Adhesive of modified barium sulfate powder and application of adhesive

Abstract

The invention relates to the technical field of polymer composite materials, and discloses a modified barium sulfate powder adhesive and application of the modified barium sulfate powder adhesive. 30-50 parts of a composite filler; 28-32 parts of a bio-based curing agent; the composite filler is composed of barium sulfate with a core-shell structure and boron nitride nanosheets, the core of the barium sulfate with the core-shell structure is barium sulfate particles, the shell layer of the barium sulfate with the core-shell structure is a bio-based amphiphilic molecule modification layer, and the boron nitride nanosheets are adsorbed on the surface of the shell layer through pi-pi stacking; according to the invention, filler-matrix phase separation is eliminated through the interface anchoring effect of core-shell filler, boron nitride nanosheets are directionally arranged to construct a three-dimensional heat conduction path, a bio-based curing agent realizes high crosslinking density and molecular level balance of a flexible chain segment, and a stepped vacuum defoaming and gradient shearing dispersion process is combined to prepare the composite material. And the characteristics of high strength, high heat conductivity, low porosity and environment friendliness are synchronously realized.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer composite materials, and specifically to an adhesive for modified barium sulfate powder and its application. Background Art

[0002] Currently, epoxy adhesives often face the dual challenges of interfacial peeling and thermal failure under extreme working conditions. The root cause lies in the insufficient compatibility between inorganic fillers and organic matrices, leading to increased stress concentration and phonon scattering.

[0003] Traditional technologies rely on petroleum-based curing agents to construct rigid crosslinked networks. Although they can temporarily maintain mechanical properties, they come at the cost of high volatile organic compound emissions and brittle fractures, and it is difficult to meet the requirements of green manufacturing and impact resistance.

[0004] On the other hand, the introduction of highly thermally conductive fillers often leads to a sharp increase in viscosity and uneven dispersion, resulting in the formation of micron-sized air gaps and filler aggregates inside the adhesive layer, which become crack initiation points during thermal cycling and severely weaken the long-term service reliability.

[0005] Existing processes attempt to alleviate the defects through single vacuum degassing or mechanical stirring, but they cannot simultaneously control the filler orientation and porosity gradient, resulting in a forced trade-off between thermal conductivity, strength, and process stability of the material. Summary of the Invention

[0006] Aiming at the deficiencies of the existing technology, the present invention provides an adhesive for modified barium sulfate powder and its application, which solves the technical problem that it is difficult to synergistically optimize the interfacial bonding strength, thermal management efficiency, environmental friendliness, and process densification of traditional epoxy adhesives.

[0007] To achieve the above objectives, the present invention is realized through the following technical solutions: The first aspect of the present invention provides an adhesive for modified barium sulfate powder, which includes the following components: 100 parts by mass of epoxy resin, 30 - 50 parts by mass of composite filler, 28 - 32 parts by mass of bio-based curing agent, and 8 - 12 parts by mass of toughening agent; Epoxy resin: The epoxy resin serves as the matrix of the adhesive, providing basic bonding performance and chemical stability. In the present invention, bisphenol A epoxy resin is used. The epoxy groups in its molecular chain can react with the curing agent to form a three-dimensional crosslinked network, endowing the adhesive layer with excellent rigidity. Traditional epoxy adhesives are difficult to balance strength and toughness and have insufficient thermal conductivity due to solely relying on the resin matrix. The present invention uses epoxy resin as a functionalized carrier and breaks through the performance bottleneck of a single resin through its synergistic effect with the composite filler.

[0008] Composite filler: The composite filler is composed of core-shell structured barium sulfate and boron nitride nanosheets (BNNS), which is the core component for the present invention to achieve performance breakthroughs. Its innovative mechanism is as follows: Core-shell structure design: A core-shell structure with barium sulfate as the core and bio-based amphiphilic molecules as the shell. Through the amphiphilic characteristics of the shell layer (the hydrophilic end anchors on the surface of barium sulfate, and the hydrophobic end is compatible with epoxy resin), the dispersibility of the filler is significantly improved, avoiding the agglomeration problem caused by traditional physical blending.

[0009] Heterogeneous interface heat conduction pathway: BNNS is directionally adsorbed on the hydrophobic end of the shell layer through π-π stacking, and together with barium sulfate, a three-dimensional continuous heat conduction network is constructed. This design uses the high thermal conductivity of BNNS to make up for the insulating properties of barium sulfate, and at the same time reduces phonon scattering through the heterogeneous interface, achieving an order-of-magnitude improvement in the overall thermal conductivity of the adhesive layer.

[0010] Bio-based curing agent: The bio-based curing agent is selected as a cashew phenol derivative. The phenolic hydroxyl group in its molecule undergoes a click chemical reaction with the epoxy group to form a strong covalent cross-linking network. Compared with traditional petroleum-based curing agents (such as polyamides), the innovation of the present invention lies in: Dual functionalization: The curing agent not only realizes the curing of the matrix, but also undergoes interfacial cross-linking through its active groups with the shell layer molecules of the composite filler (such as the epoxy groups of chitosan derivatives), enhancing the filler-matrix interfacial bonding force, thereby reducing the interfacial thermal resistance and improving the mechanical strength.

[0011] Green and environmentally friendly: The bio-based cashew phenol replaces the petroleum-based amine curing agent, reducing VOC emissions and meeting the requirements of sustainable development.

[0012] Toughening agent: The toughening agent uses carboxyl-terminated butadiene acrylonitrile rubber (CTBN). The carboxyl group at the end of its molecular chain undergoes a grafting reaction with epoxy resin, introducing a flexible phase into the cross-linking network. The innovative mechanism of the present invention lies in: Gradient modulus design: The introduction of CTBN forms a gradient modulus structure of "rigid epoxy network - flexible rubber phase", improving the toughness of the adhesive layer while avoiding the strength loss caused by traditional toughening.

[0013] Interfacial stress buffering: The rubber phase is distributed at the interface between the filler and the matrix, which can effectively absorb external stress and inhibit crack propagation, thereby prolonging the service life of the adhesive under dynamic loads.

[0014] Preferably, the mass ratio of the core-shell structured barium sulfate to boron nitride nanosheets in the composite filler is 2.8 - 3.2:1.

[0015] Preferably, the bio-based amphiphilic molecule is a modified chitosan derivative, with a grafting rate of 60% - 80% and a grafted alkyl chain length of C12 - C18. This design solves the conflict problem of hydrophilic-hydrophobic balance and interfacial directional adsorption in traditional filler modification through precise regulation of the molecular structure.

[0016] The hydroxyl groups on the chitosan molecular chain are partially substituted by epoxy groups and long-chain alkyl groups, forming an amphiphilic structure with both hydrophilic and hydrophobic properties. The optimization of the grafting rate is based on the following: Lower limit 60%: If the grafting rate < 60%, there are too many residual hydroxyl groups in the chitosan molecule, resulting in too strong hydrophilicity, which leads to a decrease in its dispersibility in the epoxy resin matrix and easily causes filler agglomeration; at the same time, the hydrophobic chain density is insufficient, making it difficult to effectively adsorb BNNS through π-π stacking.

[0017] Upper limit 80%: If the grafting rate > 80%, the rigidity of the chitosan molecule increases significantly, and the steric hindrance hinders its coating on the surface of barium sulfate, and the excessive hydrophobicity will weaken the interfacial binding force with BNNS.

[0018] The length of the alkyl chain directly affects the arrangement orientation of the amphiphilic molecule at the filler-resin interface and its interaction strength with BNNS: C12 - C14 chain length: Short-chain alkyl groups can quickly embed into the epoxy resin matrix, but the hydrophobic interaction force is weak, and the adsorption stability of BNNS is insufficient; C16 - C18 chain length: Long-chain alkyl groups form a dense adsorption layer through hydrophobic interaction, providing stable π-π stacking sites for BNNS, and at the same time, their flexible chain segments can relieve the interfacial stress during the curing process.

[0019] A grafting rate of 60% - 80% ensures that the chitosan molecule forms a gradient-polarity interface on the surface of barium sulfate (the hydrophilic end anchors the filler, and the hydrophobic end extends to the resin matrix), while the C12 - C18 chain length realizes the directional adsorption with BNNS through hydrophobic interaction, forming a "filler-shell-BNNS" ternary heat conduction pathway.

[0020] Preferably, the preparation method of this modified chitosan derivative is as follows: 1. Epoxy group grafting Dissolve 4 - 6 g of chitosan (deacetylation degree ≥ 90%) in 450 - 550 mL of acetic acid solution with a concentration of 0.8% - 1.2%, and stir until completely dissolved. Add glycidyl ether, control the molar ratio of chitosan to glycidyl ether to be 1:1.8 - 2.2, and react at 55 - 65 °C for 5 - 7 hours. After the reaction, adjust the pH to neutral (6.8 - 7.2) with 0.1 - 0.2 M NaOH solution, then carry out dialysis treatment (cut-off molecular weight 8 - 10 kDa, time 45 - 50 hours), and obtain epoxy group-modified chitosan after freeze-drying.

[0021] 2. Long-chain alkyl grafting Mix epoxy group - modified chitosan with a long - chain alkyl reagent (such as cetyltrimethoxysilane) at a molar ratio of 1:0.9 - 1.1, disperse it in 200 - 300 mL of absolute ethanol, and add 0.4% - 0.6% sulfuric acid as a catalyst. React under reflux at 75 - 85 °C for 3 - 5 hours. After the reaction, centrifuge and separate, wash with ethanol 2 - 4 times, and vacuum - dry at 45 - 55 °C for 10 - 14 hours to obtain an amphiphilic chitosan derivative (grafting rate 60% - 80%, alkyl chain length C12 - C18).

[0022] Preferably, the shell - coating process of the core - shell barium sulfate includes: Step 1: Hydrothermal synthesis of barium sulfate particles Mix a barium chloride solution with a concentration of 0.1 - 0.15 mol / L and a sodium sulfate solution with a concentration of 0.1 - 0.15 mol / L in equal volumes, and react at 75 - 85 °C for 1.5 - 2.5 hours to generate micron - sized barium sulfate particles; In the liquid - phase environment, barium chloride and sodium sulfate react through an ion - exchange reaction to form barium sulfate precipitation. The hydrothermal conditions (high - temperature environment) provide energy to drive the directional growth of barium sulfate crystals, forming uniformly - sized micron - sized particles. Under high - temperature conditions, barium ions (Ba 2+ 2 +) in the solution combine with sulfate ions (SO4 2- 2 -) and are preferentially deposited along specific crystal planes (such as the (001) plane) to form a regular crystal structure. The closed system of hydrothermal synthesis can inhibit impurity adsorption and ensure the uniform distribution of chemical active sites (such as hydroxyl groups, uncoordinated ions) on the particle surface, laying a foundation for subsequent shell - coating.

[0023] Step 2: Oriented coating of shell molecules Add a bio - based amphiphilic molecule to the reaction solution in step (1), with an addition amount of 5% - 10% of the mass of barium sulfate particles. Then, treat it at an ultrasonic frequency of 35 - 45 kHz and a power of 280 - 320 W for 15 - 25 minutes to enable the amphiphilic molecule to be coated on the surface of barium sulfate through electrostatic adsorption and covalent bonding; Bio - based amphiphilic molecules (such as modified chitosan derivatives) bind to the surface of barium sulfate particles through electrostatic attraction in an aqueous solution. The hydrophilic groups (such as hydroxyl groups, amino groups) in the molecule form hydrogen bonds or ionic bonds with the polar sites on the surface of barium sulfate, while the hydrophobic long chains (such as C12 - C18 alkyl groups) extend outward to form a stable monolayer coating. Ultrasonic treatment generates micro - jets and local high pressure through cavitation effects, promoting the dynamic rearrangement of amphiphilic molecules on the particle surface: the hydrophilic ends are tightly anchored to the surface of barium sulfate, and the hydrophobic ends form a dense shell through intermolecular forces (such as hydrophobic interaction, π - π stacking). This process is accompanied by the formation of covalent bonds (such as the condensation reaction between a silane coupling agent and the hydroxyl groups on the surface of barium sulfate), further enhancing the shell stability.

[0024] Step 3: Core-shell structure immobilization Centrifuge and separate the coated barium sulfate core-shell structure, wash it with deionized water, and dry it at 50 - 60 °C for 12 - 24 hours; Centrifugal separation uses density differences to remove unreacted free molecules and solvent impurities, retaining intact core-shell particles. Washing with deionized water removes residual ions through solvent replacement, preventing cracking of the shell layer due to salting-out effects during the drying process. During the low-temperature drying (50 - 60 °C) stage, water evaporates slowly to avoid shell layer collapse, and at the same time, the hydrophobic chain segments self-assemble through intermolecular forces to form a continuous and dense organic shell layer. Finally, the surface of the barium sulfate particles is uniformly coated, forming a stable heterogeneous structure of "inorganic core - organic shell".

[0025] Preferably, the bio-based curing agent is cardanol epoxy curing agent, and its amine value is 230 - 250 mg KOH / g; Cardanol molecules contain both phenolic hydroxyl groups and long-chain aliphatic hydrocarbon groups (C15 unsaturated alkyl chains), endowing it with dual functional characteristics: Activity of phenolic hydroxyl group: The phenolic hydroxyl group undergoes a ring-opening reaction with the epoxy group of the epoxy resin to form a strong covalent cross-linking network, providing a rigid framework for the adhesive; Long-chain flexible groups: The aliphatic hydrocarbon chains interpenetrate the cross-linking network, absorbing external stress through the entanglement and slippage of molecular chains, inhibiting crack propagation, and thus avoiding the brittle fracture problem caused by traditional rigid curing agents.

[0026] The amine value characterizes the concentration of active amino groups in the curing agent, directly determining the cross-linking density and curing rate. When the amine value is 230 - 250 mg KOH / g, it ensures sufficient amino group density, enables the full ring-opening of epoxy groups, forms a three-dimensional network with a high degree of cross-linking, endows the adhesive layer with high strength and heat resistance; and avoids excessive amino group concentration resulting in too dense cross-linking points, thus maintaining appropriate network flexibility and preventing the accumulation of internal stress caused by too fast reaction rate.

[0027] Hydrogen bonds and covalent bonds are formed between the phenolic hydroxyl group of the cardanol curing agent and the shell molecules of the composite filler (such as modified chitosan derivatives): Hydrogen bond action: The phenolic hydroxyl group combines with amino or hydroxyl groups in the shell layer, enhancing the filler - matrix interfacial bonding force; Covalent cross-linking: During the curing process, the active groups of cardanol react with the epoxy groups of the shell molecules to form a chemical bond bridge of "filler - shell - matrix", significantly reducing the interfacial thermal resistance.

[0028] Cardanol is derived from renewable resources (cashew nut shell liquid). The natural long-chain structure in its molecules can achieve the rigid - flexible balance of epoxy resin without the need to additionally introduce toughening agents, reducing the use of synthetic additives from the source and meeting the requirements of low-carbon manufacturing.

[0029] Preferably, the toughening agent is carboxyl-terminated butadiene acrylonitrile rubber (CTBN), and its acrylonitrile content is 18% - 25%; The polarity of acrylonitrile units in the CTBN molecular chain (-C≡N group) forms a dynamic match with the polarity of the epoxy resin matrix (epoxy group, hydroxyl group): Acrylonitrile content ≥ 18%: The density of polar groups is high enough to ensure the compatibility of CTBN and epoxy resin, and avoid macroscopic phase separation between the toughened phase and the matrix; Acrylonitrile content ≤ 25%: Control the proportion of polar groups, retain the flexible characteristics of the butadiene chain segment, and enable the toughened phase to form a nano-scale "island structure" through microphase separation during curing, effectively absorbing crack propagation energy.

[0030] The carboxyl group (-COOH) at the end of CTBN undergoes a ring-opening reaction with the epoxy group of the epoxy resin during curing to form a chemical bond bridge: Main chain chemical cross-linking: The carboxyl group reacts with the epoxy group to form an ester bond, anchoring the CTBN molecular chain in the epoxy cross-linking network to avoid rubber phase migration; Interface stress transfer: Chemical bonding enhances the interfacial bonding strength between the rubber phase and the resin matrix, and under external force, stress can be evenly transferred to the rubber phase instead of concentrating at the interface.

[0031] The CTBN toughened phase and the composite filler (core-shell barium sulfate / BNNS) form a multi-scale synergistic reinforcement structure: Nano-scale toughening: The nano-rubber phase of CTBN absorbs the energy of micro-cracks through deformation; Micron-scale load-bearing: Core-shell barium sulfate acts as a rigid particle to resist external loads; Cross-scale interface optimization: BNNS lamellae bridge the rubber phase and the filler phase simultaneously through physical intercalation and chemical bonding, inhibiting crack propagation across scales.

[0032] Preferably, the epoxy resin is bisphenol A epoxy resin, and the epoxy value is 0.50 - 0.55 eq / 100g; The main chain of bisphenol A epoxy resin contains benzene rings and ether bonds, and its molecular characteristics form multi-level synergy with the composite filler (core-shell barium sulfate / BNNS): π-electron effect of benzene ring: The π-π interaction between the resin benzene ring and BNNS promotes the oriented arrangement of two-dimensional lamellae in the matrix, constructing a low-thermal-resistance heat conduction path; Polarity matching of ether bond: The oxygen atom of the ether bond forms a hydrogen bond with the shell layer of core-shell barium sulfate (such as the hydroxyl group of modified chitosan), enhancing the filler-matrix interfacial bonding and inhibiting interfacial delamination.

[0033] The epoxy value of bisphenol A epoxy resin characterizes the density of epoxy groups in its molecular chain and directly affects the topological structure of the curing network: Epoxy value ≥ 0.50 eq / 100g: Sufficient epoxy group density ensures the formation of a highly cross-linked three-dimensional network with the curing agent (such as cardanol derivatives), giving the adhesive layer high strength and heat resistance; Epoxy value ≤ 0.55 eq / 100g: Control the epoxy group density to avoid excessive cross-linking and retain the moderate flexibility of the molecular chain, thereby achieving both high strength and high toughness.

[0034] Epoxy value range matches the activity of bio-based curing agents: Stoichiometric balance: The equivalent ratio of epoxy group to amino group is close to 1:1, ensuring complete curing reaction and avoiding performance degradation caused by residual unreacted monomers; Gradient curing rate: The moderate reactivity of the medium epoxy resin allows the filler to dynamically adjust its orientation during the curing process to form a penetrating BNNS thermal conductive network.

[0035] The second aspect of the present invention provides a method for preparing an adhesive of the modified barium sulfate powder according to the first aspect, comprising the following steps: Step 1: Ingredients and premix Weigh bisphenol A epoxy resin, composite filler (core-shell barium sulfate / boron nitride nanosheets), bio-based curing agent (cardanol derivative) and toughening agent (carboxyl-terminated nitrile rubber) in proportion and add them into a stirring container.

[0036] Step 2: High-speed dispersion Mechanically stir at 800-1200 rpm for 25-35 minutes to evenly disperse the components and form a viscous slurry without obvious particles.

[0037] Step 3: Vacuum degassing The mixed slurry was transferred to a vacuum degassing machine and treated at a vacuum degree of -0.095 to -0.098 MPa for 10 to 15 minutes to remove bubbles and residual volatiles.

[0038] Step 4: Filling and storage The degassed glue should be sealed and stored away from light. It should be kept below 25℃ until used. The shelf life is ≥ 4 hours.

[0039] The third aspect of the present invention provides an application of the adhesive of the modified barium sulfate powder of the first aspect for bonding electronic packaging or automotive structural parts, comprising the following steps: Step 1: Adhesive Application Apply the adhesive evenly on the surface of the substrate (such as electronic components or metal structural parts), and control the coating thickness to 0.08-0.35 mm.

[0040] Step 2: Pre-curing Pre-curing at 75-85°C for 0.8-1.2 hours allows the adhesive layer to initially cross-link and discharge low molecular weight volatiles.

[0041] Step 3: High temperature post-curing The temperature is raised to 145-155°C and maintained for 1.8-2.2 hours to complete the curing, during which a pressure of 0.4-0.6 MPa is applied to eliminate interface pores.

[0042] Step 4: Cooling and post-processing Cool naturally to room temperature to obtain a high-strength, high-thermal conductivity bonding interface, which is suitable for electronic packaging heat dissipation or durable connection of lightweight automotive structural parts.

[0043] Preferably, the substrate is metal or ceramic, which is sandblasted before coating, and has a surface roughness Ra of 1.5 to 3.5 μm.

[0044] The present invention provides an adhesive of modified barium sulfate powder and its application, which has the following beneficial effects: 1. The present invention adopts the innovative design of core-shell structure filler, and the long organic chain of the shell layer forms a chemical-mechanical dual anchoring effect with the matrix resin, which effectively eliminates the filler-matrix interface defects. At the same time, the inner core inorganic particles and the two-dimensional thermal conductive sheet layer cooperate to construct a continuous heat flow channel, realizing the simultaneous optimization of high strength and high thermal conductivity, and solving the contradiction of traditional adhesives that "strong but not conductive, conductive but not strong".

[0045] 2. The present invention adopts bio-based curing agent to replace traditional petroleum-based amine curing agent, while maintaining high cross-linking density and thermal stability, it greatly reduces volatile organic compound (VOC) emissions and meets the requirements of green manufacturing. The flexible chain segments of its molecular structure give the material excellent impact resistance, breaking through the technical bottleneck of insufficient mechanical properties of environmentally friendly materials.

[0046] 3. The present invention uses a combination of stepped vacuum degassing and gradient shear dispersion processes to precisely control the bubble discharge path and filler arrangement orientation, suppress the porosity of the adhesive layer to near the theoretical limit, and significantly improve product batch stability. It is especially suitable for high-precision electronic packaging scenarios with stringent requirements for material uniformity.

[0047] 4. The present invention introduces a multi-scale energy dissipation structure into the epoxy network through the synergistic toughening mechanism of terminal carboxyl liquid rubber and core-shell filler, so that the material can delay fracture by microcrack passivation, silver streak branching, etc. when subjected to high loads, taking into account high stiffness and anti-fatigue properties, and extending the service life of the component.

[0048] 5. The present invention has full-process controllability from molecular structure design to macro process parameters, so that product performance can be flexibly adjusted according to application scenarios. For example, the formula can be quickly switched between chip packaging that prioritizes heat dissipation and structural bonding that prioritizes strength, greatly expanding the application boundaries. DETAILED DESCRIPTION

[0049] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0050] Embodiment 1: Raw material ratio (mass parts) Bisphenol A epoxy resin (epoxy value 0.52 eq / 100g): 100 parts Composite filler (core-shell barium sulfate / BNNS=8:2): 20 parts Core-shell barium sulfate: the shell is modified chitosan (grafting rate 70%, C16 alkyl chain), particle size 1.5 μm Boron nitride nanosheets (BNNS): thickness 10-20 nm, lateral size 5-10 μm Bio-based curing agent (cardanol epoxy curing agent, amine value 240 mg KOH / g): 35 parts Toughener (carboxyl-terminated nitrile rubber, acrylonitrile content 22%): 15 parts The preparation steps are as follows: 1. Ingredients and premixes The epoxy resin and composite filler were added into a planetary mixer and premixed at 500 rpm for 5 minutes at 25°C.

[0051] 2. High-speed dispersion Add curing agent and toughening agent, increase the speed to 1000 rpm, stir for 30 minutes, and control the slurry viscosity at 12,000-5,000 mPa·s (25°C).

[0052] 3. Vacuum degassing The mixture was transferred to a vacuum tank and degassed at -0.096 MPa for 12 minutes, with the bubble residual rate being <0.5%.

[0053] 4. Filling and storage Sealed and stored in a dark place at 20℃, shelf life 4.5 hours.

[0054] Embodiment 2: Raw material ratio (mass parts) Bisphenol A epoxy resin (epoxy value 0.55 eq / 100g): 100 parts Composite filler (core-shell barium sulfate / BNNS=6:4): 25 parts Core-shell barium sulfate: The shell layer is modified chitosan (grafting rate 80%, C18 alkyl chain), particle size 2.0 μm BNNS: Surface hydroxylation treatment, thickness 8 - 15 nm Bio-based curing agent (cardanol epoxy curing agent, amine value 250 mg KOH / g): 40 parts Toughening agent (carboxyl-terminated nitrile rubber, acrylonitrile content 25%): 10 parts The preparation steps are as follows: 1. Low-temperature premixing The epoxy resin and the composite filler are premixed at 10°C at 400 rpm for 8 minutes to avoid agglomeration of BNNS.

[0055] 2. Gradient stirring The curing agent and the toughening agent are added in two portions: First stage: Stir at 800 rpm for 15 minutes; Second stage: Stir at 1200 rpm for 20 minutes until the viscosity reaches 18,000 mPa·s.

[0056] 3. Stepwise defoaming First, defoam at -0.095 MPa for 5 minutes, then defoam at -0.098 MPa for 10 minutes, and the bubble residue rate < 0.3%.

[0057] 4. Low-temperature storage The glue solution is stored in a 15°C cold storage, and the pot life is extended to 6 hours.

[0058] Example 3: Raw material ratio (parts by mass) Bisphenol A epoxy resin (epoxy value 0.50 eq / 100g): 100 parts Composite filler (core-shell barium sulfate / BNNS = 9:1): 15 parts Core-shell barium sulfate: The shell layer is modified chitosan (grafting rate 60%, C12 alkyl chain), particle size 1.0 μm BNNS: Surface silane coupling agent modification, thickness 20 - 30 nm Bio-based curing agent (cardanol epoxy curing agent, amine value 230 mg KOH / g): 30 parts Toughening agent (carboxyl-terminated nitrile rubber, acrylonitrile content 18%): 20 parts The preparation steps are as follows: 1. Step-by-step mixing First, mix the epoxy resin and the toughening agent at 600 rpm for 10 minutes, and then add the composite filler.

[0059] 2. Ultrasonic-assisted dispersion Insert an ultrasonic probe (frequency 28 kHz, power 200 W), stir at 800 rpm for 25 minutes, and the slurry viscosity is 8,000 - 10,000 mPa·s.

[0060] 3. Dynamic vacuum treatment Gradually increase the vacuum degree from -0.090 MPa to -0.097 MPa, defoam for 15 minutes, and the bubble residue rate < 0.7%.

[0061] 4. Quick filling The adhesive liquid is filled within 30 minutes, and the pot life is 3.8 hours at 25°C.

[0062] Comparative Example 1: Compared with Example 1, the difference is that the composite filler is replaced with uncoated pure barium sulfate (without core-shell structure), and the rest are the same.

[0063] Comparative Example 2: Compared with Example 1, the difference is that the bio-based curing agent is replaced with dicyandiamide (amine value 280 mg KOH / g), and the rest are the same.

[0064] Comparative Example 3: Compared with Example 1, the difference is that the epoxy value of the epoxy resin is 0.45 eq / 100g, and the rest are the same.

[0065] Comparative Example 4: Compared with Example 1, the difference is that the toughening agent is replaced with ordinary nitrile rubber (acrylonitrile content 30%), and the rest are the same.

[0066] Comparative Example 5: Compared with Example 2, the difference is that the vacuum defoaming step is cancelled, and the rest are the same.

[0067] Comparative Example 6: Compared with Example 2, the difference is that the proportion of BNNS in the composite filler is increased to 60% (core-shell barium sulfate / BNNS = 4:6), and the rest are the same.

[0068] Comparative Example 7: Compared with Example 3, the difference is that the stirring time is shortened to 15 minutes (original 25 minutes), and the rest are the same.

[0069] Comparative Example 8: Compared with Example 2, the difference is that the vacuum defoaming pressure is adjusted to -0.080 MPa, and the rest are the same.

[0070] Comparative Example 9: Compared with Example 1, the difference lies in that the grafting rate of the shell layer of the core-shell barium sulfate is reduced to 40%, and the rest are the same.

[0071] Comparative Example 10: Compared with Example 3, the difference lies in that the addition amount of the composite filler is reduced to 5%, and the rest are the same.

[0072] Test Example 1: Comprehensive evaluation of mechanical properties The test steps are as follows: 1. Sample preparation: Prepare adhesives according to the formulations of Example 1, Example 3, and Comparative Examples 1, 3, 4, 7, and 10; Coat on the sandblasted aluminum alloy substrate (Ra = 2.0 μm), and the thickness of the adhesive layer is 0.2 mm; Complete curing according to the corresponding curing conditions (Example 1: 75°C / 1h + 150°C / 2h; Example 3: 80°C / 1h + 155°C / 2h).

[0073] 2. Substrate pretreatment: All substrates are ultrasonically cleaned with acetone for 10 minutes after sandblasting and dried with nitrogen.

[0074] 3. Mechanical testing Interfacial shear strength: Use a universal testing machine (ASTM D1002), loading rate 1 mm / min, and record the maximum load; Fracture toughness (KIC): Pre-cracked single-edge notched beam specimens (ASTM D5045), tested by three-point bending method.

[0075] 4. Data recording Each group of specimens is repeated 5 times, and the average value is taken after excluding outliers.

[0076] The test results are shown in Table 1: Table 1 Summary of mechanical property data of Test Example 1 From the test data in Table 1, it can be obtained that: The micro-nano structure design of the core-shell barium sulfate is the core driving force for interface strengthening. As shown in Comparative Example 1, the uncoated barium sulfate lacks the compatibility between the grafted alkyl chains of the shell layer and the epoxy resin, resulting in micron-sized gaps at the filler-matrix interface and a 42.3% decrease in shear strength. In Example 1, the C16 alkyl chains grafted with chitosan are combined with the epoxy groups through van der Waals forces, enabling the filler to be embedded in the micropores on the substrate surface to form a "rivet-type" mechanical interlock. At the same time, the BNNS lamellae spread at the interface through π-π stacking, dispersing the shear stress from a two-dimensional plane to a three-dimensional conduction, and finally achieving a high interface strength of 51.3 MPa.

[0077] The precise matching of bio-based curing agents and epoxy values determines the thermo-mechanical stability of the crosslinked network. In Comparative Example 3, a resin with a low epoxy value was used, and the active sites of the curing agent were insufficient to completely consume the epoxy groups, resulting in a decrease in crosslink density and brittle fracture of the adhesive layer under stress. In Example 1, an epoxy resin with 0.52 eq / 100g reacted equivalently with a cashew phenol curing agent with an amine value of 240 mg KOH / g to form a dense three-dimensional network. The energy dissipation mechanism changed from single covalent bond breakage to the synergistic effect of hydrogen bond dissociation and backbone slippage, increasing the fracture toughness by 50.4%.

[0078] The correlation between toughener distribution and process defects reveals the critical conditions for the strength-toughness balance. After shortening the stirring time in Comparative Example 7, the carboxyl-terminated nitrile rubber failed to disperse sufficiently to form a sea-island structure, and there was a local enrichment of the toughener, which became the crack initiation point under stress. In Comparative Example 10, reducing the filler amount weakened the synergistic reinforcement effect of barium sulfate / BNNS, resulting in a 37% decrease in the modulus of the adhesive layer. In Example 3, ultrasonic-assisted dispersion was used to make the toughener penetrate the epoxy network in the form of nanofibers. At the same time, 15% of the composite filler constructed a "rigid-flexible" gradient transition layer in the matrix, finally achieving a fracture toughness of 2.15 MPa·m 1 / 2 and maintaining a high strength of 48.7 MPa.

[0079] Test Example 2: Thermophysical Properties and Interfacial Thermal Resistance Analysis The test steps are as follows: 1. Sample Preparation Prepare adhesives according to the formulations of Example 1, 2 and Comparative Examples 2, 6, 8, 9; Coat on a ceramic substrate (AlN, Ra = 2.5 μm) with an adhesive layer thickness of 0.15 mm; Complete curing according to the corresponding curing conditions.

[0080] 2. Thermophysical Property Testing Glass transition temperature (Tg): Differential scanning calorimeter (DSC, ISO 11357), heating rate 10°C / min, nitrogen atmosphere; Thermal conductivity: Laser flash method (ASTM E1461), sample size Φ12.7 mm × 2 mm; Interfacial thermal resistance: Infrared thermal imager, set the heating stage to 100°C constant temperature, and record the interfacial temperature rise curve.

[0081] 3. Data Acquisition Test 3 parallel samples for each group, and take the median after excluding outliers.

[0082] The test results are shown in Table 2: Table 2 Summary of Thermophysical Property Test Data for Test Example 2 It can be obtained from the test data in Table 2 that: The interfacial engineering of the core-shell filler and the directional arrangement of BNNS are the keys to constructing an efficient thermal conduction network. In Example 2, barium sulfate coated with modified chitosan eliminates the phonon scattering interface between the filler and the matrix through the hydrophobic interaction between the C18 alkyl chain and epoxy resin, while the hydroxylated BNNS sheets are oriented along the shear force direction under gradient stirring to form a thermal conduction path with a low tortuosity, increasing the thermal conductivity to 1.68 W / m·K. In contrast, in Comparative Example 6, the excessive amount of BNNS leads to sheet stacking, blocking the phonon transfer path and reducing the thermal conductivity by 33.3%; while in Comparative Example 9, the insufficient grafting rate of the shell layer results in nanoscale gaps at the interface between the filler and the resin, confirming the decisive role of shell chemical modification in regulating the interfacial thermal resistance.

[0083] The stoichiometric matching of the bio-based curing agent and epoxy resin directly determines the thermal stability of the crosslinked network. In Example 1, a cardanol curing agent with an amine value of 240 mg KOH / g is used, and its long-chain phenolic aldehyde structure forms a high-crosslinking density network with epoxy groups, significantly increasing the energy barrier of molecular chain segment movement. After replacing it with dicyandiamide curing agent in Comparative Example 2, due to the too high reactivity of amino groups, local over-crosslinking occurs, forming a loose multiphase structure and generating microcracks during thermal cycling, resulting in the interfacial thermal resistance soaring to 1.54 K·mm 2 / W. This shows that the slow-release curing characteristic of the bio-based curing agent can avoid the explosive polymerization reaction of traditional amines, ensuring thermal stability while maintaining interfacial integrity.

[0084] The hidden damage of process defects to heat conduction reveals the importance of microstructure control. In Comparative Example 8, insufficient vacuum degassing pressure results in 0.9 vol% of closed-cell bubbles remaining in the adhesive layer. These air gaps act as adiabatic barriers in the heat flow path, causing a significant deviation between the measured thermal conductivity and the theoretical value. In Example 2, the bubble residue rate is suppressed below 0.3% through stepwise degassing, combined with ultrasonic-assisted dispersion to break the van der Waals aggregation force of BNNS, ultimately achieving the synchronous optimization of thermal conductivity and interfacial strength. This proves that only by coordinating the filler modification, curing system and process control can the traditional dilemma of "high thermal conductivity necessarily sacrificing strength" of thermal conductive adhesives be broken through.

[0085] Test Example 3: Detection of Physical Properties and Process Defects The test steps are as follows: 1. Sample preparation Prepare adhesives according to the formulations of Example 2 and Comparative Examples 5 and 6; Coat on a copper foil substrate (Ra = 1.8 μm) with an adhesive layer thickness of 0.1 mm; Curing was completed according to the corresponding curing conditions (Example 2: 85°C / 1h + 155°C / 2h; Comparative Examples 5 and 6 under the same conditions).

[0086] 2. Physical Property Tests Thermal Conductivity: Laser Flash Method (ASTM E1461), sample size Φ10 mm × 1.5 mm; Bubble Residual Rate: X-ray Tomography (resolution 1 μm), statistical proportion of the closed pore volume in the adhesive layer; Viscosity Stability: Rotational Viscometer (ISO 2555), viscosity values were recorded every 30 minutes at 25°C.

[0087] 3. Data Acquisition For thermal conductivity and bubble residual rate, 5 samples were tested in each group. After excluding the highest / lowest values, the average was taken; Viscosity was continuously monitored for 2 hours, and the fluctuation range was taken.

[0088] The test results are shown in Table 3: Table 3 Summary of Physical Property and Process Defect Detection Data for Test Example 3 From the test data in Table 3, it can be obtained that: The synergistic effect of the vacuum degassing process and the core-shell filler achieved a breakthrough in the limit of adhesive layer densification. In Example 2, sub-micron bubbles were gradually discharged through stepwise vacuum degassing, reducing the porosity of the adhesive layer to 0.28%. The microporous structure on the surface of the core-shell filler served as a nano-scale exhaust channel during the degassing process, avoiding the residue of large-sized bubbles. In contrast, in Comparative Example 5, due to the cancellation of the degassing step, the closed pore bubbles in the adhesive layer expanded and contracted during the thermal cycle, triggering the propagation of microcracks and resulting in a 46.7% sharp drop in thermal conductivity. This indicates that the synergistic design of the degassing pressure gradient and filler surface modification is the core path to suppress defects.

[0089] The percolation threshold of BNNS and the dispersion kinetics together determine the effectiveness of the heat conduction path. In Example 2, the 40% BNNS ratio achieved a controllable distribution of the lamellar spacing through gradient stirring, constructing a three-dimensional continuous heat conduction network in the matrix. After increasing BNNS to 60% in Comparative Example 6, the van der Waals force between the lamellae exceeded the shear dispersion ability of epoxy resin, triggering in-plane stacking and forcing the heat flow to conduct along a tortuous path. This non-linear response confirms that the filler ratio needs to be dynamically matched with the process shear force. Blindly increasing the dosage will exceed the percolation threshold and cause performance collapse.

[0090] The spatio-temporal evolution of viscosity stability and microdefects reveals the dynamic game between process and material. In Example 2, the electrostatic stabilization of the core-shell filler inhibits the agglomeration and sedimentation of BNNS, controlling the viscosity fluctuation within 3.5% within 2 hours and ensuring the homogeneous property of the adhesive during the construction window period. In Comparative Example 5, the local enrichment of the filler is driven by the upward floating of bubbles, and the viscosity change rate is as high as 12.8%. After construction, a "resin-rich and filler-poor" interfacial layer is formed, which becomes a weak link in heat conduction and mechanical load-bearing. This proves that through the two-way optimization of "filler surface charge regulation - process parameter iteration", the present invention achieves both performance and stability in a high-solid content system, breaking through the engineering bottleneck of traditional thermal conductive adhesives.

[0091] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An adhesive for modified barium sulfate powder, characterized in that, Comprising the following components in parts by weight: 100 parts of epoxy resin; 30 - 50 parts of composite filler, the composite filler is composed of core - shell structured barium sulfate and boron nitride nanosheets, the core of the core - shell structured barium sulfate is barium sulfate particles, the shell layer is a bio - based amphiphilic molecule modified layer, and the boron nitride nanosheets are adsorbed on the surface of the shell layer through π - π stacking; 28 - 32 parts of bio - based curing agent; 8 - 12 parts of toughening agent.

2. The adhesive for the modified barium sulfate powder according to claim 1, wherein, In the composite filler, the mass ratio of core - shell structured barium sulfate to boron nitride nanosheets is 2.8 - 3.2:

1.

3. The adhesive for the modified barium sulfate powder according to claim 1, characterized in that, The bio - based amphiphilic molecule is a modified chitosan derivative, its grafting rate is 60% - 80%, and the grafted alkyl chain length is C12 - C18.

4. The adhesive for modified barium sulfate powder according to claim 1, characterized in that, The shell layer coating process of the core - shell structured barium sulfate includes: (1) Hydrothermal synthesis of barium sulfate particles: Mix an equal volume of a barium chloride solution with a concentration of 0.1 - 0.15 mol / L and a sodium sulfate solution with a concentration of 0.1 - 0.15 mol / L, react at 75 - 85 °C for 1.5 - 2.5 hours to generate micron - sized barium sulfate particles; (2) Shell layer coating: Add a bio - based amphiphilic molecule to the reaction solution in step (1), and its addition amount is 5% - 10% of the mass of barium sulfate particles. Then, process it at an ultrasonic frequency of 35 - 45 kHz and a power of 280 - 320 W for 15 - 25 minutes, so that the amphiphilic molecule is coated on the surface of barium sulfate through electrostatic adsorption and covalent bonding; (3) Separation and drying: Centrifuge and separate the coated core - shell structured barium sulfate, wash it with deionized water, and dry it at 50 - 60 °C for 12 - 24 hours.

5. The adhesive for the modified barium sulfate powder according to claim 1, characterized in that, The bio - based curing agent is a cardanol - epoxy curing agent, and its amine value is 230 - 250 mg KOH / g.

6. The adhesive for the modified barium sulfate powder according to claim 1, wherein, The toughening agent is a carboxyl - terminated nitrile rubber, and its acrylonitrile content is 18% - 25%.

7. The adhesive for the modified barium sulfate powder according to claim 1, characterized in that The epoxy resin is bisphenol A type epoxy resin, and its epoxy value is 0.50 - 0.55 eq / 100g.

8. The adhesive for the modified barium sulfate powder according to claim 1, characterized in that, The preparation method of the adhesive for the modified barium sulfate powder includes: Mix epoxy resin, composite filler, bio - based curing agent and toughening agent in proportion, stir for 25 - 35 minutes and then carry out vacuum degassing.

9. Use of an adhesive for a modified barium sulfate powder according to any one of claims 1-8, characterized in that, For bonding in electronic packaging or automotive structural parts, it includes the following steps: Coat the adhesive on the surface of the substrate, and the coating thickness is 0.08 - 0.35 mm; Pre - cure at 75 - 85 °C for 0.8 - 1.2 hours, and then post - cure at 145 - 155 °C for 1.8 - 2.2 hours, and apply a pressure of 0.4 - 0.6 MPa during the curing process.

10. The application according to claim 9, characterized in that, The substrate is metal or ceramic, and is treated by sandblasting before coating, and the surface roughness Ra is 1.5 - 3.5 μm.

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