Application of Film-fIdditive for Waterborne Two-component Polyurethane Coatings

FANG Bo1,2, LI Guo-jun1,2, ZHANG Yu-guo1,2, WANG Cong-guo1,2 (1. Guangzhou Kinte Industrial Co., Ltd., Guangzhou 510860, Guangdong, China; 2. China National Electric Apparatus Research Institute Co., Ltd., Guangzhou 510080, Guangdong, China)

Abstract: This paper briefly describes the effect of several film-forming additives applied for waterborne two-component polyurethane coatings on their application and appearance.

0 Introduction
Film-forming aids are usually high-boiling solvents, which have a lower volatilization rate than water, and will slowly evaporate after the film is formed. Film-forming aids can promote the plastic flow and elastic deformation of polymer compounds, improve their coalescence properties, and help them form films over a wide range of construction temperatures. They are a plasticizer that is easy to volatilize and disappear. In thermoplastic aqueous emulsions, in order to ensure coating film hardness and chemical resistance, the glass transition temperature of the emulsion polymer must be increased. At the same time, the minimum film formation temperature (MFT) required for the coating film is relatively high. It brings trouble to the construction and film formation of coatings at lower temperatures. The film-forming additives can just solve this problem, so that high performance and low application temperatures can not conflict.

The film formation of water-based two-component polyurethane coatings is different from that of thermoplastic water-based emulsions. Through the uniform mixing of water-based hydroxypolyol resin coatings and water-dispersible polyisocyanate curing agents, the particle spacing gradually decreases with the evaporation of water and solvents. In order to form a high-concentration stacked structure, a tighter hexagonal structure is formed, and finally a urethane is formed through the main reaction of a polyol and a polyisocyanate. The main purpose of adding film-forming additives in water-based two-component polyurethane coatings is also different from that of thermoplastic water-based emulsions. It is not to help the film formation of the coating, but to aid the construction of the coating and improve the appearance of the coating film through the addition of film-forming additives. .

In this paper, two different water-based hydroxypolyol resins from Dickson’s WD-551 and DSK’s XK-110 are used as the main component to form the film. WD-551 is a dispersion type hydroxyacrylic polyol with a hydroxyl content of 3.3. mg KOH / g, 44% solid content, belongs to high hydroxyl content resin. The coating film has high gloss, high fullness, and good physical and chemical properties. It can be used for mid-to-high-end topcoats with strict appearance and performance requirements, and because it is a dispersion-type water-based resin, it is not easy to break emulsion during production. Easy to operate. XK-110 is an emulsion type hydroxyacrylic polyol with a hydroxyl content of 2.5 mg KOH / g and a solid content of 46.5%, which is a medium-hydroxy content resin. The coating film has high gloss and fullness. Due to the low hydroxyl content and insufficient crosslinking density of the coating film, its physical and chemical properties are lacking compared to high-hydroxy resins, but in the face of products with low requirements for appearance and performance, Its lower amount of curing agent added can make it more advantageous in terms of cost performance. Because it is an emulsion-type water-based resin, the selection and use of film-forming aids in the formulation must be more careful to avoid breaking emulsion. Component B uses Covestro’s XP2655. By adding different film-forming aids, the storage stability of the coating A component, the workability (leveling and anti-sagging effect) after mixing the A and B components, and the surface of the coating film dry The speed and the appearance of the dry film are compared in order to provide some reference for the selection of film-forming additives.

1 Experimental
1.1 Raw materials and equipment
Raw materials: WD-551 from Dickson and XK-110 water-based hydroxyl polyol resin from DSM, R-980 titanium dioxide, BYK2012 dispersant, BYK028 defoamer, DEGO4100 wetting agent , PW336 leveling agent, OMG0620 thickener, film-forming aid.
Table 1 shows the basic parameters of several commonly used film-forming additives selected in this paper.

Table 1 Basic Parameters of Several Film-forming Additives

Equipment: electronic balance, disperser, sand mill, scraper fineness meter, viscosity cup (2 # cup), dry film tester, pot spray gun, etc.

1.2 Basic formula (see Table 2)
This experiment uses n (-NCO): n (-OH) = 1.5: 1 for the experiment. When the water-based hydroxyacrylic polyol resin is WD-551, it is m (A component): m (B component) = 7 1: 1; when the water-based hydroxyacrylic polyol resin is XK-110, press m (component A): m (component B) = 10: 1.

 Table 2 Basic Formula

1.3 Experimental method
Add dispersant and defoaming agent to deionized water, stir at low speed for about 5 minutes, slowly add titanium dioxide under continuous stirring, stir at medium speed for about 15 minutes, put it into a sand mill for grinding until the fineness is less than 20 μm, take out the color paste, add water-based hydroxyacrylic polyol resin, wetting agent, leveling agent, diethanolamine, thickener and film-forming aid in sequence under medium speed stirring, and continue to medium speed after all components are added Stir for 15 min to obtain component A.
Mix the AB components according to the ratio described in 1.2, add deionized water to adjust the viscosity for about 40 s (2 # cup), spray on tinplate, level at room temperature for 5 minutes, and bake at 70 ℃ for 1 h to obtain a coating film.

2 Results and discussion
2.1 Coating A component storage stability
Coating storage stability refers to coating products under normal packaging conditions and storage conditions, after a certain storage period, the physical or chemical properties of the product can reach the original specified use Required degree. After the coating products are made in the production plant, it often takes a period of time to use, and it may be stored for several months or even years. The ideal coating products should not change in quality when stored in containers. Compared with solvent-based coatings, water-based coatings are more prone to storage instability, especially after adding film-forming aids, some of them will cause damage to the emulsion, which will cause skinning, particles, and sedimentation. Different aqueous emulsions have different choices and addition amounts of film-forming auxiliaries. In this section, different film-forming auxiliaries are added to the paint A components of the two types of water-based hydroxypolyol resins prepared according to the above basic formulations to ensure stable storage. The test was carried out according to GB / T 6753.3-1986 national standard, storage temperature was 50 ℃, and the duration was 30 d.
The test results of water-based hydroxypolyol resins WD-551 and XK-110 are shown in Tables 3 and 4.

 Effect of Film-forming Aids on Storage Stability of Waterborne Coatings (WD-551) Table 3 The Effect of Film-forming Additives on Waterborne Coatings' Storage Stability (WD-551)

Note: The items in the table are evaluated according to 6 levels: 10 = none, 8 = very slight, 6 = slight, 4 = medium, 2 = severe, 0 = severe; viscosity changes are evaluated according to 6 levels: 10 is ≤5% , 8 is ≤15%, 6 is ≤25%, 4 is ≤35%, 2 is ≤35%, and 0 is> 45%.

Effect of Film-forming Additives on Storage Stability of Coatings (XK-110) Table 4 The Effect of Film-forming Additives on Coatings' Storage Stability (XK-110)

Note: The items in the table are evaluated according to 6 levels: 10 = none, 8 = very slight, 6 = slight, 4 = medium, 2 = severe, 0 = severe; viscosity changes are evaluated according to 6 levels: 10 is ≤5% , 8 is ≤15%, 6 is ≤25%, 4 is ≤35%, 2 is ≤35%, and 0 is> 45%.It can be seen from Tables 3 and 4 that the storage stability of the aqueous two-component polyurethane coatings prepared by the two water-based hydroxypolyols is very good when the film-forming assistant is not added. The problem of sedimentation is easy to occur, and the effects of different film-forming aids on the two are not the same. The problem of sedimentation becomes serious with the increase of the amount of film-forming aids, and some particles appear. Therefore, the water-based coatings prepared for different water-based hydroxypolyol resins must be fully tested for the stability of various film-forming additives when applied. As can be seen from Tables 2 and 3, WD-551 can choose DPM, TPNB, and TPM with a filming additive amount of about 5%. When using alcohol ester 12, the filming additive amount can be increased to 10%; and 5% The film-forming aids are all suitable for XK-110, and the PNB and the alcohol ester 12 can be increased to 10%.

2.2 Drying speed of coating film 

Drying of coating film can also be referred to as finger touch drying, that is, when the finger touches the coating film and feels sticky, but the coating film is not attached to the finger. There are many factors that affect the surface drying speed of the coating film. Different resins, high and low temperature and humidity, and the type and amount of film-forming aids will cause differences in surface drying speed. In the process of coating construction, too short surface drying time will adversely affect the appearance of the coating film, such as poor leveling, blistering, tares, etc., and too long surface drying time will cause a lot of inconvenience. It is also more prone to sag problems.
Different film-forming aids were added to the water-based coatings prepared for the two types of water-based hydroxypolyol resins under the same temperature and humidity conditions, and the coating films were prepared according to the experimental method, and the surface drying speed was tested. During the experiment, the air temperature was 28-30 ° C and the relative humidity was 40%. The specific test results are shown in Table 5.

Table 5 The Effect of Film-forming Additives on Tack-free Time

In combination with Table 1, the slower the evaporation rate and the higher the boiling point will be, the slower the drying speed of the coating film will be, and it will become slower with the increase of the amount. It can be used as needed when designing the coating formulation. Corresponding film-forming aids and appropriate additions to adjust the surface drying speed of the coating film.

2.3 Dry film appearance
Water-based two-component polyurethane coatings can have excellent performance and appearance, but the coating film is prone to problems such as poor leveling, orange peel, blistering, gardenia, etc. during the drying process, which not only seriously affects the appearance of the coating film, but also has some defects. The appearance will also have a certain effect on the performance of the coating film. As described in 2.2, adding a high boiling point film-forming aid to the coating formulation can adjust the surface drying speed of the coating film. By extending the surface drying speed of the coating film, problems such as poor leveling, orange peel, foaming, and gardenia can also be solved.
However, different water-based hydroxypolyol resins require different film-forming auxiliaries. If the film-forming auxiliaries are poor solvents for water-based hydroxypolyol resins, it is likely to cause the final coating film to lose light, fog, and Fullness.
The WD-551 and XK-110 two types of water-based hydroxypolyol resins involved in this experiment were tested for their gloss, leveling, and gardenia after adding 10% of different coating formulations. During the experiments in this section, the air temperature is 30-32 ° C and the relative humidity is 60%. The specific test results are shown in Tables 6 and 7.

Film Additives on the Appearance of Dry Film (WD-551) Table 6 The Effect of Film-forming Additives on the Appearance of Coatings Film (WD-551)

Note: Leveling is evaluated on 6 levels: 10 = excellent, 8 = good, 6 = medium, 4 = poor, 2 = poor, 0 = severe orange peel; zongzi problems are evaluated on 6 levels: 10 = none, 8 = Very mild, 6 = slight, 4 = moderate, 2 = more severe, 0 = severe.

Effect of Film-forming Aids on the Appearance of Dry Film (XK-110) Table 7 The Effect of Film-forming Additives on the Appearance of Coatings Film (XK-110)

Note: Leveling is evaluated on 6 levels: 10 = excellent, 8 = good, 6 = medium, 4 = poor, 2 = poor, 0 = severe orange peel; cripple problems are evaluated on 6 levels: 10 = none, 8 = Very slight, 6 = slight, 4 = moderate, 2 = more severe, 0 = severe.

In Tables 6 and 7, when the film-forming aid is not added, the measured gloss of the coating film will decrease due to poor leveling and the effect of the rafter. As the thickness of the dry film increases, the problem of the gardenia becomes more serious. The impact is also greater. Combined with Table 1, compared with Table 6 and Table 7, with the decrease of the volatilization rate of the film-forming aid and the increase of the boiling point, the leveling effect of the coating film is better, and the problem of gardenia is more slight; and different film-forming aids The effect on the coatings prepared by the two water-based hydroxypolyol resins is also different. It is especially reflected in the gloss, part of which does not affect the gloss or even enhances the gloss, and part of it will have a certain effect on the gloss. The haze of the coating film can be seen when viewed, and it is not transparent. From Table 6 and Table 7, DPM, TPM, and alcohol ester 12 are suitable film-forming aids for both resins.

3.Conclusion
This article has conducted in-depth research on the storage stability, surface drying speed, and dry film appearance of two different water-based hydroxypolyol resins without or with different film-forming additives. When preparing water-based two-component polyurethane coatings, corresponding tests must be performed according to different resins. One or more film-forming additives should be selected to combine the effects of various factors to ensure that the coating can be stored stably and the appearance and performance of the coating film can reach the maximum. Good condition.

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