Garment and Fabric Shrinkage Secrets Revealed!

Why is it that a consumer must evaluate a garments fit, when new, with an angel on one shoulder saying it will fit after laundering and a devil on the other saying to buy a size larger to accomodate shrinkage? Why do cotton fabrics shrink more than polyester fabrics? Why do wovens shrink less than knits? Why does a tumble dryer cause shrinkage when a washing machine doesn't? Have any of these questions ever crossed your mind? Here, finally, in plain english, are the answers.

In the industry, we break shrinkage into two seperate catagories. The first is called relaxation shrinkage, and is exhibited by taking a piece of unlaundered fabric, laying it in a bowl of water, and observing the dimensional changes. The first thing you will note is that it does not move much at all. Placing a fabric in a liquid suspension will cause two things to happen. The first is that the water will act as a lubricant and allow any residual energies or tensions in the fabric to relax. The second is that, if the fabric is absorbant, the fibers will swell, causing the structure that binds them to consolidate. It is old hat that polyester and nylon do not absorb, but in todays world, don't be fooled. Both fibers can now be temporarily (through topical finishes) or permanently (through molecular change) modified to absorb (think "performance fabrics"). Conversly, there are finishes that can be applied to cotton (stain resistance, water repellant) that make cotton non-absorbant. Regardless, this "relaxation shrinkage" is a very very small componant of the shrinkage you experience in a finished garment. Maybe a half a percentage, if that, is caused by relaxation.

The second measure of shrinkage is called "consolidation shrinkage". This is a consequence of combining lubrication (water and heat) with mechanical action (tumbling) that facilitates the movement of the structure as it releases the tensions that caused process deformation of the woven or knit structure. This deformation is caused by the inherent tensions when bleaching, dying, hydro-extracting, and drying the fabrics in an industrial environment. Consolidation shrinkage is the primary cause of shrinkage in home laundering.

Woven fabrics(poplins, muslins, duck,canvas,georgette,etc) are constructed by laying yarns across each other in engineered patterns. The yarns are typically tightly woven in a manner that leaves little space between the yarns. For this reason, wovens are inherently more stable (and less stretchy). There are exceptions (like gauze), where the fabric is loosely structured, but we won't concern ourselves with exceptions or this article will be longer than you will care to read (are you still reading?). Cotton wovens will, typically, have higher shrinkage than synthetics or blends, but not for the reasons you suspect. Synthetic fibers all have a heat range in which they begin to soften (glass transition temperature). When a fiber is brought to that temperature for a very brief period of time it will cool and "set" itself in whatever arrangement it is confined to. What this means is that you can process fabrics with synthetics to purposefully heat set them so that they stabilize at that specific point in time within the structure, thereby stabilizing the structure itself. You can't do this with natural fibers. Therefor, it is not the fact that cotton fabrics shrink more, it is the fact that synthetics can be manipulated to shrink less!

There are a few processes to help stabilize cotton shrinkage. In years gone by, the industry used urea formaldehyde resins to cross link the fibers and set them. This worked great, as long as nobody cared that the plant workers were dying of cancer. Once the link of urea formaldehyde and carcinogens was found the industry resorted to mechanical compression. Although there are several types of methods used, they all result in squishing the fabric temporarily together, so that when washed, it does not move as much. This is known as "pre-shrunk". The only issue is that these processes are not exacting. They cannot measure how much the fabric was stretched in previous processes and cannot measure the variability of those forces over any given length of fabric, so that the entire process is reliant on an operator setting the equipment to a fixed setting. This allows for variation yard to yard and roll to roll, especially in knit products. Add to this the fact that fabrics are then spread on a cutting table, and that the tensions involved in spreading may actually pull out some of the compaction, furthering the variability!

Knit fabrics are a story unto themselves. There are two main knit catagories, weft and warp, and most fabrics you encounter in casualwear (tees, sweats, undies, polos, etc) are weft knits. Warp knits are typically found in lingerie and foundation garments. Therefor, we will stick with weft knits here. Knit fabrics are made up of loops that are formed one within another. Unlike wovens, where yarns are laid across each other, knits require bending and forming of yarns into all kinds of constructions. This means knits have allot more space between the yarns, which lends to their being stretchy. These spaces allow room for the structures to deform more readily in processing. As fabrics run through rollers, are soaked in liquids (making them extremely heavy) and dragged over and under beams they are stretched. It is the challenge to recover those fabrics to their original form that defines how much they shrink. Therefor, knits shrink more than wovens due to their less stable structure.

Now, why does a garment shrink in the dryer but not the washer? If you have been paying attention you now know that the washing machine simply makes a garment wet and that means relaxation shrinkage is the only factor involved. Yes, there is mechanical action and yes, there may be hot water, but therin is the rub. Fabrics do not begin the process of consolidation shrinkage until they reach a moisture level near 20%, and in a washer, they are fully saturated. I had mentioned heat acted as a lubricant, in conjunction with water, but the weight of the water must be low enough to not cause drag and fiber swelling. Once the fabric begins to near a level of dryness the structure is aided by the reduced swelling, lower coefficient of friction, and the loops and yarns can begin to recover to the original state in which they were formed. This is why hanging clothes to dry results in no shrinkage- no heat or mechanical action!

As always , your questions and comments are welcome.