JP2869475B2 - 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 two-stage foaming method in which energy loss is reduced, the surface is smoothed efficiently, and the height, width and height are reduced. The present invention relates to a method for producing a polyolefin foam having excellent dimensional uniformity.

[0002]

2. Description of the Related Art As a method for producing a polyolefin foam, a mixture of a polyolefin resin, a crosslinking agent and a blowing agent is filled in a mold, and the crosslinking agent and the blowing agent are completely decomposed under pressure and heat. Then, a method is known in which the mixture is expanded at a time to a desired density by depressurizing to obtain a final foam (hereinafter, referred to as a one-stage foaming method). Further, as disclosed in Japanese Patent Publication No. 42649/1990, the mixture is filled in a primary mold, heated under pressure to decompose 15 to 60% of the foaming agent, and thereby primary expanded,
Thereafter, a method of heating the intermediate foam at normal pressure to decompose the remaining foaming agent and secondary expand to obtain a final foam having a desired density (hereinafter, referred to as a two-stage foaming method) is also known. .

However, when a high-foamed product is obtained by the above-described one-stage foaming method, the foam is expanded to a final foam having a desired density at one time. In addition, there is a problem that the foam is cracked or the like, and the commercialization rate is extremely low. Therefore, the two-stage foaming method has been developed in order to improve the reduction in the production yield of the one-stage foaming method. In this two-stage foaming method,
A product having a predetermined expansion ratio is expanded and expanded in two stages without expanding and expanding all at once, thereby eliminating factors such as cracks and the like that lower the product production yield.

[0004] However, even in this two-stage foaming method, the surface smoothness of the final foam is not sufficient. Therefore,
As a countermeasure, as described in JP-A-61-266441, the foam after completion of the second foaming step is taken out at the time of warming, and the cooling plate (in the expression of JP-A-61-266441, becomes a heating plate). However, from the viewpoint of the temperature of the foam, its function can be said to be a cooling plate.

[0005]

However, Japanese Patent Application Laid-Open No.
In the case of cooling and molding by the method of 266441, the thickness direction becomes uniform due to the compression of the foam by the cooling plate, but stress is applied to the side portion, and particularly, a bulge occurs around the center portion. is there. Furthermore, since the foam which is cooled and set in such a state is set in a state where the structure of the foam is distorted so as to be pulled toward the above-mentioned bulging portion (central portion of the side surface), the foam 4 A concave portion (hereinafter referred to as sink mark) is generated at a corner portion.

The present invention solves the above-mentioned problems, and provides a method for producing a polyolefin foam having reduced energy loss in a two-stage foaming method, and having excellent dimensional uniformity in the vertical, horizontal and height directions. The purpose is to provide.

[0007]

The method for producing a polyolefin foam of the present invention is a method for producing a polyolefin foam using an admixture comprising a polyolefin, a crosslinking agent and a foaming agent. And fill 5
By heating under a pressurized state of 0 kg / cm ² or more to decompose a part of the crosslinking agent and the foaming agent, then, the pressure-reducing pressure causes the mixture to undergo primary expansion to produce a primary foam,
The primary foam is placed in a non-closeable, closable secondary mold having a cross section corresponding to the shape of the final foam product, heated, and the remaining foaming agent is decomposed under normal pressure to form a secondary foam. Producing a secondary foam, further placing the secondary foam in a non-closeable, closable, cooling-only mold corresponding to the shape of the final foam product and having a smaller cross-section than the secondary foam;
It is characterized by compression molding under cooling.

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, compounds containing urea as a main component, metal oxides such as zinc oxide and lead oxide, lower or higher fatty acids or metal salts of lower or higher fatty acids, etc. A foaming aid and the like can be added. Further, carbon black, zinc white, titanium oxide, and other commonly used additives can be added to improve physical properties.

[0011] In the present invention, "the secondary foam is placed in a non-closeable and closable dedicated cooling mold having a cross section smaller than that of the secondary foam and corresponding to the shape of the final foam product. Molding ". In this way, cooling molding using a separate cooling mold is
Repetition of cooling and heating of the secondary mold can be eliminated and energy efficiency can be improved.Also, the cooling-only type can always be cooled with a refrigerant and maintained at a low temperature, so the cooling time is also extremely short. Because you can do it.

The expression "corresponding to the shape of the polyolefin foam" means that the internal space shape of the cooling-only type is substantially similar to the shape of the polyolefin foam. The size of the secondary foam immediately after the secondary foaming is 2 to 10% larger than the size of the desired final product. Preferably, it is 8% smaller. The reason for increasing the former to 2 to 10% is to take into account the temporal shrinkage of the foam, and to increase the amount of shrinkage in advance so that the dimensions after shrinkage can secure the desired final product dimensions. The reason why the latter is reduced to 1 to 8% is that the foam is completely adhered to the cooling-only mold to improve the molding accuracy,
This is to increase the cooling efficiency. If the amount is less than 1%, the adhesion of the foam to the mold is insufficient, and the above effects cannot be expected.
If it exceeds 8%, an excessive load is applied to the foam, and conversely, the molding accuracy is lowered. The cooling only type is not hermetically closed when closed. By doing so, it is possible to prevent molding defects due to air pockets during compression molding under cooling in this mold.

[0013]

Conventionally, when a foam is sandwiched between two cooled flat plates and molded in a slightly compressed state, although the thickness direction becomes uniform due to the compression by the flat plate, stress is applied to the lateral portions, Bulging occurs around the central portion. Further, in this case, since the foam is compression-molded in a state of being strained so as to be pulled toward the center portion of the side surface, sink occurs at the four corners of the foam over time. On the other hand, in the present invention, the compression molding under the cooling is performed in a cooling-only mold having a cross section corresponding to the shape of the final product. Therefore, the expansion of the foam is regulated not only in the thickness direction of the molded body, but also in the vertical and horizontal directions thereof, by the side portions of the cooling-only mold. The central part on the side of the body does not expand, and is cooled and formed into a uniform shape and dimensions according to the internal space of the cooling-only type.

Further, as a method for smoothing the surface of the foam and improving the dimensional accuracy, a method of cooling the secondary mold after completion of foaming as it is and cooling and molding the final foam (the inventor of the present invention previously filed the application) Japanese Patent Application No. 4-179152) is conceivable. Although this method has the advantage that extremely excellent cooling molding can be performed in a short time without requiring any manual operation,
Since heating and cooling are repeated with the same mold, energy efficiency is not sufficient. On the other hand, in the present invention, since the cooling only type is used separately from the secondary type, it is possible to eliminate the repetition of cooling and heating of the secondary type, so that the energy efficiency can be improved. The cooling time can be kept low, and the cooling time can be extremely shortened.

[0015]

The present invention will be described below in detail with reference to examples. Example 1 To 100 parts by weight of polyethylene having a melt index of 1.0 (hereinafter, referred to as parts), 5 parts of azodicarbonamide,
A composition comprising 2 parts of dicumyl peroxide and 0.5 part of zinc oxide was kneaded on a roll having a surface temperature of 100 ° C. to obtain an admixture. 6.8 kg of this mixture is placed in a primary mold (420
mm × 420mm × 42mm), 70kg / c
After heating at 160 ° C. for 30 minutes under pressure of m ² ,
The primary foam (880 mm x 880 mm x 86 mm) was decompressed at the time of high temperature heat and foamed and expanded about 10 times. This primary foam in a high temperature state is placed in a secondary mold (inner space size: 100
0 mm × 1000 mm × 100 mm), and the steam is conducted through the heat medium flow paths of the metal plates constituting the upper and lower surfaces of the mold to maintain the metal plate at 165 ° C. and heat the primary foam for 30 minutes. And secondary foaming to obtain a secondary foam.

The secondary foam was taken out of the secondary mold and its dimensions were measured. The dimensions were 1030 mm × 1030 mm ×
103 mm. The secondary foam in the high temperature state is cooled to a temperature of 20 ° C. by cooling water (100 cooling only type).
(0 mm × 1000 mm × 100 mm) and cooled for 20 minutes to perform compression molding to obtain a final foam. The final foam was aged for one week, and the length, width and height dimensions were examined. The surface was smooth and the height and width were 9
An extremely uniform foam of 80 to 985 mm and a height of 98 to 95 mm was obtained.

Comparative Example 1 A primary molding obtained by the same method as in Example 1 was cooled and pressed by a pair of heating plates 2 as shown in FIG.
(Internal space dimensions: 1000 mm x 1000 mm x 100
mm), and heating was performed for 30 minutes through the cooling mold using the heating plate 2 maintained at 165 ° C. by passing steam through the heat medium flow path 21. After heating is complete,
The steam supplied to the heating plate 2 was stopped, and at the same time, normal-temperature cooling water was allowed to flow through the coolant channels 111 and 121 of the cooling mold 1.

As a result, the side frame 11 of the cooling mold 1 is formed.
The temperature of the portion was lowered to about 50 ° C. after about 3 minutes, and the temperature of the outer peripheral portion of the foam was lowered to 90 ° C. after about 30 minutes. at this point,
The cooling water was stopped at the same time when the final foam was taken out, and steam was allowed to flow again to the heating plate 2. Thus, it took about 18 minutes for the heating plate 2 to reach 165 ° C. again. Therefore, the time required for starting the cooling → the completion of the cooling → the reheating of the cooling mold is 4 hours.
8 minutes, about 2.5 times as long as that of Example 1. After the obtained final foam was aged for one week, the length, width, and height were examined.
The foam was 83 to 988 mm and the height was 96 to 99 mm. In this example, as in Example 1, an extremely uniform foam was obtained.

Comparative Example 2 The secondary foam obtained in the same manner as in Example 1 was sandwiched between two flat plates and subjected to cooling molding for 20 minutes. as a result,
After the obtained foam was aged for one week, the dimensions were measured. The vertical and horizontal dimensions were 992 to 970 mm, and the height dimension was 92.
９８98 mm, and the dimensional variation in the vertical and horizontal directions was particularly large.

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 in accordance with the purpose and application. That is,
In the above embodiment, a cooling mold or a cooling-only mold for compression-molding one foam was used.
For example, each of the cooling molds may be a lamination type in which side molds and flat plates are sequentially laminated so that a plurality of foams can be simultaneously cooled or compression molded. In addition, these cooling types may have a coolant channel through which a coolant can flow, and the coolant may be caused to flow through this channel to perform cooling. The cross-section, size,
The number is not particularly limited, and the number of inlets and outlets is also variously selected. The shape of the internal space of the cooling mold can be not only a square plate (or a rectangular parallelepiped), but also various shapes according to the desired product shape. Furthermore,
It is preferable that the cross-linking agent and the foaming agent in the heat foaming are decomposed almost completely, but the present invention is not limited to this, and undecomposed materials may remain.

[0021]

According to the manufacturing method of the present invention, as described above, the energy loss can be reduced in the two-stage foaming method, the surface can be efficiently smoothed, and the dimensional uniformity in the vertical, horizontal and height directions can be achieved. A final foam having excellent properties can be produced.

[Brief description of the drawings]

FIG. 1 is an explanatory sectional view showing a foam molding section of an apparatus for producing a polyolefin foam used in Comparative Example 1.

[Explanation of symbols]

1; mold for cooling molding; 11; side mold; 11a; upper mold;
11b; lower mold, 111; refrigerant flow path, 12a, 12b;
Flat plate, 121; refrigerant channel, 1211; tubular body, 122;
Medium plate, 123; plate, 2; heating plate, 21;
22; metal box, 23; bottom plate, 24; tubular, 2
5; rib, 3; internal space, 4; bolt, 5; bolt,
6; secondary type.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ identification symbol FI B29K 105: 04 105: 24 C08L 23:00 (58) Fields surveyed (Int.Cl. ⁶ , DB name) B29C 39/00- 39/44 B29C 67/20 C08J 9/06 B29C 35/16

Claims (1)

(57) [Claims] 1. A method for producing a polyolefin foam using an admixture of a polyolefin, a cross-linking agent and a foaming agent, wherein the admixture is filled in a primary mold and heated under a pressure of 50 kg / cm ² or more. To partially decompose the cross-linking agent and the foaming agent, and then to firstly expand the mixture by depressurizing to produce a primary foam. Then, the primary foam corresponds to the shape of the final foam product. Into a non-closed closable secondary mold having a cross-section to be heated and heated to decompose the remainder of the foaming agent under normal pressure to produce secondary foam, thereby producing a secondary foam. The polyolefin foam is characterized in that the body is placed in a non-closed, closable and exclusive cooling mold having a cross section smaller than that of the secondary foam and corresponding to the shape of the final foam product, and compression-molded under cooling. Production method.