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As we approach GCSE season, Year 11 Chemistry students have ended the course by taking a close look at the chemical processes that transform fuels into everyday materials.
We have all come to understand the impact of plastics on the natural world. From plastic bottles piling up in landfills to microplastics polluting our oceans, synthetic materials are becoming increasingly hazardous and unsustainable. However, it is an undeniable fact that the durability of plastics has revolutionised industries like medicine and technology, whilst also contributing to long-term pollution. Most plastics and synthetic materials originate from the same source as fuels: crude oil.
So, how do we transform crude oil into these everyday materials and with rising concerns over plastic waste, can chemists provide a more sustainable alternative? In order to answer this question, we must first examine the process of cracking and polymerisation.
Crude oil is a complex mixture of many different hydrocarbon compounds of different sizes. Hydrocarbons are compounds that contain hydrogen and carbon atoms only and they form over millions of years. Crude oil is the liquid form of hydrocarbons and it is formed when marine substances decay on the seabed. After millions of years, the accumulation of more layers and the application of larger amounts of pressure means that all other substances are removed and carbon becomes concentrated. Hydrocarbons can be divided into two main groups, or as we refer to them in Chemistry, homologous series: alkanes and alkenes. A homologous series is a group of molecular compounds that have the same general formula, and functional group, differ by CH2 and exhibit a trend in chemical and physical properties. Alkanes are a group of saturated hydrocarbons; the term saturated means that they only form a single carbon-carbon bond and that there are no double bonds present. They are colourless compounds that have a gradual change in their physical properties as the number of carbon atoms in the chain increases. Alkenes, on the other hand, contain a double carbon bond, which is shown as two lines between two of the carbon atoms (C=C). Whilst alkanes are mainly used as fuels, alkenes are the building blocks of the polymer industry.
However, because crude oil naturally contains more long-chain alkanes than alkenes the process of cracking is necessary. Cracking is the chemical process by which long-chain alkanes are heated to around 470-550°C to vaporise them. The vapours are then passed over a hot powdered catalyst (something that speeds up the rate of a chemical reaction without being used up) of aluminium oxide. This process breaks covalent bonds in the molecules as they come in contact with the surface of the catalyst, causing thermal decomposition reactions. The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes. The smaller chain alkenes are more useful. One of the key products of cracking is ethene, an alkene that serves as a starting material for many plastics. Without cracking the plastic industry would lack the raw materials needed for production.
Once alkenes are obtained from cracking, they undergo polymerisation to form different synthetic materials. Polymerisation is the process where smaller monomers join to form larger polymers. An example of a polymer is DNA! DNA is made of monomers (nucleotides) that form the longer chain polymer. There are two main types of polymerisation:
Addition Polymerisation: this is used to create plastics like poly(ethene), which is found in products like shopping bags and water bottles.
Condensation Polymerisation: this is the process that creates polyesters that are widely used in things like clothing and bottles.
Addition polymerisation simply links monomers together whilst condensation polymerisation involves the formation of an ester or amide link followed by the release of a small byproduct, usually water.
Addition polymers are formed by joining up of many monomers but can only occur in monomers that contain C=C bonds (unsaturated hydrocarbons or alkenes). The double bond is necessary as one of the bonds in each C=C bond will break to form a bond with the adjacent monomer with the final polymer being formed containing single bonds only. For example, poly(ethene), also known as polythene, is formed by the additional polymerisation of ethene monomers. Addition polymers can be engineered to have distinctive properties depending on their intended use. This is done by the selection of the monomer. For example, polythene is flexible, cheap, and electrically insulating which means in low densities it can be used in plastic bags and in high densities in plastic bottles. Whereas, polypropene which comes from the additional polymerisation of the hydrocarbon propene, is flexible and strong which makes it useful in food packaging.
Despite their many conveniences, polymers have a major drawback: they cannot break down easily. Due to the formation of polymers by the joining up of many small molecules with strong covalent bonds, this makes them unreactive and chemically inert so they don’t easily biodegrade. Most waste polymers are disposed of in landfill sites but this takes up valuable land and as they are non-biodegradable, decomposers cannot naturally break them down, which causes sites to quickly fill up. Alternatively, some polymers are incinerated. However, as polymers release lots of heat energy when they burn and produce carbon dioxide this only exacerbates climate change as CO2 is a greenhouse gas.
Currently, chemists are working on the development of biodegradable polymers and biological-based alternatives that decompose faster whilst also maintaining the durability that we value in most synthetic materials today. For example, Mexican researcher Sandra Pascoe Ortiz and her team have been leading a promising innovation in the development of biodegradable plastics by using materials derived from nopal cactus. This process involves extracting the thick liquid cactus pads, that contain natural polymers which are then combined with organic additives and dried to produce thin, malleable sheets that can be moulded into various shapes. Unlike polymers that come from crude oil, this cactus-based alternative is able to break down naturally within a few weeks without leaving behind harmful microplastics that end up in our oceans and potentially our food too. The overarching goal is to find a way to completely or partially replace oil-based plastics with sustainable alternatives and to make traditional polymers more eco-friendly whilst balancing durability and functionality.
Rola Al-Hassani - Year 11 Science Prefect
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