56 Introduction to Renewable Biomaterials
Amylopectin Amylopectin
Amylose
(a) Amylose
(d)
(b)
(c)
Temperature
+ water
Gelatinization
Heat
Time (aging)
Retrogradation
Cool Gel ball
50 A°
Figure 2.11Schematic representation of the phase transitions of starch during thermal processing
and aging.
Among the various phase transitions, gelatinization is particularly important because
it is closely related to others, and it is the basis for the conversion of starch to a
thermoplastic. Furthermore, the decomposition temperature of starch is higher than
its melting temperature before gelatinization. Without physical force (shear stress), the
process of gelatinization depends mainly on water content and temperature conditions.
A previous study has shown that shear stress can result in the fragmentation of starch
granules during extrusion. Indeed, both the mechanical and thermal energies are
transferred to starch dough during extrusion in molten medium. The main objectives
of most starch-processing techniques are melting and mixing, which are adjusted to
minimize chain degradation.
Starch use in papermaking dates back to the invention of paper 2000 years ago, when
it was applied to obtain a stronger and smoother writing surface. Starch contributes
to paper manufacturing because it serves as a binding agent that can enhance the
mechanical properties of paper and improve paper manufacturing by increasing paper
pulp retention on the paper machine. Starch was also chemically modified into cationic
forms to further improve interconnections between fibers, increasing thus the paper
strength. Non-covalent binding of starch and cellulosic fiber in biocomposites were
created using a polymer suspension either by, for example, drying or hot-pressing.
2.4.3 Starch-Based Plastics
Plastic ranks as the second most used packaging material in the United States.
In contrast to paper, only 7% of plastic generated as waste is recycled. This explains why
more plastics ultimately end up in landfills than paper or any other packaging material.
China, one of the world’s largest plastic producers, noted that the largest source of its
marine pollution was from discharging wastewater to sea. In Europe alone, an estimated
2–3 million tons of plastics is used each year in agricultural applications. Polyethylene
