Both methods are based on polyaddition, in the presence of low-molecule diols as a chain extender, catalyst, or another additive (e.g., coloring, fillers, moisture scavengers, cross-linking agents) of aliphatic or aromatic diisocyanates and oligomeric diols. Two steps of traditional PU synthesis are demonstrated in Figure 1. In the first stage, the reaction between diisocyanates and polyols leads to the formation of urethane prepolymers ended with isocyanate groups followed by the extension of prepolymer chains by low molecular weight chain extenders (e.g., diamines or diols) [14; 15; 16]. Figure 1: Traditional PU synthesis by a two-stage process using diisocyanate, polyol, and a low molecular weight chain extender [14; 15; 16]. In the beginning, most of the polyols used to prepare PUs were derived from petroleum, but the high demands of resources from the manufacturing process and environmental issues increased the need for more appropriate and environmentally sustainable alternatives. Recently, this attracted considerable commercial and academic attention to renewable resources, such as vegetable oils [3; 17; 18; 19]. Furthermore, certain hazards and problems are associated with applying PU materials, especially concerning isocyanate monomers. PUs can be prepared from renewable polyols, such as lignin [20; 21] and tannins [22; 23; 24]. Lignin and tannins have gained a significant scientific and industrial interest as potential sustainable raw materials for the development of wood adhesives, since they consist of natural plant-derived polymer compounds. Wood adhesives are manufactured in large quantities and used for many large-scale applications, such as load-bearing structures, floors, furniture, doors, and windows [25]. Starting in the 1960s on a small scale, and increasing considerably starting from 2012 to 2013, some synthetic polymers derived from fossil materials due to their increasing costs were starting to be replaced by natural binders. The best case of this is the substitution of phenol-formaldehyde wood adhesives by vegetable tannin-based adhesives with and without formaldehyde [26]. The traditional petroleum-based thermosetting adhesives have numerous advantages, such as excellent adhesion properties and water resistance, ease of handling, low curing temperature, short press times, reasonable costs, etc. [6; 27; 28; 29]. They, however, have a major drawback connected to the hazardous emission of free formaldehyde and other volatile organic compounds from the finished wood-based composites [30; 31], associated with serious environmental problems and a number of significant human health-related hazards, such as such as eye, skin, and nervous system irritation, skin sensitization, nausea, and even cancer [32; 33; 34]. In 2004, the International Agency for Research on Cancer reclassified formaldehyde as \"carcinogenic to humans\" [35]. Formaldehyde is classified as a cancerous and toxic substance with a 100 mg/kg (rat) acute oral toxicity (LD\({}_{50}\)) based on the scientific laboratory's Material Safety Data Sheet (MSDS). However, polymeric diphenylmethane diisocyanate (pMDI) is considerably less toxic than formaldehyde with a total acute (LD\({}_{50}\)) oral toxicity of \(>\)2000 mg/kg (rat), based on the Demilec (Dallas, TX, USA) LLC MSDS. For that reason, formaldehyde is forbidden in a variety of applications [36]. The stricter formaldehyde regulations and the increased consumer environmental awareness have shifted the industrial interest towards more sustainable materials, and less petroleum-based adhesive industry [37; 38]. However, the bonding characteristics of bio-based adhesives should be very similar or incorporate other added values to substitute fossil adhesives with renewable equivalents effectively, and very crucially, costs must be equivalent to current unsustainable adhesives [25].