Environmental Engineering Reference
In-Depth Information
70.4 percent cellulose, 3.7-5.7 percent lignin, 17.9-22.4 percent hemicellulose, 0.9 percent
pectin, 0.3 percent wax, and 10.8 percent moisture. Natural waxy substances on the fiber
surfaces diminish fiber-matrix bonding, leading to the necessity of investigation of various
surface treatments to improve fiber-matrix adhesion of the resulting biocomposites. In
addition, hemp, as a natural fiber, starts to decompose at 3 00°C, with the consequence that its
use is limited to plastics that can be processed at lower temperatures. Fortunately, that is not
an obstacle for soy- and starch-based plastics [17].
2.4.4. Nanocomposites
. Particles are often incorporated into plastics to improve stiffness
and toughness, to enhance barrier properties or fire resistance, or simply to reduce cost.
Unfortunately, however, brittleness and opacity are sometimes imparted to the resulting
composites. Nanocomposites are a new class of plastics that incorporate nanoparticles, or
particles having at least one dimension in the nanometer range, in attempts to reduce these
undesirable side effects.
2.4.4.1. Classes
. Three types of nanocomposites are distinguished based on the number of
dimensions of the dispersed particles that are in the nanometer range. If all three dimensions
are in the nanometer range, the nanoparticles are isodimensional; examples include spherical
silica [26] and other [27] nanoparticles. Nanotubes or nanowhiskers are particles in which
only two dimensions are in the nanometer scale; examples include carbon nanotubes [28] and
cellulose whiskers [29-31]. If only one dimension is on the nanometer scale, the filler is
present in the form of sheets of a few nanometers thick. These are polymer-layered crystal
nanocomposites and are almost exclusively obtained by the intercalation of the polymer
inside galleries of layered host crystals. A wide variety of both synthetic and natural
crystalline fillers are able to intercalate as polymers. Those based on clays and other layered
silicates are most widely investigated because the starting materials are widely available and
inexpensive. In addition, the intercalation chemistry of clays has been well-studied [32, 33].
When such sheet- shaped nanofillers are successfully dispersed in a polymer, the resulting
nanocomposites exhibit markedly improved mechanical, thermal, optical, and physio-
chemical properties when compared with the pure polymer or conventional (microscale)
composites. For example, the first profound demonstration provided by Kojima and
coworkers [34-37] for Nylon6-clay nanocomposites showed that improvements can include
increased moduli, strength, and heat resistance, as well as decreased gas permeability and
inflammability.
2.4.4.2. Cellulose nanocomposites
. In recent years, natural cellulose fibers have gained
attention as reinforcing phases for polymer nanocomposites [30, 31, 38-46]. Several
approaches to the production of such microfibers are known, including chemical treatments
and steam explosion of cellulose starting materials. Their low density, gentleness toward
processing equipment, and relatively reactive surfaces hold great promise for excellent
property improvement. In addition, they are abundant and inexpensive, and most notably,
they retain their biodegradability [47].
In cellulose nanocomposites, the polymer modulus can be increased by more than three-
fold at 6 percent loading levels due to the long aspect ratio of the cellulosic fillers. These
fibers reach the percolation threshold at relatively low loading, causing the modulus to
increase rapidly. Cellulose nanocrystals can exhibit high aspect ratios, whereby the length
divided by thickness can approach 500 for some agricultural fibers such as sugar beet and the
giant reed Arundo donax. Proper treatment can also lead to the desired platelet-like
morphology similar to the very successful clay nanofillers [48].
