JP2010106077A - Crosslinking method of high molecular weight polymer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a crosslinking method of a high molecular weight polymer, the method obtaining a polymer structure maintaining a mechanical strength even at a temperature higher than the melting point of the structure by progressing a radiation crosslinking of a high molecular weight polymer such as a polyethylene using both a gamma ray and an electron beam with a radiation quantity less than that as irradiating singly with an electron beam. <P>SOLUTION: The crosslinking method of a high molecular weight polymer crosslinks a high molecular weight polymer by using the gamma ray and the electron beam as a radiation and by irradiating a high molecular weight polymer alternately with a gamma ray and an electron beam in such radiation quantities that a total radiation quantity of only a gamma ray is 100 kGy or more and 300 kGy or less; a total radiation quantity of only an electron beam is 200 kGy or more and 500 kGy or less; and the total radiation quantity of radiation quantities of a gamma ray and an electron beam is in a range of 400 kGy or more and 600 kGy or less. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for crosslinking a polymer, and more particularly to a method for crosslinking a polymer by radiation irradiation in which a plurality of gamma rays and electron beams are irradiated alternately.

Polyolefins, which are typical industrial polymers, are used in large quantities in a wide range of industrial fields (containers, insulating materials, etc.) because of their low cost and ease of processing.
However, since these polyolefins have a characteristic of thermoplastic polymers that flow when the temperature exceeds the softening point (in this case, the melting point) and can no longer maintain their original shape, they are not suitable for high temperature environments such as automotive packaging. Its use is difficult. Therefore, a method is adopted in which they are three-dimensionally cross-linked to form a three-dimensional network structure and maintain the mechanical strength at high temperatures.

  As a crosslinking method as a technique for making this polyolefin resin into a three-dimensional crosslinked structure, an organic thermistor subjected to crosslinking treatment using a silane coupling agent is disclosed in Patent Document 1. In this case, since there are many reactive groups in low-density polyethylene, it is easy to crosslink by causing a reaction with a silane coupling agent. However, when high-density polyethylene is used, the effect is not seen so much. Causes a problem that the affinity with the resin deteriorates, the silane coupling agent oozes out on the resin surface, and is also expensive.

As another crosslinking method, a radiation crosslinking method by irradiation with an electron beam or the like has been proposed.
For example, Patent Document 2 discloses crosslinking by irradiation of an olefin resin sheet containing an organic peroxide that is difficult to handle environmentally with a low voltage electron beam of 50 to 400 KeV in an oxygen concentration atmosphere of 1000 ppm or less. Yes.

  Further, Patent Document 3 discloses that a mixture containing linear polyethylene and conductive particles is irradiated with 40 to 300 kGy using an electron accelerator having an acceleration voltage of 250 kV or more, but the resin of Patent Document 3 is low. It is a density resin, that is, a resin that undergoes crosslinking even at a low dose.

  According to Non-Patent Document 1, the crosslinking of polyethylene by electron beam irradiation achieves a gel fraction of 90% at a dose of 500 kGy in the case of low density polyethylene, but in the case of high density polyethylene, the gel fraction is about 80%. In addition, since there are many crystal parts, it is difficult for the crosslinking reaction to proceed. In addition, when the crystal structure like polyethylene is included in it, it is said that a crosslinking reaction occurs in an amorphous part (amorphous part).

This crosslinking method by radiation has a very high initial cost due to the electron beam irradiation apparatus, but has the advantage that the crosslinking proceeds in a short time. Recently, as in the case of sterilization by radiation, crosslinking of rubber, polypropylene, and polyolefin resin is performed. Widely used (see, for example, Non-Patent Document 2).
JP 2000-82602 A Japanese Patent No. 3090695 JP 2003-347106 A Masahiko Imai, “Crosslinking of polyethylene film with low energy electron beam”, Convertech, February 1995, p. 36-39. Mukai Sadayoshi, Nakai Koji, “Advances in Electron Beam Applied Technology”, Nissin Electric Technical Report, Nissin Electric Co., Ltd., July 1995, Vol. 40, No. 2, p. 10-19.

As described above, radiation crosslinking techniques have been developed for various applications, but some problems have been caused by the radiation used.
Gamma ray irradiation penetrates widely and deeply to effect crosslinking. In other words, gamma rays take longer to obtain the required irradiation dose than electron beams, but they are highly penetrating and suitable for thick products. Furthermore, since gamma rays have a low dose per hour (dose rate), heat generation due to activation is sufficient. Since the product is diffused and released, there is an advantage that the heat of the product does not rise, and there is little thermal deformation and residual decomposition gas (hydrogen) in the resin. In addition, regarding the cross-linking reaction by gamma ray irradiation, the radicals generated by radiation recombine with gamma rays, the contraction stress generated there is sufficiently relaxed, and the next recombination reaction takes place. The effect of being obtained is also expected.

  However, it is not suitable for irradiation requiring a long irradiation time and a large dose, and when it is irradiated for a long time in an oxygen atmosphere, oxygen diffuses inside the resin (product) and reacts with radicals to produce peroxy radicals. Occurrence and mechanical deterioration of the resin.

  On the other hand, although cross-linking by electron beam irradiation depends on the acceleration voltage, it is difficult to uniformly cross-link thick objects because of its remarkably low penetrating power compared to gamma rays. In addition, since the dose rate per hour is high and causes a reaction due to a large amount of electron beam radiation per unit time, there is a merit that a large amount of product can be irradiated in a short time, but in order to induce a reaction in a short time, In the case of polyolefin or the like, there is a problem in that hydrogen gas generated by the cross-linking reaction may cause swell (void) in the resin or discoloration due to resin alteration.

  In view of such a situation, the present invention uses both electron beams and gamma rays for radiation crosslinking of a polymer such as polyethylene, and proceeds with radiation crosslinking at a lower irradiation dose than irradiation with an electron beam alone, exceeding the melting point. However, it is an object of the present invention to provide a method for crosslinking a high molecular polymer to obtain a high molecular structure having mechanical strength.

  That is, the first invention of the present invention is a polymer characterized in that gamma rays and electron beams are used as radiation, and the polymer is crosslinked by alternately irradiating the polymer with the radiation. A method for crosslinking a polymer is provided.

  The second invention of the present invention uses gamma rays and electron beams as radiation, and irradiates the polymer polymer in the order of at least gamma ray irradiation and electron beam irradiation. The present invention provides a method for crosslinking a polymer, which comprises crosslinking a polymer.

  Furthermore, the third invention of the present invention uses gamma rays and electron beams as radiation, and irradiates the polymer with gamma rays, followed by electron rays, and then in this order. Thus, the present invention provides a method for crosslinking a polymer, wherein the polymer is crosslinked.

  The fourth invention of the present invention uses gamma rays and electron beams as radiation, and irradiates the polymer polymer in the order of at least electron beam irradiation and gamma ray irradiation. The present invention provides a method for crosslinking a polymer, which comprises crosslinking a polymer.

  Furthermore, a fifth invention of the present invention uses gamma rays and electron beams as radiation, and the polymer polymer is irradiated in the order of irradiation of electron beams, then irradiation of gamma rays, and then this repetition. Thus, the present invention provides a method for crosslinking a polymer, wherein the polymer is crosslinked.

  In addition, a sixth invention of the present invention is the high polymer material according to any one of the first to fifth inventions, wherein the polymer is a kind of polyolefin resin selected from polyethylene, polystyrene, and polypropylene. A method for crosslinking a molecular polymer is provided.

  Further, according to a seventh aspect of the present invention, the total irradiation amount of only the gamma rays is 100 kGy to 300 kGy, the total irradiation amount of only the electron beams is 200 kGy to 500 kGy, and the total irradiation amount of the gamma rays and the electron beams is The combined irradiation amount of the high molecular weight polymer according to any one of claims 1 to 6, wherein the total irradiation dose is 400 kGy or more and 600 kGy or less.

  According to the present invention, by alternately irradiating a high-density polymer with an electron beam and a gamma ray, crosslinking at an equivalent level or more proceeds at a lower radiation dose than when irradiating the electron beam alone, Even when the melting point of the density polyethylene exceeds 140 ° C., the polymer does not melt and a high molecular polymer maintaining mechanical strength can be obtained.

  The polymer used in the present invention is a thermoplastic polymer having an amorphous part, such as polyethylene, polystyrene, polypropylene, polymethyl methacrylate, polyvinyl chloride, olefin copolymer. And other thermoplastic polymers.

  Among these thermoplastic polymers, it is suitable for crosslinking of polyolefin resins such as polyethylene, polystyrene, and polypropylene, and further suitable for polyethylene, particularly low density polyethylene and high density polyethylene.

Here, the low density polyethylene refers to polyethylene having a density of 0.910 to 0.929 g / cm ³ . A material having a density of 0.930 to 0.941 g / cm ³ is referred to as a medium density polyethylene, and a material having a density of 0.942 g / cm ³ or more is referred to as a high density polyethylene.

  In general, low-density polyethylene is produced by a high-pressure method, that is, a high-pressure radical polymerization method of 1000 atm or more, and includes long-chain branches in addition to short-chain branches such as ethylene groups. High-density polyethylene is produced by coordination anionic polymerization using a transition metal catalyst under medium and low pressures of several tens of atmospheres or less, and is linear. In many cases, the polymer is used in the form of a sheet or a part, but the polymer may be mixed with an additive according to the required performance to produce a mixture.

  The radiation crosslinking method of the present invention eliminates the problems of each of gamma rays and electron beams by alternately irradiating a plurality of radiations, for example, gamma rays and electron beams, tensile strength at a temperature exceeding the melting point of the resin, etc. The mechanical strength is maintained and crosslinking is performed to maintain the shape.

In the electron beam irradiation, considering the temperature rise of the irradiated object due to the irradiation, the radiation dose per irradiation (several seconds) is 30 to 40 kGy, and the reaction is said to be fast with a short irradiation time. The heat absorption is large and the generation of heat is large. Due to the generated heat, a large amount of hydrogen gas generated at the time of radiation crosslinking may be generated, which may cause swelling and foaming of the resin. In order to prevent this, when a large amount of radiation is required, it is irradiated in several steps.
In addition, since the penetrating power of electron beams with a large thickness is weak, it is difficult to uniformly crosslink the product beyond a certain thickness.

On the other hand, since the energy of gamma rays is smaller than that of electron beams, irradiation with gamma rays requires a longer irradiation time to obtain the same crosslinking effect, but the radical reaction is slow and the electron beam The amount of gas generated per unit time that causes problems with irradiation is small, and there are few defects such as blistering and discoloration. Also, gamma rays have strong penetrating power and are suitable for irradiation of thick parts.
However, there is a limit to the dose that can be irradiated per day, and there is a problem that it takes time to irradiate a large amount of irradiation.

  Therefore, as a result of investigating a cross-linking method that maximizes the effective effect by solving the problems of the cross-linking method of irradiating each independently by irradiating a combination of gamma rays and electron beams alternately, the following invention was achieved. It is a thing.

In other words, when gamma rays are first irradiated, the amount of radicals generated by the irradiation is small, but the crosslinking reaction proceeds gradually after irradiation, and then a lot of radicals are generated at once by electron beam irradiation. Cross-linking reaction is further promoted by capturing and bonding the radicals generated by the irradiation of gamma rays. Although there are still unclear points regarding this crosslinking mechanism, the degree of crosslinking is clearly higher when gamma ray irradiation and electron beam irradiation are alternately performed than when gamma ray irradiation alone or electron beam irradiation alone is used for the same dose.
Therefore, first irradiate gamma rays, then irradiate electron beams, and then continue to irradiate alternately in the order of gamma rays and electron beams, or irradiate gamma rays after irradiating electron beams, and then continue to irradiate electron beams and gamma rays. By irradiating radiation alternately in this order, efficient radiation crosslinking is achieved.

  The irradiation amount per irradiation of gamma rays to be irradiated is preferably 100 to 300 kGy. However, since the allowable irradiation amount per day of gamma rays is about 50 kGy, this irradiation requires a period of 2 to 6 days. . When the irradiation amount of gamma rays is less than 100 kGy, the amount of generated radicals is small and the crosslinking effect is not seen so much. When it exceeds 300 kGy, the number of days is 6 days or more, and the irradiation time is lengthened and the manufacturing cost is increased.

Next, it is desirable that the amount of irradiation per one electron beam irradiation is 200 to 500 kGy. A large number of radicals are generated by irradiation, and the radical reaction proceeds in parallel with the radicals generated by the previous irradiation of gamma rays. A cross-linking reaction proceeds efficiently due to a synergistic effect.
When the irradiation amount is less than 200 kGy, the generated radicals are not sufficient and the crosslinking is insufficient. When the irradiation amount exceeds 500 kGy, the cross-linking reaction of the polymer is sufficiently advanced, but it is efficient at a low irradiation dose. It was limited because it was not a crosslinking method. As an irradiation method, irradiation with an irradiation amount of about 20 to 40 kGy may be performed in several degrees so that the temperature of the irradiated object does not exceed 70 ° C.

  The irradiation conditions required for crosslinking are as follows. The total irradiation amount (the total irradiation amount of only gamma rays and the total irradiation amount of only electron beams) is 400 kGy or more and 600 kGy or less as shown in FIG. The total irradiation dose of 100 to 300 kGy and the irradiation with an electron beam are divided into several times so that the temperature of the irradiated object does not increase, and the total irradiation dose is 200 to 500 kGy, and the resin is generated by radicals generated by gamma rays and electron beams. Is crosslinked.

  Here, when the total irradiation amount is less than 400 kGy, the polymer cannot be sufficiently crosslinked even with the crosslinking method of the present invention, and the irradiation with the total irradiation amount exceeding 600 kGy advances the crosslinking. However, this is not preferable because it takes a long time to irradiate, resulting in a decrease in productivity and high cost.

This alternate irradiation of gamma rays and electron beams is suitable for the case where the increase in temperature due to the cross-linking reaction is suppressed and the increase in temperature is hated as compared with the case of only electron beam irradiation due to the addition of gamma rays.
There is also a method of increasing or decreasing the number of irradiations within a range of the total irradiation amount of gamma rays and electron beams in an alternating manner, such as gamma ray irradiation 150 kGy, then electron beam 150 kGy, then gamma ray 150 kGy irradiation, and then electron beam 150 kGy irradiation. Can be taken.

  In addition, when the electron beam is irradiated first, a large amount of radicals are generated at one time, and then gradually crosslinked by irradiation with gamma rays, the degree of crosslinking after irradiation is slightly lower than when gamma rays are irradiated first. However, there are few practical problems, and it is desirable to use them properly according to work efficiency and required performance.

The degree of progress of the crosslinking reaction of the polymer is evaluated by dynamic thermomechanical measurement (DMA) and differential scanning calorimetry (DSC) of the polymer after irradiation.
Dynamic thermomechanical measurement is an evaluation method in which a strain or stress that changes (vibrates) with time is applied to a sample, and the strain and stress generated thereby are measured to measure the mechanical properties.

In this example, the temperature was raised from room temperature to a temperature equal to or higher than the melting point of the resin (160 ° C.), the force of the sample repelled against the applied stress was measured, and the degree of progress of the crosslinking reaction was increased after irradiation. It evaluated by measuring the storage elastic modulus of a molecular polymer.
Here, the higher storage elastic modulus is desirable because the polymer has a repulsive force even at a high temperature, indicating that the crosslinking reaction is proceeding.

  By the way, the relationship between the amount of electron beam irradiation in the crosslinking of polyethylene and the value of dynamic thermomechanical measurement is that when irradiated with an electron beam of 1000 kGy, the storage elastic modulus was 3650 MPa, and no discoloration was seen in the appearance. When the electron beam was irradiated up to 1200 kGy, the storage elastic modulus decreased to 3620 MPa and the appearance was slightly changed to brown.

Similarly, when the gamma ray was irradiated at 1000 kGy, the storage elastic modulus was 385 MPa and no discoloration was observed in the appearance, but when the gamma ray was irradiated up to 1200 kGy, the storage elastic modulus decreased to 3840 MPa and the appearance was also changed to brown. It was.
These discoloration seems to be due to the decomposition of the resin of the irradiated object, and the crosslinking of polyethylene is considered to have a maximum dose of 1000 kGy irradiation for both electron beams and gamma rays.

Differential scanning calorimetry is an evaluation method in which the temperature of a sample portion composed of a sample and a reference material is changed to be constant and the temperature difference between the sample and the reference material is measured.
In the examples, the temperature was raised from 0 ° C. to 160 ° C., returned to 0 ° C., once the resin was returned to its original state, again raised from room temperature to 160 ° C., and the resin was recrystallized in the process. We focused on the endothermic peak.

  In differential scanning calorimetry, the progress of crosslinking can be known from the relationship between the crystal size of the polymer and the heat of fusion. That is, since crosslinking occurs in the amorphous part of the polymer (in this case, high-density polyethylene), the crosslinked structure of the amorphous part is preserved after dissolution. If cooled as it is, the crystal cannot grow freely during the crystallization due to the restriction of the molecular chain movement by the cross-linking part. As a result, the crystal size becomes smaller than that before melting, and the heat of dissolution of crystal melting also becomes smaller.

  The electron beam 1000 kGy irradiation is the maximum irradiation amount before the deterioration starts, and the value of the differential scanning calorimetry at that time is 3.27 cal / g. If the value is equal to or smaller than this value, the degree of cross-linking is equivalent to an electron beam of 1000 kGy.

Regarding the relationship between the irradiation amount of gamma rays and electron beams for polyethylene and the progress of crosslinking, first, the ease of the crosslinking reaction at low irradiation doses, the change in elastic modulus and endothermic change per unit dose of the storage modulus. Evaluate by seeking. The greater the change in elastic modulus, the easier the crosslinking proceeds even at lower doses.
On the other hand, as a measure of the degree of progress of crosslinking, the degree of progress of crosslinking (crosslinking degree A) determined from the storage modulus and the degree of progress of crosslinking (crosslinking degree B) determined from the endothermic amount are obtained and evaluated.

First, the elastic modulus change and endothermic change per unit dose of the storage elastic modulus are obtained from the following formula 1. In Equation 1, ΔE is a change in elastic modulus per unit dose of the storage elastic modulus, ΔT is a change in endothermic amount per unit dose of the storage elastic modulus, E _m is a measured storage elastic modulus, T _m is a measured endothermic amount, and E ₀ bridging The storage elastic modulus of the previous polyethylene, T ₀ is the endothermic amount of the polyethylene before crosslinking, and R _t is the total irradiation amount.

Next, FIG. 2 shows the relationship between the doses of gamma rays and electron beams applied to polyethylene and the respective storage elastic moduli, and FIG. 3 shows the relationship between each endothermic amount.
In the case of gamma ray irradiation, the storage elastic modulus shows a tendency for the storage elastic modulus to be flat from around the irradiation amount of 1000 kGy, and in the case of electron beam, the storage elastic modulus shows a tendency to be flat from the vicinity of the irradiation amount of 1000 kGy. It seems that the degree of progress of the crosslinking reaction, that is, the limit of the degree of crosslinking is shown.
In addition, it is understood that the crosslinking reaction is not sufficiently progressed in polyethylene by irradiation with a single radiation of about 600 kGy.

  Regarding the endothermic amount, similarly to the storage elastic modulus, in the case of gamma rays, the endothermic amount shows a peak near an irradiation amount of 1000 kGy, and in the case of an electron beam, the endothermic amount shows a peak near an irradiation amount of 1000 kGy. It seems to indicate the degree of progress of the crosslinking reaction, that is, the limit of the degree of crosslinking.

Therefore, in this embodiment, the storage elastic modulus obtained by electron beam irradiation is larger and the endothermic amount is smaller in the case of gamma ray irradiation. Therefore, the storage elastic modulus or endothermic amount obtained at the time of gamma ray 1000 kGy irradiation is used as a reference for the degree of crosslinking of 100%. And the degree of crosslinking A by the storage elastic modulus and the degree of crosslinking B by the endothermic amount in the following examples and comparative examples.
The degree of cross-linking A and the degree of cross-linking B are calculated using the following formula 2. In Equation 2, KA is the degree of crosslinking A due to the storage elastic modulus, KB is the degree of crosslinking B due to the endothermic amount, E _m is the measured storage elastic modulus, T _m is the measured endothermic amount, and E ₁₀₀ is when the gamma ray is irradiated alone at 1000 kGy. Storage elastic modulus, T ₁₀₀ is an endothermic amount when gamma rays are irradiated alone at 1000 kGy, E ₀ is a storage elastic modulus of polyethylene before crosslinking, and T ₀ is an endothermic amount of polyethylene before crosslinking (see Comparative Example 4).

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.
Example 1
A high-density polyethylene (specific surface area 1.5 to ^2.5 m ² / g) having a melting point of 135 ° C. was used as a test material, and was molded into a sheet having a thickness of 0.6 mm using a 160 ° C. hot press.
First, gamma rays emitted from ⁶⁰ Co were irradiated for a total of 300 kGy for 6 days for crosslinking. Since the dose rate of gamma rays is low, the specimen is kept at a temperature of 40 ° C. or lower.

  Subsequently, using an electron beam accelerator manufactured by NHV Corporation (maximum voltage of 5 Mev), the electron beam was irradiated at a current value of 20 mA at a rate of 33.3 kGy, and irradiated approximately 9 times for a total of 300 kGy. The electron beam was irradiated for several seconds, and the temperature at that time was 55 ° C. at maximum, and no swelling or discoloration was observed.

Next, evaluation of the degree of progress of crosslinking is performed by dynamic thermomechanical measurement (DMA) and differential scanning calorimetry (DSC), and storage elastic modulus change, endothermic change, crosslinking degree from the obtained storage elastic modulus and endothermic amount. A and the degree of crosslinking B were determined and determined.
The results are shown in Tables 1 and 2.

(Example 2)
Crosslinking was performed under the same conditions as in Example 1 except that the electron beam was first irradiated with 300 kGy and then the gamma ray was irradiated with 200 kGy, and the degree of crosslinking was determined from the obtained storage elastic modulus and absorption amount.
The results are shown in Tables 1 and 2.

(Example 3)
Crosslinking was carried out under the same conditions as in Example 1 except that gamma rays were first irradiated with 100 kGy and then electron beams were irradiated with 500 kGy, and the degree of crosslinking was determined from the obtained storage elastic modulus and absorption amount.
The results are shown in Tables 1 and 2.

Example 4
Crosslinking was performed under the same conditions as in Example 1 except that the irradiation was performed with 200 kGy of gamma rays first, followed by 200 kGy of electron beams, and the degree of crosslinking was determined from the obtained storage elastic modulus and absorption.
The results are shown in Tables 1 and 2.

(Example 5)
Crosslinking was performed under the same conditions as in Example 1 except that the first irradiation was performed with an electron beam of 200 kGy and the second irradiation was performed with a gamma ray of 200 kGy, and the degree of crosslinking was determined from the obtained storage elastic modulus and absorption amount.
The results are shown in Tables 1 and 2.

(Example 6)
Crosslinking was carried out under the same conditions as in Example 1 except that the first irradiation was performed with an electron beam of 500 kGy and the second irradiation with a gamma ray of 100 kGy, and the degree of crosslinking was determined from the obtained storage modulus and absorption.
The results are shown in Tables 1 and 2.

(Example 7)
Cross-linking under the same conditions as in Example 1 except that the first irradiation was performed with 150 gGy of gamma radiation, the irradiation with 150 kGy of electron beam for the second time, the irradiation of 150 kGy with gamma ray for the third time, and the irradiation of 150 kGy with electron beam for the fourth time. The degree of progress of crosslinking was determined in the same manner as in Example 1 from the storage elastic modulus and absorption amount thus determined.
The results are shown in Table 3.

(Comparative Example 1)
Crosslinking was carried out under the same conditions as in Example 1 except that irradiation with radiation was performed once with an electron beam of 400 kGy alone, and the progress of crosslinking was determined in the same manner as in Example 1 from the obtained storage elastic modulus and absorption amount. . The results are shown in Tables 1 and 2.

(Comparative Example 2)
Crosslinking was carried out under the same conditions as in Example 1 except that irradiation with an electron beam of 500 kGy was performed twice and irradiation with 1000 kGy was performed, and the degree of progress of crosslinking was determined in the same manner as in Example 1 from the obtained storage elastic modulus and absorption amount. The results are shown in Tables 1 and 2.
In addition, 37 kGy was irradiated per irradiation, and it was irradiated 27 times.

(Comparative Example 3)
Except for irradiation with an electron beam of 600 kGy twice and irradiation with 1200 kGy, crosslinking was performed in the same manner as in Comparative Example 2, and the progress of crosslinking was determined in the same manner as in Example 1 from the obtained storage elastic modulus and absorption amount. The results are shown in Tables 1 and 2.

(Comparative Example 4)
Crosslinking was carried out under the same conditions as in Example 1 except that only gamma rays were irradiated for the first time at 1000 kGy, and the degree of progress of crosslinking was determined in the same manner as in Example 1 from the obtained storage elastic modulus and absorption amount.
The results are shown in Tables 1 and 2.

(Comparative Example 5)
Crosslinking was carried out under the same conditions as in Example 1 except that only gamma rays were irradiated for the first time at 1200 kGy, and the progress of crosslinking was determined in the same manner as in Example 1 from the obtained storage elastic modulus and absorption amount.
The results are shown in Tables 1 and 2.

(Comparative Example 6)
Crosslinking was performed under the same conditions as in Example 1 except that only 400 gGy was irradiated for the first irradiation with gamma rays, and the degree of progress of crosslinking was determined in the same manner as in Example 1 from the obtained storage elastic modulus and absorption amount.
The results are shown in Tables 1 and 2.

(Comparative Example 7)
It is a sample before cross-linking in a state where no radiation is irradiated.
The measurement results of Examples 1 to 6 and Comparative Examples 1 to 8 are collectively shown in Tables 1 and 2, and the measurement results of Example 7 are shown in Table 3.

  As can be seen from Tables 1, 2 and 3, Examples 1 to 7 were irradiated with gamma rays and electron beams alone at 1000 kGy based on the results of the degree of crosslinking A by storage modulus and the degree of crosslinking B by endothermic amount. It can be seen that the degree of progress of the cross-linking reaction is the same as that of the case, and that sufficient cross-linking can be obtained with a lower dose from the change in the sculpture modulus and the change in the endothermic amount. Also, no discoloration is seen.

  On the other hand, when the electron beam of Comparative Example 1 alone was irradiated with 400 kGy, the elastic modulus change and endothermic amount change showed the same values as those in each example, but the storage elastic modulus was less than 3000 MPa and the endothermic amount was 3.69 cal / g. As shown by the crosslinking degree A and the crosslinking degree B, it can be seen that the crosslinking has not progressed so much.

  In Comparative Example 2 in which irradiation with an electron beam of 500 kGy was performed twice in one irradiation, the crosslinking seems to have progressed sufficiently as can be seen from the degree of crosslinking A and B determined from the storage modulus and endothermic amount. The total irradiation amount is as large as 1000 kGy, and this irradiation requires a lot of time and labor, resulting in a decrease in productivity and high cost.

As in Comparative Example 2, even in Comparative Example 3 where the amount of electron beam irradiation per irradiation is as large as 600 kGy, crosslinking is sufficiently advanced, but the appearance is discolored and cannot be used as a product.
This discoloration of the appearance is considered to be caused by the decomposition of the resin, such as formation of a diene structure and cleavage of the main chain by excessive (600 kGy) electron beam irradiation.

In Comparative Example 4 and Comparative Example 5 in which the irradiation amount of gamma rays in a single irradiation using only gamma rays is as large as 1000 kGy and 1200 kGy, crosslinking is sufficiently advanced, but in Comparative Example 5 in which the total irradiation amount is as large as 1200 kGy. Discoloration occurs and cannot be used as a product.
On the other hand, in the comparative example 4, it takes 20 days for irradiation, which is inefficient and increases the manufacturing cost. On the other hand, no discoloration was seen in the appearance.

In Comparative Example 6 with an irradiation amount of 400 kGy, the elastic modulus change and the endothermic amount change show the same values as those in each example, but from the degree of crosslinking A and B obtained from the storage elastic modulus and the endothermic amount, It turns out that it is not fully advanced.
The comparative example 7 which is not irradiated with radiation is not crosslinked.

It is a figure which shows the irradiation range of a radiation. It is a figure which shows the relationship between the irradiation amount of a gamma ray and an electron beam with respect to polyethylene, and a storage elastic modulus. It is a figure which shows the relationship between the irradiation amount of a gamma ray and an electron beam with respect to polyethylene, and an endothermic amount.

Claims (7)

  A method for crosslinking a polymer, wherein gamma rays and electron beams are used as radiation, and the polymer is crosslinked by alternately irradiating the polymer with the radiation.   A polymer characterized in that gamma rays and electron beams are used as radiation, and the polymer is crosslinked by irradiating the polymer with the radiation in the order of at least gamma ray irradiation and electron beam irradiation. Polymer crosslinking method.   Gamma rays and electron beams are used as radiation, and the polymer is cross-linked by irradiating the polymer with gamma rays, then with electron beams, and then in this order. A method for crosslinking a polymer.   A polymer characterized in that gamma rays and electron beams are used as radiation, and the polymer is crosslinked by irradiating the polymer with the radiation in the order of at least electron beam irradiation and gamma ray irradiation. Polymer crosslinking method.   Using a gamma ray and an electron beam as radiation, the polymer is crosslinked by irradiating the polymer with an electron beam, followed by gamma ray irradiation, and then repeating this sequence. A method for crosslinking a polymer.   The method for crosslinking a polymer according to any one of claims 1 to 5, wherein the polymer is a kind of polyolefin resin selected from polyethylene, polystyrene, and polypropylene.   The total irradiation amount of only the gamma rays is 100 kGy or more and 300 kGy or less, the total irradiation amount of only the electron beams is 200 kGy or more and 500 kGy or less, and the total irradiation amount of the total irradiation amount of the gamma rays and the electron beam is 400 kGy or more and 600 kGy or less. The method for crosslinking a polymer according to any one of claims 1 to 6, wherein the polymer is irradiated in a range.