A chalking-resistant waterborne polyurethane ink-receptive coating, its preparation method and use

Abstract

The present application relates to a kind of anti-pulverization water-based polyurethane ink-absorbing coating and its preparation method and application, belong to water-based paint technical field.The coating includes by weight parts: epoxy modified water-based polyurethane resin 4560 parts, polycarbodiimide crosslinking agent 610 parts, inorganic filler 15 parts, film-forming aid 2.54 parts, wetting and leveling agent 0.52 parts, defoaming agent 0.050.2 parts and deionized water 2030 parts.The present application is constructed rigid and tough network by epoxy modified resin and polycarbodiimide crosslinking agent, and realizes stress dispersion by means of surface modified nanosilica, improves the problem that anti-pulverization, fast ink-absorbing and water resistance in ink-absorbing coating are difficult to take into account.The obtained coating has excellent anti-pulverization, fast ink-absorbing and good water resistance, and is suitable for digital inkjet printing substrate.

Description

Technical Field

[0001] This invention belongs to the field of water-based coating technology, and relates to an anti-chalking water-based polyurethane ink-absorbing coating, its preparation method, and its application. Background Technology

[0002] Digital inkjet printing technology, with its high precision, high efficiency, and personalized output, has been widely used in advertising, decorative materials, and packaging printing. However, digital inkjet inks generally cannot be directly printed onto the substrate; an ink-absorbing layer is usually required as an intermediary layer. As the core functional layer that carries the ink and affects image quality, the performance of the ink-absorbing layer directly determines the color saturation, drying speed, and long-term durability of the printed pattern. Water-based polyurethane coatings, due to their environmental friendliness, non-toxicity, good film-forming properties, and strong adhesion, have become one of the mainstream coating systems for preparing ink-absorbing layers.

[0003] Currently available waterborne polyurethane ink-absorbing coatings still have shortcomings in overall performance. On the one hand, methods to improve ink absorption speed, such as constructing porous structures or reducing solid content, often result in a decrease in the coating's mechanical strength, manifesting as low surface hardness, poor abrasion resistance, and susceptibility to chalking and peeling under friction or outdoor environments. On the other hand, techniques aimed at improving chalking resistance and abrasion resistance, such as increasing filler content or using high-crosslinking-density resins, may affect the ink absorption pore structure, leading to slower ink drying. Furthermore, the compatibility issues between hydrophilic ink-absorbing components and hydrophobic durable components also challenge the coating's water resistance and adhesion in humid environments. These existing technologies fail to resolve the inherent contradictions between chalking resistance, ink absorption, and water resistance.

[0004] Existing technologies struggle to maintain rapid ink absorption while simultaneously imparting high resistance to chalking, water resistance, and durable adhesion to the coating. Summary of the Invention

[0005] The purpose of this invention is to provide an anti-chalking waterborne polyurethane ink-absorbing coating, its preparation method, and its application. This coating has good anti-chalking and ink-absorbing properties.

[0006] The objective of this invention can be achieved through the following technical solutions: In a first aspect, the present invention provides an anti-chalking waterborne polyurethane ink-absorbing coating, comprising the following components by weight: 45-60 parts of epoxy-modified waterborne polyurethane resin; 6-10 parts of polycarbodiimide crosslinking agent; 1-5 parts of inorganic filler; Film-forming aid 2.5-4 parts; 0.5-2 parts of wetting and leveling agent; Defoamer 0.05-0.2 parts; 20-30 parts deionized water.

[0007] The "epoxy-modified waterborne polyurethane resin" mentioned in this text refers to a waterborne polyurethane copolymer with epoxy groups in its molecular chain, prepared by introducing epoxy resin or its derivatives as reactive components through chemical synthesis. It can improve the adhesion between the resin and the substrate, and also participate in subsequent cross-linking reactions, enhancing the chemical stability and mechanical strength of the coating.

[0008] The effects of epoxy modification are twofold: first, to enhance the adhesion of the resin to the substrate (especially metals, glass, etc.); second, within the coating system, its ring-opening activity (especially under heating or in the presence of amines) can react with the amino groups on the surface of aminosilane-modified fillers, thereby indirectly incorporating the fillers into the network and enhancing interfacial bonding.

[0009] The "polycarbodiimide crosslinking agent" described in this text is a polymer or oligomer containing multiple carbodiimide functional groups on its molecular backbone. Preferably, the molecular weight of the polycarbodiimide crosslinking agent is 2000–4000 g / mol. Polycarbodiimide, containing multiple -N=C=N- structures in its molecule, can react efficiently with carboxyl groups in resin to form acylurea bonds, exhibiting high crosslinking efficiency and irreversible reaction, making it particularly suitable for constructing high-density crosslinked networks in aqueous systems. Simultaneously, the high reactivity of polycarbodiimide also enables it to interact with other nucleophilic functional groups in the system (such as amino groups from modified fillers), further enhancing the network integrity and interfacial bonding of the entire composite system.

[0010] Preferably, the inorganic filler is nano-silica with a particle size of 10-50 nm. Nano-silica in this particle size range can be uniformly dispersed in the coating, which can improve the coating's hardness and wear resistance. Because its size is much smaller than the ink absorption pores, it will not clog the ink channels, thus achieving both enhancement and ink absorption functions.

[0011] Preferably, the surface of the nano-silica is modified with a silane coupling agent. Surface modification can solve the compatibility problem between nano-silica and the resin matrix, prevent filler agglomeration, improve its dispersion stability in the system, and enhance the interfacial bonding force between the filler and the resin.

[0012] Preferably, the silane coupling agent is an aminosilane coupling agent. The amino functional groups in the aminosilane coupling agent molecule can further react with polycarbodiimide crosslinking agents or resin active groups during the coating curing process, fixing the filler in the crosslinking network in the form of chemical bonds, thereby preventing the filler from falling off.

[0013] Preferably, the weight ratio of the epoxy-modified waterborne polyurethane resin to the polycarbodiimide crosslinking agent is 5.0–9.0:1. This ensures that the resin is fully crosslinked without over-crosslinking and causing brittleness, achieving a ratio that balances high anti-chalking properties with good film toughness.

[0014] Secondly, the present invention provides a method for preparing an anti-chalking waterborne polyurethane ink-absorbing coating, comprising the following steps: (1) Mix deionized water, film-forming aid and wetting leveling agent, and stir at 25-35℃ for 15-20 minutes; (2) Add inorganic filler to the system in step (1) and stir and disperse for 30-40 minutes; (3) Heat the system of step (2) to 65-75℃, add epoxy modified waterborne polyurethane resin, stir for 20-40 minutes, and then add polycarbodiimide crosslinking agent dropwise at a rate of 1-2 ml / min. (4) Continue the reaction at 65-75℃ for 60-90 minutes; (5) Cool the system to 25-30℃, add defoamer, and filter; During the preparation process from step (1) to step (4), the pH value of the system is maintained at 7.5-8.5.

[0015] By controlling the pH of the system within a weakly alkaline range of 7.5-8.5 throughout the process, the hydrolysis of the crosslinking agent can be effectively inhibited, while ensuring that the carboxyl groups in the resin are in a suitable reactive state, thereby guaranteeing the high efficiency and directional progress of the crosslinking reaction. In step (3), the controlled-rate addition of the crosslinking agent can avoid the risk of gelation caused by local overheating or excessively high concentration of crosslinking agent, ensuring a stable and uniform reaction.

[0016] Preferably, ethanolamine is used to maintain and adjust the pH value of the system. More preferably, triethanolamine is used to adjust the pH value.

[0017] Preferably, in step (3), an organotin catalyst (such as dibutyltin dilaurate) is added at 0.1%-0.5% of the weight of the epoxy-modified waterborne polyurethane resin. The organotin catalyst can promote the reaction rate of polycarbodiimide crosslinking agent with resin and modified filler, shorten reaction time, reduce process energy consumption, and is especially beneficial for achieving full crosslinking at lower temperatures (65-75°C).

[0018] Thirdly, the present invention provides an application of an anti-chalking waterborne polyurethane ink-absorbing coating, namely, coating the surface of a substrate such as paper, plastic film, cloth, or metal with the anti-chalking waterborne polyurethane ink-absorbing coating described in the first aspect, and drying it to form a dry coating to obtain a digital inkjet printing substrate.

[0019] Through in-depth research, the inventors believe that existing technologies struggle to synergistically optimize anti-chalking, ink absorption speed, and water resistance. The underlying reason lies in the temperature and humidity cycles experienced by the coating during actual outdoor applications. These cycles cause differential expansion and contraction of the hydrophilic / hydrophobic components within the coating, generating alternating internal stress. Traditional methods of increasing initial hardness by simply raising crosslinking density or adding hard fillers often sacrifice coating toughness, potentially leading to brittle chalking under long-term internal stress.

[0020] Based on the above understanding, the present invention aims to improve this problem from the perspective of material structure design, and its synergistic effect is as follows: Rigid and toughened matrix network construction: This invention utilizes epoxy-modified waterborne polyurethane resin reacted with a specific polycarbodiimide crosslinking agent. The introduction of epoxy groups facilitates the formation of chemical bonds, enabling the final three-dimensional crosslinked network to possess both the necessary rigidity (high elastic modulus) and effective stress dissipation capability (moderate elongation at break). This rigid and tough, rather than brittle, network structure can better absorb and release internal stress caused by temperature and humidity changes, thereby significantly inhibiting the initiation of microcracks.

[0021] Stress regulation effect of nanofillers: One of the core functions of the surface-modified nano-silica is to act as stress dispersion points uniformly dispersed in the resin matrix. Its nanoscale structure and strong interfacial bonding (achieved through silane coupling agents) ensure that when the paint film is under stress, these particles can effectively hinder the propagation path of microcracks, passivate and disperse the stress at the crack tips, and prevent microcracks from penetrating and causing macroscopic damage, thereby converting the material's destructive energy into higher wear resistance and anti-chalking properties.

[0022] Preparation process: The preparation method described in this invention, which involves "first fully dispersing the filler, then controlling the temperature and speed for crosslinking," ensures the uniform distribution of the nanofiller in the resin network and achieves a stable and uniform crosslinking reaction. This avoids stress concentration defects caused by filler agglomeration or localized over-dense / over-sparse crosslinking networks, thus guaranteeing the uniformity and reliability of the coating's microstructure.

[0023] In summary, this invention aims to enhance the coating's resistance to internal stress and its ability to suppress microcracks through resin modification design, the introduction of nanofillers, and the preparation process. This allows the coating prepared by this invention to maintain rapid ink absorption characteristics while exhibiting good anti-chalking properties, water resistance, and adhesion.

[0024] The beneficial effects of this invention are: This invention constructs a network with both rigidity and toughness using epoxy-modified resin and a crosslinking agent, and achieves stress dispersion through surface-modified nano-silica. The resulting coating exhibits good anti-chalking properties, water resistance, and adhesion while maintaining rapid ink absorption characteristics. Furthermore, its water-based formulation and preparation process meet environmental protection requirements, demonstrating promising prospects for industrialization. Detailed Implementation

[0025] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with embodiments, is provided below.

[0026] Main raw materials and sources Epoxy-modified waterborne polyurethane resin: solid content 40±2%, acid value 45±5 mg KOH / g, self-made (prepared by acetone method from polyester polyol (such as polybutylene adipate diol with a number average molecular weight of 1000-3000), isophorone diisocyanate, dimethylolpropionic acid and epoxy resin E-44, and neutralized and emulsified by triethylamine).

[0027] Ordinary waterborne polyurethane resin: industrial grade, solid content 35%, commercially available.

[0028] Polycarbodiimide crosslinking agent (PCDI): 40% active ingredient content, commercially available.

[0029] Water-dispersible polyisocyanate crosslinking agent (HDI): solid content 80±2%, commercially available.

[0030] The other raw materials, such as nano silica, γ-aminopropyltriethoxysilane (KH-550), film-forming aid (dodecyl alcohol ester), wetting and leveling agent (BYK-346), defoamer (BYK-024), pH adjuster (triethanolamine), catalyst (dibutyltin dilaurate) and deionized water, are all common materials in this field.

[0031] Surface-modified nano-silica can be processed using conventional methods, such as dispersing nano-silica in an alcohol-water mixed solvent, reacting it with KH-550 at a certain temperature, and then separating, washing, and drying it.

[0032] Example 1 A method for preparing an anti-chalking waterborne polyurethane ink-absorbing coating includes the following steps: (1) Add 25 parts of deionized water, 3 parts of dodecyl alcohol ester and 1 part of BYK-346 to the reaction flask. Turn on the stirrer and set the speed to 400 rpm. Stir for 15 minutes in a water bath at 25-35℃ to form a homogeneous aqueous phase.

[0033] (2) Add 3 parts of pretreated modified nano-silica (20nm) to the system in step (1). Increase the stirring speed to 1100 rpm and disperse at high speed for 35 minutes at 30°C to obtain a uniform slurry.

[0034] (3) Raise the water bath temperature to 70°C and maintain the stirring speed at 500 rpm. Add 55 parts of epoxy-modified waterborne polyurethane resin and 0.1 parts of dibutyltin dilaurate to the slurry, and stir and mature at this temperature for 30 minutes.

[0035] Eight parts of polycarbodiimide crosslinking agent were slowly added dropwise at a rate of approximately 1.5 mL / min using a constant-pressure dropping funnel. During the addition, the pH of the system was precisely maintained at 8.0 ± 0.2 by adding a 10 wt% aqueous solution of triethanolamine.

[0036] (4) After the crosslinking agent is added, continue to react at 70°C for 65 minutes.

[0037] (5) Cool the reaction system to 25-30℃, add 0.1 parts of BYK-024, and stir at 400 rpm for 10 minutes. Finally, filter with 150-mesh nylon filter cloth to obtain the finished coating.

[0038] Example 2 A method for preparing an anti-chalking waterborne polyurethane ink-absorbing coating includes the following steps: The difference from Example 1 is that no organotin catalyst is added in step (3), and the heat preservation reaction time in step (4) is extended to 90 minutes. The other formulations and steps are the same as in Example 1.

[0039] Example 3 A method for preparing an anti-chalking waterborne polyurethane ink-absorbing coating includes the following steps: The formulation is essentially the same as in Example 1, with the following adjustments: 50 parts epoxy-modified waterborne polyurethane resin, 10 parts polycarbodiimide crosslinking agent, 2 parts modified nano-silica (20nm), 28 parts deionized water, 3 parts film-forming aid, 1 part wetting and leveling agent, and 0.05 parts organotin catalyst. The resin to crosslinking agent mass ratio in this formulation is 5:1. Other steps are the same as in Example 1.

[0040] Comparative Example 1 The difference from Example 1 is that the 8 parts of polycarbodiimide crosslinking agent in step (3) are replaced with an equal part by weight of water-dispersible polyisocyanate crosslinking agent (diluted with an appropriate amount of deionized water to a solid content of 40% before use). The other steps are the same as in Example 1.

[0041] Comparative Example 2 The preparation steps are basically the same as in Example 1, except that the 3 parts of modified nano-silica in step (2) are replaced with an equal part by weight of untreated silica. The other steps are the same as in Example 1.

[0042] Comparative Example 3 The preparation steps are basically the same as in Example 1, except that the 55 parts of epoxy-modified waterborne polyurethane resin in step (3) are replaced with an equal part by weight of ordinary waterborne polyurethane resin. The other steps are the same as in Example 1.

[0043] Comparative Example 4 The preparation steps are basically the same as in Example 1, except that the 3 parts of modified nano-silica (20nm) in step (2) are replaced with an equal part of modified silica (100nm). The other steps are the same as in Example 1.

[0044] Comparative Example 5 The difference from Example 1 is that no crosslinking agent is used, and unmodified nano-silica is used, while the remaining components and processes are the same as in Example 1.

[0045] Performance testing: Coating and Curing: Using a wire bar coater, the above coatings were evenly applied to a cleaned PET film with a thickness of 50 μm. The wet film thickness was controlled at 100 μm. The film was then transferred to an 80°C forced-air drying oven and dried for 2 minutes. After removal, it was cured for 24 hours under standard conditions of 23±2°C and 50±5% relative humidity for performance testing. The dry film thickness was approximately 15 μm.

[0046] Performance testing methods: Ink absorption speed: Following industry-standard methods, 4 μL of dye ink (black) was dropped onto the coating surface using a micro-syringe. The time required for the droplet to be completely absorbed by the coating and for the surface to become non-reflective was recorded using a stopwatch, accurate to 0.1 seconds. Five points were tested for each sample, and the average value was taken.

[0047] Abrasion resistance (anti-chalking): Using a Martindale abrasion tester at a pressure of 20N, with standard wool felt as the friction medium, pause every 50 cycles for inspection. Record the number of friction cycles at which visible powdery flakes first appear on the coating surface. If no chalking occurs after 500 cycles, record it as ">500".

[0048] Water resistance: The coating sample was completely immersed in distilled water at 25±1℃. After 72 hours, it was taken out and the surface moisture was gently absorbed with filter paper. The coating was immediately observed for bubbling, whitening, wrinkling, peeling and other phenomena. The adhesion was tested after 1 hour of recovery under standard conditions.

[0049] Adhesion: Refer to GB / T 9286-2021, use a single-blade cutter to draw a 1mm×1mm grid, apply 3M 610 tape, quickly pull it up vertically, and rate it according to the number of squares where the paint film peels off (0 is the best, 5 is the worst).

[0050] Performance retention rate after damp heat cycling: The coated samples were placed in a constant temperature and humidity test chamber and subjected to the following temperature and humidity cycling: 8 hours at 60±2℃ and 90±5% RH, followed by 16 hours at -10±2℃ and 30±5% RH, constituting one cycle (24 hours). After 10 consecutive cycles, the samples were removed and placed under standard conditions (23±2℃, 50±5% RH) for 24 hours to recover, and then their abrasion resistance was tested. The abrasion resistance retention rate was calculated as: (Number of abrasion cycles after cycling / Initial number of abrasion cycles) × 100%.

[0051] Coating appearance: Visually inspect the coating surface for uniformity, transparency, gloss, and whether there are defects such as particles and pinholes under a standard light source box (D65 light source).

[0052]

[0053] The coatings prepared in Examples 1-3 achieved anti-chalking properties (abrasion resistance >500 cycles), rapid ink absorption (1-2 seconds), excellent water resistance (no abnormalities after 72 hours), and adhesion (Grade 1). Comparative Example 1 demonstrates that polycarbodiimide crosslinking agents are indispensable for constructing a high-strength, highly water-resistant crosslinking network. Comparative Example 2 proves that surface chemical modification of fillers can effectively promote uniform dispersion and achieve interfacial bonding with the resin matrix. Comparative Example 3 demonstrates that epoxy modification plays a key role in improving resin reactivity, enhancing the overall performance of the coating, and improving interfacial adhesion.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A water-based polyurethane ink-absorbing coating with anti-chalking properties, characterized in that, By weight, it includes the following components: 45-60 parts of epoxy-modified waterborne polyurethane resin; 6-10 parts of polycarbodiimide crosslinking agent; 1-5 parts of inorganic filler; Film-forming aid 2.5-4 parts; 0.5-2 parts of wetting and leveling agent; Defoamer 0.05-0.2 parts; 20-30 parts deionized water.

2. The anti-chalking waterborne polyurethane ink-absorbing coating according to claim 1, characterized in that, The molecular weight of the polycarbodiimide crosslinking agent is 2000-4000 g / mol.

3. The anti-chalking waterborne polyurethane ink-absorbing coating according to claim 1, characterized in that, The inorganic filler is nano-silica with a particle size of 10-50 nm.

4. The anti-chalking waterborne polyurethane ink-absorbing coating according to claim 3, characterized in that, The surface of the nano-silica is modified by a silane coupling agent.

5. The anti-chalking waterborne polyurethane ink-absorbing coating according to claim 4, characterized in that, The silane coupling agent is an aminosilane coupling agent.

6. The anti-chalking waterborne polyurethane ink-absorbing coating according to claim 1, characterized in that, The weight ratio of the epoxy-modified waterborne polyurethane resin to the polycarbodiimide crosslinking agent is 5.0 to 9.0:

1.

7. A method for preparing an anti-chalking waterborne polyurethane ink-absorbing coating as described in any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Mix deionized water, film-forming aid and wetting leveling agent, and stir at 25-35℃ for 15-20 minutes; (2) Add inorganic filler to the system in step (1) and stir and disperse for 30-40 minutes; (3) Heat the system of step (2) to 65-75℃, add epoxy modified waterborne polyurethane resin, stir for 20-40 minutes, and then add polycarbodiimide crosslinking agent dropwise at a rate of 1-2 ml / min. (4) Continue the reaction at 65-75℃ for 60-90 minutes; (5) Cool the system to 25-30℃, add defoamer, and filter; During the preparation process from step (1) to step (4), the pH value of the system is maintained at 7.5-8.

5.

8. The preparation method according to claim 7, characterized in that, Ethanolamine was used to maintain and adjust the pH of the system.

9. The preparation method according to claim 7 or 8, characterized in that, In step (3), an organotin catalyst is added at a weight of 0.1%-0.5% of the epoxy-modified waterborne polyurethane resin.

10. The application of an anti-chalking waterborne polyurethane ink-absorbing coating as described in any one of claims 1 to 6, characterized in that, The anti-powdering water-based polyurethane ink-absorbing coating is coated onto the surface of substrates such as paper, plastic film, cloth, and metal, and then dried to form a dry coating, thus obtaining a digital inkjet printing substrate.

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