Abstract
Dual-functioning additives with plasticizing and antibacterial functions were designed by exploiting the natural aromatic compound eugenol and green platform chemical levulinic acid or valeric acid that can be produced from biobased resources. One-pot synthesis methodology was utilized to create three ester-rich plasticizers. The plasticizers were thoroughly characterized by several nuclear magnetic resonance techniques (1H NMR, 13C NMR, 31P NMR, HSQC, COSY, HMBC) and by electrospray ionization-mass spectrometry (ESI-MS) and their performances, as plasticizers for polylactide (PLA), were evaluated. The eugenyl valerate was equipped with a strong capability to depress the glass transition temperature (Tg) of PLA. Incorporating 30 wt% plasticizer led to a reduction of the Tg by 43 °C. This was also reflected by a remarkable change in mechanical properties, illustrated by a strain at break of 560%, almost 110 times the strain for the breaking of neat PLA. The two eugenyl levulinates also led to PLA with significantly increased strain at breaking. The eugenyl levulinates portrayed higher thermal stabilities than eugenyl valerate, both neat and in PLA blends. The different concentrations of phenol, carboxyl and alcohol functional groups in the three plasticizers caused different bactericidal activities. The eugenyl levulinate with the highest phenol-, carboxyl- and alcohol group content significantly inhibited the growth of Staphylococcus aureus and Escherichia coli, while the other two plasticizers could only inhibit the growth of Staphylococcus aureus. Thus, the utilization of eugenol as a building block in plasticizer design for PLA illustrated an interesting potential for production of additives with dual functions, being both plasticizers and antibacterial agents.
Highlights
The increasing demand for food, due to the increase in world population and the long-distance logistic chains, has prompted amplified requirements of food freshness
Three plasticizer candidates that can be derived from biobased resources, two eugenyl levulinates (ML and TL) and eugenyl valerates (MV), with aliphatic and phenolic ester functionalities, were successfully synthesized from eugenol and levulinic acid or valeric acid under classic esterification conditions
Three eugenol-based plasticizers were successfully prepared by a one-pot reaction between eugenol and the green platform chemical levulinic acid, or eugenol and valeric acid
Summary
The increasing demand for food, due to the increase in world population and the long-distance logistic chains, has prompted amplified requirements of food freshness. Innovative materials with delicately designed functions and targeting food packaging could provide solutions. Green building blocks that can be achieved from renewable biomass conversions in a commercial scale should be utilized in the design of the high-performance materials. If agricultural and food waste is used as converted biomass, green features of the building blocks will be further accentuated, contributing to a more sustainable engineering production for the society. The U.S Food and Drug Administration (FDA) has approved PLA to be used in food-contact packaging [7], the commercial utilization of PLA packaging has been increasingly realized [8]. To extend the shelf-life of food products, the microbiological contaminations inside the packaging need to be inhibited or minimized. Many antimicrobial agents have been evaluated as additives in PLA to examine the final antimicrobial performances, including catechin [10], nisin [11,12], copper nanoparticles [13], silver-based nanoclay [14], preservatives [15], lauric arginate [16], essential oil [17], thymol [18], natamycin [19], triclosan [20] and chitosan [21]
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