New waterborne catalyst for two-component waterborne polyurethane coatings

Two-component waterborne polyurethane (2K WB PU) coating technology has been commercialized since 1990, when such products were developed primarily to reduce the problem of VOC content, which was not feasible to solve using solvent-based technology.1 Since the introduction of two-component waterborne polyurethane (2K WB PU) coating technology, progress has been made to address many of the shortcomings inherent in waterborne coatings. Efforts have been made to match the performance requirements and application range of waterborne coatings with those of conventional two-component solvent-borne polyurethane coatings.
However, a difficult deficiency of two-component waterborne polyurethane (2K WB PU) systems to overcome is the drying speed, especially in high humidity conditions. This problem is particularly evident when typical polyester polyols are used as the OH component of the system. Acrylic polyols do not confer the problem of severely slowed drying times under high humidity conditions, but other properties such as flexibility, durability or resistance to chemical media may be affected. It will therefore be interesting to check whether the correct choice of catalyst can improve the drying time problems of polyester-based two-component waterborne polyurethane (2K WB PU) coatings.
The catalytic performance of generic polyurethane catalysts such as dibutyltin laurate (DBTDL) in aqueous systems is diminished, mainly due to incompatibility and hydrolytic instability in aqueous matrices. Two important characteristics of effective waterborne polyurethane catalysts are hydrolytic stability and increased water solubility. In addition to these characteristics, the waterborne polyurethane coating catalyst should be able to provide the required reactivity and assist in the formation of various formulation characteristics properties (chemical structure, functional groups, additives, % solids, etc.). Ideally an effective waterborne polyurethane catalyst should also provide consistent buildability and other performance characteristics under a variety of environmental conditions including temperature and relative humidity.
Reaxis has developed a novel waterborne catalyst with excellent hydrolytic stability compared to typical polyurethane catalysts to provide improved performance in two-component waterborne polyurethane (2K WB PU) formulations under a variety of environmental conditions. This paper focuses on in-service stability, service life, service life, drying time, solvent resistance, and the effect of humidity on cure rate.

Reactivity and Film Formation Properties
Two approaches can be used to prepare stable two-component waterborne polyurethane (2K WB PU) coating formulations.2 One approach is to use hydrophilically modified polyols to provide emulsification capability, which allows the use of typical hydrophobic polyisocyanates. The polyol droplets are typically much smaller and surround the polyisocyanate droplets to help them disperse. Emulsification occurs when the polyol droplets surrounding the larger polyisocyanate droplets form stable polyisocyanate micelles.
In addition, typical two-component waterborne polyurethane (2KWB PU) coatings can be prepared with hydrophobically modified polyisocyanate dispersions mixed with polyols. Polyisocyanates can form micelle structures without the help of polyols (Figure 1). As the formulated product sits for a longer period of time, the polyisocyanate droplets and polyol droplets begin to agglomerate and become larger in particle size. This usually leads to a decrease in starting viscosity. Because of this drop, these systems cannot be measured by measuring the increase in viscosity with time to determine the service life, which is often the case with solvent-based two-component polyurethane systems.

Once the formulated product is coated, the water begins to evaporate and the particles begin to agglomerate and form a paint film.3 The curing curve in Figure 2 was generated by measuring the relative concentrations of water and isocyanate groups (NCO) using Fourier infrared spectroscopy (FT-IR). The curves show that most of the water evaporates within the first 30 minutes and almost all of the water evaporates after 60 minutes. The main reaction that occurs at this point is the reaction of the hydroxyl (OH) group of the polyol with the NCO group of the polyisocyanate. The reactivity and selectivity of the catalyst is very important, as is the competitive reaction of the formulation with water in the mixing tank between the first 30 and 60 minutes after the construction of the paint film. It is desirable that the catalyst preferentially promotes the reaction of the NCO group with the OH group of the polyol over the reaction with water. Too much water reacting with the NCO groups will form bubbles due to the release of carbon dioxide. If the catalyst is too reactive, too much crosslinking will form before all the water evaporates, as carbon dioxide bubbles are entrapped and pinholes form.

The advantages of using a catalyst can be illustrated by a simple FT-IR experiment. Cured lacquer films with and without catalyst were analyzed to show the difference in cure completion. After two days, no isocyanate peaks were found in the cured paint film with the new REAXIS C333 catalyst. In contrast, the isocyanate peak (2265 cm-1) was still visible in the paint film prepared without the catalyst, as shown in Figure 3.

Experiment
Two polyester/hexamethyl diisocyanate (HDI) formulations with different reactivities were used in this study. Throughout this article, the two formulations are defined in the following way: Formulation 1 is composed of Bayer Bayhydrol® 2591 urethane modified polyol and Bayhydur® 2487/1 isocyanate. Formulation 2 is composed of W2K® 2002 polyester polyol and Bayhydur 302 isocyanate from American Polymers.
We further defined the formulation as high performance and standard performance based on the hydroxyl functional groups and the main chain structure of the polyol. Thus, formulation 1 (tetrafunctional carbamate functional polyol with an OH equivalent of 436) is defined as high performance, while formulation 2 (polyester polyol with an OH equivalent of 252) is defined as standard performance. bayhydrol 2591 has an equivalent of 436 at 100% solids, while W2K 2002 is 252. these formulations are shown in tables 1 and 2. based on resin solids, the The amount of catalyst used is 0.2%.

In preparing the coatings, Component A (polyol, catalyst, water, wetting additive) was mixed with Component B (isocyanate) for 1 minute. Using a Binks siphon gun, set the gun pressure to 50 psi and spray each coating onto the aluminum substrate with a dry film thickness of 1.5-2.0 mils. The coatings were allowed to dry in air for the specified time as required by the test method used. Finger touch dry time, non-dusting dry time, dry hard time, number of butanone reciprocal wipes and pencil hardness were determined in the determination of physical properties as specified by ASTM.

Results
Physical properties
The results of the comparison of physical properties illustrate the short drying time from finger touch to solid dry for the coatings on the formulated plates using Reaxis C333. They also obtained the same final physical properties as any other catalyst. Of course, the final physical properties are determined by the nature of the raw material selected and REAXIS C333 helps to obtain these final properties in a short period of time. Catalysts can reduce the time needed to obtain these end properties, but they may also degrade the end physical properties if they promote undesired side reactions. Thus, selectivity is an important property.
Table 3 illustrates that for reactive polyols all catalysts act similarly, but the solid dry time is good with REAXIS C333. We define the reactivity of a polyol as the ability to give the formulation better final film properties, all else being equal. Table 4 shows that the use of REAXIS C333 in a sub-reactive polyol system resulted in a more rapid formation of final film properties.

An important advantage of the REAXIS C333 catalyst is that it is soluble in both the organic and aqueous phases. This makes the catalyst compatible with most systems and ensures uniform distribution in the formulated product. This helps to ensure uniform curing of the coating.

Storage period stability
For practical reasons, it is important to determine the appropriate shelf life stability of the A and B components of a two-component waterborne polyurethane (2K WB PU) system. The best stability is usually seen when a catalyst is added to the A component. The use of catalysts in the B component (NCO) can lead to the formation of by-products such as diurea, urea formate, isocyanate and urea under certain conditions. Similarly the use of catalysts in component A avoids catalytic effects on the water/NCO reaction because the mixture absorbs water as it sits.

Tables 5 and 6 illustrate that the drying time and pencil hardness of the formulated products were largely unaffected by the addition of C333 to the polyol matrix (component A) after 2 weeks at 60°C. Further testing is needed to confirm the stability of the polyol matrix, but these preliminary results are very encouraging.

Certain catalysts are designed to be used in polyisocyanate matrices (B component); however, this is not the norm. As mentioned earlier, if traces of moisture get into the polyisocyanate component, it can lead to many problems. We did not find any difference in the performance of the aged and unaged formulations of the B component, except for the good pencil hardness of the REAXIS C333 system. REAXIS C333 showed good versatility in that it can be used in either component A and B as long as the B component is kept free of moisture.

Service life
With waterborne coatings, the service life is usually not measured by an increase in viscosity, since a decrease in viscosity is usually encountered when the coating is left to age. The typical method of measuring the service life of waterborne coatings is to measure the physical properties after a specified period of time.
Although REAXIS C333 promotes rapid attainment of final properties, a reasonable action time (at least 2 hours) is required after mixing the A and B components. The drying time was shortened due to certain reactions occurring in the tank, but the final properties did not change. Then, as shown in Tables 8 and 10, the difference in pencil hardness after aging placement was more significant for the system based on REAXIS C333 compared to the use of other catalysts.

Coating performance under different humidity conditions
REAXIS C333 catalyst provides excellent curing in a wide range of humidity conditions. High humidity often results in slower drying of aqueous coatings. Drying time and final physical properties of the coating remain relatively unchanged with REAXIS C333. This is advantageous to the end-user because it allows the coating to be applied under a variety of conditions. For example, consistent performance can be obtained in high humidity conditions and/or hot outdoor environments (where temperature and humidity cannot be controlled).

Selectivity of the isocyanate/water reaction and isocyanate/hydroxyl reaction
FT-IR was used to investigate the relative selectivity of C333 for promoting the reactions of isocyanates with hydroxyl groups and water. Polyisocyanate (concentration of 0.8 molar concentration) and co-reactants were mixed in dipropylene glycol dimethyl ether. The catalyst was used at a metal concentration of 200 ppm relative to the reactant solid. The negative natural logarithm of the height of the absorbance peak of NCO (-Ln) was plotted against time in minutes. The slope of the straight line was compared to determine the relative rate. Figure 4 shows that n-butanol reacts 6.7 times faster with the aliphatic NCO group in the primary position than water reacts with the NCO group. This is very advantageous for formulating two-component waterborne polyurethane coatings as it helps prevent poor film appearance due to blistering. seneker and Potter reported a selectivity of about 24 for DBTDL. figure 5 shows that the reaction of water with NCO catalyzed by DBTDL is 1.45 times faster than the reaction catalyzed by REAXIS C333.

Overview and Conclusion
Reaxis C333 is a water-soluble, hydrolytically stable catalyst that provides fast drying times and good physical properties for two-component aqueous polyurethane (2K WB PU) formulations at a variety of temperatures and humidity conditions. Many two-component waterborne polyurethane (2K WB PU) systems suffer from slow drying times and reduced physical properties at higher humidity levels, so the use of C333 provides a wider range of applications.
REAXIS C333 is unique in that it is soluble in both aqueous and organic media, thus providing a very wide formulation range that allows it to be uniformly distributed in liquid coatings, resulting in uniform curing of the entire paint film.

The activity of REAXIS C333 can be illustrated by the fact that two-component waterborne polyurethane (2K WB PU) formulations containing this catalyst retain their original physical properties and drying time after aging. Likewise, the service life and in-service stability of these formulations is excellent.
The excellent selectivity of REAXIS C333 (compared to DBTDL) promotes the reaction of isocyanates with hydroxyl groups over water, as confirmed by FT-IR tests. This is a very important advantage over typical catalysts used in other two-component waterborne polyurethane (2K WBPU) coating formulations, as it helps prevent blistering and thus optimizes the appearance of the film.
Further testing is needed to better determine and understand the benefits of using C333 in two-component waterborne polyurethane (2K WBPU) systems and related coating technologies. Preliminary studies have provided promising data that require further research