JP6608652B2 - Multilayer foam sheet - Google Patents

Description

  The present invention relates to a multilayer foamed sheet, and more particularly to a multilayer foamed sheet that can be suitably used as an interleaf for a plate-like material such as a glass plate for a substrate.

  Conventionally, when glass plates used in a liquid crystal panel are stacked and transported, a slip sheet is interposed between the glass plates for protection. Paper has been used as the slip sheet, but in recent years, a polyethylene resin foam sheet has been used (for example, Patent Document 1).

The foam sheet used as the interleaving paper has a cushioning property to protect the glass plate as a packaged object, and has a small amount of sag when cantilevered (high strength), and is interposed between the glass plates. It is required to be excellent in handleability when performing. Further, when performing the processing and manufacturing operations of the glass plate, it is necessary that to eliminate by vacuum suction the foam sheet interposed between the glass plates from the glass plate surface. At that time, if the amount of sag when the foam sheet is cantilevered is too large (the strength is weak), the part protruding from the glass plate may sag or wrinkle may occur on the foam sheet. Exclusion workability, such as exclusion, is reduced.

JP 2007-262409 A

  On the other hand, in recent years, as the glass plate for liquid crystal panels has been further reduced in thickness, the foam sheet used as a slip sheet has been required to be thinner. Therefore, there is a demand for a thin foam sheet having sufficient cushioning and stiffness as a slip sheet.

  The present invention provides a multilayer foamed sheet that has sufficient cushioning and stiffness, and can be suitably used as an interstitial paper for a glass plate for a substrate, even if it is thinner and lighter than a conventional multilayer foam sheet for slippers. It is for the purpose.

According to the present invention, the following multilayer foam sheet is provided.
[1] In a multilayer foamed sheet having an apparent density of 30 to 300 kg / m ³ and a thickness of 0.05 to 2 mm, comprising a polyethylene-based resin foam layer and a thermoplastic resin layer laminated and adhered to both sides of the foam layer.
The thermoplastic resin layer is formed from a mixed resin composition containing a polyethylene resin , a polystyrene resin, and a styrene elastomer , and in the mixed resin composition, the polyethylene resin and the polystyrene resin both have a continuous phase. A multilayer foamed sheet which is formed , wherein the weight ratio of polystyrene resin in the mixed resin composition is 25 to 60% by weight, and the weight ratio of styrene elastomer is 2 to 20% by weight .
[2] The multilayer foamed sheet as described in 1 above, wherein the weight ratio of the polystyrene-based resin in the mixed resin composition is more than 30% by weight and 60 % by weight or less .
[3] The multilayer foamed sheet according to 1 or 2 above, wherein the basis weight per side of the thermoplastic resin layer is 10 g / m ² or less.
[4] Any of the above 1-3, wherein the mixed resin composition contains a polymer antistatic agent, and the polymer antistatic agent is dispersed in the continuous phase of the polyethylene resin. A multilayer foam sheet as described in 1.
[5] The multilayer foamed sheet as described in 4 above, wherein the polymer type antistatic agent in the mixed resin composition has a weight ratio of 2 to 30% by weight.

  The multilayer foam sheet of the present invention has a sandwich structure in which a thermoplastic resin layer (hereinafter also simply referred to as a resin layer) is laminated and adhered to both sides of a polyethylene resin foam layer (hereinafter also simply referred to as a foam layer). It is what you have. Since the foamed layer is formed of a polyethylene-based resin, the multilayer foamed sheet has excellent buffering properties and can be suitably used as a slip sheet for glass plates for precision electronic devices. Furthermore, since the resin layer is formed of a mixed resin composition containing a polyethylene resin and a polystyrene resin, the polyethylene resin forms a continuous phase, and the polystyrene resin also forms a continuous phase. The multi-layer foam sheet having the resin layer has excellent rigidity and firmness compared to a conventional foam sheet having the same thickness. Therefore, the multilayer foamed sheet is excellent in following ability at the time of vacuum suction, and can be handled in the same manner as a conventional one even when the thickness is thin.

FIG. 1 is a transmission electron micrograph (magnification 15400 times) of a longitudinal section of a resin layer constituting the multilayer foamed sheet obtained in Example 1. FIG. 2 is a transmission electron micrograph of the longitudinal section of the resin layer in which the portion of FIG. FIG. 3 is a photomicrograph of the transmission electron longitudinal section of the resin layer constituting the multilayer foamed sheet obtained in Comparative Example 3 (magnification 15400 times). FIG. 4 is a transmission electron micrograph (magnification 61800 times) of the longitudinal section of the resin layer in which the portion of FIG. 3 is enlarged.

Hereinafter, the multilayer foamed sheet of the present invention will be described in detail.
The multilayer foamed sheet of the present invention comprises a polyethylene-based resin foam layer and a thermoplastic resin layer laminated and bonded by coextrusion on both sides of the foam layer, and has a sandwich structure. Furthermore, the resin layer of the multilayer foamed sheet is formed from a mixed resin composition having a specific composition as will be described later, and further, the mixed resin composition forms a specific morphology. The sheet is strong as a whole and excellent in handleability while maintaining cushioning properties.

  The thickness of the multilayer foamed sheet of the present invention is 0.05-2 mm. As described above, considering the recent reduction in the size of the substrate glass plate, the upper limit is preferably 1.5 mm, more preferably 1.3 mm, and even more preferably 1.0 mm. On the other hand, the lower limit is preferably 0.07 mm, more preferably 0.10 mm, and still more preferably 0.15 mm, in order to ensure higher buffering properties.

The apparent density of the entire multilayer foamed sheet is 30 kg / m ³ or more, preferably 35 kg / m ³ or more, more preferably 40 kg / m ³ or more. Usually, the lower the apparent density, the lower the rigidity. On the other hand, since the multilayer foam sheet of the present invention has a specific resin layer, even if the entire multilayer foam sheet has a low apparent density, it has excellent rigidity. Considering the buffering property, the upper limit of the apparent density is about 300 kg / m ³ , preferably 200 kg / m ³ .

The upper limit of the basis weight of the multilayer foamed sheet is preferably 200 g / m ² , more preferably 100 g / m ² , still more preferably 50 g / m ² , and particularly preferably 30 g from the viewpoint of handleability. / M ² . On the other hand, the lower limit is preferably about 10 g / m ² or more, and more preferably 20 g / m ² or more.

  The thickness of the entire multilayer foamed sheet in the present invention is an arithmetic average value of thickness (mm) measured at intervals of 1 cm in the width direction over the entire width of the multilayer foamed sheet.

The apparent density (kg / m ³ ) of the multilayer foamed sheet in the present invention is obtained by dividing the total basis weight (g / m ² ) of the multilayer foamed sheet by the thickness (mm) of the multilayer foamed sheet and converting the unit. can get.

  Moreover, since the width | variety of the multilayer foamed sheet | seat of this invention can be used for the packaging of a large sized glass plate, 1000 mm or more is preferable. The upper limit is approximately 5000 mm.

  In addition, the closed cell ratio of the multilayer foamed sheet of the present invention is preferably 15% or more, and more preferably 20% or more from the viewpoints of the flexibility of the foamed sheet, the surface protection of the article to be packaged, and the appropriate slipperiness.

  The closed cell ratio is calculated according to the following formula (1) using the true volume Vx of the multilayer foam sheet (cut sample) measured according to the procedure C of ASTM-D2856-70. . In addition, it is set as the cut sample for a measurement of 25 mm x 25 mm x about 20 mm by cutting out and laminating a plurality of samples of 25 mm x 25 mm x multilayer foam sheet thickness. As a measuring apparatus, an air comparison type hydrometer 930 type manufactured by Toshiba Beckman Co., Ltd. can be used.

S (%) = (Vx−W / ρ) × 100 / (Va−W / ρ) (1)
Vx: the true volume (cm ³ ) of the cut sample measured by the above method, which corresponds to the sum of the volume of the resin constituting the cut sample and the volume of the closed cell portion in the cut sample.
Va: Apparent volume (cm ³ ) of the cut sample used for measurement, which is the sum of the volume of the resin constituting the cut sample and the total volume of bubbles in the closed cell portion and the open cell portion in the cut sample. Equivalent to.
W: Total weight (g) of cut sample used for measurement.
ρ: Density of resin required by defoaming the multilayer foamed sheet (g / cm ³ )

Next, the resin that forms the foam layer of the multilayer foam sheet of the present invention will be described.
The foam layer is made of polyethylene resin A.

In the present invention, the polyethylene-based resin means a resin having an ethylene component unit of 50 mol% or more. Specific examples of the polyethylene resin include low density polyethylene (LDPE), ethylene-vinyl acetate copolymer (EVA), linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), and the like. And the like.
As the polyethylene-based resin A, a polyethylene-based resin mainly composed of low-density polyethylene is preferable because the foamability is excellent and the multilayer foamed sheet is more excellent in buffering properties.

  The foamed layer has other synthetic resins and elastomers, bubble regulators, nucleating agents, antioxidants, heat stabilizers, weathering agents, ultraviolet absorbers, hard absorbers and the like as long as the objects and effects of the present invention are not impaired. Functional additives such as a flame retardant, antibacterial agent and shrinkage inhibitor, and additives such as inorganic fillers can be added.

Next, the mixed resin composition forming the resin layer of the multilayer foamed sheet of the present invention will be described.
The resin layer is formed from a mixed resin composition containing a polyethylene resin B and a polystyrene resin. In the mixed resin composition, both the polyethylene resin B and the polystyrene resin are continuous phases (both continuous phases). Is forming. That is, since the polystyrene resin having a higher elastic modulus than the polyethylene resin forms a continuous phase in the mixed resin composition, the multilayer foam sheet of the present invention is more rigid than the conventional foam sheet. Excellent elasticity will be strong. In addition, since the polyethylene resin B and the polystyrene resin together form a continuous phase in the mixed resin composition of the resin layer, the resin layer is excellent in adhesiveness with the foam layer.

The polyethylene resin B forms a continuous phase in the mixed resin composition constituting the resin layer together with the polystyrene resin described later. The melting point is preferably 140 ° C. or lower, more preferably 120 ° C. or lower because it is easy to form a morphology showing both continuous phases. The lower limit is approximately 80 ° C.
The melting point of the polyethylene resin is based on the plastic transition temperature measurement method of JIS K7121-1987, and “(2) When measuring the melting temperature after performing a certain heat treatment” is selected as the condition adjustment of the test piece. It is a melting peak temperature measured as follows.

  Since the polyethylene resin B constituting the resin layer and the polyethylene resin A constituting the foam layer are the same type, it is difficult to destroy the bubbles in the foam layer at the time of co-extrusion, and the adhesiveness is excellent. preferable. However, different types of resins can be used.

  Since it is easy to form a morphology showing both continuous phases in the resin layer, the content of the polyethylene resin B in the mixed resin composition is preferably 20 to 75% by weight of the entire mixed resin composition, and preferably 25 to 25%. More preferably, it is 65 weight%, and it is further more preferable that it is 30-50 weight%.

  Examples of the polystyrene resin include, for example, polystyrene, rubber-modified polystyrene, styrene-α-methylstyrene copolymer, styrene-p-methylstyrene copolymer, styrene-acrylic acid copolymer, and styrene-methacrylic acid copolymer. Styrene-maleic anhydride copolymer, styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-acrylonitrile copolymer A polymer etc. are mentioned. Among these, polystyrene is preferable because a good resin layer is easily obtained.

  Moreover, it is preferable that the said mixed resin composition which comprises a resin layer contains the compatibilizer of polyethylene-type resin B and polystyrene-type resin. When the mixed resin composition contains a compatibilizing agent, the film forming property can be improved, a good resin layer can be formed, and the continuous phase of the polystyrene-based resin can be more easily formed.

  Examples of the compatibilizing agent include styrene elastomers such as styrene-butadiene copolymers, styrene-isoprene copolymers, and hydrogenated products of these copolymers. The copolymer is preferably a block copolymer.

  The content of the compatibilizer in the mixed resin composition constituting the resin layer is preferably 2 to 20% by weight of the entire mixed resin composition. The lower limit of the content is preferably 3% by weight, and the upper limit thereof is preferably 15% by weight, more preferably 10% by weight.

  In the mixed resin composition constituting the resin layer in the present invention, the polyethylene resin and the polystyrene resin together form a continuous phase. That is, since the polyethylene-based resin forms a continuous phase and the polystyrene-based resin having a large bending strength and excellent rigidity forms a continuous phase, the multilayer foamed sheet of the present invention is compared with the conventional foamed sheet, It has excellent rigidity and firmness. Therefore, the multilayer foamed sheet is excellent in following ability at the time of vacuum suction, and can be handled in the same manner as the conventional one even when the thickness is thin.

  In the resin layer, the continuous phase of the polystyrene-based resin is preferably stretched and oriented in the plane direction of the multilayer foamed sheet. Moreover, it is preferable that there are two or more continuous layers of polystyrene-based resin in the resin layer thickness direction, more preferably three or more layers, and still more preferably five or more layers.

  Examples of continuous phases are shown in FIGS. In the figure, 1 represents a continuous phase of polyethylene resin, 2 represents a continuous phase of polystyrene resin, and 3 represents an antistatic agent described later. In addition, the morphology of the mixed resin composition of the resin layer can be confirmed by observing the cross section using a transmission electron microscope or the like.

  The content of the polystyrene resin in the mixed resin composition is preferably 25 to 60% by weight of the entire mixed resin composition. If the content is too small, the stiffness of the multilayer foam sheet may be insufficient. On the other hand, when there is too much this content, there exists a possibility that a foaming layer and a resin layer may not adhere | attach, or a softness | flexibility may be lost. From this viewpoint, the lower limit of the content is preferably 30% by weight. The upper limit is preferably 50% by weight.

The basis weight of the resin layer per side of the multilayer foamed sheet of the present invention is preferably 10 g / m ² or less. When the basis weight of the resin layer is within the above range, the multilayer foamed sheet has sufficient buffer properties. From this viewpoint, the upper limit of the basis weight of the resin layer is preferably 5 g / m ² , more preferably 3 g / m ² , and further preferably ² g / m ² . In addition, from the viewpoint of forming a good resin layer, the lower limit of the basis weight of the resin layer is preferably 1 g / m ² or more. Further, from the viewpoint of handleability, it is preferable to make the basis weights of the resin layers on both sides as equal as possible.
As described above, the multilayer foamed sheet of the present invention is sufficient because the mixed resin composition having a specific composition forms a specific morphology even when the basis weight of the resin layer is small. Can exhibit high rigidity.

In the resin layer of the multilayer foamed sheet of the present invention, the mixed resin composition constituting the resin layer contains a polymer type antistatic agent, and the surface resistivity of the multilayer foamed sheet is 1 × 10 ⁷ to 1 × 10 6. ¹⁴ (Ω) is preferable, and 1 × 10 ⁷ to 1 × 10 ¹³ Ω is more preferable. A multilayer foam sheet having a surface resistivity in such a range is less likely to accumulate static charge and less likely to adhere to dust. In order to develop a sufficient surface resistivity with a small amount of addition, the polymer antistatic agent is preferably dispersed in the polyethylene resin continuous phase.

The polymer type antistatic agent is made of a resin having a surface resistivity of less than 1 × 10 ¹² Ω, preferably less than 1 × 10 ¹¹ Ω, more preferably less than 1 × 10 ¹⁰ Ω. Specifically, polyether, polyether ester amide, block copolymer of polyether and polyolefin, ionomer resin, and the like. Among these, a block copolymer of polyether and polyolefin and an ionomer resin are more preferable.

  Examples of the block copolymer include those having a structure in which a polyolefin block and a polyether block are repeatedly and alternately bonded through bonds such as ester bonds, amide bonds, ether bonds, urethane bonds, and imide bonds. .

  The ionomer resin is a metal salt cross-linked product of a copolymer of ethylene and a carboxylic acid such as acrylic acid, methacrylic acid, and maleic acid. Examples of the metal salt include alkali metal salts, alkaline earth metal salts, A typical metal salt, a transition metal salt, etc. are mentioned.

  Specific examples of such a polymer antistatic agent include, for example, “Pelestat 300”, “Pelestat 230”, “Pelestat HC250”, and “Pelestat HC250” manufactured by Sanyo Chemical Industries, Ltd. as block copolymers of polyether and polyolefin. Examples of ionomer resins such as “Peletron PVH”, “Peletron PVL”, and “Peletron HS” that are commercially available under trade names such as “ENTILA SD100” and “ENTILA MK400” manufactured by Mitsui DuPont Polychemical Co., Ltd. may be mentioned.

The content of the polymer antistatic agent in the resin layer is preferably 2 to 20% by weight, more preferably, based on the performance of the polymer antistatic agent itself, although it is preferably 2 to 20% by weight of the total mixed resin composition constituting the resin layer. Is 3 to 15% by weight, more preferably 5 to 15% by weight.
Moreover, a polymer type antistatic agent can be contained in the foamed layer. From the viewpoint of the balance between foamability during extrusion and antistatic performance of the resulting multilayer foam sheet, the content of the polymer antistatic agent in the foamed layer is preferably 2 to 15% by weight, more preferably 3 to 8% by weight.

Next, the manufacturing method of the multilayer foam sheet of this invention is demonstrated.
As a method for producing a multilayer foamed sheet according to the present invention, a coextrusion foaming method is adopted in which a molten resin forming a resin layer and a molten resin forming a foamed sheet are laminated and merged in a die and extrusion foamed. The coextrusion foaming method is preferable because the thickness of the resin layer can be reduced and a multilayer foamed sheet having a high adhesive force between the resin layer and the foamed sheet can be obtained.

  The method for producing a sheet-like multilayer foamed sheet by the coextrusion foaming method includes a method of coextrusion foaming into a sheet using a coextrusion flat die to form a sheet-like multilayer foamed sheet, and a coextrusion annular die There is a method in which a cylindrical laminated foam is obtained by co-extrusion foaming using a sheet, and then the cylindrical foam is cut open to form a sheet-like multilayer foam sheet. Among these, a method using an annular die for coextrusion is a preferable method because a wide multilayer foam sheet having a width of 1000 mm or more can be easily produced.

A method of co-extrusion using the annular die will be described in detail below.
First, the polyethylene resin A and an additive such as a bubble adjusting agent added as necessary are supplied to an extruder for forming a foamed layer, heated and kneaded, and then a physical foaming agent is injected and further kneaded. To obtain a resin melt for forming a polyethylene resin foam layer. At the same time, the polyethylene resin B, the polystyrene resin, the compatibilizer added as necessary, the polymer antistatic agent, and the like are supplied to a resin layer forming extruder and heated and kneaded. To obtain a resin melt for forming a thermoplastic resin layer. Next, the foamed layer forming resin melt and the resin layer forming resin melt are introduced into an annular die for coextrusion, laminated and coextruded to produce a multilayer foamed sheet.

  In addition, since it is excellent in especially foamability, it is preferable that MFR of the polyethylene-type resin A is 0.5-15 g / 10min. Moreover, when laminating | stacking a resin layer by coextrusion, it is preferable that MFR of the polyethylene-type resin B is more than MFR of the polyethylene-type resin A. FIG.

  Also, in order to disperse both polyethylene resin and polystyrene resin as a continuous phase in the resin layer, polystyrene resin having a melt viscosity close to that of polyethylene resin B (in other words, having a melt flow rate (MFR) close) ) And the addition of a volatile plasticizer are preferably employed. In addition to these methods, it is more preferable to use the compatibilizer.

  The melt flow rate (MFR) of the polyethylene resin B is preferably 5.0 to 15 g / 10 min, more preferably 6.0 to 14 g / 10 min, from the viewpoint of ease of coextrusion. .

Since it is easy to form the continuous phase, the melt flow rate (MFR) of the polystyrene-based resin is preferably 5.0 to 15 g / 10 min, and more preferably 6.0 to 14 g / 10 min.
Further, the MFR of the polystyrene resin and the MFR of the polyethylene resin B are preferably close to each other, and the MFR of the polystyrene resin is within the above range, and is 0.5 with respect to the MFR of the polyethylene resin B. It is preferably about ˜2 times, more preferably 0.5 to 1.5 times, and even more preferably 0.5 to 1 times.

  The MFR is a value measured based on the condition H (200 ° C., load 5 kg) of JIS K7210-1999.

  It is preferable that a volatile plasticizer is added to the resin melt for forming a resin layer from the viewpoint of forming a continuous phase of a polyethylene resin and a polystyrene resin. As the volatile plasticizer, one that has a function of reducing the melt viscosity of the resin melt and that volatilizes from the resin layer and does not exist in the resin layer after the resin layer is formed is used. By adding a volatile plasticizer into the resin melt, when the multilayer foam sheet is coextruded, the extrusion temperature of the resin melt for forming the resin layer can be brought close to the extrusion temperature of the resin melt for forming the foam layer. In addition, the melt elongation of the softened resin layer can be remarkably improved. If it does so, it will become difficult to destroy the bubble of a foam sheet by the heat | fever of a resin layer at the time of foaming, and also the elongation of this resin layer will follow the elongation at the time of foaming of a foam sheet easily. In particular, the continuous phase of polystyrene resin is multi-layer foamed in the resin layer by extruding into a cylindrical shape using an annular die and taking out the expanded foam (blow-up) while producing a foamed sheet. Rigidity can be improved by stretching and orienting the sheet in the plane direction.

  The volatile plasticizer is 1 selected from C3-C7 aliphatic hydrocarbons and alicyclic hydrocarbons, C1-C4 aliphatic alcohols, or C2-C8 aliphatic ethers. Species or two or more types are preferably used. When a so-called lubricant having low volatility is used instead of the volatile plasticizer, the lubricant may remain in the resin layer and contaminate the surface of the package. On the other hand, the volatile plasticizer is preferable from the viewpoint that the resin of the resin layer is efficiently plasticized and the volatile plasticizer itself hardly remains in the obtained resin layer.

  The boiling point of the volatile plasticizer is preferably 120 ° C. or lower, more preferably 80 ° C. or lower because it easily evaporates from the resin layer. If the boiling point of the volatile plasticizer is within this range, if the obtained multilayer foamed sheet is allowed to stand after coextrusion, it will volatilize due to heat immediately after coextrusion or further gas permeation at room temperature. The plasticizer is volatilized and removed from the resin layer. The lower limit of the boiling point of the volatile plasticizer is approximately -50 ° C.

  The volatile plasticizer is 5 parts by weight to 100 parts by weight in total of the polyethylene resin B for forming the resin layer and the polystyrene resin, and the compatibilizer and polymer antistatic agent added as necessary. It is preferable to add so that it may become 50 weight part.

  Moreover, you may add various additives to resin which forms this melt in the range which does not inhibit the objective of this invention to the resin melt for resin layer formation. Examples of the various additives include antioxidants, heat stabilizers, weathering agents, ultraviolet absorbers, flame retardants, fillers, antibacterial agents, and the like. In this case, the amount added is appropriately determined according to the purpose and effect of the additive, but is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, and more preferably 3 parts by weight with respect to 100 parts by weight of the mixed resin composition. The following are particularly preferred:

  Examples of the physical foaming agent added to the foamed layer forming resin melt include aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, normal hexane, and isohexane, and fats such as cyclopentane and cyclohexane. Organic physical foaming agents such as cyclic hydrocarbons, chlorinated hydrocarbons such as methyl chloride and ethyl chloride, fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane, nitrogen, dioxide Examples include inorganic physical foaming agents such as carbon, air, and water. In some cases, a decomposable foaming agent such as azodicarbonamide can be used. The above-mentioned physical foaming agents can be used in combination of two or more. Among these, an organic physical foaming agent is particularly preferable from the viewpoint of compatibility with a polyethylene resin and foaming properties, and among them, those mainly containing normal butane, isobutane, or a mixture thereof are preferable.

  The addition amount of the physical foaming agent is adjusted according to the type of foaming agent and the target apparent density. Moreover, the addition amount of a bubble regulator is adjusted according to the target bubble diameter. For example, in order to obtain a multilayer foamed sheet having an apparent density range using a butane mixture of 30% by weight of isobutane and 70% by weight of normal butane as a foaming agent, the amount of butane mixture added is 100 parts by weight of the base resin. 3 to 30 parts by weight, preferably 4 to 20 parts by weight, more preferably 6 to 18 parts by weight.

As the main additive added to the foamed layer forming resin melt, a bubble regulator is usually added. As the bubble adjusting agent, either an organic type or an inorganic type can be used. Examples of the inorganic foam regulator include borate metal salts such as zinc borate, magnesium borate, borax, sodium chloride, aluminum hydroxide, talc, zeolite, silica, calcium carbonate, sodium bicarbonate, and the like. Examples of the organic bubble regulator include sodium 2,2-methylenebis (4,6-tert-butylphenyl) phosphate, sodium benzoate, calcium benzoate, aluminum benzoate, and sodium stearate. A combination of citric acid and sodium bicarbonate, an alkali salt of citric acid and sodium bicarbonate, or the like can also be used as the bubble regulator. These bubble regulators can be used in combination of two or more.
In addition, the addition amount of a bubble regulator is 0.01-3 weight part per 100 weight part of base resin, Preferably it is 0.03-1 weight part.

  As the manufacturing apparatus such as the annular die and the extruder, a known apparatus conventionally used in the field of extrusion foaming can be used.

  Since the multilayer foamed sheet of the present invention has sufficient buffering properties and stiffness, it can be suitably used as an interleaf for glass plates. However, the use of the multilayer foamed sheet is not limited to the paper for glass plates, and can be suitably used widely as a packaging material for precision instruments.

  Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the examples.

  Table 1 shows polyethylene resins and polystyrene resins used in Examples and Comparative Examples, and Table 2 shows polymer type antistatic agents and compatibilizers. In addition, the melt flow rate in Table 1 is a value measured under condition H (200 ° C., load 5 kg) based on JIS K7210-1999.

  As the physical foaming agent, mixed butane composed of 70% by weight of normal butane and 30% by weight of isobutane was used.

  As the cell regulator, a cell regulator master batch comprising 20% by weight of talc (trade name “High Filler # 12” manufactured by Matsumura Sangyo Co., Ltd.) with respect to 80% by weight of low density polyethylene was used.

  As an extruder for forming a polyethylene resin foam layer, a tandem extruder composed of a first extruder having a diameter of 90 mm and a second extruder having a diameter of 120 mm was used. As an extruder for forming a thermoplastic resin layer, a diameter of 50 mm, L / D = 50 third extruders were used. Further, the respective outlets of the second extruder and the third extruder were connected to the co-extrusion annular die so that the respective molten resins could be laminated in the co-extrusion annular die.

Examples 1-2 and Comparative Examples 1-3
The polyethylene resin A in the amount shown in Table 3 and the master batch in the amount shown in Table 3 are supplied to the raw material charging port of the first extruder of the tandem extruder, heated and kneaded, and melted at about 200 ° C. A resin mixture was obtained. Next, mixed butane of the amount shown in Table 3 is press-fitted into the molten resin mixture, and then supplied to the second extruder connected to the downstream side of the first extruder, and the extruded resin temperature shown in Table 3 The foamed layer-forming resin melt was introduced into the co-extrusion annular die at a discharge amount shown in Table 3.

  At the same time, the polyethylene resin B, the polystyrene resin, the polymer antistatic agent and the compatibilizer blended as shown in Table 3 are supplied to the third extruder and heated and kneaded, and the amounts shown in Table 3 as volatile plasticizers. The mixed butane was press-fitted, kneaded, adjusted to the extrusion resin temperature shown in Table 3 to obtain a resin melt for forming a resin layer, and the resin melt for forming a resin layer was coextruded at a discharge amount shown in Table 3. Introduced into an annular die.

  The resin melt for forming a resin layer introduced into the annular die for coextrusion is introduced into the outer and inner sides of the resin melt for forming a foam layer that is introduced into the annular die for coextrusion and flows in a resin flow path in the die. The melt laminate was extruded into the atmosphere from a die having a lip diameter of 135 mm to form a three-layered cylindrical laminate foam composed of resin layer / foam layer / resin layer. The extruded cylindrical laminated foam was expanded in diameter (blow-up ratio 3.47) and cut along the extrusion direction while being taken up, and wound into a roll to obtain a multilayer foam sheet having a width of 1400 mm.

  Table 4 shows various physical properties of the multilayer foamed sheets obtained in Examples and Comparative Examples.

The morphology of the mixed resin composition in the resin layer was observed by the following method. First, a test piece was cut out from the surface of the multilayer foam sheet. After the specimen was stained with ruthenium tetroxide, it was sliced into ultrathin sections, and the ultrathin sections were observed under the condition of an acceleration voltage of 100 kV using a Hitachi transmission electron microscope (H-7100).
FIGS. 1 and 2 show transmission electron micrographs of the cross section of the resin layer of the multilayer foam sheet obtained in Example 1, and FIGS. 3 and 2 show cross section photographs of the resin layer of the multilayer foam sheet obtained in Comparative Example 3. 4 shows.
1 and 2, it can be confirmed that both the polyethylene resin 1 and the polystyrene resin 2 are formed as a layered continuous phase (sea / sea) stretched and oriented in the plane direction of the multilayer foamed sheet. Further, it can be confirmed that the polymer antistatic agent 3 is dispersed in the polyethylene resin 1.
In the multilayer foam sheet obtained in Example 2, the same morphology was confirmed in the resin layer.
On the other hand, in FIGS. 3 and 4, the polystyrene resin 2 does not form a continuous phase, but forms a dispersed phase (island) dispersed in the continuous phase (sea) of the polyethylene resin 1 (sea). / Island) can be confirmed.

The thickness of the multilayer foam sheet in Table 4 was measured by the above method (n = 5).
The total basis weight of the multilayer foamed sheet is obtained by cutting out a test piece having a width of 10 cm over the entire width of the sheet from the multilayer foamed sheet unwound from the roll and dividing the weight of the test piece by the total width of the sheet × 10 cm. Obtained (n = 5).
Moreover, the basic weight of the resin layer was calculated | required from ratio of the discharge amount of a foam layer and a resin layer based on the said whole basic weight.
The apparent density of the multilayer foam sheet was determined by dividing the total basis weight of the multilayer foam sheet by the thickness of the multilayer foam sheet and converting the unit.

The film forming properties in Table 4 were evaluated according to the following criteria.
○: No break in the resin layer of the obtained multilayer foam sheet ×: There is a tear in the resin layer of the obtained multilayer foam sheet

The amount of sagging in Table 4 was measured as follows.
(Horizontal sag)
Ten measurement test pieces each having a width of 100 mm and a length of 200 mm were cut out from 10 randomly selected portions of the multilayer foam sheet by matching the extrusion direction of the obtained sheet with the length direction of the test piece. The obtained test piece is placed on a horizontal base and fixed in a horizontal direction from the edge of the base with the length of the test piece protruding 100 mm, and the vertical direction from the top of the base to the bottom of the test piece The distance of was measured. This measurement was performed on each test piece, and the arithmetic average value of each measurement value was defined as the horizontal sag amount.
(60 ° inclination sag)
Ten test specimens each having a width of 200 mm and a length of 200 mm were cut out from 10 randomly selected locations of the multilayer foamed sheet by matching the extrusion direction of the obtained sheet with the length direction of the test piece. Place the obtained test piece on the base surface of the base inclined at 60 ° above the horizontal plane, and fix it with the length direction of the test piece protruding 100 mm in the direction extending straight from the upper end of the base. Then, the distance in the direction orthogonal to the foundation upper surface from the free tip of the hanging test piece to the virtual plane obtained by extending the foundation surface straight was measured. This measurement was performed on each test piece, and an arithmetic average value of each measurement value was defined as a 60 ° inclination sag.

1 Continuous phase of polyethylene resin 2 Continuous phase of polystyrene resin 3 Dispersed phase of polymer antistatic agent

Claims (5)

In a multilayer foamed sheet having an apparent density of 30 to 300 kg / m ³ and a thickness of 0.05 to 2 mm, comprising a polyethylene-based resin foam layer and a thermoplastic resin layer laminated and adhered to both sides of the foam layer,
The thermoplastic resin layer is formed from a mixed resin composition containing a polyethylene resin , a polystyrene resin, and a styrene elastomer , and in the mixed resin composition, the polyethylene resin and the polystyrene resin both have a continuous phase. Formed ,
A multilayer foamed sheet, wherein a weight ratio of polystyrene resin in the mixed resin composition is 25 to 60% by weight and a weight ratio of styrene elastomer is 2 to 20% by weight .
The multilayer foamed sheet according to claim 1, wherein the weight ratio of the polystyrene-based resin in the mixed resin composition is more than 30 wt% and 60 wt% or less .

The multilayer foamed sheet according to claim 1 or 2, wherein a basis weight per one side of the thermoplastic resin layer is 10 g / m ² or less.
The mixed resin composition contains a polymer type antistatic agent, and the polymer type antistatic agent is dispersed in the polyethylene-based resin continuous phase. Multi-layer foam sheet.
The multilayer foamed sheet according to claim 4, wherein a weight ratio of the polymer type antistatic agent in the mixed resin composition is 2 to 30% by weight.