European Journal of Wood and Wood Products (2023) 81:557–570
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non-wooden material was found in low quality mixed recy- cled wood (e.g., hazardous waste wood) rather than in high quality material (e.g., clean/non-hazardous waste wood) (Lesar et al. 2018). These fluctuations might be relevant to types, sources, fractions and seasons in year, collection and sorting process as well as management of waste wood at recycling facilities. Lesar et al. (2018) also indicated that companies with sophisticated sorting systems showed low content of non-wooden compounds in their waste wood materials. Nowadays, manual sorting, size, sink-float, grav- ity, magnetism, surface tension, and electric conductivity are the most popular sorting methods, which can help to sort out up to 96% of physical impurities in waste materials (Lahtela and Kaerki 2018). The chemical elements in waste wood originate from various substances accumulated from preservatives, adhe- sives, pigments, paints, coatings or lacquers. Wood preserva- tives (e.g., Chromated Copper Arsenate CCA) can be found in many waste wood samples of the collected articles. The amount of Cr, Cu and As varies widely depending on various origins of incoming recycled wood. For example, the waste wood materials from Europe (Faraca et al. 2019; Lesar et al. 2018; Edo et al. 2016; Jermer et al. 2001) tend to contain less CCA than the ones from America (Robey et al. 2018; Tolaymat et al. 2000). It can be explained by the fact that the CCA has been banned in Europe since 2006 as wood preservative (EU Directive 2006/139/CE), whereas it is still allowed in USA. Therefore, these CCA values are lower than in USA. Moreover, Jermer et al. (2001) also found that the amount of arsenic, copper and chromium in German waste wood are lower than in Sweden. This may be a result of the German wood waste ordinance which limits strictly those chemical impurities at lower values compared to Sweden [e.g., As and Cd (2 mg/kg), Cu (20 mg/kg), Cr and Pb (30 mg/kg), Hg (0.4 mg/kg), Cl (600 mg/kg)]. In addition, the amount of Pb, Hg, Cd and Cl varied sig- nificantly depending on waste wood sources. Pb was found at high level (up to 2900 ppm) in waste wood from Sweden (Edo et al. 2016) whereas Cl was found (up to 1191 ppm) in waste wood mix of Sweden, Germany and Netherlands (Jer- mer et al. 2001). The reasons for these phenomena could be due to the difference in waste wood quality among countries depending on company size, collecting seasons and deliv- eries of waste wood. These elements are commonly used in pigments, paints, coatings, lacquers for wood floor and furniture treatment and were found more in Swedish waste wood (Fjelsted and Christensen 2007; Jermer et al. 2001). Furthermore, Pb and Cd also originated from heat stabi- lizers in PVC products (Mesch 2010; Krook et al. 2004). Another reason could be due to the waste wood fraction variations. Faraca et al. (2019) proved that the fractions of waste wood affected the amount of contaminants. For instance, the amount of Cl and Pb in waste wood increases
for plywood production. This can be done only from wood logs. On the other hand, there are more advantages in low cost, simple treatment process (mechanical e.g., chipping instead of chemical methods), and less technical barriers for waste wood during the conversion of wood-based panel (particleboard, fiberboard, OSB, plywood) into particles for particleboard production compared to conversion of old fiberboard into fiber or old OSB and plywood into strands. In general, recycled plywood can be processed into strands for the production of OSB when they are well collected and sorted. However, waste wood streams are normally a mix- ture of wood-based panel products together. The difficulty in the conversion process of inhomogeneous waste wood types into proper strand size and shapes hinders the usage of this resource in the three-layer OSB panel production compared to virgin wood.
3.2 Challenges in waste wood conversion and recycled composites products
3.2.1 Contaminants in waste wood and sorting technologies
3.2.1.1 Contaminants in waste wood Material flows during recycling must ensure that the products contain only non- hazardous contamination levels. The type and threshold of contaminations described in the national legal framework determines whether waste wood may be reused for material purposes or can only be thermally utilized. To use the waste wood efficiently by sorting out as little uncontaminated wood as possible is one of the most challenging steps dur- ing recycling. Physical contaminants or material impurities in WW are usually plastic, metal, glass, textiles, concrete or stone (Edo et al. 2015; Vaermeforsk 2012; Krook et al. 2006) that may originate from different material sources depending on the end-use of wood products or the waste wood collection plant/process. Chemical contaminants on the other hand may come from wood treatments, which were applied in order to improve wood products appearance (e.g., coating pigments, paints, oils), strengthen properties (e.g., gluing agents), prevent biological decay (e.g., wood preservatives) or fire resistance (e.g., flame-retardants) indi- cated by Johan et al. (2007). Table 2 shows the research conducted in Europe and America on the analysis of physical and chemical con- tamination in waste wood. The waste wood materials were collected from different sources such as combustion plant, construction site and recycling companies. Physical and chemical impurities of waste wood fluctuate considerably from the findings. It can be seen in Table 2 that physical contaminants were found from 1 to 3% basic dry weight of total material content (Faraca et al. 2019; Lesar et al. 2018; Edo et al. 2016; Jermer et al. 2001). A higher proportion of
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