Wood-based panels are commonly used as building materials both for interior and exterior purposes. Their production and utilization has been increasing over the past few decades due to the good and useful properties and the environmental benefits they present (Hansted et al. 2019). Among the wood-panel alternatives, particle board, fiber board, and oriented strand board (OSB) are some of the most frequently used (Ayrilmis et al. 2016; Hansted et al. 2019). Specifically, for particle boards manufacturing, three layers are normally formed where larger particles are used for the core layer, improving the mechanical properties. Thinner particles are used for the two outer layers in order to obtain a smooth surface (Hansted et al. 2019).
Adhesive-bonded products make up 80% of the wood products on the global market, and of that, Urea-Formaldehyde (UF) makes up more than 81% of the resins used (Lei et al. 2008).
It was reported by the Food and Agriculture Organization Corporate Statistical Database (FAOSTAT) that in 2019 United States produced 4,346,542 m3 of particles boards, being the main producer in North America (FAOSTAT 2019).
Urea formaldehyde resin (UF) is a commonly used resin that holds together the particles and confers the required mechanical properties to the panel for its final application. Along with UF, other formaldehyde-based resins are primarily used due to the combination of their effectiveness and relatively low cost (Amini et al. 2017), as well as their ease of application and lack of color (Salari et al. 2013). One of the most important disadvantages of UF when used for interior particle board is that this adhesive is well known as a carcinogen and its use poses a human health issue during both wood composite manufacturing and use (Diop et al. 2017). The emission of formaldehyde is most often caused by unreacted formaldehyde trapped as a gas in the structure, as well as formaldehyde dissolving in water that enters the panel (Salari et al. 2013).
Despite the high toxicologic risks when using this adhesive, the global formaldehyde business is expected to reach 36.6 million tons at the end of 2026, due to the construction market being the biggest consumer of these resins (Transparency Market Research 2018).
Other than adjusting the urea to formaldehyde ratio, various fillers can be utilized to reduce the amount of resin needed. Common fillers must be insoluble in UF, these include cellulose, silica, talc, and chalk (Claub et al. 2011). Traditional fillers are made of larger particles, limiting mobility and making homogenization difficult (Dukarska and Czarnecki 2016). Thus, smaller particles, i.e. nano-sized, can induce effective properties such as improved mechanical strength and thermal resistance (Dukarska and Czarnecki 2016), as well as lower resin consumption, thus substantially reducing costs. (Lei et al. 2008). Among the nano particles used as filler for wood adhesives, nanoclay and nano-SiO2 are some of the reported on the literature (Lei et al. 2010; Zahedsheijani et al. 2012; Salari et al. 2013; Dukarska and Czarnecki 2016).
As the most abundant natural polymer in the world (Klemm et al. 2005), and due to the continuous improvement of technology designed to isolate materials to the nanoscale, cellulose has been positioned to be used in a number of high-performance applications. When reducing its size into the nanoscale, cellulose fibers can be separated into small particles generally known as nanocellulose (Klemm et al. 2011; Moon et al. 2011; Lavoine et al. 2012). These nanoparticles can be obtained by different approaches; the most commonly used are chemical and mechanical treatments to obtain cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), respectively. In recent years, nanocellulose has been increasingly studied for its many intriguing properties and immense potential. On the side of wood composites research, micro and nano fibrillated cellulose have been recently investigated as a filler for wood adhesives in particle boards (Mahrdt et al. 2016; Hansted et al. 2019; Morais Júnior et al. 2020), OSB (Veigel et al. 2011, 2012), and plywood (Kawalerczyk et al. 2020). Veigel et al. (2011) studied the effect of the addition of CNF fibers into UF for wood beams. As a result, they determined that by adding 2 wt.% of CNF, the adhesive toughness increased up to 45 %. Following the same approach, Veigel et al. (2012) reported the effect of nanocellulose reinforced UF and melamine urea formaldehyde (MUF) adhesives for particle boards and OSB manufacture. They demonstrated that by adding 1 wt.% of CNF, the fracture energy and fracture toughness can be improved for both wood panels. In addition, Mahrdt et al. (2016) described the addition of microfibrillated cellulose (MFC) to UF for particle boards resulting in better mechanical performance. These results were determined to be due to the larger particle size when adding MFC, improving the adhesive availability for bonding with other particles. Recently, Kawalerczyk et al. 2020, studied the effect of CNC addition on plywood panels to react with phenol formaldehyde (PF) resins. They found that 3 g of CNC to 100 g d.m. of resin, was the optimum ratio to help the effective transference of stress along the bond line and they observed the better mechanical properties f the panels at that ratio.
On the attempts to reduce the adhesive consumptions, some research groups have worked on the production of wood panels using nanocellulose as a complete replacement for commercial adhesives. Diop et al. (2017) demonstrated that using 20% unbleached CNF at 3 wt.% of consistency on fiber board panels improved the internal bonding and modulus of rupture when compared with 15 and 25 % CNF addition. Recently, Kojima et al. (2018) did similar work on particle boards, showing that the higher the amount of CNF added to the panel, the better the properties. As a negative aspect, when comparing particles boards made only with UF or (PF), they concluded that the properties of the board with 20wt.% CNF corresponded to those of the boards with 1 wt.% UF or PF.
Although several groups have studied the addition of CNF or MFC to wood panels showing how the properties can be improved, the actual interactions between UF and CNF, which may help to improve the final properties of the boards, are unknown.
The objective of this work was to demonstrate how UF resins and CNF from bleached and unbleached cellulose pulps interact in real time by using the Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D).