Textile Care Appearance/ Skewness

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>> Skewness, or torque, is the annoying characteristic of some fabrics and

garments to twist after laundering. As a result of skewing, side seams, sleeves,

legs, and other parts of a garment will become displaced, leading to an

unsatisfactory appearance. "AATCC Test Method 179: -- Skewness Change in Fabric

and Garment Twist Resulting from Automatic Home Laundering." This test method

determines change in skewness in woven and knitted fabrics, or twist in

garments, when subjected to repeated automatic laundering procedures commonly

used in the home. Washing and drying procedures used for shrinkage tests, and

other home laundering tests, are specified for this method. For some fabrics,

the degree of twist in the garments is not solely depended on its behavior in

the unsewn state. It also may be dependent on the manner of garment assembly.

Notice that one pant leg on this garment has skew and the other does not.

Garment twist is defined as a rotation, usually lateral, between different

panels of a garment, resulting from the release of latent stresses of the woven

or knitted fabric forming the garment, that build up in manufacturing during

laundering. Twist may also be referred to as torque or spirality. Skewness is

defined as a fabric condition resulting when filling yarns or knitted courses

are angularly displaced from a line perpendicular to the edge or side of the

fabric. Change in skewness in fabric, or twist in garment, is measured using

benchmarks applied to the specimens before laundering. The displacement measured

from the original marks is the basis for determining skewness. Two test methods

are used for determining skewness. The first method makes use of a square that

is drawn on the fabric or garment prior to laundering. After laundering the

distortion of the square is measured. The other method makes use of lines that

intersect at a right angle. After laundering, the displacement of the

intersecting lines is expressed in percentage skew. Three specimens are tested

for each sample, to increase the precision of the average. When testing fabrics,

where possible, cut specimens from different areas of the fabric so that each

specimen will contain different groups of lengthwise and widthwise yarns.

Identify the fabric face. Mark the lengthwise direction on each specimen.

Whenever dimensional change, appearance, and skewness measurements are required

for fabrics, the specimen dimensions can be the same for that used in "AATCC

Method 124: Smoothness Appearance of Fabrics After Repeated Home Laundering," or

in "AATCC Method 135: Dimensional Changes of Fabrics After Home Laundering." In

these cases, marking by skewness method one is used. Method one makes use of

benchmarks placed on fabric swatches, or on large panels of a garment. Here, a

knit fabric has a 15 by 15 inch square template placed with the vertical edges

parallel to the edge of the fabric. The parameter of the template is marked to

define the size of the specimen. Then the specimen is marked with 10 inch pairs

of benchmarks parallel to the length of the specimen and two 10 inch pairs of

benchmarks are marked perpendicular to the length of the specimen. Next, the

benchmarks are labeled as A, B, C, and D in clockwise direction, starting with

the lower left corner. A line is then drawn through each of the four sets of

adjacent benchmarks so that a square is formed. Finally, the specimen is cut

down to the 15 by 15 inch marks for laundering. When testing fabrics, test three

specimens whenever possible. When testing garments using this method, test three

garment specimens, or a total of three areas of two garments, to increase

precision of the average. Use the largest panels in the garments. After

laundering, follow the procedures found in AATCC Test Methods 124, 135, 143, and

179 for preparation and condition prior to testing. After conditioning, lay each

test specimen -- without tension -- on a flat, smooth, horizontal surface for a

measurement. Method one, which uses a square with the corners marked A, B, C,

and D, measures the sheer distortion of the square by analyzing the diagonals of

the square. The analysis of the skewness square requires the measurements of the

diagonal lines and a percent skewness calculation is made from those diagonal

lengths. Diagonal line AC is measured in inches and recorded to the nearest one

hundredth of an inch. The diagonal length for BD is also measured and recorded.

As stated, method one calculates the percentage change in skewness of the

displaced square. The lengths of the diagonal lines AC and BD are measured to

the nearest tenth of an inch. In this option, the diagonals AC and BD are used

in a formula that expresses sheer distortion of a plain. The value is expressed

to the nearest 0.1 percent, as follows. The difference between the diagonals is

multiplied by a factor of two and then divided by the sum of the diagonals. This

number is multiplied by 100 to give the percentage of skewness. For example, if

diagonal AC is 13.70 inches and BD is 13.65 inches, then the percent skewness

would be 13.70 minus 13.65 times 2. And this product, divided by 13.70 plus

13.65, and this value multiplied by 100, will give 3.7 percent skewness. A

positive percent change indicates product skewness to the left, which is

referred to as left-hand skew. A negative percent change indicates product

skewness to the right, which is referred to as right hand skew. Marking method

two uses different benchmarks as compared to method one for fabric specimens and

garments. Using a suitable marking device, a horizontal reference line is drawn

across the width of the specimen and is labeled YZ. Line YZ should be

approximately three inches above the bottom edge of the fabric specimen, or the

hem of the garment. If the bottom edge, or hem, is not straight, draw the

reference line perpendicular to the vertical axis of symmetry of the specimen.

Place a bench mark -- A -- perpendicular to, and midway, on line YZ. Place one

leg of a right angle marking device along YZ so that the second leg is on a

perpendicular upward from point A. At a distance of 20 inches up the right angle

marking device, directly above point A, draw a benchmark parallel to line YZ.

Draw a vertical benchmark starting at 19 inches from point A perpendicular to

line YZ to intersect the previous benchmark. Label the intersection of the

benchmarks S.B. If the specimen size is insufficient to mark a 500 millimeter --

20 inch -- length, mark the longest available length, which is at least 75

millimeters -- three inches -- below the upper edge of the test specimen. As

described earlier, the second method uses intersecting perpendicular lines that

can be expressed as an inverted T marking on garment panels and some fabric

specimens. The percentage skewness is obtained by how much the inverted T has

distorted. After laundering, a right angle marking ruler is placed with one leg

of the ruler placed along the line YZ, so that the vertical second leg passes up

through point B. A new benchmark is drawn along the perpendicular on line YZ.

The intersection of the new benchmark and the line YZ identifies point A prime.

The length of the line AA prime is measured and recorded to the nearest tenth of

an inch or smaller increment. This distance is the displacement of the right

angle. The length of line AB is also measured and recorded. Finally, the

skewness to the left or right is indicated. The percent skewness in this method

is a simple proportion of the amount of the displaced distance of point A as

defined by AA prime to the length of the vertical line segment AB. Skewness is

reported to the nearest 0.1 percent. The equation used is as follows: AA prime

is divided by AB and multiplied by 100 to give the percent change in skewness.

In this example, line AA prime was 0.425 inches in length and line AB was found

to be 18.85 inches. Therefore, the percent skewness is 0.425 divided by 18.85,

multiplied by 100, and equals 2.3 percent skewness. An important factor in

skewness is that the longer the garment, the more obvious the skewness. This

illustration shows a couple of pairs of knit pants made from the same fabric and

having the three percent skewness for both pairs. Normally this level of

skewness is considered acceptable. Pair A has an inseam of 10 inches, while pair

B has an inseam of 30 inches. Therefore, the side seam shown for A is displaced

0.3 inches. The side seam for B is displaced 0.9 inches. Pair B appears to have

much more skewness. However, the percentage change is the same. This example

shows that panel lengths in garments may appear to be worse than the actual

skewness measured.