Overview of flexographic printing inks
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First, the concept of flexographic printing ink
The flexographic printing ink was originally called aniline ink, which was later called rubber relief ink. It is now called “flexographic printing ink”.
The name of aniline inks is due to the use of aniline dyes in the earliest ink formulations. After 1935, due to advances in printing presses and inks, this printing method was greatly developed, and its name was finally called "flexographic ink". China has been translating its name into a rubber letterpress ink, and it was not renamed as "flexographic ink" after the international collective name in the 1980s. Since the plate on which the printing plate is made has changed, in addition to the rubber, there are photosensitive resins and the like.
Flexographic inks are typical solvent inks, namely liquid inks, which are dried by evaporation of the solvent. This ink is transferred to the flexographic plate by an anilox roller (a dent on the ink roller). In general, the viscosity of the ink is 35 mPa·s (25 ° C). Such inks can be classified into the following three types based on their color bodies: 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 (preventing sedimentation). Due to the different materials of the substrate, the resin and solvent in the ink formulation are also various, but the ink must have suitable viscosity and fluidity, dryness is also required, and there is no residual odor after drying. The adhesion of the object is good and does not work on the plate.
Second, the use of flexible printing ink
Initially, flexographic inks were only used for the printing of paper bags, and the range of use was gradually expanded. In the United States, the use of today's flexographic inks is mostly concentrated in the packaging and printing field, accounting for about 85%, such as various plastic films, metal foils, cardboard, decorative wrapping paper, glass cloth, kraft paper and so on. The use of these packaging printing inks includes those that are directly printed (i.e., printed), as well as substrates that are made into a composite structure (i.e., printed). The rest are used for books, commercials and special printing. Two-thirds of the US flexographic inks are water-based flexographic inks, and the rest are solvent-based. At present, in China, flexographic inks are also expanding their application areas and market share. Because flexographic inks can achieve good print quality on many different substrates.
Third, the application type of flexographic ink and its configuration
From the current application situation, there are two types from the printed substrate, namely, a porous substrate and a non-porous substrate. The printing of the surface of the porous substrate generally uses a water-based flexographic ink; about two-thirds of the printing of the surface of the non-porous substrate uses a solvent-based ink. The use of water-based flexographic inks at home and abroad is a growing trend.
1. Type: As mentioned above, the flexographic ink is a liquid ink that is dried by infiltration into a substrate (paper) or solvent evaporation.
2. Formulation method: The solvent used in this low viscosity (100-250 cps) liquid ink is based on the polymer dissolved in the ink. Typical solvents used are water and alcohol, as well as small amounts of glycol ethers, hydrazines and aliphatic hydrocarbons. The film-forming polymer is polyamine, nitrocellulose, rosin, shellac and acrylic vinegar, etc., usually one of these polymers is used to modify the other (for example, nitrocellulose can increase heat resistance) Polyamide 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 typically contains from 7 to 15 different components which form the general class 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 noted above, solvent-based flexographic inks are formulated primarily for printing non-porous substrate surfaces.
About 75% of the plastic inks use alcohol-soluble polyamide resins, which represent the following reference formulations:
Component% by weight
Polyamide resin 13
S/S nitrocellulose 2
*Organic pigments 15
Filling material 5
Plasticizers, waxes 5
Ethanol 55
Ester 5
100
Note: * If you make a white ink based on TIO2:, this amount can be doubled to reduce the amount of ethanol.
The following reference formulation is a "modified nitrocellulose" ink with good gloss and heat resistance for printing foils, coated cellophane and paper:
Component% by weight
Polyamide resin 10
S/S nitrocellulose 5
*Organic pigments 15
Filling material 5
Plasticizers, waxes 10
Ethanol 50
Ester 5
100
If it is necessary to print the ink for coated cellophane, the film-forming agent in the above formulation, that is, the modified rosin, should be replaced with an acrylic resin so that the heat seal can be not affected.
At present, flexible plastic films for composite packaging are developing rapidly, which requires an ink, that is, a composite ink is printed on the plastic film, and then combined with another plastic film or paper to make a good Sealed packaging for printing and heat sealing properties. The composite ink used for printing here does not touch the inner package nor the air. This composite ink does not require gloss and is not required to have scratch and friction resistance (because they are protected by a composite film). This ink contains no wax or is extremely small, and thus does not cause a decrease in the adhesion between the composite films.
In general, high performance composite inks are based on polyamide resins, and economical composite inks are based on nitrocellulose. All composite inks must have very low residual solvents because such composite inks are commonly used in packaging printing for foods such as chocolates, cookies and sugars.
In addition, from the environmental point of view, the VOC (volatile organic solvent) emitted by the ink needs to be recycled or incinerated. Of course, recycling and incineration involve huge investment costs, and the inks use air flow systems to achieve VOC collection. To this end, they each have their own limitations:
A. Solvent recovery methods: For those that are not suitable for different solvent mixtures, this limits the flexibility of the formulation.
B. Incineration involves the use of a catalyst whose "life" is shortened by components in certain inks - such as silicones. This limits the flexibility of the formulation.
2. Water-based flexographic inks: Water-based flexo plates are mainly used for absorbent substrate surfaces such as kraft paper, corrugated packaging boxes, multi-layer paper bags, etc., and are now being developed for printing on plastic films and foils. Typically, the resins used in water-based flexographic inks are modified acrylic copolymers and rosins, as well as several other products. These resins are solubilized with ammonia or an ammonia compound (or occasionally even with sodium hydroxide). Emulsion polyester is also used. This emulsion polyester is a good pigment grinding resin (because it has good wettability and high viscosity) and makes the ink have good resolubility. The emulsion also improves the fixability, toughness, gloss and some resistance of the ink.
When printing an absorbent substrate surface with such ink, such as corrugated packaging, the following typical reference ink formulations can be used:
Component% by weight
Acrylic resin 20
Organic pigments 12
Fillers, amines, defoamers and biocides 8
Water 60
100
Of course, the above formula is simplified. Since mixed raw materials are used in each non-aqueous component project, it is important to note that defoaming aids are required in water-based inks because of the stirring process during the printing process. There is also the need to add biocides to prevent the growth of microorganisms in the product.
When a non-porous substrate surface is printed with a water-based ink, the addition of a solvent or a volatile organic solvent can improve wettability. Volatile ingredients can be added up to 25% (typically 12-13% by weight of the overall ink formulation). These products are referred to as "quasi" water based inks.
Flexible organic solvents (VOC) content of 5%, sometimes as low as 1-2% of flexographic products are also available. 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 oven used in current flexographic printing, the evaporation rate is slow, but it can meet the production speed requirement. In addition, although some inks are used in some composite products, they are mainly bonded structures, and the binder (which has a higher fixing fastness) which is used together with the water-based ink in the composite product is solvent-based. Volatile organic solvent volatilization. Furthermore, water based inks often fail to withstand the high temperatures encountered during the extrusion process.