Estudio del curado de materiales compuestos por un sistema epoxi y partículas elastoméricas

En este trabajo de investigación se ha estudiado la influencia de algunos copolimeros de butadieno y acrilonitrilo sobre el curado de una resina epoxi y sobre las propiedades térmicas y termomecánicas de los materiales compuestos resultantes. Mediante curados isotérmicos se ha investigado utilizando la calorimetria diferencial de barrido (dsc) la influencia que tiene la adición de estos copolímeros sobre el curado de la resina epoxi. A partir de las diferentes experiencias se ha hallado en función del tiempo la temperatura de transición vitrea (tg) y el grado de conversión. Estos resultados han permitido obtener el tiempo de vitrificación, el grado de conversión cuando el material vitrifica y la tgoo (la mayor tg que puede tener el sistema) para los diferentes sistemas estudiados.Se ha analizado la cinetica del proceso de curado mediante diferentes metodos isotérmicos y dinámicos utilizando la tecnica DSC. Se han obtenido valores de energia de activación y de factor de frecuencia de los sistemas empleados. La relación experimental entre la tg y el grado de conversión se ha ajustado mediante la ecuación de dibenedetto.Utilizando el análisis termomecánico (tma) ha podido observarse el fenómeno de la gelificación y se ha determinado mediante la técnica dsc el grado de conversión cuando el material gelifica. Con estos datos y con los cineticos obtenidos anteriormente se han podido construir los diagramas ttt (tiempo-temperatura-transformación).Se ha estudiado el fenómeno del envejecimiento físico en muestras parcialmente curadas de resina epoxi pura y de esta misma resina con diferentes proporciones de ctbn31.Se ha investigado la solubilización del plastificante contenido en el endurecedor por parte de los elastómeros y la solubilidad existente entre la base epoxi y los copolímeros. Mediante el análisis térmico-dinámico-mecánico (dmta) se ha observado la separacion de fases existentes en los diferentes sistemas estudiado.Postprint (published version

Thermal curing of an epoxy-anhydride system modified with hyperbranched poly(ethylene imine)s with different terminal groups

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.Postprint (author's final draft

Curing of off-stoichiometric amine–epoxy thermosets

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 have been investigated. Diglycidyl ether of bisphenol A (DGEBA) 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 differential scanning calorimeter (DSC). The thermal–mechanical properties of the partially cured and fully cured materials with and without initiator have been determined by DSC and dynamic-mechanical analysis. First, off-stoichiometric DGEBA/DETA mixtures with excess of DGEBA, with and without 2MI, have been reacted isothermally at low temperatures, where only the epoxy–amine condensation takes place, because the epoxy homopolymerization has a very low curing rate. Afterward, 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 isothermal curing, amine–epoxy condensation, 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. Results show that the dual-curing character of off-stoichiometric DGEBA/DETA thermosets with 2MI as anionic initiator renders them suitable for multistage curing processes in which the degree of cure and material properties in the intermediate stage can be controlled easily and final material properties can be enhanced.Postprint (author's final draft

Time-temperature-transformation (TTT) diagram of dual-curable epoxy thermosets obtained via two sequential epoxy-amine condensations

A simple and robust strategy to generate time-temperature-transformation (TTT) diagrams for dual-curable thermosets is presented. TTT diagram is built for a dual-curable amine-epoxy formulation. Both curing stages are epoxy-amine polycondensations. Whereas the first stage is carried out at relatively low temperature using a strongly nucleophilic aliphatic amine, the second is carried out at high temperature with a weakly nucleophilic aromatic amine. The good separation between curing stages enabled the individual kinetics analysis of each stage by integral isoconversional procedures. Gelation was determined by thermomechanical analysis; the glass transition temperature - conversion relationship was determined by calorimetry and was used to draw the vitrification line. Gelation and vitrification lines were checked by means of solubility tests and temperature modulated calorimetry. The results presented in the form of a TTT diagram showed the disparate reactivity of both curing stages and a substantial storage stability at the intermediate curing stage. These materials show great potential in applications where processing flexibility and intermediate stability are crucial.Postprint (author's final draft

Curing and thermomechanical properties of off-stoichiometric anhydride-epoxy thermosets

n the present work, we report the preparation and characterization of a new family of thermosets based on off-stoichiometric anhydride–epoxy formulations in the presence of an anionic initiator. Diglycidyl ether of bisphenol A (DGEBA) and hexahydro-4-methylphthalic anhydride (HHMPA) have been used as epoxy and anhydride comonomers, respectively, and 1-methylimidazole (1MI) has been used as anionic initiator. The isothermal curing kinetics and the thermal properties of the stoichiometric and the off-stoichiometric systems have been compared. The kinetics of the isothermal curing has been analyzed by differential scanning calorimetry (DSC) using an isoconversional method and the S ¿esta ´k–Berggren equation to determine the activation energy, the frequency factor and the reaction orders. The materials obtained were characterized by DSC and dynamic mechanical analysis. Gelation during epoxy–anhydride condensation was determined by thermomechanical analysis. At the same curing temperature, the reaction is faster in the system with excess of epoxy groups. However, the glass transition temperatures of the partially cured stoichiometric system are greater. The gelation time of the off-stoichiometric system is shorter than that of the stoichiometric one. The results indicate that the dual-curing character of off-stoichiometric DGEBA/HHMPA thermosets with 1MI as anionic initiator makes them suitable for multistage curing processes with easy control of degree of cure and material properties in the intermediate stage and enhanced ¿nal material properties.Postprint (author's final draft

Optimization and testing of hybrid 3D printing vitrimer resins

The quality of photocure-based 3D printing greatly depends on the properties of the photoresin. There are still many challenges to be overcome at the material level before such additive manufacturing methods dominate the manufacturing industry. To contribute to this exciting re-search, an acrylate-epoxy hybrid and vitrimeric photoresin was studied to reveal the formulation parameters that could be leveraged to obtain improved processability, mechanical performance, and repairability/reprocessability. As the network becomes more lightly or densely crosslinked as a result of changing monomer compositions, or as its components are compatibilized to different extents by varying the types and loadings of the coupling agents, its thermomechanical, tensile, and vitrimeric behaviors are impacted. Using a particular formulation with a high concentration of dynamic ß-hydroxyester linkages, samples are 3D printed and tested for repair and recyclability. When processed at sufficiently high temperatures, transesterification reactions are triggered, allowing for the full recovery of the tensile properties of the repaired or recycled materials, despite their inherently crosslinked structure.Peer ReviewedPostprint (published version

Stress-relaxing thermosets with easily regulated bond exchange kinetics

We have carried out stress-relaxation experiments on poly(acrylate) and poly(thiol-ene) based thermosets prepared from in-house synthesized epoxy-acid adducts with acrylate (for the former) and ene functionalities (for the latter). The adducts contain disulphide bonds at their center which allow their cured thermosets to undergo disulphide metathesis when heated above a certain temperature [1]. The possibility of concurrent transesterification, albeit at a significantly lower rate, between ester and hydroxyl moieties present in the components is not discardedPeer ReviewedPostprint (published version

Preparation and characterization of dual-curable off-stoichiometric amine-epoxy thermosets with latent reactivity

A new family of dual-curable poly(hydroxyamine)-poly(ether) thermosets based on off-stoichiometric amine-epoxy formulations has been prepared and characterized. The first curing stage was a selflimiting click epoxy-amine polycondensation with an excess of epoxides and the second stage was an anionic homopolymerization of the unreacted epoxy groups, initiated by a latent base. The curing process was sequential with storage stable intermediate materials. The latency of these partially-cured intermediate materials was established not only by a thermally activated base generator, but also by the vitrification of the formulations. The intermediate and final materials exhibit a wide range of properties depending on the relative contribution of both curing stages. Intermediate materials can either be shape conformable solids, or liquids that are applicable as adhesives. Fully cured materials exhibit shapememory effect.Postprint (author's final draft

Hard epoxy thermosets obtained via two sequential epoxy-amine condensations

A new family of dual-curable poly(ß-hydroxyamine ether) thermosets based on amine-epoxy formulations has been prepared and characterized. Both curing stages consist in an epoxy-amine polycondensation, each having different reactivity. Whereas the first one is carried out at relatively low temperature using an aliphatic flexible amine, the second is carried out at high temperature with a rigid aromatic amine. The sequentiality of the curing and the stability of the intermediate materials can be established in the basis of the different nucleophilicity of the amines used. The aromatic amine, with a weakly nucleophilic character, is not able to react with epoxides during the first stage at low temperature. The intermediate and final materials exhibit a wide range of properties, thanks to the different glass transition of both epoxy-amine networks, and can be tuned by changing the relative proportion of both diamines in the formulation. The participation of the epoxy resin in both curing stages ensures the homogeneity of the prepared materials, rendering them excellent candidates for applications such as adhesives or smart materials.Postprint (author's final draft

OF THE REQUIREMENTS FOR THE DEGREE Doctor per la Universistat Politècnica de Catalunya ADDRESS PREDICTION AND RECOVERY MECHANISMS

Mitigating the effect of the large latency of load instructions is one of challenges of micro-processor designers. This thesis analyses one of the alternatives for tackling this problem: address prediction and speculative execution. Several authors have noticed that the effective addresses computed by the load instructions are quite predictable. First of all, we study why this predictability appears; our study tries to detect the high-level language structures that are compiled into predictable load instructions. We also analyse the conventional address predictors in order to determine which address predictors are most appropriate for the typical applications. Our study continues by proposing address predictors that use their storage structures more efficiently. Address predictors track history information of the load instructions; however, the requirements of the predictable instructions are different from the requirements of the unpredictable instructions. We then propose an organization of the prediction tables considering the existence of both kinds of instructions. We also show that there is a certain degree of redundancy in the prediction tables of the address predictors. We propose organizing th