ASTM standards to determine the wet strength of lyocell fibers

Determining the wet strength of lyocell is a critical quality benchmark because, unlike traditional viscose, lyocell is known for its high tenacity in a wet state. While many testers rely on BISFA (The International Bureau for the Standardization of Man-Made Fibres) for specialized regenerated fibers, ASTM International provides the foundational mechanical protocols used in North American laboratories.

The primary standard for this measurement is ASTM D3822.

ASTM D3822: Standard test method for tensile properties of single textile fibers

This is the ‘go-to’ standard for measuring the breaking force and elongation of individual fibers, including lyocell, in both dry and wet conditions.

• The procedure: Individual fibers are mounted in clamps (grips) of a constant-rate-of-extension (CRE) tensile testing machine.
• Wet conditioning: For wet strength, the standard requires the fiber to be immersed in water containing a non-ionic wetting agent (to ensure complete saturation) before being tested.
• Significance: This test calculates the wet tenacity, usually expressed in centinewtons per tex (cN/tex) or grams per denier (g/d).

ASTM D5034 & D5035: Fabric strength, grab and strip tests

If the lyocell is already in fabric form, wet strength is assessed using these standards. This is common when testing how a lyocell garment will hold up during heavy industrial laundering or high-moisture processing.

• ASTM D5034 (Grab Test): Measures the effective strength of the fabric.
• ASTM D5035 (Strip Test): Measures the breaking force and elongation of a specific width of fabric.
• Wet testing protocol: Specimens are soaked in distilled water at room temperature until thoroughly wet. They must be tested immediately (within 2 minutes of removal from the water) to prevent drying, which would artificially inflate the strength results.

Comparison: Wet vs dry performance

A key reason for performing these ASTM tests is to calculate the Wet/Dry Strength Ratio.

Table: Fiber tenacity and strength retention

Lyocell's high retention (85 per cent) is what allows it to be processed in high-speed jet dyeing machines without the fiber damage often seen in standard viscose.

Technical requirements for accuracy

To comply with ASTM standards when testing lyocell's wet properties, laboratories must control for:

• Grip selection: Because Lyocell is smooth and cylindrical, ‘flat-faced’ or rubber-coated grips are used to prevent the fiber from slipping during the test.
• Rate of extension: Usually set to a speed where the fiber breaks within 20±3 seconds to ensure the results are comparable across different labs.
• Pre-tensioning: A small amount of ‘straightening’ force is applied to the fiber before the actual test begins to remove any crimp, ensuring the gauge length is accurate.

Related standard: ASTM D4848

While not a test method itself, ASTM D4848 (Standard Terminology Related to Force, Deformation and Related Properties of Textiles) is used alongside the tests above to ensure that technical reports use the correct definitions for terms like breaking tenacity and initial modulus specifically as they apply to wet-conditioned man-made fibers.