Effects of crystallinity on solid‐state polymerization of poly(ethylene terephthalate)

There has been a widely held assumption that the solid‐state polymerization (SSP) rate of poly(ethylene terephthalate) (PET) decreases with increasing crystallinity. Several published articles that purported to prove this assumption were based on faulty experiments. Therefore, a proper experimental procedure has been used to study the true effects of crystallinity on the SSP of PET. The results show that, for PET in pellet and powder forms, the SSP rate increases with increasing crystallinity. This is because an increase in the crystallinity results in increased end‐group concentration in the amorphous phase, where SSP reactions take place, and decreased concentrations of inactive end groups trapped inside the crystals, thereby increasing the rates of end‐group collision and reactions. These positive effects outweigh the negative effect of the increased byproduct‐diffusion resistance because of the increase in crystallinity. As the particle size of PET is increased beyond a critical value of about 7 mm, the SSP rate actually decreases with increasing crystallinity because of the excessively increased byproduct‐diffusion resistance within the PET particles. However, this critical particle size is far greater than the pellet sizes of commercial PET resins. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 623–632, 2006     

Solid‐state polymerization of poly(trimethylene terephthalate)

The solid‐state polymerization (SSP) of poly(trimethylene terephthalate) (PTT) has been studied and compared with that of poly(ethylene terephthalate) (PET). Because PTT and PET share the same SSP mechanism, the modified second‐order kinetic model, which has successfully been used to describe the SSP behaviors of PET, also fits the SSP data of PTT prepolymers with intrinsic viscosities (IVs) ranging from 0.445 to 0.660 dL/g. According to this model, the overall SSP rate is ?dC/dt = 2k_a(C ? C_ai)², where C is the total end group concentration, t is the SSP time, k_a is the apparent reaction rate constant, and C_ai is the apparent inactive end group concentration. With this equation, the effects of all factors that influence the SSP rate are implicitly and conveniently incorporated into two parameters, k_a and C_ai. k_a increases, whereas C_ai decreases, with increasing SSP temperature, increasing prepolymer IV, and decreasing pellet size, just as for the SSP of PET. Therefore, the SSP rate increases with increasing prepolymer IV and increasing SSP temperature. The apparent activation energy is about 26 kcal/mol, and the average SSP rate about doubles with each 10°C increase in temperature within the temperature range of 200–225°C. The SSP rate increases by about 30% when the pellet size is decreased from 0.025 to 0.015 g/pellet. Compared with PET, PTT has a much lower sticking tendency and a much higher SSP rate (more than twice as high). Therefore, the SSP process for PTT can be made much simpler and more efficient than that for PET. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3188–3200, 2003     

Solid‐state polymerization of poly(ethylene terephthalate). I. Experimental study of the reaction kinetics and properties

The solid‐state polymerization (SSP) reaction kinetics of poly(ethylene terephthalate) were investigated in connection with the initial precursor intrinsic viscosity (IV; molecular weight). Evaluations were performed with otherwise equivalent precursors melt‐polymerized to IVs of 0.50, 0.56, and 0.64 dL/g. The changes in the molecular weight and other properties were monitored as functions of the reaction times at solid‐state temperatures of 160–230°C. Precursors with higher initial molecular weights exhibited higher rates of SSP than those with lower initial values, as discussed in connection with the levels of crystallinity and the carboxyl and hydroxyl end‐group composition. Activation energies decreased at temperatures above 200°C, and this indicated a change in the SSP reaction mechanism. At temperatures of 200–230°C, similar activation energies were required for the polymerization of all three precursors. Lower temperature polymerizations, from 160 to 200°C, required higher activation energies for all precursors, with the 0.50‐IV material requirement almost twice as high as that calculated for the higher IV precursors. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 197–212, 2003     

Solid‐state polymerization of melt‐spun poly(ethylene terephthalate) fibers and their tensile properties

The production of high modulus and high strength poly(ethylene terephthalate) fibers was examined by using commercially available melt‐spun fibers with normal molecular weight (intrinsic viscosity = 0.6 dL/g). First, molecular weight of as‐spun fibers was increased up to 2.20 dL/g by a solid‐state polymerization, keeping the original shape of as‐spun fibers. Second, the polymerized as‐spun fibers were drawn by a conventional tensile drawing. The achieved tensile modulus and strength of as‐drawn fibers (without heat setting) were 20.0 and 1.1 GPa, respectively. A heat setting was carried out for the as‐drawn fibers. Tensile properties of the treated fibers were greatly affected by the condition of the heat setting. This was related to the increase of sample crystallinity and molecular degradation during the treatments. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1791–1797, 2007     

Solid‐state polymerization of poly(ethylene 2,6‐naphthalate)

The solid‐state polymerization (SSP) of poly (ethylene 2,6‐naphthalate) (PEN) was studied and compared with that of poly(ethylene terephthalate) (PET). The SSP of PEN, like that of PET, could be satisfactorily described with a modified second‐order kinetic model, which was based on the assumptions that part of the end groups were inactive during SSP and that the overall SSP followed second‐order kinetics with respect to the active end‐group concentration. The proposed rate equation fit the data of the SSP of PEN quite well under various conditions. PEN prepolymers in pellet and cube forms with intrinsic viscosities (IVs) ranging from 0.375 to 0.515 dL/g, various particle sizes, and various carboxyl concentrations were solid‐state polymerized at temperatures ranging from 240 to 260°C to study the effects of various factors. The SSP data obtained in this study could be readily applied to the design of commercial PEN SSP processes. Because PEN and PET share the same SSP mechanism, in general, the SSP behaviors of PEN are similar to those of PET. Thus, the SSP rate of PEN increased with increasing temperature, increasing prepolymer IV, and decreasing prepolymer particle size. However, because of the much higher barrier properties of PEN, the prepolymer particle size and carboxyl concentration had much greater effects on the SSP of PEN than on the SSP of PET. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1075–1084, 2007     

Solid‐state polymerization of poly(ethylene terephthalate). II. Modeling study of the reaction kinetics and properties

Comprehensive modeling studies were used to describe the kinetics of the solid‐state polymerization (SSP) of poly(ethylene terephthalate). The validity of the model was confirmed by the successful fitting of the experimental results for molecular weight increases, at temperatures ranging from 180 to 230°C and for times up to 12 h, with one fitting parameter. The changes in the concentrations for hydroxyl end groups ([? OH]), carboxyl end groups ([? COOH]), vinyl end groups, and terephthalic acid (TPA) were simulated with the model. During SSP, the contents of not only hydroxyl and carboxyl end groups but also vinyl ester end groups and TPA monomer were predicted to decrease as a function of the SSP time and temperature. The effects of the pellet size and the molar ratio of carboxyl end groups to hydroxyl end groups were also calculated. At an end‐group molar ratio ([? COOH]/[? OH]) of around 0.7, a maximum SSP rate was obtained. As the [? COOH]/[? OH] ratio increased, the contents of the vinyl end groups and TPA monomer were predicted to increase. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 213–227, 2003     

A comprehensive model for solid‐state polycondensation of poly(ethylene terephthalate): Combining kinetics with crystallization and diffusion of acetaldehyde

A comprehensive mathematical model was established by considering the main and side reactions for solid‐state polycondensation of poly(ethylene terephthalate). The effect of temperature on chain mobility was used to estimate the rate constants of chemical reactions. The polymer crystalline fraction was modeled as containing only repeat units, thus concentrating end groups and condensates in the amorphous fraction. The diffusion coefficient of acetaldehyde was calculated by the model. The simulation results of this comprehensive model were validated by experimental data reported in literature. The model predictions were important clues for further experimental study on poly(ethylene terephthalate) solid‐state polycondensation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 3133–3144, 2002; DOI 10.1002/app.10113     

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     

Effects of the carboxyl concentration on the solid‐state polymerization of poly(ethylene terephthalate)

There are two types of polycondensation reactions in the solid‐state polymerization (SSP) of poly(ethylene terephthalate) (PET), namely, transesterification and esterification. Transesterification is the reaction between two hydroxyl ends with ethylene glycol as the byproduct, and esterification is the reaction between a carboxyl end and a hydroxyl end with water as the byproduct. The SSP of powdered PET in a fluid bed is practically a reaction‐controlled process because of negligible or very small diffusion resistance. It can be proved mathematically that an optimal carboxyl concentration for reaction‐controlled SSP exists only if k₂/k₁ > 2, where k₂ and k₁ are the forward reaction rate constants of esterification and transesterification, respectively. Several interesting observations were made in fluid‐bed SSP experiments of powdered PET: (1) the SSP rate increases monotonously with decreasing carboxyl concentration, (2) k₂ < k₁ in the presence of sufficient catalyst, (3) k₁ decreases with increasing carboxyl concentration if the catalyst concentration is insufficient, and (4) the minimum catalyst concentration required to achieve the highest SSP rate decreases with decreasing carboxyl concentration. In the SSP of pelletized PET, both reaction and diffusion are important, and there exists an optimal carboxyl concentration for the fastest SSP rate because esterification, which generates the faster diffusing byproduct, is retarded less than transesterification in the presence of substantial diffusion resistance. The optimal prepolymer carboxyl concentration, which ranges from 25 to 40% of the total end‐group concentration in most commercial SSP processes, increases with increasing pellet size and product molecular weight. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1288–1304, 2002     

Solid‐state polymerization of poly(ethylene terephthalate). III. Thermal stabilities in terms of the vinyl ester end group and acetaldehyde

Poly(ethylene terephthalate) (PET), precursors, and solid‐stated samples were evaluated in terms of changing vinyl ester (VE) concentrations. The results obtained through the application of reaction kinetics gave VE contents ranging from 0.7 to 5.5 mmol/kg of PET. As the initial intrinsic viscosity (IV) of the precursor increased, the VE content also increased, representing the different thermal histories of the samples during melt‐phase polymerization. The VE contents decreased as the solid‐state polymerization (SSP) time increased from 0 to 12 h and as the temperature of SSP increased up to 220°C. A series of acetaldehyde (AA) generation experiments were conducted from 270 to 300°C with samples solid‐stated from three precursors with different initial IVs. The rate of AA generation decreased as the final IV of the solid‐stated PET increased, and this showed that the SSP process improved the thermal stability of PET. The AA generation rates of samples that had similar final IVs but were solid‐stated from different IV precursors were also compared. When the heating temperature was low, the amount and rate of AA generation were higher for samples with higher initial precursor IVs. This tendency, however, became less clear as the generation temperatures increased, probably because interference from the dissociation reactions (occurring between the polymer chains to produce VE) increased with increasing temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 228–237, 2003     

Solid state polymerization (SSP) of low molecular weight poly(ethylene terephthalate) (PET) copolyesters compared to conventional SSP of PET

SSP, starting from very low molecular weight (MW) poly(ethylene terephthalate) (PET) precursors, is claimed to offer significant production cost advantages over conventional PET production. However, as the intrinsic viscosity (IV) of the PET precursor is reduced, there is a significant change in the crystallization behavior of PET and morphology that affects reactivity in SSP. Using small particle size PET to significantly reduce the effects of diffusion so that SSP is under chemical reaction control and using a kinetic model that describes an overall SSP rate, the effect of ethylene isophthalate substitution on the SSP rate from low MW PET precursor was determined. As the ethylene isophthalate comonomer content increases, the rate of SSP for low MW PET increases. The activation energy for SSP of low MW PET decreases with an increase in the ethylene isophthalate content. For the low MW PET copolyesters in this study, the SSP activation energy is comparable to conventional process when the comonomer content of the low MW polyester is around 7 mol % and the conventional precursor is around 3 mol %. However, even though the activation energy is reduced through the use of higher comonomer content, the overall rate of SSP for the low MW copolyesters studied is significantly slower than conventional SSP. This reduction in rate is explained by differences in crystallinity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 230–238, 2002     

Morphological changes of poly(ethylene terephthalate‐co‐isophthalate) during solid state polymerization

Poly(ethylene terephthalate‐co‐isophthalate) (PETI) prepolymer was submitted to solid state polymerization (SSP) at 184–230°C in a fixed bed reactor, to study the evolution of morphological changes during the process. Short reaction times were selected to investigate crystallization phenomena during nonisothermal (heating) and isothermal SSP phases. More specifically, multiple PETI melting behavior was observed and attributed to secondary crystallization, the rate of which increased significantly with SSP temperature. Reaction time was also found to exert a positive effect on solid‐phase perfection of secondary crystals, leading at each temperature to melting points close to the value of bottle‐grade poly(ethylene terephthalate). Finally, the mass fraction crystallinity of the SSP grades was found to comply with the crystal morphology encountered. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Solid‐state polymerization of bisphenol A polycarbonate with a spray‐crystallizing method

A novel crystallization method for the production of high‐molecular‐weight bisphenol A polycarbonate by solid‐state polymerization is suggested. In this method, a low‐molecular‐weight polycarbonate prepolymer is dissolved in a solvent and then partially crystallized with a novel spray‐crystallizing method to prepare crystallized polycarbonate particles having a very uniform and porous structure with a narrow melting region. As a result, during solid‐state polymerization, the phenol byproduct can be easily removed from the polymerizing porous polycarbonate particles, and the polymerization rate is dramatically increased. In particular, the effects of the crystallization methods on secondary crystallization during solid‐state polymerization and the melting behavior have been investigated with differential scanning calorimetry studies. The final product, a high‐molecular‐weight polycarbonate, displays a very narrow molecular weight distribution and uniform physical properties. A simultaneous process and an adequate reactor design for spray crystallization and solid‐state polymerization are also suggested. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Ultrasonic characterization of the crystallization behavior of poly(ethylene terephthalate)

On the basis of the previous observations that the ultrasonic signals are sensitive to the crystallization of polymers (Tatibouet and Piché, Polymer 1991, 32, 3147), we have expanded our efforts to study the detail relationship between the ultrasonic signals and crystallization process in this work. The nonisothermal and isothermal crystallization of virgin poly(ethylene terephthalate) (PET) and PET samples after degradation were studied by using a specially designed pressure‐volume‐temperature (PVT) device, with which an ultrasonic detector was combined. The results showed that the evolution of the ultrasonic signals not only can be used to probe the crystallization process but also can qualitatively characterize the crystallization rate, crystallinity, crystallite size, and amorphous. DSC measurement was used to verify such results. Ultrasonic signals could be as a complementary tool to polymer chain movement and well be applied to characterize the crystallization behavior. Furthermore, the ultrasonic measurement has the potential use to characterize crystallization of products in‐line during processing (i.e., injection molding, micromoulding). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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     

Crystallization and solid‐state polymerization of poly(aryl ester)s

The crystallization and solid‐state polymerization (SSP) of poly(aryl ester)s was investigated. Oligomers with different end‐groups were prepared by degradation of commercially available poly(aryl ester)s. The SSP of these oligomers was carried out after crystallization and under reduced pressure, in the presence of various catalysts. Polymers were characterized by means of their inherent viscosities and thermal properties. It has been found that Ti(OⁱPr)₄ was a better catalyst for SSP. The structures and morphologies of semicrystalline poly(aryl ester)s were investigated by X‐ray diffraction and differential scanning calorimetry (DSC). Copyright © 2004 Society of Chemical Industry     

Effect of solid‐state shearing of poly(ethylene terephtalate) on isothermal crystallization and fiber structure formation

Solid‐state shearing was applied on recycled poly(ethylene terephtalate) (PET) using twin‐screw extruder. The shearing effect on isothermal crystallization was investigated by DSC. Structure formation in melt spinning was also studied by characterizing the spun fibers. It was found that the solid‐state shearing imposes a great influence on its crystallization behavior. Crystallization peak temperature of the sheared PET (199°C) was much higher than that of the PET without shearing (188°C). Overall isothermal crystallization rate was found to be accelerated to more than 50 times by the shearing. The shearing effect remained even after fiber spinning (melt processing). Differences in crystallization behavior, fiber structure formation, and tensile properties of the fibers were found. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 787–791, 2007     

聚酯固相缩聚等温结晶特性的研究

聚酯(PET)固相缩聚(SSP)中切片的结晶性能及其演变影响固相缩聚反应,采用差示扫描量热仪(DSC)和热台偏光显微镜研究了固相缩聚反应前后PET切片的等温结晶特性。结果表明:PET切片在DSC中的等温结晶符合Avrami 方程,等温结晶温度升高,结晶速率常数K值减小,即结晶速率降低;热台偏光显微镜中不同等温结晶温度下形成了不同的球晶形态:黑十字消光图以及环形消光图;随着PET特性粘数(平均分子质量)增大,结晶速率常数K值减小,球晶生长速率减小,Avrami指数n值增大,形成更加复杂的消光图。对于固相缩聚前PET基础切片,球晶最大结晶速率在190℃左右。     

Low temperature solid‐state extrusion of recycled poly(ethylene terephthalate) bottle scraps

The processing of poly(ethylene terephthalate) (PET) involves thermal and hydrolytic degradation of the polymer chain, which reduces not only the intrinsic viscosity and molecular weight, but also the mechanical properties of recycled materials. A novel PET/bisphenol A polycarbonate/styrene–ethylene–butylene–styrene alloy based on recycled PET scraps is prepared by low temperature solid‐state extrusion. Hydrolysis and thermal degradation of PET can be greatly reduced by low temperature solid‐state extrusion because the extrusion temperature is between the glass‐transition temperature and cold‐crystallization temperature of PET. Modification of recycled PET by low temperature solid‐state extrusion is an interesting method; it not only provides an easy method to recycle PET scraps by blend processing, but it can also form novel structures such as orientation, crystallization, and networks in the alloy. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2692–2699, 2006     

Preparation and crystallization of poly(ethylene terephthalate)/SiO2 nanocomposites by in‐situ polymerization

Poly(ethylene terephthalate) (PET)/SiO₂ nanocomposites were prepared by in situ polymerization. The dispersion and crystallization behaviors of PET/SiO₂ nanocomposites were characterized by means of transmission electron microscope (TEM), differential scanning calorimeter (DSC), and polarizing light microscope (PLM). TEM measurements show that SiO₂ nanoparticles were well dispersed in the PET matrix at a size of 10–20 nm. The results of DSC and PLM, such as melt‐crystalline temperature, half‐time of crystallization and crystallization kinetic constant, suggest that SiO₂ nanoparticles exhibited strong nucleating effects. It was found that SiO₂ nanoparticles could effectively promote the nucleation and crystallization of PET, which may be due to reducing the specific surface free energy for nuclei formation during crystallization and consequently increase the crystallization rate. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 655–662, 2006