Solvent-free preparation of propionyl-modified pulp fibers for high-density polyethylene reinforcement†

With the advancement of lightweight and high-strength fiber-reinforced composites, various chemical modification methods have been proposed to improve the compatibility between fiber and plastic matrix. Among these, acetic anhydride modification is particularly notable, but its low activity necessitates the use of hazardous agents and results in suboptimal mechanical properties after reinforcement. Herein, a solvent-free and recoverable esterification approach was developed to modify pulp fibers with propionic anhydride (PAF). Subsequently, the high-density polyethylene (HDPE) was reinforced at multiple levels through capacity addition, filling, and premixing. The results showed that the optimal mechanical properties and minimum coefficient of thermal expansion (CTE) of HDPE/PAF composites were achieved at a DS value of 0.40. Compared to the unmodified fiber-reinforced composites, propionylation not only improved the discoloration of HDPE/PAF composites but also enhanced its hydrophobicity, as evidenced by an increase in the water contact angle from 89.2° to 103.4°. Moreover, compared to the neat HDPE, the tensile strength and modulus increased by a factor of 2.4 and 3.3, respectively, surpassing the improvements reported for other acid anhydride modifications. These findings indicated that the PAF could be uniformly dispersed in the HDPE matrix through a dual network structure. The successful attempts to reinforce other polyolefin matrices and achieve kilogram-scale production have demonstrated the practicality and feasibility of this approach for industrial applications.