Quantitative analyses of products from chemical recycling of polylactide (PLA) by alcoholysis with various alcohols and their applications as healable lactide-based polyurethanes

摘要

Chemical recycling is a promising approach for converting post-consumer bio-plastics, especially polyesters, into small-sized starting materials for other value-added products. In this work, a process for alcoholysis of polylactide (PLA) by various alcohols has been developed. The products are then employed as bio-based polyols in the production of highly elastic polyurethanes (PUs) with self-healing properties. Various alcohols with three carbons in the structure but different numbers and nature of hydroxyl groups, i.e., 1,3-propanediol (PDO), propylene glycol (PG), and glycerol (Gly), were employed in the alcoholysis reaction with tetrabutyl orthotitanate (TBT) as a catalyst, using a microwave reactor. Standard quantitative and qualitative analysis techniques have been developed for the characterization of the alcoholyzed PLA products, in terms of compositions, reaction yields, and structural fractions, by employing ATR-FTIR, 2D-NMR, H-1 NMR, and GC-MS spectroscopy. A mixture of hydroxyl-capped lactate sequences with different lengths was achieved as alcoholyzed PLA products, which are classified as mono-lactates, dilactates, and poly-lactates. The smallest mono-lactate is a major product for all systems, indicating that the developed process, which employs a microwave reactor, has high efficiency in the cleaving of ester bonds in long PLA chains (also at short reaction times). The yield of the mono-lactates decreases when the PLA/alcohol feed ratios were changed from 1:1 to 4:1 wt/wt, while those of the dilactates and poly-lactates increase. At similar PLA/alcohol feed ratios, the reactivity of different hydroxyls in the cleaving of the ester bonds of PLA is compared by examining the compositions of the alcoholyzed products generated when different numbers and nature of hydroxyls participate in the reaction (nucleophilicity and functionality). This provides insights into the reaction mechanisms, which are essential in determining the reaction conditions for effectively designing a process to obtain products with specific structures and properties for further use in specific applications. Additionally, lactide can be directly obtained from the alcoholysis reaction, whose content is strongly dependent on the PLA/alcohol feed ratios. The products obtained from the PG reaction was selected as a potential candidate for use as the polyol starting material for preparing highly-elastic PUs. The resulting PU products show a low modulus comparable to rubber materials, with high elongation at break, which is suitable for use as toughness enhancement agents for other polyesters, or as functional biomaterials. The materials exhibit excellent healing property, and further enhancements in the tensile strength and modulus after heat treatments. (C) 2021 Elsevier B.V. All rights reserved.