There are numerous chemical methods that have been used to modify the surface energy characteristics of natural fibers, but only a few prominent methods will be briefly mentioned here, including alkaline, liquid ammonia, and esterification.

In the alkaline or basic treatment, natural fibers are dissolved or mercerized so that they can be incorporated into thermoplastics or thermosets as reinforcement agents. It is one of the very best ways known to increase the amount of amorphous cellulose at the expense of the crystalline cellulose; this latter event occurs by com­promising the tight spacing between the cellulose chains by disruption of the hydro­gen bonding interface and loss of the crystallinity. The following reaction is a good summation of the chemistry that occurs:

Fiber-OH + NaOH a Fiber-O- Na+ + H2O (1)

The mercerization process consumes the crystalline cellulose I form as shown in the reaction above by the penetration of the alkali into the cellulose H-bonding network which forms alkali cellulose. The alkali cellulose is then washed by a water treatment to remove the unreacted alkali upon which a regenerated cellulose, cel­lulose II, is then formed. It has been reported that flax fibers have tremendously enhanced strength and stiffness as a result of the treatment.45 These results are like­ly due to two important effects resulting from the treatment: 1-increase in surface roughness which increases the friction and resultant interlocking among the fibers, and 2-increase in the number of exposed cellulose groups on the surface thus having more hydroxyl groups available for H-bonding or reaction.

The second chemical treatment, liquid ammonia, arose as an alternative to mer- cerization for cotton in the 1960s. Due to its low viscosity and surface tension, it penetrates the cotton H-bonded network very effectively to form a complex com­pound after the rupture of the H-bonded network. The ammonia is small enough to penetrate crystalline regions very well and cause cellulose I to cellulose III which can reform I after treatment with hot water.

Esterification and the associated etherification are two of the dominant meth­odologies used to derivatize cellulosics. The reaction generally proceeds via the hydroxyl groups via the introduction of organic acids or anhydrides. Many esters are possible depending on the nature of the alkylating/acylating group used. Pos­sibilities for esterification include the formation of the formate, acetate, propionate, and butyrate (1-4 carbon atoms) in addition to the longer laurate (12 carbons) and stearate (18 atoms) final products. However, the most popular esterification method is acetylation, which occurs by the use of acetic anhydride to form the natural fiber acetate and acetic acid.

With respect to nanocrystals, two routes are available to obtain nonflocculated dispersions in an appropriated organic medium:

1. Surface coating with long-chained surfactants (polar heads and long hydro­phobic tails);

2. Hydrophobic chain grafting at their surface.