Introduction of Steel Enamel Cover Coat

Enamel cover coat is coated on the ground coat layer of enamel, to achieve the required whiteness of enamel products, gloss, acid resistance, alkali resistance, alkali resistance, water resistance, atmospheric resistance and other requirements, so that the product has a bright and beautiful practical enamel surface.

Steel enamel cover coat according to the different emulsions used can be divided into antimony cover coat, titanium cover coat, tin cover coat, zirconium cover coat, cerium cover coat, etc.. In our country more commonly used steel enamel cover coat is titanium cover coat and antimony cover coat. Due to the higher cost of tin enamel frit, zirconium enamel frit melting temperature than the general enamel frit is high, and acid resistance is poor, so these two enamel frit is rarely used.

Steel enamel antimony white cover coat has a special greenish-white phase, bright colours after decoration, while antimony enamel frit has strong impact resistance. The antimony cover coat has a poor emulsion ability, and generally needs to be enamelled twice, with a thicker coating and a larger amount of enamel. Antimony may be present in the enamel in the form of toxic trivalent antimony, so it should not be used in products that come into contact with food.

Titanium cover coat emulsion ability than antimony cover coat, coated once can get a good emulsion degree, titanium cover coat chemical stability and thermal stability are better than antimony cover coat. Due to the crystal transformation of titanium oxide will affect its emulsion and colour phase, making fusing and firing are more difficult to master. White titanium enamel frit on chromium, iron and other metal colouring ions are more sensitive, porcelain surface is easy to yellow, so the chemical composition of titanium enamel frit and manufacturing process must be strictly controlled.

Ordinary daily use enamel cover coats must meet the following requirements.

① Have good emulsion, high gloss, fine porcelain surface

② Expansion coefficient should be compatible with the ground coat (slightly less than the ground coat), titanium enamel frit expansion coefficient in (270 ~ 300) x 10-7 / K antimony enamel frit in (280 ~ 340) x 10-7 / K.

③ Firing temperature should be slightly lower than the ground coat (except for the cover coat of the two enamels and one firing process), so as to avoid flooding the ground coat when firing.

④ Viscosity should be smaller than the ground coat, in order to facilitate the porcelain surface smooth and delicate.

⑤ Low surface tension.

⑥ Good physical and chemical properties such as acid resistance, alkali resistance, water resistance and heat resistance.

⑦ Good elasticity and wear resistance and other mechanical properties.

⑧ Colour enamel frit should also have beautiful colours and stable tones.

Cover coat should have these performance requirements, in addition to certain fusing and firing process parameters to ensure, mainly depends on the chemical composition of the cover coat.

The chemical composition of the cover coat for ordinary daily steel enamels ranges from:

Antimony cover coat

SiO2: 35~45%

B2O3: 5~8%

Al2O3: 6~8%

K2O+Na2O: 12~18%

CaF2+Na2SiF6： 5~15%

Sb2O3: 10~15%

Titanium cover coat

SiO2： 38~50%

B2O3： 15~20%

Al2O3： 0~3%

K2O+Na2O： 10~15%

Na2SiF6： 5~10%

TiO2: 15~20%

Normally, the ratio of SiO2 to K2O+Na2O in the formula determines the firing temperature of the cover coat. The ratio of antimony enamel frit is 2.0~2.5, antimony light cover coat is 3.5~4.0, titanium cover coat is 3.5~4.0 and titanium coloured cover coat is 2.5~3.0.

In the steel plate enamel cover coat, SiO2 content increase, can improve the chemical stability of the enamel frit, thermal stability and gloss; dosage is too high, the firing temperature of the enamel frit, surface tension increases, easy to make the porcelain surface burn shrinkage, line pattern and other defects. b2O3 content increases, can improve the thermal stability of the enamel frit, gloss and elasticity, but the emulsion of antimony enamel frit is not good. k2O + Na2O content is too high, easy to make the surface CaF2 is too high to make the antimony enamel frit surface rough, poor luster.