Master Thesis - Environmentally-friendly processing of thermosets by two-stage sequential aza-Michael addition and free-radical polymerization of amine-acrylate mixtures
Creators: name: Gonzalez, Gustavo
Publisher: name: Zenodo
Year: 2015
Subjects: Photopolymerization
Amine
Acrylate
Thermosets
Dual-curing

description: This project aims to present a novel method to prepare and characterize thermosets based on aza-Michael and radical photopolymerization dual curing of off-stoichiometric amine-acrylate formulations with custom-tailored properties after the first curing stage and of the final material. In the first stage, amino and acrylate groups react through an aza-Michael mechanism to form a partially reacted material. This reactive process proceeds at room temperature and may be catalyzed by the tertiary amines contained in the reagents or formed by the reaction of a primary amine or secondary amine with two or one equivalents of the acceptor respectively. In the second stage, the photoinitiated free radical polymerization of the excess of acrylic groups embedded within the material formed in the first stage results in a crosslinked network. The tertiary amines present in the reaction medium may also act as oxygen scavengers or as co-initiator when a type II photoinitiator is used during the second curing stage. To perform the tests, mixtures of acrylates of different functionality and structure, a simple or hyperbranched amine, and a type I photoinitiator have been used as reference materials. Methacrylates, a type II photoinitiator, and a thermal free-radical initiator have also been tested for comparison purposes. The kinetics of aza-Michael addition and the conversion reached at the end of the two stages have been studied by FTIR spectroscopy and the gelation by thermomechanical analysis (TMA). The materials obtained have been characterized by calorimetry differential scanning (DSC) and dynamomechanical analysis (DMA).<br> The methodology used in this thesis project is simple, versatile, environmentally friendly and makes it possible to fine-tune the properties of both the intermediate and final materials. By controlling the stoichiometric amine/acrylate ratio in the formulation and the amine functionality, a broad range of glass transition temperatures can be achieved, from -38ºC to 40ºC in the intermediate stage and from -5ºC to 150ºC at the end of the second curing stage. Likewise, ungelled or gelled materials can be obtained after the first curing stage. All materials are stable after aza-Michael addition and can be safely stored before further application in a second step.<br> The first chapter exposes the basic principles of cross-linked network formation, especially when free-radical polymerization is involved. Moreover, the addition of a nucleophilic to an α,β-unsaturated carbonyl compound (known as Michael reaction) and the use of hyperbranched polymers containing amines as a nitrogen donor in the Michael addition is described. The second chapter presents the materials used in this project, such as acrylates and methacrylates reactive monomers, reagent-containing amines, and the catalytic systems (initiators) as the different methods of characterization utilized and sample preparation description.<br> The third chapter is completely dedicated to the research results and discussion. First, the effect of the amine (multifunctional or difunctional) stoichiometric proportion variation on a mixture of diacrylates was analyzed to obtain thermoset by dual curing aza-Michael reaction/free radical photopolymerization. Subsequently, it was evaluated the influence of acrylate arrangement on the structure-property relationships of poly(amino ester)-poly(acrylate)s thermosets obtained by the same process of dual curing. Then, the use of methacrylate monomers was studied when this was introduced (in combination with acrylate monomers) in the preparation of formulations. Finally, it was analyzed and compared the use of type II photoinitiators on mixtures of diacrylate/multifunctional amine with type I photoinitiators and the use of thermal initiators as a solution to arrive at the areas in which the UV radiation cannot reach.