Synthesis and characterization of polyurethane-urea microcapsules containing galangal essential oil: statistical analysis of encapsulation.

Galangal essential oil (GEO) is known to possess antimicrobial activity (e.g. against Staphylococcus aureus). A way to increase oil lifetime in plants is encapsulation in polyurethane-urea (PUU) microcapsules. In this study, PUU microcapsules with GEO were synthesized by interfacial polymerization at oil-water interface in oil-water emulsion. A statistical analysis of the microcapsule size was successfully applied for characterization of the encapsulation process. Using the model of reversible aggregation, it was shown that the process of encapsulation takes place in the conditions of thermodynamic control. The polymerization conditions (agitation rate in the range 2000-10 000 rpm/min) are the key factors that affect the mean microcapsule size of primary capsules formed during encapsulation. Two complementary processes were determined the mean capsule size during a transformation of these primary microcapsules: break-up and coalescence of oil droplets in the oil-in-water emulsion. The agitation rate does not influence the coalescence of the oil droplets, but the threshold value of agitation speed (in this system 4000 rpm/min) does exist and that is what strongly increases break-up of oil droplets. The higher agitation rate resulted in smaller size of microcapsules (mean diameter decreasing from 5.6 to 4.9 µm for primary capsules and from 13.8 to 9.8 µm for secondary capsules) and with a narrower size distribution. The last mode of encapsulation allows the more effective use the shell material for encapsulating of larger amount of oil.