foi,inatioiis have been so controlled t h a t molecular elihst onieric ch:tins of s ral thousand units are formed without reduction t o an insoluble polymer. This clast o n m i c network is unique miong rubhers in that osygen is the important condensing and rrarlarigenii~ntagcnt rather than sulfur, the common vulcanizing ingrcdic~nr. RIECITANICAL PROPERTIES
HE; chemical constitution of the polysilosane clastorni~i"., o r silicone rubbers. is such that they have outstanding t h ( ~ r n i s l stability and electrical insulating characteristics. The kxtaic. unit structure has many things in common with that of gl a chemical bond may bc formed betncen the silicone poly glass itself. This unique property makes it possible t o proparo inaterials xvhicli utilize the characteristics of both sulistanccs. The study of the nic~chanical,chemical, and clectricnl propibrties of this conibination of materials forms thc basi.5 for this rc'piirt on i n g applicnlions Xvhich havv been nix1 ' of t h e sili-
T
niental unit involretl hai. bec,n givc,n t lie gc,iic,r:~l formula RR'SiO, n-here Ii and R ' represent oi,ganic. ~ Y O U I I R attached directly t o silicon (;), The compi~sitioiiof R :inti R', :I,* n . ~ l las t lie configuixtion givc,n thciii in the polynicxr, dct ivrniiiw to a h r g c c,stent the charactc~risticsof the final nitticrid Tliv initial organosilicon conipoaition may be nilidt ki! nic~tliods;three of thcni follon-:
+ Grignard reagent +organosilicun hslidc
Tlics tc~iisilcatrengi lis of v:arious vulcanized clast oniers in hoth g u m and titanium ciioside-filled compositions (100 parts filler for 100 p t r t 5 lmlymcr) foilon. fin pounds per square inch) : I'olymcr
Sntural
GR-S
GR-I
Burin S
Gum Filled
3000 1500
400 700
3000 1200
GOO 1000
GIirelectrical propcrtiw, as they do the physiral characteristics. over a temperature range of -65' t o 200" C. Since glass can be bonded t o silicone rubber lyith such a high degree of adhesion, a glass fabric impregnated with the rubber would make use of the electrical and thermal stability characterist,ics of both materials. Table ! s h o w comparative values for a Silastic 125 composition nonfabric-reinforced, and a composition of Fibergla- llCC-11-128 impregnated Tvith Silastic 125. It is
Figure 4. Volume-Time Data for Compositions of Silastic 125 nncl Fiherglas ECC-11-128 in Various Solvents
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
~iceludctivoids cau substsntially reduce t h c dielecti ic strength of tlii clectrical insulator. T h e compositivri of tmtli these materials is such t h a t carbon I raclriiig cvrinot occur after a dielectric strength breakdonn. The potential necessary t o continue a n arc is determined by t h e conmluctirity or' tlie p a t h formed. T h e resultant silica surface does :not forin :in c~lcv~tricnlly conducting on(', USES
Tlit>I~ 1i:tvc 1)et.n ni:iti>- interesting applicstiotis of t h e silicoiic , ~ t b t i c ~ i1~1~~. 1 imow y arch possible. In each instance it has been .icc.ec-:try t o cunsider the particular problcttii involved. The upplic:itiuiis ha\-e been so designed a': to take advantage of tlit: the silicone rubbers. Khencver possible gasket , I)]' 1 52 inch \rere used instead of the usual or \Vhcn Inn. temperature flesihility charncteristics arc neressary r i i i t providt3d by the ordinary syiithetic rubbers, the lioultl he c-onsitlerd. TTliere a n elastomer with rh(.rnial sttibilitj- betn.rc~ii 125' ant1 200" C. is needed, silicone tvl)ljc~rc:in serv(', L-iilcss spccisl circumitances dimand, t h e ~iliuitier i i b h r ? ,>houlci not he used n-here tlic tcmptirature of itpplicatioii is tx~tween -2.5' :I C. Thcy are not coinp t i t i v e ni:lT(31'i:ik in the ordinal of t h e r o r d , but they t l i i riialx po?!ibli: economical tlcsig *e in teniperatmc rangcis t)c,yond which t i i c ordiiiar>-rubbers s ' r v r . From ccrtniii 1inint.i of vicn- tlie physical properties are not rt(,:irl>- so outctatitliiig 8.; might initially be desired. The dcsigtiatrd pinperties given for rub 11ic:itions have frequently lid mny hear little rclationt i w i i stuted for spci~ifiratioii pur prohlcni. Tlius f:ir it hi.; -hip t o thca ac'unl requirt.inr~nts iiot been atlvissble t o m n k e a direct v b s t i t u t i o n of a n article of 4licoticL rubber in :t deiign nht3re a eolitcmImrary syrithetic had 0ec.n a t t c n i iterl. 111( w h satisfactory solution the final desigii upon a n application o the p r o p e d e s of thiq pari'r. Fortunately for the cnginrer there are marly For illuqtratioii, several typical classes of prohli,nis art' outlined ~ h i c h have been s.atisfactorily solved; t h e solutioiis should fiiitl furtlier geni~r.tl applicatiori in the process industries. t o a tlegrec
HTDRAL-LIC SEALS. Uti(. of t!ic cliaracteristics of silicone rubhas k e n the f a c t that thcy cold flon. at high temperatures.
bel'S
This property has tieen advantageously used in the design and i:onstruction of 3t:itionarj- seals for high temperature oil lines. The entire packing is rvell supported within a niechanical framework untlerii , a t h a spring loading device so t h a t the ruhhrr is a t all times under mechanical stres-. Cnder this condition it flon-s into the physical irregularities of tlii' metal surfaces present and Ycals all possible points of leakage. Packings such as this have ed :it teiiiperatures of 175' C. on luhricating oil syqtenis for ries it iq often desirable t o utilize The rubber protects the hot glass from both thermal and mechanical shock, as n-ell a5 breakage. ;Is silicone rubber adheres readily to qteel and can moment,arily withstand spot temperatures of 400' C., they have proved particularly useful in these applications. - d t t l l tool. n-ith a rubber facing t o support hot glass objects.
BELTISG.Silicone rubbers are odorless, tasteless, a n d nontoxic. These properties have macle it possihle t o huild uirecloth-reinforced belts for use in the food processing industrics. Prepared foods can b r placed directly upon t h e belt and dehydrated under such conditions that, t h e belt temperat'ure may reach 225' C. The rubber is unaffected by the water vapor or the food fluids themselves. Several of these belts have been in use for many months. In those c:~se:: where t h e mechanical strength of wire cloth is tiot necessar>-, it hns becn possible t o make processing belts from
1375
glass fabric coated with silicone rubbers. These belts have n-orketl out very \vel1 in continuous drying ovens where solverit, evnporatioii, heat treatment, and degasification processt.9 can be carried out 011 flcsiblc ruhhcr Iwlt. at clcva.tcd t ~ m p c r a t u r c s .
C'O.ITI;D FABRICS. Coatcd gl fahrics have fuuntl con-ider: r h k iiie as diaphragni niatilrials in the procesdirig indust ries \rliere t ti(. actuating a t m o s p h e r s arc wlativ tc'mpc,r;iturt.s are highcr tlixn t h o w n-hich Ot {Till rritlistand. For many u hers are P O w l l suiteid t u t h e the. m:itisriLil coated oti on(' sic i i f cnwt ruction. .I unit f:rt>ricatetl i\-ith this type of f:at)iic can rradily IJ(, formet1 :tiid fi1i:illy cwmeiirc? into a coriil)lcti.I) prc>:it :I' the 1ab0ratoi.y tests oli cotiiplctc, ~ v o u l dinclicste. It is poq.iblc t o uic. tlic-c,
tion of elcctl ical itisula.ririg tniies n-hich arc rcquirccd t o function c.:)ntiriuously :it tempcr:iturcs at l r m t 100"C'. highisr than those \vhich the prcqent-day orgnnic iiinterials n i l 1 withstand. This rcsiitancr t o high temperaturcy mean? not only tti:tt the tquipnicnt c:in he ticsigned t o function at higher temperatures, but also th:tt clrctrical units can bc locatcd on processing rquipment in positions n-licre t,he niechaiiical advantage is iniprovc4. It also ~iieansthat the units can t i e subjected t o temporary ovrrloads n-i!hout fear of t l ~ ~ ~ t r u c t of i o ntho rrihtirr iiisulatioti n-ithin the re sn-elled bj' the ot~tlinarymineral i i i w lating oils. Thcy contain. lion-ever, relatively little estractable material which rxn. iri ~ U I ' I - I , iic tlin~,,lredin tht, minibra1 oil itself. This means t,hat elrctricd itisu1:iting oils in contact with silicone rubbers do not lose thcir clectrical properties because of the acquisition of dcleterioui foxign nisiter from thib rubber. This particular property i.; especially mlua,blc where long life and high temperature operating c1iar:tcteriatics are essential in oil-filled electrical devices. LITER.4TURE CITED
Collings, IT.R.. Chem. Eng. S e u s . 23, 1616-19 (1945). Doede, C. SI.,DiK'orscia, G.. and E'anagrossi, .l..Div. of Ruhber Chem., .l.C.S., .itlantic City, 1946. Frcundlich, H., "Colloid and Capillary Cliernistry," p. G i 4 , London, Methuen 6E Co. Ltd., 1926.
Hyde, ,J. F. (to Corning Glass TT-ork?), U. S.Parent 2,371,080 (Slarch 6, 1946). Kipping, F. S.,Proc. Chem. SOC.,20, 13-16 (1901). Miller, €3. C., and Schreiher, R. S. (to Du Pont Co.), U. S. Patent 2,379,821 (July 3, 1945). I l o c h o ~E. , G.. "Clieinisti~>of Silicones," p. 61, K e n . York, John IYiley Bi Sons, Iuc., 1946. I b i d . , p. 7 2 . Itochoiv, E. G. (to General Electric C o . ) , U. 6. Patent 2,352,974 (Jul>-4 , 19-14;. Ibid.. 2,380,996 (.lug. 7, 1945). Whelarid. G. W., "Theory of Resonance," p. 78, Sew York, John IYiley 6 Sons, h e . , 1944, RECEIVEDApril 14, 1947.
