Plastic waste is a major global issue. Recycling has not made a big impact as hoped; most of the waste is still landfilled or incinerated. Hydrothermal liquefaction (HTL) is a promising technique for plastic recycling. In broad terms, it converts wet carbon-containing feedstocks (e.g., wet biomass or plastics) back into simpler molecules using elevated temperature and pressure. Sub- and supercritical water is used as solvent, reagent, and catalyst, although additional catalysts may be added. HTL efficiency depends on several factors, making optimization potentially complex. We evaluated prior literature on HTL of plastics to summarize the reaction conditions for the optimal results for several types of plastics, such as PE, PP, PET, PS, PC, PVC, and plastic mixtures. Furthermore, the proposed mechanisms were examined and summarized. The polymers with heteroatoms in the main chain, PET and PC, had maximum liquefaction efficiency at subcritical temperatures, separating into their substituent monomers, while those without had maximum liquefaction efficiency at supercritical temperatures. The polyolefins with branches, PS, PP, PVC, and LDPE, liquefied at lower temperatures than that of the branchless HDPE. Plastic and plastic–biomass mixtures showed synergy at subcritical temperatures with maximal yields of around 30%.

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塑料的水热液化：反应条件对反应效率影响的调查

塑料垃圾是一个重大的全球问题。回收并没有像希望的那样产生大的影响；大部分废物仍然被填埋或焚烧。水热液化（HTL）是一种很有前景的塑料回收技术。从广义上讲，它利用升高的温度和压力将湿的含碳原料（例如湿的生物质或塑料）转化回更简单的分子。尽管可以添加额外的催化剂，但亚临界水和超临界水用作溶剂、试剂和催化剂。 HTL 效率取决于多个因素，这使得优化可能很复杂。我们评估了有关塑料 HTL 的现有文献，总结了几种类型塑料（例如 PE、PP、PET、PS、PC、PVC 和塑料混合物）获得最佳结果的反应条件。此外，还对所提出的机制进行了检查和总结。主链中含有杂原子的聚合物PET和PC在亚临界温度下具有最大液化效率，分离成其取代单体，而那些不具有杂原子的聚合物在超临界温度下具有最大液化效率。具有支链的聚烯烃、PS、PP、PVC 和 LDPE 的液化温度低于无支化的 HDPE。塑料和塑料-生物质混合物在亚临界温度下表现出协同作用，最大产量约为 30%。