Mixing Behavior of Polyisocyanates in Waterborne 2KPU Coatings
Peng Chen, Jianming Liu

Abstract: This article described the dispersion behavior and dry film properties of 2KPU wood coating by using waterborne polyester with different types of aliphatic polyisocyanates.
Key words: waterborne, 2KPU, wood coating, polyisocyanate, polyurethane

Preface

2KPU polyurethane coating is unique in its weather-ability and durability. Its high gloss and anti-stain ability make it the first for many coating makers in different applications.

Due to the environmental concerns and the new regulations on VOC limits, free TDI content in coatings, the waterborne 2KPU is more attractive to the.

The polyisocyanates used in waterbased 2KPU coating have much effect on final coating. It is important to choose right crosslinker for different applications. This article describes the mixing and dilution behavior of waterborne 2KPU and the final film properties.

Test materials

This article uses waterborne dispersion of saturated polyester and different polyisocyanates. The authors made the polyol dispersion. The technical data are as follows:

1. waterborne dispersion of polyester

Viscosity: 250 mPa.s
Non-Volatile Component: 35.0 %
pH: 7.0
Hydroxyl Value: 130 mgKOH/g(solid)
Acid Value: 30 mgKOH/g(solid)

2. Polyisocyanates
1) AQ210: HDI modified crosslinker, made by NPU
2) 2336: HDI modified crosslinker, made by Bayer
3) 2102: HDI modified crosslinker, made by Rhodia
3) 3300: HDI trimer, made by Bayer

Technical data of above crosslinker is as in table1

Note: 1. The supplier provided the data

3. NCO content of the crosslinkers
Considering the NCO content of trimer as 100%, the relative NCO content is as in Drawing 1.

Test methods

The crosslinkers were diluted with PMA to 85% concentration for easy mixing of crosslinker with dispersion.

The coating part is the mixed with crosslinker.

coating recipe:

Mixing coating with crosslinker in 4:1 wt/wt ratio, then dilute the mixture with water. The viscosity was measured according to water added.

Film hardness was determined by pendulum method. The film was evaluated according HG2454-93 Polyurethane two packages.

Test results

1. Dilution behavior of waterborne 2KPU According to above mentioned test method, the test result of viscosity vs. solid content is shown in Drawing 2.

From Drawing-2, we can see 2336 of Bayer has the highest viscosity in the dilution process, while trimer 3300 the lowest.

As shown in Drawing-3, if the coating is applied at 40 seconds viscosity, 80% water has to be added, while 2102 need 45% of its original coating. In this case, 2102 will has more solid in the final coating.

2. Drying of waterborne 2KPU coating and its hardness
Hardness result is shown in Drawing 4 and Drawing 5
In Drawing 4, catalyst Sn++ was added, the metal Sn++ is 0.018g/100g coating. The catalyst is 10% in PMA solution.
Drying condition of the film is 20, 35% relative humidity.

In Drawing-5, effect of catalyst on drying can be seen. The crosslinker is WT2102.

From above two drawings we can conclude that different crosslinkers have different hardness curve, WT2102 of high NCO has faster hardness growth speed.

Organic Sn++ catalyst may have positive effect for earlier hardness buildup.

3. Film properties of waterborne 2KPU coating

Film properties were measured according to HG2454-93 Polyurethane Coating, Two Packages. The test results are as follows:

Table-2

Conclusions

1. It is easier to mix diluted crosslinkers with polyol dispersion, mixing process can be done by hand.
2. Crosslinker with deeper modification has higher viscosity after the mixing with polyol dispersion.
3. Deeper modification may reduce the film hardness.
4. Catalyst may shorten the through dry time of waterborne 2KPU coating.