Elsevier

Progress in Organic Coatings

Volume 55, Issue 2, 1 February 2006, Pages 160-167
Progress in Organic Coatings

Silicone-containing polymer matrices as protective coatings: Properties and applications

https://doi.org/10.1016/j.porgcoat.2005.09.012Get rights and content

Abstract

The paper addresses the technologically important problem of porous building materials protection, e.g. sandstone, limestone, marble against soiling. Protective coatings applied to the surface of porous building material should be characterized by very efficient water vapour permeability and self-cleaning properties.

The objective of this work was to examine the properties of silicone-containing polymer matrices as coating materials. Fluoropolymers, polyolefins and acrylic resins were applied as organic components of the polymer matrix. The surface properties of protective coatings obtained from silicone-containing polymer matrices were studied based on wettability (dynamic contact angle and surface free energy), morphology (atomic force microscopy, AFM) and chemical composition of the surface nanolayer (X-ray electron microscopy—ESCA/XPS). Morphology, roughness and structural regularity of protective coatings were discussed to show the effect of organic component on the surface properties. The application properties of such coatings materials were tested based on anti-soiling tests, water vapour permeability, water absorption and weathering resistance (freezing and salt resistance). Very good correlation of application properties of these coating materials and their surface properties such as high dynamic contact angle, low surface free energy, surface morphology was observed.

Introduction

Porous building materials especially when applied in highly-polluted urban or industrial areas are exposed to soiling due to environmental pollution. The most corrosive substances widely present in the atmosphere taking part in soiling processes are dusts suspensions and gaseous products such as SO2, NO, F and products of their chemical transformations in air. The most dangerous are soot particles from smoke of size <1 μm. Dust particles, gases and water act as aggressive agents contributing in the destruction of building materials by formation of soiling and hard, non-porous patina which is non-permeable barrier to salts and water vapour moving through building material. Moreover, these pollutants increase the salt content in building materials leading to micro-cracks formation due to significant changes of salt volume occurring in the crystallisation processes. The crystallisation of salts under the layer of patina can lead to delamination of this layer together with the surface layer of building materials [1]. The other dangerous factor is biological corrosion which is increased by deposition of nitrogen compounds taking part in the growth of fungi and algae [2].

The adsorption of dusts and water-soluble air pollutants decreases with decreasing wettability of the surface of porous building material measured by contact angle (CA) and surface free energy (SFE). Recently, anti-soiling coatings, especially self-cleaning ones, have become a focus of interest in the field of architectural coatings. Based on the literature survey the following solutions of anti-soiling protection of building materials reducing the cleaning efforts are possible:

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    hydrophilic coating increasing the cleanability [3]

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    oleophobic coating reducing the amount of dirt adsorbed on the surface [4], [5]

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    oleophobic-hydrophilic coating increasing the cleanability and reducing the amount of adsorbed dust [3]

The value of contact angle measured for hydrophobic surfaces (θ > 90°) is affected not only by chemical composition and structure but also by the morphology of the surface. Higher CA values measured for so-called composite surfaces are connected with the morphology of the surface [6]. High CA value for composite surfaces can be achieved only when the texture of the surface is very regular leading to low value of CA hysteresis. Lotuss Effect® surface [7] is one of the concepts of such morphology based on the lotus leafs surface self cleaning behaviour described by Barthlott [8].

Moreover, self cleaning surfaces were defined by Nun [7] with the following four parameters: contact angle, roll-off angle, contact angle hysteresis and change of colour after soiling test. Based on our earlier results, it can be stated that satisfactory anti-soiling properties of coatings were mainly connected with the following crucial parameters: high advancing contact angle (ACA) value, regular surface morphology with spherical domains.

The main goal in the area of self cleaning surfaces is to achieve permanently immobilized proper architecture of the surface layer of anti-soiling coating. One of the possibilities to solve this problem is anti-soiling coating based on the concept of silicone-organic polymer matrix [9].

Section snippets

Experimental

Silicone-containing polymer matrices formed by siloxane cage penetrated by fluoropolymer, polyolefin and acrylic resins as organic modifiers were prepared and examined. The silicone cage formation was based on the following crosslinking reactions of functional polysiloxanes:

Hybrid polymer systems dispersed in water were synthesized as precursors for formation of crosslinked polymers. The following types of hybrid polymer systems dispersed in water were synthesised:

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    type I:

Surface properties of silicon-containing polymer matrixces

The surface properties of protective coatings obtained from silicone-containing polymer matrices were studied based on wettability, surface morphology and chemical composition of the surface nanolayer.

The wettability of these coatings received from silicone-containing polymer matrices was studied based on dynamic contact angle (DCA) measurements and surface free energy (SFE) calculations. The values of measured and calculated wettability parameters are presented in Table 1. The highest values

Conclusions

The performance of the silicone-containing polymer matrix based on silicone-acrylic-polyolefin polymer system indicates that this polymer matrix shows promise for antisoiling protection of porous building materials in comparison with pure silicone or other treatments. The advantages of coatings obtained from silicone-containing polymer matrices are the consequence of their ability to crosslink in situ after treatment of substrate. Due to the fact that the crosslinking reactions through several

Acknowledgements

The authors would like to acknowledge the valuable contribution of Dr. Robert Nowakowski (Institute of Physical Chemistry Polish Academy of Science) for AFM measurements and Dr. Janusz W. Sobczak (Institute of Physical Chemistry Polish Academy of Science) for ESCA/XPS measurements.

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