Content Background
The Plastic Revolution
Natural polymers were first explored in the late 19th century, most notably cellulose. Scientists were able to modify cellulose to produce a polymer that was eventually used in things like hair brushes, jewelry, toothbrushes and cinematographic film due to its easily moldable characteristics. This polymer became known as Celluloid and is regarded as the first synthetic plastic material.9 In 1906, another scientist, Leo Baekeland, experimented with phenol and formaldehyde mixtures. In 1909, he announced his invention which he coined as Bakelite, an easily moldable and less expensive alternative to Celluloid made up of a phenol-aldehydes, creating a thermoset plastic.10 By 1910, he began his company called the General Bakelite Company in the United States and shortly after began commerically producing his product. During the 1930-1940 decade, four of today’s major thermoplastics were developed: polyvinylcholoride (PVC), polystyrene (PS), polyolefins, and PMMA (acrylic). As new plastics were developed, their popularity in usage increased in various fields due to their wide variety of characteristics dependent on their chemical makeup.11
Prior to World War II, production of many thermoplastics at the time used vegetable sources as their raw material. Henry Ford even experimented with soybean oil to create the plastic panels of his soybean car. However, war brought about a high demand for new synthetic plastics, which were more economical but durable for wartime materials like parachutes, aircraft components, helmet liners, and much more. Further production and investigation of biobased sources was halted as chemists began experimenting and creating new synthetic plastics to aid the war efforts. By the end of World War II, plastic production almost quadrupled from 213 million pounds in 1939 to 818 million pounds in 1945.12 After moving away from vegetable sources due to the war, coal was a leading source of raw material in plastic production but eventually this gradually shifted to petroleum. “Today, the plastics industry is heavily integrated with the oil industry. The development of the petrochemical industry was probably the greatest single contributing factor in the growth of the plastics industry.”13plastics production in the United States grew exponentially from 390 thousand tons in 1960 to 35.3 million tons by 2017, as shown in Figure 1.14
Chemistry of Plastic
Plastics are a type of polymer, which is made up of monomers. Monomers are individual chemical units and when those units are repeated and covalently bonded, they create a polymer. In the case of plastics, they are a type of synthetic polymer created by scientists. The monomers that are most prevalent in today’s synthetic polymers, or plastic, come from raw materials like coal, oil, and natural gas.15 Not all polymers are synthetic, though. There are many examples of polymers that exist in nature. Some of those examples include starch and cellulose, which were the first type of raw materials used in the production of plastics like Celluloid and Bakelite. Most people are probably more familiar with the terms proteins and carbohydrates. Proteins and carbohydrates are natural polymers made up of long carbon frameworks. Polymeric structures, whether natural or synthetic, can be linear, branched or crosslinked describing the structure of the bonds between monomers. The chemical structure of a polymer determines what type of properties it will have.
All polymers consist of covalent bonds since they are composed of non-metallic elements. These non-metallic elements prefer to share their electrons with other elements rather than gain or lose any electrons. The sharing of electrons is what creates the covalent bond. Carbon becomes a popular element in polymers due to the number of of valence electrons available. Carbon has four valence electrons allowing it to share those electrons with other elements. Carbon also creates strong bonds with other carbon atoms. Most plastics are based off of a carbon backbone.
There are two broad classifications of plastics: thermosets and thermoplastics. Thermoplastics are plastics that can be heated up, which melts the plastic making it able to be remolded. The chemical structure of thermoplastics is either a linear or branched polymer. Linear polymers are typically long backbone chains that look like spaghetti and branched polymers are similar but with additional shorter chains equally spaced and bonded along the backbone chain. Approximately 92% of plastics are considered thermoplastics.16 Thermosets are plastics that, once molded and set, cannot be broken down into their original form. This is due in part to their chemical structure. The chemical structure of a thermoset is a crosslinked polymer. Crosslinked polymers have a structure in which multiple backbones can be connected resembling a ladder. These types of structures and bonding are very difficult to break apart. The chemical structures determine the characteristics of each plastic ever created and those characteristics are what drive the demand of plastic for use in different products.
The physical characteristics possessed by polymers can be determined by their length and their number of cross-links if applicable. Cross-linked polymers are harder to breakdown due to their more complex structure when compared to a linear polymer. The more cross-links the polymer has, the harder it is to breakdown. This type of structure presents the challenge to how humans can manage plastic waste.17
Life Cycle
For the purposes of this unit, the life cycle of petroleum-based plastics will be further investigated in this section. The most common elements in petroleum-based plastics are carbon and hydrogen. These elements come from the raw materials of oil, natural gas and coal, which are nonrenewable resources. The fact that nonrenewable resources are used in the production of petroleum-based plastics is just one of the issues to overcome for a more sustainable plastic future.
The raw materials must first undergo a process called “cracking”. When these materials are cracked, heavy hydrocarbon molecules are broken up into lighter molecules. The cracking process can be done by heat, pressure, or catalysts. Once cracking is complete, the resulting gases are hydrocarbon monomers like ethylene or propylene18. These gases can be used or processed further to create more monomers like styrene, vinyl cholride, ethylene glycol, etc.19 Next, the raw materials must undergo one of two types of polymerization reactions. Polymerization combines monomers to create polymers. One such polymerization reaction is a condensation reaction. In this type of reaction, two monomers combine and one monomer loses a hydrogen and the other monomer loses a hydroxyl. The lost hydrogen and hydroxyl combine to form water. The remaining electrons from the monomers covalently bond and form a polymer. The variation of monomers forming polymers is what allows for various characteristics of plastics.20 The second type of polymerization reaction is an addition reaction. In this type of reaction, double bonded electrons rearrange to form single bonds with other monomers.21
Additives can be used in the production of plastics. The additives can change the mechanical, physical, or chemical properties of the final plastic product. Examples of these alterations can include protection from the effects of heat, the addition of color, flame-retardancy, and much more22. Additives do not covalently bond to the other elements, which can lead to their leaching out when plastics are disposed of in the environment leading to a myriad of harmful effects on the ecosystems they enter. After additives are blended in, processing continutes with one of four main methods: extrusion, injection molding, blow molding, and rotational molding. Each of these methods involves melting plastic pellets into a liquid form and then cooling to form a final product. The desired product determines the type of processing method used.23
There are seven major plastic groups, which are identified in Figure 3 below. Sometimes, when you look at a plastic bottle or container, you might notice a number surrounded by a recycle sign in the mold of the plastic. That symbol denotes the type of plastic polymer. Each group has its own characteristics that separates it from each other. Each group, however, is made up of different products that share similar characteristics. The name of each group is derived from the type of chemical backbone of the polymer that forms the product. For example, there is polyvinyl choloride better known as PVC. PVC’s characteristics make it suitable for pipes due to its high chemical resistance, but it is also used for other products like blood bags, leather products and medical tubing.24
Figure 3. Chart identifying the seven major plastics groups, uses, characteristics, and recyclability.25
The end of life for plastic depends largely on what category of plastics it falls into: thermosets or thermoplastics. The fate of all types of plastic will take course on one of the following paths: deposition to a landfill, improper disposal becoming anthropogenic litter on land or in bodies of water, recycling, or incineration. Figure 4 below shows the average estimated decomposition times for common plastics that end up in a marine environment. Decomposition in this case is referring to the time it takes for these common plastic items to decompose into microplastics, which never truly leave the environment they end up in. The significant amounts of time allow for the accumulation of plastic waste, while also increasing the bioavailability of microplastic waste that remains in the environment.
Figure 4. Graph depicting the average estimated decomposition times for items of debris.26
Recycling is another method of reducing plastic waste, however, as you can see in Figure 3, not all plastics are recyclable. Recycling programs began in the 1980s and there is a lot of skepticism of whether or not recycling works. Only 60% of the U.S. population has access to recycling programs in the form of curbside pick up or drop off recycling centers. When plastic is recycled, plastics are chopped up, washed, and sold to manufacturers to create new products.27
Another management option gaining traction is source reduction, which is defined as “activities to reduce the amount of material in products and packaging before that material enters the municipal solid waste management system.” This definition is limited in describing all the strategies that fall into this type of management, though. Source reduction activities include: redesigning products to minimize the quantity of materials used, optimizing the lifespan of products, or reusing or repurposing products that are already manufactured. This waste management option is something accessible and achievable by everyone and could be part of the solution to addressing plastic waste.28
Plastics and the Great Lakes
There are numerous studies on the accumulation and effect of plastic in marine systems, which in this case refers to the world’s major oceans, however studies of the Great Lakes and freshwater systems have not been as widespread. As plastic pollution becomes more concerning, though, there has been an increased awareness of studying the Great Lakes and more attention and review of the plastics accumulating in these ecosystems is being undertaken.
Summertime in Chicago is always highly anticipated each year after the brutal Midwest winters. Lake Michigan is arguably the most popular and busiest destination once sunny, summer days arrive. Residents flood the Lakeshore Trail running, cycling, rollerblading, etc. to enjoy and embrace the magnificent views that the lakefront offers. Families and individuals flock to Lake Michigan every Memorial Day weekend when the public beaches officially open. Kids and adults alike wade in the cool waters to escape the summer heat. What beachgoers may not realize is they are really swimming in somewhat of a plastic wasteland.
According to research performed at the Rochester Institute of Technology, of the 22 million tons of plastic waste entering the Great Lakes every year, 11 million tons of that plastic enters Lake Michigan alone. Lake Michigan receives the most plastic pollution of any of the five Great Lakes. To put it into perspective, imagine 100 Olympic-sized swimming pools filled with plastic water bottles being dumped into the lake each year.29 It is also estimated that 80% of litter found on the shorelines of the Great Lakes is plastic.30
According to a review of the plastic debris in the Great Lakes, survey data revealed that urban areas heavy with human and industrial activity, like Chicago, were most associated with higher concentrations of plastic debris.31 Some of the most abundant data about beach pollution along the Great Lakes comes from The Alliance for the Great Lakes Adopt-A-Beach program (AAB), which is a non-governmental organization made up of volunteers who remove debris from the environment and test water quality to assess general beach health. According to the AAB data collection, in 2012 about 77%-90% of the total shoreline debris collected was made up of plastic debris items.32 There are 24 public beaches on the Chicago lakefront spanning 26 miles along the shorelines of Lake Michigan.33 In a collection done at a north side beach in Chicago in 2018, a local volunteer group made up of students for the AAB program reportedly collected 86 pounds of litter in a one-day cleanup.34 It was not noted how much of that litter contained plastic debris, but the number is still striking. AAB has also indicated that in the data of plastic debris they collected from 2003-2014 it shows cigarette filters and plastic food wrappers and containers were the most commonly reported litter collected; implying that beach-goers could be a major factor in macroplastic debris.35 Another study also identified plastic resin pellets from plastic manufacturing as a major pollutant along the Great Lakes shorelines. This was linked to spillage during transport, eventually entering streams and storm sewers, which could be discharged into rivers eventually ending in the Great Lakes. Additionally, a study completed by Hoellein et. al (2014) analyzing anthropogenic debris in the Chicago River also suggested that rivers could represent a major transport pathway of plastic debris to the Great Lakes. Debris counts from this study were reported between 0-34 items/m2.36 Plastic debris can be classified into primary and secondary debris. Primary plastic debris is in its original form or close to it, and secondary plastic debris is plastic that has mechanically broken down into smaller pieces. Both types of debris have been found in Lake Michigan and the other Great Lakes. The effects any kind of debris has on an aquatic environment can impose health risks to aquatic animals due to ingestion or entanglement. Plastics are not biodegradable, thus do not degrade to carbon dioxide in the environment like other forms of organic matter. Over time, primary plastic structures are disrupted by mechanical forces and UV radiation resulting in smaller microplastics. Microplastics have high surface area to volume ratios and can release toxic plasticizers or additives including phthalates, bisphenol A, etc. from the original products that were synthesized to create the plastic. If fish, turtles, birds etc., ingest these toxins, they can mimic hormones and impact endocrine function and cause harmful reproductive and developmental effects to the animal and as a result affect the food web.37
In the Great Lakes, one team of scientists found 4,270 microplastics particles per kilogram of dry weight sediment in lake sediment, and up to 2,444 microplastic particles per kilogram in river sediment.38 Microplastics are generally defined as any plastic less than 5 millimeters but greater than 333 nanometers in diameter. Any plastic debris smaller than 333 nanometers is termed “microscopic plastic debris”. Common shapes of plastic debris include fragments, films, pellets, lines, fibers, filaments, and granules. The type of polymer determines the density. Some polymers are more dense than water and they sink into a body of water, and some polymers are less dense than water and they float. Macroplastic debris, plastic greater than 5 millimeters diameter, can breakdown into microplastics due to mechanical weathering, UV radiation, and some biodegradation however it is does not completely mineralize and these degradation processes can take hundreds to thousands of years.39 Microplastics are also produced in an original form. Microplastics that are directly produced are used as the resin pellets for producing macroplastics or in personal care and cleaning products as an abrasive. In 2015, the United States Congress passed the Microbead-Free Waters Act of 2015 banning the production and distribution of microbeads in cosmetics and non-prescription drugs like toothpastes. This law was developed out of direct concern of microbeads in the water supply and to follow suit with several states that had already banned their use.40 Some countries have similar legislation, but this initiative has not been adopted worldwide. While this is a very small beginning step to acknowledging human plastic waste, it is not even close to addressing the 360 million metric tons produced globally.
Microplastics can pose a serious threat to aquatic ecosystems. Most studies on the effects of microplastics have only studied the effect on oceanic life leaving the need for freshwater systems to be further investigated. In studies of the effects of microplastics in marine environments, there have been three notable findings. First, several aquatic species have been found to ingest microplastics allowing for bioaccumulation. It has been confirmed that ingested plastics can transfer trophic levels via aquatic food webs posing a threat to the entire ecosystem.41 Second, some plastics are developed with toxic additives, like phthatlates, bisphenol A (BPA), and polybrominated diphenyl ethers, which leads to the next finding. Plastic polymers themselves are not toxic, but the toxic additives can leach out especially as plastic debris slowly degrades in the environment. Again, as the toxins, which are not covalently bonded to the polymers, are leached into the environment, it can cause serious health effects to the environment and animals ingesting the plastic.42 Third, plastic debris can act as a vector for non-native species and pathogens. Microbial communities are different from the surrounding water suggesting the microplastic can serve as a new habitat for microbial growth and replication. Microbial communities attach themselves to microplastics therefore making microplastics a source of waterborne pathogens, which can affect water quality.43
Based on the available data, it is enough to suggest that plastic pollution is major environmental concern for the Great Lakes. If escalating plastic production and environmentally unfriendly waste practices continue, the Great Lakes ecosystem could be in a dire situation. Upon comparing the average concentration of microplastic debris in Lake Superior, Huron and Erie alone, it rivals the known areas of litter accumulation in the oceanic gyres.44 There are multiple entryways into combating the issues of plastic pollution in Chicago’s Lake Michigan. This rest of the unit will go on to explore various opportunities for innovation addressing plastic waste through engineering.
Future of Plastic
Currently, one of the issues at the forefront of plastic waste is its resistance to complete degradation and the accumulation of microplastics in the environment. Three quarters of plastic waste is placed into landfills and the environment each year.45 We are now seeing novel environmental and toxicological issues, like the Great Pacific Garbage Patch and ingestion of microplastics in aquatic species. Just like we are only now learning the long term challenges of plastic production, use and waste, scientists and health professionals are still learning about the effect of microplastics on life and the environment.
These challenges have raised the question on the future of plastic. Bioplastics are becoming a more widely-proposed alternative to traditional petroleum-based plastics. The term bioplastics might be insufficient in describing more sustainable alternatives to petroleum-based plastic, though. Instead, it is suggested the terms bio-based polymer and biodegradeable polymer replace bioplastic for use in the discussion of sustainable alternatives. Bio-based polymers are synthesized with renewable raw materials, but many are still commonly not biodegradable. Biodegradable polymers are able to degrade completely to carbon dioxide when exposed to microorganisms and oxygen (aerobic) processes. Bioplastics can also be degraded anaerobically (absence of oxygen). Some bio-based polymers are degradable (e.g., polylactic acid and polyhydroxyalkanoate) but not all (e.g., biopolyhethylene). Additionally, biodegradable polymers are not always bio-based (e.g., polycaprolactone)46. There are two types of bioplastics that are biodegradable: polyactic acid (PLA) and polyhydroxyalkanoate (PHA).
PLA is a thermoplasic created through bacterial fermentation resulting in lactic acid. The resulting lactic acid is polymerized. Since scientists are able to produce lactic acid relatively inexpensively, it is cheaper to produce than PHA. The cons to PLA is that it is brittle, thermally unstable, and hydrophobic. However, like all polymers, the properties can be altered by its chemical structure or blending with renewable polymers.47
In a nutrient-deficient environment, many bacteria create PHA as food and energy reserves stored in the cytoplasm. PHAs are all dissovable in carbon dioxide and water regardless of their physical properties. Again, like other polymers, properties of PHA can be altered by its chemical structure. Drawbacks to PHA are their high production costs, low yields, and low availability. A solution to the high production cost requires a blend of PHA with renewable resources like starch or cellulose. Blends actually degrade better than PHAs without renewable materials48. Additionally, in a study completed at Ohio State University, scientists claimed to have found a viable alternative to petroleum-based plastic in a blend between organic rubber and Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). “The new PHBV/NR material has mechanical properties (strength of 28 MPa and toughness of 28 J m−1) and processing windows comparable to those of some commercial plastics, such as PP and HDPE, and can replace some petroleum-based conventional thermoplastics in cast sheets and thermoforms, including those used in food packaging.”49
To truly begin combating plastic waste, it’s not simply enough to engineer bio-based, biodegradable plastics. A 2011 study from the University of Pittsburgh compared biopolymers to petroleum-based plastics and determined that the biopolymers ranked high in terms of their green design, but they also had much larger negative effects on environmental impact from production. “As shown through the LCA results, biopolymers represent decreases in fossil fuel use and global warming potential and increases in other impact categories such as eutrophication, human health impacts, and eco-toxicity. These impacts result both from fertilizer use, pesticide use, and land use change required for agriculture production as well as from the fermentation and other chemical processing steps.”50 Furthermore, in 2017, another study determined that switching to renewable energy sources to produce traditional plastics could reduce greenhouse gas emissions from plastic manufacturing by 50-75%. This is more of a decrease in harmful emissions than if we only switched from traditional plastic production to corn-based PLA, which would only result in a 25% reduction.51 Studies like these lead to a vision of multiple strategies to mitigate plastic pollution. It seems that a combination of different strategies might be the most beneficial in future plastic production and use. Using renewable energy sources in the production of plastics, legislation like the Microbead-Free Water Act of 2015, other newly instituted bans on single-use plastics, continuing engineering of bio-based and biodegradable plastics as well as alternatives, reducing dependence, and repurposing plastic are all valid in the fight against plastic pollution.
Overview of Engineering Design Process
Engineering is a process that can be defined loosely into seven stages. Engineering can be iterative. The process begins with asking a question or defining a problem. By identifying the question or problem, a list of criteria can be determined. Criteria will work as a set of goals for your design. In addition, you must develop your constraints, or factors that will limit your design. Once the criteria and constraints are developed, the research phase begins. Engineers research the problem and even use current technologies and products to influence their ideas. After extensive research, engineers brainstorm possible solutions all the while keeping their criteria and constraints in mind. By using the criteria and constraints, engineers can narrow down their possible solutions and choose the plan they will put into action. Engineers will then create a prototype of the selected plan and will then test that prototype based on the criteria of the design. After evaluating the results, engineers improve upon their design and redesign. This is a cyclic process and is repeated over and over until a desirable result is achieved.52 It is important to note that this design process could be applied to multiple stages of plastic manufacturing and disposal, and it can apply to any product not just plastics.
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