Describing things from the natural world is always more challenging than describing man-made things, mostly because of the inherent variability and complexity in nature. After perhaps biting off more than I can chew with carrots, I’m stepping it down a notch. This week, I’m going to talk about what your store-bought winter scarf is probably made out of: acrylic fibre.
Alongside the natural fibres wool and cotton, acrylic fibre is one of the most common materials for warm winter clothes. Apart from its presence in ready-made clothes (like jumpers and cardigans) and accessories (like hats, gloves and scarves) found in clothes stores everywhere, acrylic is also one of the most popular fibres of yarn found in craft stores. It is a soft, lightweight fibre, which holds dye well and is very resistant to wear as well as washing.
In terms of its chemical composition, acrylic fibre is a fibrous polymer. A polymer is a large molecule made from several repeating units, which are called its monomers. It is common for commercial polymers to be made of more than one kind of monomer, as adding a percentage of different monomers can alter the properties of the entire polymer, such as giving it heat or wear resistance.
The primary monomer in acrylic fibre is acrylonitrile. The acrylonitrile content of acrylic fibre must be greater than 85%. Fibres with an acrylonitrile content of 35-85% are referred to as modacrylic fibres. Acrylonitrile, on the right, consists of two functional groups: vinyl (two carbons connected by a double bond) and nitrile (carbon and nitrogen connected by a triple bond). The vinyl group is what undergoes the reaction to polymerise this molecule into polyacrylate — acrylic fibre. The double bond between the carbon atoms is broken in favour of each of the carbon atoms forming a bond each with a different acrylonitrile molecule.
On the left is the structure of polyacrylonitrile, a polymer which consists entirely of the monomer acrylonitrile. The square brackets represent the repeating unit of the polymer, while the n represents the number of repeating units. This value is usually not a single number, but rather a range, as the length of polymer chains can be difficult to control precisely. As noted above, acrylonitrile is not the only monomer present in acrylic fibre, so the structure on the left is not entirely representative of its chemical structure — but as acrylic fibre should be at least 85% of this monomer, it is a fair approximation. The co-monomers used in the manufacture likely vary from manufacturer to manufacturer.
Acrylic fibre is found in everything from hats and gloves to carpets and upholstery to sweaters and cardigans. This material is testament to how versatile synthetic polymers can be — a far cry from the tough, hard stuff the word “plastic” evokes in our minds.
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