Laccase mediator systems for eco‐friendly production of medium‐density fiberboard (MDF) on a pilot scale: Physicochemical analysis of the reaction mechanism

Abstract

Increasing prices of petrochemical resins and possible harmful formaldehyde emissions from conventionally produced wood composites have resulted in increased interest in enzymatic binder systems as environmentally friendly alternatives for gluing lignocellulosic products. In this study, laccase mediator systems (LMSs) were used to activate lignin on wood fiber surfaces in the pilot-scale production of medium-density fiberboard (MDF) using a dry process. Three different mediators were applied: 4-hydroxybenzoic acid (HBA), 1-hydroxybenzotriazole (HBT), and acetosyringone (AS) of which HBA performed best. The mechanical properties of the manufactured boards produced with thermomechanical pulp (TMP) fibers, laccase, and HBA fulfilled all required European standards for wood-based panels. Oxygen consumption rates of the different LMSs and (13)C NMR spectroscopy results for treated TMP fibers were obtained for qualitative and quantitative analysis of lignin activation. The results show that reactions were most effective within the first 30 min of incubation. Oxygen consumption was fastest and highest for the LMS using HBA. (13)C NMR spectroscopy indicated the highest decrease of aromatic groups in the wood fiber lignin with this LMS. The data correlated well with the quality of the MDF. The required enzymatic reaction times allowed direct integration of the LMS into standard MDF production techniques. The results indicate that application of LMSs has a high potential for environmentally friendly MDF production.

Phenolics as Mediators to Accelerate the Enzymatically Initialized Oxidation of Laccase-Mediator-Systems for the Production of Medium Density Fiberboards

“…LMSs with CA and VAl showed the most promising results. Good results for HBA were already investigated in previous studies by Euring (2008) and Euring et al (2011Euring et al ( , 2013. VAl and CA may now also be taken as serious alternatives.…”

“…However, one disadvantage is the long incubation time for laccase-only bonded MDF in comparison to other oxidative enzymes due to its limited redox potential and large molecular size (Li et al 1999;Euring 2008). To handle this issue, specific phenolic mediators have been added that ensure faster reactions within the first 20 to 30 min, enabling a lower incubation time for more cost-effective industrial applications (Euring 2008;Euring et al 2011Euring et al , 2012Euring et al , 2014Euring et al , 2015Kirsch et al 2015). Mediators, such as caffeic acid (CA), 4-hydroxy benzoic acid (HBA), and vanillic alcohol (VAl) are low-molecular compounds that act as diffusible electron carriers, enhancing the oxidative reaction of laccase (Bourbonnais and Paice 1990;Potthast et al 1995;Potthast 1998).…”

“…Its main benefits include a higher range of substrates due to the mediators' ability to oxidize even non-phenolic compounds of lignin and an accelerated reaction time (Bourbonnais and Paice 1990;Kües et al 2007). Previous studies showed LMS can be used as an alternative binding agent in MDF production, owing to the achieved standards of modulus of rupture, internal bond strength, and thickness swelling, and it can easily be integrated into existing systems (Euring 2008(Euring , 2013Euring et al 2011). Because all of the materials used for this kind of MDF manufacture are near-natural and sustainable, recycling can easily be realized in the form of composting (Hüttermann et al 2001;Euring 2008).…”

“…The mediators were chosen based on oxygen consumption measurements from previous studies where they tested positively in enhancing oxidation (Euring et al 2011;Kirsch et al 2015). The fiberboards were produced in a semi-dry process in which the LMS is sprayed directly onto wood fibers for enzymatic activation.…”

Pre-pressing and Pre-heating via Hot-Air/Hot-Steam Process for the Production of Binderless Medium-Density Fiberboards

“…LMS application for the production of wood fiber composites offers many advantages. In addition to the independence from fossil oil resources, the use of LMS causes no extra emissions (either formaldehyde or volatile organic compounds, VOCs) out of the products (Müller et al 2009;Euring et al 2011Euring et al , 2013Euring et al , 2015Euring et al , 2016.…”