Effect of antimony catalyst on solid-state polycondensation of poly(ethylene terephthalate)

The effect of antimony trioxide (Sb₂O₃) catalyst on the solid-state polycondensation (SSP) of poly(ethylene terephthalate) (PET) has been rigorously studied. It has been determined that the rate constant increases, while the activation energy decreases, linearly with increasing catalyst concentration within the range of 0-100 ppm Sb. The SSP rate reaches its maximum value at about 150 ppm Sb. The activation energies are 30.7 and 23.3 kcal/mol respectively for the uncatalyzed and fully catalyzed SSP. The frequency factor decreases with increasing catalyst concentration due to the decreased mobility of catalyzed end groups. A mechanism of Sb catalysis has been proposed to explain these observations.     

The thermal degradation of PET and analogous polyesters measured by thermal analysis-Fourier transform infrared spectroscopy

The thermal degradation of two commercial poly(ethylene terephthalate) (PET) samples and two laboratory prepared polyesters, poly(ethylene isophthalate) and poly(diethylene glycol terephthalate), was studied using thermogravimetry and thermal analysis-Fourier transform infrared spectroscopy. The commercial PET samples were copolymerised with diethylene glycol and isophthalic acid groups in different proportions, and their thermal stabilities were found to differ. Through a study of the thermal degradation of poly(diethylene glycol terephthalate) and poly(ethylene isophthalate), it was found that diethylene glycol and isophthalate units promoted thermal degradation through increased chain flexibility and more favourable bond angles, respectively. The thermal degradation of all the polyesters tested lead to the formation of non-volatile residue. Infrared spectroscopic analysis indicated that the residue consisted almost exclusively of interconnected aromatic rings.     

A comparative study of structure–property relationships in highly oriented thermoplastic and thermotropic polyesters with different chemical structures

Structure and properties of commercially available fully oriented thermoplastic and thermotropic polyester fibers have been investigated using optical birefringence, infrared spectroscopy, wide‐angle X‐ray diffraction and tensile testing methods. The effect of the replacement of p‐phenylene ring in poly(ethylene terephthalate) (PET) with stiffer and bulkier naphthalene ring in Poly(ethylene 2,6‐naphthalate) (PEN) structure to result in an enhanced birefringence and tensile modulus values is shown. There exists a similar case with the replacement of linear flexible ethylene units in PET and PEN fibers with fully aromatic rigid rings in thermotropic polyesters. Infrared spectroscopy is used in the determination of crystallinity values through the estimation of trans conformer contents in the crystalline phase. The analysis of results obtained from infrared spectroscopy data of highly oriented PET and PEN fibers suggests that trans conformers in the crystalline phase are more highly oriented than gauche conformers in the amorphous phase. Analysis of X‐ray diffraction traces and infrared spectra shows the presence of polymorphic structure consisting of α‐ and β‐phase structures in the fully oriented PEN fiber. The results suggest that the trans conformers in the β‐phase is more highly oriented than the α‐phase. X‐ray analysis of Vectran® MK fiber suggests a lateral organization arising from high temperature modification of poly(p‐oxybenzoate) structure, whereas the structure of Vectran® HS fiber contains regions adopting lateral chain packing similar to the room temperature modification of poly(p‐oxybenzoate). Both fibers are shown by X‐ray diffraction and infrared analyses to consist of predominantly oriented noncrystalline (63–64%) structure together with smaller proportion of oriented crystalline (22–24%) and unoriented noncrystalline (12–15%) structures. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 142–160, 2006     

The Current Status of Catalysis and Catalyst Development for the Industrial Process of Poly(ethylene terephthalate) Polycondensation

Abstract

In this survey of the industry, it is shown that despite the partial dissatisfaction with antimony-based polycondensation catalysts these catalysts are expected to remain the mainstay of industrial PET polycondensation catalysis. This is despite the intensive efforts invested in the search of other, stable and inexpensive non-antimony catalysts, such as those based on titanium, aluminium, and several traodtion metals such as molybdenum, cobalt and zirconium.     

Three phase model in drawn thermoplastic polyesters: comparison of differential scanning calorimetry and thermally stimulated depolarisation current experiments

Differential scanning calorimetry and thermally stimulated depolarisation current measurements are performed to quantify various phases present in amorphous and semi-crystalline polyester samples uniaxially drawn above their respective glass transition temperature. Results show the appearance of a crystalline phase induced by stretching and of a part of the amorphous phase which does not participate in glass transition. The existence of this phase—called rigid amorphous phase—is enhanced by the presence of crystallites rather than by the drawing.     

Effect of fluoropolymer antidripping agent on the microstructure and properties of polyesters

Poly(ethylene terephthalate) (PET) is of excellent mechanical properties and melt processability and is widely used as raw material for textile fibers. However, the flame retardant properties of PET were rather poor, and both reactive and additive phosphorus- and halogen-containing compounds have been employed to enhance the reaction-to-fire properties while the meltdripping behaviour during burning hasn’t been handled properly with the flame retardants. In this work, fluoropolymer was blended with both pure PET (pPET) and reactive phosphorus-containing flame retarded copolyester (fPET), and the flame retardance and char formation and mechanical properties of the resulted pPET and fPET blends were investigated. The tensile strengths of modified pPET samples were worsen whereas those of modified fPET samples were improved at low concentrations. The initial thermal degradation in nitrogen was accelerated remarkably for the two polyesters with fluoropolymer. The oxygen indices of the all modified samples were reduced while char formation and meltdripping suppression were encouraged. The apparent melt viscosity and elasticity for the two polyesters were gained much with antidripping agent. Therefore, fluoropolymer improved char formation of the two polyesters based on the gaseous phase mechanism while the partial suppression of meltdripping behavior and the decrease of mechanical properties mainly originated from the increase of melt viscosity via fibrillation for pPET.     

High-efficiency acetaldehyde removal during solid-state polycondensation of poly(ethylene terephthalate) assisted by supercritical carbon dioxide

The concentration of acetaldehyde(AA) is the main quality index of poly(ethylene terephthalate)(PET) used in food and drink packaging.A new method for AA removal has been developed by using supercritical carbon dioxide(sc CO2) during the solid-state polycondensation of PET.The influence factors of AA removal including the temperature,pressure,reaction time and the size of pre-polymer particles are systematically studied in this work.The results indicate that it is a highly efficient way to obtain high molecular weight PET with relative low concentration of AA.Correspondingly,the polymerization degree of PET could increase from 27.9 to 85.6 while the concentration of AA reduces from 0.229 × 10~(-6) to 0.055 × 10~(-6) under the optimal operation conditions of 230 °C,8 MPa and size of 0.30–0.45 mm.Thermodynamic performance tests show the increasing extent of PET crystallinity due to the fact that the plasticization of sc CO_2 is not obvious with extended reaction time,therefore the increasing crystallinity has no significant influence on AA removal.SEM observations reveal that the effects of sc CO_(2-) induced plasticization and swelling on PET increase significantly with the decrease of prepolymer size,and the surface of PET becomes more loose and porous in favor of the AA removal.     

Conformational analysis for hydrated ethylene oxide oligomer models by quantum chemical calculations

Hydrate effects on the conformations of ethylene oxide oligomers (EO-x, x = 1–8 mers) were examined using quantum chemical calculations (QCC). Conformational analyses were carried out by RHF/6-31G. The models were constructed by locating a water molecule to each ether–oxygen in the structures optimized for non-hydrate oligomers. Hydrate ratio, h (h = H₂O_mol/O_{mol in oligomer}), was set from 0 to 1.0. The six type conformations with repeated units of O–C, C–C and C–O bonds were examined. Conformational energy, E _c (HF), was calculated as difference between the energy of oligomer with water molecules and that of non-hydrogen and/or hydrogen bonding water molecules. Hydrate energies for each conformer, ∆μ _h (kcal/m.u., based on E _c in non-hydrate state), were negative and linearly decreased with the increase of h values, and such effects with the increase of h values were weaken with increasing x values. These results were consistent with our previous results calculated using the permittivity, ε (ε = 0–80.1), by QCC. In non-hydrate (h = 0), the (ttt) _x conformers were the most stable independent of x. However, in hydrate states (h = 0.44–0.67), the (tg⁺t) _x conformers were the most stable independent of x values, and in h = 1, the (tg⁺t)₈ conformer (8-mer) was most stable [∆E _c(g) = −1.3 kcal/m.u., ∆E _c(g): energy difference between a given oligomer and the (ttt) _x oligomer]. These results supported the experimental those based on NMR analyses using dimethoxyethane and triglyme solutions. Molecular lengths (l) of (tg⁺t) _x , (tg⁺g⁻) _x and (g⁺g⁺g⁺) _x conformers having higher x values significantly decreased with increasing h values. Such contraction with hydration, however, was independent of ΔE _c(g) values of each conformer.     

DSC and DMTA study of annealed cold-drawn PET: a three phase model interpretation

Cold-drawn poly(ethylene terephthalate) (PET) samples annealed at different undercoolings are studied by means of differential scanning calorimetry and dynamic mechanical thermal analysis. When heating from room temperature, the onset of the glass transition region in cold-drawn, un-annealed samples is found to be significantly lower than in the case of un-oriented PET. On the contrary, the presence of crystalline lamellae in oriented PET cause a shift (and spread out) of the glass transition region towards higher temperatures. The crystal thickening process caused by heating above the annealing temperature, is suggested to take place after a rigid amorphous phase linked to the basal surface of the lamellae has softened. It is found that the low-temperature (between 100 and 140 °C) annealed samples have a glass dispersion region ranging significantly above the annealing temperature itself. This circumstance leads to envisage vitrification as a possible mechanism able to limit lamellar thickening during the annealing process at these low temperatures.     

The role of nanoclay in the generation of poly(ethylene terephthalate) fibers with improved modulus and tenacity

The effect of nanoclay concentration on the molecular orientation and drawability of poly(ethylene terephthalate) PET was examined using thermal and vibrational spectroscopic analysis. Although drawability at 83 °C in hot air increased by the addition of nanoclay, the maximum draw ratio was independent of nanoclay concentration. The average molecular orientation of the PET chain was found to mimic the trend in mechanical property improvements. Both Young's modulus and tenacity (i.e. strength) showed the maximum improvement at a 1 wt% loading of clay, which was shown to coincide with the maximum amount of molecular orientation. Nanoclay was shown to intercalate with PET and enhanced amorphous orientation that led to modulus and strength improvements. However, at higher concentrations of nanoclay the presence of large agglomerates prevented efficient orientation to the fiber axis and acted as stress concentrators to aid in cavitation and failure during testing. Raman spectroscopy showed that the as-spun unfilled PET fibers possessed significantly more trans rotamer content of the ethylene glycol moiety than the nanocomposite fibers.     

In-situ measurement of microwave absorption properties at 2.45 GHz for the polycondensation of lactic acid

For the construction of a microwave-assisted organic synthesis plant, it is necessary to know the dielectric properties of the reaction system. Measurements of the dielectric properties of lactic acid aqueous solution, anhydrated lactic acid, oligo(lactic acid) and water, which are constituent materials in the polycondensation of lactic acid, confirm that dielectric properties decrease as reaction progresses. Calculated microwave penetration depths, obtained from the dielectric properties, show that microwaves penetrate deeply into the reaction system. This work should be useful for the development of microwave-assisted organic syntheses in the chemical industry.     

Study of various catalysts in the synthesis of poly(propylene terephthalate) and mathematical modeling of the esterification reaction

Pure terephthalic acid (TPA) was esterified with 1,3-propanediol (1,3-PDO) in the presence of various catalysts, in order to find the most effective one for this esterification reaction. The prepared oligomers were polycondensated in a second step under high vacuum and using the same catalyst (Sb(OCOCH₃)₃, Ti(OC₄H₉)₄, GeO₂₎ as before, or the well known catalyst for poly(ethylene terephthalate) (PET) production technology Sb₂O₃. The esterification reaction was monitored by measuring the distilled water as a function of time and from these data the modeling of this process was carried out. The received poly(propylene terephthalate) (PPT) samples were characterized by viscometry, carboxyl end-group content and color measurement. From this study, tetrabutoxytitanium was proved to be the most effective catalyst for the esterification reaction. When this catalyst was used in the second step a PPT polymer with the highest molecular weight was received.     

Study on the effect of dispersion phase morphology on porous structure of poly (lactic acid)/poly (ethylene terephthalate glycol-modified) blending foams

A methodology for blending foam of poly (lactic acid) (PLA)/poly (ethylene terephthalate glycol-modified) (PETG) was proposed. PLA/PETG blends were prepared through a melt blending method, using multiple functionality epoxide as reactive compatibilizer. The effects of blending ratio and compatibilizer content on the dispersion morphology, molecular structure, mechanical properties, and rheological behavior of PLA/PETG blends were studied. Then PLA/PETG blends were foamed using supercritical CO₂ as physical blowing agent, and their porous structure, pore size, as well as pore density were investigated. The results showed that the mechanical properties and rheological parameters such as melt strength and melt elasticity, as well as the porous structure of the foams dispersion morphology of PLA/PETG blends were affected strongly. The melt elasticity of PLA/PETG blends increased with increasing compatibilizer content. Dispersion phase morphology of PLA/PETG blends also had a significant effect on the pore density of all the samples. The results indicated that homogeneous and finer porous morphology of PLA/PETG foams with high expansion ratio could be achieved with a proper content of compatibilizer in the blends.     

Random terephthalate polyesters based on 1,4-butanediol and bis(hydroxyethyl ether) of bisphenol A: thermal properties and crystallization kinetics

The thermal behavior of poly(butylene-co-2,2-bis[4-(ethylenoxy)-1,4-phenylene]propane terephthalate) copolymers (PBT/BHEEBT) was investigated by thermogravimetric analysis and differential scanning calorimetry. A good thermal stability was found for all the samples. The thermal analysis carried out using DSC technique showed that the T_m of the copolymers decreased with increasing BHEEBT unit content, different from that of T_g, which on the contrary increased. Wide-angle X-ray diffraction measurements permitted to identify the kind of crystalline structure of PBT in all the semi-crystalline samples. The multiple endotherms typical of PBT were also evidenced in the PBT/BHEEBT samples, due to melting and recrystallization processes. By applying the Hoffman-Weeks' method, the T_m° of the copolymers was derived. The isothermal crystallization kinetics was analyzed according to the Avrami's treatment. The introduction of BHEEBT units decreased the PBT crystallization rate. Values of the exponent n close to 3 were obtained, independently of T_c and composition. Furthermore, the presence of a crystal-amorphous interphase was evidenced.     

Experimental studies and molecular modelling of the stress-optical and stress-strain behaviour of poly(ethylene terephthalate). Part I: Infra-red spectroscopic investigation and modelling of chain conformation and orientation changes on drawing

Quantitative infra-red data on oriented poly(ethylene terephthalate) (PET) films have been used to determine the changes in the proportions of the trans and gauche conformers of the glycol residues and the development of molecular orientation of the terephthaloyl groups as functions of draw ratio. The results are compared with predictions for the stretching of a rubber-like network based on rotational-isomeric-state (RIS) Monte Carlo (MC) modelling. It is shown that both sets of data are consistent with stretching an entangled molecular network of about 10 PET monomer units per network chain. The onset of crystallisation at a draw ratio of 2.5 affects the glycol trans-gauche conformer contents but has no detectable effect on terephthaloyl orientation.     

Transesterification in poly(ethylene terephthalate) and poly(ethylene naphthalate 2,6-dicarboxylate) blends: model compounds study

Kinetics of transesterification reaction in poly(ethylene terephthalate)-poly(ethylene naphthalate 2,6-dicarboxylate), PET-PEN, blends resulting from melt processing was simulated using model compounds of ethylene dibenzoate (BEB) and ethylene dinaphthoate (NEN). The exchange reaction between BEB and NEN was followed by ¹H NMR spectroscopy using signals from the aliphatic protons of ethylene glycol moieties at 4.66 and 4.78 ppm, respectively. The first-order kinetics was established under pseudo-first-order conditions for both reactants. Thus, the overall transesterification reaction was second order reversible. The reversibility was confirmed experimentally by heating a mixed sequence of 1-benzoate 2-naphthoate ethylene (BEN) under similar conditions. Both forward reaction of the equimolar amounts of the reagents and reverse reaction came to equilibrium at the same molar ratio of the reactants and reaction products of roughly 0.25:0.50:0.25 for BEB, BEN, and NEN, respectively. The rate equation for the transesterification reaction in the model system was modified using half-concentration of BEN, which is the only effective in the intermolecular exchange. Direct ester-ester exchange was deduced as a prevailing mechanism for the transesterification reaction under the conditions studied, and the values of equilibrium and rate constants, as well as other basic thermodynamic and kinetic parameters were determined. The use of Zn(OAc)₂ as a catalyst resulted in a significant decrease in the activation enthalpy of transesterification, which might be due to the partial switch of the reaction mechanism from primarily pseudo-homolytic to more heterolytic where Zn^II acts as a Lewis base which binds to the ester carbonyl oxygen.     

On the nature of multiple melting in poly(ethylene terephthalate) (PET) and its copolymers with cyclohexylene dimethylene terephthalate (PET/CT)

The multiple melting behavior of poly(ethylene terephthalate) (PET) homopolymers of different molecular weights and its cyclohexylene dimethylene (PET/CT) copolymers was studied by time-resolved simultaneous small-angle X-ray scattering/wide-angle X-ray scattering diffraction and differential scanning calorimetry techniques using a heating rate of 2 °C/min after isothermal crystallization at 200 °C for 30 min. The copolymer containing random incorporation of 1,4-cyclohexylene dimethylene terephthalate monomer cannot be cocrystallized with the ethylene terephthalate moiety. Isothermally crystallized samples were found to possess primary and secondary crystals. The statistical distribution of the primary crystals was found to be broad compared to that of the secondary crystals. During heating, the following mechanisms were assumed to explain the multiple melting behavior. The first endotherm is related to the non-reversing melting of very thin and defective secondary crystals formed during the late stages of crystallization. The second endotherm is associated with the melting of secondary crystals and partial melting of less stable primary crystals. The third endotherm is associated with the melting of the remaining stable primary crystals and the recrystallized crystals. Due to their large statistical distribution, the primary crystals melt in a broad temperature range, which includes both second and third melting endotherms. The amounts of secondary, primary and recrystallized crystals, being molten in each endotherm, are different in various PET samples, depending on variables such as isothermal crystallization temperature, time, molecular weight and co-monomer content.     

Recycled poly(ethylene terephthalate)/layered silicate nanocomposites: morphology and tensile mechanical properties

Various amounts (1, 3 and 5 wt%) of a non-modified natural montmorillonite clay (Cloisite^® Na⁺) or of an ion-exchanged clay modified with quaternary ammonium salt (Cloisite^® 25A) were dispersed in a recycled poly(ethylene terephthalate) matrix (rPET) by a melt intercalation process. Microphotographs of composite fracture surfaces bring evidence that particles of Cloisite^® 25A are much better dispersed in the rPET matrix than those of Cloisite^® Na⁺. Moreover, WAXS measurements indicate that the lamellar periodicity of Cloisite^® 25A is increased in the composites, which evidences intercalation of rPET between silicate layers (lamellae) of the clay. In the case of Cloisite^® Na⁺, a very small thickening of lamellae due to mixing with rPET indicates only minute intercalation.Uniaxial tensile tests show that both clays increase the modulus of the rPET composites; more effective Cloisite^® 25A accounts for a 30% increase at loading of 5 wt%. Yield strength remains practically unaffected by the used fractions of the clays while tensile strength slightly decreases with the clay content; in parallel, strain at break dramatically drops. Tensile compliance of the composites is virtually independent of applied stress up to 26 MPa. Essential part of the compliance corresponds to the elastic time-independent component, while the viscoelastic component is low corresponding only to a few percent of the compliance even at relatively high stresses. The compliance of the composites is only slightly lower than that of the neat rPET, the reinforcing effect of Cloisite^® 25A being somewhat stronger. Both clays have beneficial effect on the dimensional stability of the composites since—in contrast to the neat rPET—the creep rate does not rise at long creep periods.     

Isoconversional kinetic analysis of DSC data on nonisothermal crystallization: Estimation of Hoffman-Lauritzen parameters and thermal transitions in PET/MMT nanocomposites

The differential isoconversional method of Friedman is applied to non-isothermal melt crystallization DSC data to obtain effective activation energy ΔE. In comparison to neat PET, ΔE of intercalated 93A MMT clay nanocomposites (PCNs) decreased and highly dependent on the clay content. Hoffman-Lauritzen (H–L) secondary nucleation theory parameters, K_g and U* were evaluated using isoconversional approach of Vyazovkin and Sbirrazzuoli. Crystallization regime transition from regime III is observed at 190°C for neat PET, which shifted to higher temperature range 200°C–208 °C for PCNs. The K_g parameters for both regimes for PET are consistent with our isothermal experiments and reported in literature. However, the K_g values of nanocomposites are highly sensitive to the temperature dependent nucleation activity and ΔE of PET chain motion in melt and are not very much comparable with our isothermal results. Nevertheless, the observed results clearly indicate that the 93AMMT clay layers predominantly act as heterogeneous nucleating sites.     

Thermo-mechanical coupling and self-excited oscillation in the neck propagation of PET films

The self-excited oscillation of neck propagation during cold drawing of polymer films has been examined experimentally. On the basis of Barenblatt's model considering a thermo-mechanical coupling at the neck, the temperature rise at the neck has been studied with an infrared camera. The temperature began to rise in a range showing a negative velocity dependence of the applied load. The behavior is consistent with the view of thermo-mechanical coupling. The temperature rise was up to 80°C (>T_g) and explains the occurrence of crystallization for faster drawing rates. It has also been confirmed that the temperature rise follows the oscillation of stress due to the coupling.