J. A. Covas

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.

Effect of polyolefin structure on maleic anhydride grafting

A series of polyolefins with different ethene/propene ratios was grafted with maleic anhydride (MA) both in the melt and in solution. The MA graft content and the degree of branching/crosslinking or degradation were determined by FT-IR and rheometry, respectively. The MA graft content is low for polyolefins with high propene content, increases as the propene content decreases and reaches a plateau at propene levels below 50 wt.%. Branching/crosslinking occurs for polyolefins with low propene content, while degradation is the main side reaction for polyolefins with a high propene content. A detailed chemical mechanism is proposed to explain these results.

Evolution of morphology and of chemical conversion along the screw in a corotating twin-screw extruder

A new sampling device is used to perform near-real-time investigations of physical and chemical processes occurring inside a laboratory twin-screw extruder. Polyamide-6-ethylene propylene rubber (PA-6-EPM) blending and styrene-maleic anhydride (SMA) imidation experiments are reported in terms of morphology development and evolution of the chemical conversion along the extruder, respectively. Comparison of the results obtained using this new technique with those of classical screwpulling experiments evidenced the potential erroneous conclusions than can be drawn from the latter.

Robustness in multi-objective optimization using evolutionary algorithms

Abstract This work discusses robustness assessment during multi-objective optimization with a Multi-Objective Evolutionary Algorithm (MOEA) using a combination of two types of robustness measures. Expectation quantifies simultaneously fitness and robustness, while variance assesses the deviation of the original fitness in the neighborhood of the solution. Possible equations for each type are assessed via application to several benchmark problems and the selection of the most adequate is carried out. Diverse combinations of expectation and variance measures are then linked to a specific MOEA proposed by the authors, their selection being done on the basis of the results produced for various multi-objective benchmark problems. Finally, the combination preferred plus the same MOEA are used successfully to obtain the fittest and most robust Pareto optimal frontiers for a few more complex multi-criteria optimization problems.

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.

Monitoring polyolefin modification along the axis of a twin‐screw extruder. II. Maleic anhydride grafting

The physico-chemical phenomena developing along the screw axis of a twin-screw extruder during the grafting of maleic anhydride (MA) onto polyolefins [polyethylene (PE), ethylene–propylene rubber (EPM), and polypropylene (PP)] were investigated. For this purpose, sampling devices located along the extruder barrel were used to collect polymer samples that were subsequently characterized to follow the degrees of grafting and crosslinking or degradation. A similar evolution of MA grafting was observed regardless of the polyolefin type or MA and peroxide concentration when grafting was performed under identical conditions, that is, the same peroxide type and set temperature. A correlation between the MA grafting and the calculated peroxide decomposition was established. Chemical reactions occurred along the extruder axis until the peroxide was fully converted. More detailed quantitative measurements of the peroxide decomposition and MA grafting would allow the development of accurate process models. The final MA content depended on the polyolefin composition (PE > EPM ≫ PP). As expected for PE, crosslinking occurred in addition to grafting, but after a certain residence time, the PE network degraded. The PP viscosity reduction after MA grafting was due to the conversion of tertiary PP radicals into primary PP radicals after grafting.

Measuring uniaxial extensional viscosity using a modified rotational rheometer

Knowledge of the extensional behaviour of polymer melts is extremely important due to the industrial relevance of extensional flows in common processing techniques and sequences such as blow moulding, film blowing, fibre spinning, melt flow through extrusion dies and injection mould filling. One of the main problems both researchers and industrialists come across is the fact that, unlike shear flows, steady-state extensional flows are not easy to generate and maintain experimentally. This fact limits the extent to which one can characterise the materials and, therefore, the degree of optimisation of the productive process. In this paper, a modification to a commercially available controlled rate rotational rheometer is proposed in order to produce a cheap, easy to set-up, flexible extensional rheometer. This is based on the well-known Meissner-type extensional rheometer and makes use of the accurate velocity control and torque measurement possibilities of the rotational apparatus. In this case, the adaptation was performed on a TA Instruments Weissenberg Rheogoniometer, but the idea is applicable to most other similar devices. The feasibility of the modification will be discussed and confirmed, results being presented for two materials at different temperatures. These include the calculation of transient uniaxial extensional viscosity and a study of rupture conditions.

Effect of filler dispersion on the electromechanical response of epoxy/vapor-grown carbon nanofiber composites

The piezoresistive response of epoxy/vapor-grown carbon nanofiber composites prepared by four different dispersion methods achieving different dispersion levels has been investigated. The composite response was measured as a function of carbon nanofiber loading for the different dispersion methods. Strain sensing by variation of the electrical resistance was tested through four-point bending experiments, and the dependence of the gauge factor as a function of the deformation and velocity of deformation was calculated as well as the stability of the electrical response. The composites demonstrated an appropriate response for being used as a piezoresistive sensor. Specific findings were that the intrinsic piezoresistive response was only effective around the percolation threshold and that good cluster dispersion was more appropriate for a good piezoresistive response than a uniform dispersion of individual nanofibers. The application limits of these materials for sensor applications are also addressed.

A look inside the extruder : Evolution of chemistry, morphology and rheology along the extruder axis during Reactive Processing and blending

A simple device was recently developed for fast sampling (within a few seconds) of representative melt samples (about 2 g) on a running extruder. An array of such devices has been mounted on a twin-screw extruder. The goal of this study was to de-black-box reactive processing of polymers by studying some typical examples. - Processing of polyolefins in the presence of peroxides: when the polymer is molten and the melt temperature is sufficiently high branching/cross-linking of PE and degradation of PP occurs; the conversion follows a convex profile along the screw axis, which profile is similar to the exponential profile calculated for peroxide decomposition. - Free-radical grafting of maleic anhydride (MA) onto polyolefins: MA grafting onto PE and PP also follows a convex profile with branching/cross-linking as parallel side reaction for PE and degradation for PP; for PE degradation of the formed grafted/cross-linked gel is observed at the end of the extruder. - Reactive blending of PA-6 with EPM-g-MA: within a few seconds the in-situ compatibilization reaction, resulting in PA-6/EPM graft copolymers, is completed and the degree of rubber dispersion has changed from the mm to the sub-μm range, regardless of the MA content of EPM-g-MA and the EPM-g-MA content of the blend; PA degradation occurs along the whole extruder.

Chemical and morphological evolution of PA-6/Epm/Epm-g-MA blends in a twin screw extruder

Chemical conversion and morphological evolution of PA-6/EPM/EPM-g-MA blends along a twin screw extruder were monitored by quickly collecting small samples from the melt at specific barrel locations. The results show that the MA content of all blends decreases drastically in the first zone of the extruder, i.e., upon melting of the blend components. Significant changes in morphology are also observed at this stage. A correlation between chemistry and morphology could thus be established.