Silicone in Protective Coatings M. A. GLASER Midland Industrial Finishes Co., East W ~ ~ .%., P I Waukegan, . Ill.
Qilicwie polymers are fitiding ever increasing applications in the protectil c coatings industry. The major uses at this time arc: in the design of heat resistant, heat and corrosion resistant, and weather resistant coatings. Thew coatings are used for painting structures like smoke stacks, automotive and aircraft exhaust equipment, stows, furnaces, spare heaters, and incinerator*, In these instances, the silicone finishes have replaced either more conventionid organic finishes or inorganic finishes. Formulation guides for the development of a number of types of silicone-based protective coatings are presented, a n d the durability of the resultant finishes is discussed. \lauufacturers of protevtive and decorative coatings look forward to greatly increased usage of pol? siloxanc~sas new pol) mers heconic available. ARTLY organic finishes froin silicone resins I ~ a r earoused considerable interest since organosilicon polymers were first introduced commercially less than 10 years ago. The approxiinate relat'ive growth of silicone protective coating sales sinw 1947 has been fourfold. This modest volume of silicone b a s 4 surface coatings being manufactured will increase gradually a s new polymers and copolymers are prepared, evaluated, and adopted, understanding of this class of resins increases, and the potential users of silicone coatings become inore aware of the advantages to be gained by their usc. Some of the improvements that have been made since lY47 in silicone resins and in surface coatings made froin these resins may be noted by the comparison of a 1947 White Siliconr Fhitmrl with a 1953 White Silicone Enamel in Table I.
30 min., 4233 F.
Curing schedule
Gloss. Gardner 60°
80 75
Passro
film Abrasion resistance, Taber weal factor. mg./lOO cycles Sward hardnesv Adhesion to degreased steel Lubricating oil resistance 48 hr. Color retention, 300 hr. Atia8 single arc fadeometer C ~ Oretention, P after heat exposure of 24 hr.: T,,5noF.
Ilia 11
11R 19
]Tail-poor Excellent
Excellent Excellent Excellent
Pair-good
14;xcellenl
Gooil
Although the element silicon comprises 27.8% of the earth't; crust, and the elements silicon and oxygen combined occur to the extent of approximately 75% of the world around us, neither silicon nor its organic compounds occur in nature. The only uaturally occurring substance to have a C-Si bond is the mineral rnoissanite, a silicon carbide, and this is not ordinarily considered an organosilicon compound. I n 1840, Dumas in England suggested the possibility of organodicoii compounds. I n 1868 Ladenberg and Friedel made tetm chlorosilane. I n 1892, Combes of England prepared trichlorosilane. Chlorosilane as well as dichlorosilane were also prepared in that country shortly afterward. Kipping of University College of Sottingham, England, iq uwxlly consideled the father of
2334
silii:oiicl chemistry. Beginning x i i o u l the turn of the pi century, Kipping and his co~~rorlrcw published a series of ovrv 50 papers on organosilicon chemistry that has formed the ba&I)orie of eubsequent research in this field. Interestingly enough, Kipping was not particularly interested in the organos polymers for which he mag bc hest remembered. 111tho early 1930'5, Sullivati and Taylor of the Corning C,X ~ o r l c i ?bccame interested in the possibility of finding prodti , , intjc:lxicdiate in mechanical properties and temperature resisttinoc I)etween glass and organic resins. This interest led to the initia,tioii of a research program under &.de of Corning Glass Works :!lid LTcGregor of the Mellon Institute of Industrial Researc'li. From t,his program, and the n ~ i which y followed soon theredkr, caiiic the silicone polymere uscd in ihc coating and ot,her intl\!st'ries today. The first published 7rork.s 011 Lhc subject of silicone coating. \vas probably that of the technical committee of the Chic:qo Paint and Varnish Production Club in 1945 (f7). The Chicego club studied the compatibilities of wine of the then available: silicone resins and reported a silicont) white enamel that ret;iinivi gloss arid color reasonably well afimc~i 4 hours a t 650" F., a Fxr surpassing the perforinarm of' other heat resistant, iv i-oatings a t that time. Silicones may be thought of as hybrids between glass and organic resins. Like glass, they are formed by the chemical modiliratioii of sand or silica, To nuke glass, sand is fused with othet inorganic oxides. Silicones are prepared from sand by replacing mine of the oxygen linkages with hydrocarbon groups. Iii silicone chemistry, there are trvo important kinds of bondr. Tlir. Si--C bond, as in carborunduin, is highly resistant to heat, water, ~ n oxygen, d oxidizing only slightly even in an oxy hydrogen h l a ~ t . The Si-0 bond, as in quartz, is also highly resistant to h w t , oxygen, and water. A listing of the nomeliclat8ureused in this paper is as fo1lt)wa: General formula silanes Silane (CH, = methane) Disilane (C2He = ethancl)
I
If CH3SiH3 (C€I8)&3i CHdSICL (CHI liSiO€I (CbHd)Bi(OH)2
INDUSTRIAL A N D ENGINEERING CHEMISTRY
'L'tisilane (CIHU= propaiii ) Slcthylsilane Tetramethylsilane Xethyltrichlorosilane "rrimrthylsilanol nil)~ictirlsilanrdiol
Vol. 46, No. IB
-SiliconesHSi( 0H)1 CH3Si(OH)3 -Si -0-SiHISi- -0-Si-Ht (CHs)3-Si-O-Si-(
Silanetriol Methylsilanetriol Siloxane linkage Disiloxane Hexamethyldisiloxane
CHa)a
PREPAHATIOh
A brief mention of the three methods generally used for preparing silicone polymers may be of interest (21).
Linear polymers of this typc are the silicone oils and rubbers; n may be as high as 2000. In the preparation of dimethyldichlorosilane, other chlorosilanes were also formed. When these are separated, they form valuable building blocks for manufacturing different silicone polymem
K + HzO condense I (monofunctional) lt,SiC1 ItrSiOH R-Si-0-+
Direct Process
+
+
+
+
silane) 4 (CHI)2SiCl, 2Hs0 -+ (CH,), Si(OH)z 2HC1 (dniit~th~ 1silandiol ) 5 . x(CHS)zSi(OH)n 4 [(CH4)&0], .cHBO (dimethyls1lou:Lllc. polymer )
+
+
Itochow states that phenyldicones may be prepared LIY the direct method by means of the follom4ng reactions:
:3.
+ 2C
SiOs CaHs
+
+ C11 PCI,H,CI + Si +=
Si
+ 2CO
- -
+HzO IbSi(0H)zcondense -0(difunctional) RzSiClz - 2HC1
R
(chain unit)
+3H20
-
(trifunrtional) R Sicla --+ RSi(OH):+ - 3HC1
rondensr
oI
j R-Si-0--
I
0
I
B y using mixture6 of difunctional arid trifunctioiial chlorosilanes, hydrolyzing and (tondensing, cross-linked polymers are formed which are silicone resins (25). These resins may have the configuration
-+
-
+ 2HC1 (diphenyl+ zH2C) (diphtxri$-
(C>H3)2Sihenfollow. Olefin Addition This method depends on the fact that, under the infuoiiw of heat and/or a peroxide catalyst, an olefin will add to a compound in n-hich hydrogen is attached t o siliron.
Heat
+ H2C = CIly -
C!:$3iCHzCH;
-.+
peroxide
Trichlorosilane Ethylene
Ethyltrichlorosilane
Again, the resins may be formed by hydrolysie and polymerization. Obviously it is not possible to prepare met~hylsilicone materials from this type of reactlion. The alkyl or aryl silicones formed by any of the methods just drmribed then combine on heal irig to form linear polymers of the typp
I1
I -O--Si---.()~
It
It
II
I --.().--.Sii .-(J -Si I
1:
I
R.
which may be terminated by the introduction of end-blocking emits containing 3 alkyl or aryl groups
November 1954
i-0-
Ag -+(C~H,)&iCl~ (diphenyldichloro&400" C
$-
-+
Cl&3iH
LI
(cross-linker )
+
+
R
HCI ((8hloiol)enzene)
C&('I
silaiic~) 4. (C&)z S i c & E€& ((2bHj)2 Si(0El)z sihiiediol) z ( C ~ HSi(OH)n ~)~ [(CsH,)zSiO], silostme polymer) Grignard Method 2('HRMgC1 SiCl,
R
(end group)
+
1 2
I
-HCI
+
Si02 2C -+ Si 2CO HC1 + CHIC] HsO 2. CHIOH Cu powder 3. Si 2CHaCl 3oo" + (CHI) SiClz(dimethyldichloro1.
R may be saturated or urisaturat,ed, alkyl or xr.yl, or any desired mixture. PROPERTIES
Of the saturated organic groups, the methyls arid the phenyls have been found to confey the best heat resistance. The high met,hylsiliconeresins are fast drying and hard, but lack flexibility and adhesion. The high phenylsilicones are soft and dry to tacky films. They do, however, produce flexible and adherent films when properly cured. Many silicone resins lack compatibility with other silicorie resins when attempts are made to coldblend t'hem. It seems natural, therefore, to expect that crosslinked silicone polymers where the R groups are partly methyl and partly phenyl would be the most useful members of this olass of resins. This is generally found to be the case. Murphy, Saunders, and Smith studied the structure and oxidative changes of siloxane polymers by infrared absorption spectra (16). Polymethylphenylsiloxanes are more stable to oxidation but more volatile than the completely methyl-substituted analogs of the same viscosity. Polysiloxanes, where one or more of the R groups is an unsaturated group like vinyl or allyl, have been prepared and studied. Since the unsaturated linkages in such polymers are capable of undergoing further reactions, a large number of other polymers becomes possible. In general, polysiloxanes containing unsaturated aryl or alkyl groups have inferior heat resistance when compared to the methyl or phenyl types. The unsaturated polysiloxanes do, however, possess somc unusual rharacterktirs and are in-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2335
triguing building blocks for a different, series of silicon? copolymers. For example,
c1
OC2H6 i CH,= CH--fii-OCnH,
I
CH? = CH--Gi--Cl
01'
I
I c1
OCnHs
Vinyltrichlorosilane
Vinyltriethoxysilanc
may be hydrolyzed in the presence of alcoholic solvents (so as to avoid gellation), to give
CH2 = CHSi( OH)a Vinylsilanetriol which may then be 1. Cross-linked by heating, 2. Copolymerized with other unsaturated materials in the presence of benzoyl pcroxide, or 3. Condensed with other polysiloxanes likc methyl- or phenylsiloxane to form various siloxane copolymers. These copolymers are not ordinarily stable and must, be used immediately after they are prepared. Glass cloth treated with vinyltrichlorosilanes produces polyester laminates with superior wet flexural strengths. Another class of useful silicone derivatives is the silicone allcydh (8-10, 12, 13). Tyler ( 2 4 ) believes that, silanoi hydroxyls may combine with the excess or available hl-drosyl groups in the alkyd, possibly by the following mechanism:
I OH
'
CHOH
v bI
CHZOH
I
VC-O-YH?
%
I
+
impart bet,ter chemical resistance, presumably because thcy art. cross-linked to a greater degree. Thcse soft., heat-stable siliconc resin-based aluminum finishes will withstsand temperatures to and possibly higher than 1000" Ii'. A recently published paper reports a finish of this type that will successfully withstand a 300-hour ASTM 13117-49T salt spray test after 16-hour exposures to 500" F., 750" F., and 1000O F. ( 6 ) . This particular coating resists salt, spray corrosion when simply air-dried. The dry film thickness of the coatings tested ranged from 0.7 :tct,ually wha,t t,he author has found useful as silirorie modifiri,s.
2338
Koiivulatile matter, Yq Ford No. 4 cup viecosity a t 77' F.. RFC Curing cycle Initial 60° Gardner gloss 8Oo Gloss after 24 hr. a t J J O O F. Flexibility&after initial bake FlexibilityQ after 24 h r . at 550' I' Abrasion resistance Sward hardness Adhesion t o steel and aluniinuiii Oil and grease resistance Color retention after 24 !ir. at 550" IC, a 1 8 0 O bend around '/%-inch iiiandrel ~
-
44
45
30 inin a t 40OC 1: 80 75 Pasvej Passes Good 19
Excellent Excellent Exaellrnt
This t,ype finish may be modified to give testured effects such as hammer finishes, spatter finishes, veiling finishes, and simulated leather finishes. Composit,ions based on sjlicone coat,ings have served to upgrade the heat resistant properties of the finish on space heaters and have permitted more economical designs of the heaters themselves. These finishes have replaced porcelain in some cases. Figure 5 shows a central gas heat,cr protected on the inside and outside with silicone finishw which viithstand the 875" F. temperature8 generated by the heater'P combustion chamber without powdering or changing color. Another type of silicone coating is exemplified by the white or near-white heat resistant. enamel. White silicone enamels intended for applications where heat is involved, require great, care
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 11
.. -Siliconesin formulation and manufacture. Any contamination from milling equipment, tanks, or other coatings will result in inferior heat resistance of the finished paint. Generally, 100% silicone or very slightly modified silicone vehicles may be used. The use of more than a very small amount of conventional organic binder will cause objectionable yellow-to-brown discoloration of the finish when exposed to temperatures in the range 590' to 600' F. Satisfactory white silicone enamels designed for use in this temperature range have been developed and are used in industry today. One such enamel will withstand an exposure of 579" F. for 24 hours and vary only slightly from an unexposed control panel in color or gloss. The color will turn very slightly yellower and the Gardner 60" gloss will decrease from about 90 to 80 after the heat exposure. The pigment-to-binder ratio of silicone white enamels must be kept somewhat lower than is normal for conventional white enamels in order to prevent heat-erasing. The grades and types of white pigments used must be carefully selected to give optimum properties with the selected resin. Untreated grades of titanium dioxide are usually satisfactory. dome varieties, however, have been found to promote premature heat-crazing of the enamels in which they are used. Films of 1.0 to 1.5 mils (dry) are normally applied. Undercuring must be avoided or poor film properties and cold-checking may occur. .I special primer is sometimes employed over steel to obtain improved adhesion of these white silicone coatings, and to upgrade their resistance to heat-crazing. This last fact may be an indication that iron catalyzes the decomposition of polysiloxaneq The same silicone white enamel composition will be more craze resistant over bare aluniinurn ihan over bare steel. A typical silicone white enamel might have the following composition: Titanium dioxide, lb. Silicone resin, 50% nonvolatile matter, lb. Solvent, lb. Zinc octoate, 8%) lb. Nonvolatile matter, 70 Viscosity, Ford No. 4 Csip a t 77' F., sec. Wt./ml.. lb. Appl&adon Reduction for spray Curing schedule
250 686 70 3 __ 1.009 = 100 gal. 54 5 I
40
10 09 Spray 8-1 with Holvent 30 min. a t 450" to 500' F.
A household incinerator finishcd with a silicone white enamel is shown in Figure 6.
TABLE
Catalyst
Zn Fe
I\'.
C U R I N G CATALYbTs &ED
Form Octoate or naphthenate Octoate or naphthenate
Suggested Amounts,
%
0.5" 0.1" 0.5"
Pb
Octoate 01 naphthenate
to 0.6''
Pb
Tctraplienyllead
to 2 . 0 1
Ca
Octoate or naphthenate
t o 0.6"
Po, M n
Octoate or naphthenate
t o 0.lU
Acids -t bases o-Phenanthroline A1
.. lfiopropylate
0.2b
to 0 . P
Naphthenate or Zr, Ce octoate Rare earth driers 0 Metal weight based on silicone solids b Total weight based on ailicone solids.
November 1954
W I T H SILICONES
Coiiiinents Generally best
Figurr 4.
Silicone Prokcti, e Coating Being Sprayed on Jet Afterburner Shroud Assembly
Thus far, no mention has bccn made of the curing catalysts used with silicone resins. When preparing white silicone e ~ i m i els, which will be cured a t 60 min. a t 900' F. or when no handling after baking is required, (as for srnolre stacks), no driers :we normally required. In other e w e s , Table IV may act as a guide in {,heselection of suitable curing agents or driers for use IT-ith silicone compositions. The adhesion of properly balanced silicone coatings to stee! and aluminum substrates is usually fairly good. Sandblasting of steel and some forms of mild phosphatizing for both steel and aluminum are occasionally used to ensure complete removal of soil and to improve paint adhesion. The comparison of the :tdhesion ratings of a typical silicone enamel over sandblasted, solvent-cleaned, and phosphatized etoels is shown in Table V. The phosphate coating is adversely affected a t about 400" i:. Crazing and/or peeling of the paint film results. Solvent wasliiiig of steel, although impractical for sonic large-scale product,ion
For dark colors-gives better t o p , may cause bodying, hurt lieat resistance, craze life, and flexibility Cause gelling in 100% silicones; satisfactory in some siliconealkyds Reported t o give rapid cure and freedoin from discoloration @) Similar t o lead but action is less rapid. Satisfactory in some silicone-alkyds Cause discoloration; may be adequate for dark colors or with silicone-alkyds; good top ?tnd hardness Cure well brit destroy color and good film properties, not ordinarily used Causes discoloration, gives good dry Causes discoloration, hurts crazelife, imparts good dry Show no advantages
Figure 5. Central Gas Heater (Generates 875' F.) Coated Inside and Outside with Silicone Protective Coating
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
2339
'
TABLE
v.
ADHESIOSO S JvHITE SILICONE ESAMEL T O PRZPdRED STEELSURFACES
Metal Preparation Yandblasted Phosphatized (regular) Solvent washed
Exposure Temp_erature 5000 I;. Good-excellent Good-excellent Good Fair Good Good 400° F.
600” F. Good Poor Fair-good
Figure 6.
Household Incinerator Finished with Silicone Vi-hite Enamel
iristallations, seems t,o be a fairly good method for cleaning metal prior to painting with silicones. Some forms of mild phosphatizing make good metal treat,ment,sfor silicone paint application. Although considerable work is being done on this problem, the chemical and physical specifications for a satisfactory phosphate coat,ing in this field are not precisely known. I n the packaging of silicone coatings cans with soldered side deamF should be scrupulously avoided as bodying and subsequent gellation will most likely result. For some types of dicone paints, unlined steel drums are adequate. For other types, (drums lined lvith high baked phenolic resin coatings should be used. The proDer packaging of silicone finishes should be ~~:a~refully worked out before committing any specific finish to large-scale production. Silicone finishes are also useful for applications involving chemica81and weather resist,ance ( I ) , such as painting laboratory and process equipment and outdoor signs. Pigmented silicone finishes show remarkable resistance to chalking and fading on weathering. Outdoor silicone clears may well set new standards for the industry. One report cites a white silicone enamel that iuccessfully withstands a 2-year exposure in Florida (45’ S) with no color change, no chalking, no cracking, no checking, no dirt collection, and no mildew growth. Some silicone finishes show inferior resistance to dirt collection and water spotting on prolonged outdoor exposures, and any finishes intended for this purpose should be scrupulously teeted with respect to these two properties. Properly cured silicones have excellent resistance to tincture iJf iodine, mercurochrome, mustard, sodium hydroxide, hydrochloric acid, oleic acid, India ink, fruit acids, and boiling water ( 2 0 ) . They have relat,ively poor resistance to gasoline, aromatic hydrocarbons, and other solvents. Properly catalyzed and cured silic7one alkyds are highly resistant to such destructive factors a s steam: n-ater, detergents, hydroca,rbons, formic, acetic, nitric,
2340
sulfuric, and hydrochloric acids but will be destroyed by sodium hydroxide. Silicone resins may be modified physically or chemically to make their reeistance to solvents adequate for most in? tended uses. The use of 5 to 20 p.p.in. of cert’ain silicones to prevent or reduce foaming during varnish or resin manufacture is now well established. Batch .sizes have been safely increased approximately 30% in some cases. The silicone is added as a solution in a suitable hydrocarbon or in carbon tetrachloride, and should be the silicone least compat,ible with the vehicle being synthesized. Silicone oils (50 to 150 p.p.m.) are widely used to control interfacial tension properties of coatings (22, 23). These oils are used in dipping enamels to eliminate sillring and floiv lines, in spraying enamels to reduce orange peel, in hammer and other metallic finishes to control pattern effects, in roller coating wet ink varnishes to obtain sharper definition of printing, and in other coatings t.o confer water repellency, mar resistance, release properties, freedom froin iridescence and resistance to dust accumulation. Silicone polymers have high dielectric strengths. Dust particlcs normally carry electrostatic changes and hence will not adhere as readily to silicone surfaces. Silicone oils could be added to the mill base before grinding and dispersing to aid in the efficiency of that operation, but this procedure could result in contamination of subsequent batches, as evidenced by “eye-holes,” “cran~ling,” poor recoatability, and other undesirable phenomena. These silicone oils have low surface tensions and low interfacial tensions with many liquids. They readily wet and cling firmly to finishes, metal, glass, and porcelain. They are not easily removed by one or two rinsings with solvent. These facts should be kept in mind when using dicone oils in any coating materials. The introduction of excessive amounts of the vvrong types of silicone oils in dipping enamels may cause severe bubbling or foaming, a condition difficult t,o overcome once it has occurred. Silicone coatings have high dielectric strengths. They are also fungistatic xhen properly cured. Their use for insulating varnishrs and fungistatic varnishes where high operating tcmperatures cause the deterioration of conventional vehicles is thus indicated. Silicone coatings are fungus resistant in this type of environment because of the l o a percentage of organic material in the molecule, their water repellency, and their good film continuity a t operating temperatures. The fungist.atic nature of adequately cured silicone resins may he demonstrated by the experimenk s h o w in Table VI.
Heat Exposure before Testing, 100 Hours a t , C. Fungus Reqistance (JAN-C-173) 83 Unsatisfactory 140 Unsatisfactory (slightly better than 8.5’ C. cure) SIodified 200 Excellent Xethvlphenylu 85 Unsatisfactory hIeth;lphen~-la 140 Satisfactory Methrlphenyl’i 200 Excellent P henylrnet hyl b as CTnsati?factorv Phenylniethyla 140 Cnsstisfactori (slightly better than 86’ C. cure) Phenylmethylb 200 Satisfactory a Preponderantly methyl. b Prenonderantly phenyl. Silicone Composition Modified Modified
A modified silicone coating composition containing mlubilimd copper-8-quinolinolate fungicide was tested over steel, anodizcd aluminum, and other surfaces capable of withstanding a cuiing cycle of 3 hours a t 300” F . This composition was testcd and found to pass lunpicidal and other performancc 1 c qnn emcnts of specification JAN-C-173. This coating was not snitablc for use ovcr some types of plastic components; it was not satisfactory when air-dried, and it did not conform to the composition rc-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 11
-Siliconesquirements of the subject specification. The same fungicidal coating was also found to perform well when cured for hour at 400’ F., but darkened somewhat when cured on this schedule. These experiences indicate that silicone-based fungicidal coatings may perform satisfactorily on electronic and other equipment which permit the necessary curing bakes, even though these coatings do not conform to the composition requirements of existing military specifications. Silicone-based coating systems for wire wound resistors have good dielectric strengths and water resistance and are finding increased usage in this field. Such resistors fall between cementcoated resistors and ceramic-coated resistors in price and may be safely operated a t temperatures to 550’ F. Some ceramiccoated resistors are given an additional topcoat of silicone enamel to improve humidity and salt spray resistance. Figure 7 shows some all-silicone coated resistors.
Figure 7 . Wire Wound Resistors Coated with All-Silicone Sj-stem
In the use of methylsilicone oils for vaxcs and polishes, the oils impart good water repellency, caw of application, high gloss with minimum rubbing, and improved durability. Silicones h a w also met n-ith succcss as clear water repellent coatings for maczonry. These silkone products are pale in color, air-drying, and effective on such surfaces as brick, concrete, cinder block, pipe insulation, and gypsum. Two varieties of this coating are used, the solvent-soluble type and the watersoluble sodium met onate. Concentrations of 2 to 5 % seem to be preferr he water repellent coatings permit masonry to breathe and thus reduce condensation. Since the contact angle of water on surfaces treated with these coatings is high (up to 110”), the water will tend to collect in droplets and run off. Discoloration, staining, spotting, spalling, cracking due to freeze-thaw cycles, and efflorescence clue to the evaporation of dissolved salts (such as sodium sulfate) on the surface are reduced or eliminated when such coatings are used. They also promote improved weathering and cleanliness of exterior surfaces. The addition of small amounts of sodium methylsiliconate to casein and alkyd emulsion paints has been recommended for the purpose of improving washability and water resistance. Improvements in washability to 300% result from the addition of 0.1 to 0.6% (based on paint solid’s) of this water-soluble silicone r brushability and smoothness also occur. I n on paints, the addition of as little as 0.1% sodium promotes less foaming, better brushability and flov, better ~ashability,and improved resistance to water and water-borne stains. The applications of the products of silicone chemistry to the problems of the organic coatings industry are many and varied. Kew developments are a t hand involving silicone intermediates which will react with long chain carhosylic acids, polyols, phthalic anhydride, phenolic hydroxyl groups, and epoxy groups. Con-
November 1954
jectures as to the ultimate uses of many of these copolymers should stimulate further research in many fields. I n many cases silicone polymers have made available finishes far better than those possible from the older types of organic resins. I n some cases, silicone coatings have replaced inorganic porcelain coatings because of lower material and application costs, better resistance to chipping and cracking, and greater ease of repair. The silicone resins and oils make remarkable additives to some conventional coatings because of the low surface tension characteristics, wetting properties, and release properties imparted by even small additions. An example of a practical use of this type is found in a siliconebased “stop-off” coating for galvanizing operations where it is desired to galvanize only part of the surface area immersed in the molten zinc. This coating provides a protective film t,hat resists the chemical solutions and heat encountered in hot dip galvanizing and prevents the zinc from adhering to the base metal. The “stop-off” coating is sprayed to a film thickness of approximately 0.7 mils dry on the areas to be masked, and then air-dried for 30 minut,es or baked for 8 minutes a t 325’ F. When the masked object is placed in the galvanizing bath, the zinc will not plate on those surfaces coated with the silicone [‘stop-off” coating. The uses of organosilicon polymers and copolymers in the protective coatings industry are still in their early stages. As our understanding of the reactions and behavior of these resins is improved, and as a t least four well qualified cheniical companies step up the tempo of their work in this field, paint chemists will, perhaps, be able to make coatings that today exist only in their dreams. There are indications thst silicone-based coatings will, in the years to come, materially aid in the development of 1. Outside paints that will last 10 years or longer. 2. Outdoor varnishes that will have service lives of 5 years or more when used on wood and metal. 3. Hybrids between organic and inorganic coatings, where a low fusing inorganic composition will take over the prot’ective duties when the organic constituent,s have volatilized away. 4. Container linings that will have the long sought after combination of properties-resistance to a wide variety of foods and chemicals, excellent adhesion, and good flexibility and fabrication properties when applied t’o metals a t the desired film t’hickness. 5 , Heat and corrosion resistant coatings that will reduce the staggering annual upkeep costs in the chemical and process industries. 6. Heat and weather resistant coatings that will find increasingly prominent places in the automotive, aircraft, appliance, heating, and electrical industries. ACKNOWLEDGMENT
The author wishes to express his sincere apprecistion to R. IT. Kolderman and his colleagues of the Dow Corning Corp., 17’. ,J Dugan and his colleagues of the General Electric Co., and E H Miller, E. J. Bromstead, and G. K. Hughes of the Midland Industrial Finishes Co. for their valuable assistance in the preparation of this report. The author also wishes to acknowledge with thanks the permission of the Midland Industrial Finishes Co. to publish this paper. LITERATURE CITED
(1) British Thomson-Houston Co., Ltd., British Patent 607,022 !Aug. 24, 1948). (2) .Ibtd., 698,300 (Oct. 14, 1953). (3) IBrornstead, E. J., and Glaser, n4. A , Org. Finishing, 9, 21-6 (March 1948). (4) Durkin, A. E., and Homer, A. H., Materials & Methods. 38, NO. 3, 114-16 (1953). (5) Glaser, M. A., Am. Paint J . , 38,No.5, 74-112 (1953). (6) Glaser, h l . A , ;Modern Lithography, 22, KO. 1, 59-62 (1954).
INDUSTRIAL AND ENGINEERING CHEMISTRY
2341
( 7 ) Glaser, hl. A , , Product Eng., 21, 109-11 (February 1950). (8) Goodwin, J. T., *Jr., and Hunter, 11.J. (Dow Coriiing Corp.), U. S. Patent 2,584,340 (Feb. 5, 1952). (9) Ibid., 2,584,344. (10) Ibid.,2,589,243 (March 18, 1952). (11) Hedlund, R. C., Finish,10, No. 1 0 , 4 5 4 107-8 (1953). (12) Kress, B. H., and Hoppens, H. A., Dig. Federation P a i n t & Varnish Production Clubs, No. 333, 659-99 (1952). (13) Libbey-Owens-Ford Glass Co., Brit. Patent 694,716 (July 29, 1953). (14) MeGregor, R. R., “Silicones and Their TJses,” 3lcGraw-Hil1, New York, 1954. (15) Miller, E., and Glaser, 11.A , , Prod7~ctEng,,24, 167-74 (March 1953). (16) Murphy, C. AI., Saunders, C. E., and Smith, D. C., IND.ENG. CHEM.,42, 2462-8 (1950). (17) Ofic. Dig. Federation, Paint & T‘amish Pvoduetion Clubs, N o . 250, 4 2 4 4 ’ (1945). (18) Ibid., 262, 534-9 (1946).
(19) lbid.,334, 749-54 (1952). (20) Patterson, Ofic. Dig. Federation Paint & Varnisli Production C h b S , NO.258, 281-90, 439-42 (1946). (21) Rochow, E. G . , “-4n Introduction to the Chemistry of the Silicones,” John Wiley, Sew York, 1961. (22) Sage, C. 31. (General Electric Co.), U. S. Patent 2,523,065 (Sept. 19, 1950). (23) Shur, E. G., Ofic.Dig.E’eclcration Paint & Varnish Production Clubs, NO.323, 887-77 (1951). (24) Tyler, L. J., Dow-Corning Corp., Midland, Mich., private conimunicat,ion, 1954. (25) Warrick. E. L., Hunter, 11. J., and Barry, A , J., IND. ESG. CHEM., 4 4 , 2 1 9 6 2 0 2 (1952). (26) Williams, R.A., Luta, I. H., Hayne, W. H., Louisville Institute of Industrial Research, Louisville, Ky., “Development of
High Temperature Heat Resistant Coatings.” RECEIVED for review March 2 7 , 1R54.
ACCEPTEDScptembor 0 , 1964.
Compounding rinciples of Silicone Rubbers C . W. PFEIFER General Electric Co., Waterford, \-. Y .
Silicone rubber gums provide the rubber industrj with new polymeric materiale useful for compounding in rubbers, w-hich extend the application of elastomeric materials. Rubbers can be made that are flexible below -100’ F. and that will withstand service as high as 600’ F. Good electrical properties w-ith excellent resistance to ozone are obtainable. Sonie principles involved in the designing of a general purpose group, a low compression set group, and an extreme low temperature group of silicone compounds are described. These formulations and data are expected to be useful to many rubber compounders who are assuming the task of silicone rubber gum compounding. HE importance of the engineering possibilities of silicone rubber was recognized m early as 1942; indeed, the first United States Patent covering met,hylsiloxane elastomers was applied for in 1944 (1). Production of the silicone rubber parts for the first t,wo military applications, gaskets for turbosuperchargers and gasket,s for searchlights, was accomplished by research laboratory personnel. Shortly after the war, the applications for silicone rubber began to grow; today the list of companies fabricating silicone rubber contains over 100 names, and new ones are being added steadily. Since the industry is relatively new, technical progress is being made rapidly; the silicone rubber industry is a fine exainple of progress in the competitive enterprise system. Progress is being made in the development of improved gums, reinforcing fillers, and compounding and fabricating techniques. Silicone rubber has some unique basic properties which are responsible for its place in the industrial picture. Rubbers can be made that will withstand temperatures as high as 600’ F. without serious deterioration; rubbers can also be made that are flexible a,t - 130’ F. The oxygen, ozone, and weathering rcsistances are cxtremely good. The basic property of inertness makes it useful for many room temperature applications such as surgical rubber and pharmaceut
