Omar Khalid
Ultraviolet (UV) radiation from the sun and artificial sources, such as fluorescent lighting, can have a significant impact on various materials, including plastics like PET. While PET plastic is known to be sensitive to UV light, the question arises whether it can also block UVA rays. This query delves into the intriguing interplay between UV radiation and the unique characteristics of PET plastic, exploring potential implications for a range of applications, from packaging to UV protection.
| Characteristics | Values |
|---|---|
| PET plastic's resistance to UV rays | Regarded as fair |
| Degradation of PET plastic by UV rays | Yes, it can turn yellow |
| Avoidance of UV degradation in PET plastic | Use stabilizers, absorbers, or blockers; add carbon black at a 2% level |
| Effect of UV rays on polymers | Intensity-dependent, based on stratospheric ozone, clouds, altitude, sun height, and reflection |
| Effect of UV rays on PET plastic | Sensitive to UV light at elevated temperatures, high humidity, and in the presence of oxygen |
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What You'll Learn
PET plastic is sensitive to UV light
UV irradiation is a significant factor in the environmental degradation of plastics, and PET is no exception. When exposed to UV light, the polymer chains in PET can undergo scissions, resulting in a shortening of the chains. This was observed in a study where PET samples were exposed to UV light for 14 days, showing clear differences in the range between 2250 and 3700 cm-1. The degradation caused by UV light can also impair the enzymatic degradation of PET, making it challenging to recycle on an industrial scale.
The sensitivity of PET plastic to UV light has been demonstrated in various studies. In one experiment, two-liter PET bottles without UV-absorbing additives were crushed, stacked, and placed in open-sided crates to maximize exposure to the elements. These bottles were then left outside for a year, and the data showed that UV radiation severely damaged the PET material. Similarly, another study by PTI examined the impact of sunlight and fluorescent lighting on PET bottles and found that exposure to these light sources contributed to the development of yellowing during the melt reprocessing of the bottles.
To mitigate the negative effects of UV light on PET plastic, stabilizers, absorbers, or blockers can be added during the manufacturing process. However, this adds additional costs that may not be feasible for brand owners or converters in today's market. As a result, PET plastic is often left vulnerable to the damaging effects of UV light, which can impact its appearance, physical properties, and recyclability.
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UV light causes PET plastic to turn yellow
PET plastic is sensitive to UV light, especially in high temperatures, high humidity, and in the presence of oxygen. Exposure to UV light can cause PET plastic to turn yellow.
A study by Plastic Technologies, Inc. found that exposure to ultraviolet radiation was damaging to PET material. PET bottles were left outside for a year, and although there was not a significant amount of yellowing seen in the bottle flake, the yellowing became more pronounced after the bottles were melted down for molding. The study concluded that storing bottles outside with direct exposure to sunlight in a humid environment can have a severe effect on the properties of recycled PET.
Another study by Plastic Technologies, Inc. investigated the impact of UV light exposure on PET containers before reprocessing. Researchers exposed virgin PET bottles to sunlight for a year, while other bottles were stored inside and irradiated with fluorescent light. A third control set of bottles was stored without light exposure. The researchers found that UV light did not alter the appearance of the exposed PET bottles. However, when the exposed bottles were recycled into resin pellets and used to create new recycled PET bottles, yellowing occurred.
The yellowing of PET plastic is attributed to the breakdown or ageing of the polymer. The increase in UV light exposure initiates and accelerates a chemical reaction in the polymer, which eventually turns the plastic into a yellow color. This process can be slowed by using stabilizers, absorbers, or blockers, such as carbon black.
Other factors that can contribute to the yellowing of PET plastic include the presence of additives such as UV light blockers, acetaldehyde, oxygen scavengers, slip agents, and residual nylon from multilayer bottles. Curbside rPET bottles also tend to show increased yellowness compared to deposit-grade rPET bottles due to the presence of additives and contaminants.
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UV-absorbing additives can be added to PET plastic
PET plastic is sensitive to UV light, especially under certain conditions, such as high temperatures, high humidity, and in the presence of oxygen. This sensitivity can lead to undesirable effects, such as colour fading and shifting, off-flavours and aromas, reduced vitamin potency, and product degradation.
To mitigate these issues, UV-absorbing additives can be incorporated into PET plastic during its manufacture. These additives act as blockers, absorbing UV radiation and preventing it from damaging the PET material and its contents. This helps to maintain product quality, extend shelf life, and protect brand reputation.
There are a variety of UV-absorbing additives available, including polymer-bound formulations that do not leach out of the plastic and potential contaminate the product. These additives are designed to absorb high wavelengths of UV light, up to 400 nm, providing effective protection against both natural and artificial UV sources.
However, the use of UV-absorbing additives may come with certain challenges. For instance, migration of the additive during high-heat processing can cause plate-out, resulting in residue on the mould and potential contamination of other bottles. Additionally, the use of these additives can increase production costs and cause processing delays due to the need for frequent machinery cleaning.
Despite these considerations, UV-absorbing additives offer a valuable solution for protecting products packaged in PET plastic from the detrimental effects of UV radiation.
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UV degradation in plastics can be avoided
Ultraviolet (UV) radiation from the sun or artificial sources like fluorescent lighting can significantly degrade plastics, causing discoloration, weathering, loss of gloss, and mechanical properties (cracking). This degradation is particularly prominent in polystyrene (PS), a common material in the plastic industry. However, there are several strategies to prevent UV degradation in plastics:
Use of Stabilizers, Absorbers, or Blockers
One approach to prevent UV degradation in plastics is to incorporate UV stabilizers, absorbers, or blockers. UV stabilizers, such as hindered amine light stabilizers (HALS), inhibit or weaken the impact of UV light, enhancing the light resistance of plastic products. UV absorbers, including benzophenones, benzotriazoles, triazines, and salicylates, absorb UV radiation and convert it into low-level heat. Additionally, carbon black, titanium dioxide, and zinc oxide are effective UV light shielding agents.
Consideration of Stability
When using UV stabilizers and antioxidants, stability is crucial. Factors to consider include preventing discoloration, decomposition, reactions with other additives, and damage to machinery. For instance, acidic additives should be avoided when using HALS to prevent reactions.
Selection of Plastic Type
Certain types of plastics have better inherent resistance to UV radiation. Polymers extruded by Zeus, such as PTFE, PVDF, FEP, and PEEKTM, exhibit good resistance to UV rays. Other plastics like PET, PP, HDPE, and specific grades of PC/ABS alloys are also considered fair in terms of UV resistance.
Protection from Environmental Factors
UV degradation of plastics can be accelerated by environmental factors such as elevated temperatures, high humidity, and oxygen exposure. Therefore, protecting plastics from these factors can help mitigate UV degradation.
Addition of Carbon Black
For outdoor applications, adding carbon black at around a 2% level can effectively block UV radiation and provide protection for the plastic structure.
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The thickness of PET plastic affects its ability to block UV rays
PET plastic, or polyethylene terephthalate, is a widely used thermoplastic polymer that is known for its durability, strength, and gas impermeability. It is commonly used to make water bottles, transparent films, and textile fibres, among other products. While PET plastic is considered to have "fair" resistance to UV rays, the thickness of the plastic can indeed influence its ability to block UV rays.
The effects of UV rays on PET plastic are complex and depend on various factors such as stratospheric ozone, clouds, altitude, sun height, and reflection. Additionally, ambient temperature and humidity can accelerate the impact of UV radiation. The primary visible effects of UV exposure on PET plastic include a chalky appearance, colour shift, and a brittle structure.
The thickness of PET plastic plays a role in its ability to block UV rays because the penetration depth of UV radiation is limited. Typically, the effects of UV rays on the material are predominantly limited to the surface layer, with minimal impact beyond a depth of 0.5 mm. This means that thicker PET plastic will provide better protection against UV rays, as the radiation is less likely to penetrate through the material.
However, it is important to note that the presence of impurities, additives, and residual materials in the PET plastic can also influence its UV-blocking capabilities. For example, multilayer bottles containing low levels of nylon can cause yellowing in PET plastic, affecting its appearance and potentially its UV resistance. Additionally, additives such as UV light blockers, acetaldehyde, and oxygen scavengers can contribute to colour changes in the material when exposed to UV radiation.
To enhance the UV-blocking properties of PET plastic, stabilizers, absorbers, or blockers can be added during the manufacturing process. Carbon black, for instance, can be added at around a 2% level to provide effective protection against UV radiation. Thicker PET plastic, combined with the use of UV-blocking additives, can significantly improve the material's resistance to UV rays.
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Frequently asked questions
PET plastic is regarded as having fair resistance to UV rays. However, UV radiation from the sun and artificial sources, such as fluorescent lighting, can cause PET to turn yellow.
To avoid UV degradation in plastics, you can use stabilizers, absorbers, or blockers. One common method is to add carbon black at around a 2% level to provide protection through the blocking process.
Polymers extruded by Zeus, such as PTFE, PVDF, FEP, and PEEKTM, offer good resistance to ultraviolet rays.
