First, fiber morphology and product performance
During the preparation of the raw materials and the separation of the fibers, some of the fibers are cut, torn or crushed, and some are still present in the form of fiber bundles. Under normal conditions, most of the hot-milled fibers remain in their original form. Therefore, the fiber morphology in the raw material and the fiber morphology in the sheet are not completely the same, but they are closely related.
The strength of the sheet depends on the strength of the individual fibers themselves and the strength of bonding between the fibers. The strength of a single fiber has a significant effect on the strength of the medium density fiberboard. The strength of the individual fibers in the panel depends on the strength of the fiber cells in the material and the extent to which it is damaged during the manufacturing process. The strength of plant fiber cells, in addition to the degree of polymerization and crystallinity of cellulose, is directly related to its microscopic morphology. The greater the cell wall thickness and wall-to-cavity ratio, the higher the cell strength. Further, the smaller the angle between the microfibrils in the middle layer (S2) of the cell wall secondary layer and the fiber axis, the higher the tensile strength of the fiber.
The bonding strength between the fibers depends on the interlacing properties of the fibers and the processing conditions at the time of bonding.
Fibers with a certain aspect ratio have good bonding properties. The long and short, thick and fine fibers can be reasonably matched to fill the gap between the fibers, enlarge the contact surface and increase the strength and density of the product, but a large number of finely divided components will Resulting in a decrease in the strength of the product.
In summary, fiber morphology has a great impact on product strength, but the relationship is more complicated. The conditions required to retain the strength of the fiber itself are often contradictory to the conditions required to increase the bond strength between the fibers, so that one aspect cannot be emphasized too much.
Second, the impact of chemical composition on product quality t
The three main chemical components of the plant fiber cells and the extracts play different roles in the strength of the fiberboard. Cellulose is the skeleton and body that make up fiber cells and, therefore, is a fundamental factor in determining plate strength. Hemicellulose increases the plasticity of the fiber due to its moisture absorption and swellability, which is advantageous for improving the bonding force between the fibers. After the temperature is higher than the softening point, lignin can exert a strong bonding effect, so that the fibers are combined with each other. In addition, lignin plays an important role in improving the water resistance of products. The extract has a certain hindrance to the bonding between the fibers, which is not conducive to the strength of the product. Considering the chemical composition of the tree species, proper use of the species will have a beneficial effect on the properties of the product.
In the masson pine fiber mixed with 30% copper or maple, the product MOR can be increased by about 30%, IB is increased by 50% to 60%, and the thickness expansion rate is significantly reduced. The addition of 30% sweet gum to Chinese fir can increase IB by more than 30%. Therefore, the reasonable combination of needle and hardwood can significantly improve product quality.
Third, the effect of bark and decaying materials on the performance of medium density fiberboard
(a) bark
The bark and wood are different in composition, and the bark contains more extracts. The large bark content in the fiber raw material causes the plate property to be deteriorated, that is, the strength is lowered, the water absorption rate is increased, and the color of the board surface is uneven, thereby affecting the use range of the product.
In addition, the control of the bark content in the raw material depends on the comprehensive consideration of various factors such as the quality requirements of the product, the processing technology, the structure of the product, and whether the product is subjected to secondary processing.
After the experiment to study the increase of bark content in raw materials, how to ensure the quality of the plate, increase the amount of glue, improve and improve the plate is the easiest way, but this will increase the cost. The following study analyzes the relationship between fiber morphology and bark content. The board density is 0. 7g / c resistance, the amount of sizing is 8%. For any kind of fiber, the bark mixing ratio increased, and the MOR showed a downward trend; however, when the mixing rate was constant. Different fibers, MOR is also different. Therefore, when a certain MOR is required, the fiber length is different, the bark mixing ratio should be different, the wood fiber quality is high, and the bark allows the mixing rate to be high.
(2) Decaying materials
Corrosion can be up to 60% due to the erosion of fungi. In the production of boards, the loss of wood caused by chipping, screening, fiber separation and drying is mainly caused by decaying materials. Using decaying materials, the color of the board surface will be dark and uneven, and the physical and mechanical properties will be degraded. Of course, in dry production, the decaying material can be used as a core material. According to the results of the relevant manufacturers, since the core layer is made of decaying materials, the strength of the sheet can only be ensured by multiple adhesives.
(3) Others
Different specific gravity of raw materials, product performance is different.
In addition, different raw materials need different process conditions due to different organizational structures. Care must be taken to mix raw materials of the same or close density to produce high quality board products. Otherwise, the speed in the gas stream is different due to the difference in fiber weight, and it is impossible to obtain a uniform drying effect. At the same time, due to the difference in weight and form, it is difficult to obtain uniform and stable results in the thickness and density of the slab during fiber laying.
Fourth, the choice of raw materials
The choice of raw materials is determined by several aspects such as quality, output and price. The raw materials used are not only good in quality, but also rich in resources, and the output can meet the requirements of the scale of the plant. When selecting various residues as raw materials, the collection range must be carefully estimated. If collection and transportation are difficult, the price will inevitably be higher. In particular, when using 1 year old grass material, the seasonality of raw material supply must be considered.
The site should be close to the raw material production area, and the scale of the plant construction must be appropriate, otherwise the economic benefits of production will be affected.
When using low-quality raw materials, it should be matched with some high-quality raw materials to improve the grade rate of the products.
During the preparation of the raw materials and the separation of the fibers, some of the fibers are cut, torn or crushed, and some are still present in the form of fiber bundles. Under normal conditions, most of the hot-milled fibers remain in their original form. Therefore, the fiber morphology in the raw material and the fiber morphology in the sheet are not completely the same, but they are closely related.
The strength of the sheet depends on the strength of the individual fibers themselves and the strength of bonding between the fibers. The strength of a single fiber has a significant effect on the strength of the medium density fiberboard. The strength of the individual fibers in the panel depends on the strength of the fiber cells in the material and the extent to which it is damaged during the manufacturing process. The strength of plant fiber cells, in addition to the degree of polymerization and crystallinity of cellulose, is directly related to its microscopic morphology. The greater the cell wall thickness and wall-to-cavity ratio, the higher the cell strength. Further, the smaller the angle between the microfibrils in the middle layer (S2) of the cell wall secondary layer and the fiber axis, the higher the tensile strength of the fiber.
The bonding strength between the fibers depends on the interlacing properties of the fibers and the processing conditions at the time of bonding.
Fibers with a certain aspect ratio have good bonding properties. The long and short, thick and fine fibers can be reasonably matched to fill the gap between the fibers, enlarge the contact surface and increase the strength and density of the product, but a large number of finely divided components will Resulting in a decrease in the strength of the product.
In summary, fiber morphology has a great impact on product strength, but the relationship is more complicated. The conditions required to retain the strength of the fiber itself are often contradictory to the conditions required to increase the bond strength between the fibers, so that one aspect cannot be emphasized too much.
Second, the impact of chemical composition on product quality t
The three main chemical components of the plant fiber cells and the extracts play different roles in the strength of the fiberboard. Cellulose is the skeleton and body that make up fiber cells and, therefore, is a fundamental factor in determining plate strength. Hemicellulose increases the plasticity of the fiber due to its moisture absorption and swellability, which is advantageous for improving the bonding force between the fibers. After the temperature is higher than the softening point, lignin can exert a strong bonding effect, so that the fibers are combined with each other. In addition, lignin plays an important role in improving the water resistance of products. The extract has a certain hindrance to the bonding between the fibers, which is not conducive to the strength of the product. Considering the chemical composition of the tree species, proper use of the species will have a beneficial effect on the properties of the product.
In the masson pine fiber mixed with 30% copper or maple, the product MOR can be increased by about 30%, IB is increased by 50% to 60%, and the thickness expansion rate is significantly reduced. The addition of 30% sweet gum to Chinese fir can increase IB by more than 30%. Therefore, the reasonable combination of needle and hardwood can significantly improve product quality.
Third, the effect of bark and decaying materials on the performance of medium density fiberboard
(a) bark
The bark and wood are different in composition, and the bark contains more extracts. The large bark content in the fiber raw material causes the plate property to be deteriorated, that is, the strength is lowered, the water absorption rate is increased, and the color of the board surface is uneven, thereby affecting the use range of the product.
In addition, the control of the bark content in the raw material depends on the comprehensive consideration of various factors such as the quality requirements of the product, the processing technology, the structure of the product, and whether the product is subjected to secondary processing.
After the experiment to study the increase of bark content in raw materials, how to ensure the quality of the plate, increase the amount of glue, improve and improve the plate is the easiest way, but this will increase the cost. The following study analyzes the relationship between fiber morphology and bark content. The board density is 0. 7g / c resistance, the amount of sizing is 8%. For any kind of fiber, the bark mixing ratio increased, and the MOR showed a downward trend; however, when the mixing rate was constant. Different fibers, MOR is also different. Therefore, when a certain MOR is required, the fiber length is different, the bark mixing ratio should be different, the wood fiber quality is high, and the bark allows the mixing rate to be high.
(2) Decaying materials
Corrosion can be up to 60% due to the erosion of fungi. In the production of boards, the loss of wood caused by chipping, screening, fiber separation and drying is mainly caused by decaying materials. Using decaying materials, the color of the board surface will be dark and uneven, and the physical and mechanical properties will be degraded. Of course, in dry production, the decaying material can be used as a core material. According to the results of the relevant manufacturers, since the core layer is made of decaying materials, the strength of the sheet can only be ensured by multiple adhesives.
(3) Others
Different specific gravity of raw materials, product performance is different.
In addition, different raw materials need different process conditions due to different organizational structures. Care must be taken to mix raw materials of the same or close density to produce high quality board products. Otherwise, the speed in the gas stream is different due to the difference in fiber weight, and it is impossible to obtain a uniform drying effect. At the same time, due to the difference in weight and form, it is difficult to obtain uniform and stable results in the thickness and density of the slab during fiber laying.
Fourth, the choice of raw materials
The choice of raw materials is determined by several aspects such as quality, output and price. The raw materials used are not only good in quality, but also rich in resources, and the output can meet the requirements of the scale of the plant. When selecting various residues as raw materials, the collection range must be carefully estimated. If collection and transportation are difficult, the price will inevitably be higher. In particular, when using 1 year old grass material, the seasonality of raw material supply must be considered.
The site should be close to the raw material production area, and the scale of the plant construction must be appropriate, otherwise the economic benefits of production will be affected.
When using low-quality raw materials, it should be matched with some high-quality raw materials to improve the grade rate of the products.
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