Acetylation natural fibers

Another way to improve the adhesion is by using the process of natural fiber acetylation. Acetylation reduces the jute fiber water sorption by about 50% [77]. Liu et al. [77] investigated the effect of acetylation on cotton and rayon fibers and found that the strength of composites improved. [78] noticed the increase of fiber surface energy upon acetylation, which was explained by forming ester bonds. 

To improve the adhesion between natural fiber and polymer matrix, chemical modification of natural fibers was investigated by a number of researchers. The mechanism and utilization of selected chemical treatments is discussed in this section. There are many different methods to improve the interfacial adhesion between fiber and matrix by modifying fiber surface such as acetylation, benzoyla-tion, acrylation, permanganate, and isocyanate treatment. These treatments are described in detail by Kalia et al. [69]. 

Based on the literature discussed in this chapter, it seems that surface modification of natural fibers is absolutely necessary to improve their thermal stability, dispersion in the polymer matrix, and compatibility with the polymer matrix. Physical and chemical methods reported have significantly modified the surface properties of the fibers as well as polymer matrices to improve the dispersion of the fibers and hence various properties of the polymers. Use of silane coupling agents and acetylation... 

The acetylation treatment decreases the hydrophilic nature of the plant fibers. During the acetylation treatment the hydroxyl groups in the cellulosic fiber are replaced with acetyl functional groups. The decrease in hydrophilicity results in lower moisture intake and stronger interfacial bonding [8]. De Rosa et al. [15] obtained lower water content in acetylated okra fibers compared to untreated ones. Hill and Khalil [72] reported that the acetylation treatment enhanced the bioresistance of coir and oil palm fiber-reinforced polyester composites. 

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. 

The reactions of NaOH with cellulose in natural fiber yielded cellulose-ONa compound and removed impurities from the fiber surface [11,12]. This is confirmed by the FTIR spectroscopic analysis as shown in Figure 14.1. The FTIR spectrum of the raw wood clearly shows the absorption band in the region of3407 cm, 2917 cm and 1736 cm due to O-H, C-H and C = O stretching vibration respectively. These absorption bands are due to hydroxyl groups in cellulose, carbonyl groups of acetyl ester in hemi-cellulose and carbonyl aldehyde in lignin. 

Acetylation methods have been used for many years to improve some properties of wood cellulose such as moisture repellency, dimensional stability and resistance to environmental degradation. Their use for improving the properties of natural fibers has increased significantly in the last decade or so. The methods are based on the reaction of lignocellulosic material with acetic anhydride at elevated temperature, with or without a catalyst. The acetic anhydride reacts with the more reactive hydroxyl groups according to the equation [36],... 

Acetylation can reduce the hygroscopic nature of natural fibers and increases the dimensional stability of composites. Acetylation was used in surface treatments of fiber for use in fiber-reinforced composites [46,47]. 

Interest in natural fibers obtained from different resources to reinforce polymer so as to get the novel composites is growing rapidly because they are renewable, cheap, recyclable, and biodegradable. Research in this field has prompted surface modification of natural fibers in order to improve the compatibility between hydrophilic fibers and hydrophobic matrix [1], Major challenges for polymer scientists in the development of structural natural fiber-reinforced composites are to increase the moisture resistance, dimensional stability with minimized matrix material, and to decrease the manufacture costs of the composite materials. Different researchers have used different surface modification methods, that is, mercerization [2], benzoylation [3], silanation [4], acetylation [5], graft... 

Weight loss measurement was one of several methods used to study the biodegradation of cellulose fibers from flax by two strains of Cellvibrio [59], These aerobic cultures initially contained 0.1 g of natural fiber or fibers with different degrees of acetylation. After 13 days of incubation, weight losses of 20% to 76% were recorded these data showed that more highly acetylated fibers were more resistant to biodegradation. 

Acetylation treatment of natural fibers is originally applied to wood cellulose to stabilize cell walls against moisture, improving dimensional stability and envirormiental degradation [73]. Therefore, acetylation may contribute toward decreasing the moisture absorption of natural fibers [37]. Plasticization of cellulose-based natural fiber can be introduced by this treatment, which is related to an esterification method. During the treatment of natural fibers with acetic anhydride, the hydroxyl groups... 

Scheme 4.4 Reaction scheme of acetylation of natural fibers with (a) and without (b) catalyst. (After A.K. Mohanty [4].)...

Acetic anhydride is preferable for acetylation with cellulose because it has more reactive hydroxyl groups than acetic acid. Experimentally, natural fibers of interest are normally first soaked in acetic acid and subsequently treated with acetic anhydride. This is to promote the reaction as acetic anhydride does not swell enough cellulose for the reaction. In order to provide a combination effect of the treatment, alkali treatment may be carried out before treating natural fibers with glacial acetic acid followed by acetic anhydride [75-77]. 

In addition, many other approaches for treating the surfaces of natural fibers such as coupling agents, polymer coating, enzyme modification, and acetylation to increase the water absorption resistance through the improvement of fiber-matrix interfacial properties have been attempted [60, 147, 152, 159],... 

Acetylation rates have also been studied by Centola37 who treated natural and mercerized ramie fibers for varying times with acetic anhydride and sodium acetate and examined the reaction products chemically and by X-ray diffraction. The reagent was considered to penetrate into the interior of fibers. A heterogeneous micellar reaction was believed to occur that converted a semi-permeable elastic membrane around the micelles into the triacetate. The rate of acetylation of mercerized ramie was observed to be faster than that of unmercerized fiber. Centola concluded that about 40 % of the cellulose in native ramie is amorphous and acetylates rapidly. 

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