Polyolefin resin foamed sheet

Abstract

To manufacture a vehicle interior material that accommodates various automobile designs while ensuring high production efficiency.SOLUTION: A polyolefin resin foamed sheet is subjected to corona treatment continuously after completion of a heating-foaming step, and exhibits a maximum heating-angle change in the width direction of -10 to +10 degrees when heated for 10 minutes under an environment of the maximum melting point of the base resin +20°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyolefin resin foam sheet, and more particularly to a polyolefin resin foam sheet characterized by good wettability and excellent moldability. [Background technology]

[0002] Polyolefin resin foams have excellent flexibility, cushioning properties, heat insulation properties, and heat resistance, and have been used in a wide range of fields, including construction, electrical and electronic equipment, vehicles, etc. Demand for interior materials for vehicles, particularly for ceilings, doors, instrument panels, and other vehicle interiors, is particularly high, and polyolefin resin foams are widely used.

[0003] Vehicle interior materials are often produced by laminating a polyolefin resin foam sheet with a polyvinyl chloride resin sheet, a thermoplastic elastomer sheet, a natural or artificial cloth-like material, or a surface material such as leather to create a laminate. To improve adhesion, the polyolefin resin foam is corona-treated. Technologies have been developed to increase efficiency and yield by improving the foaming process, corona treatment process, laminate manufacturing process, and interior material manufacturing process related to this interior material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6394150 [Patent Document 2] Special Publication No. 7-4828 [Patent Document 3] Japanese Patent Application Publication No. 2-175734 [Patent Document 4] Patent No. 6757524 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent automobile designs, interior materials such as ceilings, doors, and instrument panels are often designed with complex shapes and fine irregularities to provide distinctive shapes and functionality. However, it has been reported that many problems occur when molding polyolefin resin foams with skin materials bonded to them in the manufacture of laminates with such shapes.

[0006] The present inventors have investigated the cause of the problem and have studied measures to improve the productivity of manufacturing vehicle interior materials that are compatible with a variety of vehicle designs.

[0007] To address this issue, Patent Document 1 proposes the use of a foam or laminate that has a dimensional change rate in the thickness direction after heating of -30% to -1% and a small difference between the elongation at break in the width direction and the elongation at break in the length direction. According to this invention, a foam or laminate that has a balance between flexibility and mechanical strength such as elongation at break can be obtained, but there is a problem in that it is difficult to accommodate complex and diverse shapes.

[0008] Furthermore, Patent Document 2 proposes a method for significantly reducing the tension applied to the foam during foaming, thereby minimizing the residual strain and the difference in strain between the width direction and the machine direction. While this method was effective for reducing strain in the hot air foaming method used at the time, it did not meet current requirements and was significantly inferior to the salt foaming method. Furthermore, laminates are often formed, and there has been insufficient mention or consideration of methods for improving the properties of laminates.

[0009] Regarding the salt foaming method, a method for improving dimensional stability during heat molding has been proposed in Patent Document 3 and elsewhere, but no improvement proposals have been made as to what techniques should be used to improve the properties of the laminate.

[0010] Patent Document 4 proposes a foam material that does not have problems such as deformation or wrinkling by using a laminate in which a film is laminated on at least one surface of the foam. However, such foams have many issues, such as limitations on thickness and restrictions on design.

[0011] The biggest cause of reduced productivity is the distortion that occurs in the polyolefin resin foam with a skin when it is heated and molded, resulting in poor molding stability. When investigating the causes of distortion in foam with a skin, it was found that the first is that the foam itself is distorted during the molding process, and the second is that there are uneven bonds between the skin and the foam, which causes distortion in the laminate during molding. Therefore, reducing both the distortion of the foam itself and the uneven bond between the skin and the foam can improve the productivity of vehicle interior materials. [Means for solving the problem]

[0012] The present inventors defined the heating angle change as a method for quantifying the distortion of a foam due to molding processing, clarified the relationship between the magnitude of the heating angle change and molding stability, and determined a preferable range of the heating angle change for achieving the desired molding stability.

[0013] Next, it was discovered that in order to achieve a preferred range for the amount of change in heating angle, it is important to combine the foaming method and corona treatment under specific conditions, which led to the present invention.

[0014] That is, the present invention relates to the following [1] to [3]. [1] A polyolefin resin foam sheet that is continuously subjected to a corona treatment after the completion of a heat foaming process, and that exhibits a maximum change in heating angle in the width direction of -10 to +10 degrees when heated for 10 minutes in an environment of a temperature 20°C higher than the maximum melting point of the base resin. [2] The polyolefin resin foam sheet according to claim 1, which has a wet tension of 40 mN / m or more after corona treatment. [3] The polyolefin resin foam sheet according to claim 1 or 2, characterized in that the heat-foaming step is a salt foaming step. [Effects of the Invention]

[0015] The polyolefin resin foam sheet of the present invention makes it possible to produce vehicle interior materials that are compatible with a variety of vehicle designs with high productivity. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention will be specifically described below.

[0017] <Polyolefin resin> The polyolefin resin used in the present invention is not particularly limited, but it is preferable that the polyolefin resin contains at least one of a polypropylene resin, a polyethylene resin, and a thermoplastic elastomer resin.

[0018] Examples of polypropylene-based resins used in the present invention include homopolypropylene, ethylene-propylene random copolymers, and ethylene-propylene block copolymers. If necessary, copolymers of propylene monomers with other copolymerizable monomers can also be used. The polyolefin-based resin foam sheet may contain one type of polypropylene-based resin, or a blend of two or more types. The polymerization method for these polypropylene-based resins is not particularly limited, and may be any of a high-pressure method, a slurry method, a solution method, or a gas-phase method. The polymerization catalyst may also be a Ziegler catalyst, a metallocene catalyst, or the like, but is not particularly limited.

[0019] Examples of polyethylene resins include high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), ethylene-ethyl acrylate copolymer (EEA), and ethylene-butyl acrylate copolymer (EBA). If necessary, copolymers of ethylene monomers with other copolymerizable monomers can also be used. These polyethylene resins can be used alone or in blends of two or more. There are no particular limitations on the polymerization method for these polyethylene resins, and any of high-pressure, slurry, solution, and gas-phase methods can be used. The polymerization catalyst can also be a Ziegler catalyst, a metallocene catalyst, or other suitable catalyst.

[0020] The thermoplastic elastomer resin may be any of the conventionally known thermoplastic elastomers, such as polystyrene thermoplastic elastomers (SBC, TPS), polyolefin thermoplastic elastomers (TPO), vinyl chloride thermoplastic elastomers (TPVC), polyurethane thermoplastic elastomers (TPU), polyester thermoplastic elastomers (TPEE, TPC), polyamide thermoplastic elastomers (TPAE, TPA), and polybutadiene thermoplastic elastomers.

[0021] <Foaming agent> The polyolefin resin sheet of the present invention is produced by mixing a polyolefin resin composition with a foaming agent capable of generating gas. Examples of production methods include an atmospheric foaming method in which a thermally decomposable chemical foaming agent is added to the polyolefin resin composition as a foaming agent, melt-kneaded, and foamed by heating at atmospheric pressure; an extrusion foaming method in which a thermally decomposable chemical foaming agent is thermally decomposed in an extruder and foamed while being extruded under high pressure; and a press foaming method in which a thermally decomposable chemical foaming agent is thermally decomposed in a press mold and foamed while being reduced in pressure.

[0022] In the present invention, a chemical blowing agent that decomposes upon application of heat and releases gas, i.e., a thermal decomposition type chemical blowing agent, is preferably used. Specific examples include organic blowing agents such as azodicarbonamide, N,N'-dinitrosopentamethylenetetramine, and p,p'-oxybenzenesulfonylhydrazide.

[0023] <Crosslinking agent> The polyolefin resin to be used in the resin foam of the present invention is preferably a crosslinked polyolefin resin, which can be produced using a crosslinking aid.

[0024] In the present invention, a polyfunctional monomer can be used as a crosslinking aid. Examples of the polyfunctional monomer include divinylbenzene, diallylbenzene, divinylnaphthalene, divinylbiphenyl, divinylcarbazole, divinylpyridine, and their nucleus-substituted compounds and closely related homologues, acrylic acid compounds or methacrylic acid compounds such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, butylene glycol diacrylate, and butylene glycol dimethacrylate, and vinyl esters of aliphatic dicarboxylic acids or aromatic dicarboxylic acids.

[0025] As described above, the polyolefin resin foam sheet of the present invention is based on a polyolefin resin, and although it is not essential to add all of the additives described above, a flame retardant, an inorganic additive, etc. may also be added, and the resulting mixture is melt-kneaded in an extruder or the like to produce a sheet. Thereafter, it is preferable to crosslink the resulting sheet by, for example, the following method.

[0026] <Crosslinking method> In the present invention, the degree of crosslinking is determined taking into consideration moldability and applications. Methods for crosslinking polyolefin-based resin compositions include chemical crosslinking and ionizing radiation crosslinking, and both may be used in combination. Specifically, a predetermined amount of the polyolefin-based resin composition is uniformly melt-kneaded using a kneading device such as a single-screw extruder, a twin-screw extruder, a Banbury mixer, a kneader mixer, or a mixing roll at a temperature below the decomposition temperature of the thermally decomposable foaming agent, and then formed into a sheet. The resulting sheet is then irradiated with a predetermined dose of ionizing radiation to crosslink the resin. Examples of ionizing radiation include electron beams, X-rays, β-rays, and γ-rays. The irradiation dose is generally about 1 to 300 kGy and is set depending on the desired gel fraction. Instead of crosslinking by ionizing radiation, crosslinking by peroxide or silane crosslinking may also be used.

[0027] <Foaming method> In the present invention, when a thermally decomposable chemical foaming agent is added as a foaming agent, the sheet-like material can be foamed by heating it to a predetermined temperature by some method.

[0028] The sheet is irradiated with ionizing radiation and crosslinked, and then brought into contact with a high-temperature salt bath, hot air, a heat medium, etc. to decompose the thermally decomposable foaming agent, thereby obtaining a crosslinked foam. When foaming is performed with hot air, it is called hot air foaming, and when foaming is performed with a heat medium of salt, it is called salt foaming.

[0029] In the case of salt foaming, a sodium nitrite-based salt bath is often used, and the processing temperature is determined by the foaming temperature of the foaming agent, with foaming often occurring at around 230°C. The polyolefin resin sheet to be foamed is heated and foamed by floating it on the salt bath. Stable foaming occurs due to stable heat transfer from the molten inorganic salt to the floating sheet, but the degree of foaming on the non-adhered surface of the sheet may be poor. In such cases, it is possible to heat the non-adhered surface with an infrared heater or similar to ensure a uniform foaming state.

[0030] After foaming, the surface of the foam is washed with pure water and dried to obtain a polyolefin resin foam sheet that can be used normally.

[0031] Examples of foaming methods other than salt foaming include a roll-type heat foaming method or a belt-type heat foaming method in which a sheet is supplied between a pair of heating rolls or heating belts arranged opposite to each other and heated, and a hot air constant temperature heat foaming method in which hot air is blown onto the sheet.

[0032] However, in these foaming methods, the stretching speed often varies depending on the position on the sheet, compared to salt foaming, and the amount of change in the heating angle in the width direction tends to be large.

[0033] <Corona treatment> Polyolefin resin foam sheets, when foamed by a foaming method such as the salt foaming method or the hot air foaming method, have low adhesive strength not only to the resin itself but also to adhesives, etc. Therefore, the surface of the foam sheet cannot be pasted with other materials or decorated, limiting the range of applications.

[0034] To expand the range of applications for polyolefin-based resin foam sheets, surface treatment has been widely implemented since their development. The effectiveness of surface treatment has been quantified using the wetting tension of the foam sheet surface. Surface treatment techniques such as corona treatment and plasma treatment can be used. Of these, corona treatment is preferred because it uses electricity as the energy source, is easy to control, and is readily available for industrial use.

[0035] The wetting tension of the surface of the foamed sheet due to the surface treatment can be evaluated in accordance with JIS K6768 using mixtures for wetting tension Nos. 30 to 58 manufactured by Wako Pure Chemical Industries, Ltd.

[0036] A wet tension of less than 40 mN / m is undesirable because it leaves fundamental issues unresolved, such as weak adhesion to other materials. Furthermore, attempting a surface treatment with a wet tension exceeding 60 mN / m results in the development of through-holes called discharge holes in the foam, impairing its practical utility as a foam. For these reasons, a wet tension of 40 mN / m or more and 60 mN / m or less is preferred.

[0037] <Corona treatment device> Corona treatment devices are developed and sold in Japan by several equipment manufacturers. There are no particular limitations on the device, but the basic structure is that when high-frequency, high-voltage power generated by a high-frequency power supply is applied between the electrode and treatment roll, a corona discharge occurs, and the substrate to be treated passes under this discharge, resulting in corona treatment. The distance between the electrode and treatment roll is often a few millimeters, and the voltage between the electrodes is often 15 to 50 kV, with an AC frequency of around 45 kHz.

[0038] Corona treatment of polyolefin resin foam sheets generates functional groups such as peroxide groups, hydroxyl groups, carbonyl groups, carboxyl groups, and epoxy groups on the surface of the foam sheet. These functional groups increase the surface wetting tension.

[0039] Since corona treatment is often carried out in the atmosphere, ozone may be generated, so an ozone-generating gas removal system may be installed.

[0040] <Corona treatment device location> During corona treatment, if the polyolefin resin foam sheet has moisture or inorganic substances such as salt attached to it, corona discharge often causes through holes called discharge holes to form. Therefore, after salt foaming, it is important to remove the salt components with warm water and dry the sheet.

[0041] In order to ensure the removal of salt components and dehydration and drying, it has been common practice to separate the salt foaming process from the corona treatment process. However, the inventors discovered that when the salt foaming process and the corona treatment process are performed consecutively, the wetting tension tends to increase and the decrease in wetting tension is slow, leading to the present invention.

[0042] After corona treatment, the generated functional groups are gradually lost, and the wet tension gradually decreases. As shown in the examples, when corona treatment is performed as a separate process after salt foaming, the wet tension of a typical polyolefin resin foam sheet improves from about 34 mN / m to about 45 mN / m after corona treatment. When the sheet is then stored in a warehouse, this wet tension gradually decreases, and it takes about three months for the wet tension to remain at or above 40 mN / m.

[0043] On the other hand, when salt foaming and corona treatment are carried out consecutively in one process, the wet tension of the same polyolefin resin foam sheet as mentioned above improves to about 48 mN / m after corona treatment. After that, when the sheet is stored in a warehouse, this wet tension gradually decreases, but it was found that it can be maintained at 40 mN / m or higher for up to about six months.

[0044] The interval between the salt foaming step and the corona treatment step is preferably 10 minutes or less. If the interval between the steps is longer than this, the decrease in wetting tension will be large and rapid.

[0045] <Maximum heating angle change> Recently, there has been a great demand for polyolefin resin foam sheets, which are widely used for vehicle interior materials such as car doors and instrument panels. However, automobile designs have become more complex, often requiring deep drawing, which has been known to cause many problems during molding.

[0046] The present inventors have investigated the causes of molding problems and conducted studies to produce vehicle interior materials that are compatible with a variety of car designs with high productivity, and have found that the main cause is distortion and deformation of the polyolefin resin foam sheet during molding.As a method for quantifying the amount of distortion, they defined the heating angle change amount described below and found preferable conditions for the heating angle change amount of the sheet.

[0047] <Measurement of heating angle change in the width direction> The degree to which polyolefin resin foam sheets deform during molding varies from foam sheet to foam sheet, with sheets with large deformation more likely to result in molding defects. Furthermore, even the same foam sheet can have different degrees of deformation across the width. If there is a large difference across the width, molding defects may occur in some locations. In order to quantify and quantify these conditions, squares of a certain size were cut out at each position across the width of the foam sheet, and by drawing lines connecting the midpoints of each square, the state of shrinkage and deformation due to heating was observed.

[0048] As a result, it was found that when a polyolefin resin foam sheet is heated for 10 minutes in an environment of the maximum melting point +20°C, if the maximum heating angle change at the intersection of the midpoint connecting lines of each square in the width direction is -10 to +10 degrees, more preferably -5 to +5 degrees, productivity is high and stable.

[0049] The maximum melting point of the sample was measured using DSC. If there were multiple melting points, the highest value was recorded as the maximum melting point.

[0050] <Optimization of maximum heating angle change amount> The foaming method is the most effective method for limiting the maximum heating angle change in the width direction of a polyolefin resin foam sheet to -10 to +10 degrees or less. The most preferred foaming method is the salt foaming method. On the other hand, when the hot air foaming method is used, the maximum heating angle change in the width direction may greatly exceed 10 degrees in absolute value.

[0051] It was found that the second most important factor affecting the maximum heating angle change in the width direction of the sheet, after the foaming method, is the interval between the corona treatment and the foaming process. The inventors discovered that the maximum heating angle change is smallest when the salt foaming process and the corona treatment process are performed consecutively. A connection time of 10 minutes between the salt foaming process and the corona treatment process is preferred, as this minimizes molding defects during molding. [Example]

[0052] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Polyolefin-based resin foam sheets (hereinafter also referred to as resin foam sheets) of several types of examples and comparative examples described below were produced, and measurements of physical properties and evaluations of performance were carried out. First, the measurement and evaluation methods will be described.

[0053] (Maximum melting point) The maximum melting point of the base resin was measured for each type of resin using DSC at a heating rate of 10°C per minute in accordance with JIS K7121 (1987). If multiple melting points were confirmed, the melting point on the higher side was taken as the melting point, and the highest melting point among the resins used was taken as the maximum melting point.

[0054] (Foaming method) For salt foaming using a salt bath foaming tank, tests were conducted with reference to JP-A-2-175734, and for hot air foaming using a heated hot air foaming tank, tests were conducted with reference to JP-A-6-172575. In the salt bath foaming tank, a sodium nitrite-based salt bath was used.

[0055] (Corona treatment) When the corona treatment was performed consecutively with the salt foaming process, the foam was heated to 230°C in a salt bath and then cooled and dried to room temperature in a combination of an air blower and a water bath, followed by the corona treatment. The interval between the foaming process and the corona treatment machine was adjusted by the length of the cooling and drying process between the two processes and the amount of deflection of the foam.

[0056] When the corona treatment was not performed consecutively with the salt foaming step, the foam was cooled and dried to room temperature using a combination of airflow and a water bath, then rolled up and treated the next day using a corona treatment device.

[0057] When hot air foaming was used as the foaming process, the process was arranged in the same way as the salt foaming described above, and the test was carried out.

[0058] (wetting tension) The wet tension mixture manufactured by Wako Pure Chemical Industries, Ltd. was used and evaluated in accordance with JIS K6768. Measurements were taken immediately after corona treatment, and three and six months later.

[0059] (Thickness measurement) The thickness of the resin foam sheet was measured in accordance with ISO 1923 (1981) "Measurement of linear dimensions of foamed plastics and rubber." Specifically, the resin foam sheet was placed on a flat table and measured at a 10 cm 2 A dial gauge with a circular probe having an area of ​​10g / 10cm was placed on the surface of a resin foam sheet. 2 The measurement was carried out by contacting the sample with a constant pressure of 1000 kJ / cm.

[0060] (density measurement) The density of the resin foam sheet was measured and calculated in accordance with JIS K6767 (1999) "Foam plastics - Polyethylene - Test method." Specifically, the thickness and mass of a 10 cm square test piece (resin foam sheet) were measured, and the density was calculated using the following formula. Density (g / cm 3 ) = mass of test piece (g) / [test piece area 100 (cm 2 ) × thickness of test piece (cm)]

[0061] (stretch) Measurements and calculations were made in accordance with JIS K6767 (1999) "Foamed plastics - Polyethylene - Test method."

[0062] (Angle change amount) Square samples with sides of 10 cm were cut perpendicular to the foam sheet from three locations: two at both ends and one in the center, which were in the same longitudinal position. Connecting lines were drawn with a marker to connect the midpoints of each pair of squares to create sample pieces. These sample pieces were placed in a heating furnace set at the maximum melting point +20°C for 10 minutes, after which the sample pieces were observed and quantified. The deformation was quantified by calculating the change in the heating angle in the width direction for the angle of the midpoint connecting lines, using, for example, a digital protractor (Shinwa Measurement Co., Ltd.).

[0063] Please note the following points during the exam:

[0064] 1st point: Before heating The front and back of the sample must be the same for the test. Then, measure the angle of the connecting line connecting the midpoints of the pair of lines of the square before heating. The measurement position should be the angle at the top right when looking at the square from the same direction. Make sure that it is at 90° before heating.

[0065] 2nd point: After heating The angle is measured at the same position before heating and calculated as the change in heating angle from 90° before heating. After heating, the angle may be greater than or less than 90°, so the change in heating angle may be either a positive or negative value. The maximum change in heating angle is the value with the largest absolute value among three samples at the same position in the width direction. Therefore, the maximum change in heating angle may be either a positive or negative value.

[0066] (Molding evaluation) The foam moldability was evaluated as follows. For the evaluation, the foam was heated on a vertical cylindrical female mold with a diameter of 50 mm and a depth of H, and then straight-formed using a vacuum forming machine. The foam was heated until the surface temperature reached 160°C, and straight-formed using the vacuum forming machine. The appearance, particularly the surface on the skin side, was observed and judged as good or bad. The feel was also judged by pressing the surface on the skin side with a finger. 5: It feels sufficiently flexible and has a beautiful and excellent appearance. 4: It still feels flexible and has no visible defects such as dents. 3: There is no problem with the appearance, but the flexibility has decreased or it feels like the ball is hitting the bottom, or there is no problem with the feel, but there are some defects in the appearance. 2: The product is in a significantly poor condition, with either a lack of flexibility to the touch or major defects in appearance. 1: The product is in a state where it is significantly defective in both feel and appearance and is no longer suitable for use as a product.

[0067] <Polyethylene, polypropylene resin> Polypropylene resin A "Novatec PP EG6D" manufactured by Japan Polypropylene Corporation Density: 900kg / m 3 , MFR=1.9g / 10min, Melting point=139℃ Polypropylene resin B Prime Polymer "Prime Polypro" (registered trademark) J452HP Density: 900kg / m 3 , MFR(230℃)=3.5g / 10min, melting point=162℃. Polypropylene resin C SunAllomer Co., Ltd. "PB222A" Density: 900kg / m 3 , MFR=0.75g / 10min, Melting point: 147℃ Polyethylene resin A "Novatec UJ960" manufactured by Japan Polyethylene Corporation Density: 935kg / m 3 , MFR=5.0g / 10min Polyethylene resin B "Petrosen" (registered trademark) 205 manufactured by Tosoh Corporation Density: 924kg / m 3 , MFR=3.0g / 10min, Melting point: 115℃ The density, MFR, melting point, etc. of each resin are listed as average values ​​for the types used in the test. There is some variation in each characteristic value, but as long as it is within the standard range of variation set by each manufacturer, there will be no major differences in the physical properties of the foam obtained, and it will not affect the results in any way.

[0068] <Foaming agent> - Azodicarbonamide "Vinihall AC#R" manufactured by Eiwa Chemical Industry Co., Ltd.

[0069] <Crosslinking aid> DVB: Nippon Steel Chemical DVB-570

[0070] <Antioxidants> "IRGANOX1010" manufactured by BASF Japan Ltd.

[0071] Example 1 100 parts by mass of a base resin consisting of 80 parts by mass of polypropylene-based resin A and 20 parts by mass of polyethylene-based resin were blended with 6.1 parts by mass of a blowing agent, 4.0 parts by mass of a crosslinking aid, and 0.2 parts by mass of an antioxidant. These materials were mixed in a Henschel mixer and fed into an extruder. The temperature inside the cylinder was adjusted to 180-200°C, and the mixture was melted and kneaded. The mixture was then extruded into a 1.4 mm-thick sheet to obtain a sheet-shaped polyolefin-based resin foamable composition. The resulting sheet-shaped polyolefin-based resin foamable composition was crosslinked by irradiating it with an electron beam at an accelerating voltage of 50 kGy. The crosslinked sheet-shaped polyolefin-based resin composition was then introduced into a salt bath foaming tank, the temperature of which was set at 230°C, above the decomposition temperature of the added blowing agent, and foamed. The residence time in the salt bath foaming tank was 108 seconds. Following the salt bath foaming tank, the mixture passed through a washing and drying tank, which combined multiple air-blowing tanks and water baths. The film was left in the washing and drying tank for 120 seconds, after which it was subjected to corona treatment at an inter-electrode voltage of 4 kV and an AC frequency of 30 kHz, and then wound up.

[0072] (Comparative Example 1) The electron beam cross-linked polyolefin resin foamable composition produced in Example 1 was subjected to salt foaming, washing, and drying under the same conditions as in Example 1. Thereafter, the composition was wound up without corona treatment. The next day, corona treatment was carried out under the same conditions as in Example 1.

[0073] Example 2 A polyolefin resin foam sheet was produced in the same manner as in Example 1, except that the inter-electrode voltage in the corona treatment was changed to 3.5 kV.

[0074] Examples 3 to 6 A polyolefin resin foam sheet was produced in the same manner as in Example 1, except that the resin composition, additives, and production conditions were as shown in Table 1.

[0075] Example 7 The electron beam crosslinked polyolefin resin foamable composition produced in Example 1 was passed through a vertical heated hot air foaming tank set to 250°C and foamed. The residence time in the vertical heated hot air foaming tank was 180 seconds. After leaving the heated hot air foaming tank, the composition was cooled by a cooling roll and a conveying process, then subjected to corona treatment at an interelectrode voltage of 4 kV and an AC frequency of 30 kHz, the same as in Example 1, and wound up.

[0076] (Comparative Example 2) The electron beam cross-linked polyolefin resin foamable composition produced in Example 1 was subjected to hot air foaming and drying under the same conditions as in Example 1. Thereafter, the composition was wound up without corona treatment. The next day, corona treatment was carried out under the same conditions as in Example 3.

[0077] (Comparative Example 3) The electron beam cross-linked polyolefin resin foamable composition produced in Example 1 was subjected to salt foaming, washing, and drying under the same conditions as in Example 1. Thereafter, the composition was wound up without corona treatment. After 15 minutes, corona treatment was carried out under the same conditions as in Example 1.

[0078] Example 8 A polyolefin resin foam sheet was produced in the same manner as in Example 1, except that the residence time in the salt bath foaming tank was changed to 115 seconds and the residence time in the washing and drying tank to 128 seconds.

[0079] Table 1

Claims

1. A polyolefin resin foam sheet that is continuously subjected to a corona treatment after completion of a heat foaming step, and has a maximum heating angle change in the width direction of -10 to +10 degrees when heated for 10 minutes in an environment of a temperature 20°C higher than the maximum melting point of the base resin.

2. 2. The polyolefin resin foam sheet according to claim 1, which has a wet tension of 40 mN / m or more three months after the corona treatment.

3. 3. The polyolefin resin foam sheet according to claim 1, wherein the heat-foaming step is a salt foaming step.

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