Size and strength distributions of melamine-formaldehyde microcapsules prepared by membrane emulsification

Melamine formaldehyde microcapsules containing an oil-based active ingredient were prepared using a flat membrane combined with a mechanical stirrer (also called dispersion cell) to generate oil droplets followed by in situ polymerization of melamine formaldehyde precondensate on their surface. The effects of membrane properties and process conditions on the size and size distribution of the oil droplets produced in the stage of emulsification and the final microcapsules were investigated by a static light scattering (SLS) technique. Besides, the morphology of the microcapsules, their shell thickness and mechanical properties were characterised by optical microscopy, environmental-scanning electron microscopy (ESEM), transmission electron microscopy (TEM), and a micromanipulation technique respectively. It has been found that the size distribution of the oil droplets or microcapsules, characterised by a value of coefficient of variation (CV) depended on the flux of the dispersed oil phase through the membrane and agitation speed of the stirrer in the dispersion cell. The smallest CV value of the microcapsules prepared by a membrane with a pore size of 15μm and the distance between pores of 200μm was 21.0±0.5% at a flux of the dispersed oil phase of 1.5×10 ^-5m·s ^-1 and an agitation speed of 1080rpm, and the corresponding CV value of the rupture force of the microcapsules was 18.6±0.1%. In comparison with the corresponding data obtained from the microcapsules made using conventional homogenization to generate emulsion droplets (CV value of 34.1±0.3% for diameter and 34.4±0.2% for rupture force), the distributions of the size and mechanical strength of the microcapsules have been significantly narrowed using the membrane emulsion, which clearly demonstrates its advantage.