JPS63175041A - Radiation crosslinked thick polypropylene resin foam and production thereof - Google Patents

Abstract

PURPOSE:To obtain the titled foam useful as interior automative trim materials, etc., having excellent surface smoothness free from silting of ionizing radiation even in thick material in good productivity, by irradiating a molded sheet having specific thickness made of a foamable PP resin composition with radiation. CONSTITUTION:A molded sheet made of a foamable PP resin composition is made into at least about 2mm thickness and at least one side of the sheet is irradiated through a filter plate with radiation by the use of a radiation exposure device having <=100kV accelerating voltage to give the aimed foam having 0.2-0.020g/cc apparent viscosity, at least 4mm thickness and a ratio of gel fraction of central layer of foam to gel fraction of surface layer of <=1.5.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention is characterized in that, despite having a thickness of at least 4 mm, damage caused by accumulation of ionizing radiation, discharge (discharge holes) and foaming The present invention relates to a radiation-crosslinked polypropylene resin foam having excellent surface smoothness without surface roughness, and a method for producing the same.

[Conventional technology]

Conventionally, radiation irradiation equipment used industrially for producing polyolefin resin foams usually has an acceleration voltage of about 500 to 1000 KV. When using a radiation device with an accelerating voltage of about 500 to 1000 KV, if the thickness of the polypropylene (hereinafter abbreviated as PP) resin molded sheet before foaming exceeds about 2 mm, the radiation charge irradiated by the device will be reduced. The foam tends to accumulate on the sheet, causing electrostatic breakdown, and only foams with discharge holes formed on the surface layer can be obtained.

Therefore, in order to solve the above-mentioned problems associated with an increase in the thickness of the PP resin molded sheet, it is sufficient to increase the acceleration voltage of the radiation irradiation device.
This increases the size of the equipment, which increases equipment costs (and other problems).
It has been found that as the accelerating voltage increases, the surface roughness of the resulting foam increases rapidly, degrading the quality and performance of the foam and rendering it unsuitable for industrial use.

On the other hand, as a means to solve the problem of radiation transmission due to the increase in the thickness of the PP resin molded sheet mentioned above, (1) irradiation is performed from one side of the PP resin molded sheet, and then irradiation is performed again from the opposite side, that is, the back side. There is a means. However, according to the studies of the present inventors, in this case, the charging breakdown occurs within a very short time after irradiation from the back side, and discharge holes are similarly formed in the foam, making it industrially impractical. It turns out that it is possible, and as a result, increasing the thickness of the PP resin molded sheet does not result in a good foam.

Of course, a thicker foam can be made by laminating two or more foams with a thickness of less than 4 mm.
It is said that there are many drawbacks due to the fact that it is a foam, such as problems with interfacial peeling from the bonding surface, non-uniformity of the overall physical properties of the foam due to the formation of the adhesive layer, and variations in compression properties and flexibility. ing.

[Purpose of the invention]

The purpose of the present invention is to achieve a thick layer by the above radiation irradiation method.
It is an object of the present invention to provide a PP resin foam that is free from discharge holes and surface roughness due to charging breakdown, and a method for producing the same.

[Means for solving problems]

The object of the present invention is to provide an apparent density of 0.2 to 0.020 g/cc, a thickness of at least 41, and a ratio of the gel fraction of the foam center layer to the gel fraction of the surface layer of the foam of 1. After irradiating a molded sheet made of a PP resin foam and a foamable polypropylene resin composition with a polypropylene resin composition of . This can be achieved by a manufacturing method characterized by irradiating at least one surface of the formed sheet with the radiation through a filter plate using a radiation irradiation device with a thickness of at least about 2 mm and an accelerating voltage of 1000 KV or less. .

In addition to polypropylene, the foam of the present invention also contains 1 to 15
A propylene copolymer obtained by random, block or random-block copolymerization of α-olefin in a weight%
Alternatively, it is composed of a resin composition blended with polyethylene or ethylene copolymer resin having good compatibility with these resins. In addition, other polymeric resins and additives (extenders, heat stabilizers, ultraviolet absorbers, pigments, nucleating agents) may be used as long as they are compatible and do not adversely affect the properties or cell structure of the foam. etc.) can be mixed.

The PP resin foam according to the present invention is characterized by having a thickness of at least about 4 mm, preferably 4 mm to 1Q mm, that is, a foam formed by thick radiation irradiation method, and at least Despite having a thickness of 4 mm, the PP resin foam has a smooth surface and no electrostatic breakdown, that is, no discharge holes are observed.

A discharge hole is a hole that leads from the inside of the molded sheet to the surface and is visible as a result of discharge of charges accumulated inside the molded sheet due to radiation irradiation, and can be observed with the naked eye. In addition, traces of discharge spreading like a spider's web are sometimes observed around the discharge hole. These discharge pores remain as they are even after foaming, and similar discharge pores also exist in the foam obtained by foaming a molded sheet with discharge pores, and are distinguished from the closed cells that foams originally have. form a pore.

Furthermore, the PP resin foam of the present invention has an apparent density of 0.2 to 0.020 g/cc, preferably 0.10 g/cc.
~0.33 g7cc and the ratio of the gel fraction of the foam center layer to the gel fraction of the surface layer of the foam is 1.5 or less, preferably 0.5 to 1.3.

That is, in the present invention, the apparent density of the foam is 0.
When it is larger than 2 g/cc, the soft feel disappears and the properties as a foam (for example, cushioning properties, etc.) are lost.
This is not desirable because it damages the

On the other hand, if it is less than 0.020 g/cc, it is not preferable because the recovery property against compression decreases.

As mentioned above, the PP resin foam of the present invention has a thickness of 4 mm or more, which is not found in conventional foams when using the radiation crosslinking method. The gel fraction ratio of the foam center layer to the foam layer is 1
．． It is characterized by being 5 or less. In other words, this ratio is related to the surface roughness of the foam, and if the numerical ratio is greater than 1.5, the surface layer gel of the foam decreases and the cell membrane breaks during foaming, causing surface roughness. , undesirable.

Here, the gel fraction of the surface layer of the foam is 15 to 60%.
, is preferably within the range of 25 to 45%; if it is less than 15%, surface roughness is likely to occur due to bubble breakage in the surface layer during foaming, and if it exceeds 60%, moldability decreases and Molding may cause them and is undesirable.

In order to produce such a PP resin foam of the present invention, as mentioned above, it cannot be obtained simply by increasing the radiation acceleration voltage of the device or by irradiating radiation from both sides of the PP resin molded sheet. do not have.

That is, in order to obtain the foam of the present invention, a foamable resin composition containing the above-mentioned PP resin is molded to form a sheet having a thickness of at least about 2 mm, and an accelerating voltage of 1000 K is applied to this sheet. It can be manufactured by irradiating at least one side of the molded sheet with the radiation through a filter plate using the following radiation irradiation device to impart crosslinking, and then foaming.

Note that the method of the present invention is more effective when both sides of the molded sheet are irradiated with radiation.

Hereinafter, one embodiment of the method for manufacturing the foam according to the present invention will be described in more detail.

First, the above-mentioned PP resin is mixed with various thermally decomposable blowing agents such as azodicarbonamide and dinitrosopentamethylenetetramine, crosslinking aids such as polyfunctional compounds such as divinylbenzene and diallyl phthalate, and, if necessary, various known Additives such as heat stabilizers, ultraviolet absorbers, extenders, nucleating agents, antistatic agents, fillers, etc. are appropriately blended,
A foamable resin composition is prepared, and a sheet having a thickness of at least about 2 mm, preferably 2.5 to 4.0 mm, is formed by extrusion molding or other known molding means.

Then, this thick PP resin sheet having a thickness of approximately 2 mm or more is irradiated with ionizing radiation such as X-rays, α-rays, β-rays, and γ-rays by a known method.

However, the feature of the method of the present invention is that the accelerating voltage of the radiation irradiated to the thick PP resin sheet is 1000K.
V or less, and the radiation is irradiated by interposing a filter plate on at least one side of the thick PP resin sheet.

That is, in the present invention, when the acceleration voltage of the radiation irradiated to the thick PP resin sheet exceeds 1000 KV,
The surface of the foam obtained by crosslinking and foaming after irradiation is extremely rough, making it impossible to obtain a foam of high quality and commercial value.

If only one side of the thick PP resin sheet is irradiated with radiation, but the accelerating voltage is low and the opposite side of the sheet is not irradiated with radiation, - the radiation is applied again from the opposite side (back side) of the sheet. However, in this case, it is important not to directly irradiate the radiation from the irradiation device, but to irradiate it with a filter plate interposed, and only by adopting such an irradiation means can the radiation of the present invention be achieved. It becomes possible to produce thick PP resin foams of 4 mm or more, especially as thick as 10 mm.

Examples of the filter plate include metal plates made of titanium, aluminum, iron, etc., and titanium plates are particularly preferred.

〔Example〕

Hereinafter, the effects of the present invention will be explained in more detail with reference to Examples.

In the present invention, the gel fraction, the ratio of the gel fraction of the center layer to the surface layer of the foam, the surface smoothness, and the presence or absence of discharge holes are values measured as follows.

Gel fraction: Cut the foam into cubes of about 0.5 mm on each side, and accurately weigh 0.1 g of the cut foam. Precisely weighed foam at 135°C
The sol was extracted using tetralin heated to , the weight of the foam before and after the sol extraction was measured, and the gel fraction (%) was determined according to the following formula.

Weight after extraction/weight before extraction fX 100 Also, the surface layer of the foam is 0.8-L from the surface of the cut foam.
, the surface layer is the sliced part of 0mm, and the foam piece corresponding to the foam center layer is also 0.8 to 1.0.
The above sol extraction was performed using the mm portion as the center layer, the gel fraction was measured, and the ratio was calculated.

Surface smoothness: A foam measuring 50 cm long and 5 Q cm wide was immersed in an aqueous solution of water colored with red ink and about 1% surfactant added, and after being left under reduced pressure for about 5 minutes, it was taken out and the surface layer was smoothed. Judgment was made based on the number of red ink dots that entered the bubble burst area. The larger the number, the worse the surface smoothness.

Presence or absence of discharge holes: Slice the foam as it is in the plane direction 2. Observed with the naked eye.

Examples 1-2, Comparative Examples 1-2 Polyethylene resin B in which hexene was copolymerized with polypropylene copolymer resin A containing 4% by weight of ethylene was
8% of azodicarbonamide and 3% of divinylbenzene were uniformly mixed into the mixed systems of 0% by weight and 40% by weight, respectively, and formed into a sheet having a thickness of 3.3 mm using an extruder.

After irradiating one side of this sheet with ionizing radiation of 9.4 Mrad using a radiation device with an accelerating voltage of 800 cm and a titanium filter with a thickness of 100 μm,
The opposite side of the sheet was similarly irradiated with the radiation.

Next, the irradiated sheet is foamed at a temperature of 230°C to obtain a foaming ratio of about 20 times, a thickness of about 6 mm, and a dot density of about 0.
．． A closed cell foam having a uniform cell structure of 0.05 g/cc was obtained.

The surface smoothness of the obtained foam, the ratio of the gel fraction of the center layer to the surface layer, the presence or absence of discharge holes, etc. were examined, and the results are shown in Table 1.

For comparison, a sheet was formed in the same manner using the same resin composition as in the example, and the same procedure as in the example was performed except that the titanium filter was not used and ionizing radiation was directly irradiated. A foam was obtained whose performance is shown in Table 1.

(Hereinafter, blank spaces) Table 1 [Effects of the Invention] According to the present invention, so-called thick PP resin foams having a thickness of at least about 4 mm, which could not be manufactured industrially by conventional ionizing radiation methods, For example, 1 to 3 mm because the body and its products are not available.
It has been used as a laminate by bonding two or more sheets of PP resin foam with a thickness of Automotive interior materials (molded ceilings, instrument panels, door trims, trunk mats, etc.) that were considered unusable due to reduced bending resistance and flexibility due to a certain adhesive layer, and creases when folded.
This greatly expands the range of uses of the PP resin foam.

Moreover, it can be easily made into a plurality of foamed bodies by means such as slicing, which is very advantageous industrially because productivity and cost can be reduced.

Claims (2)

[Claims] (1) The apparent density is 0.2 to 0.020 g/cc, the thickness is at least 4 mm, and the ratio of the gel fraction of the foam center layer to the gel fraction of the surface layer of the foam is 1.5 or less Radiation crosslinked thick polypropylene resin foam. (2) When a molded sheet made of a foamable polypropylene resin composition is crosslinked by irradiation with radiation and then foamed to produce a foam, the thickness of the molded sheet is at least about 2 mm, and the accelerating voltage is 1000 KV. A method for producing a radiation-crosslinked thick polypropylene resin foam, which comprises irradiating at least one surface of the molded sheet with the radiation through a filter plate using the following radiation irradiation device.