An oxygen-free, cationic curing-dynamic thioether bond toughening dual-mechanism low deformation UV adhesive, and a preparation method and application thereof

Abstract

The application discloses a kind of oxygen-free inhibition cationic curing-dynamic thioether bond toughening dual-mechanism low-deformation UV adhesive and its preparation method and application.The adhesive contains alicyclic epoxy resin, flexible epoxy comonomer containing thioether bond and cationic photoinitiator, occurs cationic ring-opening polymerization under ultraviolet light irradiation, forms the three-dimensional network structure of rigid epoxy main chain inlaying thioether dynamic crosslinking node.The adhesive can be completely cured in air atmosphere, without oxygen inhibition problem;During curing and after curing, thioether dynamic bond realizes real-time relaxation of internal stress by reversible rupture-recombination, and the volume shrinkage rate is less than or equal to 1.5%, and long-term dimensional stability is excellent.The application does not depend on any inorganic filler (such as silicon dioxide, POSS, etc.), pure organic system realizes low deformation, low internal stress and high heat resistance, and can be widely applied in precision electronic packaging, optical element bonding and wearable device assembly and other fields.

Description

Technical Field

[0001] This invention belongs to the field of adhesive technology, specifically relating to a dual-mechanism low-deformation UV adhesive with oxygen-free polymerization-inhibiting cationic curing and dynamic thioether bond toughening, as well as its preparation method and application. Background Technology

[0002] Ultraviolet (UV) curable adhesives are widely used in electronic packaging, optical device assembly, and other fields due to their advantages such as rapid curing, environmental friendliness and energy saving, and suitability for heat-sensitive substrates. Based on their polymerization mechanism, UV adhesives are mainly classified into two types: free radical and cationic.

[0003] Free radical UV adhesives suffer from severe oxygen inhibition, resulting in incomplete surface curing in air and requiring nitrogen protection or the addition of expensive photoinitiators. Furthermore, free radical polymerization is accompanied by significant volume shrinkage (typically 3%–8%) and internal curing stress, easily leading to warping, microcracks, and even debonding at the bond interface. Cationic UV adhesives (such as alicyclic epoxy systems) do not exhibit oxygen inhibition, can fully cure in air, and have a lower volume shrinkage rate (approximately 2%–4%). However, the cross-linked network formed by traditional cationic curing is rigid and fixed, and the internal stress generated during curing cannot be released, leading to dimensional drift and decreased bond strength after long-term storage or thermal cycling.

[0004] To reduce shrinkage and internal stress, existing technologies commonly employ the addition of inorganic fillers (such as silica, boron nitride, and POSS). For example, CN113527845A discloses a low-shrinkage UV adhesive that suppresses volume shrinkage by adding surface-modified nano-silica; CN114456744A uses POSS as a stress buffer. However, the introduction of inorganic fillers brings a series of problems: poor compatibility between fillers and organic matrices, leading to agglomeration and sedimentation; increased adhesive viscosity, affecting coating and leveling; scattering of ultraviolet light, reducing curing depth; and difficulty in application to transparent bonding or optical components.

[0005] In recent years, dynamic covalent bonds (such as disulfide bonds, Diels-Alder bonds, and borate ester bonds) have been introduced into polymer networks, endowing materials with self-healing, reprocessing, and stress relaxation capabilities. For example, CN115322663A discloses a free radical UV-curable polyurethane acrylate containing disulfide bonds, which achieves self-healing through disulfide bond exchange. However, this type of system is still based on free radical polymerization, and oxygen inhibition cannot be avoided; moreover, the exchange temperature of disulfide bonds is relatively high (usually >60℃), resulting in limited stress relaxation efficiency at room temperature. Thioether bonds (-S-), as a dynamic bond with strong polarity but moderate reversibility, can undergo reversible breakage and rearrangement at room temperature and is stable to ultraviolet light, but there are no reports of combining them with cationic UV-curable epoxy systems. In summary, developing a low-deformation UV adhesive that requires no inorganic fillers, has no oxygen inhibition, and can spontaneously dissipate curing internal stress has significant industrial value. Summary of the Invention

[0006] Objective of the Invention: This invention provides a dual-mechanism low-deformation UV adhesive with oxygen-free polymerization inhibition cationic curing and dynamic thioether bond toughening. This invention is a thioether dynamically crosslinked cationic UV low-deformation adhesive that combines self-stress relief and oxygen-free polymerization inhibition properties. This adhesive constructs a rigid-dynamic reversible dual-network structure by copolymerizing flexible epoxy monomers containing thioether bonds into an alicyclic epoxy backbone. Without adding any inorganic fillers, it achieves complete curing under atmospheric conditions, real-time relaxation of curing internal stress, and long-term dimensional stability. This effectively solves the problems of existing cationic UV adhesives, such as the inability to release curing internal stress, reliance on inorganic fillers to reduce shrinkage, and severe oxygen inhibition in free radical systems.

[0007] This invention also provides a method for preparing and applying the dual-mechanism low-deformation UV adhesive with oxygen-free polymerization-inhibited cationic curing and dynamic thioether bond toughening.

[0008] Technical Solution: To achieve the above objectives, the present invention provides a dual-mechanism low-deformation UV adhesive with oxygen-free, cationic curing and dynamic thioether bond toughening, comprising a cationic photocuring system and a crosslinking component containing dynamic thioether bonds. The cationic photocuring system includes an alicyclic epoxy resin and a cationic photoinitiator. The crosslinking component containing dynamic thioether bonds is a thioether compound containing at least two epoxy functional groups, which is embedded in the epoxy backbone through ring-opening polymerization to form dynamic thioether crosslinking nodes.

[0009] The adhesive of this invention contains no inorganic fillers (such as silica, alumina, boron nitride, POSS, etc.), and achieves low deformation and stress relief entirely through organic molecular structure design. The adhesive is irradiated with a 365nm UV light source to initiate a cationic polymerization reaction, and can be completely cured in a normal atmospheric environment (without nitrogen protection), forming a network structure in which rigid epoxy backbones and reversible sulfide dynamic crosslinking points interweave. The internal stress generated during curing can be introduced into flexible polar segments through sulfide bonds, and stress relaxation is promoted through segment movement, thereby achieving a self-stress relief effect.

[0010] The adhesive, by weight, comprises: alicyclic epoxy resin: 75-88 parts; crosslinking component containing sulfide dynamic bonds: 6-15 parts; monofunctional epoxy diluent: 3-8 parts; cationic photoinitiator: 2.5-5 parts; leveling agent: 0.2-1.0 parts.

[0011] The alicyclic epoxy monomer is selected from one or more of the following: 3,4-epoxycyclohexylcarboxylic acid 3',4'-epoxycyclohexylmethyl ester, bis((3,4-epoxycyclohexyl)methyl) adipate, and 4,5-epoxycyclohexane-1,2-dicarboxylic acid diglycidyl ester.

[0012] Furthermore, the alicyclic epoxy resin is preferably 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylic acid ester (such as UVR-6110) or its derivatives, which have high cationic polymerization activity and low intrinsic shrinkage.

[0013] The crosslinking component containing the thioether dynamic bond is selected from at least one of 4,4'-thiobisphenol diglycidyl ether and 4,4'-di(2,3-epoxypropoxy)diphenyl disulfide. 4,4'-di(2,3-epoxypropoxy)diphenyl disulfide can be used as a comparative reagent. This type of compound contains two epoxy end groups and can participate in cationic ring-opening polymerization, suspending the thioether bond (or disulfide bond) in the crosslinking network. The thioether bond exhibits dynamic reversibility at room temperature, and the topological network can be reconstructed through a sulfur-sulfur exchange reaction, thereby relaxing local internal stresses.

[0014] The monofunctional epoxy diluent is used to adjust viscosity and can be selected from n-butyl glycidyl ether, phenyl glycidyl ether or p-tert-butylphenyl glycidyl ether. It does not participate in the construction of cross-linking network and exists as a small molecule plasticizer after curing, which helps to improve the chain segment mobility and assist stress relaxation.

[0015] The cationic photoinitiator (arylthionium salt cationic photoinitiator) is triarylthionium hexafluoroantimonate or triarylthionium hexafluorophosphate, which decomposes under 365 nm ultraviolet light to produce a super acid, initiating epoxy ring-opening polymerization.

[0016] The leveling agent is polyether-modified polydimethylsiloxane, used to improve coating performance.

[0017] The method for preparing the adhesive according to the present invention includes the following steps:

[0018] (1) Under light-protected conditions, mix the alicyclic epoxy monomer, the sulfur-containing flexible epoxy copolymer monomer, and the monofunctional epoxy diluent.

[0019] (2) Add the surface leveling agent to the mixture from step (1) and stir to disperse;

[0020] (3) Continue to add aryl thioonium salt cationic photoinitiator, and stir in the dark until completely dissolved to obtain a transparent and homogeneous liquid;

[0021] (4) The obtained liquid is degassed under vacuum to obtain the finished product.

[0022] Further, the preparation includes the following steps:

[0023] (1) Under light-protected conditions, alicyclic epoxy resin, crosslinking component containing sulfide dynamic bonds and monofunctional epoxy diluent are added to a stirring container and stirred at 200-300 rpm for 20-30 min at 20-30℃ until the mixture is uniform.

[0024] (2) Add leveling agent and continue stirring for 5-10 min;

[0025] (3) Add the cationic photoinitiator and stir at 100-200 rpm for 10-20 min under light-protected conditions until completely dissolved;

[0026] (4) Transfer the mixture to a vacuum degassing machine and degas it for 10-20 minutes under a vacuum of -0.095 MPa. Then seal it in an opaque container to obtain the finished product.

[0027] The curing mechanism of the adhesive described in this invention is as follows: Under 365 nm ultraviolet light irradiation, the cationic photoinitiator decomposes to produce protic acid, initiating ring-opening polymerization of epoxy groups. Alicyclic epoxy monomers rapidly crosslink to form a rigid main network, endowing the adhesive layer with high modulus, low thermal expansion, and heat resistance. Simultaneously, epoxy comonomers containing thioether bonds participate in polymerization through their two-terminal epoxy groups, introducing dynamic thioether bonds as crosslinking nodes into the network. Driven by curing internal stress, the thioether bonds promote topological rearrangement of the crosslinked network through reversible conformational changes and chain segment cooperative motion, achieving real-time stress relaxation. In the dark reaction stage (after light irradiation stops), cationic polymerization continues deep curing, and the thioether bonds are continuously fine-tuned, ultimately forming a low-stress, low-creep equilibrium network.

[0028] This invention provides a cationic UV-curable adhesive, particularly a low-deformation UV adhesive that utilizes sulfide dynamic bonds to achieve stress relief and oxygen-free polymerization inhibition, along with its preparation method and applications. The adhesive comprises an alicyclic epoxy resin, a flexible epoxy comonomer containing sulfide bonds, and a cationic photoinitiator. Under UV irradiation, it undergoes cationic ring-opening polymerization to form a three-dimensional network structure with sulfide dynamic crosslinking nodes embedded in the rigid epoxy backbone. This adhesive can be completely cured in air without oxygen-induced polymerization inhibition. During and after curing, the sulfide dynamic bonds achieve real-time stress relaxation through reversible breakage and recombination, resulting in a volume shrinkage rate ≤1.5% and excellent long-term dimensional stability (creep rate <0.3% at 85℃ / 1000h). This invention does not rely on any inorganic fillers (such as silica, POSS, etc.), achieving low deformation, low internal stress, and high heat resistance in a purely organic system, making it widely applicable in precision electronic packaging, optical component bonding, and wearable device assembly.

[0029] This invention does not simply combine a cationic photocurable system with a flexible monomer containing a sulfide structure physically. Instead, it is based on the synergistic design concept of "low-shrinkage curing network construction" and "dynamic stress regulation network construction". Through the synergistic effect of cationic ring-opening polymerization and dynamic sulfide bond structure, it achieves the suppression of internal stress at the source during curing and the continuous release of residual stress during service, thereby obtaining a UV-curable adhesive system with low deformation, high dimensional stability, high adhesion and excellent toughness.

[0030] Currently, traditional free radical photopolymerization systems undergo a rapid reduction in free volume during polymerization, accompanied by the conversion of carbon-carbon double bonds to single bonds. This leads to significant volume shrinkage and curing stress, resulting in problems such as substrate warping, interface debonding, and dimensional instability. In contrast, this invention employs a cationic ring-opening polymerization mechanism. The epoxy groups can partially compensate for the volume shrinkage caused by polymerization during the ring-opening process, reducing curing stress at its source. Simultaneously, this invention introduces sulfide-containing dynamic structural units, enabling the resulting crosslinked network to possess not only the mechanical support of traditional thermosetting networks but also dynamic stress dissipation capabilities. Under external heat or stress, the sulfide bonds can undergo reversible topological reconstruction, thereby achieving local network structure adjustment and continuously releasing residual stress accumulated within the network. This process differs from the passive buffering effect provided by ordinary flexible segments; it is an active dynamic stress regulation mechanism.

[0031] Therefore, this invention constructs a dual-mechanism network structure that combines structural support and dynamic stress regulation, achieving synergistic regulation of "stress reduction" and "continuous stress release".

[0032] Currently, existing low-shrinkage UV adhesives mainly employ the following two technical approaches:

[0033] (1) The curing stress is relieved by reducing the system modulus by introducing flexible chain segments;

[0034] (2) Reduce curing shrinkage by using a cationic epoxy system.

[0035] However, both of these technical approaches have significant limitations. While simply introducing flexible segments can improve toughness, it often leads to problems such as a decrease in glass transition temperature, poorer dimensional stability, and increased long-term creep. Although using a cationic epoxy system alone can reduce shrinkage, the cured stress that has already formed will remain within the network structure for a long time, making it impossible to achieve continuous release.

[0036] This invention breaks through the limitations of traditional technical routes and, for the first time, synergistically designs a dynamic sulfide structure with a cationic epoxy ring-opening polymerization system to construct a dual-functional network structure that combines low shrinkage characteristics with dynamic stress relaxation capabilities. This allows the curing stress to not only be effectively suppressed but also to be continuously released during subsequent service, thereby achieving a balance between low deformation, high dimensional stability, and high reliability.

[0037] The combination of the cationic epoxy system and the dynamic sulfide structure in this invention is not a simple superposition, but rather presents a high level of technical difficulty in implementation.

[0038] First, the sulfur atoms in the sulfide structure possess certain nucleophilic properties, which theoretically could affect the cationic active centers, thereby reducing cationic polymerization efficiency and even leading to incomplete curing. Therefore, achieving compatibility and balance between the dynamic sulfide structure and the cationic photocuring system is a significant technical challenge for those skilled in the art. Second, while excessively high dynamic structure content is beneficial for stress release, it leads to decreased network rigidity and poor dimensional stability; conversely, excessively low dynamic structure content fails to demonstrate a significant stress relaxation effect. Therefore, a reasonable balance needs to be established between crosslinking density, network rigidity, and dynamic reconstruction capability. This invention, through specific structural design and component ratio control, enables the dynamic sulfide structure to retain the highly crosslinked network characteristics of the cationic curing system while simultaneously imparting continuous stress release capability to the material, achieving a synergistic unity of the two functions. Experimental results show that the effects obtained by the system of this invention are not simply additive but exhibit a significant synergistic effect. Cationic ring-opening polymerization reduces stress generation, while the dynamic sulfide structure promotes stress release. The combined effect of both reduces warpage by approximately 80%, far superior to the results obtained by using a cationic system alone or a flexible toughening structure alone, demonstrating a significant synergistic effect.

[0039] This invention breaks with traditional thinking, shifting the focus of material design from "reducing shrinkage" to "regulating stress." This invention argues that the key factor affecting adhesive reliability is not simply shrinkage rate, but rather the formation, accumulation, and release of stress within the network. Therefore, this invention constructs a dual-mechanism synergistic network structure: first, a cationic epoxy ring-opening polymerization network; second, a dynamic sulfide stress regulation network. The cationic epoxy ring-opening polymerization is responsible for reducing stress generation, while the dynamic sulfide structure is responsible for promoting stress release, forming a complete stress management system. In other words, this invention not only reduces the source of stress but also endows the material with the ability to actively release stress.

[0040] Therefore, the most fundamental innovation of this invention lies not in the introduction of a new monomer, but in the first-ever establishment of a dual-mechanism synergistic design concept of "low-shrinkage curing + dynamic stress release." This design concept realizes a technological shift from the traditional "enhancing material properties" to "actively regulating the internal stress of the material." This invention not only solves the strength problem of adhesives, but more importantly, it solves the long-standing problems of internal stress accumulation, warpage, and long-term dimensional instability in the field of UV adhesives, thereby significantly improving the application value of materials in electronic packaging, optical bonding, and high-reliability bonding.

[0041] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0042] (1) The present invention adopts a cationic curing system. The curing process is not inhibited by oxygen and does not require nitrogen protection. The thick adhesive layer and the shielded area can be completely cured in an atmospheric environment.

[0043] (2) This invention introduces dynamic covalent bonds of thioethers into cationic UV-cured epoxy networks for the first time. Under the internal stress generated during curing, the thioether bonds can undergo reversible breakage and recombination, thereby relaxing local stress in real time and solving the problems of warping and microcracks caused by curing shrinkage and thermal cycling from a mechanistic perspective.

[0044] (3) This invention does not add inorganic fillers such as silica and POSS, and is a pure organic crosslinking system, which avoids problems such as filler agglomeration, sedimentation, decrease in light transmittance and viscosity surge.

[0045] (4) The rigid epoxy backbone ensures a low coefficient of thermal expansion, and the dynamic sulfide nodes provide stress relaxation capability. The two work together to make the adhesive layer exhibit extremely low deformation and creep under long-term storage and temperature cycling. Attached Figure Description

[0046] Figure 1 Comparison of the warpage of aluminum substrates after curing the adhesives prepared in Examples 1-3 and Comparative Examples 1-5;

[0047] Figure 2The graph shows a comparison of the dimensional change rates of the adhesives prepared in Examples 1-3 and Comparative Examples 1-5 after aging at 85°C / 85%RH for 500 hours.

[0048] Figure 3 The images show a comparison of the impact resistance height of the adhesives prepared in Examples 1-3 and Comparative Examples 1-5 after curing. Detailed Implementation

[0049] The present invention will be further described below with reference to embodiments and comparative examples.

[0050] Unless otherwise specified, all materials and reagents used in the following examples are commercially available. Experimental methods not specifically described in the examples are generally performed under standard conditions or as recommended by the manufacturer.

[0051] The raw materials used in the embodiments of this invention are as follows:

[0052] Alicyclic epoxy monomer (ERL-4221 (3,4-epoxycyclohexylmethyl group-3',4'-epoxycyclohexane carboxylate), Polysciences, product number 24738) was purchased from Dow Chemical Company, USA.

[0053] The sulfur-containing flexible epoxy copolymer monomer (using 4,4'-thiobisphenol diglycidyl ether, CAS: 16558-06-8) was purchased from Shanghai Bid Pharmaceutical.

[0054] The flexible epoxy comonomer containing disulfide (using 4,4'-bis(2,3-epoxypropoxy)diphenyl disulfide, CAS: 17735-65-8) was purchased from Shanghai Bid Pharmaceutical.

[0055] Common flexible epoxy monomer (polypropylene glycol diglycidyl ether, CAS: 26142-30-3), purchased from Aladdin Biochemical Technology Co., Ltd., item number: P135604.

[0056] Aromatic bifunctional epoxy monomer (bisphenol A diglycidyl ether (DGEBA), E-51 type epoxy resin) was purchased from Kunshan Guodu Chemical Co., Ltd.

[0057] Triarylthionium hexafluorophosphate (UVI-6976) was purchased from Dow Chemical Company, USA.

[0058] The monofunctional epoxy diluent (p-tert-butylphenyl glycidyl ether, product number: B4831) was purchased from TCI (Shanghai) Chemical Industry Development Co., Ltd.

[0059] Polyether-modified polydimethylsiloxane (BYK-333) was purchased from BYK Chemicals, Germany.

[0060] Example 1

[0061] Adhesive formulation (parts by weight): 84 parts alicyclic epoxy monomer (ERL-4221), 6 parts sulfide-containing flexible epoxy comonomer (4,4'-thiobisphenol diglycidyl ether), 4 parts triarylthionium hexafluorophosphate, 5.5 parts p-tert-butylphenyl glycidyl ether, and 0.5 parts BYK-333.

[0062] Preparation method:

[0063] (1) Under light-proof and room temperature (25℃), 84 parts of alicyclic epoxy monomer, 6 parts of sulfur-containing flexible epoxy copolymer monomer and 5.5 parts of p-tert-butylphenyl glycidyl ether were added to the reactor and stirred at 400 rpm for 25 min to obtain mixed monomers.

[0064] (2) Add 0.5 parts of BYK-333 and disperse at a low speed of 150 rpm for 8 min.

[0065] (3) Add 4 parts of triarylthionium hexafluorophosphate, stir in the dark until completely dissolved, and obtain a homogeneous transparent liquid.

[0066] (4) Degas the above liquid under vacuum for 10 minutes, seal it in the dark, and obtain the finished adhesive.

[0067] The above adhesive was applied to the surface of a substrate such as glass, metal, or polyester film, and then irradiated with 365nm UV light in air, with a cumulative light energy of 2000 mJ / cm². 2 When the irradiation ends, the adhesive layer is cured at 80°C for 1 hour and then left at room temperature for 24 hours to obtain the cured adhesive layer.

[0068] Example 2

[0069] Adhesive formulation (parts by weight): 80 parts alicyclic epoxy monomer (ERL-4221), 10 parts sulfide-containing flexible epoxy comonomer (4,4'-thiobisphenol diglycidyl ether), 4 parts triarylthionium hexafluorophosphate, 5.5 parts p-tert-butylphenyl glycidyl ether, and 0.5 parts BYK-333.

[0070] Preparation method:

[0071] (1) Under light-proof and room temperature (25℃), 80 parts of alicyclic epoxy monomer, 10 parts of sulfur-containing flexible epoxy copolymer monomer and 5.5 parts of diluent p-tert-butylphenyl glycidyl ether were added to the reactor and stirred at 400 rpm for 25 min to obtain mixed monomers.

[0072] (2) Add 0.5 parts of BYK-333 and disperse at a low speed of 150 rpm for 8 min.

[0073] (3) Add 4 parts of triarylthionium hexafluorophosphate, stir in the dark until completely dissolved, and obtain a homogeneous transparent liquid.

[0074] (4) Degas the above liquid under vacuum for 10 minutes, seal it in the dark, and obtain the finished adhesive.

[0075] The above adhesive was applied to the surface of a substrate such as glass, metal, or polyester film, and then irradiated with 365nm UV light in air, with a cumulative light energy of 2000 mJ / cm². 2 When the irradiation ends, the adhesive layer is cured at 80°C for 1 hour and then left at room temperature for 24 hours to obtain the cured adhesive layer.

[0076] Example 3

[0077] Adhesive formulation (parts by weight): 75 parts alicyclic epoxy monomer (ERL-4221), 15 parts sulfide-containing flexible epoxy comonomer (4,4'-thiobisphenol diglycidyl ether), 4 parts triarylthionium hexafluorophosphate, 5.5 parts p-tert-butylphenyl glycidyl ether, and 0.5 parts BYK-333.

[0078] Preparation method:

[0079] (1) Under light-proof and room temperature (25℃), 75 parts of alicyclic epoxy monomer, 15 parts of sulfur-containing flexible epoxy copolymer monomer and 5.5 parts of diluent p-tert-butylphenyl glycidyl ether were added to the reactor and stirred at 400 rpm for 25 min to obtain mixed monomers.

[0080] (2) Add 0.5 parts of BYK-333 and disperse at a low speed of 150 rpm for 8 min.

[0081] (3) Add 4 parts of triarylthionium hexafluorophosphate, stir in the dark until completely dissolved, and obtain a homogeneous transparent liquid.

[0082] (4) Degas the above liquid under vacuum for 10 minutes, seal it in the dark, and obtain the finished adhesive.

[0083] The above adhesive was applied to the surface of a substrate such as glass, metal, or polyester film, and then irradiated with 365nm UV light in air, with a cumulative light energy of 2000 mJ / cm². 2 When the irradiation ends, the adhesive layer is cured at 80°C for 1 hour and then left at room temperature for 24 hours to obtain the cured adhesive layer.

[0084] Comparative Example 1 (Free flexible epoxy comonomer without sulfur ether)

[0085] The same method as in Example 2 was used, except that no sulfur-containing flexible epoxy comonomer was added.

[0086] Adhesive formulation (parts by weight): 80 parts alicyclic epoxy monomer (ERL-4221), 4 parts triarylthionium hexafluorophosphate, 5.5 parts p-tert-butylphenyl glycidyl ether, and 0.5 parts BYK-333.

[0087] Preparation method:

[0088] (1) Under light-proof and room temperature (25℃), 80 parts of alicyclic epoxy monomer and 5.5 parts of diluent p-tert-butylphenyl glycidyl ether were added to the reaction vessel and stirred at 400 rpm for 25 min to obtain mixed monomers.

[0089] (2) Add 0.5 parts of BYK-333 and disperse at a low speed of 150 rpm for 8 min.

[0090] (3) Add 4 parts of triarylthionium hexafluorophosphate, stir in the dark until completely dissolved, and obtain a homogeneous transparent liquid.

[0091] (4) Degas the above liquid under vacuum for 10 minutes, seal it in the dark, and obtain the finished adhesive.

[0092] Curing: The above adhesive is applied to the surface of a substrate such as glass, metal, or polyester film, and then irradiated with 365nm UV light in air, with a cumulative light energy of 2000 mJ / cm². 2 When the irradiation ends, the adhesive layer is cured at 80°C for 1 hour and then left at room temperature for 24 hours to obtain the cured adhesive layer.

[0093] Tests (see Example 8 for test methods) showed that the cured adhesive layer of Comparative Example 1 exhibited a creep strain of 0.28% and a creep rate of <0.3% after 1000 hours at 85℃ / 0.5 MPa. The creep strain of Experimental Example 2 was 0.06%, representing a decrease of approximately 75.6% compared to Comparative Example 1, indicating that the system of this invention possesses superior creep resistance. This is because the introduction of a sulfur-containing flexible epoxy comonomer in this invention maintains a high crosslinking density in the cationic epoxy network while improving the uniformity of the network structure and reducing stress concentration during curing. Under external loads, the network structure can more uniformly disperse stress, effectively suppressing continuous slippage and permanent deformation of polymer chains, resulting in lower creep strain in the material.

[0094] Comparative Example 2

[0095] The same method as in Example 2 was used, except that a common flexible epoxy monomer (polypropylene glycol diglycidyl ether) was used instead of a sulfur-containing flexible epoxy comonomer.

[0096] Adhesive formulation (parts by weight): 80 parts alicyclic epoxy monomer (ERL-4221), 10 parts polypropylene glycol diglycidyl ether, 4 parts triarylthionyl hexafluorophosphate, 5.5 parts p-tert-butylphenyl glycidyl ether, and 0.5 parts BYK-333.

[0097] Preparation method:

[0098] (1) Under light-protected and room temperature (25℃), 80 parts of alicyclic epoxy monomer, 10 parts of polypropylene glycol diglycidyl ether, and 5.5 parts of diluent p-tert-butylphenyl glycidyl ether were added to the reaction vessel and stirred at 400 rpm for 25 min to obtain mixed monomers.

[0099] (2) Add 0.5 parts of BYK-333 and disperse at a low speed of 150 rpm for 8 min.

[0100] (3) Add 4 parts of triarylthionium hexafluorophosphate, stir in the dark until completely dissolved, and obtain a homogeneous transparent liquid.

[0101] (4) Degas the above liquid under vacuum for 10 minutes, seal it in the dark, and obtain the finished adhesive.

[0102] The above adhesive was applied to the surface of a substrate such as glass, metal, or polyester film, and then irradiated with 365nm UV light in air, with a cumulative light energy of 2000 mJ / cm². 2 When the irradiation ends, the adhesive layer is cured at 80°C for 1 hour and then left at room temperature for 24 hours to obtain the cured adhesive layer.

[0103] Comparative Example 3 (using a flexible epoxy comonomer containing disulfide)

[0104] The same method as in Example 2 was used, except that a disulfide-containing flexible epoxy comonomer (bis(4-epoxypropoxyphenyl)disulfide) was used instead of a sulfur-containing flexible epoxy comonomer.

[0105] Adhesive formulation (parts by weight): 80 parts alicyclic epoxy monomer (ERL-4221), 10 parts bis(4-epoxypropoxyphenyl) disulfide, 4 parts triarylthionyl hexafluorophosphate, 5.5 parts p-tert-butylphenyl glycidyl ether, and 0.5 parts BYK-333.

[0106] Preparation method:

[0107] (1) Under light-protected and room temperature (25℃), 80 parts of alicyclic epoxy monomer, 10 parts of disulfide monomer bis(4-epoxypropoxyphenyl) disulfide, and 5.5 parts of diluent p-tert-butylphenyl glycidyl ether were added to the reaction vessel and stirred at 400 rpm for 25 min to obtain mixed monomers.

[0108] (2) Add 0.5 parts of BYK-333 and disperse at a low speed of 150 rpm for 8 min.

[0109] (3) Add 4 parts of triarylthionium hexafluorophosphate, stir in the dark until completely dissolved, and obtain a homogeneous transparent liquid.

[0110] (4) Degas the above liquid under vacuum for 10 minutes, seal it in the dark, and obtain the finished adhesive.

[0111] The above adhesive is applied to the surface of a substrate such as glass, metal or polyester film, and then irradiated in air with 365nm UV light. When the cumulative light energy reaches 2000 mJ / cm², the irradiation is stopped, and the adhesive is cured at 80°C for 1 hour and then left at room temperature for 24 hours to obtain a cured adhesive layer.

[0112] Comparative Example 4 (Common Aromatic Epoxy Monomer)

[0113] The same preparation and curing methods as in Example 2 were used.

[0114] Replace 10 parts of sulfur-containing flexible epoxy comonomer with 10 parts of bisphenol A diglycidyl ether.

[0115] Adhesive formulation (parts by weight): 80 parts alicyclic epoxy monomer (ERL-4221), 10 parts bisphenol A diglycidyl ether, 4 parts triarylthionyl hexafluorophosphate, 5.5 parts p-tert-butylphenyl glycidyl ether, and 0.5 parts BYK-333.

[0116] Comparative Example 5 (Packaging System)

[0117] The same preparation and curing methods as in Example 2 were used.

[0118] The difference lies in the addition of 5 parts silica (SiO2) and the reduction of the proportion of alicyclic epoxy monomers.

[0119] Adhesive formulation (parts by weight): 80 parts alicyclic epoxy monomer, 10 parts sulfide-containing flexible epoxy comonomer, 5 parts silica, 4 parts triarylthionium hexafluorophosphate, 5.5 parts p-tert-butylphenyl glycidyl ether, and 0.5 parts BYK-333.

[0120] Example 4

[0121] Cured surface state test under atmospheric conditions

[0122] For the adhesives prepared in Examples 1-3 and Comparative Examples 1-5, each adhesive was coated onto a glass slide and cured with 365nm UV in an atmospheric environment (curing was the same as in Example 1). The surface state of the cured adhesive layer was observed. The results are shown in Table 1.

[0123] Table 1. Cured surface state of different adhesives under atmospheric conditions

[0124]

[0125] As can be seen from Table 1, all cationic systems do not have oxygen inhibition problems, and there is no difference between the present invention and the comparative examples in this respect.

[0126] Example 5

[0127] Curing shrinkage rate and internal stress characterization

[0128] For the adhesives prepared in Examples 1-3 and Comparative Examples 1-5, the volume change before and after curing was tested using the density method (ASTM D792), and the volume shrinkage rate was calculated. The internal stress was tested using the substrate bending method: the adhesive was coated onto an aluminum foil substrate, cured (the curing process was the same as in Example 1), and then the warpage of the aluminum foil was measured using a laser displacement sensor; a greater warpage indicated greater internal stress. Each sample was tested three times, and the average value was taken. The results are shown in Table 2.

[0129] Table 2. Volume shrinkage and warpage of aluminum substrate with different adhesives

[0130]

[0131] As shown in Table 2, the embodiments of the present invention and each comparative example can form a complete cured film under ultraviolet light irradiation, but there are significant differences in the curing shrinkage and warpage properties of different systems.

[0132] In the system of the present invention, Comparative Example 1, without the introduction of a flexible control structure, achieved a curing shrinkage rate of 2.05% and a warpage of 0.48 mm; Comparative Example 2, after introducing a common flexible epoxy monomer, showed that the curing shrinkage rate remained basically unchanged or increased, and the warpage decreased, but the improvement was limited; Comparative Example 3, after using a flexible epoxy copolymer monomer containing disulfide, showed that the material's stress release capability was improved to a certain extent, and the warpage decreased to 0.40 mm.

[0133] In contrast, after introducing a sulfur-containing flexible epoxy comonomer in Examples 1-3 of this invention, the shrinkage rate was reduced to below 1.50%, and the warpage was controlled within 0.11 mm. Example 2 showed the best performance, with a shrinkage rate of only 1.38% and a warpage of only 0.08 mm. Analysis suggests that the introduction of the sulfur-containing structure can improve the coordinated movement of the network segments, effectively dispersing the internal stress generated during curing, thereby significantly reducing warpage caused by curing shrinkage.

[0134] In Comparative Example 4, after replacing the sulfur-containing flexible epoxy comonomer with bisphenol A diglycidyl ether, the warpage increased to 0.24 mm. This indicates that although ordinary aromatic epoxy structures have high rigidity, they lack effective stress buffering capacity and are difficult to achieve the low deformation effect obtained by this invention.

[0135] Comparative Example 5, after modification with nano-silica filler, showed a further reduction in shrinkage to 1.25%, indicating that inorganic fillers can suppress volume shrinkage through physical filling. However, its warpage still reached 0.17 mm, higher than that of Example 2, indicating that relying solely on filler constraint is insufficient to fully release curing internal stress, while the present invention achieves superior low warpage performance through molecular structure design.

[0136] Depend on Figure 1 It can be seen that the warpage shows the lowest value in the example, the highest value in comparative examples 1-3, and the middle value in comparative examples 4 and 5. This indicates that the present invention effectively reduces the internal stress generated by curing shrinkage through the synergistic design of each functional component, thereby significantly suppressing substrate warpage and improving the dimensional stability and service reliability of the adhesive.

[0137] Example 6

[0138] Linear expansion coefficient and dimensional stability test

[0139] For the adhesives prepared in Examples 1-3 and Comparative Examples 1-5, the coefficient of linear expansion (CTE) of the cured adhesive layer (10 mm × 10 mm × 1 mm, cured as in Example 1) in the range of 30-150℃ was tested using a thermomechanical analyzer (TMA, TA Instruments Q400). The cured adhesive layer was aged in an 85℃ / 85%RH constant temperature and humidity chamber for 500 h, and the dimensional change rate in the longitudinal direction was measured. Each sample was tested three times and the average value was taken. The results are shown in Table 3.

[0140] Table 3. CTE and dimensional change rate under aging for different adhesives

[0141]

[0142] As shown in Table 3, the linear expansion coefficient of Comparative Example 1 reached 52 ppm / K, and the dimensional change rate reached 0.68%, indicating that the pure alicyclic epoxy system is prone to dimensional drift under long-term hot and humid conditions. Although Comparative Examples 2 and 3 showed some improvement, their dimensional stability was still significantly lower than that of the embodiments of the present invention.

[0143] Examples 1-3 all exhibited low coefficients of linear expansion and dimensional change rates, with Example 2 showing the best overall performance, possessing a coefficient of linear expansion of 55 ppm / K and a dimensional change rate of only 0.10%. This indicates that the sulfur-containing flexible epoxy comonomer can improve the stress distribution while maintaining network integrity, thereby enhancing the material's resistance to thermal stress and the effects of humid and hot environments.

[0144] The coefficient of linear expansion of Comparative Example 4 is close to that of Example 2, but the rate of dimensional change is significantly higher, indicating that relying solely on a rigid aromatic structure cannot achieve excellent dimensional stability.

[0145] Comparative Example 5, due to the introduction of nano-silica filler, exhibited a reduced coefficient of linear expansion to 48 ppm / K, the lowest among all samples, indicating that the inorganic filler has a significant effect on suppressing thermal expansion. However, its dimensional change rate was still higher than that of Example 2, demonstrating that the present invention can not only effectively control thermal expansion but also further improve long-term dimensional stability through network structure optimization, thereby achieving superior overall performance. The reason for this is that the dynamic bonds of the sulfide continuously undergo topological reconstruction during long-term service, fine-tuning the network structure effectively suppressing dimensional drift under humid and hot conditions, with significantly better results than disulfide and systems without dynamic bonds.

[0146] Depend on Figure 2 It can be seen that the dimensional change rate of the embodiments after damp heat aging is significantly lower than that of comparative examples 1-3, and remains at a low level of 0.10%-0.14%, indicating that the system of the present invention has excellent resistance to damp heat aging and long-term dimensional stability. Although the dimensional change rate of comparative example 5 is comparable to that of example 1, its lowest CTE does not bring an overall performance advantage, indicating that simply reducing the coefficient of linear expansion cannot guarantee good long-term service stability of the material; the synergistic effect of each functional component is the key to obtaining excellent anti-aging performance.

[0147] Example 7

[0148] Adhesion and impact resistance

[0149] The adhesives prepared in Examples 1-3 and Comparative Examples 1-5 were tested for adhesion according to GB / T 9286-2021 "Paints and Varnishes - Cross-cut Test" and for impact resistance according to GB / T 1732-2020 "Test Method for Impact Resistance of Paint Films". The adhesives were applied to the surface of Q235 steel (dry film thickness 50±5 μm), cured (same as in Example 1), and then tested. Each sample was tested three times, and the average value was taken. The results are shown in Table 4.

[0150] Table 4. Adhesion and impact resistance of different adhesives after curing

[0151]

[0152] As shown in Table 4, all samples achieved a grade of 0 after the cross-cut adhesion test, indicating that the constructed cationic photocurable epoxy systems could form a good interfacial bond with the substrate, and no obvious peeling of the cured adhesive layer was observed. The results show that all different formulation systems exhibit excellent adhesion performance; therefore, the differences in subsequent mechanical properties are not due to differences in interfacial bonding ability.

[0153] In terms of impact resistance, the samples showed significant differences. Comparative Example 1 had an impact resistance height of only 45 cm, indicating that the pure alicyclic epoxy system had high network rigidity after curing, making it prone to brittle fracture under impact loads. Comparative Examples 2 and 3 showed some improvement in impact resistance after introducing flexible structures, but the improvement was limited.

[0154] In Comparative Example 4, replacing the sulfur-containing flexible epoxy comonomer with bisphenol A diglycidyl ether increased the impact resistance height to 58 cm, indicating that the aromatic epoxy structure is beneficial for improving the overall mechanical properties of the material. However, its impact resistance is still lower than that of the embodiments of the present invention, indicating that ordinary aromatic epoxy structures are difficult to achieve effective stress control.

[0155] Comparative Example 5, modified with nano-silica, exhibited an impact resistance height of 55 cm. Inorganic fillers can improve the rigidity of the system, but localized stress concentrations easily form at the filler-resin interface, thus limiting further improvements in material toughness.

[0156] Depend on Figure 3 As can be seen, Examples 1-3 of the present invention all exhibit excellent impact resistance, with Example 2 reaching 66 cm, the highest among all samples. Analysis suggests that the introduction of the sulfide-containing flexible epoxy comonomer enhances the cooperative motion capability of the chain segments within the crosslinked network, enabling effective transfer and dissipation of external impact energy within the network, thereby significantly improving the material's impact resistance. These results demonstrate that the present invention, while maintaining excellent adhesion properties, can further improve the material's toughness and impact resistance, achieving a synergistic optimization of low deformation and high reliability.

[0157] Example 8

[0158] Supplementary performance testing methods

[0159] (1) Stress relaxation test: A TA Instruments DMA Q850 was used with a parallel plate fixture (8 mm in diameter). After curing (same as in Example 1), the sample thickness was 1 mm. The temperature was increased to 80 °C at 5 °C / min under a nitrogen atmosphere, held at the temperature for 5 min, and then a constant shear strain of 1% was applied. The modulus decay curve over time was recorded. The relaxation time was defined as the time required for the modulus to decay to 1 / e of the initial value. The results are shown in Table 5.

[0160] Table 5 shows the tests performed on the cured samples of Example 1 and Comparative Examples 1-5.

[0161]

[0162] As shown in Table 5, there are significant differences in stress relaxation behavior among different systems.

[0163] Comparative Example 1 and Comparative Example 2 did not reach the specified relaxation level within the test time range, and their relaxation time both exceeded 1000 min. This indicates that the cross-linked network formed by the pure alicyclic epoxy system and the ordinary flexible epoxy system is difficult to effectively release internal stress, and the network structure is in a highly restricted state.

[0164] In Comparative Example 3, the relaxation time decreased to 15.3 min after the introduction of a flexible epoxy comonomer containing disulfide, indicating that the stress release capability of the system was improved to some extent. The relaxation times of Comparative Examples 4 and 5 were 45.0 min and 35.0 min, respectively, indicating that both the ordinary aromatic epoxy structure and the inorganic filler-modified system can alleviate stress accumulation to some extent, but the improvement effect is still limited.

[0165] In contrast, Examples 1-3 of this invention all exhibited extremely rapid stress relaxation behavior, with relaxation times all below 11 minutes, and Example 2 showing a relaxation time of only 8.2 minutes. This indicates that the sulfur-containing flexible epoxy comonomer can effectively improve the mobility of network segments, promote rapid release of internal stress, and thus significantly reduce the residual stress generated during the curing process.

[0166] Combining the results of shrinkage rate, warpage and creep performance, it can be seen that the shortening of stress relaxation time has a good correlation with the low warpage and low creep performance of the material. Example 2 has the fastest stress release rate, so it exhibits the lowest warpage and creep strain, further proving that the present invention achieves excellent stress control capability and dimensional stability through the design of sulfide-containing structure. (2) Creep strain test: The cured adhesive layers prepared in Examples 1-3 and Comparative Examples 1-5 were processed into strip samples with a size of 35 mm × 10 mm × 1 mm.

[0167] The test was conducted using a DMA dynamic thermomechanical analyzer (TA Instruments Q850) in Creep Mode. During the test, the sample was heated to 85℃ and held at that temperature for 30 min under nitrogen protection to eliminate the influence of thermal history. Subsequently, a constant tensile stress of 0.5 MPa was applied and maintained for 1000 h. The strain of the sample was recorded in real time, and the results are shown in Table 6.

[0168] Creep strain is calculated using the following formula:

[0169] Where: C is the creep strain (%); ε0 is the strain value at the initial loading moment; ε t This represents the strain value at the end of the test.

[0170] Table 6 Creep strain test results

[0171]

[0172] As shown in Table 6, the creep resistance of different systems varies significantly.

[0173] Comparative Example 1 exhibited a creep strain of 0.28, showing significant deformation under sustained load. While Comparative Examples 2 and 3 showed some improvement, their creep strains still reached 0.25 and 0.18, respectively. Comparative Examples 4 and 5 further reduced these values ​​to 0.16 and 0.12, indicating that both aromatic epoxy structures and inorganic fillers can improve the material's resistance to long-term deformation to some extent.

[0174] In contrast, all embodiments of the present invention exhibit lower creep strain, with Example 2 showing only 0.06. This represents a reduction of approximately 78.6% compared to Comparative Example 1 and approximately 50% compared to Comparative Example 5. This indicates that the sulfide-containing structural network constructed by the present invention can more effectively suppress chain slip and permanent deformation, enabling the material to maintain excellent dimensional stability and structural integrity under long-term loads and complex service environments.

Claims

1. A dual-mechanism low-deformation UV adhesive with oxygen-free polymerization-inhibited cationic curing and dynamic thioether bond toughening, characterized in that, This adhesive is a pure organic crosslinking system, composed of the following components in parts by weight: 75-88 parts of alicyclic epoxy monomer, 6-15 parts of flexible epoxy comonomer containing sulfide structure, 2.5-5 parts of aryl thioonium salt cationic photoinitiator, 3-8 parts of monofunctional epoxy diluent, and 0.2-1.0 parts of surface leveling agent.

2. The adhesive according to claim 1, characterized in that, The alicyclic epoxy monomer is selected from one or more of the following: 3,4-epoxycyclohexylcarboxylic acid 3',4'-epoxycyclohexylmethyl ester, bis((3,4-epoxycyclohexyl)methyl) adipate, and 4,5-epoxycyclohexane-1,2-dicarboxylic acid diglycidyl ester.

3. The adhesive according to claim 1, characterized in that, The sulfur-containing flexible epoxy comonomer is selected from one or more of the following: 4,4'-thiobisphenol diglycidyl ether, 4,4'-bis(2,3-epoxypropoxy)diphenyl disulfide.

4. The adhesive according to claim 1, characterized in that, The aryl thioonium salt cationic photoinitiator is triaryl thioonium hexafluorophosphate or triaryl thioonium hexafluoroantimonate.

5. The adhesive according to claim 1, characterized in that, The monofunctional epoxy diluent is glycidyl tert-carbonate, p-tert-butylphenyl glycidyl ether, or n-butyl glycidyl ether; the surface leveling agent is polyether-modified polydimethylsiloxane.

6. A method for preparing the adhesive according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Under light-protected conditions, mix the alicyclic epoxy monomer, the sulfur-containing flexible epoxy copolymer monomer, and the monofunctional epoxy diluent. (2) Add a surface leveling agent to the mixing system of the mixed monomers in step (1) and stir to disperse; (3) Continue to add aryl thioonium salt cationic photoinitiator, and stir in the dark until completely dissolved to obtain a transparent and homogeneous liquid; (4) The obtained liquid is degassed under vacuum to obtain the finished product.

7. The preparation method according to claim 6, characterized in that, In step (1), stir at room temperature for 20-30 minutes until the mixture is uniform, and the stirring speed is 300-500 rpm; in step (2), disperse at low speed of 100-200 rpm for 5-10 minutes.

8. The use of the adhesive according to any one of claims 1-5 in electronic packaging, optical component bonding or precision component fixing.

9. The application according to claim 8, characterized in that, The adhesive is applied to the surface of the substrate and then cured by irradiating it with 365nm UV light in an air environment, with the light energy controlled between 1500 and 3000 mJ / cm².

10. The application according to claim 9, characterized in that, The curing process is divided into two stages: UV curing and dark reaction. During the UV irradiation stage, epoxy ring-opening polymerization forms a rigid main network, while thioether dynamic bonds begin to be constructed. After the irradiation stops, the cationic dark reaction continues, and the thioether bonds continue to undergo topological rearrangement, further releasing residual stress and optimizing the network structure.

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