JPS5915330B2 - Method for producing radiation-resistant polyethylene - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing radiation-resistant polyethylene, and more specifically, to a method for imparting radiation resistance to polyethylene by graft polymerization.

It is. Polyethylene is a crystalline polymer that is used in large quantities as molded products, especially films. However, when exposed to high doses of radiation in the air, it hardens significantly and becomes brittle, reaching the upper limit of its use against radiation.

is thought to be several tens of Mrad or less. The cause of hardening and brittleness due to radiation irradiation is thought to be mainly due to radiation oxidation and accompanying main chain scission.
As a result of intensive research aimed at improving the radiation resistance of polyethylene, the present inventors have discovered that polyethylene film is exposed to acenaphthylene or a mixture of acenaphthylene and an ethylenically unsaturated compound while in gaseous or solution form. It has been discovered that this objective can be achieved by irradiating with ionizing radiation, and the present invention has been completed based on this knowledge.

That is, the present invention is characterized in that acenaphthylene alone or a mixture of acenaphthylene and an ethylenically unsaturated compound is brought into contact with a polyethylene film in a gaseous or solution state, and ionizing radiation is irradiated to cause graft polymerization. In the present invention, polyethylene is formed into a film, and acenaphthylene alone or a mixture of acenaphthylene and an ethylenically unsaturated compound in gaseous or solution form is formed into a film. Graft polymerization can be achieved at a high grafting rate by contacting and irradiating ionizing radiation20.Acenaphthylene is a solid compound at room temperature with a melting point of 93°C, but has a relatively high sublimation vapor pressure.

No. 5 If this acenaphthylene crystal is placed in a vacuum-sealed container containing a polyethylene film and left to stand, the inside of the container will be filled with acenaphthylene vapor, and the polyethylene film will quickly absorb the acenaphthylene vapor and reach a saturated condition in a few minutes. , turns yellow.

EO If this is irradiated with ionizing radiation such as gamma rays, most of the adsorbed acenaphthylene will undergo a graft reaction with polyethylene and be consumed, but new acenaphthylene vapor will be replenished from the crystals present in the container. Therefore, the amount of graft reaction increases almost linearly with time.
At this time, the solid phase polymerization reaction of acenaphthylene present in the form of crystals also proceeds, but since the polymerization rate is slower than the above-mentioned graft reaction, the graft reaction is hardly affected by this. It is also possible to graft react a copolymer of acenaphthylene and an ethylenically unsaturated compound to polyethylene. The ethylenically unsaturated compound used may be a solid monomer such as maleic anhydride or a maleic acid imide derivative, or a liquid monomer such as indene. In this case, acenaphthylene and the ethylenically unsaturated compound are placed in a vacuum sealed container so that they do not come into contact with each other, the vapors of both are adsorbed onto a polyethylene film, and a graft reaction is caused by irradiation with ionizing radiation.
When acenaphthylene or acenaphthylene and an ethylenically unsaturated compound are present in the form of vapor and graft-reacted with a polyethylene film, a relatively large amount of irradiation dose is required due to the low monomer vapor pressure. It has the advantage that post-treatments such as washing and drying to remove unreacted monomers after irradiation are not required.

The graft polymerization of the present invention can also be carried out by dissolving acenaphthylene or acenaphthylene and an ethylenically unsaturated compound in a solvent such as acetone or benzene, immersing a polyethylene film in this solution, and irradiating it with ionizing radiation. can. In this case, after irradiation, it is necessary to remove unreacted monomers attached to the film by washing with a solvent such as methanol, and then dry the film.
Since the radiation irradiation time for graft polymerization is short and there is no effect on the mean free path of monomer molecules, even and uniform grafting is possible. The polyacenaphthylene-grafted polyethylene film obtained according to the present invention has much better radiation resistance than ordinary polyethylene films and radiation crosslinked polyethylene films, and has only a slight resistance to radiation at high doses of 100 Mrad or more. It is only oxidized, the Young's modulus increases little, and the original flexibility of polyethylene is not lost. Generally, as the grafting ratio of polyacenaphthylene increases, the radiation resistance increases, but the effect of suppressing the increase in Young's modulus is particularly remarkable, and even with a grafting ratio of only 2%, this effect is already noticeable. appears clearly.

The reason why such a remarkable effect appears is that the graft reaction mainly proceeds to the amorphous part of polyethylene, and the oxidation of the amorphous part where oxygen can easily diffuse increases the aroma of polyacenaphthylene bonded to polyethylene through graft polymerization. This is thought to be due to the stabilizing effect of the ring.

As mentioned above, the polyethylene grafted with polyacenaphthylene obtained by the method of the present invention has excellent radiation resistance and can be used as a coating for nuclear power equipment, X-ray equipment, etc.
It can be effectively used for purposes such as insulation.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 Thickness 0.030m71 molded by inflation
L low density polyethylene film with a width of about 15C7rL,
Cut into approximately 1 m length, wrap with gauze, insert into a sox-slitter extractor, extract with acetone for 7 hours and methanol for 7 hours, air dry, vacuum dry, and cut into 8 CfL lengthwise and widthwise. This was used as a polyethylene sample.

Next, acenaphthylene crystals 414.7Tf19 purified by recrystallization and sublimation were placed in a 5mm diameter x 3CTI
Put it in a small L test tube and it will have a diameter of 15m7! Place the above two polyethylene samples (each weighing 179 lbs.) inside the test tube.

7m9 and 186.8Tf1f1) were wound and inserted so as not to come into direct contact with acenaphthylene.

Next, a glass tube was welded to the top of this test tube to facilitate sealing, and the test tube was connected to a vacuum device using a mercury diffusion pump and a liquid nitrogen trap, and the bottom of the tube was cooled with liquid nitrogen. 10-3mm
The tube was sealed. The sealed test tube was placed in a 60C0 gamma single-ray irradiation device and irradiated with gamma-ray at a dose rate of 5 x 104 R/hour for 712.9 hours at room temperature, and immediately opened to take out two polyethylene samples.

Two polyethylene samples were immersed in about 100 me of methanol, stirred for 1 hour, replaced with methanol, left overnight, taken out, dried, and weighed.

The weight increase of each sample was 64.0 m9 (grafting rate 35.6%) and 101.1 m9 (grafting rate 54.1%), respectively.
70). Moreover, the polymerization rate of acenaphthylene crystals was 13.070. Grafting rate above 35.670
sample, and as a control, an untreated polyethylene sample and an untreated polyethylene sample were subjected to 6x C0 gamma line under vacuum in a sealed state without the presence of acenaphthylene crystals at room temperature.
For cross-linked polyethylene samples obtained by irradiation for 712.9 hours at a dose rate of 104 R/hr, each sample was measured perpendicular to the machine direction with a width of 8 mT! Cut into L pieces, put two of each into a test tube with a loose stopper, (9.6 x 105
Gamma rays were irradiated in the air at a dose rate of R/hour at 100 Mrad, and the physical properties were measured after being left for a day and night, and compared with the physical properties before gamma ray irradiation.

The results are shown in Table 1. The measurement conditions for the tensile test were a specimen length of 25m77! , width 5.0, tension speed 1
The temperature was 23° C. and 00 mTIL/min. From Table 1,
Compared to the control sample, the polyacenaphthylene-grafted polyethylene sample of the present invention has an optical density increase of 1720 (1-JMoV1-1) derived from the formation of carbonyl groups caused by oxidation and an increase in the optical density of It can be seen that the weight increase rate due to the introduction of is K or less, and the resistance to oxidation due to radiation irradiation has been greatly improved. Also, the control sample showed a rapid increase in Young's modulus. Although the elongation at break has become significantly smaller and the film has hardened and become brittle, the sample of the present invention also shows a small increase in Young's modulus, indicating that it has not lost its elongation and film flexibility. Example 2 Place acenaphthylene 499.61ZfI in a test tube, add purified acetone to completely dissolve acenaphthylene, and add acenaphthylene 499.61ZfI to a thickness of 0.
A polyethylene sample similar to that used in Example 1 having a size of 0.30 mm and a weight of 223.1 mm was immersed, subjected to repeated freezing and evacuation, melting and degassing, and then sealed in a tube in the same manner as in Example 1.

This was irradiated with gamma rays at a dose rate of 4.9 x 104 R/h for 72.4 hours at room temperature, the tube was opened, the film was taken out, and acetone 100771f. After washing thoroughly with water and vacuum drying, the weight of the film increased by 32.0T! 1f
1 (grafting rate 14.3%).

Further, the polymerization rate of acenaphthylene in the solution was 4.670.

When the obtained film was irradiated with gamma rays under the same conditions as in Example 1, there was no increase in Young's modulus and flexibility was maintained.

Example 3 Acenaphthylene 221.27T1f in the same two small test tubes used in Example 1! and purified maleic anhydride 203.
9 tubes were placed separately and these were placed at the bottom of the same 15 mm diameter hard glass test tube used in Example 1.

Claims (1)

[Claims] 1. A method for producing radiation-resistant polyethylene, which comprises irradiating a polyethylene film with ionizing radiation while contacting acenaphthylene alone or a mixture of acenaphthylene and an ethylenically unsaturated compound in gaseous or solution form to cause graft polymerization.