Biomedical Engineering Reference
In-Depth Information
One biopolymer that is already used as a matrix for composite production is
poly(lactic acid) (PLA). The PLA is a thermoplastic biopolymer with lactic acid which
is derived from starch by a fermentation process as its basic monomer. High molecular
weight PLA is polymerized by the lactide ring opening polymerization to PLA
(Garlotta, 2001; Gupta et al., 2007; Lim et al., 2008).
While cellulose fiber reinforced polypropylene (PP) is already used by default for
example in the automobile industry for interior parts (Karus and Kaup, 2005), the con-
ventional use of cellulose fiber reinforced PLA is still at the beginning. But there are
also some products such as biodegradable urns, mobile phone shells or prototypes of
spare tyre covers made from natural fiber reinforced PLA at the market (Anonymous,
2007; Iji, 2008; Grashorn, 2007). Many studies deal with the use of natural fibers as
reinforcements in PLA composites. An overview about the mechanical characteristics
and application areas of natural fiber-reinforced PLA can be found for example in
Bhardwaj and Mohanty (2007), Avella et al. (2009), Ganster and Fink (2006), Jo-
noobi et al. (2010), and Graupner et al. (2009). For the improvement of the composite
characteristics it is still necessary to carry out optimization processes for fibers, PLA
matrix and the interactions of both. Moreover the processing parameters, force elonga-
tion characteristics of fibers and matrix as well as the use of additives like plasticizers
or adhesion promoters have decisive influences on the mechanical characteristics of
the composites.
For improving the mechanical characteristics of cellulose fiber reinforced PLA
some studies have been carried out which deal with an improvement of fiber/matrix
interactions. An important factor is the optimization of the fiber surface by modifica-
tion methods. Examples for surface treatments on natural fibers and their effects on
the mechanical PLA composite characteristics are given for example in the studies of
Tokoro et al. (2008), Cho et al. (2007), Hu and Lim (2007), Lee et al. (2009), Masirek
et al. (2007), Yu et al. (2010) and Huda et al. (2008). Beside fiber surface treatments,
the bonding between fiber and matrix can be optimized by additives. A well known
additive for improved fiber reinforced PP composite characteristics is anhydride ma-
leic acid (MA). Some studies tested the use of MA in cellulose fiber reinforced PLA
composites. But the most studies reported no improved tensile strength by using MA
(Bourmaud and Pimbert, 2008; Plackett, 2004). Lee and Ohkita (2004) used bamboo
fiber esterifies malefic anhydride reinforced PLA in the presence of dicumyl perox-
ide as a radical initiator. These results proved improved tensile characteristics of the
composites. Lee and Wong (2006) studied the effects of lysine-based diisocyanate
(LDI) as a coupling agent for compression molded bamboo fiber PLA composites.
The results showed improved tensile characteristics of the composites. And a study
of cotton fiber reinforced PLA with a fiber load of 40 mass% has shown that lignin
as a natural additive increased the tensile strength by 9% and the Young´s modulus
by 19%, whereas the impact strength was decreased by 17% in comparison to the
pure PLA sample (Graupner, 2008). A previous study has also shown that a mixture
of brittle and stiff hemp fibers and ductile lyocell fibers lead to improved composite
characteristics (Graupner, 2009).
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