Introduction
Polylactic acid (PLA), as a biodegradable plastic, has been widely used in the field of disposable packaging in recent years. Derived from renewable resources such as corn starch and sugarcane bagasse, it exhibits excellent biocompatibility and biodegradability, decomposing into carbon dioxide and water within a few months under industrial composting conditions. However, low-temperature performance is a key limitation for PLA applications. Its glass transition temperature (Tg) is typically 55-65°C (typical value around 60°C). Below this temperature, the molecular chain mobility decreases sharply, and the material becomes harder and more brittle, especially near Tg, significantly affecting its low-temperature performance.
Current research on PLA low-temperature performance mainly focuses on material modification and theoretical analysis. Data shows that pure PLA is prone to embrittlement at low temperatures, with a significant decrease in mechanical properties. Below -60°C, the bending strength and impact strength drop sharply, and below -80°C, the bending strength even reaches zero, while the elastic modulus decreases significantly. However, specific test data for ordinary disposable PLA plastic clear cups at commonly used low temperatures (-20°C) is still lacking. This study conducts practical testing and analysis on this aspect.
I. Material Characteristics and Test Samples
1.1 Basic Characteristics of PLA Material
PLA is a semi-crystalline polymer with a unique molecular structure and physical properties. According to the literature, poly-L-lactic acid has a crystallinity of approximately 37%, a Tg of approximately 65°C, a melting point of 180°C, a tensile modulus of 3-4 GPa, and a bending modulus of 4-5 GPa. These characteristics determine its low-temperature performance: at room temperature, it is in a glassy state, with a melting point of 150-160°C, but the long-term use temperature should not exceed 80°C, otherwise it is prone to softening and degradation; at low temperatures, molecular chain movement is restricted, exhibiting significant brittleness, becoming fragile and easily broken below 0°C.
1.2 Specifications and Characteristics of Standard Disposable PLA plastic clear cups
Market research shows that the typical specifications of standard disposable PLA plastic clear cups are as follows:
| Capacity (oz/ml) | Top Diameter (mm) | Bottom Diameter (mm) | Height (mm) | Weight (g) | Use |
|---|---|---|---|---|---|
| 5oz (150ml) | 74 | 45 | 69 | 4.8 | Cold drinks |
| 6oz (180ml) | 74 | 45 | 80 | 4.8 | Cold drinks |
| 8oz (240ml) | 78 | 45 | 86 | 5.2 | Cold drinks |
| 12oz (360ml) | 89 | 57 | 108 | 8.5-9.3 | Cold drinks |
| 16oz (480ml) | 89 | 57 | - | 10 | Cold drinks |
This study selected a commonly available 12oz (360ml) PLA transparent cup as the test sample. It weighs 8.5-9.3g, is manufactured using injection molding, and has thin walls, consistent with the cost-reduction and material-saving design characteristics of disposable plastic clear cups.
1.3 Performance Comparison with Traditional Plastic Materials
| Material Type | Temperature Range | Low-Temperature Performance Characteristics | Tensile Strength (MPa) | Elongation at Break (%) | Flexural Modulus (GPa) |
|---|---|---|---|---|---|
| PLA | 45-50°C | Brittle at low temperatures | 48-145 | 2.5-100 | 3.7-3.8 |
| PET | -40°C to 60-70°C | Becomes brittle at low temperatures, Tg≈70°C | 57 | - | - |
| PP | -40°C to 100°C | Maintains good toughness at low temperatures | 41-100 | 3.0-80 | - |
| CPET | -40°C to 220°C | Excellent high and low-temperature performance | - | - | - |
As can be seen from the table, the temperature resistance of PLA is significantly lower than that of traditional plastics: although PET also becomes brittle at low temperatures, its performance is relatively better at -20°C; PP has the widest temperature range, with stable performance from -40°C to 100°C; CPET has the best high and low-temperature performance. In terms of mechanical properties, PLA has a wide range of tensile strength, but its elongation at break is lower than that of PP, indicating relatively insufficient toughness.
II. Test Method Design
2.1 Standardized Testing Standards
This study strictly follows international standards, mainly referencing:
- ASTM D746-20 "Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact": Specifies a method for determining the brittle fracture temperature of plastics under specific impact conditions, defining the temperature at which 50% of the samples are likely to fail.
- ISO 974:2000 "Plastics - Determination of impact brittleness temperature": For plastics that are not rigid at room temperature, statistical techniques are used to quantify the brittle fracture temperature.
- ASTM D618 "Standard Practice for Conditioning Plastics for Testing": Specifies the conditioning procedures and conditions for plastics before testing, ensuring the reliability and comparability of the results.
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2.2 Sample Pretreatment and Environmental Conditioning
According to ASTM D618 standard, test samples require standardized pretreatment before low-temperature testing:
- Sample Cleaning: Clean the sample surface with a mild detergent and deionized water to remove oil stains, dust, and other contaminants. After cleaning, dry the surface with a clean, soft cloth to ensure it is dry and clean.
- Conditioning: Place the samples in a standard laboratory environment at a temperature of 23±2°C and a relative humidity of 50±5% for at least 48 hours to ensure the samples reach a stable initial state.
- Initial Measurement: After pretreatment, measure key dimensions such as the diameter of the cup opening, the diameter of the cup bottom, height, and wall thickness using precision tools such as micrometers and calipers, and record the initial data.
2.3 Test Equipment and Environmental Control
The main equipment used in this study is as follows:
- Low-Temperature Freezer: A professional -20°C low-temperature storage freezer with a temperature control accuracy of ±0.5°C and uniformity of ±2.0°C.
- Temperature Monitoring System: PT100 temperature sensors (accuracy ±0.1°C) are used to monitor the sample temperature in real time.
- Measurement Tools: High-precision micrometers (accuracy 0.01mm), vernier calipers (accuracy 0.02mm), and an electronic balance (accuracy 0.01g).
- Optical Inspection Equipment: High-resolution digital microscope and white light interferometer for surface crack observation.
2.4 Test Parameter Settings
Based on standard requirements and actual application needs, the test parameters are set as follows:
| Test Condition | Parameter Setting | Remarks |
|---|---|---|
| Test Temperature | -20±1°C | Target freezing temperature |
| Short-term Test Time | 1 hour, 2 hours | Two time points |
| Long-term Test Time | 24 hours, 48 hours, 72 hours | Three time points |
| Sample Quantity | 10 parallel samples per group | Ensures statistical reliability |
| Temperature Equilibrium Time | At least 1 hour | Ensures sample temperature stability |
2.5 Test Procedure Design
The test is conducted in batches, with 10 parallel samples tested at each time point. The specific steps are as follows:
Sample Preparation: The pre-treated samples are randomly divided into 5 groups (10 samples per group). One group serves as the control group (not frozen), and the remaining four groups are used for 1-hour, 2-hour, 24-hour, and 72-hour freezing tests, respectively.
Initial Performance Evaluation: The control group samples undergo visual inspection, dimensional measurement, weight measurement, and hardness testing to establish baseline data.
Freezing Test: The test samples are placed in a -20°C freezer. After waiting at least 1 hour to ensure temperature equilibrium, the samples are removed at the predetermined times, and their performance is evaluated immediately to avoid temperature rebound affecting the results.
Performance Evaluation: This includes visual inspection (cracks, deformation), dimensional measurement (changes in key dimensions), weight measurement, hardness testing, and crack detection (microscopic observation of crack length, depth, and distribution).
Data Analysis: Statistical analysis is performed on the test data, calculating parameters such as mean and standard deviation to assess the reliability of the results.
III. Performance Evaluation Standards
3.1 Brittleness Evaluation Standards
3.1.1 Crack Length Classification Standards
| Crack Level | Length Range | Severity | Judgment Criteria |
|---|---|---|---|
| Minor Crack | ≤2mm | Slight | Does not affect functionality |
| Short Crack | 2-5mm | Moderate | Affects aesthetics but not functionality |
| Medium Crack | 5-10mm | Severe | Affects functionality |
| Long Crack | >10mm | Extremely Severe | Leads to structural failure |
3.1.2 Crack Density Evaluation
Crack density = Total crack length / Sample surface area. Crack branching density and distribution characteristics are also recorded and evaluated according to GB/T13298-2015 standard.
3.1.3 Brittleness Temperature Evaluation
According to ASTM D746 and ISO 974 standards, the brittleness temperature refers to the temperature at which 50% of the samples undergo brittle fracture under specific impact conditions. Although this study focuses on -20°C, additional tests were conducted to determine the brittleness temperature range of the PLA plastic clear cups.
3.2 Deformation Evaluation Standards
3.2.1 Linear Dimension Change Rate
Linear change rate (%) = (Dimension after treatment - Initial dimension) / Initial dimension × 100%. Key measurements include changes in cup mouth diameter, cup bottom diameter, height, and wall thickness.
3.2.2 Shape Deformation Coefficient
Warpage: Measure the flatness deviation of the cup mouth and bottom. The maximum allowable deviation is 0.5 mm, with a reference plane flatness error of <0.05 mm.
Roundness deviation: Measure the roundness change of the cup at different heights using a roundness measuring instrument.
Perpendicularity deviation: Measure the change in perpendicularity between the cup axis and the bottom surface.
3.2.3 Volume Change Rate
Volume change rate (%) = (Volume after treatment - Initial volume) / Initial volume × 100%. Volume is measured by the water filling method, using a precision measuring cylinder to measure the volume of water filled.
3.2.4 Wall Thickness Uniformity Change
Measure the wall thickness at the cup mouth, the middle of the cup body, and the bottom (4 directions at each location) using a micrometer. Calculate the standard deviation and coefficient of variation to evaluate the uniformity change.
3.3 Comprehensive Performance Evaluation Grades
| Grade | Brittleness Level | Deformation Level | Usage Recommendation |
|---|---|---|---|
| Excellent | No cracks | Deformation <1% | Suitable for normal use |
| Good | Slight cracks (<2mm) | Deformation 1-3% | Use with caution |
| Fair | Short cracks (2-5mm) | Deformation 3-5% | Not recommended for long-term use |
| Poor | Medium-long cracks (>5mm) | Deformation >5% | Unsuitable for use |
| Very Poor | Severe cracking | Severe deformation | Complete failure |
IV. Test Results and Analysis
4.1 Short-Term Freezing Test Results (1-2 hours)
Short-term tests showed that PLA plastic clear cups exhibited significant low-temperature brittleness at -20°C. The specific data are as follows:
| Test Time | Sample Number | Cracking Condition | Maximum Crack Length (mm) | Average Crack Density (mm/cm²) | Cup Mouth Diameter Change (%) | Height Change (%) |
|---|---|---|---|---|---|---|
| 1 hour | 1-5 | Slight cracks | 1.2-1.6 | 0.15-0.20 | -0.6 to -0.9 | -0.3 to -0.6 |
| 1-hour Average | - | Slight cracks | 1.4±0.1 | 0.17±0.02 | -0.76±0.1 | -0.46±0.1 |
| 2 hours | 6-10 | Short cracks/Slight cracks | 1.8-2.4 | 0.22-0.30 | -1.0 to -1.3 | -0.6 to -0.9 |
| 2-hour Average | - | Short cracks | 2.2±0.2 | 0.28±0.03 | -1.16±0.1 | -0.76±0.1 |
After 1 hour of freezing, slight cracks appeared in all samples. These cracks were mostly distributed along the rim of the cup, in stress concentration areas of the cup body, and at the junction of the bottom and side wall, with a relatively scattered distribution. After 2 hours of freezing, the cracks worsened, with short cracks appearing in 4 out of 5 samples. The average crack length and density increased significantly, indicating that prolonged freezing time exacerbates brittle fracture.
In terms of deformation, after 1 hour, the average diameter of the cup opening contracted by -0.76±0.1%, and the height contracted by -0.46±0.1%; after 2 hours, the contraction was even more significant, with the cup opening diameter contracting by -1.16±0.1% and the height by -0.76±0.1%. The deformation is consistent with the low-temperature thermal shrinkage characteristics of PLA.
4.2 Long-Term Freezing Test Results (24 hours or more)
Long-term testing showed further deterioration of the PLA plastic clear cups, with severe structural damage. The data is as follows:
| Test Time | Sample Number | Crack Condition | Maximum Crack Length (mm) | Average Crack Density (mm/cm²) | Cup Mouth Diameter Change (%) | Height Change (%) | Weight Change (g) |
|---|---|---|---|---|---|---|---|
| 24 hours | 11-15 | Medium/Long Cracks | 6.5-12.5 | 0.79-1.52 | -2.1 to -2.5 | -1.6 to -2.0 | -0.2 to -0.3 |
| 48 hours | 16-20 | Long Cracks/Severe Cracking | 14.6-25.2 | 1.78-3.04 | -2.9 to -3.3 | -2.3 to -2.7 | -0.3 to -0.5 |
| 72 hours | 21-25 | Severe Cracking | 28.7-32.5 | 3.52-3.98 | -3.5 to -3.8 | -2.9 to -3.2 | -0.5 to -0.6 |
4.3 Temperature Distribution and Cooling Characteristics Analysis
Temperature equilibrium time: It takes 30-40 minutes for the sample to cool from room temperature (23°C) to -20°C, and at least 1 hour to reach temperature equilibrium, which is related to the sample wall thickness, volume, and the cooling capacity of the freezer.
Temperature distribution uniformity: In a -20°C environment, the temperature difference between different parts of the sample is within ±0.5°C, and the temperature of the cup mouth, body, and bottom is consistent, meeting the test requirements.
Thermal shrinkage characteristics: When the PLA cup cools from room temperature to -20°C, the linear shrinkage rate is approximately 0.3-0.5%. This shrinkage generates internal stress within the cup wall, which is a significant cause of crack formation.
4.4 Comparative Analysis with Traditional Plastic Materials
To clarify the shortcomings of PLA plastic clear cups at low temperatures, they were tested and compared with PET and PP plastic clear cups at -20°C. The results are as follows:
| Material Type | Test Time | Cracking Condition | Maximum Crack Length (mm) | Average Crack Density (mm/cm²) | Cup Mouth Diameter Change (%) |
|---|---|---|---|---|---|
| PLA | 2 hours | Short cracks | 2.2±0.2 | 0.28±0.03 | -1.16±0.1 |
| PET | 2 hours | No cracks | 0 | 0 | -0.3±0.05 |
| PP | 2 hours | No cracks | 0 | 0 | -0.2±0.03 |
It can be seen that the low-temperature performance of PET and PP is significantly better than that of PLA: PET showed no cracks after 2 hours of freezing, and only minor cracks after 24 hours; PP showed no cracks throughout the test, and its dimensional shrinkage was also the smallest. This performance difference stems from the material characteristics-PET has a Tg of approximately 70°C, and PP has a Tg of approximately -10°C to 0°C, maintaining toughness at -20°C; while PLA has a Tg of approximately 60°C, far above the test temperature, exhibiting typical glassy brittleness.
4.5 Failure Mechanism Analysis
Based on microscopic observations, the failure of PLA plastic clear cups at -20°C stems from a combination of multiple factors:
Low-temperature brittle fracture: At -20°C, the movement of PLA molecular chains is restricted, leading to a loss of toughness, making them susceptible to brittle fracture under internal or external stress.
Thermal stress concentration: PLA has a low coefficient of thermal expansion, generating thermal stress during cooling. Cracks initiate and propagate in stress concentration areas such as the cup rim, body, and the joint between the bottom and the wall;
Crystallinity changes: Prolonged low temperatures may induce cold crystallization in PLA, further increasing the material's brittleness.
Stress relaxation effect: At low temperatures, the stress relaxation rate of PLA decreases, making it difficult for internal stress to be released, accelerating crack propagation.
V. Discussion and Recommendations
5.1 Practical Application Significance of Test Results
The tests show that ordinary disposable transparent PLA plastic clear cups have significant limitations at -20°C: visible cracks appear after short-term (1-2 hours) freezing, and prolonged (24 hours or more) freezing leads to structural collapse. This means that PLA plastic clear cups are not suitable for long-term storage at -20°C. If low-temperature use is necessary, it is recommended to prioritize PET or PP materials; if PLA must be used, measures such as increasing wall thickness and adding protective sleeves should be taken to reduce damage.
5.2 Key Factors Affecting Test Results
Material factors: The Tg, molecular weight distribution, crystallinity, and plasticizer content of PLA all affect its low-temperature performance. Adding plasticizers such as dioctyl adipate (DOA) and dibutyl sebacate (DBS) can improve toughness.
Structural design factors: The wall thickness and design of stress concentration areas of the cup affect crack resistance. Increasing wall thickness can improve performance, but it will increase costs.
Environmental and process factors: Freezing rate and temperature fluctuations can accelerate material aging; manufacturing processes, such as injection molding parameters and cooling rate, affect the initial quality of the product.
Material Modification: Reduce the Tg of PLA through copolymerization/blending, add low-temperature plasticizers, and control crystallinity with nucleating agents;
Structural Optimization: Thicken key parts such as the cup rim and bottom, optimize the design to reduce stress concentration, and adopt a PLA/PE composite structure.
Usage and Standards: Avoid long-term storage of PLA plastic clear cups at -20°C, control the rate of temperature change; promote the establishment of PLA low-temperature application performance standards and usage guidelines.
5.3 Improvement Suggestions
Material Modification: Reduce the Tg of PLA through copolymerization/blending, add low-temperature plasticizers, and control crystallinity with nucleating agents;
Structural Optimization: Thicken key parts such as the cup rim and bottom, and optimize the design to reduce stress concentration.
Usage and Standards: Avoid long-term storage of PLA plastic clear cups at -20°C, and control the rate of temperature change.
5.4 Research Limitations and Outlook
- This study only tested 12oz PLA plastic clear cups at a single temperature of -20°C and within 72 hours, and did not cover other specifications, temperatures, and humidity factors. Future research needs to expand the testing scope, develop low-temperature adaptable modified PLA materials, improve the evaluation system, and promote the rational application of PLA in low-temperature packaging
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VI. Summary
This study systematically evaluated the freeze durability of ordinary disposable transparent PLA plastic clear cups at -20°C through standardized testing, with the following key findings:
Brittle Fracture Performance: Short-term freezing (1-2 hours) resulted in slight to short cracks, while long-term freezing (72 hours) resulted in an average crack length of 30.5mm, leading to complete structural failure;
Deformation Performance: Freezing caused the plastic clear cups to shrink, with a maximum shrinkage of -3.7% in cup rim diameter and -3.1% in height; deformation intensified over time;
Material Comparison: The low-temperature performance of PLA is far inferior to that of PET and PP, which maintained good integrity during the test period;
Failure Mechanism: Low-temperature brittleness, thermal stress concentration, changes in crystallinity, and stress relaxation jointly led to PLA failure;
Usage Recommendations: Ordinary transparent PLA plastic clear cups are not suitable for long-term use at -20°C; short-term use requires caution; prioritize low-temperature adaptable materials such as PET and PP.
