Improvement of non-stick feature on enamel coated parts

Enamel is an inorganic coating material applied to cast iron or metallic parts. It is baked at high temperatures and creates a glassy structure on the surface. Enamels are one of the preferred industrial coating methods since ancient times. It was developed by being inspired by the shining of precious stones in ancient times. Enamels are important coating materials required for metal parts to be resistant to external influences. It is preferred because it increases the resistance against corrosion, chemicals and external factors of the materials they are coated. In addition, the long-term preservation of the aesthetic properties of the enamel ensures that the substrate is not damaged and is protected for a long time. The contents of the enamels are developed according to the desired feature and their formulas are revised. The main component of enamels are frits. For enamel, frits must form. The properties expected from enamels are taken into account when creating frits. The surface on which the frit composition will be applied is determined according to the property and quality expected from the coating material. Frits are composed of different inorganic oxidic compounds. Inorganic metal oxides melt in high-grade furnaces to form a borosilicate glass structure and this structure is called frit. Enamel is formed by grinding the frits formed according to certain dimensions in ball mills with various additives. The enamels formed are applied according to the appropriate method and are baked at the specified temperatures and times, spread on the surface and form a coating. The enamel coating has high chemical resistance, provides a shiny and an aesthetic appearance to the parts. For this reason, the usage areas of enamels are quite common today. They are used in many areas such as ornaments, stove pipes, sinks, bathtubs, kitchenware. The most frequently used areas are kitchen utensils, cooking devices. However, the non-stick properties of enamels on surfaces that come into contact with food are not good. This study was carried out to improve the non-stick property. In the study, the 4 process inputs that will affect the wetting angle the most were determined by the cause and effect matrix method as frit rates, corundum amount, cooking temperature and cooking time. Three different levels have been determined for process inputs. The necessary trials for 4 process inputs and 3 different levels were determined and applied by the Tagucchi method. Three new frits have been formulated to increase the wetting angle. While the preliminary study of these 3 frits was carried out, many frits with high amounts of zircon dioxide, phosphorus penta oxide and aluminum oxide were tested, unlike the frits used in the market. When we use all of these materials in the same frit, the non-stick feature did not develop at the desired level. xxvi When frits were designed separately and only a high zirconia frit recipe was prepared, it was observed that there was no improvement in wetting angle, although the frit's chemical resistance was high. Likewise, when the frit recipe with only phosphorus penta oxide was prepared, the chemical resistance of the frit decreased although the wetting angle of the frit was high. Although chemical resistance improved only in high aluminum oxide frit trials, no increase in wetting angle was observed. Chemical resistance and wetting angles are balanced by defining a high amount of phosphate in frits made with high zirconia doxide and frits designed with high alumina, due to excessive deformation in the melting pot only in recipes with high phosphorus penta oxide, and also because the chemical resistance of these frits is very low. In order to balance the expansion coefficient of the final product, a frit with low expansion coefficient, high silicon and high chemical resistance is designed, and thus the high expansion coefficients of the other two frits used in the mixture will be reduced. The ratios of these frits were changed among themselves, different milling conditions and cooking conditions were determined and experiments were carried out. Frits are ground in alumina ball mills with additives determined according to the application method of the enamel and the desired color, chemical resistance, thermal shock resistance of the enamel. The grinding process has an important place in this process. The grain size measurement method of wet enamels is the Bayer technique. For the enamels to be applied as spray, the grain size is generally 0.5 – 1.5 Bayer / 150 mesh, while the grain size for the enamels to be applied as dipping is 2 – 5 Bayer / 150 mesh. In this study, the Bayer value is in the range of 0.5-1, since the application was made as a spray. The effect degrees of cleanability, gloss, color, surface, adhesion and citric acid resistance properties that can affect the non-stick properties of enamels were determined. In the degree of impact, 1 represents the least and 10 the most. It is known that the wetting angle must be increased for the development of non-stick properties. The process inputs that change the properties affecting the wetting angle in the process from the enamel preparation process to the cooking and how effective these inputs are have been determined by the cause and effect matrix. The high values in the cause-effect matrix were focused on and it was decided to work on these values while designing the experiment, in order to see the high effect. As a result of the calculations, 4 process inputs that affect the non-stick property the most were determined. Frit rate, corundum amount, cooking degree and cooking time were determined as variable parameters, while mill additives, grinding fineness, application thickness and application method were determined as fixed parameters. They were kept constant in all experiments. 3 different values were determined for variable parameters. Corundum amounts, cooking degree and cooking time were determined based on previous laboratory studies. The mixing and cooking conditions required for 3 different levels with the determined 4 process inputs were determined according to the Tagucchi experimental design method. Fixed mill additive recipe was used. The bayer value of the prepared enamels was determined as 0.5 - 1.0 Bayer range, the application method was wet spray application, and the application thickness was determined as 200 - 250 micrometers (primer + second layer).xxvii Apart from quartz or potassium feldspar, which is used for hardening in traditional wet enamel mill formulas, corundum is preferred. Since corundum is a harder material than other materials, it will not melt completely on the surface during cooking and will form bumps and voids. In this way, it is aimed to increase the wetting angle. Experimental design was prepared with different frit ratios and different corundum amounts, since the surface appearance will be distorted if corundum is used in high amounts or if it is used in a hard frit combination, if it is not fully spread on the surface. The citric acid, ETC test, color, gloss values, coating thicknesses, wetting angles were measured by grinding in alumina mills for a period determined by varying amounts of corundum and fixed mill additives. In order to compare the analyzes, measurements were made on the ETC enamel coated plate and the wetting angle value was measured as 33°. Trials numbered 2,5,8 gave the best results. The highest result among them was measured at 78.8° in experiment number 5. According to these values, it was seen that the frit usage rates were the most effective and it was concluded that the wetting angle increased when the usage rate of A frit increased. When we look at the amount of corundum, the amount is 10% in designs no. 2 and 5 with successful test results, but it is 1% in design no. 8. This shows that the corundum ratio is determined according to the hardness of the frit mixtures and the cooking temperature. This study was carried out to increase the non-stickiness of a second layer of enamel and a frit design was made accordingly. Although it seems difficult to design a single layer of enamel with high zircon, alumina and phosphorus in frits, frits with lower alkali oxide can be designed by using materials with high chemical resistance such as bismuth oxide and lower melting point than zircon and other refractories. It is thought that wetting angles can be increased by adding aluminum metaphosphate or zirconium phosphates in the mill formula by decreasing the phosphorus pentoxide ratio in the frits in order to prevent excessive use of phosphorus pentaoxide in the frit from causing excessive deformation in the frit melting pots. It can be considered as a recommendation for future studies.