Environmental Engineering Reference
In-Depth Information
(CAP), and cellulose acetate butyrate (CAB), [15] although cellulose fatty esters are under
investigation as well [16]. These later esters can be produced from wastes such as recycled
paper and bagasse, and have found applications in film substrates for photography, toothbrush
handles, selective filtration, adhesive tapes, cellophane, various semi-permeable and sealable
films, and automotive coatings [17] (http://www.innoviafilms.com/product/summary
_datasheets.htm).
2.4.2. Lignin
. The pulp and paper industries produce in the vicinity of 20 million tons per
year of lignin in the United States alone [18]. Along with cellulose and hemicellulose, lignin
constitutes a major component of the cell wall in both hardwoods and softwoods; lignin
content ranges from 15-40 percent by weight [19]. Lignins are complex heteropolymers of p-
hydroxy cinnamyl alcohols. Lignin is derived by treatment of pulp with sodium hydroxide
and sodium sulfide (kraft pulping liquor) which cleaves linkages in the protolignin structure
[20]. Glasser and Lora have recently published a survey of lignin useages [21]. Lignin can
substitute for petroleum based materials in a number of ways and an excellent overview of
these uses is available [14]. Lignin can serve as a filler in thermoplastics, thermosets, and
rubbers; it has also been converted into carbon fibers, a potentially high value-added
application. While most efforts in lignin modification have relied on chemical methods,
enzymatic grafting has been reported [22]. In the context of the integrated biorefinery, if
forest resources (hardwoods and softwoods) are to be exploited in the future, additional uses
for lignin will be needed. Presently it is typically burned as a fuel but higher value added uses
are clearly desirable but technically challenging.
For example, downregulation of lignin content is of considerable interest. Transgenic
aspen trees in which expression of a lignin biosynthetic pathway has been downregulated by
antisense inhibition have been developed [23]. However, while these trees produced up to 45
percent less lignin, this was accompanied by a 15 percent increase in cellulose. That is, the
lignin to cellulose mass ratio remained essentially unchanged.
2.4.3. Natural fiber-reinforced composites
. Natural fibers such as kenaf, flax, jute, hemp,
sisal, and henequen are attractive options for reinforcing starch- and protein-based composites
because they are renewable and sustainable, as well as low cost, with acceptable mechanical
properties, ease of separation, and biodegradability. Additionally, these fibers have excellent
thermal and sonic insulation properties. Natural fibers from grass, hemp, and ramie have been
reported as reinforcements for soy-based matrices, compared with glass fibers, and improved
in terms of physical properties via surface treatments [11, 12, 24, 25].
Although both soy plastics and natural fibers possess hydroxyl and carboxyl groups that
can interact during processing, these interactions seem to be limited and typically do not lead
to significant improvements in performance. However, enhancing these interactions through
the use of compatibilizers, molecules such as polyester amide (PEA) that interact with both
the fiber and the polymer matrix, is of current interest in the synthesis of natural fiber-
reinforced soy biocomposites. Among reinforcing fibers, pineapple leaf fiber is an attractive
option due to its high tensile strength (400-1600 MPa) and modulus (59 GPa), that result in
turn from its high cellulose content (70-82 percent) and high degree of crystallinity. In
addition, its cultivability in the southern United States and Central and South America cause
it to be widely available, and it has shown previous success in strengthening low-density
polyethelyne (LDPE), rubber, polyester, and polyhydroxybutyrate (PHB) [11, 12].
Hemp fibers and related grass fibers are additional promising natural reinforcement
agents. Hemp is an important ligno-cellulosic natural fiber that contains (by weight) 70.2-
