Quality and safety control of food packaging metal containers (1)

Quality and safety control of food Packaging Metal Containers (1)

Xin Qiaojuan

With the improvement of people's living standards and the changing concept of consumption, the requirements for product packaging are getting higher and higher. At present, packaging products are widely used in food, beverage, medical and petrochemical and petrochemical industries, and are an indispensable industry in the economy and life. According to information provided by the World Packaging Organization (WPO), international packaging sales revenue in 2011 was 592.2 billion US dollars, accounting for 2% of global GDP. Among them, Metal Packaging accounts for about 24%, second only to paper packaging and plastic packaging, and is the third largest packaging material. When it comes to metal food packaging, it is easy to think of the packaging of canned foods such as beer cans, beverage cans, octopus porridge cans, milk powder cans, ketchup cans, and canned fish. In fact, honey and edible oil are packed in 200 liter metal packaging drums. Food additives such as jams, flavors and fragrances are also an indispensable part of food manufacturing and processing, and an important line of defense for food safety.

At present, food safety has become a topic of concern to the whole society. Consumers have high expectations for the production process of food and the use of packaging materials. Here we mainly talk about the metal packaging of food.

Metal packaging is divided into two major branches: metal barrels and metal cans: in general, less than 18 liters of metal packaging made of Tinplate with a thickness of 0.14 to 0.35 mm is called a Metal Can, and a cold-rolled steel sheet with a thickness of 0.30 to 1.50 mm is used. The metal packaging of more than 16 liters produced is called a metal barrel.

Due to the corrosion of metals and the considerations of safety, health and aesthetics, metal packaging often requires coating protection and decoration, and thus forms a unique coating market - packaging coatings. Below we mainly talk about the quality and safety control of the production process of metal food packaging containers.

First, the coating of food metal packaging

Whether it is a food metal barrel or a metal can, as long as it is used to package food, since the metal itself reacts with other substances, it is generally coated with a layer of packaging paint. This coating not only protects the metal itself from corrosion, but also isolates the packaged material from the metal to prevent the two from reacting, resulting in contamination of the contents. The metal surface is decorated to make it beautiful, and the outer coating is printed with beautiful patterns and colors to promote and promote.

The packaging coating is divided into two types: inner wall and outer wall coating. As long as the outer wall coating can ensure that the metal packaging barrel (or can) is stored in the open air, the appearance does not decolorize, does not lose light (the weather resistance of the coating), has corrosion resistance, and the film after the water is sprayed. It does not fall off (strong adhesion) and other properties. The inner wall coating of the packaging metal barrel (or can) is directly in contact with the food, and the inner wall coating should be non-toxic, harmless and tasteless after being cured by coating, and meet the requirements of the US FDA food and drug packaging coating. . Straightforward, it means no double

A, corrosion-resistant, acid-resistant inner wall food coating.

1. Selection of food coating for inner wall of metal packaging steel drum

For food packaging barrels, because it is in direct contact with food, choosing the right packaging coating is a prerequisite for ensuring food quality and safety. However, there are many steel drum manufacturers that often regard cost as the first place. When choosing coatings for food, first choose cheap coatings. I don’t know that the coatings are not guaranteed, and the construction conditions are not good. The consumption per barrel is relatively high, and the exhaust gas is not only unbearable for the workers on the production site, but also the residents' lives within 2 kilometers of the vicinity will be affected. Therefore, every year, residents complain to the environmental protection bureau about the steel barrel production enterprises. In the attitude that the company is responsible for the society and responsible for the users, I sincerely hope that the steel drum enterprises will choose those coatings that are of good quality and have passed the SGS international certification and meet the US FDA food and drug coating standards. At present, most of the inner coatings used for food packaging are epoxy phenolic paints, which are one of the best metal coatings and the earliest and most common inner coatings used in the domestic steel drum industry. It is also an internal paint commonly used in foreign steel drum industry. Of course, with the development of technology and the awareness of environmental protection, many companies have begun to use water-based paints for production.

The main film-forming material for epoxy phenolic coatings is epoxy resin. The epoxy resin for the food interior coating is a bisphenol A diglycidyl ether type resin obtained by polymerizing epichlorohydrin and bisphenol A as raw materials in the presence of sodium hydroxide. Since both epichlorohydrin and bisphenol A are difunctional compounds, such epoxy resins are linear structures. The epoxy resin contains a hydroxyl group, an ether group and a very active epoxy group, wherein the hydroxyl group and the ether group have a strong polarity, so that the epoxy resin generates a strong force with an adjacent interface. Therefore, the inner coating has a strong adhesion to the surface of the steel drum. The active epoxy group gives the coating and the surface of the steel barrel a high strength bonding force.

The molecular chain of the bisphenol A diglycidyl ether type epoxy resin used in the coating of the steel drum is carbon-carbon bond and ether bond structure, and the chemical property is very stable. Therefore, its chemical resistance and acid and alkali resistance are very good. In addition, since the epoxy group of the epoxy resin is at both ends of the molecule, the internal rotation resistance of the molecular chain is small, and the epoxy resin used is subjected to low viscosity softening treatment, so that the flexibility is good. This is very advantageous for the inner coating process of the steel drum industry (coating the barrel and the bottom of the bucket, topping separately, baking and curing, and then rewinding).

The epoxy phenolic steel barrel inner coating utilizes the active epoxy group in the epoxy resin structure and the butanol etherified phenolic resin to crosslink and solidify at a high temperature to form a firm three-dimensional network structure film, so that the mechanical properties and resistance of the coating are improved. Chemical properties are further enhanced. The cross-linking curing reaction at this high temperature only releases three parts of water, no other by-products are released during the process, and the shrinkage is small, and the cross-linking curing has a very strong and dense network structure film, thereby further enhancing Adhesion, mechanical properties, corrosion resistance and chemical resistance of the coating.

At present, epoxy phenolic steel drums widely used in China are coated in steel drums produced by American PPG, DuPont of the United States and Shanghai Xugong Coatings. These coatings are certified by the German Food and Commodity Regulations and the US FDA.

2. Spray quality control of metal coatings in steel drums

Metal drums and metal cans are painted differently on the surface coating. Metal steel drums are mainly sprayed and dried after the barrel is formed by high-pressure airless painting. The quality of the inner wall of the barrel is steel drum. The manufacturer can directly control it; while the metal cans are generally printed on the double-sided printing and printing of the food packaging coatings required by the user on the double-sided printing and printing of the tinplate on the tinplate. After drying, the cans are made by the can enterprise. Perform processing operations. Large-scale canning enterprises have their own printing and coating production lines, but small and medium-sized canning enterprises, printing and coating are by external processing, so the quality of printing and coating should be strictly checked and tested. Metal cans have developed rapidly in recent years. After integration and market self-regulation, they have entered a virtuous circle. The printing and coating technology has been very mature, and it has reached a stage of reduction. New coated iron, coated aluminum, and secondary cold. New technologies such as rolling thin plates, stamping cans, punching out shaped cans and easily tearing caps are slowly becoming the mainstream of the developed canning industry. Here we mainly introduce the quality control of the paint coating in the packaging of food metal packaging steel drums.

The spraying process is a simple physical process that refers to the process of uniformly coating the coating on the surface of the steel drum to the required thickness. Although the spraying process is simple, it must meet the two requirements that it requires: a certain thickness and uniform coating, and no defects such as leakage coating, sagging and coarse grain defects. The thickness of the coating is a very important process parameter during the spraying process. The formed cured film (coating) has a certain gas permeability, which is related to the structural pores of the coating. The pores of the structure of the coating have an average diameter of 10-6 to 10-7 mm, which is larger than the molecular diameter of the gas and the liquid, so that not only the gas can pass, but also the general ion and the low molecular liquid can pass. In order to increase the water resistance and gas permeability of the coating as much as possible, the coating must be required to have a certain thickness to overlap the structural pores with each other. Usually, the minimum thickness at which the coating can be protected is called critical thickness, so the critical thickness of the steel drum coating is 0.02 mm, that is, the thickness of the paint film formed by spraying the steel drum is not less than this critical thickness. Only such a film thickness can protect the steel drum from external corrosion. In addition, the uniformity of spraying is also very important. If it is not uniform, the excessively thin places will reduce the corrosion resistance, and if it is too thick, the defects such as sagging and wrinkling will easily occur. Controlling the coating of steel drums is another important part of ensuring steel drums.

a, surface treatment before steel drum painting

In order to ensure the coating quality of the steel drum, the surface treatment must be carried out before the steel drum is painted. The degreasing, phosphating, cleaning and drying processes are used to eliminate the adhesion of foreign materials, oil stains, processing debris, dust, etc. in the production and transportation of the steel drum. Unfavorable factors in the quality of the coating, thereby improving the quality of the coating of the steel drum. The surface treatment process of the steel drum before painting is also continuously improved, from the original phosphating to the non-phosphorus conversion and zirconium treatment, because they do not contain phosphorus, the washed steel drum will not pollute the food, and the discharged wastewater will not Contaminated soil.

b. Coating process of steel drum surface coating

The paint is evenly sprayed on the surface of a clean steel drum, and after drying, a protective film with good performance can be obtained. The formation process of the entire paint film can be divided into two stages, namely the spraying process and the dry film forming process.

(1) Spraying process Spraying is the process of uniformly coating a coating on the surface of a steel drum to the required thickness. This process is typically done by an automatic sprayer. If the spray is not uniform, the excessively thin part will reduce the corrosion resistance, and if it is too thick, the sag, wrinkle and other defects will easily occur.

(2) Dry film formation process The dry film formation process is a physical and chemical synthesis process including solvent evaporation, cross-linking condensation of coatings, oxidative polymerization, etc. The whole process has both physical and chemical reactions, which can be roughly divided into two stages:

a) Solvent evaporation phase. It usually occurs in the early stage of drying, and immediately after spraying, a large amount of solvent is volatilized, and the volatilization process is carried out almost on a free surface. Due to the volatilization of the solvent, the concentration of the coating film on the surface of the steel drum changes, so that the paint loses fluidity, which is what we usually call the leveling stage.

b) Chemical reaction stage. On the surface of the steel drum, the solvent loses fluidity after volatilization. Under the action of thermal energy, part of the paint film undergoes cross-linking condensation reaction to form a network structure polymer compound, and another part of the paint base is oxidized in the air, and at the same time, polymerization is carried out to form a network. The polymer compound of the structure, this process is a relatively complicated chemical reaction process, but finally the polymer compounds having a network structure having different degrees of polymerization are formed. This polymer compound has good properties, and all the protective properties of the paint film are provided by it. Therefore, the heat and time during the drying process of the paint film on the surface of the steel drum must comply with the coating and drying process specifications provided by the paint manufacturer. If there is not enough heat and sufficient time, the chemical reaction of the coating is incomplete and the desired film quality cannot be achieved. The so-called selection of appropriate drying specifications is to set the drying temperature and drying time in order to obtain the heat required for the cross-linking condensation reaction, so that the paint film can reach the optimal structure for protecting steel drums and decorative steel drums.

c, steel drum surface coating defects

Coating film defects during or after coating are generally related to the state before the surface of the steel drum, the coating process, the quality of the coating, the coating method and operation, the coating equipment and the environment. Common defects include sag, sag and flow marks, coarse granules and sputum, poor bottom and cover, pinholes, orange peel, wrinkles, poor drying, rust, and brittleness.
For the above problems, there are solutions in the steel barrel manufacturing technology, the barrel training course and other books, not listed here. However, it is necessary for enterprises to formulate scientific and reasonable technical codes and operating procedures, improve equipment level and strengthen management. It is necessary to control the coating defects before they occur.

d, the impact of steel surface coating on the environment

In the steel drum coating process, a large amount of organic solvent is volatilized into the atmosphere, causing environmental pollution. Although some people advocate the use of inorganic solvent coatings in the steel drum industry, until now, most of the public manufacturers also use organic solvent coatings. The organic solvent gas discharge in the steel drum is divided into two parts: one is the discharge of the spray booth, but because it is the solvent gas discharge at normal temperature, the concentration is low, and it is discharged into the atmosphere through water leaching, sedimentation, filtration, etc., pollution. To a lesser extent. However, it is polluting the workshop and endangering the health of workers. Therefore, when the conditional enterprise reforms the spray drying production line, the leveling section is completely enclosed and then treated to minimize the occupational hazards to the workshop environment and employees. The second is the discharge of the paint drying room. Since the baking film of the coating is high temperature baking (170-220 ° C), the organic solvent is completely discharged from the drying room into a high temperature gas, which seriously pollutes the surrounding environment. With the increasing awareness of environmental protection, the steel drum factory uses a certain method to recover and treat the exhaust gas before it has completely searched for qualified environmentally friendly coatings that can be used as an alternative to organic solvent coating. The methods include an absorption purification method, an adsorption purification method, a combustion method (thermal combustion, catalytic combustion), and the like.

Although steel drum manufacturers have tried a lot of methods in order to obtain a good production environment, they have spent a lot of money, but they have not solved the problem fundamentally. There is still a production process in the exhaust gas. This kind of on-site production of food is really worried about the secondary pollution of the packaging container. Therefore, when producing 200 liters of food packaging drums this year, many manufacturers use water-based paints for internal and external spraying of steel drums. The superiority of water-based paints for flame retardant, non-toxic, tasteless, energy-saving and environmentally friendly, low-carbon health will be the domestic and international steel drum manufacturing industry. The trend of coating development.

【Related Links】

Quality and safety control of food packaging metal containers (1)

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Quality and safety control of food packaging metal containers (3)