## 1 Introduction  
The depletion of fossil fuels and the increasing concern about the negative effects of global warming on the environment and human health are the main driving forces for the development of a bio-economy [1]. In this respect, the resource efficiency optimisation and the valorisation of lignocellulosic biomass in different high-value products from renewable sources as an alternative to their petroleum-based equivalents are one of the key objectives for implementing the circular economy principles in the wood-based panel industry [2; 3; 4]. Significant amounts of non-hazardous solid waste and sludge are generated annually at pulp and paper facilities worldwide, which require further utilisation [5; 6; 7; 8]. This solid and liquid discharge contains residual fibres and represents a potential feedstock that can be utilised in the production of new panels [9; 10; 11; 12]. Traditional thermosetting adhesives that are used for the production of wood-based composites, such as phenol-formaldehyde, urea-formaldehyde (UF), melamine-urea-formaldehyde and melamine-formaldehyde resins, are derived from the by-products of petroleum processing [13; 14; 15]. Currently, about 95% of the total wood adhesives that are used for manufacturing engineered wood composites are based on formaldehyde [16]. UF resins are the most predominant type, accounting for almost 85% of the total worldwide, followed by melamine at 10% and phenolics at 5% [16; 17; 18]. These adhesives have been extensively used in the production of wood-based panels because of their exceptional adhesion properties and water resistance, low curing temperatures, chemical versatility and cost-effectiveness [13; 15; 19; 20; 21; 22; 23; 24]. The main disadvantage of these adhesives is the release of volatile organic compounds and formaldehyde from the finished engineered wood panels, especially in indoor applications [25; 26], causing adverse human health effects, such as skin and respiratory tract irritation, skin sensitisation, nausea, genotoxicity and sinonasal cancer [27; 28; 29; 30]. Formaldehyde emission from engineered wood panels is dependent on endogenic factors, e.g., wood species, the adhesive used, the resin content, the technological conditions employed and the type of hot press [13; 16; 31; 32], and exogenic factors, which are related to the ambient conditions, processing and ageing of wood-based panels [32; 33; 34; 35]. Thus, the growing social concern about hazardous formaldehyde emissions from engineered wood panels, along with the increased environmental consciousness related to the sustainability of the final products, have been the main factors for changing the industrial and scientific interest from the traditional petroleum-based synthetic resins to the development of renewable, bio-based and less toxic adhesives for manufacturing eco-friendly wood composites by partially or completely replacing formaldehyde in their compositions [36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48]. Different biomass sources, such as lignin [37; 49; 50], soy proteins [51; 52], starch [53; 54] and tannins [55; 56], have been used as feedstocks for the development of biobased adhesives. Lignin is a polyaromatic macromolecule and the second most abundant natural biopolymer in nature, preceded only by cellulose [57; 58]. It is also generated as waste or a by-product of the pulp and paper industry, with an annual global production of approximately 50-75 million tons [59]. The main categories of lignin are lignocellulosiconates obtained from the sulfite pulping, kraft lignin from the sulfate process (Kraft pulping) and organosolv lignin from the production of bioethanol [60; 61]. Currently, only 10% of technical lignin is further re-used at an industrial scale, while the rest is mainly burnt for energy, used to recover chemicals or disposed of as a waste [60; 62].