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(1) Relationship between morphological properties of fibers and product quality
In terms of the inherent morphology of the fiber, there are large differences in the materials. For example, the average length of the fiber cells of the coniferous fiber is 2 to 3 mm, and the aspect ratio is 63 to 110; the average length of the fiber tracheid and the tough wood fiber of the hardwood is 0.8 to 1.3 mm, and the aspect ratio is 35 to 58; while the grass material of the gramineous plant, the average length of the fiber tracheid is only 0.8-2.2 mm, and the aspect ratio is 30-130, wherein the non-fibrous cell content is high.
From the length and aspect ratio of the fibers, it seems that the fiberboard pressed by the softwood fibers is good. However, it turns out that the performance of all fiberboards pressed with coniferous materials is not necessarily optimal. This is because the fiber wall thickness of the coniferous material is tubular, and the thickness of the cell wall is relatively larger than the fiber width. The total contact area becomes smaller. On the contrary, the fiber tracheids, tough wood fibers, and ducts of hardwood are thin-walled and band-shaped, so that the contact area between the fibers is large, the interlacing property is good, and it is easy to be pressed during the hot pressing process. Fiberboard products with high density and strength.
The inherent strength of the fiber itself also has an effect on the strength of the fiberboard product. The hard fiberboard has been subjected to a bending and tensile failure test by a dyeing method, and it is observed under a microscope that 60% to 70% of the single fibers are damaged. From the test conclusions, it is considered that the inherent strength of the monomer fiber has almost a great influence on the strength of the soft fiberboard having a density of 0.25 to 0.4 g/cm3. For the strength of the density fiberboard with a density of 0.4-0.8 g/cm3, there is a significant influence. However, the strength of the high-density fiberboard having a density greater than 0.9 g/cm3 has a greater influence. This is because the inherent strength of the single fiber is related to the average length of the cellulose chain (ie, the degree of polymerization), and the breaking length of a single fiber can reach 40,000 Pm. After the fiber slab is formed into a slab, due to irregular arrangement and disordered state, and other factors are eliminated, it is assumed that the average length of the single fiber is 20 000 Pm, and then calculated by a conservative number of 40%, then the single fiber The crack length can reach 8 000 Pm. This shows the relationship between the inherent strength of the fiber itself and the strength of the fiberboard product.
(2) Relationship between fiber separation and fiberboard quality
Fiber separation refers to the degree of fiber separation after defibration, which is an aspect that indirectly reflects the quality properties of the fiber. The finer the fiber is separated, the larger the specific surface area of ​​the fiber, and the worse the drainage and gas permeability of the fiber. On the contrary, the better the water filtration and gas permeability of the fiber, but the fiber at this time tends to be relatively thick and the specific surface area of ​​the fiber is correspondingly small. After the fiber is separated, the specific surface area of ​​the fiber is inversely proportional to the drainage water. The larger the specific surface area, the larger the fine fibers and the worse the fiber filtration. The fiber separation is poor (28~48 mesh), the air resistance is small, and the fiber separation is high. The fiber has a small specific surface area (100~200 mesh). The fiber has poor gas permeability and good slab filling, but the air resistance is large. The larger the specific surface area of ​​the fiber, the smaller the volume of the fiber, and vice versa. It can be derived from this that the water filtration, gas permeability and volume of the fibers are all related to the degree of separation of the fibers. Therefore, it can be said that the fiber separation degree is an important indicator showing the quality of the fiber slurry, which directly affects the fiber production process and product quality. Practice has also proved that, within a certain range, the higher the fiber separation degree, that is, the finer the fiber, the interlacing between the slab fibers, and the strength, water resistance and product density of the fiberboard are also improved.
In addition, from the practical experience, it should be based on different types of products and process characteristics, based on the stability of product quality to control the degree of fiber separation within a certain range.
(3) Relationship between fiber sieving value and fiberboard quality
Different types of fiber raw materials, fiber shape, fiber length, fiber thickness ratio, etc., will have different degrees of influence on fiberboard quality. The method of testing fiber quality usually uses fiber separation (fiber water filtration DS and fiber enthalpy degree °SR). Due to the large differences in the fibers themselves, it is often difficult to reflect the quality of the fibers by simply determining the degree of separation of the fibers. Sometimes the two fibers have similar water filtration values, but the length and thickness of the fibers are different. Therefore, the quality of the separated fiber is additionally analyzed by testing the fiber sieving value.
The fiber sieving value is of great significance in actual production. Adjusting the fiber sieving value can improve fiber morphology and slurry properties, thereby improving the quality of fiberboard products. Research on the influence of fiber sieving score on the quality of fiberboard has been paid attention to, and the technical basis of regularity has been obtained. The method of influencing the fiber form is mainly the material species and the fiber separation method. The softwood material is better than the hardwood fiber. The chemical mechanical method is better than the heating mechanical method (ie, the hot grinding method), and the mechanical refining effect is simple. Poor.
a. Effect of fiber sieving value on fiberboard properties
The difference of the fiber sieving value affects the strength of the fiberboard. The influence of the fine fiber accountant on the strength of the hard fiberboard is obvious. The relative content of the fine fibers passing through the 200 mesh screen in the slurry is about 20%. The production practice of medium density fiberboard shows that the content of coarse fiber (fiber left on the 14 mesh net) in fiber sieving value should not exceed 20%, and the content of medium and long fiber (fiber retained in 28~100 mesh) is 56%~60. %, while the content of fine fibers (remaining in and passing through 200) should not exceed 30%.
b. Effect of fiber morphology on the quality of fiberboard products
If the different sieve values ​​and the fibers of different lengths are respectively pressed into the fiberboard, it can be seen that the quality of the product is greatly affected. When the fiber slurry has a sieving value of 30 to 60 mesh/25.4 mm, the fiberboard has the highest strength and low water absorption. This is related to the fiber morphology, in which the fibers in the range of sieve values ​​have a large aspect ratio and the best interwovenness between the fibers. In general, the longer the fiber, the better the interlacing property. However, if it exceeds a certain range, the fibers will be excessively agglomerated into agglomerates, which will cause difficulty in fiber paving and affect the quality of the product.
Due to the different fiber morphology, it also affects the chemical composition of each. For example, if there are about 95% of the fiber bundles in the coarse fiber (greater than 20 mesh), the chemical content of each component is almost similar to that of the raw material. However, the content of chemical components in other groups of sieving values ​​varied, and the fiber content of 20~200 meshes increased with the fiber morphology, and the content of lignin and five-carbon sugar increased.
It can be seen that the fiber size and the chemical composition of the fiber are different, which will bring about different degrees of influence on the quality of the fiberboard. In general, hardwood has a higher content of five-carbon sugar than softwood, so when the fiberboard is pressed with hardwood fibers, a stronger product can be obtained.
Relationship between fiber quality and fiberboard properties
The fiber quality requirements for fiberboard manufacturing are generally based on product category, production process and equipment conditions. In terms of fiber quality synthesis, the fibers to be separated are required to have a certain specific surface area and good interlacing properties, and require a certain aspect ratio, screening value and fiber filtration water permeability, gas permeability, chemical composition and fiber polymerization degree. There are strict requirements. Such as wet production, in the slab forming and hot pressing process, fiber slabs are required to have a rapid and easy dehydration function. Dry production requires that the fiber not only has an ideal interweaving property, but also requires the slab to have good gas permeability. Otherwise, the formed slabs of the two production methods will damage the slab structure during transportation and hot pressing, which will affect the intrinsic quality of the product. However, when producing low-density or soft fiberboard, the fiber can be formed into a slab without pre-pressing or light-pressing, so that the fiber not only has a certain aspect ratio, but also requires sufficient fiber. The degree of deuteration increases the interlacing and contact area between the fibers themselves.