Sofia Martinez
Plastic is a polymer that is susceptible to degradation at all stages of its product life cycle, including during its initial processing, use, disposal into the environment, and recycling. The degradation of plastics is of great ecological importance due to the persistence and potential adverse effects of plastic pollution on human health and wildlife. The process of plastic degradation involves changes in its chemical composition, such as oxidation, hydrolysis, and UV-attack, which lead to a reduction in its physical properties such as strength and flexibility. This degradation can occur through biological processes, environmental exposure, or mechanical recycling, resulting in the formation of holes, cracks, and cavities on the surface of the plastic. Technologies have been developed to both inhibit or promote degradation, such as the use of stabilizers and biodegradable additives, respectively.
| Characteristics | Values |
|---|---|
| General Term | Polymer degradation |
| Definition | Reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition |
| Degradation Stages | Initial processing, use, disposal into the environment, and recycling |
| Degradation Rate | Varies; biodegradation can take decades, while industrial processes can completely decompose a polymer in hours |
| Degradation Inhibitors | Polymer stabilizers, lubricants, and antioxidants |
| Degradation Accelerators | Biodegradable additives |
| Degradation Examples | Polypropylene garden chairs becoming brittle and breaking, polycarbonate car headlamp covers becoming yellowed and foggy, polyethylene grocery bags disintegrating in less than a year |
| Degradation Mechanisms | Hydrolysis, methanolysis, glycolysis, aminolysis, thermal-oxidation, UV-attack |
| Degradation Indicators | Changes in tensile strength and elongation, formation of cracks and pores, reduction in molecular weight, decrease in Tg (glass transition temperature), mass loss |
| Degradation by Microorganisms | Yes; specific enzymes like PET hydrolase and PCL-cutinase can break down certain polymers |
| Degradation in Marine Environments | Yes; microbial biofilms can form on the surface of plastics, leading to degradation |
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What You'll Learn
Plastic degradation rates
The degradation of plastic, or polymer degradation, is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition. Plastic degradation is of great concern due to the toxicity of fine plastic particles and the potential release of toxic by-products. Plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment, and recycling.
The rate of plastic degradation varies significantly depending on the type of plastic, the method of degradation, and the environmental conditions. Biodegradation can take decades, while some industrial processes can completely decompose a polymer in hours. For example, the degradation rate of PET plastic using LC-cutinase was found to be 230–970 times more than that of other cutinases.
The specific surface degradation rate (SSDR) is a metric used to harmonize disparate types of measurements and extrapolate half-lives for different plastics. SSDR values can range from 0 to approximately 11 μm year–1 for high-density polyethylene (HDPE) in the marine environment. Using a mean SSDR for HDPE, estimated half-lives range from 58 years for bottles to 1200 years for pipes. Media estimates for the degradation of plastic bags vary widely, with some sources claiming 10–20 years, while others state 500–1000 years.
Environmental degradation mechanisms for plastics can be classified as physical or chemical. Physical degradation refers to changes in the bulk structure, such as cracking, embrittlement, and flaking, while chemical degradation involves changes at the molecular level, including bond cleavage and oxidation of long polymer chains. Mechanical degradation, caused by shear stress, can be reduced by the addition of lubricants, while thermal degradation can be influenced by temperature, with higher temperatures increasing degradation.
Biological degradation of plastics by microorganisms, such as bacteria and fungi, is also a significant factor in the breakdown of plastics. Biofilms formed by microbial populations can contribute to the degradation of plastics in marine habitats. Additionally, enzymes like PET hydrolase and PCL-cutinase play a role in the degradation of different polymers.
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Environmental impact of plastic degradation
Plastic degradation refers to the reduction in the physical properties of plastic polymers such as strength, caused by changes in their chemical composition. Plastics are susceptible to degradation at all stages of their product life cycle, including during processing, use, disposal, and recycling. The rate of degradation varies, with biodegradation taking decades, while industrial processes can decompose polymers in hours.
The environmental impact of plastic degradation is significant and far-reaching. Plastic waste is generated at a rate of approximately 400 million metric tons per year, with an estimated 20 million tons of plastic litter ending up in the environment annually. This amount is projected to increase by 2040, affecting all land, freshwater, and marine ecosystems. Plastic pollution is a major contributor to biodiversity loss, ecosystem degradation, and climate change. It threatens human health, food and water safety, and economic activities.
In the marine environment, plastic polymers are exposed to sunlight, oxidants, and physical stress, causing them to weather and degrade over time. More than 60% of floating debris in the oceans is plastic, and this percentage is increasing annually. The specific surface degradation rate (SSDR) is a metric used to harmonize disparate types of measurements and estimate the half-lives of plastics in different environments. For example, SSDRs for high-density polyethylene (HDPE) in the marine environment range from 0 to approximately 11 μm year–1, resulting in estimated half-lives from 58 years for bottles to 1200 years for pipes.
Microbial degradation plays a crucial role in reducing the negative impact of plastics. Microbial enzymatic degradation, such as bacterial enzymes, offers an eco-friendly approach to plastic biodegradation. Studies on the biodegradation of plastics help understand how microorganisms work and their potential to reduce plastic waste. Additionally, research on enzymes like PETase and cutinases has led to the development of better enzymes to address plastic waste issues.
To address the environmental impact of plastic degradation, global efforts are needed to reduce plastic production, phase out harmful practices, and implement strong national plans and compliance mechanisms. Biodegradable plastics and the biodegradation of plastic wastes have gained increasing importance in recent years. Understanding the environmental degradation of plastics and exploring the potential of microorganisms for biodegradation are essential steps toward mitigating the ecological problems caused by plastic and microplastic pollution.
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Biological degradation of plastics
The biological degradation of plastics, or biodegradation, is a process that has attracted a lot of attention due to its potential to address the ecological problems caused by plastic and microplastic pollution. Biodegradation is a necessary process for water-soluble or water-immiscible polymers, as they eventually enter streams and cannot be recycled or incinerated.
The process of biological degradation of plastics involves the use of microorganisms, such as bacteria and fungi, to break down the polymer chains that make up plastics. This can occur through the formation of microbial biofilms on the surface of the plastic, which is called a plastisphere. Certain bacterial enzymes, such as PET hydrolase and PCL-cutinase, have been found to be particularly effective in degrading specific polymers like PET and PCL, respectively.
Research has also focused on the potential of fungal strains to degrade plastics. For example, Penicillium oxalicum NS4 (KU559906) and Penicillium chrysogenum NS10 (KU559907) have been found to degrade HDPE and LDPE, causing noticeable morphological changes on plastic sheets. The Aspergillus genus, a prominent fungal group, has also shown a strong ability to degrade plastics.
In addition to natural microorganisms, scientists have explored the design of plastics that are susceptible to microbial attack, leading to the development of biodegradable plastics. These biodegradable plastics, such as polyesters, polylactic acid (also known as polylactide), polyglycolic acid, and polyhydroxybutyrate, can be used in controlled drug releases, implantable composites, packaging, and other applications.
The use of biodegradable plastics in industries like packaging and health offers economic, environmental, and health benefits. However, effective biodegradation requires proper waste management strategies and controlled biodegradation facilities. Technologies have been developed to promote degradation, such as the use of biodegradable additives, while polymer stabilizers can be used to inhibit degradation and extend the lifespan of plastic items.
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Polymer degradation
Polymers and plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal, and recycling. Technologies have been developed to both inhibit and promote degradation. For example, polymer stabilizers are used to ensure plastic items are produced with the desired properties, extend their useful lifespans, and facilitate their recycling. On the other hand, biodegradable additives are used to accelerate the degradation of plastic waste by improving its biodegradability.
The degradation of polymers usually starts on the outer surface and gradually penetrates into the bulk of the material. The major chemical changes that occur during degradation are oxidation and chain scission, which lead to a reduction in the molecular weight and degree of polymerization of the polymer. These changes are often referred to as "aging." The effects of heat, light, air, and water are the most significant factors in the degradation of plastic polymers.
There are various environmental factors that contribute to the degradation of polymer properties, including direct or indirect sunlight, heat, oxygen, moisture, and other factors. Additionally, the microbial population and the activity of different microorganisms are influenced by environmental factors such as humidity, temperature, pH, salinity, oxygen levels, and nutrient availability. These conditions must be considered when testing the biodegradability of plastics.
The process of polymer degradation can be induced by sunlight, heat, and other atmospheric agents, leading to a modification of the polymer structure and a decrease in its mechanical characteristics. This degradation can occur through various physical and chemical changes, such as discoloration, loss of surface gloss, formation of cracks, greasy surfaces, surface erosion, and loss of properties such as resistance to traction-deformation.
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Plastic recycling
Plastics are polymers, which are materials composed of long chains of molecules. The most common plastics are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polystyrene (PS), polycarbonate (PC), and poly(methyl methacrylate) (PMMA). These polymers can undergo degradation, which is the reduction in their physical properties, such as strength, due to changes in their chemical composition. Degradation can occur during the initial processing, use, disposal, and recycling of plastics.
One common method of recycling plastics is mechanical recycling, which involves melting and reforming plastic into new items. This process can introduce polymer degradation at the molecular level, and it requires sorting waste by colour and polymer type, which is often expensive and complicated. Errors in sorting can lead to inconsistent material properties, making it less desirable for industrial use. Additionally, mechanical recycling can release microplastics into wastewater, contributing to environmental concerns.
Another approach is feedstock recycling, where waste plastic is converted into its starting chemicals through processes such as incineration or biochemical conversion. These methods require higher energy and capital costs. To address the challenges of recycling, technologies have been developed to inhibit or promote degradation. For example, polymer stabilizers are used to extend the lifespan of plastic items and facilitate recycling, while biodegradable additives accelerate the degradation of plastic waste to enhance biodegradability.
The recycling of plastics has been advocated since the early 1970s, but it only gained significant traction in the late 1980s due to economic and technical hurdles. Despite global efforts, the recycling rates for plastics lag behind those of other materials. As of 2015, only 9% of the world's plastic waste had been recycled, with a small portion recycled more than once. The plastics industry has faced criticism for lobbying for recycling programmes, even when research indicated that most plastics could not be economically recycled.
To optimize the lifespan of plastics and reduce environmental harm, it is crucial to prioritize the reduction and reuse of plastics alongside recycling efforts. Additionally, ongoing investments in technology are expanding the scope of recyclable plastic materials, and some companies are incorporating recycled plastics into their products to promote sustainability. By addressing the challenges and increasing recycling rates, we can minimize the negative impact of plastic waste on our planet.
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Frequently asked questions
The deterioration of plastic is called polymer degradation.
Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition.
Polymers will oxidise and degrade when exposed to high temperatures. They also experience significant shear stress during extrusion and moulding, which can snap the polymer chains and introduce chemical weak points.
The rate of degradation varies. Biodegradation can take decades, while industrial processes can completely decompose a polymer in hours. For example, polyethylene grocery bags have been found to disintegrate in less than a year when left outdoors.
