Glycerol-derived organic carbonates: environmentally friendly plasticizers for PLA

Abstract

Polylactic acid (PLA) stands as a promising material, sourced from renewables and exhibiting biodegradability-albeit under stringent industrial composting settings. A primary challenge impeding PLA's broad applications is its inherent brittleness, as it fractures with minimal elongation despite its commendable tensile strength. A well-established remedy involves blending PLA with plasticizers. In this study, a range of organic carbonates-namely, 4-ethoxycarbonyloximethyl-[1,3]dioxolan-2-one (1), 4-methoxycarbonyloximethyl-[1,3]dioxolan-2-one (2), glycerol carbonate (3), and glycerol 1-acetate 2,3-carbonate (4)-were synthesized on a preparative scale (∼100 g), using renewable glycerol and CO₂-derived diethyl carbonate (DEC) or dimethyl carbonate (DMC). Significantly, 1-4 exhibited biodegradability under ambient conditions within a week, ascertained through soil exposure at 25 °C-outpacing the degradation of comparative cellulose. Further investigations revealed 1's efficacy as a PLA plasticizer. Compatibility with PLA, up to 30 phr (parts per hundred resin), was verified using an array of techniques, including DSC, DMA, SEM, and rotational rheometry. The resulting blends showcased enhanced ductility, evident from tensile property measurements. Notably, the novel plasticizer 1 displayed an advantage over conventional acetyltributylcitrate (ATBC) in terms of morphological stability. Slow crystallization, observed in PLA/ATBC blends over time at room temperature, was absent in PLA/1 blends, preserving amorphous domain dimensions and mitigating plasticizer migration-confirmed through DMA assessments of aged and unaged specimens. Nevertheless, biodegradation assessments of the blends revealed that the biodegradable organic carbonate plasticizers did not augment PLA's biodegradation. The PLA in the blends remained mostly unchanged under ambient soil conditions of 25 °C over a 6 month period. This work underscores the potential of organic carbonates as both eco-friendly plasticizers for PLA and as biodegradable compounds, contributing to the development of environmentally conscious polymer systems.

Scheme 1. Exploiting CO₂ for the synthesis of organic carbonates and polycarbonates.

Scheme 2. (a) and (b) Synthetic routes and (c) experimental setup for glycerol-based organic carbonates.

Fig. 1. (a) Second-heating DSC curves of PLA/1 blends compared with neat PLA. (b) Linear correlation between the content of 1 and T_g of the blends.

Fig. 2. Temperature dependence of (a) tan δ, (b) storage modulus (E′), (c) loss modulus (E′′) curves from DMA runs illustrated for PLA/1 blends compared to neat PLA.

Fig. 3. Stress–strain behavior for PLA/1 blends.

Fig. 4. SEM images of the tensile fracture surfaces of PLA blends with 1, 2, and 4.

Fig. 5. GPC curves of pristine purchased PLA, neat PLA post thermal treatment under blending conditions, PLA/1_{20 phr}, PLA/2_{20 phr}, and PLA/4_{20 phr}.

Fig. 6. TGA/DSC analysis of PLA/1_{30 phr} blend.

Fig. 7. (a) Dynamic viscosities of PLA/1_{10 phr}, PLA/1_{20 phr}, and PLA/1_{30 phr} compared to neat PLA and PBAT measured using rotational rheometer at 170 °C. (b) Cole–Cole plots of PLA/1_{10 phr}, PLA/1_{20 phr}, and PLA/1_{30 phr}.

Fig. 8. DMA curves for unaged and aged (6 weeks) specimens of (a) PLA/ATBC_{20 phr} and (b) PLA/1_{20 phr}.

Fig. 9. Biodegradability assessments of (a) organic carbonates and (b) their blends monitored by measuring evolved CO₂ over time in a respirometer set to 25 °C and 50–55% water content with continuous air flow.