1. Design principles of the flexographic water-based ink formulation
Flexographic water-based ink is a new type of printing ink, which belongs to relief printing ink. The formula design of this article mainly refers to the current inspection methods and quality standards of various relief printing inks. The following principles should be followed in formula design:
1. The selected solvent should not affect the printing plate and rubber roller, the ink should not smell after drying, and must have good storage stability.
2. To determine the selection of ink raw materials and ink ink properties according to the type and requirements of the printed matter. Only by understanding the type and purpose of the printed matter before designing, can we choose raw materials and predetermined ink quality standards according to its characteristics.
3. The choice of pigments and dyes should be determined according to the type of substrate: for example, some packaging paper bags are often used to package stronger acidic substances, such as sour plum bags, and basic blue dyes cannot be used for coloring in ink Agent, otherwise it will change color. In the case of alkaline substances such as packaging soap, iron blue will also change color when used in ink. There are also products that are often displayed in the window. After a long period of sunlight, if the salt-based rose red and other lake pigments are used in the ink, it will be seriously discolored.
Second, the printing performance requirements of flexographic water-based ink
The properties of flexographic printing inks that need to be considered in the printing process mainly include viscosity, pH, thickness, dryness, adhesion and color. Introduced here in conjunction with the flexographic water-based ink.
1. Viscosity Viscosity is the most important control index in the application of water-based ink, because it directly affects the transfer performance of ink and the quality of flexographic printing. In addition to the viscosity and density of the resin in the binder, the viscosity is also related to the type and particle size of the colorant. In printing, low viscosity water-based ink transfers quickly, which will cause light color, large dot expansion, high-gloss dot deformation, uneven ink transfer and other disadvantages; high viscosity and slow transfer of water-based ink will affect the transfer of the anilox roller Performance, resulting in uneven ink, sometimes the color is not deep, and dirty, paste and other disadvantages. In addition, during the printing process, the temperature also has a greater impact on the viscosity of the water-based ink, usually the viscosity increases when the temperature increases, otherwise, the viscosity increases. Therefore, in the printing process, in order to maintain the consistency of the printed matter, the temperature of the printing workshop should be kept constant.
2. pH value The binder used in water-based ink is mainly alkali-soluble acidic resin, so the control of pH value is very important. Generally, the pH value should be controlled at 8.0 to 9.5. At this time, the printing performance of the water-based ink is the best and the printing quality is the most stable.
Due to the continuous volatilization of amines during the printing process, the operator must add new ink and various additives from time to time, so the pH of the water-based ink changes at any time. When the pH value of the water-based ink is higher than 9.5, the alkalinity is too strong, and the viscosity of the water-based ink will decrease, resulting in rapid drying and poor water resistance; when the pH is below 8.0, the alkalinity is too weak, water The viscosity of the base ink will increase, resulting in rapid drying and easy clogging of the printing plate and the anilox roller, causing the plate surface to be dirty and generating bubbles.
Because the influence of pH value is directly related to viscosity, it is recommended to control the two in direct printing, especially in register printing, this problem should be paid more attention to. At present, flexographic water-based inks suitable for printing in packaging paper bags and craft papers have been produced abroad. The pH value is very stable. In high-speed printing, there is no need to continuously adjust the pH value to control the viscosity, so that the printing quality is very stable.
3. Thickness The thickness of water-based ink refers to the degree to which solid powdery substances such as pigments and fillers mixed in the binder are dispersed. Thickness is also a very important quality indicator, because it is not only related to the application performance of water-based inks (including coloring power, hue, stability, etc.), but also related to the rheological properties of water-based inks. Particles that are too coarse can cause ink stacking, paste printing, etc., and the coloring power and hue of the water-based ink will also be poor; particles that are too fine will have large fluidity and will flow after printing. This flow will cause the dots to expand. , Affecting the quality of printing. In addition, the thickness of the ink will also affect its dispersion. When the dispersibility of the ink is not ideal, its printing performance will not be satisfactory. It is manifested that the surface of the printed product is not smooth, smooth and soft, and it will produce a "block" feeling, thus greatly reducing the printing effect.
4. Dryness Dryness of water-based ink is an important technical indicator. In addition to its own performance, the drying speed of water-based ink is also related to the printing speed, the drying capacity of the drying equipment and the performance of the printing material. This process is completed in two stages: the water-based ink changes from liquid to semi-solid, can no longer flow and transfer, it is the initial drying, expressed by the initial drying; the main part of the bonding material in the semi-solid water-based ink occurs physical or chemical The reaction is completely dry and solid to form a film, which is the complete drying stage of water-based ink. It is expressed by thoroughness. The fixation stage and thorough drying stage of water-based ink are collectively called fixation drying.
The principle of selecting the drying property of water-based ink is generally that the ink should be able to be fully dried before the next printing and before rewinding or stacking under the existing conditions of the printing press, but it cannot be dried on the printing plate.
Under the condition that the solvent is determined, the choice of different types of resin also has an effect on the drying properties of the ink. Because different resins have different curing speeds on the substrate and different release characteristics of the solvent, the drying properties of the ink are also different.
In practical applications, a quick-drying agent or a slow-drying agent can be added to the ink according to the actual printing speed, and the drying speed of the ink can be appropriately changed to obtain a stable printing effect. The amount of fast drying agent or slow drying agent is generally 1% -2% of the total ink volume. [Next]
3. Selection of raw materials for flexographic water-based ink
1. Selection of flexible printing ink pigment
The available colorants are divided into dyes and pigments. The dye is soluble in the binder, so the coloring power is large, the hue is bright, and the fluidity is good. But water resistance and light resistance are poor. The pigment is dispersed in the binder in the form of particles. The classification of the commonly used colorants for water-based flexible printing inks is shown in Table 1.
Table 1 Classification of flexographic water-based ink pigments
Color
dye
pigment
Water-soluble dye
Alcohol soluble dye
Colored precipitated pigment
Organic pigment
Inorganic pigment
If it is an alkaline binder, the color should have a certain alkali resistance, such as carbon black, Lisol red, disazo yellow, titanium blue, etc. are all alkaline pigments. The proportion of the pigment in the water-based ink depends on the strength of the coloring power. For organic pigments, the coloring strength is strong, the density is light, and the proportion should be smaller; for inorganic pigments, the proportion should be larger. The selected pigment should also achieve: good dispersion and no coagulation and precipitation during storage; it can show good fluidity when dispersed in the binder; in the corresponding solvent system, no discoloration or fading will occur And it has good solvent release when the system is dry.
2. Selection of linking materials in flexographic water-based ink
The flexographic water-based printing ink uses a water-soluble binder, which is composed of an alkali-soluble resin or emulsion, water and a co-solvent. The pH, viscosity, solid content and other properties of water-based binders determine the rheology, viscosity (dryness), dryness and printability of the ink to a certain extent. Water-based binder is the "heart" of water-based ink.
(1) Water-soluble resin
There are many types of water-soluble resins, such as water-based amino resins, polyvinyl alcohol hydroxycellulose, maleic anhydride rosin resins, acrylic resins, etc. With the increase in printing requirements, higher requirements have been placed on the gloss, abrasion resistance and water resistance of water-based inks. Acrylic resin is colorless and transparent, has strong adhesion, and is resistant to friction, and it can be dissolved in various solvents used in ink manufacturing, so it has been more and more widely used. Acrylic resins come in three forms: alkali-soluble, nonionic polymer, and acid-soluble polymer. In water-based inks, the alkali-soluble type is mainly used (because the non-ionic film is relatively hydrophilic after film formation and has poor water resistance; the acid-soluble type has no suitable volatile acid and has an odor). In the preparation of water-based inks, ammonia or amines or their mixed forms are usually used to make the resin an ammonium salt soluble in water. After printing on the substrate, with the volatilization of ammonia, the ink film has a certain gloss, water resistance and abrasion resistance after drying.
Acrylic resin has two forms in the flexographic water-based ink binder: One is that the acrylic alkali-soluble resin is formed into a resin solution by ammoniation, commonly known as resin oil. It is the main body of the binder, used to disperse pigments and prepare base inks. The performance and solid content of the resin directly affect the viscosity, pigment dispersion and ink transfer performance of the water-based ink; the other is an emulsion, which is acrylic acid, methacrylic acid or other monomers (such as styrene) polymerized by an emulsifier It also has a certain degree of polymerization and acid value, and these properties directly depend on the control of conditions in emulsion polymerization. It is also because the conditions of emulsion polymerization are harsh and difficult to control, the performance and stability of the emulsion are more important. The application of emulsion in water-based inks has a significant impact on improving the gloss, water resistance, and drying properties of the ink. Especially for premium water-based inks, emulsions are essential. [next]
Both water-soluble resins and water-dispersible resins are the linking materials for water-based inks, which have a great influence on the viscosity, adhesion, gloss, drying and printing suitability of inks. Rosin-modified maleic acid resins are often used in China It is used as a connecting material to make ink, but this ink can only be used to print general carton, and can not meet the requirements of gloss and water resistance required for medium and high-end packaging printing. The stability of urethane resin is better, but the printability and solubility are slightly worse. Others include maleic resin modified with styrene, waterborne amino resin, polyvinyl alcohol and carboxymethyl cellulose. These polymers The resin generally contains —COOH (carboxyl group), —OH (hydroxyl group), —NH2 (amine group) and other hydrophilic groups. After a specific process, it can become a completely water-soluble resin and can be used as a link for water-based inks. Expected, but there are clearly some unsatisfactory places. It has different effects on the printing adaptability, gloss and water resistance of different resins, see Table 2.
Table 2 Types and properties of water-based resins
project
Maleic resin
Urethane resin
Waterborne amino resin
Hydroxymethyl cellulose
Water-soluble acrylic resin
Pigment dispersion
in
in
good
good
excellent
Printing adaptability
in
in
in
good
excellent
Friction resistance
difference
in
difference
in
excellent
Ink stability
in
good
in
in
excellent
Heat resistance
difference
difference
in
in
Excellent
Water resistance
difference
difference
in
difference
excellent
Colorability
in
in
difference
in
good
luster
difference
difference
in
difference
good
It can be seen from Table 2 that the use of water-soluble acrylic modified resin as the binder for water-based inks has significant advantages in terms of gloss, abrasion resistance, heat resistance, and water resistance.
The main role of water-soluble resin in the flexographic water-based ink system is: the film-forming substance in the ink adheres the pigment to the substrate and gives the ink a considerable body; by dispersing the pigment, the ink is in the substrate It has sufficient leveling, spreadability and gloss; increase printing recoatability.
(2) Solvent in water-based binder
Since the dissolving power and wetting capacity of water are not large, the corresponding co-solvent needs to be selected. The selected co-solvent must have: can dissolve the selected resin, amine and its salt, and can be miscible with water in any proportion ; To be non-toxic, non-irritating, non-flammable and explosive; to have a certain dilution capacity, the boiling range is appropriate; have a certain volatility.
Generally, the solvent in the water-based binder should be water-soluble, soluble in the polymer and have a suitable boiling point. Different polymers use different co-solvents in order to achieve the desired rheological properties and drying properties. The available solvents are:
Alcohols: ethanol, n- (iso) propanol, butanol, etc., auxiliary water enhances the ability to dissolve the resin, improve the dispersion function of pigments and dyes, and can accelerate penetration and inhibit foaming;
Ethers: ethyl cellosolve, butyl cellosolve, etc., adjust the dryness.
3. Selection of additives in flexographic water-based ink
The printing suitability of the flexographic water-based ink on the printing machine depends largely on the use of additives in the ink. Proper additives can optimize the performance of the water-based ink. That is, it can improve and improve the ink properties, stability, printability, and imprinted surface performance of water-based inks. It includes dispersants, defoamers, leveling agents, slip agents, plasticizers, etc.
(1) Dispersion aid The manufacturing process of water-based ink is actually to disperse the pigment in the binder by mechanical force and form a dispersion of fine particles according to the designed formula, so as to obtain the ink with the expected ink properties. Due to the difference in the surface characteristics of the pigment and the surface tension of the binder, a small amount of surfactant is needed to improve the compatibility;
(2) Defoamer Since water-based inks have similar soap characteristics and are prone to foaming, foam inhibitors and defoamers need to be added to improve;
(3) Printing suitability is affected by objective conditions such as printing conditions, surface characteristics of the substrate, ambient temperature and humidity, and the length of time the water-based ink is stored. Therefore, when using water-based inks, some additives are needed Fine adjustments to obtain the best printing results.
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