ISO 21915-2:2020

Overview

ISO 21915-2:2020 specifies a laboratory method for quantitative blend analysis of cupro and lyocell fibres using a re-dyeing procedure followed by light microscopy (quantitative microscopical analysis). The method decolours dyed samples, applies a controlled pre-treatment and re-dyeing (using defined stain solutions), and then distinguishes cupro and lyocell by their different dye uptake and colour shade on longitudinal fibre views. Results are reported as mass percentages of cupro and lyocell.

Key topics and requirements

• Scope and limitations: Applicable only to cupro and lyocell, and their blends. If other fibres are present they must first be identified (ISO/TR 11827) and removed (ISO 1833 series). Not suitable when the fibre surface has been chemically or physically damaged.
• Principal steps: decolouration, alkali pre-treatment, re-dyeing at controlled temperature (40 °C ±2 °C), microscopic preparation and counting.
• Reagents and apparatus: includes sodium hydrosulphite for decolouring, 1.8% sodium hydroxide and ~2% HCl for pre-treatment/neutralization, and stain solutions (e.g., C.I. Direct Blue 71 or 78). Equipment: water bath, microtome (cut fibres ~0.4–0.8 mm, then ~0.6 mm ±0.2 mm for counting), and light microscope per ISO 20705.
• Measurement protocol: count at least 1 000 fibres by colour shade to determine number fractions; measure diameters for at least 100 fibres of each identified type; calculate mass percentages using the conventional fibre density of 1.51 g·cm−3 for both cupro and lyocell (ISO 20705).
• Accuracy and validation: interlaboratory study data are provided (Annex C) demonstrating reproducibility; the reported coefficient of determination in trials was R = 0.9994.

Applications and users

• Textile laboratories and quality control teams needing blend quantification for fibre content labeling and regulatory compliance.
• Manufacturers and suppliers of regenerated cellulose textiles seeking to verify composition of cupro/lyocell blends.
• Forensic and inspection labs performing fibre composition analysis where IR or chemical methods cannot reliably distinguish cupro from lyocell.
• Practical benefits: cost-effective microscopy-based quantification when SEM/IR (ISO 21915‑1) or chemical dissolution (ISO 1833) are impractical.

Related standards

• ISO 21915-1 - identification using SEM and spectral analysis
• ISO 21915-3 - alternative composition analysis methods (see ISO listing)
• ISO 20705 - general principles for quantitative microscopical analysis
• ISO/TR 11827 - qualitative identification of similar regenerated cellulose fibres
• ISO 1833 (relevant parts) - removal/quantitative chemical analysis of other fibres
• ISO 3696 - water quality for analytical use

This standard is essential for accurate, reproducible blend quantification using light microscopy and for ensuring correct fibre content declarations in textile products containing cupro and lyocell.