JP7680938B2 - Method for producing extruded polyethylene resin foam sheet - Google Patents

Description

The present invention relates to a method for producing an extruded polyethylene resin foam sheet, and more specifically, to a method for producing an extruded polyethylene resin foam sheet using a flammable physical foaming agent, which has a low basis weight, a small amount of residual flammable foaming agent, and is safe during transportation.

Polyethylene resin extruded foam sheets (hereinafter simply referred to as extruded foam sheets) are materials with excellent cushioning properties and are therefore used in a variety of fields as packaging materials, such as inserts for plate-shaped objects, packaging materials, and cushioning materials.

The polyethylene resin foam sheet is usually produced by feeding a polyethylene resin together with a bubble control agent and the like to an extruder, heating and kneading the resin melt to form a resin melt, injecting a physical foaming agent into the resin melt and kneading the mixture to form a foamable resin melt, extruding the foamable resin melt from a circular die into the atmosphere to form a cylindrical foam, and cutting the cylindrical foam into a sheet while taking it up with a take-up machine.

The extruded foam sheet is usually stored and shipped as a rolled product. The rolled product may also be stored in a sealed space such as a container and exported.

In the production of such foamed sheets, butane, a flammable foaming agent, is usually used as the physical foaming agent. Butane has excellent extrusion foaming properties, so it is easy to obtain extruded foamed sheets that have a low basis weight and low apparent density (high expansion ratio) as well as a thin thickness.

When butane is used, the butane dissipates from the extruded foam sheet during production, and the extruded foam sheet shrinks while being wound (wound) into a roll. For this reason, the shrunken extruded foam sheet is usually cured by heating or the like, and air is allowed to flow into the bubbles of the extruded foam sheet, thereby recovering the dimensions of the shrunken extruded foam sheet, such as its thickness and width.

On the other hand, even after the curing described above, the butane does not completely dissipate from the extruded foam sheet, and usually some of the butane remains in the extruded foam sheet after curing. Therefore, if an extruded foam sheet containing remaining butane is placed in an enclosed space such as a container during export, the butane dissipates from the extruded foam sheet and accumulates in the enclosed space, increasing the butane concentration in the enclosed space. Therefore, depending on the amount of extruded foam sheet placed in the enclosed space, the butane concentration in the enclosed space may reach the explosion limit during transportation. Therefore, it is necessary to reduce the amount of butane remaining in the extruded foam sheet when exporting, etc.

Methods for reducing the amount of butane remaining in the extruded foam sheet include extending the time during the curing process and separately carrying out a heat treatment to promote dissipation of butane (Patent Document 1).

JP 2015-979 A

However, if the curing process is carried out for a long period of time, the time until the extruded foam sheet can be shipped will be longer, which may result in a decrease in productivity. Furthermore, even if curing is carried out for a long period of time, the amount of remaining butane may not be sufficiently reduced. Furthermore, if a heat treatment is carried out, the number of manufacturing steps will increase, which may result in a decrease in productivity.

As described above, the foaming agent such as butane remaining in the extruded foam sheet immediately after production gradually dissipates to the outside over time, while the surrounding air gradually flows into the extruded foam sheet, so the dimensions of the extruded foam sheet, such as its thickness and width, tend to vary over time. Therefore, when an extruded foam sheet is wound into a roll immediately after production, the dimensions of the extruded foam sheet, such as its thickness and width, may vary depending on the part of the roll. If such dimensional variations occur, the appearance of the rolled product may deteriorate, and the rolled product may lose its wound state or become disintegrated during storage and transportation.

The present invention has been made in view of the above circumstances, and aims to provide a method for producing an extruded polyethylene resin foam sheet, which can stably produce an extruded foam sheet of a desired low basis weight when producing an extruded foam sheet using a flammable physical foaming agent such as butane (hereinafter also referred to as a flammable foaming agent), can obtain an extruded foam sheet with a small amount of remaining flammable foaming agent even if the step of reducing the amount of remaining flammable foaming agent is shortened or omitted, and can suppress dimensional variations in the thickness, width, etc. of the extruded foam sheet depending on the part of the roll even if the extruded foam sheet is wound into a roll immediately after production.

According to the present invention, there is provided a method for producing an extruded polyethylene resin foam sheet as described below.
[1] A method for producing an extruded polyethylene resin foam sheet having a basis weight of 10 g/m2 or ^more and 80 g/ ^m2 or less by extruding and foaming a foamable resin melt obtained by kneading a polyethylene resin and a physical foaming agent, comprising the steps of:
the physical blowing agent is a mixed blowing agent of dimethyl ether and butane,
The proportion of normal butane in the butane exceeds 80 mol%,
the blending amount of the mixed blowing agent per 1 kg of the foamable resin melt is 1 mol or more and 4 mol or less,
the amount of butane per 1 kg of the foamable resin melt is 0.1 mol or more and 2 mol or less,
The mixed blowing agent contains dimethyl ether in an amount of 30 mol % or more and 95 mol % or less, and contains butane in an amount of 5 mol % or more and 70 mol % or less (provided that the sum of the mixed blowing agent amounts to 100 mol %).
[2] The method for producing an extruded polyethylene resin foam sheet according to the above item 1, wherein the amount of normal butane per 1 kg of the foamable resin melt is 0.10 mol or more.
[3] The method for producing an extruded polyethylene resin foam sheet according to the above item 1 or 2, wherein the extruded polyethylene resin foam sheet has an average thickness of 0.1 mm or more and 2 mm or less and an apparent density of 25 kg/ ^m3 or more and 300 kg/m3 or ^less .
[4] The method for producing an extruded polyethylene resin foam sheet according to any one of [1] to [3] above, wherein the extruded polyethylene resin foam sheet has a length of 1 m or more in a width direction.

According to the present invention, there is provided a method for producing an extruded foam sheet using a flammable physical foaming agent such as butane, which can provide an extruded foam sheet having a low basis weight, and further can provide an extruded foam sheet having a small amount of remaining flammable foaming agent even if the step of reducing the amount of remaining foaming agent is shortened or omitted.
Furthermore, according to the present invention, even if the extruded foam sheet is wound into a roll immediately after production, dimensional variations in thickness, width, etc. of the extruded foam sheet depending on the part of the roll can be suppressed.

FIG. 1 is a schematic diagram showing an example of an apparatus for producing an extruded foam sheet used in the present invention.

The method for producing an extruded polyethylene resin foam sheet of the present invention will be described in detail below.
The polyethylene-based resin foam sheet is produced, for example, by feeding a polyethylene-based resin and an air bubble adjusting agent added as necessary to an extruder, heating and kneading the mixture to form a resin melt, injecting a physical foaming agent into the resin melt, further heating and kneading the mixture to form a foamable resin melt, extruding and foaming the foamable resin melt into the atmosphere through an annular die attached to the downstream side of the extruder to form a cylindrical foam, and cutting the cylindrical foam into a sheet while withdrawing it.

The polyethylene resin used in the present invention is one in which ethylene component units are present in a molar ratio of 50 mol% or more in the resin. The proportion of ethylene component units in the resin is preferably 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 95 mol% or more.

Examples of the polyethylene resin include low-density polyethylene, very low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, ethylene-ethyl acrylate copolymer, etc. These may be used alone or in combination of two or more.

Among these, it is preferable to use a polyethylene-based resin containing low-density polyethylene as a main component, since it has excellent extrusion foamability during production of an extruded foam sheet and can stably produce an extruded foam sheet with good cushioning properties.
The low density polyethylene means a polyethylene resin having a density of 910 kg/m ³ or more and less than 930 kg/m ³ .
In addition, in this specification, a polyethylene-based resin containing low-density polyethylene as a main component means a polyethylene-based resin in which the content of low-density polyethylene in the resin is 50% by weight or more, preferably 60% by weight or more, 70% by weight or more, more preferably 80% by weight or more, and particularly preferably 90% by weight or more.

The resin melt may contain other components, such as resins other than the polyethylene resin and elastomers, as long as the purpose and effect of the present invention are not impaired. In this case, the amount of the other components is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, and even more preferably 5 parts by weight or less, per 100 parts by weight of the polyethylene resin.

The MFR (melt flow rate) of the polyethylene resin is preferably 2 g/10 min or more and 20 g/10 min or less, and more preferably 4 g/10 min or more and 15 g/10 min or less.

The MFR is measured based on JIS K 7210-1:2014 (test temperature: 190°C, load: 2.16 kg). When two or more types of polyethylene resins are used in combination, it is preferable that the MFR of the mixture is within this range.

If the MFR is within this range, the polyethylene resin has high fluidity and the foam is easy to stretch when taken out. Therefore, it is possible to reliably obtain an extruded foam sheet with a low basis weight, thin thickness, and good appearance.

The melt tension of the polyethylene resin at 190°C is preferably 10 mN or more and 300 mN or less. If the melt tension is within this range, a foamed sheet with a low basis weight and good appearance can be obtained reliably. In consideration of this point, the melt tension is more preferably 15 mN or more, and even more preferably 20 mN or more. In addition, the melt tension is more preferably 250 mN or less.

The melt tension (hereinafter, also referred to as melt tension or MT) of a polyethylene resin at 190°C can be measured, for example, by a melt tension tester Type II manufactured by Toyo Seiki Seisakusho Co., Ltd. Specifically, a melt tension tester having a nozzle with a nozzle diameter of 2.095 mm and a length of 8 mm is used, and the resin is extruded from the nozzle in a string shape under conditions of a resin temperature of 190°C and an extrusion piston speed of 10 mm/min. This string is then hung on a tension detection pulley with a diameter of 45 mm, and wound on a winding roll with a diameter of 50 mm at a winding speed of 15.7 m/min.

A specific method for determining melt tension is, for example, as follows. The melt tension of the string-like material is measured over time as detected by a detector connected to a tension detection pulley while winding at a winding speed of 15.7 m/min. A graph with amplitude is obtained by plotting the melt tension over time on a chart with MT (mN) on the vertical axis and time (seconds) on the horizontal axis. Next, the median amplitude (X) is taken from the stable portion of the amplitude. In the present invention, this value (X) is taken as the melt tension. Note that rare, unusual amplitudes are ignored during the measurement.

However, if the string-like material hung on the tension detection pulley breaks before the winding speed reaches 15.7 m/min, the winding speed R at which the string-like material breaks is determined. Next, at a constant winding speed of R x 0.7, the median amplitude (X) is taken as the melt tension from a graph obtained in the same manner as above.

Next, the physical foaming agent used in the method of the present invention will be described.
In the method of the present invention, dimethyl ether and butane containing a specific proportion of normal butane are used as the physical foaming agent. Here, dimethyl ether is a foaming agent that can easily reduce the amount of remaining combustible foaming agent in the extruded foam sheet early without significantly impairing the extrusion foamability during extrusion. On the other hand, butane containing a specific proportion of normal butane is a foaming agent that has excellent extrusion foamability during extrusion and can take up the extruded foam sheet in a state where the polyethylene resin is moderately plasticized while suppressing the amount of remaining combustible foaming agent from becoming excessively large, thereby improving the take-up stability of the foam and making it easy to stably produce an extruded foam sheet with a low basis weight.
Therefore, according to the method of the present invention, by using dimethyl ether in combination with butane containing a specific ratio of normal butane, it is possible to obtain an extruded foam sheet having a low basis weight and further to reduce the amount of remaining flammable blowing agent.
Furthermore, in the present invention, by using the above-mentioned physical foaming agent, even when the extruded foam sheet is wound into a roll immediately after production of the extruded foam sheet, it is possible to obtain an extruded foam sheet in which dimensional variation due to the location of the roll-like material is suppressed regardless of the location of the roll.

Next, the blending of dimethyl ether, normal butane and isobutane will be described in detail.
In the method of the present invention, a mixed blowing agent of dimethyl ether and butane, which contains dimethyl ether and butane as main components, is used as the physical blowing agent.
In this specification, "containing dimethyl ether and butane as main components" means that the total content of dimethyl ether and butane in the mixed blowing agent is approximately 80 mol % or more, more preferably 90 mol % or more, and even more preferably 95 mol % or more.

In the butane used in the present invention, the proportion of normal butane in the butane must exceed 80 mol %.
If the proportion of normal butane is too low, there is a risk that the amount of the flammable blowing agent remaining in the extruded foam sheet will increase.
Since normal butane has excellent extrusion foamability during extrusion and dissipates faster from the extruded foam sheet than isobutane, the amount of butane remaining in the extruded foam sheet can be reduced quickly by making the blending ratio more than 80 mol %, and for these reasons, the blending ratio is more preferably 85 mol % or more, even more preferably 90 mol % or more, and particularly preferably 95 mol % or more.

Furthermore, the amount of normal butane per 1 kg of the foamable resin melt is preferably 0.10 mol or more.
By adjusting the amount of normal butane to be 0.10 mol or more, it is possible to obtain an extruded foam sheet having a low basis weight while easily reducing the apparent density of the extruded foam sheet (easiness to increase the expansion ratio), and it is also easy to adjust the thickness of the extruded foam sheet, so that it is easy to stably obtain an extruded foam sheet having a low basis weight and a predetermined apparent density and thickness.
For these reasons, the amount per kg of the foamable resin melt is preferably 0.20 mol or more, more preferably 0.30 mol or more, further preferably 0.40 mol or more, and particularly preferably 0.50 mol or more.
On the other hand, since the amount of the flammable foaming agent remaining in the extruded foam sheet can be further reduced, the upper limit of the amount of normal butane per kg of the foamable resin melt is preferably 2.0 mol, more preferably 1.6 mol, even more preferably 1.2 mol or less, and particularly preferably 1.0 mol.

Furthermore, in the physical foaming agent used in the present invention, the blending amount of the butane per 1 kg of the foamable resin melt must be 2 mol or less.
If the blending amount is too large, the amount of the flammable blowing agent (butane) remaining in the extruded foam sheet will be large, and a step of reducing the amount of the remaining flammable blowing agent may be required.
For these reasons, the amount per kg of the foamable resin melt is preferably 1.8 mol or less, more preferably 1.6 mol or less, even more preferably 1.2 mol or less, and particularly preferably 1.0 mol or less.
On the other hand, the lower limit of the amount of butane to be blended per kg of the foamable resin melt is preferably 0.1 mol, more preferably 0.2 mol, because this makes it easier to improve the extrusion foamability and take-up stability during extrusion and to stably obtain a desired extruded foam sheet.

The amount of isobutane per 1 kg of the foamable resin melt is preferably 0.2 mol or less, more preferably 0.1 mol or less, even more preferably 0.08 mol or less, and particularly preferably 0.06 mol or less. By setting the amount of isobutane in the above range, it becomes easier to reduce the amount of flammable foaming agent remaining in the extruded foam sheet.

The amount of dimethyl ether per kg of the foamable resin melt is preferably 0.5 mol or more, more preferably 0.6 mol or more, and even more preferably 0.8 mol or more. By setting the amount of dimethyl ether in the above range, the amount of butane can be easily reduced while maintaining the extrusion foamability during extrusion, and therefore the amount of remaining combustible foaming agent can be easily reduced at an early stage.
On the other hand, the amount of dimethyl ether per kg of the foamable resin melt is preferably 3.5 mol or less, more preferably 3.0 mol or less, and even more preferably 2.5 mol% or less. By setting the amount of dimethyl ether in the above range, the amount of the flammable blowing agent remaining in the extruded foam sheet can be further reduced.

Furthermore, the blending ratio of dimethyl ether and the blending ratio of butane in the mixed blowing agent must each be within a specific range. Specifically, the blending ratio of dimethyl ether in 100 mol% of the mixed blowing agent is 30 mol% or more and 95 mol% or less, and the blending ratio of butane is 5 mol% or more and 70 mol% or less (however, the sum of the blending ratio of dimethyl ether and the blending ratio of butane is 100 mol%).

Dimethyl ether has a significantly higher gas permeation rate for polyethylene resins than butane. Therefore, dimethyl ether dissipates from the polyethylene resin constituting the foamable resin melt extruded from the extruder while the resin melt is foaming. Therefore, if the blending ratio of dimethyl ether is too high, the foaming efficiency is deteriorated, and it becomes difficult to produce an extruded foam sheet having a low basis weight and a desired apparent density. In addition, although the polyethylene resin is plasticized by the addition of dimethyl ether, when the polyethylene resin is taken up, most of the dimethyl ether dissipates from the extruded foam sheet, and the plasticizing effect of the polyethylene resin by dimethyl ether is reduced. Therefore, if the blending ratio of dimethyl ether is too high, it becomes difficult to take up the extruded foam sheet at a high speed during production, and it becomes difficult to produce an extruded foam sheet having a low basis weight.
On the other hand, butane, particularly butane containing a high ratio of isobutane in butane, has a property of being more likely to remain in the extruded foam sheet after extrusion than dimethyl ether. Therefore, if the blending ratio of butane is too high or the ratio of normal butane in butane is too low, the amount of the flammable blowing agent remaining in the extruded foam sheet may be increased.

For these reasons, the blending ratio of dimethyl ether in 100 mol% of the mixed blowing agent is preferably 40 mol% or more and 95 mol% or less, and the blending ratio of butane is preferably 5 mol% or more and 60 mol% or less (however, the total blending ratio of dimethyl ether and butane is 100 mol%), the blending ratio of dimethyl ether in 100 mol% of the mixed blowing agent is more preferably more than 50 mol% and 95 mol% or less, and the blending ratio of butane is more preferably 5 mol% or more and less than 50 mol% (however, the total blending ratio of dimethyl ether and butane is 100 mol%), the blending ratio of dimethyl ether in 100 mol% of the mixed blowing agent is even more preferably 55 mol% or more and 90 mol% or less, and the blending ratio of butane is more preferably 10 mol% or more and 45 mol% or less (however, the total blending ratio of dimethyl ether and butane is 100 mol%).
In addition, since the amount of the flammable blowing agent remaining in the extruded foam sheet is more likely to be reduced, the blending ratio of isobutane in 100 mol % of the mixed blowing agent is preferably 10 mol % or less (however, the sum of the blending ratio of dimethyl ether and the blending ratio of butane is 100 mol %), preferably 5 mol % or less, more preferably 3 mol % or less, and particularly preferably 2 mol % or less.

In the present invention, the blending amount of the mixed blowing agent per 1 kg of the foamable resin melt must be 1 mol or more and 4 mol or less.
If the blending amount is too large, the amount of the flammable blowing agent remaining in the obtained extruded foam sheet will be large, and a step of reducing the amount of the remaining flammable blowing agent will be required. For these reasons, the upper limit of the blending amount is preferably 3.5 mol, more preferably 3.0 mol, and even more preferably 2.5 mol.
On the other hand, if the blending amount is too small, it becomes difficult to obtain an extruded foam sheet having a low basis weight, a desired low apparent density, and a thin thickness. For these reasons, the lower limit of the blending amount is preferably 1.2 mol, and more preferably 1.4 mol.

The mixed blowing agent may contain blowing agents other than dimethyl ether and butane, so long as the intended object of the present invention can be achieved. In this case, the blending ratio of the other blowing agents in the mixed blowing agent is preferably approximately 20 mol% or less, more preferably 10 mol% or less, and even more preferably 5 mol% or less, relative to the total blending ratio of dimethyl ether and butane (100 mol%). However, it is particularly preferable that the mixed blowing agent does not contain any blowing agents other than dimethyl ether and butane.

Other blowing agents include inorganic physical blowing agents such as carbon dioxide and nitrogen, and organic physical blowing agents such as aliphatic hydrocarbons such as normal pentane and isopentane, hydrofluoroolefins such as 1-chloro-3,3,3-trifluoropropene (HCFO-1233zd) and 1,3,3,3-tetrafluoropropene (HFO-1234ze), and alcohols having 1 to 4 carbon atoms such as ethanol and propanol. Two or more of these may be used in combination.

In the method for producing a polyethylene resin foam sheet of the present invention, in addition to the polyethylene resin and physical foaming agent described above, additives can be added within a range that does not impair the effects of the present invention.

Additives include, for example, cell regulators, antioxidants, heat stabilizers, weather resistance agents, UV absorbers, flame retardants, inorganic fillers, antibacterial agents, colorants, and antistatic agents.

Both inorganic and organic foam regulators can be used. Examples of inorganic foam regulators include metal borate salts such as zinc borate, magnesium borate, and borax, sodium chloride, aluminum hydroxide, talc, zeolite, silica, calcium carbonate, and sodium bicarbonate.

Examples of organic foam regulators include sodium 2,2-methylenebis(4,6-tert-butylphenyl)phosphate, sodium benzoate, calcium benzoate, aluminum benzoate, and sodium stearate. Combinations of citric acid and sodium bicarbonate, and alkali salts of citric acid and sodium bicarbonate, etc., can also be used. Two or more of these foam regulators can be mixed and used.

The amount of the bubble regulator added can be set appropriately depending on the number of bubbles to be formed in the extruded foam sheet, but is usually in the range of 0.05 parts by mass to 10 parts by mass, and preferably 0.2 parts by mass to 5 parts by mass, per 100 parts by mass of the polyethylene resin.

As the antistatic agent, for example, a polymer-type antistatic agent having a surface resistivity of less than 1× ¹⁰ Ω can be used. Examples of the polymer-type antistatic agent include polyether, polyether ester amide, block copolymer of polyether and polyolefin, ionomer resin, etc. Among these, block copolymer of polyether and polyolefin and ionomer resin are preferred, and ionomer resin is more preferred.
The block copolymer may have a structure in which polyolefin blocks and polyether blocks are repeatedly and alternately bonded via bonds such as ester bonds, amide bonds, ether bonds, urethane bonds, and imide bonds.
The ionomer resin is a resin in which the molecules of a copolymer of ethylene and an unsaturated carboxylic acid are intermolecularly crosslinked with metal ions. Examples of the unsaturated carboxylic acid include acrylic acid and methacrylic acid. Examples of the metal ions include lithium, sodium, potassium, and calcium.

Specific examples of such polymeric antistatic agents include block copolymers of polyether and polyolefin, such as "Pelestat 300," "Pelectron HS," and "Pelectron LMP" manufactured by Sanyo Chemical Industries, Ltd., and ionomer resins, such as "Entira SD100" and "Entira MK400" manufactured by DuPont-Mitsui Polychemicals Co., Ltd.

When the antistatic agent is blended, the blending ratio of the antistatic agent in the resin melt is preferably 5 to 50% by weight, based on 100% by weight of the total of the polyethylene resin and the antistatic agent. By setting the blending ratio in this range, an extrusion foam sheet can be obtained that can stably exhibit the desired antistatic performance. In order to exhibit better antistatic properties, the lower limit of the blending ratio is more preferably 6% by weight, and even more preferably 7% by weight. In addition, in order to improve the foaming properties during extrusion foaming, the upper limit of the blending ratio is preferably 40% by weight, more preferably 30% by weight, even more preferably 20% by weight, and particularly preferably 15% by weight.

Next, the physical properties of the extruded foam sheet obtained by the method of the present invention will be described.
The basis weight of the extruded foam sheet obtained by the production method of the present invention is 10 g/m2 or ^more and 80 g/ ^m2 or less. If the basis weight is within this range, an extruded foam sheet can be obtained that is lightweight and has excellent cushioning properties and can be suitably used as a packaging material, a glass plate interleaf inserted between glass substrates, etc. From the viewpoint of improving the cushioning properties, the basis weight is preferably 12 g/ ^m2 , more preferably 15 g/ ^m2 or more, and even more preferably 20 g/ ^m2 or more. Also, from the viewpoint of improving the light weight, the basis weight is preferably 50 g/ ^m2 or less, more preferably 40 g/ ^m2 or less, even more preferably 35 g/ ^m2 or less, and particularly preferably 30 g/m2 or ^less .

The basis weight can be measured as follows: First, the extruded foam sheet is cut into a predetermined size (e.g., 100 mm x 100 mm) to prepare a test piece, and the area ( ^m2 ) and mass (g) of the test piece are measured, and the mass (g) is divided by the area ( ^m2 ) to determine the basis weight (g/ ^m2 ) of the extruded foam sheet.

The apparent density of the extruded foam sheet is preferably 25 kg/ ^m3 or more and 300 kg/ ^m3 or less. When the apparent density is within this range, an extruded foam sheet can be obtained which is lightweight and has excellent cushioning properties, and can be suitably used as a packaging material, a glass plate inserting sheet to be inserted between glass substrates, and the like.
From the viewpoint of improving the cushioning property, the apparent density is preferably 30 kg/m ³ or more, more preferably 50 kg/m ³ or more, and further preferably 60 kg/m ³ or more.
From the viewpoint of improving the light weight, the apparent density is preferably 250 kg/ ^m3 or less, more preferably 200 kg/ ^m3 or less, and further preferably 180 kg/ ^m3 or less.

The apparent density of the foam sheet can be calculated as follows. First, the average thickness of the extruded foam sheet is measured as described below. The basis weight of the extruded foam sheet is also measured as described above. The basis weight of the extruded foam sheet thus measured is divided by the average thickness of the extruded foam sheet thus measured, and the unit conversion is performed to determine the apparent density [kg/m ³ ] of the extruded foam sheet.

The average thickness of the extruded foam sheet is preferably 0.1 mm or more and 2 mm or less. If the average thickness is within this range, the extruded foam sheet will have excellent cushioning properties, and when used as an interleaving sheet for glass plates, the loading efficiency will be high when stacking and transporting glass plates. Therefore, the extruded foam sheet having an average thickness within the above range can be suitably used for various applications.
From the viewpoint of providing an extruded foam sheet with high cushioning properties, the average thickness is preferably 0.12 mm or more, more preferably 0.15 mm or more, and even more preferably 0.16 mm or more.
On the other hand, from the viewpoint of increasing the loading efficiency when used as an interleaf for glass plates, the average thickness is preferably 1 mm or less, more preferably 0.80 mm or less, even more preferably 0.60 mm or less, and particularly preferably 0.50 mm or less.

The average thickness can be measured as follows. For an extruded foam sheet, three or more randomly selected locations are selected along the extrusion direction, and the average thickness is calculated by taking the arithmetic mean of the thicknesses (mm) measured at equal intervals along the width direction across the entire width of the sheet. The average thickness of the extruded foam sheet can be calculated by taking the arithmetic mean of the average thicknesses calculated at the three or more locations.

In producing an extruded foam sheet having a desired basis weight, average thickness and apparent density, the desired extruded foam sheet can be obtained mainly by adjusting the take-up speed of the extruded foam sheet, the discharge amount of the foamable resin melt, the blow-up ratio (diameter expansion ratio) of the extruded foam sheet, and the like during the production of the extruded foam sheet.
Specifically, when producing an extruded foam sheet having a small basis weight, a low apparent density, and a thin average thickness, the blow-up ratio of the cylindrical foam extruded from the extruder is preferably 2.0 to 4.0. The take-up speed of the cylindrical foam extruded from the extruder is preferably 10 m/min to 80 m/min, more preferably 20 m/min to 75 m/min. Depending on the size of the extruder and the length in the width direction of the extruded foam sheet to be obtained, the discharge rate of the foamable resin melt extruded from the extruder is preferably 50 kg/hr to 300 kg/hr, more preferably 60 kg/hr to 260 kg/hr. The blow-up ratio (diameter expansion ratio) means the ratio of the diameter of the mandrel (widening device) to the diameter of the annular die lip (diameter of the mandrel/diameter of the annular die lip).

In the case where an extruded foam sheet having a low basis weight is obtained by increasing the take-up speed during production of the extruded foam sheet (for example, in the range of 10 m/min to 80 m/min) while keeping the discharge rate of the foamable resin melt and the blow-up ratio of the extruded foam sheet within the above-mentioned ranges, if the foaming agent dissipates from the foam too quickly, the resin extruded from the extruder becomes difficult to stretch when taken up, and the foam tends to break easily.
In the present invention, by using the above-mentioned foaming agent, during production of the extruded foam sheet, the proportion of the foaming agent that dissipates early from the extruded foam sheet can be increased while ensuring the elongation of the extruded resin and maintaining the take-up stability, and the amount of the foaming agent remaining in the extruded foam sheet can be reduced early.

The length of the extruded foam sheet in the width direction is preferably 1 m or more.
By setting the widthwise length within the above range, the extruded foam sheet becomes wide and can be suitably used for various applications such as insert sheets and packaging materials. Furthermore, by winding an extruded foam sheet having a widthwise length within the above range into a roll, a large number of extruded foam sheets can be efficiently wound, thereby improving productivity. Furthermore, the upper limit of the widthwise length of the extruded foam sheet is preferably about 5 m, and more preferably 4 m.
Furthermore, according to the present invention, even when a wide extruded foam sheet having a width direction length falling within the above-mentioned range is formed into a roll-like product immediately after production, an extruded foam sheet can be obtained in which the amount of foaming agent remaining in the entire roll-like product is small and the thickness variation from part to part of the roll-like product is suppressed.
The width direction of the extruded foam sheet is a direction perpendicular to the extrusion direction and thickness direction of the extruded foam sheet.
The length of the extruded foam sheet in the width direction is measured as follows: The length of the extruded foam sheet in the width direction is measured at three or more points randomly selected along the extrusion direction of the extruded foam sheet, and the length of the extruded foam sheet in the width direction is calculated by arithmetically averaging the measured values at the three or more points.

The extruded polyethylene resin foam sheet obtained by the production method of the present invention preferably has a closed cell ratio of 1% to 50%. If the closed cell ratio is within this range, the extruded foam sheet has a good cell structure and is excellent in cushioning properties and appearance.
In order to further reduce the amount of foaming agent remaining in the extruded foam sheet and to further suppress dimensional changes in the thickness and width direction length of the extruded foam sheet over time, the extruded foam sheet preferably has a closed cell ratio of 1% or more and 10% or less, more preferably 1% or more and 8% or less, and even more preferably 1% or more and 5% or less.

The closed cell ratio of the extruded foam sheet can be measured, for example, as follows. A test piece is cut out from the extruded foam sheet, and the true volume Vx of the test piece is measured in accordance with procedure C of ASTM-D2856-70, and the closed cell ratio S (%) is calculated using the following formula (1). For example, an air comparison specific gravity meter Model 930 manufactured by Toshiba Beckman Co., Ltd. can be used as the measuring device.

S (%) = (Vx-W/ρ) x 100/(Va-W/ρ)...(1)

In the above formula (1), Va, W, and ρ are as follows:
Va: apparent volume (cm ³ ) of the foamed sheet used in the measurement
W: Mass of foam sheet in test piece (g)
ρ: density of the resin constituting the extruded foam sheet (g/cm ³ )

The test piece for measuring the closed cell ratio can be prepared by stacking several cut-out extruded foam sheets so that the apparent volume is adjusted to about 20 cm ³ .

In the extruded foam sheet obtained by the method of the present invention, the smaller the amount of the flammable blowing agent remaining in the extruded foam sheet, the more preferable. Specifically, in the extruded foam sheet immediately after production, the amount of the flammable blowing agent remaining is preferably 0.005 mol/kg or less per 1 kg of the extruded foam sheet, more preferably 0.003 mol/kg or less, and even more preferably 0.002 mol/kg or less.

In addition, in the roll-like product 48 hours after production, the amount of the flammable foaming agent remaining in the extruded foam sheet is preferably 0.002 mol/kg or less, more preferably 0.001 mol/kg or less, per 1 kg of the extruded foam sheet.
The method for measuring the amount of the flammable blowing agent remaining in the extruded foam sheet will be described in the Examples below.

Next, a specific method for producing an extruded foam sheet in the method of the present invention, and an example of a method for producing a roll-shaped extruded foam sheet by winding the extruded foam sheet (hereinafter, also simply referred to as a roll-shaped product) will be described.
In the method of the present invention, a polyethylene resin, a cell regulator, and additives that are blended as necessary are fed to an extruder, heated and melted to form a resin melt, and then a physical foaming agent is pressed into the resin melt and further kneaded to form a foamable resin melt. Next, the foamable resin melt is adjusted to a foamable temperature (resin temperature) in the extruder, introduced into an annular die, and extruded from the lip at the tip of the die into the atmosphere in a low pressure region, and the foamable resin melt is foamed to form a cylindrical foam. This cylindrical foam is expanded (blow-up) by a cylindrical widening device (mandrel), and cut open along the extrusion direction while being taken up by a take-up machine to obtain an extruded foam sheet.

FIG. 1 is a diagram showing an example of the method for producing an extruded foam sheet of the present invention.
First, a polyethylene resin is fed to an extruder 1 through a feed port 2. At this time, other components such as a bubble adjusting agent are appropriately added as necessary. A physical foaming agent is injected into the extruder 1 through a foaming agent injection port 3. In the extruder 1, the polyethylene resin is melted and kneaded with the physical foaming agent and other components to form a polyethylene resin melt.

Then, the cylindrical foam 7 formed by extruding the polyethylene resin melt from the annular die 4 attached to the tip of the extruder 1 is taken up by a take-up machine (not shown) and passed over a mandrel 5 arranged downstream of the annular die 4. The extruded foam is then cut open by a cutting means such as a cutter blade 6 attached to the mandrel 5, to obtain a sheet-like extruded foam sheet 8.

The sheet-like extruded foam sheet 8 is wound up by a take-up machine (not shown) to form a roll of extruded foam sheet. The roll is stored and/or transported in this state. When in use, the roll is unwound, and the extruded foam sheet is cut to a specified size, for example, and used for various purposes, including as interleaving paper for glass plates.

In the method of the present invention, a roll-shaped extruded foam sheet (roll-shaped product) is produced as follows.
The method for producing a roll of extruded foam sheet having a basis weight of 10 g/m2 or more and 80 g/ ^m2 or ^less by extruding and foaming a foamable resin molten material obtained by kneading the polyethylene-based resin and a physical foaming agent, and winding the extruded foam sheet into a roll, comprises:
the physical blowing agent is a mixed blowing agent of dimethyl ether and butane,
The proportion of normal butane in the butane exceeds 80 mol%,
the blending amount of the mixed blowing agent per 1 kg of the foamable resin melt is 1 mol or more and 4 mol or less,
the amount of butane per 1 kg of the foamable resin melt is 2 mol or less,
The mixed blowing agent has a blending ratio of dimethyl ether of 30 mol % or more and 95 mol % or less, and a blending ratio of butane of 5 mol % or more and 70 mol % or less (however, the sum of the blending ratio of dimethyl ether and the blending ratio of butane is 100 mol %).

For an explanation of the extruded foam sheet and the foaming agent in the manufacturing method of the roll-shaped product, please refer to the explanation of the extruded foam sheet and the foaming agent in the manufacturing method of the extruded foam sheet described above.

The length in the width direction of the roll-shaped product obtained by the method of the present invention is preferably 1 m or more since it is produced by rolling up the extruded foam sheet described above.
In the present invention, even when a wide extruded foam sheet having a length in the width direction within the above-mentioned range is formed into a roll-like product immediately after production, a roll-like product can be obtained in which the amount of residual foaming agent is small throughout the entire roll-like product and variations in thickness, width, etc. of the extruded foam sheet depending on the part of the roll-like product are suppressed.

The total length of the extruded foam sheet forming the roll-shaped product is preferably 100 m or more and 2000 m or less, and the diameter of the roll-shaped product is preferably 200 mm or more and 2000 mm or less. As the core used for the roll-shaped product, a core having a diameter (outer diameter) of 5 cm or more and 20 cm or less, more preferably 6 cm or more and 16 cm or less can be preferably used. As the core, a paper tube can be preferably used.

The roll-shaped product is obtained by winding the extruded foam sheet into a roll and has a structure in which a large number of extruded foam sheets are stacked on top of each other, and therefore the flammable foaming agent tends to remain more easily in the radial center of the roll-shaped product (i.e., a portion located near the middle between the core tube side of the roll-shaped product and the outermost surface side of the roll-shaped product, hereinafter also referred to as "mid-winding").
On the other hand, in the present invention, by using the above-mentioned physical foaming agent, the amount of the flammable foaming agent remaining in the extruded foam sheet before being wound into a roll can be reduced compared to the case where the foamable resin melt is foamed using only butane. Therefore, the amount of the flammable foaming agent remaining in the roll can be reduced to a desired amount or less immediately after the extruded foam sheet is wound into a roll or by curing for a short period of time.

In addition, when the extruded foam sheet is wound into a roll, the physical foaming agent remaining in the extruded foam sheet gradually dissipates to the outside over time, while the surrounding air gradually flows into the extruded foam sheet, so that the dimensions of the thickness, width, etc. of the extruded foam sheet wound into a roll are usually prone to fluctuate over time for each part. In addition, since the extruded foam sheet shrinks due to the dissipation of the physical foaming agent, it is usually necessary to cure the extruded foam sheet so that air can flow sufficiently into the extruded foam sheet to restore the dimensions of the shrunken extruded foam sheet, such as the thickness and width. In addition, the radial center of the roll-shaped material tends to be less prone to air flow than the surface area of the roll-shaped material (hereinafter also referred to as the "outside of the roll"). Therefore, in the past, the extruded foam sheet was loosely wound around a core to allow sufficient air to flow into the extruded foam sheet, and the dimensions of the extruded foam sheet were restored, and then the extruded foam sheet was wound into a roll to form a roll-shaped material, otherwise the dimensions of the extruded foam sheet were likely to fluctuate greatly between parts of the roll-shaped material. If such dimensional variations occur, the appearance of the rolled product may deteriorate, and the rolled product may lose its wound state or become disintegrated during storage or transportation.
On the other hand, in the present invention, an extruded foam sheet having a small basis weight is produced using the above-mentioned physical foaming agent, so that most of the physical foaming agent dissipates from the extruded foam sheet immediately after being extruded from the extruder (before the extruded foam sheet is wound into a roll).
It is believed that the dimensions of the extruded foam sheet, such as thickness and width, of the roll-shaped product obtained by taking up and winding such an extruded foam sheet are within a certain narrow range at the stage of winding, and that although air does enter the extruded foam sheet over time, the dimensions of the extruded foam sheet are unlikely to vary over time. Therefore, even if the extruded foam sheet is wound into a roll immediately after production of the extruded foam sheet, it is believed that an extruded foam sheet can be obtained in which dimensional variation between parts of the roll-shaped product is suppressed, regardless of the part of the roll.

Although the method for producing a single-layer polyethylene-based resin extruded foam sheet by extruding and foaming a foamable resin melt obtained by kneading a polyethylene-based resin with a physical foaming agent has been described above, a multi-layer extruded foam sheet can also be produced. Specifically, a resin melt for forming a resin layer obtained by kneading a polyethylene-based resin with a foamable resin melt obtained by kneading a polyethylene-based resin with a physical foaming agent is laminated in a co-extrusion die, and the foamable resin melt is co-extruded together with the resin melt for forming a resin layer to extrude and foam the foamable resin melt, thereby producing a multi-layer extruded polyethylene-based resin foam sheet having a polyethylene-based resin foam layer and a polyethylene-based resin layer laminated and bonded to the foam layer.
In this case, the resin melt for forming the resin layer is preferably a resin melt obtained by kneading a polyethylene resin and a volatile plasticizer. As the volatile plasticizer, it is preferable to use a hydrocarbon having 3 to 6 carbon atoms and/or an alcohol having 2 carbon atoms, more preferably butane, and even more preferably butane having a ratio of normal butane exceeding 80 mol%. In addition, the ratio of butane in the volatile plasticizer is preferably 50 mol% or more, more preferably 80 mol% or more. By using the volatile plasticizer, even when the mixed foaming agent is used, it is possible to suppress the occurrence of excessive corrugation in the extruded foam sheet, and it is possible to more stably obtain an extruded foam sheet having a good appearance and a desired basis weight. From the viewpoint of reducing the amount of flammable foaming agent remaining in the extruded foam sheet while increasing the take-up stability of the extruded foam sheet, it is more preferable that the ratio of the amount of volatile plasticizer per kg of the resin melt for forming the resin layer to the amount of dimethyl ether per kg of the foamable resin melt is 2 or more and 10 or less.
From the viewpoint of obtaining an extruded foam sheet having antistatic properties, the resin molten material for forming a resin layer preferably contains the above-mentioned polymeric antistatic agent. In this case, the blending ratio of the polymeric antistatic agent in the resin molten material for forming a resin layer is preferably 5 to 50% by weight, more preferably 8 to 30% by weight, based on 100% by weight of the total of the polyethylene resin and the polymeric antistatic agent.

As an apparatus for manufacturing a multi-layer extruded foam sheet, for example, a co-extrusion annular die is attached to the downstream side of an extruder for forming a foam layer, and the downstream side of an extruder for forming a resin layer is connected to the co-extrusion annular die. By co-extrusion using an apparatus, a cylindrical foam sheet having a foam layer and a resin layer can be obtained, and the cylindrical foam sheet can be produced by cutting it open as described above. The resin layer may be in a non-foamed state or a foamed state.

The resin constituting the foam layer or the resin layer may be the polyethylene resin used in the production of a single-layer polyethylene resin extruded foam sheet. The various physical properties of the multi-layer extruded foam sheet (average thickness, basis weight, apparent density, and individual cell content) can be determined by measuring the various physical properties of the multi-layer extruded foam sheet in the same manner as the measurements of the various physical properties of the extruded foam sheet.
In the multi-layer extruded foam sheet, the resin layer may be laminated on only one side of the foam layer, or on both sides of the foam layer.

In order to increase stiffness while maintaining the cushioning properties, the basis weight of the resin layer in the multi-layer extruded foam sheet is preferably 1 g/m2 or ^more and 20 g/ ^m2 or less, and more preferably 2 g/m2 or ^more and 10 g/ ^m2 or less, per side.

The basis weight of the resin layer can be calculated by multiplying the thickness of the resin layer by the density of the resin that constitutes the resin layer and converting the basis weight into units, or by allocating the basis weight of the entire multi-layer foam sheet based on the ratio of the discharge amounts of the molten resin for forming the foam layer and the molten resin for forming the resin layer.

The extruded foam sheet obtained by the method for producing a polyethylene resin foam sheet of the present invention has a small amount of residual flammable foaming agent and is highly safe during transportation, and is suitable for use as inserts for plate-like objects such as glass plates for liquid crystal panels, packaging materials, cushioning materials, and other packaging materials.

The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

A tandem extruder was used, equipped with a first extruder with a barrel inner diameter of 90 mm and a second extruder with a barrel inner diameter of 120 mm connected downstream of the first extruder. A circular die (lip diameter 94 mm) was attached to the outlet of the second extruder, and a mandrel with a diameter of 350 mm was placed downstream of the circular die. The mandrel used was equipped with a cutter blade for cutting the cylindrical foam.

The following raw materials were used in the examples and comparative examples.
(Polyethylene resin)
(1) Polyethylene resin (LDPE): Low density polyethylene, manufactured by NUC Corporation, NUC-8009, MFR (190°C, load 2.16 kg) 9.0 g/10 min, melt tension at 190°C 20 mN, melting point 107°C)

MFR and melt tension were measured using the methods described above.

(Physical foaming agent)
(1) Normal butane: manufactured by Koike Chemical Co., Ltd. (2) Mixed butane (mixture of 70 mol% normal butane and 30 mol% isobutane): manufactured by Koike Chemical Co., Ltd. (3) Dimethyl ether: manufactured by Koike Chemical Co., Ltd.

(Foam Regulator)
"Chemical foaming agent: Product name FineCell Master PO217K" manufactured by Dainichiseika Color & Chemicals Co., Ltd.

Examples 1 to 3, Comparative Examples 1 to 4
100 parts by weight of a polyethylene resin "LDPE" and 0.05 parts by weight of the cell regulator were fed to the first extruder, heated, melted and kneaded to form a resin melt, and then a physical foaming agent of the type and amount shown in Table 1 was injected into the resin melt, heated and kneaded, and the temperature was adjusted to about 200°C to form a foamable resin melt. The foamable resin melt was transferred to a second extruder with a diameter of 120 mm connected to the downstream side of the first extruder, and cooled to form a foamable resin melt with a resin temperature of 111°C.
Table 1 shows the type and amount of foaming agent used in each of the examples and comparative examples.

The foamable resin melt was extruded from the die lip of the annular die at a discharge rate of 80 kg/hr into the atmosphere and foamed to form a tubular foam. The tubular foam was widened by a tubular widening device (mandrel) with a diameter of 350 mm at a widening ratio of 3.8, and taken up by a take-up machine at a take-up speed of 52 m/min to have a basis weight shown in Table 2. The tubular foam was then cut open along the extrusion direction to obtain an extruded polyethylene resin foam sheet with a width of about 1090 mm. Next, the extruded foam sheet was wound up using a core tube (outer diameter 80 mm) to obtain a roll-shaped product with a length of 250 m in the extrusion direction and a diameter of about 250 mm.
The obtained extruded foam sheet was cured for 72 hours under the conditions of 40° C., atmospheric pressure, and free state. Thereafter, the average thickness (average thickness after curing), width direction length (width direction length after curing), basis weight, apparent density, and closed cell ratio were measured. The results are shown in Table 2.

The residual amount of the foaming agent was measured for the extruded foam sheets obtained in Examples 1 and 3 and Comparative Examples 1, 2 and 4, for a sample taken immediately after production, a sample taken from within the rolled product after curing the rolled product for one day after production under conditions of 40° C. and atmospheric pressure, and a sample taken from within the rolled product after curing for two days after production. The results are shown in Table 3.
The resulting rolled product was aged at 40° C. under atmospheric pressure for 3 days after production, and the thicknesses of the extruded foam sheet (outside, inside and inside of the roll) were measured. The results are shown in Table 4.
The obtained roll-like product was aged under conditions of 40° C. and atmospheric pressure for 3 days after production, and then the lengths (outside, middle, and inside of the roll) in the width direction of the extruded foam sheet were measured. The results are shown in Table 5.

In Example 1, the mixing ratio of the physical foaming agents was normal butane/dimethyl ether=30 mol %/70 mol %, and an extruded foam sheet having a low basis weight, low thickness, and low apparent density was obtained.
Furthermore, the amount of remaining flammable foaming agent in the obtained extruded foam sheet and roll-shaped product was extremely small.
Furthermore, the obtained roll-like product had small differences between the maximum and minimum thicknesses (R values) of the extruded foam sheet at the outside, middle, and inside of the roll, and small differences between the maximum and minimum lengths (R values) of the extruded foam sheet in the width direction.

In Example 2, the mixing ratio of the physical foaming agents was normal butane/dimethyl ether = 40 mol%/60 mol%, and the mixing ratio of dimethyl ether was reduced compared to Example 1. An extruded foam sheet with low basis weight, low thickness, and low apparent density was obtained.

In Example 3, the mixing ratio of the physical foaming agents was normal butane/dimethyl ether = 70 mol %/30 mol %, and the mixing ratio of dimethyl ether was further reduced compared to Example 1. An extruded foam sheet with a low basis weight, low thickness and low apparent density was obtained.
Furthermore, the resulting extruded foam sheet and roll-shaped product contained a small amount of remaining flammable foaming agent.
Furthermore, the obtained roll-like product had small differences between the maximum and minimum thicknesses (R values) of the extruded foam sheet at the outside, middle, and inside of the roll, and small differences between the maximum and minimum lengths (R values) of the extruded foam sheet in the width direction.

Comparative Example 1 is an example in which only a mixture of normal butane and isobutane (mixed butane, normal butane/isobutane=70 mol%/30 mol%) was used as the physical foaming agent, and dimethyl ether was not blended. An extruded foam sheet with low basis weight, low thickness, and low apparent density was obtained. However, the amount of remaining flammable foaming agent in the obtained extruded foam sheet and roll-shaped product was larger than that in the examples.
Furthermore, the obtained roll-like product had a larger difference between the maximum and minimum thickness values (R value) of the extruded foam sheet at the outside, middle and inside of the roll, and a larger difference between the maximum and minimum length values (R value) of the extruded foam sheet in the width direction, compared to the Examples.

Comparative Example 2 is an example in which only normal butane was used as the physical foaming agent, and dimethyl ether was not blended. An extruded foam sheet having a low basis weight, low thickness, and low apparent density was obtained. However, the resulting extruded foam sheet and roll-shaped product had a large amount of remaining flammable foaming agent compared to the Examples.
Furthermore, the obtained roll-like product had a larger difference between the maximum and minimum thickness values (R value) of the extruded foam sheet at the outside, middle and inside of the roll, and a larger difference between the maximum and minimum length values (R value) of the extruded foam sheet in the width direction, compared to the Examples.

Comparative Example 3 is an example in which only dimethyl ether was used as the physical foaming agent. The extruded foam sheet frequently broke when the cylindrical body was removed, making it impossible to produce an extruded foam sheet.

Comparative Example 4 is an example in which a mixture of normal butane and isobutane (normal butane/isobutane = 70 mol%/30 mol%) was used as the physical foaming agent, and the blending ratio of dimethyl ether was the same as in Example 1. An extruded foam sheet with low basis weight, low thickness, and low apparent density was obtained. However, because the blending ratio of normal butane in butane was low, the obtained extruded foam sheet and roll-shaped product had a larger amount of remaining flammable foaming agent than the Examples.

In Table 2, the average thickness, length in the width direction, basis weight, apparent density, and closed cell ratio were measured as described above.
The average thickness was calculated by measuring the thickness of the extruded foamed sheet after curing for 3 days under conditions of 40° C. and atmospheric pressure at three points randomly selected along the extrusion direction, at intervals of 50 mm along the width direction, calculating the arithmetic mean value of the thickness at each point, and then arithmetically averaging these values.
The length in the width direction of the extruded foam sheet was measured at 10 points randomly selected along the extrusion direction after curing for 3 days under conditions of 40°C and atmospheric pressure, and the length in the width direction of the extruded foam sheet was calculated to the nearest 5 mm by arithmetically averaging the measured values at the 10 points.
The basis weight was determined by cutting the extruded foam sheet to a size of 100 mm x 100 mm to prepare a test piece, measuring the area ( ^m2 ) and mass (g) of the test piece, and dividing the mass (g) by the area ( ^m2 ).

In Table 3, the amount of the foaming agent remaining in the extruded foam sheet was measured as follows.
First, about 0.3 g of a measurement sample was taken from near the center in the width direction of the extruded foam sheet immediately after extrusion or from the extruded foam sheet in a roll after aging for a predetermined period under the conditions of 40° C. and atmospheric pressure, and sealed in a measurement glass bottle. Next, the amount of each blowing agent remaining in the extruded foam sheet was quantitatively analyzed by FID method using a headspace gas chromatograph "GC353B" manufactured by GL Science Co., Ltd.
In the measurement, a column "CP-PoraPLOTQ Model CP-7553" manufactured by VARIAN was used. The measurement sample in the glass bottle was melted at a temperature of 160°C to degas the foaming agent, and the foaming agent was then introduced into the column at a temperature of 50°C to separate each foaming agent component, after which each foaming agent was quantified using an FID detector.

In Table 4, the average thickness of the extruded foam sheet was measured as follows. First, three samples were taken from each of the outside of the roll (a position where the extruded foam sheet was unwound in the extrusion direction by about 15 m from the end of the roll), the middle of the roll (a middle position between the outside and the inside of the roll), and the inside of the roll (a position about 10 mm away from the outer surface of the core tube toward the radially outward side of the roll) of the cured roll, and the thickness (mm) of each sample was measured at 50 mm intervals along the width direction over the entire width by the above-mentioned method. The average thickness of the extruded foam sheet at each position was calculated by arithmetically averaging the measured values.
The outside of the roll refers to the area near the surface of the roll-shaped material, the middle of the roll refers to the area located near the middle between the outer surface of the core tube of the roll-shaped material and the outermost surface of the roll-shaped material, and the inside of the roll refers to the area located near the winding core of the roll-shaped material.

In Table 5, the length of the extruded foam sheet in the width direction was measured at 10 randomly selected locations from the outside, middle, and inside of the rolled material after curing. The length of the extruded foam sheet in the width direction at each location was calculated to the nearest 5 mm by arithmetically averaging the measured values.

Example 4
An extruded foam sheet was produced in the same manner as in Example 1, except that the amount of normal butane was 1.0 mol/kg, the amount of dimethyl ether was 2.3 mol/kg, the discharge rate of the foamable resin melt was 74 kg/hr, and the take-up speed was 48 m/min, and the extruded foam sheet was wound up to obtain a roll-shaped product. The extruded foam sheet had an average thickness of 0.83 mm, a basis weight of 26 g/ ^m2 , an apparent density of 31 kg/ ^m3 , and a closed cell ratio of 42%.

Comparative Example 5
Except for not using dimethyl ether, changing normal butane to mixed butane, and changing the amount of mixed butane to 3.33 mol/kg, an extruded foam sheet and a roll-shaped product were produced in the same manner as in Example 4. The resulting extruded foam sheet had an average thickness of 0.97 mm, a basis weight of 27 g/ ^m2 , an apparent density of 27 kg/ ^m3 , and a closed cell rate of 49%.

The total amount of residual foaming agent in the extruded foam sheet obtained in Example 4 immediately after extrusion was 0.19 mol/kg, and the total amount of residual foaming agent in the roll-like material after curing for 2 days after production under conditions of 40° C. and atmospheric pressure was 0.080 mol/kg. On the other hand, the total amount of residual foaming agent in the extruded foam sheet obtained in Comparative Example 5 immediately after production was 0.60 mol/kg, and the total amount of residual foaming agent in the roll-like material after curing for 2 days after production under conditions of 40° C. and atmospheric pressure was 0.27 mol/kg.
As described above, the extruded foam sheet obtained in Example 4 had physical properties equivalent to those of the extruded foam sheet obtained in Comparative Example 5, but the amount of the physical foaming agent remaining in the extruded foam sheet was significantly reduced.

Furthermore, the rolled products obtained in Example 4 and Comparative Example 5 were cured at 40° C. under atmospheric pressure for 3 days after production, and the average thickness and width direction length (outside, inside, and inside the roll) of the extruded foam sheet were measured to determine the difference between the maximum and minimum values of the average thickness (R value) and the difference between the maximum and minimum values of the length in the width direction (R value). In Example 4, the R value of the average thickness was 0.02 mm, and the R value of the length in the width direction was 15 mm, while in Comparative Example 5, the R value of the average thickness was 0.24 mm, and the R value of the length in the width direction was 45 mm.
From the above, it was found that the roll-shaped material obtained in Example 4 had less dimensional variation at each portion of the roll-shaped material than the roll-shaped material obtained in Comparative Example 5.

As Example 5, a multi-layer extruded foam sheet was produced as follows.
As a manufacturing apparatus, a tandem extruder was used in which two extruders, a first extruder with a diameter of 90 mm and a second extruder with a diameter of 120 mm, were connected in series as an extruder for forming a foam layer, and a third extruder with a diameter of 40 mm was used as an extruder for forming a resin layer, and an apparatus was used in which the outlet of the second extruder and the outlet of the third extruder were connected to a co-extrusion annular die. The co-extrusion annular die has a structure in which the resin melt for forming the resin layer is merged and laminated on the inside and outside of the foamable resin melt for forming a foam layer flowing in a cylindrical shape at the die middle part, and the diameter of the lip of the die outlet is 94 mm. In addition, a mandrel with a diameter of 350 mm was arranged downstream of the annular die. As the mandrel, one provided with a cutter blade for cutting a cylindrical foam was used.

100 parts by weight of polyethylene resin "LDPE" and 0.05 parts by weight of the cell regulator were fed to the first extruder, heated, melted and kneaded to form a resin melt, and then a physical foaming agent (a mixed foaming agent of normal butane and dimethyl ether, normal butane: dimethyl ether = 30 mol%: 70 mol%) was pressed into the resin melt, heated and kneaded, and the temperature was adjusted to about 200 ° C. to form a foamable resin melt for forming a foam layer. At this time, the amount of normal butane per 1 kg of the foamable resin melt was 0.58 mol/kg, and the amount of dimethyl ether was 1.36 mol/kg. The foamable resin melt was transferred to a second extruder with a diameter of 120 mm connected to the downstream side of the first extruder, and cooled to form a foamable resin melt with a resin temperature of 111 ° C.
On the other hand, 85 parts by weight of a polyethylene resin "LDPE" and 15 parts by weight of a polymeric antistatic agent ("Pelectron LMP" manufactured by Sanyo Chemical Industries, Ltd.) were fed to the raw material inlet of the third extruder, and were heated, melted, and kneaded to obtain a resin melt. Next, normal butane was injected as a volatile plasticizer into the resin melt, and after further kneading, the resin temperature was adjusted to about 120°C to obtain a resin melt for forming a resin layer. At this time, the amount of normal butane blended per 1 kg of the resin melt for forming a resin layer was 2.9 mol/kg.

The foamable resin melt and the resin layer forming resin melt were introduced into a co-extrusion annular die, and the resin layer forming resin melt was laminated on the inside and outside of the cylindrically flowing foamable resin melt, and co-extruded into a cylindrical shape from the annular die to form a cylindrical laminated foam body having resin layers laminated on the inside and outside surfaces of the cylindrical foam layer. At this time, the discharge rate of the foamable resin melt was 67.2 kg/hr, and the discharge rate of the resin layer forming resin melt was 12.8 kg/hr.
The cylindrical laminated foam was expanded at a width expansion ratio of 3.8 by a cylindrical expanding device (mandrel) having a diameter of 350 mm, while being taken up by a take-up machine at a take-up speed of 52 m/min, and the cylindrical laminated foam was further cut open along the extrusion direction to obtain a multi-layered extruded polyethylene resin foam sheet having a width of about 1090 mm and a polyethylene resin layer laminated and bonded to both sides of the foam layer. Next, the obtained extruded foam sheet was wound up using a core tube (outer diameter 80 mm) to obtain a roll-shaped product having a foam sheet length of 250 m in the extrusion direction and a diameter of about 250 mm.

The extruded foam sheet obtained in Example 5 had an average thickness of 0.22 mm, a basis weight of the extruded foam sheet (basis weight of the entire extruded foam sheet) of 25 g/ ^m2 , a basis weight of the polyethylene resin layer per side of the extruded foam sheet of 2 g/ ^m2 , an apparent density of the extruded foam sheet of 114 kg/ ^m3 , and a closed cell ratio of the extruded foam sheet of 1%.
The total amount of the remaining foaming agent in the extruded foam sheet immediately after extrusion was 0.0004 mol/kg.
The roll-like product obtained in Example 5 was aged at 40° C. under atmospheric pressure for 3 days after production, and the average thickness and width direction length (outside, inside, and inside of the roll) of the extruded foam sheet were measured to determine the difference (R value) between the maximum and minimum values of the average thickness and the difference (R value) between the maximum and minimum values of the length in the width direction. The R value of the average thickness was 0.01 mm, and the R value of the length in the width direction was 5 mm.

Reference Signs List 1 Extruder 2 Feed port 3 Foaming agent injection port 4 Annular die 5 Mandrel 6 Cutter blade 7 Cylindrical foam body 8 Extruded polyethylene resin foam sheet

Claims (4)

A method for producing an extruded polyethylene resin foam sheet having a basis weight of 10 g/m2 or ^more and 80 g/m2 or ^less by extruding and foaming a foamable resin melt obtained by kneading a polyethylene resin with a physical foaming agent, comprising the steps of:
the physical blowing agent is a mixed blowing agent of dimethyl ether and butane,
The proportion of normal butane in the butane exceeds 80 mol%,
the blending amount of the mixed blowing agent per 1 kg of the foamable resin melt is 1 mol or more and 4 mol or less,
the amount of butane per 1 kg of the foamable resin melt is 0.1 mol or more and 2 mol or less,
The mixed blowing agent contains dimethyl ether in an amount of 30 mol % or more and 95 mol % or less, and contains butane in an amount of 5 mol % or more and 70 mol % or less (provided that the sum of the mixed blowing agent amounts to 100 mol %).
2. The method for producing an extruded polyethylene resin foam sheet according to claim 1, wherein the amount of normal butane blended per 1 kg of the foamable resin melt is 0.10 mol or more.
3. The method for producing an extruded polyethylene resin foam sheet according to claim 1 or 2, wherein the extruded polyethylene resin foam sheet has an average thickness of 0.1 mm or more and 2 mm or less and an apparent density of 25 kg/ ^m3 or more and 300 kg/m3 or ^less .
The method for producing an extruded polyethylene resin foam sheet according to any one of claims 1 to 3, wherein the extruded polyethylene resin foam sheet has a length in a width direction of 1 m or more.