Preparation of a two-component waterborne epoxy coating with early water resistance

0Introduction

In the laboratory, after the industrial coating baked at low temperature is prepared, it needs to be cured at room temperature for 7 d or 14 d.

This paper discusses the possible solutions of water-based epoxy coatings with early water resistance, and analyzes the influencing factors of coating resin composition, pigments, fillers, additives, etc.

1 Experimental part

1.

The basic formulation of the coating is shown in Table 1

(1) Slurry:

Add each component into the dispersing grinding tank in turn, stir at high speed for 10-20 min at 1000-2000 r/min, pour in zirconium beads, grind, grind to a fineness of ≤30 μm, and discharge

(2) Component A of two-component water-based epoxy coating:

Put the epoxy emulsion and slurry into the pulling cylinder, and under high-speed stirring at 500-1000 r/min, add leveling agent, defoaming agent, anti-flash rust auxiliary agent, thickener in turn, and adjust the non-volatile components with deionized water.

(3) Two-component water-based epoxy coating component B

Mix CTW-6102 water-based epoxy curing agent and deionized water evenly in proportion, and filter out the material through a 200-mesh filter screen

Table 1 Basic formulation of paint

1.
2 Preparation of the coating film

Cold-rolled steel plate is degreasing, polished with 80-grit sandpaper, and cleaned with alcohol
.
The A and B components of the two-component water-based epoxy coating are mixed evenly in proportion, and deionized water is added to adjust the viscosity of the coating to 25-35 s in 4 cups, and sprayed with compressed air
.
After spraying, flash dry at 35-40 °C for 5 min, bake at 50-55 °C for 30 min, and control the dry film thickness at (40±10) μm
.
After the test panel was cured at room temperature for 24 h, various properties of the coating film were tested
.

The pot life of the coating is tested according to GB/T 31416-2015
.

The storage stability of coatings is tested according to items 3, 4 and 5 in GB/T 6753.
3-2006
.

The pencil hardness of the coating film is tested according to GB/T 6739-2006
.

The flexibility of the coating film is tested according to GB/T 1731-2020
.

The adhesion of the coating film is tested according to GB/T 9286-1998
.

The water resistance of the coating film was tested according to GB/T 1733-1993
.

The coating film resistance to neutral salt spray is tested according to GB/T 1771-2007
.

The moisture and heat resistance of the coating film is tested according to GB/T 1740-2007
.

2 Results and discussion

2.
1 Influence of epoxy emulsion

Due to the wide variety of epoxy resins, the relative molecular weight range is relatively wide.
With the change of the relative molecular weight, the coating film flexibility, adhesion, wetting ability to the substrate, viscosity and toughness will change accordingly
.
In fact, the number of epoxidation reactions can be infinite and a huge variety of epoxy polymers have been produced
.
In this experiment, to improve the early water resistance of the coating film, one of the directions that can be considered is to introduce a small molecular weight epoxy resin into the coating film to increase the crosslinking density; the other direction can also be considered from the surface dryness.
, the introduction of a larger molecular weight epoxy resin, improve the surface drying speed of the coating film, so that the coating film has a certain initial hardness and resistance, thereby improving the early water resistance
.
Therefore, different epoxy-based epoxy emulsion paints were used in this paper to test the early water resistance and salt spray resistance of the coating film
.
The selected epoxy resin parameters are shown in Table 2, and the coating film test results are shown in Table 3
.

Table 2 Basic parameters of selected epoxy resin

Table 3 Influence of different epoxy emulsions with paint

It can be seen from Table 2 and Table 3 that after the above coatings are sprayed, baked at 50-55 °C for 30 min, and cured at room temperature for 24 h, the pencil hardness, flexibility, adhesion, and water resistance of the coatings were tested.
and salt spray resistance
.
The introduction of E-12 epoxy emulsion with larger relative molecular weight into the basic NPES-901 epoxy resin can improve the early water resistance and salt spray resistance of the coating film
.
The relative molecular weight of E-12 epoxy resin is about 1500-2000, the softening point is 85-95 ℃, the rigidity is strong, and it has a certain flexibility.
resistance, so that the coating film has the initial anti-corrosion performance
.
However, E-44 or NPES-638 epoxy resin has an epoxy equivalent weight of 150 to 200 and is liquid at room temperature.
Under short baking and curing time, it is not enough to fully cure with the curing agent, and the crosslinking is not enough, which affects the coating film.
Water resistance and salt spray resistance
.
However, its hardness increased significantly and rapidly, because although the degree of cross-linking was insufficient, the cross-linking points were dense, especially NPES-638 epoxy resin, which could provide higher initial hardness, but the flexibility also decreased
.
It can be seen from the results that the introduction of E-12 epoxy emulsion into NPPN-901 epoxy emulsion can improve the early water resistance of the coating film
.

2.
2 Influence of different types of curing agents

The structure diagrams of different epoxy curing agents are shown in Table 4
.

Table 4 Structure diagram of different epoxy curing agents

The early performance of the coating film after curing the epoxy emulsion with different kinds of curing agents is shown in Table 5
.

Table 5 Effects of different types of curing agents

It can be seen from Table 4 and Table 5 that the polyamide curing agent has a dimer acid structure with excellent flexibility.
After cross-linking and curing with the epoxy emulsion, the initial hardness of the coating film is not high and the flexibility is excellent, but the reaction rate is low after all.
If the degree of crosslinking is not reached, the resistance is insufficient, and the water resistance of the coating film is serious
.
Phenalkamines, aromatic amines and even alicyclic amines have relatively rigid benzene rings and six-membered ring structures.
After curing with the curing agent and epoxy emulsion as raw materials, the coating film has high strength, rapid increase in hardness, and slightly less flexibility.
poor
.
Under normal baking temperature and curing time, these are all advantages, but at lower curing temperature and shorter curing time, after the curing agent is mixed with epoxy emulsion, as the water volatilizes, demulsification and crosslinking, Crosslinking to a certain extent, the rigid structure of the curing agent will lead to increased Tg, poor resin fluidity, slow curing or even stop solidification
.
Although the hardness of the coating film increased, the crosslinking density did not increase accordingly, and the coating film was loose, showing poor flexibility and poor resistance to salt spray in terms of physical properties
.
In this experiment, the aliphatic amine-epoxy adduct has a relatively uniform distribution of soft blocks and hard blocks in the resin structure, which makes the coating film have a certain flexibility, which can better balance rigidity and flexibility, and obtain better The early water resistance and salt spray resistance performance
.

2.
3 Influence of the color-to-base ratio

After the resin system is determined, the pigment volume concentration (PVC) of the coating becomes an important consideration during the paint formulation stage
.
In order to make the coating formula clear at a glance and facilitate comparison, the pigment-to-base ratio P/B is replaced here
.
Table 6 discusses the various early properties of the coatings at different pigment-to-base ratios
.

Table 6 The effect of pigment-to-base ratio on the early performance of the coating film

Table 6 discusses the early water resistance of waterborne epoxy coatings at different pigment-to-base ratios
.
A certain amount of pigment and filler filling can relieve the stress generated during the curing process of the resin and increase the flexibility, adhesion and anti-corrosion performance of the coating
.
However, too many pigments and fillers will in turn affect the density of the coating film, resulting in a decrease in the anti-corrosion performance of the coating film
.
The results in Table 6 show that when used as a bottom-surface-in-one coating, when the pigment-to-base ratio is 1.
3 to 1.
5, the hardness of the coating film is higher and has better overall performance; when used as a primer, the pigment-to-base ratio is 1.
5 to 1.
8.
, The coating film and topcoat have better matching performance and have better comprehensive performance
.

2.
4 Influence of anti-rust pigments

Anti-rust pigments can be divided into: physical anti-rust pigments, chemical anti-rust pigments, and comprehensive anti-rust pigments with both physical and chemical mechanisms
.
Physical anti-rust pigments have stable chemical properties.
Commonly used non-toxic anti-rust pigments include iron oxide red, mica iron oxide, mica powder, glass flakes, aluminum powder,
etc.
Chemical anti-rust pigments are based on their own chemical activity.
In the process of metal corrosion, they react with metal ions on the metal surface to form a dense passive film to inhibit the corrosion process, or according to the principle of electrochemistry, the protection is achieved by sacrificing anode metal purpose
.
Comprehensive anti-rust pigments have both physical and chemical anti-rust mechanisms.
Commonly used comprehensive non-toxic anti-rust pigments are: flaky zinc powder, flaky zinc-aluminum alloy, composite iron-titanium powder,
etc.
In fact, the commonly used antirust pigments in waterborne epoxy coatings (except zinc rich) are: phosphate series, modified phosphate series, ion exchange pigments,
etc.
In this experiment, zinc phosphate, modified phosphate series, aluminum tripolyphosphate, and ion-exchange anti-rust pigments were mainly used to explore the effect of adding them into water-based coatings on the coating film under short baking and curing time
.

Table 7 Influence of anti-rust pigments on the early performance of coatings

The results in Table 7 show that the molybdenum-modified zinc phosphate type and ion-exchange type anti-rust pigments can endow the coatings with better early water resistance and early salt spray resistance
.
The SiO2 in the ion-exchange anti-rust pigment reacts with the OH- of the cathode, and finally a CaSiO3 deposit can be formed, which blocks the redox reaction of the cathode
.
Molybdenum-modified zinc phosphate is also an environmentally friendly active anti-rust pigment
.
Petr Kalenda et al.
found that zinc phosphomolybdate has better anti-rust effect by modifying zinc phosphate
.
Its anti-rust mechanism is: on the one hand, molybdate ions and iron ions form complexes, which play a role in anode passivation; ) to form complexes of heteropolyacids, thus stabilizing rust
.
In this experiment, the molybdenum-modified zinc phosphate antirust pigment and the ion-exchange antirust pigment have the same effect on the early anticorrosion performance of the coating, but the thermal storage performance of the coating prepared by the molybdenum modified zinc phosphate antirust pigment is better
.

2.
5 Influence of additives

In epoxy coatings, application additives can bring or enhance certain properties of the coating film.
For example, the application of tertiary amine catalysts can accelerate the curing of the resin; for example, the application of polysiloxane coupling agents can increase the performance of the coating film.
Density and adhesion to metal substrates
.
These additives were added to the coatings to investigate their effects on the early water resistance of the coatings
.
Its specific chemical structure is shown in Table 8, and its influence results are shown in Table 9
.

Table 8 Chemical structures of tertiary amines and polysiloxane additives

Table 9 Effects of additives

Table 9 shows the effect of adding tertiary amine or polysiloxane additives to the coating on the early water resistance of the coating
.
It can be seen from the results that the tertiary amine additives play a certain role, and the polysiloxane additives have no obvious effect
.
Tertiary amine additives can catalyze the reaction between epoxy and amine curing agents, increase the crosslinking density in a short period of time, make the coating film establish a certain resistance as soon as possible, and improve the initial anti-corrosion performance
.
However, it is well known that the addition of catalyst can increase the crosslinking density of the coating film and reduce the pot life of the coating.
Figure 1 examines the change of the pot life of the coating when different amounts of catalyst are added to the coating
.

Fig.
1 The effect of catalyst on the pot life of coatings

It can be seen from Figure 1 that the catalyst has a relatively large influence on the activation period of the coating.
Taking the addition of 0.
5% to 1.
0% DMP-30 catalyst as an example, the prepared coating will increase with the prolonged storage time, and the viscosity will increase significantly.
, the practicality is reduced
.
Taking into account the actual needs of on-site spraying, generally at least 4 hours of construction time should be met so that workers can complete a half-day shift, and the amount of catalyst added should be between 0.
1% and 0.
3%
.

2.
6 Influence of baking time and curing time

Different baking time and curing time have a great influence on the curing and performance of the coating film
.
Lower baking temperature and shorter baking time can only achieve initial cross-linking of the resin, and shorter curing time will not completely release the stress of the coating film, and the performance of the coating film will be reduced
.
Increasing the baking temperature, prolonging the baking time or curing time can significantly improve the anti-corrosion performance of the coating film
.
Table 10 discusses the early water resistance of the coatings at different baking temperatures and times and curing times
.

Table 10 The effect of baking and curing time on the early performance of the coating film

In this experiment, under the higher baking temperature (80-100 ℃) and sufficient curing time, the performance of the coating film is better
.
The performance of the coating film is poor under completely self-drying or very short baking and curing time
.
After the discussion and optimization of various influencing factors, under the curing conditions of 50 ℃/30 min and 24 h, the experimentally prepared two-component water-based epoxy coating can have a certain early water resistance, which can be considered for some similar engineering applications.
machinery and other practical scenarios
.

2.
7 Basic properties of waterborne epoxy coatings

After the analysis of the above-mentioned experiments under various conditions, the two-component water-based epoxy coating was prepared by selecting the best raw materials.
After baking on the ground cold-rolled steel plate for 50 ℃/30 min and curing for 24 h, the test results of the following properties were obtained, as shown in Table 11.

.

Table 11 Basic properties of two-component waterborne epoxy coatings

3 Conclusion

In this paper, the influence factors of waterborne epoxy coatings on the early anticorrosion and physical properties of coatings under the conditions of lower baking temperature and shorter curing time were discussed
.

In this system, CTW-6062 epoxy emulsion and E-12 epoxy emulsion are selected as epoxy components, and aliphatic amine-epoxy adduct CTW-6102 waterborne epoxy curing agent is used as curing agent component.
Controlled at 1.
3-1.
8, molybdenum modified zinc phosphate antirust pigment was selected, and an appropriate amount of catalyst was added.
The coating film was cured at 50 ℃/30 min.
After curing for 24 h, the performance was tested.
The coating film has good physical properties, and It has certain early water resistance and salt spray resistance
.

References (omitted)

Author | Zhu Baoying, Hu Zhong, Liu Ming, etc.

(CNOOC Changzhou Coating Chemical Research Institute Co.
, Ltd.
, Changzhou, Jiangsu, 213016)