Preliminary Study on the Mechanism of Color Change of Paulownia Wood

Abstract: The discoloration of paulownia wood is an important issue that seriously affects the processing and utilization of paulownia wood and the healthy and sustainable development of paulownia industry. It has also been a scientific problem that scholars at home and abroad are paying close attention to. In this paper, the discoloration types of Paulownia wood were identified and determined. On the basis of this, the discoloration causes, discoloration process and discoloration of Paulownia wood were studied in depth, and the change trend of wood composition in the process of discoloration of Paulownia wood was revealed. The theoretical basis.

Key words : Paulownia, discoloration, discoloration mechanism, wood

Mechanism Study on Paulownia Wood Stain

Abstract: Paulownia wood stain is a serious problem in wood process, which severely affect paulownia wood utilization and successful and sustainable development of paulownia production, it is also an international scientific difficulty that has been paid much more attention to. In this paper, the type Of paulownia stain is identified, and on the basis of it, the study is carried out on the reasons of paulownia wood stain, way, law, and the research is also on tend of the change of wood components during the process of stain, the Probe provide the theoretical evidence for paulownia wood stain control.

Key words: Paulownia, Stain, Stain mechanism, Wood

Paulownia is one of the important short-cycle oriented industrial timber species in China. It is widely used in the manufacture of furniture, wood-based panels, musical instruments, and so on. In the process of processing and use, paulownia wood is prone to discoloration, which seriously affects the deep processing and utilization of paulownia wood, which greatly limits the development of wood processing industry. From the early 1970s onwards, Sino-Japanese scholars have carried out various researches on the discoloration and prevention of paulownia materials, and have achieved certain results [3, 5, 6] . The type of discoloration of paulownia wood is considered by many scholars to be chemically discolored, and attempts have been made to extract chemical discoloration substances from them and to conduct discoloration prevention research based on these findings, but the research progress is not significant. This paper mainly studies the discoloration type, discoloration condition and discoloration law of paulownia wood. The important significance is that it is not to extract and separate one or several color-changing substances, but to find the cause from the way of color-changing substances, and to reveal the discoloration of paulownia wood. Under the action of factors, the change history of discoloration-producing substances occurs, and the discoloration mechanism of paulownia wood is clarified. In order to prevent the formation of various discoloration materials and cut off the source of discoloration materials, it provides an important scientific basis for the prevention and control of discoloration of paulownia wood.

Wood's natural beautiful texture and elegant color are one of the main reasons for its wide application in architecture and furniture. Many users like to use bright, defect-free wood. Unfortunately, some chemical, biological, and environmental factors can cause discoloration of wood, reduce surface ornamental value, and sometimes affect the structure and strength of wood. There are many factors that cause wood discoloration, and the type of discoloration is also complicated. But in general, the type of wood color can be divided into three categories:

1. Chemical discoloration: After the trees are harvested, the redox reaction occurs on the surface of the wood or inside the wood and the discoloration of the wood caused by exposure to chemicals during the production process. Including brown discoloration, oxidative discoloration of broad-leaved trees, discoloration of minerals, discoloration of iron.

2. Microbial discoloration: The initial decay of the wood and the growth of wood on the surface or inside of the wood causes the discoloration of the wood. Including initial decaying discoloration, mold discoloration, discoloration caused by discoloration bacteria, and discoloration of wood rot fungi.

3. Photochromism: Some substances in wood choose to absorb light with a wavelength greater than 290nm, and the electrons change between the energy levels, thus forming a chemical bond of photochromism, resulting in a change in the color of the wood.

1 Study on the type of discoloration of Paulownia wood

1.1 Determination of the type of microbial discoloration

1.1.1 Test method

A sample of Paulownia elongata wood having undergone discoloration was prepared, and the sample size was 10 cm × 10 cm × 5 cm (L × R × T), a total of 60 pieces.

According to the American scholar Wilcox (1964) [2] , the method for distinguishing wood microbial and non-microbial discoloration types, using a saturated aqueous solution of oxalic acid and hydrogen peroxide (hydrogen peroxide) to paint and decolorize the paulownia material, each treatment sample All are 30. The wood colorimetric values ​​before and after decolorization were measured.

The saturated aqueous solution of oxalic acid was a saturated solution, and the concentration of hydrogen peroxide (hydrogen peroxide) was 15%.

1.1.2 Results and discussion

The treatment of discolored wood with a saturated aqueous solution of oxalic acid and an aqueous solution of hydrogen peroxide (hydrogen peroxide) can clearly distinguish the type of wood microbial and non-microbial discoloration. If the discoloration wood is treated with a saturated aqueous solution of oxalic acid, the wood stain can be eliminated, and the wood discoloration type is non-microbial discoloration; and after the discoloration treatment of the discolored wood with hydrogen peroxide, the color can be eliminated. Spot, the type of discoloration is microbial discoloration.

The paulownia color-changing wood was subjected to decolorization treatment with a saturated aqueous solution of oxalic acid and hydrogen peroxide (hydrogen peroxide), and the colorimetric values ​​of the wood before and after the treatment were as shown in Table 1. The results showed that the wood color difference ΔE* was 0.32 before and after treatment with saturated aqueous oxalic acid solution, indicating that the wood color change before and after treatment was not obvious, indicating that the color spots on the paulownia material surface were not removed; and hydrogen peroxide (hydrogen peroxide) was used. After the decolorization treatment, the color difference ΔE* of the wood before and after decolorization was 11.51, and the change was obvious, indicating that the red stain on the surface of the wood after treatment was well eliminated, and the paulownia color spots could be removed. According to the method of Wilcox, it can be considered that the discoloration of the paulownia material belongs to microbial discoloration, not chemical discoloration or photochromism.

1.2 Paulownia wood photochromism test

1.2.1 Test method

(1) Surface discoloration observation method

A sample of Lancao Paulownia wood with discoloration was selected, and the sample size was 10 cm × 10 cm × 5 cm (L × R × T), a total of 60 pieces. Divided into two groups, each 30 pieces, one group is closed with transparent plastic film, the light can be freely penetrated; the other group is closed with black paper and black plastic film to ensure the light can not penetrate. Perform outdoor wind erosion tests and test color changes in stages.

(2) Different depths of paulownia wood test method

The test material that had just been harvested was immediately sawn, and then a sample of 10 cm × 10 cm × 5 cm (L × R × T) was prepared, and a total of 10 pieces were obtained. The sample was placed on a test stand to cause it to naturally change color. After 12 months, the color of the surface at different depths was measured. During the test, the surfaces of different depths were first cut by hand, the depth was measured with a vernier caliper, and then the color was measured. The color measurement method and the calculation statistical method are the same as the determination of the wood color before and after decolorization.

1.2.2 Results and discussion

Under the outdoor wind erosion conditions, the paulownia wood has a decreased brightness, a reddening degree, a yellowing degree, and an increase in chromatic aberration. The difference between light and no light is not obvious, indicating that light has a limited effect on the discoloration of paulownia wood, and paulownia discoloration is not a type of photochromism (see Figures 1 to 4).

In order to further determine the type of discoloration of the paulownia material, the color of the different depths of the paulownia material was measured. The results are shown in Table 2. It was found that the regularity of the change of the colorimetric indicators from the surface and the color was not obvious, indicating that the discoloration did not only occur on the surface of the wood, but penetrated into the interior of the wood. Studies have shown that wood chemical discoloration generally only exists in the surface layer of wood, and the discoloration caused by microbial fungi not only exists on the surface of wood, but also can penetrate into the interior of the wood, that is, there are color changes in the surface and inside [4,1] . Therefore, from the results of colorimetric colorimetric measurements of different depths of paulownia wood, the discoloration of paulownia wood is microbial discoloration. This study further confirmed the correctness of the type of discoloration of the aforementioned paulownia.

2 Study on the mechanism of discoloration of Paulownia wood

2.1 Isolation and identification of fungi from paulownia wood

2.1.1 Research methods

Separation of the color-changing fungus: One piece of the paulownia wood sample (3 cm × 2 cm × 1 cm) frozen in the refrigerator was taken for separation and culture of the paulownia wood color-changing fungus. The test was carried out by the American ASTM D2017-81 standard "Standard Method for Wood Corrosion Strengthening Test".

Simulated inoculation of paulownia wood color-changing fungi: The purified fungus was inoculated onto the sterilized paulownia wood small test piece and placed in a culture chamber for cultivation under the same conditions as in the above separation and culture test. Five replicates were performed and compared with uninoculated samples (control samples) to observe discoloration.

Identification of the species of paulownia wood color-changing fungi: The fungi isolated from the Lancao Paulownia wood color test were identified.

This work was commissioned by the Institute of Microbiology, Chinese Academy of Sciences.

2.1.2 Results and discussion

The fungus causing discoloration of paulownia wood was identified by Alternaria alternata (Fr.) Keissl and Rhizopus sp. by the Institute of Microbiology, Chinese Academy of Sciences.

The hyphae of Alternaria Alternaria are dark to black, and the conidiophores and conidia are also of similar color, often dark olive. Conidiophores are short, with septum, solitary or clumped, mostly unbranched, with spores on top. Conidia spindle-shaped or inverted rod-shaped, multi-cell, with horizontal and vertical diaphragm, brick wall. The conidia constants are chained. This genus is a common saprophytic bacteria on soil, air, and industrial materials. They are also parasites of certain cultivated plants. Rhizopus sp., commonly known as bread mold, is similar to Mucor, and is often found in spoiled food such as taro and sweet potato. They are also widely distributed in nature, and there are many Rhizopus spores in soil and air. Rhizopus is a genus that causes mold, fruit and other mildew. The mycelium of the genus Rhizopus is cotton-like, and the mycelium is topped with black sporangia. Rhizopus is highly viable, and most hyphae are aerial hyphae on the surface of the nutrient substrate, called spread. The vines are branched from the nodules to form pseudo-rooted hyphae, and the pseudo-roots penetrate the nutrient matrix to absorb nutrients.

2.2 Study on the color change law of Paulownia wood under the action of color changing fungi

2.2.1 Research methods

(1) Instrument

Temperature and humidity control room (box): Maintain temperature and humidity errors of ±1.1 °C and ±4%, respectively, preferably in accordance with the temperature and humidity of the culture chamber.

Culture room (box): The temperature is automatically controlled at 26±1.1°C, and the humidity is maintained at 70±4%.

Balance: With direct reading, the accuracy is 0.0001g.

Tray: The tray is wire-like, allowing the air around each block to flow freely during initial drying and is easy to handle.

Culture flask: round or square, at least 225ml volume, caliber at least 32mm, preferably with aluminum metal screw cap.

Other conventional equipment and glass instruments commonly used in biological tests: such as drying ovens, sterilization pots, refrigerators, nutrient bases, inoculation needles, inoculation clips, petri dishes and test tubes.

(2) Materials

Test fungus: use a color-changing bacterium isolated from paulownia wood.

Medium: Maltose agar: 2% of maltose and agar, respectively, and sterilized at 121 ° C for 15 minutes.

Sampling sample: taken from the heartwood part of Paulownia elongata wood, the height is located near the DBH of the tree, and the sample taken is representative and without defects. The size of the test piece used was 29x29x9mm, and 9mm was the growth direction of the wood. All wood blocks should have normal growth rate, density, no scars, no gum or resin, no obvious signs of fungal infection, and the test pieces used should be waterproof pen markings.

Auxiliary test piece: The bait tree used for cultivating fungi is birch, which is freshly cut and has no decay. The size is 3x29x35mm, and the longest size of the block is consistent with the direction of the tree fiber.

(3) Preparation of soil medium

Sticky sand with WHC 20-40% PH 5.0-8.0 break the clods, mix, sift, store in the can with the lid, the soil block can not wet the block to affect the sifting. Use 50mm diameter, 25mm deep cloth The funnel, put in the quick filter paper, fill the soil, shake it on the wooden table 3 (height 10 mm), scrape the top surface with a scraper. Place the loaded funnel in a 400 ml beaker, with wooden sides Block the block, add water to the beaker, slightly over the paper, wet it with the capillary, remove the air. When the top is wet, add water to the top of the funnel. Cover the beaker and let the soil absorb water for 12 hours. Cover the funnel with a damp cloth and place it on top. An inverted cup to prevent evaporation of water, pumped with a vacuum pump for 15 minutes, pumped out to the vessel, weighed W1, dried for 24 hours, 105 ± 2 ° C, said dry weight W2.

Calculate the water absorption A,%=((W1-W2)/W2)X100

Soil medium: 225 ml bottled soil 120 ml, its dry weight is not less than 90 g, weighed W3, dried 105±2 ° C, 12 hours, weighed dry weight W4, calculated soil moisture content B,%, calculated Add water: add water (g) = (1.3AB) {D / (100-1.3A)} D is the soil grams.

Prepare the culture bottle, add water, fill the soil with a long tube funnel, level it, place a piece of bait material that is not resistant to rot, and sterilize it at 121 ° C, 30 m. Be careful to keep the soil inside the bottle clean.

(4) Color change test of paulownia wood

The culture bottle containing the soil wood bait is sterilized, cooled, and a small piece (cultured 10 mm 2 ) of fungus-containing medium is cut from the culture dish and placed in a soil culture bottle to be in contact with the end of the bait. Cover the lid, that is, it should not be too tight or too loose. Place the inoculated bottle at a temperature of 26±1.1°C and a humidity of 70±4% for 3-5 weeks until the hyphae completely cover the bait, and then prepare the paulownia. Decay exposure test.

Sterilize and cool the small test piece used as the decay test, and then place one piece in each culture bottle. The cross section is placed on the top of the bait. All operations should be carried out under aseptic conditions to prevent mold infection. . Cover the cap and place it in a dark culture chamber for cultivation.

(5) Color change of paulownia wood

At different stages of decay, measure the color of some (not less than 20) Paulownia specimens. The instrument used is TC-PIIG automatic colorimetric colorimeter, using CIE L* a* b* (1976) of the International Commission on Illumination. The color of the color system is calculated, and the calculation of the colorimetric indicators is also calculated according to the formula of the color system [97].

2.2.2 Results and discussion

At the beginning of discoloration, there is mold growth on the surface of the wood, and the wood turns red and brown. As time goes by, the color inside the wood becomes deeper and deeper, and in the later stage, the wood becomes black and cracks occur. The paulownia stored under aseptic conditions, the wood has always maintained its true color, and no blackening and browning occurred. It can be seen from Fig.-5 that the color difference ΔE* of the untreated paulownia wood is very obvious. With the passage of time, the value shows an upward trend. The wood exhibits a decrease in whiteness of wood, and the early stage of wood turns from milky white to red. It gradually darkens and darkens; while the treated sterile material, the color difference remains basically the same, and the wood retains its original color. Figure 6 Luminance L* indicator shows that the brightness of the dyed material gradually decreases from about 70 to 50, and the wood brightness will continue to decrease with time; the aseptic paulownia wood brightness remains basically stable. It can be seen from Fig. 7 and Fig. 8 that the redness degree a* and the yellowing degree b* of the paulownia wood develop in a wave manner, but in general, the wood color is in the direction of darkening and deepening; The redness degree a* and yellowing degree b* index of the paulownia paulownia material remained basically stable.

Alternaria alternata (Fr.) Keissl and Rhizopus sp. grow rapidly under suitable conditions, and the hyphae quickly penetrate into the interior of the wood, and the main components of the wood such as cellulose , hemicellulose, lignin and other degrees of degradation, accompanied by discoloration. The discoloration caused by wood fungi is a complex process, mainly the result of the interaction of enzymes in the fungus and its secretions with the chemical constituents of the wood. Specifically, under suitable conditions, the color-changing fungus grows in the wood. And secrete a variety of enzymes, these enzymes promote the fungus on the wood to survive the matrix such as monosaccharides, phenols and other substances are decomposed into various products, producing a metamorphic precursor, causing brown and red on the surface and inside of the wood Black discoloration can also reduce the brightness of the wood and darken the color of the wood.

2.3 Study on the content change of Paulownia wood under the action of fungi

2.3.1 Research methods

Water content, phenylethyl alcohol, hot water, cold water, 1% NaOH extract, ash, lignin, heald cellulose, hemicellulose, cellulose, pentosan, determination according to relevant national standard methods; hot water extract The medium reducing sugar is determined according to the relevant standard method; the pH value is determined according to the national standard method.

2.3.2 Results and discussion

As can be seen from Table 3:

The water content of the normal material of paulownia wood is lower than the moisture content of the color changing material.

The benzene-ethanol extract content of the paulownia normal material is lower than the benzene-ethanol extract content of the color-changing material. It indicates that the content of fatty acids, aliphatic hydrocarbons, terpenoids and aromatic compounds in wood increased by the action of chromobacter, which may be due to the degradation of some saccharides in the process of wood discoloration, and the organic content increased.

The content of hot water and cold water extract of paulownia wood normal material is lower than that of hot water and cold water extract of color changing material. The main wood components soluble in hot water and cold water are monosaccharides, oligosaccharides, partial starches, pectins, sugar alcohols, soluble inorganic salts and some flavonoids and terpenoids. The content of hot water and cold water extract in wood discoloration may be mainly caused by increased content of monosaccharides, oligosaccharides and sugar alcohol compounds.

The 1% NaOH extract content of the paulownia wood normal material is lower than the 1% NaOH extract content of the color-changing material. In the dilute alkali solution, in addition to the compound which can be extracted by hot water and cold water, a part of the hemicellulose which has a low degree of polymerization and a relatively low degree of branching and which is weak in alkali resistance can be degraded and dissolved. The content of 1% NaOH extract of the color-changing Paulownia wood increased, indicating that a small part of the hemicellulose degradation reaction occurred under the action of fungi during the wood discoloration process.

The lignin content of paulownia wood is 21.33%, which is significantly lower than the lignin content of the color-changing material is 30.10%, while the cellulose content of the normal material of paulownia wood is 77.60%, which is much higher than the cellulose content of the color-changing material is 66.10%. . These test data show that in the process of wood discoloration caused by fungi, the fungus does not decompose or degrade the main chemical components of lignin, but decomposes or degrades certain chemical components in hemicellulose. Therefore, its content is significantly reduced. The increase in lignin content may be due to a decrease in the content of hemicellulose, which increases the relative proportion of lignin content.

In order to analyze the causes of the change of hemicellulose content in Paulownia wood, that is, which kind of glycans are mainly used in fungi, we further analyzed the α-cellulose in the cellulose of Paulownia wood before and after discoloration. As shown in Table 3, the α-cellulose content of the cellulose in the normal material of paulownia wood is 76.20%, and the α-cellulose content of the cellulose in the color-changing material is 77.02%, which is in the cellulose of Paulownia wood before and after discoloration. The α-cellulose content did not change significantly, that is, the content of cellulose and alkali-resistant hemicellulose in the wood changed little. It is indicated that cellulose having a crystalline structure and hemicellulose having a high degree of polymerization and less branching are less prone to decomposition or degradation in the process of wood discoloration caused by fungi.

The content of cellulose, hemicellulose and lignin in the main chemical components of wood was changed during the discoloration of paulownia wood caused by fungi. The content of hemicellulose changed greatly. In order to confirm this speculation, the color change was measured. The content of pentosan, the main component of hemicellulose in paulownia wood before and after. In Table 3, the pentosan content of the paulownia wood normal material was 26.13%, and the pentosan content of the color-changing material was 22.75%. The pentosan content changed greatly before and after discoloration of the paulownia wood, and the content was significantly reduced. This may indicate that the change in hemicellulose content before and after discoloration is mainly due to the change in pentosan content in hemicellulose.

In addition, before and after the fungus-induced paulownia wood is discolored, the pH value also changes accordingly. The fungi that are parasitic in the wood can release acidic volatiles such as carbon dioxide during reproduction and growth, which helps to improve the acidity of the wood. , thus accelerating the decay of the wood.

From the above test results analysis, it can be inferred that: in the discoloration caused by fungi, the main chemical component hemicellulose of wood is degraded or decomposed, which is the main food nutrient source of fungi;

3 Conclusion

The results showed that the discoloration of Paulownia wood was microbial discoloration; two discoloring fungi, Alternaria alternata (Fr.) Keissl and Rhizopus sp., were found to cause discoloration of Paulownia wood, making the whole The wood changes from pale white to dark brown; the color change law of paulownia wood under the action of fungi is that the color difference â–³E* changes very obviously. With the passage of time, the chromatic aberration increases continuously, the whiteness of the wood decreases, and the wood in the early stage of discoloration turns from milky white to red. In the later stage, it gradually darkens and darkens; the brightness L* indicator gradually decreases from about 70 to 50, and the wood brightness will continue to decrease with time; the redness a* and the yellowing b* are Wave development, but in general, the color of the wood is in the direction of darkening and deepening; according to the analysis of chemical composition test results, the lignin content is increased in the paulownia wood caused by fungi, cellulose The change in content is not obvious, and hemicellulose is obviously degraded or decomposed.

references

[1] Bailey, IW (1910). Oxidizing enzymes and their relation to "sap stain" in lumber. Botanical Gazette 50: 142-147.
[2] Wilcox., WW, Some methods used in studying microbiological deterioration of wood. USForest service research note, 1964, FPL-063.
[3] Michikazu Ota (Tada Road I), Kenzoh Taneda (farming construction), The Chemistry of Color Changes In Kiri Wood III, Mokuzai Gakkaishi, 1993, Vol.39.No.4, P.479-485.
[4] Ward JC; WYPong. Wetwood in trees: A timber resource problem. General technical report, 1980, PNW-112.
[5] Zu Bozhen, Huang Luohua, Research on the discoloration composition of Paulownia paulownia wood, 1987, Forestry Science, 23(2): 448-455.
[6] Zu Bojun, Xu Lulu, Zhou Qin, Preliminary experiment to prevent discoloration of Lankao Paulownia wood, Wood Industry, 1991, 5(3): 29-33

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