Starch is another type of naturally occurring biopolymers. In many plants, starch is the most commonly used to store energy. Many plants have specialized organs to store large amounts of starch. Examples include seeds, such as that of rice, and tubers, like those found in potato. Therefore, starch can be produced through farming, making it a renewable resource. Most plant starch consists of amylose and amylopectin. Like cellulose, both amylose and amylopectin are also made up of D-glucose units. These two differ in their structure, however, in that amylose mostly consists of a single chain of D-glucose linked through α(1->4) glycosidic bonds, while amylopectin shows branched structures through both α(1->4) and α(1->6) glycosidic bonds.[1]


external image 500px-Amylose2.svg.png

external image 500px-Amylose2.svg.png


Figure 1. Structure of amylose




external image 500px-Amylopektin_Sessel.svg.png


Figure 2. Structure of amylopectin



The earliest attempts to utilize starch for the production of plastics began in the 1970s. Although low density polyethylene (LDPE) was successfully synthesized using starch granules as particulate fillers, the requirement for highly dried starch, which was needed to avoid problems caused by volatility of water, made it financially unfeasible to commercially produce starch-based plastics. Also, due to its high hydrophilicity, starch was shown to be poorly adhesive to polyethylene. Furthermore, the starch was encapsulated by polyethylene, making them in accessible to microorganisms that could degrade starch. To combat this problem, poly(ethylene-co-acrylic acid) was blended with starch to create films but achieved limited success. Further attempts involving other additives, such as urea, were made but was not hugely successful.[1]



EAA.png


Figure 3. Core structure of poly(ethylene-co-acrylic acid)

The first commercial starch-based plastic was produced by the National Starch & Chemical Investment Corp. as an alternative to expanded polystyrene. This product was based on starch that was denatured by heating, also known as TPS. To produce TPS, a mixture of starch and an appropriate plasticizer is heated. Various non-volatile molecules with hydrogen bonding capacity can be used as plasticizers. Examples include urea, fructose, glycerol and maltitol. Once heated, starch-based thermoplastic possesses characteristics similar to those of petroleum-derived thermoplastics.[1]






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  1. ^ Belgacem MN, Gandini A. 2008. Monomers, polymers and pomposites from renewable resources. 1st ed. Kidlington (UK): Elsevier.