Abstract
In wet granulation processes, coatings or binders generally consist of mixtures of various raw materials that confer or enhance specific properties to the final product. Typically, a coating solution is composed of water, film forming polymer (such as hydroxypropyl-methylcellulose, HPMC) and filler (such as stearic acid, SA).One of the important issues in wet granulation processes is the stability of the aqueous coating (or binder) dispersion. An unstable dispersion results in the agglomeration of the colloidal particles, thereby affecting the film coating properties and eventually the coating process.In this study, we use dissipative particle dynamics (DPD) to elucidate the structure of aqueous colloidal formulations. DPD is a coarse-grained molecular dynamics simulation method where the materials are described as a set of soft beads interacting according to the Flory–Huggins (1942) model. The DPD simulation results are compared to experimental results obtained by Cryogenic-SEM and particle size distribution analysis.It is shown from the DPD simulation results that the HPMC polymer is able to form a layer that covers SA particles and thus produces stable colloids. Microcrystalline cellulose (MCC) also covers SA agglomerate but it is not able to diffuse inside its inner core. The agglomerate structure is characterized via the density distribution and the polymer chain end-to-end distance.Experimental results show similar trends; particle size distribution analysis shows that in the presence of HPMC, the majority of SA particles are below 1μm in diameter, also MCC is able to prevent the formation of big SA agglomerates and may be a better stabilizing agent than HPMC. SEM images reveal that HPMC surrounds SA agglomerates with a hatching textured film and anchors on their surface.
Highlights
Coating process involves the covering of particulate materials including seeds, agglomerates, pellets and powders with a surrounding layer of a coating agent (Salman et al, 2007)
We found that the HPMC polymer chain is composed of 10 repetitions, and the Microcrystalline cellulose (MCC) polymer is composed of 44 beads
The strength of the adsorption in our dissipative particle dynamics method (DPD) simulations can be assessed by the amount of stabilizing agent beads which are inside the agglomerate
Summary
Coating process involves the covering of particulate materials including seeds, agglomerates, pellets and powders with a surrounding layer of a coating agent (or coating material) (Salman et al, 2007). The DPD method can be used for the simulation of systems involving colloidal suspensions, emulsions, polymer solutions, Newtonian fluids and polymer melts. This method enables accessing larger spatio-temporal scales than those in the molecular dynamics. The DPD method has been used for the simulation of number of other physical systems and material interactions which are not considered in this study, including the behavior of lipid bilayer membranes (Venturoli and Smit, 1999), nanoparticles in brush polymer (Guskova et al, 2009) and flow in pores (Liu et al, 2007)
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