Factors influencing tensile properties of cellulose fibers

On the basis of previous work, a model for the structure of cellulose fibers is proposed. The effect of degree of polymerization (DP), lateral order, and orientation on tensile properties is discussed in the light of this model. A program was set up to determine separately the effects of the different factors on the tensile properties of various fibers. From this work parameters were derived to express these effects quantitatively in a manner that would be applicable to various types of cellulose fibers. This program consisted of three major lines of investigation: (a) the effect of varying stretch in viscose spinning on the tensile properties of rayon fibers, (b) the effect of hydrolytic degradation of various cellulose fibers on their tensile properties, (c) an investigation of the effect of DP, lateral order, and orientation on the tensile properties of a wide variety of commercial rayons. The first study was undertaken to obtain quantitative information about the effect of orientation on tensile properties. The second investigation was attempted to determine the effect of the degree of polymerization. In the third study the findings obtained in the course of the first two ipvestigations were applied to commercial rayon samples for the purpose of demonstrating their applicability in explaining the tensile properties of these fibers. The discussion of the results of these investigations is preceded by a description of the experimental methods applied in spinning or degrading the samples. The analytical methods used in characterizing the samples for DP, perfection of order, and degree of orientation are also discussed. In a study of the effect of molecular orientation on tensile properties, several series of tire rayons were spun under identical conditions except for variations in stretch ratio. Thus, rayon samples were obtained having the same degree of polymerization and, as proven by x‐ray investigations, having the same perfection of order but varying orientation. The results of the x‐ray orientation measurements were evaluated in different ways according to several suggestions found in the literature. It was found that the square of the orientation parameter fr = 1 − 3/2 sin2 β is linearly related to the tensile strength of the conditioned yarns. The tensile strength of the wet yarn and the elongation in the dry and wet states were also found to be related to expressions derived from this parameter. In the second study, the effect of acid hydrolysis on the tensile properties of cotton, textile rayon, high wet modulus rayons, tire yarn, Meryl, and Fortisan was investigated. Since it, was considered a neressary condition that the effect of DP on tensile strength could only be clearly separated when the degradation could be achieved under conditions which did not alter the perfection of order and the degree of orientation of the cellulose molrcules in the fibers, most of the samples were subjected t o x‐ray analysis. The results obtained clearly indicated that in all cases the hydrolysis caused the formation of a higher degree of order and orientation. This improvement of order and orientation was most pronounced in the case of tire yarn, less in the case of Meryl and textile rayon, and only slight in the case of cotton and Fortisan. Attempts to find conditions that would alter the degree of polymerization by hydrolysis without affecting the lateral order and orientation of the specimens were not successful. The effect of changes in orientation could be corrected for, however, by using the information obtained from the stretch series. This eliminated some of the discrepancies in the relationships between DP and tensile properties noted by other workers. For all samples a linear relationship between 1/DP and tensile strength was valid over a relatively wide range of DP. The slower decrease in tensile strength with DP in a low range has been interpreted as a result of a size distribution of the morphological units forming the network structure of the fibers. This distribution may result from molecular interlinks inside the fibrillar bundles forming the morphological units, and by fibrillar or molecular interlinks between the sides of these bundles as well as between their ends. The following relationships between the degree of polymerization, the perfection of order and degree of orientation, and the tensile properties were established: The letters a, b, c, d, e, f, g, and h designate constants. The “crystallinity index,” CrI, was calculated in a manner similar to that suggested by Ant‐Wuorinen. As defined above, fr is an orientation parameter. The DPL indicates the effective length of the morphological units expressed in glucose units. Finally, a large number of commercial rayon samples were tested for their degree of polymerization, their perfection of order, and their degree of orientation. With the use of the formulas derived from the preceding studies the effective limiting DP's were calculated. A comparison of calculated and observed tensile strengths of the yarns in the wet state and the elongations of the wet and dry yarns shows that the concepts outlined here are generally valid for all rayon fibers and provides an explanation of their observed tensile properties.