Page 336 - Handbook of Properties of Textile and Technical Fibres

Hemp, jute, banana, kenaf, ramie, sisal fibers 309

9.4.1 Physical modification

Surface finish of the fibers is an important parameter in manufacturing engineering. It
is a characteristic that can influence the performance of finished parts and production
costs. Different physical treatments, such as placing the fibers in boiling water with or
without pressure, can improve the cleaning of fiber surface, which then can react with
the resin more easily to form a strong interface (Ramesh et al., 2011). Polymeric coat-
ings of fibers with phenol formaldehyde or resorcinol formaldehyde resins using
different approaches are highly effective in enhancing the reinforcing character of fi-
ber, giving as high as 20%e40% improvements in flexural strength and 40%e60%
improvements in flexural modulus of the composites. These modifications improve
the fiber-matrix resin wettability and lead to improved bonding. Many physical treat-
ments including plasma treatment (Militky and Jabbar, 2015), photo oxidation by UV
irradiation (Kafi et al., 2006), ionizing radiation (Khan et al., 2006), corona (Strobel
and Lyons, 2003), cold plasma (Tamargo-Martinez et al., 2011), ozone treatment
(Militky and Jabbar, 2015), laser treatment (Stepankova et al., 2010), and atmospheric
pressure plasma (Baltazar-y-Jimenez et al., 2008) have already been employed to over-
come the incompatibility of various substrates.

9.4.2 Chemical modification

Chemical modifications are used to optimize the interface of the fibers with the matrix.
As the plant fibers contain hydroxyl groups from the cellulose and lignin, they are
amenable to modification. The chemicals may activate hydroxyl groups or introduce
new moieties that can effectively interlock with the matrix. These groups may be
involved in the hydrogen bonding in the cellulose molecules (Dhakal et al., 2014).
It is difficult to reinforce resins using plant fibers, as they possess hydrophobic surfaces
and anisotropic internal structures. To address these issues, the surface state of plant
fibers is typically modified by chemical treatments (Ma and Joo, 2010; Yu et al.,
2010; Huda et al., 2008; Sawpan et al., 2011; Goriparthi et al., 2012; Csizmadia
et al., 2013). Generally, chemical coupling agents are molecules processing two func-
tions. The first is to react with hydroxyl groups of cellulose, and the second is to react
with functional groups of matrix (Rajesh and Prasad, 2014). Environmental perfor-
mance of these composites is generally poor as a result of delamination under humid
conditions. Therefore, to develop composites with better properties and environmental
performance, it is necessary to impart hydrophobicity to the fibers by chemical treat-
ment with suitable coupling agents or by coating with appropriate resins (de Albuquer-
que et al., 2000). Because the interfacial bonding between the reinforcing fibers and the
resin matrix is an important element in realizing the mechanical properties of the com-
posites, several authors (Mukherjee et al., 1984; Samal et al., 1995; Rana et al., 1998;
Gassan and Bledzki, 1999a,b; Mukherjee et al., 1983; Zadorecki and Flodin, 1985a,b)
have focused their studies on the treatment of fibers to improve the bonding with resin
matrix. Surface modification of plant fibers has been accomplished by performing
chemical treatments, including detergent washing, dewaxing, alkali and acetic acid
treatment (Hossain et al., 2011).