Introduction

Nowadays, Environmental friendly chemicals have gained importance and industries are using techniques and chemicals with no or less impact to the environment. Chitosan as a biopolymer has been introduced to textile industries to treat textiles and protect them against water absorption. This polymer consist amine groups on its chemical structure. The chitosan is a linear polysaccharide made from monomers of 2-amino, 2-deoxy, β-D-glucopyranose (Stössel and Leuba 1984; Kumar 2000). This polymer is a cation that is prepared in alkaline condition from removal of acetyl groups connected to carbon number 2 in monomer structure (Mima et al. 1983). The chitosan polymer is non-toxic with antibacterial properties with higher reactivity (Muzzarelli et al. 1995; Sanford 1989). It exhibits a variety of physic chemical and biological properties, therefore this polymer has found numerous applications in various fields such as, agriculture (Linden et al. 2000), fabric and textiles (Enescu 2008), cosmetics (Kumar 2000), and food processing (Senel and McClure 2004) in addition to its lack of toxicity and allergenicity, its biocompatibility, biodegradability and bioactivity make it a very attractive substance for diverse applications as a biomaterial in pharmaceutical and medical fields (Shahidi et al. 1999) The chitosan was also studied as modifying agent in wood and wood composites. Treu and Larnøy (2007) applied the chitosan in Scots Pine (Pinus sylvestris) and Beech (Fagus sylvatica) as treating agent to study their resistance against soft rot fungi according to ENV 807. They treated both wood samples by applying heat under vacuum condition. Their report indicated increase of the bio-resistance against the soft rot fungi. However, the resistance of the pine wood treated with heated chitosan was higher than the beech wood. Influences of the chitosan on some properties of wood such as water absorption, bending modulus of elasticity and rupture and resistance against white rot fungus Trametes vesicular gave been studied. (Larnøy et al. 2005). The wood samples were treated with the chitosan 4 % (pH 5.5) by full cell process followed by heating at 60, 80 and 100 °C for 36, 48 and 60 h. Some samples were not heated during the treatment. Results revealed that the treated samples at 100 °C for 60 h had less water absorption than that of samples treated at 60 and 80 °C. However, all properties of both heated and non-heated samples were similar except their color. It was also indicated that the chitosan treated samples had less water absorption than that of the untreated ones. Also, the treated samples showed less bio-resistance against white rot fungi. There were also no significant differences for module of rapture and elasticity in treated and untreated samples. There was no report to indicate penetrability of the chitosan macromolecules in micro pores of the wood cell walls. It is not clear whether the chitosan exists in lumen or cell walls. Therefore, it can be thought about preparing its nano-particles to apply in wood structure. However, there is no report to indicate whether the nano-chitosan has been used to treated wood or not. Of course, there are some reports dealt with other nano-particles, e.g. nano-silver, nano-titanium (Taghiyari 2010; Sudeshna et al. 2011).Therefore, this research work was planned to prepare nano-particles of chitosan and apply them in wood to study their effects on physical and mechanical properties of wood. This idea was planned according to previous reported results of the chitosan treatment (Treu and Larnøy 2007; Witt and Treu 2009; Larnøy et al. 2005). The macro-particles of chitosan can enhance some properties of wood. There might be expected that nano-particles of the chitosan should present better properties than its macro-molecules due to smaller sizes providing easier penetration into wood cell walls.

Materials and Methods

Nano-chitosan (1 %) preparation

Chitosan (Sigma Aldrich, MW 100,000–300,000 Dalton with deacetylation degree of 85 % made from shrimp skin) was weighed 0.5 g and water (50 ml) was added and mixed gently for 8 h at room temperature. methacrylic acid(0.5 ml) was also added to distilled water (50 ml) and mixed for 8 h at room temperature. The chitosan solution was heated at 70 °C under nitrogen for 1 h to avoid oxygen. Afterwards, the methacrylic acid solution was gently dripped on the chitosan solution and then kept for 1.5 h under the nitrogen atmosphere at 70 °C. Then, potassium per-sulfide (K2S2O8) as 0.054 g was added to the solution and kept for 1.5 h at the same condition. Finally, the solution was cooled down for 2 h in ice bath and its pH was adjusted for 4 (De Moura et al. 2008).

Chitosan 1 %

Methacrylic acid 1 ml and chitosan 1 g were added to distilled water (100 ml) and mixed up for 3 h. Finally, HCl 1 M and NaOH 1 M were used to adjust the solution pH for 4.

Determination of nano-chitosan properties

Nano particle sizes

Scanning electron microscope (SEM, Philips XL30, Holland) as well as zeta meter (Zeta meter, 3000HS, Malvern, England) were used to measure sizes of the nano particles. The nano powders were fixed on a stub and coated with the gold and observed at 20 kV by the SEM.

FTIR spectroscopy

Pellets of the nano-chitosan particles as well as the chitosan were prepared and studied with a Shimadzu, FTIR 1650 Spectrophotometer, Japan to indicate any changes in the chemical structure and formation of new bonds in the nano structure.

This technique was also used for the treated wood as explained below to study consequences of reactions with the chitosan and the nano-chitosan. Wood flour (mesh 40) was prepared and dried at 103 ± 2 °C. The pellets were taken by using potassium bromide. The FTIR spectra were taken at a resolution of 4 cm−1 after 32 scans. Rubber band base line corrections were also used for all spectra.

Wood treatment

Blocks of beech wood (Fagusorientalis) and fir wood (Abies sp.) were prepared as 2 × 2 × 2 cm for determination of the physical properties and 30 × 2 × 2 cm for the mechanical properties. Afterwards, the blocks were treated with the freshly prepared chitosan 1 % and nano-chitosan 1 % for 24 h under vacuum. The treated blocks were then wrapped in Al-foils and heated at temperatures 60, 80 and 100 °C for 24 h to for the reactions. Finally, the samples were oven dried at 103 ± 2 °C.

Physical properties

Bulking effect

Bulking of the treated samples was determined according to Eq. 1 as follow:

$$ B = \frac{{{\text{V}}_{\text{t}} - {\text{V}}_{0} }}{{{\text{V}}_{0} }} \times 100 $$
(1)

where B is bulking effect (%), Vt and V0 are wood volumes after and before modification, respectively (cm3).

Density

Oven dry density of the treated samples was determined according to Eq. 2 based on ASTM D-2395.

$$ {\text{D}}\,{ = }\,\frac{{{\text{M}}_{0} }}{{{\text{V}}_{0} }} \times 1 0 0 $$
(2)

where D is oven dry density (g/cm3), M0 is oven dry weight (g) and V0 is volume after modification (cm3).

Water absorption and swelling

Weight and volume of the oven dried wood blocks were determined before and after treatment and then soaked in water for 24 h to determine their water absorption and swelling according to the following equations.

$$ {\text{A}}\, = \,\frac{{{\text{W}}_{\text{t}} - {\text{W}}_{0} }}{{{\text{W}}_{0} }} \times 100 $$
(3)

where A is water absorption (%), Wt and W0 are wet and oven dry weights respectively (g).

$$ {\text{S}} = \frac{{{\text{V}}_{\text{t}} - {\text{V}}_{0} }}{{{\text{V}}_{0} }} \times 100 $$
(4)

where S is swelling (%), Vt and V0 are wet and oven dry volumes respectively (cm3).

$$ {\text{ASE}} = \frac{{{\text{S}}_{\text{unt}} - {\text{S}}_{\text{t}} }}{{{\text{S}}_{\text{unt}} }} \times 100 $$
(5)

where ASE is anti-swelling efficiency (%), Sunt and St are swelling of untreated and treated samples respectively (%).

Mechanical properties

Bending strength

Bending module of elasticity (MOE) and rupture (MOR) were determined for treated and untreated samples according to ASTM D-143. The MOE and MOR were calculated according to the following equations:

$$ {\text{MOE}} = \frac{{{\text{P}}_{\hbox{max} } {\text{L}}^{3} }}{{4{\text{WB}}^{3} {\text{X}}}} $$
(6)
$$ {\text{MOR}} = \frac{{3{\text{P}}_{\hbox{max} } {\text{L}}}}{{2{\text{WB}}^{2} }} $$
(7)

where MOE is modulus of elasticity and MOR is modulus of rupture (N/mm2); L is span (mm); W is sample width (mm); B is samples thickness (mm); X is displacement of the maximum load; Pmax is maximum load (N).

Impact load resistance

This strength was determined according to ASTM D-256 and calculated based on Eq. 8.

$$ {\text{I}} = \frac{\text{F}}{\text{A}} $$
(8)

where I is impact load resistance (J/mm2), F is impact load (J) and A is area (mm2).

Compression parallel to grain

This strength was determined in the treated and untreated samples according to ASTM D-143 and calculated based on Eq. 9.

$$ {\text{P}}_{\text{u}} = \frac{{{\text{F}}_{\hbox{max} } }}{\text{A}} $$
(9)

where Pu is ultimate compression stress (N/mm2), Fmax is load (N) and A is area (mm2).

Bio-resistance

From the treated and the untreated samples, ten mini-blocks were cut into 2 × 1 × 0.5 cm (along the grain direction) and oven dried to weight them before any fungal inoculations. The samples were sterilized and incubated for 70 days at 25 ± 1 °C and 65 ± 5 % relative humidity. Brown rot fungus Coniophora puteana and white rot fungus Trametes vesicular were used to decay (Bravery 1978). After fungal exposure, the samples were weighed to determine dry weight to calculate mass losses after fungal attack according to Eq. 10.

$$ {\text{M}}_{\text{L}} = \frac{{{\text{W}}_{0} - {\text{W}}_{D} }}{{{\text{W}}_{0} }} \times 100 $$
(10)

where ML is mass loss (%), W0 and WD are dry weight before and after fungal attacks.

Results and discussion

Water absorption and swelling

Volumetric swelling as well as anti swelling efficiency are shown in Figs. 1 and 2 for treated and untreated samples. As shown, both chemicals could not decrease the swelling at temperatures 60 and 80 °C. An increase in the swelling in the nano-chitosan treated woods were even observed, especially in beech samples. However, treatment of the samples at 100 °C was more effective to reduce the swelling and increase the ASE in the treated woods (Fig. 1). For chitosan treated species of wood, the swelling and ASE didn’t change with increasing the temperature. However, the nano-chitosan was affected by temperature. As shown, application of the heat at 60 and 80 °C increased the swelling in the beech wood with no changes in the fir wood. However, it was different at 100 °C.

Fig. 1
figure 1

Anti-swelling-efficiency after treatment with nano-chitosan and chitosan: (a) beech, (b) fir

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Fig. 2
figure 2

Water absorption of wood after treatment with nano-chitosan and chitosan: (a) beech, (b) fir

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Water absorption of the treated samples of beech wood was not severely affected by the treatment and the reaction temperature as shown in Fig. 2a. However, there was varying effect for the nano-chitosan and slight reduction by the treatment temperature (Fig. 2b). High water absorption was achieved in the treated fir wood at 100 °C. Water absorption is an indication of porous structures in the cell walls when the swelling is decreased and the ASE is increased due to the treatment; it indicates that the cell wall polymers are affected by the treatment solutions. It is likely that both treatment solutions provided micro-spaces in the cell walls permitting penetration of the water molecules and causing water absorption. However, degradation and removal of hygroscopic chemicals in the cell walls during the treatment are responsible to reduce sorption sites and therefore providingless water absorption as well as less swelling and increased ASE.

Bulking and density

As shown in Fig. 3, there was no bulking effect of chitosan and the nano-chitosan treatments in both beech and fir woods treated at 60 and 80 °C except 100 °C (less than 0.5 %) which is negligible. However, both treatments caused slight bulking in the fir wood. treatment temperature could affect the bulking effects due to the nano-chitosan. Fir wood could achieve a bulking of about 4 % at temperature 100 °C (Fig. 3b).

Fig. 3
figure 3

Wood bulking due to treatment with nano-chitosan and chitosan: (a) beech, (b) fir

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Wood density was not affected significantly due the treatments as well as the temperature. There was only a slight reduction of density of the fir wood which was not important.

Bending strength

None of the treatment of wood with the chitosan and the nano-chitosan had significant effect on the bending strength of the two investigated species (Fig. 4a, b). Therefore the treatments couldn’t increase the strength of the woods. But there were not any reduction in the strength of the woods due to the treatments as well; this can be a good result of them.

Fig. 4
figure 4

Effect of different treatment on MOE-MOR

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Impact load resistance

It was revealed that treatment of the beech wood with the chitosan could increase this property, however, the nano-chitosan could affect this strength. It should be expressed that none of the treatment of the fir wood could affect its impact load resistance (Fig. 5). However, penetration of the chitosan into beech wood cell Lumina and its polymerization might be a reason for increase in the impact load resistance.

Fig. 5
figure 5

Effect of different treatment on Impact load resistance

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Compression parallel to grain

The nano-chitosan could only affect this strength in beech wood with no change in the fir wood after the treatment. The chitosan was also not able to affect the compression strength in both treated woods as shown in Fig. 6.

Fig. 6
figure 6

Effect of different treatment on Compression parallel to grain

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Bio-resistance

Mass losses of the treated wood are shown in Fig. 7a, b after 70 days of white- and brown-rot fugal attacks. It was revealed none of the treatment could preserve wood against both fungi. It meant that the chitosan and the nano-chitosan don’t have ability to protect cell wall polymers against the attacking fungi.

Fig. 7
figure 7

Wood mass losses in treated wood due to Coniophora puteana (a), Trametes vesicular (b)

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FTIR spectra

FTIR spectra of the nano-chitosan and the chitosan treated woods are shown in Figs. 8 and 9. It was revealed from the beech wood’s IR spectra that major structural changes were occurred at wave numbers of 3,456, 1,743, 1,496, 1,427, 1,273 and 1,041 cm−1 (Fig. 8). The peak appeared at wave number 3,456 cm−1 is assigned for O–H groups bonded to the cellulose structure. This peak was reduced in the nano-chitosan treated wood; while the peak was similar for chitosan treated and untreated woods. Any reduction in O–H groups is an indication of hydrophobicity which was occurred in the treated wood. This phenomenon was previously shown for the swelling and ASE. Any reduction in C–H structures in the hemicelluloses and the cellulose as shown at wave number 2,923 cm−1 is an indication of removal/cleavage of these bonds and/or probable substitution of the hydrogen bearing structures. Depletion of the hydrogen is probably because of hydrolysis due to accompanying acid used with the nano-chitosan. In the fir wood was also minor reduction in peak dealt with the O–H groups; while there was increase in the C–H peaks as shown in (Fig. 9). Any increase in this peak is an indication of probable substitutions of C–H with the nano structures. In the beech wood, carbonyl peak (wave number 1,743 cm−1) was reduced due probably to opening of the conjugated bonds and formation of new linkages with other groups in the hemicelluloses. Any increase in this peak indicates bonding of new carbonyl bearing structures to the hemicelluloses polymer in the fir wood. The reduction peak 1,496 cm−1 is assigned for the C–H in lignin structures is slightly shifting to higher wave numbers. Any reduction in this peak indicates probably depletion of the hydrogen and its substitution with new structures in beech wood lignin. In the fir wood, there was no change in this structure; while there were some shifting in peaks 1,512 and 1,500 cm−1 to higher wave numbers. This is probably due to cleaved aromatic structures in the lignin and formation of new structures. Such changes were determined in the beech wood. There was a reduction in the peak 1,273 cm−1 which is related to CH2 structures. These structures were not changed in the fir wood.

Fig. 8
figure 8

FTIR spectra of the nano-chitosan, chitosan-treated and untreated Beech wood

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Fig. 9
figure 9

FTIR spectra of the nano-chitosan, chitosan-treated and untreated wood fir

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Scanning electron microscopy

Determination of the nano particle sizes showed a range of 20–68 nm with an average of 39 nm. The scanning electron microscopy showed that the nano particles are mostly in polygonal structures. Such configuration with fine structures permits to the nano particles to have capabilities of better bonding as well penetration into chemical structures (Fig. 10).

Fig. 10
figure 10

Nano-chitosan prepared

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Conclusions

This research work was planned to study physical, mechanical and bio resistance properties of the nano-chitosan treated wood and compare it with the chitosan treated one. It was believed that the nano-chitosan particles should be able to react with wood cell wall polymers due to decreased sizes with higher surface to volume ratios. Therefore, it was expected that the nano-chitosan should be able to react with wood better than the chitosan.Results showed that treatment of the wood with the nano-chitosan and the chitosan 1 % under a vacuum conditioned as well as the heat could enhance the physical properties. However, any influences of the nano-chitosan were better than the chitosan. Any treatment of the wood did not affect its mechanical properties except the impact load resistance which was increased by the chitosan treatment under vacuum and heat. nano chitosan treated samples showed only slight improvement in effectiveness against the brown rot fungus Coniophora puteana and the white rot fungus Trametes vesicular. There should be expressed that the nano-chitosan is not recommended for applications where the rotting fungi are active.