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With petroleum-derived plastics becoming ubiquitous, the world is facing a huge problem with both the source and the disposal of these materials. A real alternative is emerging in the form of plastics derived from soybeans, as they are renewable and can also be made to be biodegradable.
The real drive for soy as a petroleum alternative started in America in the ‘30s and ‘40s due to shortages caused by World War II. This led to research into fuels derived from other, more abundant sources, such as soy. Along with incorporating soybean products in phenolic resins as filler, there was Henry Ford’s “Plastic Car” concept, which was a project to replace steel (another rationed resource) in automobiles. Although both of these programs succeeded, the end of WWII meant that gasoline and steel was no longer rationed, and the development of soy plastics was effectively ended.
Apart from being a finite resource, these plastics are incredibly durable so they take a long time to biodegrade; a plastic monofilament fishing line for instance, is estimated to take 600 years to biodegrade. In modern times there has been a resurgence in the use of bioplastics such as those made using soybeans. Traditional plastics are produced using petroleum, a fossil fuel.
The two main advantages of bioplastics are that they are produced from renewable sources, and that they can be made to biodegrade. While this biodegradability doesn’t apply to all bioplastics, many of them have been purposefully designed to break down in a much shorter time than conventional plastics. The renewable biological sources that they are derived from can be anything from vegetable oil to corn starch.
Typical plastics are formulated using a process known as polymerization. This is when many single molecules known as monomers are subjected to a stimulus, such as heat or change in pH, and link to together to form vast networks of connected chains called polymers. The same is true of bioplastics, but they must first be processed in order to obtain a usable material.
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In the case of soy, there are a couple of derivatives that can be used. Once processed, 60 lbs. of soy yields 11 lbs. oil, 48 lbs. meal, or 38 lbs. defatted soy flour; 20 lbs. protein concentrate; and 12 lbs. soy protein isolate. In this article we will focus on using protein isolate. The meal undergoes a series of refinements and at each turn the amount of material reduces as the protein content increases. Dehulled soybean, after defatting and meal grinding, becomes soy flour, which can be toasted to various degrees of protein solubility. This soy flour has the water/alcohol-soluble sugars leached out and is termed soy protein concentrate. Finally, the soy protein concentrate is dissolved in caustic 12 solution at pH 9 and reprecipitated by acidification at pH 4.5, becomes the purest commercially available soy protein at 90% protein. Bioplastics are long chain of monomers joined with each other by ester bond which considered as polyesters
The protein is almost ready to be turned into a plastic, but there is one more consideration. Possibly the most important factor in creating a durable soy based plastic is the moisture content. One of the early problems that Ford’s team ran into was that the plastic swelled when left out in the rain. The moisture level also affects the mechanical properties of the finished material: the less moisture, the more brittle it is. The moisture content can be controlled through addition of plasticizers such as glycerol as well as oven or air drying. Once this has been decided the protein isolate can be turned into a plastic using heat and pressure. The heat unfolds and aggregates the protein from its native state in an irreversible process known as denaturing. The pressure then fixes the proteins into their new configuration and a usable bioplastic is produced. There are multiple ways of applying heat and pressure at the same time including compression molding and extrusion.
Currently these bioplastics are limited in their applications, mostly being used for biodegradable packaging for consumer products. This has limited the share of bioplastic in the total world polymer market stands at 0.2%, but this figure will only increase. As new materials and methods are developed, bioplastics will see increased use for both commercial and engineering applications: there is a strong future in soy plastics.
Sources and Further Reading
- Mungara, P., Chang, T., Zhu, J. & Jane, J. Processing and physical properties of plastics made from soy protein polyester blends. J. Polym. Environ. 10, 31–37 (2002)
- Sun, X. S., Kim, H. & Mo, X. Sun-Plastic Performance of Soybean Protein Components-1999. 76, 117–123 (1999)
- Swain, S. N., Biswal, S. M., Nanda, P. K. & Nayak, P. L. Biodegradable Soy-Based Plastics: Opportunities and Challenges. J. Polym. Environ. 12, (2004)
- Sue, H. J., Wang, S. & Jane, J. L. Morphology and mechanical behaviour of engineering soy plastics. Polymer (Guildf). 38, 5035–5040 (1997)