Civil Engineering Reference
In-Depth Information
implies the need for removing bacteria and culture medium, typically by boiling in
alkali solution at a low concentration [52-54]. An interesting feature of bacterial cel-
lulose is that by adjusting culturing conditions it is possible to alter the microi bril
formation and crystallization. Moreover, the presence of additives may interfere with
the aggregation of the elementary i brils into the normal ribbon assembly leading to
squared cross-section microi brils [55]. Although chemically identical to plant cel-
lulose, microbial cellulose is characterized by a unique i brillar nanostructure which
determines its extraordinary physical and mechanical properties. Well-separated nano-
and microi brils of microbial cellulose create an extensive surface area which allows it
to hold a large amount of water while maintaining a high degree of conformability. h e
hydrogen bonds between these i brillar units stabilize the whole structure and confer
it its high mechanical strength [56, 57]. Microbial cellulose is characterized by high
polymerization degree and crystallinity and high stability of the single cellulose i bers.
Moreover, and dif erent to wood and plant cellulose sources, the high chemical purity
of bacterial cellulose avoids the need for chemical treatments devoted to the removal of
hemicellulose, lignin, and other plant components.
9.2
Cellulose Bionanocomposites Incorporation of Cellulose
Nanoi bers into Biodegradable Polymers: General Ef ect on
the Properties
h e i nal properties of the cellulose nanoi bers-based nanocomposites depend not only
on the aspect ratio (l/d), but also on the mechanical and percolation ef ects [4, 24] .
h e developed studies have shown that the tensile properties and transparency of the
nanocomposites increase with the aspect ratio of the cellulose nanowhiskers [25, 26]. In
addition [27], the tensile properties also depend on the orientation of the cellulose nano-
i bers inside the polymeric matrix, making critical the processing conditions. However,
other authors [26] showed that i ller orientation and distribution play an important role
in the aspect ratio. h e maximum enhancement in properties of the composites takes
place for the adequate quantity of i ller in the matrix, where the particles can form a
continuous structure; known as percolation threshold [28]. h e improvement of the
properties of nanocomposites compared with the neat matrix is also related with the
dispersion of i ller within the matrix. h e compatibility between the selected matrix
and the nanoi ller is another important factor to be taken into account [29]. h e high
polarity of cellulose surface leads to certain problems when added to nonpolar polymer
matrices; including weak interfacial compatibility, poor water barrier properties and
aggregation of i ber by hydrogen bondings [4, 30].
h e advantages of nanocomposite materials when compared with conventional
composites are their superior thermal, mechanical and barrier properties at low rein-
forcement levels (e.g.,
5 wt%), as well as their better recyclability, transparency and
low weight [31, 32]. Biodegradable polymers, in particular, may require improvement
in terms of brittleness, low thermal stability and poor barrier properties [32]. A number
of researchers have therefore explored the concept of fully bioderived nanocomposites
as a route to development of bioplastics or bioresins with better properties [33, 34].
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