Thermoplastic Polyurethane Materials Research Articles

Challenges on extrusion-based additive manufacturing of thermoplastic polyurethane

Fused filament fabrication (FFF) offers rapid production with user-friendly operation and cost-effectiveness, yet challenges in printing with thermoplastic polyurethane (TPU) persist. This study identifies causes of print defects in FFF-TPUs and their relation to process parameters, by visually analysing samples produced with various TPU materials. Extrusion temperature, printing speeds, filament cooling, overlap, and extrusion flow are subject to evaluation. Common issues include filament entanglements, under-extrusion, stringing, or geometric deviations. Among other approaches, results highlight the importance of slow, constant print speeds and careful optimization of extrusion temperature.

Porous flexible molecular-based piezoelectric composite achieves milliwatt output power density

Molecular ferroelectrics have made breakthrough progress in intrinsic piezoelectric response that can be on par with advanced inorganic piezoelectric ceramics. However, their successful applications in high-density energy harvesting and self-powered flexible devices have been great challenge, owing to the low elastic moduli, intrinsically brittle, and fracture proneness of such material systems under mechanical loading. Here, we have developed a flexible porous composite piezoelectric material by using soft thermoplastic polyurethane (TPU) and molecular ferroelectric materials. Benefiting from the porous structure of TPU, the flexible piezoelectric composites enable effectively large doping ratio (50%) of [Me3NCH2Cl]CdCl3 (TMCM-CdCl3) and highly efficient stress absorption, coupled with the excellent piezoelectric properties of TMCM-CdCl3, to realize a superior power density (636.9 µW cm−2 or 1273.9 µW cm−3). This output is 2000 times higher than that of flexible piezoelectric materials represented by poly(vinylidene fluoride) (PVDF). We believe that the outstanding performance of the porous composite piezoelectric material would pave a feasible way for real industrial applications of molecular ferroelectrics.

A scanning electron microscope evaluation of surface roughness of different clear aligners – A comparative in-Vitro study

Clear aligner are thermoplastic polyurethane materials which produces small increments of tooth movements by each aligner tray. Clinical effectiveness of the treatment largely depend on the mechanical properties of the material. The use of scanning electron microscopy (SEM) in invitro environment allows to evaluate the surface morphology of the aligners The aim of the study is to compare the surface roughness of different types of clear aligners after thermoforming and after invitro aging. A sample of 15 aligners from Group A (DURAN), Group B (ERKODUR), Group C (MONOFLEX) were taken for this study. After screening the subjects were scanned by intraoral scanner and STL file were imported to Maestero 3D software. Then resin models were printed and aligners were fabricated in the models by thermoforming machine. The first set of aligners were scanned by Scanning Electron Microscope and another set of aligners were placed in chewing simulator for simulating intraoral environment. Then the second set of aligners were sent for SEM evaluation. Mean comparison of surface roughness and root mean square surface roughness was compared by paired t test and comparison among the groups was done by one way ANOVA test. The surface roughness of both aligners increased after thermoforming. This could be due to thermoforming process which could be due to temperature variation. After the ageing of the aligners in the invitro environment, there is homogenous and smooth layer of the aligner surfaces which could be due to adhesion of biofilm and polishing effect by chewing strokes.

Preparation of phosphorus-containing organic-hybrid layered double hydroxide as a flame retardant for thermoplastic polyurethane

Developing polymers with both flame-retardant and smoke-suppressing properties is a great challenge in the field of fire-safety thermoplastic polyurethane (TPU). This study involved synthesizing a synergistic organic-hybrid layered double hydroxide (O-LDH) and phosphorus/nitrogen-containing substance (PD) inspired by the observed synergistic effect between them. The incorporation of PD significantly enhanced the flame-retardant properties of TPU in conjunction with cobalt/aluminum- layered double hydroxide. The composite containing 10 wt% O-LDH achieved a limiting oxygen index of 26.5% and met the vertical burning V-0 rating. Moreover, the heat and smoke release were decreased compared to pure TPU in cone calorimeter tests. The systematic characterization of the synergistic flame-retardant mechanism emphasized the crucial contribution of cobalt and phosphorus species in catalyzing charring effects, resulting in the compact and highly carbonized char layer in the condensed phase. The incorporation of the hybrid flame retardant significantly enhanced the advancement of the flame-retardant TPU materials.

Joint angle prediction for a cable-driven gripper with variable joint stiffness through numerical modeling and machine learning

Soft grippers in automation, particularly those with variable joint stiffness, offer promising possibilities for precise manipulation tasks. However, accurately predicting finger joint bending angles in this field poses significant challenges due to the soft and complex nature of the grippers, making modeling and angle prediction difficult. This paper presents the development of a predictive model for precisely controlling bending angles in multi-material printed soft grippers with variable stiffness, which are pivotal for delicate manipulation tasks in automation. In particular, we explore a cable-driven gripper design made of thermoplastic polyurethane and conductive polylactic acid materials, featuring integrated resistive joints for stiffness modulation through controlled Joule heating. A data-driven modeling approach, combining numerical modeling of the gripper and machine learning techniques, was employed for the development of the predictive model. We performed static structural simulations using ANSYS Workbench to measure bending angles under various conditions for developing datasets for model training. In this work, we evaluated several machine learning models such as linear regression, decision tree, and K-nearest neighbor regression models to predict the correlation between temperature, pull distance, and bending angle. The K-nearest neighbor regression model demonstrated the highest accuracy, with a mean absolute error of approximately 11%. These findings underline the importance of precise angle prediction models in enhancing the functionality and reliability of soft grippers, paving the way for their broader application in automation and robotics.

Hydrophobic carbon dots modified porous thermoplastic polyurethane for effective and durable oil/water separation

Porous thermoplastic polyurethane (PTPU) materials have shown potential for oil/water separation due to their high porosity, good elasticity and repeatability. It is urgent to find an appreciated method that modifies the surface properties of PTPU so as to improve the separation efficiency by effectively enhancing the interaction between the surface and organic pollutant. In this paper, hydrophobic carbon dots (CDs) were applied to modify PTPU (PTPU/CDs) to improve the oil/water separation performance. As a result of optimization of the preparation of PTPU and the addition of CDs, the hydrophobicity (from 120° to 138.1°), compressive strength (from 186 to 233 kPa) and saturated absorption capacity (from 2.4–23 to 4–39 g−1) were improved significantly compared with pure PTPU. In addition, in this process, the PTPU/CDs composites were endowed with bright yellow fluorescence due to the CDs, making it possible to track the oil absorption progress under ultraviolet irradiation. Furthermore, the PTPU/CDs composites demonstrated exceptional durability and environmental adaptability in the process of treating oily-water. Additionally, the absorption kinetics demonstrated that the PTPU/CDs material showed an approximately 1.5-fold increase in absorption rate compared with PTPU. When used as filter membrane, the PTPU/CDs composites could effectively separate surfactant-stabilized toluene-in-water emulsion with up to 99% efficiency, demonstrating an effective way for treatment of various oily wastewater.

The Study on the Morphology and Compression Properties of Microcellular TPU/Nanoclay Tissue Scaffolds for Potential Tissue Engineering Applications.

Thermoplastic polyurethane (TPU) materials have shown promise in tissue engineering applications due to their mechanical properties and biocompatibility. However, the addition of nanoclays to TPU can further enhance its properties. In this study, the effects of nanoclays on the microstructure, mechanical behavior, cytocompatibility, and proliferation of TPU/nanoclay (TPUNC) composite scaffolds were comprehensively investigated. The dispersion morphology of nanoclays within the TPU matrix was examined using transmission electron microscopy (TEM). It was found that the nanoclays exhibited a well-dispersed and intercalated structure, which contributed to the improved mechanical properties of the TPUNC scaffolds. Mechanical testing revealed that the addition of nanoclays significantly enhanced the compressive strength and elastic resilience of the TPUNC scaffolds. Cell viability and proliferation assays were conducted using MG63 cells cultured on the TPUNC scaffolds. The incorporation of nanoclays did not adversely affect cell viability, as evidenced by the comparable cell numbers between nanoclay-filled and unfilled TPU scaffolds. The presence of nanoclays within the TPUNC scaffolds did not disrupt cell adhesion or proliferation. The incorporation of nanoclays improved the dispersion morphology, enhanced mechanical performance, and maintained excellent biocompatibility. These findings suggest that TPUNC composites have great potential for tissue engineering applications, providing a versatile and promising scaffold material for regenerative medicine.

Failure of surface modification 3D printed polymer materials by UV/ozone irradiation

In today's technology, AM processes are widely adopted in the aerospace, energy, automotive, medicine, and agriculture industries. Fused Deposition Modeling (FDM) is one of the most remarkable methods in the AM family because of its superiorities. Besides the advantages provided, the mechanical strength of the printed parts is still not at a satisfactory level. Here, there are various secondary processes applied to polymer materials to improve both the mechanical properties and functionality of the printed part. Among these processes, the UV/O3 surface treatment method stands out as the most suitable one in terms of ease of application. In this study, two different infill orientation angles were applied to two standard test models. The fabrication process was initiated using suitable process parameters for filaments made of Polylactic Acid (PLA) and Thermoplastic Polyurethane (TPU) materials. The purpose of this investigation was to examine the mechanical strength of 3D printed polymer structures. For the same purpose, the UV/O3 (UV/Ozone) process was applied to the manufactured samples. The samples are then subjected to tensile and compression tests, Shore surface hardness measurements and Scanning Electron Microscopy (SEM) analyze for both the evaluation of mechanical properties and the examination of fracture surface structures. Consequently, significant increases of 28.33%, 25.21%, 27.90%, and 32.92% were observed in material surface hardness levels. This study is important in terms of presenting that the mechanical properties of 3D printed parts can be significantly improved with UV/O3 application, which is an effective and a practical process.

Thermal performance of thermoplastic polyurethane composites with microencapsulated piperazine pyrophosphate

This study synthesized and investigated the efficacy of a novel flame retardant, melamine formaldehyde microencapsulated piperazine pyrophosphate (MFPAPP), in improving the thermal and flame-retardant properties of thermoplastic polyurethane (TPU). When TPU was incorporated with 30wt% MFPAPP, the limiting oxygen index (LOI) value of the TPU/MFPAPP composite increased to 38.8%, achieving a V-0 rating. The thermogravimetric test (TG) results confirmed that MFPAPP significantly enhanced the thermal stability of the TPU/MFPAPP composite, as indicated by the increased char residue at 800°C, which was up to 22.4wt%. Compared with the pure TPU samples, the peak heat release rate (PHRR) and total heat release (THR) of TPU/MFPAPP30 decreased by 53% and 45%, respectively. TPU/MFPAPP10 maintained a V-0 rating after the water immersion test, whereas TPU/PAPP degraded to a V-2 rating. Scanning electron microscopy (SEM), Raman spectra, and X-ray photoelectron spectroscopy analyses revealed that MFPAPP promoted the formation of heat-resistant and dense expanded carbon layers. In summary, MFPAPP demonstrated remarkable flame retardancy and thermal stability, making it an ideal candidate for enhancing the fire safety of TPU materials.

Enhanced multi-material 4D printing hybrid composites based on shape memory polymer/thermoplastic elastomer

An improved one-shot thermoplastic elastomer reinforced shape memory polymer (SMP) four-dimensional (4D) printing method for hybrid composite samples is proposed. Through the new reinforcement method of interlayer overlapping, the problem of inability to bond between polylactic acid (PLA) and thermoplastic polyurethane (TPU) is overcome, and the composite materials had excellent bonding properties. The feature of the improved manufacturing method is that the interface between PLA and TPU materials has continuous overlapping layers, and the boundary is wavy. Herein the complex multi-material space structure can be manufactured with a three-dimensional curved bonding surface. The shape memory and mechanical properties of the PLA/TPU hybrid composite sample are significantly improved via the improved printing method. The shape memory recovery rate of PLA/TPU hybrid composite samples are all over above 99.5%, which is close to complete recovery. The fastest recovery time is 40.61% shorter than the recovery time of the pure PLA sample, showing good shape memory cycle performance. The peak recovery force is 6.92 times higher than the recovery force of the pure PLA sample. The impact strength is 9.87 times higher than the impact strength of pure PLA sample. The composites 4D printing method can be utilized to process complex parts with excellent interfacial bonding properties, shape memory properties, and mechanical properties, which has a promising application prospect in the manufacturing of PLA-based sensors and actuators.

Multi-Material Metamaterial Topology Optimization to Minimize the Compliance and the Constraint of Weight: Application of Non-Pneumatic Tire Additive-Manufactured with PLA/TPU Polymers.

In non-pneumatic tires, metamaterial cells could replace the pneumatic part of the tire. In this research, to achieve a metamaterial cell suitable for a non-pneumatic tire with the objective function of increasing compressive strength and bending fatigue lifetime, an optimization was carried out for three types of geometries: a square plane, a rectangular plane, and the entire circumference of the tire, as well as three types of materials: polylactic acid (PLA), thermoplastic polyurethane (TPU), and void. The topology optimization was implemented by the MATLAB code in 2D mode. Finally, to check the quality of cell 3D printing and how the cells were connected, the optimal cell fabricated by the fused deposition modeling (FDM) method was evaluated using field-emission scanning electron microscopy (FE-SEM). The results showed that in the optimization of the square plane, the sample with the minimum remaining weight constraint equal to 40% was selected as the optimal case, while in the optimization of the rectangular plane and the entire circumference of tire, the sample with the minimum remaining weight constraint equal to 60% was selected as the optimal case. From checking the quality of 3D printing of multi-materials, it was concluded that the PLA and TPU materials were completely connected.

A 3D-Printed Soft Haptic Device with Built-in Force Sensing Delivering Bio-Mimicked Feedback.

Haptics plays a significant role not only in the rehabilitation of neurological disorders, such as stroke, by substituting necessary cognitive information but also in human-computer interfaces (HCIs), which are now an integral part of the recently launched metaverse. This study proposes a unique, soft, monolithic haptic feedback device (SoHapS) that was directly manufactured using a low-cost and open-source fused deposition modeling (FDM) 3D printer by employing a combination of soft conductive and nonconductive thermoplastic polyurethane (TPU) materials (NinjaTek, USA). SoHapS consists of a soft bellow actuator and a soft resistive force sensor, which are optimized using finite element modeling (FEM). SoHapS was characterized both mechanically and electrically to assess its performance, and a dynamic model was developed to predict its force output with given pressure inputs. We demonstrated the efficacy of SoHapS in substituting biofeedback with tactile feedback, such as gripping force, and proprioceptive feedback, such as finger flexion-extension positions, in the context of teleoperation. With its intrinsic properties, SoHapS can be integrated into rehabilitation robots and robotic prostheses, as well as augmented, virtual, and mixed reality (AR/VR/MR) systems, to induce various types of bio-mimicked feedback.

Wear and fatigue resistance: An in-vitro comparison of three polyethylene terephthalate glycol and thermoplastic polyurethane materials for vacuum-formed retainers

To test the wear and fatigue resistance of three materials (Essix ACE®, Taglus®, and Zendura A®) for the fabrication of vacuum-formed retainers in an artificial oral environment. Wear resistance was tested by subjecting 21 retainers of each Essix ACE®, Taglus®, and Zendura A® to 12,000 wear cycles at 75N to simulate one year of retainer wear with moderate nighttime bruxing. Post-wear retainer thickness was compared to baseline measurements to calculate wear depth. Fatigue resistance was tested by flexing 15 retainers of each material at an angle of 25 degrees for 1,825 cycles to simulate one year of removing and reinserting a retainer five times per day. Retainers were visually inspected for fractures. Pairwise t-tests with correction using Tukey's method were used to determine significant differences between materials. The mean wear depths were 0.155±0.021mm, 0.168±0.031mm, and 0.096±0.033mm for Essix ACE®, Taglus®, and Zendura A®, respectively. The wear depth of Zendura A® was significantly lower than that of both Essix ACE® (P<0.001) and Taglus® (P<0.001). There was no significant difference in wear depth between Essix ACE® and Taglus® (P=0.312). Under the parameters set for the fatigue resistance test, fractures did not occur on any of the tested materials. Under the assumption of moderate nighttime bruxing for one year, Zendura A® is the most wear-resistant among the materials tested. With the assumption of retainer removal and reinsertion five times per day for one year, all three materials tested have the same ability to resist fatigue.

No Effect of EVA and TPU Custom Foot Orthoses on Mechanical Asymmetries during Acute Intense Fatigue

This study examined the impact of custom foot orthoses made of ethyl-vinyl acetate (EVA) and expanded thermoplastic polyurethane (TPU) materials, both compared to a control condition (CON; shoes only), on mechanical asymmetries during repeated treadmill sprints. Eighteen well-trained male runners executed eight, 5-s sprints (rest: 25 s) on an instrumented motorized treadmill in three footwear conditions (EVA, TPU, and CON). We evaluated the group mean asymmetry scores using the ‘symmetry angle’ (SA) formula, which assigns a score of 0% for perfect symmetry and a score of 100% for perfect asymmetry. There was no condition (all p ≥ 0.053) or time (p ≥ 0.074) main effects, nor were there any significant time × condition interactions on SA scores for any variables (p ≥ 0.640). Mean vertical, horizontal, and total forces presented mean SA values (pooled values for the three conditions) of 2.6 ± 1.9%, 2.9 ± 1.6%, and 2.4 ± 1.8%, respectively. Mean SA scores were ~1–3% for contact time (1.5 ± 0.5%), flight time (3.0 ± 0.3%), step frequency (1.1 ± 0.5%), step length (1.9 ± 0.7%), vertical stiffness (2.1 ± 0.9%), and leg stiffness (2.4 ± 1.1%). Mean SA scores were ~2–6.5% for duration of braking (4.1 ± 1.6%) and propulsive (2.4 ± 1.0%) phases, and peak braking (6.2 ± 2.9%) and propulsive (2.1 ± 1.4%) forces. In well-trained runners facing intense fatigue, wearing custom foot orthoses did not modify the observed low-to-moderate natural stride mechanical asymmetries.

Soft Pneumatic Actuators with Controllable Stiffness by Bio‐Inspired Lattice Chambers and Fused Deposition Modeling 3D Printing

This article shows how changing 3D printing parameters and using bio‐inspired lattice chambers can engineer soft pneumatic actuators (SPAs) with different behaviors in terms of controlling tip deflection and tip force using the same input air pressure. Fused deposition modeling (FDM) is employed to 3D print soft pneumatic actuators using varioShore thermoplastic polyurethane (TPU) materials with a foaming agent. The effects of material flow and nozzle temperature parameters on the material properties and stiffness are investigated. Auxetic, columns, face‐centered cubic, honeycomb, isotruss, oct vertex centroid, and square honeycomb lattices are designed to study actuators’ behaviors under the same input pressure. Finite‐element simulations based on the nonlinear hyper‐elastic constitutive model are carried out to precisely predict the behavior, deformation, and tip force of the actuators. A closed‐loop pneumatic system and sensors are developed to control the actuators. Results show that lattice designs can control the bending angle and generated force of actuators. Also, the lattices increase the ultimate strength by controlling the contact area inside the chambers. They demonstrate variable stiffness behaviors and deflections under the same pressure between 100 and 500 kPa. The proposed actuators could be instrumental in designing wearable hand rehabilitative devices that assist customized finger and wrist flexion‐extension.

Metallurgical solid waste modified thermoplastic polyurethane composites: The thermal stability and combustion properties

AbstractAs a kind of bulk industrial solid waste, the massive accumulation of iron tailings has caused serious waste of resources and environmental pollution. In this study, a silane coupling agent (KH550) was used to modify the surface of iron tailings to produce MIT, and it was compounded with ammonium polyphosphate (APP) on thermoplastic polyurethane (TPU) to prepare a series of TPU/APP/MIT composites. Thermogravimetric (TG), cone calorimetric (CCT), thermogravimetric infrared, scanning electron microscopy, and Raman techniques were also used to analyze the combustion performance, smoke toxicity, and microscopic morphology. The TG test results showed that the compounding of APP and MIT significantly improved the residual carbon value of TPU composites at 700°C. CCT test results showed that the TPU/APP/MIT composites exhibited excellent flame retardancy and smoke suppression. Compared with pure TPU, PHRR, THR, and TSR of TPU/APP15/MIT10 composite decreased by 85.56%, 87.83%, and 86.17%, respectively, the peak release rates of CO and CO2 decreased by 69.26% and 90.34%, respectively. The above studies showed that APP and MIT have excellent flame retardant and smoke suppression effect on TPU materials, providing more opportunities for the study of TPU composites and metallurgical solid waste utilization.

Hydrogen Compatibility of Polymers for Fuel Cell Vehicles

So far, hydrogen compatibility of polymer materials was investigated, focusing on gaining insight in general degradation mechanisms. However, this is not yet sufficient for purposes of safety and performance requirements, for example, in the automotive industry and especially, the corresponding advanced material development, which needs meaningful and comprehensive data for prospective long time intervals. Therefore, herein, the applicability of forced aging regimes and the suitability of different analytical techniques for clarification of the underlying mechanisms are focused on. The study analyzes the behavior of polyether ether ketone, (thermoplastic) polyurethane and fluoroelastomer materials after prolonged aging in hydrogen atmosphere under pressure as it occurs in hydrogen fuel‐cell vehicles. Material changes are investigated by electron microscopy, thermogravimetry, differential scanning calorimetry, and gas sorption measurements. Especially thermogravimetry turns out to indicate subtle material changes that are important indications for material choice.

Study on polyurethane-based porous materials and their adsorption properties

The flexible superhydrophobic thermoplastic polyurethane(TPU) porous material was prepared by heat-induced phase separation method with two cooling steps. The influence of the preparation process on the microstructure of the material was discussed in depth. The microstructure, hydrophobicity and specific surface area of porous TPU materials were analyzed in detail. The surface wettability, separation selectivity, saturated adsorption capacity and adsorption rate, mechanical properties, environmental adaptability and cyclic properties of porous TPU materials were studied.The results show that the TPU-8% porous monolithic material prepared by heat-induced phase separation method shows good performance when the polymer concentration is 8%, the phase separation temperature is 0°C, the phase separation time is 30min, and the mixing solvent ratio is 9:1.

Rehearsal simulation to determine the size of device for left atrial appendage occlusion using patient-specific 3D-printed phantoms

Left atrial appendage (LAA) occlusion (LAAO) is used to close the finger-like extension from the left atrium with occlusion devices to block the source of thrombosis. However, selection of the devices size is not easy due to various anatomical changes. The purpose of this study is patient-specific, computed tomography angiography (CTA)-based, three-dimensionally (3D) printed LAAO phantoms were applied pre-procedure to determine the size. Ten patients were enrolled prospectively in March 2019 and December 2020. The cardiac structure appearing in CTA was first segmented, and the left atrium and related structures in the LAAO procedure were modeled. The phantoms were fabricated using two methods of fused deposition modeling (FDM) and stereolithography (SLA) 3D printers with thermoplastic polyurethane (TPU) and flexible resin materials and evaluated by comparing their physical and material properties. The 3D-printed phantoms were directly used to confirm the shape of LAA, and to predict the device size for LAAO. In summary, the shore A hardness of TPU of FDM was about 80–85 shore A, and that of flexible resin of SLA was about 50–70 shore A. The measurement error between the STL model and 3D printing phantoms were 0.45 ± 0.37 mm (Bland–Altman, limits of agreement from − 1.8 to 1.6 mm). At the rehearsal, the estimations of device sizes were the exact same with those in the actual procedures of all 10 patients. In conclusion, simulation with a 3D-printed left atrium phantom could be used to predict the LAAO insertion device size accurately before the procedure.

Characterizing the interfacial fracture energy of stiff islands on stretchable films

Abstract This study characterized the interfacial fracture energy of stiff islands deposited on a thermoplastic polyurethane (TPU) film. The film can deform by &amp;gt;200%. The film was stretched using a designed fixture, and the fracture behaviors of the islands were observed using a microscope. The island–substrate interface debonding lengths associated with different levels of substrate strain were determined in the stretching tests. Because the stretchable film was a nonlinear material, the Ogden model was employed to characterize the nonlinear constitutive relation. Through the tensile tests, the material parameters in the Ogden model were determined using the reduced-gradient optimization method. On the basis of the measured debonding lengths, a finite element model was generated for the nonlinear properties of the film, and the energy release rates at the crack tip were calculated using the J-integral method. The energy release rates, representing the interfacial fracture energy, were calculated on the basis of the arrested crack associated with different crack lengths. Results reveal that the interfacial fracture energy increased from 0.14 to 0.91 kJ/m2 as the debonding length increased. The behavior is related to the rising resistance curve in TPU materials. In addition, the shearing-dominated mode slightly decreased as the debonded length increased in the stretching tests.