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Compounding is an important step in processing base polymers and is used to incorporate various additives into a polymer. For this purpose, different screw elements are used for dispersive and distributive mixing on a co-rotating twin-screw extruder. Optimising the screw configuration requires precise knowledge of the screw elements’ mixing properties, which have not been thoroughly investigated. This study analyses the mixing behaviour of individual screw elements regarding dispersive and distributive mixing using 3D CFD flow simulations with subsequent particle tracking. For distributive mixing, the particle distribution behind the screw elements in the XY plane is analysed and the mixing index MQ, which relates the standard deviation and the mean value of the triangular areas between the particles, is calculated. For dispersive mixing, the maximum shear stress on the particle path and the integral of the shear stress over the residence time of each individual particle are determined. The results show that screw element geometry and rotation speed have a significant influence on dispersive and distributive mixing. In addition, better dispersive mixing is achievable with highly viscous materials. These findings enable the optimisation of the mixing zone of a co-rotating twin-screw extruder for the efficient mixing of mineral fillers.

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.

To reduce the degradation of polylactic acid (PLA) during processing, which reduces the molecular weight of PLA and its properties, prior studies have recommended low processing temperatures. In contrast, this work investigated the impact of four factors affecting shear heating (extruder type, screw configuration, screw speed, and feed rate) on the degradation of PLA. The polylactic acid was processed using a quad screw extruder (QSE) and a comparable twin screw extruder (TSE), two screw configurations, higher screw speeds, and several feed rates. The processed PLA was characterized by its rheological, thermal, and material composition properties. In both screw configurations, the QSE (which has a greater free volume) produced 3–4 °C increases in melt temperature when the screw speed was increased from 400 rpm to 1000 rpm, whereas the temperature rise was 24–25 °C in the TSE. PLA processed at low screw speeds, however, exhibited greater reductions in molecular weight—i.e., 9% in the QSE and 7% in the TSE. Screw configurations with fewer kneading blocks, and higher feed rates in the QSE, reduced degradation of PLA. At lower processing temperatures, it was found that an increase in melt temperature and shear rate did not significantly contribute to the degradation of PLA. Reducing the residence time during processing minimized the degradation of PLA in a molten state.

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 effect of the fibrillation process through a twin-screw extruder (TSE) on properties of pulp fibers was studied, considering the degree of both fibrillation and degradation of the fibers. Never-dried refined bleached kraft pulp (NBKP) was passed through a TSE several times at a high concentration of 28 wt%. The output of fibrillated fibers had a solid content up to ca. 50 wt%, and the material was in powder form. Characterizations of the morphology, dewatering speed, sedimentation, laser light scattering, scanning electron microscopy of cellulose suspensions, and light transmittance of resin-impregnated films showed that the fibrillation degree of the pulp was enhanced with a higher number of passes. However, the results from thermogravimetry, intrinsic viscosity, and X-ray diffraction analyses indicated that some degradation occurred during the fibrillation process in the TSE. In addition, the mechanical properties of the fibrillated pulp sheets reflected the effects of treatment on the fibrillation and degradation of the cellulose. For never-dried refined NBKP pulp, the best compromise in terms of fibrillation and degradation degree is between 3 and 14 passes, depending on the envisaged properties and applications. The possibility of nanocellulose production at the reported high solid contents is of great interest for industry.

Devulcanization represents the recycling of choice for a homogenous rubber waste stream because it allows revulcanization of samples previously devulcanized, making the life of the rubber virtually endless, according to the principles of circular economy. Among the many devulcanization processes, the thermo-mechanical one is the most appealing because it is a continuous process, easy to be industrialized. In this paper a comprehensive set of analyses (FTIR, TGA, DSC, elemental analyses, Py-GC/MS, swelling tests) were carried out on a post-industrial ethylene propylene diene monomer (EPDM), thermo-mechanical devulcanized in a co-rotating twin-screw extruder with different process parameters (thermal and screw profile, rpm). Results of the swelling test according to the Flory–Rehner theory and Horikx analyses show that the higher the thermal profile and the higher the rpm, the higher is the percentage of devulcanization. The quality of the devulcanized sample in terms of sol fraction and percentage of random scissions depends on the process conditions. The screw profile concurs to the efficiency of the devulcanization: the different number of kneading elements and more in general the screw profile composition affects the percentage of devulcanization, making the results in some tests more dependent on the screw speed.

Solvent-free extrusion emulsification (SFEE) is a recently developed process for producing submicron particles of high viscosity polymers using a twin-screw extruder without hazardous solvents. Its rate dependency on mixing for catastrophic phase inversion makes the process knowingly sensitive to a variety of variables, included the manner by which it is started up. This study examined how transient dynamics at the beginning affects steady-state operations by varying the time for the surfactant-containing feed stream to reach its setpoint rate into the process, to better understand the mechanism of SFEE. The results showed that the path taken to reaching the setpoint conditions (i.e. transition time) determined process stability and whether successful emulsification occurred, not the actual setpoint conditions chosen. Combining particle size and residence time measurements with prior studies’ data related to the rate of reactions benefiting interfacial growth between the polymer/water phases and inline rheological findings, this unusual transient sensitivity was attributed to insufficient surface-active species present creating a persistent segregated regime of the flow field in the first half of the dispersion zone that exhibits an unstable phase morphology. Returning to the desired operating window was only possible once this segregated regime was purged from the process. The study offers a continuous processing perspective to this phenomenon previously observable in batch for high viscosity oil phase systems. Graphical abstract

During mechanical recycling, polypropylene typically is reprocessed using a single- or twin-screw extruder. The degradation of polypropylene during this reprocessing reduces the polymer’s molecular weight and, consequently, limits the performance of the recycled resin. This work investigated the impact of a quad screw extruder (QSE), which has greater free volume, on the reprocessing of an impact copolymer polypropylene. To mimic the recycling process, the polypropylene was subjected to three processing cycles using a QSE and a comparable twin-screw extruder (TSE) operated at three screw speeds. The reprocessed materials were characterized for their rheological, morphological, and mechanical properties. For both extruders, increasing the number of reprocessing cycles and the screw speed resulted in higher melt flow indices, decreases in zero-shear viscosity, and shifting of the crossover points for the storage and loss moduli, which indicate reductions in the molecular weight and narrowing of the molecular weight distribution of the polypropylene. The QSE exhibited greater reductions in molecular weight compared to the TSE, probably due to the higher stresses associated with the three intermeshing points along its screws. Reprocessing caused a significant reductions in the Izod impact strength of the reprocessed polypropylene, which correlated with reductions in the particle size and particle size distribution of the dispersed rubbery phase in the polypropylene during reprocessing.

In recent years, there has been a growing interest in the use of biocomposites consisting of thermoplastics and cellulosic materials. Acrylonitrile butadiene styrene is a popular thermoplastic material in engineering applications. The wood sawdust generated from manufacturing products made from jackfruit ( Artocarpus heterophyllus Lam., family Moraceae) is a promising cellulosic material for biocomposite materials due to its availability and environmental friendliness. In this paper, wood polymer composites were prepared using a twin-screw extruder with ABS and varying ratios of wood sawdust particles. The scanning electron microscopy images confirmed the proper mixing of filler material with the matrix. Mechanical tests were conducted on the composites to evaluate their impact, tensile, and flexural strength per ASTM standard. Dynamic mechanical thermal analysis (DMTA) was performed to determine the viscoelastic properties of the composites. The results showed that the mechanical properties decreased with increasing wood sawdust content. The wood sawdust content significantly affected the viscoelastic properties of the composites. Interestingly, the storage modulus was the highest in composites with 20% (by weight) wood sawdust content, while the loss modulus remained almost the same across all composites. The findings suggest that the wood polymer composites made from jackfruit wood sawdust and ABS exhibit comparable physical property to virgin polymer and could be used for various structural materials. Graphical Abstract

The incorporation of nanoparticles can significantly enhance the properties of polymers. However, the industrial production of nanocomposites presents a technological challenge in achieving the proper dispersion of nanoparticles within the polymer matrix. In this work, a novel device is presented that can be seamlessly integrated with standard twin-screw extruders, enabling the application of ultrasonic vibration to molten polymeric material. The primary objective of this study is to experimentally validate the effectiveness of this technology in improving the dispersion of nanoparticles. To accomplish this, a comparative analysis was carried out between nanocomposites obtained through conventional compounding extrusion and those processed with the assistance of ultrasonic vibrations. The nanocomposites under investigation consist of a polypropylene (PP) matrix reinforced with nano clays (Cloisite 20A) at a target loading ratio of 5% by weight. To comprehensively evaluate the impact of the ultrasound-assisted compounding, various key properties were assessed, such as the melt flow index (MFI) to characterize the flow behavior, mechanical properties to evaluate the structural performance, oxygen barrier properties to assess potential gas permeability, and microstructure analysis using Scanning Electron Microscopy (SEM) for detailed morphology characterization. The results suggested an improvement in nanoparticle dispersion when using the ultrasound device, particularly when the intensity was adjusted to 60%.