Study on wet spray coating of waterborne metal baking paint

Study on wet spray coating of waterborne metal baking paint

Chongqing Si Nabo Coating Co., Ltd. Li Xiaoming

Abstract: This paper outlines the application of waterborne metal baking varnish in the inner and outer coating of packaging steel drums. It focuses on the problems existing in the coating of steel drums with two-color paint wet-on-wet spraying (two lacquer overlapping parts, see Figure 1). Solutions. The results show that the water-based metal baking varnish can be applied to the coating of two-color packaging steel drum paint under the wet-on-wet construction. Adjusting the interfacial tension of the two lacquers can obviously make the water-based metal baking varnish be applied in the wet-on-wet construction process, and the surface tension between the primer coating and the top coating can be opened, which can completely solve the existence of the water-based metal lacquer in the wet-on-wet construction process. The lacquer film is ill.

Key words: packaging steel drum; wet-on-wet coating; water-based metal baking varnish; surface tension.

The water-based metal baking varnish is mainly composed of water-diluted or water-soluble acrylic (polyester) resin, cross-linked resin, pigment filler, deionized water, auxiliary agent and the like. The performance of the main film-forming materials determines the performance of the water-based metal baking varnish. However, since the solvent in the waterborne metal baking varnish is water, the surface tension of the water is extremely high, which causes many defects in the aqueous metal baking varnish during the construction process, so it is important to reduce the surface tension of the lacquer liquid. This drawback is especially reflected in the wet-on-wet construction process of water-based metal baking varnish, which makes the water-based metal baking varnish not wet-wet spray coating, which limits its wider application in packaging steel drum two-color paint.

The water-based metal baking varnish does not have any problem in the monochrome paint and the inner paint outside the packaging steel drum. The key is that during the painting process of the outer double-color paint, as shown in Figure 1, the two lacquer overlap parts (here is equivalent to wet There are always many paint film defects in the wet spray coating. Therefore, solving this problem plays a crucial role in the application of waterborne metal baking varnish. At the same time, it is not because the water-based metal baking paint can not be wet and wet sprayed, but the cumbersome process such as multi-pass baking and multi-pass spraying. Make construction easier and more energy efficient.

In order to make water-based metal baking varnish widely used in two-color packaging steel drums, the selection of construction technology and leveling agent was mainly investigated.

1 Experimental part

1.1 Instrumentation

SDF04 disperser, KF electronic balance, JJ500 precision electronic balance, 202A-O type electric thermostat drying oven, spraying equipment, adding device, coating -4 cup, gloss meter, measuring cup, etc.

1.2 Experimental materials

Epoxy modified acrylic resin, methyl etherified amino resin (crosslinked resin), pigment filler, leveling agent 1 (does not affect surface tension), leveling agent 2 (easy to reduce surface tension), other additives, deionized water .

1.3 Experimental steps

(1) Formulating milky white paint (first paint in two-color paint, hereinafter referred to as primer): According to the pre-designed formula, weigh quantitative deionized water in the paint tank, add PH regulator, mix at low speed, and add the main resin in sequence. And cross-linking resin, high-speed dispersion for a moderate time; then add other additives in order to make the paint liquid evenly dispersed;

(2) Preparation of purple-brown paint (post-painting in two-color paint, hereinafter referred to as topcoat): Same as above (1);

(3) Selection of substrate (cold-rolled steel sheet) and surface treatment: 3-5 pieces of pre-cut cold-rolled steel sheets are ground with 500-mesh sandpaper so that the surface of the steel sheet has no rust and other convex and concave parts, so that it is flat and ready for use.

(4) Spraying construction (imitation of the construction process of packaging steel drum two-color paint): first spray the primer, and then spray the topcoat immediately after 5-6 seconds. The two lacquers have a 5-8 cm overlap, which is equivalent to wet-on-wet spray.

(5) Baking (staged baking, imitating the baking process of packaging steel drum two-color paint): After spraying, dry for 1-2 minutes, put into constant temperature oven 100 ° C / 5 min, heat up to 160 ° C / 5 min, cool down To 140 ° C / 5 min.

(6) Paint film performance test:

2 Detection method

2.1 Status in the container

The container was opened, stirred with a stir bar, and allowed to have a precipitate at the bottom of the container. If it was easily mixed evenly after stirring, it was rated as "uniform no lumps after stirring".

2.2 Coating appearance

The test sample was visually observed under scattered sunlight. If the color of the coating film was uniform and the surface was flat, no bubbles, shrinkage cavities and other film morbidities were rated as "normal".

2.3 Viscosity

According to the provisions of the B law in GB/T1723-1993. The sample temperature is (23 ± 1) ° C;

2.4 fineness

According to the provisions of GB/T1724-1979 (1989).

2.5 film color difference

Visually, compared to the reference template. The evaluation method is agreed by the parties concerned.

2.6 gloss (60°)

According to the provisions of GB/T9754.

2.7 pencil hardness (scratch)

According to the provisions of ISO15184:1998. The pencil is a Chinese pencil 101 drawing pencil.

2.8 Impact resistance

According to the provisions of GB/T1732-1993. When the positive punching, the sample coating film is placed on the anvil of the impactor upwards for impact test, and the back coating film is placed flat on the anvil of the impactor for impact test.

2.9 bending test

According to the provisions of GB/T6742.

2.10 Cross-hatch adhesion

According to the provisions of GB/T9286-1998, the cross-track spacing is 1mm.

2.11 kerosene resistance (internal and external barrels)

According to the provisions of GB9274, 5 is carried out.

2.12 Lubricity resistance (inside and outside barrels)

According to the provisions of GB9274, 5 is carried out.

2.13 Resistance to freezing oil (internal and external barrels)

According to the provisions of GB9274, 5 is carried out.

2.14 Acid resistance (inside and outside barrels)

According to the provisions of GB9274, 5 is carried out.

2.15 Alkali resistance (inside and outside barrels)

According to the provisions of GB9274, 5 is carried out.

3 Results and discussion

3.1 Test results

See Table 1 (next page)

Table 1 Test results

3.2 Construction process

3.2.1 Two coating and one baking process

Barrel → pickling phosphating → 60 ° C / 8 min drying → primer spraying → (3 ~ 5 banknotes) → topcoat spraying → (40 banknotes) → enter the first stage drying tunnel (100 ° C / 5 min) → second Stage drying tunnel (160 ° C / 5 min) → third stage drying tunnel (140 ° C / 5 min) → natural cooling → finished product

3.2.2 Two-coating two-baking process

Barrel → pickling phosphating → 60 ° C / 8 min drying → primer spraying → (40 banknotes) → enter the first stage drying tunnel (100 ° C / 5 min) → second stage drying tunnel (160 ° C / 5 min) → Three-stage drying tunnel (140 ° C / 5 min) → natural cooling → topcoat spraying → (40 bills) → enter the first stage drying tunnel (100 ° C / 5 min) → second stage drying tunnel (160 ° C / 5 min) → third Stage drying tunnel (140 ° C / 5 min) → natural cooling → finished product

figure 1

figure 2

3.2.3 Comparison of two construction techniques

Table 2 Comparison of two construction techniques

3.3 The choice of leveling agent

3.3.1 Analysis of the defects of paint film in the two coating and one baking process

The primer is pulled out of the topcoat, that is, the primer paint migrates to the surface (see Figure 2), causing the paint film to appear. The migration of the paint liquid is mainly due to the interfacial tension. And the principle of paint dispersion is: "low surface tension paint liquid spreads to high surface tension objects." Why does the primer spread to the surface finish? The main reason is that the surface tension of the primer is lower than the surface of the topcoat.

3.3.2 Waterborne metal paint lowering surface tension preferred method

In the waterborne metallic paint, since the solvent is water, the surface tension of the water is high, so that the surface tension of the paint liquid is high. Although many coexisting auxiliaries in lacquers are surfactants, there is a tendency to reduce surface tension. However, in order to open the interfacial tension between the primer and the topcoat, the optimum and preferred auxiliaries are also leveling agents.

3.3.3 Leveling agent screening

3.3.3.1 Screening principles

Under the premise of ensuring the normal appearance of the surface of the paint film, the amount of the leveling agent added in the primer formulation is adjusted to make the surface tension tend to be stable. That is, a leveling agent that does not affect the surface tension is selected. Adjust the amount of leveling agent added in the topcoat formula so that the surface tension is lower than the surface tension of the primer, that is, the leveling agent which is easy to reduce the surface tension is selected.

3.3.3.2 Screening results

The preferred leveling agent is selected by a large number of experimental sieves:

(1) Leveling agent for primer: BYK-381 (ionic acrylic copolymer) does not affect surface tension;

(2) Leveling agent for topcoat: EFKA-3772 (fluorocarbon modified polyacrylate) easily reduces the surface tension of the aqueous coating system.

3.3.4 Primer, topcoat leveling agent addition

Table 3 Primer, topcoat leveling agent addition

4 Conclusion

The results showed that:

(1) Water-based metallic paint can be wet-wet construction under the conditions of suitable construction technology. It not only saves energy but also reduces consumption, the process is simple, and the yield is high. The scope of the promotion of water-based metallic paints is wider, and the packaging steel drum industry is green.

(2) Among many water-based paint additives, the leveling agent is more suitable for adjusting the surface tension of the water-based metal paint, and the surface tension of the paint film is more easily reduced by using the fluorocarbon-modified leveling agent. The surface tension of the two-color paint is pulled apart during the wet-on-wet construction process, and the defects of the paint film are completely eliminated.

(3) The primer may be selected from a leveling agent that does not affect the surface tension (for example, BYK-381 <ionic acrylic copolymer>); the top coat may be selected from a leveling agent that easily reduces the surface tension (for example: EFKA-3772<Fluorine) Carbon modified polyacrylate >).

references

1 China Coatings Industry Association. The latest coatings and additives manufacturing new technology and new technology and performance introduction and typical application examples of practical books. North Industrial Press, 2006: 301 ~ 316

2 Liu Yuming. Environmentally Friendly Waterborne Coatings. China Knowledge Press. 2006: 39~43

3 Jiang Yingtao. Shanghai Paint. 1 (1987).

4 Su Cisheng. Paint film surface control agent (a lecture on the theory of comprehensive coating additives theory) 1984.

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