Screen printing ink composition
Typical screen printing inks are loose slurries and have good thixotropy (when the ink is at rest, its flowability is poor, and once it is subjected to an external force, the ink fluidity increases sharply). This feature allows the ink to smoothly pass through the plate apertures during squeegee printing and quickly level up on the substrate to form the desired image. In this way, the ink film does not leave traces of the screen after drying, and does not allow the printed image to overflow and expand.
Screen printing UV ink to meet the above requirements, the viscosity of the active monomer should be small, strong retraction ability, and its amount in the ink as much as possible. Due to the large thickness of the screen printing ink film, the amount of colorant can be minimized. Even if its amount is reduced to about 5%, the ink can still obtain a relatively opaque color tone. The typical formula is as follows.
Screen printing UV ink
Reactive oligomers 35 to 60 (wt%)
Reactive monomers 15 to 50
Photoinitiator 3~7
Coloring agent 10-30
Additives 1 to 5
Total 100
The reactive oligomers in the above formulations are usually based mainly on epoxy bis-acrylates. If the adhesion is poor, they can be mixed with polyester bis-acrylate or polyurethane bis-acrylate. In applications, the active monomers often used are tripropylene glycol diacrylate and hexylene glycol diacrylate. Photoinitiators for screen printing UV inks often use two or more mixtures, such as benzophenone, thioxanthone, and benzoin ethers. In order to enhance the photo-initiating effect, organic amines are also often used as activators.
Screen printing ink properties
1. Viscosity.
Viscosity, also known as internal friction, is the resistance that occurs when one layer of fluid moves relative to another layer of fluid. It is a characteristic of the fluid inside that hinders its flow. The ink viscosity is generally expressed in terms of "poise" and "centimeter poise." The screen printing ink has a viscosity of about 4000 to 12,000 centipoise.
Excessive viscosity The ink has poor wettability with the substrate and is not easily transferred to the substrate through the screen. Difficult to print, imprinted missing ink.
If the viscosity is too small, it will cause the imprinting to expand, resulting in the consolidation of print lines and becoming waste products.
The viscosity index can be measured using a viscometer.
The relationship between viscosity change and printability is: the ink on the plate, the more stable and better the viscosity, but the transfer to the print, the viscosity becomes faster and better. Thixotropy is unfavorable to the former and beneficial to the latter, so proper thixotropy is desirable, and shear transposition is not good for printing.
Solvents, dilution penalties, or plasticizers can reduce the viscosity; fillers, pigments, and silicides can increase the viscosity.
2, thixotropy.
Thixotropy refers to the ability of a liquid to recover its original viscosity due to a decrease in the stress viscosity. In the silk screen printing process, the ink is thickened after standing for a certain period of time, the viscosity becomes large, and the ink becomes thin after stirring, and the viscosity also becomes a reversible phenomenon. Because the shape of the pigment particles in the ink is irregular, it is an irregular sphere even though it adsorbs a layer of binder. Therefore, after a certain time of standing still, the pigment particles will come into contact with each other or come close to each other, causing mutual attraction, hindering the free movement of the particles, and the ink will become thick and sticky. However, this temporarily stable structure, after being stirred by an external force, is quickly destroyed, releasing the mutual attraction between the particles, recovering the free movement of the particles, improving the fluidity, thinning the ink, and lowering the viscosity.
The thixotropy of screen printing ink is as small as possible. To eliminate this unfavorable factor, the ink must be fully agitated before printing to return it to its normal state and then printed.
The more woody pigment particles in the ink, the more porous, such as black ink, the thixotropy is large. On the contrary, if yellow ink, its thixotropy is small. There are many binders in the ink, less pigment, and less thixotropy, and vice versa. In addition, different materials have a great influence on thixotropy. For example, inks made from polymerized vegetable oils have small thixotropy, such as polymer resin as a binder, which has a high thixotropy.
3. Yield value.
The yield value is the limiting stress that applies a certain external force to the fluid, from elastic deformation to flow deformation, and is the minimum stress that must be applied when the ink starts laminar flow. The yield value is too large, the ink is hard, it is difficult to open, the inconvenience of inking, and the leveling property are poor; the yield value is too small, and the reproducibility of printed thin lines and dots is poor. The silk ink layer is thick, so the yield value cannot be too small. Screen printing ink according to different requirements, the yield value can be 1000dyn/cm2 to 3000dyn/cm2, when printing fine line drawing, the yield value should take a high value.
4. Fluidity.
Fluidity is the reciprocal of viscosity. That is, the viscosity is large, the fluidity is small, the viscosity is small, and the fluidity is large.
The fluidity of the ink can be regarded as the degree of natural flow of a certain amount of ink in a certain period of time and on a certain leveling surface without external force. The fluidity of the ink can measure the thickening of the ink. In silk screen printing ink its fluidity is generally controlled in 30 ~ 50mm. The measurement method is to take one milliliter of ink, and after measuring the pressure at 250 g for 15 minutes, the diameter can be measured.
The flow of ink is large, the imprinting is easy to expand, the thin lines with small gaps are unclear and even merged; the fluidity is small, the lines are fragile and the line is fragile, and the printing is also difficult.
5. Plasticity.
Plasticity refers to the property of being able to completely or partially retain its deformation after being deformed by external forces.
Screen printing inks are concentrated suspension gums between fluids and semi-solids, so they are both fluid and malleable. High pigment and other solids content, the ink appears thick, plasticity is large; pigments and other solids content is low, the ink is thin, plasticity is small. Screen printing inks require a certain degree of plasticity in order to maintain the accuracy of printing, otherwise the printed lines can be easily expanded.
6. Surface Tension.
The surface tension of the ink is related to the transfer properties of the ink and the stability of the print on the printing surface. This relationship is even more pronounced when printed on glossy materials, especially plastic surfaces. For example, when the surface tension of the ink is greater than the surface tension of the printing surface, the imprinting will shrink, even fisheye-like holes; if the surface tension of the printing surface and the ink is similar, and the surface tension of the screen is greater than that of the printing surface. When the surface tension, the printing will appear blocking phenomenon. These conditions can be adjusted by adding surfactants, microcrystalline wax, silicides, etc., so that the surface tension of the ink is equal to or less than the surface tension of the printing surface to obtain a good printing effect.
7. Fineness.
Fineness is an indicator of the size of the pigments and other solid raw material particles in the ink, and the degree of uniformity of the distribution of these particles in the vehicle. Fineness meter can be used to properly dilute the ink from the depth of the fineness meter groove scraped to the flat, immediately light tilt 15 observed. To see at least 15 particles in the ink within a certain scale, the value is the fineness of the ink.
The fineness of screen printing ink is generally between 15 and 45 microns. The fineness is too thick and a paste is produced in the printing, and no pattern can be printed. If the screen is thick, the fineness can also be thickened accordingly. Generally the coarsest particles should be less than a quarter of the mesh area.
The fineness of the ink is not the same as the fineness of the solid powder. The latter refers to the basic particle size of the powder. It is very small. See Table 3-12. In general, the basic particle size of the pigment is less than 1 μm. In fact, the pigment is added to the ink binder by the aggregated state of several basic particles (up to 50 μm in size), and the aggregates disperse and the dispersion becomes better. The smaller the ink particles (fineness). Generally, the fineness of the ink is about 5 μm, and the ultra fineness is 1 to 2 μm. Fineness affects the sheen of the ink film and the rheology of the ink.
Table 3-12 Particle Sizes of Several Kinds of Pigments
Pigment Name Particle Size (μm)
General pigment <0.1
Titanium dioxide 0.25
Carbon black 0.01-0.1
Fluorescent pigment 1.2～50.0
Gold and silver powder thickness: 0.1 to 2.0
Diameter: 1～200
Color foil pigment>200
8. Viscoelastic.
Viscoelasticity refers to the property that the ink is sheared and fractured after pressure is applied to the plate, and the screen plate springs up and the ink rebounds quickly. Bonding of the ink and the substrate, and the detachment of the screen, the phenomenon of rapid retraction, is a typical ink viscoelastic phenomenon.
The viscoelasticity of the ink, which has a greater effect on the screen printing, is the phenomenon of drawing. The drawing phenomenon is that when the squeegee is scraped and the screen is bouncing up, many ink filaments appear between the screen and the substrate. This is the most bogey phenomenon in silk screen printing. It is not only easy for the prints and screens to become dirty, but even for printing.
The length of the drawing is related to the viscosity of the ink. The viscosity is large and the ink is long; otherwise it is short. For this purpose, the viscosity of the ink is reduced, the viscosity of the ink is improved, and the wire drawing phenomenon is reduced.
The drawing phenomenon is also related to the time of action. In the same ink, if the separation speed is rapidly split by elasticity, the ink is short; the separation speed is very slow, and the ink flows completely like a pure liquid and does not become filamentous.
