ASTM standards to determine the tensile strength of Lyocell fibers

To determine the tensile strength of lyocell fibers (a type of regenerated cellulose) using ASTM standards, the selection of the standard depends on whether you are testing the individual fiber or the yarn itself.

1. Testing tensile strength: ASTM D2256

For evaluating the tensile properties of lyocell yarn, ASTM D2256 (Standard Test Method for Tensile Properties of Yarns by the Single-Strand Method) is the primary and most widely used standard.

• Scope: This method covers the measurement of breaking force, elongation at break, and tensile strength (tenacity) of yarns. It is suitable for monofilament, multifilament, and spun yarns.
• Procedure: A single strand of the yarn is clamped into a universal testing machine (tensile tester) and pulled at a constant rate of extension (CRE) until it fails.
• Key measurements:
• Breaking Force: The maximum force the yarn withstands before failure.
• Breaking Tenacity: Calculated as B=F/T (where F is breaking force and T is linear density in tex or denier).
• Elongation at Break: The increase in length at the moment of failure.
• Versatility: You can perform the test on straight, knotted, or looped specimens, depending on the specific performance data needed for your application.

2. Testing individual lyocell fibers: ASTM D3822

If you need to evaluate the fundamental mechanical properties of the lyocell filaments before they are spun into yarn, or to analyze the fibers themselves, ASTM D3822 (Standard Test Method for Tensile Properties of Single Textile Fibers) is the correct standard.

• Scope: This is used for measuring the tensile properties of single natural or man-made textile fibers.
• Application: It is particularly useful for R&D and quality control of the raw material (the fiber) to see how modifications in the manufacturing process (like extrusion) affect the fiber's intrinsic strength.

Critical considerations for lyocell

Like other regenerated cellulose fibers (e.g., rayon), Lyocell is highly sensitive to moisture. Its tensile properties change significantly depending on its moisture content [1.2.1]. To ensure accurate and reproducible results:

• Conditioning: Both standards require the material to be conditioned in a controlled environment- typically 21 ± 1°C and 65 ± 2 per cent relative humidity - before testing to ensure consistent moisture regain.
• Wet testing: Because lyocell and similar cellulosic fibers can weaken when wet, ASTM D2256 allows for wet testing procedures. This is vital for manufacturers to understand how the yarn will behave during downstream wet processing, such as dyeing or finishing.

Summary of best practices

• Use ASTM D2256 if your goal is to test the quality, durability, or performance of the finished yarn as it would be handled in a factory or end-use application.
• Use ASTM D3822 if you are testing the individual fiber properties for material development or fiber-level quality control.
• Always use a CRE (Constant-Rate-of-Extension) machine and ensure appropriate grips (typically pneumatic cord and yarn grips) are used to prevent slippage or premature jaw breaks, which are common challenges when testing delicate or smooth synthetic yarns.