JP2872505B2 - 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.
The present invention relates to a method for efficiently producing a polyolefin foam having a relatively large compressive stress of about 450 μm and excellent in compressive stress, having an open cell ratio of 5 to 20% and a relatively small compression set.

[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 in which the mixture is expanded at a time to a desired density by completely decomposing and then depressurizing (hereinafter referred to as one-stage foaming), and Japanese Patent Publication No. 52-834.
No. 8 and Japanese Patent Publication No. 2-42649, etc., the mixture is filled in a primary mold, heated under pressure to cause primary expansion, and then the intermediate foam is heated at normal pressure to obtain a mixture.
A method of obtaining a final foam having a desired density by performing secondary expansion (hereinafter, referred to as two-stage foaming) is known.

However, in the above-described one-stage foaming, when a high foam is obtained, the foam is expanded at a time to a final foam having a desired density. The foam may crack,
There was a problem that the commercialization rate was extremely low. For this reason, two-stage foaming has been developed to prevent a reduction in the product yield of the one-stage foaming. In the two-stage foaming, a product having a predetermined expansion ratio is not expanded and expanded at one time, but is expanded and expanded in two stages, thereby eliminating factors such as deformation and cracks which lower the product yield.

[0004]

However, in the above-mentioned two-stage foaming method, "innumerable nuclear bubbles formed by the thermal decomposition of a part of a foaming agent under a primary high pressure are reduced to 70-90 μm when decompressed and expanded.
And the secondary cells are uniformly grown to an average cell diameter of 100 to 150 μm by secondary normal pressure expansion. ”The final foam obtained is uniform. It results in fine closed cells, and such foams generally have the disadvantage of poor compression hardness and relatively large permanent set.

The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to improve the physical properties of a foam in a two-stage foam having an excellent product production yield. An object of the present invention is to provide a method for producing a foam which is rich in compressive stress and small in permanent compression strain by optimizing the amount.

[0006]

According to the method for producing a polyolefin foam of the present invention, a foaming composition containing a polyolefin, a crosslinking agent and a foaming agent is filled in a primary mold, and the primary mold is filled with 50 kg / cm ^2. After heating under the above pressure and decomposing a part of the foaming agent so as to have a decomposition rate satisfying the following formula to induce foaming, the pressure is removed at the time of high-temperature heat to cause primary expansion,
First to remove the primary mold and produce an intermediate foam
Step and thereafter, the intermediate foam obtained in the first step is placed in a secondary mold corresponding to the dimensions and shape of the final foam,
Heating the intermediate foam under normal pressure to decompose and re-expand the remainder of the foaming agent and the cross-linking agent to produce a final foam, comprising :
The average cell diameter of the final foam is 250-450 μm and
Bubble rate is characterized by 5% to 20% der Rukoto. Decomposition rate of foaming agent in the first step (%) = (9 to 12) × (10
0 / final expansion ratio (%) )

[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.

In the present invention, the pressure in the primary expansion step is
"50 kg / cm ² or more". When the pressure of the primary mold is less than 50 kg / cm ² , the foam is expanded to about 10 times when the decomposition rate of the blowing agent of the present invention is set, and the foam leaks from the primary mold during foam expansion. Arises, 1
This causes deformation of the secondary foam, and this further lowers the product ratio, which is not preferable. The secondary foaming in the second step is performed under normal pressure, and is usually processed under a pressure of 20 kg / cm ² or less. Further, the foaming agent decomposition rate in the first step is calculated as (9 to 12) × (100 / final foaming ratio).
% Is set to 250 to 4
This is because a foam having a relatively large cell diameter of 50 μm and an open cell ratio of 5 to 20% is formed, and a product rich in compressive stress and small in permanent compression set can be obtained.

Further, as a means for adjusting the amount of decomposition of the foaming agent, there is a method of adjusting the addition amount of a foaming aid such as a metal oxide. The simplest and most reliable method is a primary foaming method. There is a method in which the heating temperature is set to a relatively low temperature of about 130 to 170 ° C. and the heating time is controlled. The heating temperature in the secondary foaming is important to completely decompose and foam 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.

The "secondary mold" is not a closed mold such as a mold and used under pressure, but has a non-sealed internal space. When the intermediate foam is secondarily expanded in this internal space, the structure has a structure in which the air remaining in the internal space can be removed to the outside by the expansion pressure of the foam. One or two small holes are provided on each surface for communicating the internal space with the external atmosphere.

The size and shape of the internal space of the secondary mold correspond to the size and shape of the final foam. The term "corresponding" means that the foam has a substantially similar shape, whereby the three-dimensional expansion of the foam can be caused as uniformly as possible. Along with this, it is possible to prevent the occurrence of irregularities and dimensional errors due to shrinkage spots on the surface of the final foam due to temporal shrinkage.

Further, each of the vertical, horizontal and height dimensions constituting the internal space of the secondary mold is different from the respective dimensions immediately after foaming of the foam obtained by final foaming the intermediate foam. Preferably, the size is reduced by 1 to 10%.
In this case, “vertical, horizontal and height” is used to mean a substantially similar shape in three dimensions in the case of a three-dimensional shape that is not clearly conceptualized. By reducing this by 1 to 10%, the foamed and expanded final foam is uniformly and completely pressed against the wall surface of the internal space by its self-expanding force, so that it is shaped according to the shape of the internal space. As a result, variations in the final foam shape are reduced. In addition, the heat transfer efficiency in the heating step (the cooling step in the secondary mold performed as needed) is also improved.

If the internal space dimension is less than 1% and is small, the pressing force of the foam against the inner wall surface of the internal space due to the self-expanding force of the foam becomes insufficient, and the foam surface becomes uneven. As a result, the corners are not formed and the corners are not formed, so that it is difficult to mold the secondary molds, and the heating efficiency also deteriorates. On the other hand, if the size is reduced to more than 10%, even if the foam shrinks due to subsequent cooling, the outer size of the foam is still considerably larger than the inner size of the space, and it is difficult to remove the foam from the cooling mold. Or the center of the foam rises when the cooling mold is opened, causing deformation cracking.

[0016]

According to the present invention, since the decomposition rate of the blowing agent in the primary foaming step is set to a decomposition rate satisfying the above equation, the number density of the nuclear bubbles formed when the foaming agent is decomposed in the primary foaming step is increased. The distance between the nuclear bubbles, that is, the cell wall can be made thin. The bubble wall thus formed, together with the explosive expansion caused by depressurization, can partially induce the destruction of the bubble wall in the process of growing the fine cells during the explosive expansion. . As a result, the fine cells aggregate, the cell size increases, and the broken cell wall strands fuse to the remaining cell walls, reinforce the cell walls, are rich in compressive stress, and have relatively small permanent set. It is possible to provide a foam. For example, 15 times (ie,
In the case of a final expansion ratio of ( 1,500%) , the foaming agent decomposition rate in the primary step satisfying the above expression is 60 to 80% . this
If the decomposition rate is less than 60%, the number density of nuclear bubbles is insufficient.
Makes it difficult to induce sufficient destruction of the bubble wall
You. On the other hand, if the decomposition rate exceeds 80%, the number density of nuclear bubbles
Is too high, the cell walls are excessively broken and primary and secondary foams are deformed and cracked , and the average cell diameter is reduced.
Too much, resulting in poor compressive stress. That is, in the present invention, since the foam decomposition rate in the first step is set to a predetermined appropriate range according to the final expansion ratio, the cell diameter is relatively large, 250 to 450 μm, and the open cell rate is 5 to 20%. And a product having a high compression stress and a small permanent compression set can be obtained.

[0017]

The present invention will be described below in detail with reference to examples. Examples 1 and 2 and Comparative Examples 1 to 3 In this example, the heating time of the primary foaming was changed variously.
The final foam was manufactured by changing the decomposition rate of the blowing agent in the next foaming step. A composition consisting of 5 parts of azodicarbonamide, 2 parts of dicumyl peroxide, 0.5 part of zinc oxide and 5 parts of white kerosene was added to 100 parts by weight (hereinafter referred to as "parts") of polyethylene having a melt index of 1.0. The mixture was kneaded on a roll at a temperature of 100 ° C. to obtain a mixture. 6 kg of this mixture is placed in a primary mold (410 mm × 410 mm × 40
mm), processed under the conditions of the first step shown in Table 1 (heating was performed by passing steam through the heat medium flow path), and then depressurized at high temperature and foamed to a predetermined size. The expanded intermediate foam was removed. Then, this is fixed to a predetermined secondary mold (inner space dimension: 1000 mm × 1000 mm).
× 100 mm) and processed under the conditions of the secondary process shown in Table 1 (heating was performed by passing steam through the heat medium passage) to produce a final foam having an expansion ratio of 15 times. .

[0018]

[Table 1]

Then, 100 foams are produced for each processing condition, and the foam leaks from the primary mold at that time, and the primary foam and the final foam (secondary foam) are deformed and cracked, and each final foam is produced. The average cell diameter, 25% compressive stress, open cell ratio and 25% compression set of the foam were evaluated and measured, and 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 columns of “Foam leak in primary mold” and “Deformation and cracking of primary and secondary foams” are numbers.

The average cell diameter was measured for each foam by 10
The diameter of zero bubbles was measured and the average value was shown. The measurement of the 25% compressive stress was performed according to the method of JIS K6767. If the value obtained by such a measurement is large, the compressive stress increases, and the compressive stress is rich. The measurement of the open cell rate (%) was performed in accordance with the air pycnometer method (ASTM D2856). 2 more
The measurement of the 5% compression set was performed according to JIS K6767.

In addition to the physical properties shown in Table 1, the final foam obtained in Example 1 had a density of 0.06 g / cc,
The width is 999 to 1000 mm (the difference between the maximum value and the minimum value is 5 m
m), the thickness is approximately 98-101 mm (the difference between the maximum value and the minimum value is 3 mm), almost the same as the shape of the internal space of the secondary mold, and the outer peripheral surface is excellent in smoothness and extremely beautiful. Was something.

According to the results shown in Table 1, since the primary decomposition rate in Comparative Example 1 was as small as 50%, there was no leakage or deformation of the foam, but the average cell diameter was too small (not desired). , 25% low compressive stress, low open cell ratio, and high 25% compression set. In Comparative Examples 2 and 3,
Although the physical properties of the final foam were satisfactory, all of the foams leaked and deformed. On the other hand, in Examples 1 and 2, there was no such a problem, and an excellent performance product could be efficiently manufactured.

Examples 3 and 4 and Comparative Examples 4 and 5 In this example, the relationship between the size of the secondary mold and the size of the foam immediately after the completion of the secondary expansion was tested. That is, a predetermined amount (shown in Table 2) of the same mixture as in Example 1 was processed under the same conditions as in Example 1 using a primary mold having the predetermined mold dimensions shown in Table 2. It is. Further, secondary foaming was performed under the same conditions and equipment as in Example 1. Table 2 shows the size of the secondary mold dimensions. These results are also shown in Table 2.

[0024]

[Table 2]

In Table 2, the foam size [C] immediately after the completion of the secondary foaming expansion was forcibly taken out of the mold without cooling the foam immediately after the completion of the secondary foaming, and the height, width, and height were measured. Are measured at three locations for each of them, and are expressed as an average value. In addition, the variation in the final foam size is obtained by measuring the maximum, minimum, and vertical portions, width, and height of the foam after completion of cooling in the secondary mold, and expressing the difference. The numerical value marked with * in Table 2 is the third
It is outside the scope of the invention.

According to the above results, as shown in Table 2,
In Comparative Examples 4 and 5, where the secondary mold reduction ratio was inappropriate, the dimensional variation of the final foam was large. Further, in Comparative Example 5, a crack having a length of 200 mm and a depth of 10 mm occurred on the foam upper surface, and the removal of the final foam from the mold was cramped,
It had to be forcibly taken out. On the other hand, Examples 1 and 3
In Examples 4 and 4, the above-mentioned problems were not found (although there was variation, but very small), and the results were good. In the present invention, the present invention is not limited to the specific examples described above,
Examples can be variously modified within the scope of the present invention depending on the purpose and application.

[0027]

According to the production method of the present invention, a foam having a relatively large cell diameter and an appropriate open cell ratio, that is, a final foam having a large compressive stress and a small permanent set can be easily obtained. Obtainable. Further, according to the manufacturing method in which the secondary mold reduction ratio (the difference between the foamed dimension immediately after the completion of the secondary foaming expansion and the foamed dimension immediately after the completion of the secondary foaming expansion and the secondary mold dimension) has an appropriate range, the final The dimensional variation of the foam can be made very small, and the final foam can be easily taken out without having to forcibly take out the final foam from the secondary mold.

────────────────────────────────────────────────── (5) Continuation of the front page (51) Int.Cl. ⁶ identification code FI B29K 105: 04 105: 24 C08L 23:00 (56) References JP-A-61-146510 (JP, A) JP-A-3-3 20340 (JP, A) JP-A-5-200891 (JP, A) JP-B 2-42649 (JP, B2) JP-B 4-26301 (JP, B2) JP-B 45-29381 (JP, B1) (58) Field surveyed (Int.Cl. ⁶ , DB name) B29C 44/00-44/60 B29C 39 / 38,39 / 42 C08J 9/04-9/14 CES

Claims (2)

(57) [Claims] 1. A primary mold is filled with a foamable composition containing a polyolefin, a cross-linking agent and a foaming agent, and the primary mold is filled with 50%.
After heating at kg / cm ² or more under pressure allowed induce foaming by decomposing as the decomposition rate which satisfies the following expression a portion of the blowing agent, is the primary expansion depressurized during high temperature heat, the A first step of producing an intermediate foam by removing it from the primary mold, and then placing the intermediate foam obtained in the first step in a secondary mold corresponding to the dimensions and shape of the final foam, and in that heating the intermediate foam, a second step of producing a secondary-expanded the final foam together to decompose the remainder of the blowing agent and the cross-linking agent, Ri Tona, average of the final foam The bubble diameter is 250-450 μm and
One Ren'awaritsu manufacturing a polyolefin foam, wherein 5-20% der Rukoto. Decomposition rate of foaming agent in the first step (%) = (9 to 12) × (10
0 / final expansion ratio (%)) 2. Each of the vertical, horizontal and height dimensions constituting the internal space of the secondary mold is the same as the respective dimensions immediately after foaming of the foam obtained by final foaming the intermediate foam. 1 to
The method for producing a polyolefin foam according to claim 1, which has a size reduced by 10%.