•Local temperature is the key factor of thermomechanical devulcanization.•Devulcanization at lower screw speeds and higher temperatures is expedient.•The degradation can be minimized with the optimal ...parameters.•Extra vulcanizing agents added directly to GTR increase adhesion between phases.•Recipe development and two-step mixing helps maintain mechanical properties.
We thermomechanically devulcanized ground tire rubber (GTR) in a co-rotating twin-screw extruder at different barrel temperatures and screw speeds. We measured the soluble content and the cross-link density of the samples and then evaluated them by Horikx's analysis. The results showed that at lower temperatures, selective cross-link scission is dominant, and screw speed has no effect. At higher temperatures and screw speeds, the degradation of the polymer chains becomes more and more severe. With the appropriate parameters, cross-link scission can be maximized without severe degradation of the main chains. Two devulcanized ground tire rubber (dGTR) samples were chosen for further study. Natural rubber (NR) based rubber mixtures were produced with conventional and two-step mixing. Tensile and tear strength decreased significantly with dGTR content. These results are in agreement with Horikx's analysis, i.e. the greater the degradation of dGTR, the more mechanical properties were impaired. However, elongation at break did not follow such a trend. With two-step mixing, mechanical properties improved, especially tear strength. Additional experiments were also conducted with Horikx's analysis. The aim was to reveal the accuracy and usability of the analysis, and it showed the theoretical maximum of devulcanization.
The rapid estimation of the average shear rate encountered by the material as it flows along the screw elements of a corotating twin-screw extruder is a key point for many applications. In this ...paper, two methods of evaluation are presented that allow the calculation of the average shear rate as a function of the screw geometry, feed rate, and screw speed. A comparison is made between the approximate and exact methods. It is shown that it is crucial to take into account the shear component due to the pressure flow, especially in the left-handed screw elements.
The blend morphology model developed by Wong et al., based on Peters et al., is used to investigate the development of the disperse polymer blend morphology in twin‐screw extruder flow. First, the ...model is written in a point‐wise form suitable for using in conjunction with particle tracking. Particle tracking methods are used to generate trajectories along the flow field. Macroscopic droplet populations are placed along these trajectories and the velocity gradient tensor is extracted and applied on the point‐wise blend morphology model. Very large morphology differences arise between trajectories that pass through the middle gap and those that do not. In the global distribution of (macroscopically averaged, monodisperse) droplet sizes, two distinct peaks appear due to these different trajectories. Given enough number of screw rotations, a droplet population can reach almost every position in the twin‐screw extruder and travel along both types of particle trajectories. The effect of varying the gap size is that the largest droplets are unaffected, but the smallest droplets are smaller for a smaller gap size due to the higher maximum shear rate. The effect of varying the viscosity ratio on the global droplet size distribution is found to be nonlinear and is strongly determined by the Grace curve. The effect on polydisperse droplet populations is found to be that trajectories that do not pass through the gap evolve toward a single peak, whereas trajectories that do pass through the gap lead to a split into two peaks that ultimately rejoin as one peak. It is concluded that the initial position of a population in the twin‐screw extruder has a very large effect on the developing transient blend morphology, though future work should be done on the importance of the initial position on the steady‐state blend morphology after a very large number of screw rotations.
The properties of disperse polymer blends depend strongly on their morphology. This work presents a phenomenological model and numerical results of the evolution of this morphology in a twin‐screw extruder flow field for given initial conditions. In particular, the effects of the gap size between the screws and viscosity ratio on the morphology evolution are studied.
The use of real-time techniques to evaluate the global mixing performance of co-rotating twin-screw extruders is well consolidated, but much less is reported on the specific contribution of ...individual screw zones. This work uses on-line flow turbidity and birefringence to ascertain the mixing performance of kneading blocks with different geometries. For this purpose, one of the barrel segments of the extruder was modified in order to incorporate four sampling devices and slit dies containing optical windows were attached to them. The experiments consisted in reaching steady extrusion and then adding a small amount of tracer. Upon opening each sampling device, material was laterally detoured from the local screw channel, and its turbidity and birefringence were measured by the optical detector. Residence time distribution curves (RTD) were obtained at various axial positions along three different kneading blocks and under a range of screw speeds. It is hypothesized that K, a parameter related to the area under each RTD curve, is a good indicator of dispersive mixing, whereas variance can be used to assess distributive mixing. The experimental data confirmed that these mixing indices are sensitive to changes in processing conditions, and that they translate the expected behavior of each kneading block geometry.
3D Printing (3DP) techniques and materials applied in construction-scale are currently under constant investigation, with notable results published at research and experimental level. While their ...great importance has been discussed extensively, very scarce detailed research has been published on 3DP parameters that are incorporated into the design process at an early stage to assess their impact on 3D printing performance, focusing on geometric conformity aspects. This work aims to provide a comprehensive parametric design investigation, driven by 3DP parameters, related to infill and overhang control, which can enhance a deeper understanding of their use in construction scale of non-conventional wall components, placing emphasis on construction time performance minimization. To this end, the paper initially illustrates the development of a parametric-integrated algorithm for toolpath planning and 3DP control using an industrial robot, capable of being universally adjusted based on open-source extruders with small, medium and large nozzle diameters. In this study, earth and clay-based materials are chosen because of their lower environmental impact and recyclability compared to concrete-based materials. Secondly, important printing parameters for toolpath planning, robotic and nozzle control, as well as robotic printing time, are presented and discussed in detail through 3DP experimental tests and a non-conventional wall study. The correlation of parameters in the early stage of design allows the assessment of their effectiveness to be used in construction-scale of non-conventional geometries towards minimization of 3DP time performance.
•Review on design and construction-scale 3D printing•Parametric-integrated design approach for 3D printing using earth and clay-based materials•Development of an algorithm for design and robotic 3D printing control•Investigation of 3DP tests and a non-conventional wall study driven by geometric conformity aspects•Evaluation of parametric settings and output results of the algorithm
A recently developed model for pretreatment and acid or alkaline hydrolyses of lignocellulosic biomasses, including blue agave bagasse (BAB), in twin‐screw corotating extruders is extended to the ...case where enzymes are incorporated into the formulation. The model describes the deconstruction and degradation of BAB, as well as the production of simple sugars at the end of the extrusion process. Experiments using NaOH for delignification and alkaline hydrolysis of BAB, H3PO4 for neutralization, and different enzymatic cocktails for bioextrusion are reported. The effects of temperature, NaOH concentration, enzymatic cocktail used and its concentration, residence time, and configuration of screw elements on degree of degradation and production of sugars are assessed. It turned out that temperature, NaOH concentration, and enzymatic cocktails are the key factors in the extraction and hydrolysis of hemicellulose (H) and cellulose (C) to produce glucose (G). Good agreement between experimental data and model predictions is observed.
A combined experimental‐modeling study on the pretreatment and alkaline/enzymatic hydrolyses of blue agave bagasse in twin‐screw extruders is presented. The extruder has a modular screw configuration with different elements. Alkaline and enzymatic hydrolyses are modeled assuming that the extruder behaves as a series of continuously stirred tank reactors. Good agreement between calculated profiles and experimental data is obtained.
Determining the impacts of extrusion conditions on extrudate characteristics of whole beans flours is critical to find the suitable types of beans to use for making direct expanded products. Whole ...bean flours of four different bean types, faba, lima, pinto, and red kidney, were extruded. The influence of barrel temperature (120, 140, and 160 °C), moisture content (17%, 21%, and 25%), and screw speed (150, 200, and 250 rpm) on process and product responses was studied with a corotating twin screw extruder. The barrel temperature, moisture content, screw speed, and variety of bean had significant influence on process and product responses, back pressure (MPa), torque (N·m), specific mechanical energy (kJ/kg), expansion ratio, water absorption index (g/g), and water solubility index (%) (P < 0.05). Faba bean extrudates had a significantly higher expansion ratio compared to other beans (lima, pinto, and red kidney beans) even though faba bean contained significantly higher protein and higher crude fiber contents (P < 0.05).
Practical Application
The outputs of this research will be helpful to the food industry in the production of high nutrient‐dense food products from whole beans by maintaining the expansion and texture of the products. The data should assist to choose the suitable types of whole bean flours and the optimum processing conditions for making direct expanded extruded products.
Display omitted
•PLA’s crystallites were in-situ visualized under complex flows in extrusion.•PLA’s strain-induced crystallization was active 4°C above the Tm.•PLA’s crystallization was significantly ...enhanced with an increased strain rate.•The PLA foam structure was strongly affected by the formed crystallites.
We studied poly(lactic acid)’s strain-induced crystallization behavior during a tandem extrusion process using visualization techniques. Our visualization results showed that, despite its inherently slow crystallization kinetics in the quiescent condition, PLA crystallites were quickly formed inside the die at a temperature even above the melting temperature (Tm), as the melt temperature was decreased. Their rapid formation at a high temperature above the Tm peak, was due to the strain-induced crystallization. We also studied how the strain rate and the use of supercritical CO2 affected PLA crystallization. A significant increase in the PLA crystallization kinetics was caused by a sudden increase in the screw speeds of the extruders with a fixed barrel temperature. The crystallization enhancement, which was caused by the increased flow rate, became more pronounced at lower temperatures due to the higher degree of molecular orientation. Adding the CO2 shifted the crystallization temperature to a lower processing temperature due to its plasticization effect, but it accelerated the crystallization kinetics as well. We also observed a correlation between the content of the induced crystallites inside the extruder and the cellular structure of the extruded foams. This clearly explained the decrease in the optimal processing temperature for foaming with an increased CO2 content.
PDF
