What Are The Characteristics Of Inks For Offset Printing?
Feb 24, 2025
1. Main ingredients
2. Water resistance
3. Factors affecting gloss and methods to improve it
4.Key factors and optimization strategies for adhesion
1. Main ingredients
(1) Pigments
Function: Gives ink color and hiding power, reflects light of a specific wavelength, and directly affects the light resistance and chemical resistance of the ink.
Type: Includes inorganic pigments (such as carbon black) and organic pigments (such as azo and phthalocyanine). The latter are brightly colored and have strong coloring power, gradually replacing inorganic pigments to become the mainstream.
(2) Binders
Function: As a pigment carrier, it forms an ink film and fixes it on the surface of the substrate, determining the core properties of the ink, such as fluidity, viscosity, and drying.
Composition: Modern offset inks are mainly based on synthetic resins (such as rosin-modified phenolic resins and alkyd resins), supplemented with drying vegetable oils (such as linseed oil) or mineral oils as plasticizers or diluents.
2. Auxiliary ingredients
(1) Fillers
Function: Adjusts the viscosity and fluidity of the ink and reduces costs. White or transparent powders (such as calcium carbonate and barium sulfate) are commonly used.
(2) Additives
Function: Improve the printing suitability of inks, for example:
Desiccant: Accelerate oxidative polymerization reactions (such as cobalt and manganese salts).
Anti-adhesive: Prevent printed products from getting dirty.
Anti-emulsifier: Enhance the water resistance of inks and avoid emulsification caused by the mixing of fountain solution during printing.
3. Evolution of modern formulations
Environmental protection trend: Traditional vegetable oil binders are gradually replaced by synthetic resins, while soybean oil-based inks and UV-curable inks (containing photopolymerized resins) are promoted to reduce VOC emissions.
Functional expansion: Develop special inks for different substrates (such as plastics and metals). For example, UV inks require the addition of photoinitiators to achieve rapid curing.
1. Definition and importance of water resistance
The water resistance of offset printing ink refers to its ability to maintain color stability, non-fading, non-emulsification and strong adhesion when exposed to water or humid environment. Since the offset printing process requires the use of fountain solution (water-based solution), the ink must resist water penetration and dissolution, otherwise it is easy to cause blurring of printed products, ink layer shedding or emulsification failure.
2. Water resistance test method
(1) Immersion method: immerse the printed sample in water and observe whether the ink layer peels off, fades or penetrates, and record the time and degree of change.
(2) Filter paper pressing method: cover the surface of the printed product with wet filter paper, observe the color transfer after pressure, and evaluate the ink's resistance to water penetration.
(3) Accelerated aging test: verify the long-term water resistance of the ink by simulating a high humidity environment or cyclic water vapor exposure.
3. Key factors affecting water resistance
(1) Binder type
Synthetic resins (such as phenolic resins and alkyd resins) have stronger water resistance and film-forming stability than traditional vegetable oils (such as linseed oil), reducing water penetration.
Environmentally friendly soybean oil-based binders are modified to reduce VOC emissions while taking into account water resistance.
(2) Demulsifiers and additives
Adding demulsifiers can prevent the mixing of fountain solution and ink, maintain the viscosity stability of ink, and prevent printing failures caused by emulsification.
Desiccant (such as cobalt salts) accelerates the oxidation filming process, shortens the ink curing time, and reduces the moisture contact window.
(3) Pigments and fillers
Inorganic pigments (such as carbon black) are more water-resistant than organic pigments, but organic pigments can improve their water resistance through surface treatment (such as silicon coating).
Fillers (such as calcium carbonate) adjust the ink consistency. Excessive use may reduce the density of the ink layer and affect water resistance.
4. Technical direction for improving water resistance
(1) Formula optimization: Use highly water-resistant resins (such as UV curing resins) to replace traditional binders, and add nano-scale water-resistant additives to enhance the density of the ink layer.
(2) Process control:
Control the pH value of the fountain solution (usually 4.5-5.5) to avoid excessive acidity that damages the ink structure.
Use infrared drying or UV curing technology after printing to quickly solidify the ink film and reduce water erosion.
5. Industry standards and specifications
ISO 2836:1974: Specifies the method for evaluating the water resistance of printed products, including immersion, friction and other test processes9.
QB/T 2826-2017: Clear requirements for technical indicators such as water resistance and solvent resistance of UV offset inks.
1. Factors affecting gloss and methods to improve it
Ink film thickness and binder properties
The thickness of the ink film directly affects the gloss. A thicker ink film can retain more binders and reduce the roughness of the paper after absorption. Synthetic resins (such as phenolic resins and alkyd resins) as binders have better film stability and water resistance than plant oil-based materials, which can significantly improve the gloss.
Drying speed and process control
Fast drying (such as UV curing or infrared drying) can reduce water erosion and ink layer penetration, forming a smooth surface5. At the same time, controlling the pH value of the fountain solution (4.5-5.5) can prevent the acidic environment from destroying the ink structure, thereby maintaining the gloss.
Selection of auxiliary agents
Adding fluorocarbon surfactants can prevent the ink layer from peeling or wrinkling, while gloss paste can directly enhance the gloss of the ink. However, excessive use of anti-sticking agents and other auxiliary agents that affect the gloss should be avoided.
Binder and pigment treatment
Binder and pigment treatment
Synthetic resin binders not only improve gloss, but also enhance chemical bonding with substrates9. Inorganic pigments (such as carbon black) have better adhesion than organic pigments, but the latter can be improved by surface treatment such as silicon coating.
Surface treatment technology
Using plasma cleaning technology to treat the surface of the substrate can remove pollutants and activate surface chemical bonds, significantly improving ink adhesion.
Ink formulation and drying process
Adding desiccants (such as cobalt salts) accelerates oxidation film formation, shortens water contact time, and indirectly enhances adhesion. UV-curable inks form a dense ink layer through instant cross-linking, and have better adhesion than traditional ink
