Alexei Volkov
Polyethylene terephthalate (PET) is a type of plastic that is commonly used in single-use plastic water bottles. It is considered acid-free and BPA-free, but it is associated with health risks such as stunted growth, reproduction issues, and low energy levels. One concern surrounding PET plastic is its tendency to turn yellow when exposed to UV light, a process known as yellowing. This phenomenon has been attributed to the breakdown or ageing of the polymer, and it is more prominent when the PET material is exposed to direct sunlight, high humidity, and oxygen. The yellowing of PET bottles can be mitigated by incorporating stabilizers during the manufacturing process, but this adds to the cost.
| Characteristics | Values |
|---|---|
| Acid-free | No, the term "acid-free" isn't used in relation to plastics. Look for plastics described as "inert" or "stable" for preservation purposes. |
| UV light impact | PET is sensitive to UV light, especially in high temperatures, high humidity, and in the presence of oxygen. This can cause yellowing. |
| Health risks | Potential health risks include stunted growth, reproduction issues, low energy levels, body balance issues, and inability to process stress. |
| Additives | PET is Bisphenol-A (BPA) free. |
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What You'll Learn
PET plastic is BPA-free
Polyethylene terephthalate, or PET, is a plastic commonly used in single-use water bottles, food containers, and beverage containers. It is known to be one of the most common materials used for plastic water bottles, with around 193 billion litres of bottled water consumed globally in 2018.
PET plastic is considered BPA-free. Bisphenol-A, or BPA, is a compound found in most plastic materials, including epoxy resins, polycarbonate plastic food containers, and even some plastic water bottles. It is a chemical additive that is used in the production of hard plastics, contributing to the rigidity of the material. Due to its potential negative health impacts, consumers are increasingly looking for BPA-free products, and manufacturers are responding by creating food packaging, soft drink bottles, and plastic containers that are BPA-free.
BPA has been scrutinized for its potential negative health risks, and as a result, it has been banned in multiple countries, including Canada, France, Sweden, and Denmark. The Food and Drug Administration (FDA) in the United States banned the use of BPA in infant formula and baby bottles in 2013. However, the FDA has not ruled that BPA is unsafe for other applications, and there is ongoing debate about the safety of BPA-containing products.
While PET plastic is considered BPA-free, it is important to note that it is associated with similar health risks as those posed by BPA, such as stunted growth, reproduction issues, low energy levels, body balance issues, and an inability to process stress. Additionally, PET plastic is susceptible to yellowing caused by natural or artificial light, particularly UV radiation from the sun or artificial sources such as fluorescent lighting. This can further impact the quality and safety of the material over time.
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UV light exposure causes yellowing
The yellowing of PET plastic is attributed to a breakdown or ageing of the polymer. PET is sensitive to UV light, especially at elevated temperatures, under high humidity, and in the presence of oxygen. UV light exposure causes degradation of the PET material, which leads to yellowing.
A study by Plastic Technologies, Inc. investigated the impact of UV light exposure on PET containers. They exposed virgin PET bottles to sunlight on a rooftop for a year, and the results showed that UV light did not significantly affect the colour of the bottles. However, when these bottles were recycled into resin pellets and used to create new PET bottles, yellowing occurred. This indicates that the degradation of PET due to UV light exposure becomes apparent during the recycling process, specifically during the melting and moulding stage.
Another factor contributing to the yellowing of PET is the presence of certain additives. Additives such as ultraviolet light blockers, oxygen scavengers, nylon multilayer structures, and residual label adhesives can all influence the development of yellowing during the melt reprocessing of PET bottles.
The yellowing caused by UV light, both natural and artificial, can potentially be mitigated by incorporating improved stabilizers into the PET during its manufacture. However, this would add to the cost of producing virgin PET, which may not be well-accepted by brand owners or converters in the current market.
It is important to note that the yellowing of PET due to UV light exposure has a severe impact on the L*, b*, and IV properties of recycled PET. This highlights the importance of understanding the effects of UV light on PET materials and the need for effective measures to prevent or reduce yellowing.
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Humidity and oxygen also contribute to yellowing
The yellowing of plastic is a multifaceted process influenced by several factors, including sunlight, heat, and humidity. While UV light is often the primary cause of plastic yellowing, humidity and oxygen also play crucial roles in accelerating the colour transformation.
Humidity's Role in Plastic Yellowing
High humidity levels cause plastics to absorb moisture, creating an environment conducive to the growth of mould and bacteria. These microorganisms produce enzymes that accelerate oxidation, leading to the yellowing or browning of the plastic. Additionally, humidity can make plastic brittle, resulting in cracks that expose deeper layers to sunlight, further exacerbating photooxidation and discoloration.
Oxygen's Role in Plastic Yellowing
Oxygen, which is present in the air surrounding plastic, contributes to the degradation process known as "thermal degradation". When plastic molecules are affected by heat, they react with oxygen to form peroxides and chromophores, which are molecular structures responsible for discoloration, resulting in the plastic turning yellow.
Mitigation Strategies
To minimize colour changes and degradation in plastics, it is essential to store them in cool, dry places away from direct sunlight. Additionally, opting for UV-resistant materials can help preserve their original colour and quality over time. By understanding and addressing these factors, we can extend the lifespan and maintain the aesthetic appeal of plastic products.
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Additives can be used to mitigate yellowing
The yellowing of PET plastic can be mitigated to some degree by incorporating additives during its manufacture. The use of improved or additional stabilizers can reduce the yellowing effect of UV light on PET. However, this solution may not be cost-effective for brand owners or converters as it would add to the production cost.
UV light blockers are additives that can be used to prevent plastic yellowing. These blockers can be added to the plastic during the manufacturing process or applied as a treatment to the surface of the plastic. Another option is to use a cover that blocks UV rays, such as a dark blanket or a plastic bag, to protect the plastic from sunlight exposure.
Oxygen scavengers are also effective in mitigating plastic yellowing. This is because oxygen can contribute to the breakdown of PET, especially when exposed to UV light, high humidity, and elevated temperatures. By including oxygen scavengers as additives, the amount of oxygen available to react with the plastic is reduced, thus slowing down the yellowing process.
Additionally, nylon multilayer structures can be used as a barrier to protect the plastic from direct exposure to elements that may cause yellowing. This is particularly useful for packaging materials that need to maintain their clarity and aesthetics.
It is important to note that the effectiveness of these additives may vary depending on the specific type and quality of the plastic, as well as the environmental factors it is exposed to. Regular maintenance and proper care, such as avoiding direct sunlight and extreme temperatures, are crucial in preventing plastic yellowing.
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Recycled PET is more yellow than deposit-grade PET
The yellowness of recycled PET (rPET) is a well-known issue in the plastics industry. While all PET bottles tend to show an increase in yellowness during reprocessing, recycled PET is more yellow than deposit-grade PET. This is due to a variety of factors, including the presence of additives such as UV light blockers, acetaldehyde and oxygen scavengers, slip agents, and residual nylon from multilayer bottles. In addition, exposure to natural or artificial light sources, such as sunlight or fluorescent lighting, can cause yellowing in recycled PET. This is a problem for food and non-food rigid packaging applications, as high-quality clear, non-yellow rPET is in high demand.
The degree of yellowness in rPET can be mitigated to some extent by incorporating improved stabilizers into the PET during its manufacture. However, this adds additional costs that may not be acceptable to brand owners or converters. As a result, alternative solutions have been proposed, such as incorporating yellowed rPET into a separate stream for amber-coloured plastics. This would allow clear bottles that yellow when recycled to benefit an amber recycling stream and potentially add value to the reclaimer's operation.
Another factor that contributes to the yellowness of recycled PET is the quality of the PET bottles being recycled. Curbside rPET bottles, for example, typically result in lower-quality rPET for reuse compared to deposit-grade rPET. This is because curbside bottles are more likely to have residual contaminants such as adhesives, inks, and coatings that can contribute to yellowing. In addition, multilayer bottles can trap low levels of nylon, which can cause significant yellowing during the extrusion process.
To ensure that only high-quality PET bottles are recycled, effective sorting and collection systems are critical. This can be achieved through the use of recycling bins, stations, and collection centres. Professional plastic washing machines can then be used to thoroughly clean and treat recycled PET bottles, removing any remaining impurities that could contribute to yellowing. Nevertheless, despite these efforts, recycled PET will still exhibit more yellowness than deposit-grade PET due to the inherent challenges associated with the recycling process and the sensitivity of PET to UV light and other environmental factors.
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Frequently asked questions
The term "acid-free" is not used in reference to plastics. However, PET plastic is sensitive to UV light and can turn yellow when exposed to sunlight.
PET stands for polyethylene terephthalate, a type of plastic commonly used in single-use plastic water bottles.
No, PET plastic is BPA-free. However, it is associated with similar health risks to BPA, including stunted growth, reproduction issues, and low energy levels.
You can mitigate yellowing by incorporating improved stabilizers into the PET during its manufacture. However, this would likely add additional costs.
