97
Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 97-103 Leal. L., Skoog, J., Acrivos, A., Can. J . Chem. Eng., 49,569 (1971). Pavelic, V., Saxena, V., Chem. Eng., 76(21), 175 (1969). Princen, H. M.. Overbeek, J., Mason, S. G., J . Colloid Interface Sci., 24, 125 (1967). Princen, H. M., Mason, S. G., J . Colloid Sci., 20(4), 353 (1965). Prud'homme, R., presented at the 49th annual meeting of the Society of Rheology, Houston, Texas. 1978. Ranade, V. R., Ulbrecht, J. J., AIChE J . , 24(3), 796 (1978). Richard, J., C.I.L., MacMasterville. Quebec, private communication, 1978.
Robinson, R. V., Can. Mining Metal Bulletin, 1317 (1969). Shinnar, R., Church, J. M., Ind. Eng. Chem., 52, 253 (1960). Vermeulen, T., Williams, G. M., Langlois, G. F., Chem. ~ n g Prog., . 5(12), 85 (1955). Zieminski, S. A , , Hili, R. L., J . Chem. Eng. Data. 7(11), 51 (1962).
Received for review January 16, 1979 Accepted September 10, 1979
The Surface Activity of Silicones: A Short Review Michael J. Owen Dow Corning Corporation, Mldland, Michigan 48640
This review explores the relationship between polymer structure and surface properties of silicones, particularly polydirnethylsiloxane. Both the intrinsic surface activity of the pendant organic groups and the manner in which they are presented by the uniquely flexible siloxane backbone are considered. The relationship between these basic surface parameters and the many surface-related applications of silicones is also discussed. Comparison is made to other low-surface-energy hydrocarbon- and fluorocarbon-containing polymer systems.
Introduction The surface properties of silicones, or-more exactlyorganosiloxane polymers, are responsible for many of their applications. The origins of this unusual and useful surface activity are closely related to the unique chemistry of these polymers. Silicones consist of an inorganic siloxane backbone with pendant organic groups, usually methyl. Both the nature of this backbone and the pendant organic groups contribute to the surface activity of the polymer. It is the thesis of this paper that polydimethylsiloxane (PDMS) is a favored case of a very surface active pendant group, -CH3, whose activity is presented to best effect by the unique flexibility of the backbone. This particular combination also has thermal and oxidative stability benefits that are significant in certain applications, but this aspect will not be pursued here. Although much of the discussion is devoted to polydirriethylsiloxane, some account is also made of pendant groups other than methyl. Comparison to other polymer systems, particularly hydrocarbons and fluorocarbons, is also made. Of the silicone products in commercial use, the polydimethylsiloxanes are the most common and possess the most interesting surface properties. These polymers are colorless liquids ranging in viscosity from 6.5 X m2 s-l to over lo4 m2 s-' at 25 "C. Their general structure is (CH3)$3iO[(CH3)2SiO],Si(CH3)3,where n is approximately 0 to 2500. The fluids are insoluble in water and have an unusual combination of very low volatility, low surface tensions (between 16 and 21 mN m-I at room temperature), and low cohesive forces in surface layers. It is the low intermolecular forces that produce most of the unusual surface and bulk properties of polydimethylsiloxane. These low intermolecular forces, which result from the intrinsic surface activity of the methyl group coupled with the unique flexibility of the siloxane backbone, enable the polymer to adopt a v i ~ i e t yof configurations which present a surface that is wholly methyl. The high intermolecular forces associated with the inorganic silicate-like backbone are masked by the inert low-surface-energy organic groups and the similarity between the surface parameters of po0196-4321/80/1219-0097$01.00/0
Table I. Surface Tension ( u l v ) and Interfacial Tension against Water ( u I w ) of Low Molecular Weight Analogues of Polymers compound n-octane octamethyltrisiloxane n-perfluorononane perfluoropropylene oxide telome P
!P> C 126 153 125 152
z"d
2"orc YC rnNm-' m N m - ' 51.'ib
21.7b 17.OC 14.3d
42.5c 56.4d
13.V
-_
Actual formula CF,( CF,),[OCF( CF,)CF Values from ref 2. Values from ref 6. ref 8. e Values from ref 4. a
OCHFCF,.
2h2Values from
Table 11. Contact Angle ( B H , o ) of Water o n Solid Surfaces polymer paraffin wax poly dimethylsiloxane polytetrafluoroet hylene
0w.n
105-114" 102-110" 108-114"
b a
a Values from ref 1. Cured on glass a t temperatures > 2 0 0 'C, values from ref 11.
lydimethylsiloxane and some wholly organic methyl-containing materials is quite marked. This similarity is demonstrated in Tables 1-111, which show several of the conventional surface properties. Table I records the surface tensions of several liquids and their interfacial tension against water, Table I1 gives the contact angle of water on some solids, and Table I11 shows the critical surface tension of wetting (crJ and a surface tension called crl,extrap-20 "C. This property is the surface tension for those polymers that are liquid a t 20 "C. For those that are solids at that temperature it is an extrapolated value from studies on molten polymers a t higher temperatures. Because of the dependence of ulv on molecular weight, the highest molecular weight material reported (value in parentheses) has been chosen. Table I describes pure compounds representative of the hydrocarbon, fluorocarbon, and silicone systems that are 1980 American Chemical Society
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Tahle 111. Critical Surface Tension of Wetting (a,) and Surface Tension Extrapolated to 20 "C from Studies on Melts ( u l y Extrap -+ 20 "C) for Variety of Polymers u , 20 OC:
polymer
mN m - '
u 1 v extrap + 20
"C,b mN m"
Methyl-Containing Polymers paraffin wax 23c polydimethyl24 20.4f ( l o 4 m 2 s - ' ) siloxane polyiso27 33.6 (M, = 2700) butylene Poly34, 29 ( i ) 28.3 (M, = 3000) propylene Poly32 (i) 30.4 (MW = 4000) (propylene oxide) Poly33-44 41.1 ( M y =3000) (methyl methacrylate) Fluorine-Containing Polymers polytetra18.5 (ii) 25.7 (MW = - ) fluoroethylene polytrifluoropropyl21d (ii) 23.5g ( 3 m 2 s - ' ) methylsiloxane poly( perfluoro20e (ii) 18.6e ( 1 0 m2 s - l ) propylene oxide) poly( 1,l-dihydroperfluoro- 10.6 (ii) -octyl methacrylate) All values from ref 28 except where otherwise noted. All values from ref 32 except where otherwise noted. Reference 24. e Reference 4. f RefReference 21. erence 26. Reference 20. (i) Determined using hydrogen bonding test liquids only. (ii) Determined using n-alkane test liauids only. All others using mixed series of test liquids.
of greatest comparative interest. Compounds of similar boiling point have been selected because of the variation of surface tension with boiling point in an homologous series. The next lowest member of the PDMS series, hexamethyldisiloxane, has a boiling point as far below the comparable alkanes in Table I as octamethyltrisiloxane is above. The same conclusions can be drawn if either siloxane is considered. The perfluoro ether telomer is a direct comparison with octamethyltrisiloxane. The formula of this perfluoro ether compound is CF3(CFJ2[0CF(CF3)CF2]20CHFCF3. From the surface tension point of view the order of increasing surface activity is hydrocarbon, silicone, fluorocarbon. However, from the interfacial tension-againstwater viewpoint the order of increasing interfacial activity is fluorocarbon, hydrocarbon, silicone. The silicones do not fit the simple pattern that a reduction in surface energy means an increase in hydrophobicity and interfacial tension against water. Because of their backbone flexibility silicones can adopt various orientations at different interfaces, An orientation involving interaction of the backbone with the water phase is thought to give the relatively low interfacial tension with water. This orientation is discussed in the section on the backbone. No interfacial tension value for the perfluoro ether telomer in Table I has been reported. The next lowest member of this homologous series has a value at 25 "C of 42.7 mN m-l ( 4 ) implying an orientation with the ether linkages in the backbone toward the water phase. Unlike polydimethylsiloxane, this value is not independent of chain length. As molecular weight increases so does the interfacial tension with water, suggesting the polymer has lost some of its molecular chain freedom. For a perfluoro ether of this series having a viscosity of 0.32 m2 s-l (Du Pont trade name Krytox AZ) the interfacial tension with water is 54.8 mN m-l at 25 "C ( 4 ) . In contrast, a comparable polydimethylsiloxane of 0.35 m2 s-l viscosity shows
a value of 43.1 mN m-l at 20 "C (6) very close to that of the low molecular weight homologues. Table I1 shows that paraffin wax, polytetrafluoroethylene, and polydimethylsiloxane are of similar hydrophobicity. The contact angle value given for polydimethylsiloxane is obtained by curing at elevated temperatures, in this case at >200 "C, on a glass substrate (11). Without heat treatment water contact angles are lower, because flexibility of the siloxane chain allows water molecules to penetrate the film readily. The heat treatment may cross-link molecules in the film, so that the methyl groups cannot re-orient away from the water droplet. Commercially this type of reaction is used in textile finishes. Effective silicone water repellent treatments are cross-linked or reacted onto the fabric. Organic polymers are included in Table I11 for comparison. For example, polyisobutylene is a wholly hydrocarbon analogue of polydimethylsiloxane. Even though it has two pendant methyl groups it possesses significantly higher surface energy than PDMS. Because of the rigidity of this system, no orientations are possible that best present the available methyl groups. It is usually not enough to have intrinsically surface active pendant groups; they must also be correctly oriented. The exceptions to this generalization are those polymers with large pendant surface active groups where side chain interactions dominate the polymer configuration. Examples are the longer chain perfluoroacrylates and methacrylates. The basic conclusion from analysis of these and other polymers is that the surface activity of polydimethylsiloxane approximates that of a relatively close-packed array of methyl groups. Such an array can sometimes be obtained using wholly organic polymers, but only with difficulty and often at great cost. Moreover, organic polymers often are handicapped by thermal and oxidative instability. The fluorocarbons, often of lower surface energy than the dimethylsiloxanes, are expensive and have a high interfacial tension against both aqueous and organic solvent systems. The compatibility of silicones with organic materials can be an advantage over fluorocarbon containing species. The unique combination of surface properties of the silicones gives rise to a wide variety of applications. Some of these are discussed in the following sections.
The Pendant Organic Group It is a well-known result of Zisman's many studies (33) that the order of increasing surface energy for single carbon based moieties is CF3- --c -CF2- --* CH3-CH2-+
Much of the relative surface energy behavior of polymers can be understood with this order. It explains why perfluoro ethers such as poly(perfluoropropy1ene oxide), with many pendant CF3- groups, are of lower surface energy than the perfluoroalkanes and why polydimethylsiloxanes are of lower surface energy than the alkanes. These relationships are reflected in Figure 1,where the temperature variable is allowed for by plotting surface tension at 20 "C against the boiling point of the compound. The lower the curve lies on this plot the lower is the surface energy of that system. Table IV shows that the methyl group is the hydrocarbon entity of the lowest surface energy among pendant alkane chains. The table gives the critical surface tension of wetting of (CH&RSi- treated glass taken from the studies of Kovats and co-workers (25). Both the methyl and octadecyl situations are methyl surfaces with a similar degree of packing. Groups such as octadecyl are presumed to crystallize and produce a methyl
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 1, 1980 99 28
27 26
-
24 23 25
22
-
21
-
[’LV
mNm‘
v
PERFLUOROPROPYLENE OXIDE TELOMERS
:: 10
- 1 1 40
1 60
80
‘100
1
1
1
120 140 160 180
1
1
200 220
1
240 260
1
1
280
300
B P ‘C
Figure 1. Dependence of surface tension at 20 “C on boiling point for a variety of CH3- and CF3-containingmaterials. Table IV. Critical Surface Tension of Wetting of (CH,),RSi-Treated Glassa
-
Uc
a
20 “C
R group
mN m-’
methyl ethyl propyl hexyl decyl tetradecyl octadecyl
21.2 23.3 23.5 24.0 23.5 21.9 21.5
All values from ref’ 25.
surface akin to that found in a solid hydrocarbon such as paraffin wax. For intermediate groups such as propyl, the surface energy is higher because the less surface active -CH2groups are present in the surface. Despite this higher surface energy a propylmethylsiloxane gives a higher contact angle against water on substrates which have been treated at room temperature compared to dimethylsiloxane. This is because the bulkier propyl group adversely affects chain flexibility and impedes reorientation in the presence of water. At higher temperatures of treatment this difference disappears and at even higher temperatures the hydrophobicity declines more rapidly than that of polydimethylsiloxane because of the poorer thermal stability of these higher alkyl-substituted siloxanes. This defect is shared by the octadecyl-containing species but there are situations where the increased organic compatibility of the alkylmethylsiloxane fluids outweighs their increased surface tension and diminished thermal stability. One such application is as release agents on substrates that are subsequently painted. The inclusion of CF3- groups ought to produce lower surface energy materials than the dimethylsiloxanes. However, a hydrocarbon bridge has to be placed between the CF3- and the siloxane backbone to achieve adequate stability. These -CH2- groups offset the CF3- group’s
effect to the extent that the trifluoropropylmethylsiloxanes have higher liquid surface tension than polydimethylsiloxane. Longer-chain fluorocarbon side chains will associate in a similar manner to the longer chain hydrocarbon pendant groups and produce polymers and surface treatments of low surface energy. For example, the critical surface tension of wetting of C7F15(CH2)2Si03/2 is 15 mN m-l (24). Why is CF3- intrinsically more surface active than CH3-? Hoernschmeyer (IO) has concluded that the van der Waals forces around analogous hydrocarbon and fluorocarbon moieties are very nearly the same. He ascribes the difference in wettability to the size difference in the two groups which results in a lower concentration of attractive centers in the surface in the fluorocarbon case. For best effect CF3- has to be part of a longer fluorinated chain owing to the uncompensated dipole a t the -CF2CH2- junction. Pittman’s (22) review of the surface properties of fluoropolymers gives more detail. In the preceding discussions reference has been made to the Zisman (33)critical surface tension of wetting. This property is dependent on the contact-angle test-liquids chosen. Using water, glycerol, methylene iodide, n-hexadecane, a-bromonaphthalene, tricresyl phosphate, tertbutylnaphthalene and bis(2-ethylhexyl)sebacate, Zisman (34)found a value of 24 mN m-l for a polydimethylsiloxane fluid heat-treated on glass at 300 “C. Using the first five liquids and dioctyl phthalate for several cross-linked dimethylsiloxane surface treatments, we have found values from 21 to 23 mN m-l a t 20 “C. These values are higher than the liquid surface tension of polydimethylsiloxane, which is 20.4 mN m-l at the highest molecular weights (26). Consequently, the polymer will spread over its own adsorbed film. This “creep” of silicone fluid can be a problem if unreacted fluid is present in silicone compositions and migrates to electrical contacts. Conversely, it can be an advantage in achieving complete surface coverage in applications such as metal protection, pigment surface treatment, insulator compounds and mold release. This property is not unique to polydimethylsiloxane (for example the behavior of polyisobutylene) and is not shared by all silicone systems. Polytrifluoropropylmethylsiloxane has a lower critical surface tension of wetting than its liquid surface tension.
The Backbone The first question is, does the high surface energy siloxane backbone directly affect surface energy by “showing through” to the surface? Molecular modeling, discussed in more detail later, shows that there are numerous orientations possible where the surface is all methyl. However, there is evidence that the backbone has a small effect. This comes from the concept of splitting the surface energy into that part resulting from polar interactions (hydrogen bonding, permanent dipoles) and that resulting from dispersion forces (London, van der Waals forces) as first suggested by Fowkes ( 5 ) . Kaelble (13)has calculated a dispersion contribution of 20.5 mN m-l and a polar contribution of 1.6 mN m-’ for polymethylsiloxane using Zisman’s contact angle data (34). For our cross-linked polydimethylsiloxane coating compositions we find values of around 19.0 and 1.2 mN m-l, respectively. This polar component is higher than the 0.5 mN m-l Kaelble finds for paraffin wax and suggests an effect from the backbone. This may be due to one or more of three possibilities: (i) Some small fraction of the backbone occupies the outermost surface layer and the packing of methyl groups is not complete. (ii) The polar nature of the backbone has a depth of influence that affects
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Table V. Bond Lengths and Bond Angles for Selected Pure Compoundsa compound hexamethyldisiloxane dimethyl ether pro pane a
bond length, nm Si-0, 0.163 C-0, 0.142 C-C, 0.154
bond angle! deg Si-0-Si, 130 C-0-C, 111 C-C-C, 1 1 2
SURFACE PRESSURE
1
D
All values from ref 29.
Table VI. Glass Transition Temperature (T,) for Various Polymersa ~ o l me v r
T,. K
polydimethylsiloxane polyethylene poly methylene poly( thiodifluoromet hylene) poly( oxytetramethyleneoxyadipoyl) cis-poly( 1-pentenylene) polytetrafluoroethylene polytrifluoropropylmethylsiloxane poly(propy1ene oxide) polyisobutylene polypropylene (atactic) poly( methyl methacrylate) (atactic) a All values from ref 15. data, some conflicting”.
146 148 155 155 155 159 160,400 20 kJ mol-’ (30). This freedom to rotate about bonds is reflected in the glass transition temperatures (7’ ) of polymers. It is not only the internal mobility that Jetermines T,; polymer free-volume, attractive forces between molecules, chain stiffness, and chain length all contribute. Nevertheless a low T gis indicative of polymer flexibility and some comparative values are given in Table VI. It includes all those polymers listed by Lee and Rutherford (15)with T g
