Biopolymers - Polymer Science and Engineering - page 520

13.2.2.3 Chitin
Chitin is a skeletal polysaccharide, poly(2-acetamido-2-deoxy-D-glucopyranose),
which makes up the basic constituent of shells such as lobsters, shrimps, and
insects [4,13] . Chitosan, poly(2-amino-2-deoxy-D-glucopyranose) is obtained by
the deacetylation of chitin [13] . The extraction and modification of this natural
polymer demonstrate unique properties such as biocompatibility, antimicrobial
activity, biodegradability, film and fiber forming ability as well as heavy metal
absorption [4,13] . Chitinous fibers are manufactured into artificial skin, absorb-
able sutures, carriers for controlled delivery of drugs, and bone tissues [4,13] .
Natural polymers without modification and processing are not acceptable to
be used in many functional engineering applications, like conventional plastics. It
is the modification of these natural polymers that allows them to be made into use-
ful materials. The extraction and modification will be discussed in Section 13.3 .
This concludes the brief overview of naturally occurring polymers, one of
three classifications of biopolymers. The next section focuses on bio-based poly-
mers, the second subclassification of biopolymers.
13.3 Bio-Based Polymers (Bioplastics)
A bio-based polymer is derived fully or in part from biomass or microorganisms
as an alternative to petroleum plastics. Many of these polymers are commonly
referred to as bioplastics. A bioplastic is similar to a conventional plastic, like
polypropylene, in the fact that it is used to make commercial products. There are
three generic pathways to producing plastics from natural materials such as (1)
the extraction and modification of natural polymers from biomass; (2) the poly-
merization of bio-based monomers; and (3) the extraction of polymers produced
in microorganisms. An explanation of each pathway along with specific examples
of bioplastics, processing, properties, and applications will be detailed. A general
overview of synthetic pathways to a bioplastic is illustrated in Fig. 13.4 , along
with examples of products.
13.3.1 Extraction and Modification of Natural Polymers in Plants
Many different types of natural polymers exist in the environment, as outlined in
Section 13.2 . The extraction and/or modification of these polymers enable them
to be used as commercial materials, a bioplastic. Examples of these products
include the vulcanization of natural rubber, thermoplastic starch, and regenerated
cellulose.
13.3.1.1 Natural Rubber
As mentioned, natural rubber is collected by tapping rubber trees, Hevea brazil-
iensis, by making an incision into the bark of the tree [5,7] . Natural rubber is
unique to “bioplastics” as the tree producing the rubber is not destroyed to harvest