Comparative studies on the durability and moisture performance of wood modified with cyclic N-methylol and N-methyl compounds

Abstract

Glyoxal-based condensation resins like 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) have been used to modify wood and improve its resistance against decaying fungi. High biological durability of DMDHEU-treated wood has already been confirmed in laboratory and field tests in the past. However, the modes of protective action behind an improved decay resistance are not fully understood yet. Furthermore, it is questionable how the use of formaldehyde-poor N-methylol and formaldehyde-free N-methyl compounds instead of DMDHEU affects the moisture behavior and durability, respectively. In this study, wood blocks were treated with N-methylol (DMDHEU, methylated DMDHEU) and N-methyl compounds (1,3-dimethyl-4,5-dihydroxyethyleneurea; DMeDHEU). Untreated and modified specimens were exposed to different moisture regimes and wood-destroying fungi in order to study the indicators that control changes in the wetting ability and decay resistance. Both N-methylol and N-methyl compounds decreased the water uptake and release and increased the durability of Scots pine sapwood from ‘not durable’ (DC 5) to ‘very durable to durable’ (DC 1–2). However, high fluctuations were observed in water uptake and release as well as mass loss (ML) caused by fungal decay, when modified specimens were tested without passing through a cold-water leaching. Consequently, a significant effect of the leaching procedure according to EN 84 on the durability classification could be established. The latter appeared more pronounced for treatments with N-methyl compounds compared to N-methylol compounds. Finally, wetting ability (kwa) and resistance indicating factors (kinh) enabled a forecast of high biological durability for both treatments with N-methylol and N-methyl compounds under real service life conditions.

Biological durability and wood–water interactions of sorbitol and citric acid (SorCA) modified wood

“…In the present study, all tested collectives experienced a considerable decrease in the CWU in comparison to the untreated reference (Table 2). Similar decreases in the CWU were also observed in wood modified with cyclic N-methylol and N-methyl compounds [44]. Although the leaching procedure caused higher uptake of liquid water, it was still lower than that of the untreated control.…”

“…Corresponding studies on Scots pine sapwood modified with cyclic N-methylol (DMDHEU, mDMDHEU) and N-methyl (DMeDHEU) compounds indicated a decrease of all three W24 parameters [44] in comparison to the untreated reference samples. Similar to the improved resistance against wood-destroying basidiomycetes, a decreased water uptake of treated specimens might be attributed to a reduced spatial availability for water molecules due to CWB and cross-linking.…”

Review of Wood Modification and Wood Functionalization Technologies

“…For this reason, ultra-low formaldehyde and formaldehyde-free chemistries have been considered for wood modification. Specifically, methylated and diethyleneglycolated DMDHEU (mDMDHEU) and zeroformaldehyde N,N -dimethyl-4,5-dihydroxyethyleneurea (DMeDHEU) were studied for modifying wood [286,287]. Compared to DMDHEU, mDMDHEU molecules cause similar CWB but lower formaldehyde emissions during the polymerization reaction, which occur at a lower monomer reactivity.…”

“…In the past, decay resistance in resin treated wood was often attributed and correlated with the chemical loading expressed by the modification mass gain [262,268,306]. According to Emmerich et al [287], it is the cell wall deposition of the resin molecules (CWB), i.e., the chemical location on cellular level, rather than the chemical loading of a piece of wood itself (e.g., modification mass gain) that determines the resistance against fungal decay. Hence, the CWB was suggested as the important parameter for the decay resistance in resin treated wood.…”

“…This was clearly shown when comparing treatments with DMDHEU and DMeDHEU. During decay tests with basidiomycete monocultures, both resins increased the durability of Scots pine sapwood (Pinus sylvestris L.) from European durability class 5 ("not durable") to durability class 1 ("very durable"), even though the modification mass gain of DMeDHEU treated specimens measured one third of DMDHEU treated specimens, but identical CWB levels [287]. Emmerich et al [307] mimicked wood products with a homogeneously treated shell and an untreated core zone by a laboratory set-up.…”