A developed, eco-friendly, and flexible thermoplastic elastomeric foam from SEBS for footwear application

Micro/Nano Engineering Advances Next-Generation Flexible X-ray Detectors

The growing demands for X-ray imaging applications impose diverse and stringent requirements on advanced X-ray detectors. Among these, flexibility stands out as the most expected characteristic for next-generation X-ray detectors. Flexible X-ray detectors can spatially conform to nonflat surfaces, substantially improving the imaging resolution, reducing the X-ray exposure dosage, and enabling extended application opportunities that are hardly achievable by conventional rigid flat-panel detectors. Over the past years, indirect- and direct-conversion flexible X-ray detectors have made marvelous achievements. In particular, microscale and nanoscale engineering technologies play a pivotal role in defining the optical, electrical, and mechanical properties of flexible X-ray detectors. In this Perspective, we spotlight recent landmark advancements in flexible X-ray detectors from the aspects of micro/nano engineering strategies, which are broadly categorized into two prevailing modalities: materials-in-substrate and materials-on-substrate. We also discuss existing challenges hindering the development of flexible X-ray detectors, as well as prospective research opportunities to mitigate these issues.

Fabrication of Triblock Elastomer Foams and Gelation Studies for Oil Spill Remediation

Polymeric foamed materials are among the most widely utilized technologies for oil spill accidents and releases of oil-contaminated wastewater oil due to their porosity to absorb and separate oil/water effectively. However, a major limitation of traditional polymeric foams is their reliance on an ad/absorption mechanism as the sole method of oil capture, leading to potential oil leakage once their saturation point is exceeded. Tri-block polymer styrene-ethylene-butylene-styrene (SEBS) is a fascinating absorbent material that can bypass this limitation by both capturing oil and providing a sealing mechanism via gelation to prevent oil leakage due to its unique chemical structure. SEBS foams are produced via simultaneous crosslinking and foaming that results in an impressive expansion ratio of up to 15.2 with over 93% porosity. Most importantly, the SEBS foams show great potential as oil absorbents in spill remediation, demonstrating rapid and efficient oil absorption coupled with superhydrophobic properties. Moreover, the unique interaction between the oil and SEBS enables the formation of a physical gel, acting as an effective barrier against oil leakage. These findings indicate the potential for commercializing SEBS foam as a viable option for geotextiles to mitigate oil spill concerns from infrastructures.

Comparative Study of the Foaming Behavior of Ethylene-Vinyl Acetate Copolymer Foams Fabricated Using Chemical and Physical Foaming Processes

Ethylene-vinyl acetate copolymer (EVA), a crucial elastomeric resin, finds extensive application in the footwear industry. Conventional chemical foaming agents, including azodicarbonamide and 4,4'-oxybis(benzenesulfonyl hydrazide), have been identified as environmentally problematic. Hence, this study explores the potential of physical foaming of EVA using supercritical nitrogen as a sustainable alternative, garnering considerable interest in both academia and industry. The EVA formulations and processing parameters were optimized and EVA foams with densities between 0.15 and 0.25 g/cm3 were produced. Key findings demonstrate that physical foaming not only reduces environmental impact but also enhances product quality by a uniform cell structure with small cell size (50-100 μm), a wide foaming temperature window (120-180 °C), and lower energy consumption. The research further elucidates the mechanisms of cell nucleation and growth within the crosslinked EVA network, highlighting the critical role of blowing agent dispersion and localized crosslinking around nucleated cells in defining the foam's cellular morphology. These findings offer valuable insights for producing EVA foams with a more controllable cellular structure, utilizing physical foaming techniques.

Stearate-Coated Biogenic Calcium Carbonate from Waste Seashells: A Sustainable Plastic Filler

Waste seashells from aquaculture are a massive source of biogenic calcium carbonate (bCC) that can be a potential substitute for ground calcium carbonate and precipitated calcium carbonate. These last materials find several applications in industry after a surface coating with hydrophobic molecules, with stearate as the most used. Here, we investigate for the first time the capability of aqueous stearate dispersions to coat bCC powders from seashells of market-relevant mollusc aquaculture species, namely the oyster Crassostrea gigas, the scallop Pecten jacobaeus, and the clam Chamelea gallina. The chemical-physical features of bCC were extensively characterized by different analytical techniques. The results of stearate adsorption experiments showed that the oyster shell powder, which is the bCC with a higher content of the organic matrix, showed the highest adsorption capability (about 23 wt % compared to 10 wt % of geogenic calcite). These results agree with the mechanism proposed in the literature in which stearate adsorption mainly involves the formation of calcium stearate micelles in the dispersion before the physical adsorption. The coated bCC from oyster shells was also tested as fillers in an ethylene vinyl acetate compound used for the preparation of shoe soles. The obtained compound showed better mechanical performance than the one prepared using ground calcium. In conclusion, we can state that bCC can replace ground and precipitated calcium carbonate and has a higher stearate adsorbing capability. Moreover, they represent an environmentally friendly and sustainable source of calcium carbonate that organisms produce by high biological control over composition, polymorphism, and crystal texture. These features can be exploited for applications in fields where calcium carbonate with selected features is required.

TPU-based porous heterostructures by combined techniques

The production of thermoplastic polyurethane-based porous heterostructures combining physical foaming with fused deposition modeling is detailed in this contribution. The choice of combining these two techniques lies in the possibility of creating objects endowed with a dual-scale structure at millimeter scale by fused deposition modeling and at microscopic scale by gas foaming. Thermal stability and rheological properties of the neat polymer were studied prior to foaming to design a suitable processing protocol and three different combined techniques are proposed: pressure quench, temperature rise and direct 3D foam printing. Foam morphologies were evaluated by SEM and foamed samples were characterized by thermal and mechanical analyses to highlight the differences among the combined processing techniques. Samples foamed via pressure quench exhibit the highest degree of crystallinity and a uniform cell morphology, also resulting in the largest stiffness. The results presented in this contribution open up the possibility of producing objects with complex geometry and porosity architecture at the dual scale.

Mechanically Durable Memristor Arrays Based on a Discrete Structure Design

Memristors constitute a promising functional component for information storage and in-memory computing in flexible and stretchable electronics including wearable devices, prosthetics, and soft robotics. Despite tremendous efforts made to adapt conventional rigid memristors to flexible and stretchable scenarios, stretchable and mechanical-damage-endurable memristors, which are critical for maintaining reliable functions under unexpected mechanical attack, have never been achieved. Here, the development of stretchable memristors with mechanical damage endurance based on a discrete structure design is reported. The memristors possess large stretchability (40%) and excellent deformability (half-fold), and retain stable performances under dynamic stretching and releasing. It is shown that the memristors maintain reliable functions and preserve information after extreme mechanical damage, including puncture (up to 100 times) and serious tearing situations (fully diagonally cut). The structural strategy offers new opportunities for next-generation stretchable memristors with mechanical damage endurance, which is vital to achieve reliable functions for flexible and stretchable electronics even in extreme and highly dynamic environments.

Body-Temperature Programmable Soft-Shape Memory Hybrid Sponges for Comfort Fitting

Porous shape memory hybrids are fabricated with different matrix (silicone) hardness and different inclusion (polycaprolactone, PCL) ratios. They are characterized to obtain their mechanical response to cyclic loads (with/without pre-straining/programming) and their shape memory performances after body-temperature programming are investigated. These materials are lightweight due to their porous structures. Wetted hydrogels used in the fabrication process for creating pores are reusable and hence this process is eco-friendly. These porous shape memory hybrids exhibit the good shape memory effect of around 90% with higher inclusion (PCL) ratios, which is better than the solid versions reported in the literature. Hence, it is concluded that these materials have great potential to be used in, for instance, insoles and soles for comfort fitting, as demonstrated.

Production and characterization of thermoplastic elastomer foams based on the styrene–ethylene–butylene–styrene (SEBS) rubber and thermoplastic material

Thermoplastic elastomer foams based on styrene–ethylene–butylene–styrene (SEBS)/polypropylene (PP) were produced by using different processing techniques such as extrusion and injection molding to achieve optimized mechanical and thermal properties in terms of strength, elongation, and damping capability. Foaming is a method of introducing gas-filled cells into the material and it is considered an effective way to meet the requirements for higher impact resistance with low density and relatively low hardness. In this study, microspheres were used as a foaming agent and were introduced to the system by using an injection molding machine. They were used in different percentages and ranged from 1 and 3%. They decrease the density of the product thereby lowering the weight and cost. Besides improving damping abilities and decreasing the density, inorganic fillers such as talc, silica, and calcium carbonate were used to increase the mechanical strength, and their effectivity was also investigated. It was observed that a higher amount of foaming agent lowered the density by creating voids in the blend, as expected. The introduction of fillers increases the mechanical properties; however, the density had a negative effect even in the presence of foaming agents. About 3% density reduction can be achieved in the presence of talc and a foaming agent whereas the other fillers had an opposite effect on the density. Accordingly, the impact resistance was affected negatively because of the stiffness of the filler materials, and the highest Izod impact value was 50.2 kJ/m2. The elastic modulus values for foamed samples and filled with CaCO3, talc, and silica were 808, 681, and 552 MPa respectively. Combining foaming and thermoplastic elastomers (TPEs) offers a wide variety of possibilities to new and existing applications. In addition to low hardness and density, foaming provides better damping ability thanks to its morphological structure.

Thermo-Mechanical and Morphological Properties of Polymer Composites Reinforced by Natural Fibers Derived from Wet Blue Leather Wastes: A Comparative Study

The present work investigated the possibility to use wet blue (WB) leather wastes as natural reinforcing fibers within different polymer matrices. After their preparation and characterization, WB fibers were melt-mixed at 10 wt.% with poly(lactic acid) (PLA), polyamide 12 (PA12), thermoplastic elastomer (TPE), and thermoplastic polyurethane (TPU), and the obtained samples were subjected to rheological, thermal, thermo-mechanical, and viscoelastic analyses. In parallel, morphological properties such as fiber distribution and dispersion, fiber-matrix adhesion, and fiber exfoliation phenomena were analyzed through a scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS) to evaluate the relationship between the compounding process, mechanical responses, and morphological parameters. The PLA-based composite exhibited the best results since the Young modulus (+18%), tensile strength (+1.5%), impact (+10%), and creep (+5%) resistance were simultaneously enhanced by the addition of WB fibers, which were well dispersed and distributed in and significantly branched and interlocked with the polymer matrix. PA12- and TPU-based formulations showed a positive behavior (around +47% of the Young modulus and +40% of creep resistance) even if the not-optimal fiber-matrix adhesion and/or the poor de-fibration of WB slightly lowered the tensile strength and elongation at break. Finally, the TPE-based sample exhibited the worst performance because of the poor affinity between hydrophilic WB fibers and the hydrophobic polymer matrix.

Improvement of Impact Strength of Polylactide Blends with a Thermoplastic Elastomer Compatibilized with Biobased Maleinized Linseed Oil for Applications in Rigid Packaging

This research work reports the potential of maleinized linseed oil (MLO) as biobased compatibilizer in polylactide (PLA) and a thermoplastic elastomer, namely, polystyrene-b-(ethylene-ran-butylene)-b-styrene (SEBS) blends (PLA/SEBS), with improved impact strength for the packaging industry. The effects of MLO are compared with a conventional polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene-graft-maleic anhydride terpolymer (SEBS-g-MA) since it is widely used in these blends. Uncompatibilized and compatibilized PLA/SEBS blends can be manufactured by extrusion and then shaped into standard samples for further characterization by mechanical, thermal, morphological, dynamical-mechanical, wetting and colour standard tests. The obtained results indicate that the uncompatibilized PLA/SEBS blend containing 20 wt.% SEBS gives improved toughness (4.8 kJ/m²) compared to neat PLA (1.3 kJ/m²). Nevertheless, the same blend compatibilized with MLO leads to an increase in impact strength up to 6.1 kJ/m², thus giving evidence of the potential of MLO to compete with other petroleum-derived compatibilizers to obtain tough PLA formulations. MLO also provides increased ductile properties, since neat PLA is a brittle polymer with an elongation at break of 7.4%, while its blend with 20 wt.% SEBS and MLO as compatibilizer offers an elongation at break of 50.2%, much higher than that provided by typical SEBS-g-MA compatibilizer (10.1%). MLO provides a slight decrease (about 3 °C lower) in the glass transition temperature (T_g) of the PLA-rich phase, thus showing some plasticization effects. Although MLO addition leads to some yellowing due to its intrinsic yellow colour, this can contribute to serving as a UV light barrier with interesting applications in the packaging industry. Therefore, MLO represents a cost-effective and sustainable solution to the use of conventional petroleum-derived compatibilizers.

Development of Poly (Lactide Acid) Foams with Thermally Expandable Microspheres

This study presents the investigation of different content of thermally expandable microsphere (EMS) type of a physical blowing agent added to polylactic acid (PLA). The effects of the different doses of EMS, processing temperatures, and d-lactide content of the polylactic acid were analyzed for foam properties and structures. We characterized the different PLAs and the physical blowing agent with different testing methods (gel permeation chromatography, rotational rheometry, isothermal thermogravimetric analysis, and thermomechanical analysis). The amounts of the foaming agent were 0.5, 1, 2, 4, 8 wt%, and processing temperatures were 190 °C, 210 °C, and 230 °C. The foam structures were produced by twin-screw extrusion. We used scanning electron microscopy to examine the cell structure of the foams produced, and carried out morphological and mechanical tests as well. The result of extrusion foaming of PLA using different amounts of EMS shows that an exponentially decreasing tendency of density reduction can be achieved, described by the following equation, ρ(x)=1.062∙e-x7.038+0.03 (R² = 0.947) at 190 °C. With increasing processing temperature, density decreases at a lower rate, due to the effect that the microspheres are unable to hold the pentane gas within the polymer shell structure. The d-lactide content of the PLAs does not have a significant effect on the density of the produced foam structures.