Relation between surface roughness of free films and process parameters in spray coating.

A novel spraying apparatus was developed to obtain reproducible free sprayed films. Aqueous solutions of PolyVinyl Alcohol PVA 4-98, HydroxyPropyl MethylCellulose HPMC 603 and HPMC 615 were used as reference coating materials. The apparatus is composed by a spraying system, a closed chamber containing a rotating Teflon cylinder, a pressured air supply system, a spray solution supply system, and a computerized control system. The spraying air pressure, the cylinder rotation speed, and the cylinder-spray nozzle distance were tailored in such a manner that the roughness of the obtained free films was similar to that from reference coated particles. Optimum spraying process conditions were found for all three coating materials using design of experiments. The morphology of the sprayed films obtained using the optimum conditions is evaluated by means of scanning electron microscopy (SEM), and atomic force microscopy (AFM), and then compared with those from corresponding cast films and coating layers on particles. A match was found between the morphology of sprayed films and that from the corresponding coating layer on the particle surface. The spray apparatus produced reproducible sprayed films with tuneable roughness and/or smoothness depending on the set of processing parameters.

Characterization of physical and viscoelastic properties of polymer films for coating applications under different temperature of drying and storage.

The increasing tendency to enhance consumer products with added functionality is leading to ever more complex products. Nowadays more and more particulate products are coated to give the product specific functionalities. An appropriate approach is needed to be able to satisfy customer's requirements. In this work, three reference well-known coating agents, namely two grades of hydroxypropyl methylcellulose (HPMC) and one polyvinyl alcohol (PVA) were selected and investigated. Aqueous solutions of such polymers were obtained and viscosity and shear stress were measured function of shear rate, temperature and polymer concentration. The viscosities of the solutions appear to be mainly shear rate independent, they clearly show Newtonian behaviour. Drying and storage conditions influence on morphology and structure of the cast films were evaluated using scanning electron microscope (SEM). Dynamic mechanical thermal analysis (DMTA) experiments were carried out on HPMC and PVA cast films to assess the viscoelastic properties over wide temperature-frequency range. The time-temperature superposition principle was used to determine the shift factor, aT, and to compose a master curve. Magnitudes and profiles of storage modulus, E', loss modulus, E'', and tan delta master curves are discussed with relation to drying and storage conditions. No impact of drying temperature on the polymer properties was observed whereas the effect of storage temperature resulted to be relevant in terms of shifts in glass transition temperature and, only partially, changes in the magnitudes of E' and E''.

The use of Stokes deformation number as a predictive tool for material exchange behaviour of granules in the 'equilibrium phase' in high shear granulation.

The objective of this study was three-fold; to investigate the different mechanisms of material exchange during the equilibrium phase of the granulation process and whether these mechanisms are consistent with the mechanisms described in the growth regime map, to study how material properties and process conditions affect these exchange mechanisms, and to correlate Stokes deformation number to the exchange mechanisms. Microcrystalline cellulose (MCC), alpha-lactose, microfine cellulose (MFC), and dextrin were granulated using water as a binding agent. Once in the equilibrium phase, 5% (w/w) of the granular mass was replaced with wet tracer granules, after which the granulation process was continued. Granules were typically of a size of approximately 1mm in diameter. Therefore, these granules can also be called pellets. Tracer experiments show indeed solid material exchange can take place in the equilibrium phase of the high shear granulation process. Tracer material was equally dispersed throughout the whole batch for all materials tested. However, the granulation time needed to reach this homogeneous distribution varied with material and granulation conditions. Three different mechanisms of material exchange were identified: exchange by disintegration, where granules are rapidly crushed and formed to granules again; exchange by deformation, where abraded granule fragments immediately fuse with other granules; and exchange by distribution, where there is a prolonged period over which both tracer and standard granules stay intact, followed by uncontrolled growth and exchange of material. It was found that it is possible to shift between the mechanisms by changing the process conditions, e.g., changing viscosity or amount of binder liquid. These observations indicate that by choosing the appropriate process conditions improved distribution of small amounts of insoluble materials in the granules can be obtained. A relation exists between the exchange mechanisms and the growth regime map: the disintegration mechanism resembles 'crumb behaviour', the deformation mechanism resembles 'steady growth', and the distribution mechanism resembles 'nucleation' and 'induction growth'. Unfortunately, Stokes deformation number cannot be used as a predictive tool when low viscosity binders like water are used, due to the importance of viscosity in the equation. However, this number is one of the variables of the growth regime map. Since the exchange mechanisms correspond to the granule growth mechanisms in the regime map, alternatively colour experiments might be used to reveal the granulation regime.