JP3308294B2 - 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, and more specifically, to a method for producing a polyolefin foam having a cell diameter of 250 to 250.
It is relatively large at about 450 μm and rich in compressive stress.
The present invention relates to a method for efficiently producing a polyolefin foam foamed 10 times or more.

[0002]

2. Description of the Related Art As a method for producing a polyolefin block foam, generally, a mixture of a polyolefin resin, a crosslinking agent and a foaming agent is filled into a mold, and the crosslinking agent and the foaming agent are pressurized and heated. A method of expanding the mixture to a desired density at one time by completely decomposing and then removing the pressure (hereinafter referred to as one-stage foaming), and Japanese Patent Publication No. 52-834.
No. 8, Japanese Patent Publication No. 2-42649, etc., the mixture is filled in a primary mold and heated under pressure to cause primary expansion, and then the foam is heated at normal pressure to perform secondary expansion. A method for obtaining a foam having a desired density (hereinafter referred to as two-stage foaming) is known.

[0003]

However, in the above-mentioned one-stage foaming, when a foam of 10 times or more is obtained, the foam is expanded to a final foam of a desired density at a time, so that the resulting final foam is deformed. In addition, there is a problem that the foam is cracked when the foam is taken out from the mold, and the commercialization rate is extremely low. On the other hand, in the two-stage foaming, there is a drawback that, during the secondary foaming, the cross-linking of the foam has progressed considerably, so that the cell diameter becomes small, and as a result, a foam having a high compression stress cannot be obtained. In addition, the above-mentioned
The production method of Japanese Patent No. 42649 produces a thick foam having uniformly fine (about 100 μm) closed cells by indirectly heating an intermediate primary foam by heating a metal plate of a mold from outside. Is what you do. Furthermore, it does not mention at all the relationship between the primary and secondary expansion ratios and the cell diameter of the final foam.

The present invention overcomes the above-mentioned drawbacks, and efficiently removes a final foam foamed 10 times or more, in particular, a foam having a cell diameter of about 250 to 450 μm, which is not too small and not too large. It is intended to provide a method of manufacturing.

[0005]

Means for Solving the Problems The present inventor has found a method for obtaining a foam having a high productive efficiency, a large cell diameter and a high compressive stress by simply changing production conditions, and has completed the present invention. It was done. More specifically, foaming in a high pressure state in the primary is suppressed to a certain level or less, and in a normal pressure state in the secondary, by foaming as much as possible, while securing a desired cell diameter of the final foam,
It is intended to prevent problems such as deformation and cracking.

That is, according to the present invention, a mixture comprising a polyolefin, a crosslinking agent and a foaming agent is filled in a primary mold, and the primary mold is heated under a pressure of 50 kg / cm ² or more. The cross-linking agent and a part of the foaming agent are decomposed and then decompressed and then subjected to primary expansion, and the volume expansion is 1/7 or less of the total expansion of the final foam and the primary expansion ratio is less than 10 times. A primary foam having a specific modulus is manufactured, and then the primary foam is placed in a secondary mold and heated at normal pressure, and has a volume expansion of 7 times or more and less than 10 times the primary foam. the Rukoto to secondary expansion, 30 times or more
Having an average cell diameter of 250 to 450 μm, JI
The 25% compressive stress measured by the method of SK6767 is
It is characterized by producing a final foam of 0.48 Kg / cm ² or more .

In the present invention, “polyolefin” refers to
For example, polyethylene, an ethylene-propylene copolymer, an ethylene-butene copolymer, an ethylene-vinyl acetate copolymer, ethylene and methyl, ethyl, propyl which are usually produced by a high pressure method, a medium pressure method or a low pressure method which are commercially available Or a copolymer of butyl with each acrylate (the content of this ester; within 45 mol%), or those chlorinated to a chlorine content of 60% by weight,
Further, a mixture of two or more of these, or a mixture of these with isotactic polypropylene or atactic polypropylene, and the like.

[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.

In the present invention, the pressure in the primary expansion step is
It is 50 kg / cm ² or more. If the pressure is less than this pressure, depending on the expansion ratio, if the conditions are such that the expansion is to about 10 times, the foam will leak from the mold and cause deformation of the primary foamed product, thereby lowering the product production rate. I do. The secondary foaming is performed under normal pressure, and is usually 20 kg / cm.
It will be processed under a pressure of ² or less.

[0011] The primary expansion ratio is 1/7 or less of the total expansion of the final foam. If it is larger than this, the expansion force in the secondary foaming becomes insufficient, and it becomes impossible to improve the bubble size. As a means for obtaining such an expansion coefficient, it is most preferable to set the heating temperature of the primary foaming to a relatively low temperature of about 120 to 150C, preferably about 130 to 145C. The heating time can be arbitrarily selected depending on the desired primary expansion ratio. The heating temperature in the secondary foaming is important to completely decompose and expand the foaming agent, and it is preferable to set the heating temperature within a range that does not adversely affect the polyolefin, and is usually about 160 to 190 ° C. The heating time is usually about 20 to 60 minutes.

[0012]

According to the study of the present inventors, as shown below,
By selecting an appropriate primary expansion ratio and secondary expansion ratio , the foam is expanded to 30 times or more, and the desired cell diameter (250 to 4
50 μm) and the method of JIS K6767.
25% compressive stress measured by 0.48 kg / cm ²
As described above, it was found that the compression stress was excellent, and the deformation and cracking of the foam could be prevented. That is, when the primary expansion ratio performed under high pressure is 1/7 or less of the total expansion ratio, for example, when the final expansion ratio is 30 times and is as large as about 4.3 times or more, foaming under high pressure is mainly performed. As the expansion force in the secondary foaming becomes insufficient, the average cell diameter becomes about 100 μm.
Too small to the extent. In particular, if this is 10 times or more, the primary and secondary foams will be further deformed and cracked.

On the other hand, in the secondary foaming, even if the crosslinking is progressing, if the secondary foaming ratio is 10 times or more that of the primary foam, the cell diameter rapidly increases. This is because the secondary foaming is performed under normal pressure, so if the foaming ratio is increased to a certain degree or more, it is possible to obtain an expansion force that overcomes the restraint due to the crosslinking, and as a result, a sudden increase in the bubble diameter is induced. It is estimated that it is possible. Therefore, the secondary expansion ratio is set to less than 10 times so that the cell diameter is not excessively increased. Further, when the expansion ratio is 10 times or more, the secondary foam may be deformed and cracked. In addition, by setting the secondary foaming ratio to 7 times or more, the expansion force in secondary foaming is sufficiently secured, and a foaming diameter of an appropriate size that is not too small and about 250 to 450 μm is secured. Is what you do.

As described above, by appropriately setting the primary expansion ratio and the secondary expansion ratio, foaming in the primary high pressure state is suppressed to a certain level or less, and foaming is performed as much as possible in the secondary normal pressure state. It is to achieve the purpose. In addition, the primary pressure is 50 kg /
When it is less than cm ² , the mold clamping force is small, so that the foam easily leaks from the primary mold.

[0015]

As described above, according to the production method of the present invention, a final foam having a desired average cell diameter not too large and not too small and having excellent compressive stress can be produced without deformation or cracking. The product can be manufactured, and since the foam does not leak from the mold, the product production rate is very good.

[0016]

The present invention will be described below in detail with reference to examples. Azodicarbonamide 1 was added to 100 parts by weight (hereinafter referred to as "parts") of polyethylene having a melt index of 1.0.
A composition comprising 0 parts, 2 parts of dicumyl peroxide, 0.5 part of zinc oxide and 5 parts of white kerosene was kneaded on a roll having a surface temperature of 100 ° C. to obtain an admixture. This mixture was processed under various conditions shown in Table 1 to produce a 30-fold final foam. The results are also shown in Table 1. In Table 1, the numbers marked with * are outside the scope of the present invention. In the same table, the units of the numbers in the column of “leakage of foam in primary mold” and the column of “deformation and cracking of primary and secondary foams” are numbers.

Further, the average cell diameter is measured by measuring the diameter of 100 cells for each foam and expressing the average value. The measurement of 25% compressive stress is based on JIS K67
67 was performed.

[0018]

[Table 1]

According to the results, the primary expansion ratio was 5-1.
In the case where the average bubble diameter is as large as 1 time (Comparative Examples 1 to 3), the average bubble diameter is as small as 105 to 115 μm and the compressive stress is also small. When the primary expansion ratio was as large as 11 times (Comparative Example 3), deformation and cracking of the foam also occurred. When the secondary expansion ratio was as large as 11 times (Comparative Example 4), the average cell diameter was as large as 635 μm, and the secondary foam was deformed and cracked. Furthermore, when the primary pressure is 40 kg / cm ²
(Comparative Example 5), the leakage of the foam in the primary mold was large, and this led to the deformation and cracking of the primary and secondary foams. On the other hand, in Examples 1 to 3, there are no such disadvantages,
The desired average cell size, not too small and not too large (250
４２０420 μm), and a final foam excellent in compressive stress can be manufactured. Further, since the foam does not leak from the mold, the product production rate is very good. It should be noted that the present invention is not limited to the specific embodiments described above, but can be variously modified within the scope of the present invention according to the purpose and application.

Continuation of front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B29C 44/00-44/60 C08J 9/04-9/10

Claims (1)

(57) [Claims] 1. A primary mold is filled with an admixture comprising a polyolefin, a cross-linking agent and a foaming agent, and the primary mold is subjected to 50K.
By heating in a pressurized state of g / cm ² or more, the crosslinking agent and a part of the foaming agent are decomposed, then decompressed and then primarily expanded. Producing a primary foam having a volume expansion coefficient of 1/7 or less and a primary expansion ratio of less than 10 times, then placing the primary foam in a secondary mold and heating it at normal pressure, to at and 10 times less than the rate of swelling of 7 or more times by Rukoto by secondary expansion, more than 30 times
Foamed, average cell diameter 250-450 μm, JIS
The 25% compressive stress measured by the method of K6767 is
A method for producing a polyolefin foam, which comprises producing a final foam of 0.48 kg / cm ² or more .