JPS631977B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a crosslinked polypropylene foam that can be bonded only by heat without using an adhesive, and has so-called good thermal bonding properties.

Cross-linked polypropylene foam has excellent heat resistance, cushioning properties, and thermoformability, so in recent years it has been used in automotive interior materials, such as molded ceiling materials and instrument panel materials, by laminating it with other materials. .

That is, a low-foam polypropylene sheet, plastic cardboard, paper cardboard, resin-impregnated fibrous felt, etc. is used as a core material, and a cross-linked polypropylene foam sheet is adhesively laminated to this, and then compression molded or vacuum molded to form a specified interior material. There is. In this case, an adhesive is used to bond the core and the crosslinked polypropylene foam sheet because thermal lamination is generally difficult, or even if thermal lamination is possible, the adhesive strength is insufficient, but the crosslinked polypropylene foam sheet Adhesives for foams are limited, and relatively expensive rubber-based adhesives are used. Therefore, in addition to the cost of the adhesive, there are also costs for applying and drying the adhesive. Furthermore, there are various problems associated with the use of adhesives, such as contamination of the working environment due to the solvent in the adhesive, uneven adhesive strength due to uneven application of the adhesive, and poor adhesion due to changes in the adhesive over time. There is.

The present invention was made as a result of intensive research in view of the above circumstances, and it is an object of the present invention to provide a crosslinked polypropylene foam with good thermal adhesiveness that can be bonded only by heat without using an adhesive.

That is, the present invention provides a crosslinked polypropylene foam containing 20% by weight or more of polypropylene, in which the degree of crosslinking in a portion from the surface of the foam to a depth of 0.5 mm is 10 to 30% in terms of gel fraction; This is a heat-adhesive crosslinked polypropylene foam characterized in that the degree of crosslinking inside the foam at a depth of 0.5 mm or more from the surface of the foam is 30 to 60% in terms of gel fraction.

In the present invention, the crosslinked polypropylene foam refers to a crosslinked polyolefin foam containing 20% by weight or more of polypropylene. Polypropylene as used herein includes crystalline propylene homopolymers and copolymers.

In the present invention, a particularly preferred crosslinked polypropylene foam includes 30 to 80 parts by weight of a propylene-ethylene random copolymer containing 1 to 15% by weight of ethylene, preferably 2 to 6% by weight, and 70 to 20 parts by weight of polyethylene.
It consists of parts by weight. The melt index of propylene-ethylene random copolymer is 2
~20 is preferred.

In the present invention, the content of polypropylene is 20
The reason for setting the amount to be at least % by weight is to impart heat resistance to the crosslinked polypropylene foam. i.e. 110
This is to prevent shrinkage or deformation even if heated at ℃ for a long time.

In addition, the degree of crosslinking in the area from the surface of the crosslinked polypropylene foam to a depth of 0.5mm (hereinafter referred to as the surface layer) is limited to 10 to 30% in terms of gel fraction. This is because it deteriorates and is not practical. In addition, if it is less than 10%, it becomes difficult to obtain a molded product that conforms to the shape of the inner surface of the mold by compression molding or vacuum molding. In other words, the ability to give shape by thermoforming becomes poor.

In addition, the degree of crosslinking in the interior at a depth of 0.5 mm or more from the surface of the foam (hereinafter referred to as the inner layer) is 30 to 30 in terms of gel fraction.
The reason why we set it to 60% is because if it is less than 30%, the foam will shrink or deform significantly during thermoforming, reducing its shape retention ability, and if it exceeds 60%, foaming during thermoforming will decrease. This is because the elongation of the molded product is reduced, and the edges and corners of the molded product are cut or torn, resulting in insufficient molded products and particularly poor deep drawing workability.

In the present invention, the gel fraction indicating the degree of crosslinking is
A test piece of cross-linked polypropylene foam was immersed in tetralin at 135°C for 12 hours, then taken out and dried, and the gel content was divided by the weight before immersion, and the value was expressed as a percentage. In addition, the gel fraction of the surface layer was measured by slicing and removing the surface layer of the foam.

Next, a method for manufacturing the foam of the present invention will be described.
There are various manufacturing methods and there are no limitations, but for example, a blowing agent such as azodicarbonamide and a crosslinking aid such as divinylbenzene or triallyl isocyanurate are uniformly added to a resin containing 20% or more of polypropylene. After thermally laminating a polypropylene film without the addition of a crosslinking aid or with a smaller amount added than the above-mentioned sheet to a foamable composition sheet obtained by kneading, irradiation with radiation such as an electron beam, and heating and foaming the resultant. The foam of the present invention can be obtained by the following steps.

Alternatively, the foam of the present invention can also be obtained by irradiating the foamable composition sheet with radiation and then foaming it in a heated atmosphere with a high oxygen concentration, such as heated air.

Alternatively, a film containing a so-called radical scavenger, such as 2,2-diphenyl-1-picrylhydrazyl, which has a large radical trapping ability, is thermally bonded to the foamable composition sheet. The foam of the present invention can also be obtained by irradiating and then heating and foaming the same.

In addition, the degree of crosslinking of the surface layer can be lowered than that of the inner layer by heat-treating the surface of the crosslinked polypropylene foam already obtained in a heated gas atmosphere with a high oxygen concentration.

In the present invention, particularly preferred foamable compositions include an ethylene component of 1 to 15% by weight, preferably 2% by weight.
Contains ~6% by weight, Melt Index (MI)
2-20 propylene-ethylene random copolymer
100 parts by weight of a polypropylene resin consisting of 30 to 80% by weight and 70 to 20% by weight of polyethylene, 1 to 30 parts by weight of azodicarbonamide as a blowing agent, and an aliphatic polyhydric alcohol of trihydric or higher valence as a crosslinking agent. 0.5 to 4.0 parts by weight of acrylic ester (for example, trimethylolpropane triacrylate), and a phenolic derivative (for example, pentaerythrityl-tetrakis [3
There is a composition in which 0.1 to 5.0 parts by weight, preferably 0.3 to 1.0 parts by weight of -(3,5-deta-siabutyl-4-hydroxyphenyl)propionate is added and thoroughly kneaded.

Next, the present invention will be explained with reference to Examples and Comparative Examples.

Example 1 Medium density polyethylene (M1=4, density 0.925) 50
To 100 parts by weight of a mixture consisting of 50% by weight of propylene-ethylene random copolymer (M1 = 7, ethylene content 4% by weight), 15 parts by weight of azodicarbonamide as a blowing agent and triglyceride as a crosslinking agent. 1.8 parts by weight of methylolpropane trimethacrylate and pentaerythrityl as an anti-aging agent.
Tetrakis[3-(3,5-dithyabutyl-4-hydroxyphenyl)]propionate 0.2
Parts by weight were added, kneaded using a kneader, and then pelletized. Next, using this pellet, a foamable composition sheet having a thickness of 1.0 mm was obtained using a 90 mmφ extruder.

Next, this sheet was irradiated with an electron beam of 1.5 Mrad to effect crosslinking, and then heated air at 260°C was blown onto both the upper and lower surfaces of the sheet to cause foaming.

The foamed sheet obtained in this way has a degree of crosslinking in the surface layer (portion from the surface to a depth of 0.5 mm) of 22 in terms of gel fraction.
%, and the degree of crosslinking in the inner layer portion (at a depth of 0.5 mm or more from the surface) was 37% in terms of gel fraction.

Next, the foamed sheet thus obtained was thermally bonded to a separately prepared low-foamed polypropylene sheet as a core material, and then this laminated sheet was compression molded to obtain a molded ceiling for an automobile. This molded ceiling conforms to the inside shape of the mold,
It was a sharp molded product from Edge.

In addition, when the above laminated sheet was subjected to a peel test between the foam sheet and the core material according to the peel test method specified in JIS-K-6854, it was found that at the interface between the core material and the foam sheet over the entire laminated surface. The foam sheet was not peeled off, and the foam sheet remained adhered to the core material, and the inner surface of the foam sheet tore. Therefore, it was found that this adhesive strength was greater than the strength of the foam sheet.

Comparative Example 1 The foamable composition sheet obtained in Example 1 was
After crosslinking under the same conditions as above, the surface was covered with aluminum foil and foamed in an air constant temperature bath at approximately 260°C. The foamed sheet thus obtained had a gel fraction of 32% in the surface layer and 38% in the inner layer.

In addition, attempts were made to thermally bond this foam sheet and a low-foam polypropylene sheet as a core material in the same manner as in Example 1 by changing various temperatures, but all of the obtained laminated sheets were tested in a peel test. It peeled off easily at the material interface.

Example 2 A film with a thickness of 250 μm prepared by adding 0.6 parts by weight of trimethylolpropane trimethacrylate as a crosslinking aid to 100 parts by weight of polypropylene was thermally laminated to the foamable composition sheet obtained in Example 1.
Crosslinking was carried out by irradiation with a 1.5 Mrad electron beam.

This was then passed through a salt bath to foam. The gel fraction in the surface layer of the foam sheet thus obtained was 16%.
The gel fraction in the inner layer was 34%.

Next, this foamed sheet was thermally bonded to a low-foamed propylene sheet as a core material, and then this laminated sheet was compression molded to obtain an automobile instrument panel. This panel conformed to the inside shape of the mold and was a sharp molded product.

In addition, when the above laminated sheet was subjected to a peel test similar to that in Example 1, the interface between the foam sheet and the core material did not peel off, and the foam sheet remained adhered to the core material. The adhesive strength was found to be greater than that of the foam sheet since it was torn.

As described above, the crosslinked polypropylene foam according to the present invention has excellent thermal adhesion to other materials without the use of adhesives, and excellent thermoformability, so it is widely used as various laminated composites and molded products thereof. It has the following.

Claims (1)

[Claims] 1. In a crosslinked polypropylene foam containing 20% by weight or more of polypropylene, the degree of crosslinking in a portion from the surface of the foam to a depth of 0.5 mm is 10 to 10% by gel fraction.
30%, and a depth of 0.5mm from the surface of the foam
A heat-adhesive crosslinked polypropylene foam characterized in that the internal crosslinking degree is 30 to 60% in terms of gel fraction. 2. The crosslinked polypropylene foam is made of a mixed resin of 30 to 80 parts by weight of a propylene-ethylene random copolymer containing 1 to 15% by weight of ethylene and 70 to 20 parts by weight of polyethylene. The heat-adhesive crosslinked polypropylene foam according to item 1.