How To Break The Barrier Of Less Than 10% Utilization Of Waste Polyester Fiber Products?

Polyester is the main raw material of textile, its waste product stock exceeds 1 billion tons, but the regeneration utilization rate is less than 10%. It is difficult to degrade naturally, and has a great impact on resources and environment. Exploring recycling technology of waste polyester fiber products can effectively alleviate the shortage of oil and arable land resources, save a lot of fiber materials, and achieve sustainable economic development. Starting from the development of polyester recycling technology, this paper briefly describes the development status of polyester regeneration at home and abroad, including physical, chemical and physical chemistry methods, analyzes the development routes suitable for China's current national conditions, and finally looks forward to the development trend and construction measures of polyester regeneration technology.

1 Introduction

With the improvement of people's living standards, the quantity of waste textiles has increased substantially. How to make scientific and rational use has become the focus of attention of all countries. Japan, France, the United Kingdom, the United States and other countries have upgraded the development of textile chemical fiber recycling and low carbon economy into national strategies, and promulgated the bill on textile and clothing, footwear and home textiles recycling and processing in the environmental basic law, ecology and sustainable law. China is a big textile country in the world. The number of waste textiles is more than 20 million tons per year, but the recycling rate is less than 10%, causing serious waste of resources and solid waste pollution. In particular, the annual output of polyester fiber, which accounts for nearly 70% of the textile raw materials, has exceeded 40 million tons in 2018, and the stock of waste products has exceeded 1 million tons. The recycling and recycling of waste polyester fiber products is the most important part of the sustainable development of the national chemical fiber and textile industry. [1]

Waste polyester fiber products refer to all kinds of leftovers and waste materials produced in the process of production by using polyester textile and clothing as raw materials, including waste silk waste pieces produced in the process of chemical fiber preparation, waste fabric and scrap materials generated during the processing of waste yarn, silk, printing and dyeing produced in spinning and weaving process, and textile and clothing products and textile products produced after various consumption, such as garments, fabrics, carpets, decorations and other polyester fabrics after use. Its source and morphology also have important reference value for the selection of recycling methods, because the source often determines the impurities in waste products. For example: polyester fiber factory waste silk and polyester clothing after consumption, these two different sources of polyester fiber waste, the former can be simple doped direct melt regenerated spinning, and the latter needs to undergo crushing, granulation, melting, conditioning, viscosity adjustment, color matching and other processes to achieve regeneration. At the same time, our country does not allow second-hand clothing to enter the market at present, and the composition of waste clothing is complex, and the classification of recycling is chaotic. It also directly restricts the application rate of regeneration. Therefore, the regeneration of polyester textiles is mainly concentrated in the production waste, the leftover materials or uniform uniforms of garment factories, which are categorical, heterogeneous and fixed, which are seriously restricted and urgently needed to develop. How to realize the efficient and high value utilization of waste polyester through recycling and recycling technology has been the focus of the chemical fiber industry and the development of circular economy.

2 waste recycling technology abroad

Europe, the United States, Japan and other developed countries and regions in the world began to study the recycling and utilization of waste polyester in 1960s. The technology that has been formed mainly includes two types: physical method and chemical method.

Two point one

Physical method

Typical processes include waste identification and sorting, crushing and cleaning, granulation (for waste fiber products), drying and viscosity increasing (mainly by adding chain extender, liquid phase or solid state polycondensation), melt processing and so on. In recent years, the most representative technological progress is mainly the multi rotation super high surface vacuum extruder (MRS) technology developed by Gneuss, Germany and the VACUREMA system [3] developed by Austria Erema company, [2].

Fig. 1 Multi rotation high specific surface vacuum extrusion technology of Gneuss, Germany

Gneuss's multi rotating ultra-high surface vacuum extruder (MRS) technology can make polyester melt and achieve high devolatilization, high efficiency and energy saving, as shown in Figure 1. The VACUREMA system developed by Austria Erema company is also based on the principle of vacuum devolatilization to regenerate waste polyester. The system can realize continuous drying and melting through automatic vacuum gates. The high vacuum reactor is directly connected with the screw rod, which can avoid the exhaust of the extrusion process, shorten the length of the screw rod, enhance the regeneration efficiency, and reduce the acetaldehyde production, as shown in Figure 2.

Fig. 2 VACUREMA system for polyester regeneration developed by Erema company of Austria

Two point two

chemical method

The main idea of chemical regeneration is to make use of the reversibility of polycondensation reaction, and then dissolve the waste polyester into monomers or polymerized intermediates. After separation and purification, it can be repolymerized to high-quality regenerated polyester. Therefore, closed recycling of waste PET can be realized. For the recovery of waste textiles with high impurity content, the theory of chemical regeneration has an absolute advantage. At the same time, due to the variability of chemical recovery, a variety of products with higher added value can be developed in the process of regeneration, so as to realize the high value reuse of waste PET. However, the process of chemical recovery is relatively complex, technically difficult and expensive to produce, so there are few cases of successful commercialization worldwide.

At present, the most representative technology of industrialization is the alcoholysis regeneration method of Japanese Teijin company. Aiming at the problem of more oligomers in ethylene glycol products, Teijin company in Japan has developed a regeneration process of PET glycol depolymerization methanol transesterification ([4]) and achieved large-scale commercial production. The process consists of two steps: ethylene glycol depolymerization and methanol transesterification. PET is first dissolved in ethylene glycol, and alcoholysis products are exchanged with methanol. The process of transesterification with methanol can effectively decompose the process of ethylene glycol decomposition and complete oligomer decomposition and polymerization, and convert it to DMT uniformly. The yield of monomer is obviously improved. The purity of crude DMT after transesterification can reach more than 99% after remelting and vacuum distillation, and its quality is stable. But the obvious drawback is that the process is too long and the cost is high. In addition, there are more oligomers in the product due to the reaction equilibrium. Alcoholysis catalysts are likely to interfere with the product quality. Ethylene glycol can produce self polymerization at higher temperature and produce by-product two ethylene glycol.

Fig. 3 process flow chart of EG alcoholysis ethylene glycol alcoholysis combined methanol transesterification of Diege company

3 waste recycling technology in China

Compared with the developed countries such as Europe, America and Japan, the recycling technology of waste textiles in China has started relatively late, and there is still a significant gap between the advanced technology and maturity of the whole industry compared with that of foreign countries. However, since 12th Five-Year, the enthusiasm of R & D industry has improved significantly, and the technology has been combined with the actual situation of China. At present, polyester regeneration technology in China can be divided into mechanical method, physical method, chemical method and physical chemistry method.

Three point one

Mechanical and physical methods

The capacity of direct delivery and utilization of waste textiles is mainly concentrated in the early Wenling and Cangnan areas of Zhejiang. Due to the upgrading of technology, the technology is currently used only in cotton products and blended products. The physical regeneration of direct melt spinning after waste cleaning and drying accounts for nearly 95%, mainly for the regeneration of PET bottle flakes. The textile wastes in Cangnan District of Zhejiang reach hundreds of thousands of tons each year, and have become the national famous waste textile recycling base. Recycled fibers are widely used in furniture decoration, clothing, home textiles, toys and automotive industries.

Three point two

chemical method

The research on polyester chemical recovery technology in China is mainly focused on the laboratory stage. The research focus is on the application of alcoholysis, high efficiency catalysis and alcoholysis products in many fields. For the recycling of waste polyester textiles containing more impurities, the theory of chemical method has absolute advantage, but because of the high cost, the domestic industrialization process is slow. In recent years, Zhejiang beauty New Material Co., Ltd. has effectively integrated resources and technology, improved the process, greatly reduced the production cost of DMT chemical regenerated polyester fiber, and expanded its capacity from the current 25 thousand tons to 160 thousand tons on the basis of continuous and stable production. In addition, in 2014, Zhejiang Luyu environmental protection Co., Ltd. in 2014 built the first international 100 thousand tons of low cost BHET waste textile chemical regeneration production line and opened the production line. The production line is different from the emperor's DMT method. It does not separate the additives and dyes from the depolymerization products. Instead, it uses the powder on-line adding technology to match the color of the depolymerization liquid, reduces the production cost and maximizes the utilization of dyestuff in the waste textiles, though it can not compare with the products of the Japanese company, but other properties can better meet the application requirements. The development of low cost chemical method has also promoted the technological progress of polyester recycling industry in China.

Three point three

Physicochemical method

The physical chemistry method is aimed at the improvement and upgrading of the limitations of the physical method. By melting the recovered polyester waste, the liquid phase or solid phase can be added to enhance the viscosity. This method is mainly physical method, supplemented by chemical method to increase the molecular weight and reduce the impurity content. Under the condition of little increase in production cost, the quality of regenerated products can be effectively improved and differentiated regeneration can be realized. Ningbo Dafa Chemical Fiber Co., Ltd., excellent color environmental protection resources Polytron Technologies Inc independently developed "micro alcoholysis - devolatilization polymerization" polyester regeneration technology [5, 6] is the representative technology of the industrialization of physical chemistry in China, as shown in Figure 4. The technology first reduces viscosity of polyester melt by depolymerization, so that impurities in the melts can be effectively removed by filtration and devolatilization. At the same time, the molecular weight of polyester is homogenized to obtain low viscosity melt with low impurity content. After that, good viscosity increasing effect can be obtained by condensation polymerization. The technology achieves regenerated melt stability increasing to 0.63dL g-1 above, and viscosity fluctuation reduced to + 0.01 dL g-1.

Fig. 4 physical chemistry method "vertical falling film horizontal squirrel cage" conditioning and viscosity adjusting device

4 Summary and Prospect

Mechanical and physical methods have advantages such as short process, high efficiency, low cost, wide applicability and so on. They are the main technology of regeneration of waste textiles at present. However, there are still many problems to be solved, such as low quality and poor stability. At the same time, because of the low technical threshold, the relatively low capacity of the low end, and the uneven production technology, the application of recycled products is directly restricted. Chemical regeneration is to regenerate polymers into small molecules and regenerate them after being purified and repolymerized. The quality of the products can almost reach the original level. The technology is widely adaptable, and can realize the comprehensive utilization of many kinds of waste chemical fiber textiles. But the most prominent problem is that the cost is too high and the industrialization is blocked. The regeneration of physical chemistry method is aimed at the improvement and upgrading of the limitations of the physical method itself, mainly by physical method, supplemented by chemical method to increase molecular weight and reduce impurity content. This method is more suitable for the development of polyester recycling industry in China at present. It can improve the quality of recycled products effectively and realize differential regeneration under the condition that the production cost increases little. It is of great significance to improve the quality and utilization of recycled products.

Taking a look at the development mode of recycling and recycling economy in developed countries and China, the development of regenerated polyester fiber will focus on the efficient and recycling utilization of resources, with the principle of reduction, reuse and resource utilization as the characteristics of low input, low consumption, low emission and high efficiency. It is emphasized that according to the characteristics of recycled polyester raw materials and the application requirements of clothing, geotechnical, automobile, home textiles and other fields, the flexible regeneration processes such as tempering, viscosity adjusting, color matching, alcoholysis, decolorization and copolymerization modification should be designed to control the whole process, standardize the standards of fiber and textile products, ensure the efficient regeneration and recycling of products, and promote the development of recycled polyester industry in the direction of scale, automation, continuity, cleanliness and integration. In addition, At the same time, the joint brand enterprises should strengthen the concept propaganda and promotion, and promote the healthy development of the industry.

(author of this article: Chen Ye, Wang Shaobo, Ke Fuyou, Wang Huaping. Among them, Chen Ye, Ke Fuyou and Wang Huaping are from the State Key Laboratory of fiber material modification, School of materials science and engineering, Donghua University; Wang Shaobo is from the Key Laboratory of functional textile materials of Henan Textile College of Zhongyuan University of Technology. )

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