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2.5 COMPARISON OF VACUUM AND CONVENTIONAL DRYING QUALITY In drying, the most common defects are classified as fracture, distortion, warp, or discoloration. These defects are caused by an interaction of wood properties with processing factors such as drying temperature (Simpson 1991). Drying rate can be defined as the time that it takes to remove water from the wood to a specific or desired moisture content. In fracture or distortion, the defects that can be further defined are surface and end checks, collapse, and honeycomb. Surface checks occur early in drying when the shell of a board is stressed in tension enough to fracture the wood. End checks occur because of the rapid longitudinal movement of moisture causes the end to dry very quickly and develop high stresses which can lead to fracture. Collapse is a distortion, flattening, or crushing of wood cells caused by shrinkage. Honeycomb is an internal crack that occurs in the later stages of kiln drying when the core of a board is in tension (Simpson 1991). Warp is any deviation of the face or edge of a board from flatness or any edge that is not at right angles to the adjacent face or edge. Warp can be produced by differences between radial, tangential, and longitudinal shrinkage in the piece as it dries or by growth stresses. Warp is aggravated by irregular or distorted grain and the presence of juvenile and reaction wood. The six major types of warp are bow, crook, twist, oval, diamond, and cup (Simpson 1991) There are two types of discoloration: chemical and fungal. Chemical discoloration is the result of oxidative and enzymatic reactions with chemical compounds in wood. This can cause the degradation of the cell wall. Chemical discolorations can be presented in the wood as pinkish and yellowish hues through gray and reddish brown to dark brown shades. Fungal stains are usually bluish discolorations in the wood caused by mold fungi, decay fungi, bacteria, and others. Generally, stains may be caused by fungi that grow in the sapwood (Simpson 1991). As an alternative, vacuum drying has the potential to improve the quality of dried wood products. Vacuum drying increases water movement within the wood; as a result, the severity of the moisture gradient is reduced. Drying in these conditions can result in less internal stress, 13PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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