Issue link: https://www.e-digitaleditions.com/i/1197147
January 2020 / 49 used to judge homogeneity. This test, which involves smearing a sample of the mixed material on a white surface, is ideal for conditions that call for multiple powders with or without liquid additions. The absence of color agglomerates, patchiness, and streaks denote thorough particle dispersion throughout the mix. Figures 2, 3, and 4 show additional examples of visual checks to determine the homogenity of a mix. eration and greatly impact a powder's compressibility and flowrate. In industries such as cosmetics and pharmaceuticals, this poses a quality assurance and control nightmare as agglomeration greatly impacts the dispersion performance of these materials. Friction. Of the risks associated with prolonged mix times, friction may have the greatest impact by causing premature breakdown of components within a blend. This can not only lower end-product quality but can also result in the accumulation of fines, which are ultimately wasted product. This affects an operation's profitability. Shearing active ingredients, which creates additional friction, can spark the same untimely reactions and have detrimental results. One of these results is the aforemen- tioned triboelectric effect, which is the electrification of dissimilar materials caused by particle collisions. During prolonged mixing, particle interactions are increased, and the surface-to-surface particle contact can produce an electric charge. While most mixers are constructed of metal alloys and are good conductors of electricity, many powders are good insulators. The situa- tion creates the perfect conditions for triboelectrification, more commonly known as static electricity. Analysis can help ensure mix quality Whatever the material or problems involved, navigating through the task of establishing ideal mixing param- eters and cycle times can be tricky. Fortunately, there are some quick and easy methods and analyses you can employ — both prior to and after mixing — to help ensure that a batch doesn't exceed its mixing threshold. Establishing the bulk density of each component in a mix, as well as a blended bulk density, is a good baseline to set before establishing what your mixing parameters are. Knowing your mixture's blended bulk density also is a critical component for accurately siz- ing mixing equipment. There are several types of bulk density (aerated, poured, tapped, and others), so take care when using these values as they depend on how the material is handled. Values should be measured a minimum of three times to validate the result. Once values are determined, they can be used in conjunction with a compressibility index (measure of a powder's ability to settle) and Hausner ratio (ratio of tapped density to fluffy density) to predict powder flow characteristics. These characteristics affect the mixing process but are especially important for materials that will be pressed, molded, or vacuum sealed because they determine how the end-product will perform during packaging and after reaching the consumer. In mixes comprised of various colored pigments, a simple visual streak test, shown in Figure 1, can be FIGURE 2 Samples show a completely homogenized blend of salt and pigment mixed in a fluidizing paddle blender. FIGURE 3 Samples (left to right) show a batch's mixing progression. As the spots of oil agglomeration disappear, the powdered cosmetic foundation is homogenized. Total mix time was 4 minutes.