Page 274 - Handbook of Properties of Textile and Technical Fibres

248 Handbook of Properties of Textile and Technical Fibres

samples should be brought to moisture equilibrium from a dry condition and over a
sufficient period of time. USDA recommends a minimum of 48 h conditioning time
in the standard atmosphere before testing. Moisture regain of cotton in standard
conditions has to be within a range of 6.75%e8.25%. During processing, different
levels of relative humidity may be recommended depending on the type of process.
In addition to the predictable change in fiber dimensions, moisture content also has a
significant effect on the physical properties of cotton. The increase in fiber strength with
the increase inmoisturecontent ishighly expectedas a resultof the formation of hydrogen
bonding as discussed above. Some studies (Elmogahzy and Chewning, 2001)reported a
fiber strength increase by more than 6 cN/tex when moisture regain increased by 4%.
Other studies showed that a change in laboratory relative humidity of 3%e5% would
cause a change of 1 cN/tex in measured fiber strength. These results simply mean that
any comparison between the strength of two cotton fiber types or two cotton products
should be made at the same level of relative humidity, or more precisely at the same level
of moisture content. This represents a critical challenge in many situations where it is
difficult to maintain the same moisture content of different cotton samples.
Another key fiber property that is normally thought to be less influenced by ambient
conditions and moisture content is the specific gravity of cotton fibers. A specific
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gravity of cotton fiber of 1.54 g/cm is normally reported in most literature as one
of the more-or-less constant identities of the cotton cellulosic substance. In the
Australian Cotton Grower Report (Gordon et al., December 2009eJanuary 2010)
(2009e10), an interesting observation was made in which cotton fiber starting at the
dry state (approximately 0% moisture content) exhibited a slight increase in specific
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gravity from 1.54 to 1.56 g/cm as the moisture content was increased from 0% to
about 4%. This was attributed to the water filling effect of the internal cellulosic
structure space (in the proximity of available hydroxyl groups). Direct adsorption of
water molecules onto these hydroxyl groups resulted in more efficient molecular
packing and gives an initial increase in density. Following this brief trend, the specific
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gravity began to decrease, reaching about 1.53 g/cm at a moisture content of 8%.
After this point, the increase in moisture content continued to result in a decrease
in specific gravity in an approximately linear fashion until it reached a value of
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1.3 g/cm at saturation rate of nearly 30% moisture content.
This decrease in density past 4% moisture content was attributed to water molecules
diluting the density of cotton cellulosic structure. It was suggested that at moisture
contents in excess of 5%, cotton cellulose is as full of bound water as it can be, and
that beyond this level cotton contains more loosely bound water, or unbound water
held by surface tension effects such as capillary action.

7.12 Adjustment for moisture content in cotton fiber
strength testing

Because of the significant impact of moisture content on the dimensional and physical
properties of cotton fibers, a great deal of research was made in the direction of
minimizing this impact or controlling it. As indicated above, fiber tests are typically