Discoloration of poly(vinyl acetate) (PVAc) gluelines in wood assemblies

The bondline chromatic alteration of poly(vinyl acetate) (PVAc) in wood assemblies has an appreciable impact on the aesthetic characteristic of these products (mostly important for panels used in furniture). While the origin of this phenomenon has been explained for the cases of pure polymers and wood substrates alone, present work investigates about the origin of discoloration for the case of bondlines prepared with selected wood species (cherry and maple) and two commercial PVAc dispersions, one containing and one not containing aluminum ions. Assemblies were exposed to temperature and UV light. The comparison (carried out through GC-MS analysis) among the compounds extracted by these species before and after exposure allowed identifying specific flavonoids that were used as model-compounds to replicate the discoloration process in laboratory. Thus, using spectrophotometric measurements and TLC analysis it is shown that discoloration is related to the presence of aluminum in the dispersion, whose interaction with flavonoids in wood generates different forms of complexes that can be modified upon irradiation and emitting in the visible range. Although the specific involved compounds are species-related, it can be suggested that the identified mechanism at the basis of discoloration in PVAc-wood bondlines has general validity.