Photopolymerization Process Research Articles

Investigation of processing the alumina slurry through an economic vat photopolymerization process

Vat Photopolymerization (VP) is a 3D printing process to fabricate delicate and precise ceramic parts. LCD VP is one of the most economical 3D printing processes and is less explored for ceramic materials. This work investigated the feasibility of consolidating alumina slurry (45 vol %) using an LCD-based resin 3D printer. Results show that a lower exposure time is better for building a multilayer part than a higher exposure time to avoid scattering and delamination. However, higher exposure time is required to bond the initial layers with the build platform. Fine features, such as holes lesser than 3 mm, were not possible to print due to the scattering of the light. A heating rate of 0.1 °C/min is preferable for crack-free debinding. The final sintered part showed sintering shrinkage of 25% in the x-y direction and 30% in the z-direction with a sintered density of 94%.

Shrinkage stress evolution during photopolymerization: Theory and experiments

Photopolymerized materials are widely used for a broad range of industrial applications including coatings, three-dimensional (3D) printing, micro/nano-scale fabrications, restorative dentistry, etc. However, the service quality and life of these materials are dramatically impaired by polymerization shrinkage stress evolved during the photopolymerization process in practices. A theoretical model for mechanism exploration and accurate prediction of the shrinkage stress evolution is thus-far inaccessible and strategies for alleviating the stress based on existing studies are quite controversial. Herein, we develop a comprehensive theoretical model that can accurately capture the evolution of the shrinkage stress under various working conditions. The model is established by a time-incremental method based on 3D viscoelasticity theory coupling the evolutions of reaction kinetics and material properties during photopolymerization. Theoretical predictions of the shrinkage stress are then validated by groups of experimental measurements using a series of photopolymers and their composites under various testing conditions. We show that our model is able to correctly predict both the individual and combined effects of instrumental compliance, specimen geometry, material composition, and photocuring protocol on the shrinkage stress evolution, results of which well clarify the diverse conflicts existed in the literature. The magnitudes of the predicted stresses based on our model agree quantitatively with those of the experimental ones, far exceeding the prediction accuracy of all existing models. In addition, our model also successfully predicts the results measured in the literature using various testing devices, indicating that our model could be applied to existing setups for shrinkage stress measurement of bulk materials. Our theory-experiment-combined study not only provides a theoretical approach to accurately predict and thoroughly uncover the shrinkage stress evolution during photopolymerization, it will also guide further explorations on shrinkage stress reduction and material property optimization for photopolymers with improved service life. Additionally, the presented model on the shrinkage stress evolution paves a potentially new way for the derivation of material mechanical property evolutions and thus revealing the mechanical-chemical-coupled mechanisms during photopolymerization.

Continuous Vat Photopolymerization for Optical Lens Fabrication

Optical lenses require feature resolution and surface roughness that are beyond most (3D) printing methods. A new continuous projection-based vat photopolymerization process is reported that can directly shape polymer materials into optical lenses with microscale dimensional accuracy (< 14.7µm) and nanoscale surface roughness (< 20nm) without post-processing. The main idea is to utilize frustum layer stacking, instead of the conventional 2.5D layer stacking, to eliminate staircase aliasing. A continuous change of mask images is achieved using a zooming-focused projection system to generate the desired frustum layer stacking with controlled slant angles. The dynamic control of image size, objective and imaging distances, and light intensity involved in the zooming-focused continuous vat photopolymerization are systematically investigated. The experimental results reveal the effectiveness of the proposed process. The 3D-printed optical lenses with various designs, including parabolic lenses, fisheye lenses, and a laser beam expander, are fabricated with a surface roughness of 3.4nm without post-processing. The dimensional accuracy and optical performance of the 3D-printed compound parabolic concentrators and fisheye lenses within a few millimeters are investiagted. These results highlight the rapid and precise nature of this novel manufacturing process, demonstrating a promising avenue for future optical component and device fabrication.

Correlation between Critical Energy, Penetration Depth, and Photopolymerization Kinetics in Aluminum–Phosphate–Silicate Hybrid Materials for Vat Photopolymerization

The photopolymerization of aluminum–phosphate–silicate resins obtained from the hybrid sol–gel route for Vat photopolymerization (VPP) process was investigated. The printing parameters derived from Jacob’s work curve model, critical energy (Ec) and penetration depth (Dp), were determined as a function of laser power and MPTMS (silicate) concentration for materials with stoichiometry Si(x)-(Al + P)(1–x), 0 ≤ x ≤ 0.7. The kinetics of photopolymerization was further explored using steady- and unsteady-state photo-DSC experiments. The oxygen inhibition and primary termination had similar contributions to the polymerization process for all compositions, while the propagation and bimolecular termination constants increased with MPTMS concentration. These experimental results were used to test the validity of the Ec ∝ kt1/2/kp and Dp ∝ ϵ relationship derived from a photochemical model for VPP assuming steady-state kinetics. Both Ec ∝ (kt1/2/kp) and Dp ∝ ϵ may be used to predict critical energy and penetration values for an arbitrary resin without calculating its work curve function, according to our study.

Cost-effective bi-functional resin reinforced with a nano-inclusion blend for vat photopolymerization additive manufacturing: The effect of multiple antibacterial nanoparticle agents

Herein, a blend of nanoparticle (NP) inclusions has been arranged at various loadings into a photosensitive resin, while bi-functional three-dimensional (3D) printed specimens were fabricated through a vat photopolymerization process, to elucidate physicochemical mechanisms and synergistic effects, over the filler loading. Energy-dispersive X-ray spectroscopy (EDS), Raman, and thermogravimetric analysis (TGA) revealed the chemical/spectroscopic and thermal properties of the fabricated specimens. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) depicted the surface morphology, while SEM fractography demonstrated the morphology of tensile test specimens’ fractured surfaces. Mechanical tests exhibited a strong reinforcement mechanism. The highest reinforcement of 20.8% in the tensile strength is reported for the 2 wt.% nanocomposite. All the prepared recipes exhibited a boosted antibacterial performance, as was documented via a screening agar well diffusion method. The research herein leads towards a novel generation of low-cost bi-functional materials for 3D printing bioactive applications, where mechanical reinforcement and antibacterial performance are required in the operational environment.

Lignin nanoparticle-enhanced biobased resins for digital light processing 3D printing: Towards high resolution and tunable mechanical properties

The more widespread applicability of photopolymerization-based three-dimensional (3D) printing is limited by the availability of light-curable resins, most of which are based on fossil-derived compounds. We developed a biobased lignin-derivable resin by utilizing methacrylated derivatives of vanillin, vanillyl alcohol, and eugenol as aromatic monomers. Lignin nanoparticles (LNPs) were incorporated as functional fillers that enhance print resolution and material properties. The crosslinking degree, and thereby the tensile properties, was modulated through the use of mono- or dimethacrylated vanillin derivatives in the resin formulation. The LNPs acted as UV absorbers, conferring better control of the photopolymerization process by preventing light penetration across unintended layers, leading to enhanced print resolution. The LNPs showed excellent dispersion stability due to their size and morphology. The inclusion of up to 2 wt% of LNPs improved the ductility of the 3D printed nanocomposites through toughening mechanisms enabled by the rigid nanoparticles. Finally, exploiting the differences in crosslinking degree of the resin formulations, a multi-material model featuring both soft and rigid domains was fabricated. This study demonstrates a simple but effective strategy for the design of biobased photocurable resins with tailorable mechanical properties that are suitable for high-resolution and multi-material 3D printing.

3D printed human hair - polymer continuous fiber reinforced composites through Vat Photopolymerization process

This study gives insight into the fabrication of 3D-printed human hair polymer composites using the Stereolithography (SLA) technique. Different surface modification techniques have been introduced to overcome the problem of poor interfacial adhesion between the fiber and the matrix. Human hair fibers were modified by grafting with Methacryl Polyhedral Oligomeric Silsesquioxane (MA-POSS) and cold air plasma treatment. The effect of the surface treatment was investigated using single hair pull-out tests, and the results showed that plasma-treated human hair composites showed the best performance in terms of interfacial strength between fibers and matrix. Furthermore, the mechanical properties of 3D-printed human hair composites have been investigated. The results indicate that the ultimate tensile strength has increased by 35% in plasma-treated composites compared to nontreated human hair composites and 100% compared to composites with no reinforcement. Scanning Electron Microscopy (SEM) and an optical microscope were used to study the surface morphology of the modified fibers. Moreover, X-ray Photoelectron Spectroscopy (XPS) was performed to confirm the grafting of MA-POSS on the human hair fibers. In addition, the X-ray Computed Tomography (µ-CT) technique has been utilized to analyze voids, fiber volume, and fiber orientation.

3D Printing of Bone Substitutes Based on Vat Photopolymerization Processes: A Systematic Review

Treatment options for critically sized bone defects are currently limited to metal osteosynthesis, autologous bone grafting, or calcium-based implants to bridge the gap. Additive manufacturing techniques pose a possible alternative. The light-basedthree-dimensional printing process of vat photopolymerization (VP) is of particular interest since it enables the printing of complex scaffold architectures at high resolution. This review compares multiple vat photopolymerization processes as well as the employed resin components’ interactions with musculoskeletal cells and tissue. The results show an outstanding printing capability, exceeding the potential of other printing methods. However, despite the availability of various biocompatible materials, neither the mechanical strength of bone nor the scale necessary for clinical application has been achieved so far when relying on single material constructs. One possible solution is the development of adaptive hybrid constructs produced with multimaterial VP.

Fabrication of 3D Printed Ceramic Part Using Photo-Polymerization Process.

Ceramics are high-strength and high-temperature resistant materials that are used in various functional parts. However, due to the high strength and brittleness properties, there are many difficulties in the fabrication of complex shapes. Therefore, there are many studies related to the fabrication of ceramic parts using 3D printing technology optimized for complex shapes. Among them, studies using photo-polymerization (PP) 3D printing technology with excellent dimensional accuracy and surface quality have received the most widespread attention. To secure the physical properties of sintered ceramic, the content and distribution of materials are important. This study suggests a novel 3D printing process based on a high-viscosity composite resin that maximizes the content of zirconia ceramics. For reliable printing, the developed 3D printers that can adjust the process environment were used. To minimize warpage and delamination, the divided micro square pattern images were irradiated in two separate intervals of 1.6 s each while maintaining the internal chamber temperature at 40 °C. This contributed to improved stability and density of the sintered structures. Ultimately, the ceramic parts with a Vickers hardness of 12.2 GPa and a relative density of over 95% were able to be fabricated based on a high-viscosity resin with 25,000 cps.

Design of Reliable Remobilisation Finger Implants with Geometry Elements of a Triple Periodic Minimal Surface Structure via Additive Manufacturing of Silicon Nitride

When finger joints become immobile due to an accident during sports or a widespread disease such as rheumatoid arthritis, customised finger joint implants are to be created. In an automated process chain, implants will be produced from ceramic or metallic materials. Artificial intelligence-supported software is used to calculate three-dimensional models of the finger bones from two-dimensional X-ray images. Then, the individual implant design is derived from the finger model and 3D printed. The 3D printing process and the structures used are evaluated via model tests and the final implant design via a reliability calculation in a way to ensure that this is also possible via an AI process in the future. Using additive manufacturing with silicon nitride-based ceramics, model specimens and implants are produced via the lithography-based ceramic vat photopolymerisation process with full geometry or elements of triple periodic minimal surfaces structure. The model specimens are tested experimentally, and the loads are matched with a characteristic strength assuming a Weibull distribution of defects in the volume to generate and match failure probabilities. Calculated fracture forces of the silicon nitride-based ceramic structure was validated by comparison of simulation and tests, and the calculation can be used as a quality index for training of artificial intelligence in the future. The proposed method for individualized finger implant design and manufacturing may allow for correction of potential malpositions of the fingers in the future.

The process of photopolymerization of acrylated soybean oil-based epoxides investigated by positron annihilation lifetime spectroscopy

The process of photopolymerization of acrylated soybean oil-based epoxides investigated by positron annihilation lifetime spectroscopy

Photoinitiator or photosensitizer? Dual behaviour of m-terphenyls in photopolymerization processes

A series of eight m-terphenyl-based compounds functionalized with amino, diethylamino, piperidine, and pyrrolidine moieties were synthesized and investigated for the first time as one-component free-radical photoinitiators for acrylate and methacrylate monomers. Developed compounds were also applied as photosensitizers of iodonium salt in two-component initiating systems for free radical and cationic photopolymerization. That is why they are excellent for hybrid polymerization e.g. synthesis of interpenetrating polymer networks (IPN). The mechanical properties of produced IPN samples were measured. We investigated the effect of the amine group modification and the presence of one or two bromine atoms on the photophysical and electrochemical properties of the m-terphenyls and their consequent efficiency in polymerization processes. Two bromine substituents in the molecule slightly shift the absorption spectrum towards longer wavelengths in comparison to a compound with one bromine atom. Functionalization of amine group by the introduction of alkyl moieties strongly affects absorption properties, the wavelength corresponding with the maximum absorption values increases in the following order: amine group > diethylamine group > piperidine group > pyrrolidine group. Investigated compounds were found to be efficient in photoinitiating systems for free-radical, cationic, and hybrid photopolymerization, kinetic of polymerization processes with the use of different monomers were followed using real-time FT IR methodology. Moreover, the chemical mechanism was investigated using photolysis and ESR experiments. Developed photoinitiators were also applied for laser writing experiments. In the next step, the key printing parameters were settled, the critical energy necessary to initiate polymerization (Ec) and penetration depth of curing light (Dp), and studied systems were applied for resin used for LCD-based 3d printing.

Development of robot assisted hybrid additive manufacturing technology for the freeform fabrication of lattice structures

The extensive study and development of additive manufacturing technologies have allowed not only increased customization of the final parts but especially of their complexity. From such perspectives, intricate and mechanically efficient lattices which maximize their strength-to-weight ratio could be printed. Nevertheless, the fabrication of regular lattices, like octet cells, has been proven to be particularly limited by the layer-by-layer construction process due to the introduction of additional interfaces and/or the requirement of supports. In this work, a hybrid extrusion–photopolymerization process is coupled with a 6-axis robotic arm to allow for the fabrication of layers-less lattices thanks to the use of a custom graphical user interface and control over the printing head orientation. A highly reactive resin is employed as the ink and loaded with silica particles in order to tailor its rheological properties. A good resemblance between the printed components and the digital model is achieved, while their mechanical properties are superior to the ones of the traditionally additive manufactured octets.

Mechanistically Novel Frontal‐Inspired In Situ Photopolymerization: An Efficient Electrode|Electrolyte Interface Engineering Method for High Energy Lithium Metal Polymer Batteries

The solvent‐free in situ polymerization technique has the potential to tailor‐make conformal interfaces that are essential for developing durable and safe lithium metal polymer batteries (LMPBs). Hence, much attention has been given to the eco‐friendly and rapid ultraviolet (UV)‐induced in situ photopolymerization process to prepare solid‐state polymer electrolytes. In this respect, an innovative method is proposed here to overcome the challenges of UV‐induced photopolymerization (UV‐curing) in the zones where UV‐light cannot penetrate, especially in LMPBs where thick electrodes are used. The proposed frontal‐inspired photopolymerization (FIPP) process is a diverged frontal‐based technique that uses two classes (dual) of initiators to improve the slow reaction kinetics of allyl‐based monomers/oligomers by at least 50% compared with the conventional UV‐curing process. The possible reaction mechanism occurring in FIPP is demonstrated using density functional theory calculations and spectroscopic investigations. Indeed, the initiation mechanism identified for the FIPP relies on a photochemical pathway rather than an exothermic propagating front forms during the UV‐irradiation step as the case with the classical frontal photopolymerization technique. Besides, the FIPP‐based in situ cell fabrication using dual initiators is advantageous over both the sandwich cell assembly and conventional in situ photopolymerization in overcoming the limitations of mass transport and active material utilization in high energy and high power LMPBs that use thick electrodes. Furthermore, the LMPB cells fabricated using the in situ‐FIPP process with high mass loading LiFePO4 electrodes (5.2 mg cm‐2) demonstrate higher rate capability, and a 50% increase in specific capacity against a sandwich cell encouraging the use of this innovative process in large‐scale solid‐state battery production.

Influence of Methacrylate and Vinyl Monomers on Radical Bulk Photopolymerization Process and Properties of Epoxy-Acrylate Structural Adhesives

In this paper, epoxy-acrylate structural adhesives tapes (SATs) were obtained from Bisphenol A-based liquid epoxy resin and epoxy acrylic resins (EARs). A new method of EARs preparation, i.e., the free radical bulk photopolymerization process (FRBP), was studied in detail. The influence of methacrylic monomers (methyl methacrylate, ethyl methacrylate, butyl methacrylate, lauryl methacrylate, (2-acetoacetoxy)ethyl methacrylate) and vinyl monomers (N-vinylpyrrolidone and styrene) on the FRBP process of base monomers (i.e., butyl acrylate, glycidyl methacrylate and 2-hydroxyethyl acrylate) was investigated. The kinetics of photopolymerization process was monitored by photo-differential scanning calorimetry method. The properties of the obtained EARs (viscosity and average molecular weights), as well as monomers conversion using 1H NMR, were determined. It was revealed that styrene significantly decreases the photopolymerization rate and increases the final monomers conversion (+27%). However, the resulting tetrapolymers BA-co-GMA-co-HEA-co-STY have low molecular weights and low polydispersity (2.2). Methacrylate monomers with shorter aliphatic chains (<C4) also decrease the rate of photopolymerization due to the length of the aliphatic chain increasing. Surprisingly, the best results of adhesion to steel and shear strength were obtained for SAT based on epoxy acrylate resin with styrene (11 N/25 mm and 20.8 MPa, respectively). However, the thermomechanical properties of SAT with styrene were weaker than those with methacrylates.

Lithography based Metal Manufacturing (LMM): Influence of particle size and composition of copper powder on UV light penetration

In this study, a Mie-theory-based computation was used to investigate the influence of two main factors (the particle size and the particle oxidation) affecting UV light transmission in copper loaded suspensions. The behavior of UV light penetration was then compared with highly loaded copper photocurable formulation used in 3D printing. According to the model, large particles (diameter > 2 µm) should be selected to afford UV transmission in a diluted (<10 vol%) copper suspension regardless of the oxidation of copper particles. For loading rate higher than 20 vol% transmission dramatically drops but the photopolymerization process remains efficient within the cured layer. This might confirm the hypothesis of UV light penetration through light scattering. Additional results on oxidized copper powders exhibited curing depths and curing kinetics that might be correlated to the less important light scattering compared to oxide-free powders. Solutions are proposed to tune curing depth by modifying the oxidation state of copper particles. The wise selection of copper powders and the application of thermal pre-treatment offers the opportunity to fabricate copper parts by Lithography based Metal Manufacturing (LMM).

Impact of Beam Shape on Print Accuracy in Digital Light Processing Additive Manufacture.

Photopolymerization-based additive manufacturing requires selectively exposing a feedstock resin to ultraviolet (UV) light, which in digital light processing is achieved either using a digital micromirror device or a digital mask. The minimum tolerances and resolution for a multilayer process are separate for resolution through the Z-axis, looking through the thickness of a printed part, and resolution in the XY-axes, in the plane of the printed layer. The former depends wholly on the rate of attenuation of the incident UV light through the material relative to the mechanical motion of the build layer, while the latter is determined by a two-dimensional pattern of irradiance on the resin formed by the digital micromirror device or the digital mask. The size or the spacing of elements or pixels of this digital mask is frequently given by manufacturers as the "resolution" of the device, however, in practice the achievable resolution is first determined by the beam distribution from each pixel. The beam distribution is, as standard, modeled as a two-parameter Gaussian distribution but the key parameters of peak intensity and standard deviation of the beam are hidden to the user and difficult to measure directly. The ability of models based on the Gaussian distribution to correctly predict the polymerization of printed features in the microscale is also typically poor. In this study, we demonstrate an alternative model of beam distribution based on a heavy-tailed Lorentzian model, which is able to more accurately predict small build areas for both positive and negative features. We show a simple calibration method to derive the key space parameters of the beam distribution from measurements of a single-layer printed model. We propose that the standard Gaussian model is insufficient to accurately predict a print outcome as it neglects higher-order terms, such as beam skew and kurtosis, and in particular failing to account for the relatively heavy tails of the beam distribution. Our results demonstrate how the amendments to the beam distribution can avoid errors in microchannel formation, and better estimates of the true XY-axes resolution of the printer. The results can be used as the basis for voxel-based models of print solidification that allow software prediction of the photopolymerization process.

Study on New Dental Materials Containing Quinoxaline-Based Photoinitiators in Terms of Exothermicity of the Photopolymerization Process.

Modern dentistry places great demands on the dental composites used for filling tooth cavities or treating cavitated tooth decay. The aim of the work was to modify the properties of composites by changing the initiators and co-initiators. This was achieved by using initiators based on a quinoxaline skeleton and co-initiators that are derivatives of acetic acid, which is an advantage of these photoinitiating systems due to the elimination of aromatic amines from the photocurable composition. The composites also differed in dental fillers. The effect of the compounds on the exothermicity of the photopolymerization process, the surface morphology of the obtained materials and the maximum compressive strength were determined. The photoinitiating capacity of the two-component systems was tested by the microcalorimetric method using the multifunctional monomer TMPTA, typical for dental filler compositions. The new photoinitiating systems show particularly good efficiency of free radical polymerization initiation, which occurs by the photoinduced intermolecular electron transfer (PET) mechanism. The comparison of the tested systems with camphorquinone, a photoinitiator traditionally used in dentistry, made it possible to observe a decrease in temperature during photopolymerization without a significant decrease in the polymerization rate or increase in photocuring time, as well as a better homogeneity of the surface of the obtained polymeric materials. This indicates that dye-acetic acid derivative systems may be useful in dental applications.

3D Laser Nanoprinting of Functional Materials

Abstract3D laser nanoprinting represents a revolutionary manufacturing approach as it allows maskless fabrication of 3D nanostructures at a resolution beyond the optical diffraction limit. Specifically, it endows the printed structures novel physical, chemical, or mechanical properties not observed at macroscopic scale. However, 3D laser nanoprinting typically relies on the photopolymerization process, indicating its limitation on the printable materials and functionalities. The capability to print diverse functional materials beyond polymer will enable a lot of new device applications in nanophotonics, microelectronics, and so on. One of the strategies is to use the 3D‐printed polymer structures as skeletons for functional material deposition, while another is to mix the functional components with the photocurable molecules and print the nanocomposites. More recently, several laser nanoprinting techniques beyond photopolymerization are also developed. In this review, the cutting‐edge technical innovation is summarized and a couple of examples are highlighted showing exciting applications of the printed structures in magnetic microrobots, photonics, and optoelectronics. Finally, the vision for existing challenges and future development in this field is shared.

Additively Manufactured Lattice Materials with a Double Level of Gradation: A Comparison of Their Compressive Properties when Fabricated with Material Extrusion and Vat Photopolymerization Processes.

Natural porous materials adjust their resulting mechanical properties by the optimal use of matter and space. When these are produced synthetically, they are known as mechanical metamaterials. This paper adds degrees of tailoring of mechanical properties by producing double levels of gradation in lattice structures via cross-section variation in struts in uniformly periodic lattice structures (UPLS) and layered lattice structures (LLS). These were then additively manufactured via material extrusion (ME) and vat photopolymerization (VP). Their effective mechanical properties under compressive loads were characterized, and their stiffness contrasted with finite element models (FEM). According to the simulation and experimental results, a better correlation was obtained in the structures manufactured via VP than by ME, denoting that printing defects affect the correlation results. The brittle natural behavior of the resin caused a lack of a plateau region in the stress-strain curves for the UPLS structures, as opposed to those fabricated with ME. The LLS increased energy absorption up to 244% and increased the plateau stress up to 100% compared to the UPLS. The results presented in this paper demonstrate that the mechanical properties of lattice structures with the same base topology could be modified by incorporating variations in the strut diameter and then arranging these differently.