Process Control and Evaluation of Aqueous Latex Film Coating of Pellets in Fluidized Bed
Crilles Casper Larsen
In the present thesis aqueous latex film coating of pellets in fluidised bed was evaluated and an analysis method for estimating film thickness was developed. Furthermore, a process control strategy was developed for aqueous latex film coating based on the thermodynamics of the coating process.
Aqueous latex film coatings are dispersions of discrete polymer spheres in water and are normally divided into two groups (true latexes and pseudo latexes). Aquacoat ECD and Eudragit RS 30D are pseudo latexes and Eudragit NE 30D is a true latex. Film formation from aqueous latex film coatings arises as the polymer particles impinges onto the surface of the pellets and as the water evaporates the polymer spheres approaches each other and finally coalesce into a continuous film. A clear coherent film coating can only be obtained if the film formation is done above the minimum film-forming temperature (MFT) of the polymer. Plasticizers are added to the aqueous latex film coatings to modify the physical properties of the film polymer and thereby improving the film formation, adhesion to substrate and elongation of the coatings. The plasticizers cause a reduction in the cohesive intermolecular forces along the polymer chains with subsequent lowering of Tg and MFT. Often a curing or thermal post coating treatment stage is applied to the pellets immediately after the coating process. Curing is applied to accelerate the coalescence of the polymers particles into a continuous film coating. Curing is a heat treatment of the coated pellets performed at temperatures above the Tg of the latex film coating. The curing can be performed in the fluidised bed after coating or the coated pellets can be transferred to an oven for curing.
When applying plasticizers to the aqueous latex film coatings the MFT of the polymers are lowered and consequently the tackiness of the film coating is increased and to minimise this increased tackiness anti-adherents can be added to the polymer dispersion before coating. Also surfactants, antifoam agents, pore formers and pigments can be added to the polymer dispersion before coating.
Spherical particles e.g. pellets have the ideal shape for application of film coating due to the minimal surface area to volume ratio. Digital image analysis is generally the method of choice when the shape of pellets has to be assessed. By employing the right setup of the analysis method (Appendix I) a high amount of pellets can be assessed in a limited time and thereby increasing the overall accuracy of the estimation. Furthermore, an estimate for the film thickness on the coated pellets can be assessed by digital image analysis.
Fluidised beds are normally used for coating pellets with aqueous latex film coatings and the two most prevalent types of fluidised beds are the top- and bottom-spray coaters. When looking at aqueous latex film coating the newer types of bottom spray coaters (Precision coater® and Wurster HS®) seems superior to the conventional top-spray coater concerning coating quality. However, when looking at the coating uniformity in proportion to different sizes of pellets the top-spray principle seems superior to the bottom-spray principle.
The critical process parameters of aqueous latex film coating in fluidised bed have been identified as process airflow, atomisation airflow and pressure, inlet air temperature, product bed temperature, inlet air humidity and spray rate. To control theses variables analytically and to understand the interactions between these individually the thermodynamics of the coating process have to be employed. Several thermodynamic models for aqueous film coating in fluidised bed have been proposed ranging from the very simple almost empirical models to the very detailed and sophisticated models. By employing these thermodynamic models changes in and consequences of altering the critical process variables can be followed. Ideally, the thermodynamic models can be used to control the critical process variables employing different process control strategies.
In this dissertation three different types of control strategies are
addressed as; inlet air humidity-, temperature- and spray rate maximisation
process control strategies. The main approaches in inlet air humidity
control strategies are constant inlet air humidity by using humidification
or de-humidification. Temperature control strategies are mainly focused
on maintaining a constant product bed temperature by altering the inlet
air temperature. The spray rate maximisation control strategy employed
a temperature control feedback system along with the DUE factor developed
by Larsen et al. and the maximum outlet air humidity feasible. The scope
of this strategy was twofold, one was to maximise the spray rate, the
other was to ensure that sufficient drying capacity was present at all
times to avoid over-wetting of the product. This process control strategy
can be described as a self-optimisation of the spray rate whereby the
optimal / shortest coating time can be identified as the spray rate is