JPH02196638A - Manufacture of crosslinked polyolefin continuous foamed material - Google Patents

Abstract

PURPOSE:To carry out foaming, sliding well in the inner surface of a mold, hard to generate cracks in foamed material even in a violent foaming, having a thick non-foaming skin layer of foamed material itself and of good product and material efficiency by heat foaming a foamable crosslinked composition in the state of being covered with a plastic sheet. CONSTITUTION:A foaming agent, a crosslinking agent and, if necessary, an auxiliary foaming agent, a filler, a pigment and the like are added to polyolefin, which is mixed by a heated mixing roll or the like. Then, a foamable crosslinked composition thus prepared is put into a mold and formed at 115-155 deg.C corresponding to the kinds of resin and the crosslinking agent under pressure, and also the composition and a plastic sheet are welded by heating and the sheet is covered on the surface of composition. The formed foamable crosslinked composition is then heated under normal pressure, and decomposition of the crosslinking agent and the foaming agent is carried out simultaneously. Membranes of the foamed material thus prepared is destroyed by compression deforming by means of a uniform rate twin roll or the like, and the foams are connected to manufacture a continuous foamed material.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing a crosslinked polyolefin open cell foam.

[Conventional technology]

Conventionally, the method for producing open-cell crosslinked polyolefin cells involves partially decomposing the blowing agent and crosslinking agent in a foamable crosslinkable polyolefin composition under pressure in a closed mold, and then crosslinking the remaining blowing agent under normal pressure. The agent is decomposed to obtain closed cells,
A method has been proposed in which the closed cells are then compressed to destroy the closed cells (see Japanese Patent Publication No. 59-23545 and Japanese Patent Application Laid-Open No. 146732-1982). However, in this method, the blowing agent and crosslinking agent are decomposed by heating in a pressurized closed mold, and although a crosslinking reaction occurs, no bubbles are formed, and when the pressure is removed, This will be the first time it foams.

In other words, a method is adopted in which a crosslinking reaction is first caused and then foaming is carried out.Since so-called photo-crosslinking is carried out at the manufacturing stage of closed cell membranes, the cell membrane becomes strong and, therefore, independent cells are formed. It was difficult to make the cells open, and it was not possible to obtain a cell with an open cell rate of 100% or close to 100%.

In order to solve the above problem, the present applicant heated and shaped a foamable crosslinkable polyolefin composition into a desired shape, and then heated it under normal pressure to simultaneously decompose the crosslinking agent and the blowing agent to form bubbles. Developed a method to generate air bubbles and then apply mechanical deformation to make the bubbles communicate (Special Publication Publication No. 82-192)
No. 94, JP-A-58-121739).

[Problem to be solved by the invention]

According to the applicant's method described above, 100% or 100
It is possible to obtain a crosslinked polyolefin open cell with an open cell ratio close to %.

However, in this method, a non-crosslinked shaped product is heated and foamed under normal pressure, that is, in a mold that is not airtight, so if foaming is performed rapidly, friction between the composition during the foaming stage and the inner surface of the mold will cause Cracks occur in the resulting foam.

Therefore, in order to solve this problem, mild heating and foaming must be carried out for 100 minutes or more. As a result, production efficiency was poor, and the non-foamed skin layer and non-uniformly foamed areas near the skin became thick due to prolonged heating, resulting in poor material efficiency.

Accordingly, an object of the present invention is to provide a method for producing a crosslinked polyolefin open-cell foam that eliminates the drawbacks of the conventional methods as described above and has excellent production efficiency and material efficiency.

[Means to solve the problem]

According to the present invention, in order to achieve the above object, a foamable crosslinkable composition prepared by adding a foaming agent and a crosslinking agent to a polyolefin is heated and foamed in a non-airtight mold to generate a foam, and then mechanically deformed. A method for producing a crosslinked polyolefin open cell in which the cells are made open by adding the above-mentioned foamable crosslinkable composition to open the cells, characterized in that heating and foaming is carried out with at least a portion of the foamable crosslinkable composition covered with a plastic sheet. A method of manufacturing a body is provided.

[Operation and mode of the invention]

According to the method of the present invention, as described above, heat foaming is performed with at least a portion of the foamable crosslinkable composition covered with a plastic sheet, and the property of the plastic sheet being stretched by heating is utilized to create bubbles. The aim is to prevent the formation of cracks in the body and to efficiently produce bubbles.

That is, when heat foaming is performed with a foamable crosslinkable composition covered with a plastic sheet, the plastic sheet softens and stretches to follow the foaming of the composition. In this case, since the plastic sheet is pre-formed into a sheet shape, there is less slippage on the inner surface of the mold than in the case where the composition directly contacts the inner surface of the mold which is not airtight and is suppressed by the foaming pressure. Good.

Therefore, even with relatively rapid foaming, cracks are less likely to occur in the foam, and the unfoamed skin layer of the foam itself becomes thinner, making it possible to foam with high production efficiency and material efficiency in a relatively short period of time. This makes it possible to prevent gas from escaping from the bubbles.

In the present invention, when covering the foamable crosslinkable composition with a plastic sheet, it is necessary to cover at least a part of the composition, but it is preferable to cover the top and bottom surfaces of the composition. , this has a significant effect.

In the present invention, a plastic sheet is a sheet that, when coated on a foamable crosslinkable composition based on polyolefin, stretches without cracking as the composition expands, as is clear from the above description of its action. It is not limited to any particular one. Generally, the same resin as the base resin or a copolymer thereof is used. Among these, polyolefins, especially ethylene resins, have excellent elongation and are particularly preferred in the production of crosslinked polyolefin open-cell foams with an expansion ratio of 20 times or more. The above-mentioned ethylene resins include homopolymers and copolymers containing ethylene as a main component, such as high-pressure polyethylene, low-pressure low-density polyethylene, linear low-density polyethylene, ethylene-vinyl acetate copolymer, and ethylene-vinyl acetate copolymer. Examples include vinyl ester copolymers, ethylene-alkyl acrylate copolymers, ethylene-propylene copolymers, ethylene-α-olefin copolymers, and mixtures thereof.

The plastic sheet used in the present invention may be a single plastic, but when obtaining a crosslinked polyolefin open cell with a foaming ratio of 15 times or more, in order to prevent gas leakage,
Preferred are plastic sheets that have been crosslinked in advance by ionizing radiation or a crosslinking agent, or to which a crosslinking agent has been added. Furthermore, when obtaining a crosslinked polyolefin open cell with a foaming ratio of 25 times or more, it is necessary to add a foaming agent to the plastic sheet to increase the elongation of the sheet, since the expansion ratio of the foamable crosslinkable composition is large. preferable.

The foaming agent may be added together with a crosslinking agent, or a plastic sheet to which the foaming agent has been added may be crosslinked using ionizing radiation. Alternatively, a plastic sheet to which a foaming agent and a crosslinking agent have been added may be crosslinked before use. Further, a foaming aid, a crosslinking aid, a filler, etc. can be added to the plastic sheet as appropriate.

The amount of foaming agent added to the plastic sheet is determined as appropriate depending on the expansion ratio of the crosslinked polyolefin open cell, but is generally about 1 to 25 parts by weight per 100 parts of resin. This makes it possible to handle crosslinked polyolefin open cell foams with a foaming ratio of about 20 to 70 times. Even if a foaming agent is added to a plastic sheet, the thickness of the plastic sheet does not increase because it is pushed by the foaming pressure caused by the foaming of the foamable crosslinkable composition.

The amount of crosslinking agent added to the plastic sheet is 100% of the resin.
0.1 to 5 parts by weight per part of ffl, preferably 0.2 to 5 parts by weight
Approximately 3 parts by weight is appropriate. If the plastic sheet is not pre-crosslinked, it is desirable to select a crosslinking agent with a low decomposition temperature so that the plastic sheet is substantially pre-crosslinked.

When a plastic sheet is cross-linked using 'ffS female radiation, alpha rays, beta rays, gamma rays,
Although there are X-rays, accelerated proton beams, electron beams, neutrons, etc., high-energy electron beam irradiators are generally used.

For example, the formed sheet is crosslinked by irradiating it with a dose of 1 to 50 Mrad. In this case, in order to increase crosslinking efficiency, 0.1 to 10 parts by weight of a crosslinking auxiliary agent, such as divinylbenzene, diallylbenzene, trimethylolpropane triacrylate, diethylene glycol dimethacrylate, etc., is added to 100 parts by weight of the resin. Monomers etc. may be added.

The plastic sheet may be molded according to a conventional method. For example, a plastic resin or a plastic resin and a compounding agent are formed into a sheet using a conventional extruder, mixing roll, or the like.

When the sheet is first crosslinked, part or all of the crosslinking agent is decomposed by irradiating it with ionizing radiation or by heating the plastic sheet to which the crosslinking agent has been added in a press, oven, or heat medium bath.

The thickness of the sheet is preferably 0.1 to 4 mm.

Particularly preferably, it is 0.5 to 2 mm. If it is less than 0.1 mm, the sheet will easily tear, while if it exceeds 4 mm, heat transfer during heating will be poor and material efficiency will be poor.

As a method for obtaining a crosslinked polyolefin open cell using the plastic sheet of the present invention, a foamable crosslinkable composition prepared by adding a blowing agent and a crosslinking agent to a polyolefin is heated and foamed to form a cell, and then mechanically deformed. Any method known in the art can be applied as long as the method is not particularly limited, as long as it is a method for making bubbles communicate by adding. Among these methods, in particular, Japanese Patent Publication No. 82-19294 and Japanese Patent Application Laid-Open No. 50-12
As described in Japanese Patent No. 1739, a foamable crosslinkable composition is heated and shaped into a desired shape, and then heated under normal pressure in a non-airtight mold to simultaneously release the crosslinking agent and the blowing agent. The effects of the present invention are particularly remarkable in a method in which bubbles are formed by decomposition and then mechanical deformation is applied to make the bubbles open.

Hereinafter, preferred embodiments of the method for manufacturing an open-cell body according to the present invention will be specifically explained.

First, a blowing agent, a crosslinking agent, and if necessary a blowing aid, a filler, a pigment, etc. are added to the polyolefin, and the mixture is kneaded using a heated mixing roll or the like. Examples of crosslinking agents include dicumyl peroxide, 1,1-di-t-
Butylperoxy-3,3,5-1-limethylcyclohexane, 2,5-dimethyl-2,5-di-t-butylperoxyhexane, 2,5-dimethyl-2,5-di-t-
Organic peroxides such as butyl peroxyhexine, α,α'-bis(t-butylperoxy)diisopropylbenzene, t-butyl peroxyketone, and t-butyl peroxybenzoate are used as blowing agents. Chemical blowing agents such as azo compounds such as azodicarbonamide and barium azodicarboxylate; nitroso compounds such as dinitrosopentamethylenetetramine and trinitrosotrimethyltriamine; hydrazide compounds p and p
'Oxybisbenzenesulfonyl hydrazide, etc.; p of sulfonyl semicarbazide compounds.

Preferred examples include p'-oxybisbenzenesulfonyl semicarbazide, toluenesulfonyl semicarbazide, and the like, but the present invention is not limited thereto. Furthermore, as foaming aids to be added as necessary, compounds containing urea as a main component, metal oxides such as basic zinc carbonate, zinc oxide, and lead oxide, compounds containing salicylic acid, stearic acid, etc. as main components, That is, there are higher fatty acids or metal compounds of higher fatty acids. Furthermore, for the purpose of improving the physical properties of the composition used or lowering the price, compounding agents (fillers) that do not have a significant adverse effect on crosslinking, such as carbon black, zinc oxide, titanium oxide, calcium oxide,
Metal oxides such as magnesium oxide and silicon oxide, carbonates such as magnesium carbonate and calcium carbonate, fiber materials such as bulbs, various dyes, pigments, fluorescent substances, and other commonly used rubber compounding agents as necessary. Can be added.

Next, the plastic sheet obtained by the various methods described above is placed on at least a portion of the foamable crosslinkable composition obtained by kneading as described above, preferably on the upper and lower surfaces, and placed in a mold. The composition is heated and shaped under pressure in a press at 115 to 155°C, preferably 120 to 140°C, depending on the type of resin and crosslinking agent, and the above composition and the plastic sheet are heat-sealed to form a plastic. A sheet is coated onto the surface of the composition. Note that although the covering with the plastic sheet may be performed after this heat shaping, it is preferable to carry out the covering at the same time as the heat shaping from the viewpoint of simplifying the work. In this heat shaping step, maintaining the foamable crosslinkable composition in a state where the gel fraction is zero during shaping is a condition for obtaining an open cell body with an open cell rate of 100% or close to 100%. In this heat shaping process, a very small amount of the foaming agent causes initial decomposition, and when the shaped product is taken out of the mold, it can expand up to about twice as much. However, this is far from the concept of bubbles, and is not suitable for the present invention. There is no problem for me.

The foamable crosslinkable composition shaped as described above is then heated under normal pressure to simultaneously decompose the crosslinking agent and the foaming agent. This foaming/crosslinking step can be carried out, for example, by placing the shaped polyolefin composition into a non-airtight, that is, non-sealed mold having a desired cross-sectional shape and dimensions, and heating the metal plate of the mold from the outside. The composition is heated indirectly. Indirect heating methods include, for example, heating the metal plate by placing a heater in close contact with its outer surface, or providing heat medium flow channels in the metal plate and heating the metal plate using steam, heating oil, etc. using a jacket method. . Alternatively, the shaped polyolefin composition is placed in a non-airtight openable mold and placed in a metal bath or oil bath using rose alloy, wood alloy, etc., and one or two salts such as sodium nitrate, potassium nitrate, potassium nitrite, etc. It is directly heated in a salt bath using the above molten salt, in a nitrogen stream, or covered with an extensible iron plate or the like. After heating for a predetermined time, it is cooled to obtain a foam.

The heating temperature varies depending on the type of polyolefin used.
The temperature is set in the range of 40 to 210°C, preferably 160 to 190°C. The heating time is preferably 10 to 90 minutes, more preferably 20 to 70 minutes.

In this way, a foam can be obtained that has a foam membrane that can be easily destroyed by mechanical deformation and has a degree of crosslinking comparable to that of conventional foams (gel fraction up to about 95%). .

Furthermore, in the present invention, the heating in the foaming/crosslinking step can be carried out in two stages, which slows down the foaming and crosslinking conditions and allows the decomposition of the crosslinking agent and the blowing agent to occur more simultaneously in the two stages. be able to. That is, homogeneous heating of the foamable crosslinkable composition is performed, and non-uniform heating in the thickness direction of the composition is eliminated, thereby effectively preventing the appearance of uneven foaming and degassing. can be prevented. In this way, when heating in the foaming/crosslinking process is performed in two stages, 1. It is effective to set the heating temperature of the second stage to be higher than the heating temperature of the first stage, for example, the first stage heating is in the range of 145 to 180 °C, and the second stage heating is in the range of 170 to 210 °C. Let's do it.

The cell membrane obtained in the above manner (so-called closed cell cell) is then compressed and deformed using, for example, two constant-velocity rolls, so that the cell membrane is destroyed, the cells are made open, and an open cell cell is obtained. It will be done. By providing countless small needles on the surface of two constant velocity rolls, or by arranging rolls provided with countless small needles in front and/or after the two constant velocity rolls,
By opening numerous small holes on the surface of the foam, communication between the foam can be promoted.

By this method, RuaiBtoa Psti@C
An open cell body with an open cell ratio of 100% or close to 100% measured based on the t method (ASTM D1940-62T) is obtained.

〔Example〕

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to the Examples below.

Example 1 Ethylene-vinyl acetate copolymer (trade name Evaflex P1403, vinyl acetate content 14fflQ%, manufactured by Mitsui Polychemicals ■) was processed using a 65 mm diameter uniaxial extruder (cylinder temperature setting 110"C, die setting temperature 110°C) The mixture was melted and extruded to obtain a sheet with a thickness of 1 mm.

Next, 100 parts by weight of ethylene-vinyl acetate copolymer (trade name Evaflex P1403), 8 parts by weight of azodicarbonamide (trade name Vinifol AC#50S, manufactured by Eiwa Kasei Kogyo ■), 0.3 parts by weight of zinc white, A composition consisting of 0.6 parts by weight of dicumyl peroxide and 30 parts by weight of heavy calcium carbonate was kneaded with a mixing roll at 85°C, and then placed in a mold (460×23
0 x 38 mm) with the sheets (480 mm x 25 Q mm) arranged on the top and bottom surfaces of the kneaded product, heated under pressure at the above temperature for 35 minutes, and shaped into a foamable crosslinkable sheet covered with the sheets. did. The gel fraction of the foamable crosslinkable sheet was 0.

Next, the obtained foamable crosslinkable sheet was heated to 170°C.
A non-airtight openable mold (1000X
500x100mm) and 17
It was heated with steam at 0° C. for 70 minutes, and after cooling, it was taken out to obtain a foam.

The obtained foam was passed three times between two constant-velocity rolls set at a roll interval of 20 mm to destroy the cell membrane and to make the cells open. The resulting foam with skin after communication (
The thickness of the part (excluding the sheet) is 90 to 95 mm, and the skin layer is thin, and the thickness of the effective product excluding the skin is 85 mm.
mm, apparent density 0.09 g/cm', open cell rate 10
It was 0%.

Comparative Example 1 In Example 1, when foaming was carried out under the same foaming conditions without placing sheets on the upper and lower surfaces of the kneaded product, the foaming was too fast, cracks occurred on the surface of the foam, and a satisfactory foam was not obtained. I couldn't get it.

Example 2 High-density polyethylene (trade name Yucalon-HDJY20-
8, MFR5g/10min, density 0゜953g/cm3,
(manufactured by Mitsubishi Yuka ■) in a 65 mmφ single screw extruder (cylinder temperature set at 130°C, die set temperature at 130°C),
A sheet with a thickness of 1 mm was obtained by extrusion.

Next, the same method as in Example 1 except for using the above sheet,
Foams were produced under the following conditions. The thickness of the resulting skinned foam after communication is 90 to 95 mm, the skin layer is thin, and the effective product excluding the skin has a thickness of 85 mm and an apparent density of 0.0.
It was 9g/cm3 and the open cell rate was 100%.

Example 3 Ethylene-vinyl acetate copolymer (trade name Yucalon EVA)
-41H, vinyl acetate content 16% by weight, manufactured by Mitsubishi Yuka ■), 0.6 parts by weight of dicumyl peroxide was added, mixed in a tumbler, and extruded using the same extruder as in Example 1. A sheet with a thickness of 1 mm was obtained under these conditions.

Next, 100 parts by weight of ethylene-vinyl acetate copolymer (trade name Yucalon EVA41H) and azodicarbonamide (
Product name Vinifol AC#50S) llffi parts, zinc white 0.2 parts by weight, α.

α'-bis(t-butylperoxy)diisopropylbenzene 1.1 parts by weight, heavy calcium carbonate 15ffif
The composition consisting of the fi part was kneaded with a mixing roll at 85°C, and then placed in a mold (385°C) in a press heated to 140°C.
x 190 x 36 mm) with the sheets (400 x 210 mm) placed on the top and bottom surfaces of the kneaded product, heated under pressure at the above temperature for 40 minutes, and shaped into a foamable crosslinkable sheet covered with the sheets. did.

The gel fraction of the foamable crosslinkable sheet was 0.

Next, the obtained foamable crosslinkable sheet was heated to 170°C.
Openable molds (1000 °C) that are not airtight are heated to
x 500 x 100 mm), and the jacket type
It was heated with steam at 70° C. for 60 minutes, cooled, and then taken out to obtain a foam.

The obtained foam was passed three times between two constant-velocity rolls set at a roll interval of 20 mm to destroy the cell membrane and to make the cells open. The thickness of the resulting skinned foam after communication was 90 to 95 mm, and the thickness of the effective product excluding the skin was 85 mm, with an apparent density of 0.05 g/cm' and an open cell rate of 100%. .

Comparative Example 2 In Example 3, when foaming was performed under the same foaming conditions without placing sheets on the upper and lower surfaces of the pure product, the foaming was too fast and cracks occurred on the surface of the foam, making it impossible to obtain a satisfactory foam. I couldn't.

Example 4 Low density polyethylene (trade name Yucalon YF-30, MF
0.2 parts by weight of dicumyl peroxide was added to 100 parts by weight of Rl, Og/10 minutes, density 0.920 g/cm', manufactured by Mitsubishi Yuka ■, and after mixing in a tumbler, 65 m
Melt and knead with a mφ single-screw extruder (cylinder set temperature 110°C, die set temperature 110°C) and extrude to a thickness of 1 mm.
A sheet was obtained, and the sheet was sandwiched between hot plates of a press heated to 160° C. and heated for 30 minutes to cause crosslinking.

Next, a foam was produced using the same method and conditions as in Example 3, except for using the crosslinked sheet obtained as described above.

The thickness of the obtained skinned foam after communication is 90 to 95 mm.
The thickness of the effective product excluding the skin was 85 mm, the apparent density was 0.05 g/cm3, and the open cell rate was 100%.

Example 5 High density polyethylene (trade name Yucalon HDJY20-8
) 0.2 parts by weight of dicumyl peroxide was added to 100 parts by weight, mixed in a tumbler, and a sheet with a thickness of 1 mm was obtained using the same extruder and extrusion conditions as in Example 2.
The crosslinked sheet was heated for 30 minutes by sandwiching it between hot discs of a press heated to 0° C. to obtain a crosslinked sheet.

Next, a foam was produced using the same method and conditions as in Example 3 except for using the above-mentioned crosslinked sheet. The thickness of the obtained skinned foam after communication is 90 to 95 mm, and the thickness of the effective product excluding the skin is 85 mm, and the apparent density is 0.05 g/c.
m3, and the open cell rate was 100%.

Example 6 To 100 parts by weight of ethylene-vinyl acetate copolymer (trade name Evaflex P1403), 18 mm parts of azodicarbonamide (trade name Vinyhole AC#503) and 0.2 parts by weight of dicumyl peroxide were added and mixed in a tumbler. After mixing, use the same extruder and extrusion conditions as in Example 1 to obtain a thickness of 1.
A crosslinked sheet was obtained by sandwiching the sheet between hot plates of a press heated to 160° C. and heating it for 30 minutes.

Next, 100 parts by weight of ethylene-vinyl acetate copolymer (trade name Evaflex P1403), 18 parts by weight of azodicarbonamide (trade name Vinyhole AC#50S), 0.2 mm part of zinc white, and 0.5 parts of dicumyl peroxide. 85 parts by weight of a composition consisting of 15 parts by weight of heavy calcium carbonate
The kneaded product was kneaded with a mixing roll at 135°C, and placed in a mold (340 x 170 x 29 mm) in a press heated to 135°C.
190 mm) were placed in each case and heated under pressure for 30 minutes to shape the foamable crosslinkable sheet covered with the crosslinked sheet. The gel fraction of the foamable crosslinkable sheet was 0.

Next, the obtained foamable crosslinkable sheet was placed in a non-airtight opening/closing mold (1000 x 500
x100mm) and heated to 170℃ using a jacket type.
The mixture was heated with steam for 60 minutes, cooled, and then taken out to obtain a foam.

The resulting foam was subjected to cell communication in the same manner as in Example 1. The thickness of the resulting skinned foam after communication was 9.
The thickness of the crosslinked covering sheet after foaming was 0.5 to 1 mm. The thickness of the effective product excluding the skin was 85 mm, the apparent density was 0.03 g/Cm', and the open cell rate was 100%.

Comparative Example 3 In Example 6, when foaming was performed under the same foaming conditions without placing crosslinked sheets on the upper and lower surfaces of the kneaded product, the foaming was too fast and cracks occurred on the foam surface, resulting in unsatisfactory results. No foam was obtained.

Comparative Example 4 100 parts by weight of ethylene-vinyl acetate copolymer (trade name Evaflex P1403), 17 parts by weight of azodicarbonamide
Part by weight, zinc white 0. A composition consisting of 0.05 parts by weight of dicumyl peroxide and 0.7 parts by weight of dicumyl peroxide was kneaded using a mixing roll at 85°C, and the kneaded product was filled into a mold (340 x 170 x 29 mm) in a press heated to 125°C. The mixture was heated under pressure for 30 minutes to form a foamable crosslinkable sheet. The gel fraction of the foamable crosslinkable sheet was 0.

Next, the resulting foamable crosslinkable sheet was placed in a non-airtight opening/closing mold (1000 x 500 x 1
00 mm), heated with steam at 160° C. for 130 minutes using a jacket method, and then taken out after cooling to obtain a foam.

The resulting foam was made to have open cells in the same manner as in Example 1. The thickness of the resulting skinned foam after communication was 9.
However, because there was a hard part with non-uniform foaming inside near the epidermis, the effective thickness that could be used as a product was only 80 mm. The apparent density is 0.03g/c
m3, and the open cell rate was 100%.

Example 7 To 100 parts by weight of ethylene-vinyl acetate copolymer (trade name Evaflex P1403), 18 parts by weight of azodicarbonamide (trade name Vinyhole AC#505), 0.6 parts by weight of dicumyl peroxide, and 0.6 parts by weight of zinc white. After adding 1 part by weight and mixing in a tumbler, a sheet with a thickness of 1 mm was obtained using the same extruder and extrusion conditions as in Example 1.

Next, a foam was produced in the same manner and under the same conditions as in Example 6, except that it was covered with the sheet obtained as described above. The thickness of the resulting skinned foam after communication is 90 to 95 m.
The thickness of the effective product excluding the skin was 85 mm, the apparent density was 0.03 g/cm', and the open cell rate was 100%.

Example 8 Low density polyethylene (trade name Yucalon YF-30) 10
To 0 parts by weight, 18 parts by weight of azodicarbonamide (trade name: Vinyhole AC#50S) and 0.2 parts by weight of dicumyl peroxide were added, mixed in a tumbler, and then extruded using the same extruder and extrusion conditions as in Example 1. A sheet with a thickness of 1 mm was obtained, and the sheet was placed between hot plates of a press heated to 160° C. and heated for 30 minutes to obtain a crosslinked sheet.

Next, a foam was produced in the same manner and under the same conditions as in Example 6, except that it was covered with the crosslinked sheet obtained as described above. The thickness of the obtained skinned foam after communication was 90 to 9.
5mm, and the effective product thickness excluding the epidermis is 85mm.
, apparent density 0.03g/Cm', open cell rate 100%
Met.

Example 9 18 parts by weight of azodicarbonamide (trade name Vinyhole AC #505) was added to 100 parts by weight of ethylene-vinyl acetate copolymer (trade name Evaflex P1403),
After mixing in a tumbler, a sheet with a thickness of 1 mm was obtained using the same extruder and extrusion conditions as in Example 1. The obtained sheet was crosslinked by irradiating it with an electron beam of 7 Mrad.

A foam was produced using the same method and conditions as in Example 6, except that it was covered with the crosslinked sheet obtained in this manner, and the same open-cell foam was obtained.

The manufacturing conditions and results of each of the above Examples and Comparative Examples are summarized in Table 1.

〔Effect of the invention〕

As described above, according to the method of the present invention, the polyolefin-based foamable crosslinkable composition is heated and foamed while being covered with a plastic sheet, so it slides easily on the inner surface of the mold and is relatively rapidly Even with foaming, cracks do not occur in the foam, and the unfoamed skin layer becomes thinner, making it possible to manufacture open-cell foams with high material efficiency and production efficiency.

As a result, high production volumes can be obtained with small-scale foaming equipment. Therefore, a crosslinked polyolefin open cell can be obtained at low cost and can be suitably used for cushioning materials, packing materials, filters, heat insulating materials, coating materials, and the like.

Claims (9)

[Claims] (1) A crosslinked polyolefin in which a foamable crosslinkable composition prepared by adding a foaming agent and a crosslinking agent to a polyolefin is heated and foamed in a non-airtight mold to generate cells, and then mechanically deformed to make the cells open. A method for producing an open-celled crosslinked polyolefin foam, the method comprising heating and foaming the expandable crosslinkable composition while covering at least a portion of it with a plastic sheet. (2) At least a portion of the foamable crosslinkable composition is covered with a plastic sheet and heated and shaped, the plastic sheet is heat-fused to the shaped composition, and then heated and foamed in this state. The method of claim 1, characterized in that: (3) The method according to claim 1 or 2, characterized in that the upper and lower surfaces of the foamable crosslinkable composition are covered with plastic sheets. (4) The method according to claim 1, 2 or 3, wherein a plastic sheet to which a crosslinking agent is added is used. (5) The method according to claim 1, 2 or 3, using a plastic sheet to which a blowing agent and a crosslinking agent are added. (6) Claim 1 wherein the plastic sheet is a crosslinked sheet.
, 2 or 3. (7) The method according to claim 1, 2 or 3, wherein the plastic sheet is a crosslinked sheet to which a foaming agent has been added. (8) The method according to any one of claims 1 to 7, wherein the plastic sheet is a polyolefin sheet. (9) The method according to any one of claims 1 to 7, wherein the plastic sheet is an ethylene resin sheet.