G.F.S. Hussain

Inverse Relaxation/Stress Recovery in Cotton Fibers and Yarn

The literature on mechanical properties of textile fibers is replete with data on viscoelastic characteristics like stress relaxation, creep, creep recovery, etc. A closely associated property, &dquo;stress recovery&dquo; or &dquo;inverse relaxation,&dquo; on the other hand, does not seem to have been investigated. If an extended specimen is allowed to recover a part of the deformation and constrained to remain at a length higher than what it is likely to assume if allowed to retract fully, the stress in the specimen tends to increase. This inverse relaxation or stress recovery, which is a function of time, is initially fast but slows down later on. The stress thus rises exponentially with respect to time. Neither a discussion of the significance of stress recovery nor experimental data on this property can be found in literature, though Peters and Woods have mentioned tension build-up [ 1 ]. We record here the stress recovery data on a cotton fiber sample and a cotton yarn sample tested using the Instron Tensile Tester.

Sonic Modulus of Cotton Yarn and Its Relationship with Recovery Parameters

Effects of chemical modifications such as mercerization (with and without stretch) and crosslinking (with HCHO and DMDHEU of various concentrations) on me chanical properties such as dynamic (sonic) modulus, immediate elastic recovery (IER), work recovery (WR), and crease recovery angle (CRA) of yam samples were studied. While slack mercerization reduced the dynamic modulus and the recovery parameters, stretch during mercerization brought about a profound increase in their values. The dynamic modulus and recovery parameters also increased progressively with the severity of crosslinking in HCHO and DMDHEU. A high degree of linear correlation existed between the dynamic modulus and IER, WR, and CRA for samples treated with a given reagent; however these relationships were specific to the reagent.

Influence of Mercerization and Crosslinking on the Dynamic and Static Moduli of Cotton Yarns

’ Physical properties of textile materials under the influence of cyclic tensile forces are known as &dquo;dynamic mechanical properties.&dquo; Dynamic modulus is the foremost among the dynamic mechanical properties usually studied. Various techniques for determining the dynamic modulus have been summarized by Woo et al. [ 14], Chaikin and Chamberlain [ 11, and Tipton [ 11 ]. From published literature, very little information is available on the dynamic mechanical properties of cotton fibers and yarns. Hamburger [2], Lyons [4], Typton [ 11 ], and Woo et al. [ 14] employed sonic velocity methods to determine the dynamic modulus of raw cotton fibers. It is well known that chemical modifications such as mercerization and crosslinking treatment affect several physical properties of cotton fibers. In viscoelastic materials, where time effects such as stress relaxation and creep are significant, the ratio of dynamic to static modulus will be high compared to that in perfectly elastic materials where the ratio is unity. The more plastic a material is, the greater will be the ratio of dynamic to static modulus [4]. Our paper discusses the effect of chemical modifications such as mercerization and crosslinking on the dynamic and static moduli of cotton yarn, as well as on the ratio of these two moduli. Leas of cotton Digvijay spun to 30s count with a twist multiplier of 4.0 were dewaxed and kier boiled. These leas formed the control sample (first control) for studying the effect of mercerization. The treatment was carried out in slack as well as stretched conditions. The sample designations are MS-mercerized slack, M 8%-stretched to 8% below the original length, M 0%-stretched to original length, M + 2%-stretched to 2% over original length, and M + 4%-stretched to 4% over original length. The slack mercerized yarn constituted the control sample (second control) for crosslinking with HCHO and DMDHEU. In order to forestall the likely effects of yarn geometry on modulus measurements [7, 15], the yarn was wound on a special metallic frame and maintained at constant length with no twist loss during treatments. Form W process [9] was used for HCHO treatment at three alternative concentrations: 8% (XH 8%), 16% (XH 16%), and 22% (XH 22%). Bound HCHO in the treated sample was estimated chemically. The conventional pad-dry-cure method [ 12] was employed for DMDHEU treatments. Four different concentrations, 5% (XD 5%), 10% (XD 10%), 15% (XD 15%), and 20% (XD 20%), were studied. Nitrogen content (N%) was estimated by infrared method based on the carbonyl absorption [3]. All yarn samples were conditioned at 65% RH before modulus determination.

Changes in Cross-Sectional Dimensions of Cotton Fibers on Crosslinking and on Subsequent Wetting

Changes in cross-sectional dimensions of cotton fibers resulting from crosslinking treatments were studied. One variety of cotton, Sujata, was subjected to crosslinking in dimethyloldihydroxyethyleneurea and in formaldehyde to different add-on levels. The changes in cross-sectional area and perimeter due to crosslinking, as well as the changes in these dimensions resulting from subsequent wetting, were measured. The results were discussed in the light of available information on fiber morphology and the nature of crosslinking reactions.

Yarn Bundle Strength from Single Strand Test Data

In an earlier publication [I ] we derived a relationship to calculate fiber bundle strength from single fiber test data. With textile fibers in general, the breaking extension and breaking load are mutually independent. Assuming probability distribution functions for breaking strength and breaking extension and a linear load-extension relationship, the load developed in a bundle of N fibers at extension x was derived as