Workers in cold storage facilities spend their shifts at temperatures of −18 to −30 °C, alternating between lifting heavy packages and operating forklifts while remaining almost motionless. Conventional cold protective clothing provides the same level of thermal insulation regardless of the wearer’s physical activity. But what if clothing could adapt to changes in the body’s heat production?
This question was explored by Dr Barbara Pause, founder of Textile Testing & Innovation, a company based in Colorado, USA, during the fifth webinar in the series organised by TTK University of Applied Sciences. She introduced the use of phase change materials (PCMs) to store and release heat in response to changing thermal conditions.
The webinar series forms part of the programme leading up to the 12th European Conference on Protective Clothing (ECPC2027), which will take place at TTK University of Applied Sciences in May 2027.
How phase change materials work
Phase change materials (PCM) absorb large amounts of latent heat as they melt and release it as they solidify, while their temperature remains almost constant throughout the phase transition. Ice provides a useful example: melting ice absorbs approximately 335 J/g of energy — equivalent to the energy required to heat water by around 80 °C.
More than 500 PCMs exist, differing in their phase transition temperature ranges and heat storage capacities. Paraffins are widely used in textiles because they are stable, non-toxic, non-corrosive, readily available and relatively inexpensive. Pure paraffins can have a latent heat storage capacity exceeding 200 J/g.
More cost-effective blends, such as those containing approximately 80% octadecane, have a somewhat lower storage capacity but offer a wider melting range. Bio-based fatty acids, such as myristic acid derived from coconut oil, offer similar properties. Salt hydrates, meanwhile, are used mainly in technical applications.
From capsule to fabric
Because PCMs alternate between solid and liquid states, they must be contained to prevent leakage when they melt. One solution is microencapsulation, in which the PCM is enclosed in capsules measuring 5–30 µm in diameter. The core accounts for approximately 80% of the capsule’s volume and is surrounded by a thin polymer shell, typically made of acrylic resin. Encapsulated PCMs are available in different types with phase transition temperatures ranging from −10 to 50 °C. Each type operates within its own relatively narrow temperature range.
There are three main ways to incorporate these capsules into textiles:
- Fibres and yarns. PCM can be incorporated into wet-spun acrylic fibres or bicomponent polyester fibres. These fibres can contain 5–7% PCM by weight. Air permeability is generally maintained, but heat storage capacity remains limited.
- Coatings. Acrylic, polyurethane or silicone coatings can contain up to 50% PCM by weight, allowing them to store considerably more heat while reducing the fabric’s breathability. Screen printing, which applies the PCM-containing compound to only part of the fabric surface, offers a compromise.
- Polymeric films. PCMs can also be incorporated into polymeric films, such as silicone films. In some cases, cross-linked paraffin can be used without encapsulation.
The active thermal barrier
Contrary to a common assumption, PCM incorporated into clothing does not necessarily release its stored heat directly towards the skin. Its main function is to regulate heat transfer according to the temperatures of the surrounding environment and the different garment layers. As physical activity increases, the body produces more heat than the garment system can transfer to the environment. In this situation, the PCM absorbs some of the excess heat. When physical activity decreases and the body produces less heat, the PCM releases its stored heat, helping to reduce body cooling.
PCM therefore does not directly increase a fabric’s thermal insulation. Instead, it buffers changes in heat transfer within the garment system and helps slow the body’s cooling.
Garment design also plays an important role. A thin, dense substrate helps heat reach the PCM and supports heat absorption, while a thick, lofty insulation layer helps retain the released heat within the garment system for longer.
Garment design also plays an important role. A thin, dense substrate helps heat reach the PCM and supports heat absorption, while a thick, lofty insulation layer helps retain the released heat within the garment system for longer.A PCM selected for one garment layer may not work at all in another if its phase transition temperature does not match the temperature range of that layer. This mismatch can help explain why PCM-integrated garments do not always deliver the expected results.
Developing a garment system for cold storage workers
Dr Barbara Pause and her company, Textile Testing & Innovation, developed a garment system for cold storage workers based on actual working conditions in cold storage facilities. The work involves alternating between physically demanding tasks and forklift operation, which requires relatively little movement. This creates contrasting demands on protective clothing: the body generates substantial heat during physical work, but heat production decreases considerably when a worker is sitting on a forklift.
The aim was to adapt the thermal performance of the garment system to changes in the wearer’s physical activity. To achieve this, the team developed a computer model and software to simulate the relationships between the body’s heat production, temperatures within the garment layers and heat transfer. These calculations helped determine the appropriate PCM type, quantity and placement.
The starting point was an existing garment system consisting of three main components: underwear made from 100% wool, a work suit and a cold protective suit with a liner, insulation wadding and an outer shell. The researchers analysed how different work tasks affected the body’s heat production and modelled temperatures throughout the garment layers to identify the appropriate PCM transition temperature, quantity and location.
The software-supported method was based on workers’ actual schedules. The analysis considered 90-minute work periods followed by 30-minute warming breaks. Depending on the task — sorting, forklift operation or inspection — the body’s heat production ranged from 70 to 200 W.The researchers calculated heat balances based on 20–30 different activity profiles representing the tasks performed throughout a working day. These calculations were used to determine the amount of PCM required and its optimal position within the garment system.
The most suitable location proved to be between the liner and the insulation wadding. The thin, dense liner allows heat to reach the PCM, while the lofty insulation wadding helps retain the released heat.
The calculations indicated that the suit required approximately 30 kJ of latent heat storage capacity. This could be provided by around 155 g of hexadecane microcapsules, with a latent heat storage capacity of approximately 190 J/g. The capsules were screen-printed in a polymer compound onto a lightweight nonwoven substrate. The solution increased the suit’s weight by only 10–15%.
Results in a real working environment
Field tests compared the performance of cold protective suits with and without PCM during different work phases: forklift operation, lifting and carrying packages, and a subsequent period of forklift operation.The PCM-equipped garment maintained a more stable microclimate temperature, largely within the thermal comfort range. By contrast, the temperature inside the garment without PCM dropped considerably during forklift operation and rose sharply when the wearer handled packages.
By slightly increasing the amount of PCM, the researchers were able to extend the working period from 90 to 110 minutes. This reduced the need for warming breaks in a heated area. Dr Pause’s book, published by Cambridge Scholars Publishing, provides a more detailed account of the methodology and results.
Key conclusions
PCMs can improve the thermal comfort of protective clothing in cold working environments where workers alternate between different levels of physical activity. Their performance depends on selecting a material with a suitable phase transition temperature and positioning it correctly within the garment system. In the solution studied, placing the PCM between the liner and the insulation wadding proved most effective.
For the cold storage garment system, increasing the amount of PCM made it possible to extend the working period from 90 to 110 minutes while increasing the suit’s weight by only 10–15%. Improved thermal comfort can help create better working conditions and support workers’ ability to perform their tasks. However, the impact on productivity requires separate evaluation.
The webinar is available to watch in full HERE.
Learn more about the ECPC2027 conference and the webinar series: https://ecpc2027.ee and ECPC LinkedIn.
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