Resumen de Dual-curing thiol-acrylate-epoxy thermosets for functional applications
Nowadays, more and more applications demand materials with complex shape designs which is a difficult task when working with thermosetting polymers. The curing or fabrication of thermosets is a complex process involving drastic physical changes, from a liquid-like mixture to rigid covalently crosslinked networks. This makes difficult to control the processing of complex shapes. In addition, the final shape cannot be processed anymore due to the presence of crosslinks within the network. The rise of 3D printing procedure has provided a viable way to cope with thermosets conformability issues but its characterized by a long processing time and low productivity.
In this context, dual-curing processing has emerged as an extremely valuable tool for the design of thermosets. It consists in the combination of two independent crosslinking processes that can be carried out both sequentially and simultaneously. The benefits of dual-curing processing rely on its processing flexibility and the possibility of enhancing the properties of cured parts in single- or multi-stage processing scenarios. In particular, well-separated sequential systems are characterized by an intermediate stable material which can be exploited in two-stage applications and to obtain complex shaped final thermosets without using complex processing techniques. Control of the curing sequence can be obtained combining two curing reaction triggered by different stimuli (i.e., heat and light) or by kinetic control of the reactions. Once a suitable combination of polymerization reactions has been established, a family of thermosets with different intermediate and final properties can be obtained by selecting monomer structure and functionality and altering the monomer feed ratio.
In view of the increasing needs of easy and versatile curing process, we developed a novel dual-curing system based on the combination of two “click” reactions: the thiol-acrylate Michael addition (first stage) and thiol-epoxy reaction (second stage). Dualcuring systems combining these two reactions were recently reported in the literature, but the reaction was activated with UVlight and the curing mechanism and sequence were not studied in detail. The novelty of our work is represented by the thermal activation of both reactions using a single catalyst, and the elucidation of the curing sequence and the conditions that make it possible to control it in multi-stage processing scenarios. Furthermore, the developed system was oriented towards the development of functional applications by adjusting the network structure to the final task.
To begin with, the dual-curing chemical mechanism was comprehensively studied and a curing procedure ensuring a good separation between two process was developed. Both the thiol-acrylate and thiol-epoxy reactions can take place under similar temperature conditions. Nevertheless, a safe and robust control of the curing sequence was achieved by choosing a suitable e temperature for the first curing process and taking into consideration the different intrinsic reactivity and the temperature dependence of the two reactions. A wide range of properties was attainable with this dual-curing system just by modifying the proportion between the thiol-acrylate and thiol-epoxy bonds in the network. Furthermore, the intermediate materials were found to present a large stability time window during which they can be safely stored or processed.
Once the dual-curing system was characterized we proceeded to study possible advanced applications. First of all, we characterized the shape-memory properties of thermosets obtained introducing new components to make network structural changes addressed to the enhancement of the shape-memory effect (SME). We were able to develop shape-memory thermosets with optimal SME in all recovery conditions. Due to their remarkable behaviour in partially and fully constrained recovery conditions we propose their possible application as mechanical actuators.
In addition, we tested the suitability of the dual-curing to the preparation of two-stage adhesives taking advantage of the system developed. Adhesive strength of single-lap joints (SLJs), obtained using both liquid-like and solid-like intermediate materials, were determined. Dual-curing processing was proved to have high potentiality in adhesive bonding applications, in particular with solid-like intermediate materials. An accurate control of the bondline thickness was achieved, while liquid-like intermediate led to stronger joints with respect to the joint obtained with the direct curing of the same reactive system. A beneficial effect of the incorporation of boron nitride (BN) fillers on adhesion was also evidenced.
Finally, thiol-acrylate-epoxy dual-curing system was used to develop electroresponsive shape-memory polymers. The capability to recover the shape under an electrical stimulus was studied by combining the thermal SME of these materials with an electrically conductive layer incorporated into them. When a voltage is applied, the heat released by Joule effect in the conductive layer increases the temperature and activates the shape-memory response. In this case BN fillers were used to enhance the thermal conductivity of the shape-memory thermoset in order to obtain faster recovery process. Unconstrained recovery experiments based on direct heating in oven and internal joule heating were performed. The electro activation of the SME resulted in significantly faster recovery and, using a custom-made thermoelectric control, an accurate control of the recovery process was achieved.
In the final part of the investigation, we proposed the application of the developed electroresponsive design to a free-standing actuator design. It consists in the addition of a shape-changing layer that can be provide a bi-directional motion to the actuator. In this way, a reversible motion activated by on/off cycles of the electric stimulus can be obtained, leading to a significant step towards an efficient and autonomous operation of this actuators.
