PTFE (Polytetrafluoroethylene) envelope gaskets, also known as "PTFE encapsulated gaskets," are widely used in industrial sealing applications due to PTFE’s excellent chemical resistance, non-stick properties, and high-temperature stability. Their core design involves a PTFE outer "envelope" that encapsulates a softer, more compressible inner core (e.g., rubber, graphite, or fiber), combining PTFE’s corrosion resistance with the core’s sealing flexibility. 

 

Below is a detailed classification of PTFE envelope gaskets based on core material, PTFE envelope structure, and application-specific designs, along with their key characteristics and use cases.

 

 

 1. Classification by Inner Core Material  

The inner core is critical for achieving effective sealing (since pure PTFE is relatively rigid and prone to creep). Different core materials tailor the gasket’s compressibility, temperature resistance, and cost. 

Core Type	Key Characteristics	Typical Applications
Rubber-Core (Most Common)	- High compressibility and elasticity (excellent for irregular flange surfaces).	- General-purpose sealing (water, air, oils).
	- Cost-effective.	- Food & beverage (EPDM/Silicone core, FDA-compliant).
	- Common rubber types: EPDM, Nitrile (NBR), Silicone, Viton® (FKM).	- Chemical processing (Viton® core for oil/chemical resistance).
Graphite-Core	- Ultra-high temperature resistance (-200°C to 600°C).	- High-temperature applications (steam, hot oils, thermal fluids).
	- Excellent thermal conductivity.	- Chemical reactors, refineries, and power plants.
	- Compatible with aggressive chemicals (acids, alkalis).
	- Low creep (better than rubber).
Fiber-Core	- Made of synthetic fibers (e.g., aramid, glass fiber) or mineral fibers.	- Low-to-medium pressure sealing (pumps, valves).
	- Balances compressibility and mechanical strength.	- Applications where rubber may degrade (e.g., mild chemicals, moderate temperatures).
	- Resists edge tearing.
Metal-Core (Rare)	- Inner core of thin metal (e.g., copper, aluminum, or stainless steel).	- High-pressure piping systems (oil & gas, hydraulic lines).
	- High pressure resistance (up to 100 bar+).	- Applications requiring rigid sealing (e.g., flanges with high bolt torque).
	- Minimal creep (stable under long-term load).

 

 2. Classification by PTFE Envelope Structure  

The design of the PTFE outer layer affects the gasket’s sealing performance, installation ease, and resistance to "cold flow" (PTFE’s tendency to deform under pressure over time). 

 

 2.1 Full Envelope (Standard Type)  

Design: The PTFE sheet fully wraps the inner core, with the edges of the PTFE sealed (e.g., by heat welding or mechanical crimping) to prevent the core from leaking or being exposed to the medium. 

Advantages: Maximum protection of the core from corrosive fluids; suitable for full-face flange sealing. 

Limitation: Slightly lower compressibility than partial envelope types (due to full PTFE coverage). 

Use Case: Most industrial applications (chemical tanks, pipelines, pumps) where the medium is aggressive.

 

 2.2 Partial Envelope (Exposed Core Type)  

Design: The PTFE envelope covers only the sealing face (the area in contact with the flange) and the outer perimeter of the core; the inner bore (hole) of the gasket leaves the core partially exposed. 

Advantages: Higher compressibility (since less PTFE restricts the core’s deformation); easier to install in tight spaces. 

Limitation: The exposed core may be vulnerable to corrosion if the medium is highly aggressive. 

Use Case: Low-to-moderate corrosion environments (e.g., water treatment, HVAC systems) where compressibility is prioritized.

 

 2.3 Reinforced Envelope (Anti-Creep Type)  

Design: The PTFE envelope is reinforced with a thin layer of inert material (e.g., glass fiber, carbon fiber, or metal mesh) embedded in the PTFE matrix. 

Advantages: Significantly reduces PTFE cold flow and creep; maintains sealing integrity under long-term pressure or temperature cycles. 

Limitation: Higher cost than standard PTFE envelopes. 

Use Case: High-pressure/high-temperature applications (e.g., steam turbines, chemical reactors) where creep resistance is critical.

 

 

 3. Classification by Flange Type & Shape  

PTFE envelope gaskets are customized to match common flange designs, ensuring proper fit and sealing. 

Gasket Shape	Matching Flange Type	Key Features
Full-Face Gaskets	Full-face flanges (flanges with bolt holes covering the entire gasket area).	- Large surface area for sealing.
		- Requires alignment with all bolt holes.
Ring-Type Gaskets	Raised-face (RF) flanges or flat-face (FF) flanges (seal only the raised face).	- Smaller than full-face gaskets; lighter and easier to handle.
		- Reduces material cost.
Spiral-Wound Envelope Gaskets (Hybrid)	High-pressure flanges (e.g., ANSI Class 300+).	- Combines a PTFE envelope with a spiral-wound core (metal strip + filler).
		- Ultra-high pressure/temperature resistance (up to 1500 psi, 600°C).
Custom Shapes	Irregular flanges (e.g., oval, rectangular, or special industrial equipment).	- Tailored to unique flange dimensions.
		- Common in custom machinery (pharmaceutical reactors, semiconductor tools).

 

 4. Specialized PTFE Envelope Gaskets  

These are engineered for niche industries with strict requirements (e.g., food safety, ultra-purity, or extreme environments). 

 

 4.1 FDA-Compliant Gaskets 

Design: Uses food-grade PTFE (e.g., PTFE meets FDA 21 CFR Part 177.1550) and inner cores (EPDM, Silicone) certified for food contact. 

Use Case: Food & beverage processing (dairy, brewing), pharmaceutical manufacturing (drug synthesis), and cosmetics production.

 

 4.2 High-Purity (Ultra-Clean) Gaskets  

Design: Made with virgin PTFE (no additives) and a core of high-purity graphite or PTFE foam. The envelope is polished to minimize particle shedding. 

Use Case: Semiconductor manufacturing (ultra-pure water systems), laboratory equipment, and biotech (cell culture reactors).

 

 4.3 Low-Temperature Gaskets  

Design: Inner core of low-temperature-resistant materials (e.g., silicone rubber, expanded PTFE) to maintain flexibility at -200°C to -50°C. 

Use Case: Cryogenic applications (LNG storage, liquid nitrogen pipelines).

 

 

 Summary of Key Selection Factors  

To choose the right PTFE envelope gasket, consider: 

1. Medium Properties: Corrosiveness (dictates PTFE grade and core material). 

2. Operating Conditions: Temperature (graphite core for high temp; silicone for low temp) and pressure (reinforced envelope for high pressure). 

3. Flange Type: Full-face vs. ring-type, standard vs. custom shape. 

4. Industry Standards: FDA, ASME, or ISO compliance (for regulated sectors like food/pharma). 

 

By aligning these factors with the classifications above, you can ensure optimal sealing performance and long service life.