JP6346919B2 - Method for producing thermoplastic polyurethane expanded particles - Google Patents

Description

  The present invention relates to a method for producing foamed particles of thermoplastic polyurethane (hereinafter, sometimes abbreviated as TPU).

  TPU is a kind of thermoplastic elastomer, but exhibits properties similar to vulcanized rubber, and is excellent in wear resistance, cold resistance, and resilience. In addition, since it has high mechanical strength, it is positioned as an engineering elastomer and is used in various applications such as cushioning materials, vibration-proof materials, sporting goods, and automobile members.

Since the foamed molded body obtained by foaming this TPU can achieve weight reduction and flexibility while maintaining excellent properties such as wear resistance and impact resilience, it will be further used in sporting goods, automotive parts, etc. in the future. Application expansion is expected.
However, the manufacturing method of TPU expanded particles is still under development and has not been established.
As a manufacturing method of TPU expanded particle, it describes in patent documents 1 and 2, for example.
Patent Document 1 describes a method of producing TPU foamed particles by foaming TPU particles impregnated with carbon dioxide in a supercritical state. Specifically, first, TPU particles and water are added to the reaction kettle, and carbon dioxide is further added. Next, after controlling the temperature and pressure in the reaction kettle to bring carbon dioxide into a supercritical state, the temperature in the reaction kettle is raised to 90 to 140 ° C. and maintained at that temperature. Then, TPU particles impregnated with carbon dioxide in a supercritical state are released into the pressure tank held from the inside of the reaction kettle, the temperature is lowered to 70 ° C. or lower, and first-stage foaming is performed to obtain TPU primary foamed particles. . Thereafter, a method of producing TPU expanded particles by performing second-stage normal pressure expansion on the TPU primary expanded particles is described.

  In Patent Document 2, TPU particles mixed with 0.1 to 5 phr of cell nucleating agent are added to a pressure vessel together with a blowing agent and water, and the inside of the vessel is set to 110 to 135 ° C. and 10 to 25 bar (1.0 to 2). And a method of producing TPU foamed particles by foaming the TPU particles impregnated with the foaming agent by releasing the contents in the container to the atmosphere after the temperature is raised to 5 MPa) Has been.

Chinese Patent Publication No. 104130430 Chinese Patent Publication No. 104231592

However, when carbon dioxide in a supercritical state is used as the foaming agent as in the cited document 1, the amount of carbon dioxide impregnated into the TPU particles becomes excessively large. Therefore, when it is going to manufacture 100-300 kg / m ³ TPU foam particles in one step using carbon dioxide in a supercritical state, it is necessary to considerably reduce the temperature at the time of foaming. Residual stress tends to remain on the particles. Furthermore, since the difference between the pressure in the container and the pressure in the discharge atmosphere becomes too large, the foaming speed of the TPU particles becomes too fast, and as a result, the crystals are easily oriented in the TPU foamed particles. Therefore, in order to obtain such a good molded body by molding such TPU foamed particles in the mold, there arises a problem that the pressure of the forming steam has to be considerably increased. In addition, if the pressure difference during foaming is too large, not only will the foaming speed be too high, but the bubbles in the TPU foamed particles will tend to become finer, resulting in a decrease in in-mold moldability and the resulting TPU. There also arises a problem that physical properties such as compression set of the foamed particle molded body are lowered. For this reason, in Cited Document 1, TPU particles are foamed under a certain pressure to produce TPU foam particles with a low expansion ratio, and then foamed in a separate process, followed by two-stage foaming (two-stage foaming). However, such a two-stage foaming method has a problem that it is inferior in productivity and a problem that the uniformity of bubbles of the TPU foamed particles is lowered.

On the other hand, when the carbon dioxide is used as the foaming agent when producing the TPU foamed particles with the pressure in the pressure vessel set to 2.5 MPa or less as in the cited document 2, the amount of carbon dioxide impregnated into the TPU particles is small. Too much. Therefore, when preparing TPU expanded particles of 100 to 300 kg / m ³ , it is necessary to increase the expansion ratio by increasing the temperature during expansion, and the TPU raw material has an extremely small molecular weight (extremely high melt flow rate). It is necessary to improve the foaming ratio by using. As a result, hydrolysis of TPU is promoted, and only TPU foam particles having poor surface properties can be obtained, and mechanical properties of the obtained TPU foam particle molded body are lowered. Furthermore, the technique of the cited document 2 has a problem that the bubbles of the obtained TPU expanded particles are likely to be non-uniform.

  The present invention has been made in order to solve the above-described problems, and an object of the present invention is to provide a method for producing TPU foamed particles, which is capable of obtaining a molded article having excellent in-mold moldability and excellent surface properties. .

That is, the present invention provides the following [1] to [9].
[1] A thermoplastic polyurethane particle dispersed in an aqueous medium in a sealed container is impregnated with a foaming agent under heating, and the thermoplastic polyurethane particles containing the foaming agent are discharged together with the aqueous dispersion medium from the sealed container and foamed. A method for producing thermoplastic polyurethane foam particles having an apparent density of 100 to 300 kg / m ³ , comprising:
The foaming agent is a physical foaming agent mainly composed of carbon dioxide,
The thermoplastic polyurethane particles contain 300 to 2000 mass ppm of inorganic powder,
The method for producing thermoplastic polyurethane foam particles, wherein the pressure in the container at the time of release is more than 2.5 MPa (G) and not more than 4.0 MPa (G).
[2] The method for producing thermoplastic polyurethane foam particles according to [1], wherein the inorganic powder is talc and the talc has a 50% volume average particle diameter of 1 to 15 μm.
[3] The temperature of the aqueous medium when impregnated with the foaming agent is 80 ° C. or higher and the melting temperature Tm−20 ° C. or lower of the thermoplastic polyurethane particles, and the pressure in the sealed container is 2.5 MPa (G ) To 7.0 MPa (G) or less, and the physical foaming agent is impregnated with thermoplastic polyurethane particles by press-fitting a physical foaming agent mainly composed of carbon dioxide into a sealed container. The method for producing thermoplastic polyurethane foam particles according to [1] or [2].
[4] The temperature of the aqueous medium at the time of release is not lower than the melting temperature Tm-60 ° C. of the thermoplastic polyurethane particles and not higher than the melting temperature Tm-20 ° C. of the thermoplastic polyurethane particles. 3] The process for producing thermoplastic polyurethane foam particles according to any one of [3].
[5] The method for producing thermoplastic polyurethane foam particles according to any one of [1] to [4], wherein the pressure in the container at the time of discharge is 2.6 to 3.4 MPa (G).
[6] The method for producing thermoplastic polyurethane foam particles according to any one of [1] to [5], wherein the thermoplastic polyurethane constituting the thermoplastic polyurethane particles is an ether-based thermoplastic polyurethane.
[7] The method for producing thermoplastic polyurethane foam particles according to any one of [1] to [6], wherein the thermoplastic polyurethane particles contain 300 to 950 mass ppm of inorganic powder.
[8] The thermoplastic polyurethane foam particles according to any one of [1] to [7], wherein the thermoplastic polyurethane particles have a melt flow rate of 10 to 50 g / 10 min at 190 ° C. and a load of 10 kg. Production method.
[9] The thermoplastic polyurethane foam particles according to any one of [1] to [8], wherein the type A durometer hardness of the thermoplastic polyurethane constituting the thermoplastic polyurethane particles is 90 or less. Production method.

  According to the method for producing TPU foamed particles of the present invention, foamed particles that are excellent in in-mold moldability and capable of obtaining a molded article having excellent surface properties are obtained.

[Thermoplastic polyurethane (TPU)]
The TPU constituting the TPU expanded particles produced in the present invention (hereinafter also simply referred to as expanded particles) is a hard segment obtained by polymerizing a diisocyanate and a chain extender (a diol compound such as a short-chain glycol) via a urethane bond, and an ether. A soft segment composed of a polymer chain containing a group, an ester group, a carbonate group or the like has a structure in which they are bonded to each other. In the normal temperature region, the soft segment exhibits elasticity, and the hard segment generates a strong hydrogen bond and acts as a physical crosslinking point, thereby exhibiting elasticity close to rubber.

  In TPU, the type of soft segment has a great influence on the characteristics of TPU. The ester-based TPU is particularly excellent in mechanical strength, heat resistance, and the like, while the ether-based TPU is particularly excellent in cold resistance, hydrolysis resistance, bacteria resistance, and the like. Therefore, the type of TPU to be used can be appropriately selected according to the characteristics required for the TPU expanded particle molded body.

  The constituent elements of the TPU are not particularly limited, and are appropriately selected according to physical properties required for a TPU foamed particle molded body (hereinafter also simply referred to as a foamed molded body) obtained by in-mold molding of foamed particles. can do. Since ether-based TPU is superior in hydrolysis resistance compared to ester-based TPU, by using ether-based TPU as a raw material, hydrolysis can be performed even in a foaming method using an aqueous medium as in the production method of the present invention. The molecular weight is not easily lowered by foaming, bubbles are hard to break during foaming, and a good cell structure is maintained, so that it is easy to obtain expanded particles having good moldability.

  The TPU may be mixed with other polymers such as polyolefin resins, polystyrene resins, styrene elastomers, etc. in the TPU within a range that does not impair the purpose of the present invention. Can also be used. In addition, it is preferable that the usage-amount of these other polymers is 30 mass parts or less with respect to 100 mass parts of TPU, More preferably, it is 20 mass parts or less, More preferably, it is 10 mass parts or less. It is particularly preferable that the expanded particles do not contain any polymer other than TPU.

Moreover, it is preferable that the TPU raw material used for manufacture of the said foaming particle is 140-170 degreeC of the melting temperature. If the melting temperature of TPU is within the above range, expanded particles with better moldability can be obtained. From the above viewpoint, the melting temperature is more preferably 150 to 170 ° C.
The melting temperature is based on JIS K7121-1987, and the test piece condition adjustment is “when the melting temperature is measured after performing a certain heat treatment” (the heating rate and the cooling rate in the test piece condition adjustment are both 10 This is a value obtained as a peak apex temperature of a melting peak of a DSC curve obtained by a heat flux differential scanning calorimetry method at a heating rate of 10 ° C./min. When the DSC curve has a plurality of melting peaks, the peak apex temperature of the melting peak with the highest temperature is adopted as the melting temperature.

The TPU preferably has a type A durometer hardness of 90 or less.
If the hardness is 90 or less, a good foamed molded article can be obtained without excessively increasing the steam pressure (molding pressure) during molding. On the other hand, if the hardness is too low, depending on the molding conditions and the shape of the foamed molded product, after the foamed molded product is released from the mold, the foamed molded product tends to shrink or deform so that a so-called sink is likely to occur. Therefore, the type A durometer hardness is preferably 70 to 90, more preferably 80 to 88.
The type A durometer hardness is a value measured based on JIS K6253-3: 2012.

[Method for producing TPU expanded particles]
In the method for producing foamed particles of the present invention, TPU particles dispersed in an aqueous medium in a sealed container are impregnated with a foaming agent under heating, and TPU particles containing the foaming agent are discharged from the sealed container together with the aqueous dispersion medium to foam. And producing a TPU foam particle having an apparent density of 100 to 300 kg / m ³ , wherein the foaming agent is a physical foaming agent mainly composed of carbon dioxide, and the TPU particle is an inorganic material having a concentration of 300 to 2000 ppm by mass. Including the powder, the pressure in the container at the time of discharge exceeds 2.5 MPa (G: gauge pressure) and is 4.0 MPa (G) or less.

In the present invention, the aqueous medium in which the TPU particles are dispersed is not particularly limited, but water or the like can be used.
The foaming agent impregnated into the TPU particles is a physical foaming agent mainly composed of carbon dioxide. By using carbon dioxide as a foaming agent and controlling the pressure in the pressure vessel so that the carbon dioxide does not enter a supercritical state, TPU foam particles can be obtained without excessively miniaturizing the bubbles.
In addition to carbon dioxide, other physical foaming agents and chemical foaming agents can be used in combination as the foaming agent.
Other physical blowing agents include aliphatic hydrocarbons such as propane, butane, hexane, pentane, heptane, chlorofluoromethane, trifluoromethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane. And organic physical foaming agents such as halogenated hydrocarbons such as methyl chloride, ethyl chloride and methylene chloride, and dialkyl ethers such as dimethyl ether and diethyl ether. Moreover, inorganic physical foaming agents, such as nitrogen, argon, air, and water, are mentioned.
In this case, the blending ratio of carbon dioxide in the foaming agent is preferably 50% by mass or more, more preferably 70% by mass or more, and further preferably 90% by mass or more.

When carbon dioxide is used as the foaming agent, carbon dioxide is contained in the sealed container so that the pressure in the sealed container is 7.0 MPa (G) or less from the viewpoint of not excessively reducing the bubble diameter of the obtained foamed particles. It is preferable to press-fit, that is, the impregnation pressure is 7.0 MPa (G) or less, more preferably 5.0 MPa (G) or less, and still more preferably 4.0 MPa (G) or less. Furthermore, it is particularly preferable that the impregnation pressure is 3.4 MPa (G) or less from the viewpoint that the pressure at the time of foaming described later can be easily controlled. On the other hand, from the viewpoint of sufficiently impregnating the TPU particles with the foaming agent, the impregnation pressure is preferably 0.5 MPa (G) or more, more preferably 1.0 MPa (G) or more. From the viewpoint of easy control of the pressure at the time, it is more preferably more than 2.5 MPa (G), particularly preferably 2.6 MPa (G) or more.
Further, from the viewpoint of impregnation of the foaming agent into the TPU particles, the impregnation is preferably performed under heating (impregnation temperature). The impregnation temperature is preferably 20 ° C. or higher and TPU particle melting temperature Tm ° C. or lower, more preferably 80 ° C. or higher (Tm−20) ° C. or lower. Further, the impregnation time is appropriately set according to the pressure in the sealed container, the type and mass of the TPU particles, etc., but is set as a time during which the TPU particles can be sufficiently impregnated with the foaming agent. Therefore, it is preferably 0.05 to 3 hours, more preferably 0.1 to 1 hour.

In particular, the temperature of the aqueous medium is set to 80 ° C. or higher and the TPU particle melting temperature Tm−20 ° C. or lower, and the pressure in the sealed container exceeds 2.5 MPa (G) and 7.0 MPa (G ) It is preferable to impregnate the TPU particles with the physical foaming agent by press-fitting a physical foaming agent mainly composed of carbon dioxide into the hermetic container until the following is achieved.
Note that the melting temperature is “when the melting temperature is measured after performing a certain heat treatment” as the condition adjustment of the test piece based on JIS K7121-1987 (the heating rate and the cooling rate in the condition adjustment of the test piece are These are values obtained as the peak apex temperature of the melting peak of the DSC curve obtained at a heating rate of 10 ° C./min by a heat flux differential scanning calorimetry method. When the DSC curve has a plurality of melting peaks, the peak apex temperature of the melting peak with the highest temperature is adopted as the melting temperature.

In the present invention, TPU particles contain 300 to 2000 ppm by mass of inorganic powder. If the content of the inorganic powder in the TPU particles is too small, wrinkles are likely to occur on the surface of the expanded particles, and the bubbles are likely to be non-uniform, which may make it impossible to form a good molded product. From this viewpoint, the lower limit of the content of the inorganic powder in the TPU particles is preferably 400 mass ppm. Moreover, when there is too much content of inorganic powder, there exists a possibility that the bubble of an expanded particle may become too fine and the foaming speed may become too fast, or in-mold shaping | molding may fall. From this viewpoint, the upper limit of the content of the inorganic powder in the TPU particles is preferably 1500 ppm by mass, more preferably 1000 ppm by mass, and even more preferably 950 ppm by mass.
Although it does not specifically limit as this inorganic powder, It is preferable to use a talc, and it is more preferable to use a talc whose 50% volume average particle diameter (d50) is 1-15 micrometers. If d50 of talc is 1 μm or more, the bubble diameter of the foamed particles will not be excessively refined, so that foamed particles having excellent in-mold moldability can be obtained, and the obtained molded foam can be permanently compressed. There is no deterioration of physical properties such as strain. On the other hand, if d50 of talc is 15 μm or less, bubbles are not excessively coarsened, and thus expanded particles having excellent in-mold moldability can be obtained.

In the production method of the present invention, the pressure in the container (foaming pressure) at the time of releasing the TPU particles needs to be more than 2.5 MPa (G) and 4.0 MPa (G) or less.
When the foaming pressure is 2.5 MPa (G) or less, wrinkles are generated on the surface of the foamed particles, and the bubble diameter is not uniform.
When the foaming pressure exceeds 4.0 MPa (G), the foaming speed of the TPU particles becomes too high, and as a result, the TPU crystals are easily oriented in the foamed particles. Therefore, in order to obtain such a molded article by molding such expanded particles in the mold, the pressure of the forming steam must be increased.
From such a viewpoint, the foaming pressure is preferably 2.6 to 3.4 MPa (G).

  In the production method of the present invention, the temperature (foaming temperature) of the aqueous medium at the time of release is preferably (Tm-60) ° C. or higher and (Tm-20) ° C. or lower. By setting the foaming temperature to a range of (Tm-60) ° C. or higher and (Tm-20) ° C. or lower, foamed particles having better in-mold moldability can be obtained. From this viewpoint, the temperature (foaming temperature) of the aqueous medium at the time of release is more preferably (Tm-40) ° C. or higher and (Tm-25) or lower.

The mass of one TPU particle is appropriately set according to the size of the target expanded particle and the expansion ratio, but is preferably 0.5 to 30 mg. Within the above range, the TPU particles can be sufficiently impregnated with the foaming agent, and the foamed particles have an excellent balance between the filling ability in the mold and the moldability in the mold. From this viewpoint, the lower limit of the mass of the TPU particles is more preferably 1 mg, and even more preferably 3 mg. On the other hand, the upper limit is more preferably 20 mg, further preferably 15 mg, and particularly preferably 12 mg.
In addition, the manufacturing method of TPU particle | grains is not specifically limited, It can obtain by a well-known method. For example, the raw material TPU is melted in an extruder, the melt of TPU is extruded into a strand form from a small hole in a die attached to the tip of the extruder, and this is cut into a predetermined mass, To obtain TPU particles by underwater cut method (UWC method) that cuts immediately after extruding TPU melt into water from small holes, and hot cut method that extrudes and cuts TPU melt from small holes into the bubble phase. Can do. The mass of the TPU particles can be adjusted by adjusting the hole diameter of the small holes, the extrusion amount, and the cutting speed.

  In addition to the inorganic powder as a bubble regulator, TPU particles include commonly used antistatic agents, conductivity imparting agents, lubricants, antioxidants, ultraviolet absorbers, flame retardants, metal deactivators, Various additives such as a crystal nucleating agent, a filler, and a colorant can be appropriately blended as necessary. The addition amount of these various additives varies depending on the purpose of use of the foamed particle molded body, but is preferably 25 parts by mass or less, more preferably 15 parts by mass or less, and further preferably 10 parts by mass with respect to 100 parts by mass of the raw material TPU. It is 5 parts by mass or less, particularly preferably 5 parts by mass or less.

The apparent density of the expanded particles obtained by the production method of the present invention is 100 to 300 kg / m ³ . By the production method of the present invention, foamed particles having an apparent density of 100 to 300 kg / m ^{3 and} uniform foam and excellent moldability can be produced by one-stage foaming.
If the apparent density of the expanded particles is too low, the obtained molded product is likely to be greatly deformed and contracted when the expanded particles are molded in the mold. From this viewpoint, the apparent density of the expanded particles is preferably 150 kg / m ³ or more, more preferably 200 kg / m ³ or more. On the other hand, if the apparent density is too high, the foamed particles are less likely to be secondary foamed during in-mold molding, voids are likely to remain between the foamed particles of the obtained molded product, and a molded product having a desired buffering property is obtained. There is a risk that it will not be obtained.
The apparent density of the expanded particles is a value obtained by dividing the weight of the expanded particles by the volume of the expanded particles. The volume of the expanded particles can be determined by a submersion method.

  In the present invention, the average cell diameter of the expanded particles is preferably 100 to 500 μm. When the average cell diameter is 100 μm or more, it is difficult for bubbles to break during molding in the mold, and a foamed molded product having particularly excellent surface properties is easily obtained. In addition, when the average cell diameter is 500 μm or less, steam easily penetrates into the inside of the foamed particles at the time of molding in the mold, so that the foamed particles are sufficiently secondary foamed to obtain a foamed molded product having particularly excellent surface properties. Can do. From this viewpoint, the average cell diameter of the expanded particles is more preferably 150 to 400 μm.

  The average cell diameter of the expanded particles is a value measured as follows in accordance with ASTM D3576-77. The expanded particle is cut so as to pass through the central portion thereof and divided into two. In one cross section of each of the cut expanded particles, four line segments are drawn at equal angles from the outermost surface of the expanded particle to the outermost surface on the opposite side. Measure the number of bubbles intersecting each line segment and divide the total length of the four line segments by the total number of bubbles intersecting the line segment to obtain the average chord length of the bubbles, and further divide by 0.616 By doing this, the average cell diameter of the expanded particles is obtained.

In the expanded particles of the present invention, the melt flow rate (MFR) at 190 ° C. and a load of 10 kg is preferably 10 to 50 g / 10 minutes. When the MFR is 10 g / 10 min or more, the secondary foaming property at the time of in-mold molding becomes good, and a foamed particle molded body having a particularly good surface can be obtained. From this viewpoint, the lower limit of the MFR is more preferably 15 g / 10 minutes, and further preferably 20 g / 10 minutes. Moreover, if MFR is 50 g / 10min or less, the obtained foaming molding will be excellent in recoverability. From this viewpoint, the upper limit of MFR is more preferably 45 g / 10 minutes, and further preferably 40 g / 10 minutes.
This MFR is a value measured under conditions of a temperature of 190 ° C. and a load of 10 kg based on JIS K7210-2: 2014.

[Foamed particle compact]
A foam-molded article can be obtained by in-mold molding of the foamed particles obtained by the production method of the present invention. A conventionally known method can be adopted as the in-mold molding method.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by this.
Examples 1-6 and Comparative Examples 1-2
The raw materials used in the examples are shown below.
[raw materials]
・ TPU
TPU1 (manufactured by Covestro, ether-based thermoplastic polyurethane, grade name: Desmopan DP9385A, MFR [190 ° C., load 10 kg]: 17 g / 10 min, type A durometer hardness: 86, melting temperature: 164 ° C.)
TPU2 (manufactured by Covestro, ether-based thermoplastic polyurethane, grade name: Desmopan DP9386A, MFR [190 ° C, load 10 kg]: 26 g / 10 min, type A durometer hardness: 86, melting temperature: 165 ° C)
・ Bubble nucleating agent (inorganic powder)
Talc 1 (manufactured by Hayashi Kasei Co., Ltd., grade name: KFP-125B, d50: 8 μm)
Talc 2 (manufactured by Hayashi Kasei Co., Ltd., grade name: PK-S, d50: 12 μm)
Talc 3 (Matsumura Sangyo Co., Ltd. grade name: High filler # 12, d50: 3 μm)

[Manufacture of TPU particles]
TPU and 100 parts by mass of TPU are supplied to a twin screw extruder having an inner diameter of 20 mm with the types and amounts of talc shown in Table 1 as a bubble regulator (inorganic powder), and these are heated and kneaded. A molten TPU composition was obtained. The molten TPU composition was extruded into water from a small hole in a die attached to the tip of the extruder and cut to obtain TPU particles having an average weight of 10 mg and L / D = 1.0.

[Production of expanded particles]
50 kg of the TPU particles obtained above and 270 liters of water as a dispersion medium are charged into a 400 liter autoclave equipped with a stirrer. Furthermore, with respect to 100 parts by mass of TPU particles, kaolin 0.2 Part by mass and 0.008 part by mass of sodium alkylbenzene sulfonate as a surfactant were added to the dispersion medium.
The contents in the autoclave are heated while stirring, and after reaching the temperature shown in Table 1 (impregnation temperature), carbon dioxide is used as a blowing agent in the autoclave, and the pressure in the sealed container is shown in Table 1 (impregnation). The pressure was maintained until the pressure reached, and the pressure was maintained for 15 minutes while maintaining the pressure. Thereafter, back pressure is applied with carbon dioxide, and the pressure in the container is adjusted to be constant at the pressure (foaming pressure) shown in Table 1, while foaming is performed at the temperature of the dispersion medium (foaming temperature) shown in Table 1. The TPU particles impregnated with the agent were discharged together with the dispersion medium under atmospheric pressure to obtain expanded particles.
The obtained foamed particles were put in a sealed container, pressurized at 30 ° C. with compressed air of 0.3 MPa (G) for 12 hours, then released and left at 40 ° C. under atmospheric pressure for 48 hours.
A method for measuring the apparent density, MFR, and average cell diameter of the obtained expanded particles, and the surface properties of the expanded particles and the presence or absence of large and small bubbles were evaluated. The results are shown in Table 1. The evaluation method is shown below.
These measurements were performed after the foamed particles obtained were conditioned for 2 days under conditions of 50% relative humidity, 23 ° C. and 1 atm.

(Apparent density of expanded particles)
First, foamed particles having a mass W1 were submerged in a graduated cylinder containing water at a temperature of 23 ° C. using a wire mesh. Then, taking into account the volume of the wire mesh, the volume V1 [L] of the expanded particles read from the rise in the water level is measured, and the mass W1 [g] of the expanded particles is divided by the volume V1 (W1 / V1). Was converted to [kg / m ³ ] to determine the apparent density of the expanded particles.

(Average cell diameter of expanded particles)
50 expanded particles were randomly selected from the obtained expanded particle group. The expanded particles were cut into two parts by passing through the center. In one cross section of each of the cut expanded particles, four line segments were drawn at equal angles from the outermost surface of the expanded particles through the central portion to the outermost surface on the opposite side.
Measure the number of bubbles intersecting each line segment and divide the total length of the four line segments by the total number of bubbles intersecting the line segment to obtain the average chord length of the bubbles, and further divide by 0.616 By doing this, the average cell diameter of each expanded particle was determined. Then, the average cell diameter of the expanded particles was obtained by arithmetically averaging these values.
(MFR of expanded particles)
Based on JIS K7210-2: 2014, the measurement was performed under conditions of a temperature of 190 ° C. and a load of 10 kg. As a sample for measurement, a foamed particle was dried at a temperature of 80 ° C. for 4 hours, and then the water content was 500 mass ppm or less.

(Surface properties of expanded particles)
Visual evaluation was performed according to the following criteria.
○: Good surface properties without wrinkles on the surface of the expanded particles ×: Surface properties with wrinkles confirmed on the surfaces of the expanded particles

[Preparation of foamed particle compact]
The foamed particles produced above were filled into a cavity of a molding die having a length of 200 mm, a width of 250 mm, and a thickness of 20 mm, and heated with steam until the molding pressure shown in Table 1 was reached. Then, after cooling, the molded body was taken out from the molding die to obtain a plate-like foamed particle molded body.

(Surface property)
When the space between the foamed particles on the surface of the obtained foamed particle molded body was filled, it was evaluated as “◯”, and when it was not filled, it was evaluated as “x”.

(Fusability)
In order to evaluate the fusing property of the obtained foamed particle molded body, the fusing rate was measured. When the fusing rate was 90% or more, “◯” and when the fusing rate was less than 90%, “×” ".
The fusion rate of the foamed particle molded body was measured by the following method. A test piece was cut out from the foamed particle molded body with the length of 170 mm, the width of 30 mm, and the thickness as they were. On one surface of the test piece, a cut with a depth of about 10 mm is made in the thickness direction at a position where the vertical length of the test piece is bisected with a cutter knife, and the molded body is bent and broken from the cut portion. It was. A ratio (m / n × 100 [%]) of the number m of the foam particles whose material was broken on the fracture surface and the number n of all the foam particles present on the fracture surface was calculated. When the molded body could not be broken even when it was bent, the fusion rate was set to 100%. The above measurement was performed 5 times using different test pieces, the respective material destruction rates were obtained, and the arithmetic average of them was used as the fusion rate.

(Recovery)
The thicknesses of the center part and the four corner parts of the obtained foamed particle molded body were measured, respectively, and the case where the ratio of the thickness of the center part to the thickest part among the four corner parts was 90% or more, “◯”, 90% The case where it was less than "x" was evaluated.

  The apparent density of the obtained foamed particle molded body was measured. The results are shown in Table 1. The method for measuring the apparent density is shown below. These measurements were performed after the foamed particle molded body thus obtained was left for 2 days under conditions of a relative humidity of 50%, 23 ° C. and 1 atm.

(Apparent density of molded product)
The foamed particle compact was submerged in ethanol, and the apparent volume of the compact was determined from the rise in the water level. The apparent density [kg / m ³ ] of the foamed particle molded body was determined by dividing the mass of the foamed particle molded body by the apparent volume.

From the evaluation results shown in Table 1, in Examples 1 to 6, it is possible to produce foamed particles having good surface properties and good air bubble uniformity. A wide and good foamed molded article could be obtained.
On the other hand, in Comparative Example 1 in which the foamed particles were produced under a condition where the amount of the inorganic powder was small, the surface properties of the obtained foamed particles were poor and the bubble diameter was extremely nonuniform, which was better due to in-mold molding. A foamed molded product could not be obtained.
Further, in Comparative Example 2 in which the foamed particles were produced under a low foaming pressure of 2.0 MPa (G), the foamed molded product could be obtained by molding the obtained foamed particles in-mold, but molding was possible. The range was narrow. In addition, since the foam diameter of the foamed particles is non-uniform, the resulting foam-molded product also has a non-uniform cell diameter, white portions of the excessively fine cell diameter are whitened, and the foam molded product is inferior in marble appearance. Only obtained.

Claims (9)

Thermoplastic polyurethane particles dispersed in an aqueous medium in a closed container are impregnated with a foaming agent under heating, and the thermoplastic polyurethane particles containing the foaming agent are discharged together with the aqueous dispersion medium from the closed container and foamed to give an apparent density of 100. a method of manufacturing a thermoplastic polyurethane foam particles ~300kg / ^{m 3,}
The foaming agent is a physical foaming agent mainly composed of carbon dioxide,
The thermoplastic polyurethane particles contain 300 to 2000 mass ppm of inorganic powder,
The method for producing thermoplastic polyurethane foam particles, wherein the pressure in the container at the time of release is more than 2.5 MPa (G) and not more than 4.0 MPa (G).   The method for producing thermoplastic polyurethane foam particles according to claim 1, wherein the inorganic powder is talc, and the 50% volume average particle diameter of talc is 1 to 15 µm.   The temperature of the aqueous medium when impregnated with the foaming agent is 80 ° C. or higher and the melting temperature of the thermoplastic polyurethane particles is Tm−20 ° C. or lower, and the pressure in the sealed container exceeds 2.5 MPa (G) under the temperature condition. The thermoplastic polyurethane particles are impregnated with the physical foaming agent by press-fitting a physical foaming agent mainly composed of carbon dioxide into the sealed container until the pressure becomes 7.0 MPa (G) or less. A process for producing the thermoplastic polyurethane expanded particles according to 1 or 2.   The temperature of the aqueous medium at the time of release is not lower than the melting temperature Tm-60 ° C of the thermoplastic polyurethane particles and not higher than the melting temperature Tm-20 ° C of the thermoplastic polyurethane particles. The manufacturing method of the thermoplastic polyurethane expanded particle as described in any one of.   The method for producing thermoplastic polyurethane foam particles according to any one of claims 1 to 4, wherein an internal pressure of the container at the time of discharge is 2.6 to 3.4 MPa (G).   6. The method for producing thermoplastic polyurethane foam particles according to any one of claims 1 to 5, wherein the thermoplastic polyurethane constituting the thermoplastic polyurethane particles is an ether-based thermoplastic polyurethane.   The method for producing thermoplastic polyurethane foam particles according to any one of claims 1 to 6, wherein the thermoplastic polyurethane particles contain 300 to 950 mass ppm of inorganic powder.   The method for producing thermoplastic polyurethane foam particles according to any one of claims 1 to 7, wherein the thermoplastic polyurethane particles have a melt flow rate of 10 to 50 g / 10 min at 190 ° C and a load of 10 kg.   The method for producing thermoplastic polyurethane foam particles according to any one of claims 1 to 8, wherein the type A durometer hardness of the thermoplastic polyurethane constituting the thermoplastic polyurethane particles is 90 or less.