depolymerises the native cellulose structure and exposes the short
length crystallites [14]. Acrylic acid treatment was also reported to
be effective in modifying the natural fibers surface. A study on flax
fibers-reinforced polyethylene bio-composites by Li et al. found
that the efficiency of such a treatment was higher than alkali and
silane treatment [14].
The chemical coupling method is also one of the important
chemical methods, which improve the interfacial adhesion. In this
method the fiber surface is treated with a compound that forms a
bridge of chemical bonds between fiber and matrix. The chemical
composition of coupling agents allows them to react with the fiber
surface forming a bridge of chemical bonds between the fiber and
matrix. Most researchers found these treatments were effective
and showed better interfacial bonding [13]. Among different coupling
agents, maleic anhydride is the most commonly used. In general,
the literature reports improvements in tensile strength and
elongation at break when maleic anhydride grafted matrices are
used as compatibilizers (coupling agent) [15].
Hu and Lim [18] investigated that alkali treatment significantly
improved the tensile properties of hemp fiber reinforced polylactic
acid (PLA) compare to those untreated. Figs. 14 and 15 showed that
the composites with 40% volume fraction of alkali treated fiber
have the best tensile properties. The tensile strength and tensile
modulus of the composites with 40% treated fiber are 54.6 MPa
and 85 GPa respectively, which are much higher than neat PLA,
especially for the tensile modulus which is more than twice of that
of neat PLA (35 GPa).