## 1 Introduction
The overly high crystallinity drawback was very recently solved [9] by blocking the formation of hydrogen bonds using transition metal ion-bentonite nanoclay through in situ intercalation and, thus, converting the crystalline domains of the UF resins to amorphous polymers. Addition of 5% nanoclay to the UF yielded in excess of 50% better adhesion and almost 50% lower formaldehyde emission, thus resulting in a marked improvement in performance with a low level of crystallinity. In the same trend, the potential introduction of a very acidic pH condensation step in the preparation of UF resins, inducing the formation of occasionally considerable amounts of unon (a cyclic intramolecular urea methylene ether) in the UF resins with lower formaldehyde emission, has attracted some research interest [10; 11]. This initial work indicated that introduction of such an acid step can lead to UF resins with improved bonding strength, but also higher post-cure formaldehyde emission. With regard to the second approach, formaldehyde scavengers, capable of capturing formaldehyde either physically or chemically and forming stable products, are added to UF resins or to wood particles before pressing [12; 13]. These additives should provide long-term formaldehyde emission reduction, in principle, along the panel's life. Examples used in industry include the addition of urea in an aqueous solution or powder form, organic amines, scavenger resins, sulfites, functionalized paraffin waves, and porous absorbers such as pozzolan and charcoal [14]. Very good formaldehyde emission reduction is obtainable by adding sodium metabisulfite to the resin, by adding tannin solution to urea-formaldehyde resin, or by using different starch derivatives [15; 16]. The third approach involves treatments that are applied after pressing. Currently used methods include panel impregnation with formaldehyde-scavenging species, such as aqueous solutions of ammonia, ammonium salts, or urea. Another option is the creation of diffusional barriers in the panel surfaces that keep formaldehyde confined, by using paints, varnishes, veneers, laminates, or resin-impregnated papers [17; 18]. Alternative adhesives involve isocyanate-based adhesives and biobased wood adhesives. The concern about formaldehyde emission vapor levels from UF adhesives has brought isocyanate adhesives to the fore, where formaldehyde emission does not occur as no formaldehyde is added. pMDI (polymeric methylenediphenyl isocyanate) is an excellent adhesive and can be used in markedly smaller proportions than formaldehyde-based adhesives to bind wood composites. Another attractive option is biobased adhesives. The term biobased adhesive has come to be used in a very well specified and narrow sense to only include those materials of natural, nonmineral origin which can be used as such or after small modifications to reproduce the behavior and performance of synthetic resins. Thus, only a limited number of materials can be currently included, at a stretch, in the narrowest sense of this definition. These are tannins, lignin, carbohydrates, unsaturated oils, proteins and protein hydrolysates, dissolved wood, and wood welding by self-adhesion. An excellent review on this topic was recently published by [PERSON] et al. [1]  
The most common biobased adhesives are protein-based sourced from animal bones and hides, milk (casein), blood, fish skins, and soybeans. Soy protein is obtained from soybean and has been used for centuries as a wood adhesive. In the context of wood composite production, soy protein was added to phenol formaldehyde resin to lower formaldehyde emission, but lower water resistance is an important limitation [19]. Formaldehyde-free wood composites were obtained using an adhesive based on soy flour and glyoxal, a nontoxic, but less reactive, aldehyde [20]. It was reported that the use of soy protein combined with polyamidoamine-epichlorohydrin (PAE) resins yields a strong and water-resistant product that is commercially available for wood composites [21].