Page 254 - Handbook of Properties of Textile and Technical Fibres

228 Handbook of Properties of Textile and Technical Fibres

called “fibrillar failure (Hearle and Sparrow, September 1971).” This is due to the
fibrillar network that must be destroyed before a cotton fiber can break.
Although a unified theory describing the relationship between the internal structure
of cotton fiber and fiber tensile behavior has not been established, most studies (Hearle
and Sparrow, September 1971; Atlas of Fiber Fracture and Damage of Textiles, 2006;
Hearle and Sparrow, 1979a,b; Gould and Seagull, 2002) agreed that the orientation of
the fibrils within the walls of the fiber plays a vital role in this relationship. SEM
studies revealed that within the wall, cellulose microfibrils are helically oriented
around the fiber axis, and the angle of inclination of cellulose fibrils to the major
axis of the fiber decreases from the primary wall inward. This angle has been found
to directly influence the straining process of fiber until the rupture point (the breaking
elongation). A tensile force applied on the fiber tends to straighten the fibrils and align
them along the fiber axis. Fibers whose secondary wall deposition has been arrested by
some factor will exhibit greater elongation-to-break than fibers whose secondary wall
deposition has proceeded to the normal limit. This phenomenon is clearly observed in
single-fiber strength testing. Because fibers of a higher amount of secondary wall
deposition are also highly mature fibers, this implies that higher maturity will yield
higher absolute tensile force (tensile force not adjusted for fiber diameter or linear
density). In other words, when flat bundle test data are converted to force-to-break
of individual fibers, the breaking force increases in direct proportion to secondary
wall thickness.
In the secondary walls of cotton fibers, microfibrils routinely reverse their spiral
direction. This is a unique occurrence found only in cotton fibers and has been referred
to as a reversal point. Some studies (Gould and Seagull, 2002) suggest that reversal
points are thought to create weak areas in the secondary wall where fibers will prob-
ably break. As the secondary wall develops, the number of reversal points increases
and microfibril orientation appears to change. The reversal appearance changes with
the deposition of subsequent layers of the secondary wall. Early in secondary wall
development, cellulose microfibrils are oriented in a shallow-pitch helix and reversal
points exhibit an abrupt change in the orientation of microfibrils (Z reversals). Later in
development, microfibrils exhibit a steep-pitch helical orientation and reversals
exhibit a more gradual change in microfibril orientation (S reversals). As the secondary
wall develops, there are significant increases in total reversal frequency and in the
frequency of S reversals. The frequency of Z reversals decreases with subsequent
development of the secondary wall. As fibers develop, the total number of reversals
increases. Ultimately, almost all reversals observed in the microfibril structure become
of S-type. Other studies (Hearle and Sparrow, 1979a,b) revealed that many fiber
fractures occur adjacent to the reversal zone and not through it, indicating that the
reversal itself is strong, but, because of its existence in the fiber, it is a source of
weakness in that it is the cause of fracture in a region adjacent to the reversal.
It is well known that many genetic varieties of cotton fiber will exhibit different
levels of fiber strength, although they may have approximately the same fibrillar
orientation. This has led many researchers to focus on other factors such as fiber
convolutions and the fiber ribbon width. An early study by Hearle and Sparrow
(1979a) revealed that the removal of the convolutions contributes substantially to