Analysis of Blister on Surface of Waterborne 2K Intercoat

Wang Yanyan, Liu Hangong, Zhang Hanqing, Zhu Baoying, Xu Fei, Liu Ming, Liu Rui (CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd., Changzhou, Jiangsu 213016, China )

Abstract: The effects of resin, -NCO/-OH, cosolvent, dust-free time and curing temperature, etc. on the blister of waterborne 2k polyurethane intercoat have been analyzed. Thus, an optimal formula is obtained to eliminate the film defect.

0 Introduction
With the advancement of cleaner production in the industry, Gradually guide companies to use environmentally friendly coatings. Especially with the attention of human beings to environmental protection and the promulgation of relevant laws, for example, China ’s Ministry of Finance and the State Administration of Taxation issued the fiscal and taxation [2015] 16 “Notice on Levying Consumption Tax on Batteries and Coatings” on January 26, 2015, The notice states that a 4% consumption tax will be levied on all paints with a volatile organic compound (VOC) content of more than 420 g / l under the application state. Forcing solvent-based coatings to become environmentally friendly. The water-based coating uses water as the dispersion medium, which minimizes the emissions of volatile organic compounds (VOC) and harmful air pollutants (HAP). It can be seen that water-based coatings have become a hot spot and development direction of current coatings research.


Intermediate paint is a coating used between the primer and the topcoat, and plays an important role in connecting up and down. Poor surface condition of the middle paint will result in poor compatibility with the top paint, matte finish and other consequences, seriously reducing the protection and decorative performance of the coating. Aqueous two-component mid-coat paints are based on acrylic resin and undergo cross-linking curing reaction with isocyanate resin, but water as a dispersion medium will react side-by-side with isocyanate resin.
The reaction formula of the side reaction is:
R-NCO + H2O → R-NH2 + CO2 ↑
So the water-based two-component polyurethane paint is easy to produce bubbles during the drying film formation process, especially when the coating is thick, it is easy to foam. , Shrinkage, pinholes, defects, etc. So how to solve the foaming problem of waterborne two-component paint is an important point to promote the practical application of waterborne two-component polyurethane paint. In this paper, the resin system, -NCO / -OH, co-solvent, surface drying time, curing temperature and other factors are tested to find the reasons that affect the foaming of the middle paint, and the best formula is obtained to avoid foaming problems of the paint film.

1 Experimental part
1.1 Main raw materials
Water-based hydroxyacrylic emulsion: DSM Co., Ltd .; Water-based hydroxyacrylic emulsion: Bayer Co., Ltd .; Water-based polyurethane curing agent WT2102: Perstorp; Rutile titanium dioxide, carbon black, precipitated barium sulfate, talc, co-solvent: propylene glycol Methyl ether acetate (PMA), dispersant, wetting agent, defoamer, film-forming aid, leveling agent and thickener, rust remover, etc .: commercially available; deionized water: homemade.


1.2 Preparation of water- based two-component quick-drying intermediate coating 

The formula of water-based two-component intermediate coating is shown in Table 1.

Waterborne two-component mid-coat formulation

Beating: Firstly, disperse and grind the deionized water, dispersant, wetting agent, rutile titanium dioxide, carbon black, defoamer, and zirconium beads in Part A at a high speed of 2 000 r / min to a fineness of ≤20. Obtains water-based intermediate coating slurry.
Paint making: Then add the water-based acrylic resin, leveling agent, and wetting agent of Part B into the paint mixing kettle and mix with the water-based slurry according to the formula, and disperse uniformly at high speed at 1 000 r / min.
Construction: Mix with the polyurethane curing agent in Part C and disperse it uniformly; finally add appropriate deionized water to dilute to a suitable viscosity for construction.
Drying conditions: After flash-drying at room temperature for a period of time, bake at 70 ℃ for 40 minutes.

2 Results and discussion
2.1 Effect of film-forming resin on water-based two-component paints
Water-based two-component paints use water-based acrylic resins and polyurethanes as film-forming resins. There are two types of water-based acrylic resins: emulsion and dispersion. Emulsion type has fast drying speed and meets the performance requirements as an intermediate coating. However, water-based acrylic emulsions with different hydroxyl values ​​and glass transition temperatures have different effects on film properties. The technical specifications of the water-based acrylic emulsion used in this study are shown in Table 2.

Technical index of water-based hydroxyacrylic emulsion

The above three types of water-based hydroxyacrylic emulsions were used alone as film-forming resins, and the safety film thickness and other properties of the paint film were tested under the same conditions as the base-to-base ratio, curing agent, and -NCO / -OH. The test results are shown in Table 3.

Using single resin paint film performance test

It can be known from the above experimental results that as the hydroxyl value and glass transition temperature increase, the drying speed of the paint film increases. The safety film thickness has also increased. Emulsion B has a higher hydroxyl value and glass transition temperature, and it dries significantly faster than emulsions A and C. However, the drying speed of the paint film is too fast, which will cause the surface of the paint film to dry prematurely to form a sealing layer. The underlying water does not have enough time to evaporate. As the drying temperature increases, it suddenly vaporizes to form air bubbles. , And adhere and stabilize the formation of air bubbles on the surface of the paint film. Moreover, the increase of the hydroxyl value and the glass transition temperature will increase the hardness of the paint film, resulting in a decrease in adhesion and flexibility. Therefore, considering the properties of the paint film and the safety film thickness, the emulsions A and C are used in combination and the effects of different compounding ratios on the performance of the paint film are shown in Table 4.

Performance test of paint films at different ratios of resins A and C

From the above experimental results, it can be known that: when the resin A is added in a large amount, the safety thick film of the paint film is higher, but the hardness and the flexibility of the paint film are poor due to the hardness of the paint film; Increasing the mechanical properties of the paint film can be improved, but too much addition will affect the sandability of the paint film. When mA: mc = 6: 4, the effect is the best, the safety film thickness of the paint film is 60 μm, and the mechanical properties of the paint film can meet the requirements. The following uses resins A and C to discuss the other influencing factors according to the added amount of mA: mc = 6: 4.


2.2 Effect of -NCO / -OH on waterborne two-component paint
Two-component waterborne polyurethane coatings are cross-linked by the reaction of -NCO with polyisocyanate curing agent and -OH in water-based acrylic resin. To ensure complete cross-linking, the ratio of -NCO / -OH is generally greater than 1.0. However, the ratio of -NCO / -OH is too high, which will not only increase the cost, but also affect the performance of the paint film. The effects of different -NCO / -OH on the film properties were investigated. The test results are shown in Table 5.

Effect of different -NCO / -OH ratios on film properties

It can be known from the above experimental results: When the -NCO / -OH ratio is large, a large amount of -NCO does not participate in the reaction, and the remaining -NCO will react with water, and the generated CO2 stays in the coating film. With the curing temperature Increasing the value will cause bubbles on the surface of the paint film, and the thickness of the safety film will decrease. However, when -NCO / -OH is too low, it will affect the hardness of the paint film, resulting in a decrease in sandability. Therefore, it is recommended that -NCO / -OH be controlled at 1.2.

2.3 Effect of co-solvent on waterborne two-component paint

Co-solvent plays a key role in forming a uniform paint film on water-based paint. It can not only increase the solubility of the resin in water, but also adjust the viscosity of the coating, improve the stability of the coating, and improve the performance of the paint film. Commonly used co-solvents are generally ketones, esters, alcohol ethers, ether esters and other non-hydroxyl solvents. In summer construction, the water volatilizes quickly and the surface drying time is relatively shortened. The water inside the paint film does not have enough time to evaporate and is sealed in the paint film. During the drying of the paint film, as the temperature increases, a part of the water will break through the surface of the paint film and form bubbles on its surface. Adding an appropriate amount of co-solvent can delay the drying speed of the paint film, increase the opening time of the paint film, and improve its construction performance and appearance of the paint film. During the construction in winter, the volatilization of water is slow, and the volatilization rate of the co-solvent is faster than that of water, which will promote the evaporation of water. Prevents ills such as air bubbles in the paint film. This product uses propylene glycol methyl ether acetate (PMA) as a co-solvent to investigate the inhibitory effect of different additions on the film foaming. The test results are shown in Table 6.

Effect of Amount of PMA on Safety Film Thickness of Coating Film

Note: (1) The added amount refers to the mass percentage of PAM relative to the total paint;
(2) ○ means no foaming, △ means foaming at the edges, □ means full board foaming;
Table 6 shows that: Increased, the foaming phenomenon of the paint film is better suppressed. When the amount of PMA is 10% of the paint, the safety film thickness is 70 μm; when the amount of PMA is 15% of the full paint, the safety film thickness can reach 80 μm. When the amount of PMA added is more than 5%, although the safety film thickness of the paint film is effectively increased, the emulsion of the aqueous acrylic emulsion is broken, and the stability of the paint is poor. Therefore, the amount of co-solvent in the coating should be controlled within 5%. In order to increase the safety film thickness of the paint film, a certain amount of co-solvent can be added to the curing agent system, and the safety film thickness can reach 70 μm.


2.4 Effect of surface drying time on waterborne two-component paints
Water-based and solvent-based coatings both require a certain surface drying time during construction, which can not only meet the initial leveling of the paint film, but also control the foaming of the paint film. In this study, resins A and C were used, with the addition of mA: mc = 6: 4, -NCO / -OH = 1.2, and 15% of co-solvent was added to the curing agent. The relationship between film safety and film thickness. The test results are shown in Table 7.

Relationship between different surface drying time and safety film thickness of paint film

It can be known from Table 7 that when there is no surface drying time or when the surface drying time is very small, foaming starts when the film thickness is about 50 μm; when the surface drying time is 10 minutes, the safety film thickness can reach 60 μm; Time ≥15 min, safety film thickness can reach more than 70 μm. It can be seen that controlling a certain surface drying time can increase the safety film thickness of the paint film. At the same time, during the spraying process, the interval between each pass is 3 to 5 minutes, which can also avoid the occurrence of blistering.

3 Conclusion
For water-based two-component paint, you can use water-based acrylic emulsion with different hydroxyl value and glass transition temperature. When resins A and C are added according to mA: mc = 6: 4, -NCO / -OH = 1.2, under the condition that 15% co-solvent is added to the curing agent, when the surface drying time reaches 15min, the safety film thickness of the paint film can reach more than 70 μm. Get the ideal paint film.

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