Physical, Chemical And Biological Methods of Recycling PET And Industrialization Progress

Physical, Chemical And Biological Methods of Recycling PET And Industrialization Progress

Plastic waste is an important part of solid waste, according to statistics, the annual output of plastic packaging worldwide is 130 million tons, 50% of plastic products are disposable items, accounting for about 36% of the total plastic production,however, 79% of the recycled plastic waste is landfilled or directly dumped into the sea, and only 9% of the recycling has been utilized. However, plastics are difficult to degrade in nature due to their unique structural properties, and with the large-scale popularization and application of plastics, the problem of “white pollution” has gradually surfaced, and plastic products that are difficult to degrade naturally pose a great threat to the natural environment.

PET is an important component of recyclable plastics, accounting for about one-third of the total amount of recyclable plastics. However, PET is difficult to be completely degraded in nature, but under the stimulation of external conditions such as ultraviolet light, biological free radicals and seawater, it is very easy to be degraded into “microplastics” which are even more harmful.

The problem of microplastics in the oceans and their potential hazards was first reported by Richard Thompson of Plymouth University in 2014. With the increasing white pollution, countries around the world have introduced a series of laws and regulations to restrain the arbitrary disposal of plastic waste and regulate the recycling of plastic waste. Researchers, government officials, and business owners around the world are also actively involved in this “environmental defense war”.

A large number of advanced technologies for recycling PET and large-scale projects for recycling PET have emerged in recent years. Currently the main recycling methods for waste PET plastics are physical, chemical and biological (enzymatic).

I. Physical method

The physical method for recycling PET is an effective approach. It involves sorting and cleaning waste PET after collection. Impurities such as PP bottle caps, PVC bottle labels (some labels are made of aluminum or paper), and adhesives are separated from PET bottles. The cleaned waste PET is then crushed, washed, dried, and granulated to obtain clean PET bottle flakes.

This method is relatively simple, requiring only physical separation and crushing of waste PET. It does not involve chemical reactions and does not produce additional hazardous substances, making it environmentally friendly. The recycling process is also relatively inexpensive. The recycled material can be used for secondary processing to make plastic products or can be modified, granulated, and spun for fabric production.

Germany's BB Engineering (BBE) has combined its VacuFil recycling process with the VarioFil direct spinning system. This integration creates a one - step recycling and inline direct spinning process that recycles post - consumer PET waste and PET industrial production waste, spinning them inline into pre - oriented yarn (POY) or fully - drawn yarn (FDY).

Aiming at the athleisure fashion market, Japan's Teijin Frontier Co., Ltd. has developed a new cotton - like, high - functionality polyester fabric made from recycled raw materials. This new material combines the look and feel of cotton with excellent moisture - wicking, quick - drying, and UV protection properties.

Intco Recycling Resources Co., Ltd. responded to the national “Opinions on Further Strengthening the Control of Plastic Pollution” call in 2020 by building a 50,000 - ton food - grade PET beverage bottle recycling and reclamation project in Malaysia. It has obtained certifications for the GRS full - process traceability system, FDA, EFSA, OceanCycle, etc. The bottle - flake - grade r - PET and fiber - grade r - PET produced have stable quality and are widely used in food packaging, apparel, electronic information, and other fields. To date, INTCO RECYCLING recycles over 150,000 tons of recycled plastics annually and saves 300,000 tons of carbon emissions annually.

However, the physical method has certain limitations. It can only perform simple separation and granulation of waste PET polyester. The melting process of waste PET may produce residual hazardous substances like acetaldehyde. Furthermore, as the number of recycling times of recycled PET increases, the molecular weight, viscosity, and other physicochemical properties of the recycled PET will significantly decrease. Consequently, the recycled material is often only suitable for manufacturing lower - grade products.

II. Chemical method

Chemical recycling PET, that is, the waste PET materials through the chemical method of complete or partial degradation into the original chemical raw materials (generally reactive monomers or oligomers), to achieve recycling. Usually includes hydrolysis, amine decomposition, alcohol decomposition method, etc. Chemical recycling method because of its wide range of depolymerization agent with different reaction conditions can get a wide range of depolymerization products, can be used as a chemical raw material re-entry into the industrial cycle, so as to play a role in reducing the consumption of fossil raw materials to promote energy conservation and emission reduction and other effects.

• Hydrolysis

It is difficult to hydrolyze PET under normal temperature and pressure. Hydrolysis degradation of PET usually refers to the degradation of PET to produce terephthalic acid (TPA) and ethylene glycol (EG) under a certain temperature and pressure. This method can effectively avoid the use of organic solvents, and the operation is relatively simple, and the separation and refinement of the product is more convenient. According to the different hydrolysis environment, it can be divided into acidic hydrolysis, alkaline hydrolysis and neutral hydrolysis.

1. Acid hydrolysis method

YangW et al. from Nanjing Green Manufacturing Industry Innovation Research Institute of CSCI, aimed at the current problem that most of the strong acid or strong alkali type hydrolysis catalysts are difficult to be separated after being dissolved in the reaction medium, which leads to the high cost of production and the impact on the environment. A novel idea of acid-catalyzed hydrolysis for PET degradation was proposed, i.e., using TPA, the basic unit of PET, as an acid catalyst to promote the hydrolysis of PET. The results showed that up to 100.0% PET conversion and 95.5% TPA yield were achieved under optimized conditions. The purity of the produced TPA was as high as 99%, which was virtually indistinguishable from that of brand-new TPA, and did not require cumbersome post-treatment and purification processes. In addition, the recovered catalyst maintains high activity over eight consecutive reaction cycles.

2. Alkaline hydrolysis

Chemical recovery of PET waste by alkaline hydrolysis and glycolysis was carried out by TMPZanela et al. at the University of Paraná, Brazil. The results showed that alkaline hydrolysis is the greenest method and the method can be realized by mild reaction conditions. The method produced good yields of TPA up to 97%. Catalytic glycolysis of waste PET can be carried out with shorter reaction time and relatively low temperature, and the yield of bis(2-hydroxyethyl) terephthalate (BHET) was 75%.

3.Neutral hydrolysis method

KangMJ et al. from Korea Institute of Chemical Technology synthesized a zeolite-based catalyst ZSM-5 with abundant Brønsted acidic sites using hydrothermal method and used it for microwave-assisted hydrolysis of PET. By studying the reaction kinetics and activation energy of the PET hydrolysis reaction in the presence of the catalyst, the results showed that the ZSM-5 catalyst could realize the complete degradation of PET at 120 °C, which doubled the efficiency compared with the catalyst-free condition.

• Alcoholysis

Alcoholysis is a degradation method in which PET is degraded to dimethyl terephthalate (DMT) and EG by ester exchange reaction between PET and small molecule alcohols under the action of catalyst. This method has mild reaction conditions and is the chemical degradation and recycling technology with the most potential for large-scale industrial application. According to the different alcohols used, it can be divided into methanol method and diol method (glycolysis method).

1.Methanol method

Methanolysis of PET corresponds to DMT and EG, which can be recovered separately or directly used in the re-synthesis of PET. The process is mature and the raw material cost is low, but the equipment requirements are high. At present, most of the studies on methanolysis of PET focus on the co-solvent method and supercritical alcoholysis method.

The introduction of co-solvent can effectively reduce the reaction conditions and reaction time.2023 In 2023, JingTang et al. from Qingdao University used co-solvent to strengthen methanolysis of polyester in response to the problem of high temperature and high pressure required for traditional PET methanolysis, and systematically compared it with the ordinary methanolysis process. The results showed that the addition of co-solvent could simultaneously shorten the reaction time and significantly reduce the reaction temperature. The 100% conversion of PET could be realized in only 2h at 120℃. The mechanism of choosing acetonitrile as the optimal co-solvent for the reaction was also investigated in combination with experimental characterization and DFT simulation calculations. The addition of co-solvent can increase the specific surface area of PET and improve the mass transfer coefficient of methanol, which can promote the contact between methanol and PET; meanwhile, the addition of co-solvent acetonitrile reduces the activation energy of the reaction from 106.9 kJ/mol to 90.3 kJ/mol, which can make the reaction easier.

Supercritical Fluid Technology (SupercriticalFluidTechnology) is a technology that utilizes the special state of fluid at its critical temperature and pressure, which is characterized by both low viscosity and high diffusion coefficient of the gaseous state as well as high solubility of the liquid state, to carry out the operations of material processing, chemical reaction and separation. The related research mainly focuses on supercritical hydrolysis and supercritical methanolysis. However, the reaction conditions of supercritical hydrolysis are more demanding, and EG is decomposed in large quantities under the catalytic effect of TPA, which seriously affects the recovery rate.

QinliLiu et al. from Xi'an Jiaotong University used Response Surface Methodology (RSM) to establish a mathematical model to study the supercritical methanolysis reaction of PET, focusing on the effects of three main factors, namely, methanolysis temperature, time, and methanol-to-polyester mass ratio, on the reaction. The results showed that the increase of methanolysis temperature and time could help to increase the DMT yield, and the mass ratio of methanol to polyester had little effect on the DMT yield in the case of methanol excess. The model predictions matched the experimental results, and the DMT yield reached 99.79% under ideal conditions.

2.Diolysis

Diolysis usually refers to a class of reactions in which diols and PET molecules are esterified at a certain temperature and in the presence of a catalyst, resulting in the breakage of the PET ester bond and substitution of the hydroxyl terminus. It is also commonly referred in the literature as glycolysis.

Common diols that can be used for PET degradation include ethylene glycol, diethylene glycol, 1,4-butanediol and neopentyl glycol. Diolysis is a simple process, which can be reacted under low pressure or atmospheric pressure and requires less equipment, but the reaction requires a large number of solvents and is difficult to purify, so the study of glycolysis reaction system mainly focuses on the optimization of the degradation reaction process. The process parameters affecting PET glycolysis are mainly the choice of alcohol solvent, basic PET properties, catalyst, and degradation time and temperature. These process parameters have different effects on glycolysis efficiency and products.

ZhouX et al. from Xi'an University of Science and Technology (XUST) used three kinds of diols, neopentyl glycol (NPG), dipropylene glycol (DPG) and polypropylene glycol (PPG), to degrade the waste PET at different temperatures and catalysts for different kinds of PET substrates, and obtained a series of glycolysis products with different appearances. The ontological structure and thermal properties of the alcoholysis products were systematically investigated by a series of analytical and testing methods. The results showed that DPG had the lowest alcoholysis activity, and PPG could react with PET to form a copolymer, which had excellent stability and transparency.

Xu Hui et al, Lanzhou University of Science and Technology, disclosed a Lewis acid efficient catalytic alcoholysis of PET, the catalyst comprising a single-component tungstate or a mixture of its tungstate with Na, Ni, Zn, and Mn. The catalyst, in combination with a solvent such as monoconjugated diethylene glycol or propylene glycol, enables efficient degradation of PET. The catalyst can be used for the preparation of high-quality polyester polyols, which can then be used for the production of rigid polyurethane foams due to the drastically reduced catalyst dosage and the low subsequent impact on the products.

Chen Xisheng et al. from Shenyang University of Chemical Technology designed and synthesized high thermally stable bisimidazolium cationic zinc acetate low eutectic solvents with multiple active sites and low metal content, and applied them to the alcoholysis application of PET. The results showed that the complete degradation of PET could be realized by EG4 times the mass feeding ratio of PET, 5 wt% catalyst dosage, and reaction at 190°C for 70 min, and the BHET yield was 85.2%.

• Aminolysis

Aminolysis is mainly used to degrade PET by organic amines. Since the amine group is more active than the hydroxyl group, aminolysis is more likely to occur than alcoholysis, and the commonly used aminolysis agents are methylamine (Methylamine), ethylamine (Ethylamine), ethylenediamine (EDA), ethanolamine (MEA), and n-butylamine (BA), etc. The aminolytic degradation of PET was investigated in detail by using NaA1O2 as the catalyst and diethanolamine (DEA) as the aminolytic agent.

Wu Shijie from Hunan University used NaA1O2 as catalyst and diethanolamine (DEA) as aminolytic agent to degrade PET, and studied in detail the effects of DEA, NaA1O2 dosage, reaction temperature and time on the aminolysis efficiency of PET. The results showed that under the reaction condition of 190℃, the catalyst NaA1O2 was used as 1.6 wt% of PET, and the mass feeding ratio of PET∶EG=1∶4 was optimal for the reaction of 2h aminolysis efficiency, which could reach 99.0%. The total yield of the resulting product TPA with BHAET was 89.7%.

KazukiFukushima et al., Almaden Research Center, IBM, San Jose, California, USA, utilized 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a catalyst for the amine depolymerization of waste PET, and a series of terephthalamide derivative monomers were crystallized. Due to the hydrogen bonding between the terephthalic acid group and the amide group, these monomers have great potential in the construction of high-performance engineering materials.

III. Biological method

Bio-degradation of PET is the use of PET-degrading enzymes isolated and identified from microorganisms to hydrolyze PET macromolecules into mono-(2-hydroxyethyl) terephthalate, TPA, EG and other chemical materials, which not only solves the environmental pressure brought about by discarded PET, but also optimizes the resources and is more environmentally friendly. Biodegradation method is not mature at present, the activity and stability performance of PET degradation enzymes still need to be improved, but biodegradation of PET is more environmentally friendly, and these studies provide research ideas for the efficient biodegradation of PET in the future.

A research team from Tianjin Institute of Industrial Biotechnology, Chinese Academy of Sciences, in collaboration with a research team from Nanjing University of Traditional Chinese Medicine, developed a novel, high-throughput screening method based on fluorescence detection for directed evolution of IsPETase, and the obtained mutant DepoPETase demonstrated excellent waste PET depolymerization performance at medium temperatures, and achieved complete depolymerization of a wide range of waste PET packages. This work improves the PET hydrolysis activity and thermal stability of IsPETase by directed evolution technique and novel high-throughput screening method, which provides new ideas and methods for the molecular modification of PET hydrolase, as well as efficient enzyme catalysts for enzymatic degradation and recycling of PET.

Prof. Qingsheng Qi's team at the Institute of Microbial Technology, Shandong University developed a dynamic molecular docking-based substrate affinity analysis strategy, rationally designed and obtained a highly active PET-degrading enzyme mutant, LCC-A2, which degraded more than 90% of post-consumer PET (Pc-PET) waste in 3.3 h, and more than 99% of the products were completely degraded terminal monomers, which is currently reported to be the most efficient PET-degrading enzyme mutant reported so far.

A new closed-loop recycling strategy for PET was proposed by Christian Sonnendecker et al. from the University of Leipzig, Germany, who isolated a polyester hydrolase, PHL7, from composted soil, which enables rapid hydrolysis of PET plastics. Without any energy-intensive pretreatment, only 0.6 mg of hydrolase PHL7 per gram of PET was required at 70°C to achieve 90% degradation in 16 hours.

Carbios France and IndoramaVentures (IVL) have jointly announced a joint venture to build and operate the world's first enzymatic polyethylene terephthalate biorecovery facility (PCN) in France. The project is expected to be officially put into operation in 2025, with construction costing about 230 million euros, and can process 50,000 tons/year of PET waste, including those that cannot be mechanically recycled.

Recycling of waste plastics has become a key link in energy saving, carbon reduction and environmental and ecological governance. Through in-depth research on PET recycling technology, it can be found that technological innovation and industrialization have largely improved the efficiency and effectiveness of PET recycling. It not only helps to reduce the pollution of waste PET to the environment, but also plays a role in promoting energy saving and carbon reduction through resource utilization.

Conclusion

With the global emphasis on environmental protection and sustainable development, the innovation and industrialization of PET recycling technologies are advancing, providing strong support for reducing plastic pollution and promoting resource recycling. However, in the process of recycling, it is also crucial to improve the performance and safety of PET materials. YINSU Flame Retardant has developed a variety of PET halogen-free flame retardants in response to this demand, such as red phosphorus flame retardant PET-55D and organophosphorus flame retardant YS-F22B. These products can not only effectively enhance the flame retardant properties of PET wire drawing, PET film and other materials, but also maintain their excellent physicochemical properties, providing the possibility of PET materials in a wider range of applications. In the future, with the continuous progress of technology and environmental protection awareness, the combination of PET recycling and flame retardant technology will inject new impetus for the green development of the plastics industry.