Posted 05 Jan 2011 00:00
First, the concept of water-based flexographic inks
Water-based inks and water based polymer resin emulsion, organic pigments, solvent (mainly water) and related chemicals by a mixture of physical and chemical processes. Water-based ink has a non-volatile organic solvents, non-flammable, will not damage the printing ink manufacturers and operators of health, environmental pollution and other atmospheric properties. As a new water-based ink printing ink, solvent ink is not certain toxic and hazardous substances in print and on the residual contamination of packaged goods, improving the overall quality of the environment.
Water-based flexographic inks, printing plates based ink according to the classification of type version and with the ink drying mechanism derived from the combination of names.
Second, water-based flexographic ink formulations of the technical basis
1. Waterborne binders and salt in the mechanism
Using water-based ink water-based binders. Water-based binder basically divided into three categories: colloidal dispersion, emulsion polymers, polymer dilution water. Ink industry is currently the main type of connection used, or dilute material. Many polymer (water soluble) is not as soluble in water or only partially soluble in water, only to add an acid or base, only because of ionization and soluble in water. A typical example is the polypropylene and polyamide acids are anionic and cationic electrolytes. The pH value of aqueous solutions of these substances to its viscosity, flocculation, stability, dispersion and other closely related. Therefore, in the molecular chain of water-soluble polymer containing a certain amount of hydrophilic groups such as carboxyl, hydroxyl, amino, etc., but these polar groups can only be mixed with water to form emulsion when the majority. The acid salt can be dissolved in water, so the manufacture of water-soluble material is often linked with high acid state of synthetic resin, and then amine salt.
2. Volatilization, infiltration, or a combination of the three curing reaction mechanism of the dry film
In general, the printing process and the corresponding drying ink formulations used in the decision system. To meet the technical requirements of flexographic printing and water-based ink of their own characteristics, depending on the printing substrate, water-based flexo ink drying methods are evaporation, seepage, or a combination of the three curing reaction.
(1) evaporation drying system
For non-absorbent substrates of the flexible printing substrate, the drying method based mainly on evaporation drying, the resin / solvent ink systems. Because:
` Flexo machine fast, 80m/min to 200m/min or more have, in general, and India finished the first color to a second-color printing, the interval of only a few seconds to a few tenths. For non-absorbent substrates, in all the dry type, only the evaporation drying to meet this requirement.
a the liquid ink film only emerge as soon as the ink film, such as the use of high boiling point solvents is not instantaneous evaporation of the features to make quick drying ink. Quick-drying ink is also required for low viscosity and en thin liquid.
b For the offset printing ink, letterpress printing ink, the ink roller can be reconciled between, evenly coating the ink on the plate, and flexo ink flow itself alone, filling in the roller net adhesion eyes and transfer ink to the plate, only a low viscosity liquid that is thin compared with ene of such a nature to give. And the ink must be in a very short period of time to fill hollow eyes, it is difficult to fill if the viscosity is too large concave in the eyes, and scraper to scrape off the network is difficult to smooth the wall part of the ink. Therefore, flexo ink is generally low viscosity fluid form, a large number of solvents or water only through evaporation or infiltration manner. Of course, not too low viscosity of the line, or stamping will cause the ink dot in the deformation, the graphic representation of the poor.
Ink drying speed depends primarily on the evaporation rate of solvent inks. Ink solvent evaporation, mainly in the following factors:
` different resins on the solvent evaporation slows to different degrees. The greater solubility of the resin, the more difficult to emerge solvent, the lower evaporation rate.
a pigment in the ink the greater the proportion, the lower the evaporation rate of solvent; the radius of the smaller pigment particles, the greater the specific surface area, the lower the evaporation rate of solvent; different kinds of paint on the extrusion of solvent is not the same.
(2) penetration of absorption drying
The absorbent substrate flexo, the drying method to dry and penetrate the main absorption. Absorbent substrates such as paper and ink with absorbent. Ink connection material is a liquid membrane material, and pigment particles to make the adhesive bonding the paper surface. After the material transferred to the paper linked to penetrate to the inside of paper until the ink of the pigment particles become sufficiently close, shrinking the gap between the particles of the pigment particles generated tension between the capillary force of the paper is equal to the complete absorption of links absorption penetration process. Binder to the paper, the degree of penetration within the capillary tension is proportional to the paper, the ink of the paper depends on its degree of internal porosity, which is the tightness of paper and tissue evenness. Actual printing, you should also consider the printing pressure, embossing time, ink viscosity. Olsson basically sums up the above factors the formula ink is pressed into the paper on the depth of the relationship:
d = (p × r2 × t/2η) 1 / 2
d: depth of the ink onto the paper
p: Pressure Printing
r: capillary radius of the paper
η: viscosity of printing ink
The printing pressure is much greater than the capillary attraction, so the latter is negligible.
The penetration of the ink on the absorbent substrate absorption of ink curing and drying process is very important. Permeation too little or too light, ink adhesion is not strong, not easy to dry. However, the penetration is too big or too deep, it will cause problems through India, will also reduce the ink gloss.
In addition, the absorbent substrate, but also simultaneous evaporation drying mechanism. Therefore, absorbent substrates for flexo, dry way is volatile film-forming mechanism and double penetration.
Ink in the preparation of some special requirements, also introduce reactive groups to further improve its film properties.
Third, the impact of water-based flexographic ink formulation of the many factors
1. Printability requirements
Ink from the ink **** to the printing plate was removed to the substrates of the process is the transmission and transfer ink separation. Requires, first in the printing process can always transfer to the plate and stable ink, the ink on the second layout is always hope that a certain state to effectively transfer to the substrates. Transmission and transfer performance and the quality and precision of the printing press, and also requires a corresponding ink printability. Rheology of the ink itself, the ink has become dominant and important factor in fitness. Rheological properties including viscosity, yield value, thixotropy, fluidity, adhesion, ink thread length and so on.
The size of the ink viscosity, cohesion, must be suitable for the whole process of printing requirements.
(1) the higher the printing speed, the smaller the required viscosity of the ink. Fast printing, request the transfer fast, fast drying, that is, the ink viscosity force is small, easily separated, and the solvent is easy to escape from the surface of the ink film. Practice shows that the plastic viscosity of the ink ink transfer rate significantly, in the same printing speed under the same conditions, low viscosity divided state of the ink layer of ink than the viscosity of the transfer of benefit. Figure 1.
(2) The requirements of substrates: coated paper for the more smooth, when the ink supply is adequate, a high transfer rate, so the higher the viscosity of the ink required; structure of soft paper for offset paper should be slightly lower viscosity of the ink .
Thixotropy thixotropy or "shear thinning." Ink should have some thixotropy ink during the printing process by the ink roller rotational mechanical role, the ink flow to produce a change, the viscosity decreased, mobility increased, the ductility is also increasing, conducive to the smooth uniform ink transfer. Ink transfer to the substrates, the loss of external force, because of its thixotropic soon surrounded by a thin thickens without overspill to ensure the accuracy of imprinting. In particular, network printing, and text lines, to avoid ink on paper caused by the invasion and spreading of dot gain, line thicker. Thixotropic ink with the following factors:
(1) the nature of paint and strong surface adsorption of the ink pigment made from a large thixotropy.
(2) the shape of pigment particles when pigment particles were needle-like than spherical particles made of ink thixotropy large.
(3) the amount of pigment in general, the greater the proportion of ink pigment particles, the stronger the interaction, the greater the thixotropy of the ink.
(4) pigment particles and binder wetting ability of the low wettability, ink thixotropy large.
(5) resin compound of the resin molecular weight connecting the molecular weight, the greater the thixotropy.