Yang Hong compilation
1. The concept of flexographic printing ink
The original name of flexographic printing ink was aniline ink, which was later called rubber relief ink, and it is now uniformly called “flexographic printing ink†internationally.
The name of aniline ink is due to the use of aniline dyes in the earliest ink formulations. After 1935, due to the advancement of printing presses and inks, this printing method has been greatly developed, and its name is finally uniformly called "flexible ink". China has been translating its name into rubber relief ink, and it was not renamed to "flexible ink" until the 1980s according to the international collective name. Because the plate material used to make this printing plate has changed, in addition to the use of rubber, there are still photosensitive resins.
Flexographic ink is a typical solvent ink, that is: liquid ink, which is dried by the evaporation of the solvent. This ink is transferred to the flexographic plate through the anilox roller (the dent on the roller). In general, the viscosity of the ink is 35 mpo · s (25 ° C). Such inks can be divided into the following three categories based on their colored bodies, namely: dye type, pigment type, and dye pigment type. Generally, the dye type has a lower viscosity and the pigment type has a higher viscosity to facilitate dispersion and storage (prevention of settling). Due to the different materials of the printing materials, the resins and solvents in the ink formulations are also various, but the ink must have suitable viscosity and fluidity, and the dryness needs to be good. There is no residual odor after drying. The adhesion of the object is better, and it has no effect on the printing plate.
Second, the use of flexible printing ink
Initially, flexographic inks were only used for printing on paper bags, and later the scope of use gradually expanded. In the United States, most of the use of flexographic inks today is concentrated in the field of packaging and printing, accounting for about 85%, such as various plastic films, metal foils, cardboard, decorative packaging paper, glass cloth, kraft paper, etc. The use of these packaging printing inks includes those directly printed substrates (ie, surface printing), as well as substrates made of composite structures (ie, inner printing). The rest is used for books, commercials and special printing. In the United States, two-thirds of flexographic inks are water-based flexographic inks, and the rest are solvent-based. At present, in China, flexographic printing inks are also constantly expanding their application areas and market share. Because flexographic ink can basically achieve good printing quality on many different substrates.
3. Application types and configuration methods of flexographic ink
From the current application situation, the printing substrate can be divided into two types, namely, porous substrate and non-porous substrate. The printing on the surface of the porous substrate generally uses water-based flexographic ink; about two-thirds of the printing on the surface of the non-porous substrate uses solvent-based ink. The use of water-based flexographic inks is increasing at home and abroad.
1. Type: As mentioned above, flexographic inks are liquid inks, which are dried in two ways: penetration into the substrate (paper) or volatilization of the solvent.
2. Preparation method: The solvent used in this low-viscosity (100-250 cps) liquid ink is based on dissolving the polymer in the ink. The typical solvents used are water and alcohol, as well as small amounts of glycol ethers, unitary and aliphatic hydrocarbons. The film-forming polymers are polyphthalamide, nitrocellulose, rosin, shellac and acrylic vinegar, etc. One of these polymers is usually used to modify the other (for example: nitrocellulose can increase heat resistance ; Polyphthalamide can increase gloss and flexibility). In most flexographic inks, two or more polymers or solvents are generally used to achieve a balance in performance. The flexographic ink system generally contains 7-15 different components. These components constitute the general components of pigments, polymers, solvents and additives used in typical flexographic inks.
The following is a brief description of the preparation of solvent-based inks and water-based flexographic inks:
1. Solvent-based flexographic inks: As mentioned above, the formulation of solvent-based flexographic inks is mainly for printing non-porous substrate surfaces.
About 75% of plastic inks use alcohol-soluble polyphthalamide resin, and its representative reference formula is as follows:
Component weight% Polyphthalamide resin 13 S / S nitrocellulose 2 * Organic pigment 15 Filler 5 Plasticizer, wax 5 Ethanol 55 Ester 5 100Note: * If white ink based on TIO2: is manufactured, this amount can be doubled while reducing the amount of ethanol.
The following reference formula is "modified nitrocellulose" ink, with good gloss and heat resistance, suitable for printing foils, coated cellophane and paper:
Component weight% Polyphthalamide resin 10 S / S nitrocellulose 5 * Organic pigment 15 Filler 5 Plasticizer, wax 10 Ethanol 50 Ester 5 100If the ink for printing coated cellophane is needed, then the film-forming agent in the above formula, namely modified rosin, should be replaced with acrylic resin, so as not to affect the heat sealing.
At present, the development of flexible plastic films for composite packaging is rapid, which requires an ink, that is, the composite ink is suitable for printing on this plastic film, and then compounded with another plastic film or paper to make a good Sealed packaging for printing and heat sealing performance. The composite ink used for printing here does not contact the inner packaging nor the air. This composite ink does not require gloss, nor does it require scratch and rub resistance (because they are protected by a composite film). This ink does not contain wax or very little, so it will not cause a reduction in the adhesion between the composite films.
Generally, high-performance composite inks are based on polyphthalamide resins, and economical composite inks are based on nitrocellulose. All compound inks must have very low residual solvents, because this compound ink is usually used in packaging printing of foods such as chocolate, cookies and sugar.
In addition, from the perspective of environmental protection, the VOC (volatile organic solvent) emitted by this ink needs to be recycled or incinerated. Of course, recycling and incineration will involve huge investment costs, and this ink also needs to use air flow system to achieve VOC collection. For this reason, they each have their own limitations:
A. Solvent recovery method: it is not suitable for those made of different solvent mixtures, which limits the flexibility of the formulation.
B. The incineration method involves the use of a catalyst whose "life" will be shortened by the components in certain inks-such as: siloxane. This limits the flexibility of the formulation.
2. Water-based flexographic inks: Water-based flexographic printing plates are mainly suitable for absorbent substrate surfaces, such as kraft paper, corrugated cardboard boxes, multi-layer paper bags, etc. Now they have developed to printing on plastic films and foils. Generally, the resins used in water-based flexographic inks are modified acrylic copolymers and rosin. There are also several other products. These resins are solubilized with ammonia or an ammonia complex (or occasionally even sodium hydroxide). Emulsion polyester is also used. This emulsion vinegar is a good pigment grinding resin (because it has good wettability and high viscosity), and makes the ink have good reconstitution. The emulsion can also improve the ink's fixation, toughness, gloss and some resistance.
When printing absorbent substrate surfaces with this ink, such as corrugated paper packaging, the following typical reference ink formulations can be used:
Weight% of components Acrylic resin 20 Organic pigments 12 Fillers, amines, defoamers and biocides 8 Water 60 100Of course, the above formula is simplified. Because mixed raw materials are used in various non-aqueous components, however, it is important to note that defoaming aids are needed in water-based inks, because in the printing process, there is a stirring process. There is also the need to add biocides to prevent the growth of microorganisms in the product.
When printing non-porous substrate surfaces with water-based inks, the addition of solvents or volatile organic solvents can improve wettability. The amount of volatile components added can reach 25% (typically 12--13% of the total ink formulation weight). These products are called "quasi" water-based inks.
The volatile organic solvent (VOC) content is 5%, sometimes as low as 12% flexo products are also launched. These products often use surfactants to improve their wettability. These products also contain a small amount of glycol ether. In the case of the drying furnace used in current flexographic printing, the evaporation rate is slow, but it can meet the requirements of production speed. In addition, although some water-based inks are used in some composite products, they are mainly bonded structures. Since the binder used in water-based inks in composite products (which has a higher fixing fastness) is solvent-based, There are volatile organic solvents. Furthermore, water-based inks often cannot withstand the high temperatures encountered during the extrusion process.
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