Latent curing of epoxy-thiol thermosets

Epoxy-thiol curing is a click reaction which allows quantitative yield of the end products. The base-catalyzed reaction is rapid at low temperatures so it is most often desirable to harness reactivity by using latent catalysts. In this work, we used triazabicyclodecene tetraphenylborate (TBD·HBPh4) as a photobase generator (PB). We activated the PB either thermally or by UV light and monitored reaction kinetics by DSC and FTIR methods. Depending on the catalytic system used, the rate of the thiol-epoxy reaction was ordered as follows: Neat base > UV activated PB > thermally activated PB > uncatalyzed system. A series of isothermal and non-isothermal DSC experiments were run on non-irradiated and irradiated samples in order to study the effect of PB content and UV irradiation duration on PB activation efficiency and latency/storage stability. The data from DSC were analyzed using model-free linear isoconversional methods to estimate kinetic parameters such as activation energies. In addition, the kinetics data for both activation methods were shown to be accurately represented by multi-term Kamal models. The storage stability of the systems were studied at room temperature and was shown to fit well to the predictions of the kinetic model.Postprint (author's final draft

Sequential curing of amine-acrylate-methacrylate mixtures based on selective aza-Michael addition followed by radical photopolymerization

Dual curing systems find various uses in industry with the process flexibility they provide which allows tailoring properties at different curing stages in accordance with application requirements. A safe and efficient dual curing scheme is proposed here for a set of mixtures containing different proportions of acrylates and methacrylates. The first curing stage is a stoichiometric aza-Michael addition between acrylates and an amine, followed by photo-initiated radical homopolymerization of methacrylates and remaining acrylates. An analysis of aza-Michael reaction kinetics confirmed that amines react selectively with acrylates, leaving methacrylates unreacted after the first curing stage. It was found that acrylate-rich mixtures achieve complete global conversion at the end of the scheme. However, the highest crosslinking density and thermal resistance was observed in a methacrylate-rich formulation. The resulting materials show a wide range of viscoelastic properties at both curing stages that can be tailored to a variety of industrial application needs.Postprint (author's final draft

Acetoacetate based thermosets prepared by dual-Michael addition reactions

A novel set of dual-curable multiacetoacetate-multiacrylate-divinyl sulfone ternary materials with versatile and manipulable properties are presented. In contrast to common dual-curing systems, the first stage polymer herein consists of a densely crosslinked, high Tg network as a result of base-catalyzed multiacetoacetate-divinyl sulfone Michael addition. A more flexible secondary network forms after base-catalyzed Michael addition of remaining multiacetoacetate to multiacrylate. Curing is truly sequential as the rates of the two Michael additions are significantly different. Curing kinetics were analyzed using differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR). The materials at each curing stage were characterized using dynamic mechanical analysis (DMA) and SEM. Although some phase separation was observed in certain formulations, the incompatibilities were minimized when the molar percentage of the acetoacetate-divinyl sulfone polymer network was above 75%. Furthermore, the environmental scanning electron microscopy (ESEM) images of these materials show that the more flexible acetoacetate-acrylate phase is dispersed in the form of polymeric spheres within the rigid acetoacetate-divinyl sulfone matrix. This unique dual microstructure can potentially render these materials highly resilient in applications requiring densely crosslinked polymer architectures with enhanced toughnesPostprint (published version

New understanding of the shape-memory response in thiol-epoxy click systems: towards controlling the recovery process

Our research group has recently found excellent shape-memory response in “thiol-epoxy” thermosets obtained with click-chemistry. In this study, we use their well-designed, homogeneous and tailorable network structures to investigate parameters for better control of the shape-recovery process. We present a new methodology to analyse the shape-recovery process, enabling easy and efficient comparison of shape-memory experiments on the programming conditions. Shape-memory experiments at different programming conditions have been carried out to that end. Additionally, the programming process has been extensively analysed in uniaxial tensile experiments at different shape-memory testing temperatures. The results showed that the shape-memory response for a specific operational design can be optimized by choosing the correct programming conditions and accurately designing the network structure. When programming at a high temperature (T » Tg), under high network mobility conditions, high shape-recovery ratios and homogeneous shape-recovery processes are obtained for the network structure and the programmed strain level (eD). However, considerably lower stress and strain levels can be achieved. Meanwhile, when programming at temperatures lower than Tg, considerably higher stress and strain levels are attained but under low network mobility conditions. The shape-recovery process heavily depends on both the network structure and eD. Network relaxation occurs during the loading stage, resulting in a noticeable decrease in the shape-recovery rate as eD increases. Moreover, at a certain level of strain, permanent and non-recoverable deformations may occur, impeding the completion and modifying the whole path of the shape-recovery process.Postprint (author's final draft

Sequential heat release: an innovative approach for the control of curing profiles during composite processing based on dual-curing systems

The sequential heat release (SHR) taking place in dual-curing systems can facilitate thermal management and control of conversion and temperature gradients during processing of thick composite parts, hence reducing the appearance of internal stresses that compromise the quality of processed parts. This concept is demonstrated in this work by means of numerical simulation of conversion and temperature profiles during processing of an off-stoichiometric thiol–epoxy dual-curable system. The simulated processing scenario is the curing stage during resin transfer moulding processing (i.e. after injection or infusion), assuming one-dimensional heat transfer across the thickness of the composite part. The kinetics of both polymerization stages of the dual-curing system and thermophysical properties needed for the simulations have been determined using thermal analysis techniques and suitable phenomenological models. The simulations show that SHR makes it possible to reach a stable and uniform intermediate material after completion of the first polymerization process, and enables a better control of the subsequent crosslinking taking place during the second polymerization process due to the lower remaining exothermicity. A simple optimization of curing cycles for composite parts of different thickness has been performed on the basis of quality–time criteria, producing results that are very close to the Pareto-optimal front obtained by genetic algorithm optimization procedures.Postprint (author's final draft

Curat tèrmic i fotocurat de resines epoxi per al control químic de la contracció i millora de la degradabilitat

S’ha estudiat la copolimerització de resines epoxi amb una spirobislactona com a comonòmer per tal de controlar la contracció química durant el curat i millorar la seva degradabilitat per tal d’afavorir la seva recuperació total o parcial i la reutilització de materials un cop finalitzada vida útil. S’han fet servir dues tecnologies de curat, curat tèrmic i fotocurat amb radiació UV. S’han comparat diferents vies catalítiques, catiònica i aniònica. Es fa èmfasi en aspectes claus del processat com ara la cinètica de curat, la gelificació i la vitrificació, en la caracterització de les propietats tèrmiques i mecàniques dels materials curats i en la influència del procés químic en la cinètica i en les propietats. L’estudi s’ha dut a terme mitjançant tot un seguit de tècniques d’anàlisi tèrmic (DSC, DMTA, TMA, TGA), espectroscòpic (FTIR) i no instrumental. La diferent química dels processos de curat catiònic i aniònic, així com el tipus d’iniciador, condicionen la cinètica del procés de curat, el desenvolupament del reticle i per tant la morfologia i les propietats dels materials obtinguts. L’efecte de la spirobislactona en la cinètica de curat és complex i depèn de la via catalítica, de l’iniciador i la tecnologia de curat escollits. Aniònicament, la obertura de la spirobislactona fa disminuir la velocitat de reacció, mentre que catiònicament la velocitat pot augmentar o disminuir en funció de la interacció entre l’iniciador i les diferents espècies. La introducció de la spirobislactona disminueix la contracció química durant el curat, tant per als sistemes tèrmics aniònics com per als tèrmics catiònics, degut a l’obertura expansiva directa o indirecta, després de la formació d’un espiroortoèster intermedi, de la spirobislactona. Els copolímers epoxi-spirobislactona ténen una densitat d’entrecreuament més baixa i són més flexibles i tenaços degut a la formació d’estructures lineals flexibles que actuen d’espaiadors entre les unitats de monòmer epoxi, responsables de l’entrecreuament. Les condicions de curat tèrmic i fotocurat poden influir en el desenvolupament del reticle i per tant en les propietats del material completament curat. L’estabilitat tèrmica dels materials completament curats disminueix amb la incorporació de la spirobislactona degut a la introducció d’enllaços èster al reticle. D’aquesta manera s’obre una via de cara a la recuperació parcial de materials per degradació tèrmica controlada. La possibilitat d’hidrolitzar els enllaços èster obre una segona via de recuperació parcial o total dels materials que permet a més obtenir precursors reutilitzables dels monòmers de partida. La tecnologia de fotocurat permet processar els materials de manera més ràpida i controlada i a temperatures més baixes que el curat tèrmic. Per tant, es pot optimitzar en termes de temps i energia l’obtenció de materials amb propietats equivalents.The polymerization of epoxy resins with a spirobislactone as a comonomer has been studied with the purpose of controlling the chemical shrinkage during curing and improving their degradability to permit partial o total recovery and reuse of the materials once their service life is over. Thermal curing as well as photocuring using UV radiation have been used. The curing of these systems has been studied under cationic and anionic catalysis. The study focuses on key aspects of curing and processing such as kinetics, gelation and vitrification, analysis of thermal and mechanical properties of the cured materials and the influence of the chemical processes on kinetics and properties. Thermal analysis (DSC, DMTA, DSC, TGA), spectroscopic (FTIR) and non-instrumental techniques have been employed. The kinetics of the curing process, the network development and therefore the morphology and properties of the obtained materials are highly dependent on the different chemical processes taking place under cationic or anionic catalysis. The effect of the spirobislactone on the kinetics of the curing process is complex and depends on the catalysis, the type of initiator and the curing technique. In anionic curing the opening of the spirobislactone reduces the reaction rate, whereas in cationic curing the reaction rate can either increase or decrease depending on the interaction between the initiator and the reagents. Chemical shrinkage undergone during curing is reduced with the use of the spirobislactone, for both thermal cationic and thermal anionic systems, due to the expanding effect of the direct opening of the spirobislactone or indirectly via formation of an expandable spiroortoester. Epoxy-spirobislactone copolymers have a lower cross-linking density and are more flexible and tougher due to the formation of linear flexible structures that act as spacers between cross-linkable epoxy monomer units. Curing schedule can have an influence on the network development and therefore on the properties of the fully cured materials. The thermal stability of the fully cured materials decreases as the spirobislactone is incorporated due to the introduction of ester linkages into the network. Thus, it is possible to partially recover the materials by means of controlled thermal degradation. Hydrolysis of ester groups can also lead to total or partial recovery of reusable monomer precursors. Photocuring technology permits faster and controlled processing of materials at lower temperatures than thermal curing, therefore making it possible to reduce time and energy consumption during processing and obtention of materials with similar properties

Curat tèrmic i fotocurat de resines epoxi per al control químic de la contracció i millora de la degradabilitat

S’ha estudiat la copolimerització de resines epoxi amb una spirobislactona com a comonòmer per tal de controlar la contracció química durant el curat i millorar la seva degradabilitat per tal d’afavorir la seva recuperació total o parcial i la reutilització de materials un cop finalitzada vida útil. S’han fet servir dues tecnologies de curat, curat tèrmic i fotocurat amb radiació UV. S’han comparat diferents vies catalítiques, catiònica i aniònica. Es fa èmfasi en aspectes claus del processat com ara la cinètica de curat, la gelificació i la vitrificació, en la caracterització de les propietats tèrmiques i mecàniques dels materials curats i en la influència del procés químic en la cinètica i en les propietats. L’estudi s’ha dut a terme mitjançant tot un seguit de tècniques d’anàlisi tèrmic (DSC, DMTA, TMA, TGA), espectroscòpic (FTIR) i no instrumental. La diferent química dels processos de curat catiònic i aniònic, així com el tipus d’iniciador, condicionen la cinètica del procés de curat, el desenvolupament del reticle i per tant la morfologia i les propietats dels materials obtinguts. L’efecte de la spirobislactona en la cinètica de curat és complex i depèn de la via catalítica, de l’iniciador i la tecnologia de curat escollits. Aniònicament, la obertura de la spirobislactona fa disminuir la velocitat de reacció, mentre que catiònicament la velocitat pot augmentar o disminuir en funció de la interacció entre l’iniciador i les diferents espècies. La introducció de la spirobislactona disminueix la contracció química durant el curat, tant per als sistemes tèrmics aniònics com per als tèrmics catiònics, degut a l’obertura expansiva directa o indirecta, després de la formació d’un espiroortoèster intermedi, de la spirobislactona. Els copolímers epoxi-spirobislactona ténen una densitat d’entrecreuament més baixa i són més flexibles i tenaços degut a la formació d’estructures lineals flexibles que actuen d’espaiadors entre les unitats de monòmer epoxi, responsables de l’entrecreuament. Les condicions de curat tèrmic i fotocurat poden influir en el desenvolupament del reticle i per tant en les propietats del material completament curat. L’estabilitat tèrmica dels materials completament curats disminueix amb la incorporació de la spirobislactona degut a la introducció d’enllaços èster al reticle. D’aquesta manera s’obre una via de cara a la recuperació parcial de materials per degradació tèrmica controlada. La possibilitat d’hidrolitzar els enllaços èster obre una segona via de recuperació parcial o total dels materials que permet a més obtenir precursors reutilitzables dels monòmers de partida. La tecnologia de fotocurat permet processar els materials de manera més ràpida i controlada i a temperatures més baixes que el curat tèrmic. Per tant, es pot optimitzar en termes de temps i energia l’obtenció de materials amb propietats equivalents.The polymerization of epoxy resins with a spirobislactone as a comonomer has been studied with the purpose of controlling the chemical shrinkage during curing and improving their degradability to permit partial o total recovery and reuse of the materials once their service life is over. Thermal curing as well as photocuring using UV radiation have been used. The curing of these systems has been studied under cationic and anionic catalysis. The study focuses on key aspects of curing and processing such as kinetics, gelation and vitrification, analysis of thermal and mechanical properties of the cured materials and the influence of the chemical processes on kinetics and properties. Thermal analysis (DSC, DMTA, DSC, TGA), spectroscopic (FTIR) and non-instrumental techniques have been employed. The kinetics of the curing process, the network development and therefore the morphology and properties of the obtained materials are highly dependent on the different chemical processes taking place under cationic or anionic catalysis. The effect of the spirobislactone on the kinetics of the curing process is complex and depends on the catalysis, the type of initiator and the curing technique. In anionic curing the opening of the spirobislactone reduces the reaction rate, whereas in cationic curing the reaction rate can either increase or decrease depending on the interaction between the initiator and the reagents. Chemical shrinkage undergone during curing is reduced with the use of the spirobislactone, for both thermal cationic and thermal anionic systems, due to the expanding effect of the direct opening of the spirobislactone or indirectly via formation of an expandable spiroortoester. Epoxy-spirobislactone copolymers have a lower cross-linking density and are more flexible and tougher due to the formation of linear flexible structures that act as spacers between cross-linkable epoxy monomer units. Curing schedule can have an influence on the network development and therefore on the properties of the fully cured materials. The thermal stability of the fully cured materials decreases as the spirobislactone is incorporated due to the introduction of ester linkages into the network. Thus, it is possible to partially recover the materials by means of controlled thermal degradation. Hydrolysis of ester groups can also lead to total or partial recovery of reusable monomer precursors. Photocuring technology permits faster and controlled processing of materials at lower temperatures than thermal curing, therefore making it possible to reduce time and energy consumption during processing and obtention of materials with similar properties.Postprint (published version

Analysis of the reaction mechanism of the thiol-epoxy addition initiated by nucleophilic tertiary amines

A kinetic model for thiol–epoxy crosslinking initiated by tertiary amines has been proposed. The kinetic model is based on mechanistic considerations and it features the effect of the initiator, hydroxyl content, and thiol–epoxy ratios. The results of the kinetic model have been compared with data from the curing of off-stoichiometric formulations of diglycidyl ether of bisphenol A (DGEBA) crosslinked with trimethylolpropane tris(3-mercaptopropionate) (S3) using 1-methylimidazole (1MI) as the initiator. The model has been validated by fitting the kinetic parameters to the experimental data under a variety of reaction conditions. In spite of the experimental uncertainty and model assumptions, the main features of the curing kinetics are correctly described and the reaction rates are quantitatively reproduced.Postprint (published version

Epoxy sol-gel hybrid thermosets

Sol-gel methodologies are advantageous in the preparation of hybrid materials in front of the conventional addition of nanoparticles, because of the fine dispersion of the inorganic phase that can be reached in epoxy matrices. In addition, the use of organoalkoxysilanes as coupling agents allows covalent linkage between organic and inorganic phases, which is the key point in the improvement of mechanical properties. The sol-gel process involves hydrolysis and condensation reactions under mild conditions, starting from hydrolysable metal alkoxides, generally alkoxy silanes. Using the sol-gel procedure, the viscosity of the formulation is maintained, which is an important issue in coating applications, whereas the transparency of the polymer matrix is also maintained. However, only the proper combination of the chemistries and functionalities of both organic and inorganic structures leads to thermosets with the desired characteristics. The adequate preparation of hybrid epoxy thermosets enables their improvement in characteristics such as mechanical properties ( modulus, hardness, scratch resistance), thermal and flame resistance, corrosion and antimicrobial protection, and even optical performance among others.Postprint (published version

Structural design of CANs with fine-tunable relaxation properties: a theoretical framework based on network structure and kinetics modeling

In this work, we present a model capable of reproducing the stress relaxation dynamics of a wide range of relaxation processes in covalent adaptable networks (CANs) produced by stepwise polymerization. The proposed model captures the effective elastic response of the material subject to an initial stress by analogy with a network decrosslinking process. The combination of a recursive structural model and a kinetic model for the bond exchange reaction makes it possible to predict the expected stress relaxation profile in simple and complex systems depending on the structure of the network, the rate of bond exchange of the different components, and the presence of permanent bonds. After the basic features of the model are analyzed, its prediction capabilities are validated by simulating a number of scenarios taken from the literature. The results show that tailoring of the network architecture enables unprecedented flexibility in the design of CAN-based materials.This work was funded by the Spanish Ministry of Science and Innovation (MCIN/AEI/10.13039/501100011033) through R&D project PID2020-115102RB-C22 and also by Generalitat de Catalunya (2017-SGR-77). X. Fernández-Francos and O. Konuray acknowledge the Serra-Hunter ́ programme (Generalitat de Catalunya).Postprint (published version