Overview Of The Polylactic Acid Thermoplastic
There are several conditions for the synthesis of PLA. First of all the process requires a pH range of 5.4 to 6.4, a temperature range of 38℃ to 42℃ and lastly a low oxygen concentration. Whereas for the production of Nylon 6,6 the reaction should be carried out in a vacuum, otherwise the reaction will be driven in the reverse direction according to Le Chatelier’s principle. The principle states that “if a constraint (such as the change in pressure, temperature, or concentration of a reactant) is applied to a system in equilibrium, the equilibrium will shift so as to tend to counteract the effect of the constant.” Hexanedioic acid is also flammable, so a dust collector and ventilation are used to keep levels below explosive. The biggest advantage of the bioplastic is it’s sustainable process. For example producing 500,000 tonnes of PLA requires less than 0.5% of the annual US corn crop. Since corn is a cheap dextrose source, the current feedstock supply is more than sufficient to meet foreseeable demand.
Furthermore, there are many other alternatives for starch or sugar supply. As fermentation techniques improve, and if PLA production extends to other geographical locations, it is quite likely that other materials such as grass and even biomass could be used; meaning that there is no need to be reliant on food crops. According to NatureWorks “PLA takes advantage of a biological system to do chemistry that traditional chemical techniques cannot.” In contrast conventional synthetic polymers such as Nylon 6,6 rely on reserves of oil and gas for their monomers. The reserves of fossil fuel take millions of years to regenerate and are a declining resource. Currently scientists are stating 2052 as the expected date for when oil will be exhausted. PLA is also “carbon neutral” meaning that it comes from renewable, carbon-absorbing plants; this is another way to reduce greenhouse gas emissions. No toxic fumes will be emitted when PLA is incinerated. However, PLA does come with a few disadvantages.
PLA might be biodegradable, however, it occurs very slowly. The bioplastic will break into its constituent parts (CO2 & H2O) within three months in a controlled composting environment. This is in an industrial composting facility that needs to be heated to 140℉ and be fed a steady diet of digestive microbes. If PLA was to be placed in a normal compost bin or landfill it could take anywhere from 100-1000 years to decompose. This is because compost bins and landfills are packed so tightly that no light and very little oxygen are available to assist in the process. However, it is important to note that PLA is of different origin than regular plastic so it must be kept separate when recycling, preventing the contamination of the recycling stream. The plant-based origin of PLA means that it has to head to a composting facility instead of a recycling facility. The problem with this is that industrial grade composting facilities are quite uncommon around the world. An example of this is that in the USA there are only 113 industrial-grade composting facilities while there are 9800 municipal recycling plants across the country.
Polyesters (PLA) need to be hydrolysed (before biodegradation can occur) in a reaction with dilute acids or alkalis, then the soluble oligomers formed are metabolised by cells. Polyesters are readily attacked by alkalis but much more slowly by dilute acids. Whereas, in polyamides they are attacked by strong acids and more resistant to alkalis. Hydrolysis by water is very slow in both polyamides and polyesters. To break an ester linkage, a dilute alkali is needed. Therefore, polyesters (PLA) take part in acid hydrolysis while polyamides (Nylon 6,6) take part in alkaline hydrolysis. The polyester must be hydrolysed at 58℃ to reduce the molecular weight before biodegradation can start. Nylon 6,6 is also not biodegradable however it is recyclable at any stage of production. There are two processing methods for recycling; either chemically or mechanically. In the mechanical process the polymer is cleaned and then pelletized for further use. The chemical process is widely used and involves the polymer being depolymerised and broken down into its monomer components using hydrolysis. Polyamides are readily attacked by strong acids, but are much more resistant to alkaline hydrolysis. Hydrolysis is faster at higher temperatures. Technically, hydrolysis is a reaction with water, however, for polyamides (and polyesters), hydrolysis by water alone is too slow. Once amide linkages and the long chains are broken and you eventually end up with the original monomers hexanedioic acid and 1,6-diaminohexane.
As useful as the polymer is, there are many negative environmental impacts associated with Nylon 6,6. Hexanedioic acid (Adipic acid) is produced from KA mixed oil (cyclohexanol and cyclohexanone.) The process of creating the acid is detrimental to the environment as a side product of the reaction is Nitric Oxide. Nitric Oxide can be oxidised in the atmosphere to form Nitrogen Dioxide (N2O.) N2O is a powerful greenhouse gas, when high levels of N2O are present it can cause damage to the human respiratory tract and increase a person’s vulnerability to asthma. High levels of N2O are not only harmful to us, but to the environment as well. High levels are damaging to vegetation, as it damages foliage, decreases growth and reduced crop yields. However, it can be carefully removed by thermal or catalytic treatment units to control the nitrogen oxide levels. The net greenhouse emission of Nylon 6,6 is 7.9kg CO2 per kg polymer, while only PLA releases -0.7kg of CO2. The PLA value is negative as carbon dioxide is absorbed from air when corn is grown. Another environmental hazard associated with the production involves a large scale leak of feed materials or process fluid. If this were to ever to occur it was would be incredibly toxic to the environment, disrupting ecological systems and increasing pollution. Nylon 6,6 is not suited to natural dyes and lowest impact chemical dyes, meaning that the process of colouring the fiber also creates significant water pollution.
As well as contributing to water pollution, the production of nylon 6,6 uses very large amounts of water for cooling the fibres. “With more nylon being produced in countries with weaker environmental protections in place, this makes nylon a significant contributor to water pollution and thus water insecurity in the developing world” according to an article. Waste water should be treated by a third party who specialises in the removal of Hexamethylenediamine. Hexanedioic acid also becomes corrosive when it is mixed with water. Two of the largest sources of microplastic pollution in the ocean are nylon fishing nets and synthetic textile fibers from clothing. Nylon fishing nets are one the main reasons why hundreds and thousands of marine animals are harmed by plastic pollution. The nets can travel long distances from their origin points and can remain in the ocean for long after they have been discarded. This results in the entrapment and death of marine mammals, sea birds and fish. An estimated 640,000 tons of fishing gear is left in the ocean, with Nylon fishing nets being the largest contributor. In 2013 the University College Dublin found that 85% of the microfibres found on the shoreline were from synthetic fibres. Synthetic fibres can be placed into two categories; those made from cellulose of plants (PLA) and those that are petroleum derived (Nylon 6,6.) Several studies have found that fish consume these microfibres. When fish eat the fibres, the plastic fills their stomachs and gets stuck. This gives the fish the illusion that they are full, even though it is only the plastic that fills their stomach and they eventually starve to death. This occurs as both Polyesters and Polyamides do not break down in water. The versatility of Nylon 6,6 makes it one of the most widely used engineering thermoplastics. Nylon 6,6 is popular in every major market that uses thermoplastic materials.
The thermoplastic is the best candidate for metal replacement applications due to its balance of strength, ductility and heat resistance. PLA has similar properties aside from heat resistance. PLA is a thermoplastic that melts with ease, which makes it a material that is easy to work with. This means that is quite commonly used for 3D printing, food packaging such as product overwrap, bottled water, and lamination film. However, a material made from PLA may show signs of deformation or getting soft on a hot day. This makes it unsuitable for high temperature applications such as containers. In conclusion, both Nylon 6,6 and PLA are both advantageous thermoplastics in their own way.
Nylon is noted for its physical properties such as ductility, heat resistance, and exceptional strength whereas, PLA is known for its environmentally friendly production process. It is difficult to make an inference on whether Nylon 6,6 or PLA is the better thermoplastic. From an environmental and economic standpoint PLA is the more efficient option, as the production process is renewable and does not depend on dwindling resources such as fossil fuels. However, from a manufacturer’s viewpoint Nylon 6,6 is the better thermoplastic due to its superior physical properties when compared with PLA.
Nevertheless in a time where global warming is occurring and greenhouse gas emissions are at an all-time high is it important place significance on the environmental impacts associated with the production processes of both the materials. Nylon 6,6 is a traditional thermoplastic, with a fairly simple process that uses fossil fuel resources. Fossil fuels are one of the largest contributors to global warming, and with more reserves being used, the substance is soon to run out as it is not renewable. On the other hand PLA is derived from renewables resources such as corn, cassava roots and sugar cane. This means that the production of PLA is sustainable and will not cause further damage to the environment. Therefore, it is difficult to state which is the better thermoplastic. It depends on whether you value the environment and the sustainability of resources or the quality of the material more.
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