Digitalfire Ceramics Technical Articles
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An overview of the technical challenges a technician in the tile industry faces in making good tiles from the least expensive materials and process possible.
The ceramic tile industry is by far the largest user of clay bodies in ceramics. If you are a technician from another sector of ceramic industry the dust pressing process of compacting 8% moisture powder at 500 kg/cm2 seems foreign. You may wonder what types of body formulations are used in tile. Not surprisingly, the familiar principles apply. Tiles need to develop high fired strength in the kiln like many other kinds of ceramic ware. Dry tiles must of course have strength so they can be handled and survive glazing operations. Generally bodies are needed that fire strong at the lowest possible temperatures. Surfaces absolutely must be durable and defect free. That being said, R&D often faces constant pressure from management to lower costs, this generally means finding ways to use lesser quality materials, firing lower or making the tiles thinner. Of course this means some compromises in quality.
Of course, body formulation depends on the quality of tiles a company wants to produce: Irrespective of color, tiles fall into two broad categories. Porous after firing (wall tiles, single and double firing) and zero porosity (floor tiles). Porous bodies contain calcium or magnesium compounds so they develop new crystal phases during firing and those crystals are responsible for mechanical strength. Usually shrinkage is very low or zero but porosity is higher than 10%. For vitrified bodies mechanical strength is a product of a glassy phase developing during firing (from feldspars). These bodies contain negligible amounts of calcium compounds. Shrinkage is usually higher than 5%. Red and white bodies can be of either type.
R&D are always looking for ways to produce the best tiles possible from the most economical materials possible, so formulation is strongly affected by locally available raw materials or by materials having a low freight cost (trading is well organized in the tile supply chain). Thus, it is no surprise that each body is tailor-made. There are also contract factors at play. A machinery supplier (e.g. Italian) usually provides the complete know-how to attain a quality and quantity stated in a contract (that contract specifies the body formulation). In other cases, these machinery suppliers do allow customers in whom they have confidence leeway to formulate their own bodies (e.g. Chinese companies).
As expected, white burning tile bodies need less clay than is customary in other types of ceramic industry since they are not plastic formed (30-40% clays and/or kaolins are typical). This makes more room for feldspar (40-60%) and even other fluxing materials (e.g. dolomite or calcium carbonate or talc 0-5%, calcium carbonate can be 10 or more in case of wall tiles bodies). Finally 0-10% quartz is employed as a thermal expansion regulator (less than typical stonewares).
While white burning high temperature fired tiles are of course ideal, practicality, especially material and transportation costs, normally determines the type of body actually employed. Often the pressure to use less expensive local materials is great because materials can represent 50% of total production cost. For example, a small production unit with 10,000 sqm/calendar day output needs about 200 tons/day of materials for the body (about $15,000 US day). Sometimes a geological survey and opening of new mines and processing units are part of a tile project. Practicality works the other way around also, tile plants are built near the mines of material suppliers and agents to minimize transportation costs (USA production of tiles is close to Texas or Tennessee mines). Excluding red clays, some materials have a very wide market in tile. For instance, in the case of vitrified bodies, clays from Ukraine are used in all European countries, Russia, Middle East and even in India. Turkish feldspar is widely used in all Mediterranean countries and Russia.
If you look at the backside of a typical glazed floor tile you will find at many tile stores, most are made from brown and red burning clays fired to varying states of vitrification. Many technicians are not fully aware at how much lower temperatures these bodies can fire and yet still be strong and hard and even vitrified. However it can be difficult for R&D to adapt white engobes (reguired for a glaze base) that have dry and fired fit and adherence. Also, redder burning clays can have very narrow and volatile vitrifying ranges and properties of locally available clays can vary alot, QC must be alert and capable to adjust. Part of the secret of firing these bodies is doing it quickly. Some companies are blessed with quite remarkable local materials and abilities to process them internally (eg Turkey), others have much poorer materials and resources (eg India, Pakistan) and others can rely on a nearby well developed supplier channel (eg. Spain).
In many cases binders are employed to improve mechanical strength (of green tiles after pressing) where clays of sufficient plasticity are not available. Examples of binders are lignin sulfonate, polyacrylates, polyvinyl alcohol, starch, carboxymethyl cellulose, sodium silicate, bentonite, etc. Organic binders can pose a significant problem: these generate a black core in tiles during firing if organic matter content of clays is high. In the opinion of xxxx the best binder is sodium silicate, also used as deflocculant, and it can improve mechanical strength 2-4 bar without black coring and it is inexpensive.
As noted, one thing cannot be compromised in tile: Companies must have body materials with no particulate contaminants. Some go to the incredible effort of wet processing materials, filter pressing and dry pelletizing their own materials. However, for others who process their own materials, there is pressure from management to minimize processing to cut equipment and material costs, this can put R&D in a difficult position. Of course, like any body, many properties must be monitored. If you are going to be involved in tile you are going to need to know about monitoring material properties both to maintain product consistency and quality but also to weigh these against cost. For example:
As with any ceramic manufacture, there is a need to be practical in the lab. It makes little sense to be testing chemistry and mineralogy on fancy machines when there are particulate and plasticity issues with the body, for example. A page full of lab numbers are no substitute for understanding properties like plasticity and devitrification and the relationships between chemistry, mineralogy and physical properties. But most important are the physical properties, often only simple tests flag key changes and rationalize what to do.
By Tony Hansen
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