Double-layer acrylic acid OCA optical cement for folding screen and structural characterization method of double-layer acrylic acid OCA optical cement

Abstract

The invention discloses a double-layer acrylic acid OCA optical adhesive for a folding screen and a structural characterization method of the double-layer acrylic acid OCA optical adhesive, and belongs to the technical field of optical adhesives. The OCA optical cement comprises an upper high-creep layer and a lower high-recovery layer, and an intersoluble cross-linked structure is formed on the interface of the two layers through a gradient curing process. The upper layer formula contains 70-90 parts of 2-ethylhexyl acrylate and 10-30 parts of hydroxy butyl acrylate; the formula of the lower layer comprises 20-35 parts of isobornyl acrylate, 50-70 parts of 2-ethylhexyl acrylate and 5-15 parts of hydroxy butyl acrylate. And the longitudinal crosslinking density gradient is represented by adopting a unilateral nuclear magnetic resonance depth analysis technology. The contradiction between high adhesion and high resilience of the folding screen OCA is solved, high light transmittance, low haze, high stripping force and high creep recovery rate are achieved, and the resistance change rate is remarkably reduced after dynamic folding.

Description

Technical Field

[0001] This invention belongs to the field of optical adhesive technology, specifically relating to a double-layer acrylic OCA optical adhesive for foldable screens and its structural characterization method. Background Technology

[0002] In recent years, with the diversification of application scenarios, the portability of consumer electronics, and the intelligentization of engineering technologies, new display technologies are constantly evolving towards thinner, more flexible, higher contrast, and higher environmental tolerance. The widespread adoption of terminal products such as smartphones and foldable devices continues to drive the development of display modules towards thinner, lighter, and more integrated designs, thereby placing more stringent performance requirements on key adhesive materials—optically clear adhesives (OCA).

[0003] OCA is a transparent adhesive with excellent optical properties. Currently, the ideal OCA for foldable screens needs to possess the following characteristics simultaneously: excellent optical transparency (high light transmittance, low haze), strong adhesion to different materials (such as glass cover plates, polarizers, PET / PI functional films), and a low glass transition temperature (T). g This ensures low-temperature flexibility and high creep recovery rate to avoid permanent creases or buckling after folding, and disperses bending stress by adjusting the position of the neutral plane to protect brittle functional layers (such as ITO electrodes).

[0004] In existing technologies, single-layer homogeneous OCA cannot simultaneously meet the aforementioned mutually restrictive performance requirements. For example, high crosslinking density can provide good creep resistance and resilience, but often sacrifices initial tack and filling ability for rough interfaces; while low crosslinking density or formulations mainly composed of soft monomers have good adhesion, but are prone to irreversible deformation under long-term stress, leading to bubbles, delamination or creases in foldable screens.

[0005] Chinese patent CN117229726A discloses a high-performance double-layer acrylic OCA optical adhesive and its preparation method. This high-performance double-layer acrylic OCA optical adhesive, through a combination of different formulations in its upper and lower layers, is adapted to the substrate characteristics of different bonding surfaces, aiming to improve interfacial adhesion and suppress bubble rebound. This design improves the compatibility and bonding stability of OCA with heterogeneous materials to some extent. However, such solutions still focus primarily on improving static adhesion performance. For the dynamic bending fatigue performance and effective characterization of the double-layer structure, which are crucial in foldable screen applications, a systematic solution has not yet been developed, making it difficult to fully meet the long-term reliability requirements of high-end foldable display devices. Summary of the Invention

[0006] The present invention aims to provide a double-layer acrylic OCA optical adhesive for foldable screens and a method for characterizing its structure. Through its unique upper and lower layer formulation design and integrated molding process, the present invention significantly improves creep resistance and dynamic folding durability while ensuring excellent interfacial adhesion reliability, and effectively suppresses the formation of permanent creases. Furthermore, the present invention provides a non-destructive characterization method based on single-sided nuclear magnetic resonance (NMR) technology to accurately evaluate and verify the double-layer structure.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A double-layer acrylic OCA optical adhesive for foldable screens comprises, from top to bottom: a first release film (light release film), a double-layer OCA optical adhesive, and a second release film (heavy release film).

[0009] The upper OCA layer is a high creep layer, designed with a focus on relatively low crosslinking density, low modulus, and excellent initial tack and interfacial wettability. Its main function is to tightly adhere to substrates of different materials (such as ITO and polarizers), provide durable and strong adhesion, and buffer interfacial stress.

[0010] The lower OCA layer is a high-recovery layer, designed with high cross-linking density, high modulus and excellent creep recovery rate. Its main function is to provide elastic recovery force during folding and resist permanent deformation.

[0011] The bilayer OCA structure exhibits a gradient crosslinking density in the thickness direction, meaning that the crosslinking density and storage modulus decrease gradually from the upper layer to the lower layer. This gradient structure is not a simple physical superposition, but rather a structure with continuously transitioning properties formed through specific coating and curing processes that allow partial mutual solubility and chemical crosslinking at the interface between the two layers.

[0012] Furthermore, the preparation method of the upper OCA layer (high creep layer) is as follows, in parts by weight:

[0013] Mix 70-90 parts of isooctyl acrylate and 10-30 parts of hydroxybutyl acrylate, then add dropwise 0.01-2.0 parts of crosslinking agent (1,6-hexanediol diacrylate) and 0.2 parts of photoinitiator D1173 (α-hydroxyisobutyrylphenyl). Stir under nitrogen atmosphere at 365 nm and 10 mW / cm². 2 Irradiate with ultraviolet light for 30 seconds to obtain a prepolymer mixture, i.e., the upper resin adhesive.

[0014] Furthermore, the preparation method of the lower OCA layer (high recovery layer) is as follows, in parts by weight:

[0015] Mix 20-35 parts of isoborneol acrylate, 50-70 parts of isooctyl acrylate, and 5-15 parts of hydroxybutyl acrylate. Add dropwise 0.01-2.0 parts of crosslinking agent (1,6-hexanediol diacrylate) and 0.2 parts of photoinitiator D1173 (α-hydroxyisobutyrylbenzene). Stir under nitrogen atmosphere at 365 nm and 10 mW / cm². 2 Irradiate with ultraviolet light for 30 seconds to obtain a prepolymer mixture, i.e., the lower layer resin adhesive.

[0016] The role of the main components in the preparation of resin adhesives:

[0017] Isoborneol acrylate, isooctyl acrylate, and hydroxybutyl acrylate: These three monomers are the main components of acrylic OCA optical adhesive, which polymerize in the reaction to form the polymer matrix of the adhesive.

[0018] 1,6-Hexanediol diacrylate: This is a crosslinking agent containing two double bonds, which links polymer chains during the reaction.

[0019] α-Hydroxyisobutyrylbenzene: This is a photoinitiator. Its activity is activated by ultraviolet irradiation, which initiates the polymerization reaction of monomers and promotes the curing of acrylic OCA optical adhesive.

[0020] This invention also provides a method for preparing the double-layer acrylic OCA optical adhesive, comprising the following steps:

[0021] S1. Preparation of the lower OCA film: After vacuum degassing of the lower resin adhesive, the resin adhesive is coated between the heavy release film and the light release film using a doctor blade coating method at 365nm and 50mW / cm. 2 Irradiate with ultraviolet light for 10 minutes to form the lower OCA film.

[0022] S2, Preparation of upper OCA film: Tear off the light release film obtained in S1, pour the upper acrylic OCA resin adhesive onto the lower modified acrylic OCA optical adhesive that has been coated in the first layer, and cover it with a new light release film. Apply the second type of resin adhesive between the heavy release film and the light release film containing the adhesive layer by a scraper coating method.

[0023] S3. Gradient Curing: The "release film / upper OCA adhesive / lower OCA film / release film" structure obtained in step S2 is cured under ultraviolet light at 365 nm and 50 mW / cm². 2 Irradiate with ultraviolet light for 10 minutes to allow the upper layer of adhesive to fully cure and undergo interfacial cross-linking with the surface of the lower OCA film, ultimately forming an integrated double-layer gradient structure OCA optical adhesive.

[0024] Another core aspect of this invention lies in providing a method for characterizing the longitudinal gradient structure of the aforementioned double-layer OCA optical adhesive, employing single-sided nuclear magnetic resonance depth profiling technology. The specific steps are as follows:

[0025] a) Place the sample (double-layer OCA film after peeling off the release film) on the NMR-MOUSE (single-sided nuclear magnetic resonance) instrument sample stage.

[0026] b) Run the Profile sequence and use a high-precision lifting stage to control the sensor to scan in steps along the sample thickness direction (stepping accuracy can reach 10μm).

[0027] c) Acquire the CPMG echo attenuation signal at each depth point to obtain the lateral relaxation time at that location.

[0028] d) Process the obtained depth-relaxation time two-dimensional data to draw a depth dimension profile or a two-dimensional relaxation spectrum.

[0029] e) By analyzing the distribution and variation of lateral relaxation time at different depths, the crosslinking density gradient of the bilayer OCA in the thickness direction can be directly observed and quantified. If a gradient structure exists, the spectrum will show an asymmetric distribution of relaxation time with depth or the appearance of a transition region.

[0030] This explanation describes the modification of acrylic OCA optical adhesive by controlling the crosslinking density gradient to improve its adhesion and dynamic folding properties. The likely reason and mechanism is that the crosslinking density gradient can alter the overall adhesive and mechanical properties of the acrylic OCA. A layer with a lower crosslinking density can disperse the stress experienced by the multilayer film during bending, while a layer with a higher crosslinking density can enhance the adhesive's strength and durability, thus improving adhesion and dynamic folding properties. The crosslinking density gradient can also regulate the overall viscoelasticity and surface wettability of the OCA. These structural changes can enhance the adhesive's strength and toughness, thereby improving its dynamic folding properties.

[0031] This invention aims to significantly improve the adhesion and dynamic folding performance of acrylic OCA optical adhesive in foldable screen applications by controlling the crosslinking density structure. Its mechanism mainly includes: a corresponding relationship exists between the splitting of the neutral plane and the crosslinking density; as the film thickness in the module increases and the folding radius decreases, the use of a low-elasticity optical adhesive can make the splitting of the film's neutral planes tend to be complete; combining a thicker and harder substrate with a softer OCA can effectively generate multiple neutral planes, protecting the module in the bending direction.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] 1. This invention achieves both high resilience and high adhesion within a single adhesive film through differentiated formulations for the upper and lower layers and a gradient curing process. This solves the contradictory requirements of foldable screens for OCA, which must both adhere firmly and have good resilience, and realizes a gradient design with balanced performance.

[0034] 2. The present invention uses a gradient structure with high creep in the upper layer and high modulus in the lower layer to help generate multiple or split neutral surfaces during folding, so that the maximum strain is far away from the brittle ITO and other functional layers, which significantly improves the durability of the foldable screen and realizes the protective functional layer.

[0035] 3. This invention is the first to apply the single-sided nuclear magnetic resonance deep profiling technology system to the structural characterization of double-layer OCA, which can reveal its longitudinal gradient structure non-destructively, intuitively and quantitatively, providing a powerful tool for product development and quality control, and using advanced and reliable characterization methods.

[0036] 4. The product prepared by this invention has high light transmittance (>95%), low haze (<0.5%), strong peel force (>10N / 25mm for glass), high creep recovery rate (>90%), and excellent dynamic folding performance (observation of resistance change during folding test), thus realizing the verification of comprehensive performance. Attached Figure Description

[0037] To illustrate the embodiments of the present invention and the design techniques in the prior art in more detail, the accompanying drawings, which are essential references in describing the embodiments or the prior art, will be briefly outlined below. The present invention will be further described below with reference to the accompanying drawings.

[0038] Figure 1 This is a schematic diagram of the structure of the double-layer acrylic OCA optical adhesive of the present invention and a schematic diagram of its folding test sample.

[0039] Figure 2 The two-dimensional NMR depth-relaxation time spectra of the bilayer OCA film prepared in Example 1 and the comparative single-layer OCA film are compared.

[0040] Figure 3 This is a comparison chart showing the rate of change of resistance of the ITO film after 50 dynamic folding tests between Example 1 and the comparative example. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] like Figure 1 As shown, the basic structure of the double-layer acrylic OCA optical adhesive of the present invention is as follows: from top to bottom, it includes a first release film (light release film), an upper OCA layer (high creep layer), a lower OCA layer (high recovery layer), and a second release film (heavy release film). In application, the release film is removed, the upper high creep layer is attached to a brittle display module (such as a polarizer or ITO film), and the lower high recovery layer is attached to a cover plate (PET or CPI film).

[0043] Example 1

[0044] Fabrication of the lower OCA layer (high recovery layer):

[0045] Weigh out 33 parts of isobornyl acrylate (IBOA), 53 parts of isooctyl acrylate (2-EHA), 14 parts of hydroxybutyl acrylate (4-HBA), 0.01 parts of 1,6-hexanediol diacrylate (HDDA), and 0.2 parts of photoinitiator α-hydroxyisobutyrylphenyl (D1173). Mix and stir until completely dissolved. Then, at 365 nm and 10 mW / cm², [the solution is used]. 2 Irradiate with ultraviolet light for 30 seconds, then degas under vacuum to obtain the lower layer of adhesive.

[0046] The lower adhesive layer is applied between a PET heavy release film (75 μm thick) with a release force of 15 gf / in and a PET light release film (50 μm thick) with a release force of 5 gf / in, using a doctor blade coating device, with the wet film thickness controlled at 50 μm. Under a nitrogen atmosphere, the strength is 50 mW / cm². 2 Irradiate with a 365 nm UV-LED lamp for 10 min to cure and form the lower OCA film.

[0047] Preparation of the upper OCA layer (high creep layer):

[0048] Weigh out 80 parts of isooctyl acrylate (2-EHA), 20 parts of hydroxybutyl acrylate (4-HBA), 0.01 parts of 1,6-hexanediol diacrylate (HDDA), and 0.2 parts of photoinitiator α-hydroxyisobutyrylbenzene (D1173). Mix and stir until completely dissolved. At 365 nm, 10 mW / cm² 2 Irradiate with ultraviolet light for 30 seconds, then degas under vacuum to obtain the upper layer of adhesive.

[0049] Double-layer composite and gradient curing:

[0050] Remove the light release film surface of the lower OCA film and fix it (OCA side up). Pour the upper layer of adhesive onto its surface and quickly cover it with a new PET light release film (50μm thick) with a release force of 5 gf / in. Use a doctor blade coating device to control the total thickness to 100μm.

[0051] The operating intensity is 50 mW / cm.2 The composite structure was irradiated with a 365 nm UV-LED lamp for 10 minutes to complete the final curing. This stepwise curing process facilitates interfacial miscibility and cross-linking.

[0052] A double-layer OCA optical adhesive with a total thickness of approximately 100 μm was obtained, denoted as DL-1. The lower OCA layer is denoted as sample DL-A, and the upper OCA layer is denoted as sample DL-B.

[0053] Comparative Example 1 (Single-layer high-recovery OCA)

[0054] Using only the lower OCA layer formulation and process described in Example 1, a single-layer OCA membrane with a thickness of approximately 100 μm was prepared, denoted as SL-A.

[0055] Comparative Example 2 (Single-layer high-creep OCA)

[0056] Using only the formulation and process of the upper OCA layer in Example 1, a single-layer OCA membrane with a thickness of approximately 100 μm was prepared, denoted as SL-B.

[0057] Test Example 1

[0058] Peel force test

[0059] Peel strength testing was performed according to GB / T 2792-2014. A universal tensile testing machine was used for the 180° peel strength test. Samples prepared in the embodiments and comparative examples of this invention were cut into strips 25 mm wide, 150 mm long, and 100 μm thick. These strips were placed in a sandwich-like position between the PET and glass substrate, pressed three times with a 2 kg rubber roller, and left to stand at room temperature for 20 minutes before testing. The PET film was peeled off at a 180° angle and a speed of 300 mm / min. The peel strength is the average force during the debonding process, denoted as N / 25 mm. The average value of five sets of test data is shown in Table 1.

[0060] Table 1 Peel force test results

[0061]

[0062] Test Example 2

[0063] Initial tack test

[0064] A probe-based initial adhesion tester equipped with a 1-inch diameter stainless steel ball was used to adhere a sample with a thickness of approximately 100 μm onto PET. The stainless steel ball probe approached the sample surface at a speed of 10 mm / min, held for 1 s with a force of 1 N, and then detached from the sample surface at a separation speed of 5 mm / min. The maximum value during the separation process was recorded as the initial adhesion force. The average value of the three sets of test data is shown in Table 2.

[0065] Table 2 Initial Tack Test Results

[0066]

[0067] Test Example 3

[0068] Optical testing

[0069] A 100 μm sample was adhered to optical glass, and its transmittance and haze values ​​were measured using a dedicated transmittance and haze meter. The average value of the three sets of test data is shown in Table 3.

[0070] Table 3. Results of transmittance and haze tests

[0071]

[0072] Test Example 4

[0073] Creep recovery test

[0074] Using a rotational rheometer, a 1000 μm thick adhesive film was attached to a parallel plate with a diameter of 25 mm. A shear stress of 20 kPa was applied for 600 s, then the applied stress was removed, and the sample was allowed to recover in the fixture for approximately 600 s. Creep tests were then performed on the assembled layer sample. The maximum creep strain within 600 s after stress application and the strain recovery rate 600 s after stress removal were recorded. The average value of the three sets of test data is shown in Table 4.

[0075] Table 4. Results of creep strain and recovery rate tests

[0076]

[0077] Test Example 5

[0078] Characterization of the double-layer structure: In-depth analysis using single-sided nuclear magnetic resonance

[0079] Samples DL-1, SL-A, and SL-B were tested using a Magritek PM5 NMR-MOUSE instrument.

[0080] Set the profile sequence, scan depth range 1000-500 μm (covering the entire film thickness), step 10 μm. Acquire 256 scans at each depth point, with 80 echoes.

[0081] The results are as follows Figure 2As shown, the two-dimensional spectra of samples SL-A (monolayer high recovery) and SL-B (monolayer high creep) exhibit a uniform and symmetrical relaxation time distribution in the depth direction. In contrast, the spectrum of sample DL-1 shows a smooth and continuous gradient change in relaxation time from the top (near the light release membrane side, corresponding to the upper layer) to the bottom (near the heavy release membrane side, corresponding to the lower layer), without any sharp interfaces, demonstrating the formation of an ideal gradient crosslinking structure.

[0082] Test Example 6

[0083] Folding test

[0084] Static folding tests were performed using a FlexTest-FC instrument. Laminated samples with a PI / OCA / PI structure were prepared, with a PI thickness of 50 μm and an OCA thickness of 100 μm, measuring 15 cm (length) × 2.5 cm (width). The samples were bent to a radius of curvature of 2 mm and held for 24 hours. If no cracking or delamination occurred at the fold point, the test was considered passed. After 24 hours, the angle between the sample and the plane was recorded at different time points, with the final angle θ measured at the end of the subsequent 3-minute cycle. The results are as follows: Figure 3 As shown in the left figure.

[0085] Dynamic folding tests were conducted using the FlexTest-FC instrument. Samples were prepared using an ITO / OCA / PET structure with thicknesses of 50 μm (ITO), 100 μm (OCA), and 50 μm (PET), and dimensions of 3 cm × 3 cm. The samples were folded at a speed of 180° / s to a radius of curvature of 2 mm, held at the folded position for 1 second, and then unfolded. The change in sheet resistance of the ITO with the number of folds was measured, and the rate of change of sheet resistance (R / R0) was calculated. Here, R0 represents the initial sheet resistance before the folding test, and R represents the sheet resistance measured at different numbers of folds. The results are as follows: Figure 3 As shown.

[0086] This invention designs and prepares a double-layer acrylic OCA optical adhesive for foldable screens. In the designed double-layer OCA structure, the upper layer is a high-creep layer (low crosslink density, high adhesion), and the lower layer is a high-recovery layer (high crosslink density, high elasticity). Through a gradient curing process, interfacial chemical crosslinking is formed between the two layers, achieving a continuous transition in longitudinal crosslink density rather than simple physical superposition. This structure maintains high peel strength and significantly improves creep recovery rate, effectively resolving the contradiction between "adhesion" and "recovery" in foldable screen applications. Combined with single-sided nuclear magnetic resonance (NMR) depth profiling, the internal structure of the adhesive film was systematically characterized, verifying its structural advantages and performance. (See Tables 1-4 and Appendix...) Figure 3 It can be seen that the dual-layer gradient OCA (DL-1) of the present invention achieves the best overall performance balance.

[0087] Comparative tests between Example 1 (DL-1) and the comparative examples of single-layer OCA (SL-A, SL-B) clearly demonstrate the comprehensive performance advantages of the dual-layer gradient structure: while maintaining peel strength similar to that of a single-layer high-recovery layer, the dual-layer structure improves initial tack comparable to that of a single-layer high-creep layer, exhibiting good interfacial compatibility. The creep recovery rate of the dual-layer OCA is significantly higher than that of the single-layer high-creep layer and close to that of the single-layer high-recovery layer, indicating its excellent resistance to permanent deformation. The light transmittance of the dual-layer OCA is comparable to that of the single-layer OCA, indicating that the gradient structure does not negatively affect optical transparency.

[0088] Non-destructive testing of the adhesive film was performed using single-sided nuclear magnetic resonance depth profiling. The results showed that the monolayer OCA (SL-A, SL-B) exhibited a uniform and symmetrical relaxation time distribution along the thickness direction in the depth direction, while the bilayer OCA (DL-1) showed a smooth, continuous gradient change from the upper to the lower layer without sharp interfaces, proving the successful construction of an integrated gradient cross-linked structure. This characterization method not only verified the effectiveness of the structural design but also provided a reliable and intuitive analytical tool for product development and quality control.

[0089] Dynamic folding tests show that the resistivity change rate of the ITO film using double-layer OCA lamination is significantly lower than that of single-layer OCA during folding. Combined with theoretical analysis, the gradient modulus structure helps to form a "split neutral plane" during folding, keeping the maximum strain away from the brittle ITO layer, thus effectively protecting the functional film layer and suppressing creases and bubble rebound. This mechanism is particularly crucial in foldable screen applications, directly improving the device's durability and reliability.

[0090] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A double-layer acrylic OCA optical adhesive for foldable screens, characterized in that, It includes an upper OCA layer and a lower OCA layer stacked from top to bottom, wherein: The upper OCA layer is a high creep layer with a lower energy storage modulus than the lower layer. It is composed of 70-90 parts by weight of isooctyl acrylate, 10-30 parts by weight of hydroxybutyl acrylate, 0.01-2.0 parts by weight of crosslinking agent and 0.2 parts by weight of photoinitiator. The lower OCA layer is a high recovery layer with a higher energy storage modulus than the upper layer. It is composed of 20-35 parts by weight of isobornyl acrylate, 50-70 parts by weight of isooctyl acrylate, 5-15 parts by weight of hydroxybutyl acrylate, 0.01-2.0 parts by weight of crosslinking agent and 0.2 parts by weight of photoinitiator. The two layers are cured by gradient curing to form an interface structure with a continuous transition in crosslinking density.

2. The double-layer acrylic OCA optical adhesive according to claim 1, characterized in that, The crosslinking agent is 1,6-hexanediol diacrylate, and the photoinitiator is α-hydroxyisobutyrylbenzene.

3. The double-layer acrylic OCA optical adhesive according to claim 1, characterized in that, The total thickness is 50-200μm, with the thickness ratio of the upper and lower layers being 1:

1.

4. A method for preparing the double-layer acrylic OCA optical adhesive according to any one of claims 1-3, characterized in that, include: S1: The lower layer raw materials are mixed and coated between the heavy release film and the light release film, and cured under 365nm ultraviolet light to form the lower layer OCA film; S2: Remove the lower light release film, coat the upper material onto the surface of the lower OCA film, and cover with a new light release film; S3: Gradient curing of the composite structure, UV intensity 50mW / cm² 2 Wavelength 365nm, time 10min.

5. The preparation method according to claim 4, characterized in that, In steps S1 and S2, the coating is applied using a doctor blade, and the wet film thickness is controlled to be 50 μm.

6. A method for structural characterization of the double-layer OCA optical adhesive according to any one of claims 1-3, characterized in that, The technique employs single-sided nuclear magnetic resonance depth profiling, including: a) Place the adhesive film stripped from the release film onto the sample stage of the nuclear magnetic resonance equipment; b) Scan along the thickness direction with a step accuracy of 10μm; c) Acquire CPMG echo signals at each depth point to obtain the lateral relaxation time; d) Plot a two-dimensional spectrum of depth-relaxation time; e) Verify the continuity of crosslink density by relaxation time gradient distribution.

7. The characterization method according to claim 6, characterized in that, The relaxation time exhibits an asymmetric continuous distribution in the depth direction, with a transition zone spanning ≥20μm.

8. A foldable screen module, characterized in that, The double-layer OCA optical adhesive described in claims 1-3 is attached between the ITO film and the cover plate.

9. The foldable screen module according to claim 8, characterized in that, The upper high-creep layer of the OCA optical adhesive is attached to the surface of the ITO film, and the lower high-recovery layer is attached to the CPI colorless polyimide or PET polyethylene terephthalate film.

10. A method for applying a foldable screen, characterized in that, Using the OCA optical adhesive described in claims 1-3, a split neutral surface is formed during folding, and the maximum strain is far away from the ITO functional layer.

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