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Thermal Responsive

Thermally responsive textiles provide a response upon encounter of a pre-determined thermal stimulus. Textiles able to respond to a stimuli are defined as Smart or intelligent textiles. The response derived from the stimuli can be one to evoke a functional, protective or novel function. Thermally responsive materials will provide a physical or chemical change on incurrence of the programmed thermal range. Currently there are four types of thermally responsive materials in use or being developed for future applications under the heading of smart textiles, they are:

PCM's (Phase Change Materials)
SSM's (Shape Memory Materials) 
Thermoelectric Materials
Thermochromatic Materials
 

 

Thermal stimuli and response is useful in Technical Apparel, where thermoregulation of the body is paramount to protect the wearer and enable normal body function for uninhibited concentration and performance. Garments such as those for outdoor wear including mountaineering or rock climbing, encounter periods of extreme physical activity in combination with periods of inactivity, so the energy produced by the body dips and peaks, but the core body temperature must remain constant. The implementation of Phase Change Materials provide a intelligent solution to thermo regulation, harnessing the energy produced by the body and giving it back to the body when it is needed.

This same technology can be implemented into Sports and leisure wear, as well as Interior textiles, such as bedding, to aid the body to maintain a balanced temperature despite activity levels or environment.

In Personal Protective Equipment for workers that experience extreme temperature changes, such as those experienced by foundry workers, thermal regulation is a necessity. The materials used face a more intense challenge and provide a protective, rather than functional use, so need to be high performance. Phase Change materials used in this role require careful selection to ensure the materials used can withstand the temperatures they are exposed to and provide reliable, durable protection.

In Interiors smart textiles have been integrated into bedding for thermo regulation, in window dressing Shape Memory materials have been utilised to produce self opening and closing blinds. In construction a similar principle is applied with self opening and closing vents or greenhouse windows.

Thermo reactive materials have wide ranging applications in the Medical and Hygiene sector, enabling key-hole surgery of implants which can be extended into their full form once inside the body, such as stents used to hold open arteries.

In aerospace Shape memory alloys are being developed as self shape changing components to meet environmental requirements, adding or removing drag, increasing fuel efficiency while requiring no additional power source.

Thermochromic responses have been used in food packaging to indicate product temperature, most famously for Coors beers that used the technology as a novel indicator of when the drink was cold enough for maximum enjoyment, the same screen printing technology can be utilised in textile applications.

PCM's (Phase Change Materials) are those which transition between gas, liquid or solid states. The chemical reaction which occurs during the transition of states emits or consumes thermal energy. Microencapsulationhttp://en.wikipedia.org/wiki/Micro-encapsulation provides a stable way of incorporating this technology into textiles, and materials encapsulated, such as paraffin, are selected for the range at which they melt, solidify and their capacity for thermal storage and release; defined as Melting temperature, Crystallisation temperature and the heat storage capacity respectively.

This function is useful in thermoregulation. As PCM microcapsules encounter thermal energy it is absorbed and causes melting, this energy is then released as the PCM microcapsule re-solidifies. The energy transfer that occurs is much greater than that of a textile material alone, and application in technical apparel can provide protection from heat stress, by storing the heat away from the body, and provide heat release in a cold environment, protecting from hypothermia.

Unlike Microencapsulationhttp://en.wikipedia.org/wiki/Micro-encapsulation of active ingredients, Microencapsulated PCM’s can be applied directly into the fibre, ensuring a high durability, or to a fabric through traditional textile finishing techniques such as Padding? or Spraying?.

The heat absorbed or released by a PCM can be measured using Differential Scanning Calorimetry, DSC. The overall thermal capacity of PCM’s within a textile is this sum plus the thermal capacity of the textile alone.

The ability of PCM’s materials in textiles to sense and react thermally classes them as intelligent or smart textiles.

SSM''s ( Shape memory Materials) Shape shifting polymers have the ability to shift shapes in accordance to a stimuli, which can be thermal, photonic, electronic, magnetic or Ph. They are engineered to have a reactive range on which a mechanical change will occur; the material can hold two or three forms which it will transition between on occurrence of its stimuli.

Shape memory alloys are materials which possess the same characteristic; however they are based on alloys which often require a more extreme stimulus range, such as a high temperature so the usefulness of such in textile applications is restricted.

To be useful in textile applications the trigger temperature needs to be around that of body temperature and the application potential in garments is wide ranging. Some possibilities include that of a shape memory polyurethane coating, which when activated by body heat would shape to remove wrinkles and creases. The possibility is further extended into garments such as sportswear or PPE? of which air permeability is necessary for water vapour transfer, heat reactive materials within the fabric structure would allow the structure to expand, thus opening the interstices allowing further air/vapour transfer. Anti counterfeit measures such as labels which display text upon encountering heat is an area being used in brand protection.

Shape memory materials have a range of applications within the medical and hygiene sector, for example, implants can be put in the body through smaller incisions, such as stents to open arteries, or plates to bring bones back into alignment.

Anti counterfeit label video

The Science behind Shape Memory Materials

The shape change properties are possible because of the atomic modelling of the material. The 3 states of matter include, gas, liquid and solid, however there are two models of atomic structuring in a solid;

Martensite; occurs at a lower temperature, is more flexible, atoms can change position from pressure, allowing for deformity.

Austenite; Occurs at a higher temperature, is harder and more rigid.

Materials can be set in the required shape at the Austenite stage, at an extreme level not encountered under normal conditions, then revert into the martensite stage appearing to loose its shape. When encountering the temperature required for Austenite structure it will remember its set shape and revert to this, however this temperature is at a much lower level than that required to set it.

To understand the science behind Shape memory materials, view the video.
The science behind Shape Memory materials.
 

Thermoelectric materials convert thermal energy into electricity. Referred to as the Seebeck effect it requires temperature differences on opposing sides of the device to create a power output and it is this difference which determines the power output.

This concept is being explored as a possible power source in wearable technologies.

Developments in Thermoelectric textiles

The Centre for Nanotechnology and Molecular Materials have created a thermoelectrial material named Power felt which can convert the bodies own heat into an electrical current.

See article: Power felt gives a charge
 

Thermochromic Textiles convert thermal radiation to produce a chromatic effect, therefore altering the perceived colour to the human eye. The transition between displayed colours, in the instance of thermochromic, is reversible because it is entirely dependant upon temperature to provoke the colour change process. Other colour changing materials are available but the stimulus can vary from light, photochromic, to electrical current, electrochromic. The way this interaction occurs and is implemented into textile products varies because of the material; there are two types of material which enable this reaction;

• Liquid Crystals
• Leuco Dyes

Objects are perceived as a colour because of the wave length within the visible light section of the Electromagnetic Spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_spectrum the object does not absorb. If a material can change the way it interacts with rays within the light spectrum it will alter its perceived colour. 

Liquid Crystals

Liquid crystals are used where precision is a requirement as they can be engineered to react at a programmable temperate rate; applications of such include thermometers and LCD screens, and more novel products such as mood rings. Because of their liquid state, to be used in textile applications they are microencapsulated in an aqueous solution. To ensure maximum visual impact the microencapsulated product is often screen printed so it remains on the product surface. The temperature range at which they display different colours is engineered to meet the application, above and below the temperature range they will display black. The colour range they display follows that of the electromagnet spectrum within the visible light range, from red through to violet. The range between the red and the blue start is defined as the bandwidth. The temperature to evoke the colour change in the bandwidth can be as little as 1°C or much larger such as 20°C. 

A black background maximises visual impact, as black absorbs the light rays, ensuring the desired colour is the only one bounced back. Liquid Crystals work through a change in their molecular structure which alters the range of visible light they are able to reflect back, transitioning from black (no Reflectionhttp://missionscience.nasa.gov/ems/03_behaviors.html), through the visible light spectrum and then back to black.

Liquid Crystals in textile applications are sensitive to soiling and contact with foreign matter. This is influenced by the material used as the microcapsule shell wall, as this degenerates the capsules burst resulting in a loss of the effective ingredients. 

Leuco Dyes

Leuco dyes can use a wider range of colours although are limited to between two to three different colours in each application and with reduced accuracy in terms of temperature range. They are a dye so must be used over the secondary colour wishing to be displayed, for example a red Leudo dye over a yellow back ground, will change from red to transparent, thus displaying the yellow background, and perhaps a orange colour as an in between state. They are generally more robust and less expensive,

The dye pigments and the required solvent, which can be alcohol based such as ethanol, are microencapsulated. In a cool state the solvent will remain solid and the colour will be displayed due to electron interaction. When the temperature rises and the solvent becomes a liquid this electron interaction is disrupted and therefore no visible colour is displayed. Therefore Leuco dyes differ to that of liquid changing crystals as molecular structure does not change as a result of temperature, the solvent they are in reacts to the temperature and as a result of this mixes with the dye to provoke not a colour change, but a colour absence.

Video explaining how colour works:

Regulations and Standards

Regulations for smart textiles are available in the relevant section.

PPE? and Medical and Hygiene textiles are required to conform to the CE marking guidelines, and any performance requirements related to thermal responsiveness need to ensure efficiency in line with this.

Any product on sale to consumer needs to ensure claims are correct under the consumer claims directive EC 2005/29.

In smart textiles, such as those thermally responsive, particularly those with novel applications, this legislation is particularly relevant as it will act as a guide line to what claims can be made on a product sold to the consumer.  

Phase change materials contain compounds, such as paraffin which may influence the flammability of the product and this therefore needs to be considered in the application.