Natural fiber alkali treatment

Extensive work was carried out by Joseph et al. on many treatments of natural fiber (alkali, permanganate, peroxide (DCP and BPO), isocyanate) for improving the mechanical properties of sisal fiber LDPE composites. The results showed that in every case the mechanical properties were increased with chemical treatment but peroxide treatment showed the best results (Table 22.12) [58, 59]. 

An older method of cellulose fiber modification is mercerization [22,33-36], which has been widely used on cotton textiles. Mercerization is an alkali treatment of cellulose fibers. It depends on the type and concentration of the alkalic solution, its temperature, time of treatment, tension of the material, and the additives used [33,36]. At present there is a tendency to use mercerization for natural fibers as well. Optimal conditions of mercerization ensure the improvement of the tensile properties [33-35,37] and absorption characteristics [33-35], which are important in the composing process. 

It is known that alkali treatment improves the fiber-matrix adhesion due to the removal of natural and artificial impurities from the fiber surface as well as the change in the crystal structure of the cellulose [119]. Moreover, alkali treatment reduces fiber diameter and thereby increases the aspect ratio. Therefore, the development of a rough surface topography and enhancement in aspect ratio offer better fiber-matrix interface adhesion and an increase in mechanical properties was reported by Mohanty et al. [68]. Alkali treatment increases surface roughness resulting in better mechanical interlocking due to the amount of cellulose exposed on the fiber surface [53]. Several other studies were conducted on alkali treatment and the results are discussed elsewhere [36, 37, 41, 86]. 

Gassan J, Bledzki AK (1999) Possibilities of improving the mechanical properties of jute/ epoxy composites by alkali treatment of fibers. Compos Sci Technol 59 1303-1309 Singh B, Gupta M, Verma A (1998) Studies on adsorptive interaction between natural fiber and coupling agents. J Appl Polym Sci 70 1847-1858... 

The significant increase of melting point after 10% alkaU treatments shows that, at that concentration, the NaOH has a role to improve fiber-matrix interface. When the fiber is untreated, the fiber and matrix is not mixed well due to the hydrophobic nature of the matrix and hydrophihc nature of the fiber. Hence, the adhesion between fiber and matrix is poor and there is no heat transfer between fiber and matrix [54, 55]. As a result, the melting point of the composite is not higher than that of the matrix. After 10% alkali treatment, the adhesion gets better and then the heat transfer occurs. Hence,... 

Alkali treatment It makes the surface pores wider and fiber become thinner due to dissolution of natural and artificial impurities... 

Sometimes, degree of adhesion between fiber and matrix achieved through various primary processing of biocomposites is weak which results in poor performance of developed composites. Natural fibers comprise of waxy and fatty materials on their surface which causes improper bonding between fiber and matrix. There are various chemical treatment techniques (such as alkali-treatment, silane treatment, Benzoylation, Acetylation, etc.) which can be applied to natural fibers before primary processing to condition the surface and consequently improve the fiber matrix adhesion to develop high performance biocomposites. 

Several studies have been made to optimize the properties of natural fiber-reinforced PLA composites from the point of view of fiber-matrix adhesion. Pretreatment of fibers, such as chemical modification, seems to be the most promising approach, in which covalent bonds are formed between the fiber and matrix. One of the most common and efficient methods is alkali treatment (for example, with 2% sodium hydroxide aqueous solution) of fibers, which has been used to... 

Alkalization Mwaikambo et al. were one of the earher groups who adopted various techniques to analyze the effect of alkali treatment on different natural fibers. Even though they did not conduct mechanical testing of composites, their investigation is vital in understanding the effect of alkali concentrations on the crystallinity index of the fibers. Their studies showed that lower concentrations of alkali would result in... 

Silkworm silk is another protein-based fiber produced naturally by the silkworm, B. Mori or other varieties of moth, and it is the only naturally and commercially produced continuous filament. Silkworm silk fiber is composed of a fibroin core and a sericin casing, and is relatively rigid because the sericin causes the filaments to adhere to one another. After the desericin treatment (with alkali treatment) silk has high tenacity, high luster, and good dimensional stability. A triangle cross section is its typical stracture, which results in the luster of silkworm silk (Fig. 2.10). 

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