The Chemistry of Bioplastics
Introduction
As the world’s landfills begin to overflow with non-biodegradable, oil-based plastics, scientists are beginning to develop remarkable solutions to these seemingly overwhelming problems. One way scientists around the world can reduce the threat plastic poses to the environemnt is through the development of bioplastics. Bioplastics include plastics that are biodegradable and/or made for renewable biomass sources. Bioplastics that are made from renewable biomass are usually made from vegetable starch and glycerin and are called starch-based bioplastics. Essentially, bioplastics are built upon long polymer chains (really large molecules made of monomers) that result from biomass starch (like cornstarch) mixed together with biomass glycerol (like glycerin) under heat. Forms of bioplastic include: starch based; PHB, a product of bacteria processing glucose; PA 11, a polymer made from natural oils that is very useful and made from renewable sources but not biodegradable; and finally PLA, which is one of the most promising bioplastics. PLA is semi-biodegradable and is one of the more easier bioplastics to produce, as its manufacturing process and its qualities are similar to that of PET, a fossil-fuel-based plastic. By using corn instead of fossil fuels to create plastic, scientists and manufacturers are developing a more sustainable method in producing plastic--a product that is used countlessly throughout a day. Through replacing the world’s plastics with generally biodegradable and renewable alternatives, bioplastics and the awesome chemists who develop them are helping reduce both plastic waste and the world’s dependency on finite fossil fuels. Ultimately, bioplastics perfectly exemplify how chemistry can change the world for the better.
Composition of ...
Bioplastics, like most all plastics, are mainly comprised of three elements: hydrogen, carbon, and oxygen.
Main Chemicals, Compounds, Components
This project is examining bioplastics as a whole. However, for the sake of analyzing the chemistry and composition of a bioplastic, I will specifically investigate one of the more common bioplastics, Polylactide (or PLA).
PLA serves as an excellent example of how chemistry can help shape the future of the world with just three basic elements. These elements are shown through PLA’s chemical formula: (C3H4O2)n. The “n” in this formula represents that the molecule in the parentheses can be repeated to create a long chain-like molecule called a polymer. PLA, like all polymers, are made up of many monomers. PLA’s monomer is comprised of two oxygen atoms bonded to one methyl group. Methyl groups are three hydrogen atoms bonded to one carbon atom, they are both stable and very common in organic compounds. The methyl groups and oxygen found in PLA come from lactic acid produced from fermented corn or other vegetables. PLA’s monomer is similar in composition to that of lactic acid, hence the polymer name: polylactide or polylactic acid.
In a polymer, many of these monomers (in fact even thousands) can bond together to essentially create a long, chain-like molecule. This molecule is then “capped off”. On the ends of a PLA molecule are, on one side: two oxygen atoms, a methyl group, and a hydroxide ion (OH), and on the other side: an oxygen atom, a methyl group and a hydroxyl group (HO). Both hydroxide and hydroxyl serve as catalysts in many biological reactions and typically “cap off” biopolymers. In fact, in the lab scientists can determine how long a polymer chain will be by adding these caps. In doing so, they can alter the number of monomers in the polymer, also known as the degree of polymerization. Polymers with different degrees tend to have different properties, this is why some plastics are far stronger, flexible, or more heat resistant than others. In fact, two polymers with the exact same monomer composition but with different degrees of polymerization can exhibit different qualities!
The admirable quality of PLA is that its composition is very similar to that of PET (a fossil-fuel based plastic)--both are comprised of carbon, hydrogen, and oxygen and share a the string-like polymer structure, however PET’s monomers come from the methyl groups in oil, whereas PLA’s monomers come from the methyl groups in plants.
Chemistry's Role / Background Research
So if a polymer is just a long sting-like molecule, why doesn’t plastic look like a basket of yarn or a bowl of spaghetti, and how do all these “strings” bond together to create a cohesive shape? Polymers are not completely straight lines, in fact, plastics are rather a tangled mess of string-like polymers. In this tangled mess, molecules bind to each other as a result of entanglement and cohere into plastic. This also explains plastic’s ability to melt and remelt. When these tangles of polymers are heated, the bonds within each polymer begin to fail, creating a “soup” of monomers. When cooled, these monomers once again form polymers which bind together. This process also explains why chemists are even able to create plastics.
PLA is created by first harvesting dextrose (a carbohydrate found in many starchy vegetables) from corn. Then using bacteria, chemists ferment the dextrose to produce lactic acid. This lactic acid provides the main ingredients for the “monomer soup”, which chemists then heat to create PLA. Scientists may also use certain additives like glycerin, to alter the consistency of the plastic. Additionally, as mentioned early, chemists can alter the length of the polymer chains to achieve certain plastic qualities.
Thanks to the remarkable chemistry of bioplastics, society can begin to explore sustainable alternatives to fossil-fuel based plastics. While bioplastics may not be perfected, understanding the chemistry behind them allows scientists to develop improvements; improvements that not only clean plastics from landfills but improvements that ultimately reflect human resourcefulness and ingenuity.
And that’s why chemistry is important.
Resources
https://en.wikipedia.org/wiki/Bioplastic#cite_note-2
States the three main components of bioplastics
Explains how bioplastics are built upon long polymer chains
http://www.cheminst.ca/magazine/feature-story/many-faces-bioplastics
Mentions that certain additives produce favorable qualities in plastic
Additives alter the structure of the polymer chains, thus resulting in varying properties
Defines bioplastics
Explains why bioplastics usually serve as a better alternative to fossil fuel-based plastics
Shows that bioplastics primarily biodegrade into water and carbon dioxide.
http://www.innovativeindustry.net/types-of-bioplastic
Details the many different kinds of bioplastic
https://en.wikipedia.org/wiki/Degree_of_polymerization
Defines Degree of Polymerization as essentially the number of monomers in a polymer. Scientists can manipulate a polymer’s degree of Polymerization to affect that plastic’s qualities.
https://en.wikipedia.org/wiki/Polylactic_acid
Scientific definition of PLA
https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/polymer.htm
Overview of polymer structure and why plastics can be melted and remelted
Clarifies the science behind the entanglement of polymer chains
More base information about bioplastic chemical reactions/compositions.
Explains the process of creating PLA plastic from corn
http://green-plastics.net/news/
Resource for information on bioplastic news and recent innovations
Explains that bioplastics are certainly not perfect and have room to improve
About the Author
Drew is a junior at Billings Senior High. He is engaged in extracurriculars such as Academic Team, Speech and Debate, Business Professionals of America, Interact Club, Student Council, Senior Advocates, Spanish Club, Youth Volunteer Corps, and Pep Band. Drew enjoys learning about any subject, this makes it quite difficult for him when choosing a career. Possible career options for Drew include: politics, entrepreneurship, environmental activism, community/urban development, economics, or bioengineering. Of course, these are mostly speculative and Drew feels as if he could be all of them. Drew is also interested in many undergraduate study programs such as UChicago, MSU, Harvard, Columbia, and Amherst College but similarly is unsure as to his true wishes. One thing is true, however, Drew is very much looking forward to college. When he’s not bogged down by the monotony of public education, Drew enjoys being with friends, participating in political debates, attending plays/concerts, eating good food, listening to music, and simply seeing the light of day.