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Sulphuric Acid on the WebTM Technical Manual DKL Engineering, Inc.

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Sulphuric Acid on the Web

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Handbook of Sulphuric Acid Manufacturing
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Sulphuric Acid Plant Specifications
 

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Materials of Construction - Fluoro Rubber
November 15, 2003

Description
Viton
Naming
Associated Links

Description

Common Name FKM, FPM, FFKM
Chemical Name Fluoro Rubber
Trade Names

VITON® (registered trademark of the DuPont Dow Elastomers)

FLUOREL® (registered trademark of the 3M Company)

General

FKM is a M Class Rubber

  • Contains a saturated chain of the polymethylene type
  • Fluoro rubber of the polymethylene type having substituent fluoro and perfluoroalkyl groups on the polymer chain

Fluorocarbon elastomers are inherently compatible with a broad spectrum of chemicals.  Because of this extensive chemical compatibility, which spans considerable concentration and temperature ranges, fluorocarbons have gained wide acceptance as a material of construction for butterfly valve o-rings and seats.

Excellent resistance to mineral acids, salts solutions, oils and many aliphatic and aromatic hydrocarbons.

Attacked by caustic (sodium hydroxide) and low molecular weight organics.

There are several different grades of material available.  Care should be taken in selecting the right grade for the application.

Application

General

  • Not suitable for steam service
  • Gaskets
  • O-rings
  • Bellows

Sulphuric Acid Plants

  • Seals and gaskets in the gas cleaning system handling weak sulphuric acid. 
  • Plate heat exchanger gaskets

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Viton

DuPont produces several different grades of Viton.   There are three major families of standard Viton Fluoroelastomers: A, B and F.  The designation refer to their relative resistance to attacks by fluids and chemicals.  The differences in fluid resistance that exist are the result of different levels of fluorine in the polymer which is determined by the types and relative amounts of monomers that comprise the polymer.

Type Chemical Composition Fluorine Content Description
A -CH2-CF2-CF3CFCF2- 66% The most common fluoroelastomer composition used in O-rings.  Comprised of vinylidene fluoride (VF2) and hexafluoropropylene (HFP).
B -CH2-CF2-CF3CFCF2-CF2-CF2- 67%

Improved acid and alcohol resistance over A type.  Comprised of vinylidene fluoride (VF2), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE).

GF or F   70%

Improved fuel and alcohol resistance.  Comprised of vinylidene fluoride (VF2), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE).

GLT   65% Often referred to as “Good for Low Temperature”.  Good performance down to –40oC.  Introduced in 1976, this type was the first commercial fluoroelastomer to use the fluorinated vinyl ether monomer.
GFLT   68% Often referred to as “Good for Fluids and Low Temperature”.  Good performance down to –35oC.
AFLAS -CF2-CF2-CH3CHCH2- 55 to 60% Excellent resistance to bases, amines and solvents.
Super FKM -CF2-CF2-CF3OCFCF2-CH2-CH2- 72 to 74% Resistance to fluids is increased with the higher fluorine content
Perfluoroelastomers FFKM -CF2-CF2-CFOCFCF2- 75% Family of fully-fluorinated fluorocarbon elastomers

The chemical make-up of the polymer contributes to the differences in physical properties and chemical resistance between the materials.  As well, the compounding method, particularly the cross-linking or curing system used imparts certain physical characteristics to the material.

GLT types are peroxide cured which provides fast cure rates and imparts excellent physical properties.  As such, only the G type of fluoroelastomers are suitable for service in sulphuric acid.

Viton B is the original grade of Viton used in plate heat exchangers when they were first introduced into sulphuric acid plants.

Viton G is Alfa Laval's recommended material for hot concentrated sulphuric acid.  The material Vton G contains substantially more fluorine than other grades which improves its resistance to hot concentrated sulphuric acid.  The gasket material is also peroxide cured which provides the lowest temperature relaxation and the lowest acid absorption.

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Naming

With the wide range of materials available a naming system was created to identify the material more accurately.

Viton A-401C

1st Character (letter)

Represents the type of viton: A, B or F

A “G” prefix, in addition to a type prefix, indicates that the polymer can be crosslinked with the peroxide cure system.

An “L” designation indicates that the polymer provides slightly improved low temperature flexibility.

An “LT” designation indicates a more significant improvement in low temperature performance criteria.

2nd Character (Number)

Represents nominal Mooney Viscosity of the product – ML 1 + 10 at 121oC (250oF)

3rd Character (Number)

Represents the relative level of curative in a precompound on a scale of 10 to 1 (10 is represented by 0)

0 – High curative level (for optimum compression set)

9 to 2 – Intermediate, decreasing levels of curative (increased elongation at break, tear resistance)

1 – Low curative level (for optimum tear, flex resistance)

4th Character (Number)

Represents a slightly different version of a particular compound (e.g. Viton A-402C contains process aid, A-401C does not).

5th Character (Letter)

No letter suffix indicates that the product is a gum polymer only and containes no curatives (may contain process aid).

“C” indicates that the product is a precompound, containing accelerator and curative.

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