UV-cured Composites Research Articles

Solid Rocket Propellant Photo-Polymerization with an In-House LED-UV Prototype

Composite solid propellants have used cast molding production technology for many decades, with intrinsic limitations on production flexibility, promptness, and grain geometry, as well as environmental implications on toxicity and global carbon footprint. This traditional method involves the use of toxic chemicals, has a long processing time, requires high temperature, and the products have limited geometries. To overcome those issues, different photo-curable resins have been evaluated as possible matrices. In fact, the UV-curing process is fast and has low energy consumption. The photocuring reaction parameters of six different pristine formulations were evaluated by Fourier transform infrared spectroscopy analysis. After finding the optimal curing parameters, different composites were prepared by adding 75 or 80 wt% ammonium sulfate particles used as an inert replacement for the oxidant. The thermomechanical properties and thermal resistance of the UV-cured composites were characterized via dynamic thermal-mechanical and thermogravimetric analysis. Subsequently, the mechanical properties of the inert propellants were investigated by tensile tests. The most promising resin systems for the production of solid rocket propellants were then 3D printed by an in-house developed illumination system and the obtained object micro-structure was evaluated by X-ray computed tomography.

A novel one-step ultraviolet curing fabrication of myristic acid-resin shape-stabilized composite phase change material for low temperature thermal energy storage

In this work we present for the first time a simply, fast and efficient process for the fabrication of form-stable composite phase change material (PCM) by ultraviolet (UV)-curing technique. Simply by UV-curing of the PCM with a desired UV curable carrier for several minutes, a neoteric UV-cured composite that suitable for low temperature thermal energy storage (TES) can be fabricated. The feasibility of such a novel technique is demonstrated by investigating a case of myristic acid (MA) dispersed into a UV curable resin containing polyurethane acrylate (PUA). The results show that a crosslinked structure is formed in the composite after UV-curing due to the occurrence of photochemical reaction, which offer sufficient paths to accommodate the PCM, endowing the composite with the great ability to maintain the structure stabilization and prevent the liquid leakage. A good physical and chemical compatibility has been achieved in the composite and 40 wt% of UV-curing resin grants the composite optimum formula at which a melting temperature of 52 °C and an energy storage density over 210.8 kJ/kg at temperature ranged 25–100 °C are obtained. The results also reveal that the material mechanical strength can be linked to the UV-curing duration. For a given V-curing time of 5–30 mins, the composite mechanical strength is measured over 25–34 MPa, almost over twice as high as that of high-density polyethylene (HDPE) and styrene ethylene butylene styrene (SEBS) based composites. Owing to the high shape strength and excellent thermal cycling performance, such UV-curable composites can be utilized for modular design of TES devices, and also be used to precisely and directionally produce TES devices through additive manufacturing. This work exploits a new perspective for the energy-saving and time-effective fabrication of shape stabilized composite PCMs for low temperature thermal energy storage.

On the Influence of Flame-Retardant Additives on UV-Curable Thermosetting Glass Fiber-Reinforced Composites.

One of the main advantages of fiber-reinforced polymer (FRP) composites is the ability to reduce their weight while they exhibit exceptional properties such as high strength, stiffness, and resistance to corrosion, and reduction in their lifetime maintenance when they are compared to the metallic components. These features led fiber-reinforced polymer composites to have applications in the mechanical, construction, aerospace, automotive, medical, marine, and other manufacturing industries. However, the use of this type of material is not possible in all of these applications since, in certain sectors, the fire resistance property that the material must present is one of the key factors. For this reason, a thermosetting resin composed of ultraviolet (UV)-curable acrylic monomers has been used as a matrix, where transparent aluminum trihydrate (ATH) flame-retardant fillers were incorporated for manufacturing flame-retarded UV-curable composites. The composite parts were produced by using glass fiber-reinforced UV-curable prepregs. An exhaustive study of different types of ATH-based flame-retardant additives and the possible cooperation between them to improve the fire properties of the UV-curable composite was carried out. Additionally, the most suitable additive percentage to meet the railway sector requirements was also evaluated, as well as the evolution in the viscosity of the matrix and its processing capacity during the manufacture of the prepregs at 60 °C. The compatibility between the fillers and the matrix was assessed using a dielectric analysis (DEA). The fire properties of both the matrix and the final composite were established.

Direct-write 3D printing of UV-curable composites with continuous carbon fiber

Continuous fiber-reinforced polymer composites, with their superior combinations of high stiffness, high strength, lightweight, and corrosion resistance, have been leading contenders in various applications ranging from aerospace to ground transportation. The integration with 3D printing allows the composite products to be customized quickly and fabricated inexpensively to meet unique specifications. However, the material-process-property relationships of 3D printed thermoset composites with continuous fiber are insufficiently explored in the field. In this work, a cost-effective 3D printing method is developed based on the direct ink writing method to print UV-curable composites reinforced with continuous carbon fiber. During the printing, the fiber tow is impregnated with an acrylate ink inside the printer head and then extruded onto the printer bed, where it is cured upon UV irradiation. The influences of different material and processing variables, such as the resin viscosity, nozzle size, printing speed, and substrate temperature, on the quality and mechanical properties of composites, are investigated. The porosity, stiffness, and strength of printed symmetric laminate are also examined. Finally, the 3D printing of composite structures and the conformal coating of curved surfaces along a defined printing path are demonstrated. The findings reported in this work could contribute to the rapid design and manufacturing of composite components with unprecedented mechanical properties and emerging functionalities for the automotive, defense, and aerospace industries.

Effects of two silane surface modifications with different functional groups on the thermal conductivity and mechanical properties of UV-cured composites with high ceramic filler loading

With rapid technological advancements, efficient thermal management is becoming increasingly important to sustain the stable operation of electronic devices. In this study, aluminum nitride (AlN) fillers with various acrylate monomers were subjected to two types of silane surface treatments to prepare composites with a high loading of AlN filler (65 wt%). The acrylates—isobornyl acrylate (IBOA), 1,4-butanediol diacrylate (BDDA), and trimethylolpropane triacrylate (TMPTA)—were mixed with bisphenol A ethoxylate dimethacrylate (Bis-EMA) as an oligomer, and phenylbis (2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) as a photo-initiator in different proportions to obtain resin matrices. Pristine AlN and AlN functionalized with APTES and MPS were used as fillers. The effect of the acrylate functional group in silanes on the thermal and mechanical properties of the acrylate resin was explored. The thermal conductivities of the IBOA/AlN/APTES and IBOA/AlN/TMPTA composites with a high loading of the filler functionalized with APTES and MPS were 1.34 and 1.57 W/(m∙K), respectively, 4.15 and 5.28 times higher than that of the composite with neat resin. The enhanced filler–matrix compatibility increased the tensile strength of the composites. The findings highlighted that silane functionalization of AlN can enhance the thermal conductivity and mechanical properties of the composite.

Study of the influece of flame retardant additives on ultra-violet curing thermosetting composites

Fibre-reinforced polymer composites are materials capable of reducing weight while maintaining optimised properties in multiple applications. However, the use of composite materials is not suitable for all sectors, where the need for improved fire properties is highlighted. In the present study, a thermosetting matrix composed of UV-curable acrylic monomers and UV-transparent aluminium trihydrate (ATH) flame retardant fillers have been added to UV-transparent polyester resin for the manufacture of flame retardant composites. An in-depth study of the most suitable types of ATH-based flame retardant additives and possible synergistic combinations to improve the fire performance of the UV-curable composite has been carried out. Not only has the type of flame retardant been studied, but also the most suitable additive percentage to comply with railway sector regulations. The compatibility between the fillers and the matrix has been analysed by means of dielectric analysis, as well as the properties of the matrix and final composites against fire and mechanical properties.

Electrospun cellulose fiber-reinforced UV-curable composites with tunable properties

The goal of a carbon-neutral economy urges the search for unconventional eco-friendly methods of manufacturing conventional materials, including functional composites. This study combines the best practices of cellulose dissolution, fiber production through electrospinning, and composite preparation to develop a facile method for preparing lightweight fiber-reinforced composites. The challenge in solution electrospinning is that the solvent must be immediately removed. We demonstrated an electrospinning method using the nonvolatile cellulose solvent ionic liquid 1-butyl-3-methylimidazolium acetate (BMIMAc). Fiber fusion was effectively prevented through the rapid removal of the solvent. The overall process resulted in micro-sized fibers and a porous cellulose matrix, which was further impregnated to prepare polymer composites. The removal of ionic liquid (IL) and the grafting of Si for reducing the hydrophilicity were confirmed by solid-state nuclear magnetic resonance, Fourier-transform infrared, and water contact angle analyses. Finally, strong, tough fiber-reinforced composites were fabricated through impregnation using UV-curable acrylic resin. The composite samples containing 10 wt% of cellulose fibers had a 10% higher Young’s modulus, 50% greater tensile strength, and an almost 100% higher strain at break compared with neat acrylic resin. All in all it was shown that pure cellulose fiber can be readily produced in controlled environment and used for effective reinforcement of polymer matrix.

Synthesis and Characterization of Fiber-Reinforced Resorcinol Epoxy Acrylate Applied to Stereolithography 3D Printing

As perceived, fiber-reinforced photocuring resins possess various distinctive advantages over traditional reinforced resin systems. This includes a rapid curing rate, energy efficiency, no volatile organic compounds, high strength, good thermal stability, and chemical resistance. Printing 3D composite objects using vat polymerization techniques has been a center of interest where photocuring resins are applied. In our present study, we have synthesized resorcinol-based diglycidyl ether, that is, a resorcinol-based epoxy resin, and further acrylated to the resorcinol epoxy acrylate oligomer. This oligomer was further formulated to photocuring resins using a suitable quantity of reactive diluents and photoinitiators. Now, three types of synthetic fibers, that is, glass fibers, nylon fibers, and polyester fibers, were incorporated in this formulated resin at different loading percentages. The oligomer synthesized was analyzed for structural conformation using Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance. Further, we comparatively examined the rheological behavior of prepared formulations, and compatible formulations were applied to stereolithography 3D printers. Finally, physical, mechanical, thermal, transmittance, and morphological characteristics were comparatively analyzed for prepared UV-cured composites. The outcomes obtained during characterizations of UV-cured composites will be inevitably reflected in 3D-printed objects.

Coupling Monte Carlo Light Propagation Method and Curing Kinetic Equations to Model the Degree of Conversion Evolution of UV-Curable Composites

Coupling Monte Carlo Light Propagation Method and Curing Kinetic Equations to Model the Degree of Conversion Evolution of UV-Curable Composites

Evaluation of Reactive Diluent Impact on Stability of Systems Viscosity in UV-Curable Compositions

This study investigated the impact of various reactive diluents on viscosity of uncured mixture at 50 °C over a 12-week period. This criterion is important to predict the shelf life of uncured composition. Studied mono-functional acrylate monomers were: hydroxypropyl methacrylate (HPMA), hydroxyethyl methacrylate (HEMA), tetrahydrofurfuryl acrylate (THFA), isobornyl methacrylate (IBOMA), isobornyl acrylate (IBOA), 2-(2-Ethoxyethoxy)ethyl acrylate (EOEOEA), acryloyl morpholine (ACMO), glacial methacrylic acid (GMAA) and phosphate acrylate monomer (PAM); and various solvents: isopropanol(IP) ethyl acetate (EA), butyl acetate (BA). There was found the difference of effect on the systems viscosity in time between the mono-functional monomers and solvents. HPMA and IPA showed the most stable viscosity values in time.

3D printing of continuous fiber-reinforced thermoset composites

Abstract Current printing methods for continuous fiber-reinforced composites require the composites to quickly solidify during the printing, so the matrix is limited to thermoplastics or thermosets with fast curing mechanisms. This paper presents the first design of a 3D printer head for direct-ink-writing that can overcome the limitations on printable resins. It leverages the shear stress imposed on the fiber to enable the ready extrusion of composite filaments and is applicable to a wide variety of thermoset resins and commercially-available continuous fibers. The developed printing method can create thermally-curable composite components with covalently bonded interfaces and comparable mechanical strength as directly-molded samples. It also enables the free-standing 3D printing of UV-curable composites. Overall, the developed printing method allows the composite parts to be designed quickly to meet unique specifications, as well as provides new functions in 4D printing, biomedical printing, and the printing of functional devices.

Analysis of Mechanical Curing Properties Based on Vacuum Pressure of UV-Cured Composites

Analysis of Mechanical Curing Properties Based on Vacuum Pressure of UV-Cured Composites

Influence of an Alkoxylation Grade of Acrylates on Shrinkage of UV-Curable Compositions.

Commercially available UV curable restorative materials are composed of inorganic filler hydroxyapatite, multifunctional methacrylate, photoinitiator and alkoxylated acrylate. Especially, the application of alkoxylated monomers with different alkoxylation grade allows the reduction of polymerization shrinkage which plays the major role by application of low shrinkage composites as high quality restorative dental materials or other adhesive materials in the form of UV-polymerized self-adhesive acrylics layers (films). There are several ways to reduce polymerization shrinkage of restorative compositions, for example, by adjusting different alkoxylated acrylic monomers, which are integral part of investigated UV curable restorative composites. This article is focused on the studies of contraction-stress measured as shrinkage during UV-initiated curing of restorative composites containing various commercially available alkoxylated acrylates. Moreover, studies with experimental restorative materials and recent developments typical for UV curing technology using special photoreactive monomers are described.

Low dielectric loss and high breakdown strength photosensitive high-k composites containing perfluoroalkylsilane treated BaTiO3 nanoparticles

High-k, UV curable polymer composites with low dielectric loss and high breakdown strength were prepared. Fluorosilane coupling agent (FAS-17) was adopted to modify the surface of BaTiO3 (BT) nanoparticles. Characterization results of UV-cured composites filled with BT@FAS-17 particles indicate that the dielectric loss is dramatically decreased and the breakdown strength is significantly improved, especially for the highly-loaded composites. The underlying mechanism of these performance improvements is further studied by temperature-dependent dielectric spectra, thermally stimulated depolarization current experiments as well as the first-principle simulation, which suggest that H-bonding and additional interfacial traps introduced by FAS-17 account for the enhanced dielectric properties. This work proposes a facile and efficient technique for the fabrication of photosensitive high-k composites with low dielectric loss and high breakdown strength, which is suitable for a variety of advanced applications such as energy storage, functionally graded dielectric materials and 3D-printing fabrication of dielectrics.

Influence of the Fibre Content, Exposure Time, and Compaction Pressure on the Mechanical Properties of Ultraviolet-Cured Composites

A new process for the impregnation, consolidation, and curing of glass-fibre-reinforced polyester composites was developed to reduce manufacturing costs and secure end properties that compete with other traditional materials. This new process, based on the ultraviolet (UV) curing of prepregs, could be a viable alternative to infusion and other processes. In this paper, we showed that glass fibre composites 3 mm thick could be easily formed using suitable photoinitiating systems. We achieved improved mechanical properties through the application of favourable parameters to traditional manufacturing processes such as hand lay-up and infusion. The prepreg polymerization was monitored by dielectric analysis (DEA), and we evaluated the relationship between the UV radiation exposure time and curing degree. Both the exposure time and compaction pressure affected the fibre content of composites and interlaminar shear strength. Experimental results showed that compaction pressures higher than 4 bar are necessary to increase the mechanical properties of the UV-cured composites. Finally, the properties of the composites manufactured by this new process were compared to the properties of composites manufactured using traditional processes such as hand lay-up and infusion.

Preparation and Characterization of a Novel Terbium Complex Coordinated with 10-Undecenoic Acid for UV-Cured Coatings

A UV-cured composite containing a rare earth complex was prepared for this study. First, the photoluminescent terbium complex was synthesized with a long-chain unsaturated fatty acid (10-Undecenoic acid) by a solution precipitation method, resulting in the 10-UA-Tb(III) complex. Its structure was proven by FTIR, elemental analysis, XRD, and TGA. The results indicated that the organic acid ligand successfully coordinated with the Tb3+ ion and that the complex had a chelate bidentate structure. The emission spectrum of the 10-UA-Tb(III) complex indicated that the complex can emit a bright green light with the unique luminescence of the Tb3+ ion. Furthermore, the luminescence properties of complexes with different ratios of Tb3+ and ligand were studied, and the ratio of Tb3+ and the ligand had an obvious impact on the luminescence intensity of the 10-UA-Tb(III) complex. Subsequently, the prepared rare earth complex was doped into a UV-cured coating in different proportions to obtain a UV-cured composite. The morphology of the rare earth UV-cured composite was observed by SEM. The images showed that the rare earth complex was dispersed uniformly in the polymer matrix. Moreover, the composites could emit fluorescence. Additionally, it has good thermal stability and compatibility with the resin. Therefore, these composites should have potential applications in UV curable materials, such as luminescence coatings.

Down-conversion particles as internal UV-source assist in UV-curing systems: Physical and mechanical properties of UV-curable micro-composites

UV-curable composites always suffer from low conversion due to the low penetration depth of photons as a result of absorption of light by fillers. Herein, down-conversion particles were introduced as mineral materials within the composites to observe the alteration in final conversion along with some physical and mechanical properties. FT-IR, DMTA, micro-indentation, pendulum hardness, scratch resistance, and surface roughness analysis exhibited no reduction in the photopolymerization process in all the composites, and enhanced scratch resistance up to 5% filling. Storage modulus and cross-linked density were increased up to 5% loading, and enhanced micro-hardness up to 5% loading and decreased in 10% loaded composites. However, the surface hardness decreased because of increased surface roughness due to the addition of micron-sized fillers.

Preparation, Characterization and Application of UV-Curable Flexible Hyperbranched Polyurethane Acrylate.

A novel UV-curable hyperbranched polyurethane acrylate (FHBPUA) with excellent flexibility is successfully synthesized based on a reaction of hydroxyl terminated hyperbranched polyurethane (regarded as core) with flexible semiadduct urethane monoacrylate (regarded as arms). The structure and property of FHBPUA is firstly analyzed and then utilized as functional additives to ameliorate the UV-curing and mechanical properties of epoxy acrylate resin. The degree of branching of FHBPUA turns out to be 0.82. Its thermal decomposition process consists of three different stages, and the glass transition temperature is around 65 °C. The freestanding FHBPUA film (~30 μm thickness) can be UV-cured within 3 s, and its flexibility is up to 1 mm. With the increase of FHBPUA content to 10 wt %, the UV-curing time of UV1000 film decreases from 6 to 3 s, flexibility strikingly increases from 10 to 1 mm, and adhesive force also improves from 5 to 3 grades, meanwhile its glossiness is not influenced by FHBPUA. In addition, a certain amount of FHBPUA can improve the tensile strength and elongation at break of UV1000 film. This novel FHBPUA can be used not only to develop flexible UV-curable freestanding films but also as functional additives to perfect other UV-curable compositions like coatings, inks and 3D printed parts.

Photolatent base catalyzed Michael-addition and concomitant in situ graphene oxide reduction to obtain electrically and thermally conductive UV-cured composite

A photolatent base ((1,5-diazabicyclo [4.3.0]non-5-ene) DBN) is used for the first time as photocatalyst for the curing of a formulation, which undergoes crosslinking by Michael addition reaction between acetoacetate and acrylate groups together with the reduction of graphene oxide.The reduction degree of graphene oxide during in situ irradiation is monitored by XPS analysis. The effect of the graphene oxide concentration on the photopolymerization reaction is studied by RT-FTIR. The thermal properties of the composite are determined by DSC analyses.The crosslinked nanocomposite showed a decrease of one order of magnitude for the electrical resistance in the presence of 3 wt% of graphene oxide. In addition, the thermal conductivity is significantly increased by the in situ reduction of graphene oxide in the organic matrix catalyzed by the phototriggered base.

The Influence of Reactive Diluents on the Properties of UV Dual Cured Polyurethane-Modified Epoxy Monoacrylates Films

The UV-curable liquid compositions were developed with different oligomers (epoxy diacrylates, EDA; epoxy monoacrylates, EMA; or polyurethane-modified epoxy monoacrylates, PMEMA), reactive diluents and photoinitiators. Thin films were prepared by curing these mixtures using a high pressure mercury lamp. The change of absorption peaks of double bond and epoxy group were identified by Fourier Transform Infrared (FTIR) spectra during UV dual curing process. Gel content, mechanical, volume shrinkage and thermal characterizations of the UV-cured films were investigated. The gel content, youngs modulus and stress at break turned to maximum values as the degree of polymerization achieved the maximum value, except for the strain at break. The flexibility of UV dual cured films using PMEMA as oligomer was improved due to grafting of the soft chain segments (polyurethane). The volume shrinkage test indicated that the UV dual cured films exhibited lower internal stress and higher cohesive force than simple radical UV cured films. The thermogravimetric analysis of UV cured films revealed that UV dual cured films prepared from PMEMA showed excellent thermal stability.