The storage stability of personal care formulations such as emulsions, suspensions and foams is of prime importance to formulators. Here again, rheological additives have been used widely to prevent sedimentation of solid particulates, prevent coalescence in emulsions, and halt collapse of foams. Rheological substances can impart suspending power to the base fluid. The polymer's yield stress or high viscosity at low shear rates are both used for this purpose. Fluids that possess a yield stress may experience flow only when the imposed stress on the fluid surpasses its yield stress. Below the yield stress the fluid displays solid like properties.
Among the polysaccharides, xanthan gum has been widely used as a suspending aid. Xanthan gum has a double helical structure and undergoes significant hydrogen bonding in solution. At rest or when subjected to very low deformations, a weak three dimensional network structure is the prevailing structure which gives rise to the yield stress . When subjected to higher deformations, this structure can easily be broken down to give rheological behaviour similar to fluid (e) in Figure 1.
Other polysaccharides that exhibit yield stresses are kappa and iota carrageenans. These polysaccharides will also form weak gels and are used in personal care products for stabilization .
As discussed in the section on thickeners, cellulose ethers represent another class of polysaccharide-based rheological additives used as suspending aids.
Carboxymethylcellulose imparts a high viscosity at low shear to formulations, enabling it to effectively suspend solids. These characteristics are effectively described by
fluid (e) in Figure 1. CMC has a high capacity for water-binding, and it is generally used to effect rheology and prevent syneresis in high solids formulations .
Methylhydroxypropylcellulose has been shown to enhance shampoo lather by way of the water-binding, surface activity, and thermal gelation properties of this cellulose ether. This polymer can stabilize lather by a mechanism known as interfacial gelation .
Hydrophobically modified cellulose ethers, such as modified hydroxyethylcellulose, viscosify aqueous phases through both hydrogen-bond network formation and through the formation of three-dimensional networks due to hydrophobic interactions. This dual thickening mechanism makes modified hydroxyethylcellulose particularly effective at suspending solids . The hydrophobic moieties may also associate with surfactant micelles, making modified hydroxyethylcellulose a particularly efficient thickener for surfactant-based systems .
Modified hydroxyethylcellulose finds use in many applications, including viscosity and structure development in shampoos, conditioners, and in hand and body lotions . Typical rheological profiles for a modified hydroxethyl cellulose (HMHEC) and CMC are shown in Figure 5.
Salts of cross-linked polyacrylic acids also exhibit considerable yield stresses. However, unlike the other substances, their ensuing structures tend to be much more sensitive to electrolytes . The properties of these materials will be further discussed in the next section.
Colloidal size materials, like fumed silica, are also used for stabilization . Fumed silica can be processed to develop aggregate particles, and thus form weak three-dimensional structures. Stabilization can also be achieved directly by milling the materials to be used in the formulation to colloidal sizes to take advantage of colloidal forces for stabilization.
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