*Article* **Rheological Behavior of Spectrally Selective Coatings for Polymeric Solar Absorbers**

**Mohor Mihelˇciˇc 1, Marta Klanjšek Gunde <sup>2</sup> and Lidija Slemenik Perše 1,\***


**\*** Correspondence: lidija.slemenik.perse@fs.uni-lj.si

**Abstract:** Since the world's energy demands are growing rapidly, there is a constant need for new energy systems. One of the cleanest, most abundant, and renewable natural resources available is solar energy; therefore, the development of surfaces with high absorption of solar radiation is increasing. To achieve the best efficiency, such surfaces are coated with spectrally selective coatings, which are strongly influenced by the pigments and resin binders. Spectrally selective paints have a very specific formulation, and since the applied dry coatings should exhibit high spectral selectivity, i.e., high solar absorptivity and low thermal emissivity, the rheological properties of liquid paints are of great importance. In the present work, we studied the effect of the rheological properties of liquid thickness-insensitive spectrally selective (TISS) paints on the spectral selectivity and adhesion of dry coatings on a polymeric substrate. The results showed that the functional and adhesion properties of dry coating on polymeric substrates is strongly dependent on the rheological properties of the binder and catalyst used for the preparation of the liquid paints. It was shown that the paints with good spectral selective properties (thermal emissivity *eT* < 0.36 and solar absorptivity *aS* > 0.92) and good adhesion (5B) can be prepared for polymer substrates.

**Keywords:** spectrally selective paints; polymer solar absorbers; rheological characterization
