Silicone offers numerous excellent properties such as being non-toxic, highly biocompatible, and resistant to extreme temperatures, ozone, and UV rays. As environmental awareness increases and silicone becomes more affordable, its applications are growing significantly. Different types of silicone are tailored for various uses, each with unique properties. Below are the key performance indicators of silicone.
Physical Properties of Silicone
1. Mechanical Properties
The parameters reflecting mechanical properties include hardness, tensile strength, tear strength, elongation at break, resilience, and compression set.
- Hardness: Silicone hardness can range from SHORE-A 0 to 90, with commonly used hardness between 30 and 70. Below 30 indicates low hardness silicone, while above 70 is considered high hardness silicone.
- Tensile Strength: Typically, silicone tensile strength is below 10 MPa. Achieving 11 MPa is challenging, and 12 MPa is rare. Generally, fumed silica silicone exhibits higher tensile strength than precipitated silica silicone.
- Tear Strength: The common testing methods are the right-angle and crescent methods, yielding different results. Using the right-angle method, high tear-resistant silicone can reach nearly 50 Kgf/cm, while the crescent method can achieve over 35 Kgf/cm. High tear-resistant silicones are typically made using the fumed silica method.
- Elongation at Break: This typically decreases as hardness increases, meaning lower hardness silicones have higher elongation at break, sometimes exceeding 1,000%. Fumed silica silicone generally shows higher elongation compared to precipitated silica silicone with the same hardness.
- Compression Set: The lowest compression set for silicone can be below 5%. This parameter is closely related to resilience and is critical for products like rollers and seals.
- Resilience: High-resilience silicone can achieve a resilience value of over 70, with some reaching up to 80.
2. Thermal Properties
Silicone outperforms most rubbers in temperature resistance, ranging from -90°C to over 300°C. Regular silicone typically functions within a temperature range of -40°C to 180°C. Many applications leverage silicone’s thermal advantages, such as in cold climates where rubber parts may require silicone for proper functioning.
3. Electrical Properties
Silicone is generally an insulator unless specially modified to be conductive. Conductivity is determined by volume resistivity, measured in Ω·m.
- Insulating Silicone: Volume resistivity above 10910^9109.
- Antistatic Silicone: Volume resistivity between 10610^6106 and 10910^9109.
- Conductive Silicone: Volume resistivity below 10610^6106.
Key Physical Properties of Silicone
The most critical physical properties of silicone include:
- Hardness
- Elongation at Break
- Tensile Strength
- Tear Strength
- Resilience
- Compression Set
Below is an example of physical property data from the Shin-Etsu CHN-9000 series:
1. Hardness
Hardness refers to the material’s ability to resist indentation from external forces. Silicone offers a Shore hardness range of 0 to 80, allowing designers flexibility to achieve specific functional requirements. Mixing different ratios of base polymers, fillers, and additives can produce intermediate hardness levels. Adjusting curing time and temperature can also modify hardness without affecting other physical properties.
- Common hardness values:
- Solid silicone: Shore A 30–70
- Liquid silicone: Shore A 0–70
Liquid silicone with hardness below Shore A 0 exists but requires customization and a Shore C durometer for measurement. Liquid silicones with Shore A hardness of 80–90 are also available but are less common due to brittleness.
- Observations:
- Silicone with a hardness below 10 has low tear strength and high elongation, making it prone to deformation.
- Silicone with hardness above 40 has reduced tear strength, increasing brittleness and susceptibility to cracking.
- Optimal tear strength and elongation occur at hardness levels of 20–40 Shore A.
2. Elongation at Break
Elongation refers to the percentage increase in length when a sample breaks under tension.
- Typical ranges:
- Solid silicone: 90–1,120%
- Fluorosilicone: 159–699%
- Liquid silicone: 220–900%
Processing methods and curing agents significantly influence elongation. Low-hardness silicones often exhibit elongations exceeding 1,000%.
3. Tensile Strength
Tensile strength measures the maximum stress a material can withstand during stretching.
- Typical ranges:
- Solid silicone: 4.0–12.5 MPa
- Fluorosilicone: 8.7–12.1 MPa
- Liquid silicone: 3.6–11.0 MPa
Fumed silica silicone generally exhibits higher tensile strength than precipitated silica silicone.
4. Tear Strength
Tear strength represents the resistance to tearing or crack propagation in notched samples.
- Typical ranges:
- Solid silicone: 9–55 kN/m
- Fluorosilicone: 17.5–46.4 kN/m
- Liquid silicone: 11.5–52 kN/m
5. Resilience
Resilience quantifies the ability of a material to return to its original shape after deformation. Silicone, being a highly elastic polymer, exhibits reversible deformation and excellent resilience at room temperature.
- High-resilience silicone achieves values exceeding 70, with some reaching 80.
6. Compression Set
The compression set evaluates a silicone product’s elasticity and recovery after deformation. Factors affecting compression sets include intermolecular forces, network structure changes, and molecular displacement.
Understanding these factors is essential for optimizing silicone’s recovery capabilities in various applications.
Physical Properties of Silicone
1. Hardness
Hardness refers to the ability of a material to resist localized deformation or indentation. Silicone offers a Shore hardness range of 0 to 80, allowing designers significant freedom to select the appropriate hardness for specific applications. Adjustments in the mixing ratios of the polymer base, fillers, and additives can achieve various intermediate hardness levels. Similarly, curing time and temperature can modify hardness without altering other physical characteristics. Silicone hardness is now available up to SHORE A 0-90 degrees, with common values ranging from 50-70 degrees. Silicone below 30 degrees is considered low-hardness silicone, while above 70 degrees is high-hardness silicone.
Shore hardness, also known as Durometer hardness, measures material hardness using three categories: Shore A, Shore C, and Shore D. The distinction lies in the size of the indenters used:
- Shore A: A needle tip with a diameter of 0.8mm, suitable for soft plastics, silicone, synthetic silicone, felt, and leather.
- Shore D: A sharper needle tip (radius R0.1) for harder materials such as rigid plastics, hard silicone, hard resins, acrylic, thermoplastics, and printed circuit boards.
- Shore C: A spherical indenter with a 5mm diameter, used for foam and spongy materials.
Shore A durometers, featuring a blunt indenter and moderate spring tension, become less precise above 90 readings. For harder materials, Shore D durometers, with sharper indenters and stronger springs, are used. For extremely soft materials like gels and foam silicones, Shore 00 durometers are employed.
Testing Method
The Shore hardness tester presses into the test material and the reading on the dial indicates the hardness value. Precise testing involves strict requirements for the testing instrument, conditions, samples, and readings.
Key Testing Considerations
- All silicone must cool naturally to ambient temperature post-molding before testing.
- Standardized test specimens are necessary for consistent results.
- Specimen dimensions: flat surface, thickness ≥6mm, length/width or diameter ≥25mm.
- Molding conditions for specimens (solid molded silicone): 160 ± 5°C, 300 seconds.
- Instrument: Shore A or Shore C durometer.
Testing and Reading
- Apply pressure at 5mm/s until the durometer and silicone fully contact, maintaining 1kg pressure (achieved with a testing frame and standard weights).
- Measure hardness at five points, averaging the values for an accurate result.
Common Errors and Precautions
- Thin specimens: Supporting material hardness affects results, leading to overestimated values.
- Narrow samples: Sidewall deformation during penetration results in lower-than-actual values.
- Uneven surfaces: Non-flat specimens cannot fully contact the durometer, compromising accuracy.
2. Elongation at Break
Elongation at break refers to the percentage increase in the length of a sample relative to its original length when it fractures. For heat-cured solid silicone, elongation at break typically ranges from 90% to 1120%. Fluorosilicone ranges between 159% and 699%, while liquid silicone ranges from 220% to 900%. Variations in processing methods and curing agents significantly influence elongation. Silicone’s elongation decreases with increased hardness, making low-hardness silicone capable of exceeding 1000% elongation. Fumed silica-based silicone generally exhibits higher elongation than precipitated silica-based silicone.
Testing Capabilities
Silicone elongation test machines evaluate: tensile properties, tensile strength, deformation rate, tear resistance, seal strength, roller peel tests, 90-degree and 180-degree peels, compression tests, bending, shearing, puncture resistance, and fatigue. Standards such as GB, ISO, ASTM, JIS, and EN guide the tests, calculating parameters like ultimate force, breaking force, yield force, weld strength, tensile strength, modulus of elasticity, elongation, stress at fixed elongation, and elongation at fixed stress.
3. Tensile Strength
Tensile strength measures the maximum tensile stress a material can withstand before breaking during a tensile test.
- Heat-Cured Silicone Rubber: Tensile strength ranges from 4.0-12.5 MPa.
- Fluorosilicone Rubber: Ranges between 8.7-12.1 MPa.
- Liquid Silicone Rubber: Ranges from 3.6-11.0 MPa.
Silicone tensile strength typically stays below 10 MPa, making values of 11 MPa challenging and 12 MPa extremely rare. Generally, vapor-phase silicone has higher tensile strength than precipitation silicone.
Factors to Improve Silicone Tensile Strength and Toughness:
- High-Tensile Vapor-Phase Silicone Raw Materials
- Controlled Silicone Oil Addition: Excessive silicone oil weakens tensile performance.
- Evenly Distributed Vulcanizing Agents: Prevents reduced flow and tear resistance.
- Optimal Vulcanization Time: Insufficient time leads to softness and poor rebound strength.
Tensile strength and elongation at break are critical for assessing the structural integrity of silicone products.
- Elongation at Break: The ratio of the maximum elongation before breaking to the original length (expressed in %).
- Tensile Strength: The force required to tear a standardized silicone sample (measured in MPa).
Key Insights:
- Vapor-phase silicone outperforms precipitation silicone in elongation and tensile strength.
- Silicone generally offers better tensile and elongation properties than rubber.
Misconceptions:
- Lower hardness materials have higher elongation but do not necessarily have higher tensile strength.
Tear Strength
Tear strength measures resistance to the expansion of cuts or notches under force.
- Heat-Cured Silicone Rubber: Ranges from 9-55 kN/m.
- Fluorosilicone Rubber: Ranges from 17.5-46.4 kN/m.
- Liquid Silicone Rubber: Ranges from 11.5-52 kN/m.
Common Tear Strength Testing Methods:
- Right-Angle Testing: High-tear-resistant silicone can achieve 50 Kgf/cm.
- Half-Circle Testing: Achieves 35 Kgf/cm or more.
Procedure Overview:
Samples are clamped and pulled until torn, and results are calculated based on wall thickness and recorded in real-time.
5. Elastic Recovery
Elasticity refers to a material’s ability to return to its original shape after deformation. Silicone exhibits high elasticity, with rebound values reaching 70-80%.
Testing Methods:
- Rebound Testing Instrument: Measures height and time curves after compression release.
- Ball Drop and Pendulum Impact: Evaluate rebound percentage under specific conditions.
6. Compression Set
The compression set evaluates silicone’s ability to recover its shape after compression.
- Minimum Compression Set: Below 5%.
Testing Steps:
- Sample Preparation: Compress to a specified height and maintain conditions.
- Measurement: Calculate the deformation rate using the formula:
Testing conditions adhere to standards like ASTM D1349-99, with recommended durations of 22h and 70h under varied temperatures.
Testing Procedure
- The sample should be treated under the conditions of (23±2)°C and (50±5)% RH for 24 hours.
- Measure the original thickness of the sample, accurate to 0.02 mm.
- Place the sample into the testing apparatus, use the appropriate spacer for the sample type, and securely lock the apparatus. This can be done at room temperature.
- After clamping the sample, it must be placed in a pre-set temperature oven within 2 hours.
The testing temperature and time refer to standards or can be specified by the customer. For example, ASTM D1349-99 includes the following temperature points: -75°C, -55°C, -40°C, -25°C, -10°C, 0°C, 23°C, 40°C, 55°C, 70°C, 85°C, 100°C, 125°C, 135°C, 150°C, 160°C, 175°C, 200°C, 225°C, and 250°C.
