**6.4 Liquid carrier or solvent**

274 Ceramic Coatings – Applications in Engineering

good binding properties.

Table 3. Type and properties of binding agents most frequently used

Agent Influence on coating quality

Montmorillonite clays, i.e. bentonites, are most

Bentonite clays are widely applied, especially Nabentonite for its swelling and suspension maintaining properties and it is suitable for a gel formation.

molasses and resin have similar properties.

alcohol It doesn't get destroyed or burnt when ignited, therefore the binding agent maintains its properties.

Sodium alghynates are derivatives of certain seaweed enabling good suspension and they are relatively cheap. They reduce the possibility of heat cracks on the coating after it is dried. They are used in aqueous

Carboxymethyl cellulose may replace Na-alghynate, as it is not that sensitive to normal pH variations. It is used in aqueous coatings.

Bentonites, such as organic bentonites where Na ion is either completely or partially substituted by an organic one, are preferable since they create a thyxotropic gel in organic solvents. They are used in alcohol-based

Aluminium stearate is used as a suspension forming agent in the coatings where cyclohexane is used as a liquid carrier. In preparation of alcohol coatings, they are used as suspension maintaining agents for resins and oil

The agent maintaining a suspension for the longest time, therefore mostly

Binding agents depend on chemical composition defined by a module being determined by the ratio between the number of quartz moles (SiO2) and sodium oxide (Na2O). Binding process is based on silica acid colloid-like separation, where condensation is created due to lack of water. Special strength is obtained due to a reaction with the CO2 gas from the atmosphere.

significant. As a binding agent, bentonite is mostly used for a water-based quartz coating manufacture, due to electric charge activity on the surfaces of both the binding agent particles and refractory filler particles.

Good binding agent. It has advantages over clay as there are no cracks on the surface of a dried coating. Starch,

They might be natural (linen) and process oils (mineral, synthetic oils and alkine resins). They present relatively

Binding agent Coating type Application

Refractory clay aqueous

Bentonites aqueous

Dextrine aqueous

Sodium silicate aqueous

Oils aqueous

coatings.

coatings.

either.

Table 4. Suspension maintaining agents

used in aqueous coatings.

Shellac Colophony Resins

Sodium alghynate

Cellulose derivatives

Bentonites

Metal stearates

Sodium bentonite The role of a liquid carrier or solvent is to dissolve and transport the refractory powder to the sand surface in the form of film. Generally, three types are used: water, alcohol (flammable liquids) and chloric hydrocarbons.

The solvent is chosen depending on several factors, some of which are the following: the type of sand blend, application method, ecological factor, type of production cycle dictating the time allowed for drying and application. Mostly applied solvent is water, for it is the cheapest, but also non-toxic and safe for use. Increment of application costs for this type of coating is due to energy consumption for the coating drying process and for water elimination from the layers applied.

Solvents like alcohol mixtures (methyl, ethyl, isopropyl) with pure water are often flammable liquids. The advantage of alcohol over water is faster drying, which is important for the sand moulds; but, since these coatings are flammable, they require more attention to be paid during production, use and stock-keeping.

Isopropyl-alcohol represents technically most acceptable solvent for the flammable foundry coatings, primarily for its combustion properties being close to perfect. This type of combustion has a reduced erosion risk for the surface observed. Other alcohol solvents are less frequently applied as pure, but mostly together, even when mixed with water (up to 12,0 vol %).

Choloric hydrocarbon is air-dried, eliminating a fire risk when flammable liquids are used. Air-dried coatings are convenient, like flammable liquids: they are self-extinguishable, they have a high steam pressure, they evaporate easily without a usage of heat, but they are also expensive and toxic matters.
