Compression Strength of Fir and Beech Wood Modified by Citric Acid

Previous articles have shown that modifi cation of wood by citric acid (CA) improves dimensional stability and resistance of wood against fungi attack. However, chemical modifi cation of wood also modifi es its mechanical properties in some way. The compression strength of wood is one of its representative mechanical properties. Modifi ed wood with lower values of compression strength has limited purpose. The intention of this work is to show the effect of wood modifi cation by citric acid on the compression strength of wood. Fir wood (Abies alba Mill.) and beech wood (Fagus sylvatica L.) were impregnated by citric acid with sodium-dihydrogen-hypophosphite (NaH2PO2) as a catalyst. Part of the impregnated samples together with control samples were cured at the temperature of 140 °C for 10 hours and the remaining samples were cured in microwaves for 35 minutes. The average compression strength parallel to the grain of wood modifi ed by CA, using different regimes of curing, was compared to the strength of unmodifi ed wood. The average compression strength parallel to the grain of wood was retained after modifi cation. In the case of fi r wood, the average compression strength was even improved after modifi cation. The results indicate that wood modifi ed by citric acid may be considered for the purposes where compression strength properties are equally important as improved durability and dimensional stability of wood.

The chemical modifi cation of wood is the reaction of a chemical reagent with the hydroxyl groups of holocellulose and lignin (Militz et al, 1997).
Over 90 % of cotton textiles consist of cellulose, while the percentage of cellulose in the wood is about 40 -50 %.Large percentage of cellulose in the wood explains the fact that impregnation solutions suitable for durable-press fi nishing of cellulose textile materials can be effi ciently applied to the wood.One of them is 1.3 dimethylol-4.5-dihydroxyethyleneurea(DMDHEU), which is successfully introduced to wood (Nicholas and Williams, 1987;Militz, 1993;Krause et al, 2003;Schaffert et al, 2005;Wepner and Militz, 2005;Hill, 2006).
Due to chemical modifi cation of wood, its mechanical properties may be improved, reduced or retained.Chemical modifi cation of wood may reduce its water absorption, which can also lead to improving its mechanical properties.Mechanical properties of wood can be improved by wood acetylating (Akitsu et al, 1993;Goldstein et al, 1961;Militz, 1991).On the other hand the degradation of wood cell wall by acid chemicals and higher temperature of thermocondensation during the wood modifi cation process (i.e.etherifi cation with alkyl chloride) can cause loss of strength.Some authors reported great strength loss of wood modifi ed by DMDHEU due to acid degradation of the cell wall (Nicholas and Wiliams, 1987;Hill, 2006).Xie et al. (2007) reported great loss of tensile strength of thin veneer strips modifi ed by DMDHEU, determined by a thin-veneer strip technique (Turkulin and Sell, 2002).According to them the cause is probably in the catalyst and namely magnesium chloride, which makes complex compounds with DMDHEU directly affecting the loss of wood strength.Another factor that limits the application of DMDHEU is hydrolytic formaldehyde release at elevated temperatures of processing, so new environmentally friendly chemicals have been introduced.One of them is citric acid (CA).Katović et al. (1994) esterifi ed solid fi r and beech wood with citric acid (CA) and NaH 2 PO 2 (SHP) as a catalyst.First results reported good improvement in wood stability giving guidelines to other researchers.Hasan et al. (2006Hasan et al. ( , 2007) ) and Despot et al. (2008) reported multiple increasing of biological durability of pine sapwood modifi ed by CA.
Bischof Vukusic et al. (2006) described the effect of wood modifi cation by CA on the reduction of water uptake and swelling of small wooden blocks.They compared the results with those obtained using DMDHEU as the modifying chemical.
In the present study we also focus on microwave curing.Microwave dielectric heating is based on the activation of polar molecules in the treated medium (polarization phenomenon).Cellulose, hemicelluloses and lignin are polar polymers whose electric properties are defi ned by polarization processes that take place because of the interactions between the molecules of the wood and the external fi eld (Torgovnikov, 1993).Microwaves allow more uniform and faster heating of the whole volume, not just the surface.An additional advantage of the MW treatment lies in the fact that the energy (heat) and the mass (water) fl ow are travelling in the same direction.These advantages are additional reasons to use microwaves for drying.Furthermore, the energy consumption is 60-70 % lower in the case of microwave treatment (Metaxas and Meredith, 1982).
In addition to all above, so far there has been no report on mechanical properties of wood modifi ed by CA, although mechanical properties can also be expected to be modifi ed.The compression strength of wood is one of its representative mechanical properties.Modifi ed wood with lower values of compression strength has limited purpose.The intention of this work is to show the effect of wood modifi cation by citric acid on the compression strength of wood.

MATERIJALI I METODE
Lattices with dimensions 20 x 20 x 800 mm (T x R x L) were sawn from air-dried radial boards of fi r wood and beech wood.Samples with dimensions 20 x 20 x 40 mm (T x R x L) were sawn and signed in succession (Fig. 1).
In this work water solutions of citric acid (CA) have been applied with NaH 2 PO 2 as a catalyst.
After conditioning at 20 °C and 65 % relative humidity, part of the samples (C t and C mw ) was impregnated with citric acid solution.The part of control sam-.....Šefc, Trajković, Sinković, Hasan, Ištok: Compression Strength of Fir and Beech Wood... ples (C2 and C3) was impregnated with distilled water, and the rest (C1) was left unimpregnated.
The number of parallel samples (series) was 30 (Figure 1).
(C-t) or in microwaves for 35 minutes (C-mw).After thermocondensation the samples were conditioned in climate chamber (20 °C and 65 % relative humidity) before wood strength measurements.To separate the effect of curing procedures from the effect of chemical modifi cation on mechanical properties of wood, control samples C2 and C3 were used.
Compression strength measurements parallel to the grain were performed in accordance with HRN D. A1.045 using Wolpert -universal machine for the determination of mechanical properties.

REZULTATI I DISKUSIJA
The compression strength parallel to the grain of unmodifi ed fi r wood was 52 MPa in average and of beech wood it was 73 MPa (Table 2).
Compression strength values of unmodifi ed wood are in accordance with other authors (Wagenfuhr, 1974; Govorčin et al, 2001Govorčin et al, , 2003)).After curing at the temperature of 140 °C for 10 hours, the average compression strength of fi r wood slightly increased (Figure 2), although not signifi cantly, while the average compression strength of beech wood remained unchanged (Figure 3).
In the case of microwave treatment, the average compression strength of fi r wood was unchanged while the average compression strength of beech wood increased slightly but not signifi cantly after microwave treatment.This results show that curing itself at the tempera-  The impregnation cycle consisted of a 5-minute initial vacuum of 2 kPa.The vacuum vessel was then fi lled with treating solutions and the vacuum of 2 kPa was maintained for 3 h, followed with an 18 h soaking at atmospheric pressure (Katović et al, 2004;Bischof Vukusic et al, 2006).
Impregnated samples were air-dried and then cured in oven at the temperature of 140 °C for 10 hours DRVNA INDUSTRIJA 63 (1) 45-50 (2012) ture of 140 °C or by microwaves has almost no infl uence on the average compression strength of wood.
When fi r wood was modifi ed by CA at the temperature of 140 °C, the average compression strength increased signifi cantly from 52 MPa (unmodifi ed wood) to 57 MPa (modifi ed wood) (Figure 2, Table 2).However, the range of compression strength values of modifi ed fi r wood was doubled compared to the unmodifi ed wood.In the case of beech wood, the average increase of compression strength also occurred due to wood modifi cation, but the increase was insignifi cant (Figure 3, Table 2).The amount of CA bound to wood is signifi cantly larger in fi r wood (Katović et al, 2006).Higher average compression strength of modifi ed fi r wood can be explained by better crosslinking of CA and wood.
On the other hand, a signifi cant decrease in average compression strength of modifi ed fi r wood cured by microwaves was observed.It is possible that during MW treatment in some fi r wood samples micro-splits occurred due to low permeability of wood.The range of compression strength values of modifi ed fi r wood was doubled compared to the unmodifi ed wood.It did not happen in the case of beech wood.The compression strength of beech wood modifi ed in microwaves was unchanged.One can assume that low permeability of fi r wood in contrast to beech wood contributed to uneven distribution of chemical bound to the wood, and that both factors led to doubled range of compression strength values in modifi ed fi r wood.
These results indicate the need of control over more parameters during microwave treatment to achieve better results.

ZAKLJUČCI
Exposure of wood to the temperature of 140 °C for 10 hours or to intermittent power of 750 W microwaves for 35 minutes has little or no infl uence on changing the average compression strength of wood.
The process of fi r wood and beech wood modification by CA in which wood was cured at 140 °C for 10 hours resulted in the improvement of the average compression strength of wood.The improvement was signifi cant in fi r wood and insignifi cant in beech wood.
One of the important goals of chemical modifi cations -to get wood with unchanged strength properties, has been achieved.
When intermittent power of 750 W microwaves was used for 35 minutes for curing, there was no improvement of the average compression strength.Instead, the average compression strength of fi r wood decreased and in beech wood it was retained during the process.
In addition to improved dimensional stability and resistance of wood against fungi attack, retaining or even improving the average compression strength of wood modifi ed by CA confi rms CA as a successful chemical in wood modifi cation process.
However, further research is needed on more parameters for predicting the mechanical properties of modifi ed wood.