Abstract
To enhance compression stability and fire retardancy of densified wood, a new modification method i.e., combined nitrogen–phosphorus (NP) fire retardant pre-impregnation with surface thermo-mechanical densification is used to fabricate a certain thickness of functionalized surface layer on poplar. This combined treated wood is investigated via vertical density profile (VDP), and the compression stability is revealed by both soaking test and cone analysis. Results demonstrate that the combined treatment hardened the surface of wood and reformed the interface combination of the NP with the wood cell wall, thus making the surface tissue more close-grained. Fire retardancy was also enhanced; the total heat release and CO generation values decreased by 21.9% and 68.4%, respectively, when compared with that of solely NP-treated wood. Moreover, surface hardness increased by 15.8%, and the recovery of surface hardness and thickness were 56.8% and 77.2% lower than that of simply densified wood. It appears that this NP-involved thermal densification could be considered as an alternative approach to enhance both the compression stability and fire resistance of wood.
Highlights
Wood has been considered as one natural composite material for a long time, and it is one of the most popular materials in construction, interior decoration, and the furniture industry mainly because of its strength-to-weight ratio, renewability, and versatility [1]
This study provides a novel combined modification, which can functionalize the surface layer and endow multiple functions to the treated wood
The dendrogram of Hierarchical cluster analysis (HCA) for treated samples in regard to surface color (Figure 5a) shows two main clusters; the WNP-TM was classified into the first group, which contains WHT and WHT-TM, while the Wc and WTM were classified into the second group, further proving that the NP-TM process can lead to color generation
Summary
Wood has been considered as one natural composite material for a long time, and it is one of the most popular materials in construction, interior decoration, and the furniture industry mainly because of its strength-to-weight ratio, renewability, and versatility [1]. Fang et al (2011, 2012) combined oil-heat treatment with wood densification and claimed that the HT could efficiently improve dimensional stability, mechanical properties, and compression set recovery of THM densified veneers [25,26]. In former studies, NP has been used as an acidic-accelerating medium to intensify the effect of HT on wood It brought about significant property improvement for the NP pre-impregnated and heated wood at a low treatment temperature for a short period of time [29,33]. The NP fire retardant is used to reinforce poplar wood together with the thermal densification, aimed at fabricating a functionalized surface layer to enhance fire retardancy and compression stability of combined treated wood. It will be useful for promoting the usage of plantation timber
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