Application of Functional Additives in High Solid Acrylic Polyurethane Coatings

WANG Jian-gang, ZHANG Guo-fang, QIN Tie-han, FAN Zeng-jun
(Shijiazhuang Paint Company, Shijiazhuang 050086, Hebei, China)

Abstract: In this experiment, the preparation process and performance test method of a kind of high solid acrylic polyurethane coatings are introduced, and the effects of different functional additives (anti-sagging additives, dispersants, leveling agents and defoaming agents) in the coatings are compared. The results are briefly analyzed, and the following conclusions are drawn: (1) with the increase of polyamide wax, the anti-sagging property increases obviously, and with the increase of thixotropy, the amount and defoaming ability of defoaming agent should be strengthened accordingly; (2) dispersant has a great influence on the storage stability and anti-sagging property of coatings; (3) the type and dosage of leveling agent should be screened to prevent the formation of bubbles, seeds and other paint film defects; (4) the choice of defoaming agent is closely related to solvent, leveling agent and construction viscosity.

0 Introduction
Acrylic polyurethane coatings have excellent decorative properties, adhesion, and mechanical properties With good compatibility with other resins, it is widely used in many fields such as automobiles, ships, agricultural machinery, engineering equipment, etc. It is one of the best coatings with comprehensive performance. However, the traditional acrylic polyurethane coatings have the disadvantages of high VOC content and low construction solids (multiple thin sprays are required to achieve the required film thickness). Not only does VOC have a large volatility, pollute the environment, but also requires labor and a long construction cycle. As environmental protection laws and regulations become more and more complete, the country and the people pay more and more attention to environmental protection concepts. Coatings with high VOC content will gradually be eliminated by the industry and the market, and high solids coatings will occupy a major share in the future market.
In the preparation process of high-solids coatings, the role of functional additives is very important. In this paper, an acrylic polyurethane coating with a VOC of less than 420 g / L was prepared by screening high-solid acrylic polyurethane additives. At the same time, it has excellent weather resistance, good elasticity and high construction efficiency. Under the premise of responding to national environmental protection policies, meeting the needs of industry customers has broad market prospects and economic benefits.

1 Experimental part
1.1 Experimental raw materials and basic formula
Paint components Main raw materials: hydroxyacrylic resin, titanium dioxide R706, medium yellow, MA100, dispersant, anti-sag additive, leveling agent, defoaming agent, mixed solvent (self-made ), Isocyanate, butyl ester, PMA, T-12, etc. The experimental raw material information and basic formula are shown in Table 1.

Experimental raw materials and basic formulas

1.2 Experimental equipment
Sander, QSM-Ⅱ, Tianjin Jingkelian Material Testing Machine Co., Ltd .; Electric constant temperature blast drying oven, TST202A-1B, Chengdu Tesite Instrument Co., Ltd .; Multifunctional mixer, U450 / 80-220, Shanghai Weida Industry and Trade Co., Ltd .; Electronic Balance, TC3K, Changshu Shuangjie Testing Instrument Factory; Tu 4 # Viscometer, NDJ-5, Shanghai Pingxuan Scientific Instrument Co., Ltd .; Digital Specific Gravity Meter, YMS (0.1-5.0) , Qingdao Chuangmeng Instrument Co., Ltd.

1.3 Experimental process and performance testing
(1) Firstly, part of the hydroxyacrylic resin and anti-sagging agent are added to the reaction kettle A, and the high-speed stirring bar is stirred for 0.5 to 1.0 h to the required fineness uniformly and set aside.
(2) Slowly stir part of the resin, dispersant, solvent, and pigment batching reactor B. After uniform dispersion, stir at high speed for 0.5 to 1.0 h to the required fineness.
(3) Add the materials in reactor B to reactor A, make up the remaining resin and auxiliary solvent, stir at low speed for 30 minutes, filter through a 100-mesh sieve, and discharge the package.
Table 2 shows the coating performance testing methods.

Coating performance testing method

Mix the paint’s paint component and curing agent component in a mass ratio of 5: 1 and pour it into the paint spray tank, adjust the spray gun pressure and flow rate, and spray the paint uniformly on the sample board. The dry film thickness is 25 μm, 50 μm , 80 μm, 100 μm, etc., tested under standard conditions [temperature 23 ℃, relative humidity (50% ± 10%)].

2 Experimental results and discussion
2.1 Selection of anti-sag additive

A high-solid two-component polyurethane has a high construction film thickness. Therefore, it is necessary to first select the appropriate anti-sag additive and the appropriate addition amount.

2.1.1 Screening of different types of anti-sagging additives

Screen the four commonly used anti-sagging additives. Refer to the basic formula design A, B, C, and D for the four experimental schemes in Table 1. Performance tests such as sag, thixotropy, and coating appearance are shown in Table 3.

Test results of 4 anti-sag additives

As can be seen from Table 3, this experimental product is a topcoat product, so polyethylene wax and polyamide wax that have less effect on the appearance of the coating film are selected for experiments. Although polyethylene wax has poor thixotropy, it can prevent hard precipitation. Finally, the polyamide anti-sag additive is selected and used together with polyethylene wax. Because ordinary polyamide wax is sensitive to temperature and easy to return to coarse, an improved variety is selected.

2.1.2 Screening of the amount of anti-sagging additives
According to the experimental results in 2.1.1, polyamide wax was selected as the anti-sagging additive of the high solid acrylic polyurethane coating in this experiment. Therefore, the optimal dosage should be tested. The experimental design addition amounts are 0, 0.5%, 1%, and 1.5%, respectively, to test the anti-sag resistance, leveling property and gloss of the coating film. The specific test results are shown in Table 4. .

Test results of different anti-sag additives

By the experiment results of Table 4, when the addition amount of 1% polyamide wax additives, coatings prepared from both good sag resistance, but also excellent in coating film appearance.

2.2 Screening of dispersant
According to the actual production situation of our factory, resin and pigment mixed color grinding production process is generally used. Therefore, the dispersant should have good versatility and excellent wetting and dispersing ability, in order to reduce the blemishes such as floating color, flooding, fading after storage, and return to coarseness.
Wetting agents are substances that have a wetting effect by reducing the contact angle between the pigment and the resin solution, accelerating the liquid phase into the agglomerated pigment structure. The dispersant is adsorbed on the surface of the pigment, and maintains the proper distance between the pigments through charge repulsion and steric hindrance. This reduces uncontrolled flocculation, improves pigment spreading power, storage stability, and reduces pigment usage. There are four types of commonly used dispersants and their mechanism of action. The specific information is shown in Table 5.

Different dispersant types and mechanism

According to previous experimental experience, dispersants have a large impact on the performance of coatings such as grinding efficiency, storage stability, and color difference after storage. In this experiment, 8 kinds of dispersants were selected for experiments, among which 1 #, 3 # and 8 # are modified polyurethane polymers, and 2 #, 4 # and 6 # are saturated polycarboxylic acid polymers and polysiloxane copolymers. 5 # and 7 # are pigment block copolymer-containing high molecular block copolymers. The specific test results are shown in Table 6.

Effect of different dispersants on coating performance

According to the experimental comparison in Table 6, 1 #, 3 #, and 8 # dispersants are inferior in dispersing efficiency and storage stability. 6 # dispersants have a greater effect on the color difference of the coating after storage, so they are discarded. Although 2 #, 5 #, and 8 # dispersants have better dispersion efficiency and color difference after storage, the appearance of multi-color paints is less than that of 7 # dispersants. The coatings prepared by 7 # dispersants have the best comprehensive performance. Therefore, 7 was selected. #Dispersant scheme is the best dispersant for this experiment.

2.3 Selection of leveling agent
The choice of leveling agent is very important. This directly affects the appearance of the coating film. Therefore, different types are used in the same way as dispersants to achieve excellent appearance. Table 7 shows the results of different types of leveling agents on coating performance.

Influence of different kinds of leveling agents on coating performance

It can be seen from Table 7 that the polysiloxane-based leveling agent can provide better substrate wettability for the coating, while increasing the surface slippage and preventing the formation of Bernard swirls, and the leveling and gloss are improved. , Most used in high solid polyurethane coatings. However, excessive addition should be avoided to avoid the phenomenon of foam stabilization. The concentration and amount of additives of different additives manufacturers are not the same, so the specific addition amount will not be discussed.

2.4 Screening of defoaming agent
Because the high-solid coating has a thick film formation (dry film is greater than 80μm), it is very important to eliminate the micro-bubbles in the coating film. It is necessary to use a certain defoaming agent. In this experiment, three different defoamers were selected to test the viscosity and miscibility of the coating. The specific test results are shown in Table 8.

Effects of 3 kinds of defoamers on defoaming performance of coatings

Type A defoamer is a defoaming polysiloxane polymer. This type of defoamer exhibits excellent defoaming ability when using a small amount of incompatible materials, but it needs to be dispersed uniformly to avoid shrinkage; B The defoamer is a high polymer defoamer, which has no adverse effect on the interlayer adhesion during recoating, but the defoaming effect is not as good as that of the silicon defoamer; the C defoamer polysiloxane-polyether copolymer , The defoaming effect is moderate.
The choice of defoaming agent has a great relationship with the leveling agent. Some defoaming agents have a very good defoaming effect when no leveling agent is added. However, the addition of the leveling agent immediately weakens the function and requires attention. Also, the mixed solvents have different surface tensions and different volatilization rates, which have a great effect on the defoamer.

3 Conclusions In
this experiment, the following four conclusions were obtained through experiments and analysis of additives for high solids acrylic polyurethane coatings.
(1) With the increase of the polyamide wax, the sag resistance is significantly increased, and as the thixotropy is increased, the amount of the defoaming agent and the defoaming ability must be strengthened accordingly.
(2) The dispersant also has a great influence on the storage stability and sag resistance of the coating.
(3) The type and amount of the leveling agent need to be screened to prevent the formation of coatings such as dark bubbles and gardenia.
(4) The choice of defoaming agent has a greater relationship with the solvent, leveling agent, and construction viscosity.

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