This article reports on the development of novel composites of starch-based polymers reinforced with hydroxylapatite (HA). Two different grades of blends of starch and ethylene vinyl alcohol copolymers were reinforced with up to 30% (wt) of sintered and nonsintered HA. The initial compounding stage was carried out either in a rotating drum or by twin-screw extrusion (TSE). Compounds were injection molded using both conventional molding and a shear-controlled orientation in injection molding (Scorim) technique. It was possible to obtain composites with very good mechanical properties, approaching those of cortical bone. The best results were achieved for 30% sintered HA composites processed by TSE and Scorim, due to the in situ formation of HA fibers, and the development of molecular orientation and more compact microstructures in the moldings. This was confirmed by SEM analysis of the fracture surfaces, x-ray diffraction, and Raman spectroscopy. The biodegradability of the composites was also assessed and was found to be faster than that of the matrix. (C) 1997 John Wiley & Sons, Inc.
Differential scanning calorimetry (DSC) was used to measure the kinetic parameters of the EPON 862/W epoxy system (Shell EPON(R) Resin DPL-862/EPON CURING AGENT(R) W/EPON CURING AGENT(R) Accelerator 537). Heat of reaction, degree of cure, pre-exponential constant, and activation energy were determined using dynamic scanning at heating rates of 5, 10, and 20 degrees C/min. Using a Brookfield DV-III rotational-type rheometer, the viscosity was measured isothermally as a function of time at temperatures of 23 degrees, 35 degrees, 45 degrees, 55 degrees, 65 degrees, 80 degrees, 90 degrees, and 100 degrees C. Experimental modeling of the rheological characteristics of the resin was conducted using expressions correlating the viscosity and the degree of cure. Viscosities calculated by the model were in good agreement with data obtained experimentally. (C) 1997 John Wiley & Sons, Inc.
Although twin screw compounding extruders have become increasingly important in recent years, single screw extruders have a number of advantages. With a better understanding of their mixing limitations and the basic requirements for achieving good dispersive and distributive mixing, single screw extruders can be used for many compounding applications. This applies particularly to products requiring good distributive mixing. (C) 1997 John Wiley & Sons, Inc.
The aim of this article is to describe the development of a mathematical model for the simulation of the flow of polymer melts inside internal mixers. The rheological behavior of the polymeric fluid is assumed to be described by the Carreau equation. The flow regime is considered to be non-isothermal. The set of the governing equations are solved using the finite element method for both steady-state and transient conditions. In the steady state case, the flow equations are solved by the penalty method using the standard Galerkin technique. Petrov-Galerkin schemes based on both consistent and inconsistent streamline upwinding methods are employed to solve the energy equation and the obtained results are compared. Transient velocity, pressure, and stress fields are modeled using implicit theta method. Ln addition to implicit theta method, two versions of the Taylor-Galerkin approach are used to solve the transient energy equation. Slip-stick on the solid walls, encountered in the flow of viscous fluids, is incorporated in the model by the use of Navier's slip conditions. We describe two new methods for the inclusion of this condition in the working equation. Our simulations yield the velocity field, distribution of pressure, stress and temperature in the steady state and the variations of these parameters with respect to time under transient conditions. As an example of the applicability of the developed model, a typical mixing problem which involves convection of carbon black with flowing rubber in a domain representative of the section under the blade of a tangential rotor mixer is simulated. Concentration profiles of carbon black in the rubber matrix in this case is obtained by the solution of carbon mass continuity equation in conjunction with the flow model. This solution gives the distribution of filler volume fraction at different mixing times in the mixer. Comparison of the obtained results with the available experimental data gives some indication of the validity of the model. (C) 1997 John Wiley & Sons, Inc.
The potential of recycling polypropylene-based composites by means of a dissolution process was investigated. The composites were dissolved in an appropriate solvent, and subsequently the polymer solution, containing the fibers in suspension, was filtrated for recovery of the reinforcing agent and of the polymer matrix. Different amounts of hot solvent were employed for washing during filtration, to vary the polymer content remaining on the fibers. The recycled fibers were incorporated in the same polymer matrix, but of virgin quality. The results on the tensile and impact performance of these second generation composites were compared to those of the base material containing unused fibers. It becomes evident that a significant increase of the tensile modulus and strength occurs as the polymer phase deposited on the fibers increases. However, it appears that this interphase alteration has a negative effect on the Izod impact strength. The overall behavior is discussed in terms of fiber dispersion aspects and fiber/matrix adhesion. (C) 1997 John Wiley & Sons, Inc.
Gas-assisted injection molding can effectively produce parts free of sink marks in thick sections and free of warpage in long plates. This research investigated the basic filling phenomena in a gas-assisted injection molding process with flow visualization. A high-speed video camera was used to record the mold filling phenomena of rectangular cavities with various arrangements of gas channels. The mold-filling phenomena are compared and guidelines for layout of gas-channel ribs are drawn. (C) 1997 John Wiley & Sons, Inc.
Resistance welding has been demonstrated as a viable technique for joining thermoplastic composites. Extensive research has been conducted in previous studies on the processing and postprocessing evaluation of coupon-sized resistance welded specimens using this thermoplastic fusion bonding technique. This work focuses on the manufacturing and evaluation of large-scale resistance welds used to join carbon fiber (CF)/polyetheretherketone (PEEK) thermoplastic composite parts. Applications using the present resistance welding technology are assessed, and potential applications of resistance welding are discussed. The sequential resistance welding process is introduced as an approach to large-scale welding that increases joint quality and performance while maintaining modest pressure and weld power requirements. The single-step and alternate multiple-step (sequential) techniques are implemented for large-scale resistance welds of CF/PEEK adherends comolded with polyetherimide (PEI) (i.e., the Thermabond(R) process). Nondestructive ultrasonic evaluation, mechanical testing, fractography, and microscopy show that: (1) a higher weld uniformity was obtained with the multiple-step weld than the single-step weld; and (2) uneven heating and overheating were minimized by reducing the heating element length within the bond region through the use of the sequential resistance welding process. The different welding techniques produce a large variation in lap shear strength; direct comparison of the weld quality and lap shear strength shows that the sequential resistance welding process yields a superior bond. With the principles developed in this investigation, resistance welding call be used to join large-scale thermoplastic composite parts with consistent, high levels of performance and quality. (C) 1997 John Wiley & Sons, Inc.
In this article, three computational models have been proposed to analyze the structure of gas-assisted injection-molded parts under external loads and constraints. Through comparison of numerical experiments, model II, which treats gas-assisted injection-molded parts as thin-walled hollow structures, is demonstrated to be reasonable and efficient for analysis and simulation of such parts. Model I, which converts the parts into shell structures with equivalent reinforcing hollow beams, is shown to be acceptable only under certain restricted situations. Considered to be the most accurate approach, model III faithfully models the parts as a solid structure with a hole. However, this kind of modeling is computationally inefficient. (C) 1997 John Wiley & Sons, Inc.
Blending processes are primarily carried out in corotating intermeshing twin screw extruders. Because the knowledge about morphology development in such extruders is limited methods to obtain a desired morphology are based on empirical values and screening experiments. For the analysis of morphology evolution, a new special sampling device was developed which enables melt sampling in time periods of less than 10 seconds. With this technique integrated in an intermeshing corotating twin screw extruder, ZSK 40, it is possible to detect morphology development along the extruder screw nearly in real time. The performance of the developed method was verified by observation of morphology development in a polypropylene/polyamide-6 blend system along a screw section. A difference in morphology development caused by a variation of the screw rotational speed was observed. Furthermore, drop deformations in partially filled screw sections could be detected. (C) 1997 John Wiley & Sons, Inc.
High density polyethylene blow modling resins have been identified as a primary material for solid waste minimization and recycling. An experimental study into the effect of multiple extrusion cycles on the properties of a virgin homopolymer, virgin copolymer, natural post consumer, and mixed color post consumer blow molding resin was conducted. Rheological properties such as shear and elongational viscosity and elastic modulus were studied in the context of changes experienced during recycling. The G'-G '' (elastic storage and loss modulus) crossover point was used to measure relative changes in the polydispersity index and molecular weight distribution (MWD). It is also shown that extrudate swell and sag change after multiple extrusion passes. Environmental stress crack resistance was also measured. A rationale for the significant decrease in the environmental stress crack resistance of the virgin copolymer resin is presented. The results are analyzed in terms of known degradation mechanisms such as chain scission and crosslinking, and their relationship to the molecular structure. (C) 1997 John Wiley & Sons, Inc.
The elongation behavior of various polymer melts extruded from the single screw extruder has been studied. Low-density polyethylene, high-density polyethylene, polypropylene, and their short glass-fiber (GF)-filled composites were used. It was found that the spinning stability was good but the drawability was poor for GF-filled polymers. The elongation viscosity was measured by use of a Gottfert tensiometer (Rheotens) in the strain rate range from 0.1 to 4 s(-1). In regard to the viscosity calculations, a very convenient and straightforward method, especially for molten materials without die swell, is presented. All the sample melts showed slight tension thinning; that is, the elongation viscosity decreased along with the strain rate. It was also found that the elongation viscosity of the GF-filled polymer may increase along with the fiber content up to a critical value, and then drop if more fiber is added. The influence of the die geometry on the viscosity measurement is discussed. (C) 1997 John Wiley & Sons, Inc.
With the recent development of new catalysts based on specific metallocenes, ethylene polymers with a narrow molecular weight distribution and comonomers placed homogeneously in the polymer chain have been accessible. In particular, bridged amido complexes can be used to integrate defined long-chain branches into the macromolecule. These complexes lead to homogeneous branched polyethylenes (HBPEs). These long-chain branches provide various new properties. Polymer blends of these polymers with high-density polyethylene (HDPE) were manufactured in a twin-screw extruder and characterized by their thermal, mechanical, and processing properties. The HDPE/HBPE blends with low and high comonomer content in the HBPE have been studied over the total range of composition of the blend components, and the influence of the octene content of the HBPEs on the properties of the mixtures was shown for two different blend compositions. Furthermore, the properties of the blends for HDPE and long-chain branched HBPE were compared with HDPE blends of polyethylenes without long-chain branching but similar molecular parameters in relation to the HBPE. The differences were discussed on the basis of the density of the tie-molecules of the polymers. (C) 1997 John Wiley & Sons, Inc.
The effect of processing conditions on the performance of advanced styrenic resins (ASR) is examined. ASR systems offer a combination of high gloss and high impact, which allows them to be considered as alternatives in applications previously dominated by acrylonitrile / butadiene / styrene copolymers (ABS). Injection molding conditions were set according to the principles of statistical design of experiments. The information provided by this statistical design of experiments (DOE) method is extensive and highly organized. For the current study, a 25 full factorial model was based on five parameters at two levels of variation. With this model, the effects of injection speed, melt temperature, mold temperature, hold pressure, and cooling time on part properties were investigated. The properties measured were tensile modulus, tensile yield strength, tensile ultimate strength, and Gardner and Izod impact strengths. This study represents an initial variable screening for the establishment of a process window for this particular material. At the same time, significant process knowledge, valuable for further future optimization, was gained and structure-morphology relationships were investigated. (C) 1997 John Wiley & Sons, Inc.
A least-square finite element model was used to model the rheological behavior of both homogeneous and two-phase polymer blends, and their viscosity was calculate over a range of shear rates. The analysis was carried out for a pressure flow situation through a wide rectangular channel. Homogeneous blends were simulated by a model consisting of a large number of alternating layers of A and B components parallel to the flow direction using the adhering layers concept. Heterogeneous blends were simulated by similar alternating layers separated by very thin interlayers. In modeling the negative deviation behavior, associated with the flow characteristics of immiscible blends, the interlayers were assumed to have a viscosity more than one order of magnitude lower than either component. Viscosity values for the interlayers higher than both polymers were used, on the other hand, to represent positive deviation behavior, which is synonymous with the flow behavior of compatibilized blends. To test the accuracy of the model, the computed viscosity results were compared with the values calculated by the more widely quoted equations and also with published experimental data. (C) 1997 John Wiley & Sons, Inc.
Kinetics of reaction between ethylene-methylacrylate copolymer and poly-dimethyl siloxane rubber during melt processing has been studied from the infrared spectrophotometric data. The kinetic pattern confirms first order kinetics and the activation energy of the reaction was found to be 17 kJ/mole. (C) 1997 John Wiley & Sons Inc.
Polypropylene (PP) and Nylon6 were extruded with a single screw extruder. Nylon6 could form microfibrils in a PP matrix. When the blends were compression-molded at the temperature between the melting temperature of PP and that of Nylon6, these fibrils could be maintained in the matrix. The existence of Nylon6 microfibrils could improve the impact strength, but the tensile strength of the composites decreased. Differential scanning calorimetry (DSC) analysis show that Nylon6 microfibrils could affect the crystallization of PP slightly. (C) 1997 John Wiley & Sons, Inc.
Semirigid liquid crystalline polymers (LCPs) show some advantages with respect to rigid LCPs: in particular lower processing temperatures, and better compatibility with flexible thermoplastics, but also some disadvantages: lower mechanical properties and poor thermomechanical resistance. Both properties can be improved by adding inorganic fillers. In this work, samples of a semirigid Liquid crystalline polymer reinforced with carbon and glass fibers has been characterized. Although elastic modulus and tensile strength of the glass fiber filled LCP improve remarkably with increasing the filler content, the same properties for the carbon fiber-filled samples do not increase with increasing ti-re content of the fiber; rather a maximum tensile strength is observed at about 20% of carbon fibers. The thermomechanical and the creep behavior are also drastically improved by adding the fibers. Ln particular, the creep curve is shifted by several decades toward longer times, and the heat distortion temperature increases about 60 degrees C by adding 20% of carbon fibers. The processability is, on the other hand, significantly worsened. By comparing the results for the carbon fiber-filled samples with those obtained for the same polymer reinforced with glass fibers, it is worth noting that the properties are similar, but lower amounts of carbon fibers are needed to obtain the same mechanical and thermomechanical properties. (C) 1997 John Wiley & Sons, Inc.
The fiber degradation in the kneading disk region of a corotating twin screw extruder was studied with a closed single-stage co-rotating twin screw extruder as a model for the analysis. In a previous article, we reported that the stress in the kneading zone was proportional to the screw torque. In the present study, we measured the fiber-length distributions and the average fiber lengths under various compounding conditions. It was found that the fiber-length distributions depend upon the shear stress and the total number of rotations and that the fiber-length distributions were identical if the average fiber lengths were the same. (C) 1997 John Wiley & Sons, Inc.
An improved resin transfer molding process was implemented by the assistance of the autoclave in epoxy/carbon fiber composite manufacturing. In the autoclave-assisted resin transfer molding (ARTM) process, the mold-filling pressure and the consolidation pressure were controlled by the autoclave in a continuous fashion during the mold filling and consolidation stages. Using a smart spacer inserted between the RTM molds, the part consolidation was effectively achieved after the complete mold filling, which consequently improved processing as well as performance characteristics of advanced composite materials. Visualizing the resin-front advancement, anisotropic permeabilities of several reinforcement fabrics were collectively determined by comparing the experimental results and Darcy's law. A finite element/control volume method was used to predict the resin pressure and flow front advancement during the ARTM process. Finally, the processing variables of resin bleeding, consolidation pressure, and surface formers were investigated to identify the ARTM processing and structural characteristics in high-performance composite utilization. (C) 1997 John Wiley & Sons, Inc.