Mask image projection is an additive manufacturing (AM) technique used in photocurable materials which allows the simultaneous energy delivery in a whole area instead of a single spot. A common problem for this AM process is the uncontrolled penetration of light energy, which could cause a solidification of non - desired layers. In this paper , an optimization procedure is developed and presented in order to increase Z accuracy and resolution of printed parts through the control of the total accumulated dose and the photocuring conversion ratio of each spatial location of the manufacturing volu me. Consequently, the uncontrolled monomer - to - polymer conversion of down - facing surfaces could be reduced. A finite element strategy is used in the optimisation procedure to obtain a full discretization of the whole manufacturing domain. Furthermore, exper imental tests have been done to compare experimental results and numerical estimations. The results show that the use of the optimisation procedure increases the accuracy and resolution of printed parts along the manufacturing direction.
Konuray, A.; Ruiz, A.; Morancho, J.; Salla, J.; Fernandez-Francos, X.; Serra, À.; Ramis, X. European polymer journal Vol. 98, p. 39-46 DOI: 10.1016/j.eurpolymj.2017.11.003 Data de publicació: 2018-01 Article en revista
A new family of poly(keto ester)-poly(methacrylate) thermosets based on off-stoichiometric acetoacetate-acrylate/methacrylate formulations has been prepared and characterized. The first stage of curing is a self-limiting click Michael addition between acetoacetate and acrylate groups at room temperature and the second stage is a methacrylate radical photopolymerization. An analysis of the reaction kinetics confirmed that acetoacetate reacts selectively with acrylates, leaving methacrylates completely unreacted after the first curing stage. It was found that acrylate-rich mixtures achieve complete conversion at the end of both curing stages. In methacrylate-rich formulations the dilution effect of this monomer leads to a slow curing rate and a certain amount of unreacted acrylates in the first curing stage, which react completely after irradiation in the second curing stage. The prepared materials are gelled after acetoacetate/acrylate Michael addition and show a wide range of properties at both curing stages that can be tailored by changing the relative contribution of the Michael and radical vinyl homopolymerization reactions. The latency of the formulations at the beginning of the second stage can be regulated as desired by irradiation.
Guzman, D.; Ramis, X.; Fernandez-Francos, X.; de la Flor, S.; Serra, À. Progress in organic coatings Vol. 114, p. 259-267 DOI: 10.1016/j.porgcoat.2017.10.025 Data de publicació: 2018-01-01 Article en revista
A new dual-curing scheme was developed for thiol-acrylate-epoxy mixtures. A photo-initiated latent catalytic system was used to carry out thiol-acrylate Michael addition at 35 °C (Stage 1) followed by thiol-epoxy click reaction (Stage 2) at 80–110 °C. The intermediate materials were shown to have several days of storage stability. The use of a radical inhibitor has suppressed radical mediated acrylate homopolymerization which would otherwise lead to unreacted thiols remaining. Kinetics of Stage 2 was analyzed mathematically using isoconversional differential method and Kamal model regression. Glass transition temperatures (Tg) of samples with varying contents of epoxy and different types of acrylates were measured. Epoxy-rich formulations gave the highest final Tg. Although not as influential as the epoxy content, using higher functional and more rigid acrylate monomers resulted in higher intermediate and final Tg. The proposed curing scheme and the resulting materials could be useful in applications such as adhesives, industrial coatings with high chemical resistance, optical and electronic materials.
Dual curing is the methodology for obtaining thermosets by combining two different and compatible polymerization reactions, taking place sequentially or simultaneously. Sequential dual curing processing enables the preparation of handleable materials after a first stage of curing which, upon application of a second stimulus, further crosslink to achieve the ultimate properties.
Controlled curing sequences can be achieved by means of using latent catalysts that are activated upon irradiation or temperature, or else by thermal control of the reaction kinetics. Many possible combinations of reactions are possible, especially those including click-type polymerization reactions, which are especially suited because of their efficiency, selectivity and orthogonality. Careful choice of the polymerization reactions, monomer composition and structure makes it possible to obtain intermediate materials with well-defined rheological, thermal and mechanical properties, ranging from those of viscoelastic liquids to lightly crosslinked rubbers or glassy crosslinked networks. After the second curing stage, the materials complete the crosslinking process and reach the ultimate properties required for the end-use of the material. This unique combination of features represent significant advantages in terms of processing flexibility, facilitating manufacturing and assembly in comparison with curing systems based on a single polymerization reaction, and opens new application perspectives in diverse areas such as photo-patterning, holography, shape-memory devices or micro-fluidics.
Belmonte, A.; Lama, G.; Gentile, G.; Cerruti, P.; Ambrogi, V.; Fernandez-Francos, X.; de la Flor, S. European polymer journal Vol. 97, p. 241-252 DOI: 10.1016/j.eurpolymj.2017.10.006 Data de publicació: 2017-12-01 Article en revista
This investigation presents a new approach to obtain free-standing thermally-triggered “two-way” shape-memory actuators by realizing multilayer structures constituted by glassy thermoset (GT) films anchored to a previously programmed liquid-crystalline network (LCN) film. The GT is obtained via dual-curing of off-stoichiometric “thiol-epoxy” mixtures, thus enabling the development of complex actuator configurations thanks to the easy processing in the intermediate stage, and a compact and resistant design due to the strong adhesion between the layers obtained upon the final curing stage of the GT. A model based on the classical multilayered beam theory to predict the maximum deflection of a “beam-like” design is proposed and its reliability is verified by experimental investigation of actuators with different configurations and LCN stretching levels. The results show the capability of these actuators to bend and unbend under various consecutive heating–cooling procedures in a controlled way. The maximum deflection can be modulated through the configuration and the LCN stretching level, showing an excellent fitting with the model predictions. The model is able to predict high actuation levels (angles of curvature ˜ 180°) and the bidirectional shape-memory behavior of the device as a function of the thickness, configuration of the layers, and the LCN stretching level. This approach enables the design of free-standing two-way actuators covering a range of bending actuation from 27 to 98% of the theoretical maximum deflection.
Novel thermosets were prepared by the base-catalysed reaction between a cycloaliphatic resin (ECC) and various thiol crosslinkers. 4-(N,N-Dimethylaminopyridine) (DMAP) was used as base catalyst for the thiol–epoxy reaction. A commercial tetrathiol (PETMP) and three different thiols synthesized by us, 6SH-SQ, 3SH-EU and 3SH-ISO, were tested. 6SH-SQ and 3SH-EU were prepared from vinyl or allyl compounds from renewable resources such as squalene and eugenol, respectively. Thiol 3SH-ISO was prepared starting from commercially available triallyl isocyanurate. A kinetic study of the mixtures was performed using differential scanning calorimetry. Stoichiometric ECC/thiol/DMAP formulations were cured at 120 °C for 1 h, at 150 °C for 1 h and post-cured for 30 min at 200 °C. The materials were characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis and dynamic mechanical thermal analysis. The results revealed that the materials obtained from the synthesized thiols had higher thermal stability and glass transition temperatures than those obtained from the commercial PETMP. In addition, all the materials obtained exhibited very good transparency. This study proves the ability of multifunctional thiols to crosslink cycloaliphatic epoxy resins, leading to more flexible materials than those obtained by cationic homopolymerization of ECC or base-catalysed ECC–anhydride copolymerization.
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.
Belmonte, A.; Cesare , G.; Gentile, G.; Fernandez-Francos, X.; de la Flor, S.; Cerruti, P.; Ambrogi, V. Journal of Physical Chemistry C Vol. 121, num. 40, p. 22403-22414 DOI: 10.1021/acs.jpcc.7b04610 Data de publicació: 2017-10-12 Article en revista
In this paper, epoxy-based shape-memory liquid-crystalline lightly cross-linked networks (LCN) are synthesized and characterized with a view to the future development of two-way autonomous shape-memory actuators by coupling the LCN with an external epoxy-matrix. Carboxylic acids of different aliphatic chain lengths are used as curing agents for a rigid-rod epoxy-based mesogen. Thermal and liquid-crystalline (LC) properties of the LCN are investigated through calorimetric and X-ray diffraction analysis on unstretched and stretched samples. Structural and thermomechanical properties are studied by means of tensile and dynamic-mechanical analyses and the shape-memory capabilities are analyzed in terms of actuation strain and stress under partially- and fully constrained thermomechanical procedures. The results have shown the possibility to obtain LCN with isotropization temperatures above 100 °C, controlled degree of liquid crystallinity, and high actuation stress and strain by simply varying the aliphatic chain length of the curing agent. Moreover, by properly adjusting the programming conditions (stress level), it is possible to optimize and stabilize the actuation performance. In addition, the effects of the liquid-crystalline domains on the network relaxation and their degree of orientation after programming at the different stress levels have been discussed. Overall, proper design of chain length and stress level allows strain actuation to be modulated from low, ~60%, to high, ~160% strain levels. The results evidence the possibility of finely tuning LCN with controlled and stable actuation protocols by balancing the aliphatic chain length and programming conditions.
Novel bio-based and dual-curable thermosets were prepared from eugenol derivatives. The curing sequence combined two click reactions, a photoinduced radical thiol-ene reaction followed by a thermally activated thiol-epoxy reaction.
Eugenol was transformed into a triallyl (3A-EU) and a diallyl glycidyl derivative (2AG-EU) with high yields, and they were used as starting monomers in order to study the thiol-ene reaction and the dual-curing process, respectively. Three different thiol crosslinkers were tested, one commercially available tetrathiol derived from pentaerythritol (PETMP) and two other that were also synthesized: a trithiol derived from eugenol (3SH-EU) and a hexathiol derived from squalene (6SH-SQ).
FTIR and DSC were used to monitor both curing stages and analyze the obtained materials. The results evidenced the occurrence of side reactions that led to incomplete thiol-ene reaction. The dual-curable materials showed higher Tgs than the materials obtained by a simple thiol-ene process and presented higher mechanical and thermomechanical performance.
The influence of UV post - curing process on mechanical properties as well as photocuring conversion ratios is presented. An analytical model to determinate the conversion ratio for frontal polymerization is used t o define 3D printing parameters in order to obtain a conversion profile as homogeno us as possible. The mechanical properties of 3D p rinted coupons with and without UV post - curing process were obtained through experimental tensile and bending tests. Furthermore, the experimental conversion ratios of printed samples were obtained by means of FTIR spectrometry analysis. It was observed that conversion ratios and some mechanical properties increase because of UV post - curing treatment, enabling the chance to optimize the whole manufacturing process in function of the 3D printed part requiremen ts.
Morancho, J.; Ramis, X.; Fernandez-Francos, X.; Salla, J.; Konuray, A.; Serra, À. Journal of Thermal Analysis and Calorimetry Conference and V4 (Joint Czech-Hungarian-Polish-Slovakian) Thermoanalytical Conference p. 242-243 Data de presentació: 2017-06-07 Presentació treball a congrés
Epoxy resins are widely used in applications such as adhesives, coatings, electric laminates, encapsulation of semiconductors devices, matrix material for composites, structural components and cryogenic engineering because of their superior mechanical properties, adhesion and chemical resistance. Curing of an epoxy/amine mixture proceeds through different reaction mechanisms. In competition with the main step-wise polycondensation reaction between epoxy and amine groups, homopolymerization of epoxy groups can take place at higher temperatures and can be catalyzed by the tertiary amines formed by the polycondensation reaction or additional anionic initiators .
The main objective of this work has been to investigate the kinetics of the epoxy-amine polycondensation and the epoxy homopolymerization in off-stoichiometric epoxy/amine formulations with excess of epoxy groups, and in the presence of an anionic initiator. Diglycidyl ether of bisphenol A (DGEBA, Epikote 827) and diethylenetriamine (DETA) have been used as epoxy and amine reagents, respectively, and 2-methylimidazole (2MI) has been used as anionic initiator. This study has been carried out using a DSC (differential scanning calorimeter). The thermal-mechanical properties of the partially-cured and fully-cured materials with and without initiator have been determined by DSC and DMA (dynamic-mechanical analysis).
First, off-stoichiometric DGEBA/DETA mixtures with excess of DGEBA, with and without 2MI, have been reacted at low temperatures, where only the epoxy-amine condensation takes place, because the epoxy homopolymerization has a very low curing rate. Afterwards, samples containing 2MI have been heated at different heating rates to study the homopolymerization process of the epoxy excess. The kinetics of both processes have been analyzed with an isoconversional method to determine the activation energy and the Šesták-Berggren equation has been applied to determine the frequency factor and the orders of reaction . In the thermal curing, the activation energy and the frequency factor decrease with increasing degree of conversion, but in the homopolymerization process, both magnitudes increase with the degree of conversion. The results indicate that the dual-curing character of off-stoichiometric DGEBA/DETA thermosets with 2MI as anionic initiator makes them suitable for multi-stage curing processes with easy control of degree of cure and material properties in the intermediate stage and enhanced final material properties.
1. Fernández-Francos X, Santiago D, Ferrando F, Ramis X, Salla JM, Serra A, Sangermano M. Network structure and thermomechanical properties of hybrid DGEBA networks cured with 1-methylimidazole and hyperbranched poly(ehtyleneimine)s. J Polym Sci B Polym Phys. 2012;50:1489-503.
2. Vyazovkin S, Burnham AK, Criado JM, Pérez-Maqueda LA, Popescu C, Sbirrazzuoli N. ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data. Thermochim Acta. 2011;520:1-19.
Guzman, D.; Ramis, X.; Fernandez-Francos, X.; Serra, À. International Conference on Advanced Polymers via Macromolecular Engineering p. 201 Data de presentació: 2017-05-24 Presentació treball a congrés
Konuray, A.; Ramis, X.; Fernandez-Francos, X.; Serra, À. International Conference on Advanced Polymers via Macromolecular Engineering p. 161 Data de presentació: 2017-05-23 Presentació treball a congrés
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.
Belmonte, A.; Fernandez-Francos, X.; de la Flor, S.; Serra, À. Mechanics of time-dependent materials Vol. 21, num. 2, p. 133-149 DOI: 10.1007/s11043-016-9322-z Data de publicació: 2017-05-01 Article en revista
The shape-memory response (SMR) of “click” thiol-epoxy polymers produced using latent catalysts, with different network structure and thermo-mechanical properties, was tested on unconstrained shape-recovery processes under isothermal conditions. Experiments at several programming temperatures ( TprogTprog ) and isothermal-recovery temperatures ( TisoTiso ) were carried out, and the shape-memory stability was analyzed through various consecutive shape-memory cycles. The temperature profile during the isothermal-recovery experiments was monitored, and it showed that the shape-recovery process takes place while the sample is becoming thermally stable and before stable isothermal temperature conditions are eventually reached. The shape-recovery process takes place in two different stages regardless of TisoTiso : a slow initial stage until the process is triggered at a temperature strongly related with the beginning of network relaxation, followed by the typical exponential decay of the relaxation processes until completion at a temperature below or very close to TgTg . The shape-recovery process is slower in materials with more densely crosslinked and hindered network structures. The shape-recovery time ( tsrtsr ) is significantly reduced when the isothermal-recovery temperature TisoTiso increases from below to above TgTg because the network relaxation dynamics accelerates. However, the temperature range from the beginning to the end of the recovery process is hardly affected by TisoTiso ; at higher TisoTiso it is only slightly shifted to higher temperatures. These results suggest that the shape-recovery process can be controlled by changing the network structure and working at Tiso
Thiol-acetoacetate-acrylate ternary dual-curing thermosets were prepared by a sequential process consisting of thiol-Michael addition to acrylates at room temperature followed by Michael addition of acetoacetates to acrylates at moderately elevated temperature. The curing sequence can be controlled with the help of the different acidities of the protons on thiol and acetoacetate groups, the favorable pKa of the base used as catalyst and the self-limiting character of Michael additions. The latency of the curing steps can be regulated by selection of the right catalysts, temperature and curing conditions. The properties of the intermediate and final materials can be tuned by changing the structure of the monomers and the contribution of both Michael addition reactions.
In this work, epoxy-based shape-memory actuators have been developed by taking advantage of the sequential dual-curing of off-stoichiometric “thiol–epoxy” systems. Bent-shaped designs for flexural actuation were obtained thanks to the easy processing of these materials in the intermediate stage (after the first curing process), and successfully fixed through the second curing process. The samples were programmed into a flat temporary-shape and the recovery-process was analyzed in unconstrained, partially-constrained and fully-constrained conditions using a dynamic mechanical analyzer (DMA). Different “thiol–epoxy” systems and off-stoichiometric ratios were used to analyze the effect of the network structure on the actuation performance. The results evidenced the possibility to take advantage of the flexural recovery as a potential actuator, the operation of which can be modulated by changing the network structure and properties of the material. Under unconstrained-recovery conditions, faster and narrower recovery-processes (an average speed up to 80%/min) are attained by using materials with homogeneous network structure, while in partially- or fully-constrained conditions, a higher crosslinking density and the presence of crosslinks of higher functionality lead to a higher amount of energy released during the recovery-process, thus, increasing the work or the force released. Finally, an easy approach for the prediction of the work released by the shape-memory actuator has been proposed.
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.
An extensive characterization of a sequential dual-curing system based on off-stoichiometric “thiol-epoxy” mixtures was carried out using thiol compounds of different functionality. The intermediate and final materials obtained after each curing stages at different thiol-epoxy ratios were studied by means of thermomechanical and rheological experiments. The storage and loss modulus and the loss factor tan d were monitored during the curing process to analyse gelation and network structure build-up. The critical ratio for gelation was determined making use of the ideal Flory-Stockmayer theory and compared with experimental results. Intermediate materials obtained in the vicinity of the theoretical critical ratio did not have the mechanical consistency expected for partially crosslinked materials, did not retain their shape and even experienced undesired flow upon heating to activate the second curing reaction. The rheological results showed that the critical ratio is higher than the predicted value and that a softening during the second curing stage affects the shape-retention at this ratio. From the thermomechanical results, a wide range of intermediate and final materials with different properties and applicability can be obtained by properly choosing the thiol-epoxy ratio: from liquid-like to highly deformable intermediate materials and from moderately crosslinked (deformable) to highly crosslinked (brittle) final materials.
Morancho, J.; Fernandez-Francos, X.; Acebo, C.; Ramis, X.; Salla, J.; Serra, À. Journal of thermal analysis and calorimetry Vol. 127, num. 1, p. 645-654 DOI: 10.1007/s10973-016-5376-z Data de publicació: 2017-01 Article en revista
New hyperbranched polymers (HBP) have been
synthesized by reaction of a poly(ethylene imine) with phenyl and t-butyl isocyanates. These HBPs have been characterized by 1H-NMR (nuclear magnetic resonance of
hydrogen) and Fourier transform infrared spectroscopy. Their influence on the curing and properties of epoxy-anhydride thermosets has been studied by different techniques:
differential scanning calorimetry (DSC), dynamic
mechanical analysis (DMA), and thermogravimetry (TG). The curing kinetics has been studied with DSC. Integral
isoconversional method and the S ¿ esta´k–Berggren model have been used to determine the activation energy and the frequency factor. The kinetic parameters are very similar for all the studied systems at the middle stage of the process, but changes are observed at the beginning and at the end of the process when these modifiers are used. The HBPs reduce the glass transition temperature of the cured
materials. In addition, from the DMA analysis it can be seen that the HBP modifier obtained from phenyl isocyanate hardly changes the storage modulus, but the obtained ones from t-butyl isocyanate decrease it. TG analysis reveals a decrease in the onset temperature of the degradation process upon addition of the HBPs.
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.
Guzman, D.; Mateu, B.; Fernandez-Francos, X.; Ramis, X.; Serra, À. International Conference on Nanostructured Polymers and Nanocomposites p. 1-3 Data de presentació: 2016-09-19 Presentació treball a congrés
Serra, À.; Ramis, X.; Acebo, C.; Guzman, D.; Belmonte-Parra, A.; Fernandez-Francos, X.; de la Flor, S. Reunión del Grupo Especializado de Polímeros (GEP) de la RSEQ y RSEF p. 43-44 Data de presentació: 2016-09-05 Presentació treball a congrés
Ramis, X.; Konuray, A.; Liendo, F.J.; Fernandez-Francos, X.; Morancho, J.; Salla, J.; Serra, À. Reunión del Grupo Especializado de Polímeros (GEP) de la RSEQ y RSEF p. 261-263 Data de presentació: 2016-09-05 Presentació treball a congrés
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.
A new allyl terminated hyperbranched poly(ethyleneimine) was synthesized and characterized and then used in different proportions as multifunctional macromonomer in tetrathiol – diglycidyl ether of bisphenol A formula- tions. The curing process had a two-stage character and was composed by two click reactions: a fi rst photoin- duced thiol – ene addition followed by a thermal thiol – epoxy reaction. The thiol – ene reaction was catalyzed by a radical initiator and the thiol – epoxy curing by tertiary amines. The evolution of the fi rst part of the curing was studied by photo-DSC and FTIR and the results compared with those obtained in a photoirradiation chamber, which was used to prepare samples for thermomechanical tests. These studies showed that the thermal thiol – epoxy process prematurely began during the photoirradiation because the presence of amines in the PEI struc- ture accelerated this process. The thiol – epoxy reaction was more extensively produced when the proportion of the poly(ethyleneimine) increased in the formulation. The overlapping between both processes was greater in the photoirradiation chamber than in the photo-DSC. The intermediate material was completely cured by ther- mal treatmentin an oven.The need of adding 1-methylimidazole as catalyst to complete thethiol – epoxyreaction was derived from the calorimetric studies. The materials prepared were characterized by thermogravimetry and thermomechanical analysis.
Fernandez-Francos, X.; Konuray, A.; Belmonte, A.; de la Flor, S.; Serra, À.; Ramis, X. Polymer chemistry Vol. 7, num. 12, p. 2280-2290 DOI: 10.1039/c6py00099a Data de publicació: 2016-01-01 Article en revista
A new dual-curing system based on sequential thiol-epoxy click polycondensation and epoxy anionic homopolymerization was studied. Formulations of diglycidyl ether of bisphenol A and trimethylolpropane tris(3-mercaptopropionate) with 1-methylimidazole as a base catalyst and excess of epoxy groups were prepared and characterized. The curing process is sequential: fast thiol-epoxy polycondensation takes place first, followed by slower homopolymerization of excess epoxy groups. This makes it possible to define curing sequences with easy time-temperature control for both curing stages. The network buildup process during the first curing stage can be easily modelled assuming ideal polycondensation, which allows tailoring the structure and properties of the intermediate materials. The homopolymerization of the excess epoxy groups in the second curing stage results in a higher glass transition temperature (T-g) in comparison with the stoichiometric thiol-epoxy material, thus extending the application of thiol-epoxy thermosets to wider temperature ranges.
A novel curing methodology based on the combination of thiol-yne and thiol-epoxy click reactions has been developed. The curing process consists of a first photoinitiated thiol-yne reaction, followed by a thermal thiol-epoxy process. As alkyne substrate a new propargyl terminated hyperbranched poly(ethyleneimine) (PEIyne) has been synthesized from the reaction of commercial poly(ethylenimine) (PEI) and glycidyl propargyl ether. The evolution of the curing of different mixtures of PEIyne and diglycidylether of bisphenol A (DGEBA) with the stoichiometric amount of tetrathiol (PETMP) has been monitored by DSC. The new hybrid materials have been characterized by thermomechanical analysis, thermogravimetry and by electronic microscopy inspection and compared with neat thiol-yne and thiol-epoxy materials. The T(g)s of the complete cured materials increase with the proportion of epoxide in the formulation. The thiol-yne network improves the plasticity of the fracture of the materials.
A new sequential two steps photo and thermal process for the preparation of click thiol-ene/thiol-epoxy thermosets is described. Commercially available diglycidyl ether of bisphenol A (DGEBA), triallylisocyanurate (TAIC) and pentaerythritol tetrakis (3-mercaptopropionate) (PETMP) were combined to produce tailored materials with a 75, 50 and 25% of thiol-ene/thiol-epoxy networks. A photoinitiator was used to trigger the radical thiol-ene polymerization and a latent amine precursor was used to start the base-catalyzed thiol-epoxy click reaction. Neat thiol-ene and thiol-epoxy materials were prepared and taken as the references. The use of a latent amine precursor in adequate proportion and under suitable reaction conditions allowed us to reach a dual system with two well-defined steps, stable intermediate materials and well-controlled structure after the first curing stage and at the end of the curing process. This process overcomes some limitations observed in analogous curing systems reported previously such as the absence of latency for the second curing stage leading to unstable materials in the intermediate stage. Both chemical reactions were studied by FTIR and calorimetry. The latency of the different formulations was studied by DSC and rheometry. The materials prepared were characterized by thermal mechanical analysis and thermogravimetry.
This paper presents a new methodology to develop "thiol-epoxy" shape-memory polymers (SMPs) with enhanced mechanical properties in a simple and efficient manner via "click" chemistry by using thermal latent initiators. The shape-memory response (SMR), defined by the mechanical capabilities of the SMP (high ultimate strength and strain), the shape-fixation and the recovery of the original shape (shape-recovery), was analyzed on thiol-epoxy systems by varying the network structure and programming temperature. The glass transition temperature (T-g) and crosslinking density were modified using 3- or 4- functional thiol curing agents and different amounts of a rigid triglycidyl isocyanurate compound. The relationship between the thermo-mechanical properties, network structure and the SMR was evidenced by means of qualitative and quantitative analysis. The influence of the programming temperature (T-prog) on the SMR was also analyzed in detail. The results demonstrate the possibility of tailoring SMPs with enhanced mechanical capabilities and excellent SMR, and intend to provide a better insight into the relationship between the network structure properties, programming temperature and the SMR of unconstrained (stress-free) systems; thus, making it easier to decide between different SMP and to define the operative parameters in the useful life.
This paper presents a new methodology to develop “thiol-epoxy” shape-memory
polymers (SMPs) with enhanced mechanical properties in a simple and efficient manner via “click”
chemistry by using thermal latent initiators. The shape-memory response (SMR), defined by the
mechanical capabilities of the SMP (high ultimate strength and strain), the shape-fixation and
the recovery of the original shape (shape-recovery), was analyzed on thiol-epoxy systems by
varying the network structure and programming temperature. The glass transition temperature
(Tg) and crosslinking density were modified using 3- or 4- functional thiol curing agents and
different amounts of a rigid triglycidyl isocyanurate compound. The relationship between the
thermo-mechanical properties, network structure and the SMR was evidenced by means of
qualitative and quantitative analysis. The influence of the programming temperature (Tprog) on the
SMR was also analyzed in detail. The results demonstrate the possibility of tailoring SMPs with
enhanced mechanical capabilities and excellent SMR, and intend to provide a better insight into the
relationship between the network structure properties, programming temperature and the SMR of
unconstrained (stress-free) systems; thus, making it easier to decide between different SMP and to
define the operative parameters in the useful life.
Multiarm star polymers, with a hyperbranched poly(ethyleneimine) (PEI) core and poly(epsilon-caprolactone) (PCL) arms end-capped with acetyl groups were synthesized by ring-opening polymerization of epsilon-caprolactone from PEI cores of different molecular weight. These star polymers were used as toughening agents for epoxy/anhydride thermosets. The curing process was studied by calorimetry, thermomechanical analysis and infrared spectroscopy. The final properties of the resulting materials were determined by thermal and mechanical tests. The addition of the star polymers led to an improvement up to 130% on impact strength and a reduction in the thermal stresses up to 55%. The structure and molecular weight of the modifier used affected the morphology of the resulting materials. Electron microscopy showed phase-separated morphologies with nano-sized fine particles well adhered to the epoxy/anhydride matrix when the higher molecular weight modifier was used.
Acebo, C.; Fernandez-Francos, X.; Santos, J.; Messori, M.; Ramis, X.; Serra, À. European polymer journal Vol. 70, p. 18-27 DOI: 10.1016/j.eurpolymj.2015.06.029 Data de publicació: 2015-09-01 Article en revista
New epoxy-silica hybrid coatings were prepared by a dual process consisting of a sol-gel process using tetraethoxysilane (TEOS) or 3-glycidyloxypropyl trimethoxysilane (GPTMS) in the presence of hyperbranched poly(ethyleneimine) with ethoxysilyl groups at the chain ends (PEI-Si) followed by a homopolymerization of diglycidylether of bisphenol A (DGEBA) using 1-methylimidazole (1-MI) as anionic initiator. The influence of the amount of TEOS and GPTMS in the characteristics of the coating was examined.; Thin transparent films were obtained and their morphology was observed by transmission electron microscopy (TEM). The hydrolytic condensation was confirmed by Si-29 NMR studies. Cage-like nanometric structures were formed in case of adding GPTMS and bigger silica particles on adding TEOS to the formulation. Thermal stability was evaluated by thermogravimetry and the scratch resistance properties were also investigated, showing an improvement in resistance to break and to detachment in all the coatings containing GPTMS. (C) 2015 Elsevier Ltd. All rights reserved.
Morancho, J.; Fernandez-Francos, X.; Acebo, C.; Ramis, X.; Salla, J.; Serra, À. Central and Eastern European Conference on Thermal Analysis and Calorimetry p. 324 Data de presentació: 2015-08-27 Presentació treball a congrés
Epoxy resins are widely used in applications such as adhesives, coatings, electric laminates, encapsulation of semiconductor devices, matrix material for composites, structural components and cryogenic engineering because of their mechanical properties, adhesion and chemical resistance.
However, epoxy resins are inherently brittle due to their high cross-link density. To increase their toughness different modifiers such as rubber, thermoplastic and glass particles can be added, but they always limit the processability of the formulation. Hyperbranched polymers (HBP) are a new kind of polymers used as modifiers of epoxy resins to increase their toughness, but they maintain the processability of the formulation due to their highly branched dendritic structure, which is a key point in coatings applications [1-2].
In this paper, the influence of three hyperbranched poly(ethylene imine) polymers with different terminal groups (two of them with t-butyl terminal groups and the other one with phenyl groups) on the thermal curing of an epoxy-anhydride system has been studied. The synthetic procedure to prepare the polymeric modifiers is quite easy and it consists in reacting commercial poly(ethylene imine) with the corresponding isocyanate in chloroform solution at room temperature. The epoxy resin and the anhydride we selected are Epikote 828 and 4-methyl hexahydrophtalic anhydride, respectively. As initiator we have used N,N-dimethylbenzylamine . The techniques employed in this study have been DSC (differential scanning calorimetry) and DMTA (dynamic mechanical thermal analysis). The kinetics of the thermal curing has been analyzed using isoconversional methods.
The addition of the hyperbranched polymers with t-butyl terminal groups hardly changes the thermal curing process of the epoxy-anhydride system, but if the poly(ethylene imine) with phenyl end-groups is added, the thermal curing of the system is decelerated.
In the systems studied, all the HBPs used decrease the glass transition temperature of the epoxy-anhydride system, acting therefore as plasticizers.
 B. Voit, J. Polym. Sci. A: Polym. Chem., 38 (2000) 2505
 L. Boogh, B. Pettersson, J.-A.E. Manson, Polymer, 49 (2001) 2249
 M. Flores, X. Fernández-Francos, F. Ferrando, X. Ramis, A. Serra, Polymer, 53 (2012) 5232
Ye, Lishi; Fernandez-Francos, X.; Morancho, J.; Serra, À.; Ramis, X. Central and Eastern European Conference on Thermal Analysis and Calorimetry p. 325 Data de presentació: 2015-08-27 Presentació treball a congrés
B-stage processing is a term used for one-pot thermosetting formulations that can be partially cured or pre-dried, after being applied to a substrate and then cured under heat and pressure. B-staging has processing advantages that can be exploited in assembly processes, especially the possibility of splitting the different steps of a process in a production line, in terms of time and location, thus helping to prevent manufacturing bottlenecks. However, in B-stage processing the pre-drying process has usually tight time and temperature constraints that make necessary a careful control to avoid over-drying leading to loss of properties such as adhesion and material waste. Alternative dual-curing procedures aim at eliminating the above drawbacks while keeping and even enhancing their processing advantages [1-3]. Further advantage comes from the possibility of tailoring the material structure and properties between both curing stages by changing the formulation composition. This represents a significant advantage over conventional B-stage, in which the material properties in the intermediate stage are kinetically controlled, depending on temperature/time and vitrification. This approach opens a new way for flexible processing, not just in terms of curing stageability but also in terms of the intermediate material characteristics (adhesion, flowing ability, rigidity,…). In addition, the latent character of the second curing stage ensures good storage stability between both curing stages.
In this work, the sequential UV/thermal processing of acrylate-epoxy clearcoats is studied. Mixtures of methacrylates/acrylates with different side chain and functionality, at least one of them bearing an epoxy group, are photocured to obtain a linear or cross-linked polymer that is subsequently post-cured at elevated temperatures to obtain the final cross-linked material. A combination of a radical photoinitiator and a cationic thermal latent initiator is used in order to permit sequential controlled curing. Different formulation compositions are examined in order to analyze the possibility of custom-tailoring the structure and properties between both curing stages and at the end of the process. The sequential UV-curing and thermal curing processes are studied with photoDSC, FTIR/ATR and DSC. The thermal-mechanical properties of the materials in the intermediate and final states are assessed with DMA, DSC and TGA. The potential application of this curing process is also illustrated.
 K. Studer, C. Decker, E. Beck, R. Schwalm, Eur. Polym., J. 41 (2005) 157
 J. A. Carioscia, J. W. Stansbury, C. N. Bowman, Polymer, 48 (2007) 1526
 C. F. Carlborg, A. Vastesson, Y. Liu, W. van der Wijngaart, M. Johansson, T. Haraldsson, J. Polym. Sci. A: Polym. Chem., 52 (2014) 2604
Acebo, C.; Fernandez-Francos, X.; de la Flor, S.; Ramis, X.; Serra, À. Progress in organic coatings Vol. 85, num. August, p. 52-59 DOI: 10.1016/j.porgcoat.2015.02.023 Data de publicació: 2015-08-01 Article en revista
New dendritic modifiers have been synthesized by amidation of hyperbranched poly(ethylenimine)(PEIs) with 10-undecenoic acid to obtain hyperbranched polymers (HBPs) with different degree of modification. These HBPs have been used as toughness modifiers in a proportion of 10 and 20% in reference to the epoxy resin in diglycidyl ether of bisphenol A (DGEBA)/methyltetrahydrophthalic anhydride (MTHPA) formulations. The curing process has been studied by dynamic scanning calorimetry and by rheometry, which allow the kinetic constants and the gel and vitrification times to be evaluated. The materials obtained have been thermally characterized and their mechanical properties have been evaluated. An increase in impact resistance has been achieved and the T-g of all thermosets prepared was higher than 100 degrees C in spite of the flexible structure of the PEI modifiers. (C) 2015 Elsevier B.V. All rights reserved.
Morancho, J.; Fernandez-Francos, X.; Ramis, X.; Salla, J.; Serra, À. Journal of thermal analysis and calorimetry Vol. 121, p. 389-395 DOI: 10.1007/s10973-015-4535-y Data de publicació: 2015-06-21 Article en revista
In this work, the influence of trimethylolpropane
tris-(3-mercaptopropionate) and a vinyl epoxy
compound (4-vinyl-1-cyclohexene 1,2-epoxide) over the photocuring and subsequent thermal curing of an epoxy resin (CYRACURE UVR-6105) is studied. The photoinitiator used is CYRACURE UVI-6976. The techniques used in this study have been DSC (differential scanning calorimetry) and FTIR (Fourier transform infrared spectroscopy). In the isothermal photocuring, when the proportion of the modifiers are 5 % or greater, the photopolymerization of the epoxy resin is stopped by the thiol-ene reaction of both modifiers, due to the formation of
alkyl sulfonium salts, decreasing the maximum degree of conversion of the process. After the photocuring process, the different systems studied have been post-cured thermally
and the activation energy of this process has been determined using a differential isoconversional method. When the epoxy resin is neat or only it has been added 2.5 % of the modifiers, at the beginning of the post-curing
the activation energy decreases, but when the proportion of the modifiers is 5 % or greater, the activation energy always increases.
Composición para el entrecruzamiento de una amina con un compuesto vinílico α, β-conjugado a un grupo carbonilo de éster y posterior polimerización del compuesto vinílico y procedimientos correspondientes. Composición para el entrecruzamiento de una amina con un compuesto vinílico α,β-conjugado a un grupo carbonilo de éster y posterior polimerización del compuesto vinílico, que comprende:a) una amina con dos o más enlaces N-H,
b) un compuesto vinílico, de fórmula general (I), que contenga como mínimo dos dobles enlaces α,β-conjugados a un grupo carbonilo de éster
donde Y y Z son cada uno independientemente un hidrógeno, restos alifáticos o aromáticos o grupos más complejos, y donde X es un resto alifático o aromático o un grupo más complejo, y
c) un fotoiniciador radicalario o un iniciador térmico radicalario, aptos para iniciar la reticulación de dichos dobles enlaces α,β-conjugados a un grupo carbonilo de éster en exceso.
A new dual-curing, solvent-free process is described for the preparation of tailor-made materials from off-stoichiometric amine-acrylate formulations. The first stage reaction is a self-limiting click aza-Michael addition between multifunctional amine and acrylate monomers with an excess of acrylate groups. The second stage reaction is a photoinduced radical polymerization of the unreacted acrylate groups. By selecting the structure of the monomers and the stoichiometry of the formulations, mechanical and thermal characteristics of the intermediate and final materials can be tuned. The materials obtained after the first curing stage can be gelled or ungelled and loosely or tightly crosslinked at the end of the second curing stage. The methodology used allows to obtain storable and processable intermediate polymers and final networks with optimum properties for different applications. The presence of amines in the reaction medium overcomes the intrinsic oxygen inhibition of acrylate free-radical polymerizations, resulting in a quasi complete cure.
Tri and tetrafunctional thiol were used as curing agent for diglycidyl ether of bisphenol A (DGEBA) catalyzed by a commercially available amine precursor, LC-80. Triglycidyl isocianurate (TGIC) was added in different proportions to the mixture to increase rigidity and glass transition temperature (T-g). The cooperative effect of increasing functionality of thiol and the presence of TGIC in the formulation leads to an increased T-g without affecting thermal stability. The kinetics of the curing of mixtures was studied by calorimetry under isothermal and non-isothermal conditions. The latent characteristics of the formulations containing amine precursors were investigated by rheometry and calorimetry. The increase in the functionality of the thiol produces a slight decrease in the storage lifetime of the mixture. The materials obtained with tetrathiol as curing agent showed the highest values of Young's modulus and T-g.