Nim Bunthoeun, Opaprakasit Pakorn
School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology (SIIT), Thammasat University, Pathum Thani 12121, Thailand.
Spectrochim Acta A Mol Biomol Spectrosc. 2021 Jul 5;255:119684. doi: 10.1016/j.saa.2021.119684. Epub 2021 Mar 11.
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 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.
化学循环是一种很有前景的方法，可将消费后的生物塑料，尤其是聚酯，转化为用于其他增值产品的小尺寸起始原料。在这项工作中，已开发出一种用各种醇对聚乳酸（PLA）进行醇解的工艺。然后将产物用作生物基多元醇，用于生产具有自愈性能的高弹性聚氨酯（PU）。使用微波反应器，以钛酸四丁酯（TBT）为催化剂，将结构中含有三个碳原子但羟基数量和性质不同的各种醇，即1,3 - 丙二醇（PDO）、丙二醇（PG）和甘油（Gly），用于醇解反应。已开发出标准的定量和定性分析技术，通过采用衰减全反射傅里叶变换红外光谱（ATR - FTIR）、二维核磁共振（2D - NMR）、核磁共振氢谱（H NMR）和气相色谱 - 质谱联用（GC - MS）光谱，来表征醇解后的PLA产物的组成、反应产率和结构组分。醇解后的PLA产物是不同长度的羟基封端乳酸序列的混合物，可分为单乳酸酯、二乳酸酯和聚乳酸酯。最小的单乳酸酯是所有体系的主要产物，这表明所开发的采用微波反应器的工艺在裂解长PLA链中的酯键方面具有高效率（即使在短反应时间内）。当PLA/醇的进料比从1:1重量/重量变为4:1重量/重量时，单乳酸酯的产率降低，而二乳酸酯和聚乳酸酯的产率增加。在相似的PLA/醇进料比下，通过检查不同数量和性质的羟基参与反应时生成的醇解产物的组成（亲核性和官能度），比较了不同羟基在裂解PLA酯键中的反应活性。这为反应机理提供了见解，而反应机理对于确定有效设计工艺以获得具有特定结构和性能的产物以便在特定应用中进一步使用的反应条件至关重要。此外，丙交酯可直接从醇解反应中获得，其含量强烈依赖于PLA/醇的进料比。从PG反应获得的产物被选为用作制备高弹性PU的多元醇起始原料的潜在候选物。所得的PU产物显示出与橡胶材料相当的低模量，具有高断裂伸长率，适用于用作其他聚酯的增韧剂或用作功能性生物材料。这些材料表现出优异的自愈性能，并且在热处理后拉伸强度和模量进一步提高。
