408 JOURNAL OF COSMETIC SCIENCE DISPERSION OF PARTICULATES David Schlossman and Yun Shao, Ph.D. Kobo Products, Inc., South Plainfield, NJ Introduction The popularity of personal care formulations containing particulates with consumers depends on sensory characteristics such as their color, gloss, wear, wrinkle hiding power, transparency, softness, spreadability, blendability, and smoothness, in addition to stability in the finished product. Thus, dispersions of particulates are important to cosmetic chemists, because the desirable properties of the particulates are likely to be influenced by the degree of their dispersion in the formulation. Terms such as dispersions, slum.es, grinds, pastes, suspensions and gels are all used by cosmetic chemists to describe mixing particles in a liquid medium. The elements of particulate dispersion technology, wetting, stabilizing, and grinding are frequently cited in patents. Home, et al. claimed a novel silicone elastomer gel processed from the flow induced shearing through an orifice of a cross linked particulate to create a unique multiple particle size distribution with improved spreadability and substance (I). Gardlik, et al. patented a low residue antiperspirant gel-solid stick comprised of a particulate dispersion of antiperspirant actives held within a non-polymeric crystalline gel-solid matrix (2). Skin care compositions to improve skin appearance containing a charged particulate material dispersed in a thickened hydrophilic carrier were disclosed by Ha, et al. (3) Numerous references can be found describing methods to disperse and emulsify inorganic ultraviolet filters to enhance their SPF and efficacy in formulations (4,5,6,7). Dispersions of particulates have been extensively studied and numerous mathematical models have been proposed to describe their behavior. In practice, however, the formulation and testing of a particulate dispersion is still considered to be much of an art. In this presentation, both science and art will be acknowledged. Princiules of Disnersions Volumes of articles, chapters and textbooks have been published on the science of dispersions. Dispersion is a stepwise process. The objective is to produce in an application medium a stable and uniform milling of finely-divided particles, i.e. aggregates and primary particles. Mechanical Breakdown In order to achieve mechanical breakdown it is necessary to use energy to break down the cohesive forces, the intermolecular forces of attraction that hold the solid particles together. The primary particles always aggregate to form secondary particles due to their high surface energy such as Van der Waal's forces, electrostatic force, hydrogen bonding of surface hydroxyl groups and water bridging between the primary particles (8) The hardness and morphology of the particulate, its percent solids in the carrier and the viscosity of the pre-mix are all likely to influence the mechanical process. High pressure homogenizers, sonolators, and bead mills are all well suited to disperse particulates. Wetting The oil absorption value of a particulate will give an indication about its wetting. Theoretically, the wetting of a solid by a liquid is often described in tenns of the equilibrium contact angle formed at the solid-liquid-vapor (air) triple interface or the spreading coefficient. Both are functions of the three paired interfaces as shown in Figure l. The maximum wetting occurs when the contact angle is zero or when the spreading coefficient has a large positive value (9). Besides surface treatment, dispersing aids are frequently added to increase wetting and stabilization. They also reduce the amount of mixing and milling time, which may prevent the over milling and fracturing of the particles. The finesses of the pigment grind will be detennined by the wetting of the particles in the carrier. (9). Stabilization Unless the particulates are immobilized by the high viscosity of the slurry, after the mechanical forces that accompany the dispersion process are removed, the forces between the particles will start to come into effect that will lead to sedimentation and flocculation or stabilization. The close approach of particles can be repelled by Coulombic interactions between similarly charged particles in polar media or through steric interactions between long-chain molecules adsorbed on the particulate surfaces. These interactions are shown as Figures 2 and 3, respectively. Steric stabilization can operate in both aqueous and non-aqueous media (9). The ideal stabilizing molecule must be capable of being absorbed on the particles surface (and swollen around each particle) and being solvated and extended into the carrier. Particles possess potential attractive and repulsive energies whose actions are strongly determined by their distance of separation. As the distance between particles is increased the interaction energy is
2004 ANNUAL SCIENTIFIC SEMINAR 409 diminished. The attractive energy arises from Van der Waal's forces. Thus, the thickness of the adsorbed layer necessary for the effective stabilization of the dispersed particles increases with increasing particle sizes as shown in Figure 4 (8,9). According to theoretical calculations, dispersions of micronized pigments with particle sizes under 0.2 microns should require a thinner layer than dispersions of pigmentary grades or other filler particles greater than 1 micron. Apnlications of Dispersion Technologv In practice, most of the inorganic particulates contained in cosmetic formulations have highly polar surfaces that are hydrophilic, and are poorly wetted by organic carriers with low to medium polarity such as cyclopentasiloxanes, hydrocarbons and esters. Organic surface treatments like alkoxy titanates, silanes, methyl polysiloxanes, and alkoxy dimethicones are all known to react with and displace the air and water of hydration absorbed on a pigment surface, rendering it from hydrophilic to hydrophobic or lipophilic. We compared the importance of the surface treatment to the dispersant to pre-wet a 15 nanometer micronized titanium in cyclopentasiloxane in Figure 5. (10) Further, we developed a novel crosspolymer surface treatment to promote the wetting of pigments in multimedia. Fi gu re 6 shows the viscosity of treated iron oxide pre-mixes in mineral oil and cyclopentasiloxane. Fi gu re 7 shows the viscosity of a treated rutile titanium dioxide pre-mix in an ester. (11). Cosmetic dispersions are also made in hydrous (water, polyhydric alcohol) systems. Ammonium and sodium polyacrylates have been found to be important dispersing aids, acting to swell the particles water layer and put a stabilizing charge on their surface. According to our study, reactive polyether silanes may also be employed to enhance wetting in aqueous systems. Viscosity data for iron oxide and titanium dioxide premixes can be found in Figure 8 (11). Lastly, we compared the wetting of branched dimethicone treated rutile titanium dioxide in cyclopentasiloxane to popular hydrophobic treatments in Figure 9 by measuring their pre-mix viscosities. (11 ). Table I lists the viscosity and particles sizes of four dispersions containing a 14 nn1. methicone treated micro titanium dioxide that we compared (12). The wetting was best when polyhydroxystearic acid was used as the dispersant in isododecane. All the dispersions were stable at 50°C for one week. Experiments were made dispersing inorganic ultraviolet filters with primary particle sizes ranging from 10 to 200 nm. in silicone fluids, hydrocarbons and esters with various surface treatments and dispersants. The dispersion particle sizes were recorded and compared to their primary particle size. An index of agglomeration was calculated as the ratio of the dispersion particle size over the primary particle size (12). The data presented in Table 2 is representative of the samples prepared and shows that pigments having a smaller primary particle size contain larger indices of agglomeration. Thus, smaller primary particles are harder to disperse. We have found that adding bentonite clays, fumed silica, synthetic waxes, and polysaccharides can thicken dispersions to reduce syneresis and improve suspension of pigments and actives. Evaluation of Pigment Dispersions Both science and art play a role in dispersion testing. Twenty-five years ago the plant manager could approve a batch of nail laquer by observing the way it ran off his blade. Nowadays, viscosity samples are mixed with a mechanical mixer and incubated before testing on an instrument such as a Brookfield RVT. Draw downs are still compared by operators at various stages during processing on cards or glass plates to study transparency, color, drying time and gloss, however, greater reliance is placed on instrun1ents. An example is the particle size which is often measured by using a light scattering size analyzer. Nevertheless, it is an art to measure the size correctly, as the powder or dispersion san1ple must be diluted and sonicated to a usually low concentration in an appropriate solvent. Imaging technologies will play a greater role in evaluating dispersions in the future. Ono, et al measured particle sizes of novel dispersions of cellulose particulates with both a light scattering size analyzer and an electron microscope and calculated a coefficient of aggregation of 1.0 to 3.0 (13). Willian1s, et al. patented a microelectrical resistance tomography system comprising one or more sensors to characterize flowing dispersions including particle shape and size. Summarv The conditions to wet pigments were shown to influence their pre-mix viscosity and the dispersion particle size. Silicone/copolymers and polyhydroxystearic acid are effective stabilizers for pigment dispersions. The stability of dispersions could be improved by decreasing their particle size as the theoretical models predict. More work needs to be done to quantify the differences between the dispersions in order to learn more about their behavior and to predict their performance in finished products.
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