Causes of skinning of packaging printing inks and measures to be taken after skinning

When the concentration of printing ink increases to a certain value, its surface will be covered by a layer of molecules.

1.

As we all know, when the ink in the ink fountain forms a layer of film due to the drying of the surface during the printing process, either a stirring device is placed, or the ink is simply poured out, otherwise, the film will adhere to the roller, and the result will be Make the layout uneven, resulting in the contamination of random knife marks on the printed product

(1) The ink dries too fast;

(2) Poor ink fluidity;

(3) The precipitation of the ink has dried

However, I do not know that this phenomenon is caused by the large gradient of volatilization, evaporation, or oxidative drying of the ink system, while the printing industry believes that the structure of the ink bucket or ink fountain of the ink is not good, resulting in a non-flowing retention part, from drying.

(1) Do not use ink that dries too fast;

(2) Add medium and slow drying solvent;

(3) Replace the deteriorated ink (that is, from poor fluidity to excellent fluidity);

(4) Keep stirring in the ink fountain;

(5) Set a closed top cover on the ink fountain;

(6) Adjust the hot air blowing angle
.

Generally, people with printing experience and common sense, when the drying speed is too fast, or the ink concentration is too light, or the flow is poor, or the ink thixotropy is large, or the ink is static, they will take the method of increasing the speed of the printing machine, which not only prevents the ink It can be dried on the printing plate in advance, and it can overcome the skinning formed by the packaging and printing ink film formation, and eliminate the pattern failure of the graphics and text
.

2.
Stable packaging and printing ink graphics and text film quality

It can control the free radical polymerization, drying or solvent volatilization of the ink film of packaging and printing ink, as well as the active effect of oxidation, and can achieve the purpose of preventing skinning
.
However, this control function must be based on needs, neither affecting the drying rate of the film formation of printed graphics, nor affecting the performance and quality of the ink product, which is the minimum standard for ink manufacturing
.

For a long time, packaging and printing inks have been added with anti-skinning agents (i.
e.
polymerization inhibitors) for more than 50 years.
In the past 40 years, phenols have been used, while benzoic acid is used in inks, which is higher than the earliest ones.
Boiling point solvent capping saves time and effort
.
Because the phenolic compound itself is a proton donor, it can generate a peroxidative free radical reaction during the oxidation reaction, generate ROOH, and at the same time form a stable antioxidant free radical, capture active free radicals, and terminate the oxidative polymerization reaction— - Prevents skinning of printing inks
.

3.
Application scope and precautions of anti-skinning agent

Oxidation of packaging and printing inks, especially the volatilization speed of solvents, oils of modified resins, used pigments, fillers, dosage of driers, storage environment temperature and humidity of finished ink products, etc.
Therefore, when using an anti-skinning agent (gel agent), the above factors must be considered, and at the same time, seven basic conditions such as the amount of addition should be paid attention to:

1.
The amount of addition
.
Generally, it is calculated according to the ink material (because the main component of the crust is the main raw material of the ink) or the total amount of the packaging printing ink, and the influence of other components must also be considered
.
Usually the addition amount is between 0.
1-0.
3%
.
Because of the difference in temperature and humidity throughout the year, the speed of its crusting is different
.
Therefore, it is 0.
1% in winter, 0.
3% in summer, and 0.
2% in spring and autumn
.
It is scientific to depend on the ingredients of the formula
.

2.
Ink film dry rate
.
Whether it is the rate of oxidation or volatilization at room temperature or the rate of evaporation, we have proved in practice that in the offset printing ink made of alkyd resin, the amount of methyl ethyl ketoxime is generally 1%.
Although the drying speed is slightly longer, it is still packaged and printed.
within the specified standards
.
This is because the complex formed by the auxiliary and the drier temporarily loses the activity of the drier.
Therefore, it is different from the traditional principle of using phenolic compounds to capture active free radicals to terminate the polymerization reaction.
It has little effect on the drying rate.
If the dosage is too large, the volatilization time will be prolonged, and the disintegration of the complex will be too slow, which often affects the drying rate of packaging and printing ink film formation
.

3.
Yellowing
.
Once the anti-skinning agent exceeds 0.
3%, it will often lead to the yellowing of the printed graphic ink film, especially when used in white ink, which will cause yellowing problems after long-term storage
.
Therefore, the yellowing failure can be avoided by strictly controlling the addition amount or changing to butyraldehyde oxime
.

4.
shade
.
After adding anti-skinning agent to packaging printing ink, it often directly affects the color of the ink.
Usually, the effect of varnish is greater than that of color ink, and the effect of white or yellow ink is greater than that of dark-colored packaging printing ink
.
Generally, this phenomenon is difficult to observe by visual inspection and has nothing to do with the amount of addition
.
Although we already know that the application hue of the ink film after the packaging and printing graphic film is dried has no effect, but in the presence of some pigments (such as amphoteric pigments) and driers, the packaging and printing color graphic ink film There will be a significant difference between the tones, which will cause trouble for the toning
.
In this case, the color difference test should be done first in the continuous proofing of ink production, the purpose of which is to avoid the appearance quality of the ink film due to discoloration after printing
.

5.
Printing ink film status
.
Although the anti-skinning agent will not have any effect on the state of the printing ink film, when the packaging printing ink is stored at low temperature for a long time, crystals will be precipitated, which will cause the turbidity of the packaging printing ink after film formation and affect the gloss of the ink film
.
If the above-mentioned material is not properly designed and proportioned with the binder, pigment, filler and drier in the ink system, larger particles will appear
.
Therefore, the test should be carried out before printing
.

6.
Gloss and weatherability
.
After adding the anti-skinning agent, the gloss of the printing ink will not be affected, but also the gloss retention
.
After adding the additives, the weather resistance of the printed graphic ink film has been improved and improved
.

7.
How to use
.
The anti-skinning agent is generally added when the ink is adjusted, and is added while stirring at room temperature
.
For volatile inks, in order to control the skinning of the inks and pigments during grinding, dispersion and storage, especially during the printing process, it is advisable to add a part during ink making, and then add another part before printing
.