Chemicals Used in Ceramic Materials Production

Ceramics, in general terms, can be defined as the science, art, technology and material relating to obtaining hard products by applying various methods and processes to substances with inorganic chemical structure. Ceramics, which have a very wide range, can be divided into two main groups: traditional ceramics and engineering ceramics. Traditional ceramics include pottery products, artistic ceramics and industrial ceramics. Examples of industrial ceramic materials include building products such as wall tiles, floor tiles, porcelain tiles, wash basins, toilet bowls and brick and tile products, tableware and refractories. The production of such materials primarily uses natural raw materials such as clay, quartz and feldspar. Glazes and decorations with glassy structure may be applied to the obtained ceramic bodies depending on the product group. Engineering ceramics are also called advanced technology ceramics and are mostly produced from processed raw materials using special methods due to their higher purity. These types of ceramics, containing products such as carbides, borides, titanates and ceramic metal composites, have more specialized applications compared to traditional ceramics (armor, rockets, missiles, spacecraft, etc.) and are gaining increasing importance as their fields of application continue to expand today. In the production of traditional or engineering ceramics, the type of raw material, mixing ratios, additive materials used, shaping and firing methods vary according to the final physical, chemical and mechanical properties required from the products. Many chemical substances are used in ceramic material production from the raw material stage until the final product is obtained. This article addresses the chemical substances used during the production of traditional ceramics under two main headings: chemical substances used as raw materials in production (used in the preparation of body, underglaze, glaze and decoration paints) and additives (auxiliary substances) used during the production process to improve various properties.

1. Raw Materials Used in Ceramic Production

Raw materials are at the forefront of chemicals used in ceramic material production. Generally, raw material is the natural state of substances before processing to create a product or good. Numerous natural and synthetic raw materials are used in ceramic production. The composition of these raw materials consists of minerals.

1.1. Raw Materials Used in the Production of Ceramic Bodies, Underglazes and Glazes

Ceramic raw materials are divided into the following groups, taking into account the properties required in the production of ceramic bodies and glazes.

Plastic Raw Materials:

These are substances that can be kneaded with water, easily shaped without disintegrating, and retain their shape when dry. This group consists of clays (Table 1). 1.2. Raw Materials Used in Glaze Coloring and Decoration of Ceramics The basic oxides used in coloring ceramic glazes and preparing ceramic paints and the raw materials containing them are presented in Table 3. In addition to glaze, underglaze and ceramic paints, which are considered the main decoration materials in decorating ceramic items, certain chemical salts, precious metals and luster are also used as decorative elements. In decoration processes with salts, salt solutions are first created. Since these salt solutions decompose during firing, there is a difference between their color before application and the final color. Since the texture and color effects obtained in the firing of salt solution paints are not stable, this type of application is used more in artistic ceramics rather than industrial production, particularly in alternative firing techniques such as saggar and raku. Additionally, salt solutions are used in the production of lusters, which are also applied in industry and have a pearlescent lustrous appearance. Information on salt solutions and their uses is provided in Table 4. 2. Additives Used in the Ceramic Production Process 2.1. Additives Used During Ceramic Slip and Ceramic Glaze Preparation Various additives are used in different stages to achieve faultless results in ceramic production. These additive materials are added during the ceramic slip preparation stage to prevent ceramic slip settling, control its fluidity, prevent lumping, or increase plasticity depending on the shaping method. Table 5 contains the types and functions of additives added in ceramic production, Table 6 shows the types of binders used in ceramic slip production, and Table 7 shows the deflocculants used in ceramic slips and underglazes. These binders are used particularly to prevent problems during the shaping stages of ceramics, and the type and quantity of binder vary depending on the shaping method and the raw material used. The rheological properties of glaze suspension are affected by the particle size of various components and the flow properties change over time. To control the fluidity of the glaze suspension, it is necessary to control the thickness and consistency of the applied glaze with rheological modifiers, that is, additional chemical additives, to modify its thixotropy, prevent settling, improve wetting properties, control drying time and improve the dry strength of unfired glaze on the selected ceramic substrate. Controlling these properties means controlling the glazing process. Glaze Binders or Hardeners: The top surface of the glaze layer modified with these additives is hardened. Examples: Starch, cellulose ethers, natural gums, alginic acid, water-soluble acrylics, polyvinyl alcohol, resin solutions, sodium silicate, potassium silicate. Electrolytes: In glaze suspension, they are used to control viscosity and thixotropy with the help of deflocculating agents (dispersing agents) and flocculants. Ions passing into the glaze from glaze components are alkaline and act as deflocculants. The glaze thins and its viscosity drops below normal application values. This effect occurs during storage. Flocculation can reverse this increase in fluidity. The deflocculants used to control glaze rheological properties are given in Table 8, and flocculants in Table 9. Suspending Agents: The proportion of colloidal substances in glaze recipes (particularly clays) supports non-plastic materials in suspension. These clay-like or entirely organic materials support the floating of particles larger than the average particle size in the suspension (such as pigments) (Taylor and Bull, 1980). Bentonite: Clays in this group contain ultra-fine particles smaller than 1 μm, making their initial dispersion in water difficult, but once dispersion occurs, water penetrating the clay molecule lattice produces strong gel formation. This gel prevents the settling of other non-plastic grinding materials. For effective use, bentonite should be used in the range of 0.5-1% in glaze recipes; if used in higher amounts such as 1.5-2%, bentonites form thixotropic gels. In glazes with high flux content, bentonites create balance. Bentonites have many types such as sodium and calcium-containing montmorillonites. Cations are present in their lattices. Clay develops surface charge in water depending on the type of cation (Ca+2, Mg+2, Na+, etc.). If the clay mineral contains calcium and magnesium ions, the suspension will be prone to lumping; if it contains sodium ions, the suspension will be prone to remaining in suspension. The adhesion (cohesion) between clay particles and water is regulated by wetting agents. Wetting Agents: Particularly in underglaze decoration applications, they prevent the formation of droplets of glaze applied over oil-based underglaze decoration inks and support increased glaze spread. Foam Control Agents: Agents that prevent foam formation can be used in additive amounts in the range of 0.1-0.2%. The presence of phosphate ions is an effective foam control agent. Glaze Fixatives: These are used for applying decoration paints manually or directly or mechanically with the aid of other equipment to the surface of unfired glaze. Polyvinyl acetate and polyacrylate emulsions are sprayed onto the glaze surface as solution or emulsion. (Taylor and Bull, 1980) 2.2. Additives Used in Ceramic Decoration Applications Printing Oils • General Use Printing Oils: Minimizes viscosity, density variations and color tone shifts in the paste. • Third firing printing oils: Printing oils that provide desired drying times in decoration applications and maintain stable viscosity and density. • Relief printing oils: Printing oils that provide relief effect by giving the paste high density and low viscosity properties. Oils for Aqueous Grinding Systems These are grinding oils that ensure the stability of water-ground paste during storage. Screen Printing Oils Printing oils that minimize adhesion, drying and dusting problems and maintain viscosity. Wetting Agents They allow working with low viscosity at high solids content in paste applications. Fixatives Fixatives used in printing applications improve print quality, particularly in ceramic tile production for products requiring multiple printing applications. They are produced in concentrated form and used after dilution. Thickeners These are rheology agents used in ceramic decoration that increase the consistency of glazes and pastes used, providing the opportunity to work at desired viscosity values. Defoamers These remove foam formed for various reasons in fluid materials used in ceramic decoration, preventing potential defects on the ceramic surface. By minimizing production waste, they provide the opportunity for high-quality, low-cost production. Asst. Prof. Dr. Şerife Yalçın Yastı - Selçuk University / Faculty of Fine Arts - Ceramics Department References Arcasoy A., 1983. Ceramic Technology, Marmara University, Publication no:457. Akunova L. F., 1984. Krapinin V.A., Technology of Production and Decoration of Artistic Ceramic Products, Moscow, Higher School, p.154. Cushing Val M., 1994. Cushing Handbook, Cushing Publisher, third edition, p.23. Geçkinli E.,1992. Advanced Technology Materials, Istanbul Technical University Press, p.62,63. http://www.smaltochimica.com.tr/urunler Taylor J.R., Bull A.C.1980, Ceramic Glaze Technology, The Institute Of Ceramics By Pergamon Press p.196- 208.