JPH1044219A - Production of intermediate and high density polyethylene resin formed body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably improve the low temperature toughness, by cooling a formed body at a specified average cooling speed in the temperature range from the annealing temperature to a specified temperature, after the formed body has been annealed at a temperature low than the melting point. SOLUTION: An intermediate or high density polyethylene resin is melted and formed by a heat-press. After cooling and solidifying by water, it is cooled in the atmosphere to make a sheet. Then, the sheet is annealed at a lower temperature than the melting point and thereafter, it is cooled at an average cooling speed at most 10 deg.C/min in the range from the annealing temperature to 60 deg.C. In this case, the melting point is meant a melting peak temperature measured when a sample formed by cooling at the cooling speed of 25 deg.C/min in the range from 140 deg.C to 80 deg.C, has been subjected to a differential-thermal analysis, while heating it at the rate of 5 deg.C/min. In this way, when the cooling speed after annealing is set at most 10 deg.C/min, a non-ductile temperature lower than an unannealed sample can be obtained, and the lower temperature toughness is improved.

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 medium / high-density polyethylene resin molded article such as a polyethylene pipe for city gas conduits.

[0002]

2. Description of the Related Art Recently, medium and high-density polyethylene pipes, which are lightweight, excellent in workability, flexible and resistant to earthquakes, and have excellent corrosion resistance, have attracted attention as city gas conduits. It is replacing steel pipes that have been used in the past.

[0003] The low-temperature toughness of a polyethylene pipe is determined by the following ND.
T (Nil DutyTemperature)
e: non-ductility temperature), the NDT of the medium density material is-
The NDT of the high-density material is around -35 ° C, around 15 ° C.
Therefore, if a medium-density material is used at a temperature lower than −15 ° C. or a high-density material is used at a temperature lower than −35 ° C., brittle fracture may occur.

[0004] Here, the NDT is a laser having a depth of 2 mm and an angle of 4 mm with a notch of 0.5 mm in depth at the tip.
A Charpy test was performed using a 55 mm long, 10 mm wide, and 5 mm thick test piece provided with a 5 ° V notch under the conditions of a span of 40 mm and an impact speed of 3.8 m / sec. It is the temperature that starts to increase from the side.

[0005] Polyethylene pipes are generally produced by water-cooling and solidification immediately after extrusion molding and then air-cooled after the solidification. Makromol. Che
m. , 105, 167 (1982)], it is reported that the toughness is improved by annealing the thus produced polyethylene resin molded article at a temperature just below the melting point.

[0006]

However, when the present inventors conducted additional tests, there were cases where improvement in toughness was not recognized depending on the annealing conditions.

The present invention has been made in order to solve such problems, and an object of the present invention is to provide a method capable of stably producing a medium / high-density polyethylene molded article having excellent low-temperature toughness.

[0008]

The object of the present invention is to anneal a compact at a temperature lower than the melting point, and then to cool the temperature range from the annealing temperature to 60 ° C. at an average cooling rate of 10 ° C./min or less. And a method for producing a medium / high density polyethylene resin molded article.

Here, the melting point is measured when a sample prepared by cooling at a cooling rate of 25 ° C./min between 140 ° C. and 80 ° C. is subjected to differential thermal analysis while increasing the temperature at 5 ° C./min. It is the melting peak temperature.

The present inventors have studied the effects of the annealing conditions of the molded article for the purpose of improving the low-temperature toughness of the polyethylene resin molded article, and found that the low-temperature toughness is greatly affected by the cooling conditions after annealing. .

FIG. 1 shows the relationship between NDT when a high-density polyethylene resin sheet is annealed at 100 ° C. for 30 minutes and the cooling rate after the annealing.

If the cooling rate after annealing is set to 10 ° C./min or less, an NDT lower than that of the unannealed sample can be obtained.
It can be seen that the low temperature toughness is improved.

Although the cause is not always clear, it is considered that the entanglement (entanglement) of the polymer is weakened by slow cooling, and as a result, crazing (dispersion of cracks) is promoted.

Such effects were not obtained when annealing was performed at a temperature higher than the melting point. Control range of cooling rate is 60 ℃
The cooling rate in the lower temperature range did not affect the low temperature toughness.

The above is a method in which the polyethylene resin molded body once produced is heated and annealed again to control the cooling rate after the annealing. When the polyethylene resin is melt-molded and cooled, the temperature from the melting point to 60 ° C. Range 10
The same effect was obtained even when cooling was performed at an average cooling rate of not more than ° C / min.

This suggests that the state of entanglement of the polymer formed during cooling from the vicinity of the melting point is a dominant factor in low-temperature toughness.

The effect of the present invention was observed for a molded article made of a medium / high density polyethylene resin, but could not be confirmed for a molded article made of a low density polyethylene resin.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention can be applied to any polyethylene resin as long as it is a medium / high density polyethylene resin.

The temperature range from the annealing temperature after annealing and from the melting point after melt molding to 60 ° C. is 10 ° C./mi.
When cooling at an average cooling rate of n or less, the same effect can be obtained even if cooling is performed at a constant rate, or if a heat holding process is performed during cooling.

[0020]

【Example】

(Example 1) Using extrusion-grade high-density polyethylene resin manufactured by Nippon Polyolefin Co., Ltd., melt molding at 190 ° C. for 5 minutes using a hot press under conditions simulating the conventional method, solidify with water, and allow to cool in air. Under the conditions, a sheet having a thickness of 5 mm was prepared. This sheet was annealed using an infrared image furnace under the conditions shown in Table 1 and
5 was created. And NDT was measured by the method mentioned above.

The melt flow rate MFR of the polyethylene resin used was measured at a temperature of 190 ° C. under a load of 2.16 kg according to JIS K6760.
dg / min. In addition, the degree of branching per 1000 carbons was determined according to Reference 2 [Macromolecules,
19, 2722 (1986)] and 13C-NM
It was 1.7 pieces / 1000C when evaluated using R.

Sample 1 is a sample according to the method of the present invention which was annealed at 80 ° C. × 60 min, cooled to 60 ° C. at a cooling rate of 10 ° C./min, and allowed to cool in the air.

Sample 2 is a sample according to the method of the present invention which was annealed at 80 ° C. × 60 min, cooled to 60 ° C. at a cooling rate of 2 ° C./min, and allowed to cool in the air.

Sample 3 is a sample according to the method of the present invention, which is annealed at 120 ° C. × 30 min, cooled to 60 ° C. at a cooling rate of 10 ° C./min, and allowed to cool in the air.

Sample 4 is a sample according to the method of the present invention which was annealed at 120 ° C. × 30 min, cooled to 60 ° C. at a cooling rate of 1 ° C./min, and allowed to cool in air.

Sample 5 is a sample that has not been annealed. Table 1 shows the results.

Samples 1 annealed under the conditions of the method of the present invention
The NDT of No. 4 is −42 ° C. to −45 ° C., which is significantly lower than that of the sample 5 prepared by the conventional method without annealing, and the low temperature toughness is improved by the method of the present invention. You can see that there is.

[0028]

[Table 1]

(Example 2) Using the high-density polyethylene resin used in Example 1, the sheet-shaped samples 6 to 9 having a thickness of 5 mm were prepared under the melt molding conditions (hot press conditions) and the subsequent cooling conditions shown in Table 2. Created. The cooling was performed by changing the conditions of the cold press for rapidly cooling and maintaining the temperature at a constant temperature after hot pressing, or controlling the cooling rate in the temperature range from the melting point to 60 ° C. using an infrared image furnace. And NDT was measured by the method mentioned above.

Sample 6 was heated at 190 ° C. for 5 mi with a hot press.
n at 60 ° C at 100 ° C with a cold press
This is a sample according to the method of the present invention which was allowed to cool in the air after holding n.
The average cooling rate in the temperature range from the melting point of the sample to 60 ° C. was 1 ° C./min.

Sample 7 was heated at 190 ° C. for 5 mi with a hot press.
n is a sample according to the method of the present invention, which was subjected to melt molding, cooled in a temperature range from the melting point to 60 ° C. at an average cooling rate of 3 ° C./min, and then allowed to cool in the air.

Sample 8 was heated at 190 ° C. for 5 mi with a hot press.
n is a sample outside the present invention, which was subjected to melt molding of n and kept at 20 ° C. for 5 minutes in a cold press. From the melting point of this sample
Average cooling rate in the temperature range up to 35 ° C is 35 ° C / mi
n.

Sample 9 was heated at 190 ° C. for 5 mi with a hot press.
This is a sample that simulates the conventional method in which melt molding of n was performed, followed by solidification with water cooling and then cooling in air. At this time, the average cooling rate in the temperature range from the melting point to 60 ° C. is 20 ° C. /
min.

Table 2 shows the results. The NDT of the sample prepared by the method of the present invention was -44 ° C to -45 ° C, and the NDT of the sample prepared outside the conditions of the conventional method and the method of the present invention was -35 ° C to -45 ° C.
This is significantly lower than that at 33 ° C., indicating that the low-temperature toughness is improved by the method of the present invention.

Example 3 Using an extrusion grade medium density polyethylene resin manufactured by Nippon Polyolefin Co., Ltd., the thickness was 5 m under the melt molding conditions shown in Table 2 and the subsequent cooling conditions.
m sample sheets 10 to 13 were prepared. And NDT was measured by the method mentioned above.

The MFR and the degree of branching per 1,000 carbons of the polyethylene resin used were 0.2 dg / min, respectively, as measured by the method shown in Example 1.
It was 8.8 / 1000C.

The sample 10 was 5 m at 190 ° C. in a hot press.
in, and cold-pressed at 80 ° C for 30 mi
This is a sample according to the method of the present invention which was allowed to cool in the air after holding n.
The average cooling rate in the temperature range from the melting point of the sample to 60 ° C. was 2 ° C./min.

Sample 11 was 5 m at 190 ° C. by hot pressing.
This is a sample according to the method of the present invention which was subjected to melt molding in, cooled in a temperature range from the melting point to 60 ° C. at an average cooling rate of 3 ° C./min, and then allowed to cool in the air.

The sample 12 was 5 m at 190 ° C. in a hot press.
in, and 5 minutes at 20 ° C with a cold press
It is a sample outside the present invention retained. From the melting point of this sample, 6
The average cooling rate in the temperature range up to 0 ° C is 35 ° C / m
was in.

Sample 13 was 5 m at 190 ° C. in a hot press.
This is a sample that simulates a conventional method in which melt molding is performed in water, solidified by water cooling, and then cooled in the air. The average cooling rate in the temperature range from the melting point of this sample to 60 ° C. is 20 ° C.
/ Min.

Table 2 shows the results. The NDT of each of the samples prepared by the method of the present invention was −23 ° C. to −24 ° C., which was significantly lower than the NDT of −15 ° C. of the samples prepared outside the conditions of the conventional method and the method of the present invention. It can be seen that the low temperature toughness is improved by the method.

[0042]

[Table 2]

[0043]

As described above, the present invention is constructed as described above, so that it is possible to provide a method for stably producing a medium / high-density polyethylene resin molded article having excellent low-temperature toughness.

[Brief description of the drawings]

FIG. 1 shows a high-density polyethylene resin sheet at 100 ° C.
It is a figure which shows the relationship between NDT at the time of 0 minute annealing, and the cooling rate after annealing.

Claims (2)

[Claims] 1. A molded body is annealed at a temperature lower than its melting point, and then a temperature range from the annealing temperature to 60 ° C.
A method for producing a medium- / high-density polyethylene resin molded article, wherein the molded article is cooled at an average cooling rate of 0 ° C./min or less. 2. When cooling the polyethylene resin after melt molding, the temperature range from the melting point to 60 ° C. is 10 ° C./mi.
characterized by cooling at an average cooling rate of n or less
A method for producing a high-density polyethylene resin molded article.