Coated iron and coated aluminum for food containers are mainly used for container packaging of canned, beverage, milk powder and other products. Although there are nearly a thousand enterprises in China that manufacture coated iron, most of the production processes of these enterprises use the adhesive method, which cannot meet the food container food safety and sterilization performance requirements, so there are about seven or eight iron-coated manufacturing enterprises that can really be applied to food containers. As a food packaging material, the quality and safety of coated iron and coated aluminum are naturally guaranteed. China belongs to the food metal container production and use of large countries, the development of food containers with coated iron, coated aluminum standard system, can greatly improve China's international standard discourse in the field of canning and related industries, so the National Standards Commission decided to take the lead in the first time to develop food containers coated iron, coated aluminum testing standards.
Although the standard has not yet been released, in order to facilitate everyone to grasp the relevant testing project requirements in time, we will introduce the key detection indicators of some food containers with coated iron and coated aluminum. At present, the films used in China for coated iron and coated aluminum for food containers are mainly biaxially modified polyethylene terephthalate (PET) films. Polypropylene films are also technically feasible and have relatively few applications. In order to ensure stable adhesion, the coated iron substrate mainly uses cold-rolled electroplated chrome thin steel plate, and some coated iron substrates will also use electroplated tin thin steel plates in order to meet the color requirements or three-piece can welding requirements. For materials that require surface treatment, lubricants (oil, wax) that comply with the corresponding national food safety standards are generally used. Specific reference to the implementation standards are as follows:
GB/T 16958 Biaxially oriented polyester film for packaging;
GB/T 10003 General purpose biaxially oriented polypropylene (BOPP) film
GB/T 24180 Cold rolled chrome plated steel plate and steel strip
GB/T 2520 Cold rolled electroplated tin plate and steel strip.
The size testing items of coated iron and coated aluminum are mainly divided into thickness, width, length and inner diameter of steel coils. The thickness of coated iron is expressed in terms of substrate thickness, the thickness allowable error and thin edge requirements are also in accordance with the requirements of the substrate, and the detection refers to the corresponding national standard GB/T24180 cold-rolled electroplated chrome steel plate and steel strip. Although the requirements for width, length, inner diameter and shape of the coil are also consistent with the requirements of GB/T 24180, the coated iron is not chrome-plated iron, and these contents also have a certain relationship with the lamination and cutting process, so they need to be tested separately. Appearance requirements include de-squaredness, sickle bending, unevenness, side waves, mid-wave, warping, burrs, and lace plates. The weight and allowable deviation shall meet the requirements of GB/T 708 cold-rolled steel plate and steel strip size, shape, weight and permissible deviation. The capacity of the receiving equipment and the process conditions are limited, and some customers have higher requirements for the size of the coated iron, the supply and demand sides can negotiate. Coated aluminium products refer to coated iron to specify size, shape and weight requirements.
The requirements of the appearance quality inspection project are divided into basic requirements, weld and membrane joint requirements, and defective delivery requirements. The surface defects mentioned in the basic requirements are easily identifiable. Due to the difference in the thickness of the weld and the substrate, the nearby process instability section usually needs to be rejected, and the weld itself is easy to cause damage to the lid and can mold, so although the steel belt is allowed to be delivered with defects, there should be no welds in the steel belt. When the number of membrane joints is large, it seriously affects the downstream production efficiency, and the number of membrane joints in the laminated iron delivered to the steel belt is limited to no more than 3. There will be a certain proportion of defects in the substrate steel strip itself, in addition to this, the proportion of defects caused by the lamination should be limited.
For the physical and chemical properties of coated iron and coated aluminum, we can detect key indicators such as film indentation, film thickness and allowable deviation, cooking resistance, acid resistance, sulfur resistance, salt resistance, impact resistance, adhesion, compatibility and so on. The specific requirements are as follows:
1. Membrane indentation
The determination of the allowable range of membrane indentation is mainly determined according to the capacity and process level of the laminating equipment. When using thermal lamination or adhesive lamination, due to the deviation between the film width and the width of the substrate, the shrinkage of the film after heating, and the fluctuation of the position of the substrate or film during the conveying process, the film width after lamination is greater than or less than the width of the substrate. When there are too many substrates in the edge film (> 2.0mm), it is easy to lead to defects in the shape of the laminated iron (warping, edge waves, etc.), and the generation of edge film chip will also occur. For manufacturers who often use paint iron as a raw material, switch from coating iron to coated iron, when the edge film is not covered with the substrate (exposed iron) more (<-2.0mm), the lid making can may appear the defect of the side can lid exposed iron, in the actual application process, there will usually be edge residue, and the amount of exposed iron is less than the edge residue. If it is difficult to meet the existing film indentation allowable range of -2.0mm ~ +2.0mm, it can be solved by increasing the width of the substrate (which will reduce material utilization).
2. Film thickness and allowable deviation
There is a certain roughness on the surface of the substrate, and after the coating, a part of the film layer is embedded in the surface of the substrate, and the actual thickness is not easy to measure accurately. Therefore, the film thickness is expressed as the thickness (in μm) of the film before lamination, which is determined according to the negotiation between the supply and demand sides. The allowable deviation of film thickness is ±5%, and the test method is shown in GB/T 6672 Plastic Film and Sheet Thickness Determination Mechanical Measurement Method. After the agreed film thickness, the change in the thickness of the film layer before and after the lamination usually does not affect the processing performance, and the requirements for other resistance aspects can be specified separately through the performance requirements of impact resistance, adhesion, and cooking resistance.
3. Cooking resistance
Some of the materials are processed and formed into packaging containers, and after filling the food, they need to go through the process of cooking and sterilization. Therefore, the requirements for the cooking resistance of the material itself are put forward. At present, most of the domestic coated iron materials use modified PET films, and after this type of film coating (heating, melting and bonding + cooling), amorphous and crystalline co-exist in the film layer. After sterilization and heating, the amorphous crystals change to crystals, and there will be certain changes in hue. Especially when the heat is uneven and the crystallization rate in different regions is different, obvious crystallization spots will appear. Therefore, when there is a requirement for cooking and sterilization, it is necessary to choose a suitable film, or to pre-treat the coated iron material to avoid the occurrence of crystalline spots. When the film layer has pinholes and poor adhesion, bubbles, shedding, and corrosion may occur after cooking. Products that do not have the requirement of cooking resistance, this indicator is not required.
4. Anti-acidic
PET, PP and other films have good acid resistance and do not react with acids. However, some types of acid molecules penetrate the gaps between the membrane molecules during heating and react with the substrate (e.g., corrosion after PET-coated iron is steamed with 3% acetic acid). After a long-term application summary, for the contents of the PH value adjusted by citric acid, coated iron can reflect excellent corrosion resistance. Meets the basic requirements of 2% citric acid as acid resistance of coated iron. When the contents contain other types of acids, a compatibility test is recommended. Similar to the requirements of cooking resistance, if there are performance defects such as pinhole exposed iron and poor adhesion in the laminated iron film layer, bubbles, shedding, corrosion and other phenomena will occur after the acid resistance test, which belongs to the scope of elimination. Products with no acid resistance requirements, this indicator is not required.
5. Sulfur resistance
The use of cysteine hydrochloric acid solution, after the anti-sulfur test, the laminated surface has no bubbles, no shedding, no whitening, no corrosion. Products with no sulfur resistance requirements, this indicator is not required.
6. Salt resistance
Using 3% NaCl solution, the laminated surface has no bubbles, no shedding, no whitening, and no corrosion after salt resistance test. Products with no salt resistance requirements, this indicator is not required.
7. Corrosion resistance
After electrolytic corrosion test, it can be intuitively seen whether there are defects of pinhole exposed iron and scratched exposed iron on the surface of the film layer. The exposed iron defect of the coated iron product can be determined by the defect current value of the coating integrity, but when the coated iron material is tested for film integrity, the defect current value is very small in time (< 1mA), and there is already a risky dew point on the surface of the coated iron, which requires a more stringent inspection method, so the corrosion resistance requirements and test methods are used as the inspection method. Due to various influencing factors such as film processing, rough uniformity of the substrate surface, impurities on the surface of the substrate, impurities in the laminating process, etc., it cannot be decided to ensure that there are no dew points in the laminated iron film layer. After long-term application experience, when the corrosion resistance meets the requirements of the following table 1, the coated iron can meet the corrosion resistance requirements in practical applications.
8. Impact resistance
This index is used to judge whether the coated iron material maintains the integrity of the film layer and still has certain corrosion resistance after processing deformation such as stamping and stretching. The thinner the thickness of the substrate and the higher the hardness, the greater the probability of the substrate breaking after impact, and the height of the falling hammer is specified according to different thicknesses, as shown in the following table.
Thermoplastic film (PET, PP, etc.) will be embrittled after baking at a certain range of temperature after printing and curing, and its impact resistance will be significantly reduced (the probability of cracking in the film layer after impact is increased), and the process is irreversible, and the material with poor impact resistance is not easy to make up for the performance defect through other processes. The laminating process parameter setting of the laminated iron material will have an impact on the subsequent baking resistance, and the lamination process can be set accordingly after the material use is determined. For materials that can be printed (or otherwise subjected to baking before deformation), they should be heat treated before testing. Heat treatment conditions should not be lower than the actual baking conditions.
9. Adhesion
The sample is engraved with tic-tac-toe, and whether it is peeled off is observed through cup protrusion and cooking, and whether the adhesion of the coated iron film layer and the substrate is qualified.
When laminated iron is used for non-printing high deformation products, the height of the cup is required to be no less than 6mm, if the adhesion cannot meet the requirements of no peeling under the height of the cup, there will be certain risks in the corrosion resistance of the engraving line of the easy opening lid (EOE) and the durability of the deep-drawn DRD tank film layer. For printable high-deformation products, there is currently an application in cupping products, however, the material is printed and baked, and then after processing and deformation, the adhesion between the film layer and the substrate will be significantly reduced, and it is impossible to ensure that the material is heat treated to meet the requirements of the 6mm cup test, and the performance of the product can only be improved through the process optimization after canning. The height of the cup is 3mm at the level
The peeling of the film layer and the substrate indicates poor adhesion, which will have a greater impact on the subsequent high deformation of processing and forming and the performance stability of the product.
For materials with low deformation, whether baked or not, sterilization and no peeling at a height of 3mm cups can basically meet the requirements of practical applications. In addition, when the secondary cold-rolled thin steel plate is used as the laminated iron substrate, the cup height exceeds 3mm, which is prone to the problem of cup bursting, so the cup height is not easy to be over-required.
10. Compatibility
When the raw materials used in the laminated iron material, the laminating process change, or the use (lid type, size, contents, sterilization method, etc.) changes, it is recommended to perform a compatibility test.
The above are the requirements put forward by the National Standards Commission for the detection of coated iron and coated aluminum for food containers. Unified quality requirements play an important role in the development of the domestic laminated iron laminated aluminum industry. In order to promote the quality control level of coated iron manufacturing enterprises, let more enterprises participate in the production of food packaging containers, and ensure the safety of the canned field, it is necessary to formulate a perfect standard for coated iron and coated aluminum for food containers, and further standardize the coated iron industry.