JPH091578A - Production of polyolefin foam - Google Patents

Abstract

PURPOSE: To enable formation of a foam having a small compressive permanent strain by forming an intermediate foam at specified values of the average particle diameter of a foaming agent, the decomposition ratio of the agent, the pressure and temperature for a primary mold, and then carrying out the secondary expansion of the article. CONSTITUTION: This method of producing a polyolefin foam comprises the first step of forming an intermediate foam, by filling a primary mold with an expandable composition containing polyolefin, a cross linking agent and a foaming agent having an average particle diameter of 1-9μm, pressurizing the mold at 50-100kg/cm<2> and heating it to a temperature lower by 30-40 deg.C than the one-minute half-life temperature of the cross linking agent to decompose a portion of the foaming agent so that the decomposition rate may become (9 to 12)×(100/final expansion ratio) thereby inducing foaming, and depressurizing the primary mold when the temperature is high, to allow the primary expansion followed by taking an expanded article out of the mold, thus obtaining an intermediate foam. In the second step, the intermediate foam is placed in a secondary mold having dimensions and a shape corresponding to those of a final foam, and the mold is heated under normal pressure to decompose the remaining cross linking and foaming agents and allow the secondary expansion, thus producing a final foam having an expansion ratio of 15 or greater.

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, it has a cell diameter of from 350 to 350.
The present invention relates to a method for efficiently producing a polyolefin foam having a large size of about 550 μm, a high compression stress, and a small compression set.

[0002]

2. Description of the Related Art Generally, a method for producing a polyolefin block foam is to fill a mold with a mixture of a polyolefin resin, a cross-linking agent and a foaming agent, and pressurize and heat the cross-linking agent and the foaming agent. Is completely decomposed and then decompressed to expand the mixture at one time to a desired density (hereinafter, referred to as one-stage foaming method), and JP-B-52-83.
No. 48, Japanese Patent Publication No. 2-42649, etc., the mixture is filled in a primary mold and heated under pressure for primary expansion, and then the resulting intermediate foam is subjected to normal pressure. A method (hereinafter referred to as a two-stage foaming method) in which a final foam having a desired density is obtained by heating and secondary expansion is known.

However, when a high foam is obtained by the one-stage foaming method, since the final foam having a desired density is expanded at a time, the obtained final foam may be deformed, or
There was a problem that when the product was taken out of the mold, the foam was cracked, resulting in a very low commercialization rate. Therefore, a two-stage foaming method has been developed to prevent the reduction in product yield due to the one-stage foaming method. In this method, a product having a predetermined expansion ratio is not expanded or expanded at a time, and foaming is performed in two steps. The factors that reduce the product yield such as deformation and cracking by expanding are removed.

[0004]

However, the above-mentioned 2)
The step-foaming method is as follows: "In the first step, countless nuclear bubbles formed by thermal decomposition of a part of the foaming agent under high pressure are grown into fine cells of 70 to 90 µm at the time of decompression expansion, and are subjected to normal pressure in the second step. In foaming, the fine cells are further added to 100 to 150 μm.
Grow uniformly to an average bubble size of m. The resulting foam has a uniform and fine closed cell structure, and such foam generally has poor compression hardness and relatively high compression set. It has drawbacks.

The present invention solves the above problems and improves the physical properties of a foam in the two-stage foaming method, which has a high production yield, and the particle size of the foaming agent in the two-stage foaming method. By optimizing the pressurization pressure in the first step, the balance between the progress state of the crosslinking reaction and the decomposition state of the foaming agent, and the amount of decomposition of the foaming agent, a foam having a high compression stress and a small compression set is produced. The purpose is to provide a method of doing.

[0006]

The method for producing a polyolefin foam according to the present invention comprises the steps of filling a primary mold with a foaming composition containing a polyolefin, a cross-linking agent and a foaming agent having an average particle size of 1 to 9 μm. The primary mold is pressurized to 50 to 100 kg / cm ² , and the pressure is increased to 3 from the 1-minute half-life temperature of the cross-linking agent under pressure.
By heating to a low temperature of 0 to 40 ° C., a part of the foaming agent is decomposed so as to have a decomposition rate satisfying the following formula to induce foaming, and decompressed at the time of high temperature heat to perform primary expansion to cause the primary expansion. The first step of taking out and manufacturing the intermediate foam, and thereafter,
The intermediate foam obtained in the first step is placed in a secondary mold corresponding to the shape and size of the final foam, and the secondary mold is heated under normal pressure to remove the crosslinking agent and the foaming agent. The second step of decomposing the rest and secondary expanding the intermediate foam to produce a final foam having an expansion ratio of 15 times or more. Foaming agent decomposition rate (%) in the first step = (9 to 12) × (10
0 / final expansion ratio)

In the present invention, the above-mentioned "polyolefin"
The term, for example, commercially available high-pressure method, polyethylene produced by a medium-pressure method or low-pressure method, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-vinyl acetate copolymer, ethylene and methyl, Copolymers of ethyl, propyl or butyl with each acrylic acid alkyl ester (content of this ester; 45 mol% or less), or chlorinated compounds thereof each having a chlorine content of up to 60% by weight, and two or more of these Or a mixture of these with isotactic polypropylene or atactic polypropylene.

The "crosslinking agent" in the present invention has a decomposition temperature of at least the flow initiation temperature of the polyolefin in the above-mentioned polyolefin, and is decomposed by heating to generate free radicals to cause intermolecular formation of the polyolefin. It refers to an organic peroxide or the like which is a radical generator that causes cross-linking. Specific examples thereof include dicumyl peroxide, 2,5-dimethyl-2,5-bis-tert-butylperoxyhexane and 1,3-bis-tert-peroxy-isopropylbenzene. As the "foaming agent", one having a decomposition temperature equal to or higher than the flow initiation temperature of the polyolefin can be used, and examples thereof include azodicarbonamide and dinitrosopentamethylenetetramine.

Further, 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 and lead oxide, a lower or higher fatty acid or a metal salt of a lower or higher fatty acid, etc. The foaming auxiliary agent and the like can be added. Further, in order to improve the physical properties, carbon black, titanium oxide, and the like, as well as various compounding agents commonly used in this type of foamable composition, can be appropriately added.

The foaming agent used in the present invention must have an "average particle size of 1 to 9 μm", and 1 to 7 μm.
m, particularly preferably in the range of 1 to 5 μm. If the average particle size exceeds 9 μm, the number density of the nuclear bubbles formed during decomposition of the foaming agent becomes low, and as a result, the gap between the nuclear bubbles, that is, the bubble wall becomes thick. Therefore, the bubble aggregation phenomenon in the bubble growth process at the time of the primary expansion in the first step is hard to be induced, the giant cell which is the object of the present invention is hard to be formed, and the physical properties are deteriorated.

Further, the foaming agent having a particle size of less than 1 μm is
It is difficult to disperse it uniformly in the foamable composition, and it is difficult to obtain a foam having good appearance and properties due to aggregation. When the foaming agent has an average particle size of 1 to 5 μm, a foam having a better compression set can be obtained. still,
The average particle size of the foaming agent as used herein means a specific surface area of the foaming agent particles measured using a powder specific surface area measuring instrument (manufactured by Shimadzu Corporation, SS-100 type), and the specific surface area value is as follows. It is calculated according to the formula. Average particle diameter (μm) of foaming agent = (6 / ρ · Sw) × 10 ^{4 In} the above formula, ρ is powder density (g / cm ³ ), Sw
Is the specific surface area (cm ² / g).

The "pressure" in the first step is "50-100".
kg / cm ² ", especially 50-80 kg
The range of / cm ² is preferable. The pressure in this primary mold is 5
When it is less than 0 kg / cm ^2, it is a condition for expanding the foam up to about 10 times when the decomposition ratio of the foaming agent in the first step is a predetermined value. As a result, the expandable composition leaks from the primary mold during expansion, which causes deformation of the intermediate foam, which further lowers the commercialization rate.

On the other hand, when the pressure exceeds 100 kg / cm ² , the dispersion speed of the gas generated by the decomposition of the foaming agent is slow, so that the number density of the nuclear bubbles formed is suppressed to be low and the bubble wall becomes thick. For the same reason as when the foaming agent particle size is not less than the upper limit, it becomes difficult to form a giant cell. If the pressure is 50 to 80 kg / cm ² , no deformation or the like will occur and a giant cell can be easily formed. The pressure referred to here is the pressure applied to the entire inner surface of the closed primary mold.

Furthermore, the "heating" in the first step must be "a temperature 30 to 40 ° C. lower than the one-minute half-life temperature of the crosslinking agent used", and a temperature range of 32 to 37 ° C. is particularly preferable. . If this temperature is lower than 1 minute half-life temperature by more than 40 ° C, the crosslinking inside the foam becomes insufficient,
This causes quality problems such as cell roughness and void generation. On the other hand, this heating temperature causes the 1-minute half-life temperature of the crosslinking agent to be 30
When the temperature is lower than 0 ° C, the decomposition reaction of the foaming agent is preceded by the crosslinking reaction, and the dispersion rate of the decomposition gas decreases as the crosslinking density increases. As a result, the number density of the nuclear bubbles decreases, the bubble walls become thicker, and the bubble aggregation property deteriorates. As a result, it becomes difficult to form huge cells and the physical properties also decrease.

The "decomposition rate" of the foaming agent in the first step is "(9-12) x (100 / final foaming ratio)"
% ", Especially (10-11) x (100
/ Final expansion ratio) is preferable. That is, in the first step, after defoaming under high pressure and expanding explosively, the nuclear bubbles formed in the foaming step coalesce (assemble) with adjacent bubbles in the growth process. In other words, it is easy to form huge cells, and the decomposition rate of the foaming agent is 9 × (1
When 00 / final foaming ratio)% or more, the tendency becomes apparent,
A foam having a final expansion ratio of 15 times or more can be easily manufactured.

On the other hand, when the decomposition rate exceeds 12 × (100 / final foaming ratio)%, the expansion rate of the intermediate foam becomes excessive, resulting in defects such as deformation and cracks, and the original purpose of the two-stage foaming. Is damaged. For these reasons, in order to form huge cells without inducing product defects, the decomposition rate of the foaming agent must be within the above range.

As a means for adjusting the decomposition rate of the foaming agent, since the heating temperature range is specified in the present invention, a foaming auxiliary agent such as a metal oxide or a urea type auxiliary agent within the temperature range is specified. It is necessary to carry out by adjusting the addition amount of

In addition to the above-mentioned means, the decomposition amount of the foaming agent can be adjusted by a simpler and more reliable method, considering the heating temperature in the first step and the one-minute half-life temperature of the crosslinking agent.
It is also possible to set the temperature to a relatively low temperature of about 0 to 140 ° C. and perform the heating time. Further, the heating temperature in the second step is important to completely decompose and foam the foaming agent, and it is necessary to set it within a range that does not adversely affect the polyolefin, and is usually about 160 to 190 ° C. The heating time may be about 20 to 60 minutes.

The "secondary type" used in the present invention
Is different from the closed type used in the first step and used under pressure, and has a non-closed internal space. Then, when the intermediate foam expands secondarily in this internal space, it has a structure capable of removing the air remaining in the internal space to the outside by the expansion pressure of the foam, and is usually a suitable secondary mold. Each surface is provided with one or two small holes for communicating the internal space with the external atmosphere.

The shape and dimensions of the internal space of the secondary mold correspond to those of the final foam. The term "corresponding" means that the shapes are substantially similar to each other, so that the three-dimensional expansion of the foam can be caused more uniformly. Such uniform expansion without directivity can prevent the occurrence of unevenness, dimensional error, etc. due to shrinkage unevenness on the surface of the final foam due to aging shrinkage.

Further, as in the third aspect of the invention, foaming of a foam obtained by subjecting the intermediate foam to a secondary expansion in all of the longitudinal, lateral and height dimensions that form the internal space of the secondary mold. It is preferable that the size is reduced by 1 to 10% with respect to each size immediately after. With such a size, the foamed and expanded final foam body is uniformly contacted and pressed against the wall surface of this internal space by its self-expanding force, and is pressed into the shape of the internal space. In addition to being possible, variations in the shape of the final foam are reduced. In addition, the heat transfer efficiency in the heating process in the secondary mold or the cooling process performed as needed is also improved. In this case, the term “vertical, horizontal and height” is used to mean a substantially similar shape when viewed three-dimensionally in the case of a three-dimensional shape that is not clearly defined.

When the size of the internal space is less than 1%, the pressing force of the foam on the inner wall surface of the internal space due to the self-expansion force of the foam becomes insufficient and the surface of the foam becomes uneven. Or, since the corners are not formed, it becomes difficult to mold the secondary mold and the heating efficiency also decreases. On the other hand, when it is made smaller than 10%, even if the foam shrinks due to the subsequent cooling, the outer size of the foam is still considerably larger than the inner size of the space, and the final foam from the cooled secondary mold is It may be difficult to take it out, or the center of the foam may be lifted when the secondary mold is opened, resulting in deformation, cracking or the like.

[0023]

In the present invention, the average particle size of the foaming agent in the first step,
By optimizing the pressurizing pressure, heating temperature, the balance between the crosslinking reaction and the decomposition of the foaming agent, and the decomposition rate of the foaming agent,
It is possible to increase the number density of the nuclear bubbles formed when the foaming agent is decomposed in the process, thin the gap between the nuclear bubbles, that is, the bubble wall, and promote explosive expansion during depressurization. The bubble wall thus formed and the explosive expansion caused during depressurization are combined with each other, and in the process of growing the fine cells at the time of the explosive expansion, it is possible to partially induce the destruction of the bubble wall. As a result, the fine cells gather, and 350 to 550μ
(EN) Provided is a foam which has a large m and a cell size, and a strand of a destroyed cell wall is fused to the remaining cell wall to reinforce the cell wall, which is rich in compressive stress and has a small compression set. Is possible.

[0024]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. Examples 1 to 3 and Comparative Examples 1 to 7 100 parts by weight of polyethylene having a melt index of 1.0 (hereinafter referred to as "parts") 5 parts of azodicarbonamide having various average particle sizes, 1-minute half-life temperature of 170 ℃
2 parts of dicumyl peroxide (40% concentration), and various zinc oxide / urea type auxiliaries to adjust the decomposition rate of the foaming agent were prepared. The mixture was kneaded on a roll at a temperature of 100 ° C. to obtain a foamable mixture having various compositions.

6 kg of the above mixture was put into a primary mold (internal dimension: 4
10 mm × 410 mm × 40 mm) and processed under the conditions of the first step shown in Table 1 (heating was carried out by passing steam through the heat medium passage), and then depressurized at high temperature. Then, it was foamed and expanded to a predetermined size and taken out from the mold to obtain an intermediate foam. Then, this is put into a predetermined secondary mold (internal space dimension: 1000 mm × 1000 mm ×
100 mm), heated at 170 ° C. for 30 minutes (heated in the same manner as above), then passed through a heat medium passage in the mold and cooled to room temperature in 60 minutes, and the expansion ratio was 15 times. A final foam of

[0026]

[Table 1]

Under the above-mentioned conditions, 100 foams were obtained, and the final foams were deformed, cracked, the inner layer state (cell roughness, voids), and the average cell diameter,
Evaluation for 25% compressive stress and 25% compression set,
The measurement was performed, and the results are also shown in Table 1.

In Table 1, the numbers marked with * are outside the scope of the present invention. Further, in the same table, the foaming agent decomposition rate (%) in the first step is "0" for the foaming agent decomposition rate of the foaming mixture before the primary expansion, and the foaming agent decomposition rate of the foam after the secondary expansion is completed. Was set to “100” and calculated by the following formula. Blowing agent decomposition rate _{(%) = {(V 2} -V 1) / (V 3 -
V ₁ )} × 100 V ₁ : Volume of foamable mixture (cm ³ ), V ₂ : Volume of intermediate foam immediately after completion of primary expansion (cm ³ ), V ₃ : Final foam immediately after completion of secondary expansion volume (cm ³⁾ in addition, the deformation of the final foam, crack (number) by appearance inspection, also rough cell, void (pieces) were evaluated by breaking the inner visual inspection.

The average cell diameter (μm) was measured by observing the diameter of 100 cells in each foam with an optical microscope.
It is measured and shown as the average value. The measurement of the 25% compressive stress was performed according to the method of JIS K6767.
If the value obtained by this measurement is large, the compressive stress will be large and the compressive stress will be rich. Furthermore, the 25% compression set was also measured according to JIS K6767.

As shown in Table 1, in the examples of the present invention, there was no deformation, voids, etc., and the average cell diameter was as large as 350 to 550 μm, the compression stress was large, and the compression set was small. Is efficiently obtained. on the other hand,
When it is out of the range of the present invention, product defects frequently occur (Comparative Examples 2, 4, 7) or the cell diameter is small (Comparative Examples 1, 3, 5, 6), and a foam having excellent physical properties is obtained. I can't. Furthermore, in addition to the physical properties shown in Table 1, Examples 1-3
The final foam obtained in 1. has a density of 0.065 g / cm ³ ,
The height and width are 995 to 1000 mm (the difference between the maximum and minimum values is 5 mm) and the thickness is 98 to 101 mm (the difference between the maximum and minimum values is 3 mm), which are substantially uniform, and the shape of the internal space of the secondary mold. Almost the same, the outer peripheral surface was excellent in smoothness and was extremely beautiful.

Examples 4 and 5 and Comparative Examples 8 and 9 The dimensions of the primary mold used in Example 1 were variously changed, and accordingly, the amount of the admixture charged was changed to Examples 4 and 5 and Comparative Examples 8 and 9. A final foam having a foaming ratio of 15 times was produced in the same manner as in Example 1 except that the amounts were changed to 5.5, 6.7, 5.2 and 7.1 kg, respectively. Table 2 shows the results.

[0032]

[Table 2]

In Table 2, the foam dimension [C] immediately after the completion of secondary expansion is the foam immediately after the final expansion due to secondary expansion, which is forcibly taken out from the mold without cooling.
The dimensions are measured at three points in each of length, width, and height, and are expressed as an average value. The variation in the final foam size is expressed by the difference between the maximum and minimum portions of the vertical foam, the horizontal width, and the height of the final foam after the completion of cooling in the secondary mold. Numerical values marked with * in Table 2 are out of the range of the third invention.

According to the results shown in Table 2, the reduction ratio of the secondary mold (the value obtained by subtracting the inner dimension of the secondary mold from the size of the foam immediately after the completion of the final foaming is divided by the size of the foam immediately after the completion of the final foaming, is 100). In Comparative Examples 8 and 9 in which the (doubled value) is out of the range of the third invention, the dimensional variation of the final foam is large. Further, in Comparative Example 9, it was difficult to take out the final foam from the mold, and the foam was forcibly taken out.
A crack having a depth of 0 mm and a depth of 10 mm occurred. On the other hand, in Examples 4 and 5 which were within the scope of the third invention, the above-mentioned inconvenience was not present (there was some variation, but it was very small), and no cracks were generated, which was good.

[0035]

According to the method for producing a polyolefin foam of the first invention, the final foam having a relatively large cell diameter, abundant compressive stress and a small compression set can be easily obtained as in the second invention. Can be obtained. Further, as in the third invention, by setting the secondary type reduction ratio in an appropriate range,
The dimensional variation of the final foam can be made very small, the final foam can be easily taken out from the secondary mold, and the final foam does not crack.

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display area C08L 23:02

Claims (3)

[Claims] 1. A primary mold is filled with a foamable composition containing a polyolefin, a cross-linking agent, and a foaming agent having an average particle size of 1 to 9 μm, and the primary mold is pressurized to 50 to 100 kg / cm ² and then applied. From the 1-minute half-life temperature of the above-mentioned cross-linking agent under pressure to 30 ~
By heating to a low temperature of 40 ° C., a part of the foaming agent is decomposed so as to have a decomposition rate satisfying the following formula to induce foaming, depressurized at high temperature heat to perform primary expansion, and taken out from the primary mold. The first step of producing an intermediate foam, and then the intermediate foam obtained in the first step is placed in a secondary mold corresponding to the shape and dimensions of the final foam, and the secondary mold is placed under normal pressure. Is heated to decompose the balance of the cross-linking agent and the foaming agent, and secondarily expands the intermediate foam to produce a final foam having a foaming ratio of 15 times or more. A method for producing a characteristic polyolefin foam. Foaming agent decomposition rate (%) in the first step = (9 to 12) × (10
0 / final expansion ratio) 2. The average cell diameter of the final foam is 350 to
It is 550 micrometers, The manufacturing method of the polyolefin foam of Claim 1. 3. Each of the vertical, horizontal and height dimensions forming the internal space of the secondary mold is different from the respective dimensions immediately after foaming of the foam obtained by secondary expansion of the intermediate foam. , 1
The method for producing a polyolefin foam according to claim 1, which has a size reduced by 10%.