Canned containers made of tinplate usually need to be painted on the inner surface of the container. The purpose is to prevent the chemical interaction between the contents of the can and the tank wall and the contamination of the contents with trace amounts of harmful substances, and to protect the freshness of the contents. As well as the original flavor, the starter foods can be stored, transported and sold for a long time without affecting their food value.
First, the principle of coating the coating on the inner surface of the can container is to apply a suitable coating on the smooth surface of the tinplate to a uniform, thickness controllable coating with a roller coating machine before the tinplate is cut into a can body blank. . The coating machine can perform the coating operation independently by its own feeding device, and can also be combined with a printing machine to form a printing and coating line. The process principle is shown in Figure 1.
It can be seen from FIG. 1 that the rotating stainless steel feed roller is half immersed in the supply tray, and the coated paint is transferred to the blanket cylinder by the sizing roller. There is an impression roller underneath the blanket cylinder. When the tinplate skin runs to the rubber roller, the pressure roller squeezes one end of the iron plate, and the rubber roller and the impression roller cooperate to push the tinplate through and complete the painting operation. The thickness of the coating is controlled by the feed roller, the screed roller, and the gap between the blanket cylinder and the impression cylinder. Due to the gap between the tinplate sheet and the sheet, a small amount of paint is taken away by the impression cylinder. The squeegee is used to scrape the squeegee clean (avoiding the back surface of the iron coating). The scraped coating material flows into the splicing tray and is collected and returned through the conduit to the storage drum for recycling.
Canned seam welding can not be coated with paint, or can body billet in high-frequency resistance welding process easily break through the seam and cause waste cans. Thus, the operator should slot the blanket cylinder according to the application area of ​​the can container. The slotted rubber roller of the coating was applied on the tinplate skin. The blank iron left in the slotted part was the high-frequency resistance welding part of the can (see Figure 2).
We realized in actual operation that the size of the slot should be 1~2mm wider than the actual size of the blank, so that the size of the blank to be printed out can meet the requirements of the empty cans.
After the coated tinplate is transported to the drying room via a conveyor belt, it is cured at a high temperature in the drying room driven by the circulating chain. The drying room is generally divided into three parts: the first part is to preheat the iron sheet to baking temperature; the second part is constant temperature baking; and the third part is to fully cool the iron sheet. The whole baking time needs 10 to 15 minutes. After that, the iron sheet was unloaded by an automatic plate unloader, and the coating was transferred to the canning process after passing the inspection, passing the physical and chemical tests.
2. Process Requirements Tinplate has become a leader in canned food packaging materials for a long time, and its performance is largely due to the important protective effect of internal coatings. The performance of the coating should not only have corrosion resistance, but also good adhesion, flexibility, non-toxic and tasteless. The coating must withstand the heating during high frequency resistance welding and autoclave at 121 °C without loss of performance. In addition, the type of coating must be selected based on the contents of the can and the coating thickness of the coating must be determined.
1. Types of Coatings and Applicability Epoxy-Phenolic Resin Coatings: The coating has the widest range of applications. It has good corrosion resistance for acidic foods, and its epoxy resin component gives it good adhesion and flexibility. Its baking temperature is about 200～210°C/15min. It is mainly used for food cans such as soups, meats, fish, fruits and juices. It can also be used for coating inner and outer surfaces of crown caps and other various caps. The content ratio of the two resins can vary within a wide range. Although the adjusted formula can play a role in resistance to phosphate, it will reduce the adhesion of phenolic resin. For foods with high sulfur content, zinc oxide and aluminum powder fillers may be added: to prevent the occurrence of vulcanization spots on the inner wall of the tank. Coatings containing zinc oxide are not suitable for acidic foods and cause discoloration of individual green vegetables.
Acrylic resin coating: usually added with titanium dioxide filler material formulated into white, has good flexibility, excellent sulfur resistance, mainly used for vegetables, soups and foods that contain sulfur-cured spots. The open cans have a pleasing ceramic look on the inside.
Phenolic resin coating: good coverage against chemicals has excellent corrosion resistance. It blocks the sulphide ion and does not penetrate the coating. It is suitable for meat, fish, vegetables and soups. However, the adhesion to the tinplate surface is poor, especially when the coating thickness is greater than 3-4mm, the flexibility of the coating is poor. The baking temperature is generally not more than 190°C, otherwise the coating will be thermally degraded by the pores of the tin plating layer penetrating into the matrix. Sulfur-containing compounds naturally occur in animal and plant foods. During the hot processing of cans, these compounds decompose to produce free sulfides and hydrosulfide ions, and hydrogen sulfide is released to form a vulcanization spot on the scraped part (mechanical injury) of the inner wall of the can. Hydrogen sulphide at a pH value of less than 6 can cause the production of iron sulfide at the headspace in the tank and it can fall off and contaminate the contents of the can.
It should be pointed out that the canning phenomenon occurs on the inner wall of the can, which only affects the appearance of the inner wall and does not actually affect the food. When the consumer opens the canned food, the vulcanization spot on the inner wall of the can causes the consumer to mistakenly believe that the can has deteriorated and complains to the relevant department. Therefore, the production technologist needs to understand the use of the tinplate when printing tinplate, and select the kind of coating according to the characteristics of the canned food.
2. Coating thickness of the coating One of the main purposes of the paint coating on the inner wall of the food can is to prevent the large amount of tin from being dissolved. However, some foods require some tin in the tank to dissolve to form free tin ions to maintain the appearance and flavor of the food in the tank. If there is no free tin ion, the color of light-colored foods tends to be dark, and the color of green vegetables also changes. In the absence of oxygen in the can, tin ions and food produce tin-chromium compounds to remove the sulfides in the food. These effects will enable the cans to be stored, sold, and eaten for long periods of time.
Canned foods require trace amounts of tin to dissolve, so the thickness of the coating on the inner wall of cans is generally required to be between 3.5 and 6.0 g/m2. In this thickness range, the coating has a certain porosity, and the liquid of the can content penetrates through the pores to the surface of the tin layer, slowly dissolving the tin to form tin ions, so as to achieve the purpose of extending the shelf life of cans.
For acidic foods, such as asparagus, bamboo shoots, bamboo, coconut, etc., the cans of the inner wall coating can be "composite coating layer" process for painting. The protein content of these plants is high. During the process of pasteurization and heat sterilization of cans, the hydrogen sulfide contained in the sulfur-containing amino saddles resolves more and has stronger sulfide corrosion on the tank wall. The "composite coating layer" is a "three-coat-two-baking" method in the coating process. That is, a layer of primer with strong adhesion and large plasticity is coated on the surface of tinplate with epoxy resin and xylene resin as the base material. After baking and curing, the epoxy resin and the phenolic resin-type coating are re-coated twice. (The bottom of the can and the lid are coated on the second time, and the position of the tines mouth should be turned.) After such "three-painted and two-baked" coating, the total thickness of the coating reached 10 to 13 g/m2, so that the denseness of the coating increased, and the porosity almost disappeared. The coating not only has good adhesion, but also has good flexibility and ductility, and has good canning performance.
3. Baking conditions to obtain a good coating quality, in addition to the coating itself has sulfur, acid resistance, drying room drying conditions have a close relationship with the formation of the coating structure. Baking temperature and baking time depend on the curing conditions required for the coating of the paint. Therefore, the operator must fully understand the relationship between the temperature in the drying room and the surface temperature of the tinplate in the production, so as to ensure that the paint applied on the surface of the tinplate can be cured at the required temperature and time, thereby obtaining a high-quality coating. In general, the temperature of the drying room differs from the temperature of the tinplate surface by 20°C. The temperature difference has a close relationship with the heat capacity of the drying room, the specification of the tinplate, and the printing speed, and must be determined according to actual conditions. The commonly used test method is to select five fixed points on the surface of tinplate, attach the temperature measurement paper on the surface, and then apply the iron sheet attached with the temperature measurement paper through the hot drying room, and the maximum temperature difference between the five test points is 10 Within °C. From the temperature measuring paper, the temperature and heat distribution of the drying room can be judged to know whether the coating is heated evenly.
4. The drying frame bracket scratches the tinplate surface coating and even scratches the tin plating layer, which is a serious quality problem in the printing process. A food container is scraped with a scratched inner wall. The iron base of the scratch is exposed to the contents of the can and begins to corrode. A large amount of hydrogen is generated during the corrosion process, which increases the internal pressure in the tank and causes can deformation (commonly known as an expansion tank) to be discarded. The main causes of stent scratches are:
(1) When the iron sheet is running in the drying room, due to the impact of hot air, the iron skin and the bracket are rubbed;
(2) The chain and the bracket shake during operation, causing the iron skin and the bracket to collide with each other;
(3) The stent is subjected to high-temperature baking for a long time, and the surface is oxidized. When the paint coating is not yet dry, the oxide adheres to the surface of the iron skin to form sand grains, and the tinplate causes scratches after being stacked.
To avoid and reduce the traces of mechanical scratches, the drying rack must be kept clean and bright, and it should be cleaned periodically depending on the use of the bracket. The cleaning sodium hydroxide solution heated to 80 °C, automatic cycle rinse, rinse with water and dry. Drying chain and bracket should pay attention to maintenance, to keep the operation straight and balanced. If the sheet metal jitter is too great, the wind pressure should be reduced appropriately. The specifications of the bracket, the weight and the weight of the iron sheet should be proportional to the weight to reduce the vibration amplitude during operation.
First, the principle of coating the coating on the inner surface of the can container is to apply a suitable coating on the smooth surface of the tinplate to a uniform, thickness controllable coating with a roller coating machine before the tinplate is cut into a can body blank. . The coating machine can perform the coating operation independently by its own feeding device, and can also be combined with a printing machine to form a printing and coating line. The process principle is shown in Figure 1.
It can be seen from FIG. 1 that the rotating stainless steel feed roller is half immersed in the supply tray, and the coated paint is transferred to the blanket cylinder by the sizing roller. There is an impression roller underneath the blanket cylinder. When the tinplate skin runs to the rubber roller, the pressure roller squeezes one end of the iron plate, and the rubber roller and the impression roller cooperate to push the tinplate through and complete the painting operation. The thickness of the coating is controlled by the feed roller, the screed roller, and the gap between the blanket cylinder and the impression cylinder. Due to the gap between the tinplate sheet and the sheet, a small amount of paint is taken away by the impression cylinder. The squeegee is used to scrape the squeegee clean (avoiding the back surface of the iron coating). The scraped coating material flows into the splicing tray and is collected and returned through the conduit to the storage drum for recycling.
Canned seam welding can not be coated with paint, or can body billet in high-frequency resistance welding process easily break through the seam and cause waste cans. Thus, the operator should slot the blanket cylinder according to the application area of ​​the can container. The slotted rubber roller of the coating was applied on the tinplate skin. The blank iron left in the slotted part was the high-frequency resistance welding part of the can (see Figure 2).
We realized in actual operation that the size of the slot should be 1~2mm wider than the actual size of the blank, so that the size of the blank to be printed out can meet the requirements of the empty cans.
After the coated tinplate is transported to the drying room via a conveyor belt, it is cured at a high temperature in the drying room driven by the circulating chain. The drying room is generally divided into three parts: the first part is to preheat the iron sheet to baking temperature; the second part is constant temperature baking; and the third part is to fully cool the iron sheet. The whole baking time needs 10 to 15 minutes. After that, the iron sheet was unloaded by an automatic plate unloader, and the coating was transferred to the canning process after passing the inspection, passing the physical and chemical tests.
2. Process Requirements Tinplate has become a leader in canned food packaging materials for a long time, and its performance is largely due to the important protective effect of internal coatings. The performance of the coating should not only have corrosion resistance, but also good adhesion, flexibility, non-toxic and tasteless. The coating must withstand the heating during high frequency resistance welding and autoclave at 121 °C without loss of performance. In addition, the type of coating must be selected based on the contents of the can and the coating thickness of the coating must be determined.
1. Types of Coatings and Applicability Epoxy-Phenolic Resin Coatings: The coating has the widest range of applications. It has good corrosion resistance for acidic foods, and its epoxy resin component gives it good adhesion and flexibility. Its baking temperature is about 200～210°C/15min. It is mainly used for food cans such as soups, meats, fish, fruits and juices. It can also be used for coating inner and outer surfaces of crown caps and other various caps. The content ratio of the two resins can vary within a wide range. Although the adjusted formula can play a role in resistance to phosphate, it will reduce the adhesion of phenolic resin. For foods with high sulfur content, zinc oxide and aluminum powder fillers may be added: to prevent the occurrence of vulcanization spots on the inner wall of the tank. Coatings containing zinc oxide are not suitable for acidic foods and cause discoloration of individual green vegetables.
Acrylic resin coating: usually added with titanium dioxide filler material formulated into white, has good flexibility, excellent sulfur resistance, mainly used for vegetables, soups and foods that contain sulfur-cured spots. The open cans have a pleasing ceramic look on the inside.
Phenolic resin coating: good coverage against chemicals has excellent corrosion resistance. It blocks the sulphide ion and does not penetrate the coating. It is suitable for meat, fish, vegetables and soups. However, the adhesion to the tinplate surface is poor, especially when the coating thickness is greater than 3-4mm, the flexibility of the coating is poor. The baking temperature is generally not more than 190°C, otherwise the coating will be thermally degraded by the pores of the tin plating layer penetrating into the matrix. Sulfur-containing compounds naturally occur in animal and plant foods. During the hot processing of cans, these compounds decompose to produce free sulfides and hydrosulfide ions, and hydrogen sulfide is released to form a vulcanization spot on the scraped part (mechanical injury) of the inner wall of the can. Hydrogen sulphide at a pH value of less than 6 can cause the production of iron sulfide at the headspace in the tank and it can fall off and contaminate the contents of the can.
It should be pointed out that the canning phenomenon occurs on the inner wall of the can, which only affects the appearance of the inner wall and does not actually affect the food. When the consumer opens the canned food, the vulcanization spot on the inner wall of the can causes the consumer to mistakenly believe that the can has deteriorated and complains to the relevant department. Therefore, the production technologist needs to understand the use of the tinplate when printing tinplate, and select the kind of coating according to the characteristics of the canned food.
2. Coating thickness of the coating One of the main purposes of the paint coating on the inner wall of the food can is to prevent the large amount of tin from being dissolved. However, some foods require some tin in the tank to dissolve to form free tin ions to maintain the appearance and flavor of the food in the tank. If there is no free tin ion, the color of light-colored foods tends to be dark, and the color of green vegetables also changes. In the absence of oxygen in the can, tin ions and food produce tin-chromium compounds to remove the sulfides in the food. These effects will enable the cans to be stored, sold, and eaten for long periods of time.
Canned foods require trace amounts of tin to dissolve, so the thickness of the coating on the inner wall of cans is generally required to be between 3.5 and 6.0 g/m2. In this thickness range, the coating has a certain porosity, and the liquid of the can content penetrates through the pores to the surface of the tin layer, slowly dissolving the tin to form tin ions, so as to achieve the purpose of extending the shelf life of cans.
For acidic foods, such as asparagus, bamboo shoots, bamboo, coconut, etc., the cans of the inner wall coating can be "composite coating layer" process for painting. The protein content of these plants is high. During the process of pasteurization and heat sterilization of cans, the hydrogen sulfide contained in the sulfur-containing amino saddles resolves more and has stronger sulfide corrosion on the tank wall. The "composite coating layer" is a "three-coat-two-baking" method in the coating process. That is, a layer of primer with strong adhesion and large plasticity is coated on the surface of tinplate with epoxy resin and xylene resin as the base material. After baking and curing, the epoxy resin and the phenolic resin-type coating are re-coated twice. (The bottom of the can and the lid are coated on the second time, and the position of the tines mouth should be turned.) After such "three-painted and two-baked" coating, the total thickness of the coating reached 10 to 13 g/m2, so that the denseness of the coating increased, and the porosity almost disappeared. The coating not only has good adhesion, but also has good flexibility and ductility, and has good canning performance.
3. Baking conditions to obtain a good coating quality, in addition to the coating itself has sulfur, acid resistance, drying room drying conditions have a close relationship with the formation of the coating structure. Baking temperature and baking time depend on the curing conditions required for the coating of the paint. Therefore, the operator must fully understand the relationship between the temperature in the drying room and the surface temperature of the tinplate in the production, so as to ensure that the paint applied on the surface of the tinplate can be cured at the required temperature and time, thereby obtaining a high-quality coating. In general, the temperature of the drying room differs from the temperature of the tinplate surface by 20°C. The temperature difference has a close relationship with the heat capacity of the drying room, the specification of the tinplate, and the printing speed, and must be determined according to actual conditions. The commonly used test method is to select five fixed points on the surface of tinplate, attach the temperature measurement paper on the surface, and then apply the iron sheet attached with the temperature measurement paper through the hot drying room, and the maximum temperature difference between the five test points is 10 Within °C. From the temperature measuring paper, the temperature and heat distribution of the drying room can be judged to know whether the coating is heated evenly.
4. The drying frame bracket scratches the tinplate surface coating and even scratches the tin plating layer, which is a serious quality problem in the printing process. A food container is scraped with a scratched inner wall. The iron base of the scratch is exposed to the contents of the can and begins to corrode. A large amount of hydrogen is generated during the corrosion process, which increases the internal pressure in the tank and causes can deformation (commonly known as an expansion tank) to be discarded. The main causes of stent scratches are:
(1) When the iron sheet is running in the drying room, due to the impact of hot air, the iron skin and the bracket are rubbed;
(2) The chain and the bracket shake during operation, causing the iron skin and the bracket to collide with each other;
(3) The stent is subjected to high-temperature baking for a long time, and the surface is oxidized. When the paint coating is not yet dry, the oxide adheres to the surface of the iron skin to form sand grains, and the tinplate causes scratches after being stacked.
To avoid and reduce the traces of mechanical scratches, the drying rack must be kept clean and bright, and it should be cleaned periodically depending on the use of the bracket. The cleaning sodium hydroxide solution heated to 80 °C, automatic cycle rinse, rinse with water and dry. Drying chain and bracket should pay attention to maintenance, to keep the operation straight and balanced. If the sheet metal jitter is too great, the wind pressure should be reduced appropriately. The specifications of the bracket, the weight and the weight of the iron sheet should be proportional to the weight to reduce the vibration amplitude during operation.