What Are Stretch Textiles Made of?
Elastomeric fabrics are a blend of elastane and one or more other fibres. Elastane is a copolymer fibre made from alternating segments of polyethylene glycol and polyurethane. It is not rubber but has similar properties, including memory, which is the ability to return to its original size or shape after being stretched or distorted.
Elastane is more durable than rubber, being significantly more resistant to chemicals like salt and chlorine, as well as detergents, alkaloids (sweat) and body lotions. Elastane, like rubber however, is damaged by heat and UV radiation (sunlight). As such, elastane blended fabrics should not be dry cleaned, machine dried or hot ironed, and should definitely be drip dried in the shade. Why then do we use such a fibre for swimwear which almost always is worn out in the sun? In the past, there has been no better stretch fabric suited to the task. Recently 100% polyester knits have begun come close to the performance of elastane blends although they tend to lose memory much faster so haven't yet gained widespread industry acceptance. Polyester knits are still likely to be the way of the future because they have better chemical resistance and can even tolerate the heat of dye sublimation printing. You can machine wash and dry them, and even iron them without risk of damage.
There are as many different blends and weights of elastane knits as there are non-stretch textiles, each performing differently. Generally there are some rules of thumb:
The higher the elastane content, the greater the stretch and the stronger the rebound tension.
The looser the knit the more stretch but the lower the rebound tension.
The heavier the fabric weight the greater the rebound tension, but the lesser the working stretch range.
Fabrics with natural fibres in the blend have a lower rebound tension and may collapse (lose their rebound tension) completely when wet.
Rebound is the force that causes the stretched fabric to return to its original size. When you take, for example, a 10cm long strip of stretch fabric and pull you will feel a force pulling back. The amount of force generated to return it to its original length will change depending on how long you stretch it. It may for example stretch easily out to 13cm then begin to get 'heavy' but continue to stretch out to 17.5cm. This would be called a 75% stretch as it can increase 75% over its original length. But if you think carefully about how the wearer will put on the garment and move in it you'll soon realize that it's only the first bit of 'soft' stretch that is functional. The objective, then, is to design your garment such it both fits correctly and can stretch comfortably within that initial stretch.
That said, there may be times you might wish to exploit the extra rebound tension to help stabilize your garments. Typically you'd do this for shoulder and back straps, or anywhere else that narrow strip sections are used in the design.
Two way stretch knits have one unforgiving downfall. They do not respond well to being stretched in the 45° cross grain (not that there is technically a cross grain in knitted fabrics). Stretch fabrics require the vertical and horizontal tensions to exceed any forces applied cross grain. You will know you have excessive cross grain tension when you come to fit the garment and you end up with a series of parallel angled ripples somewhere. There is another fit issue which results in non-parallel ripples, but we'll cover that later. Just be sure you can tell the difference between them!
Another consideration, if used, is the lining fabric. This component will add both extra fabric weight and rebound tension. If used in one section only (e.g.; power netting in a front body panel), the greater part of the rebound tension will be taken out of non-lined panels before the lined panel begins to stretch which may result in a poor fitting garment.
So what does all this mean? Much to the dismay of many designers and manufacturers, it means you may need to redraft your basic blocks each time your fabric characteristics change. For example, let's say you design your blocks for the average 180gsm, 80% nylon / 20% elastane, 75% two way stretch. This fabric is generally workable to 30% before getting heavy and works well with negative ease values up to 12%. If you were to change to a 220-240gsm fabric of the same content you'd find you were dealing with only 18% workable stretch meaning that while your block would still fit the body, the wearer would experience more trouble getting it on. Furthermore, the wearer would intuitively know the heavier fabric garment was 'too tight' and possibly suspect they required a larger size.
If you redraft your block every time your fabric characteristics change, no matter how small, you'll find your clients are much happier. If you don't believe smaller changes are practical or economical, at least understand the consequences.