Abstract
When applying an adhesive to wood, the chemical heterogeneity of the wood cell walls makes it difficult to understand the contribution they make to the interfacial adhesion between the adhesive and the wood as the adhesion is a very complex physical and chemical phenomenon. This study, for the first time, directly measured the surface adhesion between cellulose, a major component of wood, and urea–formaldehyde (UF) resin adhesives. The adhesion between thin, smooth nanocellulose films, such as cellulose nanofibrils (CNFs), carboxymethylated nanofibrils (CM)–CNFs, and carboxylated cellulose nanocrystals (C–CNCs), and UF resins with two formaldehyde to urea (F/U) molar ratios of 1.0 and 1.6 was measured using two approaches: (1) direct measurement of the adhesion force between nanocellulose films and liquid droplets of the UF resins, and (2) calculation of the work of adhesion between films of the nanocelluloses and UF resins using the contact angle and the van Oss–Chaudhury–Good method. The results show that the total surface free energies, either between the different nanocelluloses or between the two UF resins are somewhat similar, indicating the similarity in their surface properties. However, the adhesion force and work of adhesion of 1.6 UF resins with different types of nanocellulose are higher than those of 1.0 UF resins, which shows that van der Waals forces are dominant in their molecular interactions. These results suggest that the adhesion between 1.6 UF resins and nanocellulose is stronger than that when 1.0 UF resins are used because the 1.6 UF resins have a more branched structure, smoother surface, and higher surface free energy.
Highlights
For a long time, wood has been used for many purposes, as a structural material in engineering, as a fuel, and in wood-based panels (Pereira Oliveira Moreira et al 2020)
The high amount of free formaldehyde present in the 1.6 UF resin is capable of facilitating the curing of the sample, resulting in a shorter gelation time, because it acts as a bridge that connects the oligomers formed in the pre-cured resin (Wibowo and Park 2020). 342 343 Table 2
This suggests that dispersion forces are dominant in the adhesion between the thermosetting UF resins and nanocellulose, which is in agreement with the results of another study (Gustafsson et al 2012), which showed that van der Waals interactions play a dominant role in the adhesive interactions between lignin, cellulose, and hemicellulose
Summary
Wood has been used for many purposes, as a structural material in engineering, as a fuel, and in wood-based panels (Pereira Oliveira Moreira et al 2020). This study is focused on the direct measurement of adhesion between cellulose, the major component of the wood cell wall, and the most extensively used wood adhesives, UF resins, which are prepared with two different F/U molar ratios of 1.0 and 1.6. To this aim, three types of nanocellulose are used in this study, namely, cellulose nanofibrils (CNFs), carboxymethylated nanofibrils (CM)– CNFs, and carboxylated cellulose nanocrystals (C–CNCs). Lai et al compared the JKR contact mechanics and AFM peak– force mode measurements combined with a colloidal probe technique to measure the interfacial adhesion between cellulose surfaces (Lai et al 2019). Where Sc represents the area of the crystalline domain and St is the area of the total domain (crystalline + amorphous)
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