Properties of Mosquito Repellent-Plasticized Poly(lactic acid) Strands

Abstract

Poly(lactic acid) (PLA) is an attractive candidate for replacing petrochemical polymers because it is fully biodegradable. This study investigated the potential of PLA as a sustainable and environmentally friendly alternative material that can be developed into commercially viable wearable mosquito repellent devices with desirable characteristics. PLA strands containing DEET and IR3535 were prepared by twin screw extrusion compounding and simultaneously functioned as plasticizers for the polymer. The plasticizing effect was investigated by thermal and rheological studies. DSC studies showed that the addition of DEET and IR3535 into PLA strands reduced the glass transition temperature consistent with predictions of the Fox equation, thus proving their efficiency as plasticizers. The rheology of molten samples of neat PLA and PLA/repellents blends, evaluated at 200 °C, was consistent with shear-thinning pseudoplastic behaviour. Raman studies revealed a nonlinear concentration gradient for DEET in the PLA strand, indicating non-Fickian Type II transport controlling the desorption process. Release data obtained at 50 °C showed initial rapid release followed by a slower, near constant rate at longer times. The release rate data were fitted to a novel modification of the Peppas-Sahlin desorption model.

Keywords: desorption; mosquito repellents; polymer strands; rheology; thermal analysis.

Figure 1

Thermogravimetric plots of: (a) neat DEET, neat PLA and PLLA/DEET blends; (b) neat IR3535, neat PLA and PLA/IR3535 blends. The legend indicates the nominal repellent contents of the PLA strands.

Figure 2

Differential scanning calorimetry traces obtained on the second heating scan for (a) neat PLA, PLA/DEET blends and (b) neat PLA, PLA/IR3535 blends.

Figure 3

Experimental data for the glass transition temperatures for blends of PLA with (a) DEET and, (b) IR3535. The continuous line shows the trend, predicted by the Fox model, for the present data.

Figure 4

Raman spectra of: (a) Spectrum for neat DEET and neat PLA. (b) Confocal Raman spectra recorded at different positions inside a PLA strand initially containing 15 wt.% DEET. The distance from the surface indicated in units of μm. The insert shows integrated peak areas for the Raman band located between 1000 and 1020 cm⁻¹ as a proxy for the concentration of DEET inside the PLA strand.

Figure 5

Complex viscosity vs. versus angular frequency curves measured at 200 °C. The effect of repellent concentration is shown in (a) for PLA/DEET blends, and in (b) PLA/IR3535 blends.

Figure 6

Variation of the complex viscosity with the concentration of the repellents DEET and IR3535. The plotted plateau values reflect measurements obtained at 200 °C.

Figure 7

Repellent release data for DEET and IR3535 trapped inside cylindrical PLA strands. The initial concentrations of the repellents are indicated. The data reflects measured mass loss data recorded on ageing blends in convention oven controlled at 50 °C over a period of 66 days. The solid lines represent prediction data obtained by Equation (3) based on the regression constants presented in Table 3.