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In the packaging printing process, the phenomenon of streaks or light color printing occasionally occurs. This failure, which is called no ink and ink release roller, is caused by the swelling of the color and filler of the packaging printing ink system. Due to gelation.
Explain from a theoretical point of view: the yield value of the ink is too high, and the ink is too short to become a cheese. From a practical point of view, too many pigments and fillers in the ink cause the ink to gel or cause water to enter the ink system during the storage process and cause partial flocculation.
People use the ink in the ink fountain continuously for bloating or gelatinization, or install a stirrer in the ink fountain, and also put an iron bar on the ink to push the ink to the ink fountain roller to maintain the fluidity of the packaging printing ink. Most operators prefer to use medium- or high-viscosity varnish or thinner to adjust the ink (but this must be used with it, otherwise it will continue to swell or gel). Some manufacturers, however, dumped the ink and replaced it with ink with good fluidity and resistance to water. Looking at this kind of failure, this issue is explored roughly around this issue, and a new method of eliminating the failure is communicated to colleagues in the packaging and printing industry.
(A) The concept of gelation and acidity of ink
The problem of gelation of packaging printing inks has been encountered in the early days of inks (oxidative drying and osmotic drying), and occasionally encountered in solvent-based (volatile drying) and water-based inks. We know that the cause of gelation is very complicated (including the process flow of ink manufacturing, such as the addition and subsequent addition of driers). With the development of science and technology and the use of new materials, we have strengthened the study of scientific theories and the control of the quality of ink component materials. When the composition of the ink changes, various problems are effectively controlled. But generally, after a failure occurs, operators often do nothing. There are six reasons for the phenomenon of ink gelation:
1. The molecular weight of the ink system is too large (viscosity is too high);
2. There are too many solid components (even small viscosity resin binders are no exception);
3 ink system acid value is too high;
4. Pigments, fillers with alkaline;
5. Too much desiccant content or improper order of addition (eg two results before and after ink production);
6. The solvent of the ink system is especially too much dry fast solvent.
From the above six reasons, it can be seen that the first two belong to the category of physics and the latter four belong to the category of chemistry (reaction). Acids are electrophilic and electron acceptors (proton donors); bases are nucleophilic and electron donors (proton acceptors). There are also so-called amphoteric solvents and resins (both proton donors and proton acceptors of water, alcohols, carboxylic acids, and nitrocellulose, etc.), followed by an aliphatic hydrocarbon solvent that does not form hydrogen bonds, and The pigments, amphoteric pigments, etc. are all related to the gelation of the ink. It is often said that when the acid-base characteristics of pigments are not compatible with the acid-base characteristics of the binders, we can use a variety of resins and pigments, fillers to adapt the acid-base characteristics, or choose the appropriate solvent to make the acid and alkali The characteristics are improved, which is a regular remedy.
When we explore the chemical (reaction) category, we find that when the stoichiometric concentration reaches a certain value (strictly speaking, a very narrow concentration range), various macroscopic properties will suddenly change. From the microscopic point of view, the active molecules of the ink system begin to associate and form aggregates of unequal size. Such aggregates are called micelles or micelles, also known as associative colloids. An important characteristic of micelles is the result of their solubilizing action, which greatly increases the solubility of the originally insoluble or sparingly soluble substances - commonly known as bloating. This speeds up the chemical reaction. The mechanism of the reaction is likely to have a close relationship with the binder, especially the pigment dispersion process.
(II) Efficient Ways to Eliminate Swelling and Avoid Gelatinization
In addition to the above-mentioned reasons, the driers in the ink system can also directly affect the viscosity, the increase in the viscosity of the ink during storage, and the precipitation of pigments and fillers. For example, a highly viscous alkyd resin is ground with a basic pigment zinc oxide and driers to promote gelation and swell of the ink during storage. This is due to the reaction of free fatty acids and basic pigments during thermal processing. Therefore, in the production of inks using high viscosity ink materials, lead, cobalt, and manganese driers should not be added during the grinding process. However, after adding 1% benzoic acid, the tendency to swell can be reduced. Another drawback of driers is that they can accelerate pigment precipitation during storage. For example, when titanium dioxide is used to make inks, zinc naphthenate is added during grinding, and the pigments tend to precipitate and form a lump.
In recent years, people have proposed such remedial measures in terms of swelling, gelation, and sedimentation around packaging printing inks. The solution to this problem is often caused by the appearance of faults. This method of passive operation not only increases the production costs and delays of the packaging and printing manufacturers, but also brings with it certain processing difficulties. The phenomenon of gelation, thickening and blocking of the ink system is related to the reaction of salt-based pigments with higher acid values ​​or binders containing free fatty acids into soaps. The binder itself agglutinates, the ink system absorbs moisture, the pigment content is too high, the binder and the pigment used are not balanced (inappropriate), the adjuvant of the binder and grease is poorly miscible, and the polymer binder is acid Absorption of dissolved structures by pigments can lead to gelation of the ink. The general remedies are nothing more than the following four methods:
1. When it is too thick or the gelation occurs due to the high degree of polymerization of the linking material, dilute with aliphatic hydrocarbons, esters, and ketones.
2. React to gelatinized inks. Rosin is dissolved in linoleic oil and its high acid content is used to relieve it.
3. Add naphthalic acid metal salts to the packaging printing ink.
4. The ink needs to add inorganic salt, can control the gel.
The improved phosphate ester titanate coupling agent produced, in addition to having excellent dispersion properties of the pigment, can also make the swollen, gelatinized packaging printing ink “return to lifeâ€. For example, in the printing process, only 3% to 5% of the anti-expansion breaker needs to be added to the swelled ink, so that the above failure can be controlled and the original structure and viscosity of the ink can be restored. We only need to add 1.5% to 2.5% anti-expansion gel breaker in pigments and fillers in ink formulations to avoid swelling and gelation of packaging printing. It is also possible to use toluene, xylene in a ratio of 1:1 to dilute with the product first, and then add the ink in the ink to grind and disperse together, which can not only greatly improve the dispersibility of the pigment, but also prevent the packaging printing ink from being produced and stored. In the printing process, due to swelling, gelation, precipitation, resulting in streaks or color fading. At the same time, the degree of branching of the ink can be greatly increased to increase the attachment fastness of the ink film, and the baking temperature and the blowing amount of the printing ink or the volatile drying type (solvent type) printing ink can be reduced, thereby effectively promoting the normal printing of the packaging printing ink. Printing fluidity.
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