Recycled Polymer
Green Paths for Polyester Plastics in a Sustainable World
Plastics have been an integral part of numerous social advancements, but their environmental impact, especially in terms of pollution and contribution to the climate crisis, cannot be ignored. As the production and demand for plastics continue to rise, it's crucial to find ways to promote their green recycling, aiming to prevent environmental accumulation and reduce carbon emissions. This review focuses on the green development paths of polyester plastics, exploring trends in mature commercial polyester plastics, newly emerging biodegradable polyester plastics, and future polyester plastics.
Introduction
Modern society is highly dependent on plastic materials. However, the life cycle of plastics, from raw material extraction to product disposal, causes significant environmental pollution and contributes to climate change. The greenhouse gas emissions from plastics were about 1.7 billion tons of carbon - dioxide - equivalent in 2015 and are projected to reach 6.5 billion tons by 2050, accounting for 15% of the global carbon emissions budget. To achieve sustainable and low - carbon economic development, countries and regions have introduced relevant policies, and the Paris Agreement has set a goal to control global temperature rise within 2 °C by 2100.
Carbon emissions are present throughout the plastic life cycle. Currently, 90% of plastics are derived from fossil fuels, and their manufacturing processes are energy - intensive and emission - prone. Additionally, due to improper disposal and a lack of effective recycling technology, waste plastics accumulate in the environment, with only about 14% being recycled. Conventional disposal methods like incineration and landfilling also result in high carbon emissions. Therefore, improving the plastic recycling rate and developing efficient reuse methods are essential for the sustainable development of the plastic industry.
This review focuses on polyester plastics, which are widely used and have great development potential. We will discuss the green development directions of different types of polyester plastics, including mature commercial ones, newly emerging biodegradable ones, and future - oriented ones.
Green Paths for Mature Commercial Polyester Plastics
Developing Bio - based Raw Materials
Polyethylene terephthalate (PET) is a widely used thermoplastic. Traditionally, its raw materials, terephthalic acid (PTA) and ethylene glycol (EG), are derived from petroleum. However, the development of bio - based polymers is an important way to reduce carbon emissions. Bio - based raw materials are obtained through "carbon sequestration" during plant growth, which can significantly reduce carbon dioxide release during the refining of fossil fuels. Life cycle assessment shows that bio - PET manufacturing can decrease greenhouse gas emissions by 82% compared with petroleum - based PET.
Bio - based PET is mostly 30% biomass - derived (Bio - PET30), with EG often coming from renewable biological sources such as ethanol, glycerol, sorbitol, sugar, and cellulose biomass. For example, ethanol can be dehydrated to prepare ethylene and then EG. Although 30% bio - based PET has been commercialized, 100% bio - based PET is still a long - term goal. The main bottleneck is the poor economy of bio - based terephthalic acid. Many techniques have been developed to directly synthesize p - xylene or terephthalic acid from biomass, but they face issues such as high costs, which limit their large - scale application.
Mechanical Recycling
Mechanical recycling through melt extrusion is currently the mainstream method for PET recycling. The recycling rate of PET bottles, which are mainly used for beverages, can reach 57% due to their easy recyclability. However, this method is limited to food - grade transparent PET. Colored PET bottles and other fiber PET materials are difficult to recycle. Moreover, after several high - temperature melting processes, the polymer chains of PET break, leading to performance degradation. Thus, mechanical recycling is a "downcycling" process, and eventually, waste plastic is generated. Therefore, while mechanical recycling is prioritized for PET, chemical recycling should be considered for PET that cannot be mechanically recycled.
Chemical Recycling
PET and other polyester materials have polarized ester groups in their polymeric main chains, which are susceptible to nucleophilic attack, facilitating chemical recycling. Chemical recycling can break the ester bonds in PET to degrade it into its initial monomers, and then chemically regenerated PET can be obtained through polycondensation. Current degradation strategies include glycolysis, alcoholysis, and alkaline hydrolysis. For example, ethylene terephthalate (BHET) can be produced by the glycolysis of EG, and after purification, it can be used to produce PET.
Although significant progress has been made in the industrialization of PET chemical recycling, the main challenge is that PET has high solvent resistance and stability, so its depolymerization usually requires harsh conditions, involving high temperatures and energy consumption. Scientists have been working on developing depolymerization methods under mild conditions. For example, Niu et al. designed a binuclear Zn catalyst based on a biomimetic catalytic strategy, which can facilitate the continuous hydrolysis of PET in a real seawater environment. Wang et al. developed a THF - promoted selective nonaqueous hydrolysis method that can degrade PET at 30 °C.
In addition to recycling PET back to monomers, upcycling PET into high - value - added products is also an important direction. For example, PET can be converted into dioctyl terephthalate (DOTP), a commonly used plasticizer, or other high - value products such as third - generation environmentally friendly solvents and new polymer materials with excellent biocompatibility.
Moreover, with the development of green hydrogen technology, catalytic hydrogenolysis has become an important method for PET recycling. Depending on the degree of hydrogenation, different degradation products can be obtained, enriching the recycling pathways. In addition to traditional chemical or thermal catalysis, cutting - edge catalytic methods such as photocatalysis, photothermal catalysis, and electrocatalysis have also attracted attention. These methods use green energy sources like solar energy or renewable - sourced electrical energy to promote the recycling of plastics.
Green Paths for Newly Emerging Biodegradable Polyester Plastics
Biological Cycle
Conventional plastics are designed with high performance and durability in mind, often at the expense of degradability. In recent years, there has been a global effort to control "white pollution", and biodegradable plastics have emerged as a promising alternative. Biodegradable plastics can be broken down into carbon dioxide, water, and biomass by microorganisms, avoiding continuous environmental harm.
Polylactide (PLA), made from lactic acid fermented by biomass, is the most mature biodegradable plastic, accounting for about 30% of all biodegradable plastics. Under the action of microorganisms, end - of - life biodegradable plastics can be degraded and enter the biological cycle. However, the degradation efficiency of these materials in natural environments varies greatly depending on factors such as the concentration of degrading enzymes, microorganisms, temperature, pH, humidity, oxygen supply, and light. For example, PLLA, a type of PLA, has a glass transition temperature of about 60 °C. When the ambient temperature is lower than this, degradation is difficult.
Additionally, the lack of complete back - end facilities such as composting plants limits the popularization of biodegradable plastics.
Chemical Recycling
Although biodegradable plastics can be biorecycled, considering their long biological cycles and poor economic efficiency, recycling and utilization are also important. Due to their easy - degradation characteristics, biodegradable plastics like PET are not suitable for mechanical recycling, so chemical recycling becomes a priority.
For PLA, its chemical recycling has received extensive attention. The conversion of PLA to its initial monomers, such as lactide, is a key research area. The high ceiling temperature of lactide polymerization has been a bottleneck in the depolymerization of PLA to lactide. However, recent studies have proposed methods to overcome this problem, such as using solvation to regulate the balance between polymerization and depolymerization.
Converting PLA into high - value - added chemicals is another important approach. For example, PLA can be converted into alkyl lactate through alcoholysis, which can be used as a green solvent or to prepare lactide monomers. In recent years, various catalytic systems have been developed to achieve efficient alcoholysis under mild conditions. Additionally, by selectively introducing additional chemical functional groups, PLA can be converted into other high - value products such as alanine or methyl methacrylate.
Another research objective is to convert PLA into new polymer materials. For example, the "depolymerization - repolymerization" strategy can directly convert waste PLA into high - performance polymer materials, shortening the recycling process.
Green Paths for Future Polyester Plastics
With the development of polymer science, new polyester materials are emerging. Future polyester plastics are more oriented towards biodegradable polyester materials from biological sources. These materials can avoid the use of non - renewable fossil fuels and prevent plastic pollution due to their degradable properties.
Polyhydroxyalkanoates (PHA) are natural polymer materials synthesized by microorganisms. They have excellent degradation and physical properties and are considered the most promising biodegradable materials. PHA can be degraded in various environments such as soil, seawater, and lake water. Chemical recycling of PHA is also a potential option, as waste PHA can be used as a bio - based carbon source to synthesize chemicals. However, the application of PHA is still in its infancy, and more research is needed on its chemical recycling.
Polyethylene furanoate (PEF) is a 100% biodegradable polymer material. It has superior functionality and is regarded as an ideal alternative to PET, with higher mechanical strength and gas barrier properties. If it replaces PET, carbon emissions can be reduced by 57%. However, its actual production faces problems such as product degradation and discoloration under high - temperature conditions. New production methods, such as ring - opening polymerization of cyclic oligomers, are being studied to solve these problems. Since PEF and PET have similar chemical structures, the recycling methods for PET can also be applied to PEF.
Materials with long alkyl chains derived from unsaturated fatty acids in vegetable oil are also being developed. These materials have properties similar to polyethylene but can be degraded due to the presence of ester bonds. Poly(δ - valerolactone) (PVL) is another promising material. Although its physical properties were previously considered poor, recent studies have shown that when its molecular weight exceeds the critical entanglement molecular weight, its properties can surpass those of polyolefin materials. PVL can achieve a closed - loop recycling process to monomers.
Conclusion and Outlook
The green development of polyester plastics currently focuses on source green and post - treatment green. For green plastic sources, several issues need to be addressed. First, the conversion efficiency and selectivity of renewable resources to polymeric monomers or polymers need to be improved. Second, the choice of renewable resources should avoid conflicts with grain and use non - grain biomass as much as possible. Third, the compatibility between the performance and degradability of new polymers and cost issues should be considered, and life cycle assessment (LCA) should be conducted. Fourth, monomers compatible with existing polymerization industries should be developed to promote large - scale production and application.
From the perspective of plastic post - treatment green, it is necessary to ensure that waste plastics are recycled instead of being released into the environment. This requires enhancing citizens' awareness of waste recycling and sorting, improving existing plastic recycling support facilities, and building biodegradable plastic composting plants. Post - treatment processes such as plastic sorting and cleaning also need to be further developed. In addition, the economy and green nature of the plastic recycling process, especially energy consumption, should be evaluated to establish a cheap, mild, and pollution - free degradation system.
The green and sustainable development of plastics is a long - term and arduous task, but it is crucial for the future of humanity. Through joint efforts, we can solve the plastic crisis and create a greener future for the Earth.