James N. Grant

Density of Modified Cottons Determined with a Gradient Column

A rapid method which compares favorably in accuracy with that of slower methods for measuring the density of cellulosic materials with a gradient column is described. Densities of several cottons before and after chemical modification by partial acetylation, carboxymethylation, aminization, and mercerization are given. Per cent acetyl can be expressed as a function of density in a linear empirical equation over a range of 13 to 42% acetyl with a precision of ± 2%. Density measurements of decrystallized, ball-mill ground, and acid hydrolyzed cottons were in agreement with the generally accepted con cept of the crystalline-amorphous cellulose phase composition in these materials. Cotton from which water was removed by solvent exchange was found to have a high density before, and a low density after, air drying.

The Role of Spiral Structure in Untreated and Treated Cottons

The untwisting of the spiral structure was observed as load was applied to the cotton fiber. Reversals are shown to be a vital structural feature which affects the twisting. A reversing spirality represents an idealized structure for obtaining optimum strength, elongation, and elastic recovery from the straight-chain molecules of cellulose. Friction between growth layers and fibrils in such a structure is suggested as a possible cause for permanent set, low intrinsic strength of highly oriented cotton, and the weak points near their reversals. The spiral structure persists through mercerization even though tension is applied; however, the X-ray angle is reduced appreciably. The high alignment achieved by resin treatment of cotton while under tension causes a reduction in elongation and increase in strength from that of slack treatment. The high alignment in cotton resin-treated with tension persists through washing with a detergent in water. Differences between properties of cotton are reduced but not eliminated by mercerization and resin treatment.

Physical Properties of Chemically Modified Cottons Part III: Effects of Mercerization

Yarns from six cottons selected for their widely different inherent fiber characteristics were mercerized (1) under sufficient tension to maintain their original length, and (2) while permitted to contract freely. Fibers were removed from the yarns and were sub jected to certain physical measurements. Moisture regain, cellulose density, linear den sity, breaking load, and elongation at break were measured on either or both fibers and yarns. Samples of the different cottons were found to differ in their response to the treatment. Those samples with a low value in a property generally displayed the greatest per cent change in that property. Large differences in the properties of the fibers and yarns were associated with the condition of mercerization whether at constant length or slack. Mercerization tended to equalize differences between the properties of fibers in a sample as well as between those of different samples.

A Progress Report on Cyanoethylated Cotton

The mechanism of cyanoethylation of cotton is discussed, and it is shown that for best effi ciency the pick-up of sodium hydroxide must not exceed 15% of the weight of the cotton. Measurements of physical properties show that any increases in the breaking strength or elon gation of yarns are results of changes in frictional forces. The physical properties of cyano ethylated fabrics reflect the properties of the yarn and are also dependent on fabric construction. Extremely high resistance to flat abrasion is obtained with high cyanoethyl substitution. The density of cyanoethylated cotton is shown to be linearly related to the nitrogen content, and the spread of values obtained has been found to give an estimate of the uniformity of treatment. Cyanoethylated cotton can be dyed by methods developed for polyacrylonitrile fibers such as those employing acid dyes in the presence of cuprous ion, and the dyed samples so produced are suitable for cross sectioning for microscopic examination. The intensity of dye color can be used as a measure of the amount of cyanoethylation.

Physical Properties of Mercerized and Decrystallized Cottons

Yarns from samples of Pima S-1, Hopi Acala, and Coker 100 Wilt varieties were scoured, chloroform extracted, mercerized, and decrystallized with anhydrous ethylamine without tension. Breaking loads for yarns immersed in solutions when compared to those in water were essentially unchanged by ethylamine but decreased appreciably by sodium hydroxide. Untreated yarns in water showed decreases in strength with in creases in water temperature. Sodium hydroxide caused yarn to shrink in length as much as 35% but ethylamine only about 17%. Yarn shrinkages were related to in creases in cross-sectional areas of fibers and to cell sizes of the two complexes. Fiber lengths after treatments were unchanged by scouring or extraction but decreased by the sodium hydroxide and the ethylamine. Mercerization at normal length caused a reduc tion in the percentage decrease in single fiber and bundle tenacities as the gauge length was increased. Strength increases during mercerization are attributed to increased uni formity of strength along the fiber length.

Abrasion and Tensile Properties of Cross-Linked Cotton Fabrics

Fiber toughness, fabric construction, and pretreatment were found to affect abrasive wear in permanent-press trouser cuffs. Samples of Pima S-2, Hopi Acala, and Delta pine 15 cottons were processed into twelve constructions of print-cloth weight fabrics. Cross-linking with the dimethylol dihydroxyethyleneurea-type resin was applied as a continuous process to fabrics after scouring and after slack mercerizing. Nitrogen con tents of treated fabrics show that type of cotton, fabric construction, and pretreatment affected the amounts of resin reacted with the fabric. Pima cotton had the lowest and Deltapine the highest level of nitrogen in almost every fabric construction. Pima cotton fabrics showed less wear during conventional abrasion test and laundering and greater crease-recovery angles than did the other cottons. Cuffs of basket-weave fabrics with 0.87% nitrogen showed less crease wear during laundering than did plain weave at 0.64% nitrogen. At comparable nitrogen content, slack-mercerized fabrics were more durable at a higher crease-recovery angle than were cuffs made from scoured fabrics.

Fiber Structure and Mechanical Properties of Untreated and Modified Cottons

A brief review of investigations relating fiber structure to mechanical properties of cotton is given. The relationship of fibril alignment, as measured by the x-ray technique, to the strength and elongation properties of cottons covering a wide range in physical properties is discussed. Alteration of mechanical properties of cotton brought about by degradation in hydrochloric acid, mercerization, decrystallization in ethylamine, resin treatment, and acetylation are related to changes in the fiber structure. The effects of stresses imposed during some treatments are discussed.

Part II: Effects of Strains During Swelling, Washing, and Drying

Yarns from samples of Pima S-1 and Hopi Acala varieties were merceriud and decrystallized when strain was controlled to provide unrestricted shrinkage in length, normal length, and 3% extension beyond normal length during ethylamine treatment. Comparisons of effects of sodium hydroxide and ethylamine at different strain conditions were made from measurements on the cellulose, tensile properties of fiber bundles and yarns, and elastic behavior of the yarns. Densities of the mercerized are in general below those of the decrystallized if occluded solvents are removed. The lower densities are in accord with the higher moisture regains for mercerized than decrystallized cottons. Strains had no influence on the crystallinity ratio during decrystallization but pre vented complete conversion to cellulose type II in mercerization. The decreases in bundle tenacities with increases in gauge length were essentially equal in the decrys tallized and untreated cottons but smaller in the mercerized. Yarn tenacities of the mercerized and untreated increased as the moisture regain was increased, white those of the decrystallized were essentially equal at standard and wet conditions. Elongation at break was decreased by strain during treatment and increased with moisture in the sample when tested. The slack mercerized or decrystallized yarns when strained to less than 2% of their elongation at break have greater recovery and less permanent set than the untreated. At high strains or stresses the reverse in recovery and permanent set was found. Yarns treated at normal length have no appreciable difference in re covery and permanent set from that of the untreated.

Degradation of Cotton Fibers and Yarns by Heat and Moisture

Strength and elongation measurements were made on single cotton fibers and on yarns which had been subjected to various temperatures from 110° to 162°C and various moisture conditions from 3% R.H. up to saturation for periods of heating from 2 to 128 hrs. Moisture contents and degrees of polymerization were also determined, the latter being used to calculate cellulose chain rupture. The simultaneous reduction in strength and elongation at break indicates that heat degradation weakens fibers by creating or intensifying weak points along the fiber. An equation similar to that derived by Sippel, relating fiber strength loss to time of heating and percentage of cellulose links broken, is discussed. Yarn strength, although not as readily affected by heat degradation as fiber strength, follows a similar pattern.

Relation of Specific Strength of Cotton Fibers to Fiber Length and Testing Method

Four cottons in commercial production, covering a range of such physical properties as strength, length, and fineness, were studied in a comparison of two methods of determining fiber strength—the individual fiber test and the Pressley flat-bundle test. Because of the combing action in bundle preparation, cotton fibers broken in the flat-bundle test represent the longer fibers found in a sample of cotton. These remaining fibers are not representative of the length of the original sample; and since fiber specific strength increases with fiber length, neither are they representative of the strength of the original sample. This increase in specific strength with increase in length is evident whether fibers are broken indi vidually or in aggregates. The relationship between the logarithm of individual fiber tensile strength and the logarithm of the specimen length used is inversely linear. The flat-bundle test represents the strength of fibers whose specimen lengths were deduced to be between 1/16 and 3/32 inch.