Development and Application of a Kind of Waterborne Wood Coatings Based on Electrostatic Spraying

WANG Pei-shu
(Dongguan Borda Industry Co., Ltd., Dongguan 523000, Guangdong, China)

Abstract: With selected emulsion and high-performance nano pigment dispersion, we made a kind of waterborne wood coatings that could be sprayed with an electrostatic bell by optimizing the coatings formulation. The utilization ratio of coatings was greatly improved. We solved the problem of bell blocking and at the same time reduced color difference.

0 Foreword
After the development of waterborne wood coatings in recent years, the performance has been gradually improved, and the coating process is also constantly changing. Some manufacturers have introduced electrostatic spin cup equipment based on the centrifugal atomization method in order to achieve the effect of automobile painting, but the current situation is that most are not ideal. After a lot of experiments, the author found that this phenomenon is basically caused by improper selection of coating emulsions and defects in the formula. For self-drying waterborne wood coating emulsions, the neutralizer or co-solvent contained in the emulsion is generally relatively volatile, and quickly evaporates during the spraying process, causing the film-forming material to gel prematurely and cause so-called “blocking”. Gun phenomenon. In this paper, by selecting an emulsion with a large number of hydrophilic groups and a slowly volatile neutralizing agent, and using a high-performance nano-pigment, by optimizing the coating formula, the problems of spray gun clogging, cleaning, and coating recycling are solved.

1 Experimental
1.1 Raw material
emulsion: self-crosslinking acrylic emulsion: NeoCryl XK12 (DSM), Setaqua 6785 (allnex), AC2782 (Alberdingk); soap-free polymer emulsion: NeoCryl XK14 (DSM), Setaqua 6716 (allnex), AC3630 (Alberdingk ); PUD dispersion: NeoRezlVR2005 (DSM), U9800 (Alberdingk), Lacper4211 (Wanhua). Emulsion with a large number of hydrophilic groups and slow volatile neutralizers: Urosin 4619 (Wanhua), WB18WF (QuiWin), WR03 (home-made).
Auxiliary: dispersant: TEGO Dispers 755 (Evonik); defoamer: TEGO Foamex 825 (Evonik); wetting agent: BYK3455 (BYK); thickener: Aquaflow XLS 530 (Ashland); wax: ACumist3405 (Honeywell) .
Colorants: Hostafine (Clariant); SuperFine (CCA); UTINTWNW (Uchem).
Equipment: Automatic detection grating + rotary encoder, PLC control cabinet, lifting reciprocator, PPH308 rotary cup, TRP501 spray gun, variable frequency speed regulating motor + gear pump, high-voltage insulation table + electrostatic isolation barrier (SAMES), TG-20 (12 000 r / min) high speed centrifuge.

1.2 Formulation of water-based one-component topcoat (see Table 1)

Waterborne one-component topcoat formulation

1.3 High-speed centrifugation experiment

Centrifuge the different emulsions and color pastes on a 12 000 r / min high-speed centrifuge for 20 minutes to check whether the samples in the test tube are evenly suspended and separated. Layer and precipitate. When the turbine air of the electrostatic spin cup reaches 0.2 to 0.3 MPa (2 to 3 bar), the rotation speed of the spin cup will reach 10 000 to 20 000 r / min. Therefore, whether the emulsion is broken at high shear forces and whether the color paste is stable is very critical. Self-crosslinking emulsion (No. 1), soap-free polymerizable emulsion (No. 2), PUD dispersion (No. 3), emulsion with a large number of hydrophilic groups and a slowly volatile neutralizing agent (No. 4) ), And adding a sample of organic yellow pulp (No. 5) and carbon black pulp (No. 6) to an emulsion with a large number of hydrophilic groups and a slow volatile neutralizer for high-speed centrifugation experiments. Shown in Figure 1.

High-speed centrifugal experimental sample group and its experimental stratification

1.4 Rotary Cup Spraying Static Parameters
Flow: 150 ~ 300 mL / min. Set the flow parameters according to the film thickness requirements of the sprayed workpiece, coating characteristics, chain speed, and temperature and humidity of the spray booth.
Molding air: 0.2 ~ 0.3 MPa (2 ~ 3 bar), set this parameter according to the conditions of sprayed workpiece structure and coating characteristics.
Turbine air: 0.2 to 0.3 MPa (2 to 3 bar). Set this parameter according to the conditions of the sprayed workpiece structure and paint characteristics.
High voltage static: 30 ~ 60 kV, set this parameter according to the conditions of sprayed workpiece structure and paint characteristics.
The number of advance and delay pulses, and the number of shots at the upper and lower points are set according to the conditions of the sprayed workpiece structure and paint characteristics.

1.5 Coating film performance test
According to Table 1, optimize the formula, spray the board according to different formulas, and test the performance after the sprayed board is left for 7 days.
Test reference standards: (1) Hardness GB / T 6739—2006; (2) Indentation HG / T 3828—2006; (3) Moisture and heat resistance GB / T 4893.2—2005; (4) Resistance to HG / T 3828—2006.

2 Results and discussion
2.1 Selection of main film-forming substances
2.1.1 Results of high-speed centrifugation experiments of emulsions (see Table 2)

High-speed centrifugal experiment results of emulsion

Centrifugation experiments simply simulate the stability of emulsions under high-speed centrifugation, and are a basic judgment for emulsion selection . From the experimental results in Table 2, many emulsions are not suitable for centrifugal atomization. Generally, when the turbine air is between 0.2 and 0.3 MPa (2 and 3 bar), the rotation speed of the rotor will reach more than 20 000 r / min. Even if the speed of the rotating cup is controlled below 10 000 r / min, the shear force of the air bearing turbine is very large. From the results in Table 2, it can be known that soap-free polymerization type emulsions, PUD dispersions, and shear-resistant emulsions with a large number of hydrophilic groups and slow-volatile neutralizing agents can be selected.

2.1.2 Experimental results of electrostatic spraying of different emulsions
From the emulsions stabilized by the centrifugation experiment, one emulsion was selected and sprayed on the electrostatic experiment line for 30 minutes. The results are shown in Figure 2.

Different emulsion electrostatic spraying effects

According to the electrostatic spraying effect shown in Figure 2, although the acrylic soap-free polymer emulsion and PUD dispersion have good resistance to mechanical shear forces, they are essentially relatively large-molecular thermoplastic resins. Under the action of molding air, it quickly dehydrates into a film and has irreversibility. This will quickly build up in the cup. As the spraying progresses, it will be thrown on the surface of the workpiece under the action of high-speed centrifugal force, forming defects. Therefore, a reversible emulsion that can be quickly redispersed by water when spraying is used to solve the above problems. This emulsion has a large number of hydrophilic groups, such as a structure like Figure 3, and is connected to a hydrophobic group to ensure the performance of the coating film. Like the dry film-forming process of ordinary emulsions, the molecular chains in the particles of this emulsion penetrate and diffuse with each other, and gradually become entangled with each other. However, after the coating film encounters water again, the neutralizer still retains a sufficient amount. The hydrophobic part of the The main chain of the molecular chain flexes quickly, and this hydrophilic group will coat the hydrophobic main chain like a surfactant, and become emulsion particles dispersed in water again. In this way, the emulsion will no longer form so easily. The spray result is shown in Figure 2 (c).

Schematic diagram of emulsion containing a large number of hydrophilic groups

2.2 Selection of color paste

2.2.1 Results of high-speed centrifugal experiment of color paste (see Table 3)

High-speed centrifugal experiment results of color paste

The paint mist speed of traditional rotary cups is generally 1 ~ 2 m / s (Figure 4). The speed can reach 6 to 11 m / s (Figure 5).
Due to the high paint mist speed and vortex effect, when spraying colored paint, the pigment must be completely deflocculated and very stable (see Table 3), so that the pigment particles can reach the surface of the workpiece and be evenly distributed, thereby reducing the color difference. As for the colorant’s own anti-floating color and the compatibility of the base coating, this is the basic requirement of the colorant.

Traditional spin cup, magnetic levitation spin cup

2.2.2 The results of electrostatic spraying experiments of colored paints

It is preferred that the emulsion with a large number of hydrophilic groups and slow volatile neutralizers is mixed with a nano-color paste. The pH value of the paint solution is adjusted to 8-9. The flow rate is 180mL / min. Continuous spraying under the conditions of 0.2 MPa (2 bar), turbine air 0.2 MPa (2 bar), and high-voltage electrostatic 50 kV for 30 min. The effect is shown in Figures 6 and 7.

Black paint spray effect
Yellow paint spray effect

The choice of neutralizing agent for this type of emulsion is very critical. The neutralizing amine used is a slow-volatile amine or a metal hard base. The commonly used slow-volatile amines are TEA (N, N-diethylethylamine). ), TBA (N, N-dibutylbutylamine), DMEA (N, N-dimethylethanolamine), DMAPAPA (N, N-dimethyldipropyltriamine) and commercially available Eastman Vantex-T, etc .; commonly used metal hard bases include NaOH and KOH. High boiling point and low odor are the best choices.

2.3 Formulation optimization
By adding the hydrolyzed oligomer of glycidyl etheroxypropyltrimethoxysilane, the test performance of the plate after 7 days is shown in Table 4.

Coating performance

The presence of a large number of hydrophilic groups will seriously affect the resistance of the coating film. The addition of a hydrolyzed oligomer of glycidyl ether oxypropyltrimethoxysilane can greatly improve the performance of the coating film. Such substances can be stably stored in aqueous coating liquids for more than 0.5 a. Hydrolytic oligomers can greatly reduce the film-forming temperature and have a lower VOC release than similar substances.

3 Conclusions
Electrostatic spraying of water-based coatings has not been effective in the application of woodware. The commonly used rotary butterfly type is limited by the width of the workpiece, and the flatness of the coating film is not ideal. The rotary cup is easy to accumulate in the cup when spraying the topcoat. Defects on the surface of the workpiece. These are often related to the unsuitability of the selected coating. An emulsion with a large number of hydrophilic groups and a slowly volatile neutralizer is a good choice. A resin with a low Tg point and small molecules is also good. Selection. Of course, these coatings need to be cured by different cross-linking methods or appropriate temperatures. However, due to the available time limit and the relationship between equipment, they are often not selected by wood electrostatic coating.

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