UV and hot melting synergistic curing bonding process for earphone structure

Abstract

The invention relates to the technical field of adhesive materials, and discloses a UV (ultraviolet) and hot-melt synergistic curing and bonding process for an earphone structure, which adopts an adhesive composition containing thermoplastic polyurethane, a light curing component, a heat curing component, a heat-conducting filler and a latent thermal initiator, the process comprises the following steps of: after melting and sizing, cooling to physically shape thermoplastic polyurethane; ultraviolet irradiation is applied to the illumination area, a photocuring reaction is initiated, and reaction heat is generated; and the reaction heat is conducted from the illumination area to the shadow area by virtue of an internal heat-conducting passage formed by the heat-conducting filler, so that the latent thermal initiator is activated, and the shadow area is driven to be thermally cured. Heat released by the light curing reaction is used as an internal heat source, full-area curing can be achieved without heating of an external drying oven, damage to a thermosensitive element is avoided, and the method is suitable for bonding of precise electronic components.

Description

Technical Field

[0001] This invention relates to the field of adhesive materials technology, specifically to a UV and hot melt co-curing adhesive process for headphone structures. Background Technology

[0002] In the manufacturing process of precision consumer electronics products such as headphones, adhesives are key materials for bonding and sealing structural components. Ultraviolet (UV) curing adhesives are widely used due to their advantages such as fast curing speed and suitability for automated production lines. However, due to the complex internal structure of headphones, with numerous irregular overlapping surfaces and chambers, light cannot reach all bonding areas. This dependence on direct light means that UV curing adhesives cannot cure in "shaded areas," causing localized bonding failures and seriously affecting the overall reliability of the product.

[0003] To address the curing issue in shaded areas, the industry often employs thermosetting or hot-melt adhesives. While these materials are not limited by light, their curing or bonding process typically requires placing the entire component in an oven for prolonged, overall heating. In today's highly integrated electronic products, headphones integrate numerous heat-sensitive components such as batteries, miniature speakers, sensors, and precision plastic parts. High-temperature baking can easily lead to performance degradation or even permanent damage to these components, significantly reducing production yield.

[0004] While some existing technologies combine UV and thermosetting mechanisms with dual-curing adhesives, they often still require an external heating step (such as an oven or local heater) to activate the thermosetting reaction, failing to fundamentally address the risk of heat damage. Furthermore, in single-component reactive adhesive systems, precisely controlling the activity of each reactive component to ensure stability and prevent premature curing during storage and high-temperature melt application, while simultaneously enabling rapid response when needed, remains a technical challenge. Improper design often results in short operating windows and poor storage stability. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a UV and hot melt co-curing bonding process for headphone structures, which solves the problem of how to achieve full-area curing of both illuminated and shaded areas without relying on external oven heating when bonding precision structures containing heat-sensitive components (such as headphones).

[0006] To achieve the above objectives, the present invention provides a UV and hot melt synergistic curing bonding process for earphone structures, comprising the following technical solution:

[0007] A UV and hot melt co-curing bonding process for earphone structures includes the following steps:

[0008] (a) An adhesive composition is provided, said adhesive composition being made from raw materials comprising, by weight, the following: thermoplastic polyurethane: 20.0-35.0 parts, photocurable oligomer: 25.0-40.0 parts, reactive diluent: 5.0-15.0 parts, epoxy resin: 10.0-20.0 parts, polyether polyol: 2.0-8.0 parts, thermally conductive filler: 10.0-25.0 parts, photoinitiator: 1.0-4.0 parts, and latent thermal initiator: 0.5-2.5 parts;

[0009] (b) The adhesive composition is heated to 80-95°C and applied to the bonding area of ​​the first substrate in a molten state;

[0010] (c) The second substrate is bonded and assembled to the first substrate, and cooled until the thermoplastic polyurethane in the adhesive composition is physically set;

[0011] (d) Apply ultraviolet light to the light-illuminated area of ​​the area to be bonded, and generate reaction heat in the light-illuminated area;

[0012] (e) The heat of reaction generated in step (d) is conducted from the illuminated area to the shaded area in the area to be bonded through the internal thermally conductive pathway formed by the thermally conductive filler, so that the adhesive composition in the shaded area is thermally cured.

[0013] By adopting the above technical solution, this invention provides a multi-stage, synergistic curing bonding process. The curing mechanism and technical effects of this process are specifically reflected in:

[0014] Initial physical setting: In step (c), the thermoplastic polyurethane component in the composition undergoes physical setting (curing) upon cooling, providing rapid initial positioning and green strength for the substrates to be bonded, facilitating subsequent processes.

[0015] Photocuring and internal heat generation: In step (d), ultraviolet light is applied to the illuminated area. After the photoinitiator absorbs the light energy, it initiates free radical polymerization or cationic polymerization of the photocurable oligomer and the reactive diluent. This polymerization reaction is a strongly exothermic process, generating a large amount of reaction heat in situ in the illuminated area, thus achieving rapid chemical curing of the illuminated area.

[0016] Internal heat conduction: In step (e), the thermally conductive filler dispersed in the composition matrix forms an efficient internal heat conduction pathway. This pathway rapidly and directionally conducts the reaction heat generated by polymerization in the illuminated area in step (d) to the shaded area where ultraviolet light cannot reach.

[0017] Thermosetting in shaded areas: The heat of reaction conducted to the shaded areas causes the temperature of the adhesive composition in those areas to rise rapidly. When the temperature reaches the activation temperature of the latent thermal initiator, the latent thermal initiator is activated (e.g., by releasing an acid or alkali), which in turn catalyzes the thermosetting reaction (e.g., ring-opening polymerization) between the epoxy resin and the polyether polyol in the shaded areas.

[0018] In summary, the process of this invention cleverly utilizes the exothermic reaction of the photocuring reaction as an internal heat source, and conducts this heat source to the shaded area through thermally conductive fillers, thereby driving the thermocuring reaction in the shaded area. This solution eliminates the need for external oven heating, avoiding damage to heat-sensitive components (such as batteries and sensors) in precision structures like headphones caused by high-temperature baking. Simultaneously, it achieves full-area deep curing in both the illuminated and shaded areas, ensuring the final strength and long-term reliability of the bond.

[0019] Preferably, the thermally conductive filler is hexagonal boron nitride with an average particle size D50 of 5-15 μm.

[0020] By adopting the above technical solution, hexagonal boron nitride (h-BN), as a filler with high thermal conductivity and excellent electrical insulation properties, can efficiently construct internal thermal conduction paths, ensuring rapid heat transfer from the illuminated area to the shaded area, while maintaining the electrical insulation properties of the adhesive, making it suitable for electronic components.

[0021] Preferably, the latent thermal initiator is a latent thermal acid generator with an activation temperature of 110-130℃; the epoxy resin is an alicyclic epoxy resin; and the polyether polyol is polytetrahydrofuran diol.

[0022] Preferably, the heat of reaction generated in step (d) causes the adhesive composition to reach a peak temperature of 120°C or higher in the illuminated area, which is higher than the activation temperature of the latent thermal initiator.

[0023] By employing the above technical solution, a precise thermodynamic and kinetic control window was constructed. The exothermic peak temperature of the photocuring reaction (e.g., above 120°C) was designed to be significantly higher than the activation temperature of the latent hot acid generator (e.g., 110-130°C), ensuring that the heat generated in the illuminated area is sufficient to activate the initiator in the shaded area, achieving reliable thermocuring. The combination of alicyclic epoxy resin and polytetrahydrofuran diol contributes to the formation of a tough and weather-resistant curing network.

[0024] Preferably, the heating temperature (80-95°C) in step (b) is lower than the activation temperature (110-130°C) of the latent thermal initiator.

[0025] Preferably, in step (a), when preparing the adhesive composition, the photoinitiator and the latent thermal initiator are added under light-shielding conditions after the other components are melt-mixed at 80-95°C and cooled to below 70°C.

[0026] By adopting the above technical solution, it is ensured that the latent thermal initiator is kept below the activation temperature during the preparation, storage and heating application (step b) of the adhesive composition, so that it is in an inert state, thereby giving the adhesive composition a good operating window and storage stability and preventing premature curing of the components.

[0027] Preferably, in step (d), the wavelength of the ultraviolet light irradiation is 365 nm or 395 nm, and the irradiance is 100-1000 mW / cm². 2 By adopting the above technical solution, conventional and efficient UV curing process parameters are defined, ensuring that the photoinitiator can be effectively excited and generate sufficient heat of reaction in a short time.

[0028] Preferably, the latent hot acid generator is an amine-blocked sulfonate.

[0029] Preferably, the amine-blocked sulfonate is dicyclohexylammonium p-toluenesulfonate, and its preparation method includes the following steps:

[0030] p-Toluenesulfonic acid monohydrate was dissolved in the first part of toluene solvent to obtain an acid solution;

[0031] Dicyclohexylamine was dissolved in the second part of toluene solvent to obtain an amine solution;

[0032] Under room temperature and continuous mechanical stirring, the amine solution and the acid solution are mixed to cause an acid-base neutralization reaction and precipitate a solid product.

[0033] After mixing, continue stirring at room temperature for 30-60 minutes to ensure complete reaction;

[0034] The mixture obtained in the above steps is subjected to vacuum filtration to collect the solid filter cake;

[0035] The solid filter cake was washed with cold toluene 1-3 times;

[0036] The washed solid filter cake is placed in a vacuum drying oven at 40-50°C and dried to constant weight to obtain the dicyclohexylammonium p-toluenesulfonate.

[0037] By adopting the above technical solution, a latent hot acid generator with stable performance and easy preparation is provided.

[0038] Preferably, the photocurable oligomer is an aliphatic polyurethane acrylate;

[0039] The method for preparing the photocurable oligomer is as follows: isophorone diisocyanate is reacted with polytetrahydrofuran diol to obtain a prepolymer with -NCO group end-capped, and then hydroxyethyl acrylate is added to carry out the end-capping reaction until the characteristic absorption peak of the -NCO group disappears.

[0040] By adopting the above technical solution, a photocurable base resin with good flexibility and excellent yellowing resistance is provided, which helps to improve the overall toughness and durability of the cured adhesive layer.

[0041] This invention provides a UV and hot melt co-curing bonding process for earphone structures. It offers the following advantages:

[0042] 1. This invention generates in-situ reaction heat in the illuminated area by ultraviolet light irradiation, and rapidly conducts this heat to the shaded area through an internal thermally conductive pathway formed by a thermally conductive filler (such as hexagonal boron nitride). This process cleverly utilizes the exothermic reaction of the photocuring reaction as an internal heat source to drive the thermal curing of the shaded area, eliminating the need for external high-temperature heating in an oven throughout the process. Therefore, this invention avoids the irreversible thermal damage that traditional thermal curing processes can cause to heat-sensitive components (such as batteries and sensors) in precision structures like headphones, significantly improving assembly yield and product reliability.

[0043] 2. The adhesive composition of this invention simultaneously comprises a photocurable component (such as a photocurable oligomer) and a thermocurable component (such as epoxy resin). Through the synergistic effect of photocuring in the illuminated area and thermocuring in the shaded area, the process of this invention overcomes the fundamental defect of traditional UV curing, which cannot cure in the shaded area, and achieves full-area deep curing of the bonding region. The resulting interpenetrating network structure endows the adhesive layer with excellent mechanical strength and long-term durability, ensuring the final bonding performance.

[0044] 3. The latent thermal initiator used in this invention has its activation temperature (e.g., 110-130°C) precisely controlled to be higher than the melt application temperature (80-95°C) and preparation cooling temperature (below 70°C) of the adhesive. This thermodynamic and kinetic window design ensures that the thermosetting components remain inert during storage, preparation, and application, giving the adhesive composition excellent storage stability and a long working life, greatly facilitating actual industrial production operations. Detailed Implementation

[0045] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0046] Preparation Examples 1-2:

[0047] Preparation Example 1: Preparation of Aliphatic Polyurethane Acrylate (UA-1) In a four-necked flask equipped with a mechanical stirrer, thermometer, reflux condenser, and nitrogen protection device, 444.6 g (2.0 mol) of isophorone diisocyanate (IPDI) and 2000 g (1.0 mol) of PTMG-1 were added. The mixture was heated to 70 °C, and 1.2 g of dibutyltin dilaurate (DBTDL) was added as a catalyst. The reaction was stirred at this temperature for 2.5 hours to obtain a polyurethane prepolymer with -NCO group end-capping. Subsequently, the temperature of the reaction system was lowered to 55 °C, and 232.2 g (2.0 mol) of hydroxyethyl acrylate (HEA) was slowly added dropwise with stirring. The temperature of the system was kept below 70 °C by controlling the dropping rate and external cooling. After the addition was complete, the reaction was continued at 65 °C for 3 hours. The reaction progress was monitored by Fourier transform infrared spectroscopy (FTIR), and the reaction was stopped when the molecule reached 2270 cm⁻¹. -1 The complete disappearance of the characteristic absorption peak of the -NCO group at the given location indicates the termination of the reaction. The final product is a colorless to pale yellow viscous liquid, which is UA-1.

[0048] Preparation Example 2: Preparation of a Latent Hot Acid Generator (TAG-1) At room temperature, 190.2 g (1.0 mol) of p-toluenesulfonic acid monohydrate was dissolved in 1000 mL of toluene to form a clear solution. Under continuous mechanical stirring, a 500 mL toluene solution containing 181.3 g (1.0 mol) of dicyclohexylamine was slowly added dropwise to the p-toluenesulfonic acid solution through a constant-pressure dropping funnel. During the addition, a large amount of white solid precipitated immediately. After the addition was complete, the reaction was continued to be stirred at room temperature for 30 minutes to ensure complete reaction. After the reaction was complete, the mixture was filtered under reduced pressure, and the white solid filter cake was collected. The obtained filter cake was washed twice with 100 mL of cold toluene to remove residual unreacted raw materials and impurities. The washed white solid was placed in a vacuum drying oven at 45 °C and dried for 4 hours to constant weight to obtain the final product, dicyclohexylammonium p-toluenesulfonate, which is TAG-1.

[0049] Examples 1-3:

[0050] Example 1: This example provides a UV and hot melt co-curing bonding process for earphone structures, including:

[0051] Preparation of adhesive composition A1:

[0052] 35.0 parts by weight of TPU-1 and 10.0 parts by weight of h-BN-1 were dried in a vacuum oven at 85°C for 4 hours. A planetary mixer with heating and vacuum functions was used, with the jacket temperature of the mixing vessel set to 90°C. The dried 35.0 parts by weight of TPU-1, 25.0 parts by weight of UA-1, 13.5 parts by weight of IBOA, 10.0 parts by weight of epoxy resin-1, and 4.0 parts by weight of PTMG-1 were added sequentially to the mixing vessel. The mixture was stirred at low speed under normal pressure until the TPU-1 particles were completely dissolved and the system formed a homogeneous molten liquid. Then, the dried 10.0 parts by weight of h-BN-1 was slowly added while stirring. After the addition was complete, the stirring speed was increased and the vacuum system was activated. The mixture was continuously stirred and dispersed under vacuum for 60 minutes to ensure uniform dispersion of the filler and removal of air bubbles. Stop heating and cool the material to 65°C. Remove the vacuum and add 2.0 parts by weight of photoinitiator-1 and 0.5 parts by weight of TAG-1 under light-shielding conditions. Stir at low speed for 15 minutes to mix evenly. Finally, discharge the prepared adhesive composition while it is still hot and dispense it into opaque, sealed containers, labeled A1.

[0053] Example 2: This example provides a UV and hot melt co-curing bonding process for earphone structures, including:

[0054] Preparation of adhesive composition A2:

[0055] The preparation process is basically the same as in Example 1, except that the mass fractions of the raw materials used are different. Specifically, it includes the following steps: 27.5 parts by mass of TPU-1 and 17.5 parts by mass of h-BN-1 are dried in a vacuum oven at 85°C for 4 hours. The temperature of the jacket of the planetary mixer is set to 90°C. The dried 27.5 parts by mass of TPU-1, 32.5 parts by mass of UA-1, 9.0 parts by mass of IBOA, 15.0 parts by mass of epoxy resin-1, and 5.0 parts by mass of PTMG-1 are added sequentially to the mixer. The mixture is stirred at low speed under normal pressure until a uniform molten liquid is formed. Then, the dried 17.5 parts by mass of h-BN-1 is slowly added while stirring. After the addition is complete, the stirring speed is increased and the mixture is continuously stirred and dispersed under vacuum for 60 minutes. Stop heating and cool the material to 65°C. Remove the vacuum and add 2.5 parts by weight of photoinitiator-1 and 1.0 parts by weight of TAG-1 under light-shielding conditions. Stir at low speed for 15 minutes to mix evenly. Finally, discharge the prepared adhesive composition while it is still hot and dispense it into opaque, sealed containers, labeled A2.

[0056] Example 3: This example provides a UV and hot melt co-curing bonding process for earphone structures, including:

[0057] Preparation of adhesive composition A3:

[0058] The preparation process is basically the same as in Example 1, except that the mass fractions of the raw materials used are different. Specifically, it includes the following steps: 20.0 parts by mass of TPU-1 and 25.0 parts by mass of h-BN-1 are dried in a vacuum oven at 85°C for 4 hours. The temperature of the jacket of the planetary mixer is set to 90°C. The dried 20.0 parts by mass of TPU-1, 40.0 parts by mass of UA-1, 5.0 parts by mass of IBOA, 20.0 parts by mass of epoxy resin-1, and 6.0 parts by mass of PTMG-1 are added sequentially to the mixer. The mixture is stirred at low speed under normal pressure until a uniform molten liquid is formed. Then, the dried 25.0 parts by mass of h-BN-1 is slowly added while stirring. After the addition is complete, the stirring speed is increased and the mixture is continuously stirred and dispersed under vacuum for 60 minutes. Stop heating and cool the material to 65°C. Remove the vacuum and add 3.0 parts by weight of photoinitiator-1 and 1.0 parts by weight of TAG-1 under light-shielding conditions. Stir at low speed for 15 minutes to mix evenly. Finally, discharge the prepared adhesive composition while it is still hot and dispense it into opaque, sealed containers, labeled A3.

[0059] Comparative Examples 1-4:

[0060] Comparative Example 1: Compared with the synergistic curing adhesive process described in Example 1, this comparative example provides a conventional UV-curing adhesive process. The adhesive composition B1 used in this process does not contain the thermosetting system components (epoxy resin-1, PTMG-1, TAG-1) or the thermally conductive filler component (h-BN-1), and the proportions of the remaining components have been adjusted accordingly. Specifically, it consists of 40.0 parts by weight of TPU-1, 48.0 parts by weight of UA-1, 9.0 parts by weight of IBOA, and 3.0 parts by weight of photoinitiator-1. Except for the difference in the adhesive composition, the remaining operational steps of this process are consistent with the process flow of Example 1.

[0061] Comparative Example 2: Compared to the synergistic curing bonding process described in Example 2, this comparative example provides a conventional UV-oven dual-curing bonding process. The adhesive composition B2 used in this process does not contain the thermally conductive filler h-BN-1, and 1.0 part by weight of the conventional medium-high temperature epoxy curing agent DICY-1 is used instead of the latent thermal acid generator TAG-1. When using this adhesive for bonding, the remaining process steps are consistent with the process flow of Example 2, but additional oven heating curing is required after UV curing.

[0062] Comparative Example 3: Compared to the synergistic curing bonding process described in Example 2, this comparative example provides a UV and hot-melt dual-curing bonding process without thermally conductive fillers. The adhesive composition B3 used in this process differs only in that it does not contain the thermally conductive filler h-BN-1, and this portion is replaced by the matrix resins TPU-1 and UA-1. Except for the difference in the adhesive composition, the remaining operational steps of this process are consistent with the process flow of Example 2.

[0063] Comparative Example 4: Compared to the synergistic curing bonding process described in Example 1, this comparative example provides a conventional hot melt bonding process. The adhesive composition B4 used in this process consists only of 100 parts by weight of TPU-1, and does not contain any photocurable or thermocurable reactive components or thermally conductive fillers. The bonding process in this method involves simply melting and applying TPU-1 particles at 160°C and then cooling and setting them; it does not include a subsequent UV irradiation step.

[0064] Test Examples 1-4:

[0065] Test Example 1: Peak Exothermic Temperature Test of UV Curing

[0066] The purpose of this test example is to verify whether the exothermic reaction of the free radical polymerization of the photocurable system in the embodiment of the present invention under UV radiation is sufficient to make the instantaneous temperature inside the adhesive layer reach or exceed the activation temperature threshold (120°C) of the latent hot acid generator (TAG-1).

[0067] Experimental steps:

[0068] Take two PC / ABS sheets with dimensions of 25mm×25mm×2mm.

[0069] The measuring end of a K-type thermocouple with a probe diameter of 0.1 mm is placed in the center of one of the plates.

[0070] Using a dispensing device, apply the adhesive composition to be tested to the area containing the thermocouple measuring end, and then cover it with another sheet to form a sandwich structure with an adhesive layer thickness of 0.3 mm, ensuring that the thermocouple measuring end is completely covered by the adhesive layer.

[0071] The prepared test components were placed on the test platform and the thermocouples were connected to a high-frequency data logger, which collected temperature data at a frequency of 10 Hz.

[0072] After the system temperature stabilizes, start the LED surface light source with a wavelength of 365nm at 500mW / cm². 2 The irradiance was vertically irradiated onto the upper surface of the test component for 30 seconds.

[0073] The data logger records the temperature change curve from the start of irradiation to 60 seconds after the end of irradiation.

[0074] The highest temperature reached by each sample during UV irradiation is read from the recorded temperature data; this is the exothermic peak temperature.

[0075] For the adhesive compositions of Examples A1, A2, A3, and Comparative Examples B1 and B3, the above steps were repeated three times for independent testing, and the results were recorded.

[0076] Experimental data:

[0077] Table 1: Peak exothermic temperatures of different adhesive compositions under UV irradiation

[0078] Adhesive samples Test 1 / ℃ Test 2 / ℃ Test 3 / ℃ Peak temperature average value / °C Example A1 129.4 131.1 128.8 129.8 Example A2 134.8 136.1 135.5 135.5 Example A3 140.2 138.9 141.3 140.1 Comparative Example B1 125.6 124.9 126.3 125.6 Comparative Example B3 133.9 132.8 134.5 133.7

[0079] Conclusion: The data from Test Example 1 show that the average exothermic peak temperature inside the adhesive layer of the adhesive compositions in Examples A1, A2, and A3 significantly exceeds 120°C under the specified UV irradiation conditions. This temperature is higher than the dissociation activation temperature of the latent thermal acid generator TAG-1 used. This result confirms a core premise of the technical solution of this invention: the heat of reaction released by the free radical polymerization reaction of the photocurable system is sufficient to form an instantaneous, high-intensity thermal pulse inside the adhesive layer. This thermal pulse can act as an internal energy source, providing the necessary thermodynamic conditions for activating the latent thermal curing reaction in the shaded area, thereby verifying the feasibility of the technical path proposed in this invention for achieving curing in the shaded area without relying on an external heat source.

[0080] Test Example 2: Curing Degree Test in Shaded Areas

[0081] The purpose of this test example is to evaluate the curing ability of the adhesive composition of the present invention in shaded areas where ultraviolet light cannot reach, and to compare it with a comparative example lacking key components, in order to verify the necessity of internal heat conduction pathways for achieving curing in cold areas.

[0082] Experimental steps:

[0083] PC / ABS test strips with dimensions of 50mm × 12.5mm × 2mm were prepared.

[0084] Apply the adhesive to be tested within a 25mm length from the end of the strip.

[0085] Take another sample strip and overlap it with the adhesive sample strip. The overlap area should be 25mm × 12.5mm. Apply slight pressure to ensure that the adhesive layer thickness is uniformly controlled at 0.3mm.

[0086] Using 12.5mm wide, completely opaque tape, precisely cover half of the overlapping area (12.5mm x 12.5mm) to divide it into a designated light and shadow area.

[0087] The prepared test components were cured according to the curing conditions of each embodiment and comparative example.

[0088] After curing, the test components were left to stand at room temperature for 1 hour.

[0089] Use tools to forcibly peel off the overlapping strips, exposing the surface of the cured adhesive layer.

[0090] Using a Shore D hardness tester, five different points were randomly selected on the surface of the adhesive layer in the original shaded area for hardness testing. The readings were recorded and the average value was calculated. If the adhesive layer was not cured and exhibited a viscous, flowing state, it was recorded as uncured.

[0091] Experimental data:

[0092] Table 2: Curing hardness of different adhesive compositions in the shaded area

[0093] Adhesive samples Test 1 / D Test 2 / D Test 3 / D Test 4 / D Test 5 / D Average hardness / D Example A1 54.5 55.8 56.1 55.2 54.9 55.3 Example A2 62.1 61.8 63.2 62.5 61.9 62.3 Example A3 65.4 64.9 66.0 65.7 65.2 65.4 Comparative Example B1 - - - - - Uncured Comparative Example B2 60.8 61.5 60.2 61.1 60.5 60.8 Comparative Example B3 14.8 15.5 16.1 15.2 14.9 15.3 Comparative Example B4 - - - - - Uncured

[0094] Note: The data for Comparative Example B2 were measured after oven heating; if not oven-dried, the shaded area is uncured, and the symbol "-" indicates that the data cannot be measured or is not applicable.

[0095] Conclusion: The data from Test Example 2 directly demonstrates the effectiveness of the present invention's technical solution in curing the shaded area. Examples A1, A2, and A3, without any external heating, all formed cured products with high Shore D hardness in their shaded areas, indicating that the crosslinking reaction had been fully carried out.

[0096] The key difference between Comparative Example B3 and Example A2 is that it lacks thermally conductive filler h-BN-1. Its shaded area has an extremely low hardness value, and the adhesive layer is in a soft solid state of incomplete curing. This indicates that when there is no internal heat conduction path, the reaction heat generated in the irradiated area cannot be effectively transferred to the shaded area to activate the latent curing system. This confirms that the function of thermally conductive filler in this invention is a necessary condition for achieving curing in the shaded area.

[0097] Comparative Examples B1 and B4, lacking a shaded area curing mechanism, remained uncured, as expected. Comparative Example B2 required external oven heating to achieve a degree of curing in the shaded area comparable to that of the embodiments of the present invention. In summary, the composition of the present invention successfully solves the problem of shaded area curing in conventional UV-curable adhesives through a synergistic mechanism of internal heat conduction, and avoids the reliance on oven heating in traditional dual-curing technologies.

[0098] Test Example 3: Overlap Shear Strength Test

[0099] The purpose of this test example is to quantify the final adhesive properties provided by the adhesive composition of the present invention after complete curing and to compare it with a comparative example to evaluate its mechanical strength as a structural adhesive.

[0100] Experimental steps:

[0101] Cut PC / ABS test strips to 100mm × 25mm × 2mm dimensions according to ASTM D1002 standard. Clean the surfaces of the strips to be bonded with anhydrous ethanol.

[0102] At the end of a strip, using a specific clamp, the adhesive to be tested is applied to form a 25mm × 12.5mm bonding area.

[0103] Cover it with another strip to form a single overlap structure. Ensure uniform adhesive layer thickness by applying pressure and using 0.3mm metal wire as a gasket.

[0104] The prepared lap shear specimens were cured according to the curing conditions of each embodiment and comparative example.

[0105] After curing, all samples were placed in a standard laboratory environment (23±2℃, 50±5%RH) for 24 hours to ensure complete curing and eliminate internal stress.

[0106] Using a universal testing machine, tensile shear tests were performed on the lap joint specimens at a beam displacement rate of 10 mm / min, and the maximum load at failure of each specimen was recorded.

[0107] Divide the maximum load by the bonding area (312.5mm²). 2 The lap shear strength (MPa) was calculated.

[0108] For each adhesive composition, five valid specimens were tested repeatedly, and the average shear strength was calculated.

[0109] Experimental data:

[0110] Table 3: Overlap Shear Strength of Different Adhesive Compositions

[0111] Adhesive samples Test 1 / MPa Test 2 / MPa Test 3 / MPa Test 4 / MPa Test 5 / MPa Average strength / MPa Example A1 12.8 12.3 12.9 12.1 12.4 12.5 Example A2 15.3 14.8 15.5 15.0 14.9 15.1 Example A3 16.0 16.5 15.9 16.3 16.1 16.2 Comparative Example B1 6.5 7.1 6.9 6.6 7.0 6.8 Comparative Example B2 14.2 14.8 14.6 14.3 14.5 14.5 Comparative Example B3 7.4 6.8 7.0 7.2 7.1 7.1 Comparative Example B4 4.3 4.8 4.4 4.6 4.4 4.5

[0112] Conclusion: The data on lap shear strength reflect the overall mechanical properties of the adhesive after complete curing. Examples A1, A2, and A3 all exhibited high shear strengths, comparable to or higher than Comparative Example B2, which required additional oven heating. This indicates that the curing mechanism of the present invention enables the adhesive to achieve the same crosslinking density and cohesive strength throughout the bonding area as conventional thermosetting processes.

[0113] The shear strength of Comparative Example B3 (without thermally conductive filler) was significantly lower than that of the Example, consistent with the conclusion in Test Example 2 that the shaded area was not fully cured. This demonstrates that the uncured area at the adhesive interface acts as a structural defect, greatly reducing the overall load-bearing capacity.

[0114] The strength values ​​of Comparative Examples B1 (UV curing only) and B4 (hot melt only) are significantly lower than those of the embodiments of the present invention, indicating that a single curing or physical bonding mechanism cannot provide sufficient bond strength for structural applications. Therefore, the results of this test example confirm that the technical solution of the present invention not only achieves full-area curing under oven-free conditions, but also that the curing is of high quality and can form a reliable bond interface with high mechanical strength.

[0115] Test Example 4: Thermal Cycling Performance Test

[0116] The purpose of this test case is to evaluate the performance stability of the adhesive of the present invention after being subjected to alternating high and low temperature shocks, and to characterize its long-term reliability in simulated actual use environment by comparing the changes in its mechanical strength before and after aging.

[0117] Experimental steps:

[0118] According to the method of Test Example 3, lap shear test specimens of each group of adhesives were prepared and fully cured.

[0119] Half of each group of cured samples was used as the initial strength test group, and the data were directly taken from the results of test example 3.

[0120] The other half of each cured sample was placed in a high and low temperature alternating damp heat test chamber for thermal cycling aging test.

[0121] The thermal cycling conditions were set as follows: heating from -40℃ to +85℃, holding at both extreme temperatures of -40℃ and +85℃ for 30 minutes each, with a heating / cooling rate of 10℃ / min. This constituted one cycle, and a total of 100 cycles were performed.

[0122] After the cycle is completed, the sample is removed from the test chamber and placed in a standard laboratory environment (23±2℃, 50±5%RH) for at least 2 hours to allow it to return to room temperature.

[0123] Using a universal testing machine, and following the test method of Test Example 3, the lap shear strength of the sample after thermal cycling was tested, and the shear strength after aging was recorded.

[0124] According to the formula: Strength retention rate (%) = (Shear strength after thermal cycling / Initial shear strength) × 100, calculate the shear strength retention rate of each group of adhesives.

[0125] Comparative examples B1 and B3 were not tested because their shaded areas were not fully cured and their initial strengths were not comparable.

[0126] Experimental data:

[0127] Table 4: Shear strength and strength retention of different adhesive compositions after thermal cycling

[0128] Adhesive samples Initial shear strength / MPa Shear strength after thermal cycling / MPa Strength retention rate / % Example A1 12.5 11.0 88.0 Example A2 15.1 13.8 91.4 Example A3 16.2 14.9 92.0 Comparative Example B2 14.5 13.0 89.7 Comparative Example B4 4.5 1.5 33.3

[0129] Conclusion: The data from Test Example 4 show that Embodiments A1, A2, and A3 of this invention all exhibited high shear strength retention rates after 100 high and low temperature cycles. This performance is attributed to the interpenetrating polyurethane-acrylate network formed by the photocurable acrylate network and the thermocurable epoxy network. This stable, fully cross-linked system effectively resists repeated shrinkage and expansion caused by temperature changes, thereby maintaining the integrity of the adhesive interface. Simultaneously, the presence of the thermally conductive filler h-BN helps reduce the overall coefficient of thermal expansion of the adhesive, decreasing the interfacial stress caused by thermal expansion mismatch between the adhesive and the PC / ABS substrate, further improving its weather resistance.

[0130] Comparative Example B4, as a typical thermoplastic material, undergoes irreversible creep and aging during thermal cycling, resulting in severe damage to its structural integrity and extremely low strength retention.

[0131] It is worth noting that the strength retention rate of the embodiment of the present invention is comparable to that of comparative example B2, which requires additional oven curing. This proves that the technical solution of the present invention simplifies the process and eliminates the oven heating step without sacrificing the long-term reliability of the final product, and can meet the usage requirements under harsh environments.

Claims

1. A UV and hot melt synergistic curing adhesive process for earpiece structures, characterized in that, The method comprises the following steps: (a) providing an adhesive composition made from raw materials comprising the following parts by weight: thermoplastic polyurethane: 20.0-35.0 parts; photo-curable oligomer: 25.0-40.0 parts; active diluent: 5.0-15.0 parts; epoxy resin: 10.0-20.0 parts; polyether polyol: 2.0-8.0 parts; thermally conductive filler: 10.0-25.0 parts; photo-initiator: 1.0-4.0 parts; latent thermal initiator: 0.5-2.5 parts; (b) heating the adhesive composition to 80-95℃, and applying the adhesive composition in a molten state to the area to be bonded of a first substrate; (c) assembling a second substrate with the first substrate, and cooling to physical setting of the thermoplastic polyurethane in the adhesive composition; (d) applying UV irradiation to the light-exposed area in the area to be bonded, generating reaction heat in the light-exposed area; (e) conducting the reaction heat generated in step (d) from the light-exposed area to the shadow area in the area to be bonded by internal thermal conduction paths formed by the thermally conductive filler, and thermally curing the adhesive composition in the shadow area.

2. The UV and hot melt co-cured bonding process for earpiece structures of claim 1, wherein, The thermally conductive filler is hexagonal boron nitride with an average particle size D50 of 5-15 μm.

3. The UV and hot melt co-cured bonding process for earpiece structures of claim 1, wherein, The latent thermal initiator is a latent thermal acid generator with an activation temperature of 110-130℃, the epoxy resin is a cycloaliphatic epoxy resin, and the polyether polyol is polytetrahydrofuran diol.

4. The UV and hot melt co-cured bonding process for earpiece structures of claim 3, wherein, The latent thermal acid generator is an amine-blocked sulfonate.

5. The UV and hot-melt co-cured bonding process for earpiece structures of claim 4, wherein, The amine-blocked sulfonate is dicyclohexylammonium p-toluenesulfonate, and the dicyclohexylammonium p-toluenesulfonate is prepared as follows: Dissolving p-toluenesulfonic acid monohydrate in a first portion of toluene solvent to obtain an acid solution; Dissolving dicyclohexylamine in a second portion of toluene solvent to obtain an amine solution; Mixing the amine solution and the acid solution at room temperature under continuous mechanical stirring to cause acid-base neutralization reaction and precipitate solid product; After completion of mixing, continuing stirring at room temperature for 30-60 minutes to ensure complete reaction; Performing vacuum filtration on the mixture obtained in the above step to collect the solid filter cake; Washing the solid filter cake with cold toluene for 1-3 times; Drying the washed solid filter cake in a vacuum drying oven at 40-50℃ to constant weight to obtain the dicyclohexylammonium p-toluenesulfonate.

6. The UV and hot-melt co-cured bonding process for earpiece structures of claim 1, wherein, The photo-curable oligomer is aliphatic polyurethane acrylate; The photo-curable oligomer is prepared by reacting isophorone diisocyanate with polytetrahydrofuran diol to obtain a pre-polymer capped with -NCO groups, and then adding hydroxyethyl acrylate for capping reaction until the characteristic absorption peak of -NCO groups disappears.

7. The UV and hot-melt co-cured bonding process for earpiece structures of claim 1, wherein, The temperature of heating in step (b) is lower than the activation temperature of the latent thermal initiator.

8. The UV and hot-melt co-cured bonding process for earpiece structures of claim 1, wherein, The wavelength of the ultraviolet light irradiation in the (d) step is 365 nm or 395 nm, and the irradiance is 100-1000 mW / cm 2 .

9. The UV and hot-melt co-cured bonding process for earpiece structures of claim 1, wherein, The reaction heat generated in step (d) causes the peak temperature of the adhesive composition in the light-exposed area to reach above 120℃, which is higher than the activation temperature of the latent thermal initiator.

10. The UV and hot-melt co-cured bonding process for earpiece structures of claim 1, wherein, In the step (a) of preparing the adhesive composition, the photoinitiator and the latent thermal initiator are added under light shielding conditions after the remaining components are melt-mixed at 80 to 95°C and cooled to 70°C or less.