Solid-state coextrusion of poly(ethylene terephthalate). II. Drawing of semicrystalline PET

The drawing of semicrystalline (33 and 50%) poly(ethylene terephthalate) (PET) films has been studied by solid-state coextrusion. Because of its brittleness and opacity, isotropic and semicrystalline PET film is of little practical use. Early attempts to cold-draw crystalline films led to fracture in contrast to deformation of amorphous PET. However, we have succeeded in systematically preparing films with extrusion draw ratios ≤4.4 from semicrystalline PET. In many cases, the properties of the drawn extrudates, as a function of extrusion temperature T_ext and extrusion draw ratio EDR, were similar to those prepared from amorphous PET. However, some remarkable differences have also been found. In the case of coextrudates prepared from isotropic 50% crystalline PET, we found that the larger the deformation, the lower the apparent resulting crystallinity. In the extreme, a 34% reduction in crystallinity after deformation was observed. For the coextrudates drawn from initially 33% crystalline PET, slightly different behavior occurred. For T_ext ≤ 90°C, all extrudates showed crystallinities lower than the original isotropic film, with a minimum at EDR = 3; for T_ext ≥ 110°C, crystallinities were slightly greater than in the original film and increased with EDR. Qualitative measurements of heats of fusion were in agreement with density gradient results for PET crystallinity. In contrast is our previous finding that extrudates from initially amorphous PET always increase in crystallinity with EDR, because of stress-induced crystallization. The results now suggest that in the T_ext range investigated, the initial spherulitic structure is at least in part destroyed on drawing. In addition, the percent crystallinity is revealed to be dependent on T_ext, with lower values at lower temperatures. Mechanical tests show that the extrudates are similar or sometimes higher in tensile modulus when compared to amorphous PET drawn under the same conditions.     

Photoconduction in poly(ethylene terephthalate). I. Mechanisms of carrier generation

Mechanisms of photocarrier generation in poly(ethylene terephthalate) (PET) have been investigated. In the wavelength range of λ ≦320 nm, the photocurrent spectra correspond closely with the optical absorption spectra of PET and the assignment of the absorption peaks revealed that photocarriers are generated through ππ^* excitations. In the wavelength range from 320 to 400 nm, photocarriers are injected from metal electrodes. The threshold energies for Al and Cu electrodes are 2.8₇ and 2.9₄ eV, respectively, indicating the presence of surface states. The simplified model gives the density of the surface states as 1.7 × 10¹⁴ cm^?2 eV^?1.     

Morphology and melting behavior of semicrystalline poly(ethylene terephthalate). II. Annealed PET

Annealing of poly(ethylene terephthalate) samples crystallized under isothermal conditions above the crystallization temperature has a marked influence on their morphology and results in increased thermal stability of the crystalline structure as indicated by the melting point increase of the samples. The morphological transformation processes induced by annealing are very complex and depend on the thermal history of the samples, i.e., crystallization temperature and heating procedure. Depending on the nature of the processes occurring during annealing, various parameters characterizing the semicrystalline state of the samples can be affected such as the degree of crystallinity, the long spacing, the thickness of amorphous and crystalline layers, the crystal perfection, the fold-surface structure, and the mosaic structure of the crystalline lamellae. Annealing involves a solid-state transformation of the original crystalline structure including crystal perfection without thickening or a melting followed by recrystallization with crystal perfection and crystal thickening. The combination of differential scanning calorimetric (DSC) measurements and small-angle x-ray scattering is a powerful analytical tool to detect morphological changes and helps in deciding on the processes which are involved in the transformation of the microstructure upon annealing.     

Recycling of poly(ethylene terephthalate) (PET)

After a survey of the mostly utilized recycling methods for fibres and bottles, the possibilities of recovering films are discussed in more detail. This presentation describes primarily the melt-recovery method, which is used by us and by other manufacturers. During this process the greatest difficulties arise from the presence of impurities and contaminations. The influence of these determining parameters on the recycling process and on the re-usability of the recovered PET are elucidated, with special emphasis on the role of the oligomers.     

Glycolytic recycle of poly(ethylene terephthalate) (PET)

In a comparison of PET waste recovery methods, glycolysis to monomer (DHET + low MW oligomers) represents a compromise between regeneration of starting ingredients by methanolysis or hydrolysis and direct remelt extrusion. It is less costly than the former and more versatile than the latter. The resultant easily filtered, low viscosity monomer is repolymerized to a useful molecular weight (MW), used as an ingredient for copolymers and used with additives for controlling luster, color, etc. Glycolytic recycle can be a batch or continuous process, the rate being dependent on temperature, catalyst, state of subdivision of the feedstock and the 2G/T ratio. In addition, the final monomer composition is controlled by the time of reaction and hold-up time after depolymerization. Low 2G/T ratios permit higher temperatures and faster reactions but result in higher MW oligomers. Side reactions occurring in the depolymerization are minimized by addition of a buffer as sodium acetate and limiting storage time at high temperatures. PET scrap suitable for glycolytic recycle includes production waste, fibers, film, flake, bottles, etc., but ingredients leading to side reactions, copolymers, end caps or color must be minimized for critical products and satisfactory rates of polymerization.     

Solid-state relaxations in poly(ethylene oxide). I. Study using the differential scanning calorimetry method

Differential scanning calorimetry (DSC) studies show that poly(ethylene oxide) (PEO) exhibits three transition regions below its melting point. The effects of annealing on the intensity and temperature of these transitions enable us to locate T < T_g (T_γ) T_g, and T_α at about 130–140. 190–240, and 263–313°K, respectively. Our results argue for a small transition T_g (L) at 190–200°K with a second T_g (U) above 233°K, the temperature of which increases on annealing. The shape of DSC derivative curves reveals that T < T_g and T_α are complex and suggests the possibility of two steps in these processes. In addition, a splitting of T_α is observed every time a multiple melting endotherm appears as a result of annealing. Up to three separate melting endotherms can be observed. One of them is related to the normal primary crystallization process. Its peak temperature increases linearly with the annealing temperature, yielding an extrapolated value for the equilibrium melting temperature T of 347°K as found before.     

Rigid amorphous fraction and melting behavior of poly(ethylene terephthalate)

The multiple melting behavior of poly(ethylene terephthalate) (PET) is generally attributed to the fusion of original crystals recrystallized during the heating at conventional scanning rate. In the present study, the triple and double melting behavior that is observed after isothermal crystallization at T _c lower and higher than 215 °C, respectively, is put in relation with the presence and absence of rigid amorphous fraction around the original primary crystal lamellae. The complex melting behavior is explained by assuming that two different morphologies of primary crystals develop during crystallization at temperatures lower than 215 °C, in a proportion that is a function of the crystallization temperature: chain cluster aggregations with a high percentage of rigid amorphous fraction on the boundaries and small crystals with a high percentage of adjacent reentry folding and reduced constraints at the amorphous/crystal interphase. These distinct morphologies differently transform upon heating at low scanning rate, originating two endotherms. On the contrary, after crystallization at T _c ?>?215 °C, all the primary crystalline structure, which probably are characterized by the same morphology made of tightly chain folded lamellae and absence of rigid amorphous fraction, undergo the same reorganization route, originating a single endotherm.     

Low-frequency glow discharge treatment of amorphous poly(ethylene terephthalate) films

The influence of treatment in a low-frequency glow discharge on the surface properties of an amorphous poly(ethylene terephthalate) (PET) film was studied. It was shown that, at identical external discharge parameters, changes in the wettability of plasma-treated PET films depended on its morphological structure—the amorphous film had higher values of the contact angle and lower values of the surface charge density than a biaxially oriented poly(ethylene terephthalate) of the PET-E brand.     

Block copolymers of poly(ethylene terephthalate)-poly(butylene terephthalate). II. Small-angle light-scattering studies

Small-angle light-scattering (SALS) studies were carried out on block copolymers of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), the synthesis and characterization of which have been reported in an earlier paper. Samples were crystallized isothermally from the melt at 95°C for predetermined crystallization times in order to follow the formation and growth of crystalline superstructure. During the early stages of crystallization, the block copolymers showed unusual H_v patterns with the four lobes along the polarizer directions, while at later stages they showed the usual H_v patterns with the four lobes at 45° to the polarizer directions. The unusual patterns are characteristic of PBT superstructure, while the usual patterns are characteristic of PET superstructure. These results show that PBT, which is the faster-crystallizing component, crystallizes first and provides nucleation sites for the crystallization of PET, which crystallizes later. Similar behavior was not observed in PET homopolymer and random copolymers of equivalent compositions. In each case the spherulite size increased with the time of crystallization. The ultimate spherulite size decreased with increasing PBT content in the block copolymer, thus showing an increase in nucleation density. It was demonstrated that light scattering is a useful tool to characterize block copolymers of two crystalline components which have different types of superstructure.     

Visualization of strain-induced structural rearrangements in amorphous poly(ethylene terephthalate)

A direct microscopic observation procedure is applied to study the deformation of amorphous PET decorated with a thin metal layer when stretching is performed at different draw rates and at temperatures below and above the glass transition temperature T _g. Analysis of the formed microrelief allows stress fields responsible for the deformation of the polymer to be visualized and characterized. When tensile drawing is performed at temperatures above T _g, inhomogeneity of stress fields increases with the increasing draw rate; at high draw rates, the stress-induced crystallization of PET takes place. In the case of drawing the polymer at temperatures below T _g, direct microscopic observations make it possible to visualize the development of shear bands that appear in the unoriented part of the polymer specimen adjacent to the neck. The shear bands are oriented at an angle of about 45° with respect to the draw direction. When necking involves the unoriented part of the polymer, shear bands abruptly change their orientation and become aligned practically parallel to the draw axis.     

Block copolymers of poly(ethylene terephthalate–polybutylene terephthalate). I. Preparation and crystallization behavior

Block copolymers of two crystallizable compounds, poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), were developed with PET as the major component and the amount of PBT varying from 1.0 to 20.0 wt %. These block copolymers were prepared by end-group coupling of preformed oligomers. All polymers prepared were of equivalent molecular weight as determined by the intrinsic viscosity method. Thermal properties were determined by differential thermal analysis (DTA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). With increasing PBT content, the block copolymers showed a general decrease in the values of glass transition temperature, melting temperature, initial decomposition temperature, and maximum decomposition temperature. The heat of fusion and heat of crystallization first increased and then decreased slightly. Rates of crystallization were determined by measuring density as a function of time of isothermal crystallization carried out at 95°C. It was found that small amounts of PBT increased the crystallization rate considerably over that of PET. Random copolymers did not show this phenomenon and behaved more like pure PET. The crystallization behavior of block copolymers was analyzed by the Avrami equation and Avrami exponents were determined. Results were explained on the basis that the faster-crystallizing PBT blocks crystallized first and provided built-in nucleation sites for the subsequent crystallization of PET, thus resulting in a relatively fast-crystallizing copolyester.     

The role of crystalline, mobile amorphous and rigid amorphous fractions in the performance of recycled poly (ethylene terephthalate) (PET)

The action of thermo-mechanical degradation induced by mechanical recycling of poly(ethylene terephthalate) was simulated by successive injection moulding cycles. Degradation reactions provoked chain scissions and a reduction in molar mass mainly driven by the reduction of diethyleneglycol to ethylene glycol units in the flexible domain of the PET backbone, and the formation of -OH terminated species with shorter chain length. The consequent microstructural changes were quantified taking into account a three-fraction model involving crystalline, mobile amorphous (MAF) and rigid amorphous fractions (RAF). A remarkable increase of RAF, to a detriment of MAF was observed, while the percentage of crystalline fraction remained nearly constant. A deeper analysis of the melting behaviour, the segmental dynamics around the glass-rubber relaxation, and the macroscopic mechanical performance, showed the role of each fraction leading to a loss of thermal, viscoelastic and mechanical features, particularly remarkable after the first processing cycle.     

Rigid amorphous fraction in poly(ethylene terephthalate) determined by dilatometry

Volumetric thermal analysis of semicrystalline poly(ethylene terephthalate), PET, with different content of crystalline phase was carried out using mercury-in-glass dilatometry. The effect of crystals on the thermal properties of amorphous phase (glass transition temperature, T _g, thermal expansion coefficients, α) were determined. At cold-crystallization (106°C, up to 4 h), crystalline content of 2.4–25.3 vol.% was achieved. Increasing content of crystalline phase broadens the glass transition region and increases T _g. The change of thermal expansion coefficient during glass transition is lower than that predicted by the two-phase model, which indicates the presence of a third fraction — rigid amorphous fraction (RAF), whose content steadily increases during crystallization. However, its relative portion (specific RAF) is significantly reduced. Further significant decrease in specific RAF appears after annealing at a higher temperature.     

Planar deformation of amorphous poly(ethylene terephthalate) by stretching and forging

The planar deformation of amorphous poly(ethylene terephthalate) (PET) was performed by stretching and by forging under comparable conditions at a series of constant temperatures, 80, 90, 100, and 110°C. The highest planar draw ratios of 4.5 × 4.5 and 3.5 × 3.5 were obtained by forging and stretching, respectively. Samples were studied before and after deformation by wide angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), density measurements, and elastic recovery at 100°C. A distinct difference in efficiency of draw between these two techniques is found, as judged mainly by the straininduced crystallization. The forging is more effective than stretching in achieving stabilized planar draw under comparable process conditions.     

Crystallization in oriented poly(ethylene terephthalate) fibers. I. Fundamental aspects

The nature of crystallization- and mobility-induced changes during annealing of melt-spun poly(ethylene terephthalate) precursor fibers of a range of orientations has been examined. The kinetics of crystallization and the accompanying orientational changes have been studied under conditions of constant, low tensile stress, with the accompanying dimensional changes and under a constraint against shrinkage in length, with the stress developed being monitored. The effects of precursor orientation and externally imposed constraints on the course of the fundamental crystallization and orientational relaxation processes are revealed. Oriented crystallization has been shown to have a significant effect on the stress developed and on the dimensions of oriented precursor fibers, with a strong tendency to spontaneously extend as a consequence of the reorientation of crystallizing segments predominantly along the preferred fiber direction. The sequence in which crystallization and major orientational relaxation, if any, occur is found to have a profound effect on the structure and thus the deformability of oriented fibers after annealing above the glass transition temperature.     

Drawing of poly(p-phenylene sulfide) by solid-state coextrusion

The effects of drawing temperature on the physical and mechanical properties of poly(p-phenylene sulfide) have been studied. A melt-quenched film was drawn by solid-state coextrusion both below (75°C) and above (95 and 110°C) the glass transition temperature T_g (85°C) of PPS. The maximum extrusion draw ratio (EDR_max) increased from 3.4 to 5.6 with increasing extrusion temperature T_e from 75 to 110°C. It was found that extrusion drawing just above the T_g of PPS (95°C) produced more stress-induced crystals. A high efficiency of draw in the amorphous region was achieved by extrusion at T_e-75°C. The tensile modulus at EDR_max decreased from 5.1 to 3.5 GPa with increasing T_e from 75 to 110°C. The low efficiency of draw for the samples extruded at 110°C is explained in terms of disentanglement and chain slippage during drawing due to a less effective network.     

Solid-state relaxations in poly(ethylene oxide). II. Study using the spin-labeling technique

Poly(ethylene oxide) (PEO) was spin-labeled with 3-chlorocarbonyl-2,2,5,5-tetramethylpyrroline-1-oxyl. Correlation times were calculated from 173 to 375°K. For estimating slow-motion rotational correlation times three models were used: Brownian diffusion, “moderate jump” diffusion, and “large jump” diffusion. Analysis of the changes in the extrema separation and spectra shape with temperature strongly suggests the presence of β and γ processes in solid poly(ethylene oxide). The Brownian diffusion model gives, for the γ relaxation, an activation energy of 8 kcal mole^?1 which is in good agreement with that found using dielectric relaxation measurements.     

γ-Irradiation of poly(ethylene terephthalate). I. Yields of gas and carboxyl groups

Poly(ethylene terephthalate) was exposed in vacuo γ-radiation from a Co⁶⁰ source at a dose rate of 0.02 Mrad/min., and an ambient temperature of 47°C. Gaseous products were analyzed in a mass spectrometer and carboxyl groups estimated by titration with NaOH and by infrared analysis. Initial G values were ? COOH = 0.77, CO₂ = 0.17, CO = 0.11, H₂ = 0.015, and CH₄ = 0.003. All these values decreased markedly with increasing dose except G(CO)₂, which, roughly, was maintained up to 5000 Mrad. It was considered whether the dependence on dose of the yields of the major reaction products could be accounted for by the following set of first-order reactions: It was found that the rate of formation of ? COOH groups at low doses was much too high to fit this simple reaction scheme. However, a better fit was obtained over a range of higher doses (ca. 100–5000 Mrad). A final conclusion could not be reached concerning the validity of the above reaction scheme.