Supercritical carbon dioxide (CO2) was used in extrusion for the improvement of polymer blend properties. Various extruder configurations using a twin-screw extruder and a single-screw extruder were designed for investigating the effects of dissolved supercritical CO2 on the viscosity and morphological properties of polyethylene, polystyrene, and their blends. The viscosities of the polymer/CO2 and the blend/CO2 solutions were measured at various concentrations of CO2 rind PE/PS blending ratios using a wedge die mounted on the twin-screw extruder. The effect of CO2 on the morphology of PE/PS blends was also investigated using a twin/single-screw tandem system. This system allowed for preferential dissolution of the CO2 into the matrix and/or dispersed polymer phase. By introducing devolatilization to the tandem system, the morphological behaviors of PE/PS blends were investigated on unfoamed filaments. In general, the mixing between two polymers was improved by the dissolution of CO2. The size reduction of the dispersed phase was explained using the viscosity ratio of the two polymers. Finally, the interface between foamed and unfoamed polymers was studied in a bilayer structure produced by using a special configuration of the tandem system. (C) 2000 John Wiley & Sons, Inc.
Structure-property relationships of injection- and compression-molded microfibrillar-reinforced PET/PA-6 blends with different weight ratio of the components have been studied. The blends were prepared by melt mixing using a single-screw extruder. Thermal, X-ray (WAXS), morphological, and static mechanical studies of the bulk samples were carried out in order to determine an optimum processing window. The upper processing temperature was established by the onset of the melting of the fibrillized PET component in the blend. Morphological studies of the injection-molded samples indicate skin-core morphology. The skin layer is composed of fibrils that are well oriented along the flow axis. The core region is composed of a small amount of randomly oriented, fibrillar PET bundles, as well as a large amount of spherical PET domains in the isotropic PA-6 matrix. SEM observations of the fracture surface of compression-molded samples show well-extended PET fibrils embedded in an isotropic PA-6 matrix. The reason for this significant difference in the morphology of the samples is the fact that in the case of compression molding it is possible to keep very accurately the desired processing molding it is possible to keep very accurately the desired processing temperature in a quite narrow interval. On the other hand, an overheating of the system due to non-isothermal shear and a viscosity dissipation are the main reasons for the melting of a part of th spherical domains in the core of the ir strongly affects the tensile properties if the bulk samples. Compared to the values of injection-molded neat PA-6, injection-molded MFC (Microfibrillar reinforced composites) blends have been increased by a factor of 2.5 and 1.7, respectively. In the case of compression-molded samples, both modulus and strength were about four times higher than those of the neat PA-6. (C) 2000 John Wiley & Sons, Inc.
The changes in temperature distributions of flowing polypropylene melt in the barrel of an injection molding machine were investigated using a designed experimental rig coupled with a temperature sensing device. The main objective was to study the effect of injection speed on melt temperature measurement. In the course of the study, the temperature of the melt changed continuously with injection time, which was associated with a number of factors, such as shear heating, heat conduction, residence time, and the flows occurring in the barrel. The experimental results obtained differed (C) 2000 John Wiley & Sons, Inc.
In this study, the finite element method ws used to investigate the effects of screw speed, entering peroxide distribution, and pressure-to-drag flow ratio on the mixing characteristics of steady non-isothermal reactive flows in a forward conveying element of a self-wiping twin screw extruder. The reaction considered was the peroxide-initiated degradation of a commodity polypropylene resin. The predicted average degree-of-freedom profiles from the simulations largely confirmed to expectations. The average flow efficiencies for all runs were found to remain at values close to that for two-dimensional flow, with fluctuations being observed in the channel intermeshing regions. No significant effect of either screw speed or peroxide distribution was found on the flow efficiencies; however, the pressure-to-drag flow ratio was found to have a significant influence. (C) 2000 John Wiley & Sons, Inc.
Microwave curing has been shown to be a viable alternative to conventional thermal curing of polymers on the basis of significant reaction, rate enhancements. Proper understanding of the microwave cure reaction kinetics has been hindered by the lack of an adequate basis relating the process to conventional thermal analysis methods. The rate enhancement obtained ill microwave curing is shown to have been obtained from the decrease in the lag time prior to initiation of cross-linking, as well as, a decrease in the Effective cure time. Phenomenologically, the shortening of the cure time is no different from a shortening achieved by a higher curing temperature, thereby providing a basis for relating the microwave cure process to thermal curing. This article attempts to relate the test results for both thermal and microwave curing by obtaining a temperature equivalent value using a phenomenological logarithmic approach. Results show that the equivalent temperature can be elucidated using a logarithmic plot of the cure times and the glass transition temperature. The values of the equivalent temperature so obtained were also consistently and significantly higher than the actual sample temperature during cure, confirming microwave curing to differ in mechanism from thermal curing. (C) 2000 John Wiley & Sons, Inc.
Reactor produced blends of polypropylene (PP) and polystyrene (PS)are obtained by graft copolymerization of styrene onto polypropylene chains. This technique generates simultaneously a graft copolymer (PP-g-PS) and polystyrene homopolymer. The resulting blends, however, have a low impact resistance and have to be modified with the addition of rubbery toughening agents, such as an ethylene propylene copolymer (EPR) or a styrene-b-ethylene-alt-butylene-b-styrene (SEBS) triblock copolymer, in a downstream compounding operation. Part 1 of this work, which has been accepted for publication in the British Journal "Plastics, Rubber, and Composites", was concerned with the compatibilization efficiency and the effect of mixing intensity on the morphological and rheological properties. Part 2 of the study deals with interactions of the components of the blends. These interactions were assessed by monitoring the crystallization behavior and mechanical properties. Mixing experiments were conducted on a proprietary twin screw mixing element evaluator (TSMEE) and also on a commercial TSE-30 extruder. Differential scanning calorimetry (DSC) was used to determine the transition temperatures and the crystallinity of the blends specimens after annealing. The results show that both the free polystyrene and the graft copolymer, PP-g-PS, act as nucleators for the polypropylene phase, thus increasing both the degree of crystallinity and the crystallization temperature of the blends. While a good correlation was found between modulus and crystallinity, the factors affecting fracture behavior were less clearly discernible. (C) 2000 John Wiley & Sons, Inc.
A control volume technique based on the finite difference method is used to characterize the flow behavior in resin transfer molding (RTM) of composite structures. Resin flow through fiber mats is modeled as a two-phase flow through porous media. The transient rime terms are considered, and the concept of the fraction volume of fluid (VOF) is applied in order to accurately describe the behavior of the free boundary at the interface between the two fluids involved in the process, the resin and the air. Flow experiments in a transparent mold were performed to verify the numerical model. Experimental results on flow behavior of EPON 826 epoxy resin into irregular mold cavity with fiberglass mats agree well with the present numerical simulation. Several parametric studies using the developed model are conducted to investigate the effects of injection pressure and mold design on resin flow pattern, mold filling time, and pressure distribution inside the mold. (C) 2000 John Wiley & Sons, Inc.
Deriving maximum benefit from the incorporation of fillers into polymers depends upon achieving uniform distribution of well wet-out individual particles. At higher loadings, the best practice is to add the filler downstream in the extruder after the base resin is fully melted. (C) 2000 John Wiley & Sons, Inc.