JP2779876B2 - 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 particularly, to a method for producing a polyolefin foam having a cell diameter of 250-4.
The present invention relates to a method for efficiently producing a polyolefin foam having a large compressive stress by making it relatively large at about 50 μm and foaming 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 cross-linking agent and a foaming agent is filled in a mold and the cross-linking agent and the foaming agent are pressurized and heated. Is completely decomposed and then decompressed to expand the mixture to a desired density at one time (hereinafter referred to as one-stage foaming), and Japanese Patent Publication No. 52-8 / 1982.
No. 348, 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. To obtain a foam having a desired density (hereinafter referred to as 2
Called step foaming. ) And 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, when taken out of the mold, there is a problem that the foam is cracked, and the commercialization rate is extremely low. On the other hand, in the two-stage foaming, at the time of the secondary foaming, since the cross-linking of the foam has progressed considerably, the cell diameter becomes small, and as a result, a foam having a high compressive stress cannot be obtained. Was.
In the manufacturing method disclosed in Japanese Patent Publication No. 2-42649, an intermediate primary foam is indirectly heated by externally heating a metal plate of a metal mold to form a uniform fine (about 100 μm
The present invention is intended to produce a thick foam having closed cells of a certain degree. However, there is no mention of 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 provides a final foam foamed 10 times or more, particularly a final foam having a cell diameter of about 250 to 450 μm which is not too small and not too large. Is to provide a method for efficiently producing the same.

[0005]

SUMMARY OF THE INVENTION The present inventor has developed a method of obtaining a foam having a high productive efficiency, a large cell diameter and a high compressive stress by merely changing the production conditions and optimizing the mold size. And found the present invention. More specifically, foaming in a high-pressure state in the primary is suppressed to a certain level or less, and foaming is performed as much as possible in a normal-pressure state in the secondary, thereby securing a desired cell diameter of the final foam and problems such as deformation and cracking. In addition, the final foam obtained by optimizing the dimensions of the primary and secondary molds has a substantially desired size and shape,
In other words, the cutting in the post-processing process for adjusting the size and shape is minimized, and the yield of the raw material is also improved.

That is, the first invention provides a method for producing a final foam which is foamed 10 times or more using an admixture of a polyolefin, a crosslinking agent and a foaming agent. By filling a similar primary mold into a primary mold and heating the primary mold in a pressurized state of 50 kg / cm ² or more, a part of the cross-linking agent and the foaming agent is decomposed and then depressurized to remove the primary pressure. To produce a primary foam having a volume expansion rate of 1/7 or less of the total expansion rate of the final foam in terms of volume expansion rate and a primary expansion rate of less than 10 times. A secondary mold having a shape substantially similar to the foam and each dimension of length, width, and height being 1 to 5% smaller than each dimension of the target final foam, and a secondary mold for the primary mold. The vertical and / or vertical ratios of the mold The horizontal ratio value and the height ratio value are placed in a secondary mold having the following range, and heated at normal pressure to a volume expansion coefficient of 7 times or more and less than 10 times with respect to the primary foam. And secondary expansion to produce the final foam. [(Height ratio value) / (vertical and / or horizontal ratio value) =
1.3-1.6]

According to a second aspect of the present invention, the average foam diameter of the final foam is 250 to 450 μm by the production method according to the first aspect.
It is characterized by the following.

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.

The term "crosslinking agent" as used in the present invention means a compound having a decomposition temperature at least equal to the flow starting temperature of the polyolefin in the above-mentioned polyolefin. 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.

[0011] In the present invention, the pressure in the primary expansion step is 50 kg / cm ² or more. Below this pressure,
Although it depends on the expansion ratio, if the expansion ratio is set to a value close to 10 times, the foam will leak from the mold and cause deformation of the primary foamed product, thereby lowering the commercialization rate. is there. The secondary foaming is performed under normal pressure, and is usually processed under a pressure of 20 kg / cm ² or less.

[0012]

According to the study of the present inventor, foaming due to decomposition of the foaming agent in the primary mold is caused by the fact that the resin is densely filled in the mold and maintained in a high pressure state. Will spread evenly over the entire surface. Therefore, it is presumed that when it becomes supersaturated, it forms nuclear bubbles in a uniformly dispersed state. Therefore, even if the amount of decomposition of the foaming agent is increased in this state, it is presumed that it does not contribute much to the growth of bubbles, but only increases the number of nuclear bubbles. On the other hand, in the foaming in the secondary mold, the decomposition gas flows into the nuclear bubbles already formed in the primary mold because the resin is in a state in which the resin can freely expand in the mold. It is considered that the bubble grows with the expansion force of.
In other words, it is assumed that the primary foaming contributes to an increase in the number of nuclear bubbles, and the secondary foaming contributes to the growth of the bubbles (expansion of the bubbles themselves) under a certain degree of expansion force.

From the above findings, the present inventors have found that the primary expansion ratio is suppressed to a certain level or less, the number of formed nuclear cells is reduced, and then the secondary expansion ratio is set as a ratio having an expansion force of a certain level or higher. It was presumed that by growing these nuclear bubbles, a uniform and relatively large cell structure having a desired cell diameter could be obtained, and intensive studies were conducted. As a result, it was found that the primary expansion ratio and the secondary expansion ratio had appropriate selection ranges as shown below. In this range, the desired cell diameter (250 to 450 μm) was obtained, and the foam was prevented from being deformed or cracked. The selection range of an appropriate primary expansion ratio refers to 1/7 or less and less than 10 times of the total expansion of the final foam. 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, about 4.3.
If it is larger than twice, foaming under high pressure is mainly performed, so that the number of nuclear bubbles increases, the expansion force in secondary foaming becomes insufficient, and the average bubble diameter becomes as small as about 100 μm. If the expansion ratio is 10 times or more,
In addition to this, there is a problem that the primary and secondary foams are deformed and cracked.

On the other hand, the selection range of the appropriate secondary expansion ratio is 7 times or more and less than 10 times. In secondary foaming,
Even if the cross-linking is progressing, if the secondary expansion ratio is 7 times or more of the primary foam, the cell diameter increases rapidly. This is because the secondary foaming is performed at normal pressure,
This is because if the expansion ratio is increased to a certain degree or more, it is possible to obtain an expansion force that can overcome the restraint caused by the crosslinking.
As a result, it is estimated that rapid expansion of the bubble diameter can be induced. However, when the expansion ratio is 10 times or more, the bubbles are too large, and there is a possibility that the bubbles may be broken. Therefore, the secondary expansion ratio is set to 7 times or more and less than 10 times so that the cell diameter is not too small or too large. Here, if the secondary expansion ratio is set to 10 times or more, there is a problem that the secondary foam may be deformed or cracked.

As means for obtaining such an expansion coefficient, the primary foaming heating temperature is set to 120 to 150 ° C., preferably 13 to 150 ° C.
Most preferably, it is set to a relatively low temperature of 0 to 145 ° C. The heating time can be arbitrarily selected depending on the desired primary expansion ratio. In addition, 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.

By the way, when the primary expansion ratio is reduced and the secondary expansion ratio is increased as described above, the secondary expansion is free expansion, so that the dimensions of the final foam may be larger than desired dimensions. Often occurs. For this reason, the dimensions of the final foam must be larger than the desired dimensions, and cutting must be performed in the final stage to obtain the desired dimensions and shapes. Therefore, there is a problem that the raw material yield is reduced by the cutting and the number of steps is increased. As a countermeasure, the inventor has tried to optimize the dimensions and shape of the mold. That is, the primary and secondary mold shapes are substantially similar to the final foam shape, and the vertical, horizontal, and height dimensions of the secondary mold are 1 to each of the intended final foam sizes. ~ 5% smaller,
Moreover, the vertical and / or horizontal ratio values, which are the dimensional ratios of the vertical, horizontal, and height of the secondary mold to the primary mold, and the height ratio value are [((height ratio value) / (Vertical and / or horizontal ratio value) = 1.3 to 1.6].

The reason why the shapes of the primary and secondary molds are substantially similar to the shape of the final foam is, needless to say, of the foam 3.
This is to make it possible to induce dimensional expansion as uniformly as possible, thereby preventing dimensional errors of the final foam. The reason for making the size of the secondary mold smaller by 1 to 5% than the size of the final foam is that the foamed and expanded final foam is uniformly pressed against the secondary mold wall by its self-expanding force. This is because molding can be performed according to the shape of the next mold. If the secondary mold is less than 1% and is small, the self-expanding force of the foam causes insufficient pressing force of the foam on the secondary mold wall, and the foam side surface becomes uneven or square. It is difficult to form the portion as the secondary mold because the portion is not formed. On the other hand, if it is smaller than 5%, it becomes difficult to take out the foam from the mold, or the center of the foam rises and becomes deformed when the mold is opened.

In addition to the above conditions, the relationship between the vertical and / or horizontal ratio values, which are the respective dimensional ratios of the secondary mold to the primary mold in the vertical, horizontal, and height directions, and the height ratio value is set. It must be as follows. [(Height ratio value) / (vertical and / or horizontal ratio value) =
1.3-1.6] That is, the expansion of the foam in the secondary mold is extremely small compared with the resistance due to friction with the bottom surface in the vertical and horizontal directions although the height direction has resistance to gravity. In addition, secondary expansion tends to occur in the height direction. In particular, in the case of foaming at a high magnification in the secondary as in the present invention, this tendency becomes remarkable, and the ratio of the height ratio value to the vertical and / or horizontal ratio value is set to 1.3 to 3.0 as in the present invention. If it does not fall within the range of 1.6, the final foam hardly conforms to the shape of the secondary mold.

For example, to obtain a 30-fold foam, the dimensions of the primary mold are 100 mm long × 100 mm wide × 10 m high.
m, the size of the secondary mold is 265 mm x 265 m
m × 40 mm, and the final foam obtained immediately after the secondary foaming is 270 mm × 270 mm × 41 mm. In this case, the size of the secondary mold is 1.times.
9% smaller and 2.4% smaller in height. The ratio of the secondary mold size to the primary mold size is as follows: aspect ratio = 2.65, width ratio = 2.65, and height ratio = 4.0. Therefore, (height ratio value) / (length and / or width ratio value) = 4.0 / 2.65
= 1.5, both are within the scope of the present invention, and a beautifully shaped final foam having almost no difference from the size of the desired final foam can be obtained. [(Height ratio value) / (vertical and /
Or the value of the transverse ratio)] is less than 1.3, the expansion in the longitudinal and / or transverse directions becomes too large, and even if the friction between the foam and the mold bottom surface is reduced by the lubricant, Often, the required amount of linear expansion cannot be satisfied, resulting in molding failure. When the ratio exceeds 1.6, the expansion margin is too small, and the expansion is restricted by the side surface of the mold at the stage where the foaming is not completed. Becomes

[0020]

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 compression stress can be produced without deformation or cracking. Manufacturable, and the desired final foam shape,
Since a product having almost the same dimensions can be obtained, the product commercialization rate is very good.

[0021]

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. The mixture was processed under various conditions shown in Tables 1 and 2 to produce a 30-fold final foam. The results are shown in Tables 3 and 4.
Note that, in Tables 2 and 3, the numbers marked with * are outside the scope of the present invention. In Tables 3 and 4, the units of the numbers in the columns of “Foam leak in primary mold” and “Deformation and cracking of primary and secondary foams” are numbers. Further, immediately after the completion of the secondary foaming, the product dimensions (E) are obtained by taking out the product from the secondary mold immediately after the completion of the foaming, and measuring the minimum values of the height, width and height of the product having no deformation or crack. , Expressed as the average. On the other hand, in the measurement of the average cell diameter, the diameter of 100 cells was measured for each foam, and the average value was shown. The measurement of 25% compressive stress is based on JIS K6
767. Note that “high compressive stress” or “rich in compressive stress” used in the present invention means that the value obtained by such measurement is large.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

According to the results, the primary expansion ratio was 5 to 1.
In the case where it was as large as 1 time (Comparative Examples 1 to 3), the average bubble diameter was as small as 105 to 115 μm and the compressive stress was also small. When the primary expansion ratio was as large as 11 times (Comparative Example 3), the foam was deformed and cracked. 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. On the other hand, the primary pressure is 40 kg / c
In the case of a low m ² (Comparative Example 5), the leakage of the foam in the primary mold occurred, and this led to a large number of deformations and cracks in the primary foam and the secondary foam.

However, even if the primary and secondary expansion ratios and processing conditions are optimized, if the dimensions of the mold are not optimized, it is difficult to obtain a foam having the desired dimensions. Occurred. For example, the reduction rate of the secondary mold is 1%
In the case of less than (Comparative Example 7), the pressing force of the foam to the inner surface of the secondary mold due to self-expansion was insufficient, and irregularities were formed on the outer surface of the foam, so that a product having a desired size could not be obtained. Here, the reduction ratio of the secondary mold = [(the target dimension immediately after the completion of the secondary foaming−the secondary mold dimension) / the target dimension immediately after the completion of the secondary foaming]. Further, when the reduction ratio was set to a value exceeding 5% (Comparative Example 8), the secondary mold was too small, and the center of the foam was lifted when the mold was opened, and product deformation was observed.

When the value of [(height ratio value) / (vertical and / or horizontal ratio value)] is less than 1.3 (Comparative Example 6), the expansion in the vertical and horizontal directions is insufficient. The desired product dimensions could not be obtained. In addition, when the above value was set to a value exceeding 1.6 (Comparative Example 9), the expansion in the vertical and horizontal directions was inhibited by the secondary mold wall surface, and deformation occurred.

On the other hand, in Examples 1 to 3, the above-mentioned disadvantages are not all present, and the final foam having a desired average cell diameter (250 to 420 μm) which is neither too small nor too large, and which is excellent in compressive stress. I was able to make my body. In addition, the product of desired dimensions was obtained without leakage of the foam from the mold, and the product commercialization rate was 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 the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B29C 67/20 B29C 44/00-44/60 C08J 9/00-9/42

Claims (2)

(57) [Claims] 1. A method for producing a final foam which is foamed 10 times or more using an admixture comprising a polyolefin, a crosslinking agent and a foaming agent, wherein the admixture is a primary mold substantially similar in shape to the final foam. The primary mold is heated under a pressurized state of 50 kg / cm ² or more to decompose a part of the crosslinking agent and the foaming agent, and then remove the pressure to perform primary expansion, thereby increasing the volume expansion coefficient. Producing a primary foam having a volume expansion coefficient of 1/7 or less of the total expansion coefficient of the final foam and a primary expansion ratio of less than 10 times, and then forming the primary foam substantially similar to the final foam. And a secondary mold in which each dimension of length, width, and height is 1 to 5% smaller than each dimension of the target final foam, and the length, width, and height of the secondary mold relative to the primary mold The vertical and / or horizontal ratio values, which are the respective height ratios, and the height Is placed in a secondary mold having the following range, and heated at normal pressure, and the secondary foam is subjected to secondary expansion at a volume expansion ratio of 7 times or more and less than 10 times with respect to the final foam. A method for producing a polyolefin foam, comprising producing a foam. [(Height ratio value) / (vertical and / or horizontal ratio value) =
1.3-1.6] 2. The final foam has an average cell diameter of 250.
The method for producing a polyolefin foam according to claim 1, wherein the thickness is from 450 to 450 µm.