JPH06184343A - Crosslinked foamed polyolefinic resin sheet and its production - Google Patents

Abstract

PURPOSE:To provide a sheet of improved surface smoothness, by performing the thermal expansion of an electron-beam-cross-linkable and expandable polyolefinic resin sheet by heating by means of an infrared heater and hot air under conditions such that the heating by the infrared heater prevails over that by hot air. CONSTITUTION:A composition prepared by mixing a PP-based polyolefinic resin with specified amounts of additives such as an antioxidant is extrusion- molded into a sheet by the T-die process or the like. This sheet is irradiated with electron beams to form an expanded resin sheet having a gel fraction of 40% or above and a specified thickness. The obtained sheet is introduced into a vertical foaming oven heated by means of two opposed infrared heaters of a surface temperature of 260-270 deg.C and hot air having a temperature of 240-250 deg.C at the inlet of the oven and expanded to obtain a crosslinked foamed polyolefinic resin sheet having a skin layer on the surface, excellent surface smoothness and an apparent density of 0.040-0.100g/cm<3>.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin resin foam sheet having a skin layer on its surface and a method for producing the same. Since the foamed sheet of the invention of the present application has a skin layer and has a smooth surface, it has good adhesion to the skin layer when adhering the skin layer, and therefore can be formed by a stamping molding method or the like. Yes, it can be used for molded products such as automobile interior moldings, bags, and video cases.

[0002]

2. Description of the Related Art Conventionally, in a so-called stamping mold method, a polyolefin resin crosslinked foamed sheet having a skin layer and a foam layer is laminated with a thermoplastic resin in a molding mold in a molten state and integrally molded by a press or the like. In order to obtain a better product appearance, many innovations have been made to polyolefin resin crosslinked foam sheets. For example,
Japanese Unexamined Patent Publication (Kokai) No. 61-119034 describes that a film made of a synthetic resin material similar to that of the foam is laminated on both sides of the foam, and JP-A 1-222293.
Japanese Patent No. 6 describes a sheet in which a foam slice product having a high gel fraction is attached to the surface of a foam. In these polyolefin resin crosslinked foamed sheets, thermoplastic resin in a high temperature molten state comes into contact with the surface of the foamed body, and further, a part of the foamed body is melted by high temperature compression and high temperature shearing, resulting in unevenness. In order to prevent the deterioration of the surface of the foamed sheet, the surface smoothness of the foamed sheet is improved, and at the same time, heat resistance near the surface is given.

[0003]

However, these conventional polyolefin-based resin crosslinked foamed sheets laminate another material on the surface of the foamed sheet, and a process therefor is required, so that only the cost is increased. Not
There is a problem that the deep drawability when molding a product having a complicated shape is poor, and in an extreme case, creases and peeling from the laminate layer occur, resulting in a high defective rate of the product. In particular, in recent years, from the standpoint of protecting the global environment, there has been a strong demand for simplification of the process, including the meaning of measures for industrial waste reduction, but the conventional ones cannot meet these demands.

[0004]

Means for Solving the Problems The inventors of the present application have conducted extensive studies on a foam sheet satisfying these demands, and as a result, have improved the heating conditions during foaming to provide a skin layer on the surface. It is possible to manufacture a foamed sheet. The foamed sheet having the skin layer of the invention of the present application not only has a smooth surface, but also gives good results in stamping moldability.

The polyolefin resin crosslinked foam of the present invention is a polyethylene resin, a mixed resin of an ethylene-propylene random copolymer having an ethylene content of 1 to 15% by weight and polyethylene, or a resin having 4 or more carbon atoms. A resin such as a so-called resin mixed with linear polyethylene in which α-olefin is copolymerized is used, but the resin is not limited to these resins. However, considering the heat resistance of the molded product, the melting temperature of the resin during stamping molding, and the flow state of the resin, a resin composition containing polypropylene as a main component is preferable.

The invention of the present application is based on the above resin, a foaming agent,
A foaming resin sheet is produced by blending a cross-linking aid, an antioxidant, etc., by extrusion molding, and then irradiating with an electron beam to cross-link to obtain a foaming resin sheet. The sheet can be manufactured by a conventional manufacturing method and is not particularly limited.

The foam cross-linking degree of the invention of this application is as follows.
Percentage of insoluble components based on the original foam weight after 24 hours of extraction with 130 ° C. p-xylene (gel fraction)
And the value is 40% or more, preferably 5
It is preferably 0% or more, and the foam density is 0.040 g / cm ³ or more, preferably 0.050 g / cm ³ or more. When the gel fraction is 40% or less, the foam is partially melted during stamping molding, resulting in surface irregularities, and when the foam density is 0.040 g / cm ³ or less, the cells are crushed during stamping molding. This is because the appearance of the molded product will be impaired.

Next, in order to heat and foam the foamable resin sheet in the foaming furnace, the foaming furnace may be a vertical type (the foaming resin sheet is introduced from the upper portion and foamed while hanging down) or a horizontal type (foaming). Resin sheet is supported in the lateral direction and foamed while moving at low speed).
It is necessary to have a foaming furnace that uses both a heating method using an infrared heater and a heating method using hot air. That is, in the case of only the infrared heater, the heated foamable resin sheet is foamed from the inner central portion of the foamable resin sheet,
Since the heat is easily dissipated at the end and the surface, the temperature is slightly lowered, and it becomes unfoamed. On the other hand, in the case of only hot air heating, since the foamable resin sheet is heated from the surface portion, not only the foamed end portion is melted but also the surface of the foamed sheet is heated up to a temperature at which the entire sheet can be foamed. Due to excessive heating, heat deterioration of the resin occurs, the appearance is deteriorated, and the physical properties of the foam tend to be remarkably impaired. In order to solve these problems, hot air and infrared heater heating are used in combination, and heating is performed so as to compensate for the disadvantages of each other, to produce a uniform foam.

By the way, the inventors of the present application, when the heating condition of the infrared heater is strengthened and at the same time the hot air temperature is set to be slightly lower to perform the foaming, the foam sheet obtained has a certain thickness. It was found that the surface skin layer of, and the surface of this foam sheet is smooth, and that stamping molding using this foam sheet gives a beautiful molded article with no unevenness, The invention of this application was completed.

In order to produce the crosslinked foamed polyolefin resin sheet of the present invention, the expandable resin sheet is entirely preheated in a foaming furnace and heated to a temperature just before decomposition of the foaming agent. Next, in foaming, conditions such as infrared heater surface temperature, heater distance, hot air temperature, and wind speed are determined in order to obtain an equal temperature distribution inside the foamable resin sheet and the entire surface. Then, at this time, it is important to foam under the condition that the infrared heating is made stronger. That is, while the foamable resin sheet surface portion is still in a non-foamed and soft state, the inside of the foamable resin sheet is rapidly foamed by an infrared heater to stretch the unfoamed portion of the surface in a two-dimensional direction. In addition, a foamed sheet covered with a skin layer free from resin deterioration is manufactured. The heating by infrared rays referred to in the invention of this application is performed under a condition that is stronger than the hot air heating, for example, to make the surface temperature of the infrared heater higher than the hot air temperature.

The operating conditions for producing the crosslinked foamed polyolefin resin sheet of the present invention are as follows:
It depends on the type or amount of the foaming agent, the thickness of the foamable resin sheet, and the color of the polyolefin resin crosslinked foamable sheet. For example, polypropylene resin as a main raw material resin and azodicarbonamide (ADCA) as a foaming agent.
A case where a yellow foamable resin sheet having a thickness of 1.5 mm is foamed by using is described.

From the upper part of the foaming furnace to a vertical foaming type furnace having a structure in which a foamable resin sheet can be introduced at a constant speed between two infrared heater panels facing each other and at a constant speed. Introducing a foamable resin sheet, first
Heat uniformly throughout the preheating chamber, then 240-250
It is introduced into the foaming chamber kept at 0 ° C. to start foaming (this point is called the foaming starting point). In the invention of this application, a skin layer is formed on the surface by making the heating condition of the infrared heater stronger than that of the hot air, and foaming the infrared heater at a temperature condition of 260 ° C. and hot air heating of 240 ° C. A sheet having excellent surface smoothness is produced. The thickness of this skin layer or foam density is
The infrared heater can be controlled by adjusting the temperature difference of hot air heating. When the foam density is 0.068 g / cm ³ , for example, a skin layer having a thickness of 23 μm is formed under the above-mentioned conditions. Not only unevenness but also wrinkles start to stand out. Next, the invention of the present application will be described with reference to Examples and Comparative Examples, but the invention of the present application is not limited thereto.

[0013]

【Example】

Example 1 70% polypropylene resin in which 4% by weight of ethylene were randomly copolymerized and 30% of linear polyethylene resin in which α6 olefin having a density of 0.920 g / cm ³ was copolymerized. 6 parts by weight of azodicarbonamide as a foaming agent was mixed with 100 parts of the mixed resin, and a foaming composition in which a crosslinking aid, an antioxidant and the like were further mixed was extruded to a sheet thickness of 1.5 mm by a T-die method, An expandable resin sheet was produced by irradiating with an electron beam so that the average gel fraction was 50%. This was placed in an infrared heater having a surface temperature of 260 ° C. and a vertical foaming furnace heated by hot air having a temperature of 240 ° C. at the hot air generating port in the furnace for foaming, whereby a smooth surface was obtained. A 3 mm foam sheet was produced. This foam has an apparent density of 0.068 g /
It was cm ³ . When a cross section of this foamed sheet was observed with a stereoscopic microscope, it was confirmed that a skin layer having an average thickness of 23 μm was formed on the surface. By laminating a vinyl chloride skin material sheet on this foamed sheet and performing stamping molding, a molded product excellent in mold followability and appearance was obtained. The results are shown in Table 1.

Example 2 The expandable foamable resin sheet obtained by the same method as in Example 1 was placed in a vertical foaming furnace heated by an infrared heater having a surface temperature of 270 ° C. and hot air at 235 ° C. By foaming, a foamed sheet having a smooth surface and a thickness of 3 mm was obtained. The apparent density of this foam is 0.068 g / c
It was m ³ . When a cross section of this foamed sheet was observed with a stereoscopic microscope, it was confirmed that a skin layer having an average thickness of 42 μm was formed on the surface. By laminating a vinyl chloride skin material sheet on this foamed sheet and performing stamping molding, a molded product excellent in mold followability and appearance was obtained. The results are shown in Table 1.

Example 3 A foamable resin sheet obtained by the same method as in Example 1 was foamed in a vertical foaming furnace heated with hot air having a surface temperature of 280 ° C. to give a smooth surface. Although a foamed sheet having a thickness of 3 mm was obtained, it had a striped pattern in the longitudinal direction, but the density was uniform and the average apparent density was 0.
It was 069 g / cm ³ . When the cross section of this foamed sheet was observed with a stereoscopic microscope, it was confirmed that a skin layer having an average thickness of 65 μm was formed on the surface. When a vinyl chloride skin material sheet was laminated on this foamed sheet and subjected to stamping molding, a molded product with a crease in the deep-drawn portion was obtained, and a molded product with a good appearance could not be obtained. The results are shown in Table 1.

Comparative Examples 1 and 2 Under the same conditions as in Examples, except that in the case of Comparative Example 1, heating was performed with an infrared heater having a surface temperature of 250 ° C. and hot air of 250 ° C., and in the case of Comparative Example, Surface temperature is 260 ℃
Was heated with an infrared heater of No. 2 and hot air of 260 ° C., and foamed in a vertical foaming furnace. The resulting foam is
In each case, the skin layer on the surface was thin, and the surface was uneven. Further, when stamping molding was performed in a state where a vinyl chloride skin material sheet was laminated, a fine uneven pattern was generated on the surface, and a molded product having a good appearance could not be obtained.
The results are shown in Table 1.

[0017]

[Table 1]

[0018]

INDUSTRIAL APPLICABILITY According to the invention of the present application, a foamed sheet obtained by heating and foaming by an infrared heater under a stronger condition than hot air heating has a skin layer formed on the surface, and the skin layer forms a skin layer. In addition to being smooth, a laminated product obtained by laminating a skin material on the foamed sheet is excellent in deep drawability and a molded product having a good appearance can be obtained. Further, when the polyolefin resin foamable resin of the invention of this application is produced, the conventional operation method may be used without using special equipment, and the product acquisition rate is high, which is an excellent effect.

Claims (2)

[Claims] 1. A foamed body having a skin layer of 20 to 60 μm on the surface thereof, a gel fraction of 40% or more, and an apparent density of 0.
A polyolefin resin crosslinked foamed sheet of 040 g / cm ^{3 to} 0.100 g / cm ³ . 2. When the electron beam crosslinkable expandable polyolefin-based resin sheet is heated and foamed by an infrared heater and hot air, the infrared heater is heated and foamed under a condition stronger than the hot air heating. The method for producing a crosslinked foamed polyolefin resin sheet according to [4].