Dr Melanie Windridge climbed Mount Everest earlier this year, and made a series of videos about science in the mountains. As part of her research on the chemistry of outdoor clothing, she spoke to Alice Coles-Aldridge from the University of St Andrews.
AUTHOR: Alice Coles-Aldridge
Despite the English gentleman’s tweed jackets worn by George Mallory and Sandy Irvine at base camp, the pair were in fact very well-prepared to ascend Mount Everest in 1924. Their ascent attire consisted of thin layers of natural fibres: silk, for wind-proofing, and cotton and wool for insulation, with an outer layer of gabardine (made from worsted wool and developed by Burberry in 1879).
Picture: Melanie Windridge
These natural fibres were ideal for trapping air next to the skin and were also significantly lighter than the typical modern down suit. However, Guy Cotter, CEO of Adventure Consultants, a mountain-guiding company to the world’s highest peaks, says, "If I was climbing Everest from the north side or climbing Everest without oxygen or winter climbing in the Himalaya I’d definitely want a down suit". Down is a key material in the modern mountain climber’s garment selection.
Materials for mountains
Down is the layer of feathers closest to a bird’s body. It traps air and heat but allows moisture to escape and evaporate. "It is not so much the chemistry as the structure that makes down so effective", says Matthew Fuller, product engineer at Mountain Equipment.
However, down cannot repel moisture and loses its insulating properties when wet. This is because moisture causes the down fibres to stick together meaning down loses its fluffiness or 'loft' and therefore its ability to trap air. Synthetic materials can be used to overcome this.
Melanie Windridge (right) on Everest.Picture: Melanie Windridge
PrimaLoft® or 'synthetic down' is a mix of 80–95 weight % polyester microfibres and 5–20 weight % thermoplastic macrofibres. Unlike natural down, PrimaLoft® keeps 96% of its insulating capability when wet due to the hydrophobic and microporous polyester fibres. Fine fibres minimise heat transfer but lack strength, particularly in wet environments where the surface tension and capillary forces associated with water are significantly greater than gravitational or other loading forces. This is why Primaloft® contains two types of fibres; polyester fibres of 3–12 µm diameter with optimal thermal performance and thermoplastic macrofibres of 12–50 µm diameter with good mechanical strength.
Another way to improve the insulating properties of down in wet environments is with a hydrophobic fluorochemical or silicone coating. This works due to the contact angle between the water droplet and the material surface. A hydrophobic coating will give a large contact angle, meaning the water droplets will bead and fall off the surface of the material.
However, this does not make down fully waterproof. For that a waterproof hard-shell is needed.
One example of this is Gore-Tex®, a waterproof hard-shell with a ‘breathable’ fabric membrane composed of microporous polytetrafluoroethylene (PTFE), more commonly known as Teflon, and hydrophilic polyurethane within the PTFE micropores. Breathability is key to climber comfort to avoid perspiration building up inside the waterproof garment. Perspiration needs to get out whilst water is simultaneously prevented from getting in.
The microporous hydrophobic outer layer permits the passage of water vapour (from sweat) but resists the infiltration of water droplets (from rain). The micropores are created by heating PTFE above its crystalline melting point after it has been expanded.
Picture: Melanie Windridge
The hydrophilic inner layer also permits the passage of water vapour and prevents bodily oils, which reduce waterproofness, from reaching the hydrophobic outer layer. The inner layer is considered hydrophilic because of its ability to transport water from inside to outside on a molecule-by-molecule basis. This is possible because the partial pressure of water vapour inside the garment exceeds that outside the garment in almost all climates. Even when it rains, insulation means temperature is higher inside the garment than outside, maintaining the vapour pressure difference.
An alternative waterproof hard-shell is Pertex®. It differs from Gore-Tex® in that is it a wicking fabric as opposed to a waterproof membrane, with its waterproofness arising from interlocking diamond or Y-shaped filaments. The breathability of Pertex® arises due to capillary action. Moisture moves from the larger filaments on the inside to the smaller filaments on the outside because the finer fibres on the outside have a greater surface area, allowing water to evaporate more quickly. This capillary action is possible due to the temperature difference between the body heat on the inside and the air temperature on the outside.
Melanie reached the summit of Everest at 4.30am (local time) on 21 May 2018.Picture: Melanie Windridge
Expeditions of the future
These materials are currently in use by mountaineers but future expeditions may make use of new technologies. The trapping of gas inside the pores of aerogels means they are the ‘ultimate insulator in many ways’ says Fuller. An aerogel is a ‘gel compromised of a microporous solid in which the dispersed phase is a gas’ (IUPAC).
Aerogels can be made from many different chemical compounds, have very low thermal conductivity and are very lightweight and strong. However, the problem with containing aerogels means they are not yet widely used in clothing. So far aerogels have been used in gloves, sleeping mats and insoles. If aerogels could be contained in fabrics, their use would rapidly expand.
Picture: Melanie Windridge
Significant advances have been made in synthetic materials, particularly when it comes to improving the water resistance of materials that do not traditionally behave well in moist environments. This is excellent for climbing in the Scottish Highlands. However, when it comes to much colder, but also much dryer places such as the Himalayas, natural materials such as down still reign supreme because ‘the warmth-to-weight ratio of down compared to other insulating materials is unparalleled’ (Matt Miller, Alpine Ascents).It may be some time before we see an all-synthetic Everest summit.
Alice Coles-Aldridge recently completed her PhD in Chemistry at The University of St Andrews. This article was provided as part of Dr Melanie Windridge’s Science of the Summit outreach campaign, which was supported by the Royal Society of Chemistry. For more information, visit her website.
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