The specialists in woven fabric geometry have been con­centrating on the details of simple woven structures for the last 50 years, but seem to have been avoiding the more highly complex structures. Elastic fabrics have been ig­nored completely. All research has been very much on an ad hoc basis, mainly within the elastic manufacturing companies. I have therefore made a few preliminary re­searches into the subject; the original reason being to an­swer some of the practical problems we were up against.

This paper is part of my work on woven narrow elastic fabric characteristics, including their geometry in relation to dyeing and finishing.

One of the first theoretical problems faced was to ration­alise the huge variety of fabrics manufactured into a small number of generalised constructions. The one studied most has been called the strap construction.

The longitudinal cross section of this strap construction is shown in Diagram 1. Although a fabric may be exceed­ingly complex in its patterning, the diagram shows the bare essentials of one type. There may be many floats, and dif­ferent warp yarns, but essentially each weft shot is separ­ated from the next by an elastomer and/or a warp yarn. If the length taken up by one weft shot in the length di­rection can be calculated and the average length taken up by each intersection of weft yarn also calculated, it is possi­ble to calculate the length taken by one pick. The length of two picks is shown in the diagram.

The length of one weft can be calculated from the deci­tex count of each filament and the assumption of hexa­gonal close packing of the filaments. From conventionally based calculations of the geometry of the warp yarn we can calculate the length taken up by the warp. Perhaps surpris­ingly, the theory and the practice agree fairly well for grey state fabric.

During the dyeing and finishing process the fabric is sub­jected to fairly severe processing conditions. It is squeezed, steamed, pulled, washed, heated, dried and cooled. This experience changes the characteristics of the fabric fairly markedly. The finish applied also increases in­ternal fabric friction and fills fabric interstices.

It is very difficult to obtain an average fabric from the loom in an average condition of relaxation to study the changes that occur during these processes. A length of fabric from a production loom has one end which has had more time to relax than the other, and each loom piece will be different from the others. The problem is to decide how to find an "average" piece of web. It is therefore worth stu­dying a number of pieces to see what happens and to see what can be learned.


















Fig. 1 Longitudinal cross-section of woven narrow elastic (strap type construction)

In order to obtain a constant degree of relaxation in a fabric it has long been the practice when working with elas­tic fabrics to carry out a "boil-off". This involves putting the fabric into a washing machine for ten to fifteen minutes with soap acting as a lubricant, and then drying the fabric before testing. This process thoroughly relaxes the fabric.

If a series of long cuttings is taken straight from the loom and part of each piece is tested for extension and modulus (modulus is the load needed to stretch a piece of fabric by a given extension, often 40% or 50%), a scatter of test results is obtained. If a "boil off" on new lengths from the long cuttings is carried out, a scatter about a different point on the graph is obtained — diagram 2. The two regions on the graph can be joined with a "best line" or by a linear re­gression. The relationship is not a true "straight line" one, but a straight line is a useful approximation. This line has been called "The Grey Fabric Relaxation Line". The remain­der of the long cuttings are then sewn end to end, and passed through the dyeing range (or any other treatment being studied). Measurements of modulus and extension can then be obtained from finished fabric before and after boil off. If these results are plotted on the graph used earlier, a "Finished Fabric Relaxation Line" can be obtained — diagram 2. Since these two relaxa­tion lines have the effect of increasing our knowledge of the effect of changes in extension on the modulus, these lines have a number of uses.

By carrying out fairly extensive work to develop relaxa­tion lines of fabrics, it has been shown that the same basic elastomer core behaves similarly whatever the cover wound on it, or its extension — within reasonable limits — with respect to its tensile properties.

For example, per end of 1880 decitex Lycra, the same re­lationship exists between modulus and extension whatever the covering details. However, it is important to mention that weavability, cockling, bowing and so on are other im­portant factors that must be taken into account when se­lecting an elastomer for practical sampling. If asked to de­velop a web to a certain extension and modulus specifica­tion, it is possible to calculate how many ends of which elastomer to use (admittedly experience can tell us what to use, but this work is an attempt to put the whole thing on a theoretical base). It is possible to predict what the effect on modulus will be from changes in extension.

From this experimental method, if the pick count is measured when testing for modulus and extension, the ef­fect on pick count can be measured if the fabric stretch and processing techniques are altered. A typical result for the changes that occur in pick count is shown in diagram 3. Region A shows the pick count at the loom state, region B after boil off, when the fabric relaxes and the picks close together. Region C indicates the pick count after dyeing and finishing: when the fabric is extended the pick count drops. The amount by which the pick count rises during boil off, region D, is an indication of the severity and perma­nency of the dyeing treatment and the potential fabric shrinkage.

As shown earlier, it is possible to predict from the gen­eral fabric model a maximum and expected pick count for grey state fabrics. If the fabric is put under greater tension in the dyeing process and the fabric is at that tension, using heat, a lighter weight fabric is obtained at lower pick count, and at lower extension, but the fabric will be likely to have a higher shrinkage.

If the fabric is stressed on the drying cylinders one result is obtained and if stressed in the steaming chamber, an­other is obtained.

By looking at the fabric relaxation lines in relation to ad­justments made on the dyeing range, it was found that we could learn about what happened on the dyeing range with greater understanding. In the past it was found that if fabric is run through dye liquor and steam chamber only, a com­pletely different modulus and extension is obtained than that obtained when the fabric is passed through the full range. We then had no way to interpret the results. Now the interrelationship between extension and modulus is un­derstood more fully, the calculation of exactly what effect each part of the process has on the fabric is possible. It has been found with problem fabrics that some difficulties could be overcome by the use of educated guesses. Also, more importantly, it is possible to predict the cost of a new fabric design more accurately before the high cost of sam­ple weaving a length of web has been incurred.
























                                  



Fig. 3 Pick count vs ultimate elongation for a woven narrow ela­stic B in grey and finished states (Signs as indicated in Fig. 2)






Julian G Ellis
March 1981