JP2024158931A - Chemically crosslinked extruded polyethylene resin foam and heat insulating resin sheet using same - Google Patents

Abstract

The present invention provides a polyethylene resin having a novel cellular structure, and by obtaining this cellular structure, a reduction in thermal conductivity is achieved.
[Solution] A chemically crosslinked, extruded polyethylene-based resin foam having a cell structure in which cells having a cell diameter of 100 μm or less are randomly accumulated and dispersed in the form of clusters among a large number of other cells having larger cell diameters, where the maximum cell diameter of each bubble in the thickness direction in the foam is defined as the cell diameter of each bubble, and the chemically crosslinked, extruded polyethylene-based resin foam has a cell structure that satisfies a cell size distribution in which cells having a cell diameter of 100 μm or less account for 50% or more of all cells, cells having a cell diameter of 400 μm or more account for 20% or less of all cells, and the extruded foam has an average cell diameter of 200 μm or less.
[Selected Figure] Figure 1

Description

Application for application of Article 30, Paragraph 2 of the Patent Act has been filed. [Publication address] https://www.furukawa.co.jp/release/2022/fun_20221031. html [Posting date] October 31, 2022 [Distribution] Presentation materials for "Introduction to high insulation foam" [Distribution destination] National Research and Development Agency, Public Works Research Institute, Cold Region Civil Engineering Research Institute [Distribution date] May 10, 2022 [Distribution] Presentation materials for "Introduction to high insulation foam" [Distribution destination] Ando-Hazama Co., Ltd. [Distribution date] May 17, 2022 [Distribution] Presentation materials for "Introduction to polyethylene foam" [Distribution destination] Nakano Kagaku Co., Ltd. [Distribution date] May 23, 2022 [Distribution] Presentation materials for "Introduction to high insulation foam" [Distribution destination] Yoshida Sangyo Co., Ltd. [Distribution date] August 18, 2022 [Distribution] Presentation materials for "Introduction to polyethylene foam" [Distribution destination] Hashizume Shoji Co., Ltd. [Distribution date] August 19, 2022 [Distribution] Presentation materials for "Introduction to high insulation foam" [Distribution destination] Totetsu Kogyo Co., Ltd., etc. [Distribution date] August 23, 2022, etc. [Distribution material] Presentation materials for "Introduction to high insulation foam" [Distribution recipient] Shimizu Corporation, etc. [Distribution date] February 22, 2023, etc. [Distribution material] Presentation materials for "Introduction to Furukawa foam products" [Distribution recipient] Sekisui House, Ltd. [Distribution date] January 20, 2023 [Distribution material] Presentation materials for "Introduction to Furukawa foam products" [Distribution recipient] Shibata Kogyo Co., Ltd. [Distribution date] February 7, 2023

The present invention relates to a chemically crosslinked extruded polyethylene resin foam and a thermal insulation resin sheet using the same.

There are several known methods for producing polyethylene sheet foams, excluding the method for producing foams having three-dimensional shapes such as injection foaming. Specifically, the methods for producing sheet-shaped polyethylene resin foams include: 1) a method for producing a non-crosslinked extruded foam in which gas is injected into molten resin while it is not crosslinked to produce an extruded foam; 2) a method for producing a crosslinked foam in which the resin to be foamed is crosslinked with electron beams to prevent bubbles from breaking during the high-temperature foaming process of the extruded material; 3) a method for producing a crosslinked foam in which the resin to be foamed is chemically crosslinked to prevent bubbles from breaking during the high-temperature foaming process of the extruded material; and 4) a method for producing a micro-foamed resin in which gas is infiltrated from the outside of a preformed resin sheet by batch foaming to create a fine bubble structure.

Here, in the manufacturing method of foaming the above-mentioned polyethylene without crosslinking, the cell walls break during foaming due to the high temperature strength of the resin, and so foams with an expansion ratio of 3 to 4 times can be produced, but there are problems in obtaining foams with a high expansion ratio. To produce foams with an even higher expansion ratio, it is necessary to crosslink the polyethylene resin in order to improve the resin strength. In the case of polyethylene resin foams, foams have been produced by either the chemical crosslinking foaming method, in which chemical crosslinking is performed during extrusion to cause foaming, or the electron beam crosslinking foaming method, in which the base resin is crosslinked by irradiating it with an electron beam and then foamed.

The electron beam crosslinking method for producing polyethylene resin foam allows, for example, the continuous production of long sheets of polyethylene resin by incorporating an electron beam irradiator into the production line, and has the advantage that the crosslinking state is easier to control compared to chemical crosslinking.

Instead of the electron beam crosslinking extrusion method, which uses electron beam crosslinking to increase the strength of the resin in a molten state, there is the extrusion chemical crosslinking foaming method, which improves the resin strength by chemical crosslinking and increases the high-temperature strength of the resin during the foaming process after extrusion, thereby preventing cell breakage during the foaming process and enabling the stable production of high-magnification polyethylene extrusion foams.

In addition to the extrusion foaming method in which foaming is performed by the above three different processes, there is also a foaming molding method in which a preformed resin sheet is immersed in a supercritical fluid (supercritical carbon dioxide) and impregnated until it is saturated, or is once removed and then pressure is released or heated to generate fine bubbles, thereby producing a microfoamed resin by batch foaming.

When foaming is performed by releasing pressure, the pressure is rapidly reduced while maintaining the plastic above its glass transition temperature (Tg) in the autoclave. When foaming is performed by raising the temperature, the plastic is first cooled to below the Tg in the autoclave, and the gas-impregnated plastic is removed and then rapidly heated. The features of this method are that a large amount of physical foaming agent (gas) is dissolved to generate a large number of bubbles, and because foaming occurs near the Tg, coarsening of the bubbles is avoided, resulting in fine bubbles. When performing pressurized batch foaming, when foaming a high-strength resin such as PET resin at a low expansion ratio, it is possible to obtain a uniformly fine foam structure and an average bubble diameter of 10 μm or less.

In contrast, in the case of extruded polyethylene resin foams, which are expanded at a high rate of 20 to 30 times during extrusion, it is difficult to obtain a uniformly fine cell structure like that of microcellular resin due to the manufacturing method. Up until now, research and development of the cell structure of sheet-shaped resin foams has basically aimed to homogenize the size of the cells throughout the foam, or to control the cell structure in a portion of a given thickness of the sheet.

By controlling the temperature and reaction conditions of the crosslinking reaction and foaming process while the foam is held in the heating furnace after extrusion, it is possible to change the cell diameter of the extruded foam after production; however, it is difficult to obtain a uniformly fine cell structure in the extruded foam while maintaining a high expansion ratio.

In the present invention, the goal is to obtain an extruded foam with a fine cell structure from a resin foam with a relatively high expansion ratio. However, in the case of non-crosslinked extruded gas foam and electron beam crosslinked extruded foam, it is difficult to reduce the overall cell diameter. Due to the characteristics of the manufacturing process, it is possible to reduce the size of the cells from the surface of the foam to a certain thickness, but it has been confirmed that it is difficult to reduce the overall size of the cells.

Therefore, the inventors, after various studies, confirmed that it is possible to reduce the average bubble size and further reduce the thermal conductivity by providing a bubble structure in which bubble groups with a smaller bubble size than a specified size are randomly dispersed in a specified ratio among bubble groups with a larger bubble size in the bubble structure of the foam, and thus came to make the present invention. Specifically, the developed sheet-like foam has two types of bubble groups, a large bubble group and a small bubble group, and a polyethylene-based resin chemically crosslinked extruded foam having a new bubble structure was obtained, which has a bubble structure in which the small bubble group is randomly dispersed among the large bubble group. Furthermore, even when the bubble structure is changed according to the present invention, the mechanical properties are maintained at the same level as foams having a conventional bubble structure, as exemplified by the compression characteristics, and the thermal conductivity is reduced and the heat insulation is improved.

By obtaining a polyethylene extruded resin foam having a cell structure in which small cell diameter groups are randomly dispersed among large cell diameter groups as described above, it is possible to reduce the average cell diameter and, as a result, the thermal conductivity.
The feature of the present invention is that the fine bubbles are present either near the boundaries between relatively large bubbles or between the boundaries between the relatively large bubbles, and a plurality of small bubbles are accumulated to form random small clusters and are dispersed, and the thermal conductivity can be reduced even if the area ratio of the fine bubbles is small. In the present invention, hereinafter, a cluster refers to an accumulation of a plurality of bubbles.

Here, in the case of polyethylene cross-linked extruded resin foam, the raw material resin, chemical foaming agent, cross-linking agent, cross-linking aid, and thickener are melt-kneaded at low temperatures using a mixing roll during extrusion, and the resulting mixture is then extruded and cross-linked to form a foam. In order to obtain such a unique structure, the cross-linking phenomenon before foaming and the strength of the cell walls of the bubbles are particularly important. For example, by randomly dispersing the cross-linking aid and thickener through kneading, the bubble walls of the bubbles near the additives are indirectly reinforced by the presence of the additives, which are more thermally stable than the polyethylene resin, making it possible to suppress bubble growth and merging.

For example, it is believed that if trimethylolpropane trimethacrylate (a multifunctional ester) is used as a cross-linking aid, a hard film can be created due to its high number of functional groups, and if a PTFE-based modifier is used as a thickener, the PTFE is stable even after extrusion processing and has the effect of preventing the cell walls from breaking or deforming during foaming. Therefore, it is presumed that the growth of the bubbles near the parts where the PTFE is present is inhibited, resulting in a smaller bubble diameter.

In the present invention, the polyethylene resins covered by the invention include low-density polyethylene, medium-density polyethylene, high-density polyethylene, and polyethylene copolymers.

Patent Document 1 provides a foam, a foam sheet, and a manufacturing method thereof that are useful as a foam for insulating folded plates, which are excellent in heat insulation, moldability in a folded plate molding machine, and water retention performance for retaining condensation water, etc. Patent Document 1 describes a chemically crosslinked polyethylene resin foam, which is made of a low-density polyethylene (A) having a density of 0.930 g/cm ^{3 or less, a high-density polyethylene (B) having a density of 0.940 to 0.960 g/cm 3 ,} and a copolymer (C) of ethylene and at least one monomer selected from the group consisting of vinyl acetate, acrylic acid, and ethyl acrylate, in which the ratios of (A) to (B) + (C) are 60 to 80% by weight and 40 to 20% by weight, and the weight ratio of (B) to (C) is 0.3 to 1.0:1.0, and the resin composition is crosslinked and foamed to have an average cell diameter of 0.4 to 0.7 mm (400 to 700 μm).
The resin foam of Patent Document 1 is not a resin foam having a cell structure in which small-diameter cells are randomly dispersed in clusters at a predetermined ratio among cells having diameters larger than a predetermined size.

The invention of Patent Document 2 aims to control the degree of crosslinking in the sheet thickness direction in the production of electron beam crosslinked polyolefin resin foam. In the method for producing a polyolefin resin foam, a foamable resin sheet is irradiated with an electron beam to crosslink it and then heated to foam, the method is characterized in that the electron beam is irradiated at least twice, a first irradiation with a lower acceleration voltage and a second irradiation with a higher acceleration voltage, and a method for producing a polyolefin resin foam and a polyolefin resin foam produced by the method are described. In particular, Patent Document 2 describes that the first irradiation is performed at an acceleration voltage at which the penetration depth of the electron beam is about 1/6 to 1/3, especially about 1/4, of the thickness of the foamable resin sheet, and the second irradiation is performed at an acceleration voltage at which the penetration depth of the electron beam reaches the entire thickness of the foamable resin sheet, thereby making the average bubble diameter from the surface layer to 1 mm below 300 μm in the foam foamed by irradiating the electron beam at least twice. Thus, in the case of electron beam crosslinking, the degree of crosslinking of the foam can be adjusted by adjusting the dose of electron beam irradiation, so it is easier to control the bubble size than in the case of chemical crosslinking. The resin foam in Patent Document 2 describes an invention in which the bubble size is controlled up to a predetermined thickness from the surface of the foam, but it is not a resin foam having a bubble structure in which small bubble groups are randomly dispersed in clusters at a predetermined ratio among bubbles with a larger bubble size than a predetermined size, as in the present invention.

Patent Document 3 provides a foam excellent in thermoformability, which is made of a non-crosslinked polyethylene resin that can be recycled, an efficient manufacturing method thereof, and a molded product excellent in cushioning and flexibility. Patent Document 3 describes a sheet-like foam for thermoforming, which is manufactured by feeding a non-crosslinked polyethylene resin and a foaming agent to an extruder, melt-kneading the mixture, extruding the mixture into a cylindrical shape through a circular die to foam the mixture, and drawing the cylindrical foam along the outer periphery of a circular mandrel. This sheet-like foam is a non-crosslinked polyethylene resin foam that is wavy, has a maximum wave height of 1 to 10 mm, a wave number of 10 pieces/200 mm or less, a center line average roughness Ra of the foam surface of 9.0 μm or more, an average cell diameter of 0.2 to 2.5 mm (200 μm to 2500 μm), a density of 0.015 to 0.05 g/cm ³ , and a thickness of 0.7 to 20 mm. The resin foam of Patent Document 3 has a wide range of average bubble diameter, from 200 μm to 2500 μm, and is not intended for the purpose of controlling the bubble diameter, and is not a resin foam having a bubble structure in which small bubble groups are randomly dispersed in clusters at a predetermined ratio among bubbles having a diameter larger than a predetermined size, as in the present invention.

Patent Document 4 aims to provide a styrene-based resin foam board that has excellent heat insulation and flame retardancy, particularly in terms of heat insulation, and satisfies Class B Type 3 as specified in JIS A9511, and can be suitably used as a building insulation material, etc. Patent Document 4 discloses a styrene-based resin foam board manufactured by extrusion foaming using a foaming agent consisting of dimethyl ether, butane, and water, characterized in that the average bubble diameter in the thickness direction of the styrene-based resin foam board in the bubbles of at least one surface layer is 0.05 to 0.20 mm, and the average bubble diameter in the thickness direction of the styrene-based resin foam board in the bubbles of the central layer is 1.45 to 2.50 times the average bubble diameter in the thickness direction of the styrene-based resin foam board in the bubbles of the surface layer. According to Patent Document 4, when comparing the bubble diameters of the surface and center parts of a polystyrene foam produced by gas foaming, it is found that the resin foam has a bimodal bubble size distribution in the thickness direction, with the bubble diameter being small in the surface part and large in the center part. Therefore, this invention is not a resin foam in which small bubble groups are randomly dispersed in a cluster shape at a predetermined ratio among bubbles with a larger bubble diameter than a predetermined size, as in the present invention.

Non-Patent Document 1 describes a technology for obtaining a microcellular resin foam with a high expansion ratio by repeating two microcellular foaming processes (mixing each agent with a base resin, kneading it in a pressure kneader, and pelletizing it to obtain pellets of a foamable resin composition. The pellets are fed from the hopper of a single-screw extruder, melt-kneaded in the extruder, and extruded through a die of a specified width to obtain a smooth foaming base sheet. Next, this foaming base sheet is passed through a heating furnace to be expanded to an expansion ratio corresponding to each test material, to obtain a sheet-like foam). Non-Patent Document 1 describes a technology for obtaining a microcellular resin foam with a high expansion ratio by repeating two microcellular foaming processes (mixing each agent with a base resin, kneading it in a pressure kneader, and pelletizing it to obtain pellets of a foamable resin composition. The pellets are fed from the hopper of a single-screw extruder, melt-kneaded in the extruder, and extruded through a die of a specified width to obtain a smooth foaming base sheet. Next, this foaming base sheet is expanded to an expansion ratio corresponding to each test material by passing it through a heating furnace to obtain a sheet-like foam). Non-Patent Document 1 describes a technology for obtaining a microcellular resin foam with a high expansion ratio of 10 to 36 times by heating and foaming a foam that has been expanded to a specified expansion ratio of 5 to 6 times by performing the first microcellular foaming process, and then foaming it at a specified temperature for the second time. In this case, even when the expansion ratio is maximum, the average bubble diameter is less than 40 μm and the thermal conductivity is 0.0382 W/m·K@0°C. In this case, the average bubble diameter is less than 40 μm, and the thermal conductivity tends to decrease as the expansion ratio increases, but even at the maximum expansion ratio, the thermal conductivity is 0.0382 W/m·K@0°C, and does not fall below 0.0350 W/m·K@0°C.

In conclusion, even when a high expansion ratio is achieved by the two-stage MC foaming process, the bubble diameter is small at 40 μm or less, which is at a level where there is no need to consider a decrease in thermal conductivity due to convection. Therefore, in the case of PPS resin foam manufactured by the fine cell foaming process, although the effect of a decrease in the amount of heat transfer due to thermal conduction through the resin part due to a decrease in resin density caused by a high expansion ratio is observed, even with a high expansion ratio, the bubble diameter is smaller overall than in the case of extrusion foaming, etc., so it is thought that the amount of heat transfer due to thermal conduction through the resin part is large and the decrease in the heat transfer coefficient is small.

From this paper, it was considered that even when foams produced by the fine cell foaming process are expanded to a high expansion ratio, they will exhibit different behaviors due to differences in cell diameter, cell wall thickness, and heat conduction mechanism.
It is found that when the bubble morphology is unimodal and the bubble diameter is around 40 μm, the thermal conductivity exceeds 0.0350 W/m·K@0° C. It is considered that a bubble size distribution in which small bubble groups are randomly dispersed at a predetermined ratio among large bubble groups is not obtained, and as a result, even when the expansion ratio is high, the bubble diameter is small and highly uniform, so that the thermal conductivity does not become 0.0350 W/m·K@0° C. or less.

Patent Documents 1 to 4 and Non-Patent Document 1 describe inventions relating to electron beam crosslinked foams, chemically crosslinked foams, non-crosslinked foams, and microcellular resin foams produced by batch foaming, and the cell structures of foams produced by each production method. However, none of these documents describe or suggest a resin foam having a cell structure as in the present invention, in which cells with smaller cell diameters are randomly dispersed in clusters at a predetermined ratio among cells with diameters larger than a predetermined size in the cell structure of the entire foam. It is clear that the resin foam proposed by the inventors has a novel structure.

In the above documents, the resin foam by electron beam crosslinking in Patent Document 2 and the non-crosslinked polystyrene resin foam in Patent Document 4 are inventions for controlling the bubble diameter. In particular, the invention of the polystyrene resin foam in Patent Document 4 is common to both in that it has a group of bubbles with a small bubble diameter and a group of bubbles with a large bubble diameter, but the invention of Patent Document 4 is to obtain a bubble structure in which a group of bubbles with a small bubble diameter is arranged in a range of a predetermined thickness in the polystyrene resin foam, and a group of bubbles with a large bubble diameter is arranged inside that. In addition, even if the fine bubble foaming process by batch foaming described in Non-Patent Document 1 is used, it is not possible to obtain a bubble structure like that of the present invention, and a decrease in thermal conductivity cannot be expected.
In other words, there has been no method for stably introducing a predetermined amount of fine bubbles into a foam having large bubbles for a high expansion ratio crosslinked extruded polyethylene resin foam crosslinked by chemical crosslinking. In contrast, the present invention is for obtaining a polyethylene resin foam having a cell structure in which small bubbles are randomly dispersed in a predetermined ratio in clusters among large bubbles, and therefore the two are different in both the type of resin and the cell structure.

Japanese Patent Application Publication No. 08-302053 Japanese Patent Application Publication No. 11-279315 JP 2002-036337 A JP 2004-277673 A

Furukawa Electric Review, February 2014, No. 133 (Example of micro-foamed resin produced by batch foaming)

The present invention provides a polyethylene extrusion crosslinked resin foam with low thermal conductivity and excellent surface quality by developing a polyethylene-based resin chemical crosslinked extrusion foam having a bubble structure in which small bubble groups are randomly dispersed in a predetermined ratio among large bubble groups of the crosslinked extrusion foam. Although a resin foam having a bubble structure in which small bubble groups are randomly dispersed in a predetermined ratio among large bubble groups has never been manufactured, a large number of small bubble groups are dispersed among large bubble groups, and this acts as a barrier to heat transfer, so that a decrease in thermal conductivity can be expected. For example, this resin foam can be expected to improve performance such as a decrease in thermal conductivity when used as a heat insulation sheet cover, and since the thermal conductivity can be made 0.0350 W/(m·K) or less, it can satisfy the specifications as a curing sheet for dam concrete. By using the resin foam of the present invention as a curing sheet for dam concrete, cracks due to water absorption and expansion of concrete due to snowfall and temperature drop can be prevented. In addition, the resin foam having a cell structure of the present invention is characterized by high dimensional accuracy achieved by roll rolling.
Therefore, an object of the present invention is to provide a chemically crosslinked extruded foam of a polyethylene resin which has a novel cell structure in a polyethylene resin and which, by obtaining this cell structure, achieves a reduction in thermal conductivity, and a heat insulating resin sheet using the same.

The chemically crosslinked extruded foam of a polyethylene-based resin of the present invention has a cell structure in which the cells constituting the extruded foam have groups of cells larger than a predetermined cell diameter, and groups of cells having cell diameters smaller than the predetermined cell diameter are randomly dispersed in clusters among the groups of cells larger than the predetermined cell diameter at a predetermined ratio.
Specifically, the chemically crosslinked extruded polyethylene-based resin foam of the present invention is a chemically crosslinked extruded polyethylene-based resin foam having a cell structure in which cells having a cell diameter of 100 μm or less are randomly accumulated and dispersed in the form of clusters among a large number of other cells having a larger cell diameter, where the maximum cell diameter of each cell in the thickness direction in the foam is defined as the cell diameter of each bubble, and the chemically crosslinked extruded polyethylene-based resin foam has a cell structure that satisfies a cell size distribution in which the proportion of cells having a cell diameter of 100 μm or less to the total number of cells is 50% or more, the proportion of cells having a cell diameter of 400 μm or more to the total number of cells is 20% or less, and further the average cell diameter of the extruded foam is 200 μm or less. This type of cell structure, in which small-diameter cells are randomly distributed (irregularly arranged) among large-diameter cell groups, is a characteristic of a cell structure obtained by adding additives in the production process of a resin foam obtained by chemically crosslinking a polyethylene-based resin, and is a differentiating feature from foams obtained by gas foaming or electron beam crosslinking foaming, and is a unique structure of chemically crosslinked extruded polyethylene-based resin foams.

The chemically crosslinked extruded polyethylene resin foam of the present invention has a thermal conductivity of 0.0350 W/m·K@0° C. or less.
In the present invention, by making the cell structure of the chemically crosslinked extruded polyethylene resin foam into the above-mentioned embodiment, the thermal conductivity is reduced and the foam can be used as a good heat insulating material.
In addition, the bubbles having a large diameter are present at the boundary between adjacent cell walls or between them to form clusters of bubbles having a small diameter, and further, the cell structure is characterized in that the clusters of the bubbles having a small diameter are randomly present in the field of view of the cross section of the foam. Here, the number of bubbles, which is the number of bubbles having a diameter of 100 μm or less, is important, and the ratio of the bubbles having a diameter of 100 μm or less to the total number of bubbles is 50% or more, and the area ratio of these bubbles does not exceed 50% even if the ratio of the bubbles having a diameter of 400 μm or more is 20% or less, and even if the ratio of the number of bubbles having a diameter of 100 μm or less exceeds 50% in terms of area ratio, and usually the area ratio of the bubbles having a diameter of 100 μm or less is much smaller than 50%.

In the chemically crosslinked extruded polyethylene resin foam of the present invention, the density of the polyethylene resin foam is preferably 22.5 to 40.0 kg/m ^3. When the density of the chemically crosslinked extruded polyethylene resin foam is in the above range, the density can be adjusted to a predetermined range by adjusting the expansion ratio while keeping the thermal conductivity low.

The polyethylene resin chemically crosslinked extruded foam of the present invention preferably has a mechanical characteristic of a compressive strength of 40 kPa or more at 25% compression deformation. By having the compressive strength of the polyethylene resin chemically crosslinked extruded foam in the above range, it is possible to produce sheets that are less likely to deform and are morphologically stable.

The polyethylene resin chemically crosslinked extruded foam of the present invention preferably has a smooth surface roughness. By reducing the surface roughness of the polyethylene resin chemically crosslinked extruded foam and making it smooth, the adhesion and heat insulation properties improve when used as an insulating resin sheet, which has the advantage of making it easier for the insulating resin sheet to adhere to the mating material used.

In the present invention, the chemically crosslinked extruded polyethylene resin foam is preferably a resin sheet for thermal insulation.

The heat insulating resin sheet is preferably a curing heat insulating sheet for dams. The heat insulating resin sheet of the present invention exhibits excellent heat insulating properties as a curing heat insulating sheet for dams.

The chemically crosslinked extruded polyethylene resin foam of the present invention is preferably produced by melt-kneading, crosslinking and foaming a polyethylene resin having a composition containing, relative to 100 parts by mass of low-density polyethylene, 15 to 20 parts by mass of a foaming agent, 0.6 to 1.4 parts by mass of a crosslinking agent, 0.4 to 3.0 parts by mass of a thickener and 0.05 to 0.5 parts by mass of a crosslinking auxiliary.
By using the components of the above composition as raw materials, it becomes easy to achieve a desired cell diameter distribution, and a desired chemically crosslinked extruded polyethylene resin foam can be obtained with good reproducibility.

In the production of the chemically crosslinked extruded polyethylene resin foam of the present invention, it is preferable to use ADCA (azodicarbonamide) as the foaming agent, DCP (dicumyl carbon oxide) as the crosslinking agent, a PTFE (polytetrafluoroethylene)-based modifier as the thickening agent, and TMPTA (trimethylolpropane triacrylate) as the crosslinking auxiliary.
By using a predetermined amount of each of the above compounds, it becomes easy to achieve a predetermined cell diameter distribution, and a desired chemically crosslinked extruded polyethylene resin foam can be obtained with good reproducibility.

The present invention also provides a method for improving the thermal conductivity of a chemically crosslinked polyethylene resin extruded foam, characterized in that the polyethylene resin foam has a bubble structure in which a group of bubbles having a diameter of 100 μm or less is randomly accumulated and dispersed in a cluster shape among a large number of other bubbles having a larger diameter, where the maximum value of the bubble diameter in the thickness direction of each bubble in the foam is defined as the bubble diameter of each bubble, and the proportion of the number of bubbles having a diameter of 100 μm or less to the total number of bubbles is 50% or more, the proportion of the number of bubbles having a diameter of 400 μm or more to the total number of bubbles is 20% or less, and the average bubble diameter of the extruded foam is 200 μm or less, thereby making the thermal conductivity of the polyethylene resin foam 0.0350 W/m·K@0°C or less. As described above, the thermal conductivity improvement method makes it possible to control the thermal conductivity of a polyethylene resin foam by controlling the bubble structure of a chemically crosslinked polyethylene resin extruded foam.

As described above, it has been difficult to uniformly refine the bubbles as a cell structure of extruded polyethylene resin foam. Therefore, in the present invention, instead of obtaining a bimodal cell structure in which the cell diameter is refined as a whole and two types of cell groups, that is, a group of large cells with a diameter of several hundred μm and a group of small cells, are arranged in a predetermined ratio in the thickness direction, it is possible to obtain a resin foam having a cell structure in which a group of small cells is randomly dispersed in a predetermined ratio near or between the cell walls of the group of large cells.

As described above, the resin foam obtained by the present invention has a novel cell structure, and by obtaining this cell structure, it is possible to realize a decrease in thermal conductivity and an improvement in surface quality.
The details of the reason for the decrease in thermal conductivity and the improvement of surface quality by roll rolling will be described separately, but the decrease in thermal conductivity is due to the convection suppression effect and radiant heat reflection effect of the bubble group with a small bubble diameter, and the surface quality improvement effect after roll rolling is due to the fact that the bubble group with a small bubble diameter has a higher rigidity than the bubble group with a large bubble diameter, and therefore the stress caused by rolling on the bubble group with a large bubble diameter works effectively. In addition, in the case of the bubble structure of the present invention, even if the area ratio of the bubble group with a small bubble diameter is smaller than that in the case of obtaining a bimodal bubble structure in which the bubble group with a small bubble diameter is arranged on the surface side in the thickness direction and the bubble group with a large bubble diameter is arranged on the center side, the bubble group with a small bubble diameter is randomly dispersed among the bubble groups with a large bubble diameter, and the feature of inhibiting heat transfer by the above-mentioned convection suppression effect and radiant heat reflection effect can be effectively functioned.

1 is a drawing-substitute photograph showing an electron microscope image of a cut section of a heat insulating resin sheet obtained in an example.

(Bubble diameter)
The chemically crosslinked extruded polyethylene resin foam of the present invention has a unique internal foaming state. That is, in the chemically crosslinked extruded polyethylene resin foam of the present invention, the lower limit of the diameter of the large cells in the thickness direction is 400 μm, and the ratio of the number of cells of the cells having a diameter of 400 μm or more to the total number of cells is 20% or less. The ratio of the large cells is more preferably 15% or less. The lower limit is not particularly limited.
On the other hand, the upper limit of the diameter of the small bubbles in the thickness direction is 100 μm, and the proportion of the number of bubbles of the small bubbles having a diameter of 100 μm or less to the total number of bubbles is 50% or more. The proportion of the small bubbles is more preferably 65% or more. The upper limit is not particularly limited. Furthermore, the average bubble diameter of the extruded foam is 200 μm or less, preferably 150 μm or less, and more preferably 100 μm or less. The lower limit is not particularly limited, but it is practical to set it to 50 μm or more.

(Thermal Conductivity)
Furthermore, the thermal conductivity of the chemically crosslinked extruded polyethylene resin foam of the present invention is not more than 0.0350 W/m·K@0° C. Because the chemically crosslinked extruded polyethylene resin foam of the present invention has this low thermal conductivity, the transfer of heat between the outside and the concrete side is suppressed, and the foam can satisfy the specifications as a curing sheet for dam concrete.

(Consideration of the relationship between bubble structure and thermal conductivity improvement effect)
Here, there are three methods of heat transfer and transport: thermal conduction, in which heat is transferred directly through the foam and then to the material in contact with the foam; thermal radiation, in which heat is radiated and transported as electromagnetic waves; and convection, in which heat is transferred by natural heat transfer due to differences in temperature distribution (density flow) or forced flow, which causes fluids in different regions of a fluid to move relative to each other.
In the resin foam of the present invention, heat is transported by heat conduction through the resin layer inside the foam, and heat radiation from the surface of the resin foam is not so great, but air bubbles exist inside the foam, and there is an air layer inside the air bubbles. Therefore, it is considered that a temperature gradient exists between one surface and the other surface of the resin foam inside this air layer, and thus micro-convection occurs inside the air bubbles.

The amount of heat transport due to this microscopic convection differs depending on the bubble diameter. If we assume that the actual bubble structure is spherical, for example, the volume of a bubble with a diameter of 400 μm and a bubble with a diameter of 100 μm will appear to differ by approximately 64 times, and the cross-sectional area will differ by 16 times.
In a resin foam having such bubble groups of different sizes, the thermal conductivity can be reduced when comparing two bubble groups, a group with a large bubble diameter of 400 μm or more and a group with a bubble diameter of 100 μm or less, for example, convection occurs inside the bubbles of the large bubble group, but convection is less likely to occur inside the small bubble group because the bubbles are small. Furthermore, the small bubble group not only has a convection prevention effect, but also has the effect of reflecting and blocking radiant heat at the bubble walls, which is thought to have an effect of reducing the thermal conductivity in this respect as well.

As a result, the thermal conductivity of a group of bubbles with a large diameter is greater than that of a group of bubbles with a small diameter. Therefore, by introducing a group of bubbles with a small diameter into a group of bubbles with a normal large diameter, it is possible to simultaneously reduce both convection and radiation in the heat transfer of the resin foam, thereby reducing the thermal conductivity.

In fact, when the thermal conductivity of a polyethylene resin foam consisting only of large-diameter bubbles and a polyethylene resin foam having a bubble structure in which small-diameter bubbles are randomly dispersed in clusters among the large-diameter bubbles described above was measured, a clear difference in thermal conductivity was observed.

(Relationship between foam cell structure and thermal conductivity)
When the structure of the foam of the present invention is compared with that of a conventional foam consisting only of large-diameter bubbles, it is found that the foam of the present invention has a cell structure which is composed of large-diameter bubbles as a whole, in which a certain proportion of small-diameter bubbles are formed among the large-diameter bubbles, and in which a certain proportion of the small-diameter bubbles are randomly accumulated and dispersed in various places.

By obtaining such a cell structure, it is possible to reduce the thermal conductivity as compared with a foam having a conventional cell structure.
・Conventional polyethylene resin foam with large air bubbles:
A polyethylene resin foam having a thermal conductivity of 0.03702 W/m·K@0°C or less, in which a cell structure is formed in which small-sized cells are randomly accumulated and dispersed at a predetermined ratio among large-sized cells of the present invention:
Thermal conductivity is 0.03384 W/m·K @ 0°C or less. In order to compare the thermal conductivity of a conventional foam with large bubble diameter and a foam of the present invention in which small bubbles are randomly dispersed in clusters among large bubbles, the ratio of the thermal conductivities of the two is 0.914, which is a decrease in thermal conductivity of approximately 9.14%.

This reduction in thermal conductivity can be achieved by obtaining a bubble structure in which small bubbles are randomly dispersed in clusters in a foam material with a large bubble structure using only foaming materials of nearly the same composition. As a result, it is considered to be of great technical significance that a reduction in thermal conductivity can be achieved while maintaining a high expansion ratio in a polyethylene resin chemically crosslinked extruded foam, where it is difficult to stably obtain a foam with a small bubble size. The above test also shows that a reduction in thermal conductivity of at least 8% or more is possible, and a sheet-shaped foam with a thermal conductivity of 0.0350 W/m·K@0°C or less can be obtained.

In a preferred embodiment, the chemically crosslinked extruded polyethylene resin foam of the present invention may be a foam obtained by extruding and crosslinking a resin composition containing polyethylene as the base resin and, if necessary, additives (foaming agent, crosslinking agent, crosslinking assistant, thickener, other additives, etc.).

(polyethylene)
The polyethylene that is the base resin of the polyethylene-based resin chemically crosslinked extruded foam of the present invention is not particularly limited, and may be any of high density polyethylene (HDPE), low density polyethylene (LDPE), and linear low density polyethylene (LLDP). In the present invention, LDPE, which has a large branched structure and high melt tension, is particularly preferred. It may also be a copolymerized polyethylene in which propylene or butylene is partially interposed.

(Foaming Agent)
For the chemically crosslinked extruded polyethylene resin foam of the present invention, any foaming agent commonly used in this field can be used, among which thermally decomposable foaming agents are preferred. Examples of the thermal decomposition type foaming agent include inorganic foaming agents such as ammonium carbonate, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate; and organic foaming agents such as azodicarbonamide (ADCA: C ₂ H ₄ N ₄ O ₂ ), barium azodicarboxylate, azobisbutyronitrile, nitrodiguanidine, N,N'-dinitropentamethylenetetramine, N,N'-dimethyl-N,N'-dinitrosoterephthalamide, p-toluenesulfonyl hydrazide, 4-toluenesulfonylselcarbazide, 4,4'-oxybisbenzenesulfonyl hydrazide, azobisisobutyronitrile, 4,4'-oxybisbenzenesulfonylsemicarbazide, 5-phenyltetrazole, trihydrazinotriazine, and hydrazodicarbonamide. Among these, azodicarbonamide, sodium hydrogen carbonate, and 4,4'-oxybisbenzenesulfonylhydrazide are preferred from the viewpoint of economic efficiency. It is more preferred to use a blowing agent containing azodicarbonamide or sodium hydrogen carbonate, since it has a wide molding temperature range and can produce a foam with fine bubbles.

The amount of foaming agent may be adjusted as appropriate, but is preferably 1 to 30 parts by mass, more preferably 5 to 25 parts by mass, and particularly preferably 15 to 20 parts by mass, per 100 parts by mass of polyethylene resin. One type or two or more types of foaming agents may be used. When two or more types are used, the total amount falls within the above range.

(Crosslinking Agent)
Examples of crosslinking agents include dicumyl peroxide (DCP: C ₁₈ H ₂₂ O ₂ ), bis(α,α-dimethylbenzyl) peroxide, bis(1-methyl-1-phenylethyl) peroxide ([C ₆ H ₅ C(CH ₃ ) ₂ ] ₂ O ₂ , molecular weight: 270.37), 1,1-ditertiarybutylperoxy-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-ditertiarybutylperoxyhexane, 2,5-dimethyl-2,5-ditertiarybutylperoxyhexyne, α,α-ditertiarybutylperoxyisopropylbenzene, tertiarybutylperoxyketone, and tertiarybutylperoxybenzoate. Among these, the crosslinking agent is preferably dicumyl peroxide, bis(α,α-dimethylbenzyl) peroxide, or bis(1-methyl-1-phenylethyl) peroxide, and particularly preferably dicumyl peroxide.

The content of the crosslinking agent is preferably 0.1 to 3.0 parts by weight, more preferably 0.4 to 2.0 parts by weight, and particularly preferably 0.6 to 1.4 parts by weight, per 100 parts by weight of the polyethylene resin. One type or two or more types of crosslinking agents may be used. When two or more types are used, the total amount falls within the above range.

(Crosslinking aid)
The crosslinking aid is preferably a polyfunctional monomer, which is a compound having multiple double bonds in the molecule. Examples of the polyfunctional monomer that can be used include divinylbenzene, trimethylolpropane trimethacrylate (TMPTA, chemical formula: C ₁₅ H ₂₀ O ₆ ), 1,6-hexanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, trimellitic acid triallyl ester, triallyl isocyanurate, and ethylvinylbenzene. Among these, trimethylolpropane trimethacrylate is preferred. The crosslinking aid also has the effect of promoting crosslinking and suppressing the growth of bubbles.
The content of the crosslinking aid is preferably 0.01 to 1.0 part by mass, more preferably 0.05 to 0.5 part by weight, and particularly preferably 0.05 to 0.4 part by weight, relative to 100 parts by weight of the polyethylene resin. One type or two or more types of crosslinking aids may be used. When two or more types are used, the total amount falls within the above range.

(Thickener)
The thickener used in the polyethylene resin chemically crosslinked extruded foam of the present invention includes PTFE (polytetrafluoroethylene): chemical formula: (C ₂ F ₄ ) _n . This is a polymer of tetrafluoroethylene, a fluororesin consisting of only fluorine atoms and carbon atoms. It is known under the trade name Teflon (registered trademark). It is chemically stable and has excellent heat resistance and chemical resistance (melting point: 327°C, density: 2.2 g/cm ³ ). Trade names include Metablen A-3800 (acrylic modified polytetrafluoroethylene), Mitsubishi Rayon Co., Ltd. polytetrafluoroethylene: manufactured by Mitsubishi Rayon Co., Ltd., trade name "Metablen A3000" (a representative example of a commercially available product using an acrylic polymer (b-1) and polytetrafluoroethylene (b-2) in combination). The molecular chain of PTFE has a larger atomic radius of fluorine atom than hydrogen, and has a resin structure in which fluorine atoms cover the carbon chains densely, so it is stable and strong even after extrusion processing, and is thought to have the effect of preventing the cell walls from breaking or deforming during foaming. In addition, in the case of Metablen A3000, which is PTFE modified with acrylic, the shear force during melt kneading extrusion causes fibrillation, which further increases the effect of improving the melt tension of the molten resin.

The content of the thickener is preferably 0.05 to 10.0 parts by weight, more preferably 0.1 to 7.0 parts by weight, and particularly preferably 0.4 to 3.0 parts by weight, per 100 parts by weight of the polyethylene resin. One type or two or more types of cross-linking aids may be used. When two or more types are used, the total amount falls within the above range.

(Other additives)
The composition according to the present invention may further contain a polyolefin resin for the purpose of improving moldability. Examples of the polyolefin resin include homopolymers, random copolymers, block copolymers, and mixtures thereof of α-olefins, or random copolymers, block copolymers, and graft copolymers of α-olefins and other unsaturated monomers, and oxidized, halogenated, or sulfonated versions of these polymers. These may be used alone or in combination of two or more. Specific examples include polyethylene, ethylene-propylene copolymers, ethylene-propylene-non-conjugated diene copolymers, ethylene-butene copolymers, ethylene-hexene copolymers, ethylene-octene copolymers, ethylene-vinyl acetate copolymers, ethylene-vinyl alcohol copolymers, ethylene-ethyl acrylate copolymers, and polyethylene resins such as chlorinated polyethylene, polypropylene, propylene-ethylene random copolymers, propylene-ethylene block copolymers, and polypropylene resins such as chlorinated polypropylene, polybutene, polyisobutylene, polymethylpentene, and (co)polymers of cyclic olefins. Among these, polyethylene resins, polypropylene resins, or mixtures thereof are preferably used from the viewpoints of cost and balance of physical properties of the thermoplastic resin.
The amount of the polyolefin resin (d) is preferably 2 to 400 parts by weight, more preferably 5 to 100 parts by weight, based on 100 parts by weight of the thermoplastic block copolymer (a). If the amount exceeds 400 parts by weight, the rubber elasticity of the resulting thermoplastic resin composition decreases.

(Cell structure and surface properties of foamed material after rolling)
Generally, the surface properties of a foam surface are considered to be inferior in the case of a foam with a high expansion ratio, which is foamed after chemical crosslinking, since the degree of crosslinking within the sheet varies more than in the case of electron beam crosslinking.
For this reason, the surface of the chemically crosslinked resin foam is rolled to improve the surface properties of the resin foam. Thus, the resin foam of the present invention has a cell structure in which small-sized cells are randomly dispersed at a predetermined ratio among cells with a large cell diameter, and a resin foam consisting only of normal cells with a large cell diameter, and is characterized in that the surface properties of the resin foam are improved after roll rolling.
As a result, the resin foam having the cell structure of the present invention has a surface property superior to that of a resin foam having a normal unimodal cell structure. Although the reason for this is not necessarily clear, it is believed that the simultaneous addition of both the thickener and the processing aid makes the cell growth suppression effect of these materials more pronounced, resulting in a cell structure in which small bubbles are randomly distributed among bubbles with a larger diameter than a predetermined size.

The reason why the properties of the resin foam having the cell structure of the present invention are superior to those of a high-magnification foam having a small cell group with a normal cell structure is that, when comparing a basic group with a large cell diameter and a group with a small cell diameter, the small cell group has a higher cell wall rigidity and the large cell group has a lower cell wall rigidity, so that deformation during roll rolling tends to concentrate in the large cell group, and differences in cell structure within the large cell group tend to cause differences in the amount of deformation between the large bubbles. As a result, in the case of a resin foam having a cell structure such as that of the present invention, when viewed as a whole foam, excessive deformation of the large cell group is suppressed by the small cell group with a high cell wall rigidity, and the surface quality after roll rolling is improved compared to the case of a resin foam consisting only of normal large cell groups.

(Rolling and surface roughness of foam after rolling)
<Rolling method>
The extrusion temperature may be appropriately set depending on the desired foaming state, and extrusion molding is usually performed at a temperature of about 160 to 190°C. After extrusion molding, rolling is performed using a molding roll die. Here, the stretching by rolling using a molding roll die can be performed, for example, by rolling in the extrusion direction (MD direction) so that the stretching ratio is 20 to 90%. Here, a stretching ratio of 20% means that the length in the rolling direction is stretched to 120% (1.2 times) by rolling, and similarly, a stretching ratio of 30% means that the length in the rolling direction is stretched to 130% (1.3 times) by rolling. In the embodiment of the present invention, a stretching of 40% was performed.

<Surface roughness of foam after rolling>
The surface roughness of the extruded foam is rougher in the TD direction than in the MD direction. This is because the foam is stretched in the MD direction while being cooled by the tension of the rolling by the forming roll die, and the surface roughness is greatly improved. Here, the bubbles are flattened in the TD direction by the compression force of the roll, so the roughness in the TD direction is also improved, but the improvement effect in the TD direction is smaller than the surface roughness in the MD direction, where the effect of flattening the bubbles by stretching is large, and as a result, the surface roughness in the TD direction tends to be larger than that in the MD direction.

1) Method for measuring bubble diameter A 5 mm thick sample cut from a cross section in the thickness direction of a foam sample was measured with a JEOL SEM measuring device at an acceleration voltage of 20 kV and a magnification of 22. At this time, in order to make the foam surface side clear from the photograph, the sample end face on the foam surface side was photographed so as to be included in the image.
A typical method of measuring bubble diameter is to obtain the bubble diameters in the X and Y directions and use the average of these measurements as the average bubble diameter, or to read the shape of the bubble in an image, then use image software to obtain the area of the bubble and determine the apparent diameter by regarding the area as a circle.

In contrast, in the case of the present invention, it is necessary to analyze the effect of bubble diameter on the thermal conductivity of the foam sheet in the thickness direction, so it is necessary to determine the bubble diameter in the thickness direction of the foam. However, since the bubble diameter in the thickness direction differs depending on the measurement position of each bubble, the measurement value of the maximum bubble diameter in the Y direction was used as the bubble diameter in each thickness direction.

Here, image processing for measuring the bubble diameter was performed using software called Click Measure. This software can output the distance in the X or Y direction between two points clicked on the image as the number of pixels. With this, the number of pixels with a reference length of 1 mm within the SEM field of view was measured, and using this as a reference, the number of pixels in the Y direction of the maximum bubble diameter part of all bubbles contained in the entire observed cross section was measured, and the bubble diameter of each bubble was calculated from the relationship with the number of pixels with a reference line length of 1 mm. For the calculation, the field of view was set to 5 mm in the X direction x 4 mm in the Y direction so that the field of view was included from the surface to the center, and the maximum value of the bubble diameter in the Y direction of all bubbles within that range was taken as the bubble diameter.

Therefore, the average bubble diameter was determined by calculating the arithmetic mean of the data on the diameters of all bubbles present within the field of view. From the acquired data, the number of bubbles with a diameter of 400 μm or more and the number of bubbles with a diameter of 100 μm or less were counted, and the existence probability relative to the total number of bubbles was calculated.
2 shows an electron microscope image of a cross section cut in the thickness direction of the test piece of Example 1. It was confirmed that the number of bubbles having a size of 400 μm or more was 20% or less, and the number of bubbles having a size of 100 μm or less was 50% or more.

2) Measurement method for foam density The density of the extruded foam was measured in accordance with JIS K7222-1999, "Method for measuring apparent density of foamed plastics and rubber."
Specifically, the apparent overall density, apparent core density, and apparent density were calculated by the following formulas, with the unit being kg/ ^m3 .
Here, m is the mass of the test piece (g), and V is the volume of the test piece (mm ³ ).
For all test pieces, the average density is calculated from the measurement results and rounded to the nearest 0.1 kg/ ^m3 .
[Note] In the case of low density closed cell materials, for example, ^less than 30 kg/m3, the air buoyancy may be within the margin of error. In that case, ρ _a is calculated by the following formula:
ma: mass of air displaced.

3) Thermal conductivity measurement method: Plate heat flow method: Steady-state method JIS R2616:2001
Thermal conductivity (λ) is a physical property that indicates how easily heat flows within a substance, and is expressed in terms of the amount of heat that moves (W), the distance that moves (m), and the temperature difference (K). When there is a temperature difference between two points, heat flows, and the ease with which that heat flows is thermal conductivity, expressed in units of Wm ^- 1K ^-1 . There are steady and unsteady methods for measuring thermal conductivity, which are used depending on the purpose. The steady method is a method in which a steady, unidirectional heat flow is created in a sample and thermal conductivity is measured, while the unsteady method is a method in which a material is heated unsteadily and the temperature response is measured.

The thermal conductivity was measured in accordance with JIS A1412-2. Specifically, the sample was sandwiched between a heating plate and a cooling plate, and after a steady state was reached, the thermal conductivity was calculated from the temperature difference (ΔT) between both sides of the test piece, the heat density q passing through the test piece measured by a calorimeter, and the sample thickness d.
λ=q·d/ΔT
Here, the heat flow density q (W/m ² ) is the heat flow rate per area of the sample, and is a value measured by a hot plate temperature control device and a temperature/heat flow measuring device.

Thermal conductivity measurements are performed in accordance with JIS R2616, with the exception that the conditioning period is longer. This is because the thermal conductivity of the material changes immediately after manufacture, gradually increases, and reaches a steady state in about 72 hours, so all data used in this test is data obtained after conditioning at room temperature for 72 hours or more.

4) 25% Compressive Strength of Foam The resin foam obtained with a thickness of 10 mm was subjected to a method conforming to JIS K6767 to determine the compressive strength at 25% compression deformation at a measurement temperature of 23°C. Specifically, the foam was sandwiched between plates of a larger area than the foam cut into 50 mm squares and expanded 30 times, and the foam was compressed by 2.5 mm at a speed of 5.0 mm/min and stopped, and the strength after 20 seconds was measured, and the strength at that time was taken as the 25% compressive strength. This strength measurement was also performed using an autograph tensile tester (model: AGS-akNX, manufactured by Shimadzu Corporation).

- Method for producing a thermal insulation resin sheet The following organic decomposition type foaming agent, crosslinking agent, crosslinking assistant, and thickener were blended with the base resin, kneaded with a pressure kneader, and pelletized to obtain pellets of a foamable resin composition. The pellets were fed from the hopper of a single-screw extruder, melt-kneaded in the extruder, and extruded through a die of a predetermined width to obtain a smooth foaming base material sheet with a thickness of 3.3 mm. Next, this foaming base material sheet was continuously passed through a heating furnace at 220°C to 240°C to foam to an expansion ratio corresponding to each test material, and a sheet-like foam was obtained.

The base resin of this crosslinked extruded foam was low-density polyethylene (LDPE), and in both cases, F120N manufactured by Ube Maruzen Polyethylene Co., Ltd. was used as the low-density polyethylene. To obtain this sheet-like foam, the crosslinking agent was Parkmil D (dicumyl peroxide, product name) manufactured by NOF Corp., the foaming agent was Uniform AZ (azodicarbonamide) manufactured by Otsuka Chemical Co., Ltd. as an organic decomposition type foaming agent, the crosslinking assistant was Sunester TMP (trimethylolpropane trimethacrylate) manufactured by Sanshin Chemical Industries Co., Ltd., and the thickener was Metablen A-3000 (PTFE-based modifier) manufactured by Mitsubishi Chemical Co., Ltd., mixed in the mass ratio (parts by mass) listed in the composition table in Table 1. Here, the sheet-like extruded resin foam shown in Table 2 was obtained. Test pieces were cut out from this foam, and measurements of density, cell diameter, thermal conductivity, and compressive strength were carried out in accordance with the above-mentioned standards.

Composition: Composition of foam, etc.

<<Bubble structure and strength of foams: density, bubble size, thermal conductivity, compressive strength, etc.>>

-Evaluation results of heat insulating resin sheets As can be seen from the above results, in the resin sheets of Examples 1 to 3, the proportion of the number of bubbles in the bubble group with a bubble diameter of 400 μm or more to the total number of bubbles is 20% or less, the proportion of the number of bubbles in the bubble group with a bubble diameter of 100 μm or less to the total number of bubbles is 50% or more, and further, the average bubble diameter is all 200 μm or less, and a bubble structure that satisfies the provisions of the claims was obtained, and as a result, good low thermal conductivity and sufficient compressive strength can be obtained. In contrast, in the resin sheets of Comparative Examples 1 to 3, the proportion of the bubble group with a bubble diameter of 400 μm or more to the total number of bubbles was more than 20%, and the proportion of the bubble group with a bubble diameter of 100 μm or less to the total number of bubbles was less than 50%. The average bubble diameter was also more than 200 μm, and as a result, a bubble structure having a bubble diameter distribution satisfying the provisions of the claims could not be obtained, and the comparative example materials all had inferior thermal conductivity compared to the inventive example materials. Although the addition of a thickener or processing aid reduces the thermal conductivity by increasing the number of bubbles with a bubble diameter of 100 μm or less, the thermal conductivity cannot reach 0.03500 W/m·K@0°C or less. However, by adding a thickener and a processing aid at the same time, the bubble growth suppression effect of these materials becomes more pronounced, and this is thought to be the cause of the bubble structure in which small bubbles are randomly distributed among bubbles with a diameter larger than a predetermined size.

The foam of Example 1 of the present invention, as seen from the SEM photograph, is characterized in that, in the field of view of the cross section of the foam, multiple small bubbles accumulate at the boundaries or between adjacent cell walls of bubbles with a large diameter, forming clusters and being dispersed randomly among the large bubbles. Furthermore, the cell structure of the polyethylene resin chemically crosslinked extruded foam of the present invention is characterized in that, in the field of view of the cross section of the foam, small bubble clusters are randomly present among the large bubbles.

Here, the clusters of small bubbles were randomly distributed in both the thickness and width directions of the cut surface. The reason for this type of bubble structure is thought to be that the cross-linking aid and thickener are randomly dispersed in the material during the raw material kneading process, and the growth of bubbles is inhibited at the dispersed positions, resulting in a state in which small bubbles are accumulated in clusters. It is thought that materials with small bubbles are dispersed in response to the dispersion state of the materials that inhibit the growth of these bubbles. It is also possible to disperse a group of bubbles with small bubbles among a group of bubbles with large bubbles in gas foaming and electron beam cross-linking foaming, but in the case of foams using extrusion gas foaming and electron beam cross-linking foaming, the accumulation of small bubbles tends to occur near the surface layer of the foam due to the characteristics of the manufacturing method.

The densities of the foams of Examples 1 to 3 were 31.4 to 32.7 kg/ ^m3 , and the densities of the polyethylene resin foams satisfied the range of 22.5 to 40.0 kg/ ^m3 . In addition, as mechanical characteristics of the polyethylene resin foams, the compressive strength at 25% compression deformation was 40.7 to 43.3 kPa, satisfying the range of 40 kPa or more, and it was found that the density of these foams as ordinary foams and the compressive strength at 25% compression deformation satisfied the prescribed values.

As described above, according to the present invention, in a chemically crosslinked extruded polyethylene foam, which has been considered difficult to reduce in thermal conductivity, a chemically crosslinked extruded foam is realized having a cell structure in which a predetermined proportion of cells have a smaller cell diameter than a predetermined size, and the cells have a larger cell diameter that is randomly dispersed in clusters. Furthermore, by satisfying the following conditions: the proportion of the number of cells having a cell diameter of 400 μm or more out of the total number of cells in the foam is 20% or less, the proportion of the number of cells having a cell diameter of 100 μm or less out of the total number of cells, and the average cell diameter of the extruded foam is 200 μm or less, a resin foam having a thermal conductivity of 0.0350 W/m·K@0°C or less can be obtained.

It was confirmed that basic properties such as density and compressive strength of the polyethylene resin foam can be maintained at the normal product level as a normal extruded resin foam. In the present invention, in the chemically crosslinked polyethylene extruded foam having the cell structure of the present invention, it is considered that the introduction of cells having a small cell diameter inhibits the growth of cells in the foaming process and prevents the deformation during roll rolling from being excessively concentrated on the cells having a large cell diameter, thereby not only suppressing the thermal conductivity of the polyethylene resin foam within a predetermined range, but also improving the surface properties of the extruded resin foam while maintaining mechanical properties such as compressive strength.

Claims (9)

A chemically crosslinked extruded polyethylene resin foam having a cell structure in which cells having a cell diameter of 100 μm or less are randomly accumulated and dispersed in clusters among a large number of other cells having a larger cell diameter, the maximum cell diameter of each cell in the thickness direction of the foam being defined as the cell diameter of each cell,
The polyethylene resin chemically crosslinked extruded foam is
The ratio of bubbles having a diameter of 100 μm or less to the total number of bubbles is 50% or more,
The proportion of bubbles having a diameter of 400 μm or more to the total number of bubbles is 20% or less,
Furthermore, the extruded foam has an average cell diameter of 200 μm or less.
A chemically crosslinked extruded foam of polyethylene resin having a cell structure that satisfies the cell size distribution. The chemically crosslinked extruded polyethylene resin foam according to claim 1, characterized in that the thermal conductivity of the chemically crosslinked extruded polyethylene resin foam is 0.0350 W/m·K@0°C or less. 3. The chemically crosslinked extruded polyethylene resin foam according to claim 2, characterized in that the density of the chemically crosslinked extruded polyethylene resin foam is 22.5 to 40.0 kg/ ^m3 . The chemically crosslinked extruded polyethylene resin foam described in claim 3 is characterized in that the mechanical characteristics of the chemically crosslinked extruded polyethylene resin foam are a compressive strength of 40 kPa or more at 25% compression deformation. A thermal insulation resin sheet using a chemically crosslinked extruded polyethylene resin foam according to any one of claims 2 to 4, which is a thermal insulation resin sheet. The heat insulating resin sheet according to claim 5, characterized in that the heat insulating resin sheet is a curing heat insulating sheet for dams. The chemically crosslinked extruded polyethylene resin foam according to any one of claims 1 to 4, characterized in that the polyethylene resin is produced by melt-kneading and crosslinking and foaming a polyethylene resin from a composition containing 15 to 20 parts by mass of a foaming agent, 0.6 to 1.4 parts by mass of a crosslinking agent, 0.4 to 3.0 parts by mass of a thickener, and 0.05 to 0.5 parts by mass of a crosslinking assistant, relative to 100 parts by mass of low-density polyethylene. The polyethylene resin chemically crosslinked extruded foam according to claim 7, characterized in that the foaming agent is ADCA (azodicarbonamide), the crosslinking agent is DCP (dicumyl carbon oxide), the thickener is a PTFE (polytetrafluoroethylene)-based modifier, and the crosslinking aid is TMPTA (trimethylolpropane tri(meth)acrylate). A polyethylene resin chemically crosslinked extruded foam is
The foam has a cell structure in which cells having a cell diameter of 100 μm or less are randomly accumulated and dispersed in clusters among a large number of other cells having a larger cell diameter, the maximum cell diameter of each cell in the thickness direction of the foam being defined as the cell diameter of each cell,
The ratio of bubbles having a diameter of 100 μm or less to the total number of bubbles is 50% or more,
The proportion of bubbles having a diameter of 400 μm or more to the total number of bubbles is 20% or less,
Furthermore, the present invention provides a method for improving the thermal conductivity of a chemically crosslinked extruded polyethylene resin foam, characterized in that the thermal conductivity of the polyethylene resin foam is 0.0350 W/m·K@0°C or less by making the extruded foam have a cell structure that satisfies a cell size distribution in which the average cell size of the extruded foam is 200 μm or less.