Polymers are long chains made up of monomers. A monomer is a unit of a molecule which joins together with another monomer to make up a polymer. Synthetic polymers can be used to create types of plastics which are often more beneficial to the environment than traditional plastics sourced from petroleum (oil) which is a finite resource. Oil plastics have many negative effects on the environment, ocean, air and animal and human health. Plastic in the ocean can cause aquatic animals to choke and suffocate.

Extraction of oil can also cause pollution in the air, ocean and environment. What is polystyrene?Polystyrene is synthetic aromatic polymer thermoplastic. Eduard Simon created polystyrene in 1839. Polystyrene is made of the polymer “styrene”, with the IUPAC systematic name “ethenylbenzene”. Styrene has the molecular formula C?H?CH=CH?. Styrene is a small monomer which is made up of carbon- to carbon double bonds, making it a vinyl monomer.

(styrene monomer. Image from https://en.wikipedia.org/wiki/Styrene#/media/File:Styrene.svg)Polystyrene is a synthetic polymer made of a monomer styrene. The chemical formula of styrene is C?H?CH=CH?. The chemical formula of polystyrene is C?H??. The two forms of polystyrene are foam and solid.

Polystyrene is a transparent glass-like solid at room temperature. Colourants can be used to change the colour of polystyrene. Polystyrene can be molded into finely detailed shapes, hence why it is used for creating parts of vacuums and objects which require fine detail. It returns to a solid state when returned to room temperature. It can be used for many household, industrial, gardening and medical products. Being a thermoplastic, polystyrene can melt to a liquid (?) when heated above 100? and return to a solid state when cooled or in room temperature.

How is Polystyrene Produced?There are three stages or processes to making polystyrene.First, benzene is added to ethene to create ethylbenzene (C?H??). This is done by mixing benzene vapour (C?H?) and ethene (C?H?) together.

An acid catalyst is added. The conditions are 650 K (K is the chemical equilibrium constant) with 20 atmospheric pressure (atm). C?H? (g) + C?H? (g)? C?H?? (g)  (chemical equation of benzene vapour with ethene gas) Secondly, styrene (C?H?) is produced from ethylbenzene (C?H??). Ethylbenzene vapour is mixed in with added steam.

Iron(III) oxide (Fe?O?) is mixed with potassium oxide as a catalyst. The conditions are 850 K (Chemical equilibrium constant). Phenylethene is formed as a gas (C?H?), also known as the monomer styrene. Hydrogen gas (H?(g)) is also produced. Lastly, styrene (phenylethene) is polymerised into polystyrene with addition polymerisation. Radical polymerisation is the polymerisation technique used. This type of polymerisation is where radical indicators are used as catalysts. For predominant polymerisation (continual thermal mass polymerisation), only heat is used to initiate it.

Dibenzoyl peroxide is used as a catalyst to initiate suspension polymerisation. Usually, polystyrene (Poly phenylethene) is a transparent thermoplastic. However, to produce a product stronger than the brittle clear polystyrene, 5-10% dissolved poly(buta-1,3-diene) rubber can be added to phenylethene.

   Degradation of Polystyrene: Polystyrene is inert, which means it doesn’t react with acids or bases, posing an issue with it degrading in the natural environment. However, polystyrene degrades when exposed to hydrocarbons or particularly chlorinated substances. Polystyrene is a thermoplastic which means its solidification and melting is reversible, i.e, once turned into solid plastic it can be melted above 100? then molded into another shaped plastic. This means, polystyrene waste can be recycled when undergoing melting or excursion and be molded into a different shaped plastic and reused for a different purpose or product.

Firstly, polystyrene waste is segregated from other waste in the bins. Due to its size (especially polystyrene foam products), the waste is compacted and the size is decreased by up to 98%. Next, larger polystyrene products undergo shredding to shred it into smaller pieces. Lastly is the melting where the polystyrene is melted and recycled into a different product. This process is a positive side of using polystyrene plastics in products as it enables it to be reused into a different thing. On the other hand, polystyrene foam takes up a large amount of space making it high in cost to transport for reuse and melting. This means only a small fraction of polystyrene is remodeled and reused.

Another downside to polystyrene is that polystyrene which has been used for food cutlery, plates and food storage are rarely recycled to make new products as they need to be cleaned and consumers are more likely to use new polystyrene rather than recycled polystyrene.  A downside to the degradation of polystyrene is that polystyrene is an inert substance. This means that it doesn’t react with acids or bases well. As a result, polystyrene takes a very long time to biodegrade in the natural environment. Polystyrene is used profusely in the world for products which are frequently disposed of, resulting in a large amount of litter in the natural environment.

 What is Polylactic AcidPolylactic acid is a thermoplastic with the chemical formula C?H?O?n which is biodegradable. It can be made from sources such as corn starch, cassava roots, sugarcane and chips/starch. Lactic acid is chiral, meaning it can form different distinct forms. Polylactic acid is a polyester which means it undergoes esterification (condensation polymerisation). Polylactic acid can be used for plastic bottles, biodegradable medical products such as rods and screws, film and 3D printing. Since polylactic acid is known as a biopolymer, it is completely biodegradable and sourced from biomass.

It is a thermoplastic, meaning it can be heated to 150-160? and simply melt into a liquid to be remolded again.  How is Polylactic Acid MadeThere are two ways of making polylactic acid. PLA can be made directly through polymerisation, where L-lactic acid is polymerised to polylactic acid.

It can also be made by condensation polymerisation where L-lactic acid is esterified to lactide, then to polylactic acid. This method with two steps produces polylactic acid with larger molecular weight. The first step to making polylactic acid is corn to dextrose (also known as D-glucose).

The corn starch is separated from the other parts of the corn using the wet milling technique. The starch is heated and acids and enzymes are added which causes hydrolysis, a process where water moles are added to a molecule or substance. D-glucose/dextrose then undergoes glycolysis.

This is where 2 moles of pyruvate, ATP (energy) and NADH (Nicotinamide adenine dinucleotide) are made, as shown below on the diagrams.Polylactic acid is made through esterification (condensation). This is because it is a polyester. Firstly, L-lactic acid is condensed into lactide. The reaction between the alcohol and carboxylic acid produces an ester.  Water (H?0) is also formed. Heat increases for this reaction to take place.      Secondly, the purified lactide is polymerised into polylactic acid using ring-opening polymerisation.

 Ring opening polymerisation is a type of chain growth polymerisation. This occurs when cyclic monomers (circle-shaped) open their chain and react with the end of a polymer, resulting in a longer chain. A tin catalyst is used to polymerise purified L,L-lactide to produce PLLA with a higher molecular weight. This two-step procedure is used over the one-step polymerisation as it produces heavier polylactic acid which is more suitable for industrial products.How does Polylactic Acid Degrade?Polylactic acid degrades by hydrolysis of the ester bonds. Hydrolysis is the opposite of esterification.

In esterification (condensation), a water (H?O) molecule is removed from the monomers. In the esterification of L-lactic acid, H?O is removed from the carboxylic group (R-C=O-OH) to form an ester (R-C=O-OR’). Being able to undergo hydrolysis, polylactic acid can easily biodegrade in 3-6 months.  When exposed to the environment and water, hydrolysis can occur in polylactic acid products. This reverses the process of making PLA (as hydrolysis is the opposite of esterification). Eventually, water enters PLA, causing it to undergo hydrolysis and unzip the polymer into monomers L-lactic acid and lactide. Enzymes and acids also help towards the breakdown of polylactic acid and cause it to breakdown a lot faster. Carbon dioxide is an acid so polylactic acid in the natural environment can be broken down by water and carbon dioxide.

  Polylactic acid hydrolysis produces carboxyl and hydroxyl groups.Overall, the use of polystyrene and polylactic acid polymer thermoplastics can be considered “better” for the environment than regular petroleum sourced plastics. This is because traditional plastics are made from a non-renewable source and the extraction of oil (petroleum) can cause air pollution, ocean pollution and spills.

Polystyrene is very slow to biodegrade in the environment, and so this is a negative effect on the environment. Since polystyrene can be made in foam form, it is bulky and lightweight. This causes it to be blown into the ocean and around the environment, potentially causing danger to animals.

Polystyrene is also very inexpensive, making it a widely used plastic in many industries. This is also a downside since there is so much polystyrene litter- especially since it is thrown away after very little use.Polylactic acid is a lot more environmentally “friendly” than polystyrene. This is because it not only is made from renewable sources like corn starch, but it is very easy to biodegrade in compost environments and in the natural environment. One downside to polylactic acid is that it can actually take up to 1000 years to degrade in a landfill as there is little oxygen or water to assist degradation or hydrolysis (the chemical process which breaks down polylactic acid into its monomers lactide and L,L-lactic acid).

Another issue is that polylactic cannot be recycled with other things in recycling bins due to being sourced from organics such as corn starch.     

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