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New tin-free catalysts as alternatives to DBTL and amine-based compounds in modern solventborne Polyurethane clearcoat systems

Dietmar Oberste and Dr. Andreas Steinert Borchers GmbH, Langenfeld (Germany)

Modern high-end automotive and industrial coatings systems are facing increasingly challenging requirements from the end users' side. Not only should they create value for the goods produced, they also are expected to enhance the durability of the finished goods, allow for shorter production cycles, and cause no or minimal impact on the environment. The majority of these requirements have been met by existing high performance 1component and 2-component polyurethane coatings systems, especially in the automotive segment. However, the increasing desire for lower energy consumption and less environmental impact poses a major challenge, even for highly sophisticated coatings systems. The objective of reducing VOC contentand the continuing development of high solids systems, call for low viscous resin binders. These types of binders are made by using low molecular weight polyol and isocyanate components. As a result, drying times for these systems often are longer. However, this problem can be avoided by choosing the proper catalysts. In the past, in addition to tertiary amines, tin-based components were used as cross-linking accelerators. Dibutyltin-dilaurate (DBTL) was the most commonly used accelerator for 1component and 2-component polyurethane coatings systems. (Figure 1)

Comsumption of organotin compounds (March 2000)

Stability agents / catalysts Preserving agents Pesticides

Intermediates Antifouling paints Wood preservatives

Figure 1-source:

,,Production and use of organotin compounds in Germany", report from Dep. of the Environment, Berlin, June 2000

This chart illustrates that the majority of organo-tin compounds are used as stabilizers and catalysts for the production of plastics and coatings. An effort is underway to replace catalysts such as DBTL in many industrial areas with less hazardous alternatives. This is due to the impurities of trioragonic compounds produced during the production process of DBTL. These compounds have shown a high toxic virility with consequences not yet fully determined. For manufacturers of additives and coatings, this created a need to develop alternative catalysts for 1-component and 2-component polyurethane systems that offer a comparable film property profile to DBTL. It has long been known that alternate metal compounds, mainly metal carboxylates, can be used as catalysts for polyurethane reactions. However, compared to DBTL these compounds provide different properties to the coatings regarding dry time, film hardness, coloring, and shelf life durability.


New Catalysts for modern high-performance PUR automotive coatings

This article offers opportunities to the reader on how to achieve a similar property profile in a large area of solvent-borne Polyurethane clearcoats. By using a newly developed product line of tin-free metal carboxylate catalysts, a comparable property profile to DBTL can be achieved. It introduces the entire product line, marketed under the trade name Borchi® Kat, and its individual products as alternatives to DBTL. The study also outlines their typical application properties compared to systems that are catalyzed with DBTL. Borchi® Kat 22 and Borchi® Kat 24 are single metal catalysts based on zinc and bismuth carboxylates. They define themselves through specific properties, different from those that DBTL offers, that allow formulators to achieve very specific characteristics in their coatings. Due to a special production process and a specific chemical modification, it was possible to optimize Borchi Kat® 24 with regard to its activity as a polyurethane catalyst. At the same time, the hazardous virility that the product poses to users was significantly lowered compared to other bismuth carboxylates. Borchi® Kat VP 0243, Borchi® Kat VP 0244 and Borchi® Kat VP 0245 are metal mixed catalysts. Various base metals were combined in a way that they offer new catalytic properties. In addition, in many systems they show a similar reactivity compared to DBTL and a similar property profile in a wide variety of applications. Borchi® Kat VP 0243 and Borchi® Kat VP 0244 were developed for usage in clearcoats, whereas Borchi® Kat VP 0245 was specially developed for use in pigmented coatings systems.

Comparison of catalyst systems

Two of the most important areas of applications for polyurethane catalysts in the coatings segments are automotive OEM and automotive refinish coatings. Products that are used in these areas have to work at the lowest possible energy consumption while offering the shortest possible throughput times. At the same time they are expected to offer optimal coating and film properties. This goal can be achieved by using modern low viscous resins and coatings systems and forced drying of the applied coating. This unfortunately requires relatively high temperatures that can easily reach 60° C to 130° C. Here the industry is looking for ways to reduce these temperatures to <100° C. The following work with a 2-component Polyurethane clearcoat, based on a modern commercially available hydroxyacrylate, shows test results that take these industry requirements into consideration. The forced drying was conducted at 80° C, a temperature that the industry is targeting. In several additional coatings systems that were tested during the developmental work and that for clarity reasons are not included in this paper, we saw comparable results.


To evaluate the application properties and the activity of the catalysts, a model was used to compare a control containing DBTL to the Borchi Kats and a non-catalyzed system. The formulations were tested for pot life (measured as a doubling of the initial flow time in a DIN-cup 4mm), the surface drying time and the through drying time (measured by means of sand drying per DIN 53150 and recorder drying according to ASTM 5895). The films were tested for hardness after forced drying (at 30 Min. / 80C) and room temperature according to pendulum hardness tester Koenig (DIN 53157). For both measurements the coatings were applied to glass with a defined film thickness of 100 µm. A comparison of the drying results at 80° C to room temperature allows for evaluation of the impact of temperature on the activity of the different catalysts. To determine the storage stability of the catalyzed coatings the catalysts were added to the master batch component and stored for 14 days at 50° C. The hardener was added and the coatings were tested according to original testing conditions to determine possible storage related changes in the catalysts' activity. The clearcoat was tested with the catalysts Borchi® Kat 22, Borchi® Kat 24, Borchi® Kat VP 0243 and Borchi® Kat VP 0244 and compared to DBTL and the non-catalyzed coating. To assure accurate addition all catalysts were diluted to 10% in butylacetate. The formulation used (Table 1) was similar to a starting formulation of Bayer Polymers AG that is used in practice.


Table 1: 2K-PUR clearcoat formulation (Bayer Polymers AG, slightly modified) Master batch: Desmophen A 870, 70% BAC Baysilone OL 17, 10% in Xylene Baysilone 3468, 10% in MPA Tinuvin 292, 10% in BAC Tinuvin 1130, 10% in BAC catalyst, 10% in BAC MPA / Solvesso 100 (1:1) Butylglycol acetate (BGA) Hardener Desmodur N 3390, 90% in BAC Total Technical data: Solid content Viscosity Drying conditions Forced drying ca. 55% ca. 25 s 10` / RT 30` / 80°C 19.49 100.00 % by weight: 51.15 0.53 0.53 5.33 10.67 0.15 10.02 2.13

The formulation for all testing systems was madefollowing the same procedure. One day before the testing was started, the catalyst solutions were added to the master batch component and thoroughly mixed. The master batch component was then split in half. One sample was put in the drying cabinet for storage stability testing and the other sample was immediately tested.

Testing results

The pot life measurements (DIN-cup 4 mm) showed processability between 1 and 3 hours depending on the catalyst used. After storage for 14 days at 50° C, no significant change in the processability of any of the tested coatings was observed. In the systems including Borchi® Kat 24 and Borchi® Kat VP 0244, both showed stronger reactivity compared to DBTL. Both resulted in shorter processability, whereas Borchi® Kat 22 allowed for a processability of the coating that was twice as long. Table 2: Pot life times of the clearcoats before and after storage (14 days at 50° C) catalyst before storage after storage none > 4,0 h > 4,0 h Borchi® Kat VP 0243 ca. 1,75 h ca. 1,75 h Borchi® Kat VP 0244 ca. 1,25 h ca. 1,25 h Borchi® Kat 22 3,0 h 3,0 h Borchi® Kat 24 1,0 h 1,0 h DBTL 1,5 h ca. 1,25 h


Figure 1: Development of the viscosity of the coatings before and after storage

Potlife of 2 comp. PU clearcoats based on different catalysts before storage

viscosity [s]

Potlife of 2comp. PU clear coats based on different catalysts after storage

viscosity [s]

150 100 50 0 0 none Borchi Kat 22 0,5 1 1,5 2 2,5 3 3,5 DBTL 4 VP 0243 Borchi Kat 24 VP 0244

200 150 100 50 0 time [h] 0 none Borchi Kat 22 0,5 1 1,5 2 2,5 3 3,5 4 VP 0243 Borchi Kat 24 time [h]

VP 0244 DBTL

The observation of the increase in viscosity of the controls provides further important information about the activity of the catalysts. The following graphs (Figure 2) compare the viscosity trends of the coatings catalyzed with DBTL, Borchi® Kat 24, Borchi® Kat VP 0243 and Borchi® Kat VP 0244. Figure 2: Viscosity behavior of the clearcoats using different catalysts

Potlife of the DBTL catalyzed 2comp. PU clearcoat

viscosity [s] 240 200 160 120 80 40 0 0 before storage 0,5 1 1,5 2 2,5 3 time [h]

Potlife of the VP 0243 catalyzed 2comp. PU clearcoat

viscosity [s] 160 120 80 40 0 0 before storage 0,5 1 1,5 2 2,5 3 3,5 time [h]

after storage

after storage

viscosity [s]

Potlife of the Borchi Kat 24 catalyzed 2comp. PU clearcoat

viscosity [s] 100 80 60 40 20 0

Potlife of the VP 0244 catalyzed 2comp. PU clearcoat

160 120 80 40 0 0 0,5 1 1,5 after storage 2 2,5 before storage 3 time [h]

0 0,5 1 before storage

1,5 2 2,5 after storage


3,5 time [h]


The graph for the coating that is catalyzed with DBTL (Figure 2) initially shows a slow increase of viscosity up to 1.5 hours. After that, a clearly stronger build up of the curve can be observed. After storage the curve shows further significant increase in the viscosity build up, which in practice leads to a reduction in processability. The sample catalyzed with Borchi® Kat 24 shows a slow increase of viscosity up to approximately 1 hour. After reaching twice the Auslaufviskositaet a rapidly occurring crosslinking process can be observed. Yet at the same time, the behavior of Borchi® Kat VP 0243 is characterized by increased catalytic activity. A moderate increase of viscosity is observed over the pot life testing of Borchi® Kat VP 0244. It also shows an even reactivity profile, yet higher activity resulting in a steeper viscosity curve. Both catalysts show little influence on their activity during storage. Another criteria of evaluation is the development of the film hardness after forced drying (30 min. at 80° C). It is an especially important parameter to determine fast processing and the final hardness of the coating, determined after one week. In conjunction with the effort to reduce the baking temperature from 130° C currently to <100°C, the film hardness of the respective coatings system must be built up fast. In addition, the coating systems are expected to offer a film hardness similar to un-catalyzed systems. This currently is not possible under every circumstance. With respect to hardness, our investigation showed that coatings samples catalyzed with DBTL built up a significantly higher initial hardness after forced drying, but a lower final hardness than the un-catalyzed coating. Several, but not all, of the catalysts tested showed this phenomenon (Figure 3). The tests conducted with Borchi® Kat VP 0244 reached a very high initial hardness and, at the same time, reached the final hardness of the uncatalyzed coatings. In a slightly diminished form, the same observations were made for the system that was catalyzed with Borchi® Kat VP 0243. Figure 3: Development of film hardness of the clear coats after forced drying

Film hardness before storage: test results of the Desmophen A 870 clear coat system after forced drying (30'/80°C)

240 film hardness [s] 200 160 120 80 40 0 none 147 182

227 192 170

216 184 152

226 193 169

216 185 165

219 189 170


DBTL VP 0243

Borchi Kat Borchi Kat 22 24 Borchi Kat 24

VP 0243 DBTL

VP 0244 none

VP 0244

Borchi Kat 22

30 min.


7d time

The most reactive catalyst, Borchi® Kat 24, shows nearly identical behavior as DBTL with respect to development of film hardness. A very high initial film hardness can be seen in the first 24 hours, but the final film hardness after one week is lower. Finally, the coating that was catalyzed with Borchi® Kat 22 shows film hardness numbers that are analog to the un-catalyzed system. The hardness testing results after storage are essentially the same as the samples tested immediately (Figure 4). Surprisingly, after storage, our tests show that the coating catalyzed with DBTL reaches a higher final film -5-

hardness than the uncatalyzed system. The coatings formulated with Borchi® Kat VP 0244 and Borchi® Kat VP 0243 show consistently high film hardness and prove their excellent storage stability. This also applies to Borchi® Kat 22 and Borchi® Kat 24 which also has a very consistent property profile after storage. Figure 4: Development of film hardness of systems after storage for 14 days at 50° C (forced drying)

Film hardness after storage: test results of the Desmophen A 870-clearcoat system after forced drying (30'/80°C)


film hardness [s]

200 160 120 80 40 0


219 190 142 165


226 196 148

228 199 162

225 188 158

220 197 167



Borchi Kat Borchi Kat 22 24

VP 0243

VP 0244 none

VP 0244

VP 0243

Borchi Kat 24

Borchi Kat 22


7d 30 1d min time .

The results for drying at room temperature showed a slightly different picture (Figure 5). Here, the films that were catalyzed with Borchi® Kat 22, Borchi® Kat 24 and DBTL achieved higher initial film hardness than the uncatalyzed coatings, whereas comparable final film hardness was observed. The other systems showed lower initial hardness values. Borchi® Kat 22 proved to be the best catalyst to reach the highest possible film hardness at room temperature. Figure 5: Development of film hardness of the clearcoats at room temperature

film hardness: test results of the catalyzed Desmophen A 870 clearcoat system at room temperature

250 film hardness [s] 200 150 100 50 0 none DBTL Borchi Kat 22 Borchi Kat 24 53 69 67 61 46 215 199 211 198 211 199


VP 0243 DBTL

VP 0244 none


7d time

VP 0244

VP 0243

Borchi Kat 24

Borchi Kat 22


After 14 days of storage at 50°C we did not observe any significant change in the profiles of film hardness of the coatings that were cured at room temperature. Borchi® Kat 22 in particular showed very consistent values. With the other catalysts slightly lower film hardness was found. Another important application property of the coatings systems is their behavior in surface and through drying. These were measured by means of dust-free drying/degree of drying 1 according to DIN 53150 and by using a drying recorder. While this property is less important to users who work with forced drying conditions, it is most important for users in the area of automotive refinish coatings. In this area it is not possible to achieve forced drying by using high temperatures, while still requiring fast blocking resistance, load capacity and processability of the coated parts. In our investigation, all catalyzed systems showed clearly shorter drying times compared to the uncatalyzed clearcoat, before and after storage of the coating. With DBTL and Borchi® Kat 24 the drying time of the surface could be cut in half with both systems showing very similar properties. Compared to the fast drying systems, the other test systems showed a slightly slower drying, yet no or minimal changes after storage (Figure 6). Figure 6: Surface drying deg. 1 (dust-free drying) of the clearcoats before and after storage, DIN 53150

Dust-free drying time (degree 1) of the catalyzed clearcoats according to DIN 53150

time [min.]

70 60 50 40 30 20 10 0 none VP 0243 VP 0244 Borchi Kat Borchi Kat 22 24 DBTL


before storage

after storage

The through drying of the coatings systems was examined by using a drying-recorder (ASTM 5895). Here also a significant reduction of the needed drying time of the uncatalyzed systems could be observed. All systems showed comparable drying times of about 4 hours, which is approximately half the drying time of the control system (Figure 7). For the through drying of the samples that were stored at 50°C, values consistent to the unstored materials were measured. Only the system catalyzed with Borchi® Kat 22 showed slightly longer drying times in contrast to the other systems, again confirming its moderate catalytic activity compared to the other products.


Figure 7: through drying of the clearcoat systems by means of recorder drying before and after storage

through drying time of the catalyzed clearcoats following ASTM 5895

time [h] 10 8 6 4 2 0 none VP 0243 VP 0244 Borchi Kat Borchi Kat 22 24 DBTL catalysts

before storage

after storage


With the ongoing goal of saving time and expenses, modern 1-component and 2-component Polyurethane coatings for industrial and automotive applications are expected to meet increasingly high standards. The critical aspects among these standards include: 1) lowering the baking temperature at forced drying of coatings system to < 100° C which requires the usage of high performance catalysts and 2) replacing tin-based products as they raise toxicological concerns. Whereas this study examined alternative metal caroboxylates based on single metal carboxylates and newly developed metal combinations. They had to be suitable for usage as polyurethane catalysts for coatings applications and show comparable properties to DBTL. The most important application parameters in the study were processing time, development of viscosity, and film hardness. Surface and through drying were determined based on an automotive OEM and an automotive refinish sample coating. The coatings were cured at room temperature and at slightly increased temperature. Moderate baking conditions were chosen (30 min. at 80° C) to address the ongoing need for lowering the temperature in the curing process. Our tests proved that it is possible to reproduce the wet property profile of the formulation containing DBTL as well as reproduce the properties of the final coating by choosing the correct metal carboxylate catalyst. In addition, it is possible to achieve very specific qualities in the coating by choosing the proper product. With Borchi® Kat 24 the user has a product that, due to its strong catalytic activity, creates an advantage when fast processing of parts is required. At the same time, the result is a film hardness very similar to what DBTL creates and the drying characteristics will be almost identical. Conversely, Borchi® Kat 22 is a moderately reactive catalyst that allows for longer processing times. Its reactivity remains consistent after storage stability creating high final film hardness at room temperature and under baking conditions. It is especially suitable for automotive refinish coatings. The two combination products Borchi® Kat VP 0243 and VP 0244 show only slightly different pot-life times compared to DBTL and offer almost identical properties and processing conditions. Both products demonstrated consistent reaction behavior and excellent storage stability. In general, Borchi® Kat VP 0244 proved to be the product of higher reactivity, reflected in a shorter pot life time and a steeper buildup in viscosity. At the same time it showed similar film hardness development compared to DBTL in both curing methods. Borchi® Kat VP 0243 showed slightly lower film hardness at forced drying, however it reached the highest overall film hardness value under room temperature conditions. Therefore, the catalysts examined offer users a wide range of possibilities as they adjust their systems based on individual requirements.




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