Abstract:
Nanocomposite strands with mosquito repellent DEET or Icaridin incorporated in a poly(ethylene-co-vinyl acetate) (EVA) matrix, with either pyrogenic
silica or an organoclay as a nanofiller, were prepared by a twin-screw extrusion
compounding technique. The nature and levels of the repellent and nanofiller
that was used affected the material phase morphology. The repellent release
was followed as a function of aging time in convection ovens set at 30 and
50 C. The experimental release data of the mosquito repellent from the microporous polymer swellable matrix strands was mathematically modeled and
fitted using a range of semi-empirical models. In the majority of case, the
Korsmeyer-Peppas power law model provided the best data fit. As expected,
the wide range of internal morphologies also resulted in quite different release
profiles. These models were found to be valuable as they provided insights into
the mechanism of repellent release from EVA swellable matrices. It was possible to differentiate between diffusion and relaxation mechanisms. Surprisingly,
strands containing nominally more than 30 wt% Icaridin showed accelerating
mass loss during the initial phase, consistent with Super Case II transport. Diffusional exponents as high as 1.81 were found. Furthermore, the internal
microporous region of the extruded EVA strands was covered by a surface
membrane that acted a diffusion barrier that, in effect, controlled the release
rate of the mosquito repellents. Some of the investigated samples exhibited
release profiles that suggest that longer lasting effective release of repellents is
possible than currently achieved by available commercially products.
