Structures And Properties Of Kapok Fiber And The Wettability And Buoyancy Characteristics Of Kapok Fibrous Assemblies

The structures and performances of the kapok fiber from the Pangzhihua area in Sichuan province have been investigated. The cross section and ends are photographed in natural status. The kapok fiber shows a unique morphology with hollow lumen, thin cell walls, and two closed ends in nature. The fiber is very light in weight with the density of 0.03g/cm³. Its crystallinity and oritation degree are lower than those of the cotton fiber, the structure of the cell wall is looser than that of cotton fiber, the moisture regain is about 10.8% which is higher than that of cotton fiber, and the resistance to alkali is weaker than that of cotton fiber.By staining of osmium tetroxide and uranyl acetate and lead salts, a clear cell structure of the ultrathin sections of the stained kapok fibers were observed using transmission electron microscopy (TEM). The ultrastructure of kapok fiber was presented in first time. The wall of kapok fibre can be divided into five layers, including skin layer, S;primary wall, W₁;secondary wall, W₂;tertiary wall, W₃;and inner skin, IS. There are transition layers between adjacent layers, namely, L₁ between W₁ and W₂, and L₂ between W₂ and W₃. An interlaced fibril-like network is the characteristics of the primary wall, W₁, with about 160²40nm in thickness;the fibrils in the secondary wall, W₂, incline to the cell axis at a constant angle of 16.3°;and the fibrils in the tertiary wall, W₃, are closely packed and aligned in at a certain angle of 36.6° to the fiber axis. The W₂ and W₃ walls have the same thickness about 500nm. The highest packing density lies in the skin layer which is the protective layer of the fiber with about 40⁷0nm in thickness. The structures of W₁ and W₃ are more compact than that of W₂. The inner skin with thickness below 40nm is relatively loose. The loosest packing density is within the transition layers. The interaction between fibrils in the transitional layers (L₁ and L₂) is weaker than that in the individual layers. The scale of the visible minimum structure units is not the same for different walls. The minimum fibril size of kapok fiber is 3.2nm⁵.0nm thick and near to the dimension of the protofibril of cotton fiber. Some larger fibrils with different sizes are also in the different walls. The ultrastructure of kapok fibre show that there is an obvious fibril structure and the geometric scale belongs to the range from protofibrils to fibrils.Kapok fiber swollen by alkali shows an evident difference among the different layers in the same cross section, and W2 and inner skin are most easily swollen and are most weak resistant to alkali. The fibrils are easily departed from the inner wall and dispersed into the lumen. It is obviously different from cotton fiber swollen by alkali.In order to measuring the wettability of the short and fine single fiber with poor wettability, a new wettability evaluation method for the single fiber measure has been developed, the systemically theoretical analysis was carried out, and the quantitative parameters were put forward. A fiber in horizontal is forced into and withdrawn from the liquid at a certain speed, and the force changes are detected simultaneously. The experimental results show that the force impulse can be found at fiber contacting with liquid and immersed into liquid, and its value depends on the wettability of the fiber. According to the force impulses of different fibers, the immerging behavior of the fibers can be obtained and categorized into four sorts. Meanwhile, the wettability of the fiber can be evaluated with the wettability factor, w, derived from the fiber immerging curve, where w is the ratio of the force increment on fiber initially contacting with the liquid to the force increment on liquid surface closing with fiber immerging into the liquid, and with the contact angle, 9, calculated from the wettability factor {0 = ix/(w+l)) supposed that the fiber is a circular cross-section. The force measurement indicates that the method can be used for various fibers with short length, or lower density than that of the liquid, or poor wettability.The influencing factors in quantitative calculations are discussed. The force impulses during the initial contacting process and the liquid surface closing process increase with the increase of the fineness of the fiber, but the fineness does not affect the wettability of the fiber. The shape of the cross section of the fiber is an important factor to the wettability calculation. By this new method, the contact angle between the kapok fiber and the distilled water is about 157° that is similar to the value measured from the sessile drop method.The capillary performance of the fibrous assemblies with positive wettability has been studied by using the dynamic capillary method, and the factors affecting the performances of the fibrous assemblies with negative wettability were deduced. The bulk density, the fineness of the single fiber, the surface characteristics of the fiber, and the liquid surface tension are the important factors to the wettability of the fibrous assemblies. The fiber profile is also a reason influencing the wicking property but less than the fineness and the bulk density do. The modified Lukas-Washburn equation is only suitable to the initial capillary process. The shapefactor of the fibrous assemblies can be obtained when the contact angle is known.Through the compression tests of the kapok fibrous assemblies, this present thesis emphasizes on the importance of the initial bulk density. At the same pressure and compressing time, the fibrous assembly in random alignment will approach to a constant bulk density despite of the different initial bulk density. If the initial bulk density of the fibrous assemblies is lower than the constant value, the compression rate decreases and the compression recovery rate increases with the increases of the initial bulk density of the fibrous assemblies.The reasonable bulk density of the kapok fibrous assemblies as buoyancy materials ranges from 0.038g/cm3 to 0.05g/cm3. In fact, if kapok fibers are immerged in the water for the long time, the moisture regain of the fibers will increase. The experimental results show that the weight of the kapok fibrous assemblies increases to 45%⁵5% after 24hr immerging in water. The theoretical model for the buoyancy evaluation of the fibrous assemblies has been established. It can be used to guide the design of fibrous assemblies as buoyancy materials.The investigation on the three composite buoyancy materials indicates that the multiple layers buoyancy material made of the kapok layers piled up with the three-dimension-crimp polyester layers forms a good and stable supporting structure, which can improve the compression recovery rate comparing to the pure kapok fibour assemblies. The buoyancy material made of kapok fibers and low melting point fiber forms a steadier structure than the pure kapok assemblies and has good resistance to the compression. It is a better composite buoyancy material with a higher buoyancy factor and low buoyancy loss. However, the mixed fibrous assembly of the kapok fibers and the three-dimension-crimp polyester fibers by carding process cannot form a good buoyancy material.In short, all the experimental results show that the typical morphology of the kapok fiber is unique with large lumen, thin cell walls, two closed ends and elaborate fibril structure. Its fibrous assembly is a good buoyancy material with good resistance to water and lower bulk density. The composite made of the kapok fibers bonded with the low melting point fibers has a stable structure and better buoyancy behavior.