JP7637533B2 - Method for producing expanded polypropylene resin particles, and method for producing in-mold expanded polypropylene resin articles - Google Patents

Description

The present invention relates to a method for producing expanded polypropylene resin particles and a method for producing in-mold expanded polypropylene resin articles.

The in-mold foamed molded product obtained by filling a mold with polypropylene-based resin foam particles and heating and molding with water vapor has the advantages of in-mold foamed molded products, such as the freedom to change the shape, light weight, and heat insulation. Due to these characteristics, in-mold foamed molded products obtained using polypropylene-based resin foam particles have been used for various applications such as heat insulation materials, cushioning packaging materials, automobile interior parts, and core materials for automobile bumpers. In recent years, further studies have been conducted to improve the quality of polypropylene-based resin foam particles used in polypropylene-based resin in-mold foamed molded products. For example, Patent Document 1 proposes that polypropylene-based resin foam particles contain a hindered amine compound in order to improve heat resistance and weather resistance. In addition, since black is preferred when used for automobile interior parts, Patent Document 2 proposes that polypropylene-based resin foam particles contain carbon black as a colorant and a sterically hindered amine ether flame retardant to ensure flame retardancy.

JP 2003-321567 A JP 2018-162370 A

However, when a hindered amine flame retardant is added to polypropylene resin foam particles as described in Patent Documents 1 and 2, there is a problem in that the yield of polypropylene resin foam particles is poor.

In order to solve the above-mentioned problems of the conventional art, the present invention provides a method for producing expanded polypropylene resin particles that can produce expanded polypropylene resin particles in a high yield, and a method for producing in-mold foam molding of polypropylene resin.

In one or more embodiments of the present invention, the present invention relates to a method for producing expanded polypropylene resin particles, the method including: (i) dispersing polypropylene resin, carbon black, and polypropylene resin particles containing a NOR-type hindered amine compound, a peroxide, and a blowing agent in an aqueous dispersant in a sealed container to obtain a dispersion; (ii) heating and pressurizing the dispersion; and (iii) releasing the dispersion after the heating and pressurization into a pressure range lower than the internal pressure of the sealed container.

In one or more embodiments of the present invention, the present invention relates to a method for producing an in-mold foamed polypropylene resin article, which includes a step of in-mold foaming of the polypropylene resin foamed beads obtained by the method for producing the polypropylene resin foamed beads.

According to the method for producing expanded polypropylene resin beads of the present invention, expanded polypropylene resin beads can be obtained with a high yield. In addition, by using the expanded polypropylene resin beads with a high yield obtained by the method for producing expanded polypropylene resin beads of the present invention, it is possible to produce polypropylene resin in-mold foamed articles with high productivity.

FIG. 1 shows an example of a differential scanning calorimeter (DSC) curve obtained when the temperature of expanded polypropylene resin particles is raised from 40° C. to 220° C. at a rate of 10° C./min., and shows the calorific value of melting peak Ql on the low temperature side and the calorific value of melting peak Qh on the high temperature side.

The inventors of the present invention have conducted extensive research to solve the above problems. As a result, they have found that polypropylene resin particles, in which carbon black and a NOR-type hindered amine compound have been added to polypropylene resin, are dispersed in an aqueous dispersant together with peroxide and a blowing agent, and then heated and pressurized to foam, thereby obtaining polypropylene resin expanded particles with a high yield. Peroxides are known to be used to adjust the physical properties of polypropylene resins, such as the molecular weight and melt flow rate, and to modify the surface of polypropylene resins so that they can be molded even at low vapor pressures. Surprisingly, the inventors of the present invention have found that the yield of polypropylene resin expanded particles can be improved by (a) using a NOR-type hindered amine compound as the hindered amine compound, and (b) dispersing polypropylene resin containing a NOR-type hindered amine compound and carbon black in an aqueous dispersion medium in which peroxide is present, and foaming the resin.

(Method for producing expanded polypropylene resin particles)
The polypropylene resin particles (hereinafter, also simply referred to as "expanded particles") contain a polypropylene resin, a NOR type hindered amine compound, and carbon black.

The composition of the polypropylene-based resin is not particularly limited, and may be, for example, a propylene homopolymer or a copolymer of propylene and another α-olefin. The copolymer of propylene and another α-olefin may be an olefin-propylene random copolymer or an olefin-propylene block copolymer. Among these, an olefin-propylene random copolymer is preferred. From the viewpoint of dimensional properties and mechanical strength, the polypropylene-based resin preferably contains 50% by weight or more of a propylene (monomer) component, more preferably 60% by weight or more, more preferably 70% by weight or more, and even more preferably 80% by weight or more.

In polypropylene-based resins, the other α-olefins copolymerizable with propylene are not particularly limited, and examples thereof include α-olefins having 2 or 4 or more carbon atoms. Specific examples include ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3,4-dimethyl-1-butene, 1-heptene, 3-methyl-1-hexene, 1-octene, and 1-decene. These other α-olefins may be used alone or in combination of two or more. Among these, ethylene and/or 1-butene are preferred in terms of availability, economy, and foamability during in-mold foaming. The polypropylene-based resins may be used alone or in combination of two or more.

The polypropylene resin is not particularly limited, but the melting point is preferably 130°C or higher and 165°C or lower, more preferably 135°C or higher and 160°C or lower, and even more preferably 140°C or higher and 155°C or lower. When the melting point of the polypropylene resin is within the above range, it tends to be easier to obtain an in-mold foamed product with an excellent balance of dimensionality, mechanical strength, and surface beauty.

In one or more embodiments of the present invention, the melting point is the peak temperature of the endothermic peak in the DSC curve obtained by using a differential scanning calorimeter (DSC) to heat a resin sample of 1 mg or more and 10 mg or less from 20°C to 220°C at a heating rate of 10°C/min, then cooling from 220°C to 20°C at a heating rate of 10°C/min, and heating again from 20°C to 220°C at a heating rate of 10°C/min.

The melt flow rate (hereinafter sometimes referred to as MFR) of the polypropylene resin is preferably 4.0 g/10 min or more and 10 g/10 min or less, and more preferably 5.0 g/10 min or more and 9 g/10 min or less. When the melt flow rate of the polypropylene resin is within the above-mentioned range, it tends to be easier to obtain an in-mold foamed product with little deformation and excellent surface beauty.

In one or more embodiments of the present invention, the MFR is a value measured using an MFR measuring device described in JIS K 7210 under the conditions of an orifice of 2.0959 ± 0.005 mmφ, an orifice length of 8.000 ± 0.025 mm, a load of 2160 g, and a temperature of 230 ± 0.2°C.

The polypropylene resin particles preferably contain 0.1 to 10 parts by weight of carbon black per 100 parts by weight of polypropylene resin, more preferably 0.5 to 8 parts by weight, and even more preferably 1 to 5 parts by weight. When the carbon black content is within this range, the colorability is good, the viscosity of the resin is less likely to decrease, and the yield of the polypropylene resin foam particles is not reduced, making it easier to obtain in-mold foamed products of good quality.

The carbon black is not particularly limited, but generally, the primary particle diameter is preferably 10 nm or more and 500 nm or less, more preferably 10 nm or more and 100 nm or less from the viewpoint of the balance between the flame retardancy and mechanical strength of the in-mold foamed body, and even more preferably 10 nm or more and 50 nm or less from the viewpoint of surface beauty. The primary particle diameter of the carbon black can be determined by taking a photograph of the cross section of the cell membrane of the polypropylene resin foamed beads at a magnification of 40,000 times with a transmission electron microscope, measuring the particle diameters of the carbon black primary particles in the X direction and Y direction in the obtained transmission electron microscope photograph, and calculating the average value.

但し、一般式（１）中、Ｒ¹およびＲ²は下記一般式（２）で表されるｓ－トリアジン部分、Ｒ³およびＲ⁴の一方は下記一般式（２）で表されるｓ－トリアジン部分、Ｒ³およびＲ⁴の他方は水素原子を表す。 Specifically, the NOR type hindered amine compound may be a compound represented by the following formula (1).

In the general formula (1), ^R1 and ^R2 each represent an s-triazine moiety represented by the following general formula (2), one of ^R3 and ^R4 represents an s-triazine moiety represented by the following general formula (2), and the other of ^R3 and ^R4 represents a hydrogen atom.

但し、一般式（２）中、Ｒ⁵は炭素数が１～１２のアルキル基、Ｒ⁶はメチル基、シクロヘキシル基またはオクチル基を表す。

In the formula (2), R ⁵ represents an alkyl group having 1 to 12 carbon atoms, and R ⁶ represents a methyl group, a cyclohexyl group, or an octyl group.

In general formula (2), specific examples of the alkyl group having 1 to 12 carbon atoms represented by ^R5 include a methyl group, an ethyl group, a propyl group, a butyl group, an n-pentyl group, an n-hexyl group, an n-heptyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, an isopropyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a 2-ethylbutyl group, an isopentyl group, a 1-methylpentyl group, a 1,3-dimethylbutyl group, a 1-methylhexyl group, an isoheptyl group, a 1,1,3,3-tetramethylpentyl group, a 1-methylundecyl group, and a 1,1,3,3,5,5-hexamethylhexyl group.

Specific examples of the s-triazine moiety represented by the general formula (2) include, for example, 2,4-bis[(1-methoxy-2,2,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazine, 2,4-bis[(1-cyclohexyloxy-2,2
,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazine, 2,4-bis[(1-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazine, and the like.

Specific examples of NOR-type hindered amine compounds represented by general formula (1) include, for example, N,N',N'''-tris{2,4-bis[(1-cyclohexyloxy-2,2,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazin-6-yl}-3,3'-ethylenediiminopropylamine; N,N',N''-tris{2,4-bis[(1-cyclohexyloxy-2,2,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazin-6-yl}-3,3'-ethylenediiminodipropylamine; N,N',N''-tris{2,4-bis[(1-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazin-6-yl}-3,3'-ethylenediiminodipropylamine; }-3,3'-ethylenediiminodipropylamine; N,N',N''-tris{2,4-bis[(1-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazin-6yl}-3,3'-ethylenediiminopropylamine; N,N',N'''-tris{2,4-bis[(1-methoxy-2,2,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazin-6yl}-3,3'-ethylenediiminopropylamine; N,N',N''-tris{2,4-bis[(1-methoxy-2,2,6,6-tetramethylpiperidin-4-yl)n-butylamino]-s-triazin-6yl}-3,3'-ethylenediiminopropylamine. These may be used alone or in combination of two or more.

The polypropylene resin particles preferably contain 0.01 to 0.50 parts by weight of a NOR hindered amine compound relative to 100 parts by weight of polypropylene resin, more preferably 0.02 to 0.40 parts by weight, and even more preferably 0.03 to 0.20 parts by weight. When the content of the NOR hindered amine compound is within this range, the heat resistance and weather resistance of the polypropylene resin particles tend to be improved, and the yield of the polypropylene resin particles tends to be improved while a good quality in-mold foamed product is obtained.

The polypropylene-based resin particles may contain a non-NOR type hindered amine compound in addition to the NOR type hindered amine compound, if necessary. From the viewpoint of easily improving the yield of the polypropylene-based resin particles, the polypropylene-based resin particles preferably contain 0.50 parts by weight or less of the NOR type hindered amine compound and the non-NOR type hindered amine compound in total, more preferably 0.40 parts by weight or less, and even more preferably 0.30 parts by weight or less, per 100 parts by weight of the polypropylene-based resin.

The polypropylene resin particles may contain, as necessary, a cell nucleating agent, a hydrophilic compound, an antioxidant, an antistatic agent, a stabilizer (or a flame retardant) other than the hindered amine compound, and the like.

Examples of hydrophilic compounds include compounds containing hydrophilic groups such as carboxyl groups, hydroxyl groups, amino groups, sulfo groups, and polyoxyethylene groups in the molecule, and derivatives thereof, including hydrophilic polymers. Preferred examples of the hydrophilic compounds include glycerin, polyethylene glycol, polypropylene glycol, and melamine. There is a tendency to easily obtain in-mold foamed molded products with excellent surface beauty. One type of hydrophilic compound may be used alone, or two or more types may be used in combination.

The content of the hydrophilic compound in the polypropylene resin particles is preferably 0.01 parts by weight or more and 10 parts by weight or less, more preferably 0.03 parts by weight or more and 5 parts by weight or less, and even more preferably 0.05 parts by weight or more and 1 part by weight or less, relative to 100 parts by weight of the polypropylene resin. When the content of the hydrophilic compound is within the above-mentioned range, it is easy to maintain good surface beauty and mechanical properties of the obtained in-mold foamed molded body while increasing the expansion ratio.

As cell nucleating agents, inorganic nucleating agents such as talc, calcium stearate, calcium carbonate, silica, kaolin, titanium oxide, bentonite, and barium sulfate are commonly used. These may be used alone or in combination of two or more. Among these cell nucleating agents, talc is preferred because it produces uniform cells.

The content of the cell nucleating agent in the polypropylene resin particles may be adjusted as appropriate depending on the desired cell diameter and the type of nucleating agent, but may be 0.001 to 2 parts by weight, or 0.01 to 1 part by weight, of the cell nucleating agent per 100 parts by weight of the polypropylene resin. When the content of the cell nucleating agent is within this range, it is easy to obtain cells that are uniform and have a size suitable for expanded particles.

The weight per particle of the polypropylene-based resin particles is preferably 0.2 mg or more and 10 mg or less, and more preferably 0.5 mg or more and 6.0 mg or less. When the weight per particle of the polypropylene-based resin particles is within the above-mentioned range, it is easy to fill the mold and the shrinkage rate of the obtained in-mold foamed molded body is easy to reduce. The weight per particle of the polypropylene-based resin particles is the average resin particle weight calculated based on the weight of 100 randomly selected polypropylene-based resin particles.

Polypropylene-based resin particles can be prepared, for example, as follows. First, polypropylene-based resin, carbon black, and NOR-type hindered amine-based compound, as well as other additives as necessary, are mixed by a mixing method such as a dry blend method or a master batch method. Next, the resulting mixture (resin composition) is melt-kneaded using an extruder, kneader, Banbury mixer (registered trademark), roll, etc., and then shredded into particles using a cutter, pelletizer, etc.

The polypropylene resin particles obtained above are dispersed in an aqueous dispersant together with a peroxide and a blowing agent in a sealed container, and the resulting dispersion is heated and pressurized. The heated and pressurized dispersion is then released into a pressure range lower than the internal pressure of the sealed container, to obtain expanded polypropylene resin particles.

Specifically, for example, polypropylene resin particles, peroxide, an aqueous dispersion medium, and if necessary, a dispersant are charged in a sealed container, and then the sealed container is evacuated if necessary, and then a foaming agent is introduced into the sealed container, and then the container is heated to a temperature equal to or higher than the softening temperature of the polypropylene resin. The amount of foaming agent added is adjusted so that the pressure in the sealed container increases by heating to about 1.5 MPa (gauge pressure) or more and 5 MPa (gauge pressure). If necessary, after heating, the pressure in the sealed container is adjusted to the desired foaming pressure by adding more foaming agent, and the temperature in the sealed container is held for more than 0 minutes and less than 120 minutes while fine-tuning the temperature to the desired foaming temperature, and then the dispersion liquid (contents in the sealed container) containing polypropylene resin particles impregnated with the foaming agent is released into a pressure range (usually atmospheric pressure) lower than the internal pressure of the sealed container to obtain polypropylene resin foamed particles. The foaming pressure is not particularly limited, but is preferably, for example, 1.5 MPa (gauge pressure) or more and 5 MPa (gauge pressure). In one or more embodiments of the present invention, "above the softening temperature of the polypropylene-based resin" means at least 10°C below the melting point of the polypropylene-based resin. The foaming temperature is not particularly limited as long as it is at least the softening temperature of the polypropylene-based resin, and is preferably, for example, at least 10°C below the melting point of the polypropylene-based resin and at most 10°C above the melting point.

To adjust the expansion ratio, the temperature of the atmosphere from which the particles are released may be adjusted to between room temperature and approximately 110°C. To obtain expanded particles with a particularly high expansion ratio, it is desirable to set the temperature of the atmosphere from which the particles are released to approximately 100°C using steam or the like.

The method of introducing the blowing agent may be a method other than the above. For example, polypropylene resin particles, peroxide, aqueous dispersion medium, and if necessary, dispersant, etc. may be charged into a sealed container, and if necessary, the sealed container may be evacuated, and the blowing agent may be introduced while heating to a temperature equal to or higher than the softening temperature of the polypropylene resin. As another method of introducing the blowing agent, polypropylene resin particles, peroxide, aqueous dispersion medium, and if necessary, dispersant, etc. may be charged into a sealed container, and then heated to near the foaming temperature, and the blowing agent may be introduced at this point.

Methods for adjusting the expansion ratio and average bubble diameter of polypropylene resin expanded particles include, for example, injecting carbon dioxide, nitrogen, air, or a substance used as a blowing agent into a sealed container before releasing into a low pressure area to increase the internal pressure in the sealed container and adjust the pressure release speed during expansion, and further, controlling the pressure by introducing carbon dioxide, nitrogen, air, or a substance used as a blowing agent into the sealed container even during release into a low pressure area, thereby adjusting the expansion ratio and average bubble diameter. It is also possible to adjust the expansion ratio and average bubble diameter by appropriately changing the temperature inside the sealed container before releasing into a low pressure area (roughly the expansion temperature).

Examples of blowing agents include saturated hydrocarbons such as propane, butane, and pentane; ethers such as dimethyl ether; alcohols such as methanol and ethanol; inorganic gases such as air, nitrogen, and carbon dioxide; and water. These may be used alone or in combination of two or more.

Among these, it is preferable that the blowing agent contains an inorganic gas such as carbon dioxide, nitrogen, or air, since this has a particularly small environmental impact and is not flammable, and it is even more preferable that the blowing agent contains carbon dioxide, since this makes it easier to obtain expanded particles with a relatively high expansion ratio.

The amount of foaming agent added varies depending on the type of foaming agent and the desired expansion ratio of the polypropylene resin foam particles, and may be adjusted as appropriate, but may be, for example, 1 part by weight or more and 20 parts by weight or less per 100 parts by weight of polypropylene resin particles.

Although the reason is unclear, the yield of polypropylene resin foam particles is improved by dispersing polypropylene resin containing a NOR-type hindered amine compound and carbon black in an aqueous dispersion medium containing peroxide and foaming it.

Examples of peroxides that can be used include peroxydicarbonates, peroxyesters, peroxyketals, diacyl peroxides, and dialkyl peroxides.

Peroxydicarbonates are not particularly limited, but examples that can be suitably used include di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, bis(4-t-butylcyclohexyl) peroxydicarbonate, di-2-ethoxyethyl peroxydicarbonate, di(2-ethylhexylperoxy)dicarbonate, dimethoxybutyl peroxydicarbonate, and di(3-methyl-3-methoxybutylperoxy)dicarbonate.

The peroxy esters are not particularly limited, but examples thereof include cumyl peroxy neodecanoate, t-butyl peroxy neodecanoate, 1-cyclohexyl-1-methylethyl peroxy neodecanoate, 1,1,3,3-tetramethylbutyl peroxy neodecanoate, t-hexyl peroxy neodecanoate, t-hexyl peroxy pivalate, t-butyl peroxy pivalate, t-butyl peroxy-3,5,5-trimethylhexanoate, 1-cyclohexyl-1-methylethyl peroxy-2-ethylhexanoate, t -Hexylperoxy-2-ethylhexanoate, t-butylperoxy-2-ethylhexanoate, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, t-butylperoxylaurate, t-hexylperoxybenzoate, t-butylperoxyisobutyrate, t-hexylperoxyisopropyl monocarbonate, t-butylperoxyisopropyl monocarbonate, t-butylperoxy-2-ethylhexyl monocarbonate, and t-butylperoxymaleic acid can be preferably used.

Peroxyketals are not particularly limited, but examples that can be suitably used include 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, 1,1-bis(t-butylperoxy)cyclododecane, 1,1-bis(t-hexylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-hexylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, and 1,1-di-t-butylperoxy-2-methylcyclohexane.

Diacyl peroxides are not particularly limited, but examples that can be suitably used include isobutyl peroxide, 2,4-dichlorobenzoyl peroxide, 3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, lauroyl peroxide, stearoyl peroxide, succinic peroxide, m-toluoylbenzoyl peroxide, and benzoyl peroxide.

The dialkyl peroxides are not particularly limited, but examples that can be suitably used include α,α'-bis(neodecanoylperoxy)diisopropylbenzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, 2,5-dimethyl-2,5-di(m-toluoylperoxy)hexane, and 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane.

The organic peroxides described above may be used alone or in combination of two or more. Among them, from the viewpoint of further increasing the yield of polypropylene-based resin expanded particles, it is preferable to use one or more selected from the group consisting of peroxydicarbonates and peroxyesters, and it is more preferable to use one or more selected from the group consisting of bis(4-t-butylcyclohexyl)peroxydicarbonate, t-butylperoxy-2-ethylhexyl monocarbonate, t-butylperoxyisopropyl monocarbonate, and t-hexylperoxyisopropyl monocarbonate.

The sealed container is not particularly limited, and may be one that can withstand the pressure and temperature inside the container during the production of expanded beads, such as an autoclave-type pressure-resistant container.

It is preferable to use only water as the aqueous dispersion medium, but it is also possible to use a dispersion medium in which methanol, ethanol, ethylene glycol, glycerin, etc. have been added to water. When the polypropylene resin particles contain a hydrophilic compound, the water in the aqueous dispersion medium also acts as a foaming agent, contributing to an improvement in the foaming ratio.

A dispersant may be added to the aqueous dispersion medium to prevent adhesion between polypropylene resin particles.

Examples of dispersants include inorganic dispersants such as tribasic calcium phosphate, tribasic magnesium phosphate, basic magnesium carbonate, calcium carbonate, barium sulfate, kaolin, talc, and clay. These dispersants may be used alone or in combination of two or more.

In the method for producing polypropylene-based expanded beads according to one or more embodiments of the present invention, a dispersing aid may be used together with the dispersing agent.

Examples of the dispersion aid include anionic surfactants such as carboxylates, such as N-acylamino acid salts, alkyl ether carboxylates, and acylated peptides; sulfonate types, such as alkyl sulfonates, n-paraffin sulfonates, alkylbenzene sulfonates, alkylnaphthalene sulfonates, and sulfosuccinates; sulfate ester types, such as sulfated oils, alkyl sulfates, alkyl ether sulfates, alkylamide sulfates, and alkyl allyl ether sulfates; and phosphate ester types, such as alkyl phosphates and polyoxyethylene phosphates. In addition, polycarboxylate-type polymer surfactants, such as maleic acid copolymer salts and polyacrylates, and polyanionic polymer surfactants, such as polystyrene sulfonates and naphthalsulfonic acid formalin condensate salts, can also be used as dispersion aids. These dispersion aids may be used alone or in combination of two or more.

The amount of dispersant and dispersion aid used can be appropriately determined depending on the type and the type and amount of polypropylene resin particles used. For example, 0.1 to 3 parts by weight of dispersant and 0.001 to 0.1 parts by weight of dispersion aid may be mixed with 100 parts by weight of aqueous dispersion medium.

In order to improve dispersibility in the aqueous dispersion medium, the polypropylene resin particles may be used in an amount of, for example, 20 parts by weight or more and 100 parts by weight or less per 100 parts by weight of the aqueous dispersion medium.

In one or more embodiments of the present invention, the yield of expanded polypropylene resin particles calculated using the following formula (1) based on the weight W0 of the polypropylene resin particles charged in a sealed container and the weight W1 of the polypropylene resin particles remaining in the sealed container after expansion is preferably 99.0% or more, and more preferably 99.5% or more.
Yield (%) of polypropylene resin expanded particles = (W0 - W1) / W0 x 100

The expansion process for obtaining expanded polypropylene resin particles from polypropylene resin particles as described above is sometimes called the "first-stage expansion process," and the expanded polypropylene resin particles obtained in this manner are sometimes called "first-stage expanded particles."

If necessary, the first-stage expanded particles can be impregnated with an inorganic gas (e.g., air, nitrogen, carbon dioxide, etc.) to apply internal pressure, and then contacted with water vapor at a specific pressure to obtain polypropylene-based resin expanded particles with an improved expansion ratio compared to the first-stage expanded particles. This expansion process in which the polypropylene-based resin expanded particles are further expanded to obtain polypropylene-based resin expanded particles with a higher expansion ratio is sometimes called a "second-stage expansion process", and the polypropylene-based resin expanded particles obtained through such a second-stage expansion process are sometimes called "second-stage expanded particles".

It is desirable to change the internal pressure of the inorganic gas impregnated into the first-stage expanded particles as appropriate, taking into consideration the expansion ratio of the second-stage expanded particles, and it may be, for example, 0.12 MPa or more (absolute pressure) to 0.6 MPa or less (absolute pressure).

The pressure of the water vapor in the second-stage expansion process may be adjusted within the range of 0.02 MPa (gauge pressure) to 0.25 MPa (gauge pressure), or within the range of 0.03 MPa (gauge pressure) to 0.15 MPa (gauge pressure), taking into account the expansion ratio of the second-stage expanded particles.

The polypropylene resin foam particles may have two melting peaks in a DSC curve obtained when the temperature is increased from 40°C to 220°C at a heating rate of 10°C/min in a differential scanning calorimeter measurement.

The polypropylene resin foam particles preferably have a DSC ratio of 10% or more and 50% or less, and more preferably 15% or more and 30% or less. When the DSC ratio is within this range, it is easy to obtain a polypropylene resin in-mold foamed article with excellent surface beauty.

In one or more embodiments of the present invention, the DSC ratio can be calculated by [Qh/(Ql+Qh)×100], where Ql is the melting peak area on the low-temperature side of the DSC curve obtained when heating 5-6 mg of polypropylene resin expanded particles from 40° C. to 220° C. at a heating rate of 10° C./min, and Qh is the melting peak area on the high-temperature side of the DSC curve obtained during the first heating. Strictly speaking, as shown in FIG. 1, the melting peak heat quantity on the low-temperature side, which is the heat quantity surrounded by the melting peak on the low-temperature side and the tangent line from the maximum point between the melting peak on the low-temperature side and the melting peak on the high-temperature side to the melting start baseline, is Ql, and the melting peak heat quantity on the high-temperature side, which is the heat quantity surrounded by the melting peak on the high-temperature side and the tangent line from the maximum point between the melting peak on the low-temperature side and the melting peak on the high-temperature side to the melting end baseline, is Qh.

Since the DSC ratio varies depending on the temperature and pressure during expansion when producing polypropylene resin expanded beads, it is possible to obtain expanded beads with the desired DSC ratio by appropriately adjusting the expansion temperature and pressure. In general, the DSC ratio tends to decrease when the expansion temperature and pressure are increased, and although this also depends on the type of polypropylene resin, additives, and type of blowing agent, specifically, the DSC ratio decreases by about 5 to 20% when the expansion temperature is increased by 1°C, and decreases by about 0.5 to 5% when the expansion pressure is increased by 0.1 MPa.

The expansion ratio of the polypropylene-based resin expanded particles is not particularly limited and may be adjusted as necessary. In one or more embodiments of the present invention, the expansion ratio of the polypropylene-based resin expanded particles is preferably 3 to 40 times, more preferably 3 to 25 times.

In one or more embodiments of the present invention, the expansion ratio of the polypropylene-based resin foamed particles is determined by measuring the weight w (g) of the polypropylene-based resin foamed particles, submerging them in a measuring cylinder containing ethanol, measuring the volume v ( ^cm3 ) based on the rise in the water level in the measuring cylinder (submersion method), calculating the true specific gravity ρb = w/v of the polypropylene-based resin foamed particles, and then calculating the ratio (ρr/ρb) to the density ρr of the polypropylene-based resin particles before expansion.

(Method for producing in-mold polypropylene resin foamed article)
A polypropylene resin in-mold foamed molded article (hereinafter, simply referred to as "in-mold foamed molded article") can be obtained by in-mold foaming of polypropylene resin expanded beads. Specifically, the polypropylene resin expanded beads are filled into a mold of a predetermined shape, heated with steam or the like, and the expanded beads are heat-fused to each other to form a mold, and then cooled with a refrigerant such as water and then removed to obtain a polypropylene resin in-mold foamed molded article. As the mold, a mold that can be closed but cannot be sealed can be used. By using the polypropylene resin expanded beads with a high yield obtained by the method for producing polypropylene resin expanded beads of the present invention, a polypropylene resin in-mold foamed molded article can be produced with high productivity.

When polypropylene resin foamed beads are foamed in a mold after applying a pressure equal to or greater than atmospheric pressure to the inside of the foamed beads, it is easy to obtain a polypropylene resin foamed in-mold product with no gaps, a beautiful surface, and little deformation. There are no particular limitations on the method for applying a pressure equal to or greater than atmospheric pressure to the inside of the foamed beads, but pressure can be applied to the inside of the foamed beads by conventional methods such as the internal pressure application method or the compression filling method.

In the internal pressure application method, polypropylene resin foam particles are held under pressure with an inorganic gas in advance to apply an internal pressure equal to or greater than atmospheric pressure to the foam particles, and the foam particles to which internal pressure has been applied are filled into a molding space such as a mold that can be closed but not sealed. The applied internal pressure is preferably 0.12 MPa (absolute pressure) to 0.40 MPa (absolute pressure), and more preferably 0.14 MPa (absolute pressure) to 0.30 MPa (absolute pressure). When the internal pressure of the polypropylene resin foam particles is within this range, it tends to be easier to obtain an in-mold foamed molded product with a beautiful appearance.

Inorganic gases used to apply internal pressure include air, nitrogen, helium, neon, argon, and carbon dioxide. These gases may be used alone or in combination of two or more. Of these, air and/or nitrogen are preferred because of their versatility.

In the compression-filling method, polypropylene resin foamed particles are compressed in a compression tank using pressurized gas, preferably to a bulk density of 1.25 to 3 times the bulk density of the foamed particles before filling, more preferably to a bulk density of 1.5 to 2.2 times the bulk density of the foamed particles before filling, and the compressed foamed particles are filled into a molding space such as a mold that can be closed but not airtight. When the compression ratio is within this range, it tends to be easier to obtain an in-mold foamed molded product with a beautiful appearance.

Pressurized gases used for compression include air, nitrogen, helium, neon, argon, carbon dioxide, etc. These gases may be used alone or in combination of two or more. Of these, air and/or nitrogen are preferred due to their versatility.

After filling a mold or the like with polypropylene resin foam particles, they are heated at a water vapor pressure of 0.15 MPa (gauge pressure) to 0.4 MPa (gauge pressure) using water vapor as a heating medium, and the polypropylene resin foam particles are fused together. The mold is then cooled with water, opened, and the polypropylene resin foamed molded body is removed.

Polypropylene resin in-mold foamed products can be used for a variety of purposes, including, but not limited to, insulation materials, cushioning packaging materials, automotive interior components, and core materials for automotive bumpers.

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

（化学式（３）中、Ｒは化学式（４）で表される）

非ＮＯＲ型ヒンダートアミン系化合物：コハク酸ジメチルおよび４－ヒドロキシ－２，２，６，６－テトラメチル－１－ピペリジンエタノールの縮合物［ＢＡＳＦ製、ＴＩＮＵＶＩＮ ６２２ ＳＦ］
＜有機過酸化物＞
有機過酸化物１：ビス（４－ｔ－ブチルシクロヘキシル）パーオキシジカーボネート［日本油脂（株）製、パーロイルＴＣＰ］
有機過酸化物２：ｔ－ブチルパーオキシ－２－エチルヘキシルモノカーボネート［日本油脂（株）製、パーブチルＥ］ The substances used in the examples and comparative examples are as follows, but were used without any particular purification.
<Polypropylene resin>
Polypropylene resin 1: Ethylene-propylene random copolymer [ethylene content 3.1% by weight, MFR = 7.2, melting point 150°C]
Polypropylene resin 2: Ethylene-propylene random copolymer [ethylene content 3.5% by weight, MFR = 8.4, melting point 145°C]
<Additives>
Carbon black [primary particle size 35 nm]
Talc [Talc Powder PK-S, manufactured by Hayashi Kasei Co., Ltd.]
Glycerin [Lion Corporation, Refined Glycerin D]
NOR-type hindered amine compound: hindered amine light stabilizer [Flamestab NOR116FF, manufactured by BASF; chemical formula (3)]

(In chemical formula (3), R is represented by chemical formula (4))

Non-NOR type hindered amine compound: Condensate of dimethyl succinate and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol [manufactured by BASF, TINUVIN 622 SF]
<Organic Peroxide>
Organic peroxide 1: bis(4-t-butylcyclohexyl) peroxydicarbonate [Perloyl TCP, manufactured by Nippon Oil & Fats Co., Ltd.]
Organic peroxide 2: t-butylperoxy-2-ethylhexyl monocarbonate [Perbutyl E, manufactured by Nippon Oil & Fats Co., Ltd.]

The evaluation methods used in the examples and comparative examples are described below.

<DSC Ratio Measurement>
The DSC ratio was measured using a differential scanning calorimeter [DSC7020, Hitachi High-Tech Science Co., Ltd.]. Specifically, the DSC ratio was calculated by [Qh/(Ql+Qh)×100], where the melting peak area on the low-temperature side of the DSC curve during the first heating step obtained when heating 5-6 mg of polypropylene resin foamed particles from 40° C. to 220° C. at a heating rate of 10° C./min was taken as Ql and the melting peak area on the high-temperature side was taken as Qh. Strictly speaking, as shown in FIG. 1, in the DSC curve, the melting peak heat quantity on the low-temperature side, which is the heat quantity surrounded by the melting peak on the low-temperature side and the tangent line from the maximum point between the melting peak on the low-temperature side and the melting peak on the high-temperature side to the melting start baseline, was taken as Ql, and the melting peak heat quantity on the high-temperature side, which is the heat quantity surrounded by the melting peak on the high-temperature side and the tangent line from the maximum point between the melting peak on the low-temperature side and the melting peak on the high-temperature side to the melting end baseline, was taken as Qh.

<Measurement of Expansion Ratio of Expanded Polypropylene Resin Beads>
Approximately 3 g to 10 g of expanded polypropylene resin particles were taken, dried at 60° C. for 6 hours, and then conditioned indoors at 23° C. and 50% humidity, and the weight w1 (g) was measured, then submerged in a measuring cylinder containing ethanol, and the volume v (cm ³ ) was measured by measuring the rise in the water level in the measuring cylinder (submersion method), and the true specific gravity ρb=w1/v of the expanded polypropylene resin particles was calculated, and further calculated as the ratio (ρr/ρb) to the density ρr of the polypropylene resin particles before foaming. In the examples and comparative examples shown below, the density ρr of the polypropylene resin particles before foaming was 0.9 g/cm ³ in all cases.

<Yield>
After obtaining expanded polypropylene resin particles in the below-described [Preparation of expanded polypropylene resin particles], the weight W1 of the unexpanded propylene resin particles remaining in the 10 L pressure-resistant autoclave was measured, and the weight of the charged propylene resin particles (1800 g) was taken as W0, and the yield was calculated from [(W0-W1) x 100/W0]. The yield was evaluated according to the following criteria.
Good: Yield is 99.0% or more. Bad: Yield is less than 99.0%.

<Molded body density>
The weight of the obtained in-mold foamed polypropylene resin article was divided by the volume and converted into units of g/L.

Example 1
[Method of producing polypropylene-based resin particles]
The polypropylene resin 1 was 100 parts by weight, and 3 parts by weight of carbon black, 0.2 parts by weight of glycerin, 0.05 parts by weight of talc, and 0.05 parts by weight of a NOR-type hindered amine compound were weighed out and dry-blended using a blender. The mixture obtained by dry-blending was melt-kneaded at a resin temperature of 220°C using a twin-screw extruder [TEM26-SX, manufactured by Toshiba Machine Co., Ltd.], and the extruded strand was water-cooled in a 2 m-long water tank and then cut to produce polypropylene resin particles (1.2 mg/grain).

[Preparation of polypropylene resin foam particles]
In a 10L pressure-resistant autoclave, 100 parts by weight (1.8 kg) of the polypropylene-based resin particles obtained above, 300 parts by weight of water, 0.3 parts by weight of kaolin [ASP170, manufactured by BASF] as a poorly water-soluble inorganic compound, 0.06 parts by weight of sodium dodecylbenzenesulfonate [Neopelex G-15, manufactured by Kao Corporation] as a surfactant, and 1.0 parts by weight of organic peroxide 1 were charged, and then 5 parts by weight of carbon dioxide was added as a foaming agent under stirring. The contents of the autoclave were heated to a foaming temperature of 156.0°C and held for 10 minutes, and then carbon dioxide was additionally injected to raise the pressure inside the autoclave to a foaming pressure of 2.3 MPa. After holding for 20 minutes at the foaming temperature and foaming pressure, the valve at the bottom of the autoclave was opened, and the mixture was released under atmospheric pressure through an opening orifice with a diameter of 3.6 mm to obtain polypropylene-based resin foamed particles with an expansion ratio of 12 times. During the release, carbon dioxide was injected to maintain the pressure in the container so that the pressure would not decrease.

[Preparation of polypropylene resin in-mold foamed article]
The obtained expanded beads were dried at 75°C. The dried expanded beads were put into a pressure-resistant container, impregnated with pressurized air, and the polypropylene resin expanded beads, which had been adjusted in advance to have an internal pressure of 0.20 MPa (absolute pressure), were filled into a mold having a length of 370 mm, a width of 320 mm, and a thickness of 50 mm. The interior of the mold chamber was then heated with steam at a predetermined pressure to fuse the expanded beads together. After cooling the interior of the mold and the surface of the molded body with water, the molded body was removed to obtain a polypropylene resin in-mold expanded molded body. This operation was performed by changing the steam pressure from 0.20 MPa (gauge pressure) to 0.35 MPa in increments of 0.01 MPa, and the minimum pressure required for sufficient fusion of the polypropylene resin expanded beads together was determined and used as the lower limit molding pressure. The obtained in-mold expanded molded body was left to stand at 23°C for 2 hours, and then aged at 75°C for 16 hours.

(Example 2, Comparative Examples 1 to 4)
Polypropylene-based resin particles, polypropylene-based resin expanded particles, and polypropylene-based resin in-mold foamed articles were prepared in the same manner as in Example 1, except that in [Preparation of polypropylene-based resin particles], the type of additives and the resin formulation were changed as shown in Table 1, and in [Preparation of polypropylene-based resin expanded particles], the foaming conditions were changed as shown in Table 1. The evaluation results of the obtained polypropylene-based resin expanded particles and polypropylene-based resin in-mold foamed articles are shown in Table 1.

From Table 1 above, it can be seen that in Examples 1 and 2, the yield of polypropylene-based resin expanded particles is good. In Comparative Examples 1 and 2, although polypropylene-based resin particles containing a NOR-type hindered amine compound and carbon black are used, the yield of polypropylene-based resin expanded particles is poor because no peroxide is present in the aqueous dispersion medium. In Comparative Example 3, even if polypropylene-based resin particles are dispersed in an aqueous dispersion medium containing peroxide and expanded, the yield of polypropylene-based resin expanded particles is poor because the polypropylene-based resin particles do not contain a NOR-type hindered amine compound. In Comparative Example 4, even if polypropylene-based resin particles containing a hindered amine compound are used and expanded in an aqueous dispersion medium containing peroxide, the yield of polypropylene-based resin expanded particles is poor because the polypropylene resin does not contain a NOR-type hindered amine compound.

Claims (8)

A method for producing expanded polypropylene resin particles, comprising the steps of:
(i) a step of dispersing a polypropylene-based resin, carbon black, polypropylene-based resin particles containing a NOR-type hindered amine-based compound, a peroxide, and a blowing agent in an aqueous dispersion medium in a sealed container to obtain a dispersion;
(ii) heating and pressurizing the dispersion; and
(iii) discharging the dispersion liquid after the heating and pressurization into a pressure region lower than the internal pressure of the sealed container ;
The polypropylene-based resin particles contain 100 parts by weight of a polypropylene-based resin, 0.1 to 10 parts by weight of carbon black, and 0.01 to 0.50 parts by weight of a NOR-type hindered amine compound . The method for producing expanded polypropylene resin particles according to claim 1, wherein the polypropylene resin particles contain 100 parts by weight of polypropylene resin, 1 part by weight or more and 5 parts by weight or less of carbon black, and 0.05 parts by weight or more and 0.20 parts by weight or less of a NOR-type hindered amine compound. The method for producing expanded polypropylene resin particles according to claim 1 or 2, wherein the amount of the peroxide added is 0.01 parts by weight or more and 10 parts by weight or less per 100 parts by weight of the polypropylene resin. The NOR type hindered amine compound is a compound represented by the following general formula (1):

In the general formula (1), R ¹ and R ² each represent an s-triazine moiety represented by the following general formula (2), one of R ³ and R ⁴ represents an s-triazine moiety represented by the following general formula (2), and the other of R ³ and R ⁴ represents a hydrogen atom:

The method for producing expanded polypropylene resin beads according to any one of claims 1 to 3, wherein in general formula (2), R ⁵ represents an alkyl group having 1 to 12 carbon atoms, and R ⁶ represents a methyl group, a cyclohexyl group, or an octyl group. The method for producing expanded polypropylene resin particles according to any one of claims 1 to 4, wherein the peroxide is one or more organic peroxides selected from the group consisting of peroxyesters and peroxydicarbonates. The method for producing expanded polypropylene resin particles according to any one of claims 1 to 5, wherein the foaming agent contains carbon dioxide. The method for producing expanded polypropylene-based resin beads according to any one of claims 1 to 6, wherein a yield of expanded polypropylene-based resin beads calculated by the following formula (1) based on a weight W0 of the polypropylene-based resin particles charged into the sealed container in the step (i) and a weight W1 of the polypropylene-based resin particles remaining in the sealed container after the step (iii) is 99.0% or more.
[Formula 1]
Yield (%) of polypropylene resin expanded particles = (W0 - W1) / W0 x 100 A method for producing an in-mold polypropylene resin foamed article, comprising a step of in-mold foaming the polypropylene resin foamed beads obtained by the method for producing the polypropylene resin foamed beads according to any one of claims 1 to 7.

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