Influence of the Spent Coffee Ground-to-Ammonium Lignosulfonate Ratio on the Performance of Wood-Fiber Biocomposites

Spent coffee grounds (SCGs) and ammonium lignosulfonate (ALS) are promising renewable constituents for wood-fiber biocomposites, but the effect of their relative proportion within the binder phase remains insufficiently understood. This preliminary study evaluated wood-fiber panels produced at a constant dry-matter matrix-to-fiber ratio of 50:50 while varying the SCG:ALS ratio from 70:30 to 30:70. The panels were hot-pressed and characterized for their physical and mechanical properties, while the corresponding SCG–ALS matrix formulations were evaluated by simultaneous TGA–DSC analysis. Increasing the SCG fraction generally improved dimensional stability and mechanical performance, whereas formulations containing 50% or more ALS showed a marked deterioration in water resistance. The SCG-rich 70:30 formulation provided the highest bending performance, with MOE and MOR values of 2156.5 and 12.83 N·mm−2, respectively. Thermal analysis revealed ratio-dependent changes in mass loss and in the principal heat-flow event, which occurred between approximately 113 and 136 °C, confirming that the SCG:ALS ratio altered the early thermal behavior of the binder system. Overall, the SCG:ALS ratio was identified as a key formulation parameter, with SCG-rich systems providing the most favorable balance between mechanical performance, dimensional stability, and internal cohesion.