|Monthly Tech-Tip |
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.
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 are glues that harden ceramic powders as they dry. They enable improved surface adherence. And slower drying.
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Additives for Ceramic Bodies
Materials that are added to bodies to impart physical working properties and usually burn away during firing. Binders enable bodies with very low or zero clay content to have plasticity and dry hardness, they can give powders flow properties during pressing and impart rheological properties to clay slurries. Among potters however, it is common for bodies to have zero additives.
By Nilo Tozzi