Abstract

The production of MFC and CNF has recently been focused on fiber raw materials which, still contain residual lignin instead of bleached chemical pulp fibers. The target of this study was to assess the energy consumption and microfibrillation rate for high lignin containing microfibrillated cellulose (LMFC) produced from non-delignified pine and spruce sawdust directly or after lignin-preserving sulfonation, and evaluate their grade by measuring strength properties of self-standing films made from the LMFCs and by determining their ability to reinforce fluting board. MFC from bleached softwood was used as a reference.

Microfibrillation rate at 80 °C was faster for spruce sawdust than for pine sawdust, i.e., bonding ability (measured as tensile strength of self-standing films) developed faster in spruce LMFC than in pine LMFC. By mild pre-sulfonation of sawdust, corresponding to the sulfonation degree of 0.5−0.6 %, the rate increased further with both softwood species. In similar grinding conditions and equal grinding time, pre-sulfonation did not affect much energy consumption but at a given specific energy consumption the grade of LMFC was better. Due to the high lignin content, mechanical properties of LMFCs could not been developed at as high level than MFC. However, the tensile strength of 100 MPa was possible to achieve for LMFC films using either the equivalent (spruce and sulfonated pine) or even clearly lower amount (sulfonated spruce) of grinding energy than in microfibrillation of bleached Kraft pulp to the same strength.

By comparing the pre-sulfonation of sawdust and the post-sulfonation of LMFC, it was concluded that the main sulfonation effect in grinding was lignin softening, which enhanced cell wall disintegration in microfibrillation. The increased bonding ability of sulfonated lignin and the dissolution of extractives were seen to be secondary effects but not without significance. Pre-sulfonation also improved redispersibility, i.e., the recovery of the mechanical properties of dried LMFCs.

The reinforcing potential was evaluated by dosing LMFCs and MFC without a retention aid into the handsheets made from fluting pulp. The tensile strength of fluting increased almost linearly with an increasing dosage and increasing microfibrillation time. Although the grade of LMFCs, in terms of viscosity and mechanical properties of LMFC films, was lower than that of MFC from bleached softwood, their ability to reinforce fluting board was shown to be equal, indicating lignin present did not interfere the formation of hydrogen bonds. With a 4 % dosage (equaling to the amount of 2–2.5 % within the handsheets) of each LMFC microfibrillated for 120 min, the tensile strength improved by 20 %. Among the tested raw materials, LMFC from spruce sawdust without pre-sulfonation has the greatest potential as a cost-effective reinforcing agent for the paper and board industry. For sulfonated LMFCs should be found applications where their better bonding ability could be better utilized.