Effect of Plasticizers on Gelatinization Properties of Cornstarch
Karen Yu
Sir Winston Churchill Secondary
Floor Location : S 189 V

Plastic is everywhere. However, most plastics on the market are not biodegradable, and when used for purposes such as packaging, they are often quickly thrown away and left to sit in a landfill. A possible solution to this problem may lie in the completely biodegradable thermoplastic cornstarch. rnThis experiment sought to investigate the effect of ethanol, propan-1-ol, and glycerol on the gelatinization properties of cornstarch when used in their pure form and when mixed in equal proportions with water. The hydrogen bonds that hold together the secondary structure of starch break when heated, and plasticizer molecules form new h-bonds with starch, linking them together in network. This forms a gel and the process is called gelatinization. rnWater is an excellent plasticizer because its small size makes it easy for the water molecules to penetrate the secondary structure of starch. However, water forms relatively weak hydrogen bonds with starch, so over time the bonds weaken and the water will recrystallize in a process known as retrogradation. This presents a problem as the material will become brittle over time, so novel plasticizers need to be used for thermoplastic starch. rnI used differential scanning calorimetry in this experiment and found that pure ethanol, propan-1-ol, and glycerol cannot gelatinize starch in the temperature range of 10-100oC. For ethanol and propan-1-ol, this is likely because they have only one hydroxyl group and thus cannot form bridges between different starch molecules. The glycerol samples may have not reached a sufficient temperature for gelatinization to occur, since the machine I used had an upper limit of 100oC. rnThe 50% glycerol samples have the highest average change in enthalpy (278kJ/g+26.9kJ/g) followed by the 50% ethanol sample (220kJ/g+41.4kJ/g), the water sample (193kJ/g + 41.0kJ/g), and the 50% propan-1-ol sample (138kJ/g+42.8kJ/g). The 50% glycerol sample, which absorbed the largest amount of energy, must have formed the weakest bonds since formation of bonds is exothermic and the breaking of bonds is endothermic, and the 50% propanol sample, which absorbed the least energy per gram, must have formed the strongest bonds. However, because all of the plasticizers resulted in endothermic reactions, this still means that the intramolecular starch bonds broken are stronger than the new plasticizer-starch bonds formed, so there is still a tendency towards retrogradation. rnThe 50% glycerol sample had the highest gelatinization onset (77.9C+5.7C), peak (80.8C+2.6C), and conclusion (97.1C+1.0C) temperatures, followed by the water sample (59.0C+0.6C; 60.1C+0.4C; 77.0C+6.7C), the 50% ethanol sample (57.2C+0.5C; 67.1C+0.4C; 76.3C+0.6C ) and the propan-1-ol sample (52.7C+2.7C; 61.8C+0.5C; 72.6C+1.6C). The reason glycerol had such high gelatinization temperatures is likely due to its larger molecule size and greater steric hindrance. Surprisingly, the presence of ethanol and propan-1-ol lowered the gelatinization temperatures, which indicates that molecular size is not the primary factor in determining gelatinization temperature. rnI am currently in the process of conducting further experimentation on the tensile and flexural strength properties of the samples to determine if the new plasticizers would be able to form films with the necessary strength to be used in consumer products.