Determining the shrinkage rate of Lyocell fibers with ASTM D2102

While ASTM D2102 is the standard test method for shrinkage of individual textile fibers in general, applying it to Lyocell requires specific technical considerations. Lyocell is a regenerated cellulose fiber (like viscose) but produced via a solvent-spinning process, giving it a high crystalline structure that reacts uniquely to thermal and aqueous stress.

The methodology for lyocell

To measure the linear shrinkage of Lyocell fibers, the lab follows the ‘Individual Fiber’ protocol to observe dimensional changes under specific conditions—usually boiling water or hot air.

• Initial length measurement (L0​): A single Lyocell fiber is secured in a measuring device under a specific mounting tension. For Lyocell, this tension is strictly controlled (typically 1±0.02 cN/tex) to remove the crimp without inducing elastic deformation.
• The treatment phase: The fiber is subjected to the shrinking medium. In the ‘Boiling Water Method,’ the fiber is immersed for 15–30 minutes. In the ‘Hot Air Method,’ it is placed in an oven, often at 150∘C or 180∘C.
• Equilibrium and re-measurement (Lf​): Lyocell is highly hygroscopic. After treatment, the fiber must be conditioned in a standard atmosphere (21±1∘C and 65±2% RH) as per ASTM D1776 before the final length is recorded.

Critical characteristics of Lyocell in ASTM D2102

Lyocell behaves differently than other man-made cellulosic fibers during this test:

• Low thermal shrinkage: Compared to synthetic fibers like polyester, Lyocell has high thermal stability. Its shrinkage in dry hot air is generally very low (often <1 per cent).
• Fibrillation sensitivity: During the aqueous (boiling water) shrinkage test, Lyocell can undergo ‘fibrillation,’ where the outer skin of the fiber peels into micro-fibrils. While ASTM D2102 measures length, the mechanical agitation involved in the treatment can alter the fiber surface, which may slightly affect the final length measurement.
• Wet swelling: Lyocell has a high degree of longitudinal and lateral swelling. ASTM D2102 focuses on the permanent linear change after drying, but the wet-to-dry cycle is what triggers the relaxation shrinkage measured by this standard.

Why this measurement matters

1. Fabric stability: Since Lyocell is frequently used in high-end apparel and bedding, knowing the fiber shrinkage helps engineers predict the Dimensional Stability of the finished fabric.
2. Blend calibration: Lyocell is often blended with synthetic fibers. If the Lyocell and the synthetic partner (like Polyester) have wildly different shrinkage rates under ASTM D2102, the final garment may develop a "puckered" or uneven texture after its first wash.
3. Process Control: It allows fiber producers to ensure that the heat-setting and drying phases of the Lyocell production line are consistent across batches.

Technical limitations

Because Lyocell fibers are extremely fine and strong, manual handling under ASTM D2102 is difficult. Many labs now use Vibroscope methods to determine the linear density (ASTM D1577) alongside an automated shrinkage tester to ensure that the mounting tension is applied with precision, reducing human error in the L0​ and Lf​ calculations.