Modification method of hollow glass beads and application of hollow glass beads in PU adhesive

Abstract

The invention belongs to the technical field of organic and inorganic composite materials, particularly discloses a modification method of hollow glass beads and application of the hollow glass beads in a PU adhesive, and aims to solve the problems of poor storage stability, high viscosity change rate and the like of a composite material due to the fact that the surfaces of the hollow glass beads contain silicon hydroxyl and free alkali metal in the prior art. According to the invention, an alkyl silane coupling agent (such as methyltrimethoxysilane and n-octyltriethoxysilane) which does not contain active hydrogen is adopted to carry out surface modification on hollow glass beads. The method comprises the following specific steps: S1, acidizing microspheres with a hydrochloric acid solution to remove surface metal ions; and S2, hydrolyzing the alkylsilane coupling agent, and bonding the hydrolyzed alkylsilane coupling agent with silicon hydroxyl on the surface of the microbead. The compatibility of the obtained modified microbead and an isocyanate prepolymer is remarkably improved, the viscosity change rate is low, and the curing phenomenon is avoided. The preparation method is simple in process and low in cost, and can be widely applied to the fields of coatings, adhesives and lightweight composite materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of organic and inorganic composite materials, and particularly relates to a modification method of hollow glass microspheres and application of the same in PU adhesives. Background Art

[0002] Hollow glass microspheres are thin-walled, sealed glass spheres whose primary components are alkali / alkaline earth metal borosilicates (including SiO2, Na2O, B2O3, and CaO), containing trace metal elements such as free sodium and calcium. Because their interiors are vacuum-filled or filled with N2 or CO2, hollow glass microspheres exhibit excellent physical and chemical properties, such as light weight, high strength, and insulation properties. They are becoming increasingly important fillers in aerospace, paint tapes, and lightweight transportation applications. Polyurethane sealants offer excellent performance, including strong molecular designability, a wide range of physical properties, and a wide range of adhesive applications, making them a key component of synthetic adhesives. Two-component polyurethane adhesives typically consist of an isocyanate component (curing agent) containing NCO and a hydroxyl component (main agent) containing hydroxyl groups. During use, the two liquid components are mixed in a specific ratio, and the -NCO and -OH groups react to form a solid polyurethane. The polar groups of the polyurethane then form hydrogen bonds and other bonding interactions with the substrate surface, achieving adhesion. Due to the structural peculiarities of polyurethane adhesives, their molecular chains contain carbamate (-NHCOO-) or isocyanate (-NCO) groups, derived from the reaction of isocyanates with hydroxyl-containing compounds or other polyols. Isocyanate components readily react with compounds containing active hydrogen atoms, such as amines, water, alcohols, acids, and bases; or with metal elements that cause isocyanate groups to self-polymerize, altering the system's physical and chemical properties. This can lead to difficulties in processing, storing, or using PU adhesives. If the hollow glass microspheres contain polar groups containing active hydrogen, such as amino or epoxy groups, or if the hollow glass microspheres themselves contain silanol groups and free alkaline metals (which may have catalytic effects), the hollow glass microsphere fillers may react with isocyanates during storage or high-temperature processing, acting as crosslinking points for the reactive adhesive and significantly increasing the molecular weight of the polymer. In severe cases, the polymer may solidify, rendering it useless.

[0003] In order to obtain a lightweight polyurethane adhesive, hollow glass microspheres (containing silanol groups on the surface) have almost no reactive groups that react with polyol (containing hydroxyl groups) components, and the polyol component has high storage stability. The storage stability of the hollow glass microspheres mixed with the polyol component adhesive is much higher than that of the hollow glass microspheres mixed with the isocyanate component. Therefore, most existing technologies add hollow glass microspheres to the PU adhesive component containing hydroxyl groups, or add a small amount to the PU adhesive component containing isocyanate groups, and introduce stabilizers, dewatering agents or other additives to briefly evaluate their storage stability. Patent CN 116694295A discloses a low-density two-component polyurethane potting structural adhesive and its application. The low-density two-component polyurethane potting structural adhesive is made by using a specific type and ratio of small molecule diols, side chain modified polyester diols, dimer acid modified polyester polyols, defoamers, catalysts, low-density fillers, stabilizers, and dewatering agents to make component A, and polyisocyanates as component B. The prepared component A is not easy to delaminate during long-term storage, and the adhesive layer is uniform and has good fluidity (the state of component A inside the test tube was observed after six months in an environment of 25±2°C). The surface of the hollow glass microspheres is treated with a silane coupling agent, and component A, which is mainly a polyol, is added. The specific viscosity change value of component A after six months is not indicated. Whether the adhesive layer is delaminated and whether it has fluidity is subjectively judged to indicate the storage stability of the polyol polymer containing hollow glass microspheres. Patent CN 118325560A discloses a low-density two-component polyurethane structural adhesive and its preparation method and application, which mainly solves the problems of poor storage stability and low bonding strength of low-density structural adhesives in the prior art. The patent shows that hollow glass microspheres are added to both components A and B, wherein a silane coupling agent is added to component A, and the hollow glass microspheres of component B are not treated. While ensuring a low final mixed density, the content of hollow glass microspheres in component B is adjusted to be as low as possible, reducing the alkalinity of component B and the likelihood of self-polymerization of NCO groups in component B, thereby improving the storage stability of component B. The maximum specific gravity of the glass microspheres in component B, which contain an isocyanate polymer, is only 14%. According to GB 6753.3-1986 (Test Method for Storage Stability of Coatings), the storage stability is simply stated as 40 days, without providing a corresponding grade for the viscosity change of material B. Patent CN 118325560A discloses a modified polyurethane composite plugging material and its preparation method. Zirconium dioxide is formed on hollow glass microspheres to form a composite material, which is then modified with a silane coupling agent and then added with cysteine for a click reaction to form a modified filler with multiple reactive groups. Modified hollow glass microspheres (carrying active groups of amino, hydroxyl and carboxyl groups) are mixed with MDI to form component B, which is then mixed with component A containing glycerol-propylene oxide polyether to prepare a polyurethane composite sealing material, aiming to enhance the pressure resistance and high temperature resistance of the composite material and reduce costs.The preparation method of this patent will cause the viscosity of component B to increase sharply, so it is necessary to add a leveling agent to component A to reduce the overall viscosity of the polyurethane material. There is also the possibility that the hollow glass microspheres will further react with MDI, which is not conducive to the long-term storage and use of component B. Similarly, patent application CN118085793A discloses a low-density, high-performance thermal conductive structural adhesive for power batteries and its preparation method. 3-13 parts by mass of modified hollow glass microspheres are added to the A / B components of the polyurethane thermal conductive adhesive, and the two components are mixed during use. The main function of the glass microspheres is to reduce the density of the colloid. There is also no mention of the storage stability of the modified hollow glass microspheres in the two components.

[0004] Few studies have examined the high-temperature and long-term compatibility of hollow glass microspheres in single-component PU adhesives (primarily containing an isocyanate component and excluding stabilizers, dehumidifiers, and other organic additives). Therefore, to further reduce the cost of hollow glass microspheres and expand their application range, including by adding other additives or operating costs, it is necessary to modify the hollow glass microspheres to meet similar application requirements. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the prior art and provide a modification method for hollow glass microspheres and their application in PU adhesives. The present invention uses an alkylsilane coupling agent that does not contain active hydrogen (such as methyltrimethoxysilane and n-octyltriethoxysilane) to modify the surface of the hollow glass microspheres after acid treatment, which effectively reduces the specific gravity of the composite material while significantly improving the viscosity change rate of the material during storage and use, and effectively reduces the use cost of the PU adhesive with an equal mass and high volume share.

[0006] In order to achieve the above objectives, one of the technical solutions of the present invention is: a method for modifying hollow glass microspheres, comprising the following steps:

[0007] S1: The hollow glass microspheres were acidified with hydrochloric acid solution, and then washed with pure water and ethanol in sequence and dried;

[0008] S2: adding an alkylsilane coupling agent without active hydrogen to an ethanol aqueous solution for hydrolysis reaction, then adding the hollow glass microspheres treated with hydrochloric acid in step S1 for coupling reaction, and drying after the reaction to obtain modified hollow glass microspheres.

[0009] In a preferred embodiment of the present invention, the concentration of the hydrochloric acid solution in step S1 is 0.5-1.5 mol / L, the weight ratio of the hollow glass microspheres to the hydrochloric acid solution is 1:(4-6), and the acidification reaction time is 0.5-2 h.

[0010] In a preferred embodiment of the present invention, in step S1, the sample is first washed with pure water until it becomes neutral, and then washed with ethanol until no chloride ions remain.

[0011] In a preferred embodiment of the present invention, the volume ratio of ethanol to water in the ethanol aqueous solution in step S2 is (8-10):1.

[0012] In a preferred embodiment of the present invention, the alkyl group of the alkylsilane coupling agent in step S2 is a C1-C12 straight chain or branched structure and does not contain amino, hydroxyl or thiol active groups.

[0013] More preferably, the alkylsilane coupling agent is at least one of methyltrimethoxysilane and n-octyltriethoxysilane.

[0014] In a preferred embodiment of the present invention, the hydrolysis reaction time in step S2 is 15-45 minutes, and the coupling reaction time is 1-3 hours.

[0015] In a preferred embodiment of the present invention, the drying temperature in step S2 is 70-90°C.

[0016] In order to achieve the above objectives, the second technical solution of the present invention is: a method for modifying hollow glass microspheres to obtain modified hollow glass microspheres.

[0017] The third technical solution of the present invention is: application of modified hollow glass microspheres in PU adhesive.

[0018] The fourth technical solution of the present invention is: a PU adhesive comprising the modified hollow glass microspheres, which is prepared by uniformly mixing MDI, modified hollow glass microspheres and fumed silica in a mass ratio of (30-50): (10-15): 1.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The present invention can effectively reduce the alkali metal elements such as Na on the surface of the hollow glass microspheres by acidification with hydrochloric acid, and at the same time activate some silanol groups on the surface of the hollow glass microspheres, thereby significantly improving the subsequent reaction activity of the hollow glass microspheres with the alkyl silane coupling agent.

[0021] 2. The surface of the acidified hollow glass microspheres of the present invention is grafted with an alkyl group that does not contain active hydrogen, which not only avoids reaction with isocyanate, but also improves the compatibility of the hollow glass microspheres with the polymer components of the PU adhesive, preventing the separation of the inorganic filler and the organic polymer during subsequent storage and use.

[0022] 3. The modified hollow glass microspheres of the present invention exhibit excellent chemical inertness in isocyanate systems, with a small viscosity change rate at high temperatures. After long-term storage, the viscosity change rate is significantly lower than that of unmodified microspheres and microspheres modified with conventional modifiers.

[0023] 4. The present invention solves the problem of system instability of hollow glass microspheres in PU adhesives caused by the precipitation of alkali metal ions and surfactant groups through hydrochloric acid pretreatment combined with modification with a specific alkyl silane coupling agent, significantly reduces the viscosity change rate and prolongs the storage life. DETAILED DESCRIPTION

[0024] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is described in more detail below with reference to specific embodiments, but the protection scope of the present invention is not limited to these embodiments.

[0025] The raw materials used in the following examples of the present invention are:

[0026] Hollow glass microspheres HGM: Zhongke Yali H32, density 0.32g / cm 3 ;

[0027] Silane coupling agent: methyltrimethoxysilane-McLean;

[0028] n-Octyltriethoxysilane-Aladdin;

[0029] (3-Aminopropyl)triethoxysilane-McLean;

[0030] Isocyanate MDI-1: MDI prepolymer (i.e., MDI and hydroxyl components are prepolymerized), isocyanate content 16.5%, viscosity 2212 mPa·s, density: 1.07 g / cm 3 , purchased from Yantai Debang Technology Co., Ltd.;

[0031] Isocyanate MDI-2: Wanhua PM200, density: 1.23 g / cm 3 , isocyanate content 31.3%, viscosity 190 mPa·s; fumed silica: H21, WACKER.

[0032] Example 1

[0033] A modified hollow glass microsphere, comprising 60 parts by weight of hollow glass microspheres and 1 part by weight of methyltrimethoxysilane, wherein the density of the hollow glass microspheres is 0.32 g / cm 3 , compressive strength of 3500psi, particle size D50 = 54um. It is prepared by the following modification method:

[0034] S1: Hydrochloric acid acidification treatment of glass microspheres: by weight, 60 parts of hollow glass microspheres were added to a hydrochloric acid solution with a concentration of 1 mol / L, and the weight ratio of microspheres to hydrochloric acid solution was 1:5. The glass microsphere suspension was obtained under stirring at a speed of 600 rpm and reacted at room temperature for 1 hour. The mixture was washed with pure water and ethanol, and the wet powder was placed in an oven at 60°C and dried to constant weight to obtain 60 parts of dry hollow glass microspheres after hydrochloric acid acidification.

[0035] S2: Prepare 150 parts of ethanol aqueous solution (volume ratio of ethanol to water is 9:1) by weight, add 1 part of methyltrimethoxysilane to the ethanol aqueous solution, and hydrolyze at room temperature for 30 minutes under stirring (speed 600 rpm); take the hollow glass microspheres acidified with hydrochloric acid in step S1 and put them into 150 parts of the above coupling agent modified solution, stir to obtain a hollow glass microsphere suspension, and after the coupling reaction for 2 hours, wash with pure water and ethanol to a pH of 7; the obtained wet powder is placed in an oven at 80°C and dried to constant weight to obtain dried modified hollow glass microspheres, named M-HGM.

[0036] Example 2

[0037] A modified hollow glass microsphere, comprising 60 parts by weight of hollow glass microspheres and 1 part of n-octyltriethoxysilane, wherein the density of the hollow glass microspheres is 0.32 g / cm 3 , compressive strength of 3500psi, particle size D50 = 54um. It is prepared by the following modification method:

[0038] S1: Hydrochloric acid acidification treatment of glass microspheres: by weight, 60 parts of hollow glass microspheres were added to a hydrochloric acid solution with a concentration of 1 mol / L, and the weight ratio of microspheres to hydrochloric acid solution was 1:5. The glass microsphere suspension was obtained under stirring at a speed of 600 rpm and reacted at room temperature for 1 hour. The mixture was washed with pure water and ethanol, and the wet powder was placed in an oven at 60°C and dried to constant weight to obtain dry hollow glass microspheres after hydrochloric acid acidification.

[0039] S2: Prepare 150 parts of ethanol aqueous solution (volume ratio of ethanol to water is 9:1) by weight, add 1 part of n-octyl triethoxysilane to the ethanol aqueous solution, and stir (speed 600 rpm) for hydrolysis reaction at room temperature for 30 minutes; take the hollow glass microspheres acidified with hydrochloric acid in step S1 and put them into 150 parts of the above coupling agent modified solution, stir to obtain a hollow glass microsphere suspension, and after the coupling reaction for 2 hours, wash with pure water and ethanol to a pH of 7; the obtained wet powder is placed in an oven at 80°C and dried to constant weight to obtain dried modified hollow glass microspheres, named N308-HGM.

[0040] Example 3

[0041] The modified hollow glass microspheres M-HGM and N308-HGM prepared in Example 1-2 were used to prepare PU adhesives. The preparation method was as follows: MDI-1 and MDI-2 were respectively stirred and evenly mixed with the modified hollow glass microspheres and fumed silica in a mass ratio of 40:12:1 to obtain PU adhesive samples.

[0042] Comparative Example 1

[0043] Considering that some coupling agent-modified microbeads require the addition of acetic acid or other acids for hydrolysis, a control group was prepared in which the microbeads were directly modified without prior acidification.

[0044] The components are the same as those in Example 1, and the modification method is as follows: 150 parts by weight of an ethanol aqueous solution (the volume ratio of ethanol to water is 9:1) are prepared, and 1 part of methyltrimethoxysilane is added to the ethanol aqueous solution; acetic acid is added to adjust the pH to 5-6, and the mixture is stirred (at a speed of 600 rpm) and hydrolyzed at room temperature for 30 minutes to obtain a modified solution; 60 parts by weight of hollow glass microspheres that have not been acidified with hydrochloric acid are added to 150 parts of the above-mentioned modified solution, stirred to obtain a hollow glass microsphere suspension, subjected to coupling reaction for 2 hours, and washed with pure water and ethanol to a pH of 7; the obtained wet powder is placed in an oven at 80°C and dried to constant weight to obtain dried modified hollow glass microspheres, which are named MH-HGM.

[0045] Comparative Example 2

[0046] A modified hollow glass microsphere, comprising 60 parts by weight of hollow glass microspheres and 1 part of a conventional modifier (3-aminopropyl)triethoxysilane, wherein the density of the hollow glass microspheres is 0.32 g / cm 3 , compressive strength is 3500psi, particle size D50=54um. It is prepared by the following modification method:

[0047] S1: Hydrochloric acid acidification treatment of glass microspheres: by weight, 60 parts of hollow glass microspheres were added to a hydrochloric acid solution with a concentration of 1 mol / L, and the weight ratio of microspheres to hydrochloric acid solution was 1:5. The glass microsphere suspension was obtained under stirring at a speed of 600 rpm and reacted at room temperature for 1 hour. The mixture was washed with pure water and ethanol, and the wet powder was placed in an oven at 60°C and dried to constant weight to obtain dry hollow glass microspheres after hydrochloric acid acidification.

[0048] S2: By weight, 150 parts of ethanol aqueous solution (the volume ratio of ethanol to water is 9:1) are prepared, 1 part of (3-aminopropyl)triethoxysilane is added to the ethanol aqueous solution, and the mixture is hydrolyzed at room temperature for 30 minutes under stirring (speed 600 rpm); the hollow glass microspheres acidified with hydrochloric acid in step S1 are placed in 150 parts of the above-mentioned coupling agent modified solution, and stirred to obtain a hollow glass microsphere suspension. After the coupling reaction for 2 hours, the suspension is washed with pure water and ethanol to a pH of 7; the obtained wet powder is placed in an oven at 80°C and dried to constant weight to obtain dried modified hollow glass microspheres, named KH550-HGM.

[0049] Comparative Example 3

[0050] Commercial unmodified hollow glass microspheres (named HGM) were used to prepare PU adhesives. The preparation method was as follows: MDI-1 and MDI-2 were respectively mixed with unmodified hollow glass microspheres and fumed silica in a mass ratio of 40:12:1 to obtain PU adhesive samples.

[0051] Comparative Example 4

[0052] The modified hollow glass microspheres prepared in Comparative Example 1-2 were used to prepare a PU adhesive. The preparation method was as follows: MDI-1 and MDI-2 were respectively mixed with the modified hollow glass microspheres and fumed silica in a mass ratio of 40:12:1, and stirred to obtain a PU adhesive sample.

[0053] The viscosity of the PU adhesive samples prepared in Example 4 and Comparative Examples 2 and 3 was evaluated and tested, and the test standard was referred to GB / T 2794-2022. Test method: The prepared PU adhesive sample was allowed to stand for 10 minutes at -0.06Mpa to remove bubbles; the initial (0h) viscosity of the mixture was tested. In order to better evaluate the stability of hollow glass microspheres in MDI, the above mixture was placed in a 65°C oven and allowed to stand for 72 hours. After the sample returned to room temperature, the viscosity at 72 hours was measured. The average viscosity of the two groups of samples was taken (the error between the two test values was <3%), and the viscosity change rate Δη was calculated as |(final viscosity - initial viscosity)| / initial viscosity * 100%. The above PU adhesive samples were allowed to stand at room temperature for 60 days and then the viscosity was tested. The measured viscosity and viscosity change results are shown in Table 1 below.

[0054] The test instrument is a digital viscosity tester, model: LICHEN NDJ-8S, with a range of 100-2000000 mPa·s. The drying oven model is: DZF-6050, with a temperature fluctuation of ±1°C.

[0055] Compared to the modified hollow glass microspheres prepared without hydrochloric acid treatment in Comparative Example 1, the modified hollow glass microspheres prepared after hydrochloric acid treatment in Examples 1-2 can reduce the viscosity change rate of the mixed system and improve the stability of the hollow glass microspheres and MDI. The surface end groups of these modified hollow glass microspheres are all alkyl groups -R-CH3, which effectively prevents the hollow glass microspheres from reacting with the PU adhesive component containing isocyanate during storage, avoiding chemical reactions in the mixed system that lead to a significant increase in the system viscosity. The hollow glass microspheres modified with the conventional KH550 modifier in Comparative Example 2, although also treated with hydrochloric acid, have a higher viscosity change rate than the unmodified hollow glass microspheres. This is because the surface groups of the KH550-modified hollow glass microspheres contain -NH2, which contains active hydrogen that can react with isocyanate. During high-temperature storage, KH550-HGM and MDI may undergo a chemical reaction, as shown in Equation 1, increasing the molecular weight of the mixed system and the viscosity.

[0056] R1-N=C=O+HGM-R2-NH2→R1-NH-CO-NH-R2-HGM Formula 1

[0057] After pre-acidified and acid-treated hollow glass microspheres were mixed evenly with MDI, there were significant differences in their high-temperature and long-term stability. The acid-treated hollow glass microspheres had higher high-temperature and long-term storage stability than the acid-treated hollow glass microspheres.

[0058] Table 1 Viscosity and viscosity change rate of PU adhesive samples prepared in Example 4 and Comparative Example 3

[0059]

[0060] The PU adhesive sample made with KH550-HGM was completely cured after 60 days of storage, and the viscosity could not be measured.

[0061] 1 g of commercial unmodified hollow glass microspheres HGM was dissolved in 5 mL of 1 mol / L hydrochloric acid and acidified for 1 hour, and the concentration of Na ion precipitation was tested. The hollow glass microspheres HGM acidified for 1 hour were then placed in 100°C hot water for 1 hour, and the concentration of Na ion precipitation was tested. At the same time, the unmodified hollow glass microspheres HGM that had not been acidified with hydrochloric acid were directly placed in 100°C hot water for 1 hour, and the concentration of Na ion precipitation was tested. Three parallel samples were made for each test, and the average value was taken. The test results are shown in Table 2.

[0062] Table 2 ICP test results of Na ion precipitation concentration

[0063]

[0064] From the ICP test results of Na ion precipitation concentration in Table 2, it can be seen that the average sodium ion precipitation amount of commercial unmodified hollow glass microspheres HGM after hydrochloric acid acidification for 1 hour is 1474ppm; compared with HGM directly placed in 100℃ hot water for 1 hour without acidification treatment, the Na ion precipitation concentration of HGM after acidification treatment after being placed in 100℃ hot water for 1 hour is reduced by 74%, indicating that hydrochloric acid acidification can reduce the alkali metal ion content in HGM.

[0065] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for modifying hollow glass microspheres, characterized in that: The following steps are involved: S1: The hollow glass microspheres were acidified with a hydrochloric acid solution, and then washed with pure water and ethanol in sequence and dried; S2: adding an alkylsilane coupling agent without active hydrogen to an ethanol aqueous solution for hydrolysis reaction, then adding the hollow glass microspheres treated with hydrochloric acid in step S1 for coupling reaction, and drying after the reaction to obtain modified hollow glass microspheres.

2. The method for modifying hollow glass microspheres according to claim 1, wherein: In step S1, the concentration of the hydrochloric acid solution is 0.5-1.5 mol / L, the weight ratio of the hollow glass microspheres to the hydrochloric acid solution is 1:(4-6), and the acidification reaction time is 0.5-2 h.

3. The method for modifying hollow glass microspheres according to claim 1, wherein: The volume ratio of ethanol to water in the ethanol aqueous solution in step S2 is (8-10):

1.

4. The method for modifying hollow glass microspheres according to claim 1, wherein: The alkyl group of the alkylsilane coupling agent in step S2 is a C1-C12 straight chain or branched structure and does not contain amino, hydroxyl or thiol active groups.

5. The method for modifying hollow glass microspheres according to claim 4, wherein: The alkylsilane coupling agent is at least one of methyltrimethoxysilane and n-octyltriethoxysilane.

6. The method for modifying hollow glass microspheres according to claim 1, wherein: The hydrolysis reaction time in step S2 is 15-45 minutes, and the coupling reaction time is 1-3 hours.

7. The method for modifying hollow glass microspheres according to claim 1, wherein: The drying temperature in step S2 is 70-90°C.

8. Modified hollow glass microspheres obtained by the method for modifying hollow glass microspheres according to any one of claims 1 to 7.

9. Use of the modified hollow glass microspheres according to claim 8 in PU adhesives.

10. A PU adhesive comprising the modified hollow glass microspheres according to claim 8, characterized in that: The product is prepared by uniformly mixing MDI, modified hollow glass microspheres and fumed silica in a mass ratio of (30-50): (10-15): 1.

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