Preparation of powder coating for hot-dip galvanized steel pipe

Abstract: The anti-corrosion advantages of hot-dip galvanized steel pipes are pointed out, and the reasons for the poor adhesion of powder coatings on the surface of hot-dip galvanized steel pipes are analyz.

0 Preface

It has long been recognized that zinc has excellent corrosion protection properties for steel coatings online coating.

Test part

1 Test raw materials

Epoxy resin A, epoxy resin B; phenolic curing agent; dicyandiamide; polyester resin A, polyester resin B, polyester resin C, polyester resin D, polyester resin E, polyester resin F; flow Leveling agent; filler; polyethylene wax A; polyamide wax B; silane coupling age.

2 Preparation of coatings

Accurately weigh the raw materials of each component according to the formula, put them into a mixer for high-speed mixing, and then extrude and cool them through a twin-screw extrud.

3 Preparation of the workpiece

Workpiece type: DN=50 mm galvanized steel pipe; pretreatment method: light sandblasting to remove impurities such as zinc oxide on the surface of the steel pipe and retain the zinc layer; spraying process: 15 min@180 ℃ preheating, 20 min@200 ℃ post-curing ; Coating thickness: 250~350 .

4 Detection method

1 Testing method of galvanized layer performance The galvanized layer performance of the hot-dip galvanized steel pipe used in the experiment shall meet the requirements for the galvanized layer in clause 9 of the standard GB/T 3091-2015 "Welded Steel Pipes for Low-Pressure Fluid Transpor.

2 Testing method of powder coating performance

Referring to the standard requirements of CJ/T 120-2016 "Plastic-coated composite steel pipes for water supply", the workpieces were made for 24 h@95 ℃ boiling adhesion testing, and a rectangle of 15 mm × 30 mm was drawn for rati.

The adhesion grade of the rectangular inner coating is evaluated according to the following grading standards:

Class 1 - the coating is clearly not able to be peeled off by prying;

Class 2 - Pryed off less than or equal to 50% of the coating;

Level 3 - more than 50% of the coating is pryed off, but the coating exhibits significant anti-prying performance;

Class 4 - The coating can be easily pryed into strips or large pieces;

Class 5 - The coating is peeled off in one piece [

It should be specially pointed out that the CJ/T 120-2016 standard tests the adhesion of the inner coating of the steel pi.

Results and Discussion

1 Analysis of the reasons for the poor adhesion of powder coatings on hot-dip galvanized steel pipes

As we all know, the adhesion of powder coatings on galvanized pipes, especially after boiling, has always been a problem in the indust.

(1) Influence of galvanizing process: Before galvanizing, the residue on the surface of the substrate is too high, which will cause the galvanized layer to fall o.

(2) The galvanized layer is too thick, gas is mixed in the galvanized layer, and it expands during baking, resulting in a large number of pinholes on the surface of the coating, and in severe cases, the coating film falls off;

(3) The galvanized layer has a certain activity, and it is easy to react with some groups in the powder coating at high temperature, which affects the curing of the coating and causes the adhesion of the coating to decrease;

(4) The secondary product of zinc, the basic double salt is easily soluble in water, and expands after boiling, which affects the adhesion of the coati.

2 Influence of powder system on the adhesion of galvanized pipe

In our common powder coating systems, a large amount of water molecules are released during the curing process of polyester hydroxyalkyl amide system, which causes pinholes, which is not conducive to thick coating; acrylic, polyurethane, fluorocarbon and other systems have high costs and are usually not used for The coating of the pipeline; so the experiment selected epoxy dicyandiamide, epoxy phenol, epoxy polyester system and polyester TGIC system for comparis.

The experimental results show that the adhesion of epoxy dicyandiamide system, epoxy phenolic system and epoxy polyester system was tested on lightly sandblasted hot-dip galvanized steel pipe after being boiled at 95 ℃ for 24.

In order to prevent the accidental occurrence of the experiment and affect the judgment of the results, the verification experiment was carried out again according to the above formula and process conditions, and the test results were consistent with the above experimen.

3 Influence of polyester types on the adhesion of galvanized pipes

The above experiments found that the polyester TGIC system had better adhesion on lightly sandblasted galvanized steel pipes after being boiled at 95 ℃ for 24.

The experimental results show that the adhesion of different types of polyesters on lightly sandblasted hot-dip galvanized steel pipes after being boiled in water at 95 ℃ for 24 hours is quite differe.

Among them, polyester resin B and polyester resin C were boiled at 95 ℃ for 24 h to test the adhesion, and the coating was peeled off as a whole, and the adhesion rating was grade 5; while polyester resin D, polyester resin F and poly Ester resin A is similar, and the adhesion is tested after being boiled at 95 ℃ for 24 hou.

4 The effect of degassing agent on the adhesion of galvanized pipe

Considering that there is gas residue in the galvanized layer of the pipe fittings, if the gas cannot be discharged in time during baking, the porosity of the coating will be increased, thereby affecting the performance of the coati.

In the experiment, polyester resin A with better boiling adhesion was selected, and the representative degassing agents on the market, benzoin, polyethylene wax A and polyamide wax B, were selected to increase the degassing performance of the coating, and the boiling adhesion was test.

The experiments are shown in Table 3 and Figure

The experimental results show that on the hot-dip galvanized steel pipe treated by light sandblasting, the addition of polyethylene wax A and polyamide wax B will affect the adhesion of boiling wat.

The flakes were peeled off and the adhesion was rated 5, not recommended for additi.

The addition of benzoin can improve the degassing performance of the coating, and has little effect on the adhesion of boiling water, so it can be added in an appropriate amou.

5 Influence of silane coupling agent on the adhesion of galvanized pipe

According to the literature, it is found that the silane coupling agent has a special YSiX3 structu.

The mechanism of action is shown in FigureX is usually a chlorine group, a methoxy group, an ethoxy group, a methoxyethoxy group, an acetoxy group, e.

, and these groups are hydrolyz.

Silanol [Si(OH)3] is generated when it is formed, which can be effectively combined with the metal substrate; Y is vinyl, amino, epoxy, methacryloyloxy, mercapto or urea, and these reactive groups can be It reacts with the reactive groups in the coating to combi.

Therefore, by using a silane coupling agent, a "molecular bridge" can be set up between the interface of inorganic substances and organic substances, connecting the two materials with disparate properties together, improving the performance of the coating, and increasing the substrate and coating properti.

The bond strength between layers [

In the experiment, polyester resin A with better boiling adhesion and degassing agent benzoin were selected, and the total mass fraction of the formula was 5‰ silane coupling agent, 10‰ silane coupling agent and blank sample to compare the boiling adhesi.

The performance test results are shown in Table 4 and Figures 5~

The experimental results show that the addition of silane coupling agent can further improve the boiled adhesion of the coating on the lightly sandblasted hot-dip galvanized steel pi.

After being boiled at 95 °C for 24 h, the adhesion was tested, and the coating could not be peeled off obviously, and the adhesion was rated as gradeHowever, the addition of 10‰ silane coupling agent will reduce the T g of the powder, affect the storage stability of the powder, and increase the formulation co.

Comprehensive consideration, it is recommended to add a silane coupling agent with a mass fraction of 5‰ in the formu.

Conclusion

(1) The hot-dip galvanized steel pipe has excellent corrosion protection performance, and the three-layer protection system of "steel pipe + galvanized layer + powder coating" composed of powder coating further prolongs the service life of the steel pi.

(2) Due to the improper control of the galvanizing process, the zinc layer is too thick, gas is mixed in the zinc layer, and the gas breaks through the coating after heating, which affects the compactness of the coati.

and other factors, the adhesion of powder coatings on the galvanized layer is greatly affect.

(3) In common powder coating systems, a large amount of water molecules are released during the curing process of polyester hydroxyalkyl amide system, which is not conducive to thick coating; acrylic, polyurethane, fluorocarbon and other systems have high costs and are usually not used for pipeline coati.

Compared with epoxy dicyandiamide, epoxy phenol, epoxy polyester system and polyester TGIC system, it is found that polyester TGIC system shows excellent boiled adhesion on sandblasted hot-dip galvanized steel pi.

(4) The adhesion of different types of polyester resins on lightly sandblasted hot-dip galvanized steel pipes after being boiled at 95 ℃ for 24 h has a great differen.

When using, it should be selected carefully through experimen.

(5) On the hot-dip galvanized steel pipe treated with light sandblasting, the addition of polyethylene wax and polyamide wax will affect the adhesion of boiled water, and it is not recommended to add .

The addition of benzoin can improve the degassing performance of the coating, and has little effect on the adhesion of boiling water, so it can be added in an appropriate amou.

(6) Silane coupling agent has a special YSiX3 structure, in which X represents an alkoxy group, and the silanol generated after hydrolysis can be effectively combined with the metal substrate; Y is other active group, which can be combined with the reactive groups rea.

On the hot-dip galvanized steel pipe treated with light sandblasting, the addition of silane coupling agent can further improve the boiled adhesion of the coati.