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

Linseed oil was satisfactorily encapsulated in urea-formaldehyde (UF) shell by one acid stage polymerization. This was confirmed by thermal analyses (TGA) and FTIR studies. Microcapsules produced by one acid stage were strong enough to bear the preparation circumstances, and were dried into a free flowing powder, which remained stable under storage for 6 months at ambient laboratory conditions. Undesirable formation of UF particles in suspension that do not contribute to the shell growth was inhibited by selecting a proper amount of poly vinyl alcohol (PVA) and sodium dodecyl benzene sulfonate (SDBS), and adjusting the pH of the process. The surface morphology of the microcapsules was particularly sensitive to the pH of the reaction medium. At low pH values (similar to 1.7) the UF nanoparticles deposited onto the microcapsule surface, thereby producing a strong shell. Conversely, microcapsules prepared at pH 3.5, did not maintain their mechanical integrity after drying because of the weak wall shell. Spherical microcapsules with diameters in the range of 50-200 mu m were obtained under mechanical agitation at 600 rpm. The size was reduced to 1-20 mu m using an ultrasonic homogenizer. Linseed oil could not be encapsulated by UF resin using the two-stage method. Under basic pH, UF nanoparticles remained in suspension and, consequently, the linseed oil was not completely microencapsulated. Results obtained in this research highlight the role of the pH of the reaction medium on the microencapsulation process of linseed oil by UF resins. (C) 2016 Elsevier B.V. All rights reserved.