Bioplastics

In Lesson 2, we made bioplastic in class that required the following ingredients:

Corn starch/ Potato starch/ Milk
Ethanoic Acid
Distilled water
Food colouring
Corn oil
Ziplock bag
Microwave oven

Scientific Concepts/ Processes behind the making of bioplastic from starch
As background knowledge, there are two general types of starch that exist: amylose and amylopectin. They both differ in structure and functional properties, where amylose is linear, with 200-2000 polymer units and forms a firm gel formation, whereas amylopectin is branched, with up to 2 million polymer units and forms a non-gelling to soft gel formation.

In this experiment, corn starch and potato starch are mostly made of amylopectin (about 80%, if not completely).

Bioplastics are made by converting sugar present in plants (starch) into plastic. There are two types of bioplastic - polyactide acid (PLA) and polyhydroxyal-kanoate (PHA). In this experiment, PLA is made.

Starch is dried from an aqueous solution (after adding corn oil and water) to form a film due to hydrogen bonding between the chains of polymers. However, amylopectin inhibits the formation of film. Thus, a strong acid is needed to break down amylopectin for the film formation.

Starch undergoes hydrolysis to form glucose, which then undergoes fermentation to form lactic acid. However, lactic acid cannot be polymerized directly to form PLA as the formation of water molecules in the process will prevent the growing chain of lactic acid molecules from staying together. Hence, lactic acid first undergoes dehydration to remove water molecules and from polyactic acid oligomers, then thermal cracking to form lactide. Lactide can then be polymerized to form a chain of lactic acid molecules also known as PLA.

Why bioplastics?
Many people around the world have created bioplastic in order to tackle the disadvantages and problems with our conventional plastic, namely:

1. The complex entanglements of polymer chains make it hard to decompose and hence non-biodegradable.
2. It relies heavily on petrochemicals (which are obtained from fossil fuels - nonrenewable resource!)
3. The recycling process is energy-consuming and costly.
4. The production of conventional plastic releases toxic chemicals.

Hence, using one of the 12 principles of Green Chemistry to use renewable feedstocks, a raw material/ feedstock should be renewable rather than depleting whenever technically and economically practicable. The materials used in this experiment, namely corn starch/potato starch and corn oil, are renewable since they can be obtained from plants.

Another one of the 12 principles of Green Chemistry used here is to have less hazardous chemical syntheses. Whenever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment. This making of bioplastic reduces or eliminates Greenhouse Gases during the production, requires less/ no petrochemicals and with the slow release of carbon dioxide gas, it allows sufficient time for plants to absorb this carbon dioxide gas production.

The last principle of Green Chemistry involved is that of design for degradation. Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment. As compared to the non-biodegradable conventional plastic, bioplastic is biodegradable and hence reduces the amount of waste on earth that persist in the environment.

Pictures of final product

Green Bioplastic (colour from food colouring)

Yellow Bioplastic (colour from food colouring)

Resources
http://www.beyondbenign.org/greenchemistry/12principles.html