Volatile substances such as perfumes, insect pheromones and volatile corrosion inhibitors can be released into the atmosphere from polymer matrices. The release characteristics of the volatile substances depend on the original concentration of the substances, and also on the type and geometry of the matrix. The design of the matrix can be done with a trial and error process involving several iterations of tool making followed by testing of the release characteristics. However, this is a costly and time-consuming method. The objective of this study is to propose and evaluate a mathematical model based on Fick's second law of diffusion. The model can be used to predict the release profiles of volatile substances from polymer matrices based on the initial volatile concentration, matrix geometry and the coefficient of diffusion of the volatile through the polymer. The alcohols I-propanol, I-butanol, I-hexanol and I-octanol and the polymer ethylene-eo-vinyl acetate (EVA) were chosen as a model system for this study. The coefficients of diffusion of all the alcohols through the EVA were determined with the time lag test using a diffusion cell and polymer sheets. Several methods of making polymer sheets were evaluated. Injection moulded disks was the most suitable method for the system under consideration. Based on the results of the time lag tests, the proposed model was used to predict the release characteristics of the different alcohols from two EVA matrix designs. Injection moulded test pieces of both designs were prepared. All the test pieces contained ca. 10% of one of the alcohols. The test pieces were aged at ambient conditions and the release of the alcohols was monitored. It was found that the proposed model gave a good prediction of the residual mass of the dispensers, never diverging more than 10% from the experimental result. The experimental results tended to show faster release than predicted. This was expected since the model does not consider the effect of concentration on the coefficient of diffusion. It was concluded that the model gave accurate predictions of the release characteristics of the system investigated. It would be a useful tool in the design and development of polymer dispensers for volatile substances. The smaller number of tool modifications and release tests required will lead to cost and time savings in the development process.
Dissertation (M Eng (Chemical Engineering))--University of Pretoria, 2006.