Effect of curing conditions on the water vapor sorption behavior of melamine formaldehyde resin and resin-modified wood

Abstract

Impregnation modification of wood with melamine formaldehyde resin reduces the adverse effects caused by moisture uptake, but the underlying modes of action are not fully understood. The present study showed that it is crucial to understand the sorption behavior of the pure resin when interpreting the behavior of resin-modified wood. Furthermore, the applied heat-curing conditions had a significant effect on the moisture uptake of resin-modified wood. At the same resin loads, dry curing conditions were more effective in causing a cell wall bulking effect than wet curing conditions. This reduced the water-accessible cell wall pore volume in dry cured wood and counterbalanced the moisture uptake by the resin. Deuterium exchange measurements suggested that the occupancy of cell wall pores reduced the number of simultaneously active sorption sites. However, there was no evidence that a swelling restraint or reduced mechanical relaxation affected the water sorption of resin-modified wood significantly.

Sorption behavior and hydroxyl accessibility of wood treated with different cyclic N-methylol compounds

“…Solid wood blocks were milled in a cutting mill (RETSCH SM 2000, Retsch GmbH, Haan, Germany) to pass through a 2 mm mesh screen. Hydrogendeuterium exchange (HDX) was performed in a dynamic vapor sorption (DVS) apparatus (DVS ET, Surface measurement Systems, London, UK) as described previously [18]. In short, approximately 20 mg of wood particles were first dried using the pre-heater at 60°C for 6 h under dry nitrogen flow (0% RH) followed by a temperature stabilization period to 25°C for 2 h. The particles were then exposed to deuterium oxide (D 2 O) vapor at a target RH of 95% for 12 h. Finally, the dry mass was determined again as described above.…”

“…All measurements were done in triplicate. The amount of exchanged H and the amount of absorbed D 2 O (both in mmol g -1 ) was calculated as described by [18] using either the dry specimen mass or the dry wood mass (excluding the mass of the added resin) as the reference mass for the calculations.…”

“…Accessible sorption sites were quantified by the HDX approach, which has previously been applied to thermally modified wood [42,43] and wood that was treated using carboxylic acid anhydrides [14,44], furfuryl alcohol [45] or melamine formaldehyde [18]. The results were illustrated by relating the mass of exchanged H or absorbed D 2 O either to the dry sample mass or the dry wood mass (Fig.…”

“…The moisture exclusion of the treated wood by the cell wall bulking and/or cross-linking effect may be counterbalanced by the ability of resins to take up moisture [18]. Monomers of glyoxal resins like DMDHEU contain free OH groups which constitute 38% of the molecular weight [7].…”

Humidity-dependence of the hydroxyl accessibility in Norway spruce wood

“…Several studies have shown that the amount of absorbed water can change independently from the OH accessibility (Stevens and Smith 1970;Rautkari et al 2013;Salmén and Stevanic 2018). There is evidence that additional factors have a strong impact on the amount of absorbed water without affecting the sorption site density, such as the degree of crosslinking in heat-treated wood (Altgen et al 2018;Willems et al 2020) or the spatial availability of wood cell walls in modified wood (Thybring et al 2020;Altgen et al 2020). Furthermore, Lindh et al (2016) showed that OH(3) groups in cellulose that are associated with the C(3) atom of the glucose units are unreactive to H-D exchange, but these OH groups may still form hydrogen bonds with water molecules.…”