Type of wood discoloration

 

Type of wood discoloration

1.

Photochemical degradation

Among the many factors that cause wood fading and discoloration, ultraviolet light and visible light are the most important, and wood placed under sunlight.

The surface will rapidly undergo chemical degradation, which will change the color of the wood surface.

î—¥ Factors affecting photochromism

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A temperature can increase the temperature to accelerate chemical reaction. Therefore, increasing the temperature accelerates the photodegradation and oxidation reactions.

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2 Water is generally used as a polar solvent, water, and allowed to penetrate into the wood cell wall expands, increasing the distance between molecules, in order to facilitate penetration of light,

Thereby producing more free radicals. When the wood moisture content is

When 0 to 3.2%, ESR intensity increases; at a moisture content of 6.3%, the radical concentration reaches a peak,

When the water content increases again

The concentration of free radicals has decreased. The main reason is that excessive amounts of water molecules in the wood can form free radical-water complexes with free radicals.

Reduced free radical concentration.

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The larger the capacity of the timber 3 species in general, the lower the erosion rate. In addition, oxygen, metal ions ( especially iron ions ) , fuels, and the like can also promote the formation of free radicals.

2

. Discoloration caused by chemical reagents

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1 ) The characteristics of chemical agents that cause discoloration. Chemical reagents cause the color change of the wood surface to be wood

An important category of color change.

Features: 1 fast response, wood discoloration in a short time; 2 mostly appear in the process, or

After processing; 3 subject to reaction conditions such as temperature, time and

The influence of PH, etc.; 4 reactions occur more often

In the heartwood.

Chemical reagents: The chemical reagents that cause the color change of the wood surface are mainly: 1 transition metal (such as

Iron ions, etc.) and oxidants (such as

KmnO4, etc.; 2 acid, alkali or other chemical aids in the process

Agent.

Chemical composition of discoloration: The main chemical components that produce color changes in wood are: tannins, yellow

Ketones as well as lignin and phenolic compounds.

(2

The mechanism of discoloration of chemical agents.

Extracts from wood are an important factor in the discoloration of wood.

At present, although there are many related studies, the structure, discoloration mechanism and all of the single chromophoric compounds

There are few studies on the effects of tree species discoloration.

1 tannins. Tannins consist of hydrolyzed tannins and condensed tannins. Hydrolyzed tannins are usually

An ester compound of gallic acid and its dimer (digallic acid or cyanoic acid) with a monosaccharide. This

The tannins can be hydrolyzed by acids and enzymes. Cholesterol tannin is catechin (flavan

Condensation of -3- ol) derivatives

Such tannins are not easily hydrolyzed by acids and enzymes, and polymer amorphous substances are produced when heated in dilute acid.

Red powder.

The nature of tannin: First, it has strong polarity and is easily soluble in hot water, ethanol, acetone and acetic acid.

Ester; the second is capable of reacting with heavy metal salts (such as lead acetate, copper acetate) and alkaline earth metal hydroxides [eg

Ca

( OH ). The solution forms a precipitate; the third is to form a complex with the transition metal, such as with FeCl3

The reaction produces a black or green precipitate; the fourth is a strong reducing agent, which is easily oxidized in the air, especially in the action of enzymes.

Underneath, a chromogenic compound is formed.

2.

Flavonoids and lignin and phenols. Flavonoids, lignin and phenols cause wood coloration and

The main chemical component of discoloration. These compounds have strong reducing power and are easily oxidized by oxidants.

The oxidation product can color or darken the wood. At the same time, it is also easy to develop color with the transition metal. Such as

The research of Nakano Junsan confirmed that the main color structure of polyphenols such as lignin is the structure of ferric iron.

Compound, epicyclic structure of o-benzene, p-benzene oxime, methide, and conjugated coniferaldehyde and chae

Ketone structure.

Tannins, flavonoids and other phenolic compounds are mostly found in heartwood.

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3 ) Iron discoloration. This is made from F4 "ion and wood tannins, phenols, flavonoids and wood

The chemical reaction of the quality causes the wood to change color, mostly grayish black. In this type of change, transition metals

Iron ions are not only easy to form a metal complex with polyphenols, but also present in iron ions.

Polyphenols are more easily oxidized to form tannin iron, iron phthalate and terpenoids. Transition gold

A similar reaction occurs in the copper ion, which produces tannin copper or copper tannate to make the wood light red.

color.

Iron ions that cause discoloration of such woods are often derived from wood processing processes, such as the process of making wood.

Knife saws, hot platens in the production of wood-based panels, and industrial water, etc.

The iron ions make the wood discolored. But generally wood has a lower tannin content (

1 % or less). Only a few

The wood species, which are often used as raw materials for the gum, have a higher tannin content. The tannin content of some tree species such as wood

table

3 . 2 is shown.

table

3 . 2 Some types of wood tannin content and structure type

material

Species

Production

Ground

Tannin content /%

Tannin category

Hardwood

Argentina, Paraguay

16

~ 28

Condensation

Chestnut

Europe, America

8

~ 14

Hydrolysis

Elm

Europe, North America

4

~ 16

Hydrolysis

Red chestnut

Fujian will

12

. 12

Hydrolysis

Camellia

Zhejiang Longquan

33

. 92

Condensation

Press wood

Australia

Heartwood phenolic compound

30 %

Hydrolyzed and condensed

Factors affecting iron discoloration:

1 wood component. The main wood component that affects iron discoloration is tannin, and the content of tannin affects wood.

The color difference changes, but flavonoids, lignin, etc. also have an effect on the color change process.

2 iron concentration. Generally, when the concentration of iron ions is low (

FeCl3 concentration 0 . 5x10-4 %), can produce discoloration.

3 time. The time required for iron to cause discoloration is very short, generally

3 minutes to change color; temperature rise

High, time will increase. At the same time, the wood moisture content increases and the discoloration increases.

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4 ) Acid discoloration. The acid discoloration of wood is tannin and flavonoid polyphenolic compounds in acid and wood.

Caused by a chemical reaction. Wood that has been leached by hot water is generally free of acid-chromic reactions, so this

The color-like reaction is not caused by lignin.

Acid-chromic wood is generally red, with

The pH value decreases and the color difference value increases. When the pH is

2

_ 5 , the wood only appears slightly discolored, usually not visible to the naked eye. In addition, contains condensation

The coniferous village with more tannins has a greater degree of discoloration, and the acidity of the wood placed in the dark is slightly discolored.

Not only does light speed up the color change, but the color difference increases.

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5 ) Alkali discoloration. Alkali discoloration is a basic compound and a small amount of tannins and flavonoids in wood.

And other phenolic compounds reacted. The wood disintegrated by hot water has no obvious reaction of alkali discoloration.

But with

The pH is increased, the color is deepened, and the colors are different, mostly brown, reddish brown. Alkali discoloration usually occurs during the use of wood products, such as wood products in contact with alkaline materials, in wet conditions

Underneath, alkali discoloration can occur. The status of discoloration of iron, acid and alkali in some wood species is listed in the table.

3 . 3 .

tree

Species

Tannin content

Whiteness decline

l %

Chromatic aberration

AE )

l

% ,

l

% FeCl3

Alkali

acid

Douglas fir

)

0

. 3

50.9

10.9

15.2

Japanese cedar

0.3

38.4

6.3

15.3

American cypress (heartwood)

0.2

28.2

7.4

4.1

Japanese red pine (heartwood)

0.1

20.4

15.3

3.3

Japanese cypress

0.1

20.4

15.3

3.3

Water walnut (heartwood)

2.1

66.2

4.7

25.6

Elm (heartwood)

5.6

68

3.1

11.9

Maple (heartwood)

0.6

58.2

10.0

16.0

Black walnut

2.0

51.6

3.1

9.5

Japanese beech (heartwood)

0.4

40.4

13.3

20.9

Japanese Paulownia (heartwood)

0.6

42.4

5.0

9.3

Birch (heartwood)

0.3

32.1

6.3

7.7

Brahma (heartwood)

0.2

20.8

5.9

5.8

Magnolia (heartwood)

0.4

21.2

6.3

1.5

Genus

( heartwood )

0.2

26.0

3.1

2.9

Teak

( heartwood )

0.4

4.9

1.5

3.6

3

. Microbial action

The mold that harms wood is a fungus belonging to the ascomycete and incomplete fungi. Most common on wood

There are Trichoderma, Penicillium, Aspergillus and the like. Wood damaged by mold, visible in black and light

Green mildew. The hazards of mold to wood fiber structure under microscope were similar to those of color-changing bacteria.

The color-changing bacteria that are harmful to wood are also fungi belonging to the genus Trichophyton and incomplete bacteria. Wood photochromic bacteria

There are many types, including blue-colored bacteria, Fusarium, grape bacillus, and color-stringed bacteria. Long-shelled genus

The fungus harms more wood.

Molds and discoloring bacteria are different from decaying bacteria that erode wood, and they are parasitic in the ray of the village.

In parietal cells and axial parenchyma cells, they survive and proliferate with the nutrients in these cells as nutrients.

Colonization. The hyphae pass from one cell to another through the pit. As its hyphae spread around and inward

Invading people, the color of the surface or interior of the wood changes. Wood color caused by mold and discoloring bacteria

The change mainly occurs in sapwood. The discoloration bacteria cause discoloration of wood, resulting in different strains and tree species.

The colors are also different, and the changed colors are blue, cyan, yellow, green, red, gray, black, and the like. Red pine

Blue discoloration, green discoloration of eucalyptus, yellow discoloration of hardwood caused by Penicillium, cited by the genus

Pine brown color change, etc. It can often be seen with the naked eye on the wood or the section of the wood.

It is blue discoloration and blue discoloration (also known as green skin).

The growth and development of wood-changing bacteria requires the moisture content of wood to be

More than 20 %, but also have appropriate temperature, humidity, air. The optimum temperature for the growth and development of the color-changing bacteria is 23 _ 35 °C at 7 °C.

Following or

When the temperature is above 40 °C, the development of the color-changing bacteria can be completely stopped. When the temperature is 50 _ 54 °C,

After a long period of time, it can kill the color-changing bacteria. ,

one

4

. Other discoloration

(2) Thermal discoloration. Thermochromism usually occurs during the drying process of wood. Wood color-fixing tree species and

Different from drying temperature, it can turn yellow, brown, red, gray and other colors. Wood that has been exposed to high temperature for a long time can be used.

It turns brown.

The thermal discoloration of wood is mainly due to the moisture migration in the wood during the drying process, and the partial water-soluble extraction.

Things such as phenols and flavonoids are transferred to the surface and are oxidized by air at high temperatures.

Discoloration.

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2 ) Enzymatic discoloration. Freshly harvested wood, sawn into wood, placed in a wet environment, oxidase guide

Causes discoloration of the wood surface. The color of the fixed tree species varies, such as the fir-edge village turns yellow, and the eucalyptus turns red-brown

Color, cedar turns black. Moisture content and humidity are important factors in the discoloration of enzymes. When the relative humidity of the environment reaches

100

At %, the enzyme discolors in the wood. Temperature also affects discoloration, and the ambient temperature is below 20 t .

slow. In addition to the discoloration of various woods described above, there is also discoloration caused by resin exudation of wood.

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