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Rheological effects with a hydrophobically modified polymer
Colloids and Surfaces A: Physicochem. Eng. Aspects 288 (2006) 165–169
Rheological effects with a hydrophobically modified polymer Dennis Miller ∗ , Matthias L¨offler Clariant Produkte (Deutschland) GmbH, Industriepark H¨ochst, 65926 Frankfurt a. M., Germany Received 12 October 2005; received in revised form 3 April 2006; accepted 6 April 2006 Available online 25 April 2006
Abstract In cosmetics, rheological behaviour is directly related to ease of product use, skin feel and physical stability as well as aesthetic perceptions. Hydrophobically modified (HM) polymers allow formulators to exploit various types of interaction with the hydrophobic side chains: polymer–polymer (intrachain and interchain association), polymer–surfactant (charged and un-charged) and polymer–oil interaction. The polymer used was ammonium acryloyldimethyltaurate/beheneth-25 methacrylate crosspolymer. It was compared in several formulation types: polymer + water (aqueous gel: traditional hair gel) polymer + water + nonionic surfactant (“spray gel”: sprayable hair gel) polymer + water + anionic surfactant (shower gel) polymer + water + oil (“cream gel”: surfactant-free O/W emulsion) polymer + water + surfactant + oil (traditional O/W emulsion) The rheological requirements for the different formulation types are discussed in terms of surfactant and electrolyte effects on polymer properties. Above a certain critical polymer concentration a yield stress is observed. By carefully adjusting the polymer concentration it is possible to obtain formulations which are pourable but can suspend solid particles. © 2006 Elsevier B.V. All rights reserved. Keywords: Cosmetics; Polymer; Rheology; Yield stress
1. Introduction Rheology is an important aspect of cosmetic formulations. The behaviour at medium and high shear shows how a product can be spread or poured. Physical stability, both for emulsions and systems with larger suspended particles, is related to the rheology at very low shear [1–5]. A further aspect is the role played by rheology in skin feel [6,7]. From the formulator’s point of view, this means that product rheology must be optimised to fit several requirements. One of the commonest ways of modifying rheology is via polymeric thickeners. A wide variety of natural and synthetic products are used in personal care formulations. In recent years there has been considerable interest in hydrophobically modified ∗
Corresponding author. Tel.: +49 69 305 2361; fax: +49 69 305 3141. E-mail addresses: [email protected] (D. Miller), [email protected] (M. L¨offler). 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.04.022
polymers [8–11]. Such thickeners consist of a hydrophilic backbone to which some hydrophobic groups have been attached. This leads to increased interactions between and within polymer chains as well as with surfactants [12,13]. Within a given chemical type, the properties of hydrophobically modified polymers can be tuned by varying the number of hydrophobic groups and their chain length. In practice, thickeners are usually optimised to give good performance over a range of formulations rather than for one specific case. In this paper we examine the behaviour of such a product in different types of personal care formulations. The results are arranged by formulation types; the more fundamental aspects are discussed together with the type of product to which they are most relevant. The formulation types considered are all aqueous systems thickened with polymer. They may also contain surfactant and/or oil as discussed below. Table 1 shows a summary of the rheological requirements.
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Table 1 Summary of formulation types Formulation type
Hair gel (aq. gel) Spray gel Opt. effects shower gel Conventional O/W emulsion Cream gel
Components
Requirements
Water
Polymer
Surfactant
X X X X X
X X X X X
X X X
• Hair gels are aqueous systems viscosified with polymer. They contain styling polymers and other additives which have little effect on the rheology. The consumer expects high viscosity and shear thinning. Hair gels may have a yield stress, but this is not a requirement. • Spray gels are sprayable hair gels. The polymer concentration must be adjusted so that there is sufficient viscosification, but the product remains spayable. Shear thinning is of particular importance, so that the viscosity is low when the mixture is forced through the spray head. Low dynamic surface tension favours droplet formation. Such products can be formulated by adding nonionic surfactant. • Optical effects shower gels contain suspended particles, such as coloured wax balls, to give the product an interesting appearance. Typically there will be 10–15% surfactant (predominantly anionic). The liquid should have a yield stress but be pourable. • Conventional O/W emulsion. Surfactant (emulsifier) stabilises the droplets against coalescence. In lotions, the polymer gives the mixture a small yield stress, which prevents creaming. If more polymer is used, the product is semi-solid. • Cream gel. In this O/W formulation there is no surfactant type emulsifier; stabilisation is entirely due to the polymer. Typically there is a “fresh” skin feel. This is thought to correspond to a breakdown in structure on applying the product. The structure breakdown is observed as a sudden viscosity drop on plots of viscosity vs. shear stress.
Oil Viscosification, shear thinning Viscosification, shear thinning Low yield stress, pourable Yield stress, shear thinning Yield stress, shear thinning
X X
acryloyldimethyltaurate/beheneth-25 methacrylate crosspolymer”. In the rest of the paper the abbreviation “AMP-S/Beh-25” will be used. The surfactants were all cosmetic raw materials. C12/14 E7 and C12/14 E3 are ethoxylated C12 /C14 alcohols with an average of 3 and 7 mol EO, respectively. Sodium laureth sulfate is a C12 /C14 based ether sulfate with 2 EO. Sodium cocoyl glutamate is the mono sodium salt of N-cocoyl glutamic acid. Trilaureth-4-phosphate is a mixture of mono-, di- and tri(alkyltetraglycolether)-o-phosphoric acid esters. The nonionic emulsifier for the conventional O/W emulsions was a 60:40 mixture of polyglyceryl-2-sesquiisostearate and PEG-10 polyglyceryl2-laurate. Flow curves were measured with a stress controlled rheometer (Bohlin CS). Yield stresses were obtained from plots of square root of shear rate vs. square root of shear stress (Casson method). The wax balls used to study creaming had a maximum diameter of 850 m. To make the emulsions, emulsifier was first dissolved in the oil. A 2% aqueous polymer gel was prepared by mixing at high shear until homogeneous. Then the required amounts of oil/emulsifer, water and polymer gel were stirred in a beaker for 20 min. 3. Results
2. Materials and methods
3.1. Hair gels and spray gels: polymer–surfactant interactions
The polymer used is made by copolymerisation of acrylamidomethylpropane sulphonic acid and an alkyl ethoxylate capped with methacrylic acid, in the presence of ammonia and crosslinking agent (Fig. 1). The chemical structure was optimised to give good thickening of aqueous gels and stabilisation of a test emulsion. The INCI name is “ammonium
Interactions of alkyl ethoxylates with polymer AMP-S/Beh25 alter the rheology. Hydrophobic ethoxylates usually give a marked increase in yield stress and viscosity. Results for C12/14 E3 are given in Table 2. In some cases, however, there is incompatibility so that the mixture may become inhomogeneous at higher surfactant concentrations. Table 2 Interaction of AMP-S/Beh-25 with C12/14 E3
Fig. 1. Structure of polymer AMP-S/Beh-25. For simplicity crosslinking is not shown.
AMP-S/Beh-25%
C12/14 E3 %
Viscosity at 1 s−1 (mP a.s)
Yield stress (Pa)
0.3 0.3 0.4 0.4
0 2 0 2
4300 8800 16400 32300
<1 6.6 4.5 29.4
D. Miller, M. L¨offler / Colloids and Surfaces A: Physicochem. Eng. Aspects 288 (2006) 165–169
Fig. 2. Effect of surfactant on viscosity of aqueous gels. Polymer: 0.6% AMPS/Beh-25. Alkyl ethoxylate: C12/14 E7 . Anionic surfactant: sodium laureth sulfate/sodium cocoyl glutamate 85:15.
For medium HLB surfactants such as C12/14 E7 , the effects are fairly small, however. This is shown in Fig. 2 where the interaction gives a slight viscosity increase. The effects depend on the concentration of the two components and in some cases addition of the surfactant may cause a small reduction in viscosity. We have used this surfactant for a model spray gel formulation. Including the surfactant gave a product that could be sprayed more uniformly, presumably because lowering the surface tension makes droplet formation easier. Anionic surfactants strongly lower the viscosity. For example, a 0.4% solution of AMP-S/Beh-25 is a viscous but pourable liquid. In the presence of 0.5% of either sodium dodecyl sulphate or a sodium sec-alkane sulphate there is very little thickening effect. The influence of an anionic surfactant was studied in more detail with an ether sulphate based mixture suitable for shower gels. Results are shown in Fig. 2. On the basis of Na+ concentration, the surfactant mixture lowers the viscosity about half as much as NaCl. Studies of counterion binding have shown that a substantial fraction of counterions are bound to the micelles of anionic surfactants [14]. In view of this, it appears that influence of anionic surfactant is probably largely an electrolyte effect. In order to thicken aqueous systems with AMP-S/Beh25, a critical polymer concentration is required. Above this concentration the viscosity rises rapidly as more polymer is added. Anionic surfactants increase the critical concentration, but effective thickening is possible, provided sufficient polymer is used. Small amounts of surfactants normally increase the viscosity of hydrophobically modified polymers by promoting intermolecular hydrophobic junctions. On the other hand, an excess of surfactant will tend to reduce viscosity as the hydrophobic side chains can interact with the micelles rather than with each other [15]. With AMP-S/Beh-25 the surfactant effects appear to be small. This may be related to the cross-linking.
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Fig. 3. Yield stress and stability to creaming of shower gel formulation with wax balls. Surfactant: sodium laureth sulfate/sodium cocoyl glutamate 85:15. Concentration: 10% surfactant with varying amounts of polymer. Creaming test: d = 850 m, ρ = 0.03 g cm−3 , time = 1 week.
3.2. Optical effects shower gels: pourable systems with yield stress Normally yield stresses are associated with high viscosity. To obtain a pourable system which can suspend particles, the composition must be adjusted carefully to give a very low yield stress. If the yield stress is larger than the shear stress associated with creaming, the system should be stable. As a rough estimate, the shear stress can be taken as the buoyancy force divided by the surface area of the particle. In this approximation, the surface of the particle is taken as the area over which the force acts. For a sphere this gives rgρ/3, where r is the radius, ρ the density difference, and g the acceleration due to gravity. A comparison between theory and experiment is shown in Fig. 3. In this figure, yield stress is plotted as a function of polymer concentration. A critical polymer concentration is required to give a yield stress. At a slightly higher concentration denoted c , creaming of the wax balls is prevented. Also shown is an estimate of c calculated by the method described in the previous paragraph. The agreement is very good, considering the approximate nature of the theory. One source of uncertainty in this theory is the difference in time scales. In the rheological experiment, shear rate is measured for a few seconds at each value of the shear stress. In our test, creaming was observed over a period of 1 week, but in practice products should be stable for at least 6 months. Another way of looking at this is to ask whether we have a real or apparent yield stress. In other words, is there some slight viscous flow below the observed yield stress or only a small elastic deformation? Creaming could be prevented by a sufficiently large viscosity, even if there is no true yield stress. In this case, the viscosity will approach a limit η0 at zero shear. If η0 is high enough, creaming will be too slow to be observable. We have estimated this limit from Stokes’ law assuming that creaming can be neglected if the particles move less than 1 cm. For a 1-week test this gives
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Fig. 4. Test for yield stress with wax balls. The critical polymer concentration required for a yield stress is denoted c .
about 400 Pa.s. Six months storage stability would correspond to about 10,000 Pa.s. Another approach is to determine the critical polymer concentration required to prevent creaming. This is approximately the critical polymer concentration above which a yield stress is observed. The test we used is shown in Fig. 4. The timescale of one week is a compromise between experimental convenience and practical stability requirements. In Fig. 5 the test is used to examine an anionic/nonionic surfactant mixture. It may be seen that small amounts of anionic surfactant increase the critical concentration, but the nonionic surfactant has little effect. These results fit well with the viscosity plots in Fig. 2. Thickening of a model shower gel with polymer and with electrolyte is shown in Fig. 6. The range of typical product viscosity shown is for shower gels with suspended bubbles or particles purchased in 2005. With AMP-S/Beh-25 the sudden rise in viscosity corresponds to physical stability. The polymer concentration can be fine-tuned to give the required pouring behaviour. NaCl is an effective thickener and, if enough is added,
Fig. 5. Critical concentration of AMP-S/Beh-25 for yield stress: effect of surfactant composition. Surfactant concentration: 10%.
Fig. 6. Model shower gel: NaCl vs. polymer. surfactant 10% sodium laureth sulfate.
the product becomes too viscous to pour properly. Even so, it does not prevent creaming. Thickening of ether sulphate solutions with NaCl is a well known phenomenon caused by the presence of rod-like micelles [16,17]. These solutions have no yield stress. 3.3. O/W emulsions and cream gels: rheology and stability The discussion in Section 3.2 on yield stress and creaming also applies to emulsions. However, because of their smaller size the droplets move much more slowly than the wax balls used to decorate shower gels. Droplet–droplet interactions also play a role. In Fig. 7, yield stress is plotted against polymer concentration. For comparison, aqueous gels are also shown. A yield stress is found when the polymer concentration exceeds a critical value. The critical polymer concentration appears to be
Fig. 7. Yield stress vs. concentration of polymer AMP-S/Beh-25. Cream gels and conventional emulsions contain 15% mineral oil. The conventional emulsions contain 2% nonionic emulsifier.
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Formulations with the polymer show a yield stress if a critical polymer concentration is exceeded. It is often very useful for the formulator to know where this critical concentration lies and how it is affected by other components, such as surfactants and electrolytes. A simple test based on the creaming of wax balls can provide this information. The structure of hydrophobically modified AMP-S polymers can be varied quite widely. This can provide additional possibilities for the formulator. References
Fig. 8. Shear thinning: comparison of formulation types with 0.6% AMP-S/Beh25. Cream gel and emulsion compositions as in Fig. 7.
the same for aqueous gels, emulsions and cream gels. Below this critical concentration emulsions and cream gels are unstable. Above the critical concentration, the order of yield stress is cream gel > emulsion > aq. gel. This indicates an interaction between polymer and oil droplets. The interaction is less in the emulsions, because of competition between surfactant and polymer for the oil/water interface. The higher yield stress for cream gels has also been found with a similar polymer that was not hydrophobically modified [18]. Plots of viscosity vs. shear rate are shown in Fig. 8 for the three formulation types at 0.6% polymer. These samples all show a yield stress and correspondingly the viscosity does not level off to a constant value at zero shear. The conventional emulsion is less viscous than the cream gel, but the difference at medium shear rates is quite small. Some experiments were also performed with trilaureth-4phosphate as emulsifier. It is predominantly nonionic, but has some anionic character because of the mono- and diphosphate esters present as minor components. In an experiment corresponding to the one shown in Fig. 8, the viscosity was lower by about a factor of three, presumably due to electrolyte effects. 4. Conclusions Hydrophobically modified polymers are useful in a variety of cosmetics formulation types. Interaction of the side chains with oil and surfactant can enhance thickening. With the polymer studied in this work, AMP-S/Beh-25, interactions with low HLB surfactants increase viscosity, but only small effects are observed with hydrophilic nonionic surfactants. Anionic surfactants tend to reduce the viscosity due to electrolyte effects. In spite of this, the polymer can be used to obtain special rheological effects in shower gels which contain anionic surfactants.
[1] H.A. Barnes, Rheology of emulsions—a review, Coll. Surf. A 91 (1994) 89–95. [2] G. Zografi, Physical stability assessment of emulsions and related disperse systems: a critical review, J. Soc. Cosmet. Chem. 33 (1982) 345–358. [3] E. Dickinson, M.I. Goller, D.J. Wedlock, Creaming and rheology of emulsions containing polysaccharaide and non-ionic or anionic surfactants, Coll. Surf. A 75 (1993) 195–201. [4] D.J. Miller, E.-M. Wiener, A. Turowski, C. Thunig, H. Hoffmann, O/W emulsions for cosmetics products stabilized by alkyl phosphates— rheology and storage tests, Coll. Surf. A 152 (1999) 155–160. [5] D.J. Miller, M. L¨offler, Rheology of cream gels and O/W emulsions stabilised by a polymeric sulphonic acid, Eur. Cosmet. (2001 (Nov/Dec)) 26–29. [6] R. Brummer, S. Godersky, Rheological studies to objectify the sensations occuring when cosmetic emulsions are applied to the skin, 2nd World Emulsion Congr. (1997) paper 4-2-129. [7] V.A.L. Wortel, C. Verboom, M.-C. Taelman, S. Leonard, J.W. Wiechers, Th. Tadros, Linking sensory and rheology characteristics: a first step to understand the influence of emulsion structure on sensory characteristics, Proc. 23rd IFSCC Congr. (2004) 129–134. [8] D.J. Miller, R. Morschh¨auser, M. L¨offler, R. Milbradt, W. Stelter, New hydrophobically modified polyelectrolyte thickeners, Cosmet. Toilet. 119 (2004) 47–50. [9] R.J. English, R.D. Jenkins, D.R. Bassett, S.A. Khan, Rheology of a HASE associative polymer and its interaction with non-ionic surfactants, ACS Symp. Ser. 765 (2000) 369–380. [10] M. Antonietti, H.P. Hentze, B. Smarsky, M. L¨offler, R. Morschh¨auser, Structure characterisation of surfactant assisted polymer thickeners by silica nanocasting, Macromol. Mater. Eng. 287 (2002) 195–202. [11] R.Y. Lochhead, C.J. Rulison, An investigation of the mechanism by which hydrophobically modified hydrophilic polymers act as primary emulsifiers, Coll. Surf. A 88 (1994) 27–32. [12] J. Selb, F. Candau, Inter- and intra-molecular aggregation of associating polymers in water, ACS Symp. Ser. 765 (2000) 95–108. [13] L. Piculell, S. Nilsson, J. Sj¨ostr¨om, K. Thuresson, How much surfactant binds to an associating polymer? ACS Symp. Ser. 765 (2000) 317–335. [14] L.R. Fisher, D.G. Oakenfull, Micelles in aqueous solution, Chem. Soc. Rev. 6 (1977) 25–42. [15] K.C. Tam, W.P. Seng, R.D. Jenkins, D.R. Bassett, Rheological and microcalorimetric studies of a model alkali-soluble associative polymer (HASE) in nonionic surfactant solutions, J. Polym. Sci. Part B 38 (2000) 2019–2032. [16] D. Balzer, S. Varwig, M. Weihrauch, Viscoelasticity of personal care products, Coll. Surf. A 99 (1995) 233–246. [17] K. Penfield, A look behind the salt curve: the link between rheology, structure, and salt content in shampoo formulation, Proc. 23rd IFSCC Congress (2004) paper P220. [18] D.J. Miller, M. L¨offler, O/W Emulsions stabilised by a polymeric sulphonic acid, Proc. 48th Sepawa Congr. (2001) 129–135.
Rheological effects with a hydrophobically modified polymer Dennis Miller ∗ , Matthias L¨offler Clariant Produkte (Deutschland) GmbH, Industriepark H¨ochst, 65926 Frankfurt a. M., Germany Received 12 October 2005; received in revised form 3 April 2006; accepted 6 April 2006 Available online 25 April 2006
Abstract In cosmetics, rheological behaviour is directly related to ease of product use, skin feel and physical stability as well as aesthetic perceptions. Hydrophobically modified (HM) polymers allow formulators to exploit various types of interaction with the hydrophobic side chains: polymer–polymer (intrachain and interchain association), polymer–surfactant (charged and un-charged) and polymer–oil interaction. The polymer used was ammonium acryloyldimethyltaurate/beheneth-25 methacrylate crosspolymer. It was compared in several formulation types: polymer + water (aqueous gel: traditional hair gel) polymer + water + nonionic surfactant (“spray gel”: sprayable hair gel) polymer + water + anionic surfactant (shower gel) polymer + water + oil (“cream gel”: surfactant-free O/W emulsion) polymer + water + surfactant + oil (traditional O/W emulsion) The rheological requirements for the different formulation types are discussed in terms of surfactant and electrolyte effects on polymer properties. Above a certain critical polymer concentration a yield stress is observed. By carefully adjusting the polymer concentration it is possible to obtain formulations which are pourable but can suspend solid particles. © 2006 Elsevier B.V. All rights reserved. Keywords: Cosmetics; Polymer; Rheology; Yield stress
1. Introduction Rheology is an important aspect of cosmetic formulations. The behaviour at medium and high shear shows how a product can be spread or poured. Physical stability, both for emulsions and systems with larger suspended particles, is related to the rheology at very low shear [1–5]. A further aspect is the role played by rheology in skin feel [6,7]. From the formulator’s point of view, this means that product rheology must be optimised to fit several requirements. One of the commonest ways of modifying rheology is via polymeric thickeners. A wide variety of natural and synthetic products are used in personal care formulations. In recent years there has been considerable interest in hydrophobically modified ∗
Corresponding author. Tel.: +49 69 305 2361; fax: +49 69 305 3141. E-mail addresses: [email protected] (D. Miller), [email protected] (M. L¨offler). 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.04.022
polymers [8–11]. Such thickeners consist of a hydrophilic backbone to which some hydrophobic groups have been attached. This leads to increased interactions between and within polymer chains as well as with surfactants [12,13]. Within a given chemical type, the properties of hydrophobically modified polymers can be tuned by varying the number of hydrophobic groups and their chain length. In practice, thickeners are usually optimised to give good performance over a range of formulations rather than for one specific case. In this paper we examine the behaviour of such a product in different types of personal care formulations. The results are arranged by formulation types; the more fundamental aspects are discussed together with the type of product to which they are most relevant. The formulation types considered are all aqueous systems thickened with polymer. They may also contain surfactant and/or oil as discussed below. Table 1 shows a summary of the rheological requirements.
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Table 1 Summary of formulation types Formulation type
Hair gel (aq. gel) Spray gel Opt. effects shower gel Conventional O/W emulsion Cream gel
Components
Requirements
Water
Polymer
Surfactant
X X X X X
X X X X X
X X X
• Hair gels are aqueous systems viscosified with polymer. They contain styling polymers and other additives which have little effect on the rheology. The consumer expects high viscosity and shear thinning. Hair gels may have a yield stress, but this is not a requirement. • Spray gels are sprayable hair gels. The polymer concentration must be adjusted so that there is sufficient viscosification, but the product remains spayable. Shear thinning is of particular importance, so that the viscosity is low when the mixture is forced through the spray head. Low dynamic surface tension favours droplet formation. Such products can be formulated by adding nonionic surfactant. • Optical effects shower gels contain suspended particles, such as coloured wax balls, to give the product an interesting appearance. Typically there will be 10–15% surfactant (predominantly anionic). The liquid should have a yield stress but be pourable. • Conventional O/W emulsion. Surfactant (emulsifier) stabilises the droplets against coalescence. In lotions, the polymer gives the mixture a small yield stress, which prevents creaming. If more polymer is used, the product is semi-solid. • Cream gel. In this O/W formulation there is no surfactant type emulsifier; stabilisation is entirely due to the polymer. Typically there is a “fresh” skin feel. This is thought to correspond to a breakdown in structure on applying the product. The structure breakdown is observed as a sudden viscosity drop on plots of viscosity vs. shear stress.
Oil Viscosification, shear thinning Viscosification, shear thinning Low yield stress, pourable Yield stress, shear thinning Yield stress, shear thinning
X X
acryloyldimethyltaurate/beheneth-25 methacrylate crosspolymer”. In the rest of the paper the abbreviation “AMP-S/Beh-25” will be used. The surfactants were all cosmetic raw materials. C12/14 E7 and C12/14 E3 are ethoxylated C12 /C14 alcohols with an average of 3 and 7 mol EO, respectively. Sodium laureth sulfate is a C12 /C14 based ether sulfate with 2 EO. Sodium cocoyl glutamate is the mono sodium salt of N-cocoyl glutamic acid. Trilaureth-4-phosphate is a mixture of mono-, di- and tri(alkyltetraglycolether)-o-phosphoric acid esters. The nonionic emulsifier for the conventional O/W emulsions was a 60:40 mixture of polyglyceryl-2-sesquiisostearate and PEG-10 polyglyceryl2-laurate. Flow curves were measured with a stress controlled rheometer (Bohlin CS). Yield stresses were obtained from plots of square root of shear rate vs. square root of shear stress (Casson method). The wax balls used to study creaming had a maximum diameter of 850 m. To make the emulsions, emulsifier was first dissolved in the oil. A 2% aqueous polymer gel was prepared by mixing at high shear until homogeneous. Then the required amounts of oil/emulsifer, water and polymer gel were stirred in a beaker for 20 min. 3. Results
2. Materials and methods
3.1. Hair gels and spray gels: polymer–surfactant interactions
The polymer used is made by copolymerisation of acrylamidomethylpropane sulphonic acid and an alkyl ethoxylate capped with methacrylic acid, in the presence of ammonia and crosslinking agent (Fig. 1). The chemical structure was optimised to give good thickening of aqueous gels and stabilisation of a test emulsion. The INCI name is “ammonium
Interactions of alkyl ethoxylates with polymer AMP-S/Beh25 alter the rheology. Hydrophobic ethoxylates usually give a marked increase in yield stress and viscosity. Results for C12/14 E3 are given in Table 2. In some cases, however, there is incompatibility so that the mixture may become inhomogeneous at higher surfactant concentrations. Table 2 Interaction of AMP-S/Beh-25 with C12/14 E3
Fig. 1. Structure of polymer AMP-S/Beh-25. For simplicity crosslinking is not shown.
AMP-S/Beh-25%
C12/14 E3 %
Viscosity at 1 s−1 (mP a.s)
Yield stress (Pa)
0.3 0.3 0.4 0.4
0 2 0 2
4300 8800 16400 32300
<1 6.6 4.5 29.4
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Fig. 2. Effect of surfactant on viscosity of aqueous gels. Polymer: 0.6% AMPS/Beh-25. Alkyl ethoxylate: C12/14 E7 . Anionic surfactant: sodium laureth sulfate/sodium cocoyl glutamate 85:15.
For medium HLB surfactants such as C12/14 E7 , the effects are fairly small, however. This is shown in Fig. 2 where the interaction gives a slight viscosity increase. The effects depend on the concentration of the two components and in some cases addition of the surfactant may cause a small reduction in viscosity. We have used this surfactant for a model spray gel formulation. Including the surfactant gave a product that could be sprayed more uniformly, presumably because lowering the surface tension makes droplet formation easier. Anionic surfactants strongly lower the viscosity. For example, a 0.4% solution of AMP-S/Beh-25 is a viscous but pourable liquid. In the presence of 0.5% of either sodium dodecyl sulphate or a sodium sec-alkane sulphate there is very little thickening effect. The influence of an anionic surfactant was studied in more detail with an ether sulphate based mixture suitable for shower gels. Results are shown in Fig. 2. On the basis of Na+ concentration, the surfactant mixture lowers the viscosity about half as much as NaCl. Studies of counterion binding have shown that a substantial fraction of counterions are bound to the micelles of anionic surfactants [14]. In view of this, it appears that influence of anionic surfactant is probably largely an electrolyte effect. In order to thicken aqueous systems with AMP-S/Beh25, a critical polymer concentration is required. Above this concentration the viscosity rises rapidly as more polymer is added. Anionic surfactants increase the critical concentration, but effective thickening is possible, provided sufficient polymer is used. Small amounts of surfactants normally increase the viscosity of hydrophobically modified polymers by promoting intermolecular hydrophobic junctions. On the other hand, an excess of surfactant will tend to reduce viscosity as the hydrophobic side chains can interact with the micelles rather than with each other [15]. With AMP-S/Beh-25 the surfactant effects appear to be small. This may be related to the cross-linking.
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Fig. 3. Yield stress and stability to creaming of shower gel formulation with wax balls. Surfactant: sodium laureth sulfate/sodium cocoyl glutamate 85:15. Concentration: 10% surfactant with varying amounts of polymer. Creaming test: d = 850 m, ρ = 0.03 g cm−3 , time = 1 week.
3.2. Optical effects shower gels: pourable systems with yield stress Normally yield stresses are associated with high viscosity. To obtain a pourable system which can suspend particles, the composition must be adjusted carefully to give a very low yield stress. If the yield stress is larger than the shear stress associated with creaming, the system should be stable. As a rough estimate, the shear stress can be taken as the buoyancy force divided by the surface area of the particle. In this approximation, the surface of the particle is taken as the area over which the force acts. For a sphere this gives rgρ/3, where r is the radius, ρ the density difference, and g the acceleration due to gravity. A comparison between theory and experiment is shown in Fig. 3. In this figure, yield stress is plotted as a function of polymer concentration. A critical polymer concentration is required to give a yield stress. At a slightly higher concentration denoted c , creaming of the wax balls is prevented. Also shown is an estimate of c calculated by the method described in the previous paragraph. The agreement is very good, considering the approximate nature of the theory. One source of uncertainty in this theory is the difference in time scales. In the rheological experiment, shear rate is measured for a few seconds at each value of the shear stress. In our test, creaming was observed over a period of 1 week, but in practice products should be stable for at least 6 months. Another way of looking at this is to ask whether we have a real or apparent yield stress. In other words, is there some slight viscous flow below the observed yield stress or only a small elastic deformation? Creaming could be prevented by a sufficiently large viscosity, even if there is no true yield stress. In this case, the viscosity will approach a limit η0 at zero shear. If η0 is high enough, creaming will be too slow to be observable. We have estimated this limit from Stokes’ law assuming that creaming can be neglected if the particles move less than 1 cm. For a 1-week test this gives
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Fig. 4. Test for yield stress with wax balls. The critical polymer concentration required for a yield stress is denoted c .
about 400 Pa.s. Six months storage stability would correspond to about 10,000 Pa.s. Another approach is to determine the critical polymer concentration required to prevent creaming. This is approximately the critical polymer concentration above which a yield stress is observed. The test we used is shown in Fig. 4. The timescale of one week is a compromise between experimental convenience and practical stability requirements. In Fig. 5 the test is used to examine an anionic/nonionic surfactant mixture. It may be seen that small amounts of anionic surfactant increase the critical concentration, but the nonionic surfactant has little effect. These results fit well with the viscosity plots in Fig. 2. Thickening of a model shower gel with polymer and with electrolyte is shown in Fig. 6. The range of typical product viscosity shown is for shower gels with suspended bubbles or particles purchased in 2005. With AMP-S/Beh-25 the sudden rise in viscosity corresponds to physical stability. The polymer concentration can be fine-tuned to give the required pouring behaviour. NaCl is an effective thickener and, if enough is added,
Fig. 5. Critical concentration of AMP-S/Beh-25 for yield stress: effect of surfactant composition. Surfactant concentration: 10%.
Fig. 6. Model shower gel: NaCl vs. polymer. surfactant 10% sodium laureth sulfate.
the product becomes too viscous to pour properly. Even so, it does not prevent creaming. Thickening of ether sulphate solutions with NaCl is a well known phenomenon caused by the presence of rod-like micelles [16,17]. These solutions have no yield stress. 3.3. O/W emulsions and cream gels: rheology and stability The discussion in Section 3.2 on yield stress and creaming also applies to emulsions. However, because of their smaller size the droplets move much more slowly than the wax balls used to decorate shower gels. Droplet–droplet interactions also play a role. In Fig. 7, yield stress is plotted against polymer concentration. For comparison, aqueous gels are also shown. A yield stress is found when the polymer concentration exceeds a critical value. The critical polymer concentration appears to be
Fig. 7. Yield stress vs. concentration of polymer AMP-S/Beh-25. Cream gels and conventional emulsions contain 15% mineral oil. The conventional emulsions contain 2% nonionic emulsifier.
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169
Formulations with the polymer show a yield stress if a critical polymer concentration is exceeded. It is often very useful for the formulator to know where this critical concentration lies and how it is affected by other components, such as surfactants and electrolytes. A simple test based on the creaming of wax balls can provide this information. The structure of hydrophobically modified AMP-S polymers can be varied quite widely. This can provide additional possibilities for the formulator. References
Fig. 8. Shear thinning: comparison of formulation types with 0.6% AMP-S/Beh25. Cream gel and emulsion compositions as in Fig. 7.
the same for aqueous gels, emulsions and cream gels. Below this critical concentration emulsions and cream gels are unstable. Above the critical concentration, the order of yield stress is cream gel > emulsion > aq. gel. This indicates an interaction between polymer and oil droplets. The interaction is less in the emulsions, because of competition between surfactant and polymer for the oil/water interface. The higher yield stress for cream gels has also been found with a similar polymer that was not hydrophobically modified [18]. Plots of viscosity vs. shear rate are shown in Fig. 8 for the three formulation types at 0.6% polymer. These samples all show a yield stress and correspondingly the viscosity does not level off to a constant value at zero shear. The conventional emulsion is less viscous than the cream gel, but the difference at medium shear rates is quite small. Some experiments were also performed with trilaureth-4phosphate as emulsifier. It is predominantly nonionic, but has some anionic character because of the mono- and diphosphate esters present as minor components. In an experiment corresponding to the one shown in Fig. 8, the viscosity was lower by about a factor of three, presumably due to electrolyte effects. 4. Conclusions Hydrophobically modified polymers are useful in a variety of cosmetics formulation types. Interaction of the side chains with oil and surfactant can enhance thickening. With the polymer studied in this work, AMP-S/Beh-25, interactions with low HLB surfactants increase viscosity, but only small effects are observed with hydrophilic nonionic surfactants. Anionic surfactants tend to reduce the viscosity due to electrolyte effects. In spite of this, the polymer can be used to obtain special rheological effects in shower gels which contain anionic surfactants.
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