JPS599336B2 - Method for producing polyolefin foam - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing polyolefin foams.

Traditionally, polyolefin crosslinked foams have excellent flexibility and strength, as well as excellent properties in heat insulation, shock absorption, and secondary processability, so they have recently been widely used in construction material packaging and other fields, and the demand for them is increasing. This is something that is rapidly increasing.

As a method for producing a polyolefin crosslinked foam,
Usually, a polyolefin admixture containing a crosslinking agent and a foaming agent, such as a polyethylene mixture, is molded into a desired shape and then heated under normal pressure to decompose the crosslinking agent and foaming agent and foam to a desired ratio. be.

However, when using this method, heating is required for both the molding process and the foaming process, and the crosslinking and foaming are performed under normal pressure, so the surface of the foam becomes yellow and the bubble diameter becomes large and uniformly fine. It is something that cannot be absorbed by the body. Another method is to obtain a foam by decomposing the crosslinking agent and foaming agent under pressure in a mold or autoclave, cooling the mixture, and then heating it again to expand it. However, although this method results in uniform and fine cell diameters, it requires two or more heating steps and cannot produce a foam continuously, resulting in a significant increase in cost. The inventors of the present invention conducted intensive research to improve these drawbacks, and as a result, they discovered a method for efficiently producing a crosslinked polyolefin foam having a white surface and uniform and fine cell diameters.

That is, in the present invention, a mixture consisting of a polyolefin, a crosslinking agent, and a blowing agent is formed into a predetermined shape, and the mixture is introduced into a closed mold whose inner surface is lubricated and has the same cross-sectional shape in the longitudinal direction, and the mixture is transferred and crosslinked while being heated. The method is characterized in that the foaming agent and the foaming agent are decomposed, the cross section is partially expanded in a similar shape, and then the cross section is further expanded while being cooled and extruded. An example of the method of the present invention will be explained in detail based on the drawings. As shown in Fig. 1, a mixture obtained by adding and kneading a crosslinking agent and a blowing agent to polyolethin is passed through an orifice, heated and pressurized to form a predetermined shape, and then introduced into a closed mold 1. At the same time, a lubricant such as silicone oil or glycerin is applied to the inner surface of the container via a porous metal body 3 from a lubricant injector 2 attached to the outside of the end of the container. The molded body is pressurized (50 to 300 k9/d) in zone 5 by providing a heating heater 4 on the outside of the container.
, after being heated (160 to 260°C) to decompose the crosslinking agent and blowing agent, it is subsequently partially expanded in zone 6,
Next, in zone 7, the foam is cooled to about 130 to 200 DEG C. by a cooler 8 installed outside, and extruded to the outside and expanded again under normal pressure to obtain foam 9.

As described above, the method of the present invention particularly crosslinks the polyolefin molded product without bringing it into contact with air and under pressure, so that it is easy to obtain a foam that is uniformly crosslinked and has uniform fine cells.

Note that the pressure after partial expansion is preferably 5 kg/Cd or more.
In particular, 10k9/CrA or more is desirable. In addition, in the method of the present invention, crosslinking and foaming are carried out continuously, and the decomposition of the blowing agent is also carried out under pressure, so that the aggregation of bubbles is suppressed and a foamed product with uniform and fine cells is obtained.

Therefore, in the method of the present invention, the pressure is reduced in the expansion zone to partially expand the composition, and in this case, the volume of the composition is limited to less than 6 times, preferably 3 times or less. be.

The shape of this expansion zone is similar in cross-section to the crosslinking zone, and it expands with a gentle slope. If the angle is too steep, it will be difficult to obtain bubbles with a uniform and fine diameter. The angle is usually 3 to 30 degrees, preferably 5 to 15 degrees. In the method of the present invention, the partially expanded foam is then cooled in a cooling zone and extruded into the atmosphere, where it undergoes secondary expansion to obtain a low-density foam.
This cooling is preferably performed to a temperature close to the melting point of the polymer. However, foams may also be obtained by extruding into the atmosphere without cooling. In addition, the heating of the present invention may utilize internal heating due to shear stress during molding, but by heating the closed mold by directly applying electricity or heating it with a heater, extremely uniform heat transfer is achieved. Therefore, crosslinking and foaming are performed at relatively low temperatures, resulting in a foam of good quality. In addition, in the method of the present invention, it is necessary to crosslink after forming into a predetermined shape, but since the crosslinking agent and blowing agent are decomposed under pressure, there is no significant effect before or after the decomposition.

However, a foam with a high degree of crosslinking can be obtained by partially expanding the foam while the crosslinking reaction is not completely completed.
．． The expansion is 5 times or more, preferably 2 times or more. In order to promote foaming in this way, a foaming aid may be used together with the foaming agent. In this way, the gel fraction is 8
0% or more, resulting in a foam with excellent heat resistance or heat processability. However, it is not preferable to make the decomposition temperature of the blowing agent significantly lower than the decomposition temperature of the crosslinking agent by adding a large amount of the blowing aid. In the present invention, the polyolefin is a polymer or copolymer of ethylene, propylene, butene, or a mixture thereof with a rubber or plastic that can be mixed in an equal amount or less.

The crosslinking agent is one that can be decomposed by heating to crosslink the polyolefin, such as organic peroxides and azides, and is usually blended in an amount of 0.5 to 5 parts by weight per 100 parts by weight of the polyolefin. It is.

Foaming agents such as PV oxybisbenzenesulfonyl hydrazide, P azodicarbonamide, and deinitron pentamethylenetetramine, which decompose and generate gas when heated, are usually used, but lower hydrocarbons and freon Volatile blowing agents such as can also be used.

Further, in the present invention, antioxidants, flame retardants, pigments, and other fillers can be used in the polyolefin.

However, if a large amount of filler is used, the bubble diameter will instantly become coarse, but in the method of the present invention, the crosslinking and foaming are performed under pressure, so even if a large amount of filler with a pressure of 30 PHR or more is used, the bubble diameter will become coarse. A foam having air bubbles is obtained. The filler is added in an amount of 3 to 100 parts by weight per 100 parts by weight of the polyolefin. In this way, the method of the present invention performs cross-linking and foaming in a closed mold without contacting with air, so the resulting foam has a low density six times that of the previous one, has strong elasticity, and has a strong skin. It is soft and has a good feel.

Further, in the method of the present invention, the polyolefin mixture is passed through an orifice prior to molding, and the temperature of the mixture is raised by shear heat generation to obtain a thick-walled foam that can be molded. According to the method of the present invention, pipe-shaped, rod-shaped, and plate-shaped foams can be obtained continuously, and in particular, semicircular and other irregularly shaped foams can be produced.
It is also possible to use an electric wire or copper pipe as a core material and cover the outside with a foam. Next, examples of the present invention will be described.

(The following parts indicate parts by weight.) Example 1 1 part of dicumyl peroxide, 15 parts of azodicarbonamide, 1 part of cadmium stearate, and zinc white were added to 100 parts of low-density polyethylene (MIL.O). Add 3 parts each,
This was kneaded in an extruder, and then the mixture was heated to 180°C through an orifice by shear heat generation.
Under pressure of 9/Cd, inner diameter 3mm, wall thickness 1.5m1
It was molded into a pipe shape.

Next, while maintaining the shape, the mold was heated to 200° C. in a closed mold lubricated with silicone oil to decompose the crosslinking agent and foaming agent. The mold is provided with an expansion zone in which the molded body expands in a similar shape in the direction of transition, and its cross section is expanded at an angle of about 10 degrees.

Therefore, when the crosslinking is completed, the molded body has an inner diameter of 4.5 mm, a wall thickness of 2.21!11, and a density of 0.337 in the mold.
/CC became the primary expansion body. This expanded body is heated to about 160℃
Cooled and extruded for secondary expansion.Inner diameter 911, wall thickness 4.
．． A 5 mm pipe-shaped foam was obtained. The foam has a density of 0.03y/CC, a cell diameter of 80μ, and a gel fraction of 8.
1%, the surface was smooth and thin, and the feel and elasticity were good.

Example 2 3 mm of the same polyethylene blend as in Example 2
Copper wire with a diameter of 1.5 mm in thickness under a pressure of 150 kg/word
Then, while maintaining this shape, it was heated to 200° C. in a closed mold lubricated with silicone oil to decompose the crosslinking agent and foaming agent and partially expand it.

The mold is provided with an expansion zone in which the molded body expands in a similar shape in the direction of transition, and the expansion zone expands at an angle of about 8 degrees in its cross section.

Therefore, the molded body has a density of 0.0 when crosslinking is completed.
It was below 5t/Cd. The expansion zone continued to increase in cross-section to a conformal shape, and the composition expanded and cooled in a conformal manner. The outer diameter of the thus obtained coating was 23 ml. The thickness of the resulting coating is 1
0m11L? Yes, with a density of approximately 0, consisting of uniform and fine bubble diameters.
．． A foam of 057/CC was obtained. As detailed above, the method of the present invention has remarkable effects such as obtaining a polyolefin foam having excellent compressive strength and having uniform and fine cell diameters.

[Brief explanation of drawings]

The drawing is a cross-sectional view showing one example of the method for producing the polyolefin foam of the present invention. DESCRIPTION OF SYMBOLS 1... Sealed container, 2... Lubricant injector, 3... Porous metal body, 4... Heater, 5, 6, 7. ...Zone, 8...
- Cooler, 9... Foam.

Claims (1)

[Scope of Claims] 1. A mixture consisting of a polyolefin, a crosslinking agent, and a blowing agent is formed into a predetermined shape, and the mixture is introduced and transferred into a closed mold whose inner surface is lubricated and whose cross-sectional shape is continuous in the longitudinal direction, A polyolefin foam is produced by decomposing a cross-linking agent and a blowing agent while heating, partially expanding the cross-sectional area in a similar shape, and then further expanding the cross-section while cooling and extruding the foam. Production method. 2.3 to 30 mm so that the inner surface of the expanding part of the closed mold expands into a similar shape along the migration direction of the polyolefin crosslinked body.
2. The method for producing a polyolefin foam according to claim 1, wherein a gentle slope of .degree. is formed.