Structural Relation of Ravon to Natural Cellulosic Fibers J
STUDY OF THE VISCOSE PROCESS JACK COMPTON Cellulose Department, Chemical Foundation, Boyce Thompson Institute for Plant Research, Inc., Yonkers, N. Y. The structural relation of rayon to5natural cellulosic fiber materials has been shown through a detailed study of the changes taking place in fiber structure during the various steps of the viscose process. The crystalline cellulose particle component of natural cellulosic materials undergoes no visible change through the aging, sulfiding, dispersion, ripening, or coagulation steps, although it does participate in the chemical reaction involved. The intercrystalline continuous fiber phase in its relation to the crystalline fiber phase is considerably altered during the various steps of the viscose process, and it renders the physical properties of rayon quite different from natural cellulosic fiber materials. The aging and sulfiding steps cause the principal changes in cuprammonium viscosity
T
HROUGH research during the past fifteen years the physical properties of rayon have undergone many decided improvements with the ultimate ideal of obtaining a higher quality rayon embodying the desired properties of both natural and synthetic cellulosic fibers. The attainment of the ideal synthetic textile fiber, however, has been greatly hindered by the lack of definite information concerning the structure of natural cellulosic fiber materials. Recent work directed toward the elucidation of the structure of natural cellulosic fibers which should be of value in this connection has pointed conclusively to a dual structure (10-12, 31, 39) consisting of crystalline microscopic ellipsoidal cellulose particles surrounded by a continuous intercrystalline material. Mild chemical treatment of cellulosic fibers, whether employed for purification or otherwise, does not visibly affect the predominating crystalline cellulose component but does affect the properties of the intercrystalline phase. Although the latter phase is apparently present in comparatively small amounts in cellulosic fibers ( l a ) ,t,he physical properties of the fiber as a whole are largely determined by the proportion and state of this component relative to that of the crystalline fiber phase. When raw or purified cellulosic fibers are dispersed in reagents such as cuprammonium hydroxide solution, the crystalline cellulose component has been shown to maintain a visible state of aggregation (7, IU, 32), whereas the intercrystalline phase swells enormously and gives rise to a gellike structure. The consistency of this gel-like structure, upon which the cuprammonium viscosity of the dispersion 1250
of the cellulosic fiber materials, the former mainly through oxidation of the intercrystalline fiber phase, the latter, in the absence of air, by a disruption of the components of the organized fiber structure. According to the dual concept of fiber structure, and hence of the resulting viscose or cuprammonium dispersions, molecular weight determinations by other than purely chemical means have no real significance. Although it is apparently impossible to produce rayon with all the physical properties characteristic of natural cellulosic fiber materials, improvements can be expected upon combining adequate protection of the natural fiber components, both during the purification treatments and in the preparation of viscose, with perfected spinning methods.
depends, is conditioned by the pretreatment of the fiber. It is on the basis of this phenomenon that one is able to correlate the cuprammonium viscosity of cellulosic fiber dispersions with many of the physical properties of natural cellulosic fibers. I n the present investigation a microscopic and chemical study has been made, on a laboratory scale, of the steps involved in the manufacture of rayon by the viscose process with the object of determining the function and fate of each of the components now recognized as fundamental in the structure of natural cellulosic fibers. I n the interest of brevity i t has been possible to acknowledge only a few of the many workers who have made contributions to certain of the conventional experimental procedures here described.
Apparatus Capillary viscometers of three designs were used for determining the viscosities of the various cellulosic fiber dispersions. Each instrument was calibrated so that the absolute viscosities in centipoise units would be mutually convertible. Viscometers of the ty e recommended by Clibbens and Geake (6) were modified by t i e addition of ground-glass stoppers in place of the rubber stoppers s ecified. These tubes were used when the concentration of the ispersed fibers in cuprammonium hydroxide solution was 0.5 per cent and above. Unless otherwise specified, all cuprammonium viscosities, in centipoises, were determined with cellulosic fiber concentrations of 0.5 per cent (0.5 gram substance in 100 cc. solution, moisture-free basis) at 25" C., T::,, cuprammonium hydroxide solution 1.2. An Ostwald viscometer was used for determining the viscosities of dilute dispersions of the cellulosic materials in which the
INDUSTRIAL AND ENGINEERING CHEMISTRY
OCTOBER, 1939
concentration of the fiber was approximately 0.1 per cent. Calibration volume was 5.0 cc. A Fenske or modified Ostwald viscometer was used for determining the viscosity of concentrated (2 per cent and above) dispersion of sodium cellulose xanthate in sodium hydroxide solution. Calibration volume was 10.0 cc. A Leitz slit ultramicroscope was used in conjunction with various lens systems and instrument settings. The standard microcell, a calibrated eyepiece micrometer ruled in 1.O-mm. squares, and the lens system (6 L objective, 10 X eyepiece), were employed in obtaining the particle counts of the viscose dispersions (Table VI). A camera holding 35-mm. Agfa ultraspeed panchromatic film was used in obtaining the photomicrographs. A Jena fritted-glass pressure filter, porosity 5/3, was utilized for fdtering the sodium hydroxide solution used in making the viscose dispersion in which particle counts were made.
Materials Raw cotton fibers (Gossypium hirsutum L.) were extracted for 24 hours with alcohol-benzene (1 to 1) in a Soxhlet extractor, thoroughly washed with distilled water, and then treated with 1per cent sodium hydroxide solution at 75" C. for 4 hours. After being washed with distilled water, the fibers were allowed to stand in dilute acid solution ( I3 3 3) for one hour, washed free of acid with distilled water, a n i air-dried at room temperature. TABLE I. MEASURABLE QUANTITIES O F CELLULOSIC MATERI.4LS USED IN THE PREPARATION OF VISCOSE Moisture Cellulosic Material Content, % Cotton fiber extd. with alcohol-bensede and NaOH 4.71 Cotton linters (purified) 4.63 Wood pulp (spruce) 5.81 * Viscosity at 25' C., in centipoises.
* 'lcP.
38.4 10.7 5.58
a-Cellulose,
%
Ash, %
98.9 99.3 96.2
