Analysis of Printing Fastness of Tinplate Based on Packaging
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With the continuous development of the social economy and the continuous improvement of people's living standards, tinplate printing products are widely used in different types of food, beverage cans, daily cosmetics, pharmaceuticals, petrochemical products with their novel binding design and special fine printing effects. And a variety of packaging containers. Therefore, the quality of tinplate printing is very important. The printing fastness of tinplate is discussed below.
First, the structure and characteristics of tinplate
1. The structure of tinplate
Tin Plate, also known as tin plated steel, is tin-plated on a thin steel plate substrate. It consists of oil film, iron oxide, tin layer, alloy layer and substrate. It will weld steel strength and formability with tin. Corrosion resistance, solderability and aesthetic appearance are combined in one material, which is corrosion-resistant, non-toxic, high-strength and ductile.
Figure 1 Tinplate structure
1-oil film 2-iron oxide 3-tin layer 4-alloy layer 5-substrate
2. Characteristics of tinplate
2.1 opacity: Tinplate has good opacity, can effectively prevent visible light and ultraviolet radiation, can protect the corrosion and deterioration of food, so that the shelf life can be extended.
2.2 Good sealing: The good sealing of tinplate makes it block the effect of air and other gases on canned products, especially food protection, when making packaging containers.
2.3 Good environmental protection: Tinplate is a material that can be completely recycled, which is conducive to protecting our living environment. In addition, tinplate can be naturally decomposed in the environment and will not cause environmental pollution.
2.4 Saving energy: Producing steel with tinplate waste is more energy efficient than producing iron with iron ore. It saves about 230m3 of natural gas per ton of scrap iron.
Second, tinplate surface performance and printing fastness
1. Tinplate surface properties
The surface properties of tinplate are also the difference between surface condition and surface quality. There may be defects such as scratches, pits, wrinkles, dust and rust on the surface of tinplate. From the structure of tinplate, there is often a very thin oil film on the surface of tinplate. And it is generally electroplated, so the surface is smoother. From the relationship between the specific surface area and the adsorption force of the solid surface, the adsorption force of the tinplate is not very good.
2. Surface properties affect printing fastness
The tinplate printing is mainly based on lithographic offset printing. The process is as follows:
The surface roughness of tinplate is one of the important factors affecting the printing fastness. The surface of the tinplate substrate is generally smooth. Although some organic coatings have strong adhesion on a smooth surface, the overall absorption is poor, and the adhesion is small, so that the adhesion and drying properties of the coating layer and the ink layer are improved. affected.
3. The influence of the oil film layer
In the tinplate structure, the surface layer is an oil film. This layer of oil film is used to prevent the tinplate from rusting during storage and rubbing during transportation, which will greatly reduce the adhesion between the coating and the ink. Therefore, the control of the amount of oil film is very important. At present, the tinplate produced in various countries in the world does not have a certain standard for the thickness of the oil film. Generally, the oil amount is controlled at 2 to 4 mg/m2, but some are above 6 mg/m2. If the amount of oil is too much, it will affect the printing quality of the tinplate, so that the coating and the ink layer cannot completely wet the surface of the tinplate, and the adhesion performance is poor, and the "pinhole" is easily formed on the ink layer after baking and solidification. The "pinhole" causes a decrease in the adhesion between the ink layer and the tinplate. Therefore, when the surface oil content of the tinplate is higher than 6 mg/m2, the tinplate is pre-baked before printing to make the excess oil film volatilize.
In addition, in addition to the smooth surface of the tinplate, the ink adsorption force is lowered and the printing fastness is deteriorated. Due to the poor adsorption performance, the drying speed of the ink is slow, and the conjunctival property is poor, thereby affecting the adhesion fastness and abrasion resistance of the ink layer. As a metal material, when tinplate is exposed to large air humidity and the temperature difference is relatively large, many fine water droplets are formed on the surface of the tinplate, which also causes the adsorption force to decrease and affect the printing fastness.
4. The effect of the oxide layer
In addition to the oil film on the surface of the tinplate affecting the adsorption force, the second oxide film in the tinplate structure also affects the adhesion of the ink. There is a thin oxide film on the surface of the tin layer of tinplate. The structure of this oxide film is unstable, and it will continue to oxidize and thicken, and even cause the ink layer on the oxide film to fall off, so that the ink adhesion performance is lost. Therefore, after the tinplate is tinned, the passivation treatment is carried out to improve the stability of the oxide layer, hinder the growth of the oxide film, and maintain the stability of adsorption to the ink.
Therefore, the surface properties of the tinplate have a great influence on the printing fastness of the ink. In addition to the good quality of the tinplate, there must be good performance coatings and inks, and it is better to treat the surface of the tinplate and improve the surface. Surface adsorption force.
Third, tinplate printing requirements for inks and coatings
Tinplate is mainly used as a tin can for packaging. Tinplate printing is mainly based on lithographic offset printing. It is printed by the physical phenomenon that oil and water are incompatible. Therefore, in addition to the printing suitability of the offset ink, the printing ink must also have properties such as heat resistance, solvent resistance, process formability, retort resistance, and abrasion resistance. The coating is also an indispensable material in the processing of tinplate.
1. Coating
The coating is mainly composed of film-forming substances, pigments, drier, auxiliary agents, solvents, etc., and can be divided into three types of coatings: inner coating, outer coating and varnish. The ink layer is well protected and the brightness of the image is increased to give the product a good metallic color. At the same time, the coatings such as food cans can effectively prevent chemical and biological contamination of the contents, reduce the possibility of leaking the cans, protect the original flavor color of the food, prolong the storage time and make the contents easy to be poured out from the container.
1.1 good performance primer coating. Primer coatings are an essential coating in the tinplate surface printing process. It mainly improves the adhesion of the surface of the tinplate, so that the ink can be firmly adsorbed on the iron surface, so that the tin coating is not easily damaged when the tinplate is bent, impacted, stretched, crimped, etc. Therefore, as a coating, it is required to have a good wet adsorption property on a metal surface, and has a high mechanical strength after drying into a film. Resins such as alkyd and epoxy have good adsorption and impact resistance due to molecular structure.
1.2 advanced coating process. In order to obtain a high-quality primer coating layer, in addition to a good performance primer, advanced technology coating technology is also indispensable. High-quality coating means applying a thick and uniform coating on the surface of the tinplate, good flowability, and smooth surface. The thickness of the coating should be controlled at about 1.5~2g/m2. If the coating is too thin, the hiding power, abrasion resistance, adsorption, gloss, etc. cannot meet the requirements. If the coating is too thick, the shrinkage stress will be caused, the adhesion will be affected, and the cost will increase, which may cause the hardening to be incomplete. quality.
The control of the coating process is mainly to control the coating speed and stability of the coating machine, the uniformity of the coating supply amount and the baking temperature, which largely depends on the advancement of the coating process, coating The accuracy and automation of the equipment.
1.3 The effect of paint viscosity. In addition to the superior performance of the paint and the advanced coating process, the viscosity of the coating will have a great influence on the uniformity of the coating thickness. Therefore, the control of the viscosity of the coating is also quite equivalent. important. Different coating viscosities will have different coating layer thicknesses under otherwise identical conditions. The higher the viscosity, the thicker the coating of the same applied amount of coating after coating; this aspect improves the protective performance of the coating, but the mechanical processing performance is reduced, the curing is poor, the adhesion is not enough, and the coating turtle is Cracking and shedding. At the same time, the leveling property of the coating is lowered, the coating film is unevenly coated, and defects such as "wrinkling" are liable to occur. On the contrary, the lower the viscosity, the thinner the coating thickness, and the poor wear resistance, adsorption, and the like.
Therefore, whether the viscosity of the coating is too high or too low will affect the quality of the coating, and controlling the viscosity of the coating is mainly achieved by adding a diluent, and the change of the ambient temperature will also change the viscosity of the coating, and the viscosity will decrease when the temperature rises. Otherwise it rises.
2. Printing ink
The composition of the tinplate printing ink is similar to that of the ordinary ink. It is mainly composed of pigments, binders, fillers and adjuvants. However, since the surface of the tinplate is smooth and the absorption is poor, the ink used for tinplate printing must be different from ordinary offset inks. In addition to the printing suitability of general offset inks, tinplate printing inks should also have impact resistance, adhesion, heat resistance, light resistance, and resistance to cooking.
2.1 good adhesion and mechanical properties. Tinplate printed substrates are very smooth and have no absorption capacity, so they should have good adhesion. At the same time, tinplate prints are generally not the final product. They must be made into various food cans, toys, metal boxes, and barrels and cans of chemical products. They need to be cut, bent, stretched, formed, etc. Has strong mechanical properties.
2.2 high temperature resistance. The surface of the tinplate is smooth, non-absorbent, and does not penetrate moisture and solvent. Drying is mainly accomplished by evaporation of the oxidized conjunctiva and ink solvent, and the drying speed is slow. Generally, it is baked to complete the drying, so the tinplate should have good heat resistance, and the high temperature baking pigment does not change color. At the same time, food, medicine and other packaging boxes must be sterilized by high temperature sterilization, and the ink should not change color and not yellow in this environment.
2.3 dryness of the ink. In the tinplate printing production, it is necessary to properly control the drying speed of the ink to ensure product quality. If the ink dries too fast, the ink transfer performance is lowered; the print is faint, the ink color is light, and even the ink on the surface of the printing plate and the ink roller is dry, and the ink is blocked. If the ink is too dry, it will cause difficulty in overprinting, adhesion, dirty on the back side, poor adhesion, low abrasion resistance, and easy scratching during transfer. Controlling the drying speed of the ink is one of the important factors to improve the adhesion fastness of the ink layer. Therefore, in recent years, a large amount of UV ink has been used in metal printing because the UV ink can easily control the drying speed and has good adhesion. And wear resistance.
3. Coating varnish
In tinplate printing, in order to improve the adaptability of post-press processing and improve the gloss of the appearance of tinplate products, glazing should be performed before the printing ink is not completely dried to form a uniform and smooth coating film to avoid bleeding. . Coating the varnish not only improves the gloss, but also enhances the flexibility, impact strength, abrasion resistance and adhesion of the surface of the ink layer. In addition, the coating film formed by the varnish after high temperature curing isolates the ink coating from the outside, so that the tinplate product is protected from physical and chemical substances.
Commonly used varnishes are solvent-based, water-based and UV varnishes. The solvent type has certain restrictions on the environment due to its volatile solvent, and its use is limited.
Water-based varnish is an environmentally-friendly varnish that has been widely used in recent years. It has high gloss, good fast film-forming performance and excellent abrasion resistance. It is an ideal varnish. However, 30% of the drying process of water-based varnish is by volatilization, 70% is absorbed by the material, and tinplate is not absorbable at all, so it is not suitable for use in tinplate printing at present.
UV varnish is mainly composed of photosensitive resin, reactive diluent, photoinitiator and auxiliary agent. UV curing is a film formation by initiating a polymerization reaction under ultraviolet irradiation. Since the surface of the tinplate is a non-absorbent surface, the post-press drying problem is one of the main technical problems of tinplate printing. Ultra-violet irradiation for instantaneous drying, fast curing speed, firm film surface after drying, good gloss, abrasion resistance and protection, no pollution to the environment, is a very widely used glazing oil in recent years. UV varnish currently has high cost, high energy consumption and irritating odor.
All kinds of varnishes have advantages and disadvantages. In actual production, the type of varnish should be correctly selected according to the characteristics and use of the printed products. However, no matter which type of varnish is used, the solid content of varnish, the purity of the oil and the viscosity of the coating, baking temperature and other technical data should be known before use to ensure the hardness (wear resistance) and gloss of the coating film. Degree and adhesion.
Fourth, other factors affecting the printing fastness of tinplate
1. Effect of baking temperature on the adhesion of tinplate
In tinplate printing, since the surface of the substrate is smooth and the drying speed is slow, it is necessary to use heating baking to accelerate the drying speed, which is one of the characteristics of tinplate printing. At the same time, because the coatings and inks used in each process are different in resin and pigment composition, the baking temperature and baking time are also different. If the baking temperature is too low, the molecules in the resin are not sufficiently cross-linked to form a film, and the adhesion is low. At the same time, the back side of the tinplate is sticky, offset, and stuck, which affects the color of the tinplate (the inner wall of the tinplate is contaminated by the dirt on the back side of the tinplate). The contents of the container). If the baking temperature is too high, the molecules are broken, the ink layer is cracked, and the mechanical processing and high-temperature sterilization performance are deteriorated and fall off. If the baking time is too short, the adhesion of the coating is not good and soft; if it is too long, the coating will become brittle and the yellowing will occur when the can is broken. Therefore, it is necessary to strictly control the baking temperature and time of each process, which will directly affect the adhesion fastness of the entire ink film layer. The baking temperature of the printing ink is controlled at 130-150 ° C on average, and the time is 8-10 min, and the baking temperature of the paint is 180-210 ° C on average, and the time is also 8-10 min.
The baking temperature is mainly controlled by baking electric heating pipes, automatic thermostats, and the like. The actual temperature for controlling baking is generally about 10 to 15 ° C lower than the theoretical standard baking temperature, and a slightly lower temperature may leave some unreacted polar groups, so that the two interfaces of the membrane ink are densely entangled. To enhance the bonding fastness between the coatings. The baking time is mainly controlled by the running speed of the machine, and the running speed is fast and the baking time is short, and vice versa.
2. Influence of printing process on tinplate printing fastness
Tinplate printing is currently mainly done with lithographic offset printing, while a small amount of dry offset printing and waterless offset printing are used. Different printing processes will also affect the printing fastness of tinplate.
First of all, no matter which printing process is used, due to the particularity of tinplate printing, printing a product often requires multiple times of coloring and drying, and it is a non-absorbent smooth surface. Different ink layer thicknesses will be different. The adhesion is fast, the ink layer is thick, and the cohesion between the ink layers is large. Invisibly offsets the adhesion between the primer coating and the surface of the tinplate, resulting in a decrease in adhesion of the entire coating layer. Therefore, the thickness of the ink layers of each color should be controlled during the construction process.
Lithographic offset printing is the main method of tinplate printing at present, but the use of dampening solution will cause the emulsification of the ink, and the surface of the tinplate is not absorbed, which will affect the drying speed, so that the bonding fastness of the ink is to some extent. And wear resistance is also affected, so it is necessary to control the relationship between ink and water balance.
Waterless offset printing is a printing method popularized and applied in tinplate printing in foreign countries. Since there is no dampening solution, the tinplate can be improved on the surface without absorption, the drying speed can be improved, the conjunctival property is good, and the bonding fastness is better than that of water. Offset printing should be strong, and the printing dot reduction is better. Therefore, there is a certain prospect in promoting waterless offset printing on tinplate.
Dry Offset, also known as letterpress offset printing, is mainly used in the printing of shaped tinplate containers. The ink layer is thicker and the printing speed is faster. Although the ink with low viscosity and good drying property is used, it is not popular because of its use. The performance of the material is not guaranteed, so the adhesion fastness of the ink layer will also cause large quality fluctuations.
In recent years, with the continuous development of packaging and printing technology and metal packaging materials, China has made great progress in metal packaging printing. But overall, there is still a certain gap in the quality of tinplate printing in China, especially in printing fastness. With the rapid development of China's economy, tinplate printing will develop towards the diversification of printing methods and the high quality of printing.