Controlled release of an antifouling agent from microcapsules with enhanced mechanical and thermal properties

Zhang, Xiaotong (2020). Controlled release of an antifouling agent from microcapsules with enhanced mechanical and thermal properties. University of Birmingham. M.Phil.

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Abstract

It is well known that wind energy is abundant at sea so many offshore wind power generators have been installed and used. However, there are a few factors affecting the efficiency and reliability of wind turbines. Biofouling caused by a complex marine environment is one of the problems influencing the state of power generation systems in seawater. Therefore antifouling coatings have to be developed and applied to their surfaces. Microcapsules, which can be incorporated into certain coating materials, are an ideal micro-carrier of antifouling agent to achieve its sustained release. This project aimed to prepare microcapsules with an antifouling agent, which can have a high payload, be mechanically strong, have good thermal conductivity, and can achieve sustained release of the active ingredient. The long-term objectives are to achieve the long-term sustained release of the active ingredient in marine environment and antifouling performance, after the microcapsules are incorporated into marine coating that will then be applied to metal surfaces.

In this project, microcapsules consisting of N-Vanillylnonanamide (capsaicin synthetic) in the core, which is an antifouling agent, and cellulose acetate butyrate (CAB) as the shell material have been prepared by solvent evaporation method, based on oil in water (O/W) emulsification in a stirred tank. The effects of formulation and processing conditions on the morphology, size, mechanical properties, thermal properties of microcapsules, release rate of the active ingredient in model liquid have been investigated. The formulation and processing conditions included type of emulsifier, emulsifier concentration, agitation speed, core/shell ratio, metallic coating on the CAB microcapsules with capsaicin synthetic. A range of experimental techniques have been used to characterise the microcapsules, including optical microscopy, Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS), Malvern particle sizing, micromanipulation, UV-VIS spectrophotometry and laser flash analysis.

It has been found that the prepared microcapsules had a matrix structure with multiple cores. Based on the experimental conditions and experimental results, using an emulsifier of polyvinyl alcohol (PVA) with concentration of 0.5%, and agitation speed of 1000 rpm to generate the O/W emulsion produced the microcapsules smaller than 60µm for potentially being incorporated into the coatings, an encapsulation efficiency and the payload reached 80.7% and 16.5%, respectively, which is desirable. As for the mechanical properties, the microcapsules showed elastic and plastic deformations under compression, but not rupture for the experimental conditions investigated due to their matrix structure. The Young’s modulus value of microcapsules, calculated by the Hertz model, decreased significantly with increasing the ratio of core/shell material from 0 to 1/2, w/w, and were all in Mega Pascal scale, which are stiffer compared with many different microcapsules prepared by other researchers. Moreover, cyclohexane was used for accelerating the release of capsaicin synthetic from microcapsules and was found to enable to predict the release profile in water by considering the solubility of capsaicin synthetic in each liquid. Ritger-Peppas Model with n = 0.43 was demonstrated to describe the release kinetics of capsaicin synthetic from the microcapsules well, and it is concluded that the release from the matrix microcapsules was driven by Fickian diffusion.

In addition, metallic coating of the microcapsules on their surface were achieved successfully by electroless plating copper after generating a layer of polydopamine based on its selfpolymerisation of dopamine. The microcapsules with copper were proved much stiffer than those without any metallic composition. The thermal conductivities were enhanced up to 7 times by incorporating copper and prolonging the time of both self-polymerising PDA and suspending microcapsules.

Thus, the microcapsules with an antifouling agent have been successfully prepared, which showed its sustained release over a few months. And the microcapsules with metal on their surface were prepared by electroless plating copper after generating a layer of polydopamine based on its self-polymerisation of dopamine, achieving the aim of enhancing thermal conductivity of the antifouling microcapsules.

Type of Work:

Thesis (Masters by Research > M.Phil.)

Award Type:

Masters by Research > M.Phil.

Supervisor(s):