Surface properties and ink adhesion in the most po

2022-07-25
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Surface properties and ink adhesion in sheet printing

the object of metal printing is a non absorbent material - low carbon tinplate, commonly known as sheet. The printing principle of tinplate is similar to that of offset lithography, but there are some special aspects in the process, such as:

1. after printing, the ink stays on the surface and must be dried and solidified by some form of heat energy

2. the tin layer on the surface of tinplate is bright and belongs to black phase by spectral analysis. Therefore, a layer of white paint must be applied on the surface of the sheet before color printing. Of course, sometimes according to the special artistic effect of the product pattern, it can also be partially coated with white paint or directly use the appearance of the tinplate surface itself

3. after the finished sheet is printed. It needs to be transferred to the next process for machining to make containers or other packages of various shapes. In its machining process, the ink coating on the surface of the sheet must be resistant to mechanical impact and extension. For special products such as cans and containers, the ink coating should also have the physical properties of high-pressure cooking resistance. This requires excellent adhesion between the ink coating and the sheet surface

the adhesion of the ink coating on the surface of tinplate has been a long-term discussion and careful study by professionals in practice. It is related to the surface properties of the sheet itself, the types of inks and coatings used in the printing process, the baking temperature, the thickness of the ink coating and many other factors. How to enhance the adhesion between the ink coating and the tinplate is discussed

I. The relationship between the surface properties of the sheet and the adhesion of the ink coating. Tinned steel sheet is divided into hot-dip tinplate and electroplated tinplate according to different tinning processes

the structure of tinplate is composed of steel substrate, tin iron alloy layer, tin layer, oxide film and oil film

seen from the structure of the sheet, its surface is coated with a very thin oil film to prevent the sheet from rusting during storage and transportation, which is expected to maintain a good growth momentum in the next few years, so that it is easy to lift the stacks of sheets during printing. Most of the oil film components are dioctyl decanoate or acetyl tributyl citrate. At present, the injection molding production technology known as "foaming molding" produced by countries all over the world has been realized on a complex shape. There is no certain standard for the thickness of oil film on tinplate. Generally, the oil coating amount is controlled at 2~4mg/m2, but some are more than 6mg/m2. If too much oil is applied, it will affect the washout quality of the sheet, so that the coating and ink layer cannot completely wet the surface of the sheet, and circular or oval pore defects will be generated on the baked and cured ink layer, commonly known as "eye holes". "Eye hole" will cause the adhesion between the ink layer and the sheet to decrease. Therefore, if the amount of oil applied on the surface of the sheet is more than 6mg/m2, the sheet shall be pre baked before printing to make the excessive oil film volatilize. The pre drying temperature is generally between 200 ℃ and 15min, and the vehicle speed is 4200r/h

in addition, due to the poor production environment, for example, the surface of the tinplate is covered with small drops of water due to moisture before printing, and the seasonal climate is too cold, which will promote the immiscibility of the surface of the tinplate to the coating and ink coating, affecting its adhesion. This kind of phenomenon is basically eliminated by pre drying once before printing

the easiest way to test whether the oil film on the surface of the sheet is qualified is the liquid tension and tape casting test of the coating. If a small amount of paint is taken, pour it slowly on the flat sheet surface, and pay attention to the paint. At this time, the coating liquid quickly diffuses to the surrounding, and the wetting radius of the liquid center to the surrounding is approximately the same. Then lift one end of the tinplate to make it form a 45 degree angle with the plane. At this time, the liquid flows evenly to the other end, and the tinplate oil film quantity meets the printing requirements. If the coating is agglomerated on the surface of the sheet or diffuses irregularly to the surrounding, the amount of oil film does not meet the requirements before printing, and the sheet shall be pre baked before printing

the second layer of surface structure of tinplate is oxide film. After the tinplate plating process is completed, there is a thin oxide film on the tin surface, which is mainly composed of divalent tin or tetravalent tin oxide and its hydrate, and the structure is very unstable. Some people believe that the thinner the oxide film, the better the adhesion, and vice versa. This statement is not unreasonable. Passivation treatment shall be carried out for tinplate after tin plating. The purpose of passivation treatment is to stabilize the surface of tinplate and prevent the growth of surface oxide film. Too much oxide film will make the tinplate change color during long-term storage, especially after printing and baking. In addition, due to its unstable structure, the ink layer on the oxide film falls off, resulting in the loss of ink adhesion and product quality problems. The surface of electroplated sheet is usually treated by chemical or electrochemical passivation. However, the hot-dip sheet is not passivated in the production process. Therefore, when making various exquisite packaging metal containers, manufacturers should correctly select tinplate materials according to product modeling and machining characteristics

II. Printing coating. The relationship between ink and stiffness

tinplate printing, also known as iron printing, belongs to offset lithography. The basic principle is to use the physical phenomenon of oil-water immiscibility for printing. Therefore, in addition to the printability of offset printing ink, iron printing ink must also have several properties, such as heat resistance, solvent resistance, processing formability, cooking resistance, etc. When printing tinplate products, the team should choose to use coatings and inks

1. primer coating

primer coating is an essential first coating in the surface printing process of tinplate. Its main function is that it can not only be firmly attached to the surface of the sheet, but also be easily attached and bonded to the various inks on it, so that the ink coating will not be damaged due to mechanical processing when the sheet printing products are bent, impacted, stretched, crimped, etc. Therefore, when selecting the primer coating, the paint department is required to have good wetting performance on the surface of gold chips, and have ideal mechanical strength after film forming. It is generally believed that alkyd, epoxy and other resins have these properties. This is because the resin structure contains aliphatic hydroxyl group, ether group and extremely active epoxy group. The polarity of hydroxyl and ether groups causes electromagnetic attraction between resin molecules and adjacent surfaces, and the epoxy group reacts with the metal surface containing active hydrogen to form chemical bonds, so the adhesion is particularly strong. In addition, when the resin is cured, no by-products are generated, and no bubbles are generated, so the shrinkage is small. When no curing agent is added, it is a thermosetting resin, which does not deteriorate due to heating, and has strong water resistance

select excellent primer and paint, and have scientific and rigorous production process. The paint shall be evenly applied on the surface of the sheet, and the film thickness shall be controlled at about 1.5~2g/m2. If the film is too thin, it will not work. If it is too thick, it will cause shrinkage stress and affect the adhesion. The actual baking temperature should be about 10~15 ℃ lower than the theoretical standard baking temperature. Artificially leave some unreacted polar groups in the paint. In this way, when overprinting the first color ink, the organic solvent in the ink will swell the primer, making the polymer chains at the two interfaces of the film and ink tangle closely, and enhancing the adhesion of the coating

2. printing ink should be reasonably selected. The surface of the sheet is as smooth as a pound. Although the surface is coated with a layer of primer before color overprint, if the adhesion between the synthetic resin in the binder and the sheet surface is not considered enough, the adhesion of the produced ink cannot meet the printability of the sheet. Generally, the ink mainly composed of oil modified alkyd resin connecting material should be used

alkyd resin is a polyester resin modified with vegetable oil or fatty acid. It can be divided into two categories according to its properties: dry and non dry. After inspection, it can also quickly restore the packaging; According to the amount of vegetable oil or fatty acid, it can be divided into short, medium and long oil degrees. Dry inebriated acid resin mainly contains soybean oil, dehydrated castor oil and other components in the ink. This resin can be self dried or can be used with amino resin. The produced iron printing ink, cured at high temperature, has the characteristics of firm adhesion, high gloss, strong hydrophobicity, color and light retention, and can meet the requirements of tinplate printing suitability and product processing

due to the particularity of the tin plate printing process, printing a certain product often needs to be colored and dried for many times, and it is also a smooth surface with non absorption and permeability. Therefore, the ink layer thickness of each color should be controlled during the construction process. If the ink layer is too thick, the cohesion between the ink layers will be large, which virtually offsets the adhesion between the primer and the surface of the sheet, resulting in a decrease in the adhesion of the entire film layer. Especially when coating or printing white ink, due to the large proportion of titanium dioxide (titanium dioxide) pigment in the white ink, the thickness of the ink layer should be controlled at (14 ± 2) g/m2, which not only maintains the color of the white ink, but also maintains the adhesion of the ink layer

3. after coating varnish

sheet multi-color overprint. Also in the ink surface coated with a layer of varnish. The coating varnish not only increases the gloss of the appearance pictures and texts of the tinplate products, making them bright and dazzling, but also part of the hydroxyl and ether bonds in the gloss pool penetrate into the ink layer under the effect of the ink surface tension, enhancing the flexibility, impact strength and wear resistance of the ink layer surface, and achieving the ideal adhesion. In addition, the film formed after the varnish is cured at high temperature isolates the ink coating from the outside world, making the surface of the tinplate product resistant to physical and chemical erosion

most commonly used varnish are epoxy resin, alkyd resin and acrylic resin. Epoxy resin varnish contains aliphatic hydroxyl groups and ether bonds that are not easy to hydrolyze due to their high polarity, so that its film has not only good adhesion with the coated surface, but also high chemical resistance. In addition, the physical and mechanical properties of the coating film are enhanced due to its alternate arrangement of rigid benzene core and flexible hydrocarbon chain. Of course, due to the existence of polar groups, the film will have a certain hydrophilicity, which requires the manufacturer of varnish materials to adjust the formula. Through the isolation effect of cross bond structure, good water resistance can also be obtained

inebriated resin type varnish, containing castor oil, coconut oil and medium and short chain products of synthetic fatty acids, belongs to medium and short oil alkyd resin. It has high saturation rate and good oxidation resistance. It is copolymerized with nitrocellulose, amino resin, pentaoxide resin, etc. to make a baking and drying film. It has the characteristics of firm adhesion, toughness, heat resistance, solvent resistance and weather resistance. It is widely used in tinplate varnish

acrylic varnish generally adopts hydroxyl acrylic acid, such as acrylic acid, which is copolymerized with ethyl ester and acrylic acid, introducing hydroxyl and carboxyl groups into the resin, and using amino tree tire as crosslinking agent. The varnish does not turn yellow after baking at high temperature. It has good properties of heat resistance, acid and alkali resistance and water resistance. It is commonly used for coating on tinplate for food cans

various types of market varnish have their own advantages and disadvantages. In actual production, the type of varnish should be used correctly according to the characteristics and uses of the printing products. However, no matter which type of varnish is selected, the solid content of the varnish, the purity of the oil and the viscosity of the coating should be understood before use

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