Volume 28, Issue 5

A statistical experimental design method known as the Taguchi method was utilized to optimize the injection molding processes of poly(butylene terephthalate) (PBT) and poly(trimethylene terephthalate) (PTT) blends. Impact strength was taken as the optimized property. The significant parameters included mold temperature, injection pressure, holding pressure, injection time and holding temperature. Results of the Taguchi analysis gave mold temperatures as major influencing factor on the impact strength. The optimal processing conditions were determined through the Taguchi method giving an increase of 13.7% in impact strength for the blend. Further analysis was done to distinguish the blends dependency on temperature. Differential scanning calorimetry curves indicated the presence of recrystallization peaks that were dependent on the temperature profile the sample had received prior to testing. Polarized optical microscopy was used to show the different sphereulitic growth patterns under varying isothermal conditions. It was seen that at 90°C sphereulitic growth contained pockets of different sized spereulites. AFM imaging was also used to indicate differences in blended polymer morphology.

In analytical analyses for single-screw extruders the simplified approach of rotating the barrel and keeping the screw fixed is often used instead of rotating the screw and fixing the barrel. Although the flow field is independent of the reference frame, as has already been shown to a satisfactory degree (Rauwendaal et al., 1997), the question of the dependence of the melt temperature rise on the reference frame is still being challenged, e.g. Campbell et al. (2001, 2008). In this work we develop a finite-volume CFD code allowing for the three-dimensional simulation of the flow and temperature rise in both reference frames. The question of frame invariance is addressed by simulating the flow of a Newtonian-like polycarbonate both in a two-dimensional cross-section of a single-screw extruder and in a three-dimensional model with two full turns of the screw. Our results show that the kinematics and the melt temperature rise are equal for screw- and barrel-rotation and thus independent of the reference frame. Furthermore, the presence of a clearance flow has a negligible influence on the temperature rise.

The shear viscosity and normal stress difference of two filled and two unfilled thermotropic liquid crystal polymer (TLCPs) were studied. The rigid and rod like molecules of TLCPs orientate differently at different shear rates. Under low shear rate, the molecules tend to align in the direction of the flow but also tumble and wagging on their own axis. The abnormal orientation of the molecules also depends upon temperature, fillers contents, aspect ratio and elastic nature of LCP molecules. These behaviors lead to unusual rheological properties of LCPs, such as negative first normal stress difference for filled LCPs at low shear rates. But with the increase of shear rate, the molecules are oriented in the direction of flow, which lead to isotropic flow at high shear rates. The complicated rheological properties and characteristic anisotropic properties of LCPs are modelled by recently developed Leonov's viscoselastic constitutive equations. Simulation has been carried out using Mathematica software and the characteristic anisotropic properties of LCPs have been identified. The experimentally measured viscosities at high shear rate have been compared with model predictions. Moreover, the normal stress differences using at high shear rates have been estimated using Leonov's model, which is experimentally not accessible.

Nanocomposites of Poly(butylene succinate) (PBS) and poly(∊-caprolactone) (PCL)-coated nano-TiO 2 (C–TiO 2 ) are prepared by melt-blending technique. The influence of high loading C–TiO 2 on the microstructure, mechanical, thermal and dynamic rheological properties of pure PBS and its nanocomposites are studied. Scanning Electron Microscope results demonstrate that the surface modification and content of TiO 2 are main factors to affect the microstructure of composites. Mechanical tests show that adding C–TiO 2 into PBS matrix is an effective way to improve the impact strength of the PBS. Impact property and tensile property are sensitive to different microstructure, the former is distance between particles and the latter is the aggregation of particles. Dynamic rheological tests show that the incorporation of C–TiO 2 particles into PBS can improve the elastic properties of PBS nanocomposites and restrict the mobility of PBS chains. Differential scanning calorimeter indicates the addition of C–TiO 2 can increase the crystallization rate but do not influence the main PBS crystals. Depending on the state of composites, the C–TiO 2 restricts or improves the mobility of PBS molecules.

Bamboo fibers (BFs) coated with titanium urushiol (BFTiU) was prepared by in situ reaction. Effects of activation time, urushiol concentration, reaction time and butyl titanate concentration etc on the coating ratio of BFTiU were investigated. On the other hand, the BFTiU was characterized by FTIR measurements, scanning electron microscope, thermo gravimetric analysis and differential scanning calorimetry. The results showed that titanium urushiol compound was coated on the BFs by coordination reaction and the coating ratio of BFTiU reached 35% at the optimal preparation condition. It also could be found that BFTiU possessed better heat resistant properties than BFs. Due to titanium urushiol coated on BFs, the elongation at break, the maximum deflection and the bending strength of BFTiU/PP composites were 33%, 30 % and 10% higher than those of BP/PP composites respectively. Particularly, the tensile strength of BFTiU/PP composites was better than that of BP/PP composites at the presence of 70% BFTiU.

Polypropylene block-copolymer (PP-B) pipes were prepared using a self-designed rotational extrusion processing system. The experimental results showed that though the crystal morphology in the prepared PP-B pipes did not change compared to conventional extrusion, the molecular chains in amorphous region apparently deviated from the axial direction. As a result, in comparison with the conventional extrusion pipes, the mechanical properties in hoop direction and the resistance to crack growth of rotation extrusion PP-B pipes were greatly improved. Furthermore, both the two mechanical properties improved monotonously with increasing mandrel rotation speed and inner wall cooling rate.

In order to improve the compatibility of bisphenol F epoxy resin with the liquid oxygen, the hexabromocyclododecane and the antimony trioxide were added to bisphenol F epoxy resin, and then cured resin was mechanically impacted according to ASTM D-2512-95. The microstructure and the surface element compositions of the specimen before and after the mechanical impact were investigated and analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. These results indicated that the addition of the antimony trioxide was favorable to help the hexabromocyclododecane to improve the compatibility of the bisphenol F epoxy resin with liquid oxygen. The liquid oxygen compatibility mechanism and the synergistic effect of the hexabromocyclododecane and antimony trioxide on the improvement of compatibility of bisphenol F epoxy resin with liquid oxygen were analyzed in detail.

Pressure variation during the coextrusion of two low density polyethylene melts was investigated. Melt streams were delivered to a die from two separate extruders to converge in a 30°° geometry to form a two layer extrudate. Melt flow in the confluent region and die land to the die exit was observed through side windows of a visualisation cell. Stream velocity ratio was varied by control of extruder screw speeds. Layer thickness ratios producing wave type interfacial instability were quantified for each melt coextruded on itself and for the combined melts. Stream pressures and screw speeds were monitored and analysed. Wave type interfacial instability was present during the processing of the melts at specific, repeatable, stream layer ratios. Increased melt elasticity appeared to promote this type of instability. Analysis of process data indicates little correlation between perturbations in extruder screw speeds and stream pressures. The analysis did however show covariance between the individual stream pressure perturbations. Interestingly there was significant correlation even when interfacial instability was not present. We conclude that naturally occurring variation in extruder screw speeds do not perturb stream pressures and, more importantly, natural perturbations in stream pressures do not promote interfacial instability.

Water-assisted injection molding (WAIM) has been widely used for tubular plastic parts due to its advantages of relatively low cost and fast cycling time. However, the non-uniform distribution of the wall thickness, especially at the sharp corner, is still a basic problem in the WAIM process. This work presents the effects of sharp corner angles on wall thickness distribution in sections near corners for various processing conditions of the WAIM process, including melt temperature, mold temperature, water delay time, water holding time, and holding pressure. Three grades of polypropylene (PP) resins with different melt flow indices were studied using seven mold geometries that varied the angle of the sharp corner section. The wall thickness distribution at the corner sections were characterized in terms of inner and outer residual wall thicknesses, hollow core ratio, and the percentage of difference between the inner and outer wall thicknesses. In addition, computational fluid dynamic simulations with Moldflow Plastics Insight version 4.1 were performed for each sharp corner angle. It was found that the wall thickness distribution of the straight tube was more uniform than those of the curve tubes. Water injection delay time and water pressure were the major parameters that had a significant impact on the hollowed core ratios, while the percent difference between inner and outer wall thicknesses was mainly influenced by melt temperature.

A recyclable plastics waste stream of electrical and electronic equipment has previously been found to contain acrylonitrile-butadiene-styrene copolymer (ABS, ∼40 wt%), high impact polystyrene (HIPS, ∼40 wt%), polypropylene (PP, ∼10 wt%) and a rest fraction consisting mainly of other styrene-based thermoplastics. In this work, one virgin and one recycled ternary blend consisting of these three components were melt-blended in an extruder to study the influence of processing conditions on the mechanical and thermal properties. The aim of the work has been to understand the inherent compatibility between ABS, HIPS and PP without added compatibilisers, in order to investigate the recycling potential of a real recyclable WEEE plastics fraction. Favourable processing conditions with respect to tensile properties of the virgin blend were found at intermediate screw rotations (40 to 80 min −1 ) and relatively low barrel temperatures (170 to 220°C), which can be understood from the low onset of thermo-oxidative degradation at 200°C. The recycled blend and recycled ABS, HIPS and PP showed higher stiffness and yield stress, but lower elongation at break than the corresponding virgin materials. The stiffness and yield stress of the blends were found mainly to follow the rule of mixtures of their components while the elongation at break exhibited adverse characteristics indicating incompatibility between ABS, HIPS and PP. The significant variations in the elongation at break of the blends appeared to be due to the ABS component. Differential scanning calorimetry showed an additional melt peak for the recycled blend compared to the virgin blend, otherwise the transitions were similar. The additional peak could be assigned to polyethylene in the PP component. The onset of the thermo-oxidative degradation was found to be at almost 190°C in the case of the recycled blend, which was high considering that it was close to that of the virgin blend and higher than expected from the rule of mixtures of the recycled components.

A variety of polymer parts used in microsystems technology is manufactured by injection molding processes. Particularly the high cooling velocity negatively affects the process and the resulting part properties. The scope of this paper is to investigate the influence of the mold temperature during the injection phase on the melt flow and the mold filling as well as on the resulting part properties. The results indicate that an increasing mold temperature supports the filling behavior, although the injection pressure has more impact. An increasing mold temperature also influences the part properties. It was found that a higher mold temperature leads to a more homogeneous and spherulitic structure as well as to an increasing degree of crystallinity. As a consequence the mechanical part properties are affected, too.

The effect of pressure on the viscosity of polymer melts is an often forgotten parameter due to the inherent difficulty to measure this quantity. Different experimental approaches have already been undertaken in literature in the past. A popular methodology to measure the pressure dependence of the viscosity is to use a capillary rheometer equipped with a counter pressure chamber in which the exit pressure can be controlled. In order to process the data, one of the key elements is the Bagley correction that is required to determine the correct entrance pressure at a specific shear rate. In all analysis approaches presented in literature on data at controlled exit pressure, the Bagley correction was always determined at atmospheric exit pressure, disgarding possible effects of an enhanced exit pressure. In this paper, a new analytical approach is presented that for the first time allows for a direct assessment of the entrance pressures obtained when capillary measurements are performed with controlled counter pressures. It is demonstrated, using polycarbonate, that the entrance pressure correction needed to obtain correct viscosity values under pressure is significantly different than the one needed to correct measurements performed at atmospheric exit pressure.