Chemical Engineering of Surface Coatings for Natural Stones

Natural stones are materials widely used in today’s architecture. White stones (such as marble) and speckled stones (such as granite) are regarded as suitable for decorative purposes and as construction materials. A major challenge with natural stones is that they degrade in nature through weathering effects, such as UV exposure, acid degradation as well as mechanical weathering through salt crystallization and ice formation inside the pores of the stones.

 

The overall objective of this work has been to develop different hydrophobization methods suitable for natural stones, with the goal of minimizing water contact with the stone substrates and consequently deterring weathering effects. Another goal was to develop testing methods for the evaluation of the protective effects of hydrophobic coatings on natural stones, when weathering was concerned.

 

In this study, a hydrophobic pore-lining coating for marble stones was produced via static and dynamic application methods. In the static systems using a total immersion method, a successful hydrophobization of marble stone products with a fluorosurfactant was verified by water contact angle and capillary absorption measurements before and after intentional UV degradation of the outmost modified layer. Optimization of treatment time and solution concentration led to marble stones that were fully protected from water absorption, even if the outermost surface was degraded by UV weathering. The functionalization method for hydrophobization of porous marble networks was further optimized by controlling the vesicle behavior of fluorosurfactants via tuning the solvent polarity and reaction temperature, leading to successful functionalization deep inside the porous network of the marble stones. This was confirmed by removing the outermost surface of the coated stones through mechanical grinding and measuring capillary absorption on the new surface, showing an increase in effective functionalization depth from µm to mm scale. The functionalization process was furthermore refined through the use of a spray coating method. The optimized spray coating conditions yielded an effective penetration of the fluorosurfactant into the pores of marble, ensuring a long-term protection against water uptake through pore wall functionalization. The final coating also displayed anti-graffiti properties, allowing for graffiti paint to repeatedly be easily washed away with a pressure washer 3-4 times. A new coating could be applied when the first coating lost its effectiveness.

 

Lastly, a durable surface coating based on polydimethylsiloxane for granite was studied to inhibit aging effects related to weathering of granite. The impact of several common weathering effects was evaluated by determining the mass loss, water contact angle, capillary water absorption and possible color alterations before and after the surface treatments. An important discovery was that the impact of the weathering tests on the pure granite was in most cases quite small, however, still noticeable and in some cases even considerable (especially concerning salt weathering). Furthermore, it was found out that sufficiently thick PDMS coatings displayed a superior stability in all the studied weathering tests compared to a reference coating, as the impact of weathering on the hydrophobic functionality of the PDMS-coated samples was minimal. Furthermore, due to the excellent water and salt blocking behavior, the PDMS-coated stones displayed an overall better stability against weathering compared to the untreated stones.