Flexible adhesive and preparation method thereof

Abstract

The invention relates to the technical field of flexible electronics, in particular to the field of realizing element bonding by using a flexible conductive material. The invention provides a flexible conductive adhesive and a preparation method thereof. The flexible conductive adhesive comprises conductive filler, a resin precursor, modified epoxy resin, a curing agent, a curing accelerator and an additive, wherein the resin precursor comprises precursor epoxy resin, an epoxy resin cured product and a viscosity regulator; the precursor epoxy resin is epoxy resin containing a benzene ring or alicyclic structure; the modified epoxy resin has a linear main skeleton, contains two or more divalent non-aromatic hydrocarbon groups of-(CH2)-, and has epoxy groups at both ends. The flexible conductive adhesive disclosed by the invention solves the technical problems of high hardness and poor flexibility of a cured product of an epoxy system adhesive, and the disclosed flexible conductive adhesive has better heat resistance and binding force, can bear 180-degree bending and passes a four-point bending test.

Description

Technical Field

[0001] The present invention relates to the field of flexible electronic technology, and in particular to the field of realizing component bonding by using flexible materials. Background Art

[0002] The concept of flexible electronics was proposed in the 1980s. People tried to replace inorganic semiconductors such as silicon with organic semiconductors to make organic electronic devices flexible. With the development of science and technology, the application of flexible electronics has gradually become richer. Including: flexible display and lighting, flexible photovoltaics, chemical and biological sensors and wearable devices. The latest research direction involves flexible stretchable micro batteries, flexible radio frequency tags, electronic skin, artificial retinas, organic transistors and wearable devices, etc., so there are more detailed requirements for flexible materials.

[0003] The bonding of various components of flexible electronic systems requires an adhesive layer, and the adhesive layer is particularly important for the bonding of cross-linked conductors and flexible substrates. (1) During the assembly and use of flexible electronic products, they will inevitably experience an environment higher than normal temperature, so good heat resistance is necessary. (2) Since flexible electronic products are constantly subjected to tension, compression, bending and deformation during use, and the two thin layers connected by the adhesive layer usually have different mechanical properties, if the bonding force is not strong enough, it will inevitably lead to relative sliding or even peeling of the two thin layers. (3) The adhesive layer itself is an integral part of the structure of the flexible electronic system, and its own bending ability has an important influence on the bending ability of the entire structure. (4) The bonding of certain parts of the flexible electronic system also requires good conductivity.

[0004] At present, the mainstream choice of conductive adhesives used in the bonding layer is the conductive adhesive whose resin system contains epoxy resin. Epoxy resin has excellent performance in mechanical properties, electrical properties, thermal properties, chemical resistance, and bonding strength, but the hardness of epoxy resin system cured products is generally high. The use of this type of conductive adhesive has a higher bonding force and good heat resistance, but the cured product of this conductive adhesive generally has a higher elastic modulus and basically cannot meet the bending requirements of flexible electronic systems. Summary of the invention

[0005] The object of the present invention is to provide a flexible conductive adhesive which can meet the requirements of the adhesive layer of each component of the flexible electronic system on bending ability while meeting the requirements of the bonding force and heat resistance.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] In one aspect, the present invention provides a flexible conductive adhesive, comprising: a conductive filler (A), a resin precursor (B), a modified epoxy resin (C), a curing agent (D), a curing accelerator (E) and an additive (F); wherein:

[0008] The conductive filler (A) is composed of nano- or micron-sized conductive powder;

[0009] The resin precursor (B) comprises: a precursor epoxy resin (B1), an epoxy resin cured product (B2), and a viscosity adjuster (B3); the epoxy resin cured product (B2) is a cured product generated by a pre-treatment curing reaction of the precursor epoxy resin (B1); the precursor epoxy resin (B1) is an epoxy resin containing a benzene ring or an alicyclic structure;

[0010] The modified epoxy resin (C) is an epoxy resin with a linear main skeleton, containing two or more divalent non-aromatic hydrocarbon groups of -(CH2)-, and epoxy groups at both ends;

[0011] The epoxy curing agent (D) described in the present invention is an imidazole curing agent or an amine curing agent.

[0012] The curing accelerator (E) is used to promote the curing reaction of the epoxy resin in the flexible conductive adhesive;

[0013] The additive (F) is used to improve the product physical properties of the flexible conductive adhesive.

[0014] In this technical solution, two types of epoxy resins are selected for compounding, namely, precursor epoxy resin (B1) and modified epoxy resin (C). After a large number of experiments, it is found that after the two epoxy resins are mixed, the modified epoxy resin (C) monomer is more likely to be physically entangled with the precursor epoxy resin (B1), increasing the spatial length of the non-filler system monomer, thereby improving the flexibility of the system. In addition, the inventors further found that when the precursor raw material without pretreatment is directly added to the flexible conductive adhesive formula, it is difficult to obtain the required flexibility. The possible reason is that the epoxy resin used to prepare the precursor and the specially modified epoxy resin form a short-chain network cross-linking under the action of the curing agent at the same time, resulting in a decrease in the flexibility of the cured product. Therefore, the technical solution also requires pretreatment of the precursor epoxy resin (B1) to partially cure the precursor epoxy resin (B1), so that the resin precursor (B) is flexible and has a certain degree of reactivity. The use of it as a precursor for the subsequent adhesive can better solve the problem that the cured product of the epoxy system adhesive has a high hardness, a high elastic modulus, is not flexible, and is not suitable for the bending ability requirements of the adhesive layer of each component of the flexible electronic system.

[0015] In some optional embodiments, the conductive filler material may include, but is not limited to: silver, silver-plated fiber, silver-plated ball, copper, silver-plated copper, gold, platinum, palladium, aluminum, silver-plated aluminum, carbon black, carbon brazing, graphite, indium tin oxide, conductive nanofiller, conductive oxide and mixtures thereof. Preferred are gold, silver, aluminum, carbon black, and conductive nanofiller.

[0016] As a preferred embodiment, the precursor epoxy resin (B1) may include one or more combinations of bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenol novolac type epoxy resin, biphenyl aralkyl type epoxy resin, phenol aralkyl type epoxy resin, aromatic glycidylamine type epoxy resin, and epoxy resin with a dicyclopentadiene structure.

[0017] These resins may be in liquid or solid form. In addition, a mixture of a liquid resin and a solid resin may be used. Here, "liquid" and "solid" refer to the state of the epoxy resin at room temperature (25° C.). From the perspective of operability and processability of subsequent use as a precursor, it is preferred that at least 70% by mass of all the epoxy resins used are liquid epoxy resins.

[0018] Specific examples of the resin include bisphenol A type (hereinafter sometimes referred to as BPA type) epoxy resins ("JER828EL", "JER827", and "JER1001" manufactured by Mitsubishi Chemical Corporation), bisphenol F type epoxy resins ("JER807" manufactured by Mitsubishi Chemical Corporation), hydrogenated structure epoxy resins ("JERYX8000" manufactured by Mitsubishi Chemical Corporation), dicyclopentadiene type multifunctional epoxy resins ("HP7200" manufactured by DXC Corporation), epoxy resins having a butadiene structure ("PB-3600" manufactured by Daisiro Chemical Industries, Ltd.), and epoxy resins having a biphenyl structure ("NC3000H" and "NC3000L" manufactured by Nippon Kayaku Co., Ltd., and "YX4000" manufactured by Mitsubishi Chemical Corporation). Among them, it is preferable to use a BPA type epoxy resin having high heat resistance and low viscosity, more preferably "JER828EL", "JER827", "JER1001", and "JER807" manufactured by Mitsubishi Chemical Corporation, and further preferably "JER828EL".

[0019] Specifically, the special modified epoxy resin is characterized by:

[0020] The main skeleton contains two or more divalent non-aromatic hydrocarbon groups of -(CH2)-, both ends of which are epoxy groups.

[0021] Here, the "main skeleton" refers to the skeleton having the longest chain among skeletons having two or more epoxy groups at both ends.

[0022] The number of -(CH2)- contained in the main skeleton is preferably 2 or more, more preferably 6-20.

[0023] The two or more -(CH2)- groups may be directly bonded, or may be bonded via an ether bond, an ester bond, an amide bond, two carbon atoms bonded via a double bond, two carbon atoms bonded via a triple bond, or a thioether bond.

[0024] The specific modified epoxy compound of the present invention is preferably a compound having an epoxy group whose main skeleton is linear rather than cyclic.

[0025] The epoxy resin that meets the special modification described above preferably has 6 to 12 -(CH2)- groups in the main skeleton, and preferably the main skeleton is a linear rather than cyclic compound with epoxy groups. After mixing with the precursor described in this article, the resin monomer is more likely to be physically entangled with the resin precursor, increasing the spatial length of the non-filler system monomer, thereby improving the flexibility of the system. When selecting the modified epoxy resin, on the one hand, the flexibility of the epoxy resin should be considered, and the viscosity of the epoxy resin should also be taken into account. When the -(CH2)- groups in the main skeleton are less than 6, the crosslinking density of the epoxy resin increases during curing, resulting in a decrease in the flexibility of the cured product; when it is greater than 12, it is easy to increase the molecular weight of the epoxy resin, resulting in an increase in viscosity, and it is not suitable for use in the adhesive described in the present invention.

[0026] The specially modified epoxy resin that meets the above description has more than two epoxy groups and both ends of the main skeleton are epoxy groups, so the reactivity of the resin is improved. After mixing with the resin precursor described in this article, the proportion of chain crosslinks with increased chain length is greater than that of network crosslinks. At the same time, the steric hindrance of internal rotation of the ether bonds in the precursor resin is small, which can greatly improve the flexibility of the cured product.

[0027] Special modified epoxy resins that meet the requirements of the present invention include: EXA-4850-15, EXA-4816, and EXA-4822 manufactured by DIC Corporation; EP-4000S, EP-4000SS, EP-4003S, EP-4010S, and EP-4011S manufactured by ADEKA; BEO-60E and BPO-20E manufactured by Shin Nippon Chemical Co., Ltd.; and YL7175-500, YL7175-1000, and YL7410 manufactured by Mitsubishi Chemical Corporation.

[0028] In the aforementioned technical solution, the resin precursor (B) also includes a viscosity adjuster (B3); in the process of preparing the resin precursor (B), the epoxy resin part of the precursor undergoes a curing reaction, which leads to an increase in viscosity. In order to ensure the stability and convenience of subsequent use as a precursor, the viscosity adjuster (B3) needs to be added.

[0029] As a preferred embodiment, the viscosity modifier (B3) includes a combination of one or more of a glycidyl ether-based reactive diluent, polypropylene glycol, and polyether polyol.

[0030] Specific examples of the viscosity modifier (B3) include benzyl glycidyl ether, tert-butyl glycidyl ether, p-tert-butylphenol glycidyl ether, C8-10-alkyl glycidyl ether, polyethylene glycol diglycidyl ether, isopropyl glycidyl ether, phenol glycidyl ether, 2-toluene glycidyl ether, o-cresol glycidyl ether, resorcinol glycidyl ether, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, 4-methoxyphenyl glycidyl ether, glycidyl propargyl ether, neopentyl glycol diglycidyl ether, 3-[(2,3)-glycidyloxy]propylmethyldimethoxysilane, polypropylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether, dimer acid diglycidyl ether, glycerol diglycidyl ether, and the like. Or tetraethylene glycol, polyethylene glycol octadecyl ether, hexaethylene glycol dodecyl ether, glycolic acid ethoxylated oil ether, or polypropylene glycol with a relative molecular weight of 200 to 600.

[0031] As a more preferred embodiment, the viscosity adjuster (B3) contains both epoxy groups and alcoholic hydroxyl groups, such as the viscosity adjuster (B3) is composed of neopentyl glycol diglycidyl ether containing epoxy groups and polypropylene glycol containing alcoholic hydroxyl groups. The reason is that compared with using only one type of viscosity adjuster, when the viscosity adjuster has both epoxy groups and alcoholic hydroxyl groups, the obtained cured adhesive has lower flexibility.

[0032] As a preferred embodiment, the epoxy curing agent (D) described in the present invention is an imidazole curing agent or an amine curing agent. Specific examples of the curing agent include: 2-heptadecylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, 2-undecylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2-phenyl-4-benzyl-5-hydroxymethylimidazole, 2-methylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole and other imidazole curing agents; and amine curing agents include: triethylenetetramine, tetraethylenepentamine, divinylpropylamine, dicyandiamide, adipic acid dihydrazide, 4,4'-diaminodicyclohexylmethane, meta-xylylenediamine and other amine curing agents.

[0033] As a preferred embodiment, the curing accelerator (E) of the present invention is a solid dispersible latent curing accelerator. The solid dispersible latent curing accelerator refers to a compound that is a solid insoluble in the above-mentioned epoxy resin at room temperature (25°C), becomes soluble by heating, and acts as a curing accelerator for the epoxy resin. Examples thereof include imidazole compounds that are solid at room temperature and solid dispersible amine adduct-based latent curing accelerators, but are not limited thereto.

[0034] The solid dispersible latent curing accelerator can be selected from epoxy adducts (amine adducts) and urea adducts. Among them, solid dispersible amine adducts are preferred, for example, epoxy adducts (amine adducts), such as PN-23 from Ajinomoto Co., Ltd. and HX-3742 from Asahi Kasei Co., Ltd.; urea adducts, such as FXE-1000, FXR-1030, and FXR-1081 from T&K TOKA Co., Ltd.

[0035] In some optional embodiments, the additive (F) can be any of various additives such as flame retardants, storage stability enhancers, fillers, diluents, solvents, pigments, flexibility imparting agents, coupling agents, antioxidants, thixotropy imparting agents, dispersants, etc.

[0036] For example, in order to achieve the excellent storage stability of the adhesive of the present invention, a storage stability improver that can be used in the present invention can be added. As a storage stability improver, for example, borate compounds, titanate compounds, aluminate compounds, zirconate compounds, and isocyanate compounds can be listed. Among them, from the viewpoint of high versatility and safety and improved storage stability, borate compounds are preferred, triethyl borate, tri-n-propyl borate, triisopropyl borate, tri-n-butyl borate are more preferred, and triethyl borate is further preferred. For example, as a thixotropy imparting agent, micro powder silicon dioxide, fatty acid amide, polyolefin polymers, etc. can be listed.

[0037] As a preferred embodiment, before pretreatment, the resin precursor (B) includes, by mass, 50 to 80 parts of the precursor epoxy resin (B1) and 10 to 50 parts of the precursor curing agent (B4); after pretreatment and adjustment, the resin precursor (B) includes, by mass, 30 to 50 parts of the precursor epoxy resin (B1), 40 to 70 parts of the precursor epoxy resin cured product (B2) and 1 to 10 parts of the viscosity adjuster (B3).

[0038] As a preferred embodiment, the flexible conductive adhesive comprises, by mass, 70 to 80 parts of the conductive filler (A), 7 to 20 parts of the precursor (B), 7 to 15 parts of the modified epoxy resin (C), 1 to 3 parts of the curing accelerator (D) and 0.3 to 1 part of the additive (E).

[0039] The compounding of the precursor resin (B) and the special modified epoxy resin (C) should preferably be such that the epoxy resin curing material (B2) and the special modified epoxy resin (C) in the precursor are maintained in equivalent amounts, so that the physically entangled network is wider and sparser. If there is too much special modified epoxy resin (C), the special modified epoxy resin (C) will be more cross-linked with the precursor epoxy resin (B1), so that the density of the physically entangled network will increase, which is not conducive to the flexibility of the final cured product. If there is too much epoxy resin curing material (B2) in the precursor, the reactivity of the precursor will be reduced, and it will not be able to stably cross-link with the special modified epoxy resin during the curing of the flexible adhesive, so that the curing effect of the cured product will be poor.

[0040] On the other hand, the present invention provides a method for preparing a flexible conductive adhesive, which is used to prepare the aforementioned flexible conductive adhesive, wherein the preparation process comprises:

[0041] The precursor epoxy resin (B1') and the precursor curing agent (B4) are mixed and partially cured at a curing temperature to obtain the precursor epoxy resin cured product (B2) and the remaining precursor epoxy resin cured product (B1) that does not participate in the curing reaction;

[0042] Adding the viscosity modifier (B3) to the solution in which the partial curing reaction is completed to obtain the resin precursor (B);

[0043] The flexible conductive adhesive is prepared according to the resin precursor (B).

[0044] As a preferred embodiment, the precursor curing agent (B4) is a curing agent having a chain structure and containing ether bonds.

[0045] Considering the purpose of pretreatment to provide flexibility, a curing agent with a long chain or a curing agent containing an ether bond is preferred. The curing agent with a long chain has a reduced degree of crosslinking with the resin, which improves flexibility. The curing agent containing an ether bond has a small steric hindrance to the internal rotation of the ether bond, which improves the flexibility of the cured product.

[0046] Furthermore, the precursor curing agent (B4) is a polyetheramine curing agent.

[0047] From the perspective of having both a long chain structure and an ether bond, a polyetheramine curing agent is preferred, wherein the main chain is a polyether structure and the terminal active functional group is an amine polymer. Specific examples of the curing agent include: D230, D2000, T5000, etc.

[0048] In some preferred embodiments, the mass fraction of the precursor epoxy resin (B1') is 50 to 80 parts, and the mass fraction of the precursor curing agent (B4) is 10 to 50 parts.

[0049] In some preferred embodiments, the flexible conductive adhesive can be prepared according to the following component mass ratio:

[0050] 70 to 80 parts of the conductive filler (A), 7 to 20 parts of the resin precursor (B), 7 to 15 parts of the modified epoxy resin (C), 1 to 3 parts of the curing agent (D), 0.1 to 0.3 parts of the curing accelerator (E) and 0.3 to 1 parts of the additive (F).

[0051] In the above-mentioned preparation method, the resin precursor (B) is obtained by pre-treating the above-mentioned preferred precursor epoxy resin (B1') and the precursor curing agent (B4) mixed system, and then adjusting the obtained mixture to a suitable operating viscosity using a viscosity adjuster (B3). The use of the resin precursor (B) in the adhesive formulation can significantly improve the flexibility of the adhesive cured product and have good heat resistance and excellent bonding strength.

[0052] The aforementioned pretreatment method may specifically be: after mixing the precursor epoxy resin (B1') and the precursor curing agent (B4) in a certain proportion, pre-curing at a certain temperature for a period of time. Then, adding a viscosity adjuster (B3) to the pre-cured mixed system of the precursor epoxy resin and the curing agent, adjusting the mixed system to a state suitable for further use, and obtaining the resin precursor (B) for standby use.

[0053] The following is an example of a preferred bisphenol A epoxy resin and a polyetheramine curing agent:

[0054] The pre-curing temperature can be selected as the best recommended temperature of 80° C. for curing the polyetheramine curing agent and bisphenol A epoxy resin. The pre-curing time is preferably at least 4 hours to allow the precursor curing agent in the precursor to react completely.

[0055] Regarding the method for confirming the amount of the precursor curing agent (B4), the following scheme can be adopted:

[0056] Define a value X: According to the average active hydrogen value of the selected curing agent and the average epoxy value of the epoxy resin, curing 100g of the corresponding precursor epoxy resin requires Xg of the corresponding precursor curing agent to react completely in theory.

[0057] In the preparation process of the resin precursor (B) of the present invention, the mixing ratio of the precursor epoxy resin (B1') and the precursor curing agent (B4) is preferably 0.3Xg to 0.5Xg for 100g of the corresponding precursor epoxy resin (B1'). That is, according to the theoretical calculation value, preferably, 30% to 50% of the precursor epoxy resin reacts with the polyetheramine curing agent.

[0058] Since the precursor curing agent reacts with part of the precursor epoxy resin, the resulting mixed system contains unreacted precursor epoxy resin and partially cross-linked precursor epoxy resin cured product, so that the mixed system has certain reactivity while being flexible, and can be used as a precursor for subsequent adhesives.

[0059] The mixed system obtained by pre-curing has a viscosity increase due to partial curing, so for the stability and convenience of subsequent use as a precursor, a viscosity adjuster (B3) needs to be added.

[0060] After the resin precursor (B) to which the viscosity modifier (B3) is added is dispersed uniformly and allowed to stand for stabilization, the viscosity of the resin precursor (B) is preferably controlled to be 1000-4000 mPa·s. Preferably, the amount of the viscosity modifier (B3) added is 0.3-1.5% by mass.

[0061] Regarding the amount of the curing agent (D), the amount of the curing accelerator (E) is preferably about 10% of the amount of the curing agent (D) based on a theoretically calculated value based on the total epoxy equivalent of the precursor epoxy resin (B1) and the special modified epoxy resin (C) in the conductive adhesive.

[0062] Regarding the step of preparing the flexible conductive adhesive according to the resin precursor (B), the step may specifically include:

[0063] The resin precursor (B) and the special modified epoxy resin (B1) are mixed evenly; a conductive filler (A), a curing agent (D) and an accelerator (E) are respectively added to the resin mixture obtained in the previous steps and mixed evenly; finally, an additive (F) is added, mixed and dispersed evenly to obtain the flexible conductive adhesive.

[0064] The above technical solution has the following advantages or beneficial effects:

[0065] The present invention discloses a flexible adhesive and a preparation method thereof. By selecting two types of epoxy resins for compounding, the modified epoxy resin monomer is more likely to be physically entangled with the precursor epoxy resin, increasing the spatial length of the non-filler system monomer, thereby improving the flexibility of the system. By pretreating the precursor epoxy resin, the resin precursor has a certain reactivity while generating flexibility. When used as a precursor of a subsequent adhesive, the hardness and elastic modulus of the cured product of the epoxy system adhesive can be further reduced, thereby improving the softness. The flexible adhesive has good heat resistance and bonding strength, can withstand 180-degree bending, and passes a four-point bending test.

[0066] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0068] Figure 1 It is a schematic diagram of the reaction between the precursor epoxy resin and the precursor curing agent during pre-curing.

[0069] Figure 2 It is a schematic diagram of the physical entanglement between epoxy resin and epoxy resin cured product in the precursor.

[0070] Figure 3 It is a schematic diagram of the physical entanglement between the precursor resin solid and the special modified epoxy resin.

[0071] Figure 4 This is a schematic diagram of the physical entanglement of the precursor epoxy resin (B1) and the specially modified epoxy resin (C).

[0072] Figure 5 It is a schematic diagram of experimental data of various embodiments and comparative examples in this article. DETAILED DESCRIPTION

[0073] The following description and accompanying drawings fully illustrate specific embodiments of the present invention so that those skilled in the art can practice them. The examples represent possible variations only. Unless explicitly required, separate components and functions are optional, and the order of operations can vary. Parts and features of some embodiments may be included in or replace parts and features of other embodiments. The scope of the embodiments of the present invention includes the entire scope of the claims, and all available equivalents of the claims. In this article, each embodiment may be represented individually or generally by the term "invention", which is only for convenience, and if more than one invention is disclosed in fact, it is not intended to automatically limit the scope of the application to any single invention or inventive concept. In this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, without requiring or implying any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method or device including a series of elements includes not only those elements, but also other elements not explicitly listed. The various embodiments herein are described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same and similar parts between the various embodiments can be referred to each other. For the methods, products, etc. disclosed in the embodiments, since they correspond to the method part disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method part description.

[0074] [Measurement of the flexibility effect of the cured adhesive]

[0075] Low elastic modulus, not only can it withstand 180-degree bending, but also pass the four-point bending test

[0076] Modulus of elasticity:

[0077] For the flexible adhesive of the present invention, when the elastic modulus of the cured product at 25° C. is, for example, 10 to 10,000 MPa, it indicates that the cured product has a flexible basis that meets the requirements. The elastic modulus can be measured by a tensile test using a TENSILON universal testing machine (RTM-500) at 25° C. and 40% RH according to JIS-K-7161.

[0078] Four-point bending test:

[0079] The four-point bending test was conducted in accordance with the national ISO international standard: ISO 16540:2015, at 25°C and 40% RH using a four-point bending tester model WDD-50E (manufactured by Wenteng Testing Instruments Co., Ltd.).

[0080] Example:

[0081] Hereinafter, the present invention will be described in more detail based on Examples, but the present invention is not limited to the following Examples. It should be noted that "parts" in the following descriptions refer to "parts by mass".

[0082] 1. Preparation of resin system precursor:

[0083] 1. First mix the epoxy resin and the polyetheramine curing agent to obtain a resin curing agent mixture

[0084] 2. Mix the selected several adjusters and heat them at 80℃ for later use.

[0085] 3. The cured resin is placed in an oven for pre-curing.

[0086] 4. Add viscosity adjuster to the pre-cured resin curing agent mixture and stir to disperse evenly.

[0087] 2. Preparation of adhesive:

[0088] According to the data in the following table, the flexible adhesive of the present invention is prepared, and the weight proportions of the components are shown in Table 1.

[0089] Table 1 Mass ratio of each component of flexible adhesive

[0090] Resin Precursor 7～20％ Special modified epoxy resin 7～15％ Curing agent 1～3％ Accelerator 0.1～0.3％ filler 70～85％ additive 0.3～1％

[0091] The specific configuration process is as follows:

[0092] 1. The resin precursor and the special modified epoxy resin are stirred and mixed evenly to obtain a resin mixture.

[0093] 2. Add conductive filler, curing agent and accelerator to the resin mixture and mix well.

[0094] 3. Finally, add the additives and mix them, using a planetary mixer to fully stir and disperse them evenly.

[0095] Figure 1 to Figure 4 The schematic diagrams of the mechanism of the reaction between the precursor epoxy resin and the precursor curing agent, the mutual entanglement state between the precursor epoxy resin and its cured product, and the material entanglement state with the special modified epoxy resin are respectively shown.

[0096] Figure 5 The experimental data of component selection and addition ratio in the specific embodiments and comparative examples of the present invention are shown, as well as the test result data of each embodiment and comparative example. Table 2 specifically shows the relevant data parameters of the pretreatment link in each embodiment and comparative example.

[0097] Table 2 Relevant data information of pretreatment process in various embodiments and comparative examples

[0098]

[0099] Result description:

[0100] During the preparation of the precursor, Figure 1 As shown in the figure, the schematic diagram of the reaction between the precursor resin and the precursor curing agent during pre-curing, the epoxy end of the partially reacted epoxy resin is connected to the flexible polyether segment, which improves the flexibility. When the precursor is mixed with the special modified epoxy resin, the precursor resin cured product is physically entangled with the special modified epoxy resin, as shown in the attached Figure 3 As shown, the precursor is physically entangled with the special modified epoxy resin, making the cross-linking network more flexible, reducing the cross-linking density of subsequent curing, and improving the flexibility of the adhesive cured product.

[0101] The results of Examples 1 and 3 and Comparative Examples 1 and 2 show that directly adding the raw materials in the precursor to the silver colloid formula without pretreatment to obtain the precursor resin cannot obtain the required flexibility. The possible reason is that the epoxy resin used to prepare the precursor and the specially modified epoxy resin are first physically entangled together to form a dense network, such as the attached Figure 4 During the curing process, under the action of the curing agent, a network cross-link with a shorter chain length is formed, and the cross-linking density increases, resulting in a decrease in the flexibility of the cured product.

[0102] The results of Example 2 and Comparative Example 3 show that the value of X is lower than the preferred value. Under the preferred pre-curing time, the amount of the flexible precursor resin cured product obtained after treatment in the precursor is too small, that is, the amount of the epoxy resin with long-chain ether bonds is low, making the cured product insufficiently flexible. At the same time, when physical crosslinking occurs between the precursor epoxy resin and the precursor epoxy resin cured product, the crosslinking chain length becomes shorter, such as the attached Figure 2 As shown, the subsequent curing cross-linking density is large and the elastic modulus is high.

[0103] The results of Example 2 and Comparative Example 4 show that the value of X is higher than the preferred value. Under the preferred precuring time, the number of epoxy resins with long-chain ether bonds obtained by the reaction after pretreatment is large, which reduces the reactivity of the precursor and cannot stably cross-link with the special modified epoxy resin during the curing of the flexible adhesive, resulting in poor curing effect of the cured product. Although the elastic modulus is low, it cannot pass the four-point bending test.

[0104] The results of Example 1 and Comparative Examples 5, 6, and 7 show that the pre-curing time is shorter than the preferred time. In the precursor, the precursor curing agent is not fully reacted, so that the amount of the modified precursor epoxy resin cured product is too small, and the special modified epoxy resin is more cross-linked with the precursor epoxy resin, so that the network density of the physical entanglement is increased. Figure 4As shown, the crosslinking density of the final cured product increases and the elastic modulus is high. In addition, the precursor epoxy resin is relatively hard without modification, which also leads to a high elastic modulus. At the same time, there is a large amount of residual curing agent in the precursor, and there is a lot of curing agent in the curing process of the flexible adhesive. The reaction is easy to be too fast, resulting in uneven curing, which makes the cured product have a poor curing effect, and ultimately leads to a relatively high elastic modulus, and it also fails to pass the four-point bending test.

[0105] The results of Examples 1 and 4 and Comparative Examples 8 and 9 show that when only one type of adjuster (containing epoxy or alcoholic hydroxyl groups) is added to the precursor, compared with two types, a mixed liquid of adjusters with a large number of long chains cannot be obtained, which reduces the flexibility of the final cured adhesive.

[0106] The results of Example 4 and Comparative Example 10 show that when no adjuster is added, the operability of the precursor is poor when used, and the epoxy resin and curing agent in the precursor and the partially cross-linked part cannot be well dispersed, making it difficult for the cured product to pass the bending test.

[0107] The results of Example 2 and Comparative Example 11 show that without adding special modified epoxy resin, there is a lack of reactive special modified epoxy resin to cross-link with the precursor epoxy curing material, resulting in poor curing effect of the adhesive. The cured product has a certain flexibility but cannot pass the four-point bending test.

[0108] The results of Example 3 and Comparative Examples 12 and 13 show that when a special modified epoxy resin other than the special modified epoxy resin described in this patent is used instead of a special modified epoxy resin, the resin itself does not have a certain flexibility, the precursor resin becomes entangled, and a cured product with qualified flexibility cannot be obtained through the subsequent curing process.

[0109] The technical solution provided by the embodiment of the present invention may have the following beneficial effects:

[0110] The present invention discloses a flexible adhesive and a preparation method thereof. By selecting two types of epoxy resins for compounding, the modified epoxy resin monomer is more likely to be physically entangled with the precursor epoxy resin, increasing the spatial length of the non-filler system monomer, thereby improving the flexibility of the system. By pretreating the precursor epoxy resin, the resin precursor has a certain reactivity while generating flexibility. When used as a precursor of a subsequent adhesive, the hardness and elastic modulus of the cured product of the epoxy system adhesive can be further reduced, thereby improving the softness. The flexible adhesive has good heat resistance and bonding strength, can withstand 180-degree bending, and passes a four-point bending test.

[0111] It should be understood that the present invention is not limited to the processes and structures described above and shown in the drawings, and various modifications and changes can be made without departing from the scope thereof. The scope of the present invention is limited only by the appended claims.

Claims

1. A flexible conductive adhesive, characterized in that: include: Conductive filler (A), resin precursor (B), modified epoxy resin (C), curing agent (D), curing accelerator (E) and additive (F); wherein, The conductive filler (A) is composed of nanometer or micrometer-sized conductive powder; The resin precursor (B) comprises: a precursor epoxy resin (B1), a precursor epoxy resin cured product (B2), and a viscosity adjuster (B3); the epoxy resin cured product (B2) is a cured product generated by a pre-treatment curing reaction of the precursor epoxy resin (B1); the precursor epoxy resin (B1) is an epoxy resin containing a benzene ring or an alicyclic structure; The modified epoxy resin (C) is an epoxy resin with a linear main skeleton, containing two or more divalent non-aromatic hydrocarbon groups of -(CH2)-, and epoxy groups at both ends; The curing agent (D) is an imidazole curing agent or an amine curing agent, and reacts with the epoxy resin in the flexible conductive adhesive to undergo a curing reaction. The curing accelerator (E) is used to promote the curing reaction of the epoxy resin in the flexible conductive adhesive; The additive (F) is used to improve the product physical properties of the flexible conductive adhesive.

2. The flexible conductive adhesive according to claim 1, characterized in that: The precursor epoxy resin (B1) includes one or more combinations of bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenol novolac type epoxy resin, biphenyl aralkyl type epoxy resin, phenol aralkyl type epoxy resin, aromatic glycidylamine type epoxy resin, and epoxy resin with a dicyclopentadiene structure.

3. The flexible conductive adhesive according to claim 1, characterized in that: The viscosity adjuster (B3) includes a combination of one or more of a glycidyl ether-based active diluent, polypropylene glycol, and polyether polyol.

4. The flexible conductive adhesive according to claim 1, characterized in that: The curing accelerator (E) is a solid dispersed latent curing accelerator; the curing accelerator (E) includes epoxy adduct system and urea adduct system.

5. The flexible conductive adhesive according to any one of claims 1 to 4, characterized in that: The resin precursor (B) comprises, by mass fraction, 30 to 50 parts of the precursor epoxy resin (B1), 40 to 70 parts of the precursor epoxy resin cured product (B2) and 1 to 10 parts of the viscosity modifier (B3).

6. The flexible conductive adhesive according to claim 6, characterized in that: The flexible conductive adhesive comprises, by mass, 70 to 80 parts of the conductive filler (A), 7 to 20 parts of the resin precursor (B), 7 to 15 parts of the modified epoxy resin (C), 1 to 3 parts of the curing agent (D), 0.1 to 0.3 parts of the curing accelerator (E) and 0.3 to 1 parts of the additive (F).

7. A method for preparing a flexible conductive adhesive, characterized in that: include: The process for preparing the resin precursor (B) according to any one of claims 1 to 6, comprising: Mixing the precursor epoxy resin (B1') and the precursor curing agent (B4), and performing a pretreatment curing reaction at a curing temperature to obtain the precursor epoxy resin cured product (B2) and the remaining unreacted precursor epoxy resin (B1); Adding the viscosity modifier (B3) to the solution in which the partial curing reaction is completed to obtain the resin precursor (B); The flexible conductive adhesive is prepared according to the resin precursor (B).

8. The flexible conductive adhesive according to claim 7, characterized in that: The precursor curing agent (B4) is a chain curing agent containing an ether bond; the mass parts of the precursor epoxy resin (B1') are 50 to 80 parts, and the mass parts of the precursor curing agent (B4) are 10 to 50 parts.

9. The preparation method according to claim 10, characterized in that: The input amount of the precursor curing agent (B4) is 0.3Xg to 0.5Xg, wherein X is the amount of the precursor curing agent (B4) calculated based on the precursor epoxy resin (B1').

10. The preparation method according to any one of claims 7 to 9, characterized in that: The step of preparing the flexible conductive adhesive according to the resin precursor (B) specifically comprises: Mixing the resin precursor (B) and the special modified epoxy resin (B1) uniformly; Adding the conductive filler (A), the curing agent (D) and the accelerator (E) to the resin mixture obtained in the previous step respectively and mixing them evenly; Finally, the additive (F) is added, mixed and dispersed evenly to obtain the flexible conductive adhesive.

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