After selecting high-quality tinplate, coatings and formulating a suitable processing plan, the final task is how to ensure the correct implementation of the anti-corrosion process in daily construction. The following issues are worth noting.
1. Baking temperature
Baking temperature is one of the key factors that restrict the quality of anti-corrosion coating film formation. The role of baking is to cross-link and polymerize the film-forming substances in the resin to form a film, and heat and evaporate most of the solvents and additives. Too low baking temperature will directly lead to insufficient cross-linking of the resin, reduced polymerization between the polymers of the coating resin, poor density and stability of the coating, making it easy for the contents to penetrate the coating and corrode the tinplate; the hardness of the coating will also be affected by insufficient cross-linking of the resin, making it very easy to produce mechanical damage on the coating surface during post-processing, which brings hidden dangers to anti-corrosion. Insufficient baking temperature may even cause some high-boiling point solvents to fail to completely evaporate and form residues, which is absolutely not allowed by the hygiene requirements of food cans. However, it is not good to have a baking temperature that is too high. A baking temperature that is too high will cause the coating film to become brittle and excessively cross-linked, and ultimately change and reduce the original chemical properties of the resin, and will also fail to achieve the desired anti-corrosion effect. Therefore, for the anti-corrosion coating used, it is necessary to ask the supplier for the corresponding technical and processing condition parameters to correctly set the baking temperature of the coating.
In order to obtain the correct baking temperature, we should also understand and master some characteristics of the drying room.
Generally speaking, the baking temperature on the coating instruction manual refers to the surface temperature of tinplate, and the surface temperature of tinplate is often 10 to 20°C lower than the indicated temperature of the drying room. This temperature difference is related to the performance of the drying room, production speed, and size of the sheet. This factor must be fully considered during construction.
To test this temperature difference, it is best to use a five-point temperature tester to measure the temperature rise curve of the drying room every six months, and adjust it to meet the requirements, while recording the difference between the displayed temperature and the actual temperature. Enterprises with limited conditions can also use temperature test paper to test different sections of the drying room. Only in this way can the baking process be correctly implemented.
2. Resins, solvents and various additives in anticorrosive coatings
In addition to the resin component, the anticorrosive coatings for the inner wall of food cans also contain other auxiliary substances, such as solvents, diluents, plasticizers, etc.
It is necessary to explain the difference between solvents and diluents here.
Solvents refer to liquids that can dissolve solid or semi-solid resins to form useful solutions. They mainly play the role of dissolving resins, also known as true solvents. Diluents cannot dissolve resins, but can adjust the viscosity and dilute the coating to adapt it to coating processing, also known as false solvents.
Solvents and diluents are for different resins. For example, xylene can dissolve polyamide resins but cannot dissolve nitrocellulose resins. Therefore, xylene is a solvent for polyamide resins, but can only be a diluent for nitrocellulose resins.
In production, we are most concerned about the master-slave relationship between the evaporation rates of solvents and diluents. This is because during the baking process, all components in the coating begin to evaporate at the same time, but due to their different evaporation rates, the coating film presents different properties during the heating process, and the final structure of the coating film mainly depends on the role of the solvent with low evaporation. Therefore, the coating should use a suitable good solvent to make it evaporate at the latest (but it must evaporate completely within the baking time), so that the resin polymer has enough time to fully cross-link and ensure that the anti-corrosion function of the coating is maximized. If the good solvent evaporates too early, it will cause the polymer macromolecule chain to curl, and the coating will be discontinuous, thereby reducing the corrosion resistance. What's more, if the solvent evaporates earlier than the diluent, the resin will not be effectively dissolved and precipitate, and eventually cause serious defects in the coating, so it should be paid attention to.
In addition to the evaporation rate, too much diluent during construction will also reduce the quality of the coating. The large amount of concentrated volatilization of the diluent is also prone to "pinhole" failures, forming iron spots, which is very unfavorable for anti-corrosion.
Plasticizers can increase the flexibility of the coating and improve the adhesion of the coating, but at the same time they will also reduce the coating's ability to resist water, alkali, acid, solvent, and oil, so the addition of plasticizers should be considered comprehensively.
In short, when adding various additives, if you add them manually without an automatic control device, you must add them frequently and in small amounts and strictly control them.
3. Other processing conditions
Experienced tinplate printers have encountered this situation. In seasons with low temperature and high humidity, especially in rainy weather in the south, there will be a thin layer of water mist on the surface of tinplate, which is easy to produce "eye holes" during coating, and eventually the contents will be corroded at the "eye holes". The solution is to transport the tinplate to the workshop a few days in advance to adapt to its temperature and humidity conditions. If necessary, the moisture of the tinplate should be dried at low temperature before coating.
In addition, the production environment of the workshop strives to be clean and dust-free. If there are dust or impurities in the coating, the coating film will produce "eye holes" centered on the dust. A filtering device can be added during coating.
The requirements for the coating rubber roller are moderate hardness, smooth surface, and corrosion resistance. After the rubber roller has been used for a period of time, the corrosion of the solvent will cause "skin" on its surface. When using this rubber roller for construction, the coating film has poor leveling and is also easy to produce "eye holes", which can be solved by grinding the rubber roller. Eliminating quality defects such as "eye holes" is of great significance to corrosion prevention.
