Wood modification with hydrophobation textile finishing agents

Wood modification with hydrophobation textile finishing agents
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Goettingen University, Germany, Chế biến gỗ, , 2007 ;
Tác giả	Nguyễn Hồng Minh
Người hướng dẫn	Prof. Dr. Holger Militz & Dr. Carsten Mai
Từ khóa
DOI   [ URL]  [{{{PDF}}} PDF]

Finishing agents are applied in textile industry to improve certain properties such as form stability and hydrophobicity. Cotton textiles mainly consist of cellulose, their chemical composition is very similar to that of wood. Improving natural wood durability for outdoor application through wood thermo chemical modification with textile finishing agents is main objective of this research. Wood modification was carried out with 18 agents from textile finishing industry (Table 1.8). The main chemical groups are: N-methylol and fatty acid condensate compounds, metal – ion paraffin emulsions, functional polydimethylsiloxanes (epoxy and amino functional silicones), fluoropolymer dispersion, polyethylene, stearylurea derivative and oxim-blocked polyisocianate. The typical wood species from Germany, i.e. Scots pine sapwood (Pinus syvestris L.) and beech (Fagus sylvatica L); and from Vietnam, i.e. rubber wood (Hevea brasinliensis L.) and paulownia (Paulownia fortunei L.) were selected for experiments. Chemical wood modification was carried out to improve the resistance of treated wood against different types of biological attacks, i.e. basidiomycetes, soft-rot decay and staining fungi. The critical requirement of the treated wood is to get high hydrophobicity, which leads to reducing hygroscopic property and improving dimensional stability of wood. It is important to optimize curing conditions for the chemical impregnated wood while maintaining strength properties and controlling crack performance. Artificial weathering (QUV) was performed to evaluate crack performance and examine fixation of the chemicals on wood.

Hydrophobicity of treated wood

Impregnated wood was cured for 16h at an approximate 120oC (depended on the chemicals applied) for fixation and reactions of the chemicals with wood composition. Water submersion test was performed as far as saturation water uptake state achieved to evaluate water uptake reduction of the treated wood in comparison to untreated wood. In the first 24h of water submersion test, water uptake of the treated wood was found very much lower (30% - 70%) than that of untreated wood because of the cell lumen and pits were filled up by the hydrophobic agents closing water path during water submersions. For this mode of action, hydrophobicity of wood is mainly influenced by hydrophobicity of the impregnated chemicals. In addition to that effect, during the followed 24h – 96h water submersion, reducing water uptake (20% - 40%) of the treated wood for a long time is caused by reactions of the chemicals which block hydroxyl groups (OH) in wood, hence hydrophilicity of wood is remained. The textile finishing agents, which impart high hydrophobic characteristic for treated wood are fatty acid N-methylol compounds (Phobotex VFN, Persistol HP), aluminum and zirconium salt containing paraffin (Hydrophobol APK, Ramasit KGT and Persistol E), quaternary ammonium siloxane (Hansa SQE 2620), fluoropolymer dispersion (Oleophobol C, Oleophobol 7713), epoxy functional siloxane (Hasan ESE 6960), oxim-blocked polyisocianate (Hydrophobol XAN) and stearylurea derivative (Siligen PW).

Hydrophobicity of wood treated with Al-FNM, AP and ES

Fatty acid modified N-methylol compounds (FNM, HP) work as a chemicals complex where reactions of N-methylol active ingredients with hydroxyl group of wood polymers blocking hydrophilic hydroxyl groups of wood, while long carbon chain (from fatty acid side) and paraffin in the compound imparted it own very high hydrophobicity for the treated wood. Metal-ion containing paraffin compounds (AP, R and E) imparted very high hydrophobicity for treated wood. Curing at elevated temperature play a role to coat and fill up well the molten paraffin compounds in the wood pits and lumen, hence closing path for water penetration. Epoxy functional siloxane compound (ES) impart advantage characteristic for hydrophobicity modified wood, in which the methyl groups contribute hydrophobic property, while epoxy ends are able to react with hydroxyl groups of wood through generated hydroxyl groups of ES at elevated temperature, that blocks hydrophilic hydroxyl groups in wood polymers causing water uptake decreased of the ES treated wood. Beech wood treated with fatty acid N-methylol (Al-FNM, 10%) and epoxy functional siloxane (ES, 14%) at 120°C for 16h result 2% equilibrium moisture uptake lower as compared to the untreated wood.

Dimension Stability of Treated Wood

Decreasing equilibrium moisture and water uptake of wood treated with Al-FNM (WPG 10%) and ES (WPG14%) caused anti shrink efficiency (ASE) by 19% and 9% respectively. A significant increased ASE (27%) was found when wood treated with Al-FNM at a higher WPG (16%). Even aluminum containing paraffin (AP) treated wood has high hydrophobicity caused by low water uptake but the dimension stability of the treated wood was not achieved (ASE ≈ 0%) when equilibrium moisture content was not decreased in comparison to the untreated wood.

Artificial Weathering

A fraction strength test of weathered wood was developed to evaluate the cracking performance of modified wood against weathered degradation. Improvement of dimension stability of the treated wood limited cracks development after exposures to weather, consequently higher fraction strength (rupture force) was gained as compared to the untreated wood. The results after 4 QUV cycles test (672h weather exposures) proved dimension stability improved of Scots pine sapwood, beech, rubber and paulownia wood treated with fatty acid N-methylol (Al-FNM) and epoxy functional siloxane (ES) as compared to the untreated wood (fraction strength was strongly decreased up to 70%). Even high hydrophobicity surface characteristics, but has no anti-shrink efficiency, the AP treated wood result mainly slowing down of crack development as compared to the control. As the naturally high demission stability wood species, fraction strength of treated rubber and paulownia wood was not much decreased after weathering.

Chemical Penetration

Chemical uptake of the wood impregnated with the textile finishing agents depends on solid content, particle size of the chemical and wood structure. Weigh percent gain was found high (up to 18%) when beech impregnated with quaternary ammonium siloxane (SQ, 40%) and amino functional polysiloxane (A8, 58%). Beech wood with 10mm grain direction can absorb relatively equal chemical quantity of the silicones (SIO, A7, SIN, A8, SQ, MSI and ES) and fatty acid N-methylol (FNM, HP) compounds. With 10% and 30% stock concentration applied, weight percent gain of those impregnated specimens can obtain about 4% and 10% respectively. Penetration was found lower with paraffin (AP, R, and E), polyethylene (VN), fluoropolymer (C and O7), polyisocianate (H) and stearylurea (PW), since their particle size is large. Wood structure (Appendix 1, 2 and 3) influences strongly to chemical penetration when other factors of impregnation process are equally conditioned. The depth of penetration of fatty acid N-methylol (FNM), aluminum containing paraffin (AP) and epoxy functional siloxane (ES) compounds was found highest in Scots pine sapwood, followed by beech, rubber and paulownia wood.

FTIR Spectra of Impregnated Wood

Penetration of Phobotex VFN/Catalyst RB (FNM), Hydrophobol APK (AP) and Hansa ESE 6960 (ES) into wood was further detected basing on IR spectrum presence of the chemical functional groups in the impregnated Scots pine sapwood. The presence of the constituents of Al-FNM (methoxy groups, primary amide from melamine, C-N stretching of CH2-N, C-O stretching of methylol group, C-O-C and C=O stretching derived from fatty acid) is mostly pronounced up to 10mm depth of penetration. The IR spectrum of C3H3N3 triazine ring of melamine in methoxymethylenmelamine structure showed a deeper penetration within 20mm. Penetration of Al-FNM compound in the longitudinal direction was found up to 11cm. Epoxy functional siloxane (ES) compound can penetrate into 22 mm in the radial direction of Scots pine sapwood. The outstanding IR spectra peak of the ES impregnated pine sapwood was caused by effects of CH3 and CH2 stretching symmetric, Si-CH3 deformation, C-O stretches symmetric of carbohydrates in wood, Si-O stretches of siloxane and Si-O-Si stretching vibration. It was observed up to 12 cm long penetration of epoxy functional siloxane in the longitudinal direction of pine sapwood. Aluminum containing paraffin (AP) compound has long carbon chain that makes it difficult for penetration into wood structure. Investigations by FTIR at the IR spectra peaks of CH3 and CH2 group in the paraffin revealed presence of paraffin in Scots pine sapwood at less than 2mm radial and 2cm in the grain direction respectively.

Spectromicroscopy Analysis

FTIR microscopy (IR microscope Hyperion 3000) has a technique limitation of small aperture size (10 µm), while cell wall thickness of Scots pine sapwood is mostly less than or equal to 10 µm. Therefore the presence of epoxy functional siloxane (ES) has not been clearly distinguished in the cell wall of Scots pine sapwood when analyzed by FTIR microscopy technique.

SEM/EDX Measurements

SEM/EDX analysis is a very efficient technique to detect penetration of the metal elements, which is contained in the chemical impregnated wood. SEM/EDX X-ray line analysis showed that epoxy functional siloxane (ES) was strongly presented at 20mm and 50mm in radial and longitudinal penetration depth in pine sapwood respectively. Microscopic distribution of the silicone showed mostly presence of silicone in the cell lumen late wood; while precipitated mainly in the void space of the bordered pits in the cell wall early wood. When Scots pine sapwood impregnated with aluminum containing paraffin (AP) and fatty acid modified N-methylol with 15% aluminum salt (Catalyst RB), signals of aluminum were not significantly found higher than that of other available inorganic elements in pine sapwood at 10mm radial penetration depth.

Mechanical Properties of Modified Wood

Wood treatments with Al-FNM, AP and ES under condition of super-heated steam curing at 120°C for 16h does not decrease strength properties (MOE, MOR and hardness) of the treated Scots pine sapwood, beech, rubber and paulownia wood.

Biodegradation Resistance

Hydrophobicity characteristic of modified wood plays an important role to decrease biological degradation. Scots pine sapwood, beech, rubber and paulownia wood were modified with fatty acid modified N-methylol (Al-FNM), epoxy functional siloxane (ES) and aluminum salt containing paraffin (AP). The results showed significantly improved resistance of the treated wood (especially Al-FNM treated wood) against wood destroying basidiomycetes, soft-rot and blue stain (indoor and out door) attacks. The aluminum salt composed in Al-FNM and AP compound played an important role to increase treated wood resistance against biological degradation.

Basidiomycetes Resistance

Fatty acid N-methylol compound (Al-FNM) was capable to reactions with hydroxyl groups in wood activated a biocidal function for high resistance (durability class 1, EN350-1) against attacks of white and brow rot decay. Wood treated with aluminum salt containing paraffin compound (AP) was able to meet standard EN350 class 1 for a period of 8 weeks testing. However, with a longer exposure time (24 weeks), the basidiomycetes resistance of the treated wood was classified as durability class 2. Increasing equilibrium moisture content of the AP treated wood after a long period incubation made more favorable for further attacks by basidiomycetes. Beech wood was treated with epoxy-functional siloxane (ES) to achieved high durability (Class 1) against attacks by white rot Trametes versicolor. However, the efficiency against brown rot Coniophora puteana of Scots pine sapwood treated with epoxy-functional siloxane was not high (durability class 2 - 3). The research also found that the used aluminum salt (in Al-FNM and AP compound) has a biocidial function which strengthens treated wood resistance against basidiomycetes degradation.

Soft-rot Resistance

Relatively high efficiency against soft-rot decay was found on the wood treated with fatty acid N-methylol compound containing aluminum salt (Al-FNM). After 32 weeks incubation in soil-bed test, Al-FNM treated Scots pine sapwood showed almost no loss in both dynamic modulus of elasticity and mass. When beech, rubber and paulownia wood were equally treated with Al-FNM, the loss in dyn.MOE and mass were of between 13 - 20%. Soft rot degradation of the untreated wood was recorded as 74 - 100%. In all the treatments, wood treated with aluminum salt containing paraffin (AP) and epoxy functional siloxane (ES) diminished soft-rot decay by about 50% in comparison to decay of untreated wood. Aluminum salt was found as active ingredient in the fatty acid modified N-methylol (Al-FNM) compounds with an important role for the resistance of treated wood against soft-rot decay.

Blue Stain Resistance

Wood treated with aluminum salt containing paraffin (AP) and fatty acid N-methylol/aluminum salt (Al-FNM) showed significantly high resistance against blue stain (Aureobasidium pullulans) infection (up to class 0 – 1). The resistance was found lower in the case of epoxy functional siloxane (ES) treated wood when the same WPG was applied. In all the cases, where Scots pine sapwood, beech, rubber and paulownia wood impregnated with aluminum salt (RB) and aluminum sulphate (Al2(SO4)3 resulted significant efficiency against blue stain A. pullulans. Wood impregnated with catalyst RB (8% WPG) and Al2(SO4)3 (7.2 - 20.6% WPG) resulted no blue stain grown on the impregnated wood. Beyond hydrophobic characteristics of wood treated with Phobotex VFN (FNM), Hydrophobol APK (AP) and Hansa ESE 6960 (ES), once again, this study showed effectiveness of the aluminum salts in resistance to A. pullulans infection.

Outdoor Weathering of Modified Wood

The results from testing the Al-FNM, ES, AP treated wood during one year outdoor exposure are comparable to the results of blue stain resistance, fraction and dimension stability of the treated wood from indoor tests. Scots pine sapwood, beech, rubber and paulownia wood treated with Al-FNM, AP resulted highly resistance against blue stain infection. The efficiency was significantly high in the reverse side of the treated specimens. Photo degradation of the Al-FNM, ES and AP treated wood due to ultraviolet light happened slower than as in untreated wood. Wood treated with Al-FNM and ES diminished cracking wood after outdoor exposures. Dimension stability of the Al-FNM and ES treatment wood was improved. Cracking performance is slower in the wood treated with paraffin compound (AP), but its dimension stability is not improved over a long time of outdoor exposure. The improvement in dimension stability happened only in wood treated with Al-FNM and ES, diminishing equilibrium moisture uptake is essential condition to prevent cracking and deformation of the treated wood. Dimension stability of the treated rubber and paulownia wood was significantly increased since they have naturally very high dimension stability, moreover reinforced by all the treatments with aluminium salt fatty acid modified N-methylol (Al-FNM), epoxy functional siloxane (ES) and aluminium salt paraffin (AP) compounds.

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Bằng sáng chế do Cộng đồng Châu Âu cấp liên quan đến luận án: European Patent EP1716995