Improvement of mechanical properties of thermally modified hardwood through melamine treatment

ABSTRACT

Specimens of beech, ash, lime and poplar were thermally modified (T) and treated with an aqueous solution of melamine (M) resin to investigate the mechanical changes after combined (double) modification (TM). Density, solution uptake, weight percent gain, bulking and equilibrium moisture content were recorded to ensure proper treatment. Samples for Brinell hardness and three-point bending were cured at 120°C under dry conditions. The WPGs of the two treatment groups M and TM were similar, but bulking of TM specimens was negative. This might indicate an incomplete penetration into the thermally modified cell wall in combination with a potential leaching of soluble hemicellulose components by the alkaline impregnation solution. The decreased hardness of heat-treated wood was substantially increased by melamine treatment (combined modification). Both modifications and their combination slightly increased the modulus of elasticity. The modulus of rupture was increased after melamine treatment, decreased after thermal modification and combined modification. The work in bending was severely reduced for all treatments. Melamine treatment of thermally modified wood was carried out successfully and some mechanical properties were improved. Double-modified wood with increased modulus of rupture (MOR) and extraordinary surface hardness would be suitable for non-structural outdoor applications such as decking and cladding.

Abbreviations: ANOVA: Analysis of variance; EMC: Equilibrium moisture content; EMC_R: Reduced (corrected) equilibrium moisture content; IB: Impact bending strength; M: Melamine treated; MOE: Modulus of elasticity; MOR: Modulus of rupture; MUF: melamine-urea-formaldehyde resin; OD: Oven dry density; R: Untreated references; RH: relative humidity; SC: Solid content; SU: Solution uptake; T: Thermally modified; TM: Thermally modified and melamine treated (double modification); WB: Work in bending; WPG: Weight percent gain

KEYWORDS:

• European hardwoods
• mechanical properties
• melamine formaldehyde resin
• thermal modification
• double modification

Acknowledgements

The authors would like to thank INEOS Melamines for providing the resin, Timura Holzmanufaktur GmbH for the thermal treatment and Christoph Stiehm for his valuable help with the statistical analysis of the data.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Georg Behr http://orcid.org/0000-0002-9630-8637

Additional information

Funding

This study was funded by the Federal Ministry for Food and Agriculture via the Agency for Renewable Resources (FNR) under reference number 22024211.