NATURAL AND MAN-MADE FIBERS

Tests To Help You Know Textiles

Ruby K. Worner.

When you judge cloth or any other textile product, your eyes and hands can tell you a lot about it from the way it looks and feels, but they cannot tell you how it will wear or whether it will require special care to give satisfaction. To know what to look for and what to expect, you need to know something of the fibers and the processes used to make textiles. Even when you are a textile expert, though, your predictions may not be right if you depend solely on your knowledge, as sometimes you must in over-the-counter buying, for many properties of the finished product are not revealed until that product is subjected to service or laboratory tests. Fortunately, tests of both types are available to aid in the production and certification of textiles of quality. Not everyone can apply the tests, but everyone can benefit from an understanding of textile properties and of how technologists and producers test them.

When you choose a textile, you usually know the general type of product you require and some of the special properties strength, the colorfastness, launderability, and so on that are essential for your purpose. The textile manufacturer, to furnish you such a product with assurance, must know which fiber or fibers, which thread, or fabric, or other construction, and which finish to use. Most producers maintain testing laboratories in their mills and finishing plants to help them check quality from the fiber through the finished material.

If a special product is needed, technologists are called in to determine, in terms of measurable properties, the specific characteristics needed, and also the manufacturing details to achieve them. For instance, say a technologist decides that a jacket to be used for hunting should be windproof, water-resistant, light in weight, and durable. He finds from laboratory correlations of service and manufacturing requirements that those needs can be met by a lightweight, closely woven cotton poplin with an efficient water-repellent finish.

But the laboratory tests, so useful in determining specific properties of textiles, are made under reproducible, controlled conditions, and so are often limited. In actual use, textiles meet unpredictable conditions so diverse and complex that it has generally been impossible to simulate all of them in the laboratory. The ultimate test of products, especially new ones, is their behavior in use, and service tests should therefore supplement laboratory evaluations.

Similarly, judgment of a laboratory test should be based on the agreement between its answers and those found in service. To illustrate: Many different laboratory machines have been developed to test resistance to abrasion and wear, but little has been known of how well the results with them agreed with behavior of the textile in use. During the war, the Army needed methods for predicting accurately how materials would wear in service. Because its technical laboratories could not supply the information, the Army designed and constructed a combat course at Camp Lee, Va., for testing military clothing and equipment under simulated battle conditions. Then it called upon the textile division at the Massachusetts Institute of Technology and the Fabric Research Laboratories to determine the relationship between the combat-course and laboratory abrasion tests. On the basis of the results, it was possible to select laboratory procedures that could help predict wear.

Certain large groups of consumers also make use of laboratory tests. Government, railroad, and institutional purchase departments apply them, usually in their own laboratories, to check deliveries for conformance with specifications.

Organizations whose business it is to protect the public often require laboratory tests to aid them in detecting misrepresentation and violations of trade-practice rulings. For instance, the Federal Trade Commission needs accurate methods for identifying, analyzing, and evaluating textiles to insure correct labeling or certification. The Customs Bureau and the Interstate Commerce Commission require methods for identifying textile products in connection with duty and tariff rates.

IN TEXTILE TESTING, wherever possible, standard atmospheric conditions and standard procedures and equipment are used. In that way, results by different investigators and on different instruments can be compared and their significance understood. Much past effort is practically worthless for guiding technologists today because the information published on the materials used or the details of test conditions was insufficient.

Especially in tests of physical properties, atmospheric conditions influence results. For example, the amount of invisible water in the fiber (in the trade usually called moisture regain) depends mostly on the relative humidity and temperature of the surrounding atmosphere to a lesser degree on the previous history of the fiber. In buying and selling on the basis of weight, moisture regain is important and transactions are often made on an oven-dry weight basis. Because of differences in moisture regain, some textiles are stiff on sunshiny days and limp on damp days. Moisture regain also affects the strength of fibers. Cotton and most other vegetable fibers tend to be stronger wet than dry, while the rayons, the man-made protein fibers, and fibers of animal origin, such as wool, tend to be weaker. Therefore, to furnish uniform conditions, practically all testing laboratories maintain rooms with a relative humidity of 65 percent at 70 F., the generally accepted standard condition.

Testing equipment and procedures must be known if results are to be understood. Consider breaking strength. Breaking-strength testers of many types and ranges provide facilities for testing from single fibers that break almost on handling to heavy ropes that require many thousands of pounds to rupture. Types of machines and procedures are selected with reference to the material and the purpose for which it is being tested.

A measure of breaking strength has little significance unless the test method is given. For fabrics, mention must be made as to whether the break was a grab break in which the action of grabbing the fabric between the fingers of two hands and pulling to break is imitated or whether it was a strip break where a strip of the fabric raveled to a width usually of 1 inch is broken. Grab tests usually give higher values than strip tests because the yarns alongside those caught in the fabric jaws give "fabric assistance." The time required to make the break after the pulling starts also affects the result.

In the same way, measurements of breaking strength on different types of instruments may have different meanings. The common breaking-strength testers for yarns, cords, and fabrics operate on one of two principles, the pendulum or the inclined-plane principle. In the pendulum type, the jaws move at a fixed rate of speed, usually 12 inches a minute, so that a stretchy textile breaks much more slowly than one with little stretch. With the inclined-plane type, the load is applied at a definite rate, so that, regardless of stretch, different textiles can be broken in approximately the same length of time. Because the rate at which the load is applied may affect the results a higher strength usually being indicated when the load is applied rapidly than when it is applied slowly different values may be obtained from the two types of machines or even from two machines of the same type but of different capacities.

In all tests of breaking strength, the capacity of the machine, the type of jaws and the distance between them, and the size and shape of the test specimen may affect results. Each should be specified.

FIBERS ARE THE building blocks of all textile structures. For their best use, their chemical nature must be known. They can be cellulosic, protein, inorganic, or resinous corresponding to the old kingdoms of plant, animal, and mineral, and to the newer chemical kingdom. On the basis of these classifications, much of their behavior with chemical agents in processing and in use can be predicted and explained.