## 1 Introduction  
Adhesives play an important role in the efficient utilization of wood resources and in the development and growth of the forest product industry. Adhesive bonding of solid wood and wood particles of various sizes is a key factor for the production of modern, functional wood products, used in a variety of applications. For centuries, wood was bonded using biobased adhesives until synthetic adhesives, mainly thermosetting ones, gradually took over in the 20 th century, as they were typically regarded as more effective, cost-efficient [1], and stable for use in humid conditions. Today, the main classes of thermosetting adhesives are amino-based, phenolic, and isocyanate resins. The utilization of these thermosetting adhesives is considered more economical, and reactive adhesives with quick curing behavior are versatile in a range of properties in the cured state. These adhesives have dominated the wood composite industry for many decades. Within this group, urea-formaldehyde (UF) resins are the most important adhesives in terms of quantity. Due to their low-cost raw materials, their rapid curing, their high dry-bond strength, and a colorless glue line, UF-based adhesives are almost exclusively used for producing wood-based materials, such as particleboard or medium-density fiberboardsfor interior applications [2]. When products are utilized in conditions exhibiting higher humidity, UF resins are usually modified with significantly more expensive compounds such as melamine, phenol, or resorcinol [2]. It has to be mentioned that the final adhesive composition in use depends on the requirements of the wood-based material such as the required strength properties, the expected moisture resistance, the production cost of the finished product, and the desired formaldehyde emission class. When it comes to formaldehyde emission from wood-based composites, it has to be mentioned that emission can originate from (i) synthetic-free formaldehyde that is not polymerized into the network, which emits during or quickly after panel production, (ii) formaldehyde released due to adhesive hydrolysis, which emits over the lifetime of the panel depending on moisture and temperature, and (iii) biogenic sources [2]. The discussion surrounding formaldehyde started as early as the mid-1960s, as reviewed by [PERSON] et al. [3], and various stages of reduced emissions were achieved in the 1970s and intensified in the 1980s when the carcinogenicity of formaldehyde in rats and mice after long-term inhalation exposure was reported. The topic of formaldehyde in indoor environments was intensively and controversially discussed by various authors, as reviewed by [PERSON] et al. [4]. Driven by the standard requirements specified by the local authorities in, e.g., Europe, Japan, and the United States of America (USA), formaldehyde content and emissions from wood-based composites have been continuously reduced over the last few decades. During the last few decades, the wood industry made great effort with many innovations in amino resin technology to gradually reduce formaldehyde emissions in order to fulfill the individual product and standard requirements for each type of composite. These can be summarized as follows: (i) low-formaldehyde-content resins, (ii) formaldehyde scavenger additives, (iii) post treatments, and (iv) alternative adhesives [1]. For the first type of approach, the first tendency has been to prepare engineered UF resins of progressively lower molar ratio, at levels much lower than 1:1 [5], which has become rather common in industry today. This is due to the attempt to minimize formaldehyde emissions from wood panels bonded with UF resins. One of the drawbacks of the much lower than 1:1 molar ratio has been identified in an increase in the tendency of the UF resin to form increasingly present crystalline domains upon hardening as a result of hydrogen between linear molecules [6; 7; 8]. At higher molar ratios, the hardened resin is amorphous, affording better adhesion and better bonding performance.