JPS6213070B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a powder coating for coating the inner surface of metal pipes, and is useful for preventing corrosion of the inner surface of cast iron pipes. When coating the inner surface of a metal tube by a powder coating method such as a fluidized dipping method, a spray method, or an electrostatic spray method, powder coatings manufactured by a melt blending method are often used. Powder coatings produced by the melt blending method are manufactured by melting and kneading the resin component and curing agent using an S-Smelder and pulverizing the mixture. There is no separation of the resin component and curing agent during use, and if the powder coating obtained by such a melt blending method is used for coating, compared to the case of coating using the powder coating obtained by the dry blending method, A high-quality coating can be obtained. However, even when coating with a powder coating obtained by the melt blending method, there are problems such as pinholes and peeling of the coating due to curing shrinkage peculiar to the resin, and deterioration of long-term durability due to shrinkage stress of the coating. In particular, when painting the inner surface of a centrifugally cast metal tube, there are pinholes in the inner surface of the tube, and the air in these pinholes cannot be vented smoothly during painting. There is a concern that pinholes may occur in the membrane. Incidentally, in order to improve the thermal expansion and contraction of a coating film (lower the coefficient of thermal expansion), it is known to mix an inorganic powder into a coating material. However, when manufacturing powder coatings using the melt-blending method, if inorganic powders are also melt-blended, the powder unit of the coating becomes a form in which the inorganic powder is wrapped in a resin composition. Units are welded together,
As a result of the immediate loss of degassing properties through the coating film, air is trapped between the coating film and the base.
There are concerns that it may easily cause foaming and pinholes in the paint film. The powder paint for coating the inner surface of metal pipes according to the present invention can solve the above-mentioned difficulties, and is a resin composition manufactured by a so-called melt blending method in which a resin component and a curing agent are melted and mixed uniformly and then pulverized. It is characterized in that it is made by adding 5 to 40% by weight of inorganic powder to the powder and uniformly mixing it using a dry blending method. In the present invention, the reason why inorganic powder is added to the resin composition powder produced by the melt blending method is to improve the thermal expansion and contraction of the coating film. The reason why this inorganic powder is added using the dry blend method is as follows.
This is to maintain a gap between the resin composition powder and the inorganic powder for a relatively long time even after application, and to effectively vent gas. The reason for limiting it to more than % by weight is that
If the amount is less than this, the degassing effect cannot be sufficiently achieved, and pinholes and foaming phenomena will occur in the coating film. The reason why the amount of inorganic powder added is limited to 40% by weight or less is that if it exceeds this amount, the inorganic powder will inhibit the fusion of the resin composition powder, which will cause pinholes to occur in the paint film. This is because it leads to. In the present invention, the inorganic powder includes silica powder,
Inorganic pigments such as talc can be used. Examples of the present invention will be described below along with comparative examples. Example A Silica powder was added by dry blending in the weight percentage shown in Table 1 to low density polyethylene resin powder to which 2% by weight of carbon black was added by melt blending method, and each of the thus obtained The powder paint was applied to the inner surface of a ductile cast iron pipe (preheated at 270°C) to a thickness of 400μ by air spray painting. The appearance of each coating film, pinhole test results (AC1500V), 3% NaC immersion test results (after 1 year), and exposure test results (after 1 year) are as shown in Table 1.

【table】

[Table] Example B Epoxy resin (Ciel #1004); 70.6 parts (parts by weight, same hereinafter), curing agent (hydrazide derivative);
4.4 parts, silica powder; 18.6 parts, titanium oxide; 6.0 parts,
A total of 100 parts of carbon black (0.1 part) and leveling agent (0.3 parts) were melt-mixed into powder using a melt blending method, and silica powder was added thereto in the weight percentage shown in Table 2 using a dry blending method. The obtained powder paints were applied to ductile cast iron pipes (preheated to 270°C) by air spray painting.
℃) was coated on the inner surface to a thickness of 400μ. The appearance of each paint film, pinhole test results (AC1500V), 3% NaC immersion test results (after 1 year), and exposure test results (after 1 year) are shown in the second page.
It was as shown in the table.

[Table] As is clear from the comparison between the above-mentioned examples and comparative examples, according to the present invention, metal tubes are powder-coated to achieve good appearance, elimination of pinholes, and excellent corrosion resistance. This is due to the excellent film-forming ability of the resin powder using the melt blending method, and the good degassing during painting due to the addition of an appropriate amount of inorganic powder using the dry blending method. It is thought that it depends on the preservation of sexuality.

Claims (1)

[Claims] 1. A resin composition powder produced by a so-called melt blending method, in which a resin component and a curing agent are melted and mixed uniformly and then pulverized, is mixed with a dry blending method for 5 to 50 minutes.
A powder paint for coating the inside of metal pipes, characterized by adding 40% by weight of inorganic powder and mixing it uniformly.