Oxidation Film Formation + Penetration Drying: A Complete Guide to the Dual Drying Mechanism of Offset Inks

In offset printing production, the drying speed and quality of printed products directly determine production efficiency, product qualification rate and the smoothness of subsequent processes, which are crucial to an enterprise's production capacity and reputation. Practitioners often encounter problems such as sticky and smudged printed surfaces, set-off, slow drying that hinders die-cutting and binding, or even ink peeling and color distortion. The drying of offset ink is not a single "solidification" process, but the result of the synergistic effect of oxidation film formation and penetration drying. Complementing each other, they jointly realize the transformation of ink from liquid to solid, which is applicable to mainstream offset inks such as solvent-based and soybean-based inks. This article disassembles the core and influencing factors of the two drying mechanisms, interprets the causes and  solutions of faults in combination with practical operation, helping practitioners understand the principle, optimize parameters,  reduce faults, and improve efficiency and quality.  

1. Basic Understanding: Why Do Offset Inks Need "Dual Drying"?

To understand dual drying, it is necessary to clarify the characteristics of offset printing process and the limitations of single drying. The core of offset printing is to transfer ink to the paper surface by utilizing the incompatibility between ink and water. The ink needs to complete both "transfer" and "curing" simultaneously—it must flow evenly and dry quickly to prevent smudging, which cannot be met by single drying.

With only penetration drying, the binder penetrates into the paper in large quantities. Although drying is fast, the surface lacks a complete film layer, which is prone to ink stickiness, poor adhesion, smudging and peeling. With only oxidation film formation, the binder cannot penetrate the paper in time, resulting in slow drying and poor combination between the film layer and the paper, which is easy to peel off.

Therefore, offset inks require dual drying: penetration drying "lays the foundation" by allowing part of the binder to penetrate the paper to initially prevent smudging; oxidation film formation "solidifies" by forming a tough film on the surface to ensure firmness. The two work synergistically to balance drying speed and quality, which is a core feature distinguishing offset printing from other processes.

2. Mechanism Disassembly 1: Penetration Drying – The Process of Ink "Seeping" into Paper

Penetration drying is the key to the initial drying of offset ink, which is a physical process in which the binder seeps into the fiber gaps of the paper under the action of capillary adsorption, without complex chemical reactions. The composition and viscosity of the binder, as well as the characteristics of the paper, directly determine the penetration effect.

Definition: After the ink is transferred to the paper surface, the binder (mineral oil, vegetable oil and a small amount of organic solvent) is adsorbed by the capillary pores of the paper and seeps into the fiber gaps, driving part of the pigment particles to penetrate together, so that the surface ink initially loses fluidity and reaches a "non-sticky" state, laying the foundation for subsequent oxidation film formation. Only part of the binder penetrates, and the rest remains on the surface for oxidation reaction, with the proportion varying according to the type of ink and paper.

Key influencing factors: First, paper characteristics—higher porosity and oil absorption lead to faster penetration; newsprint has fast penetration, while coated paper such as copperplate paper has dense coating and slow penetration; excessive paper moisture will block penetration. Second, binder viscosity—lower viscosity means better fluidity and faster penetration; viscosity needs to be adjusted according to paper oil absorption in production. Third, printing pressure—appropriate pressure can expand paper pores and promote contact, while excessive or insufficient pressure will hinder penetration.

Intuitive performance: After ink transfer, it shows liquid luster; with the penetration of the binder, the luster fades and the color deepens slightly; it becomes non-sticky within 10-30 minutes, but it is not completely cured, and smudging may still occur if rubbed hard, requiring subsequent oxidation film formation to complete drying.

3. Mechanism Disassembly 2: Oxidation Film Formation – The Process of Ink "Solidifying" into Film

Oxidation film formation is the core of complete ink drying, which is a chemical reaction process in which unsaturated fatty acids in the binder undergo oxidative polymerization under the action of oxygen to form a tough film, which is completely different from the physical action of penetration drying.

Definition: After penetration drying, the binder remaining on the paper surface (unsaturated fatty acids such as soybean oil and linseed oil) undergoes oxidative polymerization under the action of oxygen in the air—double bonds open to form peroxides, which decompose to produce free radicals; free radicals combine to form macromolecular polymers, which are intertwined to form a dense and tough film, wrapping pigment particles to achieve complete curing of the ink, with good scratch resistance, friction resistance and water resistance.

Key influencing factors: First, oxygen content—poor ventilation and dense stacking will lead to insufficient oxygen, slowing down drying; printed products should be placed separately to ensure ventilation. Second, temperature and humidity—the optimal range is 20-30℃ and 50%-60% relative humidity; abnormal temperature and humidity will affect the reaction rate, requiring temperature and humidity control equipment. Third, drier addition—metal salts such as cobalt and manganese can accelerate the reaction; the addition amount is controlled at 0.1%-0.5% of the total ink amount; excessive addition will cause film brittleness and cracking, while insufficient addition has no effect.

Intuitive performance: After initial penetration drying, the film layer is soft and may fade when rubbed; with the progress of oxidation, the film layer hardens, the luster improves and the color stabilizes; after complete curing, it is not smudged when rubbed and does not peel off when scratched. The oxidation film formation time of conventional ink is 2-4 hours, which can be shortened to 1-2 hours under optimal conditions.

4. Practical Application: The Core Connection Between Drying Mechanism and Printing Production

Mastering the dual drying mechanism can guide production optimization and solve drying faults. There are differences in the drying focus of mainstream offset inks, and most faults are caused by the imbalance between the two mechanisms, which needs targeted adjustment.

Differences in mainstream offset inks: Solvent-based offset ink is mainly penetration drying (60%-70%) and supplemented by oxidation film formation (30%-40%), suitable for newsprint and offset paper, focusing on controlling ink viscosity. Soybean environmental offset ink is mainly oxidation film formation (50%-60%) and supplemented by penetration drying (40%-50%), which is environmentally friendly and solvent-free, suitable for copperplate paper, focusing on optimizing drier addition and temperature and humidity. UV offset ink adopts photocuring mechanism, which is irrelevant to dual drying, dries quickly under UV irradiation, and requires special equipment.

Common drying faults and solutions: First, sticky and smudged surface—caused by insufficient oxidation film formation; solutions include increasing drier addition, optimizing temperature and humidity, strengthening ventilation and placing separately. Second, excessive penetration and weak color—caused by too fast penetration; solutions include replacing low-oil-absorption paper, increasing ink viscosity and adjusting printing pressure. Third, slow drying—caused by unbalanced dual mechanisms; solutions include optimizing temperature and humidity, adjusting ink viscosity, increasing drier and strengthening ventilation; auxiliary drying can be used in emergency.

Practical optimization suggestions: Adjust ink viscosity according to paper type; control workshop temperature and humidity within the optimal range; add drier reasonably; optimize stacking method and process rhythm according to ink type.

5. Conclusion

In summary, oxidation film formation and penetration drying are the "dual engines" for the drying of solvent-based and soybean-based offset inks. Their synergy and balance are the key to solving drying problems and improving production efficiency and quality, which is essential knowledge for practitioners.

With the tightening of environmental protection policies, soybean-based and low-VOC environmental offset inks are increasingly widely used. Their drying focuses on oxidation film formation, requiring higher production conditions and targeted optimization by enterprises. Although offset printing technology is constantly upgrading, the core logic of dual drying remains unchanged.

It is hoped that this article can help practitioners understand the drying mechanism, optimize production parameters and solve practical faults. We welcome industry colleagues to leave messages to share problems and consult ink adaptation schemes, so as to jointly promote the high-quality development of the printing industry.