Double crosslinking strategy to achieve high-strength, wide pH range, repeatable adhesion of underwater soybean oil-based polyurethane adhesive for detecting crack expansion in bonded joints

Abstract

Due to increasing economic and environmental constraints, the development of renewable-source polymers as alternatives to conventional petroleum-based polymers has become a critical challenge in materials science. Vegetable oils, particularly soybean oil (SO), have emerged as promising renewable resources owing to their abundance, cost-effectiveness, and ease of chemical modification. In this study, a novel polyurethane was synthesized through the combination of SO, diethanolamine (DEA), dicyclohexylmethane 4,4′-diisocyanate (HMDI), and bis(2-hydroxyethyl) disulfide (HEDS). The incorporation of HEDS as a cross-linking agent facilitated the formation of a cross-linked network structure during the chemical cross-linking pre-polymerization reaction of the polyurethane main chain. The resulting polyurethane adhesive demonstrated remarkable rebonding capability, maintaining good bond strength through up to nine rebonding cycles, and exhibited exceptional shear strength of 7 MPa. Furthermore, the adhesive-coated substrates maintained substantial shear strength after 24-h immersion in various pH solutions, indicating excellent chemical stability. The integration of carbon nanotubes (CNTs) with the block copolymer using ultrasonic cell disruption techniques enhanced the material’s electrical conductivity, thereby expanding its potential applications in non-destructive testing of adhesive sizing processes, particularly in aerospace applications. This innovative material offers a supplementary approach to the existing methodologies for detecting adhesive sizing conformance, potentially enhancing quality control in advanced manufacturing processes. The development of this soybean oil-based polyurethane represents a significant advancement in sustainable adhesive technology, offering both environmental benefits and superior mechanical properties compared to traditional petroleum-based adhesives. The incorporation of CNTs not only improves electrical conductivity but also potentially enhances the mechanical strength and thermal stability of the composite material, making it suitable for demanding industrial applications.

The polyurethane adhesive presented in this paper is an adhesive prepared from soybean oil by ammonolysis, esterification and other steps, and heat curing, and the self-repairing and repeat bonding properties of the adhesive are improved by changing the type of chain extender. This bi-dynamic polyurethane adhesive has good water and acid resistance and can be recycled. Shear strength of up to 7 Mpa allows this adhesive to be used in extreme environments for glues, such as humid, acidic and alkaline environments. The carbon nanotube composite adhesive also showed good adhesive and conductive properties, and two methods for testing the conformity of the adhesive sizing process (i.e., non-destructive testing) are presented. In conclusion, this study provides a reference for the development of soybean oil-derived multifunctional adhesives, which is of great significance in promoting the practical application of novel bio-based adhesives.