The chemistry of microplastics is important to their investigation for various reasons. First, understanding their chemical structure allows us to see how they can last so long in the environment and what pollutants bind to them. Secondly, we can observe the influences microplastics have on our own bodies, which is medically useful. Finally, by studying the chemistry of microplastics, we can take measures to make them harmless, or at least less harmful.
In general, all plastics are manufactured in the same way, using techniques known as polycondensation and polymerization. The process starts off with the refining of crude oil into collections of oils through distillation, which separates light and dense components of crude oil. This allows the extraction of hydrocarbons under the group naphtha that is used to produce plastics.
The goal of polymerization and polycondensation is to convert the raw materials into chains that can be connected and folded to form polymers that can then be made into plastics. There are two ways to perform this, and these can occur in all states of the monomer, although it is more common to perform them in the liquid and gaseous states than the solid one. In the first method, called condensation polymerization or polycondensation, two monomers combine and lose a small molecule, typically water, acid or alcohol. Then, when joined by a catalyst, one monomer loses a hydrogen (H+) while the other loses a hydroxyl (-OH) forming a covalent bond, which leads to the production of a chain of different monomers, also known as a chain of copolymers. The remaining hydrogen and hydroxyl join to form a water molecule. It is important to note that in this reaction, both monomers have a hydrogen and a hydroxyl group. The second reaction, addition polymerization, involves arranging double bonds within monomers into external single bonds with other monomers. After either of these reactions, the hydrocarbons are heated such that large molecules are broken down into smaller ones under high amounts of heat. Once this has been completed, the monomer chain moves onto polymerization. In this process, monomers combine to form a polymer, which is a high-molecule compound. Polymerization can be divided by number of monomers (homopolymerization and copolymerization) or by type of active center (radial or ionic). To start the process, catalysts called initiators are introduced to the molecules, which convert certain areas of the monomers in active centers. In radical polymerization, this results in the formation of free radicals. After this, a macromolecule is created by the addition of monomers to the active site, and then a breakage of the active side by collision. Polymers can grow in two ways: step-growth and chain growth. While chain growth involves the linkage of monomers, step-growth is characterized by the step reactions between monomer linkage.
The Plastic Family
Before exploring the chemistry of microplastics, it is important to know some general information about plastics. There are 2 major categories of plastics in the world: thermoplastics, which can be reheated and molded, and thermosets which can never be reheated again. Under these categories, plastics are divided again into 7 main groups. These are:
- Polyethylene Terephthalate (PETE, PET) - (C10H8O4)n
- High-Density Polyethylene (HDPE) - (C2H4)n
- Low-Density Polyethylene (LDPE) - (C2H4)n
- Polyvinyl Chloride (PVC) - (C2H3Cl)n
- Polystyrene, Styrofoam (PS) - (C8H8)n
- Polypropylene (PP) - (C3H6)n
These “categories” of plastics are also known as resins, and a large number of consumer and industry goods are made of up them. In order to provide a better understanding of the different plastic resins, we have defined them below:
- Polyethylene Terephthalate
Making up to 96% of all containers and bottles in the US, this is the most common plastic found on the planet. PET is a thermoplastic and is more commonly known as polyester, a material found in many textiles. It's also used in a wide range of products, such as solar panels, ropes and 3D printed objects. Its characteristic of being fairly durable and lightweight explain the high concentration of this plastic in the world.
Physically, PET is a colorless, odorless semi-crystalline solid that can vary from semi-rigid to rigid. Additionally, the intrinsic viscosity of PET varies with the product it is being adapted to. PET is produced through the polymerization of terephthalic acid (C₆H₄(CO₂H)₂) and ethylene glycol ((CH₂OH)₂) in a transesterification or esterification reaction, depending on whether terephthalic acid or dimethyl terephthalate (C10H10O4 ) is used. Ethylene glycol is an alcohol that contains to hydroxyl groups (OH-), also called a diol, and terephthalic acid is a dicarboxylic aromatic acid, which means it has a six-carbon ring and two carboxyl groups (-COOH). In the terephthalic acid reaction, these two monomers undergo polycondensation and form ester linkage bonds (CO-O). With dimethyl terephthalate, the DMT method is used where the two are reacted in the presence of a catalyst and methanol is removed from the reaction. When heated, molten PET is produced which can then be prepared as a solid to be made into plastic or spun into fibers. The large six-carbon ring in PET gives it its firmness which can be reinforced by its semi-crystalline structure. While PET is generally considered “safe plastic”, over time it discharges the metals bromide (Br) and antimony (Sb) which are toxins that can be found in plastic water bottles left unused too long.
- High-Density Polyethylene
HDPE is typically used in industrial work, specifically with pipelines and artificial lumber. To create this material, high amounts of heat are applied to petroleum through cracking. This produces ethylene gas (C2H4), which goes through addition polymerization and binds during its process forming the polymer polyethylene. HDPE specifically uses Ziegler-Natta or metallocene catalysts, or activated chloride, and is manufactured at low pressures and low temperatures. The repeating chains of ethylene can be structured into branches or in a linear arrangement, the latter of which is the case with HDPE. This allows more chains to fit into a smaller space since they can pack closer together, lending HDPE its high durability.
- Low-Density Polyethylene
LDPE is produced using the same raw materials as HDPE, except that it is created under extremely high temperatures and high amounts of pressure as opposed to the low pressure/low temperature conditions of HDPE. This causes LDPE to have a branches structure as opposed to the linear structure of HDPE, which causes it to be more malleable than HDPE. LDPE is typically used in products like trash bags, shopping bags, and plastic toys due to its ease to be manipulated.
- Polyvinyl Chloride
PVC has a wide range of uses, from canvases to door lining. This is mostly due to the fact that PVC can be manufactured to be either flexible or rigid, allowing it to be applied to many different areas. PVC is created through a type of polymerization called suspension polymerization, where the substrate vinyl chloride monomer (VCM, C2H3Cl) is subjected to high pressure and water is added to sustain the suspension. This polymerization is classified as a radial polymerization. The resulting structure of PVC is linear and antiparallel, so that the chlorides are at opposite end of each other on each adjacent strand. Due to the large amount of chlorides in the structure, PVC is denser and has different characteristics from the related polyethylene. PVC required the use of additives in order to lend it properties to withstand stress, the most infamous one being phthalate plasticizers. Research has show these become toxic when the leak into the environment, causing large amount of damage to the ecosystem.
- Polystyrene, Styrofoam
While the compound styrene (C₆H₅CH=CH₂) is naturally occuring, it is the building block for polystyrene, a plastic resin that has uses in the manufacturing industry. Polystyrene is created by linking together styrene monomers with carbon-carbon sigma bonds, which makes this polymer difficult to depolymerize. During this step, the polymer can be sorted into two categories based on its final polymer structure. This first structure, atactic, involves a random placement of phenyl groups along the polymer chain, and it prevents the polymer from crystallizing well while the second structure, syndiotactic, has a rigid structure that is produced with Ziegler-Natta catalysts in polymerization. Atactic polystyrene has more uses in the industry, where it can be modified through pouring molten polystyrene into a mold in injection molding or by heating a plastic sheet and then altering it in a process known as thermoforming. Both these methods produce sheet polystyrene, which is often used in biotechnology. Another type of polystyrene, called foam polystyrene, is made by injecting large amounts of air into the polystyrene, creating a foam that is 95-98% air. This type of plastic has been popularized by the creation of Styrofoam, which is used mainly in packaging. Foam polystyrene should not be confused with expanded polystyrene foam (EPS), which is produced by heating polystyrene beads which expand upon the release of hydrocarbon gases such as pentane. This is then molded into sheets that can be further processed to be used in industry. Extruded polystyrene (XPS) contains more gases and releases them over time, which causes to to differ from EPS.
Similar to polyethylene, polypropylene is lightweight and flexible, but slightly more rigid. It is created in a manner like polyethylene as well, using Ziegler-Natta and metallocene catalysts to perform polymerization on propylene gas (C3H6). Polypropylene can be categorized into atactic,syndiotactic and isotactic polypropylene, all of which are related to the way the the methyl groups are arranged around the polymer backbone.These different forms of polypropylene lend it different chemical properties that allow it to be easily manipulated; for example,the low viscosity of polypropylene allow it to be injection modeled easily. It also means that a large variety of consumer goods are composed of polypropylene due to its versatility.
All plastics that do not fall in the previous categories are put into the miscellaneous category. These include the polycarbonates, which contain large amount of carbonate groups in their polymer structure, and BPA ((CH₃)₂C(C₆H₄OH)₂) and BPS ((HOC6H4)2SO2). The last two are particularly important when discussing the impact of microplastics on the environment. BPA is used as a composite in polycarbonate plastics and epoxy resins and over time, it leaks out of the product. This raises health concerns since BPA and BPS both act as endocrine disruptors, affecting the chemical signals in your body.
Now that the background is set, we can begin looking at the chemistry of microplastics. Microplastics are any piece of plastic that is under 5 mm, and they can either be primary or secondary, depending on their source. Primary microplastics are manufactured, and they include microbeads, nurdles and fibers. Sorting these into the 7 categories, we get that:
Microplastics in Detail
Thus as we can see, the structure of a microplastic is dependent on the type of plastic it is derived from. PET fibers will tend to have large six-carbon aromatic rings, while microbeads will be more rigid and harder and harder to decompose. This also directly correlates to the degradation of microplastics in the environment based on their chemical structures. Microbeads made with PP and PS tend to not degrade easily, so they persist in the environment for a long while. Additionally, additives that modify the plastics to give them characteristics such as heat resistance leach into the environment after time due to the ester bonds, which are not covalent and thus can be broken by exposure to heat. This is why after time, if a plastic water bottle is left in the sun, it should not be used due to potential residues in the water.
The other type of microplastic is the secondary microplastics, which are more commonly known as fragments. These are produced when larger pieces of plastics which can result from erosion due to heat and UV rays. These can be made of any plastics as well, which brings up an important point about microplastics and plastics in general: plastics never disappear, they only get smaller. This is due to the difficulty in depolymerization plastics with their strong crystal structure and ester bonds.
- Microbeads are made of PP and PS
- Fibers are made of PET
- Nurdles are small plastic pellets that can be composed of any plastic
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