JPH05163381A - Production of expanded polymer particle - Google Patents

Abstract

PURPOSE:To easily produce the title particles having a high expansion ratio without making cells fine even when an inorg. gaseous blowing agent, such as carbon dioxide, is used in a process wherein polymer particles are impregnated with a blowing agent and spouted out of a hermetically sealed container into a low-pressure atmosphere to expand the particles. CONSTITUTION:Polymer particles contg. 0.02-5.0wt.% inorg. substance such as a metal carbonate, bisulfate, sulfate, oxide, or chloride, clay, or a natural mineral and 0.02-5.0wt.% polypropylene glycol-polyethylene glycol copolymer are impregnated with a blowing agent and spouted out of a hermetically sealed container into a low-pressure atmosphere to expand the particles.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing expanded polymer particles.

[0002]

2. Description of the Related Art Conventionally, foamable polymer particles impregnated with a volatile foaming agent are dispersed in a dispersion medium such as water in a closed container,
An atmosphere of a pressure lower than that in the container by heating one of the container particles to a temperature not lower than the softening temperature of the polymer particles while maintaining the pressure in the container at the vapor pressure of the foaming agent or higher, and then opening one end of the container. Methods of foaming polymer particles by releasing below are known.

Hydrocarbons such as propane, butane and pentane, and halogenated hydrocarbons such as trichlorofluoromethane and dichlorodifluoromethane have been used as volatile blowing agents in this method. .. However, the above compounds conventionally used as a volatile foaming agent have a risk of toxicity and flammability, or have a problem of depletion of the ozone layer like CFCs, or a danger. In reality, many of them have problems such as being expensive and impractical even though they are not so problematic in terms of environmental damage.

Much research has been done to solve such problems, and as a result of intensive research by the applicant of the present invention to solve these problems, an inorganic gas-based foaming agent that has never been considered as a foaming agent has been obtained. A method for obtaining polymer expanded particles by using the above has been previously proposed (for example, Japanese Patent Publication No. 62-61227).
JP-A-61-2741 and JP-A-61-4
No. 738, etc.).

However, when an inorganic gas type foaming agent is used, it is difficult to stably obtain expanded particles having a high expansion ratio due to the plasticizing effect of the polymer and the difference in gas permeation rate as compared with the volatile foaming agent. There was a problem.

The present inventors have conducted extensive studies to solve the above problems, and as a result, by using an inorganic gas-based foaming agent by incorporating an inorganic substance such as borax, nickel sulfate and sodium chloride in the polymer particles. Even when polymer expanded particles are produced on an industrial scale, polymer expanded particles with a high expansion ratio can be obtained, and the amount of the foaming agent used can be reduced even when a conventional volatile foaming agent is used. At the same time, the inventors filed an application for finding that it is possible to obtain expanded particles having a high expansion ratio more stably than in the method described in JP-A-61-2738 with a small amount of use (JP-A-3-1).
166238, JP-A-3-223347, etc.).

However, when polymer particles containing the above-mentioned inorganic material are impregnated with an inorganic gas-based foaming agent, particularly carbon dioxide for foaming, the bubbles of the resulting foamed particles are likely to become fine, and such foamed particles are The molded product obtained by using had problems such as reduced dimensional accuracy and poor secondary foamability during molding.

The inventors of the present invention have conducted extensive studies to solve the above-mentioned problems, and as a result, by incorporating a polypropylene glycol / polyethylene glycol polymer in the polymer particles together with the inorganic substance, the two act synergistically and the added amount of the inorganic substance is increased. Even when the amount is small, it is possible to obtain expanded particles having a high expansion ratio by using an inorganic gas such as carbon dioxide, and it is possible to solve the problem of micronization of bubbles in the obtained expanded particles, thereby completing the present invention. Came to.

[0009]

That is, the method for producing expanded polymer particles of the present invention is to prepare a polymer particle containing an inorganic substance and a polypropylene glycol / polyethylene glycol polymer in a closed container in the presence of a foaming agent. Polymer particles dispersed in a dispersion medium and heated to a temperature equal to or higher than the softening temperature of the polymer particles, and then the polymer particles impregnated with a foaming agent and the dispersion medium are discharged under an atmosphere at a pressure lower than that in the container, and the polymer particles are discharged. Is foamed.

In the method of the present invention, in the polymer particles,
It is preferable to contain 0.02 to 5.0% by weight of an inorganic substance and 0.02 to 5.0% by weight of a polypropylene glycol / polyethylene glycol polymer. Further, in the method of the present invention, an inorganic gas type foaming agent is preferable as the foaming agent.

The polymer particles used in the present invention include propylene homopolymer, propylene-ethylene random copolymer, propylene-ethylene block copolymer, propylene-butene random copolymer and propylene-ethylene-butene random copolymer. Propylene-based polymers such as coalesce, low-density polyethylene, high-density polyethylene, ethylene and a small amount of α-olefin (carbon number 4,
6, 8, etc.) and ethylene-based copolymers such as linear low-density polyethylene.

Of these, propylene-based polymers such as propylene-ethylene random copolymers, propylene-butene random copolymers, propylene-ethylene-butene random copolymers, and linear low-density polyethylene are particularly preferred. Although these polymers may be crosslinked, non-crosslinked ones are particularly preferable.

Examples of the inorganic substance contained in the polymer particles include inorganic hydroxides such as aluminum hydroxide, calcium hydroxide and magnesium hydroxide, calcium carbonate,
Inorganic carbonates such as magnesium carbonate and barium carbonate, inorganic sulfites such as calcium sulfite and magnesium sulfite,
Inorganic sulfates such as calcium sulfate, aluminum sulfate, manganese sulfate and nickel sulfate, inorganic oxides such as calcium oxide, aluminum oxide and silicon oxide, inorganic chlorides such as sodium chloride, magnesium chloride and calcium chloride, borax, talc and clay , Clay such as kaolin and zeolite, natural minerals and the like.

The above inorganic substances may be used alone or in combination of two or more, and may be added at the time of granulating the polymer particles. The inorganic substance is usually added as a granular material, but the particle size is not particularly limited. However, in general, the particle size
It is preferable to use one having a thickness of 0.1 to 150 μm, particularly 1 to 100 μm.

In the present invention, the polymer particles contain a polypropylene glycol / polyethylene glycol polymer (hereinafter abbreviated as PPG / PEG polymer) together with the above-mentioned inorganic material. The PPG / PEG polymer is a nonionic surfactant obtained by polymerizing a hydrophilic PEG and a lipophilic PPG, and is generally a block copolymer. Further, among the PPG / PEG polymers, those having a PPG weight average molecular weight of 2000 or more have less bleeding, and the prefoamed particles have particularly good moldability. On the other hand, from the viewpoint of kneading dispersibility, the PPG / PEG polymer having a weight average molecular weight of 8,000 or more, particularly 10,000 or more is preferable.

The content of the above inorganic substance in the polymer particles is preferably 0.02 to 5.0% by weight, particularly preferably 0.05 to 1% by weight, and the content of the PPG / PEG polymer in the polymer particles is preferable. Is 0.02 to 5.0% by weight, especially 0.1 to 1% by weight
Is preferred. When the content of the inorganic substance exceeds 5.0% by weight,
The resulting foamed particles tend to shrink, which is not preferable in terms of foaming moldability. On the other hand, if the content of the inorganic substance is less than 0.02% by weight, the effect of the present invention cannot be obtained. Also PPG
If the amount of the PEG polymer added exceeds 5.0% by weight, problems such as defective fusion during molding and deterioration of mechanical strength will occur, and if it is less than 0.02% by weight, the bubbles of the resulting expanded particles will be fine. The molded product obtained by using such expanded particles has low dimensional accuracy and poor secondary property during molding. In the present invention, the polymer particles containing an inorganic substance and a PPG / PEG polymer generally have a particle size of 0.3 to 5 mm, and particularly preferably 0.5 to 3 mm.

In the present invention, the step of impregnating the polymer particles with the foaming agent may be either before or after the step of dispersing the polymer particles in a dispersion medium in a closed container, but usually the polymer particles are dispersed. Simultaneous in the process. In this case, the foaming agent is once dissolved or dispersed in the dispersion medium and then impregnated into the polymer particles, and the foaming agent is heated and pressurized with stirring while placing the polymer particles, the foaming agent and the dispersion medium in a closed container. The polymer particles can be impregnated by the method described above.

The foaming agent used in the present invention is a volatile foaming agent such as propane, butane, pentane, hexane, cyclobutane, cyclohexane, trichlorofluoromethane, dichlorodifluoromethane, or an inorganic material such as nitrogen, carbon dioxide, argon or air. Any gas type foaming agent may be used,
Inorganic gas type foaming agents are preferable, and carbon dioxide, nitrogen and air are particularly preferable. When these inorganic gas type foaming agents are used, it is preferable to supply them so that the pressure inside the container is 50 kg / cm ² · G or less.

The dispersion medium for dispersing the polymer particles may be one that does not dissolve the polymer particles, and examples of such a dispersion medium include water, ethylene glycol, and the like.
Glycerin, methanol, ethanol and the like can be mentioned, but water is usually used.

When the foamable polymer particles impregnated with the foaming agent are dispersed in the dispersion medium and heated to the foaming temperature, a fusion preventing agent can be used to prevent the polymer particles from being fused. As the anti-fusing agent, an inorganic type or an organic type can be used as long as it does not dissolve in a dispersion medium such as water and does not melt by heating, but an inorganic type is generally preferable. Examples of the inorganic anti-fusing agent include aluminum oxide, titanium oxide, aluminum hydroxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, tricalcium phosphate, magnesium pyrophosphate and the like.

The fusion preventing agent has a particle size of 0.001-1.
It is preferably 00 μm, particularly 0.001 to 30 μm.
The amount of the anti-fusion agent added is 100 parts by weight of the polymer particles,
Usually, 0.01 to 10 parts by weight is preferable.

The above-mentioned inorganic anti-fusing agent is preferably used in combination with an emulsifier. As the emulsifier, anionic surfactants such as sodium dodecylbenzene sulfonate and sodium oleate are suitable. Generally, it is preferable to add 0.001 to 5 parts by weight of the emulsifier per 100 parts by weight of the polymer particles.

In the method of the present invention, secondary crystals are preferably present in the expandable polymer particles. The expandable particles obtained from the expandable polymer particles containing the secondary crystals have excellent moldability. Particularly when the polymer particles are non-crosslinked polypropylene resin or non-crosslinked linear low-density polyethylene resin, it is advantageous that the expandable polymer particles have secondary crystals.

The presence of secondary crystals indicates whether or not a high temperature peak on the higher temperature side than the intrinsic peak due to the so-called endotherm at the time of melting of the polymer appears in the DSC curve obtained by the differential scanning calorimetry of the obtained expanded particles. It can be determined by The intrinsic peak and the high temperature peak can be determined by performing differential scanning calorimetry on the same sample twice.
In this method, first, 1 to 3 mg of a sample (foamed particles) was measured by a differential scanning calorimeter at a temperature of 10 ° C./minute to 220 ° C. to obtain a first DSC curve, and then from 220 ° C. to around 40 ° C. Decrease the temperature at a rate of ℃ / min, and re-heat to 10 ℃
The second DSC curve is obtained by measuring the temperature rise up to 220 ° C./min.

By comparing the two DSC curves thus obtained, the characteristic peak and the high temperature peak can be discriminated. The intrinsic peak is an endothermic peak associated with so-called melting of the polymer, and therefore the first DSC curve and the second D
It is also a peak that appears in the SC curve, and the temperature at the top of the peak may be slightly different between the first time and the second time, but the difference is less than 5 ° C, usually less than 2 ° C. On the other hand, the high temperature peak is an endothermic peak that appears on the higher temperature side than the above-mentioned intrinsic peak in the first DSC curve. The presence of secondary crystals was confirmed by the appearance of this high temperature peak,
When no substantial high temperature peak appears, it is determined that the secondary crystal does not exist.

In the above two DSC curves, it is desirable that there is a large difference between the temperature at the apex of the characteristic peak appearing in the second DSC curve and the temperature at the apex of the high temperature peak appearing in the first DSC curve. The temperature difference is preferably 5 ° C or higher, and particularly preferably 10 ° C or higher.

FIGS. 1 and 2 show DSC curves obtained by differential scanning calorimetry of expanded particles. FIG. 1 shows expanded particles containing secondary crystals, and FIG. 2 shows expanded particles not containing secondary crystals. It is a thing. In FIGS. 1 and 2, curves 1 and 2 are DSC curves obtained by the first measurement, and curves 1 ′ and 2 ′ are DSC obtained by the second measurement.
A curve is shown.

As shown in FIG. 1, in the expanded particles having secondary crystals, in addition to the characteristic peak B in the curve 1 obtained by the first measurement, the curve 1 obtained by the second measurement was obtained. There is a high temperature peak A which is not found in ′ (only the characteristic peak B ′ appears in the curve 1 ′ obtained in the second measurement), and the existence of this high temperature peak A confirms the existence of secondary crystals. To be done. On the other hand, in the case of expanded particles containing no secondary crystal, as shown in FIG. 2, only the characteristic peaks b and b ′ appear in both curve 2 and curve 2 ′, and the high temperature peak does not appear. Is not present.

Particles in which the presence of secondary crystals are not found, such as the expanded particles shown in FIG. 2, are obtained by the heat treatment at the secondary crystallization acceleration temperature (melting point to melting end temperature) for a sufficient time. However, this is the case where foaming occurs at a temperature equal to or higher than the melting end temperature. When an inorganic gas type foaming agent is used,
Expanded particles having secondary crystals as shown by curve 1 are
For example, it can be manufactured by the following method.

In the case of the non-crosslinked polypropylene resin, generally, the polymer particles are not heated above the melting end temperature thereof in a pressure resistant container, but at a temperature of not less than about -20 ° C. and less than the melting end temperature for a sufficient time. , Usually for 5 to 90 minutes, preferably for 15 to 60 minutes. Also, in the case of particles that have been kept at such a temperature to form secondary crystals, the foaming temperature (temperature at the time of discharging) when polymer particles are discharged under a low pressure atmosphere than in the container and foamed is the end of melting. Even if the temperature is not less than the temperature, if the temperature is not higher than the high temperature peak, it is possible to obtain expanded particles having good moldability.

In the case of non-crosslinked linear low-density polyethylene, generally, the temperature of the melting point is not lower than about −15 ° C. and lower than the melting end temperature without heating the polymer particles above the melting end temperature in a pressure vessel. For a sufficient time, usually 5 to 90 minutes, preferably 5 to 30 minutes.

In addition, it is desirable that the above-mentioned temperature maintenance is performed in a plurality of times in order to facilitate temperature control.
In this case, a method of increasing the holding temperature after the first holding temperature is adopted. Then, it is desirable that the final holding temperature is the foaming temperature.

When an inorganic gas type foaming agent is used as the foaming agent in the method of the present invention, the foaming temperature at which the foamable polymer particles and the dispersion medium are discharged from the container under a low pressure atmosphere to foam the polymer is The temperature is not lower than the softening temperature of the particles, but a temperature near the melting point is particularly preferable. The suitable foaming temperature range varies depending on the type of resin. For example, in the case of non-crosslinked polypropylene resin, melting point -5 ° C or higher, melting point + 15 ° C.
The melting point is preferably −3 ° C. or higher and the melting point + 10 ° C. or lower. In the case of a polyethylene resin, the melting point is −10 ° C. or higher and the melting point + 5 ° C. or lower.

Further, the heating rate at the time of heating to the foaming temperature is preferably 1 to 10 ° C./minute, particularly preferably 2 to 5 ° C./minute. The atmospheric pressure for releasing the expandable polymer particles and the dispersion medium from the container may be lower than that in the container, but is usually atmospheric pressure.

In the present invention, the melting point of the above-mentioned polymer is the differential scanning calorimeter and about 6 mg of a sample is heated to 220 ° C. at a temperature rising rate of 10 ° C./minute, and then at a temperature lowering rate of 10 ° C./minute to about 50 ° C. It is the temperature at the apex of the endothermic peak (inherent peak) in the DSC curve obtained when the temperature is cooled to 20 ° C. and the temperature is again raised to 220 ° C. at a rate of 10 ° C./min. The melting end temperature is the second D obtained by the above measurement.
It means the melting end temperature at the endothermic peak (specific peak) of the SC curve. The softening temperature of polymer particles is
In the -D-648 method, it means a softening temperature determined under a load of 4.6 kg / cm ² .

[0036]

EXAMPLES The present invention will be described in more detail with reference to examples.

Examples 1 to 6 and Comparative Examples 1 to 4 Resins, inorganic substances and PPG / P shown in Table 1 below in an extruder.
After melt-kneading the EG polymer, it was extruded in a strand form from a die at the tip of the extruder, rapidly cooled in water, and then cut and pelletized into pellets (length 2.4 mm, cross-sectional diameter 1.1 mm).
The polyethylene glycol component content (PEG weight%) in the PPG / PEG polymer and the weight average molecular weight of the PPG / PEG polymer are also shown in Table 1.

[0038]

[Table 1]

The properties of the base resin of the polymer and the particle size of the inorganic material used in the above Examples and Comparative Examples are as follows.

Base resin: LLDPE: Density 0.925 g / cm ³ , MI = 1.0 g /
10-minute linear low-density polyethylene / Et-Pr random copolymer: ethylene content 2.
4% by weight of ethylene-propylene random copolymer, M
I = 10.0 g / 10 min-Crosslinked LLDPE: Crosslinked low-density polyethylene with 53% gel fraction-Inorganic substance-Aluminum hydroxide: Particle size 3 μm-Talc: Particle size 2 μm-Organic treated bentonite: Particle size 0.05 μm, trimethylammonium Surface treated with chloride.・ 13X type zeolite: particle size 2 μm ・ Silica: particle size 1 μm

100 kg of the pellets, 400 g of fine-grained aluminum oxide, and 220 liters of water were mixed and stirred in a closed container (volume: 400 liters) without raising the temperature to a temperature higher than the melting temperature. The temperature was held at the one-step holding temperature (only Example 5, Comparative Example 3 and others were immediately held at the two-step holding temperature). Then, the temperature was raised to the two-stage holding temperature shown in Table 2, and immediately thereafter, the foaming agent shown in the same table was supplied so that the internal pressure of the container became the pressure shown in the same table, and the same temperature was maintained.

Thereafter, while maintaining the two-stage holding temperature, a back pressure was applied with carbon dioxide to maintain the inside of the container at the pressure shown in Table 2 and one end of the container was opened to bring the polymer particles and water under atmospheric pressure. It was then discharged into a foam. Table 2 shows the average bulk expansion ratio of the obtained expanded particles, the cell diameter, and the moldability of these expanded particles.
It is also shown.

In Table 2, the dimensional accuracy of the molded product, the fusion property,
The evaluation of the secondary foaming property was performed based on the evaluation criteria shown below. The molded body using the expanded beads of Comparative Example 4 was good in both dimensional accuracy and fusion property of the particles, but since the expansion ratio was too low, the comprehensive evaluation was evaluated as x.

Dimensional accuracy The shrinkage in the surface direction after curing for 24 hours in an oven at 80 ° C. was measured and evaluated according to the following criteria. A: Shrinkage in the plane direction is less than 3% B: Shrinkage in the plane direction is 3% or more and less than 4% X: Shrinkage in the plane direction is 4% or more

Fusing Property A slice plate obtained by cutting a foamed molding so that a cross section in the width direction is 1 cm in thickness and 5 cm in width is pulled in the longitudinal direction until it is broken, and the fracture surface is observed and evaluated according to the following criteria. did. ○: Material fracture of fracture surface is 60% or more △: Material fracture of fracture surface is 40% or more and less than 60% × ・ ・ ・ Material fracture of fracture surface is less than 40%

Secondary foaming property The surface condition of the foamed molded product was observed and evaluated according to the following criteria. ○: Almost no unevenness on the surface. Δ: Partially uneven. X: There are irregularities on the entire surface.

[0047]

[Table 2]

[0048]

INDUSTRIAL APPLICABILITY As described above, in the method of the present invention, since the polymer particles containing a specific ratio of the inorganic substance and the PPG / PEG polymer are impregnated with the foaming agent to foam, the inorganic gas type foaming agent is used. It is possible to easily obtain expanded particles having a high expansion ratio by using. Even when a volatile foaming agent is used, the amount of volatile foaming agent used can be reduced as compared with the conventional method using a volatile foaming agent, and even if a small amount of volatile foaming agent is used, foaming with a high expansion ratio is achieved. It has the effect that particles can be obtained.

Further, by using the inorganic substance and the PPG / PEG polymer in combination, there is no fear that the bubbles of the foamed particles obtained will be miniaturized, the dimensional accuracy of the molded article is reduced by the miniaturization of the bubbles, and the secondary foaming at the time of molding. It has the effect of providing expanded particles having excellent properties that do not cause poor properties and the like.

[Brief description of drawings]

FIG. 1 is a graph showing a DSC curve of expanded particles having secondary crystals in the particles.

FIG. 2 DSC of expanded particles without secondary crystals in the particles.
It is a graph which shows a curve.

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[Procedure amendment]

[Submission date] July 7, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0022

[Correction method] Change

[Correction content]

The above-mentioned inorganic anti-fusion agent is used as a foaming agent.
When carbon dioxide is used, it is preferably used in combination with an emulsifier. As the emulsifier, anionic surfactants such as sodium dodecylbenzene sulfonate and sodium oleate are suitable. Generally, it is preferable to add 0.001 to 5 parts by weight of the emulsifier per 100 parts by weight of the polymer particles.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0032

[Correction method] Change

[Correction content]

In addition, when the temperature is maintained, the formation of crystals
Therefore, it is desirable to divide the temperature into a plurality of times to facilitate temperature control. In this case, a method of increasing the holding temperature after the first holding temperature is adopted. Then, it is desirable that the final holding temperature is the foaming temperature.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0033

[Correction method] Change

[Correction content]

When an inorganic gas type foaming agent is used as the foaming agent in the method of the present invention, the foaming temperature at which the foamable polymer particles and the dispersion medium are discharged from the container under a low pressure atmosphere to foam the polymer is The temperature is not lower than the softening temperature of the particles, but a temperature near the melting point is particularly preferable. The suitable foaming temperature range varies depending on the type of resin. For example, in the case of non-crosslinked polypropylene resin, melting point -5 ° C or higher, melting point + 15 ° C.
The melting point is preferably −3 ° C. or higher and + 10 ° C. or lower.
The temperature is preferably not lower than 0 ° C and not higher than the melting point + 5 ° C. In addition, cross-linked low density
In the case of polyethylene, melting point -10 ° C or higher, melting point + 50 ° C or higher
Lower is preferred.

[Procedure amendment 4]

[Document name to be amended] Statement

[Item name to be corrected] 0040

[Correction method] Change

[Correction content]

Base resin: LLDPE: Density 0.925 g / cm ³ , MI = 1.0 g /
10-minute linear low-density polyethylene / Et-Pr random copolymer: ethylene content 2.
4% by weight of ethylene-propylene random copolymer, M
I = 10.0 g / 10 minutes- Crosslinked LDPE : Crosslinked low-density polyethylene with a gel fraction of 53% -Inorganic substance-Aluminum hydroxide: Particle size 3 μm-Talc: Particle size 2 μm-Organic treated bentonite: Particle size 0.05 μm, trimethylammonium Surface treated with chloride.・ 13X type zeolite: particle size 2 μm ・ Silica: particle size 1 μm

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0041

[Correction method] Change

[Correction content]

1 kg of this pellet and 7 parts of tricalcium phosphate
5 g, sodium dodecylbenzene sulfonate 0.2
g and 3 liters of water were mixed and stirred in a closed container (volume: 5 liters), and the temperature was raised to and maintained at the one-step holding temperature shown in Table 2 without increasing the temperature to the melting temperature or higher (Example 5 , Comparative Example 3 only, and others were immediately held at the two-stage holding temperature.). Then, the temperature is raised to the two-stage holding temperature shown in Table 2,
It was kept at the same temperature. Dry ice is used as the foaming agent.
The amount shown in Table 2 when the above raw materials such as pellets are added.
I entered .

Claims (3)

[Claims] 1. Polymer particles containing an inorganic substance and a polypropylene glycol / polyethylene glycol polymer are dispersed in a dispersion medium in a closed container in the presence of a foaming agent to a temperature not lower than the softening temperature of the polymer particles. A method for producing expanded polymer particles, which comprises heating and then releasing the polymer particles impregnated with a foaming agent and the dispersion medium into an atmosphere at a pressure lower than that in the container to foam the polymer particles. 2. The expanded polymer particles according to claim 1, wherein the polymer particles contain 0.02 to 5.0% by weight of an inorganic substance and 0.02 to 5.0% by weight of a polypropylene glycol / polyethylene glycol polymer. Production method. 3. The method for producing expanded polymer particles according to claim 1, wherein the foaming agent is an inorganic gas type foaming agent.