Organic solvents-free and ambient-pressure drying melamine formaldehyde resin aerogels with homogeneous structures, outstanding mechanical strength and flame retardancy.

Atmospheric drying method for fabricating aerogels is considered the most promising way for casting aerogels on a large scale. However, the organic solvent exchange, remaining environmental pollution risk, is a crucial step in mitigating the impact of surface tension during the atmospheric drying process, especially for wet gel formed through the alkoxy-derived sol-gel process, such as melamine-formaldehyde resin (MF) aerogel. Herein, a tough polymer-assisted in situ polymerization was proposed to fabricate MF resin aerogel with a combination of mechanical toughness and strength, enabling it to withstand the capillary force during water evaporation. The monolithic MF resin aerogel through the sol-gel method can be directly prepared without additional network strengthening or organic solvent exchange. The resulting MF resin aerogel exhibits a homogeneous as well as hierarchical structure with macropores and mesopores (~6 µm and ~5 nm), high compressive modulus of 31.8 MPa, self-extinguishing property, and high-temperature thermal insulation with 97 % heat decrease for butane flame combustion. This work presents a straightforward and environmentally friendly method for fabricating MF resin aerogels with nanostructures and excellent performance in open conditions, exhibiting various applications.

Multicycling of Epoxy Thermoset Through a Two-Step Strategy of Alcoholysis and Hydrolysis using a Self-Separating Catalysis System.

Plastic has now become a contradiction between civilization and pollution that human society has to resolve. The recycling of thermosetting plastics in waste plastics is a huge challenge since they are difficult to remold like thermoplastic plastics due to their high crosslinking density. Here, a new strategy was developed to achieve multicycling of anhydride-cured epoxy thermosets. The process consisted of mild and high-efficiency alcoholysis catalyzed by potassium phosphate/low-boiling alcohol system, and subsequent fast hydrolysis to obtain degradation products rich of carboxyl groups. The degradation products were reused as curing agent to prepare new anhydride-cured epoxy thermosets without sacrifice of high strength and stability. Moreover, the new epoxy thermosets could still be repeatedly recycled using the same protocol. The insolubility of potassium phosphate in ethanol at room temperature made the separation and reuse of the catalyst more convenient. The use of low-boiling alcohol not only allowed high-efficiency degradation but also enabled easy separation from the degradation products. The excellent degradation performance was attributed to the improved swelling of the thermoset and the increased solubility of potassium phosphate induced by small amounts of water in the alcohol. This research provides a recycling method that can reintegrate thermoset waste plastics into remodeling ones under the background of circular economy.

Recycling waste thermosetting unsaturated polyester resins into oligomers for preparing amphiphilic aerogels.

The styrene-maleic acid copolymer (SMC) was obtained by selective and complete cleavage of ester groups from waste thermosetting unsaturated polyester resins (WTUPR). The degradation was performed in glycol at 180 °C for 5 h with potassium carbonate as a catalyst and the resultant potassium salt of SMC (SMC-K) can be very easily separated by precipitation using ethanol with a yield of 63.8%. The SMC-K was integrated with polyvinyl alcohol to form amphiphilic aerogels via freeze-thaw and freeze-drying processes. The aerogel exhibits a low density of 0.024 g·mL-1 due to hierarchical pore structures with a size range from nanometer to micrometer scale. Besides, the good compressibility and resilience of the aerogel are demonstrated. The amphiphilic aerogel displayed high absorption of both water and oily liquids (over 30 g.g-1 and 20 g.g-1 for water and dichloromethane respectively), together with a good recycle adsorption efficiency (>90% after 10 cycles). This work provides a new strategy on upcycling of WTUPR.

Fast microwave-assisted hydrolysis of unsaturated polyester resin into column packing for rapid purifying of dye wastewater.

Microwave-assisted selective degradation successfully converts thermosetting unsaturated polyester resins into a low-swelling (below 10 g g-1) gel material (GM) with a high yield (58-65%) in water at 100°C for only 1 h. The obtained GM possesses rough and porous structure while the content of carboxylate group obtained by cleavage of partial ester groups is more than 10%, varying with the concentration of the catalyst. It is suitable for use as packing of adsorption column to rapidly purify sewage. Super high filtering rates of 18582-27002 L h-1 m-3 without external pressure and high removal efficiency of more than 99.8% were achieved, promoting practical application for rapid removal of organic pollutants.

Autofluorescence of hydrogels without a fluorophore.

Fluorescent hydrogels have recently attracted great attention for medical diagnostics, bioimaging and environmental monitoring. However, additional phosphors or fluorophores are always required to label the hydrogels, and they suffer from marker bleaching, signal drifts, or information misrepresentation. Here we report autofluorescence that universally exists in carbonyl-containing hydrogels without any traditional fluorophore. The fluorescence is successfully employed to self-monitor the gelation process since the fluorescence signal is closely related to the internal structural change of the gels. The crosslinked structure is beneficial to the fluorescence efficiency. Specifically, the fluorescence intensity is amplified with decreasing water content of the gels. The system realizes aggregation-induced emission in a water-deficient environment. The fluorescence is quenched by the addition of some specific metal ions, which can realize the successfully erasure and rewriting of information under visible light and ultraviolet light respectively. We believe that the spontaneous fluorescence of a gel provides the most reliable basis for the detection of a gel structure and opens new prospects in the application of hydrogels.