JPH0610269B2 - Polyolefin foamed particles and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polyolefin resin foamed particle which can be formed into a molded product by heating the expanded particle in a mold with a heating medium such as steam, and a method for producing the same. More specifically, the present invention relates to a foamed polyolefin resin particle having a uniform and fine cell structure in which there is no blocking between foamed particles, and the degree of expansion is extremely small, and a method for producing the same.

Foamed polyolefin resin particles such as polyethylene used for in-mold molding are most commonly prepared by heating resin particles containing a volatile foaming agent or a pyrolytic foaming agent. However, it is difficult to obtain highly expanded particles by this method, and in addition, there are drawbacks that the expansion degree of the expanded particles greatly varies and the expanded particles tend to shrink. Further, as disclosed in West German Laid-Open Patent Publication No. 2107683, Japanese Examined Patent Publication No. 56-1344, etc., polyolefin-based resin particles containing a volatile foaming agent are dispersed in water in a pressure-resistant container, and high temperature and high pressure are applied. It is known that the foamed resin particles are produced by stirring the dispersion with the above method and then discharging it into a low pressure region.

The latter method is efficient in that the starting resin particles can be continuously impregnated with the foaming agent and foaming in the same apparatus, but the resin particles containing the foaming agent are heated to a high temperature by this method. Therefore, there is a problem that the resin particles are easily aggregated in the container, and the resin particles are released to the low pressure region at a high speed, so that the foamed particles are likely to cause blocking.
Further, in this method, the foamed particles have a large degree of foaming fluctuation and the cell structure between the particles is not uniform, which impairs moldability and produces a product having satisfactory characteristics during molding. It has the drawback that it cannot be obtained.

As a result of intensive studies conducted by the present inventors to overcome these problems in such a method, when the mixture of resin particles and water is discharged from the pressure resistant container to a low pressure region, the temperature and pressure inside the container are controlled. A method has been completed which is characterized in that it is discharged from a small-diameter opening while strictly controlling it. The small-diameter apertures through which the particles are released prevent contact between the particles when passing through the apertures, control the release rate, and consequently prevent blocking of the particles during release. Further, by controlling the internal pressure of the container at the time of discharging, the fluctuation of the discharging rate is removed, and the foamed particles having a uniform degree of foaming and a uniform cell structure of the particles are provided. The release of the particles through such small apertures has the advantage that the interior of the container can be easily maintained at a controlled pressure during release.

That is, the present invention is to disperse polyolefin resin particles having a particle diameter of 0.25 to 10 mm and a volatile foaming agent in water in the presence of a dispersant in a pressure resistant container, and impregnating the polyolefin resin particles with the volatile foaming agent. In order to allow the mixture to be heated to a temperature in the range of the crystalline melting point of the polyolefin-based resin −25 ° C. to the melting point + 10 ° C., while maintaining the pressure-resistant container at a constant temperature and pressure, the polyolefin-based resin particles used And a method for producing expanded polyolefin resin particles, characterized in that the mixture of the polyolefin resin particles and water is discharged from an opening having a diameter of 1.2 to 2 times to the low pressure region. It is intended to provide expanded polyolefin resin particles having substantially no particle blocking.

Examples of the polyolefin resin used in the present invention include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ethylene-vinyl acetate copolymer, polypropylene, ethylene-propylene random copolymer, ethylene- Examples thereof include a propylene block copolymer and an ethylene-propylene-butene terpolymer, which may be used alone.
You may use it as a mixture of 2 or more types. Further, an ultraviolet absorber, an antistatic agent, a heat stabilizer, a flame retardant, a coloring agent or a filler may be mixed with the resin. Further, a polyolefin-based resin having a crosslink generated by peroxide or electron beam irradiation may be used.

According to the production method of the present invention, by the strict control of the internal pressure of the container and the use of small openings, the obtained polyolefin resin foamed particles having a uniform cell structure between particles and substantially no blocking between particles. Can be manufactured. However, the polyolefin-based resin particles used in the present invention have an inorganic nucleating agent in an amount of 0.01 to 5.0 parts, preferably 0.01 to
It is more desirable to contain 1 part, most preferably 0.01 to 0.5 part. The inorganic nucleating agent acts to give resin expanded particles having fine and uniform bubbles having a linear diameter of 0.05 to 0.5 mm among the particles. If the amount of the inorganic nucleating agent is less than 0.01 part, it is difficult to obtain fine and uniform bubbles, and even if the amount exceeds 0.5 part, a remarkable effect cannot be obtained by adding a large amount. Examples of the inorganic nucleating agent used include talc, silica, calcium silicate, calcium carbonate, aluminum oxide, titanium oxide, diatomaceous earth, clay, baking soda, barium sulfate, and bentonite. Among them, talc, silica Is preferably used.

The polyolefin resin particles to be foamed have a particle size of 0.25 to 10 mm, preferably 0.5 to 6 mm.

Examples of the volatile foaming agent used in the present invention include boiling point-
Hydrocarbons or halogenated hydrocarbons having 50 to 120 ° C.,
Specific examples include propane, butane, pentane, hexane, heptane, cyclopentane, cyclohexane, monochloromethane, dichloromethane, monochloroethane, trichloromonofluoromethane, dichlorodifluoromethane, dichloromonofluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane. Etc., and these compounds may be used alone or in combination of two or more kinds. The charged amount of the volatile foaming agent is determined by the kind of resin, the kind of foaming agent, the desired degree of foaming, and the ratio of the gas phase space of the container to the resin in the pressure resistant container. In the present invention, the charging amount of the foaming agent is determined so that the polyolefin resin is usually impregnated with 5 to 40 parts of the foaming agent.

In the present invention, it is desirable to use a small amount of a dispersant when dispersing the polyolefin resin particles in water to prevent the resin particles from being fused to each other during heating. Examples of the dispersant used include water-soluble polymers such as polyvinyl alcohol, methyl cellulose and N-polyvinylpyrrolidone; fine powders of poorly water-soluble inorganic substances such as calcium phosphate, magnesium pyrophosphate and zinc carbonate; or such. Mixtures of fine powders of such inorganic substances with small amounts of sodium alkylbenzene sulfonate, sodium α-olefin sulfonate, sodium alkyl sulfonate or similar anionic surfactants. It is preferable to use an inorganic substance because the water-soluble polymer causes a problem of water pollution. However, since expanded particles obtained by using a large amount of an inorganic substance do not heat-bond to each other effectively during molding, it is preferable to use a small amount of anionic surfactant together with the inorganic substance. In this case, 0.1 to 3 parts of poorly soluble inorganic substance powder and anionic surfactant 0.001 to
It is preferable to use 0.5 part with respect to 100 parts of the resin.

In the method of the present invention, the mixture of materials is heated to a resin containing the volatile blowing agent in the polyolefin resin at a temperature not lower than the softening point. The suitable heating temperature varies depending on the type of polyolefin resin and the type of volatile foaming agent. However, in carrying out the present invention, the heating temperature is from the crystalline melting point of the resin −25 ° C. to the melting point +
The temperature is determined in the range of 10 ° C, preferably in the range of -20 ° C to the melting point + 5 ° C. For example, if the crystal melting point is
For low density polyethylene at 110 ° C, suitable heating temperatures are 85-120 ° C. Ethylene content 4.5%
In the case of ethylene-propylene random copolymers (wt%, likewise below) and having a crystalline melting point of 135 ° C, suitable heating temperatures are 110-145 ° C. When the heating temperature is lower than the temperature range, the degree of foaming is remarkably reduced, and when the heating temperature is too high, the expanded particles have a low proportion of closed cells, which is not preferable.

In the present invention, the mixture of heated polyolefin-based resin expandable particles in an unfoamed state and water in the container is 1.2 to 2 times the particle diameter of the polyolefin-based resin particles used thereafter. It is discharged to a low pressure region through an opening having a pore size of. If the pores are too small, the particles cannot pass through the pores, resulting in blockage. If the pores are larger than twice the particle size, they are not foamed under pressure and softened by heating. A large number of unexpanded particles are simultaneously discharged through the openings into the low pressure region, and heat fusion between particles occurs at the time of passing through the openings or after that (during foaming) to cause blocking. The size of one aperture used in the present invention is 0.0
The area is 7 to 300 mm ² , preferably 0.28 to 120 mm ² . The size of the pores should theoretically have at least the same particle size as the particle size of the polyolefin resin particles used, and in reality, it has at least 1.2 times the particle size of the particles. In order to prevent a large number of unexpanded particles from being simultaneously released into the low pressure region through the openings, and blocking is prevented, the maximum is preferably doubled.

In the present invention, since the size of the opening is preferably as small as possible, the upper limit of the size of the opening is about 2 times as large as the diameter of 10 mm of the largest polyolefin resin particles used in the present invention. Is determined to have an area of. When such small apertures are used therethrough to pass the resin particles into the low pressure region,
The resin particles can pass through the openings without coming into contact with each other, and the number of particles per unit time can be reduced so that the resin particles can be discharged without colliding with each other. As a result, it is considered that the foamed particles are less likely to be heat-sealed (blocked). The opening is usually circular,
It may be oval or polygonal. Said openings are provided, for example, by arranging a pressure-resistant orifice plate with at least one opening on the rear side of the discharge valve by means of a flange.

As one of the features of the present invention as described below, it is important to maintain the pressure inside the pressure vessel at a specified pressure when discharging particles, but by using a small opening having an area of 0.07 to 300 mm ^2. The advantage is that the pressure inside the container can be easily controlled.

In the present invention, when the mixture of resin particles and water is discharged into the low pressure region, the temperature and pressure inside the container must be kept constant.

When the internal temperature changes, the degree of foaming of the resin particles changes greatly. Therefore, it is necessary to control the temperature as tightly as possible. For example, the temperature change from the beginning to the end of the release of resin particles should preferably not be higher than 5 ° C, more preferably not higher than 3 ° C. The temperature can be easily controlled, for example when the pressure vessel is provided with a conventional jacket.

When the mixture of the resin particles and water is discharged from the container, the upper space in the container is increased, the pressure in the container is decreased, and the foaming degree of the discharged particles is decreased. In order to prevent the foaming inside the container from hindering the release of the particles, it is necessary to keep the pressure inside the container constant while the resin particles are released.

It is usually considered to introduce nitrogen gas, air or similar inert gas to maintain the pressure, but if the total internal pressure and the partial pressure of the foaming agent are maintained at a constant level, the degree of foaming will vary. It is possible to obtain extremely small expanded particles.
What has been said is to reduce the volatile blowing agent by reducing the amount of head space inside the container by an amount corresponding to the increase in space, for example in order to maintain the space at a constant volume. It can be realized by supplying the container from the outside. The need to supply the blowing agent arises when the space inside the container is not saturated with the blowing agent and when the partial pressure of the blowing agent in the container is reduced by the release of particles. Therefore, it is not necessary to supply the blowing agent when the interior space of the container is saturated with the blowing agent and excess blowing agent is present as a liquid. When a volatile foaming agent is supplied, the foaming agent can be introduced into the container from outside in the form of (1) vapor or (2) liquid. The latter method is simpler and therefore more preferred. When the foaming agent is introduced into the container by the method (2), the volatile foaming agent is immediately vaporized. The foaming agent may be introduced continuously or intermittently, in order to control the pressure so that, for example, the variability in the container is kept constant due to an acceptable variability in the degree of foaming of the resulting expanded particles. The blowing agent is introduced through the control valve. As used herein, "maintaining a constant pressure" is a concept including adjusting and maintaining the pressure within a range of an allowable variation in the foaming degree of the expanded particles.

When the mixture of resin particles and water is discharged from the pressure vessel into the low pressure region, the particles usually foam immediately and therefore spread with an angle of discharge. Therefore, if it is placed directly behind the opening through which the duct or the like discharges, it is the path of discharge of the particles that spread with a jetting angle.
It should be shaped to have a larger space. When a duct of small diameter, or the like, is placed behind the opening that discharges it, the duct becomes clogged with expanded particles or the resulting expanded particles contain large amounts of particle agglomerates.

According to the invention, the mixture of said resin particles and water is 0.07
Since the particles are emitted through a small area opening of ˜300 mm ² , the jetting angle of the particles is small, and the particles are emitted at a reduced velocity, so that the scattering distance of the particles is shortened. Therefore, the flying trajectory is compact as a whole.

Therefore, in the method according to the invention, the duct or container provided behind the discharge aperture can be made compact.

1 and 2 are explanatory views showing an example of an apparatus used in the method of the present invention.

An apparatus for carrying out the method of the invention will be briefly described with reference to the drawings.

Similar parts in the drawings are indicated by the same symbols. FIG. 1 shows a pressure resistant container (1) equipped with a heater (jacket) (4), a pressure control valve (5) and a pressure gauge (11). Water (3)
The polyolefin resin particles (2) mixed with are impregnated with the foaming agent under increased pressure in the container (1). The discharge valve as shown in the drawing for said mixture of resin particles and water.
It is discharged into the tank (9) through (6) and the orifice plate (7). There, said particles are expanded particles (10). On the other hand, the opening may be opened to the atmosphere (a space without obstruction), and in FIG. 1, the expanding duct (8) is provided directly behind the orifice plate (7) in order to make the apparatus compact. Has been. The duct (8) has a space larger than the scattering trajectory for particles to be emitted and foamed.
Instead of a divergent duct, a regular cylinder-duct can be used as long as its inner diameter is large enough.
In the figure, the opening is opened in a tank (9) having a space larger than a scattering trajectory where particles are discharged and foamed.

The method of the present invention is a polyolefin-based resin having a low density (high foaming degree), a very uniform density distribution, a uniform cell structure between particles, and excellent properties when molded in a mold. It gives resin foam particles. The expanded polyolefin resin particles obtained according to the present invention are usually used for in-mold molding. Density 0.01-0.2 g for in-mold molding
/ Cc expanded resin particles are used. Density 0.01-0.05g
Those having / cc are suitable for producing shock absorbing materials, packaging materials, heat insulating materials and the like. On the other hand, density 0.05-0.2 g
Those having / cc are suitable for structural materials, containers, heat insulating materials and the like.

The process of the invention makes it possible to produce expanded particles having a high closed cell content, ie at least 65%, usually at least 80% of said expanded particles. According to the present invention, the crystalline melting point of the polyolefin resin is −25.
By strictly controlling the heating temperature from 0 ° C to the crystal melting point + 10 ° C, the amount of the foaming agent impregnated in the resin and the viscoelasticity of the resin can be controlled to secure a high closed cell rate. The foamed particles have almost no variation in the closed cell rate. This is because during the discharge of the particles, resin particles capable of foaming are discharged through the small openings while maintaining the partial pressure of the volatile foaming agent in the gas phase of the pressure vessel at a constant level.
Thus, the particles can be effectively fused together in in-mold molding, tend to shrink little, and give a foam in-mold article with excellent properties. As described above, according to the method of the present invention, it is possible to industrially and stably produce uniform expanded particles having excellent properties. In addition, the method of the present invention is an industrial method for producing expanded particles of a non-crosslinked polyolefin resin, which has a difficulty in being formed into an expanded molded article by a general expansion method because the temperature range that gives viscoelasticity suitable for expansion is narrow. Can be manufactured in a simple manner.

Hereinafter, ethylene-propylene random which gives a useful in-mold foam molding product having higher rigidity, strength and heat resistance than polyethylene, and has better in-mold molding property than polypropylene homopolymer in a usual in-mold foam molding method. The present invention will be further described by taking a copolymer as an example. The ethylene-propylene random copolymer preferably has an ethylene content of 1 to 15% and MI (melt flow index, JIS K 6758) 0.1 to 40.
When the ethylene content is less than 1%, the copolymer becomes close to a polypropylene homopolymer, and the remarkable advantage that it can be molded in a mold under a wide range of conditions cannot be obtained.
When the ethylene content exceeds 15%, the copolymer does not have the high rigidity and strength characteristic of polypropylene, and is close to polyethylene. When the MI is less than 0.1, the copolymer has poor fluidity during foaming, and foaming becomes difficult. On the other hand, when the MI exceeds 40, the copolymer exhibits, on the contrary, excessively high fluidity, which makes it highly difficult to foam, and tends to shrink after foaming. In Example 14, an ethylene-propylene random copolymer having an ethylene content of 4.5% and MI 9 was used to give expanded beads having a high degree of expansion and a very small variation in density.

As described above, the method of the present invention provides polyolefin resin foamed particles which are free from blocking, have a high closed cell rate, have a uniform cell structure, are fine, and have a small degree of expansion.

The obtained expanded beads can be molded in a mold by a known method. For example, the obtained expanded beads are cured as they are or after being cured and dried for an appropriate period of time, and then, after being imparted with the foaming ability as they are or subsequently, they are put into a mold. After that, it is heated with a heating medium such as steam for in-mold molding. As a method of imparting foaming ability to the foamed particles,
There may be mentioned a method of impregnating the bubbles of the particles with nitrogen gas, air or a similar inorganic gas, or a method of compressing the particles to impart an increased pressure inside the bubbles to the particles. Another method is also useful in which the obtained inner volume of the mold filled with the expanded particles having the foaming ability is reduced by molding and molding. By this method, an in-mold molded product having a uniform and fine cell structure, uniform particle size, beautiful appearance, uniform density distribution and effective fusion between particles can be obtained. The heating conditions during molding differ depending on the type of resin. When steam is used as the heating medium, the foamed material may be heated with steam at 105-160 ° C for about 5 seconds to 3 minutes.

In the present specification, it is the ratio of closed cells (air bubbles closed by the bubble spacing wall and closed) to the total bubbles calculated from the following formula.

(In the formula, d is the resin density (g / cm ³ ), w is the weight of the expanded particle sample (g), V is the volume of the sample (volume measured by submersion in water) (cm ³ ), and v is the air comparison specific gravity. The true volume of the sample is measured using a meter (for example, an air-comparison specific gravity meter 930 type manufactured by Toshiba Beckman Co., Ltd.). The present invention will be described in more detail based on examples.

Example 1 Ethylene-propylene random copolymer (density 0.90 g /
cm ^3, MI9, about 4.5%) 100 parts of ethylene content (parts by weight, hereinafter the same) and powdered talc 0.05 parts were mixed and extruded the mixture in Peretsuto of about 2mm size. 100 parts of the pellet (700g) in a 4 pressure vessel with a stirrer
Was dispersed in 300 parts of water in the presence of 0.5 part of powdery basic tribasic calcium phosphate and 0.025 part of sodium dodecylbenzenesulfonate as a dispersant. While stirring the dispersion, 55 parts of dichlorodifluoromethane was added, and the mixture was heated to 136 ° C. While maintaining the pressure inside the container at 30 kg / cm ² (gauge pressure) with nitrogen gas, the mixture of pellets and water was attached to the rear end of the discharge valve with an inner diameter of 25 mm, and the diameter of the orifice plate was 3 mm (7.07 mm). ^It was released into the atmosphere through the circular orifice of No. ² , and foamed particles were obtained.The time required for the release was about 11 seconds.The obtained foamed particles had no blocking and had a closed cell ratio of 94% and a density of 94%.
0.021 g / cm ³ Bubble diameter was 0.2 m or less and uniform.

Example 2 MI 3 as ethylene-propylene random copolymer
Expanded particles were obtained in the same manner as in Example 1 except that the ethylene content was 3.5% and the heating temperature of the mixture was 141 ° C. The foamed particles obtained were completely free of blocking, had a closed cell content of 93%, a density of 0.022 g / cm ³ , and a cell diameter of
It was uniform up to 0.2 mm.

Example 3 Expanded particles were obtained in the same manner as in Example 1 except that 40 parts of n-butane was used as the volatile foaming agent, the addition amount of talc to the resin was 0.3 parts, and the heating temperature of the mixture was 115 ° C. The foamed particles obtained had no blocking, closed cell ratio 91%, density
It was 0.016 g / cm ² and the bubble diameter was 0.2 mm or less, which was uniform.

Example 4 Linear low-density polyethylene (density 0.920, M
I 2.1), foamed particles were obtained in the same manner as in Example except that the addition amount of talc was 0.01 part, dichlorodifluoromethane was 60 parts, and the heating temperature of the mixture was 117 ° C. The obtained expanded particles were uniform without blocking, with a density of 0.034 g / cm ³ and a cell diameter of 0.2 mm or less.

Example 5 An ethylene-propylene random copolymer having an ethylene content of about 4.8%, an MI of about 8.8 and a spherical volume-converted diameter of about 6 mm was used, and one opening having a 113 mm ² cross-sectional area was provided after the discharge valve. Expanded particles were obtained in the same manner as in Example 1 except that the orifice plate was attached. The expansion ratio of the obtained expanded particles was 37 times, and it was a single-particulate expanded particle in which particles having uniform and fine closed cells were not fused at all.

Comparative Example 1 Expanded particles were obtained in the same manner as in Example 1 except that a mixture of pellets and water was directly discharged from a discharge valve having an inner diameter of 25 mm (491 mm ² ) without using an orifice plate. The obtained expanded beads contained agglomerates of about 2 to 10 particles.

Example 6 Inner diameter 50 at the rear end, located behind the orifice plate
Same as Example 1, except that a divergent duct measuring 150 cm in length and 150 cm in length was used, and a wire mesh reservoir was provided (see FIG. 1) behind the duct to collect the expanded particles. Pre-expanded particles were obtained. The obtained expanded particles were uniform with no blocking, a closed cell rate of 94%, a density of 0.023 g / cm ³ , and a cell diameter of up to 0.2 mm.

Examples 7 to 13 Foamed particles were obtained in the same manner as in Example 1 except that the addition amount of the inorganic nucleating agent (powdered talc, powdered silica) shown in Table 1 was changed. The foamed particles obtained had the densities and cell diameters shown in Table 1 without any blocking.
The expanded particles obtained using talc or silica had a small cell diameter and were uniform.

The bubble diameter of the expanded particles was measured according to JIS K 6402. The symbol A in Table 1 indicates an average bubble diameter of less than 0.2 mm, and the symbol B indicates an average bubble diameter of 0.2 mm or more and less than 0.5 mm.

Example 14 100 parts (225 kg) of the same pellets as in Example 1 were used to obtain an internal volume of 10
It was dispersed in 300 parts of water in the presence of 0.5 parts of powdery basic tribasic calcium phosphate and 0.025 part of sodium dodecylbenzenesulfonate as a dispersant in a pressure resistant container having a stirrer of 00. After degassing the inside of the container with a vacuum pump while stirring, 30 parts of dichlorodifluoromethane was added, and the mixture was heated to 136 ° C. At this time, the internal pressure of the container is 26 kg
It was / cm ² (gauge pressure). Then, while maintaining the internal pressure of 26 kg / cm ² by press-fitting liquid dichlorodifluoromethane while adjusting it with a valve, while maintaining the temperature,
The discharge valve at the bottom of the container was opened, and the mixture of pellets and water was discharged into the atmospheric pressure through an opening having a diameter of 4 mm (12.57 mm ² ) of an orifice plate attached to the rear end of the discharge valve. The release of the mixture took about 30 minutes. The expanded particles obtained by the release have an average density of 0.0298 g / cm ³ and a density of 0.028.
Most of the foamed particles were contained in the range of up to 0.032 g / cm ³ , and the variation in foaming degree was extremely small and there was no blocking. In the latter half of the discharging operation, the degree of foaming did not decrease and there was no significant variation in the bubble structure throughout the discharging process.

Example 15 The pre-expanded particles having an average density of 0.0298 g / cm ³ obtained in Example 14 were dried at 60 ° C. for 24 hours and then at 60 ° C. and 9 kg / cm ^2.
It was pressurized with (gauge pressure) air for 2 hours. After that 950 ×
2.8 kg / c for filling a mold of 645 × 65 mm and molding in the mold
It was pressurized with m ² (gauge pressure) of steam. The obtained molded product had a density of 0.0198 g / cm ³ , a smooth surface and a beautiful appearance, and the particles were well fused to each other.

[Brief description of drawings]

1 and 2 are explanatory views showing specific examples of the apparatus used in the method of the present invention. (Main symbols in the drawing) (1): Pressure vessel (2): Polyolefin resin particles (3): Water (7): Orifice plate (10): Foamed particles

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takehiko Nishida 4-13, 610 Shimohozumi, Ibaraki-shi, Osaka (56) References JP-A-57-195131 (JP, A) JP-A-58-76433 (JP, A) Japanese Patent Publication Sho 49-2183 (JP, B1)

Claims (10)

[Claims] 1. A polyolefin resin particle having a particle size of 0.25 to 10 mm and a volatile foaming agent are dispersed in water in the presence of a dispersant in a pressure vessel, and the polyolefin resin particle is impregnated with the volatile foaming agent. In order to allow the mixture to be heated to a temperature in the range of the crystalline melting point of the polyolefin-based resin −25 ° C. to the melting point + 10 ° C., while maintaining the pressure-resistant container at a constant temperature and pressure, the polyolefin-based resin particles used The method for producing expanded polyolefin resin particles, characterized in that the mixture of the polyolefin resin particles and water is discharged from an opening having a diameter of 1.2 to 2 times the particle diameter of 1. 2. The manufacturing method according to claim 1, wherein the opening is an opening provided in an orifice plate. 3. The manufacturing method according to claim 1, wherein the pressure inside the pressure vessel is maintained at a constant pressure, and the partial pressure of the volatile foaming agent in the gas phase portion inside the pressure vessel is maintained at a constant level. 4. The method according to claim 1, wherein a fine powder of a poorly water-soluble inorganic substance and an anionic surfactant are used in combination as the dispersion stabilizer. 5. The polyolefin resin is low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ethylene-vinyl acetate copolymer, polypropylene, ethylene-propylene random copolymer, ethylene-propylene. The method according to claim 1, wherein the resin is a resin selected from the group consisting of a block copolymer and an ethylene-propylene-butene terpolymer. 6. The method according to claim 1, wherein the polyolefin resin contains 0.01 to 5 parts by weight of an inorganic nucleating agent with respect to 100 parts by weight of the polyolefin resin. 7. The polyolefin-based resin comprises ethylene having an ethylene content of 1 to 15% by weight and an MI of 0.1 to 40.
Claim 5 which is a propylene random copolymer
The manufacturing method described in paragraph. 8. A polyolefin resin particle having a particle size of 0.25 to 10 mm and a volatile foaming agent are dispersed in water in the presence of a dispersant in a pressure resistant container, and the volatile foaming agent is contained in the polyolefin resin particle. In order to impregnate, the crystalline resin of the polyolefin resin is heated to a temperature in the range of -25 ° C to the melting point + 10 ° C, and the inside of the pressure vessel is maintained at a constant temperature and pressure, while maintaining the gas phase portion in the pressure vessel. While maintaining the partial pressure of the volatile blowing agent constant, the mixture of the polyolefin-based resin particles and water is discharged to the low-pressure region from the pores having a diameter 1.2 to 2 times the particle diameter of the used polyolefin-based resin particles. Polyolefin-based expanded particles obtained by carrying out the above and having a closed cell ratio of at least 65% at a density of 0.01 to 0.2 g / cm ³ and having substantially no particle blocking. 9. The polyolefin resin is low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ethylene-vinyl acetate copolymer, polypropylene, ethylene-propylene random copolymer, ethylene-propylene. The expanded beads according to claim 8, which is a resin selected from the group consisting of a block copolymer and an ethylene-propylene-butene terpolymer. 10. The expanded particles according to claim 8, wherein the polyolefin resin is an ethylene-propylene random copolymer having an ethylene content of 1 to 15% by weight and an MI of 0.1 to 40.