CN115109299A

CN115109299A - Foamed particle molding

Info

Publication number: CN115109299A
Application number: CN202210272236.5A
Authority: CN
Inventors: 大井贵史; 太田肇
Original assignee: JSP Corp
Current assignee: JSP Corp
Priority date: 2021-03-22
Filing date: 2022-03-18
Publication date: 2022-09-27
Also published as: JP2022146483A

Abstract

The present invention relates to a foamed particle molded article comprising a plurality of foamed particles, the foamed particles having a foamed core layer comprising a polypropylene resin as a base resin and a coating layer covering the foamed core layer, the coating layer being in a non-foamed state, the coating layer comprising a thermoplastic elastomer as a base polymer, the foamed particle molded article having a coefficient of dynamic friction of 1.0 to 3.0, and the foamed particle molded article having a coefficient of static friction with respect to the base polymerThe ratio of the coefficient of dynamic friction of the foamed molded article is 0.4 to 0.9, and the density of the foamed molded article is 30kg/m ³ Above 300kg/m ³ The following.

Description

Foamed particle molding

Technical Field

The present invention relates to a foamed particle molded article.

Background

The foamed molded article of expanded beads obtained by in-mold molding of polypropylene resin foamed beads is excellent in chemical resistance, impact resistance, compressive strain recovery, and the like, as compared with the foamed molded article of polystyrene resin foamed beads. Therefore, polypropylene resin foamed particle moldings are used as impact absorbers, heat insulators, various packaging materials, and the like in a wide range of fields such as food containers, packaging cushioning materials for electric and electronic components, automobile bumpers, interior parts, building parts such as house heat insulators, and miscellaneous goods. However, in order to solve the difficulty in molding processing due to the crystallinity and heat resistance of propylene resin, improvement of the in-mold molding method using expanded propylene resin beads has been sought, and studies have been made on a molded article using expanded propylene resin beads coated with a resin different from the foamed layer.

Patent documents 1 and 2 disclose expanded beads having a multilayer structure including a core layer made of a polypropylene resin and an outer layer made of a polypropylene resin, for the purpose of obtaining a molded article having moldability, rigidity, and heat resistance under low pressure. For example, patent document 1 discloses a method for producing propylene resin foamed particles and a molded article, in which a relation between the melting point of the polypropylene resin in the outer layer and the melting point of the polypropylene resin in the core layer satisfies a specific relational expression, a foaming agent is impregnated into a multilayer foamed particle having a thickness of the outer layer of 30 μm or less, and the multilayer foamed particle impregnated with the foaming agent in a heat-softened state is foamed.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2004-68016

Patent document 2: japanese laid-open patent publication No. 2012-126816

Disclosure of Invention

Technical problem to be solved by the invention

As described above, when the polypropylene foamed particle molded article is used as a packaging container, for example, the polypropylene foamed particle molded article has a higher strength than the polystyrene resin foamed particle molded article. Further, the molded article of expanded polypropylene particles is light in weight and excellent in protection properties as compared with conventional plastic containers such as polyethylene containers. However, when such a packaging container is transported while the packaging material is packaged, the surface of the foamed particle molded article rubs against the packaging material and changes, which may deteriorate the appearance of the surface of the foamed particle molded article in a portion in contact with the packaging material or reduce the grip of the foamed particle molded article in a portion in contact with the packaging material. In addition, when the appearance of the molded article is deteriorated, it may be difficult to exhibit the grip performance of the initial foamed particle molded article even when the molded article is repeatedly used.

Accordingly, an object of the present invention is to provide a foamed molded article of expanded beads which has excellent grippability, is less likely to change in appearance due to friction, and can withstand repeated use.

Solution for solving the above technical problem

The present inventors have conducted extensive studies and found that the above-mentioned problems can be solved by forming the foamed particles constituting the foamed molded article of polypropylene resin foamed particles into multi-layered foamed particles, forming the coating layer of the multi-layered foamed particles of polypropylene resin into a non-foamed state and forming the multi-layered foamed particles of polypropylene resin from a thermoplastic elastomer, and setting the dynamic friction coefficient, the static friction coefficient, and the density of the molded article within specific ranges.

That is, the present invention is a foamed molded article comprising a plurality of foamed particles, the foamed particles having a foamed core layer comprising a polypropylene resin as a base resin and a coating layer covering the foamed core layer, the coating layer being in a non-foamed state, the coating layer comprising a thermoplastic elastomer as a base polymer, the foamed molded article having a dynamic friction coefficient of 1.0 to 3.0, a ratio of a static friction coefficient of the foamed molded article to the dynamic friction coefficient of the foamed molded article being 0.4 to 0.9, and a molded article density of the foamed molded article being 30kg/m ³ Above 300kg/m ³ The following.

In addition, the details are:

[1]

a molded article of foamed particles comprising a plurality of foamed particles each having a foamed core layer comprising a polypropylene resin as a base resin and a coating layer for coating the core layer,

the coating layer is in a non-foaming state,

the coating layer takes a thermoplastic elastomer as a base polymer,

the foamed molded article has a coefficient of dynamic friction of 1.0 to 3.0,

the ratio of the static friction coefficient of the foamed molded article to the dynamic friction coefficient of the foamed molded article is 0.4 to 0.9,

the molded article of the foamed particle molded article has a molded article density of 30kg/m ³ Above 300kg/m ³ The following.

[2]

The foamed molded article of foamed particles according to [1], wherein the base polymer of the coating layer has a coefficient of dynamic friction of 1 or more.

[3]

The foamed molded article of foamed particles according to [1] or [2], wherein the base resin of the core layer has a flexural modulus of elasticity of 500MPa or more and 1500MPa or less.

[4]

The foamed molded article of foamed particles according to any one of the above [1] to [3], wherein the core layer contains an antistatic agent.

[5]

The expanded bead molded article according to any one of the above [1] to [4], wherein the expanded bead molded article has a static friction coefficient of 0.7 or more and 2 or less.

[6]

The molded article of expanded particles according to any one of the above [1] to [5], wherein the base polymer of the coating layer has a static friction coefficient of 2 or more.

[7]

The foamed molded article of foamed particles according to any one of the above [1] to [6], wherein the thermoplastic elastomer is at least 1 selected from the group consisting of olefinic thermoplastic elastomers, styrenic thermoplastic elastomers and polyurethane thermoplastic elastomers.

[8]

The molded article of expanded particles according to any one of the above [1] to [7], wherein the hardness of the base polymer of the coating layer is 65 to 95 inclusive.

[9]

The molded foam particle according to any one of the above [1] to [8], wherein a mass ratio of the core layer to the clad layer (core layer/clad layer) is not less than 85/15 and not more than 99.5/0.5.

[10]

The expanded bead molded article according to any one of the above [1] to [9], wherein the 25% compressive stress of the expanded bead molded article is from 0.20MPa to 0.80 MPa.

[11]

A packaging container comprising the foamed molded article of foamed particles according to any one of the above [1] to [10 ].

Effects of the invention

According to the present invention, a foamed molded article of expanded beads which has excellent grip properties, is less likely to change in appearance due to friction, and can withstand repeated use can be obtained. Further, the foamed particle molded body can be particularly suitably used as a packaging container.

Drawings

Fig. 1 is a conceptual view of a load-displacement curve in the case of measuring a friction coefficient using the expanded bead molded article of the present invention.

Fig. 2 is a conceptual diagram of a load-displacement curve in the case where a friction coefficient is measured using a molded foam particle that does not satisfy the conditions of the present invention and a stick-slip phenomenon occurs.

Detailed Description

[ molded foam particle ]

The foamed particle molded article of the present invention is a foamed particle molded article comprising a plurality of foamed particles, the foamed particles having a foamed core layer comprising a polypropylene resin as a base resin and a coating layer covering the core layer, the coating layer being in a non-foamed state, the coating layer comprising a thermoplastic elastomer as a base polymer, the foamed particle molded article having a coefficient of dynamic friction of 1.0 to 3.0, a ratio of a coefficient of static friction of the foamed particle molded article to a coefficient of dynamic friction of the foamed particle molded article being 0.4 to 0.9, and a molded article density of the foamed particle molded article being 30kg/m ³ Above 300kg/m ³ The following.

In the present specification (invention), "a or more" is synonymous with "a and more" and indicates the numerical value (a) and a numerical value larger than the numerical value (a), and "B or less" is synonymous with "B and less" and indicates the numerical value (B) and a numerical value smaller than the numerical value (B). That is, in the present specification (the present invention), "above" and "below" both include the numerical value.

< coefficient of friction of expanded particle molded article >

In the foamed molded article of foamed particles of the present invention, the ratio of the static friction coefficient of the foamed molded article of foamed particles to the dynamic friction coefficient of the foamed molded article of foamed particles is 0.4 or more and 0.9 or less.

By setting the ratio of the static friction coefficient to the dynamic friction coefficient of the expanded bead molded article in the above range, the expanded bead molded article can be made excellent in grip performance, in particular, can withstand repeated use with appearance hardly changing due to friction. Although the reason is not clear, it is considered as follows.

In the expanded bead molded article of the present invention, since the ratio of the static friction coefficient of the expanded bead molded article to the dynamic friction coefficient of the molded article is in the above range, that is, the dynamic friction coefficient and the static friction coefficient have a value close to each other, when the object to be packed moves due to vibration or the like, a stick slip (stick slip) phenomenon due to friction is less likely to occur, and the appearance is less likely to change due to friction, and can withstand repeated use. On the other hand, since the coefficient of dynamic friction has a relatively large value, it is considered that the gripping property is excellent.

In particular, in the present invention, it is considered that the above-mentioned frictional properties can be exhibited by using a multi-layer foamed particle as the foamed particle constituting the foamed particle molded body and using a thermoplastic elastomer as the base material for the coating layer thereof in a non-foamed state.

From the above-described viewpoint, the lower limit of the ratio of the static friction coefficient of the expanded particle molded body to the dynamic friction coefficient of the expanded particle molded body is preferably 0.45 or more, more preferably 0.47 or more, still more preferably 0.60 or more, and still more preferably 0.70 or more. The upper limit of the ratio is preferably 0.80 or less, and more preferably 0.75 or less. When the coefficient of static friction of the foamed molded article of foamed particles is within the above-described preferred range, the strength and cushioning properties of the molded article are also excellent.

Further, in the expanded bead molded article of the present invention, the expanded bead molded article has a coefficient of dynamic friction of 1.0 or more and 3.0 or less. The lower limit of the dynamic friction coefficient is 1.0 or more, preferably 1.1 or more, and more preferably 1.2 or more. The upper limit of the dynamic friction coefficient is 3.0 or less, preferably 2.5 or less, and more preferably 2.0 or less.

When the friction characteristics of the foamed particle molded body satisfy the above relationship and the dynamic friction coefficient is in the above range, the packaged object is easily retained again even when the molded body is moved by vibration or the like, and the gripping force is excellent.

The dynamic friction coefficient can be measured by the method described in examples according to JIS K7125. The static friction coefficient can be measured by the method described in examples according to JIS K7125. From the obtained static friction coefficient and the dynamic friction coefficient, the ratio of the static friction coefficient of the foamed particle molded body to the dynamic friction coefficient of the foamed particle molded body can be calculated.

In the expanded bead molded article of the present invention, the coefficient of static friction of the expanded bead molded article is preferably 0.7 or more and 2 or less. The lower limit of the static friction coefficient is preferably 0.7 or more, more preferably 0.8 or more, and still more preferably 0.9 or more. The upper limit of the static friction coefficient is preferably 2 or less, more preferably 1.8 or less, and still more preferably 1.19 or less.

Fig. 1 is a conceptual view showing a load-displacement curve when the friction coefficient of the expanded bead molded product of the present invention is measured. Fig. 2 is a conceptual diagram showing a load-displacement curve when the friction coefficient of the expanded bead molded body that does not satisfy the conditions of the present invention is measured. The foamed molded article with foamed particles giving the load-displacement curve of fig. 2 had a coating layer not in a non-foamed state, a base polymer of the coating layer contained no thermoplastic elastomer, and a ratio of a static friction coefficient to a dynamic friction coefficient of the foamed molded article exceeded 0.9.

As is clear from the load-displacement curve of fig. 2, vibration occurs due to friction during the measurement of the friction coefficient, and the stick-slip phenomenon occurs. On the other hand, it is seen from the load-displacement curve of fig. 1 that vibration is not generated by friction at the time of friction coefficient measurement, so that stick-slip phenomenon is hardly generated, and gripping property is excellent.

< molded article Density of expanded particle molded article >

In the foamed molded article of the present invention, the density of the foamed molded article is 30kg/m ³ Above 300kg/m ³ The following. The lower limit of the density of the formed body is 30kg/m ³ Above, preferably 40kg/m ³ Above, more preferably 55kg/m ³ The above. Further, the upper limit of the density of the molded article is 300kg/m ³ Hereinafter, it is preferably 100kg/m ³ Hereinafter, more preferably 70kg/m ³ The following.

By setting the compact density within the above range, a compact that is light in weight and excellent in cushioning properties is obtained. Further, it is considered that the above-mentioned frictional properties can be exhibited by the contact state with the object to be packaged in the surface of the foamed particle molded body by forming the bubbles and the bubble film of the foamed particles to have the molded body density of the foamed particle molded body as described above.

[ multilayer foamed particles ]

The foamed particle molded article of the present invention is composed of a plurality of foamed particles, each of which has a foamed core layer containing a polypropylene resin as a base resin and a coating layer covering the foamed core layer, wherein the coating layer is in a non-foamed state, and the coating layer contains a thermoplastic elastomer as a base polymer.

< coating layer >

The core layer is wrapped by the wrapping layers of the multilayer foaming particles, and the wrapping layers are in a non-foaming state. By making the coating layer in a non-foamed state, it is possible to exhibit specific frictional characteristics on the surface of the foamed particle molded article when the foamed particle molded article is produced. Further, it is considered that by using a non-foamed layer as the coating layer, unevenness due to bubbles on the surface of the foamed particles can be reduced, and a foamed molded article with good appearance can be obtained.

Here, the non-foamed state includes not only a state where no bubbles are present in the coating layer at all, but also a substantially non-foamed state where very small bubbles are present only very rarely. The state where no bubble is present in the coating layer includes a state where a bubble formed temporarily is broken and disappears.

The clad layer preferably covers 50% or more, more preferably 70% or more, of the surface of the core layer, and may substantially cover 100% of the surface, but the upper limit is approximately 90%.

The method for producing the multilayer foamed particles will be described below, but the cladding layer can be brought into a non-foamed state by, for example, not including a bubble nucleating agent in the polymer that is the raw material of the cladding layer, increasing the difference in melting point between the cladding layer and the core layer, setting the mass ratio of the core layer to the cladding layer (core layer/cladding layer) to a specific range, and the like. The obtained coating layer can be formed in a non-foamed state under the condition that the core layer is foamed and the coating layer is not foamed, or under the condition that bubbles cannot be stably held even when the coating layer is foamed and bubble collapse occurs. From the above-described viewpoint, the amount of the bubble nucleating agent in the coating layer is preferably less than 2 parts, more preferably 1 part or less, per 100 parts of the base polymer of the coating layer, and it is further preferable that the bubble nucleating agent is not contained in the coating layer.

The coating layer preferably contains a lubricant such as erucamide or calcium stearate. The amount of the lubricant added is preferably 0.05 to 0.2 parts by mass based on 100 parts by mass of the base polymer of the coating layer.

(base Polymer of coating layer)

The coating layer uses thermoplastic elastomer as a base polymer. It is considered that when the covering layer is made of a thermoplastic elastomer as a base polymer, the covering layer has excellent flexibility and the surface of the foamed particle molded body has excellent grip properties. On the other hand, it is considered that the surface of the expanded particle molded article is formed in a smooth surface state by making the coating layer in a non-expanded state, and therefore, it is considered that the grip property is excellent and the stick-slip phenomenon is less likely to occur, and the surface state of the expanded particle molded article having a specific frictional property can be formed.

The phrase "the coating layer has a thermoplastic elastomer as a base polymer" means that the polymer of the coating layer has a thermoplastic elastomer as a main component. Specifically, the content of the thermoplastic elastomer in the polymer of the coating layer is preferably 90% by mass or more, more preferably 95% by mass or more, still more preferably 97% by mass or more, and still more preferably 99% by mass or more. The upper limit is not limited, and is 100 mass% or less, and the polymer of the coating layer may be composed of only the thermoplastic elastomer.

The coating layer may contain another thermoplastic resin within a range not to impair the effects of the present invention. Examples of the other thermoplastic resin include a polyolefin resin and a polystyrene resin, and the polyolefin resin is preferable, and the polypropylene resin is more preferable.

The lower limit of the coefficient of dynamic friction of the base polymer of the coating layer is preferably 1 or more, more preferably 1.2 or more, and even more preferably 1.5 or more. Further, the upper limit thereof is preferably 3 or less.

The base polymer of the coating layer preferably has a static friction coefficient of 2 or more, more preferably 2.3 or more, and even more preferably 2.5 or more. The upper limit of the static friction coefficient of the base polymer of the coating layer is preferably 3.5 or less.

In the present invention, the friction coefficient of the base polymer of the coating layer is preferably in the above range.

The coefficient of dynamic friction and the coefficient of static friction of the base polymer of the coating layer can be determined by the same method as the coefficient of friction of the foamed molded article according to JIS K7125. The test speed was 500 mm/min.

The ratio [ static friction coefficient/dynamic friction coefficient ] of the static friction coefficient of the base polymer of the coating layer to the dynamic friction coefficient of the base polymer of the coating layer is preferably 1.5 or more and 2 or less.

When the base polymer of the coating layer is crystalline and the base polymer of the coating layer has a melting point, the melting point of the base polymer of the coating layer is preferably lower than the base resin of the core layer by 5 ℃ or more, more preferably lower by 8 ℃ or more, and still more preferably lower by 10 ℃ or more. The base polymer of the cladding layer is preferably lower than the base resin of the core layer by 30 ℃ or less, more preferably lower by 20 ℃ or less, and still more preferably lower by 15 ℃ or less.

From the viewpoint of improving moldability, the lower limit of the melting point of the base polymer of the coating layer is preferably 116 ℃ or higher, more preferably 120 ℃ or higher, and still more preferably 120 ℃ or higher. The upper limit of the melting point of the base polymer of the coating layer is preferably 143 ℃ or lower, more preferably 140 ℃ or lower, and still more preferably 130 ℃ or lower. In addition, the melting point of the base polymer is based on JIS K7121: 2012 for the heat flux in the sample.

The lower limit of the heat of fusion of the base polymer of the coating layer is preferably 20J/g or more, more preferably 30J/g or more, and still more preferably 40J/g or more. The upper limit of the heat of fusion of the base polymer is preferably 100J/g or less, and more preferably 80J/g or less.

In addition, the heat of fusion of the base polymer of the coating layer is determined based on JIS K7122: 2012 was measured using a heat flux differential scanning calorimeter.

From the viewpoint of flexibility and the like, the lower limit of the flexural modulus of the base polymer of the coating layer is preferably 10MPa or more, more preferably 15MPa or more, and still more preferably 20MPa or more. The upper limit of the flexural modulus of the base polymer of the coating layer is preferably 50MPa or less, more preferably 35MPa or less, and still more preferably 25MPa or less.

The flexural modulus of the base polymer of the coating layer was measured in accordance with JIS K7171: 2016.

The lower limit of the durometer hardness of the base polymer of the coating layer is preferably 65 or more, and more preferably 70 or more. The upper limit of the durometer hardness of the base polymer of the coating layer is preferably 95 or less, and more preferably 90 or less. The durometer hardness was measured using a type a durometer. That is, the durometer hardness (HDA, shore a) of the base polymer measured using a type a durometer is preferably 65 or more, and more preferably 70 or more. Further, it is preferably 95 or less, and more preferably 90 or less. When the durometer hardness of the base polymer is within the above range, a foamed molded article with more excellent flexibility can be obtained.

Further, the durometer Hardness (HDA) of the base polymer is in accordance with JIS K7215: 1986.

The content ratio of the ethylene component unit in the base polymer is preferably 45% by mass or more, more preferably 55% by mass or more, and further preferably 65% by mass or more.

The base polymer constituting the coating layer preferably contains a lubricant such as erucamide or calcium stearate. The content of the lubricant is preferably 0.05 to 0.2 parts by mass with respect to 100 parts by mass of the base polymer of the coating layer.

(thermoplastic elastomer)

The thermoplastic elastomer of the base polymer to be the coating layer is not particularly limited, and examples thereof include olefin thermoplastic elastomer (TPO), styrene thermoplastic elastomer (TPS), polyurethane thermoplastic elastomer (TPU), and the like. That is, the thermoplastic elastomer is preferably 1 or more selected from the group consisting of an olefinic thermoplastic elastomer (TPO), a styrenic thermoplastic elastomer (TPS), and a polyurethane thermoplastic elastomer (TPU). These thermoplastic elastomers can be used alone or in combination of two or more. Among them, olefin thermoplastic elastomer (TPO) is preferable from the viewpoint of adhesiveness between the core layer and the clad layer.

[ olefin thermoplastic elastomer (TPO) ]

Examples of TPOs include thermoplastic elastomers having a polyolefin such as polypropylene or polyethylene as a hard segment and an α -olefin copolymer or an ethylene rubber as a soft segment; and a block copolymer having a hard segment composed of a polyethylene block and a soft segment composed of an ethylene/α -olefin copolymer block.

In the block copolymer having a hard segment composed of a polyethylene block and a soft segment composed of an ethylene/α -olefin copolymer block, examples of the polymer constituting the polyethylene block include an ethylene homopolymer and a copolymer of ethylene and an α -olefin having 3 to 8 carbon atoms. On the other hand, the ethylene/α -olefin copolymer block includes a block of a copolymer of ethylene and an α -olefin having 3 to 20 carbon atoms. On the other hand, examples of the α -olefin to be copolymerized with ethylene include propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 3-methyl-1-butene, 4-methyl-1-pentene and the like. Among these, the α -olefin copolymerized with ethylene is preferably propylene, 1-butene, 1-hexene or 1-octene, and particularly preferably 1-octene, from the viewpoints of industrial availability, various characteristics, economic efficiency, and the like.

The content of the polyethylene component in the block copolymer is preferably 45% or more, and more preferably 65% or more.

The proportion of the ethylene unit in the polyethylene block is preferably 95% by mass or more, and more preferably 98% by mass or more, relative to the mass of the polyethylene block. On the other hand, the proportion of the α -olefin unit in the ethylene/α -olefin copolymer block is preferably 5% by mass or more, more preferably 10% by mass or more, and still more preferably 20% by mass or more, relative to the mass of the ethylene/α -olefin copolymer block.

The content ratio of the ethylene component unit in the TPO is preferably 45% by mass or more, more preferably 55% by mass or more, and further preferably 65% by mass or more.

In addition, the proportion of the polyethylene block and the proportion of the ethylene/α -olefin copolymer block can be calculated based on data obtained from Differential Scanning Calorimetry (DSC) or Nuclear Magnetic Resonance (NMR).

Commercially available products can be used as TPO, and examples thereof include a trade name "Infuse" as an olefin block copolymer manufactured by dow chemical, a trade name "Affinity" manufactured by dow chemical, a trade name "thermmoun" manufactured by mitsubishi chemical, a trade name "MILASTOMER" manufactured by mitsubishi chemical, a trade name "TAFMER" manufactured by mitsui chemical, a trade name "sumitomo TPE" manufactured by sumitomo chemical, and "PRIME TPO" manufactured by primiman polymer.

The olefinThe lower limit of the density of the thermoplastic-like elastomer is preferably 700kg/m ³ Above, more preferably 800kg/m ³ The above. Further, the upper limit of the density is preferably 1000kg/m ³ Hereinafter, more preferably 900kg/m ³ The following. When the amount is within the above range, a molded article of expanded particles having excellent heat resistance can be easily obtained.

When the olefinic thermoplastic elastomer has crystallinity, the lower limit of the melting point of the thermoplastic elastomer is preferably 110 ℃ or higher, more preferably 115 ℃ or higher, still more preferably 116 ℃ or higher, and still more preferably 120 ℃ or higher. The upper limit of the melting point of the olefinic thermoplastic elastomer is preferably 150 ℃ or lower, more preferably 145 ℃ or lower, still more preferably 143 ℃ or lower, yet more preferably 140 ℃ or lower, and yet more preferably 130 ℃ or lower. When the melting point of the olefinic thermoplastic elastomer is within the above range, a foamed molded article of foamed particles having particularly excellent heat resistance is obtained.

In addition, the melting point of the olefinic thermoplastic elastomer is based on JIS K7121: 2012 for the heat flux in the sample.

The melting point of the olefinic thermoplastic elastomer is preferably 5 ℃ or higher, more preferably 8 ℃ or higher, and still more preferably 10 ℃ or higher lower than that of the base resin of the core layer. The melting point of the olefinic thermoplastic elastomer is preferably 30 ℃ or lower, more preferably 20 ℃ or lower, and still more preferably 15 ℃ or lower than that of the base resin of the core layer.

The heat of fusion of the olefinic thermoplastic elastomer is preferably 20J/g or more, more preferably 30J/g or more, and still more preferably 40J/g or more. The TPO has a heat of fusion of approximately 80J/g or less.

Further, the heat of fusion of the olefinic thermoplastic elastomer is determined in accordance with JIS K7122: 2012 was measured using a heat flux differential scanning calorimeter.

The lower limit of the Melt Flow Rate (MFR) of the olefinic thermoplastic elastomer is preferably 2g/10 min or more, more preferably 3g/10 min or more, and still more preferably 4g/10 min or more under the conditions of 190 ℃ and a load of 2.16 kg. The upper limit of the MFR is preferably 10g/10 min or less, more preferably 8g/10 min or less, and still more preferably 7g/10 min or less. When the amount is within the above range, the formation of the coating layer becomes easier when the multilayer expanded particles are formed. The melt flow rate of the olefinic thermoplastic elastomer is determined in accordance with JIS K7210-1: 2014, measured at a temperature of 190 deg.C and a load of 2.16 kg.

The lower limit of the durometer hardness (HDA, shore a) of the olefinic thermoplastic elastomer, measured using a type a durometer, is preferably 65 or more, more preferably 70 or more. The upper limit of the durometer hardness (HDA, shore a) is preferably 95 or less, and more preferably 90 or less. When the durometer hardness of the thermoplastic elastomer is within the above range, a foamed molded article with more excellent flexibility can be obtained.

Further, the durometer Hardness (HDA) of the olefinic thermoplastic elastomer is in accordance with JIS K7215: 1986.

From the viewpoint of flexibility and the like, the lower limit of the flexural modulus of the olefinic thermoplastic elastomer is preferably 10MPa or more, more preferably 15MPa or more, and still more preferably 20MPa or more. The upper limit of the flexural modulus is preferably 50MPa or less, more preferably 35MPa or less, and still more preferably 25MPa or less.

The flexural modulus of the olefinic thermoplastic elastomer is measured in accordance with JIS K7171: 2016.

The lower limit of the coefficient of dynamic friction of the olefinic thermoplastic elastomer is preferably 1 or more, more preferably 1.2 or more, and still more preferably 1.5 or more. The upper limit is preferably 3 or less.

The olefinic thermoplastic elastomer has a static friction coefficient of preferably 2 or more, more preferably 2.3 or more, and still more preferably 2.5 or more. The upper limit of the static friction coefficient of the olefinic thermoplastic elastomer is preferably 3.5 or less.

The dynamic friction coefficient and the static friction coefficient of the olefinic thermoplastic elastomer can be determined by the same method as the friction coefficient of the foamed molded article according to JIS K7125. The test speed was 500 mm/min.

The ratio of the static friction coefficient of the olefinic thermoplastic elastomer to the dynamic friction coefficient of the base polymer of the coating layer [ static friction coefficient/dynamic friction coefficient ] is preferably 1.5 or more and 2 or less.

From the viewpoint of improving moldability, the lower limit of the melting point of the olefinic thermoplastic elastomer is preferably 116 ℃ or higher, more preferably 120 ℃ or higher, and still more preferably 120 ℃ or higher. The upper limit of the melting point of the olefinic thermoplastic elastomer is preferably 143 ℃ or lower, more preferably 140 ℃ or lower, and still more preferably 130 ℃ or lower. In addition, the melting point of the olefinic thermoplastic elastomer is based on JIS K7121: 2012 for the heat flux in the sample.

The lower limit of the heat of fusion of the olefinic thermoplastic elastomer is preferably 20J/g or more, more preferably 30J/g or more, and still more preferably 40J/g or more. The upper limit of the heat of fusion of the olefinic thermoplastic elastomer is preferably 100J/g or less, and more preferably 80J/g or less.

From the viewpoint of flexibility and the like, the lower limit of the flexural modulus of the olefinic thermoplastic elastomer is preferably 10MPa or more, more preferably 15MPa or more, and still more preferably 20MPa or more. The upper limit of the flexural modulus of the olefinic thermoplastic elastomer is preferably 50MPa or less, more preferably 35MPa or less, and still more preferably 25MPa or less.

Further, the olefinic thermoplastic elastomer has a durometer hardness (HDA, shore a) of preferably 65 or more, more preferably 70 or more, as measured with a type a durometer. Further, it is preferably 95 or less, and more preferably 90 or less. When the durometer hardness of the olefinic thermoplastic elastomer is within the above range, a foamed molded article with more excellent flexibility can be obtained.

The thickness of the coating layer is preferably 1 μm or more, more preferably 2 μm or more, and further preferably 3 μm or more. On the other hand, the upper limit value is preferably 25 μm or less, more preferably 18 μm or less, and further preferably 15 μm or less.

Within the above range, the frictional properties of the surface of the foamed particle molded body can be specified without inhibiting the weldability of the foamed particles to each other.

The mass ratio of the core layer to the clad layer, that is, the mass ratio of the core layer to the clad layer (core layer/clad layer) is more preferably 85/15 or more, still more preferably 88/12 or more, and still more preferably 90/10 or more. Further, it is more preferably 99.5/0.5 or less, still more preferably 99/1 or less, and still more preferably 95/5 or less. By constituting the coating layer within the above range, particularly, the frictional characteristics unique to the present invention can be exhibited in the coating layer, and the foamed particles having excellent cushioning properties against the packed material can be formed by setting the core layer to a foamed state.

< core layer >

The core layer of the multilayer foamed particle is in a foamed state, and polypropylene resin is used as base resin. The phrase "polypropylene resin is used as the base resin" means that the resin constituting the core layer contains a polypropylene resin as a main component. In the present specification, the term "polypropylene-based resin as a main component" means that the content of the polypropylene-based resin in the base resin is 50 mass% or more, preferably 60 mass% or more, more preferably 70 mass% or more, and still more preferably 80 mass% or more. The upper limit is not particularly limited, but is 100 mass% or less.

(Polypropylene resin)

In the present invention, the polypropylene-based resin means a propylene homopolymer, a propylene-based copolymer or a mixture thereof, preferably a propylene-based copolymer or a mixture of a propylene homopolymer and a propylene-based copolymer, and more preferably a propylene-based copolymer.

When the polypropylene-based resin is a propylene-based copolymer or a mixture of a propylene homopolymer and a propylene-based copolymer, the content of the propylene-derived structural unit in the polypropylene-based resin is preferably 50% by mass or more, more preferably 70% by mass or more, further preferably 80% by mass or more, and further preferably 90% by mass or more. The content of the propylene-derived structural unit in the polypropylene-based resin is preferably 99% by mass or less, more preferably 98% by mass or less, still more preferably 97% by mass or less, and still more preferably 96% by mass or less.

Examples of the propylene copolymer include copolymers of propylene and ethylene and/or α -olefins such as 1-butene, 1-pentene, 1-hexene, 1-octene, and 4-methyl-1-butene having 4 to 20 carbon atoms, and copolymers of propylene and ethylene are preferable. The ethylene content in the copolymer of propylene and ethylene is preferably 2% by mass or more, and more preferably 2.5% by mass or more. Further, it is preferably 5% by mass or less, and more preferably 4.5% by mass or less.

The polypropylene resin may contain a resin or an elastomer other than the propylene homopolymer or the propylene copolymer, as long as the intended effect of the present invention is not impaired.

The content of the resin or elastomer other than the propylene homopolymer or the propylene copolymer in the polypropylene resin is preferably 20% by mass or less, more preferably 10% by mass or less, still more preferably 5% by mass or less, and still more preferably 1% by mass or less.

The flexural modulus of the polypropylene resin is preferably 500MPa or more, more preferably 850MPa or more, and still more preferably 900MPa or more. Further, 1500MPa or less is preferable, 1300MPa or less is more preferable, and 1100MPa or less is even more preferable. When the flexural modulus of elasticity is within the above range, the cell film during foaming becomes strong, and the strength of the foamed particle molded article obtained by molding the foamed particles can be further increased, and the foamed particle molded article is useful particularly as a packaging container. The flexural modulus can be determined in accordance with JIS K7171 (2008).

The melting point of the polypropylene resin is preferably 130 ℃ or higher, more preferably 132 ℃ or higher, and still more preferably 135 ℃ or higher. From the viewpoint of heat resistance, the melting point of the polypropylene-based resin is preferably 148 ℃ or lower, more preferably 145 ℃ or lower, and still more preferably 142 ℃ or lower.

Further, additives may be added to the base resin of the core layer as appropriate to such an extent that the effects of the present invention are not hindered. Examples of the additives include antioxidants, ultraviolet absorbers, antistatic agents, flame retardants, pigments, dyes, nucleating agents, and bubble nucleating agents. Among them, the core layer preferably contains an antistatic agent. The content of these additives is preferably 20 parts by mass or less, more preferably 5 parts by mass or less, per 100 parts by mass of the base resin forming the core layer. In the case where the core layer contains an antistatic agent, the content of the antistatic agent is preferably 0.2 parts by mass or more, and more preferably 0.5 parts by mass or more, with respect to 100 parts by mass of the base resin of the core layer. Further, the amount is preferably 2 parts by mass or less, and more preferably 1 part by mass or less, per 100 parts by mass of the base resin of the core layer.

The flexural modulus of the base resin of the core layer is preferably 500MPa or more, more preferably 850MPa or more, and still more preferably 900MPa or more. The upper limit of the flexural modulus is preferably 1500MPa or less, more preferably 1300MPa or less, and still more preferably 1100MPa or less. When the flexural modulus of elasticity is in the above range, the cell film during foaming becomes strong, and the strength of the foamed molded article of foamed particles obtained by molding the foamed particles can be further increased, and the foamed molded article is particularly useful as a packaging container. The flexural modulus can be determined according to JIS K7171 (2008).

The lower limit of the ratio [ core/clad ] of the flexural elastic modulus of the base resin of the core layer to the flexural elastic modulus of the base polymer of the clad layer is preferably 10, more preferably 26, and even more preferably 30. The upper limit of the ratio is preferably 50, more preferably 50, and still more preferably 50.

The lower limit of the melting point of the base resin is preferably 130 ℃ or higher, more preferably 132 ℃ or higher, and still more preferably 135 ℃ or higher. From the viewpoint of heat resistance, the upper limit of the melting point of the base resin is preferably 148 ℃ or lower, more preferably 145 ℃ or lower, and still more preferably 142 ℃ or lower.

The multilayer expanded particle preferably has 1 or more melting peaks (high temperature peaks) on the high temperature side of a melting peak (resin-specific peak) specific to the base resin, i.e., a polypropylene-based resin, in a Differential Scanning Calorimetry (DSC) curve.

These melting peaks can be obtained by the following method.

Specifically, the expanded beads were subjected to temperature rise measurement at 10 ℃/min from 23 ℃ to 200 ℃ by a differential scanning calorimeter to obtain a DSC curve having 2 or more melting peaks, and the peak having the largest heat of fusion was defined as a melting peak specific to the polypropylene-based resin (resin-specific peak), and the melting peak appearing on the higher temperature side than this was defined as a high-temperature peak.

It is considered that the DSC curve in this case is a DSC curve of the 1 st heating obtained by the above-described measurement method, and the endothermic peak due to melting specific to the resin (resin-specific peak) is an endothermic peak due to melting specific to the polypropylene-based resin constituting the expanded beads, and is a peak due to endothermic heat at the time of melting of crystals originally contained in the resin constituting the core layer of the expanded beads.

On the other hand, the endothermic peak on the high temperature side of the resin intrinsic peak (high temperature peak) means an endothermic peak appearing on the higher temperature side than the resin intrinsic peak in the 1 st DSC curve. When this high temperature peak occurs, it is presumed that secondary crystallization exists in the resin. In addition, only an endothermic peak due to melting unique to the polypropylene-based resin constituting the core layer of the multilayer foamed particle was observed in a DSC curve (DSC curve of heating 2 nd time) obtained when heating from 23 ℃ to 200 ℃ at a temperature increase rate of 10 ℃/min, then cooling from 200 ℃ to 23 ℃ at a cooling rate of 10 ℃/min, and then heating again from 23 ℃ to 200 ℃ at a temperature increase rate of 10 ℃/min. This resin-specific peak appears in both the DSC curve of the 1 st heating and the DSC curve of the 2 nd heating, and the temperature of the peak top may be slightly different between the 1 st and 2 nd heating, but usually the difference is less than 5 ℃. This makes it possible to confirm which peak is a resin-specific peak. The endothermic peak varies depending on the composition of the resin and the like.

The lower limit of the heat of fusion at the high temperature peak of the multilayer expanded beads is preferably 5J/g or more, more preferably 7J/g or more, and still more preferably 10J/g or more. The upper limit of the heat of fusion is preferably 40J/g or less, more preferably 30J/g or less, and still more preferably 20J/g or less.

When the heat of fusion at the high temperature peak is in such a range, it is considered that the expanded beads are particularly excellent in mechanical strength and excellent in-mold moldability due to the presence of secondary crystals expressed as the high temperature peak.

The lower limit of the average mass per 1 of the multilayer foamed particles (the arithmetic average of 1 of the randomly selected 200 masses measured) is preferably 0.1mg or more, more preferably 0.2mg or more, further preferably 0.3mg or more, and further preferably 0.4mg or more. The upper limit of the average mass is preferably 20mg or less, more preferably 10mg or less, further preferably 5mg or less, and further preferably 2mg or less.

< molded foam particle >

By having the frictional characteristics of the foamed particle molded article as described above, since the article to be packaged has excellent grippability, the surface is less likely to be broken, and the appearance is less likely to change, it is suitable for repeated use.

Such frictional characteristics are considered to be associated with a foamed particle molded body in which a coating layer of a foamed particle is formed of a plurality of foamed particles, the coating layer having a thermoplastic elastomer as a base polymer, and the coating layer being in a non-foamed state. Specifically, it is considered that the grip strength is exerted by the flexibility of the thermoplastic elastomer of the coating layer, and the stick-slip phenomenon is less likely to occur by the non-foamed state of the coating layer, and the frictional properties unique to the foamed molded article of expanded particles of the present invention can be exhibited. Further, it is considered that when the molded article density of the foamed particle molded article is in the above range, the foamed layer is brought into a specific foamed state, and the cushioning property to the object to be packed and the frictional property in the foamed particle molded article are exhibited.

The lower limit of the 25% compressive stress of the expanded bead molded product of the present invention is preferably 0.20MPa or more, and more preferably 0.30MPa or more. The upper limit of the 25% compressive stress is preferably 0.80MPa or less, and more preferably 0.50MPa or less.

Within the above range, the foamed molded article of foamed particles is particularly excellent in cushioning properties.

The above 25% compressive stress is based on JIS K6767: 1999 compressive stress at 25% deformation (MPa) measured when compressed at a rate of 10 mm/min.

The ratio of the exposed area of the coating layer of the expanded particles constituting the expanded particle molded article on the surface of the expanded particle molded article is preferably 50% to 80%.

In particular, when the outer shape of the foamed particles used in the present invention is cylindrical, it is preferable to set the ratio L/D of the length L of the foamed particles to the diameter D to 1.2 or more in view of bringing out the excellent frictional characteristics of the obtained foamed particle molded article and setting the occupation area ratio of the coating layer component of the foamed particles in the foamed particle molded article (the ratio of the exposed area of the coating layer of the foamed particles) to 50% or more.

The occupancy area ratio of the coating layer component of the expanded beads on the surface of the expanded bead molded body can be obtained by image analysis of the surface of the expanded bead molded body using image analysis software such as NS2K-Pro manufactured by Nano systems co. Specifically, a photograph is taken of a portion where a coating component of the foamed particles is present on the surface of the foamed particle molded body and a portion where a core component of the foamed particles is present on the surface of the foamed particle molded body, and the portion are binarized into black and white by image analysis software, and the occupied area of each portion is obtained, and the occupied area ratio of the coating component of the foamed particles can be obtained by the following expression.

The area ratio (%) occupied by the coating layer component of the foamed particles is equal to the area occupied by the coating layer component of the foamed particles/(the area occupied by the coating layer component of the foamed particles + the area occupied by the core layer component of the foamed particles) × 100

In addition, when it is difficult to distinguish the exposed portion of the coating layer of the foamed particles on the surface of the foamed particle molded body, image analysis can be easily performed by coloring the resin forming the coating layer.

< method for producing multilayer expanded beads >

The method for producing the multilayer expanded beads is not limited, but the following method is preferably used.

As a preferable production method of the multilayer foamed particles, the following method can be adopted: multilayer particles each having a core layer made of a polypropylene resin as a base resin and a coating layer made of a thermoplastic elastomer as a base polymer and covering the core layer are dispersed in a dispersion medium, and the multilayer particles are impregnated with a foaming agent and released under low pressure (dispersion medium releasing foaming method).

(production of multilayer particles)

The multilayer particle used for producing the multilayer foamed particle of the present invention has a core layer and a cladding layer.

First, the polypropylene resin is blended with other additives such as a bubble nucleating agent, if necessary, to form a resin of the core layer, and the resin is supplied into a first extruder, heated, and kneaded to form a resin melt of the core layer.

Further, another additive blended as necessary is blended with the polymer of the coating layer and supplied as a coating layer into a second extruder, and the mixture is heated and kneaded to prepare a resin melt of the coating layer. In this case, it is preferable that the bubble nucleating agent is not incorporated in the coating layer. By not blending the bubble nucleating agent, a coating layer in a non-foamed state can be easily obtained.

Next, the resin melt of the core layer and the resin melt of the cladding layer are supplied to a coextrusion die, and the flow of the resin melt of the core layer and the flow of the resin melt of the cladding layer are merged in the coextrusion die, and the two layers are laminated. The resin melt after lamination can be obtained by pelletizing the resin melt from an extruder by a strand cutting method, a thermal cutting method, an underwater cutting method, or the like.

In the production of the multilayered particle, the lower limit of the mass ratio of the core layer to the clad layer (core layer/clad layer) is preferably 85/15 or more, more preferably 88/12 or more, and still more preferably 90/10 or more. The upper limit of the ratio is preferably 99.5/0.5 or less, more preferably 99/1 or less, and still more preferably 95/5 or less. In addition, in the production of the multilayer particle, the core layer may contain additives such as zinc borate as a bubble nucleating agent, an antistatic agent, and a weather resistant agent. In addition, the coating layer may contain additives such as a lubricant, an antistatic agent, and a weather-resistant agent.

In particular, when an antistatic agent is added to the core layer, the reason is not clear, but the core layer preferably contains an antistatic agent because the effect of facilitating mold release during molding can be exhibited. The antistatic agent is not particularly limited, and examples thereof include nonionic surfactants such as hydroxyalkylamine, hydroxyalkyl monoether amine, polyoxyalkylene alkylamine, glycerin fatty acid ester, and polyoxyethylene alkyl ether; anionic surfactants such as alkylsulfonates, alkylbenzenesulfonates, and alkylphosphates; cationic surfactants such as octyl dimethylethyl ammonium ethyl sulfate, lauryl dimethylethyl ammonium ethyl sulfate, didecyl dimethyl ammonium chloride, tetraalkyl ammonium salts, and trialkyl benzyl ammonium salts. In addition, these antistatic agents can be used alone or in combination. Among them, nonionic surfactants are preferable. The content of the antistatic agent in the core layer is preferably 0.2 parts by mass or more, and more preferably 0.5 parts by mass or more, per 100 parts by mass of the base resin of the core layer. Further, the amount is preferably not more than 1 part by mass, and more preferably not more than 1 part by mass, per 100 parts by mass of the base resin of the core layer.

From the viewpoint of production stability, it is preferable that the coating layer contains a lubricant such as erucamide or calcium stearate. The content of the lubricant is preferably 0.05 to 0.2 parts by mass with respect to 100 parts by mass of the base polymer constituting the coating layer.

In this case, the mold release during molding is also facilitated.

The particle diameter of the multilayered particle is preferably 0.1mm or more, and more preferably 0.3mm or more. Further, it is preferably 3.0mm or less, more preferably 1.5mm or less. The length/diameter ratio of the multilayered particle is preferably 0.5 or more, and more preferably 1.0 or more. Further, it is preferably 5.0 or less, and more preferably 3.0 or less. The average mass per 1 (arithmetic average of 1 after measuring the mass of 200 randomly selected cells) is adjusted to be preferably 0.1mg to 20mg, more preferably 0.2mg, even more preferably 0.3mg, and even more preferably 0.4 mg. Further, it is more preferably 10mg or less, still more preferably 5mg or less, and still more preferably 2mg or less. The multilayer particle has a cylindrical outer shape.

In addition, the particle diameter, length/diameter ratio, and average mass of the multilayer particles in the strand cutting method can be adjusted by cutting the resin melt by appropriately changing the extrusion speed, drawing speed, cutting speed, and the like when extruding the resin melt.

(production of multilayer foamed particles)

As a dispersion medium for dispersing the multilayer particles obtained as described above in a closed container, an aqueous dispersion medium can be used. The aqueous dispersion medium is a dispersion medium containing water as a main component.

In the dispersion medium discharge foaming method preferably used in the present invention, a dispersant is preferably added to the dispersion medium so that the multilayer particles heated in the container are not welded to each other in the container. The dispersant may be any dispersant as long as it prevents fusion of the multilayered particles in the container, and may be used in both organic and inorganic forms, and is preferably a particulate inorganic substance in view of ease of handling. For example, can give alumina, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, iron oxide natural or synthetic clay mineral, alumina, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, can use 1 or a combination of 2 or more. Among them, natural or synthetic clay minerals are preferably used. The dispersant is preferably added in an amount of about 0.001 parts by mass or more and about 5 parts by mass or less per 100 parts by mass of the multilayered particles.

When a dispersant is used, it is preferable to use an anionic surfactant such as sodium dodecylbenzenesulfonate, sodium alkylsulfonate, or sodium oleate as a dispersion aid in combination. The dispersion aid is preferably added in an amount of about 0.001 parts by mass or more and 1 part by mass or less per 100 parts by mass of the multilayered particles.

As the foaming agent for foaming the multilayer particles, a physical foaming agent is preferably used. Examples of the physical blowing agent include inorganic physical blowing agents and organic physical blowing agents, and examples of the inorganic physical blowing agents include carbon dioxide, air, nitrogen, helium, argon, and the like. Further, examples of the organic physical blowing agent include aliphatic hydrocarbons such as propane, butane and hexane; cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; halogenated hydrocarbons such as chlorofluoromethane, trifluoromethane, 1-difluoromethane, 1-chloro-1, 1-dichloroethane, 1, 2, 2, 2-tetrafluoroethane, methyl chloride, ethyl chloride and methylene chloride. The physical blowing agents may be used alone or in combination of two or more. In addition, an inorganic physical blowing agent and an organic physical blowing agent may be mixed and used. Among these blowing agents, inorganic physical blowing agents are preferably used, and carbon dioxide is more preferably used, from the viewpoint of environmental load and handling property.

The amount of the blowing agent added is preferably 0.1 part by mass or more, and more preferably 0.5 part by mass or more, per 100 parts by mass of the multilayer particles. Further, it is preferably 30 parts by mass or less, and more preferably 15 parts by mass or less.

In the step of producing the foamed particles, as a method of impregnating the multilayer particles with the foaming agent, a method of dispersing the multilayer particles in an aqueous dispersion medium in a closed container and pressing the foaming agent while heating to impregnate the multilayer particles with the foaming agent is preferably used.

The internal pressure of the closed container at the time of foaming is preferably 0.5MPa (G) or more, more preferably 0.8MPa (G) or more. On the other hand, the upper limit of the internal pressure of the closed vessel is preferably 4MPa (G) or less, and more preferably 3MPa (G) or less. Within the above range, the expanded beads can be produced safely without risk of breakage, explosion, or the like of the sealed container. The temperature is raised to 100 ℃ to 200 ℃ inclusive, preferably 130 ℃ to 160 ℃ inclusive, and the resulting mixture is held at that temperature for about 5 minutes to 30 minutes, and then the expandable multilayer particles are released from the closed container under low pressure to foam the particles.

The lower limit of the average cell diameter of the expanded particles is preferably 20 μm or more, more preferably 50 μm or more, and further preferably 80 μm or more. The upper limit of the average cell diameter is preferably 400 μm or less, more preferably 300 μm or less, and still more preferably 200 μm or less. When the average cell diameter is within the above range, a molded article of expanded particles having excellent in-mold moldability, excellent dimensional recovery after molding, and excellent mechanical properties such as compressive properties can be obtained.

The lower limit of the apparent density of the multilayer foamed particle is preferably 10kg/m ³ Above, more preferably 30kg/m ³ As described above. Further, the upper limit of the apparent density is preferably 100kg/m ³ Hereinafter, more preferably 80kg/m ³ The following. When the amount is within the above range, the obtained expanded beads preferably have cushioning properties and frictional properties against the article to be packed.

The multilayer expanded beads obtained as described above can be further expanded by increasing the internal pressure by a pressure treatment with air and then heating the resulting expanded beads with steam or the like (two-stage expansion), and can be also expanded to a higher expansion ratio (lower apparent density).

< production of molded foam particle >

The foamed particle molded article of the present invention is a molded article comprising the multilayer foamed particles, and can be obtained by in-mold molding the foamed particles.

Specifically, the multilayer foamed particle can be obtained by in-mold molding a multilayer foamed particle having a foamed core layer containing a polypropylene resin as a base resin and a coating layer covering the core layer, the coating layer being in a non-foamed state, the coating layer containing a thermoplastic elastomer as a base.

The in-mold forming method can be performed by filling foamed particles into a forming mold and performing thermal forming using a heating medium such as steam. Specifically, after the expanded beads are filled in the molding die, a heating medium such as steam is introduced into the molding die to heat and expand the expanded beads, and the expanded beads are welded to each other, whereby a foamed article having a shape provided with a molding space can be obtained. In the in-mold molding of the present invention, it is preferable to perform the molding by a pressure molding method (for example, japanese patent laid-open No. 51-22951), in which the pressure in the cells of the expanded beads is increased by subjecting the expanded beads to a pressure treatment with a pressurized gas such as air, the pressure in the expanded beads is adjusted to a pressure higher than atmospheric pressure by 0.01MPa to 0.3MPa, the expanded beads are filled into a molding die under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied into the die to heat-weld the expanded beads. Further, the molding can be performed by a compression filling molding method (japanese patent laid-open publication No. 4-46217), in which expanded particles pressurized to an atmospheric pressure or higher are filled in a molding die pressurized to the atmospheric pressure or higher by a compressed gas, and then a heating medium such as steam is supplied into a cavity of the molding die to heat and weld the expanded particles. In addition, molding can be performed by an atmospheric pressure filling molding method (japanese patent laid-open publication No. 6-49795), in which foamed particles having a high secondary foaming force obtained under special conditions are filled into a cavity of a molding die under atmospheric pressure or reduced pressure, and then heated by supplying a heating medium such as steam to heat-weld the foamed particles. Or can be formed by a method combining the above methods (Japanese patent laid-open No. 6-22919), or the like.

In the production of the foamed molded article of expanded particles, the shrinkage ratio is preferably 3% or less, more preferably 2.5% or less.

In the obtained expanded bead molded article, the fusion ratio is preferably 80% or more, and more preferably 90% or more. When the amount is within the above range, the foamed particles are not broken when the object to be packed is gripped, and the object to be packed has more excellent grippability.

In the foamed molded article obtained as expanded beads, the flexural strength of the molded article is preferably 0.2MPa to 2MPa, and from the viewpoint of being able to form a more excellent packaging container, the flexural strength is preferably 0.3MPa to 1.5 MPa.

The compression strength (25%) of the foamed molded article of expanded particles is preferably 0.1MPa to 1MPa, and is preferably 0.2MPa to 0.8MPa from the viewpoint of the protection of the packaged article.

[ packaging Container ]

The packaging container of the present invention is constituted by the foamed molded article. Specifically, the packaging container of the present invention is composed of a foamed particle molded article composed of a plurality of foamed particles, the foamed particle molded article having a foamed core layer containing a polypropylene resin as a base resin and a coating layer covering the core layer, the coating layer being in a non-foamed state, the coating layer containing a thermoplastic elastomer as a base polymer, the foamed particle molded article having a coefficient of dynamic friction of 1.0 to 3.0, a ratio of the coefficient of static friction of the foamed particle molded article to the coefficient of dynamic friction of the foamed particle molded article of 0.4 to 0.9, and a molded article density of the foamed particle molded article of 30kg/m ³ Above 300kg/m ³ The following.

Since the packaging container of the present invention is composed of the foamed molded article, the shape thereof can be adjusted by the shape of the mold used for molding. The shape of the packaging container may be selected to be most suitable in accordance with the shape, size, weight, and material of the object to be packaged. Examples of the shape of the packaging container include a box shape, a tray shape, and a sheet shape.

The foamed particle molded body constituting the packaging container of the present invention is excellent in grippability, is less likely to change in appearance due to friction, and can withstand repeated use, and therefore the packaging container of the present invention is suitable for packaging of heavy objects or precision parts. Further, the packaging method is also suitable for packaging objects to be packaged, which are easily deteriorated or damaged by dropping or vibration during transportation.

[ examples ]

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

[ Properties and evaluation ]

The resins, expanded particles, and expanded particle molded bodies used in examples and comparative examples were subjected to the following measurements and evaluations. The results are shown in tables 1 and 2. The evaluation of the expanded beads or expanded bead molded article was carried out after the state was adjusted by leaving them under the conditions of a relative humidity of 50%, 23 ℃ and 1atm for 2 days.

(coefficient of friction)

The friction coefficient of the foamed molded article was measured by the following method in accordance with JIS K7125.

A sample of a single-sided skin (skin means a surface which is in contact with a mold during in-mold molding and is not a cut surface) having a length of 63mm X a thickness of 10mm was cut out from the foamed molded article obtained in examples and comparative examples, and this was used as a test piece. The test sample foamed particle molded article was obtained by in-mold molding with a smooth mold surface.

A soft vinyl chloride resin sheet having a smooth surface and a thickness of 3mm was placed on a horizontal surface, and the sample was formed into a skin surface (area 40 cm) ² ) Placed thereon in a downward (vinyl chloride resin sheet side) manner, and further placed thereon a weight to attain a normal force of 9.8N. In addition, the upper side of the sample was covered with a felt (felt) having a thickness of 1mm to apply a uniform pressure distribution.

The measurement was performed using a TENSILON universal material tester. The sample was moved on a soft vinyl chloride sheet at a speed of 100 mm/min in the horizontal direction by 70mm, and the load (unit: N) at that time and the moving distance were recorded to obtain a load-displacement curve.

Fig. 1 and 2 show conceptual views of load-displacement curves.

In the load-displacement curve, the initial maximum load value (static load) (F) _S ) The coefficient of static friction (. mu.) was obtained by dividing the value by the normal force (9.8N) _S ). Specifically, the formula is shown below. The local maximum value can be determined by obtaining an enlarged view of the load-displacement curve (fig. 1), for example.

Coefficient of static friction (. mu.) _S )＝F _S /9.8

In the load-displacement curve, the average value of the dynamic load in the detected measurement displacement with a displacement (moving distance) of 10mm to 60mm is set as the dynamic load (F) _D ) The coefficient of dynamic friction (. mu.) in the present invention was obtained by dividing the normal force (9.8N) by the value obtained _D ). Specifically, the formula is shown below.

Coefficient of dynamic friction (. mu.) _D )＝F _D /9.8

In the present invention, the value of the kinetic friction force immediately after the start of the relative displacement motion may be unstable because the sample is a foam, and therefore, the value from the start of the displacement of 10mm is used.

Further, a non-foamed test sample was prepared except that the test speed was changed from 100 mm/min to 500 mm/min, and the test conditions and the operation for measuring the foamed molded article of foamed particles were measured, and the coefficient of dynamic friction and the coefficient of static friction of the base polymer or the thermoplastic elastomer of the raw material of the coating layer were determined by the above formula. The accuracy of the displacement gauge is 0.001mm or more.

Table 3 shows the coefficient of dynamic friction, the coefficient of static friction, and the ratio of the coefficient of static friction to the coefficient of dynamic friction of the base polymer of the raw material for the coating layer.

(formed body Density)

The volume of the expanded bead molded body was calculated based on the outer dimension of the expanded bead molded body. The mass of the molded article of expanded particles divided by the volume is defined as a molded article density ρ (D) [ kg/m ] ³ ]。

(melting Point and Heat of fusion)

The melting point (Tm) was measured in accordance with "after a constant heat treatment, melting temperature was measured" described in JIS K7121(1987) (both heating rate and cooling rate in conditioning the test piece were 10 ℃/min).

A DSC curve was drawn by a DSC apparatus (DSC Q1000 manufactured by TA Instruments) while the temperature was increased at a heating rate of 10 ℃/min, and the peak temperature of the endothermic peak accompanying the melting of the resin on the DSC curve was defined as the melting point. When a plurality of endothermic peaks are present on the DSC curve, the melting point is defined as the peak of the endothermic peak having the largest area of the endothermic peaks.

The heat of melting is determined from the peak area from the start of melting to the end of melting of the endothermic peak accompanying melting of the resin on the DSC curve.

(flexural modulus of elasticity)

Flexural modulus of elasticity was measured according to JIS K7171: 2008, the calculation. As the test piece, a 4mm sheet was prepared by hot-pressing the test piece at 230 ℃ and a test piece cut out from the sheet and having a length of 80mm, a width of 10mm and a thickness of 4mm (standard test piece) was used. Further, the radius R of the indenter is made ₁ And radius R of the supporting table ₂ The thickness of the test pieces is 5mm, the distance between the supporting points is 64mm, and the test speed is 2 mm/min.

(Heat of fusion of high temperature Peak of expanded particles)

1 to 3mg of the inner layer portion of the expanded beads were sampled, and temperature increase from 23 ℃ to 200 ℃ was measured at 10 ℃/min by a differential scanning calorimeter (DSC Q1000 manufactured by TA Instruments), to obtain a DSC curve having 1 or more melting peaks. The resin-specific peak in the following description is denoted by a, and the high-temperature peak appearing on the higher-temperature side than this is denoted by B.

A straight line (α - β) is drawn connecting a point α corresponding to 80 ℃ on the DSC curve and a point β on the DSC curve corresponding to the melting completion temperature T of the expanded particles. The melting end temperature T is an intersection point between a DSC curve on the high temperature side of the high temperature peak B and a high temperature side base line. Next, a straight line parallel to the vertical axis of the graph is drawn from a point γ on the DSC curve corresponding to the trough between the resin-specific peak a and the high-temperature peak B, and a point intersecting the straight line (α - β) is represented as δ.

The area of the resin specific peak a is the area of a portion surrounded by a curve of the resin specific peak a portion of the DSC curve, line segment (α - δ) and line segment (γ - δ), and is defined as the heat of fusion of the resin specific peak.

The area of the high temperature peak B is the area of a portion surrounded by the curve of the high temperature peak B portion of the DSC curve, the line segment (δ - β) and the line segment (γ - δ), and is defined as the heat of fusion of the high temperature peak.

(shrinkage factor)

The shrinkage ratio [% ] of the foamed particle molded article is the ratio of the length of the long side of the foamed particle molded article to the length of the long side of the mold. Specifically, the value is determined by (the dimension of the long side of the mold [ mm ] -the length of the long side of the molded article [ mm ])/the dimension of the long side of the mold [ mm ]. times.1000. The "length of the long side of the molded article [ mm ]" means a value obtained by measuring the length of the long side of the foamed molded article of foamed particles obtained in examples and comparative examples, which was aged at 80 ℃ for 12 hours, then slowly cooled, and further aged at 23 ℃ for 6 hours.

(fusion ratio)

The fusion rate of the expanded particle molded article can be evaluated by the material failure rate at the fracture surface in the bending test. That is, the number of broken expanded particles and expanded particles peeled off from the interface were counted by visual observation by observing the curved fractured surface (fractured surface where 100 or more expanded particles exist) of the expanded particle molded body. Then, the percentage of the number of broken foamed particles to the total of the number of broken foamed particles and the number of foamed particles peeled off at the interface was defined as a fusion ratio.

(Friction test)

A sample having a skin surface on at least one surface thereof and having a length of 50mm X a width of 50mm X a thickness of 15mm was cut out from the foamed molded article, and fixed. No. 400 sandpaper 30mm in length by 30mm in width was set in contact with the surface of the skin. The surface of the sample was rubbed 500 times with sandpaper under the conditions of a load of 1000g, a test moving distance (horizontal movement) of 8mm, and a test speed of 60 times/min, and evaluated by the following criteria. In addition, refer to JIS K7204: 1999, the test was stopped every 100 times of the rubbing test, and the clogging of the sandpaper was checked to remove the powder adhering to the sandpaper.

(evaluation criteria)

A: the amount of friction (the mass of the foamed molded article of expanded particles reduced by the test) was 1mg or less, and no surface fuzzing was observed even after the friction test, resulting in good appearance.

B: the amount of friction (the mass of the foamed molded article of foamed particles reduced by the test) was 1mg or more, and fluffing was observed after the friction test, resulting in poor appearance.

(grip test)

When the above-mentioned friction coefficient measurement was performed using a sample of a single-sided skin cut out from a molded foam particle body and having a length of 63mm × a width of 63mm × a thickness of 10mm, whether stick-slip phenomenon occurred or not was evaluated by the following criteria.

(evaluation criteria)

A: neither stick-slip phenomenon nor frictional sound (sound due to friction of the foamed particle molded body in the friction coefficient measurement) was generated. (Excellent grippability)

B: the stick-slip phenomenon is generated, and the friction sound is generated. (poor grippability)

(bending Property)

Under the conditions described in examples and comparative examples, a foamed molded article of foamed particles having a plate shape of 300mm in length × 75mm in width × 25mm in thickness was produced by changing only the mold, instead of the foamed molded article having a length of 300mm × 250mm in width × 60mm produced in examples and comparative examples. Using this molded article as a test piece, a 3-point bending test was carried out in accordance with the bending test method for a large test piece described in appendix 1 of JIS K7221-2 (1999), and a stress-strain curve was obtained. The bending stress in the maximum load calculated based on the stress-strain curve was used as the bending strength of the expanded bead molded body. In the 3-point bending test, a universal tester ("Autograph (registered trademark)") was used, and the test was performed under the conditions of a distance between lower support points of 200mm and a test speed of 10 mm/min.

(compressive Strength (25% compressive stress of expanded bead molded article))

As a measure of the rigidity of the foamed molded article of expanded beads, a test piece having a length of 50mm × a width of 50mm × a thickness of 25mm was cut out from the foamed molded article of resin expanded beads obtained in examples and comparative examples without a skin layer, and the thickness was measured in accordance with JIS K6767: 1999, the compressive stress (MPa) at 25% deformation when compressed at a rate of 10 mm/min was measured.

(average bubble diameter)

The average cell diameter of the expanded beads was measured in accordance with ASTM D3576-77 as follows.

50 or more expanded particles are randomly selected from the expanded particle group. The foamed particles were cut and bisected so as to pass through the center portion thereof, and enlarged photographs of the cross sections thereof were taken, respectively. In each cross-sectional photograph, 4 line segments are drawn at equal angles from the outermost surface of the expanded beads to the outermost surface on the opposite side through the center portion. The number of cells intersecting each line segment was measured, the average chord length of the cells was determined by dividing the total length of 4 line segments by the total number of cells intersecting the line segment, the average cell diameter of each expanded particle was determined by dividing the average chord length by 0.616, and the average cell diameter of the expanded particles was determined by arithmetically averaging these values.

[ raw materials ]

The resins and elastomers used in the examples and comparative examples are as follows.

(1) Abbreviation PP1 (density 900 g/cm) ³ MFR (2.16kg, 190 ℃ C.) of 7g/10 min, melting point 141 ℃ C., flexural modulus of elasticity 950MPa, ethylene content 3.1%): manufactured by Priman polymers Inc.; polypropylene resin, ethylene-propylene random copolymer "J832 MZV"

(2) Abbreviation PP2 (density 900 g/cm) ³ MFR (2.16kg, 190 ℃)7g/10 min, melting point 134 ℃, flexural modulus of elasticity 600MPa, ethylene content 4.2%): manufactured by Priman polymers Inc.; polypropylene resin, ethylene-propylene random copolymer "F744 NP"

(3) Abbreviated as PP3 (density 900 g/cm) ³ MFR (2.16kg, 190 ℃)7g/10 min, melting point 131 ℃ bending modulusThe modulus of elasticity is 650MPa, and the content of ethylene is 3.1%): manufactured by POLYPRO corporation of Japan; polypropylene-based resin, propylene/1-butene/ethylene copolymer "FX 4 ET"

(4) Short for TPO (density 887 g/cm) ³ MFR (2.16kg, 190 ℃) of 5g/10 min, melting point 119 ℃, Shore A83 hardness, flexural modulus of elasticity 34MPa, ethylene content 68%, heat of fusion 54J/g): manufactured by dow chemical corporation; olefinic thermoplastic elastomer "INFUSE 9530"

(5) A bubble nucleating agent masterbatch; trade name "CP-130786F" (polypropylene resin (PP); J832MZV, zinc borate concentration 10 mass%)

Example 1 production of expanded particle molded article

The polypropylene resin shown in Table 1 was used alone as a base resin of a core layer, and the polypropylene resin was melt-kneaded at 180 to 240 ℃ in an extruder to obtain a resin melt for the core layer. Meanwhile, only the elastomer shown in Table 1 was used as a base polymer of the coating layer, and the elastomer was melt-kneaded at 180 to 240 ℃ in an extruder to obtain a resin melt for the coating layer.

Next, the resin melt for the core layer and the resin melt for the clad layer were supplied to a coextrusion die, laminated in the die so that the resin melt for the clad layer covered the periphery of the resin melt for the core layer, extruded into a strand shape, water-cooled, cut with a pelletizer so that the mass was about 1.0mg, and dried to obtain multilayer pellets.

The mass ratio of the resin melt for the core layer to the resin melt for the cladding layer at this time was 90/10. In addition, in the production of multilayer particles, zinc borate as a bubble nucleating agent and an antistatic agent (available from Kao corporation; an electrically releasing agent TS-8B (0.9 part by mass of a mixture of a higher alcohol, hydroxyalkyl diethanolamide, and glycerol monostearate) were supplied to an extruder as a resin melt for a core layer. Thus, the base resin contains the above-mentioned additives. Further, as the resin melt for the coating layer, a lubricant (erucamide 1000ppm (0.1 part by mass relative to 100 parts by mass of the base polymer)) was supplied to the extruder. Thus, the base polymer contains additives. Then, the resin melt for the cladding layer and the resin melt for the core layer were coextruded to obtain multilayer particles. At this time, the bubble nucleating agent was supplied as a master batch, and 1000ppm by mass of zinc borate as the bubble nucleating agent was contained only in the base resin of the core layer, while the bubble nucleating agent was not contained in the base polymer of the cladding layer.

50kg of the multilayer particles were charged into a 400L closed vessel together with 280L of water as a dispersion medium, and further, for 100 parts by mass of the multilayer particles, 0.008 part by mass of kaolin as a dispersant, 0.004 part by mass of a surfactant (sodium alkylbenzenesulfonate), and 0.0002 part by mass of aluminum sulfate as a dispersion aid were added to the closed vessel, and carbon dioxide as a foaming agent was added to the closed vessel so that the pressure in the vessel reached 1.5MPa (carbon dioxide pressure), and after heating to 150.6 ℃ (foaming temperature) with stirring and holding at the temperature for 6 minutes, the vessel contents were released to atmospheric pressure, thereby obtaining multilayer foamed particles 1. When resin particles in which the mass ratio of the discharge amounts of the resin melt for the core layer to the resin melt for the cladding layer is 90/10 were foamed, the thickness of the cladding layer in the multilayer foamed particles was 10 μm.

After applying an internal pressure of 0.18MPa (G) to a flat plate molding die 300mm in length by 250mm in width by 60mm in thickness by air, a plurality of layers of expanded beads 1 were packed so that the magnification became 10 times, steam was supplied from both sides of the die for 5 seconds to perform preliminary heating (air-discharging step), one-side heating was performed from one surface side of the die until steam at a pressure 0.08MPa (G) lower than the molding pressure 0.34MPa (G) was reached, one-side heating was performed from the other surface side of the die until steam at a pressure 0.04MPa (G) lower than the molding pressure was reached, and heating was performed until the molding pressure reached 0.34MPa (G) (main heating). After the heating, the pressure was released, and water cooling was performed until the surface pressure due to the foaming power of the molded article reached 0.04MPa (G) (water cooling time 51 seconds), and then the molded article was taken out by opening the mold. The molded article thus obtained was cured in an oven at 80 ℃ for 12 hours to obtain a molded article of expanded beads.

(examples 2 to 5)

Multilayer expanded beads and an expanded bead molded body were obtained in the same manner as in example 1, except that the raw materials and conditions shown in table 1 were changed.

Comparative example 1

Multilayer expanded particles and expanded particle molded articles were obtained in the same manner as in example 1, except that in example 1, the resin melt for the clad layer was not used, but only the resin melt for the core layer was used to obtain single-layer resin particles, and the conditions were changed as shown in table 2.

Comparative example 2

Multilayer expanded beads and expanded bead molded articles were obtained in the same manner as in example 1, except that only a polypropylene resin (propylene/1-butene/ethylene copolymer (FX4ET)) was used as the base polymer for the coating layer in example 1, and the conditions were changed as shown in table 2.

Comparative examples 3 to 5

Expanded beads and an expanded bead molded article were obtained in the same manner as in example 1, except that a mixture of an olefinic thermoplastic elastomer and a polypropylene resin was used as a base polymer for a coating layer in example 1, and the conditions were changed as shown in table 2.

Further, pellets of the olefinic thermoplastic elastomer and the polypropylene resin were placed in an extruder for coating layers at the ratios shown in table 2, and the mixture was supplied to a coextrusion die in the same manner as in example 1. In comparative examples 3 and 5, the bubble nucleating agent was also supplied to the extruder for the resin melt for the cladding layer. The coating layer contained 1000ppm by mass of zinc borate as a bubble nucleating agent.

[ Table 1]

[ Table 2]

[ Table 3]

It is found that the foamed molded article of the example has a specific frictional property and is excellent in gripping properties by forming the coating layer of the foamed particles from the thermoplastic elastomer. It is understood from the above that the foamed molded article of foamed particles of the examples does not cause stick-slip phenomenon in grip test, and thus the appearance is hardly changed by friction, and can be used repeatedly.

Claims

1. A molded article of expanded particles comprising a plurality of layers of expanded particles each having a core layer in an expanded state and a coating layer for coating the core layer, wherein the core layer comprises a polypropylene resin as a base resin,

the coating layer is in a non-foaming state,

the coating layer takes a thermoplastic elastomer as a base polymer,

the foamed molded article has a coefficient of dynamic friction of 1.0 to 3.0,

2. The foamed article according to claim 1, wherein the foamed article is a foamed article,

the base polymer of the coating layer has a coefficient of dynamic friction of 1 or more.

3. The molded article of expanded particles according to claim 1 or 2,

the flexural modulus of elasticity of the base resin of the core layer is 500MPa to 1500 MPa.

4. The foamed article according to claim 1 to 3, wherein the foamed article is a foamed article,

the core layer contains an antistatic agent.

5. The foamed article according to claim 1 to 4, wherein the foamed article is a foamed article,

the foamed molded article has a static friction coefficient of 0.7 to 2.

6. The foamed article according to claim 1 to 5, wherein the foamed article is a foamed article,

the base polymer of the coating layer has a static friction coefficient of 2 or more.

7. The foamed article according to claim 1 to 6, wherein the foamed article is a foamed article,

the thermoplastic elastomer is 1 or more selected from the group consisting of olefinic thermoplastic elastomers, styrenic thermoplastic elastomers and polyurethane thermoplastic elastomers.

8. The foamed article according to claim 1 to 7, wherein the foamed article is a foamed article,

the hardness of the base polymer of the coating layer is 65 to 95.

9. The foamed article according to claim 1 to 8, wherein the foamed article is a foamed article,

the mass ratio of the core layer to the clad layer is not less than 85/15 and not more than 99.5/0.5.

10. The molded article of expanded particles according to any one of claims 1 to 9,

the foamed molded article has a 25% compressive stress of 0.20MPa or more and 0.80MPa or less.

11. A packaging container, characterized in that,

the foamed molded article of claim 1 to 10.