Effect of cellulose microfibril (CMF) addition on strength properties of middle ply of board

Cellulose microfibrils (CMF) are a promising biobased material with unique nanospecific properties, giving them potential for use in numerous applications. Based on this, cost-effective and novel high-consistency enzymatic fibrillation (HefCel) technology was used to produce CMF, which was further used to reinforce middle ply of board structure and the results compared with those obtained without CMF addition and with addition of CMF produced by traditional Masuko grinding (VTT Native grade). The results showed an average increase in tensile index of middle ply of board of approximately 50% with VTT Native grade and approximately 15% with HefCel grade at 3% CMF dosage. According to special board measurements, ~100% improvement in Scott bond, ~117% improvement in Z-directional strength, as well as ~13% improvement in bending stiffness were achieved with VTT Native grade compared with the reference case, while addition of HefCel resulted in improvements of ~35% in Scott bond, ~40% in Z-directional strength, and ~20% in bending stiffness. Addition of HefCel CMF generated a bulkier handsheet structure compared with VTT Native CMF, which had a direct impact on the bending stiffness. The differences in the strengthening effect between HefCel CMF and VTT Native CMF are most probably due to the fibril morphology; VTT Native CMF with long and flexible fibrils provided tensile strength, while HefCel CMF consisting of short fibrils had a more substantial impact on the bending stiffness. The bending stiffness is one of the most important characteristics in board applications, indicating that HefCel CMF has potential for use as a reinforcement material in packaging applications.