Beating the heat with cooling fabric technologies

Charles Ross, outdoor industry specialist, speaks to Alice Davies about developments in cooling fabric technologies for sports and outdoor applications. 

On 18-19 April 2018, Performance Days returned to Munich for its summer edition and 10-year anniversary event. With 182 exhibitors and 22% more visitors than the previous year, this edition was the biggest yet. A full programme of presentations and workshops was also on offer, predominantly addressing the focus topic of this year’s event: cooling technologies.

One of the event’s numerous panel sessions was chaired by Charles Ross, who has worked and educated in the sports and outdoor industry for the last 30 years. Ross spoke to WTiN about his thoughts on the current approaches to cooling fabric technologies and the trends shaping future developments.

Mechanisms of cooling

Thermal management is an important feature of functional clothing, being essential to maintaining the comfort of the wearer both during activity and while at rest. While exercising, the human body converts energy into heat, which raises the core body temperature. Heat and sweat are emitted to the environment in order to cool down the body and ensure it stays within a safe and comfortable temperature range. Clothing creates a barrier between the human body and the environment so, if not properly designed, clothing can interfere with this important natural cooling mechanism – which can not only be uncomfortable for the wearer but potentially dangerous, too.

Particularly in hot conditions, the fabrics worn during sporting activities should ideally facilitate cooling to regulate the body heat generated by the activity as well as from the ambient temperature, as Ross says: “The human body functions best between 36 and 37.5°C. Within that boundary, the body works efficiently but outside of this range there are profound physical and neurological changes. So, cooling fabrics enable the body to cool within this temperature range in an efficient way,” But, Ross notes there is a risk that some technologies might be too effective and cause an ‘over-cooling’ effect, especially if the cooling function continues after the body has stopped producing excess heat, so there is a delicate balance that must be reached.

Moving moisture

The human body transfers heat to the environment by a number of different mechanisms, namely conduction, convection, radiation, and evaporation. As was highlighted at the Performance Forum at Performance Days, a number of fabric and garment features need to be considered to facilitate cooling by all of these mechanisms. Ideally, cooling fabrics should be reflective to visible light to allow heat loss by radiation, the garment should include ventilating areas such as open slots or mesh for water vapour to escape by convection, and the whole system should facilitate rapid and wide distribution of sweat close to the skin surface to create more area for evaporation. Ross explains that most of the cooling technologies in the market are focused on promoting evaporative heat loss. He says: “The main approaches to achieving a cooling effect are about moving moisture as this is a very efficient way of transferring heat. That’s why everyone wants to talk about the efficiency of evaporation when they are developing new technologies.” 

Fabric or finish?

The transmission of moisture and moisture vapour can occur via diffusion through fabric pores, migration along fibre surfaces, and absorption/adsorption within fibres. This transmission is therefore dependent on the adsorption, absorption and wicking properties which are influenced by the fibre chemistry as well as the fabric structure. Fabric developers can create cooling effects by manipulating these properties, either by altering the fibre and/or fabric structure or by altering the surface chemistry of fibres through the application of functional finishes.

Ross explains that the key developments have centred around natural fibres and finishes. He says: “The progress of synthetics has been strong, but the trend now is towards the wide array of additional benefits that natural fibres can offer.” During the Performance Days panel session, ‘Effective cooling by fibre construction’, Ross put the question to the panellists: What can a fabric do that a finish can’t? While all panellists agreed there is a lot that finishes can offer, they also appeared to be in agreement that the same functionality is often achievable through altering fibre and fabric constructions alone.

Mark Taylor, from the Performance Clothing Research Group at the University of Leeds, says: “Finishes change the nature of fabrics and fibres, so lots of companies might have a wicking finish on the base layer to try and move moisture, but you can do that mechanically with the fabric if you want”. As Esther Inwood, designer at Paramo made clear, one benefit to achieving cooling through fabric or fibre construction is the permanence of the effect. By contrast, even the most durable finishes may lose functional performance over time due to laundering, abrasion and general wear and tear. This can also present additional challenges. Gary Smith, CEO at Polartec, says: “I’m not a huge fan of finishes. Finishes go on and come off. A finish is maybe made out of something today that can be viewed as good and we find out later on that they’re not so good when they come off into the environment”. Tony Simpson, operations manager at HD Wool echoed this sentiment, stating: “We would rather work with natural fibres. Why alter a material, when that material can do the work for you?” 

Novel approaches and developments

Beyond fabrics or finishes, other novel cooling technologies have begun to enter the market, such as phase change materials, which can absorb excess energy, and plasma modification methods that can alter fibre surface properties without chemical finishes. Ross touches on the areas of development he expects the industry to focus on in the future as well as the novel approaches to cooling that he would like to see realised. He says: “In the future I expect that the science will concentrate on nanotechnologies, but I also expect to see more cross-functionality with odour control as well as a better application of phase change materials. Personally, I would like to see something in terms of electronics where the excess energy could be captured for later use.”

Sustainability is no longer a trend but a necessity, and Ross expects this will also be a key driver of future developments in cooling fabric technologies and the sports and outdoor industries more broadly. Ross believes Generation Z consumers are influencing this change in approach: He says: “As an example of generational difference, older generations have grown up wanting to own physical souvenirs of their experiences, whereas the younger generations now are more accustomed to things such as digital subscription services and there is less emphasis on ownership. There is a different mindset with the younger generations and they are increasingly conscious of the impact of what they are buying.”

Challenges and testing

A continuing challenge for developers of cooling fabric technologies is the difficulty correlating lab-tested performance with true performance in the field. A number of simple, standard test methods are commonly used to assess these fabrics in the lab. These are typically methods aimed at assessing the fabric’s moisture management properties – for example wicking ability, absorption rate and wetting/drying times, among others. More complex testing equipment is also available – which takes account of multiple factors, therefore offering a more comprehensive assessment of performance.

For example, the moisture management tester (MMT) simultaneously measures several liquid management properties by using sensors to record changes in electrical resistance as liquid is transported through a fabric, while the sweating guarded hotplate (SGHP), often referred to as the ‘skin model’, measures thermal properties and water vapour resistance of fabrics while simulating both dry and wet skin conditions. However, the major drawback of lab-based tests, even those that are more sophisticated, is that, by definition, they are performed under controlled, steady state conditions, with external factors such as temperature, humidity and air flow remaining constant – it is highly unlikely these fabrics would ever be worn under such conditions. Lab-based methods are certainly useful for comparing fabric performance in a controlled and repeatable manner, but good performance in the lab might not always correlate to good performance during conditions of wear.

A possible solution for replicating wear conditions more closely in the lab is to test garments on a sweating mannequin and in a climate chamber. This allows for factors such as garment fit, positioning of vents, body movement and variable climactic conditions to be accounted for. This approach has the advantage of still being controllable and repeatable, while providing objective assessment data. But thermal comfort is a relatively subjective sensation and Ross notes that the psychological aspect of thermal comfort, which cannot be accounted for by a mannequin, should not be underestimated. He says: “The aesthetics of the garment have a major influence, as does the fit of the garment. Of course, the fabric technology is important, but it needs to be balanced with these other factors as they have a very real influence on the wearer’s perception of cooling performance”.

Ross also comments that when humans are in conditions to require cooling, it is unlikely that they will be wearing complete body coverage and it is difficult to assess cooling technologies when the cooling effect of exposed skin is not accounted for. He says: “In hot weather we wear shorts and short sleeves, leaving areas of skin exposed. The exposed skin has a more dramatic cooling effect than the textile-covered areas and the fit and comfort of these garments can have a greater effect in our mind than their cooling properties”. For these reasons, Ross believes it is especially valuable to test cooling fabrics by wearer trials using experienced testers, and his panellists at Performance Days all agreed.

Other applications

Beyond activewear, cooling fabrics can be utilised for a number of other apparel applications, particularly apparel for wear in extreme conditions such as space and deep sea exploration or military uniforms, where the cooling effect can significantly contribute to the safety as well as performance of the wearer. Another potential benefit of fabrics with thermal management properties is that their ability to keep the body at a comfortable temperature reduces the need to change the temperature of the surrounding environment, thus they could help to reduce the energy required to cool and heat buildings.

To learn more about thermal management technologies, read WTiN’s report, ‘Enhanced thermal comfort and moisture management for activewear’.