JP5851850B2 - Returnable box - Google Patents

Description

  The present invention relates to a returnable box formed by in-mold foam molding of polypropylene resin expanded particles.

  In-mold foam molded articles obtained using thermoplastic resin foam particles have features such as shape flexibility, cushioning, light weight, and heat insulation, which are the advantages of in-mold foam molded bodies. It is used for various applications such as materials, heat insulating materials, and automobile parts.

  As a box, it is used as a food box for transporting fish, vegetables and other foods, a tool box for transporting tools, a luggage box mounted on an automobile, and the like.

  Such a box includes a box used as a so-called “return box” used for many times, in addition to a box used for one-way transportation (only one transportation). In the case of a returnable box, since it is used multiple times, it is disliked that dirt or mold adheres to it, and if dirt or mold adheres, it must be used for transportation after washing it There is. In particular, in the case of a food transport box for transporting food, it is particularly hateful for dirt and mold, and hygiene is required.

  By the way, a fish box, a vegetable box, etc. formed by foaming polystyrene resin foam particles in a mold are well known, but there are strength problems such as being easy to break if dropped during transportation, and easy to dent due to collision or pressure. For this reason, it cannot be transported and used repeatedly over a long period of time, and is mainly used for one-way use.

  On the other hand, a box formed by in-mold foam molding of polypropylene resin expanded particles is excellent in impact resistance and durability, so it is also marketed as a returnable box that is used multiple times instead of one-way. It is used as a food return box for transportation.

However, a box formed by in-mold foam molding of polypropylene resin foam particles has a conspicuous dent on the surface of the box (a dent / granular gap that occurs when the polypropylene resin foam particles are not completely fused together). There is a problem that dirt easily adheres to the dent, and once the dirt adheres, the dirt strongly sticks to the dent and is difficult to clean.
Particularly in food return boxes, mold may be generated due to spills and scraps of food to be transported and contents spilled from lunch boxes, and it is necessary to thoroughly clean from a hygienic point of view. However, if dirt or mold adheres to the dents on the inner surface of the food return box, it cannot be easily cleaned.

  In order to deal with such problems, methods to cover the surface of the foam with a film or to provide a hardened layer by melting the surface of the foam as a foam that is difficult to clean or easy to clean even when dirty are proposed. However, in these methods, the manufacturing method is complicated and the cost is increased (for example, Patent Documents 1 to 3).

JP 2000-355379 A JP 2006-206167 A JP 2008-012671 A

  An object of the present invention is to provide a returnable box, in particular, a returnable box for foods, which is formed by in-mold foam molding of polypropylene resin foamed particles and can easily remove dirt and mold.

  As a result of diligent research in view of the above problems, the present invention has found that by using specific polypropylene resin foamed particles, it becomes a returnable box that can be easily cleaned of dirt and mold, and has completed the present invention. is there.

That is, this invention consists of the following requirements.
[1] A returnable box formed by in-mold foam molding of polypropylene resin expanded particles,
Polypropylene resin foam particles
A polypropylene-based random copolymer containing 1-butene and / or ethylene as a comonomer and having a melting point of 125 ° C. or higher and 155 ° C. or lower is used as a base resin,
A returnable box satisfying the following conditions (a) and (b):
(A) In a DSC curve obtained by differential scanning calorimetry (DSC) in which the temperature is raised from 40 ° C. to 220 ° C. at a rate of temperature increase of 10 ° C./min, the low-temperature side heat of fusion (Ql) region and the high-temperature side heat of fusion ( Qh) has two regions, and the ratio {Qh / (Ql + Qh)} × 100) (%) of the high temperature side heat of fusion is 10% or more and 30% or less.
(B) A maximum value between 105 ° C. and 120 ° C. in the differential curve of the DSC curve obtained by differential scanning calorimetry (DSC) in which the temperature is raised from 40 ° C. to 220 ° C. at a temperature rising rate of 20 ° C./min. Have.
[2] The returnable box according to [1], wherein the returnable box is a food returnable box.
Moreover, this invention relates also to the manufacturing method which consists of the following requirements.
[3] A method for manufacturing a returnable box in which a polypropylene resin foamed particle is filled in a mold and then heated to obtain a foamed molded product in a polypropylene resin mold,
Polypropylene resin foam particles
Polypropylene resin particles containing butene-1 and / or ethylene as a comonomer and having a melting point of from 125 ° C. to 155 ° C. as a base resin were accommodated in a pressure vessel together with water and an inorganic gas. rear,
Disperse under stirring conditions, raise the temperature above the softening point temperature of the polypropylene resin particles, and then discharge the dispersion in the pressure vessel to a pressure range lower than the internal pressure of the pressure vessel to expand the polypropylene resin particles. Obtained by a foaming process including at least a one-stage foaming process,
And satisfying the following conditions (a) and (b):
How to make a returnable box.
(A) In a DSC curve obtained by differential scanning calorimetry (DSC) in which the temperature is raised from 40 ° C. to 220 ° C. at a rate of temperature increase of 10 ° C./min, the low-temperature side heat of fusion (Ql) region and the high-temperature side heat of fusion ( Qh) has two regions, and the ratio {Qh / (Ql + Qh)} × 100) (%) of the high temperature side heat of fusion is 10% or more and 30% or less.
(B) In the differential curve of the DSC curve obtained by differential scanning calorimetry (DSC) where the temperature is raised from 40 ° C. to 220 ° C. at a rate of temperature increase of 20 ° C./min, the maximum value is between 105 ° C. and 120 ° C. Have
[4] The method for producing a returnable box according to [3], further including a two-stage foaming step for further increasing the expansion ratio of the expanded polypropylene resin particles obtained in the one-stage foaming step.
[5] The method for producing a returnable box according to [3] or [4], wherein the one-stage foaming step releases the dispersion into a pressure region heated with heated steam.

  The returnable box formed by in-mold foam molding of the polypropylene resin expanded particles of the present invention is superior in terms of hygiene, in addition to the impact resistance and durability that are the characteristics of conventional returnable boxes, as well as easy cleaning of attached dirt and mold. It becomes a returnable box. The returnable box of the present invention is particularly suitable as a food returnable box.

In order to determine the melting point of the polypropylene resin, in differential scanning calorimetry (DSC), the temperature of the polypropylene resin particles is increased from 40 ° C. to 220 ° C. at a temperature increase rate of 10 ° C./min, and then 10 ° C./min. The temperature is decreased from 220 ° C. to 40 ° C. at a temperature decrease rate of 2 ° C., and a second temperature increase operation is performed from 40 ° C. to 220 ° C. at a temperature increase rate of 10 ° C./min. It is an example. It is an example of the DSC curve obtained from differential scanning calorimetry (DSC) which heats up from 40 degreeC to 220 degreeC with the temperature increase rate of 10 degree-C / min of the polypropylene resin expanded particle of this invention. The horizontal axis is the temperature, and the vertical axis is the endothermic amount. The portion surrounded by the dashed line of the low-temperature side melting calorie peak and the line segment AD and line segment CD is Ql, and the portion surrounded by the broken line of the high-temperature side melting calorie peak, line segment BD and line segment CD is Qh. It is an example of the differential curve of the DSC curve obtained from the differential scanning calorimetry (DSC) which heats up from 40 degreeC to 220 degreeC with the temperature increase rate of 20 degree-C / min of the polypropylene resin expanded particle of this invention. Curve A relates to the expanded polypropylene resin particles of the present invention, and curve B relates to the conventional expanded polypropylene resin particles. It is an example which shows the main body of the returnable box of this invention (a lid is not shown).

  The polypropylene resin used in the present invention is a polypropylene random copolymer containing 1-butene and / or ethylene as a comonomer.

However, the polypropylene resin used in the present invention may contain a comonomer other than 1-butene and ethylene.
Examples of such comonomers include isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3,4-dimethyl-1-butene, 1-heptene, 3 Examples include α-olefins such as -methyl-1-hexene, 1-octene, and 1-decene. Furthermore, cyclic olefins such as cyclopentene, norbornene, tetracyclo [6,2,1 ^1,8 , 1 ^3,6 ] -4-dodecene, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 1,4 -Dienes such as hexadiene, methyl-1,4-hexadiene, and 7-methyl-1,6-octadiene.

  In the present invention, among polypropylene random copolymers, propylene / ethylene / 1-butene random copolymers or propylene / ethylene random copolymers are preferable from the viewpoint of good foamability.

Further, in these propylene / ethylene / 1-butene random copolymers and propylene / ethylene random copolymers, the structural unit consisting of propylene is 90% by weight or more and 99.8% by weight or less in 100% by weight of the polypropylene resin. The structural unit composed of 1-butene and / or ethylene is preferably 0.2% by weight or more and 10% by weight or less, the structural unit composed of propylene is 92% by weight or more and 99% by weight or less, 1-butene and / or ethylene. It is more preferable that the structural unit consisting of 1 to 8% by weight.
When the structural unit composed of propylene exceeds 99.8% by weight and the structural unit composed of 1-butene and / or ethylene is less than 0.2% by weight, the molding heating steam pressure tends to be high during in-mold foam molding. is there. When the structural unit composed of propylene is less than 90% by weight and the structural unit composed of 1-butene and / or ethylene exceeds 10% by weight, the dimensional stability of the polypropylene resin in-mold foam molded product tends to decrease and the compressive strength. Tends to decrease.

  In the propylene / ethylene / 1-butene random copolymer and propylene / ethylene random copolymer, the structural unit composed of 1-butene is preferably 6% by weight or less, and 3% by weight or more and 5% by weight or less. More preferably. If the structural unit composed of 1-butene exceeds 6% by weight, the rigidity of the polypropylene random copolymer itself tends to be weak, and there is a tendency that practical rigidity such as compressive strength is not satisfied.

  In the propylene / ethylene / 1-butene random copolymer and propylene / ethylene random copolymer, the structural unit composed of ethylene is preferably 0.2% by weight or more and 4% by weight or less, and 0.2% by weight. More preferably, the content is 3.5% by weight or less. If the structural unit composed of ethylene is less than 0.2% by weight, the molding heating steam pressure tends to be high during in-mold foam molding, and if it exceeds 4% by weight, it tends to endure practical rigidity such as compressive strength. is there.

  There is no restriction | limiting in particular in the catalyst at the time of superposing | polymerizing the polypropylene resin of this invention, A Ziegler-Natta polymerization catalyst, a metallocene polymerization catalyst, etc. can be used.

  The melting point of the polypropylene random copolymer used in the present invention is preferably 125 ° C. or higher and 155 ° C. or lower, more preferably 130 ° C. or higher and 150 ° C. or lower, and 135 ° C. or higher and 148 ° C. or lower. Further preferred. If the melting point of the polypropylene random copolymer is less than 125 ° C., the dimensional stability of the polypropylene resin in-mold foam molding tends to decrease, and if the melting point exceeds 155 ° C., molding heating during in-mold foam molding is performed. Steam pressure tends to increase.

  The polypropylene random copolymer of the present invention may contain a radiation inhibitor and other additives described later, if necessary. In such a case, the melting point of the polypropylene random copolymer containing the radiation inhibitor and other additives is set as the melting point of the polypropylene random copolymer in the present invention.

Here, the melting point of the polypropylene random copolymer is measured as follows using a differential scanning calorimeter DSC [for example, DSC6200 type, manufactured by Seiko Instruments Inc.].
That is, 5 to 6 mg of a polypropylene random copolymer resin was heated from 40 ° C. to 220 ° C. at a temperature rising rate of 10 ° C./min to melt the resin, and then from 220 ° C. at a temperature decreasing rate of 10 ° C./min. From the DSC curve obtained when crystallizing by lowering the temperature to 40 ° C. and then increasing again from 40 ° C. to 220 ° C. at a temperature increase rate of 10 ° C./min, and giving such a series of temperature history, The melting peak temperature at the second temperature rise is defined as the melting point (Tm in FIG. 1).

The melt flow rate (hereinafter referred to as “MFR”) of the polypropylene random copolymer used in the present invention is not particularly limited, but is preferably 0.5 g / 10 min to 100 g / 10 min, and preferably 2 g / 10 minutes or more and 50 g / 10 minutes or less is more preferable, and 3 g / 10 minutes or more and 20 g / 10 minutes or less is more preferable.
When the MFR of the polypropylene random copolymer is within the above range, it is easy to obtain polypropylene resin expanded particles having a relatively large expansion ratio, and the polypropylene resin in-mold expanded molded product obtained by in-mold foam molding is obtained. There is a tendency that a product having excellent surface aesthetics and a small dimensional shrinkage rate can be obtained.

  Here, the value of MFR is the conditions of orifice 2.0959 ± 0.005 mmφ, orifice length 8.000 ± 0.025 mm, load 2160 g, 230 ± 0.2 ° C. using an MFR measuring instrument described in JIS-K7210. It is the value when measured below.

  In addition, when a polypropylene random copolymer contains the radiation inhibitor and other additive which are mentioned later, MFR of the polypropylene random copolymer containing the radiation inhibitor and other additive is used as the polypropylene in the present invention. It is set as MFR of the system random copolymer.

  The polypropylene resin foamed particles used in the present invention can be obtained by processing the above-mentioned polypropylene random copolymer into polypropylene resin particles and then foaming them.

  The polypropylene resin particles used in the present invention are, for example, a polypropylene random copolymer melted by using an extruder, a kneader, a Banbury mixer, a roll, etc., extruded into a strand shape, for example, before or after cooling, in a cylindrical shape Then, it is molded into a desired particle shape such as an elliptical shape, a spherical shape, a cubic shape, a rectangular parallelepiped shape, and the like, and becomes a polypropylene resin particle.

In the present invention, in addition to the polypropylene-based random copolymer, additives such as a radiation inhibitor, an antioxidant, a light resistance improver, an antistatic agent, a colorant, a flame retardant improver, and a conductivity improver are added. If necessary, polypropylene resin particles may be used. In that case, it is usually preferable to add these additives into the molten resin in the process of producing polypropylene resin particles.
Moreover, when using carbon dioxide gas, air, or water as a foaming agent described later, it is preferable to add an inorganic nucleating agent and / or a water-absorbing substance that can improve foaming properties.

  The melting point of the polypropylene resin particles used in the present invention can be determined as the melting point of the polypropylene random copolymer of the present invention by measuring the melting point using the differential scanning calorimeter described above. .

The radiation inhibitor used in the present invention is not particularly limited as long as it is a substance that suppresses heat conduction by radiation, and examples thereof include carbon black, graphite, activated carbon, titanium oxide, and barium sulfate. These may be used alone or in combination.
Among these, carbon black, graphite, activated carbon, and titanium oxide are preferable from the viewpoint of radiation suppression effect, and carbon black and activated carbon are more preferable.

  There is no restriction | limiting in particular as carbon black used by this invention, Carbon black for coloring, conductive carbon black, etc. can be used.

Although there is no restriction | limiting in particular as activated carbon used by this invention, From a dispersible viewpoint to resin, it is preferable that it is a powdered activated carbon. Specifically, powdered activated carbon having a particle size of 0.1 μm or more and 150 μm or less and a BET specific surface area of 500 m ² / g or more and 2000 m ² / g or less is preferably used.

  Although there is no restriction | limiting in particular in content of the radiation inhibitor in this invention, It is preferable to add so that it may become 0.1 to 20 weight% in 100 weight% of polypropylene resin particles. When the addition amount of the radiation inhibitor is less than 0.1% by weight, the radiation suppression effect tends to be small, and when it exceeds 20% by weight, the expansion ratio tends to be difficult to increase.

  The inorganic nucleating agent used in the present invention promotes the formation of bubble nuclei that are the starting point of foaming, contributes to the improvement of the foaming ratio, and also contributes to uniform bubble formation. Examples of the inorganic nucleating agent include talc, silica, calcium carbonate and the like.

  The content of the inorganic nucleating agent used in the present invention is preferably added so as to be 0.005 wt% or more and 0.5 wt% or less in 100 wt% of the polypropylene resin particles.

  The water-absorbing substance used in the present invention means that when the substance is added to polypropylene resin particles and the polypropylene resin particles are brought into contact with water or impregnated with a foaming agent in an aqueous dispersion, A substance that can contain water.

  Specific examples of the water-absorbing substance used in the present invention include water-soluble inorganic substances such as sodium chloride, calcium chloride, magnesium chloride, borax, and zinc borate, and a special block polymer (polyethylene glycol, polyether) having a hydrophilic segment ( Product name: Pelestat; manufactured by Sanyo Chemical Co., Ltd.), alkali metal salt of ethylene (meth) acrylic acid copolymer, alkali metal salt of butadiene (meth) acrylic acid copolymer, alkali metal salt of carboxylated nitrile rubber, Hydrophilic polymers such as alkali metal salts of isobutylene-maleic anhydride copolymer and alkali metal salts of poly (meth) acrylic acid, polyhydric alcohols such as ethylene glycol, glycerin, pentaerythritol, isocyanuric acid, melamine, etc. It is done.

  The content of the water-absorbing substance used in the present invention varies depending on the target foaming ratio, the foaming agent used, and the type of water-absorbing substance used, and cannot be described in general, but water-soluble inorganic substances and polyhydric alcohols are used. In the case of 100% by weight of polypropylene resin particles, it is preferably 0.01% by weight or more and 2% by weight or less. When a hydrophilic polymer is used, 0.05% by weight in 100% by weight of polypropylene resin particles. The content is preferably 5% by weight or less.

In the present invention, there is no limitation on the addition of the colorant, and a natural color can be obtained without adding the colorant, or a desired color can be obtained by adding a colorant such as blue, red, or black.
Examples of the colorant include perylene organic pigments, azo organic pigments, quinacridone organic pigments, phthalocyanine organic pigments, selenium organic pigments, dioxazine organic pigments, isoindoline organic pigments, and carbon black.

  The polypropylene resin foamed particles used in the present invention contain a dispersion containing polypropylene resin particles and water in a pressure vessel, and then disperse them under stirring conditions, and in the presence of a foaming agent, The temperature can be raised above the softening point temperature of the resin particles, and then the dispersion in the pressure vessel can be discharged into a pressure range lower than the internal pressure of the pressure vessel to produce polypropylene resin particles by foaming. In addition, this process may be referred to as “one-stage foaming process”, and the polypropylene resin particles obtained in this process may be referred to as “single-stage foam particles”.

More specifically, the following method is mentioned, for example.
(1) After charging polypropylene resin particles and an aqueous dispersion medium, a dispersing agent, etc., if necessary, the inside of the sealed container is evacuated and then 1 MPa (gauge pressure) or more and 2 MPa. The following (gauge pressure) foaming agent is introduced and heated to a temperature equal to or higher than the softening temperature of the polypropylene resin. By heating, the pressure in the sealed container rises to about 2 MPa (gauge pressure) or more and 5 MPa or less (gauge pressure). If necessary, add a foaming agent near the foaming temperature to adjust to the desired foaming pressure, further adjust the temperature, and if necessary, hold the foaming pressure and temperature for a predetermined time, and then By releasing into a pressure range lower than the internal pressure of the sealed container, polypropylene-based resin expanded particles can be obtained.
(2) After charging the pressure-resistant container with polypropylene resin particles, an aqueous dispersion medium, and a dispersant as necessary, the inside of the sealed container is evacuated and then heated to a temperature equal to or higher than the softening temperature of the polypropylene resin. A foaming agent may be introduced while heating.
(3) After charging polypropylene resin particles, aqueous dispersion medium, and dispersing agent if necessary into a pressure vessel, heating to near the foaming temperature, introducing a foaming agent to obtain the foaming temperature, and from the internal pressure of the sealed container Further, it is possible to obtain expanded polypropylene resin particles by releasing into a low pressure range.

Before releasing into the low-pressure region, carbon dioxide, nitrogen, air or a substance used as a foaming agent is injected to increase the internal pressure in the pressure-resistant container, and the pressure release speed during foaming is adjusted. The foaming ratio can be adjusted by introducing carbon dioxide, nitrogen, air, or a substance used as a foaming agent into the pressure-resistant container and controlling the pressure during the release to the low pressure region.
Here, the pressure range lower than the internal pressure of the pressure vessel is preferably atmospheric pressure. In this case, the equipment does not become complicated, and no special pressure adjustment in the low pressure region is required.

  The temperature in the low pressure range may be room temperature, or may be set to a temperature higher than room temperature. Examples of a method for setting the temperature higher than normal temperature include a method of heating with heated steam.

In the present invention, when the dispersion is discharged into a pressure region heated with heated steam and the polypropylene resin particles are produced by foaming, as will be described later, the DSC curve of the obtained polypropylene resin foam particles is used. This is one of the preferred embodiments because the maximum value is easily expressed in the differential curve.
From such a viewpoint, the heating temperature is preferably 60 ° C. or higher and 120 ° C. or lower, and more preferably 70 ° C. or higher and 115 ° C. or lower.

  In order to ensure foamability, it is necessary to raise the temperature to a temperature in the range of the melting point of the polypropylene resin particles −20 ° C. or higher and the melting point of the polypropylene resin particles + 10 ° C. or lower. preferable.

Examples of the blowing agent used in the present invention include aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane and hexane; aliphatic cyclized hydrogens such as cyclopentane and cyclobutane; air and nitrogen Inorganic gas such as carbon dioxide, water, etc. These foaming agents may be used alone or in combination of two or more.
Among these, it is preferable to use inorganic gas, and it is particularly preferable to use carbon dioxide gas, air, and water.

  The amount of the foaming agent used in the present invention is not particularly limited, and may be appropriately used according to the desired expansion ratio of the polypropylene resin expanded particles. The amount of the foaming agent used is preferably 3 to 60 parts by weight with respect to 100 parts by weight of the polypropylene resin particles.

  There is no particular limitation on the pressure vessel used when producing the expanded polypropylene resin particles, and any vessel that can withstand the pressure and temperature in the vessel at the time of producing the expanded polypropylene resin particles can be used. For example, an autoclave type pressure vessel can be used. It is done.

In the present invention, it is preferable to use an inorganic dispersant from the viewpoint of enhancing the dispersibility of the dispersion during the production of the polypropylene resin expanded particles and preventing adhesion between the polypropylene resin expanded particles.
Examples of such inorganic dispersants include tricalcium phosphate, tribasic magnesium phosphate, basic magnesium carbonate, calcium carbonate, basic zinc carbonate, aluminum oxide, iron oxide, titanium oxide, aluminosilicate, kaolin, and sulfuric acid. Barium etc. are mentioned.

  In the present invention, it is preferable to use a dispersion aid in combination in order to further improve dispersibility. Examples of such a dispersion aid include sodium dodecylbenzene sulfonate, sodium alkane sulfonate, sodium alkyl sulfonate, sodium alkyl diphenyl ether disulfonate, sodium α-olefin sulfonate, and the like. Among these, as a combination of the inorganic dispersant and the dispersion aid, a combination of tricalcium phosphate and sodium alkylsulfonate is preferable.

The amount of the inorganic dispersant and dispersion aid used in the present invention varies depending on the type and the type and amount of polypropylene resin used, but usually 0.2 part by weight of the inorganic dispersant with respect to 100 parts by weight of water. The amount is preferably 3 parts by weight or less and more preferably 0.001 part by weight or more and 0.1 parts by weight or less.
Moreover, in order to make the dispersibility in water favorable, it is usually preferable to use the polypropylene resin particles at 20 parts by weight or more and 100 parts by weight or less with respect to 100 parts by weight of water.

The expansion ratio of the expanded polypropylene resin particles of the present invention is not particularly limited, but is preferably 3 to 50 times, more preferably 7 to 45 times.
Here, the expansion ratio of the polypropylene resin expanded particles was determined by measuring the weight w (g) and the ethanol submerged volume v (cm ³ ) of the polypropylene resin expanded particles, and obtaining the true specific gravity ρb = w / v of the expanded particles, The expansion ratio K = ρr / ρb is calculated from the ratio with the density ρr of the polypropylene resin particles before foaming.

The bulk density of the expanded polypropylene resin particles of the present invention is not particularly limited, but is preferably 10 g / L or more and 180 g / L or less, and more preferably 12 g / L or more and 78 g / L or less.
Here, the bulk density of the expanded polypropylene resin particles is determined by measuring the weight of the expanded polypropylene resin particles in the container after the polypropylene expanded resin particles are gently put into the container and filling it. Divided by g / L.

  By the way, although the first-stage expanded particles depend on the type of the foaming agent at the time of production, the expansion ratio may not reach 10 times. Furthermore, the maximum value of the differential curve may not appear at 105 ° C. or higher and 120 ° C. or lower in the differential curve (described later) of the DSC curve at the temperature rising rate of 20 ° C./min of the first stage expanded particles.

In such a case, the single-stage foamed particles are impregnated with an inorganic gas (for example, air, nitrogen, carbon dioxide gas, etc.) and given an internal pressure, and then contacted with water vapor at a specific pressure to thereby form the single-stage foam particles. To obtain expanded polypropylene resin particles of the present invention having a maximum value at 105 ° C. or higher and 120 ° C. or lower in the differential curve of the DSC curve at a heating rate of 20 ° C./min. Can do.
As described above, the process of further foaming the polypropylene resin expanded particles to obtain a polypropylene resin expanded particle having a higher expansion ratio may be referred to as a “two-stage expansion process”.

  By passing through the two-stage foaming step, it becomes easy to obtain the foamed particles of the present invention having a maximum value at 105 ° C. or more and 120 ° C. or less in the differential curve of the DSC curve. Polypropylene-based resin foam particles obtained through such a two-stage foaming process may be referred to as “two-stage foam particles”.

In the present invention, when the pressure of water vapor in the two-stage foaming step is to obtain two-stage foamed particles having a maximum value at 105 ° C. or more and 120 ° C. or less in the differential curve of the DSC curve at a heating rate of 20 ° C./min. Very important. Therefore, the water vapor pressure in the two-stage foaming step is preferably adjusted to 0.04 MPa (gauge pressure) or more and 0.25 MPa (gauge pressure) or less in consideration of the expansion ratio of the two-stage foam particles. It is more preferable that the pressure be adjusted to 05 MPa (gauge pressure) or more and 0.15 MPa (gauge pressure) or less.
If the water vapor pressure in the two-stage foaming process is less than 0.04 MPa (gauge pressure), the maximum value may not be expressed in the differential curve, and if it exceeds 0.25 MPa (gauge pressure), the maximum value in the differential curve is Although it develops, the resulting two-stage foam particles tend to coalesce and block, and cannot be used for subsequent in-mold foam molding.

The internal pressure of the air impregnated in the first-stage expanded particles is preferably changed in consideration of the expansion ratio of the second-stage expanded particles and the water vapor pressure in the second-stage expansion process, but is 0.2 MPa or more (absolute pressure) 0.6 MPa or less (Absolute pressure) is preferred.
If the internal pressure of the air impregnated in the first-stage expanded particles is less than 0.2 MPa (absolute pressure), high-pressure steam is required to improve the expansion ratio, and the second-stage expanded particles tend to block. When the internal pressure of the air impregnated in the first-stage expanded particles exceeds 0.6 MPa (absolute pressure), the water vapor pressure for obtaining the desired expansion ratio becomes lower, and the second-stage expanded particles having no maximum value in the differential curve Tend to be.

  Thus, the polypropylene resin foamed particles obtained through two or more foaming steps as in the two-stage foaming process as compared with the single-stage foaming process have a differential curve of the DSC curve at a temperature rising rate of 20 ° C./min. , A maximum value between 105 ° C. and 120 ° C. is preferable in the present invention.

  In addition, a single-stage expanded particle having a low expansion ratio is obtained in a single-stage expansion process, and then a desired expansion ratio is obtained in a two-stage expansion process, and the differential curve of the DSC curve is 105 ° C. or higher and 120 ° C. or lower. It may be a two-stage expanded particle having a maximum value between.

  The bulk density of the two-stage expanded particles in the present invention is preferably 10 g / L or more and 45 g / L or less.

The amount of the inorganic dispersant adhered to the surface of the expanded polypropylene resin particles used in the present invention is preferably 2000 ppm or less, more preferably 1300 ppm or less, and even more preferably 800 ppm or less.
When the amount of the inorganic dispersant adhering to the surface of the polypropylene resin foamed particles exceeds 2000 ppm, the fusion property during in-mold foam molding tends to be lowered.

In the DSC curve obtained by differential scanning calorimetry (DSC) in which the polypropylene resin expanded particles used in the present invention are heated from 40 ° C. to 220 ° C. at a temperature rising rate of 10 ° C./min, Q1) region and high temperature side heat of fusion (Qh) region, the ratio of high temperature side heat of fusion {Qh / (Ql + Qh)} × 100) (%) (hereinafter referred to as “high temperature heat ratio”) However, it is preferably 10% or more and 30% or less, and more preferably 12% or more and less than 28%.
When the high-temperature calorie ratio of the polypropylene resin expanded particles is less than 10%, the polypropylene resin expanded particles tend to be open, and the polypropylene resin in-mold expanded molded product tends to shrink. If the high-temperature heat ratio of the polypropylene resin expanded particles exceeds 30%, the fusion property of the expanded foamed polypropylene resin mold (return box) decreases, and the dents and intergranularities (hereinafter simply referred to as “between the expanded particles”). "Void") tends to stand out, and dirt and mold that enter and adhere to the void tend to be more difficult to clean.

Here, the total heat of fusion (Q), low temperature side heat of heat (Ql) and high temperature side heat of heat (Qh) in the DSC curve obtained by differential scanning calorimetry (DSC) are as follows, as shown in FIG. Defined in
The total heat of fusion (Q) is the amount of heat absorbed at the temperature at which the high-temperature side melting ends (point B) from the endothermic amount at the temperature 100 ° C. at which the low-temperature side heat of fusion starts (point A) in the obtained DSC curve. Is a portion surrounded by a line segment AB and a DSC curve.
Furthermore, the point where the endothermic amount becomes the smallest between the two melting calorie regions of the low temperature side melting calorie and the high temperature side melting calorie of the DSC curve is defined as a point C, and the point where the point C intersects with the line segment AB is raised perpendicularly. , The portion surrounded by line segment AD, line segment CD, and DSC curve is the low temperature side melting heat quantity (Ql), and the portion surrounded by line segment BD, line segment CD, and DSC curve is the high temperature side melting. It is the amount of heat (Qh).

  The high temperature side heat of fusion Qh of the polypropylene resin expanded particles of the present invention is not particularly limited, but is preferably 4 J / g or more and 40 J / g or less, more preferably 7 J / g or more and 30 J / g or less, and more preferably 7 J / g or more and 19 J or less. / G or less is more preferable. When the high-temperature side heat of fusion Qh of the polypropylene resin foamed particles is less than 4 J / g, the polypropylene resin foamed particles tend to be open, and when it exceeds 40 J / g, the expansion ratio tends to be difficult to increase.

The high-temperature heat ratio and the high-temperature side heat of fusion are, for example, the retention time from temperature rise to foaming in the one-stage foaming process (maintenance time from reaching the foaming temperature until foaming), foaming temperature (at the time of foaming) Temperature), foaming pressure (pressure during foaming), and the like.
Generally, by increasing the holding time, lowering the foaming temperature, and lowering the foaming pressure, the high-temperature heat quantity ratio or the high-temperature side heat of fusion tends to increase.

  From the above, it is possible to easily find the conditions for the desired high-temperature calorie ratio and high-temperature side melting peak calorie by experimenting several times with systematically changing the holding time, foaming temperature, and foaming pressure. Can do. The foaming pressure can be adjusted by adjusting the amount of foaming agent.

The polypropylene resin expanded particles used in the present invention are 105 ° C. in the differential curve of the DSC curve obtained by differential scanning calorimetry (DSC) in which the temperature is raised from 40 ° C. to 220 ° C. at a temperature rising rate of 20 ° C./min. It is preferable to have a maximum value between 120 ° C. and below.
The differential curve of the DSC curve of the polypropylene resin expanded particles in the present invention has a maximum value (point E) in the low-temperature side melting heat quantity region, as shown by curve A in FIG.

  The maximum value (point E) indicates the heat history temperature applied to the polypropylene resin particles and greatly affects the molding heating steam pressure for obtaining the in-mold molded product.

  In the present invention, the temperature showing the maximum value of the differential curve is preferably 105 ° C. or higher and 120 ° C. or lower, and more preferably 110 ° C. or higher and 115 ° C. or lower. When the temperature showing the maximum value in the differential curve is within the temperature range, the heated steam penetrates well into the in-mold foam molded product when foaming in the mold, and as a result, even if the molding heated steam pressure is low, the mold It is thought that polypropylene resin expanded particles are firmly bonded to each other not only on the surface of the inner foamed molded body but also inside.

  When the temperature showing the maximum value in the differential curve is less than 105 ° C. or when there is no maximum value, the permeability of the heated steam when foaming in the mold tends to be insufficient, and the resulting polypropylene resin in-mold foam molding Although it may be difficult to visually distinguish in the body, it is assumed that small voids are likely to occur, and as a result, dirt and mold that enter the small voids and become conspicuous, and the cleaning tends to be more difficult. is there. Moreover, since the permeability of heated steam is insufficient, the meltability itself in the polypropylene resin mold-internally foamed molded product tends to decrease. In order to solve this problem, a high molding pressure tends to be required.

  On the other hand, when the temperature showing the maximum value in the differential curve exceeds 120 ° C., the cell film in the polypropylene resin foamed particles is broken, the shrinkage ratio of the obtained molded body is large, large wrinkles are generated, and the good There is a tendency that a molded body cannot be obtained. Moreover, when it is going to obtain the polypropylene resin expanded particle which the temperature which shows maximum value exceeds 120 degreeC, there also exists a tendency for a polypropylene resin expanded particle to unite | attach (agglomeration).

In the present invention, the expanded polypropylene resin particles having a maximum value between 105 ° C. and 120 ° C. in the differential curve of the DSC curve at a temperature rising rate of 20 ° C./min are, for example, any one of the following methods. It can be easily obtained by one or a combination.
(1) Once the polypropylene expanded resin particles are obtained, impregnated with an inorganic gas (for example, air, nitrogen, carbon dioxide gas, etc.) and applied with an internal pressure, and then brought into contact with water vapor at a specific pressure. A method that undergoes two foaming steps (a method that performs a two-stage foaming step after the one-stage foaming step),
(2) A method in which polypropylene resin particles are foamed in a heated low pressure region such as in heated steam in the single-stage foaming process (the low pressure in which the dispersion in the pressure vessel is heated with heated steam in the single-stage foaming process) To release into the area),
(3) At least 1 selected from the group consisting of glycerin, polyethylene glycol, a glycerin ester of a fatty acid having 10 to 25 carbon atoms, or a water-absorbing substance such as melamine and isoanuric acid in 100% by weight of polypropylene resin particles. A method of containing 0.01 wt% or more and 5 wt% or less of a seed hydrophilic compound.

  The average cell diameter of the expanded polypropylene resin particles used in the present invention is preferably 0.05 mm or more and 0.4 mm or less, and more preferably 0.1 mm or more and 0.3 mm or less.

  If the average cell diameter of the polypropylene resin foamed particles is less than 0.05 mm, the surface appearance of the expanded foam in the polypropylene resin mold tends to be poor, and if it exceeds 0.4 mm, it is one of the characteristics of a returnable box. There is a tendency for the heat insulation property to be lowered.

The particle weight of the polypropylene resin expanded particles used in the present invention is preferably 0.5 mg / particle or more and 1.8 mg / particle or less, more preferably 0.7 mg / particle or more and 1.2 mg / particle or less. preferable.
The particle weight of the polypropylene resin expanded particles can be easily obtained by setting the particle weight of the polypropylene resin particles to 0.5 mg / particle or more and 1.8 mg / particle or less.

As described above, the polypropylene resin particles in the present invention are obtained by extruding a once melted resin into a strand shape, and before cooling or after cooling, a desired particle shape such as a cylindrical shape, an elliptical shape, a spherical shape, a cubic shape, a rectangular parallelepiped shape, etc. Can be obtained by molding into
When the grain weight is less than 0.5 mg / grain, the dispersion of the grain weight tends to be large, and the expanded polypropylene resin particles tend to have a large variation in the expansion ratio. On the other hand, when the grain weight exceeds 1.8 mg / grain, the resulting polypropylene resin in-mold foam molded product tends to be poorly fused. That is, voids tend to stand out, and dirt and mold that enters and adheres to voids tend to be more difficult to clean.

In the present invention, the polypropylene resin foam particles are subjected to in-mold foam molding to form a polypropylene resin in-mold foam molded body.
B) After the polypropylene resin foamed particles are pressurized with an inorganic gas such as air, nitrogen, carbon dioxide, etc., impregnated with the inorganic gas in the polypropylene resin foamed particles, and given a predetermined polypropylene resin foamed particle internal pressure, A method of filling a mold and heat-sealing with steam,
B) A method in which polypropylene resin foamed particles are compressed with gas pressure and filled in a mold, and heat recovery is performed with water vapor using the recovery force of the polypropylene resin foamed particles.
C) A method such as a method in which a polypropylene resin expanded particle is filled in a mold without heat treatment and heat-sealed with water vapor can be used.

As a specific example of a method for obtaining a polypropylene resin-in-mold foam-molded article using the polypropylene resin foam particles of the present invention, for example,
The polypropylene resin foamed particles are preliminarily air-pressed in a pressure-resistant container, and after applying internal pressure (foaming ability) by press-fitting air into the polypropylene resin foamed particles, it can be closed with two molds. Filled in a molding space that cannot be hermetically sealed, molded with a water vapor of about 0.1 to 0.4 MPa (gauge pressure) as a heating medium, with a heating time of about 3 seconds to 60 seconds, and polypropylene After fusing the resin-based resin foam particles, after cooling the mold to the extent that the deformation of the polypropylene-based resin mold in the mold after taking out the mold in the mold by water cooling is suppressed, the mold is opened, Examples thereof include a method for obtaining a polypropylene resin molded in-mold foam.

  The internal pressure of the polypropylene resin expanded particles is, for example, 0.1 MPa or more and 1.0 MPa (gauge pressure) with an inorganic gas such as air or nitrogen under a temperature condition of 1 hour to 48 hours, room temperature to 80 ° C. in a pressure vessel. ) It can be adjusted by pressurizing below.

Although there is no restriction | limiting in particular as a density of the returnable box foaming molding obtained using the polypropylene resin expanded particle of this invention, 10 g / L or more and 180 g / L or less are preferable, and 12 g / L or more and 65 g / L or less are more. preferable.
Here, the density of the returnable box foamed molded product is obtained by cutting a sample into a rectangular parallelepiped from the center of each of the bottom of the returnable box and the four standing wall parts, calculating the sample volume from the product of the vertical, horizontal, and thickness dimensions, The weight is divided by the sample volume, and the density of each sample is calculated and then averaged and expressed in g / L.

In addition, although an example of the returnable box of this invention is shown in FIG. 4, there is no restriction | limiting in particular about an opening part shape or an external shape. FIG. 4 shows an example of a rectangular shape as viewed from above, but is not limited to this, and any opening such as a quadrilateral other than a triangle, a circle, an ellipse, or a polygon The shape may be sufficient, and the shape may change from the opening toward the bottom.
In addition, FIG. 4 shows an example of a rectangular shape as an external shape viewed from above, but is not limited to this, and any external shape such as a quadrilateral other than a triangle, a circle, an ellipse, or a polygon is shown. It may be a shape.

  Furthermore, although not shown in FIG. 4, a partition member for partitioning the goods to be transported is provided inside the box, a passage box standing wall or a bottom is provided with a groove or a mountain to fix the partition member, etc. It does not matter if this processing is applied.

  The returnable box of the present invention may be provided with a lid corresponding to the box body. There is no restriction on how the box body and the lid are aligned, and any means such as a method of fitting the box body and the lid or a method of dropping the lid into the box body may be used.

The returnable box of the present invention is a returnable box used for transporting various goods. Specifically, the goods to be transported include foods, tools, electrical products, glass substrates, chemicals, cosmetics, various parts, and the like. Although it is mentioned, it is not limited to these.
However, since the returnable box of the present invention is a returnable box that easily cleans attached dirt and mold, it is preferably a returnable box for food, a returnable box for medicine, and a returnable box for cosmetics. Most preferably it is.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to such examples.

In the examples and comparative examples, the substances used were as follows, but no particular purification was performed.
● Polypropylene resin: Polypropylene resin shown in Table 1 [manufactured by Prime Polymer Co., Ltd.]
● Polyethylene glycol: Lion Corporation, average molecular weight 300
● Melamine: Nissan Chemical Co., Ltd. ● Talc: Hayashi Kasei Co., Ltd., Talcan Powder PK-S

  The evaluation in the examples and comparative examples was performed by the following method.

(Quantification of copolymer composition)
Add 50 g of xylene to a polypropylene resin (about 1 g), heat and dissolve at 120 ° C., and use high-temperature centrifuge [manufactured by Centrifuge, H175] under the conditions of 12000 rpm × 30 minutes and insoluble and soluble components Sorted into After cooling the obtained soluble matter, insoluble matter was obtained by centrifugation (12000 rpm × 30 minutes).
0.4 g of orthodichlorobenzene-d ₄ was added to 50 mg of the obtained insoluble matter, and the mixture was melted by heating at 100 ° C., and ¹³ C-MNR measurement [manufactured by VARIAN, INOVA AS600] was performed at 98 ° C., and propylene, butene The copolymer composition of ethylene was quantified.

(Measurement of melting point of polypropylene resin)
The melting point was measured using a differential scanning calorimeter DSC [Seiko Instruments Co., Ltd., DSC6200 type], and obtained polypropylene resin particles 5-6 mg at a heating rate of 10 ° C./min from 40 ° C. to 220 ° C. The resin particles are melted by heating to 40 ° C., and then crystallized by cooling from 220 ° C. to 40 ° C. at a cooling rate of 10 ° C./min, and further from 40 ° C. at a heating rate of 10 ° C./min. This is a value obtained as a melting peak temperature at the second temperature increase from a DSC curve obtained when the temperature is increased to 220 ° C.

(Average cell diameter of polypropylene resin expanded particles)
Using a double-edged razor [feather, high-stainless steel double-edged], be careful not to break the bubble film (cell film), cut the center of the foamed particles, and cut the cut surface with a microscope [manufactured by Keyence Corporation] , VHX-100] and observed at a magnification of 100 times. In the obtained image, a line segment corresponding to a length of 1000 μm was drawn on the portion excluding the surface layer part, the number of bubbles n passing through the line segment was measured, and the bubble diameter was calculated at 1000 / n (μm). The same thing was done with 10 expanded particles, and the average value of the calculated bubble diameters was taken as the average bubble diameter.

(Particle weight of expanded polypropylene resin particles)
100 polypropylene resin particles were arbitrarily selected and weighed, and the particle weight per particle was calculated. The particle weight of the polypropylene resin particles was taken as the particle weight of the polypropylene resin expanded particles.

(Expansion ratio of polypropylene resin expanded particles)
After taking 3 g or more and 10 g or less of the obtained expanded particles, drying at 60 ° C. for 6 hours, adjusting the condition in a room at 23 ° C. and 50% humidity, measuring the weight w (g), and then volume by ethanol submersion method. v (cm ³ ) was measured to determine the true specific gravity ρb = w / v of the expanded particles, and the expansion ratio K = ρr / ρb was determined from the ratio to the density ρr of the polypropylene resin particles before expansion.
In the following examples and comparative examples, the density ρr of the polypropylene resin particles before foaming is 0.90 g / cm ³ .

(Calculation of the ratio of high-temperature melting heat amount of polypropylene-based expanded particles)
The ratio of the high-temperature side melting heat quantity is measured by using a differential scanning calorimeter DSC [manufactured by Seiko Instruments Inc., DSC6200 type] at a heating rate of 10 ° C./min. It calculated from the DSC curve (refer FIG. 2) obtained when it heats up to 220 degreeC.

(Reading the maximum value of the differential curve of the DSC curve of polypropylene-based expanded particles)
The maximum value of the differential curve of the DSC curve was calculated by using a differential scanning calorimeter DSC [Seiko Instruments Co., Ltd., DSC6200 type], and increasing the obtained polypropylene resin foamed particles 5-6 mg by 20 ° C./min. In the curve obtained by differentiating the DSC curve obtained when the temperature was raised from 40 ° C. to 220 ° C. at the temperature rate (see FIG. 3), the temperature at which the maximum value was reached was read.

(Density of returnable box foam moldings)
Five test pieces each having a length of 50 mm, a width of 50 mm, and a thickness of 25 mm were cut out from the vicinity of the center of the bottom portion and the four standing wall portions of the returnable box foam molded body. However, it was cut out so as not to include the skin layer of the foamed molded product in the returnable box mold.
The vertical, horizontal, and thickness dimensions of the test piece were measured with calipers, the sample volume was calculated from the product, and the density was calculated by dividing the sample weight by the sample volume. After calculating the density of each test piece, the density of the returnable box foamed molded article was averaged and expressed in units of g / L.

(Fusion rate evaluation of returnable box foam moldings)
After making a cut of about 3 mm in the thickness direction with a cutter knife into the standing wall portion of the foamed molded product in the returnable box mold, the standing wall portion was broken from the cut portion by hand, and the fracture surface was observed, The ratio of the broken polypropylene resin foam particles to the number of polypropylene resin foam particles constituting the cross section was determined and used as the fusion rate.

(Void evaluation of returnable box foam moldings)
The inside bottom surface of the obtained returnable box molded product was visually observed and evaluated according to the following criteria.
○: No void is seen on the bottom surface, or a little is seen.
X: A void is noticeable on the bottom.

(Evaluation of detergency of returnable box foam moldings)
After stirring and homogenizing a curry powder suspension prepared by mixing 0.3 g of curry powder [manufactured by House Foods Co., Ltd., curry powder granules] and 10 g of tap water, the box foamed molded product (internal volume 8 L) ) Towards the bottom of the inside. The returnable box molded body was shaken by hand, and the curry powder suspension was cast on the bottom.
The returnable box molded body was left at 23 ° C. for 24 hours without being covered, and then washed as follows. (1) The operation of pouring 8 liters of 50 ° C. tap water into a box molded body and immediately releasing it was washed twice (hereinafter referred to as “washing a”).
(2) When cleaning A is performed, the inside bottom surface of the box is observed, and if the curry color remains on the inner surface, the inner surface is rubbed 10 times with a dishwashing sponge, and then 8 liters of tap water at 50 ° C. It was poured into a returnable box molded product, immediately discharged and washed (hereinafter referred to as “washing b”).
After washing as described above, evaluation was performed according to the following criteria.
◎: Curry color cannot be confirmed on the inner surface of the return box and void after completion of cleaning a ○: Curry color cannot be confirmed on the inner surface of the return box after cleaning a, but a little curry color remains on the void Δ: After completion of cleaning a, curry color remains on the inside surface of the return box, and curry color clearly remains on the void. However, the curry color on the inner surface of the return box disappears due to the cleaning b. X: After the completion of the cleaning a, the curry color remains on the inner surface of the return box, and the curry color clearly remains on the void portion. Moreover, even if the cleaning b is performed, the curry color on the inner surface of the returnable box is not lost.

( Reference Example 1)
<Preparation of polypropylene resin particles>
Polyethylene glycol and talc as an inorganic nucleating agent were added to and mixed with a polypropylene resin (Prime Polymer Co., Ltd.) having a 1-butene content of 3.8 wt% and an ethylene content of 0.5 wt%. The resulting mixture was melt-kneaded at a resin temperature of 220 ° C. using a twin-screw extruder (manufactured by OMN Machine, TEK45), extruded into a strand, and the resulting strand was cooled with water and cut. Polypropylene resin particles (1.2 mg / grain) were produced. The melting point of the polypropylene resin particles was 148 ° C. Polyethylene glycol and talc were added so as to be 0.5% by weight and 0.2% by weight, respectively, in 100% by weight of polypropylene resin particles.
<Production of single-stage expanded particles>
In a pressure-resistant container having an internal volume of 10 L, 100 parts by weight of the obtained polypropylene resin particles, 2 parts by weight of powdery basic tricalcium phosphate as a dispersing agent, and 0.05 weight of sodium n-paraffin sulfonate as a dispersing aid 300 parts by weight of an aqueous dispersion medium containing 5 parts by weight and 7.5 parts by weight of carbon dioxide gas as a foaming agent, while stirring, the temperature is raised to the foaming temperature shown in Table 1 and held for 10 minutes, and then carbon dioxide gas is additionally injected. And it adjusted to the foaming pressure shown in Table 1, and hold | maintained for 30 minutes.
Thereafter, while keeping the internal temperature and pressure constant while injecting carbon dioxide gas, the valve at the bottom of the pressure vessel is opened, and the aqueous dispersion medium is passed through the orifice plate having a hole diameter of 3.6 mmφ to form an unsealed system (under atmospheric pressure). ) Was released into a cylindrical container to obtain polypropylene resin expanded particles (single-stage expanded particles). In addition, the inside of a cylindrical container was not heated.
<Preparation of expanded polypropylene resin particles (two-stage expanded particles)>
The obtained single-stage expanded particles were dried at 80 ° C. for 6 hours, and then impregnated with pressurized air in a pressure-resistant container to adjust the internal pressure to 0.37 MPa (absolute pressure), and then 0.08 MPa (gauge Pressure) was brought into contact with water vapor to effect two-stage foaming.
With respect to the obtained two-stage expanded particles, the expansion ratio, the calculation of the ratio of the high-temperature side heat of fusion, the presence / absence of the maximum value of the differential curve of the DSC curve, and the temperature of the maximum value were read. The results are shown in Table 1.
<Production of returnable box foam molding>
Using a polyolefin foam molding machine [Toyo Machine Metal Co., Ltd., P150N], the air pressure inside the polypropylene resin foam particles is set to 0.2 MPa (absolute pressure) in advance in a mold for obtaining a box-shaped molded body. Filled with two-stage expanded polypropylene resin particles, adjusted to a molding heating vapor pressure of 0.30 MPa (gauge pressure), compressed by 10% in the thickness direction, and thermoformed to form a box-type [inside dimension, long side 400 mm × short side 200 mm × height 100 mm, thickness 30 mm], a polypropylene-based in-mold foam molded product (return box molded product = internal volume 8 L) was obtained.
The obtained returnable box molded article was left to stand at 23 ° C. for 1 hour, then cured and dried in a constant temperature room at 75 ° C. for 3 hours, and left again at 23 ° C. for 3 days, and then evaluated for void evaluation and cleaning performance. .
The evaluation results are shown in Table 1.

( Reference Examples 2 to 9 and 11 and Example 10 )
The same procedure as in Reference Example 1 was carried out except that the type of polypropylene resin, the blending of additives, the one-stage foaming conditions, the two-stage foaming conditions, etc. were changed as shown in Table 1, respectively. Expanded particles, two-stage expanded particles, and in-mold expanded molded articles were obtained and evaluated. The results are shown in Table 1.
However, in Reference Example 9, two-stage foaming and three-stage foaming were performed under the conditions shown in Table 1.
In Example 10, in the step of producing the one-stage expanded particles, when the aqueous dispersion medium was released, 100 ° C. water vapor was continuously blown (introduced) into the non-sealed cylindrical container. And two-stage foaming was not performed.

(Comparative Examples 1-6)
Polypropylene resin particles, single-stage foamed particles, two-stage foamed, similar to Reference Example 1 except that the types of polypropylene-based resins, additive blends, one-stage foaming conditions, two-stage foaming conditions, etc. were as shown in Table 2. Particles and in-mold foam moldings were obtained and evaluated.
The results are shown in Table 2.
However, in Comparative Example 6, 50 ° C. warm water was showered (introduced) into the non-sealed cylindrical container when the aqueous dispersion medium was released in the step of producing the one-stage expanded particles. And two-stage foaming was not performed.

In Comparative Example 1, the go-to-box foamed molded product had a tendency to shrink and wrinkled on the surface. Dirt adhered to the wrinkles, resulting in poor cleanability.
In Comparative Example 2, voids in the foamed molded product in the returnable box mold were remarkable, and the presence of voids resulted in poor cleaning performance of the attached dirt.
In Comparative Example 3, the two-stage expanded particles were in an agglomerated state aggregated between the particles and could not be molded.
In Comparative Example 4, the fusion rate of the foamed molded product in the returnable box was clearly low, and the heated water vapor did not sufficiently permeate during molding. Since the void was not noticeable, it is presumed that the expanded polypropylene resin particles on the surface of the molded body were fused at an early stage of molding, and the heated steam did not sufficiently penetrate into the molded body.
In Comparative Example 5, the pressure at the molding heating vapor pressure of 0.30 MPa (gauge pressure) at the time of in-mold foam molding was too low, so that the foam particles were not fused, and an in-mold foam molded article could not be obtained.
In Comparative Example 6, the maximum value of the differential curve of the DSC curve of the polypropylene resin expanded particles was 101 ° C., and no significant voids were found in the void (visual) evaluation, but the detergency was not good. It is presumed that the dirt adhering to the small voids that were difficult to understand deteriorated the cleaning performance. Also, the fusion rate itself was in a downward trend.

Tm Melting point Ql Low temperature side heat of fusion Qh High temperature side heat of fusion E Maximum value in differential curve of DSC curve

In the present invention, it is possible to provide a returnable box, in particular, a returnable box for foods, which is formed by in-mold foam molding of polypropylene resin expanded particles and can easily clean dirt and mold.

Claims (2)

A method for producing a returnable box, in which a polypropylene resin foamed particle is filled in a mold and heated to obtain a foamed molded product in a polypropylene resin mold,
Polypropylene resin foam particles
Polypropylene resin particles containing butene-1 and / or ethylene as a comonomer and having a melting point of from 125 ° C. to 155 ° C. as a base resin were accommodated in a pressure vessel together with water and an inorganic gas. rear,
Disperse under stirring conditions, raise the temperature above the softening point temperature of the polypropylene resin particles, and then discharge the dispersion in the pressure vessel to a pressure range lower than the internal pressure of the pressure vessel to expand the polypropylene resin particles. Obtained by a foaming process including at least a one-stage foaming process,
And, Ri satisfy the condition der the following (a) and (b),
One-step foaming process is characterized that you release the dispersion into a pressure zone are heated by heating steam,
How to make a returnable box.
(A) In a DSC curve obtained by differential scanning calorimetry (DSC) in which the temperature is raised from 40 ° C. to 220 ° C. at a rate of temperature increase of 10 ° C./min, the low-temperature side heat of fusion (Ql) region and the high-temperature side heat of fusion ( Qh) has two regions, and the ratio {Qh / (Ql + Qh)} × 100) (%) of the high temperature side heat of fusion is 10% or more and 30% or less.
(B) In the differential curve of the DSC curve obtained by differential scanning calorimetry (DSC) where the temperature is raised from 40 ° C. to 220 ° C. at a rate of temperature increase of 20 ° C./min, the maximum value is between 105 ° C. and 120 ° C. Have Characterized in that it comprises a two-stage foaming step to further increase the expansion ratio of the obtained polypropylene resin expanded particles in one step expansion process, the production method of the returnable box according to claim 1, wherein.

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