JP2002347025A - Method of manufacturing polyolefin resin composition prefoamed particles - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain prefoamed particles of a polyolefin resin having a small expansion ratio unevenness and a uniform grain size without using a volatile foaming agent in the case of manufacturing the prefoamed particles of the polyolefin resin having the low expansion ratio in such a manner that the low expansion ratio is 2 to 10 times or more preferably 3 to 8 times. SOLUTION: A method for manufacturing polyolefin resin composition prefoamed particles comprises the steps of dispersing the polyolefin resin composition particles containing less than 0.05 pt.wt. of a hydrophilic polymer in 100 pts.wt. of the polyolefin resin in a water dispersion medium in a sealed container, heating the composition particles to a softening temperature or higher of the composition, pressurizing the particles by an inorganic gas, and then uniformly contacting the discharged composition particles with a saturated water vapor in the case of foaming by discharging the particles in an atmosphere lower than an internal pressure in the container.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a pre-expanded polyolefin resin composition particle and a method for producing the same.
More specifically, water is used as a main foaming agent to obtain an expansion ratio of 2
It belongs to the technical field of polyolefin-based resin composition pre-expanded particles which can be suitably used as a raw material of an in-mold expanded molded product of about 10 to 10 times, preferably a low magnification area, for example, 3 to 8 times, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, polyolefin resin composition particles have been dispersed in an aqueous dispersion medium in a closed container, and a volatile foaming agent has been added thereto. It is well known that a pre-expanded polyolefin resin composition particle is obtained by heating the product to a temperature higher than the softening temperature and then releasing the product into a low-pressure atmosphere (for example, Japanese Patent Application Laid-Open No. 58-199125). However, when the pre-expanded particles are produced by the method, the use of the volatile foaming agent not only increases the cost but also makes it difficult to control the expansion ratio due to the large impregnation unevenness of the volatile foaming agent. In addition, the magnification of the expanded particles is not uniform, and pre-expanded particles having good characteristics cannot be obtained. When the volatile foaming agent is a flammable gas such as propane or butane, there is a problem in safety such as combustion and explosion.

Japanese Unexamined Patent Publication No. 60-229936 discloses a method of producing pre-expanded particles by discharging polyolefin resin particles containing a nitrogen-containing inorganic gas as a blowing agent into a low-pressure atmosphere. ing. In this method, the use of a volatile foaming agent eliminates the cost and safety problems, but pre-expanded particles having a uniform particle size cannot be obtained due to variation in magnification due to uneven temperature of the atmosphere to be released. .

Japanese Patent Application Laid-Open No. Sho 60-221440 discloses that ethylene-propylene random copolymer resin particles having an ethylene content of 1 to 12% by weight are dispersed in water, an inorganic gas is introduced, and then heating is performed. , Outflow speed 200 ~
It describes the production of pre-expanded particles by discharging into a low pressure atmosphere at 500 m / sec. However, in this method, it is difficult to maintain the outflow speed at the time of foaming because the outflow speed at the time of release into a low-pressure atmosphere is too large, and the variation in magnification of the obtained pre-expanded particles is deteriorated.

Japanese Patent Application Laid-Open No. H11-106546 discloses that a polyolefin resin containing 100 parts by weight of a polyolefin resin and 0.05 to 20 parts by weight of a hydrophilic polymer is heated to form water-containing particles, and then heated in a low-pressure atmosphere. It describes that the pre-expanded particles are produced by bringing the released particles into contact with a gas at 60 ° C. or higher when the particles are released into the air. However, in this method, the uneven dispersion of the obtained pre-expanded particles is deteriorated due to the poor dispersion of the hydrophilic polymer in the resin.

[0006]

Means for Solving the Problems Accordingly, the present inventors have:
In view of the prior art, when manufacturing pre-expanded particles of a polyolefin-based resin having a low expansion ratio, water is used as a main blowing agent without using a volatile blowing agent, and a uniform polyolefin having a small particle size with little variation in magnification. As a result of intensive studies on the method of producing the pre-expanded particles of the resin, the pre-expanded particles obtained by the method of bringing the particles into contact with saturated steam when releasing the particles into an atmosphere having a pressure lower than the internal pressure in the closed container are obtained. As compared with the case where the particles are not brought into contact with saturated water vapor, the expansion ratio is improved, and it has been found that pre-expanded particles having a uniform particle size with less variation in magnification can be obtained, and the present invention has been completed.

That is, according to the present invention, particles of a polyolefin resin composition comprising 100 parts by weight of a polyolefin resin and less than 0.05 parts by weight of a hydrophilic polymer are dispersed in an aqueous dispersion medium in a closed vessel, and the particles are dispersed in the polyolefin resin. When heated to a temperature equal to or higher than the softening temperature of the system resin composition and pre-foamed by discharging into the atmosphere at a lower pressure than the internal pressure in the closed container, the released particles are brought into contact with saturated steam. The method for producing the pre-expanded polyolefin resin composition particles (Claim 1), increasing the internal pressure in the closed vessel by introducing nitrogen, air or an inorganic gas mainly composed of these, and then reducing the atmosphere to a pressure lower than the internal pressure 2. The method according to claim 1, wherein the foam is prefoamed by discharging the saturated steam into the polyolefin resin set. Object of the softening temperature or higher, for producing a polyolefin resin composition pre-expanded particles according to claim 1, characterized in that the melting point or less (claim 3).

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a method for producing pre-expanded particles having a low expansion ratio from a polyolefin resin composition particle containing 100 parts by weight of a polyolefin resin and less than 0.05 parts by weight of a hydrophilic polymer. used.

The polyolefin resin contains 50 to 100% by weight of an olefin monomer unit, and more preferably 70 to 10% by weight.
The resin contains 0% by weight, and 0 to 50% by weight, and more preferably 0 to 30% by weight of a monomer unit copolymerizable with the olefin monomer. Since the olefin monomer unit is contained in an amount of 50% by weight or more, a molded article that is lightweight and has excellent mechanical strength, workability, electrical insulation, water resistance, and chemical resistance can be obtained. The monomer unit copolymerizable with the olefin monomer has adhesiveness,
It is a component used for modifying transparency, impact resistance, gas barrier properties, and the like. In order to obtain the effect of use, it is preferably used in an amount of 2% by weight or more, more preferably 5% by weight or more. .

Specific examples of the olefin monomer include:
Ethylene, propylene, butene, pentene, hexene,
C2-C8 alpha-olefin monomers, such as heptene and octene, and cyclic olefins, such as a norbornene monomer, are mentioned. These may be used alone,
More than one species may be used in combination. Of these, ethylene,
Propylene is preferred because it is inexpensive and the properties of the resulting polymer are improved. Specific examples of the monomer copolymerizable with the olefin monomer include a vinyl alcohol ester such as vinyl acetate, an alkyl group such as methyl methacrylate, ethyl acrylate, and hexyl acrylate having 1 to 6 carbon atoms (meth). Examples include alkyl acrylate, vinyl alcohol, methacrylic acid, and vinyl chloride. These may be used alone or in combination of two or more. Among them, vinyl acetate is preferred from the viewpoint of adhesiveness, flexibility and low-temperature characteristics, and methyl methacrylate is preferred from the viewpoint of adhesiveness, flexibility, low-temperature characteristics and thermal stability.

The polyolefin resin may have a melt index (MI) of, for example, 0.2 to 50 g / 10 min for a polypropylene resin, and more preferably 1 to 30 g / min.
10 minutes is preferable, and the flexural modulus (JIS K
7203) is, for example, 5,000 to 20,000 kg / cm ² G for a polypropylene resin,
000~16000kg / cm ² G, as the melting point, for example, one hundred twenty-five to one hundred sixty-five ° C. in polypropylene resin, more preferably from 130 to 160 ° C..

When the MI is less than 0.2 g / 10 minutes,
If the melt viscosity is too high, pre-expanded particles with a high expansion ratio are difficult to obtain, and if it exceeds 50 g / 10 minutes, the melt viscosity with respect to the elongation of the resin at the time of expansion is low and the foam tends to break, and the pre-expanded particles with a high expansion ratio Tends to be difficult to obtain. If the bending strength is less than 5000 kg / cm ² G, the mechanical strength and heat resistance become insufficient, and
When it exceeds g / cm ² G, the flexibility and cushioning properties of the obtained foamed molded article tend to be insufficient. Further, when the melting point is less than 125 ° C., the heat resistance is insufficient, and when it exceeds 165 ° C., the adhesiveness at the time of molding and the secondary foaming power tend to be insufficient.

Specific examples of the polyolefin resin include, for example, an ethylene-propylene random copolymer,
Ethylene-propylene-butene random terpolymer,
Polypropylene resins such as polyethylene-polypropylene block copolymer and homopolypropylene; low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer And polyethylene-based resins; polybutene, polypentene and the like. The polyolefin-based resin may be used in a non-crosslinked state, or may be used after being crosslinked by peroxide or radiation. These polymers may be used alone or in combination of two or more. Among them, a polypropylene-based resin is preferable, in which the variation in magnification is smaller than that of other polyolefin-based resins, and the molded article produced from the obtained pre-expanded particles has good mechanical strength and heat resistance.

The above hydrophilic polymer is ASTM D5
A polymer having a water absorption of 0.5% or more as measured in accordance with No. 70, and is a so-called hygroscopic polymer, a water-absorbing polymer (water that is several to several hundred times its own weight without being dissolved in water. It is a concept including a polymer that absorbs and is not easily dehydrated even when pressure is applied) and a water-soluble polymer (a polymer that dissolves in water at normal or high temperature). In the molecule of the hydrophilic polymer, a hydrophilic group such as a carboxyl group, a hydroxyl group, an amino group, an amide group, an ester group, and a polyoxyethylene group may be contained.

Examples of the hygroscopic polymer include a carboxyl group-containing polymer, a polyamide, a thermoplastic polyester elastomer, and a cellulose derivative. These may be used alone or in combination of two or more. Examples of the water-absorbing polymer include, for example, a crosslinked polyacrylate polymer, a starch-acrylic acid graft copolymer, a crosslinked polyvinyl alcohol polymer, a crosslinked polyethylene oxide polymer, an isobutylene-maleic acid copolymer, and the like. Is raised. These may be used alone or in combination of two or more.

Examples of the water-soluble polymer include poly (meth) acrylic acid-based polymers, poly (meth) acrylate-based polymers, polyvinyl alcohol-based polymers, polyethylene oxide-based polymers, and water-soluble cellulose derivatives. Is raised. These may be used alone or in combination of two or more.

The amount of the hydrophilic polymer used depends on the type of the hydrophilic polymer, but is less than 0.05 part by weight of the hydrophilic polymer based on 100 parts by weight of the polyolefin resin. When the amount of the hydrophilic polymer is 0.05 parts by weight or more, the dispersibility of the hydrophilic polymer in the polyolefin-based resin composition becomes poor, and the variation in magnification of the obtained expanded particles becomes large.

When the polyolefin resin composition used in the present invention contains a filler, that is, an inorganic filler and / or an organic filler, pre-expanded particles having uniform cells and high expansion ratio can be obtained. This is preferable from the viewpoint.

Specific examples of the inorganic filler include talc, calcium carbonate, calcium hydroxide and the like. Among these inorganic fillers, talc is
This is preferable because pre-expanded particles having a small variation in magnification, uniform cells, and a relatively high expansion ratio are provided.

The organic filler is not particularly limited as long as it is solid at a temperature equal to or higher than the softening temperature of the polyolefin resin. Specific examples of the organic filler include a fluororesin powder, a silicone resin powder, and a thermoplastic polyester resin powder.
The fillers may be used alone or in combination of two or more.

With respect to the average particle diameter of the filler, pre-expanded particles having uniform cells can be obtained, and a molded article having excellent mechanical strength and flexibility can be obtained from the pre-expanded particles. From the point, 50μm or less, further 10μ
m, preferably 0.1 μm or more, more preferably 0.5 μm or more from the viewpoint of secondary aggregation and handling workability.

The amount of the filler to be used is such that polyolefin-based resin 1 can be used in order to obtain pre-expanded particles having a relatively high expansion ratio.
The amount is preferably 0.001 part by weight or more, more preferably 0.005 part by weight or more, based on 00 parts by weight. Also, when forming the pre-expanded particles, the pre-expanded particles exhibit excellent fusibility. In order to obtain a molded article having excellent mechanical strength and flexibility, the amount is 3 parts by weight or less, preferably 2 parts by weight or less.

The polyolefin resin composition containing a hydrophilic polymer, a filler and the like in the polyolefin resin is usually melt-kneaded using an extruder, kneader, Banbury mixer, roll, or the like. Then, cylindrical,
It is preferable to form a desired resin composition particle shape that can be easily used for preliminary foaming, such as an elliptic column, a sphere, a cube, and a rectangular parallelepiped. The conditions for producing the resin composition particles, the size of the resin particles, and the like are not particularly limited. For example, the resin composition particles obtained by melt-kneading in an extruder are usually 0.1 wt.
5 to 10 mg / particle.

In the present invention, the polyolefin resin composition particles are dispersed in an aqueous dispersion medium in a closed container,
The resin composition particles are heated to a temperature equal to or higher than the softening temperature of the polyolefin-based resin composition, and preferably, an inorganic gas is introduced into the closed container, and the pressure in the closed container is set to 0.6 to
After pre-foaming by discharging into the atmosphere at a pressure lower than the internal pressure in the closed container after holding at 7.5 MPa, the pre-foamed polyolefin resin composition is obtained by contacting the released particles with saturated steam. The particles are made.

The aqueous dispersion medium in which the resin composition particles are dispersed may be any solvent which does not dissolve the polyolefin resin composition, and is usually water or water and ethylene glycol, glycerin, methanol, ethanol, isopropyl alcohol, or the like. A mixture with at least one kind is exemplified, but water is preferred from the viewpoint of environment, economy and the like.

When dispersing the resin composition particles in the aqueous dispersion medium in the closed container, as the inorganic dispersant, for example, inorganic salts such as tribasic calcium phosphate, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, etc. Clays such as bentonite and kaolin; Of these, tribasic calcium phosphate is preferred because of its strong dispersing power.

Examples of the surfactant include anionic surfactants such as sodium dodecylbenzene sulfonate, sodium n-paraffin sulfonate, sodium α-olefin sulfonate, and sodium alkylnaphthalene sulfonate; benzalkonium chloride; And cationic surfactants such as alkyltrimethylammonium chloride and dialkyldimethylammonium chloride. Of these
Sodium n-paraffin sulfonate is preferred because it gives good dispersing power and is easily biodegradable.

The amount of the inorganic dispersant and the surfactant used is not particularly limited, and any commonly used amount may be used.
The amount is preferably 0.05 to 10 parts by weight based on 0 parts by weight, and the surfactant is 0.000 part by weight based on 100 parts by weight of resin particles.
5 to 1 part by weight is preferred.

The amount of the resin composition particles dispersed in the aqueous dispersion medium is preferably 3 to 100 parts by weight, more preferably 10 to 50 parts by weight, based on 100 parts by weight of the aqueous dispersion medium. When the amount of the resin composition particles is less than 3 parts by weight, the productivity is reduced and the method is not economical. When the amount exceeds 100 parts by weight, particles tend to fuse in the container during heating.

The temperature at which the resin composition particles, the inorganic dispersant, and the surfactant are dispersed in an aqueous dispersant in a closed container and heated is preferably a temperature not lower than the softening temperature of the polyolefin resin composition to be used. Is the melting point + 20 ° C or less,
Further, the temperature is preferably from the melting point + 5 ° C. or higher to the melting point + 15 ° C. For example, in the case of an ethylene-propylene copolymer having a melting point of 145 ° C, 145 to 165 ° C, furthermore, 150 to 16
0 ° C. is preferable, and if it is lower than 145 ° C., it is difficult to foam,
If the temperature exceeds 165 ° C., the mechanical strength and heat resistance of the obtained pre-expanded particles are not sufficient, and the particles tend to be easily fused in the container.

The melting point of the polyolefin resin composition is from 40 ° C. to 2 ° C. by DSC (differential scanning calorimetry).
The peak of the melting peak that appears when the temperature is raised to 20 ° C. at a rate of 10 ° C./min, cooled to 40 ° C. at a rate of 10 ° C./min, and then raised again to 220 ° C. at a rate of 10 ° C./min. Temperature.

As the inorganic gas, nitrogen, air or these are mainly used (usually 50% by volume or more, more preferably 70% or more).
Volume%), an inert gas such as argon, helium, xenon, or an inorganic gas containing a small amount of water vapor, oxygen, hydrogen, ozone (50% by volume or less, further 30% by volume or less) can be used. Nitrogen and air are preferred from the viewpoints of performance, productivity, safety, and environmental compatibility.

As described above, the pressure in the closed vessel after the introduction of the inorganic gas is preferably 0.6 to 7.5 MPa, more preferably 1.0 to 7.0 MPa. When the pressure is less than 0.6 MPa, the effect of introducing the inorganic gas is reduced, and unfoamed beads tend to be obtained. If it exceeds 7.5 MPa, the cell diameter in the pre-expanded particles becomes finer, the closed cell ratio decreases, and the shrinkage, shape stability, and mechanical strength of the molded product are impaired. The timing of introducing the inorganic gas may be before, during, or after heating the closed container.

The pressure lower than the internal pressure in the closed container may be a pressure lower than the internal pressure in the closed container, and a pressure near the atmospheric pressure is usually selected. The term "atmosphere" refers to a space containing the trajectory of the mixture of the discharged particles and the aqueous dispersion medium, but generally refers to a pipe or a duct-like device which is isolated from the outside air.

The above-mentioned saturated steam is used for producing pre-expanded particles having a uniform particle size with reduced and reduced shrinkage of expanded particles by contacting the released particles with the expanded particles. It is preferably adjusted to a temperature equal to or lower than the melting point of the raw resin composition used for the pre-expanded particles, preferably within a temperature range of 90 to 110 ° C. When the temperature exceeds 110 ° C., the glass transition temperature or the melting point of the resin is reached and the bubbles of the pre-expanded particles are broken or the particles are fused to each other. If the temperature is lower than 90 ° C., water in the expanded particles is rapidly condensed, and the particles shrink greatly, and the variation in magnification is large.

The saturated steam pressure is preferably a saturated steam pressure equal to or lower than the melting point of the raw material resin of the pre-expanded particles,
It is preferable that the pressure be adjusted to 150 kPa. When contacting the pre-expanded particles with saturated steam, immediately after the outlet, several saturated steam blowing ports are installed,
A preferred method is one in which saturated water vapor uniformly contacts the pre-expanded particles to be released. The uniform variation of the saturated water vapor reduces the variation in magnification of each particle.

In this case, a small amount of water spray may be used at the time of jetting steam, or an operation such as adjusting the cooling rate of the pre-expanded particles by spraying water after contact with saturated steam may be used. There is no problem.

When the resin particles are discharged from the closed container to the low pressure region, it is preferable that the resin particles be discharged through a throttle plate and that the internal pressure of the closed container is maintained as constant as possible by introducing an inorganic gas or the like into the closed container. . As the diaphragm, an orifice type, a nozzle type, a venturi type, and the like can be used, and a combination thereof can also be used. The discharge ports provided in these diaphragms are of such a size that the resin particles to be discharged are not clogged, and the opening area and the cross-sectional shape are not limited as long as they achieve a predetermined discharge speed. is not.

As described above, by keeping the pressure in the closed container as constant as possible and discharging the resin through the diaphragm, the release speed and the release amount of the resin particles can be kept constant, so that the saturated steam uniformly contacts each particle. As a result, uniform pre-expanded particles with less variation in magnification can be obtained.

The pre-expanded particles obtained by the method of the present invention are treated in a pressure vessel under heating and pressure for a certain period of time to carry out air inclusion, etc., and then filled in a mold for in-mold foam molding, followed by steam heating molding. Then, a foamed molded article according to the mold may be manufactured. The foamed molded article thus obtained has a small variation in magnification of the pre-expanded particles used as a raw material, and therefore has a small variation in density within the molded article, a small dimensional shrinkage and a small shape deformation, and therefore has a very high commercial value.

[0041]

The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples.

<Examples 1 to 3> Ethylene-propylene random copolymer (density of 0.1) as a polyolefin resin.
90 to 0.91 g / cm ³ , ethylene content 3% by weight,
145 ° C., MI = 5.5 g / 10 min, flexural modulus 1
0000 kg / cm ² G) 100 parts by weight and talc (average particle diameter 7 μm) 0.3 part by weight were added, and the mixture was fed to a 50 mmφ single screw extruder, melt-kneaded, and extruded from a cylindrical die having a diameter of 1.5 mmφ. After cooling with water, cut with a cutter, and resin particles (1.8 mg /
Grain). The obtained particles had a softening temperature of 63 ° C., a melting point of 145 ° C. and a density of 0.90 to 0.91 g / cm ^{3 as} measured according to JIS K7112.

100 parts by weight of the obtained resin particles, 0.3 part by weight of tribasic calcium phosphate as an inorganic dispersant, and 0.00% of sodium n-paraffin sulfonate as a surfactant
5 parts by weight and 300 parts by weight of water are charged into a pressure-resistant sealed container,
The contents of the vessel were heated to 153.0 ° C. while stirring. At this time, the pressure in the pressure vessel was about 0.5 MPa. Thereafter, the pressure in the container was brought to the pressure shown in Table 1 by air pressurization over 10 minutes, and after holding for 20 minutes, the valve at the lower part of the closed container was opened and the aqueous dispersion (resin particles and the aqueous dispersion medium) was removed. It was discharged into a low pressure vessel through an orifice having one opening. At the time of discharge, a water vapor blowing port was provided immediately after passing through the orifice, so that water vapor having a vapor pressure of about 100 kPa was in contact with the water dispersion to be discharged. The blown steam temperature at this time was 98.0 ° C. by a temperature recorder. During the discharge, heated air was introduced into the closed container, and the temperature and the internal pressure in the closed container were kept constant.

<Comparative Examples 1 and 2> The same operation as in Example 1 was performed, and pre-expanded particles were obtained without blowing steam when releasing resin particles.

Example 4 Ethylene-propylene random copolymer (density 0.91 g / cm ³ , ethylene content 3% by weight, melting point 145 ° C., MI = 5.5 g / 10 min, flexural modulus 10,000 kg / cm to ² G) 100 parts by weight, the hydrophilic polymer (ethylene - the carboxyl groups of methacrylic acid copolymer salified with sodium ions, ionomers crosslinked between molecules (ethylene units 85 wt% methacrylic acid units 15 wt% , And 0.04 parts by weight of methacrylic acid units forming a salt) and 0.3 part by weight of talc (average particle size: 7 μm), and fed to a 50 mmφ single screw extruder. After melt-kneading, the mixture was extruded from a cylindrical die having a diameter of 1.5 mmφ, cooled with water, cut with a cutter, and resin particles from a columnar polyolefin resin (1.8 mg / ) To give a. The resin particles subjected to the same procedure as in Example 1 to obtain pre-expanded particles.

Comparative Example 3 Pre-expanded particles were obtained in exactly the same manner as in Example 4, except that the amount of the hydrophilic polymer added was 0.1 part by weight.

<Comparative Example 4> The amount of the hydrophilic polymer added was 2
Except for the weight part, the same operation as in Example 4 was performed,
Pre-expanded particles were obtained.

(Expansion ratio (dry ratio)) Pre-expanded particles 3
After weighing about 10 g and drying at 60 ° C. for 6 hours or more, measuring the weight w, measuring the volume v by the submerged method, determining the true specific gravity ρb = w / v of the pre-expanded particles, The expansion ratio K = ρr / ρb was determined from the ratio of the density to the density ρr.

(Content ratio) The dried pre-expanded particles were
The expansion ratio measured by using the pre-expanded particles in a state where the pressure inside the pre-expanded particles was left higher than the atmospheric pressure by leaving the pre-expanded particles under atmospheric pressure of Pa for 2 hours, and the contraction of the pre-expanded particles was completely recovered.

(Variation in magnification) The variation in magnification was determined by the following equation. K _av magnification variation _{(%) = (σ m)} / K av × 100 wherein is, JIS Z8801 standard sieve (3.5,
7, 8, 9, 10 the weight fraction W _i of each residue foamed particles upon sieved with a mesh eight), the expansion ratio K _i, formula; K _av = Σ {K _i × Average expansion ratio determined by _Wi ). Σ _m in the formula is JIS Z8801 standard sieve (3.5, 4, 5, 6, 7, 8, 9, 10 mesh 8).
Weight fraction W _i of each residue expanded particles when sieved by species), the expansion ratio K _{i, wherein; σ m = √Σ {W i} × (K av -
_Ki ) The standard deviation of the magnification determined in ² ｝.

[0051]

[Table 1]

As is apparent from Table 1, when pre-expanded polyolefin resin particles having a low expansion ratio are to be obtained, if the steam is not blown in, the expansion ratio is low, but the variation in the expansion ratio is large. Was found to be mixed with unexpanded pre-expanded particles, and was unsuitable for use in in-mold foam molding.

When 0.04 parts by weight of the hydrophilic polymer is added to 100 parts by weight of the polyolefin resin, the foaming ratio and the dispersion are the same as those of the system without the addition of the hydrophilic polymer. Is increased to 0.1 parts by weight and 2 parts by weight, the variation in magnification is large even when steam is blown in when releasing particles. In other words, it can be seen that, in order to obtain polyolefin resin pre-expanded particles in the low magnification region, for example, 3 to 8 times, which is the object of the present application, the method can be achieved with little variation in magnification according to the method of the example.

[0054]

According to the method of the present invention for producing pre-expanded particles of a polyolefin resin composition comprising water as a main blowing agent, the water vapor is brought into contact with the resin composition particles to reduce the variation in magnification. Pre-expanded particles having a small and practically sufficient appearance can be obtained. Particularly when the expansion ratio is low, for example, when obtaining pre-expanded particles of 3 to 8 times suitable for in-mold foam molding, the effect of the present invention is remarkable, and a good in-mold foam molded article can be obtained.

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Claims (3)

[Claims] 1. A polyolefin-based resin composition particle comprising 100 parts by weight of a polyolefin-based resin and less than 0.05 parts by weight of a hydrophilic polymer is dispersed in an aqueous dispersion medium in a closed container, and the resin particles are dispersed in the polyolefin-based resin particles. When heated to a temperature equal to or higher than the softening temperature and pre-expanded by discharging into an atmosphere at a lower pressure than the internal pressure in the closed container, the released particles are brought into contact with saturated steam to obtain an expansion ratio of 2 to 10 times. A method for producing pre-expanded particles of a polyolefin-based resin composition, comprising: 2. The method according to claim 1, wherein the internal pressure in the closed vessel is increased by introducing nitrogen, air or an inorganic gas mainly composed of nitrogen and air, and then pre-foamed by releasing the gas into an atmosphere having a pressure lower than the internal pressure. A method for producing the pre-expanded particles of the polyolefin resin composition according to the above. 3. The method for producing pre-expanded polyolefin resin composition particles according to claim 1, wherein the temperature of the saturated steam is not lower than the softening temperature of the polyolefin resin composition particles and not higher than the melting point.