Synergistic Reinforcement and Toughening of Urea–Formaldehyde Resin via a Hydrogen‐Bond Network Induced by Quaternary Ammonium Salts

ABSTRACT Urea–formaldehyde (UF) resin suffers from high brittleness, and conventional toughening methods, such as blending with flexible polymers, often lead to a reduction in stiffness. In this study, a novel strategy was developed to simultaneously enhance both the toughness and strength of UF resin by introducing quaternary ammonium salts (QASs). The incorporation of QASs, particularly chitosan‐based quaternary ammonium salt (HACC), enabled the establishment of a hydrogen‐bond network in UF resin, wherein chloride ions (Cl − ) served as strong hydrogen bond acceptors. These ions competed with the native acceptors present in UF, leading to the partial replacement of inter‐resin hydrogen bonds with stronger UF‐QASs interactions. The formation of such a hydrogen‐bond network reduced the chemical crosslinking density of the resin while serving as sacrificial bonds that dissipated energy under stress, thereby enhancing toughness and resulting in obvious plastic deformation upon impact. Concurrently, the cationic nature of QASs facilitated electrostatic adsorption onto negatively charged lignocellulose surfaces, increasing the ζ ‐potential of lignocellulose, suppressing fiber pull‐out, and strengthening the fiber‐matrix interface. For UF‐HACC, remarkable improvements in tensile strength (58.41%) and impact toughness (21.84%) can be achieved, along with significantly enhanced water resistance. This work presents a facile and effective strategy for developing high‐performance UF resins.