Effect of Curing Agent in Polyurethane Coatings on Whitening of Coating Films

The whitening of the polyurethane coating film mainly occurs in the polyurethane transparent primer
.
For the phenomenon of whitening of the polyurethane transparent primer coating film
.

The effects of different types of curing agents (polydiisocyanates) and different amounts of curing agents on the whitening phenomenon of polyurethane transparent primer coatings were discussed, and the reasons for the whitening were analyzed at coating online coatingol.

foreword

After the polyurethane clear primer is applied, the condition of the coating film is normal at first, and the coating film becomes white after 1 month, 2 months or more

This phenomenon not only brings the trouble of rework to the user, but also troubles the formulator

The phenomenon of whitening of the polyurethane transparent primer coating generally occurs in the process of changing seasons from winter to spring, which is mainly reflected in this process:

(1) The drying speed of the curing agent matching the primer is faster;

(2) Oil workers are accustomed to increase the drying speed by increasing the amount of curing agent used;

(3) The process of transition from lower temperature to higher temperature

In this paper, the detection method of cold and heat cycle (placed at room temperature for 0.

1.

Short oil alkyd resin, dispersant, defoamer, methyl ethyl ketone anti-skinning agent, bentonite anti-settling agent, 1 250# talc, xylene, butyl ester, curing agent (polydiisocyanate), thinner, stearic acid Zinc

Component A: put resin, defoamer, dispersant and solvent in sequence in the dispersing tank, disperse at 800 r/min for 10 min; then add powder under low-speed dispersion, disperse at 1 000 r/min for 15 min, fineness ≤ 50 μm to make the first component of polyurethane clear primer
.

Component B: Add diisocyanate and solvent to the dispersing tank and stir to make the curing agent for polyurethane transparent primer

1.
3 Preparation of polyurethane transparent primer coating film

Substrate: black walnut sticker board (20 cm × 30 cm);

Matching paint: n (main paint—OH): n (curing agent—NCO)=1:1, add thinner to dilute the viscosity to 14 s (Iwata 2# cup);

Spraying: wet-on-wet, 200 g/m2×2 times;

Drying: 48 h (temperature: 25～30 ℃, humidity: <70%)
.

1.
4 Test of whitening phenomenon of polyurethane transparent primer coating film

The coating film prepared according to 1.
3: placed at room temperature for 0.
5 h, then placed at -20 °C for 2 h, then placed at room temperature for 0.
5 h, and then placed at 60 °C for 2 h
.
2 Results and discussion

2.
1 Influence of different types of curing agents

The whitening of the coating film with the same main paint and different types of curing agents is shown in Figure 1
.

The curing agents used in Figure 1 are: (a) domestic TDI trimer; (b) imported TDI trimer; (c) domestic chemical TDI adduct; (d) imported thin film TDI adduct; (e) HDI trimer
.

The left side of each figure is without the cooling and heating cycle, and the right side is the appearance of the paint sample after 48 hours of natural drying and 5 cooling and heating cycles
.

It can be seen from Figure 1 that the whitening in Figure 1(a) and Figure 1(b) is the most serious, and the whitening in Figure 1(c) is slightly white.
Figure 1(d) and Figure 1(e) It is normal, and the scoring results are shown in Table 3
.

Note: 5 points: no whitish; 4 points: slight whitish; 3 points: whitish; 2 points: more severe whitish; 1 point: severe whitish
.

From the analysis of the nature of the curing agent, TDI trimer curing agent and -OH(a) (b) (c) (d) (e) have the fastest cross-linking speed;

The domestic chemical method TDI adduct curing agent is a post-trimerization treatment to reduce TDI, and there is a certain trimer, so the cross-linking speed is also faster; the imported TDI adduct curing agent has a relatively slow cross-linking speed, and HDI trimer Curing agent crosslinks the slowest
.

According to the film-forming mechanism of the two-component polyurethane coating, the curing agent R-NCO and the main paint R-OH are cross-linked and cured to form a film, but in fact, the curing of the -NCO/-OH two-component polyurethane coating is not simple -NCO/-OH reaction cross-linking
.

During the curing process of the coating film, it gradually changes from a liquid state to a solid state, the viscosity increases sharply, the molecular movement becomes more and more difficult, and it is difficult for the groups to fully contact
.

Even if the -NCO is excessive, it is impossible to ensure that the -OH is paired correctly, but the penetration of water molecules in the air to the coating film promotes the moisture-curing cross-linking reaction of -NCO.
The higher the activity of -NCO, the easier it is to react with water
.

The reaction of isocyanates with water is carried out stepwise
.
The first step is the reaction of the isocyanate with water to form carbamic acid
.
The second step is the decomposition reaction of carbamate to generate amine and carbon dioxide, carbamate is unstable
.

Under normal conditions, it will decompose immediately and quickly release CO2
.
The third step is the rapid reaction of the resulting amine with the isocyanate to form the urea
.
The reaction formula is expressed as follows:

In the reaction process of isocyanate and water: CO2 generated in the second step reaction in a short time can only be filled in the interior of the coating film, especially the gap between the powder and the resin in the primer, resulting in the appearance of a phase between the resin and the powder in the coating film.
separation
.

According to the principle of refraction of light, the refractive index of CO2 gas is about 1.
0, while the refractive index of resin is 1.
55.
Therefore, the difference in refractive index between resin and CO2 gas causes whitening inside the coating film
.

Based on the above inferences, in the early stage of the film-forming process of the polyurethane transparent primer, the curing agent R-NCO and the main paint R-OH are cross-linked and cured to form a film.
This stage is a normal film-forming process, which lasts for a long time.
There is no whitening phenomenon;

The second stage is mainly that the remaining or unreacted curing agent R-NCO reacts with water to generate CO2, which leads to the separation of the resin and powder in the coating film, and the coating film appears white
.

According to the test results of the test, it can be known that the curing agent using TDI trimer has a high-NCO activity and a more intense reaction with water, so the phenomenon of whitening is more serious
.

Especially in winter, the polyurethane transparent primer often uses a quick-drying curing agent, so the coating film is prone to whitening in the later period
.

2.
2 Influence of different usage amounts of the same curing agent

Figure 2 shows the whitening of the coating film with the same main paint and different amounts of curing agent
.

Figures 2(a) to 2(d) are respectively: (a) is n(-NCO):n(-OH)=0.
65:1; (b) is n(-NCO):n(-OH)= 0.
8:1; (c) is n(—NCO):n(—OH)=1:1; (d) is n(—NCO):n(—OH)=1.
3:1
.

The left side of each figure is without the cooling and heating cycle, and the right side is the appearance of the paint sample after 48 hours of natural drying and 5 cooling and heating cycles
.

As can be seen from Figure 2, it is obvious that the whitening phenomenon becomes more and more serious with the increase of the amount of curing agent.
Figure 2(a) is normal, Figure 2(b) is slightly white, and Figure 2(c) and Figure 2(c) are normal.
2(d) is severe whitening, and the scoring results are shown in Table 4
.

This whitening phenomenon is more in line with the above principle.
The more the remaining curing agent R-NCO, the more CO2 produced by its reaction with water, and the more serious the whitening phenomenon of the coating film
.

Generally, according to the usage habits of oil workers, when the weather is relatively cold, the amount of curing agent will be increased to improve the drying speed, which just promotes the further whitening of the coating film
.
3 Conclusion

There are many factors that affect the late whitening of polyurethane transparent primers, among which the type and amount of curing agent play a key role
.

The higher the -NCO activity of the curing agent, the faster the reaction with water, the greater the possibility of causing the primer to turn white; at the same time, the greater the amount of curing agent used, the greater the possibility of reacting with water, resulting in the whitening of the primer.
the more serious the phenomenon
.
This requires the user to consider the possibility of blushing when increasing the drying speed of the primer
.