Sustainable synthesis of rosin-based epoxy resin and application in paper sizing

The development of high-performance bio-based paper sizing agents is challenging. By chemically modifying the abundant natural resource rosin and using a recyclable deep eutectic solvent (DES) [CoCl 2 ·6 H 2 O][ChCl] 2 as the catalyst to catalyze both the dimerization of rosin and the subsequent epoxidation of rosin dimer, a bio-based epoxy resin sizing agent containing active epoxy groups and a hydrophobic polycyclic structure unit was synthesized. The structure of the epoxy resin was characterized using Fourier transform infrared (FT-IR) and 1 H nuclear magnetic resonance (NMR) spectroscopy. The thermogravimetric (TG) analysis showed that the epoxy resin was stable below 150 °C. Combined with quantitative calculations, the existence of coordination and hydrogen bonds in the DES matrix and the stability of DES catalyst were confirmed. A detailed study was conducted on the various factors influencing the properties of the epoxy resin emulsion. Under the conditions of a composite emulsifier (AEO-9: SLES = 1: 4, mass ratio) dosage of 10 % of the emulsion weight, an oil-water mass ratio of 1:1.25, a temperature of 130 ℃, and a stirring speed of 800 rpm, a highly stable resin emulsion with a high solid content (48.02 wt%) was prepared. The prepared epoxy resin emulsion was used for both surface sizing and internal sizing of the paper, and the sizing properties were evaluated using scanning electron microscopes, water resistance, tensile strength, tear resistance, and folding endurance. Surface sizing mainly relies on the hydrophobic surface covering of the sizing agent. The prepared epoxy resin has excellent film-forming properties, which enhances the water resistance of the paper. Internal sizing is achieved through the formation of covalent bonds between the epoxy groups of the resin and the hydroxyl groups of cellulose, forming a dense network structure, which helps to enhance water resistance. Furthermore, whether for surface sizing or internal sizing, the polycyclic structure units in the epoxy resin further enhance the water resistance and mechanical properties of the paper. By applying surface sizing and internal sizing, the water resistance of the paper can increase by 80.2 % and 50.4 % respectively, and the tensile strength can increase by 110.7 % and 48.2 % respectively. The applicability study shows that the straws made from the sized paper have great potential to replace plastic straws. This work shares a green preparation method for a novel bio-based sizing agent and its application, providing a promising and sustainable alternative to petroleum-based sizing agents for paper production.