A flexographic sheet overprint ink composition and a method of making the same

Abstract

The present application relates to the technical field of ink composition, and discloses a flexographic paper surface printing ink composition and a preparation method thereof, which comprises pigment particles, an aliphatic polyurethane resin, an alcohol-soluble nitrocellulose resin, an additive and a composite alcohol solvent; the polar functional groups in the molecular chain of the aliphatic polyurethane resin are combined with the surface active sites of the pigment particles and form a flexible layer to build a coated pigment core; the alcohol-soluble nitrocellulose resin is distributed on the periphery of the coated pigment core and builds a rigid film-forming matrix, and the two together constitute a heterogeneous toughness-enhancing structure; the present application realizes mechanical stress absorption and deformation compensation by using the three-dimensional anchoring of the rigid matrix to the flexible core, solves the bottleneck problem of brittle cracking and falling of the high-hardness ink film, and maintains the stability of the resin dissolution by cooperating with the volatilization gradient of the alcohol solvent, thereby eliminating the risk of plate drying under high-speed printing conditions.

Description

Technical Field

[0001] This invention relates to a flexographic paper surface printing ink composition and its preparation method, belonging to the technical field of ink compositions. Background Technology

[0002] Currently, alcohol-soluble composition systems have become the mainstream technology in this field due to their compliance with environmental protection and low emission requirements. Existing technologies typically select alcohol-soluble nitrocellulose resin to construct the ink film skeleton, utilizing the excellent fast drying properties and pigment wetting characteristics of nitrocellulose resin to achieve high-efficiency production. However, with the continuous increase in printing speed, high-speed transfer conditions place higher demands on the physical resistance of the ink film and the rheological stability of the system. The ink film formed by nitrocellulose resin has high hardness but insufficient flexibility, and is prone to cracking and peeling when the paper substrate is bent or rubbed.

[0003] To improve ink film toughness, polyurethane resin is typically added to the system. However, aside from equipment improvements, existing technologies have shortcomings in ink component control and surface phase construction. For example, Chinese invention patent application CN104212234A discloses an alcohol-soluble flexographic printing ink that solves the problems of plate adhesion and heat resistance by multi-level dispersion of nitrocellulose resin, plasticized polyurethane, and modified rosin resin. However, in actual working conditions, this solution relies on the random physical blending of multiple resins, with long-chain polyurethane molecules floating in a random coiled form within the continuous solvent phase. Under conventional preparation processes... Polyurethane resin and nitrocellulose resin are in a simple physical blending state. The long-chain molecules of polyurethane are free in the continuous phase of the solvent in a random coiled form. During the high shear rate transfer process of flexographic printing, these free flexible long chains undergo shear-induced orientation and instantaneous entanglement between molecules. This physical interaction at the molecular level causes an abnormal increase in the viscosity of the ink system, resulting in plate drying and clogging, which damages the integrity of the printed image. Simply increasing the solvent ratio or adjusting the linearity of the resin molecular weight often leads to secondary problems such as deterioration of hiding power and decrease in physical resistance.

[0004] Therefore, how to solve the dynamic rheological interference of free flexible resin under high-speed shearing conditions, eliminate the problem of uneven stress transmission and film quality degradation caused by the randomness of component distribution, and thus reconcile the contradiction between high-speed printing and high strength resistance, has become the technical problem to be solved by this invention. Summary of the Invention

[0005] To address the problems mentioned in the background art, the technical solution of the present invention is as follows: A flexographic paper surface printing ink composition, based on 100% of the total weight of the flexographic paper surface printing ink composition, comprises the following components: The composition includes 8.0% to 15.0% pigment particles, 12.0% to 20.0% aliphatic polyurethane resin, 15.0% to 25.0% alcohol-soluble nitrocellulose resin, 2.0% to 5.0% additives, and a composite alcohol solvent to make up to 100%. The polar functional groups in the aliphatic polyurethane resin molecular chain are bonded to the active sites on the surface of the pigment particles through hydrogen bonds, and a flexible layer is formed on the surface of the pigment particles to construct a pigment core. Alcohol-soluble nitrocellulose resin is distributed around the coated pigment core and cross-linked to form a rigid film-forming matrix. The rigid film-forming matrix and the coated pigment core together constitute a heterogeneous toughness-enhancing structure. In the ink film formed by curing the ink composition of flexographic paper, the heterogeneous toughness enhancement structure uses a rigid film-forming matrix to three-dimensionally anchor the coated pigment core, so that when the rigid film-forming matrix is ​​subjected to mechanical external force, the stress is transmitted to the flexible layer, and the elastic deformation of the flexible layer is used to absorb and buffer the stress. The composite alcohol solvent is composed of a variety of alcohol monomers with different evaporation rate gradients. It is used to maintain the dissolution equilibrium of aliphatic polyurethane resin in the ink layer during the transfer of flexographic paper surface printing ink composition to paper substrate, thereby inhibiting the precipitation and drying of polyurethane molecular chains on the printing plate surface. Furthermore, the mass ratio of aliphatic polyurethane resin to pigment particles is 1.2 to 1.8; the thickness of the flexible layer is 50 nm to 200 nm, used to generate a steric hindrance effect between pigment particles to suppress particle agglomeration; the nitrogen content of the alcohol-soluble nitrocellulose resin is 10.7% to 11.3%, and the ratio of the elastic modulus of the rigid film-forming matrix formed by the alcohol-soluble nitrocellulose resin to the elastic modulus of the coated pigment core satisfies the viscoelastic matching rule, which is limited to the elastic modulus of the rigid film-forming matrix being 10 to 15 times that of the elastic modulus of the flexible layer; Aliphatic polyurethane resins have a number average molecular weight of 15,000 to 25,000 and a glass transition temperature of -50°C to -30°C. The isocyanate groups at the ends of the aliphatic polyurethane resin molecular chains covalently bond with the hydroxyl or carboxyl groups on the surface of the pigment particles to improve the interfacial anchoring strength of the coated pigment core.

[0006] Preferably, the alcohol-soluble nitrocellulose resin has an apparent viscosity of 300 mPa·s to 600 mPa·s at 25°C; the alcohol-soluble nitrocellulose resin provides a higher hardness contribution value in heterogeneous toughness-reinforced structures than aliphatic polyurethane resins.

[0007] Preferably, the complex alcohol solvent is composed of ethanol, isopropanol and n-propanol; based on the total weight of the complex alcohol solvent as 100%, the content of ethanol is 50.0% to 70.0%, the content of isopropanol is 15.0% to 25.0%, and the content of n-propanol is 5.0% to 15.0%.

[0008] Preferably, the additives consist of polyethylene wax, antistatic agent, defoamer and leveling agent; the average particle size of the polyethylene wax is 2μm to 5μm, and the polyethylene wax is distributed on the surface of the ink film after the flexographic paper surface printing ink composition is cured, so as to reduce the sliding friction coefficient of the ink film surface.

[0009] Preferably, the fineness of the pigment particles in the flexographic paper surface printing ink composition is no greater than 5 μm, and the pigment particles are centrally symmetrically distributed in the coated pigment core to maintain the mechanical isotropy of the heterogeneous toughness-enhancing structure.

[0010] Preferably, at a shear rate of 1000 s -1 up to 3000s -1 Under the specified conditions, the absolute value of the change rate of apparent viscosity of the flexographic paper surface printing ink composition with shear time is less than 0.01 mPa·s / s, the adhesion level of the flexographic paper surface printing ink composition on the paper substrate surface is Grade 1, and after being subjected to 50 cycles of 180-degree crease friction test, no cracks with a width greater than 0.1 mm are generated on the ink film surface.

[0011] A method for preparing a flexographic paper surface printing ink composition includes the following steps: Step 1101: Mix and pre-disperse pigment particles, aliphatic polyurethane resin and some composite alcohol solvent, and utilize the polar functional groups in the aliphatic polyurethane resin molecular chain to generate adsorption on the surface of pigment particles. Step 1102: Grind the pre-dispersed material to a fineness of no more than 5 μm, and form a flexible coating layer on the surface of the pigment particles to construct a coated pigment core and obtain a color paste; Step 1103: Add alcohol-soluble nitrocellulose resin, additives and the remaining composite alcohol solvent to the pigment paste, and stir at room temperature to allow the alcohol-soluble nitrocellulose resin to form a rigid film-forming matrix around the pigment core.

[0012] Compared with the prior art, the beneficial effects of the present invention are: 1. In flexographic paper surface printing ink compositions, rheological decoupling is achieved through specific constraints on the process sequence. During the pre-dispersion stage, the aliphatic polyurethane molecular chains are freed from their random coil state and targeted for adsorption and anchoring on the surface of lightfast organic pigment particles through the synergistic effect of a shear stress field and temperature field of specific intensity. This mechanism significantly reduces the concentration of free flexible long chain segments in the continuous solvent phase and eliminates the orientation entanglement of flexible chain segments under high-speed shear transfer conditions. The resulting technical result is that the ink composition maintains linear stability of fluid viscosity during high-speed plate transfer, eliminating the shear thickening risk that is easily generated in traditional two-resin blend systems and solving the plate clogging problem in high-speed printing.

[0013] 2. A heterogeneous surface stress dissipation structure is constructed to achieve synergistic enhancement of ink film hardness and toughness. During the film-forming and curing process of the composition, alcohol-soluble nitrocellulose resin constructs a rigid continuous skeleton, while pigment particles with a polyurethane flexible layer on the surface form a flexible core. When the ink film is subjected to mechanical stress such as external folding or friction, the stress transmitted by the rigid skeleton is effectively absorbed and buffered by the flexible interface layer on the pigment surface. This heterogeneous distribution mode that combines rigidity and flexibility enables the ink film to maintain high hardness while having excellent crack resistance, avoiding the detachment defects caused by the brittleness of the single nitrocellulose system, and improving the adhesion of the composition to the paper substrate surface.

[0014] 3. Significantly improves the wetting efficiency and dispersion stability of pigments. By utilizing the hydrogen bonding between the polar groups in the aliphatic polyurethane molecular chain and the active sites on the pigment surface, a stable coating structure is formed on the surface of the pigment particles. This structure generates a strong steric hindrance effect between pigment particles, effectively inhibiting secondary agglomeration of particles during grinding and storage. This optimization of interface characteristics not only shortens the grinding cycle but also ensures the high homogenization of pigment particles in the ink system, thereby improving the opacity and gloss saturation of printed images. Attached Figure Description

[0015] Figure 1 This is a schematic diagram illustrating the multi-step preparation process and surface film formation mechanism of the flexographic paper surface printing ink composition of the present invention. Figure 2 This diagram illustrates the relationship between the ink composition preparation process and the construction of the heterogeneous system from the operational perspective of this invention.

[0016] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0017] The technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0018] A flexographic paper surface printing ink composition, based on 100% of the total weight of the flexographic paper surface printing ink composition, comprises the following components: The composition includes 8.0% to 15.0% pigment particles, 12.0% to 20.0% aliphatic polyurethane resin, 15.0% to 25.0% alcohol-soluble nitrocellulose resin, 2.0% to 5.0% additives, and a composite alcohol solvent to make up to 100%. The polar functional groups in the aliphatic polyurethane resin molecular chain are bonded to the active sites on the surface of the pigment particles through hydrogen bonds, and a flexible layer is formed on the surface of the pigment particles to construct a pigment core. Alcohol-soluble nitrocellulose resin is distributed around the coated pigment core and cross-linked to form a rigid film-forming matrix. The rigid film-forming matrix and the coated pigment core together constitute a heterogeneous toughness-enhancing structure. In the ink film formed by curing the ink composition of flexographic paper, the heterogeneous toughness enhancement structure uses a rigid film-forming matrix to three-dimensionally anchor the coated pigment core, so that when the rigid film-forming matrix is ​​subjected to mechanical external force, the stress is transmitted to the flexible layer, and the elastic deformation of the flexible layer is used to absorb and buffer the stress. The composite alcohol solvent is composed of a variety of alcohol monomers with different evaporation rate gradients. It is used to maintain the dissolution equilibrium of aliphatic polyurethane resin in the ink layer during the transfer of flexographic paper surface printing ink composition to paper substrate, thereby inhibiting the precipitation and drying of polyurethane molecular chains on the printing plate surface. Furthermore, the mass ratio of aliphatic polyurethane resin to pigment particles is 1.2 to 1.8; the thickness of the flexible layer is 50 nm to 200 nm, used to generate a steric hindrance effect between pigment particles to suppress particle agglomeration; the nitrogen content of the alcohol-soluble nitrocellulose resin is 10.7% to 11.3%, and the ratio of the elastic modulus of the rigid film-forming matrix formed by the alcohol-soluble nitrocellulose resin to the elastic modulus of the coated pigment core satisfies the viscoelastic matching rule, which is limited to the elastic modulus of the rigid film-forming matrix being 10 to 15 times that of the elastic modulus of the flexible layer; Aliphatic polyurethane resins have a number average molecular weight of 15,000 to 25,000 and a glass transition temperature of -50°C to -30°C. The isocyanate groups at the ends of the aliphatic polyurethane resin molecular chains covalently bond with the hydroxyl or carboxyl groups on the surface of the pigment particles to improve the interfacial anchoring strength of the coated pigment core.

[0019] Preferably, the alcohol-soluble nitrocellulose resin has an apparent viscosity of 300 mPa·s to 600 mPa·s at 25°C; the alcohol-soluble nitrocellulose resin provides a higher hardness contribution value in heterogeneous toughness-reinforced structures than aliphatic polyurethane resins.

[0020] Preferably, the complex alcohol solvent is composed of ethanol, isopropanol and n-propanol; based on the total weight of the complex alcohol solvent as 100%, the content of ethanol is 50.0% to 70.0%, the content of isopropanol is 15.0% to 25.0%, and the content of n-propanol is 5.0% to 15.0%.

[0021] Preferably, the additives consist of polyethylene wax, antistatic agent, defoamer and leveling agent; the average particle size of the polyethylene wax is 2μm to 5μm, and the polyethylene wax is distributed on the surface of the ink film after the flexographic paper surface printing ink composition is cured, so as to reduce the sliding friction coefficient of the ink film surface.

[0022] Preferably, the component ratio of the flexographic paper printing ink composition satisfies the stress distribution coefficient λ, and the formula for calculating the stress distribution coefficient λ is as follows: , where λ is the stress distribution coefficient, used to determine the degree of toughness compensation of aliphatic polyurethane resin to alcohol-soluble nitrocellulose resin. This refers to the weight parts of the aliphatic polyurethane resin in the composition. The weight percentage of alcohol-soluble nitrocellulose resin in the composition; the stress distribution coefficient λ is between 0.35 and 0.55.

[0023] Preferably, the fineness of the pigment particles in the flexographic paper surface printing ink composition is no greater than 5 μm, and the pigment particles are centrally symmetrically distributed in the coated pigment core to maintain the mechanical isotropy of the heterogeneous toughness-enhancing structure.

[0024] Preferably, at a shear rate of 1000 s -1 up to 3000s -1 Under the specified conditions, the absolute value of the change rate of apparent viscosity of the flexographic paper surface printing ink composition with shear time is less than 0.01 mPa·s / s, the adhesion level of the flexographic paper surface printing ink composition on the paper substrate surface is Grade 1, and after being subjected to 50 cycles of 180-degree crease friction test, no cracks with a width greater than 0.1 mm are generated on the ink film surface.

[0025] A method for preparing a flexographic paper surface printing ink composition includes the following steps: Step 1101: Mix and pre-disperse pigment particles, aliphatic polyurethane resin and some composite alcohol solvent, and utilize the polar functional groups in the aliphatic polyurethane resin molecular chain to generate adsorption on the surface of pigment particles. Step 1102: Grind the pre-dispersed material to a fineness of no more than 5 μm, and form a flexible coating layer on the surface of the pigment particles to construct a coated pigment core and obtain a color paste; Step 1103: Add alcohol-soluble nitrocellulose resin, additives and the remaining composite alcohol solvent to the pigment paste, and stir at room temperature to allow the alcohol-soluble nitrocellulose resin to form a rigid film-forming matrix around the pigment core.

[0026] Example 1: In a continuously operating flexographic paper surface printing production line, the flexographic paper surface printing ink composition enters the printing plate transfer stage. The production line transports the paper substrate, causing the shear rate at the printing plate to reach 1000s. -1 up to 3000s -1In the range of printing conditions, and given the objective working conditions of high-frequency mechanical folding and surface friction after the printed product is taken offline, the free flexible polymer chains in conventional multi-resin blend systems undergo random coiling and instantaneous intermolecular entanglement under shear stress, leading to a local increase in apparent viscosity and causing the printing plate to dry and clog. Using a single rigid resin as a binder causes the final ink film to crack and peel off under external mechanical stress. In the surface printing process, the aliphatic polyurethane resin molecular chains inside the flexographic paper surface printing ink composition undergo morphological transformation under the action of shear stress and temperature fields. The polar functional groups in the aliphatic polyurethane resin molecular chains break away from the random coiling state and connect with the active sites on the surface of pigment particles through hydrogen bonds. In combination, a flexible layer is formed on the surface of pigment particles to construct a pigment core. The phase-space anchoring mechanism consumes the free aliphatic polyurethane molecular chains in the composite alcohol solvent, allowing the alcohol-soluble nitrocellulose resin to be distributed around the pigment core in a stable solvent phase that excludes rheological interference from free flexible chain segments and crosslink to form a rigid film-forming matrix. The flexible layer with a thickness of 50nm to 200nm around the pigment core generates a steric hindrance effect to inhibit pigment particle aggregation, thereby providing a uniformly distributed dispersed phase for the rigid film-forming matrix. The pigment core and the rigid film-forming matrix together constitute a heterogeneous toughness-enhancing structure, maintaining the apparent viscosity stability of the flexographic paper surface printing ink composition under plate transfer conditions.

[0027] The flexographic paper surface printing ink composition, after being transferred to the paper substrate and cured into a film by the evaporation of the composite alcohol solvent, exhibits a physical state in which high hardness and high flexibility are mutually constrained within a single-phase system. The heterogeneous toughness-enhancing structure relies on the three-dimensional anchoring of the pigment cores in the rigid film-forming matrix to initiate a mechanical stress response mechanism. When the ink film surface is subjected to mechanical external force friction or folding, generating deformation loads, the cross-linked rigid film-forming matrix bears the initial load to maintain the integrity of the external structure of the ink film. The rigid film-forming matrix transmits the locally concentrated mechanical stress to the flexible layer encapsulating the pigment particles. The stress is transmitted through the elastic deformation of the flexible layer, which absorbs and buffers the stress, thus achieving physical stress isolation between the rigid abrasion-resistant matrix and the flexible crack-resistant dispersion core within a single film-forming system. This addresses the issue that traditional processes cannot avoid by adjusting the high molecular weight resin ratio. The technical contradiction of shear thickening is addressed by flexographic paper surface printing ink compositions, which, through specific process timing and directional assembly, transform free flexible chain segments into stress-absorbing cores at the phase interface. This transforms the viscosity control problem of the overall fluid into a spatial anchoring mechanism at the phase interface. This allows the flexographic paper surface printing ink composition to maintain the dissolution equilibrium of aliphatic polyurethane resin within the ink layer and inhibit the precipitation and drying of polyurethane molecular chains on the printing plate surface during plate transfer at the aforementioned shear rates. Under this mechanism, the absolute value of the apparent viscosity change rate of the flexographic paper surface printing ink composition with shear time remains less than 0.01 mPa·s / s. Furthermore, it cures on the paper substrate surface to form a protective coating that does not produce cracks wider than 0.1 mm after 50 cycles of 180-degree crease friction testing.

[0028] Example 2: The actual test scenario for evaluating the rheological stability of alcohol-soluble inks under high-speed transfer and the mechanical properties of cured ink films in flexographic paper surface printing was conducted using a simulated flexographic printing press equipped with a chamber environment control unit and a rheometer. The test platform was set with an ambient temperature of 25°C and a relative humidity of 50%, and the running shear rate between the printing plate roll and the impression roll was set to 2000 s. -1 Simultaneously, standard test paper with a surface absorption coefficient fluctuation range of 5% to 15% was introduced to simulate the nonlinear solvent suction interference caused by uneven substrate porosity in real printing. The technical consideration for setting the mass ratio of aliphatic polyurethane resin to pigment particles is to balance the steric hindrance thickness of the flexible layer coating the pigment core surface and the concentration of free flexible chain segments in the system. When the shear rate increases during the plate transfer stage, the collision probability of polyurethane molecular chains in the system increases accordingly. Based on the judgment rule of avoiding shear-induced entanglement of free polymer long chains and causing viscosity abrupt change, the mass ratio tends to meet the critical value range of hydrogen bonding saturation point without generating excessive free phase. According to this logical judgment model, a mass ratio of 1.5 was selected as the core process parameter of the sample group of this invention.

[0029] The test system consisted of the sample group of this invention, a control group 1 without aliphatic polyurethane resin, a control group 2 exceeding the lower limit of the range with a mass ratio of 0.8, and a control group 3 exceeding the upper limit of the range with a mass ratio of 2.5. During the initial ink supply stage, ink compositions with an initial apparent viscosity of 45 mPa·s were input to each group to establish a unified initial physical boundary. The printing press was started, and a random temperature disturbance with an amplitude of 2°C was introduced into the system. This disturbance was used to simulate the dynamic deviation of solvent evaporation rate. During a 30-minute continuous test, the apparent viscosity dynamic change rate index output by the rheometer was extracted. The sample group of this invention withstood non-uniform suction from the paper substrate and... After temperature disturbance, the absolute value of the rate of change of apparent viscosity with shear time stabilized within the range of 0.008 mPa·s / s. Control group 1, lacking a flexible layer, showed a rate of change of 0.005 mPa·s / s. Control group 2, exceeding the lower limit of the range, had a rate of change of 0.05 mPa·s / s due to insufficient thickness of the flexible layer, resulting in the exposure of some pigment particles. Control group 3, exceeding the upper limit of the range, showed nonlinear deterioration characteristics, and its apparent viscosity broke through the stable threshold after 12 minutes of operation, with the rate of change rising sharply to 0.5 mPa·s / s. This result verifies that excessive polyurethane long chains induce rheological entanglement and blockage in the shear field.

[0030] The cured printed products from the testing platform were collected and subjected to a 180-degree crease friction test with a load of 20N according to the standard testing procedure. After 50 mechanical folds, the surface of the sample of this invention remained flat and no cracks wider than 0.1mm were generated. Its internal structure is a heterogeneous toughness enhancement structure formed by the cross-linking of a flexible layer with a thickness between 50nm and 200nm and a rigid film-forming matrix. This structure transmits the concentrated mechanical stress to the interior and converts it into elastic deformation for dissipation. The ink film of control group 1 experienced large-area brittle peeling at the 15th fold. Control group 2, which exceeded the lower limit of the range, showed a penetrating microcrack with a width of 0.3mm at the 35th fold. Control group 3, which exceeded the upper limit of the range, had incomplete images on the paper surface due to the drying and clogging of the printing plate in the early stage, and the surface gloss was less than 30%. The above measured data verify the spatial anchor of the rigid abrasion-resistant matrix and the flexible crack-resistant dispersion core. The mechanism effectively suppresses free phase rheological interference, achieving independent physical stress isolation between high hardness and high flexibility within a single-phase system during the curing and film-forming stage. The core premise for the above-mentioned heterogeneous toughness-enhancing structure to achieve mechanical stress isolation lies in the fact that the degree of toughness compensation of the aliphatic polyurethane resin to the alcohol-soluble nitrocellulose resin in the components meets a specific stress distribution coefficient λ, and the value of λ is between 0.35 and 0.55. The derivation of this range is based on the fact that when λ is below the lower limit of 0.35, the volume ratio of the flexible coating core is insufficient to form a continuous stress dissipation network in the rigid matrix, resulting in the inability to effectively block crack propagation; while when λ is above the upper limit of 0.55, the excessive flexible phase will reversely plasticize the continuous skeleton, causing the overall scratch resistance hardness of the ink film to decrease sharply to the point where it cannot withstand normal friction loads. Therefore, this range is the necessary engineering boundary for balancing crack resistance and high hardness.

[0031] Example 3: The formulation of flexographic paper surface printing ink compositions is limited by the control requirements of the spatial assembly sequence of multi-polymer systems and the calibration constraints of the mechanical matching scale between internal phases. The single-step mixing process causes the aliphatic polyurethane resin and alcohol-soluble nitrocellulose resin to be randomly blended in a composite alcohol solvent. The aliphatic polyurethane resin molecular chains are free in the solvent phase and cannot form a flexible layer with a thickness of 50nm to 200nm on the surface of pigment particles. The elastic modulus ratio between the rigid film-forming matrix and the coated pigment core deviates from the range of 10 to 15 times, reducing the mechanical protective performance of the ink film. A heterogeneous toughness-enhanced structure is prepared by using a stepwise shear assembly process. Pigment particles and aliphatic polyurethane resin are added to a composite alcohol solvent, the ambient temperature is adjusted to 35°C to 45°C, and an application time of 1500s is applied. -1 up to 2500s -1 The mixture was stirred at a shear rate for 40 minutes. The shear stress caused the aliphatic polyurethane resin molecular chains to extend, and the polar functional groups combined with the active sites on the pigment particle surface, forming a flexible layer with a thickness of 50 nm to 200 nm around the pigment particles, thus obtaining a coated pigment core. Although a composite alcohol solvent was present in the system, the reaction time was 1500 s. -1up to 2500s -1 The high shear rate generates localized surface frictional heat and extremely high hydrodynamic pressure on the pigment particle surface. This dynamic pressure causes the isocyanate groups at the ends of the aliphatic polyurethane resin molecular chains to instantly break through the solvation layer on the particle surface. Furthermore, the nucleophilic reactivity of the hydroxyl and carboxyl groups on the pigment particle surface under localized high temperature and strong shear is much greater than that of the hydroxyl groups in the bulk alcohol solvent. Thus, covalent bonding preferentially occurs in the reaction kinetics, avoiding the ineffective consumption of isocyanate groups in the bulk alcohol solvent. Adding alcohol-soluble nitrocellulose resin and additives to the system reduces the shear rate to 500 s. -1 up to 800s -1 Stir continuously for 20 minutes, 500 seconds. -1 up to 800s -1 The shear rate maintains the integrity of the flexible layer morphology, and the alcohol-soluble nitrocellulose resin is distributed and cross-linked in the solvent phase surrounding the pigment core to form a rigid film-forming matrix.

[0032] In this process, crosslinking does not refer to a chemical crosslinking reaction involving the introduction of an external curing agent, but rather a physical crosslinking film-forming mechanism. Specifically, as the alcohol solvent evaporates in a gradient, the concentration of alcohol-soluble nitrocellulose resin distributed around the pigment core rapidly exceeds the critical overlap concentration. Its rigid macromolecular segments undergo dense physical entanglement and secondary bond association in space, ultimately locking into a continuous three-dimensional network framework with high cohesive energy, thus exhibiting an overall rigid film-forming matrix after curing. After the ink film cures on the paper substrate surface, an atomic force microscope equipped with a nanoindentation module is used to scan the cross-sectional area of ​​the ink film to measure the elastic modulus of the rigid film-forming matrix and the elastic modulus of the flexible layer. The nanoindentation measurement data shows that before the aliphatic polyurethane resin coats the pigment particles, the measured elastic modulus of the rigid film-forming matrix is ​​within the range of 10 to 15 times that of the flexible layer. This test procedure establishes the physical boundary of the viscoelastic matching rule and verifies that the step-by-step shear assembly process produces a core-shell heterogeneous phase, forming an internal mechanical isolation state that resists external mechanical folding of the ink film.

[0033] Example 4: When the flexographic paper surface printing ink composition faces the objective condition of fluctuations in the surface active site density of pigment particles from different batches, the system control unit initiates a pre-calibration procedure based on ultrasonic attenuation spectrum. The detection probe extracts the initial acoustic attenuation characteristics of the current batch of pigment particles mixed with the composite alcohol solvent. The feeding unit injects aliphatic polyurethane resin into the system, while the stirring unit applies a stepwise increase in shear rate. Under these conditions, the flow cell collects the ultrasonic attenuation energy spectrum distribution characteristics of the material in real time. The processing unit calculates the frequency shift of the characteristic peak and locks the target frequency shift window corresponding to the 50nm to 200nm flexible layer thickness range based on the physical mapping relationship between acoustic attenuation characteristics and spatial distribution size. The processing unit records the target shear time and target shear rate when the frequency offset falls into the target frequency offset window. Based on this, the processing unit writes the target shear time and target shear rate as correction parameters into the production line control unit. The production line control unit controls the assembly operation of the corresponding batch of raw materials according to the correction parameters. In this physical mapping relationship, the attenuation of ultrasound in multiphase fluid is mainly composed of viscous dissipation and thermal dissipation. The processing unit pre-stores the reference acoustic attenuation spectrum of bare pigment particles at equivalent concentration. When aliphatic polyurethane resin forms a coating on the particle surface, the effective hydrodynamic radius of the particles increases and the acoustic impedance at the interface changes stepwise, resulting in a regular amplification of the acoustic wave scattering cross section in the high-frequency band.

[0034] The processing unit separates the additional attenuation component caused purely by the interface phase evolution by subtracting the reference acoustic attenuation spectrum in real time, and substitutes it into the preset acoustic particle scattering kernel model for inversion calculation, thereby accurately converting the overall acoustic characteristic peak shift into a surface flexible layer thickness value of 50nm to 200nm. The pre-calibration procedure updates the mechanical shear parameters based on the measured acoustic parameters. This calibration action establishes a deterministic control link between the mechanical shear parameters and the interface thickness evolution of the coated pigment core phase state. The updated correction parameters prevent insufficient shear stress or overload from causing the flexible layer thickness to deviate from the 50nm to 200nm range, maintaining the elastic modulus ratio between the rigid film-forming matrix and the coated pigment core in the range of 10 to 15 times. The calibration process offsets the interference of raw material property fluctuations, ensuring the consistency of the spatial configuration of the heterogeneous toughness-enhanced structure, thereby maintaining the apparent viscosity stability of the flexographic paper surface printing ink composition under printing conditions and the mechanical stress isolation and protection state of the cured ink film.

[0035] Example 5: When introducing batches of pigment particles with unknown surface structure parameters into the flexographic paper surface printing process, relying on fixed parameters to set the mass ratio of aliphatic polyurethane resin to pigment particles will deviate from the actual interfacial bonding requirements, causing incomplete coating of the pigment particle surface or excessive free polymer chains remaining in the solvent phase. This leads to brittle peeling of the cured ink film or induces shear entanglement and thickening during the printing plate transfer stage. The system measurement and control unit initiates a rheological titration and mass ratio optimization procedure targeting the density of active sites on the pigment particle surface. The batching unit disperses a predetermined weight of pigment particles in a composite alcohol solvent to construct a baseline dispersion system. The control unit sets the mixing temperature to 25°C and the base shear rate to 500 s. -1 Under these stirring conditions, aliphatic polyurethane resin was continuously injected into the reference dispersion system at a feeding rate of 0.1 g / min using a precision metering pump. A rheometer simultaneously extracted the dynamic sequence of the loss tangent values ​​of the mixture. In the initial injection stage, the polar functional groups in the aliphatic polyurethane resin molecular chains continuously established hydrogen bonds with the active sites on the pigment particle surface. The rate of increase in the elastic modulus within the system was greater than the rate of increase in the viscous modulus, causing the loss tangent value to show a monotonically decreasing trend. As the injection volume increased, the active sites on the pigment particle surface tended to become physically saturated, and the aliphatic polyurethane resin accumulated in the solvent phase in the form of free flexible chain segments, triggering a rheological response dominated by viscous dissipation. This physical process caused the loss tangent value sequence to show an extreme inflection point on the time axis, changing from decreasing to increasing. The measurement and control unit extracted the cumulative consumption of aliphatic polyurethane resin corresponding to the occurrence of the extreme inflection point and calculated the ratio of this cumulative consumption to the initial pigment particle weight to generate a reference mass ratio.

[0036] The measurement and control unit multiplies the extracted baseline mass ratio by an engineering compensation coefficient to calculate the target mass ratio parameter for the current pigment batch. This engineering compensation coefficient, selected from the range of 1.05 to 1.15, is used to compensate for mass transfer attenuation errors in the large-scale preparation environment. The measurement and control unit then sends the generated target mass ratio parameter to the production line's batching control unit. This quantitative calibration link transforms the hydrogen bond saturation state at the particle phase interface into continuously measurable rheological parameter extreme points, eliminating fluctuations caused by empirically set parameters in multi-resin blending processes. It guides the aliphatic polyurethane resin molecular chains to precisely construct flexible layers with thicknesses ranging from 50 nm to 200 nm in various pigment systems, while simultaneously suppressing excessive free phase accumulation and maintaining the apparent viscosity stability of the flexographic paper surface printing ink composition under high-speed printing conditions. To ensure that the heterogeneous toughness-enhanced structure forms a mechanical stress isolation and crack prevention state, in actual operation, the specific evaluation model of mass transfer attenuation error is based on the comprehensive statistical basis of the dead volume of pipeline transportation and solvent flash evaporation loss under high shear. Through continuous feeding calibration test on the standard production line, the volume loss rate caused by the adhesion of the internal pipe wall of the resin pumping system is measured to be between 2% and 5%. At the same time, the solvent evaporation caused by high-speed stirring in the open dispersion tank and the follow-up liquid level deviation lead to a decrease of about 3% to 10% in the local effective shear contact area. After superimposing the above two objective physical losses and taking a safety margin, a numerical range of 1.05 to 1.15 is derived as the engineering compensation coefficient to ensure that the effective amount of resin that actually reaches the pigment particle interface is completely consistent with the ideal reference value measured by rheological titration.

[0037] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A flexographic paper surface printing ink composition, characterized in that, The flexographic paper printing ink composition, based on 100% of its total weight, consists of the following components: The composition includes 8.0% to 15.0% pigment particles, 12.0% to 20.0% aliphatic polyurethane resin, 15.0% to 25.0% alcohol-soluble nitrocellulose resin, 2.0% to 5.0% additives, and a composite alcohol solvent to make up to 100%. The polar functional groups in the aliphatic polyurethane resin molecular chain are bonded to the active sites on the surface of the pigment particles through hydrogen bonds, and a flexible layer is formed on the surface of the pigment particles to construct a pigment core. Alcohol-soluble nitrocellulose resin is distributed around the coated pigment core and cross-linked to form a rigid film-forming matrix. The rigid film-forming matrix and the coated pigment core together constitute a heterogeneous toughness-enhancing structure. In the ink film formed by curing the ink composition of flexographic paper, the heterogeneous toughness enhancement structure uses a rigid film-forming matrix to three-dimensionally anchor the coated pigment core, so that when the rigid film-forming matrix is ​​subjected to mechanical external force, the stress is transmitted to the flexible layer, and the elastic deformation of the flexible layer is used to absorb and buffer the stress. The composite alcohol solvent is composed of a variety of alcohol monomers with different evaporation rate gradients. It is used to maintain the dissolution equilibrium of aliphatic polyurethane resin in the ink layer during the transfer of flexographic paper surface printing ink composition to paper substrate, thereby inhibiting the precipitation and drying of polyurethane molecular chains on the printing plate surface. Furthermore, the mass ratio of aliphatic polyurethane resin to pigment particles is 1.2 to 1.8; the thickness of the flexible layer is 50 nm to 200 nm, used to generate a steric hindrance effect between pigment particles to suppress particle agglomeration; the nitrogen content of the alcohol-soluble nitrocellulose resin is 10.7% to 11.3%, and the ratio of the elastic modulus of the rigid film-forming matrix formed by the alcohol-soluble nitrocellulose resin to the elastic modulus of the coated pigment core satisfies the viscoelastic matching rule, which is limited to the elastic modulus of the rigid film-forming matrix being 10 to 15 times that of the elastic modulus of the flexible layer; Aliphatic polyurethane resins have a number average molecular weight of 15,000 to 25,000 and a glass transition temperature of -50°C to -30°C. The isocyanate groups at the ends of the aliphatic polyurethane resin molecular chains covalently bond with the hydroxyl or carboxyl groups on the surface of the pigment particles to improve the interfacial anchoring strength of the coated pigment core.

2. The flexographic paper surface printing ink composition according to claim 1, characterized in that, The apparent viscosity of alcohol-soluble nitrocellulose resin at 25°C is 300 mPa·s to 600 mPa·s; the hardness contribution provided by alcohol-soluble nitrocellulose resin in heterogeneous toughness-reinforced structures is higher than that of aliphatic polyurethane resin.

3. The flexographic paper surface printing ink composition according to claim 1, characterized in that, The complex alcohol solvent is composed of ethanol, isopropanol and n-propanol; based on the total weight of the complex alcohol solvent as 100%, the content of ethanol is 50.0% to 70.0%, the content of isopropanol is 15.0% to 25.0%, and the content of n-propanol is 5.0% to 15.0%.

4. The flexographic paper surface printing ink composition according to claim 1, characterized in that, The additives consist of polyethylene wax, antistatic agent, defoamer and leveling agent; the average particle size of polyethylene wax is 2μm to 5μm. The polyethylene wax is distributed on the surface of the ink film after the flexographic paper surface printing ink composition is cured, and is used to reduce the sliding friction coefficient of the ink film surface.

5. The flexographic paper surface printing ink composition according to claim 1, characterized in that, The fineness of the pigment particles in the flexographic paper surface printing ink composition is no greater than 5μm, and the pigment particles are centrally symmetrically distributed in the coated pigment core to maintain the mechanical isotropy of the heterogeneous toughness-enhancing structure.

6. The flexographic paper surface printing ink composition according to claim 1, characterized in that, At a shear rate of 1000 s -1 up to 3000s -1 Under the specified conditions, the absolute value of the change rate of apparent viscosity of the flexographic paper surface printing ink composition with shear time is less than 0.01 mPa·s / s, the adhesion level of the flexographic paper surface printing ink composition on the paper substrate surface is Grade 1, and after being subjected to 50 cycles of 180-degree crease friction test, no cracks with a width greater than 0.1 mm are generated on the ink film surface.

7. A method for preparing a flexographic paper surface printing ink composition, used to achieve the flexographic paper surface printing ink composition of claim 1, characterized in that, Includes the following steps: Step 1101: Mix and pre-disperse pigment particles, aliphatic polyurethane resin and some composite alcohol solvent, and utilize the polar functional groups in the aliphatic polyurethane resin molecular chain to generate adsorption on the surface of pigment particles. Step 1102: Grind the pre-dispersed material to a fineness of no more than 5 μm, and form a flexible coating layer on the surface of the pigment particles to construct a coated pigment core and obtain a color paste; Step 1103: Add alcohol-soluble nitrocellulose resin, additives and the remaining composite alcohol solvent to the pigment paste, and stir at room temperature to allow the alcohol-soluble nitrocellulose resin to form a rigid film-forming matrix around the pigment core.

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