High‐Strength and Flexible Ultra‐Thin Fiberboards With a Sandwich Structure

ABSTRACT Ultra‐thin fiberboard (UTF) is a new type of wood‐based panel. It is increasingly used in furniture and surface finishing, yet achieving high strength and flexibility remains challenging. To address this, sandwich‐structured UTFs were fabricated using a laminated design with different adhesives, and relationships between flexibility and strength were systematically evaluated. In sandwich‐structured UTFs (denoted E‐pMDI‐E and E‐PF‐E), the middle layer consisted of wood fibers bonded with polymeric diphenylmethane diisocyanate (pMDI) or phenol‐formaldehyde resin (PF). The upper/lower layers consisted of wood fibers bonded with ethylene–vinyl acetate (EVA) emulsion. Compared with EVA‐only UTF, the sandwich‐structured UTFs showed superior performance, with E‐pMDI‐E exhibiting the best overall properties. Regarding flexibility, E‐pMDI‐E achieved a 39% increase in flexibility coefficient. Mechanically, both bending strength and tensile strength were improved by 50%. SEM analysis revealed that E‐pMDI‐E had enhanced interfacial bonding, reducing delamination and minimizing crack propagation compared to EVA‐only UTF and E‐PF‐E. XPS and FTIR analyses confirmed that pMDI reacted chemically with wood fibers to form urethane bonds, establishing strong interfacial bonding. Overall, the sandwich‐structured design effectively balances strength and flexibility in UTFs, providing experimental and theoretical guidance for applications in special‐shaped furniture, large‐curved surface finishing, and customized interior design.