JP2023019516A - Polypropylene-based resin foam particle and method for producing the same - Google Patents

Abstract

To provide polypropylene-based resin foam particles which have a wide range of a moldable molding pressure and are excellent in releasability.SOLUTION: Polypropylene-based resin foam particles have a foam core layer composed of a polypropylene-based resin (C), and a coating layer composed of a polypropylene-based resin (S), wherein a melting point of the polypropylene-based resin (C) is 125°C or higher and 170°C or lower, and the resin foam particles satisfy the following (1) to (4). (1) The polypropylene-based resin (S) has a melting peak at a low temperature side and a melting peak at a high temperature side. (2) A top temperature of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than a melting point of the polypropylene-based resin (C). (3) A top temperature of the melting peak on the high temperature side of the polypropylene-based resin (S) is equal to or higher than 138°C. (4) A difference between the melting point of the polypropylene-based resin (C) and the top temperature of the melting peak on the high temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to expanded polypropylene resin particles and a method for producing the same.

Expanded bead molded articles obtained by in-mold molding polypropylene resin expanded beads are excellent in chemical resistance, impact resistance, compressive strain recovery, etc. It is used in a wide range of fields, such as food containers, packaging or cushioning materials for electrical and electronic parts, automobile bumpers and interior materials, building materials such as housing heat insulating materials, and miscellaneous goods.

Polypropylene-based resin foamed beads are produced by, for example, filling a molding die with polypropylene-based resin foamed beads and heating with a heating medium such as steam to cause secondary expansion of the foamed beads and melt the surfaces of the foamed beads. It is produced by an in-mold molding method, in which the resin is fused to a desired shape and molded into a desired shape. In the in-mold molding of polypropylene-based resin expanded beads, the temperature of heating by the heating medium is mainly set in consideration of the melting point of the resin forming the foamed layer.

Polypropylene-based resin foamed particles are difficult to mold due to the crystallinity and heat resistance of the propylene-based resin, and therefore, improvements in moldability have been studied. For example, in Patent Document 1, a core layer formed of a polypropylene resin and an outer layer formed of a polypropylene resin are formed, and the melting point of the polypropylene resin of the outer layer and the melting point of the polypropylene resin of the core layer have a specific relationship. and the thickness of the outer layer is less than or equal to a specific value. has the effect of being excellent in mutual fusion bonding between expanded particles at a low molding heating temperature (molding pressure).

However, the expanded polypropylene-based resin particles described in Patent Document 1 may be inferior in releasability depending on the molding conditions and the type of base resin. In particular, when foamed particles having a low apparent density are used, or when the molding heating temperature is high, the releasability may be poor. Therefore, from the viewpoint of further increasing the productivity of expanded bead molded articles, expanded bead having good releasability in a wide molding heating temperature range is desired. In order to improve the releasability of foamed particles, for example, Patent Documents 2 and 3 disclose a molding apparatus in which comb teeth are provided on a mold or a movable member in a molding space. Further, for example, Patent Document 4 discloses a mold in which pores or slits having a cross-sectional shape whose opening area expands toward the surface in contact with expanded particles are formed on at least one side of the inner wall surface of the mold. A method for producing an expanded bead molded article using the material is disclosed.

JP-A-2004-68016 JP 2001-328135 A Japanese Patent Application Laid-Open No. 2002-172642 Japanese Unexamined Patent Application Publication No. 2015-214110

However, Patent Documents 2 to 4 require a dedicated device, and even when a versatile device is used, the range of molding heating temperature at which the expanded bead molded article can be molded is wide. There is a demand for the realization of expanded polypropylene-based resin particles that are excellent in releasability.

Accordingly, an object of the present invention is to provide polypropylene-based resin expanded beads which have a wide range of moldable heating temperature and excellent releasability, and a method for producing the same.

The present inventors have found that the above problems can be solved by adopting the configuration described below, and have completed the present invention.
That is, the present invention is as follows.
<1> Expanded polypropylene resin beads having a foam core layer composed of a polypropylene resin (C) and a coating layer composed of a polypropylene resin (S), wherein the polypropylene resin (C) Expanded polypropylene resin particles having a melting point Tmc of 125° C. or more and 170° C. or less and satisfying the following (1) to (4).
(1) The polypropylene resin (S) is heated by heat flux differential scanning calorimetry from 30° C. to 200° C. at a heating rate of 10° C./min, then 200° C. at a cooling rate of 10° C./min. The second DSC curve obtained by cooling from ° C. to 30 ° C. and heating again at a heating rate of 10 ° C./min has a melting peak on the low temperature side and a melting peak on the high temperature side,
(2) the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is lower than the melting point Tmc of the polypropylene resin (C);
(3) The peak temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is 138° C. or higher.
(4) The difference [Tmc−Tmsh] between the melting point Tmc of the polypropylene resin (C) and the apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is −10° C. or more and 15° C. or less. .
<2> The expanded polypropylene resin particles according to <1>, wherein the melting peak temperature Tmsl of the polypropylene resin (S) on the low temperature side is 125°C or higher and 135°C or lower.
<3> The difference [Tmc−Tmsl] between the melting point Tmc of the polypropylene resin (C) and the peak temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is more than 0° C. and 25° C. or less. , <1> or <2>.
<4> The ratio [ΔHh/ΔHt] of the heat of fusion (ΔHh) of the melting peak on the high temperature side of the polypropylene resin (S) to the total heat of fusion (ΔHt) of the polypropylene resin (S) is 0.35. The polypropylene-based resin expanded beads according to any one of <1> to <3>, which is 0.80 or less.
<5> The difference [Tmsh−Tmsl] between the peak temperature Tmsh of the melting peak on the high temperature side and the peak temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is 10° C. or more and 20° C. or less, <1 > The expanded polypropylene resin particles according to any one of <4>.
<6> The expanded polypropylene resin beads according to any one of <1> to <5>, wherein the polypropylene resin (C) has a melting point Tmc of 130°C or higher and 150°C or lower.
<7> The polypropylene-based resin (S) according to any one of <1> to <6>, wherein the polypropylene-based resin (S) is a propylene-based copolymer containing a propylene component as a main component and containing an ethylene component and a butene component. Expanded polypropylene resin particles.
<8> The expanded polypropylene resin beads according to any one of <1> to <6>, wherein the expanded polypropylene resin beads have a bulk density of 10 kg/m ³ or more and 30 kg/m ³ or less.
<9> A method for producing expanded polypropylene resin particles, comprising: a core layer composed of a polypropylene resin (C); and a coating layer composed of a polypropylene resin (S) covering the core layer. a step of granulating the multi-layered resin particles, a step of dispersing the multi-layered resin particles in a dispersion medium in a closed container, and a step of impregnating the multi-layered resin particles dispersed in the dispersion medium with a foaming agent. and a step of foaming the multilayer resin particles containing the foaming agent to produce foamed beads, wherein the polypropylene-based resin (C) has a melting point Tmc of 125° C. or higher and 170° C. or lower, and the following ( A method for producing expanded polypropylene resin particles that satisfies 1) to (4).
(1) The polypropylene resin (S) is heated by heat flux differential scanning calorimetry from 30° C. to 200° C. at a heating rate of 10° C./min, then 200° C. at a cooling rate of 10° C./min. The second DSC curve obtained by cooling from ° C. to 30 ° C. and heating again at a heating rate of 10 ° C./min has a melting peak on the low temperature side and a melting peak on the high temperature side,
(2) the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is lower than the melting point Tmc of the polypropylene resin (C);
(3) The peak temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is 138° C. or higher.
(4) The difference [Tmc−Tmsh] between the melting point Tmc of the polypropylene resin (C) and the apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is −10° C. or more and 15° C. or less. .

ADVANTAGE OF THE INVENTION According to this invention, the range of the molding heating temperature which can be shape|molded is wide, and the polypropylene-type resin foamed bead which is excellent in mold release property in the whole range can be provided.

2 is a diagram showing a second DSC curve of the polypropylene-based resin of the coating layer used in Example 1. FIG. FIG. 4 is a diagram for explaining how to obtain a high-temperature peak of expanded beads; 2 is a diagram showing a second DSC curve of the polypropylene-based resin of the coating layer used in Comparative Example 1. FIG.

[Expanded polypropylene resin particles]
The expanded polypropylene resin beads of the present invention (hereinafter also simply referred to as expanded polypropylene resin beads or expanded beads) comprise a foamed core layer (hereinafter also simply referred to as a foamed core layer) composed of a polypropylene resin (C), Polypropylene-based resin expanded beads having a coating layer (hereinafter also simply referred to as a coating layer) composed of a polypropylene-based resin (S), wherein the polypropylene-based resin (C) has a melting point Tmc of 125° C. or higher and 170° C. or lower. and satisfies the following (1) to (4).
(1) Polypropylene-based resin (S) is heated by heat flux differential scanning calorimetry from 30°C to 200°C at a heating rate of 10°C/min and then to 200°C at a cooling rate of 10°C/min. In the second DSC curve obtained by cooling from to 30 ° C. and heating again at a heating rate of 10 ° C./min, it has a low temperature side melting peak and a high temperature side melting peak,
(2) the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is lower than the melting point Tmc of the polypropylene resin (C);
(3) The apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is 138° C. or higher.
(4) The difference [Tmc−Tmsh] between the melting point Tmc of the polypropylene resin (C) and the peak temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is −10° C. or more and 15° C. or less.

The polypropylene-based resin foamed beads of the present invention are foamed beads having a multi-layer structure in which the polypropylene-based resin (C) and the polypropylene-based resin (S) having a specific relationship are used as a foam core layer and a coating layer, respectively. It has a wide range of possible molding pressures, that is, excellent moldability and excellent releasability.
Specifically, the inventors have found that, in order to achieve both moldability and releasability, it is preferable to use expanded polypropylene-based resin particles that satisfy the above (1) to (4).
Conventionally, in foamed beads provided with a coating layer composed of a polypropylene resin having a lower melting point than the polypropylene resin constituting the foam core layer, in order to improve the fusion bondability, the releasability tends to be impaired. there were. In particular, when the molding heating temperature is high, the releasability tends to be significantly impaired, probably because the resin of the coating layer softens and easily adheres to the molding die. In addition, even when foamed particles having a low apparent density are used, the releasability tends to be significantly impaired. In the present invention, it was found that the foamed polypropylene-based resin particles, by satisfying the above (1) to (4), are excellent in fusion bondability and can achieve both moldability and releasability.

<Coating layer>
The coating layer of the foamed polypropylene resin particles covers the foam core layer. The covering layer preferably covers substantially the entire outer surface of the foam core layer, and may completely cover the entire outer surface of the foam core layer. Specifically, the coating layer preferably covers 50% or more of the surface area of the core layer, more preferably 70% or more of the surface area, and substantially 100% of the surface area. It may cover 90% or less of the area.

The coating layer is non-expanded from the viewpoint of the appearance of the expanded polypropylene resin bead molded article obtained using the expanded polypropylene resin beads of the present invention (hereinafter also referred to as expanded polypropylene resin bead molded article or expanded bead molded article). It is preferably in a state or substantially non-foamed state, more preferably in a non-foamed state. By non-foamed or substantially non-foamed is meant substantially no cellular structure. It also includes a state in which a bubble once formed is broken and the bubble disappears. In addition, the coating layer may be in a microfoamed state as long as the intended effect of the present invention is not impaired.

(Polypropylene resin (S))
The coating layer of the expanded polypropylene-based resin particles is composed of the polypropylene-based resin (S). In the present specification, the polypropylene-based resin refers to a polymer containing 50% by mass or more of structural units derived from propylene. Examples of polypropylene-based resins include propylene homopolymers, propylene-based copolymers, and mixtures thereof. In addition, the polypropylene resin may contain other polymers such as resins other than polypropylene and elastomers, but the content thereof is preferably 20% by mass or less, more preferably 15% by mass or less. It is preferably 10% by mass or less, and particularly preferably 5% by mass or less.

≪(1) Double peak≫
Based on JIS K 7121: 1987, the polypropylene resin (S) adopts "(2) when measuring the melting temperature after performing a certain heat treatment" as conditioning, and 10 test pieces that have been conditioned. A DSC curve obtained by heating from 30°C to 200°C at a heating rate of °C/min has a melting peak on the low temperature side and a melting peak on the high temperature side. Specifically, a resin that had been conditioned at 23° C. and 50% RH for 24 hours or more was used as a test piece, and the resin was heated from 30° C. to 200° C. at a heating rate of 10° C./min by heat flux differential scanning calorimetry. Then, in the second DSC curve obtained when cooling from 200 ° C. to 30 ° C. at a cooling rate of 10 ° C./min and heating again at a heating rate of 10 ° C./min, the melting peak on the low temperature side and the melting peak on the high temperature side of melting peaks. Here, having a melting peak on the low temperature side and a melting peak on the high temperature side means that the melting peak on the low temperature side and the melting peak on the high temperature side have peak apex temperatures (melting points) that are different from each other, and the heat of fusion of each is 10 J/g or more. A method for measuring the heat of fusion will be described later. A part of the melting peak on the low temperature side and the melting peak on the high temperature side may overlap. In this specification, the melting peak on the low temperature side and the melting peak on the high temperature side in the second DSC curve of the polypropylene resin (S) may be collectively referred to as a double peak.
In addition, in the second DSC curve of the polypropylene resin (S), when three or more melting peaks appear, the peak temperature is on the lowest temperature side and the heat of fusion is 10 J / g or more. is defined as the heat of fusion on the low temperature side, and the melting peak having the peak temperature on the highest temperature side and the heat of fusion of 10 J/g or more is defined as the melting peak on the high temperature side.
If the polypropylene resin (S) does not have a melting peak on the low temperature side and a melting peak on the high temperature side (double peak) in the second DSC curve, and has only a single melting peak, the polypropylene resin It may not be possible to achieve both moldability and releasability of the expanded beads.

<<(2) Melting peak apex temperature Tmsl on the low temperature side>>
The apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is lower than the melting point Tmc of the polypropylene resin (C) described below. When Tmsl is lower than Tmc, even at a low molding heating temperature, the fusion bondability is excellent and the moldability is improved. From the same point of view, Tmsl is preferably 135° C. or lower, more preferably 132° C. or lower, and even more preferably 130° C. or lower. On the other hand, Tmsl is preferably 125° C. or higher, more preferably 126° C. or higher, and still more preferably 127° C. or higher, from the viewpoint of further improving the releasability (hereinafter simply referred to as releasability) of the expanded polypropylene resin particles. is.

<<(3) Peak temperature Tmsh of melting peak on high temperature side>>
The apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is 138° C. or higher. When Tmsh is less than 138°C, the releasability of the expanded particles may be impaired especially under conditions of high molding heating temperature. From this point of view, Tmsh is preferably 139° C. or higher, more preferably 140° C. or higher, and even more preferably 141° C. or higher. From the viewpoint of further improving moldability, the temperature is preferably 170° C. or lower, more preferably 160° C. or lower, even more preferably 155° C. or lower, even more preferably 150° C. or lower, and even more preferably 145° C. or lower.

<<(4) Difference between melting point Tmc and Tmsh of polypropylene resin (C)>>
The difference [Tmc−Tmsh] between the melting point Tmc of the polypropylene resin (C) and the peak temperature Tmsh of the melting peak of the polypropylene resin (S) on the high temperature side, which will be described later, is −10° C. or more and 15° C. or less. If the difference [Tmc−Tmsh] exceeds 15° C., the releasability of the expanded beads may be impaired when a resin having a relatively high melting point is used. From this point of view, the difference [Tmc−Tmsh] is preferably 12° C. or less, more preferably 10° C. or less, and even more preferably 7° C. or less. If the difference [Tmc−Tmsh] is less than −10° C., it may be impossible to achieve both moldability and releasability. The difference [Tmc−Tmsh] is preferably −8° C. or higher, more preferably −5° C. or higher, from the viewpoint of improving releasability while suppressing deterioration of moldability.

<<Difference between melting point Tmc and Tmsl of polypropylene resin (C)>>
The difference [Tmc−Tmsl] between the melting point Tmc of the polypropylene resin (C) described later and the peak temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is, as described above, 0° C. from the standpoint of moldability. exceed. From the viewpoint of further improving the moldability of the expanded beads, the difference [Tmc−Tmsl] is more preferably 5° C. or higher, still more preferably 10° C. or higher. On the other hand, from the viewpoint of further improving releasability and suppressing peeling between the foam core layer and the coating layer, the difference [Tmc−Tmsl] is preferably 25° C. or less, more preferably 22° C. or less, and even more preferably 20°C or less.

<<Difference between Tmsh and Tmsl>>
The difference [Tmsh-Tmsl] between the peak temperature Tmsh of the melting peak on the high temperature side and the peak temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is from the viewpoint of achieving both moldability and releasability in a well-balanced manner. , preferably 10°C or higher, more preferably 13°C or higher, and preferably 20°C or lower, more preferably 17°C or lower, and even more preferably 15°C or lower.

<<The ratio of the heat of fusion ΔHh of the melting peak on the high temperature side to the total heat of fusion ΔHt>>
The ratio [ΔHh/ΔHt] of the heat of fusion ΔHh of the melting peak on the high temperature side to the total heat of fusion ΔHt of the polypropylene resin (S) is preferably 0.00, from the viewpoint of achieving both moldability and releasability in a well-balanced manner. 35 or more, more preferably 0.38 or more, still more preferably 0.40 or more, and preferably 0.80 or less, more preferably 0.70 or less, still more preferably 0.60 or less, still more preferably 0.50 or less.

The heat of fusion ΔHl of the melting peak on the low temperature side of the polypropylene resin (S), the heat of fusion ΔHh of the melting peak on the high temperature side, and the total heat of fusion ΔHt are obtained as follows in the case of the DSC curve illustrated in FIG. . Let α be the point on the DSC curve at a temperature of 80° C., and let β be the point on the DSC curve corresponding to the melting end temperature. A straight line parallel to the vertical axis of the graph is drawn from the point γ on the DSC curve corresponding to the valley between the melting peak _Pl on the low temperature side and the melting peak _Ph on the high temperature side, and the straight line connecting the points α and β Let δ be the point of intersection with (α−β). In addition, due to the fact that the melting peak on the low temperature side and the melting peak on the high temperature side partially overlap, the point γ on the DSC curve that corresponds to the valley between the melting peak _Pl on the low temperature side and the melting peak _Ph on the high temperature side is If it is not clear, the position of point γ can be obtained by referring to the DSC differential curve (DDSC) near the temperature. Point γ indicates 0 in DDSC.
The area (A) of the melting peak _Pl on the low temperature side is the heat of fusion _ΔHl of the melting _peak Pl on the low temperature side. It is obtained as the area of the portion surrounded by the line segment (γ-δ).
The area (B) of the melting peak _Ph on the high temperature side is the heat of fusion ΔHh of the melting peak _Ph on the high temperature side, and the DSC curve showing the melting peak _Ph on the high temperature side, the line segment (δ-β), It is obtained as the area of the portion surrounded by the line segment (γ-δ).
The total heat of fusion ΔHt is the sum of the area (A) of the melting peak _Pl on the low temperature side and the area (B) of the melting peak _Ph on the high temperature side [A + B (that is, ΔHl + ΔHh)]. It is the area enclosed by the straight line (α-β) and the DSC curve in the section between points α and β.

≪ΔHl≫
The heat of fusion ΔHl of the melting peak on the low temperature side of the polypropylene resin (S) is preferably 25 J/g or more, more preferably 30 J/g or more, and still more preferably 35 J/g or more, from the viewpoint of further improving moldability. And, from the viewpoint of more reliably ensuring releasability, it is preferably 50 J/g or less, more preferably 45 J/g or less, and still more preferably 43 J/g or less.

≪ΔHh≫
The heat of fusion ΔHh of the melting peak on the high temperature side of the polypropylene resin (S) is preferably 15 J/g or more, more preferably 21 J/g or more, and still more preferably 26 J/g or more, from the viewpoint of further improving mold releasability. More preferably, it is 30 J/g or more, and from the viewpoint of ensuring moldability more reliably, it is preferably 45 J/g or less, more preferably 40 J/g or less, and even more preferably 35 J/g or less.

≪ΔHt≫
The total heat of fusion ΔHt of the polypropylene resin (S) is preferably 57 J/g or more, more preferably 65 J/g or more, and still more preferably 70 J/g or more, from the viewpoint of achieving both moldability and releasability in a well-balanced manner. and is preferably 85 J/g or less, more preferably 80 J/g or less, still more preferably 75 J/g or less.

≪Melt flow rate (MFR)≫
The MFR of the polypropylene-based resin (S) is preferably 2 g/10 min or more, more preferably 4 g/10 min or more, and still more preferably 5 g/10 min, from the viewpoint of easy formation of the coating layer and suppression of peeling from the foam core layer. and preferably 10 g/10 min or less, more preferably 9 g/10 min or less, still more preferably 8 g/10 min or less.
The MFR of the polypropylene resin (S) is measured under conditions of a temperature of 230° C. and a load of 2.16 kg in accordance with JIS K7210-1:2014.

The polypropylene-based resin (S) is not particularly limited as long as it satisfies the above (1) to (3) and satisfies the above relationship (4) with the melting point Tmc of the polypropylene-based resin (C), but the main component is a propylene component. A propylene-based copolymer containing an ethylene component and a butene component is preferable.

When the polypropylene-based resin (S) is a propylene-based copolymer containing a propylene component as a main component and an ethylene component and a butene component, the ethylene component content in the copolymer is 3.0% by mass or more. It is preferably 6.0% by mass or less, more preferably 3.5% by mass or more and 5.5% by mass or less, and even more preferably 4.0% by mass or more and 5.0% by mass or less. The butene component content in the copolymer is preferably 5.0% by mass or more and 18% by mass or less, more preferably 7.5% by mass or more and 15% by mass or less, and 9.0% by mass. More preferably, the content is not less than 12% by mass and not more than 12% by mass. However, the total of the propylene component, the ethylene component and the butene component is 100% by mass. The content of the monomer component in the copolymer can be determined by IR spectrum measurement. A propylene-based copolymer having an ethylene component content and a butene component content within the above ranges has a crystal structure that satisfies the above conditions (1) to (4) and exhibits a melting peak on the low temperature side and a melting peak on the high temperature side. It is easy to become what you have.
From the same viewpoint, the ratio of the butene component content to the ethylene component content [butene component content/ethylene component content] is preferably 1.5 or more, more preferably 1.8 or more, and 2.0. More preferably, it is 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less.
In the present invention, FL7540L and FL7320L manufactured by TPC are exemplified as resins that can be used as the polypropylene-based resin (S).

The polypropylene-based resin (S) may contain additives as long as they do not impair the intended effects of the present invention.

<Foam core layer>
The expanded core layer of the expanded polypropylene-based resin beads is composed of the polypropylene-based resin (C). From the viewpoint of improving the moldability of the expanded beads and the rigidity of the molded article, etc., the closed cell ratio of the expanded beads is preferably 80% or more, more preferably 90% or more, and 95% or more. is more preferred. The closed cell content of the expanded beads can be measured using an air comparison hydrometer based on ASTM-D2856-70 Procedure C.

(Polypropylene resin (C))
<<Melting point Tmc>>
The melting point Tmc of the polypropylene-based resin (C) is 125° C. or higher, preferably 130° C. or higher, more preferably 135° C. or higher, and still more preferably 140° C. or higher, from the viewpoint of rigidity, heat resistance, etc. of the expanded bead molded article. , more preferably 145° C. or higher, and 170° C. or lower, preferably 165° C. or lower, more preferably 160° C. or lower, even more preferably 155° C. or lower, and even more preferably 150° C. or lower. According to the present invention, by using the polypropylene-based resin (S) as the coating layer, the polypropylene-based resin (C) having the melting point Tmc in the wide range described above can be used to separate the moldability of the foamed polypropylene-based resin particles. It can be compatible with type. Conventionally, when a coating layer is provided in order to improve fusion bondability in foamed beads using a polypropylene-based resin having a particularly high melting point as a base resin, mold release is possible over the entire moldable heating temperature range. It was difficult to make the properties good. According to the present invention, even when a polypropylene-based resin having a relatively high melting point is used as the foam core layer, both moldability and releasability can be achieved.
The melting point Tmc of the polypropylene resin (C) is based on JIS K7121: 1987, and the condition adjustment of the test piece is "(2) When measuring the melting temperature after performing a certain heat treatment". A DSC curve is obtained by heating the subsequent test piece from 30° C. to 200° C. at a heating rate of 10° C./min. When a plurality of melting peaks appear in the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point. In this case, the area of each melting peak can be obtained by the same method as for the heat of fusion of the polypropylene-based resin (S).

≪Flexural modulus≫
The flexural modulus of the polypropylene resin (C) is preferably 500 MPa or more, more preferably 600 MPa or more, and more preferably 600 MPa or more, from the viewpoint of achieving both moldability and releasability while maintaining the mechanical strength of the expanded bead molded product. It is preferably 700 MPa or more, more preferably 800 MPa or more, and preferably 1600 MPa or less, more preferably 1300 MPa or less, and still more preferably 1100 MPa or less.
The flexural modulus of polypropylene resin can be obtained based on JIS K7171:2016.

≪MFR≫
From the viewpoint of foamability, the MFR of the polypropylene resin (C) is preferably 2 g/10 min or more, more preferably 4 g/10 min or more, still more preferably 5 g/10 min or more, and preferably 15 g/10 min or less. It is more preferably 12 g/10 min or less, still more preferably 10 g/10 min or less.
The MFR of the polypropylene resin (C) is a value measured under conditions of 230° C. and a load of 2.16 kg by the same method as for the MFR of the polypropylene resin (S).

The polypropylene resin (C) has a melting point Tmc in the above range, satisfies the above relationship (2) with the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S), and the polypropylene resin (S) Although it is not particularly limited as long as it satisfies the above relationship (4) with the apex temperature Tmsh of the melting peak on the high temperature side, it is preferably a mixture of a polypropylene homopolymer and a propylene copolymer or a polypropylene copolymer.

The polypropylene copolymer is preferably an ethylene-propylene copolymer, an ethylene-propylene-butene copolymer, and a mixture of a copolymer thereof and a polypropylene homopolymer, more preferably an ethylene-propylene copolymer. The polymer is an ethylene-propylene-butene copolymer, more preferably an ethylene-propylene copolymer.

The polypropylene-based resin (C) may contain other polymers such as resins other than the propylene homopolymer or propylene-based copolymer, or elastomers, as long as the desired effects of the present invention are not impaired. The content of the other polymer in the polypropylene resin (C) is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and even more preferably 3% by mass or less, Most preferably, the content is 0% by mass, that is, the polypropylene resin (C) is preferably composed only of the polypropylene resin as a polymer.

In the foam core layer, if necessary, a cell control agent, a flame retardant, a flame retardant auxiliary, a cell nucleating agent, a plasticizer, an antistatic agent, an antioxidant, an ultraviolet inhibitor, a light stabilizer, a conductive filler, Additives such as antibacterial agents can be added.

The mass ratio (mass% ratio) between the resin forming the foam core layer and the resin forming the coating layer is preferably 99.0% from the viewpoint of maintaining the rigidity of the molded body and improving moldability and releasability. 5:0.5 to 80:20, more preferably 99:1 to 85:15, still more preferably 97:3 to 90:10. The mass ratio is represented by the ratio of the resin constituting the foam core layer to the resin constituting the coating layer.

[Physical properties of expanded polypropylene resin particles]
<Bulk density>
The bulk density of the expanded polypropylene resin particles is preferably 10 kg/m ³ or more, more preferably 15 kg/m ³ or more, from the viewpoint of achieving a good balance between the mechanical strength and lightness of the expanded particle molded product, and , preferably 100 kg/m ³ or less, more preferably 70 kg/m ³ or less, still more preferably 50 kg/m ³ or less, still more preferably 30 kg/m ³ or less, and particularly preferably 20 kg/m ³ or less.
The bulk density of expanded particles is determined as follows. Expanded particles are randomly taken out from the group of expanded particles and placed in a graduated cylinder with a capacity of 1 L. A large number of expanded particles are accommodated up to the scale of 1 L so as to be in a state of natural accumulation, and the mass of the accommodated expanded particles is W1 [g]. is divided by the storage volume V1 (1 [L]) (W1/V1), and the unit is converted to [kg/m ³ ] to obtain the bulk density of the expanded beads.
Conventionally, when the bulk density of the expanded particles is low, the releasability tends to be impaired. According to the present invention, even foamed particles having a low bulk density of, for example, 30 kg/m ³ or less have good releasability.

The foamed beads show a melting peak (that is, resin-specific peak) due to melting specific to polypropylene resin and a high temperature It is preferable to have a crystal structure that exhibits one or more melting peaks (that is, high temperature peaks) on either side. A DSC curve is obtained by differential scanning calorimetry (DSC) according to JIS K7121:1987 using 1 to 3 mg of foamed particles as a test sample. The resin-specific peak is a melting peak due to melting specific to the polypropylene-based resin constituting the foam core layer, and is considered to be due to endothermic heat absorption during melting of the crystals inherent in the polypropylene-based resin. On the other hand, the melting peak on the high-temperature side of the resin-specific peak (that is, the high-temperature peak) is a melting peak appearing on the high-temperature side of the resin-specific peak in the DSC curve. When this high temperature peak appears, it is presumed that secondary crystals are present in the resin. The foamed beads were heated from 23°C to 200°C at a temperature increase rate of 10°C/min (that is, the first heating), and then cooled from 200°C to 23°C at a cooling rate of 10°C/min. , and then again heated from 23°C to 200°C at a rate of temperature increase of 10°C/min (that is, the second heating). Since only the melting peak due to melting is seen, it is possible to distinguish between the resin specific peak and the high temperature peak. The temperature at the top of this resin-specific peak may slightly differ between the first heating and the second heating, but the difference is usually within 5°C.

<The heat of fusion ΔH2 at the high temperature peak>
The heat of fusion ΔH2 at the high-temperature peak of the expanded polypropylene resin beads is preferably 5 J/g or more, more preferably 8 J/g or more, and still more preferably 10 J/g or more, from the viewpoint of mechanical strength and moldability of the expanded beads. Yes, and preferably 40 J/g or less, more preferably 30 J/g or less, even more preferably 20 J/g or less, and even more preferably 18 J/g or less.

A method for measuring the heat of fusion ΔH2 at the high temperature peak of the foamed polypropylene resin particles will be described using the DSC curve exemplified in FIG. A straight line L1 is obtained by connecting the point α on the DSC curve at a temperature of 80° C. and the point β at the melting end temperature T of the expanded beads. Next, a straight line L2 parallel to the vertical axis of the graph is drawn from the point γ on the DSC curve corresponding to the valley between the resin-specific peak _P1 and the high temperature peak _P2 , and the point where the straight line L1 and the straight line L2 intersect be δ. The point γ can also be said to be a maximum point existing between the resin-specific peak _P1 and the high-temperature peak _P2 .
The area (2) of the high temperature peak _P2 of the expanded beads is the heat of fusion ΔH2 of the high temperature peak _P2 , and the DSC curve showing the high temperature peak _P2 , the line segment (δ-β), and the line segment (γ-δ ) and the area enclosed by The area (1) of the resin-specific peak _P1 of the expanded bead is the heat of fusion ΔH1 of the resin-specific peak _P1 , and the DSC curve showing the resin-specific peak _P1 , the line segment (α-δ), and the line It is calculated as the area of the part enclosed by the minute (γ-δ).

[Method for producing expanded polypropylene resin particles]
The method for producing expanded polypropylene resin particles is not particularly limited, but for example, it can be produced by a method including the following steps (A) to (D).
Step (A): The polypropylene-based resin (C) constituting the core layer and the polypropylene-based resin (S) constituting the coating layer are melt-kneaded and co-extruded into a strand, which is then cut into a non-foamed core. a granulation step of obtaining multilayer resin particles having a layer and a coating layer covering the core layer;
Step (B): a dispersing step of dispersing the multilayer resin particles in a dispersion medium in a closed container;
Step (C): A foaming agent impregnation step of impregnating the multilayered resin particles with a foaming agent, and Step (D): The expandable multilayered resin particles impregnated with the foaming agent are extruded from the sealed container to a pressure lower than the pressure in the sealed container. A foaming step in which at least the core layer is foamed to form a foamed core layer by discharging together with a dispersion medium under an atmosphere of pressure to produce foamed particles.

<Step (A)>
In step (A), for example, an extruder having a core layer forming extruder, a coating layer forming extruder, and a multilayer strand forming die installed on the exit side of these extruders can be used. . The core layer-forming extruder is supplied with the polypropylene resin (C) constituting the core layer and additives added as necessary, and melt-kneaded to form a core layer-forming melt-kneaded product, and the coating layer is formed. Into the extruder, the polypropylene-based resin (S) constituting the coating layer and optional additives are supplied and melt-kneaded to obtain a melt-kneaded material for forming the coating layer. The melt-kneaded material for forming the core layer and the melt-kneaded material for forming the coating layer are introduced into a multi-layer strand forming die and joined together to form a non-foamed core layer and a non-foamed coating layer covering the core layer. Forms a complex with a sheath structure. Then, the composite is extruded in a strand form from a small hole of a die attached to the tip of the extruder, cooled in water, and then cut to a predetermined mass with a pelletizer (strand cut method). It is possible to obtain a multilayer resin particle having a core layer in a state and a coating layer covering the core layer. As a method for cutting the extruded composite, in addition to the above methods, an underwater cut method in which the composite is extruded into water and cut, a hot cut method in which the composite is cut immediately after being extruded into the air, etc. are adopted. can also

(Particle diameter of multilayer resin particles)
The particle diameter of the multilayer resin particles is preferably 0.1 to 3.0 mm, more preferably 0.3 to 1.5 mm.

(Mass of multilayer resin particles)
The average mass of the multilayered resin particles is preferably adjusted to 0.1 to 20 mg, more preferably 0.2 to 10 mg, even more preferably 0.3 to 5 mg, and even more preferably 0.5 mg. 4-2 mg.

(Mass ratio of coating layer and core layer)
When producing a multilayer resin particle, the mass ratio of the coating layer and the non-foamed core layer (coating layer/core layer) is from 0.5/99.5 to 0.5/99.5 from the viewpoint of fusion bonding of the foamed beads. It is preferably 20/80, more preferably 1/99 to 15/85, even more preferably 3/97 to 10/90.

(Additive)
Multi-layered resin particles having a non-foamed core layer and a coating layer are optionally added with a cell control agent, a flame retardant, a flame retardant aid, a cell nucleating agent, a plasticizer, an antistatic agent, an antioxidant, and an ultraviolet ray. Additives such as inhibitors, light stabilizers, conductive fillers, antimicrobial agents, etc. can be added. When adding additives, they can be added in step (A). Inorganic powders such as talc, mica, zinc borate, calcium carbonate, silica, titanium oxide, gypsum, zeolite, borax, aluminum hydroxide, carbon, etc.; , an amine-based nucleating agent, and an organic powder such as a polyfluoroethylene-based resin powder. When a cell control agent is added, the content of the cell control agent in the multilayer resin particles is preferably 0.01 to 1 part by mass with respect to 100 parts by mass of the multilayer resin particles.

<Step (B)>
In the step (B), for example, the multi-layered resin particles can be dispersed in a dispersion medium using, for example, a stirrer in a container such as an autoclave that can be sealed and can withstand heat and pressure.

The dispersion medium is not particularly limited as long as it does not dissolve the multilayer resin particles, and examples thereof include water, ethylene glycol, glycerin, methanol, and alcohols such as ethanol, among which water is preferred.

In step (B), it is preferable to further add a dispersant to the dispersion medium in order to prevent fusion between the multilayered resin particles. Examples of dispersants include organic dispersants such as polyvinyl alcohol, polyvinylpyrrolidone and methylcellulose; sparingly soluble inorganic salts such as aluminum oxide, zinc oxide, kaolin, mica, magnesium phosphate and tricalcium phosphate. These can be used alone or in combination of two or more. Among these, sparingly soluble inorganic salts are preferred, and kaolin is more preferred, for ease of handling. When a dispersant is added, it is preferable to add about 0.001 to 5 parts by mass of the dispersant to 100 parts by mass of the multilayer resin particles.

A surfactant may be further added to the dispersion medium. Examples of surfactants include sodium dodecylbenzenesulfonate, sodium alkylsulfonate, sodium oleate, sodium lauryl sulfate, sodium polyoxyethylene alkyl ether phosphate, sodium polyoxyethylene alkyl ether sulfate, and others commonly used in suspension polymerization. anionic surfactants, nonionic surfactants, etc., which are commonly used. When adding a surfactant, it is preferable to add about 0.001 to 1 part by mass of the surfactant with respect to 100 parts by mass of the multilayer resin particles.

<Step (C)>
In the step (C), for example, the polypropylene-based resin (C) constituting the core layer is heated to a softening temperature or higher and impregnated with a foaming agent to obtain expandable multilayer resin particles.

The foaming agent is not particularly limited as long as it can foam the multilayer resin particles. Examples of foaming agents include inorganic physical foaming agents such as air, nitrogen, carbon dioxide, argon, helium, oxygen, and neon; aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, and normal hexane; and cyclohexane. , cyclopentane and other alicyclic hydrocarbons, chlorofluoromethane, trifluoromethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane, methyl chloride, ethyl chloride, methylene chloride and other halogenated hydrocarbons , dimethyl ether, diethyl ether, organic physical foaming agents such as dialkyl ethers such as methyl ethyl ether. Among these, inorganic physical blowing agents that do not deplete the ozone layer and are inexpensive are preferred, nitrogen, air and carbon dioxide are more preferred, and carbon dioxide is particularly preferred. These are used alone or in combination of two or more.

The amount of foaming agent to be added is determined in consideration of the desired bulk density of foamed particles, the type of polypropylene resin, the type of foaming agent, and the like. The agent is preferably 5 to 50 parts by mass, and the inorganic physical blowing agent is preferably 0.1 to 30 parts by mass, more preferably 0.5 to 15 parts by mass.

The heating temperature in the step (C) is preferably the melting point of the polypropylene resin (C) or higher and the melting point +80°C or lower, specifically, preferably 100°C to 230°C. The time for holding at the heating temperature is preferably 1 minute or more, more preferably 20 minutes or more, and preferably 100 minutes or less, more preferably 60 minutes or less.

<Step (D)>
In step (D), for example, the foaming agent-impregnated and heated expandable multilayer resin particles in step (C) are released from the sealed container into an atmosphere having a pressure lower than the pressure in the sealed container. Then, at least the core layer is foamed to form a foamed core layer, thereby producing foamed particles.
Specifically, while maintaining the pressure in the sealed container at a pressure equal to or higher than the vapor pressure of the foaming agent, one end of the sealed container under the water surface is opened to disperse the expandable multilayer resin particles impregnated with the foaming agent. The medium is released from the closed container together with the medium into an atmosphere having a lower pressure than the pressure inside the closed container, usually under atmospheric pressure, and at least the core layer of the expandable multilayer resin particles is expanded to form a foamed core layer. A multi-layered foamed bead having a layer and a coating layer covering the foamed core layer can be produced. Alternatively, after the expandable multilayer resin particles that have undergone the step (C) are cooled and taken out, the expandable multilayer resin particles are heated with a heating medium such as hot air or steam to expand them, thereby producing expanded beads. can also

In step (D), the temperature during foaming is usually preferably 110°C to 170°C. Moreover, the pressure in the sealed container is preferably 0.5 MPa (G) or more and 5 MPa (G) or less.

The above steps (B) to (D) are preferably carried out as a series of steps in a single closed container. It is also possible to perform a separate step such as performing the step of.

In addition, a crystal structure in which a resin-specific melting peak (resin-specific peak) and one or more melting peaks (high-temperature peaks) appear on the high temperature side appear in the first DSC curve obtained by differential scanning calorimetry (DSC) of expanded beads. The foamed beads having the properties are obtained, for example, as follows.
When heating in the expanded bead manufacturing process, the temperature is maintained at (melting point of polypropylene resin (C) −20° C.) and below (melting end temperature of polypropylene resin (C)) for a sufficient time, preferably about 10 to 60 minutes. A one-stage holding step is performed. After that, the temperature is adjusted from (melting point of polypropylene resin (C) −15° C.) to (melting end temperature of polypropylene resin (C) +10° C.). Then, if necessary, a two-stage holding step is performed in which the temperature is maintained for a sufficient time, preferably about 10 to 60 minutes. Then, the foamable resin particles containing the foaming agent are released from the sealed container under low pressure to be foamed, thereby obtaining the foamed particles having the crystal structure described above. Foaming is preferably carried out in a closed container at (melting point of polypropylene resin (C) −10° C.) or higher, (melting point of polypropylene resin (C)) or higher (melting point of polypropylene resin (C) +20° C. ).

The expanded beads obtained as described above are pressurized with air to increase the internal pressure, then heated with steam or the like to expand (two-step expansion), and further expanded with a high expansion ratio (low apparent density). It can also be a particle.

[Polypropylene-based resin expanded particle molded product]
A polypropylene resin expanded bead molded article obtained by using the polypropylene resin expanded beads of the present invention (hereinafter also simply referred to as a polypropylene resin expanded bead molded article or expanded bead molded article) is obtained by using the polypropylene resin expanded beads of the present invention. It can be obtained by molding in a mold.

The in-mold molding method can be carried out by filling a mold with foamed particles and heating and molding using a heating medium such as steam. Specifically, after the foamed particles are filled in the mold, a heating medium such as steam is introduced into the mold to heat and foam the foamed particles and fuse them together to form a molding space. It is possible to obtain an expanded bead molded article having a shaped shape. In addition, in the in-mold molding in the present invention, the expanded beads are preliminarily pressurized with a pressurized gas such as air to increase the pressure inside the cells of the expanded beads so that the pressure inside the expanded beads is 0.01% higher than the atmospheric pressure. After adjusting the pressure to 0.3 MPa higher, the foamed particles are filled into the mold under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied into the mold to heat and fuse the foamed particles. Molding by a compression molding method (for example, JP-B-51-22951) is preferable. In addition, after filling a mold pressurized to a pressure higher than the atmospheric pressure with compressed gas with expanded particles pressurized to a pressure higher than the pressure, a heating medium such as steam is supplied into the cavity to heat the expanded particles. It can also be molded by a compression filling molding method (Japanese Patent Publication No. 4-46217) in which heat is fused. In addition, foamed particles with high secondary foaming power obtained under special conditions are filled into the cavity of the mold under atmospheric pressure or reduced pressure, and then heated by supplying a heating medium such as steam. , a normal pressure filling molding method (Japanese Patent Publication No. 6-49795) in which foamed particles are heated and fused, or a method combining the above methods (Japanese Patent Publication No. 6-22919).

<Density>
From the viewpoint of balancing mechanical strength and light weight, the density of the polypropylene-based resin expanded particle molded product is preferably 10 kg/m ³ or more, more preferably 15 kg/m ³ or more, and still more preferably 20 kg/m ³ . It is preferably 150 kg/m ³ or less, more preferably 100 kg/m ³ or less, even more preferably 50 kg/m ³ or less, and particularly preferably 35 kg/m ³ or less.
The density of the expanded bead molded article is calculated by dividing the mass (g) of the expanded bead molded article by the volume (L) obtained from the outer dimensions of the molded article and converting the result into units. If it is not easy to determine the volume from the outer dimensions of the molded body, the volume of the molded body can be determined by the submersion method.

<50% compressive stress>
From the viewpoint of rigidity, the 50% compressive stress of the polypropylene-based resin expanded bead molded article is preferably 50 kPa or more, more preferably 100 kPa or more, still more preferably 150 kPa or more, and preferably 500 kPa or less, more preferably 400 kPa or less. , and more preferably 300 kPa or less.
The 50% compressive stress of the expanded bead molding is measured according to JIS K6767:1999.

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

The following measurements or evaluations were performed on the resins, foamed beads and foamed beads used in Examples and Comparative Examples. The physical properties of the expanded beads were measured by using the expanded beads that had been left standing for 24 hours under the conditions of 50% relative humidity, 23° C., and 1 atm. The physical properties of the expanded bead molded article were measured and evaluated using a molded article that had been left to stand for 12 hours under the conditions of relative humidity of 50%, 80°C, and 1 atm to condition the expanded bead molded article. rice field.

[Measuring method]
<Polypropylene resin>
(Monomer component content)
The monomer component content of the polypropylene resin was obtained by a known method of determining from IR spectrum. Specifically, Polymer Analysis Handbook (Edited by Japan Society for Analytical Chemistry Polymer Analysis Research Round-table Conference, publication date: January 1995, publisher: Kinokuniya Bookstore, page numbers and item names: 615-616 "II. 2.3 2.3.4 Propylene/Ethylene Copolymers”, 618-619 “II.2.3 2.3.5 Propylene/Butene Copolymers”), i.e. ethylene and butene It was obtained by a method of quantifying from the relationship between the absorbance corrected by a predetermined coefficient and the thickness of the film-like test piece. More specifically, first, a polypropylene resin was hot-pressed in an environment of 180° C. to form a film, and a plurality of test pieces having different thicknesses were produced. Then, by measuring the IR spectrum of each test piece, the absorbance (A ₇₂₂ , A ₇₃₃ ) derived from ethylene at 722 cm ⁻¹ and 733 cm ⁻¹ and the absorbance (A ₇₆₆ ) at 766 cm ⁻¹ derived from butene were read. rice field. Next, for each test piece, the ethylene component content in the polypropylene resin was calculated using the following formulas (1) to (3). The value obtained by arithmetically averaging the ethylene component contents obtained for each test piece was defined as the ethylene component content (unit: mass %) in the polypropylene resin.
( _K'733 )c = 1/0.96 {( _K'733 )a-0.268( _K'722 )a} (1)
( _K'722 )c = 1/0.96 {( _K'722 )a-0.268( _K'722 )a} (2)
Ethylene component content (%) = 0.575 {(K' ₇₂₂ )c + (K' ₇₃₃ )c} (3)
However, in formulas (1) to (3), K'a: apparent extinction coefficient at each wavenumber (K'a = A / ρt), K'c: extinction coefficient after correction, A: absorbance, ρ: resin density (unit: g/cm ³ ), t: thickness of film-like test piece (unit: cm).
Also, for each test piece, the content of the butene component in the polypropylene resin was calculated using the following formula (4). The value obtained by arithmetically averaging the butene component contents obtained for each test piece was defined as the butene component content (%) in the polypropylene resin.
Butene component content (%) = 12.3 ( _A766 /L) (4)
However, in Formula (4), A means the absorbance, and L means the thickness (mm) of the film-like test piece.

(DSC characteristics of polypropylene resin (S))
In the second DSC curve of the polypropylene resin (S), the peak shape, the peak temperature Tmsl of the melting peak on the low temperature side, and the peak temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) are obtained as follows. rice field. Based on JIS K7121: 1987, as the condition adjustment of the test piece, "(2) When measuring the melting temperature after performing a certain heat treatment" is adopted, and the conditioned test piece is heated at 10 ° C./min. A DSC curve was obtained by ramping the temperature from 30°C to 200°C. Specifically, about 3 mg of resin that has been conditioned at 23 ° C. and 50% RH for 24 hours or more is used as a test piece, and the test piece is measured by a heat flux differential scanning calorimeter (manufactured by Shimadzu Corporation, model number: DSC- 60 A) at a heating rate of 10°C/min from 30°C to 200°C, then cooled at a cooling rate of 10°C/min from 200°C to 30°C, again at a heating rate of 10°C/min. A DSC curve was obtained by heating from 30° C. to 200° C. and the shape of the melting peak was observed. Here, the double peak shape means that two melting peaks having different peak apex temperatures (melting points) appear, and the heat of fusion of the two melting peaks is 10 J/g or more. . The heat of fusion is a value determined by the method described later. Since PP2 had a single peak shape in which only one melting peak appears, the apex temperature of the melting peak is shown only in the column of the apex temperature of the melting peak on the low temperature side. A second DSC curve of PP1 is shown in FIG. A second DSC curve of PP2 is shown in FIG.

(Amount of heat of fusion ΔH of polypropylene resin (S))
The heat of fusion of the polypropylene-based resin (S) was determined as follows. A point on the DSC curve at a temperature of 80° C. was defined as α, and a point on the DSC curve corresponding to the melting end temperature was defined as β (see FIG. 1). A straight line parallel to the vertical axis of the graph is drawn from the point γ on the DSC curve corresponding to the valley between the melting peak _Pl on the low temperature side and the melting peak _Ph on the high temperature side, and the straight line connecting the points α and β The point at which (α−β) intersects is δ. The area (A) of the melting peak _Pl on the low temperature side is the heat of fusion _ΔHl of the melting _peak Pl on the low temperature side. It was obtained as the area of the portion surrounded by the line segment (γ-δ). The area (B) of the melting peak _Ph on the high temperature side is the heat of fusion ΔHh of the melting peak _Ph on the high temperature side, and the DSC curve showing the melting peak _Ph on the high temperature side, the line segment (δ-β), It was obtained as the area of the portion surrounded by the line segment (γ-δ).
The total heat of fusion ΔHt is the sum of the area (A) of the melting peak _Pl on the low temperature side and the area (B) of the melting peak _Ph on the high temperature side [A + B (that is, ΔHl + ΔHh)]. It was obtained as the area surrounded by the straight line (α-β) and the DSC curve in the section between points α and β.
As for PP2, since it had a single peak shape in which only one melting peak appeared, the heat of fusion of the melting peak was indicated only in the column of the heat of fusion of the melting peak on the low temperature side.

(Melting point of polypropylene resin (C))
The melting point Tmc of the polypropylene-based resin (C) was based on JIS K7121:1987, and adopted "(2) the case of measuring the melting temperature after performing a certain heat treatment" as the state adjustment of the test piece. Specifically, about 3 mg of resin that has been conditioned at 23 ° C. and 50% RH for 24 hours or more is used as a test piece, and the test piece is measured by a heat flux differential scanning calorimeter (manufactured by Shimadzu Corporation, model number: DSC- 60 A) at a heating rate of 10°C/min from 30°C to 200°C, then cooled at a cooling rate of 10°C/min from 200°C to 30°C, again at a heating rate of 10°C/min. A DSC curve was obtained by raising the temperature from 30° C. to 200° C. at , and the peak temperature of the melting peak accompanying the melting of the resin on the DSC curve was obtained.

(MFR)
The MFR of the polypropylene resin was measured according to JIS K7210-1:2014 under conditions of a temperature of 230° C. and a load of 2.16 kg.

(flexural modulus)
The flexural modulus of polypropylene resin conforms to JIS K7171: 2016, heat presses at 230 ° C. to produce a sheet with a thickness of 4 mm, and from the sheet, length 80 mm × width 10 mm × thickness 4 mm (standard test piece ) was used. The radius R1 of the indenter and the radius R2 of the support base were both 5 mm, the distance between fulcrums was 64 mm, and the test speed was 2 mm/min.

<Expanded particles>
(The bulk density)
The bulk density of the expanded beads was determined as follows. Expanded particles are randomly taken out from the group of expanded particles after conditioning, placed in a graduated cylinder with a capacity of 1 L, and a large number of expanded particles are accommodated up to the 1 L scale so as to be in a state of natural accumulation. The bulk density of the expanded beads was obtained by dividing W1 [g] by the storage volume V1 (1 [L]) (W1/V1) and converting the unit into [kg/m ³ ].

(High temperature peak heat quantity)
One expanded bead was collected from the expanded bead group after conditioning. Using these foamed particles as a test piece, the test piece is heated from 23° C. to 200° C. at a heating rate of 10° C./min with a differential thermal scanning calorimeter (specifically, model number: DSC-60A manufactured by Shimadzu Corporation). A DSC curve was obtained upon warming. An example of a DSC curve is shown in FIG. The DSC curve exemplified in FIG. 2 has a resin-specific peak _P1 and a high temperature peak _P2 on the high temperature side. A straight line L1 is obtained by connecting the point β at the temperature T. Next, a straight line L2 parallel to the vertical axis of the graph is drawn from the point γ on the DSC curve corresponding to the valley between the resin-specific peak _P1 and the high temperature peak _P2 , and the point where the straight line L1 and the straight line L2 intersect is δ. The area (2) of the high temperature peak _P2 of the expanded beads is the heat of fusion ΔH2 of the high temperature peak _P2 , and the DSC curve showing the high temperature peak _P2 , the line segment (δ-β), and the line segment (γ-δ ) was obtained as the area of the part surrounded by Tables 3 and 4 show the values obtained by performing the above measurements on five expanded beads and arithmetically averaging them.

<Expanded particle molding>
(Mold density)
The density of the expanded bead molded product was determined by randomly cutting out three test pieces each having a rectangular parallelepiped shape of 25 mm long, 25 mm wide, and 100 mm thick, excluding the skin layer during molding. The mass and volume of the piece were measured, and the density of the three test pieces was calculated by dividing the mass by the volume for unit conversion, and the arithmetic average was obtained.

(50% compressive stress)
The 50% compressive stress of the expanded bead molded product was determined by removing the skin layer from the expanded bead molded product and cutting out a rectangular parallelepiped test piece measuring 50 mm long × 50 mm wide × 25 mm thick. Using A&D RTF-1350, the load at 50% strain when compressed at a speed of 10 mm / min based on JIS K6767: 1999 is obtained, and this is divided by the pressure receiving area of the test piece. 50% compressive stress [kPa] was obtained by calculating as follows.

[Evaluation method]
<Range of Moldable Molding Pressure (Moldability)>
The moldability of the expanded beads was evaluated as follows. In <Preparation of expanded bead molded article> described later, the molding pressure (steam pressure) is changed in increments of 0.01 MPa (G) to produce an expanded bead molded article. The molding pressure at which all evaluations of appearance and recoverability were "A" was defined as the range of molding pressure at which molding is possible. Tables 3 and 4 also show the points of the molding pressure (the number of molding pressures at which all the evaluations were "A"). Since the molding temperature is adjusted by the molding pressure, the wider the range of the molding pressure from the lower limit to the upper limit, the wider the range of the molding heating temperature that can be molded.
(fusion)
The fusion bondability of the expanded bead molded article was evaluated by the following method. The expanded bead molded product is bent and broken, the number of expanded beads present on the fracture surface (C1) and the number of broken expanded beads (C2) are determined, and the ratio of broken expanded beads to the above expanded beads (C2/C1 × 100) was calculated as the material destruction rate. Measurements were performed five times using different test pieces, and the respective material destruction rates were determined, and the arithmetic mean was calculated to evaluate the fusion bondability according to the following criteria.
A: Material destruction rate of 90% or more B: Material destruction rate of 20% or more and less than 90% C: Material destruction rate of less than 20% (appearance (degree of gap))
Draw a rectangle of 100 mm x 100 mm in the center of the expanded bead molding, draw a line diagonally from the corner of the rectangular area, and count the number of voids (gap) of 1 mm x 1 mm or more on the line, The surface appearance of the expanded bead molded article was evaluated according to the following criteria.
A: The number of voids is less than 3 B: The number of voids is 3 or more and less than 5 C: The number of voids is 5 or more (recoverability)
The thicknesses of the central portion and the four corner portions of the resulting expanded bead molded article were measured, and the ratio of the thickness of the central portion to the thickest portion among the four corner portions was calculated, and the recoverability was evaluated according to the following criteria.
A: When the thickness ratio is 95% or more B: The thickness ratio is 90% or more and less than 95% C: The thickness ratio is less than 90%

<Range of molding pressure for good mold releasability (mold releasability)>
In the evaluation of <Range of molding pressure that can be molded (moldability)>, the releasability is evaluated according to the following criteria in the range of molding pressure that can be molded. The molding pressure was set to give good moldability. In addition, the points of the molding pressure (the number of molding pressures at which the evaluation was "A") are also shown in the table. It means that the wider the range from the lower limit value to the upper limit value of the molding pressure at which mold releasability is good, the wider the molding heating temperature range at which mold releasability is good.
A: Can be released without using a release assist such as a release pin or release air B: Cannot be released without a release assist such as a release pin or release air

[material]
Tables 1 and 2 show polypropylene-based resins (S) and polypropylene-based resins (C) used in Examples and Comparative Examples. In the table, PP1 is a propylene-based copolymer containing ethylene, propylene and butene, and has a grade name of FL7540L manufactured by TPC.
In the table, PP5 is a mixture of an ethylene-propylene random copolymer (ethylene 3.1% by mass, melting point 142°C) and an ethylene-propylene random copolymer (ethylene 1.4% by mass, melting point 153°C). resin (the mass ratio is 50% by mass:50% by mass when the total of both is 100% by mass). PP6 is ^a mixed resin (mass The ratio is ethylene-propylene random copolymer:linear low-density polyethylene=67% by mass:33% by mass, with the total of both being 100% by mass.

Examples 1-5 and Comparative Examples 1-9
<Preparation of expanded particles>
Using the types of polypropylene resins (C) shown in Tables 3 and 4, melt kneading was performed in a core layer forming extruder at a maximum set temperature of 245° C. to obtain a core layer forming resin melt-kneaded product. Also, the types of polypropylene resin (S) shown in Tables 3 and 4 were melt-kneaded in an extruder for forming a coating layer at a maximum set temperature of 245° C. to obtain a melt-kneaded resin material for forming a coating layer. Next, each resin melt-kneaded product was extruded from the tip of the coextrusion die from the core layer forming extruder and the coating layer forming extruder. At this time, the melted and kneaded resins are combined in the die to form a sheath-core composite composed of a non-foamed core layer and a non-foamed coating layer covering the outer surface of the core layer. let me The composite was extruded in the form of a strand through a hole in a mouthpiece attached to the tip of the extruder, cooled with water while being taken up, and then cut into pieces having a mass of about 1.0 mg with a pelletizer. Thus, a multilayer resin particle comprising a core layer and a coating layer covering the core layer was obtained. The mass ratio of the core layer to the coating layer in the multilayer resin particles was set to core layer:coating layer=95:5 (that is, the mass ratio of the coating layer was 5%). In the production of the multi-layered resin particles, zinc borate as a cell control agent was supplied to the extruder for forming the core layer, and 500 ppm by mass of zinc borate was contained in the polypropylene-based resin.

(One-step foaming process)
1 kg of multi-layered resin particles are placed in a 5-L closed container together with 3 L of water as a dispersion medium. Parts by mass and 0.004 parts by mass of a surfactant (sodium alkylbenzenesulfonate) were added into the closed container. After adding carbon dioxide as a foaming agent into the closed container, the closed container was closed and heated to the foaming temperature shown in Tables 3 and 4 while stirring the inside of the closed container. The internal pressure of the container (that is, impregnation pressure, carbon dioxide pressure) at this time was as shown in Tables 3 and 4. After holding at the same temperature for 15 minutes, the contents of the container were discharged to atmospheric pressure to obtain expanded beads. The expanded beads were dried at 23° C. for 24 hours. Thus, expanded beads having the bulk ratios shown in Tables 3 and 4 were obtained.

(Two-step foaming process)
Next, the foamed particles are placed in a pressure vessel (specifically, a metal drum), and air is forced into the pressure vessel to increase the pressure inside the vessel, impregnating the air into the air bubbles, thereby producing the foamed particles. increased the internal pressure inside the bubbles. Next, steam was supplied to the expanded beads (single-stage expanded beads) removed from the pressure vessel so that the pressure in the pressure vessel (that is, the drum pressure) reached the pressure shown in Tables 3 and 4, and the particles were heated under atmospheric pressure. Tables 3 and 4 show the pressure inside the cells (that is, internal pressure) in the single-stage expanded beads taken out of the pressure-resistant container. As described above, the apparent density of the single-stage expanded beads was lowered, and expanded beads (two-stage expanded beads) having the bulk ratio shown in Tables 3 and 4 were obtained.

<Preparation of expanded bead molding>
The foamed beads were dried at 23° C. for 24 hours and used for the production of the foamed beads. First, after applying an internal pressure of 0.25 MPa (G) to the expanded beads with compressed air, the cracking amount was adjusted to 20% (that is, 12 mm), and a vertical type having a cavity of 300 mm long × 250 mm wide × 60 mm thick The molding machine was filled with expanded particles, the molds were clamped, and an exhaust process was performed in which steam was supplied from both sides of the molds for 5 seconds to preheat. The vertical molding machine is a molding machine in which the opening direction of the mold is the vertical direction (height direction). Thereafter, steam was supplied from one side of the mold until the pressure reached 0.08 MPa (G) lower than the predetermined molding pressure, and heating was performed on the other side. Next, steam is supplied from the other side of the mold until a pressure 0.04 MPa (G) lower than the molding pressure is reached, and after one heating is performed, heating is performed until a predetermined molding pressure is reached (that is, main heating). did After the completion of heating, the pressure was released, and after cooling with water until the surface pressure due to the foaming force of the molded body reached 0.04 MPa (G), the molded body was released from the mold to obtain an expanded bead molded body.

As can be seen from Table 3, the expanded beads of the present invention have a wide range of moldable molding pressure and good releasability over the entire molding pressure range. When the foamed bead molded article is removed from the mold, if the molded article sticks to the mold, a mold release assist such as a mold release pin or mold release air is generally used. According to the method, the release assist is unnecessary, so the manufacturing efficiency can be improved.
As can be seen from Table 4, the polypropylene resin (S) does not have a double peak, is a single peak, and does not satisfy the above requirement (1). It has been difficult to achieve both good moldability and moldability. As can be seen from the comparison between Comparative Examples 1 and 9, the expanded beads having a low bulk density tended to be particularly poor in releasability.
Comparative Examples 2, 3, and 8, in which the peak temperature of the melting peak on the high temperature side of the polypropylene resin (S) was less than 138°C and did not satisfy the above requirement (3), were inferior in releasability. .
The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene resin (C) and the apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) exceeds 15 ° C. Comparison that does not satisfy the above requirement (4) Example 5 was also inferior in releasability.
The apex temperature of the melting peak on the high temperature side of the polypropylene resin (S) is less than 138 ° C., does not satisfy the above requirement (3), and the melting point Tmc of the polypropylene resin (C) and the polypropylene resin (S) The difference [Tmc-Tmsh] between the melting peak on the high temperature side and the apex temperature Tmsh exceeded 15° C., and Comparative Example 4, which did not satisfy the above requirement (4), was remarkably inferior in releasability.

The expanded beads of the present invention have a wide range of moldable molding pressure and excellent releasability. Useful.

Claims (9)

Polypropylene-based resin expanded beads having a foamed core layer composed of a polypropylene-based resin (C) and a coating layer composed of a polypropylene-based resin (S),
The polypropylene resin (C) has a melting point Tmc of 125° C. or higher and 170° C. or lower,
Expanded polypropylene resin particles satisfying the following (1) to (4).
(1) The polypropylene resin (S) is heated by heat flux differential scanning calorimetry from 30° C. to 200° C. at a heating rate of 10° C./min, then 200° C. at a cooling rate of 10° C./min. The second DSC curve obtained by cooling from ° C. to 30 ° C. and heating again at a heating rate of 10 ° C./min has a melting peak on the low temperature side and a melting peak on the high temperature side,
(2) the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is lower than the melting point Tmc of the polypropylene resin (C);
(3) The peak temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is 138° C. or higher.
(4) The difference [Tmc−Tmsh] between the melting point Tmc of the polypropylene resin (C) and the apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is −10° C. or more and 15° C. or less. . 2. The expanded polypropylene resin beads according to claim 1, wherein the melting peak temperature Tmsl of the polypropylene resin (S) on the low temperature side is 125° C. or higher and 135° C. or lower. The difference [Tmc−Tmsl] between the melting point Tmc of the polypropylene resin (C) and the peak temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is more than 0° C. and 25° C. or less. 3. The expanded polypropylene resin particles according to 1 or 2. The ratio [ΔHh/ΔHt] of the heat of fusion (ΔHh) of the melting peak on the high temperature side of the polypropylene resin (S) to the total heat of fusion (ΔHt) of the polypropylene resin (S) is 0.35 or more and 0.35 or more. The polypropylene-based resin expanded beads according to any one of claims 1 to 3, which are 80 or less. Claims 1 to 4, wherein the difference [Tmsh-Tmsl] between the peak temperature Tmsh of the melting peak on the high temperature side and the peak temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is 10°C or more and 20°C or less. Expanded polypropylene resin particles according to any one of . The expanded polypropylene resin particles according to any one of claims 1 to 5, wherein the polypropylene resin (C) has a melting point Tmc of 130°C or higher and 150°C or lower. The polypropylene-based resin expanded beads according to any one of claims 1 to 6, wherein the polypropylene-based resin (S) is a propylene-based copolymer containing a propylene component as a main component and containing an ethylene component and a butene component. . The expanded polypropylene resin beads according to any one of claims 1 to 7, wherein the expanded polypropylene resin beads have a bulk density of 10 kg/m ³ or more and 30 kg/m ³ or less. A method for producing expanded polypropylene resin particles, comprising:
a step of granulating multilayer resin particles having a core layer made of a polypropylene resin (C) and a coating layer made of a polypropylene resin (S) and covering the core layer;
dispersing the multilayer resin particles in a dispersion medium in a closed container;
a step of impregnating the multilayer resin particles dispersed in the dispersion medium with a foaming agent;
a step of foaming the multilayer resin particles containing the foaming agent to produce foamed beads;
The polypropylene resin (C) has a melting point Tmc of 125° C. or higher and 170° C. or lower,
A method for producing expanded polypropylene resin particles that satisfies the following (1) to (4).
(1) The polypropylene resin (S) is heated by heat flux differential scanning calorimetry from 30° C. to 200° C. at a heating rate of 10° C./min, then 200° C. at a cooling rate of 10° C./min. The second DSC curve obtained by cooling from ° C. to 30 ° C. and heating again at a heating rate of 10 ° C./min has a melting peak on the low temperature side and a melting peak on the high temperature side,
(2) the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is lower than the melting point Tmc of the polypropylene resin (C);
(3) The peak temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is 138° C. or higher.
(4) The difference [Tmc−Tmsh] between the melting point Tmc of the polypropylene resin (C) and the apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is −10° C. or more and 15° C. or less. .

Images