Volume 14, Issue 3

The paper treats the case where blends of polystyrene (PS), poly (dimethylsiloxane) (PDMS) and a diblock copolymer of PS and PDMS are used as model materials. This modelsystem is predicted to be “stable” in discrete blends in simple shear flow. Stable in the sence that the block copolymer can not be removed form the interface during simple shear flow. A hypothesis stating that coalescence of drops may be subdued by “stable” diblock copolymer at the interfaces in 3D flows are tested with a Brabender mixing chamber. For well dispersed systems absence of coalescence would confine the occurrence of bicontinuous structures to a phase inversion point volume or prevent bicontinuous phase structure (IPS) formation all together. Addition of PS-b-PDMS diblock copolymer do contract the volume fraction interval where IPS are found but a relatively broad interval is still observed. The phase inversion volume predicted by theory is outside the observed IPS interval for one of the investigated blend pairs.

Several studies have shown that deformation and breakup of dispersed droplets is easier in extensional than in shear flow field and this is particularly true for systems with high viscosity ratios. The simplest way to generate a significant extensional flow field in compounding equipment is to use converging flow conditions. In this work, the mixing efficiency of converging flow has been investigated as a function of the entry profile and the flow rate by using a capillary rheometer equipped with dies of different geometries. The material used for this study was a bimodal polyethylene presenting inhomo-geneities due to the high viscosity ratio between the low and high molecular weight fractions. The results indicated that the mixing performance depended strongly on the die geometry and flow rate. A critical flow rate was observed for each particular entry profile at which the highest efficiency was observed. This critical flow rate was found to correspond to the onset of melt flow instabilities.

Earlier, poly(∊-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) were found to be miscible on molecular level at 8 up to 28 wt.% of acrylonitrile (AN) in SAN copolymer [1]. The combination of PCL and different copolymers of SAN thus offers an opportunity to investigate the effect of miscibility on some key properties of the blends, e.g. rheological or mechanical. In this paper blends containing SAN with 25 wt.% of AN (miscible SAN-25) and 35 wt.% of AN (immiscible SAN-35) were used for this purpose. Blends of PCL with various contents of SANs up to 40 wt.% were prepared by melt mixing in a Brabender laboratory mixer. The miscibility of the mixed PCL/SAN blends was investigated by differential scanning calorimetry (DSC). The blends rheological characteristics were determined on a Rheoflixer capillary rheometer at various temperatures. Rabinowitsch and Bagley corrections and the power-law relation were used. The extrudates were observed under a scanning electron microscope. In addition, mechanical testing was performed on a tensile testing machine. It was found that blends of PCL with a minor amount of miscible SAN-25 provide a better balance of rheological and mechanical properties than those of pure PCL. The immiscible blend shows properties inferior to the miscible one. Immiscible SAN-35 increases viscosity, shear thinning and extrudate swell of PCL more than the miscible type. In contrast to the miscible blends containing SAN-25, which were without any form of unstable flow at shear rates of 10 to 10 000 s −1 , melt fracture occurs in the immiscible blends with SAN-35. Positive synergism in tensile strength and elongation at break can be seen for 90/10 PCL/SAN-25 blend processed at 130°C. The addition up to 30 wt.% of SAN-25 miscible with PCL increases elongation at break.

Highly filled calcium carbonate (CaCO 3 )/polypropylene (PP) blends were compounded using an intermeshing corotating twin-screw extruder. The effect of screw configuration, rotation speed, flow rate, filler feed port position and barrel temperature on dispersion of calcium carbonate into polypropylene was investigated. A method based on image analysis of optical transmission micrographs obtained from 25 micron thick films was found to be quick and reliable for evaluating the state of dispersion in the filled systems. The dispersion state affected the mechanical performance of the systems. In particular, falling weight and unnotched Izod impact resistance decreased significantly as dispersion got poorer. Mechanical properties were correlated with the relative volume concentration of agglomerates in the blend.

An experimental study is presented of the residence time distribution, the melting/fusion process, power consumption, melt temperature and blending in a Kobe Steel Nex-T continuous mixer. Attention is given both to phenomena occurring in the mixing chamber and the associated bottom extruder. The variables considered include throughput rate, rotor speed, exit orifice position and mixing chamber temperature.

This paper presents a methodology for the design of profile dies based on the Cross Flow Minimization Method. The flow balancing is described for cases where the flows are balanced by varying the die land length combined with the flow separation. Three different approaches were utilized in the die analysis. First, the flow in two proposed profile dies was calculated by substituting the flow domain by a set of simple geometries. The second approach applied the Cross-Sectional Calculation method, while the last method is a fully 3D FEM non-isothermal calculation. The results are verified by comparison with experimental data. It is shown that each of the calculation procedures may be the choice of preference depending on the geometrical complexity.

Experimental data relating to capillary flow of hard-metal carbide powders compounded with various polymeric binders are presented. Such powder—binder mixtures are used for the production of sintered hard-metal carbide components, mainly by injection moulding. In addition to measurements of their rheological behaviour in a capillary rheometer, the compounds were also subjected to thermo-gravimetric analysis in order to assess the ability of the binder to debind properly. The binder composition was varied from paraffin wax and its combinations with polyethyleneglycols to multi-component binders consisting of polyethyleneglycol, polyethylene, ethylene/vinyl acetate copolymer, and paraffin wax. Two kinds of hard-metal carbide powders differing in particle size distribution (uni-modal, bi-modal) were used. Although the compounds containing the simpler binder types had acceptable flow properties, their debinding behaviour was unsatisfactory. The multi-component compounds exhibited considerably better debinding; their flow was stable within a wide range of shear rates. Flow instabilities appearing at high solids loadings can limit the processability of the compounds used.

Numerical simulations have been undertaken for the film-casting process with viscoelastic fluids. Viscoelasticity is described by an integral constitutive equation of the K-BKZ type with a spectrum of relaxation times, which fits well experimental data for shear and extensional viscosities and the normal stresses measured in shear flow. Non-isothermal conditions are considered by applying the Morland-Lee hypothesis, which incorporates the appropriate shift factor and pseudo-time into the constitutive equation. A one-dimensional model derived from the conservation of momentum is used to approximate the thickness, while the stress free-surface condition is used to approximate the width. The resulting system of differential equations is solved using the finite element method and the Newton-Raphson iterative scheme. The method of solution was first checked against the Newtonian and Maxwell results for different film geometries. The simulations are compared to available experimental data and previous simulations in terms of film thickness, film width, and film temperature. Agreement between the experiments and the current simulations is considered good with subtle differences. Agreement is also considered good between the current one-dimensional simulations and previous two-dimensional simulations for viscoelastic fluids, in terms of width and thickness. The one-dimensional model is advantageous since the algorithm is relatively simple, convergence is almost guaranteed, and the computing time is short.

The paper addresses the buckling of fibrous reinforcements which occurs due to motion of the amorphous matrix material during solidification. The motion of the matrix material is caused by a variation in cooling rate in different portions of a composite while the sample is cooling. In the course of this study, samples were prepared with a fiber reinforcement sandwiched between premolded sheets of polycarbonate. Samples were either processed in a two-cavity, “dogbone” shape mold or in a rectangular, single-cavity mold matched die compression mold. In order to visualize the motion of the molten matrix material, strands of polycarbonate blended with carbon black were mounted into the matrix. The buckling behavior demonstrated sensitivity, to the amount of reinforcement observed with single and multiple filaments of carbon and Kevlar and bundles of carbon fiber tows. This paper provides scaling considerations indicating the sensitivity of the observed effects to processing conditions and concludes with a discussion of the implications for load carrying capabilities of composites.

The paper summarises the main experimental and theoretical results from a long-term programme of research (SAFIRE) to produce and apply long glass fibre compounds to the extrusion of pipes, sheets and profiles and to injection, blow and roto moulding. The overall objective is to obtain the processing speeds associated with short fibre reinforced thermoplastics with the reinforcement efficiencies associated with prepositioned or prepreg thermoset composites. Extrusion and injection moulding are now in the commercial domain, with industrial scale trials underway in the other technologies. Long glass fibres are defined by their ability to form lace-like mat structures within the polymer melt which persist into the solid state. Such structures, which greatly increase both melt strength and solid state thermo mechanical properties, can be formed with fibre volume concentrations (c) as low as .01. The formation of mat structures depends on the number N of virtual touches per filament. A minimum of around five touches is generally needed. From earlier work N is given as A.c. l/d. A varies with mean fibre orientation in the mat: for the random in-plane case it is approximately 8/π 2 , so that in contrast with typical fibre suspensions (c < d/l) extremely strong particle-particle interactions are involved in the melt state. In the solid state, tensile strength is measured and modelled in terms of number average fibre length (l) and diameter (d), polymer yield strength, fibre distribution efficiency, interfacial shear strength and a specially defined matrix stress magnification factor M. The role of patented fibre management devices in optimising these variables as they appear in the solid state is defined and described.

Thermoforming is characterized by large deformations of the polymeric material being processed. The simulation is complicated by the fact that an elastic material model cannot principally describe the thermoforming deformation precisely. In this respect non-linear and time-dependent viscoelastic behavior of the polymeric material has to be taken into account. In our research we used the K-BKZ model capable of describing viscoelasticity and large deformations. The parameters of this model were determined experimentally using a simple testing method. The method is derived from plug-assisted thermoforming. Computer analysis was used to evaluate the experiments: The tests performed were simulated using FEM (Finite Elements Method) repeatedly with different material parameters. The material parameters leading to a minimum sum of squares of differences between experimental and simulated values estimate the tested material behavior. It has been found that the final shape and the wall thickness distribution show a significant dependence on the material properties in case of deep thermoforming.

A new ternary planar random fiber composite (PRFC) is studied. It is a discontinuous glass fiber-reinforced, alumina trihydrate (ATH)-filled, melamine-formaldehyde (MF)-based composite of high fire resistance. Composites were manufactured from a commercially available molding compound using a Chopped Strand Mat (CSM), and compared with other ones obtained by varying the composition. Also, a comparison is made for grades with a high content of ATH but with different fiber architectures, realized by replacing the CSM mat by a needled mat. The amount and type of tensile damage is shown. An elastic-damage type parameter is discussed and used. In-situ Scanning Electron Microscopic (SEM) observations on polished surfaces of samples mounted in a tensile stage and subject to tensile load, and observations on as-obtained surfaces are made. Stress-strain behavior depending on damage is analyzed, and possibilities of improvement of mechanical properties are postulated.