Ninguna ciencia, en cuanto a ciencia, engaña; el engaño está en quien no sabe.
(Miguel de Cervantes)
domingo, 10 de julio de 2016
The Chemistry of a Football Shirt – Euro 2016 Edition
With Euro 2016 in full swing, it seemed a good time to update this look at the chemicals that make up your average football shirt. Even if the tournament isn’t the kind of event to fill you with excitement, it’s still intriguing from a chemistry perspective to examine the different chemical materials used and the properties that they lend the finished shirt.
In the past, before polymers were widely used in clothing, football shirts were made from cotton, or even woollen materials. These had the obvious disadvantage of being a little on the warm side of things, and additionally soaked up any sweat produced, making them rather uncomfortable to wear. The first team to buck the trend of cotton shirts, and wear shirts made of an artificial material, were Bolton Wanderers in 1953’s FA Cup Final. Sadly, the precise material used doesn’t seem to be recorded anywhere that I could find, only being described as a ‘shiny material’.
It seems similarly hard to track down when exactly polyester shirts came into common use, but by the 1990s polyester shirts were the norm for the majority of clubs. Polyester is actually a name for a large range of polymers; polymers are long, chain-like molecules formed from many smaller molecules, often referred to as monomers. In the case of simple polymers, such as polyethene, the monomers are all the same, but in the case of polyesters, two different sets of molecules are needed: an alcohol and a carboxylic acid. The polymerisation reaction can be carried out in a number of ways – the most commonly utilised uses a diol (a molecule with two alcohol functional groups) and a dicarboxylic acid (a molecule with two carboxylic acid functional groups). It proceeds via a condensation reaction, giving off water as a byproduct.
Polyethylene terephthalate, often abbreviated to PET, is the most commonly used polyester. It has a huge range of uses – from plastic bottles, to food trays, to thin plastic films, and of course in clothing. In clothes, polyesters have a large advantage over the more traditional cotton fibres in that they absorb much less water. Cotton can absorb 7% of its weight in water, whereas polyester only absorbs about 0.4% of its weight. This makes it much less likely to get soaked in sweat during a game of football. Instead, the sweat can run down the fibres of the shirt and evaporate; because of this it is referred to as a ‘wicking’ fabric, or more generally as one that is ‘breathable’. It’s also durable, and doesn’t crease easily.
Whilst some football shirts are 100% polyester, it’s also common for other fibres to be woven in with it to alter its properties. Elastane is another polymer that’s often utilised – more commonly known as spandex or lycra. In its manufacture, a prepolymer is first formed from glycol and diisocyanate compounds, reacted in a 1:2 ratio. This prepolymer is then reacted further with a diamine, to produce a liquid of the elastane polymer. The liquid is subsequently spun in a cylindrical cell, and heated in the presence of nitrogen gas, to convert it to solid polymer strands.
Whilst elastane is not as breathable as polyester, it has other beneficial properties. One of these is that it can be stretched to approximately 600% of its length before eventually rupturing, a trait that’s very useful in the modern game where shirt-tugging is commonplace. It also easily returns to its original shape.
A final type of polymer commonly used in the manufacture of football shirts is polyurethane. Again, this is the name for a class of polymers, rather than a specific polymer; we’ve actually already mentioned polyurethanes in a previous post, as they’re also one of the polymers used in the manufacture of footballs. Polyurethanes are built up from compounds called isocyantes and polyols. The middle parts of these molecules can be varied to give different polyurethanes with differing properties. In football shirts, they’re often the material that the letters, numbers, and sponsors on the shirt are made from, although fabrics or other materials can also on occasion be used. They can be thermally bonded onto the shirt using a heat-sealer, and unlike other fabrics, they have the benefit of being water resistant.