Nolifrit Detailed Classification of Enamel Frit 2

Steel Enamel Cover Coat

Cover coat is coated on the ground coat layer to meet the requirements of whiteness, gloss, acid resistance, alkali resistance, water proof and atmospheric resistance required by the enamel product, so that the product has a smooth and beautiful enamel surface.

Steel Enamel Cover coat can be divided into Antimony cover coat, Titanium cover coat, Tin cover coat, Zirconium cover coat, and Cerium cover coat according to the different opacifiers used. The most commonly used cover coat in China are Titanium cover coat and Antimony cover coat. Because of the high cost of Tin cover coat and the melting temperature of Zirconium cover coat is higher than that of ordinary cover coat, and the acid resistance is relatively poor, these two cover coats are rarely used.

Antimony white cover coat has a special bluish white hue, which is brightly colored after decoration, and it has strong impact resistance. While the opaque ability of the Antimony cover coat is poor, generally it needs to be coated twice, the coating layer is thicker, and the amount of enamel frit used is larger. Antimony in porcelain enamel may be present in a toxic trivalent state, so it can’t be used in food contact products.

(Nolifrit Antimony white cover coat frit: applied in subway panels)

The opacity of the Titanium cover coat is stronger than that of Antimony cover coat. A good opacity can be obtained by applying in only one time. The chemical stability and thermal stability of the Titanium cover coat are better than that of Antimony cover coat. Since the crystal transformation of Titanium oxide affects its opacity and hue, both melting and firing of Titanium cover coat are difficult to operate. Titanium white cover coat is sensitive to metallic ions such as Chromium and Iron, and its enamel surface is easily to be yellowish. Therefore, the chemical composition and manufacturing process of the Titanium cover coat must be strictly controlled.

(Nolifrit Titanium ivory cover coat frit: applied in enamel cookware)

The chemical composition range of the common cover coat formulation is shown in the table below:

In cover coat, when the SiO2 content is increased, the chemical stability, thermal stability and glossiness will be improved. If the SiO2 content is too high, the firing temperature and the surface tension of cover coat will be increased, and the surface will be easily collapsed. Defects such as cracks will easily happen.

The increase of B2O3 content can improve the thermal stability, glossiness and elasticity of the cover coat, but it is unfavorable to the opacity of the Antimony cover coat.

If the content of K2O and Na2O is too high, the chemical stability and glossiness of the cover coat will be reduced, and it will promote the formation of rutile crystal form during the firing of Titanium cover coat, which makes the hue of the enamel surface yellowish.

If the CaF2 content is too high, the surface of Antimony cover coat will be rough and the glossiness will be poor.

Appropriate amount of P₂O₅ in the Titanium cover coat can promote the formation of anatase crystal form during the firing process, making the Titanium cover coat bluish white, which is beneficial to visual whiteness.

Introducing a small amount of Al₂O₃ and MgO, can improve the opacity of the Titanium cover coat, is conducive to the stability of the hue, and improve the coating process performance of enamel slip.

NaSiF₆ can also improve the coating process performance of enamel slip and opacity.