LAS acid reactive binder: Wettability and adhesion behaviour in detergent granulation

Abstract

Linear alkylbenzene sulphonic acid (LAS acid or HLAS) is a reactive binder used in industrial detergent granulation processes. The neutralisation of HLAS with sodium carbonate particles (commercial soda ash) during granulation generates sodium linear alkylbenzene sulphonate (LAS-Na), one of the world's most used anionic surfactants for washing powder formulations. The reaction neutralises the HLAS to increasing degrees, depending on the amount of acid reacted to form the salt. Hence, during granulation the binder has the dual function of forming physical links between the particles and participating in the neutralisation reaction to form the LAS-Na along with some water. The experimental investigation reported here is aimed at establishing whether the extent of HLAS neutralisation and the binder water content influences the wettability and adhesive strength of individual liquid bridges holding particles together in the early stages of granulation. Since LAS acid is a reactive binder, the characterisation of the wetting and adhesive behaviour is complicated by their strong dependency on the kinetics of the binder–powder reaction. Hence, a novel approach to reactive binder studies is presented here, with the experiments carried out using inert particles and acid that had been partially pre-neutralised with Na 2CO 3. Wettability and adhesive force studies have been carried out using a micromanipulator system developed at University College London. Contact angle and spreading velocity measurements show that the higher the degree of neutralisation the lower is the tendency of the liquid to wet the particles. Moreover, it will be shown that particle wetting is strongly dependent on binder water content and relative humidity (RH). A “sticky regime map” is presented as a function of neutralisation and water content to distinguish between wetting and non-wetting conditions. This highlights the importance of controlling the environmental conditions, both during the granulation process and the storage of the product materials.