Unexpected differences between thermal and photoinitiated cationic curing of a diglycidyl ether of bisphenol A modified with a multiarm star poly(styrene)-b-poly(ε-caprolactone) polymer

The effect of adding a multiarm star poly(styrene)-b-poly(ε-caprolactone) polymer on the cationic thermal and photoinitiated curing of diglycidyl ether of bisphenol A was studied. This star-polymer decelerated the thermal curing of diglycidyl ether of bisphenol A and modified the final structure of the epoxy matrix. The photocuring was influenced significantly by the addition of the multiarm star. When the proportion of this modifier added was 5%, much more time was necessary for complete photocuring (160 min at 40ºC). In the presence of 10% of modifier, the degree of photocuring reached was very low (0.196 at 120°C). A subsequent thermal post-curing was necessary to cure completely the system. During photocuring in presence of poly(styrene)-b-poly(ε-caprolactone), the formation of dormant species, which are reactivated when the temperature increases, takes places. The kinetics of the thermal curing and the photocuring was analyzed using an isoconversional method due to the complexity of the reactive process. Applying this method, it has been confirmed the dependence of activation energy on the degree of conversion. The fracture morphology analyzed by scanning electron microscopy exhibited a second phase originated during photocuring by the presence of the modifier

Dual-curable stereolithography resins for superior thermomechanical properties

Stereolithography (SL) stands out as a relatively fast additive manufacturing method to produce thermoset components with high resolutions. The majority of SL resins consist of acrylate monomers which result in materials with cur-ing-induced shrinkage problems and this, in addition to the incomplete and non-uniform conversions reached in the SL process, results in poor mechanical properties. To address this issue, a dual-curing formulation was developed by mixing an epoxy monomer into a commercial multi-acrylate SL resin: the first curing stage is acrylate free-radical photopolymerization at ambient temperature, and the second curing stage is cationic epoxy homopolymerization at higher temperatures. The fully dual-cured materials are macroscopically homogeneous, with nanoscale domains observed by Atomic Force Mi-croscopy (AFM), and with unimodal tan delta peaks observed in Dynamic Mechanical Analysis (DMA). The uncured material was storage stable at ambient conditions for at least 9 weeks since the epoxy part was virtually unreactive at these temper-atures. With the dual-cured materials, a nearly 10-fold increase in Young’s modulus was achieved over the neat acrylate resin. At the thermal curing stage, the presence of diperoxyketal thermal radical initiator to the liquid formulation facilitated the polymerization of unreacted acrylates that remained from the SL process simultaneously with epoxy homopolymerization and helped the material attain improved properties

Comparative Analysis of Stochastic Network Build-up Methods for the Curing of Epoxy-Anhydride Thermosets

10.1016/j.eurpolymj.2014.01.015The network build-up of epoxy¿anhydride thermosets has been studied using two different stochastic network build-up methods based on the random combination of primary chains or simple fragments. Explicit expressions for relevant statistic averages in the pregel and postgel states have been derived for both methods. The application of both methods to a living polymerization kinetic model leads to strong divergences in the vicinity of the critical conversion at gelation due to growing differences between the real primary chain distribution and the most probable distribution in the fragment model. However, the application of both methods to an initiator regeneration kinetic model leads to identical results because of the distribution of primary chains throughout the whole curing process is a most probable one

New pegylated hyperbranched polyesters as chemical modifier of epoxy resins in UV cationic photocuring

A new hyperbranched polymer (HBPpeg) has been obtained via pegylation of an aromatic hyperbranched polyester. This polymer has been used as modifier of a commercially available cycloaliphatic epoxy resin in its cationic UV photocuring, using arylsulfonium salts as photoinitiator. The addition of the HBP slows down the curing as well as reduces the overall conversion achieved at room temperature, due to its interaction with the photoinitiator, but fully cured materials were achieved for formulations containing up to 10 phr of the HBP. The amphiphilic structure of the HBPpeg allows it to phase-separate in the epoxy matrix, which was confirmed by means of FESEM. The thermomechanical characteristics and the thermal stability have been also studied.Peer ReviewedPostprint (published version

Pollution control for industrial coal-fired boilers

SIGLEAvailable from British Library Document Supply Centre- DSC:4362.6438(IEACR--25) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

A new two-stage curing system: thiol-ene/epoxy homopolymerization using an allyl terminated hyperbranched polyester as reactive modifier

An allyl terminated hyperbranched polyester (HBP) was added to an epoxy formulation containing a trithiol compound to perform a thiol-ene click reaction. By this procedure a flexible thioether network was formed. The photoirradiation of the reactive mixture, which contained a cationic photoinitiator, converted the thioether network in a multifunctional thermal macroinitiator, capable to initiate the cure of the cycloaliphatic epoxy resin(CE) in a second thermal stage. Depending on the proportion of HBP, thermal or photocuring of the epoxy resin took place in different extent, leading to networks with different structures. The photocuring procedure was followed by FTIR and the thermal second stage by DSC. The materials obtained were characterized by DMTA, TGA and SEM. The addition of HBP-Allyl and the trithiol to the formulation allowed increasing the Tg on comparison with the neat epoxy thermoset. The system proposed constitutes a new two-stage dual photo-thermal curing procedure for cycloaliphatic epoxy resins with a thermal latent character.Postprint (published version

Motion control in free-standing shape-memory actuators

In this work, free-standing shape-memory thermally triggered actuators are developed by laminating 'thiol-epoxy'-based glassy thermoset (GT) and stretched liquid-crystalline network (LCN) films. A sequential curing process was used to obtain GTs with tailored thermomechanical properties and network relaxation dynamics, and also to assemble the final actuator. The actuation extent, rate and time were studied by varying the GT and the heating rate in thermo-actuation with an experimental approach. The results demonstrate that it is possible to tailor the actuation rate and time by designing GT materials with a glass transition temperature close to that of the liquid-crystalline-to-isotropic phase transition of the LCN, thus making it possible to couple the two processes. Such coupling is also possible in rapid heating processes even when the glass transition temperature of the GT is clearly lower than the isotropization temperature of the LCN, depending on the network relaxation dynamics of the GT and the presence of thermal gradients within the actuators. Interestingly, varying the GT network relaxation dynamics does not affect the actuation extent. As predicted by the analytical model developed in our previous work, the modulus of the GT layer is mainly responsible for the actuation extent. Finally, to demonstrate the enhanced control of the actuation, specifically designed actuators were assembled in a three-dimensional actuating device able to make complex motions (including 'S-type' bending). This approach makes it possible to engineer advanced functional materials for application in self-adaptable structures and soft robotics

Epoxy/anhydride thermosets modified with end-capped star polymers with poly(ethyleneimine) cores of different molecular weight and poly(ε–caprolactone) arms

Multiarm star polymers, with a hyperbranched poly(ethyleneimine) (PEI) core and poly(ε-caprolactone) (PCL) arms end-capped with acetyl groups were synthesized by ring-opening polymerization of ε-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