Protection of Dolomite Marble against Salts Weathering with Titanium Dioxide Nanocomposite Coatings

Stone and marble are main building materials of historic monuments and are subject to the action of environmental factors, leading to weathering. Weathering can be physical, chemical or biological and decay of stone is usually the result of a combination of these types. Pollution in the atmosphere is an important factor in causing decay. The presence of moisture increases weathering rate as water holds in solution and transports weathering agents on the material surface [1-5]. Typical such agents are sulphate, chloride, nitrite and nitrate ions that reacting with the hydrogen ions of water act as acids, attack the carbonate material and lead to the formation of sulphate, chloride and nitrate salts [6]. Crystallization and hydration of weathering products result in their expansion causing the degradation of the building materials. In most cases the stone surfaces are gradually covered by salts and black crusts containing calcium, magnesium, sodium, potassium sulphates, nitrates and other constituents. Also the water can easily penetrate and remain into the building stone materials, resulting in a destructive influence due to the absorption and evaporation of the moisture that affects their volume and causes cracks leading to the deterioration of the structure. Under these conditions, the stone surfaces disintegrate into powder and the building materials gradually lose their mechanical strength and their artistic form. In the case of marbles the main mechanism of deterioration is the sulphation of their surfaces, leading to the formation of gypsum layers on the stone surface, due to the solid state diffusion of Ca²⁺ [7-14].

As a result of the exposure in environmental weathering, moisture presence, salts precipitation and crystallization are main deterioration problems of the building materials of historic monuments. The presence of moisture in a material is due to capillary rise and falling damp. The total moisture content and the contributions of these forms in a specific material will depend on the amount and nature of the salts in the soils beneath, on the humidity and temperature. For salt weathering to occur there must be a combination of conditions of permeable material, available moisture, available soluble salts and evaporation. The presence of salts changes the liquid transport properties of marbles, influencing also their structure and surface stability. Repeated wetting and drying with seasonal changes lead to the cyclic precipitation of salts and the progressive decay of the building material. When the rate of evaporation from the material surface is low, the evaporative front may be at or very near the surface, in which case salt crystals will grow from the surface, as efflorescence. When the rate of evaporation is much greater, the evaporative front will be inside the wall and salts will crystallize within the pores of the material, as sub-florescence [15,16].

Polymeric coatings are widely used for the consolidation and protection of the building materials of historic monuments. These protective coatings must combine various properties such as hydrophobicity, transparency, durability, blocking liquid water entry and let it escape as vapour, chemical stability and stability to the alterations of climatic conditions and to the negative effects of the ultraviolet radiation from sunlight, compatibility with the surface of monuments, good penetration, solidification strength, appropriate simultaneous physical and mechanical properties, service life, aging history and reversibility of the application [17-19].

These polymers usually consist of synthetic organic polymers and hybrid organic-inorganic materials. Polymers based on various acrylics, known as Paraloids, are widely used in the conservation of monuments. As surface hydrophobicity increases lowering the surface energy, fluoropolymers are also known as protective materials because of their photostability, oil and water repellency, antifouling properties and minimized critical surface tension [2,18].

Hybrid organic-inorganic materials have extraordinary properties deriving from the combination of the different building blocks. In such systems, polymer blocks lead to good adhesion, toughness; flexibility and ease of processing and inorganic blocks improve the mechanical properties such as abrasion resistance, optical properties and heat resistance. The final properties of the hybrid systems are not only determined by the volume fractions of organic and inorganic components, but also affected by several parameters such as size, morphology and size distribution of inorganic particles, homogeneity of dispersion of organic and inorganic phases, amount of phase separation, morphology of hybrid system, molecular weight of organic polymer and its solubility in the sol-gel solution, and the number of reactive groups and coupling agents. Unfortunately some of these properties are competitive with each other [18].

The alkoxysilanes such as Methyltrimethoxysilane (MTMOS) and Tetraethoxysilane (TEOS) have widely been used as protective materials due to its low viscosity and its ability to form Siloxane (Si-O-Si) bonds. The alkyl-modified alkoxysilanes-based coatings are known as organic-inorganic hybrids systems because of their alkyl groups as organic component and silicon backbone as inorganic one. Many works reported the synthesis of organic-inorganic hybrids from various polymers and silica nanocomposite coatings containing different kinds of nanoparticles such as metal oxides and hydroxides, SiO₂, ZnO, ZrO₂, TiO₂, Ca(OH)₂, Mg(OH)₂, Ba(OH)₂, Sr(OH)₂, carbonates, silicon-containing compounds, organoclay compounds [18,20-27].

Many commercial formulations of stone consolidants based on alkoxysilanes (mainly MTMOS and TEOS) were synthesized. Such products are Wacker (OH, OH100), Conservare (OH100, H100), Silex (OH, H), Tegovakon (V, T), Funcosil (OH, 100, 300, 500), Brethane, Rhodorsil (RC-70, RC-80, RC-90). All these products contain monomer or oligomeric TEOS or MTMOS, solvents and a catalyst in various compositions. Some of these also contains a water-repellent component [20].

Aim of the present work was the study of the protective properties of nanocomposite coatings synthesized by adding TiO₂ nanoparticles to TEOS based commercial products, Rhodorsil RC-70 and RC-90, on a dolomite marble substrate against salts weathering.

White Greek dolomite marble of Thassos (86% dolomite, 12% calcite, 2% quartz, porosity 0.45%) was used as substrate. The dimensions of the specimens were 5x10x2 cm and before treatment were washed with deionised water and acetone, dried and weighted. Five series of specimens of dolomite marble were prepared. Bare marble and marble coated with four types of coatings, Rhodorsil RC-70, Rhodorsil RC-90, Rhodorsil RC-70 with 3% TiO₂ nanoparticles and Rhodorsil RC-90 with 3% TiO₂ nanoparticles. The preparation of coated marble specimens was realized by immersion in the corresponding solution for 30 min. The commercially available product Rhodorsil RC-70 contained 70% oligomeric TEOS, 30% white spirits and a neutral organoditin-siloxane catalyst. Rhodorsil RC-90 contained 70% oligomeric TEOS, methylphenylsilicone (water-repellent component), 24% white spirits, 6% toluene and a neutral organoditin-siloxane catalyst. TiO₂ nanoparticles with a diameter of 24nm (Degussa TiO₂ P-25, ~70% anatase and ~30% rutile), were used for mixing with the above polymeric materials. The mixtures were stirred vigorously for 20 min in order a homogeneous dispersion to be prepared. After the evaporation of the solvent the specimens were dried and weighted for the calculation of the polymer uptake by marble substrate. The morphologies of the surfaces were investigated by scanning electron microscopy (SEM, JEOL JSM 840 Α with Oxford EDS-WDS analyser).

Capillary absorption measurements were performed by the gravimetric sorption technique. The dried weighted specimens were placed on a filter paper pad (1cm of Whatman paper, No 4) partially immersed in distilled water or in three different saturated solutions of sodium chloride (NaCl solubility at 20ËšC 359gl^-1), sodium sulphate (Na2SO4 solubility at 20ËšC 195gl^-1) and 50%-50% mixture of these salts. The capillary tests were carried out for a period of seven days and the specimens were extracted and weighted to determine the amount of water absorbed by capillary forces and the amount of salts crystallized. After drying, the specimens were weighted again to determine the desorbed and the remained water. The investigation of the behaviour of the coatings, the moisture presence and distribution, the form and the amount of the salts crystallized on the surface as well as the height of the capillary rise were performed also by optical observation and X-Ray Diffraction (XRD). XRD measurements were carried out using a Rigaku Mini Flex II diffractometer with CuK_α radiation.

The accelerated aging tests were carried out in an aging cabinet (Votsch Industrietechnik, VCC 0034), for a period of three months, in 40ËšC temperature, 90% relative humidity, UV light conditions (253, 7nm, 25W). The investigation was performed by contact angle, colorimetric measurements and Fourier Transformation Infra Red (FTIR) measurements before and after exposure in aging conditions. Contact angle measurements were carried out by the sessile drop method using Attension Theta optical tensiometer apparatus and distilled water. Water droplets (5-7μL) were delivered to different points of each specimen and from a height sufficiently close to the substrate so that the needle remained in contact with the liquid droplet. Then, the delivery needle was withdrawn with minimal perturbation to the drop and the image of the drop was captured immediately for static contact angle measurement. Colorimetric measurements for the examination of the effect of the polymer-particle mixtures on the optical appearance of the marble and the effect of exposure in aging conditions were carried out using a portable reflectance spectrophotometer MiniScan^® XE Plus (HunterLab Associates Inc., USA). The same three homogeneous spot areas of 4mm in diameter of each specimen were examined, before and after treatment, before and after exposure in aging conditions. The results were evaluated by the use of La b ⁎ coordinates of the CIE 1976 scale. The total colour difference (ΔΕ ⁎) is defined as;


L⁎: brightness (0 for black-100 for white), a⁎: red-green component (positive for red and negative for the green colours), b⁎: yellow-blue component (positive for yellow and negative for the blue colours).

FTIR measurements were carried out using a Spotlight 400 FT-IR Imaging System Perkin-Elmer spectrophotometer.

The results presented in this work are the average of the results obtained on three different samples of the same type of specimen.

The results of the calculation of the polymer uptake by marble substrate are shown in table 1.

Element	Spectrum 1 (wt %)	Spectrum 2 (wt %)
C	69.61	63.18
O	10.04	7.2
Mg	4.07	21.04
Si	7.78	0.49
Ca	1.16	8.02
Ti	7.34	0.07
Total	100	100

Element	Spectrum 3 (wt %)
C	78.47
O	3.96
Mg	0.2
Si	12.3
Ca	0.08
Ti	4.99
Total	100

Figure 4: SEM cross-section image of RC-70 coated marble.

In both cases of composite coatings titanium agglomerates were formed. Basic difference between the two composite coatings is that in the case of RC-70 + TiO₂ (Figure 1c) the agglomerates formed were not coated by the polymer. In the case of RC-90 + TiO₂ (Figure 1d), nanoparticles were best dispersed and agglomerates appeared coated by the polymer.

The difference in the surface morphology of RC-90 coating (Figure 1b) after adding TiO₂ nanoparicles (Figure 1d) is attributed to the incorporation and dispersion of the nanoparticles in the polymer structure. In both cases of coatings nanoparticles addition increased hydrophobicity due to the total change of surface morphology in the case of RC-90 or due to the presence of titanium agglomerates in the surface coating in the case of RC-70.

These observations were confirmed with the SEM-EDS analysis results (Figures 2-3). In figure 2, RC-70 + TiO₂ coating, titanium agglomerates not coated by the polymer (spectrum 1) are shown; while in the area of spectrum 2 the titanium presence is minimum. In figure 3, RC-90 + TiO₂ coating, titanium agglomerates coated by the polymer (spectrum 3) are shown.

In all cases of coated marbles the coating thickness was ~ 15μm (Figure 4). The cross-section images of the other coated marbles were similar.

The gravimetric results of the capillary tests are shown in table 2. All values in salts solutions were greater than the corresponding in water as in the first ones salts crystallization was observed also. The water absorption and salts crystallization was greater in the case of sodium chloride, lower in the case of sodium sulphate and intermediate in the case of salts mixture, as it was expected due to the lower solubility of sodium sulphate. All coatings used decreased water absorption. The results were better for Rhodorsil RC-90 than Rhodorsil RC-70, attributed to the presence of methyl-phenyl resin in the first one. The addition of TiO₂ decreased water absorption and salts crystallization also.

 

Figure 6: XRD analysis of the powder of the salts crystallized on the surfaces of the various series of specimens (A: Marble, B: RC-70, C: RC-90, D: RC-70+TiO2, E: RC-90+TiO2).

 

 

Figure 9: FTIR diagrams of a) RC-70 and RC-70+TiO2, b) RC-70 before and after exposing in aging conditions, c) RC-70+TiO2 before and after exposing in aging conditions, d) RC-90 and RC-90+TiO2, e) RC-90 before and after exposing in aging conditions, f) RC-90+TiO2 before and after exposing in aging conditions.

In the IR spectra of figure 9 the absorption bands corresponding to the Si-O bond, (1080-1130cm^-1), are distinctly observed in all cases. In the curves of the aged RC-70 (Figure 9b) and aged RC-90 (Figure 9e), they are wider and weaker, as expected. In figures 9a and 9d, it is shown that the addition of titanium oxide, as a result the formation of a better layer on the marble surface, as indicated from the stronger absorption bands. The influence of exposure in aging conditions is lower in the cases of RC-70 + TiO₂(Figure 9c) and RC-90 + TiO₂ (Figure 9f) coatings.

From the results of the present work followed that the main actions of the addition of TiO₂ nanoparticles in the polymer coating are the anti-UV protection and the improvement of hydrophobictity, in accordance with the results of previous work [24] where we have found that the addition of titanium dioxide pigments in a widely used consolidant, Paraloid B72, acts predominantly as a physical UV absorber and significantly improved the photostabilization of films against UV irradiation. These results are also in accordance with the results of other works where it was found that organic-inorganic hybrid coatings prepared with TiO₂ nanoparticles had hydrophobic properties and improved corrosion resistance of metals [28,29]. Also super-wetting/antiwetting surfaces with extremely high contrast of surface energy and liquid adhesion have fabricated with TiO₂-based surfaces [30].