Waste and Biomass Valorization
fractions and the process was evaluated [18]. By steam refin- ing of waste MDF a liquid and a solid fraction is generated. The liquid fraction contains solubilized carbohydrates and lignin as well as a high amount of nitrogen and acids (acetic and formic). The pH of the liquid extract of around 8 is high in comparison to liquid fractions of steam treated native lig- nocellulosic material, due to a high amount of ammonium hydroxide following the degradation of the urea–formalde- hyde resins [12, 13]. As the fiber fraction accounts for the major fraction after the steam refining treatment, the devel- opment of an economically viable recycling pathway for the fibers is of prime interest for the advancement of material recycling of waste MDF. The reuse of the separated waste MDF fibers in produc- tion of new MDF is the most apparent possible recycling pathway. However, due to a combination of effects such as fiber shortening, changes in the chemical compositions of the fibers and resin residues found on their surface, a deterioration of the mechanical properties in comparison with MDF made from fresh wood can be observed when using hydrothermal or steam-based fractionation processes [19–24]. Recently, Moezzipour et al. [25] have reported that such negative effects on the fibers can be reduced using electrical heating instead of hydrothermal treatments, lead- ing to improved mechanical properties of the newly pro- duced MDF. Other previously investigated potential uses for recovered waste MDF include the production of cellulose nanocrystals [26, 27], wood polymer composites (WPC) [28–30], bio-ethanol [31–33], bio-oil and biogas [34–37], or the substitution of particles in the middle layer of parti- cle boards [38] as well as insulation or oil spill absorbance applications [39]. Another potential recycling path might be the utilization of steam refined waste MDF fibers in paper packaging appli- cations, such as corrugated boards, allowing for extended cascading of the wood fibers. Corrugated board is the pack- aging material with the highest production volume world- wide [40] and due to the continuing increase in e-commerce, the demand is expected to remain high even in times of uncertain global trade relations [41]. Corrugated board con- sists of at least one wave-like element called flute and one flat sheet called liner or linerboard [42]. The overall struc- ture of a corrugated board is making use of the engineering beam principle, in which the fluting acts as a supporting structure for the two load-bearing planes. As hardwood fib- ers are shorter and stiffer than softwood fibers, which makes the papers easy to corrugate but still gives them a rigid struc- ture, the flute is usually made from recycled pulp or from virgin hardwood neutral sulphite semi-chemical pulp. The flat liners are called test liner if manufactured from recycling fibers, or kraft liner if manufactured out of virgin softwood kraft pulps [40]. In Germany, 65% of all packaging mate- rial for transportation is made out of corrugated board and
the material are processed, might prove a viable recycling concept to fill this void. The fiber fraction represents the major share of the material after the steam treatment, and finding an economic and ecological application for the fibers will be a key element in successful valorization. Because the amount of waste MDF and the demand for fiber based packaging solutions is steadily rising, a deployment of the steamed fibers in packaging applications could be a step in solving both challenges simultaneously. Thus, an investiga- tion into the fiber and resulting paper properties was under- taken to identify problems and evaluate the viability of the concept.
Introduction
Medium density fiberboards (MDF) are an engineered wood product made out of lignocellulosic fibers, which are blended with resins and hot-pressed into panel shape [1, 2]. Roughly 80% of the manufactured panels are processed fur- ther into furniture or flooring applications [3]. Due to the ris- ing demand, the worldwide production volume of MDF has increased continuously, from roughly 8 million m 3 in 1995 to almost 100 million m 3 in 2018 [4]. At the same time, shifts in consumer behavior, such as the perception of furniture as a fashion item instead of a durable commodity, have led to increasingly shorter life cycles [5]. Consequently, a rising amount of waste MDF is accumulating. Assuming an aver- age life span of approximately 14 years, a total waste MDF volume of almost 50 million m 3 can be calculated just for the year 2016 [6]. In Europe, due a combination of factors such as government subsidies, challenges in sorting of the waste material stream and stability of the waste material supply chain, a major part of the available waste wood is incin- erated for energy generation [7]. Following the principles laid out in the Waste Framework Directive of the European Parliament [8], a material recycling should always be given priority over energy recovery or land filling, as by reusing the material before energy recovery, the biomass can be used in a cascade, improving the resource efficiency [9]. Fractionation is a necessary step to enable a material recycling of waste MDF. Using hydrothermal or steam-based treatments it is possible to hydrolyze the urea–formaldehyde (UF) based resins [10–14], which make up the majority of the resins used in MDF production [15]. To compare chemi- cal and structural changes in differing steam-based treat- ments of lignocellulosic material, the severity factor, which combines the two main process parameters treatment dura- tion and temperature, can be used [16, 17]. In a previous study steam refining was used to fractionate standard waste MDF, the applicability of the severity factor in steaming of waste MDF was confirmed and the influence of differ- ing treatment severities on the chemical composition of the
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