Application and Progress of PET Bottle Recycling Technology (I)

In 2005, the demand for bottle-grade PET in China was estimated at 650,000 tons, and the consumption of PET tons was over 20 billion. Among them, the proportion of PET bottles for beverages and beer was as high as 87%. A large number of disposable PET bottles brought about great recycling. The problem is that the recycling of recycled PET bottles has become a topic of increasing concern in the world. Experts Chen Shihong, Yang Huiyi, and Zhang Yuxia from the Materials Science and Engineering Department of Beijing Technology and Business University and the Institute of Light Industry Plastics Processing recently introduced the progress of PET bottle recycling technology.

Polyethylene terephthalate (PET) is a widely used thermoplastic polyester. It is expected that the global PET production capacity in 2005 will increase by 32.1% over 2002, from 47 500 kt to 52 850 kt, while the output will increase by 24.8% from 38 130 kt to 4l 180 kto. The increase in PET production has exceeded in 2002 The increase in its consumption is expected to be as high as 2 200 kt by 2005.

PET bottles have been widely used because of their light weight, transparency, and high strength. The proportion of polyester used in PET bottles is approximately 25% to 60%. In 2002, the PET used in bottles in the world was approximately 8,000 kt. It is estimated that the polyester used in PET bottles in the world will exceed 10 000kto in 2005. Including beverages, soy sauce and other condiments, alcohol and other food applications, and non-food applications, such as detergents, cosmetics, medicine \ and other packaging.

China's bottle-grade PET demand in 2005 is estimated to be 600-650 kt (excluding Taiwan, Hong Kong, and Australia), and the consumption of PET bottles will exceed 20 billion, of which drinks and beer bottles account for 87%.

In summary, the consumption of a large number of disposable PET bottles has brought serious recycling problems. The recycling of post-consumer PET bottles has become a topic of increasing concern in the international community.

Since 1994, the PET bottle recycling market has grown at a double-digit rate, doubling every four years in Europe. In Switzerland, some new PET bottles are recycled by 50%; materials are processed. In 2002, about 1 500 kt of post-consumer PET bottles were recycled in the world, and 5% of them were reprocessed into bottles. In 2003, a total of 612 kt PET bottles were recycled in Europe, an increase of 163 kt over 2002, an increase of 36%, of which the proportion of recycling through “bottle and bottle” recycling increased from 8.1% to 11.1%.

In the United States, from 1995 to 2003, the recovery rate of PET bottles dropped by nearly 20 percentage points. This is mainly because the increase in the consumption of PET bottles is mostly from disposable non-carbonated beverage bottles, such as mineral water bottles, and these bottles often It is used outside the home and cannot be collected in time. It is estimated that the recycling volume of bottles in 2005 will increase by about 4 times from 2000.

At present, PET bottle recycling technology has reached a very high level, mainly physical recycling and chemical recycling. Physically recovered PET bottles are mostly used for the production of fibers, but some are directly used for the production of plastic products, including the production of non-food packaging containers; chemical recovery is based on the physical recovery of clean PET hydrolysis, alcoholysis or alkali Solutions, etc., whose main products are used for re-polymerization of PET or for the production of other chemicals.

First, chemical recovery

Chemical recovery includes: depolymerization chemical recovery monomer small molecule, and then polymerization to obtain polymer; chain extension chemical recovery two methods to increase the relative molecular weight of the regeneration product.

Depolymerization chemical recovery

PET is a polyester formed by polycondensation of dibasic acid and glycol. There are two methods for its synthesis: one method is the polycondensation of terephthalic acid (TPA) and ethylene glycol (EC), and the reaction temperature is 240°C to 260°C. , Pressure 0.3MPa-0.5MPa; Another method is the transesterification of dimethyl terephthalate (DMT) with ethylene glycol. The reaction produces ethylene terephthalate and further polycondensation. Both reactions are reversible reactions. In the case of excess water or alcohol, the reverse reaction occurs. Therefore, the post-consumer PET can be decomposed into monomers or oligomers under certain conditions, and then this monomer can be used to synthesize food-grade PET resin. By using ethylene glycol instead of water, an aromatic polyol can also be obtained, which can be used together with isocyanate or unsaturated dibasic acid bodies to synthesize PET or other polymers.

Glycol alcoholysis

In the presence of ethylene glycol, heating PET, under the action of the catalyst, long-chain PET can be cracked into hydroxyl-terminated short-chain components, mainly dihydroxyethyl terephthalate (terephthalic acid Ethylene glycol), mixed ethylene glycol, ethylene terephthalate, and some free ethylene glycol (this process can produce bishydroxypropyl terephthalate if propylene glycol is used).

CCUCLU, etc. For the purpose of recovering monomers or preparing intermediates for other polymers, the PET flakes catalyzed by zinc acetate in the xylene of 170° C.-245° C. are used for the alcoholysis, PET/glycol molar The ratio is 1/0.5 to 1/3. In the heterogeneous reaction, the depolymerization product is transferred from the dispersed PET/glycol droplets into the xylene medium to achieve the glycol alcoholysis equilibrium. Tested at three temperatures (170°C, 220°C, 245°C), the best results at 220°C and 80% (molar content) of bis-2-hydroxyethyl in very pure crystalline form. Terephthalate monomer and 20% dimer.

The advantages of glycol alcoholizing glycols in xylenes are that they promote the mixing of the reaction medium at higher PET/glycol ratios, as well as recover excess glycol at high concentrations from the xylene phase at low temperatures.

Chen-ho Chen et al studied the effect of glycol alcoholysis conditions on the alcoholysis conversion rate of the recovered PET beverage bottles. The results showed that the optimal conditions for alcoholysis of r-glycol were: alcoholysis temperature 190 °C, alcohol When the time of solution is 1.5 h and the amount of alcoholysis catalyst (cobalt acetate) is 0.002 mol, the alcoholysis conversion rate can reach almost 100%. With the increase of the alcoholysis temperature, the prolongation of the alcoholysis time and the increase of the amount of the catalyst, the conversion rate of the alcoholysis of the PET greatly increases, but if there is no catalyst or the temperature is lower than 150°C, there is almost no conversion. Experiments also show that among these three influencing factors, the influence of the catalyst is the largest, followed by the time of alcoholysis and the temperature of the alcoholysis. o DSC and 叮-IR analysis show that the main product of alcoholysis is. Ethylene terephthalate and oligomers.

Diglycol alcoholysis

Medhat S Farahat uses diethylene glycol for recovery: PET bottles undergo alcoholysis. The resulting alcoholysis product oligoester was subjected to polyesterification with maleic anhydride (MAll) and p-hydroxybenzoic acid (PHBA) to obtain a modified unsaturated polyester. The effect of the added molar ratio of PHBA and MAH on the mechanical properties of the modified unsaturated polyester was studied. The results show that the elastic modulus and the maximum compressive strength of unsaturated polyesters follow. The content of MAH and PHBA increased linearly. Elastic modulus with MAH. The increase in the content is mainly due to the increase in the number of double bonds that can be cured in the main chain of the polyester, which greatly increases the crosslink density, which will make the cured polyester harder and more resistant to deformation caused by external stress.

The increase of PHBA content will cause the increase of ordered micro areas, and will also increase the resistance to external stress of the cured polyester. When the MAH content exceeds a certain amount, the maximum compressive strength value starts to decrease. This is because the cross-linking density increases to a certain extent and the solidified polyester becomes brittle and the mechanical properties decrease.

Medhat S. Farahat and DavidE. Nikles used manganese acetate as a transesterification catalyst and alcoholysis reaction of diethylene glycol with recycled PET beverage bottle flakes to produce a UV-curable acrylated oligoester for use as a binder for the production of solvent-free magnetic tapes. In order to obtain low molecular weight polyols with different relative molecular masses, PET/DMG is used in two molar ratios, namely 1/2.15 and 1/1.03. Acrylic acid esterification reaction of acryloyl chloride with hydroxyl groups to modify oligomerization Ester polyols. The results show that when the acrylated oligoester is cured alone or mixed with some commercial monomers, the resulting cured film has a maximum tensile strength of 45 MPa and a maximum elastic modulus of 2.0 gPa. When PET/diethylene glycol is The mechanical performance at 1/1.03 is better than that of PET/diethylene glycol at 1/2.15.

Alkaline solution

The alkaline hydrolysis of PET is 4%. 20% NaOH or KOH. Aqueous solution. The reaction product was ethylene glycol and terephthalic acid disodium salt (TPA-Na2), which was heated to 3400C to vaporize and recover ethylene glycol. Neutralization of the reaction mixture with sulfuric acid gives pure terephthalic acid. Alkaline hydrolysis process is very effective for the recovery of post-consumer PET bottles and the regeneration of monomeric terephthalic acid and ethylene glycol. Its main advantage is that it can recover highly contaminated used PET. This method is very useful for recycling containers of PET bottles and other materials, and is beneficial to solve the problem of plastic pollution, because now terephthalic acid has replaced dimethyl terephthalate as the main monomer material for preparing PET.

C. P. Karayannidis, etc., chopped the recycled PET bottles into 6 mm square sheets, and recycled PET's synthetic monomers terephthalic acid and ethylene glycol by alkaline hydrolysis. The depolymerization reaction was carried out in a solution of 12-2000 (2NaOH aqueous solution and 100-C-120°C anhydrous KOH in 2-methoxyethanol (CH3-O-CH2-CH2-OH) solution using an autoclave reactor. Disodium terephthalate and dipotassium terephthalate (TPA-K2) were separated by sulfuric acid to separate terephthalic acid with high concentration.The 'H NMR spectral analysis of terephthalic acid showed that it contained 98% of its content. Terephthalic acid and 2% isophthalic acid impurities The purity of terephthalic acid was determined by measuring its acidity and polymerizing it with ethylene glycol using tetrabutyl titanate as a catalyst.