Andrew Falco discusses the use of Pueraria montana in the production of biodegradable low density polyethylene plastic.
Andrew Falco
Plastics
According to the United Nations Environment Programme (UNEP), from the 1950s to the modern day, 8.3 billion tons of plastic were produced. 60% of this, 4.98 billion tons, end up scattered throughout the world. This number will continue to grow because humans produce approximately 300 million tons of plastic waste each year (UNEP, 2022). According to PlasticOceans.org (2021), low-density polyethylene (LDPE) is one of the most used plastic forms, also meaning it is one of the most commonly polluted. For this reason, this study focuses on LDPEs.
Plastic waste and its consequences impact all ecosystems. Large pieces of plastic can transport species to environments where they don’t belong (Rochman et al., 2013). Due to plastic’s buoyancy, it can float in water. Invasive species can wander onto the debris and use it as a boat to transport them to new habitats. These species displaced by plastics can damage the entire ecosystem via the food web. This interruption can cause changes to the environment and its inhabitants, threatening extinction for one or more species. According to Rochman et al. (2013), another consequence of plastic waste is damage done to ecologically and commercially important species. This harm is mainly through the consumption of or entanglement in plastic (Rochman et al., 2013). This information was corroborated by a report from the Convention on Biological Diversity in 2012, which stated that the majority of aquatic and terrestrial life is harmed by plastic waste.
In addition to being harmful to the environment, plastics are also detrimental to human health. According to Proshad (2017) and Halden (2010), plastics contain hazardous, and at times toxic, substances. These substances include but are not limited to heavy metals, flame retardants, phthalates, bisphenols, and fluorinated compounds. As a result of its components, plastics have been linked to many health issues. As a matter of fact, the chemical ingredients of more than 50% of plastics are hazardous (Linther et al, 2011). These toxic materials in plastics can also interrupt key physiological processes such as cell division (Rochman, 2013). This may lead to disease and reduce organisms' ability to escape from predators or reproduce.
Pueraria montana
Pueraria montana, commonly known as the kudzu vine, is native to Southeast Asia. It is a semi-woody vine characterized by oval-shaped or lobed leaves (Nature Conservancy, 2019). The vines connect to a central crown where the vine’s starch content is stored. The kudzu vine has a starch content of 83%, which would make it a good base for bioplastic. This is because the polysaccharides within Pueraria montana, amylose and amylopectin, are responsible for gelatinization, the process of a substance becoming gelatinous, and retrogradation, when starch molecules take on a crystalline arrangement. Both are required for bioplastic formation. Inside the crown root, the aforementioned polysaccharides are stored and used to maintain the structure of the plant (NYIS, 2019).
The vine was brought to the U.S. in 1930 for erosion control (Nature Conservancy, 2019). The Pueraria montana grew to become an invasive species, afflicting three-fifths of the United States (NYIS, 2019). According to Forseth and Innis (2004), it is estimated that the Pueraria montana vines currently cover three million hectares (7,413,161,444 acres) of land. To provide context, there is enough kudzu to cover the entire landmass of the Netherlands. It continues to cover more land over time, growing up to a foot per day (Nature Conservancy, 2019). It is because of this land coverage and overbearing growth that the Pueraria montana was declared an invasive weed in 1953 by the United States Department of Agriculture (Bergmann & Swearingen, 2005).
Pueraria montana has substantial effects on the environment. According to Forseth and Innis (2004), the kudzu vine can overtop trees, alter nitrogen levels, and emit isoprene. All of these effects result in the vine altering the environment to its needs and outcompeting other organisms.
The kudzu vine also has economic effects on the United States. Kudzu has cost many industries an estimated $100 million annually due to damage and loss of productivity (Harron et al., 2020). The main industries affected by this invasive vine are the forestry, railroad, and agriculture industries.
Although there are methods of eliminating Pueraria montana, they are often more detrimental than beneficial. The main method of disposing of these vines is the consistent use of herbicides over long periods of time (Nature Conservancy, 2019). The excessive use of herbicides would prove detrimental to the environment as it would pollute the soil. Additionally, many herbicides, namely Roundup, have been linked to cancer. Reportedly, people who work with herbicide seem to be at increased risk of non-Hodgkin lymphoma (Cressey, 2015). This evidence makes it clear that there is no obvious way to dispose of the Pueraria montana.
Problem, Hypothesis, and Gap
There is a problem with the overuse of plastics despite a surplus of evidence demonstrating how harmful said plastics are. There is also an issue with the overbearing growth of the Pueraria montana. This vine can disrupt and destroy entire environments. Although there have been attempts to dispose of the vine, these methods have been consistently shown to be ineffective. This study seeks to reduce the population of both Pueraria montana and plastic debris within the environment. It is hypothesized that when compared, Pueraria montana-based bioplastics will prove to be an effective alternative for LDPEs.
Methodology
To answer this query a bioplastic was created using the Pueraria montana as its base. The bioplastic would then be compared to a low-density polyethylene sample in a variety of assays. These tests were: biodegradability, durability, elasticity, tensile strength, cut resistances, heat resistance, cold resistance, and electrical resistance.
The bioplastic production took place in my home kitchen. The majority of tests were also performed at my home, with one or two being performed at my high school. The materials used in this study were either purchased commercially, such as the materials used in the bioplastic production, borrowed from my high school, such as the multimeter used in the electrical resistance assay, or already in my possession, such as the materials in the durability test.
In the creation of the Pueraria montana bioplastic, a ratio of 4mL water, to 1mL Pueraria montana starch, to ⅓mL acetic acid, to ⅓mL glycerin was combined, mixed, and heated to 100 degrees celsius. Once the mixture had become a gel, it was removed from the heat source. The gel was then spread out onto a baking sheet and left to cool and solidify in the open for a period of two weeks. This process was repeated eight more times to create nine sheets of Pueraria montana based bioplastics. Green food dye was applied to give it a distinguishable color. All samples were then cut into two-by-four centimeter pieces, which were used for every test.
This experiment consisted of nine assays that tested the most important qualities of plastic. The Pueraria montana based bioplastics and LDPE samples, plastic Ziploc bags, were used for these tests. The featured tests were biodegradability, durability, elasticity, tensile strength, heat resistance, cold resistance, and electrical conductivity.
Biodegradability is broadly defined. According to the Oxford Dictionary, if a substance can decompose because of a natural cause, no matter how many ways it can or how rare the method is, the substance is considered biodegradable. This study focuses on biodegradability in a saline solution since the majority of plastics end up in the ocean, which is a saltwater environment. To have the majority of plastics biodegrade, they must be able to degrade in the environment where most plastics are. In the biodegradability test, the Pueraria montana bioplastic and low-density polyethylene plastic samples were massed and placed in a saline solution with a salt concentration of 35% for twenty four hours. The 35% salt concentration emulates the conditions of the ocean. After twenty four hours passed, the samples were removed from the saline solution and massed again. The initial mass (M1) was then subtracted by the final mass (M2). The difference was then divided by the initial mass and then multiplied by one hundred producing the biodegradability percentage.
Plastics are durable, allowing for a wide variety of uses. An alternative to plastics must also be durable. For the purposes of this test, durability was defined as the amount of force distributed over the area that the plastic can resist before breaking. Only area was considered as both the bioplastic and LDPE had negligible thicknesses. To test the durability of the bioplastic and LDPE samples, the top surface area of the initial samples was calculated. Afterward, samples were propped up and held taut using thin ropes and metal stakes. Force was applied evenly over the surface to the samples until they had ruptured, through the use of bricks. The amount of force was divided by the area. This produced the durability measurement in grams per centimeter squared. This assay faced one major limitation, the lack of proper equipment. There are machines used in the industrial setting to properly test the durability of plastic. However, having no access to such machinery, simpler materials, the ropes, bricks, and stakes, were used.
The third test was the elasticity test. LDPE exhibits high elasticity, the ability of an object to stretch and return to its normal state. It is one of the main reasons that people rely on plastics such as plastic bags. To test the elasticity of the plastics, samples were stretched to their limit, before the sample would have broken. Stretching was done by hand, with one hand at each end pulling the sample apart. The samples were then left to return to a comfortable position for half an hour. The new length was then measured. The change in length (ΔL) was then calculated. And subtracted from the initial length (L1). The difference was divided by the initial length and multiplied by 100 to produce the elasticity percentage.
The next test was the tensile strength test. Tensile strength is the ability of plastic material to withstand the maximum amount of tensile stress while being pulled or stretched without failure. To test this property, samples were hooked onto a spring weight. A controlled amount of force, 110 pounds of force, was applied to the opposite side of the sample. The amount of weight the sample could bear before tearing was multiplied by the acceleration of gravity, as the sample was being pulled downward due to how the apparatus was set up, to calculate the tensile strength.
The fifth test was the cut resistance test. Due to a lack of proper materials for this assay, such as the industry standard tomodynamometer test machine, a rudimentary methodology was used. Four samples for each kind of plastic were held taut on a surface and sliced with a pocket knife in a single line across the sample until it cut all the way through. The average number of cuts needed to rupture the sample was used to determine the cut resistance.
The sixth test was the heat resistance test. Most plastics are exposed to extreme conditions, mainly heat. If the Pueraria montana based bioplastics are an effective alternative, then they must be able to withstand extreme heat. When burned, low-density polyethylene plastics release products, such as dioxins and sulfur dioxide, that are toxic to humans. To get around this, appropriate safety precautions, such as wearing a mask and being highly alert when performing this test, were taken. In this test, samples were placed in an oven at 232℃. The samples were checked at regular intervals of three-minutes for deformities caused by the heat.
The cold resistance test consisted of placing the samples in a freezer at -38℃. The samples were then continually checked on in intervals of three-minutes for deformities, such as cracks, caused by the freezing temperatures. This test is important as plastic materials are often left in freezing conditions for the storage of food. For the Pueraria montana-based bioplastic to be an effective alternative, it must be able to weather the cold.
The final test was electrical conductivity. One of the most important qualities of plastic, and why it is used so widely, is that they do not conduct electricity. This is mainly important for machinery, which must be constructed out of materials that are poor conductors of electricity and heat. In this test, a multimeter was used to test the sample’s electrical conductivity across multiple voltages, 200 volts to 1000 volts. Ideally, this test would have used a four-prong probe system to get the most accurate readings for conductivity. However, due to budgetary restraints, this test needed to be modified to use more accessible materials, hence the multimeter.
Most of the assays were repeated 3-4 times and the mean of the results was calculated, except for the durability, heat resistance, cold resistance, and electrical resistance tests, which were only done once. When applicable, a t-test was used in order to determine if there was statistical significance between the means. When performing the t-tests, the data relating to the bioplastic was used for the experimental group, and the data relating to the plastic sample was used for the control group. For this study, statistically significant was defined as a p-value less than 0.05.
Data, Results, and Discussion
When the bioplastic was created it had an area of 289 cm2, a mass of 4.5 grams, a volume of 28.9 cm3, and a density of 0.16 grams/cm3. The bioplastic was transparent. Green food dye was applied to give it a distinguishable color. Qualitatively, the plastic was taut and difficult to pull apart. In addition, the plastic had adhesive properties as it would stick to itself and other substances, similar to tape. The bioplastic had a faint yet very unpleasant odor, which may have been caused by the acetic acid used in the production of the bioplastic.
To simplify the data collection, the assays were organized into groups. The biodegradability test was a stand alone group. The first group was the physical characteristics tests: elasticity, durability, tensile strength, and cut resistance. The second group was the elemental tests: heat resistance, cold resistance, and electrical conductivity.
In the first test, the low-density polyethylene had a biodegradability of 0%, meaning no mass was gained or lost. However, the Pueraria montana-based bioplastics had an average biodegradability percentage of -789%. While that number may seem shocking, there is a simple explanation for this: the bioplastic absorbed the water. The absorption was due to the differing solute potentials of the solution and the bioplastic. Although exact calculations for the solute potential were unavailable, it can be inferred that the bioplastic had a greater solute potential than the saline solution, as the bioplastic had large amounts of solutes involved in its production. A subsequent t-test resulted in a p-value of less than 0.0001, displaying that the data was statistically significant. Overall, the LDPE proved to be superior in this category since it did not absorb excessive amounts of water.
In the durability test, the LDPEs were able to sustain 11,793 grams of force over an area of 256 centimeters squared until it broke. This resulted in a durability of 46.1 grams of force/cm2. The bioplastic was able to sustain 5,896 grams of force over an area of 289 centimeters2. This resulted in a durability of 20.4 grams of force/cm2. The LDPE was proven more durable than the bioplastic.
The elasticity test then followed. The LDPEs had an average elasticity of 91%. This is a very good percentage, however, it pales in comparison to the bioplastics which was 100%. This means that without failure, the bioplastic was able to be stretched and return to its initial length. Thus, the bioplastic was superior in this category.
In the tensile strength test, the bioplastic tore apart due to a force of 3,924 newtons. The LDPE was much stronger in this test. The plastic broke due to a force 17,658 newtons. In this test, the bioplastic was ineffective in comparison to the LDPE.
In the cut resistance test, the low-density polyethylene plastic had an average cut resistance of 3.5 cuts. The bioplastic on the other hand had an average cut resistance of 1.75 cuts. The associated t-test resulted in a p-value greater than 0.05, showing that the data was not statistically significant. This comes down to the rudimentary methodology used in this test. There are a variety of confounding variables that may have affected the results of this test, such as the amount of force applied and the sharpness of the blade to name a few. In this test, the Pueraria montana based bioplastic was less effective.
In the heat resistance test, the bioplastic lasted 15 seconds in the heat without deforming. When removed, the Pueraria montana based bioplastic had shriveled up, become more brittle, and darkened in color. The LDPE however lasted a full three-minute interval. Only one three-minute interval was used for two main reasons. The first is that the LDPE lasted longer than 15 seconds so there was no reason to continue as it had proven to be more heat resistant than the bioplastic. The second reason was to mitigate the health risks. This demonstrates a limitation in this study as the heat resistance data for the LDPE was not completed to its fullest potential. Despite that, the low-density polyethylene was superior to the bioplastic in terms of heat resistance.
The data from the cold resistance test was inconclusive. Both the LDPE and the Pueraria montana based bioplastics were able to withstand the cold for extended periods. This brings up another limitation in this study: time constraints. Ideally, the plastics would be placed at extreme temperatures for as long as necessary. However, due to the short period of time allotted to perform this study, the samples had a limited amount of time in the cold and could not be observed constantly. However, the assumption can be made that due to its initial gelatinous form before it solidified, the bioplastic would have most likely frozen first.
In the electrical conductivity test, the bioplastic conducted electricity at a minimum voltage of 200 millivolts. The LDPE sample did not conduct electricity at any of the voltages used. This can be explained by the fact that plastics have nonpolar covalent bonds, which display no ionic characteristics and no free electrons. Charge is evenly distributed, resulting in no net electrical charge. As a result, the Pueraria montana based bioplastic was ineffective in this test.
Limitations
There were two very impactful limitations in this research. The first of these limitations was budgetary restraints. Unfortunately, the proper equipment for many of these tests, such as the four-prong probe, was unaffordable, meaning many of the tests had to be modified to use readily available household materials. To further validate the results presented, the assays should be repeated with the appropriate equipment. Secondly, the materials used to create the bioplastic in this study were in limited supply, creating a smaller amount of samples. As a result, fewer trials than desired were performed. Another factor linked to these restraints was the type of Pueraria montana starch used. To obtain the starch in its purest form, a ninety-day extraction process is required. Due to the time constraint, commercially available kudzu vine starch, which had already been through the extraction process but to an unknown extent, was purchased. Nevertheless, attempts were made to mitigate the effects of these limitations. This included looking at similar research studies from the past fifteen years to validate the results and findings of this study.
Conclusion
As shown by the data presented, the Pueraria montana based bioplastic was not an effective alternative to traditional low-density polyethylene, refuting the hypothesis. The bioplastic was only effective in two tests while it was ineffective in six and one was inconclusive.
However, there was one major observation that was very concerning. Some of the bioplastic samples had grown bacteria. Out of the first six samples, two had shown signs of bacterial growth. This was a concern as bacteria is one of the leading causes of infection. Upon reflection of this issue, it was noted that the way the plastics were made was similar to how agars are created. An agar is an inert, non-nutritive substance made up of polysaccharides that serves as a breeding ground for bacteria. Since the plastic was similar to an agar, it would be very effective for growing bacteria. However, none of the three samples produced in the second batch showed any signs of bacterial growth and are still in good condition. This led to the conclusion that the bacterial growth observed in the first group of samples was more a result of mishandling the bioplastics during the solidification process rather than any natural causes. This does not, however, eliminate the fact that the bioplastic can potentially be hazardous if mishandled.
In future research, more assays should be performed to gain a more holistic picture of the properties of this novel bioplastic. As mentioned prior, only nine tests were conducted. This number pales in comparison to the millions of qualities of LDPE. In addition, different materials should be used in bioplastic production to identify if there are more optimal ways to produce this bioplastic. Based on the qualities shown in this study, this bioplastic does have some applications. It was determined that the Pueraria montana based bioplastic may have multiple applications such as a bandage, parafilm, and even a rubber alternative due to the determined properties. Furthermore, if improvements can be made to the Pueraria montana based bioplastic used in this study, it may show greater potential as an alternative to LDPE. Overall, this study represents a foot in the door for Pueraria montana bioplastics.
References
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