Sulfur-modified polyurethane adhesives: Green synthesis process and disassembly-responsive characteristics

Polyurethane adhesives have been widely used in industrial production and daily life for their excellent properties. With the urgent need for social progress, the intelligent responsiveness of polyurethane adhesives has become more and more important. Here, we demonstrate a stimulus-responsive polyurethane adhesive. Starting from the structural design, sulfur (S₈) is added into the polyurethane adhesive for the first time using the inverse vulcanized mechanism, and the controllable disassembly response of the polyurethane adhesive under the condition of thermal stimulation is realized. In addition, the stimulus response of the adhesive can also be activated through the permeation process of methoxide anion (CH₃O⁻), and the rapid disassembly response property is displayed in the methanol solution of CH₃ONa, to realize the recovery of the bonding substrate. Simultaneously, the implementation of the inverse vulcanized mechanism imparts the adhesive with a diminished water absorption swelling rate and heightened cross-link density. On the surface of the metal substrate, it shows excellent bonding properties, with a lap shear adhesion strength reaching 2.8 MPa. It is worth noting that the adhesive also exhibits excellent underwater durability. After 36 h of underwater soaking treatment, its adhesion can still be maintained at 2.4 MPa. This work reveals the mechanism of rapid disassembly of polyurethane adhesives at the molecular level. By density functional theory (DFT) calculation, it is found that the minimum bond energy of the S-S bond in a polyurethane adhesive system is 134 KJ·mol⁻¹, and the disassembly response can be triggered by heating to 53.79℃. At the same time, the mechanism of the disassembly response induced by CH₃O⁻ is further analyzed using an electrostatic potential (ESP) diagram. This work utilizes sulfur (S₈) and castor oil (CO) as raw materials, which can be sourced from industrial or agricultural by-products. It breaks away from traditional polyurethane adhesive preparation methods and offers a "green" strategy that can be synthesized through a solvent-free one-pot process, thus providing a new way of thinking about obtaining sustainable polymers from industrial and agricultural by-products.