Products Obtained by Extraction of Particulate Material from Plant Matter
Plants are complex composite structures, which mainly consist of fibers, particulates, and amorphous regions. In some cases, such as wood discussed above, the plant composite itself can be turned into a useful particulate filler, but this is rare. The most useful parts of a plant for use in composites are the fibers and particulates, and these have to be extracted from the rest of the plant matter before they can be used. Starch, lignin, and cellulose are the most abundant organic materials being investigated for use as particulates, together with silica obtained from rice hulls and are discussed in this section. Fibers are outside the scope of this work.
Starch is an inexpensive and renewable natural polysaccharide based on D-glucose. It actually consists of two different polymers: amylose (AM) and amylopectin (AP). Amylose is a linear polysaccharide and has a molecular weight of 0.2-2 million daltons. Amylopectin is a highly branched polymer of glucose with a massive molecular weight (100-400 million daltons).
In nature, starch is found in plants as semicrystalline beads with a size range of 2-100 pm in diameter, depending on the plant. These beads have a complex, onionlike structure, containing densely packed polysaccharides and a small amount of water. The inner structure of starch is believed to be formed from two regions - crystalline and amorphous lamellae - which together form the growth rings. The principal component of the crystalline region in native starch granules is thought to be amylopectin lamellae, packed together to form a double helix crystal structure.
One of the attractions of starch (but also a limitation) is the fact that it is totally biodegradable in a wide variety of environments, and degradation or incineration of starch products would recycle atmospheric CO2 trapped by starch-producing plants and would not increase potential global warming.
There are various uses for starch-derived products in polymers. Thus it can be used as a particulate filler, or as a polymer, either alone or in blends. If the semicrystalline bead form is preserved in the polymer matrix, then a particulate filler is arrived at. For various reasons, starch beads have limited application as particulate fillers, and most work has focused on producing thermoplastic starch polymers or blends, rather than using it in particulate form. Starch can be made into a thermoplastic polymer by destroying the crystalline structure; this can be done by pressure, heat, mechanical work, and plasticizers such as water, glycerine, or other polyols. The principal use for such products is for making biodegradable film.
There are two approaches to using starch beads as a filler in polymers. The first makes use of the bead in its semicrystalline form, while the second extracts the crystalline material from the bead as a nano-sized filler. The second approach is covered in ? Chap. 23, “Nanofillers” and only the first is discussed here. More details on nano-starch can also be found in the review by Lin et al. (2011).