JPH01284370A - Production of metal tube lined with polyvinylidene fluoride - Google Patents

Abstract

PURPOSE:To form a polyvinylidene fluoride film having uniform appearance on the entire inside of a metal tube by forming a molten polyvinylidene fluoride film on the inside of the metal tube, isolating the interior of the tube from the open air and cooling the tube to a temp. below the crystallization temp. of the polymer. CONSTITUTION:Polyvinylidene fluoride powder is adhered to the inside of a preheated metal tube such as a steel tube by sending under pressure, suction or other method and a perfectly molten polyvinylidene fluoride film is formed by heating to the m.p. of the polymer or above. Caps of asbestos cement or the like are fixed on both ends of the tube to isolate the interior of the tube from the open air and the tube is uniformly cooled to a temp. below the crystallization temp. of the polymer. A polyvinylidene fluoride film having uniform appearance is formed on the entire inside of the metal tube, no residual strain remains and blistering is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for manufacturing polyvinylidene fluoride coated metal tubes.

(Prior Art) Polyvinylidene fluoride is tough, has excellent corrosion resistance, chemical resistance, and heat resistance, and has excellent melt processability compared to other fluorine-based polymers. Therefore, metal pipes such as steel pipes whose inner surfaces are coated with such polyvinylidene fluoride are suitably used for piping in chemical plants and the like.

Conventionally, this type of coated metal pipe is produced by applying polyvinylidene fluoride powder to the inner surface of a metal pipe such as a steel pipe using a powder coating method, and then heating it to form a molten film of polyvinylidene fluoride. It is manufactured by cooling a metal tube to an appropriate temperature with both ends open.

(Problems to be Solved by the Invention) However, in such a manufacturing method, the cooling rate is different between the center part and the end part of the metal tube, and the crystal size of the solidified film is different. As a result, the end portions of the tube have a relatively glossy coating, but the center portion of the tube has a dull coating, resulting in a patchy coating as a whole. Furthermore, there is a problem in that large residual stress remains in the resulting coating, and when hot water is passed through the coated metal tube, blisters are likely to occur in the coating.

The present invention solves the above-mentioned problems, and its purpose is to efficiently manufacture polyvinylidene fluoride-coated metal tubes that form a coating that has a uniform appearance as a whole and that do not generate blisters during use. The purpose is to provide a method for manufacturing well.

(Means for Solving the Problems) The method for manufacturing a polyvinylidene fluoride coated metal tube of the present invention includes:
It is characterized by forming a molten coating of polyvinylidene fluoride on the inner surface of a metal tube, insulating the inside of the metal tube from outside air, and cooling it to a temperature lower than the crystallization temperature of the polyvinylidene fluoride, The above objectives are thereby achieved.

The polyvinylidene fluoride used in the present invention is produced by a suspension polymerization method or an emulsion polymerization method, and is manufactured by Melt FloivRat.
e (per ASTM 01238) at 235°CX5
2 to 30 g/10 minutes under the condition of 000 g is preferable. When the amount is less than 2 g/10 minutes, it is difficult to form a film without using a solvent or plasticizer. On the other hand, if it is 30 g/10 minutes or more, the mechanical strength such as impact strength of the coating decreases.

Incidentally, an inorganic filler such as glass, carbon, or ceramic may be added to polyvinylidene fluoride. These inorganic fillers are mixed in the form of fibers, powder or flakes. Such inorganic fillers increase the strength of the coating resin and
It is more effective in preventing the occurrence of blisters.

A powder coating method is generally employed to form a molten coating on the inner surface of a metal pipe such as a steel pipe using such polyvinylidene fluoride.

Specifically, the particle size is about 1 to 300 μm and the melting point is about 200 μm.
Prepare polyvinylidene fluoride powder at ~350°C.

This powder is then applied to the inner surface of a metal pipe such as a steel pipe that has been preheated to 200 to 350''C.The metal pipe is usually subjected to surface treatments such as alkaline degreasing, pickling, and blasting.

Further, a primer may be applied in advance. Methods for adhering powder to the inner surface of the metal tube include a pressure feeding method, a suction method, a spraying method by inserting a paint gun, and a fluidized dipping method.

If the polyvinylidene fluoride is completely melted only by preheating the metal tube, there is no need to reheat it.

However, usually after applying the powder to the inner surface of the metal tube,
This is heated in an oven at a temperature higher than the melting point of polyvinylidene fluoride, e.g. 200 to 350"C or higher, preferably 250 to 2
Heat to 80°C to form a complete molten coating.

After forming a molten coating of polyvinylidene fluoride on the inner surface of the metal tube, the inside of the metal tube is isolated from the outside air. As a method of shutting off, a method generally employed is to cover both ends of the metal tube with caps.

This method facilitates isolation from the outside air. The material used for the cap is one that is heat resistant and has good heat insulation properties, such as asbestos cement or calcium silicate.

In this way, the metal tube isolated from the outside air is uniformly cooled to a temperature lower than the crystallization temperature of polyvinylidene fluoride, preferably 10° C. lower than the crystallization temperature. Cooling methods include leaving the metal tube in an atmosphere at a constant temperature, immersing the metal tube in water at a constant temperature or warm water, and uniformly blowing air or water onto the metal tube. The method of attachment etc. will be adopted.

The above-mentioned crystallization temperature is defined as the exothermic peak temperature due to crystallization when polyvinylidene fluoride is cooled from a molten state by differential thermal analysis.

The cooling rate of the metal tube is, if uniform cooling is performed,
Either rapid cooling or slow cooling may be used, but in order to easily achieve uniform cooling, cooling is preferably performed at a rate of 5° C./second or less. After cooling the metal tube, the caps at both ends are removed to obtain a polyvinylidene fluoride coated metal tube.

(Function) In the method of the present invention, the interior of the metal tube is cooled while being isolated from the outside air, so that the molten coating of polyvinylidene fluoride formed on the inner surface of the metal tube is not affected by the outside air. Therefore, this molten coating is uniformly cooled throughout in conjunction with the cooling of the metal tube itself, and there is almost no difference in cooling rate.

(Example) Examples and comparative examples of the present invention are shown below.

After the inner surface of a steel pipe with a nominal diameter of 50 A and a length of 50 cm was subjected to grid blasting treatment, it was cleaned by blowing compressed air. Apply a phenolic primer to the inner surface of this steel pipe and
After drying at C for 30 minutes, it was baked at 200°C for 30 minutes to perform surface treatment.

The above steel pipe was left in an oven at 250°C for 30 minutes, then taken out, and a powder coating gun was inserted into the inner surface of the steel pipe.
Polyvinylidene fluoride powder (average particle size 50 μm) is applied to the inner surface of the steel pipe.
, MeltFlow Ra at 235°CX5000g
te 15 g/10 min, crystallization temperature 140°C)
It was attached by spraying.

This steel pipe was again left in an oven at 250'C for 30 minutes to completely melt the polyvinyl fluoride, and the steel pipe was taken out of the oven, and both ends of the steel pipe were covered with caps made of asbestos cement with a thickness of 15 mm. The interior was isolated from the outside air. In this state, the steel pipe was left in an atmosphere at 22°C and cooled until the steel pipe temperature reached 120°C. The temperature of the steel tube when it was taken out from the oven was 245°C, and the time required to cool it to 120°C was 10 minutes.

After that, remove the cap, cool the steel pipe to room temperature,
A polyvinylidene fluoride coated steel pipe with a film thickness of about 600 μm was obtained.

Accident ±1 Same as Example 1 except that a steel pipe with a nominal diameter of 100A and a length of 100cm was used, and the steel pipe with the inside isolated from the outside air was cooled by immersing it in 80°C warm water for 30 seconds. I went to

The temperature of the steel pipe before being immersed in hot water was 240°C, and the temperature of the steel pipe after being taken out from hot water was 110°C. The film thickness of the obtained polyvinylidene fluoride coated steel pipe was about 500 μm.

■ Except that the steel pipe was cooled without caps on both ends.
The same procedure as in Example 1 was carried out.

The coatings of the polyvinylidene fluoride coated steel pipes obtained in Example 1.2 and Comparative Example were visually observed.

As a result, the coating according to Example 1 is not very glossy overall, and the coating according to Example 2 is relatively glossy overall. However, all the coatings had a uniform appearance with no spots as a whole. On the other hand, the coating according to the comparative example was relatively glossy at the end of the tube, lacked gloss at the center of the tube, and had an uneven and uneven appearance as a whole.

Furthermore, the polyvinylidene fluoride coated steel pipe obtained in Example 1.2 and Comparative Example was connected to a part of a pipe line through which hot water at 85°C always flows, a practical test was conducted for one month, and then the coated steel pipe was was removed and the film was visually observed. As a result, no similar abnormalities were observed in the coating prepared in Example 1.2. On the other hand, in the coating according to the comparative example, many blisters were generated over a range of 15 cm inward from both tube parts.

(Effect of the invention) The method for manufacturing a polyvinylidene fluoride coated metal tube of the present invention includes:
With the structure described above, the molten polyvinylidene fluoride film formed on the inner surface of the metal tube is cooled uniformly over the entire surface, with almost no difference in cooling rate.

Therefore, a coating with a uniform appearance is formed over the entire inner surface of the metal tube, and large residual stresses due to differences in cooling rates do not remain in the coating, and blisters do not occur even when hot water is passed through the coating. There's nothing to do.

Furthermore, the method of the present invention does not require cooling under special cooling conditions that reduce the residual stress in the coating, and can be carried out by either rapid cooling or slow cooling. Therefore, production efficiency does not decrease.

Claims (1)

[Claims] 1. A molten coating of polyvinylidene fluoride is formed on the inner surface of a metal tube, the interior of the metal tube is isolated from the outside air, and the film is cooled to a temperature lower than the crystallization temperature of the polyvinylidene fluoride. A method for manufacturing a polyvinylidene fluoride coated metal tube.