Thermoplastic Behavior Research Articles

Melt‐Extruded Thermoplastic Liquid Crystal Elastomer Rotating Fiber Actuators

AbstractUntethered soft fiber actuators are advancing toward next‐generation artificial muscles, with rotating polymer fibers allowing controlled rotational deformations and contractions accompanied by torque and longitudinal forces. Current approaches, however, are based either on non‐recyclable and non‐reprogrammable thermosets, exhibit rotational deformations and torques with inadequate actuation performance, or involve intricate multistep processing and photopolymerization impeding scalable fabrication and manufacturing of millimeter‐thick fibers. Here, the melt‐extrusion and drawing of a 50 m long thermoplastic liquid crystal elastomer fiber with a ≈1.3 mm diameter on a large scale is reported. With the responsive thermoplastic material, rotating actuators are fabricated via easily exploited programming freedom resulting in large, reversible rotational deformations and torques. The actuation performance of the twisted fibers may be controlled by the programmed twisting density without complicated preparation steps or photocuring being required. The thermoplastic behavior enables fabrication of plied fibers, demonstrated as a triple helical twisted rope constructed from individual rotating fibers delivering up to three times as great rotational and longitudinal forces capable of reversibly opening and lifting a screw cap vial. Besides the programmability, the thermoplastic material employed lends itself to be completely reprocessed into other configurations with self‐healing properties in contrast to thermosets.

A Sustainable Multi‐Dimensional Printable Material

AbstractPolymeric materials are usually tailored for specific functionality. A single polymer exhibiting multiple simultaneous functionalities often requires intricate molecular architecture, which is difficult to manufacture at scale because of its complex synthesis routes. Herein, a facile, partly renewable composition―prepared via reactive melt processing―that induces tunable functionalities such as 3D printability, shape recovery, and self‐healing while exhibiting satisfactory mechanical properties is reported. The system with a hydrogen‐bonded 3D network consists of thermally reversible nano‐scale agglomerates of sustainable, rigid phenolic oligomers and crystallizable flexible polymer. Local molecular mobility and temperature‐dependent relaxation and recovery of the non‐equilibrium networked states enable exploiting these simultaneous functionalities. Transitions involving solidification and structure stabilization at ambient temperature spanning several hours after preheating only at 70 °C directly contrast typical thermoplastic or thermoplastic elastomer behaviors. Results from this study can inform the design of future rheology modifiers and materials for soft robotics.

Potentiality of sustainable corn starch-based biocomposites reinforced with cotton filter waste of spinning mill

The textile sector is among the leading industries globally in terms of releasing pollutants and producing waste. Despite being reusable, many wastes are squandered by disposing to landfills or incineration, creating a serious environmental threat. Because the cost of raw materials makes up a significant portion of the total product cost, manufacturers can obtain significant profits by exploiting waste generated during the manufacturing process. Herein, an attempt has been taken to utilize cotton filter waste (CFW) (collected from the humidification plant of the spinning mill) as reinforcement in manufacturing biocomposites with the corn starch (CS) matrix. Starch was considered to be the most suitable matrix as it is sustainable, abundant, natural, biodegradable, and, more importantly, capable of showing thermoplastic behavior under high temperatures. Sheets of corn starch composites reinforced with different wt% of cleaned cotton filter waste were fabricated using hand layup and compression molding techniques. The 50 wt% cotton waste was found to be optimum loading in terms of tensile strength, Young's modulus, bending strength, toughness, impact strength, and thermal Conductivity of the biocomposites. SEM micrographs revealed good interfacial adhesion (bonding) in matrix and filler interfaces, with the most substantial bonding for composites containing 50% fibers that concomitantly enhanced the mechanical properties of composites. The obtained biocomposites are deemed to be a sustainable alternative to non-degradable synthetic polymeric materials like Styrofoam for packaging and insulation applications.

Enhancing the thermoplastic behavior and mechanical performance of recycledHDPE/CaCO3composites using oxidized polyethylene

AbstractThere is a growing interest in sustainable materials based on recycled or natural feedstocks for the circular economy. Ideally, thermoplastic materials are anticipated to show appropriate melt‐flow behavior for thermo‐mechanical recycling, good melt stability, and mechanical performance. Thermoplastic polyolefins such as polyethylenes are used as single‐use packaging materials and generate environmental concerns. This article reports the melt‐processing and characterization of composite materials based on recycled high‐density polyethylene (rHDPE) and naturally abundant calcium carbonate (CaCO3) mineral at various mass ratios such as 70/30, 50/50, and 30/70. Oxidized polyethylene (OPE) is used as an additive, with 20 wt% replacement of rHDPE, to improve the materials' mechanical performance and thermoplastic behavior. Various characteristics such as tensile, viscoelastic, melt‐flow, phase morphology, and thermomechanical recyclability of the materials are analyzed. It reveals that composites with high CaCO3content compatibilized with OPE show improved tensile strength and modulus. For example, the composite based on 10/70/20 (wt%) of rHDPE/CaCO3/OPE shows an average tensile strength of 27.4 MPa and Young's modulus of 4.5 GPa, in contrast to the rHDPE sample, which shows an average tensile strength of 12.4 MPa and Young's modulus of 967 MPa. Notably, adding OPE decreases the melt‐viscosity of the rHDPE/CaCO3materials, thus improving the composites' melt‐flow and thermoplastic behavior. This study indicates the possibility of developing natural CaCO3‐rich polymer composites with thermoplastic behavior for real‐life applications with superior mechanical and melt‐flow properties suitable for thermomechanical recycling.

Investigation of the Influence of Fiber Content, Processing Conditions and Surface Roughness on the Polymer Filling Behavior in Thermoset Injection Molding

A completely opposite injection molding filling behavior of thermosets and thermoplastics by an effective and useful method developed by the authors was found. Specifically, for the thermoset injection molding, there is a strong slip between the thermoset melt and wall surface, which is not found for the injection molding of thermoplastic materials. In addition, the variables, such as the filler content, the mold temperature, the injection speed, and the surface roughness that could lead to or influence the slip phenomenon of thermoset injection molding compounds, were also investigated. Furthermore, microscopy was conducted to verify the correlation between the mold wall slip and fiber orientation. The results obtained in this paper open challenges in the field of the calculation, analysis, and simulation of mold filling behavior of highly glass fiber-reinforced thermoset resins in the injection molding process with consideration of wall slip boundary conditions.

The effect of cooling rates on crystallization and low-velocity impact behaviour of carbon fibre reinforced poly(aryl ether ketone) composites

Crystallization of poly(aryl ether ketone) (PAEK) plays an important role in the impact resistance of composite laminates but it is still unclear how the cooling rate affects the performance. In this paper, carbon fibre (CF) reinforced new copolymer PAEK and commonly used poly(ether ether ketone) (PEEK) were fabricated by furnace cooling and water cooling. The effects of cooling rates on the crystallization behaviour of thermoplastics, interfacial bonding strength, interlaminar fracture toughness and low-velocity impact performance of composites were studied. Results show that the crystallization and the crystalline size of PAEK are smaller than those of PEEK at the same cooling rate. In composites, the lower crystallinity and smaller crystals of the matrix are conducive to improving the matrix ductility and interfacial bonding strength. Under the water-cooling condition, the impact damage area CF/PAEK is about 71% lower than that of CF/PEEK, while the CAI is about 14% higher.

Research on inverse identification of johnson-cook constitutive parameters for turning 304 stainless steel based on coupling simulation

The constitutive model plays a decisive role in the accuracy and reliability of the simulation results. Because the material flow characteristics obtained by the traditional experimental method are difficult to describe thermoplastic deformation behavior in the cutting process accurately, and the inverse identification is usually carried out based on a two-dimensional orthogonal cutting model, ignoring the actual cutting state, which leads to significant deviations in the results of parameter identification. Therefore, a three-dimensional (3-D) turning finite element simplified model which conforms to the actual cutting scenario was established. Based on this model, a coupling simulation was used to inverse identify the Johnson-Cook (J-C) parameters for turning 304 stainless steel. Firstly, the automatic finite element modeling and calculation of 3-D turning is implemented by Python. Then, the inverse identification framework was built based on the ISIGHT. The initial yield strength of J-C parameters was obtained from quasi-static compression experiments. With the objective function of minimizing the error between the experimental cutting force and the simulated cutting force, the inverse identification of J-C parameters (strain reinforcement factor, strain rate sensitivity, thermal softening index, and hardening index) was carried out by a multi-island genetic algorithm. Finally, compared with cutting forces, chip morphology, and residual stresses, the feasibility of the inverse identification method and the accuracy of the constitutive model is demonstrated, which also provides a reference for the optimal identification method of constitutive parameters of difficult-to-machine materials.

Vat 3D Printing of Bioderivable Photoresins – Toward Sustainable and Robust Thermoplastic Parts

Vat photopolymerization 3D printing (3DP) of thermoplastic materials is exceedingly difficult due to the typical reliance on cross-linking to form well-defined, solid objects on timescales relevant to 3DP. Additionally, photoresin build materials overwhelmingly rely upon nonrenewable feedstocks. To address these challenges, we report the vat 3DP of bioderivable photoresins that produced thermoplastic parts with highly tunable thermal and mechanical properties. The photoresins were formulated from two monomers that are easily obtainable from lignin deconstruction: 4-propylguaiacyl acrylate (4-pGA) and syringyl methacrylate (SMA). These bioderivable materials generated printed parts that ranged from soft elastomers to rigid plastics. For example, for 4-pGA-based materials, the breaking stresses varied from 0.20 to 20 MPa and breaking strains could be tuned from 4.7% up to 1700%, whereas 3D-printed SMA-based materials resulted in higher breaking stresses (∼30 MPa) and Tgs (∼132 °C). Notably, parts printed from these bioderivable formulations exhibited thermoplastic behavior and were largely soluble in common organic solvents─expanding the application and repurposing of the 3D-printed parts. We highlight this feature by reusing a 3DP part via solvent casting. Overall, the tunable properties and thermoplastic behavior of the lignin-derivable photoresins showcase renewable lignin resources as promising biofeedstocks for sustainable 3DP.

Poly(hexylene vanillate): Synthetic Pathway and Remarkable Properties of a Novel Alipharomatic Lignin-Based Polyester

The negative environmental effects of conventional polymeric materials have intensified the research toward the production of biobased counterparts. Vanillic acid (VA), a vanillin oxidation product, is a relatively new and most promising aromatic monomer for the synthesis of biobased polyesters. In this work, the synthesis of a new, high molecular weight alipharomatic polyester, poly(hexylene vanillate) (PHV), using 4-(6-hydroxyethoxy)-3-methoxybenzoic acid via a two-stage melt polycondensation method is reported. The success of the polymerization was confirmed using nuclear magnetic resonance spectroscopy (NMR) and Fourier-transformed infrared spectroscopy (FTIR), while the number-average molecular weight (Mn) was estimated by size-exclusion chromatography (SEC). Moreover, the thermal behavior of PHV was determined with differential scanning calorimetry (DSC), and insights on the thermal stability and degradation mechanism of PHV were given. The mechanical and thermomechanical properties of the synthesized material were investigated, revealing a thermoplastic elastomer behavior, with elastic recovery at room temperature, associated with the formation of a mesophase status. Finally, rheological tests were applied, indicating that PHV is easily processable and fully recoverable upon yielding and flow cessation.

Rate and Load Effects on Scratch Behavior of Thermoplastics by Berkovich Indenter

Rate and Load Effects on Scratch Behavior of Thermoplastics by Berkovich Indenter

A numerical assessment of partitioned implicit methods for thermomechanical problems

Implicit partitioned methods for solving thermomechanical problems possess various advantages over monolithic schemes, such as flexibility and simplicity of implementation, at the cost of performance and robustness. This contribution’s primary purpose is to explore suitable partitioned implicit techniques for solving the thermomechanical problem with different sources of coupling. The spatial and time-discrete problem is reformulated as an equivalent root-finding problem using single-field solvers. This work considers the fixed-point method, Aitken relaxation, multisecant quasi-Newton methods, Newton–Krylov method, and polynomial vector extrapolation techniques in cycling mode, which are selected from the relevant literature. These techniques are employed to solve three different problems, including thermoelastic and thermoplastic constitutive behaviors, as well as thermomechanical contact. The methods are compared based on the CPU runtime and the number of residual evaluations, considering as well the memory requirements and the implementation effort. Overall, Broyden’s method can be regarded as the best performing and most advantageous method, with the Aitken relaxation also providing a very efficient and simple alternative. This work brings new insights into the application of partitioned implicit methods to thermomechanical problems and can motivate the selection of these techniques, which are not currently widespread in the community.

EVALUATION OF RESOURCE CHARACTERISTICS OF POWER EQUIPMENT PARTS UNDER THERMAL CYCLIC LOADING

The problem of assessing the strength and resource of critical engineering facilities, the operational properties of which are characterized by multiparametric non-stationary thermo-mechanical effects, is discussed. The main degradation mechanisms of structural materials (metals and their alloys) characteristic of such objects are considered. The basic requirements for mathematical models of these processes are formulated. A mathematical model describing the processes of thermoplastic deformation and accumulation of fatigue damage during degradation of the material by the mechanism of thermal fatigue has been developed from the position of mechanics of the damaged medium (MPS). The IPU model consists of three interrelated parts: relations that determine the cyclic thermoplastic behavior of the material, taking into account the dependence on the destruction process; equations describing the kinetics of fatigue damage accumulation; strength criteria of the damaged material. The variant of the defining relations of thermoplasticity is based on the idea of the yield surface and the principle of gradiency of the velocity vector of plastic deformations to the yield surface at the loading point. This version of the equations of state reflects the main effects of the process of cyclic thermoplastic deformation of the material for arbitrary complex trajectories of combined thermomechanical loading. The variant of kinetic equations of fatigue damage accumulation is based on the introduction of a scalar damage parameter, is based on energy principles and takes into account the main effects of formation, growth and fusion of microdefects under arbitrary complex loading conditions. As a criterion of the strength of the damaged material, the condition of reaching the critical value of the damage value is used. To assess the degree of reliability and determine the limits of applicability of the developed defining ratios of MPS, calculations of the processes of thermoplastic deformation and accumulation of fatigue damage were carried out and the numerical results obtained were compared with the data of field experiments on the example of a specific applied problem. A numerical analysis of the characteristic features of the thermal fatigue durability of a compact sample with stress concentrators simulating the operation of parts in the nozzle box of a steam turbine of a nuclear power plant (NPP) is carried out. The results of calculations of fatigue damage accumulation processes during thermal pulsations are compared with experimental data. It is shown that the developed MPS model qualitatively and with the accuracy necessary for engineering calculations quantitatively describes experimental data and can be effectively used to assess the thermocyclic fatigue durability of structures with multiaxial disproportionate paths of combined thermomechanical loading.

Determination of the frequency- and temperature-dependent stiffness and damping properties of thermoplastics for the prediction of the vibration and heating behaviour during ultrasonic welding

The precise and realistic simulation of the vibration and heating behaviour of thermoplastics in the ultrasonic welding process has so far been associated with great challenges. In particular, the determination of the required frequency- and temperature-dependent mechanical stiffness and damping properties in the high-frequency vibration range is only insufficiently possible according to the current state of the art, which prevents an early and valid numerical prediction of the weldability in the development process of new joining components. In order to provide more precise input data (storage and loss modulus) for describing the material behaviour of thermoplastics in the ultrasonic welding process in the future, a novel measurement concept was implemented that is based on the adaptation of simulation results to real structure-borne sound measurements. The test rig concept was successfully commissioned and calibrated at room temperature and the calculation routine for material data determination was implemented. On the basis of the generated material data, an increase in the prediction quality of the vibration behaviour in a frequency range of 1 Hz to 22.5 kHz of rectangular specimens at room temperature could already be achieved compared to the state of the art using dynamic mechanical analysis and a time–temperature shift approach. Measurements at different ambient temperatures up to 60 °C were also carried out. Although the prediction quality of the vibration behaviour was slightly improved at 60 °C, there is still a need for optimisation with regard to the test specimen geometry and the further development of the evaluation routine in order to increase the analysable temperature range on the one hand and the quality of the generated material data on the other.

Rapid inverse calibration of a multiscale model for the viscoplastic and creep behavior of short fiber-reinforced thermoplastics based on Deep Material Networks

In this work, we propose to use deep material networks (DMNs) as a surrogate model for full-field computational homogenization to inversely identify material parameters of constitutive inelastic models for short fiber-reinforced thermoplastics (SFRTs).Micromechanics offers an elegant way to obtain constitutive models of materials with complex microstructure, as these emerge naturally once an accurate geometrical representation of the microstructure and expressive material models of the constituents forming the material are known. Unfortunately, obtaining the latter is non-trivial, essentially for two reasons. For a start, experiments on pure samples may not accurately represent the conditions present in the composite. Moreover, the manufacturing process may alter the material behavior, and a subsequent modification is necessary. To avoid modeling the physics of the manufacturing process, one may identify the material models of the individual phases of the composite based on experiments on the composite. Unfortunately, this procedure requires conducting time-consuming simulations.In the work at hand, we use Deep Material Networks to replace full-field simulations, and to carry out an inverse parameter optimization of the matrix model in a SFRT. We are specifically concerned with the long-term creep response of SFRTs, which is particularly challenging to both experimental and simulation-based approaches due to the strong anisotropy and the long time scales involved. We propose a dedicated fully coupled plasticity and creep model for the polymer matrix and provide details on the experimental procedures.

A Highly Dynamic Covalent Polymer Network without Creep: Mission Impossible?

AbstractDynamic covalent polymer networks provide an interesting solution to the challenging recyclability of thermosets and elastomers. One of the remaining design constraints, however, is balancing thermal reprocessability in the form of material flow with dimensional stability during use. As a result, many chemistries are being investigated in order to improve bond reactivity control and material robustness. This Minireview highlights a number of promising concepts, with a particular emphasis on disconnecting chemical reactivity in low and high temperature regimes to obtain creep resistant, yet highly dynamic polymer networks. In addition, we will highlight the impact of sharp reactivity changes when applying extrapolation‐based approaches during rheological analysis. As a result, we are confident that abandoning the myth of “permanent” reactivity will aid in the development of sustainable polymeric materials that can truly combine the benefits of thermoplastic and thermoset behaviour.

A Highly Dynamic Covalent Polymer Network without Creep: Mission Impossible?

Dynamic covalent polymer networks provide an interesting solution to the challenging recyclability of thermosets and elastomers. One of the remaining design constraints, however, is balancing thermal reprocessability in the form of material flow with dimensional stability during use. As a result, many chemistries are being investigated in order to improve bond reactivity control and material robustness. This Minireview highlights a number of promising concepts, with a particular emphasis on disconnecting chemical reactivity in low and high temperature regimes to obtain creep resistant, yet highly dynamic polymer networks. In addition, we will highlight the impact of sharp reactivity changes when applying extrapolation-based approaches during rheological analysis. As a result, we are confident that abandoning the myth of "permanent" reactivity will aid in the development of sustainable polymeric materials that can truly combine the benefits of thermoplastic and thermoset behaviour.

Evaluation of Thermal and Thermo-mechanic Properties of Composites Based on Styrene-butadiene Copolymer (SBS)-seed Shell Particles

Nowadays, the use of agricultural wastes is a high impact research area, due the fact that this kind of materials can provide some interesting properties to a polymer matrix. Among these materials, the seed shell are materials with no industrial applications, so it is necessary to find a use in the aim to avoid the pollution. The Styrene-butadiene copolymer (SBS) is a wide used material due it thermoplastic behavior. In present work, the inclusion of two different species of seed shell particles: (jatropha curcas seed particles (JCSP) and pistachio (PSP)) into a Styrene-butadiene copolymer (SBS) and their effect on the thermal behavior of the resulting composites have been studied. The composites were prepared by melting fussion process at different concentrations ranging from 2 to 10 phr (parts per hounded of resin). The obtained composites were characterized by means of Thermogravimetric analysis (TGA), Dynamic Mechanical Analysis (DMA) and infrared Spectroscopy (FTIR).

3D-Printed PLA Medical Devices: Physicochemical Changes and Biological Response after Sterilisation Treatments.

Polylactic acid (PLA) has become one of the most commonly used polymers in medical devices given its biocompatible, biodegradable and bioabsorbable properties. In addition, due to PLA’s thermoplastic behaviour, these medical devices are now obtained using 3D printing technologies. Once obtained, the 3D-printed PLA devices undergo different sterilisation procedures, which are essential to prevent infections. This work was an in-depth study of the physicochemical changes caused by novel and conventional sterilisation techniques on 3D-printed PLA and their impact on the biological response in terms of toxicity. The 3D-printed PLA physicochemical (XPS, FTIR, DSC, XRD) and mechanical properties as well as the hydrophilic degree were evaluated after sterilisation using saturated steam (SS), low temperature steam with formaldehyde (LTSF), gamma irradiation (GR), hydrogen peroxide gas plasma (HPGP) and CO2 under critical conditions (SCCO). The biological response was tested in vitro (fibroblasts NCTC-929) and in vivo (embryos and larvae wild-type zebrafish Danio rerio). The results indicated that after GR sterilisation, PLA preserved the O:C ratio and the semi-crystalline structure. Significant changes in the polymer surface were found after HPGP, LTSF and SS sterilisations, with a decrease in the O:C ratio. Moreover, the FTIR, DSC and XRD analysis revealed PLA crystallisation after SS sterilisation, with a 52.9% increase in the crystallinity index. This structural change was also reflected in the mechanical properties and wettability. An increase in crystallinity was also observed after SCCO and LTSF sterilisations, although to a lesser extent. Despite these changes, the biological evaluation revealed that none of the techniques were shown to promote the release of toxic compounds or PLA modifications with toxicity effects. GR sterilisation was concluded as the least reactive technique with good perspectives in the biological response, not only at the level of toxicity but at all levels, since the 3D-printed PLA remained almost unaltered.

Mechanical and Physical Characterizations of a Three-Phase TiAl Alloy during Near Isothermal Forging

TiAl alloy is a high temperature structural material with excellent comprehensive properties in the range of 750–900 °C. However, its engineering application is limited by its poor plasticity and hot working properties at room temperature. Based on the above background, a novel three-phase Ti-40Al-6V-1Cr-0.3Ni (at.%) alloy was designed and fabricated in the present study. The as-cast ingot was subjected to near-isothermal forging, and the thermoplastic deformation behavior, microstructure evolution and mechanical properties were systematically studied. Near-isothermal forging shows excellent forming capability, and the forging disk is flawless without cracking. The core of the forging disk shows the greatest degree of deformation, and the microstructure is composed of fine equiaxed grains and residual (α2/γ) lamellae. The hardness of the B2 phase and the hardness difference between the B2 phase and γ phase are reduced by the Cr and V elements added in the alloy. The wrought alloy exhibits excellent mechanical properties at room temperature and elevated temperature, respectively. The uniform fine microstructure, low nanohardness of the B2 and γ phase and the property matching of each phase can be accounted for the excellent mechanical properties.

Thermoplastic Films Based on Polyelectrolyte Complexes of Arabino Glucurono‐Xylan and Polyethylenimine

AbstractThis work investigates film formation using polyelectrolyte complexes (PECs) of xylan and polyethylenimine (PEI). Different xylan/PEI mass ratios (from 50/50 to 90/10) and pH values (5.0 and 7.0) are analyzed and both parameters have an effect on complex size and film properties. Furthermore, the mass ratio corresponding to the neutralization charge depends on pH. When xylan/PEI mass ratios are near the neutralization charge ratio, a coacervation process is observed. In this case, separation between the coacervate and the supernatant dilute phase is possible. In the supernatant, both concentrations, xylan and PEI, depend on pH. The complex suspensions are used to obtain films by PECs casting/evaporation process. For films obtained from the whole suspension, by increasing the xylan content the stress at break is improved (up to 23 MPa for 85/15, at pH 7.0) and the strain at break is decreased (to 2.5%). When only coacervate is used to prepare films, the tensile strength is higher, reaching a value of 37 MPa for 85/15 mass ratio. Dynamic mechanical thermal analysis shows that the glass transition temperature (Tg) of the films is affected by composition. The thermoplastic behavior of a 50/50 xylan/PEI film is confirmed using a hot‐compression molding.