Preparation and Construction Application of Low-temperature Baking Water-based Coatings for Auto Parts (Ⅰ)

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Table 1 Raw material information for experiment

Table 2 Formula of water-based acrylic modified epoxy ester coating

2.
3 Coating construction and performance test

The prepared paint is adjusted to the construction viscosity by mixing with water, and the sample or workpiece is sprayed.
Among them, the laboratory standard sample is cold-rolled steel plate, which is polished before spraying, without degreasing and phosphating process.
The baking condition of the laboratory model is 80°C for 20 minutes.
After the workpiece is respectively degreasing and phosphating treatment according to the process of the painting line, it is painted.
Perform performance tests on the painted samples and workpieces, and use the corresponding national standard methods for testing.

3.
Results and discussion

3.
1 Resin selection

Water-based resin is the base material for film formation, which determines the main properties of the paint film.
High-performance resin is the most critical factor for the success of water-based metal anticorrosive paint.
In the formulation of water-based metal anticorrosive paint, resins with excellent adhesion and good barrier properties must be selected.
Waterborne epoxy ester resin has a wide range of applications in the field of metal protection due to its excellent mechanical properties and anti-rust properties.
Epoxy ester resin is made of epoxy resin and vegetable oil fatty acid through esterification reaction.
Therefore, epoxy ester has many excellent characteristics of epoxy resin and drying oil at the same time: the epoxy component has high corrosion resistance.
Properties; the fatty acid component of vegetable oil can be oxidized and cross-linked to further improve the protective performance.
Therefore, the current mainstream water-based epoxy ester products are self-emulsified products.
The main technical route of the self-emulsification method of vinyl modification method is: free radical copolymerization of epoxy ester resin and acrylate mixed monomer with predetermined acid value to form a copolymerized product, and the copolymerized product is neutralized with an organic amine and then subjected to water After dilution, an aqueous epoxy ester emulsion is obtained.
The aqueous epoxy ester emulsion prepared by the method has good storage stability, easy adjustment of product performance, good film-forming properties, easy construction, and because it can be air-crosslinked, it has good water resistance and salt spray resistance.
However, waterborne epoxy esters also have some shortcomings in application, such as slow drying speed of paint film, poor weather resistance, insufficient alkali resistance and other problems.
On the other hand, acrylic emulsion has the advantages of fast drying speed, high hardness, excellent alkali resistance, and good weather resistance.
These characteristics just make up for the lack of performance of water-based epoxy ester products.

In this project, water-based epoxy ester emulsion is selected as the main film-forming material, an appropriate amount of acrylic emulsion is added to improve the hardness and dryness of the coating, and then a pigment slurry ground with a dispersant is added to prepare a low-temperature baking water-based coating.
Since the performance of the coating mainly depends on the two resins of waterborne epoxy ester and acrylic emulsion, this paper studies the effects of different proportions of resin on the drying speed, hardness, alkali resistance, water resistance and corrosion resistance of the coating.
The experimental results See Table 3 below.
From the data in Table 3, it can be seen that when the acrylic emulsion content is less than 35wt% of the total resin, the coating drying speed is slower and the alkali resistance of the coating does not meet the requirements; when the acrylic emulsion content is too high, the epoxy ester in the coating The relative content decreases, the degree of coating cross-linking decreases, and the water resistance and salt spray resistance of the corresponding coating decrease.
Considering the coating performance and overall cost, the mass ratio of water-based epoxy ester/acrylic emulsion is 65/35.

Table 3 The effect of different emulsion ratios on coating performance

Note: a emulsion ratio = m (water-based epoxy ester)/m (acrylic emulsion); b surface drying time is measured in accordance with GB/T1728-79/89, the film thickness is 25μm, measured at 25°C.

3.
2 Auxiliary selection

3.
2.
1 Dispersant selection

In the production of paint, one of the most important steps is the uniform distribution of solid pigments in the liquid binder solution.
In the dispersion process, it is necessary to grind the pigment agglomerates to maximize the color performance of the pigment and meet the optical requirements, such as color point, color strength, chroma and transparency.
The wetting and dispersing agent wets the surface of the pigment to prevent the dispersed pigment particles from agglomerating again.
If the pigment grinding step is not optimal, many defects will occur, such as flocculation, gloss reduction, color shift, floating/floating, Bernard swirl, and precipitation.

Through the grinding process, the polymeric wetting and dispersing agent is anchored to the surface of the pigment through the attraction of functional groups.
Due to their steric hindrance and electrostatic repulsion, the proper distance between the pigments is maintained, which reduces the tendency of uncontrolled flocculation, thereby ensuring sufficient stability inside the coating.

In this project, block-type acrylic polymer is used as the dispersant, and NUOSPERSEFX-600 and DISPERBYK-190 are used in combination to prepare water-based coatings.
The fineness of the coating meets the requirements, and the coating can be stored stably for more than 6 months, and stored in an oven at 50°C for 1 month without hard sinking.
The experimental results show that the use of this type of composite dispersant can grind the pigments and fillers to the specified fineness, and the prepared coatings have excellent storage stability.

3.
2.
2 Selection of leveling agent

After the coating is applied, there is a process of flowing and drying to form a film, and then a smooth, smooth and uniform coating film is gradually formed.
Whether the coating film can achieve the characteristics of flatness and smoothness is called leveling.
Poor leveling performance is: brush marks, roll marks, orange peel, shrinkage holes, sagging and pinholes, etc.
The occurrence of these phenomena reduces the decorative and protective functions of the coating.
To improve the leveling of the coating, it is necessary to consider adding a suitable leveling agent.
It is usually necessary to add a leveling agent, which can significantly reduce the surface tension of the coating, improve the substrate wetting ability of the coating and the fluidity of the paint film, and eliminate the Benard vortex.
So as to prevent blooming.

In this project, the imported leveling agent BYK348 and the domestic leveling agent were selected, and compared with the paint without leveling agent.
Table 4 shows that after adding the leveling agent, the phenomenon of blooming in the paint is significantly improved.
The leveling agent has good leveling properties, and at the same time, the hardness, water resistance and corrosion resistance of the coating are better.
Therefore, the domestic leveling agent B is selected as the leveling agent.

Table 4 The leveling effect of different leveling agents and their influence on the performance of the coating film

Table 4 The leveling effect of different leveling agents and their influence on the performance of the coating film

Figure 1 The leveling effect of different leveling agents

Figure 1 The leveling effect of different leveling agents: 1# blank sample; 2# imported leveling agent BYK348; 3# domestic leveling agent B

3.
2.
3 Thickener selection

Water-based coatings contain a large amount of pigments and fillers, which can be stabilized in the coating system under the action of dispersants.
However, under the action of gravity, pigments, fillers, and other solid substances have a tendency to sink to the bottom of the container.
Taking into account the stability and storage period of the coating, it is very important to reduce the sedimentation rate of the coating.
Pigment sedimentation is a phenomenon of low shear rate: the shear rate involved is less than 10-2s-1, so the viscosity of the coating at this low shear rate is very important to prevent pigment precipitation.
Under normal circumstances, the thickening effect is achieved by selecting a thickener to increase the viscosity under low shear, thereby delaying the sedimentation of pigments and fillers.
On the other hand, in the construction state, especially for the spray construction process, it is hoped that the viscosity of the coating will be reduced, which is beneficial to construction and leveling.
Combining the two requirements, the coating needs to have a certain degree of thixotropy.
The thixotropic effect can be achieved by adding a specific thickener.

Table 5 Thickening effect of different thickeners

Table 5 Thickening effect of different thickeners

Rheology additives can be divided into three types of high, medium, and low shear viscosity in terms of performance: low-shear viscosity flow additives can provide viscosity in the tank, prevent pigment settling, and effectively control the standing of the paint.
Flowing additives that provide medium and high shear viscosity can improve the fullness and leveling of water-based coatings, while the flow additives that increase the high shear viscosity can help leveling and prevent Splash and improve roller and brush construction performance.
Therefore, the type and amount of flow aid must be selected according to the construction requirements and system.
Generally, the type of flow aid is selected according to the different construction methods, and it is often matched by flow aids with a variety of shear viscosities.
In order to achieve the required construction effect.

Table 5 lists the effects of different thickener systems on coating thickening and thixotropy.
In this project, a combination of polyurethane associative thickener and bentonite are used as rheology additives.
CoapurTM830W and bentonite are used in concert to provide a balance of high, medium and low shear viscosity.
The prepared coating can not only prevent pigment sedimentation under low shear viscosity, but also can improve the fullness and leveling of the coating under medium and high shear viscosity, and has achieved good results.

3.
2.
4 Selection of defoamer

In the paint manufacturing process, the dispersion of pigments and fillers, the stirring during the thickening and mixing process, the vibration of the finished paint during the filtration and packaging process, the mixing and stirring of the paint and the color paste during the toning process, the stirring of air, the air is in water When the coating is wrapped and stabilized by a surfactant, bubbles will be generated in the coating.
If the bubbles in the paint solution are not eliminated in time, unacceptable defects will be formed after the paint film dries.
Table 6 shows the defoaming performance of different defoamers.
It can be seen that the silicone-based defoaming effect is better, and the relative amount is small, but it is prone to shrinkage; the mineral oil-based defoaming agent has a slightly worse defoaming effect.
So the addition amount needs to be increased.
Considering the defoaming effect, film shrinkage and addition amount, BYK025 is selected as the defoaming agent for this project.

Table 6 Types and performance comparison table of defoamers

Table 6 Types and performance comparison table of defoamers

3.
2.
5 Anti-flash rust agent

Water-based industrial paint mainly uses water as a thinner, which is a new type of environmentally friendly anti-rust and anti-corrosion paint that is different from oil-based industrial paint.
Because the drying time of self-drying/low-temperature baking type coatings is longer, rust spots are prone to appear when water contacts with iron-containing substrates, which are generally referred to as "flash rust".
Flash rust is caused by the contact of the moisture in the paint with the iron surface (such as nail heads, steel, etc.
).
When the paint is applied in a general environment (such as a humid climate), the possibility of such flash corrosion is greater.
In order to solve the problem of flash rust, it is necessary to add anti-flash rust additives to the formulation.

At present, there are two main applications of anti-flash rust agents on the market.
The first category is low-priced nitrite.
It is well known that nitrite has severe biological toxicity.
Excessive use and discharge can cause acute poisoning of environmental organisms, and due to its good water solubility, water resistance and long-term corrosion resistance of the paint film There are also adverse effects.
The other type is organometallic salt and modified zinc chelate or a combination of the two.
Because this type of product is currently produced by no one in China, the product is imported, and the cost is high, which restricts the size of this type of product.
Scope application.

Table 7 lists the effects of different types of anti-flash rust agents.
In this project, an organic chelate compound ZT-709 containing a variety of corrosion inhibitor components is selected.
This anti-flash rust agent can effectively inhibit the flash rust phenomenon that occurs during the drying process of the water-based paint after it is applied to the iron substrate.
.
The product will not have any adverse effects on the paint film, and during the drying process, most of it is transformed into a water-insoluble complex, which is equivalent to producing a passivation film on the coated metal surface, thereby improving the coating.
The corrosion resistance of the layer, the formed complex can effectively improve the rust resistance and salt spray resistance of the coating dry film while preventing flash rust.

Table 7 Types and performance comparison table of anti-flash rust agent

Table 7 Anti-flash rust agent types and performance comparison table

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