Moisture-Induced Deformation Problems in Paper Coffee Cups with Lids

I. Analysis of Paper Cup Types and Material Fundamentals

1.1 Structural Classification and Characteristics of Paper Coffee Cups with Lids

• Single-layer cups: Made from a single layer of paper material, with a simple structure and an easy manufacturing process. Advantages include low cost and light weight; disadvantages include poor insulation, making them prone to scalding when holding hot drinks, and limited moisture resistance due to the lack of an additional insulating layer, resulting in a thinner feel.
• Double-layer cups: Consist of inner and outer layers of paper material, with a space between the layers forming an air insulation layer. This design makes the paper cup stronger, able to withstand higher temperatures, less prone to deformation, provides better insulation, and offers a more comfortable grip. For example, double-layer coffee cups have an air pocket between the outer and inner layers to improve insulation, eliminating the need for an additional cup sleeve.
• Special structure cups: Represented by corrugated paper coffee cups with lids, a popular choice in the disposable coffee cup industry. They consist of three layers of cellulose paper and a top layer with an embossed structure. The embossed structure creates air spaces between the cup walls, so the hand only touches the outer "corrugated" surface, resulting in lower thermal conductivity and heat transfer rates compared to single-layer and double-layer cups. 

1.2 Materials and Performance of Paper Coffee Cups with Lids

• Paper-based material: Food-grade kraft paper or bleached cardboard is commonly used. It possesses good hardness, durability, and shape retention capabilities, allowing it to withstand the weight of liquids without collapsing. Kraft paper is made from unbleached wood pulp, offering high strength and a rustic texture; bleached cardboard has a smooth surface, making it easy to print brand logos or designs. In terms of specifications, the basis weight of ordinary paper cup base paper is usually 180-230 g/m², while categories requiring higher strength, such as coffee cups for catering, can have a basis weight of 250-300 g/m² (e.g., commonly 280 g/m²). To further enhance performance, a mixture of softwood pulp (long fiber content ≥60%) and hardwood pulp is often used in manufacturing. Long fibers increase longitudinal strength, while short fibers fill the pores, avoiding the easy softening and collapse problems caused by pure hardwood pulp.
• Coating material: This is crucial for the moisture resistance of paper coffee cups with lids. Most modern disposable paper coffee cups with lids are coated with PE (polyethylene). This thin plastic film forms a waterproof barrier, preventing liquid from penetrating the paper fibers. The thickness of the PE coating is usually 15-20 μm. Hot drink cups, which need to withstand higher temperatures, can have a coating thickness of 25-30 μm, and the inner and outer coatings need to be evenly applied to avoid localized leakage and softening. In international standards, the PE coating weight specification is 10-30 gsm, and it can be applied in a single or double layer. In recent years, to meet environmental protection needs, bio-based coating materials (such as PLA) have been gradually adopted. PLA is made from renewable resources such as corn starch and sugarcane, is biodegradable, and easily decomposes in industrial composting facilities, reducing plastic waste while retaining waterproof performance comparable to PE coatings. 

1.3 Material Differences Between Hot and Cold Drink Cups

• Material and Coating Differences: Hot drink cups typically use thicker cardboard as the base material, with an inner layer coated with food-grade PE plastic film or PLA biodegradable coating. This allows them to withstand the heat of hot liquids and prevent the cup walls from softening and deforming. In terms of coating thickness, the inner PLA coating of hot drink cups can reach 30 micrometers, and the PE coating is also thicker, able to withstand temperatures above 90°C; cold drink cups have a PE coating thickness of only 15 micrometers, unable to withstand hot water above 70°C. Traditional cold drink cups often have a wax coating on the inner wall, which is stable in a 0-5°C environment and enhances moisture resistance.
• Structural Design Differences: Hot drink cups (such as coffee and milk tea cups) only require inner lamination to meet waterproofing needs; cold drink cups, due to the low temperature of the liquid, easily form condensation on the outer surface. If this condensation penetrates the cup wall, it can cause the paper cup to soften. Therefore, both the inner and outer sides need lamination to prevent condensation from affecting the structural stability of the paper cup.

1.4 Paper Cup Manufacturing Process and Quality Control

• Manufacturing Process: Paper cup manufacturing requires combining cardboard with a liquid-resistant coating. The cardboard is first cut into a specific shape, then coated with a plastic or wax lining, and finally formed into a cup shape using a heat-sealing process. The heat-sealing process not only shapes the cup but also strengthens the seams to prevent leakage. In the coating process, to achieve a waterproof effect, the cup is inverted and immersed in a molten polyethylene solution, or a spray coating method is used to apply the plastic coating. The plastic then quickly solidifies to form a protective barrier. The entire process requires precise control of temperature and coating thickness to ensure a uniform and complete coating.
• Quality Control: According to the national standard GB/T 27590-2022 "Paper Cups," paper cup raw materials must meet strict requirements: paper cup base paper should comply with QB/T 4032 regulations; PE, PP, and PET coated paper and cardboard must comply with GB/T 36392 regulations; PLA and PBS coated paper and cardboard must comply with corresponding standards; paper coffee cups with lids must be odorless, and the use of recycled materials is prohibited; raw material additives must comply with regulations, and additives not within the scope of GB 9685, such as fluorescent whitening agents, industrial paraffin, and industrial talc powder, are strictly prohibited in paper cup production and processing.

 

II. Systematic Classification and Quantification Standards for Moisture Conditions

2.1 Liquid Water Contact Conditions

• Core testing methods: Liquid filling tests and immersion tests are key methods for evaluating the water resistance of paper coffee cups with lids. The liquid filling test requires filling the paper cup with liquid at a specified temperature (hot or cold water), observing leakage and deformation, and checking the paper cup wall thickness and coating adhesion density; the immersion test involves immersing the paper cup upside down in water at a specific temperature for 30 minutes, then removing it and checking for leakage and coating peeling. Subsequently, a liquid filling test is performed, and after filling the cup, it is observed for 1 hour to check for penetration.
• National standard requirements: GB/T 27590-2022 has clear regulations on the leakage performance of paper cold drink cups. During testing, the test solution should be selected according to the standard and added to approximately 6mm below the rim of the cup or to the capacity mark. The paper cup should be placed on two parallel strips above a dry glass plate (or flat plate) (to prevent water accumulation at the bottom of the cup from affecting the results), and after standing for the specified time, observe whether there are any watermarks on the glass plate (or flat plate). The test conditions vary for different types of paper coffee cups with lids. Hot drink cups require testing with water at 90 °C ± 5 °C for 30 minutes, while cold drink cups require testing with water at 23 °C ± 1 °C for the same duration. 

2.2 Exposure to High Humidity Environments

• Environmental Definition and Standard Conditions: High humidity environments typically refer to environments with a relative humidity exceeding 60%. Before laboratory testing, paper coffee cups with lids must be placed in an environment with a temperature of 23°C ± 1℃ and a relative humidity of 50±2% for at least 24 hours (in accordance with GB/T 10739), and the testing must be completed in this environment.
• Storage and Usage Environment Requirements: In actual storage, the relative humidity of the paper cup environment needs to be controlled between 40% and 60%.  Humidity below 40% can easily lead to embrittlement and cracking of the paper cup edges, while humidity above 60% can easily cause moisture absorption, softening, and mold growth. Relevant research shows that when evaluating the condensation resistance of paper coffee cups with lids, they should be placed in a constant temperature and humidity chamber at 98% relative humidity and 30℃ for 72 hours. During this period, the outer printed pattern should not smudge or fade, the moisture absorption and weight gain rate of the paper base material should be ≤8.3%, and there should be no signs of softening of the adhesive layer at the bottom seal.
• Impact of High Humidity: In high-humidity environments, paper fibers absorb moisture, leading to volume expansion and decreased strength; the coating material may soften or degrade, reducing waterproof performance; and it may also promote mold growth, affecting the hygiene and safety of the paper cup.

2.3 Condensation Caused by Temperature Changes

• Condensation Formation and Impact: When a paper cup contains a low-temperature liquid, the surface temperature of the cup is lower than the environmental dew point temperature, and water vapor in the air condenses into small water droplets on the cup surface (i.e., condensation). Most double-walled paper cups have a single-sided coating (PE film) on the inside, with no coating on the outside. When containing cold drinks, condensation will wet the cup body, leading to softening, decreased stiffness, and ultimately deformation; when single-layer paper coffee cups with lids contain hot drinks, the large temperature difference between the inside and outside of the cup also easily leads to condensation, causing the paper cup to lose its rigidity and collapse.
• Countermeasures: Some high-end cold beverage cups utilize a special design, incorporating an anti-fog coating (such as hydrophilic siloxane, 5-8 μm thick) in the outer layer. This causes condensation to form a uniform water film on the cup surface instead of water droplets, reducing softening problems caused by localized water accumulation and mitigating the damage that condensation causes to the paper cup structure.

2.4 Hygroscopic Process During Long-Term Air Exposure

• Paper Hygroscopic Properties: In a typical indoor environment (relative humidity 30%-70%), paper coffee cups with lids will absorb or release moisture depending on changes in ambient humidity. Paper exhibits a hysteresis effect in moisture absorption; that is, at the same relative humidity, the amount of water absorbed during adsorption is lower than the amount of water absorbed during desorption. This leads to a gradual increase in water content after the paper cup undergoes humidity cycles. Different paper pulp materials show significant differences in hygroscopic properties. For example, at 25°C, when the relative humidity increases from 20% to 100%, the water content of cotton increases from 3.8% to 24%; in the relative humidity range of 40%-90%, the water content of flax pulp paper changes from 5.4% to 9.2%, and the water content of white machine-made pulp paper changes from 8.0% to 12.0%.
• Long-Term Exposure Effects: Long-term air exposure causes the paper fibers to gradually damage their internal structure due to swelling from moisture absorption and shrinkage from drying, leading to changes in paper cup dimensions and a decrease in strength and stiffness. At the same time, the coating material ages, reducing its waterproof performance and affecting the service life of the paper cup.

III. Paper Cup Deformation Mechanism and Evaluation Standards

3.1 Water Absorption and Swelling Mechanism of Paper Materials

• Molecular and Structural Changes: The main component of paper is cellulose molecules, which are connected by hydrogen bonds to form a network structure. When exposed to water, water molecules break the hydrogen bonds between cellulose molecules, allowing the cellulose molecules to move freely, causing the paper to swell; after the water evaporates, the cellulose molecules reform hydrogen bonds, but the connection points are misaligned, causing the paper to wrinkle. The specific process is divided into two stages: moisture absorption and drying. During moisture absorption, water enters the gaps between fibers, causing the fibers to swell and expand, widening the gaps, resulting in the appearance of ruffled edges and wavy patterns on the paper cup; during drying, the water evaporates, the fibers harden and shrink, causing wrinkles and tight edges on the paper cup.
• Irreversible and Directional Characteristics: When liquid water contacts the paper surface, localized "roughening" occurs, characterized by fiber swelling and lifting, representing an irreversible change in fiber morphology. Further penetration of water molecules leads to localized deformation of the fiber network and stress release. After water evaporation, "displacement" occurs between the fibers, preventing the restoration of the original network structure. Furthermore, paper swelling upon water absorption is directional; the longitudinal expansion rate is small (fibers are mostly aligned longitudinally), while the transverse expansion rate is large. This anisotropy is a significant reason for the shape change of paper coffee cups with lids after exposure to moisture.

3.2 Changes in Stress Distribution of Paper Cup Structure

• Influence of Humidity on Material Properties: In a high-humidity environment, the hydration degree of cellulose microfibers increases, leading to a decrease in the mechanical strength of the paper, an increase in thickness and density, and a decrease in the elastic modulus. At the same time, the compressive yield strength of the paper decreases, the slope of the initial elastic stage decreases, the material softens, and it becomes more prone to deformation.
• Stress Distribution and Weak Points: In a dry state, the stress distribution in the paper cup is relatively uniform, mainly bearing the vertical stress generated by the weight of the liquid and the radial stress generated by gripping. After exposure to moisture, uneven water distribution leads to stress concentration, and the connection between the cup bottom and the cup body, the edge of the cup mouth, and areas with incomplete coating become weak points. Especially the cup bottom, which bears the entire weight of the liquid, a decrease in strength after exposure to moisture will directly lead to the loss of stability of the paper cup.

3.3 Quantitative Evaluation Standards for Deformation Degree

• Stiffness Test and Standards: Stiffness is a core indicator for evaluating the deformation resistance of double-walled paper cups. During testing, the maximum deformation is usually set to 10% of the cup body diameter (e.g., for a paper cup with a diameter of 70mm, the deformation is set to 7mm) to avoid excessive deformation leading to paper cup damage. After starting the stiffness tester, the equipment applies pressure to the cup wall at a constant speed, recording the pressure-displacement curve. When the deformation reaches the set value, the recorded pressure value is the stiffness of the cup body (unit: N). A higher value indicates better stiffness. According to GB/T 27590-2022, the stiffness of the cup body is measured by applying a uniform force at a speed of 50.0 mm/min along the opposite sides of the cup body (at approximately 2/3 of the cup height). The maximum force at a total side wall deformation of 9.5 mm is defined as the cup body stiffness, and different cup capacities have specific requirements (e.g., premium paper coffee cups with lids with a nominal capacity ≤ 250 mL require a stiffness ≥ 3.00 N, and those with a capacity of 250-400 mL require a stiffness ≥ 3.40 N).
• Deformation and Leakage Performance: Deformation testing mainly measures the changes in diameter, height, and verticality of the paper cup under specific conditions. For example, in the heat resistance test, the paper cup needs to be kept at 85℃~95℃ for 1-2 hours, and the deformation rate must be ≤ 5%; in terms of leakage performance, the new national standard requires that the static leakage test (soaking in 90℃ hot water for 30 minutes) results in ≤ 5 drops of leakage, and the dynamic leakage test (simulating drinking at a 45° tilt) shows no liquid leakage.

3.4 Deformation Thresholds of Different Types of Paper Coffee Cups with Lids

• Stiffness Requirements Differences: Hot drink cups (for liquids ≥ 60℃) have higher stiffness requirements, with 180 mL capacity hot drink cups requiring a stiffness ≥ 7.0 N, and 250 mL capacity cups requiring ≥ 8.0 N; cold drink cups (for liquids ≤ 40℃) have relatively lower stiffness requirements, with 180 mL capacity cups requiring ≥ 5.0 N, and 250 mL capacity cups requiring ≥ 6.0 N.
• Actual Deformation Resistance: Test data shows that ordinary paper coffee cups with lids filled with 90°C hot water for 5 minutes will indent by 1.2 cm, while paper coffee cups with lids using a triple-layer fiber structure (forming an "armor-like" structure through high-temperature pressing, which can disperse pressure) only deform by 0.3 mm; double-layer paper coffee cups with lids, due to the air insulation layer between the inner and outer layers, have better deformation resistance than single-layer paper coffee cups with lids, and the degree of deformation is 30%-50% less under the same moisture conditions.
• Coating Thickness Influence: Coating thickness directly affects the deformation threshold. Hot beverage cups have a PE coating thickness of 25-30 μm, which allows them to withstand high temperatures and maintain a stable shape; cold beverage cups have a PE coating thickness of only 10-15 μm, and are prone to softening and deformation when containing liquids above 70°C.

 

IV. Usage Scenarios and Risk Assessment

4.1 Analysis of Daily Usage Scenarios

• Coffee purchase scenario: Consumers usually finish drinking hot coffee within 10-30 minutes after purchase. The main problem with paper coffee cups with lids in this scenario is condensation on the outer surface due to the hot beverage. With coffee temperatures of 80-90°C and ambient temperatures of 10-30°C, the large temperature difference leads to rapid formation of a large amount of condensation on the outside of the cup. If not evaporated promptly, it will penetrate the paper fibers, causing the paper cup to soften and deform.
• Home use scenario: Paper coffee cups with lids are used less frequently but may be stored for longer periods, usually in cabinets or drawers, with an ideal relative humidity of 40%-60%. If the storage environment is poorly ventilated or near a water source (such as a kitchen sink), the humidity may rise to over 70%, leading to slow moisture absorption by the paper coffee cups with lids, affecting their performance over time.
• Office environment: Paper coffee cups with lids are often used for hot beverages, with high usage frequency. The ambient temperature is 20-25°C, and the relative humidity is 40%-60%, which has little impact on the paper coffee cups with lids. However, air conditioning dehumidification may cause the ambient humidity to be too low (<40%), leading to the edges of the paper coffee cups with lids becoming brittle and prone to cracking.
• Takeaway scenario: Paper coffee cups with lids need to go through packaging, transportation, and delivery. Rain or snow may cause the paper coffee cups with lids to get wet, and vibrations and compression during transportation can also exacerbate deformation, resulting in a higher risk of moisture damage. 

4.2 Risk Assessment of Outdoor Activity Scenarios

• Camping and Picnic Scenarios: Outdoor temperatures fluctuate drastically (daytime > 30℃, nighttime < 10℃), causing repeated thermal expansion and contraction of the paper coffee cups with lids, accelerating material aging; relative humidity may exceed 90% in the early morning and after rain, leading to rapid moisture absorption by the paper coffee cups with lids. Some outdoor-specific paper coffee cups with lids use PP material combined with virgin wood pulp, with built-in lamination technology and tightly embossed bottoms to prevent leakage, thus improving reliability.
• Sports Event Scenarios: Large-scale events such as marathons and music festivals involve a large number of paper coffee cups with lids, which are often discarded indiscriminately. In rainy weather, they are easily soaked by rain and trampled, quickly losing their functionality and potentially causing environmental pollution.
• Hiking and Mountaineering Scenarios: Backpack space is limited, requiring lightweight and high-strength paper coffee cups with lids.  They also need to withstand unexpected situations such as accidental immersion in water and prolonged exposure to rain, resulting in higher risks of moisture damage and structural instability.

4.3 Storage Environment Requirements and Recommendations