ISO standards to determine the solubility of lyocell fibers

Determining the solubility of lyocell fibers is essential for quantitative chemical analysis, especially when identifying the composition of blended textiles. The International Organization for Standardization (ISO) uses specific solvent mixtures - primarily formic acid and zinc chloride - to dissolve lyocell while leaving other fibers (like cotton or flax) intact. The following ISO standards define the protocols for these solubility tests:

Primary solubility standards (ISO 1833 Series)

The ISO 1833 series focuses on ‘quantitative chemical analysis.’ Specific parts address the use of reagents that dissolve lyocell.

• ISO 1833-6:2018
• Scope: Binary mixtures of lyocell (or viscose, cupro, and modal) with cotton.
• Solvent: Formic acid and zinc chloride.
• Method: A preliminary test is conducted to ensure the lyocell is fully soluble in the reagent. The lyocell is dissolved, and the remaining cotton residue is weighed to determine the percentage of each fiber.
• ISO 1833-22:2020
• Scope: Binary mixtures of lyocell (or viscose, cupro, and modal) with flax fibers.
• Solvent: Formic acid and zinc chloride.
• Method: After pre-treatment to remove non-cellulosic matter from flax, the lyocell is dissolved at 70°C. The remaining flax is then washed, dried, and weighed.

The Standard Reagent: Formic Acid / Zinc Chloride

The most common standard solvent used for dissolving lyocell in an ISO-compliant laboratory is a specifically prepared mixture.

Reagent preparation (per ISO 1833-6):

• Anhydrous Zinc Chloride: 20 g (minimum assay > 98%).
• Anhydrous Formic Acid: 68 g.
• Distilled Water: Made up to 100 g total.

Dissolution conditions:

• Standard temperature: 40°C for 2.5 hours for common rayon.
• Lyocell specific: For high-tenacity or treated lyocell, the standard recommends increasing the temperature to 70°C for 20 minutes if dissolution at 40°C is incomplete.

Alternative quantitative standards

In cases where lyocell is blended with synthetic fibers, different acids are used to determine solubility by ‘difference.’

Advanced identification standards

To differentiate lyocell from other chemically identical cellulose fibers (like viscose) during solubility tests, ISO utilizes microscopy.

• ISO 21915-2:2020: Specifies the light microscopy method for the qualitative and quantitative analysis of lyocell and cupro blends.

ISO 21915-1:2020: Uses Scanning Electron Microscopy (SEM) and spectral analysis for identification before chemical dissolution