I. Essential Characteristics: Physical Behavior Determined by Linear Molecular Structure

1. Liquid Stability
   It remains in a liquid state over an extremely wide temperature range (-60℃ to 300℃), with minimal viscosity change with temperature (temperature-viscosity coefficient: 0.6–0.8). For example, the viscosity of 100 cSt silicone oil only increases by approximately 3 times at -40℃, while mineral oil has already solidified.
2. Outstanding Surface Activity
   It has an ultra-low surface tension of 21 mN/m (water: 72 mN/m), demonstrating excellent spreadability and water repellency, and can quickly form an ultra-thin lubricating film on the substrate surface.
3. Chemical Inertness and Physiological Safety
   It shows high inertness to chemicals such as acids, alkalis, oils, and solvents. Moreover, it has passed skin irritation tests and is widely used in the cosmetics and medical fields.
4. Limitations of Physical Film Formation
   The physical film formed by solvent volatilization or high-temperature evaporation (e.g., heat treatment after dip-coating) is essentially a physically adsorbed layer of silicone oil molecules on the substrate surface. Its thickness is usually less than 1 μm, and it is prone to falling off under external forces, temperature changes, or medium erosion.

II. Performance Boundaries: Core Limitations in Sealing Coating Applications

1. Inherently Insufficient Adhesion

• Only weak van der Waals forces exist between silicone oil molecules and substrates (metals, plastics, glass), without chemical bonding. For instance, the adhesion on metal surfaces is only 0.1–0.3 N/cm, far lower than the 1–5 N/cm of curable coatings.
• During long-term use, vibration, temperature cycles, or medium scouring can easily cause the film to peel off. For example, the peeling rate of the silicone oil lubricating film in the engine compartment exceeds 50% after 1,000 hours at 100℃.

2. Unsustainable Sealing Performance

• Physical films have nanoscale pores and cannot completely block the penetration of gases or liquids. For example, the water vapor transmission rate of a 0.5 μm-thick silicone oil film is 5 g/(m²·24h), while that of curable coatings can be reduced to 0.1 g/(m²·24h).
• Silicone oil volatilizes at high temperatures, leading to film thinning. Taking 100 cSt silicone oil as an example, the volatilization loss can reach 15% after 24 hours at 150℃, which may cause sealing failure in long-term use.

3. Weak Mechanical Properties

• The film has a tensile strength of less than 0.1 MPa and an elongation at break of more than 500%, showing typical "high elasticity but low strength" characteristics. In dynamic sealing scenarios (e.g., reciprocating moving parts), the film is prone to shear damage.
• Low-viscosity silicone oil (e.g., 50 cSt) may undergo an "extrusion effect" under high pressure (>0.5 MPa) and leak from the sealing gap.

III. Typical Applications: Limited Value Scenarios of Physical Film Formation

1. Temporary Protection and Lubrication

• Short-term storage of precision instruments: Apply low-viscosity silicone oil (e.g., 10 cSt) on the surface of optical lenses and electronic components to form a moisture-proof and dust-proof film, which can maintain a protective effect for 3–6 months.
• Metal processing lubrication: As an additive in cutting fluids (addition amount: 0.1–1%), it reduces the friction coefficient between tools and workpieces (from 0.3 to 0.15), but thorough cleaning is required after processing to avoid residue.

2. Dynamic Interface Assisted Sealing

• Rotary shaft sealing: Apply high-viscosity silicone oil (e.g., 12,500 cSt) on the surface of mechanical seals (e.g., O-rings). Its low surface energy reduces frictional resistance, and it fills micro-gaps to improve short-term sealing performance.
• Thread anti-seizure: Apply silicone oil to the joints of metal threads to prevent seizing at high temperatures (e.g., engine spark plugs), but regular reapplication is necessary.

3. Special Functional Coatings

• Hydrophobic and anti-fog treatment: Dip-coat glass with silicone oil followed by heat treatment (250–300℃) to form a semi-permanent hydrophobic film (contact angle >110°), which is used in scenarios such as glasses and car rearview mirrors.
• Release isolation: Spray silicone oil (concentration: 1–5%) on the surface of rubber and plastic molds, which can reduce the release force by 40–60%, but the dosage must be controlled to avoid oil contamination on the product surface.

IV. Performance Enhancement: Breaking Physical Limits through Formula Optimization

1. Thickening and Gelation
   Add fumed silica (3–10%) or organobentonite to thicken silicone oil into a semi-solid paste (e.g., thermal conductive silicone grease), improving anti-flow properties and short-term sealing performance. However, silicone oil may still exude at long-term high temperatures.
2. Surface Modification
   Introduce groups such as fluoroalkyl (-CF₃) or amino (-NH₂) to enhance adhesion to specific substrates. For example, the adsorption capacity of amino-modified silicone oil on cotton fibers is 3 times higher than that of conventional silicone oil.
3. Composite Coating Design
   Use it in combination with curable silicone oil: the bottom layer is a curable coating that provides adhesion and strength, while the surface layer is conventional silicone oil that provides lubricity. For example, in engine sealing, a "addition-curable primer + linear silicone oil topcoat" combination is adopted to balance sealing performance and friction properties.

V. Decision-Making Suggestions: Key Criteria for Scenario Adaptation

• Sealing medium: It is only suitable for non-polar liquids (e.g., mineral oil) or dry gases, and ineffective for polar solvents (e.g., water, alcohols) or corrosive media (e.g., acids, alkalis).
• Operating temperature: The long-term service temperature should be lower than the flash point of the silicone oil (e.g., the flash point of 100 cSt silicone oil is approximately 200℃), and the short-term peak temperature should not exceed 250℃.
• Sealing life: The effective protection period of the physical film is usually several days to several months, requiring regular inspection and reapplication.
• Safety risks: Low-viscosity silicone oil (<50 cSt) has a low flash point (e.g., the flash point of 0.65 cSt silicone oil is -1℃), so open flames or high-temperature heat sources should be avoided.

Summary

The physical film-forming property of conventional linear dimethyl silicone oil determines its application boundaries in the sealing field:

✅ Applicable scenarios: Temporary protection, dynamic lubrication, special functional coatings (e.g., hydrophobic, release).

      ❌ Inapplicable scenarios: Long-term static sealing, high-pressure/high-temperature environments, contact with polar media.

To achieve permanent sealing, a curable silicone oil system (condensation-curable, addition-curable, or UV-curable) must be selected to form a tough film with a three-dimensional network structure through chemical crosslinking. The value of conventional silicone oil lies more in the precise control of physical behavior rather than the durability of chemical film formation.

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