Effect of CO2 on crystallization kinetics of poly(ethylene terephthalate)

The effect of CO₂ on the isothermal crystallization kinetics of poly(ethylene terephthalate), PET, was investigated using a high‐pressure differential scanning calorimeter (DSC), which performed calorimetric measurements while keeping the polymer in contact with presurized CO₂. It was found that the crystallization rate followed the Avrami equation with values of the crystallization kinetic constant dependent on the crystallization temperature and concentration of CO₂ in PET. The presence of CO₂ in the PET increased its overall crystallization rate. CO₂ also decreased the glass transition temperature, T_g, and the melting temperature, T_m. As a result, the observed changes in crystallization rate caused by CO₂ can be qualitatively predicted from the magnitude of T_g depression and that of the equilibrium melting temperature, T_m⁰.     

Sorption and transport of CO2 in poly(ethylene terephthalate) crystallized by sorption of high-pressure CO2

Sorption and permeation of CO₂ in poly(ethylene terephthalate) crystallized by sorption of high-pressure CO₂ were examined below 1 atm at temperatures from 15 to 65°C. A large solubility and a high permeability of CO₂ in this specimen were observed compared to poly(ethylene terephthalate) crystallized by thermal annealing to a similar degree. A large unrelaxed volume is expected to be left in the specimen after removal of high-pressure CO₂ compared to the data of other PET samples. The thermal history during the measurements up to 65°C, which causes relaxation of the specimen, was shown to decrease CO₂ solubility. On the other hand, permeation data after annealing show not only decreased permeability but also increased apparent diffusivity. The results mean a lower mobility of gases sorbed in the unrelaxed volume than that of ordinarily dissolved gases, which corresponds to the partial immobilization model.     

Sorption and transport of CO2 above and below the glass transition of poly(ethylene terephthalate)

At temperatures at least 30°C above the glass transition (T_g) the sorption and transport of carbon dioxide in poly(ethylene terephthalate) (PET) can be described conveniently using Henry's law and Fick's law with a constant diffusion coefficient. Below T_g Fick's law with a concentration- dependent diffusion coefficient, coupled with a sorption isotherm which is concave toward the pressure axis adequately describes the observed sorption and transport data. Physical interpretations of the quantitative deviations from Henry's law and the form of the concentration dependence of the diffusion coefficient is provided by a model which hypothesizes dual modes of sorption and separate non zero mobilities of two populations of sorbed species in local equilibrium. The implications of the observed temperature variations of the phenomenological model parameters are discussed. Dilatometric parameters for PET, polycarbonate, and poly(acrylonitrile) (PAN) are shown to correlate well with a simple. relationship developed to explain the existence of the “extra” mode of sorption responsible for deviations from Henry's law for CO in glassy polymers. In the temperature range from T_g to + 20°C, deviations from Fickian behavior are also apparent. These effects are consistent with a transition in the nature of the polymer from an elastic solid below T_g to a viscous liquid above T_g In the narrow temperature range slightly above T the time scale for chain rearrangements apparently approaches that for the diffusion process. The polymer's viscoelastic response to the probing molecule, therefore, causes deviations from the classical time lag predictions. These deviations disappear 30°C above T_g.     

Sorption and transport of methanol in poly(ethylene terephthalate)

This paper reports the sorption and diffusion characteristics of methanol vapor in polyethylene terephthalate (PET). Amorphous PET, semicrystalline, biaxially oriented annealed and non-annealed samples have been studied for equilibrium sorption and kinetics of methanol. At activities of methanol less than 0.30, uptake shows Fickian kinetics and isotherm follows the Dual Mode model. Diffusion coefficients increase with penetrant concentration and are of the order of 10⁻¹⁰ cm²/s. Hysteresis during desorption and increase in solubility during resorption suggest methanol induced conditioning effects which may have detrimental effects on the barrier efficacy of PET. At activities greater than 0.30, swelling and relaxation effects occur and the isotherms show Flory-Huggins behavior for all three samples. Uptake follows two-stage kinetics fit by the Berens-Hopfenberg model. Greater polymer chain stability due to annealing reduces the extent of relaxation and improves the barrier efficacy over amorphous and non-annealed, oriented PET. For amorphous PET, at 80% activity and above, an induction time is observed which is absent in the semicrystalline films, suggesting strong relaxation effects in the amorphous phase of PET.     

Sorption and transport of benzene in poly(ethylene terephthalate)

The kinetics and equilibria of benzene sorption in poly(ethylene terephthalate) were measured at 40°C, 50°C, and 60°C, with benzene activities ranging from 0.02 to 0.3. At most experimental conditions, diffusion was found to be Fickian; however, evidence of non-Fickian transport was found at the highest activity levels. Values of the diffusion coefficient of benzene range from 10^-14 cm²/s at 40°C to 10^?12 cm²/s at 60°C in the limit of low concentrations. Nonlinear isotherms observed for benzene sorption were successfully interpreted in terms of the dual mode model for sorption in glassy polymers, whereby the sorbed penetrant exists as two populations: one sorbed according to Henry's law and the other following a Langmuir isotherm. Non-Fickian transport data were correlated with a model that superimposes diffusion of both the Henry's law and Langmuir populations (the “partial immobilization” model) upon first-order relaxation of the polymer matrix.     

Depolymerization of poly(ethylene terephthalate) resin under pressure

The process of depolymerization of PET resin by EG glycolysis under pressure is investigated. The kinetics of this pressurized depolymerization of PET resin is discussed. It was found that the rate of depolymerization is dependent of temperature, pressure, and concentration ratio of EG to PET. The rate of depolymerization is proportional to the square of EG concentration and faster than that under atmospheric pressure. Glycolyzed products under pressure consist of the PET monomer, BHET, and oligomers, mostly dimer and trimer. An equilibrium between BHET and oligomers is attained quickly soon after the depolymerization step is completed in the case of a higher ratio of EG/PET used. In the case of lower ratio of EG/PET, the final product now consists of higher molecular weight of oligomers rather than monomer, dimer, and trimer.     

Blends of poly(ethylene terephthalate) and poly(butylene terephthalate)

Commercial grade poly(ethylene terephthalate), (PET, intrinsic viscosity = 0.80 dL/g) and poly(butylene terephthalate), (PBT, intrinsic viscosity = 1.00 dL/g) were melt blended over the entire composition range using a counterrotating twin‐screw extruder. The mechanical, thermal, electrical, and rheological properties of the blends were studied. All of the blends showed higher impact properties than that of PET or PBT. The 50:50 blend composition exhibited the highest impact value. Other mechanical properties also showed similar trends for blends of this composition. The addition of PBT increased the processability of PET. Differential scanning calorimetry data showed the presence of both phases. For all blends, only a single glass‐transition temperature was observed. The melting characteristics of one phase were influenced by the presence of the other. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 75–82, 2005     

Effect of blend rheology on the transport property of oriented poly(ethylene terephthalate) blends

Rheological studies were performed to delineate appropriate stretching windows, and poly(ethylene terephthalate)/poly(ethylene-co-vinyl alcohol) blend films were extruded biaxially within such processing windows. The morphology and oxygen permeability properties of these films, with and without a compatibilizer, were characterized. The intent of this study was to achieve a blend oxygen permeability value (OPV) of less than 1. At a fixed draw ratio and draw temperature, appropriate rheological matching could lead to an increase in the aspect ratio of the EVOH phase and, thereby, of oxygen barrier properties. This study concludes that by melt viscosity matching it is not possible to obtain blends with OPV of less than 1. The model of Fricke, used to predict blend permeability, was found to be more accurate at the higher draw temperatures, with the measured values deviating increasingly negatively from the predicted values as the draw temperature is decreased. © 1995 John Wiley & Sons, Inc.     

Powdered poly(ethylene terephthalate)

The influence of the conditions of preparation on the properties of powdered poly(ethylene terephthalate) was followed from the point of view of its specific surface. The powdered poly(ethylene terephthalate) prepared by reprecipitation from the melt of 6-caprolactam has a porous and structured surface, and consequently, also a large specific surface in comparison with the powedered poly(ethylene terephthalate) prepared by mechanical milling. The specific surface value is influenced by the cooling rate of the initial homogeneous melt of poly(ethylene terephthalate)-6-caprolactam, by the concentration of poly(ethylene terephthalate) in this melt and by its molecular weight, by the water temperature at the extraction of 6-caprolactam from the tough mixed melt, by the drying temperature of the powdered poly(ethylene terephthalate), and by the content of residual 6-caprolactam in the powdered product. In the examined area, the specific surface value of the powdered poly(ethylene terephthalate) prepared by reprecipitation from the melt of 6-caprolactam ranged from 10 to 110 m²·g^?1.     

Flame-retardant poly(ethylene terephthalate)

Poly(ethylene terephthalate) containing hexabromobenzene, tricresyl phosphate, or a combination of triphenyl phosphate and hexabromobenzene, pentabromotoluene, or octabromobiphenyl was extruded or spun at 280°C into monofilaments or low-denier yarn, respectively. Only combinations of the phosphorus- and halogen-containing compounds resulted in flame-retardant poly(ethylene terephthalate) systems, without depreciating their degree of luster and color quality. The melting temperature, the reduced viscosity, and the thermal stability above 400°C of these flame-retardant systems were in most cases comparable to those of poly(ethylene terephthalate) itself. Phosphorus-bromine synergism was proposed with flame inhibition occurring mostly in the gas phase.     

Effect of ammonium salts on poly(ethylene terephthalate) materials

The study of the effect of solutions of ammonium salts of low concentrations on polyester fibre material showed that modification of the material with ammonia formed by thermal hydrolysis of ammonium salts differs from modification with ammonia obtained by the reaction of equimolar quantities of ammonium nitrate and sodium hydroxide and is similar to the process that takes place when ammonia is added to the working solution. Of the salts examined, ammonium fluoride in the concentration of 0.02–0.35 M at 130°C and 30-min process time is the most active modifying agent. In these conditions, ammonium fluoride and the products of its high-temperature hydrolysis — hydrogen fluoride and ammonia — have the strongest catalytic effect on hydrolysis of cyclic oligomers with formation of their linear form. On the whole, ammonium fluoride has a stronger modifying effect on polyester material than aqueous ammonia. __________ Translated from Khimicheskie Volokna, No. 1, pp. 17–21, January–February, 2007.     

Phase equilibrium characteristics of supercritical CO2/poly(ethylene terephthalate) binary system

The solubility of carbon dioxide in poly (ethylene terephthalate) (PET) at high pressure and elevated temperature conditions was investigated for a better understanding of the phase equilibrium characteristics of supercritical CO₂/PET binary system and useful data for the process development of the supercritical fluid dyeing. Based on the principle of pressure decaying, a novel experimental apparatus suitable to high pressure and high temperature measurement was established. The solubilities of CO₂ in PET were measured with the apparatus at temperatures of 110, 120, and 130°C and pressures up to 30.0 MPa. The results show that the solubility of CO₂ in PET increases with the increase of pressure and CO₂ density, respectively, at a constant temperature, whereas it decreases with the increase of temperature at a constant pressure. The Sanchez‐Lacombe equation of state (S‐L EOS) was used to correlate the experimental data. The calculated results are in good agreement with the experimental ones. The average absolute relative derivation (AARD) is less than 3.91%. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Crystallization in blends of poly(ethylene terephthalate) and poly(butylene terephthalate)

Blends composed of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) were melt-mixed in a Brabender cam mixer at different mixing speeds. The glass transition (T_g) and the crystallization behavior of the blends from glassy state were studied using DSC. It was found that although the blends had the same composition and exhibited the similar T_g, their properties of crystallization could be different; some exhibited a single crystallization peak and some exhibited multiple crystallization peaks depending upon experimental conditions. Results indicated that the behavior of crystallization from glassy state were influenced by entanglement and transesterification of chains. The crystallization time values were obtained over a wide range of crystallization temperature. From curve fitting, the crystallization time values and the temperature, at which the crystallization rate reaches the maximum, were found.     

Effect of drawing temperature on mesomorphic transitions of oriented poly(ethylene terephthalate) fibers exposed to supercritical CO2

Samples of partially oriented yarn (POY) PET fibers were uniaxially drawn at 23, 68 °C (cold drawing) and 100 °C (hot drawing) and then exposed to the supercritical carbon dioxide (scCO₂) without tension at a temperature of 80 °C and a pressure of 220 bar. The effect of drawing temperature on the evolution of mesomorphic phases and the structural changes under exposure to scCO₂ were investigated by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), and wide-angle X-ray diffraction. The orientation factor of the samples was measured using a polarizing microscope. A good correlation was obtained between the results of various analyses. The results illustrate that evolution of structure strongly depends on both process temperature and post-treatment by scCO₂ exposure. The latter process leads to plasticization and reduced glass transition temperature of the samples, thus inducing structural changes in the fibers.     

Effect of poly(ethylene glycol) on the solid‐state polymerization of poly(ethylene terephthalate)

Poly(ethylene glycol) (PEG) and end‐capped poly(ethylene glycol) (poly(ethylene glycol) dimethyl ether (PEGDME)) of number average molecular weight 1000 g mol^?1 was melt blended with poly(ethylene terephthalate) (PET) oligomer. NMR, DSC and WAXS techniques characterized the structure and morphology of the blends. Both these samples show reduction in T_g and similar crystallization behavior. Solid‐state polymerization (SSP) was performed on these blend samples using Sb₂O₃ as catalyst under reduced pressure at temperatures below the melting point of the samples. Inherent viscosity data indicate that for the blend sample with PEG there is enhancement of SSP rate, while for the sample with PEGDME the SSP rate is suppressed. NMR data showed that PEG is incorporated into the PET chain, while PEGDME does not react with PET. Copyright © 2005 Society of Chemical Industry     

Effect of shearing on crystallization behavior of poly(ethylene terephthalate)

The shear‐induced crystallization behavior of PET was investigated by measuring the time‐dependent storage modulus (G′) and dynamic viscosity (η′) with a parallel‐plate rheometer at different temperatures and shear rate. The morphology of shear‐induced crystallized PET was measured by DSC, X‐ray, and polarizing optical microscopy. When a constant shear rate was added to the molten polymer, the shear stress increased with the time as a result of the orientation of molecular chains. The induction time of crystallization is decreased with frequency. Moreover, the rate of isothermal crystallization of PET was notably decreased with increasing temperature. The shape of spherulites is changed to ellipsoid in the direction of shear. In addition, aggregation of spherulites is increased with increasing frequency. Particularly, the row nucleation morphology could be observed under polarized light for ω = 1. From the results of DSC, the melting point and enthalpy have a tendency to decrease slightly with increasing frequency. The crystallite size and perfectness decreased with frequency, which was confirmed with X‐ray data. The unit length of the crystallographic unit cell of the PET increased and the (1 0 3) plane peak increased with increasing frequency. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2640–2646, 2001     

Effect of morphology on barrier properties of poly(ethylene terephthalate)

The effects of morphology on the barrier properties of poly(ethylene terephthalate) (PET) have been investigated. Various levels of crystallinity can be developed in PET as a result of thermal exposure, orientation, and heat setting. The morphologies of the crystalline phase are affected by the conditions of their formation. As a result of morphological differences, samples with equivalent levels of crystallinity have been found to exhibit different oxygen barrier properties. These differences are most apparent at low and intermediate levels of crystallinity. For thermally crystallized systems, at the same crystalline content, increasing superstructure size in the crystalline phase leads to greater tortuosity for the permeant molecules, resulting in lower permeability. For stretched and heat set PET, transport properties can be correlated with birefringence as well as overall orientation, measured in terms of fraction of molecules in the trans or extended chain conformation. At high levels of crystallinity, where the spherulites become volume filling, permeation takes place primarily through the interlamellar regions of the crystalline phase and is controlled by level of crystallinity, independent of the mode of crystallization. The barrier properties of PET, before spherulitic impingement occurs, are governed by the size and number of spherulites as well as by the amorphous orientation present in non‐crystalline regions. POLYM. ENG. SCI., 45:400–409, 2005. © 2005 Society of Plastics Engineers     

High-impact poly(ethylene terephthalate) blends

Rubbers of different kind were tested as toughening agents of poly(ethylene terephthalate) (PET), noting significant morphological and mechanical differences. In particular, good results were obtained by using an ethylene–ethyl acrylate–glycidyl methacrylate copolymer. The resulting blend evidenced good particle distribution, and the latter was related to chemical interactions between the rubber epoxy groups and PET terminal groups, including the effect of low molecular weight and polymeric amine catalysts, and to extrusion conditions. © 1995 John Wiley & Sons, Inc.     

Finite strain behavior of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG)

Uniaxial and plane strain compression experiments are conducted on amorphous poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG) over a wide range of temperatures (25-110 °C) and strain rates (.005-1.0 s⁻¹). The stress-strain behavior of each material is presented and the results for the two materials are found to be remarkably similar over the investigated range of rates, temperatures, and strain levels. Below the glass transition temperature (θ_g=80 °C), the materials exhibit a distinct yield stress, followed by strain softening then moderate strain hardening at moderate strain levels and dramatic strain hardening at large strains. Above the glass transition temperature, the stress-strain curves exhibit the classic trends of a rubbery material during loading, albeit with a strong temperature and time dependence. Instead of a distinct yield stress, the curve transitions gradually, or rolls over, to flow. As in the sub-θ_g range, this is followed by moderate strain hardening and stiffening, and subsequent dramatic hardening. The exhibition of dramatic hardening in PETG, a copolymer of PET which does not undergo strain-induced crystallization, indicates that crystallization may not be the source of the dramatic hardening and stiffening in PET and, instead molecular orientation is the primary hardening and stiffening mechanism in both PET and PETG. Indeed, it is only in cases of deformation which result in highly uniaxial network orientation that the stress-strain behavior of PET differs significantly from that of PETG, suggesting the influence of a meso-ordered structure or crystallization in these instances. During unloading, PETG exhibits extensive elastic recovery, whereas PET exhibits relatively little recovery, suggesting that crystallization occurs (or continues to develop) after active loading ceases and unloading has commenced, locking in much of the deformation in PET.     

Properties of poly(ethylene terephthalate)/poly(ethylene naphthalate) blends

Blends composed of poly(ethylene terephthalate) (PET) as the majority component and poly(ethylene naphthalate)(PEN) as the minority component were melt-mixed in a single screw extruder at various PET/PEN compound ratios. Tensile and flexural test results reveal a good PET/PEN composition dependence, indicating that the compatibility of the blends is effective in a macrodomain. In thermal tests, single transitions for T_g, T_m and T_c (crystallization temperature), respectively, are observed from DSC as well as single T_g from DMA except for 50/50 blends. These results suggests that the compatibility is sufficient down to the submicron level. Moreover, isothermal DSC tests along with Avrami analysis indicate that PET's crystallization is significantly retarded when blended with PEN. Results in this study demonstrate that PEN is a highly promising additive to improve PET's spinnability at high speeds.