JPH0570622A - Polypropylene-based resin crosslinked foamed body - Google Patents

Abstract

PURPOSE:To obtain the title formed body useful for heat insulating material, etc., having excellent surface peel strength, molding and processing properties by controlling distribution of expansion ratio in the thickness direction in a shape such as chervon of rising from both surface layers toward the central part. CONSTITUTION:For example, 100 pts.wt. polyolefinic resin such as PE is blended with 15 pts.wt. blowing agent such as azodicarbonamide and 1 pt.wt. zinc flower, irradiated with ionizing radiation such as electron rays and crosslinked to give the objective foam wherein distribution of expansion ratio in the thickness direction is controlled substantially in a chevron or trapezoidal shape of rising from both surface layers toward the central part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin resin crosslinked foam, and more particularly to a sheet-shaped polyolefin resin crosslinked foam excellent in skin peel strength and molding processability.

[0002]

2. Description of the Related Art Polyolefin resin crosslinked foams are obtained by blending a polyolefin resin with a thermal decomposition type foaming agent and other additives, and extruding the composition into a sheet at a temperature lower than the decomposition temperature of the foaming agent. It is produced by crosslinking a foamable resin composition sheet and then heating it to a temperature equal to or higher than the decomposition temperature of a foaming agent for foaming.

Crosslinking methods include an irradiation crosslinking method of irradiating ionizing radiation such as an electron beam, a chemical crosslinking method using an organic peroxide, and a foaming method of a vertical or horizontal heating furnace. Among them, there are a method of foaming by heating with hot air (gas phase method) and a method of immersing in a heated liquid bath (liquid phase method).

The polyolefin resin cross-linked foam thus obtained is used in a wide range of fields as a heat insulating material, a cushioning material and the like. However, the conventional polyolefin resin crosslinked foam has poor moldability, and it is extremely difficult to mold it into a complex and deep shape, for example. The reason is that, according to the conventional manufacturing method, the distribution of the expansion ratio in the thickness direction of the sheet-shaped foam cannot be controlled, and the expansion ratio in the thickness direction of the obtained crosslinked foam becomes uneven, As a result, the moldability may vary. Further, the peeling strength of the skin is low, and there is a problem of breakage such as tearing at the time of molding at high temperature.

Conventionally, in order to improve the molding processability of a polyolefin resin cross-linked foam, a method of improving mechanical properties by blending linear low-density polyethylene (L-LDPE) has been proposed (special feature: Fair 2-5757
No. 6, Japanese Patent Publication No. 2-57577). However, in the methods disclosed in these publications, the distribution of the foaming ratio in the thickness direction cannot be controlled, and the moldability varies.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a polyolefin resin crosslinked foam which is superior in molding processability to the conventional one by solving the above problems.

As a result of intensive studies, the inventors of the present invention have found that the distribution of the expansion ratio of the sheet-shaped foam produced by the conventional manufacturing method is not uniform in the thickness direction, whereas the skin layers on both sides are It has been found that if the foaming ratio distribution is controlled so as to increase in the shape of a mountain or a trapezoid toward the center, there will be no variation in moldability. Further, if the distribution of the expansion ratio in the thickness direction is controlled to have such a shape, the skin peeling strength of the foam will be improved and the moldability will be improved. The present invention has been completed based on these findings.

[0008]

That is, the gist of the present invention is as follows.
The sheet-like polyolefin resin crosslinked foam is characterized in that the distribution of the expansion ratio in the thickness direction is a shape in which a peak shape or a trapezoid shape is substantially increased from both surface layers toward the center. ..

The present invention will be described in detail below. The polyolefin resin cross-linked foam of the present invention is basically produced by a method of crosslinking a foamable resin composition sheet containing a polyolefin resin and a thermal decomposition type foaming agent, and then heat-foaming.

The polyolefin resin used in the present invention includes low density polyethylene, medium to high density polyethylene, linear low density polyethylene, polypropylene, butene-propylene copolymer, ethylene-propylene copolymer, ethylene-acetic acid. Examples thereof include vinyl copolymers, vinyl chloride-ethylene copolymers and ethylene-acrylate copolymers, and these can be used alone or in combination of two or more.

As the foaming agent, azodicarbonamide,
A thermal decomposition type foaming agent such as oxybenzenesulfonyl hydrazide, nitrosopentamethylenetetramine, etc. that decomposes by heat to generate bubbles is used. Although the number of parts to be added is changed depending on the desired expansion ratio, it is generally about 2 to 25 parts by weight with respect to 100 parts by weight of the polyolefin resin.

The polyolefin resin includes, as desired, a foaming aid such as a metal salt of a fatty acid or zinc white that improves the decomposability of the foaming agent, a crosslinking aid such as trimethylolpropane trimethacrylate, an antioxidant, a colorant, Various additives such as flame retardants and fillers can be added. When chemically cross-linking, an organic peroxide such as dicumyl peroxide or benzoyl peroxide is added.

Examples of the cross-linking method include a method of irradiating ionizing radiation such as an electron beam, a chemical cross-linking method using an organic peroxide, or a water-cross-linking method in which a cross-linking reactive vinyl methoxysilane or the like is grafted or copolymerized with a resin. And the like, and the foaming method includes a vapor phase method and a liquid phase method.

As a method for controlling the foaming ratio distribution of the foam in the thickness direction so as to be substantially mountain-shaped or trapezoidal from both surface layers toward the center, Such a method can be cited.

(1) There is a method of controlling the ambient temperature at the time of foaming by heating. When heating and foaming by a gas phase method using a vertical furnace or a horizontal furnace, the atmospheric temperature in the furnace is adjusted to 230 ° C. or lower. Further, when heating and foaming by a liquid phase method using a salt bath or the like, the temperature of the liquid bath is 230 ° C.
Adjust as follows.

When the foamable resin composition sheet is rapidly heated and foamed at a high temperature, the foaming of the surface layer proceeds as compared with the degree of foaming inside the sheet, and for example, as shown in FIG. It becomes a distribution, and the expansion ratio of the central part becomes low. Such a sheet foam has low skin strength and poor moldability. Further, since the distribution of the foaming ratio becomes non-uniform, the moldability varies.

On the other hand, when the atmospheric temperature during heat-foaming is adjusted, for example, as shown in FIG. 1, both surface layers having a low foaming ratio increase substantially in the shape of a mountain or a trapezoid from the center toward the center. The uniform distribution of the foaming ratio is obtained, and the skin strength is also increased.

(2) There is a method of rapidly cooling the surface layer of the foam immediately after foaming. When the foamable resin composition sheet is foamed in a high-temperature atmosphere, and immediately after that, the surface layer is rapidly cooled, the heat generated by the foaming is used for the internal foaming, so that the foaming ratio of the interior becomes higher and It is possible to obtain a foaming ratio distribution in the thickness direction of a mold or a trapezoidal shape.

The surface layer may be rapidly cooled by blowing cold air onto the foam sheet immediately after foaming, immersing the foam in cold water, passing the foam through a cooling roll, or the like. However, in order to quench only the surface layer, the contact with these cooling media is controlled for a very short time, for example, within about 10 seconds. Since the contact time with the cooling medium changes depending on the type of the cooling medium, the ambient temperature at the time of foaming, etc., it is desirable to conduct experiments in advance to determine preferable conditions.

[0020]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[Example 1] Low density polyethylene (LDP)
E: MI = 4.0, density 0.923) To 100 parts by weight, 15 parts by weight of azodicarbonamide and 1 part by weight of zinc white were added and kneaded with a Henschel mixer. This mixture was put into a 65 mmφ extruder (L / D = 26), melt-kneaded and extruded from a T-die, thickness 1.5 mm, width 450
mm foamable resin composition sheet was molded.

The foamable resin composition sheet was irradiated with an electron beam of 3 Mrad to be crosslinked, and foamed in a rigid foaming furnace at an ambient temperature of 230 ° C. to obtain a sheet-like foam having a thickness of 5 mm and a width of 1200 mm. ..

The result of measuring the distribution of the foaming ratio in the thickness direction of the obtained foam is shown in FIG. Table 1 shows the measurement results of physical properties.

[Example 2] The same as Example 1 except that the foaming was carried out in a horizontal foaming furnace.

Example 3 The procedure of Example 1 was repeated except that the foaming was performed in a salt bath at 220 ° C.

Example 4 The procedure of Example 1 was repeated except that 80 parts by weight of polypropylene and 20 parts by weight of LDPE were blended and used as the resin component.

[Example 5] The same procedure as in Example 4 was carried out except that the foaming was carried out in a horizontal foaming furnace.

[Example 6] The procedure of Example 4 was repeated except that the foaming was performed in a salt bath at 220 ° C.

[Example 7] The foaming temperature was set to 240 ° C,
Immediately after foaming, the same procedure as in Example 1 was carried out except that the surface of the foam was cooled by immersing in water at 40 ° C. for 7 seconds.

[Embodiment 8] With a foaming temperature of 240 ° C.,
Immediately after foaming, the same procedure as in Example 6 was carried out except that the surface of the foam was cooled by immersing in water at 40 ° C. for 7 seconds.

[Comparative Example 1] The procedure of Example 1 was repeated except that the foaming temperature was 260 ° C. The result of measuring the distribution of the expansion ratio in the thickness direction of the obtained foam is shown in FIG.

[Comparative Example 2] The procedure of Example 8 was repeated except that the cooling time was 16 seconds.

Table 1 shows the results of measuring the physical properties of the sheet-like foams obtained in the above Examples and Comparative Examples. The methods for measuring the physical properties are as follows.

<Vacuum Formability (H / L)> The foam is set with a far infrared heater so that the surface temperature of the foam is 150 to 160 ° C., and vacuum forming is performed using a cylindrical female die. It is carried out, and is displayed by the ratio of the depth H and the diameter L at the time of molding.
The larger the value of H / L, the better the moldability.

<Outer skin strength (MD)> A cloth tape having a width of 25 mm was adhered to the surface of the foam, and a roll of 4 Kg was reciprocated twice on the surface of the foam, and then the tape and the outer skin were separated by a tensile tester.
The strength at that time is measured.

[0036]

[Table 1]

From Table 1, the skin strength is shown in Example 1.
It can be seen that in Nos. 8 to 8, significantly improved as compared with Comparative Examples 1 and 2.

When the foaming temperature was controlled to 230 ° C. or less as in Example 1, the foaming ratio distribution of the foam was mountain-shaped or trapezoidal as shown in FIG. In the case where the temperature is set to 260 ° C., the foaming ratio in the central portion is lowered to form a valley shape, and the foaming ratio near the surface layer is increased. It can be seen that the difference in the distribution of the expansion ratio between the two affects the skin strength.

It can be seen that even when the atmospheric temperature at the time of foaming is as high as 240 ° C. as in Examples 7 and 8, the skin strength is improved by rapidly cooling the skin layer. However, when cooled for a long time, as in Comparative Example 2, the central layer is cooled and the skin strength is reduced.

[0040]

According to the present invention, by controlling the distribution of the foaming ratio in the thickness direction of the sheet in a mountain shape or a trapezoidal shape, the peeling strength of the skin is high and the molding processability (H) is high.
It is possible to stably obtain a foam having good / L).

[Brief description of drawings]

FIG. 1 is a view showing a foaming ratio distribution in a thickness direction of a sheet-shaped foamed body of Example 1.

FIG. 2 is a view showing a foaming ratio distribution in a thickness direction of a sheet-shaped foamed product of Comparative Example 1.

Continuation of front page (51) Int.Cl. ⁵ Identification code Office reference number FI Technical display area B29L 7:00 4F C08L 23:00

Claims (1)

[Claims] 1. A sheet-like polyolefin resin characterized in that the distribution of the expansion ratio in the thickness direction is substantially mountain-shaped or trapezoidal from both surface layers toward the center. Crosslinked foam.