Unexpected efficiency of cyclic amidine catalysts in depolymerizing poly(ethylene terephthalate)

This article describes studies on the catalytic activity of several nitrogen‐based organic catalysts for the depolymerization of poly(ethylene terephthalate) (PET), in which a few cyclic amidines work more effectively than a potent, bifunctional guanidine‐based catalyst 1,5,7‐triazabicyclo‐[4,4,0]‐dec‐5‐ene (TBD) in the presence of short chain diols that play a role in activation of carbonyl groups through hydrogen bonding. Further studies prove that the catalytic efficiency at the above specific conditions depends only on the extent of activation of a hydroxyl group rather than simply the pK_a of the bases. For glycolysis with excess short‐chain alkanediols, 1,8‐diazabicyclo[5.4.0]undec‐7‐ene is the best catalyst. In contrast, TBD shows outstanding catalytic activity in depolymerizations of PET with mono‐alcohols and longer‐chain diols. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Glycolysis of poly(ethylene terephthalate) (PET) using basic ionic liquids as catalysts

The glycolysis of poly(ethylene terephthalate) (PET) was studied using several ionic liquids and basic ionic liquids as catalysts. The basic ionic liquid, 1-butyl-3-methylimidazolium hydroxyl ([Bmim]OH), exhibits higher catalytic activity for the glycolysis of PET, compared with 1-butyl-3-methylimidazolium bicarbonate ([Bmim]HCO₃), 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) and 1-butyl-3-methylimidazolium bromide ([Bmim]Br). FT-IR, ¹H NMR and DSC were used to confirm the main product of glycolysis was bis(2-hydroxyethyl) terephthalate (BHET) monomer. The influences of experimental parameters, such as the amount of catalyst, glycolysis time, reaction temperature, and dosages of ethylene glycol on the conversion of PET, yield of BHET were investigated. The results showed a strong influence of the mixture evolution of temperature and reaction time on depolymerization of PET. Under the optimum conditions of m(PET):m(EG): 1:10, dosage of [Bmim]OH at 0.1 g (5 wt%), reaction temperature 190 °C and time 2 h, the conversion of PET and the yield of BHET were 100% and 71.2% respectively. Balance between the polymerization of BHET and depolymerization of PET could be changed when the reaction time was more than 2 h and contents of catalyst and EG were changed.     

Microwave assisted ecofriendly recycling of poly (ethylene terephthalate) bottle waste

Glycolysis of poly (ethylene terephthalate) bottle waste was carried out using microwave energy. A domestic microwave oven of 800 W was used with suitable modification for carrying out the reaction under reflux. The catalysts used for the depolymerization in ethylene glycol (EG) were zinc acetate and some simple laboratory chemicals such as sodium carbonate, sodium bicarbonate and barium hydroxide. Comparison of results was made from the point of view of the yield of bis (2-hydroxyethylene) terephthalate (BHET) and the time taken for depolymerization. It was observed that under identical conditions of catalyst concentration and PET:EG ratio, the yield of BHET was nearly same as that obtained earlier by conventional electric heating. However, the time taken for completion of reaction was reduced drastically from 8 h to 35 min. This has led to substantial saving in energy.     

The kinetics of diethylene glycol formation from bis-hydroxyethyl terephthalate with antimony catalyst in the preparation of PET

This research discussed the effect of the addition of antimony catalyst on diethylene glycol (DEG) formation in poly(ethylene terephthalate) (PET) synthesis. It was found that antimony catalyst increased DEG formation in the preparation of PET, in particular, during the esterification stage and also during the prepolycondensation stage. To further discuss the effect of antimony catalyst on DEG formation in the preparation of PET, this research also focused on the kinetics of DEG formation during PET synthesis from purified bishydroxyethyl terephthalate (BHET) monomer with antimony catalyst. The rate expression of DEG formation from BHET monomer and antimony catalyst was described. It was found that the activation energy of BHET monomer with antimony catalyst in DEG formation is lower than that of BHET monomer without the addition of catalyst. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1797–1803, 1999     

Poly(ethylene terephthalate) copolymers containing 5‐nitroisophthalic units. III. Methanolytic degradation

The methanolysis of poly(ethylene terephthalate) (PET) copolymers containing 5‐nitroisophthalic units was investigated. Random copolyesters containing 10 and 30 mol % of such units were prepared via a two‐step melt copolycondensation of bis(2‐hydroxyethyl) terephthalate (BHET) and bis(2‐hydroxyethyl) 5‐nitroisophthalate (BHENI) in the presence of tetrabutyl titanate as a catalyst. First, the susceptibility of these two comonomers toward methanolysis was evaluated, and their reaction rates were estimated with high‐performance liquid chromatography. BHENI appeared to be much more reactive than both BHET and bis(2‐hydroxyethyl) isophthalate. The methanolysis of PET and the copolyesters was carried out at 100 °C, and the degradation process was followed by changes in the weight and viscosity, gel permeation chromatography, differential scanning calorimetry, and ¹H and ¹³C NMR spectroscopy. The copolyesters degraded faster than PET, and the rate of degradation increased with the content of nitrated units. The products resulting from methanolysis were concluded to be dimethyl terephthalate, dimethyl 5‐nitroisophthalate, ethylene glycol, and small, soluble oligomers. For both PET and the copolyesters, an increase in crystallinity was observed during the degradation process, indicating that methanolysis preferentially occurred in the amorphous phase. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 76–87, 2002     

Depolymerisation of poly(ethylene terephthalate) fibre wastes using ethylene glycol

Poly(ethylene terephthalate) [PET] fibre wastes from an industrial manufacturer was depolymerised using excess ethylene glycol [EG] in the presence of metal acetate as a transesterification catalyst. The glycolysis reactions were carried out at the boiling point of ethylene glycol under nitrogen atmosphere up to 10 h. Influences of the reaction time, volume of EG, catalysts and their concentrations on the yield of the glycolysis products were investigated. The glycolysis products were analysed for hydroxyl and acid values and identified by different techniques, such as HPLC, ¹H NMR and ¹³C NMR, mass spectra, and DSC. It was found that the glycolysis products consist mainly of bis(hydroxyethyl)terephthalate [BHET] monomer (>75%) which was effectively separated from dimer in quite pure crystalline form.     

Preparation and characterization of poly(butylene terephthalate)/poly(ethylene terephthalate) copolymers via solid‐state and melt polymerization

To increase the T_g in combination with a retained crystallization rate, bis(2‐hydroxyethyl)terephthalate (BHET) was incorporated into poly(butylene terephthalate) (PBT) via solid‐state copolymerization (SSP). The incorporated BHET fraction depends on the miscibility of BHET in the amorphous phase of PBT prior to SSP. DSC measurements showed that BHET is only partially miscible. During SSP, the miscible BHET fraction reacts via transesterification reactions with the mobile amorphous PBT segments. The immiscible BHET fraction reacts by self‐condensation, resulting in the formation of poly(ethylene terephthalate) (PET) homopolymer. ¹H‐NMR sequence distribution analysis showed that self‐condensation of BHET proceeded faster than the transesterification with PBT. SAXS measurements showed an increase in the long period with increasing fraction BHET present in the mixtures used for SSP followed by a decrease due to the formation of small PET crystals. DSC confirmed the presence of separate PET crystals. Furthermore, the incorporation of BHET via SSP resulted in PBT‐PET copolymers with an increased T_g compared to PBT. However, these copolymers showed a poorer crystallization behavior. The modified copolymer chain segments are apparently fully miscible with the unmodified PBT chains in the molten state. Consequently, the crystal growth process is retarded resulting in a decreased crystallization rate and crystallinity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 882–899, 2007.     

The kinetics of diethylene glycol formation in the preparation of polyethylene terephthalate

For revealing diethylene glycol (DEG) formation in poly(ethylene terephthalate) (PET) synthesis, this research focused on finding the stage most critical for DEG formation. It is found that the esterification stage was the most critical stage for DEG formation during production of PET through the direct esterification process. In addition, the kinetics of the formation of DEG (ether bond), which is mainly produced from hydroxyl end groups of ethylene glycol (EG) and bis-hydroxyethyl terephthalate (BHET) oligomer, was investigated. The results show that the reactivity of BHET-OH functional group is greater than that of EG-OH functional group in the reaction to produce ether bonds. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3073–3080, 1998     

In-Situ Formation of BHET/Titanium Compound Nanocomposite and its Catalysis for Polyester

Summary: The tetrabutylorthotitanate (TBOT) was hydrolyzed by H₂O produced during the esterification of pure terephthalic acid (PTA) and ethylene glycol (EG), and the bis(2-hydroxyethyl) terephthalate (BHET)/titanium compound nanocomposite was in-situ formed. The effect of TBOT on the esterification and its product property has been investigated. The results show that the butyl alcohol from the hydrolysis of TBOT is almost distilled out with H₂O and there has no effect on the chemical structure of BHET caused by the introducing of TBOT. A kind of novel titanium compound is manufactured during the esterification under the existence of TBOT, which shows slice-like morphology from SEM micrographs and special XRD pattern with new diffraction peaks between 2-Θ = 6.9° and 10.2°. It is found that the BHET/titanium compound nanocomposite can act as the catalyst of polymerization of poly(ethylene terephthalate) (PET). The PET resins synthesized by in-situ formed catalyst have almost the same physicochemical properties with the commerced resins and have good spinnability.     

Poly(ethylene terephthalate) copolymers containing nitroterephthalic units. III. Methanolytic degradation

The methanolytic degradation of poly(ethylene terephthalate) (PET) copolymers containing nitroterephthalic units was investigated. Random poly(ethylene terephthalate‐co‐nitroterephthalate) copolyesters (PETNT) containing 15 and 30 mol % nitrated units were prepared from ethylene glycol and a mixture of dimethyl terephthalate and dimethyl nitroterephthalate. A detailed study of the influence of the nitro group on the methanolytic degradation rate of the nitrated bis(2‐hydroxyethyl) nitroterephthalate (BHENT) model compound in comparison with the nonnitrated bis(2‐hydroxyethyl) terephthalate (BHET) model compound was carried out. The kinetics of the methanolysis of BHENT and BHET were evaluated with high‐performance liquid chromatography and ¹H NMR spectroscopy. BHENT appeared to be much more reactive than BHET. The methanolytic degradation of PET and PETNT copolyesters at 80 °C was followed by changes in the weight and viscosity, gel permeation chromatography, differential scanning calorimetry, scanning electron microscopy, and ¹H and ¹³C NMR spectroscopy. The copolyesters degraded faster than PET, and the degradation increased with the content of nitrated units and occurred preferentially by cleavage of the ester groups placed at the meta position of the nitro group in the nitrated units. For both PET and PETNT copolyesters, an increase in crystallinity accompanied methanolysis. A surface degradation mechanism entailing solubilization of the fragmented polymer and consequent loss of mass was found to operate in the methanolysis of the copolyesters. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2276–2285, 2002     

Synthesis and characterization of azo dyestuff based on bis(2-hydroxyethyl) terephthalate derived from depolymerized waste poly(ethylene terephthalate) fibers

This work aimed at effectively utilizing the chemically depolymerized waste poly(ethylene terephthalate)(PET) fibers into useful products for the textile industry.PET fibers were glycolytically degraded by excess ethylene glycol as depolymerizing agent and zinc acetate dihydrate as catalyst.The glycolysis product,bis(2-hydroxyethyl) terephthalate(BHET),was purified through repeated crystallization to get an average yield above 80%.Then,BHET was nitrated,reduced,and azotized to get diazonium salt.Finally,the produced diazonium salt was coupled with 1-(4-sulfophenyl)-3-methyl-5-pyrazolone to get azo dyestuff.The structures of BHET and azo dyestuff were identified by FT1 R and ~1H NMR spectra and elemental analysis.Nylon filaments dyed by the synthesized azo dyestuff with the dye bath pH from 4.14 to 5.88 showed bright yellow color.The performances of the dyestuff were described with dye uptake,color fastness,K/S,L~*,a~*,b~*.and △E~* values.     

Ultrafast 99% Polyethylene terephthalate depolymerization into value added monomers using sequential glycolysis-hydrolysis under microwave irradiation

Effective and efficient hybrid depolymerisation technologies are emerging as high potential sustainable routes with considerable benefits over conventional recycling methods for the achievement of circular economies for plastics. Herein, combined green and fast glycolysis-hydrolysis depolymerization of polyethylene terephthalate (PET) was carried out under microwave irradiation (MW) with excellent efficiencies. In MW assisted glycolysis of PET, the catalytic activity of two deep eutectic solvents (DES) based on (choline chloride-urea (DES 1)) and (choline chloride-thiourea (DES 2)) was evaluated and compared. Optimised glycolysis conditions were determined using Box Behnken Design (BBD) to attain maximum weight loss of PET, low crystallinity and increased carbonyl index of residual PET. DES volume of 4 mL, 5.5–6 mL of ethylene glycol, and 0.5 min MW irradiation time resulted in a prominent rise in PET weight loss and carbonyl index of residual PET. DES 2 showed an improved catalytic activity than that of DES 1 which is associated to its stronger interaction with EG and PET polymer chains during the course of the reaction. Residual PET obtained post glycolysis reaction was further depolymerized using MW assisted hydrolysis in the presence of weakly basic Na₂CO₃ and EG. Within 3-minute, the proposed sequential depolymerization technologies facilitated ≈99% conversion of PET to terephthalic acid (TPA), monohydroxyethyl terephthalate (MHET), and bis (2-hydroxyethyl) terephthalate (BHET) monomers produced at a yield of 62.79–80.66%, 17.22–34.79% and 0.54–0.59% respectively. Application on post-consumer PET sample also revealed very satisfactory results with 96.77–98.25% PET conversion and 60.98–78.10% yield of TPA.     

反相高效液相色谱法分析超临界甲醇解聚聚对苯二甲酸乙二醇酯的产物

配合超临界甲醇解聚聚对苯二甲酸乙二醇酯的工艺开发,采用高效液相色谱法对聚对苯二甲酸二乙酯的超临界甲醇解聚固体产物进行了分离、定性和定量分析。采用反相色谱体系,色谱柱为Zorbax-C8柱,流动相为甲醇－水（70／30,V／V）,紫外检测器。该法具有高色谱分辨率、简便、准确、重复性好等特点。     

Synthesis,Characterization and Catalytic Behaviour of Silica Supported Zinc Salicylaldimine Complex

Zinc salicylaldimine complex immobilized on silica gel was used as a promising catalyst for the transesterification reaction of dimethyl terephthalate (DMT) and ethylene glycol (EG).The catalyst was characterized by Fourier transform infra‐red spectroscopy (FT‐IR), thermogravimetric analysis (TGA) and atomic absorption spectroscopy (AAS). The product bis‐(2‐hydroxyethyl)terephthalate (BHET)was confirmed by mass and ¹H‐NMR studies. In comparison to zinc acetate i.e., homogeneous catalyst, a polymer supported catalyst showed better stability, catalytic activity and ease of separation from the reaction product. The catalyst can be reutilized during successive catalytic cycles.     

Preparation of copolyesters from diols and bisphenols by the solution polycondensation with TsCl/DMF/Py as a condensing agent

A novel synthetic method for the preparation of copolyesters comprised of diols and bisphenols using tosyl chloride (TsCl)/DMF/pyridine (Py) as a condensing agent has been developed. A variety of combinations of monomers could produce relatively high molecular weight copolymers, and better results were obtained by initial oligomerization of diols followed by bisphenols. In order to demonstrate usefulness of this method, copolymers comprised of IPA/TPA (50/50), bis(2‐hydroxyethyl)terephthalate (BHET),and several bisphenols were prepared and compared to the poly(ethylene terephthalate) (PET) modified by TPA and 2,2‐bis(4‐hydroxyphenyl)propane (BPA) diacetate in terms of their thermal properties. The length of mesogenic unit segments in the thermotropic IPA/TPA (50/50)‐BHET/ 4,4′‐dihydroxybenzophenone (4,4′‐DHBP) (50/50) copolymer was changed by initial reaction of BHET followed by dropwise addition of 4,4′‐DHBP in the two‐stage polycondensation and also by varying the amounts of BHET used at the initial and final stages in the three‐stage copolycondensation, and the results were studied by NMR and their thermal properties. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1270–1276, 2000     

Optimization process for glycolysis of poly (ethylene terephthalate) using bio-degradable & recyclable heterogeneous catalyst

Because of characteristics including simplicity of processing, light weight, recyclability, and low cost of production, plastic production and usage have risen every day. As a result, there is now more waste plastic generated every day, and it will be opening up a brand-new field of study for researchers to investigate and solve these issues. An ecologically friendly approach is needed to solve these problems. One approach is to recycle this kind of waste. There are several ways to recycle used plastics, but practically all of them have good and bad points. About a few decades ago, the glycolysis of used PET polymers gained industrial attention. Since used poly (ethylene terephthalate) (PET) plastics may be recycled using the most advantageous and promising techniques. This works an optimization parameter of chemical recycling of PET waste without utilizing any solvent as a reaction medium by changing a number of variables, such as catalyst types and the molar ratio of EG: PET, catalyst ratio and also recycled catalyst and reagent. The recovered Bio-catalyst (OPA/BLA) still maintained excellent catalytic efficiency for PET Glycolysis after six consecutive cycles. Optimized reaction condition was PET:EG (1:16) molar ratio 1% w/w catalyst at 192–200 °C reaction temperature obtaining 60.32% Yield of BHET product at 98.40% of PET conversion. Final product was confirmed by FT-IR, ¹H NMR and GC-MS data.     

Electrical conductivity of poly(ethylene terephthalate)/expanded graphite nanocomposites prepared by in situ polymerization

Nanocomposites based on poly(ethylene terephthalate) (PET) and expanded graphite (EG) have been prepared by in situ polymerization. Morphology of the nanocomposites has been examined by electronic microscopy. The relationship between the preparation method, morphology, and electrical conductivity was studied. Electronic microscopy images reveal that the nanocomposites exhibit well dispersed graphene platelets. The incorporation of EG to the PET results in a sharp insulator‐to‐conductor transition with a percolation threshold (?_c) as low as 0.05 wt %. An electrical conductivity of 10^?3 S/cm was achieved for 0.4 wt % of EG. The low percolation threshold and relatively high electrical conductivity are attributed to the high aspect ratio, large surface area, and uniform dispersion of the EG sheets in PET matrix. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Segmented block copolymers of poly(ethylene glycol) and poly(ethylene terephthalate)

A new series of segmented copolymers were synthesized from poly(ethylene terephthalate) (PET) oligomers and poly(ethylene glycol) (PEG) by a two‐step solution polymerization reaction. PET oligomers were obtained by glycolysis depolymerization. Structural features were defined by infrared and nuclear magnetic resonance (NMR) spectroscopy. The copolymer composition was calculated via ¹H NMR spectroscopy. The content of soft PEG segments was higher than that of hard PET segments. A single glass‐transition temperature was detected for all the synthesized segmented copolymers. This observation was found to be independent of the initial PET‐to‐PEG molar ratio. The molar masses of the copolymers were determined by gel permeation chromatography (GPC). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4448–4457, 2004     

Sub- and supercritical glycolysis of polyethylene terephthalate (PET) into the monomer bis(2-hydroxyethyl) terephthalate (BHET)

Sub- and supercritical glycolysis of polyethylene terephthalate (PET) with ethylene glycol (EG) to bis(2-hydroxyethyl) terephthalate (BHET) was investigated for the purpose of developing a PET recycling process. Supercritical glycolysis was carried out at 450 °C and 15.3 MPa while subcritical glycolysis was carried out at 350 °C and 2.49 MPa or at 300 °C and 1.1 MPa. High yields (gt; 90%) of the monomer BHET were obtained in both super- and subcritical cases. For the same PET/EG weight ratio of about 0.06, the optimum reaction time was 30 min for supercritical glycolysis and 75 and 120 min for two cases of subcritical glycolysis. GPC, RP-HPLC, ¹H NMR and ¹³C NMR, and DSC were used to characterize the polymer and reaction products. Supercritical glycolysis will be suitable to a process requiring a high throughput due to its short reaction time.     

结晶度对微波作用下PET水解解聚的影响

本文对微波作用下PET的中性水解解聚反应中原料结晶度的影响进行了研究.