Relationship Between Carbon Graphite Porosity and Sealing Performance: From Permeability to Impregnation Compensation Design

Porosity is an important indicator of the densification of carbon graphite materials, directly affecting the permeability and sealing performance of seal components. In our long-term materials R&D, Huahao Sealing Co., Ltd. (霍邱县华豪密封件有限公司) has deeply studied the relationship between porosity and sealing performance, establishing scientific impregnation compensation design methods. Based on experimental data, this article systematically analyzes this relationship.

1. Characteristics of Carbon Graphite Pores

1.1 Pore Types

Pores in carbon graphite materials are classified into three types:

1.Open Pores: Connected to the material surface, the main channels for permeation. Account for 60-80% of total porosity.

2.Closed Pores: Completely enclosed within the material, do not affect permeation. Account for 15-30% of total porosity.

3.Through Pores: Completely penetrate the material, the most severe permeation channels. Account for 5-10% of total porosity.

1.2 Pore Formation Mechanism

During the pressing-baking-graphitization process of carbon graphite materials, voids between aggregate particles, pores formed by volatilization of binder pitch volatiles, and volume shrinkage during graphitization together form a complex pore structure.

1.3 Porosity Measurement

• Total porosity: measured by density method, P_total = (1 - ρ_bulk/ρ_true) × 100%
• Open porosity: measured by water absorption or mercury intrusion
• Pore size distribution: measured by mercury intrusion porosimetry (MIP) or nitrogen adsorption (BET)

2. Effect of Porosity on Permeability

2.1 Permeability Calculation

Permeability K reflects the material's resistance to fluid permeation, following Darcy's law:

Q = K · A · ΔP / (μ · L)

Where:

• Q: volumetric flow rate (m³/s)
• K: permeability (m²)
• A: permeation area (m²)
• ΔP: pressure differential (Pa)
• μ: fluid dynamic viscosity (Pa·s)
• L: permeation path length (m)

2.2 Relationship Between Porosity and Permeability

Experimental data shows permeability has a cubic relationship with open porosity:

K = K0 × (P_open / P0)³

Where K0 is the reference permeability, P_open is open porosity, and P0 is reference open porosity.

Measured data:

• Open porosity 5%: permeability 5×10⁻¹² m²
• Open porosity 10%: permeability 4×10⁻¹¹ m²
• Open porosity 15%: permeability 1.5×10⁻¹⁰ m²
• Open porosity 20%: permeability 5×10⁻¹⁰ m²

When open porosity rises from 5% to 15%, permeability increases 30-fold.

2.3 Sealing Grade and Permeability

Different sealing scenarios have different permeability requirements:

• General liquid sealing: K<10⁻¹⁴ m²
• Gas sealing: K<10⁻¹⁵ m²
• High vacuum sealing: K<10⁻¹⁶ m²
• Ultra-high vacuum sealing: K<10⁻¹⁷ m²

3. Compensation Effect of Impregnation Processes

3.1 Impregnation Principle

Impregnation reduces effective open porosity by filling open and through pores, thereby reducing permeability. Impregnation efficiency depends on:

• Impregnant viscosity (lower is better)
• Impregnation pressure (higher is better)
• Impregnation temperature (affects viscosity and wettability)
• Holding time (affects impregnation depth)

3.2 Permeability Reduction Effects of Different Impregnation Processes

| Impregnation Process | Pre-impregnation Open Porosity | Post-impregnation Open Porosity | Permeability Reduction |

|----------------------|------------------------------|--------------------------------|----------------------|

| Phenolic resin | 15% | 2-3% | 100-500x |

| Furan resin | 15% | 1-2% | 500-1000x |

| Antimony | 15% | <1% | 1000-5000x |

| Copper | 15% | <0.5% | 5000-10000x |

| Resin + resin + metal (triple) | 15% | <0.1% | >10000x |

3.3 Effect of Impregnation Depth

The penetration depth of impregnant in graphite d = √(K·t·ΔP/μ)

• Phenolic resin impregnation (μ=0.5 Pa·s, ΔP=0.1 MPa, t=1h): d≈5 mm
• Antimony impregnation (μ=0.003 Pa·s, ΔP=2 MPa, t=4h): d≈15 mm

For thick-walled seals (>15 mm), single impregnation cannot fully penetrate; multiple impregnation or preheated impregnation processes are needed.

4. Recommended Porosity for Different Sealing Scenarios

4.1 Ordinary Water Media Sealing

• Recommended material: phenolic resin-impregnated graphite
• Open porosity: 2-3%
• Permeability: <10⁻¹⁴ m²
• Applicable pressure: ≤1.0 MPa

4.2 Oil Media Sealing

• Recommended material: furan resin-impregnated graphite
• Open porosity: 1-2%
• Permeability: <5×10⁻¹⁵ m²
• Applicable pressure: ≤3.0 MPa

4.3 Gas Sealing

• Recommended material: antimony-impregnated graphite
• Open porosity: <1%
• Permeability: <10⁻¹⁵ m²
• Applicable pressure: ≤10 MPa

4.4 High Vacuum Sealing

• Recommended material: triple-impregnated graphite
• Open porosity: <0.1%
• Permeability: <10⁻¹⁶ m²
• Applicable vacuum: ≤1×10⁻⁶ Pa

5. Porosity Control Processes

5.1 Raw Material Selection

• High-density aggregate (density >2.0 g/cm³)
• High-quality coal tar pitch binder (residual carbon rate >50%)
• Control particle size distribution of ingredients, optimize packing density

5.2 Forming Process

• Isostatic pressing: pressure ≥150 MPa, open porosity reduced by 3-5%
• Mold pressing: pressure ≥100 MPa
• Extrusion: suitable for long tubular products

5.3 Baking Process

• Slow heating: room temperature→800°C, heating rate <10°C/h
• Holding time: 800°C for 24 hours, ensuring complete volatiles removal
• Pitch impregnation: after baking, pitch impregnation followed by re-baking, cycled 2-3 times

5.4 Graphitization Process

• Temperature: 2500-2800°C
• Holding: 48-72 hours
• Graphitization promotes pore merging and restructuring, total porosity reduced by 2-3%

Conclusion

The porosity of carbon graphite materials is a key factor affecting sealing performance, and proper impregnation processes can significantly reduce permeability. Huahao Sealing Co., Ltd. provides porosity-controlled carbon graphite seals with minimum open porosity down to 0.1% and permeability up to 10⁻¹⁶ m², meeting high-vacuum and gas sealing requirements. Please contact our technical team — we will provide the optimal porosity control solution based on your sealing conditions.