Papers by Author: José A. Covas

Abstract: The present study investigates the morphology of polyamide (PA6) and polypropylene (PP) blends in the presence of an organoclay during extrusion. Physical PA6 /PP blends and blends of PA6/PP and PP modified with maleic anhydride, both filled with a montmorillonite (MMT) modified alkyl ammonium organoclay, were prepared in a modular co-rotating twin screw extruder. Samples were collected along the extruder and analyzed by X-ray diffraction (XRD), oscillatory rheology, and scanning and transmission electron microscopy (SEM and TEM, respectively). Intercalation and some degree of clay exfoliation seem to have been achieved. Generally, the presence of the organoclay yields a finer dispersed phase along the extruder.

Abstract: The performance of parts produced by Free Form Extrusion (FFE) may be limited by poor mechanical properties, due to poor bonding between the individual extruded filaments. In this work, an analytical solution is proposed for the transient heat transfer during filament deposition, taking into account contacts between filaments. The developed code using Matlab® allows to study the influence of the main process variables during filament deposition and may assist the process optimization.

Abstract: The main objective of this work is to develop conductive yarns to be used as electrical wiring in e-textiles with the typical mechanical properties of a textile yarn. Present work deals with the study of conductive polymer composites filaments of PP (polypropylene) with CB (carbon black), carbon black of high conductivity (CBHC) and CF (carbon fibers) .The novelty of this work resides in creating oriented filaments using traditional fiber processing techniques together with a specially designed drafting machine. In the authors’ opinion, the composite conductivity could be improved with the orientation of the (nano)carbon-based fillers by melt drawing after extrusion in order to facilitate the flow channels creation.

Abstract: A prototype modular single screw extruder fitted with a screw extracting device is used to monitor melting of an immiscible polymer blend (PP/PA6, with different weight ratios) in this widely used processing equipment. As anticipated, the phenomena observed are much more complex than those involved in extruding PP or PA6, when the well known Maddock/Tadmor mechanism is valid. Consequently a hybrid melting mechanism, involving Maddock/Tadmor and Dispersive melting sequences, is proposed.

Abstract: This work presents a methodology to improve the final thickness distribution of thermoformed parts. The conventional plug is replaced by a new version containing concentric layers with independent displacements. The results show that thickness gradients can be significantly reduced, with a simultaneous increase of the minimum values. Varying the relative displacement of the inner layers can bring about significant differences in the final thickness distribution, thus enabling the production of parts with prescribed thickness profiles that maximise their mechanical performance.

Abstract: Polymer extrusion is one of the most important polymer processing techniques. Modelling of the phenomena developing inside single screw extruders enabled the development of commercial software which can be used to study the performance of existing systems and to define the equipment characteristics for new applications. Although mixing is one of the most important performance measures of extrusion, it is usually disregarded in the available programs. Consequently, in this work mathematical models for quantifying mixing in single screw extruders are developed and subsequently inserted in a global computer modelling program of single screw extrusion. The mixing model quantifies distributive and dispersive mixing and coalescence mechanisms in a liquid-liquid system. This is accomplished by calculating the variation of the thickness of drops considered to be suspended in the polymeric matrix, taking into account the rate and drop break-up time and the coalescence probability. The influence of several parameters, such as viscosities, initial drop dimension, screw speed and barrel temperature, are examined

Abstract: The effect of the dispersion state of carbon nanotubes in polycarbonate on the rheological, mechanical and electrical properties of melt-extruded compounds is presented. The dispersion state was monitored by means of grey values distribution histograms of optical micrographs of the composites. Increasing the processing residence time, and hence the deformation induced by the surrounding polymer, increased the level of dispersion of CNTs in the matrix. This, in turn, resulted in a large improvement of the CNTs reinforcement effect and decreased composite electrical resistivity. Rheological behaviour is in agreement with these observations.

Abstract: Dynamic vulcanisation of EPDM/PE blends using the resol/SnCl2 system was studied in a co-rotating twin screw extruder using a series of sampling devices. In order to study the effect of the processing conditions on chemical development along the extruder and on the mechanical properties of the extrudate material, the same recipe (having the same polyolefin/EPDM ratio) was compounded under different processing conditions. The results showed that both chemical evolution and mechanical properties depend on the processing conditions.