Reduction of fractures in dried clay-like materials due to specific surfactants
Introduction
Clay-like row materials are widely used in buildings and ceramics industries for manufacturing of various products (bricks, roof and wall tiles) or for production of sanitary and tableware (Pampuch, 1988, Raabe and Bobryk, 1997). These raw clay-based materials before molding into required shape are mixed with an appropriate amount of water to get a plastic mass. The products after molding and leveling the moisture distribution are subjected to drying and next to firing. Unfortunately, the capillary-porous structure of clay-like materials is prone to shrinkage and cracking during drying, which is very disadvantageous as it weakens the mechanical strength of the products and significantly reduces the possibility of their use (Banaszak, 2007, Banaszak and Kowalski, 2002, Banaszak and Kowalski, 2005, Kowalski, 2003).
The reasons for material cracking are the stresses induced in the material during drying. The stresses arise if the moisture and/or temperature distributions in dried material become nonlinear (Augier et al., 2002, Kowalski et al., 1992, Kowalski et al., 2000, Kowalski and Rybicki, 2000, Scherer, 1986). Such an inconvenient moisture distribution is created, for example, by intense moisture removal from the surface and by hindered moisture transport inside of the material.
In order to improve moisture transport inside the dried body, and thus to assure more uniform distribution of moisture in the material and thus avoid its cracking, the authors propose wetting the raw clay with water containing surface active agents (surfactants). These agents have the ability to stimulate the surface tension between water and the pore walls and thus to improve moisture transport inside the material (Cottrell, 1970, Wert and Thomson, 1974).
To our knowledge no earlier publications exist which describe using surfactants for improving drying of clay-like materials. One can find, however, works where surfactants are used to improve manufacturing xerogels. Matos et al. (2006), for example, obtained a xerogel with high mesoporosity after synthesis with application of three different surfactants (non-ionic, cationic and anionic) with concentrations varying between 0.1% and 15% (wt./wt.). The addition of surfactants affected the porous structures of the carbon xerogels. Mosquera et al. (2008) proposed an innovative strategy to obtain crack-free gels by using a surfactant as a template for the silica pores. A neutral surfactant – n-octylamine – which weakly interacts by hydrogen bonding with the silica precursor was used. This allowed it to be removed by simple drying in ambient air. The syntheses promote the formation of a crack-free uniform mesoporous silica gel.
Surfactants are compounds composed of both lipophilic and hydrophilic fragments The number of each varies, but most commonly there is one of each kind. The more common surfactants are anionic (a hydrophobic chain attached to an acidic group like carboxylate, sulphate or sulphonate), cationic (a hydrophobic chain attached to a group like quaternary ammonium), o nonionic (a hydrophobic chain attached to a polyalkoxylate chain). Surfactants tend to be scarcely soluble in water as free molecules or ions, but they are able to form stable colloidal aggregates called micelles. The presence of ionic surfactant micelles depends on surfactant concentration being above the Critical Micelle Concentration (CMC). A concentration equal to, or below, CMC is always dissolved as monomers, the excess is present as micelles.
The surfactants with concentration over CMC are often used to remove impurities or organic compounds from aqueous solutions by means of two-step adsorption on surfactant micelles. The first step flocculates the micelles and creates sites with inverted polarity on the aggregate. The second step binds pollutant molecules (Creagh et al., 1994, Nystrom and Manttari, 1994, Paulenowa et al., 1998, Scamehorn et al., 1986, Scamehorn et al., 1994). Purkait et al. (2004), for example, used surfactants in cloud point extraction to remove pigments from waste water.
The results of research with surfactant below CMC show that this parameter has very essential influence on the surface tension between moisture and the pore walls and thus on moisture transport inside capillary-porous materials during drying (Cottrell, 1970, Chen et al., 1998). Sacchi et al. (2001) described the results of extraction techniques available to obtain water and solutes from the argillaceous rocks. The paper focuses on the mechanisms involved in the extraction processes, the consequences on the isotropic and chemical composition of the extracted pore water.
The main goal of this work was to investigate the effect of the dodecyl sulfate sodium salt (SDS) and the fluoric (FC 4430) surfactants on the reduction of the drying induced stresses in dried materials. These surfactants applied in a prescribed amount are able to decrease the surface tension between moisture and the skeleton, and thus to reduce the material fracture during intensive drying. The acoustic emission (AE) method is applied to monitor on line the development of crack formation during drying.
The experiments were carried out on cylindrical samples which were molded of clay wetted with water of different surfactant concentration. The samples after leveling the moisture distribution were subjected to convective drying in hot air at temperature 120 °C in a dryer chamber. Apart from AE monitoring the samples were visually observed and photographed during drying through the glass window in the chamber wall. The samples after drying were subjected to compression tests to show the influence of different surfactant concentration on the material strength.
Section snippets
Experimental
Fig. 1 presents the scheme of the experimental equipment used for the tests. The drying processes were carried on in the laboratory chamber dryer Zalmed SML42/250/M (4).
Drying variables such as the air temperature and the relative humidity, and the reduction of sample mass were measured every half minute during drying. All these variables were transmitted to the computer (8) provided with the software for data acquisition. The air in the drying chamber was stationary. The temperature and the
Results and discussion
All drying experiments were performed at constant air temperature of 120 °C. Such a temperature ensures a high drying rate. Each test was repeated at least three times.
The relative air humidity in the dryer chamber was very low at temperature 120 °C, but it changed slightly during drying (Fig. 2).
Initially it was ca. 2.5%, and at the beginning of the constant drying rate period (CDRP) it increased up to ca. 3.7% due to intensive water evaporation from the sample surface. Then, by the end of CDRP
Surfactants solutions below critical micelle concentration (CMC)
The structure of surfactant solutions depends on the surfactant concentration. The limit value of the surfactant concentration, at which the molecules appear in a monomolecular form is called the critical micelle concentration (CMC). The surfactant solution with concentration below the CMC is always dissolved as monomers, the excess of surfactants over CMC value may appear in the surfactant solution as associates called micelles (Fig. 12).
This phenomenon is caused by the dual nature of
Mechanical tests
Apart from the measurement of the AE descriptors, that is, the total AE energy and the total number of AE signals, which can be considered as valuable parameters of quality assessment, the mechanical compression tests were carried on to show the influence of the surfactant concentration on the mechanical strength of the samples after drying. The individual samples of different surfactant concentration were subjected to compression test on the universal strength measuring instrument COMETECH
Conclusions
The results presented in this paper allow us to state that drying of a product made of clay-like material containing a prescribed amount of added surfactant considerably reduces the tendency of this material to cracking during intensive drying at high air temperatures. The best quality of the dried samples was achieved for clay saturated with water containing rather low concentration of surfactants, both the SDS and the fluoric one. In the first series of tests the biggest decrease of AE energy
Acknowledgments
This work was sponsored by the Poznan University of Technology, scientific project No. 32-137/2012 DS-BT. Special thanks to Firm Brickyard CERPOL Kozłowice for providing the clay materials for the present research.
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