CN113304704B - Self-reinforcing thermal expansion microsphere and preparation method thereof - Google Patents

Abstract

The invention provides a self-reinforcing thermal expansion microsphere and a preparation method thereof. Comprises the following steps: (1) Reacting polyisocyanate and hydroxyl acrylate in the presence of a catalyst to prepare a double-bond-containing terminal-NCO group prepolymer, and then adding an isocyanate sealing agent into the obtained prepolymer for sealing to obtain sealed polyurethane; (2) Carrying out suspension polymerization on the obtained blocked polyurethane, an olefinic polymerizable monomer and a foaming agent to prepare the self-reinforcing thermal expansion microsphere; when the thermal expansion microsphere prepared by the invention expands, the crosslinking is greatly promoted by the reaction of the isocyanate subjected to pyrolysis end capping and the hydroxyl of the hydroxyl acrylate, so that the microsphere still keeps excellent high temperature resistance effect at a lower initial foaming temperature and has good repeated compression durability, thereby widening the application range of the microsphere.

Description

Self-reinforcing thermal expansion microsphere and preparation method thereof

Technical Field

The invention relates to the field of preparation of expanded microspheres, in particular to a self-reinforcing thermal expansion microsphere and a preparation method thereof, and particularly relates to a thermal expansion microsphere with excellent thermal stability and repeated compression durability and a preparation method thereof.

Background

The thermal expansion microsphere is a core-shell structure microsphere which takes thermoplastic resin as a shell and takes a low molecular alkane foaming agent as a core, the boiling point of the foaming agent is generally lower than the softening point of the shell, when the temperature rises, alkane is gasified, the internal pressure is increased, the shell layer softens the microsphere to expand, when the temperature is recovered to the temperature before heating, the microsphere is kept in the expanded state, the temperature for starting expansion of the thermal expansion microsphere is the initial foaming temperature Tstart, and the temperature for expanding to the maximum is the maximum expansion temperature Tmax.

The thermal expansion microspheres are generally used as foaming materials to be applied to the fields of coatings, wallpaper, printing ink and the like, and can effectively reduce the product density and save the cost. In recent years, with the progress of light weight, low cost, and high performance in the fields of automobiles, aerospace, electronics, and the like, thermally expandable microspheres have come to be applied to the foaming process of thermoplastic resins, rubbers, and thermoplastic elastomers, and have been given advantages such as light weight, heat insulation, vibration damping, high specific strength, and low price.

In some fields, such as thermosetting resins and thermosetting elastomers, the initial foaming temperature of the microspheres is lower than the curing temperature of the resin, which requires that the microspheres have a lower Tstart, and the viscosity of the resin matrix is too high during processing, which requires higher temperature and high shear, and the good high temperature durability and repeated compression durability become important properties of the microspheres compared to mixing, calendering, extrusion and injection molding. However, the existing high temperature resistant expandable microspheres generally focus on improving the expansion effect by the type and ratio of the thermoplastic resin and the foaming agent of the expandable microspheres, and patent CN107001911A improves the heat resistance of the microspheres by introducing methacrylonitrile, so that the maximum expansion temperature of the expandable microspheres reaches more than 190 ℃, and patent CN101679537A discloses a method for expanding the microspheres, in which methacrylamide is replaced by acrylamide and fluorine-containing acrylate monomers, but the initial foaming temperature Tstart of the microspheres is increased after the glass transition temperature of the thermoplastic resin of the microspheres is increased.

In addition, some researches find that the thermodynamic heat of a microsphere shell layer can be improved by introducing a polyurethane chain segment into the microsphere shell layer, in patent CN110041465A, a micron-sized elastic microsphere is prepared by preparing polyurethane acrylate and vinyl monomer through copolymerization, the obtained microsphere has high elasticity and monodispersity, but the prepared microsphere is of a solid structure and does not have thermal expansion performance, in patent CN104014287A, an expandable micro inner shell layer is prepared by using polyurethane/olefinic polymerizable monomer with a branched structure, so that the elasticity and mechanical properties of the microsphere wall material are improved, and a thermal expansion microsphere with low Tstart is prepared, however, the introduction of the polyurethane chain segment can reduce the heat resistance of the microsphere, in patent CN111675824A, an expandable microsphere capable of being crosslinked at high temperature is disclosed, in patent CN111675824A, a blocked isocyanate curing agent is introduced into the microsphere shell layer, when the expandable microsphere shell layer is foamed, the blocked by the blocked isocyanate curing agent is exposed, so that isocyanate groups are subjected to perform crosslinking reaction with shell hydroxyl or amino groups, so that the microspheres have excellent heat resistance and solvent resistance, but a large amount of macromolecular isocyanate curing agents which are not involved in the reaction is introduced into the shell layer is reduced, and the microsphere can further influence the preservation performance of the foaming agent, and the microsphere can be escaped. Particularly, the low Tsat microspheres require isobutane with a lower boiling point as a foaming agent, so that higher requirements are put on the gas barrier property of the shell layer.

The invention content is as follows:

the invention provides a self-reinforcing thermal expansion microsphere and a preparation method thereof, so that the self-reinforcing thermal expansion microsphere has low initial foaming temperature, excellent high-temperature durability and good repeatable compression durability.

In order to realize the purpose of the invention, the invention adopts the following technical scheme:

the invention provides a preparation method of a thermal expansion microsphere in a first aspect, which comprises the following steps:

(1) Preparing blocked polyurethane: reacting polyisocyanate and hydroxyl acrylate under the condition of a catalyst to prepare a double-bond-containing terminal-NCO prepolymer, and then adding a sealing agent into the obtained prepolymer to carry out end-capping reaction;

(2) Carrying out suspension polymerization on the obtained blocked polyurethane, hydroxyl acrylate, olefinic polymerizable monomer and foaming agent to prepare the self-reinforcing thermal expansion microsphere;

wherein the molar ratio of isocyanate groups in the polyisocyanate to hydroxyl groups in the hydroxyacrylate in step (1) is 5 to 1;

preferably, the molar ratio of polyisocyanate to blocking agent in step (1) is 1: (1.5-2);

in some embodiments of the present invention, in the step (1), the hydroxy acrylate, the polyisocyanate and the catalyst are contacted to react under a nitrogen atmosphere, and in some preferred embodiments, the reaction can be performed at 60 to 90 ℃ for 1 to 5 hours to obtain the double bond-containing terminal-NCO prepolymer;

further, reducing the temperature of the reaction system to 40-60 ℃, then adding a sealing agent, uniformly stirring, slowly heating to 60-80 ℃, reacting for 1-3h, cooling to below 40 ℃, and discharging to obtain sealed polyurethane;

in some specific embodiments, the amount of the catalyst added is 0.01% to 0.5% based on 100% by mass of the total mass of the polyisocyanate and the hydroxy acrylate.

In a specific embodiment of the present invention, the number average functional group of the polyisocyanate is not less than 2, and the molecular weight is 100 to 500 daltons, the polyisocyanate is selected from diisocyanate or a trimer of diisocyanate, the diisocyanate is selected from one or more of 2, 4-toluene diisocyanate, 2, 6-toluene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate and 4, 4-dicyclohexylmethane diisocyanate, and the diisocyanate trimer is selected from one or more of 2, 4-toluene diisocyanate trimer, hexamethylene diisocyanate trimer, isophorone diisocyanate trimer or a HDI and TDI mixed trimer.

In a particular embodiment of the invention, the blocked polyurethane has a weight average molecular weight of between 200 and 2000 daltons.

The researchers of the invention find that if the molecular weight of the blocked polyurethane is too high, the viscosity of the blocked polyurethane is increased, so that the blocked polyurethane is difficult to mix with a foaming agent and a vinyl monomer, meanwhile, the polyurethane with high molecular weight can thicken an oil phase, the oil phase is dispersed unevenly in the subsequent dispersion process due to the excessively high viscosity of the oil phase, and the particle size distribution coefficient of the finally synthesized microspheres is increased.

The hydroxyl acrylic ester is selected from one or more of hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, 4-hydroxybutyl acrylate and 2-hydroxypropyl acrylate; in some embodiments, the catalyst is selected from one or more of organic tertiary amine catalysts, organic tin catalysts or imidazole catalysts, the organic tertiary amine catalysts are selected from triethylamine, N-dimethyl benzylamine, N-dimethyl cyclohexylamine, dimethyl ethanolamine and triethanolamine, the organic tin catalysts are selected from dibutyltin dilaurate, stannous octoate and dibutyltin dienate, and the imidazole catalysts are selected from 2-methylimidazole and 2-ethyl-4-methylimidazole.

The blocking agent is selected from one or more of phenol, 2-chlorophenol, catechol, n-butyl alcohol, tert-butyl alcohol, acetanilide and methyl ethyl ketone oxime.

In step (2) of the present invention, a crosslinking agent and an initiator may also be added.

In the step (2), the blocked polyurethane is mixed with hydroxy acrylate, olefinic polymerizable monomer, foaming agent, optional cross-linking agent and optional initiator to obtain an oil phase mixture, and the oil phase mixture is dispersed in a water-based dispersion medium for suspension polymerization; in some preferred embodiments, the suspension polymerization is preferably carried out under nitrogen pressure of 0.2 to 2MPa, with stirring at 500 to 500rpm and at a temperature of 40 to 90 ℃ for 10 to 30 hours.

In the specific embodiment of step (2) of the present invention, the molar ratio of the blocked polyurethane to the hydroxy acrylate is 1 (0.5-1).

In some preferred embodiments of the invention, the blocked polyurethane is added in step (2) in an amount of from 5% to 25% by weight based on the mass of the ethylenically polymerizable monomers.

In a specific embodiment of step (2) of the present invention, the ethylenically polymerizable monomer is selected from one or more of nitrile monomers, acrylic monomers, acrylamide monomers, styrene monomers, maleimide monomers, vinyl monomers or vinyl acetate monomers; in some preferred embodiments, the nitrile monomer is present in an amount of 20 to 90% by weight, the vinyl monomer is present in an amount of 0 to 50% by weight, and the acrylic monomer is present in an amount of 0 to 60% by weight, based on the total weight of the ethylenically unsaturated monomers taken as 100%.

In some embodiments, the nitrile monomer may be selected from acrylonitrile, methacrylonitrile; the vinyl monomer can be selected from vinylidene chloride; the acrylic monomer may be selected from methyl methacrylate, butyl acrylate or ethyl acrylate.

In a specific embodiment of the present invention, the crosslinking agent is selected from one or more of (meth) acrylic acid alcohol ester selected from (di) ethylene glycol di (meth) acrylate, 1, 4-butanediol dimethacrylate, pentaerythritol triallyl ether or trimethylolpropane tri (meth) acrylate, and (meth) acrylic acid allyl ether selected from pentaerythritol triallyl ether, trimethylolpropane diallyl ether, glycerol triallyl ether; the foaming agent is selected from linear or branched alkanes with the number of carbon atoms of 4-8, preferably one or more of n-butane, isobutane, isopentane, n-hexane or cyclohexane; the initiator is selected from peroxide initiators and/or azo compound initiators. Peroxide initiators and azo compound initiators are among the initiators commonly used in the art and will not be described in detail herein.

In the invention, the mass of the foaming agent added in the step (2) is 10-25% of the sum of the masses of all the blocked isocyanate, the hydroxyl acrylate and the olefinic polymerizable monomer.

The addition amount of the cross-linking agent is 0.1-1% of the sum of the masses of all the blocked isocyanate, the hydroxyl acrylate and the olefinic polymerizable monomer.

The addition amount of the initiator is 1 to 5 percent of the sum of the mass of all the blocked isocyanate, the mass of the hydroxyl acrylate and the mass of all the olefinic polymerizable monomers.

In the present invention, the aqueous dispersion medium refers to an aqueous solution containing a dispersant, an electrolyte, a co-dispersant or an inhibitor, etc. therein as understood by those skilled in the art; in some embodiments, the dispersant, electrolyte, co-dispersant or inhibitor may be mixed with water to provide an aqueous dispersion medium;

preferably, the deionized water accounts for 70-90 wt% of the weight of the aqueous dispersion medium, the inhibitor accounts for 0.1-5 wt% of the aqueous dispersion medium, the dispersion aid accounts for 0.5-2 wt% of the aqueous dispersion medium, the electrolyte accounts for 5-20 wt% of the aqueous dispersion medium, and the dispersant accounts for 0.5-5 wt% of the aqueous dispersion medium.

The dispersant mainly plays a role in suspension dispersion and prevents secondary condensation of oil phase droplets in the dispersion process, and in some specific embodiments, the dispersant is selected from starch, cellulose, silica colloid, metal salt, oxide or hydroxide, for example, one or more of water-soluble organic polymer starch, silica colloid, methyl cellulose, carboxymethyl cellulose, polyvinylpyrrolidone, magnesium hydroxide, aluminum hydroxide, calcium carbonate, magnesium carbonate, barium carbonate or white carbon black can be selected.

Further, when the dispersing agent is starch, carboxymethyl cellulose, silica colloid and aluminum hydroxide, the pH value of the aqueous dispersion medium is controlled to be 1-6, preferably 3-6; when the dispersant is magnesium hydroxide, calcium carbonate, magnesium carbonate or barium carbonate, the pH value of the aqueous dispersion medium is controlled to be 5-12, preferably 6-10.

In some embodiments, to reduce the dispersion of the monomer in the aqueous phase, the electrolyte may be selected from one or more of sodium chloride, potassium chloride, calcium chloride, lithium chloride, sodium sulfate, potassium sulfate, or lithium sulfate.

In some embodiments, the dispersion aid additive is selected from one or more of sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, sodium dodecyl sulfate, sodium styrene sulfonate, sodium carboxymethyl cellulose, sodium hydroxymethyl cellulose, alkyl trimethyl ammonium chloride, dialkyl dimethyl ammonium chloride, and polyethylene glycol.

In some specific embodiments, the inhibitor is selected from one or more of potassium dichromate, sodium dichromate, potassium nitrite, sodium nitrite, or sodium bisulfite.

In some embodiments, the oil phase mixture is dispersed in the aqueous dispersion medium under high shear to form oil phase droplets, and suspension polymerization is performed after a stably dispersed suspension is formed; the high-speed shearing process can be performed in a dispersion machine or a homogenizer, and the specific operation thereof belongs to the well-known technology in the field and is not described in detail herein.

In some embodiments, the dispersion process is preferably emulsified for 2-10 minutes at 6000-10000rpm by a homomixer, and in some embodiments, the suspension polymerization may be performed at 0.2-2mpa,40-90 ℃ for 10-30 hours, and in particular, the reaction may be performed with stirring at 50-500 rpm; after the reaction is finished, cooling the obtained product to below 40 ℃, separating, washing and drying to obtain the thermal expansion microspheres; in some embodiments, deionized water or alcohol can be used for washing, and suction filtration or centrifugal separation is performed to obtain a product; the drying treatment may be freeze drying, spray drying or oven drying, such as vacuum drying at 50-80 deg.C for 1-3 days.

In some embodiments, the method of preparing expanded microspheres further comprises adsorbing a filler on the surface of the microspheres to provide improved dispersibility and flowability of the microspheres during use or expansion.

The filler can be organic particles or inorganic particles, the organic particles are selected from magnesium stearate, calcium stearate, zinc stearate, barium stearate, calcium stearate, polyacrylamide, polyimide, lauric amide, nylon, polyethylene wax and polytetrafluoroethylene, and the inorganic particles are selected from talc, mica, bentonite, carbon black, molybdenum disulfide, tungsten disulfide, carbon fluoride, calcium fluoride, boron fluoride, silicon dioxide, aluminum oxide, calcium carbonate, calcium hydroxide, magnesium phosphate, barium sulfate, titanium dioxide and zinc oxide.

The filler can be selected from one or more of the above.

The filler can be added into mother liquor in the process of synthesizing the microspheres, and can also be added into dry powder microspheres.

The filler accounts for 5 to 30 percent of the total mass of the microsphere.

In the polyisocyanate in the step (1), the molar ratio of isocyanate groups to hydroxyl groups of hydroxyl acrylate is 5-3; then, according to the molar weight of the prepolymer and the isocyanate blocking agent of 1:1.5-2, adding a sealing agent to seal the polyurethane prepolymer, so that polymerizable double bonds are introduced into a prepared polyurethane molecular chain, meanwhile, a terminal isocyanate group in the polyurethane prepolymer is protected by the sealing agent, the sealed isocyanate has low-temperature stability, the sealing agent does not exceed the unsealing temperature of the sealing agent in the subsequent dispersing and microsphere synthesizing process, and in the expansion process, the sealing agent is subjected to high-temperature deblocking to release the isocyanate group to react with hydroxyl in hydroxyl acrylate in a microsphere shell layer, so that the crosslinking degree of the microsphere shell layer is improved, the microsphere has a self-reinforcing function in the expansion process, the excellent effect is that the high-temperature durability and the repeated compression durability of the microsphere are obviously improved, and meanwhile, the microsphere can keep a lower initial expansion temperature.

The second aspect of the invention provides a thermal expansion microsphere, which is prepared by the method, and the particle size of the thermal expansion microsphere is 5-50 μm.

In some specific implementation methods, the thermal expansion microspheres provided by the invention have lower initial expansion temperature and high-temperature durability, the initial expansion temperature of the thermal expansion microspheres is 80-120 ℃, the maximum expansion temperature is 150-230 ℃, and the microsphere retention time of the thermal expansion microspheres is more than or equal to 30 minutes at 200 ℃.

In some embodiments, the thermally expandable microspheres provided herein have a repeated compression durability of 80% or more.

Detailed Description

In order to better understand the present invention, the following examples are provided to further illustrate the content of the present invention.

The following methods were used in the following examples:

(1) Particle size and particle size distribution of thermally expandable microspheres

The average particle size is expressed as the particle size in volume diameter D50 and the particle size distribution is expressed as the span, as determined by laser light scattering of the sample on a Bettersize 2000LD laser particle size analyzer.

(2) Expansion Properties of thermally expandable microspheres

The test was performed by Mettler TMA/SDTA2+ LN/600, adding 0.4mg of the sample into an aluminum crucible and placing an aluminum lid thereon, applying a load of 0.06N on the aluminum lid, measuring the high-degree directional displacement during heating of the sample, the sample heating rate being 15 ℃/min, defining the temperature at which the displacement starts in the positive direction as Tstart, the temperature at which the displacement starts in the negative direction as Tmax, and recording Tstart, tmax of the thermally expanded microspheres during expansion;

wherein Tstart represents the initial foaming temperature of the expanded microspheres, and Tmax represents the maximum foaming temperature of the expanded microspheres.

(3) High temperature durability of thermally expandable microspheres

Defining: in the present invention, the high temperature durability of the expanded microspheres is defined as the time for the volume of the expanded microspheres to begin to collapse after the expansion at 200 ℃

The test is carried out through Mettler TMA/SDTA2+ LN/600, 0.2mg of sample is added into an aluminum crucible, an aluminum cover is placed on the aluminum crucible, a load of 0.06N is applied on the aluminum cover, the heating rate of the sample is 20 ℃/min, the sample is rapidly heated to 200 ℃, the high-degree direction displacement in the heating process of the sample is measured, the timing is started when the displacement in the positive direction reaches the maximum, and the timing is finished when the displacement is reduced to 80% of the maximum displacement.

(4) Repeated compression durability of thermally expandable microspheres

The expanded microspheres can be dried by using an oven, a double-cone vacuum dryer, a positive air rake dryer or infrared drying.

0.2mg of the expanded microspheres was charged into an aluminum crucible, covered with an aluminum cover, and subjected to a pressure of 2.5N at 25 ℃ by a Mettler TMA/SDTA2+ LN/600 test to determine the thickness L1 of the sample at that time, and then the applied pressure was raised from 2.5N to 18N at a rate of 10N/min, and then the pressure was lowered from 18N to 2.5N at a rate of 10N/min, and after repeating the raising and lowering of the pressure 7 times, the thickness L2 of the sample was determined under the application of 2.5N, and then the ratio between the thicknesses L1 and L2 of the above-mentioned sample was calculated from the following formula, and the calculation result was defined as the repeated compression durability of the sample:

durability against repeated compression (%) = (L2/L1) × 100

(5) Weight average molecular weight

The weight average molecular weight of the polymer, as determined by Gel Permeation Chromatography (GPC), relative to styrene standards.

Example 1

(1) Putting 23.22g of hydroxyethyl acrylate and 0.42g of N, N-dimethyl benzylamine into a 300ml three-neck flask, and uniformly mixing; 60.95g of 2, 4-tolylene diisocyanate, in which-NCO: OH =3.5, wherein the catalyst accounts for 0.5% of the total mass of hydroxyethyl acrylate and 2.4-toluene diisocyanate, and the prepolymer is obtained by reacting at 90 ℃ for 1h in a nitrogen atmosphere;

cooling the system temperature to 40 ℃, dropwise adding 45.74g of methyl ethyl ketone oxime into the system while stirring, wherein the molar ratio of isocyanate to a sealing agent is 1.5;

(2) Mixing 9g of the blocked polyurethane prepared above, 1.62g of hydroxypropyl acrylate, 100g of acrylonitrile, 62g of methacrylonitrile, 18g of vinylidene chloride, 47.65g of isobutane, 1.9g of azobisisobutyronitrile and 1.9g of diethylene glycol dimethacrylate to obtain an oil phase mixture;

mixing 31.5g of magnesium hydroxide, 12.6g of sodium dodecyl sulfate, 100g of sodium chloride, 26g of potassium chloride, 18.9g of sodium nitrite and 441g of deionized water to obtain an aqueous dispersion medium, adjusting the pH value of the aqueous dispersion medium to 11 by using a sodium hydroxide solution, and stirring and mixing the oil phase mixture and the aqueous dispersion medium at a high speed in a homogenizing and dispersing machine to form a stably dispersed suspension;

and injecting the suspension into a high-pressure reaction kettle, introducing nitrogen gas at 0.7MPa and 70 ℃, reacting for 20 hours at the stirring speed of 500rpm, cooling the obtained product to 40 ℃, washing with water, and drying in vacuum at 50 ℃ for 3 days to obtain powdery thermal expansion microspheres, adding silicon dioxide accounting for 10wt% of the mass of the sample, and fully mixing to obtain a product, namely the thermal expansion microspheres 1.

Example 2

(1) Putting 21.73g of hydroxypropyl acrylate and 0.23g of triethylamine into a 300ml three-neck flask, and uniformly mixing; thereto was added 55.57g of isophorone diisocyanate, where-NCO: OH =3, wherein the catalyst accounts for 0.3% of the total mass of hydroxypropyl acrylate and isophorone diisocyanate, and the prepolymer is obtained by reacting for 2h at 80 ℃ in a nitrogen atmosphere;

reducing the temperature of the system to 50 ℃, dropwise adding 40g of phenol while stirring, wherein the molar weight ratio of the isocyanate to the blocking agent is 1.7, slowly heating to 70 ℃ after dropwise adding, and reacting for 2h to prepare blocked isocyanate (the weight-average molecular weight is 501.43 g/mol);

(2) Mixing 18g of the blocked polyurethane prepared above, 3.33g of hydroxyethyl acrylate, 36g of acrylonitrile, 90g of vinylidene chloride, 54g of methyl methacrylate, 20.13g of isobutane, 6.04g of bis (4-tert-butylcyclohexyl) peroxydicarbonate and 0.36g of trimethylolpropane trimethacrylate to obtain an oil phase mixture;

mixing 12.6g of magnesium hydroxide, 4g of sodium dodecyl benzene sulfonate, 2.3g of sodium styrene sulfonate, 31.5g of sodium chloride, 31.5g of potassium dichromate and 548g of deionized water to obtain an aqueous dispersion medium, adjusting the pH value of the aqueous dispersion medium to 10 by using a sodium hydroxide solution, and stirring and mixing an oil phase mixture and the aqueous dispersion medium at a high speed in a homogenizing and dispersing machine to form a stably dispersed suspension;

and injecting the suspension into a high-pressure reaction kettle, introducing nitrogen into the high-pressure reaction kettle, reacting for 25 hours at the temperature of 0.2MPa and 50 ℃ and at the stirring speed of 50rpm, cooling the obtained product to 40 ℃, washing the product with water, and drying the product in vacuum at the temperature of 50 ℃ for 4 days to obtain dry thermal expansion microspheres, adding silicon dioxide accounting for 10wt% of the mass of the sample, and fully mixing the mixture to obtain the thermal expansion microspheres 2.

Example 3

(1) Putting 18.02g of hydroxypropyl methacrylate and 0.08g of 2-methylimidazole into a 300ml three-neck flask, and uniformly mixing; thereto was added 62.56g of diphenylmethane diisocyanate, in which-NCO: OH =4, wherein the catalyst accounts for 0.1% of the total mass of hydroxypropyl methacrylate and diphenylmethane diisocyanate, and the reaction is carried out at 70 ℃ for 3h under the nitrogen atmosphere to obtain a prepolymer;

reducing the temperature of the system to 60 ℃, dropwise adding 35.2g of tert-butyl alcohol into the system while stirring, wherein the molar weight ratio of the isocyanate to the blocking agent is 1;

(2) Mixing 36g of the blocked polyurethane prepared above, 2.84g of 4-hydroxybutylacrylate, 72g of acrylonitrile, 36g of vinylidene chloride, 40g of methyl methacrylate, 32g of butyl methacrylate, 43.77g of isobutane, 8.75g of lauroyl peroxide and 0.44g of 1, 4-butanediol dimethacrylate to obtain an oil phase mixture;

mixing 10.5g of 30wt% colloidal silicon dioxide, 10.5g of polyethylene glycol, 56.07g of sodium chloride, 0.63g of sodium nitrite and 552.3g of deionized water to obtain a water-based dispersion medium, adjusting the pH value of the water-based dispersion medium to 3 by using a hydrochloric acid solution, and stirring and mixing the oil phase mixture and the water-based dispersion medium at a high speed in a homogenizing and dispersing machine to form a stably dispersed suspension;

and injecting the suspension into a high-pressure reaction kettle, introducing nitrogen at 1MPa and 70 ℃, reacting for 15 hours at the stirring rotation speed of 100rpm, cooling the obtained product to 40 ℃, washing with water, and performing vacuum drying at 50 ℃ for 4 days to obtain thermal expansion microspheres, adding silicon dioxide with the mass of 10wt% of the sample, and fully mixing to obtain the thermal expansion microspheres 3.

Example 4

(1) Putting 17.3g of 4-hydroxybutyl acrylate and 0.014g of 2-ethyl-4-methylimidazole into a 300ml three-neck flask, and uniformly mixing; 125.52g of hexamethylene diisocyanate trimer was added thereto, in which-NCO: OH =5, wherein the catalyst accounts for 0.01% of the total mass of the hydroxyethyl acrylate and the isophorone diisocyanate, and the prepolymer is obtained by reacting for 34h at 60 ℃ in a nitrogen atmosphere;

cooling the system to 50 ℃, dropwise adding 29.65g of n-butanol into the system while stirring, wherein the molar weight ratio of isocyanate to the sealant is 1;

(2) Mixing 45g of blocked polyurethane prepared above, 2g of hydroxypropyl methacrylate, 88g of acrylonitrile, 20g of methacrylonitrile, 54g of vinylidene chloride, 18g of ethyl acrylate, 34.05g of isobutane, 11.35g of benzoyl peroxide and 0.227g of trimethylolpropane triacrylate to obtain an oil phase mixture;

mixing 84g of 30wt% colloidal silicon dioxide, 9.45g of alkyl trimethylammonium chloride, 94.5g of sodium chloride, 28.35g of sodium nitrite and 413.7g of deionized water to obtain an aqueous dispersion medium, adjusting the pH value of the aqueous dispersion medium to 4 by using sodium hydroxide, and stirring and mixing the oil phase mixture and the aqueous dispersion medium at a high speed in a homogenizing and dispersing machine to form a stably dispersed suspension;

and injecting the suspension into a high-pressure reaction kettle, introducing nitrogen at the temperature of 2MPa and 90 ℃, reacting for 10 hours at the stirring speed of 300rpm, cooling the obtained product to 40 ℃, washing with water, and drying in vacuum at the temperature of 50 ℃ for 4 days to obtain thermal expansion microspheres, adding silicon dioxide accounting for 10wt% of the mass of the sample, and fully mixing to obtain the thermal expansion microspheres 4.

Comparative example 1

This comparative example differs from the preparation process of example 1 only in that no blocked polyurethane is added to the oil phase mixture; the thermal expansion microspheres prepared by the method are marked as thermal expansion microspheres 1-1.

Comparative example 2

This comparative example differs from the preparation process of example 2 only in that 55.57g of isophorone isocyanate was placed in a 300ml three-necked flask, heated to 50 ℃ and 40g of phenol was added dropwise with stirring, after completion of the dropwise addition, the temperature was slowly raised to 70 ℃ to react for 2 hours to obtain a blocked isocyanate free from double bond introduction, and the blocked isocyanate was added to the oil phase mixture; the thermal expansion microspheres prepared by the method are marked as thermal expansion microspheres 2-1.

Comparative example 3

This comparative example is different from the preparation method of example 3 only in that 72g of hydroxypropyl methacrylate and 0.08g of 2-methylimidazole were uniformly mixed in a 300ml three-necked flask, 62.56g of diphenylmethane diisocyanate was added thereto, and a reaction was carried out at 70 ℃ for 3 hours under a nitrogen atmosphere to obtain a polyurethane prepolymer; the obtained polyurethane prepolymer replaces the blocked isocyanate in the embodiment 3; the thermal expansion microsphere prepared by the method is marked as thermal expansion microsphere 3-1.

Comparative example 4

This comparative example differs from the preparation method of example 4 only in that 0.227g of trimethylolpropane triacrylate was substituted for the blocked polyurethane of example 4; the thermal expansion microspheres prepared by the method are marked as thermal expansion microspheres 4-1.

The heat-expandable microspheres obtained above were examined for particle size, particle size distribution, foaming properties, high-temperature durability, and repeated compression durability, and the examination results are shown in table 1.

TABLE 1

Claims (20)

1. A preparation method of self-reinforcing thermal expansion microspheres is characterized by comprising the following steps:

(1) Preparing blocked polyurethane, namely reacting polyisocyanate and hydroxy acrylate under the condition of a catalyst to prepare a double-bond-terminated NCO prepolymer, and then adding a blocking agent into the obtained prepolymer to carry out end-capping reaction;

(2) Carrying out suspension polymerization on the obtained closed polyurethane, hydroxyl acrylate, olefinic polymerizable monomer and foaming agent to prepare the self-reinforcing thermal expansibility microsphere; in the step (2), a cross-linking agent and an initiator are also added;

the molar ratio of isocyanate groups in the polyisocyanate to hydroxyl groups in the hydroxyl acrylate in the step (1) is 5.

2. The method of claim 1,

in the step (1), the molar ratio of the polyisocyanate to the blocking agent is 1: (1.5-2).

3. The method of claim 1, wherein in step (1), the hydroxy acrylate, the polyisocyanate and the catalyst are heated and reacted in a nitrogen atmosphere to prepare the polyurethane prepolymer.

4. The process according to claim 3, wherein the heating temperature is 60-90 ℃ and the reaction time is 1-5 hours.

5. The method as claimed in claim 1, wherein the temperature of the reaction system is reduced to 40-60 ℃, then the sealing agent is added, the mixture is stirred uniformly, the temperature is increased to 60-80 ℃, the reaction is carried out for 1-3h, the mixture is cooled to below 40 ℃, and the sealed polyurethane is obtained.

6. The method according to claim 1, wherein the catalyst is added in an amount of 0.01 to 0.5% based on 100% by mass of the total amount of the polyisocyanate and the hydroxyacrylate.

7. The process according to claim 1, wherein the polyisocyanate has a number average functionality of 2 or more and a molecular weight of 100 to 500 daltons,

the polyisocyanate is selected from diisocyanate or trimer of diisocyanate, the diisocyanate is selected from one or more of 2, 4-toluene diisocyanate, 2, 6-toluene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate and 4, 4-dicyclohexylmethane diisocyanate, and the diisocyanate trimer is selected from one or more of 2, 4-toluene diisocyanate trimer, hexamethylene diisocyanate trimer, isophorone diisocyanate trimer or HDI and TDI mixed trimer.

8. The method according to claim 1, wherein the hydroxy acrylate is selected from one or more of hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, 4-hydroxybutyl acrylate and 2-hydroxypropanyl acrylate.

9. The process according to claim 1, characterized in that the blocked polyurethane has a weight-average molecular weight of between 200 and 2000 daltons.

10. The method according to claim 1, wherein the catalyst is selected from one or more of organic tertiary amine, organic tin or imidazole catalyst, the organic tertiary amine catalyst is selected from one or more of triethylamine, N-dimethyl benzylamine, N-dimethyl cyclohexylamine, dimethyl ethanolamine and triethanolamine, the organic tin catalyst is selected from one or more of dibutyl tin dilaurate, stannous octoate and dibutyl tin dienoate, and the imidazole catalyst is selected from one or more of 2-methylimidazole and 2-ethyl-4-methylimidazole;

the blocking agent is selected from one or more of phenol, 2-chlorophenol, catechol, n-butyl alcohol, tert-butyl alcohol, acetanilide and methallyl oxime.

11. The method according to claim 1, wherein in the step (2), the blocked polyurethane is mixed with the hydroxy acrylate, the ethylenic polymerizable monomer, the foaming agent, the crosslinking agent, and the initiator to obtain an oil phase mixture, and the oil phase mixture is dispersed in an aqueous dispersion medium to perform suspension polymerization;

emulsifying for 2-10 minutes at 6000-10000rpm by a homomixer in the dispersing process;

the suspension polymerization nitrogen is pressurized to 0.2-2Mpa, the stirring speed is 500-500rpm, and the reaction is carried out for 10-30h at the temperature of 40-90 ℃.

12. The method according to claim 1, wherein the molar ratio of blocked polyurethane to hydroxyacrylate is 1 (0.5-1).

13. The method of claim 1, wherein the ethylenically polymerizable monomer is selected from one or more of nitrile monomers, acrylic monomers, acrylamide monomers, styrenic monomers, maleimide monomers, vinylic monomers, or vinyl acetate monomers.

14. The method according to claim 1, wherein the total mass of the ethylenically polymerizable monomers is 100%, and the ethylenically polymerizable monomers are selected from the group consisting of 20 to 90% of nitrile monomers, 0 to 50% of vinyl monomers, and 0 to 60% of acrylic monomers.

15. The method of claim 11, wherein the cross-linking agent is selected from one or more of ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, ethylene glycol diacrylate, diethylene glycol diacrylate, 1, 4-butanediol dimethacrylate, pentaerythritol triallyl ether, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, trimethylolpropane diallyl ether, or glycerol triallyl ether;

the foaming agent is selected from linear or branched alkane with 4-8 carbon atoms;

the initiator is selected from peroxide initiators and/or azo compound initiators.

16. The method of claim 15, wherein the blowing agent is selected from one or more of n-butane, isobutane, isopentane, n-hexane, or cyclohexane.

17. The method according to claim 11, wherein the aqueous dispersion medium is an aqueous solution containing a dispersant, a dispersion aid, an electrolyte, and an inhibitor;

the water-based dispersion medium comprises the following components in parts by mass based on 100% of the total mass of the water-based dispersion medium: 0.5-5% of dispersing agent, 0.5-2% of dispersing auxiliary agent, 5-20% of electrolyte, 0.1-5% of polymerization inhibitor and 70-90% of deionized water;

the dispersing agent is selected from one or more of starch, cellulose, silica colloid, aluminum hydroxide, magnesium hydroxide, calcium carbonate, magnesium carbonate or barium carbonate;

the dispersing auxiliary agent is selected from one or more of polyvinyl alcohol, polyvinylpyrrolidone, sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, sodium vinyl sulfonate and sodium methyl cellulose;

the electrolyte is selected from one or more of sodium chloride, potassium chloride, calcium chloride, lithium chloride, sodium sulfate, potassium sulfate and lithium sulfate;

the inhibitor is selected from one or more of potassium dichromate, sodium nitrite, potassium nitrite, sodium bisulfite and potassium permanganate.

18. The method as claimed in claim 17, wherein when the dispersant is starch, carboxymethyl cellulose, colloidal silica or aluminum hydroxide, the PH of the aqueous dispersion medium is controlled to be 1 to 6; when the dispersant is magnesium hydroxide, calcium carbonate, magnesium carbonate or barium carbonate, the pH value of the aqueous dispersion medium is controlled to be 5-12.

19. The method of claim 1, wherein the method of preparing expanded microspheres further comprises adsorbing a filler on the surface of the microspheres;

the filler is selected from organic particle filler or inorganic particle filler.

20. The expanded microspheres produced by the method according to any one of claims 1 to 19, wherein the expanded microspheres have a repetitive compression resistance of 80% or more, a particle size of 5 to 50 μm, an initial expansion temperature of 80 to 120 ℃ and a maximum expansion temperature of 150 to 230 ℃, and a retention time of 200 ℃ for 30 minutes or more.