1 Introduction

Chitosan and its derivatives emerged as good fungicides are widely used and developed quickly in medicine, agriculture, food, cosmetic, etc(Jiang, 2001; Hong et al., 2002; He et al., 2004; Yang et al., 2005). Their application in wood/bamboo industry started in 20^th century, mainly in wood preserving, dyeing, gluing and the production of wood-polymer composites(Duan et al., 2004; Furukawa et al., 2002; Tomonori et al., 1995; 1996; Sang et al., 2004). Recently, the application of chitosan-metal complexes(CMC)in bamboo preservation have been researched(Sun et al., 2004). The leachability of CMC from bamboo wood and decay-resistance of bamboo wood treated with CMC against white-rot fungus Coriolous versicolor were reported in our previous researching work. In this report, we'll focus on their resisting effects on the yellow-rot fungus Poria placenta. As a kind of construction materials, mechanical and gluing properties are also important. Therefore, the mechanical properties such as bending strength, bending modulus, compression strength vertical to grain, and the gluing performances were studied and reported in this paper.

2 Experimental 2.1 Materials

4-year-old bamboo wood was collected from Lin'an city, Zhejiang Province in November, 2004.The segment of 2 to 4 m from down on, with green and yellow faces planed off, were chosen and machined into specimens. The dimensions of the specimens were 20 by 20 by 5 mm(length by width by thickness) and free of knots. In the decay-resistance test, feeder strips were needed, with the size of 20 by 20 by 3 mm (length by width by thickness).

The yellow-rot fungus Poria placenta, bought from the Chinese Academy of Forestry, was used in this experiment.

Chitosan-copper complex(CCC, with copper ion 10.89%) and chitosan-zinc complex(CZC, with zinc ion 11.86%)were synthesized by reacting chitosan with the corresponding metal salt. CCB(copper, chrome and boron complexes)was prepared with the ratio of K₂CrO₇ to CuSO₄·5H₂O to H₃BO₃·7H₂O being 42.95 to 38.33 to 18.72.ACQ was bought from Yingtan wood preservative company, Yingtan, Jiangxi. Zinc chloride and copper chloride were bought from Shanghai reagents Co., Ltd.

2.2 Decay-resistance test methods 2.2.1 Preparation of specimens

The feeder strips should be immersed into water before using. Bamboo specimens should be sanded and gotten rid of burrs in order to get accurate mass changes, weigh specimens and group them according to their weight (weight differences among all the specimens should not exceed 0.5 g) and mark the specimens, six specimens for each group. Then the specimens would be oven-dried at 60 ℃ for 2 h, 80 ℃ for 2 h and then (105±2) ℃ for about 8 h until the weight of the specimens was constant. Bring the specimens into desiccator to cool them. Weigh the specimens to the nearest 0.001 g, the weight (T₁)was referred to as the initial or untreated weight of the specimens. After weighting the specimens were kept in desiccator until they were impregnated with preservative.

2.2.2 Preparation of test cultures

For both stock test-tube and petri dish cultures of the test fungi, use a nutrient medium as follows: Prepare malt solution 500 mL with the Baume density at 1.03, add 8.75 g agar into malt solution, and heat the solution until agar melted, then steam-sterilize the flasks at 121 ℃ and 0.1 MPa for 30 min. For the flasks applied in bamboo resisting tests, sand and bamboo powder substrate was used. The substrate was prepared by mixing brown sugar, corn flour, bamboo powder(size:0.4~0.75 mm) and clean sand(size:0.4~0.75 mm)according to the proportion of 1:8.5:15:150(mass ratio). Then the prepared substrate was carefully poured into 500 mL flask through a funnel, 180 g of the substrate per flask. Level the substrate and place directly on the substrate three feeder strips, then add 100 mL malt solution(with the Baume density at 1.03)slowly and carefully through a funnel, steam-sterilize at 121 ℃ and 0.1 MPa for 1 h.

2.2.3 Preparation and impregnation of specimens

Place the specimens to be treated with a given concentration of preservative in a suitable beaker with spaces between and weigh them down to prevent eventual floating on the treating solution. Place the beaker in vacuum desiccator, attach the apparatus to the suction pump and reduce the pressure in the vacuum desiccator to 0.1 kPa and hold this pressure for 30 min. Suck the prepared solution of preservative into the beaker slowly. After sufficient solution had been added, slowly admitted air into the desiccator to break the vacuum. Remove the beaker from the desiccator and cover with a glass. Leave the specimens submerged in the treating solution for 2 h. Remove the specimens from the solution individually, wipe lightly to remove the surface preservative solution, and immediately weigh to the nearest 0.001 g(T₂). Calculate the metal ion retention according to the following formula

Where:G=(T₂-T₁)=grams of treating solution absorbed by the specimens(initial weight of specimens before treatment substracted from the initial weight plus the treating solution absorbed), C=grams of metal ion in 100 g of treating solution, V=volume of block in cubic centimeters.

2.2.4 Control blocks

For each fungus, six control specimens were used, and they would put through all steps of the fest.

2.2.5 Leaching test

Expose the specimens to leaching by distilled water in a constant temperature room maintained at (28±1) ℃, 50 mL distilled water for each specimen. Place the beaker containing specimens and distilled water into a vacuum desiccator, and evacuate to a pressure of 0.1 MPa and hold this pressure for 1.5 h. Then slowly admit air into the desiccator to break the vacuum. Remove the beaker from the desiccator and cover with a glass. Leave the specimens submerged in the water. At every 24 h interval for a period of two weeks, remove the leaching water from the beaker. After fourteen days, remove the specimens from the water and weather them for five days, then oven-dried to constant weight and weigh them(T₃).

2.2.6 Decay-resistance test

1) Activation of the fungus: Inoculate Poria placenta on to malt and agar culture, and cultivate for 9 d at the temperature of (27±2) ℃ and relative humidity of (70±5)%. 2) Inoculation:After the sterilized sand culture were thoroughly cooled, cut fungus inoculum sections(with a diameter of 1 cm)from near the leading edge of mycelium in petri dish cultures. Aseptically place sections of inoculum in the center of the feeder strips of the sand culture. Cover the flasks with special films and incubate at the temperature of (27±2)℃ and relative humidity of (70±5)% until the feeder strips were covered by mycelium(approximately 2 to 3 weeks), then the culture bottles were ready to receive the specimens. 3) Sterilization of the treated specimens : Place the specimens by retention groups into petri dishes containing a filter paper soaked with 5 mL of water and steam-sterilize at 121 ℃ and 0.1 MPa for 20 min. 4) Incubation and duration of test: After cooling, aseptically place the specimens, with a long face centered in contact with the mycelium-covered feeder strip, in the previously prepared culture bottle. Then again cover the culture bottles with special films and incubate at the temperature of (27±2) ℃ and relative humidity of (70±5)% for three months. 5) Handling specimens after exposure to test fungi: At the end of the incubation period, remove the specimens from the culture bottles. Carefully brush off the mycelium and place the specimens on racks and weathering for five days, then oven-dry them to constant weight and weigh them(T₄).

Calculation of mass losses as follows

2.3 Test methods for the mechanical and gluing performances

The mechanical and gluing performances were determined according to the Chinese standard GB/T 15780-1995(testing methods for physical and mechanical properties of bamboos) and GB/T 17657-1999(test method of evaluating the properties of wood-based panels and surface decorated wood-based panels).

3 Results and discussion 3.1 Decay-resistance results

Decay-resistance results of bamboo wood treated with CCC, CCB or ACQ are seen in Tab. 1.

As the retention and fixation of copper ion in bamboo specimens had been studied in the previous research, this paper will mainly discuss on the resistance effects of fungicides against Poria placenta.

The resisting effects of CCC against Poria placenta were inferior to the effects against Coriolous versicolor, which resulted from the resistance of Poria placenta against copper ion. But the resisting effects of CCC against Poria placenta increased gradually as the retention of copper ion in the specimens increased, indicating again that high concentrations of CCC solutions could effectively prevent bamboo wood from being destroyed by decay fungi. When the retention of copper ion exceeded 5.141 kg·cm^－3, mass losses of the treated specimens dropped to 6.1%. CCB could resist against Poria placenta better than CCC. When the retention of copper ion in the specimens exceeded 3.320 kg·cm^－3, mass losses of the treated specimens were lower than 6.7%. The resistance effects of ACQ against Poria placenta were inferior to the effects against Coriolous versicolor, which may also result from the resistance of Poria placenta against copper ion. Among the above three preservatives, CCB was the best, then ACQ and CCC. But CCC had much prospective of being widely used in the future for its low toxicity, leachability and better preserving effects.

Decay-resistance results of bamboo wood treated with CZC and ZnCl₂ are seen in Tab. 2.

CZC had poorer resisting effect on Poria placenta than CCC and CCB, the mass losses of the treated specimens exceeded 28.2%. And the mass losses of the treated specimens decreased little with the increase of zinc ion retention. The lowest mass loss of the specimens treated with CZC was 26.4% when the zinc retention exceeded 5.091 kg·cm^－3.However, the resisting effect of CZC against Poria placenta was slightly better than that of ZnCl₂, mass losses of the later were between 30.8% and 39.2%, approaching to the mass losses of controls.

3.2 Mechanical and Gluing Performances

The mechanical strength including bending modulus, bending strength and compression strength vertical to grain was tested according to the Chinese standard GB/T 15780-1995 (testing methods for physical and mechanical properties of bamboos).

As can be seen from Tab. 3, the mechanical strength of specimens treated with fungicides decreased comparing with the controls. CCC had the least influence on the bending strength and bending modulus of specimens among the three fungicides, and CZC had the least influence on the compression strength vertical to grain. CCB had significant effect on bending strength and the compression strength vertical to grain, respectively decreased 25.8% and 46.7%.

CCC and CZC had little influences on the gluing strength of the specimens. The gluing strength of specimens treated with CCC and CZC were 1.286 2 N and 1.122 4 N respectively, approaching to those of the controls. But CCB had great influences on the gluing strength of the specimens. The gluing strength of specimens treated with CCB decreased to 24.6%.

4 Conclusions and suggestions

Decay-resistance effects of chitosan-copper complex and chitosan-zinc complex against Poria placenta were inferior to the effect against Coriolous versicolor. The mass losses of specimens treated with CCC varied from 4.1% to 32.7%. And only when the retention of copper ion exceeded 5.141 kg·cm^－3, mass losses of the treated specimens dropped below 6.1%. The mass losses of specimens treated with CZC were from 30.8% to 39.2%, approaching to the mass loss of the controls. The lowest mass loss of the specimens treated with CZC was 26.4% when the zinc retention exceeded 5.091 kg·cm^－3.CCB could resist against Poria placenta better than CCC. The resistance effect of ACQ against Poria placenta was also inferior to the effect against Coriolous versicolor. Among the above three preservatives, CCB was the best, then ACQ and CCC. But CCC had much prospective of being widely used in the future for its low toxicity, leachability and better preserving effects.

The mechanical strength of specimens treated with the test fungicides decreased comparing with the controls. CCC and CZC had the least influence on mechanical and gluing performances among the test fungicides.

Chitosan-metal complexes as non-toxic preservatives could effectively resist most of the decay fungi and mold fungi, and had little influences on the mechanical and gluing performances of the treated bamboo wood. But their effective resistances against those fungi always need higher concentrations of the treating solutions, which caused lots of troubles to the impregnation of wood or bamboo. Therefore, chitosan-metal complexes are promising preservatives but needing further research.