(9d) An Investigation into the Effects of Time and Shear Rate on the Spreading of Liquid Binders in a Granular Layering Process

Layering of active ingredients onto a core by the repeated application of coating binder and active powder is a viable production route for many applications in the chemical and pharmaceutical industries. In the current work, the mechanism by which a surrogate binder material (composed of a 50/50 wt% solution of PEG 4000 and water) spreads on the surface of core particles (18-20 mesh non-pareils) was investigated. The binder material contained a blue dye (2 wt%) that provided a visual marking of the presence of the binder.

A series of experiments was performed in which the same amount of binder (8 ml) was distributed onto equal-weight batches of core particles (350 g) under varying conditions of shear for different periods of time. Three mixing levels (low medium and high) corresponding to rotation rates of 600, 900, and 1200 rpm were used in a standard Braun mixer (model: K600 CombiMax). The experiments were performed for 4 different mixing times (10s, 20s, 40s, and 80s) and each of the three shear rates. To ensure that the excess water in the core particles was quickly absorbed, thus "freezing" the coated PEG in place, the non-pareils were first over coated with a layer of anhydrous magnesium sulfate prior to being coated with the binder solution.

In order to analyze the content and distribution of blue dye on the surfaces of the coated core particles, samples of the coated cores were prepared and analyzed using imaging analysis. The analysis technique involved taking a sample of approximately 200 core particles, placing them on a wire screen and capturing an image of the sample using a Pixelink firewire color CCD camera (model: PLA 742). A typical image is shown in Figure 1 for a low shear setting and 10 s mixing time.

The image analysis algorithm was developed using Visilog^TM software in order to determine the spread of the binder solution on the population of core particles. This was achieved by splitting the color image into its component colors (Red Green Blue), identifying the individual cores using various edge detection and blob analysis algorithms, and finally analyzing the level of color on each of the images of the individual cores.

Results will be presented showing how the distribution of binder material changes with the level of mixing (coating) time and changes in shear rate.