Selection of Filled PTFE Sealing Materials (Glass Fiber / Carbon Fiber / Graphite / Bronze) and Typical Applications

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Sealing application - Seal selection based on operating conditions and filled PTFE materials
1. Selection of Seals according to Five Critical Operating Parameters
The first step in choosing a seal is to systematically evaluate five basic operating variables:

1. Type of Movement
Reciprocating motion / rotational motion / static sealing
Material matching and surface polish are crucial in rotary applications as they are particularly sensitive to counterface wear and formation of abrasive particles.

2. Value of PV and Extrusion Gap
High pressure, big extrusion gaps and combination of both, raise the extrusion and cold flow deformation risk greatly .
In such instances, preference should be given to materials with high anti-extrusion and creep resistance.

3. Hardness and Counterface Material
Lower hardness shafts, cylinders or valve seats are more prone to abrasive wear by filled PTFE materials.
Hence, material matching should be a trade-off between wear resistance and counterface protection.

4. The Chemical Compatibility of the Medium
Corrosive media such as strong alkalis or hydrofluoric acid (HF) can have a major impact on the appropriateness of the material and long-term stability.
In harsh situations, chemical resistance is a major selection factor.

5. Stick-slip behaviour and friction sensitivity
Low friction and reliable motion behaviour are commonly required in low speed reciprocating systems and servo hydraulic applications.
In such situations, it is important to minimise stick-slip (crawling).

Creep, wear, chemical resistance, conductivity and thermal conductivity are the characteristics on which engineering seal selection is based.

2. Four Types of Commonly Used Filled PTFE Materials: Properties and Applications
This comparison is typical for general engineering trends. The real performance depends on the type of filler, morphology (fibre or powder), concentration, sintering procedure and manufacturing quality.

2.1 PTFE filled with glass
Key benefits:

Greatly reduces creep (cold-flow)

Enhances resistance to wear and compressive strength

Frequently used as a general reinforcing grade

Major limitations:

More aggressive on mating surfaces, particularly in rotary applications

Poor compatibility with HF and strong alkalis

Typical sealing uses

Reciprocating seals for hydraulic and pneumatic use

Anti-extrusion rings and back-up rings.

Wear rings / guide rings

Structural sealing elements (e.g. valve seats)

On note selection:
Glass filled PTFE is the best material for high pressure reciprocating systems where dimensional stability and creep resistance are important. But this is often not desirable in rotational applications with soft shafts because of its abrasive character.

2.2 PTFE gefüllt mit Kohlenstoff / Carbon-Graphit
PTFE filled with carbon is any system with carbon powder, carbon fibre or carbon-graphite mixes. Wordings need to be verified at the selection time.

The main benefits are:

Reduces creep and hardens.

Enhanced thermal conductivity

Excellent dry running properties of carbon-graphite mixtures

Can confer anti-static (electrically conductive) characteristics

Carbon fibre particular benefits:

Similar improvement in creep resistance as glass fibre but usually with lower loading »

Suitable for alkaline and hydrofluoric acid conditions where glass fibre is not suitable

Improved wear resistance under water lubricated circumstances

Typical sealing applications are:

Pneumatic Seals

Dry or boundary lubrication reciprocating seals

Rings and guide strips to wear

Dynamic sealing parts of low friction

Limitations

Performance may suffer in high oxidising situations.

Conductivity may be detrimental in electrical insulation applications.

Selection note: 
Carbon-graphite systems are favoured in low-lubrication or dry-running circumstances, where low wear and controlled deformation are desired. In chemically aggressive or water-lubricated systems, carbon fibre alternatives are favoured.

2.3 PTFE Filled with Graphite
Major benefits:

Very low COF (coefficient of friction)

Soft counterface materials gentle on

High chemical resistance

Good for medium to high loads and speeds

Typical applications for sealing:

Low Speed Reciprocating Systems with Anti-Stick-Slip Performance (e.g. servo-hydraulics, precision cylinders)

Non-lubricated rotary shaft seals with flexible shafts

Sealing applications for corrosive media (pumps, valves, rotating machinery)

Spring seal jackets energise seal systems

Restrictions:

Performance may be degraded in highly dry air situations

Under water-lubricated conditions, carbon-graphite or carbon fibre composites may have greater wear resistance.

Selection note: 
Graphite filled PTFE is appropriate for applications requiring ultra low friction and compatibility with soft counterfaces, especially in harsh environments.

2.4 PTFE with bronze filling
Bronze filled PTFE usually contains 40% to 60% bronze.

The key benefits:

Excellent anti-extrusion properties

High load carrying capacity

Enhanced thermal conductivity and heat dissipation

Resistance to creep under high pressure

Typical applications for sealing are:

Piston and rod seals in high hydraulic pressure

Back-up rings, anti-extrusion

Guide rings and worn strips in high PV situations

Heavy duty rotating or sliding seals

Limitations:

Limited chemical compatibility with some medium (some chemicals might attack bronze)

Not suited for electrical insulating purposes

Mating materials must be selected carefully to avoid compatibility problems.

Note of choice:
Bronze filled PTFE is the preferred material for high pressure systems where extrusion resistance is the primary consideration. Not suitable for chemically harsh or electrically sensitive applications.

3. Quick Material Selection Guide for Components
Anti-extrusion / back-up rings (high pressure, huge gaps)
Bronze-filled PTFE is preferable. Glass-filled is utilised depending on the chemical conditions.

Dynamic seals for hydraulic rod/piston (universal high pressure):
used often filled bronze
Low friction or soft shaft systems -> choose graphite or carbon-graphite

Pneumatic seals (dry or low lubrication): 
Carbon filled or carbon graphite systems favoured

Soft counter-faces Low wear requirements:
Best: Graphite impregnated PTFE
Exercise caution when using glass-filled PTFE

Strong alkali or hydrofluoric acid (HF) environments: 
Not good for glass fibre, best for carbon fibre or carbon based systems

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