WO2019014959A1 - Low-density foaming thermoplastic polyurethane elastomer and preparation method therefor - Google Patents

Abstract

The present invention relates to a low-density foaming thermoplastic polyurethane elastomer and a preparation method therefor. The preparation method comprises primary physical foaming and N-grade (N≥1) foaming under the heating effect of a heating medium. The density of particles of the foaming thermoplastic polyurethane elastomer prepared by the method of the present invention is lower than 120 g/L, and an article thereof has much better physical properties, including rebound resilience, tensile strength, compression modulus, etc.

Description

Low-density foamed thermoplastic polyurethane elastomer and preparation method thereof Technical field

The invention relates to a method for preparing a foamed thermoplastic polyurethane elastomer, in particular to a method for preparing low-density foamed thermoplastic polyurethane elastomer particles, and belongs to the technical field of foam materials.

Background technique

A plastic foam is used to fill a large amount of air bubbles inside the plastic by physical or chemical means to obtain a polymer foamed material. Foam materials have the advantages of low density, thermal insulation, high specific strength, and cushioning. Therefore, they are widely used in the packaging industry, industry, agriculture, transportation, military industry, aerospace industry and daily necessities. Commonly used foam types are polyurethane (PU) soft and rigid foam, polystyrene (PS) foam, polyethylene (PE) foam, polypropylene (PP) foam and the like. However, polyurethane (PU) foams tend to remain isocyanate during the foaming process, which is harmful to the human body, and the foamed material cannot be recycled. Styrofoam products are difficult to degrade and are prone to "white pollution" problems. The UN Environment Organization has decided to stop using PS foam products. Polyethylene foam is inexpensive, but it also has the problem of difficult degradation, and it is poor in wear resistance and elasticity. It is mainly used in low-end fields such as packaging materials.

Thermoplastic polyurethane elastomer (TPU) has a wide range of hardness, excellent wear resistance, mechanical strength, water resistance, oil resistance, chemical resistance, mold resistance, environmental friendliness, and recycling. The molded foamed article can be obtained by filling the foamed thermoplastic polyurethane particles into a mold and performing heat molding using steam. In addition to the excellent properties of the original matrix, the product has excellent resilience, shape diversity, and reduced density, and can be used over a wide temperature range. Based on the above advantages, TPU foaming materials have a very wide application prospect in many industrial fields (automotive industry, packaging materials) and daily life (shoe materials, pillows, mattresses). According to related materials It is reported that the bulk density of the foamed thermoplastic polyurethane elastomer material currently on the market is 120-300, and there is a problem of high density, which severely limits its application in the field of lightweight foaming materials.

Summary of the invention

The present invention provides a method for preparing low density foamed thermoplastic polyurethane elastomer particles in view of the deficiencies of the existing foamed thermoplastic polyurethane elastomer materials.

The technical solution of the present invention to solve the above technical problems is as follows:

A method for preparing a low-density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: physical foaming of thermoplastic polyurethane elastomer particles having a Shore hardness of 70A-85D to obtain primary foamed thermoplastic polyurethane elastomer particles;

2) Re-foaming: the primary foamed thermoplastic polyurethane elastomer particles obtained in the step 1) are placed in a pressure-resistant container, pressurized to P1 by a gas, and kept under pressure; and then the primary foamed thermoplastic polyurethane elastomer is used using a gaseous heat medium. The pellets were heated to re-foam; the operation of step 2) was carried out N times, N ≥ 1, to obtain low-density foamed thermoplastic polyurethane elastomer particles.

Further, the low density foamed thermoplastic new polyurethane elastomer particles have a bulk density of less than 120 g/L, preferably less than 100 g/L, particularly preferably less than 60 g/L.

Further, the thermoplastic polyurethane elastomer particles in step 1) have a Shore hardness of from 70 A to 85 D, preferably from 85 A to 80 D, most preferably from 90 A to 75 D, which is obtained according to ASTM D2240.

Further, P1 in the step 2) is 0.1 to 1.0 MPa, preferably 0.2 to 0.8 MPa, and most preferably 0.3 to 0.8 MPa.

Further, the pressurized gas used in the step 2) is any one or a mixture of air, nitrogen, carbon dioxide, oxygen, and argon.

Further, the gaseous heat medium described in the step 2) is any one of air and water vapor, preferably water vapor.

Further, the temperature of the gaseous heat medium used in the step 2) is 50 to 150 °C.

Further, the heating time in the step 2) is from 5 to 60 s, preferably from 5 to 30 s, and most preferably from 8 to 20 s.

Further, the time for holding the pressure in the step 2) is not less than 1 h, preferably 6-20 h, and most preferably 8-16 h.

Further, the thermoplastic polyurethane elastomer particles in the step 1) have a rheological softening point of from 100 to 210 ° C, preferably from 110 to 180 ° C, most preferably from 110 to 160 ° C, which is obtained by a constant pressure capillary rheometer test.

Further, the specific process of the primary foaming in the step 1) is one of the following modes:

1) 100 parts by weight of thermoplastic polyurethane elastomer particles, 10-60 parts by weight of physical gas blowing agent, 100-500 parts by weight of water are added to the autoclave, the pressure in the autoclave is maintained at 1-20 MPa, and the temperature is raised to thermoplastic polyurethane elasticity. The rheological softening point of the body particles, the holding pressure is 1-300 min, and the material in the autoclave is discharged into the receiving tank to obtain a primary physical foaming thermoplastic polyurethane elastomer material, and the holding time is preferably maintained in the above step 1) 10-200min, particularly preferably 30-100min;

2) 100 parts by weight of thermoplastic polyurethane elastomer particles, 10-60 parts by weight of physical gas foaming agent are put into the extruder, the control head pressure is 0.2-30MPa, the head temperature is 100-200 ° C, and the melt is orally After the foaming of the mold, underwater pelletizing, a primary physical foaming thermoplastic polyurethane elastomer material is obtained. Wherein, the physical gas blowing agent is preferably added in an amount of 10 to 50 parts by weight, particularly preferably 20 to 40 parts by weight;

Further, the physical gas blowing agent is a mixture of one or more of carbon dioxide, nitrogen, propane, n-butane, isobutane, n-pentane, isopentane.

The beneficial effects of the invention are:

1) The thermoplastic polyurethane particles used in the present invention have a high hard segment content, exhibit good plasticity and rapid setting ability, and impart the ability of the primary foamed particles to be further expanded and expanded.

2) The primary foamed thermoplastic polyurethane elastomer is expanded one or more times by a re-foaming process to obtain a low-density foamed thermoplastic polyurethane elastomer material, and the foaming density is controllable, and the hair prepared by the prior art is solved. The thermoplastic polyurethane elastomer particles have a high density and a serious shrinkage.

3) The density of the foamed thermoplastic polyurethane elastomer particles prepared by the method of the invention is less than 120 g/L, It should be emphasized that the technical solution still has more excellent physical properties, including resilience, tensile strength, compressive modulus and the like, after reducing the density of the foamed thermoplastic polyurethane elastomer particles.

Detailed ways

The principles and features of the present invention are described in the following examples, which are intended to illustrate the invention and are not intended to limit the scope of the invention.

Example 1:

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane granules having a Shore hardness of 85 A (

E185), 10kg of n-butane, 100kg of water is added to the 500L autoclave, the temperature is raised to 130 °C, the pressure in the autoclave is maintained at 1Mpa, and the material in the autoclave is discharged after maintaining the temperature and pressure in the autoclave for 300 minutes. Receiving a can, thereby preparing primary foamed thermoplastic polyurethane elastomer particles having a bulk density of 150 g/L;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material is placed in a 1000 L pressure vessel, and the pressure of the closed vessel is raised to 0.6 MPa by air pressure for 1 hour; the primary foam is heated by using 100 ° C hot air. The thermoplastic polyurethane elastomer particles were heated again for 5 s to be foamed again to obtain a final foamed thermoplastic polyurethane elastomer material having a density of 100 g/L.

The foamed thermoplastic polyurethane elastomer particles obtained above were filled in a mold having a length of 300 mm × a width of 250 mm × a thickness of 50 mm. The water was compressed by 10% in the thickness direction of the mold to bond the particles, and finally a foamed thermoplastic polyurethane molded foamed article was obtained, and the properties thereof were evaluated as shown in Table 1.

Example 2:

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane granules having a Shore hardness of 90 A (

M90), 60kg carbon dioxide, 200kg water was added to the 500L autoclave, the temperature was raised to 150 °C, the pressure in the autoclave was kept at 10Mpa, and the contents of the autoclave were discharged to the receiving tank after maintaining the temperature and pressure in the autoclave for 100 minutes. a primary foamed thermoplastic polyurethane elastomer material having a bulk density of 110 g/L;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 1500 L pressure vessel, and the pressure of the closed vessel was increased to 1 MPa by air pressure for 20 hours; the primary foamed thermoplastic was heated by 150 ° C steam. Polyurethane elastomer particles, heating time 10s, foaming, to obtain foamed thermoplastic polyurethane elastomer particles, density of 80g / L; repeat the above steps 2) foaming process conditions for secondary foaming, to obtain the final foam thermoplastic Polyurethane elastomer material with a density of 10 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Example 3:

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane granules having a Shore hardness of 95 A (

E695), 30kg of n-butane, 500kg of water was added to the 1000L autoclave, the temperature was raised to 155 ° C, the pressure in the autoclave was kept at 5 MPa, and the contents of the autoclave were discharged after maintaining the temperature and pressure in the autoclave for 10 minutes. Receiving a can, thereby preparing a primary foamed thermoplastic polyurethane elastomer material having a density of 95 g/L;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material is placed in a 2500 L pressure vessel, and the pressure of the closed vessel is increased to 0.8 MPa by carbon dioxide pressurization for 10 hours; the primary foam is heated by steam at 50 ° C. The thermoplastic polyurethane elastomer material was heated again for 60 s to obtain a final foamed thermoplastic polyurethane elastomer material having a density of 30 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Example 4:

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane elastomer particles having a Shore hardness of 55D (

E655D), 10kg of isobutane physical gas foaming agent is put into the extruder, the control head pressure is 1MPa, the head temperature is 100°C, the underwater pelletizer water temperature is 20°C, and the melt is foamed by the die. Lower pelletizing to obtain a primary foamed thermoplastic polyurethane elastomer material;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 1000 L pressure vessel, and the pressure of the closed vessel was raised to 0.9 MPa by nitrogen pressure for 1 hour; the primary foam was heated using 100 ° C hot air. The thermoplastic polyurethane elastomer material was heated again for 30 s to be foamed again to obtain a final foamed thermoplastic polyurethane elastomer material having a density of 60 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Example 5:

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane elastomer particles having a Shore hardness of 55D (

E655D), 30kg of n-pentane physical gas foaming agent is put into the extruder, the control head pressure is 15MPa, the head temperature is 150°C, the underwater pelletizer water temperature is 40°C, and the melt is foamed by the die. Lower pelletizing to obtain a primary foamed thermoplastic polyurethane elastomer material;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 1500 L pressure vessel, and the pressure of the closed vessel was increased to 0.7 MPa by air pressure for 15 hours; the primary foam was heated by 140 ° C steam. The thermoplastic polyurethane elastomer material is re-expanded by heating for 15 s to obtain a first-stage foamed thermoplastic polyurethane elastomer material having a density of 45 g/L; the above-mentioned step 2) foaming process conditions are repeated, and the first-stage foamed thermoplastic polyurethane is elasticized. The body was subjected to secondary foaming to obtain a final foamed thermoplastic polyurethane elastomer material having a density of 15 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Example 6

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane elastomer particles having a Shore hardness of 85D (

I85D), 60kg of isopentane physical gas foaming agent is put into the extruder, the control head pressure is 30MPa, the head temperature is 200°C, the underwater pelletizer water temperature is 80°C, and the melt is foamed by the die. The lower pellets are used to prepare a primary foamed thermoplastic polyurethane elastomer material.

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 2500 L pressure vessel, and the pressure of the closed vessel was increased to 0.1 MPa by carbon dioxide pressurization for 10 hours; the primary foam was heated by 150 ° C steam. The thermoplastic polyurethane elastomer material was heated for 5 s and subjected to secondary foaming to obtain a final foamed thermoplastic polyurethane elastomer material having a density of 25 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Example 7

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane granules having a Shore hardness of 80 A (

M80), 25kg carbon dioxide, 200kg water was added to the 500L autoclave, the temperature was raised to 125 ° C, the pressure in the autoclave was kept at 8 MPa, and the contents of the autoclave were discharged to the receiving tank after maintaining the temperature and pressure in the autoclave for 60 minutes. a primary foamed thermoplastic polyurethane elastomer material having a density of 125 g/L;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 1000 L pressure vessel, and the pressure of the closed vessel was raised to 0.6 MPa by air pressure for 10 hours; 100 ° C was used. The primary foamed thermoplastic polyurethane elastomer material was heated by steam and re-expanded by heating for 5 s to obtain a final foamed thermoplastic polyurethane elastomer material having a density of 85 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm × a width of 250 mm and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Example 8

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane granules having a Shore hardness of 85 A (

M85), 30kg carbon dioxide, 200kg water was added to the 500L autoclave, the temperature was raised to 130 °C, the pressure in the autoclave was kept at 8Mpa, and the contents of the autoclave were discharged to the receiving tank after maintaining the temperature and pressure in the autoclave for 100 minutes. a primary foamed thermoplastic polyurethane elastomer material having a density of 130 g/L;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 1500 L pressure vessel, and the pressure of the closed vessel was raised to 0.5 MPa by air pressure for 12 hours; the primary foam was heated by steam at 120 °C. The thermoplastic polyurethane elastomer material is heated for 10 s and re-expanded to obtain a foamed thermoplastic polyurethane elastomer material having a density of 80 g/L; the above-mentioned step 2) foaming process conditions are repeated for secondary foaming to obtain a final hair A thermoplastic polyurethane elastomer material with a density of 15 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Example 9

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane granules having a Shore hardness of 85 A (

E185), 32kg carbon dioxide, 200kg water was added to the 500L autoclave, the temperature was raised to 130 °C, the pressure in the autoclave was kept at 8.5Mpa, and the material in the autoclave was discharged to the receiving after maintaining the temperature and pressure in the autoclave for 60 minutes. a can, thereby producing a primary foamed thermoplastic polyurethane elastomer material having a density of 110 g/L;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 1500 L vessel, and the pressure of the closed vessel was increased to 0.3 MPa by air pressure for 12 hours; the primary foamed thermoplastic polyurethane was heated by steam at 125 ° C. The elastomer material is re-expanded by heating for 8 s to obtain a foamed thermoplastic polyurethane elastomer material having a density of 80 g/L; the above-mentioned step 2) foaming process conditions are repeated for secondary foaming to obtain a final foamed thermoplastic polyurethane. Elastomeric material with a density of 60 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Example 10:

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane granules having a Shore hardness of 90 A (

E690), 40kg carbon dioxide, 100kg water was added to the 500L autoclave, the temperature was raised to 135 °C, and the pressure in the autoclave was kept at 8.5Mpa. After maintaining the temperature and pressure in the autoclave for 100 minutes, the material in the autoclave was discharged to the receiving. a can, thereby producing a primary foamed thermoplastic polyurethane elastomer material having a density of 145 g/L;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material is placed in a 1500 L pressure vessel, and the pressure of the closed vessel is increased to 1 MPa by air pressure for 16 hours; the primary foaming thermoplastic is heated by 130 ° C water vapor. Polyurethane elastomer material, heating time 15s for re-expansion, to obtain a foamed thermoplastic polyurethane elastomer material, density of 60g / L;

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. Using water vapor to compress 10% in the thickness direction of the mold to bond the particles Molding, finally obtaining a foamed thermoplastic polyurethane molded foam article. The performance was evaluated as shown in Table 1.

Example 11

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane granules having a Shore hardness of 70 A (

E270), 20kg of butane, 200kg of water was added to the 500L autoclave, the temperature was raised to 125 ° C, the pressure in the autoclave was kept at 7 MPa, and the contents of the autoclave were discharged to the receiving after maintaining the temperature and pressure in the autoclave for 120 minutes. a can, thereby producing a primary foamed thermoplastic polyurethane elastomer material having a density of 160 g/L;

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 1000 L pressure vessel, and the pressure of the closed vessel was raised to 0.6 MPa by air pressure for 12 hours; the primary foam was heated by steam at 80 ° C. The thermoplastic polyurethane elastomer material, heating time 10s, re-foaming, to obtain a foamed thermoplastic polyurethane elastomer material, density of 130g / L;

The foamed thermoplastic polyurethane elastomer particles obtained in the step 2) were subjected to secondary foaming according to the process conditions and operation of the step 2) to obtain a secondary foamed thermoplastic polyurethane elastomer material having a density of 75 g/L.

The secondary foamed thermoplastic polyurethane elastomer particles obtained above were subjected to tertiary foaming according to the process conditions and operation of the step 2) to obtain final foamed thermoplastic polyurethane elastomer particles having a density of 30 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1. Example 12

A method for preparing a low density foamed thermoplastic polyurethane elastomer, comprising the steps of:

1) Primary foaming: 100 kg of thermoplastic polyurethane elastomer particles having a Shore hardness of 75 A (

M75), 30kg nitrogen physical gas foaming agent is put into the extruder, the control head pressure is 10MPa, the head temperature is 180°C, the underwater pelletizer water temperature is 20°C, the melt is foamed by the die and the underwater pelletizing A primary foamed thermoplastic polyurethane elastomer material is obtained.

2) Re-foaming: The above-mentioned primary foamed thermoplastic polyurethane elastomer material was placed in a 2500 L pressure vessel, and the pressure of the closed vessel was increased to 0.2 MPa by nitrogen pressure for 10 hours; the primary foam was heated by 100 ° C steam. The thermoplastic polyurethane elastomer material was heated for a period of 5 s and re-foamed to obtain a final foamed thermoplastic polyurethane elastomer material having a density of 55 g/L.

The foamed thermoplastic polyurethane elastomer material obtained above was filled in a mold having a length of 300 mm, a width of 250 mm, and a thickness of 50 mm. The water is compressed by 10% in the thickness direction of the mold to bond the particles to form a foamed thermoplastic polyurethane molded foam product. The performance was evaluated as shown in Table 1.

Table 1 Performance test data of the foamed thermoplastic polyurethane elastomer particles and molded foam articles obtained in Examples 1-12

Note: In this technical solution, the bulk density test standard is ISO3944, the rebound test standard is ISO8307, the tensile strength test standard is ISO1798, and the compression modulus test standard is ISO844.

It can be seen from the data in Table 1 that the density of the foamed thermoplastic polyurethane elastomer prepared by the invention is reduced by 17-80% compared with the existing products in the market, and the rebound resilience is improved by more than 18%, and the tensile strength is improved. More than 130%.

The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalents, improvements, etc., which are within the spirit and scope of the present invention, should be included in the protection of the present invention. Within the scope.

Claims (12)

1. A method for preparing a low-density foamed thermoplastic polyurethane elastomer, comprising the steps of:

   1) Primary foaming: physical foaming of thermoplastic polyurethane elastomer particles having a Shore hardness of 70A-85D to obtain primary foamed thermoplastic polyurethane elastomer particles;

   2) Re-foaming: the primary foamed thermoplastic polyurethane elastomer particles obtained in the step 1) are placed in a pressure-resistant container, pressurized to P1 by a gas, and kept under pressure; and then the primary foamed thermoplastic polyurethane elastomer is used using a gaseous heat medium. The particles are heated and foamed again;

   The operation of the step 2) is carried out N times, N ≥ 1, that is, low density foamed thermoplastic polyurethane elastomer particles are obtained.
2. The production method according to claim 1, wherein the low density foamed thermoplastic polyurethane elastomer particles have a bulk density of not more than 120 g/L.
3. The preparation method according to claim 1, wherein the pressurized gas used in the step 2) is any one or a mixture of air, nitrogen, carbon dioxide and oxygen.
4. The preparation method according to claim 1, wherein the P1 in the step 2) is 0.1 to 1.0 MPa.
5. The preparation method according to claim 1, wherein the gaseous thermal medium in the step 2) is any one of air and water vapor.
6. The preparation method according to claim 1, wherein the temperature of the gaseous heat medium used in the step 2) is from 50 to 150 °C.
7. The preparation method according to claim 1, wherein the heating time in the step 2) is from 5 to 60 s.
8. The preparation method according to claim 1, wherein the pressure holding time in the step 2) is not less than 1 h.
9. The preparation method according to claim 1, wherein the thermoplastic polyurethane elastomer particles in the step 1) have a Shore hardness of 85A to 80D.
10. The method according to claim 1, wherein the thermoplastic polyurethane elastomer particles in the step 1) have a rheological softening point of from 100 to 210 °C.
11. The preparation method according to any one of claims 1 to 10, wherein the specific process of the primary foaming in the step 1) is one of the following modes:

    1) 100 parts by weight of thermoplastic polyurethane elastomer particles, 10-60 parts by weight of physical gas blowing agent, 100-500 parts by weight of water are added to the autoclave, the pressure in the autoclave is maintained at 1-20 MPa, and the temperature is raised to thermoplastic polyurethane elasticity. The rheological softening point of the body particles, the holding pressure is 1-300 min, and the material in the autoclave is discharged into the receiving tank to obtain the primary foamed thermoplastic polyurethane elastomer particles;

    2) 100 parts by weight of thermoplastic polyurethane elastomer particles, 10-60 parts by weight of physical gas foaming agent are put into the extruder, the control head pressure is 0.2-30MPa, the head temperature is 100-200 ° C, and the melt is orally After foaming, the pellets were submerged and pelletized to obtain primary foamed thermoplastic polyurethane elastomer particles.
12. The preparation method according to claim 11, wherein the physical gas foaming agent is one or more of carbon dioxide, nitrogen, propane, n-butane, isobutane, n-pentane and isopentane. mixture.