JP2872519B2 - Method for producing polyolefin foam - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyolefin foam having excellent moldability by so-called two-stage foaming. More specifically, a part of the intermediate foam produced in the primary foaming step, which has undergone particularly advanced crosslinking, (hereinafter, referred to as a “crosslinked part”) is cut, and the self-extension of this part in the secondary foaming step is performed. The present invention relates to a method for producing a polyolefin foam which facilitates the improvement of the moldability of a final molded product and does not require a post-treatment such as shaping by cutting.

[0002]

2. Description of the Related Art As a method for producing a polyolefin block foam, a mixture of a polyolefin resin, a crosslinking agent and a foaming agent is filled in a mold, and the crosslinking agent and the foaming agent are completely decomposed under pressure and heat. Then, the mixture is expanded to a desired density at a time by removing the pressure (hereinafter referred to as one-stage foaming), or Japanese Patent Publication No. 2-42649.
As disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. HEI 7-86, the mixture is filled in a primary mold, heated under pressure to decompose 15 to 60% of the foaming agent, thereby causing primary expansion, and thereafter, the foam is constantly subjected to regular expansion. A method of heating under pressure to decompose and expand the remaining foaming agent to obtain a foam having a desired density (hereinafter referred to as two-stage foaming).
It has been known.

However, when a high-foamed product is obtained by the above-described one-stage foaming, it is necessary to expand the foam into a final foam having a desired density at one time.
There is a problem in that the final foam obtained is deformed, or when the foam is taken out of the mold, the foam is cracked, resulting in an extremely low commercialization rate. Therefore, two-stage foaming has been developed in order to improve the reduction in the production yield of the one-stage foaming. In the two-stage foaming, a product having a predetermined expansion ratio is not foamed and expanded at once, but is foamed and expanded in two stages, thereby eliminating factors such as cracks and the like, which lower the product production yield.

[0004]

However, even in such a two-stage foaming method, the formability of the final foam, particularly the formability of the corners thereof, cannot be said to be sufficient. That is, in the two-stage foaming, the heating and foaming process is performed in two stages, so that the intermediate foam to be subjected to the secondary foaming process has its skin portion, particularly its corner, at the stage of primary foaming. Some have significantly advanced crosslinking. When such an intermediate foam is placed in a secondary mold and foamed by heating under normal pressure, self-elongation due to the foaming expansion pressure is hardly induced at the corners and the like due to the advanced crosslinking state. As a result, it becomes difficult to press-mold the inner wall surface of the secondary mold at the bridging progression portion such as a corner,
A state of poor molding called "ear drop" (a state in which the corner of the final molded product is not sharply formed but rounded) is likely to occur.

The present invention has been made to solve the above-mentioned problems. For example, a notch is formed in a portion of the intermediate foam where the cross-linking proceeds, thereby facilitating the expansion and expansion in the secondary foaming step and improving the moldability of the final molded article. It is an object of the present invention to provide a method for producing a polyolefin foam, which aims at improvement.

[0006]

The method for producing a polyolefin foam according to the present invention is directed to a method for producing a polyolefin foam using a foamable composition containing a polyolefin, a crosslinking agent and a foaming agent. Is filled in a primary mold, heated under pressure to decompose part of the foaming agent to produce an intermediate foam, and then cut at least into the skin near the corners of the intermediate foam, Or at least the skin at the corners is removed, and then the intermediate foam is placed in a secondary mold having an internal shape corresponding to the shape and dimensions of the final foam product, and the intermediate foam is removed under normal pressure. The final foam is produced by heating to decompose the remaining foaming agent.

[0007] In the present invention, the term "polyolefin" refers to, for example, polyethylene, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-ethylene copolymer produced by a commercially available high-pressure method, medium-pressure method or low-pressure method.
Vinyl acetate copolymers, copolymers of ethylene with acrylic acid esters of methyl, ethyl, propyl or butyl (content of this ester; within 45 mol%), or chlorine content of up to 60% by weight, respectively, And a mixture of two or more of these, or a mixture thereof with isotactic polypropylene or atactic polypropylene.

[0008] The term "crosslinking agent" as used in the present invention has a decomposition temperature at least equal to the flow starting temperature of the polyolefin in the above-mentioned polyolefin, and is decomposed by heating to generate free radicals to form free radicals between the molecules. Organic peroxides and the like, which are radical generators that cause cross-linking. For example, dicumyl peroxide, 2,5-dimethyl-2,5-bis-tert-butylperoxyhexane, 1,3-bis-tert-peroxy-isopropylbenzene, and the like. "Blowing agent" in the present invention
The term "having a decomposition temperature equal to or higher than the flow start temperature of the polyolefin" means, for example, azodicarbonamide, dinitrosopentamethylenetetramine and the like.

In the present invention, in order to control the foaming state, a compound containing urea as a main component, a metal oxide such as zinc oxide or lead oxide, a lower or higher fatty acid, or a metal salt of a lower or higher fatty acid may be used. A foaming aid and the like can be added. Further, carbon black, zinc white, titanium oxide, and other commonly used compounding agents can be added to improve physical properties.

[0010] The position of the above-mentioned "cut" is preferably selected according to the size of the intermediate foam, the expansion ratio, the type of resin, and the like. For example, in the case of a plate having a relatively small thickness, it is sufficient to make a cut near the corner on the lateral outer peripheral surface of the intermediate foam, but in the case of a plate having a relatively large thickness, In the case of having a complicated shape, it is preferable to make a cut in the vicinity of a corner on other surfaces (upper surface, lower surface, etc.). It is preferable to determine the size and number of cuts in the same manner, but basically, the depth is 5 m.
About m ~ 50mm, the length is more than the thickness of the foam,
It is appropriate to provide a predetermined number in at least one of the thickness direction of the intermediate foam and the direction perpendicular thereto in accordance with the size and the like of the intermediate foam. The effect of the present invention can be further exhibited by making a cut not only in the vicinity of the corner of the intermediate foam but also in the entire skin (or the side outer peripheral surface).

[0011] Further, the above-mentioned "when the skin at the corner is removed"
In addition, the size of the skin to be removed
It is preferable to select according to the size of the intermediate foam, the expansion ratio, and the like, as in the case of adding. The removal of the skin may be performed only on the rounded portion of the corner of the intermediate foam. However, when this is performed over the entire skin (or the side outer peripheral surface), the final molded product is removed. The moldability is dramatically improved.

[0012]

According to the present invention, a cut is made in the skin near the corner of the intermediate foam obtained in the primary foaming step of the two-stage foaming,
After removing the skin at the corner, the intermediate foam is placed in a secondary mold to perform secondary foaming. This makes it possible to easily perform self-extension even in a portion such as the above-mentioned corner portion where self-expansion during expansion and expansion of the intermediate foam is difficult, and the inner layer portion of the intermediate foam having relatively slow crosslinking progress. And the like also appear on the epidermis from the cuts and the like along with the foaming expansion and contribute to the self-elongation of the corners and the like while constituting a part of the corners and the like. Therefore, the intermediate foam foams and expands uniformly throughout the entire foam, and the foaming and expanding force causes the foam to expand.
The final foam is then pressed against the inner wall of the forming mold and, as such, without any "ears". In this case, post-processing such as shaping by cutting is not required, so that the manufacturing efficiency can be improved.

[0013]

The present invention will be described below in detail with reference to examples. (1) Production of polyolefin foam Example 1 First, polyethylene 100 having a melt index of 1.0
5 parts by weight of azodicarbonamide, 2 parts by weight of dicumyl peroxide and 0.5 part by weight of zinc oxide
Was kneaded on a roll having a surface temperature of 100 ° C. to obtain an admixture. Next, 6 kg of this mixture was filled in a primary mold (410 mm × 410 mm × 40 mm),
After heating at 160 ° C. for 30 minutes under a pressure of kg / cm ^2, the pressure was released at a high temperature and an intermediate foam (880 mm × 880 mm × 86 mm) expanded and expanded about 10 times was obtained.

Then, the entire skin on the side outer peripheral surface of the obtained intermediate foam was cut to a thickness of 5 mm. Further, the intermediate foam is subjected to a secondary molding die (inner space dimension: 1000 mm ×
1000mm x 100mm) and put at 165 ℃
After heating for 2 minutes to cause secondary foaming, the mixture was cooled for 30 minutes to produce the polyolefin foam according to Example 1.

Example 2 A skin near each corner of the lateral outer peripheral surface of the above-mentioned intermediate foam was cut by a length of 10 mm in the direction of each side centering on a two-side joint of each side of the intermediate foam. A polyolefin foam of Example 2 was produced in the same manner as in Example 1, except that the polyolefin foam was cut off to a thickness of 5 mm. Example 3 Without cutting the side outer peripheral portion of the intermediate foam, the upper surface (or lower surface) of the intermediate foam and the two-face joint of each side surface were substantially parallel to the entire outer peripheral portion, and the depth was about 10 mm. A polyolefin foam of Example 3 was produced in the same manner as in Example 1, except that three notches were formed at intervals of about 8 mm.

Example 4 A side face outer peripheral portion of the intermediate foam is not cut, and as shown in FIG. 1, a two-face joint (11 or the like) between side surfaces (21, 22 or the like) of the intermediate foam 1 is formed. About 10 m in length perpendicular to each two-sided joint (11 etc.) toward each side (21, 22 etc.)
m, about 10 mm depth cut (31-33 etc.) about 8
A polyolefin foam of Example 4 was produced in the same manner as in Example 1 except that three pieces were inserted at mm intervals.

Example 5 First, the amount of the mixture used was 12 kg, and the primary mold (410 mm × 410 mm × 80 m
An intermediate foam (880 mm × 880 mm × 172 mm) was obtained in the same manner as in Example 1 except that m) was used. Then, as shown in FIG. 2, near the corners (61a, 62a, etc.) of this intermediate foam, each two-sided joint (11a to 11a)
15a, etc.), approximately 10 mm in length and 10 m in depth
m, and two cuts (31a to 36a etc.) arranged at intervals of about 5 mm are formed in the two-sided joints (11a to 1a).
5a, etc.). Furthermore, this intermediate foam is placed in a secondary mold having a depth of twice (inner space dimension: 100 mm).
0 mm × 1000 mm × 200 mm), and secondary foaming was carried out.
Was produced. Comparative Example While the intermediate foam obtained by primary foaming was used as it was,
A final foam product according to a comparative example was obtained in the same manner as in Example 1 except that the final foam product was placed in a secondary mold and foamed secondarily.

(2) Evaluation of Moldability of Polyolefin Foam Each of the above polyolefin foams was observed, and the presence or absence of “eardrops” was examined. In this example, "eardrops" refers to a state in which the corners of the polyolefin foam are not sharply formed and have a radius of curvature of 20 mm or more.
According to this, in the polyolefin foam of the comparative example, ears were formed at all corners, indicating poor moldability. In addition, the appearance of this foam is so poor that a post-process such as shaping is required for use as a final product.

On the other hand, all of the polyolefin foams of the examples exhibited no dropout and showed good moldability. In particular, in each of the polyolefin foams of Example 1 in which the entire outer peripheral portion of the outer peripheral portion of the intermediate foam was removed and Example 3 in which a cut was made to surround the entire outer peripheral portion, the corners were particularly sharp and the moldability was high. Excellent. In addition, even in the case of an intermediate foam having a relatively large thickness as in Example 5, a polyolefin foam having excellent moldability can be easily obtained by making a cut in the vicinity of a corner, such as on the upper surface. Was. Furthermore, since the polyolefin foam of this example is excellent in moldability, post-processing such as shaping is not required, and production efficiency can be improved.

The present invention is not limited to the specific embodiments described above, but may be variously modified within the scope of the present invention in accordance with the purpose and application. That is,
In the first to fourth embodiments, the surface other than the outer peripheral surface of the intermediate foam may be removed. In the fifth embodiment, a cut is made only on the lateral outer peripheral surface of the intermediate foam. May be. Further, the “cut” and the “removal” may be used in combination for one intermediate foam.

[0021]

As described above, according to the production method of the present invention, the intermediate foam in the two-stage foaming can be uniformly expanded and expanded throughout, so that the final foam can be produced without "eardrops". can do. Also, by a relatively simple operation such as making a cut in the skin of the intermediate foam,
Since a final foam having excellent moldability can be obtained, and post-processing such as shaping is not required, production efficiency can be improved.

[Brief description of the drawings]

FIG. 1 is a partial perspective view of an intermediate foam according to a fourth embodiment.

FIG. 2 is a partial perspective view of an intermediate foam according to a fifth embodiment.

[Description of sign]

1, intermediate foam, 11, 11a to 15a; two-sided joint,
21, 22; side surface, 31-33, 31a-40a;

Claims (1)

(57) [Claims] 1. A method for producing a polyolefin foam using a foamable composition containing a polyolefin, a crosslinking agent and a foaming agent, comprising: filling the foamable composition in a primary mold and heating under pressure. Decomposing a part of the foaming agent to produce an intermediate foam, then cut into the skin near at least a corner of the intermediate foam, or remove at least the skin of the corner,
Then, the intermediate foam is placed in a secondary mold having an internal shape corresponding to the shape and dimensions of the final foam product, and the intermediate foam is heated under normal pressure to decompose the remaining foaming agent. A method for producing a polyolefin foam, comprising producing a final foam by the following method.