Fiber Testing Terminology
Yocum-McColl Laboratories
Fiber Diameter Measurement and Distribution
Fiber diameter is measured in microns. One micron is equal to 1/1,000,000th
of a meter or 1/25,400th of one inch. Mean Fiber Diameter (MFD) is
in common use internationally. MFD, Standard Deviation (SD) and Coefficient of
Variation (CV) all relate to the (approximate) normal distribution of the
animal fiber diameters. SD characterizes dispersion of individual measurements
around the mean.
In a normal population, 66% of the individual values lie within one SD of the
mean, 95% within two SD’s and 99% within 2.6 SD’s. Since SD tends to increase
with increasing MFD, some people prefer to use CV (=SD*100MFD) as a method of
comparing variably about different sized means.
Comfort Factor
Comfort factor is the percentage of fibers over 30 microns subtracted from 100
percent. Ten percent of fibers over 30 microns corresponds to a comfort factor
of 90 percent.
Curvature
Fiber curvature is related to crimp. Average Fiber Curvature (AFC) is determined
by the measurement of two millimeter (2mm), (0.0787 inch or 5/64 inch),
snippets in degrees per millimeter (deg/mm). The greater the number of degrees
per inch, the finer the crimp. For wool, low curvature is described as less
than 50 deg/mm, medium curvature as the range of 60-90 deg/mm, and high
curvature as greater than 100 deg/mm.
Typical values might be illustrated by a 30 micron Crossbred wool fleece with
typically low curvature and broader crimp with a frequency of approximately two
crimps/cm. In contrast, a 21 micron Merino fleece typically has a medium
curvature and a medium crimp with a frequency of approximately four crimps/cm.
A 16 micron Superfine Merino fleece typically has a high curvature and a fine
crimp with a frequency of approximately seven crimps/cm.
Definition of Medullation
A medullated fiber is an animal fiber that in its original state includes a
medulla. A medulla in mammalian hair fibers is the more or less continuous
cellular marrow inside the cortical layer in most medium and coarse alpaca
fibers. By definition (ASTM), a kemp fiber is a medullated fiber in which the
diameter of the medulla is 60% or more of the diameter of the fiber.
Medullation Measurement
Medullation measurement can be performed using either a projection microscope or
the OFDA 100. Using IWTO nomenclature, a kemp fiber is classified as an
“objectionable fiber” when measured on the OFDA 100. The OFDA 100 measures
opacity and therefore only white or light colored fibers can be measured. A
reasonable assumption is that colored fibers have similar levels of medullated
fibers as their white and pastel counterparts.
Spinning Fineness
This number (expressed in microns) provides an estimate of the performance of
the sample when it is spun into yarn by combining the measured mean fiber
diameter (MFD) and the measured coefficient of variation (CV). The original
theory comes from Martindale, but the formula used comes from Butler and
Dolling and normalizes the equation so that the spinning fineness is the same
as the MFD when the CV is 24%. As the CV is reduced in comparable MFD fleeces
the spin fineness is less.
Length and Strength
Length is measured in millimeters (mm) and the reported measurements readjusted
to an annual growth period. Strength is measured in Newtons/Kilotex (N/ktex)
and is the force (measured in Newtons) required to break a staple of a given
thickness (measured in Kilotex). On the earth’s surface, one kilogram exerts a
force of 9.8 Newtons (- 1kg * acceleration due to gravity measured in
meters/second2). Kilotex indicates thickness in terms of mass per
unit length expressed in kg/km.
Intrinsically, alpaca fibers appear to be very strong, an average of 50 N/ktex
or better is not unusual. From a processing point of view, a mean staple
strength greater than 30 N/ktex is considered adequate for processing wool on
today’s high-speed equipment.
Position of Break
Truly sound fibers break in the middle section of the staple. Intrinsically,
alpaca fibers appear to be very strong, in the 50 N/ktex range. A mean staple
strength greater than 30 N/ktex is considered adequate for processing wool on
today’s high-speed equipment.
Resistance to Compression
The resistance to compression (RTC) of alpaca fibers is measured in kilopascals
(Kpa). A pascal (Pa) is a unit of pressure equivalent to the force of one
Newton per square meter. In the commercial sector, RTC values >11 kPa are
considered high, 8 to 11 kPa medium, and <8 kPa is low. The intrinsic
resistance to compression of alpaca is low because of the relatively low levels
of crimp. Thus, alpaca is not suited to end-uses that require high resistance
to compression (or high bulk).
Clean Yield
Yield is based on bone-dry, extractives-free wool (alpaca) fiber or wool
(alpaca) base (WB). Many different “commercial” yields are used in the
international marketing of wool fibers. These are values calculated to predict
the amount of clean fiber obtained after commercial scouring and/or after
combing. Allowances are typically made for grease, ash, vegetable matter, and
moisture. Various percentages of moisture are added to these calculations of
commercial yield, which in some cases (very clean wool or some alpaca yields)
may result in the clean yield exceeding 100%.