ISO standards to determine the flammability of viscose yarns

Determining the flammability of viscose yarns, a highly flammable cellulosic fiber relies on established ISO standards that focus on flame spread, ignition, and burning behavior. Because viscose has a low Limiting Oxygen Index (LOI), safety compliance is paramount, particularly for apparel and protective textiles.

ISO flammability standards

• ISO 15025 (Protective Clothing): This is the primary standard for evaluating the limited flame spread of vertically oriented materials. It uses a controlled small flame applied to a vertically suspended specimen. It is critical for assessing whether treated, flame-retardant (FR) viscose meets safety benchmarks for industrial and high-heat environments. Metrics measured include:
• Afterflame time: How long the yarn/fabric continues to burn after the source is removed.
• Afterglow time: The duration of glowing combustion.
• Char length: The vertical extent of damaged material.
• Hole formation/Flaming debris: Observations that indicate failure in protective applications.

• ISO 6940 & ISO 6941 (General Textiles): These are foundational standards for assessing the burning behavior of vertical textile specimens.
• ISO 6940: Determines the ease of ignition of a fabric or yarn when exposed to a defined flame.

• ISO 6941: Measures the rate of flame spread. It records the time taken for flames to travel between specific markers on a vertical sample. These standards provide a consistent basis for classifying textiles based on their reaction to small ignition sources.

• ISO 12952 (Bedding): If your viscose yarns are destined for bedding, this two-part standard applies:
• ISO 12952-1: Tests response to a smoldering ignition source (e.g., a cigarette).
• ISO 12952-2: Tests response to an open flame ignition source.

Important testing considerations for viscose

• Sample preparation: Because viscose is highly absorbent, test specimens must be conditioned in a controlled environment (typically 20±2°C and 65±5 per cent relative humidity per ISO 139) for at least 24 hours prior to testing. Moisture content significantly alters flammability results.

• FR vs standard viscose: Standard viscose is inherently flammable. If the end-use requires compliance (such as children's sleepwear or workwear), you are likely testing FR Viscose, fiber modified at the molecular level (e.g., phosphorus-based dope injection). These fibers are designed to meet the criteria in ISO 15025, whereas standard, untreated viscose will almost always fail to show limited flame spread characteristics.

• End-use specifics: Always check for region-specific requirements. For instance, European markets often use EN 14878 for children's sleepwear or EN 13773 for curtains/drapes, which reference the aforementioned ISO test methods as their core procedural framework.