Fiber strength


Strength is very often the predominant characteristic. This can be seen from the fact that nature produces countless types of fibers, most of which are not usable for textiles because of inadequate strength. The minimum strength for a textile fiber is approximately 6 cN/tex (about 6 km breaking length). Since binding of the fibers into the yarn is achieved mainly by twisting, and thus can exploit only 30 - 70% of the strength of the material, a lower borderline of about 3 cN/tex is finally obtained for the yarn strength, the minimum strength of a yarn. Fiber strength will increase in importance in future, since most new spinning processes exploit the strength of the material less well than older processes.

Some significant breaking strengths of fibers are:

  • polyester fiber 35-60 cN/tex
  • cotton 15-40 cN/tex
  • wool 12-18 cN/tex

In relation to cotton, the strength of fiber bundles was measured and stated as the Pressley value. The following scale of values was used (93 000 p.s.i = 93):

  • 93 and above = excellent
  • 87-92 = very strong
  • 81-86 = strong
  • 75-80 = medium
  • 70-74 = fair
  • 70 and below = weak

Conversion to physical units should be avoided because the measuring procedure is not very exact.

Today the fiber bundles are commonly tested with HVI instrumentation. Depending on the used calibration standard (USDA- or HVI-calibration cottons) the strength is expressed in g/tex (cN/tex).

For the commonly used HVI-CC calibration the following scale of values is used (1/8 in. gauge strength g/tex)  [27]:

  • 32 and above = very strong
  • 30-32 = strong
  • 26-29 = base
  • 21-25 = weak
  • 20 and below = very weak

Except for polyester and polypropylene fiber, fiber strength is moisture-dependent. It is important to know this in processing and also in testing. Since fiber moisture is dependent upon the ambient-air conditions, it depends heavily on the climatic conditions and the time of exposure before operation. Whereas the strength of cotton, linen, etc., increases with increasing moisture content, the reverse is true for polyamide fiber, viscose and wool.