Drying collapse is also called drying shrinkage, which is a serious drying defect in wood drying. The wood surface is concave, which makes the board surface extremely uneven, resulting in the decline of drying qualification rate. We often say that in order to cure a disease, we must find the root cause of the disease before we can prescribe the right medicine. So next, we will analyze the principle of shrinkage formation, so as to find the right remedy and find a solution to the problem.
Definition of collapse
Collapse is the irregular contraction caused by the collapse of the cells due to the water evaporation tension caused by the rapid movement of water during the drying of wood. It is not the result of the gap between microfibrils and micelles closing to each other after losing water, but the change of cell morphology. In the process of wood drying, shrinkage usually begins at the stage of free water evaporation (different from drying shrinkage), and increases with the decrease of moisture content. The macroscopic manifestation of wood shrinkage is that the surface of the board is irregular and the local part is concave inward and the cross section is irregular; The microscopic appearance is usually that polygonal or circular cells collapse inward, and the cells become flat and narrow. When the shrinkage is serious, there will be fine cracks on the cell wall. Shrinkage not only increases the shrinkage rate and loss of wood, but also causes deformation when wood is dried because it does not occur in all parts of wood or all cells of a certain tissue. Shrinkage is often accompanied by internal crack and surface crack. The results show that although most of the wood will shrink to different degrees during drying, the wood of some tree species is more prone to shrink. Even the same wood has different shrink degrees due to different growth positions. Generally, the heartwood is more prone to shrink than sapwood, the early wood is more late, the trunk base and tip are more likely to shrink than the middle, and the young wood is more mature; The wood growing in swamp area is easier to shrink than the wood growing in dry area, the wood with high content of invading body and the wood with many occluded pits
Mechanism of collapse
From the perspective of wood, shrinkage generally occurs on wood with low density, such as poplar, birch, eucalyptus, etc. It is easy to occur near the medullary core, because the cells in the medullary core belong to soft parenchymal tissue, and there is also the problem of low density. If the density is low, the wood material will be less, and the strength to bear external forces will be relatively weak, and collapse will easily occur.
From the perspective of moisture content, wood shrinkage generally occurs in the early stage of drying and the period with high moisture content. Why do you say this? Because the moisture content is usually high in the early stage of drying, the pores in the wood that are connected with the outside of the wood are almost occupied by water molecules. During the drying process, because the wood temperature is high, the water in the wood moves outward in the form of gas or liquid. Once the pressure formed by water vaporization of wood is insufficient, and the external air cannot enter the wood, this is a local vacuum in the cell cavity. Therefore, when the wood density is small, the pressure bearing capacity of the cell wall will be weakened and collapse will occur. It is concluded that the internal and external pressure imbalance of wood (local vacuum) exceeds the pressure bearing capacity of wood cell wall. In addition, while drying. The environment is high temperature and high humidity. In this environment, the flexibility and plasticity of wood are improved, which also makes the ability of cell wall to withstand external pressure decrease.
Methods of preventing wood from drying and collapse
These are the fundamental reasons for the formation of shrinkage. In order to avoid the occurrence of such defects, the collapsible wood species can be dried by air drying for a period of time or by low temperature. For example, the vacuum drying kiln makes full use of the principle of vacuum to reduce the boiling point of water to achieve low-temperature drying of wood. It can effectively solve the problem of wood shrinkage. Even in the early stage of drying, wet heat treatment can be properly carried out to reduce the stress of shrinkage. At the same time, it is also a method to avoid defects by drying the plates with pulp core separately.
Treatment of wood after Collapse
According to the research on Eucalyptus wood, which is most prone to collapse in Australia, we found that the higher the drying temperature, the greater the degree of collapse. In the drying process, when the pressure of water evaporation tension on wood cells is greater than the transverse compressive strength of wood cells, wood cells can shrink. Therefore, the shrinkage of wood cells has a strong selectivity, which shows that the shrinkage generally occurs in a specific part of some wood, a specific tissue or a local part of a certain tissue, and the thin-walled cells of some broad-leaved trees are prone to collapse. The wood cells that have been shrunk can be completely or partially recovered by humidity control treatment or soaking treatment. It can be seen that the shrunk wood cells are recoverable. Therefore, for the wood prone to shrinkage, during the drying process, strengthening the post conditioning treatment can make the shrunken cells recover completely or partially, thus further reducing the collapse
Extended reading about collapse
According to the basic theory of wood cell shrinkage, a common problem involved in the study of wood shrinkage characteristics by scholars from various countries is how to evaluate the shrinkage characteristics and degree of wood. The quantitative indicators are summarized as follows:
- drying shrinkage (SR) refers to the percentage of size difference of wood before and after drying in the size of wood before drying. The higher the dry shrinkage rate, the greater the degree of wood shrinkage. From the change curve of the dry shrinkage rate of wood with the moisture content, we can analyze the typical difference between the easy to shrink wood and the non easy to shrink wood. Liu Yuan [8] pointed out that the wood shrinkage has stages when studying the shrinkage characteristics of Eucalyptus wood. The shrinkage rate is also an index to compare the severity of shrinkage of sapwood.
- shrinkage area (s) the area change before and after the cross-sectional drying of the board is an important indicator to reflect the utilization rate of wood. The larger the shrinkage area, the lower the utilization rate of the board.
- shrinkage depth (H) the thickness difference in the vertical direction of the cross-section of the dry plate is an indicator of the regularity of the cross-section of the plate. Compared with the shrinkage area, the shrinkage depth has a greater impact on the utilization rate of the plate, because most of the plates are always used after planing.
- The shrinkage factor (CF) is an index that reflects the changes of the cross-sectional area and the peripheral length of the plate, and also comprehensively evaluates the internal crack of the plate. The numerical value is the ratio of the square of the plate cross-sectional perimeter (including the internal crack perimeter) to the plate cross-sectional area (excluding the internal crack). The shrinkage factor is the main index to evaluate the shrinkage defect. No matter whether the specimen is cracked or not, CF can be used to evaluate the shrinkage more accurately and effectively in most cases, especially for those plates with internal cracks. (5) Volume dry shrinkage (VS): without considering longitudinal dry shrinkage, this index has the same meaning as the shrinkage area. However, for small-diameter timber with large longitudinal dry shrinkage, the volume dry shrinkage shall be used to evaluate. To sum up, shrinkage is a very serious defect of wood drying. The research on wood cell shrinkage includes the structural characteristics of wood, the physicochemical and mechanical properties of wood, the statics, kinematics and dynamics of wood drying process. A set of theories on the shrinkage mechanism and shrinkage characteristics of wood have also been preliminarily formed to guide the wood drying process, which is of great theoretical significance for improving the utilization rate of wood.
