Secondary fibers

Fig. 4.2.4. a-d Changes in the UV absorption spectra ol various cell wall layers of hybrid poplar during differentiation, showing the initial (/), middle (2), and later (5) stages of lignification. V-SIV Secondary wall of vessel F-SW secondary wall of fiber FF-CC cell corner of middle lamella between fibers VF-CC cell corner of middle lamella between vessel and fiber, e UV absorption spectra of mature fiber secondary wall (F-SIV) and mature cell corner of middle lamella between fibers (FF-CC) obtained from the same poplar fiber. The UV spectrum shown by (FF-CC)-(F-SW) is the difference spectrum between the cell corner middle lamella and fiber secondary wall. (Takabe et al. 1987)... 

In normal wood tissue, the fiber secondary wall consists of three fairly distinct layers. The outermost layer or Si is very thin (0.1-0.2 jjim) and exhibits an average microfibril angle (for the layer as a whole) of about 50-70° (2). The bulk of the secondary wall is made up of the S2 layer, which is typically several micrometers thick (Figure 18). Here the microfibrils are usually oriented to the fiber axis at a relatively small angle (5-20°). The thickness and small microfibril angle... 

Figure 18. TEM of normal layering (SfSJS ) in the fiber secondary wall ofloblolly pine transverse sections. (Reproducedfrom Ref 41. Copyright 1971, Springer-Verlag.) Key A, portion of two earlywood fibers (ML = middle lamella) and B, lignin skeleton of region simitar to that...

Sculpturing. Pits. Softwood and hardwood fibers have closed ends, but a special wall feature facilitates movement of the tree s sap stream from one fiber to another, from fibers to vessel elements, and from fibers to ray cells. This special feature is a small opening or recess in the fiber secondary wall known technically as a pit. 

Fiber secondary wall exterior to the G-layer is just as lignified, if not more so, than in normal wood fibers. 

The sensory afferents of the muscle spindle comprise group la fibers (primary endings), which send branches to every intrafusal fiber in the muscle spindle, and group n fibers (secondary endings), which innervate the nuclear chain endings only. The larger primary endings (la) surround the center of the muscle spindle much like a coil, whereas the smaller secondary end-... 

By refining, the shape of the fibers is changed. They may be shortened, spht lengthwise, collapsed or fibrillated. Refining is done either at low consistency (virgin fibers, secondary fibers) of about 3-6% or at high consistency (mainly secondary fibers) of about 30% and more. 

Part III of the book discusses different properties of fibers. Fiber properties ean be classified into primaiy and secondary properties. Primary properties are those that fibers must possess so they can be converted into useful products. Examples of primary properties are aspect ratio, strength, flexibility, cohesiveness, and uniformity. Secondary properties are those that are desirable and can improve consumer satisfaction with the end-products made from the fibers. Secondary properties include, but are not hmited to physical shape, density, modulus, elongation, elastic recovery, resilience, thermal properties, electrical properties, color and optical properties, moisture regain, resistance to chemical and environmental conditions, resistance to biological organisms, and resistance to insects. Chapter 14 provides an introduction to these primary and secondary properties. 

However, due to the very high strength of the wood fiber secondary wall, the material resulting from mechanical homogenization of wood fibers also contains a substantial part of larger fibril bundles and even residual fibers and fiber fragments (Figure 8.4). 

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