bluuebunny/crossref_metadata_2025_split huggingface dataset:
Studies on structural foam processing. III. Bubble dynamics in foam extrusion through a converging die
Authors: Yoo Hee Ju , Han Chang Dae 
Published: December 2004

AbstractAn experimental and theoretical study was carried out to achieve a better understanding of bubble dynamics in foam extrusion through a converging die. For the experimental study, a number of converging channels were constructed of aluminum, with glass windows on both sides. Bubble dynamics in the flow channel were recorded on movie film as a gas‐charged molten polymer was extruded. The dies had various converging angles (30, 45, 60, 90, and 150 degrees), and the polymer was polystyrene. As blowing agent, sodium bicarbonate (generating CO2) was used. It was found that the gas bubbles moving along the centerline of the channel grow initially at the upstream end of the die, and then start to collapse as the gas‐charged molten polymer approaches the exit plane of the die. In order to help interpret the experimental results, a theoretical analysis was made of bubble dynamics in a converging channel, in which a thread‐like bubble was assumed to flow along the centerline of the converging channel and the Coleman‐Noll second‐order fluid model was assumed to describe the rheological behavior of the polymer melt. Some mathematically convenient simplifying assumptions not‐withstanding, the theoretical analysis corroborates the experimental observations. The practical significance of the present investigation is discussed in connection with controlling the cell structure in extruded foam products.
Rheological aspects of thermoplastic foam extrusion
Authors: Kraynik A. M. 
Published: December 2004

AbstractSeveral rheological aspects of thermoplastic foam extrusion are described by a phenomenological model of the flow in an extrusion die. The macroscopic effects of the phase change, from a homogeneous polymer melt that contains a blowing agent, to a foam, are described in terms of two dimensionless parameters. α is defined as the ratio of the pressure gradient in the melt phase to the average pressure gradient in the foam phase. Θ represents the ratio of the pressure drop in the melt phase to the pressure drop in the foam phase. The position at which the phase change initiates, the exit contribution to the Bagley ends pressure correction, and the true wall shear stress in the melt phase are related to α and Θ. The quality of the foam produced is discussed in terms of the extrusion conditions and their effect on the foaming position in the die. It is demonstrated that the ends pressure correction cannot be neglected on the basis of a long die alone. Asymptotic conditions for which the phase change has negligible effect on the melt flow rate are predicted. α is calculated from the data of Han and Villamizar, who measured the pressure distribution in the die and observed the phase change directly. The prediction of α from theory is complicated by the lack of suitable constitutive relations for the foam phase. Since Θ only requires knowledge of the pressure at which the phase change initiates, it is relatively easy to evaluate.
Structural and mechanical properties of polybutadiene‐containing polyurethanes
Authors: Brunette C. M. , Hsu S. L. , Macknight W. J. , Schneider N. S. 
Published: December 2004

AbstractA series of segmented polyurethanes based on hydroxylterminated polybutadienes (HTPBD) and their hydrogenated derivatives (HYPBD) has been synthesized. Thermal, mechanical, and spectroscopic studies were carried out over a wide temperature range to elucidate the structure‐property relationships existing in these polymers. Both thermal and dynamic mechanical response showed a soft segment Tg at −74°C for the unsaturated polyurethanes and at −69°C for the hydrogenated samples. In addition, two hard segment transitions are observed by differential scanning calorimetry (DSC) at 40 and 75°C and a softening region by thermal mechanical analysis (TMA) at 190°C. The low Tg, very close to that of the free HTPBD and HYPBD and independent of hard segment content, indicated that these polymers were well phase separated. Results of infrared analysis revealed that at room temperature, 90‐95 percent of the urethane N‐H groups formed hydrogen bonds. Since hydrogen bonding resides only within the hard segment domain in these butadiene‐containing polyurethanes the extent of H‐bonding served as additional evidence for nearly complete phase segregation. From dynamic mechanical studies, the plateau modulus above the soft segment Tg and stress‐strain behavior depended upon the concentration of hard segments. A slight increase in the modulus, a moderate increase in stress (σb), and decrease in elongation accompanied a higher hard segment content. The thermal and mechanical response of these polyurethanes appears to be consistent with behavior observed for other phase segregated systems. Variations in behavior resulting from hydrogenation of the precursor prepolymer are discussed.
Analysis of the tensile stress‐strain behavior of elastomers at constant strain rates. I. Criteria for separability of the time and strain effects
Authors: Hong S. D. , Fedors R. F. , Schwarzl F. , Moacanin J. , Landel R. F. 
Published: December 2004

AbstractConsidering the case where the relaxation time spectrum is preserved at finite deformations, a theoretical analysis of the tensile stress‐strain relation of elastomers at constant strain rates has been carried out. The finite strain effect is taken into account by replacing the Cauchy strain by a general strain function, ƒ(ϵ), in the Boltzmann superposition integral. The analysis shows that there are two cases where the time and strain effects are separable when: (1) the segment of the stress relaxation modulus which coincides with the experimental time of stretching can be represented by a single power law; and (2) the general strain function, ƒ(ϵ), is linearly proportional to the Cauchy strain. Separability of the time and strain effects, therefore, can be achieved by adjusting the stretching time (or strain) and temperature, if the relaxation time spectrum remains unchanged by the deformation. The tensile stress‐strain relations derived from the theoretical analysis were applied to analyze data on a crosslinked styrene butadiene rubber obtained in the temperature range −40 to 60°C. Γ(ϵ), which describes the strain dependence of tensile stress, Bϵ, the ratio of isochronal stresses at different strains, and ai, slope of a segment of the relaxation modulus Ei(t) on log t plot, were obtained directly from the experiment. Values of Γ(ϵ), Bϵ and ai obtained at −40°C are quite different from those obtained at −30°C or higher. Results obtained from our analysis are generally in agreement with those obtained by an empirical method for analyzing the experimental data.
Haloaldehyde polymers. XXIII: Thermal and mechanical properties of chloral polymers
Authors: Kubisa P. , Corley L. S. , Kondo T. , Jacovic M. , Vogl O. 
Published: December 2004

AbstractSamples of chloral homopolymers and copolymers with aromatic isocyanates of high chloral content were prepared by cryotachensic polymerization as films of 0.1 to 5 mm thickness. Thermal, mechanical, and electrical properties of these polymers were measured. Polychloral is unlike most other linear polymers as it does not show any melting or glass transition below 220°C, at which temperature the polymer starts to degrade by depolymerization to monomer. The polymer is of moderate crystallinity and shows properties characteristic of engineering plastics.
Processing of polyolefin blends
Authors: Ho Wu‐Jing , Salovey R. 
Published: December 2004

AbstractThe morphology of binary and ternary polyolefin blends of polypropylene (PP), ethylene‐propylene‐diene terpolymer (EPDM) and polyethylene (PE) following processing by injection and compression molding has been examined by optical and scanning electron microscopy. Internal surfaces were generated by low temperature fracture and etching with cyclohexane. In binary blends, droplets of EPDM are elongated in the flow direction within 400μm of the mold surface in injection molding, yielding a skin region which is distinct from an isotropic core containing spherical EPDM inclusions. Spherical droplets of EPDM or PE in binary blends with PP increase in size with increasing compression molding time. In ternary blends, spherical inclusions containing both EPDM and PE are dispersed in PP. With increasing compression molding time, EPDM separates from PE and concentrates at the outer edges of the PE inclusion, effectively isolating PE from the PP matrix.
Crazing and acoustic emission in poly(methyl methacrylate)/acetone and polystyrene/methanol systems
Authors: Jakus Karl , Ritter John E. , Larsen Clifford A. 
Published: December 2004

AbstractCraze initiation and growth in the poly(methyl methacrylate)/acetone and polystyrene/methanol systems was studied using photographic and acoustic emission techniques. The poly(methyl methacrylate) system was the only one that produced detectable acoustic emission; however, it was found that craze initiation occurs predominantly in the first half of a sample's lifetime while acoustic emission is detected predominantly in the second half. The detected acoustic emission in the poly(methyl methacrylate) system is believed to be due to the rupture of craze fibrils. Craze initiation in both polymer systems was found to be dependent on stress and time with higher stresses initiating more crazes at earlier times. Although the craze growth rate in both systems exhibited extreme variability, the average growth rate tended to increase with craze size and applied stress. In the poly(methyl methacrylate) system, craze velocity was independent of time but in the polystyrene system it decreased with time. None of the existing craze growth theories were capable of explaining these results.
Kinetic treatments of glass transition phenomena and viscoelastic properties of glasses
Authors: Aklonis J. J. 
Published: December 2004

AbstractPhenomenological treatments of viscoelasticity and recent models proposed to explain the kinetic aspects of glass transition phenomena are shown to be based on the same underlying physical considerations. This realization suggests the possible unification of these two areas by a single model. This possibility is explored for the simple case of a glass subjected to a single temperature jump followed by a stress relaxation experiment. Presently, sufficient data on any single chemical system to support a critical test of the viability of this model does not exist. Nevertheless, data from several sources on various materials indicate that the qualitative aspects of viscoelastic behavior of glasses at temperatures well below Tg are in accord with the predictions of the unified model.
Fourier transform infrared characterization of conformational changes in amorphous poly(ethylene terephthalate) during volume recovery
Authors: Moore R. S. , O'loane J. K. , Shearer J. C. 
Published: December 2004

AbstractSamples of melt‐quenched, amorphous poly(ethylene terephthalate), APET, annealed for up to four successive 30‐min intervals at 50°C, were analyzed by Fourier transform IR (FTIR) spectroscopy at room temperature. Results confirm earlier observations that the fraction of the gauche conformation of the ethylene glycol moiety increases upon annealing. This suggests a change toward a more random structure during the concomitant process of volume recovery. Spectra obtained after annealing for times to and beyond the onset of embrittlement, as judged from mechanical response, indicated that several distinct changes in band intensities occur during this time interval. Spectroscopic results for times longer than that required for embrittlement suggest an increase in motions associated with in‐plane vibrations located in and near the aromatic portions of the chains, as well as a decrease in out‐of‐plane vibrations. The overall results suggest that this method has potential for evaluating fast as well as slow recovery processes.
Characterization of glassy state relaxations by low pressure carbon dioxide sorption in poly(methyl methacrylate)
Authors: Fechter J. M. H. , Hopfenberg H. B. , Koros W. J. 
Published: December 2004

AbstractPoly(methyl methacrylate) (PMMA) microspheres were swollen in methanol vapor, the swelling penetrant was quickly evacuated, and the subsequent relaxation of the polymer under vacuum was monitored by determination of the rate of settling of rapidly measured low pressure pseudoequilibrium CO2 sorption isotherms. The decrease in CO2 sorption capacity occurred very rapidly at short times, but the final stages of consolidation were protracted. In all cases, the microspheres were under vacuum between isotherm determinations. A single relaxation time is not sufficient to describe the relaxation processes characterized by the time‐dependent sorption measurements. The sorption relaxation curves are similar in shape to volume relaxation curves for glassy polymers following imposition of a large pressure or temperature step change. The observed similarity between the sorption relaxation curves and classical volume relaxation curves is consistent with the notion that the excess sorption capacity introduced by methanol preswelling results from excess unrelaxed volume introduced into the glass by the quickly removed alcohol. As the excess volume relaxes, the excess sorption capacity decreases. One can, therefore, monitor the subtle process of consolidation using CO2 as a probe of the excess volume introduced by the swelling perturbation. Subatmospheric pressure CO2 sorption isotherms measured between 20 and 40°C for two different diameter microsphere samples (5436 Å and 1453 Å) for pressures up to 700 mm Hg were concave to the pressure axis. Such general isotherm shapes can be described by a two‐term expression consisting of a Henry's law term and a Langmuir term. The Langmuir term, which arises due to unrelaxed volume in the nonequilibrium glass, is responsible for the observed concavity in the sorption isotherm. Annealing the sample, increasing the temperature of the sorption experiment closer to the Tg of the PMMA or permitting the preswollen sample to relax tends to reduce the concavity of the isotherm relative to the corresponding case for sorption in the preswollen sample measured at 25°C. This trend is presumably a consequence of the reduction in unrelaxed volume in the glass which attends either annealing, approaching the glass transition temperature of the polymer, or consolidation of excess volume following exposure to a swelling penetrant.
Non equilibrium annealing behavior of poly(vinyl acetate)
Authors: Bair H. E. , Johnson G. E. , Anderson E. W. , Matsuoka S. 
Published: December 2004

AbstractThe water absorbed by poly(vinyl acetate), PVAc, at 23°C was found in two states. The first, which can account for up to 4 weight percent, was bound to the polymer. The second was in a freezable or clustered form. The latter type of water had no effect on PVAc's glass temperature, whereas, the former kind plasticized Tg. In annealing studies, the enthalpic and dielectric response of PVAc when held at a fixed temperature increment, ΔT, below Tg, was observed to be independent of the amount of bound water. The time dependence of the shift in the dielectric relaxation spectrum and the recovery of the enthalpy towards its equilibrium value as PVAc approached its equilibrium glassy state from a lower temperature as compared to a higher temperature was initially slower. This delayed response to expansion was of the order of the polymer's average relaxation time at the lower temperature. A model was proposed to explain this asymmetric behavior based upon changes in the polymer's free volume as well as its occupied volume.
Sub‐Tg annealing studies of UV‐cured poly(urethane) acrylate coatings on optical fibers: A correlation of annealing behavior with microbending loss
Authors: Levy Nicole 
Published: December 2004

AbstractThree types of UV‐curable acrylate network systems were coated on optical fibers and studied calorimetrically. It was found that an endothermic transition of ΔH ∼2‐14 cal/g is superimposed on the glass transition of these coatings. This transition is present in the fiber coatings, but not in sheet form. The time‐dependence of sub‐Tg annealing of these crosslinked coatings (following rapid quenching from a temperature above the end temperature of the endotherm) was found to correlate with a mechanical‐optical property of the coated fibers; namely, the contribution of the coating to the microbending loss of the optical fiber.
The curing of unsaturated polyester resins in adiabatic reactors and heated molds
Authors: Rojas Alfredo J. , Borrajo Julio , Williams Roberto J. J. 
Published: December 2004

AbstractThe influence of several process parameters on the curing of multipurpose unsaturated polyesters with styrene, in heated molds, is discussed. The polymerization kinetics was studied in a quasi‐adiabatic reactor taking into account corrections for heat losses. The following expression resulted: dx/dt = k′[I](1 − x)3 exp(−16.6/RT), where x is the conversion of unsaturated bonds and [I] is the initial initiator concentration (benzoyl peroxide); cobalt octoate was used as an accelerator. Specimens were cured in two kinds of heated molds (one jacketed and the other electrically heated), and temperature profiles recorded. The temperature increase at the midplane showed a maximum when plotted as a function of the reaction rate (Arrhenius preexponential factor), in agreement with previous theoretical predictions. The temperature increase was enhanced for a thermally initiated reaction.
A theoretical analysis of non‐isothermal flow in wire‐coating co‐extrusion dies
Authors: Basu Santanu 
Published: December 2004

AbstractA theoretical study of non‐isothermal superimposed flow of two polymer melts in wire coating co‐extrusion dies has been carried out. Numerical methods have been employed to solve the coupled momentum‐ and energy‐balance equations. Various combinations of three polymers—namely, high density polyethylene (HDPE), polystyrene (PS) and low density polyethylene (LDPE) have been studied and least squares curve fitted quadratic polynomials have been used for constitutive equations for all three polymers in non‐Newtonian high shear rate regions. A multitude of thermal and mechanical boundary conditions can be treated by this algorithm. It was found that temperature and velocity profiles in the die depend significantly on the arrangement of the polymers. Maximum temperature rise has been noted to increase sharply with wire velocity but it can be reduced by increasing the die radius. When the thickness of the outer layer is increased from zero, the shear stress at the wall undergoes a dramatic change (if the viscosities of the polymers are different) at small values of the flow rate ratio and it reaches an asymptotic value at large values of flow rate ratio. It was also found that viscosity ratio at the interface can be reduced by changing the initial temperatures of the liquids. It was observed in some cases that large errors in the calculation of rheological and thermal variables for this problem can be made if temperature rise due to viscous dissipation is not considered.
Optimal thermal design of compression molds for chopped‐fiber composites
Authors: Barone M. R. , Caulk D. A. 
Published: December 2004

AbstractBecause heat is convected by the motion of material in the cavity of a compression mold, the time‐averaged heating load on the cavity surface is nonuniform. In rapid production of large, thin parts, this can lead to large variations in cavity surface temperature when the mold is heated by the usual uniform distribution of heating lines. In this paper, a new method is developed for optimizing the mold heating design so that this nonuniform heating requirement can be satisfied with a minimum variation in cavity surface temperature. Oil heating is considered specifically, but the method can also be used for stream or electric heat. The optimal position and power supply for each heating line in the mold is determined by combining mathematical programming techniques with an analysis of the steady temperature field in the mold. The nonuniform heating load on the cavity surface is represented by a time‐averaged steady heat transfer coefficient calculated from the transient temperature distribution in a polyester sheet molding compound as it fills the mold cavity. The design method is applied to an example mold for a large flat panel. At a one‐minute cycle, the optimal heating design dramatically reduces nonuniformity in cavity surface temperature compared with a conventional distribution of heating lines. The optimal design is remarkably simple, uses only conventional equipment, and involves only half the customary number of heating lines. Nevertheless, it still has sufficient flexibility to adjust for changes in cycle time without sacrificing uniformity in cavity surface temperature.
Non‐equilibrium processes in poly(vinyl chloride) glasses vitrified at elevated pressures
Authors: Prest W. M. , O'Reilly J. M. , Roberts F. J. , Mosher R. A. 
Published: December 2004

AbstractThe conformational and enthalpic changes that occur in poly(vinyl chloride) (PVC) glasses that have been vitrified from the melt under pressure have been examined by Fourier transform infrared spectroscopy and quantitative differential scanning calorimetry. It is shown that these pressures freeze in the high energy states that are characteristic of the vitrification temperature and increase the apparent glass transition temperature of the polymer. In addition, pressures in excess of the vitrification pressure, cause intermolecular effects that can be relaxed out below Tg. Both of these processes create characteristic endothermic and exothermic changes in the apparent heat capacity of the glass that appear over a period of time and are sensitive functions of the glass formation processes as well as the subsequent annealing history. The endothermic events are interpreted as the stress perturbed volumetric relaxation process white the exotherms are associated withh the release of the frozen in stresses.
New model of curing process kinetics of diurethane crosslinked natural rubber
Authors: Jurkowski B. , Kubis J. , Jȩdraszak A. 
Published: December 2004

AbstractA new model for the curing kinetics of diurethane crosslinked natural rubber is elaborated. On the basis of rheometric data, calculation of rate constants of both formation and degradation of the following bond types: urea‐urea, urethane‐urethane, and urea‐urethane, was enabled and thermal activation energies of these processes were determined. Activation energy values for formation of the bonds above are: 61.5 kJ/ mole, 187.6 kJ/mole, and 28.9 kJ/mole, respectively. For degradation, activation energies are: 293.9 kJ/mole, 195.5 kJ/mole, and 163.3 kJ/mole, respectively. An algorithm for carrying out of these calculations on a computer was developed.
Charpy impact fracture testing—a precise measure of ductile‐brittle transition in a filled polymeric system
Authors: Cahill R. W. 
Published: December 2004

AbstractThe fracture toughness of a filled nitrile rubber/phenolic polymeric system was assessed by means of ductile‐to‐brittle transition temperature (DBTT) as measured by the Charpy impact method to search for sampling location dependencies of toughness in structures. This investigation established the relationship between the Charpy DBTT and exposure to selected relative humidity (RH) for the filled polymer from the three lots. Charpy fracture tests of specimens subjected to a low RH environment (15 percent) had higher DBTTs than their counterparts exposed to a high RH condition (70 to 75 percent). Charpy specimens of this material exhibited a 9 to 22°F change in DBTT which depended upon the manufacturer and the conditioning period for a corresponding change of 60 percent RH. The change in DBTT with relative humidity was found to be reversible; initial fracture behavior could be restored to samples from the low RH (15 percent) station by equilibrating at a high RH (75 percent) level.