Utilization of Birch Bark-Derived Suberinic Acid Residues as Additives in High-Density Fiberboard Production

This study investigates the use of suberinic acid residues (SARs), derived from birch outer bark, as a bio-based additive in high-density fiberboard (HDF). Boards with target densities of 800 kg m−3 were produced with SAR contents of 0, 1, 5, 10, 20, and 50%. Standardized tests evaluated mechanical properties: screw withdrawal resistance, modulus of elasticity, modulus of rupture, and internal bond, as well as moisture resistance through surface water absorption, water absorption, and thickness swelling. Density profiles were also analyzed. SAR content influenced HDF performance in a concentration-dependent manner. The most notable improvements in mechanical properties occurred at 5% SAR, where fine particles likely enhanced internal bonding and stiffness. However, higher SAR levels led to reduced mechanical strength, possibly due to an excessive particle surface area exceeding the adhesive’s bonding capacity. Moisture resistance declined with increased SAR, attributed to its hydrophilic nature and process parameters, although SAR-modified boards still outperformed those with other biodegradable additives like starch. SAR also affected the density profile, improving core densification at moderate levels. Overall, SAR shows potential as a renewable additive for enhancing HDF performance, particularly at low concentrations, balancing mechanical strength and environmental benefits.