Chapter 14
Silicone Wettability and Its Significance in Beauty Products Downloaded by COLUMBIA UNIV on February 16, 2015 | http://pubs.acs.org Publication Date (Web): December 5, 2013 | doi: 10.1021/bk-2013-1148.ch014
Michael S. Ferritto1 and Michael J. Owen*,2 1Dow
Corning Corporation, Midland, Michigan 48640, U.S.A. Molecular Institute, Midland, Michigan 48640, U.S.A. *E-mail:
[email protected].
2Michigan
Silicones have been used in cosmetic and personal care products for over 50 years. Although polydimethylsiloxane (PDMS) remains the principal silicone polymer in these applications, a wide range of other silicone types are now utilized. Perceived benefits of incorporation of silicones into beauty products can sometimes be difficult to quantify. However, considerable data on fundamental properties that are relevant to silicone attributes in this area are available such as surface energy and hydrophobicity. Much of this information comes from contact angle measurement of liquids such as water, a useful index of hydrophobicity, and n-hexadecane, an indication of oleophobicity. These data are reviewed and compared with other oils and polymers of interest in the light of fundamental attributes of significance such as low intermolecular forces and polymer backbone architecture and flexibility.
Introduction The central attribute that accounts for the widespread use of silicone fluids in cosmetic and personal care products is their ability to spread upon human skin and hair. This is a consequence of the low surface tension or energy exhibited by common silicone polymers, a lower surface energy than can be attained by typical hydrocarbon-based oils. In this chapter we review the surface energies of some different classes of silicones that are useful in cosmetic and personal care applications and explore the origins of this useful wetting behavior.
© 2013 American Chemical Society In Polymers for Personal Care and Cosmetics; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
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It is conventional to refer to the surface tension of liquids and the surface energy of solids. The surface tension of an oil or liquid can be directly measured provided that the viscosities are not too high. For a solid such as skin or hair direct determination is not possible so recourse is made to a variety of contact angle approaches. These methods are not discussed in detail here. Numerous reviews are available in the literature; see, for example, (1). Most of the available pertinent data for solids is either critical surface tensions of wetting (γC) (2) or surface energies (γSV) obtained by the Owens and Wendt (3) approach. The former property is confusingly called a tension not an energy because it is the surface tension (γ) of the liquid that just wets the solid (contact angle, θ = 0). The latter property has the benefit of separating the polar (γp) and dispersion force (γd) components of the solid substrate. A definitive value for the surface energy of human skin or hair is not to be expected because of the range of skin and hair types, the variety of treatments possible, environmental factors such as relative humidity and variations in the methods of measurement and choice of contact angle test liquids. However, most reports suggest that values in the range of 20-30 mN/m are commonly encountered for both substrates. For example Schott (4) notes that γC for untreated human skin falls in the range of 22-30 mN/m while Robbins (5) quotes surface energy ranges from a variety of contact angle approaches of 25-28 mN/m for virgin hair, 24-26 mN/m for conditioned hair and 28-30 mN/m for chemically bleached hair.
Wetting by Silicones – Liquid Surface Tension Characteristics This 20-30 mN/m surface energy range that encompasses many skin and hair types is a particularly interesting one for silicone materials. The most common silicone is polydimethylsiloxane (PDMS) and its surface tension lies at the lower end of this range (16-21 mN/m, depending on molecular weight) implying that spreading of PDMS is likely to occur on these substrates. One can anticipate spreading if the Spreading Coefficient (S) is positive:
where S is solid, L is liquid and V is vapor (air). No values appear to be available for the interfacial tension γSL for PDMS and hair or skin but comparison with known values of PDMS/other polymer interfacial tensions suggests it will only be a few mN/m. For example, γSL for polypropylene (γC = 29 mN/m)/PDMS is 3.2 mN/m. Thus, a positive spreading coefficient can be predicted for a PDMS of surface tension 20 mN/m for almost all the surface energy ranges quoted in the previous paragraph. Most hydrocarbon oils have a surface tension higher than 30 mN/m and should exhibit a negative spreading coefficient on surfaces such as skin and hair. Table 1 shows some surface tensions of common oils and selected silicone fluids to illustrate this point. 220 In Polymers for Personal Care and Cosmetics; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
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Table 1. Liquid Surface Tension of Various Oils and Silicones Oil
Surface Tension (mN/m)
Temp. (°C)
Ref.
Castor oil
39.0
20
(6)
Olive oil
36
20
(7)
Peanut oil
35.5
20
(8)
Cottonseed oil
35.4
20
(6)
Coconut oil
33.4
20
(6)
Corn oil
33.4
20
(8)
Soybean oil
31.2
25
(9)
PDMS
21.3 (∞)(f)
20
(10)
PMTFPS(a)
24.4 (∞)
25
(10)
PHDFDMS(b)
18.5 (Mn ~ 19,600)
25
(11)
PDES(c)
25.7 (Unknown MW)
RT
(12)
POMS(d)
30.4 (600-1000 cS)
25
(13)
PTDMS(e)
35.0 (700-1200 cS)
25
(13)
(a) PMTFPS is polymethyltrifluoropropylsiloxane, the most common fluorosilicone polymer. (b) PHDFDMS is polyheptadecafluorodecylmethylsiloxane, a more highly fluorinated fluoropolymer. (c) PDES is polydiethylsiloxane. (d) POMS is polyoctylmethylsiloxane. (e) PTDMS is polytetradecylmethylsiloxane. (f) ∞ indicates the surface tension of a hypothetical infinite molecular weight material obtained by extrapolation using the LeGrand and Gaines equation (14). For POMS and PTDMS only single values are available so no extrapolation is possible.
The second law of thermodynamics indicates that systems will change spontaneously in the direction of minimum total free energy. Hence, for a polymer such as PDMS which contains both high (partially ionic siloxane backbone) and low (pendent methyl groups) surface energy entities, it is expected that the latter will accumulate in the surface and dominate surface behavior. This assertion is supported by the small polar component of solid surface energy that is measured by the Owens and Wendt approach (1.1 mN/m) (3). Langmuir’s principle of the independence of surface action (15) extends this concept by envisioning separate surface energies for each of the different parts of complex molecules and asserting that the surface energy of such a material is determined by the composition and orientation of the outermost groups, independent of the underlying components. Zisman and his colleagues (2) applied this principle to their critical surface tension of wetting studies of polymers to show that the surface energy of hydrocarbon and fluorocarbon substituent groups decreases in the order of –CH2221 In Polymers for Personal Care and Cosmetics; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
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> -CH3 > -CF2- > -CF3. The same order can be seen for the liquid surface tension of hydrocarbon- and fluorocarbon-containing low molecular weight oligomers as shown in Figure 1. This order of component surface activity offers a simple explanation of the difference in surface energy between organic oils and silicones reflected in Table 1. Methylene –CH2- groups dominate the structure of the former with relatively few methyl (CH3-) groups. For example, palmitic acid (C16H32O2), the most common fatty acid constituent of vegetable oils, has 14 methylene groups for every methyl one. Conversely, for silicones at least one methyl group is generally found along the siloxane backbone on each monomer unit.
Figure 1. Dependence of surface tension at 20°C on boiling point for a variety of hydrocarbon- and fluorocarbon-containing compounds. Reproduced with permission from reference (16). Copyright 1980 American Chemical Society. The trends in liquid surface tension values of the silicones in Table 1 can also be accounted for by this Zisman order simplification. For instance, in PTFPS one of the methyl groups is replaced by a CF3(CH2)2- group. CF3- is of lower intrinsic surface energy than CH3- but the two –CH2- group are of higher surface energy, so the higher surface tension of PMTFPS than PDMS, although somewhat unexpected, can be rationalized. One might suggest that dispensing with the ethylene bridge between the fluorocarbon group and the silicon atom would result in an interesting low surface energy polymer but the bridge is required to ensure adequate thermal and hydrolytic stability. The PHDFDMS polymer is not a commercial material and is not of interest to cosmetic and personal care product formulators but it is included in the table to demonstrate that a more highly fluorinated fluorosilicone where the methyl group is replaced by a CF3(CF2)7(CH2)2- entity does have a lower surface tension than PDMS of similar molecular weight. The higher surface tension of PDES is similarly accounted for by the inclusion of the extra methylene groups. This is an interesting polymer that is attracting interest in personal care applications. It is more compatible than PDMS with many hydrocarbon oils but, like PTFPMS, although its surface tension is increased 222 In Polymers for Personal Care and Cosmetics; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
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compared to PDMS, it is still substantially below that of common hydrocarbon oils. It is not just low intermolecular forces between methyl groups that accounts for the unusual properties of PDMS. The extremely flexible siloxane backbone also plays an important role. The bulkier alkyl and fluoroalkyl groups detract from this flexibility and retaining one methyl group per silicon atom is a way of salvaging some of the flexibility benefit. The diethylsiloxane is the only other disubstituted siloxane polymer that is produced in commercial quantities. To the present time this production has been almost wholly in Russia but increasing attention is now being paid to PDES in Europe and the U.S.A.. The low surface tension advantage of silicones over hydrocarbon oils is not shared by polyalkylmethylsiloxane homopolymers such as POMS and PTDMS. These polymers have CH3(CH2)7- and CH3(CH2)13- groups, respectively, replacing a methyl group on each silicon so they are much more hydrocarbon-like than PDES. Of course, it is possible to prepare polyalkylmethylsiloxane-polydimethylsiloxane copolymers to tailor desired surface tension and compatibility characteristics.
Wetting of Silicones – Contact Angles and Solid Surface Energy Aspects Table 2 lists advancing water and n-hexadecane contact angles on some selected hydrocartbon, fluorocarbon and silicone substrates. The water contact angles are a straightforward index of hydrophobicity; likewise, n-hexadecane contact angle values offer a simple measure of oleophobicity. The table also contains γd dispersion force components of surface energy calculated from the Girifalco, Good, Fowkes, Young equation (ii) (17), with the exception of polyethylene and polypropylene, taking γLV of n-hexadecane as 27.7 mN/m:
As expected, the contact angle of n-hexadecane is essentially zero on these somewhat higher surface energy polymers. The γd values for these polymers are taken from the Physical Properties of Polymers Handbook (18) determined by the Owens and Wendt approach using water and methylene iodide as the contact angle test liquids. These γd values reflect the surface energy trend established by Zisman and co-workers from critical surface tension of wetting studies. n-perfluoroeicosane is a solid fluoroalkane, C20F42; its surface is a close-packed hexagonal array of CF3- groups so it effectively sets the lower limit for the surface energy that can be achieved with fluorocarbon-based materials. Expectedly, none of the fluorosilicones have lower surface energies than n-perfluoroeicosane but the most highly fluorinated one, PHDFDMS, is reasonably close to the limit. The most interesting aspect of the water contact angle values is the relatively large spread quoted for PDMS. It is surprising that there is not a more definitive value for a polymer so noted for its hydrophobicity. See (10) for a more detailed discussion of this topic. 223 In Polymers for Personal Care and Cosmetics; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
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Table 2. Solid Surface Properties of Selected Polymers Polymer
θwater (°) [Ref.]
γn-hexadecane (°) [Ref.]
γd (mN/m)
Polyethylene
94 (18)
