As a kind of natural fiber, kapok fiber shows the highest hollowness and the lowest specific mass that any microchemical fiber is incomparable. Kapok fibers consist of natural microtubules with fine tube structure (ca. 8-10 pm in diameter and ca.

0. 8-1.0 pm in wall thickness) (Chung et al. 2008), and the hollow ratio can reach 97 %. One end of the fiber that tapers to one point is closed, and the other end is bulbous shape and may be closed tightly (Xiao et al. 2005). As a single-cell fiber, cotton fiber looks like ribbons, rolled in a helicoidal manner around the axis, while kapok fiber is not convoluted. Figure 6.1 shows the SEM micrographs of longitudi­nal and cross-sectional view of kapok fiber. A longitudinal view of kapok fiber shows smooth cylindrical surface, while a cross section reveals a wide open lumen (Mwaikambo and Bisanda 1999). Kapok fiber shows a unique hollow structure, and this feature is expected to enlarge its specific surface area, endowing the fiber with outstanding moisture transfer property and making it an ideal environment-friendly natural thermal fiber (Feng et al. 2006).

From the fine structure of the walls of kapok fiber, five layers, i. e., cuticle (S), primary wall (W1), secondary wall (W2), tertiary wall (W3), and inner skin (IS), have been clearly observed in lateral and longitudinal cross sections. The W1 is characterized by an interlaced fibril-like network (Xiao et al. 2006), while the fibrils of W2 and W3 are arranged angled or parallel to the fiber axis. The thickness of W1 is about 200 nm, and this thickness seems the same for W2 and W3 (ca. 500 nm). The cuticle S is the protective layer of kapok fiber and shows the highest packing density. In addition, the fibrils of W1 and W3 are closely packed and accordingly, the structure of them is more compact than that of W2. However, the structure of IS is relatively loose, and the fibrils are easily escaped from IS and then dispersed in the lumen. Between the adjacent layers, a transition layer with the low packing density is present. In transitional layers, the interactions between the fibrils are weaker than those in the individual layers. As for different walls, the variety in the fibril size from protofibrils to fibrils is observed for the smallest structural units. The smallest fibril size is found to be 3.2-5.0 nm in different walls (Shi et al. 2010).

The main components of kapok fiber are cellulose, lignin, and xylan (Fengel and Przyklenk 1986; Gao et al. 2012). The outer cell wall layer contains less lignin and more of the minor polysaccharides mannan and galactan and more proteins than the main part. There is a high mineral content in the outer layer which obviously influ­ences the surface properties of the kapok fiber. Kapok fiber includes a high ratio of syringyl/guaiacyl units (4-6) and a high level of acetyl groups (13.0 %) as com­pared with normal plant cell walls (about 2-4 %) (Chung et al. 2008). The bulk density of the kapok fiber is 0.30 g/cm3, the crystallization degree is 35.90 %, and the specific birefringence is 0.017 (Xiao et al. 2005). The kapok fiber shows the well-resolved spectrum of cellulose I, and the crystallinity is lower than cotton fiber (Cao et al. 2010).

To enhance the intrinsic properties or alter the surface characteristics, natural kapok fiber is usually pretreated including (1) chemical treatment, such as alkali/ acid treatment, solvent treatment, oxidation treatment and acetyl treatment, and
(2) physical treatment, such as ultrasonic treatment and radiation treatment, by which the surface impurities can be removed and the interfacial properties will be improved.

Solvent treatment is a popular method to change the surface property of kapok fiber. Previous studies have shown that the kapok fiber has lost their silky luster after solvent treatment. By comparing the spectra of untreated and solvent-treated kapok fiber, the increase in absorption bands at 3,410 and 2,914 cm-1 can be observed, and this information is an indication of the removal of plant wax from the surface of kapok fiber. Except for the above absorption bands, there is no significant variation in other bands for water-treated and chloroform-treated kapok fiber. But for NaOH — treated fiber, the absorption bands at 1,740 and 1,245 cm-1 show a remarkable reduction in their intensities. This is ascribed to the fact that the alkali treatment can remove all the esters linked with aromatic ring of lignin, resulting in a significant de-esterification of kapok fiber. For NaClO2-treated kapok fiber, the absorption bands around 1,602 and 1,504 cm-1 nearly disappear, owing to the cleavage of the aromatic ring in lignin (Wang et al. 2012a).

Furthermore, for untreated, water-treated, HCl-treated, NaOH-treated, NaClO2- treated, and chloroform-treated fiber, the crystallinity index is determined to be 35.34 %, 33.93 %, 22.17 %, 32.00 %, 26.97 %, and 27.17 %, respectively. This result implies that the crystalline region of lignocellulose in kapok fiber shows no remarkable change for water-treated kapok fiber, while HCl, NaClO2, and chloro­form treatment will change the aggregate structure and expand the proportion of amorphous region of kapok fiber. But for NaOH-treated kapok fiber, there appears no remarkable reduction in the crystallinity when compared to NaClO2 treatment, even though a significant de-esterification occurs for kapok fiber during this process (Wang et al. 2012a).

Liu and Wang (2011) investigated the effect of mercerization on microstructure of kapok/cotton yarns, with the findings that the chemical compositions of fiber showed no appreciable changes after mercerization treatment, but this treatment could decrease the crystallinity of kapok/cotton yarns, transforming partial cel­lulose I to cellulose II. Chen and Xu (2012) found that the ultrasonic treatment with water had little influences on the morphological structure and chemical com­ponent of the kapok/cotton-blended yarns, except for some loss of kapok flocks. Via the combination process of chlorite-periodate oxidation, kapok fiber was found to harbor a certain amount of polysaccharides, together with lowered lignin content. Although a distorted hollow shape and rough surface were observed, the characteristic fine hollow shape was still maintained in all of the chemically oxidized kapok fiber (Chung et al. 2008). To provide the functions or facilitate further modification, some polymerizable monomers had been grafted onto the kapok fiber by Co60 y-ray radiation-induced graft copolymerization, such as styrene, glycidylmethacrylate (GMA), and acrylic acid (AA) (Cho et al. 2007; Kang et al. 2007).