JPS6358090B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a crosslinked polyethylene pipe having excellent creep properties, particularly notched creep properties. It has been well known that crosslinking polyethylene improves physical properties such as heat resistance, creep resistance, environmental stress cracking resistance, and chemical resistance. Taking advantage of these physical properties, crosslinked polyethylene can be used as a wire coating. It is widely used in the fields of , pipes, films, shrink tubes, etc. In addition to the conventional method of using a chemical crosslinking agent and the method of using an electron beam, methods for obtaining crosslinked polyethylene include, in recent years, Japanese Patent Publication No. 1711/1983, etc., in which polyethylene is processed into a silane compound in the presence of a radical generator. A so-called water-crosslinking method has become known, in which silane-modified polyethylene is grafted with silane-modified polyethylene, and this silane-modified polyethylene is cross-linked by exposing it to a moisture-containing atmosphere in the presence of a silanol condensation catalyst. Moreover, this crosslinking method has industrial value, as the cost of crosslinking equipment is significantly lower than conventional methods using chemical crosslinking agents or methods using electron beams, and the crosslinking work is relatively easy. Because of this, it has recently attracted particular attention as a new crosslinking method for polyethylene and is being studied in various fields. By the way, when these cross-linked polyethylenes are used under severe stress load conditions such as pipes, and especially when the pipe surface is scratched (notched) due to friction or impact with earth, sand, concrete, etc. during installation, It has been observed that cross-linked polyethylene pipes often fail within a shorter period of time than their expected service life, and cross-linked polyethylene still remains problematic in areas where severe creep properties are required. be. In view of the above-mentioned current situation, the present inventor conducted various studies on methods for producing cross-linked polyethylene pipes with excellent creep properties, and as a result, the present inventor grafted a silane compound onto polyethylene having a specific density and molecular weight to produce silane-modified polyethylene. Next, after forming this into a pipe, it was discovered that the objective could be achieved by water-crosslinking it to a specific gel fraction, thereby completing the present invention. That is, the method for manufacturing a crosslinked polyethylene pipe with excellent creep properties according to the present invention has a density of 0.942 to 0.949.
g/cm ³ and a viscosity average molecular weight of 0.5×10 ⁵ or more, in the presence of a radical generator, the general formula
RSiR′ nY _3-o (where R is an ethylenically unsaturated hydrocarbon group or a hydrocarbonoxy group, R′ is an aliphatic saturated hydrocarbon group, Y is a hydrolyzable organic group, and n is 0 or 1
or 2. ) is heated and grafted with an ethylenically unsaturated silane compound to produce silane-modified polyethylene, and then the silane-modified polyethylene is formed into a pipe, and then exposed to a water atmosphere in the presence of a silanol condensation catalyst to achieve a gel fraction of 65%.
It is characterized in that it is crosslinked in more than one manner. The polyethylene used in the present invention has a density of
It has a weight of 0.942 to 0.949 g/cm ³ . Furthermore, the polyethylene used must have a viscosity average molecular weight of 0.5×10 ⁵ or more, preferably 0.9×10 ⁵ or more. If the density and viscosity average molecular weight are outside this range, crosslinked polyethylene with excellent creep properties, especially notched creep properties, cannot be obtained. The above density was determined by the method using a density gradient tube specified in JIS K6760, and the viscosity average molecular weight Mv was determined by substituting the intrinsic viscosity [η] determined in an orthodichlorobenzene solution at 140°C into the following formula. It is something. [η] = 3.92×10 ^-4・v ^0.733If polyethylene is a mixture described below, the formula [η] _A+B+ …=[η] _A・W _A + [η] _B・W _B +… [η] _A+B+ ... determined by [η] was taken as the intrinsic viscosity.
However, W _A , W _B , . . . are the weight fractions of each component A, B, . The polyethylene used in the present invention may be either an ethylene homopolymer or a copolymer of ethylene and an α-olefin such as propylene or butene-1, as long as it satisfies the above density and viscosity average molecular weight ranges. , or a mixture thereof. Specifically, for example, when it is an ethylene homopolymer obtained by medium-low pressure ionic polymerization, a copolymer of ethylene and α-olefin, or a mixture of both, or by high-pressure radical polymerization with either of them, Examples include a mixture with an ethylene homopolymer. Among these, preferred are ethylene homopolymers produced by medium-low pressure ionic polymerization, copolymers of ethylene and α-olefin, and mixtures of both. In the present invention, first, the polyethylene is
A silane-modified polyethylene is obtained by heating and grafting an ethylenically unsaturated silane compound in the presence of a radical generator. The ethylenically unsaturated silane compound in the present invention has the general formula RSiR' nY _3-o (where R is an ethylenically unsaturated hydrocarbon group or a hydrocarbonoxy group, R' is an aliphatic saturated hydrocarbon group, Y represents a hydrolyzable organic group, n
represents 0 or 1 or 2. ), specifically, for example, R is vinyl, allyl, isopropenyl, butenyl, cyclohexenyl, γ-(meth)acryloyloxypropyl, R' is methyl, ethyl, propyl,
Decyl, phenyl, and Y are methoxy, ethoxy, formyloxy, acetoxy, propionyloxy, alkyl or arylamino. Particularly preferred are compounds represented by CH ₂ =CHSi(OA) ₃ (where A is a hydrocarbon group having 1 to 8 carbon atoms), specifically vinyltrimethoxysilane, vinyltriethoxysilane. It is also vinyltriacetoxysilane. Further, as the radical generator, any compound capable of generating free radical sites in the polyethylene under grafting reaction conditions and having a half-life shorter than 6 minutes at the reaction temperature can be used, and as mentioned above, All the compounds described in Japanese Patent Publication No. 48-1711 etc. are applicable. Typical radical generators include organic peroxides such as dicumyl peroxide, t-butyl peroxyoctate, and benzoyl peroxide, and azo compounds such as azoisobutyronitrile and methylazoisobutyrate. It will be done. The amount of the ethylenically unsaturated silane compound used is 0.1 parts by weight per 100 parts by weight of the polyethylene.
~15 parts by weight, preferably 0.5-10 parts by weight,
The amount of the radical generator used is 0.01 to 5 parts by weight, preferably 0.01 to 2 parts by weight. The grafting reaction of the ethylenically unsaturated silane compound onto the polyethylene is carried out by adding about 0.5 to 10 parts by weight of the silane compound and about 0.01 to 2.0 parts by weight of the radical generator to the polyethylene, and using, for example, an extruder or a Banbury mixer. This can be carried out by a known method by heating and kneading at a temperature at which decomposition of the radical generator occurs, generally about 150 to 200°C. The silane-modified polyethylene in the present invention has a content of the silane compound unit of 0.001 to 10% by weight,
Preferably 0.01 to 10% by weight, particularly preferably 0.5 to 10% by weight
5% by weight. Further, the melt flow rate (MFR) is preferably 1 g/10 minutes or less. This silane-modified polyethylene is molded using the normal molding method.
It is generally formed into a pipe by extrusion. The silane-modified polyethylene pipe is then crosslinked by exposing it to a water atmosphere in the presence of a silanol condensate catalyst. As silanol condensation catalysts, those that can be used as catalysts that promote dehydration condensation between silanol in silicone are generally targeted.
All compounds described in Publication No.-1711 etc. are applicable. Representative silanol condensation catalysts include dibutyltin dilaurate, dibutyltin diacetate, bibutyltin dioctoate, stannous acetate, cobalt naphthenate, ethylamine, dibutylamine, and the like. The silanol condensation catalyst is generally dispersed in the silane-modified polyethylene pipe before molding, but it can also be applied or impregnated into the silane-modified polyethylene pipe after molding as a solution or dispersion. can also be used. The amount of these silanol condensation catalysts used is generally about 0.001 to 10 parts by weight, preferably 0.1 to 5 parts by weight, based on 100 parts by weight of the silane-modified polyethylene.
Parts by weight. Exposure to a water atmosphere involves exposing the silane-modified polyethylene pipe to water (liquid or steam) at room temperature to about 200°C, usually about room temperature to 100°C, for 10 seconds or more.
The contact may be made for about one week, usually for about one minute to one day. In the present invention, the silane-modified polyethylene pipe is crosslinked by the method to increase the gel fraction to 65%.
% or more is essential, and preferably 70% or more. If the gel fraction does not reach 65%, the creep properties will be poor even if polyethylene whose density and viscosity average molecular weight satisfy the above ranges is used. Note that the gel fraction was measured by the following method. A cross-linked polyethylene sample is extracted in a Soxhlet type extractor at boiling point temperature for 10 hours using xylene as a solvent, and the weight of the extracted residue is expressed as a percentage according to the following formula. Gel fraction (%) = extraction residual weight (g) / sample weight before extraction (g)
×100 The present invention will be explained in more detail with reference to Examples below. Example 1, Comparative Examples 1 to 4 Low-pressure polyethylene with a density of 0.955 g/cm ³ and a viscosity average molecular weight of 0.90×10 ⁵ and high-pressure polyethylene with a density of 0.919 g/cm ³ and a viscosity average molecular weight of 0.39×10 ⁵ For each 100 parts by weight of polyethylene having the mixing ratio, mixture density and viscosity average molecular weight shown in Table 1,
After adding 2 parts by weight of vinyltrimethoxysilane and 0.06 parts by weight of dicumyl peroxide and mixing in a Henschel mixer, the mixture was heated to 200°C.
The average residence time is 2 in an extruder with L/D24 set to
Pellets of silane-modified polyethylene were obtained by extrusion for 1 minute. To each of these pellets, a masterbatch containing 1 part by weight of dibutyltin dilaurate in 100 parts by weight of polyethylene having the same composition ratio as the above two types of polyethylene used to obtain each pellet was added at a weight ratio of 20:1. A pipe with a nominal diameter of 13 mm and a wall thickness of 2.0 mm specified in JISK6762 was obtained at 200°C using an extruder with a diameter of 40 mm and L/D = 24, and this was immersed in hot water at 98°C for 48 hours. It was made into a cross-linked pipe. The gel fraction of each pipe was as shown in Table 1. Next, a dumbbell punched out from this pipe and with a razor notch 0.55 mm deep in the center perpendicular to the tensile direction was attached to a tensile creep test machine, and the dumbbell was tested for creep at a temperature of 80°C and a tensile load of 40 kg/cm ² . The time until breakage was measured. The results are shown in Table 1.
It was shown to.

[Table] Comparative Examples 5 to 7 Same as Example 1 except that medium-low pressure polyethylene with a density of 0.950 g/cm ³ and a viscosity average molecular weight shown in Table 2 was used, and the tensile load in the creep test was 50 Kg/cm ² . In the same manner, silane-modified polyethylene was obtained, a crosslinked polyethylene pipe was produced, and the gel fraction and creep rupture time were measured. The results are shown in Table 2.

[Table] Examples 2 and 3, Comparative Examples 8 and 9 Using medium-low pressure polyethylene with a density of 0.948 and a viscosity average molecular weight of 0.90×10 ⁵ , the amount of dicumyl peroxide added was changed, and the dumbbells were not notched in the creep test. Silane-modified polyethylene was obtained in the same manner as in Example 1, except that the tensile load was 50 Kg/cm ² , a cross-linked polyethylene pipe was produced, and the gel fraction and creep rupture time were measured. The results are shown in Table 3.

[Table] Example 4 80% by weight of medium-low pressure polyethylene with a density of 0.948 g/cm ³ and a viscosity average molecular weight of 0.90×10 ⁵ and a density of 0.920
g/cm ³ , a viscosity average molecular weight of 0.80×10 ⁵ and a mixture of 20% by weight of medium-low pressure ethylene-butene-1 copolymer, a density of 0.942 g/cm ³ , a viscosity average molecular weight of
Silane-modified polyethylene was obtained in the same manner as in Example 1, except that 0.90×10 ⁵ polyethylene was used and the amount of dicumyl peroxide added was 0.07 parts by weight, and a cross-linked polyethylene pipe was produced. The rupture time was measured. The gel fraction was 73% and the creep rupture time was 580 hours. Examples 5 and 6, Comparative Examples 10 and 11 The same medium-low pressure polyethylene and high pressure polyethylene used in Example 1 were mixed in the proportions shown in Table 4, and silane-modified polyethylene was obtained in the same manner as in Example 1. , cross-linked pipe (nominal diameter 13 mm, wall thickness
2.5mm) was obtained. Next, a short tube 50 cm in length was cut from this pipe, one end was sealed in an atmosphere of 80°C, nitrogen gas was introduced from the other end to maintain the internal pressure at 11.0 kg/cm ² , and the time required for the pipe to break was measured. did. The results are shown in Table 4.

【table】

Claims (1)

[Claims] 1 Density 0.942-0.949g/ ^cm3 , viscosity average molecular weight 0.5
×10 ⁵ or more polyethylene in the presence of a radical generator to form a compound with the general formula RSiR' nY _3-o (where R is an ethylenically unsaturated hydrocarbon group or a hydrocarbonoxy group, and R' is an aliphatic saturated hydrocarbon group). group, Y represents a hydrolyzable organic group,
n represents 0, 1 or 2. ) is heated and grafted with an ethylenically unsaturated silane compound to produce silane-modified polyethylene, and then the silane-modified polyethylene is formed into a pipe, and then exposed to a water atmosphere in the presence of a silanol condensation catalyst to achieve a gel fraction of 65%. A method for producing a crosslinked polyethylene pipe with excellent creep properties, which is characterized by crosslinking as described above.