Photocuring Kinetics Research Articles

High-performance naphthalene-based photocurable epoxy resin for advanced printed circuit boards coatings by optimizing fluorodiamine-mediator extension chemistry

Despite the great progress in alkali developing photocurable resins for electronic packaging, the continuous development of high-frequency communication technology still poses a great challenge to the high heat resistance, toughness, robustness, and dielectric properties of photocurable resins. In this paper, a simple and feasible method is proposed to prepare a series of naphthalene-based photocurable resins with excellent thermal/mechanical properties and performance by introducing four selected fluorodiamine molecules into 1,6-naphthalene diglycidyl ether (NDE), respectively, and employing maleic anhydride (MA) as the alkali developing functional component. Among them, the effects of MA on the photocuring kinetics and thermo-mechanical properties of the resins were systematically investigated. Most importantly, the high functionality molecules obtained by diamine chain extension effectively improved the properties of the photocured films. As a result, two of the cured films exhibited excellent toughness (3.00 ± 1.39 MJ cm−3 and 4.55 ± 1.16 MJ cm−3), excellent heat resistance (212 and 195 °C), low dielectric constants (2.84 and 3.25), and low dielectric losses (0.0024 and 0.0068). Finally, the fast alkali development capability possessed by these two resins was verified, demonstrating their potential for technical application in real packaging boards and providing an effective strategy for the design and manufacture of high-performance naphthalene-based inks.

"Dissolve-on-Demand" 3D Printed Materials: Polymerizable Eutectics for Generating High Modulus, Thermoresponsive and Photoswitchable Eutectogels.

Solvent-free photopolymerization of vinyl monomers to produce high modulus materials with applications in 3D printing and photoswitchable materials is demonstrated. Polymerizable eutectic (PE) mixtures are prepared by simply heating and stirring various molar ratios of N-isopropylacrylamide (NIPAM), acrylamide (AAm) and 2-hydroxyethyl methacrylate (HEMA). The structural and thermal properties of the resulting mixtures are evaluated by 1D and 2D NMR spectroscopy as well as differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). UV photocuring kinetics of the PE mixtures is evaluated via in situ photo-DSC and photorheology measurements. The PE mixtures cure rapidly and display storage moduli that are orders of magnitude greater than equivalent copolymers cured in an aqueous medium. The versatility of these PE systems is demonstrated through the addition of a photoswitchable spiropyran acrylate monomer, as well as applying the PE formulation as a stereolithography (SLA)-based 3D printing resin. Due to the hydrogen-bonding network in PE systems, 3D printing of the eutectic resin is possible in the absence of crosslinkers. The addition of a RAFT agent to reduce average polymer chain length enables 3D printing of materials which retain their shape and can be dissolved on demand in appropriate solvents.

Improving the Performance of Photoactive Terpene-Based Resin Formulations for Light-Based Additive Manufacturing.

Photocurable liquid formulations have been a key research focus for the preparation of mechanically robust and thermally stable networks. However, the development of renewable resins to replace petroleum-based commodities presents a great challenge in the field. From this perspective, we disclose the design of photoactive resins based on terpenes and itaconic acid, both potentially naturally sourced, to prepare photosets with adjustable thermomechanical properties. Biobased perillyl itaconate (PerIt) was synthesized from renewable perillyl alcohol and itaconic anhydride via a scalable solvent-free method. Photoirradiation of PerIt in the presence of a multiarm thiol and photoinitiator led to the formation of networks over a range of compositions. Addition of nonmodified terpenes (perillyl alcohol, linalool, or limonene) as reactive diluents allowed for more facile preparation of photocured networks. Photosets within a wide range of properties were accessed, and these could be adjusted by varying diluent type and thiol stoichiometry. The resins showed rapid photocuring kinetics and the ability to form either brittle or elastic materials, with Young's modulus and strain at break ranging from 3.6 to 358 MPa and 15 to 367%, respectively, depending on the chemical composition of the resin. Glass transition temperatures (Tg) were influenced by thioether content, with temperatures ranging from 5 to 43 °C, and all photosets displayed good thermal resistance with Td,5% > 190 °C. Selected formulations containing PerIt and limonene demonstrated suitability for additive manufacturing technologies and high-resolution objects were printed via digital light processing (DLP). Overall, this work presents a simple and straightforward route to prepare renewable resins for rapid prototyping applications.

Glycerol Acrylate-Based Photopolymers with Antimicrobial and Shape-Memory Properties.

In this paper, for the first time, photopolymers were synthesized from glycerol acrylates with different numbers of functional groups, 2-hydroxy-3-phenoxypropyl acrylate, glycerol dimethacrylate or glycerol trimethacrylate, without and with the addition of vanillin styrene. The photocuring kinetics were monitored by real-time photorheometry. The mechanical, rheological, thermal, antimicrobial and shape-memory properties of the photopolymers were investigated. All polymers synthesized demonstrated antibacterial activity against Escherichia coli and Staphylococcus aureus, as well as antifungal activity against Aspergillus flavus and Aspergillus niger. 2-Hydroxy-3-phenoxypropyl acrylate-based polymers showed thermoresponsive shape-memory behavior. They were able to maintain their temporary shape below the glass transition temperature and return to their permanent shape above the glass transition temperature. Synthesized photopolymers have potential to be used as sustainable polymers in a wide range of applications such as biomedicine, photonics, electronics, robotics, etc.

Photoactivity, conversion kinetics, nanoreinforcement, post-curing, and electric/dielectric properties of functional 3D printable photopolymer resin filled with bare and alumina-doped ZnO nanoparticles

The addition of nanoparticles (NPs) into a polymer matrix represents a simple yet efficient tool for introducing or enhancing functional properties. 3D printing is a processing technique with unprecedented control over the micro- and macroscopic structure. Their combination creates a versatile fabrication tool for both professional and hobby users. However, some less intuitive effects brought about by NPs must also be considered. This study has thoroughly investigated the addition of bare and alumina-doped zinc oxide NPs into a representative free-radical photo-polymerization resin. The alumina doping suppressed the characteristic surface plasmon resonance of ZnO NPs. This absorption band was found crucial for the NP photoactivity and, subsequently, the conversion kinetics, post-curing, thermomechanical, and electric/dielectric properties of the nano-filled resins. Furthermore, three experimental techniques for assessing the photocuring kinetics – real-time FTIR, photo-DSC, and Jacobs working curves, were compared and resolved. Each yielded a completely different conclusion due to the photothermal effect and nanoreinforcement. The presented data shall aid with designing and testing of functional photopolymer nanocomposites for 3D printing and other low-intensity curing applications.

Photo-Curing Kinetics of 3D-Printing Photo-Inks Based on Urethane-Acrylates.

In this study, photo-curing kinetics for urethane-acrylate-based photo-inks for 3D printing were evaluated using a photo-differential scanning calorimetry analysis. Initially, the photopolymerization kinetics of di- and monofunctional monomers were separately studied at different temperatures (5–85 °C). Later, the photo-curing kinetics and mechanical properties of photo-inks based on different monomer mixtures (40/60–20/80) were evaluated. The results showed that urethane-dimethacrylate (UrDMA) and urethane-acrylate (UrA) had no light absorption in the region of 280–700 nm, making them a proper crosslinker and a reactive diluent, respectively, for the formulation of 3D-printing photo-inks. The kinetics investigations showed a temperature dependency for the photo-curing of UrDMA, where a higher photopolymerization rate (Rp,max: from 5.25 × 10−2 to 8.42 × 10−2 1/s) and double-bound conversion (DBCtotal: from 63.8% to 92.2%) were observed at elevated temperatures (5–85 °C), while the photo-curing of UrA was independent of the temperature (25–85 °C). Enhancing the UrA content from 60% to 80% in the UrDMA/UrA mixtures initially increased and later decreased the photopolymerization rate and conversion, where the mixtures of 30/70 and 25/75 presented the highest values. Meanwhile, increasing the UrA content led to lower glass transition temperatures (Tg) and mechanical strength for the photo-cured samples, where the mixture of 30/70 presented the highest maximum elongation (εmax: 73%).

Curing kinetics of dually-processed acrylate-epoxy 3D printing resins

In this work, the curing kinetics of a new ternary acrylate/epoxy/anhydride thermoset system which is used for dual-curing 3D-printing applications is studied. The first curing stage is an acrylate free-radical photopolymerization that can also be triggered using a suitable thermal radical initiator. The second curing stage is an epoxy-anhydride copolymerization initiated at higher temperatures by a nucleophilic tertiary amine. Isothermal acrylate photocure kinetics was modeled using a first order rate expression. The thermal curing of both reaction stages was studied under by non-isothermal integral isoconversional and model fitting methods. Furthermore, they were successfully reproduced using a simulation based on the obtained kinetic parameters. Reduced master curves and isokinetic relationships were used to validate the kinetic model selected and the reliability of the employed methodology. The conditions for obtaining a thermal and sequential dual curing can be established with the help of isokinetic temperature.

Influence of Vanillin Acrylate-Based Resin Composition on Resin Photocuring Kinetics and Antimicrobial Properties of the Resulting Polymers.

The investigation of the influence of vanillin acrylate-based resin composition on photocuring kinetics and antimicrobial properties of the resulting polymers was performed in order to find efficient photocurable systems for optical 3D printing of bio-based polymers with tunable rigidity, as well as with antibacterial and antifungal activity. Two vanillin derivatives, vanillin diacrylate and vanillin dimethacrylate, were tested in photocurable systems using phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator. The influence of vanillin acrylate monomer, amount of photoinitiator, presence and amount of dithiol, and presence of solvent on photocuring kinetics was investigated by real-time photoreometry. Polymers of different rigidity were obtained by changing the photocurable resin composition. The photocuring kinetics of the selected vanillin acrylate-based resins was comparable with that of commercial petroleum-based acrylate resins for optical 3D printing. Polymers based on both vanillin acrylates showed a significant antibacterial activity against Escherichia coli and Staphylococcus aureus. Vanillin diacrylate-based polymer films also demonstrated an antifungal activity in direct contact with Aspergillus niger and Aspergillus terreus. Vanillin diacrylate-based dual curing systems were selected as the most promising for optical 3D printing of bio-based polymers with antibacterial and antifungal activity.

Curing kinetics of acrylate-based and 3D printable IPNs

In this work, the kinetics of curing of a new family of interpenetrating polymer networks (IPNs) obtained by co-formulation of a photocurable acrylic resin with a thermocurable epoxy/anhydride mixture is studied. The first curing stage is an acrylate free-radical photopolymerization at ambient temperature, and the second curing stage is an epoxy-anhydride copolymerization initiated with a nucleophilic tertiary amine at higher temperatures. The presence of a diperoxyketal thermal radical initiator added to the liquid formulation facilitates uniform and complete acrylate conversion during thermal second curing stage and opens the possibility of curing the same materials by thermal curing only. The thermal curing kinetics was studied by integral isoconversional and model fitting procedures, whereas photocuring kinetics was satisfactorily fitted to a first order expression. Thermal curing and storage stability at room temperature were successfully simulated by using the isoconversional kinetic parameters. These ternary formulations have potential as 3D printable thermosets.

Photopolymerization of Acrylated Epoxidized Soybean Oil: A Photocalorimetry-Based Kinetic Study.

Photocure kinetics of acrylated epoxidized soybean oil (AESO) was studied via photocalorimetry without adding any diluent/comonomer, in the presence of two different photoinitiators, namely, 2,2-dimethoxy phenylacetophenone and 1-hydroxycyclohexyl phenyl ketone. The effect of varying photoinitiator concentration, light intensity, and temperature on the extent of crosslinking was calculated from the ratio of experimentally measured reaction enthalpy to the theoretical enthalpy of reaction (ΔHtheoretical). Photocuring of AESO was observed to be a second-order reaction exhibiting autocatalytic behavior. Nevertheless, due to the occurrence of vitrification, incomplete crosslinking (α ≠ 1) was observed in most curing conditions. Rate constants and activation energies were determined using both nonlinear model-fitting and model-free isoconversional methods. Activation energy, as determined from the model-free isoconversional method, was observed to increase as the reaction proceeded, indicating the shift in cure mechanism from kinetic-controlled to diffusion-controlled. Finally, the reaction termination mechanism was observed to be a combination of second-order and primary radical termination mechanisms.

Boehmite Nanofillers in Epoxy Oligosiloxane Resins: Influencing the Curing Process by Complex Physical and Chemical Interactions

In this work, a novel boehmite (BA)-embedded organic/inorganic nanocomposite coating based on cycloaliphatic epoxy oligosiloxane (CEOS) resin was fabricated applying UV-induced cationic polymerization. The main changes of the material behavior caused by the nanofiller were investigated with regard to its photocuring kinetics, thermal stability, and glass transition. The role of the particle surface was of particular interest, thus, unmodified nanoparticles (HP14) and particles modified with p-toluenesulfonic acid (OS1) were incorporated into a CEOS matrix in the concentration range of 1–10 wt.%. Resulting nanocomposites exhibited improved thermal properties, with the glass transition temperature (Tg) being shifted from 30 °C for unfilled CEOS to 54 °C (2 wt.% HP14) and 73 °C (2 wt.% OS1) for filled CEOS. Additionally, TGA analysis showed increased thermal stability of samples filled with nanoparticles. An attractive interaction between boehmite and CEOS matrix influenced the curing. Real-time infrared spectroscopy (RT-IR) experiments demonstrated that the epoxide conversion rate of nanocomposites was slightly increased compared to neat resin. The beneficial role of the BA can be explained by the participation of hydroxyl groups at the particle surface in photopolymerization processes and by the complementary contribution of p-toluenesulfonic acid surface modifier and water molecules introduced into the system with nanoparticles.

Photoinitiator‐free photopolymerization of acrylate‐bismaleimide mixtures and their application for inkjet printing

ABSTRACTThere is growing interest in using photoinitiator‐free systems in coating applications such as inkjet‐printing, because residual photoinitiator can alter the properties of the resulting polymer. Bismaleimides (BMI) offer the opportunity to polymerize acrylates without the addition of photoinitiators, as this class of molecules can serve both as polymerizable monomer and as photoinitiator together with electron donor systems, like vinyl ether monomers or acrylates. The UV‐induced copolymerization of a low‐molecular‐weight BMI with various acrylate monomers and oligomers without any photoinitiator was characterized. The BMI‐acrylate systems show comparable polymerization speeds to widely used acrylic systems with photoinitiator. Superior thermal stability as well as thermomechanical properties are achieved by enhancing acrylics with BMI. Such photoinitiator‐free systems lend themselves to be used for low‐migration coatings as well as for high‐temperature applications. Here, a characterization of selected BMI‐acrylate mixtures regarding their photocuring kinetics and their application as inks for inkjet printing is shown. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47789.

Dental restorative composites containing methacrylic spiroorthocarbonate monomers as antishrinking matrixes

ABSTRACTIn this work, the synthesis of two new antishrinking methacrylic monomers of spiroorthocarbonate type, SOC‐IP‐UDMA and SOC‐UDMA, is discussed, along with studies of their performance as polymeric matrix in dental composites. The monomers were photoactive with components of a conventional resin composite. In order to study the photocuring kinetics of these SOC monomers, a real‐time test was performed using FTIR spectroscopy. It was found that SOCs promoted higher double bond conversions (DC) than conventional methacrylic monomer bis‐GMA (48%), reaching a DC value of 68% for SOC‐UDMA and 75% for SOC‐IP‐UDMA. The shrinkage stress that accompanies curing of dental composites was measured by a universal testing machine. These new monomer SOCs are capable of reducing the shrinkage up to 51.1% and 27.7% for both methacrylic monomers when they were compared with bis‐GMA as control. Also, a dynamical mechanical analysis was conducted on the dental composites, obtaining a higher modulus and a range of T g values from 80 to 100 °C. Others parameters like flexural properties, solubility, and sorption water were determined, obtaining values better than or equal to the parameters established by the standard ISO 4049. These monomers can be considered as alternative matrixes to replace bis‐GMA in dental composites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47114.

Photopolymerized Triazole-Based Glassy Polymer Networks with Superior Tensile Toughness.

Photopolymerization is a ubiquitous, indispensable technique widely applied in applications from coatings, inks, and adhesives to thermosetting restorative materials for medical implants, and the fabrication of complex macro-scale, microscale, and nanoscale 3D architectures via additive manufacturing. However, due to the brittleness inherent in the dominant acrylate-based photopolymerized networks, a significant need exists for higher performance resin/oligomer formulations to create tough, defect-free, mechanically ductile, thermally and chemically resistant, high modulus network polymers with rapid photocuring kinetics. This study presents densely cross-linked triazole-based glassy photopolymers capable of achieving preeminent toughness of ≈70 MJ m-3 and 200% strain at ambient temperature, comparable to conventional tough thermoplastics. Formed either via photoinitiated copper(I)-catalyzed cycloaddition of monomers containing azide and alkyne groups (CuAAC) or via photoinitiated thiol-ene reactions from monomers containing triazole rings, these triazole-containing thermosets completely recover their original dimensions and mechanical behavior after repeated deformations of 50% strain in the glassy state over multiple thermal recovery-strain cycles.

Biobased isosorbide methacrylate monomer as an alternative to bisphenol A glycerolate dimethacrylate for dental restorative applications

ABSTRACTIn this study, a new biobased isosorbide urethane methacrylic monomer [isosorbide‐derived urethane dimethacrylate (Is‐UDMA)] was evaluated as a replacement for currently used bisphenol A glycerolate dimethacrylate (Bis‐GMA) based dental restorative materials. Dental composites were prepared at different Is‐UDMA and Bis‐GMA concentrations. For these composites, the photocuring kinetics, volumetric shrinkage, viscoelastic properties, water sorption, and solubility were evaluated. The photocuring kinetics, followed by real‐time IR spectroscopy, showed higher double‐bond conversion (DC) values for the formulations containing the Is‐UDMA monomer; the highest DC (82%) was achieved by the formulation prepared with only the Is‐UDMA monomer. The volumetric shrinkage was reduced to 23.7% as compared with the dental resin formulated with Bis‐GMA. The viscoelastic properties of the formulations containing both Is‐UDMA and Bis‐GMA monomers in a 50:50 composition were superior to the rest of the tested formulations, including those prepared with pure polymers. This behavior was explained in terms of a compromise between crosslinking and rigidity (or flexibility) of the resulting polymer network. A preliminary test on microleakage in a dental enamel demonstrated that the new Is‐UDMA monomer is a potential replacement for the Bis‐GMA monomer in dental restorative materials. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44591.

UV-Curable bismaleimides part I: Synthesis and photo-cure kinetics

Abstract A new series of hydroxyl-substituted bismaleimide (BMI) oligomers were successfully prepared based on 2,2′-Bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA), 2,2-Bis(amino-4-hydroxyphenyl)hexafluoropropane (BIS-AP-AF), and different diamines. To develop BMI oligomers with improved UV-curing efficiency, the methacryloyl groups were incorporated into the oligomers using. The hydroxyl-substituted BMIs as well as the methacrylated BMIs oligomers were characterized by 1 HNMR, 13 C NMR, Fourier transform infrared (FT-IR), mass spectroscopy, and gel permeation chromatography (GPC). The photo-cure kinetics of the methacrylated BMI oligomers was investigated by real-time FT-IR, photo-DSC, and photo-rheometry. Although the presence of oxygen (O 2 ) delayed the polymerization of methacryloyl moieties, it did not affect the polymerization of the maleimide groups. In general, the overall conversion of these systems was reduced in the O 2 atmosphere compared to the nitrogen (N 2 ) atmosphere. In addition, the maleimide groups exhibited higher conversions than the methacrylic groups in both O 2 and N 2 atmospheres .

Surface-dependent effect of functional silica fillers on photocuring kinetics of hydrogel materials

Two types of silica: precipitated (P, prepared in non-polar media, a new type, submicrometer sized) and fumed (F, nanosized), both unmodified and surface modified are investigated as functional fillers for potential applications in nanocomposites with poly(2-hydroxyethyl methacrylate) matrix. Special attention is paid to the kinetics of composite formation in an in situ photopolymerization process. Silica-containing formulations polymerize faster; this effect is much stronger for silica P having much larger particle size than silica F. Surface treatment leads to further acceleration of the polymerization in case of silica P but to retardation in case of silica F; the effect of modification of the filler surface on properties of composites is different for each of the silicas. The obtained results are discussed in terms of effects of curvature of silica particles, surface properties, solvation cell, interphase region, viscosity changes, and morphology of the resulting composites. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3472–3487

UV-cured methacrylic-silica hybrids: Effect of oxygen inhibition on photo-curing kinetics

The kinetic behavior of innovative photopolymerizable UV-cured methacrylic–silica hybrid formulations, previously developed, was studied and compared to that of a reference control system. The organic–inorganic (O–I) hybrids proposed in this study are obtained from organic precursors with a high siloxane content mixed with tetraethoxysilane (TEOS) in such a way to produce co-continuous silica nano-domains dispersed within a cross-linked organic phase, as a result of the hydrolysis and condensation reactions. The kinetics of the radical photopolymerization mechanism induced by UV-radiations, in presence of a suitable photoinitiator, was studied by calorimetric, FTIR and Raman spectroscopic analyses, by varying the composition of the mixtures and the atmosphere for reactions. The well known effect of oxygen on the kinetic mechanism of the free radical photopolymerization of the methacrylic–siloxane based monomers was found to be strongly reduced in the hybrid system, especially when a proper thiol was used. The experimental calorimetric data were fitted using a simple kinetic model for radical photopolymerization reactions, obtaining a good agreement between the experimental data and the theoretical model. From the comparison of the kinetic constants calculated for control and hybrid systems, it was possible to assess the effect of the composition, as well as of the atmosphere used during the photo-polymerization process, on the kinetic of photopolymerization reaction.

Particle clustering in photocurable nanocomposites: Dependence of curing kinetics and viscoelastic properties

ABSTRACTThe aim of this article is to investigate the effect of nanoparticle clustering on the mobility of nanoparticles in nanocomposites, using spectroscopic methods (Brillouin and Raman). Special attention is paid to the effect of particle clustering on photocuring kinetics. The model system was poly(2‐hydroxyethyl acrylate) filled with fumed nanosilica in concentration range encompassing the percolation threshold. Results obtained from Brillouin spectroscopy show substantial changes in the sound velocity and the attenuation coefficient with increasing filler content. The damping of acoustic waves reaches the maximum at the percolation threshold (∼15 wt %), which is related to changes in the mechanism of acoustic wave propagation. The formation of the cocontinuous silica phase strongly affects the curing kinetics of the monomer/silica system: the polymerization rate is the highest at a silica content corresponding to the percolation threshold. These results correlate well with the results of AFM surface roughness analysis. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39895.

Photocure kinetics and control of LC microdroplets for acrylate-based polymer dispersed liquid crystal with different methacrylate contents

This paper reports on the relationship between the photocure kinetics and the morphological properties of UV-curable acrylate-based polymer dispersed liquid crystal (PDLC) systems with the various 2-ethylhexylacrylate (2-EHA) and 2-ethylhexylmethacrylate (2-EHMA) contents using photo-differential scanning calorimetry (photo-DSC), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). Photo-DSC analysis using an autocatalytic kinetics model demonstrated that the cure extent and cure rate gradually decreased and then leveled off as the concentration of 2-EHMA increased up to 25 wt%, which was well confirmed by using FTIR spectroscopy. The morphological observations revealed that increasing the 2-EHMA content increased the size of the LC droplets within the acrylate-based polymer matrix. Notably, there was a marked increase in the number of LC droplets larger than 10 μm in PDLC composites containing more than 20 wt% 2-EHMA, suggesting that the addition of more than 20 wt% 2-EHMA to the acrylate-based PDLC compounds is not desirable. The presence of 2-EHMA clearly affected the photocure behaviors of acrylate-based PDLC compounds, which interestingly means that the slow reactivity of methacrylate monomer could be simply used to control the phase separation rate and the microstructures of LC droplets of acrylate-based PDLC systems. Open image in new window