(76e) An Investigation into the Effect of Process and Operational Parameters on the Spray Coating of Detergent Powders through Contact Spreading in Tumbling Drums

The liquid spray coating of powders and particles is a process commonly used in many industries, from pharmaceutical to detergent manufacture, and is often achieved using a tumbling drum. The current design and scale-up of these processes is based largely on trial and error as a result of minimal research in the area and the complex interactions occurring within the drum. Furthermore, research has tended to focus on the drop deposition mechanism rather than contact spreading, which is thought to be significant in tumbling drums.

A novel image analysis technique to quantify liquid contact spreading which can be used to analyse the effect of process and formulation conditions has been developed. The set-up has been specially designed to isolate the contact spreading mechanism by removing the droplet spray and directly introducing coated particles to the system. The analysis produces average RGB values for all pixels within an individual particle. The variation in the coating can be calculated from knowledge of the mean red % and the standard deviation of the data (CoV = standard deviation / mean % red * 100). The images are captured over a significantly wide time frame, allowing both the rate of decay and the final end point of the resulting graph to be determined. This end point never represents a perfectly homogenous system, and is therefore called the asymptotic end point

The effect of liquid viscosity, tumbling regime, powder porosity, and material composition are parameters that have been analysed. Characterisation of the materials has allowed the effect of surface roughness, size distribution, material porosity, and density to be captured.

The effect of liquid viscosity has been shown to be very strong; high viscosity solutions require longer tumbling times to reach a similar CoV. Tumbling regime showed a strong effect, yet when analysing the data on a revolutions per minute basis, there was no effect of tumbling speed. Increased particle porosity showed mixed results; for high viscosity coating solutions much greater tumbling times were needed to reach a CoV comparable to non-porous systems using a similar viscosity coating solution. However, for low viscosity coating solutions, no difference between the porous and no-porous systems was seen.

The effect of particle porosity, size distribution and drum scale on liquid contact spreading, using five different sizes of tumbling drum is reported here. Drums were filled with a fixed volume of powder, chosen to represent a 10 % fill level. To isolate the liquid contact spreading from the initial drop deposition, particles are pre-coated with a PEG/dye solution and are added to the main powder bulk whilst the drum is rotating. The particle coating variability is determined as a function of time for each process condition, and also as a function of number of drum revolutions.

Liquid viscosity has been shown to greatly affect the rate at which contact spreading occurs. Higher viscosity solutions produce stronger liquid bridges, reducing the volume of coating liquid that can be readily transferred to other particles. For the case in which the strength of the liquid bonds is greater than the collision energy in the system, numerous collisions over a longer time period are required to disrupt the bridges and allow the liquid to redistribute. When the liquid bridge strength is relatively low i.e. for low viscosity coating solutions, tumbling is quickly able to overcome the bonds and allow the liquid to move around the bed. This results in a quick decrease in the coefficient of variation (CoV), and a short time to reach the asymptotic CoV value.

For cases investigating the effect of material porosity, is was difficult to separate the effects of porosity from those associated with the material density. High viscosity coating solutions tumbled with porous material took an exceptionally long time to tend towards the asymptote. The large liquid bridge strength coupled with the low material density results in poor coating; the impacts between the beads are reduced due to the reduction in weight of the material. Low viscosity systems for porous and non-porous particles showed a similar CoV at similar times.

Sodium percarbonate, a material which more closely approximates the characteristics of the commercially available detergents, showed similar results. The light particles and strong liquid bridges formed when using the high viscosity liquid resulted in minimal movement of liquid around the bed. This could be a consequence of three parameters; 1) the reduction is material density (as previously mentioned), 2) aggregation seen in the system as a result of a much wider size distribution (fines were quickly agglomerated with coating liquid and prevented the spread of liquid through the rest of the bed, meaning large tumbling time resulted in only a small decrease in the coefficient of variation), and 3) absorption of the liquid coating agent into the porous material.

Drum scale has been tested using 5 drums of differing size; all made from stainless steel with one face end constructed from transparent acrylic. No effect of scale was seen, when analysing the data against number of drum revolutions, as opposed to tumbling time. All the data from all 5 drums collapsed onto one line, suggesting the number of contacts seen per revolution is the same for both small and larger systems.

The results show that the basic mechanisms governing contact liquid spreading are of extreme importance for developing robust and reliable understanding of the tumbling drum process.