bluuebunny/crossref_metadata_2025_split huggingface dataset:
A mechanism for extrusion instabilities in polymer melts
Authors: Fyrillas M. , Georgiou G. , Vlassopoulos D. , Hatzikiriakos S. 
Published: April 2004

AbstractA mechanism for explaining some of the instabilities observed during the extrusion of polymer melts is further explored. This is based on the combination of non‐monotonic slip and elasticity, which permits the existence of periodic solutions in viscometric flows. The time‐dependent, incompressible, one‐dimensional plane Poiseuille flow of an Oldroyd‐B fluid with slip along the wall is studied using a non‐monotonic slip equation relating the shear stress to the velocity at the wall. The stability of the steady‐state solutions to one‐dimensional perturbations at fixed volumetric flow rateis analyzed by means of a linear stability analysis and finite element calculations. Self‐sustained periodic oscillations of the pressure gradient are obtained when an unstable steady‐state is perturbed, in direct analogy with experimental observations.
A study of cell nucleation in the extrusion of polypropylene foams
Authors: Park Chul B. , Cheung Lewis K. 
Published: April 2004

AbstractAn investigation has been performed of the cell nucleation and initial growth behaviors in the foam processing of polypropylene (PP) in both the linear and branched forms. These materials were foamed in extrusion with the two blowing agents, CO2 and isopentane. The cell density generally increased with an increased content of the blowing agent, for both CO2 and isopentane. The effect of processing pressure on the cell density was distinct when CO2 was used, whereas no pressure effect was observed in the foam processing with isopentane. The cell morphologies for the two PPs were found to be significantly different. A slightly lower nuclei density was observed in the branched PP foams than in the linear PP foams. However, the phenomenon of cell coalescence was observed much less in the branched PP foams. Most cells in the branched PP foams were closed, whereas in the linear PP foams they were connected to each other. The experimental results indicated that the branched structure played an important role in determining the cell morphologies through its effects on the melt strength and/or melt elasticity.
A comparison between apparent viscosity and dynamic complex viscosity for polypropylene/maleated polypropylene blends
Authors: Li Shucai , Järvelä Pentti K. , Järvelä Pirkko A. 
Published: April 2004

AbstractApparent shear and dynamic complex viscosity measurements at 200°C and 230°C, for a series of blends of a homopolypropylene (PPVC) with maleic anhydride polypropylene (PP‐g‐MA) and blends of an ethylene‐propylene copolymer (PPSC) with PP‐g‐MA, are reported. The data were taken using a capillary rheometer and a dynamic mechanical thermal analyzer. The results are analyzed in terms of the Cox‐Merz rule. Data taken at 230°C for both matrices (viz. PPVC and PPSC) follow the Cox‐Merz rule. However, deviations are noted in the blends, especially in compositions wherein PP‐g‐MA content is above 10 wt%. Cross phase separation and charges in the dispersed phase size (of PP‐g‐MA) in the two types of measurement appear to be the cause of the failure of the Cox‐Merz rule.
Hybrid effects on mechanical properties of polyoxymethylene
Authors: Hashemi S. , Elmes P. , Sandford S. 
Published: April 2004

AbstractThe present study investigated fracture and various mechanical properties of polyoxymethylene (POM) hybrids in tension and in flexure. The hybrids examined consisted of short glass fibers (GF) and spherical glass beads (GB). Comparisons are made between experimentally observed values and predictions based on the rule‐of‐hybrid mixtures for hybrid strength, modulus, impact strength, fracture toughness, and strain energy release rates. Results indicated that tensile strength, flexural modulus, and fracture toughness of POM/GB/GF hybrid composites can be estimated from the following rule‐of‐hybrid mixtureswherePPOM/GBandPPOM/GFare the measured properties of the POM/GB and POM/GF composites, and χPOM/GBand χPOM/GFare the hybrid ratio (by volume) of the glass bead and that of glass fiber, respectively. In view of this, none of the aforementioned properties show any signs of a hybrid effect. Flexural strengths, impact strengths, and strain energy release rate all showed the existence of a negative hybrid effect where negative deviation from the rule‐of‐mixtures behavior was observed. The latter was closer to the estimation based on the inverse rule‐of mixtures.
Synthesis and mechanical properties of poly(styrene‐b‐lsobutylene‐b‐styrene) block copolymer lonomers
Authors: Storey Robson F. , Chisholm Bret J. , Lee Youngkwan 
Published: April 2004

AbstractLinear and three‐arm star poly(styrene‐b‐isobutylene‐b‐styrene) (PS‐PIB‐PS) block copolymer ionomers possessing various counterions and levels of sulfonation were synthesized by sulfonating the polystyrene blocks of PS‐PIB‐PS block copolymers. Analysis of compression‐molded films of the ionomers showed that the incorporation of sulfonate groups into the polystyrene blocks of these materials resulted in an increase in tensile strength, a decrease in elongation at break, and a persistence of elastic properties to much higher temperatures as compared with unsulfonated precursors. Among all counterions studied, zinc resulted in the strongest ionomers and potassium yielded the most easily processed ionomers. Dynamic mechanical analysis showed that the block copolymer ionomers possessed a phase‐separated morphology; however, anomalous relaxations observed during the first heating cycle, but that were substantially reduced or completely absent in subsequent cycles, implied that strong ionic interactions were causing reduced processability and the formation of nonequilibrium morphologies. The observed relaxations were interpreted to be domain rearrangements brought about by the thermal energy supplied by the dynamic experiment. Annealing of films of relatively low ionic contents yielded viscoelastic behavior that was consistent with an equilibrium morphology characterized by phase‐separated, partially sulfonated polystyrene domains of the same density and size as the polystyrene domains of the unsulfonated precursor. Compression molded films of high ionic content yielded a higher rubbery plateau modulus than the unsulfonated precursor, suggesting a different morphology. Solution‐cast films of zinc ionomers exhibited two values for the rubbery modulus, a higher values at temperature below the Tg of polystyrene and a lower value at temperature above the Tg of polystyrene. Thermogravimetric analysis revealed the major mass‐loss process of the parent, linear block copolymer at 417°C (mid‐point) and of the tetramethyl ammonium ionomer at 431°C.
Fracture and impact strength of poly(vinyl chloride)/methyl methacrylate/butadiene/styrene polymer blends
Authors: Takaki Akira , Yasui Hideo , Narisawa Ikuo 
Published: April 2004

AbstractThe Izod impact strength of poly(vinyl chloride)/methyl methacrylate/butadiene/styrene(PVC/MBS)polymer blends can be changed significantly with different levels of MBS and/or MBS particle size. The following results were obtained by investigating the fracture of PVC/MBS test specimens: (1) The dependence of the Izod impact strength of PVC/MBS blend on MBS particle size confirms a maximum around a MBS particle size of 2000 Å. When MBS particle size is smaller than 2000 Å, the Izod impact strength increases with MBS particle size, and crazing occurs mainly in this region. When MBS particle size is larger than 2000 Å, then the Izod impact strength, in contrast, decreases with increasing MBS particle size, and both crazing and shear yielding occur, mainly in this region. (2) Tensile experiments of PVC/MBS blends carried out under various conditions showed that the amount of energy absorption increases with decreasing MBS inter‐particle distance and with increasing MBS particle size when crazing is the main energy absorbing mode. The MBS inter‐particle distance dominates the energy absorption when shear yielding is the main energy absorbing mode. (3) Therefore, the Izod impact strength of PVC/MBS blends and the maximum around a MBS particle size of 2000 Å can be explained as follows: Below 2000 Å, the energy absorption by crazing dominates the total energy absorption, and the energy absorption by crazing increases with MBS particle size. Above 2000 Å, the energy absorption by shear yielding is dominant, and the energy absorption by shear yielding increases with decreasing inter‐particle distance, that is to say, decreasing MBS particle size.
Ultimate properties of polycarbonate blends: Effects of inclusion plastic deformation and of matrix phase continuity
Authors: Kolar̆ík Jan , Lednický Frantis̆ek , Locati Giancarlo , Fambri Luca 
Published: April 2004

AbstractTensile energy to break (TEB) of polycarbonate (PC) blends has been increased by exploiting (i) the plastic deformation of dispersed or co‐continuous brittle polymer(s) and (ii) higher matrix phase continuity in ternary blends with two dispersed minority components. Monotonic dependence of yield strength on blend composition and micrographs of drawn specimens concurrently evidence a strong interfacial adhesion sufficient for transmission of yield and/or break stress. Good adhesion at created interfaces, was achieved through partial miscibility of PC with other components, i.e., poly(styrene‐co‐acrylonitrile) (PSAN) and poly(styrene‐co‐methyl methacrylate) (PSMMA), which was evaluated from the shifts of the glass transition temperatures of conjugate phases. Binary and ternary blends with PC contents higher than 35% show extensive plastic deformation after yielding. TEB and strain‐at‐break of ternary blends with 50% or 40% PC pass through a maximum at compositions with equal fractions of PSAN and PSMMA, which is in qualitative accord with model calculations of the matrix phase continuity. Thus, the blend PC/PSAN/PSMMA = 40/30/30 preserves 70% of TEB of neat PC.
Mechanical behavior of CaCO3 particulate‐filled β‐crystalline phase polypropylene composites
Authors: Tjong S. C. , Li R. K. Y. , Cheung T. 
Published: April 2004

Abstractβ‐crystalline phase polypropylene (PP) composites containing 5, 10, 20, 30, and 40% (by weight) of CaCO3 filler were prepared by injection molding. The β‐form PP was produced by adding a bicomponent β‐nucleator consisting of equal amounts of pimelic acid and calcium stearate. The morphology, static tensile, and impact properties of these composites were investigated in this study. Scanning electron microscopy (SEM) observations revealed that the β‐spherulites of the polymer matrix of the composites exhibit curved lamellae and sheaf‐like structures. The fillers were observed to disperse within the inter‐lamellar spacings of the β‐PP composite containing 10% calcium carbonate addition. However, the filler particles tend to link together to form larger aggregates when the filler content reaches 20%. Static tensile measurements showed that the elastic modulus of the composites increases with increasing filler content but the yield strength decreases with increasing filler addition. The falling weight Charpy impact test indicated that the β‐PP polymer exhibits the highest critical strain energy release rate (Gc) value. However, there was a drastic drop in Gc of the β‐PP composites with increasing filler content. The results are discussed and explained in terms of materials morphology.
A nonlinear regression approach to model the physical aging of poly(ether imide)
Authors: Woo E. M. , Kuo Sen‐Maw 
Published: April 2004

AbstractDifferential scanning calorimetry (DSC) was used to measure the relaxation enthalpy of glassy amorphous poly(ether imide) (PEI) at three high isothermal aging temperatures between 190 and 200°C. A three‐parameter model based on the Williams‐Watts relaxation function was utilized to describe the aging process of PEI, and nonlinear regression approach was used to estimate the model parameters. The prediction according to this model was found to be more accurate in comparison with the two‐parameter model based on the same Williams‐Watts relaxation function. Furthermore, the peculiar aging behavior at the highest aging temperature of 200°C was explained from the molecular view point of transition from sub‐Tg β‐relaxation to α‐relaxation near Tg.
Wall thickness distribution in plug‐assist vacuum formed strawberry containers
Authors: Aroujalian A. , Ngadi M. O. , Emond J‐P. 
Published: April 2004

AbstractAn experimental study on the influence of processing parameters namely film temperature, plug velocity, and temperature on wall thickness distribution in plug‐assist vacuum thermoformed fresh strawberry container using high‐impact polystyrene is presented. Film temperatures of 118, 125, 136, 150, and 165°C, plug velocities of 0.15, 0.20, and 0.27 m/s, and plug temperatures of 25, 60, 100, 123, and 135°C were used in the thermoforming. Increasing the plug velocity resulted in improved wall thickness distribution due to elastic deformation of the plastic sheet during thermoforming. Decreasing the stretching time and the temperature difference between the plastic film and plug was important for good wall thickness distribution. Better wall thickness was obtained at the plug velocity of 0.27 m/s and the plug temperature of 123°C.
Optimization of the coathanger manifold via computer simulation and an orthogonal array method
Authors: Chen C. , Jen P. , Lai F. S. 
Published: April 2004

AbstractA well‐designed coathanger die may deliver a polymer melt through the die more uniformly. However, it is difficult to optimize the manifold profile because of the complexity of flow distribution with regard to problems of die geometry, temperature, shear, and viscoelastic effects. The empirical methods have been widely used to design the manifold of a flat slit die. But this approach is time consuming and causes die material waste due to the iterative design process. With computer simulation, die designers can repeat modifications to obtain the optimal manifold shape easily. In this research, the Taguchi method was used to investigate the influences of the materials, die geometry, and processing conditions on optimizing the manifold profile. The comparison of the thickness distributions with and without modified manifold showed that the modified manifold improves the thickness uniformity significantly.
Interaction characterization of PA1010/PP blends using PP‐g‐AA as compatibilizer
Authors: Zhang Xiao‐Min , Yin Jing‐Hua 
Published: April 2004

AbstractThe morphology of polyamide 1010/polypropylene blends was found to significantly depend upon the concentration of the compatibilizer[polypropylene‐grafted‐acrylic acid (PP‐g‐AA)]. A significant reduction in phase size was observed because of the interaction that existed between the PP‐g‐AA and polyamide. These interactions have been confirmed by several methods. The tensile mechanical properties and impact behavior of the prepared blends were investigated and correlated with scanning electron microscope (SEM) analysis of the fracture surfaces. It was found that PP‐g‐AA as the compatibilizer has a profound effect upon the properties of the blends. This behavior is attributed to a series of chemical and physico‐chemical interactions taking place between the two components.
Incorporation of density relaxation in the analysis of residual stresses in molded parts
Authors: Ghoneim H. , Hieber C. A. 
Published: April 2004

AbstractThermo‐rheologically/piezo‐rheologically simple viscoelastic constitutive equations are adopted for the material behavior of a generic polystyrene, in both the deviatoric and dilatational domains, in order to investigate the effect of density relaxation on the development of the thermal residual stresses in a thin injectionmolded strip. A preliminary study is undertaken to assess the ability of the proposed dilatational viscoelastic constitutive equations to capture some of the density‐relaxation behavior such as the isobaric volume relaxation following a sudden quench from above the glass‐transition temperature and upon constant rates of cooling at different temperatures and pressures. In this preliminary study, different combinations of relaxation functions and shift factors are investigated. An appropriate combination is selected and used for the residual‐stress analysis. The numerical simulation of the development of the stresses in a one‐dimensional cavity qualitatively predicts the correct stress profile across the thickness of the molded part, as well as the dependency of this profile on some of the material properties and molding conditions. In general, the investigation presented in this paper suggests that density relaxation plays an important role in the development of residual stresses in molded parts.
Morphology and mechanical properties of injection molded articles with weld‐lines
Authors: Kim Jin Kon , Song Ju Ho , Chung S. T. , Kwon T. H. 
Published: April 2004

AbstractThe effect of weld‐lines on the morphology and mechanical properties of injection molded articles made of neat poly(butylene terephthalate) (PBT) and glass fiber‐reinforced PBT was investigated. The weld‐line was introduced to a molded article by using a rectangularly shaped insert inside a mold cavity, and tensile specimens were prepared at various positions through the entire molded article. The weld‐line position was further checked by a short‐shot experiment. Although the maximum tensile stress for specimens of neat PBT with a weld‐line is almost the same as that without a weld‐line, the maximum tensile stress and the elongation at break for fiber‐reinforced PBT with a weld‐line were found to be about half of those without the weld‐line. This is attributed to the fact that the fibers near the weld‐lines are oriented parallel to the weld‐line direction (or perpendicular to the tensile force direction) due to stretching flow. Finally, we compared experimental results of flow pattern and fiber orientations with numerical simulations. We found that the predictions of flow fronts and fiber orientations are in good agreement with experimental results.
The effect of block structure of an ethylene terephthalate/hydroxybenzoate copolymer on its ability to form compatible blends
Authors: Stachowski M. J. , Dibenedetto A. T. 
Published: April 2004

AbstractA thermotropic liquid crystalline polymer (TLCP) containing aliphatic linkages exhibits properties of both a random‐coil and a rigid‐rod. By varying its block structure, one can change its ability to form compatible blends with other polymers. In this work, the ability of the TLCP p(ET/HBA60) with a high degree of block structure, X‐7GTM, to form compatible blends with polycarbonate and aromatic TLCPs is compared to that of a p(ET/HBA60) with a more random molecular structure, RodrunTM LC‐3000. It was found that both materials formed compatible blends through transesterification of the block structure and that a higher degree of blockiness enhanced the rate of transesterification. NMR spectra and differential thermal analysis were used to quantify the degree of transesterification and scanning electron microscopy was utilized to examine the effects of the blending process on the degree of compatibilization and the morphology of the resulting blends.
Stress relaxation of elongated strip of poly(vinylidene fluoride) in ethyl acetate vapor
Authors: Phillips J. C. 
Published: April 2004

AbstractPoly(vinylidene fluoride) films in ethyl acetate vapor were studied at 30°C for vapor pressures of p = 0, 12, 30, 40, 66, 85 torr and elongations ε = 4.5%, 7%, 9.5%, 19%, 29%, and 44%. A cyclic experiment was also performed at ε = 7% and p = 40 torr for three sorption/desorption cycles. Assuming, as a first approximation, that the stress relaxation of the “dry” and “wet” polymers is proportional to the elastic strain, εel, empirical calculations were performed and compared to experimental results. In general, the presence of a vapor or gas in a polymer matrix enhances the stress relaxation by softening or plasticizing the polymer and transforming a portion of the elastic strain, εel, into the plastic strain, εpl. As the transformation continues, the sorption and stress of the wetted elongated sample change simultaneously with time until an optical “overshoot” and a mechanical or stress “undershoot” is observed. This result seems to be the consequence of the differential change of the stress of the “dry” and “wetted” samples with sorption time, τ = t − to(t = time; to = initial sorption time), which depends on the differential time dependencies of the transformations of the elastic and plastic strains.
A fracture mechanics approach to weld‐line fracture in an amorphous cellulose acetate propionate
Authors: Pecorini Thomas J. 
Published: April 2004

AbstractThe fracture properties of molded articles containing weld‐lines are analyzed by a fracture mechanics approach, treating the weld‐line as a sharp crack. The theoretical length of the weld‐line crack is estimated from diffusion properties and a simple model of cooling in a mold. These estimated weld‐line crack lengths are found to be in excellent agreement with weld‐line crack lengths calculated from measured fracture properties. Above a critical melt temperature, this weld‐line crack length will fall below a critical crack length, whereby the weld‐line will no longer initiate brittle fracture. This critical crack length is a function of the material's inherent yield strength and fracture toughness. The embrittling effect of weld‐lines can also be easily removed by surface treatments such as surface milling or solvent polishing. Weld‐lines are, therefore, determined to be near‐surface flaws, which embrittle notch sensitive materials such as most amorphous polymers.
Residual stress distribution modification caused by weathering in polypropylene and polystyrene
Authors: Tong Li , White J. R. 
Published: April 2004

AbstractA study has been made of changes in residual stress distributions caused by weathering in polypropylene and polystyrene. Chemi‐crystallization has a major effect in polypropylene and an analysis based on the volumetric changes that occur on crystallization has been developed. Close to the surface, fractional crystallinity changes up to 4% are caused by photodegradation (X‐ray measurements by Rabello and White (18)). It is estimated that this would cause a tensile residual stress of ∼2MNm−2 to form at the surface if there were no preexisting residual stresses; in the case examined here, the effect of this shrinkage stress was to reduce the compressive residual stress to a small value (<1MNm−2). Additional changes caused by stress relaxation prior to completion of chemi‐crystallization resulted in net tensile stresses near to the surface of the photo‐degraded polypropylene. The changes occur almost symmetrically in polypropylene even if the molding is exposed on one surface only. A similar analysis has been made for thermoplastics in which the changes occur only at the exposed surface, comparing the results with measurements made on a glass fiber reinforced grade of polypropylene. In this case the analysis predicts that the stress changes by 2–3MNm−2 near the surface, enough to develop tensile stresses up to 2MNm−2 there if the compressive residual stress at the beginning is fairly small, in fairly good agreement with observed changes. Glass fiber reinforced polypropylene does not relax as readily as unfilled polypropylene and better agreement is to be expected without any allowance for stress relaxation. The analysis for one‐sided chemi‐crystallization allows calculation of the resulting distortion in terms of the curvature: this was estimated at 0.33m−1, compared to the measured value of 0.44 m−1. Volumetric changes also occur in noncrystalline polymers and a similar analysis based on data obtained with polystyrene (17) confirms that these changes can explain the observed development of tensile residual stresses on weathering. In the example studied here surface stress changes of 2–3MNm−2 are predicted and this accounts for a large proportion of the change in residual stress obtained by direct measurement.