JP2006307177A - Polyproylene based resin foam particle, process for production of polyproylene based resin foam particle molded propduct and polyproylene based resin foam particle molded product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide polypropylene based resin foam particles capable to produce a molded product showing excellent appearance with little voids, even when molded under a low compression rate, and excellent moisture barrier property, with a very short cooling time. <P>SOLUTION: The foam particles comprise a base resin containing polypropylene based resin, and have cell walls forming multiple foams, in which the foam particles are in a spherical form having holes penetrating from one end to the other end with a diameter H<SB>D</SB>(mm) and have a specifically defined maximum diameter D<SB>0</SB>(mm), average radius direction foam diameter L<SB>CV</SB>and average circumferential direction foam diameter L<SB>CH</SB>, with a L<SB>CV</SB>/L<SB>CH</SB>ratio ≥1.05, a H<SB>D</SB>/D<SB>0</SB>ratio of 0.08-0.4 and a H<SB>D</SB>/L<SB>CV</SB>ratio of 0.1-10. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing polypropylene expanded particles (hereinafter sometimes referred to as expanded particles), a polypropylene resin expanded particle molded body (hereinafter also referred to as a molded body), and a molded body obtained by the method. .

  In general, polypropylene-based expanded particles are obtained by dispersing polypropylene-based resin particles (hereinafter sometimes referred to as resin particles) in water, impregnating with a physical foaming agent under pressure and heating, and then lowering the resin particles to low pressure. It is manufactured by discharging to a zone (dispersion medium release foaming method). As another method, after the polypropylene resin is melted in an extruder and mixed with a foaming agent, the foamed particles are produced by extruding and foaming from a die or by cutting to an appropriate length while foaming. It is also possible to do. In addition, the polypropylene resin foamed particle molded body is generally a method in which foamed particles are filled in a mold or guided between the belts and guided into a tunnel-shaped passage, and then heat-sealed using steam. (In-mold molding method). In the former case, the inside of the mold is in the mold, and in the latter case, the inside of the tunnel-shaped passage is in the mold.

  The molded body is used in various fields as an energy absorbing material, a heat insulating material, a cushioning material, a lightweight box, etc. due to its excellent performance. These energy absorbing materials, heat insulating materials, cushioning materials, lightweight boxes, etc. are required to have a beautiful and smooth surface, and for that purpose, air and water do not flow within the range of common sense, that is, Water shielding performance is required as a natural quality. That is, during in-mold molding, it is required that water does not permeate when the foamed particles are heat-sealed without gaps. However, this excellent water shielding property prolongs the cooling time during in-mold molding with steam, leading to the problem that productivity is not easily improved.

  That is, when steam is introduced and the foamed particles are heated and subjected to secondary foaming to fuse the foamed particles, the secondary foaming narrows the gap between the foamed particles and makes it difficult for the steam to enter. As a result, a large amount of steam is required to fuse the expanded particles without gaps and improve the water shielding performance, and the cooling time during molding becomes longer, which hinders improvement in productivity.

In order to solve this problem and improve the water shielding performance by fusing the foamed particles without any gaps using steam, at the same time to shorten the cooling time, the mold using the irregular shaped foam particles with through holes is used. A method of performing in-molding has been proposed (Patent Document 1). In this method, the foamed particles are subjected to pressure treatment to give an internal pressure of 1.3 to 7 kgf / cm ² , and then the pressure-treated foamed particles are molded in a mold to obtain a molded body.

  However, this method has a problem that the production process is increased because it is necessary to impart foaming ability to the foamed particles by pressure treatment prior to in-mold molding. That is, if the foamed particles are exposed to a high-pressure atmosphere and a gas such as air or nitrogen is added to the foamed bubbles to increase the pressure inside the foam (hereinafter also referred to as pressure treatment), the foamed particles are not separated from each other. It was not possible to obtain a molded article that was fused and excellent in water shielding performance. Dare to perform in-mold molding without pressure treatment, it is necessary to increase the steam heating temperature and forcibly molding, so that only compacts with large shrinkage can be obtained, or molding with reduced shrinkage was performed In some cases, only a molded body in which voids remained was obtained. In this case, when filling the foamed particles into the mold to increase the compressibility to compensate for the lack of foaming ability, it is possible to obtain a molded article having excellent water shielding performance in which the foamed particles are fused without gaps. However, since the compression ratio is too high, the original expansion ratio of the expanded particles is greatly reduced, and there is a problem that only a molded product having a low expansion ratio (high density) can be obtained. Further, the effect of shortening the cooling time is not so large. There wasn't.

Japanese Patent Laid-Open No. 2002-248645

  The present invention has been made to solve the above-mentioned problem, and when the foamed particles are filled in the mold without being subjected to pressure treatment and molded in-mold using steam, the cooling time is short and used. It is an object of the present invention to provide foamed particles capable of producing a molded article with little reduction in the foaming ratio with respect to the foaming ratio of the foamed particles to be produced and having few voids. Another object of the present invention is to provide a molded article having a low porosity produced using the expanded particles, and a method for producing the molded article.

According to the present invention, the following polypropylene-based resin expanded particles, a method for producing a polypropylene-based resin expanded particle molded body, and a polypropylene-based resin expanded particle molded body are provided.
[1] A foamed particle comprising a base resin containing a polypropylene resin and comprising a cell wall defining a plurality of cells, wherein the foamed particle is a spherical body having a through-hole extending from one end to the other end. And having a maximum diameter D ₀ (mm) defined by the following, an average bubble diameter L _{CV in the} radial direction, an average bubble diameter L _{CH in} the circumferential direction, and a diameter H _D (mm) of the through hole, and the ratio L _CV Expanded particles characterized in that / L _CH is 1.05 or more, ratio H _D / D ₀ is 0.08 to 0.4, and ratio H _D / L _CV is 0.1 to 10 Provided.
D _{0, L} CV, _{L CH} and _{H D} defined the maximum diameter _{D 0,} the average cell diameter _{L CV} radial diameter _{H D} of the circumferential average cell diameter _{L CH,} and the through-hole, as follows Measured:
(A) arbitrarily collecting 10 expanded particles;
(B) cutting each of the collected expanded particles at a center position of a straight line connecting one end and the other end of the through hole with a surface perpendicular to the straight line to obtain an annular cross section having an outer peripheral edge and an inner peripheral edge;
(C) The cross section is projected on a screen or photographed using a microscope.
(D) In the Projected section or photographed cross-section on said screen, intersects the two points 2 points _{D 01, D 02} and the inner periphery of the outer peripheral edge, and, between the two points _{D 01, D 02} Draw the first straight line so that the distance is maximum,
(E) measuring the distance Dn ₀ between the two points D ₀₁ , D ₀₂ ;
(F) a straight line perpendicular to the first straight line, intersects the two points 2 points _{d 01, d 02} and the inner periphery of the outer peripheral edge, and the distance between two points _{d 01, d 02} is the maximum Draw a second straight line so that
(G) about the intersection P1 of the first and second straight line, draw a radius _Dn 0/4 of the circle C1,
(H) Count the number of intersections Nn _H between the circle C1 and the cross-section projected on the screen or the bubble wall on the photographed cross-section,
(I) about the respective intersections P2, P3 of the outer periphery of the first straight line and the circle C1, a circle C2, C3 of the radial _Dn 0/8,
(J) Count the number of intersections between the first straight line and the bubble walls located on each of the circles C2 and C3, and select the larger number Nn _V.
(K) The average diameter Ln _CV is obtained from the following equation (1).
Ln _CV = 0.405 × (Dn ₀ / Nn _V ) (1)
(L) determining the average diameter _{Ln CH} from the following equation (2),
Ln _CH = 0.810 × Dn ₀ × sin (π / Nn _H ) (2)
(M) The length Hn _D (mm) of the longest straight line among the straight lines connecting two points on the inner peripheral edge forming the through hole is measured in the cross section projected on the screen or the cross section photographed. ,
(N) The maximum diameter D ₀ is an arithmetic average of Dn ₀ of 10 collected expanded particles.
(O) The average cell diameter L _{CV in the} radial direction is an arithmetic average of Ln _CV of 10 foam particles collected.
(P) Circumferential average cell diameter L _CH is an arithmetic average of Ln _CH of 10 foam particles collected.
(Q) The diameter H _{D of the} through hole is an arithmetic average of Hn _D of the 10 collected foam particles.
[2] The expanded particles according to [1], wherein the apparent density is less than 36 g / L.
[3] A method for producing a foamed particle molded body comprising filling the foamed particles according to the above [1] or [2] into a molding die without pressurizing and molding with a steam.
[4] A foamed particle molded body obtained by the method according to [3].

  By using the foamed particles of the present invention, it is possible to obtain a foamed particle molded body having excellent water shielding performance and appearance even if in-mold molding is performed without pressure treatment of the foamed particles. A molded body can be obtained without greatly reducing the magnification, and / or the cooling time can be shortened as compared with the conventional method.

  Furthermore, when the foamed particles according to the present invention are used, it is easy for the steam to penetrate into the foamed particle bubbles, so that more steam than the conventional foamed particles penetrates into the bubbles, and more cooling than usual during the cooling. Condensation heat increases by the amount of penetration, and the cooling rate and volume shrinkage rate increase. As a result, the cooling time until the molded body is sufficiently cooled and taken out is shortened.

  In addition, according to the method for producing a foamed particle molded body of the present invention, the polypropylene-based resin having excellent water shielding performance by filling the foamed particles in a mold without pressure treatment and molding the mold with steam. It is possible to obtain a foamed particle molded body at a lower cost and faster than the conventional one, and further molding without lowering the original foaming ratio of the foamed particles (that is, without significantly increasing the original bulk density of the foamed particles). The body can be obtained or / and the cooling time can be shortened compared to conventional methods.

  The polypropylene resin expanded particle molded body obtained by the method of the present invention is manufactured at a lower cost and faster than the conventional one, has a low porosity, excellent water shielding performance, and excellent fusion property between compressed particles and compressive strength. It is a foam suitable as a cushioning material, packaging material, various containers and the like.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.
1 is a plan view of the expanded particles of the present invention, FIG. 2 is a sectional view taken along the line II-II of FIG. 1, and FIG. 3 is a sectional view showing another example of the expanded particles of the present invention. . In the figure, 1 represents expanded particles, 2 represents a through-hole, 2a and 2b represent one end and the other end of the through-hole, 10 represents a bubble wall, and 11 represents a bubble.

  The foamed particles of the present invention are made of a base resin containing a polypropylene-based resin, and the foamed particles are composed of a bubble wall 10 that defines a plurality of bubbles 11 as shown in FIG. Further, as shown in FIG. 1, the expanded particle 1 is a spherical body having a through hole 2 extending from one end 2a to the other end 2b. The spherical body referred to in the present invention includes a true sphere, an oval sphere, an oval, a prism, a cylinder, a cube, a rectangular parallelepiped, a truncated cone, a truncated pyramid, and the like, and these are deformed and are asymmetric solid shapes. Is also included. Corners such as prisms, cylinders, cubes, rectangular parallelepipeds, truncated cones and truncated pyramids are preferably rounded.

  FIGS. 3A to 2D are cross-sectional views cut along a direction perpendicular to the through-holes with examples of the shape of the spherical body. The spherical body has a hollow or hollow oval cross section as shown in FIG. 3A, a hollow elliptical cross section as shown in FIG. A hollow triangle having a round cross-section as shown in FIG. 3 and a hollow polygon having a round cross-section as shown in FIG. Further, when viewed in a plan view as shown in FIG. 1, the outer shape of the spherical body may be as shown in FIGS. 3 (a) to 3 (d).

The foamed particles of the present invention have a long diameter D ₀ described later when the maximum length of the foamed particles in the direction matching the straight line connecting the one end 2a and the other end 2b of the through hole is Lmax (see FIG. 1). When the ratio to Lmax (D ₀ / Lmax) is in the range of 0.8 to 1.25, the gap between the expanded particles does not increase when the mold is filled, This is preferable because a small number of molded articles can be obtained.

The expanded particles of the present invention have a maximum diameter D ₀ (mm) defined by the following, an average bubble diameter L _{CV in the} radial direction, an average bubble diameter L _{CH in} the circumferential direction, and a diameter H _D (mm) of the through hole. The ratio L _CV / L _CH is 1.05 or more, the ratio H _D / D ₀ is 0.08 to 0.4, and the ratio H _D / L _CV is 0.1 to 10.

The maximum diameter D ₀ , the radial average bubble diameter L _CV , the circumferential average bubble diameter L _CH , and the through hole diameter H _D are measured as follows:
(A) Collecting 10 expanded particles arbitrarily.
(B) As shown in FIG. 1, each collected expanded particle is cut at a center position of a straight line (length L) connecting one end 2 a and the multi-end 2 b of the through hole 2 with a plane perpendicular to the straight line. An annular cross section having an outer peripheral edge B1 and an inner peripheral edge B2 is obtained.
(C) The cross section is projected on a screen or photographed using a microscope.
(D) intersects two points D ₀₁ and D _{02 on the} outer peripheral edge B1 and two points F ₀₁ and F _{02 on the} inner peripheral edge B2 in the cross-section projected on the screen or the photographed cross-section (FIG. 2); The first straight line E1 is drawn so that the distance between the two points D ₀₁ and D ₀₂ is maximized. In this case, when there are two or more straight lines that maximize the distance between the two points D ₀₁ and D ₀₂ , the straight line that maximizes the distance between the two points F ₀₁ and F ₀₂ of the inner peripheral edge B2 ( A straight line E1 is selected as long as two or more straight lines are present).
(E) The distance Dn ₀ between the two points D ₀₁ and D ₀₂ is measured.
(F) A straight line that is orthogonal to the first straight line E1, intersects the outer peripheral edge B1, the two points d ₀₁ and d _02, and the two points f ₀₁ and f _{02 of the} inner peripheral edge B2, and the two points d ₀₁ and d The second straight line E2 is drawn so that the distance between ₀₂ becomes maximum. In this case, when there are two or more straight lines that maximize the distance between the two points d ₀₁ and d ₀₂ , the straight line that maximizes the distance between the two points f ₀₁ and f ₀₂ of the inner peripheral edge B2 ( If there are two or more straight lines, any one may be selected) as a straight line E2.
(G) about the intersection P1 of the first and second linear E1, E2, draw a radius _Dn 0/4 of the circle C1.
(H) counting the number Nn _H of intersection of the circular C1 and said screen on Projected section or photographed cross-section on the cell walls (as shown in Figure 2 by black circles).
(I) about the respective intersections P2, P3 of the outer periphery of the first straight line E1 and the circle C1, a circle C2, C3 of the radial _Dn 0/8.
(J) Count the number of intersections (indicated by black triangles in FIG. 2) between the first straight line E1 and the bubble walls located on each of the circles C2 and C3, and select the larger number Nn _V (e.g., an Nn _V 3 of circle C2 in the case of FIG. 2, for Nn _V circle C3 is 2, and selects the third Nn _V circle C2).
(K) The average diameter Ln _CV is obtained from the following equation (1).
Ln _CV = 0.405 × (Dn ₀ / Nn _V ) (1)
(L) determining the average diameter _{Ln CH} from the following equation (2).
Ln _CH = 0.810 × Dn ₀ × sin (π / Nn _H ) (2)
(M) The length Hn _D (mm) of the longest straight line among the straight lines connecting two points on the inner peripheral edge B2 forming the through-hole 2 in the cross-section projected on the screen or the cross-section photographed. taking measurement.
(N) the maximum diameter _{D 0} is the arithmetic mean of Dn ₀ of ten expanded beads taken.
(O) The average cell diameter L _{CV in the} radial direction is an arithmetic average of Ln _CVs of 10 collected expanded particles.
(P) The circumferential direction average cell diameter L _CH is an arithmetic average of Ln _CH of 10 collected foam particles.
(Q) The diameter H _{D of the} through hole is an arithmetic average of Hn _D of the 10 collected foam particles.

Here, the equation (1) is derived as follows.
Generally, the relationship of the following formula (3) is established between the number N of bubble walls on the line segment L ₀ (length L ₀ ) and the average bubble diameter L _d .
L _d = 1.62 × (L ₀ / N) (3)
Thus, simply, (3) instead of expression of _L d, _{L 0} and N, by substituting _Ln CV, _Dn 0/4 and Nn _V respectively, is derived (1).

The equation (2) is derived as follows.
N _H length l ₀ of the surrounding regular polygon sides inscribed in a circle of radius D _0/4 is given by the following equation (4).
_{l 0 = 2 × (D 0} /4) × sin (2π / 2N H) × N H (4)
Since the length of the circumference of the polygon on the N _H side inscribed in the circle C1 as shown in FIG. 2 can be approximated to the length l ₀ of the regular polygon on the N _H side inscribed in the circle C1, simply, By substituting Ln _CH , ₁₀ and Nn _H for L _d, L ₀ and N in the formula (3), the formula (2) is derived.

In the present invention, the ratio L _CV / L _CH is 1.05 or more, the ratio H _D / D ₀ is 0.08 to 0.4, and the ratio H _D / L _CV is 0.1 to 10. This is very important. Since the expanded particles of the present invention satisfy this condition, secondary expansion is efficiently performed with a smaller amount of steam than the conventional expanded particles. That is, it is possible to give a molded article excellent in appearance and water shielding properties without performing pressure treatment. Further, the amount of steam required for in-mold molding can be reduced, and a molded body can be obtained without greatly reducing the original expansion ratio of the expanded particles. Furthermore, a molded body can be easily obtained even with a short cooling time.

When the ratio L _CV / L _CH is less than 1.05, the secondary foamability and heating efficiency in pressureless molding are not improved. Therefore, the ratio L _CV / L _CH is preferably 1.07 or more, and more preferably 1.10 or more. On the other hand, if the ratio L _CV / L _CH is too large, the heat transfer efficiency is increased, but the bubble shape becomes too long, and the obtained molded body may be easily bent. Therefore, the ratio L _CV / L _CH is preferably 3 or less.

In addition, from the viewpoint of increasing the effect of rapidly invading the steam into the foamed particles and increasing the closed cell ratio, the radial average cell diameter L _CV is preferably 30 μm or more, more preferably 50 μm or more, More preferably, it is 100 μm or more, and most preferably 130 μm or more. Further, since the thickness of the cell walls that are suitable for secondary expansion is obtained, it is preferable that L _CV is 1000μm or less, and more preferably less 700 .mu.m.

Further, from the viewpoint of increasing the closed cell ratio, the circumferential average bubble diameter L _CH is preferably 28 μm or more, more preferably 47 μm or more, and further preferably 95 μm or more. L _CH is preferably 950 μm or less, and more preferably 600 μm or less, because a cell wall thickness suitable for secondary foaming can be obtained.
In order to easily obtain the effect of the present invention, the number of bubbles on the first straight line E1 in the cross section of FIG. 2 is preferably 5 to 40, and more preferably 6 to 30.

In the foamed particles of the present invention, when the ratio H _D / D ₀ of the diameter H _D (mm) of the through hole to the long diameter D ₀ (mm) is less than 0.08, the through hole is too small and the heating efficiency is improved. In addition, the cooling effect due to adiabatic expansion becomes poor. When H _D / D ₀ exceeds 0.4, the through hole is too large, and it becomes difficult to obtain a molded article having excellent water shielding performance. Therefore, it is preferable that H _D / D ₀ is 0.1 to 0.25. Furthermore, when the ratio H _D / L _CV of the diameter H _D (mm) of the through holes to the average bubble diameter L _CV (mm) is less than 0.1, the resulting molded body is easily cramped by the bubbles, or the heating efficiency And the cooling effect due to adiabatic expansion becomes poor. When H _D / L _CV exceeds 10, it becomes difficult to obtain a molded article having excellent water shielding performance. _Therefore, it is preferred that the H D _{/ L CV} 0.5 to 8, more preferably from 1.2 to 5.5.

The size of the diameter H _D of the through hole is even not limited as long as it can steam passes, improvement of heating efficiency, in order to obtain a cooling effect due to adiabatic expansion is preferably at least 0.16 mm, 0.2 mm or more Is more preferable. The upper limit is preferably 1.8 mm, more preferably 1.6 mm or less, and more preferably 1.2 mm or less for the reason that the mold can be uniformly filled and the heating efficiency and cooling efficiency during molding are increased. More preferably, it is 0.9 mm or less.

  In the cross section shown in FIG. 2, the through hole 2 is preferably sized and shaped so that the inner peripheral edge B1 is inside the circumference of the circle C1. In addition, the inner peripheral edge B1 of the through hole is preferably present outside the respective circumferences of C2 and C3, but the length of each arc of C2 and C3 that overlaps the inner peripheral edge B1 is the circle C2, As long as it is within 30% of the length of each circumference of C3, it may overlap with the circumference of inner circumference B1 and the circumferences of circles C2 and C3.

Furthermore, in order to obtain a molded article excellent in high foaming, the apparent density D _t (g / L) is more preferably 20 to 35 g / L, and still more preferably 23 to 34 g / L. However, when the foamed particles of the present invention are used, even if the apparent density D _t (g / L), which is difficult to be molded by the prior art, is less than 36 g / L, the foaming ratio is not reduced so much. A molded article excellent in high foaming is obtained.

The apparent density D _t of the expanded particles in this specification is determined as follows. First, the expanded particles are allowed to stand for 10 days in a temperature-controlled room under conditions of atmospheric pressure, relative humidity of 50%, and 23 ° C. Next, 500 or more foamed particles left for 10 days in the same temperature chamber were collected, and after measuring the weight W1 (g) of the foamed particle group, the above weight was put in a graduated cylinder containing methanol at 23 ° C. The measured expanded particle group is submerged using a wire mesh or the like, the volume V1 (cm ³ ) of the expanded particle group is read from the rising amount of the meniscus of ethanol, and the apparent density D _t of the expanded particle is calculated by the following equation (5). .
D _t = 1000 × W1 / V1 (5)

  By using the foamed particles of the present invention, it is possible to obtain a foamed particle molded body having excellent water shielding performance and appearance even if in-mold molding is performed without pressure treatment of the foamed particles. A molded body can be obtained without greatly reducing the magnification, and / or the cooling time can be shortened as compared with the conventional method.

Such remarkable effects of the expanded particles of the present invention are presumed to be obtained by the following mechanism. That is, since the average bubble diameter L _CV in the radial direction is larger than the average bubble diameter L _{CH in the} circumferential direction (L _CV / L _CH is 1.05 or more), the bubbles are elongated in the radial direction, and They are arranged radially around the through holes. Moreover, the through-hole exists in a moderately small size (L / D ₀ is 0.08 to 0.4 and L / L _CV is 0.1 to 10). Due to the shape and arrangement of the bubbles, the foamed particles of the present invention are superior in secondary foamability in in-mold molding compared to conventional foamed particles (L _CV / L _CH is less than 1.00). It is considered a thing.

  More specifically, when steam is introduced into a mold filled with foamed particles, the foamed particles are heated from the outside by the steam, and at the same time, the steam passes through the outer skin and gradually penetrates into the bubbles inside the foam. The particles are also heated from the inside. Thereafter, the foamed particles undergo secondary foaming by releasing the pressure within the mold so as to be substantially equal to the atmosphere. At this time, the permeated steam is considered to contribute to the secondary foaming as a foaming agent together with the air in the bubbles originally possessed by the foamed particles. Therefore, in order to improve the secondary foamability and fuse the foamed particles without gaps to obtain a molded article having excellent water shielding properties, it is desirable to allow steam to penetrate into the foamed particles more quickly and uniformly. In the present invention, through-holes are formed in the expanded particles as a first means for that purpose.

  In other words, if through-holes are formed in the foam particles, (i) steam passes through the through-holes during in-mold molding, so the contact area with the steam per foam particle increases, so heating by steam The efficiency is improved, and as a result, it is not necessary to heat more than necessary for secondary foaming of the foamed particles. Furthermore, (ii) even after the foamed particles are uniformly filled in the mold, secondary foaming must be performed in excess of the amount of the through-holes, so that the volume of adiabatic expansion during secondary foaming increases. As a result, the cooling effect due to adiabatic expansion inside the expanded particles is increased, and the water cooling time is shortened.

  However, in order to obtain a molded article having excellent water shielding properties by performing in-mold molding (hereinafter sometimes referred to as pressureless molding) without subjecting the foam particles to pressure treatment, through holes are formed in the foam particles. It is not enough.

In the expanded particles of the present invention, the elongated bubbles are arranged radially from the center (P ₁ ) of the through hole toward the outside, that is, the bubble shape extends from the center of the vertical sectional hole to the outer periphery. Since it is elongated in the direction of the direction, the number of bubble membranes that must be permeated when steam enters the inside of the foamed particle from the through-hole, so that the steam can enter the inside of the foamed particle more rapidly than the conventional foamed particle. it is conceivable that. In other words, it is considered that the smaller the number of bubbles between the through hole and the outer periphery of the expanded particle, the more the steam can enter the expanded particle than the conventional expanded particle. Due to the shape and arrangement of the bubbles, it is considered that the expanded particles of the present invention are excellent in secondary foamability in in-mold molding. That is, even if pressureless molding is performed, a foamed particle molded body excellent in water shielding performance and appearance can be obtained, and a molded body can be obtained without greatly reducing the original foaming ratio of the foamed particles.

  Furthermore, in the case of the foamed particles of the present invention, the steam easily penetrates into the foamed particle bubbles, so that more steam penetrates into the foam than the conventional foamed particles, and more steam penetrates than usual when cooled. Condensation heat is increased by the amount, and the cooling rate and volume shrinkage rate are increased. As a result, the cooling time until the molded body is sufficiently cooled and taken out is shortened.

  The base resin of the expanded particles of the present invention is a polypropylene resin, and as the polypropylene resin, a propylene homopolymer, a propylene block copolymer or a propylene random copolymer is used. Here, the propylene-based random copolymer and the propylene-based block copolymer are copolymers of propylene and other comonomers each containing 60 mol% or more of a propylene component. Examples of other comonomers copolymerized with propylene include α-olefins other than propylene such as ethylene, 1-butene, 1-petene, and 1-hexene.

The propylene-based block copolymer may be a binary copolymer such as a propylene-ethylene block copolymer or a propylene-butene block copolymer, or a ternary copolymer such as a propylene-ethylene-butene block copolymer. It may be a polymer. The propylene random copolymer may be a binary copolymer such as propylene-ethylene random copolymer, propylene-butene random copolymer, propylene-ethylene random copolymer, or propylene-ethylene-butene. A terpolymer such as a random copolymer may be used.
The proportion of the comonomer component other than propylene in the copolymer is preferably 0.05 to 15% by weight, particularly preferably 0.1 to 10% by weight.

  The polypropylene resin constituting the expanded particles of the present invention may be a crosslinked polypropylene resin or a non-crosslinked polypropylene resin, but is preferably a non-crosslinked propylene resin that can be easily recycled.

  The base resin used in the present invention is preferably a propylene random copolymer having a melting point of 165 ° C. or lower, considering the productivity and equipment cost when molding expanded particles among the polypropylene resins. A propylene-based random copolymer at 135 to 160 ° C. is preferred.

Furthermore, in order to elongate the bubbles around the through-holes and arrange them radially around the through-holes, the bubbles are elongated in the direction from the center of the expanded particle cross section toward the outer periphery due to the expansion of the foaming agent at the stage where the bubbles grow during foaming. It is necessary to maintain the oriented bubbles in that state. For that purpose, the melt flow rate (MFR) is preferably 0.1 to 60 g / 10 min, more preferably 0.2 to 50 g / 10 min, further 0.5 to 35 g / 10 min. The one having 2 to 25 g / 10 minutes is particularly preferred.
The melt flow rate (MFR) is a value measured under test condition 14 (230 ° C./2.16 kgf load) of JIS K7210.

  Moreover, when further flexibility is requested | required of a molded object, it is preferable to add 5-40 wt% of elastomers, such as ethylene-propylene rubber, to the above-mentioned base resin. In the present invention, a thermoplastic resin other than the polypropylene resin and a thermoplastic elastomer can be added to the polypropylene resin and used within the range not impairing the object of the present invention. However, in this case, the addition amount of the thermoplastic resin or thermoplastic elastomer other than the polypropylene resin is preferably at most 30 parts by weight per 100 parts by weight of the polypropylene resin.

Moreover, it is desirable that the expanded particles of the present invention have a high temperature peak. The range of the amount of heat at the high temperature peak is preferably 2 J / g to 45 J / g, more preferably 5 J / g to 40 J / g. Further, the range of the high temperature peak heat amount tends to change depending on the heat of fusion of the base resin, and the ratio of the heat amount H _H of the high temperature peak to the total heat amount H _T in the case of foamed particles (H _H / H _T ) is 0. Most preferred is in the range of .05 to 0.5.

The high temperature side peak calorific value H _H of the expanded particles in this specification is a DSC curve obtained by heating 1 to 8 mg of expanded particles to 220 ° C. at a rate of 10 ° C./min using a differential scanning calorimeter (FIG. 4). It corresponds to the area of the high temperature side peak b in and can be obtained as follows. First, as shown in FIG. 4, a straight line connecting a point I on the DSC curve at 80 ° C. and a point II on the DSC curve indicating the melting end temperature of the resin is drawn. Next, a straight line passing through point III of the DSC curve corresponding to the valley between the intrinsic endothermic peak a and the high temperature side peak b and perpendicular to the temperature on the horizontal axis of the graph is drawn to a straight line connecting point I and point II. Let the intersection be IV. The area of the portion (FIG. 4: hatched portion) surrounded by the straight line connecting points IV and II, the straight line connecting point III and point IV, and the DSC curve connecting point III and point II is high. This corresponds to the endothermic amount of the side peak.

Further, the total peak heat H _T herein, corresponds to the sum of the areas of the area of the specific endothermic peak a and the high-temperature side peak b. The area of the intrinsic endothermic peak a is a portion surrounded by a straight line connecting the points IV and I, a straight line connecting the points III and IV, and a DSC curve connecting the points III and I (FIG. 4: white portions). ). Incidentally, methods of modulating the high-temperature side peak heat _{H H} of the expanded beads is disclosed in, for example, JP-2001-151928 and the like.

In the present invention, in order to adjust the bubble diameter, it is preferable to add a bubble regulator to the base resin. Examples of the air conditioner include inorganic substances such as talc, calcium carbonate, borax, zinc borate, and aluminum hydroxide. The amount added is preferably 0.001 to 10 parts by weight, preferably 0.01 to 5 parts by weight per 100 parts by weight of the base resin since the average cell diameter L _CV in the radial direction can be easily maintained at 30 μm or more. Is more preferable.
In addition, when adding a foam control agent to the base resin, the foam control agent can be kneaded into the base resin as it is, but usually a master batch of the foam control agent is made in consideration of dispersibility and the like. It is preferable to knead the base resin.

In addition, a coloring pigment and a dye can be added to the base resin. In this case as well, the addition amount is determined on the basis of maintaining the _LCV at 30 μm or more.
In addition, when adding a color pigment and dye to the base resin, it is preferable to make a master batch of the color pigment and dye in consideration of dispersibility and knead this with the base resin.

  (I) The foamed particles of the present invention can be obtained, for example, by preparing resin particles having unfoamed through holes and then foaming the resin particles by a dispersion medium discharge foaming method. Further, (II) the foamed particles have a cross-sectional shape in which a cylindrical string-like product can extrude a foamable molten resin composition obtained by melting unfoamed resin particles using an extruder and mixing with a foaming agent. It can also be produced by extrusion foaming by extruding and foaming from a die and cutting the foam in the middle of foaming or after completion of foaming into an appropriate length.

In any of the methods (I) and (II), in order to obtain the expanded particles of the present invention, the pressure difference between the high pressure and the low pressure when discharging or extruding from the high pressure at which foaming does not occur to the low pressure at which foaming occurs is obtained. It is preferably 400 kPa or more, preferably 500 to 15000 kPa. Further, the diameter H _{D of the} through hole, the maximum diameter D ₀ of the expanded particles, and the average bubble diameter L _CV in the radial direction are such that the H _D / D ₀ ratio is 0.08 to 0.4, and the H _D / L _CV ratio is It is necessary to adjust so that it may become 0.1-10.

Diameter H _D of the through hole, in general, the more expansion ratio of the foamed particles obtained is large, the foam when the larger size or extrusion foaming of the through hole of the unfoamed resin particles having a through hole The larger the diameter of the member for forming the through hole, the larger the value. In addition, the maximum diameter D ₀ of the expanded particles is generally such that the larger the expansion ratio of the obtained expanded particles, the greater the thickness of the resin particles having through holes or the greater the thickness of the expanded particles having through holes, Indicates a large value. Moreover, the average cell diameter L _CV in the radial direction can be adjusted by controlling the size of the average cell diameter of the expanded particles to be obtained. The size of the average cell diameter is usually adjusted by the type and amount of the foaming agent, the foaming temperature, and the amount of the foam regulator added. The expansion ratio is usually adjusted by the amount of foaming agent added, the foaming temperature, and the differential pressure during foaming. In an appropriate range, generally, the larger the amount of the foaming agent added, the higher the foaming temperature, and the larger the differential pressure, the larger the expansion ratio of the resulting expanded particles.

  In the method described in (II) Extrusion Foaming, for example, the method described in European Patent No. 588321, European Patent No. 968077, etc., the outlet shape of the die is changed so as to obtain a cylindrical foam. Can be manufactured.

In the (I) dispersion medium releasing foaming method, the resin particles are dispersed in water in a closed container such as an autoclave together with a physical foaming agent, and heated to a temperature equal to or higher than the softening temperature of the resin particles, and foamed in the resin particles. Next, while maintaining the pressure in the sealed container at a pressure equal to or higher than the vapor pressure of the foaming agent, once under the water surface in the sealed container is released, and the resin particles and water are simultaneously released from the container. It can be obtained by discharging in a low-pressure atmosphere.
At that time, even if the resin particles are not spherical, the resin particles heated and plasticized in the closed container can be changed to spherical by the action of the surface tension on the resin particles of the dispersion medium.
Note that the through holes of the foam particles are usually larger than the through holes of the resin particles.

  Furthermore, when the foaming agent is impregnated and released into the low pressure region at the foaming temperature, the pressure in the high pressure region in the sealed container can be increased to 0.5 MPa (G) or more so that the bubbles around the through hole are centered on the through hole. It is preferable to make it vertically oriented, and more preferably 1.5 MPa (G) or more.

  As the blowing agent used in the dispersion medium release foaming method, propane, isobutane, butane, isopentane, pentane, cyclopentane, hexane, cyclobutane, cyclohexane, chlorofluoromethane, trifluoromethane, 1,1,1,2-tetra Organic physical foaming agents such as fluoroethane, 1-chloro-1,1-difluoroethane, 1,1-difluoroethane, 1-chloro-1,2,2,2-tetrafluoroethane, nitrogen, carbon dioxide, argon, Examples thereof include inorganic physical foaming agents such as air. Among these, an inexpensive inorganic gas-based foaming agent that does not destroy the ozone layer is preferable, and nitrogen, air, and carbon dioxide are particularly preferable. Also, it can be used in a mixed system of two or more of these foaming agents. When producing foamed particles having a small apparent density (high foaming ratio), a mixed foaming agent of carbon dioxide and butane is used. preferable.

The amount of the foaming agent used is appropriately selected according to the relationship between the apparent density of the foamed particles to be obtained and the foaming temperature. Specifically, in the case of the above foaming agent excluding nitrogen and air, the amount of foaming agent used is usually 2 to 50 parts by weight per 100 parts by weight of resin particles. In the case of nitrogen and air, an amount is used in which the pressure in the sealed container is within the pressure range of 10 to 70 kgf / cm ² G.

  In the closed container, water is preferable as a dispersion medium for dispersing the resin particles, but any dispersion medium that does not dissolve the resin particles can be used. Examples of such a dispersion medium include ethylene glycol, glycerin, Examples include methanol and ethanol.

  In the closed container, when the base resin particles are dispersed in the dispersion medium and heated to the foaming temperature, an anti-fusing agent can be used to prevent the resin particles from fusing each other. As the anti-fusing agent, any inorganic or organic one can be used as long as it does not dissolve in water or the like and does not melt even when heated. In general, an inorganic one is preferable.

  As the inorganic anti-fusing agent, powders such as kaolin, talc, mica, aluminum oxide, titanium oxide, and aluminum hydroxide are suitable. The anti-fusing agent preferably has an average particle size of 0.001 to 100 μm, particularly 0.001 to 30 μm. Moreover, the addition amount of the anti-fusing agent is usually preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the resin particles.

  Further, anionic surfactants such as sodium dodecylbenzene sulfonate and sodium oleate and aluminum sulfate are preferably used as a dispersion aid. The dispersion aid is usually preferably added in an amount of 0.001 to 5 parts by weight per 100 parts by weight of the resin particles.

The resin particles used for producing the foamed particles can be obtained as follows.
The base resin added with the air conditioner is heated and kneaded in an extruder and extruded from a die having a cross-sectional shape similar to the cross-sectional shape of the target expanded particle as a plurality of hollow strands (having through holes). The resin particles can be produced by means such as cooling the strands through water and then cutting them appropriately.

  In addition, the cylindrical resin particle which has a through-hole inside can be manufactured by using the extruder die | dye which has the (circular) slit similar to the cross-sectional shape of a desired resin particle in molten resin exit. Furthermore, in order to prevent the strand hole extruded as a cylinder having a through hole from being crushed, a pressure adjusting hole for maintaining the pressure inside the through hole of the strand at atmospheric pressure or higher is provided inside the slit. It is preferred to use a die. It should be noted that the pressure adjustment hole is connected to a gas press-fitting device to supply air or the like to the inside of the strand through-hole, or the inside of the through-hole of the strand is communicated with the atmospheric pressure portion, so that the inside of the through-hole is at atmospheric pressure or higher. Can be kept at a pressure of.

  The weight of the resin particles is preferably 0.02 to 20 mg, since it is easy to maintain the through holes of the resin particles and the expanded particles, and the uniform filling property of the expanded particles in the mold can be secured. -6 mg is more preferred.

The diameter of the through hole formed in the resin particle is such that there is no mixture of resin particles with the through hole closed due to the mutual attachment of the strands coming out of the die, and the ratio L _CV / L _CH is 1.05 or more. 0.05 to 0.24 mm is preferable, 0.05 mm to 0.23 mm is more preferable, and 0.06 to 0.20 mm because particles can be easily obtained and a molded body having a desired water barrier property can be obtained. Is more preferable. The diameter of the through hole formed in the resin particles, means the maximum length of the through-holes obtained in the diameter H _D similar to the measuring method of the through hole of the expanded beads.

  The melt flow rate (MFR) of the polypropylene resin for producing the resin particles is selected in the same manner as the polypropylene resin constituting the foamed particles because the MFR does not change greatly during the resin particle production process. do it.

The polypropylene resin expanded particle molded body of the present invention is obtained by filling the expanded particle of the present invention into a mold without pressure treatment and heat molding with steam, and the porosity is 0 to 0. 11% by volume.
When the porosity exceeds 11% by volume, the obtained molded product has insufficient fusion between the expanded particles, and compression strength, fusion strength, etc. required when used as a buffer material, packaging material, various containers, etc. May not be obtained, and the appearance is inferior to a conventionally known molded body.

  The water permeability coefficient of the foamed particle molded body of the present invention is preferably 0 cm / sec. Even when the water permeability coefficient is not 0 cm / sec, the required compressive strength, fusion strength and the like may not be obtained as in the case where the porosity exceeds 11% by volume. In particular, unless the water permeability coefficient is 0 cm / sec, it cannot be used as a container for storing a liquid such as a fish box.

The porosity (A) in this specification is calculated by the following equation (6).
A (%) = [(BC) / B] × 100 (6)

However, B is a volume (cm ³ ) calculated from the outer dimensions of the molded body, C is a volume (cm ³ ) excluding the voids of the molded body, and the molded body is submerged in a liquid (for example, alcohol). The volume can be determined by measuring the volume as an increase in the amount.

  The water permeability coefficient in this specification is based on JIS A1218. The foamed particle molded body of the present invention (length 120 mm, width 120 mm, thickness 50 mm) is used instead of sand as a sample, and a square tube is used instead of a cylinder in which a sample is placed. Is obtained by performing water permeability measurement by a constant water level equation.

  According to the method of the present invention using the expanded particles, the expanded particles are filled in a mold without pressure treatment, and steam is introduced into the mold and heat-molded. A molded article of the present invention of volume% can be obtained. That is, after filling in a mold that can close the foamed particles but cannot be sealed, by introducing steam into the mold, the foamed particles are heated and foamed and fused together to form a molding space. It is possible to obtain a molded body suitable for the shape.

  In the method of the present invention, the heating temperature and the heating are sufficiently considered in consideration of the shape of the expanded particles filled in the mold, the melting point of the base resin, the expansion force of the expanded particles in the mold, and the contraction of the molded body. Heating conditions such as time are appropriately determined.

  In addition, after cooling and foaming the foamed particles, the resulting molded product can be cooled in the mold by using a water-cooling method, but by using the vaporization heat of the steam to cool it. Is preferred.

In the method of the present invention, the ratio (D _t / D _b ) of the apparent density D _t (g / L) to the bulk density D _b (g / L) is preferably 1.6 to 2.6. It is more preferably 6 to 2.1, and still more preferably 1.6 to 1.9. When the ratio (D _t / D _b ) is within this range, a molded article having excellent water shielding performance can be easily obtained, and the cooling time can be shortened as compared with the conventional method.

When the ratio (D _t / D _b ) is less than 1.6, the molded product obtained by in-mold molding has good appearance and physical properties, but the expanded volume of the foamed particles is small, so steam insulation Since the cooling effect by expansion becomes small, there is a possibility that the shortening of the molding cycle aimed at by the present invention cannot be achieved.
On the other hand, when the ratio (D _t / D _b ) exceeds 2.6, voids communicating with each other are formed in the molded body obtained by in-mold molding, resulting in an increase in water permeability. There is a possibility that a product having the same appearance and physical properties cannot be obtained.

Bulk density D _b of the expanded particles in the present specification is determined as follows.
First, the expanded particles are allowed to stand for 10 days in a temperature-controlled room under conditions of atmospheric pressure, relative humidity of 50%, and 23 ° C. Next, 500 or more foamed particles left for 10 days in the same constant temperature room were collected, and after measuring the weight W ₂ (g) of the foamed particle group, the above-mentioned foamed particles were measured in an empty graduated cylinder. The group is put in, the volume V ₂ (cm ³ ) of the expanded particle group is read from the scale of the graduated cylinder, and the bulk density D _b of the expanded particle is calculated by the following equation (7).
D _b = 1000 × W ₂ / V ₂ (7)

  In the present invention, a method is adopted in which foamed particles are filled in the mold so that the compression ratio is 4 to 25% by volume, preferably 5 to 20% by volume, and then molded in-mold with steam. By doing so, a desired molded article can be obtained.

  When the compression rate is less than 4% by volume, if the foamed particles are not provided with an appropriate internal pressure, the in-mold molding results in insufficient fusion between the expanded particles, and the porosity is 0 to 11% by volume. There is a possibility that the molded product cannot be obtained. On the other hand, when the compression ratio exceeds 25% by volume, the density of the obtained molded body is remarkably increased with respect to the bulk density of the expanded particles, and the original expansion ratio of the expanded particles cannot be effectively used.

  The compression ratio is adjusted by filling the foamed particles in the mold (cavity) with an amount of foamed particles exceeding the cavity volume. When filling the mold with foam particles, do not close the mold completely in order to exhaust the air in the mold from the mold or to efficiently fill the mold with foam particles. The opening portion of the mold is called cracking. The cracking is finally closed when the steam is introduced after the foam particles are filled in the mold, and as a result, the filled foam particles are compressed.

The compression rate in this specification is calculated | required by following (8) Formula. In the formula, a represents the weight (g) of the expanded particles filled in the mold, b represents the bulk density (g / L) of the expanded particles, and c represents the volume (L) in the mold.
Compression rate (%) = [(a / (b × c)) − 1] × 100 (8)

  According to the present invention, since a molded body is produced by the method described above, it is possible to obtain a molded body having a porosity of 0 to 11% by volume without pressing the foamed particles, and shorten the molding cycle. You can also.

  Next, the present invention will be described in more detail with specific examples. However, the present invention is not limited to the examples.

The polypropylene resins used in the examples and comparative examples are shown below.
A) Propylene-ethylene random copolymer (hereinafter also referred to as rPP), MFR: 7 g / 10 min, melting point 143 ° C.
B) Propylene-ethylene block copolymer (hereinafter also referred to as bPP), MFR: 10 g / 10 min, melting point 160 ° C.
C) Propylene homopolymer (hereinafter also referred to as hPP), MFR: 10 g / 10 min, melting point 165 ° C.

Examples 1-7, Comparative Examples 1-7

  Each base resin shown in Table 1 and zinc borate as a foam regulator are melt-kneaded in an extruder, and then a cylindrical strand (columnar in Comparative Examples 1 and 2) is extruded from a die having a slit. After quenching in water, it was cut to a predetermined length to obtain resin particles having an average weight of 2 mg per piece. Zinc borate was added in a master batch so that the blending amount was 0.05% by weight (in Example 3, Comparative Examples 2, 3, and 6 it was 0.01% by weight). The length ratio of the resin particles to the diameter of the resin particles and the diameter of the through holes of the resin particles are shown in Table 1.

  Next, using the foaming agent described in Table 1, 3 g of kaolin as an anti-fusing agent, 0.06 g of sodium dodecylbenzenesulfonate as a surfactant, and 1 kg of the above resin particles were added to 3 liters of water (Example 3, In Comparative Examples 2, 3, and 6, 0.01 g of aluminum sulfate was further added to water as a dispersion aid) and the temperature was raised while stirring in a closed container (autoclave having a volume of 5 liters). For 15 minutes. Next, while applying a back pressure equal to the equilibrium vapor pressure (by introducing a high-pressure gas of the same kind as the used blowing agent into the autoclave), the lower end of the sealed container is opened while the pressure is maintained, and the resin particles and Water was simultaneously released under atmospheric pressure to foam the resin particles to obtain expanded particles having through holes or no through holes. After the expanded particles were dried at room temperature, various physical properties were measured. The results are shown in Table 2.

  The foamed particles were molded in-mold under the molding conditions shown in Table 3 (molding steam pressure, cooling time, foaming surface pressure) to obtain a cuboid shaped product having a length of 20 cm, a width of 25 cm, and a thickness of 5 cm. The foamed particles were filled in the mold without performing an internal pressure application treatment with a pressurized gas. Table 3 also shows the compression rate of the expanded particles filled in the mold. The density, porosity, and water permeability coefficient of the obtained molded body were measured, and the appearance was observed. Furthermore, based on these, the results of evaluation of appearance, evaluation of cooling efficiency, and comprehensive evaluation are shown in Table 3. The cooling time indicates the time required to cool the molded body to such an extent that the molded body is no longer expanded even when the molded body is removed from the mold. The foaming surface pressure means the maximum pressure with respect to the inner surface of the molding die indicated by the molded body during molding. In addition, since the amount of resin increases as the density of the obtained molded body increases, the cooling time requires a longer time. Therefore, the cooling efficiency was evaluated in consideration of the density of the molded body. In the table, ○ indicates good and × indicates poor.

From the results in Table 3, it can be seen that if the foamed particles of the present invention are used, a molded article having an excellent appearance with a low porosity and an excellent water shielding property can be produced without performing a pressure treatment. In addition, it can be seen that even if the compression ratio is reduced, a molded article having an excellent appearance with a low porosity and an excellent water shielding property can be produced in a very short cooling time. In addition, since the compression rate at the time of molding is small (the degree of compression of the foamed particles required at the time of pressureless molding is small), the density of the obtained molded body is not so large with respect to the bulk density of the foamed particles, The original expansion ratio of the expanded particles can be used effectively.

It is a top view of the expanded particle of the present invention. It is sectional drawing which follows II-II of FIG. 1, and is explanatory drawing of the measuring method of radial direction average bubble diameter L _CV and circumferential direction average bubble diameter _LCH . It is sectional drawing similar to FIG. 2 which shows the example of the cross-sectional shape of the expanded particle of this invention. It is a figure which shows an example of the DSC curve of a polypropylene resin expanded particle.

Claims (4)

A foamed particle comprising a base resin containing a polypropylene resin and comprising a cell wall defining a plurality of bubbles, the foamed particle being a spherical body having a through-hole extending from one end to the other end inside , Having a maximum diameter D ₀ (mm) defined by the following, an average bubble diameter L _{CV in the} radial direction, an average bubble diameter L _{CH in} the circumferential direction, a diameter H _D (mm) of the through hole, and a ratio L _CV / L Expanded particles, wherein _CH is 1.05 or more, ratio H _D / D ₀ is 0.08 to 0.4, and ratio H _D / L _CV is 0.1 to 10;
D _{0, L} CV, _{L CH} and _{H D} defined the maximum diameter _{D 0,} the average cell diameter _{L CV} radial diameter _{H D} of the circumferential average cell diameter _{L CH,} and the through-hole, as follows Measured:
(A) arbitrarily collecting 10 expanded particles;
(B) cutting each of the collected expanded particles at a center position of a straight line connecting one end and the other end of the through hole with a surface perpendicular to the straight line to obtain an annular cross section having an outer peripheral edge and an inner peripheral edge;
(C) The cross section is projected on a screen or photographed using a microscope.
(D) In the Projected section or photographed cross-section on said screen, intersects the two points 2 points _{D 01, D 02} and the inner periphery of the outer peripheral edge, and, between the two points _{D 01, D 02} Draw the first straight line so that the distance is maximum,
(E) measuring the distance Dn ₀ between the two points D ₀₁ , D ₀₂ ;
(F) A straight line orthogonal to the first straight line, intersecting the outer peripheral edge with the two points d ₀₁ , d ₀₂ and the inner peripheral edge, and the distance between the two points d ₀₁ , d ₀₂ being the maximum Draw a second straight line so that
(G) about the intersection P1 of the first and second straight line, draw a radius _Dn 0/4 of the circle C1,
(H) Count the number of intersections Nn _H between the circle C1 and the cross-section projected on the screen or the bubble wall on the photographed cross-section,
(I) about the respective intersections P2, P3 of the outer periphery of the first straight line and the circle C1, a circle C2, C3 of the radial _Dn 0/8,
(J) Count the number of intersections between the first straight line and the bubble walls located on each of the circles C2 and C3, and select the larger number Nn _V.
(K) The average diameter Ln _CV is obtained from the following equation (1).
Ln _CV = 0.405 × (Dn ₀ / Nn _V ) (1)
(L) determining the average diameter _{Ln CH} from the following equation (2),
Ln _CH = 0.810 × Dn ₀ × sin (π / Nn _H ) (2)
(M) The length Hn _D (mm) of the longest straight line among the straight lines connecting two points on the inner peripheral edge forming the through hole is measured in the cross section projected on the screen or the cross section photographed. ,
(N) The maximum diameter D ₀ is an arithmetic average of Dn ₀ of 10 collected expanded particles.
(O) The average cell diameter L _{CV in the} radial direction is an arithmetic average of Ln _CV of 10 foam particles collected.
(P) Circumferential average cell diameter L _CH is an arithmetic average of Ln _CH of 10 foam particles collected.
(Q) The diameter H _{D of the} through hole is an arithmetic average of Hn _D of the 10 collected foam particles.   The expanded particle according to claim 1, wherein the apparent density is less than 36 g / L.   A method for producing a foamed particle molded body, comprising filling the foamed particles according to claim 1 or 2 into a mold without subjecting to pressure treatment, and molding the mold with steam.   A foamed particle molded body obtained by the method according to claim 3.

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