Alexei Volkov
Photodegradable plastics are designed to break down when exposed to sunlight. This process, called photodegradation, involves the breakdown of complex materials into simpler ones due to light exposure. While photodegradable plastics can take anywhere from two months to two years to decay, they may still pose a hazard to wildlife and contribute to solid waste volume. The challenge lies in ensuring that these plastics receive sufficient sunlight before ending up in landfills, where they may remain intact for extended periods. To address this issue, researchers are exploring various strategies, including the addition of compounds that promote biodegradation and the design of plastics with controlled lifetimes and specific degradation rates.
| Characteristics | Values |
|---|---|
| Definition | Photodegradable plastics are plastics that will decompose when exposed to sunlight. |
| Process | Photodegradation involves the breakdown of complex materials into simpler ones due to light exposure. |
| Initiation | The process of photodegradation is initiated when light strikes a molecule, potentially dislocating electrons and causing the destruction of the molecule. |
| Light Type | Ultraviolet (UV) light is generally more effective in degrading plastics compared to other forms of light. |
| Plastic Type | Aromatic-based polymers with benzene-like ring structures are more susceptible to photodegradation, while those lacking an aromatic structure are less affected by light. |
| Additives | Some additives, known as Totally Degradable Plastic Additives (TDPAs), are incorporated into plastics to promote biodegradation. Examples include cobalt(III) and manganese(III) octadecanoates (stearates). |
| Degradation Time | Photodegradable plastics typically take between two months to two years to decay, which is significantly faster than traditional plastics. |
| Limitations | Critics argue that photodegradable plastics may not always be exposed to sunlight after disposal, as they are often quickly covered in landfills. |
| Environmental Impact | Photodegradable plastics can help reduce the accumulation of plastic waste in the environment, but their potential consumption by other organisms may have unknown effects on the food chain. |
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What You'll Learn
The role of light and UV radiation
The mechanism behind photodegradation is based on the impact of light on molecules. When light, especially UV radiation, strikes a molecule in the plastic, it can initiate various reactions that lead to the destruction of that molecule. For example, UV light can dislocate electrons in chemical bonds, weakening the molecular structure of the plastic. This process is similar to the effect of UV light on human skin, where it can cause sunburns or skin damage.
The type of plastic also plays a role in its susceptibility to photodegradation. Aromatic-based polymers, which have benzene-like ring structures, are more prone to photodegradation compared to non-aromatic structures. Polyethylene, polypropylene, polyvinyl chloride, and polystyrene are examples of commonly used plastics that are less susceptible to photodegradation. However, chemists are working on mechanisms to convert these non-photodegradable plastics into forms that can be decomposed by light.
The intensity and duration of light exposure are also important factors. Higher light intensities can accelerate the photodegradation process, while insufficient light exposure may result in slower or incomplete degradation. This is particularly relevant for plastics that end up in landfills or scattered environments, where they may not receive adequate sunlight for effective photodegradation.
Additionally, the onset and rate of photodegradation can be controlled during the design stage of the plastic. By manipulating various parameters, such as chromophores, initiators, antioxidants, and absorbed light intensity, manufacturers can create photodegradable plastics with specific lifetimes and degradation rates. This tunability is advantageous for different applications, as each may require a unique lifetime for practical purposes.
In summary, the role of light and UV radiation in the breakdown of photodegradable plastics is fundamental. UV light initiates the photodegradation process by breaking down the molecular structure of plastics. The type of plastic, light intensity, and duration of exposure all influence the effectiveness of photodegradation. Designing photodegradable plastics with controlled lifetimes and degradation rates offers a potential solution to the environmental challenges posed by persistent plastics.
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The use of additives
However, the majority of plastics do not have these additives, and instead contain additives that prevent their degradation, contributing to the persistence of plastics in the environment. This is because plastics are desirable for their strength and inertness to chemicals, microorganisms, and light, which makes them difficult to dispose of in an eco-friendly way.
To address this issue, some additives are known as Totally Degradable Plastic Additives (TDPAs). These additives are added to conventional polymers such as poly(ethene) and poly(propene) and are used in products such as shopping bags.
In addition to the use of additives, there are other strategies to address the problem of plastic waste. One strategy is to recycle plastics, although technical, economic, and social factors have limited the success of this approach. Another strategy is to redesign plastics so that they degrade into environmentally benign products. This can be achieved by fabricating plastics with starch as a filler, which makes them more biodegradable.
Overall, the use of additives in photodegradable plastics is a complex issue that requires careful consideration of the advantages and limitations of different approaches.
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Degradation in landfills
The majority of garbage that ends up in landfills is biodegradable, but most landfills are too tightly packed for biodegradation to occur. Landfills are fundamentally anaerobic environments, with very little oxygen present, and few if any microorganisms. This means that any biodegradation occurs very slowly, and the plastic may remain intact for years.
Photodegradable plastics are designed to break down when exposed to sunlight. However, critics question the effectiveness of photodegradable plastics in landfills, as they are rapidly covered with other materials and deprived of sunlight. Even if they do break down, it may only be into smaller pieces of plastic, contributing to the growing problem of microplastics.
Some photodegradable plastics contain additives that attract bacteria, fungi, and other microbes, which break down the plastic into organic molecules. However, the majority of plastics do not have these additives and are resistant to microbial attacks.
The type of light that falls on a material also affects the rate of photodegradation. Ultraviolet (UV) light is more effective at degrading plastics than other forms of light. Therefore, photodegradable plastics in landfills may only break down if they receive a good dose of UV before they are covered.
Overall, while photodegradable plastics have the potential to reduce the amount of plastic waste in landfills, their effectiveness is limited by the lack of sunlight and the presence of other materials that cover them.
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The impact on the environment
Plastics have two great environmental handicaps. Firstly, the vast majority are made from non-renewable fossil fuels, oil, and natural gas. Secondly, their strength, inertness to chemicals, microorganisms, and light also make them difficult to dispose of in an eco-friendly way.
Photodegradable plastics are designed to break down when exposed to sunlight. This process, called photodegradation, is initiated by sunlight breaking down complex materials into simpler ones. Aromatic-based polymers are particularly susceptible to photodegradation. Ultraviolet (UV) light is more effective at degrading plastics than most other forms of light.
Photodegradable plastics are useful in situations involving littering, such as grocery sacks blowing into national parks. However, they take a long time to decay (anywhere from two months to two years) and are therefore still hazardous to wildlife and contribute to solid waste volume. In addition, most plastics wind up in landfills or scattered in the environment, where they may not be exposed to sunlight.
To reduce the build-up of plastics in the environment, two general strategies have been adopted: recycling plastics and redesigning them to degrade into environmentally benign products. However, only about 8% of plastics are recycled, and most plastics are loaded with additives that prevent their degradation.
Bioplastics, which are made from organic materials, are more eco-friendly than traditional plastics. They produce fewer greenhouse gas emissions and do not increase carbon dioxide levels when they break down because the plants they are made from have already absorbed that carbon dioxide. In addition, bioplastics are biodegradable, breaking down into water, carbon dioxide, and compost by microorganisms.
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The design of photodegradable plastics
Photodegradable polymers are typically designed using one of two methods. The first method involves incorporating a photosensitive degradable chromophore, such as a carbonyl group, into the backbone of the polymer chain. This chromophore absorbs UV radiation, trapping energy that ruptures nearby bonds and breaks the polymer chain into smaller fragments. These smaller fragments are then more susceptible to further degradation.
The second method involves mixing an additive into the polymer to initiate or facilitate degradation reactions. Examples of these additives include cobalt(III) and manganese(III) octadecanoates (stearates), which are added at concentrations of 1-2% during the extrusion stage. These additives promote free radical attack on the polymer, initiating its breakdown.
The ideal photodegradable plastic should have an onset of degradation that is reliably predictable, tunable, and rapid once initiated. However, achieving this ideal design is challenging due to the variability in environmental parameters such as light intensity, temperature, and other factors that influence degradation rates.
Additionally, critics have raised concerns about the potential negative impacts of photodegradable plastics. The additives used to make plastics photodegradable may have toxic or environmentally harmful effects. Furthermore, the slow degradation process of photodegradable plastics (ranging from two months to two years) means they still pose a hazard to wildlife and contribute to solid waste volumes. As most plastics are disposed of in sanitary landfills, critics also question the effectiveness of photodegradable plastics that may never be exposed to sunlight after disposal.
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Frequently asked questions
Photodegradable plastics are plastics that will decompose when exposed to sunlight.
Photodegradable plastics break down through a process called photodegradation. This is when light causes certain materials to decompose. Ultraviolet (UV) light is the most effective form of light for degrading plastics.
Photodegradable plastics typically take anywhere from two months to two years to break down.
Photodegradable plastics are useful in situations involving littering. They are also a solution to the accumulation of plastics in the environment.
Photodegradable plastics will mostly end up in landfills where they are rapidly covered with other materials, preventing them from receiving sunlight. They also take a long time to break down, during which they can be consumed by other organisms and enter our food chain.
