Polypropylene-based resin foamed particles and method for producing the same

Abstract

The present application provides a polypropylene resin foamed particle capable of being formed, having a wide range of forming pressure and excellent demolding property, having a foamed core layer composed of a polypropylene resin (C) and a coating layer composed of a polypropylene resin (S), the polypropylene resin (C) having a melting point of 125°C or higher and 170°C or lower, the polypropylene resin foamed particle satisfying (1)-(4) below: (1) the polypropylene resin (S) has a melting peak on the low-temperature side and a melting peak on the high-temperature side; (2) the apex temperature of the melting peak on the low-temperature side of the polypropylene resin (S) is lower than the melting point of the polypropylene resin (C); (3) the apex temperature of the melting peak on the high-temperature side of the polypropylene resin (S) is 138°C or higher; and (4) the difference between the melting point of the polypropylene resin (C) and the apex temperature of the melting peak on the high-temperature side of the polypropylene resin (S) is -10°C or higher and 15°C or lower.

Description

Technical Field

[0001] This invention relates to polypropylene resin foamed particles and their manufacturing methods. Background Technology

[0002] In-mold molding of polypropylene resin foam particles produces foamed particle molded bodies, which are used as impact-absorbing materials, thermal insulation materials, and various packaging materials. They are widely used in food containers, packaging or cushioning materials for electrical and electronic components, automotive bumpers or interior components, building components such as residential thermal insulation materials, and general merchandise.

[0003] Polypropylene resin foamed particle molded articles are manufactured, for example, by in-mold molding. In this method, polypropylene resin foamed particles are filled into a molding die and heated with a heating medium such as steam. This causes the foamed particles to foam a second time, melting their surfaces and fusing them together to form the desired shape. In the in-mold molding of polypropylene resin foamed particles, the heating temperature using the heating medium is primarily set considering the melting point of the resin constituting the foam layer.

[0004] Because polypropylene resin foam particles present molding and processing difficulties due to the crystallinity and heat resistance of propylene resins, research has been conducted to improve their molding processability. For example, Patent Document 1 describes a method for manufacturing polypropylene resin foam particles by impregnating multilayer resin particles with a foaming agent to induce foaming. The multilayer resin particles consist of a core layer formed of polypropylene resin and an outer layer formed of polypropylene resin. The melting point of the polypropylene resin in the outer layer has a specific relationship with the melting point of the polypropylene resin in the core layer, and the thickness of the outer layer is below a specific value. The polypropylene resin foam particles obtained by this manufacturing method exhibit excellent fusion properties between the foam particles at low molding heating temperatures (molding pressures).

[0005] However, the polypropylene resin foam particles described in Patent Document 1 sometimes exhibit poor release properties depending on the molding conditions or the type of base resin. This is particularly true when using foam particles with low apparent density or at higher molding temperatures. Therefore, from the viewpoint of further improving the productivity of foam particle molded articles, foam particles with good release properties over a wider range of molding temperatures are required. To improve the release properties of foam particles, for example, Patent Documents 2 and 3 disclose molding apparatuses with comb teeth provided in movable parts within the mold or molding space. Furthermore, for example, Patent Document 4 discloses a method for manufacturing foam particle molded articles using a mold in which at least one side of the inner wall surface of the mold has fine holes or slits with a cross-sectional shape having an opening area that gradually increases toward the surface in contact with the foam particles.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2004-68016

[0009] Patent Document 2: Japanese Patent Application Publication No. 2001-328135

[0010] Patent Document 3: Japanese Patent Application Publication No. 2002-172642

[0011] Patent Document 4: Japanese Patent Application Publication No. 2015-214110 Summary of the Invention

[0012] The technical problem that the invention aims to solve

[0013] However, patent documents 2 to 4 require specialized equipment, and it is desirable to achieve polypropylene resin foam particles that can be formed into foam particles with a wide range of molding heating temperatures and excellent demolding properties, even when using a general-purpose equipment.

[0014] The technical problem of the present invention is to provide polypropylene resin foam particles with a wide range of molding heating temperatures and excellent demolding properties, and a method for manufacturing the same.

[0015] Solution to the above technical problems

[0016] The inventors discovered that the above-mentioned technical problems could be solved by adopting the configuration shown below, thereby completing the present invention.

[0017] That is, the present invention is as follows.

[0018] <1> A polypropylene resin foamed particle has a foamed core layer made of polypropylene resin (C) and a coating layer made of polypropylene resin (S), wherein the melting point Tmc of the polypropylene resin (C) is above 125°C and below 170°C, and the polypropylene resin foamed particle satisfies the following (1) to (4).

[0019] (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in the second DSC curve obtained by heating the resin from 30°C to 200°C at a heating rate of 10°C / min, cooling it from 200°C to 30°C at a cooling rate of 10°C / min, and then heating it again at a heating rate of 10°C / min.

[0020] (2) The peak 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).

[0021] (3) The peak temperature Tmsh of the melting peak on the high-temperature side of the polypropylene resin (S) is above 138°C.

[0022] (4) The difference 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) [Tmc-Tmsh] is above -10℃ and below 15℃.

[0023] <2> like <1> The polypropylene resin foam particles have a melting peak temperature Tmsl of 125°C to 135°C on the low-temperature side of the polypropylene resin (S).

[0024] <3> like <1> or <2> The polypropylene resin foam particles wherein the difference 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) [Tmc-Tmsl] exceeds 0°C and is below 25°C.

[0025] <4> like <1> ~ <3> The polypropylene resin foam particles according to any one of the claims, wherein the ratio of the heat of fusion (ΔHh) of the high-temperature side of the polypropylene resin (S) to the total heat of fusion (ΔHt) of the polypropylene resin (S) [ΔHh / ΔHt] is 0.35 or more and 0.80 or less.

[0026] <5> like <1> ~ <4> The polypropylene resin foam particles according to any one of the claims, wherein the difference [Tmsh-Tmsl] between the peak temperature Tmsh of the high-temperature side and the peak temperature Tmsl of the low-temperature side of the polypropylene resin (S) is above 10°C and below 20°C.

[0027] <6> like <1> ~ <5> The polypropylene resin foam particles according to any one of the claims, wherein the melting point Tmc of the polypropylene resin (C) is above 130°C and below 150°C.

[0028] <7> like <1> ~ <6> The polypropylene resin foam particles according to any one of the claims, wherein the polypropylene resin (S) is a propylene copolymer with propylene as the main component and containing ethylene and butene components.

[0029] <8> like <1> ~ <7> The polypropylene resin foamed particles according to any one of the claims, wherein the bulk density of the polypropylene resin foamed particles is 10 kg / m³. 3 Above 30kg / m 3 the following.

[0030] <9> A method for manufacturing polypropylene resin foam particles includes: a step of granulating multilayer resin particles having a core layer made of polypropylene resin (C) and a coating layer made of polypropylene resin (S) covering the core layer; a step of dispersing the multilayer resin particles in a dispersion medium in a sealed container; a step of impregnating the multilayer 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 foam particles, wherein the melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, and the polypropylene resin foam particles satisfy the following (1) to (4).

[0031] (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in the second DSC curve obtained by heating the resin from 30°C to 200°C at a heating rate of 10°C / min, cooling it from 200°C to 30°C at a cooling rate of 10°C / min, and then heating it again at a heating rate of 10°C / min.

[0032] (2) The peak 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).

[0033] (3) The peak temperature Tmsh of the melting peak on the high-temperature side of the polypropylene resin (S) is above 138°C.

[0034] (4) The difference 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) [Tmc-Tmsh] is above -10℃ and below 15℃.

[0035] Invention Effects

[0036] According to the present invention, it is possible to provide polypropylene resin foam particles that can be molded over a wide range of molding heating temperatures and have excellent release properties throughout the entire range. Attached Figure Description

[0037] Figure 1 This is a graph showing the second DSC curve of the polypropylene resin used in Example 1 for the coating layer.

[0038] Figure 2 This is a diagram used to illustrate the method for determining the high-temperature peak of bubbling particles.

[0039] Figure 3 This is a graph showing the second DSC curve of the polypropylene resin used in Comparative Example 1 for the coating layer. Detailed Implementation

[0040] [Polypropylene resin foam particles]

[0041] The polypropylene resin foamed particles of the present invention (hereinafter also referred to as polypropylene resin foamed particles or foamed particles) are polypropylene resin foamed particles having a foamed core layer (hereinafter also referred to as foamed core layer) made of polypropylene resin (C) and a coating layer (hereinafter also referred to as coating layer) made of polypropylene resin (S), wherein the melting point Tmc of polypropylene resin (C) is 125°C or higher and 170°C or lower, and the polypropylene resin foamed particles satisfy the following (1) to (4).

[0042] (1) In the second DSC curve obtained by heating the polypropylene resin (S) from 30°C to 200°C at a heating rate of 10°C / min, then cooling it from 200°C to 30°C at a cooling rate of 10°C / min, and then heating it again at a heating rate of 10°C / min, there are melting peaks on the low temperature side and melting peaks on the high temperature side.

[0043] (2) The peak temperature Tmsl of the melting peak on the low-temperature side of polypropylene resin (S) is lower than the melting point Tmc of polypropylene resin (C).

[0044] (3) The peak temperature Tmsh of the melting peak on the high-temperature side of polypropylene resin (S) is above 138℃.

[0045] (4) The difference between the melting point Tmc of polypropylene resin (C) and the peak temperature Tmsh of the melting peak on the high-temperature side of polypropylene resin (S) [Tmc-Tmsh] is above -10℃ and below 15℃.

[0046] The polypropylene resin foaming particles of the present invention utilize a multi-layer structure of foaming particles in which polypropylene resin (C) and polypropylene resin (S) with a specific relationship are used as the foaming core layer and the coating layer, respectively. This results in a wide range of molding pressures, i.e., excellent formability and excellent demolding properties.

[0047] Specifically, it was found that in order to balance formability and release properties, polypropylene resin foam particles that meet the above (1) to (4) can be used.

[0048] Previously, in order to improve weldability, foamed particles with a coating layer made of polypropylene resin with a lower melting point than the polypropylene resin constituting the foamed core layer tended to have poor mold release properties. In particular, under high molding heating temperatures, the resin in the coating layer may easily soften and adhere to the molding die, thus significantly impairing mold release properties. Furthermore, even when using foamed particles with low apparent density, there is a tendency for significantly impaired mold release properties. The present invention has found that by making the polypropylene resin foamed particles satisfy the above (1) to (4), excellent weldability can be achieved, and both moldability and mold release properties can be taken into account.

[0049] <Covering layer>

[0050] A coating layer of polypropylene resin foam particles coats a foam core layer. The coating layer preferably covers substantially the entire outer surface of the foam core layer, but may also completely cover the entire outer surface of the foam core layer. Specifically, the coating layer preferably covers more than 50% of the surface area of ​​the core layer, more preferably more than 70%, and may also cover substantially 100% of the area, or less than 90%.

[0051] From the viewpoint of the appearance of the polypropylene resin foamed particle molded article (hereinafter also referred to as polypropylene resin foamed particle molded article or foamed particle molded article) obtained using the polypropylene resin foamed particles of the present invention, the coating layer is preferably in a non-foamed state or substantially non-foamed state, more preferably non-foamed state. A non-foamed state or substantially non-foamed state means having almost no bubble structure. Furthermore, it also includes a state where temporarily formed bubbles burst and disappear. Additionally, the coating layer may also be in a micro-foamed state without impairing the intended effect of the present invention.

[0052] (Polypropylene resin (S))

[0053] The coating layer of the polypropylene resin foam particles is composed of polypropylene resin (S). In this specification, polypropylene resin refers to a polymer in which the content of structural units derived from propylene is 50% by mass or more. Examples of polypropylene resins include propylene homopolymers, propylene copolymers, or mixtures thereof. Furthermore, the polypropylene resin may also contain other resins or elastomers other than polypropylene, but their content is preferably 20% by mass or less, more preferably 15% by mass or less, further preferably 10% by mass or less, and particularly preferably 5% by mass or less.

[0054] (1) Twin Peaks

[0055] Polypropylene resin (S) underwent state conditioning according to JIS K 7121:1987, specifically by measuring the melting temperature after a certain heat treatment (2). The DSC curve obtained by heating the condition-conditioned test piece from 30°C to 200°C at a heating rate of 10°C / min showed both a melting peak on the low-temperature side and a melting peak on the high-temperature side. Specifically, in the second DSC curve obtained by using resin conditioned at 23°C and 50% RH for more than 24 hours as a test piece, heating the resin from 30°C to 200°C at a heating rate of 10°C / min using differential scanning calorimetry, then cooling it from 200°C to 30°C at a cooling rate of 10°C / min, and then heating it again at a heating rate of 10°C / min, both melt peaks were observed on the low-temperature side and the high-temperature side. Here, "melting peaks on both the low-temperature and high-temperature sides" refers to melting peaks on the low-temperature and high-temperature sides having different peak apex temperatures (melting points), and each having a heat of fusion of 10 J / g or more. The method for measuring the heat of fusion will be described later. Parts of the melting peaks on the low-temperature and high-temperature sides may overlap. In this specification, the melting peaks on both the low-temperature and high-temperature sides in the second DSC curve of the aforementioned polypropylene resin (S) are sometimes collectively referred to as "double peaks."

[0056] In addition, in the second DSC curve of the aforementioned polypropylene resin (S), if more than three melting peaks appear, the melting peak with a peak temperature on the lowest temperature side and a melting heat of 10 J / g or more is set as the melting heat of the low temperature side, and the melting peak with a peak temperature on the highest temperature side and a melting heat of 10 J / g or more is set as the melting peak of the high temperature side.

[0057] If polypropylene resin (S) has only a single melting peak in the second DSC curve instead of both a low-temperature melting peak and a high-temperature melting peak (double peak), it may be impossible to simultaneously achieve both the formability and release properties of polypropylene resin foam particles.

[0058] (2) The peak temperature Tmsl of the melting peak on the low-temperature side

[0059] The peak temperature (Tmsl) of the melting peak on the low-temperature side of polypropylene resin (S) is lower than the melting point (Tmc) of polypropylene resin (C) described later. By keeping Tmsl below Tmc, excellent weldability and improved formability are achieved even at lower molding heating temperatures. Furthermore, from the same viewpoint, 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, from the viewpoint of further improving the release properties (hereinafter also simply referred to as release properties) of the polypropylene resin foam particles, Tmsl is preferably 125°C or higher, more preferably 126°C or higher, and even more preferably 127°C or higher.

[0060] (3) The peak temperature Tmsh of the melting peak on the high-temperature side

[0061] The peak temperature (Tmsh) of the melt flow on the high-temperature side of the polypropylene resin (S) is 138°C or higher. When Tmsh is less than 138°C, the foamed particles, especially under high molding heating temperatures, risk impaired mold release properties. From this perspective, Tmsh is preferably 139°C or higher, more preferably 140°C or higher, and even more preferably 141°C or higher. Furthermore, from the viewpoint of further improving moldability, Tmsh 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.

[0062] (4) Difference between the melting points Tmc and Tmsh of polypropylene resin (C)

[0063] The difference between the melting point Tmc of the polypropylene resin (C) and the peak temperature Tmsh of the high-temperature side of the polypropylene resin (S), described later, [Tmc-Tmsh], is -10°C to 15°C. When the difference [Tmc-Tmsh] exceeds 15°C, there is a risk of impaired mold release properties when using resins with higher melting points in the foamed particles. From this perspective, the difference [Tmc-Tmsh] is preferably 12°C or less, more preferably 10°C or less, and even more preferably 7°C or less. Furthermore, when the difference [Tmc-Tmsh] is less than -10°C, it may be impossible to simultaneously achieve good formability and good mold release properties. From the viewpoint of suppressing a decrease in formability while improving mold release properties, the difference [Tmc-Tmsh] is preferably -8°C or more, and even more preferably -5°C or more.

[0064] The difference between the melting point Tmc and Tmsl of polypropylene resin (C)

[0065] As described above, from the viewpoint of formability, the difference [Tmc-Tmsl] between the melting point Tmc of the polypropylene resin (C) and the peak temperature Tmsl of the low-temperature side of the polypropylene resin (S) exceeds 0°C. From the viewpoint of further improving the formability of the foamed particles, the difference [Tmc-Tmsl] is more preferably 5°C or more, and even more preferably 10°C or more. On the other hand, from the viewpoint of further improving demolding properties and suppressing the peeling of the foamed core layer from 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.

[0066] The difference between TMSH and TMSL

[0067] From the viewpoint of achieving a good balance between formability and release properties, the difference between the peak temperature Tmsh of the high-temperature side and the peak temperature Tmsl of the low-temperature side of the polypropylene resin (S) [Tmsh-Tmsl] is preferably 10°C or more, more preferably 13°C or more, and preferably 20°C or less, more preferably 17°C or less, and even more preferably 15°C or less.

[0068] The ratio of the heat of fusion of the high-temperature melting peak ΔHh to the total heat of fusion ΔHt

[0069] From the viewpoint of achieving a good balance between formability and releaseability, the ratio of the melting heat ΔHh of the high-temperature side of the polypropylene resin (S) to the total melting heat ΔHt [ΔHh / ΔHt] is preferably 0.35 or more, more preferably 0.38 or more, even more preferably 0.40 or more, and preferably 0.80 or less, more preferably 0.70 or less, even more preferably 0.60 or less, and even more preferably 0.50 or less.

[0070] exist Figure 1 In the case of the illustrated DSC curve, the heat of fusion ΔHl of the low-temperature melting peak, the heat of fusion ΔHh of the high-temperature melting peak, and the total heat of fusion ΔHt of the polypropylene resin (S) are calculated as follows. The point on the DSC curve at 80°C is designated as α, and the point on the DSC curve corresponding to the melting end temperature is designated as β. Furthermore, the heat of fusion of the low-temperature melting peak P on the DSC curve is... l and the melting peak P on the high-temperature side h Draw a straight line parallel to the vertical axis of the graph corresponding to the point γ of the valley. The point where this line intersects the line (α-β) connecting points α and β is designated as δ. Additionally, due to the overlap between the melting peak on the low-temperature side and part of the melting peak on the high-temperature side, the melting peak P on the DSC curve on the low-temperature side... l and the melting peak P on the high-temperature side h If the point γ corresponding to the valley is unclear, the location of point γ can be determined by referring to the differential curve (DDSC) of the DSC near that temperature. Point γ is shown as 0 in the DSC.

[0071] Melting peak P on the low-temperature side l The area (A) is the melting peak P on the low-temperature side. l The heat of fusion ΔHl can be used as the melting peak P shown on the low-temperature side. l The area enclosed by the DSC curve and line segments (α-δ) and (γ-δ) is used to calculate the area.

[0072] Melting peak P on the high-temperature side h The area (B) is the melting peak P on the high-temperature side. hThe heat of fusion ΔHh can be used as the melting peak P shown on the high-temperature side. h The area enclosed by the DSC curve and line segments (δ-β) and (γ-δ) is used to calculate the area.

[0073] The total heat of melting ΔHt is the melting peak P on the low-temperature side. l The area (A) and the melting peak P on the high-temperature side h The total area (B) of [A+B (i.e., ΔHl+ΔHh)] is in Figure 1 In this context, is the area enclosed by the DSC curve within the interval between the line (α-β) and points α and β.

[0074] ΔHl

[0075] From the viewpoint of further improving formability, 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 even more preferably 35 J / g or more. Furthermore, from the viewpoint of more reliably ensuring demolding properties, it is preferably 50 J / g or less, more preferably 45 J / g or less, and even more preferably 43 J / g or less.

[0076] ΔHh

[0077] From the viewpoint of further improving mold release properties, the heat of fusion ΔHh of the high-temperature side melt peak of the polypropylene resin (S) is preferably 15 J / g or more, more preferably 21 J / g or more, even more preferably 26 J / g or more, and even more preferably 30 J / g or more. Furthermore, from the viewpoint of more reliably ensuring formability, it is preferably 45 J / g or less, more preferably 40 J / g or less, and even more preferably 35 J / g or less.

[0078] ΔHt

[0079] From the viewpoint of achieving a good balance between formability and releaseability, the total heat of melt ΔHt of the polypropylene resin (S) is preferably 57 J / g or more, more preferably 65 J / g or more, even more preferably 70 J / g or more, and preferably 85 J / g or less, more preferably 80 J / g or less, and even more preferably 75 J / g or less.

[0080] Melt Flow Rate (MFR)

[0081] From the viewpoint of ease of forming the coating layer and suppression of peeling of the foamed core layer, the MFR of polypropylene resin (S) is preferably 2 g / 10 min or more, more preferably 4 g / 10 min or more, even more preferably 5 g / 10 min or more, and preferably 10 g / 10 min or less, more preferably 9 g / 10 min or less, and even more preferably 8 g / 10 min or less.

[0082] The MFR of polypropylene resin (S) was measured according to JIS K7210-1:2014 at a temperature of 230°C and a load of 2.16 kg.

[0083] There are no particular limitations on the polypropylene resin (S) as long as it satisfies (1) to (3) above and satisfies the above relationship (4) with the melting point Tmc of the polypropylene resin (C), but it is preferred to be a propylene copolymer with propylene as the main component and containing ethylene and butene components.

[0084] When the polypropylene resin (S) is a propylene copolymer with propylene as the main component and containing ethylene and butene, the ethylene content in the copolymer is preferably 3.0% by mass or more and 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. Furthermore, the butene 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 even more preferably 9.0% by mass or more and 12% by mass or less. The total content of propylene, ethylene, and butene is 100% by mass. Additionally, the content of monomer components in the copolymer can be determined by IR spectroscopy. Propylene copolymers with ethylene and butene content within the above ranges are more likely to be copolymers that satisfy the above (1) to (4) and have a crystal structure exhibiting melting peaks at both low and high temperatures.

[0085] Furthermore, from the same viewpoint, the ratio of butene content to ethylene content [butene content / ethylene content] is preferably 1.5 or more, more preferably 1.8 or more, even more preferably 2.0 or more, and preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less.

[0086] In this invention, examples of resins that can be used as polypropylene resins (S) include FL7540L and FL7320L manufactured by TPC Corporation.

[0087] Polypropylene resins (S) may contain additives to a extent that does not impair the intended effects of the present invention.

[0088] <Foamed Core Layer>

[0089] The foamed core layer of the polypropylene resin foamed particles is composed of polypropylene resin (C). From the viewpoint of improving the formability of the foamed particles and the rigidity of the molded body, the independent bubble rate of the foamed particles is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. Furthermore, the independent bubble rate of the foamed particles can be measured using an air comparison hydrometer based on ASTM-D2856-70 step C.

[0090] (Polypropylene resin (C))

[0091] Melting Point TMC

[0092] From the viewpoint of rigidity and heat resistance of the foamed particle molded body, the melting point Tmc of the polypropylene resin (C) is 125°C or higher, preferably 130°C or higher, more preferably 135°C or higher, even more preferably 140°C or higher, and even more preferably 145°C or higher, and is 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 aforementioned polypropylene resin (S) as a coating layer and employing a polypropylene resin (C) having a melting point Tmc of the aforementioned wide range, both the formability and demolding properties of the polypropylene resin foamed particles can be balanced. Conventionally, especially in foamed particles using polypropylene resins with high melting points as the base resin, when a coating layer is provided to improve weldability, it is difficult to achieve good demolding properties across the entire range of molding heating temperatures that allow for molding. According to the present invention, even when using a polypropylene resin with a relatively high melting point as the foamed core layer, both formability and demolding properties can be balanced.

[0093] Based on JIS K7121:1987, and using "(2) measuring the melting temperature after certain heat treatment" as the condition adjustment of the test piece, the temperature of the condition-adjusted test piece was increased from 30°C to 200°C at a heating rate of 10°C / min, thereby obtaining a DSC curve. The melting point Tmc of polypropylene resin (C) was determined by taking the vertex temperature of the melting peak obtained along with the melting of the resin on the DSC curve. In addition, when multiple melting peaks appear in the DSC curve, the vertex temperature of the melting peak with the largest area is set as the melting point. In this case, the area of ​​each melting peak can be determined by the same method as for the melting heat of polypropylene resin (S) described above.

[0094] Flexural Modulus

[0095] From the viewpoint of maintaining the mechanical strength of the foamed particle molded body while taking into account its formability and demolding properties, the flexural modulus of polypropylene resin (C) is preferably 500 MPa or more, more preferably 600 MPa or more, even more preferably 700 MPa or more, even more preferably 800 MPa or more, and preferably 1600 MPa or less, more preferably 1300 MPa or less, and even more preferably 1100 MPa or less.

[0096] The flexural modulus of polypropylene resins can be determined based on JIS K7171:2016.

[0097] MFR

[0098] From the viewpoint of foaming properties, the MFR of polypropylene resin (C) is preferably 2 g / 10 min or more, more preferably 4 g / 10 min or more, even more preferably 5 g / 10 min or more, and preferably 15 g / 10 min or less, more preferably 12 g / 10 min or less, and even more preferably 10 g / 10 min or less.

[0099] The MFR of polypropylene resin (C) was measured using the same method as that used for the MFR of polypropylene resin (S) at 230°C and a load of 2.16 kg.

[0100] As long as the melting point Tmc of the polypropylene resin (C) is within the above range, satisfies the above relationship (2) with the peak temperature Tmsl of the melting peak on the low-temperature side of the polypropylene resin (S), and satisfies the above relationship (4) with the peak temperature Tmsh of the melting peak on the high-temperature side of the polypropylene resin (S), there are no particular limitations. A mixture of polypropylene homopolymer and propylene copolymer or a polypropylene copolymer is preferred.

[0101] As a polypropylene copolymer, it is preferably an ethylene-propylene copolymer, an ethylene-propylene-butene copolymer, or a mixture of copolymers thereof with a polypropylene homopolymer, more preferably an ethylene-propylene copolymer or an ethylene-propylene-butene copolymer, and even more preferably an ethylene-propylene copolymer.

[0102] The polypropylene resin (C) may also contain other polymers, such as resins or elastomers, besides the propylene homopolymer or propylene copolymer, to a extent that does not impair the objective effect of the present invention. The content of the aforementioned other polymers in the polypropylene resin (C) is preferably 20% by mass or less, more preferably 10% by mass or less, further preferably 5% by mass or less, even more preferably 3% by mass or less, and most preferably 0% by mass; that is, the polypropylene resin (C) preferably consists only of polypropylene resin as a polymer.

[0103] Additives such as bubble regulators, flame retardants, flame retardant additives, bubble nucleating agents, plasticizers, antistatic agents, antioxidants, UV protectants, light stabilizers, conductive fillers, and antibacterial agents can be added to the foamed core layer as needed.

[0104] From the viewpoint of maintaining the rigidity of the molded article while improving its formability and release properties, the mass ratio (by mass%) of the resin constituting the foamed core layer to the resin constituting the coating layer is preferably 99.5:0.5 to 80:20, more preferably 99:1 to 85:15, and even more preferably 97:3 to 90:10. The mass ratio is expressed by the ratio of the resin constituting the foamed core layer to the resin constituting the coating layer.

[0105] [Physical properties of polypropylene resin foam particles]

[0106] <Bulk density>

[0107] From the perspective of achieving a good balance between the mechanical strength and lightweight of the foamed particle molded body, the bulk density of polypropylene resin foamed particles is preferably 10 kg / m³. 3 The above, preferably 15 kg / m 3 The above, and preferably 100 kg / m 3 The following is more preferably 70 kg / m 3 The following is a further preferred value: 50 kg / m 3 The following is a further preferred value: 30 kg / m 3 The following is particularly preferred: 20 kg / m 3 the following.

[0108] The bulk density of the foamed particles is determined as follows. Foamed particles are randomly selected from the particle group and placed into a 1L graduated cylinder, allowing them to accumulate naturally until the 1L mark is reached. The mass W1[g] of the accumulated foamed particles is divided by the volume V1(1[L]) (W1 / V1), and the units are converted to [kg / m³]. 3 From this, the volume density of the bubbling particles can be calculated.

[0109] Previously, when the bulk density of the foamed particles was low, the release properties were easily compromised. According to the present invention, even at a bulk density of, for example, 30 kg / m³, the release properties are improved. 3 The following foamed particles with low bulk density also have good release properties.

[0110] The foamed particles preferably have the following crystal structure: in the DSC curve obtained by heating the foamed particles from 23°C to 200°C at a heating rate of 10°C / min, a melt peak inherent to the polypropylene resin (i.e., the resin-inherent peak) and one or more melt peaks (i.e., high-temperature peaks) appearing on the high-temperature side of the resin are present. 1–3 mg of foamed particles were used as a test sample, and DSC curves were obtained using differential scanning calorimetry (DSC) according to JIS K7121:1987. The resin-inherent peak refers to the melt peak caused by the inherent melting of the polypropylene resin constituting the foamed core layer, which can be considered as a peak caused by the endothermic melting of the crystals originally present in the polypropylene resin. On the other hand, the melt peak on the high-temperature side of the resin-inherent peak (i.e., the high-temperature peak) refers to a melt peak appearing in the DSC curve closer to the high-temperature side than the aforementioned resin-inherent peak. The presence of this high-temperature peak suggests the presence of secondary crystallization in the resin. Furthermore, in the DSC curves obtained when the foamed particles are heated from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating), cooled from 200°C to 23°C at a cooling rate of 10°C / min, and then heated again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating), only the melting peak caused by the inherent melting of the polypropylene resin constituting the foamed particles is observed. Therefore, it is possible to distinguish between the inherent resin peak and the high-temperature peak. The temperature of the peak of this inherent resin peak may sometimes differ slightly between the first and second heating, but the difference is generally within 5°C.

[0111] <Heat of fusion at the high temperature peak ΔH2>

[0112] From the viewpoint of the mechanical strength and formability of the foamed particles, the melting heat ΔH2 of the high-temperature peak of the polypropylene resin foamed particles is preferably 5 J / g or more, more preferably 8 J / g or more, even more preferably 10 J / g or more, 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.

[0113] use Figure 2 The illustrated DSC curve illustrates the method for measuring the melting heat ΔH2 of the high-temperature peak of polypropylene resin foam particles. A straight line L1 is obtained by connecting point α (temperature 80°C) on the DSC curve with point β (melting point T of the foam particles). Next, a straight line L2 parallel to the vertical axis of the graph is drawn from point γ on the DSC curve corresponding to the valley between the resin's intrinsic peak P1 and the high-temperature peak P2. The point where lines L1 and L2 intersect is designated as δ. Point γ can also be considered a maximum point existing between the resin's intrinsic peak P1 and the high-temperature peak P2.

[0114] The area (2) of the high-temperature peak P2 of the foamed particles is the melting heat ΔH2 of the high-temperature peak P2, which is calculated as the area of ​​the portion enclosed by the DSC curve showing the high-temperature peak P2, the line segment (δ-β), and the line segment (γ-δ). In addition, the area (1) of the resin-intrinsic peak P1 of the foamed particles is the melting heat ΔH1 of the resin-intrinsic peak P1, which is calculated as the area of ​​the portion enclosed by the DSC curve showing the resin-intrinsic peak P1, the line segment (α-δ), and the line segment (γ-δ).

[0115] [Manufacturing method of polypropylene resin foam particles]

[0116] There are no particular limitations on the manufacturing method of polypropylene resin foam particles. For example, they can be manufactured by methods including the following steps (A) to (D).

[0117] Process (A): Granulation process, the polypropylene resin (C) constituting the core layer and the polypropylene resin (S) constituting the coating layer are melted and kneaded separately and co-extruded into wire bundles, which are then cut to obtain multilayer resin particles with a non-foamed core layer and a coating layer covering the core layer.

[0118] Process (B): Dispersion process, in which the above-mentioned multilayer resin particles are dispersed in a dispersion medium in a closed container;

[0119] Process (C): Foaming agent impregnation process, in which the foaming agent is impregnated into multiple layers of resin particles; and

[0120] Process (D): Foaming process, in which foamable multilayer resin particles impregnated with foaming agent are released from a sealed container together with a dispersion medium into an atmosphere at a lower pressure than that inside the sealed container, so as to foam at least the core layer to form a foamed core layer, thereby manufacturing foamed particles.

[0121] <Process (A)>

[0122] In process (A), for example, an extruder having a core-forming extruder, a coating-forming extruder, and a multilayer wire harness forming die provided at the exit side of these extruders can be used. In the core-forming extruder, polypropylene resin (C) constituting the core layer is supplied with additives added as needed, and the mixture is melt-kneaded to form a core-forming melt-knead. In the coating-forming extruder, polypropylene resin (S) constituting the coating layer is supplied with additives added as needed, and the mixture is melt-kneaded to form a coating-forming melt-knead. The core-forming melt-knead and the coating-forming melt-knead are introduced into the multilayer wire harness forming die and brought together to form a composite having a core-sheath structure, consisting of a non-foamed core layer and a non-foamed coating layer covering the core layer. Then, the composite is extruded into a wire bundle shape through a small orifice of a die attached to the front end of an extruder. After cooling in water, it is cut into specified masses using a granulator (wire bundle cutting method). This yields multilayer resin particles with a non-foamed core layer and a coating layer covering the core layer. In addition to the above methods, other methods for cutting the extruded composite include underwater cutting, which involves extruding the composite into water, and thermal cutting, which involves extruding the composite into air and then immediately cutting it.

[0123] (Particle size of multilayer resin particles)

[0124] The particle size of the multilayer resin particles is preferably 0.1 to 3.0 mm, more preferably 0.3 to 1.5 mm.

[0125] (Mass of multilayer resin particles)

[0126] The average mass of the multilayer resin particles is preferably adjusted to 0.1–20 mg, more preferably to 0.2–10 mg, even more preferably to 0.3–5 mg, and even more preferably to 0.4–2 mg.

[0127] (Mass ratio of cladding layer to core layer)

[0128] When manufacturing multilayer resin particles, from the viewpoint of the weldability of foamed particles, the mass ratio of the coating layer to the non-foamed core layer (coating layer / core layer) is preferably 0.5 / 99.5 to 20 / 80, more preferably 1 / 99 to 15 / 85, and even more preferably 3 / 97 to 10 / 90.

[0129] (additive)

[0130] In multilayer resin particles having a non-foamed core layer and a coating layer, additives such as bubble regulators, flame retardants, flame retardant auxiliaries, bubble nucleating agents, plasticizers, antistatic agents, antioxidants, UV protectants, light stabilizers, conductive fillers, and antibacterial agents can be added as needed. When adding additives, they can be added in step (A). Examples of bubble regulators include inorganic powders such as talc, mica, zinc borate, calcium carbonate, silica, titanium dioxide, gypsum, zeolite, borax, aluminum hydroxide, and carbon; and organic powders such as phosphoric acid nucleating agents, phenolic nucleating agents, amine nucleating agents, and polyvinyl fluoride resin powders. When adding bubble regulators, the content of the bubble regulator in the multilayer resin particles is preferably 0.01 to 1 part by weight relative to 100 parts by weight of the multilayer resin particles.

[0131] <Process (B)>

[0132] In process (B), for example, in a sealed container such as an autoclave that can withstand heating and pressurization, a mixer can be used to disperse multilayer resin particles in a dispersion medium.

[0133] The dispersion medium can be any dispersion medium that does not dissolve the multilayer resin particles, and there are no particular limitations. Examples include water, ethylene glycol, glycerol, methanol, ethanol, and other alcohols, with water being the preferred choice.

[0134] In step (B), to prevent the multilayer resin particles from fusing together, it is preferable to further add a dispersant to the dispersion medium. Examples of dispersants include organic dispersants such as polyvinyl alcohol, polyvinylpyrrolidone, and methylcellulose; and insoluble inorganic salts such as alumina, zinc oxide, kaolin, mica, magnesium phosphate, and tricalcium phosphate. These can be used alone or in combination of two or more. From the perspective of ease of processing, insoluble inorganic salts are preferred, and kaolin is more preferred. When adding a dispersant, it is preferable to add approximately 0.001 to 5 parts by weight relative to 100 parts by weight of the multilayer resin particles.

[0135] Surfactants can also be further added to the dispersion medium. Examples of surfactants include sodium dodecylbenzene sulfonate, sodium alkyl sulfonate, sodium oleate, sodium lauryl sulfate, sodium polyoxyethylene alkyl ether phosphate, sodium polyoxyethylene alkyl ether sulfate, and other anionic and nonionic surfactants commonly used in suspension polymerization. When a surfactant is added, it is preferable to add about 0.001 to 1 part by weight relative to 100 parts by weight of the multilayer resin particles.

[0136] <Process (C)>

[0137] In process (C), for example, the polypropylene resin (C) constituting the core layer can be heated to a temperature above which softens, thereby impregnating the foaming agent to obtain foamable multilayer resin particles.

[0138] There are no particular limitations on the foaming agent as long as it can foam 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, n-butane, isobutane, n-pentane, isopentane, and n-hexane; alicyclic hydrocarbons such as cyclohexane and cyclopentane; halogenated hydrocarbons such as chlorofluoromethane, trifluoromethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane, chloromethane, chloroethane, and dichloromethane; and organic physical foaming agents such as dialkyl ethers such as dimethyl ether, diethyl ether, and methyl ethyl ether. Among these, inorganic physical foaming 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 can be used alone or in combination of two or more.

[0139] The amount of foaming agent added can be determined by considering the desired bulk density of the foamed particles, the type of polypropylene resin, and the type of foaming agent. Generally speaking, relative to 100 parts by weight of multilayer resin particles, organic physical foaming agent is preferably 5 to 50 parts by weight, and inorganic physical foaming agent is preferably 0.1 to 30 parts by weight, more preferably 0.5 to 15 parts by weight.

[0140] The heating temperature in step (C) is preferably above the melting point of the polypropylene resin (C) and below +80°C, specifically, preferably 100°C to 230°C. The time at which this heating temperature is maintained is preferably 1 minute or more, more preferably 20 minutes or more, and preferably 100 minutes or less, more preferably 60 minutes or less.

[0141] <Process (D)>

[0142] In step (D), for example, foamed particles are manufactured by releasing, from a sealed container, foaming agent-impregnated and heated foaming multilayer resin particles that have passed through step (C) into an atmosphere with a lower pressure than that inside the sealed container, thereby foaming at least the core layer to form a foamed core layer.

[0143] Specifically, the pressure inside the sealed container is maintained above the vapor pressure of the foaming agent, while one end of the sealed container below the water surface is opened. Foaming multilayer resin particles impregnated with the foaming agent, along with a dispersion medium, are released from the sealed container into an atmosphere with a lower pressure than that inside the sealed container, typically to atmospheric pressure. This causes at least the core layer of the foaming multilayer resin particles to foam, forming a foamed core layer. This allows the production of foamed particles with a multilayer structure consisting of a foamed core layer and a covering layer encapsulating the foamed core layer. Alternatively, the foaming multilayer resin particles from step (C) can be cooled and removed, and then heated using a heating medium such as hot air or steam to cause them to foam, thereby producing foamed particles.

[0144] In process (D), the temperature during foaming within the sealed container is preferably 110°C to 170°C. Furthermore, the pressure within the sealed container is preferably 0.5 MPa(G) to 5 MPa(G).

[0145] It is preferable that the above-mentioned processes (B) to (D) be performed as a series of processes in a single sealed container. However, it is also possible to take out the multi-layer resin particles in each process, put them back into the sealed container, and perform the next process, and set them as separate processes.

[0146] Furthermore, foamed particles with a crystal structure exhibiting a resin-inherent melting peak (resin-inherent peak) and more than one melting peak (high-temperature peak) on its high-temperature side can be obtained, for example, as described below.

[0147] During the heating process in the foamed particle manufacturing process, a holding step is performed, in which the temperature is maintained for a sufficient time at a temperature above (melting point of polypropylene resin (C) - 20°C) and below (melting end temperature of polypropylene resin (C)), preferably for about 10 to 60 minutes. Then, the temperature is adjusted from (melting point of polypropylene resin (C) - 15°C) to (melting end temperature of polypropylene resin (C) + 10°C). Furthermore, a second holding step is performed as needed, maintaining this temperature for a further sufficient time, preferably about 10 to 60 minutes. Next, by releasing the foaming resin particles containing the foaming agent from a sealed container and foaming them under low pressure, foamed particles having the above-described crystal structure can be obtained. Foaming is preferably performed in a sealed container at a temperature above (melting point of polypropylene resin (C) - 10°C), more preferably at a temperature above (melting point of polypropylene resin (C)) and below (melting point of polypropylene resin (C) + 20°C).

[0148] The foamed particles obtained as described above can also be foamed by heating them with steam or the like after pressurizing them with air to increase their internal pressure (two-step foaming), thereby producing foamed particles with a higher foaming ratio (lower apparent density).

[0149] [Polypropylene Resin Foamed Particle Molding]

[0150] The polypropylene resin foamed particle molded body (hereinafter also referred to as polypropylene resin foamed particle molded body or foamed particle molded body) obtained using the polypropylene resin foamed particles of the present invention can be obtained by in-mold molding of the polypropylene resin foamed particles of the present invention.

[0151] In-mold forming involves filling foamed particles into a molding die and heating them using a heating medium such as steam. Specifically, after filling the molding die with foamed particles, a heating medium such as steam is introduced into the die, thereby heating the foamed particles to cause them to foam and fuse together, thus obtaining a foamed particle molded body with a shape that has been given a forming space. Furthermore, the in-mold forming in this invention is preferably performed by the following pressure forming method (for example, Japanese Patent Publication No. 51-22951): the foamed particles are pre-pressurized using a pressurized gas such as air to increase the pressure inside the bubbles of the foamed particles; after adjusting the pressure inside the foamed particles to a pressure 0.01 to 0.3 MPa higher than atmospheric pressure, the foamed particles are filled into the molding die under atmospheric pressure or reduced pressure; then, a heating medium such as steam is supplied into the die, thereby heating and fusing the foamed particles. In addition, molding can be performed using the following compression filling molding method (Japanese Patent Publication No. 4-46217): after filling a molding die pressurized to atmospheric pressure or above with compressed gas, a heating medium such as steam is supplied to the mold cavity for heating, thereby causing the foam particles to heat and fuse. Alternatively, molding can be performed using the following atmospheric pressure filling molding method (Japanese Patent Publication No. 6-49795): after filling the mold cavity of a molding die with high secondary foaming power obtained under special conditions under atmospheric pressure or reduced pressure, a heating medium such as steam is supplied for heating, thereby causing the foam particles to heat and fuse. Molding can also be performed using a method combining the above methods (Japanese Patent Publication No. 6-22919), etc.

[0152] <Density>

[0153] From the perspective of achieving a good balance between mechanical strength and lightweight, the preferred density of polypropylene resin foamed particle molded articles is 10 kg / m³. 3 The above, preferably 15 kg / m 3 The above is further optimized to 20 kg / m 3 The above, and preferably 150 kg / m3 The following is more preferably 100 kg / m 3 The following is a further preferred value: 50 kg / m 3 The following is particularly preferred: 35 kg / m 3 the following.

[0154] The density of the foamed particle molded body is calculated by dividing the mass (g) of the foamed particle molded body by the volume (L) obtained from the external dimensions of the molded body and then converting the units. Alternatively, if it is difficult to determine the volume from the external dimensions of the molded body, the volume can be determined using the water immersion method.

[0155] <50% compressive stress>

[0156] From a rigidity point of view, the 50% compressive stress of the polypropylene resin foamed particle molded body is preferably 50 kPa or more, more preferably 100 kPa or more, even more preferably 150 kPa or more, and preferably 500 kPa or less, more preferably 400 kPa or less, and even more preferably 300 kPa or less.

[0157] The 50% compressive stress of foamed particle molded bodies was measured based on JIS K6767:1999.

[0158] Example

[0159] The present invention will be further described in detail below with reference to the embodiments, but the present invention is not limited to these examples in any way.

[0160] The following measurements and evaluations were performed on the resin, foamed particles, and foamed particle molded articles used in the examples and comparative examples. The physical property measurements of the foamed particles were performed using foamed particles that had undergone state conditioning by standing for 24 hours at 50% relative humidity, 23°C, and 1 atm. Furthermore, the physical property measurements and evaluations of the foamed particle molded articles were performed using molded articles that had undergone state conditioning by standing for 12 hours at 50% relative humidity, 80°C, and 1 atm after demolding.

[0161] [Measurement Method]

[0162] <Polypropylene Resins>

[0163] (Monomer content)

[0164] The monomer content of polypropylene resin was determined using a known method based on IR spectroscopy. Specifically, the content was determined quantitatively using a method described in the *Handbook of Polymer Analysis* (edited by the Polymer Analysis Research Conference of the Japan Society for Analytical Chemistry, published January 1995, Kinokuniya Shoten, pages 615-616 "II.2.32.3.4 Propylene / Ethylene Copolymer", 618-619 "II.2.32.3.5 Propylene / Butene Copolymer"), specifically by relating the absorbance of ethylene and butene, corrected for by a specified coefficient, to the thickness of a thin-film test piece. More specifically, firstly, polypropylene resin was hot-pressed at 180°C to form a thin film, creating multiple test pieces of varying thicknesses. Then, the IR spectrum of each test piece was measured, and the absorbance at 722 cm⁻¹ originating from ethylene was read. -1 and 733cm -1 Absorbance (A) 722 A 733 ), 766cm derived from butene -1 Absorbance (A) 766 Next, for each test piece, the ethylene content in the polypropylene resin was calculated using the following formulas (1) to (3). The ethylene content in the polypropylene resin was obtained by arithmetic averaging of the ethylene content obtained from each test piece (unit: mass%).

[0165] (K′ 733 c = 1 / 0.96{(K′) 733 )a-0.268(K′ 722 )a}…(1)

[0166] (K′ 722 c = 1 / 0.96{(K′) 722 )a-0.268(K′ 722 )a}…(2)

[0167] Ethylene content (%) = 0.575{(K′) 722 )c+(K′ 733 )c}…(3)

[0168] In equations (1) to (3), K′a represents the apparent absorbance coefficient at each wavenumber (K′a=A / ρt), K′c represents the corrected absorbance coefficient, A represents the absorbance, and ρ represents the density of the resin (unit: g / cm³). 3 ), where t represents the thickness of the thin film-like test piece (unit: cm).

[0169] Furthermore, for each test piece, the butene content in the polypropylene resin was calculated using the following formula (4). The arithmetic mean of the butene content obtained from each test piece was taken as the butene content (%) in the polypropylene resin.

[0170] Butene content (%) = 12.3 (A) 766 / L)…(4)

[0171] In Equation (4), A represents absorbance and L represents the thickness (mm) of the thin film test piece.

[0172] (DSC characteristics of polypropylene resin (S))

[0173] The peak shapes, the peak temperatures Tmsl on the low-temperature side and Tmsh on the high-temperature side of the melting peak in the second DSC curve of polypropylene resin (S) were determined as follows. Based on JIS K7121:1987, the condition of measuring the melting temperature after certain heat treatment (2) was used as the condition adjustment of the test piece. The condition-adjusted test piece was heated from 30°C to 200°C at a heating rate of 10°C / min, thereby obtaining the DSC curve. Specifically, approximately 3 mg of resin, after being conditioned at 23°C and 50% RH for more than 24 hours, was used as a test piece. A differential scanning calorimeter (DSC-60A, Shimadzu Corporation) was used to heat the test piece from 30°C to 200°C at a heating rate of 10°C / min, then cooled it from 200°C to 30°C at a cooling rate of 10°C / min, and finally heated it again from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the shape of the melt peak was observed. Here, a bimodal shape refers to two melt peaks exhibiting different peak temperatures (melting points), and both melt peaks having a fusion heat of 10 J / g or more. The fusion heat is a value obtained by the method described later. Furthermore, regarding PP2, since it has a single-peak shape with only one melt peak, the peak temperature of the melt peak on the low-temperature side is only displayed in the column for the peak temperature of the melt peak. Figure 1 The second DSC curve of PP1 is shown in the figure. Figure 3 The second DSC curve of PP2 is shown in the figure.

[0174] (Heat of melting of polypropylene resin (S) ΔH)

[0175] The heat of fusion of polypropylene resin (S) is determined as follows. The point on the DSC curve at 80°C is designated as α, and the point on the DSC curve corresponding to the melting point is designated as β (refer to...). Figure 1 Furthermore, the melting peak P on the low-temperature side of the DSC curve...l and the melting peak P on the high-temperature side h Draw a straight line parallel to the vertical axis of the graph corresponding to the valley point γ. The point where this line intersects the line (α-β) connecting points α and β is designated as δ. The melting peak P on the low-temperature side... l The area (A) is the melting peak P on the low-temperature side. l The heat of fusion ΔHl is represented by the melting peak P shown on the low-temperature side. l The area enclosed by the DSC curve and the line segments (α-δ) and (γ-δ) is used to determine the melting peak P on the high-temperature side. h The area (B) is the melting peak P on the high-temperature side. h The heat of fusion ΔHh is represented by the melting peak P shown on the high-temperature side. h The area enclosed by the DSC curve and line segments (δ-β) and (γ-δ) is used to calculate the area.

[0176] The total heat of melting ΔHt is the melting peak P on the low-temperature side. l The area (A) and the melting peak P on the high-temperature side h The total area (B) of [A+B (i.e., ΔHl+ΔHh)] is in Figure 1 In this context, the area enclosed by the DSC curve within the interval between the line (α-β) and points α and β is used to calculate the area.

[0177] Additionally, for PP2, since it has a single peak shape with only one melting peak, the melting heat of the melting peak is only displayed in the column for the melting heat of the melting peak on the low-temperature side.

[0178] (Melting point of polypropylene resin (C))

[0179] The melting point Tmc of polypropylene resin (C) is determined based on JIS K7121:1987, using "(2) the condition of measuring the melting temperature after certain heat treatment" as the condition conditioning of the test piece. Specifically, about 3 mg of resin that has been condition-conditioned at 23°C and 50% RH for more than 24 hours is used as the test piece. The test piece is heated from 30°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter (manufactured by Shimadzu Corporation, model: DSC-60A). Then, it is cooled from 200°C to 30°C at a cooling rate of 10°C / min. The temperature is then increased from 30°C to 200°C again at a heating rate of 10°C / min to obtain a DSC curve. The melting point Tmc is determined as the peak temperature of the melting peak obtained with the melting of the resin on the DSC curve.

[0180] (MFR)

[0181] The MFR of polypropylene resins was measured according to JIS K7210-1:2014 at a temperature of 230℃ and a load of 2.16 kg.

[0182] (Flexural modulus)

[0183] The flexural modulus of polypropylene resin was measured according to JIS K7171:2016, using a 4mm thick sheet produced by hot pressing at 230°C, and a test piece (standard test piece) cut from this sheet with a length of 80mm × width of 10mm × thickness of 4mm. Furthermore, the radius of the indenter R1 and the radius of the support platform R2 were both 5mm, the distance between the support points was 64mm, and the test speed was 2mm / min.

[0184] <Foaming Particles>

[0185] (bulk density)

[0186] The bulk density of the foamed particles is determined as follows. Foamed particles are randomly selected from the conditioned particle group and placed into a 1L graduated cylinder. A large quantity of foamed particles is allowed to accumulate naturally until the 1L mark is reached. The mass W1[g] of the accumulated foamed particles is divided by the volume V1(1[L]) (W1 / V1), and the units are converted to [kg / m³]. 3 From this, the volume density of the bubbling particles can be calculated.

[0187] (Peak heat at high temperature)

[0188] One foamed particle was collected from the state-conditioned foamed particle group. This foamed particle was used as a test piece, and a DSC curve was obtained when the test piece was heated from 23°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter (specifically, a Shimadzu Corporation model: DSC-60A). Figure 2 An example of a DSC curve is shown below. Figure 2 The DSC curve shown in the figure has a resin-specific peak P1 and a high-temperature peak P2 on its high-temperature side. A straight line L1 is obtained by connecting the point α at the temperature of 80°C on the DSC curve with the point β at the melting end temperature T of the foamed particles. 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. The point where the straight line L1 intersects the straight line L2 is set as δ. The area (2) of the high-temperature peak P2 of the foamed particles is the melting heat ΔH2 of the high-temperature peak P2, which is calculated as the area of ​​the part enclosed by the DSC curve showing the high-temperature peak P2, the line segment (δ-β), and the line segment (γ-δ). The above measurements were performed on 5 foamed particles, and the values ​​obtained by arithmetic mean are shown in Tables 3 and 4.

[0189] <Foamed Particle Molding Body>

[0190] (Density of the molded body)

[0191] To determine the density of the foamed particle molded body, the outer skin layer from the foamed particle molded body is removed, and three test pieces are randomly cut out in the shape of a cuboid with a length of 25mm, a width of 25mm, and a thickness of 100mm. The mass and volume of each test piece are measured, and the mass is divided by the volume and the units are converted. The density of the three test pieces is then calculated, and their arithmetic mean is taken as the density of the foamed particle molded body.

[0192] (50% compressive stress)

[0193] For the 50% compressive stress of the foamed particle molded body, the skin layer was removed from the foamed particle molded body, and a test piece was cut out in the shape of a cuboid with a length of 50 mm × width of 50 mm × thickness of 25 mm. The test piece was tested using RTF-1350 manufactured by A&D Corporation, based on JIS K6767:1999, to determine the load at 50% strain when compressed at a speed of 10 mm / min. This load was then divided by the area of ​​the test piece under pressure to calculate the 50% compressive stress [kPa].

[0194] [Evaluation Method]

[0195] <The range of forming pressures that can be formed (formability)>

[0196] The formability of the foamed particles was evaluated as follows. In the section on "Fabrication of Foamed Particle Molded Bodies" described later, foamed particle molded bodies were produced by varying the molding pressure (vapor pressure) every 0.01 MPa (G). The range of molding pressures within which all evaluations of weldability, appearance, and resilience were rated "A" is considered the range of molding pressures that can be formed. Furthermore, the number of points for this molding pressure (the number of molding pressures rated "A") is also shown in Tables 3 and 4. Since the molding temperature can be adjusted by the molding pressure, the wider the range from the lower limit to the upper limit of the formable molding pressure, the wider the range of molding heating temperatures that can be formed.

[0197] (Weldability)

[0198] The weldability of the foamed particle molded body was evaluated using the following method. The foamed particle molded body was bent and fractured. The number of foamed particles present on the fracture surface (C1) and the number of broken foamed particles (C2) were determined. The ratio of broken foamed particles to the total number of broken foamed particles (C2 / C1×100) was used as the material failure rate. Five measurements were performed using different test pieces, and the respective material failure rates were calculated. The arithmetic mean of these rates was then used to evaluate the weldability according to the following criteria.

[0199] A: The material failure rate is over 90%.

[0200] B: The material failure rate is above 20% but less than 90%.

[0201] C: Material failure rate is less than 20%.

[0202] (Appearance (degree of gap))

[0203] Draw a 100mm × 100mm rectangle in the center of the foamed particle molded body. Draw lines from the corners of the rectangular area to the diagonal and count the number of gaps (voids) larger than 1mm × 1mm on the line. Evaluate the surface appearance of the foamed particle molded body according to the following criteria.

[0204] A: The number of gaps is less than 3

[0205] B: The number of gaps is more than 3 but less than 5

[0206] C: The number of gaps is 5 or more.

[0207] (Resilience)

[0208] The thickness of the central part and the four corner parts of the foamed particle molded body were measured separately, and the ratio of the thickness of the central part to the thickness of the thickest part in the four corner parts was calculated. The resilience was evaluated by the following criteria.

[0209] A: When the thickness ratio is above 95%

[0210] B: Thickness ratio is 90% or higher but less than 95%.

[0211] C: Thickness ratio less than 90%

[0212] <Range of molding pressure with good release properties (release properties)>

[0213] Within the range of formable forming pressures in the above evaluation of <the range of formable forming pressures (formability)>, the demolding performance is evaluated using the following criteria, and forming pressures rated "A" are considered to have good demolding performance. Furthermore, the number of points for these forming pressures (the number of forming pressures rated "A") is also shown in the table. The wider the range from the lower limit to the upper limit of the forming pressure with good demolding performance, the wider the range of forming heating temperatures indicating good demolding performance.

[0214] A: Demolding is possible even without using demolding aids such as release pins or release air.

[0215] B: Demolding is impossible without using demolding aids such as release pins or release air.

[0216] [raw material]

[0217] The polypropylene resins (S) and (C) used in the examples and comparative examples are shown in Tables 1 and 2. Additionally, in the tables, PP1 is a propylene copolymer comprising ethylene, propylene, and butene, manufactured by TPC Corporation, and graded FL7540L.

[0218] Furthermore, in the table, PP5 is a mixture of ethylene-propylene random copolymer (ethylene 3.1% by mass, melting point 142°C) and ethylene-propylene random copolymer (ethylene 1.4% by mass, melting point 153°C) resins (mass ratio of 50% by mass: 50% by mass, based on the total of the two being 100% by mass). Additionally, PP6 is a mixture of ethylene-propylene random copolymer (ethylene 4.2% by mass, melting point 134°C) and linear low-density polyethylene (density 0.91 g / cm³). 3 A mixture of resins (melting point 110℃) with a mass ratio of ethylene-propylene random copolymer: linear low-density polyethylene = 67% by mass: 33% by mass, where the total mass percentage of both is 100%.

[0219] Table 1

[0220]

[0221] Table 2

[0222]

[0223] Examples 1-5 and Comparative Examples 1-9

[0224] <Making of Foamed Particles>

[0225] Using the polypropylene resins (C) of the types shown in Tables 3 and 4, a core-forming resin melt-knead was obtained by melting and kneading in a core-forming extruder at the highest set temperature of 245°C. Furthermore, the polypropylene resins (S) of the types shown in Tables 3 and 4 were melt-kneaded in a coating-forming extruder at the highest set temperature of 245°C to obtain a coating-forming resin melt-knead. Next, each resin melt-knead was extruded from the front end of a co-extrusion die from both the core-forming and coating-forming extruders. At this time, the resin melt-kneads were combined within the die to form a sheath-core type composite consisting of a non-foamed core layer and a non-foamed coating layer covering the outer surface of the core layer. The composite was extruded in a wire bundle shape through a fine hole in the interface attached to the front end of the extruder. While pulling the wire bundle, it was water-cooled and then cut using a granulator to achieve a mass of approximately 1.0 mg. Thus, multilayer resin particles consisting of a core layer and a coating layer covering the core layer are obtained. The mass ratio of the core layer to the coating layer in the multilayer resin particles is set as core layer:coating layer = 95:5 (i.e., the mass ratio of the coating layer is 5%). In addition, during the manufacture of the multilayer resin particles, zinc borate, as a bubble regulator, is supplied to the extruder for core layer formation, so that the polypropylene resin contains 500 ppm of zinc borate by mass.

[0226] (One-step foaming process)

[0227] One kilogram of multilayer resin particles and three liters of water (as the dispersion medium) were placed together in a 5-liter sealed container. Then, 0.3 parts by mass of kaolin (as a dispersant), 0.0001 parts by mass of aluminum sulfate (as a dispersing aid), and 0.004 parts by mass of surfactant (sodium alkylbenzene sulfonate) were added to the sealed container relative to 100 parts by mass of the multilayer resin particles. Carbon dioxide was added as a foaming agent to the sealed container, which was then sealed. While stirring, the container was heated to the foaming temperatures shown in Tables 3 and 4. The pressures inside the container at this temperature (i.e., impregnation pressure and carbon dioxide pressure) are shown in Tables 3 and 4. After maintaining this temperature for 15 minutes, the contents of the container were released to atmospheric pressure, yielding foamed particles. These foamed particles were then dried at 23°C for 24 hours. The resulting foamed particles with the volume ratios shown in Tables 3 and 4 were obtained.

[0228] (Two-step foaming process)

[0229] Next, the foamed particles are added to a pressure vessel (specifically, a metal drum), and air is forced into the vessel, thereby increasing the pressure inside and impregnating the air within the bubbles, thus increasing the internal pressure within the bubbles of the foamed particles. Then, steam is supplied to the foamed particles (one-step foamed particles) removed from the pressure vessel to bring the pressure inside the vessel (i.e., the drum pressure) to the pressures shown in Tables 3 and 4, and the particles are heated at atmospheric pressure. The pressure (i.e., internal pressure) within the bubbles of the one-step foamed particles removed from the pressure vessel is the value shown in Tables 3 and 4. This reduces the apparent density of the one-step foamed particles, resulting in foamed particles with the volume ratios shown in Tables 3 and 4 (two-step foamed particles).

[0230] <Fabrication of Foamed Particle Moldings>

[0231] In the manufacture of foamed particle molded articles, foamed particles obtained by drying at 23°C for 24 hours are used. First, the foamed particles are pressurized to an internal pressure of 0.25 MPa (G) using compressed air, and then filled into a longitudinal forming machine with a mold cavity of 300 mm long × 250 mm wide × 60 mm thick, where the cracking amount is adjusted to 20% (i.e., 12 mm). A preheating process is then performed by supplying steam from both sides of the mold for 5 seconds after mold closing. Note that a longitudinal forming machine refers to a forming machine where the mold opening direction is vertical (height direction). Then, steam is supplied from one side of the mold for single-sided heating until a pressure 0.08 MPa (G) lower than the specified forming pressure is reached. Next, steam is supplied from the other side of the mold for single-sided heating until a pressure 0.04 MPa (G) lower than the forming pressure is reached, and then heating (i.e., main heating) is performed until the specified forming pressure is reached. After heating, the pressure is released and the material is water-cooled until the surface pressure generated by the foaming force of the molded body becomes 0.04 MPa (G). Then, the material is demolded from the mold to obtain the foamed particle molded body.

[0232] Table 3

[0233]

[0234] Table 4

[0235]

[0236] As shown in Table 3, the foamed particles according to the present invention can be formed under a wide range of molding pressures and exhibit good demolding properties throughout the entire range of molding pressures. When removing the foamed particle molded body from the mold, in cases where the molded body is unintentionally stuck to the mold, demolding aids such as release pins or release air are generally used. However, the foamed particles according to the present invention do not require demolding aids, thus improving manufacturing efficiency.

[0237] As shown in Table 4, the polypropylene resin (S) has a single peak instead of a bimodal peak. Comparative Examples 1, 7, and 9, which do not meet the above condition (1), are unable to balance mold release and formability. A comparison between Comparative Examples 1 and 9 shows that mold release is particularly prone to deterioration in foamed particles with lower bulk density.

[0238] Comparative Examples 2, 3 and 8, which do not meet the above condition (3) because the peak temperature of the melting peak on the high-temperature side of the polypropylene resin (S) is less than 138°C, have poor demolding properties.

[0239] The demolding properties of Comparative Example 5, which does not meet the above condition (4), are also poor because the difference between the melting point Tmc of polypropylene resin (C) and the peak temperature Tmsh of the high-temperature side of polypropylene resin (S) [Tmc-Tmsh] exceeds 15°C.

[0240] The demolding properties of Comparative Example 4 were significantly worse, as the peak temperature of the high-temperature side of polypropylene resin (S) was less than 138°C, which did not meet the above condition (3), and the difference between the melting point Tmc of polypropylene resin (C) and the peak temperature Tmsh of the high-temperature side of polypropylene resin (S) [Tmc-Tmsh] exceeded 15°C, which did not meet the above condition (4).

[0241] Industrial applicability

[0242] The foamed particles of the present invention have a wide range of molding pressures and excellent demolding properties. Therefore, even when using general-purpose devices instead of special devices, they exhibit excellent demolding properties. Thus, the foamed particles of the present invention are useful.

Claims

1. A polypropylene resin foamed particle, comprising a foamed core layer composed of polypropylene resin (C) and a coating layer composed of polypropylene resin (S), characterized in that, The melting point Tmc of the polypropylene resin (C) is above 125°C and below 170°C. The polypropylene resin foam particles satisfy the following (1) to (4): (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in the second DSC curve obtained by heating the resin from 30°C to 200°C at a heating rate of 10°C / min, cooling it from 200°C to 30°C at a cooling rate of 10°C / min, and then heating it again at a heating rate of 10°C / min. (2) The peak 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 above 138°C. (4) The difference between the melting point Tmc of the polypropylene resin (C) and the peak temperature Tmsh of the high-temperature side of the polypropylene resin (S) [Tmc-Tmsh] is above -5℃ and below 7℃.

2. The polypropylene resin foam particles as described in claim 1, characterized in that, The peak temperature Tmsl of the melting peak on the low-temperature side of the polypropylene resin (S) is above 125°C and below 135°C.

3. The polypropylene resin foam particles as described in claim 1 or 2, characterized in that, The difference 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) [Tmc-Tmsl] exceeds 0°C and is below 25°C.

4. The polypropylene resin foam particles as described in claim 1 or 2, characterized in that, The ratio of the heat of fusion ΔHh of the high-temperature side of the polypropylene resin (S) to the total heat of fusion ΔHt of the polypropylene resin (S) [ΔHh / ΔHt] is 0.35 or more and 0.80 or less.

5. The polypropylene resin foam particles as described in claim 1 or 2, characterized in that, The difference between the peak temperature Tmsh of the high-temperature side and the peak temperature Tmsl of the low-temperature side of the polypropylene resin (S), [Tmsh-Tmsl], is above 10°C and below 20°C.

6. The polypropylene resin foam particles as described in claim 1 or 2, characterized in that, The melting point Tmc of the polypropylene resin (C) is above 130°C and below 150°C.

7. The polypropylene resin foam particles as described in claim 1 or 2, characterized in that, The polypropylene resin (S) is a propylene copolymer with propylene as the main component and containing ethylene and butene components.

8. The polypropylene resin foam particles as described in claim 1 or 2, characterized in that, The bulk density of the polypropylene resin foam particles is 10 kg / m³. 3 Above 30kg / m 3 the following.

9. A method for manufacturing polypropylene resin foamed particles, characterized in that, have: A process for granulating multilayer resin particles having a core layer made of polypropylene resin (C) and a coating layer made of polypropylene resin (S) covering the core layer. The process of dispersing the multilayer resin particles in a dispersion medium within a sealed container. The process of impregnating the foaming agent into the multilayer resin particles dispersed in the dispersion medium; The process of manufacturing foamed particles by foaming the multilayer resin particles containing the foaming agent. The melting point Tmc of the polypropylene resin (C) is above 125°C and below 170°C. The polypropylene resin foam particles satisfy the following (1) to (4): (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in the second DSC curve obtained by heating the resin from 30°C to 200°C at a heating rate of 10°C / min, cooling it from 200°C to 30°C at a cooling rate of 10°C / min, and then heating it again at a heating rate of 10°C / min. (2) The peak 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 above 138°C. (4) The difference between the melting point Tmc of the polypropylene resin (C) and the peak temperature Tmsh of the high-temperature side of the polypropylene resin (S) [Tmc-Tmsh] is above -5℃ and below 7℃.

Images