Fibers in a Different Light

Ines V. de Gruy, Mary L. Rollins.

Not long ago the scope of textile microscopy was limited to the study of natural fibers. Now it has broadened to include examination of the many new synthetic fibers. Thus, the microscopist helps the buyer, who knows little about what has been done to a piece of cloth and scarcely realizes how much research goes into the selection of a particular cotton for a specified use.

One of the most important uses of the microscope in textiles is to identify fibers. The problem of distinguishing different types of fibers in a yarn or fabric is a common one in both research and trade. It is often surprising how many different fibers may be revealed in a single yarn said to be composed of only one type. For example, in yarns claimed to be 100 percent wool, the microscope has shown rayon and cotton fibers mixed in as adulterants.

Several approaches may be used in the identification of fibers in an unknown sample. Staining methods are the most common, the color reaction being based on the chemical character of the fiber. For example, to distinguish between fibers of viscose rayon and acetate rayon, staining with Congo red is employed. The stain produces an intense red color in viscose rayon but does not affect acetate rayon. The stains and procedures for applying them are similar to those used in clinical biology in the study of disease organisms, and for the identification of fibers in the pulp and paper industries.

Characteristic markings and shapes also furnish clues to the identity of fibers. The distinctions show up well in cross sections of the fibers, because a beam of light passing through a thin slice of a bundle of fibers under the microscope reveals significant details not apparent in the whole fiber. For that reason, the cross-sectioning technique is probably the most valuable method for fiber identification. A simple tool for sectioning textile fibers is the hand microtome developed by J. I. Hardy of the Bureau of Animal Industry. The device, about the size of a microscope slide, consists of two metal plates fitting together to hold the sample in a slot. An auxiliary plunger for propelling the bundle of fibers is attached by means of a small screw, and cross sections about 4 microns thick are cut with a razor blade. These thin slices are examined under the microscope at magnifications from 100 to 500 for the study of the fiber characteristics.

EACH of the natural fibers has a distinctive cross-sectional shape. Cotton fibers resemble kidney beans; flax fibers are polygonal and show a small dot or line for the central canal. Other vegetable fibers whose outside edges are sharp, straight lines include jute, sisal, abaca, tula, palma, and hemp, but the number of sides of the polygonal cross section and the shapes of the central canal openings are different.

Another characteristic peculiar to some of these vegetable fibers is their occurrence in fibrovascular bundles, which also have distinctive shapes, depending on the types of plant stems from which they come. Among the animal fibers, cross sections of wool are round or oval and vary considerably in size. Silk fibers, extruded in pairs by the silkworm, are regular, and usually somewhat triangular in shape.

Of the synthetic fibers, the commonest are the many rayons manufactured commercially from regenerated or modified cellulose. These fibers have characteristic shapes and markings by which the microscopist learns to recognize them in unknown samples. The cross sections of viscose filaments are irregular in both size and shape and have serrated edges, while cuprammonium rayon filaments are small and almost round. Acetate fibers are irregularly lobed and appear fused; Fortisan, a very fine saponified acetate, is scalloped.

Protein synthetics derived from agricultural products soybean, peanut, casein, egg, and zein fibers are not so easily distinguishable in cross-sectional size and shape, although they have different physical properties.

Nylon and Vinyon are examples of synthetics of purely chemical origin. Nylon filaments are round and regular; Vinyon filaments are flat and peanut-shaped.

To GET A BETTER UNDERSTANDING of the complexities of the microscopist's study of fibers, let us consider the small cotton fiber as it comes from the boll.

When the flower opens, the cotton fibers, which are single cells, begin to be produced on the surface of the seed. There may be as many as 10,000 fibers to the seed better than a quarter of a million to the boll. In the unopened boll, the fiber attains its maximum length in 16 or 17 days. At this stage, the lumen, or central canal, of the fiber is large and filled with fluids from the living protoplasm of the cell. Subsequently, the lumen becomes progressively smaller as the cell wall thickens by deposition of a layer of cellulose each day. When the boll opens at the end of the growing period, the fiber dries and the cell, no longer distended by plant juices, collapses into a shriveled, twisted, flattened tube, often more ribbonlike than cylindrical, and as much as 4,000 times longer than wide.

Cross sections of the fibers from any one seed show that the thickness of the cell wall varies considerably from fiber to fiber. Fibers that for some reason or other did not attain full growth have collapsed into flat, transparent ribbons, which in cross section are thin-walled and often curiously curled and misshapen. These underdeveloped fibers are said to be immature. Occasionally a sample of cotton will show a few thin-walled fibers, which failed to collapse on drying, so that in cross section they look like doughnuts. The normal fibers, having completed their growth, are so nearly filled with cellulose that when they dry their cross sections are oval to circular, or have collapsed to bean-shaped contours. These thick-walled fibers are called mature.

It is one of the chores of the microscopist to compare cotton samples with respect to maturity and to detect any departure from the expected varietal characteristics. The extent of cell-wall development is an inherited characteristic of cotton fibers, but maturity is even more affected by such environmental factors as soil fertility, fluctuations of rainfall and temperature during the growing period, and the time of harvest. Normal commercial cottons of better than average grade contain less than 25 percent immature fiber, while frost-killed samples can be as much as 60 percent immature. These facts may often be deduced from microscopical examinations. Such information is invaluable in manufacturing where the strength of yarns is affected by the fibers from which they are made.

THE LENGTH and fineness of the fiber denote the growth group or species to which it belongs. Sea-island and Egyptian cottons are extremely fine and long; upland varieties are characteristically much shorter and often much coarser; Asiatics are extremely coarse and short. It is impossible to identify the variety of an unknown cotton sample by microscopic examination, but experience teaches the microscopist certain distinguishing features by which particular varieties may sometimes be spotted. For instance, S X P cotton (a commercial variety bred by crossing an American and an Egyptian variety), besides being fine, often shows in cross section one or two fibers containing an amber-colored deposit in the lumen. Stoneville cotton has fibers whose cross sections have greater length than' breadth, while Rowden fiber sections are conspicuously round.

Useful as the cross-sectioning technique is, it reveals almost nothing of the internal structure of the fiber. By more specialized methods, which involve swelling of the cellulose by chemicals, microscopists have investigated the morphology of the cotton fiber to obtain a better understanding of its "architecture." An over-all knowledge of the inherent structure of cotton often helps the chemist to predict the result of a treatment before he applies it; if he is familiar with the basic fiber properties and reactions, he will have some idea of the reason why a process does or does not work.

Many questions remain unanswered, but agreement has been reached regarding the fundamental concepts of the gross or broad microscopic structure of the cotton fiber. No microscopist questions the existence of three major parts the primary wall, the secondary wall, and the lumen. Opinions differ, however, on details of the chemical composition and physical structure of the parts.

When the fibers are treated on a microscope slide with such reagents as cuprammonium hydroxide, sulfuric acid, phosphoric acid, or trimethylbenzylammonium hydroxide, all of which are solvents for cellulose, swelling of the fiber before it dissolves shows clearly its structural details. When the fiber is completely immersed in the liquid, it begins to twist and turn on itself, and the primary wall, which encases the fiber, is ruptured by the swelling pressure of the cellulose in the secondary wall beneath it.

The primary wall breaks, often into a spiral pattern, and peels back to form constricting collars or bands. The "ballooning" thus produced is characteristic, and is similar in appearance to bead necklaces in which the "beads" are separated by constricted parts of the primary wall. In the balloons may be seen the laminated structure of the secondary wall, whose layers indicate the age of the fiber. During swelling, the lumen wall reacts in much the same way as does the primary wall, both being much more resistant to attack than the secondary wall.

The cellulose of the primary wall is made up of a network of branching fibrils of cellulose, thought to spiral about the fiber at an angle of approximately 70 to the fiber axis. In addition, it contains both wax and pectin.

The secondary wall is almost pure cellulose, believed to be deposited in alternate compact and porous layers; these are made up of branching fibrils, which spiral at an angle of approximately 30 to the fiber axis. Between the primary and secondary walls is a thin layer often referred to as the "winding," because it is seen as a coarse spiral thread wrapped around the fiber after the primary wall peels off during swelling.

The lumen contains the protoplasmic residue, which is composed for the most part of coagulated proteins. The pattern of fiber structure revealed by swelling techniques contributes information useful in interpreting the results of chemical treatment.

AS A RESEARCH TOOL, the microscope provides a means of comparing treated fibers, yarns, and fabrics with untreated specimens for a rapid evaluation of experimental results during the development of laboratory methods for the modification of cotton.

While there are well-established procedures for the more routine phases of such work, new research developments make continuing demands on the ingenuity of the microscopist. Whether it is the chemist's objective to impregnate a cloth completely or just to force a compound to penetrate the edge of a yarn, microscopical techniques, both old and new, are applied in the study of the results.