Experimental Design and Evaluation of Biocide Release from Microcapsules

Licentiate thesis, 2014

To protect a painted house façade from mold or algae, anti-growth agents are usually mixed in the paint. The protection of paint systems used in the past relied on heavy metals for protection and are banned today due to their negative impact on the environment. Today, less harmful heavy metal-free biocides are used and these are mixed directly in the paint. However, contemporary paint systems lose their anti-growth protection long before the end of their intended lifetime. This problem is related to the high diffusivity of the biocide inside the paint matrix. The protection is lost prematurely as the biocides are leached and rinsed from the coating by water at a high rate. A promising improvement can be achieved by encapsulating the biocides and thereby reducing the release rate from the coating. In this thesis the biocide 2-n-octyl-4-isothizolin-3-one (OIT) has been encapsulated in core-shell microcapsules and homogenous polymeric microspheres respectively, using the internal phase separation method. The main scientific objective of this work is to explore the mechanisms affecting controlled release from microparticles. By understanding how, and to which extent, different parameters influence the release of biocides, a prolonged and controlled surface protection of paint and other coatings can be achieved, which is the general purpose of the work. Moreover, the aim is to design a release methodology in order to evaluate the release of biocides. The internal phase separation method is a straightforward technique and suitable for the encapsulation of hydrophobic biocides. The formulation has been tuned in order to sustain the release of OIT to a surrounding medium. The effect of two parameters; porosity and surface modification, on the release have been the focus of this thesis. Moreover, the porosity has been subdivided into the macroscopic porosity of the coating matrix and the microscopic porosity or free volume of the polymer matrix of the microparticles. It was found that the microscopic porosity is highly affected by the evaporation rate of the volatile solvent during the encapsulation. A slow evaporation rate gave a slower release rate of OIT. In addition, it was shown that the macroscopic porosity was significantly dependent on the drying time of the coating. Longer drying times (several weeks) gave a substantial decrease in macroscopic porosity and release rate of OIT. Regarding the surface modification, the assembly of a polyelectrolyte multilayer (PEM) on the surface of the microparticle resulted in a considerable decrease of the release rate of OIT. This is ascribed to the high charge density and hydrophilicity of the PEM barrier in which OIT has a low solubility. The use of these surface-modified microparticles in coatings rendered the release of OIT more or less independent of the drying of the paint. The PEM was therefore identified as the rate-determining barrier in that system.