JPH0367613B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a polyolefin foam, and more particularly, it relates to a method for producing a polyolefin foam that is energy efficient and low cost, has uniform fine closed cells, and has uniform physical properties. The present invention relates to a method for producing thick polyolefin foam.

BACKGROUND OF THE INVENTION Polyolefin foams are widely used for various purposes such as cushioning materials and heat insulating materials due to their excellent physical properties.

Conventionally, a pressure foaming method has generally been adopted as a method for producing polyolefin foams, and this pressure foaming method is generally divided into a pressure foaming method and a two-stage foaming method. However, with the pressure one-stage foaming method, it is difficult to obtain a high expansion ratio, and it is also difficult to obtain a smooth molded product, and if you try to finish a smooth molded product, the thickness will become thinner and the material yield will be poor. There are some difficulties. For this reason, a two-stage foaming method is generally employed to produce polyolefin foams with high expansion ratios.

This two-stage foaming method is a method as described in, for example, Japanese Patent Publication No. 45-29381 and U.S. Patent No. 3,098,832. Under pressure, the crosslinking reaction is carried out at the decomposition temperature of the crosslinking agent while suppressing the decomposition of the blowing agent or keeping the decomposed gas dissolved in the resin, and the resin is cooled and solidified. The foamable sheet material thus obtained is expanded and foamed by reheating under normal pressure. Compared to the above-mentioned one-stage pressurization method, this method allows for easier three-dimensional expansion, and therefore a foam with fine cells and a high expansion ratio can be obtained. However, even if a foamable sheet material is heated and foamed under normal pressure without putting it into a mold, only a product with uneven thickness and large irregularities on the surface can be obtained, and it is difficult to remove the skin layer from such products. Even if an attempt is made to obtain a hexagonal foam by shaving, the material yield is poor and costs are high.

For this reason, the applicant uses an unsealed mold having a cross-sectional shape and dimensions corresponding to the shape and dimensions of the final product in the above-mentioned second step of atmospheric pressure heating, and externally heats the metal plate of the mold. By heating, the intermediate primary foam in the mold is indirectly heated, the remaining foam is decomposed and foamed in a short time, and a polyolefin foam with a lower density and uniform thickness and physical properties is produced. We have developed a manufacturing method and have already applied for a patent (Japanese Patent Application Laid-Open No. 191029/1983).

However, when using such a mold, the second
After the next step, the mold and foam are heated to about 150℃, which makes it difficult to handle when removing the foam from the mold. After completion, the mold must be cooled before removing the foam. Therefore, energy efficiency is poor, and since the mold is heated and cooled repeatedly, production efficiency is relatively low, and the heat medium flow path of the mold is easily damaged, necessitating repairs.

Problems to be Solved by the Invention Therefore, an object of the present invention is to solve the above-mentioned problems and to produce a material having uniform fine closed cells and uniform physical properties with high energy efficiency and production efficiency and at low cost. An object of the present invention is to provide a method for producing a thick polyolefin foam.

Means for Solving the Problems In order to achieve the above object, the method for producing a polyolefin foam according to the present invention is to produce a foamable polyolefin resin composition containing a crosslinking agent and a chemical blowing agent in a primary mold. Heating under pressure for a certain period of time, and removing the pressure while the above blowing agent is partially decomposed, creates intermediate 1.
a step of obtaining a secondary foam, and a step of heating the obtained intermediate primary foam in a secondary mold under normal pressure to foam the foaming agent so that it remains partially undecomposed. , The foam in this state is further heated under pressure in close contact with the inner surface of the heating plate of the secondary mold to decompose the remaining foaming agent, and the mold is opened without cooling to obtain a foam. It is characterized by consisting of.

Effects and Modes of the Invention As is clear from the foregoing, the method of the present invention is to partially decompose the blowing agent by heating under pressure according to the usual method, and then (a) the obtained intermediate primary heating the foam in a secondary mold under normal pressure to cause the foaming agent to partially foam, preferably 1 to 60% of the foaming agent is in an undecomposed state; (b) in this state; The foam is further heated under pressure in close contact with the inner surface of the heating plate of the secondary mold to decompose the remaining foaming agent, and the secondary mold is opened without cooling to obtain a foam product. It is a major component that has not been seen before.

To explain the effects of each of these elements and their correlation, first, unlike conventional methods, the method of the present invention does not require cooling the secondary mold after completion of crosslinking and foaming as in (b) above. It is opened to obtain a foam product. For this purpose, it is convenient for handling that the foam after crosslinking and foaming has residual expansion force, so that the foam naturally expands and floats after the mold is opened, and this is due to the above (a). satisfied by the element. In other words, it is necessary for 1 to 60% of the blowing agent to be in an undecomposed state by heating under normal pressure, so that the undecomposed blowing agent is decomposed by subsequent heating under pressure, and the above-mentioned Expansion surplus is generated. For this purpose, it is necessary that at least 1% of the blowing agent remains in an undecomposed state by heating under normal pressure. On the other hand, residual blowing agent is 60%
If it exceeds this value, the expansion of the foam will be too large when the mold is opened, and the foam will tend to crack or crack, which is not preferable.

Moreover, by heating under the pressurized condition described in (b) above, a polyolefin foam having uniform physical properties and thickness can be obtained. That is, as explained in the prior art, if the foam is simply foamed under normal pressure, the heat transfer will not be uniform, so the resulting foam will have non-uniform thickness and physical properties. However, if the foam in which the undecomposed foaming agent remains is further heated while it is in close contact with the inner surface of the secondary mold, the foaming agent decomposes and becomes pressurized. The foam is heated in a compressed state, which results in good heat transfer and uniform temperature distribution, resulting in a foam with uniform physical properties such as density and compression hardness, as well as thickness.

Even if the foam is heated in a compressed state as described above, if the foam is taken out after cooling the secondary mold, the foam will be cooled and solidified as it is. However, if the secondary mold is opened without cooling as in the method of the present invention, the foam expands to a size similar to the shape and dimensions of the mold cavity, resulting in a polyolefin with a lower density and higher expansion ratio. A foam is obtained.

In addition, from the viewpoint of ease of taking out the foam, it is preferable to provide a taper on the side wall so that the opening side becomes wider.In this way, when the mold is opened, the foam expands and expands along the taper. It comes out naturally.

To outline the present invention, first, a blowing agent, a crosslinking agent, and if necessary a blowing aid, a pigment, etc. are added to a polyolefin resin, and a kneading machine such as a mixing roll, a kneader, or a kneader/ruder is used to mix the resin. Knead at a temperature of 70-130℃ depending on the type. The amount of blowing agent and crosslinking agent used is
This is determined as appropriate depending on the expansion ratio of the target product, but in general, the amount of blowing agent is 2 parts per 100 parts by weight of resin.
-40 parts by weight, and the crosslinking agent is suitably about 0.05 to 5 parts.

Next, the obtained crosslinkable foamable composition is filled into a primary mold, which is sealed under pressure and heated to 120 to 180°C.
Preferably at 140-170℃ for 10-50 minutes, preferably 15
Heating for ~35 minutes to decompose part of the blowing agent and part of the crosslinking agent, and the undecomposed blowing agent is preferably 40~35 minutes.
With 90% remaining, depressurize at high temperature and remove intermediate 1.
Next, remove the foam. Normally, once the type of resin, crosslinking agent, blowing agent, and blowing aid used are specified, the remaining amount of undecomposed blowing agent depends on the heating temperature and heating time, so in actual operation, These relationships may be measured in advance, and the heating time and heating temperature may be selected so that the remaining amount of undecomposed foaming agent is as described above. This method can also be applied to normal pressure heating, which will be described later.

The intermediate primary foam obtained as described above is then placed in an unsealed secondary mold, preferably having tapered side walls, such as a rectangular shape having tapered side walls such that the top opening area is larger than the bottom area. A box-shaped secondary mold is placed in a metal plate covered with two heating plates each having a heating means such as a heater on its outer surface or a channel for a heat medium such as steam or heating oil. By heating at 140-200℃, preferably 150-180℃ for 10-50 minutes, preferably 15-30 minutes, the undecomposed blowing agent is heated to 1-60℃.
Heat under normal pressure until % remains. That is, the foaming is performed so that 1 to 60% of the undecomposed foaming agent remains in the foam when the mold cavity is fully expanded.

Next, it is usually heated in the secondary mold at the above temperature under compression until most of the remaining undecomposed foaming agent is decomposed. Thereafter, when the secondary mold is opened without cooling, the foam expands and comes out of the secondary mold.

The polyolefin resin referred to in the present invention includes, for example, polyethylene, poly-1,2-butadiene, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene- Vinyl acetate copolymers, copolymers of ethylene and methyl, ethyl, propyl, and butyl acrylates or methacrylates containing up to 45%, or chlorinated versions of these (chlorine content 60% by weight) %), or mixtures of two or more of these, or mixtures of these with polypropylene having an atactic or isotactic structure.

The crosslinking agent in the present invention is one that has a decomposition temperature at least higher than the flow initiation temperature of the polyolefin in the polyolefin, and is decomposed by heating to generate free radicals and create crosslinking between or within the molecules. Various organic peroxides are radical generators that generate 5-ditertiarybutylperoxyhexane, 2,5-
Dimethyl-2,5-ditertiarybutylperoxyhexine, α,α-ditertiarybutylperoxydiisopropylbenzene, tertiarybutylperoxyketone, tertiarybutylperoxybenzoate, etc. can be used, and at that time, The most suitable organic peroxide may be selected depending on the type of polyolefin used.

The blowing agent that can be used in the present invention is a chemical blowing agent having a decomposition temperature higher than the melting temperature of the polyolefin, such as azo dicarbonamide, barium azodicarboxylate, etc.; dinitrosopenta, a nitroso compound, etc. Methylenetetramine, trinitrosotrimethyltriamine, etc.;
hydrazide-based compounds such as p, p'-oxybisbenzenesulfonyl hydrazide; sulfonyl semicarbazide-based compounds such as p, p'-oxybisbenzenesulfonyl semicarbazide, toluenesulfonyl semicarbazide, etc. be.

In the present invention, a foaming aid can be added depending on the type of foaming agent. Examples of foaming aids include compounds containing urea as a main component, metal oxides such as zinc oxide and lead oxide, compounds containing salicylic acid and stearic acid as main components, ie, higher fatty acids or metal compounds of higher fatty acids.

In the present invention, for the purpose of improving the physical properties of the composition used or reducing the cost, we use compounding agents (fillers) that do not have a significant adverse effect on crosslinking, such as zinc oxide, titanium oxide, calcium oxide, magnesium oxide, Metal oxides such as silicon, carbonates such as magnesium carbonate and calcium carbonate, fiber materials such as pulp, various dyes, pigments, fluorescent substances, and other commonly used rubber compounding agents may be added as necessary. can.

EXAMPLES The present invention will be specifically described below with reference to Examples.

Example 1 Formulation: Polyethylene (trade name Yucalon YF-30)
100 parts by weight Azodicarbonamide 14 〃 Zinc flower 0.3 〃 Zinc stearate 0.3 〃 Dicumyl peroxide 0.6 〃 After uniformly dispersing and kneading the above mixture, the kneaded product was put into a compression molding machine with a taper on the side wall. Fill the primary mold (bottom part 225 x 155 mm, height 16 mm), seal it by applying an external pressure of 10 kg/cm2 ^{or more} , and heat it to 155℃.
The mixture was heated for 17 minutes at a high temperature, and the pressure was removed to remove the primary foam. This primary foam had expanded to a size of 430 x 295 mm corresponding to the bottom part and a thickness of 35 mm (expansion ratio of about 8 times), and about 75% of the foaming agent remained undecomposed.

This high-temperature primary foam is immediately molded into a rectangular box-shaped secondary mold with tapered side walls (top opening size 680 x 520 mm, bottom size 600 x 440 mm,
45mm in height) and heated to 175°C with both top and bottom sides covered by two heating plates with steam channels.
After heating for 15 minutes, the foam expanded to fill the inner volume of the secondary mold (the residual rate of undecomposed foaming agent at this stage was about 34%). After the foam in this state was further heated at the above temperature for 5 minutes, the secondary mold was opened to obtain a white foam having uniform, fine closed cells.

The dimensions of the foam immediately after removal are 780 x 600 mm in the area corresponding to the top opening of the secondary mold, 690 x 500 mm in the area corresponding to the bottom, and 52 mm thick. There were hardly any left. After natural cooling, the final product had an apparent density of 0.03 g/cm ³ and an expansion ratio of about 30 times.

In the above, the expansion ratio and foaming agent residual rate were calculated as follows (the same applies to the examples described later).

Foaming ratio: The dimensional ratio of the foam to the primary mold (thickness x bottom surface).

Foaming agent residual rate: The residual rate of the final product is set to 0.

For example, the magnification of the foam immediately after removal from the secondary mold is (690 x 500 x 52) / (225 x 155 x 16) ≒ 32 times,
The remaining blowing agent is 0%. The magnification of the primary intermediate foam is (430 x 295 x 35) / (225 x 155 x 16) ≒ 8 times,
The remaining blowing agent is (32-8)/32×100≒75%.
The magnification of the foam after the second normal pressure foaming is (600×440×
45)/(225×155×16)≒21 times, and the remaining blowing agent is (32−21)/32×100≒34%.

Example 2 Formulation: Polyethylene (trade name Yucalon YF-30)
100 parts by weight Azodicarbonamide 14 〃 Zinc flower 0.6 〃 Dicumyl peroxide 0.6 〃 After uniformly dispersing and kneading the above mixture, the bottom surface
155℃ using a primary mold of 245 x 180 mm and height 32 mm
A primary foam was obtained by heating under pressure in the same manner as in Example 1 above, except that the mixture was heated for 35 minutes. This primary foam had expanded to a base size of 440 x 320 mm and a height of 70 mm (expansion ratio of approximately 7 times), and approximately 82% of the foaming agent remained undecomposed.

Next, the process was exactly the same as in Example 1 above, except that this primary foam was placed into the two rectangular box-shaped secondary molds used in Example 1 above, with their openings aligned. Then, heating under normal pressure (remaining rate of undecomposed blowing agent at the end of this stage was about 55%) and heating under compression were performed.

The dimensions of the foam immediately after removal are: 885 x 680 mm in the center (corresponding to the opening of the secondary mold), 800 x 580 mm in the upper and lower surfaces (corresponding to the bottom of the secondary mold), and 115 mm in thickness. There was almost no foaming agent remaining. The final product was a white foam having uniform fine closed cells with an apparent density of 0.03 g/cm ³ and an expansion ratio of about 30 times.

Example 3 A primary foam was obtained using the same formulation and conditions as in Example 1.

This primary foam was placed in the same secondary mold as in Example 1 and heated at 175°C for 15 minutes using the same heating means, and then the secondary foam expanded to the full internal volume of the secondary mold was taken out. Ta. The residual rate of undecomposed foaming agent at this stage was about 34%.

This secondary foam was placed in a tertiary mold with the same shape and dimensions as the secondary mold, and after being sealed, heated at 175°C for 5 minutes using the same heating means as the secondary mold, and then replaced with the tertiary mold. Upon opening, a white foam having uniform fine closed cells was obtained. The dimensions of the foam immediately after removal were the same as in Example 1.

Effects of the Invention According to the method of the present invention, as is clear from the above, the following effects and advantages can be obtained.

(b) Excluding cooling by natural cooling of the final product, etc.
There is no cooling process in its operation process, so it is energy efficient.

(b) Only the heating medium is flowed through the heat medium flow path of the secondary mold, so the flow path is not severely damaged, and there is no need for a switching device between heating medium and cooling medium, so equipment costs are reduced. Become. In addition, together with the above-mentioned good energy efficiency, the overall production cost can be reduced.

(c) The foam after heating under compression has some expansion capacity left, and preferably by using a tapered mold for the secondary mold, the foam will come out of the secondary mold by itself, making it easier to work. Good sex.

(d) In the final heating stage, the foam is heated in a compressed state and is heated while in close contact with the heated metal plate of the secondary mold, so heat transfer is good.
Furthermore, the temperature distribution in the thickness direction is uniform. Therefore, a thick foam having uniform physical properties and thickness, and having uniform fine closed cells can be easily obtained. In addition, the blowing agent present on the surface of the foam is completely decomposed, and the surface turns white (if pigments are added, the color changes accordingly).
As a result, the epidermis, which was previously scraped off, can be fully utilized as a product.

Claims (1)

[Claims] 1. A foamable polyolefin resin composition containing a crosslinking agent and a chemical blowing agent is heated under pressure in a primary mold for a certain period of time, and the foaming agent is removed after it is partially decomposed. pressing to obtain an intermediate primary foam, and then heating the obtained intermediate primary foam in a secondary mold under normal pressure so that the blowing agent remains partially undecomposed. The foaming step is performed, and the foam in this state is further heated under pressure in close contact with the inner surface of the heating plate of the secondary mold to decompose the remaining foaming agent, and the mold is opened without cooling to foam. A method for producing a polyolefin foam, comprising the steps of: obtaining a polyolefin foam.