Volume 19, Issue 3

Twin-screw extruders are often used to distribute and disperse additives into polymers. The mixing efficiency of the extruders highly depends on the geometry of the kneading blocks of the mixing section. In this paper, the impact of some geometrical parameters, such as the stagger angle and the width of the discs, are investigated by three dimensional time dependent finite element calculations. Results are obtained with the finite element software Polyflow. The robustness and the accuracy of the mesh superposition technique is evaluated. It appears that conclusions obtained by the numerical experiments can be used to improve the geometry of the kneading blocks. The mixing efficiency is evaluated by comparing the residence time and the total shear distributions of a large set of virtual particles launched in the flow domain.

Deformation and breakup mechanisms of an immiscible drop were investigated experimentally in a screw channel flow, which mimics the flow in the metering section of the single screw extruder. It was observed that the deformation/breakup mechanisms are far distinct from the well-known routes for the linear flow, mainly because of the complex flow history that can be characterized by the repetition of strong/weak shear, the continuously varying shear direction and the acceleration/deceleration. The experimental observations include: comet-like thread formation with the large bulbous head and the long narrow tail; the initiation of the breakup at the entrance of the low-shear zone; breakup by asymmetric end pinching; breakup by the growth of the capillary wave, propagating from the tail toward the head; monotonically decreasing droplet size distribution from the head to tail. The mechanism for the comet-like thread formation has been investigated by using the numerically obtained flow field.

A computer code, previously developed by the authors for the automatic die design, is used to optimise the flow distribution in a profile extrusion die using two alternative strategies: one based on length optimisation and the other on thickness optimisation. The numerical predictions are then compared with experimental data gathered during extrusion experiments. The numerical predictions and the experimental results agree within the experimental uncertainty thus showing the effectiveness of the computer code, the optimisation algorithm and the design strategies implemented. Generally speaking, measured and predicted values of pressure drop and flow rate are in good agreement (within 8 % and 6 %, respectively). It also is confirmed that optimisation based on thickness control leads to final profiles that are more prone to distortion.

The influence of sparse long chain branching on the onset and propagation of isothermal draw resonance in fiber spinning of polyethylene melts was investigated. Six polyethylene melts were used in this study: three sparsely branched metallocene polyethylenes, a linear low-density metallocene polyethylene, a conventional linear low-density polyethylene, and a conventional low-density polyethylene (LDPE). The sparsely branched metallocene polyethylenes have almost identical shear rheology and molecular weight distributions, but strain harden to different extents under extensional deformation because of slight differences in the amount of sparse long chain branching. Critical draw ratios and the ratios of minimum to maximum diameter were found to be different for each of these polyethylenes. The two linear low-density polyethylenes, which have no long chain branching, had critical draw ratios similar to those of the sparsely branched polyethylenes, but failed (necked to the point of filament breakage) during monofilament extrusion at draw ratios significantly lower than those measured for the sparsely branched polyethylenes. In contrast, the LDPE, which has the highest degree of branching and largest molecular weight distribution, had a much higher critical draw ratio than that obtained for the other five polyethylenes. These results suggest that the degree of extensional strain hardening, arising from differences in long chain branching, has a significant effect on the onset and propagation of draw resonance in isothermal fiber spinning. In the case of LLDPE, broadening the MWD seemed to affect the drawability of LLDPE, but had no effect on the critical draw ratio.

This paper addresses the development and application of a two-dimensional, three component model for the dry spinning process of polymer fibers. The model presented is an expansion of a previously developed two-component dry spinning model [1] that incorporated both viscous and viscoelastic effects in the constitutive equation for the spin fluid, along with two-dimensional effects to account for radial variations in the fiber temperature and composition profiles along the spin line. The model presented in this paper includes the effects of water in the spin dope through incorporation of the diffusion equations for ternary mass transfer. Mass and energy balances on the gas side are also taken into account so that the effects of ambient conditions can be investigated. Calculations are based on the system: cellulose acetate-acetone-water. Results are presented for profiles of solvent concentration, temperature, and glass transition temperature profiles for typical industrial operating conditions. Force and stress distributions in the fiber at various points along the spinline, analyzed in terms of contributions from the viscoelastic and viscous terms in the constitutive equation, illustrate the role of these stresses in the freezing in of structure along the spinline. Model predictions are shown to be in good agreement with the principal characteristics of dry spinning.

In this work, predictions based on the 2D dry spinning model developed in Part I of this work [2] along with a die swell subroutine supplied by an industrial company, are used to compare with fiber concentration and elongation to break data from an industrial spinline. Die swell ratio predictions agree well with measured values and solidification along the spinline is shown to be due to homogeneous glass transition in the absence of phase separation. Concentration profiles along the spinline are well fit by the model using previously determined prefactors for the diffusion coefficients. A good correlation of elongation at break data is found to occur with a characteristic variable σ GTP , the ratio of the rheological force of the viscoelastic Giesekus contribution to the total rheological force (Giesekus + viscous) at the glass transition point. Moreover, the regression obtained from this fit can be further used to make predictions for elongation at break under different operating conditions. Finally, the effects of spinning conditions and model parameters on fiber mechanical properties were investigated.

A concept of potential crosslinker was introduced into the synthesizing process of highly oil-absorptive resin which is traditionally prepared by single chemical crosslinking. The resin was dissolved in DMAc and then span into fiber. The fiber was heated after spinning to obtain three-dimensional network structure. The effects of potential crosslinker content, crosslinked time and crosslinked temperature on the absorptive properties of fiber were studied. The structure of oil-absorptive fiber was analyzed by FTIR, DMA and SEM.

In this first paper, we have revisited Avrami's model and cast its basic equations into a differential system. This system is integrated numerically, which avoids unnecessary simplifying assumptions generally used in order to get analytical expressions. This allows us to introduce the variations of nucleation and growth parameters as a function of processing ones (temperature, cooling rate, shear rate, etc.). Our analysis shows that it is necessary to take into account the variation of the initial number of potential nuclei with temperature, which was usually ignored. Finally, an outpout of our calculations is the size distribution of the morphological entities, i. e., a quantitative information on microstructure.

Crystallization of thin polypropylene films was performed in isothermal, constant cooling-rate and mixed conditions. The experiments were first analyzed using the classical procedures based on simplified forms (Avrami, Ozawa) of the general Kolmogoroff-Avrami-Evans (KAE) theory. These analyses, which can be applied over an unusually wide transformation range, show that the crystallizations are actually 2 D. Then, a procedure has been established for the determination of the nucleation and growth parameters involved in the theoretical model presented in the first paper of this series. These parameters have been introduced into the model in order to predict the crystallization behavior in isothermal, constant-cooling-rate and mixed-conditions: transformed fraction, number of activated nuclei, final size distribution of semi-crystalline entities. A very good agreement is generally found between predictions and experimental results.

In this study, extrusion blow molding processability of polypropylene/polyethylene (PP/PE) blends was related to their viscoelastic behavior and morphology under shear flow. The propylene block-copolymer (PP-B) and blends comprising the PP-B and the linear low-density polyethylene (PE), weight fraction of the PE (ϕ PE ) was up to 20 wt.%, were mainly used for this study. Viscoelastic behavior under shear flow and swelling ratio of the extruded strands were measured by using a capillary type rheometer. Morphology of the strands was observed by TEM. In case of the PP-B, swelling ratio highly depended on shear rate. Coarse aggregated droplets were observed irrespective of shear rate under the measurement. The droplets were ethylene-propylene copolymers (EP) included in the PP-B aggregated at conversing section of the capillary. However, in case of the PP-B/PE blends, swelling ratio showed lower dependence on shear rate as the ϕ PE was higher. When the shear rate was low, large droplets were observed and they were highly deformed in the flow direction. On the other hand, when shear rate was high, the droplets were smaller and showed no deformation in the flow direction because the droplets were broken up due to occurrence of high shear stress. Wall thickness of blow molded bottles produced from the PP-B/PE blends was not affected by the throughput rate. It was proportional to the die-core gap. On the other hand, wall thickness of bottles of the PP-B depended on the throughput rate and it was not proportional to the die-core gap. These results suggest that extrusion blow molding processability of polymer blends is good when the dependence of swelling ratio on shear rate is low.

In order to simulate the heat transfer phenomenon during rotational molding an alternative approach for the melt front treatment has been used. It was considered that a polymer particle adheres to the inner mould wall or to the polymer layer when the temperature reaches the melting point of polymer. The result of this alternative adhesion of new particles on molten polymer layer is the progress of melt front until all of particle are added to the inner surface of the polymer in the mold. The simulation was based on an explicit finite volumes scheme which, allows to proceed with a fixed mesh of the whole domain. A numerical test, then, has been performed to illustrate the capabilities of the proposed model. The results of this numerical test showed the temperature evolution of external mold surface, in the molten polymer layer and on the interface molten polymer layer – air/polymer mixture region. These results have been then interpreted.

During injection molding process, the surface of flowing resin in contact with the mold surface is cooled first followed by the cooling of inner part. This results in the variation of microstructure in moldings through the thickness direction. It has been reported that these phenomena result in the layered structure of injection moldings. Particularly, because of the rapid rate of crystallization, PBT injection moldings have well-defined skin and core layers. The purpose of this study is to investigate the effect of mold temperature on the development of structure and property variations of PBT injection moldings in the thickness direction. Tensile tests were performed using dumbbell specimens that were stamped out from the sliced specimens of moldings. Neck phenomenon occurred irrespective of mold temperature and distance from the surface of moldings. The start point of neck propagation shifted to higher strain with increasing distance from the surface. Tensile modulus increased with increasing distance from the surface. In order to discuss these results from a structural viewpoint, the microstructure of each layer was evaluated by density, wide-angle X-ray diffraction and Fourier transform infrared absorption measurements. The delay in necking start points with increasing distance from the surface is due to the higher crystallinity of the inner layer compared to that of the layer near the surface. Two crystal forms of PBT (α- and β-forms) were developed in injection moldings. The region near the surface, where content of α-form is larger than that of β-form, becomes thinner with increasing mold temperature. It is known that the crystal modulus of α-form is lower than that of β-form. Therefore the portion of moldings with lower tensile modulus and yield strength becomes smaller. It is suggested that the border between the skin and core layers, where tensile property changes, shifts to the surface of PBT injection moldings as mold temperature increases.

Resuming our previous work on β-nucleated polypropylenes, namely on their impact strength, our present work directs attention towards a flexural and tensile behavior of β-nucleated polypropylenes (β-iPP). For the purposes of this work, commercially available isotactic polypropylene was modified using various amounts of a specific nucleating agent based on N, N′-dicyclohexylnaphtalene-2,6-dicarboxamide (NU100). From the prepared blends test specimens were prepared by injection molding. Structural characterization (crystallinity and the content of β-phase) was carried out by X-ray diffractometry. A multipurpose mechanical property tester was used for determination of tensile and flexural properties. It was found that β-phase content decreased both tensile and flexural strength; on the other hand, the presence of β-phase dramatically raised cold drawability of the injection-molded specimens; addition of 0.03 wt.% of NU100 in the injection-molded parts led to a more than 800 % increase of the draw ratio of specimens tested at the testing speed of 100 mm/min. Furthermore, tensile testing carried out using various strain speeds (10 and 100 mm/min) revealed that higher ductility of the material containing β-phase is particularly pronounced by faster deformation processes, such as fast drawing or impact load.

The creep performance of plastics is one of the main criteria for a variety of applications which requires a strict dimensional stability, including electrical/electronic parts and various automotive applications. However, obtaining or generating accurate creep data is very difficult for both resin suppliers and product designers because the creep property is time dependent, which means it takes over several weeks or even a year of time span. Hence a faster and more reliable method predicting creep behavior of plastics is needed when designing plastic products that will encounter long-term loading. This work presents an easy method for predicting the long-term creep behavior of plastics using the short-term creep experimental data. The flexural creep experiments of three plastics, ABS (Acrylonitrile butadiene styrene), PC/ABS (PC and ABS alloy) and LFT (Long fiber reinforced thermoplastics), are performed. For a quick and reliable prediction of long-term creep behavior of plastics, time regional power-law model and time-stress modulus are introduced. To analyze the reliability and validity of the proposed method, the creep analyses based on the proposed method are performed. The comparison between the FEA results and the creep tests has shown that the new method can track both primary and secondary creep behavior over a long time. The creep analyses have been carried out by commercial FE Code.