A light-curing adhesive, its preparation method and application

Abstract

This invention discloses a photocurable adhesive, its preparation method, and its applications. The photocurable adhesive, by weight, comprises the following components: 0.5-15 parts of photocurable resin, 78-95 parts of reactive diluent, and 0.1-5 parts of photoinitiator. The reactive diluent includes a first monomer and a second monomer, wherein the glass transition temperature of the homopolymer of the first monomer is ≥60℃, and the glass transition temperature of the homopolymer of the second monomer is ≤10℃. The tensile strength of the photocurable resin is 100psi-4000psi. In the photocurable adhesive provided by this invention, the reactive diluent improves the viscosity of the system, the elastic recovery rate of the cured adhesive layer, and optimizes the tensile storage modulus; the photocurable resin improves the hygrothermal properties of the adhesive. The synergistic effect of these two components allows the adhesive viscosity to meet the requirements of inkjet printing while ensuring that the adhesive layer formed after photocuring possesses good peel strength, suitable tensile storage modulus, and excellent low-temperature impact resistance.

Description

Technical Field

[0001] This invention relates to the field of photocurable materials technology, and in particular to a photocurable adhesive, its preparation method, and its application. Background Technology

[0002] Photopolymer inkjet printing is a novel precision molding process that combines inkjet printing technology with the properties of photopolymer materials. With its advantages of high precision, high efficiency, low cost, and ability to process complex patterns, it has shown widespread application demand in precision manufacturing fields such as semiconductors and consumer electronics in recent years, becoming one of the key development technologies in high-end manufacturing. Currently, the core principle of photopolymer inkjet printing is as follows: low-viscosity liquid photopolymer adhesive is precisely sprayed onto the substrate surface through an inkjet printhead, followed by rapid curing under ultraviolet light, ultimately forming a functional adhesive layer with specific mechanical properties. This process can be widely used in high-end manufacturing scenarios such as precision dispensing, micro-area coating, and complex patterned packaging.

[0003] In the consumer electronics sector, including mobile phones, tablets, and laptops, the rapid iteration of products towards thinner, lighter, more precise, and higher-performance designs has placed higher standards on the manufacturing precision and reliability of core components. Among these, the precise application and sealing of precision rotating components (such as hinges for foldable phones and laptops) has become a critical manufacturing step. The sealing requirements for these precision rotating components not only demand high precision and efficiency in the application process but also impose special requirements on the mechanical properties and environmental adaptability of the sealant layer. Specifically, photocurable sealants used for sealing precision rotating components in consumer electronics must meet three core performance indicators: First, they must have a high elastic recovery rate within certain elastic limits to prevent fatigue and cracking of the sealant layer due to long-term deformation, ensuring long-term seal reliability. Second, they must have a low tensile storage modulus at low temperatures to improve the product's low-temperature impact resistance. Third, the sealant should have a certain resistance to damp heat aging to adapt to the usage requirements of consumer electronics products in different temperature and humidity environments, preventing the sealant layer from becoming brittle and debonding due to environmental changes, thus affecting the component's sealing performance and product lifespan.

[0004] However, most currently available UV-curable adhesives fail to simultaneously meet the rheological properties required for UV-curable inkjet printing and the specific mechanical properties required for demanding applications (such as precision rotating components in consumer electronics). While some adhesives can meet the low viscosity requirement and ensure printing accuracy, their elastic recovery rate after curing is insufficient, making it difficult to adapt to the repeated deformation caused by the rotation of the shaft in precision rotating components of consumer electronics, which can easily lead to sealing failure. Other adhesives, while emphasizing elasticity, have high viscosity, which cannot adapt to the continuous jetting requirements of inkjet printing, resulting in decreased printing accuracy and making them unsuitable for precision sealing applications. Summary of the Invention The present invention aims to at least solve one of the aforementioned technical problems existing in the prior art. Therefore, a first objective of the present invention is to provide a photocurable adhesive. A second objective of the present invention is to provide a method for preparing such a photocurable adhesive. A third objective of the present invention is to provide applications of such a photocurable adhesive.

[0005] The inventive concept of this invention is as follows: This invention provides a photocurable adhesive comprising a photocurable resin, a reactive diluent, and a photoinitiator. By limiting the glass transition temperature range of the homopolymers of different monomers in the reactive diluent and the tensile strength of the photocurable resin, the adhesive layer formed after photocuring has excellent elastic recovery rate and low-temperature impact resistance. Moreover, after being treated under humid heat conditions and thermal cycling conditions, the peel strength still meets the usage requirements, demonstrating good reliability.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a photocurable adhesive, comprising, by weight, the following components: 0.5 to 15 parts of photocurable resin, 78 to 95 parts of reactive diluent, and 0.1 to 5 parts of photoinitiator; wherein the reactive diluent comprises a first monomer and a second monomer, wherein the homopolymer glass transition temperature of the first monomer is ≥60°C, and the homopolymer glass transition temperature of the second monomer is ≤10°C; and the photocurable resin has a tensile strength of 100 psi to 4000 psi.

[0007] In the photocurable adhesive provided by this invention, the reactive diluent (used in combination with the first monomer and the second monomer) improves the viscosity of the system, the elastic recovery rate of the cured adhesive layer, and optimizes the tensile storage modulus. The photocurable resin improves the hygrothermal properties of the adhesive. Their synergistic effect ensures that the adhesive viscosity meets the requirements of inkjet printing while giving the cured adhesive layer good peel strength, suitable tensile storage modulus, and excellent low-temperature impact resistance. The reactive diluent in this invention includes a first monomer and a second monomer. The first monomer has a high homopolymer glass transition temperature, providing rigidity to the adhesive; the second monomer has a low homopolymer glass transition temperature, providing flexibility. The combination of these two different reactive diluent monomers imparts good elastic recovery rate to the photocured adhesive layer. Furthermore, this invention limits the tensile strength of the photocurable resin to 100psi to 4000psi, ensuring that the cured adhesive layer has both good elastic recovery rate and peel strength. Excessive tensile strength of the photocurable resin leads to a decrease in the low-temperature impact resistance of the adhesive layer, while insufficient tensile strength results in problems such as rebound and insufficient peel strength.

[0008] Preferably, the photocurable resin has a tensile strength of 200 psi to 3800 psi; more preferably, the photocurable resin has a tensile strength of 300 psi to 3500 psi; and even more preferably, the photocurable resin has a tensile strength of 400 psi to 3200 psi.

[0009] Preferably, the photocurable resin is a (meth)acryloyloxy type photocurable resin; more preferably, the photocurable resin includes at least one of acrylate resin, methacrylate resin, polyurethane (meth)acrylate resin, and epoxy acrylate resin.

[0010] Preferably, the reactive diluent is an acrylate monomer.

[0011] Preferably, the glass transition temperature of the homopolymer of the first monomer is ≥62°C; more preferably, the glass transition temperature of the homopolymer of the first monomer is ≥64°C; even more preferably, the glass transition temperature of the homopolymer of the first monomer is 65°C~95°C; and even more preferably, the glass transition temperature of the homopolymer of the first monomer is 65°C~88°C.

[0012] Preferably, the glass transition temperature of the homopolymer of the second monomer is ≤9°C; more preferably, the glass transition temperature of the homopolymer of the second monomer is ≤7°C; even more preferably, the glass transition temperature of the homopolymer of the second monomer is -60°C to 7°C; and even more preferably, the glass transition temperature of the homopolymer of the second monomer is -54°C to 5°C.

[0013] Preferably, the first monomer comprises at least one of isobornyl acrylate (IBOA), 4-tert-butylcyclohexyl acrylate (TBCHA), isobornyl methacrylate, cyclohexyl methacrylate, dicyclopentadiene (meth)acrylate, and acrylmorpholine.

[0014] Preferably, the second monomer comprises at least one of hydroxyethyl acrylate, hydroxypropyl acrylate, tetrahydrofuran acrylate, (2-ethyl-2-methyl-1,3-dioxopentyl-4-yl)acrylate, alkyltetrahydrofuran acrylate (POTHFA), ethoxyethoxyethyl acrylate (EOEOEA), 2-phenoxyethyl acrylate (PHEA), and caprolactone (CA).

[0015] Preferably, the photoinitiator comprises at least one of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphonate, isopropylthioxanthone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, 1,1'-(methylenedi-4,1-phenylene)bis[2-hydroxy-2-methyl-1-propanone], 1-hydroxycyclohexylphenyl ketone, and bis(2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propane).

[0016] Preferably, the light-curing adhesive further includes additives.

[0017] Preferably, the additives include at least one of silane coupling agents, adhesion promoters, antioxidants, and light stabilizers.

[0018] Preferably, the silane coupling agent comprises 3-(2,3-epoxypropoxy)propyltrimethoxysilane, 3 3-Oxypropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, 3-(2-aminoethylamino)propyltriethoxysilane, 3 (2 (Aminoethyl)aminopropylmethyldimethoxysilane, methacryloyloxypropyltrimethoxysilane (KH570), methacryloyloxypropyltriethoxysilane, methacryloyloxypropylmethyldimethoxysilane, 3 Isocyanate-propyltrimethoxysilane, 3 Isocyanate-based propyltriethoxysilane, 3 Mercaptopropyltrimethoxysilane, 3 At least one of mercaptopropyltriethoxysilane.

[0019] Preferably, the adhesion promoter comprises a photocurable phosphate ester containing a (meth)acryloyloxy functional group; more preferably, the adhesion promoter comprises at least one of 2-hydroxyethyl methacrylate phosphate, 2-oxyphosphine tri(oxy-2,1-ethylene) acrylate, and 2-methyl-2-oxyphosphine tri(oxy-2,1-ethylene) acrylate.

[0020] Preferably, the antioxidant comprises at least one of 2,6-di-tert-butyl-4-methylphenol (BHT), tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid], pentaerythritol ester (antioxidant 1010), isooctyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1135), tris(2,4-di-tert-butylphenyl) phosphite (antioxidant 168), and triphenyl phosphite (TPP).

[0021] Preferably, the light stabilizer comprises at least one of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole, bis(2,2,6,6-tetramethylpiperidinyl), 2-hydroxy-4-methoxybenzophenone, and 2-hydroxy-4-n-octyloxybenzophenone.

[0022] Preferably, the amount of the photocurable resin is 1 to 14 parts by weight; more preferably, the amount of the photocurable resin is 3 to 13 parts by weight; and even more preferably, the amount of the photocurable resin is 6 to 12 parts by weight.

[0023] Preferably, the reactive diluent has a mass fraction of 80-93 parts; more preferably, the reactive diluent has a mass fraction of 81-91 parts; and even more preferably, the reactive diluent has a mass fraction of 83-88 parts.

[0024] Preferably, the mass ratio of the first monomer to the second monomer is 1:(1~4); more preferably, the mass ratio of the first monomer to the second monomer is 1:(1~3); even more preferably, the mass ratio of the first monomer to the second monomer is 1:(1~2.5); and still more preferably, the mass ratio of the first monomer to the second monomer is 1:(1.5~2.5). By limiting the mass ratio of the first monomer to the second monomer, the adhesive possesses both rigidity and flexibility, giving the cured adhesive layer a high elastic recovery rate.

[0025] Preferably, the mass fraction of the first monomer is 20 to 40 parts; more preferably, the mass fraction of the first monomer is 22 to 38 parts; and even more preferably, the mass fraction of the first monomer is 25 to 35 parts.

[0026] Preferably, the second monomer has a mass fraction of 50-65 parts; more preferably, the second monomer has a mass fraction of 50-63 parts; and even more preferably, the second monomer has a mass fraction of 50-61 parts. Preferably, the glass transition temperature of the photocurable adhesive is 15~25℃; more preferably, the glass transition temperature of the photocurable adhesive is 18~24.7℃; even more preferably, the glass transition temperature of the photocurable adhesive is 19.3~24.4℃. The glass transition temperature of the photocurable adhesive is the temperature corresponding to the maximum value of the loss factor tanδ.

[0027] Preferably, the photoinitiator has a mass fraction of 0.4 to 4.5 parts; more preferably, the photoinitiator has a mass fraction of 0.8 to 4.2 parts; even more preferably, the photoinitiator has a mass fraction of 1 to 4 parts; and even more preferably, the photoinitiator has a mass fraction of 1 to 3 parts.

[0028] Preferably, the mass fraction of the auxiliary agent is 1 to 5 parts; more preferably, the mass fraction of the auxiliary agent is 1.2 to 4.5 parts; even more preferably, the mass fraction of the auxiliary agent is 1.5 to 4 parts; and even more preferably, the mass fraction of the auxiliary agent is 1.8 to 3 parts.

[0029] In a second aspect, the present invention provides a method for preparing the photocurable adhesive described in the first aspect, comprising the following steps: mixing the various components according to their mass parts and then degassing to obtain the photocurable adhesive.

[0030] Preferably, the preparation method is as follows: first, the photoinitiator is mixed with the reactive diluent, and after the photoinitiator is completely dissolved in the reactive diluent, the photocurable resin and additives are added and mixed.

[0031] Preferably, the mixing method includes stirring.

[0032] Preferably, the stirring is carried out under heating.

[0033] Preferably, the stirring speed is 500-1000 rpm; more preferably, the stirring speed is 600-900 rpm; and even more preferably, the stirring speed is 650-850 rpm.

[0034] Preferably, the stirring time is 20-50 min; more preferably, the stirring time is 22-45 min; and even more preferably, the stirring time is 25-40 min.

[0035] Preferably, the degassing method is vacuum degassing.

[0036] Preferably, the degassing time is 1-5 min; more preferably, the degassing time is 1.2-4.5 min; even more preferably, the degassing time is 1.5-3 min.

[0037] Preferably, the temperature corresponding to a viscosity of 15 mPa·s for the photocurable adhesive is less than 50°C; more preferably, the temperature corresponding to a viscosity of 15 mPa·s for the photocurable adhesive is 42~49°C. If the viscosity of the photocurable adhesive is higher than 15 mPa·s at 50°C, inkjet printing will result in process defects such as uneven inkjet printing, failing to meet production requirements. The photocurable adhesive of the present invention has a viscosity of 15 mPa·s corresponding to a temperature less than 50°C, which meets the preferred standard for inkjet printing technology.

[0038] Thirdly, the present invention provides an adhesive layer, which is obtained by curing the light-curing adhesive described in the first aspect by ultraviolet light irradiation.

[0039] Preferably, the wavelength range of the ultraviolet light is 200nm~400nm; preferably, the wavelength range of the ultraviolet light is 250nm~380nm.

[0040] Preferably, the irradiation time is 10-30 seconds; more preferably, the irradiation time is 12-25 seconds; and even more preferably, the irradiation time is 13-18 seconds.

[0041] Preferably, the intensity of the ultraviolet light is 100~300mW / cm. 2 More preferably, the intensity of the ultraviolet light is 120~280mW / cm². 2 More preferably, the intensity of the ultraviolet light is 150~250mW / cm². 2 .

[0042] Preferably, the elastic recovery rate of the adhesive layer at 25°C is ≥80%. The elastic recovery rate is tested by releasing the adhesive layer after holding it at 50% strain for 10 minutes.

[0043] Preferably, the tensile storage modulus of the adhesive layer at -20°C is less than 1000 MPa; more preferably, the tensile storage modulus of the adhesive layer at -20°C is less than 900 MPa; even more preferably, the tensile storage modulus of the adhesive layer at -20°C is less than 850 MPa; and even more preferably, the tensile storage modulus of the adhesive layer at -20°C is 600~810 MPa; the length of the adhesive layer is 15~25 mm, the width is 4~6 mm, and the thickness is 0.5~2 mm; the tensile testing frequency is 1~3 Hz. The application scenario for sealing the shaft of precision rotating components is generally -20°C. The adhesive layer of this invention has a good tensile storage modulus at -20°C, which can meet the stringent requirements of precision rotating components for the low-temperature impact resistance of adhesives.

[0044] Preferably, the peel strength of the adhesive layer is ≥6 N / cm; more preferably, the peel strength of the adhesive layer is ≥7 N / cm; and even more preferably, the peel strength of the adhesive layer is ≥8 N / cm. The peel test method involves coating the UV-curable adhesive of the first aspect onto a stainless steel substrate, curing it with ultraviolet light, and then peeling it at 180° at a speed of 200~400 mm / min.

[0045] Preferably, the peel strength of the adhesive layer after damp heat treatment is ≥3 N / cm; more preferably, the peel strength of the adhesive layer after damp heat treatment is ≥3.2 N / cm; even more preferably, the peel strength of the adhesive layer after damp heat treatment is ≥3.5 N / cm; and even more preferably, the peel strength of the adhesive layer after damp heat treatment is ≥4 N / cm. The damp heat treatment involves placing the adhesive layer in an environment of 50~60℃ and 90%~99%RH (relative humidity) for 9~11 days.

[0046] Preferably, the peel strength of the adhesive layer after thermal cycling treatment is ≥3 N / cm; more preferably, the peel strength of the adhesive layer after thermal cycling treatment is ≥4 N / cm; even more preferably, the peel strength of the adhesive layer after thermal cycling treatment is ≥5 N / cm. The thermal cycling treatment involves repeatedly placing the adhesive layer at -40℃ to -30℃ for 1 hour and then at 80℃ to 90℃ for 1 hour, for a total of 9 to 11 days.

[0047] Fifthly, the present invention provides the application of the photocurable adhesive described in the first aspect or the adhesive layer described in the fourth aspect in precision rotating components.

[0048] The beneficial effects of this invention are: (1) In the photocurable adhesive provided by the present invention, the reactive diluent (the combination of the first monomer and the second monomer) improves the viscosity of the system, the elastic recovery rate of the cured adhesive layer, and optimizes the tensile storage modulus. The photocurable resin improves the hygrothermal performance of the adhesive. The synergistic effect of the two makes the adhesive viscosity meet the requirements of inkjet printing, while the adhesive layer formed after photocuring has good peel strength, suitable tensile storage modulus, and excellent low-temperature impact resistance. The reactive diluent in the present invention includes the first monomer and the second monomer. The homopolymer of the first monomer has a high glass transition temperature, which provides rigidity to the adhesive; the homopolymer of the second monomer has a low glass transition temperature, which provides flexibility to the adhesive. The combination of the two different reactive diluent monomers can give the photocured adhesive layer a good elastic recovery rate. In addition, the present invention limits the tensile strength of the photocurable resin to 100psi to 4000psi, which can make the photocured adhesive layer have both good elastic recovery rate and peel strength. However, if the tensile strength of the photocurable resin is too high, the low-temperature impact resistance of the adhesive layer will decrease. If the tensile strength of the photocurable resin is too low, the adhesive layer will have problems such as rebound and insufficient peel strength.

[0049] (2) The photocurable adhesive of the present invention, after being cured by ultraviolet light, forms an adhesive layer with good elastic recovery rate and low temperature impact resistance. After being subjected to wet heat treatment and cold heat cycle treatment, the adhesive layer still maintains excellent peel performance, which can meet the stringent requirements of precision rotating components. Detailed Implementation

[0050] To enable those skilled in the art to better understand this application, the present invention will be further described in detail below with reference to embodiments. However, it should be understood that the following embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims.

[0051] In the description of this invention, it should be noted that unless specific conditions are specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available products. Furthermore, to better illustrate this invention, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this invention can be practiced even without certain specific details. In some embodiments, raw materials, methods, and means well known to those skilled in the art are not described in detail in order to highlight the main points of this invention.

[0052] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprising" shall be understood to include the stated elements or components without excluding other elements or other components.

[0053] The materials used in the embodiments of this invention are as follows: Polyurethane acrylate 1: Model: CN996NS, tensile strength: 3100 psi, source: Sartomer Company; Polyurethane acrylate 2: Model: BR5541M, tensile strength: 440 psi, source: Bomar Company; Polyurethane acrylate 3: Model: CN8881NS, tensile strength: 4186 psi, source: Sartomer Company; Polyurethane acrylate 4: Model: CN966H90NS, tensile strength: 77 psi, source: Sartomer Company; Reactive diluent 1: Name: Isobornyl acrylate (IBOA), Source: Sartamomer Company; Reactive diluent 2: Name: Tetrahydrofuran acrylate (POTHFA), Source: Sartamomer Company; Reactive diluent 3: Name: Ethoxyethoxyethyl acrylate (EOEOEA), Source: Sartamomer Company; Reactive diluent 4: Name: 2-Phenoxyethyl acrylate (PHEA), Source: Sartamomer Company; Reactive diluent 5: Name: 4-tert-butylcyclohexyl acrylate (TBCHA), Source: Sartamomer Company; Reactive diluent 6: Name: Caprolactone acrylate (CA), Source: Sartamomer Company; Reactive diluent 7: Name: Cyclotrimethylolpropane methyl acetal acrylate (CTFA), Source: Sartamer Company; Additive 1: Name: 2-Methyl-2-acrylate oxyphosphine tris(oxy-2,1-ethylidene) ester (3PMA), Source: Osaka Organic Co., Ltd.; Additive 2: Name: Methacryloxypropyltrimethoxysilane (KH570).

[0054] The peel strength of this invention was tested at 25°C.

[0055] Example 1 This embodiment provides a photocurable adhesive, which, by weight, comprises 9 parts of photocurable resin (polyurethane acrylate 1), 88 parts of reactive diluent (35 parts of reactive diluent 1, 44 parts of reactive diluent 2 and 9 parts of reactive diluent 3), 2 parts of photoinitiator, 1 part of additive 1 and 1 part of additive 2.

[0056] The preparation method of the photocurable adhesive is as follows: a photoinitiator and an active diluent are added to a sealed reactor equipped with a mechanical stirrer and a heating jacket and stirred. After the photoinitiator is completely dissolved in the active diluent, a photocurable resin is added and heated and stirred until the photocurable resin is uniformly dispersed. Then, additive 1 and additive 2 are added and stirred at a shear rate of 500~1000 rpm for 30 min. Finally, vacuum degassing is performed for 2 min to obtain the photocurable adhesive.

[0057] Examples 2-4 and Comparative Examples 1-6 each provide a light-curing adhesive, which is prepared in the same way as in Example 1. The components of the light-curing adhesive are shown in Table 1.

[0058] Table 1. Components (parts by weight) of the adhesives used in the examples and comparative examples.

[0059] The glass transition temperatures of the homopolymers of reactive diluents 1 to 7 in Table 1 are shown in Table 2.

[0060] Table 2 Glass transition temperatures of homopolymers containing reactive diluents

[0061] Performance testing 1. Test the viscosity of the adhesives in Examples 1-4 and Comparative Examples 1-6. Temperature scanning tests were performed on the adhesives of Examples 1-4 and Comparative Examples 1-6 using an Anton Paar rheometer (PP-25 rotor). The samples were first held at 0°C for 1 min, then heated at a rate of 10°C / min. The viscosity-temperature curves were recorded, and the temperature at which the viscosity decreased to 15 mPa·s was determined. Based on the requirements of inkjet printing processes in consumer electronics, if the adhesive viscosity remains higher than 15 mPa·s at 50°C, it will lead to process defects such as uneven adhesive application, failing to meet production needs. Therefore, the temperature at which the adhesive viscosity reaches 15 mPa·s was tested to determine whether it meets the preferred criteria for inkjet printing processes.

[0062] 2. Test the properties of the adhesive layers after curing of Examples 1-4 and Comparative Examples 1-6. 2.1 Test elastic recovery rate The adhesives from Examples 1-4 and Comparative Examples 1-6 were coated onto the Teflon-plated molds, and then cured with a 365nm UV lamp at 200mW / cm². 2 The adhesive was irradiated for 15 seconds to obtain a 1 mm thick adhesive layer, which was then cut into strips 20 mm long and 5 mm wide. At 25°C, using a TA DMA850 instrument in strain mode, the strips were stretched to 50% strain, held for 10 minutes, and then released. The strain rate ε after 10 minutes was measured. The formula for calculating the elastic recovery rate E (%) is as follows: .

[0063] The higher the elastic recovery rate of the spline, the more the corresponding adhesive meets the standards for use at the shaft of a precision rotating component.

[0064] 2.2 Testing the tensile storage modulus Samples were obtained using the same preparation method described above. On a TA DMA850 instrument, after being held at -35℃ for 5 min, the tensile storage modulus of the samples was recorded from -35℃ to 90℃ at a rate of 3℃ / min and a frequency of 1 Hz, obtaining the tensile storage modulus data at -20℃. Simultaneously, the temperature corresponding to the maximum value of the loss factor tanδ was taken as the glass transition temperature of the adhesive. Based on the application scenario of sealing the shaft of precision rotating components (typically -20℃), the tensile storage modulus of the samples at -20℃ was tested to characterize whether they possess good low-temperature toughness, thereby determining whether they meet the stringent requirements for low-temperature impact resistance of adhesives in precision rotating components for consumer electronics.

[0065] 2.3 Test peel strength (1) Sample preparation: The adhesives of Examples 1-4 and Comparative Examples 1-6 were coated on SUS316 stainless steel substrates to prepare samples with an adhesive layer width of 10 mm and an adhesive layer thickness of 1.5±0.5 mm. The samples were cured by irradiation with a 365 nm UV-LED light source of 200 mW / cm² for 15 seconds, and then the peel strength test was performed.

[0066] (2) Conventional peel strength test: On an electronic universal testing machine, the sample solidified on the SUS316 stainless steel substrate was peeled at 300 mm / min at 180°. The average force value of the middle stable section was divided by the width of the sample to obtain the peel strength.

[0067] (3) Peel strength test after wet heat treatment: After placing the sample prepared in step (1) in an environment of 55℃ and 95%RH for 10 days, the peel strength is tested using the test method in step (2).

[0068] (4) Peel strength test after hot and cold cycling treatment: The sample prepared in step (1) is placed at -40℃ for 1 hour and then at 80℃ for 1 hour, and the cycle is repeated for 10 days. Then the peel strength is tested using the test method in step (2).

[0069] By testing peel strength, the reliability of the adhesive layer after UV curing can be determined.

[0070] The test results are shown in Table 3.

[0071] Table 3 Performance test results of different adhesives

[0072] In Table 3, "●" indicates an elastic recovery rate of 90% or higher, "◎" indicates an elastic recovery rate of 80%–90%, "○" indicates an elastic recovery rate of 70%–80%, "×" indicates an elastic recovery rate of less than 70%, " / " indicates that the sample is too hard to peel 180°, "×" indicates that the adhesive layer is damaged during peeling and cannot be tested, and " / " indicates that the glass transition temperature of the adhesive is too low and cannot be tested.

[0073] As shown in Table 3, the temperature corresponding to a viscosity of 15 mPa·s for the photocurable adhesives prepared in Examples 1-4 of this invention is less than 50℃, which can meet the viscosity process requirements of inkjet printers. The adhesive strips formed by UV curing of the photocurable adhesives prepared in Examples 1-4 of this invention have suitable tensile storage modulus (all less than 1000 MPa) at -20℃, indicating that they have excellent low-temperature impact resistance. After being treated with humid heat and thermal cycling conditions, the peel strength of the adhesive strips formed by UV curing of the photocurable adhesives prepared in Examples 1-4 of this invention still meets the usage requirements (conventional peel strength ≥7 N / cm, peel strength after humid heat treatment ≥4 N / cm, peel strength after thermal cycling treatment ≥5 N / cm), showing good reliability.

[0074] Of Examples 1-4, the elastic recovery rate of Example 4 is relatively weak. Compared to Example 2, although Example 4 contains IBOA, it does not use TBCHA monomer with a homopolymer glass transition temperature of 65°C, but instead uses CTFA monomer with a homopolymer glass transition temperature of 35°C. This results in an insufficient amount of monomers with a homopolymer glass transition temperature >60°C, providing insufficient rigidity to the adhesive and thus a weak elastic recovery rate.

[0075] Compared to Example 1, the adhesive in Comparative Example 1 only contains reactive diluent and lacks polyurethane (meth)acrylate. Although the adhesive in Comparative Example 1 meets the requirements for low-temperature impact resistance and conventional initial peel strength, its elastic recovery rate is slightly lower. Furthermore, due to the lack of a UV-curable resin, the small molecule reactive diluent lacks cross-linking after UV curing, and the adhesive layer undergoes bulk damage after being subjected to external force following treatment under humid and hot conditions.

[0076] Compared with Example 1, the elastic recovery rate and conventional peel strength of the adhesive in Comparative Example 2 (which uses a light-cured resin with excessively high tensile strength) are slightly improved. However, due to the excessively high tensile strength, the adhesive strip formed after curing the adhesive in Comparative Example 2 has an excessively high tensile modulus at -20°C, which affects the low-temperature impact resistance of the adhesive strip and makes it unable to meet the stringent requirements of low-temperature impact resistance for adhesives in precision rotating components of consumer electronics.

[0077] Compared with Example 1, the elastic recovery rate and conventional peel strength of the adhesive in Comparative Example 3 (which used a light-cured resin with too low tensile strength) were significantly reduced, and the peel strength was poor after treatment under humid heat conditions and cold and heat cycling conditions.

[0078] Compared to Example 1, the glass transition temperature of the homopolymer of the reactive diluent used in Comparative Example 4 was greater than 60°C, which resulted in an excessively high low-temperature tensile storage modulus after adhesive curing, making the strips too stiff to be tested for peel strength and elastic recovery rate. In contrast, the glass transition temperature of the homopolymer of the reactive diluent used in Comparative Example 5 was very low. This low glass transition temperature endowed the adhesive strip with excellent flexibility, but the excessively low crosslinking density also significantly reduced the tensile modulus of the strip. This structural weakness resulted in an elastic recovery rate of less than 70% after curing, and due to insufficient cohesive strength, the adhesive layer itself could not effectively transfer stress, leading to damage during the peel test. As can be seen from Example 1 and Comparative Examples 4 and 5, the glass transition temperature of the homopolymer of the reactive diluent is greater than 60°C, which can provide rigidity to the adhesive, while the glass transition temperature of the homopolymer of the reactive diluent is less than 10°C, which can provide flexibility to the adhesive. When two different reactive diluents (the first monomer and the second monomer) are mixed and matched, their corresponding homopolymer glass transition temperature ranges are different, thus giving the cured adhesive layer a good elastic recovery rate.

[0079] Compared with Examples 1 to 3, the content of photocurable resin in Comparative Example 6 is too high. Although the increase in the content of photocurable resin has a certain effect on improving the elastic recovery rate, the viscosity of the adhesive and the tensile storage modulus at -20°C will increase, which will affect the smoothness of inkjet printing and the low-temperature impact resistance.

[0080] In summary, the synergistic effect of the reactive diluent and the photocurable resin in the photocurable adhesive of this invention enables the adhesive viscosity to meet the requirements of inkjet printing while ensuring that the photocured adhesive layer possesses good peel strength, suitable tensile storage modulus, and excellent low-temperature impact resistance. The high glass transition temperature of the first monomer in the reactive diluent provides rigidity to the adhesive, while the low glass transition temperature of the second monomer provides flexibility. The combination of these two different reactive diluent monomers imparts good elastic recovery to the photocured adhesive layer. Furthermore, the present invention limits the tensile strength of the photocurable resin to 100 psi to 4000 psi, ensuring that the cured adhesive exhibits both good resilience and peel strength. Excessive tensile strength of the photocurable resin leads to a decrease in the low-temperature impact resistance of the adhesive layer, while insufficient tensile strength results in problems with insufficient resilience and peel strength. The photocurable adhesive of this invention, after being cured by ultraviolet light, forms an adhesive layer with good elastic recovery rate and low-temperature impact resistance. After being subjected to wet heat treatment and cold heat cycling treatment, the adhesive layer still maintains excellent peel performance, which can meet the stringent requirements of precision rotating components and can be well used in precision rotating components of consumer electronics (such as hinges of foldable screen mobile phones, laptop hinges, etc.).

[0081] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A light-curing adhesive, characterized in that, The product comprises, by weight, the following components: 0.5-15 parts of photocurable resin, 78-95 parts of reactive diluent, and 0.1-5 parts of photoinitiator; the reactive diluent comprises a first monomer and a second monomer, wherein the homopolymer glass transition temperature of the first monomer is ≥60°C, and the homopolymer glass transition temperature of the second monomer is ≤10°C; the photocurable resin has a tensile strength of 100 psi to 4000 psi.

2. The photocurable adhesive according to claim 1, characterized in that, The photocurable resin is a (meth)acryloyloxy type photocurable resin; And / or, the reactive diluent is an acrylate monomer; And / or, the photoinitiator comprises at least one of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphonate, isopropylthioxanthone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, 1,1'-(methylenedi-4,1-phenylene)bis[2-hydroxy-2-methyl-1-propanone], 1-hydroxycyclohexylphenyl ketone, and bis(2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propane).

3. The photocurable adhesive according to claim 1, characterized in that, The photocurable adhesive also includes additives, which include at least one of silane coupling agents, adhesion promoters, antioxidants, and light stabilizers.

4. The photocurable adhesive according to claim 1, characterized in that, The mass ratio of the first monomer to the second monomer is 1:(1~4).

5. The photocurable adhesive according to claim 1, characterized in that, The glass transition temperature of the light-curing adhesive is 15~25℃.

6. The method for preparing the photocurable adhesive according to any one of claims 1 to 5, characterized in that, Includes the following steps: After mixing the components according to their respective weight proportions and degassing, the light-curing adhesive is obtained.

7. An adhesive layer, characterized in that, The adhesive layer is obtained by curing the light-curing adhesive according to any one of claims 1 to 5 by ultraviolet light irradiation.

8. The adhesive layer according to claim 7, characterized in that, The wavelength range of the ultraviolet light is 200nm~400nm; And / or, the irradiation time is 10~30s; And / or, the intensity of the ultraviolet light is 100~300mW / cm. 2 .

9. The adhesive layer according to claim 7, characterized in that, The elastic recovery rate of the adhesive layer at 25°C is ≥80%; And / or, the tensile storage modulus of the adhesive layer is less than 1000 MPa at -20°C; the width of the adhesive layer is 4~6 mm, the length is 15~25 mm, and the thickness is 0.5~2 mm; the frequency of the tensile test is 1~3 Hz; And / or, the peel strength of the adhesive layer is ≥6 N / cm; And / or, the peel strength of the adhesive layer after wet heat treatment is ≥3N / cm; the wet heat treatment method is to place the adhesive layer in an environment of 50~60℃ and 90%~99%RH for 9~11 days; And / or, the peel strength of the adhesive layer after hot and cold cycling treatment is ≥3N / cm; the hot and cold cycling treatment is to place the adhesive layer at -40℃ to -30℃ for 1 hour and then at 80℃ to 90℃ for 1 hour, and repeat the cycle for 9 to 11 days.

10. The application of the light-curing adhesive according to any one of claims 1 to 5 or the adhesive layer according to any one of claims 7 to 9 in a precision rotating assembly.

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