Posted 19 Dec 2010 21:32
Another feature is the wood moisture content directly affects the size of the value of the intensity and volume of wood, wood moisture content of the water contained in wood and wood dry weight ratio, also known as water content, take a piece of wood weigh weight, assumed to be 4.16Kg, drying it to the absolute dry state, then the weight is 3.4Kg, then the wood dry weight 3.4Kg, the weight of water contained 4.16-3.4 = 0.76Kg. Piece of wood moisture content was: moisture content (w%) = (weight of wood moisture content - dry wood weight) / (dry weight of wood) x100% = 0.76/3.4x100% = 22.3% of new cutting wood, intercellular full of water, the moisture content of wood at 100% or more, stacked in the field, the cell cavity before the water evaporated, then the total weight of wood, but the volume and intensity have not changed. To a certain time, the cell cavity are evaporating the water is completed, can the cell wall there is also full of water, this situation is called "fiber saturation." This is about 30% moisture content, for convenience, to 30% moisture content requirements for "fiber saturation point." With water to evaporation in the cell wall, causing cell wall changes, this time, wood will not only reduce the weight and volume began to shrink, the intensity began to increase. Change of strength with moisture content of wood is because the gel fibers that the cell wall is "water" in it. Evaporation, the gel reduced plasticity, cementation strength increases, and fiber to resist the role of external forces, the tensile strength parallel to grain moisture content change has little effect on the compressive strength and bending strength parallel to grain greater impact. For example, the fiber saturation point of pine compressive strength parallel to grain of about 3KN/CM2. Wood due to shrinkage caused by the reduced water content of the phenomenon called shrinkage, shrinkage is also called "anisotropy" for example, dropped from the fiber saturation point moisture content of 0%, parallel to grain shrinkage is very small, 0.1 to 0.3% , stripes and radial shrinkage of 3.66%, most unexpectedly large tangential shrinkage 9.63%, 13.8% volume shrinkage, so the wood grain is not straight when the uneven surface and internal water evaporation rate is inconsistent, the extent of the part of the shrinkage not the same, there have bent, twisted and other irregular deformation, shrinkage cracks will appear uneven. Intensity changes and shrinkage of wood for the inconvenience the use of wood, we can not eliminate the objective existence of adverse changes, but it can grasp the changes of awareness, control the change. Timber of water can be evaporated into the air, the air will be sucked in water, the latter phenomenon as "absorption", hygroscopic characteristics of wood, wood moisture content is mainly the relative saturation point, its moisture content is too high, or too low the basic physical properties of wood will give a negative factor. Example photo shows relationship between relative humidity and air humidity factor. Corresponding to an air humidity and relative humidity, there is a wood moisture content value, which is called "equilibrium moisture content." Example: The average humidity of 32 local interior. , Relative humidity 55%. Found from the figure was 10% equilibrium moisture content, furniture grade timber, usually 15% moisture content, general wood products (including wooden packaging), the departments concerned as 18% ~ 25% for the standard products, for wood under 18% in moisture content, wood rot fungi can not survive reproduce.