Formation of titanium dioxide core-shell microcapsules through a binary-phase spray technique.

Core-shell microcapsules consisting of a titanium dioxide shell and a hydrophobic solvent core have been prepared with diameters of a few micrometers and a narrow size distribution using a simple and fast airbrush technique. These microcapsules were prepared at room temperature in a single-step process in which an oil with a dissolved titanium alkoxide precursor was forced together with an aqueous solution, containing a surface-active polymer, through a narrow spray nozzle using a nitrogen gas propellant. Several different parameters of chemical, physical, and processing origin were investigated to find an optimal recipe. Two different alkanes, one ketone, and four alcohols were tested and evaluated as core materials, alone or together with the antifungal biocide 2-n-octyl-4-isothiazolin-3-one (OIT). Long-chain alcohols were found suitable as core oil due to their low solubility in water and surface activity. The addition of the surface-active polymers in the water phase was important in aiding the formation and stabilization of the titanium dioxide shell. An impressive loading of 50 wt% of the semi-hydrophobic OIT was possible to encapsulate using this simple and applicable procedure.

Controlled release of a microencapsulated arduous semi-hydrophobic active from coatings: Superhydrophilic polyelectrolyte shells as globally rate-determining barriers.

Polymethylmethacrylate-based microcapsules containing the antimicrobial agent 2-n-octyl-4-isothiazolin-3-one (OIT) decorated by an anchored polyelectrolyte brush consisting of an amphiphilic diblock copolymer of polymethylmethacrylate-block-poly(sodium methacrylate) type have been formulated via a coacervation technique. The polyelectrolyte brush surface provided the microcapsule with a high and stable surface charge density. This enabled further surface modification of the colloidal particle with a thin and dense polyelectrolyte multilayer using the layer-by-layer technique. The addition of the highly charged and hydrophilic polyelectrolyte multilayer assembled on the microcapsule surface resulted in a considerable decrease of the release rate of the encapsulated OIT in aqueous suspension, corresponding to a 40 times reduction of the effective OIT diffusion coefficient in the polymethylmethacrylate matrix. Moreover, the release of encapsulated or freely dispersed OIT from coatings as a function of the matrix density was evaluated and analyzed within the framework of applied diffusion models. Encapsulation of OIT in polyelectrolyte multilayer composite microcapsules was found to significantly prolong the release and render the release rate more or less independent of the matrix density. In addition, the long-term antimicrobial properties of the coatings were evaluated in terms of their susceptibility for biofouling using the fungus and common biofouler Aspergillus niger as model organism. The results clearly demonstrated that the use of encapsulated OIT gave a significantly prolonged surface protection and allowed for the determination of the critical surface flux. The polyelectrolyte multilayer has therefore been recognized as the rate-determining barrier for OIT. The matrix density has a minor influence on the release rate of encapsulated OIT from these microcapsules and this concept may very well be expanded to cover a broad range of hydrophobic and semi-hydrophobic biocides.

Use of microcapsules as controlled release devices for coatings.

Biofouling of surfaces is a considerable problem in many industrial sectors and for the public community in general. The problem is usually approached by the use of functional coatings and most of such antifouling coatings rely on the effect of biocides. However, a substantial drawback is the poor control over the release of the biocide as well as its degradation in the paint. Encapsulation of the biocides in microcapsules is a promising approach that may overcome some of the problems associated with the more traditional ways of incorporating the antifouling agent into the formulation. In this review, we summarize more than a decade of microcapsule research from our lab as well as from other groups working on this topic. Focus will be on two coacervation-based encapsulation techniques; the internal phase separation method and the double emulsion method, which together enable the encapsulation of a broad spectrum of biocides with different physicochemical properties. The release of the biocide from core-shell particles and from encapsulated biocides in coatings is treated in detail. The release behaviour is interpreted in terms of the physicochemical properties of the core-shell particle and the coating matrix. In addition, special attention is given to the experimental release methodology and the implementation of proper diffusion models to describe the release. At the end of the review examples of antifouling properties of some coatings against common biofoulers are presented.