Synthesis of thermally degradable epoxy adhesives

Fast-reprocessing, Post-adjustable, Self-healing Covalent Adaptable Networks with Schiff base and Diels-Alder Adduct

Covalent adaptable networks (CANs) are a new type of polymers which possess excellent performance of thermosets and reprocessability of thermoplastics. Nevertheless, it is still a challenge to realize rapid reprocessing and post-adjusting (adjust properties after preparation). Herein, for the first time, a method of combining Schiff base and Diels-Alder adduct in one network was developed to achieve rapid reprocessing and post-adjusting. Through the dissociation of the Diels-Alder adduct at high temperatures, the cross-link densities of the networks were reduced, thereby accelerating the rearrangement of the networks and realizing the rapid reprocessing and self-healing. Moreover, the reconnecting degree of network after dissociation of Diels-Alder adduct could be easily controlled by annealing, as a result, the properties of the obtained CANs were post-adjustable. This work provides a simple and promising approach of achieving excellent reprocessing and post-adjusting for CANs via the synergism of an associative dynamic chemistry with a dissociative dynamic chemistry. This article is protected by copyright. All rights reserved.

Comparative Study of Different Methods of Synthesis and Their Effect on the Thermomechanical Properties of a Halogenated Epoxy-Based Flame-Retardant Resin

The work presented in this paper deals with the comparative synthesis of diglycidyl ether-based tetrabromobisphenol-A(TBBPA) using both conventional and nonconventional methods in order to explore materials for better industrial applications with respect to effective yield, cost, and time consideration. The conventional method involved the polycondensation of TBBPA and epichlorohydrin in the presence of an alkali catalyst. The nonconventional routes adopted for the synthesis of the material involved ultrasonication, microwave irradiation, and UV light exposure. The Fourier transform infrared spectroscopy spectra of all the synthesized materials of the resin were found to be identical, and the X-ray diffraction analysis showed the material as amorphous. The mechanical studies of the resins revealed that all these resins synthesized by different methods are strong and possess high viscosity. Based on the overall thermal, rheological, and excellent hydrophobic properties, it can serve as an excellent flame retardant.

Stimuli-responsive temporary adhesives: enabling debonding on demand through strategic molecular design

Stimuli-responsive temporary adhesives constitute a rapidly developing class of materials defined by the modulation of adhesion upon exposure to an external stimulus or stimuli. Engineering these materials to shift between two characteristic properties, strong adhesion and facile debonding, can be achieved through design strategies that target molecular functionalities. This perspective reviews the recent design and development of these materials, with a focus on the different stimuli that may initiate debonding. These stimuli include UV light, thermal energy, chemical triggers, and other potential triggers, such as mechanical force, sublimation, electromagnetism. The conclusion discusses the fundamental value of systematic investigations of the structure-property relationships within these materials and opportunities for unlocking novel functionalities in future versions of adhesives.

Simultaneous Recovery of Matrix and Fiber in Carbon Reinforced Composites through a Diels-Alder Solvolysis Process

Efficient and comprehensive recycling of fiber-reinforced thermosets is particularly challenging, since the irreversible degradation of the matrix component is necessary in order to separate the fiber component in high purity. In this work, a new approach to fully recyclable thermoset composites is presented, based on the thermal reversibility of an epoxy-based polymer network, crosslinked through Diels-Alder (DA) chemistry. Carbon fiber composites, fabricated by compression molding, were efficiently recycled through a simple solvolysis procedure in common solvents, under mild conditions, with no catalysts. Specifically, the purity of reclaimed fibers, assessed by thermogravimetric analysis and scanning electron microscopy, was very high (>95%) and allowed successful reprocessing into second generation composites. Moreover, the dissolved matrix residues were directly employed to prepare smart, thermally healable coatings. Overall, DA chemistry has been shown to provide a convenient strategy towards circular economy of thermoset composites.

A High-Lignin-Content, Removable, and Glycol-Assisted Repairable Coating Based on Dynamic Covalent Bonds

Conventional thermoset coatings cannot be easily repaired and removed owing to their highly crosslinked structure. The investigation of repairable or removable coatings has been receiving extensive attention, but few reported coatings possess both features. In this work, a repairable and removable coating was developed through the curing of a modified Kraft lignin (L-COOH) with poly(ethylene glycol) diglycidyl ether (PEG-epoxy) in the presence of zinc catalyst. The L-COOH was prepared by functionalization of Kraft lignin with carboxylic acid groups. The cured material had a high lignin content (>47 wt %). At elevated temperatures (>140 °C), dynamic transesterification in the cured network was activated, which resulted in fast stress relaxation and imparted excellent repairability. If the vitrimer system was used as a coating for tin plates, it provided adequate hardness and adhesion properties. In addition, the lignin-PEG coating could be easily removed from the tin plate by using a mild (0.01-0.1 m) NaOH aqueous solution owing to the unique swelling ability of the coating in alkaline aqueous solution. With the assistance of ethylene glycol, this coating could achieve stress-free repairability in 15 min. This work demonstrates the first lignin-based repair- and removable epoxy coating based on vitrimer chemistry.

Fluoride degradable and thermally debondable polyurethane based adhesive

A polyurethane-based adhesive has been produced that can undergo multiple thermal debonding/bonding cycles and also irreversibly debond through depolymerisation on contact with fluoride ions.

Healable capacitive touch screen sensors based on transparent composite electrodes comprising silver nanowires and a furan/maleimide diels-alder cycloaddition polymer

A healable transparent capacitive touch screen sensor has been fabricated based on a healable silver nanowire-polymer composite electrode. The composite electrode features a layer of silver nanowire percolation network embedded into the surface layer of a polymer substrate comprising an ultrathin soldering polymer layer to confine the nanowires to the surface of a healable Diels-Alder cycloaddition copolymer and to attain low contact resistance between the nanowires. The composite electrode has a figure-of-merit sheet resistance of 18 Ω/sq with 80% transmittance at 550 nm. A surface crack cut on the conductive surface with 18 Ω is healed by heating at 100 °C, and the sheet resistance recovers to 21 Ω in 6 min. A healable touch screen sensor with an array of 8×8 capacitive sensing points is prepared by stacking two composite films patterned with 8 rows and 8 columns of coupling electrodes at 90° angle. After deliberate damage, the coupling electrodes recover touch sensing function upon heating at 80 °C for 30 s. A capacitive touch screen based on Arduino is demonstrated capable of performing quick recovery from malfunction caused by a razor blade cutting. After four cycles of cutting and healing, the sensor array remains functional.