Flame-Retardant Butyl Compound CN-FR (UL94 V-0): A Specification Guide

When a drawing calls for a "flame-retardant butyl," the specification almost always resolves to a single line: UL94 V-0. It is one of the most quoted and least understood ratings in materials procurement. UL94 is a vertical burning test for plastics and elastomers, and V-0 is its strictest vertical-bar classification. Understanding exactly what it measures tells you what a grade like Garmy's CN-FR guarantees — and, just as importantly, what it does not.

In the vertical burning test, a specimen bar is clamped vertically and a calibrated flame is applied to the lower end for ten seconds, removed, then applied a second time. The classification depends on how the specimen behaves after each flame removal:

1. Afterflame time — How long the specimen continues to burn once the igniting flame is withdrawn. V-0 demands the shortest self-extinguishing behaviour of the three vertical ratings
2. Afterglow — Whether the specimen keeps smouldering after visible flame stops. V-0 requires the glow to die out quickly
3. Dripping — Whether flaming particles fall and ignite cotton placed beneath the specimen. Under V-0, no flaming drips are permitted. This is the criterion that most often separates V-0 from V-1 and V-2
4. Burn-up to the clamp — No specimen may burn all the way up to the holding clamp

The hierarchy is straightforward: V-0 is stricter than V-1, which is stricter than V-2. V-2 permits flaming drips; V-1 forbids them but allows longer afterflame; V-0 is the tightest of the three on both counts.

• V-0 is a self-extinguishing rating, not a fireproof rating — The material still burns while a flame is on it. What V-0 certifies is that it stops burning quickly once the ignition source is gone and does not spread fire by dripping
• It is a material-level classification, not a system rating — A V-0 sealing bead inside a wall or an enclosure does not by itself confer any fire-resistance rating on the assembly. Assembly-level performance is governed by separate standards
• Thickness matters — UL94 results are reported at a specific specimen thickness. A grade qualified at one thickness is not automatically equivalent at a thinner section
• It is the common denominator across industries — Rail, EV, electrical enclosure, and electronics specifications all reference UL94 as a baseline screening criterion, which is why it is the rating stamped on CN-FR

For a sealing compound, this matters more than it first appears. A butyl bead is often the only continuous organic material running through an otherwise metallic joint. If that bead propagates flame or drips burning material, it turns a sealed joint into a fire path. A V-0 grade removes that failure mode by design.

Butyl rubber's polymer backbone is a saturated isobutylene-isoprene chain — chemically stable, exceptionally gas-tight, and highly weather-resistant, but, like all hydrocarbon elastomers, combustible. Flame retardancy is therefore not an inherent property of the base polymer; it is engineered in at the compounding stage. Garmy's CN-FR is a purpose-built formulation, not a standard grade with an additive dropped in at the end of the batch.

Flame-retardant systems in an elastomer generally act through three complementary mechanisms, and a well-designed compound uses more than one:

• Gas-phase interruption — The retardant releases species that interfere with the free-radical chain reactions sustaining the flame above the material surface, starving combustion of its propagating radicals
• Endothermic cooling and dilution — Mineral retardants decompose endothermically, absorbing heat from the burning zone and releasing inert vapour that dilutes the fuel-air mixture at the surface
• Condensed-phase char formation — A protective carbonaceous layer forms on the surface, insulating the bulk material from heat and blocking the outward diffusion of combustible volatiles. Char also physically resists the flow that produces flaming drips

The engineering challenge is that flame-retardant loading works against the properties buyers actually specify butyl for. Push the loading too far and you lose tack, elongation, and adhesion. The value of a dedicated FR grade lies in holding those properties while meeting V-0. Here is how CN-FR sits against Garmy's standard compound grades:

Grade	Primary Application	Specific Gravity	Peel Strength (N/cm)	Heat Resistance	Cold Resistance
CN-FR	Flame-retardant membrane (UL94 V-0)	1.45 ± 0.1	81.07	120°C	-40°C
HY-1	Self-adhesive waterproofing membrane	1.45 ± 0.1	81.07	120°C	-40°C
HY-2	Self-adhesive waterproofing membrane	1.65 ± 0.1	58.91	120°C	-40°C
CN-1	Self-adhesive waterproofing membrane	1.40 ± 0.1	62.45	120°C	-40°C
SD-1	Butyl waterproof tape	1.65 ± 0.1	42.82	110°C	-40°C
S-3	Butyl waterproof tape	1.65 ± 0.1	36.86	110°C	-40°C

The important reading of this table is the CN-FR row against HY-1. CN-FR carries the same 1.45 ± 0.1 specific gravity, the same 81.07 N/cm peel strength, and the same 120°C / -40°C service window as the highest-adhesion standard grade — while adding the UL94 V-0 classification. In other words, the flame-retardant grade is not a downgrade you accept for compliance. That is the whole point of formulating an FR grade rather than modifying a standard one.

If your program needs a UL94 V-0 sealing compound without sacrificing adhesion or temperature range, CN-FR is the grade to evaluate first.

Flame-retardant sealing is not a niche requirement. It is mandatory wherever a fire inside an enclosed or occupied space would be difficult to escape, expensive to contain, or catastrophic to a high-value asset. Four segments account for most CN-FR enquiries.

• Rail and rolling stock — Rail vehicle interiors are among the most fire-regulated environments in transport, and Garmy's butyl compound line is developed with reference to EN 45545, the European railway fire safety standard. Sealing and damping materials inside a carriage body sit directly in the occupant space, so a self-extinguishing, non-dripping grade is a baseline requirement rather than an upgrade
• EV battery packs and e-mobility — Pack enclosures need a moisture and dust seal that survives the thermal environment of a battery. Because thermal-runaway scenarios dominate EV safety engineering, procurement teams increasingly screen every organic material in the pack for flame behaviour. CN-FR's -40°C to +120°C range covers the sealing duty; the V-0 rating clears the screening gate
• Electrical enclosures and switchgear — Cable entries, gland plates, and door perimeters need a sealing compound that will not carry flame from an internal arc fault into the surrounding installation. This is a classic V-0 use case: the seal must stop burning the moment the ignition source stops
• Building envelope and interior construction — Flame-retardant self-adhesive membranes are used where a waterproofing layer runs through a fire-rated zone, in service risers, and in commercial interiors where fire-load limits apply to concealed materials. CN-FR is formulated specifically as a flame-retardant membrane compound for exactly this duty

A practical specification workflow for a CN-FR programme looks like this:

1. Confirm the governing standard first. UL94 V-0 is a screening rating. Rail work will additionally reference EN 45545; electrical work will reference its own product standards. Establish which document actually governs acceptance before choosing a grade
2. State the specimen thickness. Because UL94 results are thickness-dependent, agree the qualified thickness with your supplier rather than assuming the rating transfers to any section
3. Define the mechanical duty alongside the fire duty. Peel strength, service temperature, and substrate matter as much as the fire rating. CN-FR's 81.07 N/cm peel and -40°C to +120°C range should be checked against your joint, not assumed
4. Request lot documentation. Garmy produces under IATF 16949, ISO 9001, and ISO 14001 quality and environmental systems and supplies a batch Certificate of Analysis, so FR grade traceability can be evidenced through the audit chain
5. Qualify on a representative sample. Test on your actual substrate and joint geometry — a flame-retardant compound behaves differently on painted steel, bare aluminium, and polymer housings

Garmy has manufactured butyl compounds since 1999 and can supply CN-FR in 20 kg PE bags or 1,000 kg pallets with batch CoA for your qualification programme.

Q: Does UL94 V-0 mean CN-FR is fireproof?

A: No. V-0 is a self-extinguishing classification, not a fireproof or fire-resistance rating. A V-0 material will still burn while a flame is applied to it. What the rating certifies is that it stops burning quickly once the ignition source is removed, does not glow persistently, and does not release flaming drips that could spread fire to material below. For assembly-level fire resistance you need a system-level test, not a material classification.

Q: What is the difference between V-0, V-1, and V-2?

A: All three are vertical burning classifications, and V-0 is the strictest. V-2 allows flaming drips that ignite the cotton indicator beneath the specimen. V-1 does not allow flaming drips but permits a longer afterflame time than V-0. V-0 is the tightest on both afterflame duration and dripping. For sealing compounds in occupied or enclosed spaces, V-0 is the level normally specified.

Q: Does CN-FR give up performance compared with the standard grades?

A: On the published specifications, no. CN-FR carries a specific gravity of 1.45 ± 0.1, a peel strength of 81.07 N/cm, heat resistance to 120°C, and cold resistance to -40°C — identical to HY-1, the highest-peel standard grade in the range, while adding the UL94 V-0 classification. That parity is the reason for formulating a dedicated flame-retardant grade rather than modifying a standard compound.

Q: Is CN-FR suitable for rail applications under EN 45545?

A: Garmy's butyl compound line is developed with reference to EN 45545, the European railway fire safety standard, and CN-FR is the flame-retardant grade in that line. EN 45545 acceptance, however, depends on the hazard level, the component category, and the specific test set applicable to your part — so the correct route is to share your governing requirement with our technical team and qualify CN-FR against it rather than treating the UL94 rating as automatic compliance.

Q: Can Garmy supply CN-FR with documentation for an OEM qualification?

A: Yes. Garmy has produced butyl compounds since 1999 at a 4,200 m² owned plant in Eumseong, Chungbuk, Korea, with an annual output of over 3,400 tonnes, under IATF 16949, ISO 9001, and ISO 14001 certification and holding the Hyundai SQ mark. CN-FR ships in 20 kg PE bags or 1,000 kg pallets with a batch Certificate of Analysis, and we currently export to six countries.

Q: Can the flame-retardant formulation be customised?

A: Yes. Custom formulation is available for specific OEM requirements across the butyl compound range, including colour (black, grey, or custom) and viscosity grade. If your programme has a defined fire standard, substrate, and service temperature, our technical team can advise whether CN-FR as specified is the right starting point or whether a tailored variant is warranted.

Talk to a Garmy specialist about the right flame-retardant butyl grade for your programme.