- ▸Baked graphite has 10%-20% porosity; after vacuum-pressure impregnation with copper-based alloys (e.g., tin bronze CuSn10), residual porosity drops below 1% — the pores are filled by a continuous metal network
- ▸Measured bronze-impregnated graphite: compressive strength 260 MPa, flexural 72 MPa, Shore hardness HS 80, thermal conductivity 85 W/(m·K), thermal expansion 3.2×10⁻⁶/°C
- ▸Copper-based impregnation suits water, low-lubricity media and dry-running-prone services: frequent starts, intermittent flooding, labyrinth seal insert rubbing heat
- ▸Media compatibility: stable in water, H₂S (<500 ppm), CO, olefins and paraffins; not suitable for strongly oxidizing media or air above 450°C
- ▸Selection boundaries: general corrosion up to 180°C takes resin impregnation, 220-500°C hot oils take antimony, water/low-lubricity/dry-running takes copper
1. What Copper Impregnated Graphite Is
Copper impregnated graphite is a composite sealing material made by vacuum-pressure impregnating baked or graphitized carbon graphite with pure copper or copper alloys (tin bronze, aluminum bronze, etc.). Pure copper itself conducts heat at roughly 390 W/(m·K); once locked into the graphite pores it does three jobs at once: sealing the pores, reinforcing the structure, and conducting heat away.
The base should be fine-grain isostatic graphite, grain size around 8 μm, base density about 1.82 g/cm³. Porosity before impregnation is 10%-20%; after vacuum-pressure impregnation (impregnation rate ≥ 95%) residual porosity can be pressed below 1% — the physical precondition for a leak-tight copper-impregnated ring.
1.1 Typical Properties of Bronze-Impregnated Graphite
Using fine-grain graphite impregnated with CuSn10 tin bronze as an example, measured values:
- Compressive strength 260 MPa, flexural strength 72 MPa
- Shore hardness HS 80 (roughly Brinell HB 120)
- Thermal conductivity 85 W/(m·K)
- Thermal expansion coefficient 3.2×10⁻⁶/°C — paired with carbon steel (about 12×10⁻⁶/°C), clearance opens rather than closes as temperature rises
- Oxidation resistance in air up to 450°C; in non-oxidizing media the graphite base allows up to 600°C
- Machining precision to ±0.005 mm by CNC grinding
For scale, compare resin- and antimony-impregnated grades: resin impregnation typically gives 90-100 MPa compressive strength, antimony about 190 MPa, while copper-based impregnation reaches 260 MPa — the highest load capacity of the three.
2. Where Copper Impregnated Graphite Fits
2.1 Water and Low-Lubricity Media
Water is a poor lubricant; the seal face film is thin and breaks easily. Copper-impregnated graphite has high hardness and good heat conduction, so friction hot spots drain quickly through the metal network and the face resists scoring. Seal rings and bushings for clean-water and cooling-water pumps are its routine applications.
2.2 Dry-Running-Prone Services
Three typical scenarios:
1.Frequently started pumps: at every start the face film has not yet formed — boundary or even dry friction. The copper network provides a low-friction metallic contact surface and rapid heat removal, reducing start-up wear.
2.Intermittently flooded equipment: the film builds and breaks repeatedly with frequent thermal shock. Graphite's 70-150 W/(m·K) conductivity range plus fast conduction through the copper network is the basis of thermal-crack resistance.
3.Labyrinth seal inserts: in centrifugal compressor shaft-end and inter-stage seals, when a graphite insert rubs a chrome-plated sleeve, the "graphite wears, sleeve survives" outcome depends on fast heat removal — 85 W/(m·K) conductivity and a low expansion coefficient exist for exactly this.
2.3 Media Compatibility
Copper impregnated graphite is stable in water, steam, oils, CO, olefins and paraffins. The graphite base itself is not corroded by H₂S, and the copper phase is unaffected below 500 ppm H₂S — which makes it suitable for sealing sulfur-bearing hydrocarbon gases such as cracker gas and field gas. The limits are equally clear: strongly oxidizing media (concentrated nitric acid, chromic acid, etc.) attack both the copper phase and the graphite, and continuous service in air is capped by the 450°C oxidation limit.
3. Boundaries Against Antimony and Resin Impregnation
The three impregnation families divide along three lines: media chemistry, temperature, and lubrication condition.
3.1 Resin Impregnation (Phenolic F, Epoxy H, Furan K)
- Temperature ceiling: about 180-220°C for phenolic and furan, about 180°C for epoxy
- Strengths: broad chemical resistance, lowest cost, low density
- Limits: resins do not tolerate high temperature and give mid-range strength (roughly 90-100 MPa compressive); unsuitable for dry-running tendencies and frequent starts
3.2 Antimony Impregnation (D)
- Temperature ceiling: about 500°C in non-oxidizing media
- Strengths: retains strength at temperature — measured 190 MPa compressive, 72 MPa flexural, 92 HS, conductivity about 70 W/(m·K)
- Limits: antimony is brittle; wear resistance is good but friction compatibility is inferior to copper-based metals; conductivity is below copper impregnation; not the choice for water-dominated media
3.3 Copper Impregnation (Copper/Bronze)
- Temperature ceiling: about 450°C in air, higher in non-oxidizing media
- Strengths: highest load capacity of the three (260 MPa), highest conductivity (85 W/(m·K)), best friction compatibility — wears without damaging the counter-face
- Limits: not for strongly oxidizing media; the copper phase is sensitive to some chemical media, so each medium must be checked before selection
3.4 One-Line Selection Guide
- Corrosive media, ≤ 180°C, normal lubrication: resin impregnation (furan for acids/alkalis, phenolic for general use)
- Hot oils, thermal-oil and molten-salt pumps at 220-500°C: antimony impregnation
- Water-dominated, low lubrication, frequent starts, dry-running tendency, compressor labyrinth inserts: copper impregnation
- 500-600°C extreme temperature in non-oxidizing media: switch to a carbonized grade that does not rely on metal impregnation
4. Summary
The value of copper impregnated graphite is that the copper-based metal network solves three problems at once — pore sealing, reinforcement and heat conduction: residual porosity below 1%, 260 MPa compressive strength, HS 80 hardness and 85 W/(m·K) conductivity, the strongest load and heat performance among the three common impregnations. Its position sits between resin impregnation (corrosion resistance, low cost, low temperature) and antimony impregnation (high temperature, high strength), on the side of harsh service and insufficient lubrication. When selecting, first confirm the medium is safe for the copper phase, then that the temperature stays within 450°C, and finally that the lubrication condition genuinely calls for metal impregnation — pass all three checks, and copper impregnated graphite is usually the most durable answer for water and low-lubricity services.