Polymeric thickeners are widely used to stabilise cosmetic formulations but most of them are not very suitable for fluid formulations due to low reproducible results in a low viscosity range, and complicated handling procedures. The purpose of this study is to demonstrate the effectiveness of two new thickening-emulsifying polymers manufactured by inverse emulsion polymerisation to support fluid and stable formulations. A second objective of the study is to see how the synthesis parameters could affect the application properties of two polymers developed on a similar chemical structure. Finally, the aim is to clarify how to choose between these two polymers according to the technical brief and to advise the best handling procedure.
Materials and methods
Materials
Two polymers obtained by inverse polymerisation synthesis are compared (see Table 1). One advantage of inverse emulsion polymerisation synthesis is that it provides ready to use, preneutralised liquid polymers which ensure easy handling for the users. The gelling mechanism of these polymers based on electric repulsions is very quick, taking less than three minutes under common mechanical stirring. As soon as the water is added, the O/W emulsion inverts itself, leading to expansion of the polymeric chains and development of a microgel network. Another strength of the inverse emulsion polymers is to promote monodisperse spherical small microgel size ?2 ?M1 (the microgel size measured by static and dynamic scattering technique2 is smaller than the one obtained with same polymer manufactured by precipitation polymerisation, which is around 8 ?M). Because of this small size, the microgel network is denser and is able to stabilise a large amount of oils without any surfactant addition.
Methods
The compared technical performance of the two polymers was determined by different experiments:
• Thickening power in deionised water with a particular interest for the low viscosity range.
• Thickening power in deionised water as a function of pH.
• Polymer stabilising properties in challenging conditions with some electrolytes (NaCl content).
• Sensory profile in cream gel formulas conducted by sensorial evaluation with ten experts.
• Oil stabilising effect in milky gel formulas manufactured by a cold procedure (without any surfactant addition).
All formulas were manufactured under regular mechanical stirring (simple serrated disc stirrer).
Results
In the first part, the technical performance of the two polymers are compared in order to determine the strengths and differences of each polymer along with a hypothesis to understand how the synthesis parameters can influence the application properties. In a second part, an explanation of the formulation characteristics for each polymer will be provided with process advice to achieve the best stabilising effect for fluid formulas.
Compared thickening power in water
The two polymers have different visco/dose curves but they are both adapted for fluid formulas as they exhibit a low thickening effect in appropriate conditions; FL is ideal for designing fluid formulas as it shows a low slope curve whatever the use level, but INS is also interesting as it promotes a low slope curve with very low viscosities below 1% of use level. Furthermore, the evaluation of different batches shows a good reproducibility of the viscosity in the fluid range (as seen in Figure 2), which is not the case for other liquid polymers on the market. Despite their similar chemical structure, the difference of thickening effect between FL and INS in the fluid formulation range can be explained by a difference in the microgel size as shown in Figure 3. INS provides smaller microgels than FL, so at low level the FL network fills in the space faster than INS100, which explains the higher viscosity. This difference is also evidence of the superior oil stabilising effect of FL under the same handling conditions. Both polymers demonstrate good stability of viscosity in extreme pH conditions, from pH 3 to pH 10, as seen in Figure 4a, and both allow easy formulation in very acidic or alkaline media according to the selected active ingredients.
Compared stabilising effect with electrolytes
Most of the cosmetic formulations contain electrolytes (included in preservatives or active ingredients). For this reason, the thickening and stabilising effect of the two polymers has been studied with NaCl as an electrolyte model (two NaCl doses). Increasing levels of polymers were added to determine the minimum dose needed to achieve a stable formula (more than one month at room temperature and 45°C). As shown in Figure 4b, the viscosity provided by FL is higher than the one obtained with INS. Furthermore, FL demonstrates better stabilising than INS. For instance, only 2% of FL is required to achieve a stable formula with 0.4% of NaCl, whereas 3% of INS is required under the same conditions. FL is the preferred grade with electrolytes. As the thickening effect of the two polymers is based on electrostatic repulsion, both are sensitive to electrolyte content: salt induces a screening effect which decreases the microgel size and consequently the viscosity and the stability. The microstructure of the microgel adds to size difference, explaining the increased effectiveness of FL versus INS: the movement of polymeric chains are easier in FL microgel than in INS, leading to less screening effect by NaCl and consequently to lower size reduction of the microgel, lower viscosity drop and better stability. This effect is intensified by the larger size of microgels for FL as illustrated in Figure 5.
Recommended procedure for oil stabilisation
As expected, through the network organisation at low levels, the stabilising effect of the FL grade is more effective and does not require any specific handling procedure, as direct introduction process into the fatty or water phase can both be used. As FL also withstands high shear, mixing equipment can be freely chosen. FL is able to stabilise various kinds of oils, with low viscosity, giving stable and fluid milky gels as shown in Figure 6. Thanks to their convenient shearthinning profiles, the milky gels obtained show excellent flowing properties, even for high polymer levels (Fig. 7). On the contrary, it is possible to achieve extra-fluid formulas with INS but a simple process is required to refine the dispersion and prevent instability. The best procedure is described in Figure 8: the INS is first introduced in the fatty phase and 30% to 50% of the total amount of water is added to obtain a viscous concentrated formula which helps to refine oil droplets. The primary formula is diluted a second time with the rest of water to achieve a stable extra-fluid milky gel.
Compared sensory profile in cream-gels formulation
The sensory profiles of the two polymers seen in Figure 9 are close. As expected due to their identical chemical structures, FL and INS give fresh and light feel upon application, leaving a soft after-feel.
Conclusion
On account of their low slope viscosity curve and the good reproducibility in the low viscosity range, FL and INS are effective polymers to develop fluid formulas in a wide pH range. Due to their chemical composition and the polymerisation process, the two polymers exhibit strong emulsifying-stabilising properties which enables a good stability of low viscosity formulations with a cold process. Because of different synthesis parameters, FL and INS provide complementary behaviour. The process should be adapted according to the selected polymer, especially for extra-fluid milky gels. Recommendations can be given to add the polymer to the fatty phase and first make a concentrated preparation prior to diluting it. Furthermore, some guidelines emerge from this study to select the right polymer according to the technical brief (see Figure 10).
References 1 Structural, Elastic and Dynamic Properties of Swollen Polymers Networks: Advances in Polymer Science, 44, 27, 1982. 2 A. Roso, S. Basset, O. Braun, P. Mallo. Influence of the process of polymerization on the final properties of thickeners for cosmetics: IFSCC Osaka, 2006.