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Abstract: The printability, film forming quality and post-treatment performance of flexographic water-based ink mainly depend on the microstructure and film forming mechanism of the polymer binder. Conventional homogeneous acrylic lotion has common defects such as poor ink film density, insufficient water resistance, easy backtack at high temperature stacking, and weak adhesion of thin film substrate, which makes it difficult to adapt to the quality requirements of high-speed flexographic printing and high-end packaging printing. Aiming at the technical bottleneck of single performance of traditional linear acrylic lotion, which is difficult to give consideration to the printing fluidity and the comprehensive performance of ink film, this study uses the semi continuous seed lotion polymerization process to build a self crosslinking core-shell acrylic lotion system with gradient soft and hard structure, organically combines the rigid structure of the core layer, the flexible function of the shell layer and the room temperature self crosslinking mechanism, systematically explores the influence of the core shell monomer ratio, the amount of carboxyl functional monomer, and the amount of internal crosslinking monomer on the particle size distribution, Zeta potential, storage stability, and film formation kinetics of the lotion, and deeply analyzes the influence of the core shell microstructure and self crosslinking reaction on the color development, dot reproduction, substrate adhesion, water resistance, and friction resistance of flexographic ink The regulation mechanism of the anti backtack and anti backtack properties. The research results show that the optimized core-shell self crosslinking structure can effectively solve the industry pain points of traditional ink and wash, such as low viscosity, difficult film formation, easy film rebound, and easy whitening when wet. On the premise of ensuring low viscosity of ink and adapting to high-speed transfer of mesh rollers, it significantly improves the density of ink film and the level of later crosslinking and solidification, greatly reduces the dot expansion rate, and adapts to multi substrate flexographic printing scenarios of paper and plastic film. It provides important theoretical basis and experimental support for the formula upgrade and structural modification of high-performance universal flexographic water-based ink.

1. Research background

Flexographic printing has become the mainstream process in the packaging printing field due to its advantages of light pressure printing, convenient plate making, wide substrate adaptability, and green and low pollution. Water based flexographic printing ink uses water as a dispersing medium, with extremely low VOC emissions and high production safety. It is currently the core direction for the green upgrade of packaging printing. As the core binder of water-based ink, polymer lotion plays a key role in pigment dispersion and bearing, wetting and spreading, film bonding, wear resistance and protection. Its microstructure, particle size characteristics, glass transition temperature and cross-linking degree directly determine the rheological properties of ink and the final print quality. The traditional homogeneous acrylic lotion has a single structure, and the soft and hard monomers are evenly distributed. There are significant performance checks and balances in the formula design: although the highly flexible lotion has good film forming performance and strong adhesion, the ink film is soft, and the high temperature stack is very easy to get back stuck and dirty; High hardness lotion has excellent backtack resistance, but poor film-forming property, easy cracking at low temperature, insufficient wetting of base material, and adhesion attenuation.

In addition, the conventional linear acrylic lotion has no cross-linking structure, and the degree of freedom of the molecular chain is too high after the film is formed. The water molecules are easy to penetrate into the ink film, resulting in white printing, poor water resistance, and insufficient friction resistance, which cannot meet the requirements of high-end packaging ink for weather resistance, moisture resistance, and wear resistance. At the same time, the common lotion system has limited re solubility, and it is prone to process problems such as dry blocking of anilox roll, missed printing of pictures and texts, pinhole defects, etc. when printing for a long time. In view of the above shortcomings, this study introduced the core-shell layered structure+room temperature self crosslinking system, and through the precise design of the microstructure of latex particles, it broke the balance of traditional lotion performance, and realized the two-way improvement of the printing suitability of flexographic ink and the use performance of finished products.

2 Structure design and preparation process of lotion

In this study, semi continuous seeded lotion polymerization technology was used to construct gradient core-shell acrylic lotion with hard core and soft shell structure step by step. In the test, the ratio of hard monomer and soft monomer is used to regulate the glass transition temperature of core layer and shell layer, acrylic acid (AA) is used as a hydrophilic functional monomer to improve the dispersion stability of lotion and the wetting ability of base material, and EGDMA is used as an internal crosslinking monomer to build a molecular pre crosslinking network. Relying on self crosslinking functional groups, secondary crosslinking curing after film formation at room temperature is achieved. The experiment systematically investigated three core parameters through the single variable method: the mass ratio of core shell soft and hard monomers, the addition amount of acrylic acid carboxyl monomer, and the amount of EGDMA cross-linking monomer, and explored the influence of each parameter on the particle size, particle size distribution uniformity, Zeta potential stability, lotion appearance and storage stability, finally obtaining the optimal preparation process of self crosslinking core shell acrylic acid lotion with regular structure, uniform particle size, and excellent stability.

The core innovation of this study is the differentiated design of the core-shell structure: the core layer adopts a high Tg hard monomer ratio to construct a rigid support skeleton, providing hardness support and anti sticking foundation for the ink film; The shell adopts low Tg soft monomer and hydrophilic functional monomer to ensure the film forming fluidity, substrate wettability and ink transfer performance of lotion; With a small amount of self crosslinking monomers, the lotion maintains low viscosity flow characteristics in the wet film state, triggers the crosslinking reaction after the film is dried, builds a compact three-dimensional network structure, and greatly improves the comprehensive performance of the ink film.

3 Core test results and performance impact mechanism

3.1 Regulation of microstructure on basic performance of lotion

The experimental characterization results show that the reasonable core-shell layered structure can significantly optimize the particle size distribution and colloidal stability of lotion. Compared with homogeneous lotion, core-shell lotion has more uniform particle size, better dispersion, higher absolute Zeta potential, stronger electrostatic stability effect of colloid, and is not easy to settle in layers after long-term storage. Appropriate introduction of carboxyl monomer can improve the surface hydrophilicity and pigment compatibility of lotion, and enhance the uniformity of ink wash system; However, excessive acrylic monomers tend to lead to strong hydrophilicity of the lotion, and the water resistance of the film will be greatly reduced after the film is formed, resulting in defects such as whitening of the ink film, water absorption and swelling. The EGDMA crosslinked monomer can form a pre crosslinked structure in the latex particles to limit the molecular chain slip, provide a structural basis for the later self crosslinked film formation, and effectively improve the loose film formation and high porosity of ordinary acrylic lotion.

3.2 The Enhancing Effect of Core Shell Structure on the Printing Suitability of Flexographic Ink

The flexographic printing process has strict requirements for ink viscosity, flowability, thixotropy, and solubility, and needs to be adapted to the working conditions of quantitative ink transfer and high-speed shear transfer of the mesh roller. This study confirmed that the core shell structure lotion can achieve the rheological advantage of “low viscosity and high flow”, and the soft shell structure ensures that the ink has excellent flow spread under high-speed shear, the ink transfer is uniform, the machine flow is good, and the printing defects of flying ink, dragging ink, uneven thickness are not easy to occur; The hard core rigid structure can effectively suppress wet film sagging and dot diffusion, significantly reduce dot expansion rate, make printed graphic edges clear and layered, and greatly improve printing precision. At the same time, the system has excellent solubility and can quickly re swell and disperse after long-term shutdown and drying, effectively solving common industry problems such as clogging of the mesh roller, ink residue accumulation, and plate dirt in the flexographic printing production process.

3.3 Enhancement effect of self crosslinking system on the comprehensive performance of ink film

The self crosslinking reaction is the core mechanism for improving the water resistance, wear resistance, and anti sticking properties of ink films. In the process of film forming and drying of lotion, the internal cross-linking functional groups undergo spontaneous cross-linking reaction, and the linear molecular chains bond with each other to form a dense three-dimensional network structure, which greatly reduces the porosity of the ink film, blocks the penetration of water molecules, oxygen and corrosive media, and completely improves the defects of traditional ink whitening when meeting water and poor water resistance. The test results show that the ink film prepared by self crosslinking core-shell lotion is compact and uniform, without pinhole defects, and its water resistance and dry wet friction resistance are significantly better than those of ordinary homogeneous lotion system. At the same time, the rigid core skeleton combined with the cross-linked network structure can effectively improve the hardness and anti adhesion ability of the ink film. Under high-temperature stacking and stacking storage conditions, there is no phenomenon of back adhesion and dirt, which solves the problem of product adhesion caused by a high proportion of soft monomers.

3.4 Advantages of Multi substrate Adaptation Performance

This study systematically evaluated the adhesion and adaptability of ink on paper substrates and plastic film substrates. The flexible structure of the shell and polar functional groups can effectively wet the surface of rough paper fibers and low surface energy films, improve the mechanical occlusion and interface bonding force, so that the ink has excellent adhesion and spreading film forming effect on common flexographic substrates such as paper, BOPP, PE, etc. Compared with the traditional single lotion system, the versatility of the substrate is greatly improved, which can simultaneously adapt to the scenarios of carton rough printing and soft packaging fine printing.

4 Technical Shortcomings and Optimization Strategies in Research

At the same time, the study pointed out the performance balance problem of self crosslinking core-shell acrylic acid lotion system: too high the amount of crosslinking monomer will lead to the increase of gel ratio, particle size and viscosity of lotion, which is not conducive to high-speed flexographic printing and ink transfer; If the crosslinking monomer is too low, the crosslinking density will be insufficient, and the improvement of water and wear resistance will be limited. Excessive carboxyl monomers can reduce water resistance, while insufficient amounts can lead to poor pigment dispersion stability and ink coagulation. In response to the above issues, research has proposed optimization schemes such as precise proportioning control, gradient crosslinking modification, and micro compounding of functional monomers to maximize crosslinking efficiency while ensuring printing fluidity, achieving a balance and unity between printing suitability and ink film performance.

5 Research Conclusions and Application Value

By constructing a core-shell self crosslinking acrylic lotion system, the research broke through the bottleneck of the performance balance of the traditional homogeneous acrylic lotion in the application of flexographic ink wash, and clarified the mechanism of the influence of core-shell structure, functional monomers, and cross-linking components on the colloidal properties of lotion, ink rheological properties, dot printing quality, ink film water resistance, wear resistance, and backtack resistance. The layered structure design of the hard core and soft shell effectively balances the flowability of ink on the machine and the rigidity of film formation. The self crosslinking reaction significantly improves the density and durability of the ink film, and can simultaneously solve multiple industry pain points such as high-speed printing blockage, dot expansion, poor water resistance of the ink film, and easy re sticking of the finished product. This study provides a systematic theoretical support and experimental basis for the microstructure design, formulation modification, and multi substrate adaptation optimization of water-based ink connectors for flexographic printing. It has important guiding significance for the industrial development of high-performance, universal, and green flexographic ink.

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