Environmental Engineering Reference
In-Depth Information
A particular feature of bamboo is that its stem (culm) grows from the ground at
its full diameter and to its full height in a single growing season of 3-4 months.
Each new shoot grows vertically up to its mature height. Branches extend from the
nodes and leaves then appear. During the following 2 years, the outer wall of the
stem slowly hardens. Bamboo culms are considered mature after 3 years of growth.
While the shoots (new culms that come out of the ground) of some species of
bamboo are edible (if properly prepared; it contains a toxin that has to be degraded
before consumption), bamboo is mostly utilised for its natural resistance and
fibres. Due to its high strength-to-weight ratio in its natural form, bamboo is tra-
ditionally used as construction material for houses or scaffolding; it also has
applications in musical instruments, fishing rods, papermaking and textiles. For
the latter application, because of the nodes fibres are too short to be processed
directly and most bamboo textiles are made from viscose (see Section 6.3.1.2)
obtained from bamboo cellulose; the fibres are dissolved in chemicals and then spun
into new and highly resistant fibres.
6.3 Isolated and Modified Biopolymers as Biomaterials
Plants are wonderful chemical reactors that are able to make extremely complex
macromolecules. These compounds are located in the cell wall (e.g. cellulose,
lignin, hemicelluloses and pectin), constitute the plant's energy reserves (e.g.
starch) or even have specific functions (e.g. proteins). Most of these biopolymers
are useful to make industrial biomaterials and a significant amount of research has
been and continues to be conducted on them [9].
In order to develop valuable applications, extraction methods for these biopoly-
mers are necessary. The isolation of starch is one of the most simple, as it requires
relatively straightforward physical methods for extraction and purification (wet
milling being the most common). In contrast, the cell wall components require
chemical treatment to break the covalent bonds of the LCC to liberate the cellu-
lose fibrils. This can either be done with sulphuric acid containing solutions of
hydrogen sulphites or with solutions of sodium hydroxide and sodium sulphate.
These hot treatments reduce biomass chips to fibres by using a mild mechanical
action. To further purify the raw pulp, a multi-stage refining procedure is needed
in which alkali and oxidising agents are used to remove residual lignin. Finally,
extractions with cold or hot alkali are used to remove pentanes and oligosaccha-
rides. Pure cellulose (99%) can finally be obtained.
The characteristics of the isolated biopolymers depend on their structure.
Cellulose and amylose are linear polymers whereas amylopectin, pectin and
hemicelluloses are branched polymers. Pectin and amylopectin contain carbox-
ylic groups that can significantly interact with water molecules. Amylose has a
helix structure whereas cellulose molecule looks like a ribbon. The interactions
with water and other neighbouring molecules are therefore different.
One of the most important aspects of these biopolymers is the fact that they cannot
be used directly for thermoplastic applications. They are not reticulated polymers
