Enhancing wood hardness through surface impregnation via Michael addition: a study on penetration patterns of three North American hardwoods

High-density wood is crucial for interior applications, especially for engineered wood flooring, as mechanical properties such as hardness are correlated with density. However, high-density wood is scarce and expensive. Densification methods allow low or moderate-density woods to replace harder species, transforming them into high-performance, high-value products. The objective of this project was to increase the surface hardness of three abundant North American hardwoods, sugar maple (Acer saccharum Marsh.), yellow birch (Betula alleghaniensis Britt.), and red oak (Quercus rubra L.), through lateral impregnation of the Michael Addition formulations. Samples were densified using three formulations based on Michael addition reactions with acrylate and malonate components. Results showed that the pattern of formulation penetration into lumens and vessels depends on viscosity over curing time. Chemical retention was higher in red oak, and formulations with lower initial and overtime viscosity retained more chemicals. As expected, an asymmetric density profile was observed via X-ray densitometry and microtomography in comparison to bulk densification methods. Passive chemical densification was confirmed by confocal Raman spectroscopy in densified wood. All densified woods showed a significant increase in Brinell hardness compared to untreated wood, although no significant differences appeared between samples with different formulations. The highest hardness was reported for sugar maple, which has a higher initial hardness, while the increase in surface hardness was greatest for red oak. Overall, samples densified with formulations containing a mixture of diacrylate and triacrylate with moderate viscosity (around 27 cP) and a glass transition temperature (around 55 °C) showed slightly higher surface hardness.