Synthesis and characterization of lignin-carbonate prepolymers as formaldehyde-free wood adhesives

As most conventional wood adhesives are still based on petrochemicals and harmful formaldehyde, there is an increasing demand for the development of sustainable alternatives. Due to its chemical functionalities and polymeric properties, lignin is a suitable feedstock for bio-based polymers and adhesives. However, it shows relatively low reactivity due to its complex and sterically unfavorable structure. For this reason, functionalization reactions prior to use are necessary. In this work, a lignin functionalization approach with a cyclic organic carbonate compound is demonstrated. In a first step, a bifunctional cyclic carbonate “BCC” was synthesized via CO 2 insertion into poly(ethylene glycol) diglycidyl ether (PEGDGE). This BCC was used as a crosslinking agent for kraft lignin to produce partially crosslinked prepolymers. The development of the chemical structure of the products over the synthesis time was investigated by FTIR, SEC, 13 C NMR and 31 P NMR. The results show successful crosslinking via new bonds formed by reaction of the cyclic carbonate groups in the BCC with the OH groups in the lignin, resulting in increasing average molecular weight. Furthermore, the potential of the cyclocarbonated lignin prepolymers as formaldehyde free adhesive was demonstrated with Automated Bonding Evaluation System (ABES) tests. Tensile shear strength values up to 6.3 N·m −2 were achieved at 180 °C and a pressing time of 30 min. ● PEGDGE was converted into a bifunctional cyclic carbonate “BCC” via CO 2 insertion. ● BCC and softwood kraft lignin react to form a succesfully crosslinked prepolymer. ● First in-depth characterization of lignin-carbonate prepolymer synthesis was conducted. ● Lignin-carbonate prepolymers were successfully tested as formaldehyde-free adhesives.