Raw material type and fiber form
BNF HARDWARE CO., LTD , https://www.bnfhardware.cn
The fiber plant of the raw material used for the production of medium density fiberboard generally requires a cellulose content of more than 30%. There are many kinds of plant fiber raw materials, which can be roughly divided into two categories: lignocellulosic materials and non-wood fiber raw materials.
(1) Wood fiber
The main production of fiberboard is wood fiber raw materials. It includes harvesting residues (eg small diameters, branches, firewood), material residues (truncated), processing residues (side skins, cores, broken veneers and other scraps), as well as recycled waste wood. Wait. It can also directly produce wood chips produced by forest areas or wood processing enterprises.
There are many kinds of trees used for production. Most of them are red pine, larch, spruce, birch, eucalyptus, ash, eucalyptus, etc.; in the south, they are mostly masson pine, Chinese fir, sweet gum, etc., and various wild shrubs. Articles, vines, etc.
(2) Non-wood fiber
In order to protect the ecological environment and protect forest resources, the state strictly limits the amount of wood harvesting, and the supply of lignocellulosic raw materials, which are the main raw materials for medium-density fiberboard, is becoming increasingly scarce. Therefore, non-wood fiber raw materials occupy an increasingly important position. Cotton stalks, bagasse, bamboo, and reeds in gramineous plants have been applied. In addition, grass raw materials, bast fibers, and hair fibers have also become more and more important. At the same time, recycling waste and renewable resources can also achieve good economic benefits.
Second, the main cell content of the raw materials and fiber morphology (a) fiber and fiber morphology
Any plant is made up of countless cells. These cells are different in shape and structure during their growth due to their different positions, functions and physiological functions in plants. They are usually divided into thick-walled cells and parenchyma cells.
Thick-walled cells: The function of increasing mechanical strength in plants, giving plants a certain tensile strength, flexural strength and compressive strength. These cells are tracheids of softwood, wood fibers and ductal cells of hardwood, and fibroblasts of grasses. These cells are thin and long, with thick cell walls and narrow lumens and small ends. They are known as fibroblasts (abbreviated as fibers). Fiber is the most important and useful component of medium density fiberboard.
All other cells in plants except fiber are commonly referred to as non-fibrous cells. For example, catheters, wood rays, parenchyma cells in wood, epidermal cells in grasses, stone cells, and sieve tubes are called "cells."
Non-fibrous cells are thin because they are thin, and they are easily broken up to form fragments when the fibers are separated, or become fine dust in the dry processing process due to the short shape, which slows the molding speed and reduces the quality of the finished product. Therefore, the non-fibrous cell content is measured. One of the important criteria for the quality of raw materials.
Fiber morphology mainly refers to the length, width, aspect ratio, wall thickness and morphological characteristics of various cells.
(2) Main cell content and fiber morphology of wood
1. The main cell content of needles and broadleaf wood cells are not the same. The composition of various types of cells in different species of coniferous wood changed little, and the tracheids accounted for 90% to 95% of the whole wood volume. The main tissues among different tree species in broadleaf wood vary greatly, such as high fiber content of more than 80% and low level of only 16%. Therefore, when using hardwood as a raw material, attention must be paid to the fiber content of different tree species. The main cell contents of needle and hardwood are shown in Table 1--1 and 2.
2. Fiber morphology The fiber morphology of commonly used wood raw materials is shown in Table I-3. It can be clearly seen that the coniferous tracheids have a large length, a large aspect ratio, and a thick cell wall compared to the hardwood wood fibers. The fiber morphology of each tree species varies widely and varies with age, site conditions, and location within the tree. In the same tree species, the early fiber has a smaller length than the late fiber, a small aspect ratio, and a thin cell wall. In addition, the fiber length of the same plant gradually increased from the tree base to _L, and the tree tip began to shorten and the branches were shorter. There are also variations in different growth wheels, from the medulla, the fiber length gradually increases, and tends to be stable after the maturity. The mature period of conifers is more than 6U years, and the broad-leaved trees mature earlier. For example, poplars can reach the maximum fiber length after 10 years.
3. Uniformity of fiber morphology When considering the fiber length, only the average value is considered to be incomplete. The frequency and distribution of various long widths of fibers in the same raw material is one of the considerations in determining the raw material matching scheme in the production of fiberboard.
(3) Main tissue content and fiber morphology of grass stalk fiber raw materials
1. Main cell content Gramineous plants include grasses and bamboos, and their stems generally have distinct nodes and internodes. There are solid parts between the festival, such as sugar cane, cotton stalks, etc.; there are also hollow ones, such as bamboo, reed, and Luzhu.
1d is the cell diameter, d = L-4w.
Gramineous plants are used as fiber raw materials, mainly for their stems. There are three kinds of tissues on the transverse section of the stalk: epidermal tissue, basic parenchyma and vascular tissue. The epidermal tissue contains long and short cells, short and thin and has both cork and silice. Basic parenchyma has a large proportion, consisting of parenchyma cells with distinct intercellular spaces between cells. The vascular tissue is distributed in the basic parenchyma, composed of fibroblasts and ducts according to the vascular bundle arrangement: in a circle. Mostly for the heart: early _ punishment distribution, named solid table 11 gives the content of fiber cells and non-fibrous cells of several gramineous plant materials.
It can be seen from Table 1--4 that the non-fibrous cells of gramineous plants are much higher than wood, and the bamboos are close to 20% to 3 U%, and other grasses are higher.
2. Fiber Morphology Characteristics The fiber morphological characteristics of several species of gramineous plant materials are listed in Table 5. It can be seen from the table that the fibers of the bamboo and bagasse are relatively slender, and the other plant fibers are relatively thin and short, and the average length is between 1.0-1. 5 mm, and the average width is 10^}20 t.m. between.
Third, the influence of fiber morphology on the main performance of the board (a) strength
During the processing, the raw materials are cut and separated, and the fibers are cut, torn and crushed, but most of the hot-milled fibers retain their original form. Therefore, although the fiber form in the raw material and the slab is different in concept, the strength of the related plate product is determined by the strength of the monomer fiber itself and the bonding strength between the fibers. The strength of the monomer fiber itself has a significant influence 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 of damage it has suffered during the manufacturing process. The strength of plant cells, in addition to the degree of polymerization and crystallinity of cellulose, is directly related to its microscopic morphology. The greater the thickness of the cell wall and the ratio of wall to cavity, the higher the strength. In addition, the secondary wall of the cell wall
The bonding strength between the fibers depends on the interlacing properties and the process conditions at the time of bonding. The relationship between fiber morphology and fiber interlacing properties is as follows: First, fibers with large length and large aspect ratio have good bonding properties; second, fibers with thin cell walls and relatively small wall cavity are separated in fibers. It is easy to be flattened during hot pressing, and it is band-shaped, with good flexibility and large contact area. Thirdly, the reasonable combination of length, length and thickness can fill the gap between fibers, increase the contact surface, and improve the product. Density and bond strength.
According to the analysis, the fiber morphology has a great influence on the strength of the product, but the relationship is also 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. Due to the very high content of the tracheid tube, the fiber has high intrinsic strength, large length and large aspect ratio, so it is generally believed that the softwood is superior to the hardwood. In addition, a small amount of hybrid cells also has a role in increasing product density and increasing the contact area between fibers.
(2) Water resistance and expansion shrinkage rate
The structural form of various fibers in the raw material has a certain influence on the moisture absorption and water absorption capacity of the fiberboard. The intracellular capillary (cell cavity) and microcapillary (the gap between the microfibers in the cell wall) are not only a channel for moisture transfer, but also a storage mechanism for moisture. The size and number of various capillary sizes, as well as the structure of the pits on the cell walls, affect the ability of the fibers to absorb water.
The secondary wall S: layer is the main component of the whole cell, and the angle between the microfibers of this layer and the fiber axis is small. When the fibers are hygroscopic or absorbent, the lateral (including radial and chordwise) expansion rates are large and the axial direction is very small. Since the axial direction of the fibers in the plate is parallel to the plate surface, the arrangement in both the length and width directions is substantially uniform. Therefore, the thickness expansion ratio of the medium density fiberboard is much larger than the length direction, and the expansion ratio in the length and width directions is not much different. The greater the density of the plates, the greater the number of fibers arranged in the same thickness and the greater the rate of thickness expansion.
The microfiber angulation of the S2 layer in various cell walls varies depending on the species, site conditions, and the location of the cells in the plant. In addition, the thickness of the S: layer in various cell walls is also different. Therefore, the raw materials are different, and the product's swelling and shrinkage rate is also significantly different.