European Journal of Wood and Wood Products (2023) 81:557–570
567
wood fiber decreases with the increase in recycled fiber con- tent. This formaldehyde reduction could be related to the amount of urea pre-polymers, urea and ammonia generated during the degradation of amino-plastic resin in recycled fiberboard reacting with formaldehyde as formaldehyde scavengers. Another explanation might be the release of melamine during hot-pressing acting as formaldehyde scav- engers (Sugita et al. 1990; Martin et al. 1992). Furthermore, the findings of Costa et al. (2014), Martins et al. (2007) and Mirski, and Dorota (2011a) indicate that formaldehyde emission of particleboards and OSB (middle layer) produced from 100% recycled wood meets the standard values of EN 120 (< 8.0 mg/100 g o.d). The type of adhesives affects the formaldehyde emission strongly. According to Hameed et al. (2018b), the particleboard produced from UF showed higher amount of formaldehyde content than TF/PMDI. Moreover, the amount of formaldehyde content reduced notably when the ratio of TF/PMDI increased. Wang et al. (2008) added that formaldehyde release of particleboards made from waste wood decreases linearly when the PMDI/PF ratio increases. The same tendency was found in the study of Roffael et al. (2016) with UF and PMDI for fiberboard production from secondary wood fibers. On the other hand, the hydrothermal process of waste wood particles contributed to reduce the amount of formaldehyde emission. Moreover, the formaldehyde content of thermally treated wood particles is comparable or almost the same as the formaldehyde content of the virgin ones. Waste wood particles treated at 150 °C with 30% water retention/20 mins; 45% water retention/10 mins; and 60% water retention/8 mins (Lykidis and Grigoriou 2011) and 6 bar/156 °C/45 min (Lykidis and Grigoriou 2008) reduce considerably the formaldehyde content in the produced particleboards compared to control panel and comply with emission class E1. Roffael (1995), Michanickl (1996a, b) and Dix et al. (2001a, b) found the same. It can be explained by the fact that the increase in temperature during hydrothermal treatments speeds up the degradation of adhesives in waste wood particles and therefore, urea and other derivatives of hardened urea-formaldehyde will be activated as formaldehyde catchers (Roffael and Kraft 2005). It can be noticed that the formaldehyde emission of wood-based panels manufactured from waste wood could be reduced using thermal hydrolysis process to handle waste wood particles or formaldehyde-free adhesives.
a homogeneous material. Therefore, there are still some limitations that need to be overcome before waste wood can be used as raw material for wood composites production. Considering the research questions put forward in this review of waste wood utilization, the following conclusions can be drawn: • It is not surprising that most of previous investigations focused on recycled wood of construction and demoli- tion, furniture and packaging since they are the most popular waste wood stream resources. The potential municipal waste resource was missing in the research. • There are not enough published data and results available based on research with material derived from plywood and OSB as compared to particleboard and fiberboard even though most of the wood-based panel products in the world are plywood. It is controversial between pro- duction volume, usage and recycling. • Due to the rather strict national and/or Europe-wide regu- lations controlling recycling topics, European institutions and European research institutes are currently the fore- runners in waste wood studies. However, the European member states are lacking a common legislation scheme about the recycling of wood regarding classification and thresholds. Waste wood resources in other continents such as Asia, Africa and Australia would be of high research interest in the future. In addition, more stud- ies about waste wood ordinances should be conducted especially for countries outside Europe. • The advantages in technical and mechanical treatment process of waste wood into particles indicated that par- ticleboard was the primary option for the production of wood-based panel compared to fiberboard and OSB. The present literature analysis has confirmed that currently, there appears to be hardly any research on the use of waste wood materials in the production of plywood. • Physical and mechanical contaminants of waste wood resources would not be a problem for wood composites recycling if they were well managed. This management can be done beforehand at recycling companies or facili- ties via steps of collection, separation and sorting into certain grades. In general, every contaminant could be detected and eliminated by appropriate sorting tech- niques. On the other hand, the focus on improvement of sorting methods will bring the future perspective val- ues for cleaning waste wood mix. Moreover, changing ingredients of coating pigments, paints, preservatives etc., which contain less harmful substances contributing to reduce contaminants in recycled wood, would be an option as well. • Particleboard and the core layer of OSB panel products could be substituted up to 100% by waste wood particles. However, the contaminants and the low slenderness ratio
4 Conclusion
Over the last two decades, many efforts have been put into studying the properties of waste wood resource and its application on material use. Evidently, waste wood is not
1 3
Made with FlippingBook Digital Proposal Creator