Analysis on the quality problems in the production and use of paint iron

Analysis on the quality problems in the production and use of paint iron

The light industry standard QB/T 2763-2006 "coated tin (or chrome) steel sheet" (referred to as coating iron) was released on August 19, 2006 by the National Development and Reform Commission and implemented on December 1, 2006. This standard is for the light industry standard QB/T 3773-1999 issued by the former National Light Industry Bureau (the national standard GB8230-87 before the 1999 standard rectification) "epoxy phenolic coated tinned (or chrome plated) steel sheet" Revised. During the revision process, we investigated various representative coating iron products. From the test results, we can see that the overall quality level of coating iron has been significantly improved. Most of the products can meet the original standard QB/T 3773- The technical indicators specified in 1999; according to the revised standard QB/T 2763-2006, acid resistance (except for chrome-plated iron due to test solution is not suitable), sulfur resistance, appearance, adhesion, film thickness and plate thickness The compliance rate is higher, 100%, 98%, 94%, 90%, 92%, respectively, while the compliance rate of corrosion resistance and impact resistance is relatively low, 75% and 88% respectively. The cause of the corrosion problem of food and beverage cans seen in recent years is related to the poor corrosion resistance, impact resistance, adhesion and other properties of the coating iron; of course, in addition to the quality of the coating iron, improper use can lead to the tank wall. Severe corrosion or perforation. The following is an analysis of the cases in the test.

First, the quality problems caused by the production of paint iron

1, raw materials

(1) Tinplate in the paint iron standard QB/T 2763-2006 specifies the iron requirements for food and beverage cans, that is, tinplate should meet the requirements of GB/T 2520 grade I tinned steel sheet, and its surface energy is suitable for the whole sheet. Printed. However, sometimes companies use low-quality or tinplate that is incompatible with food corrosion characteristics in order to reduce costs, resulting in abnormal corrosion of the tank wall.

Canned sauce, coconut juice and other cans are not corrosive, but due to the poor quality of tinplate tinplate, the nominal tin plating capacity of 2.8 g/m2 is only 2.25 g/m2, and the iron point is much more than normal iron. Although the film thickness is 6-8 g/m2, the cans have a large amount of rust in the expansion ring of the top and bottom cover after several months of storage; and the tin content of canned food such as cherry juice, rare fruit and aloe vera is 1.1 g/ M2 tinplate, the coating film is very thin, the minimum coating thickness of the top cover and the can body of the jar is 3.3 g/m2, 2.3 g/m2, and the other two products are 5-6g/m2. Soon, the cans of the jars of Zhenguolu were covered with intensive corrosion points. After a few months of canning of cherry juice and aloe vera, corrosion perforations appeared on the tank and leaked. The amount of dissolved iron reached 20ppm.

The surface condition of the tinplate is poor. For example, the oxide film on the surface of the tinplate is too thick (coated after one year of low-chromium iron storage for canned mushrooms), or the oil film is too thick, which affects the adhesion of the coating film and is not resistant to corrosion caused by processing. .

The quality of canned products caused by the irreversible corrosion characteristics of tinplate and food, such as luncheon meat, pork ribs, chicken, sweet corn, broad beans and other sulphur-containing food cans, all the indicators of coating iron used meet the original standard requirements, but canned Severe sulphide spots appear after storage and even sterilization. The reason is that the tinplate has a low degree of passivation and the surface chromium content is only about 1 mg/m2, which also proves that the surface of the tinplate is passivated (the 311 passivation should be used). The important role of sulphide corrosion. If the iron is used for a corrosive acidic food, there is no problem because the hydrated oxide of chromium and chromium in the passivation film dissolves quickly in the acidic solution and does not have a corrosion resistance. The above cases of sulphide corrosion also reflect the problem of poor correspondence between the original standard sulphur resistance test method and the actual use effect (these coating irons can pass through the sulfur test), so the standard QB/T 2763-200 anti-sulfur test The method has been revised to use the internationally accepted cysteine ​​hydrochloride method, which can effectively predict the sulfur resistance of coating iron.

(2) The quality of the coating and solvent has a direct impact on the leveling, the compactness of the coating film and even the hygiene problems of the food in the can.

When using a coating of unqualified quality, the coating will not be fully cured even under the normal coating and baking conditions, and the coating will not be fully cured after the film is sterilized, which will cause the coating iron to fail to meet the hygienic standard: epoxy phenolic The problem of excessive consumption of free phenol, free aldehyde and potassium permanganate of coating iron was solved after the improvement of the coating formulation. In addition, the above phenomenon occurs due to the self-polymerization of the paint for a long time. Any coating film that cannot be completely cured not only affects the pore properties of the polymer coating film, but more importantly, the organic matter that is not involved in the reaction in the coating film will migrate to the tank to bring hidden dangers to food safety.

The ratio of the solvent and the impurities in the coating also cause problems in the quality of the coating iron. For example, the lack of high-boiling components in the mixed solvent affects the solubility of the coating and its wettability and adhesion to the surface of the tinplate. At the same time, small bubbles formed by the large amount of low-boiling solvent in the coating during baking also affect the small bubbles. Micropores and impact resistance of the coating film. Another example is the coating iron with obvious impurities, the discontinuous gap between the coating film, and the phenomenon that the fat sea sponge is corroded and perforated after several months; when the wax content as a lubricant in the paint is high, the coating film is dense. Small bubbles affect the pore and impact resistance of the coating film, and the problem of corrosion spots on the top and bottom covers after the ketchup is installed.

The type of coating is not suitable for the corrosion problems caused by food characteristics. For example, a fully open lid made of aluminum paste paint iron is used for acidic foods with a pH of less than 5, and the phenomenon of corrosion perforation in rivets and scribe lines is an example. Because the aluminum paste is easily soluble in acidic decomposing, the corrosion resistance of the material is lowered, and the rivet and the scribe line itself are the parts where the coating film is weak and the stress is concentrated, so it is easy to cause concentrated corrosion.

2, processing technology

(1) The important effect of the coating process on the uniform density of the coating film cannot be ignored.

In order to save energy, some companies often use unreasonable processing techniques, such as the use of corrosive acidic foods, such as iron, which should be double-coated. The physical coverage can effectively reduce the number of micropores. The chemical bonding also enhances its compactness and adhesion. However, the thick coating film obtained by single-layer coating has a high viscosity and poor leveling property, and the micropores of the coating film are inevitably increased. Therefore, a large area of ​​diffusion corrosion spots or dense is generated on the tank wall after the canning. Corrosion point, serious film peeling phenomenon also occurs. Canned tomatoes such as ketchup and syrup have experienced such problems this year. Therefore, the coating process of the coating iron must be based on the corrosiveness requirements of the installed product, and the cost can be saved under the premise of ensuring the quality.

The double-coated coating iron also has an effect on the workability of the coating film due to the improper thickness of each layer of the coating film. For example, the coating iron consisting of aluminum paste and organosol is thicker and softer than the aluminum paste coating film, and the organosol coating film with good processability is thin, so the coating iron is not rubbed, and the cover is fully covered when the cover is made. The coating film is damaged and has micropores. Corrosive perforations are formed in the defect area after the canned fish in the tomato sauce.

(2) Corrosion problems caused by insufficient baking. The coating does not reach enough heat to carry out sufficient cross-linking reaction, which not only affects the pore property of the coating film, but also affects the flexibility of the coating film. When the coating is applied, the coating film cannot meet the micro-cracking caused by the new geometry after the deformation of the substrate, that is, the coating Potential corrosion sites behind the tank.

Asparagus, beef and other canned foods with insufficient baking iron, and obvious iron sulfide appeared in the bottom cover expansion ring and the buried head soon after storage. The above corrosion conditions are consistent with the results of the intensive corrosion points that occur during the impact resistance test. After the coating iron is baked again, the impact resistance is improved.

The judgment and mastery of the degree of baking should not be limited to the results of the sterilization test, or only the strength of the adhesion. Often coatings with appropriate thickness, adhesion, and resistance to acid can also cause intensive corrosion spots after coating the ketchup. Because the coating has not yet reached the optimum degree of curing, if the baking heat is increased and the curing is sufficient, the corrosion resistance will be There has been a noticeable improvement. Therefore, it is necessary to comprehensively measure the quality of the coating iron which is highly corrosive, especially the corrosion resistance and impact resistance of the oven. The corrosion resistance index specified by QB/T 2763-2006 has a good practical significance. The correlation between the test results and the corrosion of canned foods is good. Any material that does not reach the corrosion resistance is prone to corrosion problems after being filled. Therefore, it is necessary to strictly control the baking process according to the coating supplier and ensure the normal operation of the drying room to control the quality of the coating iron. In short, the quality of tinplate, paint and solvent is closely related to the performance of coating iron. Although the polymer organic coating has the function of blocking the contact between the food and the metal, the thickness of the coating film is limited, especially for the commonly used epoxy phenolic coating, which is mostly 5 to 10 micrometers, and the coating film is too thick to weaken the adhesion. The coating film is always more or less microporous, and it is inevitable that the corrosive component in the food is in contact with the metal through the micropores of the coating film when the can is stored. Therefore, the coating quality and surface passivation condition of tinplate must be considered in the processing of coating iron, and the coating iron for strong corrosive food or embossed cans should also have an appropriate amount of tin plating. In the case of saving tin resources and material costs, and using tinplate tinplate with low tin plating capacity, paint iron producers must compensate for the reduction in tin plating and reduce the corrosion resistance by relying on the compactness of the coating film, relying on qualified raw materials and specifications. Reasonable processing technology is a necessary condition to ensure the quality of coating iron.

Second, the corrosion problem caused by improper use of paint iron

1. The influence of canning on the performance of coating iron The mechanical damage caused by improper adjustment of the maintenance of the can equipment will weaken the anti-corrosion effect of the coating iron, and the corrosion condition of the tank wall depends on the degree of damage of the coating film and the food loaded. Characteristics.

(1) The application is slightly damaged, and the surface scratches that do not damage the tin layer will increase the micropores of the coating film, but have little effect on the corrosion of the tank wall of general corrosive products, especially the tank body is plain iron, top and bottom. The canned fruit canned with coating iron, due to the cathodic protection provided by the large area of ​​the tin-melting reaction of the can body, the tin layer at the scratch of the bottom cover coating film does not cause corrosion problems.

(2) When the coating film is severely scratched, and the coating iron has dew or dew, the defect of the food through the coating film is electrochemically reacted with tin or iron to dissolve tin or iron, and the corrosion characteristics of the food It is determined that the block-shaped diffusion corrosion spots are even caused to peel off the coating film, or to penetrate the iron and perforate, both of which cause the food and beverage cans to lose the commercial value.

Ketchup and Red Bull beverages are products with a strong tendency to dissolve tin. The uniformity and uniformity of the coating film are required. Therefore, if there is severe abrasion, the contents will corrode with the tin of the incomplete film and spread to the surroundings. As the iron also dissolves, the adhesion decreases and the coating film falls off.

Asparagus and luncheon meat are sulfur-containing foods that are inclined by iron. When the top cover film is severely scratched, in the condition that there is residual oxygen in the headspace, the hydrogen sulfide released by the protein during the sterilization of the cans will be coated with the film. The iron dissolved in the defect produces iron sulfide. Although the aluminum paste coating iron has good sulfur resistance, it is especially necessary to pay attention to the maintenance adjustment of the can making equipment because the coating film is soft and easy to be damaged during the can making.

When the chrome-coated coating is severely damaged, corrosion perforation is usually caused in some highly corrosive acidic foods. For example, the coated chrome-plated iron used for the canned bottom cover of sugar water strawberry has a low coating thickness of only 5-7 g/m 2 and is severely scratched, and as a result, the bottom cover is perforated after several months of canning. The characteristics of such pitting corrosion are determined by the electrochemical properties of the metal. The chrome-plated iron is an absolute anode. Although the coating of the chrome-plated coating is excellent with the adhesion of the substrate, the chrome plating is extremely Thin, mostly at 50-80mg/m2, the number of micropores is much higher than that of tin-plated iron. Once the metal is exposed by scratches, the probability of iron is high, and the surface chromium has no cathodic protection to the iron at the micropores. Iron corrosion is easy to proceed in depth and eventually leads to perforation. For the application of chrome-plated iron, it is particularly important to control the thickness and integrity of the coating film. The measurement of the coating thickness of chrome-plated iron and the acid-proof test method have new regulations in QB/T2763-2006.

Severe mechanical damage can sometimes cause stress corrosion and cracking of cans. For example, the recently detected canned body of the mushroom can be partially cracked and the leakage is caused by typical stress corrosion. The main reason is that the wire is more scratched, and the scratch is deeper when the plate is cut. After the canning, obvious stress corrosion occurs in the pressing part of the tin layer, and one or more places can be seen on the tank wall. Split in a straight line. This kind of corrosion problem has also occurred in foreign countries. According to their research report: sulfur-containing and medium-corrosive canned foods packed with paint iron, when tinplate is sensitive to stress corrosion, residual stress during empty can processing, free tin of tinplate When the amount is low or the plating of ferrochrome is uneven and the coating film is poorly covered, stress corrosion and cracking are liable to occur.

2, problems caused by real can processing

(1) When the sealing machine is improperly adjusted or the mold is worn, the coating operation of the top cover coating film in the hot state after the canning is damaged, and the damage of the coating film at the time of fully opening the cover rivet is higher than that of the empty can manufacturing more serious.

Corrosion caused by damage to the outer coating film and metal exposure. Because the canister is stored with oxygen, the moisture, salt, sulfur or other corrosive components in the air react with the molten iron at the defect of the curling film, and finally produce reddish brown or orange yellow rust. This rust is very common this year.

Improper sealing and sealing of the fully-opened rivet has obvious indentation and deformation, the height of the cover surface and the degree of burying become large, and the inner coating film is seriously damaged. After filling, the concentrated corrosion or even perforation problem occurs at the defect. This year in the fruit There have been cases of canned vegetables.

(2) After cooling, excessive cooling and insufficient drying, beverage cans packed with plastic film shrinkage, local rust of the outer wall caused by residual moisture on the surface of the can, and residual cleaning after wiping off the oil outside the canister after sterilization and cooling The agent causes the problem of rust at the rivet of the fully open cover.

(3) The micro-leakage caused by poor sealing or the low vacuum of the can caused by the improper canning process causes excessive residual oxygen in the tank to promote corrosion of the inner wall of the tank.

(4) The food raw materials are not fresh (not processed in time), or there are residual pesticides in the raw materials. When the sulphuric acid escapes too much during sterilization, serious sulphide spots may occur, such as mushrooms, oxtail soup, steamed beef, etc. This type of corrosion problem; in addition, improper handling of raw materials, such as Bailing mushroom, canned coconut dioxide in the canned coconut, resulting in significant sulphide corrosion.

In summary, the causes of abnormal corrosion caused by canned food and beverage in paint iron packaging are various, sometimes caused by various factors. Therefore, in addition to selecting qualified raw materials suitable for food characteristics, reasonable processing technology, and strict control of the quality of coating iron, the standard empty can and solid can processing has the same important role in preventing and slowing the corrosion of the tank wall.

Finally, the relevant issues in the implementation of the coating iron standard are explained:

a) Printing iron standard QB/T 2763-2006 printing error correction: P56.7.1 third line should be: acid sodium disodium (Na2HP・12HP20) 7.20g.

b) Paint Iron Standard 2763-2006 If it is not available locally, contact the Light Industry Standardization Institute for mail order. contact number

c) The 3M adhesive tape used in the adhesion test can be contacted by Zhao Juan, the 3M agent of Beijing Yongjiaxin Printing Co., Ltd., by mail order, or by telephone number 13269660158.

d) Corrosion resistance and film thickness measuring instrument can be contacted by the China Canning Association Technical Center for mail order, contact telephone number, and the instrument can also be used for empty tank defect current measurement.