In recent years, with the continuous development of my country's economy, PET foam materials have been widely used in the fields of construction, packaging, electronics, automobiles, etc., and the market demand has continued to grow. However, compared with the international advanced level, there is still a certain gap in my country's PET foam material industry, mainly in terms of product performance, production efficiency and environmental protection level. In order to cope with market competition and environmental protection requirements, my country's PET foam material industry needs to strengthen technological innovation, improve product performance and production efficiency, and pay attention to environmental protection and sustainable development. In addition, it is also necessary to strengthen cooperation with upstream and downstream industries to form a complete industrial chain and improve overall competitiveness. Driven by policy support and market demand, my country's PET foam material industry is expected to achieve greater development and become the world's leading PET foam material production and application base.

1、Development trend of PET foam material industry

PET foam material has excellent physical and chemical properties and is currently a hot topic for domestic and foreign scholars. Many foreign companies have successfully prepared PET foam materials, which have a very broad market prospect. Japan Water Chemical Industries Co. first used the extrusion process to manufacture" CelPet" PET Thin slice，This is because it has low density, excellent oil and chemical resistance, and good impact resistance, and is used as a raw material for manufacturing packaging materials, motor parts, pallet parts, etc. In addition, the company also produces PET foam materials that can be used at 220°C. In addition, the company also produces PET foam boards that can be recycled at 220°C and have good heat resistance. The performance comparison of "CelPet" and PP and PS foam sheets is shown in Table 1.

Table 1 Performance comparison of CelPet, PP and PS foam sheets

Guit uses low-cost recycled raw materials (PET plastic beverage bottles and fabrics) and automated batch extrusion technology to produce a renewable structural foam core (GPET) that has lower resin absorption than lightweight wood and is not subject to market demand. Armacell offers a range of ARMAFORM polyester foams for sandwich composites. Armacell offers a range of ARMAFORM PET foams for sandwich composites, also made from recycled PET beverage bottles. The newly developed ARMAFORM PETAC115 has 10-20% higher shear elongation, fracture toughness and resilience than the original ARMAFORMPET100.

Kerdyn, a PET structural foam board developed by BASF in Germany, has been widely used in composite materials because of its excellent properties such as high temperature resistance and chemical corrosion resistance.[3]。The multifunctional polyester foam core material AIREX produced by Swiss 3 AC posites is suitable for train compartment floors, wall panels, interior panels, roofs, wind turbine accessories, ships, vehicles, etc. Its maximum operating temperature is 150℃ and the maximum operating temperature is 100℃. It will not deform during operation and will not produce exhaust gas. The newly developed new PET foam material AIREXT92 has a shear elongation 2-3 times higher than PET,it breaks the limitation of PET being"too brittle "in use and opens up a new field of PET foam materials［4］.The PET microporous foam reflective board (MCPET) developed by Furukawa Electric Company of Japan has strong reflection and scattering properties due to its microporous structure. Without adding any light source, the lighting brightness can reach 40%-60%; at the same time, MCPET can also be used as a large-area LED display panel of more than 42 inches, solving the problems of low diffuse reflectivity of metal reflective boards, blue light caused by absorbing red light, and reflection only in a certain direction. At present, the method of producing PET foam boards abroad is mostly batch method. MicroGREEN Company of the United States uses supercritical carbon dioxide as a foaming agent, and foams PET solid flakes in batches after saturating them with supercritical carbon dioxide, and then heats them into continuous foaming, thereby obtaining high-density, high-strength, high-density, low-cost PET composite foam materials. At present, there are three companies in the world, including Airex, Armacell, and Gruit, which can produce PET foam boards using continuous extrusion technology of supercritical fluid.

At present, the global PET continuous extrusion foaming process is mainly adopted by some companies in developed countries such as Europe, America, Japan, and Sweden. Since the domestic PET foaming industry started relatively late and there are not many companies developing and manufacturing, there is still a long way to go with the markets of advanced countries such as Europe and America. At present, except for Sinopec Yizheng Chemical Fiber Research Institute and Beijing Business School, the research and development of PET foaming materials in my country is still in the laboratory exploration stage. Until 2019, my country has made a major breakthrough in this research: Beijing University of Chemical Technology and Nanjing Chuangbo Mechanical Equipment Co., Ltd. completed the world's first PET micro-foaming experiment, and applied it to the research of PET and other polymer foaming, and applied it to the industrialization of foaming products.

2、Progress in PET Foaming Technology

2.1Self-Aggregation Method

The self-polymerization method is to use the activity of the end of the PET chain segment to produce a high molecular weight and high strength polymer material through polymerization reaction. Self-polymerization reaction can be further divided into solid phase polycondensation and melt polycondensation. Solid phase polycondensation refers to further polycondensation under the protection of inert gas such as vacuum or nitrogen when the temperature is greater than or equal to the glass transition temperature (Tg) and lower than the melting point (Tm). Companies such as Bepex in the United States, Buhler in Switzerland, and Zimmer in Germany have established their own process flows and a complete set of continuous production lines. Solid phase polycondensation has significant advantages in molecular weight and properties, but its reaction temperature is low, the reaction speed is slow, and the reaction time is long. The melt polycondensation process refers to adding a reactor at high temperature after the polycondensation reactor of PET to make the polycondensation reaction last longer, thereby obtaining a macromolecular polymer. During the melt polycondensation reaction, since the small molecular substances generated during the polymerization process cannot be quickly discharged, it will cause local overheating or degradation.

2.2Introduction Of Chain Extender Method

The introduction of chain extenders can not only increase the relative molecular weight of PET, but also increase the branching degree of PET, broaden the molecular weight distribution, and thus improve the viscosity and strength of PET. Because of its obvious effect, it has become a research hotspot for scholars at home and abroad. Commonly used are bisoxazolines and imine carbonates containing tertiary amine structures, or epoxy compounds such as isocyanates and anhydrides. These two materials can interact with the carboxyl groups at both ends of PET to reassemble the fragments that are broken after heating.

Hao Yuanzeng et al.［6］studied the chain extension and viscosity-increasing effect of oxazoline and epoxy chain extenders and found that epoxy chain extenders had better viscosity-increasing effect than oxazoline chain extenders. In addition, with the increase in the amount of epoxy chain extender, the viscosity of PET after chain extension increased rapidly, while oxazoline chain extenders had almost no viscosity-increasing effect.NascimentoChristineＲ.et al. ［7］introduced chain extenders OXZ and ANP in the solid phase polycondensation process and studied the effects of these two chain extenders. The results showed that OXZ broadened the molecular weight distribution, the molecular weight of PET was significantly improved, and the molecular weight of PET increased with the increase of OXZ dosage, while ANP made the PET polymer form a branched structure; after thermodynamic analysis, the two modified PET structures were stable and the thermal degradation performance did not change significantly.Zhang Suwen et al. ［8］used 2,2-bis(2-oxazoline) (BOZ) and pyromellitic dianhydride (PMDA) as chain extenders to study the changes in PET viscosity and carboxyl value before and after extrusion. The results showed that the chain extension effect of BOZ alone was not obvious, while PMDA alone could effectively increase the viscosity of PET; the product obtained by combining the two had high viscosity, low carboxyl value, and the best chain extension effect. Under the optimal process conditions, when the introduction amount of BOZ and PMDA was 0.2% (mass fraction), a PET product with a viscosity of 0.90dL/g could be obtained.

3、Productionstatusof PET foam

In the entire PET industry chain, the upstream suppliers are raw materials, such as polyethylene terephthalate (PET), and some auxiliary materials, such as foaming agents and flame retardants; the midstream is the polyester foam producer; the company's downstream products include: transportation equipment materials, aerospace materials, wind turbine blades, building energy-saving materials, sports equipment, sports equipment, etc.

PET is an important raw material for the PET foaming industry. PET foaming enterprises in developed countries such as Europe and the United States are at the forefront of the world. Their high-performance, cost-effective, renewable (recyclable) PET foaming materials are the main suppliers of China's wind power industry. The PET foaming process is monopolized by foreign companies such as 3 AC, Armacell, and Diab. Among them, 3 AC of Switzerland is a benchmark enterprise in the field of PET foaming, and its market share in China ranks first in the world. Armacell and Diab of the United States are also the world's largest PET foaming material manufacturers; the PET foaming materials produced by 3 AC, Armacell, and DIABY have a combined market share of more than 80% worldwide, and the market share in China is about 90%.

At present, China's wind power market has occupied about 1/3 of the world's share, and domestic PET structural foam materials are mainly used in the field of wind power generation. In 2020, due to the outbreak of the new coronavirus, the supply of wind turbine cores was in short supply, the epidemic situation in Italy and Ecuador continued to escalate, and the blockades of various countries caused the upstream balsa wood and PVC foam of wind power blades to be in short supply. PET is an ideal alternative material. Therefore, some specialized companies began to enter this industry and began to seize the market. In response to the current situation of insufficient supply of PET foam raw materials for wind power generation, many domestic companies have broken through technical barriers by cooperating with scientific research institutions. Shanghai Yueke New Materials has cooperated with East China University of Science and Technology to successfully develop the world's first batch and continuous production line for PET structural foam materials. Shanghai Yueke New Materials Co., Ltd. cooperated with East China University of Science and Technology to successfully develop my country's first batch and continuous production technology for PET structural foaming in May 2019. At present, Shanghai Yueke New Materials Co., Ltd. (annual output of 100km3/year) has two PET structural foam plastics (annual output of 100km3/year), and its flame retardancy, smoke resistance, dripping resistance and toxicity indicators all meet the requirements of the German "Flame Retardant and Fireproof Specifications for Rail Vehicle Materials" (DIN5510-2), and have obtained GL certification; Anhui Dongyuan New Materials Co., Ltd. completed the annual production of 15,000 tons of PET foaming production line and the second phase of the annual production of 75,000 tons of PET foam core material in August 2019, and started construction in July 2020; Anhui Dongyuan New Materials Co., Ltd. has put into use 7 lines (co-developed with Beijing University of Chemical Technology, PET supercritical carbon dioxide anhydrous foaming, and PET foam plastic products produced by Nanjing Chuangbo Machinery Co., Ltd. can reach B1 grade); in April 2021, the 110km3/a PET foam core material project of Chazhi New Materials (Jiangsu) officially started.

4、Conclusion

In summary, the PET foam material industry has made significant progress in recent years, with growing market demand and expanding product application areas. However, compared with the international advanced level, there is still a certain gap in my country's PET foam material industry, and it is necessary to further improve product performance, production efficiency and environmental protection level. In future development, my country's PET foam material industry needs to strengthen technological innovation, improve product performance and production efficiency, and pay attention to environmental protection and sustainable development. In addition, it is also necessary to strengthen cooperation with upstream and downstream industries to form a complete industrial chain and improve overall competitiveness. Driven by policy support and market demand, my country's PET foam material industry is expected to achieve greater development and become the world's leading PET foam material production and application base.