Control of PLA chemical structures by using GMA and DCP during reactive processing and its influence on PLA foamability

Abstract

This study aimed to chemically modify poly(lactic acid), PLA, by reactive processing as a strategy to increase its melt strength, making it viable for producing foams. For this purpose, modification reactions were carried out in a Haake torque rheometer according to a 2² factorial design, where the concentrations of dicumyl peroxide (DCP) and glycidyl methacrylate (GMA) were varied. The levels evaluated were 0.5 and 1.5 phr (peroxide concentration) and 1 phr and 3 phr (GMA concentration). Reference samples with a single reagent were also investigated. The modified PLAs were characterized, and foams were produced by incorporating supercritical CO₂ followed by fast depressurization. The results showed that the predominant reactions differed depending on the relative levels of peroxide and GMA, resulting in crosslinking, and/or insertion of branches, and/or linear reactions. Rheological analyses showed that PLA with low levels of peroxide and GMA (PLA_0.5_1) and high levels of both (PLA_1.5_3) displayed a more elastic behavior, which can be correlated to long chain branching formation with longer relaxation times. Consequently, these samples presented smaller cell sizes with homogeneous distribution, while unmodified and less branched PLA foams comprised coalescent and heterogeneous cells with thick walls. The study showed an optimization of the use of reagents once the sample containing the lowest levels of DCP and GMA presented the smallest cell size, attributed to the rheological properties and influence of crystallinity. As far as we know, the correlation between GMA and DCP concentrations on the final cellular structures of PLA foams has never been explored before.