Last reviewed: May 2026 - MLC editorial team - UK trade engineering perspective
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Kalrez (DuPont) and Chemraz (Greene Tweed) are the two FFKM perfluoroelastomer brands every UK process-industry engineer specifies. Both materials handle temperatures exceeding 260°C and resist aggressive chemicals where alternatives like Viton or nitrile would degrade within hours. The choice between them often comes down to compound family, availability in the UK market, and adherence to existing plant standards. For many applications, these brands are interchangeable, but nuances such as specific chemical resistance profiles and supplier relationships can influence procurement decisions. Engineers and maintenance staff must consider factors like cost, delivery times, and technical support when selecting between Kalrez and Chemraz for their O-rings and seals.
What they have in common
- Both are FFKM (perfluoroelastomer) chemistry - fully fluorinated backbone, ~270-330°C peak temperature, broad chemical compatibility.
- Both certified to FDA 21 CFR 177.2600 for food/pharmaceutical contact (specific compounds only).
- Both publish compound-by-compound chemical compatibility charts on their websites.
Header comparison
Where they differ
Compound family range
When comparing Kalrez and Chemraz along the compound family range axis, Kalrez offers approximately 20 standard compounds tailored to specific applications such as steam, semiconductor, and oil and gas industries. In contrast, Chemraz provides around 25 standard compounds with a slightly broader portfolio in semiconductors.
| Kalrez | ~20 standard compounds, organised by application (steam / semiconductor / O&G) |
|---|
| Chemraz | ~25 standard compounds, slightly broader semiconductor portfolio |
|---|
Semiconductor industry preference
In the UK semiconductor industry, Kalrez 7075 is frequently specified for lithography equipment due to its robust performance in demanding environments. Conversely, Chemraz 526 excels in plasma chamber and etch applications, offering superior resistance to aggressive chemicals.
| Kalrez | 7075 widely specified for UK semiconductor / lithography equipment |
|---|
| Chemraz | 526 strong in plasma chamber / etch applications |
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Chemical / pharma preference
Kalrez 4079 is a versatile choice for pharmaceutical reactor seals, widely accepted in the industry due to its generalist approach. In contrast, Chemraz 555 and 615 offer broader explicit FDA 21 CFR documentation, providing additional regulatory compliance support.
| Kalrez | 4079 the generalist; widely accepted in pharma reactor seals |
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| Chemraz | 555 + 615 cover similar duty; broader explicit FDA 21 CFR documentation |
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Steam / pure water peak
On the steam-in-place cycling front, Kalrez 6375 stands out with its purpose-built design for peak steam performance, whereas Chemraz offers a comparable NPT range at a slightly lower cost, making it a viable alternative despite minor differences in application specifics.
| Kalrez | 6375 - purpose-built for steam-in-place cycling |
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| Chemraz | NPT range - comparable steam resistance, slightly cheaper |
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Hardness range (Shore A)
When comparing Kalrez and Chemraz by their hardness range on the Shore A scale, Kalrez operates between 60 to 90 Shore A, whereas Chemraz spans a slightly wider spectrum from 55 to 95 Shore A, offering greater flexibility in application requirements.
| Kalrez | 60-90 Shore A across compounds |
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| Chemraz | 55-95 Shore A - slightly broader hardness range |
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UK trade availability
In the UK trade market for elastomeric sealing materials, Kalrez by DuPont is distributed through James Walker, AESSEAL, and AcumenSeals, offering broad coverage across various industrial sectors. Chemraz from Greene Tweed is available via its UK service centre, alongside AcumenSeals and James Walker, providing a similar reach but with fewer direct distributors.
| Kalrez | DuPont UK distribution + James Walker + AESSEAL + AcumenSeals. |
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| Chemraz | Greene Tweed UK service centre + AcumenSeals + James Walker. |
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Trade price (BS 1806 -114)
At trade prices, Kalrez O-rings cost £35 to £65 each, while Chemraz rings are typically 10-15% cheaper at £30 to £55 per unit. For the BS 1806-114 specification, this price difference can impact material selection in workshops.
| Kalrez | £35-65 per O-ring (4079) at trade prices |
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| Chemraz | £30-55 per O-ring (615) at trade prices - typically 10-15% cheaper |
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Decision framework: when to pick each
| Scenario | Recommendation |
|---|
| Semiconductor plasma etch chamber | Chemraz 526 or Kalrez 7075 - both qualified by major semiconductor OEMs. Specify whichever your fab tool was built around. |
| Pharmaceutical reactor with steam-in-place | Kalrez 6375 - designed for SIP cycling; longer rated life than general-duty 4079. |
| Oil + gas downhole (high H2S) | Kalrez 8475 - explicit H2S rating to 200°C, NACE compliant. |
| General chemical / corrosive duty | Either - Kalrez 4079 vs Chemraz 615 are functionally equivalent. Choose by price + lead time. |
| Cost-sensitive FFKM substitute | Markez or Simriz - both also FFKM, typically 30-50% cheaper than Kalrez or Chemraz, slightly narrower temperature window. |
| Replacement on existing seal | Match the existing compound - cross-substitution between FFKM brands works for general duty but voids OEM warranty in semiconductor and pharma. |
Real trade scenarios
- Plasma etch chamber (Lam Research / Applied Materials): typically Chemraz 526 or Kalrez 7075 - verify the OEM's recommended part number.
- Pharma reactor SIP cycling: Kalrez 6375 outperforms general-duty FFKM by 2-3x cycle life.
- Hydrochloric acid pump: either Kalrez 4079 or Chemraz 615 - both compatible to 200°C.
- Steam injection well: Kalrez 8475 - H2S + high-temperature rated.
Related reference on MLC
Tools + parts for these comparisons (Amazon UK)
Frequently asked questions
Is Kalrez or Chemraz better?
Class-for-class equivalent. Both are FFKM, both rated to 260-325°C depending on compound, both broadly chemically inert. The choice usually comes down to OEM spec, UK availability, and existing plant standardisation.
Why are FFKM O-rings so expensive?
Manufacturing process for fully fluorinated elastomers is complex (fluorination of the carbon backbone). Annual global production is small compared to Viton or nitrile. Trade O-ring (BS 1806 -114) is £30-65 vs £1-3 for nitrile.
Can I substitute a different FFKM brand?
For general duty - yes, dimensionally and chemically equivalent. For OEM-spec applications (semiconductor tools, pharma cGMP, hazardous-area certified pumps) - only if the OEM has cross-approved the substitute. Always check the equipment manual.
What's the difference between Kalrez 4079 and 6375?
4079 is the general-purpose compound - broad chemistry, 260°C peak. 6375 is specifically formulated for steam cycling - better compression-set recovery after repeated SIP cycles, slightly narrower chemistry.
Where do I buy Kalrez or Chemraz O-rings in the UK?
Kalrez: AESSEAL (Rotherham), James Walker, AcumenSeals, DuPont UK direct. Chemraz: Greene Tweed UK, AcumenSeals, James Walker. AcumenSeals (Sunderland) stocks both and ships within 24h on standard BS 1806 / AS568 sizes.
Are there cheaper FFKM alternatives?
Markez (Marco Rubber), Simriz (Freudenberg), Isolast (Trelleborg), Perlast (PPE) - all FFKM, 20-40% cheaper than Kalrez or Chemraz. Slightly narrower temperature ranges and chemistry compatibility. Acceptable for general duty; not always qualified for semiconductor or pharma.
Does temperature rating differ by compound or by brand?
By compound, not by brand. Kalrez 7075 (327°C peak) and Chemraz 526 (327°C peak) are equivalent. Kalrez 4079 (260°C) and Chemraz 615 (260°C) are equivalent. The brand alone doesn't tell you the temperature limit.
How do I identify a Kalrez or Chemraz O-ring?
Visually: both are black, dense FFKM elastomer. Brand identification requires the compound code on the packaging - there's no visual way to tell them apart once installed. Always store with original packaging.
Are FFKM compounds RoHS compliant?
Yes - all standard Kalrez and Chemraz compounds are RoHS compliant (no lead, mercury, cadmium, hexavalent chromium). REACH compliance also published per compound.
How long do FFKM O-rings last in service?
Typically 12-48 months in continuous service depending on temperature, chemistry, and compression. At rated peak temperature, lifetime drops to 3-12 months. Below 200°C in general chemical service, FFKM regularly achieves 5+ years.
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Frequently Asked Questions
Is Kalrez or Chemraz better?
Class-for-class equivalent. Both are FFKM, both rated to 260-325°C depending on compound, both broadly chemically inert. The choice usually comes down to OEM spec, UK availability, and existing plant standardisation.
Why are FFKM O-rings so expensive?
Manufacturing process for fully fluorinated elastomers is complex (fluorination of the carbon backbone). Annual global production is small compared to Viton or nitrile. Trade O-ring (BS 1806 -114) is £30-65 vs £1-3 for nitrile.
Can I substitute a different FFKM brand?
For general duty — yes, dimensionally and chemically equivalent. For OEM-spec applications (semiconductor tools, pharma cGMP, hazardous-area certified pumps) — only if the OEM has cross-approved the substitute. Always check the equipment manual.
What's the difference between Kalrez 4079 and 6375?
4079 is the general-purpose compound — broad chemistry, 260°C peak. 6375 is specifically formulated for steam cycling — better compression-set recovery after repeated SIP cycles, slightly narrower chemistry.
Where do I buy Kalrez or Chemraz O-rings in the UK?
Kalrez: AESSEAL (Rotherham), James Walker, AcumenSeals, DuPont UK direct. Chemraz: Greene Tweed UK, AcumenSeals, James Walker. AcumenSeals (Sunderland) stocks both and ships within 24h on standard BS 1806 / AS568 sizes.
Are there cheaper FFKM alternatives?
Markez (Marco Rubber), Simriz (Freudenberg), Isolast (Trelleborg), Perlast (PPE) — all FFKM, 20-40% cheaper than Kalrez or Chemraz. Slightly narrower temperature ranges and chemistry compatibility. Acceptable for general duty; not always qualified for semiconductor or pharma.
Does temperature rating differ by compound or by brand?
By compound, not by brand. Kalrez 7075 (327°C peak) and Chemraz 526 (327°C peak) are equivalent. Kalrez 4079 (260°C) and Chemraz 615 (260°C) are equivalent. The brand alone doesn't tell you the temperature limit.
How do I identify a Kalrez or Chemraz O-ring?
Visually: both are black, dense FFKM elastomer. Brand identification requires the compound code on the packaging — there's no visual way to tell them apart once installed. Always store with original packaging.
Are FFKM compounds RoHS compliant?
Yes — all standard Kalrez and Chemraz compounds are RoHS compliant (no lead, mercury, cadmium, hexavalent chromium). REACH compliance also published per compound.
How long do FFKM O-rings last in service?
Typically 12-48 months in continuous service depending on temperature, chemistry, and compression. At rated peak temperature, lifetime drops to 3-12 months. Below 200°C in general chemical service, FFKM regularly achieves 5+ years.