CN106032398A - A kind of preparation method of long chain branched polypropylene - Google Patents

Abstract

The invention provides a preparation method of long-chain branched polypropylene, which comprises the following steps: a. Weigh 100 parts by weight of polypropylene, 0.01-0.4 parts by weight of initiator, 0.1-0.3 parts by weight of antioxidant, and 1 to 10 parts by weight of acrylate monomers; b, adding the initiator, antioxidant, and acrylate monomers with multifunctional groups into an organic solvent for premixing to obtain a premixed solution, and then mixing the premixed solution with polypropylene Mix to obtain a mixture; c put the mixture into an extruder, and pass supercritical carbon dioxide into 1/5 to 1/3 of the screw of the extruder, and melt and extrude to obtain long-chain branched polypropylene . In the method, by feeding supercritical carbon dioxide into the screw rod, the actual reaction temperature is greatly reduced, and the grafting ability of the acrylate monomer with multifunctional groups is greatly improved, thereby significantly improving the melt strength of polypropylene. The present invention also provides a long chain branched polypropylene.

Description

A kind of preparation method of long chain branched polypropylene

technical field

The invention relates to the technical field of polymer materials, in particular to a long-chain branched polypropylene with high melt strength and a preparation method thereof.

Background technique

At present, in the world, the output of polypropylene (PP) is second only to polyethylene (PE) and polyvinyl chloride (PVC), and it is widely used. Compared with other general-purpose thermoplastics, PP has the characteristics of low price, low density, non-toxic, easy processing, chemical corrosion resistance and excellent mechanical properties. However, the molecular structure of PP is a linear structure, the softening point and melting point are very close, and the melt strength is low, which leads to uneven product thickness during thermoforming, edge curling and shrinkage during high-speed extrusion coating, and extrusion foaming. When the cells collapse, this greatly limits the application range of PP. In order to improve the competitiveness of polypropylene in plastic applications, its melt strength needs to be improved.

High melt strength polypropylene (HMSPP) is a new type of polypropylene material developed for the low melt strength of PP. HMSPP can be obtained by increasing the relative molecular weight of polypropylene, broadening the molecular weight distribution, and introducing long-chain branched structures. Usually the method of introducing long-chain branched structure is more effective. At present, HMSPP is prepared by melt grafting. For example, the patent application with publication number CN1986589A discloses a preparation method of high melt strength polypropylene resin, which adopts the method of grafting polypropylene by melt extrusion of acrylate compounds to obtain HMSPP.

However, the reaction temperatures in the second to fourth zones of the melt extrusion method are all higher than 200°C, which can easily induce β breakage of the PP molecular chain, resulting in the degradation of the PP resin, thereby affecting the overall performance of the product.

Contents of the invention

In view of the above problems, the purpose of the present invention is to provide a long-chain branched polypropylene with less degradation in the reaction process and a preparation method thereof, so as to solve the technical problems in the prior art.

The invention provides a kind of preparation method of long-chain branched polypropylene, it comprises the steps:

a Weigh 100 parts by weight of polypropylene, 0.01 to 0.4 parts by weight of initiator, 0.1 to 0.3 parts by weight of antioxidant, and 1 to 10 parts by weight of acrylate monomer with multifunctional groups;

b. Adding the initiator, antioxidant, and acrylate monomer with multifunctional groups into an organic solvent for premixing to obtain a premixed solution, and then mixing the premixed solution with polypropylene to obtain a mixture;

c. Add the mixture into an extruder, and feed supercritical carbon dioxide into 1/5-1/3 of the screw of the extruder, and melt and extrude to obtain long-chain branched polypropylene.

The organic solvent in step b is at least one of acetone, ethanol, and chloroform, and the weight ratio of the organic solvent to the initiator is 1:50˜1:150.

In step c, the temperature of the first half of the screw is 190°C to 210°C, and the temperature of the second half of the screw is 150°C to 190°C.

The grafting ratio of the acrylate monomer with multifunctional groups to the polypropylene is 1%-5%.

The mass of the supercritical carbon dioxide in step c accounts for 5%-15% of the total mass of the mixture, and the injection pressure of the supercritical carbon dioxide is 8MPa-20MPa.

In step c, supercritical carbon dioxide is fed into 1/4 of the screw rod of the extruder.

The polypropylene is homopolypropylene, copolymerized polypropylene or a blend of the two in any proportion, and the melt index of the polypropylene is 0.1g/10min-50g/10min.

The initiator is dibenzoyl peroxide, dicumyl peroxide, bis(4-tert-butylhexanoic acid) peroxydicarbonate, cumene hydroperoxide, di-tert-butyl peroxide, peroxide At least one of bis-2-ethyl acetate dicarbonate and tert-butyl peroxyacetate.

The antioxidant is tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid]pentaerythritol ester, β-(3,5-di-tert-butyl-4-hydroxyphenyl) Octadecyl propionate, N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl)hexamethylenediamine, thiodiethylenebis[3- At least one of (3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tris[2,4-di-tert-butylphenyl]phosphite, the multifunctional group The acrylate monomer is at least one of trimethylolpropane triacrylate, pentaerythritol triacrylate, 1,6-hexanediol diacrylate, and 1,4-butanediol diacrylate.

The present invention also provides a long-chain branched polypropylene obtained by the above preparation method, the melt strength of the long-chain branched polypropylene is 19CN-31CN, and the melt index is 2.0g/10min-5.5g/10min.

Compared with the prior art, in this method, the actual reaction temperature is greatly reduced by introducing supercritical carbon dioxide at 1/5 to 1/3 of the screw, which significantly inhibits the degradation of polypropylene during the reaction process and greatly The grafting ability of acrylate monomers with multifunctional groups is improved, thereby significantly improving the melt strength of polypropylene. Specifically, during the reaction process, supercritical carbon dioxide increases the free volume of polypropylene melt, effectively weakens the cage effect of the reaction unit, increases the reaction probability of the reaction unit, thereby improving the grafting rate; , supercritical carbon dioxide has the density of liquid and the diffusion coefficient of gas, which can be used as the carrier of the reaction unit to increase the probability of mutual reaction; in addition, the supercritical carbon dioxide reduces the viscosity of polypropylene melt, thereby reducing the reaction rate. The temperature effectively suppresses the β scission on the polypropylene molecular chain, so as to obtain long-chain branched polypropylene with high grafting rate, low degradation rate, low melt index and high melt strength. The method has simple process and low energy consumption, and the obtained long-chain branched polypropylene shows excellent characteristics in terms of color, processability, mechanical properties and the like. The invention also provides a long-chain branched polypropylene prepared by the preparation method.

detailed description

The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the implementation manners in the present invention, all other implementation manners obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

The invention provides a kind of preparation method of long chain branched polypropylene, it comprises the steps:

a Weigh 100 parts by weight of polypropylene, 0.01 to 0.4 parts by weight of initiator, 0.1 to 0.3 parts by weight of antioxidant, and 1 to 10 parts by weight of acrylate monomer with multifunctional groups;

b. Adding the initiator, antioxidant, and acrylate monomer with multifunctional groups into an organic solvent to pre-mix to obtain a premixed solution, and then mixing the premixed solution with polypropylene to obtain a mixture;

c. Add the mixture into an extruder, and feed supercritical carbon dioxide into 1/5-1/3 of the screw of the extruder, and melt and extrude to obtain long-chain branched polypropylene.

In step a, the polypropylene may be homopolypropylene, copolymerized polypropylene or a blend of the two in any proportion, and the melt index of the polypropylene is 0.1 g/10min to 50 g/10min. The initiator is used for initiating the grafting reaction between the polyfunctional acrylate monomer and polypropylene. The initiator can be dibenzoyl peroxide, dicumyl peroxide, bis(4-tert-butylhexanoic acid) peroxydicarbonate, cumene hydroperoxide, di-tert-butyl peroxide, peroxide Oxidation of at least one of bis-2-ethyl acetate dicarbonate and tert-butyl peroxyacetate. The antioxidant can be tetrakis [β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol ester, β-(3,5-di-tert-butyl-4-hydroxyphenyl ) octadecyl propionate, N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyl) hexamethylenediamine, thiodiethylenebis[3 -at least one of (3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris[2,4-di-tert-butylphenyl]phosphite. The acrylate monomer with multifunctional groups can be trimethylolpropane triacrylate, pentaerythritol triacrylate, 1,6-hexanediol diacrylate, 1,4-butanediol diacrylate at least one.

In step b, the premixed solution can be mixed with polypropylene by a high-speed mixer to obtain a mixture. The organic solvent is at least one of acetone, ethanol, and chloroform. The weight ratio of the organic solvent to the initiator is 1:50˜1:150.

In step c, the mixture can be fed into the extruder at a speed of 1kg/h-10kg/h from the feed port. The mass of the supercritical carbon dioxide accounts for 5%-15% of the total mass of the mixture. The purity of the supercritical carbon dioxide is 99.9%. The injection pressure of the supercritical carbon dioxide is 8MPa-20MPa. The temperature of the first half of the screw of the extruder is 190°C to 210°C, and the temperature of the second half of the screw is 150°C to 190°C.

Under the action of an initiator, the acrylate monomer with multifunctional groups undergoes melt grafting reaction with the polypropylene. Since supercritical carbon dioxide is introduced into 1/5 to 1/3 of the screw, the actual reaction temperature is greatly reduced, thereby significantly inhibiting the degradation of polypropylene during the reaction, thereby improving the performance of acrylic acid with multifunctional groups. Grafting ratio of ester monomers. Preferably, feed supercritical carbon dioxide at 1/4 of the screw rod, so as to ensure sufficient melting of polypropylene and preliminary mixing of each component in the mixture before feeding supercritical carbon dioxide, and simultaneously It can ensure that the supercritical carbon dioxide has enough time to uniformly disperse and dissolve in the mixture after the supercritical carbon dioxide is introduced, so that the reaction temperature in the second half of the screw can be fully reduced, and the grafting reaction can be promoted. The grafting ratio of the acrylate monomer with multifunctional groups to the polypropylene is 1%-5%.

The present invention also provides a long-chain branched polypropylene obtained by the above-mentioned preparation method. The melt strength of the long-chain branched polypropylene is 19CN-31CN, and the melt index is 2.0g/10min-5.5g/10min.

Compared with the prior art, this method makes the actual reaction temperature significantly reduce (the temperature in the first half of the screw rod is 190° C. to 210° C. , the temperature in the second half of the screw is 150°C to 190°C), which significantly inhibits the degradation of polypropylene during the reaction process, greatly improves the grafting ability of acrylate monomers with multifunctional groups, and thus significantly improves the polypropylene the melt strength. Specifically, during the reaction process, supercritical carbon dioxide increases the free volume of polypropylene melt, effectively weakens the cage effect of the reaction unit, increases the reaction probability of the reaction unit, thereby improving the grafting rate; , supercritical carbon dioxide has the density of liquid and the diffusion coefficient of gas, which can be used as the carrier of the reaction unit to increase the probability of mutual reaction; in addition, the supercritical carbon dioxide reduces the viscosity of polypropylene melt, thereby reducing the reaction rate. The temperature effectively suppresses the β scission on the polypropylene molecular chain, so as to obtain long-chain branched polypropylene with high grafting rate, low degradation rate, low melt index and high melt strength. The method has simple process and low energy consumption, and the obtained long-chain branched polypropylene shows excellent characteristics in terms of color, processability, mechanical properties and the like.

The preparation method of the long-chain branched polypropylene of the present invention is described below in conjunction with specific examples:

Example 1:

Weigh 0.05 parts of dibenzoyl peroxide, 0.1 parts of tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol ester and 4 parts of 1,6-hexanediol diacrylic acid ester;

Add dibenzoyl peroxide, tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol, 1,6-hexanediol diacrylate into 5 mL of acetone and pre-mix Obtain the premixed solution, then put 100 parts of polypropylene and the premixed solution into the high-speed mixer, mix at a speed of 300r/min for 10min to obtain the mixture, wherein the weight ratio of acetone to the initiator dibenzoyl peroxide is 1 :80;

Add the mixture into the extruder; wherein, the screw speed is set to 30r/min, the temperature in the first half of the screw is set to 190°C, and the temperature in the second half of the screw is set to 160°C; 5 wt. Part supercritical carbon dioxide, the injection pressure is set to 10MPa, extruded and granulated to obtain long-chain branched polypropylene.

Example 2:

Weigh 0.05 parts of dicumyl peroxide, 0.1 parts of thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] and 6 parts of trimethylolpropane Triacrylate;

Add dicumyl peroxide, thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and trimethylolpropane triacrylate into 3 mL of ethanol Premixed to obtain a premixed solution, then put 100 parts of polypropylene and the premixed solution into a high-speed mixer, and mix at a speed of 300r/min for 10min to obtain a mixture, wherein the weight ratio of ethanol to the initiator dicumyl peroxide is 1:50;

Add the mixture into the extruder; wherein, the screw speed is set to 70r/min, the temperature in the first half of the screw is set to 200°C, and the temperature in the second half of the screw is set to 160°C; 10 wt. Part supercritical carbon dioxide, the injection pressure is set to 12MPa, extruded and granulated to obtain long-chain branched polypropylene.

Example 3:

Weigh 0.2 parts of di-tert-butyl peroxide, 0.3 parts of N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl)hexamethylenediamine and 10 parts of 1, 4-Butanediol diacrylate;

Di-tert-butyl peroxide, N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl)hexamethylenediamine and 1,4-butanediol diacrylic acid The ester was pre-mixed with 25mL of acetone to obtain a pre-mixed solution, and then 100 parts of polypropylene and the pre-mixed solution were put into a high-speed mixer, and mixed at a speed of 300r/min for 10 minutes to obtain a mixture, wherein, acetone and initiator di-tert-peroxide The weight ratio of butyl is 1:100;

Add the mixture into the extruder; wherein, the screw speed is set to 60r/min, the temperature in the first half of the screw is set to 210°C, and the temperature in the second half of the screw is set to 170°C; 12 wt. Part supercritical carbon dioxide, the injection pressure is set to 14MPa, extruded and granulated to obtain long-chain branched polypropylene.

Example 4:

Weigh 0.08 parts of di-tert-butyl peroxide, 0.3 parts of N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl)hexamethylenediamine and 10 parts of 1, 4-Butanediol diacrylate;

Di-tert-butyl peroxide, N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl)hexamethylenediamine and 1,4-butanediol diacrylic acid The ester was pre-mixed with 8 mL of chloroform to obtain a pre-mixed solution, and then 100 parts of polypropylene and the pre-mixed solution were put into a high-speed mixer, and mixed at a speed of 300 r/min for 10 min to obtain a mixture, wherein chloroform and the initiator di-tert-peroxide The weight ratio of butyl is 1:150;

Add the mixture into the extruder; wherein, the screw speed is set to 90r/min, the temperature of the first half of the screw is set to 200°C, and the temperature of the second half of the screw is set to 150°C; 10 wt. Part supercritical carbon dioxide, the injection pressure is set to 10MPa, extruded and granulated to obtain long-chain branched polypropylene.

Example 5:

Weigh 0.1 part of tert-butyl peroxyacetate, 0.5 part of β-(3,5-di-tert-butyl-4-hydroxyphenyl) octadecyl propionate and 8 parts of pentaerythritol triacrylate;

Add tert-butyl peroxyacetate, parts of β-(3,5-di-tert-butyl-4-hydroxyphenyl) octadecyl propionate and pentaerythritol triacrylate into 8 mL of ethanol and pre-mix to obtain a pre-mixed solution. Then put 100 parts of polypropylene and the premix solution into a high-speed mixer, and mix at a speed of 300r/min for 10 minutes to obtain a mixture, wherein the weight ratio of ethanol to initiator tert-butyl peroxyacetate is 1:60;

Add the mixture into the extruder; among them, the screw speed is set to 70r/min, the temperature of the first half of the screw is set to 210°C, and the temperature of the second half of the screw is set to 160°C; 8 weights are injected into 1/5 of the screw Part supercritical carbon dioxide, the injection pressure is set to 12MPa, extruded and granulated to obtain long-chain branched polypropylene.

Embodiment 6:

Weigh 0.1 part of bis(4-tert-butylhexanoic acid) peroxydicarbonate, 0.3 part of tris[2,4-di-tert-butylphenyl] phosphite and 10 parts of pentylene glycol triacrylate;

Add bis(4-tert-butylhexanoic acid) peroxydicarbonate, tris[2,4-di-tert-butylphenyl] phosphite and pentylene glycol triacrylate into 8 mL of chloroform and pre-mix to obtain a pre-mixed solution, Put 100 parts of polypropylene and the premixed solution into the high-speed mixer afterwards, mix 10min with the speed of 300r/min to obtain the mixture, wherein, the weight of chloroform and initiator peroxydicarbonate bis(4-tert-butylhexanoic acid) The ratio is 1:120;

Add the mixture into the extruder; wherein, the screw speed is set to 100r/min, the temperature in the first half of the screw is set to 190°C, and the temperature in the second half of the screw is set to 150°C; 15 wt. Part supercritical carbon dioxide, the injection pressure is set to 18MPa, extruded and granulated to obtain long-chain branched polypropylene.

Embodiment 7:

Weigh 0.05 parts of cumene hydroperoxide, 0.1 parts of tetrakis [β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol ester and 8 parts of trimethylolpropane triacrylate;

Add cumene hydroperoxide, tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol and trimethylolpropane triacrylate into 4 mL of acetone and pre-mix to obtain a pre-mixed Mix solution, then put 100 parts of polypropylene and premix solution into a high-speed mixer, mix at a speed of 300r/min for 10min to obtain a mixture, wherein the weight ratio of acetone to initiator cumene hydroperoxide is 1:70 ;

Add the mixture into the extruder; wherein, the screw speed is set to 80r/min, the temperature in the first half of the screw is set to 200°C, and the temperature in the second half of the screw is set to 170°C; 7 wt. Part supercritical carbon dioxide, the injection pressure is set to 12MPa, extruded and granulated to obtain long-chain branched polypropylene.

Embodiment 8:

Take by weighing 0.15 parts of two 2-ethyl acetates of peroxydicarbonate, 0.2 parts of thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] and 4 parts Part 1,6-hexanediol diacrylate;

Bis-2-ethyl acetate peroxydicarbonate, thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and 1,6-hexanedi Alcohol diacrylate was pre-mixed with 19 mL of ethanol to obtain a pre-mixed solution, and then 100 parts of polypropylene and the pre-mixed solution were put into a high-speed mixer, and mixed at a speed of 300 r/min for 10 min to obtain a mixture. The weight ratio of oxidized dicarbonate bis-2-ethyl acetate is 1:100;

Add the mixture into the extruder; wherein, the screw speed is set to 90r/min, the temperature in the first half of the screw is set to 200°C, and the temperature in the second half of the screw is set to 150°C; 12 wt. Part supercritical carbon dioxide, the injection pressure is set to 16MPa, extruded and granulated to obtain long-chain branched polypropylene.

Comparative example 1:

Weigh 0.05 parts of dibenzoyl peroxide, 0.1 parts of tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol ester and 10 parts of 1,4-butanediol diacrylic acid ester;

Add dibenzoyl peroxide, tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol and 1,4-butanediol diacrylate into 5 mL of acetone for pre-mixing Obtain the premixed solution, then put 100 parts of polypropylene and the premixed solution into the high-speed mixer, mix at a speed of 300r/min for 10min to obtain the mixture, wherein the weight ratio of acetone to the initiator dibenzoyl peroxide is 1 :80;

Add the mixture into the extruder; among them, the screw speed is set to 50r/min, the temperature in the first half of the screw is set to 210°C, and the temperature in the second half of the screw is set to 190°C; do not inject supercritical carbon dioxide, extrude and granulate .

Comparative example 2:

Weigh 0.1 part of dicumyl peroxide, 0.2 part of thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate and 6 parts of trimethylolpropane tris Acrylate;

Add dicumyl peroxide, thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and trimethylolpropane triacrylate into 8mL ethanol for pre- Mix to obtain the premixed solution, then put 100 parts of polypropylene and the premixed solution into the high-speed mixer, mix at a speed of 300r/min for 10min to obtain the mixture, wherein the weight ratio of ethanol to the initiator dicumyl peroxide is 1:60;

Add the mixture into the extruder; among them, the screw speed is set to 40r/min, the temperature in the first half of the screw is set to 190°C, and the temperature in the second half of the screw is set to 190°C; without injecting supercritical carbon dioxide, extrusion granulation .

The products obtained in Examples 1 to 8 and Comparative Examples 1 and 2 were tested for mechanical properties, melt index and melt strength, and the results are shown in Table 1. Among them, tensile performance test: use Instron5567 universal material testing machine; impact strength test: use XJ-50Z combined impact testing machine; melt index test: use RL-11B melt flow rate instrument, the test condition is temperature 230 ℃, Load 2.16kg; Melt strength test: ATS Faar melt strength tester is used.

Table 1

It can be seen from Table 1 that compared with Comparative Examples 1 and 2, the mechanical properties and melt strength of the long-chain branched polypropylene obtained in Examples 1 to 8 are improved, while the melt index is decreased. This shows that the long-chain branched polypropylene obtained by the preparation method has excellent processability and mechanical properties, which is beneficial to industrial application.

The above description is a preferred embodiment of the present invention, and it should be pointed out that for those skilled in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications are also considered Be the protection scope of the present invention.

Claims (10)

1. a preparation method for long-chain branched polypropylene, comprising the steps of: a Weigh 100 parts by weight of polypropylene, 0.01 to 0.4 parts by weight of initiator, 0.1 to 0.3 parts by weight of antioxidant, and 1 to 10 parts by weight of acrylate monomer with multifunctional groups; b. Adding the initiator, antioxidant, and acrylate monomer with multifunctional groups into an organic solvent for premixing to obtain a premixed solution, and then mixing the premixed solution with polypropylene to obtain a mixture; c. Add the mixture into an extruder, and feed supercritical carbon dioxide into 1/5-1/3 of the screw of the extruder, and melt and extrude to obtain long-chain branched polypropylene. 2. a kind of preparation method of long-chain branched polypropylene as claimed in claim 1 is characterized in that, organic solvent described in step b is at least one in acetone, ethanol, chloroform, and described organic solvent and described The weight ratio of the initiators is 1:50-1:150. 3. A preparation method for long-chain branched polypropylene as claimed in claim 1, characterized in that, in step c, the temperature of the first half of the screw is 190°C to 210°C, and the temperature of the second half of the screw is 150℃～190℃. 4. a kind of preparation method of long-chain branched polypropylene as claimed in claim 1, is characterized in that, the grafting rate of described polypropylene of melting grafting described polyfunctional group acrylate monomer is 1% ~5%. 5. A preparation method for long-chain branched polypropylene as claimed in claim 1, wherein the quality of the supercritical carbon dioxide described in step c accounts for 5% to 15% of the total mass of the mixture , the injection pressure of the supercritical carbon dioxide is 8MPa-20MPa. 6. a kind of preparation method of long-chain branched polypropylene as claimed in claim 1 is characterized in that, in step c, feeds supercritical carbon dioxide at 1/4 place of the screw rod of extruder. 7. a kind of preparation method of long-chain branched polypropylene as claimed in claim 1, is characterized in that, described polypropylene is homopolypropylene, copolymerized polypropylene or the blend that both mix with arbitrary ratio, The melt index of the polypropylene is 0.1g/10min-50g/10min. 8. a preparation method of long-chain branched polypropylene as claimed in claim 1, is characterized in that, described initiator is dibenzoyl peroxide, dicumyl peroxide, dicumyl peroxide, bis(4 -at least one of tert-butylhexanoic acid), cumene hydroperoxide, di-tert-butyl peroxide, bis-2-ethyl acetate peroxydicarbonate, and tert-butyl peroxyacetate. 9. A preparation method for long-chain branched polypropylene as claimed in claim 1, wherein the antioxidant is tetrakis [beta-(3,5-di-tert-butyl-4-hydroxyphenyl) Propionic acid] pentaerythritol ester, β-(3,5-di-tert-butyl-4-hydroxyphenyl) octadecyl propionate, N,N'-bis-(3-(3,5-di-tert-butyl Base-4-hydroxyphenyl)propionyl)hexamethylenediamine, thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tris[2,4 -at least one of di-tert-butylphenyl] phosphite, the acrylate monomer with multifunctional groups is trimethylolpropane triacrylate, pentaerythritol triacrylate, 1,6-hexanediol At least one of diacrylate and 1,4-butanediol diacrylate. 10. A long-chain branched polypropylene obtained by a preparation method as claimed in any one of claims 1 to 9, wherein the melt strength of the long-chain branched polypropylene is 19CN to 31CN, and the melt index is 2.0 g/10min～5.5g/10min.