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Abstract: With the upgrading of the green printing industry and increasingly strict VOC control, water-based inks have gradually replaced traditional solvent based inks with advantages such as low toxicity, low odor, environmental safety, and adaptability to flexible printing processes, becoming the mainstream printing consumables for flexographic printing, gravure printing, digital inkjet printing, and textile printing. However, water-based ink systems have high surface tension, pigment powders are prone to agglomeration, and low surface energy substrates are difficult to wet. Under high-speed printing conditions, they are prone to quality defects such as uneven spreading, incomplete dots, offset printing, dry plate clogging, coating shrinkage, and insufficient adhesion. Wetting and dispersing agents, as the core functional components of water-based ink formulations, play a dual role in regulating interfacial tension, dynamically wetting the substrate, and stabilizing pigment depolymerization. They directly determine the ink grinding fineness, storage stability, droplet forming quality, film density, and final printing quality. To systematically elucidate the structure-activity relationship between the structure of additives and ink properties, this article selects four types of wetting and dispersing additives, namely anionic, nonionic, organosilicon, and alkynediol, which are mainstream in industry. The study systematically explores the influence of additive types, compounding ratios, and addition amounts on the particle size distribution, dynamic surface tension, rheological properties, film-forming morphology, adhesion, friction resistance, and high-speed printing adaptability of water-based inks, revealing the synergistic effect mechanism of wetting dispersion and the mechanism of controlling film-forming defects. The test results show that there are performance shortcomings in a single auxiliary agent. The silicone wetting agent has excellent dynamic spreading, but it is easy to cause film shrinkage and reduce the adhesion between layers. The anionic dispersant has good dispersion stability, but foam is on the high side. High speed printing is easy to break ink; The composite system of alkyne diol wetting agent and polymer block dispersant can achieve comprehensive advantages of low foaming and efficient wetting, uniform pigment dispersion, and low residual migration, effectively optimizing ink dot reduction rate, improving coating density and friction resistance, and significantly improving image quality uniformity and production continuity under high-speed printing conditions. This study can provide theoretical basis and technical support for the formulation optimization, additive selection, and industrial production of high-performance low foaming water-based inks.

1 Introduction

Water based ink is mainly dispersed in deionized water, combined with water-based resins such as acrylic acid and polyurethane, organic/inorganic pigments, and functional additives. It conforms to the industry development trend of green printing and low-carbon production, and is widely used in food packaging, paper printing, plastic flexible packaging, textile digital printing, label printing and other fields. Compared to traditional solvent based inks, water-based inks have no flammable or explosive risks, extremely low VOC emissions, and are safe and environmentally friendly in production and construction. However, water-based media have inherent surface tension and strong polarity, and have poor interface compatibility with low surface energy plastic substrates such as PET, BOPP, and PE. During high-speed printing, ink droplet spreading resistance is high and wetting hysteresis is significant. At the same time, organic pigments and carbon black powder in ink have a large specific surface area and high surface energy. They are prone to soft and hard agglomeration in the aqueous phase, resulting in low grinding efficiency, uneven particle size distribution, and rough settling during storage, which seriously restricts the improvement of printing accuracy and coating quality.

In the ink formulation system, wetting agents and dispersing agents complement each other’s functions and are indispensable. Wetting aids mainly act on the gas-liquid and solid-liquid interfaces, rapidly reducing the static and dynamic surface tension of the system, and solving printing problems such as poor substrate wetting, ink droplet shrinkage, bleeding, and missing dots; Dispersants mainly adsorb on the surface of pigment powders, suppressing particle aggregation through electrostatic repulsion and steric hindrance effects, achieving stable dispersion of ultrafine particle size, ensuring ink fineness, fluidity, and long-term storage stability. Under the low speed traditional printing conditions, a single auxiliary agent can basically meet the basic production needs, but modern printing equipment is developing in the direction of high speed, refinement and continuity. A single wetting agent is easy to cause foam accumulation, coating defects, auxiliary agent migration, and a single dispersant has problems such as insufficient wetting of the base material, uneven spreading, and low image and text clarity, which seriously affect the printing yield.

At present, there are many studies on the modification of water-based ink resins, pigment selection, and curing processes in China. However, systematic research on the matching mechanism of wetting/dispersing additives, the synergistic mechanism of compounding, and the effect of additives on the microstructure of film formation and printing mechanical properties is relatively weak. The applicability range, defect causes, and compatibility rules of various additives have not been fully clarified. Based on this, this article systematically conducts single factor and compound experiments on different types of wetting and dispersing additives to explore the effects of additive systems on the rheological properties, pigment dispersion state, film-forming microstructure, interfacial adhesion, friction resistance, and high-speed printing stability of water-based inks. The optimal additive formulation system is clarified, and the regulation mechanism of additives is elucidated, providing theoretical support for the industrial research and application of high-end refined water-based inks.

2 Experimental section

2.1 Experimental raw materials

The main raw materials of the test include water-based acrylic lotion resin, phthalocyanine blue organic pigment, carbon black pigment, deionized water, ethylene glycol moisturizing cosolvent; The auxiliary agents selected include anionic polymer dispersants, nonionic fatty alcohol wetting agents, polyether modified organic silicon wetting agents, alkynediol low foaming wetting agents, matched with defoamers, thickeners, fungicides and other auxiliary materials. All raw materials meet the industrial production standards of low VOC, high stability, and environmental compliance of water-based inks. The printing substrate uses ordinary printing paper, PET film, and BOPP film to simulate the actual working conditions of paper packaging and soft packaging printing.

2.2 Ink preparation process

Adopting a segmented preparation process of pre dispersion, high-speed sanding, and low-speed paint mixing. Firstly, deionized water, moisturizing solvent, wetting agent, dispersant and some lotion resins are mixed and stirred evenly to prepare a stable transparent auxiliary base liquid; Batch feeding of pigment powder, high-speed shear pre dispersion, breaking down large particle aggregates; Transfer to a horizontal sand mill for ultrafine grinding, control the grinding speed and cycle time, monitor the fineness and particle size in real time until the system D90 meets the standard; After grinding, add the remaining lotion, thickening agent and defoamer to fine tune the viscosity and workability of the system, filter it and leave it standing for defoaming, seal it and maintain it at constant temperature for standby.

2.3 Performance Testing and Characterization

Dynamic surface tension test: The maximum bubble pressure method is used to test the dynamic surface tension of different additive system inks and evaluate the instantaneous wetting ability of high-speed printing;

Particle size and dispersion stability testing: Laser particle size analyzer is used to test the D50 and D90 particle sizes of ink, and the layering, settling, and coarsening of the system are observed by standing at room temperature and high-temperature thermal storage;

Rheological performance testing: using a rotational rheometer to test ink viscosity and thixotropy, and evaluate printing rheological compatibility;

Microscopic morphology analysis of film formation: Scanning electron microscopy (SEM) is used to observe the surface smoothness, pores, and aggregated particles of the coating film, and to analyze the density of the film formation;

Printing performance testing: Conduct high-speed flexographic and gravure printing tests on the machine to evaluate dot restoration, overlay accuracy, graphic clarity, and bleeding degree;

Coating mechanical performance testing: testing ink coating adhesion, friction resistance, water resistance, and evaluating film durability.

3 Experimental results and performance analysis

3.1 The influence of different additives on the surface tension and dynamic wetting properties of ink

The dynamic wetting performance is the core indicator that determines the spreading and forming of high-speed printing ink and the accuracy of dot restoration. The experimental results show that there are significant differences in the interfacial regulation ability of different types of wetting agents. Polyether modified organosilicon wetting agent has the best effect in reducing static surface tension, achieving ultra-low interfacial tension spreading and excellent adaptability to low surface energy PET and BOPP substrates. However, the dynamic response speed lags behind, and the control of interfacial tension is unstable under high-speed continuous printing conditions, which can easily lead to uneven ink droplet size and inconsistent virtual and real graphics and text; At the same time, organic silicon molecules have strong mobility and are prone to accumulate on the surface of the coating, causing local shrinkage and pinhole defects, significantly reducing interlayer adhesion and recoating properties of the coating.

The traditional anionic wetting agent has moderate interfacial activity and stable wetting efficiency, but its molecular structure is easy to form a stable hydration film. A large number of micro foam are easily generated under high-speed mixing and printing shearing. The foam is small and persistent, and it is difficult to eliminate naturally, which will cause printing white spots, ink shortage, and ink break, and cannot adapt to high-speed continuous printing production. Ordinary non-ionic wetting agent foam is low and the system is stable, but the interfacial tension reduction ability is limited, the dynamic wetting speed is slow, the ink drop shrinks significantly after landing, the dot diameter is small, the edge serration is serious, and the image and text clarity and color saturation are insufficient.

Alkylene glycol type low foam wetting agent has symmetric molecular structure, fast interface adsorption speed and sensitive dynamic response, which can reduce the dynamic surface tension of the system in a very short time, adapt to the instantaneous wetting demand of high-speed printing, and has extremely low foaming property, excellent foam inhibition effect, and no foam accumulation defects. It is the optimal wetting substrate for high-speed printing of water-based ink.

3.2 Effect of dispersing agents on pigment particle size and storage stability

The uniformity of pigment dispersion directly determines the coloring strength, glossiness, and storage stability of ink. Ink systems without high-quality dispersing agents have insufficient wetting of pigment powder, incomplete disintegration of aggregates, and a large number of submicron sized particles remaining in the system. The grinding fineness is difficult to meet the standard, and short-term storage results in increased particle size, bottom powder accumulation, and upper clear liquid layering. Anionic polymer block dispersants, with their multi-point anchoring adsorption properties, can firmly adsorb on the surfaces of organic pigments and carbon black powders. They suppress particle aggregation through a dual mechanism of electrostatic repulsion and steric hindrance, significantly optimize particle size distribution, reduce ink viscosity, and improve system fluidity and grinding efficiency.

Experimental comparison shows that a single wetting agent cannot achieve long-term stability of pigments, only improve the interfacial spreading effect, and has no inhibitory effect on powder aggregation and depolymerization; A single dispersant can stabilize pigment particles, but the substrate has insufficient wetting ability and poor printing spreading effect. The complex system of wetting and dispersing agents can achieve complementary advantages, ensuring the ultrafine and uniform dispersion of pigments, long-term non coarse settling, and solving the problem of poor wetting of low-energy substrates. The ink storage stability and printing adaptability are synchronously improved.

3.3 Effects of Additive Systems on Ink Rheology and High Speed Printing Performance

Water based ink needs to have suitable viscosity and thixotropy to meet the requirements of high-speed ink application, ink uniformity, and transfer printing. The type and amount of additives directly control the rheological properties of ink: adding an appropriate amount of dispersant can break down pigment aggregation, release system wrapped moisture, effectively reduce ink viscosity, improve fluidity and transfer uniformity; Excessive dispersant can lead to excess charge in the system, excessive particle repulsion, decreased ink thixotropy, thinning after standing, and the printing process is prone to defects such as sagging, dot enlargement, and blurry graphics and text.

Adding an appropriate amount of wetting agent can optimize the forming state of ink droplets, improve the fullness and edge clarity of the dots; Insufficient wetting agent leads to severe shrinkage of ink droplets, incomplete dots, and dull colors; Excessive wetting agent results in low surface tension and unconstrained spreading of ink droplets, leading to severe bleeding, dot diffusion, and decreased printing accuracy. The optimal compounding system can accurately regulate the rheological parameters of ink, making the ink both shear thinning and static stable thixotropy. The ink is evenly distributed during high-speed printing, stable transfer printing, no ink flying or breaking, and the dot reduction rate is significantly improved.

3.4 Effects of Additives on the Microstructure and Durability of Ink Film Formation

The process of ink drying and film formation is a composite process of resin fusion, particle accumulation, and water evaporation. Residual and uneven dispersion of additives can directly damage the micro density of the coating film. The single anionic auxiliary foam residue is easy to form micropore and pinhole defects in the film, resulting in rough surface of the film, low gloss and poor friction resistance; Organic silicon additives migrate and accumulate on the surface of the coating, forming an interface isolation layer that significantly reduces the adhesion between the ink and the substrate. In the later stage, problems such as peeling, detachment, and water resistance may occur.

The high-quality wetting/dispersing composite system can evenly disperse pigment particles in the resin matrix, with tightly packed particles, no agglomeration, no pore defects, continuous and smooth film formation, and a dense and uniform structure. Microscopic morphology observation shows that the coating surface of the composite additive system is smooth and has very few defects, and the resin film has excellent continuity; Macro performance testing shows that the adhesion, friction resistance, and water resistance of the coating have been significantly improved. The printed graphics and text are wear-resistant, do not fade, and do not turn white when in contact with water. The long-term service stability is stronger.

4. Mechanism of synergistic effect and defect regulation of additives

The wetting and dispersing agents in water-based ink systems have functional differentiation and synergistic performance characteristics, forming a complete “powder depolymerization interface wetting stable film formation” control system. Dispersants mainly act on the solid-liquid interface of pigments, achieving pigment powder disintegration, uniform dispersion, anti coarsening and anti settling through anchoring adsorption and spatial electrostatic stabilization mechanisms, and solving internal system problems such as poor ink fineness, high viscosity, and unstable storage; Wetting agents mainly act on the gas-liquid and substrate solid-liquid interface, quickly regulating dynamic surface tension and solving external printing problems such as poor ink droplet formation, difficult substrate spreading, and low dot reduction.

A single additive system has inherent performance balancing contradictions: high dispersion stability additives generally have weak wetting ability and poor printing spreading; High wetting efficiency additives generally have high foam, easy migration and many film forming defects. The composite system can break the performance gap, relying on the high-speed dynamic wetting ability of low foaming acetylene glycol wetting agents to ensure printing accuracy, and relying on the strong dispersion stability of polymer block dispersants to ensure system stability. The two work together to achieve a comprehensive balance of ink flowability, dispersibility, wetting, film-forming, and printing stability, solving a series of common industry problems such as ink agglomeration, foaming, shrinkage, bleeding, and poor adhesion at the micro level.

5 Key issues and optimization strategies for formula application

5.1 Performance defects caused by imbalanced dosage of additives

There is a strict optimal range for the amount of wetting and dispersing agents added. Insufficient addition of dispersants, incomplete coating of pigments, incomplete aggregation and depolymerization, poor ink fineness, low coloring power, and easy return to coarse settling during storage; Excessive addition of free dispersant residue system reduces the water resistance and adhesion of the coating, causing problems such as sagging and dot expansion. Insufficient addition of wetting agent, poor wetting of substrate, shrinkage of ink droplets, and blurred graphics and text; Excessive addition leads to low surface tension, severe bleeding, large hydrophilic residue in the coating, and decreased friction and water resistance. The optimal industrial addition range is 0.2% to 0.8% for wetting agents and 1.0% to 3.0% for dispersants, which can be fine tuned according to pigment concentration and substrate type.

5.2 Auxiliary compatibility conflict and foam control

Anionic dispersants have charge compatibility conflicts with some cationic preservatives and thickeners, which can easily cause system flocculation, thickening failure, and ink delamination; High foam wetting agent is incompatible with high-speed printing process, and foam accumulation will directly lead to batch printing defects. Formula optimization should follow the principle of low foam adaptation, with priority given to non-ionic and alkyne diol based low foam wetting systems, combined with polymer dispersants to avoid charge antagonism issues and achieve bubble free, stable, and continuous printing production.

5.3 Effects of Additive Migration on Film Durability

Small molecule wetting and dispersing agents have strong mobility and are prone to migrate and accumulate on the surface of the coating during the film-forming process, resulting in a high hydrophilicity of the coating surface and a decline in weather and friction resistance. Long term storage of graphics and text can easily cause yellowing, discoloration, and peeling. The use of macromolecular low migration additives and structural anchoring dispersants can enhance the adsorption firmness of additives in pigment and resin systems, reduce free migration residues, and ensure the long-term stability of ink coatings.

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