O. S. Carneiro

Production and Assessment of Polycarbonate Composites Reinforced with Vapour-Grown Carbon Fibres

Abstract Vapour-grown carbon fibres were produced from methane in a flow reactor. The fibres were compounded with poly-carbonate in a co-rotating twin-screw extruder and the composites were subsequently injection moulded. The rheological characteristics of the polycarbonate composites reinforced with these vapour-grown carbon fibres were determined by using capillary and rotational rheometry. The tensile properties of the injection-moulded specimens were marginally better than those of the un-reinforced polycarbonate, but the impact resistance was severely diminished by the addition of the carbon fibres. This was attributed to the presence of polycyclic aromatic hydrocarbons on the surface of the fibres, that may impair the impact resistance of polycarbonate by chemical stress cracking.

Experimental and theoretical study of twin-screw extrusion of polypropylene

The flow of a polypropylene in a self-wiping corotating twin-screw extruder was characterized by measuring the pressure, temperature, and residence time along the screw profile. The influence of the operating conditions (feed rate, screw speed, barrel temperature) and screw profile was studied. Flow modeling was performed using the Ludovic© software and measured and calculated pressure, temperature, residence time, and energy consumption were compared. The values of the temperature close to the melting zone were overestimated by the model, which considers instantaneous melting upon the first restrictive screw element. If the program assumes that melting occurs at the screw location identified experimentally, a correct description of the temperatures along the screw profile is produced. The influence of the processing conditions (feed rate, screw speed, barrel temperature, screw profile) is well described by the model. These results put in evidence the importance of including an adequate melting model in the modeling of the twin-screw extrusion process.

Computer aided rheological design of extrusion dies for profiles

Abstract A global methodology for the rheological design of profile extrusion dies is proposed. This methodology accounts for: (i) flow defects due to maximum admissible stresses, pressure drop and melt temperature increase; (ii) post-extrusion phenomena (shrinkage upon cooling, draw-down promoted by pulling and swelling after die exit) and (iii) flow balancing. The part of the methodology, that is concerned with flow balancing, was implemented and is here illustrated in two case studies, each one leading to the adoption of a different constructive solution. The software is based on a finite-volume method, which performs the required three-dimensional simulations, and is also briefly described.

Flow balancing in extrusion dies for thermoplastic profiles. Part III: Experimental Assessment

Abstract A computer code, previously developed by the authors for the automatic die design, is used to optimise the flow distribution in a profile extrusion die using two alternative strategies: one based on length optimisation and the other on thickness optimisation. The numerical predictions are then compared with experimental data gathered during extrusion experiments. The numerical predictions and the experimental results agree within the experimental uncertainty thus showing the effectiveness of the computer code, the optimisation algorithm and the design strategies implemented. Generally speaking, measured and predicted values of pressure drop and flow rate are in good agreement (within 8 % and 6 %, respectively). It also is confirmed that optimisation based on thickness control leads to final profiles that are more prone to distortion.

Flow patterns in twin-screw extruders

Abstract Flow visualisation studies in an intermeshing co-rotating twin-screw extruder operated under starve-fed conditions are presented. Different numbers of kneading discs and relative staggering angles were used in the definition of the mixing blocks. Conveying and left-handed elements were also utilised. The images recorded provided information on flow patterns, on the degree of filling of the screw channels and on the number of filled turns. Local flow times were estimated. Partial residence time distribution (RTD) functions were determined by inserting especially designed collecting devices along the barrel, and measuring the evolution with time of the concentration of a tracer.

Online monitoring techniques for studying evolution of physical, rheological and chemical effects along the extruder

Abstract During extrusion and/or compounding, polymeric systems may be subjected to a complex thermo-mechanical-chemical environment, therefore monitoring the evolution of physical, rheological and chemical effects along the extruder is an important tool assisting process understanding and optimisation. Online monitoring concepts that allow sample collection, rheology measurements and RTD characterisation at specific locations along the screw axis of an extruder are presented. Each practical set-up is presented, its operation is described and the results obtained are validated experimentally. Finally, examples of the use of the tools developed for the study of specific polymer systems are presented and discussed.

Flow Balancing in Extrusion Dies for Thermoplastic Profiles: Part I: Automatic Design

Abstract To achieve a specified geometry for an extruded profile, together with a minimal degree of internal stresses, flow balancing of the die is required. To attain this objective, the flow along a profile die channel must be accurately described, and this demands a computational code able to predict complex 3D non-isothermal flow patterns. In this work new methodologies for flow balancing are implemented and illustrated. The design code developed to carry out the automatic search of a final geometry consists of an optimisation routine coupled with geometry and mesh generators and a 3D computational code based on the finite volume method. The issues discussed and described here encompass recent developments, namely the implementation of two alternative optimisation algorithms for the automatic search of a final solution, the enhancement of the strategies previously developed to balance the flow, some improvements in the routine used to generate the mesh and the adoption of a progressive mesh refinement technique. The examples show that the proposed methodology performs well and the solution is attained in just a few minutes of calculation without any user intervention during the calculation process.

Flow Balancing in Extrusion Dies for Thermoplastic Profiles: Part II: Influence of the Design Strategy

Abstract To achieve a specified geometry for an extruded profile with minimal level of stress gradient induced by pulling, flow balancing of the die is required. To fulfil this requisite, a set of operating conditions and polymer rheological properties is considered during the design step. However, fluctuations of the operating conditions and/or slight variations of the polymer rheological properties are expected to occur during long-term production. Their effect on the performance of an extrusion die will depend, among other things, on the sensitivity of the flow distribution within the die. In this work, an extrusion die is optimised (balanced) using four different design methodologies and the final shapes of the die are compared in terms of their absolute quality (when used in the optimal conditions) and stability to the factors considered. For this purpose, a finite-volume based computational code is used to perform the required simulations of the non-isothermal three-dimensional flows, under conditions defined by a statistic Taguchi technique. The influence of some operating conditions on the flow distribution is assessed and the effect of the polymer melt rheology is also investigated. It was concluded that the use of different design methodologies lead to different results in terms of flow balancing and sensitivity to the factors considered and that the most balanced and stable extrusion die was that generated by the strategy based on the parallel zone thickness control.

Monitoring the Evolution of Morphology of Polymer Blends Upon Manufacturing of Microfibrillar Reinforced Composites

Abstract Microfibrillar reinforced composites (MFC) based on HDPE/PET blends were prepared under conditions relevant for direct scale-up to an industrial process. The evolution of the morphology and of the linear viscoelastic response of the blend along the axis of a co-rotating twin screw extruder and at several locations along the extrusion line was monitored. Major changes in the average particle size and size distribution of the disperse phase occurred upon melting of the components, whilst a much slower evolution rate was evident downstream in the extruder. Simultaneously, G′ and G″ increased along the extruder. Pellets showing well oriented PET fibrils embedded in a HDPE matrix with poor adhesion between both were obtained. This MFC showed the typical improvement expected in mechanical performance when compared with the matrix.

The effect of processing conditions on the characteristics of electrically conductive thermoplastic composites

Carbon black (CB) thermoplastic composites are extensively used in antistatic, electrostatic dissipative, and semiconductive applications, where a relatively low electrical conductivity and low material costs are required. Achieving the adequate mixing levels during compounding is a major practical requirement. The present work studies the preparation by twin-screw extrusion of conductive composites containing ca. 20% weight-in-weight (w/w) CB, by investigating the effects of the processing conditions and of small variations in the fillers feeding rate on the evolution of the relevant processing parameters and rheological response along the extruder axis and of the electrical properties.