Anisotropic Conductive Film with Conductive Microspheres and Preparation Method Thereof

Abstract

The present invention discloses a conductive microsphere for an anisotropic conductive film and a preparation method thereof, belonging to the technical field of conductive microspheres. The conductive microsphere for the anisotropic conductive film comprises the following raw materials in parts by weight: 80-100 parts of styrene, 0.5-1.5 parts of an initiator, 6-8 parts of a modified crosslinking agent, 2-8 parts of a dispersant, 10-12 parts of carbon nanotubes, 15-20 parts of graphene, 80 parts of deionized water, and 800 parts of toluene; in the present invention, intermediate 1 is synthesized by reacting 2,2'-diallyl bisphenol A with epichlorohydrin, and the modified crosslinking agent is prepared by reacting intermediate 1 with γ-aminopropyltriethoxysilane. The conductive microsphere for the anisotropic conductive film prepared by the present invention has good electrical conductivity and hardness.

Description

Technical Field

[0001] The present invention relates to the technical field of conductive microspheres, and particularly relates to a conductive microsphere for an anisotropic conductive film and a preparation method thereof. Background Art

[0002] An anisotropic conductive film (ACF) is a material widely used in the field of electronic packaging. Its core lies in being able to conduct electricity in the vertical direction while insulating in the horizontal direction, thereby achieving high-precision electrical connection between electronic components. This material usually consists of an insulating resin matrix and conductive microspheres dispersed therein. The conductive microspheres play a key role. They contact the upper and lower metal contacts under the conditions of pressurization and heating to form a conductive path. However, many current conductive microspheres on the market have a certain rebound problem after heating and pressurization, resulting in a decrease in the contact area with the metal contacts, thereby reducing the conductive performance of electronic products.

[0003] In addition, properties such as the coating uniformity and crushing strength of conductive microspheres are also crucial for the conductive performance and reliability of anisotropic conductive films. Although common existing chemical plating of polymer microspheres can achieve the chemical plating of metal coatings, there is a problem of relatively small interfacial bonding force between the metal coating and the polymer microsphere core. During the compression process, the crushing strength of the conductive microspheres is weak, and the metal layer is prone to falling off. These problems have restricted the application of anisotropic conductive films in high-end electronic devices. Therefore, developing a conductive microsphere for anisotropic conductive films that can solve the above problems is of great significance for meeting the requirements of modern electronic devices for high-performance connection materials.

[0004] Chinese Patent No. CN109735257A discloses a heat-resistant conductive microsphere for an anisotropic conductive adhesive film and a preparation method thereof. This method uses methyl methacrylate, methacrylic acid, and hexachlorocyclotriphosphazene as monomers, polyvinylpyrrolidone as a dispersant, ethanol and water as solvents, and azobisisobutyronitrile as an initiator to prepare monodisperse methyl methacrylate-methacrylic acid-hexachlorocyclotriphosphazene cross-linked copolymer microspheres by dispersion polymerization, and then roughening, sensitizing, activating, and electroless nickel plating to obtain a heat-resistant conductive microsphere with a core-shell structure. Although its particle size distribution is uniform, it is easy to break and has poor coating uniformity. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a conductive microsphere for an anisotropic conductive film and a preparation method thereof.

[0006] To achieve the above purpose, the present invention is realized through the following technical solutions:

[0007] A conductive microsphere for an anisotropic conductive film, comprising the following raw materials in parts by weight:

[0008] Styrene: 80 - 100 parts, initiator: 0.5 - 1.5 parts, modified crosslinking agent: 6 - 8 parts, dispersant: 2 - 8 parts, carbon nanotubes: 10 - 12 parts, graphene: 15 - 20 parts, deionized water: 80 parts, toluene: 800 parts;

[0009] The modified crosslinking agent is prepared by the following method:

[0010] S1: Under nitrogen protection, add toluene, 2,2'-diallylbisphenol A, and epichlorohydrin into the reactor, stir, heat up to 80 - 100 °C, react for 2 - 4 h, cool down to 60 °C, slowly dropwise add sodium hydroxide solution, continue to react for 1 - 2 h after dropping, and perform post-treatment to obtain intermediate 1;

[0011] S2: Add DMF, intermediate 1, and γ-aminopropyltriethoxysilane into the reactor in sequence, stir and mix evenly, heat up to 80 - 120 °C, react for 4 - 6 h, and perform post-treatment to obtain the modified crosslinking agent.

[0012] In step S1, the feeding mass ratio of the toluene, 2,2'-diallylbisphenol A, and epichlorohydrin is 30:(10 - 12):(5 - 7);

[0013] In step S2, the feeding mass ratio of the DMF, intermediate 1, and γ-aminopropyltriethoxysilane is 50:(6 - 8):(5 - 7).

[0014] The initiator is one of azobisisobutyronitrile and ammonium persulfate.

[0015] The dispersant is one of polyethyleneimine, polyvinylpyrrolidone, and polyvinyl alcohol.

[0016] A preparation method of conductive microspheres for an anisotropic conductive film, comprising the following steps:

[0017] S1: Weigh by weight: Styrene: 80 - 100 parts, initiator: 0.5 - 1.5 parts, modified crosslinking agent: 6 - 8 parts, dispersant: 2 - 8 parts, carbon nanotubes: 10 - 12 parts, graphene: 15 - 20 parts, deionized water: 80 parts, toluene: 800 parts;

[0018] S2: Add toluene, styrene, initiator, and dispersant into the reactor, introduce nitrogen, heat up to 80 - 100 °C, carry out polymerization reaction for 10 - 16 h, then slowly dropwise add the modified crosslinking agent, and continue to react for 6 - 8 h to obtain a mixed solution A;

[0019] S3: Add carbon nanotubes and graphene into deionized water, mix them evenly by ultrasonic treatment to obtain mixed solution B. Slowly add mixed solution A into mixed solution B, stir and mix evenly to obtain a precursor solution. Atomize the precursor solution through an ultrasonic spray device and spray it into a high-temperature furnace, and keep it for 4 - 6 h to form precursor microspheres;

[0020] S4: Disperse the precursor microspheres in a nickel plating solution, stir and perform ultrasonic treatment, control the temperature at 75 - 80 °C and pH = 4.5 - 5.0, complete electroless plating, filter, wash and dry to obtain conductive microspheres for anisotropic conductive films.

[0021] The concentration of nickel salt in the nickel plating solution is 10 - 30 g / L, the concentration of complexing agent is 10 - 15 g / L, and the concentration of reducing agent is 15 - 20 g / L.

[0022] The nickel salt is nickel sulfate, and the reducing agent is sodium hypophosphite.

[0023] The complexing agent is carboxylated chitosan oligosaccharide, and the carboxylated chitosan oligosaccharide is prepared by the following method:

[0024] Dissolve 10 g of chitosan oligosaccharide in 1000 mL of deionized water, cool it to 0 - 5 °C in an ice bath; add 1 g of NaBr and 0.16 g of TEMPO and stir in sequence, slowly dropwise add 100 mL of NaClO solution; at the same time, adjust the pH to 10.0 - 10.5 with 1 M NaOH solution, after reacting for 2 h, add 100 mL of absolute ethanol to quench the reaction, and stir for 10 min; adjust the solution to neutral with 0.1 M HCl; transfer the reaction solution to a dialysis bag, dialyze with deionized water for 48 h, and freeze-dry the dialysis solution to obtain carboxylated chitosan oligosaccharide.

[0025] Due to the adoption of the above technical solutions, the beneficial effects of the present invention include:

[0026] (1) In the present invention, intermediate 1 is first synthesized by the reaction of the hydroxyl group in 2,2'-diallylbisphenol A with the chlorine atom in epichlorohydrin, and then the epoxy group in intermediate 1 undergoes a ring-opening reaction with the amino group in γ-aminopropyltriethoxysilane to prepare a modified crosslinking agent.

[0027] (2) In the present invention, in the presence of NaBr and NaClO, TEMPO is oxidized to generate highly active oxoammonium cations (TEMPO +), selectively oxidize the primary hydroxyl group (-CH2OH) at the C6 position of chitosan oligosaccharide to generate an aldehyde intermediate (-CHO). Subsequently, under alkaline conditions (pH 10.0 - 10.5), the aldehyde group is further oxidized to a carboxylic acid group (-COOH) by NaClO. Ethanol terminates the reaction by reducing the residual oxidant, and finally, salt by-products are removed by dialysis and freeze-dried to obtain carboxylated chitosan oligosaccharide. The key to this reaction lies in the high selectivity of the TEMPO system for primary hydroxyl groups, while amino groups and secondary hydroxyl groups are basically unaffected.

[0028] (3) The modified cross-linking agent prepared in the present invention has a core structure of 2,2'-diallylbisphenol A. Its bisphenol A group has high thermal stability (decomposition temperature > 200 °C) and can withstand the high temperature (130 - 180 °C) in the thermocompression process of the anisotropic conductive film (ACF), avoiding performance deterioration caused by cross-linking agent degradation. The introduced γ-aminopropyltriethoxysilane generates silanol groups through hydrolysis and forms stable chemical bonds with conductive microspheres or resin matrices, significantly enhancing the interfacial adhesion force and avoiding the shedding of conductive particles.

[0029] (4) Carboxylated chitosan oligosaccharide contains a large number of carboxyl groups, which can provide more active sites and higher reaction activity. These carboxyl groups can form stable complexes with metal ions; the amino and hydroxyl groups of chitosan oligosaccharide can also participate in the complexation reaction, thus significantly improving the complexation ability of the complexing agent. This enhanced complexation ability can better fix and disperse conductive microspheres and improve the performance of the conductive film.

[0030] (5) The present invention uses ultrasonic spray technology to atomize the precursor solution into uniform droplets through high-frequency vibration. When the droplets rapidly dehydrate and shrink in a high-temperature furnace, a wrinkled surface is formed. The wrinkled structure disperses stress through the surface uneven topography, thereby improving the mechanical ability of the microspheres. Detailed implementation mode

[0031] The following is further described in conjunction with embodiments, but the present invention is not limited to these embodiments.

[0032] Example 1 Preparation of the modified cross-linking agent:

[0033] S1: Under nitrogen protection, add 300 g of toluene, 100 g of 2,2'-diallylbisphenol A, and 50 g of epichlorohydrin to the reactor, stir, heat up to 80 °C, react for 4 h, cool down to 60 °C, slowly dropwise add 40 g of 48 wt% sodium hydroxide solution over 20 min, continue to react for 2 h after dropping, after the reaction is completed, cool to room temperature, slowly add 1 M dilute hydrochloric acid solution until the pH value of the reaction solution is neutral. Let it stand for stratification, separate the aqueous phase, retain the organic phase, carry out vacuum distillation at 70 °C for 2 h, then add 50 ml of saturated sodium chloride solution for washing, repeat three times, and vacuum dry at 70 °C for 3 h to obtain intermediate 1;

[0034] S2: Add 500 g of DMF, 60 g of Intermediate 1, and 50 g of γ-aminopropyltriethoxysilane into a reactor in sequence, stir to mix evenly, heat up to 80 °C, react for 6 h, cool to room temperature, perform vacuum distillation at 60 °C for 4 h, then add 500 ml of ethyl acetate for extraction, repeat three times, then add 50 g of anhydrous magnesium sulfate for drying for 5 h, filter, and obtain the modified crosslinking agent through column chromatography separation. Its structural formula is as follows:

[0035]

[0036] The data of its nuclear magnetic resonance hydrogen spectrum are as follows:

[0037] 1 H NMR (500 MHz, Chloroform-d) δ 7.16 – 7.01 (m, 4H), 6.74 (d, J =7.6 Hz, 2H), 5.87 (ddt, J = 16.8, 10.1, 6.7 Hz, 2H), 5.27 – 4.87 (m, 4H),4.77 (tt, J = 6.6, 4.5 Hz, 2H), 4.11 (dd, J = 12.3, 5.2 Hz, 2H), 3.97 (d, J =5.7 Hz, 2H), 3.95 – 3.69 (m, 16H), 3.29 (dtd, J = 6.7, 1.5, 0.8 Hz, 4H), 2.96(ddd, J = 13.2, 6.5, 4.9 Hz, 2H), 2.70 (dqd, J = 8.5, 6.8, 4.7 Hz, 6H), 1.74– 1.46 (m, 10H), 1.21 (t, J = 7.4 Hz, 18H), 0.71 (t, J = 9.1 Hz, 4H).

[0038] Example 2 Preparation of the modified crosslinking agent:

[0039] S1: Under nitrogen protection, add 300 g of toluene, 110 g of 2,2'-diallylbisphenol A, and 60 g of epichlorohydrin into a reactor, stir, heat up to 90 °C, react for 3 h, cool down to 60 °C, slowly dropwise add 45 g of 48 wt% sodium hydroxide solution over 20 min, continue to react for 1.5 h after dropping, after the reaction ends, cool to room temperature, slowly add 1 M dilute hydrochloric acid solution until the pH value of the reaction solution is neutral. Let it stand for layering, separate the aqueous phase, retain the organic phase, perform vacuum distillation at 60 °C for 3 h, then add 50 ml of saturated sodium chloride solution for washing, repeat three times, and dry in vacuum at 60 °C for 4 h to obtain Intermediate 1;

[0040] S2: Add 500 g of DMF, 70 g of Intermediate 1, and 60 g of γ-aminopropyltriethoxysilane into a reactor in sequence, stir to mix evenly, heat up to 100 °C, react for 5 h, cool to room temperature, perform vacuum distillation at 70 °C for 3 h, then add 500 ml of ethyl acetate for extraction, repeat three times, then add 50 g of anhydrous magnesium sulfate for drying for 5 h, filter, and obtain the modified crosslinking agent through column chromatography separation.

[0041] Example 3 Preparation of the modified crosslinking agent:

[0042] S1: Under nitrogen protection, add 300 g of toluene, 120 g of 2,2'-diallylbisphenol A, and 70 g of epichlorohydrin into a reactor, stir, heat up to 100 °C, react for 2 h, cool down to 60 °C, slowly dropwise add 50 g of 48 wt% sodium hydroxide solution dropwise for 20 min, continue to react for 1 h after dropping, after the reaction is completed, cool to room temperature, slowly add 1 M dilute hydrochloric acid solution until the pH value of the reaction solution is neutral. Let it stand for stratification, separate the aqueous phase, retain the organic phase, perform vacuum distillation at 50 °C for 4 h, then add 50 ml of saturated sodium chloride solution for washing, repeat three times, and perform vacuum drying at 50 °C for 5 h to obtain Intermediate 1;

[0043] S2: Add 500 g of DMF, 80 g of Intermediate 1, and 70 g of γ-aminopropyltriethoxysilane into a reactor in sequence, stir to mix evenly, heat up to 120 °C, react for 4 h, cool to room temperature, perform vacuum distillation at 80 °C for 2 h, then add 500 ml of ethyl acetate for extraction, repeat three times, then add 50 g of anhydrous magnesium sulfate for drying for 5 h, filter, and obtain the modified crosslinking agent through column chromatography separation.

[0044] Example 4 Preparation of carboxylated chitosan oligosaccharide

[0045] Dissolve 10 g of chitosan oligosaccharide in 500 mL of deionized water, cool it to 0 - 5 °C in an ice bath; add 1 g of NaBr and 0.16 g of TEMPO in sequence and stir for 10 min to form an activation system; slowly dropwise add 100 mL of NaClO solution (take 10 mL of sodium hypochlorite stock solution (10 wt%) and dilute it to 100 mL with deionized water), with a dropping rate of 5 mL / min; at the same time, adjust the pH to 10.0 - 10.5 with 1 M NaOH solution, add 80 mL of absolute ethanol to quench the reaction after reacting for 2 h, and stir for 10 min; adjust the pH of the solution to 7.0 with 0.1 M HCl to avoid protonation of carboxylic acid; transfer the reaction solution to a dialysis bag (MWCO 1000 Da), dialyze with deionized water for 48 h (change water every 8 h), and freeze-dry the dialysate at -50 °C for 48 h to obtain white powdery carboxylated chitosan oligosaccharide.

[0046] Example 5 Preparation of conductive microspheres for anisotropic conductive films:

[0047] S1: Weigh by weight: styrene: 800 g, initiator (azobisisobutyronitrile): 5 g, modified crosslinking agent (prepared in Example 1): 60 g, dispersant (polyethyleneimine): 20 g, carbon nanotubes: 100 g, graphene: 150 g, deionized water: 800 g, toluene: 8000 g;

[0048] S2: Add toluene, styrene, initiator, and dispersant into the reactor, stir, introduce nitrogen, heat up to 80 °C, carry out polymerization reaction for 16 h, then slowly add dropwise the modified crosslinking agent, add dropwise for 10 min, continue the reaction for 8 h, and cool down to room temperature to obtain the mixed solution A;

[0049] S3: Add carbon nanotubes and graphene into deionized water, ultrasonically mix for 10 min to obtain the mixed solution B, slowly add the mixed solution A into the mixed solution B, stir and mix evenly to obtain the precursor solution, atomize the precursor solution through an ultrasonic spraying device (ultrasonic frequency is 120 kHz) and spray it into a high-temperature furnace at 220 °C, with a residence time of 6 h to form precursor microspheres;

[0050] S4: Carry out electroless plating on the precursor microspheres, disperse the precursor microspheres in 1000 ml of electroless nickel plating solution (nickel sulfate 10 g / L, complexing agent (prepared in Example 4) 10 g / L, sodium hypophosphite 15 g / L), stir at 150 rpm and ultrasonically disperse at 75 kHz for 1 h, control the temperature at 75 - 80 °C, pH = 4.5 - 5.0, filter, wash with 1000 ml of absolute ethanol, and vacuum dry at 70 °C for 5 h to obtain the conductive microspheres for the anisotropic conductive film.

[0051] Example 6 Preparation of Conductive Microspheres for Anisotropic Conductive Film:

[0052] S1: Weigh by weight: styrene: 900 g, initiator (ammonium persulfate): 10 g, modified crosslinking agent (prepared in Example 2): 70 g, dispersant (polyvinylpyrrolidone PVP K25): 50 g, carbon nanotubes: 110 g, graphene: 180 g, deionized water: 800 g, toluene: 8000 g;

[0053] S2: Add toluene, styrene, initiator, and dispersant into the reactor, stir, introduce nitrogen, heat up to 90 °C, carry out polymerization reaction for 13 h, then slowly add dropwise the modified crosslinking agent, add dropwise for 15 min, continue the reaction for 7 h, and cool down to room temperature to obtain the mixed solution A;

[0054] S3: Add carbon nanotubes and graphene into deionized water, ultrasonically mix for 10 min to obtain the mixed solution B, slowly add the mixed solution A into the mixed solution B, stir and mix evenly to obtain the precursor solution, atomize the precursor solution through an ultrasonic spraying device (ultrasonic frequency is 120 kHz) and spray it into a high-temperature furnace at 220 °C, with a residence time of 5 h to form precursor microspheres;

[0055] S4: Electroless plate the precursor microspheres. Disperse the precursor microspheres in 1000 ml of electroless nickel plating solution (nickel sulfate 15 g / L, complexing agent (prepared in Example 4) 15 g / L, sodium hypophosphite 18 g / L), stir at 150 rpm, disperse ultrasonically at 75 kHz for 1 h, control the temperature at 75 - 80 °C, pH = 4.5 - 5.0, filter, wash with 1000 ml of absolute ethanol, and dry in vacuum at 70 °C for 5 h to obtain the conductive microspheres for anisotropic conductive film.

[0056] Example 7 Preparation of conductive microspheres for anisotropic conductive film:

[0057] S1: Weigh by weight: styrene: 1000 g, initiator (ammonium persulfate): 15 g, modified crosslinking agent (prepared in Example 3): 80 g, dispersant (polyvinyl alcohol PVA - 2499): 80 g, carbon nanotubes: 120 g, graphene: 200 g, deionized water: 800 g, toluene: 8000 g;

[0058] S2: Add toluene, styrene, initiator, and dispersant to the reactor, stir, introduce nitrogen, heat up to 100 °C, carry out polymerization reaction for 10 h, then slowly dropwise add the modified crosslinking agent, the dropping time is 20 min, continue the reaction for 6 h, and cool to room temperature to obtain the mixed solution A;

[0059] S3: Add carbon nanotubes and graphene to deionized water, mix evenly by ultrasonic for 10 min to obtain the mixed solution B. Slowly add the mixed solution A to the mixed solution B, stir and mix evenly to obtain the precursor solution. Atomize the precursor solution through an ultrasonic spraying device (ultrasonic frequency is 120 kHz) and spray it into a high-temperature furnace at 220 °C, with a residence time of 4 h to form precursor microspheres;

[0060] S4: Electroless plate the precursor microspheres. Disperse the precursor microspheres in 1000 ml of electroless nickel plating solution (nickel sulfate 30 g / L, complexing agent (prepared in Example 4) 15 g / L, sodium hypophosphite 20 mg / L), stir at 150 rpm, disperse ultrasonically at 75 kHz for 1 h, control the temperature at 75 - 80 °C, pH = 4.5 - 5.0, filter, wash with 1000 ml of absolute ethanol, and dry in vacuum at 70 °C for 5 h to obtain the conductive microspheres for anisotropic conductive film.

[0061] In the examples and comparative examples of this application, when carrying out electroless plating, control the pH within the specified range by 0.1 M dilute sulfuric acid or 0.1 M sodium hydroxide.

[0062] Comparative Example 1

[0063] A kind of conductive microspheres for anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the modified crosslinking agent is replaced with divinylbenzene of equal weight.

[0064] Comparative Example 2

[0065] A conductive microsphere for an anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the modified crosslinking agent is replaced with an equal weight of Intermediate 1 prepared in Step S1 of Example 2.

[0066] Comparative Example 3

[0067] A conductive microsphere for an anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the modified crosslinking agent is replaced with an equal weight of vinyltriethoxysilane.

[0068] Comparative Example 4

[0069] A conductive microsphere for an anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the modified crosslinking agent is replaced with an equal weight of a modified crosslinking agent prepared by the following method:

[0070] S1: Under nitrogen protection, add 300 g of toluene, 110 g of 2-allyl-4-methylphenol, and 60 g of epichlorohydrin to the reactor, stir, heat up to 90 °C, react for 3 h, cool down to 60 °C, slowly add 45 g of 48 wt% sodium hydroxide solution dropwise for 20 min, and continue to react for 1.5 h after dropping. After the reaction is completed, cool to room temperature, slowly add 1 M dilute hydrochloric acid solution until the pH value of the reaction solution is neutral. Let it stand for stratification, separate the aqueous phase, retain the organic phase, distill under reduced pressure at 60 °C for 3 h, then add 50 ml of saturated sodium chloride solution for washing, repeat three times, and dry in vacuum at 60 °C for 4 h to obtain Intermediate 1;

[0071] S2: Add 500 g of DMF, 70 g of Intermediate 1, and 60 g of γ-aminopropyltriethoxysilane to the reactor in sequence, stir and mix evenly, heat up to 100 °C, react for 5 h, cool to room temperature, distill under reduced pressure at 70 °C for 3 h, then add 500 ml of ethyl acetate for extraction, repeat three times, then add 50 g of anhydrous magnesium sulfate for drying for 5 h, filter, and obtain the modified crosslinking agent through column chromatography separation.

[0072] Comparative Example 5

[0073] A conductive microsphere for an anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the complexing agent in the nickel plating solution is replaced with an equal concentration of EDTA.

[0074] Comparative Example 6

[0075] A conductive microsphere for an anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the complexing agent in the nickel plating solution is replaced with an equal concentration of unmodified chitosan oligosaccharide.

[0076] Comparative Example 7

[0077] A conductive microsphere for an anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the complexing agent in the nickel plating solution is replaced by a complexing agent prepared by the following method with the same concentration:

[0078] Disperse 10 g of chitosan ultrasonically in 500 mL of deionized water, cool it in an ice bath to 0 - 5 °C; add 1 g of NaBr and 0.16 g of TEMPO in sequence and stir for 10 min to form an activation system; slowly dropwise add 100 mL of NaClO solution (take 10 mL of sodium hypochlorite stock solution (containing 10 wt% available chlorine), dilute it to 100 mL with deionized water), the dropping rate is 5 mL / min; at the same time, adjust the pH to 10.0 - 10.5 with 1 M NaOH solution, after reacting for 2 h, add 80 mL of absolute ethanol to quench the reaction, stir for 10 min; adjust the solution to pH = 7.0 with 0.1 M HCl to produce precipitation; filter and dry at 50 °C for 5 h to obtain carboxylated chitosan.

[0079] Comparative Example 8

[0080] A conductive microsphere for an anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the complexing agent in the nickel plating solution is replaced by a complexing agent prepared by the following method with the same concentration:

[0081] Dissolve 10 g of chitosan oligosaccharide in 500 mL of deionized water, stir until completely dissolved, add 10 mL of concentrated sulfuric acid to adjust the pH to 1.5 - 2.0 to form an acidic reaction system; add 5 g of potassium permanganate to the solution in batches (add 1 g each time, the batch interval is 3 min), control the temperature at 45 °C, after stirring and reacting for 6 h, add 80 mL of absolute ethanol to quench the reaction, stir for 10 min; filter to remove the precipitated manganese dioxide (MnO2), adjust the pH of the filtrate to neutral 7 with 5% NaOH solution; transfer the reaction solution to a dialysis bag with a cut-off molecular weight of 1000 Da, dialyze with deionized water for 48 h, and change the dialysis solution every 8 h. After dialysis, the solution is pre-frozen at -50 °C and dried in a freeze dryer for 48 hours to obtain white powdery carboxylated chitosan oligosaccharide.

[0082] Comparative Example 9

[0083] A conductive microsphere for an anisotropic conductive film, the raw material composition and process are basically the same as those in Example 6, the difference is that the complexing agent in the nickel plating solution is replaced by a complexing agent prepared by the following method with the same concentration:

[0084] Dissolve 10 g of chitosan oligosaccharide (degree of deacetylation ≥ 90%, molecular weight 3000 Da) in 500 mL of deionized water, and cool it to 0 - 5 °C in an ice bath; sequentially add 1 g of NaBr and 0.16 g of TEMPO and stir for 10 min to form an activation system; slowly dropwise add 100 mL of NaClO solution (take 10 mL of sodium hypochlorite stock solution (10 wt%) and dilute it to 100 mL with deionized water), with a dropping rate of 5 mL / min; simultaneously adjust the pH to 10.0 - 10.5 with 1 M NaOH solution, add 80 mL of absolute ethanol to quench the reaction after reacting for 2 h, and stir for 10 min; adjust the solution to pH = 7.0 with 0.1 M HCl to avoid protonation of carboxylic acid; transfer the reaction solution to a dialysis bag (MWCO 1000 Da), dialyze with deionized water for 48 h (change water every 8 h), and freeze-dry the dialysate at -50 °C for 24 h to obtain carboxylated chitosan oligosaccharide in the form of a white powder.

[0085] Comparative Example 10

[0086] A heat-resistant conductive microsphere for an anisotropic conductive film prepared by using the raw material composition, ratio and method of Example 1 of the Chinese invention patent with the publication number CN109735257A.

[0087] The materials used in the examples and comparative examples of this application are as follows except as otherwise specified:

[0088] The molecular weight Mn of polyethyleneimine is 10000; chitosan oligosaccharide is produced by Zhejiang Jinke Pharmaceutical Co., Ltd., with a purity of 99%, a degree of deacetylation ≥ 90%, and a molecular weight of 2000 Da; chitosan is produced by Zhejiang Jinke Pharmaceutical Co., Ltd., with a purity of 99% and an average molecular weight of 25000 daltons; the carbon nanotube model is: FT900, with a tube diameter of 10 - 25 nm and a purity of 99.9%, purchased from Jiangsu Tiannai Technology Co., Ltd.; the graphene model is LB1G3 - 54, purchased from Jiangsu Tiannai Technology Co., Ltd. The ultrasonic spray device used in this application is the UAM4000L type spray device of Hangzhou Chifei Ultrasonic Equipment Co., Ltd.

[0089] Perform volume resistivity tests on the anisotropic conductive films prepared in Examples 5 - 7 and Comparative Examples 1 - 10 using conductive microspheres, and the experimental method is carried out according to GB / T 1410 - 2006 "Determination of Volume Resistivity and Surface Resistivity of Solid Insulating Materials"; perform hardness tests on the anisotropic conductive films prepared in Examples 5 - 7 and Comparative Examples 1 - 10 using conductive microspheres, and the experimental method is carried out according to GB / T 4340.1 - 2009 "Metallic Materials - Vickers Hardness Test - Part 1: Test Method"; use a Malvern dynamic light scattering instrument to test the particle size distribution and calculate the average particle size and coefficient of variation C.V., and the test results are shown in Table 1.

[0090] Table 1

[0091]

[0092] As can be seen from Table 1, the coefficient of variation of the particle size of the conductive microspheres for anisotropic conductive films prepared in Examples 5, 6, and 7 is less than 2.3%, and the volume resistivity is less than 2.5×10 -3 Ω·cm, and the K value is greater than 350 kgf / mm 2 , having excellent electrical conductivity and hardness.

[0093] The modified crosslinking agent added in Comparative Example 1 was divinylbenzene of equal weight. From the data in Table 1, it can be seen that the volume resistivity was 5.0×10 -3 Ω·cm, and the K value was 250 kgf / mm 2 . The electrical conductivity was poor and the hardness was low. This was because divinylbenzene had a high double-bond crosslinking density and was prone to forming a rigid network, which might lead to an increase in the brittleness of the prepared conductive microspheres. However, the crosslinking agent of the present invention, through the synergistic effect of allyl and silane groups, could not only form a moderately crosslinked network but also relieve stress through the flexibility of the siloxane bond, making the conductive microspheres not easily broken.

[0094] Comparative Example 2 was a comparative example different from Example 6. The difference was that γ-aminopropyltriethoxysilane was not grafted onto the added crosslinking agent. From the data in Table 1, it could be seen that the electrical conductivity was poor and the hardness was low. This was because the crosslinking agent without grafted silane led to insufficient crosslinking degree, making the microspheres prone to deformation at high temperatures; at the same time, without grafted silane, the microspheres were prone to agglomeration and had poor dispersion performance, resulting in stress concentration inside the microspheres and reducing the mechanical and electrical conductivity of the conductive microspheres.

[0095] Comparative Example 3 was a comparative example different from Example 6. The difference was that the added modified crosslinking agent was replaced with vinyltriethoxysilane. From the data in Table 1, it could be seen that the electrical conductivity was poor and the hardness was low. This was because the rigid bisphenol A skeleton of 2,2'-diallylbisphenol A could provide high crosslinking density and thermal stability, and its allyl double bond could copolymerize with styrene to form a stable three-dimensional crosslinked network, thereby improving the performance of the conductive microspheres.

[0096] Comparative Example 4 was a comparative example different from Example 6. The difference was that 2,2'-diallylbisphenol A in step S1 was replaced with 2-allyl-4-methylphenol during the preparation of the modified crosslinking agent. From the data in Table 1, it could be seen that the volume resistivity was 3.2×10 -3 Ω·cm, and the K value was 320 kgf / mm 2, it has poor electrical conductivity and low hardness. The structure of 2 - allyl - 4 - methylphenol is relatively simple, and the cross - linked network of the monophenol structure is weak, resulting in a decrease in the mechanical strength of the conductive microspheres. The rigidity of the 2,2'-diallylbisphenol A structure helps to improve the toughness of the material, while the structure of 2 - allyl - 4 - methylphenol may lead to poor toughness of the material and brittle fracture easily.

[0097] Comparative Example 5 is a comparative example different from Example 6. The difference is that the complexing agent is replaced by EDTA. It can be seen from the data in Table 1 that the volume resistivity is 4.3×10 -3 Ω·cm, and the K value is 310 kgf / mm 2 The complexing agent prepared by the present invention has a large number of carboxyl groups, which can provide more complexing sites and can significantly improve the complexing ability.

[0098] Comparative Examples 6, 7, 8, and 9 are respectively replacing the complexing agent with unmodified chitosan oligosaccharide, carboxylated chitosan, carboxylated chitosan oligosaccharide prepared by potassium permanganate oxidation, and carboxylated high - molecular - weight chitosan oligosaccharide. It can be seen from Table 1 that their hardness and electrical conductivity are not as good as those prepared in the examples. The reason analysis is as follows:

[0099] Uncarboxylated chitosan oligosaccharide has weak complexing ability for nickel ions due to the lack of strong coordination groups (only containing hydroxyl and amino groups), resulting in an increase in the free Ni 2+ concentration in the plating solution; this causes the grain coarsening, porosity increase, and resistivity increase of the coating. At the same time, its lack of carboxyl groups and insufficient pH buffering ability will lead to a decrease in the reduction efficiency of sodium hypophosphite. The long molecular chain of carboxylated chitosan will form a dense adsorption layer on the surface of the microspheres, covering a large number of active sites, which will hinder the interfacial reduction of Ni 2+ and prevent the uniform deposition of nickel ions; in addition, the dissociation rate of its complex is low, and the nickel release is insufficient, resulting in a gradient distribution of nickel in the coating, leading to a significant deterioration of the coating adhesion and electrical conductivity uniformity. The strong oxidizing property of potassium permanganate causes the molecular chain of chitosan oligosaccharide to break and the distribution of carboxyl groups to be uneven, reducing the effective complexing sites. At the same time, it may introduce Mn 2+ impurities, which compete with Ni 2+ for adsorption, forming a non - conductive NiMnO3 heterophase, destroying the continuity of the coating, and resulting in a significant reduction in conductivity and hardness. Carboxylated high - molecular - weight chitosan oligosaccharide is prone to form micelles to wrap nickel ions due to its too long molecular chain, and its complex has high thermodynamic stability, and its ability to dissociate and release Ni ions at a process temperature of 75 - 80℃ is poor, resulting in a significant decrease in the density and deposition rate of the coating. Therefore, its hardness and conductivity decrease simultaneously.

[0100] Comparative Example 10 is a heat-resistant conductive microsphere for an anisotropic conductive film prepared by using the raw material composition, ratio and method of Example 1 of a Chinese invention patent with the publication number CN109735257A. It can be seen from the data in Table 1 that the volume resistivity is 5.6×10 -3 Ω·cm, and the K value is 230 kgf / mm 2 , and the electrical conductivity is poor.

[0101] As described above, the above are only the preferred embodiments of the present invention and are not used to limit the present invention; however, for those of ordinary skill in the art, without departing from the scope of the technical solution of the present invention, any equivalent changes made by using the technical content disclosed above, such as slight modifications and evolutions, are equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the substantial technology of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. An anisotropic conductive film conductive microsphere, characterized in that, It comprises raw materials in the following parts by weight: Styrene: 80 - 100 parts, initiator: 0.5 - 1.5 parts, modified crosslinking agent: 6 - 8 parts, dispersant: 2 - 8 parts, carbon nanotubes: 10 - 12 parts, graphene: 15 - 20 parts, deionized water: 80 parts, toluene: 800 parts; The modified crosslinking agent is prepared by the following method: S1: Under nitrogen protection, toluene, 2,2'-diallylbisphenol A, and epichlorohydrin are added to a reactor, stirred, heated to 80 - 100 °C, reacted for 2 - 4 h, cooled to 60 °C, and a sodium hydroxide solution is slowly added dropwise. After the addition is complete, the reaction continues for 1 - 2 h, and intermediate 1 is obtained after post-treatment; S2: DMF, intermediate 1, and γ-aminopropyltriethoxysilane are successively added to the reactor, stirred and mixed evenly, heated to 80 - 120 °C, reacted for 4 - 6 h, and the modified crosslinking agent is obtained after post-treatment.

2. The anisotropic conductive film according to claim 1, wherein In step S1, the mass ratio of the charged toluene, 2,2'-diallylbisphenol A, and epichlorohydrin is 30:(10 - 12):(5 - 7); In step S2, the mass ratio of the charged DMF, intermediate 1, and γ-aminopropyltriethoxysilane is 50:(6 - 8):(5 - 7).

3. The anisotropic conductive film according to claim 1, characterized in that The initiator is one of azobisisobutyronitrile and ammonium persulfate.

4. The anisotropic conductive film according to claim 1, wherein the conductive microspheres The dispersant is one of polyethyleneimine, polyvinylpyrrolidone, and polyvinyl alcohol.

5. A method for preparing conductive microspheres for an anisotropic conductive film according to any one of claims 1-4, characterized in that, It comprises the following steps: S1: Weigh by parts by weight: styrene: 80 - 100 parts, initiator: 0.5 - 1.5 parts, modified crosslinking agent: 6 - 8 parts, dispersant: 2 - 8 parts, carbon nanotubes: 10 - 12 parts, graphene: 15 - 20 parts, deionized water: 80 parts, toluene: 800 parts; S2: Toluene, styrene, initiator, and dispersant are added to a reactor, nitrogen is introduced, heated to 80 - 100 °C, and a polymerization reaction is carried out for 10 - 16 h. Then, the modified crosslinking agent is slowly added dropwise, and the reaction continues for 6 - 8 h to obtain a mixed solution A; S3: The carbon nanotubes and graphene are added to deionized water, ultrasonically mixed evenly to obtain a mixed solution B. The mixed solution A is slowly added to the mixed solution B, stirred and mixed evenly to obtain a precursor solution. The precursor solution is atomized by an ultrasonic spraying device and sprayed into a high-temperature furnace, and stays for 4 - 6 h to form precursor microspheres; S4: The precursor microspheres are dispersed in a nickel plating solution, stirred and ultrasonically treated, the temperature is controlled at 75 - 80 °C, pH = 4.5 - 5.0, chemical plating is completed, filtered, washed, and dried to obtain conductive microspheres for an anisotropic conductive film.

6. The preparation method according to claim 5, characterized in that, The concentration of nickel salt in the nickel plating solution is 10 - 30 g / L, the concentration of complexing agent is 10 - 15 g / L, and the concentration of reducing agent is 15 - 20 g / L.

7. The preparation method according to claim 6, characterized in that, The nickel salt is nickel sulfate, and the reducing agent is sodium hypophosphite.

8. The preparation method according to claim 6, characterized in that, The complexing agent is carboxylated chitosan oligosaccharide, and the carboxylated chitosan oligosaccharide is prepared by the following method: Dissolve 10 g of chitosan oligosaccharide in 500 mL of deionized water and cool it to 0 - 5 °C in an ice bath; add 1 g of NaBr and 0.16 g of TEMPO and stir in sequence, then slowly add 100 mL of NaClO solution dropwise; at the same time, adjust the pH to 10.0 - 10.5 with 1 M NaOH solution, after reacting for 2 h, add 100 mL of absolute ethanol to quench the reaction and stir for 10 min; adjust the solution to neutral with 0.1 M HCl; transfer the reaction solution to a dialysis bag and dialyze it with deionized water for 48 h, and the dialysate is freeze-dried to obtain carboxylated chitosan oligosaccharide.

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