Effect of the preparation method on the structure of linseed oil-filled poly(urea-formaldehyde) microcapsules

Abstract

tLinseed oil was satisfactorily encapsulated in urea-formaldehyde (UF) shell by one acid stage polymer-ization. This was confirmed by thermal analyses (TGA) and FTIR studies. Microcapsules produced by oneacid stage were strong enough to bear the preparation circumstances, and were dried into a free flowingpowder, which remained stable under storage for 6 months at ambient laboratory conditions. Undesir-able formation of UF particles in suspension that do not contribute to the shell growth was inhibited byselecting a proper amount of poly vinyl alcohol (PVA) and sodium dodecyl benzene sulfonate (SDBS), andadjusting the pH of the process.The surface morphology of the microcapsules was particularly sensitive to the pH of the reactionmedium. At low pH values (∼1.7) the UF nanoparticles deposited onto the microcapsule surface, therebyproducing a strong shell. Conversely, microcapsules prepared at pH 3.5, did not maintain their mechanicalintegrity after drying because of the weak wall shell. Spherical microcapsules with diameters in the rangeof 50?200 m were obtained under mechanical agitation at 600 rpm. The size was reduced to 1?20 musing an ultrasonic homogenizer. Linseed oil could not be encapsulated by UF resin using the two-stagemethod. Under basic pH, UF nanoparticles remained in suspension and, consequently, the linseed oil wasnot completely microencapsulated. Results obtained in this research highlight the role of the pH of thereaction medium on the microencapsulation process of linseed oil by UF resins.