JPS605265A - Preparation of hot water resistant silane crosslinked polyethylene-coated steel pipe - Google Patents

Abstract

PURPOSE:To form a silane crosslinked polyethylene film excellent in hot water resistance to the inner surface of a steel pipe, by a method wherein special polyethylene with a specific hot melt index is applied to the inner surface of the steel pipe and succeedingly crosslinked. CONSTITUTION:The inner surface of a heated steel pipe is coated with a polymer which is at least one of a silane grafted polyethylene powder or a silane copolymerized polyethylene powder obtained by copolymerizing the main chain of polyethylene with silane such as vinyltriexthoxysilane and of which the melt index is 2-10. In the next step, a thermal crosslinking catalyst such as zinc stearate or dibutyl tin laurate is used in an amount of 0.1-0.01wt% on the basis of the above mentioned polyethylene to subject the film to silane-crosslinking. By this method, a film reduced in blister generation and having hot water resistance and good adhesiveness is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a silane-crosslinked polyethylene coated steel pipe for hot water use.

Conventionally, coated steel pipes with a polyvinyl chloride or polyethylene coating formed on the inner surface of the steel pipe have been used as water-resistant piping materials, but these have poor heat resistance and are easily softened and deformed by hot water, causing the coating to deteriorate. There are some drawbacks that make it useless as an instrument.

Therefore, as a solution to such shortcomings.

For example, steel pipes coated with water-crosslinked polyethylene on the inner surface as seen in Japanese Utility Model Application Publication No. 57-190980,
Furthermore, steel pipes whose inner surfaces are coated with cross-linked PPS resin have been proposed.

However, in the former method, blisters are likely to occur in the coating film, resulting in poor hot water resistance and the inability to maintain corrosion protection on the inner surface of the steel pipe for a long period of time. In the case of the latter, although hot water resistance can be ensured by applying a thick coating, PPS resin is expensive and requires a large amount of energy and production time to crosslink, making it difficult to supply it at a low price. There is a drawback that it cannot be done.

The present inventors aimed to eliminate the conventional drawbacks as described above (
As a result of extensive research aimed at providing a manufacturing method for coated steel pipes that have excellent hot water resistance and can be supplied at low cost, we have developed silane-grafted polyethylene or silane copolymerization with a melt index within a certain range. It was discovered that polyethylene cross-linked with silane is less prone to blistering and has excellent hot water resistance.In addition, these polyethylene resins have significantly lower costs than the PPS resins mentioned above, and have a lower cost than the above-mentioned PPS resins. The present invention was completed by focusing on the fact that the cost and production time are significantly lower.

That is, 1. The gist of the present invention is that at least one kind of silane-grafted polyethylene or silane-copolymerized polyethylene powder having a melt index of 2 to 10 is applied to the inner surface of a heated steel pipe, and then cross-linked with silane. The present invention relates to a method for manufacturing a silane-crosslinked polyethylene coated steel pipe for hot water use.

The silane-grafted polyethylene used in the present invention is
Ethylene-based resins such as polyethylene, ethylene-based copolymers containing ethylene as the main monomer component, or chlorinated polyethylene, and the general formula RR' 5iY2 (wherein R is bonded to silicon through a silicon-carbon bond, and carbon, hydrogen and optionally oxygen, and each Y represents a hydrolyzed I selected from alkoxy, alkoxy-alkoxy, acyloxy and oxime groups.
the presence of a compound that generates free radicals with a silane represented by a monovalent hydrocarbon group having no aliphatic unsaturation, and R' represents a monovalent hydrocarbon group having no aliphatic unsaturation; It is obtained by bonding the above-mentioned silane to an ethylene resin through the reaction described below.It should be noted that the polyethylene to be grafted T, LT-D (1)-0,
90~0.92> or I7MD(ρ-0,92~0.9
4) Polyethylene gives good results in the present invention. In addition, the grafting rate of polyethylene is lo ~ 85
% is preferable.

The silane copolymerized polyethylene used in the present invention is obtained by copolymerizing silane to the main chain of polyethylene.

Examples of the silane include vinyltrier 1, xysilane, vinyltrimethoxysilane, vinylmethyljethoxysilane, vinylphenyldimethoxysilane, and the like.

The silane grafted polyethylene or silane copolymerized polyethylene has a melt index of 2 to 2.
It is preferably in the range of 10.

If the melt index is less than 2, the flowability will be poor and surface irregularities will form on the coating film formed on the inner surface of the steel pipe.As a result, blisters will occur within a short period of time, making it difficult to prevent corrosion on the inner surface of the steel pipe. It becomes difficult to maintain for a long period of time. On the other hand, if the melt index is greater than 10, even if the molecular chains are bonded by crosslinking, the network constituent units will collapse into blocks and will move easily against hot water, and as a result,
Unless the degree of crosslinking is significantly increased, the effects of crosslinking will be difficult to develop, and in this case also there will be the disadvantage that blisters will occur in a short period of time.

The crosslinking catalyst used for silane crosslinking in the present invention is:
For example, one that is a solid powder at room temperature is good.

Preferred are alkalis of Group 1 of the periodic table, alkaline earth metal soaps of Group 2, and solid organic tin oxides such as dialkyltin oxides. Among these, it is particularly preferable to use a catalyst for thermal crosslinking; when a catalyst for thermal crosslinking is used, the crosslinking process is simpler and the degree of crosslinking is higher than when using a catalyst for water crosslinking, so manufacturing costs are reduced. There is an advantage that it can be done. Examples of the catalyst for thermal crosslinking include zinc stearate, dibutyltin oxide, and the like.

The appropriate amount of the crosslinking catalyst used is 0.1-0.011 parts by weight per 100 parts by weight of silane-grafted polyethylene or silane-copolymerized polyethylene.

Next, a specific example of the manufacturing method of the present invention will be explained.

Silane-grafted polyethylene or silane-free base polyethylene is pulverized into a powder of about 10 to 3 o mesh, and to this the powder of the above-mentioned crosslinking catalyst is added to 100 parts by weight of the polyethylene, about 0.1 to 0. Coating IIQ is formed by adding and mixing the mixture, coating it on the inner surface of a steel pipe preheated to 180 to 220'C, and repeating melting. The thickness of the coating film is 3
It is preferable to set it as 00-700 micrometers. In addition, when coating is performed when the temperature of the steel pipe is about 180"C, it is preferable to once raise the temperature of the steel pipe to about 220"C after coating.

Note that the manufacturing method of the present invention is not limited to the above example.

By the way, 1. In the present invention, a blend of 10% or less of normal polyethylene with the silane grafted polyethylene or silane copolymerized polyethylene may be used. 1. If the content of normal polyethylene is 10% or less, the hot water resistance will not be affected too much. It doesn't come.

This is because 5 ordinary polyethylene improves the flowability of the resin as a whole, allowing smooth coating film formation.

As explained above, in the manufacturing method of the present invention, at least one type of silane-grafted polyethylene or silane-copolymerized polyethylene having a melt index of 2 to 10 is crosslinked with silane, so it has excellent hot water resistance and , it is possible to provide inexpensive coated steel pipes.

In addition, when a thermal crosslinking catalyst is used for silane crosslinking, the crosslinking process is simple and the degree of crosslinking is high.

Manufacturing costs can be further reduced.

An example of the present invention will be shown below.

[Example 1] A silane-grafted polyethylene pellet with a melt index of 2 was ground into a powder of 13 mesosinus or less, and zinc stearate powder was added as a thermal crosslinking catalyst to this powder at a weight of 0.001 weight per 100 weight parts of the polyethylene. The mixture is mixed, applied to the inner surface of a steel pipe that has been heated to 200°C, and then kept at 200°C for 5 minutes. This process was repeated five times to form a coating film with a thickness of 380 μm.

[Example 2] Silane-grafted polyethylene pellets with a melt index of 2.4 were used in place of the silane-grafted polyethylene pellets with a melt index of 2 used in the example process, and the film thickness was adjusted in the same manner as in Example 1. A coating film of 380 μm was formed.

[Example 3] Silane-grafted polyethylene pellets with a melt index of 10 were used in place of the silane-grafted polyethylene pellets with a melt index of 2 used in Example 1, and dibdeltin was used in place of zinc stearate used in Example 1. Use Oki-go ID.

A coating film having a thickness of 380 μm was formed in the same manner as in Example 1.

[Example 4] Instead of the silane-grafted polyethylene pellets with a melt index of 2 used in Example 1, pellets made by blending 90% vinyltrimethoxysilane-grafted polyethylene with a melt index of 2 and 10% of normal polyethylene were used. , the film thickness was 380 mm using the same method as in Example 1.
A coating film of μ was formed.

Next, a comparative example with respect to the above example will be shown.

[Comparative Example 1] Silane-grafted polyethylene pellets with a melt index of 0.9 were used in place of the silane-grafted polyethylene pellets with a melt index of 2 used in Example 1,
And cyphthyltin oxide was used in place of the zinc stearate used in Example 1.

A coating film having a thickness of 380 μm was formed in the same manner as in Example 1.

[Comparative Example 2] The same method as in Example 1 was carried out except that vinyltrimethoxysilane-grafted polyethylene pellets with a melt index of 1.3 were used in place of the silane-grafted polyethylene pellets with a melt index of 2 used in Example 1. A coating film having a thickness of 380 μm was formed.

[Comparative Example 3] Silane-grafted polyethylene pellets with a melt index of 2 used in Example 1 were replaced with vinyl trime-1-kidisilane-grafted polyethylene pellets with a melt index of 15, and the same method as in Example 1 was used. A coating film with a film thickness of 380 μm was formed by this method.

In addition to Comparative Examples 1 to 3 above, three ordinary polyethylene coated steel pipes were used as Comparative Example 4.

Regarding Examples 1 to 4 and Comparative Examples 1 to 4, in order to examine hot water resistance, the steel pipes were immersed in hot water at 95°C.

It was taken out every 100 hours and the time until the blister burst was measured.

Also, regarding Examples 1 to 4 and Comparative Examples 1 to 3.

In addition to measuring the crosslinked gel fraction, the beer peel force at 180° C. was also measured to examine the adhesion of the coating film.

The above results are shown in the table below.

From the table above, it can be seen that all of the examples of the present invention are less likely to generate blisters than the comparative examples, and have excellent hot water resistance as well as excellent adhesiveness.

riOQ-

Claims (1)

[Scope of Claims] 1) At least one kind of silane-grafted polyethylene or silane-copolymerized polyethylene powder having a melt index of 2 to 10. A method for producing a silane-crosslinked polyethylene coated steel pipe for heat-resistant water use, which comprises coating the inner surface of a heated steel pipe and then crosslinking with silane. 2) The method for producing a silane-crosslinked polyethylene-coated steel pipe for hot water use according to claim 1, wherein the silane crosslinking is carried out using a thermal structure catalyst.