The Study on the Effect of Agents on the Properties of Waterborne Two-component Polyurethane High Gloss Coatings

SONG Li-qiang, ZHANG Ge-hui, XU Xu, ZHOU Ze-qian, DONG Li-zhi
(Hebei Chenyang Industrial and Trade Group Co., Ltd., Baoding 072550, Hebei, China)

Abstract: The effects of different wetting-dispersing agents and leveling agents on waterborne two-component polyurethane high gloss coatings are analyzed. The test results show that the wetting-dispersing agents and leveling agents have an important influence on the performance of waterborne two-component polyurethane high gloss coatings, especially on the apparent effect and gloss of coatings.

0 Introduction
Waterborne two-component polyurethanes not only have solvents Type two-component polyurethane has excellent performance, at the same time has the characteristics of non-toxic, odorless, pollution-free, safe and environmentally friendly, has become one of the research focuses of the coatings industry in recent years, especially in high-end furniture, industrial And other markets. Aqueous two-component polyurethane is composed of water-based hydroxy resin and water-based isocyanate. Its performance is mainly determined by the composition and structure of water-based hydroxy resin. Aqueous two-component polyurethanes are composed of polymer polyols and polyisocyanates. The polymer polyols affect the dispersibility of polyisocyanates in the aqueous phase, the diffusion and cross-linking of molecules during film formation, and the final properties of the coating film. There are many types and preparation methods of polymer polyols. Among them, acrylic polyols have the advantages of molecular weight, glass transition temperature, and hydroxyl content, which are easy to adjust. Two different types of acrylic polyols can be obtained by using different polymerization processes. Among them, the dispersion type polyol hydroxyl resin is the research focus of the current coating industry. Similar to other water-based coatings, except for the construction process and the construction environment, the resin, wetting and dispersing agent, leveling agent, defoaming agent, thickening agent in the formula Agents, co-solvents, curing agents, etc. all have a great impact on the properties of the coating film, especially the physical and chemical properties of the water-based two-component polyurethane high-gloss series products. The main problem of waterborne two-component polyurethane high-gloss coatings is the gardenia problem. I personally think that the root cause of gardenia is the leveling problem, so this article will only discuss the effect of wetting and dispersing agents and leveling agents on water-based two-component polyurethane high-gloss white paint Do key discussions.

1 Experimental part
1.1 Experimental raw materials to
prepare high-gloss products, it is necessary to select resins with high hydroxyl content. In this experiment, Covestro’s water-based acrylic polyol dispersion A2601 was selected as the main resin, and in order to improve its leveling, hardness, and fullness Degree and other properties, mixed with water-based polyurethane polyol dispersion XP2755.
For the curing agent, Covestro’s XP2655 (70%) was selected, and the solvent was PGDA. Tego901W and BYK-024 were selected as the defoamer, WT105A and PUR40 were selected as the thickener, and Texanol was selected as the co-solvent. n (-NCO): n (-OH) = 1.5, so mA: mB = 4: 1.
The test uses spraying, the ratio is mA: mB: m water = 100: 25: (20-25), the viscosity of the finished paint is controlled 25-30 s, the construction viscosity is 20-25 s, and the wet film thickness is 120-140 μm. In the activation period test, the viscosity change and the spray test gloss change are measured every 1h. The test environment temperature is 28 ℃ and the humidity is 60%. The basic formula is shown in Table 1.

Basic formula

1.2 Experimental equipment
The main equipment used in the experimental process is shown in Table 2.

Experimental equipment

1.3 Preparation of paint
(1) Add raw materials 1-4 to the preparation tank in sequence, stir 600-800 r / min for 10 minutes, then slowly add 5 and clean the side of the tank with 6, after all the addition is 800-1000 r / min Stir for 20 min until homogeneous, then grind with a grinder to a fineness ≤ 15 μm.
(2) Add raw materials 7 to 8 to the paint mixing tank, stir at 600 to 800 r / min for 5 minutes, add raw materials 9 to 12, and stir for 10 minutes, and then add a mixture of 1 to 6, 800 to 1,000 r / min. Stir for 20 minutes, and then adjust the viscosity to 70-80 s (23-25 ​​° C) with 13.
(3) After adding components 1 and 2, it can be stirred and mixed uniformly at a low speed of 400 to 600 r / min.
After the preparation of component A and component B is completed, the spray method is used when making the board. The mixing ratio is mA: mB: m water = 100: 25: (20-25), and the viscosity is controlled to 20-25 s (T-4 cup ).
1.4 Coating performance testing and
characterization methods See Table 3 for coating performance testing and characterization methods.

Method for testing and characterizing coating film

2 Analysis and discussion
2.1 Selection of wetting and dispersing agent
Wetting and dispersing is a very important process in the paintmaking process. Whether the application of wetting and dispersing agent is proper is directly related to the dispersion efficiency of pigments and fillers, the viscosity and fluidity of pigment pastes, and the development Color, storage state, gloss of coating film, hiding power, chemical resistance, water resistance, etc. Strictly speaking, the dispersion of pigments and fillers can be specifically divided into wetting, dispersing and stabilizing. Wetting refers to the process by which wetting agents reduce the surface tension between media such as water / resin and pigments. Dispersion is the process of opening pigment particles that have been agglomerated together to form a stable state by mechanical external forces and preventing them from flocculating again.

Wetting agents are mainly used to reduce the surface tension of the system. Non-ionic and anionic wetting agents are mostly used in coatings. The dispersant adsorbs on the surface of the pigment to generate charge repulsion and steric hindrance, which prevents the pigment from flocculating again and keeps the dispersion stable. Poor dispersion, affecting gloss, hiding power, and coating resistance. Good dispersion can make the coating film dense and smooth and improve the resistance. At the same time, the coating is stable during the storage period, preventing the precipitation, flocculation, phase separation (water separation) and other undesirable phenomena during storage. The wetting strength of a wetting agent depends on the ability and structure to reduce interfacial tension. The greater the wetting strength, the greater the density of adsorption. The amount of wetting agent depends on the adsorption density and the adsorption state. If the powder density is small and the specific surface area is large, the amount of wetting agent is large. The key to the wetting effect of the wetting agent at the solid / liquid interface is the amphiphilic chemical structure of the wetting agent molecule, the lipophilic functional group of the wetting agent and the powder (the surfactant can The chemical reaction of the powder is the most ideal), and the hydrophilic chain end of the wetting agent can spread in water to make the powder stable. In order to improve the compatibility with the system, the general powder surface will be treated with hydroxylation (except calcium carbonate) and other corresponding treatments, so that the coating system is more stable.
The evaluation of the dispersant is mainly carried out from the aspects of grinding efficiency, viscosity (viscosity reduction), color development, and coating film resistance (such as water resistance). In the aqueous two-component polyurethane highlight system, the dispersant’s foam stability, The effect of gloss is also an important consideration.
During the coating film formation process, most of the water-based dispersants will migrate to the surface of the coating film under the influence of surface tension, resulting in the coating film surface being more sensitive to media such as water, which will have a side effect on the scrub resistance of the coating film. The screening scheme of the dispersant is shown in Table 4.

Screening scheme of wetting and dispersing agents

The choice of dispersant is mainly through testing the activation period (testing for changes in viscosity, gloss and other properties every 1 h), the gloss of the coating film (60 °) and appearance (with or without haze, pinholes, etc.), and the resistance of the coating film. (Water resistance, alcohol, acid, alkali, coffee, hot water, etc.) and coating storage stability (storage at 50 ℃ for 15 days). 。 Its test results are shown in Table 5. Comprehensive test results, ⑧ is the best, ④ can be used as an alternative.

Wetting and dispersing agent test results

Note: 1. The gloss is measured and compared at 60 °.2. The coating film appearance, storage stability, and coating film resistance are compared on a scale of 1 to 10. The larger the number, the better the result.3. Dry film thickness of coating film is measured by dry film thickness.

2.2 Selection of leveling agent

Leveling agent plays a decisive role in the leveling of polyurethane bright white topcoat, especially the silicone leveling agent is very effective in promoting the leveling effect of the facade. It is through the strong incompatibility with the system, migrates to the surface of the coating film to form a monomolecular layer, improves the fluidity of the coating, shortens the leveling time of the coating film, plays a role in eliminating the short-wave orange streaks of the coating film, and improves the coating. Smoothness and gloss of the film; The low level of incompatibility between the polymer polymer leveling agent and the coating system promotes the leveling of the vertical surface of the coating film and eliminates the long-wave orange streaks of the coating film. The two are used together to play a synergistic effect, which can have better short-wave leveling and better long-wave leveling, thereby improving the coating film leveling effect, making the coating film uniform and satisfying the mirror effect of the coating film. , But adding too much will increase the risk of sagging, and the amount should be determined through experiments. The addition of macromolecules can improve the release of solvents, accelerate the release of solvents, and promote the instant leveling of coatings. It also helps to eliminate the long-wave orange streaks of the coating film and has a great benefit in improving the leveling of the facade.
The leveling agent screening scheme is shown in Table 6, and the wetting and dispersing agents are all scheme ⑧.

Leveling agent screening program

The test results are shown in Table 7.

Leveling agent test results

Note: 1. The gloss is measured and compared at 60 °2. Appearance of coating film, storage stability, and resistance to coating film are compared on a scale of 1-10. The larger the number, the better the result.3. The coating film’s resistance to tares is measured by the thickness of the dry film, that is, the maximum film thickness of the taresThe choice of leveling agent mainly depends on the appearance of the coating film, that is, whether there are pinholes, zongzi, haze and other bad phenomena. Water-based two-component polyurethane is easy to pickle, mainly due to poor leveling, which causes the thickness of the coating film to vary. Thick spots will be easy to pick up. Based on comprehensive test results, a leveling agent that reduces dynamic surface tension and static surface tension is used in combination, and the leveling agent with the best leveling effect is the best solution. The solution ⑥ can be used as an alternative. The product construction effect is shown in Figure 1.

Application of water-based two-component polyurethane high-gloss coating in Sany Machinery

3 Conclusions
(1) In waterborne two-component polyurethane high-gloss coatings, the wetting and dispersing agent will affect its activation period and gloss. It is necessary to choose the best dispersing agent according to the system.
(2) The leveling agent has the greatest influence on the apparent effect of waterborne two-component polyurethane high-gloss coatings. Wetting and leveling agents that do not affect the gloss and have a good leveling effect need to be selected according to the system, and the static surface tension of the system must be reduced at the same time And dynamic surface tension.

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