Abstract
In this study, the interactions between a phenol–formaldehyde resorcinol (PRF) adhesive and water-extractable wood constituents were investigated using combined in-situ FTIR spectroscopy and rheology analysis for a simultaneous examination of the progress of chemical reactions and coherent changes in rheological properties during adhesive curing. Complementary evolved gas analysis and pyrolysis gas chromatography/mass spectroscopy (Py-GC/MS) were performed to detect differences in the final crosslinking and chemical composition of the cured adhesive, respectively. The rheological and chemical analysis results correlated with the tensile shear strength of wood-PRF assemblies. The results showed that adhesive curing was significantly affected by the presence of acidic wood extractives. In particular, the acidic extractives of chestnut wood led to a delay in the resin curing and less final crosslinking of the cured adhesive. This was most likely caused by a reduction in the catalytic effect of the base-catalyzed curing of the adhesive rather than by direct chemical reactions between the extracts and adhesive. These findings can be useful for adapting the resin formulation to the chemistry of acidic wood species.
Highlights
Wood bonding is considered to be a challenging process as wood has a complex hierarchical structure and is highly variable in terms of species-specific chemical and structural properties
Some additional in-situ rheology-FTIR measurements were performed with some standard compounds, such as gallic acid representing an acidic compound in the extract chemistry
It has been revealed that even the addition of a minor amount of acidic chestnut extract (< 1%) to the phenol resorcinol formaldehyde (PRF) adhesive influenced the adhesive curing prominently by resulting in prolonged curing reactions and less final crosslinking of the adhesive
Summary
Wood bonding is considered to be a challenging process as wood has a complex hierarchical structure and is highly variable in terms of species-specific chemical and structural properties. The actual reason behind this is the potential physical (e.g. the alterations in surface phenomena: pH, wetting behaviour etc.) or chemical interactions (e.g. chemical interference of the adhesive curing by the wood chemistry) between the chemical wood constituents, in particular the wood extractives, and the adhesives. These interactions between the wood extractives and the adhesives are known to be influential on the wood bonding performance (Roffael 2016; Özparpucu et al 2020). Wang (1992) reported inhibitory effects of white oak extractives on phenolic adhesive curing. Prolonged curing times for the phenolic adhesives were reported when gluing oak and Kapur (Hse and Kuo 1988)
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