JP2013227460A - Crosslinked polyethylene resin article, and polyethylene resin and polyethylene resin composition thereof, used for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a crosslinked polyethylene-based resin article crosslinked by radiation, without essentially requiring the use of an additive resin, and capable of achieving an excellent creep resistance when it is formed as a tubular material, and to provide a polyethylene resin and its resin composition used for manufacturing the same.SOLUTION: A crosslinked polyethylene resin article crosslinked by radiation is characterized in that when a relaxation time spectrum function is obtained under a specific condition, the curve of the spectrum function has a minimum value, the minimum value is present between 50 to 100 seconds of the relaxation time, and its value (H(λ)Pa) is ≤10,000.

Description

  The present invention relates to a crosslinked polyethylene resin, a polyethylene resin used in the polyethylene resin, and a resin composition.

  In recent years, plastic piping materials with excellent corrosion resistance and workability have been used as pipes for water supply and hot water supply, especially using cross-linked polyethylene pipes with excellent pressure resistance and creep resistance at high temperatures. Has become the mainstream. The crosslinking method of such a crosslinked polyethylene pipe includes a peroxide crosslinking method (crosslinked polyethylene pipe by this crosslinking method: PEXa) and a silane crosslinking method (crosslinked polyethylene pipe by this crosslinking method: PEXb). A cross-linked polyethylene pipe (PEXb) obtained by a silane cross-linking method, which is advantageous in that it can be produced using the same equipment as that for producing a polyethylene pipe, has been used.

  Both the above PEXa and PEXb are called chemical crosslinking methods because they use additives to cause crosslinking. In PEXa, an organic peroxide is added as a reaction initiator, and in PEXb, an organic peroxide as a reaction initiator and a silane compound as a coupling agent are usually added. PEXa has a problem of low productivity because it involves a crosslinking reaction during extrusion. In addition, since both compounds are blended in common with both of them, there is a problem in hygiene such that the residual substance after the reaction elutes in the accumulated water in the cross-linked polyethylene pipe even though the amount is small, and this causes odor. Further, PEXb has a problem that the shelf life of the material is short in terms of moisture management of the silane-modified resin material.

  On the other hand, a crosslinking method for producing a crosslinked polyethylene tube includes radiation crosslinking, and the crosslinked polyethylene tube formed thereby is called PEXc. With this cross-linking method, cross-linking is possible without using additives such as organic peroxides and silane compounds. Can be obtained.

JP 2011-246520 A

By the way, in the cross-linked polyethylene pipe generally used for water supply and hot water supply pipes, from the viewpoint of pressure resistance and creep resistance, polyethylene obtained by cross-linking high density polyethylene (0.938 g / cm ³ or more) to a high gel fraction is used. Used. However, the radiation crosslinking has a problem that it is difficult to obtain a high gel fraction when high-density polyethylene is used as a raw material. Moreover, since the antioxidant prescribed | regulated in order to lengthen a long lifetime inhibits bridge | crosslinking, there also existed a problem that it became difficult to bridge | crosslink. Therefore, in order to obtain a gel fraction that sufficiently satisfies pressure resistance and creep resistance, a high dose has to be irradiated, and it has been difficult to obtain an electron beam cross-linked polyethylene tube with a low dose and a high gel fraction.

  In order to solve the above problems, the present inventor previously provided a radiation-crosslinked polyethylene tube excellent in creep resistance in a high temperature range by blending an ethylene vinyl acetate copolymer in combination with a polyethylene resin. Thereafter, in order to further improve the characteristics and meet the technical requirements described above, the present inventor has advanced research on resin materials applied to radiation-crosslinked polyethylene pipes, and in particular, does not necessarily require a resin blend as described in Patent Document 1 above. In addition, the construction of a technology capable of achieving high performance with a polyethylene resin alone was an issue.

  In view of the above points, the present invention is a polyethylene-based cross-linked polyethylene resin that can achieve excellent creep resistance when used as a tube material without requiring the use of an additive resin. An object of the present invention is to provide a crosslinked resin, a polyethylene resin used in the production thereof, and a resin composition thereof.

In view of the above problems, the present inventor has analyzed the performance of piping made of a crosslinked resin and the physical properties of the radiation-crosslinked resin constituting the resin, and proceeded with the accumulation of data. As a result, it was discovered that there is a correlation between the creep resistance of the pipe material and the physical properties of the crosslinked resin obtained by a specific measurement method. Moreover, it discovered that the physical property of the said radiation crosslinking resin (resin crosslinked body) was achieved by the polyethylene resin which has specific molecular weight distribution etc. The present invention has been completed based on these new findings.
That is, the present invention has the following means.

[1] A cross-linked polyethylene resin cross-linked by radiation,
When the relaxation time spectrum function is obtained under the following conditions, the curve of the spectrum function has a minimum value, and the minimum value exists between the relaxation time of 50 seconds and 100 seconds, and its value (H (λ) Pa) Is a polyethylene resin cross-linked product, characterized in that it is 10,000 or less.
“Conditions: For a sheet-like resin cross-linked body, a dynamic viscoelasticity measuring tester using a parallel plate, with a distortion of 1.0% and an angular frequency of 0.01 to 100 (under 190 ° C. in air) Measure the dynamic viscoelastic spectrum (storage elastic modulus G ′ (ω) and dynamic loss G ″ (ω) with ω as the horizontal axis) while changing it stepwise until 1 / s), and convert this Obtained relaxation time-relaxation time spectral function "
[2] The polyethylene resin cross-linked product according to [1], which is a pipe material for supplying hot water and hot water.
[3] There is a maximum point of the relaxation time spectrum (H (λ) Pa) in the relaxation time of 1 second to 10 seconds, and the relaxation time spectrum (H (λ) Pa) of the maximum point is over 10,000. The polyethylene resin crosslinked product according to [1] or [2].
[4] The polyethylene resin is a homopolymer of ethylene, a copolymer of ethylene and an α-olefin monomer of 5 mol% or less, and 1 mol% or less having only carbon, oxygen and hydrogen atoms in the ethylene and functional groups. The polyethylene resin crosslinked product according to any one of [1] to [3], which is selected from a copolymer with a non-olefin monomer.
[5] The crosslinked polyethylene resin according to any one of [1] to [4], wherein the mass gel fraction is 60% or more.
[6] A resin crosslinked product produced by extrusion molding and crosslinking a resin composition containing a polyethylene resin,
The resin has a density of 0.938 to 0.953 g / cm ³ , MFR of 0.01 to 3.0 g / 10 minutes, Mn of 30,000 or more, Mw / Mn of 15.0 or less, and Log. ₁₀ (Mw) The cumulative content in the range of 4 or less is 3.0% by mass or more and 9.0% by mass or less, and the polyethylene resin cross-linking according to any one of [1] to [5] body.
[7] Density is 0.938 to 0.953 g / cm ³ , MFR is 0.01 to 3.0 g / 10 min, Mn is 30,000 or more, Mw / Mn is 15.0 or less, and Log ₁₀ ( Mw) A polyethylene resin having a cumulative content in a range of 4 or less of 3.0% by mass or more and 9.0% by mass or less.
[8] The resin component is a homopolymer of ethylene, a copolymer of ethylene and an α-olefin monomer of 5 mol% or less, and 1 mol% or less of ethylene and a functional group having only carbon, oxygen and hydrogen atoms. The polyethylene resin according to [7], which is selected from a copolymer with a non-olefin monomer.
[9] A resin composition comprising the resin according to [7] or [8] and 0.5 to 2.0% by mass of an antioxidant.
[10] The resin composition according to [9], wherein the antioxidant includes a phenolic antioxidant, an amine antioxidant, or a combination thereof.
[11] The resin composition according to [9] or [10], wherein at least one of the antioxidants is an antioxidant having an ester structure or a carbonyl structure in its molecular structure.

  The crosslinked resin of the present invention is a crosslinked polyethylene resin crosslinked by radiation, and can achieve excellent creep resistance when used as a pipe material without requiring the use of an additive resin. The polyethylene resin and the resin composition of the present invention are useful as a raw material for the crosslinked resin having the above excellent physical properties.

It is a graph which shows the molecular weight distribution (frequency) of the raw material resin used by the Example and the comparative example. It is a graph which shows the molecular weight distribution (cumulative) of the raw material resin used by the Example and the comparative example. It is a graph which shows the relaxation time-relaxation time spectrum function of the resin crosslinked body produced by the Example and the comparative example. It is sectional drawing (JIS-K-6787 appendix 2 FIG. 1) which shows typically the test state in a heat-resistant creep characteristic test.

  The crosslinked resin body of the present invention can be suitably applied to piping for hot water supply and hot water supply, and has specific physical properties defined by a relaxation time spectrum function. Hereinafter, the present invention will be described focusing on preferred embodiments thereof. First, a resin that is a raw material of the crosslinked resin and the composition thereof will be described, and then the physical properties of the crosslinked resin will be described.

<Polyethylene>
The raw material for the crosslinked resin of the present invention is a polyethylene resin. Here, the polyethylene is preferably one defined in JIS K6922-1. Specifically, a homopolymer of ethylene, a copolymer of ethylene and an α-olefin monomer of 5 mol% or less, and a non-olefin of 1 mol% or less having only carbon, oxygen and hydrogen atoms in the functional group of ethylene and ethylene. It is preferably one selected from a copolymer with a monomer.

・ Density, MFR
The polyethylene resin used in the present invention preferably has a density of 0.938 to 0.953 g / cm ³ . This is from the viewpoint of both pressure resistance and flexibility of the tube. If this value is too small, the tensile yield strength of the material will be low and the pressure resistance of the tube will be insufficient. On the other hand, if this value is too large, the elastic modulus of the material will be high and the flexibility of the tube will be insufficient.

  Moreover, it is preferable to limit MFR to 0.01-3.0 g / 10min, 0.01-1.0 are more preferable, and 0.01-0.5 are especially preferable. This is from the viewpoint of both the moldability of the tube and the crosslinkability by irradiation. If this value is too small, the molecular weight of PE tends to increase, which is advantageous for radiation crosslinking, but the load on the extruder increases and the moldability becomes unstable. In addition, if this value is large, the flowability of the resin during molding increases, so the load on the extruder is reduced and the molding stability is improved. Therefore, it becomes difficult to obtain a crosslinked PE tube having an appropriate gel fraction. On the other hand, if this value is too high, an appropriate gel fraction cannot be obtained after irradiation, and drawdown may occur during molding, which may hinder the dimensional stability of the pipe.

  The number average molecular weight Mn of the polyethylene resin preferably applied in the present invention is preferably 30,000 or more. Here, not the weight average molecular weight Mw generally used for expressing the physical properties of the resin, but the number average molecular weight Mn is specified, Mn is more sensitive to the size of low molecular weight components than Mw. Because it is a good value. A value of Mn of 30,000 or more indicates that when considering the molecular weight distribution of polyethylene suitable for radiation crosslinking, a large proportion of molecules with a large molecular weight occupies a large proportion and a small proportion of molecules with a small molecular weight occupies. Yes. Therefore, if this value is too small, the final reached gel fraction becomes small and the creep resistance at high temperatures is impaired. In addition, the upper limit of Mn is not particularly defined, but if it is too high, the fluidity of the resin will deteriorate, the load on the extruder will increase, the moldability will become unstable, and the productivity will decrease. In manufacturing a pipe, it is desirable that Mn is not extremely high. From this point of view, Mn is practically 100,000 or less, and practically 50,000 or less.

  Further, the value (polydispersity = PDI) obtained by dividing the weight average molecular weight measured by the GPC by the number average molecular weight is preferably 15.0 or less. If it is larger than this, a sufficient gel fraction cannot be obtained at the time of radiation irradiation crosslinking, and the creep resistance at high temperatures is impaired. There is no lower limit of PDI, but if this value is too low, the change in viscosity of the polyethylene resin when the molding temperature changes becomes too large, and the moldability and dimensional stability become unstable. Is preferably 3.0 or more.

Further, the cumulative content of components having a molecular weight of Log ₁₀ Mw = 4 or less in the molecular weight distribution curve obtained by GPC for the polyethylene resin is preferably 3.0% by mass or more and 9.0% by mass or less. More preferably, it is 0 mass% or more and 8.5 mass% or less. This indicates the content of low molecular weight components contained in the base resin. If this value is larger than the above upper limit value, the reached gel fraction will be low even after radiation irradiation crosslinking, so that a sufficient gel fraction cannot be obtained and the creep resistance at high temperatures will be impaired. On the other hand, if it is smaller than the lower limit value, the flowability of polyethylene during molding is lowered, so that moldability and productivity are extremely lowered.

  These numerical limits are numerical representations of the shape of the molecular weight distribution. The polyethylene selected as the base resin is not particularly limited in its production method, but is produced in a single-stage or multi-stage polymerization process using a single site catalyst system, or in a single stage using a heterogeneous Ziegler catalyst system. Desirably, it is a polyethylene produced by a multi-stage polymerization process or a polyethylene produced using a Philips catalyst. That is, it is desirable that the molecular weight distribution is in a range of PDI ≦ 15.0, and that it has a mountain shape without a bump or a distorted bulge.

-Manufacturing method As polyethylene (polyolefin) resin which has resin density, MFR, and molecular weight distribution in the range mentioned above, it can obtain by multi-site catalysts, such as a Ziegler-Natta catalyst, for example. In particular, regarding the molecular weight distribution (contents up to Mn, PDI, Log Mw = 4), the adjustment can be performed as follows.

  The polyethylene is preferably polymerized using a heterogeneous Ziegler catalyst and a Philips catalyst. In general, examples of the catalyst used in the production of polyethylene include a heterogeneous Ziegler catalyst, a homogeneous Ziegler catalyst (so-called single site catalyst), a Philips catalyst, and a standard catalyst. From the viewpoint of obtaining the specific polyethylene, polyethylene polymerized by a heterogeneous Ziegler catalyst or a Philips catalyst is preferable. A heterogeneous Ziegler catalyst is a catalyst obtained by adding triethylaluminum as a co-catalyst to titanium tetrachloride supported on magnesium chloride or magnesium hydroxide chloride. In the polymerization using this, slurry polymerization or gas phase polymerization is the mainstream. In any case, the molecular weight distribution can be controlled variously by selecting a single-stage polymerization or a multi-stage polymerization process. The Phillips catalyst is a catalyst in which a chromium compound such as chromium acetate is supported on silica and calcined in air. In polymerization using this, polymerization is generally performed by a solution polymerization process.

  Since the two types of catalysts described above have many different types of polymerization active sites distributed, for example, compared with a uniform Ziegler catalyst, polyethylene having a broad molecular weight distribution can be obtained. This point is suitable for satisfying the range of Mw / Mn defined in the present invention. Also, these catalysts are relatively easy to increase in molecular weight by changing the design of the catalyst and the polymerization conditions as compared with the other two types of catalysts. In this respect, LogM ≦ 4 and Mn ≧ 30,000, that is, suitable for increasing the high molecular weight component by decreasing the low molecular weight component.

  If the polyethylene prescribed | regulated in this invention is obtained, you may superpose | polymerize using any of the above-mentioned two types of catalysts and the manufacturing process which can be taken. Examples of the polyethylene having specific physical properties obtained by such a production method include “NIPOLON HARD 8600A” (trade name) manufactured by Tosoh Corporation.

-Blending of resin In the present invention, it is preferable that the base resin consists essentially of a polyethylene component. Specifically, when the polyethylene component of the base resin is 100 parts by mass, the content of other resins is preferably 5 parts by mass or less, and this ratio is more preferably 3 parts by mass or less. The amount is particularly preferably 1 part by mass or less. For example, it may be blended with ethylene-vinyl acetate copolymer (EVA) or the like, but in the present invention, it is preferable that the base resin does not contain ethylene-vinyl acetate copolymer (EVA). It is preferable that it is not blended with resin other than polyethylene including this resin. This is because blending with EVA resin and other resins having an ester bond structure in the molecule can be expected to assist in radiation crosslinking, but on the other hand, it can cause acetic acid odor and acrylic odor. This is because it may affect the From this point of view, the base resin in this patent is most preferably composed of only a polyethylene component. The method for evaluating the resin component is not particularly limited. However, when the low molecular weight component is excluded and compared, the component having a molecular weight up to Log Mw = 4 is excluded and the composition of the resin component is evaluated. May be.

<Antioxidant>

  In addition to the base resin, an antioxidant may be added to the resin composition in the present invention. In particular, Japanese tap water contains a small amount of hypochlorous acid for the purpose of sterilization, and this promotes oxidative deterioration of the pipe. Therefore, from the viewpoint of long life, the addition of an antioxidant is essential.

  On the other hand, antioxidants inhibit crosslinking during radiation crosslinking. Although the crosslinking process by radiation is a radical reaction, the antioxidant inactivates this radical and thus competes with the crosslinking reaction. This competition also consumes antioxidants during crosslinking. Therefore, from the viewpoint of obtaining a hot water supply pipe that can be used for a long time, not only the polyethylene suitable for the radiation crosslinking as described above is selected, but also the antioxidant that is difficult to inhibit the crosslinking reaction and is not easily consumed by radiation. It is also important to select the agent.

  The present inventors have found that, particularly in phenolic antioxidants and amine-based antioxidants, there is little inhibition of radiation crosslinking, and the consumption of antioxidants by radiation is low. Among them, the present inventors have found that the above effects are great in phenol-based and amine-based antioxidants having an ester group or a carbonyl group in the molecule. It is conceivable that the ester group or carbonyl group is stabilized by forming a resonance structure with the radical in the molecule, and has an effect of temporarily suppressing the antioxidant from trapping the radical.

  When applied to this system, based on antioxidants containing ester groups or carbonyl groups suitable for radiation irradiation crosslinking treatment, antioxidants (water extraction resistance and chlorine water resistance) with a view to water supply and hot water use May be blended with a more excellent antioxidant.

  0.5-2.0 mass% is preferable and, as for the quantity of the antioxidant mix | blended in a system, 0.7-1.5 mass% is more preferable. If it is less than this, the amount of antioxidant remaining after consumption by radiation irradiation is too small, and the long-term life is shortened. Further, if it is more than this, the inhibition of crosslinking at the time of radiation crosslinking becomes too large, and the radiation dose necessary for achieving a gel fraction sufficient to obtain creep resistance at high temperatures becomes high, so the productivity is high. In addition to being significantly reduced, the resin is also deteriorated by high-dose irradiation, which also adversely affects the long-term life.

  According to a preferred embodiment of the present invention, by blending an antioxidant as described above, long-term performance (oxidation degradation resistance, resistance to resistance) after the antioxidant is partially consumed by the crosslinking treatment by radiation irradiation. The antioxidant remains to the extent that the chlorine aqueous solution is maintained, and a cross-linked polyethylene pipe for hot and cold water supply with a suitable gel fraction can be provided. In addition, although an electron beam can be used suitably for a radiation, you may utilize actinic radiation, such as a gamma ray, as needed. Hereinafter, embodiments according to electron beam irradiation will be described in detail.

<Irradiator>
The device for irradiating the electron beam may be of any type as long as the necessary acceleration voltage and irradiation dose can be obtained. The acceleration voltage is preferably 500 keV or more, and preferably 2 MeV or more in the case of a polyethylene tube having a thickness of 1 mm. From the viewpoint of productivity, it is most preferable to use 5 to 10 MeV.

<Irradiation method>
The irradiation method is to wrap a polyethylene tube around a drum and irradiate it as a bundle (bundle irradiation method), or to hang a polyethylene tube on a rotary drum, mechanically route it, There is a method by irradiation (line irradiation method). Either irradiation method can produce a crosslinked polyethylene tube.
<Irradiation dose>
The optimum value of the dose varies depending on the type of polyethylene as the base resin and the type of additive added, but when using the materials described in this patent, hot water and hot water supply satisfying the desired physical properties in the range of 60 to 270 kGy. An electron beam cross-linked polyethylene tube is obtained. If the irradiation dose is too low, the gel fraction does not reach the target value. If the irradiation dose is too high, excessive consumption of the added antioxidant and deterioration of the resin are caused, and the long-term life is reduced.

<Resin crosslinked body>
-Gel fraction The gel fraction is an index representing the degree of crosslinking. The crosslinked resin of the present invention preferably has a mass gel fraction of 60% or more, and more preferably 65% or more, in order to sufficiently satisfy “creep resistance at high temperatures”. The upper limit of the gel fraction is not particularly limited, but is practically 90% or less. If the gel fraction is too low, the creep resistance at high temperatures is impaired. The gel fraction can be measured by the method described in Examples below.

<Relaxation time spectrum>
In the resin crosslinked product of the present invention, the minimum value of the relaxation time spectrum function H (λ) / Pa obtained under the following conditions exists between the relaxation times of 50 seconds to 100 seconds, and the value is 10,000 Pa or less. It is a feature.

“Condition: A sample punched out from a 13A cross-linked pipe into a 25Φ sheet shape was subjected to a dynamic viscoelasticity measuring tester using a parallel plate, with a distortion rate of 1.0% and an angle of 190 ° C. in air. Dynamic viscoelastic spectrum (storage elastic modulus G ′ (ω) and dynamic loss G ″ (ω) with ω as a horizontal axis) while changing the frequency stepwise from 0.01 to 100 (1 / s) Was measured and converted to obtain relaxation time-relaxation time spectrum function "

また、複素弾性率は緩和弾性率のフーリエ変換によって、下のような関係式に表される。

一方、緩和弾性率Ｇ（ｔ）と緩和時間−緩和時間スペクトル関数（Ｈ（τ））の間には、下式のような関係がある。ここでｔは時間、τは緩和時間を表す。

上述の２つの関係式から、貯蔵弾性率Ｇ’（ω）および動的損失Ｇ”（ω）とＨ（τ）の関係が導き出されるため、Ｇ’（ω）およびＧ”（ω）の測定結果からＨ（τ）に換算することで緩和スペクトル関数を求める。
ちなみに、Ｇ’（ω）およびＧ”（ω）とＨ（τ）との関係は、下式によって与えられる。

以上の操作によって得られた緩和時間−緩和時間スペクトル関数において、本発明による架橋ポリエチレン管は、緩和時間スペクトル関数Ｈ（λ）（Ｐａ）の最小値が、緩和時間５０秒〜１００秒の間に存在し、その値が１０，０００Ｐａ以下であり、８，０００Ｐａ以下であることがより好ましい。この下限値は特に制限されないが、１，０００Ｐａ以上であることが実際的である。なお、上記の数式においてλは、Ｌｎ（τ）である。
また、本発明においては、上記と同様の観点より、その樹脂架橋体について、緩和時間１秒〜５０秒（好ましくは１秒〜１０秒）の間における緩和時間スペクトル（Ｈ（λ）Ｐａ）が、１０，０００超であることが好ましい。さらに、緩和時間１秒〜５０秒（好ましくは１秒〜１０秒）の間に、緩和時間スペクトル（Ｈ（λ）Ｐａ）の極大点があり、該極大点の緩和時間スペクトル（Ｈ（λ）Ｐａ）が１０，０００超であることが好ましい。 Incidentally, conversion from a dynamic viscoelastic spectrum to a relaxation time-relaxation time spectrum function is performed according to the following equation.
The ratio of the storage elastic modulus G ′ (ω) and the dynamic loss G ″ (ω) is defined as the loss tangent tan δ (ω). Here, using Euler's theorem e ± iθ = cos θ ± isiniθ, the complex The elastic modulus G * (iω) is expressed by the following equation.

The complex elastic modulus is expressed by the following relational expression by Fourier transform of the relaxation elastic modulus.

On the other hand, there is a relationship between the relaxation modulus G (t) and the relaxation time-relaxation time spectrum function (H (τ)) as shown in the following equation. Here, t represents time and τ represents relaxation time.

Since the relationship between the storage elastic modulus G ′ (ω) and the dynamic loss G ″ (ω) and H (τ) is derived from the above two relational expressions, the measurement of G ′ (ω) and G ″ (ω) A relaxation spectrum function is obtained by converting the result into H (τ).
Incidentally, the relationship between G ′ (ω) and G ″ (ω) and H (τ) is given by the following equation.

In the relaxation time-relaxation time spectrum function obtained by the above operation, the crosslinked polyethylene tube according to the present invention has a minimum value of the relaxation time spectrum function H (λ) (Pa) between the relaxation time of 50 seconds and 100 seconds. It exists and the value is 10,000 Pa or less, and it is more preferable that it is 8,000 Pa or less. Although this lower limit is not particularly limited, it is practical that it is 1,000 Pa or more. In the above formula, λ is Ln (τ).
Moreover, in this invention, the relaxation time spectrum (H ((lambda)) Pa) between relaxation time 1 second-50 seconds (preferably 1 second-10 second) is about the resin crosslinked body from the viewpoint similar to the above. Preferably it is greater than 10,000. Further, there is a maximum point of the relaxation time spectrum (H (λ) Pa) between the relaxation time of 1 second and 50 seconds (preferably 1 second to 10 seconds), and the relaxation time spectrum (H (λ)) of the maximum point. It is preferred that Pa) is greater than 10,000.

In the present invention, the reason why the excellent effect is obtained by defining the relationship between the relaxation time-relaxation spectrum function of the crosslinked resin as described above includes an unclear part, but is estimated as follows.
Electron beam crosslinking is a technique in which radicals are instantaneously generated in the system by ionizing radiation, and the crosslinking reaction proceeds by the action thereof. The characteristic behavior of the relaxation time spectral function as described above, which is seen in the crosslinked resin of the present invention, is that a component having a large entanglement density and molecular weight among polyethylene materials forms a crosslinked body predominantly. It can be considered that the region with a relaxation time of 50 to 100 seconds corresponds to this location. This means that the relaxation time in the relaxation time spectral function of the amorphous polymer (PE is considered to be an amorphous polymer because PE is placed at a temperature higher than the melting point [190 ° C.] in this measurement) and the entanglement of the polymer chain. It is presumed from the fact that there is a correlation between the density and the molecular weight, and that the minimum value is not observed in this relaxation time region in the case of crosslinking by a method other than the electron beam crosslinking method.

  In the measurement of the post-crosslinking sample, the minimum value of the relaxation time spectrum function is present at this point. This is because the PE in this region is cross-linked by an electron beam, so that a longer relaxation time region (= apparent molecular weight is larger). It means that it has moved to (region). Therefore, a sample having a smaller minimum value of the relaxation time spectrum function in this relaxation time region means that there are more components that have shifted to a longer relaxation time region--that is, insoluble components in xylene in the weight gel fraction measurement. It is thought that this indicates that the degree of crosslinking has increased.

  According to a preferred embodiment of the present invention, necessary physical properties (creep resistance and long-term characteristics at high temperatures) can be obtained with a minimum irradiation dose, so that the cost for electron beam irradiation can be reduced and productivity can be improved. To do. In addition, since the consumption of antioxidants during irradiation can be minimized, the decomposition residue of antioxidants and additives present in the system can be minimized, and it can be used for hot water and hot water with better hygiene. Piping can be obtained. In addition, since the relaxation time-relaxation spectrum function is limited to a specific region as described above, the piping formed of the crosslinked resin has an advantage in industrial use that it has excellent long-term creep characteristics.

  The present invention will be described in more detail based on examples, but the present invention should not be construed as being limited thereby.

(Examples and comparative examples)
(1) Material Polyethylene listed in Table 1 below was used as a base resin. Further, tetrakis [methylene-3- (3 ′, 5′-di-t-butyl-4-hydroxyphenyl) propionate] methane 0.53% by weight as an antioxidant is added to the base resin, did.

(2) Production of polyethylene pipe A single-screw extruder with a screw diameter of 80 mm and L / D = 25, a die and a nipple connected to the discharge port, and a resin composition are extruded as a tubular melt, and a resin temperature of 200 ° C. The molten tubular resin composition extruded in step 1 was molded and cooled to obtain a polyethylene tube having an outer diameter of 17.2 mm and a thickness of 2.1 mm.

(3) Electron Beam Irradiation With an electron beam irradiator (electron beam irradiator using a repetitive acceleration method), an acceleration voltage of 10 MeV, a current value of 10 mA, a single irradiation dose of 15 kGy × 10 passes (total irradiation dose of 150 kGy), and a temperature at room temperature ( After each irradiation pass, air cooling was performed until the temperature returned to room temperature), and irradiation with an electron beam was performed under conditions of atmospheric air.

(4) Evaluation [density]
The density was measured by D method of JIS K 7112.

[MFR]
MFR was measured by a method according to JIS K 7210 test temperature 190 ° C. and load 21.18N.

[Measurement of molecular weight and polydispersity (PDI)]
The weight average molecular weight, number average molecular weight, and polydispersity (Mw / Mn) were measured under the following conditions by gel permeation chromatography.
(Measurement condition)
Equipment: GPC / V2000 manufactured by Water
Column: Shodex AT-G + AT-806MS × 2 Solvent: o-dichlorobenzene Temperature: Column and injector 145 ° C.
Concentration: about 1.0 g / L
Flow rate: 1.0 mL / min
Molecular weight calculation: Universal calibration using standard PS

[Measurement of gel fraction]
The sample used for measuring the gel fraction was sliced or scraped from a crosslinked polyethylene tube to a thickness of 0.1 to 0.2 mm. The mass of the sample was 0.750 ± 0.050 g. The mass of a clean and dried basket with a lid was measured with a sensitivity of 1 mg (mass m ₁ ). Next, the sample was put in the cage, and the mass of the cage with the sample was measured with a sensitivity of 1 mg (mass m ₂ ). The basket with the sample was placed in the flask, and a sufficient amount of xylene was added to ensure that it was kept immersed. The solvent was vigorously boiled for 8 hours ± 5 minutes. The sample residue and basket were removed from the boiling solvent and dried in a vacuum oven at 140 ° C. ± 2 ° C. under a negative pressure of at least 85 kPa for 3 hours. After cooling, the cage and the residue were measured with a sensitivity of 1 mg (mass m ₃ ). Gel fraction G ₁ is calculated by the following the following formula A.
G ₁ = (m ₃ −m ₁ ) / (m ₂ −m ₁ ) × 100 Formula A

[Measurement of relaxation time-relaxation time spectral function]
For a sample punched out into a 25Φ sheet from a 13A cross-linked pipe, the angular frequency was measured using a parallel plate with a dynamic viscoelasticity measurement tester under a distortion rate of 1.0% and in air at 190 ° C. Measure the dynamic viscoelasticity spectrum (storage elastic modulus G ′ (ω) and dynamic loss G ″ (ω) with ω as a variable) while changing stepwise from 0.01 to 100 (1 / s), The relaxation time-relaxation time spectrum function was calculated by converting this, and the calculation formula is as described above, from which the minimum value of the relaxation time spectrum function at the relaxation time of 50 to 100 seconds was read.

[Hot internal pressure creep test (creep property)]
About the sample of each Example and comparative example produced as mentioned above, the hot internal pressure creep test was done according to JISK6787 appendix 2, and it was evaluated whether 170 hours and 8760 hours were cleared. The outline of the test is shown below.
Both ends of the test piece were sealed (see FIG. 4), and a pressurizing device for applying a constant internal pressure inside, a water bath or an oven for keeping the temperature of the test piece constant were used. All test specimens were collected from tubes that had passed 15 hours or more after production. Both ends of the test piece were cut so as to be perpendicular to the tube axis, and the effective length of the test piece was a minimum of 250 mm. The number of test pieces was at least three. The test piece was filled with water set at a temperature 0 to 5 ° C. higher than the test temperature (95 ° C. (170 hours method) or 110 ° C. (8760 hours method)), placed in a water bath or oven at the test temperature, 15 Let stand for a minute. Then, it connected with the pressurization apparatus and pressurized so that it might become the regulation pressure calculated | required by Annex 2 Table 1. FIG. In this state, each test piece was left for a specified time. In the table, “Fair” was given when there was a failure, and “Good” was given when there was no failure. The performance of the cross-linked polyethylene pipe confirmed by this test is clarified by the classification according to the service temperature and the maximum service pressure of the PN15 pipe in Table 1 described in JIS-K6769. Guaranteed use.

  For reference, molecular weight distributions (FIGS. 1 and 2) and relaxation time spectra (FIG. 3) of representative examples and comparative examples are shown. In addition, the uncrosslinked resin in FIG. 3 is a value obtained by measuring the resin before crosslinking in Example 1 under the same conditions as described above (measurement temperature is 190 ° C.).

  From the above, it can be seen that the radiation-crosslinked polyethylene pipe satisfying the relaxation time-relaxation time spectrum function defined in the present invention achieves high hot internal pressure creep properties. Moreover, the polyethylene resin satisfying the resin density, MFR, and molecular weight distribution defined in the present invention can realize the production of radiation-crosslinked polyethylene exhibiting the above excellent performance without depending on the blending of EVA or the like.

Claims (11)

A crosslinked polyethylene resin crosslinked by radiation,
When the relaxation time spectrum function is obtained under the following conditions, the curve of the spectrum function has a minimum value, and the minimum value exists between the relaxation time of 50 seconds and 100 seconds, and its value (H (λ) Pa) Is a polyethylene resin cross-linked product, characterized in that it is 10,000 or less.
“Conditions: For a sheet-like resin cross-linked body, a dynamic viscoelasticity measuring tester using a parallel plate, with a distortion of 1.0% and an angular frequency of 0.01 to 100 (under 190 ° C. in air) Measure the dynamic viscoelastic spectrum (storage elastic modulus G ′ (ω) and dynamic loss G ″ (ω) with ω as the horizontal axis) while changing it stepwise until 1 / s), and convert this Obtained relaxation time-relaxation time spectral function "   The cross-linked polyethylene resin according to claim 1, which is a pipe for water supply and hot water supply.   A relaxation time spectrum (H (λ) Pa) has a maximum point within a relaxation time of 1 second to 10 seconds, and the relaxation time spectrum (H (λ) Pa) of the maximum point is more than 10,000. 3. A crosslinked polyethylene resin according to 1 or 2.   The polyethylene resin is a homopolymer of ethylene, a copolymer of ethylene and 5 mol% or less α-olefin monomer, and 1 mol% or less of non-olefin having only carbon, oxygen and hydrogen atoms in the functional group of ethylene and ethylene. The polyethylene resin crosslinked body according to any one of claims 1 to 3, which is selected from a copolymer with a monomer.   The cross-linked polyethylene resin according to any one of claims 1 to 4, wherein the mass gel fraction is 60% or more. A resin crosslinked product produced by extrusion molding and crosslinking a resin composition containing a polyethylene resin,
The resin has a density of 0.938 to 0.953 g / cm ³ , MFR of 0.01 to 3.0 g / 10 minutes, Mn of 30,000 or more, Mw / Mn of 15.0 or less, and Log. The cumulative content in the range of ₁₀ (Mw) 4 or less is 3.0 mass% or more and 9.0 mass% or less, The polyethylene resin crosslinked body according to any one of claims 1 to 5. Density is 0.938 to 0.953 g / cm ³ , MFR is 0.01 to 3.0 g / 10 min g / 10 min, Mn is 30,000 or more, Mw / Mn is 15.0 or less, and Log ₁₀ (Mw) A polyethylene resin having a cumulative content in a range of 4 or less of 3.0 mass% or more and 9.0 mass% or less.   The resin component is a homopolymer of ethylene, a copolymer of ethylene and an α-olefin monomer of 5 mol% or less, and 1 mol% or less of a non-olefin monomer having only ethylene, carbon, oxygen and hydrogen atoms in the functional group. The polyethylene resin according to claim 7, which is selected from a copolymer with a monomer.   The resin composition formed by mix | blending the resin of Claim 7 or 8 and 0.5-2.0 mass% antioxidant.   The resin composition according to claim 9, wherein the antioxidant includes a phenol-based antioxidant, an amine-based antioxidant, or a combination thereof.   The resin composition according to claim 9 or 10, wherein at least one of the antioxidants is an antioxidant having an ester structure or a carbonyl structure in its molecular structure.

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