JP2020033446A - Foamable composite resin particle - Google Patents

Abstract

To provide foamable composite resin particles which can produce a foamable particle molding having excellent toughness in a short molding cycle.SOLUTION: Foamable composite resin particles contain a composite resin of a polyethylene-based resin and a polystyrene-based resin and a physical foaming agent. In an infrared absorption spectrum obtained by measuring a cross section of the foamable composite resin particles in a center direction from the surface, a ratio D/Dof absorbance Din a peak top of a peak including a wave number of 698 cmto absorbance Din a peak top of a peak including a wave number of 2850 cmsatisfies relationships (1) to (3): (1) the maximum value A1 of D/Din a range of 0-30 μm from the surface of 1 to 5; (2) the value A2 of D/Din a range of 100-110 μm from the surface smaller than the maximum value A1; and (3) the value A3 of D/Din the central part larger than the above A2.SELECTED DRAWING: Figure 4

Description

  The present invention relates to expandable composite resin particles made of a composite resin containing a polyethylene resin and a polystyrene resin.

  The foamed particle molded body is obtained by molding the foamed particles in a mold and fusing them together. BACKGROUND ART Expanded particle molded articles are utilized in a wide range of applications such as packaging materials, building materials, and shock absorbing materials, utilizing their excellent properties such as cushioning, light weight, and heat insulation. As the foamed particle molded body, a base resin made of a polyolefin resin such as polypropylene or polyethylene, a base resin made of a polystyrene resin such as polystyrene, or the like is used.

  BACKGROUND ART A polyolefin-based resin foamed particle molded article has excellent impact resistance and toughness, and is therefore used as a packaging material or a packaging material for precision parts and heavy-weight products. Further, since the polyolefin resin foamed particle molded article has excellent heat resistance and oil resistance, it is also used as an automobile member such as a shock absorber, a bumper, and a floor spacer.

  On the other hand, foamed polystyrene resin particles are generally inexpensive, have low molding vapor pressure during in-mold molding, and have good workability, and are therefore widely used in the market. Further, since the expanded polystyrene resin particle is excellent in rigidity, the expansion ratio can be increased in accordance with the application, and thus it is advantageous in terms of lightness.

  However, the polyolefin-based foamed resin particle molded article and the polystyrene-based resin foamed particle molded article each have the following problems. The foamed polyolefin-based resin particles have lower rigidity than the foamed polystyrene-based resin particles. In addition, since the polyolefin-based resin foamed particle molded article has a high steam temperature during in-mold molding, equipment such as a special mold is required at the time of production. On the other hand, the expanded polystyrene resin particles have lower toughness than the expanded polyolefin resin particles.

  In recent years, the development of a foamed particle molded article having both toughness, which is an advantage of a foamed particle molded article using a polyolefin-based resin as a base resin, and rigidity, which is an advantage of a foamed particle molded article using a polystyrene-based resin as a base resin, has been developed. It has been demanded. Therefore, foamable particles, foamed particles, and foamed particle molded articles using a composite resin of a polyolefin-based resin and a polystyrene-based resin as a base resin have been developed (see Patent Documents 1 to 4).

JP 2011-256244 A JP 2011-042718 A JP 2013-112765 A JP 2014-196444 A

  However, there is room for improvement in the conventional foamed particle molded product using a composite resin as a base resin as described in Patent Documents 1 to 4, for example, from the viewpoint of moldability. Specifically, in order to sufficiently increase the toughness of the foamed particle molded body, it is necessary to increase the fusion bonding of the foamed particles. However, if the heating temperature is increased in order to increase the fusion property, the cooling time after heating during molding becomes longer, and as a result, the molding cycle of the foamed particle molded article may be lengthened. On the other hand, if the molding cycle is shortened, the fusion property of the expanded particles becomes insufficient, and the appearance of the expanded particle molded product may be deteriorated, or the desired physical properties required for the composite resin such as toughness may be impaired.

  The present invention has been made in view of such a background, and an object of the present invention is to provide expandable composite resin particles capable of obtaining a foamed particle molded article having excellent toughness even in a short molding cycle.

One embodiment of the present invention is an expandable composite resin particle containing a composite resin of a polyethylene resin and a polystyrene resin, and a physical foaming agent,
Infrared absorption spectrum obtained by measuring a cross section passing through the center of the expandable composite resin particle every 10 μm from the surface of the expandable composite resin particle to the center by microscopic infrared spectroscopic analysis, including a wave number of 2850 cm ⁻¹ . An expandable composite resin in which the ratio D ₆₉₈ / D ₂₈₅₀ of the absorbance D ₆₉₈ at the peak top containing the wave number 698 cm ⁻¹ to the absorbance D ₂₈₅₀ at the peak top satisfies the following relationships (1) to (3): particle.
(1) The maximum value A1 of the ratio D ₆₉₈ / D ₂₈₅₀ is 1 to 5 when the distance from the surface of the expandable composite resin particles is in the range of 0 to 30 μm.
(2) the value A2 of the ratio D ₆₉₈ / D ₂₈₅₀ distance from the surface of the expandable composite resin particles in the range of 100~110μm is smaller than the maximum value A1.
(3) The value A3 of the ratio D ₆₉₈ / D ₂₈₅₀ at the center of the expandable composite resin particles is larger than the value A2.

The expandable composite resin particles contain a composite resin of a polyethylene-based resin and a polystyrene-based resin, and a physical blowing agent, and the ratio D ₆₉₈ / D ₂₈₅₀ in the infrared absorption spectrum satisfies the above relationship, and The polystyrene resin of the resin particles has a specific inclined structure.

  As described above, the expandable composite resin particles satisfying the above-mentioned relationship can shorten the cooling time at the time of molding the foamed particle molded article, thereby shortening the molding cycle. Furthermore, the expandable composite resin particles make it possible to obtain a foamed particle molded article excellent in the fusion property between foamed particles even if the cooling time during molding is shortened. The desired excellent toughness required for the foamed particle molded article using the composite resin as the base resin can be exhibited.

FIG. 4 is an infrared absorption spectrum of the expandable composite resin particles in Example 1 and Comparative Example 1. FIG. 9 is an infrared absorption spectrum of the expandable composite resin particles in Example 2 and Comparative Example 2. FIG. 9 is an infrared absorption spectrum of the expandable composite resin particles in Example 3 and Comparative Example 3. FIG. 3 is an enlarged view of an infrared absorption spectrum of the expandable composite resin particles of Example 1, Example 2, and Comparative Example 1. FIG. 4 is an enlarged view of an infrared absorption spectrum of the expandable composite resin particles of Example 3 and Comparative Example 2. FIG. 2 is an explanatory view schematically showing a gradient structure of a polystyrene resin ratio in a cross section of the expandable composite resin particles of Example 1. FIG. 4 is an explanatory view schematically showing a gradient structure of a polystyrene resin ratio in a cross section of the expandable composite resin particle of Comparative Example 1. FIG. 9 is an explanatory view schematically showing a gradient structure of a polystyrene resin ratio in a cross section of the expandable composite resin particle of Comparative Example 3.

  Hereinafter, preferred embodiments of the expandable composite resin particles will be described. In this specification, when a numerical value or a property value is sandwiched before and after using “to”, it is used as including the values before and after. Further, in a preferable range, a more preferable range, a further preferable range, and the like of the numerical values and the physical property values, all combinations of the upper limit and the lower limit that determine these ranges can be selected. Therefore, a combination of a preferred upper limit and a lower limit, a more preferred combination of an upper limit and a lower limit, the most preferred combination of the upper limit and the lower limit is the most suitable, but is not necessarily limited to these combinations Absent. Further, the use of “consisting of” or “consisting of” in this specification does not mean “consisting of” or “consisting of”.

[Expandable composite resin particles, composite resin expanded particles, expanded particle molded article]
The expandable composite resin particles are heated and heated by a heating medium such as steam to form expanded composite resin particles. Therefore, the composite resin expanded particles are a particulate foam obtained by pre-expanding the expandable composite resin particles and using the composite resin as a base resin. Hereinafter, the “expandable composite resin particles” may be referred to as “expandable particles”, and the “composite resin expanded particles” may be referred to as “expanded particles”.

  The foamed particle molded body is formed by molding a large number of foamed particles in a mold, and a large number of foamed particles are mutually fused. In-mold molding, for example, by filling a large number of foamed particles in the mold, introducing a heating medium such as steam into the mold, and heating the foamed particles to mutually fuse in the mold by heating. After that, it is cooled by leaving it to cool to obtain a molded article having a desired shape. Hereinafter, the foamed particle molded product may be referred to as a “molded product”.

<Expandable particles>
The expandable particles are, for example, a particulate composite resin impregnated with a foaming agent, and include the composite resin and a foaming agent. The composite resin is a resin obtained by impregnating and polymerizing a polyethylene resin with a styrene monomer, and contains a polyethylene resin and a polystyrene resin. The composite resin has a different concept from a mixed resin obtained by melt-kneading a polymerized polyethylene resin and a polymerized polystyrene resin.

(Composition of composite resin)
The mixing ratio of the polyethylene resin and the polystyrene resin in the composite resin can be adjusted as appropriate. By increasing the content of the polyethylene resin in the composite resin, the toughness of the molded article can be improved. On the other hand, by increasing the content ratio of the polystyrene resin in the composite resin, it is possible to improve the retention of the blowing agent in the expandable particles and the rigidity of the molded body. The mixing ratio of the polyethylene resin and the polystyrene resin in the composite resin can be determined from the mixing composition of the raw materials at the time of producing the expandable particles.

  From the viewpoint of sufficiently increasing the toughness of the molded article, the content of the polyethylene resin is preferably 5% by mass or more, more preferably 10% by mass or more, and further preferably 12% by mass or more. preferable. From the same viewpoint, the content of the polystyrene resin in the composite resin is preferably 95% by mass or less, more preferably 90% by mass or less, and further preferably 88% by mass or less. In addition, each content of a polyethylene resin and a polystyrene resin is represented by the content ratio of each resin with respect to the total amount of a polyethylene resin and a polystyrene resin.

  On the other hand, from the viewpoint of sufficiently increasing the rigidity of the molded article, the content of the polyethylene resin is preferably 50% by mass or less, more preferably 40% by mass or less, and preferably 35% by mass or less. Is more preferred. From the same viewpoint, the content of the polystyrene resin is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 65% by mass or more.

  Further, by increasing the ratio of the polystyrene resin in the composite resin as described above, the retention of the blowing agent comprising a hydrocarbon compound such as butane is improved. Therefore, the expandable particles enable storage at a high storage temperature and extend the shelf life. As a result, the expandable particles can be stored for a long period of time at room temperature, for example, while keeping the foaming power sufficiently in a closed container. Therefore, it is not necessary to foam the expandable particles within a short time after the production of the expandable particles, and the transport and storage in the state of the non-bulky expandable particles become possible.

  When the expandable particles are actually stored for a long period of time and then expanded to obtain expanded particles, it is possible to reduce the variation in the apparent density of the expanded particles. Furthermore, the in-mold moldability of the foamed particles is improved, and the molded body is excellent in appearance and in the fusion property between the foamed particles.

  In the expandable particles, the weight ratio of the xylene-insoluble component by Soxhlet extraction is preferably 40% or less (including 0). In this case, the expandability of the expandable particles can be further improved. Further, the weight ratio of the xylene-insoluble portion of the expandable particles is more preferably 5 to 30%, and further preferably 5 to 20%. In this case, the rigidity and toughness of the molded body can be further improved. The xylene-insoluble matter is a crosslinked polyethylene resin in the composite resin.

(Infrared absorption spectrum)
The infrared absorption spectrum is obtained by measuring a cross section passing through the center of the expandable particle every 10 μm from the surface of the expandable particle to the center by microscopic infrared spectroscopy (hereinafter sometimes referred to as microscopic IR). It is. The central direction of the expandable particles is, for example, a direction from the surface of the expandable particles toward the center on a straight line passing through the center of the cross section of the expandable particles. Further, the “straight line passing from the surface to the center” is not a virtual cross section passing through the center of the expandable particle, but is a virtual line or a line drawn when measuring the microscopic IR as necessary. Hereinafter, the distance from the surface in the center direction of the expandable particles may be referred to as “depth”.

The expandable particles satisfy a predetermined relationship in the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the infrared absorption spectrum. The absorbance D ₆₉₈ means the absorbance value at the peak top of the peak including the wave number of ₆₉₈ cm ⁻¹ . This peak is derived from out-of-plane bending vibration of the benzene ring mainly contained in the polystyrene resin. The absorbance D ₂₈₅₀ means the value of the absorbance at the peak top of the peak including the wave number of ₂₈₅₀ cm ⁻¹ . This peak is derived from CH-H stretching vibration of the methylene group contained in both the polyethylene resin and the polystyrene resin. Therefore, the absorbance ratio D ₆₉₈ / D ₂₈₅₀ is the absorbance at the peak top of the peak derived from the out-of-plane bending vibration of the benzene ring with respect to the absorbance D ₂₈₅₀ at the peak derived from the CH vibration of the methylene group. D ₆₉₈ ratio is meant.

In the infrared absorption spectrum of the above, effervescent maximum value A1 of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the range of depth 30μm from the surface of the particles, the value of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the range of the depth 100～110Myuemu A2 The value A3 of the absorbance ratio _D698 / _{D2850 at} the center satisfies the following relationships (1) to (3).
(1) A1 is 1 to 5. That is, the relationship of 1 ≦ A1 ≦ 5 is satisfied.
(2) A2 is smaller than A1. That is, the relationship of A2 <A1 is satisfied.
(3) A3 is larger than A2. That is, the relationship of A3> A2 is satisfied.

  Expandable particles containing a composite resin and a foaming agent and satisfying the relations of (1) to (3) are capable of shortening a molding cycle during the production of a molded article and producing a molded article having excellent rigidity and toughness. enable. The reason is considered as follows.

  Among the relations (1) to (3), in the case of the expandable particles that satisfy the relation (1) in particular, it is considered that a polystyrene resin having excellent rigidity is present in a large amount in the surface layer of the expandable particles. In addition, the surface layer of the expandable particles refers to a range from the surface of the expandable particles (that is, the depth 0) to a depth of 30 μm.

  It is considered that the foamed particles obtained by foaming the foamable particles satisfying the relationship (1) above have a distribution structure corresponding to the distribution structure of the polystyrene resin. That is, it can be considered that a large amount of polystyrene resin is present in a portion of the expanded particles corresponding to the surface layer of the expandable particles (hereinafter, sometimes referred to as “surface layer portion of the expanded particles”). As a result, for example, during the production of a molded article by in-mold molding, since the excessive expansion force of the expanded particles is suppressed by the polystyrene resin present in the surface layer portion of the expanded particles, The shape of the molded body is stabilized. Thereby, the cooling time can be reduced.

  In general, during the in-mold molding of the foamed particle molded body, the surface pressure (hereinafter, sometimes referred to as surface pressure) due to the foaming force of the foamed particle molded body is reduced to a predetermined pressure, and the shape of the molded body is stabilized. After that, the foamed particle molded body is released from the mold, for example. The expandable particles, when the expandable particles obtained from the expandable particles are molded in a mold, it is possible to increase the speed from the start of cooling of the expandable particle molded body until the surface pressure decreases to a predetermined pressure. it can. Further, even if the internal temperature of the foamed particle molded article after the completion of cooling is higher than that of a conventional product, it is possible to prevent the internal deformation such as shrinkage from occurring in the foamed particle molded article. As a result, the expandable particles can shorten the molding cycle.

  Furthermore, in the expandable particles satisfying the above-mentioned relations (2) and (3), the ratio of the polystyrene resin existing around the depth of about 100 μm, that is, between the surface layer and the central portion of the expandable particles is reduced to the surface layer. And locally less than at the center. Therefore, the expandable particles obtained by expanding the expandable particles have a locally high rigidity due to a high ratio of the polystyrene resin in the surface layer portion. On the other hand, since the toughness of the foamed particles as a whole is maintained, it is considered that a molded article obtained by molding the foamed particles in a mold can exhibit excellent toughness.

  In general, conventional expandable particles using a composite resin as a base resin are considered to have a gradient structure in which the polystyrene resin simply decreases or increases from the center toward the surface. On the other hand, as described above, the expandable particles satisfying (1) to (3) have a characteristic inclined structure in which the amount of the polystyrene resin near the position at a depth of 100 μm is smaller than that in the surface layer or the central portion. . It is considered that such an inclined structure contributes to shortening of a molding cycle and improvement of toughness.

  In the infrared absorption spectrum of the expandable particles, when A1 <1, the amount of the polystyrene resin existing in the surface layer of the expandable particles is small, and the excessive expansion force of the expandable particles obtained by expanding the expandable particles is reduced. There is a possibility that suppression is difficult. As a result, the effect of shortening the molding cycle may not be sufficiently obtained. In addition, the expansion force can also be referred to as secondary foamability. On the other hand, when A1> 5, the toughness of the molded body may be insufficient. From the viewpoint of shortening the molding cycle, it is preferable to satisfy the relationship of A1 ≧ 1.5, and more preferably to satisfy the relationship of A1 ≧ 1.8. On the other hand, from the viewpoint of further increasing toughness, it is preferable that A1 ≦ 3, and it is more preferable that A1 ≦ 2.5.

  In the case of A2> A1 and A3 <A2, it is difficult to sufficiently exhibit the toughness of the expanded particles as a whole, or the time required for cooling during the production of a molded article becomes longer, so that the molding cycle becomes longer. Or

  From the same viewpoint, it is preferable that the relationship of A1−A2 ≧ 0.5 is satisfied, and it is more preferable that the relationship of A1−A2 ≧ 1 is satisfied. Further, it is preferable that the relationship of A3-A2 ≧ 1 is satisfied, and it is more preferable that A3-A2 ≧ 1.2.

In the infrared absorption spectrum, the value A0 of the absorbance ratio D ₆₉₈ / D _{2850 in} the range of 0 to 10 μm from the surface of the expandable composite resin particles is the absorbance ratio in the range of 0 to 30 μm from the surface. It is preferably smaller than the maximum value A1 of _D698 / _D2850 . That is, it is preferable to satisfy A0 <A1. This means that, in other words, it is preferable that the measurement region of the infrared absorption spectrum showing the maximum value A1 of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ is not within a range of 10 μm in depth from the surface.

In the following, a range from the surface of the expandable particle to a depth of 10 μm may be referred to as “outermost layer”. In addition, the value A0 of the absorbance ratio in the outermost layer is the value of the absorbance ratio D ₆₉₈ / D _{2850 at} the position closest to the surface of the expandable particle among the positions at which the infrared absorption spectrum can be obtained in the cross section passing through the center of the expandable particle. Mean value.

  In this case, the amount of polystyrene resin in the outermost layer is reduced, so that the fusibility is further improved. Therefore, the molding temperature can be lowered while maintaining the fusion property between the foamed particles constituting the molded body, and as a result, the molding cycle at the time of molding can be further shortened. From the viewpoint of further enhancing this effect, A1−A0 ≧ 0 is more preferable, and A1−A0 ≧ 0.5 is more preferable.

Further, in the infrared absorption spectrum, it is preferable that the maximum value A1 of the absorbance ratio _D698 / _D2850 in a range from the surface to a depth of 30 μm is smaller than the value A3 of the absorbance ratio _D698 / _{D2850 in} the central part. That is, it is preferable that A1 <A3. In this case, the amount of the polystyrene resin in the surface layer is smaller than that in the center of the expandable particles. That is, the amount of the polyethylene resin existing in the surface layer is larger than that in the center. As a result, in the molded article, the toughness which is an excellent property of the polyethylene resin is more easily exhibited, and the toughness of the molded article is improved.

Further, the infrared absorption spectrum has a minimum value A4 of an absorbance ratio D ₆₉₈ / D ₂₈₅₀ within a range of 30 to 100 μm from the surface of the expandable composite resin particle, and the minimum value A4 is less than 1. It is more preferably 0.1 or more and less than 1. In this case, there is a region where the amount of the polystyrene resin is small locally between the center of the expandable particles and the surface layer. As a result, in the foamed particles obtained by foaming the expandable particles, the proportion of the polystyrene resin in the portion corresponding to the surface layer of the foamed particles is high, although the rigidity is locally high. At the outermost surface where they are fused together, the amount of polyethylene-based resin relatively increases. Therefore, in this case, the toughness is easily maintained as a whole of the foamed particle molded body, and the effect of shortening the molding cycle and the effect of improving the toughness are further increased. From the viewpoint of shortening the molding cycle and further increasing the effect of improving toughness, the minimum value A4 is more preferably 0.1 to 0.9, and even more preferably 0.4 to 0.8.

The measurement of the absorbance ratio D ₆₉₈ / D _{2850 in} the cross section of the expandable particle can be performed based on JIS K0117 (2017). Specifically, first, a sample is prepared by cutting the expandable particles in half. Next, using a FT / IR-4600 type Fourier transform infrared spectrophotometer manufactured by JASCO Corporation and an IRT-5200 type infrared microscope manufactured by JASCO Corporation, a line was cut at a cut surface passing through the center of the expandable particles by a microscopic transmission method. An infrared absorption spectrum of absorbance can be obtained by performing mapping measurement in a shape. From this infrared absorption spectrum, the absorbance ratio D ₆₉₈ / D ₂₈₅₀ can be calculated. Detailed measurement methods will be described in Examples.

(Molecular weight of acetone-soluble component in composite resin)
The weight average molecular weight of the acetone-soluble component in the composite resin is preferably 100,000 to 300,000. In this case, shrinkage of the expandable particles during expansion can be prevented. Further, it is possible to improve the fusing property between the foamed particles during in-mold molding. As a result, the dimensional stability of the molded body can be improved. From the same viewpoint, the weight average molecular weight of the acetone-soluble component in the composite resin is more preferably 100,000 or more, and further preferably 150,000 or more. Further, the weight average molecular weight of the acetone-soluble component in the composite resin is more preferably 250,000 or less. A method for measuring the weight average molecular weight of the acetone-soluble component in the composite resin will be described later in Examples.

(Morphology of composite resin)
The morphology of the composite resin in the expandable particles includes a morphology (sea-sea structure) in which a polyethylene-based resin and a polystyrene-based resin form a co-continuous phase, and a polyethylene-based resin forms a dispersed phase (island phase) and a polystyrene-based resin forms a continuous phase. (Sea phase) or a morphology (sea-island structure) in which a polyethylene resin forms a continuous phase and a polystyrene resin forms a dispersed phase. Among these, a sea-island structure or a sea-sea structure is preferable. In this case, more excellent toughness can be obtained as compared with the morphology (Shimakai structure) in which the polystyrene-based resin forms the continuous phase and the polyethylene-based resin forms the dispersed phase. In addition, from the viewpoint of obtaining good toughness, it is more preferable that the polyethylene resin has a continuous phase and the polystyrene resin has a morphology (sea-island structure) of forming a dispersed phase.

  The morphology of the surface layer of the expandable particles can be observed by the following method. Specifically, first, a sample for observation including the surface layer, that is, a range from the surface to a depth of 30 μm is cut out from the expandable particles. Next, the observation sample is embedded in an epoxy resin and stained with ruthenium tetroxide, and then an ultra-thin section is prepared from the sample using an ultramicrotome. The ultrathin section is placed on a grid, the morphology of the cross section at the center of the expandable particles is observed with a transmission electron microscope (JEM1010 manufactured by JEOL Ltd.) at a magnification of 10,000, and a cross-sectional photograph (TEM photograph) is taken. From the cross-sectional photograph, the dark gray portion is the polyethylene resin, and the light gray portion is the polystyrene resin. Then, in the TEM photograph, when the polyethylene-based resin and the polystyrene-based resin form a co-continuous phase, the morphology is determined to be a sea-sea structure, the ethylene-based resin forms a dispersed phase, and the polystyrene-based resin is When a continuous phase is formed, the morphology is determined to be an islands-in-sea structure, and when a polyethylene-based resin forms a continuous phase and a polystyrene-based resin forms a dispersed phase, the morphology is determined to be a sea-island structure.

(Physical foaming agent)
Examples of the physical foaming agent include an organic physical foaming agent. Examples of the organic physical foaming agent include saturated hydrocarbon compounds having 3 to 6 carbon atoms such as propane, n-butane, isobutane, cyclobutane, n-pentane, isopentane, neopentane, cyclopentane, n-hexane, and cyclohexane; Examples thereof include lower alcohols such as ethanol; ether compounds such as dimethyl ether and diethyl ether. As the physical foaming agent, one kind of compound may be used, or two or more kinds of compounds may be used. Hereinafter, the physical blowing agent may be referred to as a “blowing agent”.

  As the blowing agent, it is preferable to use a saturated hydrocarbon compound having 3 to 6 carbon atoms, and it is more preferable to use 30 to 80% by mass of isobutane and 20 to 70% by mass of another hydrocarbon having 4 to 6 carbon atoms. . However, the total amount of isobutane and other hydrocarbons having 4 to 6 carbon atoms is 100% by mass. By adjusting the ratio of isobutane and other hydrocarbons having 4 to 6 carbon atoms as described above, the expandable particles can be sufficiently impregnated with the blowing agent and can be sufficiently retained. In addition, during in-mold molding, the foaming power of the foamed particles can be improved. Further, in the molded article, the fusion property between the expanded particles can be further improved. More preferably, the proportion of isobutane in the blowing agent is 40% by mass or more and 75% by mass or less.

  The average particle diameter of the expandable particles is preferably 1.0 to 2.0 mm. By adjusting to this range, the effect obtained by adjusting each value of the absorbance ratio in the infrared absorption spectrum described above is further enhanced. That is, it is possible to more reliably obtain a molded body having excellent rigidity and toughness while shortening the molding cycle. Further, the holding ability of the foaming agent can be enhanced, and the filling property of the foamed particles during molding can be enhanced. In light of further enhancing this effect, the average particle size of the expandable particles is more preferably 1.1 to 1.9 mm, and still more preferably 1.2 to 1.8 mm. The average particle diameter of the expandable particles means a particle diameter (that is, d63) at a volume integrated value of 63% in a particle size distribution obtained by a method described later.

  Further, the content of the blowing agent in the expandable particles is preferably 3 to 10% by mass. In this case, the expandability of the expandable particles can be further improved, and shrinkage during expansion can be prevented. Furthermore, at the time of in-mold molding, the fusion property between the foamed particles can be further improved, and the dimensional stability of the molded body can be improved. More preferably, the content of the foaming agent is 4% by mass or more and 9% by mass or less.

<Production method of expandable particles>
The expandable particles are produced, for example, by performing a dispersion step, a polymerization step, and a foaming agent impregnation step. In the dispersion step, core particles containing a polyethylene resin as a main component are dispersed in an aqueous medium to obtain a dispersion. In the polymerization step, a styrene-based monomer is added to the dispersion, and the core particles are impregnated with the styrene-based monomer and polymerized. In the foaming agent impregnating step, particles during and / or after polymerization are impregnated with a foaming agent. Thus, expandable particles can be obtained. Hereinafter, embodiments of each step will be described.

<Dispersion process>
In the dispersing step, core particles containing a polyethylene resin are dispersed in an aqueous medium containing, for example, a suspending agent, a surfactant, a water-soluble polymerization inhibitor, and the like.

(Nuclear particles)
Examples of the polyethylene resin used for the core particles include low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, and ethylene-acrylic acid alkyl ester copolymer. A polymer, an ethylene-methacrylic acid alkyl ester copolymer, or the like can be used. The polyethylene resin may be a single polymer, or a mixture of two or more polymers.

  Preferably, the polyethylene resin is mainly composed of linear low-density polyethylene. The linear low-density polyethylene preferably has a branched structure having a linear polyethylene chain and a short-chain branched chain having 2 to 6 carbon atoms. Specific examples include an ethylene-hexene copolymer and an ethylene-octene copolymer.

  The polyethylene resin is preferably a linear low density polyethylene polymerized using a metallocene polymerization catalyst. In this case, a molded article having particularly excellent toughness can be obtained.

  The polyethylene resin preferably has a melting point Tm of 95 to 115 ° C. In this case, the polyethylene resin can be sufficiently impregnated with the styrene monomer during the production of the expandable particles, and the suspension system can be prevented from becoming unstable during polymerization. As a result, it becomes possible to manufacture a molded article having both the excellent rigidity of the polystyrene resin and the excellent toughness of the polyethylene resin at a higher level. More preferably, the melting point Tm of the polyethylene resin is 100 to 110 ° C. The melting point Tm of the polyethylene resin can be measured by differential scanning calorimetry (DSC) based on JIS K7121-1987.

  The melt mass flow rate (MFR) of the polyethylene resin at 190 ° C. and 2.16 kgf is preferably from 0.5 to 4.0 g / 10 min, more preferably from 1.0 to 3.0 g / 10 min from the viewpoint of foamability. preferable. The MFR of the polyethylene resin at 190 ° C. and 2.16 kgf is a value measured by the condition code D based on JIS K7210-1999. In addition, a melt indexer (for example, model L203 manufactured by Takara Kogyo Co., Ltd.) can be used as the measuring device.

  The degree of crystallinity of the polyethylene resin is preferably 20 to 35%. In this case, it is possible to further improve the retention and foaming properties of the foaming agent. This is considered to be because when the crystallinity of the polyethylene resin is within the above range, gas molecules (specifically, a foaming agent) are unlikely to spread the polymer chains of the polyethylene resin. . As a result, it can be inferred that the permeability of the foaming agent decreases and the retention of the foaming agent increases as described above. From the same viewpoint, the crystallinity of the polyethylene resin is more preferably 20 to 30%. The crystallinity of the polyethylene resin is determined from the heat of fusion measured by differential scanning calorimetry (DSC) based on JIS K7122-1987.

  The core particles can contain a dispersion diameter enlarger. As the dispersion diameter enlarging agent, for example, at least one selected from polystyrene, acrylonitrile-styrene copolymer, rubber-modified polystyrene, ABS resin, and AES resin can be used. Preferably, an acrylonitrile-styrene copolymer is good. Further, the amount of the acrylonitrile component in the acrylonitrile-styrene copolymer is preferably 20 to 40% by mass.

  Further, the melt mass flow rate (MFR) of the dispersion diameter expanding agent at 200 ° C. and 5 kgf is preferably from 1 g / 10 min to 20 g / 10 min, more preferably from 2.5 g / 10 min to 15 g / 10 min. The MFR of the dispersion diameter enlarger at 200 ° C. and 5 kgf is a value measured by the condition code H based on JIS K7210-1999. As an apparatus for measuring the MFR, a melt indexer (for example, Model L203 manufactured by Takara Kogyo Co., Ltd.) can be used.

  The content of the dispersion diameter-enlarging agent in the core particles is preferably 1 to 10 parts by mass, more preferably 3 to 7 parts by mass, based on 100 parts by mass of the polyethylene resin. When the content of the dispersion diameter-enlarging agent is within the above range, the polyethylene-based resin and the polystyrene-based resin can easily form a morphology showing a co-continuous phase (sea-sea structure). Further, when the content is within the above range, the dispersion diameter of the polystyrene resin (dispersed phase) is easily increased in the morphology (sea-island structure) in which the polyethylene resin forms the continuous phase and the polystyrene resin forms the dispersed phase. As a result, the foaming agent holding performance of the foamable particles can be sufficiently improved. Further, from the viewpoint of maintaining good toughness and rigidity of the molded article, it is preferable that the content of the dispersion diameter-enlarging agent be in the above range.

  The core particles can be produced by blending an additive, which is added as necessary, with the polyethylene resin, melt-kneading the blend, and then granulating. Examples of the additive include a foam adjusting agent, a pigment, a slip agent, an antistatic agent, a flame retardant, and the like, in addition to the above-described dispersion diameter expanding agent. Melt kneading can be performed by an extruder. In order to perform uniform kneading, it is preferable to extrude after mixing the resin components in advance. The mixing of the resin components can be performed using a mixer such as a Henschel mixer, a ribbon blender, a V blender, a Ladyge mixer, or the like. The melt-kneading is preferably performed using a high dispersion type screw such as a dalmage type, a mudock type, a unimelt type, or a twin screw extruder. In this case, the retention of the foaming agent of the foamable particles and the foaming moldability can be improved. Further, in this case, the dispersion diameter increasing agent can be uniformly dispersed in the polyethylene resin. As a result, it is possible to further increase the rigidity while maintaining the toughness characteristic of the polyethylene resin of the molded body.

  As the cell regulator, for example, organic substances such as higher fatty acid bisamides and higher fatty acid metal salts; and inorganic substances such as talc, silica, zinc borate and alum can be used. It is preferable that the compounding amount of the organic bubble regulator is in the range of 0.01 to 2 parts by mass based on 100 parts by mass of the resin for core particles. It is preferable that the compounding amount of the inorganic cell regulator is in the range of 0.1 to 5 parts by mass with respect to 100 parts by mass of the resin for core particles. By adjusting the blending amount of the cell regulator within the above range, the cell size of the foamed particles and the molded article is improved, the cells are less likely to be broken at the time of molding in the mold, and the appearance of the molded article is further improved.

  Granulation of the core particles can be performed by, for example, a strand cut method, an underwater cut method, a hot cut method, or the like. The method can be performed by another method as long as it is a method capable of obtaining core particles having a desired particle diameter.

  The average particle diameter of the core particles is preferably 0.1 to 1.5 mm, more preferably 0.3 to 1.2 mm. By adjusting the average particle size of the core particles to the above range, it becomes easy to adjust the average particle size of the expandable particles to the above-described desired range. When using an extruder to granulate the core particles, for example, by adjusting the cut speed at the time of cutting while extruding the resin from a hole having a diameter within the range of the desired particle diameter, to adjust the particle diameter of the core particles can do.

  The average particle diameter of the core particles can be measured, for example, as follows. That is, the core particles are observed with a micrograph, the maximum diameter of each core particle is measured for 200 or more core particles, and the arithmetic mean value of the measured maximum diameters is defined as the particle size of the core particles.

  In the dispersion step, the core particles are dispersed in an aqueous medium to prepare a dispersion. The dispersion of the core particles in the aqueous medium can be performed, for example, in a closed container equipped with a stirrer. The aqueous medium includes, for example, deionized water.

  The core particles are preferably dispersed in an aqueous medium together with a suspending agent. In this case, when adding the styrene monomer, the styrene monomer can be uniformly suspended in the aqueous medium.

  Examples of the suspending agent include tricalcium phosphate, hydroxyapatite, magnesium pyrophosphate, magnesium phosphate, aluminum hydroxide, ferric hydroxide, titanium hydroxide, magnesium hydroxide, barium phosphate, calcium carbonate, and magnesium carbonate. And fine-particle inorganic suspending agents such as barium carbonate, calcium sulfate, barium sulfate, talc, kaolin and bentonite. Further, for example, an organic suspending agent such as polyvinylpyrrolidone, polyvinyl alcohol, ethylcellulose, and hydroxypropylmethylcellulose can also be used. Preferably, tricalcium phosphate, hydroxyapatite, and magnesium pyrophosphate are good. As the suspending agent, one kind of substance may be used, or two or more kinds of substances may be used.

  The amount of the suspending agent to be used is 0. 0 in terms of solid content with respect to 100 parts by mass of the aqueous medium of the suspension polymerization system (specifically, all water in a system containing water such as a reaction product-containing slurry). The amount is preferably from 05 to 10 parts by mass. More preferably, the amount is 0.3 to 5 parts by mass. By setting the amount of the suspending agent within the above range, the styrene-based monomer can be stably suspended, and the expansion of the particle size distribution of the expandable particles can be suppressed.

  A surfactant can be added to the aqueous medium. As the surfactant, for example, it is preferable to use an anionic surfactant, a nonionic surfactant, a cationic surfactant, or an amphoteric surfactant. These surfactants can be used alone or in combination of two or more.

  Examples of the anionic surfactant include sodium alkyl sulfonate, sodium alkylbenzene sulfonate, sodium lauryl sulfate, sodium α-olefin sulfonate, and sodium dodecylphenyl oxide disulfonate.

  As the nonionic surfactant, for example, polyoxyethylene nonylphenyl ether, polyoxyethylene lauryl ether, or the like can be used.

  As the cationic surfactant, an alkylamine salt such as coconutamine acetate and stearylamine acetate can be used. Also, quaternary ammonium such as lauryltrimethylammonium chloride and stearyltrimethylammonium chloride can be used.

  As the amphoteric surfactant, alkyl betaines such as lauryl betaine and stearyl betaine can be used. Further, an alkylamine oxide such as lauryldimethylamine oxide can also be used.

  Preferably, an anionic surfactant is used. More preferably, an alkali metal salt of an alkylsulfonic acid having 8 to 20 carbon atoms (preferably a sodium salt) is preferred. Thereby, the suspension can be sufficiently stabilized.

  If necessary, an electrolyte composed of inorganic salts such as lithium chloride, potassium chloride, sodium chloride, sodium sulfate, sodium nitrate, sodium carbonate, sodium bicarbonate, etc. can be added to the aqueous medium.

  It is preferable to add a water-soluble polymerization inhibitor to the aqueous medium. The water-soluble polymerization inhibitor hardly impregnates the core particles and dissolves in the aqueous medium. Therefore, in the aqueous medium in which the water-soluble polymerization inhibitor is present, the styrene-based monomer impregnated in the core particles is polymerized, but is absorbed by the styrene-based monomer and the core particles in the form of fine droplets present in the aqueous medium. The polymerization of the styrene-based monomer existing near the surface of the core particles being suppressed is suppressed.

By using the water-soluble polymerization inhibitor and adjusting the amount of addition of the styrene-based monomer and the rate of temperature increase during the later-described polymerization, the respective values of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the infrared absorption spectrum of the expandable particles are desired. And the production of expandable particles having the above-mentioned characteristic inclined structure becomes possible. The reason is considered as follows. That is, particularly, by using the water-soluble polymerization inhibitor, the polymerization of the styrene monomer in the aqueous medium not impregnated into the core particles and the styrene monomer near the surface being absorbed by the core particles are suppressed. As a result, it is considered that the amount of polystyrene resin near the surface of the expandable particles is smaller than that in the central portion. In this way, the above-mentioned characteristic inclined structure is formed, and expandable particles capable of producing a molded article having both excellent rigidity and toughness in a short molding cycle can be obtained.

As the water-soluble polymerization inhibitor, sodium nitrite, potassium nitrite, ammonium nitrite, L-ascorbic acid, citric acid and the like can be used. As a water-soluble inhibitor, sodium nitrite is preferable from the viewpoint that it is easy to adjust each value of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ within the above-mentioned desired range. From the same viewpoint, the addition amount of the water-soluble polymerization inhibitor is 0.001 to 100 parts by mass with respect to 100 parts by mass of an aqueous medium (specifically, all water in a system containing water such as a reaction product-containing slurry). 0.1 parts by mass is preferable, and 0.005 to 0.06 parts by mass is more preferable. In this case, the ratio of the polystyrene resin on the surface of the expandable particles can be easily reduced, and the value A0 of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ can be easily adjusted to the above-described desired range.

<Polymerization step>
In the polymerization step, the styrene-based monomer is impregnated into the core particles and polymerized in an aqueous medium.

  For example, styrene can be used as the styrene monomer. Further, as the styrene monomer, a monomer copolymerizable with styrene and styrene can be used in combination. The styrene-based monomer may be one type or two or more types.

  Examples of monomers copolymerizable with styrene include the following styrene derivatives and other vinyl monomers. Examples of the styrene derivative include α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-methoxystyrene, pn-butylstyrene, and p-methylstyrene. -T-butylstyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, 2,4,6-tribromostyrene, divinylbenzene, styrenesulfonic acid, sodium styrenesulfonate and the like.

  Other vinyl monomers include acrylates, methacrylates, hydroxyl-containing vinyl compounds, nitrile-containing vinyl compounds, organic acid vinyl compounds, olefin compounds, diene compounds, vinyl halide compounds, vinylidene halide compounds. And a maleimide compound.

  Examples of the acrylate include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and the like. Examples of the methacrylate include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, and the like.

  Examples of the vinyl compound having a hydroxyl group include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate. Examples of the vinyl compound containing a nitrile group include acrylonitrile and methacrylonitrile.

  Examples of the organic acid vinyl compound include vinyl acetate and vinyl propionate. Examples of the olefin compound include ethylene, propylene, 1-butene and 2-butene.

  Examples of the diene compound include butadiene, isoprene, and chloroprene. Examples of the vinyl halide compound include vinyl chloride and vinyl bromide.

  Examples of the vinylidene halide compound include vinylidene chloride. Examples of the maleimide compound include N-phenylmaleimide and N-methylmaleimide.

  From the viewpoint of enhancing the expandability of the expandable particles, the polystyrene resin is preferably a polystyrene or a copolymer of styrene and an acrylate. As the acrylate, butyl acrylate is preferred. The content of the butyl acrylate component in the composite resin is preferably 0.5 to 10% by mass, more preferably 1 to 8% by mass, and more preferably 2 to 5% by mass based on the entire composite resin. It is more preferred that there be.

  The polymerization of the styrene monomer can be carried out in the presence of a polymerization initiator. In this case, crosslinking may occur with the polymerization of the styrene monomer. In the polymerization of the styrene monomer, a polymerization initiator is used, but a crosslinking agent can be used in combination as needed. When using a polymerization initiator and a crosslinking agent, it is preferable to dissolve the polymerization initiator and the crosslinking agent in the styrene monomer in advance.

  The polymerization initiator is used in a suspension polymerization method of a styrene monomer. For example, a polymerization initiator that is soluble in a vinyl monomer such as a styrene-based monomer and has a 10-hour half-life temperature of 50 to 120 ° C. can be used.

  Examples of the polymerization initiator include cumene hydroxy peroxide, dicumyl peroxide, t-butylperoxy-2-ethylhexanoate, t-butylperoxybenzoate, benzoyl peroxide, t-butylperoxyisopropyl carbonate, Organic peroxides such as amyl peroxy-2-ethylhexyl carbonate, hexyl peroxy-2-ethylhexyl carbonate, and lauroyl peroxide; azo compounds such as azobisisobutyronitrile; As the polymerization initiator, one type may be used, or two or more types may be used. The polymerization initiator is preferably used in an amount of 0.01 to 3 parts by mass with respect to 100 parts by mass of the styrene monomer.

  As the cross-linking agent, one that does not decompose at the polymerization temperature but decomposes at the cross-linking temperature is used. For example, those having a 10-hour half-life temperature higher by 5 ° C. to 50 ° C. than the polymerization temperature can be used.

  As the crosslinking agent, for example, peroxides such as dicumyl peroxide, 2,5-t-butyl perbenzoate, and 1,1-bis-t-butyl peroxycyclohexane can be used. One type of crosslinking agent may be used, or two or more types may be used. The compounding amount of the crosslinking agent is preferably 0.1 to 5 parts by mass based on 100 parts by mass of the styrene monomer. The polymerization initiator and the crosslinking agent may be the same compound.

  The polymerization initiator can be added to the core particles. In this case, the core particles can be impregnated with a polymerization initiator dissolved in a solvent. As the solvent, for example, aromatic hydrocarbons such as ethylbenzene and toluene; and aliphatic hydrocarbons such as heptane and octane are used. A bubble regulator can be added to the solvent.

  In addition, a plasticizer, an oil-soluble polymerization inhibitor, a cell regulator, a flame retardant, a dye, and the like can be added to the styrene-based monomer as needed.

  As the plasticizer, for example, fatty acid esters such as glycerin tristearate, glycerin trioctoate, glycerin trilaurate, sorbitan tristearate, sorbitan monostearate, and butyl stearate can be used. Also, acetylated monoglycerides such as glycerin diacetomonolaurate, oils and fats such as hardened tallow and hardened castor oil; organic compounds such as cyclohexane, liquid paraffin, 1,2-cyclohexanedicarboxylic acid diisononyl ester and the like can be used.

  As the oil-soluble polymerization inhibitor, para-t-butylcatechol, hydroquinone, benzoquinone, and the like can be used.

  As the cell regulator, for example, fatty acid monoamide, fatty acid bisamide, talc, silica, polyethylene wax, methylene bisstearic acid, methyl methacrylate copolymer, and silicone can be used. As the fatty acid monoamide, oleic acid amide, stearic acid amide, or the like can be used. As the fatty acid bisamide, ethylene bisstearic acid amide and the like can be used.

  The mixing ratio of the core particles and the styrene monomer is preferably adjusted so that the mass ratio of the polyethylene resin and the polystyrene resin (polyethylene resin: polystyrene resin) becomes 10:90 to 50:50, It is more preferable to adjust so as to be 12:88 to 40:60, and even more preferable to adjust so as to be 15:85 to 35:65.

  When the core particles are impregnated with the styrene-based monomer and polymerized, the whole amount can be added all at once, but can be added in plural times. In the case of adding in a plurality of times, the styrene-based monomer can be divided into a plurality of monomers such as a first monomer and a second monomer, and these monomers can be added to the aqueous medium at different timings and temperatures. By adding in a plurality of times, it is possible to prevent the formation of a resin mass in the polymerization step.

Note that the styrene-based monomer to be blended can be added in two portions, and the first monomer and the second monomer can be added to the aqueous medium at different temperatures. In this case, the value of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the infrared absorption spectrum is adjusted to the above-mentioned desired value by adjusting the rate of temperature rise after the start of the addition of the second monomer, as in the second polymerization step described below. Foamable particles satisfying the relationship can be produced. Note that “after the start of the addition of the second monomer” means after the start of the addition of the second monomer, and includes during the addition of the second monomer. The above-mentioned polymerization initiator, cross-linking agent and the like can be dissolved in the monomer added at any timing.

  In the polymerization step, the polymerization temperature varies depending on the type of the polymerization initiator used, but is preferably from 60C to 105C. The crosslinking temperature varies depending on the type of the crosslinking agent used, but is preferably from 100C to 150C.

  The polymerization step is performed, for example, in a closed container containing an aqueous medium containing core particles, a polymerization initiator, a styrene-based monomer, a water-soluble polymerization inhibitor, and the like. The temperature control in the polymerization step can be performed, for example, in the following two steps. First, a first polymerization step of maintaining the temperature in the closed container at the first polymerization temperature and a second polymerization step of maintaining the temperature in the second polymerization temperature higher than the first polymerization step are performed. In the first polymerization step, the temperature in the vessel can be raised to the first polymerization temperature, for example, at a first heating rate of 15 to 50 ° C / h.

On the other hand, in the second polymerization step, the temperature can be raised from the first polymerization temperature to the second polymerization temperature, for example, at a second temperature rising rate of less than 15 ° C./h. By lowering the second temperature rising rate as described above, each value of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the infrared absorption spectrum of the expandable particles can be adjusted to a desired range, and the characteristic tilt structure described above is obtained. Can be produced. From the same viewpoint, the second heating rate is preferably equal to or lower than 12 ° C / h, more preferably equal to or lower than 9 ° C / h, and still more preferably equal to or lower than 8.5 ° C / h. In addition, the second heating rate is generally 2 ° C./h or more, and preferably 3 ° C./h or more, from the viewpoint of productivity. When the polymerization is performed three or more times, it is preferable that the above relationship be satisfied in the polymerization step in the final stage. In other words, it is preferable to control the above-mentioned second heating rate in the N-th step (where N is a natural number of 2 or more), which is the final stage.

  In particular, in order to form a polystyrene resin gradient structure unique to the present invention, using a water-soluble polymerization inhibitor, the foamable particles as a whole, the outermost layer of the polystyrene resin than the central polystyrene resin. In addition, it is preferable that the temperature is increased and the rate of temperature rise in the second polymerization step is reduced in the polymerization step so that the polystyrene resin in the surface layer of the expandable particles is locally increased. By combining the above methods, the production of expandable particles having a gradient distribution structure of the specific polystyrene resin in the present invention becomes easy.

  Further, when the second heating rate is 5 to 14 ° C./h, it is easy to decrease the abundance ratio of the polystyrene-based resin in the outermost layer of the expandable particles and increase the abundance ratio of the polyethylene-based resin. Therefore, in this case, when the foamed particles obtained by foaming the expandable particles are fused by in-mold molding, toughness is particularly exhibited in the fused portion, and the bending characteristics of the entire molded body, such as bending properties, are improved. Toughness can be improved.

<Blowing agent impregnation step>
In the blowing agent impregnation step, the particles are impregnated with a physical blowing agent during or after the polymerization of the styrene monomer. The particles may be impregnated with a physical blowing agent both during and after polymerization. That is, the addition of the foaming agent can be performed on the particles in the course of polymerization, on the particles on which polymerization has been completed, or on both of them. Specifically, during the polymerization, a physical foaming agent is pressed into a container for accommodating the particles after the polymerization to impregnate the particles with the physical foaming agent.

  The impregnation temperature of the physical foaming agent is preferably in the range of (Tg-10) ° C to (Tg + 40) ° C, where Tg (unit: ° C) is the glass transition temperature of the polystyrene resin, and (Tg-5) C. to (Tg + 25) .degree. C. In this case, the retention of the foaming agent can be further improved. In addition, coagulation of the expandable particles can be prevented. The polystyrene resin is, for example, a styrene homopolymer, or a copolymer of a styrene monomer and a monomer copolymerizable with a styrene monomer.

  The glass transition temperature (Tg) of the polystyrene resin is, for example, a midpoint glass transition temperature measured as follows. That is, first, 1.0 g of expandable particles are added to a flask containing 200 ml of xylene, and the mixture is heated with a mantle heater for 8 hours to perform Soxhlet extraction. The extracted xylene solution is dropped into 600 ml of acetone, decanted and evaporated to dryness under reduced pressure to obtain a polystyrene resin as an acetone-soluble matter. About 2 to 4 mg of the obtained polystyrene-based resin, a heat flux differential scanning calorimetry is performed according to JIS K7121-1987 using a 2010 type DSC meter manufactured by TIA Instruments. Then, the midpoint glass transition temperature of the DSC curve obtained at a heating rate of 10 ° C./min can be determined.

  After the impregnation with the foaming agent, the foamable particles are dehydrated and dried, and the foamable particles can be coated with a surface coating agent as necessary. Examples of the surface coating agent include zinc stearate, triglyceride stearate, monoglyceride stearate, and hardened castor oil. Further, an antistatic agent or the like can be used as a functional surface coating agent. The addition amount of the surface coating agent is preferably 0.01 to 2 parts by mass based on 100 parts by mass of the expandable particles.

  As described above, expandable particles can be obtained by performing the dispersion step, the polymerization step, and the foaming agent impregnation step.

<Expanded particles>
The expanded particles can be obtained by heating and expanding the expandable particles with a heating medium. Specifically, the expandable particles can be expanded by introducing a heating medium such as steam into a pre-expanding machine to which the expandable particles have been supplied. The bulk density of the expanded beads is preferably from 10 to 200 / m ^3, more preferably 15~100kg / m ^3.

<Foam particle molded body>
By molding the foamed particles in a mold by a known molding means, a foamed particle molded body can be obtained. Preferably the apparent density of PP bead molding is 10 to 200 / m ^3, more preferably 15~100kg / m ^3.

(Example 1)
Hereinafter, Examples and Comparative Examples of the expandable particles will be described. In this example, expandable particles are prepared, and the expandable particles and the molded body are formed using the expandable particles.

  The expandable particles use a composite resin containing a polyethylene resin and a polystyrene resin as a base resin. In the expandable particles, the composite resin is impregnated with a blowing agent made of a saturated hydrocarbon compound. Hereinafter, a method for producing the expandable particles of this example will be described.

(1) Preparation of core particles Linear low-density polyethylene (“Nipolon Z HF210K” manufactured by Tosoh Corporation, ethylene-vinyl acetate containing 15% by mass of vinyl acetate) obtained by polymerization using a metallocene polymerization catalyst as a polyethylene resin. A copolymer (EVA: “Ultracene 626” manufactured by Tosoh Corporation) was prepared, and an acrylonitrile-styrene copolymer (“AS-XGS, manufactured by Denka Corporation, weight average molecular weight: 19,000”) was used as a dispersion diameter enlarger. Acrylonitrile component amount: 28% by mass, MFR (200 ° C., 5 kgf: 2.8 g / 10 min) was prepared, and linear low-density polyethylene (“Nipolon Z HF210K” manufactured by Tosoh Corporation) 15 kg, ethylene-vinyl acetate 5 kg of a polymer (EVA: “Ultracene 626” manufactured by Tosoh Corporation) and 1 kg of a dispersion diameter enlarger were mixed with Henshi. Rumikisa; was put into (manufactured by Mitsui Miike Engineering Corporation Type FM-75E), to obtain a resin mixture was mixed for 5 minutes.

  Next, the resin mixture was melt-kneaded at a temperature of 230 to 250 ° C using a 26 mmφ twin-screw extruder (specifically, model TEM-26SS manufactured by Toshiba Machine Co., Ltd.). The melt-kneaded product was extruded and cut into 0.3 to 0.4 mg / piece (average 0.35 mg / piece) by an underwater cutting method to obtain core particles containing a polyethylene resin.

(2) Preparation of Expandable Particles To a 3 L autoclave equipped with a stirrer, 1000 g of deionized water was added, and 6.0 g of sodium pyrophosphate was further added. Thereafter, 12.9 g of powdery magnesium nitrate hexahydrate was added, and the mixture was stirred at 40 ° C for 30 minutes. This produced a magnesium pyrophosphate slurry as a suspending agent.

  Next, 1.04 g of sodium lauryl sulfonate (10% by mass aqueous solution) as a surfactant, 0.1 g of sodium nitrite as a water-soluble polymerization inhibitor, and 150 g of core particles were added to the suspension.

  Then, 2.0 g of benzoyl peroxide ("Nipper BW" manufactured by NOF Corp., water-diluted powder product) as a polymerization initiator and 0.25 g of t-butylperoxy-2-ethylhexyl monocarbonate ("NOF Corp." Perbutyl E ") and 5.1 g of 1,1-di (tert-butylperoxy) cyclohexane (" Luperox 331M70 "manufactured by Arkema Yoshitomi) as a crosslinking agent were dissolved in the styrene monomer. Then, the melt was poured into the suspension in the autoclave while stirring at a stirring speed of 500 rpm (dispersion step). As the styrene monomer, a mixed monomer of 335 g of styrene and 15 g of butyl acrylate was used.

  Next, after the inside of the autoclave was replaced with nitrogen, the temperature was raised and the temperature was raised to 88 ° C. over 1 hour and 30 minutes. After the temperature was raised, the temperature was maintained at 88 ° C. for 30 minutes. Thereafter, the stirring speed was reduced to 450 rpm, the mixture was cooled to 80 ° C. in 30 minutes, and kept at a polymerization temperature of 80 ° C. for 5 hours (first polymerization step). Next, the temperature was raised to a temperature of 120 ° C. over 5 hours, and the temperature was maintained at a temperature of 120 ° C. for 5 hours (second polymerization step).

  Thereafter, the mixture was cooled to a temperature of 90 ° C. over 1 hour, the stirring speed was reduced to 400 rpm, and the temperature was maintained at 90 ° C. for 3 hours. Then, when the temperature reached 90 ° C., 20 g of cyclohexane and 65 g of butane (a mixture of about 20% by mass of normal butane and about 80% by mass of isobutane) were added to the autoclave over about 1 hour as a blowing agent (impregnation step). Further, the temperature was raised to a temperature of 105 ° C. over 2 hours, kept at a temperature of 105 ° C. for 5 hours, and then cooled to a temperature of 30 ° C. over a period of about 6 hours.

  After cooling, the contents were taken out, and nitric acid was added to dissolve magnesium pyrophosphate adhering to the surface of the resin particles. Thereafter, dehydration and washing were performed with a centrifugal separator, and water adhering to the surface was removed with a flash dryer to obtain expandable particles having an average particle diameter (d63) of about 1.7 mm.

  To 100 parts by mass of the obtained expandable particles, 0.008 parts by mass of N, N-bis (2-hydroxyethyl) alkylamine as an antistatic agent was added, and 0.12 parts by mass of zinc stearate was further added. A mixture of 0.04 parts by mass of glycerin monostearate and 0.04 parts by mass of glycerin distearate was added, and these were used to coat the expandable particles.

The average particle diameter of the expandable particles obtained as described above, the absorbance ratio D ₆₉₈ / D _{2850 in} the cross section of the composite resin particles measured by micro IR, the morphology of the surface layer, the ratio of xylene-insoluble components, and the weight of the polystyrene resin The average molecular weight (Mw) was determined as follows.

`` Average particle size ''
The particle size distribution of the composite resin particles was measured using a particle size distribution measuring device “Millitrac JPA” manufactured by Nikkiso Co., Ltd. Specifically, first, 30 g of the composite resin particles were dropped freely from the sample supply feeder of the measuring device, and a projected image was captured by a CCD camera. Next, calculation and combination processing were sequentially performed on the captured image information, and measurement was performed under the conditions of an image analysis method of outputting a particle size distribution result. Thereby, each particle diameter (d63) mm at a volume integrated value of 63% in the particle size distribution was obtained.

"Absorbance ratio D ₆₉₈ / D _{2850 in} cross section of expandable particles measured by microscopic IR"
After cutting in half in a cross section passing through the center of the expandable particles, the cut surface was sliced to prepare a 10 μm thick sample for micro IR. In the sample for microscopic IR, a measurement region partitioned every 10 μm in the center direction was set along a straight line passing from the surface of the expandable particle (that is, the edge of the sample for microscopic IR) to the center of the expandable particle. . Then, mapping measurement for measuring these measurement regions was performed by infrared spectroscopy using the microscopic transmission method, and an infrared absorption spectrum was obtained. For the infrared spectroscopy, a FT / IR-4600 type Fourier transform infrared spectrophotometer manufactured by JASCO Corporation and an IRT-5200 type infrared microscope manufactured by JASCO Corporation were used. The absorbance D ₆₉₈ at the peak top of the peak containing the wave number ₆₉₈ cm ⁻¹ and the absorbance D ₂₈₅₀ at the peak top of the peak containing the wave number 2850 cm ⁻¹ in the infrared absorption spectrum were measured. Then, the ratio of the absorbance D ₆₉₈ to the absorbance D ₂₈₅₀ , that is, the absorbance ratio D ₆₉₈ / D ₂₈₅₀ was calculated. The conditions of the mapping measurement in microscopic transmission method, microscopic optical path: transmitting, measuring area: 4000cm ^-1 ~600cm ^-1, detector: MCT, Resolution: 8 cm ^-1, number of integrations: 32 times, microscopic aperture: 10 [mu] m × 100 μm, 0 degree. In addition, a sample was measured after performing background measurement of the apparatus in a region where no measurement sample was present. The results of the infrared absorption spectrum of the expandable particles of this example are shown in FIGS. The measured value is an arithmetic average value obtained by performing the above measurement on 10 samples.

In this example, the value A0 of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the outermost layer is specifically, in the infrared absorption spectrum obtained by the mapping measurement, in the range from the surface of the expandable particles to a depth of 10 μm. _Is the value of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the infrared absorption spectrum with as the measurement region. Specifically, the maximum value A1 of the absorbance ratio D ₆₉₈ / D _{2850 in} the surface layer is, specifically, in a range from the surface of the expandable particles to a depth of 10 μm, in a range from 10 μm to 20 μm, and in a range from 20 μm to 30 μm. _Is the maximum value among the values of the absorbance ratio D ₆₉₈ / D _{2850 in} the infrared absorption spectrum, each of which is a measurement region.

Value A2 of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the range of depth 100~110μm is specifically absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the infrared absorption spectrum of the range of depth 100~110μm the measurement region Value. The value A3 of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the central portion, specifically, a value of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the infrared absorption spectrum of the range of depth 850~860μm the measurement region. The minimum value A4 of the absorbance ratio D ₆₉₈ / D ₂₈₅₀ in the range of 30 μm to 100 μm in depth from the particle surface is specifically the absorbance ratio D of the infrared absorption spectrum whose measurement region is in the range of 30 to 100 μm in depth. ₆₉₈ / D _{2850 is} the minimum value. Table 1 shows the values of A0 to A4 in the expandable particles of this example.

"Surface morphology"
The morphology was performed by observing the cross section of the expandable particles with a transmission electron microscope (TEM). JEM1010 manufactured by JEOL Ltd. was used as the TEM. Specifically, first, an observation sample was cut out from a part of the surface layer of the expandable particles (specifically, a part within a range of 5 μm from the surface). After embedding this observation sample in an epoxy resin and dyeing it with ruthenium tetroxide, an ultra-thin section was prepared from the sample using an ultramicrotome. The ultrathin section was placed on a grid for TEM, and a photograph of a cross section of the surface layer of the expandable particles at a magnification of 10,000 or 50,000 was taken to observe the morphology. In a transmission electron micrograph (that is, a TEM photograph), a dark gray portion is a polyethylene resin, and a light gray portion is a styrene resin. In the TEM photograph, when the polyethylene resin and the styrene resin form a co-continuous phase, the polymorphology is determined to be a sea-sea structure, the polyethylene resin forms a dispersed phase, and the polystyrene resin forms a dispersed phase. Formed a continuous phase, the morphology was determined to be an islands-in-sea structure, and when the polyethylene-based resin formed a continuous phase and the polystyrene-based resin formed a dispersed phase, the morphology was determined to be a sea-island structure. Table 1 shows the results.

"Ratio of xylene insolubles"
Specifically, first, about 1 g of expandable particles were collected, weighed (W ₀ ) to the fourth decimal place, and placed in a 150 mesh wire mesh bag. Then, about 200 ml of xylene was put into a 200 ml round bottom flask, and the sample put in a wire mesh bag was set in a Soxhlet extraction tube. Soxhlet extraction was performed by heating the flask with a mantle heater for 8 hours. After completion of the extraction, the mixture was cooled by air cooling. After cooling, the wire net was taken out of the extraction tube, and the sample was washed together with about 600 ml of acetone. Next, the acetone was volatilized and dried at a temperature of 120 ° C. After the drying, a sample collected from the wire mesh was collected as “xylene-insoluble matter”, and its weight (W ₁ ) was measured to the fourth decimal place. The ratio of the xylene-insoluble component was calculated by the ratio of the weight (W ₁ ) of the xylene-insoluble component to the weight (W ₀ ) of the expandable particles (W ₁ / W ₀ ; percentage (%)).

"Weight average molecular weight (Mw) of acetone-soluble component in composite resin"
First, Soxhlet extraction was performed in the same manner as described above. Next, the extracted xylene solution was dropped into 600 ml of acetone, decanted, and evaporated to dryness under reduced pressure. The acetone-soluble matter extracted in this way mainly corresponds to the polystyrene resin. The weight average molecular weight of the acetone-soluble component was measured by a gel permeation chromatography (GPC) method (mixed gel column for polymer measurement) using polystyrene as a standard substance. Specifically, using a measuring device (HLC-8320GPC EcoSEC) manufactured by Tosoh Corporation, eluent: tetrahydrofuran (THF), flow rate: 0.6 ml / min, sample concentration: 0.1 wt%, column: TSKguardcolumn The measurement was carried out under the measurement condition that one SuperH-H and two TSK-GEL SuperHM-H were connected in series. That is, the weight average molecular weight was determined by dissolving acetone-soluble components in tetrahydrofuran, measuring by gel permeation chromatography (GPC), and calibrating with standard polystyrene.

(3) Preparation of Expanded Particles Next, the expandable particles obtained as described above were aged in a low-temperature warehouse at 6 ° C. for 1 day, and then composite resin expanded particles having a bulk density of about 20 kg / m ³ were prepared. Specifically, the expandable particles were first placed in a normal pressure batch foaming machine having a volume of 30 L, and steam was supplied into the foaming machine. As a result, the expandable particles were expanded to a bulk density of about 20 kg / m ³ to obtain expanded particles having a bulk expansion ratio of 50 times.

For the bulk density (kg / m ³ ) of the expanded particles, prepare a 1 L measuring cylinder, fill the empty measuring cylinder with the expanded particles up to the 1 L marked line, and measure the mass (g) of the expanded particles per 1 L. Asked to do.

(4) Production of molded article (composite resin foamed particle molded article) First, the foamed particles obtained as described above were aged at room temperature for 1 day. Next, the foamed particles were formed into a rectangular parallelepiped molded article of 300 mm × 75 mm × 25 mm using a mold molding machine (DSM-0705VS manufactured by DABO). After the obtained molded body was dried at a temperature of 40 ° C. for one day, it was further cured at room temperature for one day or more.

In this manner, by molding the expanded beads having a bulk density of about 20 kg / m ^3, expansion ratio 50 times, to obtain a molded product of an apparent density of 20 kg / m ^3. The foaming ratio of the molded article can be calculated by dividing the mass of the molded article by its volume to calculate the apparent density (kg / m ³ ) and using the following equation (5).
Expansion ratio (times) = 1000 / apparent density (kg / m ³ )

  Next, for the foamed resin molded article, the vacuum cooling time during molding, the fusing property, and the toughness were measured as follows.

`` Vacuum cooling time during molding ''
A molded product having a size of 300 mm x 300 mm x a thickness of 50 mm was molded by a mold molding machine (AD / 0907 manufactured by Kasahara Kogyo KK). The molding conditions are as follows. First, after heating for 20 seconds at a steam pressure of 0.07 MPa (gauge pressure, hereinafter represented by the symbol “G”), water cooling was performed for 5 seconds. Furthermore, vacuum cooling was performed at a reduced pressure of -0.08 MPa (G), and when the surface pressure gauge attached to the inner surface of the mold reached 0.02 MPa (G), the mold was opened to release the molded body. The time from the start of vacuum cooling to the release was measured as the vacuum cooling time. It should be noted that a shorter vacuum cooling time results in a shorter molding cycle.

`` Fusibility ''
In a plate-shaped molded body having a length of 300 mm x a width of 75 mm x a thickness of 25 mm, a cut of 2 mm in depth is cut across one surface (that is, one surface having a length of 300 mm and a width of 25 mm) at the center in the length direction, and traverses the entire width. To prepare a test piece. Next, the test piece was bent in a direction to widen the cut, until the test piece was broken, or until both end portions of the test piece were in contact with each other. Next, the cross section of the test piece was observed, and the number of foamed particles that were broken inside and the number of foamed particles that peeled off at the interface were visually measured. Next, the ratio of the foamed particles that fractured inside and the foamed particles that peeled off at the interface was calculated, and this was expressed as a percentage, and was defined as the fusibility (%).

"Bending test"
A molded body having a length of 300 mm, a width of 75 mm and a thickness of 25 mm was used as a test piece and subjected to a three-point bending test (span 200 mm) in accordance with JIS K 7221-2: 1999. As a result, the test piece was evaluated as “◎” when not broken, “○” when the strain at break was 15% or more, and “X” when the strain at break was less than 15%.

(Example 2)
In this example, in the second polymerization step, expandable particles, expanded particles, and a molded article were produced in the same manner as in Example 1, except that the temperature was raised to 120 ° C. over 10 hours. Further, the same evaluation as in Example 1 was performed. The results of the infrared absorption spectrum of the expandable particles of this example are shown in FIGS.

(Example 3)
In this example, in the second polymerization step, expandable particles, expanded particles, and a molded article were produced in the same manner as in Example 1, except that the temperature was raised to 120 ° C. over 3 hours. Further, the same evaluation as in Example 1 was performed. The results of the infrared absorption spectrum of the expandable particles of this example are shown in FIGS.

(Example 4)
(1) Preparation of core particles A linear low-density polyethylene (“Nipolon Z HF210K” manufactured by Tosoh Corporation) prepared by polymerization using a metallocene polymerization catalyst was prepared as a polyethylene resin. In addition, acrylonitrile was used as a dispersion diameter expanding agent. -A styrene copolymer ("AS-XGS, Denka Co., Ltd., weight average molecular weight: 109,000, acrylonitrile component amount: 28 mass%, MFR (200 ° C, 5 kgf: 2.8 g / 10 min) was prepared. 20 kg of linear low-density polyethylene (“Nipolon Z HF210K” manufactured by Tosoh Corporation) and 1 kg of a dispersing agent are added to a Henschel mixer (manufactured by Mitsui Miike Kakoki; model FM-75E), mixed for 5 minutes, and mixed. A mixture was obtained.

  Next, the resin mixture was melt-kneaded at a temperature of 230 to 250 ° C using a 26 mmφ twin-screw extruder (specifically, model TEM-26SS manufactured by Toshiba Machine Co., Ltd.). The melt-kneaded product was extruded and cut into 0.15 to 0.25 mg / piece (average 0.20 mg / piece) by an underwater cutting method to obtain core particles containing a polyethylene resin.

(2) Preparation of Expandable Particles A magnesium pyrophosphate slurry as a suspending agent was prepared in the same manner as in Example 1. Next, 2.0 g of sodium lauryl sulfonate (10 mass% aqueous solution) as a surfactant, 0.2 g of sodium nitrite as a water-soluble polymerization inhibitor, and 75 g of core particles were added to the suspension.

  Subsequently, 1.72 g of t-butyl peroxy-2-ethylhexyl monocarbonate ("Perbutyl E" manufactured by NOF Corporation) and 0.86 g of t-hexyl peroxybenzoate ("Perhexyl Z" manufactured by NOF Corporation) as polymerization initiators. ) And 0.63 g of α-methylstyrene dimer (“NOFMER MSD” manufactured by NOF Corporation) as a chain transfer agent were dissolved in the first monomer (styrene-based monomer). Then, the melt was poured into the suspension in the autoclave while stirring at a stirring speed of 500 rpm (dispersion step). Note that a mixed monomer of 60 g of styrene and 15 g of butyl acrylate was used as the first monomer.

  Next, after the inside of the autoclave was replaced with nitrogen, the temperature was raised and the temperature was raised to 100 ° C. over 1 hour and 30 minutes. After the temperature was raised, the temperature was kept at 100 ° C. for 60 minutes. Thereafter, the stirring speed was reduced to 450 rpm, and the mixture was kept at a polymerization temperature of 100 ° C. for 7 hours and 30 minutes (first polymerization step). In addition, 350 g of styrene as the second monomer (styrene-based monomer) was added into the autoclave over 5 hours when 60 minutes had elapsed after the temperature reached 100 ° C. Next, the temperature was raised to 125 ° C. over 3 hours, and kept at 125 ° C. for 5 hours (second polymerization step).

  Thereafter, the same operation as in Example 1 was performed to produce expandable particles, expanded particles, and a molded article. Further, the same evaluation as in Example 1 was performed.

(Comparative Example 1)
In this example, in the second polymerization step, expandable particles, expanded particles, and a molded body were produced in the same manner as in Example 1 except that the temperature was raised to 120 ° C. over 2 hours. Further, the same evaluation as in Example 1 was performed. The results of the infrared absorption spectrum of the expandable particles of this example are shown in FIGS.

(Comparative Example 2)
In this example, in the second polymerization step, expandable particles, expanded particles, and a molded article were produced in the same manner as in Example 1 except that the temperature was raised to 120 ° C. over 30 minutes. Further, the same evaluation as in Example 1 was performed. The results of the infrared absorption spectrum of the expandable particles of this example are shown in FIGS.

(Comparative Example 3)
In this example, expandable particles, expanded particles, and a molded article were produced in the same manner as in Example 1, except that sodium nitrite was not used as a water-soluble polymerization inhibitor. Further, the same evaluation as in Example 1 was performed. FIG. 3 shows the results of the infrared absorption spectrum of the expandable particles of this example.

(Comparative Example 4)
In this example, expandable particles were produced in the same manner as in Example 4, except that the temperature was raised to 125 ° C. over 2 hours in the second polymerization step.

(Results of Examples and Comparative Examples)
Table 1 shows the polymerization conditions and evaluation results of the expandable particles produced in Examples 2 to 4 and Comparative Examples 1 to 4 in the same manner as in Example 1. In Table 1, “○” means that the inequality relationship is satisfied, and “x” means that the inequality relationship is not satisfied.

As can be seen from Table 1, the expandable particles whose absorbance ratio D ₆₉₈ / D ₂₈₅₀ satisfies the predetermined relationship as in Examples 1 to 4 can shorten the molding cycle (specifically, the vacuum cooling time). In addition, it is possible to produce a molded article having excellent fusion property and toughness. Since the molded articles of Examples 1 to 4 are excellent in fusing properties, they can also exhibit excellent rigidity unique to polystyrene resins. It is considered that such expandable particles can be obtained by adjusting the rate of temperature rise, the water-soluble polymerization inhibitor, the resin composition, and the like in the second polymerization step.

  On the other hand, the expandable particles of Comparative Examples 1, 2, and 4 can obtain molded articles having excellent toughness, but have a longer molding cycle. Further, the expandable particles of Comparative Example 3 can shorten the molding cycle, but the toughness of the molded body becomes insufficient.

As described above, the reason why the expandable particles of the examples are superior in toughness and molding cycle as compared with the comparative example is considered to be due to the difference in the gradient structure of the ratio of the polystyrene resin (see FIGS. 1 to 5). In the following description, with reference to FIGS. 6 to 8, the internal structure of the expandable particles 1, 8, 9 will be described in terms of the central layer 11, 81, 91, the intermediate layers 12, 82, 92, the surface layers 13, 83, Although described separately in 93, it is considered that the actual internal structure of the expandable particles 1, 8, and 9 cannot be distinguished as a layer. The surface layers 13, 83, and 93 have a depth of 30 μm from the surfaces 10, 80 and 90 of the expandable particles 1, 8 and 9. The intermediate layers 12, 82, and 92 are located inside the surface layers 13, 83, and 93. The central layers 11, 81, and 91 are located inside the intermediate layers 12, 82, and 92, and are regions including the centers O ₁ , O ₈ , and O ₉ of the expandable particles ₁ , ₈ , and ₉ .

  6 to 8 are image diagrams schematically showing the gradient structures of the proportions of the polystyrene resin in the expandable particles 1, 8, and 9 of Example 1, Comparative Example 1, and Comparative Example 3, respectively. Does not necessarily correspond to the sex particles. 6 to 8, the number of polystyrene resins is schematically shown by the density of the hatched dot density.

As schematically shown in FIG. 7, in the expandable particles 8 of Comparative Example 1, for example, in the order of the center layer 81, the intermediate layer 82, and the surface layer 83, the ratio of the polystyrene resin is increased from the particle center O ₈ toward the surface 80. Is gradually getting lower. It is considered that the reason why the expandable particles 8 of Comparative Example 1 have such a gradient structure is that the second heating rate in the polymerization step is too high, and the amount of polystyrene resin cannot be increased near the surface layer. Therefore, in Comparative Example 1, although the foamed particle molded article obtained from the foamed particles obtained by foaming the foamable particles has excellent fusion bonding property and toughness, it is considered that the vacuum cooling time during the molding of the foamed particles becomes longer. . Although not shown in the figure, in Comparative Example 2, it is considered that the effect of shortening the molding cycle cannot be obtained as in Comparative Example 1, because the second temperature-raising temperature is too high.

As schematically shown in FIG. 8, in the expandable particles 9 of Comparative Example 3, for example, in the order of the center layer 91, the intermediate layer 92, and the surface layer 93, the ratio of the polystyrene resin is increased from the particle center O ₉ toward the surface 90. Is thought to gradually increase. The reason why the expandable particles of Comparative Example 3 have such a gradient structure is that the water-soluble polymerization inhibitor was not added to the aqueous medium in the dispersion step, and the styrene monomer near the resin particle surface in the polymerization step. It is considered that polymerization was excessively promoted. Therefore, in Comparative Example 3, although the vacuum cooling time is shortened, it is considered that a molded article having excellent fusion property and toughness cannot be obtained.

On the other hand, in the expandable particle 1 of Example 1, as schematically shown in FIG. 6, the polystyrene resin in the center layer 11 including the particle center O ₁ is the largest, and is then indicated by the absorbance ratio value A1. It is considered that the polystyrene resin in the surface layer 13 is increased as shown in FIG. Then, as indicated by the value A2 of the absorbance ratio, it is considered that the intermediate layer 12 having less polystyrene resin than these layers exists between the central layer 11 and the surface layer 13. Thus, it is considered that the expandable particles 1 of Example 1 have a characteristic inclined structure in which the ratio of the polystyrene resin in the intermediate layer 12 is reduced. The same applies to the third and fourth embodiments.

In such an inclined structure, the presence of the surface layer 13 having more rigid polystyrene resin than the intermediate layer 12 suppresses the expansion force remaining inside the molded body, for example, in in-mold molding, and It is thought that the time can be reduced. Furthermore, since the intermediate layer 12 in which the ratio of the polystyrene resin is locally reduced exists between the surface layer 13 and the center layer 11, the molded article is formed despite the high ratio of the polystyrene resin in the surface layer 13. Is considered to be able to show excellent toughness. As typified by such a characteristic tilted structure, the expandable particles of the examples in which the absorbance ratio D ₆₉₈ / D ₂₈₅₀ satisfies the predetermined relationship in the infrared absorption spectrum, the toughness is obtained even when the cooling time is reduced. This makes it possible to produce a molded article excellent in quality.

Further, although not shown, the expandable particles in Examples 1, 3, and 4 have a value A0 of the absorbance ratio _D698 / _D2850 in the outermost layer smaller than the value A1 of the absorbance ratio in the surface layer. Therefore, in the surface layer portion of the foamed particles in these examples, that is, in the portion corresponding to the surface layer in the foamable particles, the polystyrene resin on the surface is less than the portion slightly inside the surface, such a feature It is considered that a typical inclined structure is formed. Therefore, in these examples, since the surface layer portion contains a large amount of polystyrene resin, the effect of shortening the molding cycle can be obtained. Furthermore, since the polystyrene resin is reduced on the surface, the toughness of the molded article obtained by fusing the foamed particles to each other can be improved. Although not shown, in Example 2, the ratio of the polystyrene-based resin on the outermost surface was the highest, followed by the polystyrene-based resin in the center layer. It is believed that a layer exists.

DESCRIPTION OF SYMBOLS 1 Expandable composite resin particle 11 Center layer 12 Middle layer 13 Surface layer

Claims (7)

In a foamable composite resin particle containing a composite resin of a polyethylene resin and a polystyrene resin, and a physical foaming agent,
Infrared absorption spectrum obtained by measuring a cross section passing through the center of the expandable composite resin particle every 10 μm from the surface of the expandable composite resin particle to the center by microscopic infrared spectroscopic analysis, including a wave number of 2850 cm ⁻¹ . An expandable composite resin in which the ratio D ₆₉₈ / D ₂₈₅₀ of the absorbance D ₆₉₈ at the peak top containing the wave number 698 cm ⁻¹ to the absorbance D ₂₈₅₀ at the peak top satisfies the following relationships (1) to (3): particle.
(1) The maximum value A1 of the ratio D ₆₉₈ / D ₂₈₅₀ is 1 to 5 when the distance from the surface of the expandable composite resin particles is in the range of 0 to 30 μm.
(2) the value A2 of the ratio D ₆₉₈ / D ₂₈₅₀ distance from the surface of the expandable composite resin particles in the range of 100~110μm is smaller than the maximum value A1.
(3) The value A3 of the ratio D ₆₉₈ / D ₂₈₅₀ at the center of the expandable composite resin particles is larger than the value A2. 2. The foam according to claim 1, wherein in the infrared absorption spectrum, the value A0 of the ratio D ₆₉₈ / D _{2850 in} a range of 0 to 10 μm from the surface of the expandable composite resin particle is smaller than the maximum value A1. 3. Composite resin particles. 3. The expandable composite resin particle according to claim 1, wherein in the infrared absorption spectrum, the maximum value A1 is smaller than a value A3 of the ratio D ₆₉₈ / D _{2850 at} the center. The infrared absorption spectrum has a minimum value A4 of the ratio D ₆₉₈ / D ₂₈₅₀ within a range of 30 to 100 μm from the surface of the expandable composite resin particle, and the minimum value A4 is less than 1. The expandable composite resin particles according to claim 1.   The expandable composite resin particle according to any one of claims 1 to 4, wherein the maximum value A1 is 1.5 to 3.   The expandable composite resin particle according to any one of claims 1 to 5, wherein the average particle diameter of the expandable composite resin particle is 1 mm to 2 mm.   The composite resin contains 5 to 50% by mass of a polyethylene resin and 50 to 95% by mass of a polystyrene resin (however, the total of the polyethylene resin and the polystyrene resin is 100% by mass), The expandable composite resin particles according to any one of claims 1 to 6.

Images