JPH0438221B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a low-density cross-linked polyolefin foam, and particularly to a method for producing a cross-linked polyolefin foam having uniform microcells without causing surface deterioration due to oxidation. Regarding the method. (Prior art and problems to be solved) Conventionally, polyolefins are blended with a blowing agent, a blowing aid, and a crosslinking agent, and a part of the blowing agent is decomposed by applying pressure and heating for a certain period of time, and a crosslinking reaction is caused by the crosslinking agent. It is a well-known method to produce a low-density crosslinked polyolefin foam by performing secondary foaming to form a primary foam and then heating it further to decompose the remaining foam. (For example, see Special Publication No. 45-29381). For this secondary foaming, the primary foam is placed in a closed mold and heated in a molten salt bath, or heated in a metal bath or oil bath without being placed in a mold.
Alternatively, a method of directly heating with steam is known. However, none of these methods are efficient, and to further improve these methods, the primary foam is placed in an unclosed mold with a cross-sectional shape and dimensions that correspond to the shape and dimensions of the final product. There is also known a method of performing secondary foaming by placing the foam in a mold and heating the mold from the outside. (For example, Tokukai Sho
57-191029). However, when using this method, a degraded layer due to oxidation may occur on the surface of the secondary foamed product, or voids may occur at the corners of the mold, so it is not always possible to obtain a uniform product. . (Problems to be Solved) Therefore, the present inventors improved the above-mentioned drawbacks and developed a method for producing a crosslinked polyolefin foam having low-density and uniform microcells without forming a deteriorated layer due to oxidation on the surface. The present invention was completed as a result of various studies, and the object of the present invention is to provide a method for producing a crosslinked polyolefin foam having low density and uniform microcells without a deteriorated layer. (Means for Solving the Problems) That is, the present invention involves adding a foamable resin composition containing a polyolefin resin, a foaming agent, a crosslinking agent, and, if necessary, a foaming aid and other additives, into a mold. The foaming agent and crosslinking agent in the foamable resin composition are partially decomposed by heating under pressure and the pressure is removed under high temperature to produce a primary foam having an expansion ratio of 25 to 40% of the final product from the mold. The first step is to take out the high temperature primary foam taken out from the first step, and the first step is to put the primary foam taken out from the first step into a closed mold that has a shape and dimensions corresponding to the shape and dimensions of the final product and is equipped with a vacuum device. This is a method for producing a crosslinked polyolefin foam, which comprises a second step of heating under a reduced pressure of 200 mmHg or more to decompose and foam the remaining foaming agent, and then cooling to produce a low-density foam. However,
The crosslinked polyolefin foam in the present invention is, for example, polyolefin such as low density polyethylene, high density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrytate copolymer, or chlorinated ethylene ionomer, and dicumyl peroxide. , a crosslinking agent such as di-t-butyl peroxide, and a foaming agent such as azodicarbonamide or dinitrosopentamethylenetetramine, which is crosslinked and foam molded.The final degree of crosslinking is preferably 70 to 80% in terms of gel concentration. It is. It is preferable to use a foaming auxiliary agent in this foamable composition depending on the type of foaming agent. Examples of the foaming auxiliary agent include metal oxides such as zinc oxide and lead oxide, fatty acids or their metal salts, and urea. There are compounds as main components, and if necessary, commonly used compounding agents such as fillers and pigments can be added. The foamable resin composition thus obtained is placed in a primary foaming mold and the foaming agent is gasified at the same time as the crosslinking agent decomposes or after the crosslinking agent starts to decompose, and the unfoamed foaming agent is Primary foaming is performed so that 75% remains, and the pressure is removed at high temperature. The obtained primary foam has an expansion ratio of 25-40% of the final product. Next, this is placed in a closed mold having a shape and dimensions corresponding to the shape and dimensions of the final product and equipped with a vacuum device to perform secondary foaming. An example of a mold used for secondary foaming is shown in FIG. That is, it is composed of an upper mold 2 and a lower mold 3, and the lower mold is preferably a double mold consisting of an inner mold 4 and an outer mold 3. Then, small holes 5 for gas venting are provided in the inner mold and the outer mold.
A small hole 6 for degassing in the outer mold is connected to a vacuum device via a valve 7. The above-mentioned primary foam, which is still at a high temperature, is placed inside this mold and the inside of the mold is connected to a vacuum device to perform secondary foaming while reducing the pressure. In this specification, the degree of vacuum refers to the pressure of evacuation, and for example, a degree of vacuum of 100 mmHg means that the pressure inside the mold is (atmospheric pressure - 100 mmHg). The foaming conditions for secondary foaming are a temperature of 160 to 170℃;
The remaining foaming agent is decomposed and foamed at a vacuum level of 200mmHg or higher. As shown in the comparative example below, those obtained by heating under a reduced pressure of 200 mmHg or less cannot achieve the object of the present invention. In the present invention, since a closed mold as described above is used, there is extremely little discoloration due to oxidative deterioration compared to foams obtained using conventional open molds, and the mold is well packed, resulting in uniform thick crosslinked polyolefin foam. It is possible to produce foams. In addition, it is preferable to apply talc powder, clay powder, etc. to the inner surface of the mold in advance for secondary foaming. Next, the present invention will be explained using examples. Examples 1 to 3 100 parts by weight of polyethylene, Yucalon LM-31 (MFR 8.0 density 0.918 g/cm ³ , manufactured by Mitsubishi Yuka Co., Ltd.) produced by a high-pressure method, 17 parts by weight of azodicarbonamide as a blowing agent, and a crosslinking agent 0.7 parts by weight of dicumyl peroxide, 0.15 parts by weight of activated zinc white, 3.
0.84 to 1.0 parts by weight of 3,5-trimethylcyclohexane, 0.5 parts by weight of a urea foaming aid (Selton NP), 0.5 parts by weight of stearic acid as a foam regulator,
Mix 0.5 parts by weight of zinc stearate and reduce the surface temperature.
After kneading on rolls at 95 to 115°C and uniformly dispersing, the kneaded product was filled into a mold (27.5 x 305 x 305 mm) in a compression molding machine, and external pressure was applied to seal it.
The mixture was heated at ℃ for 46 minutes, the pressure was removed at high temperature, and the primary foam was taken out. The primary foam has thickness, width,
It had expanded about 2.1 times both vertically and vertically. This high-temperature primary foam was immediately placed in a closed mold (85 x 850 x 850 mm) with talc powder sprinkled as a lubricant and equipped with a vacuum device, and heated with steam at 165°C for 35 minutes using a jacket method. , the degree of vacuum was 200 mmHg in Example 1, and 300 mm in Example 2.
Hg, as in Example 3, was pulled at 560 mmHg and taken out after cooling.
A closed-cell foam with a white, smooth skin and little discoloration due to deterioration was obtained. Similarly, in Examples 2 and 3, foams with increased whiteness were obtained by further increasing the degree of vacuum. The resulting foam had a uniform thickness of 85 m/m, almost the same cross-sectional dimensions as the secondary heating mold, and a density of 0.030 g/cm ³ . Comparative Examples 1 and 2 In the same manner as in Example 1, the primary foam at high temperature was immediately placed in a closed mold (85 x 850 x 850 mm) with talc powder sprinkled as a lubricant and equipped with a vacuum device, and the jacket was placed in the mold. The method was heated with steam at 165℃ for 35 minutes, and the vacuum degree was 50mm as Comparative Example 1.
Hg, as in Comparative Example 2, the vacuum level was 100mmHg, and after secondary foaming, each was cooled and taken out. In Comparative Examples 1 and 2, there was relatively little discoloration due to oxidative deterioration, which was not much different from the conventional open mold heating method. It became a foam. Comparative Example 3 A primary foam in a high temperature state that was primarily foamed in the same manner was
Immediately, the mold was placed in a conventional open mold (85 x 850 x 850 mm) on which talc powder was sprinkled as a lubricant, heated with steam at 165°C for 35 minutes, cooled, and then taken out.
The result is a relatively highly colored foam. The thickness density is the same as in the example. At the secondary foaming stage, compared to foams obtained using conventional open molds, foams obtained using closed molds and vacuum evacuation (especially under vacuum of 200 mmHg or higher) have very little discoloration due to oxidative deterioration. The first thing is
This is clearer than the color difference meter measurement (yellowness) results in the table.

[Table]
(Effects) As described above, in producing a crosslinked polyolefin foam by two-stage foaming, the present invention
To easily produce a uniform thick crosslinked polyolefin foam of, for example, about 50 mm, without forming a deteriorated layer due to oxidation on the surface by using a closed mold in the secondary foaming step and foaming under reduced pressure. I can do it.

[Brief explanation of drawings]

Fig. 1 is a plan view of the mold used in the present invention, Fig. 2 is a plan view of the mold used in the present invention;
The figure is a sectional view of the mold. 1... Primary foam, 2... Upper mold, 3... Lower mold (outer mold), 4... Inner mold, 5, 6... Small holes for gas venting, 7... Valve, 8... Packing.

Claims (1)

[Claims] 1 Polyolefin resin with foaming agent and crosslinking agent,
Further, a foamable resin composition containing a foaming aid and other compounding agents as necessary is placed in a mold and heated under pressure to partially decompose the blowing agent and crosslinking agent in the foamable resin composition. The first step is to remove the pressure from the mold under high temperature to remove the primary foam that has an expansion ratio of 25 to 40% of that of the final product. Place it in a sealed mold with a shape and dimensions corresponding to that of
A method for producing a crosslinked polyolefin foam comprising a second step of heating under reduced pressure of Hg or higher to decompose and foam the remaining blowing agent, followed by cooling to produce a low-density foam.