Will Plastic Chinese Food Takeout Containers Become Brittle in the Refrigerator?
author: Iris
2025-11-26
I. Low-Temperature Characteristics of Common Plastic Lunch Box Materials
1.1 Main Plastic Types and Their Glass Transition Temperatures
Plastic Chinese food takeout containers on the market are mainly made of several basic polymers, each with its own unique characteristics at low temperatures. Polypropylene (PP), as one of the most common lunch box materials, has a glass transition temperature (Tg) between -18°C and 0°C. This means that at refrigerator temperatures (0-10°C), PP material can still maintain relatively stable properties, but at freezing temperatures (below -18°C), its molecular chain movement begins to be restricted.
Polystyrene (PS) has a glass transition temperature as high as 100°C, which makes PS close to its glass transition temperature at room temperature. In a refrigerator environment, the mobility of PS molecular chains is significantly suppressed, exhibiting obvious low-temperature brittleness. Polyethylene terephthalate (PET) has a glass transition temperature of approximately 70-80°C and also readily becomes brittle at low temperatures, especially under extreme freezing conditions of -20°C to -40°C, where PET containers may rupture due to impact or pressure.
Polyethylene (PE), including high-density polyethylene (HDPE) and low-density polyethylene (LDPE), has extremely low glass transition temperatures of -90°C and -110°C, respectively. Despite this, PE materials still exhibit a decrease in strength and stiffness at low temperatures, indicating that even materials with good low-temperature resistance will experience performance changes under extreme conditions.
1.2 Influence of Material Structure on Low-Temperature Performance
The molecular structure of different plastic materials determines their sensitivity to temperature changes. Amorphous polymers such as PS, PET, and polycarbonate (PC) have disordered molecular chain arrangements, making them more prone to brittleness at low temperatures. In contrast, semi-crystalline polymers such as PP and PE, due to their partially ordered molecular structure, exhibit relatively stable properties at low temperatures, but are still affected.
The rigidity and flexibility of molecular chains are key factors influencing the low-temperature performance of materials. Polymers containing aromatic rings or large side groups (such as PS and PET) typically have higher glass transition temperatures and are more prone to brittleness at low temperatures. Polymers containing flexible segments (such as PE) have lower glass transition temperatures and exhibit better low-temperature toughness.
II. Mechanism of the Influence of Low-Temperature Environments on the Performance of Plastic Food Containers
2.1 Restricted Mobility of Molecular Chains and Glass Transition
When plastic food containers are placed in a refrigerator, the decrease in temperature significantly affects the mobility of polymer molecular chains. At room temperature, polymer molecular chains can undergo thermal motion, giving the material flexibility and elasticity. However, when the temperature drops below the glass transition temperature, the movement of molecular chains is "frozen," and the material transitions from a highly elastic state to a glassy state.
The essence of this transition is that the chain segments need to overcome an energy barrier to move, and this energy barrier originates from intermolecular van der Waals forces, hydrogen bonds, and the repulsion/overlap of electron clouds. At low temperatures, molecular chains lack sufficient thermal energy to overcome energy barriers, causing materials to lose their ductility and become rigid and brittle. This change at the molecular level is directly reflected in macroscopic properties, manifesting as an increase in the elastic modulus, a decrease in elongation at break, and a reduction in impact strength.
2.2 Temperature Dependence of Mechanical Properties
The phenomenon of plastic food containers becoming brittle in refrigerators can be explained by the temperature dependence of their mechanical properties. Studies show that as temperature decreases, the elastic modulus of plastics increases significantly, while toughness indicators such as elongation at break and impact strength decrease. This change exhibits different degrees in different materials.
Taking EPDM insulation as an example, when the test temperature drops from room temperature to -60°C, its tensile strength increases from 6 MPa to 33 MPa, while its elongation at break drops sharply from 670% to 42%. Similar phenomena have been observed in other polymers: the tensile strength of polymers increases as temperature decreases from room temperature to 77 K (approximately -196°C), but decreases again as temperature continues to decrease to 20 K; Young's modulus continuously increases with decreasing temperature, while the fracture strain continuously decreases.
This temperature dependence of mechanical properties is closely related to the molecular structure of the polymer. At low temperatures, the thermal motion of molecular chains is restricted. When subjected to stress, the material cannot dissipate energy through molecular chain sliding, untangling, or "craze" (neck fission), but instead releases energy through molecular-scale fracture or crack propagation.
2.3 Effect of Crystallinity Changes
For semi-crystalline polymers such as PP and PE, low temperatures also affect their crystallinity. Studies have found that during cooling, the crystallinity of the polymer increases, leading to reduced compatibility of low molecular weight components. The increase in crystalline regions increases the rigidity and decreases the toughness of the plastic, making the material more brittle at low temperatures.
Taking PP as an example, at low temperatures, its crystallinity increases, its transparency decreases, and the degree of molecular chain folding increases, leading to a significant decrease in impact toughness. This change in crystallinity not only affects the material's optical properties but, more importantly, alters its mechanical properties, making it more susceptible to brittle fracture under external forces.
III. Actual Performance of Different Plastic Food Containers in Refrigerators
3.1 Low-Temperature Behavior of Polypropylene (PP) Food Containers
PP food containers perform relatively well in refrigerator environments, thanks to their low glass transition temperature (-18°C to 0°C). Under refrigeration conditions (0-10°C), PP material maintains good toughness and strength, and is not prone to significant brittleness. The operating temperature range of conventional PP is -6°C to 120°C, while modified PP can operate from -18°C to 110°C.
However, PP food containers still exhibit some degree of brittleness under freezing conditions. Although PP maintains relatively good mechanical properties at low temperatures, its impact toughness decreases. This means that PP take out boxes for food are more prone to breakage when subjected to impact while frozen. In practical use, it has been found that PP Chinese food takeout containers exhibit a certain tendency to become brittle when stored at refrigerated temperatures for extended periods, requiring careful handling to avoid excessive roughness.
3.2 Low-Temperature Fragility of Polystyrene (PS) Chinese Food Takeout Containers
PS Chinese food takeout containers exhibit significant low-temperature brittleness in refrigerator environments. Due to its high glass transition temperature of 100°C, PS is close to its glass transition temperature at room temperature and far below this critical temperature at refrigerator temperatures. PS material is characterized by its high rigidity and brittleness, which becomes even more pronounced at low temperatures.
Studies show that PS is considered brittle at 68°F (approximately 20°C) and becomes extremely brittle below its freezing point. Although PS has relatively good low-temperature performance and is suitable for storing refrigerated foods, its hardness and brittleness are amplified at low temperatures. In practical applications, PS Chinese food takeout containers are often used for storing cold foods such as salads, fruits, and cold dishes, but are not suitable for long-term storage under frozen conditions.
3.3 Low-Temperature Characteristics of Polyethylene Terephthalate (PET) Food Containers
PET food containers exhibit significant brittleness at low temperatures. When the temperature drops below its glass transition temperature (70-80°C), the molecular chain mobility of PET decreases significantly, making the material more rigid and brittle. At temperatures between -20°C and -40°C, PET plastic containers may crack or break due to impact or pressure.
Although PET maintains good dimensional stability at low temperatures, its impact resistance decreases significantly. This brittleness limits the application of PET in extreme low-temperature environments, especially requiring extra care in frozen food packaging.
3.4 Low-Temperature Resistance of Polyethylene (PE) Food Containers
PE food containers, including HDPE and LDPE, have relatively good impact resistance at low temperatures, but their strength and stiffness decrease significantly. HDPE exhibits particularly excellent low-temperature resistance, typically maintaining good mechanical properties between -50°C and -60°C. It does not become brittle at freezing temperatures and possesses high impact and crack resistance.
LDPE's low-temperature resistance is slightly inferior to HDPE, but it still retains some toughness around -40°C. LDPE's embrittlement temperature is -50°C, meaning it retains some flexibility above this temperature. However, studies have found that LDPE's relative elongation at break at -45°C is only 50%, far lower than the over 300% of linear low-density polyethylene (LLDPE).
3.5 Performance of Other Plastic Materials
Besides the main materials mentioned above, there are other types of plastic food containers on the market. Polyvinyl chloride (PVC) becomes brittle at low temperatures, with a decrease in both flexural and tensile strength. PVC's low-temperature resistance typically decreases significantly below 0°C, making it prone to breakage.
Although polycarbonate (PC) has a high glass transition temperature (147°C) and can maintain good mechanical properties at low temperatures, its toughness may decrease at extremely low temperatures, causing the material to become brittle and increasing the risk of cracking and breakage.
IV. Identification and Assessment of Brittleness
4.1 Appearance Changes
When plastic Chinese food takeout containers become brittle, obvious changes in appearance will occur. The most obvious manifestation is the appearance of cracks or silver streaks (fine cracks) on the surface. These defects usually appear immediately after removal from the mold or during processing. In addition, plastic Chinese food takeout containers may also exhibit discoloration, loss of luster, and changes in transparency.
For transparent PS or PET Chinese food takeout containers, transparency may decrease after brittleness, and whitening may occur on the surface. PP Chinese food takeout containers experience increased crystallinity at low temperatures, which also leads to reduced transparency. Although these appearance changes do not directly affect usability, they are often early signs of material performance degradation.
4.2 Changes in Physical Properties
Brittle plastic take-out boxes for food will undergo significant changes in physical properties. The most prominent manifestation is reduced flexibility, making them prone to breakage. High-quality PP Chinese food takeout containers should possess moderate flexibility, be able to withstand moderate bending without breaking. However, if the box is brittle, cracks appear with light pressure, or burrs are sharp on the edges, this may indicate material aging or an unqualified formulation.
Brittle plastic lunchboxes exhibit typical brittle fracture characteristics under external force: a smooth fracture surface with no obvious plastic deformation, accompanied by a distinct cracking sound, and the fracture surface often shows petal-like or pinhole-like features. This fracture mode is completely different from ductile fracture, which involves deformation, elongated fibers, or scales at the fracture surface.
4.3 Mechanical Performance Testing Indicators
From a materials science perspective, the brittleness of plastic lunchboxes can be quantitatively assessed through a series of mechanical performance indicators. Decreased fracture strain, reduced toughness (absorbed fracture energy), and reduced impact strength are three observable mechanical manifestations of embrittlement.
Specifically, when a plastic lunchbox becomes brittle, its elastic modulus increases, meaning the material becomes harder; simultaneously, the elongation at break decreases significantly, indicating a loss of ductility; and the decrease in impact strength indicates that the material is more prone to fracture under sudden external impact. The degree of change in these indicators depends on the type of plastic, temperature conditions, and usage time.
V. In-depth Analysis of the Brittleness Mechanism
5.1 Molecular Chain Motion and Energy Dissipation Mechanism
The fundamental reason why plastic Chinese food takeout containers become brittle in the refrigerator is that low temperatures restrict the movement of molecular chains, altering the material's energy dissipation mechanism. At room temperature, plastic materials can dissipate energy from external forces through the sliding, untangling, or "craze" formation of molecular chains. These mechanisms allow the material to undergo plastic deformation without fracturing under stress.
However, at low temperatures, the thermal motion of molecular chain segments is significantly restricted, and the material loses its ability to dissipate energy through the aforementioned mechanisms. When external forces are applied, the material cannot adapt to stress through molecular chain movement; instead, it releases energy through molecular-scale fracture or the rapid propagation of existing cracks. This transition from plastic deformation to brittle fracture is the essential reason why plastic bento take-out boxes become brittle in the refrigerator.
5.2 The Influence of Temperature on Molecular Chain Segment Motion
The influence of temperature on the movement of plastic molecular chain segments can be explained by the glass transition theory. The glass transition temperature (Tg) is the boundary temperature at which a polymer transforms from "brittle rubber" to "tough plastic." When the ambient temperature is below the Tg, the molecular chains are almost "frozen," only able to vibrate locally, exhibiting rigidity and brittleness, much like glass, and are not easily deformed.
The Tg values vary greatly among different plastic materials, determining their performance in a refrigerator environment. For example, PS has a Tg of 100°C, far higher than refrigerator temperatures, thus exhibiting significant brittleness under both refrigeration and freezing conditions. In contrast, PE has a Tg below -100°C and does not enter the glassy state even under freezing conditions, thus exhibiting better toughness.
5.3 Influence of Internal Stress and Crystallinity
Besides temperature directly affecting molecular chain movement, the refrigerator environment can also cause plastic food containers to become brittle through other mechanisms. The presence of internal stress accelerates the embrittlement process. During the production of plastic food containers, residual internal stress is generated due to uneven mold cooling. In low-temperature environments, these internal stresses become more concentrated, becoming the starting point for crack initiation and propagation.
For semi-crystalline polymers, low temperatures also lead to increased crystallinity. Studies have shown that increased polymer crystallinity during cooling reduces the compatibility of low molecular weight components. The rigidity of crystalline regions is much higher than that of amorphous regions; increased crystallinity means a reduction in deformable amorphous phases, making the overall material more brittle. This "cold crystallization" phenomenon is particularly pronounced in PP and PE food containers.
5.4 The Role of Time Factors
The brittleness of plastic food containers in refrigerators is a gradual process that develops over time. Studies have shown that plastics become brittle and prone to cracking under prolonged low-temperature conditions, resulting in a sharp decline in physical properties. This time dependence is related to several factors.
First, prolonged exposure to low temperatures leads to the migration and volatilization of plasticizers. Plasticizers lower the glass transition temperature of polymers, maintaining the material's flexibility. When plasticizers migrate out of the material, the local Tg increases, and the surface becomes brittle first. This "deplasticization" phenomenon is particularly evident in materials containing large amounts of plasticizers, such as PVC.
Second, even at low temperatures, the molecular chains of plastics still undergo slow movement and rearrangement. This molecular chain rearrangement occurring at low temperatures leads to changes in the internal structure of the material, potentially causing stress concentration and the formation of microcracks. Over time, these microcracks gradually propagate, eventually leading to macroscopic failure of the material.
VI. Practical Usage Recommendations and Precautions
Based on the above research and analysis, to extend the lifespan of plastic Chinese food takeout containers in refrigerators and reduce brittleness, we offer the following recommendations:
Choose the appropriate material type: For long-term refrigeration or freezing storage, prioritize PE or modified PP Chinese food takeout containers. These materials have lower glass transition temperatures and maintain good toughness at low temperatures. Avoid using PS and ordinary PET Chinese food takeout containers for long-term freezing storage.
Choose the appropriate material type: For long-term refrigeration or freezing storage, prioritize PE or modified PP Chinese food takeout containers. These materials have lower glass transition temperatures and maintain good toughness at low temperatures. Avoid using PS and ordinary PET Chinese food takeout containers for long-term freezing storage.
Control the operating temperature range: Strictly adhere to the operating temperature range indicated on the lunch box. Generally, PP Chinese food takeout containers can be used in the range of -18°C to 110°C, PE Chinese food takeout containers have a wider temperature resistance range, while PS and PET Chinese food takeout containers should be avoided for long-term storage below 0°C.
Precautions for Use: Before placing hot food into plastic food containers, allow them to cool to room temperature to avoid stress concentration caused by rapid temperature changes. When removing food containers from the refrigerator, warm them slowly to avoid sudden temperature changes. Handle with care during storage and use to avoid impact.
Regular Inspection and Replacement: Regularly check food containers for signs of damage, such as cracks or deformation. If brittleness is detected, replace them immediately to prevent breakage during use, which could lead to food contamination or personal injury.
Through a comprehensive study of the performance of various common plastic food containers in a refrigerator environment, we can draw the following conclusions: Almost all types of plastic food containers are at risk of becoming brittle in the refrigerator, but the degree varies depending on the material properties. PS and PET food containers are most prone to brittleness, PE food containers perform best, and PP food containers are in the middle.
The fundamental reason why low temperatures cause plastics to become brittle is that the movement of molecular chain segments is restricted. The material loses its ability to dissipate energy through plastic deformation and instead releases energy through brittle fracture.
Brittleness is a gradual process related to various factors such as temperature, time, and usage. Prolonged use in low-temperature environments accelerates this process.
By selecting suitable materials, controlling usage conditions, and conducting regular inspections, the embrittlement process of plastic bento take-out boxes can be effectively slowed down, extending their lifespan.
Understanding the embrittlement phenomenon and mechanism of plastic Chinese food takeout containers in refrigerators not only helps consumers use and choose Chinese food takeout containers correctly but also provides a scientific basis for lunch box manufacturers to improve product performance. With the continuous development of materials science, it is believed that more new plastic materials with excellent low-temperature performance will be applied to lunch box production in the future, providing consumers with safer and more durable products.
Will Plastic Disposable Food Boxes Age over Time?
Which Saves More Space: Round or Square Plastic Chinese Take-Out Boxes?
Related Article
You searched for 4 oz Chinese take out boxes — but what you actually need may not be a paper box at all. The folded paper container with the wire handle looks iconic, but for 4 oz portions of sauce, dressing,
4 oz Chinese Take Out Boxes Alternative — Clear PP Portion Cups
Looking for plastic to-go containers with lids in bulk? Every container we ship includes a matching lid — not as an optional add-on,
Plastic To-Go Containers with Lids Wholesale | Bulk PP Food Boxes
SEND MESSAGE