Effects of binder addition politics on the wet agglomeration process

Wet agglomeration is achieved by adding a liquid in an agitated powder mixture. The liquid is often pure water or a dispersion of a solid binder (typically a polymer) in water to enhance the agglomeration between particles. The process is carried out in a granulator, which is designed to obtain a good contact between solid and liquid. Monitoring the agglomeration process and study the kinetics of the process are necessary to understand the basic mechanisms of agglomeration. A way to perform this tasks is the direct observation of the granules growth with a sensor that takes digital images of the powder bed during all the process. Images can then be processed by digital image analysis to obtain information about granules size. In particular texture analysis (TA) can be an interesting tool since image texture contains the information on the granules size. In particular from the histograms of grey levels intensities of the images it is possible to get information on the contrast which is related to the average size of the granules. In this work texture analysis was used in order to study the granulation kinetics of a system constituted by cellulose microcrystalline (MCC) and xanthan gum (XG). In particular XG has been added in two different ways: as liquid (pre-dispersed in the water) or as solid (pre-mixed in the MCC). Granulation experiments were performed in an orbital mixer. The experiments have shown that, to obtain granules with the same size, a much larger amount of XG (up to 10 time) was required when pre-mixed in solid form. From kinetics analysis it appeared that the growth was also faster in this case even if with an initial delay, probably due to the hydration time of the polymer. Solid binder hydration indeed is a missing step in the case of pre-dispersion in water. Because of the larger amount of solid binder when pre-mixed in MCC, the final granules were also mechanically more stable and a good agreement between texture analysis and sieve analysis was always found independently of the liquid binder addition rates. Instead in the case of XG pre-dispersed in water the agreement between TA and sieving was found only for high binder addition rates, suggesting that with low XG amount the binder addition rate can impact on the strength of the final granules as can be deduced by the final PSDs which resulted modified by the sieving operation.