10 Things to Consider When Buying Ceramic Binder Materials
Binders for Ceramic Bodies
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Check now All Articles An overview of the major types of organic and inorganic binders used in various different ceramic industries. By Nilo Tozzi Binders are substances that improve the mechanical strength of green ceramic bodies so they can pass through production steps, before firing, without breakage. In many cases, binder additions to bodies are essential (without them some production processes would be impossible). For instance, in the pressing process of powders, adding organic binders makes possible a forming method that is independent of the plasticity). There are a wide variety of binders used in traditional ceramics, including natural products, like cellulose or clays, and synthetic products, like polyacrylates or polyvinyl alcohol. A normal body binder must have several characteristics: Inorganic binders have a couple of very important characteristics: they are inexpensive and are not subject to attack by microorganisms. Another big bonus is that they never cause black coring. Its main use is in slip deflocculation. However it also improves the mechanical strength of dry pieces when employed in pressing and extruding operations. Behavior does change according to chemical composition of sodium silicates but, in the case of pressed tiles, often it is the best when the properties range toward some tendency to black core. Available products have different compositions because they are obtained from extremely plastic natural minerals (called smectites). Particles are off-white with colloidal sizes. When we add these binders to slips in the range 0.5-5% the mechanical strength of pieces proportionally increases. These binders are less effective than others but they have an important characteristic: they don t migrate during drying so we have less problem during glazing procedures. This is a very plastic natural material mineralogically known as montmorillonite. It is used in the range 0.5-3.0% however it raises the viscosity of slips during milling (the maximum allowed percentage depends on characteristics of the material and on the permissible viscosity value). It improves mechanical strength of green and dry bodies and it also does not migrate during drying. Often organic binders are made from polymers with more or less long chains where polar groups are present. Most organic binders are soluble in water and behavior is like a surfactant (it improves contact between liquid and solid phases). Short chain binders are adsorbed on the surface of particles and during drying water elimination from hydroxyl groups produces tridimensional hydrogen bonds (among the molecules of binder distributed on the surface of the particles). The development of chemical bonds contributes to a stronger tridimensional structure and mechanical strength improves proportionally to the amount of organic binder. Long chain binders have poor solubility in water but are emulsifiable. During drying they are not absorbed at the surface of particles but they are able to form tridimensional hydrogen bonds. Usually organic binders do not improve the strength of green pieces before drying. Drying strength rises proportional to the added amount of binder (actually it can even reach values 30% higher). Theoretically, organic binders burn off on firing at low temperatures with minimal residue. Nevertheless, these binders are increasing the organic matter content in bodies, experience has shown that pressed tiles employing organic binders are quite subject to black coring problems. Often organic binders are decomposed by bacteria and we have to add an anti-bacterial agent to stabilize slips.
Organic binders are also used in glazes and engobes to ensure good adhesion to the ceramic surface, prevent sedimentation and improve rheological properties. The most popular are:
Usually it is used as a binder for glazes, during glazing operations, before the screen printer (a water solution of polyvinyl alcohol is sprayed on the surface to be decorated). It is a strong surfactant and binding power is connected to its ability to wet particles (products having a low molecular weight exhibit low viscosities and they have a minimal effect on the viscosity of glazes or body slips). It is stable because it does not ferment. Usually suppliers propose water solutions of polyvinyl alcohol. Starches are powdered forms of a group of carbohydrates producing colloidal emulsions in water having strong binding properties (however some times modified starches are supplied as liquids). Often they are not fully soluble in water because of their high molecular weight (this characteristic prevents migration during drying). It is possible to mix starch and dry ceramic powders (after which the mix can be wetted, formed and dried). Starches quickly ferment. Chemical derivatives have properties like esters of cellulose but are not stable against bacteria. It is a white-yellow powder soluble in water. It can migrate during drying (thus the distribution in the body matrix may not be uniform). There are different types with different molecular weights. Carboxymethylcellulose products with medium or high molecular weights are stronger binders but they increase viscosity of slips so they cannot be used above certain percentages (thus not fully developing their binding properties). These products improve the plasticity and mechanical strength of dry bodies and completely burn out during firing, however they are expensive. NFJ Product Page A yellowish powder obtained by treating certain starches with small amounts of acid. Dextrin is a strong binder and is some times used to prepare glaze grains for dry application or as a "glue" for glaze slips to improve adherence to the ceramic body. Dextrin also improves the plasticity of clay slips. Wax emulsions behave like inter-particle lubricants when bodies are wet and like binders when dry. They are widely used for the production of technical alumina components. Polyethylene glycols having low molecular weights are viscous liquids often used as plasticizers or lubricants. Those having high molecular weights are waxy solids that are used as binders and plasticizers in pressing. They are water soluble and often used as basic mediums for the preparation of printing colors. Lignosulfonates are yellowish powders having variable compositions and also variable molecular dimensions (because they are polymers that can be modified by the addition of organic or inorganic groups to the molecule). They are anionic derivatives of lignin, water soluble and surfactants. Lignosulfonates are very effective in increasing mechanical green and dry strength in ceramic pieces. In addition, they act as lubricants during extrusion or pressing operations.
Additions of lignosulfonates to ceramic slips can vary from 0.1 to 2.0% and they are relatively inexpensive. For pressed tiles, 1% lignosulfonate can double mechanical strength (however often a black core appears).
Lignosulfonates are often used to reduce shrinkage yet maintain after-forming mechanical strength (because reduced amounts of plastic clays are needed). A derivative of cellulose (when treated with methylene chloride and alkali under pressure). The composition is variable depending on the length of chains and methylcelluloses are non-ionic polymers, water soluble at low temperature. They are very stable against microorganisms but they have tendency to form foam.
Methylcelluloses have different viscosities in water (depending the length of chains) and strong deflocculating properties.
Often methylcelluloses are used as temporary binders in refractory production and other technical ceramics because they are at the same time lubricant and wetting agents and plasticizers.
Hydroxyethilcellulose is a similar product having less tendency to form foam. Mixtures of paraffins and carnauba wax are widely used as binders for the production of special pieces obtained by cold isostatic or normal pressing. The nature of mixtures is determined by the dimensional tolerance needed and the shapes of edges. Mixtures as liquid emulsions are blended for specific purposes and they also have binding, plasticizing and lubricating properties. Sodium and ammonium salts of polyacrylate acid are water soluble and they are mainly used as strong deflocculants. They can also act like binders but their cost curtails usage for this purpose. Polyacrylate esters are not soluble but they have a similar behavior. Binders for Ceramic Bodies
Description
Article
Inorganic Binders
Sodium Silicate
Magnesium Aluminum Silicates
Bentonite
Organic Binders
Polyvinyl Alcohol
Starches
Carboxymethylcellulose
Dextrin
Wax Emulsions
Polyethylene Glycols
Lignosulfonates
Methylcellulose
Paraffins
Polyacrylates
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Ceramic Binder
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Ceramic Binder
Binders are glues that harden ceramic powders as they dry. They enable improved surface adherence. And slower drying.
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A classic ceramic glaze (a water based slurry of feldspar, clay and silica powders) can can be applied to the surface of porous ceramic bisque (or even leather hard or dry ware) and it will dry durable enough to be able to handle the ware for further processing. However, in many applications (e.g. where the powder contains little plastic clay or high dry durability or surface adherence are needed), an organic binder must be employed.
Binders are basically glue, they are designed to harden and strengthen bodies and glazes as they dry. Ceramic powders containing binders can exhibit remarkable surface adherence and durability on drying. Binders are especially important in glazes, bonding them to even dense bisque surfaces. Binders in enamels can even bond them to metal. The mechanism of a binder can be as simple as a glue that hardens and adheres particles to each other (and any surface they touch). Body binders make it possible to form powders that would not otherwise hold a shape (e.g. dust pressing is a common method for these). In fact, bodies having zero clay content (e.g. dental porcelain) can still be hardened using a binder. Glaze binders make it possible to use slurries with very low plastic clay content (either no clay or highly processed clay materials that have little binding power). Conversely, binders can be so effective that a high-clay engobe or slip that would normally shrink, crack and fall off can be multi-layered on the same bisque surface (by the addition of CMC gum).
Binders come at a cost. They can be very expensive. They slow down drying. They make slurries messy and difficult to process and handle. Cleanup is much more difficult. They generate CO2 as they decompose during firing. In some industries, like tile, binders are avoided where possible, or carefully chosen and monitored (e.g. sodium silicate is used as a binder). As noted, binders are needed to adhere glazes to dense non-porous surfaces (e.g. already-vitrified bone china ware, metal).
Orton cones are a good example of the use of a binder. Cones are made from ceramic materials, yet resist re-wetting when immersed in water. Once they do slake, the slurry is very sticky and very slow to dewater on a plaster bat. Another common example are prepared commercial hobby and pottery glazes. The binder dramatically slows drying, but they have very good brushing properties. It also enables them to adhere to even already-fired glazes or non-porous surfaces. And the dried surface is hard and difficult to remove, even with water.
The world of binders is typically outside the scope of what a typical potter needs to know. Potters are able to use blends of natural materials and minerals (using age old processes) and they adapt their methods to the idiosyncrasies these materials present. Industry, on the other hand, needs to optimize processes, so binders are much more applicable.
If a material name or description includes the word “gum”, that does not mean it is a binder in the above explained sense. Veegum, for example, is a clay. CMC gum is a glue.
Depending on time, temperature, pH, binder can be attacked by microbes or molds. If this happens store in a cooler place, make smaller batches, adjust the pH to make a less friendly environment, or add a biocide (i.e. Tektamer, NaN3). The shelf-life of commercial brushing glazes can be affected for this reason.
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Make frit melt fluidity test balls using CMC gum as a binder
Traditionally we have made GBMF test balls of non-plastic materials (e.g. frit) using Veegum as the binder. However, Veegum interacts with materials enough to affect melting. CMC gum is better. I use 10g frit (the right amount for one ball), 0.1g CMC (1%) and ~3g of water. Of course, you need a scale accurate to 0.01g to be able to weigh out only 0.1g (otherwise consider doing 50g). The procedure that seems to work best is to shake the frit/CMC mix in a small bag, pour it into a bowl, pour in a tiny amount of water and stir and smear with a spoon. With the right amount of work and water the thickened material will become cohesive and can be formed by hand. Does cornstarch work? No, it does not gel like this and the mix is not plastic either. Psyllium? Yes, but it has a flakey texture and demands more water. For better formability use 1.5% CMC gum - however, the ball will dry slower and much harder. Other powders will behave differently (needing more or less water, forming better or poorer, dewatering slower or faster). You will need a plaster surface to absorb the excess water (which invariably happens).
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