Injection-molded part thin-wall spray paint and preparation method thereof

Abstract

The invention relates to the technical field of paint, in particular to injection-molded part thin-wall spray paint and a preparation method thereof. The problems of low low-temperature curing speed and poor mechanical property of the existing spray paint are solved. The preparation method comprises the following steps: by taking polyurethane as a base material, introducing a polyether-polyester block copolymer into a polyurethane molecular chain for modification, and modifying nano silicon dioxide by using a silane coupling agent, so as to prepare a special microcapsule antioxidant; the raw materials are mixed according to a specific ratio to form a spraying coating, and after electrostatic spraying, UV curing and thermocuring are combined. The method realizes rapid curing of paint spraying, improves mechanical properties such as paint adhesion, hardness, wear resistance and the like, and is suitable for preparation of thin-wall paint spraying of injection molded parts.

Description

Technical Field

[0001] The invention relates to the technical field of coatings, in particular to a thin-wall spray paint for injection molded parts and a preparation method thereof. Background Art

[0002] In the fields of electronic housings, automotive interiors, etc., the surface spraying of thin-walled injection molded products must meet the requirements of both lightweight and high decorativeness. However, due to the thin and easily deformed characteristics of the wall, traditional curing coatings often have problems such as poor adhesion and paint film cracking during application. Polyurethane-based coatings have attracted attention due to their advantages such as wear resistance and weather resistance, but the existing technology still has pain points such as high curing temperature, insufficient coating toughness, and complex processes, making it difficult to meet the industry's growing demand for high-quality production.

[0003] At present, the existing technology for spraying thin-walled injection molded parts has obvious defects. Commonly used epoxy / acrylic coatings need to be cured at a high temperature above 120°C, which can easily cause thin-walled parts to warp and deform; most coatings are difficult to balance flexibility and hardness, and cannot adapt to complex product usage scenarios; traditional UV curing equipment is expensive, and when processing deep-cavity injection molded parts, there is a problem of poor coating coverage, which affects the overall quality and performance of the product.

[0004] Faced with these current situations, the industry urgently needs to solve two key technical problems: one is how to achieve rapid curing of coatings under low temperature conditions to avoid deformation of thin-walled parts due to high temperature; the other is how to effectively improve the mechanical properties of polyurethane coatings and achieve coordinated optimization of adhesion, hardness and wear resistance.

[0005] Therefore, a thin-wall spray painting of injection molded parts and a preparation method thereof are proposed. Summary of the invention

[0006] The purpose of the present invention is to design a thin-wall spray paint for injection molded parts and a preparation method thereof. With polyurethane as the base material, in order to solve the problems of slow low-temperature curing and poor mechanical properties of spray paint, a special microcapsule antioxidant is prepared by introducing a polyether-polyester block copolymer into the polyurethane molecular chain, and modifying nano-silicon dioxide with a silane coupling agent; a spray paint is mixed in a specific ratio, and after electrostatic spraying, UV curing and thermal curing are combined. The method realizes rapid curing of the paint spray, and at the same time improves the mechanical properties of the spray paint, such as adhesion, hardness, and wear resistance, and is suitable for the preparation of thin-wall spray paint for injection molded parts.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] In one aspect, the present invention provides a method for preparing a thin-wall spray paint for an injection molded part, the method comprising the following steps:

[0009] S1 introduces polyether-polyester block copolymer into the polyurethane molecular chain to obtain modified polyurethane;

[0010] S2 uses the silane coupling agent KH-550 to modify nano-silica to obtain functional nano-silica;

[0011] S4 coats antioxidants and ultraviolet absorbers with microcapsules to obtain microcapsule antioxidants;

[0012] S4 mixes and stirs modified polyurethane, functional nano-silica, a leveling agent and microcapsule antioxidants to obtain Component A; mixes and homogenizes an isocyanate curing agent, a TPO-L photoinitiator, wear-resistant fillers and ethyl acetate to obtain Component B; mixes Component A and Component B to obtain a spray coating;

[0013] S5 electrostatically sprays the above spray coating, UV cures for 2 min - 6 min after spraying, and finally thermally cures at 60 °C for 15 min - 25 min to obtain a thin-walled spray painting for injection molded parts.

[0014] Preferably, by weight parts, the preparation method of the modified polyurethane is as follows: Add 95 - 105 parts of polyurethane prepolymer to a reaction kettle, heat up to 80 °C - 90 °C, stir until dissolved under nitrogen protection, then add 100 parts of polyester diol and 0.5 - 1 part of dibutyltin dilaurate, heat up to 120 °C - 140 °C, react for 1.5 h, when the acid value drops below 10% of the initial value, add polytetramethylene ether glycol, the molar ratio of polyester diol to polytetramethylene ether glycol is 2, continue to heat up to 160 °C - 180 °C, react for 4 h, stop the reaction when the hydroxyl value is close to the theoretical value, then cool down and add triphenyl phosphite, stir evenly and then vacuum dry to obtain the modified polyurethane; select a polyurethane prepolymer with hydroxyl groups at both ends of the molecular chain, and its number average molecular weight is controlled at 2000 - 3000 g / mol; the polyester diol is polyethylene adipate glycol.

[0015] Preferably, by weight parts, the preparation method of the functional nano-silica is as follows: Slowly add 90 - 100 parts of nano-silica to 150 parts of an ethanol aqueous solution, the weight ratio of ethanol to water is 8:1, perform ultrasonic dispersion for 30 min - 60 min to obtain a dispersion; Slowly add 1 - 5 parts of the silane coupling agent KH-550 to the dispersion, perform water bath stirring reaction at 60 °C - 80 °C for 3 h, after the reaction ends, filter and separate the solid product, wash the vacuum drying oven to obtain the functional nano-silica; the particle size of the nano-silica is usually 10 - 100 nm.

[0016] Preferably, by weight parts, the preparation method of the microcapsule antioxidant is as follows: Add 10-20 parts of antioxidant 1010 and 5-10 parts of ultraviolet absorber UV-326 into acetone, stir and dissolve in a water bath to form a core material solution; Add 25 parts of gelatin and 25 parts of gum arabic into 80 parts of distilled water respectively, stir and dissolve in a water bath at 55°C to form a gelatin solution and a gum arabic solution; Slowly add the core material solution into the gelatin solution, stir and emulsify at 55°C for 50 min to form an emulsion; Then slowly add the gum arabic solution into the emulsion, continue to stir for 15 min, adjust the pH value to 4, keep the temperature at 45°C, then slowly add 8 parts of formaldehyde, continue to react for 1 h-2 h, after the reaction is completed, adjust the pH value to neutral, separate the solid product, wash and dry in vacuum to obtain the microcapsule antioxidant; The wear-resistant filler is micron-sized aluminum oxide particles with a particle size of 1 μm-5 μm.

[0017] Preferably, by weight parts, the specific process of mixing in S4 is as follows: Add 35-45 parts of modified polyurethane into the reaction kettle, stir to make the modified polyurethane in a uniformly dispersed state; Slowly add 1-5 parts of functional nano-silica, continuously stir for 15 min-25 min to obtain a dispersion solution; Add 0.1-1 part of polydimethylsiloxane and 1-3 parts of microcapsule antioxidant in sequence, continue to stir for 15 min to obtain component A; Add 25-35 parts of toluene diisocyanate into another reaction kettle, slowly add 1-3 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO-L photoinitiator) under stirring conditions, add 5-10 parts of wear-resistant filler after stirring for 10 min, add 10 parts of ethyl acetate after uniformly dispersing, stir for 15 min to obtain component B; Finally, slowly mix component A and component B together, stir at 150 rpm-200 rpm for 20 min-30 min to obtain the spraying coating.

[0018] Preferably, before electrostatic spraying in S5, the thin-walled surface of the injection molded part is cleaned, and the speed of electrostatic spraying is 3 m / min-5 m / min.

[0019] On the other hand, the present invention provides a thin-walled spray paint for injection molded parts. The synthetic raw materials of the thin-walled spray paint for injection molded parts include: modified polyurethane, functional nano-silica, leveling agent, microcapsule antioxidant, isocyanate curing agent, TPO-L photoinitiator, wear-resistant filler and ethyl acetate; The modified polyurethane includes polyurethane prepolymer, polyether diol and polyester diol; The functional nano-silica includes nano-silica and silane coupling agent; The microcapsule antioxidant includes antioxidant and ultraviolet absorber.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1. The modified polyurethane obtained by introducing a polyether-polyester block copolymer into the polyurethane molecular chain combines the characteristics of polyether and polyester. The polyether segment has good flexibility and low surface energy, and can form a continuous and relatively dense network structure in the paint film, reducing the channels for water molecules to enter the interior of the paint film. During the spraying process, the siloxane groups in the leveling agent chemically react with the polyurethane molecules to form covalent bonds. This covalent bond bonding method effectively reduces the porosity of the paint spray and improves the density. At the same time, the leveling agent molecules are oriented on the surface of the paint film, reducing the surface tension of the paint film surface. The combination of the two optimizes the microstructure of the paint spray and improves the overall water resistance.

[0022] 2. Functional nano-silica has a high specific surface area and can form a large number of physical entanglements and chemical bonds with the modified polyurethane, enhancing the adhesion between the paint film and the substrate. At the same time, the photoinitiator TPO-L can achieve UV / thermal dual-triggered low-temperature curing. Under UV irradiation, TPO-L rapidly decomposes to generate free radicals, initiating the polymerization reaction. During the subsequent low-temperature thermal curing process, the remaining active groups continue to react to ensure the complete curing of the paint spray, solving the problem of low-temperature rapid curing. The synergistic effect of the two enables the paint film to be rapidly cured at low temperature and ensures that the cured paint film has good adhesion.

[0023] 3. The modified polyurethane provides the basis for film formation and flexibility, providing a platform for the functions of other components to play. Functional nano-silica has characteristics such as high hardness and high specific surface area, and can be filled into the voids between the polymer molecular chains, increasing the intermolecular interaction force, thereby improving the hardness of the paint film. And its interaction with the modified polyurethane helps to maintain the stability of the paint film. The isocyanate curing agent forms a three-dimensional network structure through cross-linking reaction with the modified polyurethane, tightly connecting each component together to form a solid whole. The wear-resistant filler can improve the hardness and wear resistance of the paint film, reduce the wear and scratches on the surface of the paint film, and further improve the comprehensive performance of the paint film.

[0024] 4. Introducing microcapsule antioxidants into the paint spray. During the use of the paint film, with the action of environmental factors, the microcapsules will slowly release antioxidants and ultraviolet absorbers. The antioxidant can inhibit the oxidation reaction of the paint film and delay the aging process of the paint film. The ultraviolet absorber can absorb the ultraviolet energy and reduce the damage of ultraviolet rays to the paint film. The wear-resistant filler can reduce the wear and scratches on the surface of the paint film. The synergistic effect of the two enables the paint film to resist mechanical actions such as external friction and chemical actions such as oxidation during long-term use, thereby enhancing the durability of the paint film. Description of the Drawings

[0025] Figure 1 It is the weather resistance performance diagram of Experimental Example 11 and Comparative Examples 11-15 in the present invention. Detailed implementation manners

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] Specifically refer to Figure 1 , the present invention provides an injection-molded part thin-wall painting and its preparation method, and the technical solution is as follows:

[0028] Example 1

[0029] S1 Add 100 parts of polyurethane prepolymer to the reaction kettle, heat up to 85°C, stir until dissolved under nitrogen protection, then add 100 parts of polyester diol and 0.8 part of dibutyltin dilaurate, heat up to 130°C, and react for 1.5 h. When the acid value is reduced to less than 10% of the initial value, add polytetramethylene ether glycol. The molar ratio of polyester diol to polytetramethylene ether glycol is 2. Continue to heat up to 170°C and react for 4 h. When the hydroxyl value is close to the theoretical value, stop the reaction. Then cool down and add triphenyl phosphite, stir evenly and dry under vacuum to obtain modified polyurethane;

[0030] S2 Slowly add 95 parts of nano-silica to 150 parts of ethanol aqueous solution. The weight ratio of ethanol to water is 8:1, and perform ultrasonic dispersion for 45 min to obtain a dispersion; Slowly add 3 parts of silane coupling agent KH-550 to the dispersion, and stir and react in a water bath at 70°C for 3 h. After the reaction ends, filter and separate the solid product, wash it in a vacuum drying oven to obtain functional nano-silica;

[0031] S3 Add 15 parts of antioxidant 1010 and 8 parts of ultraviolet absorber UV-326 to acetone, stir and dissolve in a water bath to form a core material solution; Add 25 parts of gelatin and 25 parts of gum arabic to 80 parts of distilled water respectively, stir and dissolve in a water bath at 55°C to form a gelatin solution and a gum arabic solution; Slowly add the core material solution to the gelatin solution, stir and emulsify at 55°C for 50 min to form an emulsion; Then slowly add the gum arabic solution to the emulsion, continue to stir for 15 min, adjust the pH value to 4, keep the temperature at 45°C, and then slowly add 8 parts of formaldehyde, continue to react for 1.5 h. After the reaction ends, adjust the pH value to neutral, separate the solid product, wash and dry under vacuum to obtain microcapsule antioxidant; The wear-resistant filler is micron-sized aluminum oxide particles with a particle size of 3 μm;

[0032] S4 Add 40 parts of modified polyurethane into the reaction kettle, and stir to make the modified polyurethane in a uniformly dispersed state; slowly add 3 parts of functional nano-silica, and continuously stir for 20 min to obtain a dispersion solution; sequentially add 0.5 part of polydimethylsiloxane and 2 parts of microcapsule antioxidant, and continue to stir for 15 min to obtain Component A; add 30 parts of toluene diisocyanate into another reaction kettle, slowly add 2 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide under stirring conditions, add 8 parts of wear-resistant filler after stirring for 10 min, add 10 parts of ethyl acetate after uniformly dispersing, and stir for 15 min to obtain Component B; finally, slowly mix Component A and Component B together, and stir at 180 rpm for 25 min to obtain the spray coating;

[0033] S5 Clean the surface of the thin wall of the injection molded part, and electrostatically spray the above spray coating at a speed of 4 m / min. After spraying, cure it by UV for 4 min, and finally cure it thermally at 60 °C for 20 min to obtain the painted thin wall of the injection molded part.

[0034] Example 2-12

[0035] Refer to the parameter conditions in Example 1, and the specific differences are shown in Table 1.

[0036] The parameter conditions of Example 1, Example 4, Example 7 and Example 10 are the same.

[0037] Table 1 Parameter conditions of Examples 1-12

[0038]

[0039] Comparative Example 1 Refer to the parameter conditions in Example 1, the difference is that only polyether diol is added during the polyurethane modification process.

[0040] Comparative Example 2 Refer to the parameter conditions in Example 1, the difference is that only polyester diol is added during the polyurethane modification process.

[0041] Comparative Example 3 Refer to the parameter conditions in Example 1, the difference is that only polyurethane is added and it is not modified.

[0042] Comparative Example 4 Refer to the parameter conditions in Example 1, the difference is that the leveling agent is not added.

[0043] Experimental Example 1 Water resistance test

[0044] Determine the water resistance of Examples 1-3 and Comparative Examples 1-4 according to Method A in GB / T 1733, and the obtained results are shown in Table 2.

[0045] Table 2 Water resistance test of Examples 1-3 and Comparative Examples 1-4

[0046] Example Water resistance for 168 h Example 1 No loss of gloss, discoloration, blistering, wrinkling, peeling Example 2 No loss of gloss, discoloration, blistering, wrinkling, peeling Example 3 No loss of gloss, discoloration, blistering, wrinkling, peeling Comparative Example 1 Slight loss of gloss, wrinkling Comparative Example 2 Slight wrinkling Comparative Example 3 Obvious loss of gloss, discoloration, slight blistering Comparative Example 4 Obvious loss of gloss, wrinkling

[0047] As can be seen from Table 2, the spray paint of the examples has good water resistance. In Comparative Example 1, only polyether diol was added during the polyurethane modification. Compared with the polyurethane modified with polyether-polyester block copolymer in the examples, its water resistance will decrease because the polyester segment has better polarity and regularity, which can improve the intermolecular force and the compactness of the spray paint. The lack of the polyester segment will make the structural integrity and compactness of the spray paint slightly worse. In Comparative Example 2, only polyester diol was added for polyurethane modification, lacking the polyether segment. Since the polyether segment has good flexibility and hydrolysis resistance, it can improve the toughness and water resistance of the spray paint. The lack of the polyether segment will lead to insufficient toughness of the spray paint and easily appear micro-cracks and other defects under long-term water immersion. In Comparative Example 3, the unmodified polyurethane has relatively poor performance, and its molecular chain regularity, flexibility, polarity, etc. are not as good as those of the modified polyurethane, and the compactness, toughness and water resistance of the paint film will be affected. In Comparative Example 4, the lack of the leveling agent will affect the leveling property of the coating on the surface of the injection-molded part, resulting in the surface of the paint film not being smooth enough and having many microscopic defects. Water is easy to penetrate into the interior of the paint film through these defects, thereby reducing the water resistance of the paint film.

[0048] In summary, introducing polyether-polyester block copolymer into the polyurethane molecular chain combines the flexibility and hydrolysis resistance of the polyether segment and the polarity, regularity and higher cohesive energy of the polyester segment. The modified polyurethane improves the water resistance from the internal structure of the spray paint, while the leveling agent starts from the surface state of the spray paint. The leveling agent makes the surface of the paint film smooth, provides better external protection for the dense structure formed by the modified polyurethane, and reduces the chance of external water contacting the interior of the paint film; the good internal structure formed by the modified polyurethane provides a stable foundation for the leveling agent to play its role. The two cooperate with each other to jointly improve the water resistance of the paint film, so that the paint film can still maintain good performance under long-term contact with water and does not show phenomena such as loss of gloss, color change, blistering, wrinkling, and peeling.

[0049] Comparative Example 5 refers to the parameter conditions in Example 4, with the difference that the nano-silica is not modified.

[0050] Comparative Example 6 refers to the parameter conditions in Example 4, with the difference that the functional nano-silica is not added.

[0051] Comparative Example 7 refers to the parameter conditions in Example 4, with the difference that the photoinitiator is not added.

[0052] Comparative Example 8 refers to the parameter conditions in Example 4, with the difference that UV curing is not carried out after spraying.

[0053] Comparative Example 9 refers to the parameter conditions in Example 4, with the difference that the cleaning treatment is not carried out before spraying.

[0054] Experimental Example 2 Adhesion Test

[0055] According to the standard of GB / T 9286-1998, the cross-cut method was used to conduct adhesion tests on Examples 4-6 and Comparative Examples 5-9. The higher the grade, the worse the adhesion. The obtained results are shown in Table 3.

[0056] Table 3 Adhesion Tests of Examples 4-6 and Comparative Examples 5-9

[0057] Example Adhesion / grade Example 4 0 Example 5 0 Example 6 0 Comparative Example 5 1 Comparative Example 6 2 Comparative Example 7 2 Comparative Example 8 2 Comparative Example 9 2

[0058] It can be found from Table 3 that the adhesion ability of the examples is very strong. In Comparative Example 5, the nano-silica is not modified. The binding force between the unmodified nano-silica and the matrix is relatively weak. In the adhesion test, due to its limited interaction with components such as modified polyurethane, the overall adhesion of the paint film decreases. In Comparative Example 6, the functional nano-silica is not added, lacking the strengthening effect of nano-silica on the paint spray. The cohesion of the paint film and the binding force with the matrix will be affected. Nano-silica can fill in the paint spray, increasing the density of the paint film and the contact area with the matrix. In Comparative Example 7, the photoinitiator is not added. The photoinitiator plays a key role in decomposing to generate free radicals to initiate the polymerization reaction during the UV curing process. Without the photoinitiator, the paint spray cannot be effectively cured, the molecular chains cannot be fully cross-linked, and the adhesion is poor. In Comparative Example 8, after spraying, UV curing is not carried out. Even if there is a photoinitiator, without UV irradiation, the paint spray cannot be fully cured, and the paint spray is in an uncured state, and the adhesion decreases significantly. In Comparative Example 9, the oil stain on the surface of the injection-molded part will form an isolation layer between the paint film and the surface of the injection-molded part, hindering the direct contact and combination between the paint film and the surface of the injection-molded part, resulting in a decrease in adhesion; the presence of dust and impurities will make the contact between the paint film and the surface of the injection-molded part not tight, reducing the effective contact area between the paint film and the surface of the injection-molded part, thereby reducing the adhesion.

[0059] In summary, for the functional nano-silica modified by silane coupling agent KH-550, its surface properties are improved, enabling better combination with components such as modified polyurethane; when the nano-silica is filled in the coating, on the one hand, it increases the hardness and wear resistance of the paint film, and on the other hand, it enhances the adhesion between the spray paint and the substrate, providing a good basic structure for the paint film. The photoinitiator TPO-L rapidly decomposes under UV irradiation to generate free radicals, initiating the polymerization reaction of components such as modified polyurethane and isocyanate curing agent, causing the molecular chains in the coating to crosslink and form a three-dimensional network structure. UV curing provides the energy condition for the decomposition of the photoinitiator, promoting the rapid progress of the polymerization reaction. Under the action of UV light, the free radicals generated by the photoinitiator can efficiently initiate the polymerization reaction, enabling the spray paint to cure in a short time and solving the problem of rapid curing at low temperature. The synergistic effect of functional nano-silica, photoinitiator, and UV curing not only improves the adhesion of the spray paint but also enhances the comprehensive properties such as the hardness and wear resistance of the spray paint.

[0060] Comparative Example 1 referred to the parameter conditions in Example 1, with the difference that only polyether diol was added during the polyurethane modification process.

[0061] Comparative Example 2 referred to the parameter conditions in Example 1, with the difference that only polyester diol was added during the polyurethane modification process.

[0062] Comparative Example 3 referred to the parameter conditions in Example 1, with the difference that only polyurethane was added without modification.

[0063] Comparative Example 5 referred to the parameter conditions in Example 4, with the difference that the nano-silica was not modified.

[0064] Comparative Example 6 referred to the parameter conditions in Example 4, with the difference that functional nano-silica was not added.

[0065] Comparative Example 10 referred to the parameter conditions in Example 7, with the difference that isocyanate curing agent was not added.

[0066] Comparative Example 11 referred to the parameter conditions in Example 7, with the difference that wear-resistant filler was not added.

[0067] Experimental Example 3 Hardness and Scratch Resistance Test

[0068] The hardness of Examples 7-9, Comparative Examples 1-3, Comparative Examples 5-6, and Comparative Examples 10-11 was tested using the pencil hardness method; the scratch resistance of Examples 7-9, Comparative Examples 1-3, Comparative Examples 5-6, and Comparative Examples 10-11 was tested using a five-finger scratch tester; the obtained results are shown in Table 4.

[0069] Table 4 Hardness and Scratch Resistance Test of Examples 7-9, Comparative Examples 1-3, Comparative Examples 5-6, and Comparative Examples 10-11

[0070] Example Hardness Scratch resistance Example 7 3H Good Example 8 3H Good Example 9 3H Good Comparative Example 1 HB Medium Comparative Example 2 1H Medium Comparative Example 3 2B Unqualified Comparative Example 5 1H Medium Comparative Example 6 2B Unqualified Comparative Example 10 1B Unqualified Comparative Example 11 1H Medium

[0071] As can be seen from Table 4, the examples have good hardness and scratch resistance. In Comparative Example 1, only polyether diol was added during the polyurethane modification. The molecular structure of polyether diol is relatively soft. When it modifies polyurethane alone, although it can endow the paint film with certain flexibility, its effect on improving hardness is limited. Therefore, the hardness of the paint film is relatively low and it is easily scratched. In Comparative Example 2, only polyester diol was added for polyurethane modification. Polyester diol has a certain rigidity and can improve the hardness of the paint film. However, compared with the polyurethane modified by polyether-polyester block copolymer, its comprehensive performance is not ideal. In Comparative Example 3, only unmodified polyurethane was added, and its own performance is relatively poor and the hardness is low. In this case, the paint film lacks sufficient strength and hardness to resist external scratching, has poor scratch resistance, and obvious scratches are likely to appear. In Comparative Example 5, the nano-silica was not modified. The dispersibility of unmodified nano-silica in the paint spray and its binding force with other components are not as good as those of the modified nano-silica. This will affect its enhancement effect on the hardness and scratch resistance of the paint film, resulting in a decrease in the hardness of the paint film and general scratch resistance. In Comparative Example 6, functional nano-silica was not added. Without the enhancement of nano-silica, the hardness and compactness of the paint film will be affected, and the scratch resistance is poor, and it is more likely to be damaged when scratched. In Comparative Example 10, isocyanate curing agent was not added. Isocyanate curing agent plays a key role in the formation of the paint film. It can undergo cross-linking reactions with components such as modified polyurethane to form a three-dimensional network structure, thereby improving the hardness and strength of the paint film. Without isocyanate curing agent, the paint film cannot form an effective cross-linked structure, has poor hardness, and very poor scratch resistance, and hardly has the ability to resist scratching. In Comparative Example 11, wear-resistant filler was not added. Wear-resistant filler can directly improve the wear resistance and hardness of the paint film. Without wear-resistant filler, the hardness of the paint film will decrease, the scratch resistance is general, and the resistance ability when facing scratching is weak.

[0072] In summary, the modified polyurethane provides the basic properties of the paint film and the reaction basis with other components. Functional nano-silica enhances the compactness, hardness and adhesion of the paint film. Isocyanate curing agent enables the paint film to form a strong network structure through cross-linking reactions. Wear-resistant filler directly improves the wear resistance and hardness of the coating. These four components cooperate with each other and work synergistically, making the coating have excellent properties in terms of hardness, scratch resistance, etc.

[0073] Comparative Example 11 refers to the parameter conditions in Example 7, with the difference that wear-resistant filler is not added.

[0074] Comparative Example 12 refers to the parameter conditions in Example 10, with the difference that the antioxidant and ultraviolet absorber are not coated.

[0075] Comparative Example 13 referred to the parameter conditions in Example 10, with the difference that only antioxidants were encapsulated in the microcapsule antioxidant.

[0076] Comparative Example 14 referred to the parameter conditions in Example 10, with the difference that only ultraviolet absorbers were encapsulated in the microcapsule antioxidant.

[0077] Comparative Example 15 referred to the parameter conditions in Example 10, with the difference that the microcapsule antioxidant was not added.

[0078] Experimental Example 4 Durability Test

[0079] The weather resistance of Examples 10 - 12 and Comparative Examples 11 - 15 was determined according to the SAE J1960 standard. The test requirements were as follows: after xenon lamp irradiation, the light retention rate at 60° > 95%; after xenon lamp irradiation, the color difference was less than 1. The abrasion resistance of Examples 10 - 12 and Comparative Examples 11 - 15 was tested by observing whether there was substrate exposure after scrubbing 10,000 times. The obtained results are shown in Table 5, and the weather resistance of Example 11 and Comparative Examples 11 - 15 is as Figure 1 shown.

[0080] Table 5 Durability Test of Examples 10 - 12 and Comparative Examples 11 - 15

[0081] Example Gloss retention rate / % Color difference Wear resistance Example 10 97 0.75 No showing through Example 11 98 0.71 No showing through Example 12 96 0.73 No showing through Comparative Example 11 90 1.2 Slight showing through Comparative Example 12 86 1.5 Slight showing through Comparative Example 13 88 1.3 No showing through Comparative Example 14 87 1.1 No showing through Comparative Example 15 80 1.9 Slight showing through

[0082] From Table 5 and Figure 1 it can be found that the weather resistance and abrasion resistance of the examples are relatively good, improving the durability of the paint film. The absence of wear-resistant fillers in Comparative Example 11 led to a significant decrease in the abrasion resistance of the paint film, and it was easy to expose the substrate in the scrubbing test; at the same time, the wear-resistant fillers have a certain supporting and strengthening effect on the structure of the paint film, and their absence will affect the performance of the paint film in the weather resistance test, resulting in a decrease in the light retention rate and an increase in the color difference. In Comparative Example 12, the antioxidants and ultraviolet absorbers were not encapsulated, and they would react quickly and could not play a stable role in the paint film, being easily affected by external factors and becoming ineffective. Therefore, under xenon lamp irradiation, the light retention rate of the paint film decreased, the color difference increased, and due to the overall performance decline of the paint film, the abrasion resistance was also affected. In Comparative Examples 13 - 14, only antioxidants or ultraviolet absorbers were encapsulated respectively, and they could not effectively resist oxidation and ultraviolet damage simultaneously, resulting in the weather resistance of the paint film being inferior to that of the microcapsule antioxidant containing both. In Comparative Example 15, without adding the microcapsule antioxidant, the paint film lost the key protective components against oxidation and ultraviolet rays. Under xenon lamp irradiation, the light retention rate decreased significantly, the color difference increased significantly, the performance of the paint film was severely damaged, and the abrasion resistance also decreased sharply.

[0083] In summary, the antioxidants and ultraviolet absorbers in the microcapsule antioxidants can effectively inhibit the oxidation reaction and ultraviolet degradation of the paint film during use, delay the aging of the paint film, and thus maintain the stable performance of the paint film. The wear-resistant filler can enhance the hardness and wear resistance of the paint film, reduce the wear and scratches on the surface of the paint film caused by external factors. At the same time, the wear-resistant filler can also improve the compactness of the paint film to a certain extent, prevent harmful substances such as oxygen and ultraviolet rays from entering the interior of the paint film, and cooperate with the microcapsule antioxidants to improve the weather resistance of the paint film. The synergistic effect of the two enables the paint film to have good performance in terms of weather resistance and wear resistance. The microcapsule antioxidants protect the paint film matrix from the inside to prevent its aging; the wear-resistant filler enhances the resistance of the paint film from the surface, reduces wear and the erosion of external substances, and the two cooperate with each other to jointly improve the durability of the paint film.

[0084] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A preparation method for spraying paint on thin-walled injection molded parts, characterized in that: The preparation method comprises the following steps: S1: Introduce a polyether-polyester block copolymer onto the polyurethane molecular chain to obtain a modified polyurethane; S2: Modify nano-silica with a silane coupling agent to obtain functional nano-silica; S3: Coating antioxidants and ultraviolet absorbers with microcapsules to obtain microcapsule antioxidants; S4: Mix and stir the modified polyurethane, the functional nano-silica, a leveling agent, and the microcapsule antioxidants to obtain Component A; Mix an isocyanate curing agent, a photoinitiator, wear-resistant fillers, and ethyl acetate uniformly to obtain Component B; Mix Component A and Component B to obtain a spray coating; S5: Electrostatically spray the spray coating and cure it to obtain the thin-walled spray painting of the injection molded part.

2. The preparation method of thin-wall spraying paint for injection molded parts according to claim 1, wherein: By weight, the preparation method of the modified polyurethane is as follows: Add 95 - 105 parts of polyurethane prepolymer into a reaction kettle, heat up to 80°C - 90°C, stir until dissolved under nitrogen protection, then add 100 parts of polyester diol and 0.5 - 1 part of dibutyltin dilaurate, heat up to 120°C - 140°C, react for 1.5 h, then add polytetramethylene ether glycol. The molar ratio of the polyester diol to the polytetramethylene ether glycol is 2. Continue to heat up to 160°C - 180°C and react for 4 h. Then cool down and add triphenyl phosphite, stir evenly and then dry under vacuum to obtain the modified polyurethane.

3. The preparation method of a thin-walled spray painting for an injection molded part according to claim 1, characterized in that: By weight, the preparation method of the functional nano-silica is as follows: Slowly add 90 - 100 parts of the nano-silica into 150 parts of an ethanol aqueous solution with a weight ratio of ethanol to water of 8:1, perform ultrasonic dispersion for 30 min - 60 min to obtain a dispersion; Slowly add 1 - 5 parts of the silane coupling agent KH-550 into the dispersion, stir and react in a water bath at 60°C - 80°C for 3 h. After the reaction ends, filter and separate the solid product, wash it and then dry it in a vacuum drying oven to obtain the functional nano-silica.

4. The preparation method of thin-wall painting for an injection molded part according to claim 1, characterized in that: By weight, the preparation method of the microcapsule antioxidant is as follows: Add 10 - 20 parts of antioxidant 1010 and 5 - 10 parts of ultraviolet absorber UV-326 into acetone, stir and dissolve in a water bath to form a core material solution; Add 25 parts of gelatin and 25 parts of gum arabic into distilled water respectively, stir and dissolve in a water bath at 55°C to form a gelatin solution and a gum arabic solution; Slowly add the core material solution into the gelatin solution, stir and emulsify at 55°C for 50 min to form an emulsion; Then slowly add the gum arabic solution into the emulsion, continue to stir for 15 min, adjust the pH value to 4, keep the temperature at 45°C, then slowly add 8 parts of formaldehyde, continue to react for 1 h - 2 h. After the reaction ends, adjust the pH value to neutral, separate the solid product, wash it and dry it under vacuum to obtain the microcapsule antioxidant; The wear-resistant filler is micron-sized aluminum oxide particles with a particle size of 1μm - 5μm.

5. The preparation method of thin-wall spraying paint for an injection molded part according to claim 1, characterized in that: By weight parts, the specific process of mixing in S4 is as follows: Add 35 - 45 parts of the modified polyurethane into a reaction kettle, and stir to make the modified polyurethane in a uniformly dispersed state; Slowly add 1 - 5 parts of the functional nano - silica, and continuously stir for 15 min - 25 min to obtain a dispersion solution; Sequentially add 0.1 - 1 part of the leveling agent and 1 - 3 parts of the micro - capsule antioxidant, and continue to stir for 15 min to obtain Component A; Add 25 - 35 parts of the isocyanate curing agent into another reaction kettle, slowly add 1 - 3 parts of the photo - initiator under stirring conditions, add 5 - 10 parts of the wear - resistant filler after stirring for 10 min, add 10 parts of ethyl acetate after uniform dispersion, and stir for 15 min to obtain Component B; Finally, slowly mix Component A and Component B together, and stir at 150 rpm - 200 rpm for 20 min - 30 min to obtain the spraying coating.

6. The preparation method of a thin-walled spray painting for an injection molded part according to claim 1, wherein: Before electrostatic spraying in S5, the thin - wall surface of the injection - molded part is cleaned. The speed of electrostatic spraying is 3 m / min - 5 m / min; After electrostatic spraying, it is first UV - cured for 2 min - 6 min, and finally heat - cured at 60 °C for 15 min - 25 min.

7. A thin-walled painting of an injection-molded part, characterized in that: The thin - wall paint of the injection - molded part is prepared by the preparation method according to any one of claims 1 - 6; The synthetic raw materials of the thin - wall paint of the injection - molded part include: modified polyurethane, functional nano - silica, leveling agent, micro - capsule antioxidant, isocyanate curing agent, photo - initiator, wear - resistant filler and ethyl acetate; The modified polyurethane includes polyurethane prepolymer, polyether diol and polyester diol; The functional nano - silica includes nano - silica and silane coupling agent; The micro - capsule antioxidant includes antioxidant and ultraviolet absorber.

Images