SILICONE RESINS..
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Use in Protective and Decorative Finishes J A M E S R. P A T T E R S O N GENERAL ELECTRIC C O M P A N Y , S C H E N E C T A D Y . N . Y .
SILICOhE resins are a recent addition to the list of \chicles atid binders of interest to the protectiie atid decoratiie coatings iiidustrj The? offer seieral properties which are riot obtained with other resins and pro\ide a n unusual conibination of resistance characteristics. Their use w i l l permit the attainment of new leiels in resistance to heat, chemicals, and weather. \ hen their cost is lower, the? will find general ure in niaiij tjpes of finishes. Se?era1 niodified silicone resins can be made, arid those modified with alli>d resins are particularly promising. The? possess most of the desirable characteristics and few of the disadrantages of the indi,idual members.
l-'os>ihlythe outstaiitliIig propc'rties of silicone finishes are thcir cxscellent resistance to h w t , iveather, and various caheniicals IYhcri pigmeiited tvitli heat tirig pigments, these finishes s and 500" F. for several hunn.irhstaritl 1000' F.for a ie\v d r d hours. K i t h a moi thorough selection of pigments, more itable proportions t o be used, aiid a knoivledgc of the most bcttcsr understanding of s cone rrsin behavior, even greatcr heat I nee is anticipated. Apparently the ultimate iailule of a silicone redin at high temperaturcbs is tlirough osidation of the organic radical.?, leaving silica as a residue. .Use there seems to be an intermediate stage caused by volatilization of lou--molecular-weight polymers, and resulting in slirinkagt: and inibrittlement of the film. Discoloration, esccpt that which may be attributrd to the pigment, is not a part of this failurcl. ?;or is any loss of glosa apparent until the ILICOSE resins are organosilicoii conipounds n-hich are befilial stagc,s arc reached. Csually the cvidencc oi film failure is lieved t o have a liasic molecular structure of recurring cwc~kirigor checliing. aiid loss of adhesion to the substrate. i n quartz. -Sigroupings similar to the arrangemelit, -Si-()T h e effiacta of high tc,mpeiatures on alkyd siliconcs are sonieHon-ever, in silicones orgmic radicals sucli as methyls and phenyla \\.hat tliffvrent iii that di,woloration doc^ o('cur. Possibly this is a are attached to the silicon atoms t o provide solubility, toughness, ~ ~ c ~ofu lthe t cnrtmiiizatiori of thv fatty acid portion of thc alkyd. and flesibility. T h e strength and stability of the silicoii-tonis that t h r presence of the silicone resin exerts oxygen and silicon-to-cartion linkages plup the preseiice of lit tie ti strong protectivi: action over the alkyd. An allcl-d silicone deor no chemical unsaturatioii is thought t o be responsible in large signed for heat resistaricv will discolor somewhat a t 480" F., but part for the unusual rcsistarice characteristics of silicoiics (Fiyurcl \vi11 retain ita aclhc,sioii and toughness for several hours. d blend 1). ormaldehyde resin is of the same t>-peof aI!q T h e mechaiiiuin of lieat cwing is beli d t o Lie nio>tly :i ('ross con-iticraljly less heat r the condensation of .residual hydroxyl groups. s has not tieen fully The \veather resistan reniain from the hydrolysis oi chlorosilaiic~si l l iiivc>tigated, but thclri; is ,strong cvidcLncc pointing to unusual t h e formation of the rcaiii. dwatiility. Possilily the longwt weather test is t h a t of a white enaincxl n-liicli h a i I i c ~ nesposcd four >-ears 011 a Schen PROPERTI E S fence facing south a t 45' from t h e vertical. Dirt collection is tli(j only cvidciice of f:tilure; there has Ijcen no chccking, cracking, Biliconc r c k i . > ra11gc1from hard, brittlc, gl is a straight silicone resin chrtlliing, or lo+ of gloss. This c~namc~l viscous oils. Usually the>- are coloi,lcss or pigiiwiitcd with rutile-titanium oxide. .inothclr set of panels of easily soluble in aliphatic arid aromatic hydrocarbon solvents. These solutions are IOK in viscosity at high resin coiimmt rations. a I)litliuloc>-aiiiiie l)lur~-tintetl chalking-typp matasc-titanium iixitic. in a silicorit. rehin has tieen exposed in Srheiicctady for nearly They et piginerits readily, arid many of tlic comnion pigiiic'nt Y tlvo y t w n \vitliout dinning any fading and chalking. T h e alkyd can be uscd n-ith them. S o special care rictd be cxcr( wsin coiit 1.01s in thi, t w t h a w faded and chalked badly. selecting pigment-dispersing equipment. Silicone paints li:~v(\ taiice is bring supported I~:vitlc,nc.c~ oi this i~ucc~lleiit \wather r been made successfully with roller, pebble, and ball mills. by tests in Florida. These havct These paints may be made not been sufficiently prolongcd to have a very high clean-cut R R to be convincing, hut they ai'e gloss. arid to have a n excellent R R I I I I t'olloiving t h r pattern of previous purity of color and tint. T h e -Si-O-Ssi -0 -Si -0 -Si I I I observations. These exposures latter is possible because of the R R R may indicate the feasibility of usabsence of color in the resin and ing such pigments as the niolybthe freedom from resin discoloraRI RI O \ date oranges more estensivcly fol, tion during baking or h e a t cur- S I -0 -SI-0 -SiI -O-Si- I outdoor finishes. This particular ing. Also, this permits the use I R R R tt'st has not been long enough of some of the loiver cost white to prove utility, but after four pigments which, because of their R R months in Florida a molybdate inferior opacity and inability to RI I -SI -0 - S I -0 -Si -0 -Si orange in an alkyd resin has obscure vehicle discoloration, I I I I darkened considerably nhile the have not been extensively R R R R same pigment in a silicone resin utilized in other types of baking Figure 1. Probable 3Iolecular Arrangement of has changed little. enamels. a Cross-Linked Silicone Resin
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1376
November 1947
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1377
hnothcr quality of ,-ilicoiie coatings, and possibly one which conti,itiutcs to 11-eather resistance, is lo^ temperature flexibility. They liavc~shown good results n-hen givrn bending and mechanical shock t w t s a t tc,mperatures as low as -50°F. Resistance to burning is still another quality. This n-ould tie espcctcd from a chitmical structure I,ased o n silicon. PIGMENT.iTIO\
Tlie limited aniouiit of n-ork n hicli h 1s bcwi tlonc' o n the pigmentation of silicone resins indicatcs that many wtablished conccyits must bc modified and new oiies reckoned \\-it h. For instance), pigments ivhirh previously may have, t x w i con-idered unsuit:itile under certain conditions ran he uscd in siliconc i ~ ~ s i n s n i t h satisfactory results. Specifically the so-called cahalliing t y p w of titanium oxide.? may hv inore generally cniploycd in outdoor firiishcis, low-cost estcnded irliite pigments may be usc4 in ing enamels, some organic colors can be high tcmpc~raturer utilized at highcr t e ratures than w r e previously feasible, and nevi pigments may he developed or adopted from oth, r industries i t tainatjlv \vi t h thri;r i i c ~ w t o nievt t h r ni'lv li~vclsof pcrfomian re.-11is.
Alkydm eIa m i n e , 2 hours
Alkydsilicone, 2 hours
Silicone, 24 hours
Figure 2. High 'Temperature ( f 8 0 " F . ) Discoloration (below) of White Enamels i n Comparison with Original Color ( n b o r e )
Tlicare is no intention of elainiing uiiusual \ v w t her for any arid all silicone paints. Some pignients n-
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iits as sylenc.. Onc mema of ovcxrcoiiiing \vi11 he 11y niot1ific:iiioii with all~yds, Tlie -ilicc to a1cohol.s. I'iwihl?- iodine. i. t h c most potent stainer t o lie c ~ n c o u n t e i ~1jy d
Silicone coating< adhere well to glass anti such metals as -tee1 nnd aluminum, hut in this respect they are surp finishes. Thcir adhesion is greatly improved by modification with alkyds. Since their. adhesion to primers is much better than t o metal-, they are often applied ovrr' sp illy pignicnted alkyd ix.sins. Lsu~illythis is not desirable for high tcmpcrature resistk in the system. ing finishes, as the alltl-d then beconies a ~ e a link This is n o t necessarily true, however, in systems for JTithstanding alkaline conditions. .Ipparently the silicone top coat fully shields the much less alkali-resisting alkyd. Further evidence of this unusual protective action has been observed in the high resistance to acids of silicone paints containing zinc osidr as a portion of the pigment.
Since. t h c firat stag(= of film f:iilurc. at elrvatrd teniperaturcs sho\v a.s eracaking o r chc~kirigancl loss of atlhc-ion, it ~voultltic c~spcc~t t d that micaccwuq a n d acicular pigiiicrits nould lie I i i ~ n e ting finis1ic.s Thi- is truo a s ihon.n n.lieri aluniinuni pon-dcr, mica dust, anti asliestiiie arc UPIYI. I l o i ~ r ~ v ti ~t r , lie anticipated that, with titanium olitlc, high temperam-iw inversely n-it11 thcl contrnt of pignicnt; tance oi a titanium ositlc cnanic.1 i k iniprovcxd at t volume, ~ v l i c ~other i pignicnts arid estendcis, i i i c osidc, lithoponc~,:intimony osid(,, calcium sulfate, :ind calcium carbonntc, are added. Furtlicsr study may modify tions and iiiay provide :in vsplnnstion. Possibly this c~xplanation \\-ill tio associatrtl with c h r m i c d tiwitmciit of thc pigment, or Tvith t h c cfiect of the pigmcnts at tllevated teiiipc~i'aturc~s 0 1 1 thc t1ic~riii:~lstability and lar rearr:ingc~nicntof t h r rt,siii. d on priwnt kno\vledgc~,th(x i i i o ~ theat-resistant silicone finiqhes arcs t l i c w pigmc~ntotlwith the conventional amount of :iluniinuni pon-tlc,r. Tlicw~f i n i - h c will n-itlistanti 1000 F. for :I f(,n- hours and 750 F~for as much as 100 hour> h f o l c t licre is c~onsidC~r:il,li~ film f;iilui~c~ a n t i lo+ of :itltiocion. T h r l i ( l h t \\-bite c~nami~ ai'(> l ~ t h i ) ~corit:iiiiing tit:tiiiuiii osidix c~striidctlwith cal-
Alkyd-melamine
Figure 3.
Alkyd-silicone
Silicone
Resistance to Immersion in 3qc Sulfuric Acid a t 100" F. for 100 Hours
I N D U S T R I A L A N D E N G I N-E E R I N G C H E M I S T R Y
1378
ilkjdmelamine. 24 hours
ilhgdailicone.
Ciilicone. 100
48 hours
hours
Figure I.
Vol. 39, No. 11
placed iri a n electrically heated, ventilated o w n , u u t i rheir COIJdition was observed after the stated time intervals. The coniposite flexibility and impact resist'ance ratings were dctermined b? averaging the effect~son the films of brnding them over a 1/8-incD mandrel, and of bumping t'hem with a falling '/e-inch iveighted ball. These tests were made at' 77" F. T h e film integrity or crazing was observed on a sample which had no treatment except the shock of rapid cooling to room temperature. Usually silicone resins must be baked or heat-cured. For Sonir uses, such as in stack paints, they can be applied and cured by the subsequent exposure to heat \$-hen the stack is in uw. T o areelwatr heat curing, a catalyst may be desirable. There a r t many materials, particularly soluble salts of metals, which have catalytic effect. Several of these-for instance, the ,salts of lead tin, and calcium-are too active for safe handling. Others, suct : t ~iron, cobalt, and zinc salts, can be used. Iron is preferred fo, curing speed arid hardness, b u t it diminishes heat resistance, ana badly discolors whites and light tints. Cobalt and zinc provide less curing speed and hardness than iron, but are much leas dvlrterious t,o heat resistance and color. Cobalt, however, does give n-hite paints a blue tint. The amounts of metal required on the basis of silicone resin solids are 0.1 to 0.2% cobalt or irnn, and 0.5 to 1.0% zinc.
Composite Flexibility a n d biliii Integrit> Inipact Resistance Rating cium sulfate. Su(all eilaniels have h s c s i i held iti X U F. for 500 Color after after Heat Exposure hours witliout macliing or cllt&ing. I l s o , there. \\.a5 little diiI000 Hr. 1 50 100 500 1000 hr. hr. hr. hr. hr. C r a w Color Pigmentation" coloration and loss of gloss. Adhesion and flexibility were im5 4 3 4 4 a 0 paired but not sufficiently t o he regartled as failure. Other good white pigmentations are lithoponc and antimony oside wit11 1 3 4 , : 5 3 P enough titanium oxide t o provide the required opacit>-. 5 0 1 0 Among the other pigments n-liich have bren used successfi~ll~ 3 0 with silicone resins are carbon black, Hack iron oxide (hut not for 2 0 I 0 high temperature use), red iron oxide, cadmium red, litho1 anti 1 0 1 0 toluidine reds ( h u t not for iuse above 300" F.:I, phthalocyaniiic, , 1 0 blue and gree, iron blur, ultraniarinil blur, chrome grccxn, chrome 1 0 yellow, and lead molybdate. r\ntiniony oxide, 25% PYC go titanium oxide R-10 zinc Table I gives the results of exposure to 500" F. for siliconu oxide, 20% PVC enamels containing various pigments. The sarne lot of silicoric 50 titanium oxide R-50 zinc oxide, 2 5 9 I'VC resin was used in all enamels, and the curing wtalyst was zinc 50 titanium oxide R-50 zinc coctoate. Single coati (between 1.0 and 1.1 riiils t,hick) of tlicse sulfide, 25% PVC
pl~liwlt o t h i n
~RYP.
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