The Fine Structure of the Kapok Fiber

Abstract

The fine structure of kapok fiber was investigated by transmission electron microscopy. The walls of the fibers contained five layers, observable both in lateral and in longitudinal cross-sections: the cuticle, S; primary wall, W1; secondary wall, W2; tertiary wall, W3; and inner skin, IS. W1, with a thickness of about 200 nm, was characterized by an interlaced fibril-like network. The fine structure of kapok fiber showed that there was an obvious fibril structure. The fibrils of W2 were arranged at a certain angle to the fiber axis; and those of W3 were closely packed and aligned parallel to the fiber axis. The thicknesses of W2 and W3 were the same, about 500 nm. The highest packing density lay in the cuticle, which is the protective layer of the fiber, while the structures of W1 and W3 were more compact than that of W2. Fibrils were easily separated from the relatively loose IS, and dispersed in the lumen. Moreover, there was a transition layer of low-packing density between adjacent layers. The interactions between the fibrils in the transitional layers were weaker than those in the individual layers. The sizes of the smallest visible structural units were not the same for the different walls and the geometric scales were in the range between that of protofibrils and fibrils. The smallest fibril size was 3.2-5.0 nm, which is near the dimensions of the protofibrils of the cotton fiber. Some larger fibrils of different sizes were also observed in the different walls.