Synthesizing Bioplastic

Bioplastics are plastics that are derived from renewable biomass, such as corn. This contrasts with conventional plastics, which are made with petroleum, a non-renewable resource.

Bioplastics are more sustainable because they come from renewable sources. However, using food crops such as corn to produce bioplastics poses a problem - crops require large amounts of water, land, fertiliser and other materials to grow. The production of bioplastics is also energy-intensive. Some bioplastics also produce methane gas when they decompose. Methane gas is a greenhouse gas many times more potent than carbon dioxide.

However, unlike petroleum-based plastics, bioplastics are biodegradable. Although bioplastics may require as much energy to produce as petroleum-based plastics, they will not clog landfills and are more environmentally-friendly in this aspect.

Want to know more about different types of plastics and their pros and cons? Visit the website at this link: http://www.explainthatstuff.com/bioplastics.html


Unlike bioplastics, conventional petroleum-based plastics are non-biodegradable

During this practical, we synthesized bioplastic using corn starch, potato starch and synthesized starch as the biomass material.

Method
> Add one spoon of starch into a ziplock bag. Add corn oil and mix by rubbing the bag. There should be just enough oil to make the flour clump together.
> Add a little water and a few drops of food colouring to colour the plastic. Mix well.
> Microwave the open ziplock bag on high for 25 seconds.
> Remove from microwave and mould with wooden block while still hot.

Before microwaving

After microwaving

The plastic that we synthesized (which was made using potato starch) is soft, bouncy and easily broken apart, very unlike plastic we are used to. The plastic we synthesized also disintegrates when wet, due to its slightly polar nature. Therefore, other additives are needed to give starch-based plastics the desirable properties of plastic. Often, flexibilisers and plasticisers such as sorbitol and glycerine are added. Starch-based plastics are already in the market and are called thermoplastic starch (TPS).

Starch is made of amylose (linear glucose polymer) and amylopectin (highly branched glucose ploymer). The amylose/amylopectin ratio is different for starch from different plant sources.




When heated with water, starch is gelatinised or destructurised: hydrogen bonding sites such as hydroxyl groups bind more water, irreversibly dissolving the starch. Penetration of water increases the randomness of the general starch granule structure and decreases the number and size of crystalline regions. When heated, crystalline regions become diffuse so that the chains begin to separate into an amphorous form.


Crystalline regions are ordered and have characteristic geometry but amorphous regions do not.

During the gelatinisation of starch, 3 main things happen to the starch granule: 1. granule swelling 2. crystal or double helical melting 3. amylose leaching.

Water is first absorbed in the amorphous space of starch, which leads to a swelling phenomenon during heating and then transmitted through connecting molecules to crystalline regions.^[3] Water enters tightly bound amorphous regions of double helical structures to swell amylopectin, thus causing crystalline structures to melt and break free.^[4] Stress caused by this swelling phenomenon eventually interrupts structure organization and allows for leaching of amylose molecules to surrounding water.

The result is a colloidal solution - polymers dispersed in a medium of water - that gels when cooled. This is due to to a process called retrogradation. When cooled, cooked starch molecules will rearrange to form a more ordered crystalline structure and inter-molecular bonding occurs between chains of amylose and amylopectin, with water embedded within the molecule.

Due to strong associations of hydrogen bonding, longer amylose molecules will form a stiff gel.^[6] Amylopectin molecules with longer branched structure, increases the tendency to form strong gels. High amylopectin starches will have a stable gel, but will be softer than high amylose gels.

Molecules that are capable of hydrogen bonding with hydroxyl groups such as water, sorbitol and glycerine act as plasticisers and change the properties of the gel formed.

The corn oil in our practical probably acted similarly to glycerin, making the plastic more pliable by acting as a lubricant at a molecular level. I am not sure about this, because corn oil and glycerin are quite different. But there must be a reason why the plastic was 'bendy'. Classmates that added less oil got a less flexible plastic.

More can be deduced by changing the proportion of reactants and observing the properties of the plastic.

This year, one of the prize-winning projects are the Google Science Fair was about synthesizing bioplastic from banana peels. Check it out here!

Sources:
http://www.sciencehq.com/chemistry/crystalline-and-amorphous-solids.html
http://en.wikipedia.org/wiki/Starch_gelatinization
http://polymerinnovationblog.com/thermoplastic-starch-a-renewable-biodegradable-bioplastic/
http://green-plastics.net/videos/35-howto/52-video-brandon121233