Chemistry Reference
In-Depth Information
8.2.1
Phase-change materials
Phase-change materials can absorb heat, resulting in an increase in the tem-
perature of the material. In reverse, the temperature of the material will
decrease during a cooling process. For typical textile materials, energy of
about one kilojoule per kilogram of heat is absorbed while its temperature
rises by one degree Celsius. Comparing heat absorption during the melting
process of a phase-change material (PCM) with those in a normal heating
process, a much larger amount of heat is absorbed if a PCM melts. A
paraffin-PCM, for example, absorbs approximately 200 kilojoules per kilo-
gram of heat if it undergoes a melting process. In order for a textile to absorb
the same amount of heat, its temperature would need to be raised by 200 K.
The heat absorbed by the paraffin in the melting process is released into the
surrounding area in a cooling process which starts when the PCMs reach
their crystallisation temperature. After comparing the heat-storage capaci-
ties of textiles and PCMs, it is obvious that by applying a paraffin-PCM to
textiles; the heat-storage capacities can be substantially enhanced.
In textiles developed for temperature-balance control using the PCM
principle, the PCM material is either present in a solid or a liquid form in
the textile structure and immobilised in a specific way. Active wear needs
to provide a thermal balance between the heat generated by the body and
the heat released into the environment. Only by using PCMs can the wearer
remain at a constant temperature. In other words, a steady state should be
obtained in heat exchange between the body and the textile structure.
Normal active-wear garments do not always fulfil this requirement. The
heat generated by the body during strenuous activity is often released into
the environment too slowly, thus resulting in a thermal-stress situation due
to heat accumulation in the body. However, during periods of rest, the body
generates less heat. Owing to this insufficient heat-exchange rate, hypother-
mia is likely to occur in extreme situations.
In order to ensure a suitable and durable active thermal-insulation effect,
a phase-change material should be incorporated in the textile material in
the correct way and in appropriate quantities. The selected PCM is normally
incorporated in a textile substrate by micro-encapsulation. Another pos-
sibility is deposition of the phase-change material at the textile surface, but
this results in poorer mechanical stability and lifetime. Selecting a suitable
substrate requires consideration of whether the textile structure is able to
carry the necessary PCM quantity to provide the required heat transfer rate
to and from the microcapsules. Further requirements of the textile substrate
in a garment application include sufficient breathability, high flexibility and
mechanical stability; in other words, the advantageous properties of con-
ventional textiles should not be lost. The substrate with incorporated PCM-
microcapsules needs to be suitably integrated into the garment design.
Certain design principles need to be taken into account
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