Plastic for seals: choosing the right materials

Which plastic is suitable for seals depends above all on the medium to be sealed, the temperature, the pressure and the type of movement. PTFE is a frequent choice for aggressive chemicals and large temperature differences. PEEK offers higher mechanical strength, while PE-UHMW has advantages in wear and low temperatures.

For many classic seals, however, a thermoplastic plastic is not required, but rather an elastomer such as EPDM, NBR or silicone. These materials can be compressed elastically and largely return to their original shape after loading. The most important question is therefore initially not PTFE or PEEK, but: Is an elastic or a dimensionally stable seal required?

Which plastic is suitable for seals

The following materials in particular come into consideration for technically demanding sealing elements:

• PTFE for aggressive media, low friction and high or low temperatures
• PEEK for high temperatures, high pressures and mechanical loading
• PVDF for acids, alkalis and chemically stressed systems
• PE-UHMW for abrasion, wear and low temperatures
• POM-C for dimensionally stable support, guide and scraper elements
• EPDM for water, weathering and many hot water applications
• NBR for contact with oils and fuels
• Silicone for high and low temperatures
• FKM for oils, fuels, chemicals and elevated temperatures

There is no blanket ranking. A chemical-resistant sealing material can be unsuitable under high mechanical loading. A particularly elastic material can in turn fail against the medium used.

Anyone wishing to select the right sealing material must therefore consider the complete operating conditions.

Plastic seal or rubber seal?

In general usage, elastomers are frequently referred to as rubber and thermoplastics as plastic. Technically, however, both groups belong to the polymeric materials.

The decisive difference lies in the mechanical behaviour.

Elastomers can be heavily deformed under pressure and build up a restoring force in the process. This restoring force ensures that the seal rests against the sealing surfaces and compensates for unevenness. Elastomers are therefore frequently used for flat gaskets, O-rings, diaphragms and flexible sealing profiles.

Thermoplastic sealing materials such as PTFE or PEEK are significantly more dimensionally stable and less elastic. They are particularly suitable for:

• high temperatures
• aggressive media
• high pressures
• low friction
• wear-stressed seals
• support and guide rings
• precisely machined sealing elements

The question of plastic or rubber for seals can therefore be answered on the basis of the required restoring force. If the seal must adapt strongly and compensate for unevenness, an elastomer is frequently required. If, on the other hand, the component must withstand pressure, temperature or chemical loading, a thermoplastic high-performance plastic may be more suitable.

The most important selection criteria for sealing materials

A sealing material must function permanently under real operating conditions. Data sheet values at room temperature are frequently not sufficient for a reliable selection.

Medium to be sealed

The medium is one of the most important selection criteria. Acids, alkalis, solvents, fuels, oils, steam and cleaning agents attack materials in different ways.

An unsuitable plastic seal can:

• swell
• become brittle
• lose strength
• become discoloured
• dissolve
• lose its elasticity
• become permanently deformed

PTFE possesses a very broad media resistance. PEEK combines high chemical resistance with better mechanical properties. PVDF is resistant to many acids, alkalis and chlorine-containing media. NBR, on the other hand, is particularly suitable for oils, while EPDM is frequently used for water and weathering.

Since concentration, temperature and exposure time change the resistance, the chemical resistance of plastics should always be checked for the specific application. PTFE is regarded as almost universally chemical-resistant, while PEEK combines high chemical resistance with greater structural strength.

Temperature

A seal must withstand both the maximum and the minimum operating temperature. What is decisive is not only short-term temperature peaks, but also continuous loading and possible temperature changes.

High temperatures can cause a plastic to:

• become softer
• lose its compressive strength
• creep more
• age faster
• lose its restoring force

At low temperatures, materials can harden or become brittle. PTFE, PEEK and PE-UHMW can also be of interest for very low temperatures. Silicone also remains elastic over a broad temperature range.

Further information for cooling systems, freezer warehouses or cryogenic applications can be found in the article on plastics at low temperatures. There, PTFE and PE-UHMW in particular are classified as suitable materials for very low temperatures.

Pressure

As pressure increases, the requirements for dimensional stability, extrusion resistance and creep resistance rise. A soft or strongly creeping sealing material can be pressed out of the sealing gap.

For high pressures, the following are among those that come into consideration:

• PEEK
• filled PTFE
• metal-backed PTFE seals
• composite seals
• elastomer seals with support rings

Pure PTFE is chemically and thermally very high-performing, but possesses a significantly lower mechanical strength than PEEK. This is also shown by the technical data sheets of the two materials.

Static or dynamic seal

In a static seal, the sealing surfaces do not move against each other during operation, or only to a minor extent. Typical examples are flange gaskets, housing seals and cover seals.

In a dynamic seal, at least one surface slides or rotates. These include:

• piston seals
• rod seals
• shaft seals
• rotary seals
• scrapers

Dynamic seals place higher demands on friction, wear and heat generation. A material can be well suited for a static flat gasket and yet wear quickly under permanent movement.

Restoring capacity

A seal frequently functions by being compressed and thereby generating a permanent counterforce. Elastomers possess a significantly better restoring capacity for this than PTFE, PEEK or POM-C.

PTFE can become plastically deformed under permanent load. If the restoring force drops, the seal can leak. PTFE seals are therefore frequently preloaded by design, combined with elastic elements or manufactured from modified materials.

Friction and wear

In dynamic seals, the coefficient of friction influences:

• the required drive force
• heat generation
• the starting resistance
• the wear rate
• the service life of the mating surface

PTFE possesses very good sliding properties and is therefore frequently suitable for moving seals. Pure PTFE, however, is mechanically comparatively soft. Filled PTFE types can improve wear, compressive strength and creep behaviour.

Food and pharmaceutical conformity

A polymer is not automatically food-safe just because the plastic is fundamentally used in the food industry. What is decisive is:

• the specific material formulation
• fillers and additives used
• colour
• manufacturing process
• contact time
• contact temperature
• the food
• existing conformity certificates

PTFE, PEEK, PE, PP and POM-C are available in suitable grades for food applications. The actual suitability, however, must be demonstrated on the basis of the specific product documentation. The comparison of food-safe plastics and their approvals explains more about this.

When is PTFE suitable for seals?

PTFE is one of the most important thermoplastic sealing materials. The material possesses very low water absorption, a low coefficient of friction and high resistance to numerous chemicals.

Typical applications for a PTFE seal are:

• valve seals
• pump seals
• flange gaskets
• piston and rod seals
• seals in chemical plants
• sealing elements in food technology
• applications with solvents
• seals at high or low temperatures

For individually machined sealing rings, sealing plates and valve parts, PTFE sheets cut to size can be used as semi-finished products.

Advantages of a PTFE seal

PTFE offers in particular the following advantages:

• very high chemical resistance
• low moisture absorption
• low friction
• non-stick behaviour
• broad temperature application range
• good electrical insulation
• high purity of suitable material grades

Disadvantages of a PTFE seal

The greatest disadvantage is the low elastic recovery. PTFE does not behave like a classic rubber seal. If the material is permanently compressed, it can creep and not fully return to its original shape.

This can lead to problems when:

• the pressure fluctuates strongly
• the sealing surfaces work
• the connection is frequently heated and cooled
• a permanent preload is required
• a large sealing gap is present

In such applications, filled PTFE compounds or spring-supported sealing systems may be more suitable.

When is PEEK better than PTFE?

PEEK is a mechanically highly resilient high-performance thermoplastic. It possesses high strength, stiffness, temperature resistance and chemical resistance. A PEEK seal is therefore particularly suitable for applications in which PTFE would be mechanically too soft or too prone to creep.

PEEK comes into consideration, among other things, for:

• high operating pressures
• high temperatures
• rapid pressure changes
• mechanically stressed valves
• pumps
• compressors
• hydraulic systems
• high-pressure fittings
• support and guide rings

For precisely milled or turned sealing elements, PEEK sheets for thermally and mechanically stressed components can be used.

PTFE or PEEK for seals?

The decision PTFE or PEEK for seals can be made in simplified form on the basis of the most important priority.

PTFE is frequently more suitable when:

• the broadest possible media resistance is required
• low friction is decisive
• there is no high mechanical loading
• very low water absorption is required
• non-stick behaviour is important

PEEK is frequently more suitable when:

• high compressive strength is required
• the seal must hold its shape under load
• elevated temperatures and mechanical loading come together
• tight tolerances are required
• a more creep-resistant sealing material is needed

A more detailed decision-making aid is offered by the technical article PEEK or PTFE in direct comparison.

When is PVDF suitable for seals?

PVDF possesses high resistance to numerous acids, alkalis and halogen-containing media. The material is therefore frequently used in chemical plant construction, in water treatment and in the semiconductor industry.

PVDF is suitable, for example, for:

• seals in chemical piping systems
• valve components
• pump parts
• sealing discs
• corrosion-resistant flange systems
• applications with high purity

Compared with PTFE, PVDF is stiffer and more mechanically resilient. However, it does not achieve its universal chemical resistance and broad temperature application range. Whether a PVDF seal is suitable must therefore be checked on the basis of the specific medium and concentration.

When is PE-UHMW suitable as a sealing material?

PE-UHMW, frequently also referred to as PE 1000, possesses very high wear resistance, low water absorption and good sliding properties. The material is less suitable as a classic elastic seal, but can be used for scrapers, sealing strips, guides and wear-stressed sealing elements.

Typical applications are:

• scraper strips
• sealing strips
• guide rings
• slides
• wear-stressed sealing elements
• components in cooling and freezer systems
• seals at low temperatures

For such components, PE 1000 sheets cut to size can be used.

PE-UHMW is chemically resistant to many media and remains tough even at low temperatures. At high temperatures, high pressures or very tight dimensional tolerances, however, the material reaches its limits more quickly.

Is POM-C suitable for seals?

POM-C is not a classic material for elastic flat gaskets. However, the plastic possesses high dimensional accuracy, good machinability and comparatively high strength.

POM-C is therefore frequently used for the following components:

• support rings
• guide rings
• scrapers
• valve parts
• seal housings
• seal carriers
• spacer rings

POM-C sheets cut to size are particularly suitable for precisely machined components with tight tolerances.

POM-C, however, is not suitable for all media. Particularly with strong acids, oxidising agents and high temperatures, alternatives must be checked. For a sole static flat gasket, the material also lacks the elastic restoring force of an elastomer.

Is PA6 suitable for seals?

PA6 is also not a universal sealing material. Polyamide possesses high toughness and wear resistance, but absorbs significantly more moisture than PTFE, PEEK or POM-C.

Water absorption can lead to the following changes:

• dimensional change
• lower stiffness
• altered strength
• altered bearing clearance
• altered sealing effect

PA6 can be used for guide rings, scrapers or mechanically stressed support elements. For precise seals under changing humidity, however, the material is frequently less suitable.

Which elastomers are suitable for seals?

For flexible seals, elastomers are frequently more suitable than thermoplastic plastics. The selection depends above all on the medium and the temperature range.

Sealing material	Typical strength	Typical limitation
NBR	Oils, greases and fuels	Limited weathering and ozone resistance
EPDM	Water, hot water, weathering and ozone	Unsuitable for many mineral oils
FKM	Oils, fuels, chemicals and elevated temperatures	Higher costs, limited low-temperature flexibility
Silicone	High and low temperatures, good flexibility	Limited abrasion and tear resistance
SBR	Water, simple technical applications	Limited oil and weathering resistance
PUR	Abrasion and mechanical loading	Not suitable for every chemical medium

When is NBR suitable for seals?

NBR is a widespread material for seals in contact with mineral oils, lubricants and fuels.

Typical applications are:

• oil seals
• hydraulic seals
• machine seals
• fuel systems
• flat gaskets in plant construction

For cut flat gaskets and interlayers, NBR rubber sheets can be used.

NBR should not be selected as a blanket choice for outdoor applications, ozone exposure or hot water. Here, EPDM may be more suitable.

When is EPDM suitable for seals?

EPDM possesses high resistance to weathering, UV radiation, ozone and many water-based media. The material is frequently used for seals in buildings, water systems and outdoor areas.

Typical applications are:

• water pipes
• hot water applications
• roof seals
• window and façade seals
• seals in outdoor systems
• flange gaskets for water-based media

For individually cut seals, EPDM rubber sheets or round EPDM rubber discs are available.

EPDM, however, is usually not suitable for mineral oil, petrol or diesel fuel. For such media, NBR or FKM frequently come into consideration.

When is silicone suitable for seals?

Silicone remains elastic over a broad temperature range and is therefore suitable for applications with strong thermal fluctuations. The material is used, among other things, in food technology, medical technology, electrical engineering and equipment construction.

Silicone sheets for temperature-resistant seals can be processed into flat gaskets, pads and elastic interlayers. According to the product information, the silicone sheets offered are designed for a broad temperature range.

Typical applications are:

• oven and drying systems
• food machinery
• electrical housings
• laboratory equipment
• cooling and heating systems
• covers subjected to temperature changes

Silicone, however, possesses lower abrasion and tear resistance than many other elastomers. For dynamically highly stressed seals, the suitability must therefore be checked carefully.

Which plastic is suitable for flat gaskets?

The right plastic for flat gaskets depends above all on the medium, temperature, surface pressure and surface condition.

Typical material decisions are:

• PTFE for aggressive chemicals and large temperature differences
• EPDM for water, weathering and many hot water applications
• NBR for oils and lubricants
• Silicone for high and low temperatures
• FKM for oils, chemicals and elevated temperatures
• PEEK for highly stressed, dimensionally stable sealing elements

A flat gasket must be able to compensate for small unevenness in the sealing surfaces. The harder and more dimensionally stable the material is, the higher the requirements usually are for surface quality, preload and component precision.

Which plastic is suitable for static seals?

In static seals, the components do not move against each other after assembly. As a result, friction and wear play a lesser role than in dynamic seals.

More important are:

• restoring force
• surface pressure
• creep behaviour
• setting behaviour
• media resistance
• temperature resistance

EPDM, NBR, FKM and silicone are suitable for many static seals because they elastically compensate for surface unevenness. PTFE is used when the chemical or thermal loading exceeds the possibilities of classic elastomers.

Which plastic is suitable for dynamic seals?

In dynamic seals, movement, friction and wear occur. In addition to the sealing effect, the tribological loading must therefore also be taken into account.

Suitable materials can be:

• PTFE and PTFE compounds
• PEEK
• PUR
• special NBR or FKM grades
• material composites
• spring-supported sealing systems

PTFE is particularly interesting because of its low coefficient of friction. Under high surface pressure or strong load changes, however, filled PTFE materials or PEEK support rings may be required.

Which seal is suitable for high temperatures?

For high temperatures, the following materials in particular come into consideration:

• PTFE
• PEEK
• silicone
• FKM
• special high-temperature elastomers

The pure maximum temperature from the data sheet is not sufficient for the selection. In addition, pressure, medium, duration of use and mechanical loading must be considered.

A silicone seal can remain flexible at high temperature, but is not as mechanically resilient as PEEK. PTFE is chemically very resistant, but possesses a lower restoring force. PEEK is mechanically stable, but significantly less elastic than classic rubber seals.

Which seal is suitable for hot water and steam?

EPDM is frequently used for hot water. With steam, however, the requirements rise significantly. Temperature, pressure and duration of steam exposure must be assessed together.

Possible materials are:

• suitable EPDM grades
• PTFE
• special FKM types
• high-temperature elastomers
• graphite or composite seals

NBR is usually not the first choice for hot water and steam. PEEK can be used for mechanically stressed sealing components in steam systems, but does not automatically take on the function of an elastic flat gasket.

Which seal is suitable for acids and alkalis?

PTFE is a particularly safe starting point for many acids and alkalis. PVDF, PP and PE can also be suitable depending on the medium, concentration and temperature.

EPDM is resistant to many diluted acids and alkalis, but not universally. FKM possesses high media resistance, but can be unsuitable against certain alkalis, amines or hot water.

A plastic seal for acids or a plastic seal for alkalis should therefore never be selected on the basis of the polymer designation alone. The resistance test must include the specific medium, the concentration, the temperature and the exposure time.

Which sealing material is suitable for food?

For seals with food contact, PTFE, PEEK, silicone, EPDM, NBR, PE and POM-C come into consideration, among others. What is decisive, however, is whether the specific material grade is approved for the intended contact.

The following must be checked:

• EU conformity
• FDA conformity
• migration limits
• cleaning media
• contact temperature
• contact time
• traceability
• colour and additives

A white or natural-coloured seal is not automatically food-safe. The term FDA-compliant plastic for seals also says little without specific manufacturer documentation.

The article on food-safe plastics in comparison explains the differences between material suitability, conformity and application-related testing.

Typical mistakes when selecting a plastic seal

Considering only the maximum temperature

A seal can withstand a temperature briefly, but under simultaneous pressure it can creep strongly or lose its restoring force.

Always regarding PTFE as the best solution

PTFE possesses very high chemical resistance, but is not particularly elastic. For simple water or oil seals, a suitable elastomer can be technically and economically more sensible.

Assessing the medium without concentration

A diluted acid can barely attack a material, while the same substance in high concentration is unsuitable.

Treating dynamic and static seals the same

A flange gasket and a moving piston seal place completely different demands on friction, wear and recovery.

Considering only the base material

Filled PTFE behaves differently from pure PTFE. EPDM, NBR or silicone can also possess strongly differing properties depending on the formulation.

Assuming food conformity

The fundamental suitability of a polymer does not replace a declaration of conformity for the specific material grade.

Underestimating manufacturing tolerances

Thickness, flatness, cut quality and dimensional tolerances influence the sealing effect. Particularly with thin flat gaskets or precise installation spaces, the manufacturing must match the design.

Decision matrix for the right sealing material

Operating condition	Frequently suitable material
Water and weathering	EPDM
Mineral oil and lubricants	NBR or FKM
Aggressive chemicals	PTFE, sometimes PVDF or PEEK
High mechanical loading	PEEK or material composite
High and low temperatures	PTFE or silicone
Dynamic seal with low friction	PTFE compound
High abrasion	PUR or PE-UHMW
Food contact	Suitable PTFE, PEEK, silicone, EPDM or NBR grade
Precise support ring	PEEK or POM-C
Hot water	Suitable EPDM grade
Steam	PTFE or specially approved elastomer grade
Low temperatures	PTFE, PE-UHMW or silicone

Selecting the sealing material on the basis of the operating conditions

For aggressive media and large temperature ranges, PTFE is frequently the first material to be checked. At high pressures and mechanical loadings, PEEK can be the more dimensionally stable solution. PE-UHMW is suitable for wear-stressed scrapers and sealing strips, while POM-C comes into consideration above all for precise support and guide rings.

If the seal requires a high elastic recovery, EPDM, NBR, FKM or silicone are frequently more suitable than a hard thermoplastic. EPDM is suitable above all for water and weathering, NBR for oil, silicone for large temperature differences and FKM for demanding media and temperature combinations.

The material decision must therefore not be made on the basis of the polymer name alone. Only the joint consideration of medium, concentration, temperature, pressure, movement, sealing gap and required restoring force leads to a reliable selection.

Which plastic is best suited for seals?

PTFE is particularly suitable for aggressive media, low friction and high and low temperatures. PEEK is frequently more dimensionally stable at high pressures and mechanical loading. If elastic recovery is required, materials such as EPDM, NBR, FKM or silicone are usually more suitable.

When is PTFE better than a rubber seal?

PTFE is frequently more suitable when aggressive chemicals, solvents, very high or low temperatures and low friction play a role. A rubber seal, on the other hand, usually offers higher elasticity and restoring force and can better compensate for unevenness in the sealing surfaces.

Which seal is suitable for high temperatures?

For high temperatures, PTFE, PEEK, silicone and suitable FKM grades come into consideration in particular. The medium to be sealed, the operating pressure, the duration of the temperature loading and the required elasticity are also decisive.

Is PEEK or PTFE better for high pressures?

PEEK is frequently more suitable at high pressures because it possesses higher strength, stiffness and creep resistance than pure PTFE. PTFE, on the other hand, offers advantages in chemical resistance and friction. In many sealing systems, both materials are combined with each other.

Which sealing materials are suitable for food?

PTFE, PEEK, silicone, EPDM, NBR, PE and POM-C are available in food-compliant grades. What is decisive is not only the polymer name, but the specific formulation and valid conformity documentation for the intended contact duration and temperature.