Wood discoloration and prevention and control

1 Wood color and its type

Wood discoloration, in simple terms, is the color change that occurs on the surface due to the action of the environment (sunlight, oxygen, moisture, temperature) and microbes (fungi). Wood, sawn timber, and wood products may be discolored. After the trees are felled, the logs are prone to discoloration under the ends of the logs and under the intact bark. After the logs are processed, the sawn timber (sheets, squares) is also prone to blue, brown, mildew, etc. during storage and processing. After wood is made into wood products, discoloration may still occur during use. There are many types of wood color changes, from wood (white/light yellow/light brown, etc.) to pink, red, blue, green, gray, dark gray, brown, taupe, dark brown, black, and so on.

Wood discoloration can be divided into two categories due to their different causes, one is chemical discoloration, including tannin discoloration and oxidative discoloration; the other is fungal discoloration, including mildew and blue (also known as cyanosis, edge Village color change). Among them, fungal discoloration is more common, and the impact is more serious. Generally speaking, wood discoloration refers to the discoloration of fungi.

2 Chemical discoloration of wood

Wood of many species, when exposed to high moisture content or exposed to moist air for a prolonged period of time, can be discolored, not caused by fungal infection of wood, but by chemical reactions of certain components in the wood. , called chemical discoloration.

Oxidative condensation of tannins, pigments, alkaloids, sugars, phenols and other organic matter in wood, the most important of which is the oxidation of phenolic substances in wood. Phenolic compounds have a benzene ring structure and are easily oxidized, which is the cause of chemical discoloration. Phenolic compounds are colorless before oxidation, some are soluble in water; after oxidation, they form water-insoluble condensates, the color is red, reddish brown, brown, so chemical discoloration is also called oxidative discoloration.

Some woods contain tannins, also known as plant tannins, which are a mixture of polyphenols. When exposed to iron under humid conditions, the tannins react with iron to form a chemical reaction (complexation reaction) to form iron tannic acid. . The iron tannic acid is black and is the main raw material for making ink, thus making the color of the wood black. Depending on the amount of iron and the length of time the wood is in contact with the iron, the color of the wood changes from light grey to blue-black. Similarly, this chemical discoloration occurs in wood when the wood is immersed in water with high iron content. In addition, the wood is in contact with copper or copper alloy, and the tannins in the wood react chemically with copper to form copper tannin, which also discolors the wood (light red).

Chemical discoloration often occurs during the drying of wood. This is mainly due to the slow drying speed of the wood, especially at the point of contact with the skid.

The chemical discoloration is characterized by a shallow depth of discoloration and a uniform discoloration.

3 Mildew of wood

Moldy wood will discolor the surface and sapwood of the wood, but the mildew has a lighter range of discoloration, and its discoloration is produced by colored bolsters. Since the fungus's stalks only grow on the surface of the wood, the moldiness of the wood is limited to the surface of the wood or a layer that is very shallow near the surface.

Moldy often makes the wood green, white, black, and occasionally other colors. The discoloration caused by mold is often flocculated or spotted. In warm and humid climates, or in poorly ventilated environments, it is easy for mold fungus deposited on the surface of wood to proliferate and grow mildew.

The fungi causing moldy wood are Trichodermas P., Penicillium sPP., AsPergllluss PP·, Mucro, and the like. The most important species of Trichoderma fungus is Trichoderma viride, and the surface of the wood infected with this fungus is green. There are many kinds of fungi of Penicillium and Aspergillus, the most common ones are Aspergillus niger. After the wood is infected with this mold, the surface is black spots, sometimes connected into pieces.

The adaptability and endurance of mold to the environment and substrate is stronger than that of orchids and decaying bacteria. The resistance of molds to chemical drugs is also strong, even when it comes into contact with certain toxic chemicals.

Growth, so mildew can still be found on some preservative treated wood.

As a result of the mildew of the wood, the surface of the wood is only discolored, and the range of discoloration is shallow, so it can be removed by a brush or removed by shaving off the surface layer. Mildew has little effect on the quality of the wood itself and is therefore generally not considered a defect. However, when the mold infects the wood, it can increase the permeability of the liquid to the wood, thereby promoting the formation of blue.

4 Wood blue change

Wood blue change usually refers to the discoloration of all the sapwoods that appear in the wood. The blue change is a general term for the color change of the wood side village. In addition to changing to blue, it also includes other color changes, such as black, pink, green, and so on.

The fungi causing the blue change of wood are Botryo-diplodia sp., Ceratocystls sp., and Diploid {DIPloda sP. ), the most serious damage to rubber wood is Botryodiplodla theobromae Pat.

4·1 characteristics of blue change

Blue change can occur in both softwood and hardwood, but usually only occurs in sapwood.

Under suitable conditions, blue is more likely to occur on the surface of the sawn timber and at the ends of the logs.

If the conditions are appropriate, the blue bacteria can penetrate from the surface of the wood into the interior of the wood, resulting in deep discoloration.

Light-colored wood is more susceptible to blue-stained bacteria, such as rubber wood, red pine, masson pine, willow, maple and so on.

Blue change does not result in loss of wood structure (does not affect wood strength), but finished products made from blue-variable wood are difficult to receive.

4.2 Reasons why wood is prone to blue

The discoloration of wood is caused by the reproduction and growth of color-changing fungi on wood. The discoloration of wood is affected by the following factors:

1) Moisture: Only when the moisture content of wood is higher than 20%, microorganisms such as color-changing fungi can reproduce and grow. Therefore, if the harvested wood can be dried immediately to a moisture content of 20% or less, and the moisture content is kept below 20% during processing and use, the blue color can be prevented.

2) Nutrients (Nutrition): The most suitable nutrients required for various wood fungi are different, but all fungi can get the nutrients they need from the wood. Carbohydrates in wood, namely starch and monosaccharides, are the energy needed to grow blue bacteria. In addition, trace substances (inorganic salts, ammonia compounds, etc.) in wood are also necessary for fungal growth, but the amount required is extremely small. Because the starch and monosaccharide content of rubber wood is much higher than other woods, rubber wood is more prone to blue color than other woods.

3) Air: Most fungi are good bacteria and must grow in the presence of oxygen. However, they require a small amount of oxygen. When the oxygen content in the air reaches 1%, the fungus can grow. Therefore, it is unrealistic to use air (oxygen) to control the growth of blue bacteria.

4) Temperature: Wood microorganisms can grow in a certain temperature range, and have the most suitable growth temperature, maximum growth temperature and minimum growth temperature. The most suitable temperature for fungal growth is

20 ~ 30CC, the temperature is lower than 10CC, higher than 35C, the growth rate of fungi is slow. Low temperature (cold) can not kill the fungus, only to inhibit, make it dormant. High temperatures (such as kiln drying, high temperature sterilization) can kill fungi in wood.

4·3 Blue damage

4·3·1 Blue changed wood is more perishable

Usually, the wood first changes blue and then decays. Sometimes, only the obvious decay defects formed in the late blue period may be seen. It can also be said that discoloration is a harbinger of decay.

4.3.2 Discoloration increases the permeability of wood

Due to the penetration of the blue fungal hyphae, many small pores are formed, which increases the permeability of the wood. The hygroscopicity of the blue-modified wood after drying increases, and the decaying bacteria are easy to grow and reproduce after moisture absorption.

4.3.3 If not prevented, the mycelium of the blue bacterium can penetrate deep into the wood to form an internal discoloration. The internal discoloration of the wood is due to the fact that the surface of the wood infected with the color-changing fungus is very dry, and the surface of the dried wood does not have enough water to supply the fungus, allowing the fungus to develop into a colored hyphae. Thus, the bacteria on the surface of the wood are colorless (not yet developed into colored hyphae), so no discoloration is observed on the surface of the wood. However, the dry wood surface does not prevent the hyphae from spreading inside the wood. If the interior of the wood is moist, the hyphae will continue to multiply and develop inside the wood, thus becoming a colored hyphae, causing discoloration inside the wood.

4·3·4 Reduce the value of wood

Due to discoloration, the appearance of wood is not good-looking, users often refuse to accept such discolored wood or wood products, especially as wood used in decorative wood, furniture and other areas where wood appearance is more important, or require price reduction, so Commercially, controlling wood discoloration is an important aspect of maintaining the value of wood products.

4.4 Blue Change Prevention and Control

1) After felling, the logs should be processed as soon as possible. The sooner the better, because the coloring bacteria and molds harm fresh logs, fresh sawn timber, undried and semi-dried plates, so the logs should be sawed and processed as soon as possible to reduce biological factors. The possibility of the end of the log (fresh surface).

2) The processed wood should be dried as soon as possible

After sawing and processing the logs, the surface of the exposed fresh wood increases, and the moisture content is suitable for the reproduction and growth of the color-changing bacteria. Therefore, the moisture content of the wood should be reduced to less than 20% as soon as possible, that is, the wood should be dried as soon as possible. .

3) Timely treatment of wood (logs, sawn timber) with anti-tarnishing agents, if the logs after felling cannot be sawed in time, but are easy to change blue (such as rubber wood, some pine, maple, etc.), Then it needs to be treated against blue.

If the sawn timber after sawing wood cannot be dried in time, or air-dried (naturally dried, air-dried) before kiln drying, it should be treated with anti-blue varnish in time. If the wood has changed color, the anti-blue coloring agent cannot remove the discoloration. However, the anti-blue coloring agent prevents colonization (formation of colonies) and growth of fungi on undiscolored wood.

a. Anti-blue change/mildew treatment method The processing method of the log is very simple, and the anti-blue agent can be sprayed to the end of the log. In the area where the bark is peeled off, spray a blue-proof agent to prevent the blue-stained bacteria from invading the wood from the side skin.

There are two methods for treating sawn timber: one is that the sawn timber is immediately immersed in the dipping tank, and the immersion time is usually 15 seconds to 30 seconds. Another method is to stack the sawn sawn timber, bundle it, and use a forklift to soak the village sinker in the tank for 2 to 3 minutes (subject to each wood is immersed in the anti-blue modifier). Then hang out and re-stack, at this time, a spacer is added between the layers of wood to allow air to flow between the layers of wood.

b. Anti-blue change/mildew treatment capacity

The wood is treated with a blue-proof/mold inhibitor, and the throughput is usually calculated as the area of ​​the wood. Therefore, the amount of treatment is related to the size of the wood, the concentration of the drug used, and the like. For example, the wood size is 2m long, 10cm wide and 5cm thick. The area of ​​each wood is: (2 X0.1) X 2 ten (2 X 0.05) X 2 ten (0.IX 0.05) X 2 = 0.61 m 2 , 100 wood per m 3 , with a total area of ​​61 m 2 .

For example, wood is treated with Antiblu 20EC. Antiblu20EC is used at a concentration of 1.0%, so 1 liter of Antiblu 20EC concentrate can be formulated into a 100 liter anti-blue/mold inhibitor working solution, ie 100 kg solution. The required treatment volume is about 2009/m3, which is 0.2 kg/m2. Therefore, the amount of wood that can be treated with 1kg Antiblu20EC concentrate (100 kg anti-blue/mold inhibitor working solution) is: 100kgM2009/m' 500m2. The area of ​​1 cubic meter of wood is 61m2, so 1kg An-tiblu 20EC concentrate can handle wood 50O/61=8.2m3.

c. Wood post treatment

After anti-blue/mold treatment, appropriate stacking and drying methods must be followed (to avoid) secondary discoloration of the wood. The treated wood code should be placed in a well-ventilated, rain-proof place, such as a dry air shed, to prevent rain from drenching the wood. It also helps to reduce drying defects such as cracking, distortion, and the like.

After the wood is protected against blue/mildew, it is recommended to use the following stacking method in drying practice:

The width of the wood pile should not exceed 1.8m, otherwise the drying of the wood in the middle of the pile will be slow. The length of the pile depends on the length of the wood, the height of the drying shed and the size of the space.

The distance between adjacent piles should generally be greater than 45 cm), the pile foundation should be cement floor, or it can be kept dry during drying, and there are no seeds, garbage and other debris.

There must be a shed (top) above the wood pile to ensure that the village pile is not wet under any circumstances.

There should be a support under the pile. It is recommended to use removable cement bricks with a height of 0.3 m or so, and the distance between the supports should be equal to the distance of the spacers.

The spacers shall be of the same thickness and planed wood and shall be completely air-dried or kiln-dried to reduce the cracking of the wood they support. The stack of spacers should be aligned up and down. In addition, if all the outer sides (outer edges) of the pile are vertical, it is advantageous for uniform drying of the wood at the edge of the pile.

Do not use spacers less than 25mm thick.

The distance between the plates should be 15-50mm so that the air can flow vertically.

5 anti-blue agent / anti-mold agent

Sodium trichlorophenol (NaPCP) has been used in China to control (prevent) blue and mildew. Chlorinated phenols such as tetrachlorophenol, pentachlorophenol, sodium pentachlorophenol have been used for wood protection for more than 50 years, and it is very effective to inhibit wood blue, mildew and decay. However, since the discovery of chlorinated phenolic compounds, there are carcinogenic mountains. After the xin compound, chlorinated phenolic compounds are gradually banned in more and more countries.

The dangers of sodium pentachlorophenol include: carcinogenicity, damage to the central nervous system, blood system disorders, damage to the liver and kidneys, stomach pain, vomiting, nausea, fever, irritating eyes, skin, and respiratory system.

Countries that ban uranium pentachlorophenol are: Sweden, Germany, Spain, Chile, Denmark, Switzerland, Italy, Greece, Indonesia, Japan, New Zealand, Australia, United States (restricted use).

Since the 1980s, many research institutes around the world have carried out the screening, development and screening of high-efficiency, low-toxicity, non-polluting protective agents against wood blue and mildew, and nearly a thousand compounds have been screened. At present, new compounds widely accepted in most countries include: thiocyanatomethylbenzothiazole (TCMTB), methylidene disulfate (MBT), 3-iodo-2-propynylformate (IPbc), quaternary ammonium salt (such as: didecyl dimethyl ammonium chloride (DDAC), octahydroxyquinolin copper, triazole compounds (such as: acaconazle, hexaconzole), chlorothalonil and the like.

When choosing an anti-blue agent/mold inhibitor, consider the various properties of the agent, including:

1 anti-blue change / anti-mildew effect; 2 toxicity of drugs on humans and animals; 3 volatilization of drugs; 4 corrosiveness of drugs; 5 stability of drugs; 3 environmental/ecological effects of drugs; 3 flammability of drugs; The cost of the pharmacy (especially the cost of treatment - the cost of processing each cubic meter of wood); 3 is easy to use.

The main trade name of the anti-blue modifier/mold inhibitor and the manufacturer's attached text are now for reference.


Main product name and manufacturer's product name of anti-blue modifier/mold inhibitor
Busan 1030 Hickson International Limited
Busan 30 Hickson International Limited
Busan 30WB Hickson International Limited
Busan ills Hickson International Limited
Antiblu 20EC Koppers Arch Wood Protection Group
Antiblu 375 Koppers Arch Wood Protection Group
Antiblu CC Koppers Arch Wood Protection Group
Antimould Koppers Arch Wood Protection Group
NP-IKoppers Arch Wood Protection Group
Ecobrite Diachem Industries
F-2 Walker Brothers
Nytek GD MAAG GmbH
PQ-8 Chapman Chemical Company
Rodewood 200EC Diachem lAb
Timbercote 11 NaPier International Technology
Chapco SA-1 Chapman Chemical Company
AP-143 CSI
Defence Hoechst
Passport ICI
Prevento Bayer
LFZ Safer
Sta-Brlte Chapman Chemical Company

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