Zefang Xiao

Dynamic water vapour sorption properties of wood treated with glutaraldehyde

The dynamic water vapour sorption properties of Scots pine (Pinus sylvestris L.) wood samples were studied to investigate the modifying effects of glutaraldehyde. Pine sapwood was treated with solutions of glutaraldehyde and a catalyst (magnesium chloride) to obtain weight per cent gains of 0.5, 8.6, 15.5, and 21.0%, respectively. The sorption behaviour of untreated and treated wood was measured using a Dynamic Vapour Sorption apparatus. The results showed considerable reduction in equilibrium moisture content of wood and the corresponding equilibrium time at each target relative humidity (RH) due to glutaraldehyde treatment. The moisture adsorption and desorption rates of modified and unmodified wood were generally faster in the low RH range (up to approximate 20%) than in the high range. Modification primarily reduced the adsorption and desorption rates over the high RH range of 20–95%. Glutaraldehyde modification resulted in a reduction in sorption hysteresis due to the loss of elasticity of cell walls.

Effects of chemical modification on the mechanical properties of wood

Chemical modification has been recognized as an efficient strategy for dimensionally stabilizing wood and protecting it from environmental damage, such as deterioration due to weathering and fungal decay during the service period. Studies reported in the literature mainly concern the establishment of workable modification techniques, testing methodologies, and assessment of the durability of modified wood. The development of wood modification techniques has recently been reviewed; limited information is however given on the effects of chemical modification on the mechanical properties of wood that are of importance to it as an engineering material. This paper reviews the effects of wood modification, typically by heat treatments and impregnation with low molecular weight resins, reactive monomers, or hot melting paraffins on the mechanical properties of wood. The modifying variables associated with mechanical properties of wood such as wood species, treating temperature and time, catalyst, type of solvent, weight percent gain, and molecular structures of the modifying agent were analysed and the results interpreted. The reasons for changes in the mechanical properties of wood are discussed.ZusammenfassungChemische Modifikation wird als ein wirksames Verfahren zur Verbesserung der Dimensionsstabilität und zum Schutz gegen umweltbedingte Schäden wie zum Beispiel Holzabbau aufgrund von Bewitterung oder Pilzbefall während der Gebrauchsdauer angesehen. In der Literatur vorhandene Studien befassen sich hauptsächlich mit geeigneten Behandlungsverfahren, Prüfmethoden und der Beurteilung der Dauerhaftigkeit von modifiziertem Holz. Die Entwicklung von Holzbehandlungsmethoden wurde kürzlich beschrieben, jedoch gibt es nur wenig Informationen hinsichtlich der Einflüsse einer chemischen Modifikation auf die mechanischen Eigenschaften von Holz im Hinblick auf seine Nutzung als Bau- und Werkstoff. In diesem Artikel werden die Einflüsse einer chemischen Modifikation, üblicherweise durch Hitzebehandlung oder Imprägnierung mit niedermolekularem Harz, reaktiven Monomeren oder heiß schmelzenden Paraffinen, auf die mechanischen Eigenschaften von Holz untersucht. Einflussgrößen auf die mechanischen Eigenschaften von Holz wie Holzart, Behandlungstemperatur und –dauer, Katalysator, Art des Lösungsmittels, prozentuale Gewichtszunahme und molekulare Struktur des Modifiziermittels wurden untersucht und die Ergebnisse diskutiert. Gründe für die Änderungen der mechanischen Eigenschaften wurden erörtert.

The fungal resistance of wood modified with glutaraldehyde

Abstract Scots pine sapwood (Pinus sylvestris L.) and European beech wood (Fagus sylvatica L.) were treated with glutaraldehyde (GA) in aqueous solution in the presence of magnesium chloride as a catalyst to evaluate the durability improvement towards staining and rot fungi. The GA modified specimens were dipped in a spore suspension of the blue stain fungus Aureobasidium pullulans and incubated for 8 weeks. The growth on both pine and beech wood was restrained, when the weight percent gain (WPG) of the specimens was above 7%. Under this condition, GA-modified beech wood did not suffer any mass loss after incubation with the white rot fungus Trametes versicolor. The threshold to prevent decay of beech and pine specimens towards the brown rot fungus Coniophora puteana was at a WPG of only 3%. GA treatment to a WPG over 6% protected the Scots pine stakes from soft rot decay during 32 weeks’ exposure according to ENv 807 (2001).

Esterification of wood with citric acid: The catalytic effects of sodium hypophosphite (SHP)

Abstract Sodium hypophosphite (SHP) has been recognized as the most efficient catalyst in the esterification reaction of cellulosic fabrics with citric acid (CA), but both the high cost and the environmentally harmful property of SHP call for optimization of its application. In this study poplar wood (Populus adenopoda Maxim.) was treated with CA to various weight percent gains (WPGs) and the effect of SHP on the resulting properties of treated wood was investigated. Esterification with CA can occur also in the absence of SHP, as evidenced by the resistance to water leaching of CA. Wood treated with CA alone to 36% WPG exhibited 7% bulking, 50% anti-swelling efficiency, 30% reductions of the modulus of rupture, and 50% lower impact strength. Treatments with CA in the presence of SHP provided wood properties comparable to wood treated with CA alone. Thus the application of SHP can be questioned from the point of view of an economic production and environmental protection.

Modification of poplar wood with glucose crosslinked with citric acid and 1,3-dimethylol-4,5-dihydroxy ethyleneurea

Abstract Poplar wood (Populus adenopoda Maxim) has been modified by glucose (GLC) in the presence of citric acid (CA) or 1,3-dimethylol-4,5-dihydroxy ethyleneurea (DM) as crosslinker agents. GLC can penetrate easily the wood cell walls, and after crosslinking, the cell wall is bulked. At 20% GLC concentration level, the leaching ratio of GLC incorporated into wood decreased from 73% to 15% in the presence of 10% CA or DM. The crosslinking efficiency of DM was higher than that of CA. The fixed chemicals in the cell walls caused ca. 6% permanent bulking, namely, a 43% reduction in volumetric swelling after water saturation. FTIR spectroscopy shows that GLC can be activated in the presence of a catalyst; however, the reaction of GLC with a wood polymer is very limited. Scanning electronic microscopy (SEM) reveals that some of the chemicals remained located in the cell lumens. The findings demonstrate that GLC alone is not efficient, but joint treatment of GLC with crosslinkers is a feasible way of wood modification.

Coupling pattern and efficacy of organofunctional silanes in wood flour-filled polypropylene or polyethylene composites

The coupling efficacies of four silanes, i.e. n-propyl-trimethoxysilane, γ-amino-propyltrimethoxysilane, γ-methacryloxypropyl trimethoxy silane, and allyltrimethoxysilane, were investigated in wood flour-filled polypropylene or polyethylene composites. Compared to untreated composites, treatment of wood flour with these silanes alone did not cause any improvement in the mechanical strength of the ensuing composites. In the presence of dicumyl peroxide, composites treated with γ-methacryloxypropyl trimethoxy silane and allyltrimethoxysilane exhibited an increase up to 90, 60, and 50% in flexural, tensile, and impact strength, respectively. Moreover, creep deformation was inhibited during the creep recovery test. The improvement was obviously greater in polyethylene-based composites than in polypropylene-based composites. This study demonstrates that the coupling efficacy of silane depends not only on the establishment of covalent bonds between silane and the matrix, but also on the chemical structure of the matrix.

Degradation of chemically modified Scots pine (Pinus sylvestris L.) with Fenton reagent

Abstract The Fenton reaction is supposed to play a key role in the initial wood degradation by brown rot fungi. Wood was modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) and glutaraldehyde (GA) to various weight percentage gains in order to study if these types of modifications are able to reduce wood degradation by Fenton reagent. Veneers modified with higher concentrations (1.2 and 2.0 mol l-1) of both chemicals exhibited minor losses in mass and tensile strength during treatment with Fenton reagent, which shows restrained oxidative degradation by hydroxyl radicals. The decomposition rate of H2 O2 was lower in the Fenton solutions containing modified veneers than in those containing unmodified controls. More CO2 evolved in systems containing unmodified veneers than in systems with modified veneers, indicating that modification protected wood from mineralisation. The reason for the enhanced resistance of modified wood to the Fenton reaction is attributed to impeded diffusion of the reagent into the cell wall rather than to inhibition of the Fenton reaction itself. The results show that wood modification with DMDHEU and GA is able to restrain the degradation of wood by the Fenton reaction and can explain why modified wood is more resistant to brown rot decay.

Combustion behavior of Scots pine (Pinus sylvestris L.) sapwood treated with a dispersion of aluminum oxychloride-modified silica

Abstract Treatment of wood with aqueous dispersions of silica that have been modified with aluminum oxychloride (AlOCl) can impart wood water repellence and increased resistance to fungal decay. This study is a comparative survey on the effects of treatment with modified and non-modified silica dispersions in terms of the combustion behavior of the Scots pine (Pinus sylvestris L.) to evaluate the fire risk of this wood utilized as a construction material. The thermogravimetric results showed that treatments with the silica dispersions did not change the pyrolysis temperature of wood polymers, i.e. there was no synergetic interaction between silica and cell wall polymers during pyrolysis. Cone calorimetry (CONE) indicated that the silica-treated wood required longer time for ignition than the untreated control, but wood chars were similar. Treatments with both silica dispersions led to considerable reduction in the heat release (HR) and smoke production, but the cationically modified silica was more efficient than the unmodified silica. These findings were interpreted that incorporation of modified silica did not substantially influence the pyrolysis of cell wall polymers because they have not penetrated the cell wall; they have rather reduced the fire risk via forming a barrier against oxygen access and a thermal protection shield.

Radiata pine wood treatment with a dispersion of aqueous styrene/acrylic acid copolymer

Abstract Wood of radiata pine (Pinus radiata Don) was treated with an aqueous styrene/acrylic acid (St/AA) copolymer dispersion leading to weight percent gains (WPGs) of 10–42%. The reactivity of St/AA in wood and the modifying effects on wood properties were investigated. The St/AA precipitated in the cell lumens and condensed under catalysis at elevated temperatures was shown via scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Treatment did not cause positive bulking efficiency and anti-swelling efficiency, because St/AA cannot diffuse into the cell walls due to its high molecular mass. The modulus of elasticity and rupture (MOE and MOR, respectively), and the surface hardness of the wood were improved by 24.4, 22.2, and 40.6%, respectively. Compression strength increased by 81.6 and 48.2% in radial and longitudinal directions, but the impact strength was hardly influenced by the treatment. In addition, the treated wood was slightly more hydrophobic than untreated controls as shown by reduced moisture content and water uptake. Accordingly, treatment with St/AA leads to mechanical reinforcement of wood and enhances its water resistance, and as a consequence, it has an application potential to improve the wood quality.

Vaporization heat of bound water in wood chemically modified via grafting and crosslinking patterns by DSC and NMR analysis

Abstract Radiata pine wood (W) was modified with acetic anhydride and glutaraldehyde (GA) resulting in WAc and WGA to various weight percent gains (WPGs), whereas in WAc the effect is due to grafting and in WGA, crosslinking. The heat of vaporization of bound water (BW) of the modified woods was studied by differential scanning calorimetry (DSC) and the mass loss (due to water loss) of the samples by thermogravimetry (TG). The temperature program was in both cases from 25 to 40°C with 10°C min−1. The adsorbed or condensed water in wood were observed via low-field nuclear magnetic resonance (LFNMR). At a comparable WPG level, the LFNMR analysis showed that the interaction of water with WGA was stronger than that with WAc. In both modified woods, a considerable reduction in the vaporization heat of BW was visible due to cell wall hydrophobization and bulking. The reduction of condensed water in micropores was lower for WGA than WAc, probably because BW needs more energy to evaporate from the crosslinked stiff WGA cell walls.