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.
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|
|Volume Resistivity||>10ᴵ⁶ ohm/cm|
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.
The addition of suitable fillers to PTFE can enhance the compression resistance, increase wear resistance, reduce deformation under load and increase thermal conductivity.
|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.|
Common acids and bases.
|Reagent||% Concentration||Exposure temp ⁰C||Exposure Time||% Weight Increase|
|Hydrochloric acid||10||25||12 months||NIL|
|Nitric Acid||10||25||12 months||NIL|
|Sulphuric Acid||30||25||12 months||NIL|
|Sodium Hydroxide||10||25||12 months||Nil|
|Ammonium Hydroxide||10||25||12 months||NIL|
|Solvent||Exposure Temperature ⁰C||Exposure Time||% Weight Increase|
|Carbon Tetrachloride||25||12 months||0.6|
|Ethyl alcohol (95%)||25||12 months||NIL|
|Ethyl acetate||25||12 months||0.5|
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.