Guide To PTFE Gasket Material

A Guide To PTFE

An in-depth look at PTFE, and the gasket materials that are made from this versatile fluorocarbon polymer.

On this page: Operating Temperature | Why use PTFE | Dielectric PTFE | PTFE Fillers | PTFE Properties | PTFE Chemical Resistance | RAM’s PTFE Materials Available | Price Fluctuations

PTFE (Polytetrafluoroethylene) is a unique material. It is one of the most valuable engineering materials available due to its versatility. PTFE has an unmatchable profile of unique properties; a fluorocarbon polymer with outstanding chemical and thermal resistance, so it can be used to seal against virtually any chemical under a huge range of operating temperatures. As a semi-crystalline plastic that doesn’t absorb UV light, it also shows excellent resistance to sunlight.

Temperature Range Of PTFE

The operating temperature of Virgin PTFE (PTFE with no fillers) ranges from -75⁰C to +260⁰C. It is the best known non-stick material with a coefficient friction comparable to wet ice on wet ice. Most people commonly utilise PTFE by using a commonly seen trademark: Teflon, which is a PTFE coating for non-stick household items.

Why Use PTFE As A Seal?

• It is chemical resistant. Pure PTFE can be used in the most corrosive of environments and will not contaminate the most sensitive medium. PTFE is completely insoluble and has very high intrinsic purity and can be manufactured with no contamination for ultra-pure or corrosive applications.
• It is completely resistant to attack by all chemicals with the exception of molten alkali metals and a few fluorine compounds at elevated temperatures and pressures. It is unaffected by lubricants, hydraulic fluids, aircraft/rocket fuel and atmospheric conditions.
  

Is PTFE Safe in Electrical Environments?

The dielectric constant of PTFE is largely independent of temperature and will not leave a carbon ‘track’ when subject to arcing. Unaffected by weathering and ageing, PTFE is not wetted nor will it absorb water. It can be used as an external electrical insulator.

Dielectric Constant	2-13 over a wide range of frequencies
Dissipation Factor	0.0004
Volume Resistivity	>10ᴵ⁶      ohm/cm
Dielectric Strength	>16kV/mm

PTFE and PTFE Fillers.

PTFE can be made into sheet material, washers, spaces, rings, tubes, coating for pump interiors, drilling-components and dip tubes. It is truly an incredibly versatile product and maintains all the unique properties of PTFE as discussed above when utilised in any format.

It is a waxy powder like substance that cannot be melted but is pressed and formed into shapes.

The fillers increase creep resistance, hardness, chemical resistance, and compliance requirements for specific industries.

PTFE can have fillers added blending with base PTFE resin to enhance specific properties.

Glass fibre is often added in different percentages and different fibre lengths, generally glass fibre increases wear resistance with less deformation under load, creating a harder material with lower thermal expansion.

Porous PTFE – porous PTFE is manufactured for filtration, with excellent control over pore size, water entry pressure etc.

Carbon Fibre filled PTFE – normally at a ratio of 5% CF to 95% PTFE, Carbon filled PTFE is often used where Glass fibre fails, better resistance to pressure under load as it is a harder material, used in strong alkali and hydrochloric acid which is where glass fibre can fail.

Graphite filled PTFE – Excellent in high speed contact applications where it offers little wear, including when wearing against soft metals.

Bronze filled PTFE – excellent creep resistance and high thermal conductivity.

Glass Moly-filled PTFE (Glass fiber; Molybdenum DiSulfide) – makes a very stiff, rigid material, chemically unreactive and further reduces friction.

Mineral filled PTFE – for FDA compliance

Polymide PTFE – good friction against soft metals.

Stainless Steel Filled PTFE – stiff, hard PTFE material.

PTFE Composites And Their Properties

The addition of suitable fillers to PTFE can enhance the compression resistance, increase wear resistance, reduce deformation under load and increase thermal conductivity.

Filler	Properties	Applications
Glass Fibres	Improved mechanical wear properties and dimensional stability.	Bearings, Anti-extrusion rings, valve seats, seals and gaskets.
Carbon	Improved surface hardness and pressure resistance.	Dynamic seals, water contact applications.
Graphite	Wear resistance and good thermal conductivity.	Special Seals
Molybdenum Disulphide	Wear resistance and reduced friction.	Special Seals
Bronze	Increased Hardness, compressive strength and dimensional stability.	Special bearings, valve seats, liners etc.

Virgin PTFE’s Chemical Resistance,

Common acids and bases.

Reagent	% Concentration	Exposure temp ⁰C	Exposure Time	% Weight Increase
Hydrochloric acid	10	25	12 months	NIL
		50	12 months	NIL
		70	12 months	NIL
	20	100	8h	NIL
		200	8h	NIL
Nitric Acid	10	25	12 months	NIL
		70	12 months	0.1
Sulphuric Acid	30	25	12 months	NIL
		70	12 months	NIL
		100	8h	NIL
		200	8h	0.1
Sodium Hydroxide	10	25	12 months	Nil
		70	12 months	0.1
	50	100	8h	NIL
		200	8h	NIL
Ammonium Hydroxide	10	25	12 months	NIL
		70	12 months	0.1

 

Common Solvents

Solvent	Exposure Temperature ⁰C	Exposure Time	% Weight Increase
Acetone	25	12 months	0.3
	50	12 months	0.4
	70	2 weeks	NIL
Benzene	78	96 h	0.5
	100	8 h	0.6
	200	8 h	1.0
Carbon Tetrachloride	25	12 months	0.6
	50	12 months	1.6
	70	2 weeks	1.9
	100	8h	2.5
	200	8h	3.7
Ethyl alcohol (95%)	25	12 months	NIL
	50	12 months	NIL
	70	2 weeks	NIL
	100	8h	0.1
	200	8h	0.3
Ethyl acetate	25	12 months	0.5
	50	12 months	0.7
	70	2 weeks	0.7
Toluene	25	12 months	0.3
	50	12 months	0.6
	70	2 weeks	0.6

We use PTFE in the following forms to manufacture gaskets and seals:

Pure PTFE (Virgin) – any washers and components.

Filled PTFE – PTFE with graphite (carbon), glass and mineral – we source by customer requirement if any other fillers are required.

Expanded PTFE – Softchem®, Goretex® Grr, an expanded PTFE sheet. Softer material that compresses similarly to a foam, specifically targeted for sealing flange joints in pipelines and hydraulic and pneumatic systems.

We also use branded PTFE products, these are the Gylon and Uniflon products we use;

Gylon® and Uniflon PTFE and Chemical Resistance at its best

Garlock manufactures a PTFE product called Gylon®. A unique and superior material through higher temperature/pressure combinations. Better sealability and torque retention, minimizing gasket creep and cold flow problems. The main benefits of Gylon® over conventional PTFE and PTFE/Filler blends are found in the areas of environmental improvement and overall cost savings. Gylon conforms to FDA regulations and is often used in chemical processing and hydrocarbon processing industries.

There are 3 variants of Gylon®

Style 3500 – Strong Acids (exception: hydrofluoric), solvents, hydrocarbons, water, steam, chlorine.

Style 3504 – Moderate concentrations of acids and caustics, hydrocarbons, solvents, water, refrigerants, cryogenics, hydrogen peroxide.

Style 3510 – Best fluid resistance available. Strong Caustics, moderate acids, chlorine gases, water, steam, hydrocarbons, hydrogen and aluminium fluoride.

Flexitallic Manufactures a Product called Uniflon, (formerly Novus Uniflon).

Uniflon is a blended PTFE and filler material; see below for information regarding their use and compatibility.

Uniflon is available in five variants:

Uniflon 50 – Specifically designed for use in low bolt loaded irregular flanges. Typical flanges include glass lined, ceramic, plastic coated or uneven and badly distorted flanges. It is suitable for sealing all chemicals across the whole pH range with the exception of molten alkali metals.

Uniflon 51 – General purpose grade for sealing applications across the whole pH range. It is particularly suitable for use with strong acids (except hydrofluoric acid) and alkalis. Other applications include solvents, fuels, water, steam, and chlorine.

Uniflon 53 – General purpose grade for sealing applications across the whole pH range. It is suitable for use with hydrofluoric acid, but not pure liquid hydrogen fluoride. It can also be used with alkalis, solvents, fuels, water, steam and chlorine.

Uniflon 58 – Specifically designed for use in low bolt loaded irregular flanges. Typical flanges include glass lined, ceramic, plastic coated or uneven and badly distorted flanges. It is suitable for sealing all chemicals across the pH range with exception of molten alkali metals and fluorine gas. Uniflon 58 is very similar to 50 however it has a pure PTFE core.

Uniflon 60 – Specifically designed for use in low bolt loaded irregular flanges. Typical flanges include glass lined, ceramic, plastic coated or uneven and badly distorted flanges. It is suitable for sealing all chemicals across the pH range with exception of molten alkali metals and fluorine gas. Different to Uniflon 58 and 50 as it has controlled Porosity.

Price Fluctuations on the Global Market

PTFE is much more expensive than other sealing materials, so it is important to consider first whether a more cost-efficient suitable material can be used. The price can fluctuate due to the raw material demand being high. In 2011 Virgin PTFE saw an increase of 185%. However recently the prices have stabilised with new Flourspar mines opening in Africa offsetting the pressure on China’s mines. There are other options matching PTFE’s characteristics however even with price increases these are still more expensive. Technavios’s analysts forecast the global PTFE market to grow at a CAGR of 5.03% over the period 2014 – 2019.

When there is consistent risk of gasket failure due to strong chemicals, corrosion and heat when in conjunction, PTFE can often be a perfect solution. Therefore, the cost can remain high because there are often no other suitable alternatives.

For more information about PTFE please contact us.

Commonly Used Gasket Materials By Sector:

Oil and Gas

Generally this industry will utilise spiral wounds, ring type joints, Kammprofile gaskets, non-asbestos material including brand names Klinger and Flexitallic (previously called Novus). Composite materials including mica and graphite, all these materials are utilised in high temperature, high pressure environments.

Automotive

This industry will generally utilise silicone as a high temperature polymer that can be supplied in O-ring, sheet and liquid form. Nitrile and viton that are fuel and oil resistant are other often used polymers. EPDM is commonly used for door seals and plastic foams are used for seat fillers. Within the combustion areas of the engines, graphite and non-asbestos (including brand names Flexitallic and Klinger) materials will be used, these are resistant to fuels and oils combined with high temperatures (approx. +350⁰C). Finally, a cork composite material, often natural cork blended with nitrile can be used as a cheaper alternative to pure nitrile rubber. Classic cars now use a range of modern materials since asbestos is no longer allowed as a fibre within gasket formulations.

Aerospace

This industry will utilise many different gasket materials. The certification requirements that need to be supplied along with the gaskets are often the most critical aspect within Aerospace. Products supplied in this area are supported by Certificates of Conformity, including batch and cure dates with full traceability. Supply chain, and specific standards like RR SABRE can be met by RAM when producing gaskets for the Aerospace industry.

Food Processing

Temperature limits in food production are also required to be taste and taint free. Silicone is most often utilised in this environment as a stable high-temperature polymer that also inhibits bacterial growth. For steam and high pressure environments non-asbestos material and graphite is required. EPDM is used in potable water applications which has great UV and environmental resistance over a number of years and seals effectively. PTFE with low co-efficient friction and excellent chemical resistance is used as an envelope to line flange gaskets made from rubber or non-asbestos.

Drinking Water/Water

Potable water in the UK often requires WRAS approval (Water Regulations Advisory Scheme). WRAS approved EPDM is stocked by RAM in the form of sheet, O-rings, and cord. Other weather seal materials that have very good UV resistance are EPDM, neoprene, non-asbestos and graphitewill be used when steam is present along with higher pressures.

Electrical

Enclosure gaskets require weather seals, UV resistance and general weather fluctuations and conditions mean an effective seal is required against weather elements. Cork, Nebar Cork, Neoprene, EPDM, nitrile foams and rubbers are used here. For conductive gasket materials and EMEI shielding products conductive foam can be used, wired silicone (solid/foam) and branded composite gasket materials can be supplied. Plastic shim is also used as a spacing and insulating material within the electrical industry. Gaskets can be selected to help meet IP tests. The test applies to the assembly not the gasket itself.

Petrochemical

Flange Gaskets within this industry are often made with non-asbestos gasket material, including brands such as Flexitallic and Klinger. For higher pressure spiral wounds, Kammprofile, mica and graphite are used. Quality and safety in a volatile environment is paramount. Fuel, oil and corrosion resistant high grade rubbers such as nitrile, viton, silicone and PTFE in foam and solid form are also used. The quality of the rubber in high corrosion environments is very important. Correct supply of quality materials ensures an effective seal. PTFE as a low co-efficient friction material with excellent chemical resistance is supplied as envelopes for pipe flange gaskets, in sheet form or foamed to create a compressible material for sealing uneven flange faces.

Pharmaceutical

Silicone rubber is often used within pharmaceutical environments. Platinum and peroxide curing of silicone for medical grade applications are often required. RAM can cut cured materials in a clean environment. For flange gaskets and equipment seals PTFE is used in the form of envelopes as a low bacterial growth, clean liner to pipe gaskets. Envelopes were developed as a cheaper alternative to solid PTFE for flange gaskets. Solid PTFE and foamed PTFE in the form of Softchem or other brand names are used when a compressible PTFE is required. Expanded PTFE is often used for sealing uneven flanges against very aggressive chemicals. Modified PTFE’s are available for special environments that enhance the capabilities of PTFE.

Pumping

Plastic shim is commonly used for pneumatic air compressors, boosters and vacuum pumps. Plastic shim provides good corrosion resistance and low co-efficient friction. EPDM and neoprene are used for pipe gaskets. When corrosive liquids are being used nitrile, viton, PTFE and Non-asbestos can also be used.

Rail

For enclosures and components within internal public spaces a FAR 25.853 approval as discussed above may be required. Silicone, EPDM, nitrile can be supplied in solid or foam form. Manway gaskets in non-asbestos, viton and EPDM to seal against fuel, oil and water are most commonly utilised.

Rubber has a shelf life, most of our materials used for gaskets have some rubber component. The shelf life refers to the time the gasket is still usable for service. So, if the rubber is nearing the end of its shelf life, it is still considered suitable for service use. If it has exceeded its shelf life, in order to extend the service life, the gaskets should be re-made.

Storage

Gaskets should be stored in a cool, ambient location, between 4⁰C and 27⁰C Excessive humidity (40% to 75% relative humidity), direct sunlight and any chemical vapours should not be present. Low UV producing lights and minimal dust. These optimum storage conditions can increase a gaskets shelf life significantly.

Material Type and Average Recommended Shelf Life*

Material Type	Recommended Shelf Life
Silicone	20 years
Viton	20 years
Neoprene	5-10 years
EPDM	5-10 years
Butyl	5-10 years
Nitrile	5-10 years
SBR	3-5 years
Natural Rubber	3-5 years
Natural Fibre	2-5 years
Non-Asbestos Fibre	5-10 years

*Shelf-life varies for specific materials and shelf-life for specific materials can be requested. This is a general guide.

Non-Asbestos Fibre

Non-asbestos fibre materials are not designed to be stored for extended periods of time. The complex nature of the materials used, reinforcing fibres, rubber binder and filler mean the material is susceptible to storage changes. The fibre component is the most stable but gradually dries out, weathers and deteriorate. The filler is the biggest culprit, the filler optimizes gasket performance however upon manufacture involves an irreversible chemical process which allows for a shelf life of between 5-10 years.

RAM’s Products

RAM has been cutting gaskets and materials for nearly 50 years. In that time material shipping, delivery and manufacturing methods have improved greatly to increase product turnaround and re-ordering of materials Using LEAN principles in our manufacture and ordering of goods allows RAM to store only the optimal volumes of stocked materials.

Material Supply Chain

We cut huge quantities of materials which allows us to turn over our stock material quickly. Specific Batch and Cure dates can be requested when the longest possible shelf-life is required. As companies across the supply chain adopt LEAN principles, lower and more regular order volumes allows for the quality of the products to be maintained.

Many gaskets require conformance for their application. RAM can provide conformance for any parts – just ask or tell us what conformance you require. Conformance can be supplied as part of the material type or the manufacture of the goods.

Common types of conformance are;

Potable Water (WRAS) approval.

UL – predominantly a US safety requirement mark for products in the USA. However UL ratings cover European kitemarks.

US-EU is a kitemark for USA manufactured products destined for the EU. There are different types of UL rating depending on the location of the product.UL provides Fire Safety and British Construction products with testing for UL, British and EN standards.

FAIR – First Article Inspection Report. Often Aerospace and MOD require FAIR approval.

FoodSafe EU1935:2003, FDA CFR part 177 Section 2600 Allowable Substances,

For more information on gasket conformance please look here: LINK TO CONFORMANCE

For more information on UL ratings for gaskets and seals: LINK TO UL RATINGS

Gasket’s are usually made from a flat and flexible sealing material. There are many types of material for different applications. Common materials include cork, rubber, non-asbestos fibre, graphite, silicone, paper, PTFE, shim and many more. For high temperature and high pressure some gaskets are made entirely of metal such as RTJ (ring type joints) and Spirally Wound gaskets.

Ram Gaskets is dedicated to the world of gaskets – please get in touch with your application, we can help source the correct material and supply the right parts.

A general guide to gasket materials; their properties and applications.

Let’s first look at some unit conversions:

1 bar = 14 p.s.i = 100,000 newtons/sq. Metre = 1 pascal
0°(C)entigrade = 32°(F)arenheit
–273.15°(C)entigrade = 1 (K)elvin

divide gasket materials into a few broad categories:

Low Pressure:

Less than 1 bar/ 14psi / 100,000N/m²(1 pascal)

Temperatures below 100 centigrade (212 farenheit)

Simple lids & dust seals can utilise foam rubber.
If your application involves mineral oils only then consider paper or cork.

If there is water present or or some other liquid or gas look at rubber.
For food, electrical & chemicals you may need a special rubber or PTFE.
Or if you have good flanges and bolt loading then a solution may be non-asbestos jointing.

Temperatures above 100 centigrade (212 farenheit) and less than 220 centigrade (428 farenheit)

Consider Special Rubbers or PTFE.
Or if you have good flanges and bolt loading then look at non-asbestos gasket material.

High Pressures and Temperatures Above 220 Centigrade (428 Farenheit)

Temperatures up to 450 centigrade (842 farenheit) but STEAM to 230 centigrade (446 farenheit)

First consider non-asbestos jointing.
If your application involves process cycling and steam look at graphite.

Special Applications

For temperatures above 450 centigrade with no oxygen present consider graphite.

If you have extreme temperatures but little pressure eg. turbochargers use mica.

Extreme conditions in pipework can be accommodated with spiral wound gaskets.

If you have a specific material requirement, you may require a food quality or pharmaceutical grade material.

Specifying Gasket Material By Function

The following table, which gives a useful guide to specifying gasket materials by their uses, might also be helpful:

Function	Material Types	Applications	Considerations
Anti Vibration	Rubber | Neoprene | Nitrile | Cork	Electronic | Electrical Motors | Industrial Plant Machinery | Automotive	Aerospace and Defence
Flame Retardant	Silicone Foam – UL94, HT-800, HT-6360 | FPC, Fireblock Material | EC 2130 (Flame rated EMI) | R10400M (Flame rated Silicone Foam)	Residential and commercial lighting gaskets | Commercial Signage | Commercial cooking equipment and ventilation | Enclosure gaskets | Industrial Controls | Handheld Electronics | Rugged Cushion Pads | Flame rated EMI gaskets| Telecommunications Base Stations	FAR 25.83 – an FAA Flame test for aircraft interiors. Materials must be self-extinguishing under the vertical burn test
Dust	Poron® | Silicone Foam | Neoprene Foam	Clean Rooms | HEPA Filters | HVAC/air handling | LCD Displays | Touch Panel displays | Portable Electronics	Compression Cycling | Temperature | Outgassing | Cushioning | Test Requirements – Common testing for Dust applications include; NEMA Type 1, IP5X and IP6X (IP50 and IP60 have no water sealing requirements)
High Temperature	Rubber Materials – Urethane (less than 121⁰C) | Natural Rubber (less than 121⁰C) | Neoprene (less than 149⁰C) | Nitrile (less than 149⁰C) | EPDM (less than 204⁰C) | Fluorosilicone (204⁰C – 260⁰C) | Fluorocarbon (204⁰C-260⁰C) | Silicone (204⁰C-260⁰C) Higher Temperature Composite Materials – Non-asbestos jointing (Klinger Flexitallic etc;) | Ceramic Fibre (up to 1000⁰C) | Firefly (up to 1000⁰C) | Graphite (up to 450⁰C) | Metallic Gaskets (Spiral Wounds/Ring type joints)	Any application where heat is above ambient temperature. Composite gaskets and metallic considered when high temperatures and subsequent high pressure are involved	Heat aging is critical with rubber products as heat aging will gradually degrade all elastomers. The following testing all have heat aging requirements. ASTM D 1056 | ASTM D 573 | UL 50 | UL 48 | UL 508 | UL 157
FoodSafe, FDA, Antimicrobial	Platinum Cured Silicone | Silicone Rubber/Foam | EPDM (white/blue) | Natural Rubber (white) | PTFE | Nitrile (white) | WRAS approved EPDM (potable water) | WRAS approved Aramid Fibre | PTFE Sheet Jointing | Nitrile Sheet Jointing BS7531 Grade Y	Food Manufacturing | Food Processing Plants | Pharmaceutical Machinery | Clean rooms	Just because it is white or blue in colour doesn’t mean it is food safe. FDA approved materials and FoodSafe approved materials EU 19235:2004 | FDA 21 CFR | USP Class VI | NSF/ANSI Standard 51 | Mil-STD 810 | EN1186
Electrical	Nitrile Cork | Nebar Cork | Neoprene Cork | Polyethylene Foam | EPDM Foam | Silicone Rubber | EMI Shielding Rubber	Transformer gaskets | Oil and Water Tanks | IP rating (ingress protection)	IP Rating | NEMA | UL / IP rating | (See above Environmental Gasket Column)