(300e) Adhesion Force and Wetting Behaviour of Las Acid Bridges: A Comparison of Different Neutralisation States in Relation to Detergent Granulation

To model granulation processes satisfactorily, it is necessary to predict the behaviour of colliding binder-coated primary particles. Micro-mechanistic approaches to determining granule and binder interactions have gained favour over recent years, since it is the interfacial properties that are the governing parameters. The challenge is to relate what is observed at the solid-solid and solid-liquid interfaces to multi-particle granules. Information about the very first stage of agglomeration can be derived from the analysis of the individual strength of a liquid bridge holding two particles together.

A novel device, called an Environmentally Controlled Micro Force Balance (EC-MFB), has been developed and used to observe the interaction behaviour of reactive liquid binder, LAS acid, with sodium carbonate crystals (Na2CO3), in relation to the manufacture of detergent granules. LAS acid has been found to react with sodium carbonate at the solid-liquid interface, producing a complex mixture of reactant products and phase changes, ultimately resulting in a solid bridge holding the crystals together. The aim of the experimental study reported here is to investigate whether LAS acid phase changes influence the adhesive strength of individual liquid bridges.

The EC-MFB consists of a micromanipulation stage fitted with an optical microscope and an environmental chamber. The electronic controls are run by Lab-view software and an image analyser system is used to acquire and process images taken during bridge stretching and breakage. Since the measurements of geometry, strength and wettability of the acid on alkaline particles were affected by the crystal-binder reaction, the experiments reported in this study have been carried out using LAS acid in various partially neutralised states on inert particles (i.e. glass ballotini). The reactive binder wettability has been investigated through direct observation and contact angle measurements. The higher is the degree of neutralisation, the lower is the tendency of the liquid to wet the particle, however changes in temperature and humidity could also affect the binder's wettability. A comparison of the results for the different neutralized liquids show how the force exerted by a liquid bridge was largely influenced by the wetting behavior of the liquid on the particles. Low Neutralized binders that wetted both particles well produced the highest forces.

Further work is now underway to study the effects of changing the surface energy of the particles and, hence, the wetting behaviour of the binder. In order to do this, the glass ballotini will be silanised to varying degrees of hydrophobicity. This will give us a better idea of how to control the detergent granulation process once the relationship between acid neutralisation and wetting behaviour is known.