Production of wood-based panel from recycled wood resource: a literature review

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 during 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) indicated by Johan et al. (2007).

Table 2 shows the research conducted in Europe and America on the analysis of physical and chemical contamination 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 non-wooden material was found in low quality mixed recycled 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, gravity, 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, adhesives, pigments, paints, coatings or lacquers. Wood preservatives (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 significantly 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 (Jermer 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 deliveries 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 stabilizers 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 when the waste wood particles, which are lower than 4 mm (fine fraction) increases (Edo et al. 2016; Vaermeforsk 2012; Jermer et al. 2001). This is probably due to the crushing and chipping process resulting in more surface coating materials removed from the waste wood surface increasing the amount of heavy metal and Cl in the fine fraction after sieving. There are limited studies conducted on finding organic compounds of waste wood in the recycling process. Faraca et al. (2019) was the only reference found in analyzing PCP, PCB and PAH of waste wood materials. The finding showed that those substances are mainly coming from old furniture. In the past, PCB was used as plasticizers in the coating ingredients of paints and flame retardant (Butera et al. 2014; Jartun et al. 2009a, b). Nowadays, these substances are slowly replaced by other ingredients in paints and coating recipes applied to wood surface treatment. Therefore, high quality waste wood was found containing less of these components and complies with European standards for organic pollutants.

Different sorting technologies have been developed to detect and eliminate chemical contaminants in waste wood particles such as atomic absorption spectroscopy (CV, GF, or HG-AAS), inductively coupled plasma spectrometry (ICP-OES, ICP-MS), energy dispersive X-ray fluorescence (ED-XRF) and near infrared (NIR) spectroscopy (Mauruschat et al. 2016; Fellin et al. 2014, 2011; Hasan et al. 2011a, b; Williams 1976). It is noticed that atomic absorption spectroscopy and inductively coupled plasma spectrometry are the methods used to detect the chemical contaminants of waste wood or biomass in the laboratory indicated by EU Commission decision 2009/894/EC. Other sorting techniques such as energy dispersive X-ray fluorescence (ED-XRF) and near infrared (NIR) spectroscopy have recently shown advantages in the sorting process of waste wood due to fast detection and high sorting efficiency. Plastics and wood preservatives can be detected and sorted by these techniques easily. For example, Hasan et al. (2011a, b) stated that the application of ED-XRF in online sorting could eliminate 92–96% of wood preservatives (CCA) and alkaline copper quaternary (ACQ) in recycled wood at recycling plants. This method also showed high efficiencies with certain limitations for the elemental analysis of six different groups (origin, type, material, visually detected pollution, pollutant macro category and pollutant specification) from wood residues in wood recycling plants (Fellin et al. 2014). On the other hand, Fellin et al. (2011) stated the positive results on the application of infrared spectroscopy for the detection of pollutants in wood residues. Moreover, Mauruschat et al. (2016) indicated that near infrared (NIR) spectroscopy and automatically pneumatic nozzles can distinguish four types of plastic granulate in WPC. In addition, this study also showed the possibility to distinguish between untreated and treated wood at different moisture contents containing inorganic and organic preservatives. However, these technologies need more investigations/improvements prior to being used on an industrial scale besides focusing on the development of new sorting techniques.

Principally, the detection and sorting of most contaminants in waste wood could be conducted by appropriate methods. However, the waste wood after sorting and processing could contain certain contaminants. Depending on types and contents, those physical and chemical contaminants in waste wood resources will be classified whether they are problematic for the later wood-based panel products.

Different investigations based on hydrothermal treatment processes were conducted at various schedules to separate waste wood into particles and use them for the production of particleboard (Andrade et al. 2015; Lykidis and Grigoriou 2011, 2008). The findings indicated that the particleboards manufactured from treated waste wood particles show stable dimensions. However, the mechanical properties (MOE, MOR and IB) and physical properties (thickness swelling and water absorption) of the panel decrease when the temperature increases. This finding is also consistent with the results of Michanickl (1996a) and Boehme and Michanickl (1998). It can be explained by the degradation of holocellulose and lignin in recycled wood resulting in reduction of the mechanical properties of boards due to the temperature increase (Yilgor et al. 2001). Moreover, these findings correspond to the research of Goldstein (1973) that the treatment temperature range should be between 110 and 170°C for recycled particles of particleboards. Additionally, Lykidis and Grigoriou (2011, 2008) concluded that the panel made from hydrothermally treated waste wood particles at around 150 °C shows better quality than others.

The effects of different waste wood ratio on the physical and mechanical properties of particleboard were investigated by Azambuja et al. (2018a, b), Laskowska and Mamiński (2018), Zamarian et al. (2017), Martins et al. (2007) and Suffian et al. (2010). The research results stated that it is possible to use 100% recycled wood particles in the production of panel products with UF as binder. In general, the higher the wood mix ratio applied, the lower the mechanical properties (MOE, MOR and IB) and the higher the hygroscopic properties (thickness swelling and water absorption) of panel achieved. The reasons could be due to the decrease in slenderness ratio of waste wood particles formed during the chipping process, which leads to the limitation of contact area between the particles (Arabi et al. 2011). In addition, the physical contaminants from surface coating materials (e.g., polypropylene, polyethylene, polyvinylchloride) of different waste wood-mix resources ( e.g., construction and demolition, furniture, packaging) cause negative effects on glue bonding of panel production, resulting in the decline of strength properties of boards. Moreover, Czarnecki et al. (2003) confirmed that waste wood particles containing PF resin could hinder UF curing due to its alkaline character resulting in reducing MOE, MOR and IB of recycled particleboard. On the other hand, Azambuja et al. (2018b) proved that the strength properties of produced particleboard containing up to 50% of waste wood mix are comparable to the one made from fresh wood particles and their values meet the standard of panel type P2. In general, the properties of the particleboards can be controlled based on the waste wood ratio in wood mixture.

Adhesive types also strongly affect the properties of particleboards. Several investigations focused on PF, TF and PMDI adhesives instead of UF for the improvement of particleboard properties using 100% waste wood particles (Hameed et al. 2018a; Laskowska and Mamiński 2018; Yang et al. 2007; Wang et al. 2007). The key findings illustrated that the MOE, MOR, and IB increase and TS and WA decrease when the percentages of those glues in waste wood mixture increase. This may be due to the differences in bonding properties (e.g., impregnation/absorbing ability only on surface or deeply inside middle lamella of wood) of UF, PF, TF and PMDI adhesives with wood particles/fiber during the curing process affecting the physical and mechanical properties of produced panels. Better bonding ability will result in better board properties. Laskowska and Maminski (2018) and Yang et al. (2007) stated that boards made from PF showed better properties than UF boards, whereas Wang et al. (2007) indicated that panels produced from PMDI showed higher properties than PF ones. Furthermore, Hameed et al. (2018a) demonstrated that the combination of TF and PMDI at the ratio of 30%:70% and 40%:60% in particleboard manufactured from waste wood material complied with the standard values of type P2 strength.

In the sector of fiberboard produced from the mixture of waste wood fiber and virgin wood, Hong et al. (2018), Roffael et al. (2016), Mantanis et al. (2004) and Krzysik et al. (1997) found that the strength properties (MOE, MOR and IB) of the panel decrease tendentially with the increase in waste wood fiber content. It could be explained by the fact that the handling process (e.g., hammering, cooking, refining) of recycled fiberboard into fiber resulted in shortening the fiber length of recycled fiber leading to the reduction in mechanical properties. There was a controversial finding in hygroscopic properties of investigated fiberboard. Hong et al. (2018) and Krzysik et al. (1997) found that TS and WA of investigated fiberboard increased with the higher proportion of recycled fiber content, whereas Roffael et al. (2016) and Mantanis et al. (2004) stated that TS and WA were improved and decreased when more recycled fibers are used. However, the difference could be explained by the fact that the adhesive content in recycled fiberboard contributes to the increase in TS and WA. Moreover, the interaction of cross-linking of the lignocellulose fibers with existing UF-pre-polymers in UF resin could be a reason for this effect (Andrews et al. 1985). Another possibility may be the effects of contaminants from adhesives, coating layers or surface laminate types (e.g., polyethylene terephthalate) in recycled fiberboard. Therefore, the findings indicated that it is only feasible to substitute 20% (Hong et al. 2018) to 25% (Mantanis et al. 2004) of recycled fiber in the UF wood mixture to produce fiberboards, reaching mechanical and physical properties comparable to virgin wood fibers.

At industrial scale, fiberboard recycling is facing a major problem of effectively collecting, sorting and disintegrating the wood fibers. Recycled fiberboards from off-cuts, machining errors, and transport and storage losses contain different types of wood adhesives and coating surface materials. These cause difficulties in applying appropriate technologies (e.g. mechanical, thermo-hydrolytic and chemical) to disintegrating fiberboard waste wood into reclaimed fiber completely in a single step used for fiberboard production. Therefore, the combination of mechanical, thermal and chemical technologies is requested. However, this combination will lead to the quality reduction in recovered fibers (Irle et al. 2019; Buschalsky and Mai 2021).

In addition, most collected waste wood resources at recycling centers or companies are mixtures of different wood types such as particleboard, fiberboard, plywood and OSB. Fiberboard accounts for about 5–15% of the amount of these waste wood resources and normally is not easy to separate from the mixture by traditional sorting methods (Fechter 2021). This amount will generate challenges (e.g. dust during chipping process of waste wood into particles and higher consumption of adhesives at gluing stage) when using it for the manufacture of industrial particleboards. For the time being, sorting technologies are developed that can sort out most of the fines from the waste wood mixture. Great effort is made to increase the recovery of MDF by different technologies besides improved sorting such as steaming at high pressure, ohmic heating or microrelease.

For the production of OSB from waste wood, Mirski and Dorota (2011a, b) stated that 75% of recycled wood particles could replace virgin ones in the core layer of OSB with MUPF and PMDI, complying with the mechanical and physical property values of standard EN 300. On the other hand, Schild et al. (2019) indicated that the substitution up to 100% of unsorted waste wood particles in the core layer of OSB with PF is possible and the MOE, MOR and IB of the boards comply with standard requirements, except for TS and WA. However, MOE and MOR decrease when the waste wood content increases, whereas IB, thickness swelling and water absorption increase with the increase in waste wood proportions. These effects could be due to the inhomogeneous distribution of strands and particles and particles contaminants (e.g., wood preservatives, resonated waste wood particles, paints) in the face and core layers of OSB resulting in high-density variations in core layer and the whole board.