CN112109268A

CN112109268A - Preparation method of TPU (thermoplastic polyurethane) foamed beads and product thereof

Info

Publication number: CN112109268A
Application number: CN202010946933.5A
Authority: CN
Inventors: 王光阜; 张生; 宋红玮; 杨冲冲; 王仁鸿
Original assignee: Miracll Chemicals Co Ltd
Current assignee: Miracll Chemicals Co Ltd
Priority date: 2020-09-10
Filing date: 2020-09-10
Publication date: 2020-12-22
Anticipated expiration: 2040-09-10
Also published as: CN112109268B

Abstract

The invention discloses a preparation method of TPU (thermoplastic polyurethane) foamed beads and a product thereof. The preparation method comprises the following steps: (1) respectively metering TPU particles and water, continuously adding the metered TPU particles and water into a reaction kettle, and stirring to form uniform suspension; (2) respectively metering the suspension and the foaming agent, and respectively conveying the suspension and the foaming agent to a mixing position through a pump to form a mixture; (3) conveying the mixture to heat exchangeHeating to foaming temperature; (4) conveying the mixture reaching the foaming temperature into a multi-stage impregnation container through a pipeline, gradually impregnating the foaming agent into TPU particles, and finally reaching the dissolution balance; (5) and continuously discharging the TPU particles with balanced foaming agent dissolution into a receiving tank through a pore plate at the tail end of a pipeline connected with the outlet of the multistage impregnation container for foaming to obtain the TPU foamed beads. The density of the TPU foamed beads is 0.1-0.3 g/cm³The difference between the maximum dimension and the minimum dimension of the cells is not more than 20 μm.

Description

Preparation method of TPU (thermoplastic polyurethane) foamed beads and product thereof

Technical Field

The invention relates to the technical field of thermoplastic polyurethane elastomer foaming materials, in particular to a preparation method of TPU foaming beads and a product thereof.

Background

The polymer foam material generally uses plastic, rubber or thermoplastic elastomer as a matrix, forms a large amount of bubbles in the matrix material through a physical or chemical method, has a series of advantages of low density, heat insulation and sound insulation, high specific strength, buffering and the like, and therefore has wide application in the fields of packaging industry, agriculture, transportation industry, military industry, aerospace industry, daily necessities and the like. The thermoplastic polyurethane elastomer (TPU for short) has the advantages of wide hardness range, excellent wear resistance, mechanical strength, water resistance, oil resistance, chemical corrosion resistance, mold resistance, environmental friendliness, recyclability and the like. The molded foam article can be obtained by filling the TPU expanded beads into a mold and performing heat molding using water vapor. The product has excellent rebound resilience, shape diversity and low density, and can be used in a wide temperature range while maintaining the excellent performance of the original base material. Based on the above advantages, TPU molded foam products have very wide application prospects in many industrial fields (such as automobiles and packaging materials) and daily life fields (such as shoe materials, pillows and mattresses).

The preparation technology of TPU expanded beads is disclosed in CN101370861A, CN1041194030A, TW201815922A, CN103951965A, the above technical solutions are all batch kettle pressure foaming processes, the overall technical route is to add TPU particles, water, dispersant, foaming agent and required additives into an autoclave, form suspension by stirring, then raise the temperature and pressure to impregnate the foaming agent into the TPU particles, and finally release the suspension to normal pressure environment through a discharge valve at the bottom of the autoclave for foaming to obtain TPU expanded beads, such technical route is conventionally adopted in the current industrial production, but has the following disadvantages: the pressure in the kettle is always reduced in the whole discharging process from the beginning of pressure relief discharging to the end of discharging, so that the pressure drop of the TPU particles discharged in the later stage is smaller than that of the TPU particles discharged in the earlier stage, and the problem of uneven density and cell size among the TPU foamed beads is caused. Although high-pressure gas such as nitrogen is introduced into the autoclave during the discharging process to maintain the pressure in the autoclave and increase the discharging speed, the effect of improving the stability of the TPU expanded bead quality due to the pressure drop difference is still not significant.

To solve the above problems, those skilled in the art have subsequently disclosed various improved technical solutions, including, for example, the following patent methods.

Patent publication No. CN104974370A discloses that thermoplastic polyurethane elastomer, melt viscosity modifier, filler and anti-aging agent are premixed, extruded, melted and granulated to obtain blended particles, then adding the blended particles into a high-pressure kettle, introducing high-pressure fluid, heating, carrying out medium-temperature high-pressure impregnation to reach dissolution balance, gradually relieving pressure, then introducing saturated steam and high-pressure air into the kettle for foaming to obtain high-foaming thermoplastic polyurethane beads, although the technical proposal does not directly discharge the blending particles with balanced impregnation and dissolution of the foaming agent into the normal pressure environment for foaming, the problem of uneven cell size of the blending particles caused by the difference of pressure drop during discharging is avoided, however, high-temperature steam and high-pressure air are required to be introduced into the kettle again for foaming in the follow-up process, so that the problems of complicated process and low production efficiency exist, and the stability of quality among different batches cannot be guaranteed. In addition, the volume of the pre-expanded beads can expand greatly in a short time, so that a reaction kettle with large volume and high pressure resistance is required, the equipment investment is high, and the production space is occupied.

The patent with publication number CN108359123A discloses that heating thermoplastic polymer particles in a spouted bed, then introducing carbon dioxide to boost pressure, maintaining the carbon dioxide circulation and the polymer particle spouting state, and finally performing pressure relief to obtain thermoplastic polymer expanded beads, although the foam pores of the expanded beads obtained by the technical scheme are compact and uniform, and are more uniform than the foam pores of the expanded beads prepared by an intermittent kettle pressure foaming process, because the expansion of the thermoplastic polymer beads is in the spouted bed, the volume of the expanded thermoplastic polymer beads is huge after the thermoplastic polymer beads are foamed in a short time, a spouted bed with huge volume is needed to contain the thermoplastic polymer expanded beads, so the equipment investment is high, and the production space is occupied; meanwhile, after foaming, the thermoplastic polymer foaming beads need to be transferred away from the spouted bed and cleaned, and feeding foaming production cannot be continuously carried out, so that the production efficiency is affected, and the stability of the quality of the polymer foaming material in different batches cannot be ensured.

The patent specification with publication number CN110027157A discloses a continuous preparation method of a foaming material, which comprises filling a foaming agent into a material to be foamed in a first reaction zone, conveying to a second reaction zone, applying a first pressure to the foaming material and heating to a foaming temperature, conveying the foaming material heated to the foaming temperature to a third reaction zone, and removing the first pressure to obtain a final foaming material. Similarly, the patent specification CN1103353A discloses a foaming process and system for continuously producing polyurethane foam boards, the prepared polyurethane foam boards are thermosetting, are not easy to degrade, and have the problem of environmental pollution, and the technical scheme is only suitable for producing single-sized products such as boards.

Disclosure of Invention

The invention aims to solve the technical problems of unstable density, uneven cell size, high equipment investment, large occupied space and low production efficiency of the TPU foamed beads in the prior production technology of the TPU foamed beads. The invention provides a preparation method of TPU expanded beads, which can be used for continuously preparing the TPU expanded beads, improving the density stability of the prepared TPU expanded beads, improving the size uniformity of foam holes, reducing the equipment investment, saving the space, improving the production efficiency and further preparing products with various shapes by a steam molding process.

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

a method for preparing TPU expanded beads comprises the following steps:

(1) respectively metering TPU particles and water, continuously adding the TPU particles and the water into a reaction kettle, and stirring to form uniform suspension;

(2) respectively metering the suspension and the foaming agent, and respectively conveying the suspension and the foaming agent to a mixing position through a pump to form a mixture;

(3) conveying the mixture to a heat exchanger, and heating to a foaming temperature;

(4) conveying the mixture reaching the foaming temperature into a multi-stage impregnation container through a pipeline, gradually impregnating the foaming agent into TPU particles, and finally reaching the dissolution balance;

(5) and continuously discharging the TPU particles with balanced foaming agent dissolution to a receiving tank through a pore plate at the tail end of a pipeline connected with the outlet of the multi-stage impregnation container for foaming, and drying to obtain the TPU foamed beads.

The distribution state of the TPU particles in the suspension in the step (1) can influence the heat transfer and the temperature distribution uniformity in the subsequent heating process, and further influence the density difference among the final TPU foaming beads. Preferably, in the step (1), the mass ratio of the TPU particles to the water is 1: 1-5.

When the mass ratio of the TPU particles to the water is more than 1:1, the temperature among the TPU particles is not uniform in the subsequent heating process, and the density among the final TPU foaming beads is different; when the mass ratio of TPU to water is less than 1:5, the yield of the TPU expanded beads finally obtained per unit production time is reduced, resulting in an increase in production costs.

Wherein the TPU granules can be added to the reaction kettle by a metering system, for example, the flow rate or mass of the TPU granules can be measured to precisely control the amount of the TPU granules added, preferably by using a loss-in-weight feeding system. The water can be metered and controlled through a flow meter or a water meter.

In the step (2), the foaming agent is preferably one or a mixture of carbon dioxide, nitrogen, propane, n-butane, isobutane, n-pentane and isopentane.

In step (2), the suspension may be specifically delivered by a positive displacement pump, for example, a reciprocating diaphragm pump, a reciprocating piston pump or a centrifugal pump may be selected, and the mass flow rate of the suspension may be controlled by a flow meter, a flow valve, a flow indicator controller and the rotation speed of the pump, so as to adjust the impregnation time of the foaming agent in the TPU particles in the later stage.

Tests have found that the foaming temperature is closely related to the melting point Tm of the TPU particles, and can affect the density, cells and the like of the finally obtained TPU foamed beads. Preferably, in the step (3), the foaming temperature is 10 to 50 ℃ lower than the melting point Tm of the TPU particles.

When the difference between the foaming temperature and the melting point Tm of the TPU particles is more than 50 ℃, the density of the finally obtained TPU foamed beads is higher or the TPU particles are not foamed; when the difference between the foaming temperature and the melting point Tm of the TPU particles is less than 10 ℃, the finally obtained TPU foamed beads are easy to have the problems of hole breakage or foam collapse.

The melting point of the TPU particles can be obtained by a Differential Scanning Calorimetry (DSC) test, which comprises the following specific test steps of heating 5-10mg of the TPU particles from-100 ℃ to 225 ℃ at a heating rate of 20 ℃/min, keeping the temperature for 2 minutes, then cooling from 225 ℃ to-100 ℃ at a speed of 10 ℃/min, finally heating from-100 ℃ to 225 ℃ at a speed of 10 ℃/min, and taking the melting peak temperature of the DSC curve obtained at the time of heating for the 2 nd time as the melting point of the TPU particles.

In step (4), the mixture has a first average flow rate in the pipe and a second average flow rate in the multistage impregnation vessel, and the second average flow rate is preferably less than the first average flow rate.

In the step (4), the dipping time is 1-30 minutes. The impregnation time can be controlled by adjusting the parameters of the mass flow rate of the suspension, the mass flow rate of the foaming agent, the inner diameter of the pipeline, the length, the diameter and the number of stages of the multistage impregnation container and the like. When the dipping time is less than 1 minute, the foaming agent is not uniformly dissolved in the TPU granules, so that the expansion effect of the TPU granules is poor; when the dipping time is more than 30 minutes, the TPU particles are easy to degrade, the melt strength is reduced, the cells of the TPU foamed beads are broken, the foamed beads are contracted, and the density is higher.

In a preferred example, in step (4), the multistage impregnation vessel comprises a plurality of jacketed pipes which are arranged in series and horizontally. Wherein the number of the stages of the multistage impregnation vessel is determined by the number of the jacketed pipes, and preferably the number of the stages of the multistage impregnation vessel is 2-6. In order to ensure the temperature stability of the mixture in the process of dipping in the multistage dipping container, the jacketed pipe is preferably provided with an insulating layer, such as a polyurethane foam insulating layer or a jacketed heat conducting oil insulating layer. In addition, the average flow velocity of the mixture can be adjusted by controlling the inner diameter of the pipeline and the inner diameter of the jacketed pipeline of the multistage impregnation container, the inner diameter of the pipeline is selected to be smaller than the inner diameter of the jacketed pipeline of the multistage impregnation container, the flow velocity of the mixture in the pipeline can be increased, the turbulent flow state is kept, the inner diameter of the jacketed pipeline of the multistage impregnation container is large, the impregnation time of TPU particles and the foaming agent can be prolonged, and the impregnation is more uniform.

Further preferably, the flow velocity of the water in the jacketed pipe of the multistage impregnation vessel is greater than or equal to the free flow velocity of the TPU particles.

Preferably, the flow pattern of the suspension and the mixture is turbulent at all times.

In order to ensure the uniform dispersion of TPU particles in suspension and mixture in water, the suspension and mixture need to maintain turbulent flow state in the whole pipeline system, and Reynolds parameters Re > 4000 can be ensured by adjusting the flow rate, the suspension density, the density and the diameter of the pipeline. When the flow pattern is changed from turbulent flow to laminar flow, the flow velocity of the TPU particles in the pipe decreases and they accumulate along the pipe wall, causing pipe clogging, the concentration of the TPU particles in water decreases, and the liquid phase consisting of water and blowing agent tends to flow along the middle of the pipe, which results in insufficient contact time between the blowing agent and the TPU particles, and finally, the TPU particles expand poorly.

Preferably, the TPU particles are one or a mixture of polyester type, polyether type, polycaprolactone type and polycarbonate type.

Preferably, the TPU particles have a Shore hardness of 70-95A and a melting point of 110-200 ℃. Wherein the Shore hardness is measured by ASTM D2240.

When the Shore hardness of the TPU particles is lower than 70A, the finally prepared foam material has low supporting strength and is easy to shrink; when the shore hardness of the TPU particles is higher than 95A, the softness and feel of the final foam is poor.

When the melting point of the TPU particles is less than 110 ℃, the heat resistance of the finally prepared foam is deteriorated; when the melting point of the TPU particles is higher than 200 ℃, higher-pressure steam is needed during steam molding, the sintering strength among the TPU foamed beads is low, and the production cost is high.

In the step (5), the pressure before the orifice plate influences the expansion ratio, the density and the like of the prepared TPU expanded beads. Preferably, in step (5), the pressure in front of the orifice plate is maintained at 30-200 bar.

When the pressure in front of the orifice plate is lower than 30bar, the prepared TPU foamed beads have low expansion ratio and high density; when the pressure in front of the perforated plate is higher than 200bar, it is easy to cause excessive expansion and, on the contrary, shrinkage problems when the TPU particles are foamed.

In a preferred embodiment, in step (5), the orifice plate is in the form of a nozzle or venturi tube, and the orifice size is larger than the size of the TPU particles. The orifice of the orifice plate can be round, oval or polygonal, the size of the orifice plate can be designed to be small enough to ensure that the pressure in front of the orifice plate is kept between 30 and 200bar, and the size of the orifice needs to be 10 to 50 percent larger than the size of TPU particles to prevent the TPU particles from blocking the orifice plate in the discharging process. For example, when the inner diameter of the pipe connected to the orifice plate is 2 inches, the orifice size of the orifice plate is preferably 3/8 inches.

In the step (5), the drying device can be a boiling fluidized bed or a drying bin, and the drying temperature is preferably 50-80 ℃.

The invention also provides the TPU expanded bead prepared by the preparation method.

Preferably, the density of the TPU expanded beads is 0.1 to 0.3g/cm³The difference between the maximum dimension and the minimum dimension of the cells is not more than 20 μm.

The invention also provides a molded foam product prepared from the TPU expanded beads, and the molded foam product has more excellent tensile strength compared with a molded foam product prepared from the TPU expanded beads prepared by the traditional kettle pressure foaming process. The molded foam article is preferably prepared by steam molding.

Compared with the prior art, the invention has the main advantages that:

1. according to the technical scheme, the TPU particles, water and the foaming agent are continuously added, and continuous pressure relief discharging is carried out through conveying of the pump, so that pressure drops among the TPU particles in different time periods in the whole discharging process are consistent, and the problem that the density and the cell size are uneven due to the difference of the pressure drops in the traditional kettle pressure foaming process is solved.

2. The relative amount of TPU expanded beads discharged into the receiving tank in the same time is reduced by the technical scheme of the invention, so the volumes of the receiving tank and the subsequent drying equipment are reduced, and the equipment investment is reduced.

3. In the traditional kettle pressure foaming process, TPU particles, water, a foaming agent or an optional auxiliary agent are all added into a high-pressure kettle, the temperature rise in the high-pressure kettle or the impregnation process of the foaming agent has the nonuniformity of liquid-solid heat transfer and gas-solid mass transfer.

4. The technical scheme of the invention improves the uniformity of the density and the cell size of the prepared TPU foamed bead, can be used for continuous industrial production, and has high production efficiency and reduced equipment investment and production cost.

5. In the process of steam molding of the TPU expanded beads, on one hand, the surface of the expanded beads is softened by heating with steam, on the other hand, the air in the pores of the expanded beads is heated to expand the volume of the expanded beads, the expanded beads continuously fill the gaps of the expanded beads and are contacted with each other, and finally, the expanded beads are cooled to form interface sintering among the expanded beads. Because the density and the cell size of the TPU expanded beads prepared by the technical scheme of the invention are more uniform, the expanded beads are more uniformly expanded in the steam molding process, so that the defects of gaps among the expanded beads are less, the welding area among the expanded beads is more, and the molded foam product prepared from the TPU expanded beads prepared by the traditional kettle pressure foaming process has more excellent tensile strength.

Drawings

FIG. 1 is a process flow and system schematic of the process for making the TPU expanded beads of this invention;

in the figure: the method comprises the following steps of 1, 2, 3, 4, 7, 5, 16, 10 and 13, 6, 8, a three-way mixing valve, 9, a heat exchanger, 11, a multistage impregnation container, 11-1, a jacket pipeline, 12, an orifice plate, 14, a pressure gauge, 15, a thermometer, 17, a receiving tank, 18, a flow meter, 19, a flow valve and 20, wherein the TPU particles are TPU particles, the water is 2, the reaction kettle is 3, the pumps are 4, the pumps are 7, the pipelines are 13, the foaming agent is 6, the three-way;

FIG. 2 is an SEM photograph of the cell structure of the TPU expanded beads prepared in example 1 of the present invention;

FIG. 3 is an SEM photograph of the cell structure of the TPU expanded beads prepared in comparative example 1 of the present invention;

FIG. 4 is a photograph of a cross section of a molded foam article prepared in example 1 of the present invention;

FIG. 5 is a photograph of a cross section of a molded foam article prepared in comparative example 1 of the present invention.

Detailed Description

The invention is further described with reference to the following drawings and specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The following examples are conducted under conditions not specified, usually according to conventional conditions, or according to conditions recommended by the manufacturer.

The process flow and the system of the preparation method of the TPU expanded beads of the invention are shown in figure 1 and comprise the following steps:

(1) respectively metering TPU particles 1 and water 2, continuously adding the metered TPU particles and the water into a reaction kettle 3, and stirring to form uniform suspension;

(2) the suspension and the foaming agent 6 are respectively metered and respectively conveyed to a three-way mixing valve 8 through

pumps

4 and 7 in pipelines 5 and 16 to form a mixture; the suspension and the foaming agent 6 are respectively metered and controlled by a flow meter 18, a flow valve 19 and a flow indicator controller 20;

(3) conveying the mixture to a heat exchanger 9, and heating to a foaming temperature;

(4) conveying the mixture reaching the foaming temperature into a multi-stage impregnation container 11 through a pipeline 10, gradually impregnating the foaming agent into TPU particles, and finally reaching the dissolution balance;

the multistage impregnation vessel 11 comprises a plurality of jacketed pipes 11-1 which are connected in series and horizontally; the inner diameters of the

pipelines

10 and 13 are smaller than the inner diameter of the jacket pipeline 11-1;

in the jacketed pipe 11-1, the flow velocity of the water 2 is greater than or equal to the free flow velocity of the TPU particles 1;

(5) continuously discharging TPU particles with balanced foaming agent dissolution to a receiving tank 17 through a pore plate 12 at the tail end of a pipeline 13 connected with the outlet of a multi-stage impregnation container for foaming, and finally drying to obtain TPU foamed beads; the pressure and the temperature at the front end of the pore plate 12 in the pipeline 13 are monitored through a pressure gauge 14 and a temperature gauge 15; the orifice plate 12 takes the form of a nozzle or venturi tube with an orifice size larger than the size of the TPU particles 1.

In the above process flow and system, the flow pattern of the suspension and the mixture is always turbulent.

The following examples all employ the above process flow and system.

Example 1

A preparation method of TPU expanded beads comprises the following steps:

(1) respectively metering polyether type TPU (thermoplastic polyurethane) particles 1 and water 2 with Shore hardness of 75A, Tm of 160 ℃ by a weight loss feeding system and a liquid flowmeter, controlling the mass ratio of the polyether type TPU particles 1 to the water 2 to be 1:1, continuously adding the mixture into a 200L reaction kettle 3, and setting the stirring speed of the reaction kettle to be 45Hz to form uniform suspension of the TPU particles and the water;

(2) the suspension is conveyed to a three-way mixing valve 8 through a pipeline 5 by a pump 4 according to the mass flow rate of 150g/s, and simultaneously, carbon dioxide 6 serving as a foaming agent is conveyed to the three-way mixing valve 8 through a pump 7 according to the mass flow rate of 15g/s and is mixed with the suspension to form a mixture;

(3) conveying the mixture to a heat exchanger 9 to heat the mixture to 110 ℃ to reach a set foaming temperature;

(4) the mixture brought to the foaming temperature is conveyed to a four-stage impregnation vessel 11 through a pipeline 10, so that the impregnation time of the mixture in the four-stage impregnation vessel is 20 minutes, and carbon dioxide is dissolved and balanced in the TPU particles;

(5) the TPU particles having carbon dioxide brought into a dissolution equilibrium were passed through an orifice 12 having an orifice diameter of 3/8 inches at the end of a pipe 13 having a diameter of 2 inches, the pressure before the orifice 12 was controlled to 75bar, and finally discharged into a receiving tank 17 to finally obtain TPU expanded beads. The density of the TPU expanded beads obtained was tested to be 0.185g/cm³The cell size is 30-50 μm, and the SEM photograph of the cross-section cell structure of the obtained TPU expanded beads is shown in FIG. 2.

The TPU expanded beads obtained in the above manner were molded into a molded foam article of 200 mm. times.200 mm. times.20 mm by using an ETPU steam molding machine, the test tensile strength was 1.8MPa, and the photograph of the cross section of the molded foam article thus prepared is shown in FIG. 4, where the number of void defects in the tested cross section was 1.

Example 2

A preparation method of TPU expanded beads comprises the following steps:

(1) respectively metering polyester type TPU (thermoplastic polyurethane) particles 1 and water 2 with Shore hardness of 95A, Tm of 200 ℃ by a weight loss feeding system and a liquid flow meter, controlling the mass ratio of the polyester type TPU particles 1 to the water 2 to be 1:5, continuously adding the polyester type TPU particles 1 to the water 2 into a 200L reaction kettle 3, and setting the stirring speed of the reaction kettle to be 45Hz to enable the TPU particles and the water to form uniform suspension;

(2) the suspension is conveyed to a three-way mixing valve 8 through a pipeline 5 by a pump 4 according to the mass flow rate of 1500g/s, and meanwhile, foaming agent nitrogen 6 is conveyed to the three-way mixing valve 8 through a pump 7 according to the mass flow rate of 125g/s and is mixed with the suspension to form a mixture;

(3) conveying the mixture to a heat exchanger 9 to heat the mixture to 180 ℃ to reach a set foaming temperature;

(4) the mixture brought to the foaming temperature is conveyed to a six-stage impregnation vessel 11 through a pipeline 10, so that the impregnation time of the mixture in the six-stage impregnation vessel is 30 minutes, and nitrogen is dissolved and balanced in TPU particles;

(5) the TPU particles having nitrogen brought into a dissolution equilibrium were passed through an orifice 12 having an orifice diameter of 3/8 inches at the end of a pipe 13 having a diameter of 2 inches, the pressure before the orifice 12 was controlled to 200bar, and finally discharged into a receiving tank 17 to finally obtain TPU expanded beads. The density of the TPU expanded beads obtained was found to be 0.1g/cm³The size of the foam hole is 50-70 μm.

The TPU expanded beads obtained above were produced into molded foam articles of 200mm by 20mm by using an ETPU steam molding machine, and the tensile strength was measured to be 1.5 MPa.

Example 3

A preparation method of TPU expanded beads comprises the following steps:

(1) respectively metering polyether type TPU (thermoplastic polyurethane) particles 1 and water 2 with Shore hardness of 85A, Tm of 165 ℃ by a weight loss feeding system and a liquid flowmeter, controlling the mass ratio of the polyether type TPU particles 1 to the water 2 to be 1:2, continuously adding the mixture into a 200L reaction kettle 3, and setting the stirring speed of the reaction kettle to be 45Hz to enable the TPU particles and the water to form uniform suspension;

(2) the suspension is conveyed to a three-way mixing valve 8 through a pipeline 5 by a pump 4 according to the mass flow rate of 900g/s, and simultaneously, carbon dioxide 6 serving as a foaming agent is conveyed to the three-way mixing valve 8 through a pump 7 according to the mass flow rate of 90g/s and is mixed with the suspension to form a mixture;

(3) conveying the mixture to a heat exchanger 9 to heat the mixture to 125 ℃ to reach a set foaming temperature;

(4) the mixture reaching the foaming temperature is conveyed into a third-stage impregnation container 11 through a pipeline 10, so that the impregnation time of the mixture in the third-stage impregnation container is 15 minutes, and carbon dioxide is dissolved and balanced in TPU particles;

(5) the TPU particles having carbon dioxide brought into a dissolution equilibrium were passed through an orifice 12 having an orifice diameter of 3/8 inches at the end of a pipe 13 having a diameter of 2 inches, the pressure before the orifice 12 was controlled to 95bar, and finally discharged into a receiving tank 17 to finally obtain TPU expanded beads. The density of the TPU expanded beads obtained was tested to be 0.155g/cm³The size of the foam pores is 45-60 mu m.

The TPU expanded beads obtained above were produced into molded foam articles of 200mm by 20mm by using an ETPU steam molding machine, and the tensile strength was measured to be 1.7 MPa.

Example 4

A preparation method of TPU expanded beads comprises the following steps:

(1) measuring polycaprolactone type TPU (thermoplastic polyurethane) particles 1 with Shore hardness of 80A, Tm of 175 ℃ and water 2 by a weight loss feeding system and a liquid flow meter respectively, controlling the mass ratio of the polyether type TPU particles 1 to the water 2 to be 1:5, continuously adding the mixture into a 200L reaction kettle 3, and setting the stirring speed of the reaction kettle to be 45Hz to enable the TPU particles and the water to form uniform suspension;

(2) the suspension is conveyed to a three-way mixing valve 8 through a pipeline 5 by a pump 4 according to the mass flow rate of 150g/s, and a foaming agent n-butane 6 is conveyed to the three-way mixing valve 8 through a pump 7 according to the mass flow rate of 10g/s and is mixed with the suspension to form a mixture;

(3) conveying the mixture to a heat exchanger 9 to heat the mixture to 155 ℃ to reach the set foaming temperature;

(4) the mixture brought to the foaming temperature is transferred through a pipe 10 to a five-stage impregnation vessel 11, so that the impregnation time in the five-stage impregnation vessel is 25 minutes, and the n-butane is in equilibrium dissolved inside the TPU granules;

(5) the TPU particles with n-butane in equilibrium of dissolution were passed through an orifice 12 having an orifice diameter of 3/8 inches at the end of a pipe 13 having a diameter of 2 inches, the pressure before the orifice 12 was controlled to 50bar, and finally discharged into a receiving tank 17 to finally obtain TPU expanded beads. The density of the TPU expanded beads obtained was found to be 0.165g/cm³The size of the foam hole is 30-35 μm.

The TPU expanded beads obtained above were produced into molded foam articles of 200mm by 20mm by using an ETPU steam molding machine, and the tensile strength was measured to be 1.6 MPa.

Example 5

A preparation method of TPU expanded beads comprises the following steps:

(1) polycarbonate type TPU (thermoplastic polyurethane) particles 1 with Shore hardness of 90A, Tm of 195 ℃ and water 2 are respectively metered by a weightlessness feeding system and a liquid flow meter, the mass ratio of the polycarbonate type TPU particles 1 to the water 2 is controlled to be 1:1, then the polycarbonate type TPU particles and the water 2 are continuously added into a 200L reaction kettle 3, and the stirring rotating speed of the reaction kettle is set to be 45Hz, so that the TPU particles and the water form uniform suspension;

(2) the suspension is conveyed to a three-way mixing valve 8 through a pipeline 5 by a pump 4 according to the mass flow rate of 1500g/s, and meanwhile, the blowing agent n-pentane 6 is conveyed to the three-way mixing valve 8 through a pump 7 according to the mass flow rate of 375g/s and is mixed with the suspension to form a mixture;

(3) conveying the mixture to a heat exchanger 9 to heat the mixture to 165 ℃ to reach the set foaming temperature;

(4) the mixture reaching the foaming temperature is conveyed to a secondary impregnation container 11 through a pipeline 10, so that the impregnation time of the mixture in the secondary impregnation container is 1 minute, and n-pentane is dissolved and balanced in TPU particles;

(5) the TPU particles with n-pentane in equilibrium were passed through an orifice 12 having an opening diameter of 3/8 inches at the end of a 2-inch diameter pipe 13, the pressure before the orifice 12 was controlled at 30bar, and finally discharged into a receiving tank 17, to finally obtain TPU expanded beads. The density of the TPU expanded beads obtained was tested to be 0.3g/cm³Foam size of 35 ℃50μm。

The TPU expanded beads obtained above were produced into molded foam articles of 200mm by 20mm by using an ETPU steam molding machine, and the tensile strength was measured to be 2.5 MPa.

Example 6

A preparation method of TPU expanded beads comprises the following steps:

(1) respectively metering polyether type TPU (thermoplastic polyurethane) particles 1 and water 2 with Shore hardness of 85A, Tm of 145 ℃ by a weight loss feeding system and a liquid flowmeter, controlling the mass ratio of the polyether type TPU particles 1 to the water 2 to be 1:1, continuously adding the mixture into a 200L reaction kettle 3, and setting the stirring speed of the reaction kettle to be 45Hz to enable the TPU particles and the water to form uniform suspension;

(2) the suspension is conveyed to a three-way mixing valve 8 through a pipeline 5 by a pump 4 according to the mass flow rate of 800g/s, and simultaneously foaming agent carbon dioxide 6 is conveyed to the three-way mixing valve 8 through a pump 7 according to the mass flow rate of 200g/s and is mixed with the suspension to form a mixture;

(5) the TPU particles having carbon dioxide brought into a dissolution equilibrium were passed through an orifice 12 having an orifice diameter of 3/8 inches at the end of a pipe 13 having a diameter of 2 inches, the pressure before the orifice 12 was controlled to 75bar, and finally discharged into a receiving tank 17 to finally obtain TPU expanded beads. The density of the TPU expanded beads obtained was tested to be 0.13g/cm³The size of the foam hole is 40-50 μm.

Comparative example 1

100kg of the same TPU granules as in example 1 are introduced into a batch autoclave process100kg of water is added into a 500L high pressure kettle, the stirring speed is set to be 45Hz, so that the components form uniform suspension, then the high pressure kettle is heated to 110 ℃ and carbon dioxide is introduced into the high pressure kettle, so that the pressure in the high pressure kettle reaches 100bar, after the temperature and the pressure in the high pressure kettle are kept constant for 20 minutes, the suspension in the high pressure kettle is quickly discharged into the normal pressure environment through a discharge valve at the bottom of the high pressure kettle, high pressure nitrogen is introduced into the high pressure kettle in the discharging process, the pressure in the kettle is kept constant at 100bar in the discharging process, and finally the TPU foamed beads are obtained. The density of the TPU expanded beads obtained was tested to be 0.212g/cm³The size of the foam hole is 30-80 μm.

Using an ETPU steam forming machine, the TPU expanded beads obtained above were produced into molded foam articles of 200 mm. times.200 mm. times.20 mm in tensile strength of 1.2MPa in the same manner as in example 1. The photograph of the cross section of the molded foam article thus prepared is shown in FIG. 5, and the number of void defects in the cross section tested was 5.

The characterization test method comprises the following steps:

1. the density of the TPU expanded beads is tested according to ASTM D792-2008;

2. the cell size of the TPU expanded beads is quenched by liquid nitrogen, and the cell structure is tested and analyzed by using a Scanning Electron Microscope (SEM);

3. the tensile strength is tested according to GB/T6344-2008 standard;

4. void defects were tested as follows: a cross section having a length of 10mm was cut out in the length or width direction of the molded foam article using scissors, and the number of holes having a gap between expanded beads of more than 0.5mm in the cross section was counted.

Furthermore, it should be understood that various changes and modifications can be made by one skilled in the art after reading the above description of the present invention, and equivalents also fall within the scope of the invention as defined by the appended claims.

Claims

1. A method for preparing TPU expanded beads is characterized by comprising the following steps:

2. The preparation method according to claim 1, wherein in the step (1), the mass ratio of the TPU particles to the water is 1: 1-5.

3. The preparation method according to claim 1, wherein in the step (2), the foaming agent is one or a mixture of carbon dioxide, nitrogen, propane, n-butane, isobutane, n-pentane and isopentane.

4. The production method according to claim 1, wherein in the step (3), the foaming temperature is 10 to 50 ℃ lower than the melting point Tm of the TPU particles.

5. The method of claim 1, wherein in step (4), the mixture has a first average flow rate in the pipe, a second average flow rate in the multi-stage impregnation vessel, and the second average flow rate is less than the first average flow rate;

the dipping time is 1-30 minutes.

6. The process according to claim 1 or 5, wherein in step (4), the multistage impregnation vessel comprises a plurality of jacketed pipes arranged in series and horizontally.

7. The method of claim 6, wherein the jacketed pipe is provided with an insulating layer.

8. The process according to claim 6, wherein the flow rate of water in the jacketed pipes of the multistage impregnation vessel is greater than or equal to the free flow rate of the TPU particles.

9. The method of claim 1, wherein the suspension and the mixture are in a turbulent flow form at all times.

10. The preparation method according to claim 1, wherein the TPU particles are one or more of polyester type, polyether type, polycaprolactone type and polycarbonate type, and have Shore hardness of 70-95A and melting point of 110-200 ℃.

11. The method according to claim 1, wherein in the step (5), the pressure before the orifice plate is maintained at 30 to 200 bar.

12. The process according to claim 1 or 11, wherein in step (5), the orifice plate is in the form of a nozzle or venturi tube, the orifice size being larger than the size of the TPU particles.

13. The TPU expanded beads prepared by the preparation method according to any one of claims 1 to 12, wherein the density of the TPU expanded beads is 0.1 to 0.3g/cm³The difference between the maximum dimension and the minimum dimension of the cells is not more than 20 μm.

14. A molded foam article prepared from the TPU expanded beads of claim 13.