Disposable plastic drink cups, widely used containers in daily life, have deformation issues that directly affect their safety and convenience. Understanding the conditions under which deformation occurs is crucial for the correct selection and use of disposable plastic drink cups. From material properties and physical principles to actual application scenarios, the factors leading to the deformation of disposable plastic drink cups are multifaceted. The following will analyze various specific situations in detail.
I. Deformation Caused by High-Temperature Environments
1.1 Differences in Material Heat Resistance
Disposable plastic drink cups are mainly made of thermoplastic plastics such as polystyrene (PS), polypropylene (PP), and polyethene terephthalate (PET). These materials have significant differences in heat resistance.
Polystyrene (PS) cups have relatively poor heat resistance, with a long-term use temperature range of 0-70℃, and deformation occurs at 60-80℃. The heat distortion temperature of PS is 70-90℃ (0.45MPa), and the glass transition temperature is 80-105℃. However, due to its brittleness, it is rarely used in high-temperature environments in practice. In high-temperature environments, PS cups may experience softening and collapse of the cup wall, bulging of the cup bottom, or overall twisting and deformation.
Polypropylene (PP) cups have better heat resistance and can be used at temperatures above 100℃, reaching 120℃ under light load. The maximum continuous use temperature under no load can reach 120℃, and the short-term use temperature can reach 150℃. The heat distortion temperature of PP is 60-100℃ (1.82MPa), and it can exceed 100℃ under a 0.45MPa load. The melting point of PP is as high as 167℃, and the glass transition temperature is between -10℃ and -20℃, which makes the structural dimensions of PP cups relatively stable in the temperature range from room temperature to boiling water temperature.
Polyethene terephthalate (PET) cups have poor heat resistance and can only withstand temperatures of about 70℃; exceeding this temperature easily leads to deformation. The glass transition temperature of PET is 67-81℃, and its melting point is 244-260℃. However, due to its relatively low glass transition temperature, it is prone to deformation when in contact with high-temperature liquids.
1.2 Specific Manifestations of Thermal Deformation
The deformation of disposable plastic drink cups caused by high temperatures manifests in various ways, mainly including:
Cup wall softening and collapse: When the temperature exceeds the glass transition temperature of the material, the mobility of the plastic molecular chains increases, and the material begins to soften. At this point, the cup wall cannot withstand its own weight and the pressure of the liquid inside, leading to inward collapse. Especially with PS and PET cups, when containing liquids above 70℃, the cup walls will noticeably soften, and a slight pressure with the hand will reveal significant deformation.
- Cup bottom bulging deformation: High temperatures cause the cup bottom material to expand. If the cup bottom is improperly designed or the material's heat resistance is insufficient, the cup bottom will bulge upwards. This deformation not only affects the stability of the cup but may also lead to liquid spillage. In practical use, when PS cups contain hot water above 80℃, the bottom often shows significant bulging.
- Overall twisting deformation: When different parts of the cup are heated unevenly, different degrees of expansion occur, leading to overall twisting deformation. This is common when the cup is partially immersed in hot water or heated unevenly in a microwave oven.
- Cracking and breakage: If the temperature changes rapidly or exceeds the material's limit temperature, the plastic cup may crack or break. Especially during alternating use of hot and cold liquids, thermal stress can cause cracks to form inside the material, ultimately leading to cup failure.
1.3 High-Temperature Deformation Test Standards
According to relevant standards, the heat resistance test for disposable plastic drink cups usually uses the following methods:
Thermal Shock Resistance Test: According to ISO 22088-3 standard, a temperature cycling test (-20℃ to 100℃) is performed for at least 50 cycles, and observed for cracks.
Actual Use Temperature Test: The cup is filled with liquid at a specified temperature (usually 80℃ or 95℃) and held for a certain period (e.g., 30 minutes), observing for deformation, leakage, etc.
II. Deformation Caused by Low-Temperature Environment
2.1 Low-Temperature Brittleness Characteristics of Materials
Low-temperature environments significantly affect the mechanical properties of disposable plastic drink cups, making them brittle and prone to cracking or breaking under stress. Different plastic materials have different glass transition temperatures and low-temperature properties.
Polystyrene (PS) exhibits significant brittleness at low temperatures, with a brittle temperature of approximately -30℃. PS itself is a hard, brittle material lacking ductility, and it breaks near the yield point during stretching. In low-temperature environments, the brittleness of PS is even more pronounced, and it may crack or even shatter with slight impact.
Polypropylene (PP) has a glass transition temperature between -10℃ and 0℃, and becomes brittle like glass at refrigeration temperatures. When the temperature approaches its critical embrittlement point, the toughness of PP decreases significantly, transforming from a soft plastic to a brittle plastic. This is why PP plastic bottles are prone to breaking after refrigeration.
Polyethylene terephthalate (PET) also becomes brittle at low temperatures. Ordinary PET is extremely prone to brittle fracture under low-temperature freezing conditions and is not suitable for long-term low-temperature storage. Although the theoretical glass transition temperature of PET is 67-81℃, its toughness decreases significantly in low-temperature environments, especially when subjected to external impact, making it prone to cracking.
2.2 Manifestations of Low-Temperature Deformation
The deformation of disposable plastic drink cups due to low temperatures mainly manifests as:
Brittle cracking: Low temperatures weaken the movement of plastic molecular chains, reducing the material's toughness and increasing its brittleness. When subjected to external forces, such as handling, placement, or slight collisions, the cups are prone to cracking. This cracking usually occurs suddenly without obvious warning signs.
Stress concentration fracture: In the weak parts of the cup, such as the rim, bottom, or connections of the cup wall, low temperatures exacerbate stress concentration, making these areas more susceptible to fracture. Especially when stacked, the bottom cups bear the weight of the cups above, making them more likely to crack at low temperatures.
Cold shrinkage deformation: Low temperatures cause plastic materials to shrink. If the shrinkage is uneven, internal stress will be generated, leading to cup deformation. This deformation may manifest as rim shrinkage, body bending, or overall shape change.
Microcrack formation: Although not directly observable with the naked eye, tiny cracks may form inside the plastic cup in low-temperature environments. These microcracks may expand during subsequent use, especially during temperature changes or when subjected to stress, leading to cup failure.
2.3 Factors Affecting Low-Temperature Environments
Refrigeration environment (0-10℃): At refrigeration temperatures, the toughness of PP cups decreases significantly. According to experimental observations, PP plastic bottles become particularly brittle after refrigeration and may break with a single drop. This is because the refrigeration temperature is close to the glass transition temperature of PP, causing it to transform from a soft plastic to a brittle plastic.
Freezing environment (below -18℃): In freezing environments, all types of disposable plastic drink cups become more fragile. PET cups are prone to brittle fracture at -18℃, especially when filled with liquid and frozen. Due to the expansion of the liquid upon freezing, the cups are more likely to break.
Rate of temperature change: Rapid temperature changes are more dangerous than constant low temperatures. When a cup is suddenly moved from a low-temperature environment to a high-temperature environment, or vice versa, thermal stress is generated, leading to material cracking. This phenomenon is particularly evident when hot water is immediately poured into a cup after refrigeration.
External force: In low-temperature environments, the load-bearing capacity of plastic drink cups decreases. Even normal handling and placement can cause the cups to break. Especially when stacked, the pressure on the bottom cup, particularly at low temperatures, is more likely to cause breakage.
III. Deformation Caused by Load-Bearing Weight
3.1 Structural Design and Load-Bearing Capacity
The structural design of disposable plastic drink cups primarily focuses on holding liquids rather than bearing external weight; therefore, their load-bearing capacity is limited. According to the national standard GB18006.1, the standard load for disposable plastic drink cups is 3 kilograms, meaning that when a 3-kilogram weight is placed on the cup, the height change of the cup within one minute should not exceed 5% of its original height.
However, actual market research shows a low pass rate for the load-bearing performance of disposable plastic drink cups. A surprise inspection by the Shanghai Municipal Bureau of Quality Supervision found that only 2 out of 6 different brands and models of disposable plastic drink cups could bear a load of 3 kilograms, resulting in a failure rate as high as 66.7%. This indicates that the actual load-bearing capacity of most disposable plastic drink cups is lower than the standard requirements.
The load-bearing capacity of a cup is closely related to its structural design. Cup wall thickness is an important factor affecting load-bearing capacity; thickening the cup wall can significantly improve compressive strength. For example, aviation cups with a thickened design can withstand approximately 5 kilograms of vertical pressure without deformation. Some specially designed cups, such as those with a triangular support system, can even withstand 50 kilograms of pressure without breaking.
The reinforced structure of the cup rim is also important. A 0.8 mm-thickened ring design at the rim can lock the overall structure like a steel reinforcement ring, improving the stability of the cup. The inclination angle of the cup body also affects the load-bearing capacity; a 15° optimal inclination angle can form a triangular support system, effectively dispersing pressure. The anti-slip layer design at the bottom of the cup counteracts stacking pressure through friction, improving stability.
3.2 Manifestations of Load-Bearing Deformation
Deformation of disposable plastic drink cups caused by load-bearing weight mainly manifests as:
Bottom indentation: When the load-bearing weight exceeds the cup's capacity, a noticeable indentation will appear at the bottom of the cup. This indentation may be localised or involve the entire bottom of the cup. The degree of indentation depends on the magnitude and duration of the load-bearing weight.
Cup wall bending: Under vertical pressure, the cup wall will bend inward. If the cup wall is too thin or the material strength is insufficient, the cup wall may show significant wrinkles or permanent deformation.
Overall flattening: When the load is too heavy, the cup may be completely flattened. Especially, some cups made of softer materials may be completely flattened and lose their functionality when subjected to a weight of more than 3 kg.
Structural damage: In extreme cases, excessive load may lead to structural damage to the cup, such as the bottom falling off, the cup wall cracking, or the rim tearing. This damage is usually irreversible.
3.3 Factors Affecting Load-Bearing Capacity
Material type: Different materials have significantly different mechanical strengths. PP material has higher compressive strength than PS. The third-generation improved PP material, through molecular structure reorganisation, increases compressive strength by 40% while reducing the cup wall thickness by 15%. Although PS material has higher hardness, it is brittle and prone to brittle fracture under load.
Manufacturing process: The manufacturing process of the cup also affects its load-bearing capacity. Injection-moulded cups are usually stronger than thermoformed cups. The precision of the mould, cooling speed, and material distribution all affect the strength of the final product.
Method of use: The load-bearing capacity of the cup is also related to the method of use. If the weight is evenly distributed, the load-bearing capacity of the cup will increase; if the weight is concentrated at one point, it is easy to cause stress concentration, leading to local damage.
Environmental conditions: Temperature also affects the load-bearing capacity. At high temperatures, plastic materials soften, and the load-bearing capacity decreases; at low temperatures, the material becomes brittle, and although the hardness may increase, the toughness decreases, making it prone to brittle fracture.
IV. Deformation Caused by Chemical Environment
4.1 Chemical Corrosion Mechanisms
The impact of the chemical environment on disposable plastic drink cups is mainly achieved through chemical corrosion and dissolution. Different chemical substances have varying degrees of impact on plastic materials; some may cause material softening, while others may lead to swelling or dissolution.
Acidic and Alkaline Environments: Both acidic and alkaline environments affect the performance of plastic drink cups. Acidic beverages (such as lemon juice, carbonated drinks) corrode the plastic surface, accelerating the release of harmful substances; alkaline liquids (such as soda water, soapy water) break down the plastic molecular chains, making the cup brittle and prone to cracking. Studies have shown that both acidic and alkaline pH values of the contact solution enhance the surface degradation of plastics, potentially exacerbating the release of microplastics.
Influence of Organic Solvents: Organic solvents have a more severe impact on plastic drink cups. Experiments have shown that when disposable plastic drink cups come into contact with ethyl acetate, a serious dissolution reaction occurs. When ethyl acetate was poured into a PS cup, the liquid immediately foamed and made a "hissing" sound, and the bottom of the cup instantly disappeared; although the reaction was not as violent with PP cups, the bottom became soft after 40 minutes, showing a significant physical curvature; when a foamed plastic sheet encountered ethyl acetate, a 3cm thick sheet was burned through in 2 seconds.
Oily Substances: Although oily substances do not dissolve plastics immediately like organic solvents, long-term contact can lead to plastic swelling, changing the physical properties of the material. Especially under high-temperature conditions, oils can accelerate the leaching of additives from the plastic, affecting the strength and stability of the cup.
4.2 Manifestations of Chemical Deformation
The deformation of disposable plastic drink cups caused by the chemical environment mainly manifests as:
Surface Corrosion: Long-term contact with chemical substances can lead to corrosion marks on the surface of the cup, appearing as a rough surface, loss of gloss, or the appearance of spots. This corrosion is usually progressive; it may not be obvious initially, but it gradually worsens over time.
Swelling Deformation: Some chemical substances are absorbed by plastic molecules, causing the material to swell. Swelling changes the size and shape of the cup, potentially leading to a larger opening, thicker cup walls, or overall deformation.
Softening Deformation: Chemical substances may damage the molecular structure of the plastic, leading to material softening. Softened cups may deform under normal use pressure, such as the collapse of the cup wall or indentation of the cup bottom.
Dissolution damage: In extreme cases, strong solvents may cause partial or complete dissolution of plastic drink cups. For example, the instantaneous dissolution of PS cups when in contact with ethyl acetate is a typical example of chemical dissolution damage.
4.3 Chemical Risks in Daily Use
In daily life, disposable plastic drink cups often come into contact with chemical substances, requiring special attention:
Food and beverage scenarios: The reaction between cooking wine and vinegar used in cooking produces ethyl acetate. Although the concentration is not high, long-term contact may affect plastic drink cups. Especially when packaging food containing a lot of liquid, the organic acids and oils in the liquid may cause the cup to deform.
Cleaning and disinfection: Using cleaning agents containing alcohol, bleach, etc., to clean plastic drink cups may cause damage to the cups. High-concentration disinfectants, in particular, may cause surface corrosion or degradation of material properties.
Special uses: If disposable plastic drink cups are used to hold non-food liquids, such as medicines, cosmetics, cleaning agents, etc., chemical compatibility needs to be considered. These liquids may contain chemical components that are harmful to plastics.
V. Deformation Caused by Ultraviolet Radiation
5.1 Photoaging Mechanism
Ultraviolet radiation is an important factor causing aging and deformation of disposable plastic drink cups. The effect of ultraviolet radiation (wavelength 200-400nm) on plastic materials is mainly achieved through two mechanisms: photo-oxidative degradation and photo-induced crosslinking.
- Photo-oxidative degradation: Ultraviolet radiation has extremely high energy and can directly break chemical bonds such as C-C and C-H in plastic molecular chains, generating free radicals. These free radicals quickly combine with oxygen to form peroxy radicals (ROO•), which further generate hydroperoxides (ROOH). Hydroperoxides decompose under light or heat, causing chain scission and generating new double bonds and carbonyl groups. These conjugated structures absorb visible light, causing the material to yellow.
- Photo-induced crosslinking: In some cases, ultraviolet radiation can cause the formation of new chemical bonds between plastic molecules, leading to molecular chain crosslinking. This crosslinking increases the hardness of the material, but at the same time reduces its toughness, making the material brittle. Studies show that ultraviolet light with wavelengths between 290-320nm is the most damaging to PET, which happens to be the main wavelength range of ultraviolet light from the sun that reaches the Earth's surface.
5.2 Deformation Caused by Ultraviolet Radiation
The deformation of disposable plastic drink cups caused by ultraviolet radiation mainly manifests as:Company profile
- Yellowing: This is the most intuitive manifestation. With prolonged exposure to ultraviolet light, the plastic drink cups gradually turn yellow, affecting their appearance. The degree of yellowing is related to the intensity of ultraviolet light and the exposure time.
- Surface Powdering: Ultraviolet radiation causes the molecular chains on the plastic surface to break, forming fine powdery substances. This phenomenon creates a layer of "white powder" on the surface of the cup, which can be wiped off with a hand.
- Brittleness and Deformation: Photoaging reduces the mechanical properties of plastic materials, especially reducing toughness and increasing brittleness. When subjected to external forces, aged cups are more prone to cracking or breaking.
- Decreased Mechanical Properties: Long-term ultraviolet radiation leads to a significant decrease in the tensile strength, bending strength, and other mechanical properties of the plastic. Experiments show that after 500 hours of exposure under UV radiation intensity of 0.75W/m² and a wavelength of 340nm, the impact strength of the plastic drink cups is significantly reduced.
5.3 Factors Affecting Ultraviolet Radiation
- Exposure Time: The cumulative effect of ultraviolet radiation is significant; the longer the exposure time, the greater the damage to the cups. Cups used outdoors or stored in direct sunlight for extended periods will age faster.
- Ultraviolet Intensity: The intensity of ultraviolet radiation varies in different regions and seasons. In tropical regions or during summer, the ultraviolet intensity is high, and the ageing rate of the cups will accelerate.
- Material Type: Different plastic materials have different sensitivities to ultraviolet light. PS and PP are more sensitive to ultraviolet light and are prone to photoaging; PET has relatively good photostability, but it will also degrade under the combined action of high temperature and ultraviolet light.
- Temperature Factors: High temperatures accelerate the damaging effect of ultraviolet light on plastics. In high-temperature environments, ultraviolet radiation intensifies the movement of plastic molecular chains, making them more susceptible to breakage and oxidation reactions.
Ⅵ. Other Factors Causing Deformation
6.1 Stress Relaxation Phenomenon
Stress relaxation refers to the phenomenon where the stress in a material gradually decreases over time under constant strain. For disposable plastic drink cups, prolonged exposure to certain stresses (such as pressure from stacking) can lead to creep and stress relaxation of the material.
In practical use, stacked disposable plastic drink cups bear the weight of the cups above them. Over time, the bottom cups will undergo slow deformation, which may be permanent. This stress relaxation phenomenon is particularly noticeable in high-temperature environments.
6.2 Manufacturing Defects
Defects in the manufacturing process of disposable plastic drink cups can also lead to deformation during use:
Uneven wall thickness: If the cup has uneven wall thickness distribution during injection molding or thermoforming, weak points are prone to deformation or cracking during use.
Internal stress: If internal stress generated during the manufacturing process is not fully released, it may cause the cup to deform or crack during use. Temperature changes can exacerbate this deformation.
Material defects: Using recycled materials or low-quality raw materials may lead to unstable performance of the cups and make them prone to deformation.
6.3 Improper Use and Storage
Improper use: Using disposable plastic drink cups to hold liquids exceeding their temperature resistance range, heating unsuitable cups in a microwave oven, or using cups to bear weight beyond their design capacity can all lead to deformation.
Improper storage: Stacking too high can cause the bottom cups to bear excessive pressure; storing in a humid environment may lead to material degradation; and storing with sharp objects may cause scratches, creating stress concentration points.
Repeated use: Disposable plastic drink cups are designed for single use. Repeated use accelerates material aging, leading to decreased performance and increased susceptibility to deformation.
The deformation of disposable plastic drink cups is the result of the combined effects of multiple factors, mainly including temperature effects (high-temperature softening and low-temperature embrittlement), load-bearing weight (deformation due to exceeding the design load), chemical environment (acid-base corrosion and solvent dissolution), ultraviolet radiation (photoaging leading to performance degradation), and improper use and storage.
Different materials of disposable plastic drink cups have different sensitivities to various deformation factors: PP material has better heat resistance and mechanical strength, suitable for hot drinks; PS material is inexpensive but has poor heat resistance and is prone to brittle fracture; PET material has high transparency but poor heat resistance and low-temperature toughness. To reduce the problem of deformation in disposable plastic drink cups, consumers are advised to choose products that meet national standards and are made of suitable materials when purchasing them. They should also pay attention to temperature control during use, avoid contact with harmful chemicals, and store the cups properly. At the same time, manufacturers should improve product quality, strictly adhere to relevant standards, and reduce the occurrence of deformation problems at the source. Only through the joint efforts of consumers and manufacturers can the safety and reliability of disposable plastic drink cups during use be ensured.
