Engineering / Rubber Properties / Compression Set

# Compression Set

## Compression Set (CS)

The Compression Set (CS) of soft materials, such as elastomers, is the permanent deformation that remains when an applied force is subsequently removed. As elastomers are compressed over time, they lose the ability to return to their original thickness. Compression Set Testing measures the ability of a material to return to its original thickness after a compression force is applied at a set deflection and temperature.

### Importance

Compression set measurements are important when determining the best elastomeric materials to be used. For example, in the case of a shock isolation pad, the ability to protect an accidentally dropped unit may be compromised as the CS measurement becomes more significant. Compression set is critical for the sealing functionality, so the better the CS, the better the long term sealing.

### Compression Set Measurement

In practical terms, a material that has a zero compression set is one that returns to 100% of its original thickness, while a material that has a 100% compression set is one that does not return any of its original thickness.

The compression set (ASTM D395) of a material is determined using one of two testing methods, Compression Set A and Compression Set B.

### Test Method A

Compression Set Under Constant Force in Air applies a 1.8kN force to the specimen for a set time and temperature.

### Test Method B

Compression Set Under Constant Deflection in Air applies a force necessary to the specimen to compress it to 25% of its original height for a set time and temperature.

Compression sets A and B are defined as the percentage of original specimen thickness after it has been left in normal conditions for 30 minutes.

O-ring compression set testing is performed using the similar ASTM D1414 B Method.

### Calculating Compression Set

Calculating the compression set for a material is accomplished using formulae that are specific to each of the test methods.

### Test Method A

CA, the compression set A, is given by CA = [(to – ti) / to] * 100 where to is the original specimen thickness, and ti is the specimen thickness after testing.

### Test Method B

CB, the compression set B, is given by CB = [(to – ti) / (to – tn)] * 100 where to is the original specimen thickness, ti is the specimen thickness after testing, and tn is the spacer thickness or the specimen thickness during the test.

Example:

Testing compression = (t0 – ti) = 0.25cm – 0.177cm = 0.073cm (29% compression)

Permanent compression = (t0 – tn) = 0.221cm – 0.177cm = 0.044cm

CB = {(0.25 – 0.221)/(0.25 – 0.177)}/100 = 39.72%

## FAQ

Can you estimate O-Ring Lifespan Based on Compression Set test?

Compression set indicates the permanent deformation of an O-ring after compression and release, affecting its ability to maintain a seal. A high compression set suggests material fatigue and reduced sealing effectiveness, which can lead to leaks. However, compression set alone does not directly determine an O-ring’s lifespan.

Key Factors Influencing O-Ring Lifespan:

1. Environmental Conditions: Elevated temperatures and chemical exposure can accelerate material degradation, increasing compression set and reducing lifespan.
2. Material Properties: Different rubber materials (e.g., Nitrile, EPDM, FKM, Silicone, etc.) have varying resistance to compression set and aging factors such as ozone, UV light, oils, chemicals etc..
3. Operational Factors: The level of compression and whether the O-ring is in a static or dynamic application impact its performance. Excessive compression or dynamic movement can lead to a higher compression set.
4. Design and Installation: Proper sizing and correct installation are crucial. Incorrect installation or poorly designed glands can cause excessive compression, leading to higher compression set.

To estimate an O-ring’s lifespan, compression set should be evaluated alongside environmental conditions, material properties, and operational factors. Accelerated aging tests and real-world performance data provide insights into how compression set affects lifespan. Regular monitoring and inspection of O-rings in service help predict and extend their useful life, ensuring reliable sealing performance. This holistic assessment allows for more accurate lifespan predictions and proactive maintenance strategies.

How is the compression set of the rubber being tested, and is the procedure consistent with industry standards such as ASTM D395?
1. Test Procedure Overview The compression set of rubber is tested to evaluate its ability to retain its elastic properties after prolonged compression and exposure to heat. This is crucial for understanding the material’s durability and performance in applications where it will be subjected to continuous stress or deformation.
2. Industry Standards Compliance We ensure our testing procedures comply with the relevant industry standards, particularly ASTM D395 (Standard Test Methods for Rubber Property—Compression Set). This standard provides guidelines for conducting compression set tests under various conditions, including the following aspects:
• ASTM D395:

• Methods: Two main methods (Method A: Compression set under constant force, Method B: Compression set under constant deflection).
• Conditions: Specified temperature ranges, typically 70°C to 150°C, and durations from 22 hours to 168 hours.
• Measurement: Percentage of original deflection retained after recovery.
1. Testing Procedure
• Sample Preparation: Rubber samples are made to specified dimensions and pre-conditioned if required.
• Compression: Samples are compressed to a predetermined deflection or force using calibrated testing fixtures and equipment.
• Exposure: Compressed samples are subjected to a controlled environment, maintaining specific temperatures and durations based on the standard methods.
• Recovery: After the exposure period, samples are allowed to recover at room temperature for a specified time.
• Measurement: The final thickness of the samples is measured, and the compression set is calculated as a percentage of the original deflection.
1. Equipment and Calibration
• Fixtures: We use standardized fixtures for consistent sample compression.
• Temperature Control: Environmental chambers or ovens with precise temperature control are used.
• Measurement Tools: Calibrated micrometers or other precise measuring instruments ensure accurate deflection measurements.
1. Documentation and Reporting
• Data Logging: All test conditions, sample dimensions, and measurements are meticulously recorded.
• Compliance Verification: Results are reviewed against acceptance criteria defined by ASTM D395 to ensure compliance.
• Reporting: A detailed report is generated, including all relevant test data, calculations, and any deviations from standard procedures if applicable.
1. Continuous Improvement We regularly review our testing procedures and equipment calibration processes to ensure they remain aligned with the latest revisions of ASTM D395. Any updates or changes in the standards are promptly integrated into our testing protocols.
What temperature range is used during the compression set testing, and how is the temperature controlled and monitored to ensure it reflects the intended application environment?
1. Temperature Range

The temperature range used for rubber compression set testing typically aligns with the expected application conditions and is guided by industry standards such as ASTM D395 and ISO 815. Common temperature ranges for these tests are as follows:

• Standard Testing Temperatures:
• Low Temperatures: -10°C to 0°C, used for applications requiring performance in cold environments.
• Moderate Temperatures: 23°C (room temperature), often used as a baseline or for ambient condition assessments.
• Elevated Temperatures: 70°C to 150°C, typical for assessing performance in high-heat applications. Some tests may go up to 200°C for specific high-temperature uses.

The specific temperature range for a test is selected based on the intended application of the rubber product. For instance, automotive seals might be tested at higher temperatures reflecting engine conditions, while rubber components for outdoor equipment might be tested at both low and high temperatures to simulate seasonal variations.

1. Temperature Control

2.1. Equipment Used

• Environmental Chambers: These chambers provide precise temperature control, capable of maintaining both low and high temperatures required for the tests.
• Ovens: High-temperature tests are often conducted in calibrated ovens that can maintain consistent heat over extended periods.

2.2. Calibration and Accuracy

• Calibration: All equipment used for temperature control is regularly calibrated according to standard protocols to ensure accurate and reliable temperature maintenance.
• Accuracy: The temperature control systems are designed to maintain the set temperature within ±1°C of the desired range, ensuring uniform exposure of all samples.

2.3. Monitoring Systems

• Sensors: Temperature sensors placed within the testing chamber or oven continuously monitor the internal environment.
• Data Logging: Advanced data loggers track temperature fluctuations in real-time and record them throughout the testing period. This data ensures that the samples are exposed to the correct temperature consistently.

2.4. Compliance Verification

• Standard Protocols: Testing is conducted according to protocols specified by ASTM D395 or ISO 815, which provide guidelines on the duration and temperature conditions for different types of rubber materials.
• Validation: Before starting the test, the temperature control and monitoring systems are validated to confirm they can achieve and maintain the target temperature range.
1. Reflecting Intended Application Environment

3.1. Application-Specific Conditions

• Customized Testing: The selected temperature range reflects the real-world conditions in which the rubber product will be used. For example, rubber parts for outdoor applications might be tested across a broader temperature range to simulate seasonal temperature variations.

3.2. Environmental Simulation

• Realistic Scenarios: Testing often involves simulating the operational environment as closely as possible. For instance, rubber components used in engine compartments might undergo testing at temperatures that mimic those in running engines.

3.3. Stress Testing

• Extreme Conditions: In some cases, temperatures beyond the typical range may be used to stress-test the material and understand its limits under extreme conditions.
1. Reporting and Analysis

4.1. Documentation

• Temperature Data: All temperature data, including any deviations from the target range, are meticulously documented.
• Test Conditions: Detailed records of the test conditions, including temperature settings and durations, are included in the test report.

4.2. Result Interpretation

• Comparative Analysis: Results are compared against established benchmarks or specifications to determine if the material meets the performance criteria under the tested temperature conditions.

By rigorously controlling and monitoring the temperature during compression set testing, we ensure that the results accurately reflect how the rubber will perform in its intended application environment. This approach helps in predicting long-term durability and reliability of the rubber material under expected operational conditions.