When adjusting hardness in silicone potting compounds, low-viscosity dimethyl silicone oil is a more direct and effective choice, while vinyl silicone oil is typically used to control crosslinking density or as a base polymer. The following is a detailed analysis and recommendations:
One. Dimethyl silicone oil: The core choice for hardness adjustment
1. Mechanism of Action
Dimethyl silicone oil (PDMS) is a non-reactive plasticizer that softens potting compounds by physically filling and reducing crosslink density. Its molecular chains do not participate in the curing reaction; they act only as a "diluent" dispersed in the system, weakening the compactness of the polymer network and thus reducing hardness.
2. Recommended Viscosity
Common Range: 50~1000 cSt (25℃).
Low viscosity (50~200 cSt): Excellent flowability, suitable for applications requiring rapid penetration and degassing (e.g., electronic component potting), but excessive addition may lead to decreased strength. Medium to high viscosity (500~1000 cSt): Effectively reduces hardness while maintaining certain mechanical properties, commonly used in industrial applications requiring strength (e.g., power module packaging).
Typical example: 200 cSt dimethyl silicone oil is often used in electronic potting compounds. Adding 5% to 10% can reduce the Shore A hardness by 5 to 10 degrees.
3. Precautions
Addition Ratio: For every 2% of dimethyl silicone oil added, the Shore A hardness decreases by approximately 1 degree. However, excessive amounts (above 20%) may lead to oil seepage and decreased weather resistance.
Compatibility: A dimethyl silicone oil with good compatibility with the base polymer (such as methyl vinyl silicone rubber) should be selected to avoid phase separation.
Two. Vinyl silicone oil: The regulator of crosslinking density
1. Mechanism of Action
Vinyl silicone oil molecules contain vinyl groups (-CH=CH₂) at both ends or in the side chains, which can undergo a hydrosilylation reaction with hydrogen-containing silicone oils under the action of a platinum catalyst to form a three-dimensional cross-linked network. Its main function is to control the cross-linking density, rather than directly adjusting the hardness.
2. Relationship between viscosity and hardness
Low-viscosity vinyl silicone oils (e.g., 20~55 mPa·s):
Can improve the flowability and filler dispersibility of potting compounds, but the hardness may increase after crosslinking due to the high density of crosslinking points. For example, when 1000 cSt vinyl silicone oil is used in combination with hydrogen-containing silicone oil, the crosslinking density can reach 0.2 mol/cm³, resulting in higher hardness.
High-viscosity vinyl silicone oils (e.g., 10,000~20,000 mPa·s):
These have longer molecular chains, resulting in a more flexible network after cross-linking, which can reduce hardness. For example, using high-viscosity vinyl silicone oils in battery potting compounds can control the hardness to Shore A 40~60.
3. Application Scenarios
Base Polymer: Vinyl silicone oil is the main raw material for addition-type potting compounds, and its molecular weight and vinyl content directly affect the final properties. For example, terminal vinyl silicone oil (Vi-PDMS) is commonly used in electronic potting compounds, while side-chain vinyl silicone oil (Vi-PMVS) is suitable for industrial applications requiring high tear strength.
Auxiliary Adjustment: Hardness can be indirectly optimized by adjusting the ratio of vinyl silicone oil to hydrogen-containing silicone oil. For example, reducing the amount of hydrogen-containing silicone oil can reduce the crosslinking density, thereby reducing hardness.
Three. Comprehensive Recommendations
1. Hardness Reduction: Dimethyl silicone oil is preferred.
Applicable Scenarios: Needs significant softening of potting compounds, such as stress relief for electronic components and flexible encapsulation.
Operating Steps:
① Select a dimethyl silicone oil with a hardness of 50~1000 cSt (200~500 cSt recommended);
② Initially add 5%~10%, gradually testing the hardness change;
③ Vacuum degassing is required during mixing to avoid residual air bubbles affecting performance.
2. Hardness Fine-tuning: Combined with Vinyl Silicone Oil
Applicable Scenarios: Requires precise control of crosslinking density, such as thermally conductive potting compounds and high-temperature applications.
Operating Steps:
① Select a low-viscosity vinyl silicone oil (e.g., 1000~5000 mPa・s) as the base polymer;
② Control the crosslinking density by adjusting the amount of hydrogen-containing silicone oil (typically 3~5 phr);
③ Add a small amount of dimethyl silicone oil (<5%) to further optimize the hardness.
3. Performance Balance
Strength and Hardness: Excessive addition of dimethyl silicone oil can reduce tensile strength and tear resistance. It is recommended to use it in combination with reinforcing fillers (such as fumed silica).
Temperature Resistance and Insulation: The cross-linked network of vinyl silicone oil can improve temperature resistance (-50~200℃) and insulation properties (volume resistivity > 1×10¹⁵ Ω・cm), making it suitable for applications with high reliability requirements.
Summary
Reducing Hardness: Low-viscosity dimethyl silicone oil (50~1000 cSt) is preferred. Softening is achieved through physical dilution, with an addition amount typically between 5% and 15%.
Controlling Crosslinking Density: Vinyl silicone oil is used to construct a three-dimensional network. Hardness can be indirectly optimized by adjusting its molecular weight, vinyl content, and ratio with hydrogen-containing silicone oil.
Practical Applications: It is recommended to combine the properties of both. For example, in addition-cure potting compounds, use vinyl silicone oil as the base polymer while adding a small amount of dimethyl silicone oil for fine-tuning to achieve the best performance balance.
