JP2014001334A - Polypropylene-based resin foam particle, and polypropylene-based resin foam particle molded body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide foam particles which are polypropylene-based resin foam particles that have a low apparent density (high foaming ratio) and are excellent in lightness, and can obtain a polypropylene-based resin foam particle molded body which is excellent in hue and is uniform.SOLUTION: Polypropylene-based resin foam particles contain a specific sulfate compound and a coloring agent, and have an apparent density of 7 to 80 g/L. An average foam diameter (Ls) in a surface layer section of the foam particles is 70 μm or more, and a ratio (La/Ls) between an average foam diameter (La) in the whole foam particles and the average foam diameter (Ls) in the surface layer section of the foam particles is 1.0 to 2.2.

Description

  The present invention relates to expanded polypropylene resin particles, and more particularly, to expanded polypropylene resin particles used to obtain colored expanded polypropylene resin particles and colored expanded polypropylene resin particles.

  Polypropylene-based foamed resin particles can be molded into various shapes according to the application, and have excellent mechanical properties, heat resistance, shock-absorbing properties, and workability. It has been widely used for absorbent materials. As a method for producing the foamed particles, a so-called dispersion medium releasing foaming method may be mentioned as a suitable method. In the so-called dispersion medium releasing foaming method, after the polypropylene resin particles are dispersed in an aqueous dispersion medium in a sealed container, the resin particles are impregnated with a physical foaming agent under heating to obtain expandable resin particles. The foamable resin particles are produced by discharging the foamable resin particles together with the aqueous dispersion medium under a lower pressure than in the sealed container to cause foaming.

  However, in recent years, the use of the expanded particles has been expanded, and a molded article having a higher expansion ratio and improvement in physical properties have been demanded. As a result, attempts have been made to improve the dispersion medium discharge foaming method as disclosed in Patent Document 1 and Patent Document 2.

  Patent Document 1 discloses the use of resin particles obtained by blending a metal borate such as zinc borate as a foam regulator when producing polyolefin resin foam particles. Patent Document 1 describes that polyolefin foamed resin particles having a uniform cell diameter can be produced by this method.

  Patent Document 2 discloses a method for producing polypropylene resin expanded particles, in which a water-absorbing substance having no cell nucleus forming action and a cell nucleus agent are mixed and used. In Patent Document 2, by using these water-absorbing substances and a cell nucleating agent in combination, it becomes easy to control the apparent density and the cell diameter. Is obtained.

International Publication No. 1998/025996 JP 2009-167236 A

  According to the method described in Patent Document 1, it is possible to obtain expanded polypropylene resin particles having a low apparent density and a uniform cell diameter using an inorganic physical foaming agent. However, in order to obtain expanded particles having a lower apparent density (high expansion ratio), the amount of metal borate added must be increased. As a result, the metal salt of borate added in a large amount also acts as a cell nucleating agent, resulting in a new problem that the cell diameter of the obtained expanded particles becomes fine.

  Further, in the production method of Patent Document 2, in order to obtain expanded particles having a lower apparent density, a large amount of the water-absorbing substance has to be added. As a result, when melt-kneading the polypropylene resin and the water-absorbing substance, the extrusion stability is lowered and it becomes difficult to obtain resin particles stably, or by using a large amount of the water-absorbing substance. Problems such as increased manufacturing costs occurred. Thus, the manufacturing method of patent document 2 has left the subject regarding industrially stably obtaining expanded particles.

  Therefore, the present inventors tried to blend a specific sulfate compound into polypropylene resin particles. By blending this sulfate compound, it becomes possible to obtain resin particles stably without lowering the extrusion stability, with a smaller apparent density (higher expansion ratio) than before, and a large and uniform bubble diameter. It became possible to obtain an expanded foam particle.

  However, in the case of expanded particles obtained by the method described in Patent Document 2 or a method of blending a specific sulfate compound, there is a new problem that the color of the colored expanded particle molded body obtained from the expanded particles is lowered. Occurred.

  As a result of studying the factors that reduce the color of the colored foamed particle molded body, the present inventors have found that in the case of foamed particles obtained by the method described in Patent Document 2 or a method of blending a specific sulfate compound, It has been found that although the overall average bubble diameter is large, the bubbles in the surface layer portion of the expanded particles remain fine. The problem that the bubbles in the surface layer portion of the expanded particles become finer is particularly noticeable when the expanded particles are colored, and it is not easy to obtain an expanded particle molded body having a desired color. In other words, if molding is performed using foam particles in which the air bubbles in the surface layer portion are miniaturized, the color of the resulting foam particle molded product becomes thin, so that color unevenness on the surface becomes remarkable and the appearance is impaired. The commercial value of the foamed particle molded body may be reduced.

  The present invention solves the above-mentioned problems, and despite the fact that it is a polypropylene resin foamed particle having a low apparent density (high foaming ratio) and excellent in lightness, it is possible to prevent the bubbles in the surface layer portion of the foamed particle from being refined. It is an object of the present invention to provide expanded particles capable of obtaining a molded polypropylene resin expanded particle having a good color and uniformity.

According to the present invention, the following polypropylene resin expanded particles are provided.
[1] A polypropylene system having an apparent density of 10 to 80 g / L, containing a sulfate compound represented by the following general formula (I) (hereinafter sometimes referred to as sulfate compound (I)) and a colorant. The foamed resin particle has an average cell diameter (Ls) in the surface layer portion of the expanded particle of 70 μm or more, an average cell diameter (La) in the entire expanded particle and an average cell diameter (Ls in the surface layer portion of the expanded particle) ) And a ratio (La / Ls) of 1.0 to 2.2.
M ^I M ^III (SO ₄ ) ₂ ... (I)
(However, M ^I represents a monovalent cation and M ^III represents a trivalent cation.)

[2] The polypropylene resin foamed particles according to the above item 1, wherein the content of the sulfate compound (I) is 0.01 to 1 part by weight with respect to 100 parts by weight of the polypropylene resin.
[3] The above 1 or 2, wherein the colorant is carbon black, and the content of the carbon black is 0.1 to 5 parts by weight with respect to 100 parts by weight of the polypropylene resin. Expanded polypropylene resin particles.
[4] A polypropylene resin foamed particle molded body having an apparent density of 7 to 80 g / L obtained by in-mold molding of the foamed particles according to any one of 1 to 3 above.

  By containing the specific sulfate compound (I), the polypropylene resin expanded particles of the present invention have a low apparent density and a large expanded cell diameter. Further, since the reduction of the cell diameter of the foamed particle surface layer portion is prevented, the polypropylene resin foamed particle molded body obtained by in-mold molding the colored foamed particle of the present invention has the color of the molded body. Reduction and occurrence of color unevenness are prevented.

FIG. 1 is a photomicrograph of the cross section of the expanded particles obtained in Example 1. FIG. 2 is a photomicrograph of the cross section of the expanded particles obtained in Comparative Example 1. FIG. 3 is a drawing showing an example of the DSC curve of the first heating.

Hereinafter, the polypropylene resin expanded particles of the present invention will be described in detail.
The polypropylene resin expanded particles of the present invention (hereinafter also simply referred to as expanded particles) are composed of a polypropylene resin, and the polypropylene resin contains a sulfate compound (I) represented by the following general formula (I). To do. By containing the sulfate compound (I), the foamed particles of the present invention have a large cell diameter of the whole foamed particles even though the apparent density is small.
M ^I M ^III (SO ₄ ) ₂ (I)
In the general formula (I), M ^I represents a monovalent cation and M ^III represents a trivalent metal ion.

Examples of the polypropylene resin used in the present invention include a propylene homopolymer or a copolymer with another olefin copolymerizable with propylene. Examples of other olefins copolymerizable with propylene include ethylene, 1-butene, isobutylene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3,4-dimethyl-1-butene, and 3-methyl. Examples thereof include α-olefins having 4 to 10 carbon atoms such as -1-hexene. The copolymer may be a random copolymer or a block copolymer, and may be not only a binary copolymer but also a ternary copolymer.
In addition, the content of other olefins copolymerizable with propylene in the copolymer is preferably 25% by mass or less, and more preferably 15% by mass or less. These polypropylene resins can be used alone or in combination of two or more.

The lower limit of the melting point of the polypropylene resin is preferably 120 ° C., more preferably 125 ° C. in consideration of in-mold moldability. On the other hand, the upper limit of the melting point is usually about 170 ° C. Further, the polypropylene resin preferably has a melt flow rate (MFR) of 0.3 to 100 g / 10 min, particularly 1 to 90 g, considering the heat resistance of the foamed molded product and the foaming efficiency at the time of producing foamed particles. / 10 minutes are preferred. MFR is JIS
It is a value measured under test condition M of K7210 (2010).

  The melting point of the polypropylene resin is determined as follows in accordance with JIS K7121-1999. That is, about 2 to 10 mg of a polypropylene resin to be measured by a differential scanning calorimeter is heated to 220 ° C. at a temperature increase rate of 10 ° C./min, and immediately after reaching the temperature of 220 ° C., a cooling rate of 10 ° C./min. After cooling from 220 ° C. to 30 ° C., the temperature at the top of the endothermic peak (inherent peak) in the DSC curve obtained when heating from 30 ° C. to 220 ° C. again at a heating rate of 10 ° C./min is the melting point of the polypropylene resin. Asking. When two or more endothermic peaks derived from the polypropylene resin are observed, the temperature at the top of the largest endothermic peak is defined as the melting point of the polypropylene resin.

  The polypropylene resin can contain other thermoplastic resin components and elastomer components within a range that does not impair the effects of the present invention. Examples of other thermoplastic resin components and elastomer components include vinyl acetate resin, thermoplastic polyester resin, acrylic ester resin, methacrylic ester resin, polystyrene resin, polyethylene resin, polyamide resin, fluororesin, and ethylene-propylene. Examples thereof include rubber, ethylene-propylene-diene rubber, ethylene-acrylic rubber, chlorinated polyethylene rubber, and chlorosulfonated polyethylene rubber. The content of resins other than these polypropylene resins is preferably 30% by weight or less, and more preferably 15% by weight or less.

The sulfate compound (I) contained in the polypropylene resin is represented by the following general formula (I).
M ^I M ^III (SO ₄ ) ₂ ... (I)
(In the general formula (I), M ^I represents a monovalent cation and M ^III represents a trivalent metal ion)
Examples of trivalent metal ions include aluminum, gallium, indium, thallium, titanium, vanadium, chromium, manganese, iron, cobalt, rhodium, iridium, etc., and monovalent cations include potassium, sodium, Examples include rubidium, cesium, ammonium, thallium and the like. Among these, aluminum and iron are preferable as the trivalent metal ion, and potassium, sodium, and ammonium are preferable as the monovalent cation. As specific sulfate compounds (I), potassium aluminum sulfate and aluminum ammonium sulfate are highly safe substances that are also used in food additives, and foam particles having a uniform and large cell diameter are stable. It is preferable from the point obtained.

  Further, as the sulfate compound (I), those having the property of high solubility in water in a high temperature region are preferable. The sulfate compound (I) having high solubility in water in the high temperature region is excellent in water absorption in the high temperature region. Therefore, when resin particles containing the sulfate compound (I) having such characteristics are used, the apparent density can be reduced in the foamed particle production process by the dispersion medium discharge foaming method described later. The reason why the apparent density of the expanded particles can be further reduced is that when the resin particles dispersed in the aqueous dispersion medium are impregnated with the physical foaming agent, the sulfate compound (I) is resinized from the aqueous dispersion medium. It is considered that water is efficiently drawn into the particles and works like a foaming agent when the water taken into the resin particles foams.

  The content of the sulfate compound (I) is preferably 0.01 to 1 part by weight with respect to 100 parts by weight of the polypropylene resin constituting the expanded particles. When the content is too small, there is a possibility that desired low apparent density expanded particles cannot be obtained. On the other hand, if the content is too large, the water vapor in the foamed particles condenses immediately after the foaming of the resin particles, and the foamed particles are likely to be contracted due to the reduced pressure inside the foamed particles. From this viewpoint, the content is preferably 0.02 to 0.8 parts by weight, more preferably 0.05 to 0.5 parts by weight. In the present specification, the content and addition amount of the sulfate compound (I) are calculated as the weight of the anhydride of the sulfate compound (I).

  The upper limit of the average particle diameter of the sulfate compound (I) is preferably 500 μm, more preferably 200 μm, and still more preferably 100 μm. On the other hand, the lower limit of the average particle diameter is preferably 1 μm. Within this range, the dispersibility of the sulfate compound (I) in the polypropylene resin becomes good, and it becomes easy to obtain expanded particles having a uniform cell diameter. In addition, the average particle diameter of sulfate compound (I) can be measured by the centrifugal sedimentation type particle size measuring method.

  The sulfate compound (I) is preferably added as an anhydride when the resin particles are produced. When the compound represented by the general formula (I) is an anhydride, the polypropylene resin particles (hereinafter also simply referred to as resin particles) used for the production of the expanded particles are produced using an extruder. No water is released from the sulfate compound (I) during melt-kneading in the extruder, preventing the extrusion stability from being lowered and the dispersibility of various additives from being lowered. . Further, when foaming the resin particles, it is prevented that excessive bubbles are generated in the foamed particles due to water desorbed from the sulfate compound (I). Expanded particles are obtained.

  In the dispersion medium releasing foaming method, foaming is performed after the water in the aqueous medium is drawn into the resin particles. Therefore, even when the sulfate compound (I) anhydride is used during the resin particle production, foaming is performed. The sulfate compound (I) contained in the particles is considered to exist not only in the anhydrous state but also as a hydrate.

The foamed particles of the present invention are those in which bubbles in the surface layer portion are prevented from being refined, and the average cell diameter (Ls) of the surface layer portion is required to be 70 μm or more. If the average cell diameter (Ls) is too small compared to the average cell diameter (La) of the entire foamed particles, the color of the colored foamed particles may be light. From this viewpoint, the average cell diameter (Ls) of the surface portion of the expanded particle is preferably 80 μm or more, more preferably 85 μm or more.
On the other hand, the upper limit of the average cell diameter (Ls) of the surface portion of the expanded particle is preferably substantially the same as the average cell size (La) of the entire expanded particle, more preferably 130 μm, and still more preferably 120 μm. is there.

  The average cell diameter (La) of the entire expanded particles is preferably 70 to 500 μm, more preferably 85 to 450 μm, still more preferably 100 to 400 μm, 110 to 300 μm, and particularly preferably 120 to 200 μm. If the average cell diameter of the entire foamed particles is too small, the thickness of the cell membrane is also thinned, so that it becomes easy to form continuous cells at the time of molding, and the in-mold moldability may be deteriorated. On the other hand, if the bubble diameter is too large, the thickness of the bubble film is increased, which may deteriorate the fusion between particles during in-mold molding.

The lower limit of the ratio (La / Ls) between the average cell diameter (La) of the entire expanded particle and the average cell size (Ls) of the expanded particle surface layer is preferably closer to 1.0, but is usually 1.5.

  On the other hand, the upper limit of the ratio (La / Ls) is 2.2. The ratio (La / Ls) being too large means that the average cell diameter (Ls) of the surface portion of the expanded particle is too small relative to the average cell size (La) of the entire expanded particle. When in-mold molding is performed using foamed particles having a ratio (La / Ls) that is too large, the color of the resulting foamed particle compact may be reduced or color unevenness may occur. From this viewpoint, the upper limit of the ratio (La / Ls) is preferably 2.0, and more preferably 1.9.

  As described above, the foamed particles of the present invention have an average cell diameter (Ls) of the surface layer portion of 70 μm or more, and the ratio (La / Ls) is within the above range. The resulting foamed particle molded body is not colored thinly and does not cause color unevenness. Therefore, even if the addition amount of the colorant is reduced, it is possible to easily obtain a dark colored foamed particle molded body having a good degree of color development.

In the present invention, the average cell diameter (Ls) of the surface part of the expanded particle and the average cell diameter (La) of the entire expanded particle are measured as follows.
For the measurement of the average cell diameter (La) of the entire foamed particles, ten foamed particles are arbitrarily selected from the obtained foamed particle group, and the foamed particles are cut into approximately two equal parts and foamed. A particle cross section is obtained, and the cross section is enlarged and projected with a microscope. In this enlarged projection view, four straight lines passing from the surface of the foamed particle to the other surface and passing through the center of the bubble cross section are drawn at approximately equal angular intervals. Subsequently, the sum of the lengths of the four straight lines (L μm) is divided by the sum of the number of bubbles on the four straight lines (Na) [L / Na] to obtain the calculated value. The average is the average cell diameter (La: μm) of the expanded particles.

  For the measurement of the average cell diameter (Ls) of the surface layer portion, first, a value (A μm) obtained by dividing the total length (L μm) of the four straight lines by 40 is obtained. And the range of A micrometer from the surface of an expanded particle is made into a surface layer part (two places with respect to one straight line). The total number of bubbles on the four straight lines (Ns) existing in the surface layer part is obtained, and the total length of straight lines (8 Aμm) of the four surface layer parts is calculated as four Dividing by the sum of the number of bubbles on the straight line (Ns) [8A / Ns], the obtained value is taken as the average bubble diameter (Ls: μm) of the surface layer portion of the expanded particles.

  The expanded particles of the present invention contain a colorant and are colored. Since the foamed particles have a large cell diameter in the surface layer portion, even when they are colored, the color does not become thin, and the finally obtained polypropylene resin foamed particle molded body has color unevenness and color. The appearance is not impaired and the appearance is not impaired.

  The colorant used in the present invention is not limited. Examples of the colorant include azo dyes, anthraquinone dyes, azine dyes, quinoline dyes, and perinone dyes, carbon black, and azo pigments. And pigments such as copper phthalocyanine pigments. Of these, carbon black is preferred because it provides calm black foam particles.

  The content of the colorant is preferably 0.1 to 5 parts by weight with respect to 100 parts by weight of the polypropylene resin. If the amount of the colorant is too small, the foamed particles may not be colored as desired. On the other hand, even if the amount of the coloring agent is excessively mixed, it is not possible to develop a beautiful color. Moreover, when there is too much content, in the case of a dye, it will become difficult to impregnate the whole quantity in a resin particle, and in the case of a pigment, it will become difficult to disperse | distribute uniformly. From this viewpoint, the content is preferably 0.3 to 4 parts by weight, and more preferably 0.5 to 5 parts by weight. If it is in the said range, the foamed particle molded object with a favorable color will be obtained.

  The expanded particles of the present invention have a low apparent density (high expansion ratio), and the lower limit of the apparent density is 10 g / L. If the lower limit is too low, mechanical properties such as compressive strength may be too low. From this viewpoint, the lower limit is preferably 15 g / L, and more preferably 20 g / L. On the other hand, the upper limit of the apparent density is 80 g / L. If the upper limit is too large, the advantage of lightness as a foam may be impaired. From this viewpoint, the upper limit is preferably 75 g / L.

  The apparent density of the expanded particles was measured by preparing a graduated cylinder containing water at 23 ° C., and leaving the graduated cylinder in the graduated cylinder at a relative humidity of 50%, 23 ° C. and 1 atm for about 2 days. The weight W) of the expanded particle group is sunk using a wire mesh or the like, and the volume V (L) of the expanded particle group is read from the rise in the water level. It can be determined by dividing (W / V) from the volume V (L) and the weight W (g) of the expanded particles in the graduated cylinder.

In the expanded particles of the present invention, an endothermic curve peak specific to a crystalline polypropylene-based resin in the DSC curve during the first heating obtained by differential scanning calorimetry (hereinafter also referred to simply as “DSC measurement”). It is preferable that one or more endothermic curve peaks (high temperature peaks) are present on the higher temperature side than (vertical peaks). Such foamed particles have a high closed cell ratio, good moldability within the mold, and can provide a foamed particle molded body having excellent mechanical strength.
The high temperature peak heat amount is preferably 1 to 50 J / g, and more preferably 2 to 30 J / g. The high temperature peak of the expanded particles can be adjusted by a well-known method, and specifically, the adjusting method is disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-151928.

The high temperature peak heat quantity of the expanded particles is measured as follows by a measuring method based on JIS K7122: 1999.
1 to 3 mg of expanded particles are collected, and the temperature rise is measured from 30 ° C. to 220 ° C. at 10 ° C./min with a heat flux differential scanning calorimeter. An example of the DSC curve obtained by such measurement is shown in FIG.
As a measuring device, DSCQ1000 manufactured by TA Instruments Inc. can be used.

In the DSC curve of FIG. 3, a specific peak a derived from the polypropylene resin constituting the expanded particles and a high temperature peak b are shown on the high temperature side of the specific peak, and the heat quantity of the high temperature peak b corresponds to the peak area. Specifically, it can be obtained as follows.
First, a straight line (α−β) connecting a point α corresponding to 80 ° C. on the DSC curve and a point β on the DSC curve corresponding to the melting end temperature T of the expanded particles is drawn. The melting end temperature T is the intersection of the DSC curve on the high temperature side of the high temperature peak b and the high temperature side baseline.
Next, a straight line parallel to the vertical axis of the graph is drawn from the point γ on the DSC curve corresponding to the valley between the intrinsic peak a and the high temperature peak b, and the point intersecting with the straight line (α−β) is defined as σ. . The area of the high-temperature peak b is the portion surrounded by the curve of the high-temperature peak b portion of the DSC curve, the line segment (σ-β), and the line segment (γ-σ) (the portion hatched in FIG. 3). This is the area, which corresponds to the amount of heat at the high temperature peak.

  In addition, although the high temperature peak b is recognized by the DSC curve at the time of the 1st heating measured as mentioned above, after obtaining the DSC curve of the 1st time, it is once 30 degreeC from 220 degreeC to 10 degreeC / min. It is not recognized in the second DSC curve obtained when the temperature is lowered to 220 ° C. at 10 ° C./min. In the DSC curve during the second heating, only an endothermic curve peak unique to the polypropylene resin constituting the expanded particles is observed.

The polypropylene resin constituting the expanded particles of the present invention can contain a cell nucleating agent. The cell nucleating agent is added for the purpose of uniformly dispersing in a polypropylene resin, adjusting the number of bubbles in the foamed particles, and generating more uniform cells. There is no particular limitation.
Examples of the cell nucleating agent used in the present invention include inorganic powders such as talc, calcium carbonate, aluminum hydroxide, zinc borate, kaolin, mica, silica, carbon black, barium sulfate, titanium oxide, clay, aluminum oxide, and diatomaceous earth. Can be illustrated. Moreover, organic type powders, such as a phosphoric acid type | system | group nucleating agent, a phenol type nucleating agent, an amine type nucleating agent, and polytetrafluoroethylene (PTFE), are mentioned.
In addition, since a pigment such as carbon black used at the time of coloring also functions as a cell nucleating agent, the cell nucleating agent may not be added to the colored product.

  The content of the cell nucleating agent is preferably 0.001 to 5 parts by mass, more preferably 0.005 to 2.5 parts by mass, and further 0.01 to 1 part by mass with respect to 100 parts by mass of the polypropylene resin. preferable.

  The average particle diameter of the cell nucleating agent is preferably 0.01 to 50 μm, more preferably 0.1 to 30 μm. The average particle diameter can be measured by a centrifugal sedimentation particle size measuring method.

  As long as the effects of the present invention are not impaired, additives such as slip agents, antistatic agents, flame retardants and the like can be blended with the polypropylene resin. When such an additive is used, the blending amount thereof is preferably in the range of 0.01 to 5 parts by mass with respect to 100 parts by mass of the polypropylene resin.

  Next, the manufacturing method of the polypropylene resin expanded particle of this invention is demonstrated. The production method includes at least a resin particle production process and a foamed particle production process. In the resin particle production process, resin particles containing the sulfate compound (I) are produced, and in the foamed particle production process, the resin particles are foamed to produce foamed particles.

[Process for producing expanded particles]
In the foamed particle manufacturing process, polypropylene resin particles containing the sulfate compound (I) are dispersed in an aqueous dispersion medium in a sealed container, a physical foaming agent is injected, heated and heated to form resin particles. After impregnating with a physical foaming agent to form expandable resin particles, the expandable resin particles are discharged together with an aqueous dispersion medium under a lower pressure than in a closed container and foamed to obtain expanded particles (dispersion medium release foaming). Method).

  An aqueous dispersion medium is used as a dispersion medium for dispersing the resin particles in the sealed container. The aqueous dispersion medium is a dispersion medium mainly containing water. The proportion of water in the aqueous dispersion medium is preferably 60% by mass or more, more preferably 70% by mass or more, and still more preferably 80% by mass or more. Examples of the dispersion medium other than water in the aqueous dispersion medium include ethylene glycol, glycerin, methanol, ethanol, and the like.

  When the resin particles are dispersed in the aqueous dispersion medium and heated, an anti-fusing agent can be added to the aqueous dispersion medium in order to prevent the resin particles from fusing together. Any anti-fusing agent can be used, regardless of whether it is organic or inorganic, as long as it does not dissolve in the aqueous dispersion medium and does not melt by heating. Generally, an inorganic anti-fusing agent is used. Is done. As the inorganic anti-fusing agent, powders such as mica, kaolin, aluminum oxide, titanium oxide, and aluminum hydroxide are suitable. The anti-fusing agent preferably has an average particle size of 0.01 to 100 μm, more preferably 0.1 to 30 μm. When an anti-fusing agent is used, it is preferable to use an anionic surfactant such as sodium dodecylbenzene sulfonate, sodium alkyl sulfonate, or sodium oleate as a dispersion aid. The anti-fusing agent is preferably added in an amount of about 0.01 to 2 parts by weight per 100 parts by weight of the resin particles, and the dispersion aid is added in an amount of about 0.001 to 1 parts by weight per 100 parts by weight of the resin particles. preferable.

  In the foamed particle manufacturing process for obtaining the foamed particles of the present invention, the sulfate compound (I) blended in the resin particles effectively draws the water in the aqueous dispersion medium into the resin particles and is taken in. Since it acts as a foaming agent, it is possible to easily obtain expanded particles having a lower apparent density than conventional ones. However, in the case of the expanded particles to which the sulfate compound (I) is added, although the apparent density is reduced, the bubbles in the surface layer portion of the expanded particles become finer. As a countermeasure, the addition of an electrolyte to the aqueous dispersion medium prevents the bubbles in the surface layer from becoming finer.

  The reason why the bubbles in the surface part of the expanded particle are made fine is that water is drawn into the resin particle by the sulfate compound (I), and the water inside the expanded particle acts as a foaming agent, whereas the surface layer part of the resin particle This water is considered to be easily cooled because it exists in the surface layer part, and does not act as a foaming agent but acts as a bubble nucleating agent in the state remaining as water. Furthermore, the addition of an electrolyte to the aqueous dispersion medium prevents the bubbles from becoming finer in the surface part of the foamed particles. The reason for the addition of the electrolyte is that the osmotic pressure of the aqueous dispersion medium changes due to the addition of the electrolyte. The entrained water is discharged into the aqueous dispersion medium, and the water in the surface layer portion is reduced. As a result, the water that can be the cell nuclei existing in the surface layer portion of the resin particles is reduced. It is thought that the bubble diameter is prevented from being reduced.

  The timing of addition of the electrolyte and the addition temperature are important. The time for adding the electrolyte to the aqueous medium is preferably during the temperature rise of the aqueous dispersion medium. For example, if an electrolyte is added to the aqueous dispersion medium when the raw materials are charged, the effect of reducing the apparent density cannot be obtained. This is because the water in the resin particles is discharged to the aqueous dispersion medium side by the osmotic pressure before the water is sufficiently taken into the resin particles, and the time that the resin particles are exposed to the aqueous electrolyte solution is long. The reason is that the water in the resin is excessively discharged. Therefore, from the viewpoint of preventing the resin particles from being exposed to the aqueous electrolyte solution for an excessively long time, it is preferable to add the electrolyte after the holding step for forming a high temperature peak described later in the foamed particle manufacturing step.

  The addition temperature of the electrolyte is preferably such that the temperature of the aqueous dispersion medium is [100 ° C. to foaming temperature], more preferably [100 ° C. to foaming temperature−10 ° C.], and still more preferably [100 [° C. to foaming temperature −5 ° C.]. If an electrolyte is added within this temperature range, before the electrolyte is added, the solubility of the sulfate compound (I) in the resin particles in water increases as the temperature rises, and water is absorbed by the entire resin particles. The Therefore, only water existing in the surface layer portion can be removed by the electrolyte, and water that acts as a foaming agent can be left inside the resin particles.

  In practice, the temperature of the aqueous dispersion medium in the expanded particle production process is increased at a rate within a certain range (for example, 1 ° C./min to 5 ° C./min). The addition temperature can also be in an appropriate range.

  The amount of electrolyte added is important because the osmotic pressure depends on the molar concentration. Specifically, it is preferable to add so that the molar concentration of the electrolyte with respect to the aqueous dispersion medium is 0.1 to 1 mol / L, and more preferably 0.12 to 0.8 mol / L. If the molar concentration is too low, the effect of removing the water in the resin particle surface layer portion is weak, so that the effect of expanding the bubble diameter in the surface layer portion may not be obtained. On the other hand, if the molar concentration is too high, not only the water in the resin particle surface layer portion but also the water in the resin particles is excessively removed, so that the effect of reducing the apparent density may not be obtained.

  Moreover, since the osmotic pressure depends on the molar mass, the molar mass of the electrolyte is preferably 500 g / mol or less, more preferably 250 g / mol or less, and still more preferably 100 g / mol or less. If the added amount of the electrolyte is the same, the bubble roughening effect on the surface layer due to the osmotic pressure becomes smaller as the molar mass of the added electrolyte increases. Although depending on the substance to be added, generally, the larger the addition amount, the more easily the dispersion system is affected, and it becomes difficult to obtain good foamed particles.

As described above, by considering the relationship between the addition time and amount of the electrolyte and the discharge action of water from the resin particle surface layer part, the cell diameter of the foam particle surface layer part can be controlled, and the surface layer part can be controlled. Foamed particles having a cell diameter of 70 μm or more can be obtained.

  As the electrolyte, any substance that dissolves in an aqueous dispersion medium and ionizes into a cation and an anion can be used without limitation, for example, lithium chloride, sodium chloride, magnesium chloride, potassium chloride, calcium chloride. Inorganic salts such as ammonium chloride, sodium sulfate, magnesium sulfate, potassium sulfate, aluminum sulfate, ammonium sulfate, sodium nitrate, magnesium nitrate, potassium nitrate, calcium nitrate, sodium carbonate, magnesium carbonate, potassium carbonate, calcium carbonate, ammonium carbonate, or Examples thereof include alkali metal salts of carboxylic acids that are soluble in water, such as potassium acetate, sodium acetate, sodium behemate, sodium benzoate, and disodium oxalate. Moreover, as this electrolyte, you may use individually or may be used in mixture of 2 or more types. In particular, sodium chloride is preferable because a desired effect can be obtained with a small amount of added weight, and a large amount can be obtained at a low cost.

  Physical foaming agents for foaming the resin particles include inorganic physical foaming agents and organic physical foaming agents. Examples of the inorganic physical foaming agent include carbon dioxide, air, nitrogen, helium, argon, and water. Examples of the organic physical foaming agent include aliphatic hydrocarbons such as propane, butane and hexane, cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane, chlorofluoromethane, trifluoromethane, 1,1-difluoromethane, -Halogenated hydrocarbons such as -chloro-1,1-dichloroethane, 1,2,2,2-tetrafluoroethane, methyl chloride, ethyl chloride, methylene chloride, and the like.

  These physical foaming agents may be used alone or in combination of two or more. In addition, an inorganic physical foaming agent and an organic physical foaming agent may be mixed and used. However, from the viewpoint of environment and handleability, it is preferable to use an inorganic physical foaming agent, and among the inorganic physical foaming agents, it is more preferable to use carbon dioxide, air, nitrogen, and water. In addition, when using an inorganic type physical foaming agent, since the bubble diameter of a foamed particle tends to be refined | miniaturized especially, the difficulty of obtaining the foam particle with a large cell diameter of a surface layer part becomes high.

  The amount of the physical foaming agent to be added is appropriately determined according to the target apparent density. For example, in order to obtain foamed particles having an apparent density of 10 to 80 g / L in one step without going through a two-stage foaming step, The equilibrium vapor pressure in the sealed container immediately before the start of foaming is 0.5 to 6.0 MPa (G) ((G) represents gauge pressure), preferably 1.0 to 5.0 MPa (G), more preferably 2 It is desirable to add a physical foaming agent so that it may become 0.5-4.0 MPa (G). Moreover, when using carbon dioxide (dry ice) as a physical foaming agent, it can also add so that it may become 4-30 mass parts with respect to 100 mass parts of resin particles.

  As a method of impregnating the resin particles with the foaming agent in the foamed particle manufacturing process, the resin particles are dispersed in an aqueous dispersion medium in a sealed container, and the resin particles are impregnated with the physical foaming agent while being pressurized and heated. Is preferably used.

  The expanded particle having a high temperature peak in the DSC curve is a temperature that is 20 ° C. lower than the melting point (Tm) of the polyolefin resin without increasing the temperature to the melting end temperature (T) of the polyolefin resin or higher during heating in the expanded particle production process. As described above, it is stopped at an arbitrary temperature (Ta) within the range of less than the melting end temperature (T) and held at the temperature (Ta) for a sufficient time, preferably about 10 to 60 minutes (one-stage holding step), and then the melting point The temperature is adjusted from a temperature 15 ° C. lower than (Tm) to an arbitrary temperature (Tb) in the range of the melting end temperature (T) + 10 ° C., and if necessary, a further sufficient time, preferably about 10 to 60 minutes. The foamable resin particles can be obtained by a method of releasing the foamed resin particles from the inside of the sealed container under a low pressure and foaming.

  In the case of producing expanded particles having a low apparent density, it is preferable that the expanded particles are produced by the above-mentioned dispersion medium releasing foaming method, and the obtained expanded particles are aged under atmospheric pressure and then subjected to so-called two-stage expansion. According to the two-stage foaming, the foamed particles are filled into a pressurizable sealed container, and an operation of increasing the internal pressure of the foamed particles to 0.01 to 0.6 MPa (G) by pressurizing with a gas such as air is performed. Thereafter, the foamed particles are taken out from the container and heated using a heating medium such as steam, whereby foam particles having a low apparent density can be easily obtained.

  Since the expanded particles obtained by the above method are those in which the cell diameter of the expanded particle surface layer is prevented from being reduced, the polypropylene resin obtained by molding the colored expanded particles of the present invention in-mold The foamed particle molded body is one in which the color reduction of the molded body and the occurrence of color unevenness are prevented.

  The expanded particle production process is disclosed in, for example, Japanese Patent Publication No. 49-2183, Japanese Patent Publication No. 56-1344, Japanese Patent Publication No. 62-61227, and the like. These known foaming methods can be employed. In addition, when the contents in the sealed container are released from the sealed container to a low pressure region in order to obtain expanded particles, back pressure is applied to the sealed container with the used foaming agent or an inorganic gas such as nitrogen or air. It is preferable to discharge the contents so that the pressure in the container does not drop rapidly. In this case, the apparent density of the obtained expanded particles can be made more uniform.

  A conventionally well-known method is employ | adopted for in-mold shaping | molding of the expanded particle in this invention. For example, using a pair of molding dies, filling the foamed particles into the mold cavity under atmospheric pressure or reduced pressure, closing the mold and compressing to reduce the mold cavity volume by 5 to 95%, then Supplying water vapor as a heat medium into the mold for heating, compression filling molding method and cracking molding method (for example, Japanese Examined Patent Publication No. 46-38359) for heating and fusing the foam particles, Pre-pressurize to increase the pressure inside the foamed particles, improve the secondary foamability of the foamed particles, and fill the foam cavities in the mold cavity under atmospheric pressure or reduced pressure while maintaining the secondary foamability The mold is closed, and then a heating medium such as water vapor is supplied into the mold and heated to heat and melt the foamed particles (for example, Japanese Patent Publication No. 51-22951). In the cavity pressurized above Compression filling molding that fills the foamed particles pressurized under the above-mentioned pressure using differential pressure and then heats the foamed particles by supplying a heat medium such as water vapor into the cavity. After filling the cavities of a pair of molds under atmospheric pressure or reduced pressure, the foam particles having a high secondary foaming power obtained under the law (Japanese Patent Publication No. 4-46217) and other special conditions, Next, a normal pressure filling molding method (Japanese Patent Publication No. 6-49795) in which a heating medium such as water vapor is supplied and heated to heat-fuse the foamed particles, or a method combining the above methods (Japanese Patent Publication No. 6-22919). Issue gazette).

  The apparent density of the foamed particle molded body of the present invention thus obtained is 7 to 80 g / L. Since the foamed particle molded body is a molded body having a higher expansion ratio than conventional ones, the foamed particle molded body has excellent flexibility, light weight, and buffer characteristics. The lower limit of the apparent density of the foamed particle molded body is preferably 10 g / L, more preferably 15 g / L. On the other hand, the upper limit of the apparent density of the foamed particle molded body is preferably 75 g / L, more preferably 70 g / L.

  The apparent density d (g / L) of the foamed particle molded body is measured as it is if it is a rectangular parallelepiped molded body, and cut into a rectangular parallelepiped if it is an irregular shaped molded body. This is obtained by measuring the vertical, horizontal, and height dimensions of the test piece, multiplying it to calculate the volume V (L), and dividing the weight W (g) of the test piece.

The resin particles containing the sulfate compound (I) used in the foamed particle production process can be obtained by the following method.
[Resin particle manufacturing process]
In the resin particle production process, the amount of the polypropylene resin, the sulfate compound (I), the colorant, and other additives such as a cell nucleating agent blended as necessary. Then, the mixture is fed to an extruder, heated, melted and kneaded to obtain a resin melt, and then the resin melt is extruded and granulated to produce resin particles.
At this time, in order to perform uniform kneading, the sulfate compound (I), the colorant, and the like can be masterbatched, and the polypropylene resin, the sulfate compound (I), and the colorant can be used in advance. Etc. can be mixed using, for example, a mixer such as a Henschel mixer, a ribbon blender, a V blender, or a ladyge mixer. Moreover, in order to disperse polypropylene resin etc. more uniformly, it can melt-knead, for example using high dispersion type screws, such as a dalmage type, a Maddock type, a minimelt type, and a twin screw extruder.

  A conventionally known method can be adopted as a method for obtaining resin particles by granulating the resin melt. For example, a method of producing resin particles by extruding a resin melt into a strand or a sheet and cutting or crushing it into a desired size, extruding the resin melt, a strand cut method, a hot cut method, Alternatively, resin particles can be obtained by pelletizing by an underwater cutting method or the like.

  In the strand cutting method, for example, resin particles are produced by extruding a resin melt in a strand form from a die having a large number of fine holes attached to the tip of an extruder, and quenching and cutting the extruded strand. The Rapid quenching of the strand immediately after extrusion can be performed by placing it in water preferably adjusted to 50 ° C. or less, more preferably 40 ° C. or less, and further preferably 30 ° C. or less. The sufficiently cooled strand is pulled up from the water and cut into a suitable length to obtain resin particles having a desired size.

  The particle diameter of the resin particles is preferably 0.1 to 3.0 mm, and more preferably 0.3 to 1.5 mm. The length / diameter ratio of the resin particles is preferably 0.5 to 5.0, more preferably 1.0 to 3.0. Moreover, it is preferable that the average mass per piece (average value per piece obtained by simultaneously measuring 200 randomly selected masses) is adjusted to be 0.1 to 20 mg, more preferably. It is 0.2 to 10 mg, more preferably 0.3 to 5 mg, particularly preferably 0.4 to 2 mg.

  In addition, in the strand cutting method, the resin particle diameter, length / diameter ratio, and average mass are adjusted by appropriately changing the extrusion speed, the take-up speed, the cutter speed, etc. when extruding the resin melt. Can be performed.

  In the resin particle production process, the sulfate compound (I) is blended with the resin particles. As the sulfate compound (I), those having excellent water absorbability and high temperature dependency of absorption (temperature dependency of solubility in water) are preferable as described above.

  The sulfate compound (I) is preferably blended in the resin particle production process in an anhydrous state. The anhydride of the sulfate compound (I) is low in water content and low in water absorption at room temperature, so that it hardly contains water due to deliquescence and hygroscopicity. Accordingly, since the extrusion stability in the resin particle production process is excellent, the resin melt can be stably extruded and granulated to obtain resin particles. Furthermore, since there is little mixing of the water into the resin melt at the time of resin particle manufacture, it is also prevented that excessive bubbles due to water are generated in the resin particles.

As described above, the anhydride of the sulfate compound (I) is preferably one having a high temperature dependency of solubility in water. Specifically, the temperature dependency represented by the following formula (1): Those having the following are preferred.
[Solubility at 20 ° C.] × 5 <[Solubility at 80 ° C.] (1)

  Further, the solubility of the anhydride of the sulfate compound (I) in water at 80 ° C. (anhydride g / 100 g water) is preferably 30 or more, and more preferably 50 or more. When foamed particles are produced using resin particles containing the sulfate compound (I) having a solubility (80 ° C.) of 30 or more, water is effectively drawn into the resin particles during heating in the foamed particle production process, It is possible to more easily obtain expanded particles having a uniform and large cell diameter and a small apparent density.

  Furthermore, the solubility of the anhydride of the sulfate compound (I) in water at 20 ° C. (anhydride g / 100 g water) is preferably 20 or less. Since the anhydride of the sulfate compound (I) having a solubility of 20 or less rarely exhibits remarkable deliquescence and hygroscopicity at room temperature, water is mixed into the polypropylene resin melt when the resin particles are produced. This is further prevented. As a result, the extrusion stability is further improved, and furthermore, the sulfate compound (I) anhydride is easily dispersed uniformly, so that the bubble diameter uniformity is improved. From this point of view, the solubility of the sulfate anhydride in water at 20 ° C. is preferably 15 or less, and more preferably 10 or less.

Examples of the anhydride of the sulfate compound (I) that satisfies the formula (1) include potassium aluminum sulfate anhydride and aluminum ammonium sulfate anhydride.
Both potassium aluminum sulfate anhydride and aluminum ammonium sulfate anhydride have a characteristic that the solubility in water at room temperature is low, and the solubility in water becomes very high as the temperature increases. Specifically, potassium aluminum sulfate anhydride has a solubility in water at 20 ° C. of 6 (anhydrous g / 100 g water) and a solubility at 80 ° C. of 70 (anhydrous g / 100 g water). Aluminum ammonium sulfate anhydride has a solubility in water at 20 ° C. of 6 (anhydrous g / 100 g water) and a solubility at 80 ° C. of 75 (g / 100 g water).
In the present invention, the solubility means the maximum weight of the anhydride of the sulfate compound (I) that can be dissolved in 100 g of water at a constant temperature (20 ° C. or 80 ° C.), and is expressed in grams.

  Hereinafter, the present invention will be specifically described by way of examples. However, the present invention is not limited to the examples.

  As a polypropylene resin, an ethylene-propylene random copolymer (ethylene component content: 2.8% by weight, melting point: 143 ° C., melt mass flow rate: 5 g / 10 min (JIS 7210230 ° C., load 2.16 kg, abbreviated PP1)) used.

  As an anhydride of the sulfate compound (I), potassium aluminum sulfate anhydride (average cell diameter: 5 μm, Taiace K20 manufactured by Daimei Chemical Industry, hereinafter sometimes referred to as K20) was used. The solubility of K20 in water at 20 ° C. is 6 (anhydrous g / 100 g water), and the solubility at 80 ° C. is 70 (anhydrous g / 100 g water).

  Carbon black (hereinafter sometimes referred to as CB) was used as the colorant.

  A master batch was prepared for each of the sulfate compound (I) anhydride and carbon black. Specifically, 90% by weight of polypropylene-based resin and 10% by weight of anhydride of sulfate compound (I) or carbon black are supplied to a φ30 mm twin-screw extruder, melt-kneaded and extruded into a strand shape, Each strand was cooled with water and then cut with a pelletizer to prepare each master batch.

Examples 1-8, Comparative Examples 1-6
[Production of resin particles]
Each masterbatch is added to 100 parts by mass of the polypropylene resin so that the anhydride of the sulfate compound (I) and the colorant have the blending amounts shown in Tables 1 and 2 (without adding the cell nucleating agent) ), Supplied to a single-screw extruder with an inner diameter of 40 mm, melted and kneaded at a set temperature of 230 ° C. to form a resin melt, the resin melt was extruded into a cylindrical strand shape, this strand was water-cooled, The resin particles were then cut so that the resin particle mass was 1 mg.

[Production of expanded particles]
1000 g of the resin particles, water as an aqueous dispersion medium (adjusted so that the total amount of the electrolyte solution is 3000 g), 3 g of kaolin as a dispersant, and sodium dodecylbenzenesulfonate as a dispersion aid 0.04 g and carbon dioxide (dry ice) 60 g as a blowing agent were charged into an autoclave having an internal volume of 5 liters (L) equipped with a stirrer to obtain the equilibrium vapor pressures shown in Tables 1 and 2, and stirred. While heating to a temperature 5 ° C. lower than the foaming temperatures shown in Tables 1 and 2 (average rate of temperature increase of 3 ° C./min), the temperature was held for 15 minutes (one-stage holding step). Next, the temperature was raised to the foaming temperatures shown in Tables 1 and 2 and held at that temperature for 15 minutes (two-stage holding process), and then the contents in the autoclave were released under atmospheric pressure to obtain expanded particles.

  As the electrolyte, an aqueous electrolyte solution (100 mL) was prepared so that the molar mass of the entire aqueous dispersion medium was as shown in Tables 1 and 2, and the aqueous electrolyte solution was press-fitted with a plunger pump. The electrolyte was added as shown in Tables 1 and 2.

  The foamed particles thus obtained were washed with water and centrifuged, and after curing, the physical properties of the foamed particles were measured. The results are shown in Tables 1 and 2.

The obtained foamed particles were filled in a flat plate mold having a length of 250 mm, a width of 200 mm, and a thickness of 50 mm, and subjected to in-mold molding by pressure molding by steam heating to obtain a plate-like foamed particle compact. In the heating method, steam is supplied for 5 seconds with the drain valves on both sides open, and after preliminary heating (exhaust process), one heating is performed at a pressure 0.04 MPa (G) lower than the main heating pressure, Furthermore, after one-way heating was performed in the reverse direction at a pressure 0.02 MPa (G) lower than the main heating pressure, heating was performed at the forming steam pressure shown in Table 2. After the heating was completed, the pressure was released, and water cooling was performed until the surface pressure due to the foaming force of the molded body became 0.04 MPa (G), and then the mold was opened and the molded body was taken out of the mold. The obtained molded body was cured in an oven at 80 ° C. for 12 hours and then gradually cooled to room temperature to obtain a polypropylene resin foam molded body. The physical properties of the foamed molded product were evaluated, and the results are shown in Tables 1 and 2.

  Example 1 is an example in which expanded particles were obtained using potassium aluminum sulfate as the sulfate compound (I) and sodium chloride as the electrolyte. The obtained expanded particles satisfied the average cell diameter (La) of the entire expanded particles and the ratio of the cell diameters (La / Ls), the L value measured for the expanded particle molded body was small, and the color was good. .

  Example 2 was an example in which the type of electrolyte was changed, and the foamed particles of the present invention were obtained even when a different electrolyte was used. The obtained foamed particles are larger in both the overall bubble diameter and the cell diameter in the surface layer portion than in Example 1, but are within the range of the bubble diameter defined in the present invention, and the L The value was good.

  Example 3 was an example in which the type of electrolyte was changed, and the foamed particles of the present invention were obtained even when different electrolytes were used. The obtained expanded particles are larger in overall bubble diameter and surface area than in Example 1, but are within the range of the bubble diameter defined in the present invention, and the L value of the expanded particle molded body. Was good.

  Example 4 is an example in which the molar concentration of the electrolyte is made lower than that of Example 3, the effect of expanding the bubbles in the surface layer portion is small, and the cell diameter of the expanded particle surface layer portion is slightly smaller. Although La / Ls was increased and the L value of the foamed particle molded body was increased, the color was good.

  Example 5 is an example in which the temperature at which the electrolyte was added to Example 3 was lowered and added after holding one stage to obtain expanded particles having a higher apparent density than Example 3. Even if the apparent density of the expanded particle or the expanded particle molded body changes, if the bubble diameter satisfies the predetermined range of the present invention, the L value measured for the expanded particle molded body is small and the color is good. It was.

  Example 6 is an example in which the addition concentration of the electrolyte was made lower than that in Example 5 and added after one stage retention, and the foaming effect of the surface layer was reduced, and foamed particles having a small cell diameter in the surface layer part were obtained. It is an example. Although the cell diameter of the surface layer portion was smaller than that of Example 5, the L value measured for the foamed particle molded body was small and the color was good if the range defined by the present invention was satisfied.

  Comparative Example 1 is an example manufactured without adding an electrolyte. The obtained foamed particles have a small average cell diameter (Ls) in the surface layer portion and a large ratio (La / Ls), and the foamed particle molded body obtained by molding the foamed particles in the mold has a high L value and a decreased color. .

  Comparative Example 2 is an example in which an electrolyte is added during raw material charging. The obtained foamed particles have a large cell diameter at the surface part of the foamed particles, and the ratio (La / Ls) is satisfactory, but the apparent density is large, so that the light weight is not satisfied.

  Comparative Example 3 is an example in which non-electrolyte sucrose was added. The foamed particle surface layer portion of the obtained foamed particles had a small cell diameter and a large ratio (La / Ls), and the foamed particle molded body obtained by molding the foamed particles in-mold was deteriorated in color.

  Comparative Example 4 is an example manufactured by lowering the molar concentration of the electrolyte. The cell diameter of the surface layer portion of the obtained expanded particles was small, the ratio (La / Ls) was large, and the expanded particle molded body obtained by molding the obtained expanded particles in-mold was lowered in color.

  Comparative Example 5 is an example in which the molar concentration of the electrolyte was lowered and the electrolyte was added after two-stage holding. The cell diameter of the surface layer portion of the obtained expanded particles was small and the ratio (La / Ls) was large, and the expanded particle molded body obtained by molding the expanded particles in the mold had a decreased color.

  Comparative Example 6 is an example manufactured by increasing the molar concentration of the electrolyte. The obtained expanded particles had a large apparent density and did not satisfy the lightness.

  Comparative Example 7 is an example prepared by adding zinc borate without adding sulfate anhydride or electrolyte. Although the obtained expanded particles satisfied the ratio (La / Ls), the average cell diameter (Ls) of the surface layer portion was too small, the L value of the expanded particle molded body was large, and the color was lowered.

The physical property evaluation methods of the expanded particles and the expanded molded particles were as follows.
[Apparent density of expanded particles]
The obtained expanded particles were dried in an oven at 40 ° C. for 12 hours. After drying, the foamed particle group was pressurized with air in a 0.2 MPa (G) pressure vessel for 12 hours. The foamed particle group after completion of pressurization whose weight was measured in advance was submerged in the water in the graduated cylinder using a wire mesh, and the volume of the foamed particle group was obtained from the rise in the water level, and divided by the weight of the foamed particle group, g / L The apparent density [g / L] of the foamed particles was determined by converting the unit to.

[Average bubble diameter (La) of the entire expanded particles, average cell diameter of the expanded particle surface layer portion (Ls), ratio (La / Ls)]
It was measured by the method described above.

[Color]
The color of the molded body was measured using a simple spectral color difference meter “NF333” manufactured by Nippon Denshoku Industries Co., Ltd. As the measurement mode, L * a * b was selected, the light source was C, the viewing angle was 10 °, the L value of the molded body surface was measured at n = 10, and the average value was obtained. Note that the larger the L value, the lower the color. When colored black, it can be said that when the L value is 22 or less, it is a good colored foamed particle molded body.

Claims (4)

A polypropylene resin expanded particle having an apparent density of 10 to 80 g / L, which contains a sulfate compound (I) represented by the following general formula (I) and a colorant, in the surface layer portion of the expanded particle The average cell diameter (Ls) is 70 μm or more, and the ratio (La / Ls) of the average cell diameter (La) in the whole expanded particle to the average cell diameter (Ls) in the surface layer portion of the expanded particle is 1.0 to Polypropylene-based resin expanded particles characterized by being 2.2.
M ^I M ^III (SO ₄ ) ₂ (I)
(However, M ^I represents a monovalent cation and M ^III represents a trivalent cation.)   2. The expanded polypropylene resin particles according to claim 1, wherein the content of the sulfate compound (I) is 0.01 to 1 part by weight with respect to 100 parts by weight of the polypropylene resin.   The polypropylene according to claim 1 or 2, wherein the colorant is carbon black, and the content of the carbon black is 0.1 to 5 parts by weight with respect to 100 parts by weight of the polypropylene resin. Resin foam particles. A polypropylene resin foamed particle molded body having an apparent density of 7 to 80 g / L obtained by in-mold molding of the foamed particles according to claim 1.