M.T. Martyn

Effect of hydroxyapatite morphology/surface area on the rheology and processability of hydroxyapatite filled polyethylene composites

The commercial success of hydroxyapatite (HA) filled polyethylene composite has generated growing interest in improving the processability of the composite. A number of synthetic procedures and post synthesis heat treatment of HA has lead to the availability of powders with widely varying morphological features. This paper addresses the effect of morphological features of HA on the rheology and processability of an injection-moulding grade HA-HDPE composite. The results showed that low surface area HA filled composite exhibited better injection processing characteristics through improved rheological responses. The effect of reducing the surface area of the filler is to require less polyethylene to wet the filler and allows more polyethylene to be involved in the flow processes. These changes reduced the temperatures and pressures required for successful processing.

Stress measurements for contraction flows of viscoelastic polymer melts

The work presented in the second of a series of three papers providing a comprehensive in-process study of polyolefin melt flows through the convergent section of slit dies. Here we focus on stress fields developed in several linear and branched polyolefin melts. Accurate measurement of stress optical coefficients derived from flow induced birefringence has enabled determination of shear and first normal stress difference fields in these polymers in an abrupt entry convergent flow geometry. The progression of stress growth with increase in melt flow rate is also described. The dimensionless stress ratio (centre line tensile stress to slit wall shear stress) has been identified as a quantity which correlates with the relative vortex size in the linear and branched polyolefins. Fully developed entry flows on melt passage into the slit section are also studied and reflect relaxation characteristics of each polymer.

Flow visualisation of polymer melts in abrupt contraction extrusion dies : quantification of melt recirculation and flow patterns

This paper is the first part of a series of three in which we present experimental and analytical data covering the development of entry profiles, normal and shear stress fields, velocity fields and the in-process extensional viscosity measurement of branched and linear polyolefins. Here, we report accurate in-process measurements of the natural flow profiles of selected branched and linear polyolefins obtained from screw-driven extrusion flows through an abrupt contraction geometry. Recirculation areas, vortex centres and detachment lengths were quantified and their dependence on flow rate and process temperature studied. Significant recirculation areas were observed for the branched melts. Both the size and the development of these features were dependent on flow rate and temperature. Recirculation areas of the linear polymers were less prominent and independent of flow conditions. Correlation has been observed between rheological characteristics of the low density polyethylenes and their entry behaviour.

Visualisation and analysis of interfacial instability in coextrusion of LDPE melt

Abstract Details presented cover the study of interfacial instability of a low density polyethylene melt flow in a coextrusion flow visualisation cell. The cell design splits a single melt feed into two meltstreams using a flow divider. The meltstreams are made to converge at an angle of 30° and flow into a common die land of 1.0 mm height. The flows in the confluent region and die land to the die exit were observed through the side windows of the cell. The relative stream velocities, hence layer thickness ratio, of the two streams were varied using an adjustable restriction plate in the bottom channel. Stress and velocity fields were quantified using stress birefringence and particle image velocimetry techniques. Wave type interfacial instability was observed in the extrudate when the major–minor stream layer thickness ratio exceeded 2 : 1. Flow at immediate entry to the die land appeared stable when the extrudate exhibited instability. However, a disturbance was observed in the flow towards the exit of the die land. The frequency of the disturbance was quantified and found to be the same as that of the wave disturbance in the extrudate. A modified Leonov model and Flow 2000 software were used to simulate the melt flow through the geometry. A total normal stress difference (TNSD) sign criterion has been used to predict the presence of the wave interfacial instabilities in the coextrusion geometry.

Measurement of apparent extensional viscosities of polyolefin melts from process contraction flows

The extensional characteristics of branched and linear polyolefin melts have been evaluated in slit die flow cells with abrupt contraction ratios of 4:1 and 15:1 on a single screw extruder. Apparent extensional viscosities of the melts in planar flows have been obtained by two routes, the first employing extensional strain rate data measured from particle velocimetry, the second using a continuum mechanics analysis based on the entry flow profile. The influence of flow geometry on apparent extensional viscosity of the polymer melts has been investigated. The measured in-process apparent extensional viscosities of the branched and linear polyolefin melts are found to be in good agreement, despite differences in the strain histories imposed by two contraction geometries (centre line extensional stress and corresponding average axial strain rates differ). Particle velocimetry was found to be better than the continuum mechanics approach in obtaining extensional strain rates due to a lower susceptibility to experimental errors. It appears that the in-process methods of assessing apparent extensional viscosity, with an abrupt 180° entry slit die, are geometry independent for the range of materials, strain and strain rates covered and provide a useful technique for ranking process-typical extensional behaviour of melts.

Micromoulding : extreme process monitoring and inline product assessment

Abstract Advances in micromoulding technology are now allowing mass production of complex, three-dimensional functional products having sub-milligram masses and carefully tailored surface finishes. In order to create a viable manufacturing process for these components, accurate process monitoring and product evaluation are essential in order to highlight process problems and production of substandard parts. The present study describes work implementing a suite of sensors on a commercial micromoulding machine for detailed process interrogation. Evaluation of demoulded products is performed with a single camera based system combined with custom software to allow for three-dimensional characterisation of products during the process.

Rheological characterisation of hydroxyapatite filled polyethylene composites. Part 1 – Shear and extensional behaviour

Abstract The shear and extensional properties of injection moulding grade hydroxyapatite–polyethylene composites developed for orthopaedic applications have been studied. The composite was prepared without processing aids owing to concerns over the potential biological responses to such additives. The composite containing 20 vol.-% hydroxyapatite filler showed typical pseudoplastic behaviour. However, that containing 40 vol.-% hydroxyapatite filler tended to exhibit yield. The Maron–Pierce equation was found to be useful in predicting the viscosities of the composite systems. The activation energy of the composite and the unfilled polymer were equal, indicating that the 20 vol.-% system exhibits the same flow mechanism as the unfilled polymer. A qualitative assessment of extensional properties was made following Cogswells method. The extensional stress of the unfilled polymer decreases with increasing temperature whereas the composites behave in a complex manner. For all the systems the Trouton ratios tend to increase with apparent shear rates. The Trouton ratio also indicates that at higher temperatures the flow of these composites is dominated by extensional properties.

Rheological characterisation of hydroxyapatite filled polyethylene composites. Part 2 – Isothermal compressibility and wall slip

Abstract Rheological characterisation of hydroxyapatite–high density polyethylene (HA–HDPE) composites has been performed in terms of isothermal compressibility and wall slip. Addition of HA to the polymer melt decreases the compressibility of the melt. The unfilled HDPE was found to exhibit wall slip at shear stresses as low as 0·10 MPa. The flow curves of the composites showed three distinct regions: a gradient at low shear rates; a plateau region; and a gradient at higher shear rate. An increase in rheometer pressure seems to suppress the slip in composites. The 40 vol.-% HA–HDPE composite exhibited two critical shear stresses, one corresponding to wall slip, which occurs in the lower shear rate region of the flow curve, and the other corresponding to a plateau, which is identified with the stick–slip behaviour of unfilled HDPE reported in the literature. The plateau shear stress increased with filler volume fraction and this effect is attributed to the decreased compressibility of the melt. A good correlation with a negative correlation coefficient was found to exist between compressibility and shear stress in the plateau region. The slip observed in unfilled HDPE and at low shear rates in the 40 vol.-% HA–HDPE systems has been explained in terms of a low molecular weight polymer layer formed at the melt/wall interface. The large interfacial slip observed in the plateau region is attributed to complete disentanglement of adsorbed chains from free chains at the melt/wall interface at and beyond the plateau region.

Theoretical and experimental investigation of interfacial instability phenomena occurring during viscoelastic coextrusion

Abstract The fully viscoelastic finite element method (FEM) together with the flow visualisation technique were employed to quantify the effect of die design on wave type interfacial instabilities in coextrusion. It has been shown experimentally that minor channel geometry has a strong impact on wave type interfacial instabilities and the results can be correlated through novel criteria called Total Stress Difference (TSN) which takes into account the bulk change in total stress in the normal as well as the tangential direction with respect to the interface. It has also been shown that a specific type of high stress area occurs around the interface at the end of the converging section. This total stress has been found to be non-monotonic along the interface and is related to the onset of zigzag type interfacial instabilities when the total stress achieved 200 kPa.

Modelling of viscoelastic coextrusion flows in multi-manifold flat dies

Abstract A recently proposed modification of the viscoelastic Leonov model is employed as a stress calculator in FEM analysis with a full u-v-p-t numerical scheme for coextrusion flow in multimanifold flat dies with 30° and 90° entrance angles. It is shown that the predicted stresses, interface location and streamline fields are in good agreement with the measurements. It is also shown that extensional viscosity has to be used in the modelling of the coextrusion flow to confirm experimental data.