PTFE rings

PTFE rings, full name Polytetrafluoroethylene rings, are annular industrial components primarily made from polytetrafluoroethylene or modified composite materials based on this polymer. Depending on their function, PTFE rings are mainly categorized into several types: sealing rings (for dynamic or static seals), guide rings / wear rings (for withstanding side loads and maintaining piston concentricity), and packing rings (such as Raschig rings used as tower packing). Leveraging the PTFE material's excellent self-lubricating properties, exceptional chemical stability, and wide temperature adaptability, PTFE rings can operate stably for long periods under harsh conditions such as oil-free lubrication, strong corrosion, high or low temperatures. They are widely used in industries like chemical processing, machinery, aluminum electrolysis, food, and pharmaceuticals.

Physical and Chemical Properties

1. Physical Properties

The density of PTFE rings is approximately 2.10 to 2.30 g/cm³, with a melting point around 327°C. They have a wide continuous operating temperature range, typically from -180°C to +260°C, although some sources suggest the continuous use temperature should not exceed 180-250°C. Their thermal conductivity is relatively low, around 0.244 W/(m·K), which is more than 300 times lower than that of metals, meaning frictional heat is not easily dissipated. The water absorption rate is extremely low, less than 0.01%, ensuring dimensional stability and no swelling when exposed to water. The friction coefficient is very low; both dynamic and static friction coefficients can be as low as 0.04, and when sliding against metal, it is approximately 0.09-0.12.

2. Chemical Properties

PTFE rings possess exceptional chemical stability, resisting corrosion from nearly all chemicals, including strong acids, bases, and oxidants, except for a very few substances like molten alkali metals, elemental fluorine, and chlorine trifluoride at elevated temperatures. They also exhibit excellent electrical insulation, with high volume resistivity and a low dielectric constant. Furthermore, they have good weatherability and are not prone to aging, but their radiation resistance is poor.

3. Mechanical Properties

Regarding mechanical properties, tensile strength is about 25-27.6 MPa, flexural modulus is approximately 0.70 GPa, and compressive strength can reach over 25 MPa with specific formulations. The coefficient of linear thermal expansion is relatively high, around 15 × 10⁻⁵ /°C, which is about ten times that of steel. It is important to note that PTFE exhibits "cold flow" (creep under load) and has relatively low hardness, making the material soft. Therefore, in practical applications, this is often improved by adding fillers, using modified PTFE compounds (e.g., adding glass fiber, carbon fiber, graphite, molybdenum disulfide, bronze powder, or silica) to enhance wear resistance, creep resistance, and thermal conductivity.

Application Areas

Depending on the specific type, PTFE rings are widely used in the following scenarios:

1. Sealing Ring Applications

· Chemical and Petroleum Industry: Used in shaft seals and static seals for valves, pumps, and reactors, leveraging their resistance to strong acids and bases.

· Aluminum Electrolysis Industry: Specially used in vacuum lifting pots, maintaining vacuum environment pressure stability in molten aluminum transfer systems, with temperature resistance up to 260°C, adaptable to 160kA to 500kA series electrolytic cells.

· General Machinery: Used as piston rings, O-rings, V-rings, and combination seal rings (such as Stepseals and Glyd rings) in hydraulic and pneumatic systems.

2. Guide Ring / Wear Ring Applications

· Hydraulic Cylinders: Installed on pistons as guide rings (wear bands) to withstand side loads, prevent metal-to-metal contact between the piston and cylinder bore, and protect the expensive cylinder barrel.

· High-Pressure Systems: Used in high-pressure hydraulic systems with pressures > 400 bar and reciprocating speeds > 5 m/s to ensure piston concentricity.

3. Packing Ring Applications

· Chemical Towers: Used as Raschig ring packing in drying towers, absorption towers, scrubbers, and regeneration towers, offering excellent acid and heat resistance, capable of withstanding various inorganic acids (except hydrofluoric acid), organic acids, and organic solvents.

4. Back-up Ring Applications

· Hydraulic and Pneumatic Systems: Used as back-up rings for O-rings to prevent extrusion of the seal into the gap under high pressure, suitable for high-reliability applications such as aerospace and military equipment.

Advantages Compared to Other Products

1. Fully Self-Lubricating, Low Friction

PTFE rings have an extremely low friction coefficient (as low as 0.04), with similar static and dynamic coefficients. This eliminates "stick-slip" phenomena during low-speed motion, making them particularly suitable for precision positioning and high-speed reciprocating applications.

2. Extremely Strong Corrosion Resistance

They can function effectively in harsh chemical media such as strong acids, bases, and organic solvents – a capability unmatched by metal sealing rings or ordinary rubber seals.

3. Wide Temperature Adaptability

They maintain stable performance across a broad temperature range from -180°C to +260°C, suitable for extreme conditions like cryogenic treatment and high-temperature baking.

4. Mating Surface Friendly

PTFE molecules tend to transfer to the mating surface, forming a lubricating film approximately 0.02-0.03 μm thick. This protects the shaft or cylinder bore surface, reducing wear.

5. Clean and Contamination-Free

Being free of oil or grease, they do not suffer from aging or deterioration, making them particularly suitable for industries requiring cleanliness or avoiding contamination, such as food, pharmaceutical, and electronics.

6. Advantages Compared to Composite Materials

Compared to phenolic resin-based composites, PTFE rings perform better in high-speed (>5 m/s) and low-friction applications, with lower friction coefficients and less heat generation. Additionally, PTFE's chemical stability surpasses that of most composite materials.

. Usage Precautions

1. Poor Thermal Conductivity

PTFE material has a low thermal conductivity coefficient, so frictional heat is not easily dissipated. In high-speed or heavy-load conditions, the inability to dissipate frictional heat can lead to temperature increases and degraded sealing performance. Therefore, such sealing rings are only suitable for applications with low PV values, and cooling measures may be necessary. Alternatively, consider modified PTFE filled with bronze powder or carbon fiber to improve thermal conductivity.

2. High Coefficient of Thermal Expansion

PTFE's linear expansion is about ten times that of steel. When designing and installing, sufficient clearance (typically 0.1-0.3 mm is recommended) must be预留 to prevent seizure or sticking due to expansion at high temperatures.

3. Poor Creep Resistance (Cold Flow)

Pure PTFE is prone to permanent deformation under load – this is one of its major drawbacks. For heavy-load applications, it is essential to choose filled or modified materials, such as those containing glass fiber, carbon fiber, graphite, or bronze powder. Dynamic loads are generally recommended to be kept below 5 MPa, and static loads below 10 MPa.

4. Installation Precautions

· Avoid Forced Installation: PTFE rings have relatively low hardness; use non-metallic tools or specialized fixtures during installation to avoid scratches or deformation.

· Lubrication: Apply a lubricant compatible with the medium (e.g., silicone oil, petroleum jelly, or fluorine-based lubricants) before installation to reduce frictional resistance.

· Heat Assembly: Heating the ring to around 120°C allows it to expand for easier installation, reducing mechanical stress.

· Check Alignment: Ensure proper alignment during installation to avoid localized stress concentration.

· O-Ring Installation: For static seals, ensure no twisting or folding; for dynamic seals, avoid excessive stretching (PTFE elastic elongation ≤ 15%).

5. Requirements for Mating Shaft

To achieve optimal wear performance, the surface hardness of the mating shaft or cylinder bore should ideally be no less than HRC 50. The surface roughness Ra should be controlled within 0.2-0.4 μm for dynamic seals or ≤ 1.6 μm for general seals, avoiding burrs that could scratch the sealing ring.

6. Regular Inspection

Inspect the ring every 6 months or so for cracks, permanent indentations, or deformation, and replace it promptly to ensure sealing reliability.

Selection Guide

To ensure a PTFE ring suits specific operating conditions well, consider the following aspects during selection:

1. Operating Condition Assessment

· Functional Requirement: Clarify whether it is for sealing, guiding/positioning, or packing?

· Load Conditions: Dynamic or static? Load magnitude (light/heavy)? Operating speed or PV value?

· Ambient Temperature: Continuous operating temperature range? Are extreme temperature fluctuations involved?

· Contact Media: Chemical composition and concentration of the media? Does it contain particles?

2. Material Selection

· Pure PTFE: Suitable for general corrosive environments and light-load conditions.

· Filled/Modified PTFE:

· Glass Fiber Filled: Improves wear resistance and dimensional stability.

· Carbon Fiber Filled: Enhances thermal conductivity and creep resistance; suitable for water-based media.

· Graphite/Molybdenum Disulfide Filled: Further reduces the friction coefficient and improves self-lubrication.

· Bronze Powder Filled: Improves thermal conductivity and hardness; the preferred choice for hydraulic wear bands.

· Silica/Glass Powder Filled: Increases dimensional stability.

· Metal-Backed PTFE Composites: For heavy-load or high-pressure conditions, consider composite structures with a metal backing.

3. Structural Form Selection

· Sealing Rings: Choose O-rings, V-rings, combination seals, etc., based on the seal type.

· Guide Rings: Determine cross-sectional dimensions according to standards like ISO 10766. Angled cuts (scarf cut) are suitable for reciprocating pistons; straight cuts are less common in hydraulics.

· Back-up Rings: Choose continuous, single-turn cut, or spiral-cut forms based on the pressure level.

· Packing Rings: For Raschig rings, select appropriate sizes (commonly 25-75 mm) based on the tower diameter.

4. Dimensional Design

· Clearance: 预留 clearance for thermal expansion based on estimated temperature rise, typically 0.1-0.3 mm.

· Width Calculation: Guide ring width = Radial load / (Piston diameter × Allowable surface pressure). A safety factor of 2.0 is recommended.

· Tolerance Control: Dimensional tolerances for precision-machined parts should be controlled within ±0.02 mm.

5. Adaptation to Special Conditions

· High-Speed Conditions (>5 m/s): Prioritize PTFE filled with bronze or carbon fiber.

· High-Pressure Conditions (>40 MPa): Consider adding metal back-up rings to prevent extrusion.

· Water-Based Media: Avoid cotton-phenolic composites (which absorb water and swell); choose synthetic fiber composites or carbon fiber-filled PTFE.

· Food/Pharmaceutical Industries: Select pure PTFE or specialized food-grade materials that meet hygiene standards.

Summary

PTFE rings are high-performance annular industrial components based on polytetrafluoroethylene, categorized by function into various types including sealing rings, guide/wear rings, packing rings, and back-up rings. Their core advantages lie in their extremely low friction coefficient enabling self-lubricating operation, exceptional chemical stability resisting corrosion from various media, wide temperature range adapting to harsh environments, and being friendly to mating surfaces. However, the drawbacks of PTFE – poor thermal conductivity, high thermal expansion, and weak creep resistance – require engineers to comprehensively consider the operating conditions during selection and design. This involves rationally choosing filler materials and structural forms, precisely calculating clearances, and strictly following installation guidelines. Correctly selecting and applying PTFE rings not only effectively enhances equipment sealing reliability and service life but also reduces long-term maintenance costs, playing an irreplaceable role in industries such as chemical processing, machinery, aluminum electrolysis, food, and pharmaceuticals.