Development and application of powder coatings for steel piles!

Abstract: Through the selection of epoxy resin, curing agent, auxiliaries, pigments and fillers, precise design of formula, repeated adjustment of test formula, combined with construction technology, field application, developed and produced the steel pile external anti-corrosion sintered epoxy powder that meets the requirements paint

The ocean is not only a huge treasure house of resources, but also an indispensable space environment for human survival and development

With the continuous improvement of China's economic strength and the needs of rapid economic development in coastal areas, the sea-crossing bridges and coastal bridges, which are the infrastructure for economic development, are constantly being built

Statistics show that there are a large number of sea-crossing bridges built and under construction in China, and the economic benefits it brings will far exceed the construction of the bridge itself

The Shanghai Donghai Bridge extends the activity radius of Shanghai people to the sea by more than 30 kilometers, which is a historic breakthrough

Another example is the construction of the Hangzhou Bay Sea-crossing Bridge, which has improved the layout of highways in the Yangtze River Delta region and the main national highways, relieved the pressure on the self-flow of Shanghai, Hangzhou and two expressways, and is conducive to the active integration of cities in the Hangzhou Bay area with Shanghai

Expanding opening up has promoted cooperation and exchanges in the Yangtze River Delta region

As the support of the sea-crossing bridge, in the marine environment, corrosion is its fatal weakness

Since the sea-crossing bridge is built in a very harsh marine environment, the serious salt spray corrosion of the ocean atmosphere is a subject that must be paid attention to in the design and construction of the sea-crossing bridge

In order to ensure safety and save maintenance costs, a reasonable anti-corrosion coating scheme should be designed for the corrosion protection of steel structures according to the type, structure, material, size and geographical location of the bridge across the sea

This project is based on the original double-layer epoxy powder research and technical achievements, and according to the characteristics of the steel pile application environment and the required coating characteristics, a formula technology improvement is carried out

Steel pile powder coating performance characteristics

Bottom layer:

①It has strong boiled adhesion and good bending resistance;

②The coating is tough and wear-resistant, with good impact and scratch resistance, and can resist the stress of the soil;

③Convenient coating and quick curing;

④ It can play a protective role in a large temperature range, and has a long-lasting anti-corrosion effect;

⑤It has excellent resistance to cathodic disbondment, chemical and electrochemical corrosion resistance, water resistance, acid, alkali and salt resistance

Intermediate layer (if required):

①Have a certain elastic function, absorb and slow release energy;

②It has excellent chemical resistance, corrosion resistance and insulation, solvent resistance, cathodic disbondment resistance, leveling and adhesion, as well as flexibility, impact resistance and other mechanical properties

Surface layer:

①Construction at the same time with the previous coating, one-time molding;

②The coating has excellent abrasion resistance, erosion resistance, impact resistance and scratch resistance, excellent mechanical properties such as chemical resistance, corrosion resistance, insulation, solvent resistance, leveling and adhesion, and flexibility ;

③The coating is tough, wear-resistant, impact-resistant, and can resist the stress of the soil;

④It has certain weather resistance

According to the requirements of the above standards, through the selection of epoxy resin, curing agent, additives, pigments and fillers, accurate design of formula, repeated adjustment of test formula, combined with construction technology and field application, developed and produced the steel pile external anti-corrosion layer that meets the requirements.
Fusion bonded epoxy powder coating
.

1.
Experimental part

1.
1 Selection of resin

Epoxy resin has the characteristics of good mechanical properties, high bonding properties, low curing shrinkage, and excellent comprehensive properties, and is widely used in the field of anti-corrosion
.

The type and relative molecular weight of epoxy resin have a great influence on the performance of coatings.
Bisphenol A epoxy resin molecules contain highly polar aliphatic hydroxyl and ether bonds
.

It has high wettability and adhesion to the substrate, and has high chemical resistance, and because its structure is alternately arranged with rigid benzene rings and flexible hydrocarbon bonds, it endows the coating with better physical properties.
mechanical properties
.

In order to improve the high temperature permeability and adhesion of the coating, when designing the formulation of the primer powder coating, the ordinary bisphenol A epoxy resin was not used, but the modified epoxy resin 1 with medium relative molecular weight was used for film formation.
substance
.

The ether bond and CC bond of the modified epoxy resin 1 make its macromolecules flexible
.
The benzene ring imparts heat resistance and rigidity to the polymer, and the isopropyl group also imparts a certain rigidity to the macromolecule
.

The modified epoxy resin 1 has a flexible long-chain structure, which endows the coating with good impact resistance, and its structure is shown in formula (1)
.

When designing the formula of the powder coating for the surface layer, in order to improve the adhesion, hardness and chemical corrosion resistance of the coating film, a certain amount of special modified epoxy resin 2 is added to the formula to match with the modified epoxy resin 1.
The resin molecule The structure not only contains more benzene rings and hydroxyl groups, but also has a longer flexible chain
.

It has excellent compatibility with the corresponding curing agent, high reactivity, strong adhesion, fast curing, high cross-linking density, and strong resistance to medium penetration
.

The coating film has excellent mechanical properties such as rigidity, heat and humidity resistance, chemical resistance, solvent resistance, cathodic disbondment resistance and impact resistance, and its structure is shown in formula (2)
.

When the addition amount of special modified epoxy resin 2 is too large, although the hardness, heat resistance, abrasion resistance and chemical corrosion resistance of the coating film will be significantly improved, it will also lead to the brittleness of the coating film, which will affect the coating film.
Film low temperature bending properties
.

The two resins were tested with a number of formulas with different ratios, and the reasonable matching ratio of the two resins was finally determined according to the test results
.

1.
2 Selection of curing agent

After the epoxy resin is determined, the performance of the curing agent determines the anti-corrosion performance of the powder coating
.
The bottom epoxy powder coating uses decyl hydrazine and phenolic resin as curing agent from the beginning
.

As far as the polymer phenolic compound is used as the curing agent, it has been proved by a large number of experiments that the latter has the best effect
.
The structure of the polymer phenolic curing agent is shown in formula (3)
.

It can be seen from its molecular structure
.
It is actually a long-chain resin terminated with a phenolic hydroxyl group, which has a similar structure to the cyclic compound resin
.

From the perspective of its synthesis mechanism, in essence, bisphenol A reacts with low molecular weight epoxy resin in the presence of a catalyst to generate high molecular weight epoxy resin, and then reacts with excess bisphenol A to generate hydroxyl-containing epoxy resin.
compound
.

Therefore, when used as a curing agent for epoxy powder coatings, it has better compatibility with resins
.
In addition, part of the free bisphenol A is equivalent to a bifunctional resin when participating in the reaction, and its activity is relatively large, which is easy to cause the ring-opening reaction of epoxy groups, and also increases the crosslinking density
.

Therefore, the polymer phenolic curing agent has high reactivity, and the resulting coating film has extremely high gloss, excellent anti-corrosion performance and mechanical properties
.

The curing mechanism of epoxy powder coatings cured by polymer phenolic compounds is complex, mainly as shown in formula (4) and formula (5):

(1) etherification reaction of phenolic hydroxyl group and epoxy group:

(2) etherification reaction of secondary hydroxyl group and epoxy group:

The curing mechanism shows that no small molecules are released during the curing process, and the coating surface does not have disadvantages such as "explosion of bubbles", and the performance will not decrease due to the increase of coating thickness
.

In addition, the curing agent contains a large amount of -OH, which greatly promotes the adhesion to the substrate
.
Through a large number of tests, it has been confirmed that the phenolic curing agent-cured pipeline powder coating has excellent boiling adhesion and -30 °C 3 ° bending resistance on the flat plate or the outer wall of the pipeline
.

The surface layer powder coating curing agent is a multifunctional functional curing agent, which is a new type of synthetic resin curing agent containing multiple functional groups.
Very active and fast curing
.

It has a very high crosslinking density, giving the coating film excellent corrosion resistance, chemical resistance, solvent resistance, as well as good leveling, flexibility and excellent adhesion and impact resistance
.

1.
3 Selection of curing accelerator

2-Methylimidazole is a commonly used curing accelerator for powder coatings, and formula (6) is the process of epoxy ring-opening by 2-methylimidazole active hydrogen
.

The addition amount of 2-methylimidazole seriously affects the curing temperature and curing time of the entire coating system.
According to a large number of laboratory data and combined with the spraying conditions on the construction site, the addition amount of imidazole in the powder coating formulations for the bottom layer and the surface layer was determined respectively.
, to ensure the bonding force between layers
.

1.
4 Selection of additives

In order to improve the density of each coating and prevent pinholes, a certain proportion of degassing agent and a certain amount of leveling agent are added to the formula
.
In improving the fluidity, flexibility and curing degree of the powder, an appropriate amount of functional additives was chosen to be added 1
.

In order to improve the hardness and smoothness of the surface layer powder and enhance the scratch resistance of the coating, a certain amount of functional additives 2 are added to the formula
.

1.
5 Choice of pigments and fillers

The selection of pigments and fillers greatly affects the final performance of the coating.
Suitable pigments and fillers can not only improve the physical and mechanical properties and chemical resistance of the coating, but also improve the construction performance and storage performance
.
This project selected wollastonite, sericite powder and a functional filler as the main fillers
.

Wollastonite is an inorganic needle-like mineral, non-toxic, chemically resistant, thermally stable and dimensionally stable, with glass and pearl luster, low water and oil absorption, and excellent mechanical and electrical properties
.

It has a certain reinforcing effect, so that the coating can get better mechanical strength, increase durability, enhance adhesion and corrosion resistance, as well as good coverage and adhesion
.

Mica powder has a unique two-dimensional flake structure, which can effectively improve the permeability resistance, weather resistance, wear resistance and corrosion resistance of the coating film.
In addition, it does not contain any radioactive elements.
It is a new type of functional green filler
.

Functional fillers enable the cured product to have excellent insulating properties and arc resistance, increase thermal conductivity, improve hardness and wear resistance, change adhesion and improve flame retardant properties, and reduce the linear expansion coefficient and curing shrinkage rate of the cured product.
Eliminate internal stress and prevent cracking
.

According to the characteristics of different fillers, different proportions are matched, and two suitable proportions are selected from a large number of test results, which are applied to the bottom and top layer formulations
.
Pigments are determined according to color requirements
.

2.
Formulation composition and performance

2.
1 Formulation composition

The formula of the fusion-bonded epoxy powder coating for the outer anti-corrosion layer of the steel pile is shown in Table 1
.

2.
2 Performance testing

The sintered epoxy powder coating for the outer anti-corrosion layer of steel piles has been approved by the national authoritative department according to the enterprise standard of China National Petroleum Corporation Q/SY 1038-2007 or Q/CNPC 38-2002 (buried steel pipeline double-layer sintered epoxy powder coating See Table 2 and Table 3 for the performance indicators of coating technical specification) testing
.

3.
Production technology and construction technology

3.
1 Production process

The main production process of the product is: raw material → weighing → premixing → melt extrusion → tableting → coarse crushing → grinding (classification, screening) → mixing with functional additives 1 → testing → packaging → storage

In the specific operation, first prepare the required raw materials, and accurately weigh various raw materials according to the formula
.
According to the feeding sequence of resin→filler, pigment→filler→resin, it is put into the mixer and mixed, melt extruded [1st zone 65-70°C, second zone 90-100°C, third zone 70-80°C], and then press Slicing, crushing, grinding, screening, adding functional additives 1 at the same time, and finally passing the sample screening test, weighing and packaging, and then identifying the library
.

3.
2 Construction technology

(1) Before coating the outer surface of the steel pipe, an appropriate method must be used to remove the oil, grease and other dirt and impurities attached to the surface
.
Surface treatment equipment and components entering the steel pipe must not contain substances that contaminate the surface of the steel pipe
.

(2) For steel pipes transported by sea, the outer surface of the steel pipes shall be inspected for salt content
.
If the content exceeds 50mg/m2, it needs to be treated with phosphoric acid or deionized water
.
The residual salt content of the outer surface should be measured after phosphoric acid or deionized water treatment, at least twice per shift, and the salt content should not exceed 50mg/m2
.

(3) Before spraying (throwing) to remove rust, the steel pipe should be preheated to remove moisture, and the preheating temperature should be 40-60℃
.

(4) The spraying (throwing) rust removal on the outer surface of the steel pipe shall reach the Sa 2.
5 level specified in GB/T 8923
.
The depth of the anchor pattern on the surface of the steel pipe should be 40-100 microns, and meet the recommended requirements of the epoxy powder manufacturer
.

(5) After spraying (blasting) shot to remove rust, the rust and dust remaining on the outer surface of the steel pipe should be cleaned, and the existence of dust and other pollutants is not allowed
.
The steel pipe surface should be sprayed within 4 hours after pretreatment
.
When the surface of the steel pipe is rusted or the surface is contaminated, the surface pretreatment must be repeated
.

(6) After spraying and removing rust, clean and dry compressed air should be used to blow the inner surface of the steel pipe to remove the remaining steel shot/sand and dust
.
The surface dust degree shall meet the requirements of the current national standard GB/T 1 8570.
3 (Evaluation test for surface cleanliness of steel surface treatment before coating coating Part 3: Dust assessment on steel surface before coating coating (pressure-sensitive adhesive tape method)) Level 2
.

(7) The surface defects that may affect the quality of the anti-corrosion layer should be repaired.
For the steel pipes with serious defects or the defects cannot be removed, a consensus should be obtained through consultation with the owner
.

(8) The compressed air used for coating must be clean, dry and free of oil
.

(9) A non-polluting heat source should be used to uniformly heat the steel pipe
.

(10) Apply the anti-corrosion layer according to the coating process determined by the test.

(11) The curing of the coating should be cured at 220℃ for 3min
.

(12) The length reserved at both ends of the steel pipe should be (85 ± 5) mm, and the surface of the reserved section shall not be coated
.
If the owner has requirements, it shall fully comply with the relevant requirements of the owner
.

(13) The cured anti-corrosion layer should be cooled by water cooling method in time
.

4.
Conclusion

Through the selection of raw materials, especially the selection of epoxy resin and curing agent, combined with a large number of experimental results, a reasonable matching ratio of resin and curing agent is obtained, which makes the powder coating have excellent adhesion and anti-cathodic disbonding performance
.

Combined with the on-site construction process, by adjusting the formula of the multi-layer powder coating, constantly changing the curing temperature and gelling time, to ensure that there is no problem in the bonding force between the coatings; standardize the production process, and develop and produce a steel pile exterior anti-corrosion powder coating with excellent performance And passed the inspection of the latest oil and gas industry standards
.

This coating has been applied in many units since its successful development, and has achieved good results in the anti-corrosion of bridge steel piles
.

The multi-layer fusion-bonded epoxy powder coating has excellent mechanical properties, wear resistance, scratch resistance and chemical corrosion resistance.
It is very suitable for anti-corrosion applications on the outer surface of bridges and wharf steel piles.
Fang's unanimous praise
.

Its excellent characteristics have gradually been recognized by people.
With the further improvement of its popularity and the continuous expansion of the market, it will bring greater benefits
.

Source: Shengli Oilfield Fangyuan Anticorrosion Material Co.
, Ltd.