Liquid Resin Research Articles

Preparation and Validation of a Longitudinally and Transversely Stiffened Panel Based on Hybrid RTM Composite Materials.

In the face of the difficulty in achieving high-quality integrated molding of longitudinally and transversely stiffened panels for helicopters by resin-matrix composite materials, we combine the prepreg process and the resin transfer molding (RTM) process to propose a hybrid resin transfer molding (HRTM) for composite stiffened panel structures. The HRTM process uses a mixture of prepreg and dry fabric to lay up a hybrid fiber preform, and involves injecting liquid resin technology. Using this process, a longitudinally and transversely stiffened panel structure is prepared, and the failure modes under compressive load are explored. The results show that at the injection temperature of the RTM resin, the prepreg resin dissolves slightly and has little effect on the viscosity of the RTM resin. Both resins have good miscibility at the curing temperature, which allows for the overall curing of the resin. A removable box core mold for the HRTM molding is designed, which makes it convenient for the mold to be removed after molding and is suitable for the overall molding of the composite stiffened panel. Ultrasonic C-scan results show that the internal quality of the composite laminates prepared using the HRTM process is good. A compression test proves that the composite stiffened panel undergoes sequential buckling deformation in different areas under compressive load, followed by localized debonding and delamination of the skin, and finally failure due to the fracture of the longitudinal reinforcement ribs on both sides. The compressive performance of the test specimen is in good agreement with the finite element simulation results. The verification results show that the HRTM process can achieve high-quality integrated molding of the composite longitudinally and transversely stiffened panel structure.

Effect of Surface Coating and Plasma Treatment on Mechanical Properties of Wood Plastic Composites

Mechanical properties of plasma-irradiated and surface-coated wood plastic composites (WPCs) have been investigated in this paper. WPCs were developed by injection molding technique using wood fiber (WF) as reinforcement and polypropylene (PP) as matrix. The short, discontinuous WF was compounded with thermoplastic PP at varying weight fractions of 0 wt%, 25 wt% (WP25), and 50 wt% (WP50) to yield tensile test specimens in accordance with JIS K7139-A32 standards. Subsequently, plasma treatment was performed on the test-pieces, followed by surface coating by immersion in acrylic resin liquid containing homogeneously dispersed TEMPO-oxidized cellulose nanofibers (CNF). The results indicate an increase in surface roughness after plasma irradiation, but surface coating of the specimens with acrylic paint and CNF decreased their surface roughness by ∼50% in comparison to the untreated specimens. Plasma treatment and surface coating also increased the tensile strength of neat PP, WP25 and WP50 specimens by 5.4–7.1%, 3.5–3.7% and 3.0–3.6%, respectively, whereas their fracture strains tended to decrease. Compared to the untreated specimens, the surface-coated specimens generally displayed higher tensile strength. This finding is a corroboration that the observed increase in strength is highly contingent on the adhesion between the specimen surface and the coating layer than on the improvement in surface roughness. Thus, it is inferable that surface coating could be of great importance in enhancing the mechanical performance of WPCs.

Effect of the grafting degree of (3glycidyloxypropyl)trimethoxysilane on the corrosion resistance of waterborne epoxy/Ti3C2 nanocomposite coatings

Modification of MXene is usually necessary for their application in corrosion resistant coatings. Nevertheless, the influence of the grafting degree of modifier is not concerned in the past. In this work, modified Ti3C2 nanosheets (G-Ti3C2) with different grafting degree of (3-glycidyloxypropyl)trimethoxysilane (GPS) were obtained by changing GPS dosage, and further incorporated into waterborne epoxy resin coatings (WEP). GPS modification led to deteriorated dispersion in water but improved dispersion stability of Ti3C2 nanosheets in waterborne epoxy resin at appropriate GPS-grafting degree presumably due to the interaction between the liquid epoxy resin droplet and G-Ti3C2 nanosheets. The GPS grafting degree was also critical to get the corrosion resistant nanocomposite coatings. The best corrosion resistance, i.e., with impedance modulus four orders of magnitude higher than that of pure WEP after 30 days' immersion in salt solution as well as the lightest corrosion in salt spray test, was achieved using the Ti3C2 nanosheets modified at the GPS-to-Ti3C2 mass ratio of 30:1. Better dispersion of G-Ti3C2 nanosheets and stronger interaction between epoxy resin matrix and G-Ti3C2 nanosheets should be responsible for the excellent corrosion resistance of the coatings at optimum GPS grafting degree.

Elastohydrodynamic lubrication of soft-layered rollers and tensioned webs in roll-to-plate nanoimprinting

This work presents the development of a numerical model for the elastohydrodynamic lubrication of roll-to-plate nanoimprinting with flexible stamps. Roll-to-plate nanoimprinting is a manufacturing method to replicate micro- and nanotextures on large-area substrates with ultraviolet-curable resins. The roller is equipped with a relatively soft elastomeric layer, which elastically deforms during the imprint process. The elastic deformation is described by linear elasticity theory. It is coupled to the pressure build-up in the liquid resin film, which is described by lubrication theory. The flexible stamp, which is treated as a tensioned web, is pre-tensioned around the roller. The elastic deformation of the tensioned web is described by the large-deflection bending of thin plates equations, considering its non-negligible bending stiffness. A Fischer–Burmeister complementarity condition captures the contact mechanics between the tensioned web and the roller. The governing equations combine in a coupled elastohydrodynamic lubrication model, which is fully described by a set of non-dimensional numbers. These are used in a parameter study to investigate the effect on the pressure and film height distributions. It is shown that the bending stiffness of the tensioned web results in an additional hydrodynamic pressure peak and corresponding minimum in the film height, near the inlet of the roller contact. An increase of the bending stiffness corresponds to a decrease in film height. The numerical results are compared with benchmarks from literature and experimentally validated with layer height measurements from flat layer imprints. Good agreement is found between the numerical and experimental results.

Effect of accelerated weathering on the performance of natural fibre reinforced recyclable polymer composites and comparison with conventional composites

This work investigated the long-term performance of unidirectional (UD) flax fibre reinforced recyclable polymer composites (FFRPs), after accelerated weathering for a total of 56 days, in laboratory environment. The employed recyclable polymer matrices were a bio-based epoxy and a liquid thermoplastic acrylic resin. The performance of the developed composites, were compared with traditional UD glass fibre composites (GFRPs), employing a standard petro-based epoxy resin, in terms of flexural and viscoelastic properties, while visual inspection and Scanning Electron Microscopy (SEM) were employed to investigate further the effects of ageing after exposure. It was revealed that the reinforcing fibres dominated the performance of the composites after ageing, whilst the choice of polymer matrix exhibited less influence.

3D Printing of Ionic Liquid Polymer Networks for Stretchable Conductive Sensors

AbstractStretchable conductive materials have attracted great attention due to their potential applications as strain sensors, wearable electronics, soft robotics, and medical devices. The fabrication of these materials with customized object geometries is desirable, but the methods to achieve them are still highly limited. Additive manufacturing via vat photopolymerization can generate sophisticated object geometries, but there is still a significant need to print with materials that afford improved conductivity, mechanical properties, elastic recovery, and durability. Herein, stretchable strain sensors with a range of 3D printed designs are reported using vat photopolymerization. Ionic liquid resins are optimized for their printability using Sudan‐I as a photoabsorber and used to fabricate 3D objects that are subjected to compression, stretching, and bending loads that are detected as real‐time changes in current. Additionally, the self‐adhesive nature of these materials enables mechanically damaged structures to be mended together to regain its function as a strain sensor. These ionic liquid resins are compatible with commercial 3D printers, which enhances their applicability for on‐demand production of customized devices.

Digital Light Processing-3D Printing of Thermoset Materials with High Biodegradability from Amino Acid-Derived Acrylamide Monomers.

Six acrylamide resins, derived from l-phenylalanine and l-leucine, are designed for application in digital light processing (DLP) printers to obtain biodegradable thermoset polymers. The acrylamide copolymers are prepared under light irradiation at 405nm and thermal post-curing processes. Low molecular weight poly(ethylene glycol)diacrylate (PEGDA) and N,N-dimethylacrylamide (DMAM), both liquid resins, are used as co-monomers and diluents for the amino acid-derived acrylamide solubilization. The presence of two phenylalanine units and two ester groups in the acrylamide monomer accuses a fast degradation rate in hydrolytic medium in 90 days. The residual products leached in the aqueous media prove to be non-cytotoxic, when 3D-printed samples are cultured with osteoblast cells (MG63), which represents an advantage for the safe disposal of printer waste materials. The scaled-up pieces derived from l-phenylalanine and diethylene glycol, as amino acid-derived acrylamide (named compound C), PEGDA and DMAM, present high dimensional stability after DLP printing of complex structures used as testing samples. Layers of 50µm of thickness are well cohesive having isotropic behavior, as demonstrated with tensile-strain measurements performed in X-Y-Z (plane) directions. The compound C, which contains phenylalanine amino acid, reveals a promising potential to replace non-biodegradable acrylate polymers used in prototyping systems.

Three-Dimensional Printing of Cardiac Defects Using Stereolithography: A Single-Centre Experience

Three-dimensional (3D) printing is increasingly being used for congenital heart disease to allow an in-depth understanding of complex cardiac anatomy; however, access to 3D printing is hindered by lack of access. This study investigated the feasibility of rapid prototyping (RP)using a readily available on-site 3D printer. Patients with complex cardiac anatomy were prospectively identified. All patients underwent ECG-gated cardiac computed tomography (CT) examination as part of standard preprocedural work-up. Contrasting timing was modified to allow maximum contrast enhancement of the relevant anatomy. Using the CT dataset, 3D mesh was generated using open-source 3D software. The 3D models were printed using liquid resin stereolithography technology. To assess the accuracy of the model produced, measurement of the mid ascending aorta was compared with measurement taken from the CT. Over an 18-month period, 10 patients with complex cardiac defects underwent further assessment with 3D printed models. The average processing time to generate a 3D mesh was 6 hours and the time for printing the model varied between 4 and 8 hours depending on the model size. The aortic measurements on the 3D model had excellent correlation with measurements taken from CT (33.4±3.3 mm vs 34±3.6 mm; Pearson correlation 0.938; p<0.005). 3D printing using a CT dataset via readily available 3D printers is feasible and enables an in-depth appreciation of complex cardiac anatomy, which helps to guide patient management.

Determination of the slip resistance of interspersed synthetic resin flooring with a convolutional neural network

The ramp slip test is a reliable and widely used method to evaluate the slip resistance of interspersed synthetic resin floors, common in industrial and parking areas. Following EN 16165 and using a ramp with varying inclinations, the floor R-class can be determined. This test can, however, only be performed in a laboratory, and not on-site. Here, we propose a method to determine the floor R-class from its surface topography, which can easily be measured on-site. In this study, floors of various slip resistances were prepared following the conventional method of spreading quartz sand onto a liquid resin. The interspersed surface creates a 3-dimensional topography, which is characteristic for the anti-slip properties. After the resin hardening, the R-class was measured and, in parallel, a convolutional neural network was developed and trained with the numerous photographs taken from the specimen surfaces. The used convolutional neural network classifies 95% of the surfaces into the correct R-class. This result makes it possible, for the first time, to determine the R-class slip resistance on-site with a high probability of correctness.

Phthalonitrile functionalized resoles – use of 2,3-dicyanohydroquinone as a versatile monomer for resins with very high thermal stability

In the aerospace industry, composite materials based on high-performance fibers and polymer matrices are widely used. Indeed, they can display very important mechanical and thermal properties with a very large density-to-stiffness balance. Usually, thermal protection systems are made from composite materials containing a char precursor for the polymer matrix. Temperature-resistant organic polymers are still quite rare and generally require very restrictive manufacturing and use processes. Recently, phthalonitriles have gained interest due to their very good thermostability and high char yield. However, phthalonitrile resins are generally solid at room temperature and often have a narrow processing window. In this work, synthesis methods allowing access to phthalonitrile resins were developed from 2,3-dicyanohydroquinone. These methods propose to consider together phenolic chemistry (phenolic reactive positions and aldehyde) and self-crosslinkable phthalonitrile chemistry (hydroxyl-mediated from phenolic and nitriles). It was thus possible to synthesize liquid resins at room temperature, leading to polymeric networks with a very high thermostability (temperature at 5% mass loss > 450 °C and char yield ≈ 70 %).

Computational Fluid Dynamics Modeling of Top-Down Digital Light Processing Additive Manufacturing.

Digital light processing (DLP) as a vat photopolymerization technique is one of the most popular three-dimensional (3D) printing methods, where chains are formed between liquid photocurable resin molecules to crosslink them and solidify the liquid resin using ultraviolet light. The DLP technique is inherently complex and the part accuracy depends on the process parameters that have to be chosen based on the fluid (resin) properties. In the present work, computational fluid dynamics (CFD) simulations are presented for top-down DLP as photocuring 3D printing. The effects of fluid viscosity, travelling speed of build part, travelling speed ratio (ratio of the up-to-down traveling speeds of build part), printed layer thickness, and travel distance considering 13 various cases are scrutinized by the developed model to obtain a stability time of fluid interface. The stability time describes the time it takes for the fluid interface to show minimum fluctuations. According to the simulations, a higher viscosity leads to prints with higher stability time. However, lower stability times in the printed layers are caused by a higher traveling speed ratio (TSR). The variation in settling times with TSR is extremely small in comparison to that of viscosity and travelling speed variations. As a result, a declining trend can be detected for the stability time by increasing the printed layer thickness, while by enhancing the travel distance values, the stability time demonstrated a descending pattern. In total, it was revealed that it is essential to choose optimal process parameters for achieving practical results. Moreover, the numerical model can assist in the optimizing the process parameters.

Bildbasierte Bestimmung der räumlichen und zeitlichen Verteilung des Brechungsindex während der Aushärtung von Polymeren in der additiven Fertigung von Optiken

Abstract In many fields and applications a spatially resolved measurement of the refractive index is desirable. This also applies to the additive manufacturing of optical components and is especially true in the special case of photopolymerization of liquid resins by UV irradiation. The key point here is that the resulting refractive index of the optical component to be printed depends on the degree of cure of the polymer. The latter is directly related to the UV irradiance used for curing. Thus, a local variation of the irradiance can enable a local variation of the refractive index of the additively manufactured optical component. In order to set a defined refractive index distribution, it is desirable to measure the refractive index during the photopolymerization of the resin in a temporally and spatially resolved manner. In this article, a method is discussed with which this can be achieved. The basic principle is that the polymer under investigation is brought into contact with a prism and, based on the principle of total reflection at the interface between prism and polymer, the refractive index of the sample can be inferred.

Recycling Glass and Carbon Fibers for Reusable Components in the Automotive Sector through Additive Manufacturing

This work explores the use of additive manufacturing (AM) to reprocess recycled glass and carbon fibers in the automotive sector. It aims to foster exploitation of recycled Glass Fiber Reinforced Polymers (rGFRPs) and recycled Carbon Fiber Reinforced Polymers (rCFRPs) through two manufacturing workflows: indirect Fused Filament Fabrication (FFF) and UV-assisted Direct Ink Writing (UV-DIW). An industrial case study on vehicle components has been considered by prototyping one real component. After the tensile tests, some molds were fabricated with a FFF 3D printer for the indirect 3D printing process to cast an epoxy-based thermosetting resin with rGFs and rCFs. The second technology consisted in fabricating the parts by hardening in-situ a photo- and thermal-curable thermosetting acrylic liquid resin with rGFs. These results validate the use of AM and recycled composites for applications in the automotive sector. These approaches may be implemented for customizable components for batches below 100 vehicles as the first step for their exploitation.

Radical-Mediated Ring-Opening Photopolymerization for Semicrystalline Thermoplastic Additive Manufacturing

Conventional approaches to stereolithographic additive manufacturing (SLA) employ liquid resin formulations based on multifunctional (meth)acrylates and epoxides that afford cross-linked polymeric networks upon polymerization. Nevertheless, the utilization of resins that yield semicrystalline thermoplastics provides facile access to physical and mechanical properties that are otherwise difficult to attain, such as high toughness and resistance to solvent swelling as well as additional postfabrication processing options that can be used to extend the life cycle of three-dimensional printed polymers. Here, we report the SLA-based additive manufacturing of semicrystalline thermoplastics utilizing the radical-mediated ring-opening photopolymerization of seven- and eight-membered cyclic allylic sulfides. Photopolymerization of resin formulations incorporating seven- and eight-membered cyclic allylic sulfides and crystallization of the resultant (co)polymers were examined using Fourier transform infrared spectroscopy, photorheology, and isothermal photo-differential scanning calorimetry. The formulated resins were found to polymerize and subsequently crystallize rapidly upon irradiation under ambient conditions. The mechanical properties of stereolithographically printed copolymers approached the bulk copolymer mechanical properties, demonstrating unusually strong interlayer adhesion atypical of layerwise, stereolithographic printing of semicrystalline polymers. Finally, high-quality, recyclable, semicrystalline thermoplastic parts were printed and melted down, demonstrating the potential for these semicrystalline materials to provide a path for improved sustainability of polymeric parts produced via SLA.

Volumetric Printing of Thiol‐Ene Photo‐Cross‐Linkable Poly(ε‐caprolactone): A Tunable Material Platform Serving Biomedical Applications (Adv. Mater. 19/2023)

Volumetric 3D Printing In volumetric 3D printing, the whole 3D object is developed simultaneously (i.e., volume-at-once) by means of light-induced crosslinking of a (rotating) liquid resin, as reported by Sandra Van Vlierberghe and co-workers in article number 2210136. The 3D-structure illustrated in the image is based on the chemical network structure of the reported thiol–ene crosslinked poly(caprolactone) (PCL) network, which can be considered a homogeneous 3D network consisting of telechelic alkene-functionalized PCL and tetra-functional thiol crosslinker junctions.

Cytotoxicity of dental self-curing resin for a temporary crown: an in vitro study.

Residual monomer tests using high-performance liquid chromatography and cytotoxicity tests were performed to analyze the effect on the oral mucosa of a self-curing resin for provisional crown production. A cytotoxicity test was performed to confirm whether leaked residual monomers directly affected oral mucosal cells. The cytotoxicity of the liquid and solid resin polymers was measured using a water-soluble tetrazolium (WST) test and microplate reader. In the WST assay using a microplate reader, 73.4% of the cells survived at a concentration of 0.2% liquid resin polymer. The cytotoxicity of the liquid resin polymer was low at ≤0.2%. For the solid resins, when 100% of the eluate was used from each specimen, the average cell viability was 91.3% for the solid resin polymer and 100% for the hand-mixed self-curing resin, which is higher than the cell viability standard of 70%. The cytotoxicity of the solid resin polymer was low. Because the polymerization process of the self-curing resin may have harmful effects on the oral mucosa during the second and third stages, the solid resin should be manufactured indirectly using a dental model.

Effects of reactive and non-reactive tackifying agents on mechanical neat resin and composite performance for preforming processes and Liquid Resin Infusion (LRI) techniques

Preforming processes can be used for automated manufacturing of fiber reinforced polymers. Different technologies are used for processing of dry textile fabrics into 2D or 3D preforms. Due to missing tack of dry fabrics, auxiliary binder systems are used for fixation of the fabrics onto a substrate material and in order to achieve sufficient adhesion between layers. In this study, seven reactive and non-reactive tackifying agents have been dissolved in neat resin samples of three epoxy resin systems, showing different degrees of solubility and a variation on neat resin tensile properties (ΔσAVG = 20%) as well as a reduction on thermal properties (up to ΔTg = −18 °C). Subsequently, fiber reinforced polymers were manufactured using Liquid Resin Infusion techniques in order to characterize the influence of binder systems on water absorption (cs,max = 1.38 wt%) and Interlaminar Shear Strength (ILSS). It was shown that ILSS properties are negatively affected by non-reactive tackifying agents (up to −27%).

Biocompatibility enhancement via post-processing of microporous scaffolds made by optical 3D printer.

Providing a 3D environment that mimics the native extracellular matrix is becoming increasingly important for various applications such as cell function studies, regenerative medicine, and drug discovery. Among the most critical parameters to consider are the scaffold's complicated micro-scale geometry and material properties. Therefore, stereolithography based on photopolymerization is an emerging technique because of its ability to selectively form volumetric structures from liquid resin through localized polymerization reactions. However, one of the most important parameters of the scaffold is biocompatibility, which depends not only on the material but also on the exposure conditions and post-processing, which is currently underestimated. To investigate this systematically, microporous scaffolds with pore sizes of 0.05mm3 corresponding to a porosity of 16,4% were fabricated using the stereolithography printer Asiga PICO2 39 UV from the widely used resins FormLabs Clear and Flexible. The use of various polymers is usually limited for cells because, after wet chemical development, the non-negligible amount of remaining monomers intertwined in the photopolymerized structures is significantly toxic to cells. Therefore, the aim of this research was to find the best method to remove monomers from the 3D scaffold by additional UV exposure. For this purpose, a Soxhlet extractor was used for the first time, and the monomers were immersed in different alcohols. A Raman microspectroscopy was also used to investigate whether different post-processing methods affect DC (cross-linking) to find out if this specifically affects the biocompatibility of the scaffolds. Finally, mesenchymal stem cells from rat dental pulp were examined to confirm the increased biocompatibility of the scaffolds and their ability to support cell differentiation into bone tissue cells.

Electrically-assisted void reduction for synergistic improvement in strength and toughness of fiber-reinforced composites

In the pursuit of designing fiber-reinforced composites with synergistic increased strength and toughness, a novel electric field-driven resin infiltration method is proposed. Specifically, during epoxy resin solidification, the void content of the composites is effectively reduced by 77.1%, from 2.14% to 0.49%, and the tensile strength, interlaminar shear strength, and interlaminar fracture toughness of the composites increase from 539.9 MPa to 748.7 MPa (equivalent to 38.7%), from 40.9 MPa to 57.3 MPa (equivalent to 40.1%) and from 2.1 kJ/m2 to 3.0 kJ/m2 (equivalent to 42.9%) under the applied electric field, respectively. The improvement of the mechanical properties is primarily ascribed to the fact that the electric field-induced Maxwell force drives the liquid resin to flow into intra-tows and inter-tows, thereby improving the wettability of the resin to the fiber tows and correspondingly reducing the void content of the composite, which is supported by the finite element simulation of the rheological behavior of the resin. Interestingly, the application of an electric field can reduce the curing temperature of the composites by nearly 10 °C compared to the baseline composites, while maintaining the same mechanical properties. Therefore, it is an economical, and promising approach to fabricating composites with excellent mechanical properties.

Synthesis of a Liquid Lignin-Based Methacrylate Resin and Its Application in 3D Printing without Any Reactive Diluents.

3D printing of bio-based and renewable polymers such as lignin has gained research attention during the last few decades. We report on the synthesis and characterization of a liquid lignin-based photopolymer and its application in additive manufacturing (AM). Wheat straw soda lignin is liquified in an oxyalkylation reaction with propylene oxide under alkaline conditions and modified with methacryloyl chloride to obtain a lignin-based methacrylate resin. Ninety percent of the functional hydroxyl groups are grafted during the synthesis. The photopolymerization efficiency was evaluated by real-time-NIR-photorheology experiments with two different photoinitiators, leading to double bond conversions (DBC) of ≥80%. 3D-printing experiments of the methacrylated lignin were performed with the hot lithography technology. For the first time, a light-curable lignin derivative with a lignin content of over 30% was successfully 3D printed via vat photopolymerization without any reactive diluents, which is a significant improvement over current state-of-the-art solutions. This outstanding result is a motivating proof of concept and a promising starting point for the in-depth evaluation of bio-based precursors as an alternative to nonrenewable derivatives for 3D printing.