Biopolymers - Food and Industrial Bioproducts and Bioprocessing - page 265

The introduction of cellulose into a graft-copolymer matrix and common thermoset
polymers (Gindl and Jeronimidis, 2004) was shown to improve the mechanical properties of
the composite materials (Mohanty et al ., 2000). Fiber surface modification by physical and
chemical means (Felix and Gatenholm, 1993), and the use of coupling agents to overcome
the incompatibility between cellulose and a hydrocarbon polymer matrix as well as to
improve interfacial interactions were applied (Nair and Thomas, 2003). When cellulose is
used as a natural fiber in a biodegradable polymer matrix, so called eco-composite materials
are formed; these have drawn increasing attention due to environmental considerations. The
processes required to prepare eco-composites are very similar to fiber glass composites and
have been reviewed by Bogoeva-Gaceva and co-workers (2007). A brief review covering
composites of biodegradable and non-degradable polymers with natural fibers was given by
Netravali and Chabba in 2003.
Chemical modification of cellulose is another important research topic, as is industrial
application area. The high number of hydroxyl groups present on each repeating unit of
cellulose gives potential to be chemically modified through all possible alcohol-involved
organic reactions, typically esterifications and etherifications. Moreover, click chemistry is
currently being investigated to functionalize cellulose (Liebert et al ., 2006 ; Zhao et al ., 2010 ).
Cellulose acetate (Figure 11.3) is the most widely known and used example of esterified
cellulose derivative. It is used for potentially biodegradable fibers and films. The degradation
rates of cellulose acetate can be controlled with the degree of substitution (Buchanan et al .,
1993 ). Parkesin TM , a moldable nitro derivative of cellulose (Figure 11.3), has been used as a
replacement for ivory. Moreover, celluloid, invented in the 1860s, is a nitro-cellulose
derivative that uses camphor as a softener to improve its flexibility (Mooney, 2009).
The crystal structure and three-dimensional network prevent cellulose from behaving as
a polyol for PU syntheses. To overcome this problem a liquefaction process in the presence
of organic solvents was developed and resulted in products suitable for PU synthesis. Yan
and co-workers (2008) liquefied corn stalk, an agricultural by-product, and tested it for the
synthesis of PU foams blown by water. The report revealed that such polyurethane foams
had excellent mechanical and thermal properties and could be used as heat insulating
materials.
In addition to its renewability and satisfactory mechanical properties, cellulose presents
biocompatibility, which makes it possible to use in some pharmaceutical applications.
Different chemical modifications allow different applications of cellulose, such as oxycellulose
for controlled drug delivery matrices, sodium carboxymethyl cellulose as emulsifying agents,
and cellulose acetate phthalate for tablet coatings (Kamel et al ., 2008 ).
Just like starch, cellulose has also been shown to exhibit some important electro-active
properties. For instance, cellophane was found to be a piezoelectric material that transforms
electrical energy into mechanical energy. Nevertheless, some cellulosic blends, for example
the blend of cellulose xanthate and propylene oxide-grafted hydroxyethyl cellulose with
PEG, showed conductivities around 10 −5 and 10 −4 S/cm (Finkenstadt, 2005 ).
11.2.3 Polymers from lactic acid and lactide
An important feature of starch is its potential enzymatic hydrolysis into glucose and
subsequent fermentation into lactic acid. Poly(lactic acid) (PLA) can be obtained from this
fermentation product via direct condensation or via its cyclic lactide form (l-, d-, or meso-
lactide) (Mecking, 2004 ; Lichtenthaler, 2006 ) (Figure 11.4 ). PLA chemistry was investigated
by Carothers in 1932 and since then it has been extensively studied by means of efficient