Polymer Testing最新文献

In environments with high humidity or water exposure, the performance of polyurethane materials could be adversely affected by water and its penetration, which could compromise their long-term utility. The influence of water on polyurethane materials is affected by water transport and various factors. This article summarizes the factors affecting the water absorption of polyurethane, introduces research methods for water transport in polyurethane, analyzes the pathways of water transport, and reviews the influence of water on the mechanical properties of polyurethane and its composite materials. The ultimate goal of this paper is to furnish a comprehensive theoretical foundation and a valuable reference for the research and practical application of polyurethane materials in water environments.

The sustainability and additive manufacturing of dielectric insulators are the development direction of the power system. Introducing dynamic covalent bonds in light-based 3D printing have attracted considerable attention as the reversible crosslinks allow for the reprocessing of printed objects. However, there generally exists a trade-off between mechanical strength, glass transition temperature (T_g), and reconfigurability for dynamic covalent networks. The reconfiguring process of the dynamic covalent network often requires high mobility of molecular chains and large free volumes, which in turn decreases the mechanical strength, T_g, and electrical insulating performance. Herein, we demonstrate a novel strategy for developing a kind of mechanically robust and sustainable vitrimer by building a rigid-flexible coupling inter-penetration network (IPN). Specifically, a two-stage curing approach was used to prepare high-performance 3D-printing vitrimers by using the plant oil-epoxy hybrid resin, which brings a lot of ester bonds and β-hydroxyl ester for the crosslinking network. Computational techniques with molecular dynamics calculation are used for the design and optimization of the crosslinking network, and then the optimized IPN is prepared by digital light processing 3D printing and subsequent heat curing. In the IPN, the epoxy backbone is rigid to enhance the T_g and tensile strength, while the plant-based methacrylate is flexible to guarantee topological rearrangement at elevated temperatures. Compared to reported epoxy vitrimers, the resultant IPN exhibits simultaneous high T_g (111 °C), outstanding tensile strength and toughness (tensile strength of 70 MPa, elongation at break of 17.58 %), good topological rearrangement, and excellent dielectric properties (permittivity less than 4, breakdown strength of 49.3 kV/mm). This work provides a new strategy for balancing the strength, toughness, electrical insulating and sustainability of 3D-printed thermosets.

The melamine-formaldehyde (MF) resin adhesive was modified by graphene oxide (GO), the chemical structure, wettability, bonding performance, tensile properties, curing performance and thermal properties of the modified resin were analyzed, and the toughening mechanism was also discussed in this study. The results showed that: (1) The MF resin with a high molar ratio possessed stable methylene ether bonds, which could easily generate parallel folding in space to form a π-π stacking supramolecular self-assembly special structure, with the potential of enhancing the toughness of molecular structures. (2) GO contained a large number of oxygen-containing reactive functional groups, which could further lower the curing temperature of the MF resin. A dense cross-linked network structure improved the thermal stability of the resin. (3) The bonding strength and toughness of the resin were significantly improved when the content of GO was 0.1 wt%. However, due to the large specific surface area and the intense π-π interaction between sheets, GO was easy to agglomerate, and the properties of the resin with GO content of 0.4 wt% degraded sharply. (4) The crystallinity of the MF resin modified by GO decreased, and the surface energy and plastic deformation energy increased due to the increased fracture crack path and fracture surface of the resin, which was the macro-reason for the improvement of toughness. (5) The strong π-π interaction between GO sheets and π-π accumulation between triazine rings were like parallel “springs” in the molecular structure of the resin, which might be the internal reason for the improvement of toughness. In addition, it was also proved that this special structure could limit the activity of hydroxymethyl and the release of free formaldehyde in the resin.

Polyurethane elastomers (PUEs) will experience different strain rates in different application scenarios. Therefore, it is of great significance to study the mechanical properties of PUE under a wide range of strain rates and establish a constitutive model that considers strain rates with high accuracy and few parameters. In this study, the quasi-static and dynamic compression tests of two types of PUEs (PUE55 and PUE85) were carried out, and investigated the strain rate effect of the materials. Based on the Mooney-Rivlin hyperelastic model and the Prony series, a compressible visco-hyperelastic constitutive model for PUE was established. Different from the conventional constant relaxation time in Prony series, two relaxation times that vary exponentially with principal stretch were proposed based on the relaxation test to describe the strain rate effect of the material at low and high strain rate respectively. In addition, using the visco-hyperelastic constitutive model to obtain the model inputs of the Simplified rubber/foam model in LS-DYNA, the impact process of the Metal/PUE composite projectile was reproduced under different impact conditions through the finite element simulation. Simulation results verified the visco-hyperelastic model in generating numerical model material parameters and the rationality of the Simplified rubber/foam model in describing PUEs.

Surface-modified nanoparticles are commonly used to improve the mechanical properties and wear resistance of polytetrafluoroethylene (PTFE). However, fewer studies have been devoted to quantitatively revealing the action mechanism of graphene (Gr) modified with different functional groups on the mechanical and tribological properties of PTFE. Herein, the effects of four functional groups (−OH, −NH₂, −COOH, and −COOCH₃ functional groups) on the surface of Gr nanosheets on the mechanical and tribological properties of PTFE nanocomposites are studied using molecular dynamics simulations. The results indicate that the incorporation of functional groups to the Gr surface is able to significantly improve the mechanical properties and wear resistance of the nanocomposites, and the COOH-functionalized Gr nanosheet shows the best reinforcing effect due to the synergistic effect of its own high surface roughness and strong interfacial interaction between itself and the matrix. It is also found that the friction coefficient of the nanocomposites is obviously increased by the inclusion of functionalized Gr nanosheets, and the greater the surface roughness of the functionalized Gr nanosheet, the more significant the growth of the friction coefficient of the nanocomposites. The pull-out test and confined shear simulation reveal that due to the increased interfacial shear strength and the isolation of functional groups, an inhomogeneous transfer film is formed at the friction interface, leading to a decreased anti-friction property. This study provides some guidance for the future design and development of polymer nanocomposites with excellent mechanical and tribological performance for use in extreme service conditions.

Rigid polypropylene is mechanically recycled but flexible polypropylene is mostly used in energetic valorization because of the poor properties of the recycled polymer. A recycled polypropylene-based composite with outstanding properties for flexible food packaging was developed. For the first time, the influence of maleated polypropylene copolymer addition and the fumed silica/copolymer ratio on the packaging properties of recycled flexible polypropylene under the effects of silica hydrophilicity was investigated. The structural, morphological, thermal, mechanical, melt flow, overall migration, water vapor barrier and sealing properties of the developed nanocomposites were analyzed. Prominently, the addition of 1:1 maleated polypropylene and hydrophobic nanosilica improved the global performance of all tested methods. The recycled polypropylene had an overall migration to olive oil of 17 mg dm⁻², exceeding the limit allowed for food packaging, but the developed added-value composite reduced it to the tolerance limit according EU legislation. The seal strength was drastically increased by 50 % with adhesive peeling, high thermal stability, and well-dispersed particles without affecting the ductility.

Epoxy/glass fibre composites possess excellent mechanical and electrical properties and are widely utilised in electrical and electronic power equipment. However, the composites exhibit relatively poor thermal conductivity, causing the temperature of the composites to increase during the operation of power equipment, resulting in a significant reduction in the electrical breakdown strength. Although the effects of thermal aging on polymeric materials have been widely studied, its influence on electrical strength mechanisms has not been investigated at the molecular level. In this study, epoxy/glass fibre composite specimens were subjected to accelerated thermal aging treatment for 360 h at 180 °C. Functional groups, molecular chain dynamics, and electrical breakdown are characterised using infrared spectroscopy, dielectric spectroscopy, and breakdown measurement. Subsequently, electrical breakdown mechanism and life prediction of the thermally aged composites are discussed. During thermal aging, the epoxy resin molecular chains undergo continuous oxidation and chain scission, which generate numerous polar functional groups and short chains and an increase in the free volume. This triggers an enhancement in the chain segmental dynamics, thereby significantly reducing the activation energy of the epoxy resin. After 360 h, activation energy decreased from 0.78 eV to 0.67 eV. The DC breakdown voltages of the specimens decreased from 168.28 kV/mm to 134.91 kV/mm. An insulation life prediction model for thermally aged epoxy/glass fibre composites is established based on the time-temperature equivalence theory. The prediction results indicate that the service life of the operational composites is approximately 11.2 years at 353 K, which is consistent with engineering experience.

Polylactic acid (PLA) is a biodegradable polymer from renewable resources with mechanical properties comparable to traditional polymers, but with a higher cost. A solution to this issue is the production of bio-based composites to partially replace the PLA matrix with industrial wastes characterized by a zero-cost, e.g., linoleum, to also valorize them in a circular economy perspective. Linoleum heterogeneous nature deriving from the simultaneous presence of lignocellulosic and inorganic fillers and oil/rosin binders, made the evaluation of matrix/filler compatibilization strategies necessary. Two approaches were considered, one from the filler perspective with NaOH and silane treatments, and the other one from the matrix perspective by adding a chain extender (C.E.). The first approach marginally improved tensile stiffness (by 1.6 %) compared to neat PLA but caused a significant decrease of 32.8 % in strength. Considering this, the costs and disposal of the chemicals and the increased environmental impact of the process, this approach was discarded. One the contrary, the introduction of C.E. does not modify the manufacturing process and increases tensile stiffness and elongation at break of 7.2 % and 415.5 % compared to neat PLA with a tolerable reduction in strength, i.e., 16.6 %, thus being a suitable way to exploit linoleum as zero-cost filler.

The intricate failure modes and the yet unclear rate dependency of carbon fiber reinforced plain weave composite materials pose a challenge to mechanics researchers. This study establishes an energy-based evolution mechanism for the compressive failure modes of plain weave composite materials as the strain rate varies. This mechanism illustrates how the rate dependency of failure modes arises from the competitive relationship between strain potential energy and deformation kinetic energy. At low loading rates, the specimen exhibits a progressive crushing failure mode characterized by low peak stress and significant geometric deformation. As the loading strain rate increases, the energy required for this geometric deformation also increases. When the energy expenditure surpasses that needed to elevate the stress level of the specimen, it transitions to an instantaneous failure mode with high peak stress. In this mode, the specimen fractures into multiple small fragments immediately upon failure, lacking the large geometric deformations observed at lower rates. Through calculating this energy mechanism, a transition strain rate of 180 s⁻¹ was determined for both failure modes. The accuracy of this mechanism was further verified by tests conducted near the critical strain rate. The energy-based evolution mechanism for failure modes provides a simplified and concise framework for simplifying complex models of composite material failures.

Using the solution casting procedure, poly (vinyl alcohol)/carboxymethyl cellulose/polypyrene/milled multiwall carbon nanotubes, PVA/CMC/PPy/x wt% milled MWCNTs blended polymers were formed. X-ray diffraction and scanning electron microscopy were employed to inspect the structure and morphology of the resulted blends. The lowest direct and indirect optical band gaps are (5, 4.3) eV and (4.37, 3.38) eV, respectively, achieved when the MWCNTs content in the doped blend was 0.25 wt %. By incorporating varying quantities of milled MWCNTs into the PVA/CMC/PPy blended polymer, consistent enhancements were observed in the optical dielectric constant and optical conductivity values. The blend with 0.25 wt% MWCNTs exhibited the maximum values of refractive index. The maximum electric dielectric constant and energy density values were attained as x = 0.15. The temperature impacted the dielectric constants and energy storage values. All blends fit with the CBH model. The impact of MWCNTs doping level and the temperature on the impedance spectroscopy and electric modulus of the host blend was explored. The sample with x = 0.15 has the smallest relaxation time. The impact of MWCNTs doping level on the dc conductivity, activation energy and conductivity mechanism of the host blend was explored. The doped blends with x = 0.15 is viable materials for energy storage purposes.