ASTM C1166 Flame Propagation
ASTM C1166: Flame Propagation of Elastomeric Gaskets and Accessories
A curtain wall is mainly made of glass, aluminum, and sealed joints, with rubber gaskets around the panels. These gaskets run continuously around each panel, from floor to floor. This makes them one of the few continuous combustible paths in an otherwise non-combustible façade. ASTM C1166 is used to check whether a gasket allows flame to spread along this path or self-extinguishes.
MatX Lab performs flame propagation testing on dense and cellular elastomeric gaskets, glazing seals, and accessory profiles.
What the Method Assesses
The test determines the flame propagation behavior of dense and cellular elastomeric gaskets and accessories: how far the flame travels along a specimen after an ignition source is applied, and whether burning continues once that source is removed.
Both material forms are in scope. Dense extruded profiles and cellular sponge sections behave differently under flame, since the cellular form has far more surface area and much lower thermal mass, and the method covers both.
The result is assessed against criteria set by whatever specification or code invokes the test, rather than by the method itself. Building codes, curtain wall specifications, and manufacturers’ own qualification requirements each set limits on propagation distance and after-flame, and those limits vary.
The Test Arrangement
| Element | Arrangement |
| Specimen form | Dense or cellular elastomeric profile, as supplied |
| Orientation | Vertical so that flame can propagate upward along the specimen |
| Ignition source | Defined gas burner flame applied to the specimen for a set period |
| Measured quantities | Distance of flame propagation, afterflame duration, dripping behavior |
| Replicates | Multiple specimens per material, per the invoking specification |
| Conditioning | Defined temperature and humidity before test |
Vertical orientation is the point. Flame climbs readily, so a vertical test represents the worst realistic case for a gasket running up a mullion. A material that self-extinguishes in that configuration will generally do so in easier ones, which is why we test the harder case.
Why Gasket Geometry Complicates the Result
A gasket is not a flat coupon. It is an extruded profile with fins, bulbs, hollow sections, and different wall thicknesses, all of which affect how it burns.
Thin fins catch fire and burn away quickly because they have little thermal mass. Hollow bulbs can act like chimneys, drawing air through the section and keeping the flame going when it might not on a solid profile. Thick sections are harder to ignite, but once alight, they hold heat.
The consequence is that flame propagation on a gasket is a property of the profile as much as of the compound. Two products made from the same compound in different profiles can give different results, and results from one profile do not transfer to another.
Testing the actual production profile is therefore the only meaningful approach, and a flat slab of the compound will mislead.
Compound Factors
Elastomer type sets the baseline. EPDM, the most common curtain wall gasket material, burns readily without flame retardant additives. Silicone and fluor elastomer behave better inherently. Neoprene sits between them.
Flame retardant loading usually determines the result. It can also change between qualification and production when cost pressures come into play. A compound reformulated to reduce cost can fail a test that its predecessor passed, even if it looks the same.
Plasticizer content affects flame performance but also helps with low-temperature flexibility. Every gasket compounder must manage this trade-off.
Carbon black loading, filler type, and cure system can also affect the result. These differences may seem small, but they can matter when a compound is close to the pass limit.
Aging Changes the Answer
A gasket that passes the test when supplied may not pass after years in a façade.
Plasticizer can migrate out over time, additives can bloom on the surface, and UV exposure can degrade the polymer on the exposed face. As the compound changes, the flame retardant system may also become less effective.
Where a program addresses in-service performance rather than as-supplied performance, testing after accelerated aging gives the more useful answer. That is a separate exposure program run before the flame test. Plan it as part of the same job.
Specimen Requirements
- Supply the production profile in continuous lengths, with enough material for the required specimens and a few spares. A profile cut from a corner, splice, or molded corner piece will not represent the extruded run.
- Identify the compound, its flame-retardant system (if known), and the production lot. Compound changes are the most common reason a previously passing product fails, and lot traceability is what makes that diagnosis possible.
- State the gasket’s orientation in service when the profile is asymmetric, since the exposed face affects the result.
- Say whether the question is as-supplied or aged performance, because the two need different programs.
ASTM C1166 FAQs
Does the method set its own pass criteria?
No. It sets out how to run the test and what to measure. The building code, curtain wall specification, or qualification document that calls for the test sets the limits for flame propagation and after-flame.
Does this cover cellular sponge gaskets as well as dense ones?
Yes, both forms are within scope. They behave differently under flame, so we test and report them separately rather than assuming they're equivalent.
Why did my reformulated compound fail?
Flame retardant loading is usually the first thing to check, followed by plasticizer content. A compound reformulated to cut costs or improve low-temperature flexibility can lose flame performance, even without any visible or noticeable change.