Lost foam casting water-based coatings are composed of refractory aggregates and auxiliary materials. Among them, aggregate accounts for 90-92%, and auxiliary materials account for 8-10%. In addition to the commercial coatings that need to be added defoamers and preservatives, there are only three main ingredients in the attached materials:
(1) Binder; (2) Suspending agent; (3) Wetting agent.
The classification or division of the three agents of bonding, suspension and wetting is actually the subjective behavior of scholars. Objectively, among the substances that have been shortlisted for lost foam casting coating materials, the role of individuals is not single. For example: bentonite and carboxylic acid-based cellulose (CMC) are usually used as binders, but in lost foam casting water-based coatings, because they cannot overcome the hydrophobicity of the smooth surface of the polystyrene foam model, they are used as adhesives. As a result, the paint cannot be applied to the surface of the foam model. However, the fine particles of bentonite swell in water to form a hydration film, stick to the CMC macromolecular chain, and overlap into a three-dimensional network structure to prevent refractory aggregate particles from accumulating and sinking, thereby improving the suspension performance of the float Therefore, these two substances with binding function are used as suspending agents. As a binder material, it must first have an affinity for the smooth surface of the foam model, and at the same time it must have good low-temperature strength and high-temperature strength. Regardless of domestic paint or foreign paint, the preferred binder is: polyvinyl acetate emulsion (commonly known as white latex). White latex can significantly increase the viscosity of the coating while reducing the suspension of the coating. However, white latex has a slight corrosive effect on the surface of the foam model, which improves the wettability of the paint, thereby improving the adhesion performance of the paint, that is, the coating performance of the paint. This is incomparable to any other adhesive. According to this feature, the amount of milky white glue added should be determined according to the roughness of the foam model surface. For a smooth foam model, the amount of white latex added in the coating can be as high as 5-6%, and for the foam type with resistance wire cutting and splicing, the amount of white latex added in the coating can be as little as 1%.
The choice of binder for lost foam coatings should not only consider the low temperature strength, but also the high temperature strength of the coating. White latex has good low-temperature strength, but it will coking at 300℃-400℃ and lose its binder effect. Therefore, when white latex is used as a binder, a high-temperature binder should be added.
Among many high-temperature binders, phosphates, especially sodium tripolyphosphate, not only have good high-temperature performance, but also a wetting agent, which can reduce the surface tension of the coating and improve the coating performance and fluidity. The high-temperature strength of sodium tripolyphosphate can promote the low-temperature sintering and ceramization of refractory aggregates, forming a ceramic shell that is highly resistant to erosion by high-temperature molten metal.
Sodium tripolyphosphate can accumulate and precipitate refractory aggregates and destroy the suspension performance of the coating. The ratio should be less than more.
The various excipients in the coating have an interactive effect, some are positive interactions, appropriate ratios can play a synergistic enhancement effect, and some are negative interactions, and the imbalance of the ratio will play an antagonistic and offsetting effect. Therefore, the effect of the coating formulation is not the sum of the independent effects and effects of various auxiliary materials. The total effect of a reasonable formula is greater than the sum of independent effects, and the total effect of an irrational formula is less than the sum of independent effects.
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