Conclusions
Acknowledgment
I t appears that the polyphenyl ether, m-bis(rn-phenoxyphenoxy)benzene, will be a prime candidate for use as a hydraulic fluid and possibly a n engine oil or a gear box lubricant where operating temperatures u p to 500’ or 550’ F. will be encountered. The useful range of this fluid can be extended to 700’ to 800’ F. for applications where small quantities of insolubles formed by the fluid can be tolerated. Although rnbis(m-phenoxyphen0xy)benzene has outstanding resistance to high temperature oxidation, the oxygen content of systems using this fluid above 500’ F. should be kept low to retard viscosity increases. No major problems with either acidity or corrosion, except with beryllium copper, can be expected a t temperatures a t least to GOO’ F. even in the presence of oxygen. The high resistance of this fluid to shear-induced degradation to a t least 800’ F. indicates that it would be stable in applications where the fluid is recirculated and/or subjected to severe shear stresses. rn-Bis(m-phenoxyphen0xy)benzene has two other characteristics which suggest applications where other fluids have encountered or would encounter problems. The radiation resistance of this fluid, as determined by changes in viscosity and oxidation resistance of irradiated fluids, is superior to many other fluids and so consideration for uses in environments subjected to high levels of radiation is warranted. Another characteristic, the bulk modulus, is high enough to put m-bis(mphenoxyphen0xy)benzene virtually in a class by itself. I n hydraulic systems containing long tubing runs, actuator position control would be more positive and less subject to oscillations when this relatively incompressible fluid is used, as compared to the more compressible esters, mineral oils, and silicones. Not all the characteristics of rn-bis(m-phenoxyphenoxy)benzene are good. The base stock appears to be only a fair lubricant. However, the antiwear behavior can be appreciably enhanced by the addition of tricresyl phosphate. One major problem with this fluid for some applications is that it has a pour point of about plus 40’ F. Of course, this property is no problem at all where the fluid can be heated. Research efforts are now being directed toward lowering the pour point without making appreciable sacrifices in other desirable characteristics.
The authors are indebted to many members of the Air Force Materials Laboratory, Research and Technology Division, for their assistance. literature Cited
Bamberger, E. N., Moore, C. C., “Evaluation of Effect of Six Lubricants on Rolling Contact Fatigue Life of M-50,” Aeronautical Systems Division, Wright-Patterson AFB, ASD-TR61-429 (November 1961). Christian, J. B., “Micro Lubricant Test Methods,” Wright Air Development Center, WADC-TR-55-449, Part 3 (May 1956). General Services Administration, “Federal Test Method Standard No. 791-Lubricants, Liquid Fuels, and Related Products; Methods ofTesting,” Dec. 15, 1955. Hopkins, V., Benzing, R. J., IND.ENG.CHEM.PROD.RES. DEVELOP. 2, 71 (1963). Hopkins, V., St. John, A. D., Wilson, D. R., “Lubricating Behavior and Chemical Degradation Characteristics of Experimental High Temperature Fluids and Lubricants,” Wright Air Development Division, WADD-TR-60-855, Part I1 (January 1962); Part I11 (January 1963). Hopkins, V., Wilson, D. R., “High Temperature Shear Stability of Potential Hydraulic Fluids,” Proceedings of USAF Aerospace Fluids and Lubricants Conference, San Antonio, Tex., April 16-19, 1963. Hopkins, V., Wilson, D. R., IND.ENG. CHEM.PROD.RES. DEVELOP. 3, 38 (1964). Hopkins, V., Wilson, D. R., Lubrication Eng. 20, NO. 8 , 305 (August 1 OLA\ 1 ,“T,.
Hopkins, V., Wilson, D. R., Bolze, C., ASME J . Basic Eng. 86, No. 3, 463 (1964). Hopkins, V., Wilson, D. R., Klaus, E. E., ASLE Trans. 7, No. 2, i86 (April 1964). Klaus, E. E., Fenske, M. R., Tewksbury, E. J., “Fluids, Lubricants, Fuels and Related Materials.” Aeronautical Systems Division Report WADD-TR-60-898, Part I1 LFebruary 1962). Mahoney, C. L., Barnum, E. R., in Synthetic Lubricants,” by R. C. Gunderson and A. W. Hart, p. 406, Reinhold, New York, 1962. Mahoney, C. L., Barnum, E. R., Soari, W. S., Sax, K. J., Kerlin, W. W., “Nuclear Radiation Resistant High Temperature Lubricants,” Wright Air Development Center, WADC-TR59-173 (September 19:”. Republic Aviation Co., Investigation of Techniques for 1000” F. Hydraulic Systems,” Aeronautical Systems Division, WrightPatterson AFB, ASD-TDR-62-674, Part I (September 1962), Part I1 (March 1963). Ullmann, F., Sponagel, P., Justus Liebig’s Ann. Chem. 350, 83 (1906); C. A . 1,436 (1907). for review December 23, 1966 RECEIVED ACCEPTEDApril 3, 1967 Work supported primarily by the Air Research and Development Command, USAF, under contracts AF-33(616)-3182, -5617, -6854, -7218, -7590, and AF-33(657)-10295.
SYNTHESIS AND PROPERTIES OF SILOXANEPO LY ET H ER CO POLY M ER SU R FACTA N T S BERNARD KANNER, W. G. REID, AND I. H. PETERSEN Silicones Division, Union Carbide Corp., Tonawanda, N . Y .
HE unique behavior of methylsiloxane-oxyalkylene coTpolymers in stabilizing flexible polyurethane foam has prompted us to investigate the surface properties of these materials in aqueous and nonaqueous media. Although the unusual surface activity of siloxane polymers has long been known, only preliminary reports have appeared to the synthesis and behavior of surfactants based on methylsiloxanes ( 3 , 6 ) . The first siloxane-polyether copolymers were synthesized by the reaction of alkoxymethylsiloxane polymers with hydroxyterminated polyethers ( 2 ) .
88
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
Me
Me
I I -Si-0-Si-OEt I I Me
+ HO(CH&HRO),R‘
--c
Me Me
Me
I I -Si-0-Si-O(CH&HRO),R‘ I I Me
Me
+ EtOH
Properties of a series of methylsiloxane-oxyalkylene copolymers are compared with typical hydrocarbon surface active agents. The siloxane surfactants were characterized by surface tensions as low as 20 to 2 1 dynes per cm. in aqueous solution and relatively small micelles (aggregation numbers of 3.4 and 4.7). As a result of low aqueous surface tensions, certain of these copolymers were excellent wetting agents for low energy hydrophobic surfaces such as polyethylene. Unlike hydrocarbon derivatives, methylsiloxanepolyether copolymers were also surface active in nonaqueous polypropylene glycol systems, a contributing factor in the stabilization of polyurethane foam.
While many of these copolymers are water-soluble and exhibit typical surface active properties, the SiOC bonds in these compounds are subject to hydrolysis particularly in the presence of acids or bases. Hydrolytically stable methylsiloxane-polyether copolymers have been prepared by the catalyzed addition of hydrosiloxane polymers to the unsaturated ethers of polyoxyalkylene glycols according to the general equation (7) : Me
Me
I
I
-Si-0-Si-H
I
Me
Pt
f CHFCH-CHZ
(OCH2CHR),OR'+
I
catalyst
Me Me
Me
I I -Si-O-Si-(CHz) I I Me
a(OCHzCHR),OR'
Me
This paper describes the properties of such a series of methylsiloxane-polyoxyethylenecopolymers and compares them with conventional hydrocarbon nonionic surface active agents. Experimental
For convenience the following abbreviations are used to designate the structural formulas of silicone surfactants: Me = C H I ; E t = CZHS; 9 = CeHb; EO = -CzHdO-;
D
M = Me3SiOl/z; M ' = -CH2(Me)zSiOliz;
D'
=
=
MezSiO;
-CHZ(Me)SiO
T h e copolymers were synthesized by the addition of methylsiloxanes, containing reactive Si-H groups, to the allyl or vinyl ether of a methoxypolyethylene glycol using chloroplatinic acid catalyst. T h e siloxane polymers were relatively pure materials of discrete chemical composition and boiling point rather than equilibrates. T h e allyl ether starting materials were prepared by the Williamson ether synthesis, making the sodium salt of commercially available methoxypolyethylene glycols react with allyl chloride. Allyl ethers were also prepared from purified fractions of methoxypolyethylene glycols obtained by molecular distillation. Purity of the allyl ethers was confirmed by bromine number analysis. T h e vinyl ether starting materials were commercially available products which were redistilled before use. Lower molecular weight copolymers were distillable liquids having properties summarized in Table I. Higher molecular weight nondistillable surfactants were prepared by the same general procedure (Table 11).
of 0.5 hour. The reaction was exothermic, with the temperature rising to 110' C. After addition was complete, heating was continued a t 150" C. for 4.5 hours. After cooling, the product was fractionally distilled through a 12-inch column packed with Helipak Hastelloy B (0.092 x 0.175 X 0.175 inch) under reduced pressure. A forerun consisting largely of H (8.0 grams) was collected along unreacted (Me) &OSi(Me) 2 with 134 grams (86% conversion) of adduct [b.p. 88'/0.25 mm., nD26' = 1.4232, d25' = 0.9084 gram per ml., MRD (molar refraction) calculated = 86.64, observed = 86.511. Preparation of [ (Me) &Si (Me) 2 0 1 2Si (Me)CHZ(CzH40)14.?Me (11, Table 11). Synthesis of [ (Me)3CSi(Me)20]2Si(Me)H. I n a 1-liter, round-bottomed, three-necked flask were placed 100 ml. of benzene, 30.8 grams of MeSiHClZ (0.268 mole), and 42.3 grams (0.536 mole) of pyridine. From an addition funnel a solution of 70.9 grams (0.536 mole) of (Me)3CSi(Me)zOH ( 8 ) in 100 ml. of benzene was added to the flask a t 5' to 15' C. over a 30-minute period. The reaction mixture was stirred for 2 hours a t room temperature, washed several times with water, and distilled through a 12-inch column packed with medium size Helipak Hastelloy B. The product (b.p. = 103' C./lO mm.; nD25 = 1.4153) was obtained in 84% yield (74 grams). Calculated for C13H34Si302; C, 50.9; H, 11.2; Si, 27.5; SiH, 73 cc. per gram. Found: C, 50.5; H, 11.0; Si, 27.0; SiH, 75 cc. per gram. ADDITION TO CHF=CH-CHZO(CZH~O) 13.sMe (111). To a 100-ml. flask were added 13.6 grams (0.02 mole) of 111, 6.13 13 ml. of grams (0.02 mole) of [(Me)3CSi(Me)zO]zSi(Me)H, toluene, and one drop of an ethanolic solution of H2PtC16. 6 H 2 0 (3 x 10-6 mole of Pt). The flask was heated a t 110' C. for 18 hours. Toluene was removed by distillation and 17.9 grams of liquid product I1 remained. Calculated for C44.6H97.~Si301~.8:C, 54.29; H, 9.93; Si, 8.54. Found: C, 54.8; H, 10.2; Si, 7.5.
Preparation of Me (CHZ)~S~(M~)~OS~(M~)ZCHZ(CZH~O Me (IV, Table 11). Me(CH2)7Si(Me)zOSi(AMe)zH (V). T o a 1-liter, three-necked flask were added 160 ml. of acetone, 31.5 grams of NH4HCO3, and 13 grams of water. After the flask had been cooled to 10' C., a solution of 83.2 grams (0.362 mole) of Me(CH&Si(Me)Z(OOCMe) in 80 ml. of acetone was added over 5 minutes. After stirring 1 hour a t 23' C., 100 ml. of water was added. The mixture was then extracted with 250 ml. of benzene and the benzene was washed with water until neutral. The benzene layer which contained the product, Me(CH&Si(Me) zOH, was dried by azeotropic distillation. The silanol was not isolated but was made to react with a solution of 37.6 grams of (Me)zSi(Cl)H and 31.5 grams of pyridine in 100 ml. of benzene a t 1' to 20' C. The benzene layer was filtered, washed to neutrality, and dried. Distillation through a 12-inch Helipak column gave 69.6 grams of V (b.p. 106-07" C./lO mm., n$ = 1.4169; MRD calculated = 76.96; obsd. = 76.96). Calculated for C12H30Si20: C, 58.46; H, 12.26; Si, 22.78; SiH, 90.8 cc. per gram. Found: C, 58.4; H, 12.1; Si,22.2; SiH, 9 0 c c . p e r g r a m .
I n~ .a ADDITION O F V TO C H F C H C H Z O ( C ~ H ~ O ) I ~ . S M 100-ml. distillation flask were placed 10.0 grams (0.0406 mole) Preparation of ( M ~ ) ~ S ~ O S ~ ( M ~ ) Z C ~ H ~ ( (I,O C ~ H )Z O Mgrams ~ (0.0374 mole) of C H F C H C H Z O ( C Z H ~ ) I ~ . S of~V, 25.4 Table I). In a 500-ml. flask equipped with a reflux condenser, Me, and 25 ml. of toluene. Two drops of an ethanolic soluaddition funnel, and thermometer were placed 75 grams (0.51 tion of HzPtC16.6H20 (6 X 10-6 mole of Pt) were added and mole) of (Me)aSiOSi(Me)zH and 6 drops of an ethanolic soluthe reaction mixture was heated to 110' C. for 18 hours. tion of chloroplatinic acid (containing 4.7 X 10-6 mole of Toluene and excess V were removed by vacuum distillation. platinum). The solution was stirred and heated to 70' C. The product (IV) was obtained as a liquid residue (34.0 From the addition funnel 81 grams (0.51 mole) of CHz= grams). Calculated for C43.6H93.2Si2016.8: C, 56.52; H, CH-CHz(0CzH4)~OMe was added to the flask over a period 10.06; Si, 6.06. Found: C, 56.3; H, 9.9; Si, 5.8. Methylsiloxane-Oxyethylene Copolymers
VOL. 6 NO. 2
JUNE 1967
89
Table 1.
Distilled Methylsiloxane-Oxyethylene Copolymers
O
% si
%"
MRD
G./Ml. nDpk 0.8314 1 , 4 0 2 5
Calcd. 64.58
%C Obsd. Calcd. Obsd. 64.62
Calcd. Obsd. Calcd. 1 0 . 9 7 1 1 . 0 25.48
0.8798
1.4124
70.98
70.90
47.95
47.9
1 0 . 4 6 1 0 . 5 2 2 . 4 3 21.9
89.6
0.8784
1.4162
75.58
75.61
49.94
50.0
1 0 . 6 3 1 0 . 6 21.23 2 1 . 1
88.2
0.9084 1.4232
86.64
86.51
50.61 5 0 . 4
dZ5,
B.P., C . ( M m . ) MeaSiOSi(Mez)CzHaOEt' 94 (40) MepSiOSi(M ~ Z ) C Z H ~ O C Z H ~ OMe 105-6(15) Me&OSi( M ~ z ) C ~ H ~ O C Z H ~ OMe 119-122 (15) Me&Osi(Mez)C3H~(OC2Ha)zOMe ( I ) b 88 ( 0 . 2 5 ) Me3SiO [ Si(Mez)O]3Si( Me& CZH~OCZH~OM~ 96-99 ( 0 . 1 5 ) M ~ ~ S ~ - C ~ H ~ ( O C Z H ~ ) 160-170 Z O M ~ (0.13) Structure
Conversion, Mole
0.9208
1 , 4 0 8 1 126.90 126.74 40.63 4 0 . 8 1 ,4402
10.45
10.4 18.2
9.38 9.0 10.20 1 0 . 3 12.35
Obsd. % 24.9 95.5
18.2 86.0 90.2 12.5 82.7
I 0
0
Me = CH3; E t = CzHa.
b
Preparation described in Experimental.
Table II. Nondistillable Methylsiloxane-Oxyethylene Structure
MesSiOSi(M ~ Z ) C H Z ( C Z H ~ O ) ~ . ~ ~ M ~ MesSiOSi(Mez)CHz(CzH40)17,1Me MeSi-CHz( CzH40 )8.87Me
Copolymers
% Si
%H
%C
Calcd.
Obsd.
9.87 9.58 9.65
0 bsd. 9.7 9.5 9.7
5.93 5.89
5.7 5.4
9.38 9.11 9.06 8.99 9.34 9.33 8.86 9.43 9.15 9.15 9.93 10.06
9.5 9.1 9.1 9.0 9.5 9.4 9.1 9.3 9.4 9.1 10.2 9.9
17.99 22.4 20.3 18.88 15.29
17.4 21.4 19.6 18.8 15.2
14.82 12.01 12.75 10.62 8.54 6.06
14.1 11.2 13.1 9.9 7.5 5.8
Calcd.
Obsd.
Calcd.
52.38 53.24 53.74
52.7 53.0 54.0
47.0 44.0 44.9 44.4
I
cdhq)~,6~Me Me&O( Me~Si0)3Si( Mez)CHz(CzH40)~.6Me 46.68 VfeaSiO(MezSiO)sSi(M ~ z ) C H Z ( C Z H & ~ ) ~ . ~ ~43.51 M~ 'MeaSiO(MezSiO)s].zSi(Me)CHz(CzH40)12,46Me 44.32 Vfe3Si0(Me~SiO),Si( M~Z)CHZ(CZH~O)~~.~O 44.63 M~ r?esSiO(MezSiO)3Si(Mez)CHz(CzH40)11,sMe MerSiO(MezSiO)],Si(Mez)CHz(CzH40)13,6Me 48.38 '( MezSiO)a(MeSi(O)CHz(CzH40)lo,zM~)l 46.44 50.04 VfesSiO [MeSi(O)CHz(C Z H ~ O ) ~ . &Me3 ~ZM~] .MeSi(O)CHz(CzH40)3Me]4 49.06 49.97 .MeSi(0)CH~(CzH40)4Me] 4 MesCSiMezO)zSi(Me)CH2(CzH40)14.~Me(11)' 54.29 CsHli)MezSiOSi(Me2)CHz(C~H40)~4.sMe (IV)" 56.52 11, IV. Preparation described in Experimental.
48.8 45.8 50.5 49.0 49.7 54.8 56.3
Q
Other copolymers of higher molecular weight which could not be directly purified by distillation were similarly stripped under reduced pressure to remove solvent and unreacted hydrosiloxane. The conversions to copolymers were 80 to 99%. For the study of surfactant properties the presence of unreacted polyether in the product was not detrimental. Surface tension measurements were made by du Nouy tensiometer and Cassel maximum bubble pressure methods with good agreement between the two methods (4,5 ) except a t high dilutions in the region of critical micelle concentration. In this concentration range the Cassel gave consistently higher readings possibly due to the high surface to volume ratio characteristic of this bubble method. Critical micelle concentrations were taken as the concentration corresponding to the inflection point of the curves of surface tension us. log concentration. Micelle size or aggregation number was determined from molecular weight measurements made with a Mechrolab vapor pressure osmometer, Model 301 (Hewlett Packard, F and M Scientific Division). Measurements were made on 1 to 10 weight aqueous solutions of surfactants a t 37' C. The numbers obtained were essentially constant over the concentration range studied. 90
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
A test was devised to measure the relative ability of surfactant solutions to wet low energy surfaces such as polyethylene. A 0.02-ml. drop of a 1 weight 70 aqueous surfactant solution was placed by a microsyringe on a clean piece of plastic sheet. After 3 minutes the largest diameter of the drop was measured with a Vernier caliper. The increase in diameter of the surfactant solution drop over that of a drop of distilled water was taken as a measure of wetting ability. Results and Discussion
Aqueous Surface Tensions. In Figures 1, 2, and 3 are shown the surface tension-concentration curves for a number of methylsiloxane-polyether and related surfactants in aqueous solution. For comparison several typical hydrocarbon surfactants are included in Figure 3. I t is apparent from these curves that the limiting surface tensions and critical micelle concentrations are significantly lower for methylsiloxanepolyether copolymers. T h e lowest surface tensions range from 20 to 21 dynes per cm. a t 25' C. with critical micelle concentrations as low as 10-6mole per liter. The most surface active copolymers are generally based on relatively small siloxane hydrophobes having only 2 to 5 silicon atoms. Siloxane branching and polyether chain length
6or
70 r
Y
N In
501
s40t E
-*
\
\
In
2ol '
10
I0-5 10-4 10-3 CONCENTRATION (MOLES/Kg 1
10-6
I
I6 5 I0-4 IO-^ CONCENTRATION (MOLES / K g l
10-6
10-1
10-2
I
1
I
I
I
IO
MD3 M'-E08,2Me
10-2
10.1
Figure 3. Comparison of surface tension vs concentration for hydrocarbon and siloxane-polyether copolymers
Figure 1. Surface tensions vs. concentration for linear siloxane-polyether copolymers Table 111. 70
r
Wetting Properties of Siloxane and Hydrocarbon Surfactants on Polyethylene Aqueous Surface Tension, Dynes per Cm., 2 5 O c. Cloud 1 W t . yo Spreading o n Point, Surfactant Solution Polyethylene, 7G ' C.
*\
MM '(EO)e..,Mea / ~ ~ , " ~
_
M3T '(E0)13.2Me D3D'(EO)a.oMe D3D'(EO)lo.2Me MM '( EO)e.sMe MD3M '(E0)11.8Me MDqM ?.aMe I
I
10
22.2 20.1 20.5 20.9 21 4 20.2 20.4 20.0 30.4 23.4
33.6 26.3 25.0 30.0 26.1
have relatively minor effects on surface tension lowering. The introduction of higher alkyl groups such as n-octyl and tertbutyl results in decreased ability to lower surface tension. The low surface tensions of methylsiloxane-polyether copolymers are directly related to the properties of the siloxane hydrophobe. The surface tension of liquid methylsiloxane polymers is 5 to 8 dynes per cm. lower than comparable hydrocarbon polymers ( 6 ) . This may result from the favorable orientation of the siloxane methyl groups at an interface or the low mutual attraction and high chain flexibility characteristic of methylsiloxane polymers. Micelle formation was inferred from the marked inflections of surface tension-concentration curves for siloxane-polyether copolymers in aqueous solution. Aggregation number or actual micelle size, as determined by vapor phase osmometer measurements, gave values of 4.7 for (Me)aSiOSi(Me)2(CH2)3(C2H40)7.2Me and 3.4 for (Me) 3SiO [Si(Me) 2 0 1 3 Si(Me) 2(CH2)3(C2H40) 16.aMe. Although low, these values are comparable to aggregation numbers of 4.4 to 13 reported for a series of octylphenol- and nonylphenol-ethylene oxide adducts in water (7). Wetting. T h e spreading of a liquid on a solid is determined by the spreading coefficient:
s
= 7s - (71
+
Ydl)
A liquid will wet out a solid-i.e., spread a thin film (approaching a zero contact angle)-if the spreading coefficient is posi-
127 >300 (wets) >300(wets) >300 (wets) >300 (wets) 281 198
214 51 144 42 51 86
38 31
