The production and reactants used to produce Polystyrene and Polylactic acid determine their functionality for different uses that are essential in our daily lives. These also dictates how the products degrade as well as what substances it degrades to. This is important to determine the significant implication it has on our environment and the society. Polystyrene is the most common material used today. It is applied in numerous ways but due to its non-renewable resources that causes negative impacts in our environment as well as having shortage of it, it increases the demand for the development of biodegradable plastics. Polylactic acid is a biodegradable plastic. It is made from renewable sources and thus, its supply is continuous. It can biodegrade – stated it is at appropriate environment, in a shorter amount of time compared to polystyrene. This will increase free space in landfill, decreases pollution and helps the epidemical problem of climate change.
Polylactic acid is initially derived from corn. Corn is high in carbohydrates – specifically 19% carbohydrates and water mostly, this means that it would have greater glucose or dextrose as carbohydrate is capable of disintegrating into dextrose. This is more prefered in the fermentation process, where dextrose produces pyruvate which then results in lactate through the process of glycolysis. Corn is a great source since it can grow in great quantities and is fairly affordable, this means it is more reliable as a long term source. But there is a negative impact in the use of corn as a dextrose source. In the process of collecting corn, carbon dioxide is released by the combined harvester – requires fuel, which is used to collect corn kernels from a corn farm. If the demand for PLA were to increase, the production and collection of corn also increase and thus, increases the release of carbon dioxide into our environment. As we all know carbon dioxide is a greenhouse gas that traps heat into our atmosphere, heating up our surroundings. But in return to this release of carbon dioxide, the corn absorbs carbon dioxide from its environment. This is because for plants to obtain necessary nutrients, it needs carbon dioxide as well as water and the energy supplied by the sun to produce glucose. This lowers their contribution to the climate change.
Polylactic acid as mentioned prior is harvested from corn and fermented to produce lactate. This fermentation is anaerobic, meaning oxygen is absent in the process of producing energy in the form of ATP from the initial reactant of glucose or in this case dextrose. Firstly, dextrose must be derived from the corn. Carbohydrate is an important role in this beginning process because it can be breakdown into dextrose which can then convert to different substances through chemical process and finally lactate. So carbohydrates must be extracted from the corns, this is done by segregating the carbohydrates from water – corn is mostly constructed of water, through wet milling. To break down the carbohydrates into dextrose, it is hydrolysed which is the breaking of bonds through the addition of water:
(insert equation1)
Dextrose must then be separated from the other by-products that were produced through through wet milling. This can be done through precipitation, deriving a more refined dextrose. This is important because the other by-products may cause another reaction that will interrupt the main production of lactic acid.
Once dextrose is secured, it can undergo glycolysis. This is the process which disintegrates the dextrose into a pair of three containing carbon molecule called Pyruvate. With it produces two types of by products which are two molecules of ATP (energy) and two molecules of NADH. This NADH molecule will lose two electrons (oxidise) to provide two electrons to pyruvate (the reductant). Therefore, pyruvate is reduced (gain of electrons) to produce lactate. This is done by breaking the C=O in pyruvate and attracting the surround hydrogen atoms which is from the surrounding water that is produced as byproduct of glycolysis. This produces lactate which is the conjugate base (difference of a proton of hydrogen) of lactic acid. If the lactate earns another hydrogen from surrounding water, lactic acid will be produced.
(equation 2)
Throughout these process we must also consider the production of NAD+ from the oxidation of NADH. The NAD+ will initiate another glycolysis reaction with the other dextrose molecules and thus, producing more lactic acid. This reaction can be self-sustaining as long as there is sufficient supply of dextrose because one of the byproduct of the reaction is also one of the reactant. Therefore greater yields of lactic acids will be produced with the condition that there is enough corn to convert to dextrose. This is helpful for great manufacturing of plastics which are in great demand as it is applied in our daily lives. During this process of production of lactic acid, it also releases various types of by- products. To address these by-products to produce a refined lactic acid, a base is added to the solution. These bases neutralizes the acidic by-products. Ammonia (NH3) is preferably used because it is able to produce a more refined lactic acid. The addition of this will form ammonium lactate which will then be reacted with butanol – this is condensation reaction which is the removal of a water molecule, producing an ester called butyl lactate. With the butyl lactate it also produces ammonia and water. They water molecule will then break the ester (hydrolysis) and ultimately the lactic acid as well as a butanol will be produced as a hydrogen from the water molecule is added across the ester bond.
(insert equation 3)
Once lactic acid is derived through anaerobic fermentation it is able to undergo two types of polymerization reaction, either ring-opening polymerization or condensation polymerization. Ring opening polymerization is prefered because it does not release water unlike direct polymer condensation. This condensation reaction or esterification release water in each reaction which causes hydrolysis on product (equilibrium reaction), returning it back to its initial reactant. This disturbs the molecular weight of the products and consequently the quality of the product. A lower molecular weight means it is more vulnerable to melt or boil in a high temperature conditions, this limits its applications. Therefore ring-opening polymerization is more generally used for the production of polylactic acid because it can produce a higher molecular weight product. For ring-opening polymerization to occur, the lactic acid must alternate to a cyclic di-ester called lactide. For lactide to be produced from lactic acid, lactic acid must firstly undergo condensation reaction. This can be done through the addition of an acid catalyst thus, will remove a water molecule from the lactic acid. After this reaction oligomerization can occur, where two identical monomers are added together to form a dimer. The two monomers will bond through covalent bonding -sharing of electrons. This is because once a water molecule is obtained from the lactic acid, the single bonded oxygen will be negative because the hydrogen present prior has been removed. Therefore, they will really want to connect and bond, this dimer will form rings through cyclisation and then producing the lactide. Afterwards, refined lactide will need to be obtained for ring-opening polymerization to occur.
(insert equation 4)
Lactide will then undergo metal catalysed ring-opening polymerization (ROP). Lactide can either undergo cation or anion polymerization. Both processes produces a low molecular polymer but the addition of metal increase its molecular weight and refinement, that is much more preferable. The low molecular weight polymer that is produced prior to the addition of a metal catalyst is due to the certain reactants that are added to the lactide that allows its ring to open. These reactants can also interrupt the multiplication of the polymer to become longer and therefore a lower molecular weight. The inclusion of a metal catalyst – the most common being tin(II) octoate, increases the molecular weight. This catalyst is also proven to be safe and doesn’t produce toxic chemicals that may impact the consumers. A higher molecular weight means that it is a longer molecule (contains more carbon) that has higher boiling and melting point. This makes its uses more diverse. But the chains are still able to slide past each other making it a thermoplastic. This means that is can be recycled through reshaping it, this makes it more beneficial for the environment.
(insert equation 5)
The degradation process of polylactic acid is hydrolysis and again this is done through the addition of water across the bond of the monomers. This process will break down the long polymer into lower molecular weight polymer. Afterwards, the presence of certain microorganisms will help disintegrate it further producing some organic material, as well as some greenhouse gases – CO2 and methane gas. But the microorganism that enables disintegration of the low molecular weight PLA cannot be found in the general landfills – where the PLA usually ends up because it has similar physical features as petroleum based plastics. This causes an uprise of concerns because without these microorganisms the polymer could exist for more than expected and thus will act like any other petroleum based plastic by taking up space. Also the release of greenhouse gases through this degradation process is also quite alarming as it can accelerate the change in our environments climate. Although polylactic acid is biodegradable and is derived from a renewable source, it still has its negative consequence that could affect the environment and our society. But this problem could be addressed by increasing the growth of corn which takes away carbon dioxide from the environment for photosynthesis.
The production of styrene from ethylbenzene came about during 1937 by people from Dow Chemical Company and notably the chemist Robert Dreisbach. There, they also began to trial the polymerization of styrene and eventually, during 1938 they successfully created polystyrene. Polystyrene is derived from petroleum, a nonrenewable source because it cannot be restore. This means that it will eventually ran out, especially because it is used in many other things like energy source. This factor increases the emphasize of a new source for plastic and styrofoams. Petroleum is used as the main source for the production of polystyrene because it is a cheaper source and provides different types of hydrocarbons. The unsaturated hydrocarbons from petroleum can be manipulated to form long polymer. The benzene rings from petroleum is also important because after some reaction processes it alternates to a phenyl groups which gives the polystyrene its firm quality. To derive petroleum, crude oil must be extracted from the ground. This procedure has some great negative impacts to the people and environment. The purification of petroleum from the derived oil can cause air pollution because it releases harmful substances. Also, the process of obtaining petroleum as well as moving it to different locations can possibly cause an oil spill. An oil spill is highly destructive to our environment. It can contaminate the soil and destructing our nature. It can also possibly contaminate our water sources and if intake, we could be exposing ourselves to cancer and infections. Petroleum contains benzene and styrene that is highly dangerous to human health. Oil contamination can also be caused through the method fracking. This method requires the introduction of chemicals, sand and water into the well where the crude oil extraction is occurring. This method will help the withdrawal of oil and natural gas. Some of the chemicals inserted into soil can stay there to contaminate it, destroying the nature. And in general the manufacturing of styrofoam – which is made out of polystyrene, requires a lot of energy and this will increase the carbon dioxide emission.
The extraction of natural gas is important for the production process of polystyrene because it is used as the initial reactants to create the monomer for the polymer. The monomer for polystyrene is styrene which is derived from ethylene and benzene, these two alkenes are derived from petroleum and are very dangerous. These two alkenes will then produce ethylbenzene and finally styrene. Both styrene and benzene are unsaturated (alkene), this means they have the minimum amount of hydrogen attached to the carbon, creating the double to satisfy the four bonds which the carbon requires. To create the ethylbenzene, an ethyl group replaces a hydrogen that is attached to a C=C. The ethyl group is created by breaking the double bond of the ethylene. This is done by the addition of hydrogen in one of the CH2, turning it into CH3, while the opposing CH2 will be connected to the benzene group. Therefore the require four bonds is achieved, making this group saturated. This is done through the attendance of Aluminium Chloride (AlCl3).
(equation 7)
Once ethylbenzene is obtained, styrene can be created. This is done by removing two hydrogen (dehydrogenation) from the ethyl group, CH2CH3. Surplus steam is added into the ethylbenzene to prevent clogging around the catalyst which is iron (III) oxide, Fe2O3. Therefore, the catalyst is able to do its job effectively. A hydrogen is removed from the CH2 group and another hydrogen from the CH3 group of ethyl group. This produces an unsaturated product called styrene, ready to be polymerized.
Addition polymerisation requires unsaturated monomer, so when added together, the monomer’s double bond can break and they will bond to create a very long polymer. The certain type of addition polymerization used for this polymerization is called radical polymerization. Free radicals are added to initiate the polymerization and thus operates as the catalyst. When the styrene monomers are added together the double bond in the vinyl group (CH=CH2) is broken as it attaches to another vinyl group from another styrene. This addition polymerization continues, forming a long polystyrene.
(equation 8)
The phenyl group in this polymer is responsible for the physical features of polystyrene. We know that polystyrene is usually known as styrofoam, which can be used as an insulator or as disposable utensils. Therefore, polystyrene needs to firm and slightly strong to be able to function properly. The phenyl group in polystyrene is responsible for this feature as it prevents the polymer chains from moving and consequently it will remain in its position, giving that strong and firm nature. This means it also prevents the chain from getting into a bundle that may make the product too hard, that it cannot be recycled to another product.
The use of polystyrene comes along with many negative environmental and health factors. One of the significant negative factors of the production process of polystyrene is the use of HCFC-22, Chlorodifluoromethane to assist the reaction. Before the use of HCFC-22, chlorofluorocarbon, CFCs was used. CFC was even more detrimental to the ozone layer compared HCFC-22 and because it is so harmful is has been banned from utilization since 1980’s. HCFC-22 is a colourless, greenhouse gas and is thought to be less harmful to earth. But because it is a greenhouse gas it does trap heat into our atmosphere, increasing our climate. Increasing our climate would mean an uprise in sea level, this will endanger the lives and homes of those who live in a smaller islands. It has recently been discovered that HCFC-22 like CFCs is detrimental to the ozone layer (CFC being worst). Our ozone layer is extremely significant to life on earth as it protects us from the strong impact of ultraviolet radiation from the sun. Ultraviolet radiation has the ability to cause skin cancer on humans, harm their immune system and also demolish the nature. Our ozone layer prevents this from occurring but due to our emission of chemicals such as HCFC-22, it is gradually being destroyed. If this continues, we could be exposed to more UV light than normal.
The degradation process of polystyrene is also another alarming factor for the environment. It takes a very long time for polystyrene to degrade into substance that are less harmful and less space consuming. Polystyrene commonly ends up in landfills and because it is filled with air, making lightweight and large, it takes up large amount of space. When it is disintegrated into smaller pieces, it could easily be distributed by the wind into different places like the ocean where it can be mistaken as food. If the sea life takes polystyrene internally, it could be harmful to their system due to the presence of benzene and styrene. If the intake of polystyrene is increased, there is a significant possibility that these contaminated creatures can be eaten by humans. Therefore, we are also indirectly putting polystyrene into our body. The styrene which makes up polystyrene is known to cause cancer in the blood (leukemia) and causes mutation (DNA alteration) in the central and peripheral nervous system (this system is responsible for sending messages about a change in the environment and also relies messages from the hypothalamus on how this problem can be addressed to several glands). If these systems are damaged, it could accelerate to larger health problems acute functional impairment. Also the appliance of polystyrene into food containers is also harmful because when heated with microwaves, toxic substance that can contaminate our food. This also increase the risk of cancer development.
The clear difference between polylactic acid and polystyrene is that PLA is obtained from renewable source while PS is derived from a nonrenewable source. This makes a great difference in their durability and cost of production. Polystyrene is cheaper than polylactic acid because PLA requires the bulk yielding of corns to keep up with the plastic demand. While petroleum that makes up polystyrene is already inside earth, it doesn’t need to be grown but rather be extracted. This can also be a negative factor because petroleum can rapidly ran out because its supply cannot keep up with the great demand, since we can’t just grow it out. Also polystyrene has much more negative impact to the world and the people. It’s implication can obviously overcome the implication of polylactic acid. Although polylactic acid does have its negativity, that its degradation process isn’t completely refined and it still releases carbon dioxide, polylactic is still much more promising for future uses. It doesn’t release as much carbon dioxide like polystyrene does and the farming of corn makes up with this small release of carbon dioxide. The problems associated with polylactic acid can be addressed through investment and time. While the problems associated with polystyrene is too large to be addressed. It’s corresponds with too much obstructive implications. Polystyrene is associated with cancer, health risks and the destruction of our ozone layer – a very important component of earth. The risk of petroleum extraction and transportation is also very high as spills could occur, contaminating our land and oceans. Polylactic, although not completely environmental friendly yet, it is still much prefered for the environment and human health. Development of polylactic acid should be emphasized, so its negative implication can be eliminated.