JP7148790B2 - Expandable composite resin particles - Google Patents

Description

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

The foamed bead molded product is obtained by molding the foamed beads in a mold and fusing them to each other. Expanded bead moldings are used in a wide range of applications such as packaging materials, building materials, and impact absorbing materials, taking advantage of their excellent shock-absorbing properties, light weight, heat insulating properties, and the like. As the expanded bead molded article, those whose base resin is polyolefin resin such as polypropylene or polyethylene, or those whose base resin is polystyrene resin such as polystyrene are used.

Polyolefin-based resin foamed particles are excellent in impact resistance and toughness, so they are used as packing materials and packaging materials for precision parts and heavy products. In addition, since the polyolefin resin expanded particle molded product is excellent in heat resistance and oil resistance, it is also used as a shock absorber, a bumper, a floor spacer, and other automotive parts.

On the other hand, polystyrene resin expanded bead molded articles are generally inexpensive, have a low molding vapor pressure during in-mold molding, and have good workability, and are widely used in the market. In addition, since the polystyrene-based resin expanded bead molded article is excellent in rigidity, the expansion ratio can be increased depending on the application, which is advantageous in terms of light weight.

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

In recent years, the development of expanded bead moldings that combine toughness, which is an advantage of expanded bead moldings using polyolefin resin as a base resin, and rigidity, which is an advantage of expanding bead moldings using polystyrene resin as a base resin, has been made. It has been demanded. Accordingly, expandable particles, expanded particles, and expanded particle moldings using a composite resin of a polyolefin resin and a polystyrene 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, conventional expanded bead molded articles using a composite resin as a base resin, such as those described in Patent Documents 1 to 4, have room for improvement in terms of moldability. Specifically, in order to sufficiently improve the toughness of the expanded bead molded article, it is necessary to improve the fusion bondability of the expanded bead. However, if the heating temperature is increased in an attempt to improve the fusion bondability, the cooling time after heating during molding becomes longer, and as a result, the molding cycle of the expanded bead molded article may become longer. On the other hand, if the molding cycle is shortened, the adhesion of the foamed beads becomes insufficient, and the appearance of the molded foamed beads may deteriorate, and the desired physical properties such as toughness required of the composite resin may be impaired.

The present invention has been made in view of such a background, and an object thereof is to provide expandable composite resin particles from which an expanded particle molded article having excellent toughness can be obtained even with a short molding cycle.

One aspect 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 blowing agent,
By microscopic infrared spectroscopic analysis, the infrared absorption spectrum obtained by measuring the cross section passing through the center of the expandable composite resin particle every 10 μm from the surface of the expandable composite resin particle toward the center includes a wavenumber 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 of the peak including the wavenumber 698 cm ^-1 to the absorbance D ₂₈₅₀ at the peak top of the peak satisfies the following relationships (1) to (3) particle.
(1) The maximum value A1 of the ratio D ₆₉₈ /D ₂₈₅₀ is 1.5-5 when the distance from the surface of the expandable composite resin particles is in the range of 0-30 μm.
(2) The value A2 of the ratio D ₆₉₈ /D ₂₈₅₀ when the distance from the surface of the expandable composite resin particles is in the range of 100 to 110 μm 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 resin and a polystyrene resin and a physical blowing agent, and the ratio D ₆₉₈ /D ₂₈₅₀ in the infrared absorption spectrum satisfies the above relationship, and the expandable composite resin particles The polystyrene resin of the resin particles has a specific gradient structure.

Thus, the expandable composite resin particles that satisfy the above relationship can shorten the cooling time during molding of the expanded bead molded article, thereby shortening the molding cycle. Furthermore, the expandable composite resin particles make it possible to obtain an expanded bead molded article having excellent adhesion between the expanded particles even if the cooling time during molding is shortened. The desired excellent toughness required for an expanded bead molded article using a composite resin as a base resin can be exhibited.

1 is an infrared absorption spectrum diagram of expandable composite resin particles in Example 1 and Comparative Example 1. FIG. FIG. 2 is an infrared absorption spectrum diagram of expandable composite resin particles in Example 2 and Comparative Example 2; FIG. 2 is an infrared absorption spectrum diagram of expandable composite resin particles in Reference Example 1 and Comparative Example 3. FIG. 2 is an enlarged view of infrared absorption spectra of expandable composite resin particles of Example 1, Example 2, and Comparative Example 1. FIG. 2 is an enlarged view of infrared absorption spectra of expandable composite resin particles of Reference Example 1 and Comparative Example 2. FIG. FIG. 2 is an explanatory view schematically showing the gradient structure of the polystyrene-based resin ratio in the cross section of the expandable composite resin particles of Example 1. FIG. FIG. 2 is an explanatory view schematically showing a gradient structure of the polystyrene-based resin ratio in the cross section of the expandable composite resin particles of Comparative Example 1; FIG. 3 is an explanatory view schematically showing a gradient structure of polystyrene-based resin ratios in the cross section of the expandable composite resin particles of Comparative Example 3;

Preferred embodiments of the expandable composite resin particles are described below. In this specification, when a numerical value or a physical property value is sandwiched before and after the "~", it is used to include the values before and after it. In addition, in the preferred ranges, more preferred ranges, and further preferred ranges of numerical values and physical property values, all combinations of upper limits and lower limits that determine these ranges can be selected. Therefore, a combination of a preferred upper limit value and a lower limit value, a more preferred combination of an upper limit value and a lower limit value, and a more preferred combination of an upper limit value and a lower limit value are most preferred, but are not necessarily limited to these combinations. do not have. In addition, when the terms "consist of" and "consist of" are used in this specification, they do not mean "consist of" or "consist of only".

[Expandable Composite Resin Particles, Expanded Composite Resin Particles, Expanded Bead Mold]
The expandable composite resin particles are heated by a heating medium such as steam to expand to become expanded composite resin particles. Accordingly, the expanded composite resin particles are formed by pre-expanding the expandable composite resin particles, and are particulate foams having the composite resin as the base resin. Hereinafter, "expandable composite resin particles" may be referred to as "expandable particles", and "composite resin expanded particles" may be referred to as "expanded particles".

The foamed bead molded product is formed by molding a large number of foamed beads in a mold, and the foamed beads are fused together. In-mold molding is performed by, for example, filling a mold with a large number of foamed particles, introducing a heating medium such as steam into the mold, and heating the mold to fuse the foamed particles to each other in the mold. After that, it is cooled by standing to cool or the like to obtain a molded body having a desired shape. Hereinafter, the expanded bead molded article may be referred to as a "molded article".

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

(Composition of composite resin)
The mixing ratio of the polyethylene-based resin and the polystyrene-based resin in the composite resin can be appropriately adjusted. By increasing the content ratio of the polyethylene-based 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-based resin in the composite resin, it is possible to improve the retention of the foaming agent in the expandable particles and the rigidity of the molded article. The blending ratio of the polyethylene-based resin and the polystyrene-based resin in the composite resin can be obtained from the blending composition of the raw materials when 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 point of view, 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 even more preferably 88% by mass or less. In addition, each content of a polyethylene-type resin and a polystyrene-type resin is represented by the content ratio of each resin with respect to the total amount of a polyethylene-type resin and a polystyrene-type 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 35% by mass or less. is more preferred. From the same point of view, 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.

In addition, by increasing the ratio of the polystyrene resin in the composite resin as described above, the retention of the foaming agent made of a hydrocarbon compound such as butane is improved. Accordingly, the expandable particles allow storage at high storage temperatures as well as extended shelf life. As a result, the expandable particles can be stored for a long period of time, for example at room temperature, while maintaining sufficient foaming power in a sealed container. Therefore, it is not necessary to expand the expandable particles within a short period of time after the production of the expandable particles, and it becomes possible to transport and store the expandable particles in the form of non-bulky expandable particles.

If the expandable beads are actually stored for a long period of time and then expanded to obtain expanded beads, it is possible to reduce the variation in the apparent density of the expanded beads. Furthermore, the in-mold moldability of the expanded particles is improved, and the molded article has excellent appearance and adhesion between the expanded particles.

In the expandable particles, it is preferable that the weight ratio of the xylene-insoluble matter obtained by Soxhlet extraction is 40% or less (including 0). In this case, the expandability of the expandable particles can be further improved. Further, the weight ratio of xylene-insoluble matter in the expandable particles is more preferably 5 to 30%, more preferably 5 to 20%. In this case, the rigidity and toughness of the compact can be further improved. The xylene-insoluble matter is the 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 by microscopic infrared spectroscopic analysis (hereinafter sometimes referred to as microscopic IR) at every 10 μm from the surface of the expandable particle toward the center. is. The center direction of the expandable bead is, for example, the direction from the surface toward the center of the expandable bead on a straight line passing through the center of the cross section of the expandable bead. In addition, the "straight line passing through the center from the surface" does not actually belong to the cross section passing through the center of the expandable particles, but is an imaginary line or a line drawn at the time of microscopic IR measurement 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 _D698 / _D2850 in the infrared absorption spectrum. Absorbance D ₆₉₈ means the absorbance value at the peak top of the peak containing wavenumber 698 cm ⁻¹ . This peak is derived from the out-of-plane bending vibration of the benzene ring mainly contained in the polystyrene resin. Also, the absorbance D ₂₈₅₀ means the absorbance value at the peak top of the peak containing the wave number of 2850 cm ⁻¹ . This peak is derived from C—H stretching vibration of methylene groups 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 top derived from the CH stretching vibration of the methylene group. D means the ratio of ₆₉₈ .

In the infrared absorption spectrum described above, the maximum value A1 of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the range from the surface of the expandable particle to a depth of 30 μm, and the value A2 of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the range of 100 to 110 μm. , the value A3 of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ at the center satisfies the following relationships (1) to (3).
(1) A1 is 1.5 to 5; That is, it satisfies the relationship 1.5≤A1≤5 .
(2) A2 is smaller than A1; That is, it satisfies the relationship A2<A1.
(3) A3 is greater than A2; That is, it satisfies the relationship A3>A2.

Expandable particles containing a composite resin and a foaming agent and satisfying the relationships (1) to (3) shorten the molding cycle during the production of molded articles and produce molded articles having excellent rigidity and toughness. to enable. The reason for this is considered as follows.

Among the relationships (1) to (3), in the expandable particles that particularly satisfy the relationship (1), it is considered that a large amount of polystyrene-based resin having excellent rigidity exists in the surface layer of the expandable particles. The surface layer of the expandable particles refers to the range from the surface of the expandable particles (that is, depth 0) to a depth of 30 μm.

The expanded beads obtained by expanding the expandable beads satisfying the above relationship (1) are considered to have a distribution structure corresponding to the distribution structure of the polystyrene-based resin described above. That is, it is considered that a large amount of polystyrene-based resin is present in the portion of the expanded bead corresponding to the surface layer of the expandable bead (hereinafter sometimes referred to as "the surface layer portion of the expanded bead"). As a result, for example, during the production of a molded product by in-mold molding, the excessive expansive force of the expanded beads is suppressed by the polystyrene-based resin present in the surface layer of the expanded beads, so that it can be quickly cooled after molding. The shape of the compact is stabilized. This makes it possible to shorten the cooling time.

In general, cooling during in-mold molding of an expanded bead molded body reduces the surface pressure (hereinafter sometimes referred to as surface pressure) due to the foaming force of the expanded bead molded body to a predetermined pressure, and stabilizes the shape of the molded body. After that, the expanded bead molding is released from the molding die, for example. When the expandable particles obtained from the expandable particles are molded in a mold, the expandable particles can increase the speed from the start of cooling to the reduction of the surface pressure to a predetermined pressure. can. Further, even if the internal temperature of the expanded bead molded article after cooling is higher than that of the conventional product, internal deformation such as shrinkage can be prevented from occurring in the expanded bead molded article. As a result, the expandable particles can shorten the molding cycle.

Furthermore, in the expandable particles that satisfy the above relationships (2) and (3), the ratio of the polystyrene resin existing between the surface layer and the center portion of the expandable particle is around 100 μm in depth, and the surface layer It is locally less than the central part. Therefore, the expandable particles obtained by expanding the expandable particles have a high proportion of the polystyrene-based resin in the surface layer portion, so that the rigidity is locally high. On the other hand, since the toughness of the expanded beads as a whole is maintained, it is considered that the molded article obtained by in-mold molding of the expanded beads can exhibit excellent toughness.

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

In the infrared absorption spectrum of the expandable particles, when A1<1, the amount of polystyrene-based resin present in the surface layer of the expandable particles is small, and the expanded particles obtained by expanding the expandable particles have an excessive expansion force. It may become difficult to control. As a result, the effect of shortening the molding cycle may not be sufficiently obtained. It should be noted that the expansive force can also be referred to as secondary foamability. On the other hand, when A1>5, the toughness of the compact may be insufficient. From the viewpoint of shortening the molding cycle, the relationship A1≧1.5 shall be satisfied. From the same point of view, it is more preferable to satisfy the relationship A1≧1.8. On the other hand, from the viewpoint of further increasing the toughness, A1≦3 is preferable, and A1≦2.5 is more preferable.

Further, when A2>A1 and A3<A2, it becomes difficult to sufficiently exhibit the toughness of the foamed beads as a whole, and the molding cycle becomes longer due to the longer time required for cooling during the production of the molded article. There is a risk of

From the same point of view, it is preferable to satisfy the relationship A1-A2≧0.5, and it is more preferable to satisfy the relationship A1-A2≧1. Further, it is preferable to satisfy the relationship A3-A2≧1, and more preferably 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 particle 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. In other words, it is preferable that the measurement region of the infrared absorption spectrum showing the maximum value A1 of the absorbance ratio _D698 / _D2850 is not within the range of 10 µm deep from the surface.

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

In this case, since the amount of the polystyrene-based resin is reduced in the outermost layer, the adhesion is further improved. Therefore, it is possible to lower the molding temperature while maintaining the fusion between the expanded particles forming the molded article, and as a result, the molding cycle during molding can be further shortened. From the viewpoint of enhancing this effect, A1-A0≧0 is more preferable, and A1-A0≧0.5 is even more preferable.

Furthermore, in the infrared absorption spectrum, it is preferable that the maximum value A1 of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the range from the surface to a depth of 30 μm is smaller than the value A3 of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the center. That is, it is preferable that A1<A3. In this case, the amount of polystyrene-based resin in the surface layer is less than that in the central portion of the expandable particle. That is, more polyethylene-based resin is present in the surface layer than in the central portion. As a result, in the molded article, toughness, which is an excellent characteristic of the polyethylene-based resin, is more likely to be exhibited, and the toughness of the molded article is improved.

In addition, the infrared absorption spectrum has a minimum value A4 of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ within the 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 preferably 0.1 or more and less than 1. In this case, there is a local region with a small amount of polystyrene-based resin between the center of the expandable particle and the surface layer. As a result, in the expanded beads obtained by expanding the expandable beads, the ratio of the polystyrene-based resin in the portion corresponding to the surface layer of the expanded beads is high and the rigidity is locally high. The polyethylene-based resin is relatively abundant on the outermost surface where the members are fused together. Therefore, in this case, the toughness of the expanded bead molded article as a whole is easily maintained, and the effect of shortening the molding cycle and the effect of improving the toughness are further enhanced. 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 absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the expandable particle cross section can be measured based on JIS K0117 (2017). Specifically, first, a sample is prepared by cutting the expandable particles in half. Then, using an FT/IR-4600 Fourier transform infrared spectrophotometer manufactured by JASCO Corporation and an IRT-5200 infrared microscope manufactured by JASCO Corporation, a line is formed on the cut surface passing through the center of the expandable particle by a microscopic transmission method. An infrared absorption spectrum of the absorbance can be obtained by measuring the mapping in the shape. The absorbance ratio D ₆₉₈ /D ₂₈₅₀ can be calculated from this infrared absorption spectrum. A detailed measurement method will be described in Examples.

(Molecular weight of acetone-soluble matter in composite resin)
The weight-average molecular weight of the acetone-soluble matter 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 fusion between the expanded particles during in-mold molding. As a result, the dimensional stability of the molded article can be improved. From the same point of view, the weight-average molecular weight of the acetone-soluble matter in the composite resin is more preferably 100,000 or more, more preferably 150,000 or more. Further, the weight-average molecular weight of the acetone-soluble matter in the composite resin is more preferably 250,000 or less. A method for measuring the weight-average molecular weight of the acetone-soluble matter 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 in which polyethylene resin and polystyrene resin form a co-continuous phase (sea-sea structure), and a polyethylene resin forms a dispersed phase (island phase) and polystyrene resin forms a continuous phase. There is a morphology (island-sea structure) forming a (sea phase), or a morphology (sea-island structure) in which a polyethylene-based resin forms a continuous phase and a polystyrene-based resin forms a dispersed phase. Among these, a sea-island structure or a sea-sea structure is preferable. In this case, superior toughness can be obtained as compared with the morphology (island-sea structure) in which the polystyrene-based resin forms the continuous phase and the polyethylene-based resin forms the dispersed phase. Further, from the viewpoint of obtaining good toughness, it is more preferable to exhibit a morphology (sea-island structure) in which the polyethylene resin forms a continuous phase and the polystyrene resin forms a dispersed phase.

The morphology in the surface layer of the expandable particles can be observed by the following method. Specifically, first, a sample for observation is cut out from the expandable particles, including the surface layer, that is, the range from the surface to a depth of 30 μm. Next, this observation sample is embedded in epoxy resin and stained with ruthenium tetroxide, and then an ultrathin section is prepared from the sample using an ultramicrotome. This ultra-thin section is placed on a grid, and the morphology of the central section of the expandable particles is observed with a transmission electron microscope (JEM1010 manufactured by JEOL Ltd.) at a magnification of 10000 times to take a cross-sectional photograph (TEM photograph). 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 the dispersed phase, and the polystyrene-based resin When the continuous phase is formed, the morphology is determined to be an island-sea structure, and when the polyethylene-based resin forms the continuous phase and the polystyrene-based resin forms the dispersed phase, the morphology is determined to be a sea-island structure.

(physical blowing agent)
The physical foaming agent includes, for example, an organic physical foaming agent. Examples of organic physical blowing agents 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; lower alcohols such as ethanol; ether compounds such as dimethyl ether and diethyl ether; As a physical foaming agent, one type of compound can be used, or two or more types of compounds can be used. Henceforth, a physical foaming agent may be called a "foaming agent."

As the foaming 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 mass% isobutane and 20 to 70 mass% other hydrocarbons 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. Furthermore, in the molded article, the fusion between the expanded particles can be further improved. More preferably, the proportion of isobutane in the foaming agent is 40% by mass or more and 75% by mass or less.

The average particle size of the expandable particles is preferably 1.0 to 2.0 mm. By adjusting within this range, the effect obtained by adjusting each value of the absorbance ratio in the infrared absorption spectrum described above is enhanced. That is, it is possible to more reliably obtain a compact having excellent rigidity and toughness while shortening the molding cycle. Furthermore, it is possible to enhance the retention of the foaming agent and to enhance the filling of the foamed particles during molding. From the viewpoint of further enhancing this effect, the average particle size of the expandable particles is more preferably 1.1 to 1.9 mm, even more preferably 1.2 to 1.8 mm. The average particle size of the expandable particles means the particle size (that is, d63) at a volume integrated value of 63% in the particle size distribution determined by the method described below.

Also, the content of the foaming 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, during in-mold molding, the fusion between the expanded particles can be further improved, and the dimensional stability of the molded product can be improved. More preferably, the content of the foaming agent is 4% by mass or more and 9% by mass or less.

<Method for producing expandable particles>
The expandable particles are manufactured by performing, for example, a dispersion step, a polymerization step, and a blowing agent impregnation step. In the dispersing step, core particles containing a polyethylene-based 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 impregnation step, the particles are impregnated with a foaming agent during and/or after polymerization. In this way, expandable particles can be obtained. An embodiment of each step will be described below.

<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 particle)
Polyethylene-based resins used for core particles include, for example, 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, and the like can be used. As the polyethylene-based resin, one kind of polymer may be used, but a mixture of two or more kinds of polymers may also be used.

Preferably, the polyethylene-based 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 branched chain having 2 to 6 carbon atoms. Specific examples include ethylene-hexene copolymers and ethylene-octene copolymers.

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

The melting point Tm of the polyethylene resin is preferably 95 to 115°C. In this case, the polyethylene-based resin can be sufficiently impregnated with the styrene-based monomer during 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 polystyrene resin and the excellent toughness of polyethylene resin at a higher level. More preferably, the polyethylene-based resin has a melting point Tm of 100 to 110°C. The melting point Tm of the polyethylene-based 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 0.5 to 4.0 g/10 minutes, more preferably 1.0 to 3.0 g/10 minutes, from the viewpoint of foamability. preferable. The MFR of polyethylene resin at 190° C. and 2.16 kgf is a value measured under condition code D based on JIS K7210-1999. As a measuring device, a melt indexer (for example, Model L203 manufactured by Takara Kogyo Co., Ltd.) can be used.

The crystallinity of the polyethylene resin is preferably 20 to 35%. In this case, the retention and foamability of the foaming agent can be further improved. This is probably because when the crystallinity of the polyethylene-based resin is within the above range, it becomes difficult for the gas molecules (specifically, the foaming agent) to spread the polymer chains of the polyethylene-based resin. . As a result, it can be inferred that the permeability of the foaming agent is lowered and the retention of the foaming agent is enhanced as described above. From the same point of view, the crystallinity of the polyethylene resin is more preferably 20 to 30%. The degree of crystallinity of the polyethylene-based resin is obtained from the heat of fusion measured by differential scanning calorimetry (DSC) based on JIS K7122-1987.

The core particles can contain a dispersion diameter enlarging agent. At least one selected from, for example, polystyrene, acrylonitrile-styrene copolymer, rubber-modified polystyrene, ABS resin, and AES resin can be used as the dispersion diameter enlarging agent. Acrylonitrile-styrene copolymer is preferred. Also, the acrylonitrile content in the acrylonitrile-styrene copolymer is preferably 20 to 40% by mass.

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

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 with respect to 100 parts by mass of the polyethylene resin. When the content of the dispersion diameter-enlarging agent is within the above range, it becomes easy to form a morphology in which the polyethylene-based resin and the polystyrene-based resin exhibit a co-continuous phase (sea-sea structure). Further, within the above range, it becomes easy to increase the dispersion diameter of the polystyrene resin (dispersed phase) 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 ability of the expandable particles to retain the blowing agent can be sufficiently improved. Also from the viewpoint of maintaining good toughness and rigidity of the molded body, it is preferable to set the content of the dispersion diameter enlarging agent within the above range.

The core particles can be produced by blending the polyethylene resin with additives added as necessary, melt-kneading the blend, and then granulating the mixture. Examples of additives include, in addition to the above dispersion diameter enlarging agents, cell control agents, pigments, slip agents, antistatic agents, flame retardants, and the like. Melt-kneading can be performed by an extruder. In order to perform uniform kneading, it is preferable to mix the resin components in advance and then perform extrusion. The resin components can be mixed using a mixer such as a Henschel mixer, a ribbon blender, a V blender, and a Loedige mixer. Melt-kneading is preferably carried out using a high-dispersion type screw such as a Dulmage type, Maddock type or Unimelt type screw or a twin-screw extruder. In this case, it is possible to improve the retention of the foaming agent and the foam moldability of the expandable particles. Moreover, in this case, the dispersion diameter enlarging agent can be uniformly dispersed in the polyethylene resin. As a result, it becomes possible to further increase the rigidity while maintaining the toughness that is characteristic of the polyethylene-based resin of the molded article.

Examples of cell-controlling agents include 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. The amount of the organic foam control agent to be blended is preferably in the range of 0.01 to 2 parts by mass with respect to 100 parts by mass of the resin for the core particles. The amount of the inorganic foam control agent to be blended is preferably in the range of 0.1 to 5 parts by mass with respect to 100 parts by mass of the resin for the core particles. By adjusting the blending amount of the cell control agent within the above range, the cell size of the foamed particles and the molded product is improved, the cells are less likely to be destroyed during molding in the mold, and the appearance of the molded product is improved.

Granulation of core particles can be performed by, for example, a strand cut method, an underwater cut method, a hot cut method, or the like. Any other method may be used as long as it can obtain core particles having a desired particle size.

The average particle size 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 within the above range, it becomes easier to adjust the average particle size of the expandable particles within the desired range described above. When an extruder is used to granulate the core particles, the particle size of the core particles is adjusted, for example, by adjusting the cut speed during cutting while extruding the resin through holes having a diameter within the desired particle size range. can do.

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

In the dispersing 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 carried out, for example, in a closed vessel equipped with a stirrer. Aqueous media include, for example, deionized water.

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

Suspending agents include tricalcium phosphate, hydroxyapatite, magnesium pyrophosphate, magnesium phosphate, aluminum hydroxide, ferric hydroxide, titanium hydroxide, magnesium hydroxide, barium phosphate, calcium carbonate, and magnesium carbonate. , barium carbonate, calcium sulfate, barium sulfate, talc, kaolin, and bentonite. Organic suspension agents such as polyvinylpyrrolidone, polyvinyl alcohol, ethylcellulose, and hydroxypropylmethylcellulose can also be used. Tricalcium phosphate, hydroxyapatite, and magnesium pyrophosphate are preferred. 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 0.00 wt. 05 to 10 parts by mass is preferred. More preferably 0.3 to 5 parts by mass. By setting the amount of the suspending agent within the above range, it is possible to stably suspend the styrene-based monomer and to suppress broadening of the particle size distribution of the expandable particles.

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

Examples of anionic surfactants that can be used include sodium alkylsulfonate, sodium alkylbenzenesulfonate, sodium laurylsulfate, sodium α-olefinsulfonate, and sodium dodecylphenyloxide disulfonate.

Examples of nonionic surfactants that can be used include polyoxyethylene nonylphenyl ether and polyoxyethylene lauryl ether.

As cationic surfactants, alkylamine salts such as coconut amine acetate and stearylamine acetate can be used. Quaternary ammoniums such as lauryltrimethylammonium chloride and stearyltrimethylammonium chloride can also be used.

As amphoteric surfactants, alkylbetaines such as laurylbetaine and stearylbetaine can be used. Alkylamine oxides such as lauryldimethylamine oxide can also be used.

Preferably, an anionic surfactant is used. Alkali metal salts (preferably sodium salts) of alkylsulfonic acids having 8 to 20 carbon atoms are more preferred. This allows the suspension to be sufficiently stabilized.

In addition, 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, if necessary.

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

Along with the use of a water-soluble polymerization inhibitor, 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 value by adjusting the amount of styrene-based monomer added and the rate of temperature increase during polymerization, which will be described later. can be adjusted within the range of , making it possible to manufacture expandable particles having the characteristic gradient structure described above. The reason for this is considered as follows. That is, in particular, the use of a water-soluble polymerization inhibitor suppresses the polymerization of styrene monomers in the aqueous medium that are not impregnated into the core particles and styrene monomers near the surface that are being absorbed into the core particles. As a result, it is thought that the amount of polystyrene-based resin near the surface of the expandable particles is less than that in the central portion. In this way, the characteristic gradient structure described above is formed, and expandable particles are obtained which enable the production of moldings having both excellent stiffness and toughness in a short molding cycle.

As the water-soluble polymerization inhibitor, sodium nitrite, potassium nitrite, ammonium nitrite, L-ascorbic acid, citric acid and the like can be used. Sodium nitrite is preferable as the water-soluble inhibitor from the viewpoint that each value of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ can be easily adjusted within the above desired range. From the same point of view, the amount of the water-soluble polymerization inhibitor added is 0.001 to 0.001 to 100 parts by mass of the aqueous medium (specifically, all the water in the system containing water such as the reaction product-containing slurry). 0.1 part by mass is preferable, and 0.005 to 0.06 part by mass is more preferable. Also, in this case, the proportion of the polystyrene resin on the surface of the expandable particles can be easily reduced, and the value A0 of the absorbance ratio _D698 / _D2850 can be easily adjusted within the desired range described above.

<Polymerization process>
In the polymerization step, core particles are impregnated with a styrene-based monomer and polymerized in an aqueous medium.

Styrene, for example, can be used as the styrene-based monomer. Moreover, as a styrene-based monomer, a monomer copolymerizable with styrene and styrene can be used in combination. One type of styrene-based monomer may be used, or two or more types may be used.

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

Other vinyl monomers include acrylic acid esters, methacrylic acid esters, hydroxyl group-containing vinyl compounds, nitrile group-containing vinyl compounds, organic acid vinyl compounds, olefin compounds, diene compounds, halogenated vinyl compounds, and halogenated vinylidene compounds. , and maleimide compounds.

Examples of acrylic acid esters include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate. Examples of methacrylic acid esters include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate.

Vinyl compounds containing hydroxyl groups include, for example, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate. Examples of vinyl compounds containing nitrile groups include acrylonitrile and methacrylonitrile.

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

Examples of diene compounds include butadiene, isoprene, and chloroprene. Vinyl halide compounds include, for example, vinyl chloride and vinyl bromide.

Vinylidene halide compounds include, for example, vinylidene chloride. Examples of maleimide compounds include N-phenylmaleimide and N-methylmaleimide.

From the viewpoint of enhancing the expandability of the expandable particles, the polystyrene-based resin is preferably polystyrene or a copolymer of styrene and an acrylic acid ester. As the acrylate ester, 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 with respect to the entire composite resin. It is even more preferable to have

Polymerization of the styrenic monomer can be carried out in the presence of a polymerization initiator. In this case, cross-linking may occur together with the polymerization of the styrenic monomer. A polymerization initiator is used in the polymerization of the styrenic monomer, and a cross-linking agent can be used in combination, if necessary. When using a polymerization initiator and a cross-linking agent, it is preferable to dissolve the polymerization initiator and the cross-linking agent in the styrene-based monomer in advance.

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

Examples of polymerization initiators include cumene hydroxyperoxide, dicumyl peroxide, t-butylperoxy-2-ethylhexanoate, t-butylperoxybenzoate, benzoyl peroxide, t-butylperoxyisopropyl carbonate, t- Organic peroxides such as amylperoxy-2-ethylhexyl carbonate, hexylperoxy-2-ethylhexyl carbonate, lauroyl peroxide; and azo compounds such as azobisisobutyronitrile can be used. As the polymerization initiator, one type may be used, or two or more types may be used. It is preferable to use the polymerization initiator in an amount of 0.01 to 3 parts by mass with respect to 100 parts by mass of the styrene-based 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, one having a 10-hour half-life temperature higher than the polymerization temperature by 5° C. to 50° C. can be used.

As the cross-linking agent, peroxides such as dicumyl peroxide, 2,5-t-butylperbenzoate, 1,1-bis-t-butylperoxycyclohexane and the like can be used. One type of cross-linking agent may be used, or two or more types may be used. The amount of the cross-linking agent compounded is preferably 0.1 to 5 parts by mass with respect to 100 parts by mass of the styrene-based monomer. The polymerization initiator and the cross-linking agent may be the same compound.

A polymerization initiator can also be added to the core particles. In this case, the core particles can be impregnated with a polymerization initiator dissolved in a solvent. Examples of the solvent include aromatic hydrocarbons such as ethylbenzene and toluene; and aliphatic hydrocarbons such as heptane and octane. A foam control agent can be added to the solvent.

Moreover, a plasticizer, an oil-soluble polymerization inhibitor, a cell regulator, a flame retardant, a dye, etc. can be added to the styrene-based monomer as necessary.

Examples of plasticizers that can be used include fatty acid esters such as glycerin tristearate, glycerin trioctoate, glycerin trilaurate, sorbitan tristearate, sorbitan monostearate, and butyl stearate. In addition, acetylated monoglycerides such as glycerine diacetomonolaurate, oils such as hydrogenated beef tallow and hydrogenated castor oil, organic compounds such as cyclohexane, liquid paraffin, and 1,2-cyclohexanedicarboxylic acid diisononyl ester can also be used.

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

Examples of cell regulators that can be used include fatty acid monoamides, fatty acid bisamides, talc, silica, polyethylene wax, methylenebisstearic acid, methyl methacrylate copolymers, and silicones. As the fatty acid monoamide, oleic acid amide, stearic acid amide, and the like can be used. As the fatty acid bisamide, ethylenebisstearic acid amide or the like can be used.

The mixing ratio of the core particles and the styrene-based monomer is preferably adjusted so that the mass ratio of the polyethylene-based resin to the polystyrene-based resin (polyethylene-based resin:polystyrene-based resin) is 10:90 to 50:50. More preferably, the ratio is adjusted to 12:88 to 40:60, and more preferably adjusted to 15:85 to 35:65.

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

In addition, the styrenic 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, as in the second polymerization step described later, by adjusting the rate of temperature increase after the start of addition of the second monomer, the absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the infrared absorption spectrum can be adjusted to the desired value described above. Expandable particles can be produced that satisfy the relationship. In addition, "after the addition of the second monomer is started" means after the start of the addition of the second monomer, including during the addition of the second monomer. The above-described polymerization initiator, cross-linking agent, etc. can be dissolved in the monomer added at any timing.

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

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

On the other hand, in the second polymerization step, the temperature can be raised from the first polymerization temperature to the second polymerization temperature at a second heating rate of less than 15° C./h, for example. By lowering the second heating 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 gradient structure described above can be obtained. It becomes possible to manufacture expandable particles having From the same point of view, the second heating rate is preferably 12° C./h or less, more preferably 9° C./h or less, and even more preferably 8.5° C./h or less. From the viewpoint of productivity, the second heating rate is approximately 2° C./h or more, preferably 3° C./h or more. When the polymerization is performed in three or more steps, it is preferable that the above relationship is satisfied in the final polymerization step. That is, it is preferable to control the above-mentioned second temperature increase rate in the Nth step (where N is a natural number of 2 or more) which is the final stage.

In particular, in order to form the gradient structure of the polystyrene-based resin unique to the present invention, a water-soluble polymerization inhibitor is used so that the polystyrene-based resin in the outermost layer of the expandable particles as a whole is higher than the polystyrene-based resin in the center. It is preferable to reduce the amount of the polystyrene-based resin in the surface layer of the expandable particles by slowing down the rate of temperature increase in the second polymerization step in the polymerization step so as to locally increase the amount of polystyrene-based resin. By combining the above methods, it becomes easy to produce expandable particles having a gradient distribution structure of the specific polystyrene resin of the present invention.

Furthermore, when the second heating rate is 5 to 14° C./h, it becomes easy to reduce the abundance ratio of the polystyrene-based resin and increase the abundance ratio of the polyethylene-based resin in the outermost layer of the expandable particles. Therefore, in this case, when the expanded particles obtained by expanding the expandable particles are fused together by in-mold molding, toughness is exhibited particularly at the fused portion, and the bending properties of the entire molded body are improved. It can improve toughness.

<Blowing agent impregnation step>
In the step of impregnating the foaming agent, the particles are impregnated with the physical foaming agent during or after the polymerization of the styrene-based monomer. Physical blowing agents may be impregnated into the particles both during and after polymerization. That is, the blowing agent can be added to the particles that are in the process of polymerization, to the particles that have completed polymerization, or to both. Specifically, during polymerization, a physical blowing agent is press-fitted into a container containing the particles after polymerization, and the particles are impregnated with the physical blowing agent.

The impregnation temperature of the physical foaming agent is preferably within 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)°C. In this case, retention of the foaming agent can be further improved. In addition, aggregation of expandable particles can be prevented. Polystyrene resins are, for example, styrene homopolymers and copolymers of styrene monomers and monomers copolymerizable with styrene monomers.

The glass transition temperature (Tg) of polystyrene resin is, for example, the 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 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 component. 2 to 4 mg of the obtained polystyrene resin is subjected to heat flux differential scanning calorimetry according to JIS K7121-1987 using a 2010 type DSC measurement device manufactured by TA Instruments. Then, the midpoint glass transition temperature of the DSC curve obtained under the condition of a heating rate of 10° C./min can be obtained.

After impregnation with the blowing agent, the expandable particles are dehydrated and dried, and if necessary, the expandable particles can be coated with a surface coating agent. Examples of surface coating agents include zinc stearate, triglyceride stearate, monoglyceride stearate, and hydrogenated castor oil. Also, an antistatic agent or the like can be used as a functional surface coating agent. The amount of the surface coating agent to be added is preferably 0.01 to 2 parts by mass with respect to 100 parts by mass of the expandable particles.

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

<Expanded particles>
Expandable particles can be obtained by heating the expandable particles with a heating medium to expand them. 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 are supplied. The bulk density of the expanded particles is preferably 10-200 kg/m ^{3 , more preferably 15-100 kg/m 3 .}

<Expanded particle molding>
An expanded bead molded article can be obtained by in-mold molding of the expanded bead by a well-known molding means. The apparent density of the expanded bead molded product is preferably 10-200 kg/m ³ , more preferably 15-100 kg/m ³ .

(Example 1)
Examples and comparative examples of expandable particles are described below. In this example, expandable particles are produced and used to produce expanded particles and molded articles.

The expandable particles use a composite resin containing a polyethylene-based resin and a polystyrene-based resin as a base resin. Moreover, in the expandable particles, the composite resin is impregnated with a blowing agent comprising a saturated hydrocarbon compound. The method for producing the expandable particles of this example will be described below.

(1) Preparation of core particles As a polyethylene-based resin, linear low-density polyethylene polymerized using a metallocene polymerization catalyst ("Nipolon Z HF210K" manufactured by Tosoh Corporation, ethylene-vinyl acetate containing 15% by mass of vinyl acetate) A copolymer (EVA: "Ultrasen 626" manufactured by Tosoh Corporation) was prepared, and an acrylonitrile-styrene copolymer ("AS-XGS" manufactured by Denka Co., Ltd., weight average molecular weight: 109,000) was prepared as a dispersion diameter enlarging agent. , Acrylonitrile component amount: 28% by mass, MFR (200 ° C., 5 kgf: 2.8 g / 10 min) was prepared.Then, linear low-density polyethylene (manufactured by Tosoh Corporation "Nipolon Z HF210K" 15 kg, ethylene-vinyl acetate 5 kg of a polymer (EVA: "Ultrasen 626" manufactured by Tosoh Corporation) and 1 kg of a dispersion diameter enlarging agent are put into a Henschel mixer (manufactured by Mitsui Miike Kakoki Co., Ltd.; model FM-75E) and mixed for 5 minutes to form a resin mixture. got

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 pieces of 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 1000 g of deionized water was put into an autoclave having an inner volume of 3 L equipped with a stirring device, and 6.0 g of sodium pyrophosphate was further added. After that, 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 laurylsulfonate (10 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 this suspension.

Then, 2.0 g of benzoyl peroxide as a polymerization initiator ("Nyper BW" manufactured by NOF Corporation, water-diluted powder product) and 0.25 g of t-butylperoxy-2-ethylhexyl monocarbonate (manufactured by NOF Corporation " Perbutyl E”) and 5.1 g of 1,1-di(tert-butylperoxy)cyclohexane (“Luperox 331M70” manufactured by Arkema Yoshitomi Co., Ltd.) as a cross-linking agent were dissolved in a styrenic monomer. Then, the dissolved substance was put into the suspending agent in the autoclave while being stirred at a stirring speed of 500 rpm (dispersion step). A mixed monomer of 335 g of styrene and 15 g of butyl acrylate was used as the styrene-based monomer.

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

After that, the temperature was cooled to 90° C. over 1 hour, the stirring speed was lowered to 400 rpm, and the temperature was kept 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 as foaming agents into the autoclave over about 1 hour (impregnation step). Further, the temperature was raised to 105° C. over 2 hours, maintained at 105° C. for 5 hours, and then cooled to 30° C. over 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. After that, it was dehydrated and washed with a centrifugal separator, and water adhering to the surface was removed with a flash dryer to obtain expandable particles having an average particle size (d63) of about 1.7 mm.

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

The average particle size 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 microscopic IR, the morphology of the surface layer, the proportion of xylene insolubles, 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 analyzer “Millitrack JPA” manufactured by Nikkiso Co., Ltd. Specifically, first, 30 g of the composite resin particles were allowed to freely fall from the sample supply feeder of the measuring device, and the projected image was taken with a CCD camera. Next, the captured image information was sequentially subjected to arithmetic and combination processing, and the measurement was performed under the conditions of the image analysis method for outputting the particle size distribution results. As a result, each particle size (d63) mm at a volume integrated value of 63% in the particle size distribution was determined.

"Absorbance ratio D ₆₉₈ /D ₂₈₅₀ in cross section of expandable particles measured by microscopic IR"
After cutting the expandable particle in half at the cross section passing through the center, the cut surface was sliced to prepare a sample for microscopic IR with a thickness of 10 μm. In this sample for microscopic IR, 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, a measurement area was set that was sectioned every 10 μm toward the center. . Then, mapping measurement was performed to measure these measurement regions by infrared spectroscopic analysis of the microscopic transmission method, and infrared absorption spectra were obtained. For the infrared spectroscopic analysis, an FT/IR-4600 type Fourier transform infrared spectrophotometer manufactured by JASCO Corporation and a type IRT-5200 infrared microscope manufactured by JASCO Corporation were used. The absorbance D ₆₉₈ at the peak top containing a wavenumber of 698 cm ⁻¹ in the infrared absorption spectrum and the absorbance D ₂₈₅₀ at the peak top containing a wavenumber of 2850 cm ⁻¹ were measured. Then, the ratio of the absorbance _D698 to the absorbance _D2850 , that is, the absorbance ratio _D698 / _D2850 was calculated. The conditions for the mapping measurement by the microscopic transmission method are as follows: microscopic optical path: transmission; measurement area: 4000 cm ^-1 to 600 cm ^-1 ; detector: MCT; resolution: 8 cm ^-1 ; ×100 μm, 0 degrees. In addition, the sample was measured after the background measurement of the apparatus was performed 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. 1 and 4. FIG. The measured value is an arithmetic mean value obtained by performing the above measurements on 10 samples.

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

Specifically, the value A2 of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the depth range of 100 to 110 μm is the absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the infrared absorption spectrum with the depth range of 100 to 110 μm as the measurement region. value. The value A3 of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ at the central portion is specifically the value of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ in the infrared absorption spectrum with the depth range of 850 to 860 μm as the measurement region. Specifically, 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 the absorbance ratio D of the infrared absorption spectrum whose measurement region is in the range of 30 to 100 μm in depth. It is the smallest value among ₆₉₈ / _D2850 . Table 1 shows the values of A0 to A4 in the expandable particles of this example.

"Surface Morphology"
Morphology was determined 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 portion of the surface layer of the expandable particles (specifically, a portion within a range of 5 μm from the surface). This observation sample was embedded in an epoxy resin and stained with ruthenium tetroxide, and then an ultrathin section was prepared from the sample using an ultramicrotome. This ultra-thin section was placed on a grid for TEM, and cross-sectional photographs of the surface layer of the expandable particles were taken at a magnification of 10,000 or 50,000 to observe the morphology. In a transmission electron micrograph (that is, a TEM photograph), a dark gray portion is a polyethylene-based resin, and a light gray portion is a styrene-based resin. Then, in the TEM photograph, when the polyethylene-based resin and the styrene-based resin form a co-continuous phase, the polymorphology is determined to be a sea-sea structure, the polyethylene-based resin forms the dispersed phase, and the polystyrene-based resin Formed a continuous phase, the morphology was determined to be an island-sea structure, and when polyethylene-based resins formed the continuous phase and polystyrene-based resins formed the dispersed phase, the morphology was determined to be a sea-island structure. Table 1 shows the results.

"Proportion of xylene-insoluble matter"
Specifically, first, about 1 g of expandable particles was sampled, its weight (W ₀ ) was measured to the fourth decimal place, and placed in a 150-mesh wire mesh bag. Next, about 200 ml of xylene was placed in a 200 ml round-bottomed flask, and the sample in the wire mesh bag was placed in a Soxhlet extraction tube. Soxhlet extraction was performed by heating the flask with a heating mantle for 8 hours. After the extraction was completed, it was cooled by air cooling. After cooling, the wire mesh was taken out from the extraction tube, and the sample was washed together with the wire mesh with about 600 ml of acetone. After evaporating the acetone, the film was dried at a temperature of 120°C. A sample recovered from the inside of the wire mesh after this drying was collected as a "xylene-insoluble matter", and its weight (W ₁ ) was measured to four decimal places. The ratio of the xylene-insoluble matter was calculated as the ratio of the weight (W ₁ ) of the xylene-insoluble matter to the weight (W ₀ ) of the expandable particles (W ₁ /W ₀ ; percentage (%)).

"Weight-average molecular weight (Mw) of acetone-soluble matter in composite resin"
First, Soxhlet extraction was performed in the same manner as described above. Then, 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 manner mainly corresponds to polystyrene resin. The weight-average molecular weight of the acetone-soluble matter was measured by a gel permeation chromatography (GPC) method (mixed gel column for polymer measurement) using polystyrene as a standard substance. Specifically, using a measurement 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 performed under the measurement condition that 1 SuperH-H and 2 TSK-GEL SuperHM-H were connected in series. That is, the weight average molecular weight was determined by dissolving acetone-soluble matter in tetrahydrofuran, measuring by gel permeation chromatography (GPC), and calibrating with standard polystyrene.

(3) Production of Expanded Beads Next, the expandable particles obtained as described above were aged in a low-temperature warehouse at 6° C. for one day to produce composite resin expanded particles having a bulk density of about 20 kg/m ³ . Specifically, first, the expandable particles were placed in a normal pressure batch foaming machine with a volume of 30 L, and steam was supplied into this 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.

The bulk density (kg/m ³ ) of the expanded particles is measured by preparing a 1 L graduated cylinder, filling the empty graduated cylinder with the expanded particles up to the 1 L marked line, and measuring the mass (g) of the expanded particles per 1 L. than to do.

(4) Production of molded article (composite resin expanded bead molded article) First, the expanded beads obtained as described above were aged at room temperature for 1 day. Then, using a molding machine (DSM-0705VS manufactured by DABO Co., Ltd.), the foamed particles were molded into a rectangular solid of 300 mm×75 mm×25 mm. The molded body obtained was dried at a temperature of 40° C. for 1 day, and then cured at room temperature for 1 day or longer.

In this manner, foamed particles having a bulk density of about 20 kg/m ³ were molded to obtain a molded product having an expansion ratio of 50 times and 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 formula (5).
Expansion ratio (times) = 1000/apparent density (kg/m ³ )

Next, the foamed resin molded article was measured for vacuum cooling time, fusion bondability, and toughness at the time of molding as follows.

"Vacuum cooling time during molding"
A molded product of 300 mm×300 mm×50 mm in thickness was formed using a molding machine (manufactured by Kasahara Kogyo Co., Ltd. AD/0907). 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 symbol "G"), water cooling was performed for 5 seconds. Furthermore, vacuum cooling was carried out 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 and the molded body was released. The time from the start of vacuum cooling to mold release was measured as the vacuum cooling time. In addition, if the vacuum cooling time is shortened, the molding cycle will be shortened as a result.

"Fusible"
In a plate-shaped compact with a length of 300 mm, a width of 75 mm, and a thickness of 25 mm, a 2-mm-deep incision is made on one surface (that is, one side with a length of 300 mm and a width of 25 mm) at the center in the length direction, and is cut across the entire width. A test piece was prepared by putting it in the Next, the test piece was bent in the direction of widening the notch until the test piece was broken or until both ends of the test piece abutted against each other. Next, the cross section of the test piece was observed, and the number of foamed particles broken inside and the number of foamed particles peeled off at the interface were visually counted. Next, the ratio of the foamed beads that broke inside and the foamed beads that peeled off at the interface was calculated, and expressed as a percentage to obtain the fusion bondability (%).

"Bend test"
A three-point bending test (span of 200 mm) was performed in accordance with JIS K 7221-2:1999 using a molded body of length 300 mm×width 75 mm×thickness 25 mm as a test piece. As a result, "⊚" indicates that the test piece did not break, "◯" indicates that the strain at break is 15% or more, and "x" indicates that the strain at break is less than 15%.

(Example 2)
In this example, expandable beads, expanded beads, 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 in the second polymerization step. Furthermore, 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. 2 and 4. FIG.

( Reference example 1 )
In this example, expandable beads, expanded beads, 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 in the second polymerization step. Furthermore, 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. 3 and 5. FIG.

( Example 3 )
(1) Production of Core Particles As a polyethylene-based resin, linear low-density polyethylene (“Nipolon Z HF210K” manufactured by Tosoh Corporation) obtained by polymerization using a metallocene polymerization catalyst was prepared. -Styrene copolymer (manufactured by Denka "AS-XGS, weight average molecular weight: 109,000, acrylonitrile content: 28% by 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 dispersion diameter enlarging agent) are put into a Henschel mixer (manufactured by Mitsui Miike Kakoki Co., Ltd.; model FM-75E) and mixed for 5 minutes to form a resin. 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 pieces of 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 was prepared as a suspending agent in the same manner as in Example 1. Next, 2.0 g of sodium laurylsulfonate (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 this suspension.

Then, 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 a polymerization initiator. ) and 0.63 g of α-methylstyrene dimer (NOFUMER MSD, manufactured by NOF Corporation) as a chain transfer agent were dissolved in the first monomer (styrene-based monomer). Then, the dissolved substance was put into the suspending agent in the autoclave while being stirred at a stirring speed of 500 rpm (dispersion step). A mixed monomer of 60 g of styrene and 15 g of butyl acrylate was used as the first monomer.

Then, after the inside of the autoclave was replaced with nitrogen, the temperature was started to rise to 100° C. over 1 hour and 30 minutes. After the temperature was raised, this temperature of 100° C. was maintained for 60 minutes. After that, the stirring speed was lowered to 450 rpm, and the polymerization temperature was kept at 100° C. for 7 hours and 30 minutes (first polymerization step). In addition, 350 g of styrene as a second monomer (styrene-based monomer) was added into the autoclave over 5 hours after 60 minutes had passed since the temperature reached 100°C. Then, the temperature was raised to 125° C. over 3 hours, and the temperature was kept at 125° C. for 5 hours (second polymerization step).

After that, by performing the same operations as in Example 1, expandable beads, expanded beads, and molded articles were produced. Furthermore, the same evaluation as in Example 1 was performed.

(Comparative example 1)
In this example, expandable beads, expanded beads, and a molded article were produced in the same manner as in Example 1, except that the temperature was raised to 120° C. over 2 hours in the second polymerization step. Furthermore, 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. 1 and 4. FIG.

(Comparative example 2)
In this example, expandable beads, expanded beads, 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 in the second polymerization step. Furthermore, 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. 2 and 5. FIG.

(Comparative Example 3)
In this example, expandable particles, expandable particles and molded articles were produced in the same manner as in Example 1, except that sodium nitrite was not used as a water-soluble polymerization inhibitor. Furthermore, 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 3 , except that the temperature was raised to 125° C. over 2 hours in the second polymerization step.

(Results of Examples , Reference Examples and Comparative Examples)
Table 1 shows the polymerization conditions and the evaluation results in the same manner as in Example 1 for the expandable particles produced in Examples 2-3, Reference Example 1 and Comparative Examples 1-4. In addition, "○" in Table 1 means that the relationship of the inequality is satisfied, and "X" means that the relationship of the inequality is not satisfied.

As is known from Table 1, the expandable particles whose absorbance ratio D ₆₉₈ /D ₂₈₅₀ satisfies a predetermined relationship as in Examples 1 to 3 shorten the molding cycle (specifically, vacuum cooling time). It is also possible to manufacture a molded product that is excellent in fusion bondability and toughness. The moldings of Examples 1 to 3 are excellent in fusion bondability, so that they can exhibit the excellent rigidity peculiar to polystyrene resins. Such expandable particles are considered to be obtained by adjusting the temperature increase rate, water-soluble polymerization inhibitor, resin composition, etc. in the second polymerization step.

In contrast, with the expandable particles of Comparative Examples 1, 2 and 4, molded articles having excellent toughness can be obtained, but the molding cycle becomes longer. Further, the expandable particles of Comparative Example 3 can shorten the molding cycle, but the toughness of the molded product is insufficient.

As described above, the reason why the expandable particles of Examples are superior to those of Comparative Examples in terms of toughness and molding cycle is thought to be due to the difference in the gradient structure of the proportion of the polystyrene resin (see FIGS. 1 to 5). In the following description, with reference to FIGS. 93, the internal structure of the actual expandable particles 1, 8, and 9 is considered to be indistinguishable as layers. Surface layers 13, 83, 93 extend from surfaces 10, 80, 90 of expandable particles 1, 8, 9 to a depth of 30 μm. The intermediate layers 12 , 82 , 92 are inside the surface layers 13 , 83 , 93 and are in a range including positions 100 μm deep from the surfaces 10 , 80 , 90 of the expandable particles 1 , 8 , 9 . The central layers 11 , 81 , 91 are regions inside the intermediate layers 12 , 82 , 92 and containing the centers O ₁ , O ₈ , O ₉ of the expandable particles 1 , 8 , 9 .

6 to 8 are image diagrams schematically showing gradient structures of polystyrene-based resin ratios in expandable particles 1, 8, and 9 of Example 1, Comparative Example 1, and Comparative Example 3, respectively. does not necessarily match the sexual particles. In FIGS. 6 to 8, the density of hatched dots schematically indicates the amount of polystyrene resin.

As schematically shown in FIG. 7, in the expandable particle 8 of Comparative Example 1, the ratio of polystyrene-based resin, for example, from the particle center O ₈ toward the surface 80 in the order of the central layer 81, the intermediate layer 82, and the surface layer 83 is gradually decreasing. The reason why the expandable particles 8 of Comparative Example 1 have such a gradient structure is considered to be that the second heating rate in the polymerization step is too fast and the polystyrene resin cannot be increased near the surface layer. Therefore, in Comparative Example 1, although the expanded bead molded article obtained from the expanded bead obtained by expanding the expandable bead has excellent fusion bondability and toughness, it is considered that the vacuum cooling time during molding of the expanded bead is long. . Also, although not shown in the figure, in Comparative Example 2, the second heating temperature is too fast, so it is considered that the effect of shortening the molding cycle cannot be obtained as in Comparative Example 1.

As schematically shown in FIG. 8, in the expandable particle 9 of Comparative Example 3, the ratio of polystyrene-based resin, for example, from the particle center O ₉ toward the surface 90 in the order of the central layer 91, the intermediate layer 92, and the surface layer 93 is expected to increase gradually. The reason why the expandable particles of Comparative Example 3 have such a gradient structure is that no water-soluble polymerization inhibitor was added to the aqueous medium in the dispersing step, and the styrene monomer near the surface of the resin particles in the polymerization step. This is probably because the polymerization is excessively accelerated. Therefore, in Comparative Example 3, although the vacuum cooling time is shortened, it is considered that a compact having excellent fusion bondability and toughness cannot be obtained.

On the other hand, in the expandable particles ₁ of Example 1, as schematically shown in FIG. It is thought that the polystyrene-based resin in the surface layer 13 is increased so that As shown by the absorbance ratio value A2, it is considered that the intermediate layer 12 containing 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 gradient structure in which the proportion of the polystyrene-based resin in the intermediate layer 12 is reduced. The same applies to Examples 3 and 4.

In such an inclined structure, the presence of the surface layer 13, which has a higher rigidity polystyrene resin than the intermediate layer 12, suppresses the expansion force remaining inside the molded body in, for example, in-mold molding, and allows it to cool. It is considered possible to shorten the time. Furthermore, since there is the intermediate layer 12 between the surface layer 13 and the central layer 11, in which the proportion of polystyrene resin is locally reduced, the molded body can exhibit excellent toughness. As typified by such a characteristic gradient structure, the expandable particles of the example, which satisfy the predetermined relationship in the infrared absorption spectrum in terms of the absorbance ratio D ₆₉₈ /D ₂₈₅₀ , can maintain the toughness even if the cooling time is shortened. It is possible to manufacture a molded product excellent in

Furthermore, in the expandable particles of Examples 1 and 3 and Reference Example 1 , although not shown, the absorbance ratio D ₆₉₈ /D ₂₈₅₀ value A0 in the outermost layer is smaller than the absorbance ratio value A1 in the surface layer. Therefore, the polystyrene-based resin on the surface of the expanded bead in these examples, that is, the portion corresponding to the surface of the expandable bead, is less than the portion slightly inside the surface. It is thought that a typical tilted structure is formed. Therefore, in these examples, since the polystyrene-based resin is abundant in the surface layer portion, the effect of shortening the molding cycle can be obtained. Furthermore, since the polystyrene-based resin is less on the surface, it is possible to improve the toughness of the molded product obtained by fusion of the foamed particles. Although illustration is omitted, in Example 2, the proportion of polystyrene-based resin on the outermost surface is the highest, followed by the polystyrene-based resin in the center layer, and between these, the proportion of polystyrene-based resin is lower than that of the two. It is assumed that there are layers.

1 Expandable Composite Resin Particle 11 Central Layer 12 Intermediate Layer 13 Surface Layer

Claims (7)

In expandable composite resin particles containing a composite resin of polyethylene resin and polystyrene resin and a physical blowing agent,
By microscopic infrared spectroscopic analysis, the infrared absorption spectrum obtained by measuring the cross section passing through the center of the expandable composite resin particle every 10 μm from the surface of the expandable composite resin particle toward the center includes a wavenumber 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 of the peak including the wavenumber 698 cm ^-1 to the absorbance D ₂₈₅₀ at the peak top of the peak satisfies the following relationships (1) to (3) particle.
(1) The maximum value A1 of the ratio D ₆₉₈ /D ₂₈₅₀ is 1.5-5 when the distance from the surface of the expandable composite resin particles is in the range of 0-30 μm.
(2) The value A2 of the ratio D ₆₉₈ /D ₂₈₅₀ when the distance from the surface of the expandable composite resin particles is in the range of 100 to 110 μm 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 foaming according to claim 1, wherein in the infrared absorption spectrum, the value A0 of the ratio D ₆₉₈ /D ₂₈₅₀ in the range of 0 to 10 μm from the surface of the expandable composite resin particles is smaller than the maximum value A1. composite resin particles. 3. The expandable composite resin particles according to claim 1 or 2, wherein in the infrared absorption spectrum, the maximum value A1 is smaller than the value A3 of the ratio _D698 / _D2850 at the central portion. 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 particles, and the minimum value A4 is less than 1. The expandable composite resin particles according to any one of claims 1 to 3. The expandable composite resin particles according to any one of claims 1 to 4, wherein the maximum value A1 is 1.5 to 3. The expandable composite resin particles according to any one of claims 1 to 5, wherein the expandable composite resin particles have an average particle diameter of 1 mm to 2 mm. The composite resin contains 5 to 50% by mass of polyethylene resin and 50 to 95% by mass of polystyrene resin (however, the total of polyethylene resin and polystyrene resin is 100% by mass), The expandable composite resin particles according to any one of claims 1 to 6.

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