Will Plastic Disposable Food Boxes Age over Time?
author: Iris
2025-11-26
1. Introduction
With the rapid development of the food delivery industry and the fast pace of life, plastic disposable food boxes have become an indispensable food packaging container in modern life. Statistics show that nearly 400 million tons of plastic waste are generated each year globally, a significant portion of which consists of disposable food boxes. These containers are often discarded carelessly or improperly stored after use, undergoing long-term physical, chemical, and biological processes in the natural environment, leading to aging and degradation.
The aging of plastic disposable food boxes not only affects their performance but, more importantly, may release harmful substances, posing a potential threat to food safety and human health. Studies have shown that 65℃ is the "safety red line" for plastic disposable food boxes; exceeding this temperature, different types of plastic will release different harmful substances. When the temperature exceeds 65℃, plastic takeout containers will release 16 harmful components, including bisphenol A (BPA). Furthermore, the surface of aged plastic disposable food boxes will show cracks and abrasions invisible to the naked eye; these cracks accelerate the release of harmful substances.
This paper aims to systematically analyze the aging phenomena of plastic lunch boxes made of different materials (PET, PP, PS, etc.) under various environmental conditions, providing consumers with scientific usage and storage recommendations.
2. Research on the Influence of Environmental Factors on the Aging of Plastic Lunch Boxes
2.1 Analysis of the Impact of High Temperature Environment
High temperature is one of the most important environmental factors leading to the aging of plastic lunch boxes. In actual use and storage, plastic lunch boxes may face various high-temperature situations, including microwave heating, holding hot food, and high-temperature storage in summer.
2.1.1 Short-term High Temperature Effects (e.g., Microwave Heating)
Microwave heating is a common high-temperature usage scenario in daily life. Studies have shown that the microwave heating tolerance of plastic lunch boxes made of different materials varies significantly. PP material is the only plastic that can be microwaved, with a melting point as high as 167℃, far exceeding the conventional heating temperature of microwave ovens, and can even withstand a high temperature of 130℃ for a short period. However, even for PP lunch boxes, prolonged high-temperature heating should be avoided; it is recommended to use low or medium power for no more than 3 minutes.
In contrast, PS material has poor thermal stability, deforming above 75℃ and softening significantly at 100℃. Under the high temperatures of microwaves, the disposable food boxes become unusable, and there is even a possibility of styrene monomers or oligomers transferring into the food. While PET material has high transparency, its heat resistance is limited, and microwave heating is generally not recommended.
2.1.2 Effects of Long-Term High-Temperature Storage
The effects of long-term high-temperature storage on plastic burger take out boxes are more complex and persistent. Studies have found that when temperatures exceed 65℃, plastic disposable food boxes release 16 harmful components, including bisphenol A (BPA). Under high temperatures, the molecular chain movement within the plastic intensifies, potentially leading to the release of plasticizers, BPA, and other additives.
While PP material has better temperature resistance, continuously storing food above 80℃ accelerates the release of low-molecular-weight compounds. Studies show that PP material will naturally oxidize and yellow after 2-3 years, its impact strength will decrease by more than 50%, and storage in direct sunlight will accelerate aging; a sour smell indicates degradation.
Temperature has multifaceted effects on the properties of plastics. High temperatures cause plastic materials to soften, reducing their mechanical strength and impact resistance; while low temperatures may cause plastics to become brittle, increasing the risk of breakage. In actual storage, prolonged exposure of plastic lunch boxes to high-temperature environments should be avoided.
2.2 Analysis of the Impact of Direct Sunlight
Direct sunlight is a major cause of photoaging in plastic lunch boxes. Ultraviolet radiation from sunlight has a strong destructive effect on plastic materials, triggering a series of photochemical reactions that lead to material performance degradation.
2.2.1 The Effects of Ultraviolet Radiation
Ultraviolet radiation is one of the main physical factors causing plastic aging. Ultraviolet radiation from the sun is a major factor in the aging of plastic materials, and its energy is sufficient to break the chemical bonds in plastic molecules. Ultraviolet radiation can damage the molecular structure of plastics, causing oxidation, degradation, and yellowing.
Different wavelengths of ultraviolet radiation have different destructive effects on plastics. Ultraviolet light with wavelengths in the 290-400 nm range plays a crucial role in the direct photo-oxidation of plastics because its energy can break the C-C (375 kJ/mol), C-H (420 kJ/mol), and C-O (326 kJ/mol) bonds. The breaking of these bonds directly generates alkyl radicals, initiating subsequent chain reactions.
2.2.2 Photo-oxidation Process
The photo-oxidation reaction is a complex chain reaction process, mainly consisting of three stages: chain initiation, chain propagation, and chain termination. In the chain initiation stage, after absorbing ultraviolet light energy, the polymer forms an excited state at the carbonyl group or other structural defects, abstracting hydrogen atoms from the polymer backbone to generate reactive carbonyl alkyl radicals.
In the chain propagation stage, the alkyl radicals react with oxygen to form peroxide radicals. These peroxide radicals then abstract hydrogen atoms from another polymer chain (or from distant sites within the same polymer chain), forming hydroperoxides and new alkyl radicals. The hydroperoxides decompose under light or heat, producing alkoxy and hydroxyl radicals, which continue to attack the polymer chains. The chain termination stage occurs when two free radicals combine to form a stable molecule, ending the chain reaction. The final products of photooxidation typically include oxygen-containing functional groups such as carbonyl compounds, alcohols, aldehydes, and ketones. The formation of these products leads to changes in the physical and chemical properties of the plastic.
2.2.3 Differences in Photoaging Behavior of Different Materials
Different plastic materials exhibit significantly different photoaging behaviors under direct sunlight. Studies show that PET material has good photoaging resistance, resisting UV erosion for a certain period and maintaining high mechanical strength and transparency. However, PET itself has weak UV blocking ability, and prolonged exposure to sunlight easily leads to aging, yellowing, and even degradation.
PP material has better photoaging resistance than PE, but under high-intensity UV irradiation, it will still undergo oxidative degradation, leading to a decrease in material strength. Untreated PP cans are prone to embrittlement and discoloration under UV irradiation. PE material has weak UV blocking ability and is easily aged and degraded under sunlight.
In practical applications, it was found that after 3 months of light exposure, PE microplastics exhibited infrared absorption peaks corresponding to the C=C stretching vibrations of hydroxyl groups, methyl groups, and benzene rings, while the infrared absorption peaks corresponding to ether bonds disappeared. This indicates that PE microplastics are unstable under natural light conditions and their structure is highly susceptible to changes. PP microplastics showed relatively better stability, but even after 12 months of light exposure, they still developed functional groups such as carbonyl and hydroxyl groups.
2.3 Analysis of the Impact of Humid Environments
The impact of humid environments on plastic disposable food boxes is mainly achieved through the action of moisture, including direct hydrolysis and indirect promotion of oxidation.
2.3.1 The Effect of Humidity on Aging
Humidity is one of the important environmental factors affecting the aging of plastics. In high-humidity environments, plastics easily absorb moisture and expand, leading to a decrease in mechanical properties. Studies have shown that the higher the humidity, the faster the aging rate of plastics. For example, polymethyl methacrylate (PMMA) ages faster at a relative humidity of 75%, while its aging rate is relatively slower at a relative humidity of 30%.
Humidity-induced degradation reactions mainly include hydrolysis, oxidation, and hydrolytic oxidation. For hydrophilic plastics or plastics in humid environments, the role of water cannot be ignored. Water may penetrate the plastic interior, causing swelling and disrupting intermolecular forces. Simultaneously, impurities and ions in the water may participate in chemical reactions, accelerating plastic aging.
2.3.2 Hydrolysis Mechanism
Hydrolysis refers to the process by which water molecules react with the plastic molecular chains, leading to chain breakage. Different types of plastics have different sensitivities to hydrolysis. Polymers containing ester or amide bonds, such as PET and polyamide (nylon), are prone to hydrolysis.
Taking PET as an example, in a humid environment, water molecules can attack ester bonds, causing them to break and generate carboxyl and hydroxyl groups. This process leads to a decrease in molecular weight and a decline in mechanical properties. Studies have shown that chain breakage in PET at 60℃ and 100% relative humidity is 5 times that at 45% relative humidity.
For polyamide (nylon) plastics, in humid environments, water molecules form hydrogen bonds with the amide groups on the molecular chains, causing the plastic to absorb water and swell, resulting in dimensional changes. Furthermore, metal ions in the water may catalyze the hydrolysis of nylon, causing the molecular chains to break and further reducing the plastic's performance.
2.3.3 The Impact of Mold Growth
In high-humidity environments, plastic black take-out boxes are also susceptible to mold growth. Studies have shown that in high-humidity environments, plastic containers are easily damaged by mold, thus affecting their lifespan. Mold growth not only affects the appearance of plastic disposable food boxes but may also produce harmful metabolites, impacting food safety.
Controlling the humidity of the storage environment is crucial. Generally, the storage humidity for plastic products should be between 30% and 80%. If the storage humidity is too high, the plastic products may absorb moisture, leading to problems such as deformation, aging, and discoloration; if the storage humidity is too low, the plastic products may become dry and brittle. It is recommended to store plastic products in an environment with a relative humidity of 40%-60% to maintain their performance and appearance.
3. Comparative Study on Aging Behavior of Plastic Disposable Food Boxes of Different Materials
3.1 Aging Behavior Analysis of PET Material
PET (polyethylene terephthalate) plastic disposable food boxes exhibit unique aging behaviors under various environmental conditions.
3.1.1 Aging Behavior under High Temperature Environments
The heat resistance of PET material is relatively limited, which is determined by its molecular structure. The glass transition temperature of PET is approximately 70℃. Under high temperature environments, molecular chain relaxation easily occurs, leading to can deformation or reduced strength. Studies have shown that PET plastic containers can withstand temperatures of 60℃ to 80℃ for short periods, but if the temperature exceeds 80℃, the PET container may begin to soften, leading to structural instability.
Under long-term high temperature environments, the performance of PET will significantly decline. For example, in environments continuously exposed to temperatures above 50℃, PET may experience molecular chain relaxation, leading to can deformation or reduced strength. Furthermore, high temperatures may accelerate the aging process of PET, making it brittle or causing it to lose transparency.
Of particular note is that PET may undergo hydrolysis at high temperatures, especially in humid environments. Hydrolysis leads to the breakage of PET molecular chains, reducing its mechanical properties and heat resistance. Research data shows that chain breakage in PET at 60℃ and 100% relative humidity is five times greater than at 45% relative humidity.
3.1.2 Aging Behavior Under Light Exposure
The aging of PET under light exposure is mainly caused by photo-oxidation. Studies show that PET can degrade through thermal oxidation in natural environments, but under environmental conditions, hydrolytic breakage and photo-oxidation induced by ultraviolet light are more common.
Studies on PET microplastics revealed that after 12 months of light exposure, no new infrared absorption peaks appeared in its infrared spectrum, but the infrared absorption peaks at 1625 cm⁻¹ and 1693 cm⁻¹ were significantly enhanced. These two peaks represent the stretching vibrations of C=C bonds and carboxyl groups on the aliphatic chains, respectively. Therefore, after 12 months of light exposure, PET microplastics develop C=C double bonds and carboxyl groups.
While PET material exhibits good resistance to light aging and can resist UV erosion for a certain period, maintaining high mechanical strength and transparency, its ability to block UV rays is relatively weak. Prolonged exposure to sunlight can easily lead to aging, yellowing, and even degradation.
3.1.3 Aging Behavior in Humid Environments
PET is prone to hydrolysis in humid environments, which is one of its main aging mechanisms. Hydrolysis causes the PET molecular chains to break, generating carboxyl and hydroxyl groups, thereby reducing the material's molecular weight and mechanical properties.
Studies have shown that the hydrolysis rate of PET is closely related to temperature and humidity. Under high temperature and high humidity conditions, the hydrolysis reaction is significantly accelerated. For example, at 60℃ and 100% relative humidity, the chain breaking rate of PET is five times that at 45% relative humidity.
In practical use, PET disposable food boxes are not suitable for long-term storage in humid environments, especially for holding and storing foods with high water content for extended periods. Furthermore, washed PET disposable food boxes should be thoroughly dried before storage to reduce the occurrence of hydrolysis.
3.2 Analysis of Aging Behavior of PP Material
PP (polypropylene) is currently recognized as a relatively safe food-grade plastic, but its aging behavior exhibits unique characteristics.
3.2.1 Aging Behavior under High Temperature Environments
PP material possesses excellent thermal stability, with a melting point as high as 167℃ and a typical operating temperature range of -6℃ to 120℃. Modified PP can even withstand extreme environments ranging from -18℃ to 110℃. However, even PP material with excellent heat resistance will age under prolonged high-temperature environments.
Studies have shown that although PP material is heat-resistant up to 120℃, continuously containing food at temperatures above 80℃ will accelerate the release of low-molecular-weight compounds. Under high-temperature environments, tertiary carbon atoms on the PP molecular chain are easily oxidized, forming peroxides, which in turn trigger chain reactions, leading to molecular chain breakage and performance degradation.
Thermal aging tests show that the mechanical properties of PP material change significantly during the aging process. With increasing aging time, the tensile and flexural strengths of PP materials initially increase and then decrease, reaching peak values at 920 hours and 680 hours, respectively, while the notched impact strength of the cantilever beam gradually decreases.
3.2.2 Aging Behavior under Light Irradiation
PP materials are relatively sensitive to ultraviolet radiation and are prone to photo-oxidation reactions. Studies have shown that PP materials have better resistance to photoaging than PE, but under high-intensity ultraviolet irradiation, oxidative degradation still occurs, leading to a decrease in material strength.
The mechanism of PP photoaging is mainly due to the large number of tertiary carbon atoms in its molecular chain, which have a strong ability to lose electrons. In the presence of oxygen, only a small amount of energy is needed to break the C-H bond, forming active tertiary carbon free radicals. These free radicals react with oxygen to generate peroxy free radicals, initiating a chain reaction, ultimately leading to molecular chain breakage and performance degradation.
Studies have found that the thermal stability of PP materials decreases significantly during aging. After ultraviolet irradiation, its initial degradation temperature decreases by 100℃, and the maximum thermal degradation rate increases by nearly 5 times. This indicates that photoaging not only affects the mechanical properties of PP but also significantly impacts its thermal stability.
3.2.3 Aging Behavior in Humid Environments
PP material ages relatively less in humid environments, primarily due to its non-polar molecular structure, which provides good water resistance. However, under specific conditions, humid environments can still affect the aging of PP.
Studies show that humidity's impact on PP aging mainly manifests in promoting oxidation reactions. In high-humidity environments, water molecules can accelerate oxygen diffusion within the material, thereby accelerating the oxidation reaction. Simultaneously, humidity may also affect the migration and precipitation of additives in PP, indirectly influencing its aging behavior.
In practical applications, PP disposable food boxes perform better than other materials in humid environments, but their sealing performance may be affected. Silicone sealing rings are prone to mold growth in humid environments, requiring regular cleaning and replacement.
3.3 Aging Behavior Analysis of PS Material
Due to its unique molecular structure, PS (polystyrene) exhibits significantly different aging behavior compared to PET and PP under various environmental conditions.
3.3.1 Aging Behavior under High Temperature Environment
PS material has poor heat resistance, which is its biggest weakness. Studies show that the continuous use temperature of PS is only around 60℃, and should not exceed 80℃. It begins to soften at 75℃, and above 80℃, it releases styrene monomers, which may harm the central nervous system with long-term ingestion.
The aging mechanism of PS under high temperature is mainly thermal oxidation and molecular chain breakage. High temperature causes increased movement of PS molecular chains and weakens the intermolecular forces, making chain breakage more likely. At the same time, high temperature also promotes the release of styrene monomers, which is the most important safety concern during the aging process of PS.
In practical applications, it has been found that some PS disposable food boxes, when filled with 60℃ hot soup, showed styrene migration exceeding the standard by 3 times, highlighting its high-temperature risk. Therefore, PS disposable food boxes are strictly prohibited from being used to hold hot food or for microwave heating.
3.3.2 Aging Behavior under Light Exposure
PS material is highly sensitive to ultraviolet radiation and is prone to photo-oxidation. Studies have shown that PS exhibits different aging characteristics under UV irradiation in air, pure water, and seawater, with the most significant aging occurring in air.
Research on PS microplastics revealed significant changes in the infrared spectrum after 3 months of light exposure, with new absorption peaks appearing at 1060, 1254, 1540, and 1576 cm⁻¹, corresponding to C-O stretching vibrations and C=C stretching vibrations of aromatic hydrocarbons, respectively. After 6 months of light exposure, new absorption peaks appeared at 1733 cm⁻¹ and 3673 cm⁻¹, corresponding to C=O stretching vibrations and O-H stretching vibrations, respectively.
Photoaging of PS leads to surface cracking and embrittlement, while also releasing harmful monomers such as styrene. Studies indicate that under UV irradiation, PS undergoes photo-oxidation, producing oxygen-containing functional groups such as carbonyl and hydroxyl groups, which significantly affect the material's properties.
3.3.3 Aging Behavior in Humid Environments
PS material ages relatively less in humid environments, mainly due to its non-polar molecular structure, which gives it good water resistance. However, under certain conditions, humid environments can still affect the aging of PS.
Studies have found that in humid environments, a water film easily forms on the PS surface, which affects ultraviolet light penetration and thus the rate of photoaging. Simultaneously, dissolved oxygen and other impurities in the water may accelerate the oxidation reaction of PS.
In practical applications, PS disposable food boxes are prone to deformation and mold growth in humid environments. Especially PS disposable food boxes stored for extended periods in high humidity environments develop white mold spots on the surface, affecting aesthetics and hygiene.
3.4 Comprehensive Comparison of Aging Behavior Among Materials
Through analysis of the aging behavior of PET, PP, and PS under different environmental conditions, the following comparative results can be summarized:
3.4.1 Comparison of Aging Speeds
According to research data, there are significant differences in the aging speeds of different materials. Under the same aging conditions, PS material ages the fastest, followed by PE, while PET and PP are relatively slower.
Specific aging time comparisons show that under xenon lamp exposure conditions, the complete aging time for PET nonwoven fabric is 1960 hours, while for PP nonwoven fabric it is only 320 hours. This indicates that PET's aging resistance is far superior to PP.
In natural environments, the degradation time of different materials also varies greatly. Studies show that if all are landfilled in the same environment, PP and PS plastic fast disposable food boxes require approximately 20 to 30 years and 40 to 50 years to degrade, respectively, while EPS (expanded polystyrene) plastic fast disposable food boxes require over a hundred years to degrade.
3.4.2 Comparison of Aging Characteristics
The aging characteristics of different materials also vary:
- PET: During aging, C=C double bonds and carboxyl groups are mainly produced. The surface is prone to cracking and decreased transparency, but mechanical strength remains relatively good.
- PP: During aging, mechanical properties change complexly. Tensile strength and flexural strength initially increase and then decrease, while impact strength continuously decreases. It is prone to yellowing due to photo-oxidation.
- PS: Aging rate is the fastest, prone to molecular chain breakage, producing a large number of carbonyl and hydroxyl groups, surface cracking and brittleness, and releasing styrene monomers.
3.4.3 Environmental Adaptability Comparison
Different materials have different adaptability to various environmental factors:
- High Temperature Resistance: PP > PET > PS. PP can withstand high temperatures of 120-130℃, PET can withstand 60-80℃ for short periods, and PS can only withstand 60-80℃.
- UV Resistance: PET > PP > PS (untreated). PET has better resistance to photoaging, PP requires the addition of light stabilizers, and PS is most prone to photoaging.
- Moisture Resistance: PP ≈ PS > PET. PP and PS have better water resistance due to their non-polar structure, while PET is prone to hydrolysis.
- Chemical Stability: PP > PET > PS. PP can withstand most chemicals, PET is resistant to acids and alkalis but not to organic solvents, and PS is not resistant to aromatic solvents.
Based on these comparative results, the appropriate material should be selected according to the specific usage environment and requirements when choosing and using plastic lunch boxes.
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