It is a common assumption that plastic does not degrade. But is this true? Does plastic really not degrade? To answer these questions, let us first understand where plastic is derived from and how it is made.
Plastic is derived from various sources, such as crude oil, natural gas, coal, plants, and animals. It is then converted into synthetic or semi-synthetic materials, such as plastic resins. These raw materials are heated, melted, and combined to form a polymer, which is then cooled and shaped into a variety of plastic products.
The majority of plastic derived today is from petroleum, which is the byproduct of a few million years of unknown chemistry. So the question is, why does petroleum-derived plastic not biodegrade if it is made of a biomaterial?
Well, during the production process, petroleum is converted into a new substance (Polyolefins), which the organisms that ordinarily decompose organic matter are unable to identify, making it very difficult to degrade naturally.
Scientifically speaking, propylene, a chemical derivative of petroleum, is the source of most plastics. When heated in the presence of a catalyst, individual chemical components of propylene, known as monomer, binds together by forming very long carbon-carbon bonds. Polyolefins (long chains of monomers) are formed as a result. These chains are new for organisms, and breaking them becomes very hard for them.
What is plastic degradation?
Plastic degradation is the breaking down of plastic polymers by its interaction with air, solar radiation, heat, and physical abrasion. This results in the loss of strength and toughness of the plastic and can cause it to break down and crumble over time.
In most cases, when plastics are released into the environment, they do not naturally degrade. This is not surprising given that many polymers are extremely popular and widely used because of their incredibly high stability and durability. Plastics can break down in the environment through four different processes: photodegradation, thermo-oxidative degradation, hydrolytic degradation, and biodegradation by microbes. In general, thermo-oxidative degradation follows photodegradation as the first stage of plastic’s natural degradation. The activation energy necessary to start the incorporation of oxygen atoms into the polymer is provided by ultraviolet radiation (UV rays) from the sun. When the polymer chains are sufficiently tiny in size to be metabolised by microbes, the plastic becomes brittle and shatters into smaller pieces. The carbon in the polymer chains is either turned into carbon dioxide by these bacteria or incorporated into biomolecules.
What is the concern about the biodegradability of plastics?
The main concern about the biodegradability of plastics is that they do not break down easily in the environment because plastic is composed of polymers with a molecular structure that is resistant to degradation. Additionally, chemical stabilisers are added to prevent degradation. The bond that holds polymers together is not recognized by microorganisms, making it very hard to degrade, causing potential environmental damage. They accumulate in landfills and oceans and take a very long time to break down. Plastic trash might release harmful chemicals into soil and water or break into tiny bits that animals, fish, and birds can consume, making it part of the food chain. The problem here is not only plastic pollution but also the way it is managed. There are many ways to responsibly discard or degrade plastic, but for this, it is necessary to have the appropriate system and processes in place.
Factors Affecting Biodegradation
A number of variables can affect how soon plastics can degrade. This includes their chemical characteristics, phase structure (amorphous or crystalline), molecular weight, and miscibility with other substances. Furthermore, the biodegradation process carried out by microorganisms is also impacted by the presence of hydrolyzed and oxidised chemicals. The hydrophobicity or hydrophilicity compatibility of the microorganism’s surface with the plastic film surface, the polymeric linkages, and the degree of plastic surface roughness are additional parameters that influence the rate of the biodegradation process. It is also influenced by environmental conditions, including temperature, humidity, and availability of liquid water on the surface.
How long does it take for plastic to degrade?
- Types of plastic degradation:
Synthetic polymers are likely to be degraded by processes that are started by ultraviolet (UV) radiation. The lifetime of polymeric material, used for various applications, is determined by UV radiations ranging from 290 to 400 nm and sunlight is the source of such radiations.
- Photo-oxidative degradation:
- Thermal degradation Both thermal and photochemical degradation are categorized as oxidative processes or oxidative degradation since they are thought to be related processes. The first major difference between both is in the sequence of initial steps, while the second difference is in the site of the reaction. While photochemical degradation only affects the polymer surface, thermal degradation involves processes that affect the entire polymer. Thermal deterioration occurs either accidentally or through depolymerisation processes.
- Ozone degradation Ozone that is normally present in the atmosphere causes polymeric degradation. When oxidative reactions are not taking place, polymers live longer. Ozone is a minor part of the atmosphere but it significantly affects polymers. Ozone causes the creation of reactive oxygen species, which destroys polymeric materials (ROS).
- Mechanical degradation Mechanical degradation of polymers is an irreversible process that leads to the breakage of polymer molecules because of high mechanical stresses caused by high flow velocities or elongational deformations.
- Catalytic degradation Oils and gases are produced when polyolefins are catalytically degraded. The quality of the goods produced (after the pyrolysis of plastics) has improved thanks to the use of this degrading technique, and it also offers the chance to produce the desired products. There have been reports of several different kinds of polymer degradation catalysts, such as Pt-Mo and Pt-Co, supported by SiO2.
Is there any way to decompose plastic and polythene?
Additive to make plastic biodegradable
There are several additives that can be added to plastic to make it biodegradable. These additives include starches, enzymes, microbes, and certain chemicals that can break down the structure of the plastic over time. Adding starch, enzymes, and microbes to plastic can help speed up the degradation process. Certain chemicals, such as biodegradable additives or polylactic acid, can also be added to plastic to help it biodegrade more quickly. One such chemical which encourages sustainability and helps in degrading plastic is BioX, which is a biodegradable additive for plastics. It is based on the mixture of transition metal salts and metal oxides, it effectively accelerates the degradation of polyolefins in the presence of sunlight, makes plastic disposal easy and eco-friendly, is effective at low concentrations, is compatible with most hydrocarbon polymers, non-toxic, human-safe, and RoHS-compliant.
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