Alternatives to DBTL catalysts in polyurethanes a comparative study. Dieter Guhl, TIB Chemicals AG, Mannheim, Germany

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1 Alternatives to DBTL catalysts in polyurethanes a comparative study Dieter Guhl, TIB Chemicals AG, Mannheim, Germany Abstract DBTL as the most versatile metal catalyst in polyurethanes catalysis is facing difficulties in its use resulting from the toxicological profile and from changes in classification and labelling. Due to the unique features of DBTL the task to replace the catalyst is quite challenging. This study shows the performance of the most promising replacement catalysts based on bismuth carboxylates in different solventborne 2 K polyurethane coatings. The results of the application trials show that a replacement of DBTL is possible, but it is difficult to predict the performance of the Bismuth catalysts relative to the DBTL performance. DBTL the workhorse in polyurethane chemistry Numerous catalysts have been suggested for the polyurethane reaction. For the Urethane reactions [Figure 1] in coatings metal based catalysts are the dominant group of catalysts. Various metals compounds have been explored like salts or carboxylates of Lead, Mercury, Zinc, Manganese, Iron, Zirconium, Bismuth, Aluminium, Nickel and others. N + H [Figure 1] NH The biggest practical importance have gained the organotin catalysts and within this group especially the Dibutyltindilaurate (DBTL) [Figure 2]. It is not wrong to state, that today DBTL is the workhorse of the urethane catalysis. Whenever there is a need for acceleration of the urethane reaction, DBTL is the first choice for testing. There is of course reason why this chemical has gained such a position. DBTL offers several advantages when compared with other catalysts: - high activity with aromatic and aliphatic isocyanates - high catalytical activity at very low concentrations - good solubility in and compatability with most common formulations - reasonable stability against air, light and moisture - high tolerance against influence from other ingredients of the coatings formulation - reasonable low influence on yellowing. Today there are numerous applications in polyurethane coatings where DBTL or other organotin catalysts are used.

2 A few examples are - 2 K systems based on aliphatic or aromatic isocyanates - 1 K systems - Deblocking of blocked isocyanates - Curing of PU powder coatings - Formulation of polyurethane dispersions (PUDs) - Synthesis of PU-prepolymers The main areas of application beside others are car refinishing, other transportation, industrial and wood coatings. [Figure 2]: idealized structure of Dibutyltindilaurate The excellent catalytical performance of DBTL is based on its Lewis Acid properties. The literature shows that complexation of the tin center in the DBTL to the NC-group is one of the key steps in the catalysis of the urethane reaction [Figure 3]. N 2 + Bu Sn Bu 2 N Bu Sn Bu H HN 1 1 N H Bu Bu Sn 2 2 [Figure 3]: catalysis of the PU reaction by DBTL

3 [Figure 3] shows a simplified version of the catalysis mechanism. Several side reactions have been omitted. The goal to develop a replacement product for DBTL requires substances which should show similar properties in order to achieve a similar catalytical effect. The toxicological challenge It is unquestionable that there is a market tendency to replace DBTL as PU catalyst. It is also clear that with very limited exceptions the driving force for the replacement activities is not the technical performance. The driving forces are the ongoing toxicological discussions. Latest since the ban of the tributyltin compounds from use in antifouling paints, the organotin products are in public opinion regarded as critical substances. The following issues are raising concerns about the further use of DBTL: 1. Change in Labelling The European Chemicals Bureau of the European Commision has decided to change the classification of Dibutyltindilaurate and other Dibutyltin based products. The key fact is that DBTL is classified for reproductive toxicity with epr. Cat. 2 and for mutagenic toxicity with Muta. Cat. 3, which has not been the case in the past. As a result the labelling of DBTL changes from Xn = harmful and N = dangerous for the environment to T = toxic and N. This adds the risk phrases 6/61 and 68 to the labelling. [Figure 4] This labelling will come effective with the 3 th or 31 st adaption to the technical progress of EC-Directive 67/548/EEC to be set into force on 1 st of june 29. New Classification and Labelling of DBTL Current (TIB Chemicals AG) Classification Xn, 22-48/22 Xi, 36/38 N, 5-53 NEW (ECB) epr. Cat. 2, 6-61 Muta. Cat. 3, 68 T, 48/25 Xn, 22 Xi, 38 N, 5-53 Labelling Xn,N : 22-36/38-48/22-5/53 S: 23e-26-36/ T,N : / /53 S: Transport (dangerous goods) Class 6.1 (T3) P III UN 2788 GANTIN CMPUND LIQUID, NS (DIBUTYLTIN DILAUATE) [Figure 4]: comparison of old and new labeling of DBTL 2. potential limitation of the use of organotin chemicals There is a public discussion in the different EU member states about a limitation on the use of organotin products in several applications. ecently a report prepared by the risk assessment consultant firm isk & Policy Analysts (PA) for the European Commision

4 Directorate Enterprise and Industry suggested the limitation of the use of Dibutyltin compounds in certain applications. This report is discussed critically at the moment. Under these circumstances it seems for all users of organotin products advisable to be prepared for the time, when these products cannot be applied anymore as they can be today. The alternatives to DBTL As mentioned numerous metal compounds have been evaluated in the past as catalysts for the urethane forming reaction. Basically the search for DBTL replacement catalysts has not lead to really new substances. Most of the proposed alternatives have already been known for a number of years. Some of them were used in niche applications, where the performance of DBTL or other organotin catalysts was not sufficient to meet the specific technical requirements. Now a number of those compounds are reevaluated and sometimes reformulated for their potential as DBTL replacement products. The following catalysts show some potential as DBTL replacement products: - Dioctyltindilaurate - Bismuth Carboxylates - Zinc Carboxylates - Chelates of Zirkonium and Aluminium. Additionally there are mixed-metal catalysts being evaluated trying to combine individual positive features of different metal compounds. Dioctyltindilaurate (DTL) DTL is chemically quite close to DBTL. The only difference is the chain length of the alkyl groups attached to the tin center. It can be expected that due to the very similar chemistry the catalytical performance is quite close to DBTL. n the other hand the chemical similarity suggests that the toxicological profile is also quite close to DBTL. Currently DTL is not regarded as a dangerous material, but this may change in the future. Bismuth Carboxylates The key components of interest are Bismuth ctoate (Bi-C) and Bismuth Neodecanoate (Bi-NDE). They are known for quite some time and found application in polyurethane elastomers and sealants. Due to the high viscosity of the % material, most commercial types are diluted with solvents or carboxylic acids. Additionally combination products with other metal soaps are on the market to modify certain properties. As one of these examples a combination of Bismuth and Zinc Carboxylate (Bi-Zn- Car.) has been evaluated in comparison with the pure Bismuth Soaps. From the toxicological point of view Bismuth products are generally recognized as low toxic materials. The use of Bismuth compounds as pharmaceuticals or as ingredients for cosmetics is well known.

5 The performance of the alternatives For this comparative study we focussed on the performance of Bismuth Catalysts relative to DBTL. ccasionally other potential replacement catalysts were tested. The goal was to collect information about the behaviour of the catalysts in different coatings formulation to identify similarities and deviations in catalytical performance. As starting point a simple solventless, non-pigmented model formulation based on an oleochemical polyol and aliphatic isocyanates [Figure 5]. Formulations of this type can be used as a starting point for thick film floor coatings or casting systems. Part A: Sovermol 75,HZ = 32 gr (oleochemical polyol) Catalyst, %.12 % Part B: Vestanat HB 264 LV, NC = 23 % 11 gr (HDI- Biuret) Technical Data: Solids Content: % NC/H ratio: 1,4 : 1 Conditions: Curing Temperature: C Curing Temperature SubCASE: 3 C Film Thickness: 15 µm [Figure 5: Model Formulation ] Initial Tests were made using a SUBCASE [Figure 6] device, which measures pot life and curing behaviour via dielectric polarization. Using this device allows to screen large numbers of catalysts quickly under reproducible conditions. Due to the need of relative thick layers a transfer to real world coating formulations is sometimes difficult, but the results give reasonable predictions. [Figure 6]: SUBCASE device for testing curing behavior The results are printed in a table as Polarization vs. Time [Figure 7]. The curves clearly show that Bismuth Neodecanoate has a different curing profile than DBTL, although twice the Bismuth concentration has been applied. The potlife is shorter, but the curing takes longer

6 than DBTL. The comparison of DBTL and DTL confirms that at the same catalyst level DTL has the same curing profile as DBTL only slower, which is expected due to similarity of their chemistry. The lower metal content of DTL reduces the catalytical activity. Twice the metal concentration of the Bi-catalyst seems not sufficient to overcome the lower Lewis- Acidity of Bismuth compared to Tin DTL (,25%) DBTL (,25%) DBTA (,25%) Bi-NDE (,5%) HG-Kat (,25%) Dielectric Polarization Time [sec] [Figure 7]: curing profiles measured with SubCASE These initial findings were confirmed by traditional measurements, but it was detected that the results strongly depend on the moisture content in the system. The polyol contains up to.3 % moisture. Trials with reduced water content resulted in different Bismuth Catalyst performance. The lower the moisture content the quicker the Bismuth catalyst in comparison to DBTL [Figure 8]. DBTL performance was not affected by the moisture content. pot life [min.] Pot life vs. moisture content DBTL Bi-NDE Bi-C Bi/Zn-Carb. 5.3 <.1 <.1 moisture content [%] [Figure 8]: influence of moisture content on pot life, catalyst concentration:.12 %

7 A parallel development is found with the hardness development. The lower the moisture content, the quicker the shore hardness development [Figure 9] after one day curing time. Measuring the shore A hardness after 7 days shows the similar values for all catalysts. No matter which initial moisture content the formulations had, after 7 days the shore A hardness around 85 was found for all catalysts. Shore A hardness after 1 d vs. moisture content 9 8 Shore A hardness DBTL Bi-NDE Bi-C Bi/Zn-Carb. <.1 <.1.3 moisture content [%] [Figure 9]: initial shore A hardness vs. moisture content, catalyst concentration:.12 % The presence of moisture in a polyurethane formulation raises automatically the question, whether it is possible to decrease the carbon dioxide formation by a more selective catalyst. Carbon Dioxide is formed as by product from the reaction of water with isocyanate [Figure 1]. + H NH N + H2 NH H - C2 NH 2 + NH 2 NH NH [Figure 1]: reactions of isocyanate with hydroxyl groups and with water

8 The Carbon Dioxide is responsible for bubble formation in castings and coatings. From the pictures it is visible that Bismuth catalysts are not catalyzing the NC/H 2 reaction as strong as DBTL does [Figure ]. [Figure 11]: IPDI-Trimer + 5 % water + Bi-NDE (left bottle) / DBTL (right bottle) [Figure 12]: Casting polyol / IPDI-Trimer with.5 % Bi- Neodecanoat (left) and,5 % Bi/Zn- Carboxylate (right) After these basic evaluations in a simple model formulation, it should be interesting to see whether the first findings could be confirmed in more sophisticated formulations, which are representative for certain polyurethane coating applications. Solubility and Stability of Bismuth Catalysts in different solvents Due to the small amounts of catalysts which are needed for efficient acceleration of the reaction it is well known practice to dilute the catalyst in the solvent in order to increase the exactness of the catalyst dosation.

9 DBTL is soluble in all common PU solvents at nearly any ratio. Usually 1% or 1% stock solutions are prepared and added to the A-component. These solutions are stable for a couple of month. Bismuth based catalysts have to be handled with greater care. 1% solutions are generally not advisable for Bismuth Catalysts, because they tend to form precipitates after a certain storage time or at elevated temperatures. For most solvents with exception of Acetone 1% solution of the catalyst are more advisable [Figure 13]. Solvent 1 % content Bi-C 1 % content Bi-C Toluene 12,2 NTU 1,15 NTU Xylene 116 NTU 1,7 NTU White Spirit,8 NTU 1,3 NTU Butylacetate 13 NTU 3,1 NTU MPA 4112 NTU 2,7 NTU Methylpentanone 17 NTU 2,75 NTU Acetone 192 NTU 2,2 NTU [Figure 13]: Turbidity measurement of diluted Bi-C after 96 h at 27 C The reason for the precipitation is not completely clear at the moment. Moisture as the most potential reason can be excluded. For the tests the moisture content in all solvents was below ppm. Study 1: Performance in Wood Coatings ne of the most common polyurethane systems for wood coatings is the combination of hydroxyfunctional alkyd resins with aromatic isocyanates. In order to reduce the effect of pigments and other additives on the catalytical performance the system was simplified to a clear coat [Figure 14]. Part A: okrapol 315, HZ = 15 5 gr (Alkyd polyol) Xylene 2 gr Toluene 5 gr Butylacetate 2 gr Methoxypropylacetate 5 gr Catalyst.5 gr =,34 % Part B: Desmodur L75, NC = 13.3 % 44.4 gr Technical Data: Solids Content: 4 % NC/H ratio: 1 : 1 Flow Time [sec.] 14 (23 C) Conditions: Curing Temperature: 21 C Humidity: 42 % Film Thickness: µm [Figure 14]: Wood Coating Formulation

10 Following results were achieved: Compared with an uncatalyzed version, which has potlife longer than 8 hours, the DBTL shortens it significantly to 27 min. Bismuthoctoate and a mixed metal Bi/Zn octoate catalyst shorten the potlife even more [Figure 15]. Potlife 5 4 [min.] 3 2 without Cat DBTL Bi-C Zn/Bi-Car. Potlife [min.] [Figure 15]: pot life The curing time of µm coatings on Q-Panels is measured according the DIN The time S1 confirms the tendency of the pot life findings. Again the Bismuth based catalysts are quicker than DBTL at the same catalyst level. For the curing through [Figure 16] the differences nearly disappear between the different catalysts. Trials to elongate the pot life by lowering catalyst concentration increase the through cure time. After 7 days storage at ambient temperature film stability is the same for all catalysts. 12 Curing Times 8 [min.] without Cat DBTL Bi-C Zn/Bi-Car. S S [Figure 16]: curing times of wood coating formulation The paint film stability was good for all tested catalysts. 15 MEK double rubs did not damage any of the films. Also 85 gloss measurements were for all 4 plates between units [Figure 17].

11 Gloss without Cat DBTL Bi-C Zn/Bi-Car. [Figure 17]: Gloss measurement at 85 Tests with other aromatic isocyanates and different types of alkydpolyoles basically confirm the findings with this system. Bismuth based catalysts show shorter potlife and a similar curing profile as here demonstrated. Study 2: Industrial Coating Industrial Coatings is one of the important application fields of polyurethanes. For testing a 2 K solvent-borne system based on a polyacrylate polyol and an aliphatic isocyanate (HDI Trimer) was choosen [Figure 18]. Part A: Desmophen A 365, HZ = 15 38,3 gr (Acrylate polyol) Ti 2, util 29,83 gr Bentone 38 1% 1,96 gr Baysilone L 31, 1 %,18 gr Butylacetate 6,47 gr Methoxypropylacetate 6,47 gr Catalyst, 1 % in Butylacetate,36 gr =,4 % Part B: Desmodur N339, NC = 19.6 % 13,56 gr (HDI-Trimer) MPA/BuAc 1 :1 3,14 gr Technical Data: Solids Content: 66 % NC/H ratio: 1 : 1 Flow Time [sec.] 2 (23 C) Conditions: Curing Temperature: 21 C Humidity: % Film Thickness: 12 µm Source: Bayer Material Science, 621 [Figure 18]: Industrial Coating Formulation Comparing DBTL with different Bismuth catalysts showed the following results [Figure 19]. The Bismuth catalyst concentration was twice the amount of DBTL, because preliminary tests showed that otherwise similar results were not possible.

12 Properties No cat.,4 % DBTL,8 % Bi-NDE,8 % Bi/Zn-Carb.,8 % Bi-C Pot life [min.] > Drying Time S1 [min.] Drying Time S5 [min.] Pendulum Hardness 1d [sec.] Pendulum Hardness 7d [sec.] > [Figure 19]: performance data DBTL vs. Bismuth catalysts The results indicate that in this application DBTL is very well replaceable by different Bismuth Catalysts. The differences between the individual compounds are quite small. Study 3: Corrosion Protection Coating This formulation is quite close to the previous one [Figure 2]. The hardener is the same, polyol again a polyacrylate polyol, however with lower H number, pigments are different but with a similar binder / pigment ratio. Part A: Desmophen A 16, HZ = 53 43,15 gr (Acrylat polyol, 6 % FK) Ti 2, util 13,5 gr Ironoxide 1,61 gr Chromoxide, green 8,72 gr Talkum 7,19 gr Bentone 38 1% 7,19 gr Additive Byk 141,36 gr Stabilizer, Tinuvin 292,36 gr Solvesso 4,93 gr Catalyst, 1 % in Butylacetate,36 gr =,4 % Part B: Vestanat HT 25 L, NC = 19.6 % 13,8 gr (HDI-Trimer) Technical Data: Solids Content: 68 % NC/H ratio: 1,4 : 1 Conditions: Curing Temperature: 23 C Humidity: 54 % Film Thickness: µm Source: Bayer Material Science, 5242, modified version [Figure 2]: Corrosion Protection formulation

13 Despite the similarities in the formulation the catalyst performance is quite different. The Bismuth Catalysts do not reach the speed of the DBTL even not at double concentration [Figure 21]. Pot Life 5 4 [min.] 3 2 no cat,4 % DBTL,4 % Bi-,4 % Bi-C,4 % Bi/Zn-,8 % Bi-,8 % Bi-C Pot Life [Figure 21]: Pot Life of catalysts at different concentrations The measurement of the drying profiles shows that in this formulations the Bismuth catalysts are slower than DBTL and it does not make a big difference, when the catalyst concentration is doubled compared to DBTL [Figures ]. Drying Time Development [min.] no cat,4 % DBTL,4 % Bi-NDE,4 % Bi-C,4 % Bi/Zn-Car S1 S2 S3 S4 S5 [Figure 22]: Drying time development at.4 % catalyst level

14 [min.] Curing Time Development with increased Bi-Catalyst concentration no cat,4 % DBTL,8 % Bi-NDE,8 % Bi-C S1 S2 S3 S4 S5 [Figure 23]: Drying time development at increased Bi-concentration The pendulum hardness development on the other hand is quite similar to the previous study [Figure 24]. The initial hardness is for the same Bismuth concentration around the DBTL value, for the higher concentration lower values were detected. The hardness values after 7 days of storage are quite similar for all catalysts. Pendulum Hardness Development d 3 d 7 d 12 [sec.] no cat,4 % DBTL,4 % Bi-NDE,4 % Bi-C,4 % Bi/Zn- Car,8 % Bi-NDE,8 % Bi-C [Figure 24]: pendulum hardness development

15 Study 4: Clear Coat for Car efinishing or Transportation Coating In the following formulation [Figure 25] the basic system is again the combination of a acrylic polyol with an HDI based hardener. This time the Acrylic Polyol comes with 4,1 % of H groups, the highest amount tested so far. The system contains no pigments. The curing was tested under room temperature conditions and under forced drying conditions to see how the different catalysts respond to this change. Part A: Synocure 852 BA 8, HZ = ,36 gr (Acrylic Polyol) Tinuvin 292,27 gr Tinuvin 9 (8% in xylene) 3,4 gr MPA 3 gr Butylacetate 13,82 gr Xylene 13,82 gr Catalyst,25,1 % Part B: Vestanat HT 25 L, NC = 19,6 % 19,15 gr (HDI-Isocyanurate) Technical Data: Solids Content: 54 % NC/H: 1:1 Flow Time [sec.] 2 (23 C) Conditions: Curing Temperature: 21 C Humidity: 42 % Film Thickness: 12 µm Source: Evonik Degussa [Figure 25]: Car efinishing Coating Formulation Measuring the Pot Life brings no surprises this time. Twice the Bi-NDE amount matches the data for DBTL. It was expected and found that DTL gives longer pot life than DBTL [Figure 26]. Pot Life [min.] ,25 % DBTL,25 % DTL,5 % DBTL,5 % Bi- NDE,5 % Bi/Zn-Car.,1 % Bi- NDE,1 % Bi/Zn-Car. Pot Life [Figure 26]: pot life for car refinishing paint

16 This impression is confirmed by the pendulum hardness development. An important factor for refinishing paints is the initial hardness of the system, because this determines the speed of handling of the workpieces. In order to meet the DBTL performance the Bismuth Catalyst concentration has to be raised significantly to 4 times the tin catalyst demand [Figure 27]. [sec.] Pendulum Hardness, drying at 23 C,25 % DBTL,25 % DTL,5 % DBTL,5 % Bi-NDE,5 % Bi/Zn- Car.,1 % Bi-NDE,1 % Bi/Zn- Car. 1 d d [Figure 27]: Hardness Development at room temperature drying Under conditions of forced drying at 8 C Bi-NDE shows at the same concentration as DBTL higher initial hardness. Unfortunately the catalyst concentration was chosen higher than for the test at room temperature, which does not allow a direct comparison [Figure 28]. Pendulum Hardness after 3 min. 8 C 18 [sec.] without Cat,17 % DBTL,17 % Bi-NDE,17 % Bi/Zn-Car. 1 d d [Figure 28]: hardness development under forced drying conditions The gloss measurements at 6 did not show any differences between the catalyzed and the non catalyzed films. All values are above 9%.

17 Summary In this study 5 different PU formulations have been studied. Three of them are based on an acrylate polyole /HDI-Trimer combination, whereas the other ingredients varied: different pigmented or transparent systems. From the performance data the following can be concluded: DTL needs higher catalyst loading than DBTL but can mimic the DBTL performance. Bismuth catalysts need in most cases higher catalyst loadings than DBTL in order to have a similar pot life. Twice the DBTL concentration seems to be a reasonable starting point. Bismuth catalysts can give shorter initial drying times than DBTL, the through cure is longer or equal to DBTL. Hardness development is difficult to predict. Bismuth Neodecanoate seems to be more active than the ctoate or a combination with Zinc Carboxylate. More tests with other isocyanates are necessary to explore the potential of Bismuth catalysts further.