JP2001269980A - Method for recycling crosslinked polyethylene resin composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for recycling a crosslinked polyethylene resin composition as a remolding material or molded articles without the gels and the flow mottles. SOLUTION: A crosslinked polyethylene resin composition is supplied to the resin inlet of the extruder, and fed to the downstream side while the heat and the shearing power are added to the composition. At the same time, the high pressure gas is injected into the barrel of the extruder at least through one aperture opened on the barrel and dissolved in the composition. After that, the composition is discharged from the extruder. This recycling method for the crosslinked polyethylene resin composition is characterized by the above procedures.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a crosslinked polyethylene resin composition used for heat-shrinkable films and wire coatings, which is made of a uniform raw material which does not contain granular unmelted portions and has no flow spots. It relates to a method of reproducing as an object.

[0002]

2. Description of the Related Art Cross-linked polyethylene resin compositions used for insulating coating materials such as heat-shrinkable films and electric wires include:
Melt fluidity is reduced due to the crosslinked structure of the molecule. Among these cross-linked polyethylene resin compositions, those having a high degree of cross-linking are difficult to recycle by a usual extrusion method. For this reason, an uncrosslinked thermoplastic polymer is added to the crosslinked polyethylene resin composition, and at a temperature equal to or higher than the melting temperature of the thermoplastic polymer, a shear force is applied using a twin screw kneading extruder or a stone mill type kneading extruder. Attempts have been made to obtain a material that can be melt-molded by kneading while stirring (Japanese Patent Laid-Open No. 10-36600). However,
By using such a method, although it is possible to perform extrusion molding, the resulting molded product has granular unmelted portions (hereinafter, referred to as gel) and flow unevenness due to non-uniformity of flow characteristics. Therefore, it has been difficult to use it for precision molded products and films requiring transparency.

In the case of a crosslinked polyethylene resin composition having a relatively low degree of crosslinking, extrusion can be performed without the addition of a thermoplastic polymer.
There was the same problem as in the case of the highly crosslinked polyethylene resin.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for regenerating a crosslinked polyethylene resin composition as a regenerated raw material or a molded product free from gels and spots.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems. As a result, they have found that the above-mentioned problems can be solved by heating and kneading while dissolving a gas in a high-pressure state in a crosslinked polyethylene resin composition, and have reached the present invention. That is, the present invention supplies a crosslinked polyethylene resin composition to a resin supply port of an extruder, feeds the composition to the downstream side while applying heat and shearing force, and at least one location in a barrel of the extruder. A method for regenerating a crosslinked polyethylene resin composition, comprising: injecting a gas in a high-pressure state from a gas introduction hole provided in the composition, dissolving the gas in the composition, and then discharging the gas from an extruder. Things.

Another object of the present invention is to provide a method for regenerating the above crosslinked polyethylene resin composition, wherein the gas is carbon dioxide.

Further, the present invention provides a method for regenerating any of the above crosslinked polyethylene resin compositions, wherein the extruder is a co-directional twin screw extruder.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The polyethylene resin in a state before crosslinking of the crosslinked polyethylene resin which is a main component of the crosslinked polyethylene resin composition which can be regenerated by the present invention is a homopolymer of ethylene and / or ethylene and a small amount of ethylene. Is a copolymer with a monomer. The other monomers referred to herein are not particularly limited, but metal ion neutralization of α-olefins such as propylene and 1-hexene, vinyl acetate, alkyl (meth) acrylate, (meth) acrylic acid, and (meth) acrylic acid. Objects and the like. And the cross-linked polyethylene resin composition referred to in the present invention is a chemical method using a cross-linking aid in the polyethylene resin, a lightly introduced cross-linked structure using a method such as a method using ionizing radiation, A highly crosslinked structure is introduced by using a method such as a chemical method using a crosslinking aid in the polyethylene resin, a method using ionizing radiation, and / or a compound obtained by mixing an uncrosslinked polyethylene resin with , Etc. These further contain thermoplastic resins other than polyethylene resins; stabilizers such as antioxidants, light stabilizers, and ultraviolet absorbers; fillers such as talc, calcium carbonate, mica, and titanium oxide; and small amounts of coloring agents. You may go out.

[0009] Of the above-mentioned crosslinked polyethylene resin compositions, the melting point is usually high in many cases, and according to the present invention, the processing temperature conditions of the extruder are set so as to exceed the melting point, so that the recycled processing can be performed. Is possible. Therefore, the present invention can be applied to such a crosslinked polyethylene resin composition having a low degree of crosslinking by itself. On the other hand, among the above-mentioned crosslinked polyethylene resin compositions, the melting point does not clearly appear as the degree of crosslinking increases,
Fluidity decreases. When applying the present invention to such a crosslinked polyethylene resin composition having a high degree of crosslinking, the load applied to the extruder increases, and kneading may become difficult. In such a case, a predetermined amount of an uncrosslinked polyethylene-based resin is mixed in advance with the crosslinked polyethylene-based resin composition, and the mixture is extruded at a temperature exceeding the melting point of the uncrosslinked polyethylene-based resin as described above. In addition, the load on the extruder can be reduced, and the target reprocessing can be easily performed.

In the method for regenerating the crosslinked polyethylene resin composition of the present invention, the kneading of the crosslinked polyethylene resin composition is carried out while dissolving a gas having a plasticizing effect on the resin composition under high pressure. Is a major feature. By dissolving the gas in the crosslinked polyethylene resin composition as described above, the fluidity of the crosslinked molecules can be significantly improved. Then, by kneading in a state in which such fluidity is improved, the gel derived from the crosslinked structure can be crushed, and unevenness in melt fluidity can be eliminated.

The gas used in the present invention is not particularly limited as long as it is a substance which is a gas at normal temperature and normal pressure. For example, nitrogen, carbon dioxide, nitrous oxide, argon, neon, helium, oxygen And an organic gas such as a fluorocarbon gas and a low molecular weight hydrocarbon gas. These may be used alone or in combination of two or more. In addition, among these gases, the influence on the environment is small, and the inorganic gas which does not need to be recovered, particularly carbon dioxide from the meaning that the solubility and the plasticity-imparting effect in the crosslinked polyethylene resin composition are superior to other gases. Is most preferred.

The extruder used in the present invention can feed the crosslinked polyethylene resin composition from the upstream side where the resin supply port is located to the downstream side where the discharge port is located while applying heat and shearing force to the crosslinked polyethylene resin composition. It is not particularly limited as long as it has a gas introduction hole capable of injecting a gas in a high-pressure state at at least one position in the length direction of the barrel, and it may be a single screw extruder or a twin screw extruder. Is also good. In the case of a twin screw extruder, the directions of rotation of the screws may be the same or different. However, among these, the co-axial twin-screw extruder has a large L / D value (value obtained by dividing the length of the screw by the diameter), and has a high effect of kneading the gas and the crosslinked polyethylene resin composition. Most preferred.

The shape of the screw arranged in the barrel of the extruder is not particularly limited. For example, as shown in FIG. As shown in -B, a shape in which kneading blocks are combined in multiple stages in a spiral staircase with a specific twist angle (hereinafter, referred to as a kneading block assembly) can be assumed. Also, segments having different shapes can be used in combination. That is, a screw having both a flight-shaped portion and a kneading block assembly can be assumed. The number of spiral flights and kneading block assemblies is not particularly limited, but one to three are generally used.

The means for supplying the crosslinked polyethylene resin composition to the extruder used in the present invention is not particularly limited, but when a single-screw extruder having good quantitative supply properties is used, a hopper can be mainly used. When using a twin-screw extruder, it is desirable to use various quantitative feeders. The shape of the crosslinked polyethylene resin composition to be charged into the resin supply port is not particularly limited, but when using a hopper or a quantitative feeder, the average particle size is several to
A pellet having a shape of several tens of mm is preferable.

Various dies for extrusion can be attached to the discharge port located downstream of the extruder. The extruded molded product can be cut into several mm and regenerated as a raw material, or can be used as it is. It is also possible to connect to an injection plunger of an injection molding apparatus or to a raw material supply section of another extruder. Further, a fixed amount supply device such as a gear pump may be attached.

As a press-in device for press-injecting the above-mentioned inert gas into the barrel of the extruder used in the present invention, a compressor can be used when nitrogen or argon which is a non-coagulable gas is used. When carbon dioxide or nitrous oxide which is a cohesive gas is used, it is possible to cool the gas into a liquid state and then press-fit it with a pressure-resistant pump. The gas injection pressure must be at least higher than the pressure in the barrel at the location of the gas inlet. Further, as the position of the gas introduction hole for injecting the high-pressure gas into the extruder, when the position of the most upstream part of the barrel of the extruder is 0 and the position of the most downstream part of the barrel is L, 0.3L to It is preferably provided at a position of 0.8 L, more preferably at a position of 0.5 L to 0.8 L. This is because, if it is too close to the supply section, the cross-linked polyethylene resin composition is in a non-molten state, so that there is a problem that gas is blown out from the resin supply port of the extruder. Also, if the position of the gas inlet is too close to the outlet, the kneading of the molten resin and the gas is insufficient,
There are problems that the gas cannot be sufficiently dissolved in the molten resin to achieve the purpose of the present invention, and that the gas blows out to the outlet.

As a means for avoiding the problem that the gas blows out from the raw material supply port or the discharge port, a screw located at the upstream side and / or the downstream side of the gas injection hole is partially provided with a reverse screw portion, and a reverse screw portion is provided. Reverse feed of crosslinked polyethylene resin composition by screw part (flow from downstream to upstream)
And antagonize the forward feed (flow from the upstream to the downstream) of the polyethylene resin by the forward thread portion, so as to increase the filling density of the polyethylene resin into the barrel near this point and enhance the gas sealing effect. Is preferred.

[0018]

Next, the present invention will be described in detail with reference to Examples and Comparative Examples for the apparatus for manufacturing a resin molded article of the present invention. However, these are merely examples, and the present invention is not limited to these. It is not limited. In the following Examples and Comparative Examples, the melt flow rate (MFR) used as a parameter indicating the flowability of the resin is JIS K6760 which is generally used for polyethylene resins.
Not measured using the method specified in
Test condition 17 in K7210 (load 10 kgf,
(Temperature 230 ° C.). This is because it is difficult to evaluate the flowability of a crosslinked polyethylene-based resin composition having reduced flowability due to cross-linking by the method defined in JIS K6760. On the other hand, the measurement of the internal haze as an indicator of the flow spot of the films obtained in the examples and the comparative examples was obtained by subtracting the value of the external haze from the value of all the hazes measured according to ASTM D1003.

[Embodiment 1] FIG. 1 shows an outline of an apparatus used in carrying out the present invention. A hopper 4 is attached to the resin supply port, a strand die 8 with an inner diameter of 3 mm is attached to the discharge port, and a high-pressure gas injection device is connected to a gas introduction hole 300 mm downstream from the resin supply port (about 350 mm to the discharge port). An extruder (screw diameter: 26 mm, screw effective length: 650 mm, L / D: 25) was used. In order to prevent gas from blowing out to the resin supply port and the resin discharge port, the shape of the screw was as shown in FIG.
That is, the screw shape of the gas introduction hole portion is a forward screw flight shape (indicated by the flight being continuous from upper left to lower right), and the phases of 13 kneading blocks are provided on the upstream and downstream sides. Reverse screw section consisting of a kneading block assembly shifted by 30 ° (indicated by the top of each kneading block filled in black being continuous from lower left to upper right) (1)
(Length: 04 mm). All other parts of the screw had the same thread flight shape as the gas inlet hole.

As the crosslinked polyethylene resin composition, linear low-density polyethylene (Sumikacene α · FZ101-0 manufactured by Sumitomo Chemical Co., Ltd., density: 0.925 g / c)
m ³ , MFR: 7.0 g / 10 min) with a thickness of 250 μm
m sheet, and crosslinked by irradiating an electron beam with 5.0 Mrad (acceleration voltage 250 kV) using an electron beam irradiation apparatus. In addition, the MFR value of this cross-linked polyethylene was reduced to about 1.3 g / 10 min. The crosslinked polyethylene sheet was pulverized into a square of about 3 mm × 3 mm using a crusher. Next, the crosslinked polyethylene ground product thus obtained was supplied from the hopper into the barrel of the extruder to drive the screw, thereby applying heat and shearing force. The barrel temperature was 200, 2 from the hopper side to the downstream side of the extruder.
05, 210, 210 and 210 ° C., and the screw rotation speed was set to 80 rpm.

At the same time, while cooling and liquefied carbon dioxide is pressurized to a pressure of 80 kg / cm ^{2 by} a plunger pump, it is injected into the barrel through a gas introduction hole,
The carbon dioxide gas vaporized in the barrel and the crosslinked polyethylene were melt-kneaded. Then, it was extruded into a strand from a strand die set at 210 ° C. The obtained strand was a foam having uniform and fine cells, and it was confirmed that the gas and the molten resin were sufficiently kneaded.

The foam thus obtained was again pulverized by a crusher into a shape of about 3 mm square, and this was subjected to film formation with a thickness of about 80 μm using a conventional film forming apparatus equipped with a T-die in a single screw extruder. Was formed.

Example 2 Linear low-density polyethylene (Sumikacene α · FZ101-0 manufactured by Sumitomo Chemical Co., Ltd.)
Density: 0.925 g / cm ³ , MFR: 7.0 g / 10
min) sheet (250 μm thickness)
Irradiation with an electron beam (acceleration voltage: 250 kV) was performed by setting the MFR value to 0.2 g / 10 min and adding 100 parts by weight of uncrosslinked polyethylene (Sumikasen α manufactured by Sumitomo Chemical Co., Ltd.).
A film having a thickness of about 80 μm by the same method as in Example 1 except that the crosslinked polyethylene resin composition obtained by mixing 10 parts by weight of FZ101-0) is supplied to the hopper of the co-rotating twin screw extruder described above. Was formed.

Comparative Example 1 A film having a thickness of about 80 μm was formed in the same manner as in Example 1 except that carbon dioxide was not injected.

Comparative Example 2 A film having a thickness of about 80 μm was formed in exactly the same manner as in Example 2 except that carbon dioxide was not injected.

Table 1 shows the internal haze values of the films obtained in Examples 1 and 2 and Comparative Examples 1 and 2, and the state of the gel visually observed.

[0027]

[Table 1]

From Table 1, it can be seen that using the method for regenerating a crosslinked polyethylene resin of the present invention, the molding regenerated by heating and kneading while dissolving a high-pressure gas in the crosslinked polyethylene resin composition in an extruder. The product has a low internal haze value and, as is evident from the result of visual observation, shows that the generation of gel is small and the uniformity of the resin is good.

[0029]

As described above, the method for regenerating a crosslinked polyethylene resin of the present invention makes it possible to regenerate a crosslinked polyethylene resin composition as a useful material. That is, the regenerated material obtained by the present method requires precision molded products and transparency, which were difficult due to the occurrence of gels and flow spots caused by non-uniformity of flow characteristics in those obtained by the conventional method. Application to films and the like is possible. As described above, the present invention opens up the way as a useful recycling resource for waste crosslinked polyethylene resin which has been conventionally buried or incinerated and discarded, and can be said to be useful.

[Brief description of the drawings]

FIG. 1 is a schematic view of an apparatus used in Examples and Comparative Examples.

FIG. 2 is a schematic view of a main part of a screw of an extruder used in Examples and Comparative Examples.

FIG. 3 is a view showing a shape of a screw that can be used in the present invention.

[Explanation of symbols]

 1. Recycling device for crosslinked polyethylene resin 2. Barrel 3. Screw 4. Hopper 5. Reduction gear 6. Motor 7. Breaker plate 8. Strand dice 9. Gas cylinder 10. Cooler 11. Pressure-resistant plunger pump 12. Safety valve 13. Pressure gauge 14. Pressure indicator 15. Valve 16. Check valve 17. Gas inlet

Continued on the front page (72) Inventor Takashi Moriyoshi 2-1-1 Minamijomishima-cho, Tokushima City, Tokushima Prefecture Inside the Faculty of Engineering, Tokushima University (72) Inventor Yasuhiro Uozaki 2-1-1 Minamijomishima-cho Tokushima City, Tokushima Tokushima University Within the Faculty of Engineering (72) Inventor Tsutomu Nakanishi 2217-43 Hayashi-cho, Takamatsu-shi, Kagawa Prefecture Inside the High-Temperature-High-Pressure Fluid Research Laboratory (72) Inventor Kazuaki Hata 2217-43 Hayashi-cho, Takamatsu-shi, Kagawa Pref. Reference) 4F207 AA04 AG20 KA01 KA11 KE06 KF02 KK04 KK13 KW33 4F301 AA13 AC01 BA21 BC02 BC13 BC23 BC26 BD08 BE01 BE18 BE29 BF12 BF16 BF32 BG07 BG17 BG19 CA09 CA11 CA32 CA51 CA73

Claims (3)

[Claims] 1. A crosslinked polyethylene-based resin composition is supplied to a resin supply port of an extruder and fed to the downstream side while applying heat and shearing force to the composition, and at least one portion is provided in a barrel of the extruder. A method for regenerating a crosslinked polyethylene resin composition, comprising: injecting a high-pressure gas through a gas introduction hole provided therein, dissolving the gas in the composition, and then discharging the gas from an extruder. 2. The method for regenerating a crosslinked polyethylene resin composition according to claim 1, wherein the gas is carbon dioxide. 3. The method for regenerating a crosslinked polyethylene resin composition according to claim 1, wherein the extruder is a co-directional twin screw extruder.