Photoresponsive UV-curable polyurethane resin composition and photoresponsive polyurethane coating

Abstract

The invention discloses a photoresponse type UV curing polyurethane resin composition and a photoresponse type polyurethane coating, the composition comprises the following components by mass: 20-40 parts of isocyanate, the isocyanate comprises at least one of isophorone diisocyanate IPDI and toluene diisocyanate TDI, the photoresponse type UV curing polyurethane resin composition comprises a photoresponse type UV curing agent, and the photoresponse type polyurethane coating comprises a photoresponse type UV curing agent, a photoresponse type UV curing agent, a photoresponse type UV curing agent and a photoresponse type UV curing agent. The polyhydric alcohol comprises diol and beta-thymidine, and the mole number of hydroxyl groups of the beta-thymidine accounts for 1%-20% of the total mole number of hydroxyl groups of the polyhydric alcohol; 15 to 30 parts of an end-capping reagent; and 1-5 parts of a photoinitiator. The invention aims to prepare the polyurethane resin which can be subjected to performance adjustment after being cured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high molecular materials, and particularly relates to a light-responsive UV-cured polyurethane resin composition and a light-responsive polyurethane coating. BACKGROUND

[0002] UV-cured polyurethane materials are widely used in the fields of coatings, adhesives, inks and the like due to their advantages of fast curing speed, low energy consumption and small environmental pollution. Traditional UV-cured polyurethane is usually prepared by the reaction of isocyanate and polyol, and then the introduction of an acrylate group, and the curing process is mainly achieved by the free radical polymerization of the acrylate group initiated by ultraviolet light. However, the performance of conventional UV-cured polyurethane coating is usually fixed after curing and is difficult to be adjusted according to actual needs. The existing patent with the patent number CN117551237A discloses a high-thermal-conductivity high-hardness UV polyurethane composition, a preparation method thereof and a protective coating, which adjusts the hardness by adjusting the adding content of resin and graphite phase iron-doped carbon nitride, and cannot realize the hardness adjustment after curing, and thus there is an urgent need for a polyurethane resin material which can realize the performance adjustment after curing. SUMMARY

[0003] The main purpose of the present application is to provide a light-responsive UV-cured polyurethane resin composition and a light-responsive polyurethane coating, and to solve the technical problem that the performance of the prepared polyurethane resin cannot be adjusted after curing.

[0004] To achieve the above-mentioned purpose, the light-responsive UV-cured polyurethane resin of the present application comprises the following components by mass:

[0005] Isocyanate: 20-40 parts, the isocyanate comprising at least one of isophorone diisocyanate IPDI and toluene diisocyanate TDI;

[0006] Polyol: 30-60 parts, the polyol comprising diol and beta-thymidine, and the molar number of hydroxyl groups of the beta-thymidine accounting for 1%-20% of the total molar number of hydroxyl groups of the polyol;

[0007] Blocking agent: 15-30 parts;

[0008] Photoinitiator: 1-5 parts.

[0009] The preparation method thereof comprises the following steps:

[0010] Step 1, reacting isocyanate and polyol at 60-80 DEG C for 2-4 hours to obtain a prepolymer;

[0011] Step 2, adding a blocking agent to the prepolymer obtained in step 1, and reacting at 70-90 DEG C for 1-3 hours to introduce UV-curable acrylate groups;

[0012] Step 3, cool down the reactants in step 2 to 40-50℃, then add the photoinitiator, mix well, to obtain the photoresponsive UV-curable polyurethane resin composition.

[0013] Optionally, the diol includes at least one of polyester diol, polyether diol, 1,6-hexanediol, 1,4-butanediol.

[0014] Optionally, the polyester diol, polyether diol has a number average molecular weight of 500-3000 g / mol.

[0015] Optionally, the molar ratio of NCO groups of the isocyanate in step 1 to OH groups of the polyol is (1.5-2.5) : 1.

[0016] Optionally, the molar ratio of OH groups of the end-capping agent in step 2 to unreacted NCO groups in the prepolymer prepared in step 1 is 1 : (0.9-1.0).

[0017] Optionally, the end-capping agent is one or more of hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate.

[0018] Optionally, a catalyst is also added in step 1, the catalyst is dibutyltin dilaurate, and the addition amount is 0.01%-0.1% of the mass of the isocyanate.

[0019] In addition, in order to achieve the above-mentioned purpose, the present application also provides a photoresponsive polyurethane coating, which is formed by coating and ultraviolet curing the photoresponsive UV-curable polyurethane resin as described in any one of the above; the pencil hardness of the coating can be adjusted, and the adjustable range of the pencil hardness is 2H to 6H, and the pencil hardness is adjusted by applying different doses of ultraviolet light irradiation.

[0020] In addition, in order to achieve the above-mentioned purpose, the present application also provides a performance adjustment method of the photoresponsive polyurethane coating as described above, which comprises the following steps:

[0021] Step 1 - coating and preliminary curing, coating the photoresponsive UV-curable polyurethane resin composition as described in any one of the above on the surface of the substrate, and performing preliminary curing with ultraviolet light with a wavelength of 365-380 nm, the irradiation energy is 80-150 mJ / cm², to form a coating with an initial hardness;

[0022] Step 2 - Performance Activation: Based on the target performance requirements, the pre-cured coating is subjected to secondary irradiation with ultraviolet light of wavelength 254nm-266nm and irradiation energy of 200-600mJ / cm² to activate the photochemical coupling reaction of β-thymidine and improve the coating hardness.

[0023] Optionally, the energy of the secondary irradiation is positively correlated with the increase in coating hardness.

[0024] Furthermore, in order to achieve the above objectives, the present invention also provides the application of the above-described photoresponsive polyurethane coating in the preparation of protective coatings or decorative coatings with adjustable properties.

[0025] Beneficial effects:

[0026] (1) β-thymidine was introduced as a green and efficient photoresponsive unit. The hydroxyl groups in its molecule can directly react with isocyanates and are perfectly embedded in the polyurethane backbone, solving the problems of complex synthesis and poor matrix compatibility of traditional photoresponsive groups. Compared with traditional UV-cured polyurethane, the polyurethane resin obtained overcomes the disadvantage of fixed performance in a single step. Users can precisely and gradiently set the hardness (2H-6H) of the same coating by simply adjusting the dose of secondary ultraviolet light according to the final application scenario, realizing the "dynamic adjustability" and "performance customization" of the coating performance.

[0027] (2) The resin synthesis method of the present invention is only a conventional two-step polyurethane synthesis reaction, without the need for complex synthesis steps or special equipment, and is easy to realize industrial production. Its coating and curing process is fully compatible with existing UV curing processes, without the need to modify existing production lines, and the technology transfer threshold is low.

[0028] (3) It expands the application scenarios of UV coating in high-end and intelligent fields. Specifically, for plastics or automotive paint, it can be cured first to form a coating with good toughness (impact resistance), and then the surface can be hardened by light before use (scratch resistance). Theoretically, it can be patterned by light irradiation through a mask to create areas with different hardness on the same coating, or used for information storage. It can be used as a resin for 3D printing. The printed object can change the local hardness and shape under specific light stimulation to achieve four-dimensional (space + time) deformation or functional change.

[0029] (4) Downstream application manufacturers can first use the same coating and then meet the differentiated performance requirements of different customers or different batches of products through simple post-processing (lighting), which simplifies inventory management and production process and greatly enhances the added value and market competitiveness of products. Attached Figure Description

[0030] Figure 1This is a schematic flowchart of an embodiment of a method for preparing a photoresponsive UV-curable polyurethane resin composition according to the present invention.

[0031] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0032] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0033] See Figure 1 This invention provides a method for preparing a photoresponsive UV-curable polyurethane resin composition, the method comprising the following steps:

[0034] Step 1 - Prepolymerization reaction: Isocyanate and polyol are reacted at 60-80°C for 2-4 hours to obtain a prepolymer; wherein the isocyanate includes at least one of isophorone diisocyanate (IPDI) and toluene diisocyanate (TDI), and the polyol comprises diol and β-thymidine.

[0035] The specific reaction mechanism in step 1 involves the addition reaction between isocyanate (-NCO) and the hydroxyl groups (-OH) of the glycol to form urethane bonds (-NH-COO-), creating the basic framework of the polyurethane. This determines the basic mechanical properties (such as toughness and strength) of the subsequent resin coating. Simultaneously, the hydroxyl groups of β-thymidine account for 1%-20% of the total hydroxyl groups in the polyol. β-thymidine then partially replaces the glycol in the reaction with the isocyanate; that is, the hydroxyl groups on the β-thymidine molecule also react with the isocyanate, causing them to be firmly attached to the polymer backbone through chemical bonds, rather than simply being physically mixed. This chemical bonding ensures that β-thymidine is uniformly distributed in the material and does not migrate or precipitate, guaranteeing the stability and repeatability of the photosensitivity effect. The roles of β-thymidine include: firstly, participating in chain building as a polyol; and secondly, acting as a "photoresponse switch" for subsequent performance regulation. Based on the photochemical coupling reaction that β-thymidine can undergo under light conditions, the performance of the subsequent resin coating can be regulated through light exposure, particularly in terms of adjustable hardness and transparency.

[0036] Furthermore, by controlling the molar ratio of the NCO groups of the isocyanate component to the total OH groups of the polyol and capping agent components to (1.5-2.5):1, some isocyanate functional groups are retained, ensuring that the ends of the prepolymer molecules generated after the reaction are unreacted -NCO groups, thus preparing for the next capping reaction.

[0037] Step 2 - End-capping reaction: An end-capping agent is added to the prepolymer obtained in Step 1, and the reaction is carried out at 70-90°C for 1-3 hours to introduce UV-curable acrylate groups. End-capping is achieved by reacting the hydroxyl groups on the end-capping agent with the -NCO groups at the ends of the prepolymer, thereby grafting (meth)acrylate double bonds onto them. In the presence of a photoinitiator, this can be excited by ultraviolet light to undergo a free radical polymerization reaction, causing the linear prepolymer molecules to crosslink into a three-dimensional network structure, thus achieving rapid curing from liquid resin to solid coating, and ultimately realizing photocurable properties. This reaction only reacts with the -NCO groups at the ends of the prepolymer and does not affect the β-thymidine already embedded in the main chain, effectively protecting the photoresponsive units. Preferably, the molar ratio of the end-capping agent to the NCO groups of the isocyanate is 1:(0.9-1.0). More preferably, the capping agent is one or more of hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, pentaerythritol triacrylate, and trimethylolpropane triacrylate.

[0038] Step 3 involves cooling the reactants from Step 2 to 40-50°C, then adding the photoinitiator and mixing thoroughly to obtain a photoresponsive UV-curable polyurethane resin. The photoinitiator is heat-sensitive; adding it at high temperatures may cause premature decomposition or side reactions, leading to ineffectiveness or gelation during storage. Adding it at a lower temperature (40-50°C) ensures the product's storage stability. The photoinitiator, LKK, absorbs UV light energy and generates free radicals, thereby initiating the polymerization and crosslinking reaction of the acrylate double bonds introduced in Step 2. This step does not involve a chemical reaction; it only ensures the photoinitiator is uniformly dispersed in the resin system to guarantee the uniformity of the curing process. Preferably, the photoinitiator includes one or more of 1173, 184, and TPO.

[0039] The effective combination of the above three steps yields a smart material that can be rapidly UV cured and molded, and whose properties can be further regulated by irradiation with specific wavelengths of light after curing.

[0040] Furthermore, a catalyst, specifically dibutyltin dilaurate, is added in step 1. While the reaction between isocyanate and hydroxyl groups can occur naturally, the reaction rate is very slow without a catalyst and requires higher temperatures (typically >100°C, which leads to a series of problems: high energy consumption, easy yellowing, and exacerbated side reactions). Therefore, by using a catalyst, the activation energy of the reaction is lowered, allowing it to proceed at moderate temperatures (e.g., 60-80°C). Given the reactive nature of isocyanate, in addition to the main reaction with hydroxyl groups, various side reactions may occur, such as: reaction with water to form urea and release carbon dioxide (leading to bubbles in the resin); formation of biuret; and self-polymerization to form trimers. Therefore, selecting a suitable catalyst can preferentially and efficiently catalyze the main reaction, namely the addition reaction between isocyanate and hydroxyl groups. This makes the reaction process more specific, with fewer byproducts, thus ensuring the controllability of the prepolymer molecular weight and the uniformity of product quality, and avoiding problems such as bubbles and gelation.

[0041] Meanwhile, the synthesis strategy of this invention relies on a slight excess of isocyanate to generate a prepolymer with -NCO terminals. If the reaction is incomplete, a large amount of unreacted hydroxyl groups and isocyanate will remain in the system, leading to a disordered molecular chain structure and making it impossible to precisely control the terminal groups and molecular weight of the prepolymer. Therefore, a catalyst is used to ensure that the reaction between isocyanate and hydroxyl groups can proceed efficiently and fully within a predetermined time and temperature, achieving complete consumption of the hydroxyl groups of the polyol. This lays a solid foundation for the subsequent end-capping reaction with hydroxyethyl acrylate on the prepolymer chain.

[0042] Furthermore, too low a catalyst dosage results in insufficient catalytic effect; too high a dosage leads to over-catalysis, potentially causing the reaction to be too rapid, with concentrated exothermic reactions leading to runaway reactions, or even affecting the regularity of the molecular chains due to excessively fast reaction rates, or causing residual catalyst to affect the later performance of the product. This invention controls the addition of dibutyltin dilaurate to 0.01%-0.1% of the isocyanate mass, achieving significant effects with minimal dosage, demonstrating its high efficiency.

[0043] Furthermore, the photoresponsive UV-curable polyurethane resin composition comprises, by weight: 20-40 parts isocyanate, 30-60 parts polyol, 15-30 parts end-capping agent, and 1-5 parts photoinitiator.

[0044] Further, the diol includes at least one of polyester diol, polyether diol, 1,6-hexanediol, and 1,4-butanediol; preferably, the number average molecular weight of the polyester diol and polyether diol is 500-3000 g / mol.

[0045] Furthermore, the present invention also provides a photoresponsive polyurethane coating, wherein the coating is formed by coating and curing the photoresponsive UV-curable polyurethane resin composition with ultraviolet light; the pencil hardness of the coating is adjustable, and the adjustable range of the pencil hardness is 2H to 6H, and the pencil hardness is controlled by applying different doses of ultraviolet light irradiation.

[0046] The specific steps in the coating preparation and initial curing stage include:

[0047] Step 1: Substrate pretreatment; the purpose is to ensure good adhesion between the coating and the substrate. Then, depending on the substrate type (e.g., plastic, metal, glass, wood), clean and dry it, and if necessary, perform physical sanding or chemical priming to remove contaminants and improve surface energy.

[0048] Step Two: Coating. The purpose is to uniformly apply the resin composition to the substrate surface to form a smooth wet film. Various methods can be used, such as blade coating, spin coating, spray coating, and roller coating. The appropriate parameters are controlled according to actual needs to ensure the wet film thickness, which in turn determines the final dry film thickness of the coating.

[0049] Step 3: Preliminary UV curing. The purpose is to transform the coating from a liquid to a solid state, thereby acquiring basic mechanical properties. Specifically, preliminary curing is performed using ultraviolet light with a wavelength of 365nm-380nm. This wavelength is efficiently absorbed by the photoinitiator to initiate the polymerization reaction of the acrylate double bonds. The irradiation energy is 80-150mJ / cm², which is sufficient to ensure that the acrylate double bonds are fully cross-linked, forming a stable three-dimensional network and creating a coating with initial hardness, such as 2H. In this step, β-thymidine basically does not react.

[0050] Furthermore, the properties of the pre-cured coating are adjusted, specifically as follows:

[0051] Step Four: Performance Activation. Specific wavelength ultraviolet light irradiation is used to stimulate a photochemical reaction in the initially cured coating, thereby actively improving the coating hardness. Specifically, a secondary irradiation with ultraviolet light at a wavelength of 254nm-266nm and an energy of 200-600mJ / cm² is used to activate the photochemical coupling reaction of β-thymidine and enhance coating hardness. This 254nm±10nm ultraviolet light is effectively absorbed by β-thymidine, stimulating it to undergo an intermolecular [2+2] cycloaddition photocoupling reaction. The energy range of 200-600mJ / cm² controls the degree of photochemical reaction. Higher energy results in more β-thymidine coupling reactions, a denser density of new cross-linking points, and a more significant increase in coating hardness. Therefore, the energy of the secondary irradiation is positively correlated with the increase in coating hardness.

[0052] Furthermore, if the application scenario does not require high hardness, the coating can be used directly after step three. However, if increased hardness is required, i.e., the application scenario demands high hardness, a second irradiation is performed in step four. By precisely controlling the irradiation dose, the coating hardness can be accurately increased from the initial 2H to 3H, 4H, 5H, or even 6H, achieving on-demand customization.

[0053] The photoresponsive polyurethane coating produced by this invention separates the traditional "one-time" UV curing process into two time-separable steps: "curing" and "performance curing." Simultaneously, different wavelengths of light selectively trigger different chemical reactions within the coating (acrylate polymerization vs. β-thymidine coupling), without interference between the two processes. The final coating properties (such as hardness) are not a fixed value, but rather a variable that can be adjusted through subsequent "light instructions" (the dose of secondary irradiation).

[0054] Furthermore, in order to achieve the above objectives, the present invention also provides the application of the above-described photoresponsive polyurethane coating in the preparation of protective coatings or decorative coatings with adjustable properties.

[0055] Furthermore, to better illustrate the effect of the photoresponsive UV-curable polyurethane resin composition of the present invention, specific examples are described below.

[0056] Example 1

[0057] Preparation of a photoresponsive UV-curable polyurethane resin composition:

[0058] (1) Mix IPDI (1 mol) with polyol components. The total amount of hydroxyl groups in the polyol is 1.8 mol, including hexanediol (1.62 mol of hydroxyl groups) and β-thymidine (0.18 mol of hydroxyl groups). Add dibutyltin dilaurate (0.05% of the mass of IPDI) and react at 70°C for 3 hours.

[0059] (2) Add 1.8 mol of hydroxyethyl acrylate to the reaction system of step (1) and react at 80°C for 2 hours;

[0060] (3) After cooling to 40°C, add photoinitiator 1173 (3% of the total mass of the system) and mix evenly to obtain the final product.

[0061] Comparative Example 1

[0062] To highlight the key role of β-thymidine, a resin composition identical to that in Example 1 was prepared, but β-thymidine was completely replaced by an equimolar amount of hexanediol (i.e., the amount of hydroxyl groups in hexanediol was 1.8 mol), while the other raw materials, amounts, and preparation processes remained unchanged.

[0063] Performance testing and characterization

[0064] The resins obtained in Example 1 and Comparative Example 1 were respectively coated onto the surface of a glass substrate and initially cured using 365nm ultraviolet light at an irradiation energy of 100mJ / cm². Then, they were irradiated a second time using 254nm ultraviolet light at irradiation energies of 200mJ / cm², 400mJ / cm², and 600mJ / cm², respectively. The cured coatings underwent the following performance tests:

[0065] Pencil hardness: Tested according to GB / T 6739-2006 standard.

[0066] Transmittance and haze: The transmittance (%) and haze (%) of the coating in the 400-800 nm visible light region were measured using a UV-Vis spectrophotometer equipped with an integrating sphere.

[0067] Adhesion: Tested using the cross-cut adhesion test according to GB / T 9286-2021 standard.

[0068] Solvent resistance: The number of wiping cycles was recorded when the coating showed obvious wear or the substrate was exposed, using the methyl ethyl ketone (MEK) wiping method.

[0069] The performance indicators of Example 1 and Comparative Example 1 are shown in Table 1.

[0070]

[0071] As shown in Table 1, under the same primary and secondary light irradiation, in Example 1 using β-thymidine polyol, as the secondary irradiation energy increased from 200 to 600, the pencil hardness of the prepared polyurethane resin composition increased from 2H to 4H and 6H. It can be seen that the hardness increase is positively correlated with the secondary irradiation energy; the higher the energy, the higher the hardness. In contrast, under the same treatment conditions, the hardness of Comparative Example 1 (without β-thymidine) remained at B without any change. Therefore, the traditional polyurethane resin without β-thymidine does not have the ability to adjust the performance again by light irradiation after curing as in Example 1.

[0072] Regarding light transmittance: Example 1 (92.1%-93.5%) > Comparative Example 1 (90.3%-91.1%), the coating obtained by introducing β-thymidine to replace the diol showed a significant increase in light transmittance. The optical transparency of a material depends on the loss of light passing through it, mainly including scattering loss and absorption loss. The mechanism for suppressing scattering loss is that β-thymidine is not simply physically blended, but rather chemically grafted onto the polyurethane backbone via a covalent bond through the reaction of its hydroxyl group (-OH) with isocyanate (-NCO). This chemical bonding fundamentally ensures perfect molecular-level compatibility between β-thymidine and the polyurethane matrix, avoiding optical phase separation caused by poor compatibility, thereby eliminating scattering caused by the phase interface. Furthermore, the [2+2] cycloaddition reaction of β-thymidine under secondary illumination adds new covalent crosslinking points within the already formed cured network. This reaction is a precise intermolecular coupling and does not produce macroscopic defects such as bubbles or micropores. Therefore, the newly added crosslinking points further enhance the rigidity of the network without introducing new scattering sources. Furthermore, the UV absorption characteristic peak of β-thymidine is mainly around 260-270 nm. However, for the visible light region (400-800 nm), the β-thymidine molecule structure lacks strong absorbing groups. Therefore, when it is embedded in the polymer, it does not significantly absorb visible light, ensuring the high transmittance of the substrate. As shown in the example data, with the increase of the secondary irradiation energy at 254 nm, the transmittance of the coating actually slightly increases (from 92.1% to 93.5%). This is because the high-energy short-wave ultraviolet light acts as a "photocleaner," degrading residual trace amounts of photoinitiator fragments or incompletely reacted small molecules, which are usually weak absorption sources in the visible light region. Degrading these substances further reduces absorption loss. Furthermore, regarding haze, the haze value of Example 1 (0.5-1.0) is significantly greater than that of the corresponding Example 1 (0.4-0.5). High light transmittance does not necessarily mean the material is "clear." Haze is an important indicator of the clarity of transparent materials. Low haze means that light mainly undergoes forward scattering after passing through the material, resulting in less image distortion. The coating obtained by this invention has extremely low haze (<1%), which directly confirms the above analysis regarding scatter suppression. Due to its highly uniform internal structure, lack of phase separation, and absence of inorganic particles, light undergoes almost no directional scattering (i.e., Rayleigh scattering or Mie scattering) when passing through it, and most of the light propagates in a straight line, thus achieving a perfect combination of high light transmittance and low haze.

[0073] Regarding adhesion: Example 1 (Grade 0) is superior to Comparative Example 1 (Grade 1); regarding solvent resistance: Example 1 (>100 wipes) is significantly superior to Comparative Example 1 (<90 wipes). This indicates that the chemical bonding of β-thymidine not only brings photoresponsiveness but may also enhance the density of the polymer network and the interfacial bonding with the substrate through its rigid molecular structure and the formation of additional cross-linking points, thereby comprehensively improving the mechanical strength, durability, and optical transparency of the coating. In other words, the coating obtained by introducing β-thymidine to replace diol exhibits significantly increased light transmittance, adhesion, and solvent resistance. Therefore, by introducing β-thymidine to replace part of the diol, various properties of the original resin composition are effectively improved.

[0074] Example 2

[0075] Preparation of a photoresponsive UV-curable polyurethane resin composition:

[0076] (1) Mix TDI (1 mol) with a polyol component (total hydroxyl content of 0.68 mol), the polyol component being polyethylene glycol-400 (hydroxyl content of 0.612 mol) and β-thymidine (hydroxyl content of 0.068 mol), add dibutyltin dilaurate (0.03% of the mass of TDI), and react at 65°C for 3.5 hours;

[0077] (2) Add 0.646 mol of hydroxypropyl acrylate to the reaction system of step (1) and react at 75°C for 2.5 hours;

[0078] (3) After cooling to 40°C, add photoinitiator TPO (accounting for 2.5% of the total mass of the system) and mix evenly to obtain a photoresponsive UV-curable polyurethane resin composition.

[0079] The aforementioned resin was coated onto the surface of a plastic substrate and initially cured using 380nm ultraviolet light at an energy of 120mJ / cm². A second curing was then performed using 266nm ultraviolet light at an energy of 400mJ / cm². Test results showed that the coating's pencil hardness increased from 2H after initial curing to 5H, with a light transmittance of 92.8%, haze of 0.6%, adhesion grade 0, and solvent resistance (number of wipes) >100, meeting the requirements for a high-hardness protective coating.

[0080] Example 3

[0081] Preparation of a photoresponsive UV-curable polyurethane resin composition:

[0082] (1) A mixture of IPDI and TDI (molar ratio 1:1, total 1 mol) was mixed with a polyol component (total hydroxyl content 0.61 mol), the polyol component being polytetrahydrofuran diol (PTMG-1000, molecular weight 1000, molar content 0.29 mol, hydroxyl content 0.58 mol) and β-thymidine (hydroxyl content 0.0305 mol), and dibutyltin dilaurate (0.08% of the total mass of the isocyanate component) was added. The mixture was reacted at 75 °C for 2.5 hours.

[0083] (2) Add 0.64 mol of hydroxyethyl methacrylate to the reaction system of step (1) and react at 85°C for 1.5 hours;

[0084] (3) After cooling to 40°C, add photoinitiator 184 (accounting for 3.5% of the total mass of the system) and mix evenly to obtain a photoresponsive UV-curable polyurethane resin composition.

[0085] The aforementioned resin was coated onto the surface of a metal substrate and initially cured using 365nm ultraviolet light at an energy of 150mJ / cm². A second curing was then performed using 254nm ultraviolet light at an energy of 600mJ / cm². Test results showed that the coating's pencil hardness increased from 3H after initial curing to 6H, with a light transmittance of 92.8%, haze of 0.7%, adhesion grade 0, and solvent resistance (number of wipes) >100, meeting the requirements for a high-hardness protective coating.

[0086] Example 4

[0087] Preparation of a photoresponsive UV-curable polyurethane resin composition:

[0088] (1) Mix IPDI (1 mol) with a polyol component (total hydroxyl content of 1.34 mol), the polyol component being polybutylene adipate diol (PBA-500, molecular weight 500, molar content of 0.637 mol, hydroxyl content of 1.274 mol) and β-thymidine (hydroxyl content of 0.067 mol), add dibutyltin dilaurate (0.01% of the mass of IPDI), and react at 60℃ for 4 hours;

[0089] (2) Add 1.273 mol of hydroxyethyl acrylate to the reaction system of step (1) and react at 70°C for 3 hours;

[0090] (3) After cooling to 40℃, add photoinitiator 1173 (accounting for 2% of the total mass of the system), and mix evenly to obtain a photoresponsive UV-curable polyurethane resin composition. The above resin is coated onto the surface of a wood substrate and initially cured using 380nm ultraviolet light with an irradiation energy of 80mJ / cm²; then, it is irradiated a second time using 266nm ultraviolet light with an irradiation energy of 200mJ / cm². Test results show that the pencil hardness of the coating increases from 2H after initial curing to 4H, the light transmittance is 95.1%, the haze is 0.5%, the adhesion is grade 0, and the solvent resistance (number of wipes) is >100, making it suitable for use as a high-transparency decorative hard coating.

[0091] Examples 2, 3, and 4 above used different isocyanates (TDI, IPDI / TDI mixtures), different polyols (polyether glycol, PTMG), different end-capping agents (hydroxypropyl acrylate, hydroxyethyl methacrylate), and photoinitiators, but all successfully produced photoresponsive coatings with excellent performance. Therefore, it can be seen that the technical solution of this invention does not depend on any specific raw material, and its core principle (introduction of β-thymidine) is applicable to various polyurethane systems, possessing good versatility and industrialization potential.

[0092] Furthermore, the above embodiments all demonstrate a two-stage process: the first stage involves completing the shaping and curing process to obtain an initial coating with usable value; the second stage involves "upgrading" or "setting" the performance according to requirements. It is evident that this invention overcomes the limitation of traditional UV-cured materials where "performance is fixed in one go," creating a smart material whose performance can be "reprocessed." This provides a completely new material platform for cutting-edge applications such as 4D printing, adaptive coatings, and rewritable optical devices.

[0093] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or system that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or system. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or system that includes that element.

[0094] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0095] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A photoresponsive UV-curable polyurethane resin composition, characterized in that, The composition comprises an acryloyloxy-terminated polyurethane prepolymer generated by reacting the following components: Isocyanate: 20-40 parts; Polyol: 30-60 parts, wherein the polyol comprises diol and β-thymidine, and the molar number of hydroxyl groups of the β-thymidine accounts for 1%-20% of the total molar number of hydroxyl groups in the polyol; Capping agent: 15-30 parts; Photoinitiator: 1-5 parts; Its preparation method includes the following steps: Step 1: React isocyanate and polyol at 60-80℃ for 2-4 hours to obtain prepolymer; Step 2: Add a capping agent to the prepolymer obtained in Step 1 and react at 70-90°C for 1-3 hours to introduce UV-curable acrylate groups. Step 3: Cool the reactants from Step 2 to 40-50°C, then add the photoinitiator and mix thoroughly to obtain a photoresponsive UV-curable polyurethane resin composition.

2. The photoresponsive UV-curable polyurethane resin composition according to claim 1, characterized in that, The diol includes at least one of polyester diol, polyether diol, 1,6-hexanediol, and 1,4-butanediol; the polyester diol and polyether diol have a number average molecular weight of 500-3000 g / mol.

3. The photoresponsive UV-curable polyurethane resin composition according to claim 1, characterized in that, In step 1, the molar ratio of the NCO group of the isocyanate to the OH group of the polyol is (1.5-2.5):

1.

4. The photoresponsive UV-curable polyurethane resin composition according to claim 1, characterized in that, In step 2, the molar ratio of the OH group of the capping agent to the unreacted NCO group in the prepolymer prepared in step 1 is 1: (0.9-1.0).

5. The photoresponsive UV-curable polyurethane resin composition according to claim 1, characterized in that, The capping agent is one or more of hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, pentaerythritol triacrylate, and trimethylolpropane triacrylate.

6. The photoresponsive UV-curable polyurethane resin composition according to claim 1, characterized in that, In step (1), a catalyst is also added, which is dibutyltin dilaurate, and the amount added is 0.01%-0.1% of the mass of the isocyanate.

7. A photoresponsive polyurethane coating, characterized in that, The coating is formed by coating and curing with ultraviolet light using the photoresponsive UV-curable polyurethane resin composition according to any one of claims 1-6; the pencil hardness of the coating is adjustable, and the adjustable range of pencil hardness is 2H to 6H, which is achieved by applying different doses of ultraviolet light irradiation.

8. A method for performance control of a photoresponsive polyurethane coating as described in claim 7, characterized in that, Includes the following steps: Step 1 - Coating and Preliminary Curing: The photoresponsive UV-curable polyurethane resin composition according to any one of claims 1-6 is coated onto the surface of the substrate, and preliminary curing is performed using ultraviolet light with a wavelength of 365nm-380nm and an irradiation energy of 80-150mJ / cm² to form a coating with initial hardness. Step 2 - Performance Activation: Based on the target performance requirements, the pre-cured coating is subjected to secondary irradiation with ultraviolet light of wavelength 254nm-266nm and irradiation energy of 200-600mJ / cm² to activate the photochemical coupling reaction of β-thymidine and improve the hardness of the coating.

9. The performance regulation method as described in claim 8, characterized in that, The energy of the secondary irradiation is positively correlated with the increase in coating hardness.

10. The application of the photoresponsive polyurethane coating of claim 7 in the preparation of protective coatings or decorative coatings with adjustable properties.

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