Polyethylene terephthalate (PET, or polyester for short) is mainly used for the production of fibers, PET bottles, and films. The source of waste polyester is mainly two parts, one part is the waste and scrap produced during the production process; the second part is the waste polyester packaging, such as polyester bottles, polyester film and so on. The first type of waste polyester is relatively clean and can be reused directly; the second type of waste is often contaminated and must be separated and removed before it can be recycled.
The treatment of waste polyester mainly adopts two methods of recovery and reduction of the source, and the recovery is divided into physical recovery and chemical recovery. Chemical recovery is the depolymerization of solid polymer materials into smaller molecules, intermediate materials, or direct conversion to monomers. For PET, chemical recovery can break polyester chains into low molecular weight ethylene terephthalate (BHET) intermediates or completely degrade to PTA or terephthalate II Methyl ester (DMT) and ethylene glycol (EG). As the field of foods does not allow the use of physically recycled PET, advances in polyester chemical recovery technology are very important. Only the development of economic chemical recovery technology can effectively promote the reuse of this resin. At present, the cost of recycling is not as competitive as the production of PET from new raw materials, so it cannot be applied on a large scale.
For PET, the traditional chemical recovery mainly has the following specific methods: methanol alcoholysis, hydrolysis and glycol alcoholysis.
(1) Methanol alcoholysis The principle of methanol alcoholysis is the reaction between waste PET and methanol to obtain DMT and ethylene glycol. DMT can be converted to terephthalic acid or directly used as PET raw material. The chemical reaction of methanol alcoholysis is the reverse reaction of DMT and EG transesterification to produce PET. Although traditional methanol alcoholysis process is relatively simple, the purification of the product is much more complicated. If the PET used for depolymerization is of low quality, the mixture of methanol, ethylene glycol and DMT must be separated. If there are comonomers in the PET sheet, the difficulty of separation increases accordingly. Some processes remove the alcoholylated product from the reactor as a vapor rather than as a liquid.
(2) Hydrolysis The principle of hydrolysis is mainly PET reacts with water and degrades PET into PTA and EG. When the temperature is higher than 100 °C, PET will be hydrolyzed, and the hydrolysis speed will increase with temperature. However, the deep hydrolysis of TPA (high purity terephthalic acid) and EG must be carried out under acid-base catalysis or high temperature and pressure conditions.
(3) Ethylene glycol alcoholysis The principle of glycol alcoholysis is the reaction of PET with ethylene glycol to obtain BHET and/or oligomers. This method was introduced by DuPont. PET waste and excess EG (molar ratio of 1:4) were subjected to atmospheric pressure reaction at about 170-190 °C. The catalyst used acetate, the reaction was carried out for 2.5-3 h, PET depolymerized into BHET and EG, and the temperature was lowered to 100 °C. Impurities were filtered, a polymerization inhibitor was added, and EG was distilled off under reduced pressure. Then, BHET was dissolved in hot water (90° C.), insolubles and oligomers were removed by filtration, cooled and crystallized, and white needle-shaped BHET crystals were filtered.
Ethylene glycol alcoholysis is relatively low in cost because it is partially degraded rather than completely degraded. With this method, the impurities in the PET resin must be removed, otherwise the recycled PET cannot be used for food packaging. The resulting high purity BHET from the reaction can be used directly to produce fiber grade polyesters. The general polyester production plant adds a set of ethylene glycol depolymerization equipment based on the original equipment. The product obtained by the depolymerization is mixed with fresh monomer in proportion. Polymerized fiber polyester can be obtained and used There is essentially no difference between the PET performance recovered by the method and the performance of the new PET material.
The treatment of waste polyester mainly adopts two methods of recovery and reduction of the source, and the recovery is divided into physical recovery and chemical recovery. Chemical recovery is the depolymerization of solid polymer materials into smaller molecules, intermediate materials, or direct conversion to monomers. For PET, chemical recovery can break polyester chains into low molecular weight ethylene terephthalate (BHET) intermediates or completely degrade to PTA or terephthalate II Methyl ester (DMT) and ethylene glycol (EG). As the field of foods does not allow the use of physically recycled PET, advances in polyester chemical recovery technology are very important. Only the development of economic chemical recovery technology can effectively promote the reuse of this resin. At present, the cost of recycling is not as competitive as the production of PET from new raw materials, so it cannot be applied on a large scale.
For PET, the traditional chemical recovery mainly has the following specific methods: methanol alcoholysis, hydrolysis and glycol alcoholysis.
(1) Methanol alcoholysis The principle of methanol alcoholysis is the reaction between waste PET and methanol to obtain DMT and ethylene glycol. DMT can be converted to terephthalic acid or directly used as PET raw material. The chemical reaction of methanol alcoholysis is the reverse reaction of DMT and EG transesterification to produce PET. Although traditional methanol alcoholysis process is relatively simple, the purification of the product is much more complicated. If the PET used for depolymerization is of low quality, the mixture of methanol, ethylene glycol and DMT must be separated. If there are comonomers in the PET sheet, the difficulty of separation increases accordingly. Some processes remove the alcoholylated product from the reactor as a vapor rather than as a liquid.
(2) Hydrolysis The principle of hydrolysis is mainly PET reacts with water and degrades PET into PTA and EG. When the temperature is higher than 100 °C, PET will be hydrolyzed, and the hydrolysis speed will increase with temperature. However, the deep hydrolysis of TPA (high purity terephthalic acid) and EG must be carried out under acid-base catalysis or high temperature and pressure conditions.
(3) Ethylene glycol alcoholysis The principle of glycol alcoholysis is the reaction of PET with ethylene glycol to obtain BHET and/or oligomers. This method was introduced by DuPont. PET waste and excess EG (molar ratio of 1:4) were subjected to atmospheric pressure reaction at about 170-190 °C. The catalyst used acetate, the reaction was carried out for 2.5-3 h, PET depolymerized into BHET and EG, and the temperature was lowered to 100 °C. Impurities were filtered, a polymerization inhibitor was added, and EG was distilled off under reduced pressure. Then, BHET was dissolved in hot water (90° C.), insolubles and oligomers were removed by filtration, cooled and crystallized, and white needle-shaped BHET crystals were filtered.
Ethylene glycol alcoholysis is relatively low in cost because it is partially degraded rather than completely degraded. With this method, the impurities in the PET resin must be removed, otherwise the recycled PET cannot be used for food packaging. The resulting high purity BHET from the reaction can be used directly to produce fiber grade polyesters. The general polyester production plant adds a set of ethylene glycol depolymerization equipment based on the original equipment. The product obtained by the depolymerization is mixed with fresh monomer in proportion. Polymerized fiber polyester can be obtained and used There is essentially no difference between the PET performance recovered by the method and the performance of the new PET material.
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