Tensile shear strength of UF- and MUF-bonded veneer related to data of adhesives and cell walls measured by nanoindentation

Abstract

The tensile shear strength of veneer lap joints was characterised. The joints were produced with an Automated Bonding Evaluation System (ABES) using urea-formaldehyde (UF) as well as melamine-urea-formaldehyde (MUF) adhesive formulated for particleboard production. At a fixed heating temperature of 1108C, a systematic increase in bond strength was observed for both adhesives with increasing cure time. The absolute bond strength was significantly higher for MUF compared to UF. Nanoindentation experiments with the same specimens used for ABES revealed a very hard, stiff and brittle character of the UF resin, whereas the MUF proved significantly less hard and stiff, and less brittle. Wood cell walls in contact with adhesive, i.e., where adhesive penetration into the cell wall was assumed, showed significantly altered mechanical properties. Such cell walls were harder, stiffer and more brittle than unaffected reference cell walls. These effects were slightly more pronounced for UF than for MUF. Comparing UF and MUF, the micromechanical properties of cured adhesive and interphase cell walls confirm earlier observations that tougher adhesives can lead to higher macroscopic bond strength. In strong contrast to that, no obvious correlation was found between micromechanical properties and the strong cure time dependence of macroscopic bond strength.

Micromechanical properties of the interphase in pMDI and UF bond lines

“…With regard to nanoindentation results (Fig. 2), it is obvious that the micromechanical properties of wood cell walls in contact with UF are severely different from reference cell walls due to adhesive penetration (Bolton et al 1988;Stöckel et al 2010). For the presented results, both the modulus and the hardness of cell walls increase very clearly, whereas the work of indentation decreases compared with the reference (Fig.…”

“…Cell wall penetration of adhesive leads to a modification of wood structure and in consequence to a change in certain properties, e.g., reduced swelling and shrinkage due to changing environmental moisture content, which may be beneficial to the overall adhesive bond performance (Frihart 2009). By means of nanoindentation Stöckel et al 2010), it was shown that cell wall penetration of some adhesives (i.e., UF and MUF) not only provides mechanical support to damaged cells but also massively influences mechanical cell wall properties resulting in an embrittlement of the wood structure in the interphase region in case of UF and MUF. It was observed that this effect was not significant for polyurethane (PUR) but very clear for phenol-resorcinol-formaldehyde (PRF) adhesives (Gindl et al 2004b, c;.…”

Influence of the adhesive formulation on the mechanical properties and bonding performance of polyurethane prepolymers

“…It is known that PUR adhesives reach lower WFP at equal shear strength (Niemz and Allenspach 2009;Clauß et al 2011c), in particular under wet conditions (Vick and Okkonen 1998). Possible reasons for this could be changes in wood properties due to inter-or intracellular adhesive penetration Konnerth et al 2007b;Stöckel et al 2010) or gross penetration through the cellular network (Kamke and Lee 2007). The high gross penetration of the ductile polyurethane possibly contributes to delaying wood failure by reinforcing the wood.…”

Bonding of THM modified Moso bamboo (Phyllostachys pubescens Mazel) using modified soybean protein isolate (SPI) based adhesives

“…Most industrial wood adhesives are believed to bind wood elements through a combination of mechanical interlocking as the resin seeps into and sets inside pores (Mackay 1998;Packham 2003;De Maura and Hernandez 2005), and chemical interaction with cell wall constituents (Smith 1971;Gindl and Gupta 2002;Konnerth et al 2006;Frihart 2009;Hunt et al 2010;Stockel et al 2010). These studies have shown that the low-molecular weight components in wood-purpose adhesives including urea formaldehyde (UF), phenol formaldehyde (PF), melamine formaldehyde (MF), polymeric methylene di-isocyanate (PMDI) are also capable of infiltrating and chemically interacting with water and polymeric constituents of the cell wall structure of wood or fibres and modify their properties such as hardness and stiffness.…”