Introduction
Approximately 400 million tonnes of plastic is produced globally per annum . Plastics have a significant role in the lives of human beings, from its uses in producing automobiles to household goods to packaging. Plastics, generically known as polymers, are macromolecules that consist of repeating sub-units known as monomers. Conventional plastics, such as polyethylene (usually used for shopping bags and milk containers), are often made from fossil fuels, a non-renewable resource. This is an unsustainable method of production of plastics, one which uses a high quantum of energy and generates an even higher quantity of greenhouse gases, such as CO2 and CH4, gases that are considered to be a few of the causes of global environmental challenges. An alternative to conventional plastics are bioplastics – plastics that are derived from renewable resources that are potentially better for the environment due to their lower carbon footprint. While not commonly used in modern industry yet, bioplastics are not new.
A bioplastic is a polymer derived from organic biomass sources, such as sugar cane and corn starch. In comparison, a biodegradable plastic is a bioplastic that always decomposes into carbon dioxide, water and methane - in the presence of bacteria and under aerobic and anaerobic conditions. Aerobic biodegradation follows the chemical reaction:
C_POLYMER+O_2→CO_2+H_2 O+C_RESIDUE+C_BIOMASS
Whilst anaerobic biodegradation follows another similar reaction:
C_POLYMER→CO_2+CH_4+H_2 O+C_RESIDUE+C_BIOMASS
Many biodegradable plastics are usually aliphatic polyesters, composed of straight chain hydrocarbons. Most plastics decompose within a certain amount of time, from a few years to hundreds of years, however, for practicality, a degradable plastic is one which degrades in a shorter period of time - within a few weeks or months. Degradation is influenced by quite a few factors: the availability of oxygen, the amount of water, the temperature, and the chemistry of the soil, both organic and inorganic. Most degradable biopolymers further undergo biodegradation, where enzymes present within microorganisms are necessary to carry out the biochemical processes required to break down the polymer. Microbial degradation consists of two main processes: depolymerisation, and mineralisation. Extracellular enzymes and the properties of the soil (presence of macroorganisms, light, heat, and water) are responsible for the depolymerisation step, as it occurs outside of the microbe due to the size and insoluble nature of a polymer chain. Once smaller particles are formed, mineralisation of the monomers occurs within the microbe, producing adenosine triphosphate (ATP) within the organism, as well as gases such as CO2, CH4, N2 and H2, which are released into the environment.
Degradation of plastics is a physiochemical process, one which is induced by heat, light, moisture, and biochemical reactions. This investigation tested five different plastics to determine the Young’s Modulus of each plastic in a store-bought compost bin over a period of time of six months. According to the International Standards Organization, degradation is an irreversible process leading to significant alteration in the chemical or physical structure of a material. Furthermore, Young’s Modulus (measured in Nmm-2) is a mechanical property that indicates the stiffness of a material. It is measured by the formula:
Young^' s Modulus (E)=(Stress (σ))/(Strain (ɛ)).
For any degradation of a hydrocarbon, the chemical structure of a polymer naturally breaks down into a lower molecular form and ultimately into its monomers and other by-products (mainly CO2 and H2O). This chemical breakdown of the crystallinity of a polymer results in the polymer’s strain value to decrease - which increases its Young’s Modulus and so it is an effective measure of the degradability of a material.
I decided to carry out this research because recently I participated in the reef clean of a small island in Thailand. In one day, approximately two tonnes of plastic waste were collected. Most, if not all, of the plastic was packaging made from conventional plastics – plastics that would take hundreds of years to degrade naturally. The use of plastics is evidently very important to daily human life, and so I wanted to explore sustainable alternatives. This introduced me to the concept of biodegradation. Because of the possible correspondence between biodegradation and Young’s Modulus, this investigation measured the Young’s Modulus of plastics before and after degradation in a home compost bin. Three categories of degradable and non-degradable plastics were chosen for the test:
- Bioplastic and Degradable
- Non-renewable but Degradable
- Non-renewable and Non-degradable
