Effect of the incorporation of β-acid rich hop extract on degradation in soil of polylactic acid (PLA) sheets

Abstract

The assessment of the degradation process is an important step concerning the so-called bioplastics. Poly(lactic acid) (PLA) is currently the most commercially used bioplastic from this class of materials, and has been extensively studied as support for the development of more sustainable active packaging. However, studies approaching the impact of the incorporation of antimicrobial additives into the polymer matrix on the degradability features is still poorly assessed. In this sense, the degradation in soil of active PLA sheets incorporated with a commercial β-acid rich hop extract (KBAE) in different concentrations (0.1%, 1%, 2.5% and 5% w/w) was investigated. The sheets were buried for 180 days and studied in terms of weight loss, macro and micro changes (MEV), molecular alterations (FTIR), thermal degradation profile (TGA) and crystallinity (XRD). No differences were observed in total mass loss between control and active sheets. However, the presence of KBAE interfered with how the hydrolytic degradation occurred. FTIR spectra evidenced changes in polymer structure, especially induced by the higher concentrations of KBAE. DRX diffractograms indicated an increase in crystallinity for all samples but also revealed that the presence of hop β-acids limited the bulk hydrolysis, probably due to the lower diffusion of water molecules among the polymer chains. Supported by TGA findings, the control sheet was mainly affected by inner degradation (bulk hydrolysis), while PLA active sheets degraded mainly through surface erosion. Therefore, the KBAE incorporation affected how the degradation occurred.