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Introduction
At first glance, it seems that polyethylene is a simple polymer structure. But the analysis of polyethylene is a complicated matter because of the wide range of types and different manufacturing processes associated with the said material. Experts are in agreement that there is no precise method that can be used to classify polyethylene. However, for the sake of simplicity, polyethylene can be classed as low, medium, or high density. The conventional classification system, therefore, uses the manufacturing process to categorize different variations of polyethylene correctly. For example, there are two ways to produce polyethylene, and these are called high pressure and low pressure operations. The subsequent byproduct of the said operations can be grouped into three and listed as follows:
- Low-density polyethylene ( LDPE );
- High-density polyethylene ( HDPE );
- Linear low-density polyethylene ( LLDPE ).
The popularity of polyethylene is rooted in the fact that this polymer is cheap, flexible, durable, and chemically resistant It is important to take a closer look at the manufacturing process in order to develop innovative and sustainable ways to manufacture polyethylene.
It is interesting to note that polyethylene production started late as compared to other industrial materials. LDPE is used to make films and packaging materials. On the other hand, HDPE is often used in the manufacture of containers, plumbing, and automotive fittings. HDPE is associated with these types of products because of higher stiffness compared with LDPE and LLDPE.
It must be pointed out that extrusion coating of paper and paperboard is an application segment that proves to be a growth area for LDPE. The explanation for LDPEs popularity is based on innovations in packaging technology for paperboard coating and paper and foil composite structures. Another reason is that LDPE is easier to process than LLDPE. At the same time LDPE has good strength and clarity.
Before going any further there is a need to clarify the difference between low-density and high-density polyethylene.
For example low-density polyethylene is ideal for products with demanding performance requirements. A good example would be stretch wrap, food packaging, and hygiene and medical applications. There is a high demand for packaging products that came from low-density polyethylene because it offers flexibility and flex crack resistance, particularly when it comes to liquids that move freely within a package.
However, if there is a need for a tougher packaging material, it can be argued that HDPE is more suited for the task. There are products that require puncture resistance. Others require tear resistance. In another application of for the polyethylene polymer, manufacturers developed the high-density polyethylene. The benefits of HDPE can be summed up in the following statement: an excellent combination of stiffness and environmental stress crack resistance. The high-molecular weight of an HDPE polymer provided the toughness required in pipe applications.
Background
Due to low price, processing ease and a variety of products that can be derived from it, polyethylene has become the most used plastic. Aside from chemical resistance polyethylene also exhibited another beneficial property and that is electrical resistance. This property led to the wide use of polyethylene in wire coatings and dielectrics. Polyethylene can be easily processed using all thermoplastic methods. Consider the following observation Blow molded containers are seen in every supermarket & these containers replace heavier ones made of glass and metal. Therefore, aside from the cost and ease of use, polyethylene also offers lightweight materials and makes it easier to transport goods from source of origin to supermarkets or stores.
The commercial production of polyethylene started late. There were failed experiments in the 1920s that did not produce practical applications for the said material. However, there was a chance observation that was made in 1933 when an ICI research team discovered that traces of a waxy polymer could be formed when ethylene and benzaldehyde were subjected to high temperatures.
The most important implication of the said ICI research was the realization that the product exhibited partial crystallinity and as a result the measurement of the density of the byproduct was used to classify it into different types of polyethylene. The said research team also discovered that the polymerization of the material at high temperatures resulted in polymer chains that were branched and resulted in densities of 915-925 kg/m3. But twenty years later another group of researcher discovered that the use of a different catalyst and polymerization at low pressure and temperature produced HDPE.
The utilization of the Phillips and Ziegler catalysts produced HDPE products that were tough and chemically resistant. But the said industrial material is more expensive to produce. At the same time, an HDPE material is opaque and not transparent and therefore it is not an ideal packaging material.
In 1978 Union Carbide created the Unipol process that enabled the company to produce linear low-density polyethylene or LLDPE. The Unipol process is cheaper. But more importantly it enabled manufacturing firms to manipulate the molecular structure of the linear product. For example, manufacturing companies could produce tougher films that are stronger and yet flexible.
The functional value of polyethylene can be appreciated through the study of its chemical structure. It is an example of a polymer molecule. It must be pointed out that a polymer molecule is composed of a large number of repeating units joined together by covalent bonds. For instance, all plastics and rubbers are polymers. A better way to appreciate the importance of polyethylene when in the context of industrial use is to point out that a polyethylene like HDPE has the same chemical composition of a paraffin wax. However, the main difference is in their molecular size. Aside from the major difference when it comes to molecular size, polyethylene molecules like HDPE are much longer than those of the paraffin wax. Furthermore, the high strength of polymers comes from the long-chain nature of polymer molecules.
Table 1. Main Applications of Polyethylene.
When it comes to LDPE, the short-chain branches suppress crystallinity and explain why the density in the solid phase is low. The short-chain branches also explain why the material is highly flexible. As a result it is relatively easy to process this material in its molten state. However, the plastic material derived from this process is relatively soft and weak.
When it comes to HDPE, the process to produce this type of linear polymer was discovered in 1950. HDPE is produced through the use of reactors at pressures that are lower than those used for the production of LDPE. The said commercial process produced linear polyethylene with a higher crystallinity in the solid phase. In addition, the non-branching HDPE can form a dense mass. As a result the byproduct is harder and stronger.
When it comes to the production of LLDPE the plastics contain little chain branching. LLDPE is a tough material that possessed good clarity, high elongation, high hot tack and a low melting point. Thus, the plastics exhibit good flex life, low warpage, and improved stress-crack resistance. For example, films for ice, trash, garment and produce bags are durable as well as puncture and tear resistant.
Table 2. Advantages and Disadvantages of Polyethylene.
It can be argued that the advantages of using polyethylene far outweigh the disadvantages of using the same. There is a high demand for industrial products that can protect and secure important items and foodstuff. The contribution of polyethylene can be seen in cost-efficiency, use of lightweight materials and durability.
But, there is one aspect of the polyethylene manufacturing process that must be highlighted in order to address sustainability issues. It must be made clear that in order to produce polyethylene, manufacturing firms must use ethylene as a feedstock. The conventional method of production necessitates the use of natural gas and crude oil. These are examples of fossil fuels and are non-renewable sources of energy and organic compounds.
The sustainability issue that surrounds polyethylene production is linked to the fact that someday all fossil fuel reserves will be depleted. Therefore, the inability to acquire this particular feedstock means that the world will stop producing plastics. It is therefore important to investigate current technological breakthrough that addressed this particular problem.
State-of-the-Art
Companies that manufacture polyethylene rely on one important ingredient and that is ethylene. The application of specific pressure and temperature to ethylene will result in the production of polyethylene. But there is one major problem when viewed in the context of sustainability and environmental impact. The feedstock needed to produce ethylene is natural gas or crude oil. These are examples of fossil fuels.
There is no need to elaborate the fact that natural gas and crude oil are problematic source of energy and organic compounds because it will be depleted in the future. This particular resource is finite and therefore it is prudent to search for alternative feedstock for the production of ethylene.
The alternative source must be renewable. It must come from a crop produced by plants or a substance produced by living organisms. At the same time the feedstock that will be used to produce ethylene must be predictable and cost-effective. One of the alternative solutions suggested by experts in the field of polyethylene is to use ethanol instead of natural gas or crude oil.
Ethanol is a form of natural alcohol and it is derived from the fermentation of organic sources of energy such as grains and fruits. In recent years the production of ethanol was made possible through the use of sugar beets, sugarcane, sweet sorghum, corn, and wheat. But there is one major problem when it comes to the use of grains and fruits and it is the need for arable land that must be converted to farmlands that in turn must be planted with a single crop.
It must be pointed out that thousands of acres of farms planted with a single crop are capital and labour intensive. As a consequence scientists had to look for more radical solutions to the given problem. They turned their attention to genetically modified microorganisms also known as cyanobacteria.
The genetically modified cyanobacteria will be cultured in a bioreactor to produce a large mass of algae. The end result of the process is the production of ethanol. Therefore, ethanol as a form of energy source can be used as a feedstock to produce ethylene and afterwards ethylene will be used to produce polyethylene.
The first thing that has to be established is the viability of the said technology. Thus, it must be made clear that there is already a genetically modified microorganism that can produce ethanol. The most critical factor is the use of a genetically modified cyanobacteria strain. It is technically labelled as Synechocystis sp. PCC 6803 strain. This particular strain of genetically modified organism can photoautotrophically convert carbon dioxide to bioethanol.
There is a need to clarify the fact that this particular strain enables the direct synthesis of ethanol. Nevertheless, a word of caution must be thrown out because the synthesis of ethanol is not guaranteed. It must be made clear that synthesis of ethanol does not occur naturally. Manufacturing companies that will utilize this technology must learn to manipulate cyanobacteria. The correct conditions must be met in order to create the optimum algal growth that will lead to the production of ethanol.
Scientists discovered that microalgae placed in a dark environment could survive even without light because the said microorganism utilized the glycogen stored in their bodies. In these conditions the microorganism produces no ethanol. But a different effect can be observed when cyanobacteria are placed in a dark and anaerobic environment. Due to the absence of oxygen the oxidative reaction of starch to carbon dioxide does not proceed. At the end, the culture can produce hydrogen gas, carbon dioxide, lactic acid, formic acid, acetic acid, ethanol and other products. The behavior of microalgae under the said conditions was used as the basis for the genetic modification of cyanobacteria so that it can directly synthesize ethanol.
Manufacturing companies will not immediately embrace this technology until their respective corporate leaders are convinced that the new system is practical and cost-efficient. The theoretical framework outlined earlier is not enough to persuade these corporate leaders to invest in this new technology. They will not invest huge sums of money in order to construct a new facility that utilizes a radically different process without assurance that it is already a proven system. The new system must guarantee the delivery of ethanol on a large-scale basis. It is therefore important to point out that in 2012 a company called Algenol was able to prove that cyanobacteria can directly synthesize ethanol.
Algenol is a state-of-the-art industrial biotechnology company located in Florida, USA. The said research firm boasts of laboratories in Florida as well as in Berlin, Germany. Algenol made the assertion that their patented system was documented to have produced 6000 gallons of ethanol per acre of microalgae. In order to fully understand the importance of this claim it has to be highlighted that an acre of corn can only produce 400 gallons of ethanol. In the same manner an acre of sugarcane can only produce 1000 gallons of ethanol.
The next step is to verify if there is a way to convert ethanol to ethylene. Without ethylene there can be no production of polyethylene. According to experts in this field, the process of conversion calls for the dehydration of ethanol. Another advantage of this process is cost-efficiency because the conversion of ethylene from ethanol is simpler. For example, the end product that can be seen in the bioreactor is water and ethylene, some unreacted ethanol and trace amounts of other substances. Thus, there is no need for distillation equipment and therefore lower cost for the company.
The patented system can be characterized as an efficient system because it uses renewable materials. For instance, the direct synthesis of ethanol requires microorganisms, water, carbon dioxide and sunlight. Furthermore, the sustainability of the system was established when it was proven that the microalgae can replicate without the need to purchase new cultivars. In other words, once the manufacturing companies purchased the technology from companies like Algenol, they will be supplied with cultivars from where microorganisms grow and replicate. The replication process continues as longs as there is enough food and other critical ingredients needed by the cyanobacteria to synthesize ethanol.
Algenol also made the assertion that the 6000 gallons of ethanol per acre is within production range. The basis for this claim can be see in the fact that genetically modified cyanobacteria strain is a fast growing photosynthetic prokaryote with high rates of photoconversion of carbon dioxide into photosynthate and biomass. In other words there is evidence to show that ordinary cyanobacteria can produce higher yields of ethanol compared to feedstock like corn and sugarcane.
Therefore, genetically altered microorganisms are expected to produce a much higher yield. Even if biofuel companies used unenhanced microalgae the expected bioethanol yield is already higher than agricultural feedstock like corn and sugarcane. Thus, the genetically modified cyanobacteria strain is expected to generate more.
There is another reason why genetically modified cyanobacteria can be a reliable tool for the direct synthesis of ethanol. According to scientists, it has the capacity for stable genetic enhancement and availability of molecular tools. Thus, genetic engineers can further improved the performance of the said microorganism in order to produce better yields.
Another evidence that Algenol was able to develop a reliable system was the discovery that cyanobacteria can thrive in a defined inorganic medium with no organic carbon source required. Thus, manufacturing companies will not have to spend an additional amount with regards to maintaining the cultivar. In other words the system can thrive with minimal input from the company. For instance, Algenol said that there is no need for materials aside from water, sunlight and carbon dioxide. This system can be considered sustainable because all the needed ingredients are readily available and renewable.
There are other factors to consider, for instance, manufacturing companies will be required to purchase the technology from Algenol. At the same time these manufacturing companies are required to construct photobioreactors because it only through an enclosed and anaerobic environment that a cyanobacteria can be expected to synthesize ethanol.
Manufacturing companies should also take heed that there are at least three key aspects that must be considered prior to the construction of photobioreactors. There is a need for ample space to construct the bioreactor, there must be sunlight in the said area and there must be ready access to a water source.
Land is indeed important but there is no need for hundreds of acres of land to initiate this project. In fact, the bioreactor can be constructed in such a way that the microalgae mass is equivalent to several acres of cultivars. The location of the bioreactor must be established first because planners will be able to determine the type of water source that can be supplied to the bioreactor. There is a specific strain of cyanobacteria that can thrive on saltwater or freshwater.
Discussion
The proposed alternative solution will create a sustainable system that will insure the continuous production of polyethylene even after fossil fuels like natural gas and crude oil had been depleted. If one will consider the cost of fossil fuel as well as the sustainability of the process it is easy conclude that there is no other way other than to embrace this new technology. There are so many advantages to the use of renewable sources of energy and organic compounds that makes the cutting-edge technology attractive for investors. However, there are still various obstacles that can prevent the use of the technology espoused by Algenol.
The first problem is that the technological breakthrough claimed by Algenol was only publicized in 2012. In other words there are many businessmen that are not yet aware of the tremendous possibilities offered by the company. At the same time there is no successful model that manufacturing companies can emulate. It has to be made clear that Algenol is a research company not a manufacturing firm that produces polyethylene through the manipulation of ethylene. Algenol has the motivation to exaggerate their claims in order to entice companies to purchase their technology.
Another problem is that manufacturing companies will have to change their thinking process when it comes to a new way of producing polyethylene. There will be many changes that have to be made in order to initiate the process of using cyanobacteria instead of fossil fuels. One can just imagine the drastic changes in a system that relied on high temperature and high pressure to produce ethylene and then transition it to a system that uses microorganisms.
At the same time there is the problem of additional investment. Manufacturing companies will have to invest in new technology. A great deal of money will have to be invested in order to acquire the said technology. Thus, manufacturing companies must carefully evaluate the benefits and risk of adopting the Algenol technology. Furthermore, there is a need to invest in a company-sponsored study in order to verify the claims made by Algenol.
There are obstacles and challenges along the way. But an overview of the Algenol technology can easily convinced businessmen that it is practical to think about long-term growth. The depletion of fossil fuels will end the current manufacturing processes enjoyed by polyethylene production companies. There is therefore the need to think ahead. There are problems and risks involved, however, a sustainable mindset will help convince investors that there is no better way forward other than to use sustainable sources of energy and organic compounds.
The main challenge is the need to prove that it is a profitable system. At this point there is a high demand for plastic products. Therefore, companies that produce polyethylene are expected to make money. There is a need to sustain the production processes 365 days a year. These companies cannot afford to slow down. But the shift to a new technology requires time to think and analyse the new system. The adoption of a new system may require the disruption of business operations. Thus, it is a challenge for corporate leaders to lose money in the first year of operation and consider the long-term impact of renewable systems.
Conclusion
There are different types of polyethylene. But the most popular types are HDPE, LDPE and LLDPE. The type of plastic produced through the utilization of polyethylene results in products that can protect and secure items and foodstuff. Chemical resistance and impact strength are just some of the advantages of polyethylene. Furthermore, the manipulation of the polymerization process enabled scientist to produce different types of plastics based on their densities. Thus, there are plastics that are durable but not flexible. There are plastics that are flexible and yet puncture resistant. It is therefore easy to understand why there is a growing demand for plastics. If compared to other materials like wood, metal and glass, the advantages of plastics can be seen in its low cost, weight, and durability.
One can just imagine the added cost of transporting products that are weighed down by heavy packaging material. It is also important to point out that polyethylene can be produced in mass quantities. In the case of glass and metal the time needed to process this type of packaging material takes much longer. It is therefore important to sustain this way of life. It is difficult to imagine a supermarket or store without plastics. Food will not be protected from various external forces if there is no plastic that can be used as a packaging material. In the same manner electrical appliances and other equipment can be damaged without the use of plastic components.
The use of plastics derived from polyethylene provides a way to increase the profitability of a business enterprise. Consider for instance the ability to transport clothes and bags with the assurance that it will reach its intended recipient without incurring damage. In the past businessmen used cloth, wood crates or bottles to transport liquids, foodstuffs and various items. There is a need to maintain the integrity of the said products. In other words companies are compelled to invest in packaging materials and other mechanisms to ensure the safe and reliable delivery of their finished products. Consider also for instance the safety issues surrounding electrical devices. Without the availability of polyethylene there is no cheap and reliable way to coat wires in order to protect the people that will use electrical equipment. Thus, the convenience of polyethylene-based products must be highlighted in order to understand the implication if polyethylene manufacturing firms can no longer produce plastics.
It is therefore critical to develop a sustainable system. But at the heart of the polyethylene production process is the feedstock taken from fossil fuel such as natural gas and crude oil. There will come a time when these sources of energy and organic compounds will be depleted. Thus, it is important to consider the alternative solution proposed by companies like Algenol. Manufacturing companies must invest in a company-sponsored study in order to determine the viability of the said alternative solution. The independent study will provide answers to questions posed by business leaders. The independent study will also determine if the new technology can be adopted successfully into the present system.
References
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Algenol Biofuels. Harnessing the Sun to Fuel the World. Algenol Research. Chung, Chan. Extrusion of Polymers: Theory and Practice.
OH: Hanser Gardner Publications, 2000. Dealy, John and Ronald Larson. Structure and Rheology of Molten Polymers.
OH: Hanser Gardner Publications, 2006. Halford, N. (2011). Energy crops. Cambridge: Cambridge University Press.
Khanal, Samir. Bioenergy and Biofuel from Biowastes and Biomass. Reston, VA: American, Society of Civil Engineers, 2010.
Kjellin, Mikael. Surfactants from Renewable Resources. New Jersey: John Wiley & Sons, 2010.
Richardson, Terry and Eric Lokensgard, Industrial Plastics: Theory and Applications. New York: Delmar Learning, 2004.
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Soroka, Walter. Illustrated Glossary of Packaging Terminology. IL: Institute of Packaging Professionals, 2008.
Vasile, Cornelia and Mihaela Pascu. Practical Guide to Polyethylene. UK: Rapra Technology Ltd., 2010.
Wang, Lawrence. Environmental Bioengineering. New York: Humana, 2010.
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