Nolifrit Detailed Classification of Enamel Frit 1

Steel Enamel Ground Coat

Ground coat is directly coated on metal body. Which plays a decisive role in the intrinsic quality and appearance quality of the enamel product.

There are many types of enamel ground coats, and there are many classification methods. According to the material of the metal substrate, it can be divided into steel enamel ground coat, cast iron enamel ground coat, stainless steel enamel ground coat, copper and copper alloy enamel ground coat, aluminum and aluminum alloy enamel ground coat, etc.; According to the classification of the adhesive used, it can be divided into Cobalt ground coat, Nickel ground coat, Cobalt-Nickel ground coat, Antimony ground coat and Antimony-Molybdenum ground coat.

For Cobalt ground coat, Cobalt oxide is used as adhesive. The introduction amount of Cobalt oxide at 0.5% can achieve very good adherence. In the design of the ground coat formulation, the amount of cobalt oxide introduced depends on the adherence requirements or the thickness of the steel sheet. For the ground coat of daily used enamelware, the introduction amount of Cobalt oxide is 0.5% to 0.8%; For ground coat on cast iron sanitary ware or acid-resistant industrial enamel, the introduction amount of cobalt oxide is 0.2% to 2.6%.

For Nickel ground coat, Nickel oxide is functioned as the main adhesive. The adherence of Nickel oxide is slightly worse than that of Cobalt oxide. Therefore, the amount of introduction of Nickel oxide is slightly higher than that of Cobalt oxide, generally 1% to 2%.

The Cobalt-Nickel ground coat is a ground coat in which Cobalt oxide and Nickel oxide are simultaneously introduced as adhesive. This kind of ground coat has a strong adherence ability, and its superiority is that a good adherence can be achieved with only a small amount of adhesive introduced into the ground coat.

(Nolifrit Cobalt-Nickel ground coat frit: )

In Antimony ground coat, Antimony oxide is used as the main adhesive. Generally, a part of Antimony oxide is melted into the ground coat during its manufacturing, and Antimony oxide can be also worked as milling additives when the ground coat is milling into slip. The Antimony ground coat also has good adherence.

The Antimony-Molybdenum ground coat simultaneously introduces a certain amount of Antimony oxide and Molybdenum oxide as adhesive. The introduction of Antimony oxide and Molybdenum oxide can be carried out by both fusion addition and grinding addition. Practice has proved that the adherence of Antimony-Molybdenum ground coat is no less than that of Cobalt and Nickel ground coat, but the cost is lower than that of Cobalt and Nickel ground coat.

In addition to good adhesion, the ground coat must also have the following characteristics:

①The firing temperature of common ground coat should be slightly higher than cover coat firing temperature of 20~30 °C (except for the ground coat used in “two coat one firing” process), if the firing temperature of ground coat is lower than that of cover coat, during the firing of cover coat, ground coat will be melted and cover coat will be boiled and change its color.

②The firing range of ground coat is usually around 60 °C. The firing range of the ground coat is mainly determined by its chemical compositions. For the ground coat which is mixed by high temperature(hard), medium temperature and low temperature(soft) enamel frits, its firing range will be wider than that of any single ground coat frit.

③ The ground coat melt should have better ability to impregnate the metal, its surface tension should be small, viscosity should be low, and the flow performance should be good. And the iron oxide on the surface of the steel sheet can be dissolved in a large amount, so that the ground coat and metal are firmly combined.

The ability of ground coat melt to impregnate the metal depends mainly on the content of oxides such as B₂O₃.

④ The ground coat should have an expansion coefficient compatible with the metal [the expansion coefficient 3α of the ground coat is (280~320)×10^-7/K].

When the enamel product is fired, the ground coat and the metal body are firmly bonded together, and if the expansion coefficients of the two are not compatible, the enamel layer may be cracked or peeled off. Since the compressive strength of the enamel is greater than its tensile strength, the expansion coefficient of the enamel should be smaller than the expansion coefficient of the metal body, so that the enamel product is subjected to compressive stress after being fired, rather than being subjected to tensile stress.

⑤ The ground coat must also have good elasticity and compressive strength.

In order to achieve the above characteristics, the chemical composition of the ground coat is very important. The chemical composition range of the common ground coat formulation is shown in the table below:

Under normal circumstances, the ratio of SiO₂, K₂O and Na₂O in the formulation determines the firing temperature of ground coat, the low temperature ground coat is 2.25% to 2.5%, and the high temperature ground coat is 2.7% to 3.0%.

When the content of SiO₂ and Al₂O₃ in the formulation is increased, the firing temperature of the ground coat will increase, and its mechanical strength will be enhanced. 

When B₂O₃ content is increased, the ground coat melt’s dissolution of FeO and the ability to impregnate the metal during the firing are enhanced. , which is beneficial to the elasticity and can reduce the firing temperature of the ground coat; 

K₂O is beneficial to improve the elasticity of ground coat; 

CaF₂ can increase the high temperature fluidity of ground coat, but over use will cause pinholes and bubbles to be generated on the enamel surface. Too little use will cause focal spots; 

MnO₂ and Fe₂O₃ can improve the adherence of ground coat.