Performance of screen printing ink and application of vinly resin

Jul. 01, 2020

Vinyl Resin for Silk Screen Ink, Providing adhesion to the substrates, such as PVC and PET etc; Having excellent dispersibility to the pigments.

Measurement of ink viscosity

 

There are many methods to measure the viscosity of liquid, such as capillary method, concentric circle method, falling ball method, rotation method, funnel method and some special methods.

 

Funnel method: the principle of this method is simple and easy to operate. It is used when the measurement accuracy is not high. The relative viscosity is expressed by the time required for a certain volume of ink to pass through a circular hole with a certain ink diameter.

 

Rotary method: the viscosity of silk screen printing ink is relatively large and strict, so the measurement accuracy is high. Rotary viscometer is often used, such as Brookfield Viscometer in the United States.


Measurement environment

 

The specific measurement of ink viscosity shall be carried out under the standard environment, with the specified temperature of 25 ° C, so as to prevent deviation caused by temperature. The higher the temperature, the lower the viscosity, and vice versa (the same ink).


The relationship between viscosity, external force and temperature

 

Ink has a non-Newtonian viscosity, the dispersion density of its solid particle nucleus is large enough to almost contact each other. When combined with organic carrier, it can form a network structure, so the viscosity is very large. It must be destroyed after the external force, so that the viscosity becomes smaller and flow occurs. After the external force is removed, the viscosity becomes larger and the original network structure is restored. On the one hand, because the organic carrier itself is a polymer group colloid, the increase of temperature can destroy its agglomeration and make it "thin". On the other hand, because the internal energy of the core group composed of solid-liquid phase increases, its activity ability increases and the viscosity changes, which is a reversible process.


Non Newtonian viscosity

 

Non Newtonian viscosity refers to the viscosity that does not conform to the formula of F = Newtonian viscosity. It is caused by the change of material structure, so it is also called structural viscosity. Most colloidal suspensions are non Newtonian fluids.


Performance of silk screen printing ink

 

1. Viscosity

 

Viscosity, also known as internal friction, is the resistance produced when one layer of fluid moves relative to another. It is a kind of characteristic that the fluid interior obstructs its flow. Ink viscosity - generally expressed by "poise" and "centipoise". The viscosity of screen printing ink is about 4000-12000 centipoise. Viscosity is too large, ink on substrate wettability is poor, it is not easy to transfer to substrate through screen. Cause printing difficulties, lack of ink. If the viscosity is too small, the imprint will be enlarged, and the lines of the printed matter will be merged and become waste. Viscosity index can be measured by viscometer. The relationship between viscosity change and printability is: the more stable the viscosity of ink on the printing plate, the better, but when it is transferred to the printing piece, the faster the viscosity changes. Thixotropy is bad for the former and good for the latter, so proper thixotropy is desirable, while shear thickening is not good for printing.

 

Adding solvent, diluent or plasticizer can reduce viscosity; adding filler, pigment and silicide can improve viscosity.


2. Thixotropy

 

Thixotropy refers to the ability of a liquid to reduce its stress viscosity and then restore its original viscosity. In the process of silk screen printing, it is a reversible phenomenon that the ink thickens after a certain period of static time, the viscosity becomes larger, it becomes thinner after agitation, and the viscosity becomes smaller. Because, the shape of the pigment in the ink is irregular, although adsorbed a layer of binder, it is also an irregular ball. Therefore, after a certain period of static time, the pigment particles will contact or be close to each other, resulting in a temporary stable structure. After being stirred by external forces, the pigment particles will be destroyed quickly, and the mutual attraction between the particles will be relieved. The free movement of the particles will be restored, the fluidity will be improved, the ink will be diluted, and the viscosity will be reduced. The smaller the thixotropy of screen printing ink, the better. In order to eliminate this disadvantage, before printing, the ink should be fully stirred to make it return to normal, and then printing.

The more irregular and porous the pigment particles in the ink, such as black ink, the greater the thixotropy. On the contrary, if yellow ink, its thixotropy is small. There are more binders, less pigments and less thixotropy in the ink. On the contrary, thixotropy is large. In addition, the different binders have great influence on thixotropy. For example, the ink made of polymerized vegetable oil has a small thixotropy. For example, the polymer resin is used as the binders, with a large thixotropy. Fumed silica is a good thixotropic agent. When higher thixotropic property is needed, fumed silica with larger specific surface area is used. Fumed silica is hydrolyzed from silicon tetrachloride in hydrogen oxygen flame: SiCI4 + 2h2 + O2 = SiO2 + 4hcl.

3. Yield value

 

Yield value is to add a certain amount of external force to the fluid, from elastic deformation to flow deformation limit stress, is also the minimum stress that must be applied when ink starts laminar flow. The yield value is too large, the ink is hard, it is not easy to open, the ink is inconvenient to transport, and the leveling is poor; the yield value is too small, and the reproducibility of printing fine line and dot is poor. The screen ink is thick, so the yield value cannot be too small. According to different requirements, the yield value of screen printing ink can be from 1000dyn / cm2 to 3000dyn / cm '. When printing fine lines, the yield value should be higher.

4. Fluidity

 

Fluidity is the reciprocal of viscosity. That is: viscosity is large, fluidity is small; viscosity is small, fluidity is large.

 

The fluidity of ink can be seen as the degree of natural flow of a certain amount of ink in a certain period of time and on a certain flat surface under the action of no external force. The fluidity of ink can measure the thickness of ink. In the screen printing ink, its fluidity is generally controlled at 30-50mm. The measuring method is to take a milliliter of ink and measure its diameter after 15 minutes under the pressure of 250g. The ink has large fluidity and easy to enlarge the imprint, which makes the fine lines with small gap indistinct and even merge; the ink is also difficult to print due to the small fluidity and easy to break lines in the imprint.

6. Surface tension

The surface tension of the ink is related to the transfer performance of the ink and the stability of the imprint on the substrate. This relationship is more prominent when printed on smooth materials, especially on plastic surfaces. For example, when the surface tension of the ink is greater than that of the printing surface, the imprint will shrink, or even appear 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 the surface tension of the printing surface, the printing will appear blocking phenomenon. These conditions can be adjusted by adding surfactants, microcrystalline paraffin, silicide, etc., so that the surface tension of the ink is equal to or less than the surface tension of the substrate, so as to achieve good printing effect.

7. Fineness

 

Fineness refers to the size of pigment and other solid raw material particles in the ink, and the evenness of distribution of these particles in the binder. You can use a fineness meter to scrape the properly thinned ink from the deepest part of the fineness meter groove to the flat part, and immediately tilt the light for 15 ° observation. Look at the ink particles at least 15 in a certain scale range, its value is the fineness of the ink.

 

The fineness of screen printing ink is generally between 5-15 μ M. The fineness is too large, in the printing process, there will be a paste, not a pattern. If the screen is thick, the fineness can also be correspondingly thickened. Generally, the coarsest particles should be less than 1 / 4 of the mesh area. The fineness of ink is different from that of solid powder. The latter refers to the basic particle size of powder, which is very small. See Table 1. The basic particle size of general pigments is less than 1 μ M. in fact, pigments are added to ink binders in a state of aggregation (size up to 50 μ m) in which several basic particles are adhered together. After grinding, the aggregates are dispersed, and the better the dispersion, the smaller the ink particles (fineness). The fineness of general ink is about 5 μ m, and the fineness can reach 1-2 μ M. Fineness will affect the gloss of the ink film and the rheological properties of the ink.

8. Viscoelasticity

 

Viscoelasticity refers to the property that ink is sheared and broken after being pressed by scraper, the screen plate springs up and ink rebounds rapidly. Ink and substrate bonding, and screen separation, the phenomenon of rapid retraction, is a typical ink viscoelastic phenomenon. The viscoelasticity of ink has a great influence on screen printing. Drawing phenomenon is that the scraping board scraped, screen springing up, there are many ink filaments between screen and substrate. This is the most taboo phenomenon in silk screen printing. It is not only easy to make the printed matter and screen plate dirty, but also impossible to print.

 

The drawing length is related to the viscosity of the ink. The viscosity is large, the ink filament is long; otherwise, it is short. For this reason, viscosity reducing agent or molecular weight of resin are often added to reduce ink viscosity, improve ink viscoelasticity and reduce drawing phenomenon. The drawing phenomenon is also related to the time of the force. For the same ink, if the separation speed is very fast and elastic, the ink filament is short; if the separation speed is very slow, the ink flows down completely like a pure liquid and does not become silk.

9. Dryness

 

Screen printing not only requires that the ink on the screen can not dry the conjunctiva for a long time, but also requires that after printing, the faster the drying on the substrate, the better. This can keep the imprint clean, save space, speed up the printing speed and improve the quality. The drying requirements of silk screen printing on ink are particularly prominent in multi-color continuous overprinter. The commonly used drying methods include natural drying, hot air drying, ultraviolet drying, electron beam irradiation drying, infrared drying, microwave drying and other forms.

 

A simple method can be used to measure the drying degree of ink. That is, to make a 1.5cm wide, 4cm long screen printing plate, which is well exposed to the image, and to print the ink on the substrate. The drying time of the ink on the substrate can be determined by observing the ink and taking the non stick paper as the standard and calculating the time since printing. For screen printing, the ink of slow drying and quick drying is the ideal screen printing ink, and the ink of light fixation, heat fixation and heat printing cold fixation is produced.

10. Hardness

 

Hardness refers to the degree of pressure deformation of the ink layer, which can be measured by a hardness tester used in the paint and coating industry. Starting from the application of printed matter, the hardness can also be measured by scratch resistance. For example, with the same force, the pencil of different hardness (H) can be used to carve the fully dry (48 hours after printing) ink film, and the H value of the pencil that can carve the ink film is the hardness of the measured ink. Hardness is very important for decoration and printing of rigid bodies such as metal, hard plastic and glass.

 

 

 

11. Light resistance

 

Light fastness refers to the stability of ink imprinting color under sunlight. It depends on the light resistance of the pigment, the thickness of the ink layer, the properties of the binders and fillers. Packaging printing and outdoor advertising have high requirements for light fastness of ink. Outdoor advertising usually takes 3-5 years, and the color of printed matter has no obvious change. The illumination of light will cause chemical (such as oxidation or reduction) reaction and physical (crystal) change of pigment, resulting in pigment darkening, fading and even complete fading. For the determination of light resistance, "blue standard of class 8 light fastness" is often used together with the printed sample in the color fading instrument to expose for several hours (20 or 40 hours), and then take out the contrast to find out which standard the sample is close to, which is the light resistance level of the sample. The "blue standard of 8 grade 8 fastness to sunlight" is to dye 8 kinds of blue dyes with different fastness and the same concentration on wool fabric. Under the same amount of light, they will have 8 kinds of different fading situations, the first grade fading is the most serious and the eighth grade fading is the least easy.

12. Heat resistance

 

Depending on the requirements of thermal processing, the printing ink should not be pulled off during hot pressing. The detection method is to stack a piece of printed matter with a clean aluminum foil, treat it with the actual production temperature, pressure and time, separate the printed matter and the aluminum foil after cooling, and observe the ink transfer to the aluminum foil.

 

13. Chemical tolerance

 

The ability of ink to resist water, acid, alkali and solvent is collectively referred to as chemical resistance. The determination standard and method should be based on the application goal. At present, the commonly used methods are immersion method and wet wipe method. The soaking method is to soak the dried printed matter in the selected water, acid, alkali and solvent, take it out after a certain time, and compare the change degree of the printed matter before and after soaking. The wet wipe method is to wet the cloth with the selected solution, and then wipe the dry printed matter back and forth several times (50-100 times), and then compare the differences before and after the wet wipe.

 

Others such as ink color, glossiness and transparency (or Concealer) to be measured, readers can refer to the relevant printing ink standards.

Fixation fastness of screen printing ink

 

Ink is the most important material in screen printing and plays an extremely important role in the whole printing process. Because the object of screen printing is quite wide, such as paper, metal, plastic, glass, wood, ceramics and various textiles and so on. Therefore, it is necessary to master the performance of ink for screen printing. One of the most prominent problems of silk screen printing ink is the fastness of ink to substrate after printing. If there is no solid fastness, there will be no printing and great waste. The problem of fixation fastness relates to the bonding mechanism of ink and substrate. At present, it is generally believed that the reasons that affect the fastness of ink fixation are as follows.