WO2023067953A1 - ポリエチレン系樹脂発泡粒子及びその製造方法 - Google Patents
ポリエチレン系樹脂発泡粒子及びその製造方法 Download PDFInfo
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- WO2023067953A1 WO2023067953A1 PCT/JP2022/034281 JP2022034281W WO2023067953A1 WO 2023067953 A1 WO2023067953 A1 WO 2023067953A1 JP 2022034281 W JP2022034281 W JP 2022034281W WO 2023067953 A1 WO2023067953 A1 WO 2023067953A1
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- linear low
- density polyethylene
- polyethylene
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- density
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
- C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
- C08J9/16—Making expandable particles
- C08J9/18—Making expandable particles by impregnating polymer particles with the blowing agent
Definitions
- the present disclosure relates to polyethylene-based resin expanded particles and a method for producing the same.
- Polyethylene-based resin foamed particles are widely used as shock-absorbing packaging materials due to their excellent formability and shock-absorbing properties.
- Polyethylene-based resin expanded particle molded articles are suitable for applications such as packaging materials for electrical and electronic equipment, since they have properties that do not easily damage packaged objects.
- Such an expanded bead molded product can be obtained, for example, by an in-mold molding method in which the polyethylene resin expanded beads are subjected to secondary expansion in a mold and the surfaces thereof are melted to fuse with each other.
- Polyethylene-based resin foamed particles have conventionally been made from cross-linked polyethylene-based resin foamed particles that have been foamed after cross-linking low-density polyethylene.
- crosslinked polyethylene-based resins are disadvantageous in terms of recyclability compared to non-crosslinked polyethylenes, since recycling methods are significantly limited.
- linear low-density polyethylene it is easy to obtain expanded particles that can be molded in a mold even without cross-linking. Particles have been considered (see Patent Documents 1 to 5).
- Patent Documents 1 and 2 organic physical foaming agents such as freon and butane are used as foaming agents.
- inorganic physical foaming agents have been desired from the viewpoint of reducing the burden on the environment.
- Patent Documents 1 and 2 when an inorganic physical blowing agent is used, it is difficult to stably produce expanded beads with a low apparent density, and even when expanded beads with a low apparent density are obtained, moldability is poor. There was a risk of a decline.
- Patent Documents 3 and 4 there is room for improvement in the flexibility of the molded article, and for applications such as packaging materials for precision equipment, etc., a molded article with excellent flexibility is required.
- Patent Documents 3 and 4 there is room for improvement in the surface smoothness of the molded article.
- Patent Document 5 as well, there is room for improvement in the flexibility of the molded article, and there is also room for improvement in moldability.
- An object of the present invention is to provide an expanded bead that can be molded in a mold to form an expanded bead molded article with high flexibility and excellent surface smoothness, and that can be produced under excellent molding conditions, and a method for producing the same.
- One aspect of the present invention is a polyethylene-based resin foamed particle using non-crosslinked linear low-density polyethylene as a base resin
- the linear low-density polyethylene has a density of 0.915 g/cm 3 or more and less than 0.930 g/cm 3
- the linear low-density polyethylene has a heat of fusion of 80 J/g or more and 100 J/g or less
- the linear low-density polyethylene has a melting point of 120° C. or higher and 130° C. or lower
- the linear low-density polyethylene has a melt flow rate of 0.8 g/10 min or more and 1.5 g/10 min or less under conditions of a temperature of 190° C. and a load of 2.16 kg
- the polyethylene-based resin foamed beads, wherein the foamed beads have an average cell diameter of 50 ⁇ m or more and 180 ⁇ m or less.
- Another aspect of the present invention is a dispersing step of dispersing polyethylene-based resin particles having non-crosslinked linear low-density polyethylene as a base resin in a dispersion medium in a closed container; a foaming agent impregnation step of impregnating the resin particles with an inorganic physical foaming agent in the closed container; a foaming step of expanding the resin particles by discharging the resin particles impregnated with the foaming agent from the closed container into an atmosphere having a pressure lower than the internal pressure of the closed container;
- the linear low-density polyethylene has a density of 0.915 g/cm 3 or more and less than 0.930 g/cm 3 ,
- the linear low-density polyethylene has a heat of fusion of 80 J/g or more and 100 J/g or less,
- the linear low-density polyethylene has a melting point of 120° C.
- an expanded bead molded article with high flexibility and excellent surface smoothness can be produced under excellent molding conditions.
- the foamed beads can be produced using an inorganic physical foaming agent as a foaming agent. Therefore, it is possible to avoid using an organic physical foaming agent and reduce the environmental load.
- FIG. 1 is an explanatory diagram showing an example of the second DSC curve of linear low-density polyethylene.
- FIG. 2 is an explanatory diagram showing a method of calculating the high-temperature peak area of expanded beads.
- FIG. 3 is an explanatory diagram showing the relationship between the heat of fusion and the melting point of linear low-density polyethylene.
- the foamed beads of the present disclosure use non-crosslinked linear low-density polyethylene as a base resin and contain linear low-density polyethylene in a foamed state.
- Linear low-density polyethylene (PE-LLD) is a copolymer of ethylene and ⁇ -olefin that exhibits a linear shape.
- the ⁇ -olefin constituting the copolymer usually has 4 to 10 carbon atoms.
- the linear low-density polyethylene is preferably a copolymer of ethylene and an ⁇ -olefin having 6 to 8 carbon atoms, more preferably a copolymer of ethylene and an ⁇ -olefin having 6 carbon atoms. In this case, it is possible to easily adjust the density, melting point, and heat of fusion of the linear low-density polyethylene within the ranges specified in the present disclosure.
- linear low-density polyethylene examples include ethylene-1-butene copolymer, ethylene-1-pentene copolymer, ethylene-1-hexene copolymer, ethylene-4-methyl-1-pentene copolymer, ethylene
- a preferred example is a -1-octene copolymer.
- other monomers other than ethylene and ⁇ -olefins having 4 to 10 carbon atoms are further copolymerized as the copolymer component of the linear low-density polyethylene within a range that does not impair the object and effect of the present disclosure.
- the content of other monomers is preferably 5% by mass or less with respect to the total 100% by mass of ethylene, an ⁇ -olefin having 4 to 10 carbon atoms, and the other monomer, and 3 mass % or less, more preferably 0.
- the linear low-density polyethylene is preferably a random copolymer or a block copolymer, more preferably a random copolymer.
- the linear low-density polyethylene which is the base resin of the expanded beads, may contain one type of linear low-density polyethylene, or may contain two or more types of linear low-density polyethylene.
- PE-VLD very low density polyethylene
- PE-LD low density polyethylene
- PE-MD medium density polyethylene
- PE-HD high density polyethylene
- the physical properties such as the density, heat of fusion, melting point, and MFR of the linear low-density polyethylene specified in the present disclosure are values obtained by using the mixed resin containing the other polyethylene as a measurement sample.
- the content of polyethylene other than linear low-density polyethylene in the base resin is preferably 30% by mass or less, more preferably 20% by mass or less, and 10% by mass or less. More preferably, it is 5% by mass or less, and particularly preferably 3% by mass or less. preferable.
- the expanded particles preferably do not contain high-density polyethylene.
- linear low-density polyethylene which is the base resin of the expanded beads
- the linear low-density polyethylene is non-crosslinked.
- crosslinked low-density polyethylene has been used as the base resin of expanded beads from the viewpoint of excellent foamability and moldability.
- linear low-density polyethylene exhibits moderate foamability and moldability even without cross-linking, because its melt viscosity changes slowly with changes in temperature.
- the physical properties such as the density, heat of fusion, melting point, and melt flow rate of the linear low-density polyethylene are within the predetermined ranges specified in the present disclosure, so that even without cross-linking, foamability and moldability are improved. Further improve. Therefore, the expanded beads of the present disclosure, which use non-crosslinked linear low-density polyethylene having predetermined physical properties as a base resin, exhibit excellent foamability and moldability while enabling recycling of the resin.
- non-crosslinked means that the insoluble content of the foamed particles obtained by hot xylene extraction is 5% by mass or less. From the standpoints of easier recycling of the expanded beads and higher flexibility of the resulting expanded bead molded product, the proportion of insoluble matter in the expanded beads obtained by the hot xylene extraction method is preferably 3% by mass or less in the expanded beads. , 0 is most preferred.
- the xylene-insoluble content of the foamed beads obtained by the hot xylene extraction method (that is, the content insoluble by the hot xylene extraction method) is obtained by the following method.
- the insoluble matter (% by mass) obtained by the hot xylene extraction method is obtained from the weight L (g) of the foamed particles accurately weighed and the dry weight M (g) of the insoluble matter according to the following formula (II).
- the xylene solution containing insoluble matter is preferably filtered quickly with a wire mesh.
- Insoluble matter (mass%) by hot xylene extraction method (M / L) ⁇ 100 (II)
- the linear low-density polyethylene has a density of 0.915 g/cm 3 or more and less than 0.930 g/cm 3 , a heat of fusion ⁇ H1 of 80 J/g or more and 100 J/g or less, and a melting point of 120° C. or more and 130° C. or less. and a melt flow rate (that is, MFR) at a temperature of 190° C. and a load of 2.16 kg is 0.8 g/10 min or more and 1.5 g/10 min or less. Further, the average cell diameter of the expanded beads is 50 ⁇ m or more and 180 ⁇ m or less.
- the foamed beads of the present disclosure use linear low-density polyethylene, which has a high melting point and a low MFR in spite of its low density and low heat of fusion, as a base resin.
- the average cell diameter of the expanded particles is moderately small.
- excellent moldability means that a molded article having excellent fusion bondability, secondary foamability, and shape recoverability after molding can be molded under excellent molding conditions.
- Excellent molding conditions mean that the molding heating temperature is low (specifically, low pressure) and the molding heating temperature range (specifically, the molding pressure range) is wide.
- Linear low-density polyethylene has a density of 0.915 g/cm 3 or more and less than 0.930 g/cm 3 .
- the density of the linear low-density polyethylene is less than 0.915 g/cm 3 , the molded article tends to shrink and deform immediately after being removed from the mold, so the molded article can be molded under excellent molding conditions. become difficult. Specifically, there is a possibility that the range of molding heating temperature will be narrowed.
- the density of the linear low-density polyethylene is preferably 0.918 g/cm 3 or more, more preferably 0.920 g/cm 3 or more.
- the density of the linear low-density polyethylene is preferably 0.928 g/cm 3 or less, more preferably 0.925 g/cm 3 or less.
- the density of the linear low-density polyethylene is 0.918 g/cm 3 or more and 0.928 g/cm 3 or less from the viewpoint of further improving the moldability of the expanded beads and improving the flexibility of the obtained molded article. more preferably 0.920 g/cm 3 or more and 0.925/cm 3 or less.
- the density of linear low-density polyethylene is measured, for example, by the B method (pycnometer method) described in JIS K7112:1999.
- the heat of fusion ⁇ H1 of the linear low-density polyethylene is 80 J/g or more and 100 J/g or less.
- the heat of fusion ⁇ H1 of the linear low-density polyethylene is less than 80 J/g, moldability is significantly deteriorated and molding becomes difficult.
- moldability In polyethylene-based resin foamed particles, moldability generally tends to improve as the average cell diameter increases. Molding is difficult even when the average cell diameter is relatively large.
- the heat of fusion ⁇ H1 of the linear low-density polyethylene is preferably 82 J/g or more, more preferably 85 J/g or more.
- the heat of fusion ⁇ H1 of the linear low-density polyethylene exceeds 100 J/g, the foamability may be lowered. Moreover, molding at low temperature (specifically, low pressure) becomes difficult, resulting in poor moldability and reduced flexibility of the molded product. From the viewpoint of further improving foamability, moldability, and flexibility, the heat of fusion ⁇ H1 of the linear low-density polyethylene is preferably less than 100 J/g, preferably 95 J/g or less, and 90 J/g. The following are more preferable.
- the heat of fusion ⁇ H1 of the linear low-density polyethylene should be 82 J/g or more and 95 J/g or less from the viewpoint of further improving the expandability and moldability of the expanded beads and further improving the flexibility of the resulting molded product. is preferred, and 85 J/g or more and 90 J/g or less is more preferred.
- the heat of fusion ⁇ H1 of linear low-density polyethylene is measured using a heat flux differential scanning calorimeter based on JIS K7122:2012, for example.
- the test piece is conditioned at a heating rate and a cooling rate of 10°C/min.
- Heat flux DSC ie, differential scanning calorimetry
- the heat of fusion value can be determined.
- the sum of the areas of the plurality of melting peaks is taken as the heat of fusion.
- FIG. 1 shows an example of a DSC curve of linear low-density polyethylene.
- the heat of fusion draws a straight line connecting the point ⁇ of 80° C. on the DSC curve and the point ⁇ on the DSC curve indicating the melting end temperature Te, and is surrounded by the straight line and the DSC curve. It is represented by the area of the region (hatched portion in FIG. 1).
- the melting point of the linear low-density polyethylene is 120° C. or higher and 130° C. or lower. If the melting point of the linear low-density polyethylene is less than 120° C., the moldability of the expanded beads will be poor, making molding difficult. Specifically, the molded article tends to shrink and deform immediately after it is removed from the mold, resulting in poor recoverability. In polyethylene-based resin foamed particles, moldability generally tends to improve as the average cell diameter increases. When the melting point of the linear low-density polyethylene is less than 120° C. as described above, molding becomes difficult even when the average cell diameter is relatively large.
- the melting point of the linear low-density polyethylene is preferably 121°C or higher, more preferably 122°C or higher. Further, when the melting point of the linear low-density polyethylene exceeds 130° C., molding at a low molding temperature reduces the fusion between the foamed particles, resulting in poor moldability. In addition, the flexibility of the expanded bead molding is reduced. From the viewpoint of further improving the moldability of the expanded beads and the viewpoint of further improving the flexibility of the molded article, the melting point of the linear low-density polyethylene is preferably 128° C. or lower, more preferably 125° C. or lower. preferable. The melting point of the linear low-density polyethylene is preferably 121° C. or higher and 128° C. or lower, more preferably 122° C. or higher and 125° C., from the viewpoint of further improving the moldability of the expanded beads and the viewpoint of further improving the flexibility of the molded product. The following are more preferable.
- the melting point of linear low-density polyethylene is measured, for example, by the plastic transition temperature measuring method specified in JIS K7121:1987.
- the test piece is heated from 30 ° C. to 200 ° C. at a heating rate of 10 ° C./min, and then cooled at 10 ° C./min.
- Condition the specimen by cooling at a speed to 30°C.
- differential scanning calorimetry is performed by heating from 30° C. to 200° C. at a heating rate of 10° C./min to obtain a DSC curve.
- the peak temperature of the endothermic peak of the obtained DSC curve be the melting point.
- the apex temperature of the endothermic peak with the highest apex is taken as the melting point.
- the MFR of linear low-density polyethylene is 0.8 g/10 min or more and 1.5 g/10 min or less. If the MFR of the linear low-density polyethylene is less than 0.8 g/10 min, the secondary foamability of the expanded particles may be lowered, and the surface smoothness of the molded article may be impaired. From the viewpoint of further improving the secondary foamability and improving the surface smoothness, the MFR of the linear low-density polyethylene is preferably 0.9 g/10 min or more, and is 1.0 g/10 min or more. is more preferred. In addition, when the MFR of the linear low-density polyethylene exceeds 1.5 g/10 min, the open cell ratio of the expanded particles tends to increase, and the moldability may deteriorate.
- the MFR of the linear low-density polyethylene is preferably 1.4 g/10 min or less, more preferably 1.3 g/10 min or less. From the viewpoint of further improving the surface smoothness of the molded article and further improving the moldability of the expanded beads, the MFR of the linear low-density polyethylene is preferably 0.9 g/10 min or more and 1.4 g/10 min or less. , 1.0 g/10 min or more and 1.3 g/10 min or less.
- the MFR of linear low-density polyethylene is measured in accordance with JIS K7210-1:2014 under conditions of a temperature of 190°C and a load of 2.16 kg.
- the linear low-density polyethylene is preferably a polymer polymerized with a metallocene-based polymerization catalyst.
- the density, heat of fusion, melting point, and MFR of the obtained linear low-density polyethylene are more easily controlled within the above ranges as compared with those polymerized with a Ziegler-Natta polymerization catalyst or the like. .
- the density, heat of fusion, melting point, and MFR are the type of ⁇ -olefin that constitutes the copolymer (specifically, linear low-density polyethylene), its content, the molecular weight of the copolymer, the molecular weight distribution, etc. can be controlled by changing
- the heat of fusion ⁇ H1 of the linear low-density polyethylene and the melting point Tm of the linear low-density polyethylene satisfy the relationship of the following formula (I).
- Formula (I) indicates that the heat of fusion ⁇ H1 of the linear low-density polyethylene is within the above range (that is, 80 J/g or more and 100 J/g or less), while the melting point is relatively high. show. In this case, it is possible to produce an expanded bead molded article having high flexibility at a lower molding heating temperature, and to exhibit the effect of being able to mold in a wide range of molding heating temperatures. Tm>0.2 ⁇ H1+102 (I)
- FIG. 3 shows a graph of the linear low-density polyethylene used in Examples and Comparative Examples described later, with the heat of fusion ⁇ H1 on the horizontal axis and the melting point Tm on the vertical axis.
- the example plots are shown in black.
- linear low-density polyethylene is a polymer polymerized with a metallocene-based polymerization catalyst, it tends to easily satisfy the above formula (I). Although the reason for this is not clear, it is believed that the crystallinity of the resin in linear low-density polyethylene polymerized with a metallocene catalyst is moderately suppressed.
- the density, heat of fusion, melting point, and MFR of linear low-density polyethylene can be measured, for example, from linear low-density polyethylene used as a raw material for expanded beads. Further, for example, when the raw material is unknown, the density, heat of fusion, melting point, and MFR can be measured for a test piece prepared by defoaming foamed particles with a hot press or the like.
- the foamed particles may contain polymers other than polyethylene as long as the objects and effects of the present disclosure are not impaired.
- examples of other polymers include thermoplastic resins other than polyethylene, such as ethylene-vinyl acetate copolymers, polypropylene and polystyrene, and elastomers.
- the content of the other polymer in the expanded beads is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. It is particularly preferred that the particles contain substantially only polyethylene as polymer.
- a coating layer may be formed on the foamed beads to cover the surface thereof.
- the coating layer is preferably composed of, for example, a polyethylene-based resin having a lower melting point than the polyethylene-based resin that constitutes the expanded beads.
- the expanded beads have an average cell diameter of 50 ⁇ m or more and 180 ⁇ m or less. If the average cell diameter of the expanded particles is less than 50 ⁇ m, the moldability may be significantly deteriorated and it may not be possible to obtain a good molded product. From the viewpoint of further improving moldability, the average cell diameter of the expanded beads is preferably 75 ⁇ m or more, more preferably 100 ⁇ m or more. On the other hand, if the average cell diameter exceeds 180 ⁇ m, the surface smoothness of the molded article may be impaired. In addition, the flexibility of the molded body becomes insufficient, and there is a risk that the cushioning performance will be insufficient depending on the application.
- the average cell diameter of the expanded beads is preferably 160 ⁇ m or less, more preferably 140 ⁇ m or less, and even more preferably 120 ⁇ m or less.
- the average cell diameter of the expanded beads is preferably 75 ⁇ m or more and 160 ⁇ m or less, more preferably 100 ⁇ m or more and 140 ⁇ m or less, from the viewpoint of further improving the moldability of the expanded beads and improving the surface smoothness and flexibility of the molded article. is more preferably 100 ⁇ m or more and 120 ⁇ m or less.
- the foamed particles are produced, for example, by a so-called dispersion medium releasing foaming method using an inorganic physical foaming agent such as carbon dioxide.
- an inorganic physical foaming agent such as carbon dioxide.
- the cell diameter of the foamed particles tends to become small, and the moldability tends to deteriorate.
- the expanded beads of the present disclosure have good moldability even though the average cell diameter is small as described above. As a result, a molded article having excellent flexibility and surface smoothness can be molded under excellent molding conditions.
- the average cell diameter of expanded particles is measured, for example, as follows.
- the foamed bead is roughly divided into two halves, and an enlarged photograph of the entire cut surface is taken using a microscope such as a scanning electron microscope.
- a microscope such as a scanning electron microscope.
- four line segments are drawn at equal angles (45°) from the outermost surface of the expanded bead to the outermost surface on the opposite side through the center, and the number of cells intersecting with these line segments is measured.
- the cell diameter of the expanded beads is calculated by dividing the total length of the four line segments by the total number of cells intersecting the line segments. The same operation is performed for 20 expanded beads, the arithmetic mean value of the cell diameter of each expanded bead is calculated, and this value is taken as the average cell diameter.
- the average cell diameter of the foamed particles can be adjusted to the above range by using an inorganic physical foaming agent as a foaming agent in, for example, the dispersion medium release foaming method described below.
- the above range can be adjusted by changing the type of cell control agent, the amount of cell control agent added, the foaming method, the amount of foaming agent added, the foaming conditions such as the foaming temperature, the presence or absence of the two-step foaming process, and the conditions thereof. can do.
- the amount of the cell adjustment agent added is increased, the number of cells increases, so that the average cell diameter can be reduced.
- the average cell diameter can be increased.
- the closed cell ratio of the expanded beads is preferably 80% or more, more preferably 85% or more, and more preferably 90% or more. It is even more preferable to have The closed cell content of expanded beads can be measured based on ASTM-D2856-70 Procedure C.
- the heat of fusion of the high-temperature peak (hereinafter referred to as "high-temperature peak heat amount") is 10 J / g or more and 50 J / g or less.
- the heat of fusion at the high-temperature peak is more preferably 12 J/g or more and 40 J/g or less, and further preferably 15 J/g or more and 30 J/g or less.
- the high-temperature peak heat quantity of the expanded beads is measured using a heat flux differential scanning calorimeter, for example, based on JIS K7122:2012. Specifically, it can be calculated by the following method. First, heat flux DSC is performed using 1-3 mg of expanded beads to obtain a DSC curve. At this time, the measurement start temperature is 30° C., the measurement end temperature is 200° C., and the heating rate is 10° C./min. When the foamed beads have a high temperature peak, the DSC curve has, as shown in FIG. 2, an intrinsic peak ⁇ Ha and a high temperature peak ⁇ Hb having an apex on the high temperature side of the apex of the intrinsic peak ⁇ Ha.
- the melting end temperature T is the end point on the high temperature side of the high temperature peak ⁇ Hb, that is, the intersection of the high temperature peak ⁇ Hb and the baseline on the higher temperature side than the high temperature peak ⁇ Hb in the DSC curve.
- the straight line L2 After drawing the straight line L1, draw a straight line L2 parallel to the vertical axis of the graph passing through the maximum point ⁇ existing between the characteristic peak ⁇ Ha and the high temperature peak ⁇ Hb.
- the straight line L2 divides the characteristic peak ⁇ Ha and the high-temperature peak ⁇ Hb.
- the amount of heat absorbed by the high-temperature peak ⁇ Hb can be calculated based on the area of the portion surrounded by the portion forming the high-temperature peak ⁇ Hb in the DSC curve, the straight lines L1, and the straight lines L2.
- the foamed beads are once cooled, and when the DSC curve is obtained again, only the characteristic peak ⁇ Ha appears in the second DSC curve, and the high temperature peak ⁇ Hb is No DSC curve appears.
- the bulk density of the expanded particles is preferably 10-50 kg/m 3 .
- the physical properties and lightness of the resulting molded article can be enhanced in a well-balanced manner. From this point of view, the bulk density is more preferably 12-40 kg/m 3 , even more preferably 15-30 kg/m 3 .
- the bulk density of the expanded beads is obtained, for example, as follows. Expanded particles are randomly taken out from the group of expanded particles and placed in a graduated cylinder with a volume of 1 L. A large number of expanded particles are accommodated up to the scale of 1 L so as to be in a state of natural accumulation, and the mass of the accommodated expanded particles is W2 [g]. is divided by the storage volume V2 (1 L) (W2/V2), and the unit is converted to [kg/m 3 ] to obtain the bulk density of the expanded particles. Also, the bulk magnification [times] of the expanded beads can be obtained by dividing the density [kg/m 3 ] of the base resin constituting the expanded beads by the bulk density [kg/m 3 ].
- Expanded beads are produced by a so-called dispersing medium release foaming method (that is, direct foaming method) using an inorganic physical foaming agent. manufactured.
- polyethylene resin particles are dispersed in a dispersion medium in a closed container.
- the polyethylene-based resin particles are hereinafter appropriately referred to as "resin particles".
- the resin particles contain non-crosslinked linear low-density polyethylene as a base resin.
- Non-crosslinked linear low-density polyethylene is as described above.
- the resin particles are manufactured, for example, by the following resin particle manufacturing process. Specifically, an extruder is supplied with non-crosslinked linear low-density polyethylene and additives such as a cell control agent, which are added as necessary, and melt-kneaded. The melt-kneaded product is extruded in the form of a strand through a die orifice provided at the tip of the extruder, and the strand-shaped extrudate is cooled by, for example, being submerged in water. After that, the resin particles can be produced by cutting the strand-like extrudate with a pelletizer so that the resin particles have a predetermined weight (strand cut method).
- Resin particles can also be produced by extruding the melt-kneaded product in water after melt-kneading and cutting the resin particles with a pelletizer immediately after extrusion so that the weight of the resin particles reaches a predetermined weight (underwater cutting method). .
- the average weight per resin particle is preferably 0.2 to 10 mg, more preferably 0.5 to 5 mg.
- the shape of the resin particles is cylindrical, spherical, prismatic, or oval.
- the shape of the expanded beads is a shape corresponding to (specifically, a shape similar to) the shape of the resin particles before foaming.
- a cell adjusting agent can be added in advance to the resin particles in order to adjust the bulk density and cell diameter of the expanded particles to appropriate values.
- cell control agents include inorganic substances such as talc, calcium carbonate, borax, zinc borate, aluminum hydroxide and silica; and polymers such as polytetrafluoroethylene, polyethylene wax, polycarbonate and crosslinked polystyrene. .
- the amount of the cell control agent to be added is preferably 0.001 to 5 parts by mass, more preferably 0.005 to 3 parts by mass, based on 100 parts by mass of the linear low-density polyethylene. It is more preferably 0.01 to 2 parts by mass.
- the resin particles may further contain crystal nucleating agents, coloring agents, flame retardants, flame retardant aids, plasticizers, antistatic agents, antioxidants, UV inhibitors, light stabilizers, conductive fillers, antibacterial Additives such as agents can be added.
- the resin particles are dispersed in the dispersion medium in a closed container.
- a pressure-resistant container such as an autoclave is used as the closed container.
- the dispersion medium is, for example, a liquid, and specifically an aqueous medium such as water.
- a dispersant can be added to the dispersion medium. Examples of dispersants include fine particles of aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, kaolin, mica and clay. These dispersants are usually used in an amount of about 0.001 to 1 part by weight per 100 parts by weight of the resin particles.
- an anionic surfactant such as sodium dodecylbenzenesulfonate, sodium alkylbenzenesulfonate, sodium laurylsulfate, and sodium oleate together as a dispersing aid.
- the amount of the dispersing aid added is preferably 0.001 to 1 part by mass with respect to 100 parts by mass of the resin particles.
- the resin particles are impregnated with an inorganic physical foaming agent in a closed container.
- the inorganic physical foaming agent is pressurized into a closed container in which resin particles are dispersed in a dispersion medium, and the resin particles are impregnated with the foaming agent under heat and pressure.
- Carbon dioxide, nitrogen, argon, helium, air, etc. are used as inorganic physical blowing agents. One of these may be used, or two or more may be used. Carbon dioxide, nitrogen, and air are preferred as the inorganic physical blowing agent, and carbon dioxide is more preferred, from the viewpoints of being able to prevent destruction of the ozone layer, thereby reducing the burden on the environment, and being inexpensive.
- the organic physical blowing agent may be used together with the inorganic physical blowing agent, but from the viewpoint of reducing the environmental load, the blending amount of the organic physical blowing agent is 100 mass, which is the total amount of the inorganic physical blowing agent and the organic physical blowing agent. %, preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 0 (that is, using only the inorganic physical blowing agent).
- the amount of foaming agent used can be adjusted as appropriate according to the bulk density of the foamed particles.
- the amount of foaming agent to be used is determined in consideration of the type of linear low-density polyethylene, the type of foaming agent, and the like.
- the amount of foaming agent used is usually 0.5 to 30 parts by mass, preferably 1 to 15 parts by mass, per 100 parts by mass of the resin particles.
- the foaming agent impregnation step Before the foaming agent impregnation step, during the foaming agent impregnation step, or after the foaming agent impregnation step, it is preferable to perform a high-temperature peak forming step of maintaining the dispersion medium in which the resin particles are dispersed at a predetermined temperature. In this case, the above-mentioned high temperature peak can be easily generated.
- the high temperature peak forming step is preferably performed after the blowing agent impregnation step. Specifically, the high-temperature peak forming step is performed as follows.
- the resin particles dispersed in the dispersion medium are heated in a sealed container at a temperature lower than the melting end temperature Te° C. of the linear low-density polyethylene to partially or mostly melt the linear low-density polyethylene. Then, a sufficient time at a temperature Ta ° C. that is 30 ° C. lower than the melting point Tm ° C. of the linear low-density polyethylene (that is, Tm-30 ° C. or higher) and less than the melting end temperature Te ° C. (for example, 1 to 60 minutes) ) to recrystallize the crystalline portion of the melted linear low-density polyethylene. In this way, high temperature peaks can be produced.
- a high temperature can also be obtained. peaks can be generated.
- the holding temperature range of Tm-30° C. or more and less than Te° C. is an appropriate range when an inorganic physical foaming agent is used as the foaming agent.
- resin particles impregnated with a foaming agent are released from a closed container into an atmosphere with a lower pressure than the internal pressure. Thereby, the resin particles can be expanded to obtain expanded particles.
- foamed particles are obtained by releasing resin particles containing a foaming agent together with a dispersion medium from inside a sealed container into a low-pressure atmosphere at a foaming temperature of Tb°C.
- the foaming temperature Tb is preferably in the range of 15° C. lower than the melting point Tm of the low-density polyethylene (that is, Tm ⁇ 15° C.) or higher and the melting end temperature Te or lower.
- the magnitude of the above-mentioned high-temperature peak heat quantity can be adjusted.
- the high-temperature peak heat quantity of the expanded beads tends to increase as the holding time at the holding temperature Ta increases.
- the high-temperature peak heat quantity of the expanded beads tends to increase as the expansion temperature Tb decreases.
- the expanded beads obtained as described above are pressurized with air or the like to increase the internal pressure of the cells, then heated with steam or the like to expand (that is, a two-stage expansion process), and then further reduced in apparent density. It can also be a low expanded bead (ie a double expanded bead).
- An expanded bead molded article is obtained by molding the expanded bead in a mold. That is, after the foamed particles are filled in the mold, a heating medium such as steam is introduced into the mold to heat the foamed particles for secondary foaming and to fuse them together to shape the molding space. can be obtained.
- a known in-mold molding method is exemplified. Specifically, cracking molding method (see, for example, Japanese Patent Publication No. 46-38359), pressure molding method (see, for example, Japanese Patent Publication No. 51-22951), compression filling molding method (for example, Japanese Patent Publication No. 4-46217) No. 6-49795), normal pressure filling molding method (see, for example, Japanese Patent Publication No.
- the expanded beads are compressed with a pressurized gas such as air before being filled into the mold in order to compensate for the foamability (specifically, the secondary foamability).
- the foamed particles are pressurized in advance to increase the pressure inside the cells of the foamed particles, and after applying a predetermined internal pressure, the foamed particles are filled in a mold and molded.
- the expanded beads of the present disclosure have good expandability and moldability, a good molded article can be produced without performing the pressure treatment.
- the internal pressure of the expanded particles to be filled in the mold is preferably 0.01 MPa (G; gauge pressure) or less, more preferably 0 MPa (G).
- the molded product is composed of a large number of the foamed particles fused together, and exhibits excellent flexibility. In addition, since the molded body has a smooth surface, it has a beautiful appearance.
- the density of the molded body is preferably 10 to 50 kg/m 3 , more preferably 15 to 40 kg/m 3 , and 18 to 30 kg/m 3 . is more preferred.
- the density of the molded body is calculated by dividing the weight (g) of the molded body by the volume (L) obtained from the external dimensions of the molded body and converting the unit. If it is not easy to determine the volume from the outer dimensions of the molded body, the volume of the molded body can be determined by the submersion method.
- the product S ⁇ E [unit: MPa ⁇ %] of the tensile strength S [unit: MPa] and the tensile elongation rate E [unit: %] of the molded product is preferably 10 MPa ⁇ % or more.
- the molded article can reliably exhibit excellent flexibility.
- the durability of the molded article is improved, it can be suitably used for applications such as cushioning materials that are repeatedly used.
- the product of tensile strength S and tensile elongation E (that is, S ⁇ E) is more preferably 12 MPa ⁇ % or more.
- the upper limit of the above product is approximately 50 MPa ⁇ %.
- the tensile strength S and tensile elongation E of the molded product are measured according to JIS K6767:1999. Specifically, first, using a vertical slicer, a piece of 120 mm ⁇ 25 mm ⁇ 10 mm is cut out from the molded product so that all the surfaces are cut out surfaces. A No. 1 dumbbell-shaped test piece is produced from this cut piece using a jigsaw. Then, it is obtained by performing a tensile test at a tensile speed of 500 mm/min using the test piece.
- Tensile strength S is the maximum tensile stress at the time of stretching
- tensile elongation E is the elongation at break.
- the tensile strength S of the compact is preferably 0.25 MPa or more, more preferably 0.30 MPa or more. In this case, the molded article can more reliably exhibit excellent flexibility. From the same point of view, the tensile elongation E is more preferably 35% or more, more preferably 40% or more.
- the compressive stress at 50% strain of the compact is preferably 50 to 300 kPa.
- the molded article can exhibit excellent strength while being flexible. From this point of view, the compressive stress at 50% strain of the compact is more preferably 80 to 250 kPa, even more preferably 100 to 200 kPa.
- the compressive stress at 50% strain of the compact can be obtained, for example, as follows. First, a test piece of 50 mm long ⁇ 50 mm wide ⁇ 25 mm thick is cut out from the center of the molded body so that the skin layer on the surface of the molded body is not included in the test piece. Next, based on JIS K6767:1999, a compression test can be performed at a compression rate of 10 mm/min to determine the compressive stress at 50% strain of the compact.
- the expanded beads of the present disclosure use the specific linear low-density polyethylene as a base resin, and have cell diameters within the specific range. Therefore, a molded article obtained using the expanded particles exhibits excellent flexibility and surface smoothness. As a result, the molded article exhibits excellent cushioning properties and surface protective properties, and is suitable as a cushioning packaging material. In particular, it is suitable as a packaging material for electrical and electronic equipment.
- PE1 to PE9 shown in Table 1 were used as base resins for expanded beads.
- PE7 is a mixed resin in which PE3 and PE6 are mixed at 70% by mass:30% by mass
- PE9 is a mixed resin in which PE2 and PE6 are mixed at 90% by mass:10% by mass.
- Abbreviations in Table 1 are as follows. "PE-LLD” stands for linear low density polyethylene.
- the melting points of the linear low-density polyethylenes were measured by the plastic transition temperature measuring method specified in JIS K7121:1987.
- pellets of linear low-density polyethylene were prepared as test pieces.
- 2 mg of the test piece is heated from 30 ° C. to 200 ° C. at a heating rate of 10 ° C./min, and then at a cooling rate of 10 ° C./min.
- the specimens were conditioned by cooling to 30°C.
- differential scanning calorimetry was performed by heating from 30° C. to 200° C. at a heating rate of 10° C./min to obtain a DSC curve.
- the apex temperature of the endothermic peak of the obtained DSC curve was taken as the melting point.
- a heat flux differential scanning calorimeter "DSC7020" manufactured by Hitachi High-Tech Science Co., Ltd. was used as a measuring device.
- the flow rate of nitrogen gas during DSC measurement was 30 ml/min.
- the heat of fusion of the linear low-density polyethylene was measured using a heat flux differential scanning calorimeter based on JIS K7122:2012.
- the test piece was conditioned at a heating rate and a cooling rate of 10°C/min.
- heat flux DSC that is, differential scanning calorimetry
- the heat of fusion value was determined.
- the DSC curve of PE1 is shown in FIG. As shown in FIG.
- the heat of fusion draws a straight line connecting the point ⁇ of 80° C. on the DSC curve and the point ⁇ on the DSC curve indicating the melting end temperature Te, and is surrounded by the straight line and the DSC curve. It is represented by the area of the region (hatched portion in FIG. 1).
- a heat flux differential scanning calorimeter "DSC7020" manufactured by Hitachi High-Tech Science Co., Ltd. was used as a measuring device.
- the MFR of linear low-density polyethylene was measured in accordance with JIS K7210-1:2014 under conditions of a temperature of 190° C. and a load of 2.16 kg.
- Examples 1 to 4, Comparative Examples 1 to 7 (Production of resin particles) 100 parts by mass of each linear low-density polyethylene shown in Tables 2 and 3 and 0.02 parts by mass of zinc borate as a cell regulator were supplied to an extruder, and these were melt-kneaded and melted in the extruder. A resin was obtained.
- As the zinc borate "zinc borate 2335" manufactured by Tomita Pharmaceutical Co., Ltd. was used. Then, the molten resin in the extruder was extruded in the form of strands through the pores of the die. The strand-shaped extrudate was cooled in water and cut by a pelletizer to obtain resin particles each weighing 1.6 mg. The ratio L/D of the length L to the diameter D of the resin particles was set to 1.8.
- the bulk density of the single-stage expanded particles was measured as follows. The measurement was carried out on the single-stage expanded beads which had been condition-controlled by allowing the single-stage expanded beads to stand in an atmosphere of 23° C., 50% relative humidity, and 1 atm pressure for 2 days.
- Single-stage expanded particles were randomly taken out from the group of single-stage expanded particles after conditioning and placed in a graduated cylinder with a volume of 1 L, and a large number of single-stage expanded particles were accommodated up to the 1 L scale so as to form a natural pile state. Measure the mass W2 [g] of the accommodated single-stage expanded particles, divide this mass W2 by the accommodation volume V2 (that is, 1 L]) (W2/V2), and convert the unit to [kg/m 3 ]. Thus, the bulk density of the single-stage expanded particles was determined. The results are shown in Tables 2 and 3.
- the bulk magnification [times] of the two-stage expanded beads is obtained by dividing the density [kg/m 3 ] of the base resin constituting the expanded beads by the bulk density [kg/m 3 ] of the two-stage expanded beads. rice field.
- the foamed particles were divided into two halves, and an enlarged photograph of the entire cut surface was taken using a microscope such as a scanning electron microscope.
- a microscope such as a scanning electron microscope.
- four line segments were drawn at equal angles (45°) from the outermost surface of the expanded bead to the outermost surface on the opposite side through the center, and the number of cells intersecting with these line segments was measured.
- the cell diameter of the expanded beads was calculated by dividing the total length of the four line segments by the total number of cells intersecting the line segments. The same operation was performed on 20 expanded beads, and the arithmetic mean value of the cell diameters of the expanded beads was calculated, and this value was taken as the average cell diameter.
- the closed cell ratio of the expanded beads was measured using an air comparison hydrometer based on ASTM-D2856-70 procedure C. Specifically, it was obtained as follows. The foamed particles having a bulk volume of about 20 cm 3 after conditioning were used as a measurement sample, and the apparent volume Va was accurately measured by the ethanol soaking method as described below. After sufficiently drying the measurement sample whose apparent volume Va was measured, according to procedure C described in ASTM-D2856-70, the measurement sample measured by Beckman Model 1000 Air Comparison Pycnometer manufactured by Tokyo Science Co., Ltd. A true volume value Vx was measured.
- High-temperature peak heat quantity of the expanded beads was measured by the method described above. That is, heat flux DSC was performed using about 2 mg of the expanded beads, and the peak area of the high temperature peak in the obtained DSC curve was taken as the high temperature peak heat quantity of the expanded beads.
- the measurement start temperature was 30°C
- the measurement end temperature was 200°C
- the heating rate was 10°C/min.
- a heat flux differential scanning calorimeter manufactured by Hitachi High-Tech Science, model number: DSC7020 was used as a measuring device.
- the molding pressure (specifically, molding steam pressure) is set to 0.07 to 0.
- a compact was formed by changing the pressure between 0.13 MPa (G) by 0.01 MPa.
- the molded body was cured by allowing it to stand in an oven adjusted to 80° C. for 12 hours.
- the condition of the molded body was adjusted by allowing the molded body after curing to stand in an atmosphere of relative humidity of 50%, temperature of 23° C. and atmospheric pressure of 1 atm for 24 hours.
- the secondary foamability, fusion bondability, and recoverability of the molded body were evaluated on a scale of 5 according to the following criteria.
- the steam pressure at which a molded product that passed all the items was obtained was defined as the molding pressure at which the above-mentioned acceptable product could be molded.
- the wider the range from the lower limit to the upper limit of the steam pressure that can be molded the wider the range of molding heating temperature that can be molded, which means that the moldability is excellent.
- the number of voids is less than 5 4: The number of voids is 5 or more and less than 10 3: The number of voids is 10 or more and less than 15 2: The number of voids is 15 or more and less than 20 1: The number of voids 20 or more
- the molded product was bent and broken in the longitudinal direction so as to be divided into substantially equal parts.
- the surfaces exposed by the breakage were visually observed, and the number of expanded beads with exfoliated interfaces and the number of internally broken expanded beads were counted.
- the total number of foamed beads present on the exposed surface that is, the number of foamed beads whose interfaces are separated from each other and the number of foamed beads that are broken inside the foamed beads.
- the percentage of the number of foamed particles was calculated. A value obtained by expressing this ratio in percentage (%) was defined as a fusion rate.
- the fusion property was evaluated in 5 stages according to the following criteria. 5: The fusion rate is 80% or more 4: The fusion rate is 60% or more and less than 80% 3: The fusion rate is 40% or more and less than 60% 2: The fusion rate is 20% or more and less than 40% 1: Fusion rate is less than 20%
- Thickness ratio is 99% or more 4: Thickness ratio is 98% or more and less than 99% 3: Thickness ratio is 96% or more and less than 98% 2: Thickness ratio is 90% or more and less than 96% 1: Thickness ratio is less than 90%
- the two-stage expanded particles obtained in the two-stage expansion process were dried at 23° C. for 24 hours.
- the flat plate mold (specifically, a flat plate-shaped cavity) is adjusted to have a cracking amount of 20% (specifically, 8 mm).
- the two-stage foamed particles were filled in the mold provided with.
- the dimensions of the mold are 200 mm long, 65 mm wide and 40 mm thick.
- the mold is clamped and preheated by supplying steam for 5 seconds with the drain valves on both sides of the mold open, and then 0.01 MPa (G ) was supplied while heating was performed.
- the molded article density (kg/m 3 ) was calculated by dividing the weight (g) of the molded article by the volume (L) obtained from the outer dimensions of the molded article and converting the unit.
- Tensile strength S and tensile elongation E of the molded product were determined according to JIS K6767:1999.
- a No. 1 dumbbell-shaped test piece was produced from this cut piece using a jigsaw.
- a tensile test was performed at a tensile speed of 500 mm/min. The maximum tensile stress at the time of stretching was measured as the tensile strength S, and the elongation at break was defined as the tensile elongation E.
- the surface of the molded article was observed, and the surface property was evaluated based on the following criteria.
- Comparative Example 1 the melting point of linear low-density polyethylene is too low. Therefore, the recoverability after molding was poor, and the moldability was insufficient. As shown in Table 1, no acceptable product was obtained by molding in the molding pressure range of 0.07 to 0.13 MPa (G), and the moldability was poor. In Comparative Example 1, although the foamed particles had a sufficiently large average cell diameter, moldability was poor as described above.
- Comparative Example 5 is an example similar to Comparative Example 4, in which linear low-density polyethylene is used and the average cell diameter of the expanded particles is reduced. Since the cell diameter was smaller than that of Comparative Example 4, the moldability was further deteriorated, and a satisfactory product could not be obtained by molding in the molding pressure range of 0.07 to 0.13 MPa (G).
- Comparative Example 6 is an example in which the same linear low-density polyethylene as in Example 1 was used to produce expanded beads having a large average cell diameter. In Comparative Example 6, the average cell diameter of the expanded particles was too large, so the surface smoothness was insufficient. In addition, the flexibility of moldability was slightly lowered. In Comparative Example 7, the heat of fusion of the linear low-density polyethylene was too low. Cann't get a good product.
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Abstract
Description
上記直鎖状低密度ポリエチレンの密度が0.915g/cm3以上0.930g/cm3未満であり、
上記直鎖状低密度ポリエチレンの融解熱量が80J/g以上100J/g以下であり、
上記直鎖状低密度ポリエチレンの融点が120℃以上130℃以下であり、
上記直鎖状低密度ポリエチレンの、温度190℃、荷重2.16kgの条件でのメルトフローレイトが0.8g/10min以上1.5g/10min以下であり、
上記発泡粒子の平均気泡径が50μm以上180μm以下である、ポリエチレン系樹脂発泡粒子にある。
上記密閉容器内において、上記樹脂粒子に無機物理発泡剤を含浸させる発泡剤含浸工程と、
上記発泡剤を含浸させた上記樹脂粒子を上記密閉容器から該密閉容器の内圧よりも低圧の雰囲気下に放出することにより、上記樹脂粒子を発泡させる発泡工程と、を有し、
上記直鎖状低密度ポリエチレンの密度が0.915g/cm3以上0.930g/cm3未満であり、
上記直鎖状低密度ポリエチレンの融解熱量が80J/g以上100J/g以下であり、
上記直鎖状低密度ポリエチレンの融点が120℃以上130℃以下であり、
上記直鎖状低密度ポリエチレンの、温度190℃、荷重2.16kgの条件でのメルトフローレイトが0.8g/10min以上1.5g/10min以下である、ポリエチレン系樹脂発泡粒子の製造方法にある。
発泡粒子の熱キシレン抽出法によるキシレン不溶分(つまり、熱キシレン抽出法による不溶分)は次の方法で求められる。まず、精秤した発泡粒子約1g(その正確な重量をL(g)とする)を150mLの丸底フラスコに入れ、100mLのキシレンを加え、マントルヒーターで加熱して6時間還流させることで不溶物を含むキシレン溶液を得る。次いで、100メッシュの金網で不溶物を含むキシレン溶液を濾過することで不溶物を得る。この不溶物を80℃の減圧乾燥器で8時間以上乾燥し、乾燥した不溶物重量M(g)を測定する。熱キシレン抽出法による不溶分(質量%)は、精秤した発砲粒子の重量L(g)と、不溶物の乾燥重量M(g)とから下記式(II)により求められる。なお、不溶物を含むキシレン溶液の濾過は金網で速やかに行うことが好ましい。
熱キシレン抽出法による不溶分(質量%)=(M/L)×100・・・(II)
発泡粒子の成形性をより高めるとともに、得られる成形体の柔軟性がより向上するという観点から、直鎖状低密度ポリエチレンの密度は0.918g/cm3以上0.928g/cm3以下であることが好ましく、0.920g/cm3以上0.925/cm3以下であることがより好ましい。
発泡粒子の発泡性、成形性をより高めるとともに、得られる成形体の柔軟性をより向上させるという観点から、直鎖状低密度ポリエチレンの融解熱量ΔH1は82J/g以上95J/g以下であることが好ましく、85J/g以上90J/g以下であることがより好ましい。
発泡粒子の成形性がより向上する観点、成形体の柔軟性がより向上する観点から、直鎖状低密度ポリエチレンの融点は、121℃以上128℃以下であることが好ましく、122℃以上125℃以下であることがより好ましい。
成形体の表面平滑性をより向上させるとともに、発泡粒子の成形性をより向上させる観点から、直鎖状低密度ポリエチレンのMFRが0.9g/10min以上1.4g/10min以下であることが好ましく、1.0g/10min以上1.3g/10min以下であることがより好ましい。
Tm>0.2×ΔH1+102 ・・・(I)
直鎖状低密度ポリエチレンが、メタロセン系重合触媒により重合された重合体である場合には、上記式(I)を満足しやすい傾向がある。その理由は明らかではないが、メタロセン触媒により重合された直鎖状低密度ポリエチレンは、樹脂の結晶性が適度に抑制されるためであると考えられる。
発泡粒子の成形性をより向上させるとともに、成形体の表面平滑性及び柔軟性がより向上する観点から、発泡粒子の平均気泡径は75μm以上160μm以下であることが好ましく、100μm以上140μm以下であることがより好ましく、100μm以上120μm以下であることがさらに好ましい。
また、密閉容器内で分散媒中に分散した樹脂粒子を、Tm-30℃以上、融解終了温度Te℃未満の範囲内の温度まで十分な時間をかけてゆっくりと昇温させることによっても、高温ピークを生じさせることができる。
なお、上述のTm-30℃以上かつTe℃未満という保持温度範囲は、発泡剤として無機物理発泡剤を用いた場合の適切な範囲である。
発泡粒子の型内成形においては、その発泡性(具体的には、二次発泡性)を補うために発泡粒子は、成形型内への充填前に、空気等の加圧気体により発泡粒子を予め加圧処理して発泡粒子の気泡内の圧力を高め、所定の内圧を付与してから成形型内に充填させて成形されることが多い。一方、本開示の発泡粒子は発泡性、成形性が良好であることから、上記加圧処理を行わずに良好な成形体を製造することができる。具体的には、成形型内に充填する発泡粒子の内圧が0.01MPa(G;ゲージ圧)以下であることが好ましく、0MPa(G)であることがより好ましい。
表1中の略号などは以下の通りである。「PE-LLD」は直鎖状低密度ポリエチレンを表す。
直鎖状低密度ポリエチレン(PE1~PE9)の融点は、JIS K7121:1987に規定されたプラスチックの転移温度測定方法により測定した。まず、試験片として、直鎖状低密度ポリエチレンのペレットを準備した。「(2)一定の熱処理を行った後、融解温度を測定する場合」に従い、試験片2mgを10℃/minの加熱速度で30℃から200℃まで加熱し、次いで10℃/分の冷却速度で30℃まで冷却して試験片の状態調節を行った。その後、10℃/分の加熱速度で30℃から200℃まで加熱する示差走査熱量測定を行い、DSC曲線を得た。得られたDSC曲線の吸熱ピークの頂点温度を融点とした。なお、測定装置としては、日立ハイテクサイエンス社製の熱流束示差走査熱量測定装置「DSC7020」を用いた。DSC測定時の窒素ガスの流量は、30ミリリットル/分とした。
直鎖状低密度ポリエチレン(PE1~PE9)の融解熱量は、JIS K7122:2012に基づき、熱流束示差走査熱量計を用いて測定した。まず、「一定の熱処理を行った後、融解熱を測定する場合」に従い、加熱速度及び冷却速度を10℃/分として試験片の状態調節を行った。その後、加熱速度を10℃/分に設定して熱流束DSC(つまり、示差走査熱量測定)を行い、DSC曲線を取得した。得られたDSC曲線に基づき、融解熱量の値を決定した。なお、図1にPE1のDSC曲線を示す。図1に示されるように、融解熱量は、DSC曲線上の80℃の点αと融解終了温度Teを示すDSC曲線上の点βとを結ぶ直線を引き、当該直線とDSC曲線とによって囲まれる領域(図1の斜線部分)の面積で表される。なお、測定装置としては、日立ハイテクサイエンス社製の熱流束示差走査熱量測定装置「DSC7020」を用いた。
直鎖状低密度ポリエチレン(PE1~PE9)のMFRは、JIS K7210-1:2014に準拠して、温度190℃、荷重2.16kgの条件で測定した。
(樹脂粒子の製造)
表2、表3に示す各直鎖状低密度ポリエチレン100質量部と、気泡調整剤としてのホウ酸亜鉛0.02質量部とを押出機に供給し、押出機内でこれらを溶融混練して溶融樹脂を得た。ホウ酸亜鉛としては、富田製薬社製の「ホウ酸亜鉛2335」を用いた。次いで、押出機内の溶融樹脂をダイの細孔からストランド状に押出した。ストランド状押出物を水中で冷却し、ペレタイザーにより切断し、1個当たりの質量が1.6mgの樹脂粒子を得た。なお、樹脂粒子の直径Dに対する長さLの比L/Dは1.8とした。
内容積5Lのオートクレーブ内に、分散媒としての水3Lと樹脂粒子1kgを入れた。さらに、オートクレーブ内に分散剤としてのマイカ0.3質量部、界面活性剤としてのドデシルベンゼンスルホン酸ナトリウム水溶液0.03質量部(有効成分として)を添加した。マイカとしては、ヤマグチマイカ社製のA-11を用い、ドデシルベンゼンスルホン酸ナトリウム水溶液としては、第一工業製薬社製のネオゲン(登録商標)S-20Fを用いた。
次に、一段発泡粒子から二段発泡粒子を製造する二段発泡工程を行った。まず、圧力容器内で一段発泡粒子に圧縮空気を含浸させて、発泡粒子の内圧を表2、表3に示す値にした。内圧を付与した一段発泡粒子を加圧発泡機に充填した後、表2、表3に示す圧力(ゲージ圧)のスチームにより一段発泡粒子を加熱してさらに発泡させた(二段発泡工程)。これにより、二段発泡粒子を得た。なお、二段発泡粒子の熱キシレン抽出法による不溶分の割合は、0であった。二段発泡粒子の嵩密度、嵩倍率、平均気泡径、高温ピーク熱量を次のようにして測定した。測定結果を表2、表3に示す。
二段発泡粒子の嵩倍率を上述の一段発泡粒子と同様にして測定した。
発泡粒子を構成する基材樹脂の密度[kg/m3]を二段発泡粒子の嵩密度[kg/m3]で除すことにより二段発泡粒子の嵩倍率[倍]を求めた。
発泡粒子を略二等分し、走査型電子顕微鏡等の顕微鏡を用いて切断面全体の拡大写真を撮影した。断面拡大写真において、発泡粒子の最表面から中心部を通って反対側の最表面に至る線分を等角度(45°)で4本引き、これらの線分と交差する気泡数を測定した。4本の線分の合計長さを線分と交差する全気泡数で除することにより、発泡粒子の気泡径を算出した。同様の操作を20個の発泡粒子について行い、各発泡粒子の気泡径の算術平均値を算出し、この値を平均気泡径とした。
発泡粒子の独立気泡率は、ASTM-D2856-70手順Cに基づき空気比較式比重計を用いて測定した。具体的には、次のようにして求めた。状態調節後の嵩体積約20cm3の発泡粒子を測定用サンプルとし、下記の通りエタノール没法により正確に見掛けの体積Vaを測定した。見掛けの体積Vaを測定した測定用サンプルを十分に乾燥させた後、ASTM-D2856-70に記載されている手順Cに準じ、東京サイエンス社製Beckman Model1000 Air Comparison Pycnometerにより測定される測定用サンプルの真の体積の値Vxを測定した。そして、これらの体積値Va及びVxを基に、下記の式(III)により独立気泡率を計算し、サンプル5個(N=5)の平均値を発泡粒子の独立気泡率とした。
独立気泡率(%)=(Vx-W/ρ)×100/(Va-W/ρ)・・・(III)
ただし、
Vx:上記方法で測定される発泡粒子の真の体積、即ち、発泡粒子を構成する樹脂の容積と、発泡粒子内の独立気泡部分の気泡全容積との和(単位:cm3)
Va:発泡粒子を、エタノールの入ったメスシリンダーに沈めた際の水位上昇分から測定される発泡粒子の見掛けの体積(単位:cm3)
W:発泡粒子測定用サンプルの重量(単位:g)
ρ:発泡粒子を構成する樹脂の密度(単位:g/cm3)
前述した方法により発泡粒子(具体的には、二段発泡粒子)の高温ピーク熱量を測定した。即ち、約2mgの発泡粒子を用いて熱流束DSCを行い、得られたDSC曲線における高温ピークのピーク面積を発泡粒子の高温ピーク熱量とした。熱流束DSCにおける測定開始温度は30℃、測定終了温度は200℃、加熱速度は10℃/分とした。測定装置として、熱流束示差走査熱量計(日立ハイテクサイエンス社製、型番:DSC7020)を用いた。
実施例、比較例の発泡粒子を後述の「・成形圧を0.01MPaずつ変化させることによる型内成形」に記載の方法により、成形圧を0.07~0.13MPa(G)の範囲で0.01MPaずつ変化させて成形体を試験的に作成する型内成形を行い、成形可能範囲を下記の基準で評価した。その結果を表2、表3に示す。
3:0.07~0.13MPa(G)の範囲内に合格品を成形可能な成形圧が3点以上存在する場合。
2:0.07~0.13MPa(G)の範囲内に合格品を成形可能な成形圧が2点存在する場合。
1:0.07~0.13MPa(G)の範囲内に合格品を成形可能な成形圧が1点存在する場合。
0:0.07~0.13MPa(G)の範囲内に合格品を成形可能な成形圧が存在しない場合。
後述の「(発泡粒子成形体の製造)」の方法において、成形圧(具体的には、成形スチーム圧)を、0.07~0.13MPa(G)の間で0.01MPaずつ変化させて成形体を成形した。離型後の成形体を80℃に調整されたオーブン内に12時間静置することにより成形体の養生を行った。養生後の成形体を相対湿度50%、温度23℃、気圧1atmの雰囲気にて24時間静置することにより、成形体の状態調節を行った。次いで、成形体の二次発泡性、融着性、回復性を下記の基準により5段階で評価した。各項目では評価5が合格である。全ての項目で合格となる成形体が得られるスチーム圧を、上述の合格品を成形可能な成形圧とした。成形可能なスチーム圧の下限値が低いものほど、成形可能な成形加熱温度が低いことを意味し、成形性が優れていることを意味する。また、成形可能なスチーム圧の下限値から上限値までの幅が広いものほど、成形可能な成形加熱温度の範囲が広いことを意味し、成形性が優れていることを意味する。
発泡粒子成形体の中央部に100mm×100mmの矩形を描き、次いで、この矩形のいずれかの角から対角線を描いた。この対角線に重なるように形成され、一辺1mmの正方形よりも大きいボイド(つまり、発泡粒子間の間隙)の数を数えた。ボイドの数に基づいて、以下の基準により二次発泡性を5段階で評価した。
5:ボイドの数が5個未満
4:ボイドの数が5個以上10個未満
3:ボイドの数が10個以上15個未満
2:ボイドの数が15個以上20個未満
1:ボイドの数が20個以上
成形体を長手方向に略等分となるように折り曲げて破断させた。破断により露出した面を目視観察し、発泡粒子同士の界面が剥離している発泡粒子の数と、内部で破断した発泡粒子の数とを数えた。そして、露出した面に存在する発泡粒子の総数、つまり、発泡粒子同士の界面が剥離している発泡粒子の数と、内部で破断した発泡粒子の数との合計に対する発泡粒子の内部で破断した発泡粒子の数の割合を算出した。この割合を百分率(%)で表した値を融着率とした。融着率に基づいて、以下の基準により融着性を5段階で評価した。
5:融着率が80%以上
4:融着率が60%以上80%未満
3:融着率が40%以上60%未満
2:融着率が20%以上40%未満
1:融着率が20%未満
発泡粒子成形体におけるひけ、つまり、成形体の中央が周囲よりもくぼんでいる状態の有無を評価した。具体的には、得られた成形体の中央部分と四隅部分の厚みをそれぞれ測定し、四隅部分のうち最も厚みが厚い部分に対する中央部分の厚みの比を算出した。厚み比に基づいて、以下の基準により回復性を5段階で評価した。
5:厚み比が99%以上
4:厚み比が98%以上99%未満
3:厚み比が96%以上98%未満
2:厚み比が90%以上96%未満
1:厚み比が90%未満
まず、二段発泡工程において得られた二段発泡粒子を23℃で24時間乾燥させた。次いで、二段発泡粒子に内圧付与を行うことなく(つまり、内圧は0)、クラッキング量を20%(具体的には、8mm)に調節した平板成形型(具体的には、平板形状のキャビティを備える金型)内に二段発泡粒子を充填した。成形型の寸法は、縦200mm、横65mm、厚み40mmである。次に、成形型を型締めし、成形型の両面にあるドレン弁を開放した状態で水蒸気を5秒間供給して予備加熱を行った後、成形型の一方の面側から0.01MPa(G)のスチームを供給して一方加熱を行った。次いで、成形型の反対側の面から0.01MPa(G)のスチームを供給して一方加熱を行った後、表2、表3に示す成形圧の水蒸気で8秒間加熱した。この8秒間の加熱が本加熱である。本加熱終了後、放圧し、成形型内面に取り付けられた面圧計の値が0.02MPa(G)に低下するまで水冷した後、成形体を離型した。成形体を80℃のオーブン内で12時間静置することにより養生した。成形体を相対湿度50%、温度23℃、大気圧1atmの条件にて24時間静置することにより、成形体の状態調節を行った。このようにして成形体を製造した。次に、以下のようにして成形体の密度、引張強さ、引張伸び率、50%ひずみ時圧縮応力を測定すると共に、成形体の表面平滑性を評価した。測定結果、評価結果を表2、表3に示す。
成形体密度(kg/m3)は、成形体の重量(g)を成形体の外形寸法から求められる体積(L)で除し、単位換算することにより算出した。
成形体の引張強さS及び引張伸び率Eは、JIS K6767:1999に準拠して求めた。まず、バーチカルスライサーを用いて成形体から全ての面が切り出し面となるように、120mm×25mm×10mmの切り出し片を切り出した。この切り出し片から糸鋸を用いてダンベル状1号形状の試験片を作製した。次いで、試験片を用いて、500mm/分の引張速度で引張試験を行った。測定される引張り時の最大引張り応力を引張強さS、破断時の伸び率を引張伸び率Eとした。
成形体の表面にあるスキン層が試験片に含まれないように、成形体の中心部から縦50mm×横50mm×厚み25mmの試験片を切り出した。JIS K6767:1999に基づき、圧縮速度10mm/分にて圧縮試験を行い成形体の50%ひずみ時圧縮応力を求めた。なお、50%ひずみ時圧縮応力の測定に用いた試験片の密度を上記成形体密度の測定と同様の方法により求め、「切り出し密度」として表2、表3に示した。
成形体の表面を観察し、表面性を下記基準に基づいて評価した。
A:成形体の表面に粒子間隙がほとんどなく、かつ金型転写、成形痕などに起因する凹凸が目立たない良好な表面状態を示す。
B:成形体の表面に粒子間隙がやや認められるか、あるいは、金型転写、成形痕などに起因する凹凸がやや認められる。
C:成形体の表面に粒子間隙が認められるか、あるいは、金型転写、成形痕などに起因する凹凸が認められる。
比較例3では、直鎖状低密度ポリエチレンの融解熱量が大きすぎる。そのため、合格品を得るための成形圧が高くなっていた。また、成形体の柔軟性も不十分であった。
比較例4では、直鎖状低密度ポリエチレンのMFR及び融解熱量が大きすぎる。そのため、成形可能範囲が狭く、成形性が不十分であった。また、成形体の柔軟性も不十分であった。さらに、発泡粒子の平均気泡径が大きすぎるため、成形体の表面平滑性が低かった。
比較例6は、実施例1と同様の直鎖状低密度ポリエチレンを用いて平均気泡径の大きな発泡粒子を製造した例である。比較例6では、発泡粒子の平均気泡径が大きすぎるため、表面平滑性が不十分であった。また、成形性の柔軟性がやや低下していた。
比較例7では、直鎖状低密度ポリエチレンの融解熱量が低すぎるため、発泡粒子の平均気泡径が大きいにもかかわらず、0.07~0.13MPa(G)の成形圧範囲での成形により合格品を得ることができなかった。
Claims (7)
- 無架橋の直鎖状低密度ポリエチレンを基材樹脂とするポリエチレン系樹脂発泡粒子であって、
上記直鎖状低密度ポリエチレンの密度が0.915g/cm3以上0.930g/cm3未満であり、
上記直鎖状低密度ポリエチレンの融解熱量ΔH1が80J/g以上100J/g以下であり、
上記直鎖状低密度ポリエチレンの融点Tmが120℃以上130℃以下であり、
上記直鎖状低密度ポリエチレンの、温度190℃、荷重2.16kgの条件でのメルトフローレイトが0.8g/10min以上1.5g/10min以下であり、
上記発泡粒子の平均気泡径が50μm以上180μm以下である、ポリエチレン系樹脂発泡粒子。 - 上記直鎖状低密度ポリエチレンの融解熱量ΔH1と、上記直鎖状低密度ポリエチレンの融点Tmとが下記式(I)の関係を満足する、請求項1に記載のポリエチレン系樹脂発泡粒子。
Tm>0.2×ΔH1+102 ・・・(I) - 上記直鎖状低密度ポリエチレンが、メタロセン系重合触媒により重合された直鎖状低密度ポリエチレンを主成分とする、請求項1又は2に記載のポリエチレン系樹脂発泡粒子。
- 上記直鎖状低密度ポリエチレンが、エチレンと炭素数6~8のα-オレフィンとの共重合体である直鎖状低密度ポリエチレンを主成分とする、請求項1~3のいずれか1項に記載のポリエチレン系樹脂発泡粒子。
- 上記発泡粒子の熱流束示差走査熱量測定において、上記発泡粒子を10℃/分の加熱速度で30℃から200℃まで昇温した際に得られる1回目のDSC曲線に、上記基材樹脂に固有の融解ピークである固有ピークと、該固有ピークよりも高温側に現れる融解ピークである高温ピークとが現れ、該高温ピークの融解熱量が15J/g以上50J/g以下である、請求項1~4のいずれか1項に記載のポリエチレン系樹脂発泡粒子。
- 上記発泡粒子の嵩密度が10~30kg/m3である、請求項1~5のいずれか1項に記載のポリエチレン系樹脂発泡粒子。
- 密閉容器内において、無架橋の直鎖状低密度ポリエチレンを基材樹脂とするポリエチレン系樹脂粒子を分散媒に分散させる分散工程と、
上記密閉容器内において、上記樹脂粒子に無機物理発泡剤を含浸させる発泡剤含浸工程と、
上記発泡剤を含浸させた上記樹脂粒子を上記密閉容器から該密閉容器の内圧よりも低圧の雰囲気下に放出することにより、上記樹脂粒子を発泡させる発泡工程と、を有し、
上記直鎖状低密度ポリエチレンの密度が0.915g/cm3以上0.930g/cm3未満であり、
上記直鎖状低密度ポリエチレンの融解熱量が80J/g以上100J/g以下であり、
上記直鎖状低密度ポリエチレンの融点が120℃以上130℃以下であり、
上記直鎖状低密度ポリエチレンの、温度190℃、荷重2.16kgの条件でのメルトフローレイトが0.8g/10min以上1.5g/10min以下である、ポリエチレン系樹脂発泡粒子の製造方法。
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