Phenomenological characterization of sequential dual-curing of off-stoichiometric "thiol-epoxy" systems: Towards applicability

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.Postprint (author's final draft

Rheological and mechanical characterization of dual-curing thiol-acrylate-epoxy thermosets for advanced applications

Mechanical and rheological properties of novel dual-curing system based on sequential thiol-acrylate and thiol-epoxy reactions are studied with the aim of addressing the obtained materials to suitable advanced applications. The crosslinking process is studied by rheological analysis in order to determine conversion at gelation and the critical ratio. These parameters are used to discuss the intermediate material structure for each acrylate proportion and their possible application in the context of dual-curing and multi-step processing scenarios. Results from dynamo-mechanical analysis and mechanical testing demonstrate the high versatility materials under investigation and revealed a wide range of achievable final properties by simply varying the proportion between acrylate and thiol group. The intermediate stability between curing stages has been analysed in terms of their thermal and mechanical properties, showing that these materials can be stored at different temperatures for a relevant amount of time without experiencing significant effects on the processability. Experimental tests were made to visually demonstrate the versatility of these materials. Qualitative tests on the obtained materials confirm the possibility of obtaining complex shaped samples and highlight interesting shape-memory and adhesive properties.Postprint (published version

Preparation of poly(thiourethane) thermosets by controlled thiol-isocyanate click reaction using a latent organocatalyst

Different poly(thiourethane) thermosets were prepared by means of a thiol-isocyanate click reaction starting from three diisocyanates with different structure (isophorone diisocyanate, IPDI, 4,4'-methylene bis(cyclohexy isocyanate), HMDI and hexamethylene diisocyanate, HDI, and a tetrathiol, pentaerythritol tetrakis(3-mercap- topropionate), PETMP. The curing process has been catalyzed by basic and acid catalysts. The use of a thermally activated base generator (1-methylimidazolium tetraphenylborate, BG1MI), which is an organocatalyst, allowed a better control on the curing initiation than those traditionally used dibutyltin dilaurate (DBTDL) or tertiary amines. The curing evolution was investigated by DSC and FTIR spectroscopy. The materials obtained were characterized by thermomechanical and mechanical tests. Homogeneous ma- terials were obtained in all cases. The chemical structure of the network was correlated with the thermal and mechanical data determined. T g values in the range of 75 to 150 °C were obtained. The thermal degradation of these materials has also been investigated and a complex degradation mechanism, with three different steps was observed in all cases. These materials showed a higher thermal stability than their oxygen counterparts did. The thermosets obtained using the latent organocatalyst have similar characteristics to the ones obtained by using the common DBTDLPostprint (author's final draft

Recyclable poly(thiourethane) vitrimers with high Tg. Influence of the isocyanate structure

Networked poly(thiourethane) materials with Tgs around 130 °C, derived from two aliphatic isocyanates (isophorone diisocyanate, IPDI and 4,4'-methylene bis(cyclohexyl isocyanate), HMDI) and one aromatic diisocyanate (toluene-2,4-diisocyanate, TDI) have been prepared with the same trithiol as comonomer (trimethylol propane tris(3-mercaptopropionate), S3) in stoichiometric proportions in the presence of dibutyltin dilaurate (DBTDL) as the catalyst. The higher reactivity of TDI allowed the preparation of this material in absence of catalyst. The evolution of the curing process has been followed by FTIR. Thermomechanical studies have been performed to determine their viscoelastic properties and their vitrimeric behaviour. The materials were able to reach a complete relaxation stress state thanks to the exchange process of the thiourethane moiety. Among them, TDI derived material experimented the fastest relaxation. The materials were also characterized by thermogravimetry and tensile tests. The recycled materials obtained by grinding the original thermosets and hot-pressing the powder have been fully characterized by mechanical, thermomechanical and FTIR studies, which allowed to confirm their recyclability without appreciable changes in the network structure. The presence of DBTDL in the materials has been proved to be necessary to reach a good recyclability.Postprint (author's final draft

Actuator behaviour of tailored poly(Thiourethane) shape memory thermosets

n this work, a new family of poly(thiourethane) shape memory thermosetting actuators was developed and characterized. These materials can be easily prepared from mixtures of two different aliphatic diisocyanates and a trithiol in the presence of a latent catalyst, allowing an easy manipulation of the formulation. Rheological studies of the curing process confirm the latent character of the formulations. The glass transition temperatures and the mechanical properties can be modified by varying the proportion of diisocyanates (hexamethylene diisocyanate, HDI, and isophorone diisocyanate, IPDI) with stoichiometric amounts of trimethylolpropane tris(3-mercaptopropionate). The shape-memory behavior was deeply investigated under three different conditions: unconstrained, partially constrained, and fully constrained. Tests were performed in single cantilever bending mode to simulate conditions closer to real complex mechanics of thermomechanical actuators under flexural performances. The complex recovery process in single cantilever bending mode was compared with that obtained using tensile mode. The results evidenced that the amount of recovery force in fully constrained conditions, or energy released during the recovery process in partially constrained, can be modulated by simply changing the proportion of both diisocyanates. A simple model based on Timoshenko beam theory was used for the prediction of the amount of work performed. The reported results are an important guideline to design shape-memory materials based on poly(thiourethane) networks, establishing criteria for the choice of the material depending on the expected applicationPeer ReviewedPostprint (author's final draft

Electro-responsive shape-memory composites obtained via dual-curing processing

In this work, electro-responsive shape-memory actuators were developed by incorporating a conductive heater in a dual-curing thiol-acrylate-epoxy shape-memory polymer (SMP). A conductive heater, consisting of an electrically conductive silverink track printed on Kapton® substrate, was assembled to the SMP, taking advantage of the dual-curing processing. The shape-memory effect (SME) was activated by the heat dissipated by the Joule effect in the conductive track. Boron nitride agglomerates were dispersed in the thiol-acrylate-epoxy layers to increase thermal conductivity and achieve faster shape-recovery. A thermoelectric control unit was developed to control the shape recovery of the electro-responsive actuators and provide different activation strategies. The electrically activated SME was investigated and compared to a traditional SME based on an external heating source given by the dynamic mechanical analyzer (DMA) apparatus. Electro-responsive actuators were found extremely faster than the conventional SMPs based on external heating. The fastest recovery was obtained by the 15% boron nitride actuator, which recovered the 100% of the original shape in only 8 s. The thermoelectric controlling device provided an optimal control of the shape recovery speed based on the pulse width modulation of the heating current under the application of a low voltage (5 V).Peer ReviewedPostprint (published version

Characterization of sequential dual-curing of thiol-acrylate-epoxy systems with controlled thermal properties

In this work, a novel sequential dual-curing system based on thiol-acrylate-epoxy formulations is developed and characterized. Both reaction stages are thermally activated and sequentially relies on difference in the kinetics of the reactions involved. The viability of this system is successfully tested for low curing temperatures and the kinetic of the entire process is completely characterized. Differential scanning calorimetry and Fourier transform infrared spectroscopy analysis revealed an adequate separation between the reactions that leads to a wide interval of time during which the intermediate material can be considered stable. Applicability tests on the resulting thermosets showed the high capability of the intermediate materials to be conformed in complex shapes that can be fixed by means of the second curing stage. This system also shows great potential to be used as adhesive bonding taking advantage of the two-step curing process.Postprint (author's final draft

Sequential photo-thermal curing of (meth)acrylate-epoxy thiol formulations

In this work, a novel dual-curing procedure has been developed. It is a combination of a first radical UV-initiated thiol-(meth)acrylate reaction, followed by a second thermal thiol-epoxy step catalysed by a base. Since (meth)acrylates can lead to homopolymerization by radical mechanism, the amount of thiol has to be optimized to reach cross-linked materials with Tgs above room temperature and good mechanical performance. It should be considered, that if the amount of thiol in the intermediate materials is too low, epoxy homopolymerization can take place during the second step. The use of glycidyl methacrylate in combination with trifunctional meth (acrylates) allows this system to gel in the 1st stage and avoids possible dripping or exudation of free monomers during the storage of the intermediate materials. Moreover, this compound reacts in both stages acting as a covalent coupling between (meth)acrylates and epoxy networks. We selected trimethylolpropane tris(3-mercaptopropionate) as the thiol, 2,2-dimethoxy-2-phenylacetophenone as UV initiator and 1-methylimidazole as the base catalyst. The curing evolution was studied by DSC and FTIR. All the materials obtained were characterized by thermogravimetry, thermomechanical analysis and tensile tests.Peer ReviewedPostprint (published version

Preparation of New Vitrimeric Materials Based on Glycidyl Vanillin-imines

The current exponential growth of the human population is associated with a degradation of the environment. In terms of lack of degradability and persistence in the environment, increasing attention is being paid to polymers. Among them, thermosets, that become hardened in the curing, lead to infusible networks with high strength and solvent resistance and superior mechanical and thermal properties. On the downside, their use raises a severe environmental concern because the covalent linkages in the network are irreversible, preventing recycling. An alternative that overcomes this issue is the use of dynamic covalent chemistry, which relies on chemical processes causing reversible formation and breaking of covalent bonding in response to stimuli and allows obtaining easily recyclable thermosets while retaining the overall polymer structure. Incorporating reversible groups like imines in the network structure can allow the reprocessing and recycling of these materials, maintaining their mechanical performance. The associative mechanism of the imine interchange leads to these materials being included in the group of vitrimers. In our work, the inclusion of the imine groups in the network is based on the use of a diglycidyl diimine obtained from vanillin. This renewable product is condensed with 4,4’-oxydianiline to form the bisphenol-diimine, and then it is reacted with epichlorohydrin to obtain the epoxy-imine monomer with the following structure.Peer ReviewedObjectius de Desenvolupament Sostenible::13 - Acció per al ClimaPostprint (published version

Synthesis and characterization of new fully bio-based poly(acylhydrazone) vanillin vitrimers

In the present century the need to recycle thermosets to reach a sustainable environment has led a huge number of researchers to develop new reshapable networked polymers. In this scenario, vitrimers, materials able to change their topology via exchange of the covalent bonds, usually activated by thermal stimulus, are attracting growing interest. They can potentially be reshaped, self-welded, repaired and recycled at high temperature without appreciable degradation [1]. These materials behave as thermosets at room temperature but becomes malleable at high temperature when the exchange reaction occursPeer ReviewedPostprint (published version