JP2002331566A - Manufacturing method for polyethylene resin foamed tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method for obtaining continuously a foamed tube having a high foaming multiplication rate and having uniform and minute foams and a smooth surface. SOLUTION: A polyethylene resin of a density of 0.91-0.93 g/cm<3> is modified by heating it to a temperature or above at which a free radical is formed in the existence of a radical forming agent or by irradiating it by an ionizing radiation, so that the weight-average molecular weight thereof may rise to be 1.5-10 times larger than that before modification. A foam nucleus forming agent is added to the modified resin. A modified resin composition thus obtained is melted by an extruder 1 and then a carbonic acid gas is dissolved therein by 2-5 pts.wt. to 100 pts.wt. of the modified resin composition. This molten material is bled into an atmosphere of atmospheric pressure from a foamed tube die 5 connected to the extruder 1 and formed in the shape of the tube, while it is foamed. Immediately after this forming, the outer peripheral surface of the obtained foamed tube T is air-cooled by an air-cooling device 9.

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 polyethylene resin foam tube by continuously producing a foam tube by extruding a low density polyethylene resin with carbon dioxide gas as a clean foaming agent. .

[0002] The foam tube according to the present invention is suitably used as a foam layer covering a pipe such as a synthetic resin pipe, a copper pipe, a steel pipe or the like for the purpose of imparting heat insulation to the pipe. Since the foamed tube according to the present invention is made of polyethylene resin and has both heat insulation and flexibility, it is particularly suitably used for coating a crosslinked polyethylene pipe.

2. Description of the Related Art Conventionally, as a method for producing a thermoplastic resin foam, a thermal decomposition type chemical foaming agent is kneaded into a resin composition,
A chemical foaming method in which the resin composition is foamed by heating the resin composition at a temperature higher than the decomposition temperature of the foaming agent; Alternatively, a gas foaming method is known in which a volatile liquid is dissolved in a resin melt, and then the melt is discharged into a low-pressure region and foamed. In the chemical foaming method, a foam having uniform and fine closed cells can be obtained. Occurs.

On the other hand, in the gas foaming method, when the foaming agent used is a low-boiling organic substance such as butane or pentane, there is a danger of explosion since explosive gas is generated during foam production. When the blowing agent used is dichlorodifluoromethane (CFC R-12), there is little danger of explosion, and the bubble film can be rapidly cooled and solidified by the latent heat of vaporization accompanying vaporization at the time of foaming. Since the permeability is small, it is easy to obtain a foam having a high expansion ratio, but due to environmental problems such as destruction of the ozone layer, CFC-based gases are being totally eliminated.

In order to solve the above problems, carbon dioxide gas,
A method using an inorganic gas such as nitrogen gas or air or water as a foaming agent has been proposed. These blowing agents are clean and do not cause the above-mentioned problems, but it is difficult to stably obtain a foam having a high expansion ratio.

JP-A-6-32929 discloses a method in which a crosslinked polyolefin-based resin composition is melted, an inorganic gas is dissolved in the resin melt, and the resin composition is extruded into a low-pressure region and foamed. ing. However, as a result of actual studies by the present inventors, when carbon dioxide gas is used as an inorganic gas and a foamed tube is manufactured as a foam, simply cross-linking the resin prevents foam breakage during depressurization. It was not possible to obtain only a foamed tube having a rough surface and poor appearance.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to overcome the above-mentioned disadvantages of the prior art, and to provide a clean inorganic gas.
In particular, it is an object of the present invention to provide a method for continuously obtaining a foamed tube having a high foaming ratio, uniform fine bubbles, and a smooth surface using carbon dioxide gas.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that the density is 0.91 to 0.91.
0.93 g / cm ³ low density polyethylene resin
The resin is denatured by heating it to a temperature or higher at which free radicals are generated in the presence of a radical generator, or by irradiating the resin with ionizing radiation to increase its weight average molecular weight to 1.5 to 10 times that before denaturation. The modified low-density polyethylene resin is added with a foam nucleating agent, and the resulting modified resin composition is melted by an extruder, and then carbon dioxide gas is added to the modified resin composition with respect to 100 parts by weight of the modified resin composition. 2 to 5 parts by weight are melted, and this carbon dioxide-containing modified resin melt is discharged into an atmospheric pressure atmosphere from a foam tube mold connected to an extruder to form a tube while the modified resin melt is foamed, Immediately after the molding, by cooling the outer peripheral surface of the obtained foamed tube, it has been found that a foamed tube having a high expansion ratio, uniform and fine bubbles, and a smooth surface can be continuously produced. .

In the above manufacturing method, it is preferable to cool the outer peripheral surface of the foam tube while applying an internal pressure to the foam tube. The addition amount of the cell nucleating agent is preferably 0.3 to 1.0 part by weight based on 100 parts by weight of the modified low-density polyethylene resin.

Hereinafter, the present invention will be described in detail.

The polyethylene resin used in the present invention is a low-density polyethylene resin having a density of 0.91 to 0.93 g / cm ³ . The low-density polyethylene-based resin is denatured by heating it to a temperature at which free radicals are generated in the presence of a radical generator or by irradiating the resin with ionizing radiation. Although it does not specifically limit as a radical generator, For example, 1,1-bis (t-
Butylperoxy) cyclohexane, t-butylperoxymaleic acid, t-butylperoxy-3,5,5-
Trimethylhexane, cyclohexane peroxide,
t-butyl peroxyallyl carbonate, t-butyl peroxyisopropyl carbonate, 2,2-bis (t-butylperoxy) octane, t-butylperoxyacetate, 2,2-bis (t-butylperoxy) butane , T-butyl peroxybenzoate, n-
Butyl-4,4-bis (t-butylperoxy) berlate, di-t-butylperoxyisophthalate, dicumyl peroxide, t-butylcumyl peroxide,
Organic peroxides such as 2,5-dimethyl-2,5-di (t-butyl peroxide) hexane and the like can be mentioned.

If necessary, a compound having at least one carbon-carbon unsaturated bond may be added to the low-density polyethylene resin as a polymerization aid together with the radical generator.
To modify a low-density polyethylene resin with a radical generator, the low-density polyethylene resin, a radical generator and, if necessary, a polymerization aid are supplied to an extruder for modification, and this is subjected to radical generation. It extrudes at a temperature higher than the decomposition temperature of the agent to generate free radicals to crosslink the low density polyethylene resin. The resin thus modified may be pelletized and then supplied to an extruder for producing a foamed tube, or an extruder for producing a foamed tube may be directly connected to the extruder for producing a modified May be directly supplied to an extruder for producing a foamed tube.

When the low-density polyethylene resin is modified by irradiating it with ionizing radiation, the low-density polyethylene resin is formed into a sheet, and the sheet is irradiated with an electron beam or γ.
The resin is cross-linked by irradiation with ionizing radiation such as a wire, and then is pelletized and supplied to an extruder for producing a foamed tube.

In this way, the low-density polyethylene resin is modified to increase its weight average molecular weight (hereinafter abbreviated as "Mw") to 1.5 to 10 times that before modification. In addition,
Mw is a value measured by high temperature gel permeation chromatography (GPC).

When the increase ratio of Mw is less than 1.5 times,
Only a foam having a low viscosity at the time of extrusion foaming and a low magnification and having many open cells can be obtained. On the other hand, if the increase ratio of Mw exceeds 10 times, the melt viscosity becomes too high to make extrusion impossible, or even if extrusion is possible, it is difficult to obtain a foam having a smooth surface by melt fracture. M by denaturation
The preferable range of the increase ratio of w is 1.5 to 3.0.

By the way, even if an unmodified low-density polyethylene resin having the same Mw as that obtained by the above modification is used as a raw material resin, a foam having a high expansion ratio cannot be obtained. The reason for this is not clear, but is presumed to be due to the fact that the branched chains of the modified resin are longer than the unmodified resin. In other words, it is presumed that the longer the branched chain, the higher the elongational viscosity during foaming, the less likely it is for foam to break during foaming, and a foam having a high expansion ratio can be obtained.

Next, a cell nucleating agent is added to the modified low-density polyethylene resin. As the bubble nucleating agent,
Although not particularly limited, calcium carbonate, talc, clay, magnesium oxide, zinc oxide, carbon black,
Silicon dioxide, titanium oxide, citric acid, baking soda, orthoboric acid and talc, alkaline earth metal salts of fatty acids and the like,
The particle size of the cell nucleating agent is preferably 500 μm or less. When a cell nucleating agent having a size of 500 μm or more is used, the cell diameter tends to be too large.

The amount of the cell nucleating agent is preferably 0.3 to 1.0 part by weight based on 100 parts by weight of the modified low density polyethylene resin. When the addition amount is less than 0.3 parts by weight, the effect as a cell nucleating agent is small, and only a coarse cell is difficult to be obtained because the number of cells is small.
On the other hand, if the addition amount exceeds 1.0 part by weight, the dispersion of the cell nucleating agent is not sufficiently performed, partial aggregation occurs, and there is a possibility that coarse bubbles are generated in that portion. A more preferred range for the amount is 0.3 to 0.7 parts by weight.

The above-mentioned modified resin composition may, if necessary,
Additives such as fillers, antioxidants, pigments and flame retardants may be added.

In the present invention, carbon dioxide is used as a foaming agent. This is because carbon dioxide gas is clean, has a small load on the environment, and has higher solubility in low-density polyethylene-based resins than other inorganic gases.

After the modified resin composition comprising the modified low-density polyethylene resin and the cell nucleating agent is melted by an extruder, carbon dioxide is dissolved in the resulting modified resin melt.

The supply amount of the above-mentioned carbon dioxide gas depends on the modified resin composition 1
It is 2 to 5 parts by weight with respect to 00 parts by weight. If the supply amount is less than 2 parts by weight, a tube having a high expansion ratio cannot be obtained. On the other hand, if the supply amount exceeds 5 parts by weight, the above-described method for producing a foamed tube cannot prevent foam breakage, and the surface of the foamed tube becomes rough and the appearance becomes poor.

Next, the carbon dioxide-containing modified resin melt is discharged from a foam tube mold connected to an extruder into an atmospheric pressure atmosphere, and the modified resin melt is formed into a tube while being foamed. At this time, the modified low-density polyethylene resin has a higher elongational viscosity than the unmodified low-density polyethylene resin, and is less likely to break during foaming. Thus, a foam tube having a high foaming ratio can be obtained. However, since the temperature of the foamed tube immediately after being extruded is still high, the foam breaks from the foamed tube surface, and the surface becomes rough and the appearance becomes poor. Therefore, immediately after releasing the carbon dioxide-containing modified resin melt from the foam tube mold to the atmospheric pressure atmosphere,
The outer peripheral surface of the obtained foam tube is cooled, and the temperature of the foam tube is quickly lowered to prevent foam from breaking from the surface.
Thereby, a foamed tube having a high expansion ratio and a smooth surface can be continuously obtained.

The method of cooling the outer peripheral surface of the foam tube is not particularly limited. For example, in order to uniformly cool the outer peripheral surface of the foam tube, a ring-shaped air cooling device having a diameter larger than the outer diameter of the obtained foam tube is used. Is arranged concentrically outside the foaming tube, and air is applied to the outer peripheral surface of the foaming tube from an air cylinder through a plurality of inward ventilation holes of the device to cool the foaming tube. It is preferable that the air cooling device is located as close as possible to the outlet of the foaming tube mold because the bubble breaking prevention effect is greater as the closer to the outlet of the foaming tube mold.

As a method for cooling the outer peripheral surface of the foam tube, the above-described cooling with air is preferable. this is,
For example, using a cooling water tank as a cooling device and immersing the foam tube in the water in the water tank to perform rapid cooling, the elongational viscosity becomes too high, and the growth of bubbles stops, and with it This is because the expansion ratio does not increase.

The cooling of the outer peripheral surface of the foam tube is preferably performed while applying internal pressure to the foam tube. This is because when the internal pressure is applied to the foam tube, the foam tube is cooled while the foam tube is about to expand outward, and the bubbles are solidified, so that the surface of the foam tube becomes smoother.

The method for applying the internal pressure to the foam tube is not particularly limited. For example, an air passage is provided in the inner core of the foam tube mold from the bridge portion of the mold to the tip surface of the core, and the bridge portion of the mold is provided. There is a method in which air is supplied to the ventilation path through the air supply tube to be sent into the inside of the foam tube, and an inner pressure is applied by sealing a tip portion of the foam tube. In this method, by controlling the internal pressure,
There is an advantage that the inner diameter, outer diameter, and thickness of the foam tube can be controlled, and a desired foam tube shape can be obtained.

The magnitude of the internal pressure is preferably 2 to 5 kg /
cm ² . If the internal pressure is less than 2 kg / cm ² , the above-mentioned effects due to the internal pressure can hardly be obtained. on the other hand,
When the internal pressure exceeds 5 kg / cm ² , the inner diameter and the outer diameter of the foam tube become too large, and the thickness becomes extremely thin.

[0028]

According to the production method of claim 1, when the carbon dioxide-containing modified resin melt is extruded and foamed, the foamed tube obtained immediately after the resin melt is released from the foamed tube mold into the atmospheric pressure atmosphere. Cooling the outer peripheral surface of the foam tube, it is possible to quickly lower the temperature of the foam tube and prevent the foam from breaking from the surface, thereby producing a foam tube having a high foaming ratio, uniform and fine bubbles, and a smooth surface. It can be manufactured continuously.

According to the manufacturing method of the second aspect, by cooling the outer peripheral surface of the foam tube while applying the internal pressure, the foam tube is cooled while the foam tube is about to expand outward, and the bubbles are solidified. A foam tube having a smoother surface can be obtained.

According to the method of claim 3, since the cell nucleating agent is added in an amount of 0.3 to 1.0 part by weight based on 100 parts by weight of the modified low-density polyethylene resin, a foamed tube having finer cells is added. Can be obtained.

[0031]

Embodiments of the present invention will be described below in detail with reference to the drawings.

In FIG. 1 showing an example of the method for producing a foamed tube according to the present invention, the extruder (1) for producing a foamed tube is manufactured by using a raw material supply hopper (14) at a rear end thereof and by using another extruder. A modified resin composition is prepared by adding a cell nucleating agent to the modified low density polyethylene resin. Extruder (1)
Is composed of two stages. In the first stage, the modified resin composition is heated and melted. Next, a carbon dioxide gas as a foaming agent is supplied to the foaming agent supply section between the first stage and the second stage.
In (2), the modified resin composition was supplied from the carbon dioxide gas cylinder (3) to the modified resin melt using a metering pump (4) at a supply amount of 2 to 5 parts by weight with respect to the extruded amount of the modified resin composition. Dissolve in the melt. In the blowing agent supply section (2), it is preferable to temporarily reduce the resin pressure by increasing the depth of the screw groove so that the carbon dioxide gas can be stably supplied into the extruder. In the second stage, a screw is preferably provided with a known mixing element such as dalmage to efficiently dissolve carbon dioxide gas in the modified resin melt.

Subsequently, the obtained carbon dioxide-containing modified resin melt is sent to a foam tube mold (5) directly connected to the extruder (1) to form a foam tube (T). The foam tube mold (5) consists of an inner core (6) and an outer mold (7),
The inner core (6) is held by an outer mold (7) by a bridge (8). The temperature of the outer mold (7) can be adjusted by a band heater or a heat medium such as oil. The temperature of the modified carbon dioxide-containing modified resin melt is adjusted to the optimum foaming temperature, and the outer mold (7) is larger than the foam tube mold (5). Release to atmospheric pressure and extrude foam. The optimum foaming temperature is preferably (melting point -10
C) to the melting point. It is preferable that the foam tube mold (5) is provided with a known thickness deviation adjusting mechanism.

Next, immediately after the carbon dioxide-containing modified resin melt is discharged from the foam tube mold (5) into the atmospheric pressure atmosphere, the outer peripheral surface of the obtained foam tube (T) is cooled.
For this cooling, a ring-shaped air cooling device (9) having a diameter larger than the outer diameter of the foam tube (T) is arranged concentrically outside the foam tube.
This is performed by applying air from the air cylinder (13) to the outer peripheral surface of the foam tube (T) through a plurality of inward ventilation holes of the device (9). The cooled foam tube (T) is continuously taken off by a take-off machine (10).

With the cooling of the outer peripheral surface of the foam tube (T),
Apply internal pressure to the foam tube (T). In this method, the bridge part of the mold (5) is attached to the inner core (6) of the foam tube mold (5).
An air passage (11) is provided from (8) to the core tip surface, and the air passage (1) is passed from the air cylinder (12) through the bridge (8) of the mold (5).
Air is supplied to 1) to be sent into the inside of the foam tube (T), and the end of the foam tube (T) is crushed and sealed.

[0036]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to examples, but the examples are illustrative and do not limit the present invention.

Example 1 Low-density polyethylene resin ("LF" manufactured by Nippon Polychem Co., Ltd.)
440 HB ", density: 0.925 g / cm ³ , melting point 1
13 ° C., Mw = 75,000) 100 parts by weight, and dicumyl peroxide (manufactured by NOF CORPORATION) as an organic peroxide.
0.2 parts by weight of "Perhexa D") is extruded (not shown)
The resin was extruded into a strand at 200 ° C., and the obtained strand was pelletized by a pelletizer. Mw of the modified low-density polyethylene resin thus obtained by high-temperature GPC was 151,0.
It was rising to 00.

The above modified low density polyethylene resin 100
In parts by weight, talc (average particle size 15 μ
m) 0.6 parts by weight were dry blended, and the resulting modified resin composition was supplied to the extruder (1) of FIG. 1 from the hopper (14). The first stage of the extruder (1) was set at 160 ° C., and the modified resin composition was heated and melted. At this time, the second stage of the extruder (1) and the foam tube mold (5) are
It was set to 0 ° C. The extrusion rate of the modified resin composition was about 10 kg / h.

Next, a metering pump is supplied from the blowing agent supply section (2).
Using (4), 3 parts by weight of carbon dioxide gas is supplied to the extruder (1) with respect to 100 parts by weight of the extruded amount of the modified resin composition, and in the second stage, the carbon dioxide gas is introduced into the modified resin composition. Dissolved. At this time, the second stage and the foam tube mold (5)
Was reset to 108 ° C., and the carbon dioxide-containing modified resin melt was discharged from the foam tube mold (5) into an atmosphere of atmospheric pressure to extrude and foam.

Immediately after the carbon dioxide-containing modified resin melt was discharged from the foam tube mold (5) to the atmospheric pressure, the outer peripheral surface of the obtained foam tube (T) was cooled by a ring-shaped air cooling device (9). ), And the foaming tube (T) was continuously taken out by a take-up machine (10). The foamed tube (T) thus obtained had a foaming ratio of about 5 times, uniform and fine bubbles, and a smooth surface.

Example 2 Low density polyethylene (“LF440” manufactured by Nippon Polychem Co., Ltd.)
HB ", density: 0.925 g / cm ³ , melting point 113
(C, Mw = 75,000) into a sheet having a thickness of 2 mm. Both surfaces of the obtained sheet were irradiated with an electron beam having an irradiation voltage of 500 keV and an irradiation dose of 4 Mrad, and then pelletized by a pelletizer. The Mw of the modified low-density polyethylene resin thus obtained by high-temperature GPC was 142,
000.

Thereafter, the same operation as in Example 1 was performed except that the above modified low-density polyethylene was used. The foamed tube (T) obtained had an expansion ratio of about 5 times, uniform and fine bubbles, and a smooth surface.

Example 3 Immediately after the carbon dioxide-containing modified resin melt obtained in the same manner as in Example 1 was discharged from the foam tube mold (5) into the atmosphere of atmospheric pressure, a ring-shaped air cooling device (9) was used. Cool the outer surface of the foam tube and mold from the air cylinder (12).
Air is supplied to the air passage (11) provided in the inner core (6) through the bridge portion (8) of (5), and the obtained foamed tube is obtained.
(T) inside the tube.
The tip of (T) was crushed and sealed. Thus, a foamed tube (T) was obtained in the same manner as in Example 1 except that the outer peripheral surface of the foamed tube (T) was cooled while applying an internal pressure of 3 kg / cm ² to the foamed tube (T).

The expanded tube (T) has an expansion ratio of about 7
Twice, the bubbles were uniformly fine, and the surface was even smoother.

COMPARATIVE EXAMPLE 1 Immediately after the carbon dioxide-containing modified resin melt obtained in the same manner as in Example 1 was discharged from the foam tube mold (5) into the atmosphere of atmospheric pressure, the outer peripheral surface of the foam tube (T) was removed. A foamed tube (T) was obtained in the same manner as in Example 1 except that cooling was not performed.

The expanded tube (T) has an expansion ratio of about 3
And the surface was roughened by foam breakage.

Comparative Example 2 A foamed tube (T) was obtained in the same manner as in Example 1 except that the supply amount of carbon dioxide gas was changed to 7 parts by weight based on 100 parts by weight of the modified resin composition.

This foam tube (T) has an expansion ratio of about 4
At the same time, more intense roughness due to foam breaking was observed on the surface.

[0049]

Since the method for producing a foamed tube of the present invention is constituted as described above, it is possible to continuously produce a foamed tube having a high expansion ratio, uniform and fine bubbles, and a smooth surface. it can.

Further, since carbon dioxide gas is used as the foaming agent, there is no risk of decomposition residue as in the case of the chemical foaming method, and there is no risk of environmental pollution such as destruction of the ozone layer such as fluorocarbon gas.

[Brief description of the drawings]

FIG. 1 is a vertical longitudinal sectional view showing an example of a method for manufacturing a foam tube according to the present invention.

[Explanation of symbols]

 (1): Extruder for foam tube production (2): Foaming agent supply unit (3): Carbon dioxide gas cylinder (4): Metering pump (5): Foam tube mold (6): Inner core (7): Outer mold (8): Bridge (9): Air cooling device (10): Take-off machine (11): Ventilation path (12): Air cylinder (13): Air cylinder (14): Hopper (T): Foam tube

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Claims (3)

[Claims] 1. A low-density polyethylene resin having a density of 0.91 to 0.93 g / cm ³ is heated to a temperature at which free radicals are generated in the presence of a radical generator, or the resin is ionized by radiation. To increase the weight-average molecular weight of the modified low-density polyethylene resin by 1.5 to 10 times, and add a cell nucleating agent to the modified low-density polyethylene-based resin. Is melted in an extruder, and 2 to 5 parts by weight of carbon dioxide gas is dissolved in 100 parts by weight of the modified resin composition. Forming a tube while releasing the same modified resin melt into a tube by blowing it into an atmospheric pressure atmosphere from the mold, and immediately after this forming, cooling the outer peripheral surface of the obtained foamed tube to continuously obtain a foamed tube Features Method of manufacturing that the polyethylene resin foamed tube. 2. The method for producing a polyethylene resin foam tube according to claim 1, wherein the outer peripheral surface of the foam tube is cooled while applying an internal pressure to the foam tube. 3. The addition amount of the cell nucleating agent is 0.3 to 1.0 with respect to 100 parts by weight of the modified low density polyethylene resin.
The method for producing a polyethylene-based resin foam tube according to claim 1 or 2, wherein the amount is set to parts by weight.