How to filter film gravure ink
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2. The compatibility of aqueous emulsion with film is the key
Generally, inks generally contain two types of resins, namely water-soluble resins (commonly known as aqueous solutions) and water-dispersible resins (commonly known as aqueous emulsions). The main component that determines the compatibility of ink with film is an aqueous resin in an emulsion state, that is, an aqueous emulsion. . Therefore, the compatibility of the aqueous emulsion with the film is a very important factor in our screening of gravure ink. According to the three principles of solubility coordination: the polarity is similar, the solubility parameter δ is similar and a new hydrogen bond can be generated, and the solubility parameter δ is a key factor in the solubility three factors. The solubility parameter of the aqueous emulsion should be the same as the solubility parameter of the film. The difference is controlled within ±15% to ensure compatibility of the aqueous emulsion with the film.
At this stage, we do not have to demand that aqueous emulsions are compatible with polar and non-polar films. Just as the benzene-containing gravure ink was replaced with a benzene-free gravure ink, different types of ink were used to adapt to the polar film and the non-polar film, respectively. To be on the safe side, we must also give the ink supplier a transition period.
It is important to note that only theoretically valid analysis can help with screening. Nowadays, the practice of many ink suppliers is to provide some printing ink to the gravure printing enterprises, so that the gravure printing enterprises can test them by themselves. Gravure companies can only test on polar and non-polar films separately. After printing, the adhesion of the gravure ink and the range of applicable substrates can be determined by tape method. In fact, this practice of gravure companies is not reliable. It is reliable to first analyze the resin used by the ink supplier for the gravure ink. If it is a water-based acrylic resin, then we can know the polarity range of the gravure ink and then pass it. The laboratory means to detect the approximate range of its solubility parameter δ. Thus, it is basically known whether such gravure inks are suitable for polar films or non-polar films, or for both polar and non-polar films. Ink suppliers generally do not tell gravure companies about this information for two reasons, one is for technical secrecy and the other is for themselves. However, if the gravure printing enterprise wants to test the ink, it must establish its own laboratory means to measure the information parameters.
3. Confirm that the aqueous emulsion is the first difficulty
The performance of the aqueous emulsion determines the key parameters of the adhesion of the gravure ink on the film, the drying performance, and the resistance of the ink film, so it is confirmed that the aqueous emulsion is a crucial part. However, for the determination of the compatibility of aqueous emulsions with films, ink-and-wash companies are still mostly experimentally used, rather than by methods for determining the solubility of emulsion resins. In fact, the solubility parameters of aqueous emulsions are measurable, but they have not yet become the norm for domestic ink design.
Under the condition that the solubility parameter of the aqueous emulsion cannot be confirmed, it takes time to help judge. The drying conditions of the aqueous emulsion are required to determine the drying conditions. It is also necessary to wait for one week or two weeks, and then test with a tape method to judge the compatibility of the ink with the film according to the change in the amount of ink transferred from the tape. However, we must also pay attention to avoid the misunderstanding in the actual judgment. For example, after drying and placing the ink film for 24 hours, all of the ink film was pulled down by the tape. After the ink film was left for 48 hours, the ink film was pulled down by the tape. The ink film after 72 hours of storage was mostly retained. In this case, we must pay attention to it. In fact, the aqueous emulsion is compatible with the film, but the ink film is not completely dried, as long as the drying problem is solved.
Therefore, I would like to give you a suggestion here: Don't veto easily. Gravure ink is still a growing "baby". It can't be easily vetoed because it is still immature, and the baby and the bath water are poured together. In fact, some inks still have a lot of advantages, just need to do some improvement on it. But many people simply don't know where the direction of improvement is, and then directly veto these inks, which is precisely the sorrow of our gravure world.
Drying properties characterized by initial dry length
The initial dry length is an effective indicator of the drying performance of volatile inks. Therefore, it is still effective to measure the dryness of the gravure ink by using the scraper fineness meter to determine the dry length of the ink. This method can be used to know the ink before the machine. The speed of drying is fast, and it can be judged how high the probability of failure in printing on the machine. However, the initial dry length measurement has limitations on the evaluation of the drying performance of gravure ink. The results of initial dry length measurement show that the drying speed of gravure ink is slow, how to strengthen the drying of gravure ink? The long-term solution is to modify the gravure printing machine, lengthen the drying tunnel and increase the drying temperature, but this will easily cause the film. Deformation, inaccurate overprinting, resulting in a large amount of waste. Moreover, the desiccant technology and the catalyst-accelerated resin cross-linking technology cannot be simulated by the initial dry length, that is, the drying performance of the gravure ink of the two technologies cannot be detected in the laboratory, and can only be tested on the machine.
In addition, attention should be paid to the important role of surface dryness in the application of film gravure inks. The film gravure ink is volatilized in the dry section of the surface to allow moisture to escape, and there is no other way. All other methods can only be followed by a drying tunnel to produce a chemical reaction under the influence of temperature, including desiccant technology. This is directly related to the structure of the gravure press, but many gravure printer manufacturers do not understand the ink, and do not understand the difference between solvent ink and ink, so their transformation of the gravure machine is basically wrong.
When measuring the drying performance of the film gravure ink in the laboratory, you can use the oven method (note the temperature and time control), or the hair dryer method with less control precision, and the heat-resistant adhesive commonly used in the ink industry. Blocking) test method. It should be noted that the maturation process in thin film gravure ink applications is very important. This is because no one can guarantee that the gravure printing company will reverse the printed and non-printed surfaces of the film in the actual production process, but this situation will not be discovered until two days later, and this time is the delivery date. Already very close, everything is too late. In gravure ink applications, the aging process helps us quickly determine if the printed side of the film is being used incorrectly.
Concern for viscosity
Many of the film gravure inks currently on the market contain thickeners, because many gravure companies want high ink viscosity, and the ink is diluted by adding more water to increase the amount of ink, but this traditional gravure thinking It is possible for us to go into the wrong zone. Because the water added by the gravure printing enterprise in the printing production and the water in the gravure ink (the water in the gravure ink is no longer the water in the original sense after the water-based resin is aminated), the two are different. The latent heat of evaporation is different, and the drying speed is also different. The more water the gravure printing company adds in the printing production, the slower the drying of the gravure ink. That is to say, dilution with water will increase the difficulty of drying the gravure ink.
Also pay attention to the uniformity of the viscosity of the gravure ink and the viscosity of the gravure ink. This is for the ink supplier, there must be an exact method to control the viscosity of the gravure ink. It is worth noting that the viscosity cup must be measured under the stable pH condition of the gravure ink. If the pH value is unstable, the measurement result of the viscosity cup cannot reflect the real situation.
Determination of pH stability
In the experiment, the method for determining the pH stability of the film gravure ink is as follows: an appropriate amount (about 100 g) of the gravure ink sample is taken, and after being left open at room temperature for 24 hours, 48 hours, and 72 hours, the pH value is monitored. If the change is small, the pH is stable. The pH stability is related to the resin amination process in the process of manufacturing gravure ink. At present, there are three processes for aqueous resin amination: ammonia, organic amines and inorganic bases. The stability of the three processes after resin amination is different. Larger. The amination process of the gravure ink can be screened by measuring the pH stability. If the pH value of the gravure ink drops significantly after being left open for 24 hours at room temperature, it can be confirmed that ammonia water exists in the resin amination system; the change is not significant in 24 hours, and the pH value changes slightly after 48 hours, and the resin amine can be judged. The organic amine process is adopted, and the boiling point is high, but the volatilization still exists; the pH is basically unchanged, and even after 72 hours, the pH value is still relatively stable, indicating that the resin amination is carried out by an inorganic alkali solution process.
The determination of pH stability also helps us to infer the stability of the film gravure ink during printing. The pH-stabilized film gravure ink has stable transfer performance during printing and stable viscosity, so the color density is also stable.
Determination of organic solvent content
It is difficult to avoid the inclusion of organic solvents in gravure ink, but the organic solvent content is required to be less than 5%. It is necessary to eliminate the design of drying with organic solvents, especially the use of lower boiling alcohol solvents to help dry, because the amount of organic solvent added by gravure ink is strictly limited, otherwise there will be VOCs in gravure ink. The problem of emissions, which is the same as the industry's original promotion of gravure ink is the South.
The gas chromatograph can be used to determine the organic solvent content in the film gravure ink, which is very simple and very reliable. Many gravure flexible packaging companies have to test the solvent residue of the product, basically equipped with a gas chromatograph, can carry out this test.
Surface tension control
Controlling the surface tension of the film gravure ink is mainly to control the problem of gravure printing. Earlier we mentioned the problem of the dirty version, that is, the surface of the printing plate is not clean. When this problem is encountered, the gravure printing company does not push the responsibility to the captain (such as the blade is not installed, the blade pressure is not enough), etc. On the plate-making factory (think that the surface of the plate is too rough). In fact, this kind of understanding is wrong. The problem of contamination is actually caused by the low surface tension of the ink. The gravure printing company can directly measure the surface tension of the film gravure ink used in the laboratory, and thereby judge whether it is easy to cause the problem of the stain.
Film gravure inks generally do not have a problem of fouling because the surface tension of the film gravure ink is generally higher than 32 mN/m, but additives and surfactants may reduce the surface tension of the film gravure ink. To determine the surface tension of the film gravure ink, a special liquid surface tension meter, such as K100 from KRUSS, Germany, is good.
In addition, fineness, color density (under controlled viscosity conditions), ink film abrasion resistance and scratch resistance measurement methods, gravure ink is the same as solvent ink, and will not be described herein.
Content that cannot be covered in laboratory testing
The main contents of the film gravure ink laboratory test are introduced in the previous section. The laboratory test after laboratory identification will greatly reduce the risk of failure and reduce the test cost. However, there are some contents that cannot be covered in laboratory tests, such as: 1 desiccant technology in ink drying; 2 ink film resin cross-linking technology in ink drying; aqueous solution of water-based solution and ink in 3 color paste Chemical stability after mixing. On-machine testing is essential for content that cannot be covered in these laboratory measurements.
In the application of film gravure ink, the gravure printing enterprise should be in an active rather than passive position, because the gravure printing enterprise bears both the risk of experimental failure and the pressure of the government to collect VOCs sewage charges. It can be said that the production cost is Social responsibility is shouldered. Therefore, gravure printing enterprises should learn to screen and learn to cooperate. It is necessary to use the smallest possible test cost to obtain the required film gravure ink, and completely solve the problem of gravure VOCs emission.