USA Science and Engineering Festival: The Blog

Some of you may find this surprising, but I don’t mind ironing.  Unlike giving a lecture, writing a column or appearing on TV or radio, you get immediate gratification.  You see the results of your efforts.  Wrinkles that were there before are gone.  I suspect, however, that not everyone shares my enthusiasm for this task.  The textile industry realizes this as well and has responded by producing a variety of “durable press” fabrics that can withstand wrinkles.  But withstanding allegations of toxicity is more of a challenge.  In this case the hullabaloo is about formaldehyde, the chemical used to fashion garments that can come straight out of the washing machine and sidestep the ironing board.

Wrinkling is a direct consequence of the molecular structure of cellulose, the main component of cotton.  This polymer is made up of repeating units of glucose, but the important feature of cellulose, as far as wrinkling is concerned, is that adjacent molecules can form weak associations with each other.  These “hydrogen bonds” are responsible for maintaining the shape of the fabric.  But when cotton is moistened, water molecules insert themselves between the long chains of cellulose, cleaving the hydrogen bonds.  The cellulose molecules can now move relative to each other, and as the fabric dries and the water molecules evaporate, the hydrogen bonds reform holding the fabric in its new shape, which is usually wrinkled.  Another factor in wrinkling is the thickness of the cotton fibers.  Fabric woven with very fine cotton thread will crease less than fabric made with more coarse thread.

Heat can also disrupt hydrogen bonds, which explains how ironing works.  The weight of the iron flattens the fabric and the novel shape is then retained as the material cools.  Ironing with steam is especially effective because the added water molecules serve as an internal lubricant, breaking hydrogen bonds and allowing cellulose molecules to slide past each other.  As heat is applied, the water evaporates, hydrogen bonds reform, and we have a smoothened fabric.  At least until it gets moist from perspiration.

The earliest attempts to reduce wrinkling made use of starch, possibly as early as 800 BC.  Like cellulose, starch is made of repeating units of glucose, and the two substances have an affinity for each other.  A starch solution readily penetrates into cotton fibers and when the fabric dries the molecules of starch bind to each other forming a hard network, stiffening the fiber.  An analogy would be a canvas water hose that can be easily folded when empty, but becomes stiff when filled with water.  The problem with starch, though, is that it comes out in the wash and has to be applied each time.

The early part of the twentieth century saw the introduction of a number of synthetic resins such as urea-formaldehyde which presented textile manufacturers with an opportunity to “stuff” fibers with a substance that would not wash out as readily as starch.  It worked reasonably well, but when the chemical structure of cellulose became clarified in the late twenties, chemists came up with another idea.  Given that wrinkling is caused by the movement of the cellulose molecules relative to each other, why not find a way to form bonds between cellulose molecules that are not as easily disrupted by moisture as hydrogen bonds?

Formaldehyde was just the molecule for this task.  It readily reacts with the hydroxyl groups on cellulose to form cross-links, much like the rungs of a ladder.  The idea is to form the fabric into the desired shape and then treat it with formaldehyde to retain that shape.  Manchester textile manufacturer Tootal Broadhurst Lee was the first company to use this process commercially back in the 1930s, producing the world’s first “wrinkle-free” garment, the “Tootal” tie.  But it wasn’t long before a wrinkle appeared in the novel technology.  The garments released the irritating odour of formaldehyde.  Not only was this smell unpleasant, but formaldehyde was also responsible for the allergic contact dermatitis reactions that emerged with the use of the durable press fabrics.  Later there would be increased concern about formaldehyde treatments as studies began to show that at least in animals the chemical was a carcinogen.

By the 1990s significant improvement had been made in technology to reduce formaldehyde release from treated fabrics.  During manufacture the fabric was subjected to huge rollers that squeezed out excess formaldehyde followed by heat treatment in an oven to cure the formaldehyde and prevent it from being released.  Another method that requires less formaldehyde, known as “vapour phase technology,” involves hanging moistened clothing in an airtight chamber and treating it with a gaseous mixture of formaldehyde and sulphur dioxide to form the required cross-links.

A number of other cross-linking agents have also been introduced, with the most widely used ones being dimethyloldihydroxyethyleneurea (DMDHEU) and ethylene-urea/melamine-formaldehyde (EUMF).  While effective at producing wrinkle-free fabrics, they, like formaldehyde, can still cause allergic reactions.  Exposure to permanent-press fabrics should always be considered as a possible cause of dermatitis that has no obvious trigger.

Although the amount of formaldehyde released from permanent press fabrics is unlikely to affect health except for rare cases of allergic dermatitis, manufacturers have made determined efforts to reduce formaldehyde exposure.  The textile industry uses a “Sealed Jar Test” to measure the amount of formaldehyde released from one gram of fabric under controlled conditions.  Since the early days of durable press, the amount released has been decreased by a factor of ten.  Cross-linking agents that do not release formaldehyde at all, such as dimethylurea glyoxal (DMUG), have also been developed but they cost more and do not perform quite as well.

The advent in the 1960s of polyester fabrics with a greatly reduced tendency to wrinkle took some of the pressure off producing wrinkle-resistant cotton fabrics.  But since not everyone likes the feel of polyester, even when blended with cotton, the prospect of producing a truly permanent-press cotton fabric still looms in front of manufacturers’ eyes.  They’re getting there though.  Recently I bought some shirts that barely need the touch of an iron.  And as far as formaldehyde exposure from such fabrics goes, well, I think there are far bigger wrinkles in the fabric of life to worry about.