Wood Deformation Over Time Due to External Force

glassy state, when the wood temperature (T) is between the glass transition temperature and the viscous liquid temperature (T f ) (T  (T g & T f ) then the wood is in a highly elastic state, when the wood temperature T is greater than the viscous liquid transition temperature T f then the wood is in a viscous liquid state. When the wood temperature is lower than the viscous temperature, the molecular energy is very low, the movements of the chains are frozen and we cannot measure the movements of the chains that show deformation. Therefore, from the microscopic point of view, the deformation of the polymer in the glassy state is very small. When the temperature increases, the thermal motion energy and the free volume of the molecules of the polymer gradually increase, when the temperature reaches the viscous temperature, the movements of the molecular chains begin to be stimulated, at this time forming a glass transition region of the dynamic state of amorphous polymer (lignin), when the temperature is greater than the temperature T f then the wood transforms into a viscous liquid form for polymers like viscous liquids to produce viscous liquid movements.

Many research results show that lignin has the glass transition characteristic of amorphous polymers. When lignin is heated to reach the glass transition temperature T g , lignin quickly plasticizes. Factors affecting the plasticization temperature of lignin are the origin, method of separation of molecular weight, and moisture content of lignin. If the moisture content of lignin is low, the transformation temperature is high, conversely, if the moisture content of lignin is high, its glass transition temperature decreases.

The mechanism as well as the process of lignin crystallization is very important in wood modification technology by compression and direct heating. In the process of pressing to create board thickness, when the temperature reaches the crystallization temperature, thanks to the thermoplastic effect of lignin, the board thickness can be quickly created with small pressure. Crystallization of cell walls: the components of wood that can be plasticized include cellulose, the non-crystalline region of cellulose and hemicellulose, which have a very strong compatibility with the swelling properties of wood. Water cannot penetrate the crystalline region of cellulose, but NH3 solution can penetrate. From there, the inside of the micellecellulose swells. That is why we can see that we can only plasticize the components of the cell wall with chemicals, while temperature has little effect. We can see that lignin is a very important component related to the plasticization ability of wood.

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Deformations in wood during thermo-mechanical treatment

Wood is a material that has both elastic solid and sticky liquid properties. Wood is a type of high molecular material that, when subjected to external force, produces three types of deformation: instantaneous elastic deformation, post-elastic deformation and plastic deformation.

As we know, wood is made up of countless cells, the cell wall is made up of two main components: cellulose and lignin. Cellulose has a shaped structure (microfiber) that people liken to iron ribs. This is the main component that produces the internal force of wood. Lignin is a glue with an amorphous structure, people liken it to cement that sticks to iron ribs to create concrete - which is the cell wall. Due to such structure, lignin is only a secondary component that produces internal force. Elastic deformation is due to cellulose producing internal force that creates elastic deformation; lignin, cellulose and hemicellulose produce plastic deformation (permanent deformation). Permanent deformation represents the plasticity of wood.

+ Instantaneous elastic deformation:

Figure 2.10. Deformation of wood over time under external force

“Source: Sahbi Ouertani 2014”

When subjected to external force, the deformation produced corresponds to the rate of increase of load, called instantaneous elastic deformation. This deformation follows Hooke's law. When the load ends, the wood immediately creates an elastic deformation that gradually decreases over time, called elastic plastic deformation (post-elastic deformation). It is caused by the cellulose molecular chains being bent or stretched. This type of deformation is also inversely proportional. Compared to elastic deformation, it has a time delay. The cellulose molecular chains slide against each other when subjected to external force, causing this deformation, called plastic deformation. This is a reversible deformation. From this, it can be seen that wood is a material that has both elastic and

plastic deformation. Figure 2.11 shows the dependence of deformation of wood material on the time of force application.

+ Plastic deformation of wood:

The plastic deformation of wood is relatively small, so there are certain limitations during processing. Wood is a high molecular material, its plasticity is the result of deformation and relative displacement between macromolecules under the influence of external forces. At normal temperature, to increase the plasticity of wood, chemicals must be added to weaken the binding force between molecules. In addition, through the effect of temperature, it makes the substrate (cellulose, lignin) of wood plasticize, which process can also increase the plasticity of wood. This property is called thermoplasticity of wood.

Lignin is a thermoplastic substance, because it is amorphous, its melting point is not fixed. Different plant species have different plasticizing and melting temperatures. The plasticizing temperature of lignin is closely related to humidity. Its thermoplasticizing point in the dry state is 127-193 o C, and in the wet state it drops significantly to about 77-128 o C.

Hemicellulose, due to water absorption, its plasticizing point also decreases, similar to the case of lignin. The core substance of wood is cellulose, whose plasticizing point is greater than 232 0 C. Its crystalline region is not affected by water, and the glassy state of cellulose decreases with increasing humidity.

According to Hilis (1984), for wood in a water-saturated state at temperatures of 70-80 o C and at 80-100 o C, two continuous thermoplastic regions are formed. It is believed that around 70-80 o C is the glass transition point of hemicellulose, 80-100 o C lignin; wood in a wet state when heated has obvious thermoplasticity. [30]

Thus, the ability of wood to deform depends on the humidity and temperature of the wood; in addition, the plastic deformation of wood also depends on chemicals. Therefore, to increase the plasticity of wood during processing, people act on lignin and hemicellulose by methods such as: heat and moisture treatment, using chemicals. Under normal conditions, wood has relatively small plastic deformation, so it is very limited in the wood processing process, especially the wood compression process. Under normal conditions, to increase the plasticity of wood, it is necessary to add plasticizers to weaken the binding force between molecules and increase the plastic deformation of wood.

From some structural characteristics and properties of wood we can draw the following conclusions:

- When compressing or pressing wood along the grain, thick-walled and thin-walled wood cells are compressed to the same degree, so the strength of the components that make up the wood remains different in the new state. Thus, we cannot increase the properties of wood along the grain.

- Wood should be pressed in the direction of the grain, for coniferous and broadleaf wood, it should be pressed in the radial direction, for diffused wood, it should be pressed in both the radial and tangential directions [59]

- In the process of pressing wood across the grain to increase the permanent deformation of the wood to increase the compression level of the wood, it is necessary to plasticize the wood cell walls, which means making the matrix substances (lignin, hemicellulose) change state under the influence of temperature and humidity or reduce their content in the wood, transforming into another form under the influence of external chemicals.

Factors affecting the quality of wood treated by thermo-mechanical methods

- Humidity: Wood humidity has a great influence on the quality of compressed wood, because if the humidity is too low, not enough for the wood to plasticize and the plasticizing time and temperature, the compression pressure will increase and the possibility of causing small cracks in the cell wall will increase, causing the compressed wood after pressing to increase the rate of moisture absorption, making the compressed wood increase its capacity and reduce its mechanical properties. Some research results show that increasing the wood humidity to near the saturated moisture content of the wood fibers increases the ability to plasticize the wood. However, this affects the drying time to stabilize the size of the compressed wood. If the wood humidity after plasticizing by steaming or boiling the wood is too high, it can also cause cracks and fissures in the cell wall. Compressing wood with a humidity of about 15-20% can also cause cracks in the cell wall. Wood moisture is related to the internal friction coefficient (characterizing the amount of bound moisture in the wood), the internal friction coefficient affects the elasticity of compressed wood.

- Compression ratio: Compression ratio is closely related to the density and durability of wood. Each type of wood has a maximum compression ratio that the wood can withstand. If the process of compressing wood with a high compression ratio can lead to the destruction of the wood cell walls, then the density of the wood will increase, but the durability of the wood will decrease. On the other hand, when compressing wood with a low compression ratio (<30%), the wood has a low ability to bounce back but the mechanical properties are not improved.

- Temperature: Wood temperature has a direct influence on the plasticization process of wood and is closely related to wood humidity. If the temperature increases, the plasticity of wood is increased and reduces the compressive pressure, limiting cracks and breaks in the cell wall. This influence has been presented in the section on the transformation of the crystalline state of lignin and hemicellulose: Lignin is a thermoplastic substance, because it is an amorphous substance, so the melting point is not fixed. Different tree species have different plasticization and melting temperatures. The plasticization temperature of lignin is closely related to humidity. Its thermoplasticity point in the dry state is 127-193 o C, but in the wet state it drops significantly to about 77-128 o C. Hemicellulose absorbs water, so its plasticization point also drops, similar to the case of lignin. The core substance of wood is cellulose, the plasticizing point is greater than 232 0 C, its crystalline region is not affected by water, the glassy state of cellulose decreases with increasing humidity. Therefore, in the process of wood compression, it is necessary to choose a reasonable temperature range so that lignin and hemicellulose quickly change from crystalline state to sticky liquid state, creating favorable conditions for the wood compression process.

- Time: The plasticization of the cell wall is intended to increase the plastic deformation or permanent deformation so that the wood can be compressed easily. The deformation of wood is closely related to time. When wood is subjected to external force, the deformation produced corresponds to the rate of increase in load, called instantaneous elastic deformation. This deformation follows Hooke's law. When the load ends, the wood immediately creates an elastic deformation that gradually decreases over time, called elastic deformation (post-elastic deformation). It is caused by the cellulose molecular chains being bent or stretched. This type of deformation is also inversely proportional. Compared to elastic deformation, it has a time delay. The cellulose molecular chains slide over each other, this deformation is called plastic deformation, which is a reversible deformation. Therefore, wood is a material that has both elastic and plastic deformation. Thus, during the wood compression process, there must be enough time for the wood to change from elastic deformation to plastic deformation (section DE in figure 2.11).

- Compression direction: When compressing wood, the compression direction has a great influence on the ring veins of broadleaf wood, because it can be compressed in both radial and tangential directions. Some research results of Prof. Khukhranxki show that dispersed vein wood and coniferous wood compressed in the radial direction have greater strength than in the tangential direction [53]

Dimensional stabilization of wood treated by thermo-mechanical methods

- Moisture absorption of wood

Moist wood has a very large internal surface area, which can reach hundreds of square meters per gram of wood. It is clear that to have such a large internal surface area, wood must have a very large number of small capillaries in the cell walls. The porosity of wood is not only the cell interior as we usually understand it, but also the system of capillaries and microcapillaries in the cell walls. It is divided into open porosity - in the form of connected capillaries and closed porosity - in the form of unconnected holes. In wood, the wood cell walls are connected to each other thanks to the microcapillary system, which ensures the connection between separate cells - transverse pores. Microcapillaries with a diameter equal to or larger than the diameter of water molecules play a particularly important role in the process of moisture absorption of wood. [73] When wood is chemically treated or treatment agents penetrate the wood cells, they will interact with wood components in one form or another, causing changes in the structure, connections, and properties of the wood. The impact of the agents mainly affects the cross-links (hydrogen bridges) between the components, especially and mainly the hydrogen bonds between cellulose molecules. When the treatment agents impact the wood components, there will be changes between the components such as the replacement of some functional groups, the distance between the components in the wood will change, causing the physical and mechanical properties to change accordingly. The change in functional groups (mainly the OH group) will cause changes in water absorption and moisture absorption. Therefore, if there is an impact or a chemical agent is used to treat wood so that it can change the structure or replace the hydroxyl group in the wood with a large hydrophobic group, the material will absorb less water and swell less [10]. Wood shrinks and expands when the amount of water absorbed in the microcapillaries in the cell wall decreases or increases, the root cause of which is the free OH - ions in the non-crystalline region of cellulose adsorbing water components in the air and simultaneously forming countless hydrogen bonds with water molecules. When the amount of water absorbed increases, the number of hydrogen bonds formed in the capillaries increases, increasing the width of the capillaries, increasing the thickness of the cell wall, and the wood expands. Hemicellulose absorbs water very strongly, followed by lignin, and finally cellulose.

- Limit moisture absorption of compressed wood

Hygroscopicity and dimensional change in wood can be limited by cross-linking or by sealing the microcapillaries in the cell walls, both of which can be reduced.

moisture absorption and increase wood dimensional stability. Thus, to improve wood dimensional stability, it is necessary to reduce the water absorption of cellulose, hemicellulose and lignin, the essence of which is to reduce or replace the OH groups in these components with hydrophobic functional groups or fill the microcapillaries with water-inert substances. The principle of wood dimensional stabilization is to maintain the inherent superior properties of wood but must reduce the moisture absorption and release of wood, that is, to stabilize the wood dimensional. It can be divided into 2 treatment methods: Treatment that is only contained in the non-crystalline area of cellulose in the cell wall; and not treating the cell wall but only filling and depositing chemicals into the cell cavity.

According to Militz, Becker, Homan (2004), some models of wood dimensional stability treatment can be shown as in Figure 2.12 [31]

Figure 2.11. Method of stabilizing wood dimensions

“Source: Militz, Becker and Homan 2004”

- Limit the rebound of compressed wood

The elasticity of wood depends on the coefficient of internal friction. The smaller the coefficient of internal friction, the faster the wood can return to its original shape and size; the larger the coefficient, the lower the ability of the wood to return to its original shape and size. One of the

The cause of wood's rebound is essentially due to the moisture absorption capacity of hemicellulose and lignin. Therefore, to limit this phenomenon, it is possible to reduce the amount of OH in the wood or replace the OH groups with hydrophobic functional groups.

According to the dimensional stability model in Figure 2.13, heat treatment to stabilize the size of compressed wood is one of the modification methods that essentially reduces the number of -OH groups to create cross-links between them.

When heat treating wood at high temperature after compression, due to the high temperature acting on hemicellulose, hemicellulose undergoes chemical changes to form a polycondensate compound with poor hygroscopicity. At the same time, heating causes the water to be absorbed and the distance between the molecular chains of the non-crystalline area of the cell wall cellulose to shorten, forming new hydrogen bridge combinations, thereby improving the dimensional stability of the wood. The heat treatment mechanism makes the distance between the molecular chains of the non-crystalline area of the wood cell wall cellulose smaller, the free -OH radicals form a relative force on each other and have the opportunity to combine with hydrogen bridges, causing the total number of hydrogen bridge combination points of the crystalline area to increase, thereby increasing the orientation of the molecular chains of the non-crystalline area of the cell wall cellulose, and improving the dimensional stability of the treated wood.

- Heat treatment method

In compressed wood technology, the process of dimensional stabilization is often carried out by heat treatment with the treatment temperature corresponding to the Tg temperature of the wood. The amount of oxygen present in the heat treatment process to stabilize the size of compressed wood will affect the quality of compressed wood; and based on the way the compressed wood comes into contact with the air environment, the heat treatment process is divided into 3 common types of heat treatment as follows: Contact heat treatment: Using the heat energy of the contact press surface and heating to heat the wood to the required temperature; Conventional heat treatment: Using heat in the atmospheric environment (conventional convection oven type) to heat and heat the wood to the required temperature; Heat treatment in a vacuum environment: The wood is placed in a vacuum environment and heated to the required temperature.

Basis for assessing the quality of flooring

Currently we only evaluate the quality of flooring through standards.

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