JP3358868B2 - Expanded polypropylene resin particles and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to expanded polypropylene resin particles and a method for producing the same.

[0002]

2. Description of the Related Art Polypropylene resin foams have excellent physical properties such as cushioning properties and heat insulation properties, and are therefore used in various fields such as packaging materials, cushioning materials, heat insulation materials and building materials. Have been. The polypropylene resin foam is used after being formed into a shape according to the purpose of use. As a method of obtaining a foam having a desired shape, an in-mold molding method of filling foamed particles in a mold, heating with steam or the like and fusing the foamed particles together to obtain a foam having a predetermined shape is complicated. This is a preferred method that can be obtained relatively easily even in the form of a product, and the in-mold foam molded product obtained by the in-mold molding method is used for many applications.

[0003] By the way, a foamed molded article in a polypropylene resin mold has a property of being easily charged with static electricity. However, in recent years, as the field of application of foamed molded articles in polypropylene resin molds has expanded, the electrification of static electricity on foamed molded articles has become a major problem depending on the application, and it is necessary to impart antistatic ability to foamed molded articles. Has occurred.

[0004] For this reason, an antistatic agent is impregnated into foamed particles used for producing a foamed molded article of a polypropylene resin, or an antistatic agent is spray-coated on the surface of the foamed particles to impart an antistatic ability to the foamed particles. Have been tried. However, when the antistatic agent is impregnated or spray-applied, the antistatic agent is impregnated in the vicinity of the particle surface or only adheres to the surface, so the amount of the antistatic agent is hardly sufficient. The foamed molded article obtained by using such foamed particles has a disadvantage that the antistatic ability is poor in persistence and a complicated operation for impregnating or spraying the foamed particles with an antistatic agent is required. there were.

On the other hand, attempts have been made to incorporate an antistatic agent into resin particles, which are raw materials for producing expanded particles. In order to produce resin particles, generally, a base resin is melted in an extruder, and extruded from the extruder into a strand,
Then, a method of cutting the resin is adopted. When an antistatic agent is contained in the resin particles, the work process is complicated because the antistatic agent may be kneaded at the same time as the base resin is melted in the extruder. In addition to the above, there is an advantage that a large amount of an antistatic agent can be contained in the resin particles, so that the durability of the antistatic ability is excellent.

However, if the antistatic agent is previously kneaded into the raw material resin for producing the foamed particles, it is conventionally difficult to obtain foamed particles having an excellent antistatic ability having a surface resistivity of 1 × 10 ¹² Ω or less. In particular, in the case of a non-crosslinked polypropylene resin, the fusion property between the foamed particles during molding is reduced, and poor fusion of the particles is likely to occur in the foamed molded article. When a foamed molded article is used as a packaging material or the like, the foamed molded article is cut to form a concave portion, and the product to be packaged is often stored in the concaved portion. However, it is not possible to expect an antistatic effect on the cut surface of the foamed molded article, or it takes an extremely long time to exhibit a certain antistatic ability. And so on.

[0007] The above problems are mainly caused by the bleeding of the antistatic agent contained in the foamed particles on the surface of the foamed particles. The antistatic agent kneaded in the resin needs to have appropriate compatibility with the resin. If the compatibility of the antistatic agent with the resin is too good (too high), the antistatic agent may bleed. And the antistatic effect is not exhibited. Conversely, if the compatibility of the antistatic agent with the resin is too poor (too low), the antistatic agent bleeds in large quantities in a short period of time, causing a reduction in the sustainability of the antistatic ability and stickiness of the foamed particles and the foamed molded product surface. In addition, there arise problems such as a decrease in fusion property of the particles when molding the expanded particles.

For the purpose of imparting antistatic performance to the foamed molded article, the antistatic agent contained in the foamed particles may bleed after molding the foamed particles.
If a large amount of the antistatic agent bleeds in the state of the foamed particles, there may be disadvantages such as impairing the fusibility when molding the foamed particles as described above, but a favorable result may be produced. Absent. Therefore, before molding the expanded particles, a large amount of antistatic agent does not bleed,
It is preferable to control the compatibility of the antistatic agent with the resin so that the bleeding speed is such that a minimum necessary amount of the antistatic agent bleeds after molding. However, conventionally, it has been difficult to control the antistatic agent to have an appropriate compatibility with the resin.

The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, they are foamed particles obtained from a polypropylene resin containing a specific amount of a nonionic surfactant and having a specific crystal structure, and Since the foamed particles also have a specific crystal structure, the antistatic agent bleeds at an appropriate speed, and the foamed molded article obtained from the foamed particles exhibits excellent antistatic ability over a long period of time. The present invention has been found to be able to obtain a foam molded article excellent in the fusion property of the particles without inhibiting the fusion property of the foam particles at the time of molding, thereby completing the present invention.

[0010]

In other words, the polyolefin resin foamed particles of the present invention are prepared by adding a nonionic surfactant having an average molecular weight of 200 to 1,000 having antistatic ability to 0.1 to 0.1%.
In a DSC curve containing 5.0% by weight and obtained by differential scanning calorimetry (however, a DSC curve obtained when 2 to 6 mg of resin particles are heated to 220 ° C at 10 ° C / min by a differential scanning calorimeter) Heat of fusion 50-95J
/ G foamed polypropylene resin particles obtained by foaming resin particles having a polypropylene resin as a base resin, wherein the foamed particles have a DSC curve obtained by differential scanning calorimetry (provided that the foamed particles are 2 to 6 mg). Has a crystal structure in which a high-temperature peak appears on the higher temperature side than the intrinsic peak, together with a characteristic peak in a DSC curve obtained when the temperature is raised to 220 ° C. at 10 ° C./min by a differential scanning calorimeter. Is 10 to 30 J / g.

The method for producing expanded polypropylene resin particles according to the present invention comprises 0.1 to 5.0% by weight of a nonionic surfactant having an antistatic ability having an average molecular weight of 200 to 1000, and a differential scanning calorimeter. D obtained by measurement
A polypropylene resin having a heat of fusion of 50 to 95 J / g in an SC curve (a DSC curve obtained when 2 to 6 mg of resin particles are heated to 220 ° C. at 10 ° C./min by a differential scanning calorimeter) is used as a base resin. The resin particles to be dispersed in a dispersion medium in the presence of an inorganic gas-based blowing agent in a closed container and the resin particles impregnated with the blowing agent at a temperature equal to or higher than the temperature at which the resin particles soften, and the dispersion medium are placed in the container. A DSC curve obtained by differential scanning calorimetry after releasing the resin particles under a lower pressure to expand the resin particles (however, obtained by heating 2 to 6 mg of expanded particles to 220 ° C at 10 ° C / min by a differential scanning calorimeter) (DSC curve), a characteristic feature is to obtain expanded particles having a crystal structure in which a high-temperature peak appears on the higher temperature side with respect to the intrinsic peak together with the intrinsic peak, and the calorific value of the high-temperature peak is 10 to 30 J / g. And

In the present invention, as the nonionic surfactant, a higher fatty acid glycerin ester or a higher alkylamine is preferable.

The foamed particles of the present invention can be obtained by impregnating polypropylene resin particles with a foaming agent and foaming. As the polypropylene resin as the base resin,
For example, polypropylene, ethylene-propylene block copolymer, ethylene-propylene random copolymer,
Examples thereof include a butene-propylene random copolymer and an ethylene-butene-propylene random copolymer. Among them, an ethylene-propylene random copolymer and an ethylene-butene-propylene random copolymer are particularly preferable. These resins may be non-cross-linked or cross-linked, but are preferably non-cross-linked from the viewpoint of recycling.

The polypropylene resin particles have a heat of fusion of 50 to 95 J / g on a DSC curve obtained by differential scanning calorimetry. This DSC curve means that 2 to 6 mg of the resin particles was measured by a differential scanning calorimeter.
FIG. 1 is a DSC curve obtained when the temperature was increased to 220 ° C. at a temperature increasing rate of 200 ° C./min, and the heat of fusion was, as shown in FIG. DS) until the temperature returns to the baseline at
The part surrounded by the C curve and the baseline (the shaded part)
Is the amount of heat corresponding to

The polypropylene resin particles contain 0.1 to 5.0% by weight, preferably 0.1 to 5.0% by weight, of a nonionic surfactant having an average molecular weight of 200 to 1000 having antistatic ability.
It is contained in an amount of 3 to 3.0% by weight. Nonionic surfactants have better thermal stability than cationic and amphoteric surfactants,
The resin is melted at a high temperature of 150 to 220 ° C. and kneaded into the resin, or the antistatic ability is deteriorated by heat generated when the foamed particles are filled in a mold and heat molded, or the resin is colored. Nothing. In addition, antistatic properties are superior to anionic surfactants, and nonionic surfactants are less likely to elute into water from resins than anionic, cationic or amphoteric surfactants. For this reason, when extruding the resin into which the surfactant is kneaded into water in the form of a strand and cooling, or when foaming the resin particles, the resin particles and the foaming agent are dispersed in a dispersion medium such as water in a closed container, Even when the resin particles are impregnated with a foaming agent and then the resin particles are discharged from the container and foamed, there is no fear that the surfactant is eluted from the resin and the antistatic ability is reduced.

Examples of the nonionic surfactant include an N, N- (2-hydroxyethyl) alkylamine represented by the following formula (1), a higher fatty acid ester of monoglycerin represented by the following formula (2), and a diester represented by the following formula (3): A higher fatty acid ester of glycerin, a higher sorbitan fatty acid ester represented by the following chemical formula (4) or (5), and an alkyl diethanolamide represented by the following chemical formula (6) are preferable, and a higher alkyl amine and a higher fatty acid glycerin ester are particularly preferable.

[0017]

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[0018]

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[0019]

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[0020]

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[0021]

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[0022]

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The above surfactants can be used alone or in combination of two or more. When a nonionic surfactant having an average molecular weight of less than 200 is used, the durability of the antistatic ability becomes poor. When a surfactant having an average molecular weight of more than 1000 is used, the time required for expressing the antistatic ability is two weeks. It is not preferable because it becomes longer than the above. The content of the surfactant in the resin particles is 0.1 to 5.0.
When the amount is less than 0.1% by weight, sufficient antistatic ability is not provided, and when the amount of the antistatic agent exceeds 5% by weight, the fusion property of the expanded particles becomes poor, and In many cases, it becomes impossible to obtain a suitable foamed molded article.

The resin particles containing the above surfactant have a diameter (D) of 0.5 to 2 mm and a length (L) of 0.5 to 2 mm.
3 mm and the ratio of length to diameter (L / D) is 1.5 to 2.
About 5 is preferable.

As a method for obtaining a resin particle containing a surfactant, a polypropylene resin and a nonionic surfactant are melt-kneaded in an extruder, and then the melt-kneaded product is extruded into a strand from an extruder, and cooled. A method of cutting a strand into granules can be used. Specifically, a nonionic surfactant is added to the resin at a ratio of 2 to 20% by weight, and the nonionic surfactant is added to the resin by a three-roller, a kneader, an extruder or the like.
Heated to 0 to 250 ° C, melt-kneaded to create a masterbatch, then the final surfactant content is 0.1 to
The master batch and the polypropylene-based resin containing no surfactant are melt-mixed so as to be 5.0% by weight, and are extruded into a strand form from an extruder as described above, and cut to obtain resin particles. it can. According to the above method (master batch method), the surfactant can be uniformly dispersed and contained in the resin particles. If necessary, a colorant such as a pigment and various additives can be added together with the surfactant when producing the resin particles.

The expanded particles of the present invention were prepared by adding 2 to 6 mg of expanded particles at a temperature rising rate of 10 ° C./min by a differential scanning calorimeter to 220 mg.
As shown in FIG. 2, the DSC curve obtained when the temperature was raised to 0 ° C. shows, as shown in FIG.
A high-temperature peak 2 which is higher in temperature than the intrinsic peak 1 appears, and has a crystal structure in which the amount of heat of the high-temperature peak 2 (the amount of heat corresponding to the hatched portion in FIG. 2) is 10 to 30 J / g.

The above-mentioned intrinsic peak 1 and high-temperature peak 2 are the DSC curves obtained by performing the first measurement under the above conditions (FIG. 2).
After the first measurement, after cooling to around room temperature,
The discrimination can be made by comparing the DSC curves (not shown) obtained by performing the second measurement under the same conditions. The unique peak 1 is an endothermic peak unique to the polypropylene resin constituting the expanded particles, and the unique peak 1 is the first D peak.
This is a peak that appears in both the SC curve and the second DSC curve (however, the temperature of the peak is between the first and second DSC curves).
It may be slightly different from the first time. ).

On the other hand, the high temperature peak 2 is defined as the first DS
This endothermic peak appears on the higher temperature side than the intrinsic peak 1 in the C curve, and does not appear in the second DSC curve. Foamed particles for which the high-temperature peak 2 does not appear in the first DSC curve have poor moldability when foamed and molded in a mold, and it is difficult to obtain a foamed molded body having good properties. The first D
It is desirable that the difference between the temperature of the peak of the high temperature peak 2 appearing in the SC curve and the temperature of the peak of the unique peak in the second DSC curve is large, and the difference between the two is 5 ° C. or more, preferably 10 ° C. or more. .

The expanded polypropylene resin particles having a high temperature peak 2 in the DSC curve have a different crystal structure from the expanded particles having no high temperature peak 2. As described above, the unique peak 1 appears almost similarly in the first DSC curve and the second DSC curve, whereas the high-temperature peak 2 appears only in the first DSC curve and measured under the same conditions. The crystal structure in which the high-temperature peak 2 appears is not caused by the crystal structure of the base resin itself, but is represented by the polypropylene resin in the form of expanded particles, since the crystal structure does not appear in the second DSC curve of the second round. This is considered to be due to the crystal structure of the expanded particles. Expanded polypropylene resin particles having a high-temperature peak 2 together with an intrinsic peak 1 can be obtained by adjusting foaming conditions such as a foaming temperature and a holding time at the foaming temperature.

The foamed particles of the present invention are prepared by dispersing a surfactant-containing resin particle produced by the above-described method or the like in a dispersion medium in the presence of a foaming agent in a closed container, and the resin particles are softened. The resin particles are impregnated with a foaming agent at a temperature (foaming temperature) or higher, and then one end of the container is opened to allow the resin particles and the dispersion medium to be exposed to an atmosphere at a lower pressure than the inside of the container (usually large). (Atmospheric pressure) to foam the resin particles.

In the above foaming method, when the resin particles are heated to the foaming temperature, the melting end temperature of the resin particles: Te
Without raising the temperature above (° C.), the foaming temperature is raised to the melting point of the resin: Tm (° C.)-20 ° C. or higher, and the melting end temperature: Te.
By setting the temperature to a temperature lower than (° C.) and appropriately adjusting the rate of temperature rise to the foaming temperature, foamed particles having the above-described intrinsic peak 1 and high-temperature peak 2 can be obtained. In addition, the foaming temperature is appropriately adjusted within the above range, and a sufficient time (normally about 5 to 45 minutes) at a temperature less than Te and near Te before heating the resin particles to the foaming temperature. ), And once held, the value of the melting energy of the high temperature peak can be adjusted. In addition, the value of the calorific value at the high temperature peak is more greatly affected by the foaming temperature than the holding time near Te.

The above melting point: Tm is defined as DS of resin particles.
The melting end temperature: Te, which is the temperature of the peak of the unique peak appearing in the C curve, is the temperature at which the tail of the unique peak appearing in the DSC curve of the resin particles returns to the baseline position on the high temperature side. Since the second DSC curve of the expanded particles and the Tm and Te indicated by the DSC curves of the resin particles are substantially the same temperature and the heats of fusion are also substantially the same, the second DSC curve of the expanded particles is different from the second DSC curve of the expanded particles. Approximate values of Tm, Te and heat of fusion shown by the DSC curve of the resin particles are obtained.

The foaming agents used to obtain the foamed particles include propane, butane, pentane, hexane, cyclobutane, cyclopentane, cyclohexane, trichlorofluoromethane, dichlorodifluoromethane, chlorofluoromethane, trifluoromethane, 1,2 Volatile blowing agents such as, 2,2-tetrafluoroethane, 1-chloro-1,1-difluoroethane, 1,1-difluoroethane, 1-chloro-1,2,2,2-tetrafluoroethane, nitrogen, Inorganic gas foaming agents such as carbon dioxide, argon, and air are used, and among them, an inorganic gas-based foaming agent containing as a main component an inexpensive inorganic gas, which has no risk of destruction of the ozone layer, is preferable. As the inorganic gas-based blowing agent, nitrogen, air, and carbon dioxide are particularly preferable, and a mixture of an inorganic gas blowing agent as a main component and a volatile blowing agent can also be used.

The amount of the foaming agent used is about 1 to 50 parts by weight per 100 parts by weight of the resin particles for the foaming agents other than nitrogen and air.
It is press-fitted into a closed container at a pressure of 0 to 60 kgf / cm ² G. The actual amount of the foaming agent used is selected in consideration of the relationship between the bulk density of the foam particles to be obtained and the foaming temperature, the heat of fusion, and the like.

The dispersion medium for dispersing the resin particles in the closed container may be any one that does not dissolve the resin particles. Examples of such a dispersion medium include water, ethylene glycol, glycerin, methanol, ethanol, and the like, but water is usually used.

When heating the resin particles dispersed in the dispersion medium to the foaming temperature, an anti-fusing agent can be added to the dispersion medium to prevent fusion between the resin particles. As the anti-fusing agent, any inorganic or organic one can be used as long as it does not dissolve in a dispersion medium such as water and does not melt by heating. In general, an inorganic anti-fusing agent is preferable. Powders such as kaolin, talc, mica, aluminum oxide, titanium oxide, and aluminum hydroxide are suitable as the inorganic anti-fusion agent. It is preferable to use an anionic surfactant such as sodium dodecylbenzenesulfonate or sodium oleate as a dispersing aid.

The anti-fusing agent has an average particle size of 0.001 to 0.001.
Those having a thickness of 100 μm, particularly 0.001 to 30 μm, are preferred. The addition amount of the anti-fusing agent is usually preferably 0.01 to 10 parts by weight per 100 parts by weight of the resin particles. The surfactant is preferably added in an amount of usually 0.001 to 5 parts by weight per 100 parts by weight of the resin particles.

[0038]

The present invention will be described below in further detail with reference to examples and comparative examples. Examples 1 to 5 and Comparative Examples 1 to 7 After adding 5% by weight of the surfactants shown in Table 1 to the polypropylene resins shown in Table 1, kneading them sufficiently with a pressure kneader heated to 140 to 150 ° C. After cooling, the mixture was granulated with a square pelletizer to obtain a master batch of granular surfactant. Similarly, a master batch of aluminum hydroxide was granulated. Next, these master batches and the same type of resin containing no surfactant were mixed in such a manner that the content of aluminum hydroxide was 0.05% by weight and the content of surfactant was a value shown in Table 1. And mix in an extruder for 20
The mixture was heated and melted at 0 to 220 ° C., kneaded, extruded into strands, and then granulated with a pelletizer to obtain polypropylene resin particles having a diameter (D) of 1 mm and a length (L) of 2 mm.

[0039]

[Table 1]

1 kg of the surfactant-containing resin particles obtained as described above was placed in a closed container having an internal volume of 5 liters,
4 g of kaolin as an anti-fusing agent and 0.3 g of sodium dodecylbenzenesulfonate as a dispersing aid were mixed.
Dispersed in 000 cc of water, using 40-90 g of carbon dioxide as a foaming agent in a container with dry ice, and without stirring, without raising the temperature to a temperature higher than the melting end temperature of the resin,
It was kept at a holding temperature of 138 to 151 ° C. for 15 minutes. Thereafter, the temperature is further raised to the foaming temperature of 143 to 156 ° C. and maintained for 15 minutes. Thereafter, carbon dioxide is introduced into the vessel to apply a back pressure equal to the equilibrium vapor pressure of the foaming agent, thereby keeping the vessel internal pressure constant. Then, one end of the container was opened, and the resin particles and the dispersion medium were simultaneously released under the atmospheric pressure to foam the resin particles.

Table 2 shows the properties of the obtained expanded particles. The foamed particles were aged at room temperature under normal pressure for 48 hours, aged, filled in a mold, and heat-molded with steam. The properties of the obtained foamed molded product are shown in Table 2.

[0042]

[Table 2]

* 2: The state of the foamed particles was determined according to the following criteria by observing the shape of the foamed particles, the presence or absence of shrinkage, the size of bubbles in the particles, and the like.・ ・ ・: No deformation, shrinkage of the shape, and no fine bubbles were observed. Δ: Some particles in which deformation, shrinkage, and miniaturization of bubbles have occurred. X: There are extremely many particles in which deformation, shrinkage, and fineness of air bubbles have occurred.

* 3: The surface smoothness of the foamed molded article was observed by observing the surface of the foamed molded article. ×: Poor surface smoothness, large irregularities and wrinkles. Was evaluated.

* 4: The fusion property of the particles in the foamed molded article was determined by pulling a test piece obtained by cutting the molded article to a length of 150 mm, a width of 50 mm, and a thickness of 10 mm at a speed of 500 mm / min using a tensile tester. Break and observe the state of the fracture surface. ○: Breakage between materials occurs, and there is almost no cutting between particles. ×: Cut between particles. Was evaluated.

* 5: The surface resistance of the skin layer of the molded product is
After molding, the molded body was cured for one day under the conditions of a temperature of 20 ° C. and a relative humidity of 65%, and the measurement was performed.

* 6: The surface specific resistance of the cut surface of the molded body was measured after the molded body was cut and cured for 10 days at a temperature of 20 ° C. and a relative humidity of 65%.

[0048]

As described above, the expanded polypropylene resin particles of the present invention contain a nonionic surfactant having a molecular weight of 200 to 1000 and having an antistatic ability at a specific ratio and have a specific heat of fusion. It is a foamed particle composed of polypropylene resin particles, and also has a crystal structure in which a unique peak and a high temperature peak appear in a DSC curve obtained by differential scanning calorimetry of the foamed particle, and the melting energy of the high temperature peak is in a specific range. Thereby, the foamed particles of the present invention have excellent fusion bonding property during molding, and the foamed molded article obtained from the foamed particles of the present invention has excellent antistatic ability, and also has a sustained antistatic ability. Has an excellent effect. According to the present invention, when the foamed particles contain an antistatic agent, to ensure the fusion of the foamed particles at the time of molding, and to impart excellent antistatic ability to the obtained foamed molded article. Both of the problems can be satisfied simultaneously in the expanded polypropylene resin particles in which it was difficult to achieve both.

Further, according to the method of the present invention, there is no problem of variation in expansion ratio when using an inorganic gas-based blowing agent, and effects such as improvement of expansion ratio and uniformization of cell diameter can be expected. Expanded particles can be produced reliably.

[Brief description of the drawings]

FIG. 1 shows a DSC curve obtained by differential scanning calorimetry of a resin particle containing a surfactant before foaming.

FIG. 2 shows a DSC curve obtained by differential scanning calorimetry of expanded particles.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-28239 (JP, A) JP-A-60-110934 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08J 9/16 C08L 23/10

Claims (6)

(57) [Claims] 1. An average molecular weight having an antistatic ability of 200 to 200.
DS containing 0.1 to 5.0% by weight of a nonionic surfactant of 1000 and obtained by differential scanning calorimetry
C curve (however, D obtained when 2 to 6 mg of resin particles were heated to 220 ° C. at 10 ° C./min by a differential scanning calorimeter)
(SC curve) are expanded polypropylene resin particles obtained by foaming resin particles containing a polypropylene resin having a heat of fusion of 50 to 95 J / g as a base resin, and the expanded particles are obtained by differential scanning calorimetry. DSC curve (However, 2 to 6 mg of expanded particles were measured by a differential scanning calorimeter.
(A DSC curve obtained when the temperature is raised to 220 ° C. at 0 ° C./min), has a crystal structure in which a high-temperature peak appears on the higher temperature side than the specific peak along with a specific peak, and the calorific value of the high-temperature peak is 10 to 30 J / G foamed polypropylene resin. 2. The expanded polypropylene resin particles according to claim 1, wherein the nonionic surfactant is a higher fatty acid glycerin ester. 3. The expanded polypropylene resin particles according to claim 1, wherein the nonionic surfactant is a higher alkylamine. 4. A DS containing 0.1 to 5.0% by weight of a nonionic surfactant having an antistatic ability having an average molecular weight of 200 to 1000 and obtained by differential scanning calorimetry.
C curve (however, D obtained when 2 to 6 mg of resin particles were heated to 220 ° C. at 10 ° C./min by a differential scanning calorimeter)
The resin particles having a heat of fusion of 50 to 95 J / g as a base resin in an SC curve) are dispersed in a dispersion medium in the presence of an inorganic gas-based blowing agent in a closed container to soften the resin particles. A resin curve impregnated with a foaming agent and a dispersion medium at a temperature higher than the temperature are discharged under low pressure from the container to foam the resin particles, and a DSC curve obtained by differential scanning calorimetry (provided that the expanded particles are 2 to 6 mg) Has a crystal structure in which a high-temperature peak appears on the higher temperature side than the intrinsic peak, together with a characteristic peak in a DSC curve obtained when the temperature is raised to 220 ° C. at 10 ° C./min by a differential scanning calorimeter. A method for producing expanded polypropylene resin particles, characterized by obtaining expanded particles having a calorific value of 10 to 30 J / g. 5. The method for producing expanded polypropylene resin particles according to claim 4, wherein the nonionic surfactant is a higher fatty acid glycerin ester. 6. The method according to claim 4, wherein the nonionic surfactant is a higher alkylamine.