Why Do PET Transparent Plastic Cups Break When Frozen?
author: Iris
2025-12-11
1. Introduction
PET (polyethylene terephthalate), a widely used thermoplastic polyester material in the food and beverage packaging industry, holds a significant market position due to its excellent transparency, mechanical strength, and chemical stability. However, the problem of PET transparent plastic cups cracking in freezing environments not only causes product loss but also poses potential safety hazards, especially in cold chain logistics and frozen food packaging.
The phenomenon of PET transparent plastic cup cracking during freezing involves multiple disciplines, including materials science, thermodynamics, and fluid mechanics. Research data shows that when a 500ml bottle of water is completely frozen, its volume expands by 9%, generating an internal pressure exceeding 2.5 MPa in a sealed PET bottle, equivalent to the water pressure at a depth of 200 meters in the deep sea. This high pressure not only causes the bottle to bulge and deform but also accelerates the fracture of plastic molecular chains, ultimately leading to cracking. This report starts from the basic characteristics of PET material, deeply analyzes the thermodynamic changes during the freezing process and the cracking patterns under different filling states, and combines structural design and manufacturing processes to construct a theoretical framework for the cracking mechanism, providing a scientific basis and improvement directions for the low-temperature application of PET transparent plastic cups.
2. Basic Characteristics and Low-Temperature Performance of PET Material
2.1 Molecular Structure and Basic Physical Properties of PET Material
PET is formed by the polycondensation reaction of terephthalic acid (TPA) and ethylene glycol (EG). The molecular chain structure unit contains rigid benzene rings, flexible methylene groups (-CH₂-CH₂-), and polar ester groups. The benzene ring and ester group form a conjugated system, enhancing the rigidity of the molecular chain, making it difficult for the flexible methylene groups to fully exert their effect, resulting in relatively strong rigidity of PET macromolecules at room temperature.
Key physical property parameters of PET material are as follows: glass transition temperature (Tg) is 70-85°C, melting point (Tm) is 245-260°C, melting enthalpy is 140 J/g, decomposition temperature is 425-445°C; density is 1.33-1.45 g/cm³, possessing excellent transparency and impact resistance, making it an ideal choice for beverage packaging. However, this molecular structure also poses a risk of low-temperature brittleness – the short, flexible chain segments and benzene rings need to move as a whole. While the highly rigid molecular chains provide good mechanical properties, they also make the material prone to losing toughness at low temperatures.
2.2 Changes in Mechanical Properties of PET Materials in Low-Temperature Environments
Low temperatures significantly alter the mechanical properties of PET materials, primarily resulting in increased brittleness, decreased toughness, and deteriorated impact resistance. When the temperature drops below 0°C, the mobility of PET molecular chains weakens, and the material gradually loses its toughness; at -20°C, the tensile strength increases by 15%-20% compared to room temperature, but the elongation at break decreases significantly, and the breakage rate in low-temperature drop tests increases noticeably; at -40°C, the impact strength may decrease by more than 50% compared to room temperature.
The root cause of PET's low-temperature brittleness lies in the restricted movement of molecular chains: when the temperature is below the glass transition temperature, the mobility of molecular chains decreases significantly, and the material becomes rigid and brittle. In extremely low-temperature environments of -20°C to -40°C, PET plastic containers are prone to cracking and breaking under impact or pressure. However, some studies show that PET still maintains durability in freezing environments; this difference is related to the material grade, additive formulation, and testing conditions.
2.3 Differences in Low-Temperature Performance of Different Grades of PET Materials
The low-temperature performance of PET is affected by factors such as molecular weight, crystallinity, and additives, and different grades of products show significant differences. Copolymerization modification, by randomly inserting aliphatic polyester units into the aromatic PET polymer chain, can effectively improve low-temperature performance – introducing flexible chains such as aliphatic dicarboxylic acids and polyethers into the molecular chain can increase the flexibility of the molecular chain. Studies show that when the adipic acid content reaches 15%, low-temperature resistant PET can be used normally at -20°C, and as the adipic acid content increases, the material viscosity and polycondensation reaction rate increase, while Tg and Tm gradually decrease.
Crystallinity is a key factor affecting the cold resistance of PET: PET with high crystallinity is more prone to embrittlement at low temperatures because the crystalline regions restrict the movement of molecular chains; PET with low crystallinity has better flexibility and impact resistance at low temperatures. Molecular weight distribution also affects cold resistance; PET with a narrower distribution of molecular chains has more uniform movement and better low-temperature performance. Furthermore, adding polyolefin plasticizers, impact modifiers, or blending with polyolefin materials can improve low-temperature flexibility and reduce brittleness.
3. Thermodynamic Analysis during Freezing
3.1 Temperature Gradient and Heat Conduction during Freezing
When PET transparent plastic cups are frozen, the temperature drops from room temperature (approximately 25°C) to -18°C or lower. The temperature changes differently in different parts, forming a complex temperature field. The temperature gradient and heat conduction characteristics directly affect the fracture behavior.
The freezing rate significantly affects PET performance: rapid freezing (cooling rate > 10°C/minute) causes a large temperature gradient within the material—PET's thermal conductivity is only 0.24 W/(m·K), so the surface temperature drops sharply while the interior cools slowly. This temperature difference induces thermal stress. The thicker the transparent plastic cup wall, the stronger the thermal stress, leading to fracture when it exceeds the material's limit. In a -20°C environment, the impact resistance of PET transparent plastic cups decreases significantly, making them prone to cracking under external force. Slow freezing (cooling rate < 1°C/minute) allows the material more time for stress relaxation, reducing the risk of fracture.
3.2 Thermal Stress Generation Mechanism and Stress Concentration Analysis
During the freezing of PET plastic cups for weddings, thermal stress is a significant factor in fracture, stemming from the material's thermal contraction and temperature gradient. When PET cools from room temperature to freezing temperature, thermal contraction occurs. If this contraction is constrained, thermal stress is generated. Based on linear elastic theory, thermal stress σ = EαΔT (E is the elastic modulus, α is the thermal expansion coefficient, and ΔT is the temperature change). For PET, with a temperature drop from 25°C to -18°C (ΔT ≈ 43°C), α = 80 × 10⁻⁶/K, and E = 2500 MPa, the calculated thermal stress is approximately 86 MPa, which is close to the yield strength, demonstrating that freezing generates significant thermal stress.
Stress concentration is a key trigger for localized fracture: the geometric shape of the PET transparent plastic cup and wall thickness defects can exacerbate stress concentration. The corners of the transparent plastic cup bottom, the bottle neck threads, and areas with sudden changes in wall thickness are common stress concentration points, where local stress can reach several times the material's strength. Computer simulations show that in the petal-shaped bottom design of PET bottles, stress concentration is most significant in the valley regions, with the maximum principal stress being higher than in other areas. The reduced material toughness at low temperatures further amplifies this effect, accelerating fracture.
3.3 Phase Transition Expansion Effect (Water Freezing)
The phase transition expansion of water during freezing is the primary factor causing the PET transparent plastic cup rupture. Studies confirm that water increases in volume by approximately 9% when it freezes, generating immense pressure in sealed containers. According to the formula ΔV/V₀ = 9.05 × 10⁻² (where ΔV is the volume change and V₀ is the initial volume), a 500ml PET transparent plastic cup filled with water and frozen will experience a volume increase of approximately 45ml, translating to an internal pressure exceeding 2.5 MPa under sealed conditions. This is equivalent to the water pressure at a depth of 200 meters in the ocean, causing the bottle body to bulge and deform, accelerating molecular chain breakage.
Research from the University of Amsterdam in the Netherlands reveals a more complex mechanism: when the top surface of the water freezes first, the remaining water freezes from the outside inward, forming pockets of liquid water surrounded by ice; when these pockets freeze, they exert immense pressure. In experiments, this pressure was approximately 260 MPa, enough to dent high-strength steel and four times the pressure that a small glass vial can withstand, further confirming the decisive role of phase transition expansion.
3.4 Temperature Change Patterns under Different Freezing Conditions
The temperature changes of PET transparent plastic cups vary under different freezing conditions, significantly impacting rupture. In a household refrigerator (-18℃), it takes 2-4 hours for a PET transparent plastic cup to freeze completely. Below -20℃, the bottle body is prone to cracking upon impact, and at -40℃, the impact strength is reduced by more than 50% compared to room temperature. Lower temperatures in commercial freezers (-25℃ to -30℃) exacerbate PET brittleness, and PET plastic containers are prone to rupture in extremely low-temperature environments (-20℃ to -40℃). Ultra-low temperatures (below -40℃) have an even more severe impact on PET transparent plastic cups; at these temperatures, the molecular chain segments of PET products have difficulty moving, resulting in extreme brittleness and limiting their application range.
The freezing rate also affects temperature changes: rapid freezing leads to larger temperature gradients and thermal stress, while slow freezing provides more time for stress relaxation. At the same time, the cooling rate affects the crystallinity of PET—when cooling is slow, the temperature of the blow molding mold is high, which is conducive to the formation of crystal nuclei and crystal growth, resulting in increased crystallinity; when cooling is fast, the degree of supercooling increases, and the crystallization rate is slower than the rate of temperature decrease, resulting in lower crystallinity.
4. Influence of Liquid Filling State on Fracture Behavior
4.1 Fracture Mechanism in Fully Filled State
When a PET plastic cup for wedding is completely filled with liquid and sealed before freezing, fracture is primarily driven by the expansion of the liquid during phase transition. The volume expansion of the liquid (mostly water) during freezing is constrained by the container, generating extremely high pressure within the sealed space. The pressure can reach over 2.5 MPa when water is completely frozen, acting on the transparent plastic cup wall and causing stress concentration at weak points, leading to fracture. Experimental observations show that when PET bottles filled with water are frozen, fractures often occur in the thin-walled or defective areas of the bottom or side walls.
Carbonated beverages pose an even higher risk when frozen: low temperatures reduce the solubility of carbon dioxide, causing gas to be released and form bubbles, further increasing internal pressure. PET bottles must withstand both the pressure from water freezing and the pressure from carbon dioxide, significantly increasing the risk of fracture. Furthermore, a fully filled PET transparent plastic cup may "explode" during thawing – the outer layer of ice melts while the inside remains solid, preventing the melted water from escaping. The sudden release of pressure triggers a fracture; there have been recorded cases of this in Brazil.
4.2 Gas-Liquid Interface Effects in Partially Filled State
When a partially filled PET transparent plastic cup is frozen, a gas space remains at the top, and the gas-liquid interface alters the pressure distribution and fracture behavior. The liquid first freezes at the transparent plastic cup wall and the liquid surface. The gas space can partially alleviate the expansion pressure through compression, resulting in relatively lower internal pressure. However, the risk of fracture still exists, stemming from the temperature gradient and stress concentration at the gas-liquid interface.
Research from the University of Amsterdam in the Netherlands found that in partially filled containers, the top surface of the water freezes first, forming an ice cap that hinders the expansion of the water below. The remaining water continues to freeze under the ice cap, forming an upwardly convex ice surface, creating stress between the ice cap and the transparent plastic cup wall. Simultaneously, the thermal conductivity of gas is much lower than that of liquid, resulting in a larger temperature gradient near the gas-liquid interface, leading to localized thermal stress concentration. Experiments show that partially filled PET transparent plastic cups often exhibit top deformation and side wall bulging after freezing, with the highest risk of fracture occurring at filling levels of 70%-90% – the gas space is insufficient to completely alleviate the expansion pressure, while the liquid volume is sufficient to produce a significant expansion effect.
4.3 Influence of Pure Thermal Stress in Empty Transparent Plastic Cups
When empty transparent plastic cups are frozen, there is no liquid phase change expansion, and cracking is entirely caused by the thermal contraction of the PET material due to temperature changes. The thermal expansion coefficient of PET is 80-100 × 10^-6/K. When the temperature drops from room temperature to -18°C, the linear contraction is approximately 0.3%-0.4%. If the contraction is constrained, tens of megapascals of thermal stress will be generated (typical PET transparent plastic cup wall thickness is 0.3-0.5 mm).
Cracking of empty transparent plastic cups mostly occurs at stress concentration points, such as the corners of the transparent plastic cup bottom and the threads of the bottle mouth. Optimizing the bottle shape and wall thickness distribution can reduce stress concentration. Uneven wall thickness will exacerbate stress concentration and is prone to cracking at low temperatures. Experiments show that although the risk of cracking in empty transparent plastic cups during freezing is lower than in filled transparent plastic cups, cracking may still occur after repeated freezing and thawing. Plastic bottles that have undergone three freeze-thaw cycles will develop spiderweb-like microcracks on the surface, and their tensile strength will decrease by 38%. In addition, tiny scratches, bubbles, and other surface defects during the manufacturing process will be magnified at low temperatures, becoming the starting points for cracking. The bottom and shoulder areas are prone to defects and are high-risk areas for cracking.
4.4 Differences in the Impact of Different Liquid Types
Different liquids have different physical properties (freezing point, density, viscosity), which significantly affect the cracking of PET transparent plastic cups:
Water-based liquids (pure water, fruit juice, etc.): Phase change occurs around 0°C, with a volume expansion of 9%. Food-grade PET that meets the GB 4806.7-2016 standard does not release harmful substances when containing liquids with a pH > 2.5, but strongly acidic liquids, such as lemon juice (pH ≈ 2.0), may accelerate PET aging and reduce low-temperature toughness.
Carbonated beverages: Low temperatures cause carbon dioxide to precipitate, and PET bottles need to withstand both freezing pressure and gas pressure. This double pressure significantly increases the risk of cracking.
Alcoholic beverages: High-alcohol concentration beverages (vodka, etc.) have low freezing points (below -20°C) and will not completely freeze in a normal refrigerator, resulting in a low risk of cracking; low-alcohol beverages (beer, etc.) contain a large amount of water, and their freezing point is close to 0°C, so they may still crack. Oils: Oils do not undergo phase transition expansion during freezing, thus having little impact on PET transparent plastic cup breakage. However, increased viscosity at low temperatures may lead to localized stress concentration, and prolonged contact with some oils can cause PET to swell, reducing its mechanical properties.
Carbonated beverages: Low temperatures cause carbon dioxide to precipitate, and PET bottles need to withstand both freezing pressure and gas pressure. This double pressure significantly increases the risk of cracking.
Alcoholic beverages: High-alcohol concentration beverages (vodka, etc.) have low freezing points (below -20°C) and will not completely freeze in a normal refrigerator, resulting in a low risk of cracking; low-alcohol beverages (beer, etc.) contain a large amount of water, and their freezing point is close to 0°C, so they may still crack. Oils: Oils do not undergo phase transition expansion during freezing, thus having little impact on PET transparent plastic cup breakage. However, increased viscosity at low temperatures may lead to localized stress concentration, and prolonged contact with some oils can cause PET to swell, reducing its mechanical properties.
5. Usage Environment and Operating Factors
5.1 Influence of Freezing Rate
The freezing rate affects the PET transparent plastic cup breakage behavior by altering the temperature change process and stress development pattern:
Rapid freezing (cooling rate > 10℃/minute): The surface temperature drops sharply, while the internal temperature decreases slowly, creating a large temperature gradient and generating high thermal stress; the material does not have enough time for stress relaxation, and the instantaneous stress easily exceeds the tolerance limit, leading to breakage.
Slow freezing (cooling rate < 1℃/minute): The temperature gradient is small, and the thermal stress is lower, but slow cooling facilitates the formation of crystal nuclei and crystal growth, increasing crystallinity and exacerbating brittleness at low temperatures.
Slow freezing (cooling rate < 1℃/minute): The temperature gradient is small, and the thermal stress is lower, but slow cooling facilitates the formation of crystal nuclei and crystal growth, increasing crystallinity and exacerbating brittleness at low temperatures.
Different freezing equipment has significantly different rates: household refrigerators have a rate of 1-5℃/hour, requiring 3-6 hours for a 500ml PET bottle to freeze completely; commercial rapid freezing equipment has a rate of >10℃/minute, requiring only 30 minutes to 1 hour for freezing. At the same time, the freezing rate affects the ice formation pattern—rapid freezing forms small ice crystals, while slow freezing forms larger ice crystals, and larger ice crystals exert stronger mechanical pressure on the PET transparent plastic cup.
5.2 Temperature Cycling and Fatigue Effects
Temperature cycling (repeated freezing and thawing) causes cumulative fatigue damage to PET material, reducing its resistance to breakage:
Damage mechanism: In each cycle, PET undergoes thermal expansion and contraction, generating alternating stress. Under long-term action, microcracks are generated internally and propagate. Experiments show that after three freeze-thaw cycles, a spiderweb-like network of microcracks forms on the surface of the plastic bottle, and the tensile strength decreases by 38%.
Influencing factors: The larger the temperature cycle amplitude (e.g., room temperature to -20℃ compared to 0℃ to -20℃), the stronger the thermal stress and the more severe the damage; the higher the cycle frequency, the faster the accumulation of fatigue damage. Specially designed PET transparent plastic cups (material modification + structural optimization) have excellent fatigue resistance. One type of ice transparent plastic cup maintained its "unbreakable" drop resistance after 300 freeze-thaw cycles (room temperature to -20℃). Crystalline Structure Changes: Repeated heating and cooling can lead to changes in the crystallinity of PET, with some crystalline regions undergoing recrystallization, and these microstructural changes subsequently affect mechanical properties.
Influencing factors: The larger the temperature cycle amplitude (e.g., room temperature to -20℃ compared to 0℃ to -20℃), the stronger the thermal stress and the more severe the damage; the higher the cycle frequency, the faster the accumulation of fatigue damage. Specially designed PET transparent plastic cups (material modification + structural optimization) have excellent fatigue resistance. One type of ice transparent plastic cup maintained its "unbreakable" drop resistance after 300 freeze-thaw cycles (room temperature to -20℃). Crystalline Structure Changes: Repeated heating and cooling can lead to changes in the crystallinity of PET, with some crystalline regions undergoing recrystallization, and these microstructural changes subsequently affect mechanical properties.
5.3 Superposition Effect of Mechanical Stress (Stacking, Compression, etc.)
PET transparent plastic cups often experience mechanical stresses such as stacking, compression, and impact during use. These stresses, superimposed with thermal stress and internal pressure stress during the freezing process, significantly increase the risk of breakage:
Stacking Stress: At room temperature, PET has good compressive strength and can withstand a certain stacking height; at low temperatures, its toughness decreases, and the same stacking pressure may lead to breakage. Stacking stability is related to the bottom design and wall thickness distribution.
Compression Stress: External pressure during packaging and transportation can be relieved by elastic deformation at room temperature; at low temperatures, the ability for elastic deformation decreases, and compressive stress may lead to plastic deformation or direct breakage.
Collision and Impact Stress: Collisions or drops during handling and loading/unloading. At room temperature, PET has good impact resistance; at low temperatures, the impact strength decreases significantly. At -18℃, the drop resistance is 41% lower than at room temperature, and the same impact is more likely to cause breakage.
Compression Stress: External pressure during packaging and transportation can be relieved by elastic deformation at room temperature; at low temperatures, the ability for elastic deformation decreases, and compressive stress may lead to plastic deformation or direct breakage.
Collision and Impact Stress: Collisions or drops during handling and loading/unloading. At room temperature, PET has good impact resistance; at low temperatures, the impact strength decreases significantly. At -18℃, the drop resistance is 41% lower than at room temperature, and the same impact is more likely to cause breakage.
The superposition effect is particularly evident in the following scenarios: material shrinkage during freezing is subject to external constraints; stacked PET disposable plastic cups with lids simultaneously experience thermal stress and compressive stress during freezing; vibration, impact, and temperature changes occur simultaneously during transportation, and the total stress exceeds the material's strength limit, leading to breakage.
5.4 Storage Conditions and Environmental Factors
Storage conditions and environmental factors affect the aging degree and low-temperature performance of PET materials, changing the risk of freezing breakage:
Storage Temperature: High temperatures accelerate the fracture of PET molecular chains and oxidation reactions, reducing low-temperature toughness. It is recommended to store in a dry environment at room temperature (15-25℃), avoiding direct sunlight.
Humidity: In high-humidity environments, PET (water absorption rate 0.1%-0.2%) may absorb moisture, and ice formation during freezing increases the risk of breakage; at the same time, humidity changes surface properties (friction coefficient, electrostatic characteristics), affecting storage and transportation behavior.
Light Exposure: Ultraviolet radiation causes photo-oxidation reactions in PET, leading to molecular chain breakage and performance degradation. PET transparent plastic cups exposed to sunlight for a long time have significantly reduced low-temperature toughness and should be stored away from light. Chemical environment: Organic solvents cause PET to swell, reducing its density and mechanical strength; strong acids and bases accelerate hydrolysis reactions, leading to molecular chain breakage. Contact with these substances should be avoided during storage.
Humidity: In high-humidity environments, PET (water absorption rate 0.1%-0.2%) may absorb moisture, and ice formation during freezing increases the risk of breakage; at the same time, humidity changes surface properties (friction coefficient, electrostatic characteristics), affecting storage and transportation behavior.
Light Exposure: Ultraviolet radiation causes photo-oxidation reactions in PET, leading to molecular chain breakage and performance degradation. PET transparent plastic cups exposed to sunlight for a long time have significantly reduced low-temperature toughness and should be stored away from light. Chemical environment: Organic solvents cause PET to swell, reducing its density and mechanical strength; strong acids and bases accelerate hydrolysis reactions, leading to molecular chain breakage. Contact with these substances should be avoided during storage.
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