.

Fig. 2: Thermal radiation and reflection: the emissivity is crucial
© ZAE Bayern

Fig. 3: Coating: metallic pigments in the carrier matrix
© ZAE Bayern

Fig. 4: Emissivities of various fabrics in the infrared spectral range
© ZAE Bayern

Fig. 5: Production unit
© ZAE Bayern

Fig. 6: Coated fabric with low-e properties
© ZAE Bayern
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Adding value to surfaces

Material properties

The light fastness and abrasion resistance of various fabrics coated using the large-scale unit were tested in accordance with DIN standards. The light fastness achieved a value of 7 (excellent) on the eight-point scale, and tended towards the highest light fastness level of 8. The rubbing fastness results were also good. A representative selection of some of the colour effects that can be achieved while fulfilling the low-e properties described can be seen in Fig. 6; further colours and tones are also possible here.

Alongside silvercoloured low-e fabrics with optimised properties, various colours in different gradations can now also be manufactured, with emissivities of around 0.3. The spectrum of solar irradiation ranges from ultraviolet via visible light to the nearinfrared range, and around 45% of the irradiation is in the near-infrared range. On the other hand, a surface at room temperature emits in the middle- to far-infrared ranges. The emissivity is a measure of material-dependent radiation behaviour; it is around 0.9 for standard construction materials. Low-e coatings make it possible to selectively reduce the emissivity in the infrared range. This reduces thermal radiation and, at the same time, increases the thermal reflection. This can be visualised using an infrared camera (Fig. 2). The body heat from the hand is more strongly reflected by the fabric on the left with low-e coating than by the fabric on the right without low-e coating.

Coating: Metallic and colour pigments in a PU carrier matrix

In physical terms, the electrical conductivity and the thermal radiation properties of a body are closely related. For this reason, metallic particles that are included in a carrier matrix lead to high reflectivities in the infrared range and, unfortunately, also in the visible spectral range. The addition of colour pigments (Fig. 3, left) or a covering with colour pigments (Fig. 3, right) can suppress the metallic colour effect and yield the desired colour. The researchers attained emissivities, ε, of around 0.1 for a silvery colour effect and around 0.3 for an arbitrary colour effect using aluminium particles. The spectral emissivity, ελ, measured for two coated fabrics is shown in Fig. 4 as a function of wavelength over the range of thermal radiation as compared to an uncoated fabric. The resulting thermal emissivity, ε, is also shown. While the emissivity of the conventional, uncoated fabric is 0.95, this value can be significantly reduced to 0.3 using a coating with a freely chosen colour or to 0.1 using a silvercoloured coating.

Coating method

Flat, disc-shaped aluminium particles with a diameter in the micrometre range have proved to be especially suitable for coloured, low-emissivity coatings. These particles can be successfully suspended in polyurethane along with suitable colour pigments, and then attached to fabrics by using a doctor blade. Experience which can be applied for industrial production is now being gathered with a prototype large-scale unit (Fig. 5).

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