Biomedical Engineering Reference
In-Depth Information
Interestingly, no vertebrate has the capacity to digest cellulose enzymatically. Herbivores
subsist largely on cellulose, not because they can digest it themselves, but because their
digestive tracks contain microbes that produce cellulose-hydrolyzing cellulases; various
cellulase types are also synthesized by fungi. When compared to starch, cellulose is relatively
resistant to biodegradation. Each cellulose molecule is an unbranched polymer of 10
3
-10
6
d-glucose units (Flieger
et al
., 2003 ).
Although paper has been known for more than 2000 years, the sensitivity of the
mechanical properties towards water limits the use of cellulose as a material in many
applications. Moreover, cellulose cannot be thermally processed into plastics because of its
hydrogen-bonded structure. However, microfibrils with diameters ranging from 2 to 10 nm
and lengths reaching several micrometers can be isolated from native cellulose fibers
through a combination of chemical and mechanical methods. The viscose process has been
known for more than 100 years and although it is far from being environmentally friendly
(Figure 11.3) it dominates other methods for isolating cellulose fibers. Cellulose processed
in this way finds applications from textile fibers to technical fibers for cords of high
performance tires (rayon) or for film (cellophane) production, which is suitable for food
packaging (Guerra
et al
., 2005). An environmentally friendly method, the Lyocell process,
was developed in the 1980s as an alternative to the viscose process, in which
N-methylmorpholine-N-oxide (NMMO) monohydrate is used as a more convenient solvent
system, resulting in a process practically free from emissions since solvent recovery is
almost complete (Klemm
et al
., 2005 ).
Their mechanical properties, with a Young's modulus of
140 GPa, close to that of
Kevlar, make cellulose fibers ideally suited as reinforcing elements in composite materials
(Gandini, 2008). Moreover, cellulose ultrathin films are also of great interest for their
extreme wetting behavior with aqueous systems and for being extremely stable against
oxidation and thermal degradation (Schaub
et al
., 1993 ; Kontturi
et al
., 2006 ). These features
make cellulose interesting for two distinct types of materials, cellulose composites and
thermoplastically processable cellulose derivatives.
∼
Viscose
Blends or
composites
Cellulose
Fibers & Films
+ CS
2
+ NaOH
Biodegradable
or
non-degradable
polymer
+ NMMO
Lyocell
OH
O
O
O
HO
OH
O
O
2
N
OO
O
O
+ HNO
3
+ H
2
SO
4
O
H
3
C
O
O
CH
3
O
O
O
O
HO
HO
OH
OH
Nitrocellulose
Celluloseacetate
Figure 11.3
Modification possibilities of cellulose.
Search WWH ::
Custom Search