Accelerating the Curing of Hybrid Poly(Hydroxy Urethane)-Epoxy Adhesives by the Thiol-Epoxy Chemistry

Abstract

The polyaddition between dicyclic carbonates and diamines leading to poly(hydroxy urethane)s (PHUs) has emerged as the preferred method for the synthesis of green, non-isocyanate polyurethanes. However, when proposed for use as structural adhesives, the long times for completion of aminolysis of the 5membered cyclic carbonates under ambient conditions force the use of complementary chemistries to accelerate the curing process. In this work, a system that combines an amino-terminated PHU (NH 2 −PHU−NH 2 ), an epoxy resin, and a thiol compound was employed to develop high-shear strength PHU-epoxy hybrid adhesives able to cure at room temperature in short times. A NH 2 −PHU−NH 2 prepolymer synthesized by using a substoichiometric quantity of dicyclic carbonates was mixed with a bisphenol A-based epoxy resin for the preparation of the structural adhesive. While this adhesive showed good lap-shear strength and shear resistance under static load and temperature, the curing process was slow. In order to speed up the curing process, a thiol (trimethylolpropane tris(3-mercapto propionate)) was added and its impact on the curing process as well as on the adhesive properties was evaluated. The trifunctional thiol additive allowed for faster curing in the presence of the 1,1,3,3-tetramethylguanidine basic catalyst. Moreover, a combination of NH 2 −PHU−NH 2 and the thiol as curing agents for the epoxy resin resulted in adhesives with superior toughness, without any deterioration of the ultimate lap-shear strength or shear resistance under load and temperature, making these adhesives suitable for high-demand applications in the automotive industry.

Recyclable bio‐based epoxy resins containing hybrid cross‐linking networks

“…The test results (Figure 4C) showed that the tensile strength of the samples after remolding once was 80 MPa, which was slightly lower than that of the original specimen (89 MPa). Excitingly, the tensile strength of the samples could still maintain 60 MPa after remolding twice, which is higher than the existing reported tensile strength of thiol‐epoxy thermoset materials 19,25–30 . This indicates that the EIA‐based epoxy thermoset material has excellent processability properties.…”

“…The glass transition temperature ( T g ) was determined by the peak tan δ of the DMA test, as shown in Figure 3B. The T g increased continuously with increasing BDB values and was as high as 105.2°C for EB7T3, which is higher than that reported by other researchers for thiol‐epoxy thermosets 19,25–30 . Compared with the existing reported literature, the EIA‐based epoxy thermosets with hybrid cross‐linked network structure had the highest tensile strength while maintaining high T g (Figure 3C).…”

“…The T g increased continuously with increasing BDB values and was as high as 105.2 C for EB7T3, which is higher than that reported by other researchers for thiol-epoxy thermosets. 19,[25][26][27][28][29][30] Compared with the existing reported literature, the EIA-based epoxy thermosets with hybrid cross-linked network structure had the highest tensile strength while maintaining high T g (Figure 3C). The tensile strength of most thiol-epoxy thermosets in previous studies was about 40 MPa, while the EIA-based epoxy thermosets had a tensile strength of up to 105.9 MPa, which was more than twofold higher.…”

“…Excitingly, the tensile strength of the samples could still maintain 60 MPa after remolding twice, which is higher than the existing reported tensile strength of thiol-epoxy thermoset materials. 19,[25][26][27][28][29][30] This indicates that the EIA-based epoxy thermoset material has excellent processability properties.…”

Evaluation of the adhesive properties of vanillin‐derived polyhydroxy urethanes

“… 14 The cohesive force in the PHUs was found to be dominant over the adhesive strength between the PHU and aluminum layer. Even though a few articles explored the applicability of PHUs as adhesives, the dicarbonates utilized for synthesizing these PHUs mainly depend on non‐renewable material and often involve further modifications or crosslinking to achieve better adhesive strength 46–51 …”