Evolution of ordered structure of TPU in high-elastic state and their influences on the autoclave foaming of TPU and inter-bead bonding of expanded TPU beads

Development of Eco-Friendly and High-Strength Foam Sensors Based on Segregated Elastomer Composites with a Large Work Range and High Sensitivity

Achieving a high-strength piezoresistive foam with high sensitivity and a large workable range remains a major challenge. To realize these goals, we developed a facile, novel, and eco-friendly strategy for constructing segregated microcellular structures fabricated using coating, heat compression molding, and supercritical CO2 (ScCO2) foaming. The segregated poly(ether block amide) (PEBA)/carbon nanostructure (CNS) composites were fabricated via compression molding. This effectively improved the foamability and cell morphology of PEBA/CNS composites. Moreover, compared with the randomly distributed structure, the segregated structure also endowed the foams with better conductivity and sensing capability. Subsequently, the ScCO2 foaming was employed to fabricate segregated PEBA/CNS composite foams. The foaming gave composites a large compressibility and reduced their percolation threshold. Under 1 wt % CNS loading, via tuning the expansion ratio of foam from ∼2.1 to 4.1, the compression stress at 50% compression strain of foam varied from ∼3.3 to 0.5 MPa, and the conductivity changed from 4.89 × 10-3 to 1.93 × 10-6 s/m, implying a tunable conductivity. Additionally, the adjustable conductivity enabled the sensitivity of segregated composite foams to be regulated. The segregated PEBA/CNS foam (FCNS1-4.1) exhibited a good combination of high sensitivity (GF = 3.5), large work range (80% strain), and high compression strength (∼0.5 MPa at 50% strain) as well as a stable, reproducible, and durable sensing response under a low CNS content (∼0.11 vol %). Furthermore, the ΔI/I0 of FCNS1-4.1 (75.6% porosity) reached a high value of ∼810 and exhibited an ultrahigh sensitivity of ∼3706 (Δ I / I 0 ε ) from 60 to 80% strain. Moreover, the foam sensor could be used as a sensing function sole for monitoring diverse human motions. Therefore, the segregated PEBA/CNS composite foams with outstanding piezoresistive performances show promising potential applications in monitoring human motions as wearable electronics and provides a new design strategy for a new generation of foam sensors with high performance.

Progress in Photocurable 3D Printing of Photosensitive Polyurethane: A Review

In recent years, as a class of advanced additive manufacturing (AM) technology, photocurable 3D printing has gained increasing attention. Based on its outstanding printing efficiency and molding accuracy, it is employed in various fields, such as industrial manufacturing, biomedical, soft robotics, electronic sensors. Photocurable 3D printing is a molding technology based on the principle of area-selective curing of photopolymerization reaction. At present, the main printing material suitable for this technology is the photosensitive resin, a composite mixture consisting of a photosensitive prepolymer, reactive monomer, photoinitiator, and other additives. As the technique research deepens and its application gets more developed, the design of printing materials suitable for different applications is becoming the hotspot. Specifically, these materials not only can be photocured but also have excellent properties, such as elasticity, tear resistance, fatigue resistance. Photosensitive polyurethanes can endow photocured resin with desirable performance due to their unique molecular structure including the inherent alternating soft and hard segments, and microphase separation. For this reason, this review summarizes and comments on the research and application progress of photocurable 3D printing of photosensitive polyurethanes, analyzing the advantages and shortcomings of this technology, also offering an outlook on this rapid development direction.

Biodegradable Polylactic Acid and Its Composites: Characteristics, Processing, and Sustainable Applications in Sports

Polylactic acid (PLA) is a biodegradable polyester polymer that is produced from renewable resources, such as corn or other carbohydrate sources. However, its poor toughness limits its commercialization. PLA composites can meet the growing performance needs of various fields, but limited research has focused on their sustainable applications in sports. This paper reviews the latest research on PLA and its composites by describing the characteristics, production, degradation process, and the latest modification methods of PLA. Then, it discusses the inherent advantages of PLA composites and expounds on different biodegradable materials and their relationship with the properties of PLA composites. Finally, the importance and application prospects of PLA composites in the field of sports are emphasized. Although PLA composites mixed with natural biomass materials have not been mass produced, they are expected to be sustainable materials used in various industries because of their simple process, nontoxicity, biodegradability, and low cost.

Autoclave foaming and steam-chest molding of polypropylene/polybutene-1 blend bead foams and their crystallization and mechanical properties

Expanded polypropylene (EPP) foams have showed wide applications in our daily life, such as automotive and packaging. Usually, autoclave foaming combined with steam-chest molding is the main artwork to prepare the high-precision EPP foam products. However, the foaming behavior of EPP and the excessive pressure required for molding still need to be further improved, which is great significance for energy saving and cost saving, etc. Herein, this study finds that adding a certain amount of polybutene-1 (PB-1) into the PP can help to reduce the temperature and pressure required for foaming/molding, and to broaden the foaming temperature. For example, in order to make the foam beads bonding well and with the expansion ratio of 20, the molding pressure should be higher than 2.7 bar for Neat PP foams, but just 1.5 bar for PP/PB-1 mixtures. Moreover, the effects of PB-1 content on the crystallization properties and foaming/molding behaviors of the PP/PB-1 bead foams are illustrated, and then the mechanical properties are also studied. Furthermore, the low-pressure foaming strategy presented here is beneficial for reducing the barriers of energy consumption and promoting the development of new bead foam materials.

Supercritical Fluid Microcellular Foaming of High-Hardness TPU via a Pressure-Quenching Process: Restricted Foam Expansion Controlled by Matrix Modulus and Thermal Degradation

High-hardness thermoplastic polyurethane (HD-TPU) presents a high matrix modulus, low-temperature durability, and remarkable abrasion resistance, and has been used in many advanced applications. However, the fabrication of microcellular HD-TPU foam is rarely reported in the literature. In this study, the foaming behavior of HD-TPU with a hardness of 75D was investigated via a pressure-quenching foaming process using CO₂ as a blowing agent. Microcellular HD-TPU foam with a maximum expansion ratio of 3.9-fold, a cell size of 25.9 μm, and cell density of 7.8 × 10⁸ cells/cm³ was prepared, where a high optimum foaming temperature of about 170 °C had to be applied with the aim of softening the polymer's matrix modulus. However, the foaming behavior of HD-TPU deteriorated when the foaming temperature further increased to 180 °C, characterized by the presence of coalesced cells, microcracks, and a high foam density of 1.0 g/cm³ even though the crystal domains still existed within the matrix. The cell morphology evolution of HD-TPU foam was investigated by adjusting the saturation time, and an obvious degradation occurred during the high-temperature saturation process. A cell growth mechanism of HD-TPU foams in degradation environments was proposed to explain this phenomenon based on the gas escape through the defective matrix.

A Study on Preparation and Property Evaluations of Composites Consisting of TPU/Triclosan Membranes and Tencel®/LMPET Nonwoven Fabrics

This study investigated eco-friendly antibacterial medical protective clothing via the nonwoven process and characteristic evaluations. Firstly, Tencel^® fibers and low melting point polyester (LMPET) fibers (re-sliced and granulated from recycled PET bottles) were mixed at different ratios and then needle punched at diverse needle rolling depths. The influences of manufacturing parameters on the Tencel^®/LMPET nonwoven fabrics were examined in terms of mechanical properties, water vapor transmission rate, and stiffness. Next, Tencel^®/LMPET nonwoven fabrics were combined with thermoplastic polyurethane (TPU)/Triclosan antibacterial membranes that contained different contents of triclosan using melt processing technology. The resulting Tencel^®/LMPET/TPU/Triclosan composites were characterized via different measurements; an optimal bursting strength of 86.86 N, an optimal horizontal tensile strength of 41.90 N, and an optimal stiffness along the MD and CD of 8.60 cm were recorded. Furthermore, the Tencel^®/LMPET/TPU/Triclosan composites exhibited a distinct inhibition zone in the antibacterial measurement, and the hydrostatic pressure met the requirements of the EN 14126:2003 and GB 19082-200 disposable medical protective gear test standards.

Supercritical CO2 Foaming of Poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

The supercritical carbon dioxide foaming characteristics of the biodegradable polymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) are studied for environmentally friendly packaging materials. The effect of the 4HB composition of the P(3HB-co-4HB) copolymers on the foaming conditions such as pressure and temperature is studied and the density and the expansion ratio of the resulting P(3HB-co-4HB) foam are together evaluated. The increase in the 4HB content reduces the crystallinity and tan δ value of P(3HB-co-4HB) required for the growth of the foam cells. Therefore, the foaming temperature needs to be lower to retain a suitable tan δ value of P(3HB-co-4HB) for foaming. It was found that P(3HB-co-4HB) with less crystallinity showed better formability and cell uniformity. However, foaming is not possible regardless of the foaming temperature when the 4HB content of P(3HB-co-4HB) is over 50%, due to the high tan δ value. A lower foam density and higher expansion ratio can be obtained with crystalline P(3HB-co-4HB) of low 4HB content, compared with non-crystalline P(3HB-co-4HB) of high 4HB content. The expansion ratio of P(3HB-co-4HB) foams can be increased slightly by using a chain extender, due to the lowing of crystallinity and tan δ. This is most effective in the case of P(3HB-co-4HB), whose 4HB content is 16%.