Rheology of siloxane-stabilized water in silicone emulsions



Using silicone copolymers in personal care products can improve the aesthetic performance of formulations. During their manufacture, distribution and topical application they are subject to various mechanical stresses. In this study rheology was used to measure their effects. A number of water in silicone (w/Si) emulsions were prepared in which the oil phase consisted of cyclomethicone. The surfactant used was a branched type silicone copolymer. Both viscoelastic and viscometry measurement were performed on model systems and on commercial products. Experimental data were obtained using a Bohlin rheometer. The measurements were taken applying shear rates in the range of 0.46-58 l s-1 and for the strain sweep frequencies of 0.1 Hz, 1 Hz and 10 Hz were applied. Oscillation tests were performed in the 0.1 Hz to 10 Hz range. All measurements were taken at 35deg;C, representing the approximate temperature encountered during topical application. The effect of surfactant concentration on viscoelastic properties was examined. It was shown that with increasing surfactant concentration the elastic moduli G' and the viscous moduli G" increased. Furthermore, the emulsions showed a transition from a predominantly elastic to a predominantly viscous response as the surfactant concentration increased. The effect of varying the water phase volume fraction on viscometry and viscoelastic measurements was also examined. With increasing water phase volume fraction the viscosity of the emulsions, as well as the yield stress, increased. The Cross and Sisko models were applied. From the Dougherty and Krieger equation φeff was calculated. It was found the the data derived from the Sisko model gave more reliable results. Results obtained from commercial samples showed a high proportion of elasticity; oscillation tests and viscometry experiments suggested that tumbling had the biggest impact on the theological profiles; viscosity, η, shear stress, σ, elastic module, G', and dynamic viscosity, η', dropped to a minimum in these samples. Results from the two commercial samples were compared and it was observed that, although both were w/o emulsions, different rheological behaviour could be observed. L'utilisation de copolymeres de silicone dans les produits d'hygiene personnelle peut ameliorer les caracteristiques esthetiques des formulations. Durant leur fabrication, leur distribution et leur application externe elles sont soumises a diverses contraintes mecaniques. Dans cette etude on a utilise la rheologie pour mesurer leurs effets. On a prepare un certain nombre d'emulsions eau/Si dans lesquelles la phase huile est constituee de cyclomethicone. Le tensioactif utilise est un copolymere de silicone de type ramifie. On a effectue des mesures de viscoelasticite et de viscosimetrie sur des systemes modelises et sur des produits du commerce. Les donnees experimentales ont ete obtenues avec un rheometre Bohlin. Les mesures ont ete prises avec des cisaillements compris entre 0.46 et 58 l.s, et pour la fatigue on a applique des frequences de 0.1 Hz, 1 Hz et 10 Hz. Les essais d'oscillation ont ete effectues dans le domaine compris entre 0.1 et 10 Hz. Toutes les mesures ont ete effectuees a 35°C, figurant la temperature approximative existant durant l'application externe. On a etudie l'effet de la concentration du tensioactif sur les proprietes viscoelastiques. On a montre que lorsque la concentration en tensioactif augmente les modules elastiques G' et les modules visqueux G" augmentent. En outre, les emulsions presentent une transition d'une transition d'une reponse majoritairement elastique a une reponse majoritairement visqueuse a mesure que la concentration en tensioactif augmente. On a aussi etudie l'effet de la variation de la fraction volumique de la phase aqueuse sur les mesures de viscosimetrie et de viscoelasticite. Lorsque la fraction volumique de la phase aqueuse augmente, viscosite et la contrainte d'ecoulement des emulsions augmentent. On a applique les modeles de Cross et Sisko, φ eff a ete calcule a partir de l'equation de Dougherty et Krieger. On a constate que les donnees derivees du modele de Sisko donnent des resultats plus fiables. Les resultats obtenus a partir des echantillons du commerce presentent une forte proportion d'elasticite; les essais d'oscillation et de viscosimetrie suggerent que c'est la centrifugation qui a l'impact le plus important sur les profils rheologiques; la viscosite, η, la contrainte de cisaillement, σ, le module elastique, G', et la viscosite dynamique, η', ont atteint leurs minima pour ces echantillons. Les resultats des deux echantillons du commerce ont ete compares et on a constate que, bien que les deux soient des emulsions e/h, on observait des comportements rheologiques differents.