Boiling Water Shrinkage (BWS) is the percentage reduction in the length of a polyester filament after immersion in boiling water for a fixed period. It is calculated as:

BWS (%) = (Original Length – Length After Boiling) / Original Length × 100%

This indicator reflects the thermal setting level, molecular orientation, and dimensional stability of polyester fibers. It directly affects fabric performance, finishing quality, and final product stability.

1. How Boiling Water Shrinkage Is Tested

Typical test conditions simulate dyeing and washing processes:

  • Temperature: 100°C

  • Duration: approx. 30 minutes

  • Sample: fixed-length polyester filament

Higher shrinkage indicates greater internal stress; lower shrinkage suggests better heat-setting and stability.

2. Key Factors Affecting BWS

a. Raw Material & Spinning Process

  • Polyester melt quality

  • Spinning temperature

  • Drawing ratio and drawing method

b. Post-Processing Conditions

  • False-twist texturing

  • Heat-setting temperature & time

  • Tension applied during heat setting

3. Why BWS Matters in Production

For Fabrics

  • High BWS → shrinkage after washing, deformation, and dimensional instability

  • Low BWS → stiffer hand feel, reduced elasticity

For Downstream Processing

Dyeing and weaving mills must adjust machine settings based on BWS to avoid size deviation and quality defects.

4. How to Reduce Boiling Water Shrinkage

The goal is to release internal stress in the fiber through controlled heat treatment.

(1) Strengthen Heat Setting (Most Effective)

  • Increase heat-setting temperature within material limits

  • Extend heat-setting time to ensure complete stress relaxation

  • Use medium-low tension to avoid generating new internal stresses

(2) Optimize Spinning & Drawing

  • Adjust the total drawing ratio

  • Use multi-stage drawing (e.g., pre-draw + main draw)

  • Improve cooling efficiency during spinning for more uniform crystallization

5. Typical Boiling Water Shrinkage Values

Polyester Type Typical BWS (%) Notes
DTY 3–6% Standard textured yarn
FDY 6–8% Microfiber types: lower by ~1%
Low-shrinkage polyester <3% (premium products ≤1%) For high-end and industrial applications
POY 15–50% Not heat-set; considerable variation

6. How BWS Influences End-Use Performance

  • High or unstable BWS: fabric shrinkage, deformation, higher processing loss

  • Overly low BWS: limited elasticity, stiffness, reduced comfort for knitted fabrics

Conclusion:
Boiling Water Shrinkage is not about “the lower the better.”
It must be stable, consistent, and matched to the intended application.