Organic phosphorus modified hyperbranched polyethylene, modified boron nitride nanosheet, flame-retardant heat-conducting adhesive and preparation thereof

Abstract

The invention discloses organophosphorus modified hyperbranched polyethylene, a modified boron nitride nanosheet, a flame-retardant heat-conducting adhesive and preparation of the organophosphorus modified hyperbranched polyethylene and the modified boron nitride nanosheet. The organophosphorus modified hyperbranched polyethylene is obtained by catalyzing ethylene and an organophosphorus functional monomer in an anhydrous organic solvent by an alpha-diimine palladium catalyst through one-pot copolymerization based on a chain walking mechanism; the structure of the organophosphorus functional monomer is shown as a formula I. The modified boron nitride nanosheet is prepared by attaching the organic phosphorus modified hyperbranched polyethylene to the surface of a boron nitride nanosheet. The flame-retardant heat-conducting adhesive is an adhesive which takes epoxy resin as a resin matrix and the modified boron nitride nanosheet as a nanofiller and has heat-conducting and flame-retardant functions. The flame-retardant heat-conducting adhesive has excellent bonding, heat-conducting and flame-retardant properties at the same time. # imgabs0 #

Description

Technical Field

[0001] The invention relates to an organophosphorus modified hyperbranched polyethylene, a modified boron nitride nanosheet, a flame retardant heat conductive adhesive and a preparation method thereof. Background Art

[0002] In recent years, various electronic components have become increasingly miniaturized, high-frequency, thin, flexible, and integrated. The use of various high-power electric drive equipment and facilities will become increasingly common, which has put forward urgent demands for common technologies such as thermal management, interface bonding, flame retardancy, fire protection, and packaging protection. As one of the thermal conductive interface materials, high-performance flame retardant thermal conductive adhesives have the functions of promoting interface heat transfer, stable bonding, packaging protection, and flame retardancy, and are important key materials that effectively meet the above application needs.

[0003] To date, many methods for preparing thermally conductive adhesives have been developed and reported. One of the basic ideas is to use various low-dimensional thermally conductive nanofillers to fill and compositely modify the adhesive matrix resin. The low-dimensional thermally conductive nanofillers mainly include graphene, carbon nanotubes, and boron nitride nanosheets (BNNS). The latter has aroused widespread research interest because it not only has excellent in-plane thermal conductivity, but also has outstanding electrical insulation, flame retardant properties, and high temperature and chemical corrosion resistance. Therefore, using BNNS to fill and modify the matrix resin is an important idea for preparing high-performance flame-retardant thermally conductive adhesives with multiple functions such as excellent thermal conductivity, bonding and flame retardancy.

[0004] The key to using BNNS to prepare high-performance flame-retardant thermally conductive adhesives is to achieve efficient stripping of the former and to achieve surface functionalization modification to promote its uniform dispersion in a specific matrix resin and form a strong interface effect. So far, a series of BNNS preparation and surface functionalization modification methods have been reported, mainly including: chemical vapor deposition (CVD) method, chemical oxidation-reduction method, supercritical assisted stripping method, ball milling method and liquid phase stripping method. Among them, the preparation of BNNS by CVD requires high temperature and specific equipment, and the obtained BNNS is difficult to adapt to the composite application requirements of thermally conductive adhesives; the surface of BNNS obtained by the supercritical method lacks effective functional groups, and it is difficult to uniformly disperse in the matrix resin and form a strong interface effect; the preparation of BNNS by chemical oxidation method has the disadvantages of long process route and the need to involve toxic chemicals; the ball milling process is time-consuming and the obtained BNNS has many structural defects; the preparation of BNNS by liquid phase stripping method has the advantages of simple process, easy liquid phase processing and application, and few structural defects of the obtained BNNS, but it has specific requirements for the solvent type; and so on. Therefore, how to achieve efficient exfoliation of BNNS and functional modification through a relatively simple and feasible process, promote its uniform dispersion in a specific matrix resin and obtain strong interfacial effects to obtain high-performance flame-retardant thermally conductive adhesives, is a key application technology that needs to be broken through in this field. Summary of the invention

[0005] The first object of the present invention is to provide organophosphorus modified hyperbranched polyethylene, modified boron nitride nanosheets, flame retardant thermal conductive adhesive and preparation method thereof, wherein the obtained flame retardant thermal conductive adhesive has excellent bonding, thermal conductivity and flame retardant properties.

[0006] The technical solution adopted by the present invention is described in detail below.

[0007] In a first aspect, the present invention provides an organophosphorus-modified hyperbranched polyethylene, wherein the organophosphorus-modified hyperbranched polyethylene is obtained by one-pot copolymerization of ethylene and an organophosphorus functional monomer in an anhydrous organic solvent using an α-diimine palladium catalyst based on a "chain walking" mechanism;

[0008] The structure of the organophosphorus functional monomer (DOPA) is shown in Formula I:

[0009] .

[0010] The organophosphorus modified hyperbranched polyethylene of the present invention is composed of approximately spherical hyperbranched polyethylene and multiple organophosphorus terminal groups.

[0011] The α-diimine palladium catalyst is selected from the α-diimine palladium catalyst 1 or 2 having the following structural formula:

[0012]

[0013] Both of the above can be synthesized in the laboratory according to the following literature:

[0014] [1] Johnson LK, Killian CM, Brookhart MJ Am. Chem. Soc., 1995, 117, 6414; [2] Johnson LK, Mecking S., Brookhart MJ Am. Chem. Soc., 1996, 118, 267.

[0015] The organophosphorus functional monomer can be synthesized in the laboratory with reference to the following literature:

[0016] [1] Shieh J., Wang C., Lv X. Polymer, 2001, 42, 7617-7625; [2]XingW., Hu Y. Fire Safety Science, 2012, 21, 65-70.

[0017] The anhydrous organic solvent is selected from one of the following: anhydrous dichloromethane, anhydrous chloroform, anhydrous chlorobenzene, preferably anhydrous dichloromethane.

[0018] In the one-pot copolymerization reaction system, the feed concentration of the organophosphorus functional monomer is 0.1-2.0 mol / L, preferably 0.5-1.0 mol / L, based on the total volume of the anhydrous solvent; the initial concentration of the α-diimine palladium catalyst is 0.5-50 g / L, preferably 2-20 g / L, based on the total volume of the anhydrous solvent.

[0019] The copolymerization reaction temperature is controlled at 5-30°C, preferably 15-25°C; the ethylene pressure is controlled at 0.01-0.5MPa, preferably 0.05-0.1MPa; and the polymerization time is controlled at 2-48h, preferably 12-24h.

[0020] After the copolymerization reaction is completed, the purification process can be carried out according to the following steps to obtain the organophosphorus-modified hyperbranched polyethylene:

[0021] (a) After the polymerization reaction is completed, the reaction solution is exposed to air to terminate the reaction;

[0022] (b) removing the solvent by blowing cold air to obtain a preliminary polymerization product;

[0023] (c) dissolving the obtained product in tetrahydrofuran, adding a small amount of hydrogen peroxide and concentrated hydrochloric acid aqueous solution, and stirring for 1 to 5 hours to dissolve the catalyst particles remaining in the polymer product;

[0024] (d) subsequently precipitating with methanol and removing the solvent;

[0025] (e) removing the solvent by blowing with cold air, dissolving the obtained product in tetrahydrofuran, and removing the free organophosphorus functional monomer by polymer precipitation using methanol, repeating 2 to 3 times until the upper layer liquid is colorless and transparent;

[0026] (f) The obtained product was vacuum dried at 30-60 °C for 24-48 h to obtain organophosphorus-modified hyperbranched polyethylene.

[0027] In a second aspect, the present invention provides a modified boron nitride nanosheet, wherein the modified boron nitride nanosheet is a boron nitride nanosheet having the organophosphorus-modified hyperbranched polyethylene described in the first aspect attached to its surface.

[0028] In a third aspect, the present invention provides a method for preparing the modified boron nitride nanosheets according to the second aspect, comprising the following steps (1):

[0029] Hexagonal boron nitride, the organophosphorus modified hyperbranched polyethylene described in the first aspect and an organic solvent are mixed, and the obtained mixture is sealed and ultrasonicated to obtain a BNNS initial dispersion, and further low-speed centrifugation and static treatment are performed to obtain a BNNS dispersion containing excess organophosphorus modified hyperbranched polyethylene, and excess free polymer is removed by high-speed centrifugation or vacuum filtration. Modified boron nitride nanosheets are obtained after drying at room temperature, and the organophosphorus modified hyperbranched polyethylene is attached to the surface of the boron nitride nanosheet.

[0030] In the above step (1), the hexagonal boron nitride powder is selected from analytically pure or chemically pure; and the organic solvent is selected from one of the following analytically pure or chemically pure solvents: chloroform, tetrahydrofuran, chlorobenzene, n-heptane, and dichloromethane.

[0031] In the above step (1), the concentration of the hexagonal boron nitride powder is 0.1-1500 mg / mL, preferably 0.5-1000 mg / mL, based on the total volume of the organic solvent; the mass ratio of the organophosphorus-modified hyperbranched polyethylene to the hexagonal boron nitride powder is 0.0005-0.5:1, preferably 0.005-0.5.

[0032] In the above step (1), the ultrasonic treatment is preferably carried out at an ultrasonic power of 300-1000 W and a constant temperature of 15-35 °C, and the continuous ultrasonic treatment time is preferably 12-150 h; the low-speed centrifugation is preferably carried out at room temperature and 2000-5000 rpm, and the centrifugation time is preferably 25-60 min; the static treatment time is preferably 1-4 h; the vacuum filtration is carried out through a microporous filter membrane with an average pore size of 0.1-0.5 μm, and the material thereof is selected from one of polytetrafluoroethylene, polyvinylidene fluoride or alumina; the high-speed centrifugation is recommended to be carried out at room temperature and 5000-20000 rpm, and the centrifugation time is preferably 5-60 min.

[0033] In a fourth aspect, the present invention provides a flame retardant thermally conductive adhesive, which is an adhesive having thermal conductivity and flame retardancy, and has epoxy resin as a resin matrix and the modified boron nitride nanosheets described in the second aspect as nanofillers.

[0034] Preferably, in the flame retardant thermally conductive adhesive, the mass percentage content of modified boron nitride nanosheets is 2-30%.

[0035] In a fifth aspect, the present invention provides a method for preparing the flame retardant thermally conductive adhesive according to the fourth aspect, comprising the following steps:

[0036] (A) The modified boron nitride nanosheets are ultrasonically dispersed in an organic solvent, and then mixed evenly with an epoxy resin and a curing agent. The resulting mixture is vacuum degassed and then evenly coated between bonding substrates or in a casting mold. The flame retardant thermal conductive adhesive is obtained after the solvent is volatilized and cured.

[0037] In the above step (A), the epoxy resin is selected from bisphenol A epoxy resin, bisphenol F epoxy resin, glycidyl ether epoxy resin or alicyclic epoxy resin, and its specification is analytical grade or chemical grade; the curing agent is selected from curing agent 593, ER-630, ER-840 or ER-1050, and its specification is analytical grade or chemical grade; the mass ratio of the epoxy resin to the curing agent is preferably 1-6:1.

[0038] In the above step (A), the degassing treatment is recommended to be carried out at a rotation speed of 60-300 rpm, and the degassing time is preferably 5-60 min; the curing is recommended to be carried out at 40-80 °C, and the curing time is preferably 4-16 h. The bonding substrate is one of the following materials: glass, wood, ceramic, plastic film.

[0039] Compared with the prior art, the present invention has the following outstanding advantages and beneficial effects:

[0040] First, based on the unique "chain walking" polymerization mechanism of α-diimine palladium catalyst, it is used to catalyze the copolymerization of ethylene and organophosphorus functional monomers under mild conditions, and organophosphorus-modified hyperbranched polyethylene is synthesized by a simple one-pot process. Compared with existing hyperbranched flame retardants, it has the advantages of simple process and controllable and adjustable composition ratio.

[0041] Second, by utilizing the non-covalent CH-π and π-π synergistic effect between the synthesized organophosphorus-modified hyperbranched polyethylene and BNNS, the former can be used to effectively promote the liquid phase exfoliation of the latter with high exfoliation efficiency; and simultaneously achieve non-covalent and stable modification of its surface to obtain low-defect BNNS with surface organophosphorus modification.

[0042] Third, the synergistic effect of these two types of flame retardants, organophosphorus-modified hyperbranched polyethylene and BNNS, can give the epoxy resin matrix excellent flame retardant properties.

[0043] Fourth, by means of the organophosphorus-modified hyperbranched polyethylene, the obtained BNNS can be effectively promoted to be uniformly dispersed in the epoxy resin matrix, and a strong interfacial effect can be obtained, so that the obtained epoxy resin composite adhesive exhibits excellent thermal conductivity and bonding properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 : Schematic diagram of the preparation of organophosphorus modified hyperbranched polyethylene;

[0045] Figure 2:Schematic diagram of the preparation of modified boron nitride nanosheets using organophosphorus modified hyperbranched polyethylene;

[0046] Figure 3 :A is the sample of Example 1 1 HNMR spectrum, where methyl, methine, and methylene represent methyl (-CH3), methine (-CH=), and methylene (-CH2-) respectively; B is the 31 P NMR spectrum; C is the GPC elution curve of the samples of Example 1 and Comparative Example 1; D is the melt rheology curve of the sample of Example 1;

[0047] Figure 4 :A is the appearance of the BNNS dispersion obtained in Example 2; B is the appearance of the BNNS dispersion obtained in Comparative Example 2; C is a high-resolution TEM photo of the BNNS obtained in Example 2; D is an electron diffraction pattern of the BNNS obtained in Example 2; E is an AFM photo and thickness distribution curve of the BNNS obtained in Example 2; F is the thickness distribution of the BNNS obtained in Example 2; G is a WAXRD curve of the BNNS and hexagonal boron nitride powder obtained in Example 2; H is a Raman spectrum of the BNNS and hexagonal boron nitride powder obtained in Example 2; I is a TGA curve of the BNNS (BNNS exfoliated with HBPE@DOPA) obtained in Example 2, the BNNS (BNNS exfoliatedin DMF) obtained in Comparative Example 3, and the organophosphorus-modified hyperbranched polyethylene (HBPE@DOPA) prepared in Example 1; JL is an EDS element distribution diagram of the BNNS obtained in Example 2.

[0048] Figure 5 : AC is a cross-sectional SEM photograph of the composite thermally conductive adhesive sample obtained in Example 3; DF is a cross-sectional SEM photograph of the composite thermally conductive adhesive sample obtained in Comparative Example 3; G is the lap stress-strain curve of the composite adhesive sample obtained in Example 3; H is the lap stress-strain curve of the composite adhesive sample obtained in Comparative Example 3.

[0049] Figure 6 : A is the thermal conductivity of the composite thermally conductive adhesive obtained in Example 4 and Comparative Example 4; B is the thermal conductivity improvement ratio of the composite thermally conductive adhesive obtained in Example 4 and Comparative Example 4.

[0050] Figure 7 : A is a vertical combustion test picture of the sample of Comparative Example 5; B is a vertical combustion test picture of the sample of Comparative Example 6; C is a vertical combustion test picture of the sample of Example 5; D is a vertical combustion test picture of the sample of Comparative Example 7. E is the vertical combustion time of Example 5 and Comparative Example 6; F is the heat release curves of Example 5 and Comparative Examples 5 and 6. DETAILED DESCRIPTION

[0051] The present invention is further described in detail below with reference to specific embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0052] If no specific conditions are specified in the examples of the present invention, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be obtained by conventional technical means or purchased commercially.

[0053] Example 1, Comparative Example 1

[0054] 1. Sample preparation

[0055] (1) Example 1

[0056] Under ethylene protection, 10 mL of anhydrous dichloromethane and 4.8 g of organophosphorus functional monomer DOPA (0.0159 mol) were added to a 250 mL glass reaction bottle and stirred for 30 min to form a uniform solution; then 0.25 g of α-diimine palladium catalyst 1 was dissolved in 10 mL of anhydrous dichloromethane and further injected into the above reaction bottle; then the polymerization reaction was continued for 24 h under ethylene pressure of 0.1 MPa at 25 °C by stirring. After the polymerization reaction was completed, the reaction solution was exposed to air to terminate the polymerization reaction, and the solvent was removed by air purge to obtain the preliminary product; the obtained product was further dissolved in an appropriate amount of tetrahydrofuran, a small amount of H2O2 and hydrochloric acid aqueous solution were added dropwise, stirred for 4 h to dissolve the residual catalyst particles, and then precipitated with methanol; the obtained product was further dissolved in a small amount of tetrahydrofuran and precipitated with methanol, and the process was repeated 3 times to remove the unreacted functional monomer DOPA; the obtained product was further vacuum dried at 60 °C for 48 h to obtain organophosphorus modified hyperbranched polyethylene. (Note: In the above reaction system, the feed concentration of the functional monomer DOPA is 0.80 mol / L based on the total volume of the solvent; the initial concentration of the catalyst 1 is 12.5 g / L based on the total volume of the solvent)

[0057] (2) Comparative Example 1

[0058] The steps of Example 1 were followed, except that the concentration of the functional monomer DOPA was changed from 0.80 mol / L to 0.40 mol / L, and the other process parameters remained unchanged.

[0059] 2. Characterization and testing

[0060] (1) 1 H NMR spectroscopy analysis

[0061] A 500 MHz ANANCE III NMR spectrometer (Bruker, Switzerland) was used for the measurement. The solvent was deuterated chloroform and the test temperature was room temperature.

[0062] (2) 31 P NMR spectroscopy analysis

[0063] A 500 MHz ANANCE III NMR spectrometer (Bruker, Switzerland) was used for the measurement. The solvent was deuterated chloroform and the test temperature was room temperature.

[0064] (3) Gel permeation chromatography analysis

[0065] The test was carried out on a CHR7100 gel permeation chromatograph (Malvern, UK) at a test temperature of 35 °C, using chromatographic grade tetrahydrofuran as the mobile phase and polystyrene as the standard. The sample to be tested was dissolved in tetrahydrofuran to prepare a solution with a concentration of about 5 mg / mL, and then filtered through a needle filter membrane with a pore size of 0.2 μm for testing.

[0066] (4) Rheological properties analysis

[0067] The test was carried out on a HR-2 rotational rheometer (TA, USA) at a test temperature of 25 °C. The rotor model used was a flat plate pp25 with a gap value of 0.5 mm and an angular frequency range of 0.1-100 rad / s.

[0068] 3. Comparison and analysis of test results

[0069] Figure 3 A shows the sample obtained in Example 1 1 H NMR spectrum, the results in the figure confirm that the synthesized polymer sample is composed of a spherical hyperbranched polyethylene skeleton with a high branch density (123 branches / 1000C) and multiple organophosphorus end groups, where the concentration of organophosphorus end groups (concentration of organophosphorus end groups = n 有机磷基团 / (n 有机磷基团 +n 乙烯单元 )×100%) is 5.8 mol%; further, Figure 3 B shows the sample 31 P NMR spectra further confirmed that the organophosphorus terminal groups had been successfully introduced into the ends of hyperbranched polyethylene; Figure 3 C gives the GPC elution curve of the sample, indicating that the molecular weight distribution of the sample is unimodal and its molecular weight distribution range is narrow (PDI=1.6); Figure 3 D shows the melt rheological curve of the sample, which shows that the sample does not have shear thinning phenomenon, confirming that it has a hyperbranched nonlinear chain morphology. In addition, we also 1HNMR and GPC were used to analyze the sample of Comparative Example 1. The results showed that the sample also had a hyperbranched chain structure, and the organophosphorus terminal groups were successfully introduced, with a ratio of 1.8 mol%, which indicated that the concentration ratio of the organophosphorus terminal groups in the obtained organophosphorus-modified hyperbranched polyethylene structure could be effectively regulated by regulating the DOPA functional monomer feed concentration. In summary, by comparing the analysis of Example 1 and Comparative Example 1, the results showed that the organophosphorus-modified hyperbranched polyethylene product could be successfully prepared using the process, and its organophosphorus ratio could be regulated by the monomer feed ratio.

[0070] Example 2, Comparative Example 2

[0071] 1. Sample preparation

[0072] (1) Example 2

[0073] Step 1: Prepare an organophosphorus-modified hyperbranched polyethylene sample according to the method of Example 1.

[0074] Step 2: In a cylindrical glass bottle with a capacity of 100 mL, 640 mg of hexagonal boron nitride, 160 mg of organophosphorus-modified hyperbranched polyethylene obtained in step 1 above, and 80 mL of chloroform solvent were added in sequence. The mixture was sealed and placed in an ultrasonic pool (model KQ-800 DE) with a power of 300 W for continuous ultrasonication at room temperature for 48 h; the product was centrifuged at 3000 rpm for 45 min, and after standing for 0.5 h, about 2 / 3 of the volume of the upper centrifuge was collected to obtain the initial dispersion of BNNS; then, a PTFE membrane with an average pore size of 0.1 µm was used for vacuum filtration, and fresh chloroform solvent was used for repeated elution to remove excess polymer. The product was dried at room temperature to finally obtain BNNS powder with surface organophosphorus modified. (Note: In the above system, the feed concentration of hexagonal boron nitride powder is 8 mg / mL based on the total volume of organic solvent; the mass ratio of the organophosphorus-modified hyperbranched polyethylene to hexagonal boron nitride powder is 0.25:1)

[0075] (2) Comparative Example 2

[0076] The process is carried out with reference to step 2 of Example 2, except that in Example 2, organophosphorus-modified hyperbranched polyethylene is introduced, while in Comparative Example 2, organophosphorus-modified hyperbranched polyethylene is not introduced.

[0077] 2. Characterization and testing

[0078] (1) High-resolution transmission electron microscopy (HR-TEM) analysis

[0079] A small amount of BNNS dispersion (0.1 mg mL -1 ) was dropped onto the surface of a copper mesh covered with a non-porous carbon support film and analyzed after infrared drying.

[0080] (2) Atomic force microscopy (AFM) analysis

[0081] The test was carried out on a Dimension ICON instrument (Bruker, USA) in tapping mode. A small amount of BNNS dispersion (0.02 mg mL -1 ) was dropped onto the surface of a clean silicon wafer and analyzed after infrared drying.

[0082] (3) Wide-angle X-ray diffraction (WAXRD) analysis

[0083] The X-ray diffractometer was used (PNAlytical, Netherlands). The X-ray source was Kα rays (λ = 1.54 Å) from a Cu target. The test voltage and current were 40 kV and 40 mA, respectively. The scanning speed was 10° / min and the scanning range was 10–80°. The BNNS dispersion was dropped into the sample groove of a glass slide and tested after the solvent was completely evaporated.

[0084] (4) Raman spectroscopy analysis

[0085] The Raman spectrometer was produced by Renishaw with a wavelength of 532 nm and a scanning range of 400–3000 cm -1 .

[0086] (5) Thermogravimetric analysis (TGA)

[0087] The test was carried out on a TGA55 thermogravimetric analyzer (TA, USA). The test heating program was as follows: first, the temperature was increased from room temperature to 100 °C at a rate of 20 °C / min, and then the temperature was kept constant at this temperature for 10 min, and then the temperature was continued to be increased from 100 °C to 800 °C at a rate of 20 °C / min. The test was carried out in a nitrogen atmosphere, and the sample amount was 5-10 mg.

[0088] (6) EDS element distribution determination

[0089] The results were carried out on a scanning electron microscope (SEM-4000).

[0090] 3. Comparison and analysis of test results

[0091] like Figure 4As shown in A, the dispersion of BNNS obtained in Example 2 in chloroform solvent exhibits a uniform, milky white appearance, preliminarily indicating that a relatively high concentration of BNNS is dispersed in the solvent; Figure 4 Figures C and D show the high-resolution transmission electron microscopy image and electron diffraction pattern of the BNNS sample, respectively, from which it can be found that the BNNS has few structural defects and a complete lattice structure. Figure 4 Figure E shows the AFM image of one of the BNNS in the sample and its thickness distribution curve, which again shows that its structure is intact and its thickness is 1.35 nm, corresponding to a thickness of <5 layers; further, as Figure 4 As shown in F, the thickness of the obtained BNNS is mainly distributed in 3–5 layers; in addition, WAXRD ( Figure 4 G) and Raman spectra ( Figure 4 The results of H) together confirmed that hexagonal boron nitride has been successfully exfoliated into BNNS with the assistance of organophosphorus-modified hyperbranched polyethylene; Figure 4 The thermogravimetric analysis results of I showed that 20.6% of the organophosphorus-modified hyperbranched polyethylene was non-covalently stably adsorbed on the surface of the obtained BNNS. This result can also be seen from the EDS element distribution diagram ( Figure 4 J–L) was confirmed. In contrast, Figure 4 Figure B shows the appearance of the BNNS dispersion obtained in Comparative Example 2. As shown in the figure, the dispersion is almost transparent, indicating that the proportion of BNNS therein is very low, indicating that the stripping efficiency of BNNS is very low in the absence of organophosphorus-modified hyperbranched polyethylene. Therefore, Figure 4 The results show that: through the method described in Example 2, the organophosphorus-modified hyperbranched polyethylene can be used to effectively promote the liquid phase exfoliation of hexagonal boron nitride, and the surface organophosphorus-modified BNNS can be successfully obtained.

[0092] Example 3, Comparative Example 3

[0093] 1. Sample preparation

[0094] (1) Example 3

[0095] Step 1: Prepare surface organophosphorus-modified BNNS powder by using organophosphorus-modified hyperbranched polyethylene according to the method of Example 2.

[0096] Step 2: Ultrasonic dispersion of 0.30 g of the surface organic phosphorus-modified BNNS powder obtained in step 1 above in an appropriate volume of chloroform, followed by sequential addition of 1.30 g of epoxy resin E51 and 0.33 g of curing agent 593, vacuum degassing for 5 min under stirring at 120 rpm, and then casting on the surface of two glass slides, curing at 80 °C for 8 h after solvent evaporation to obtain an adhesive sample. (Note: In the above system, the mass ratio of epoxy resin to curing agent is 3.9:1; in the obtained composite adhesive, the mass percentage of BNNS is 15%)

[0097] (2) Comparative Example 3

[0098] Step 1: Prepare BNNS powder with reference to the aforementioned Example 2, except that in the aforementioned Example 2, the exfoliation of BNNS is carried out in the presence of organophosphorus-modified hyperbranched polyethylene using chloroform as solvent, while in this comparative example 3, N,N-dimethylformamide (DMF) is used as solvent and the exfoliation is carried out directly without the assistance of polymer.

[0099] Step 2: proceed with reference to Example 3, except that in Example 3, the BNNS is obtained by exfoliation assisted by organophosphorus-modified hyperbranched polyethylene, while the BNNS used in this Comparative Example 3 is obtained by direct exfoliation using DMF in Step 1.

[0100] 2. Characterization and testing

[0101] (1) Overlap bonding performance test

[0102] The test was carried out on a 3369 universal material mechanical testing machine (Instron, USA) at room temperature and a tensile rate of 5 mm / min. The test samples were sandwich-type overlapped samples with a bonding area specification of 25.6 mm in length, 20 mm in width, and 30 μm in thickness.

[0103] (2) Scanning electron microscopy (SEM) analysis

[0104] The images were taken on a Regulus 8100 field emission scanning electron microscope (HITACHI, Japan). The samples were fractured by liquid nitrogen and then surface-sprayed with gold.

[0105] 3. Comparison and analysis of test results

[0106] Figure 5 Figures A–C show the cross-sectional SEM photos of the composite thermal conductive adhesive sample obtained in Example 3. As shown in the figure, in this composite system, BNNS is evenly dispersed in the epoxy resin matrix, and the interface contour between the two is fuzzy, and the interface between the surface BNNS and the matrix is ​​strong. In contrast, Figure 5D–F show the cross-sectional SEM images of the composite sample obtained in Comparative Example 3. It can be seen from the figure that the BNNS in the sample is severely agglomerated and has poor dispersion performance. At the same time, the interface contour is clear, and the interface interaction between the surface BNNS and the matrix is ​​weak. Figure 5 G and H respectively give the lap stress-strain curves of the composite adhesive samples obtained in Example 3 and Comparative Example 3, where the lap shear strength of the sample obtained in Example 3 is 5.6 MPa, while the corresponding strength of the sample in Comparative Example 3 is 3.6 MPa. At the same time, the elongation at break of the two samples is 3.0% and 1.8%, respectively, indicating that compared with Comparative Example 3, the composite sample obtained in Example 3 has better interfacial bonding performance. This is because in this system, the organophosphorus-modified hyperbranched polyethylene can effectively promote the uniform dispersion of BNNS in the epoxy matrix while obtaining a strong interfacial effect. Therefore Figure 5 The results show that the epoxy-based BNNS composite thermally conductive adhesive obtained by the method described in Example 3 has better interface bonding performance than the BNNS filling system obtained by simple solvent stripping.

[0107] Example 4, Comparative Example 4

[0108] 1. Sample preparation

[0109] (1) Example 4

[0110] Step 1: Prepare organophosphorus-modified BNNS powder according to the method of Example 2.

[0111] Step 2: Referring to the method of Example 3, a series of BNNS / epoxy resin composite thermal conductive adhesives (the filling mass percentage of BNNS is 0, 5, 10, 15, and 20%) are prepared by serially changing the BNNS addition ratio. The difference is that in Example 3, the obtained composite material is obtained by curing between two glass slides, while in this Example 4, each obtained adhesive is obtained by curing in a cylindrical mold made of polytetrafluoroethylene.

[0112] (2) Comparative Example 4

[0113] Step 1: Referring to Comparative Example 3, hexagonal boron nitride was directly exfoliated using DMF solvent to obtain BNNS powder.

[0114] Step 2: Referring to the method of Comparative Example 3, a series of BNNS / epoxy resin composite thermal conductive adhesives were prepared by changing the BNNS filling ratio in series (the BNNS filling mass percentage was 0, 5, 10, 15, and 20% in sequence). Consistent with Example 4, these samples were also obtained by curing a polytetrafluoroethylene cylindrical mold of the same specifications.

[0115] 2. Characterization and testing

[0116] (1) Determination of thermal diffusion coefficient (α)

[0117] The test was carried out on a LFA467 laser thermal conductivity meter (NETZSCH, Germany) at a test temperature of 25 °C. A disc with a diameter of 12.7 mm and a thickness of 1.5 mm was used as the test sample. A carbon layer was sprayed on the surface of the sample to avoid laser reflection. Three points were randomly selected on each sample for measurement, and the standard deviation was taken.

[0118] (2) Specific heat capacity (C p ) Determination

[0119] The specific heat capacity (C) of the samples was measured by a DSC214 differential scanning calorimeter (NETZSCH, Germany). p ), nitrogen atmosphere, atmosphere flow rate 50 mL / min, the sample was heated from 0 °C to 50 °C at a heating rate of 10 °C / min, the sample amount was 10 mg, and the reference sample was sapphire.

[0120] (3) Density (ρ) determination

[0121] The density was measured using a ME-204 density meter (METTLER TOLEDO, Switzerland). The sample was first placed on a tray in air and weighed, then placed on a tray in water and weighed, and the density value was read.

[0122] (4) Determination of thermal conductivity (λ)

[0123] The determination is carried out according to the following formula (1):

[0124] λ = α × ρ × C p (1)

[0125] Where: α is the thermal diffusivity of the composite material, C p is the specific heat capacity of the sample, and ρ is the sample density.

[0126] 3. Comparison and analysis of test results

[0127] Figure 6 Figures A and B respectively give the thermal conductivity and thermal conductivity improvement ratio of the BNNS / epoxy resin composite system obtained in Example 4 and Comparative Example 4 at different BNNS filling ratios. As shown in the figure, as the BNNS filling ratio gradually increases, the thermal conductivity of both systems increases accordingly, but compared with Comparative Example 4, the thermal conductivity of the composite material obtained in Example 4 is significantly better. For example, corresponding to a BNNS filling ratio of 20wt%, the thermal conductivity of the composite material obtained in Example 4 is 1.73 Wm -1 K -1 , which is 7 times higher than that of pure epoxy resin, while the thermal conductivity of the sample obtained in Comparative Example 4 is only 0.91 W m-1 K -1 , which is only 3.2 times higher than that of pure epoxy resin. Therefore, this shows that the epoxy-based BNNS composite thermal conductive adhesive with excellent thermal conductivity can be prepared by the method described in Example 4.

[0128] Example 5, Comparative Examples 5-7

[0129] 1. Sample preparation

[0130] (1) Example 5

[0131] Step 1: Prepare organophosphorus-modified BNNS powder according to the method of Example 2.

[0132] Step 2: Referring to the method of Example 3, a BNNS / epoxy resin thermal conductive composite material with a BNNS filling mass percentage of 20% is prepared, which is obtained by curing a polytetrafluoroethylene strip mold.

[0133] (2) Comparative Example 5

[0134] 1.60 g of epoxy resin E51 and 0.40 g of curing agent 593 were added to an appropriate amount of chloroform solvent, vacuum degassed for 5 min under stirring at 120 rpm, and then cast into a polytetrafluoroethylene strip mold. After the solvent evaporated, the adhesive sample was cured at 80 °C for 8 h to obtain.

[0135] (3) Comparative Example 6

[0136] Step 1: Referring to Comparative Example 3, hexagonal boron nitride was directly exfoliated using DMF solvent to obtain BNNS powder.

[0137] Step 2: Referring to the method of Comparative Example 3, a BNNS / epoxy resin thermal conductive composite material with a BNNS filling mass percentage of 20% is prepared, which is obtained by curing a polytetrafluoroethylene strip mold.

[0138] (4) Comparative Example 7

[0139] 1.54 g of epoxy resin E51, 0.38 g of curing agent 593 and 0.08 g of organophosphorus-modified hyperbranched polyethylene (prepared as described in Example 1) were added together into an appropriate amount of chloroform solvent, vacuum degassed for 5 min under stirring at 120 rpm, and then cast into a polytetrafluoroethylene strip mold. After the solvent evaporated, the mixture was cured at 80 °C for 8 h to obtain an adhesive sample.

[0140] 2. Characterization and testing

[0141] (1) Vertical burning (UL-94) test

[0142] The test was carried out on a HVUL2 vertical combustion test system (ATLAS, USA) in accordance with GB / T 2408 “Determination of the combustion performance of plastics - Horizontal and vertical methods”. The sample specifications were 125 mm in length, 13 mm in width, and 1.6 mm in thickness. Each sample was tested 3 times.

[0143] (2) Limiting oxygen index (LOI) test

[0144] The test was carried out on a JB-3 oxygen index instrument (Jiubin, China) in accordance with GB / T 2406.2 “Determination of combustion behavior by oxygen index method”. The test temperature was room temperature. The sample specifications were 80 mm in length, 10 mm in width, and 4 mm in thickness. Each sample was measured 3 times.

[0145] (3) Microcalorimeter test

[0146] The test was carried out on a MCC-3 microcalorimeter (DEATAK, USA) according to ASTM D7309-2007. The sample was heated from room temperature to 600 °C at a heating rate of 1 °C / s, and the sample amount was 30–50 mg.

[0147] 3. Comparison and analysis of test results

[0148] Figure 7 The flame retardant performance results of each sample obtained in Example 5 and Comparative Examples 5-7 are given. Figure 7 As shown in Figure A, the pure epoxy resin (Comparative Example 5) continued to burn after ignition, and the sample burned out in about 100 seconds. During this period, the molten dripping phenomenon was serious, indicating that its flame retardant performance was very poor. By introducing 20% ​​BNNS (obtained by direct stripping in DMF solvent, Comparative Example 6), the flame retardant performance of the composite system was improved to a certain extent compared with the pure epoxy resin, but Figure 7 B shows that the sample was still burning 90 seconds after ignition, indicating that it did not meet the fire protection requirements. Figure 7 D shows that by introducing organophosphorus modified hyperbranched polyethylene into epoxy resin, the flame retardant properties of the obtained sample (Comparative Example 7) are also improved to a certain extent. However, the sample is basically burned out after the combustion duration reaches 100 s. The difference is that by introducing organophosphorus modified BNNS, the obtained composite sample (Example 5) shows significantly improved flame retardant properties, such as Figure 7 The C of the sample shows that it is completely extinguished about 1 second after ignition. Figure 7 Figures E and F compare the heat release rate results of each sample during the combustion test, and also show that the sample in Example 5 has better flame retardant properties than Comparative Examples 5 and 6.

Claims

1. An organophosphorus modified hyperbranched polyethylene, characterized in that: The organophosphorus modified hyperbranched polyethylene is obtained by one-pot copolymerization of ethylene and organophosphorus functional monomers in an anhydrous organic solvent catalyzed by an α-diimine palladium catalyst based on a "chain walking" mechanism; The structure of the organophosphorus functional monomer is shown in Formula I: 。 2. The organophosphorus modified hyperbranched polyethylene according to claim 1, characterized in that: The α-diimine palladium catalyst is selected from the α-diimine palladium catalyst 1 or 2 having the following structural formula: 。 3. The organophosphorus modified hyperbranched polyethylene according to claim 1, characterized in that: The anhydrous organic solvent is selected from one of the following: anhydrous dichloromethane, anhydrous chloroform, and anhydrous chlorobenzene.

4. The organophosphorus modified hyperbranched polyethylene according to claim 1, characterized in that: In the one-pot copolymerization reaction system, the feed concentration of the organophosphorus functional monomer is 0.1-2.0 mol / L based on the total volume of the anhydrous solvent; and the initial concentration of the α-diimine palladium catalyst is 0.5-50 g / L based on the total volume of the anhydrous solvent.

5. The organophosphorus modified hyperbranched polyethylene according to claim 1, characterized in that: The copolymerization reaction temperature is controlled at 5-30°C; the ethylene pressure is controlled at 0.01-0.5 MPa; and the polymerization time is controlled at 2-48 h.

6. A modified boron nitride nanosheet, characterized in that: The modified boron nitride nanosheets are boron nitride nanosheets on which the organophosphorus-modified hyperbranched polyethylene described in any one of claims 1 to 5 is attached.

7. A method for preparing the modified boron nitride nanosheets according to claim 6, characterized in that: The preparation method comprises the following steps: Hexagonal boron nitride, organophosphorus modified hyperbranched polyethylene and an organic solvent are mixed, and the obtained mixture is sealed and ultrasonicated to obtain a BNNS initial dispersion liquid, and further subjected to low-speed centrifugation and static treatment to obtain a BNNS dispersion liquid containing excess organophosphorus modified hyperbranched polyethylene, and excess free polymer is removed by high-speed centrifugation or vacuum filtration, and the modified boron nitride nanosheets are obtained by drying at room temperature, and the organophosphorus modified hyperbranched polyethylene is attached to the surface of the boron nitride nanosheets.

8. A flame retardant thermally conductive adhesive, characterized in that: The flame retardant heat-conductive adhesive is an adhesive with heat-conducting and flame-retardant functions, which uses epoxy resin as a resin matrix and the modified boron nitride nanosheets described in claim 6 as nanofillers.

9. The flame retardant thermally conductive adhesive according to claim 8, characterized in that: In the flame-retardant thermally conductive adhesive, the mass percentage content of the modified boron nitride nanosheets is 2-30%.

10. A method for preparing the flame retardant thermally conductive adhesive according to claim 8 or 9, characterized in that: The preparation method comprises the following steps: The modified boron nitride nanosheets are ultrasonically dispersed in an organic solvent, then evenly mixed with an epoxy resin and a curing agent, and evenly coated between bonding substrates or in a casting mold after vacuum degassing treatment. The flame retardant thermal conductive adhesive is obtained after the solvent is volatilized and cured.

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