Transparent Resins Research Articles

Geometrical and Optical Properties of Optical Components Manufactured by Means of Stereolithography

Additive manufacturing (AM) is a fast-growing technology that supports the rapid fabrication of prototypes. Already, employing AM technologies in early product development stages can help to accelerate the time from conceptualisation phase to the market entry. Furthermore, advances in the field of materials, such as highly transparent resins in the visible wavelength spectrum, also open application potentials for the design and fabrication of optical components. Three highly transparent resins from different suppliers are studied in this work regarding their influence on the geometrical and optical properties of additively manufactured components. The different materials were processed using a masked stereolithography 3D printer. Geometrical and optical properties of the generated test samples were analysed by means of laser scanning microscopy and spectrophotometry, respectively. A clear correlation between optical properties of the resin and the solidified samples was identified. It was further found that while spin coating does not significantly affect the geometrical properties, a strong influence on the resulting optical properties could be observed. The highest transmission properties, in the range of up to 90%, were determined for samples that were spin-coated on both surfaces. Thus, applying a spin coating operation to additively manufactured optical components is recommended for the highest transmission properties.

Preparation and characterization of chiral carbon dots with red emission

Methods for the synthesis of chiral carbon dots (CDs) possessing bright long-wavelength emission are highly desirable. Here, we develop red-emissive chiral CDs from hydrothermal treatment of L-/D-lysine and o-phenylenediamine (OPD). Pure chiral CDs are obtained by filtration and dialysis. Ultraviolet-visible (UV–vis) absorption spectra, photoluminescence (PL) spectra, PL quantum yield (QY), and PL decay of the chiral CDs are thoroughly studied. The chiral CDs exhibit bright red fluorescence with PL QY in the range of 26.6%–42.2% in different solvents. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) indicate that the chiral CDs have a mean particle size of 2.7 nm, and consist of graphite-like carbons and abundant functional groups at their surfaces. Circular dichroism spectroscopy shows new Cotton bands at about 300 and 610 nm for chiral CDs differed from those of L-/D-lysine, revealing the generation of optical chirality for CDs. Then transparent optical resins made of CDs/epoxy resin composites are prepared, possessing red fluorescence and circular dichroism, broadening potential applications of solid materials with chiroptical properties. Finally, we discuss the possible origins of chirality and fluorescence of the CDs. The exploitation of highly red-emissive chiral CDs may promote the development and application of chiral CDs.

Towards microwave volumetric additive manufacturing: Generation of a computational multi-physics model for localized curing

Visible light-based volumetric additive manufacturing (VAM) technology has recently enabled rapid 3D printing of optically transparent resins in a single step. There is now strong interest in extending the design space of VAM to include opaque, scattering and composite materials. Microwave energy can penetrate more deeply than visible light into a broader family of materials. For microwaves to be useful for VAM, however it is necessary to have a fundamental understanding of material dielectric properties, microwave field propagation and localization. Here we present a multi-physics microwave beam formed-thermal diffusion model that addresses these needs. The model demonstrates its ability to optimize power delivery and curing time to obtain better thermal control. We validate the model with a proof-of-concept single-antenna experimental system operating at 10 GHz that is able to cure a wide variety of materials, including both optically translucent and opaque epoxy resins loaded with conductive additives with a minimum curing spot of 5 mm. While available microwave hardware operating at 40 Watt power cures the resins in 2.5 min, the model estimates the ability to cure in as less as 6 s at 1 Kilowatt power levels. This computational model and experiments lay the foundation for a future multi-waveguide microwave-based VAM system.

Study of the possibility of application of optically transparent epoxy resins for restoration of vitric enamels

Objects of decorative and applied arts of historical value, over time, undergo physical and mechanical changes caused by the influence of external factors. For example, vitreous enamels can lose their luster, crack, and archaeological objects can also suffer due to the corrosion processes of glass and metal base. Therefore, it seems an important task to develop approaches to the restoration of products with vitreous enamels. In the work on the results of the use of studies of optically transparent epoxy resins as a restoration material when working with objects of cultural investigation, which consists in the restoration of vitreous enamels. To assess the admissibility of using the material as a restorative, an accelerated test method was also developed and tested.

High performance epoxy composites modified by a ladder-like polysilsesquioxane

Developing transparent epoxy resins (EPs) with high mechanical properties, thermally stable, and superior dielectric properties is of vital importance for electronics and communication application. Here we synthesized a novel ladder-like polysilsesquioxane containing phenyl and amino groups (N-PPSQ) via a facile co-condensation process for EP modification. The N-PPSQ features a highly regular ladder-like structure with several reactive amino groups anchored, as confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and nuclear magnetic resonance silicon spectroscopy (29Si NMR). Different EP/N-PPSQ composites with optimized additive contents were prepared. Compared to the pure EP, 10 wt% N-PPSQ addition helps to increase the impact strength of EP by a maximal 73 % while simultaneously improving the tensile and flexural properties. With maintained high transparency and enhanced hygrothermal stability, the EP/N-PPSQ composite shows lower dielectric constant and dielectric loss than pure EP. The work demos a fine solution to improving the toughness, dielectric properties and hygrothermal stability without sacrificing the mechanical and optical properties of EP.

Synthesis of poly(methylsilsesquioxane-co-dimethylsiloxane) liquids and deep-ultraviolet transparent elastic resins through co-solvent-free water-rich hydrolytic polycondensation of organoalkoxysilanes

A co-solvent-free hydrolytic polycondensation of mono- and dimethylmethoxysilanes at high H2O/Si ratios (≳10) gave methylsilsesquioxane-rich (80 mol%) liquids heat-curable to stiff, tough, and deep-UV transparent resins showing full elastic recovery.

Durable 3D murine ex vivo retina glaucoma models for optical coherence tomography.

Durable and standardized phantoms with optical properties similar to native healthy and disease-like biological tissues are essential tools for the development, performance testing, calibration and comparison of label-free high-resolution optical coherence tomography (HR-OCT) systems. Available phantoms are based on artificial materials and reflect thus only partially ocular properties. To address this limitation, we have performed investigations on the establishment of durable tissue phantoms from ex vivo mouse retina for enhanced reproduction of in vivo structure and complexity. In a proof-of-concept study, we explored the establishment of durable 3D models from dissected mouse eyes that reproduce the properties of normal retina structures and tissue with glaucoma-like layer thickness alterations. We explored different sectioning and preparation procedures for embedding normal and N-methyl-D-aspartate (NMDA)-treated mouse retina in transparent gel matrices and epoxy resins, to generate durable three-dimensional tissue models. Sample quality and reproducibility were quantified by thickness determination of the generated layered structures utilizing computer-assisted segmentation of OCT B-scans that were acquired with a commercial HR-OCT system at a central wavelength of 905 nm and analyzed with custom build software. Our results show that the generated 3D models feature thin biological layers close to current OCT resolution limits and glaucoma-like tissue alterations that are suitable for reliable HR-OCT performance characterization. The comparison of data from resin-embedded tissue with native murine retina in gels demonstrates that by utilization of appropriate preparation protocols, highly stable samples with layered structures equivalent to native tissues can be fabricated. The experimental data demonstrate our concept as a promising approach toward the fabrication of durable biological 3D models suitable for high-resolution OCT system performance characterization supporting the development of optimized instruments for ophthalmology applications.

UV polymerization fabrication method for polymer composite based optical fiber sensors

Optical fiber (OF) sensors are critical optical devices with excellent sensing capabilities and the capacity to operate in remote and hostile environments. However, integrating functional materials and micro/nanostructures into the optical fiber systems for specific sensing applications has limitations of compatibility, readiness, poor control, robustness, and cost-effectiveness. Herein, we have demonstrated the fabrication and integration of stimuli-responsive optical fiber probe sensors using a novel, low-cost, and facile 3D printing process. Thermal stimulus–response of thermochromic pigment micro-powders was integrated with optical fibers by incorporating them into ultraviolet-sensitive transparent polymer resins and then printed via a single droplet 3D printing process. Hence, the thermally active polymer composite fibers were grown (additively manufactured) on top of the commercial optical fiber tips. Then, the thermal response was studied within the temperature range of (25–35 °C) and (25–31 °C) for unicolor and dual color pigment powders-based fiber-tip sensors, respectively. The unicolor (with color to colorless transition) and dual color (with color to color transition) powders-based sensors exhibited substantial variations in transmission and reflection spectra by reversibly increasing and decreasing temperatures. The sensitivities were calculated from the transmission spectra where average change in transmission spectra was recorded as 3.5% with every 1 °C for blue, 3% for red and 1% for orange-yellow thermochromic powders based optical fiber tip sensors. Our fabricated sensors are cost-effective, reusable, and flexible in terms of materials and process parameters. Thus, the fabrication process can potentially develop transparent and tunable thermochromic sensors for remote sensing with a much simpler manufacturing process compared to conventional and other 3D printing processes for optical fiber sensors. Moreover, this process can integrate micro/nanostructures as patterns on the optical fiber tips to increase sensitivity. The developed sensors may be employed as remote temperature sensors in biomedical and healthcare applications.

Chain microstructure comparison of two transparent polypropylene resins fractionated by temperature rising elution fractionation

Chain microstructure comparison of two transparent polypropylene resins fractionated by temperature rising elution fractionation

A Regression Approach to Model Refractive Index Measurements of Novel 3D Printable Photocurable Resins for Micro-Optofluidic Applications.

In this work, a quadratic polynomial regression model was developed to aid practitioners in the determination of the refractive index value of transparent 3D printable photocurable resins usable for micro-optofluidic applications. The model was experimentally determined by correlating empirical optical transmission measurements (the dependent variable) to known refractive index values (the independent variable) of photocurable materials used in optics, thus obtaining a related regression equation. In detail, a novel, simple, and cost-effective experimental setup is proposed in this study for the first time for collecting the transmission measurements of smooth 3D printed samples (roughness ranging between 0.04 and 2 μm). The model was further used to determine the unknown refractive index value of novel photocurable resins applicable in vat photopolymerization (VP) 3D printing techniques for manufacturing micro-optofluidic (MoF) devices. In the end, this study proved how knowledge of this parameter allowed us to compare and interpret collected empirical optical data from microfluidic devices made of more traditional materials, i.e., Poly(dimethylsiloxane) (PDMS), up to novel 3D printable photocurable resins suitable for biological and biomedical applications. Thus, the developed model also provides a quick method to evaluate the suitability of novel 3D printable resins for MoF device fabrication within a well-defined range of refractive index values (1.56; 1.70).

Design of P-decorated POSS towards flame-retardant, mechanically-strong, tough and transparent epoxy resins

Epoxy resins (EPs) are known for their durability, strength, and adhesive properties, which make them a versatile and popular material for use in a variety of applications, including chemical anticorrosion, small electronic devices, etc. However, EP is highly flammable due to its chemical nature. In this study, phosphorus-containing organic–inorganic hybrid flame retardant (APOP) was synthesized by introducing 9, 10-dihydro-9-oxa-10‑phosphaphenathrene (DOPO) into cage-like octaminopropyl silsesquioxane (OA-POSS) via Schiff base reaction. The improved flame retardancy of EP was achieved by combining the physical barrier of inorganic Si-O-Si with the flame-retardant capability of phosphaphenanthrene. EP composites containing 3 wt% APOP passed the V-1 rating with a value of LOI of 30.1% and showed an apparent reduction in smoke release. Additionally, the combination of the inorganic structure and the flexible aliphatic segment in the hybrid flame retardant provides EP with molecular reinforcement, while the abundance of amino groups facilitates a good interface compatibility and outstanding transparency. Accordingly, EP containing 3 wt% APOP increased in tensile strength, impact strength, and flexural strength by 66.0 %, 78.6 %, and 32.3 %, respectively. The EP/APOP composites had a bending angle lower than 90°, and their successful transition to a tough material highlights the potential of this innovative combination of the inorganic structure and the flexible aliphatic segment. In addition, the relevant flame-retardant mechanism revealed that the APOP promoted the formation of a hybrid char layer containing P/N/Si for EP and produced phosphorus-containing fragments during combustion, showing flame-retardant effects in both condensed and vapor phases. This research offers innovative solutions for reconciling flame retardancy & mechanical performances and strength & toughness for polymers.

Graphene Oxide Based Transparent Resins For Accurate 3D Printing of Conductive Materials

AbstractDigital Light Processing (DLP) allows the fast realization of 3D objects with high spatial resolution. However, DLP is limited to transparent resins, and therefore not well suited for printing electrically conductive materials. Manufacturing conductive materials will significantly broaden the spectrum of applications of the DLP technology. But conductive metals or carbon‐based fillers absorb and scatter light; inhibiting thereby photopolymerization, and lowering resolution. In this study, UV transparent liquid crystal graphene oxide (GO) is used as precursor for generating in situ conductive particles. The GO materials are added to a photopolymerizable resin via an original solvent exchange process. By contrast to earlier contributions, the absence of drying during the all process allows the GO material to be transferred as monolayers to limit UV scattering. The absence of UV scattering and absorption allows for fast and high‐resolution 3D printing. The chosen resin sustain high temperature to enable an in situ efficient thermal reduction of GO into reduced graphene oxide (rGO) that is electrically conductive. The rGO particles form percolated networks with conductivities up to 1.2 × 10−2 S m−1. The present method appears therefore as a way to reconcile the DLP technology with the manufacturing of 3D electrically conductive objects.

Entering a New Dimension in Powder Processing for Advanced Ceramics Shaping.

Filigree structures can be manufactured via two-photon polymerization (2PP) operating in the regime of nonlinear light absorption. For the first time, it is possible to apply this technique to the powder processing of ceramic structures with a feature size in the range of the critical defect sizes responsible for brittle fracture and, thus, affecting fracture toughness of high-performance ceramics. In this way, tailoring of advanced properties can be achieved already in the shaping process. Traditionally, 2PP relies on transparent polymerizable resins, which are diametrically opposed to the usually completely opaque ceramic resins and slurries. Here a transparent and photocurable suspension of nanoparticles (resin) with very high mass fractions of yttria-stabilized zirconia particles (YSZ) is presented. Due to the extremely well-dispersed nanoparticles, scattering of light can be effectively suppressed at the process-relevant wavelength of 800nm. Sintered ceramic structures with a resolution of down to 500nm are obtained. Even at reduced densities of 1-4g cm-3 , the resulting compressive strength with 4.5GPa is equivalent or even exceeding bulk monolithic yttria-stabilized zirconia. A ceramic metamaterial is born, where the mechanical properties of yttria-stabilized zirconia are altered by changing geometrical parameters, and gives access to a new class of ceramic materials.

Durable flame-retardant, strong and tough epoxy resins with well-preserved thermal and optical properties via introducing a bio-based, phosphorus-phosphorus, hyperbranched oligomer

The manufacture of transparent, durable fire-safe, strong yet tough epoxy resins (EPs) remains a great challenge. Herein, a novel hyperbranched, P-P synergetic, bio-based flame retardant (DIT) was synthesized, which was composed by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), bio-derived itaconic anhydride (ITA) and trihydroxymethylphosphine oxide (THPO) moieties. Adding 2 wt% DIT enabled the EP/2DIT sample to achieve a UL-94 V-0 rating and a limiting oxygen index (LOI) of 36.6%. The peak heat release rate and total smoke production of EP/4DIT sample with 4 wt% DIT were 23.8% and 19.9% lower than those of virgin EP sample. Notably, the EP/DIT sample featured durable flame retardancy because of the oligomeric structure of DIT. The high flame-retardant efficiency of DIT was mainly due to the synergism between DOPO and THPO. DIT simultaneously strengthened and toughened EP thermoset at a low addition (≤2 wt%), and 2 wt% of DIT increased the elongation at break, tensile strength, flexural strength and impact strength of EP/2DIT sample by 35.8%, 40.5%, 24.2% and 21.2%. Moreover, the glass transition temperature (Tg) of EP/2DIT sample was equal to that of EP sample, and the high transparent was maintained. Hence, this work offers an all-rounded design for the creation of durable flame-retardant EPs with superior mechanical properties, thermal resistance and transparency by introducing hyperbranched, P-P, bio-based flame retardants, which shows huge potential and broad prospect.

A P/Si-containing polyethylenimine curing agent towards transparent, durable fire-safe, mechanically-robust and tough epoxy resins

Epoxy resin (EP) features superior comprehensive performances, but inherent flammability seriously limits its industrial applications. Current flame-retardant strategies often bring about enhanced flame retardancy but deteriorated mechanical, thermal and optical properties and neglect flame retardant durability. Herein, a phosphorus/silicon-containing polyethylenimine (PES) is designed as a multifunctional curing agent for EP. Replacing polyethylenimine (PEI) with PES maintains high visible light transmittance of EPs and improves the UV-blocking properties. The EP cured by 30 wt% PES (EP/30PES) exhibits enhanced mechanical properties compared with the EP cured by 15 wt% PEI (EP/15PEI). EP/30PES achieves a limiting oxygen index (LOI) of 35.0% and a UL-94 V-0 rating, and its peak smoke release rate (PSPR) reduces by 63.8% relative to that of EP/15PEI. Hence, the great flame retardancy, smoke suppression and mechanical performances of EP/30PES enable it to outperform previous flame-retardant EP/aliphatic amine counterparts. Meanwhile, EP/30PES shows durable flame retardancy, which still achieves a UL-94 V-0 classification after acid/alkali/aging resistance tests. This work provides an advanced design of multifunctional curing agents to create durable fire-safe EPs with great UV-shielding and mechanical performances.

A phosphorus/silicon-based, hyperbranched polymer for high-performance, fire-safe, transparent epoxy resins

The implementation of current flame-retardant strategy is often accompanied by the deterioration of other properties, e.g., mechanical and optical properties. Herein, a flame-retardant, hyperbranched polymer was synthesized by using trichlorovinylsilane (TVS), triethylenetetramine (TETA) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) as raw materials, which was named as TTD and applied to epoxy resin (EP). Our result shows that TTD is able to catalyze and participate in the curing of EP. Furthermore, the EP-TTD samples exhibit high transparency, glass-transition temperature and good UV-blocking properties at a low TTD addition (≤ 4 wt%). Notably, TTD enhances the toughness and robustness of EP. The elongation at break, tensile strength, flexural strength, and impact strength of EP containing 4 wt% TTD are 54%, 42%, 29%, and 29% higher than those of pure EP. Meanwhile, the incorporation of 7 wt% TTD endows EP with a limiting oxygen index (LOI) of 40.0%, a UL-94 V-0 rating, and a 13% reduction in total smoke production (TSP), indicating the significantly enhanced fire safety. During burning, TTD exerts promoting carbonization and quenching effects to suppress the heat transfer and smoke release. Therefore, this work provides an effective strategy to fabricate high-performance, fire-safe epoxy resins using P/Si-containing, hyperbranched polymers.

Phosphorus-free hyperbranched polyborate flame retardant: Ultra-high strength and toughness, reduced fire hazards and unexpected transparency for epoxy resin

The design of high transparent epoxy resins (EPs) with excellent flame retardancy and mechanical strength is still intractable, while the current use of phosphorus-based flame retardant usually destroys the transparency and color of EPs. This work reports a phosphorus-free hyperbranched polyborate (HBPB) as a multifunctional flame retardant additive for EPs. The HBPB was synthesized via a facile, one-pot, solvent- and catalyst-free polycondensation with branched aliphatic backbone and borate group, which features good compatibility within resin matrix and does not affect its curing processability. Compared to neat EP, the as-constructed EP composite with 9% addition of HBPB displays the high fire resistance (30.2% LOI value and UL-94 V0 rating), and the fire hazards such as smoke, heat and toxic gases release are significantly reduced without introducing phosphorus group. Surprisingly, the EP material can well maintain its high transparency and color with ultra-high toughness (21.38–40.50 kJ m−2) and flexural strength (137.03–197.08 MPa) achieved, especially the glass-transition temperature can be improved from 150 °C to 170 °C, making it superior to previously described counterparts. Highlighting the compelling results, this work provides a promising candidate on seeking the halogen-free solutions to alleviate the high price of phosphorus-based FRs, and paves a useful strategy toward designing high transparent, high-performance polymeric materials.

Controlling Light in Scattering Materials for Volumetric Additive Manufacturing.

3D printing has revolutionized the manufacturing of volumetric components and structures in many areas. Several fully volumetric light‐based techniques have been recently developed thanks to the advent of photocurable resins, promising to reach unprecedented short print time (down to a few tens of seconds) while keeping a good resolution (around 100 μm). However, these new approaches only work with homogeneous and relatively transparent resins so that the light patterns used for photo‐polymerization are not scrambled along their propagation. Herein, a method that takes into account light scattering in the resin prior to computing projection patterns is proposed. Using a tomographic volumetric printer, it is experimentally demonstrated that implementation of this correction is critical when printing objects whose size exceeds the scattering mean free path. To show the broad applicability of the technique, functional objects of high print fidelity are fabricated in hard organic scattering acrylates and soft cell‐laden hydrogels (at 4 million cells mL−1). This opens up promising perspectives in printing inside turbid materials with particular interesting applications for bioprinting cell‐laden constructs.

Multifunctional flexible optical waveguide sensor: on the bioinspiration for ultrasensitive sensors development

This paper presents the development of a bioinspired multifunctional flexible optical sensor (BioMFOS) as an ultrasensitive tool for force (intensity and location) and orientation sensing. The sensor structure is bioinspired in orb webs, which are multifunctional devices for prey capturing and vibration transmission. The multifunctional feature of the structure is achieved by using transparent resins that present both mechanical and optical properties for structural integrity and strain/deflection transmission as well as the optical signal transmission properties with core/cladding configuration of a waveguide. In this case, photocurable and polydimethylsiloxane (PDMS) resins are used for the core and cladding, respectively. The optical transmission, tensile tests, and dynamic mechanical analysis are performed in the resins and show the possibility of light transmission at the visible wavelength range in conjunction with high flexibility and a dynamic range up to 150 Hz, suitable for wearable applications. The BioMFOS has small dimensions (around 2 cm) and lightweight (0.8 g), making it suitable for wearable application and clothing integration. Characterization tests are performed in the structure by means of applying forces at different locations of the structure. The results show an ultra-high sensitivity and resolution, where forces in the μN range can be detected and the location of the applied force can also be detected with a sub-millimeter spatial resolution. Then, the BioMFOS is tested on the orientation detection in 3D plane, where a correlation coefficient higher than 0.9 is obtained when compared with a gold-standard inertial measurement unit (IMU). Furthermore, the device also shows its capabilities on the movement analysis and classification in two protocols: finger position detection (with the BioMFOS positioned on the top of the hand) and trunk orientation assessment (with the sensor integrated on the clothing). In both cases, the sensor is able of classifying the movement, especially when analyzed in conjunction with preprocessing and clustering techniques. As another wearable application, the respiratory rate is successfully estimated with the BioMFOS integrated into the clothing. Thus, the proposed multifunctional device opens new avenues for novel bioinspired photonic devices and can be used in many applications of biomedical, biomechanics, and micro/nanotechnology.

3D Printing of Biocompatible Scaffolds for Eye Tissue Engineering

Eye tissue engineering is becoming a promising field in the study and treatment of vitreoretinal diseases, and novel hydrogels are under investigation for vitreous body replacement and 3D cell culture applications. In order to handle a not self-standing hydrogel and to provide it with eye shape, a rigid scaffold that is transparent, non-toxic, and easy to move is needed. To this purpose, we produced bowl-shaped samples by stereolithography (SLA) starting from different commercial transparent resins. We optimized the 3D printing protocol by tuning parameters such as exposure time, layer thickness, and light blocker content added to the resins. The specimens that showed the best dimensional accuracy were chosen to perform biocompatibility tests at different time-point and transparency tests under an optical microscope. Preliminary results showed that the cell culture medium conditioned by the first tested resin formulations caused a massive cell death in the MS5 cell line tested, whereas different formulations did not impact on cell viability. Furthermore, the as built resin samples did not allow cell observation under inverted microscope. It was demonstrated that post-processing progressive grinding remarkably improved cell visualization.