Polyethylene Chemistry and Molecular Structure

What are Plastics:       By: Fadi Alqouza

Plastics consist of chains made of molecules bonded together. These chains are classified as polymers. Thus, many plastics begin with poly, for example polyethylene, polystyrene, and polypropylene. Also, well many types of coatings, sealants and glues are composed of plastics as well.

What is Polyethylene?

Polyethylene is a light, versatile resin which consists of polymerisated ethylene. Polyethylene is a part of the polyolefin resins family. Polyethylene is the most used plastic in today’s modern globe, being produced into products ranging from clear food wrap and shopping bags to detergent bottles and automobile fuel tanks. Polyethylene is also slit or spun into synthetic fibres or modified in labs to possess elastic properties like of a rubber.

Chemical Composition and Molecular Structure:

Ethylene is a hydrocarbon with gaseous properties and possess a simple structure of C₂H₄. Ethylene is found naturally as a natural gas or can be distilled from petroleum. The molecules that compose Ethylene are essential to make two methylene units, CH₂. The methylene units are bonded together by a double bond between the carbon atoms, forming a structure of CH₂=CH₂.

Figure 1:

Figure 2:

Figure 1&2: The structure of methylene once broken for Ethene, is repeated thousands of times in a single molecule, to make polyethylene.

Ethylene with the structure of C₂H₄ is a highly stable molecule with two carbon atoms and a double bond. Polymerisation is a key factor is producing the polymer of polyethylene, as polymerisation involves the reaction monomer molecules in a chemical reaction to form a polymers chain/chains ore could result in three dimensional networks. Due to the influence of polymerisation catalysts the double bond can be broken, and the result is a single bond structure that can be used to bond a carbon atom in another ethylene molecule. Thus, the repeating of this process results in a polymeric molecule as shown in figure 1.

The simple structure of Ethylene (CH₂=CH₂), is repeated thousands of times in a single molecule, to make polyethylene as shown in figure 1. The polymeric molecule, in which hydrogen atoms are connected to a single carbon backbone, which can be produced in linear or branched forms. Branched versions are known as low-density polyethylene or linear versions that are known as high-density polyethylene.

How is Polyethylene produced?

Low density polyethylene:

Low density polyethylene is manufactured from gaseous ethylene under extremely high pressures, around 350 megapascals. Also, its synthesised under high temperatures, up to 350 °C with the aid of oxide catalysts This method produces a polymeric structure with both long and short branches. The low-density polyethylene has a melting point that is approximately 110 °C. This is due to the branches that prevent the polyethylene molecules from packing closely together hard and crystalline arrangements, however it make the molecules very flexible. The main usage of low-density polyethylene in today modern society are packaging film, trash, grocery bags, agricultural mulch, wire and cable insulation, squeeze bottles, toys, and housewares.

High-density polyethylene

High-density polyethylene is usually manufactured using Zousing Ziegler-Natta and metallocene catalysts or activated Chromium oxide accompanied with low pressure and temperature. Also due to the lack of branches in the structure of High-density polyethylene, allow the polymeric molecule chains to pack closely and forming a high crystalline material of high strength resulting in a High dense molecule. Due to the structure being packed closer, it gives it a melting point that is higher by 20 °C than low density polyethylene. Due to these features the High-density polyethylene can be synthesised into blow-moulded bottles for milk, household cleaners, blow-extruded grocery bags, construction film, agricultural mulch, injection-moulded pails, caps, appliance housings, and toys.

How is Polyethylene disposed of?

Figure 3: Showcases different types of polyethylene products and their recycling code.

In modern society, plastic production has domestically increased in recent years, increase by 325.5 million tons, also it is believed to double by 2036,

By 2014, Americans disposed about 33.6 million tons of polyethylene products, however only 3.192 million tons are recycled which amounts to 9.5% and 5.04 million was combusted to produce electricidal and heat energy which also amounts to 15%.

The rest of the disposed polyethylene ends up in landfills, the disposed polyethylene products takes 500 years to decompose, and threatens the environment as leaks pollutants the soil and water. Scientist estimate that 165 million tons of polyethylene products have already contaminated our oceans threatening the health and safety of marine life. They also estimate that an average of 8.8 million tons of polyethylene to enter our ocean each year. The products include polyethylene fibres no longer then 5 millimetres longs, which possess the risk of being ingested by marine wildlife.

Recycling of High and Low-Density Polyethylene 

High-density polyethylene is currently recycled at most recycling centres around the globe, as polyethylene has an easy structure to break down and recycle and is given code #2 as shown in figure 3.

Step 1: The plastic is cleaned and decontaminated, to remove any microbes that might present on the product.

Step 2: plastic polymers are separated by using sink-float separation, as high-density polyethylene has a density of 0.93 to 0.97 g/cm³. This density is lower than of PET’S which is 1.43-1.45 g/cm³ and has the recycling code of #1 as shown in figure 3.

Step 3: High-density polyethylene is then shredded and melted down to further refine the polymer. The plastic (polyethylene products)’[] is then cooled into pellets which can be used in manufacturing.

Recycling of Low-density Polyethylene:

Low-density polyethylene is currently

Step1: Personal at recycling centres, melt low density polyethylene products to eliminate any contaminations that might be present on the product and is given the code #4.

Step 2: Once the low-density polyethylene products have been melted, its later compressed into thin plastic sheets.

Step 3: Plastic manufacturers purchase the recycled low-density polyethylene and remelt into products that treat carpet and manufacturing.

Advantages and disadvantages of polyethylene.

Evaluation:

Economic cost:

Modern society demand for polyethylene is increasing each at 4%, by 2018 99.6 million metric tons was calculated to be produced scene it’s discovery, and a value of 164 billion dollars were spent. However due to the ever-increasing consumer needs and manufacturing activity Polyethylene will continue to be the most widely used plastic product in the global. Due to polyethylene’s versatility, easy processability, low cost and recyclability, its development will only increase due to the improvements on the polymerisation catalysts, thus it will lead to higher need, thus increasing it’s over cost. However due to its high decompose time many polyethylene products, like plastic bags have become subject to environmental regulations and bans.

Social cost:

The social cost of polyethylene is decreasing as more and more products that were made using metals and other plastics are becoming out dated and polyethylene is beginning used more due to its versatility, easy processability, low cost and recyclability, thus products were individuals would have to pay 1000 dollars decrease to 800 dollars or even less. However, that does not mean the actual cost of the polyethylene is going to decrease, as polyethene is becoming 2-5 cents more expansive each year, this is due to more products are need and the resources needed to manufacture polyethylene are becoming more and more limited.

Environmental impact:

The disposed polyethylene gets sent to landfills around the word, however the use of land fills is bad for the environmental, as the disposed polyethylene products takes 500 years to decompose. Further leaks do occur which threaten the environment even more as they pollutants the soil and water. Scientist estimate that 165 million tons of polyethylene products have already contaminated our oceans threatening the health and safety of marine life. They also estimate that an average of 8.8 million tons of polyethylene to enter our ocean each year. The products include polyethylene fibres no longer than 5 millimetres longs, which possess the risk of being ingested by marine wildlife. Also, the combustion of polyethylene to extract the crude oil and produce energy, also produces carbon dioxide which speeded up the rate of global warming.

Political implications:

Due to the manufacturing and disposal of polyethylene produces carbon dioxide which speeded up the rate of global warming. Also, most of its products are endangering our land and marine wildlife. Government officials are increasing the taxes paid of the production of these products to slow the rate that our planet is deteriorating, as of now most plastic bags are recyclable and cost 6-9 cents more per bag to produced, thus most large and low business are making environmental friendly bags with decompose easily and are reusable and have a strong properties to carry heavy objects.

Benefits to society:

Polyethylene products have a very smooth surface and is easy to clean and maintain, also well the products are light to transport and easy to install, also having a cost and this way cheaper than steel, fibreglass or concrete. This ensures the society to same both time and money. Due to its versatility, easy processability, low cost and recyclability, the products are ideal for every day items, also due nature, they can be moulded into anything needed. For example, blow-moulded bottles for milk, household cleaners, blow-extruded grocery bags, construction film, agricultural mulch, injection-moulded pails, caps, appliance housings, and toys, trash, agricultural mulch, wire and cable insulation and squeeze bottles.

Conclusions and Recommendations:

In my honest opinion, polyethylene is a very useful, versatile and highly effective polymer, which current modern society can’t function without. As polyethylene possess many properties, most plastic products are nowadays manufactured using it. Although Polyethylene is recycled and can be reused, it’s not an environmentally friendly product, as it can take up to 500 years to decompose in landfill sites and consists of crude oil, which is a non-renewable source of fuel, thus its production cost is increasing and oneday we might not be able to manufacture anymore. Also, the production and burning of polyethylene products, produce large amount of Carbon Dioxide, which contributes to global warming and the melting of the ice caps and the landfills leak and endanger our wild life. With this in consideration, I soul hearted believe that we should invited a more ecofriendly product that’s an alternative to polyethylene, which still achieves the same purposes and holds the same benefits to society.

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