ISO standards to determine the flammability of Rayon fibers

Determining the flammability of rayon (a regenerated cellulose fiber) involves several ISO standards that measure different aspects of combustion, such as ease of ignition, flame spread rate, and heat release.

Rayon behaves similarly to cotton, as it ignites easily and burns rapidly with a yellow flame, leaving a light, feathery gray ash. Because it doesn't melt (unlike polyester), it is particularly susceptible to rapid ‘surface flash’ in certain fabric constructions.

Core flammability standards

1. ISO 6940: Ease of ignition

This standard determines how long a small flame must be applied to a vertical textile specimen to cause sustained combustion.

• Method: A small gas flame is applied to the surface or bottom edge of the fabric for specific time intervals (e.g., 1s, 5s, 10s).
• Result: It records the minimum ignition time required. This is the first line of defense in safety testing.

2. ISO 6941: Flame spread properties

Once ignited, how fast does the fire travel? ISO 6941 measures the time taken for a flame to pass between marker threads at specific distances.

• Method: Large vertical specimens are ignited at the bottom.
• Result: It calculates the vertical flame spread rate. For rayon, this rate is typically high unless the fiber has been treated with Flame Retardant (FR) chemicals.

3. ISO 15025: Protection against flame

Commonly used for personal protective equipment (PPE), this standard tests the fabric's reaction to brief contact with a small igniting flame.

• Procedure A (surface ignition): Flame is applied to the face of the fabric.
• Procedure B (edge ignition): Flame is applied to the bottom edge.
• Criteria: It looks for hole formation, flaming debris, and afterglow time

Specialized & comparative standards

ISO 12952:  This standard tests how rayon sheets react to a smoldering cigarette or small open flame.

ISO 4589-2: This determines the minimum oxygen concentration needed to support combustion.

ISO 1182: This standard is used for building materials; rayon typically fails this as it is inherently combustible.

Technical factors for rayon

When testing rayon specifically, three variables significantly alter the test results regardless of the ISO standard used:

1. Fabric weight: Lightweight rayon (like challis) burns much faster than heavy rayon upholstery due to the higher oxygen-to-fiber surface ratio.
2. Surface finish: Napped or brushed rayon surfaces can exhibit ‘surface flash,’ where the flame travels across the fuzz of the fabric almost instantaneously.
3. Moisture regain: Rayon is highly absorbent (11-13 per cent regain). Bone-dry rayon in a lab setting will burn significantly faster than rayon in a humid environment.