- Research article
- Open Access
TEOS/PDMS-OH hybrid material for the consolidation of damaged pottery
© Zhao et al.; licensee Chemistry Central Ltd. 2013
Received: 31 January 2013
Accepted: 12 March 2013
Published: 16 April 2013
The use of organic–inorganic hybrid compounds containing tetraethoxysilane (TEOS) and hydroxyl-terminated polydimethylsiloxane (PDMS-OH) is one of the most promising approaches for using alkoxysilane-based formulations to improve the effectiveness of the consolidation of traditional silicate artifacts, such as ancient stone. Based on analysis of existing damaged potteries influenced by the crystallization of NaCl salt, the hybrid we prepared in this study provided a crack-free and homogeneous gel on the premise of appropriate PDMS-OH content. The consolidants were applied to samples prepared following procedures that simulate old pottery and the effect of the protective products was evaluated by characterizing the surface morphology, the determination of the consolidant uptake, color changes, water vapour permeability and several wet-dry cycles with salt. The best formulation found for the hybrid in the present work was made up of 10% PDMS-OH with TEOS, which showed a significant increase in compressive strength, with a value of 3.50 MPa at 3 mm depth of consolidation, similar to the sample treated with Primal SF(the commercial protective agents used for comparison purposes), which had 3.20 MPa in compressive strength at 2 mm depth of consolidation. Except for small color changes and excellent water vapor permeability, there was still no significant change, and the destructive effects from NaCl crystallization for the hydrophobic surface of the sample treated with hybrid material was different to what happened for the hydrophilic case in the original. Thus, this study has revealed, for the first time, the addition of an appropriate amount of PDMS-OH to TEOS helps to improve the mechanical properties, hydrophobic behavior and salt resistance of damaged pottery effloresced by the NaCl crystals; in addition small color changes and excellent water vapor permeability should also be taken into consideration.
At present, the employment of appropriate protective materials for consolidation is one of the most direct and effective methods to slow down the destruction of ancient silicate artifacts damaged by the salt decay. Cocca, M  studied the only Acrilem IC15 with excellent chemical-physical properties and durability in different commercial products such as Primal AC33, Primal B-60A, Acrilem IC15 and Acrilem IC79 for conservation of colored pottery, Vaz, M.F  also discussed the effectiveness of impregnation treatment with acrylic Paraloid B-72 on the properties of old Portuguese ceramic tiles. At the same time, the characterization and analysis of nanolimes, acrylic and vinyl polymers in nanocontainer solutions, as well as particle-modified consolidants have been applied to ancient stone conservation [3–5]. As we know, tetraethoxysilane and its oligomers were the main components in a series of commercial formulations known as stone consolidants, which were intended to preserve decayed stone of historical buildings [6–11]. Zárraga R., Kim, E.K., et al [12–14] studied stone consolidants with the addition of hydroxyl-terminated polydimethylsiloxane, silica nanoparticles and (3-glycidoxypropyl) methyldiethoxysilane as well as polyhedral oligomeric silsesquioxane to tetraethoxysilane. The results indicated organic–inorganic hybrid compounds provided the necessary flexibility to resist the stress imposed by capillary pressure and an important hydrophobic character was imparted to the stone.
There have been abundant reports about salt decay and the consolidation of damaged stone, fresco, wall painting, statues, tombs, ancient buildings and so on [15–32], but there is a scarcity of literature involved in the conservation of damaged pottery induced by soluble salt. It was necessary to study the properties of protective materials and the performance of a protective effect on damaged pottery samples, and evaluate negative effects induced by salt decay in the treated sample. Specifically, based on analysis of the existing damaged pottery influenced by the soluble NaCl salt, we used FTIR and SEM techniques to investigate how a TEOS-based consolidant formulation was influenced by the addition of PDMS-OH when DBTL was used as a catalyst. The simulated samples were prepared as two types to avoid the possibility of destroying scarce and precious relics during the protective process in our experiment. Mechanical tests, water absorption and water vapor permeability, as well as negative effects induced by wet-dry cycles with the salt measurements on the treated simulated samples, were also performed in order to determine the modification effects of this important consolidation.
Analytical grade tetraethoxysilane (TEOS) was obtained from Shanghai Lingfeng Chemcal Reagent Co., Ltd, hydroxyl-terminated polydimethylsiloxane (PDMS-OH) was purchased from Shanghai Meryer Co., Ltd, which had a viscosity of 35-45cst (molecular mass 700–1500) and an OH percentage of 3-4% w/w. Dibutyltindiauate (DBLT) was purchased from Aladdin Reagent Co. Ltd, which was used as a catalyst for the sol–gel reaction at a neutral pH.
Commercial protective agents Primal SF (methyl acrylate and methyl methacrylate copolymer, solid content 40%, Beiluo restoration technology) and WD10 (Dodecyltrimethoxysilane, WD silicon Co., Ltd.) were also used for comparison purpose.
Synthesis of hybrid materials
A series of five hybrid sols were prepared using as starting materials 10 ml of TEOS, and 1, 3, 5, 10 and 20% (w/w) PDMS-OH, respectively. Ultrasonic agitation of TEOS/PDMS-OH mixtures was performed for 24 h in order to obtain homogeneous solutions. After that, 1% (w/w) of DBTL catalyst was added to each sol, allowing ultrasonic agitation for five more minutes. Sol–gel reaction and drying occured by simple exposure of the cast sols to laboratory conditions at relative humidity of 60% and temperature of 25°C until a constant weight was reached.
Simulated pottery materials
Fourier transform infrared (FTIR) spectra of the gels were measured on a Spectrum 100 (Perkin-Elmer, Inc.) instrument equipped with an attenuated total reflectance (ATR) detector at a resolution of 4 cm-1.
The morphologies of the hybrid materials, the treated fired-clay simulated samples and the evaluation of salt effects were investigated using a JSM-6300 scanning electron microscope at accelerating voltage with secondary electrons under various magnifications to obtain the clear display (JEOL Ltd.).
The consolidant uptake, or the amount of polymer retained was measured gravimetrically. The pristine sample was weighed (W0) after being completely dried and then treated with a consolidant solution with immersion method for 24 h and dried at room temperature until a complete reaction occurred. The treated sample was weighed (Wt) after complete drying and the applied consolidant uptake was estimated using the expression: consolidant uptake (%) = (Wt-W0)/W0×100, where Wt and W0 were the masses of the samples treated with and without consolidant, respectively (in grams).
Compressive strength test was carried out on the overlying protected samples up to about 10 mm depth by means of an Instron-5500R universal testing instrument operating with a crosshead speed of 0.5 mm/min at a temperature of 25°C. The value of each sample was decided by the numbers of sample into the total value of the test results. Multiple measurements for each test were performed to avoid the absolute error of measurement and the standard deviation was always lower than 5%.
The color changes between untreated simulated samples and treated samples were evaluated by measuring the spectrum reflectance with the CIE 1976 L*a*b* color parameters, using a commercial colorimeter (Minolta CM700d).
The contact angles were measured with a 5 μL water droplet at ambient temperature with an optical contact angle meter (DSA100 of Kruss Instruments Ltd, Germany). The reported values of contact angles were averages of five measurements made on different points of the sample surface.
Water vapor permeability in the sample slabs was measured using the standard cup test . The saturated relative humidity (RH) maintained with each slab as a cover on a cup was sealed up, and the external RH was kept at 60% in this test. The mass of the system was recorded regularly and the resistance to water vapor diffusion coefficients was calculated. It is expressed as the ratio between the weight variation of the whole system in 24 h and the area of the sample surface with diameter of 25 mm and thickness of 2 mm, according to the formula to calculate the vapor diffusion resistance μ: μ=(P×δL)/[M/(t×s×d)], in which P was the vapor pressure at the testing temperature (Pa), δL was the vapor constant in the air (7.02×10-7 kg/m•h•Pa), M was the vapor pervasion mass (kg), t was the testing time (h), s was the sample area (m2), and d was the sample thickness (mm). The permeability test was carried out on five samples for each series, and the mean permeability values and their standard deviations were evaluated using statistical analysis (standard deviations were always lower than 5%).
The influence of the environmental conditions in the damage procedure during preliminary tests showed that the use of a high, but still realistic, temperature could enhance salt damage, while the use of wet-dry cycles was found to be more effective than a continuous immersion of the specimens in salt solution . About 3% of dry salt in the specimen was expressed by way of immersing the specimen (50*50*20 mm) in 10% (wt%) NaCl solution for 24 h. Once 80% of the water had evaporated, the specimens were re-wetted, by capillary rise from the bottom, with a quantity of demi-water equal to the quantity of solution used in the first wet-dry cycle. The experimental condition was selected as 4 h at 20°C 50% RH followed by 4 h at 20°C 96% RH, in which crystallization-dissolution cycles more frequently than what was obtained by rewetting and drying the specimen for a long time. Any appearance of damage (type and seriousness) and efflorescence was recorded.
Results and discussion
Characterization of hybrid materials
Efficiency of applied consolidants
Properties of the treated powder sample
Water absorption (%)
Apparent porosity (%)
Bulk density (g/cm3)
Consolidated depth and compressive stress
3.20 MPa at 2 mm depth
3.50 MPa at 3 mm depth
Evaluation of negative effects induced by the salt
Based on analysis of existing damaged pottery influenced by NaCl salt decay, we used FTIR and SEM techniques to investigate how a TEOS-based consolidant formulation was influenced by the addition of PDMS-OH when DBTL was used as catalyst. The sample treated with TEOS containing 10% PDMS-OH showed a significant increase in compressive strength – the value was 3.50 MPa at 3 mm depth of consolidation, which is similar to the sample treated with Primal SF, which had 3.20 MPa in compressive strength at 2 mm depth. On a hydrophobic surface of sample treated with hybrid material, in contrast to what happened for the hydrophilic case in the original, there were still no significant changes or damaging effects caused by the NaCl crystallization. We have demonstrated that the addition of PDMS-OH to TEOS helps to achieve effective consolidation of damaged pottery as the PDMS-OH content improved the mechanical properties, hydrophobic behavior and salt resistance of the pottery; in addition small color changes and excellent water vapor permeability should also be taken into consideration.
This work was financially supported by National Basic Research Program of China (973 Program) (Grant No. 2012CB720901) and key program of the Natural Science Foundation of China (Grant No. 51232008). The authors gratefully thank the Longxian Museum for kindly supplying the damaged pottery.
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