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2K waterborne clearcoat—a competition between crosslinking and side reactions
Progress in Organic Coatings 45 (2002) 205–209
2K waterborne clearcoat—a competition between crosslinking and side reactions W. Collong a , A. Göbel a , B. Kleuser a , W. Lenhard a,∗ , M. Sonntag b a
DuPont Performance Coatings, Wuppertal, Germany b Bayer AG Leverkusen, Germany
Received 1 September 2001; accepted 6 March 2002
Abstract The combination of OH-functional polymers with di- or trifunctional polyisocyanates is widely used as crosslinking chemistry in high-performance two component clearcoats. This curing chemistry had always been limited by strong side reactions. Thus use of solvents in formulation was limited to non-OH-/NH-functional components. Some years ago Bayer came up with the strange idea to formulate 2K isocyanate waterborne paints. They carried out a lot of investigations to establish the proportion of crosslinking and side reactions. Nevertheless the formulation of two-component waterborne PUR paint remains a highly sophisticated task. This presentation shows the basic evaluation carried out and communicated by Bayer [Aqueous two-component polyurethane systems for coatings, in: Proceedings of the IUPAC Symposium, Warsaw, July 2000; Aqueous 2-Pack Polyurethane Coatings for Automotive Refinish and Transportation, Bayer Publication; Prog. Org. Coat. 40 (2000) 99] and additional experiences we have gathered in DuPont Performance Coatings [DE 19 607 672, EP 654 051, EP 654 053, EP 654 055, EP 358 979, EP 542 105, EP 578 940, EP 669 352, EP 751 179, EP 469 389, EP 456 210, etc.]. The focus of DuPont experiments is on a high-performance waterborne clearcoat system for car refinish. Especially, the demands of refinish customers have led us to focus on miscibility, sprayability and pot life issues. Kinetic as well as thermodynamic effects are discussed. Finally, a formulation is described which fulfils most of our customer needs. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Waterborne; Clearcoat; Refinish; Analytical data; Urea; Urethane
1. Basic chemical challenges in 2K waterborne PUR chemistry Isocyanates as high reactive chemicals create several chemically different products when combined with OHand NH-functional substances (Fig. 1). Desired products and side products are formed in different amounts. In homogeneous solutions the quantities only refer to the speed of reaction and concentration as shown for the predominant reactions in Table 1. In polymers and especially in waterborne polymer dispersions reactions are also influenced by other effects: because of complex structures, solubility parameters and accessibility of functional groups it is impossible to get thermodynamic figures for each of these effects. Normally, we can only measure the total effect to some extend. Tests were carried out to determine the different reactions. Some additional analytical data led us to a better under∗
Corresponding author. Tel.: +49-202-529-2210; fax: +49-202-529-35-2210. E-mail address: werner [email protected] (W. Lenhard).
standing of the chemical background of the technical findings. IR spectroscopic measurements were made. They will be discussed later on. The method of paint preparation also has a strong influence on the final products of chemical reaction. Stirring waterborne OH-/NH-functional polymers with isocyanates is a highly sensitive process. To evaluate the mixing parameters tests were done by mixing different isocyanates under different conditions with water. The quality of dispersion can be seen by the opacity of the mixture (Fig. 2). Hydrophobic isocyanates need high shear mixing to be dispersed in water, whereas hydrophilic modified isocyanates can be stirred in water by hand. The mixing process with hydrophilic isocyanates is accompanied by an initially strong increase of viscosity. Having mixed the polymer dispersion and the isocyanate successfully two main questions arise: 1. Which reactions predominate during water evaporation after application? 2. Which reactions occur between mixing the two components and application? (potlife).
0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 2 ) 0 0 0 5 2 - 8
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W. Collong et al. / Progress in Organic Coatings 45 (2002) 205–209
Fig. 1. Isocyanate reactions.
Table 1 Relative reactivity of isocyanates depending on “steric” properties Reaction
Relative speed in homogenous solution (kg/mol h)a
Relative speed in dispersion (kg/mol h)b
R–NCO + R OH = urethane R–NCO + HOH = urea
0.08 0.10
0.017 0.018
a b
Fig. 3. NCO and water decrease in a 2K PUR film at 23 ◦ C.
Mixture related to real concentration in waterborne clearcoat. Real waterborne clearcoat.
Analytical data gave us promising results: drying at room temperature (23 ◦ C) shows nearly complete water evaporation after 30 min accompanied by only an approximate 5% decrease of isocyanate functionality. Reaction with water at room temperature is slow in spite of the extremely high OH-number of water (3117) compared to OH-numbers (100–200) in polymers (Fig. 3). Analytical results during potlife show a predominant increase of urea groups (22%) and some urethane content (8%) after 5 h accompanied by a dropping pH value (Fig. 4). All results depend on temperature, the choice of isocyanate components and polymer dispersions. Especially, under condition of only 15 min evaporation at room temperature followed by 25 min at 60 ◦ C urethane predominates over urea. This was proven by solid C13 NMR. In the beginning the reaction with water (forming urea) is higher than after heating. This refers to the decreasing amount of water during evaporation. Considering all analytical results we can summarize: we may get a proper clearcoat film at room temperature using polymer dispersions and isocyanates adjusted to crosslink
Fig. 2. Dispersability of polyisocyanates in water.
Fig. 4. Decrease of NCO content and pH value during potlife.
in the presence of water. This means especially by means of the polyisocyanate finding the right balance between hydrophylic and hydrophobic behavior to get it through the continuous phase of water in close contact to the OH polymer as quickly as possible (Fig. 5).
Fig. 5. Emulsification of polyisocyanates in water.
W. Collong et al. / Progress in Organic Coatings 45 (2002) 205–209
On the other hand, we have to take into account the required water evaporation time (Fig. 3). If we leave the balance between water release and film forming/crosslinking we are going to run into quality problems. We established that blistering, e.g., is strongly influenced by this balance. This seems to be predominant compared to the generation of CO2 being a product of the reaction of isocyanate with water. We take 20–50% excess of isocyanate to get decent crosslinking because of side reactions (forming urea structures).
2. Specially designed waterborne clearcoat for automotive refinish To develop such a complicated system you need a realistic chance to find a customer. This market was recently created by European VOC regulation. Especially, big customers can only reach the tough reduction targets using either ultra-high solid (2.1 pounds per gallon VOC) or waterborne clearcoat. Both systems have their advantages and drawbacks. Waterborne has the above-mentioned risks of side reactions. Ultra-high solid seems to need special development of low molecular weight reactive substances. The main hurdle for this development is the European restriction for the marketing of new chemical substances. Evaluating these conditions at present nobody can make a reasonable forecast about the market for only one developmental direction. With the above-mentioned results, we have a realistic chance of developing a waterborne clearcoat suitable for refinish application. To formulate a refinish clearcoat we have different tools: typical polyols are polyacrylates, urethanized polyesters and polyesters as clearcoat binders. In waterborne they show more differences from each other (especially in drying behavior) than known from solventborne systems. We learned that polyacrylates show the fastest film formation during drying, whereas polyesters dry slowly. Also initial film hardness is different, polyacrylates having higher hardness than polyesters. Hardeners are low molecular weight hydrophobic HDI and hydrophylic HDI and IPDI trimers. Solvent content is restricted by VOC regulation. The target is to remain below 2.1 p/gal (250 g/l). Therefore, we have to choose carefully which solvents support mixing process, application and film forming. Additives for waterborne clearcoats are limited by availability. This is one of the more challenging tasks in the development of waterborne clearcoat. To make a product for this special application we have to meet the needs of our customers. More or less empirical methods describe only the influence on different physical and chemical properties. This means for R&D: transfer of empirical descriptions to chemical and physical properties. 1. Mixing in ratios 3 parts polyol component:1 part isocyanate component or 2:1 by volume means formulate
207
both components in a manner that the relation by moles is 1 OH to 1.3 NCO in the above-mentioned parts by volume. 2. Mixing by hand using a stick means using a hardener composition which has good miscibility with water but high affinity to the polyol to get an excellent mixture of both resins! Every abnormal viscosity behavior has to be avoided during the mixing process. This happens by using only hydrophylic isocyanates which show an initial increase of viscosity in combination with polyols. Targeted behavior was found by formulating a cross-linker containing a good balance of hydrophobic low molecular weight isocyanates, special hydrophilic modified isocyanates and special solvents, whereas the hydrophilic part helps to get the hardener into the dispersion the hydrophobic part has a higher affinity to the polyol and equalizes the viscosity influence of the hydrophylic component. Solvent composition supports both mechanisms. The polyol dispersion needs a mixing viscosity and spray viscosity to enhance the miscibility with hardener. The adjustment to spray viscosity is done after mixing of the polymers. 3. Potlife of at least 1 h is necessary. Potlife in waterborne clearcoat does not mean the same as in solventborne systems. In both potlife is the maximum time you can use the ready to spray mixture resulting in constant properties in application, wet and dry film properties. Potlife depends strongly on spray viscosity in solventborne systems. In waterborne viscosity remains more or less constant far beyond the potlife. Nevertheless side reactions with water become predominant not only on the surface of the isocyanate particles formed by the mixing process but also by penetrating inside (Fig. 6). Urea polymers are created by this reaction as shown before. Crosslinking density is reduced when isocyanatefunctionality of hardener decreases. Chemical resistance is lowered. Lower gloss and haze influence appearance of clearcoat after drying in a negative manner. This behavior of waterborne 2K clearcoat does not impair the spray application by increasing viscosity. Therefore, we need a stable potlife to be sure the resulting film properties are unchanged even with small deviations from recommended application time. This means we have to develop a formulation providing a mixture of OH- and NCO-functional polymers that prevents
Fig. 6. Isocyanate in water during potlife.
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W. Collong et al. / Progress in Organic Coatings 45 (2002) 205–209
Fig. 7. Emulsification of polyisocyanates in OH polymer (mixed clearcoat dispersion.
the water from reacting with NCO-groups of the hardener by sterical hindrance and hydrophobic behavior. This happens if the NCO-hardener is sufficiently dispersible in water to form particles. Additionally high affinity between hardener and more hydrophylic OH polymer is needed to end up with a dispersion of NCO-hardener covered by OH polymer (Fig. 7). This process should be finished as fast as possible to minimize the time of direct contact between isocyanate and water. This is not only done by proper polymer choice but also to a large extent by the paint formulation. Emulsifying components like solvents and polymeric additives either in the base material or in the hardener influence the potlife strongly.
One example has been evaluated using polymer thickener based on PUR chemistry. Potlife as well as spray viscosity stabilization are influenced by this additive. (Figs. 8–10). These results show the sophisticated task of the formulator. For whichever reason the best film properties in gloss and NCO stability were reached with quantities of thickener that lead to increasing viscosity during potlife. In this example, potlife only reaches 30–45 min, whereas the target is 60–90 min. On the basis of these results we decided to focus our development for car refinishes on a clearcoat that provides good application and dry film properties. Good application properties encompasses mixing process, spray application and potlife. We used a modified urethane polyester as OH polymer in combination with a mixture of low molecular weight hydrophobic and hydrophylic polyisocyanate as cross-linker. Formulation was adjusted to get a mixture of three parts base material with one part hardener resulting in a OH/NCO balance of 1/1.5. Viscosity was adjusted by a mixture of water with special solvents. Paint material can be mixed by hand as usual in paint shops of car refinishers. Reactivity is balanced to a potlife of 90 min. As we have seen above the most critical factor is water evaporation after spraying. Best film properties are achieved if most of water has evaporated before film forming process and crosslinking
Fig. 8. Viscosity as a function of thickener content.
Fig. 9. NCO content as a function of thickener content.
W. Collong et al. / Progress in Organic Coatings 45 (2002) 205–209
209
Fig. 10. Gloss as a function of thickener content.
heavy duty trucks and busses. Thus, main customers today are found in this area.
Table 2 Properties of waterborne refinish clearcoat Solid content (ready for use) VOC Potlife Overspray absorption Vertical stability Blister free up to Drying time Adhesion on basecoat Sandability/polishability Appearance Direct gloss Hardness after aging Chemical resistance Weather resistance
35% 2.0 90 min Excellent 120 m 120 m 60 min at 60 ◦ C or 16 h at room temperature Excellent Good the next day Excellent 90 Good Good Excellent
3. Conclusion Waterborne clearcoat has a reasonable chance of being accepted in the car refinishing market. Referring to the present profile first application were successfully performed on busses and heavy duty trucks, whereas solventborne systems may be corrected to customer needs waterborne clearcoat properties are much more influenced by the choice of polymers. Acknowledgements
have started. Table 2 shows some properties of the present refinish clearcoat. This formulation is the best compromise of latitude in application, film quality and drying time. Acceleration in drying either by faster curing or raising physical drying reduces latitude in application by reduced threshold for blistering and overspray absorption. Blistering is dominated by water release after film forming, whereas overspray absorption is effected by poor resolving of clearcoat particles in the low amount of organic solvents in the clearcoat. Both lack of overspray absorption and blistering at low film thickness are unacceptable for customers, whereas slow drying is a minor problem especially in the after-market of
This work was strongly supported by a lot of partners. Special thanks to Dr. Joachim Probst (Bayer) who supported us by doing a lot of analytical work. Further Reading [1] J. Probst, M. Melchiors, E. Juerens, M. Sonntag, Aqueous two-component polyurethane systems for coatings, in: Proceedings of the IUPAC Symposium, Warsaw, July 2000. [2] M. Sonntag, E. Juergens, Aqueous 2-Pack Polyurethane Coatings for Automotive Refinish and Transportation, Bayer Publication. [3] M. Melchiors, et al., Recent developments in aqueous two-component polyurethane coatings, Prog. Org. Coat. 40 (2000) 99. [4] DE 19 607 672, EP 654 051, EP 654 053, EP 654 055, EP 358 979, EP 542 105, EP 578 940, EP 669 352, EP 751 179, EP 469 389, EP 456 210, etc.
2K waterborne clearcoat—a competition between crosslinking and side reactions W. Collong a , A. Göbel a , B. Kleuser a , W. Lenhard a,∗ , M. Sonntag b a
DuPont Performance Coatings, Wuppertal, Germany b Bayer AG Leverkusen, Germany
Received 1 September 2001; accepted 6 March 2002
Abstract The combination of OH-functional polymers with di- or trifunctional polyisocyanates is widely used as crosslinking chemistry in high-performance two component clearcoats. This curing chemistry had always been limited by strong side reactions. Thus use of solvents in formulation was limited to non-OH-/NH-functional components. Some years ago Bayer came up with the strange idea to formulate 2K isocyanate waterborne paints. They carried out a lot of investigations to establish the proportion of crosslinking and side reactions. Nevertheless the formulation of two-component waterborne PUR paint remains a highly sophisticated task. This presentation shows the basic evaluation carried out and communicated by Bayer [Aqueous two-component polyurethane systems for coatings, in: Proceedings of the IUPAC Symposium, Warsaw, July 2000; Aqueous 2-Pack Polyurethane Coatings for Automotive Refinish and Transportation, Bayer Publication; Prog. Org. Coat. 40 (2000) 99] and additional experiences we have gathered in DuPont Performance Coatings [DE 19 607 672, EP 654 051, EP 654 053, EP 654 055, EP 358 979, EP 542 105, EP 578 940, EP 669 352, EP 751 179, EP 469 389, EP 456 210, etc.]. The focus of DuPont experiments is on a high-performance waterborne clearcoat system for car refinish. Especially, the demands of refinish customers have led us to focus on miscibility, sprayability and pot life issues. Kinetic as well as thermodynamic effects are discussed. Finally, a formulation is described which fulfils most of our customer needs. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Waterborne; Clearcoat; Refinish; Analytical data; Urea; Urethane
1. Basic chemical challenges in 2K waterborne PUR chemistry Isocyanates as high reactive chemicals create several chemically different products when combined with OHand NH-functional substances (Fig. 1). Desired products and side products are formed in different amounts. In homogeneous solutions the quantities only refer to the speed of reaction and concentration as shown for the predominant reactions in Table 1. In polymers and especially in waterborne polymer dispersions reactions are also influenced by other effects: because of complex structures, solubility parameters and accessibility of functional groups it is impossible to get thermodynamic figures for each of these effects. Normally, we can only measure the total effect to some extend. Tests were carried out to determine the different reactions. Some additional analytical data led us to a better under∗
Corresponding author. Tel.: +49-202-529-2210; fax: +49-202-529-35-2210. E-mail address: werner [email protected] (W. Lenhard).
standing of the chemical background of the technical findings. IR spectroscopic measurements were made. They will be discussed later on. The method of paint preparation also has a strong influence on the final products of chemical reaction. Stirring waterborne OH-/NH-functional polymers with isocyanates is a highly sensitive process. To evaluate the mixing parameters tests were done by mixing different isocyanates under different conditions with water. The quality of dispersion can be seen by the opacity of the mixture (Fig. 2). Hydrophobic isocyanates need high shear mixing to be dispersed in water, whereas hydrophilic modified isocyanates can be stirred in water by hand. The mixing process with hydrophilic isocyanates is accompanied by an initially strong increase of viscosity. Having mixed the polymer dispersion and the isocyanate successfully two main questions arise: 1. Which reactions predominate during water evaporation after application? 2. Which reactions occur between mixing the two components and application? (potlife).
0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 2 ) 0 0 0 5 2 - 8
206
W. Collong et al. / Progress in Organic Coatings 45 (2002) 205–209
Fig. 1. Isocyanate reactions.
Table 1 Relative reactivity of isocyanates depending on “steric” properties Reaction
Relative speed in homogenous solution (kg/mol h)a
Relative speed in dispersion (kg/mol h)b
R–NCO + R OH = urethane R–NCO + HOH = urea
0.08 0.10
0.017 0.018
a b
Fig. 3. NCO and water decrease in a 2K PUR film at 23 ◦ C.
Mixture related to real concentration in waterborne clearcoat. Real waterborne clearcoat.
Analytical data gave us promising results: drying at room temperature (23 ◦ C) shows nearly complete water evaporation after 30 min accompanied by only an approximate 5% decrease of isocyanate functionality. Reaction with water at room temperature is slow in spite of the extremely high OH-number of water (3117) compared to OH-numbers (100–200) in polymers (Fig. 3). Analytical results during potlife show a predominant increase of urea groups (22%) and some urethane content (8%) after 5 h accompanied by a dropping pH value (Fig. 4). All results depend on temperature, the choice of isocyanate components and polymer dispersions. Especially, under condition of only 15 min evaporation at room temperature followed by 25 min at 60 ◦ C urethane predominates over urea. This was proven by solid C13 NMR. In the beginning the reaction with water (forming urea) is higher than after heating. This refers to the decreasing amount of water during evaporation. Considering all analytical results we can summarize: we may get a proper clearcoat film at room temperature using polymer dispersions and isocyanates adjusted to crosslink
Fig. 2. Dispersability of polyisocyanates in water.
Fig. 4. Decrease of NCO content and pH value during potlife.
in the presence of water. This means especially by means of the polyisocyanate finding the right balance between hydrophylic and hydrophobic behavior to get it through the continuous phase of water in close contact to the OH polymer as quickly as possible (Fig. 5).
Fig. 5. Emulsification of polyisocyanates in water.
W. Collong et al. / Progress in Organic Coatings 45 (2002) 205–209
On the other hand, we have to take into account the required water evaporation time (Fig. 3). If we leave the balance between water release and film forming/crosslinking we are going to run into quality problems. We established that blistering, e.g., is strongly influenced by this balance. This seems to be predominant compared to the generation of CO2 being a product of the reaction of isocyanate with water. We take 20–50% excess of isocyanate to get decent crosslinking because of side reactions (forming urea structures).
2. Specially designed waterborne clearcoat for automotive refinish To develop such a complicated system you need a realistic chance to find a customer. This market was recently created by European VOC regulation. Especially, big customers can only reach the tough reduction targets using either ultra-high solid (2.1 pounds per gallon VOC) or waterborne clearcoat. Both systems have their advantages and drawbacks. Waterborne has the above-mentioned risks of side reactions. Ultra-high solid seems to need special development of low molecular weight reactive substances. The main hurdle for this development is the European restriction for the marketing of new chemical substances. Evaluating these conditions at present nobody can make a reasonable forecast about the market for only one developmental direction. With the above-mentioned results, we have a realistic chance of developing a waterborne clearcoat suitable for refinish application. To formulate a refinish clearcoat we have different tools: typical polyols are polyacrylates, urethanized polyesters and polyesters as clearcoat binders. In waterborne they show more differences from each other (especially in drying behavior) than known from solventborne systems. We learned that polyacrylates show the fastest film formation during drying, whereas polyesters dry slowly. Also initial film hardness is different, polyacrylates having higher hardness than polyesters. Hardeners are low molecular weight hydrophobic HDI and hydrophylic HDI and IPDI trimers. Solvent content is restricted by VOC regulation. The target is to remain below 2.1 p/gal (250 g/l). Therefore, we have to choose carefully which solvents support mixing process, application and film forming. Additives for waterborne clearcoats are limited by availability. This is one of the more challenging tasks in the development of waterborne clearcoat. To make a product for this special application we have to meet the needs of our customers. More or less empirical methods describe only the influence on different physical and chemical properties. This means for R&D: transfer of empirical descriptions to chemical and physical properties. 1. Mixing in ratios 3 parts polyol component:1 part isocyanate component or 2:1 by volume means formulate
207
both components in a manner that the relation by moles is 1 OH to 1.3 NCO in the above-mentioned parts by volume. 2. Mixing by hand using a stick means using a hardener composition which has good miscibility with water but high affinity to the polyol to get an excellent mixture of both resins! Every abnormal viscosity behavior has to be avoided during the mixing process. This happens by using only hydrophylic isocyanates which show an initial increase of viscosity in combination with polyols. Targeted behavior was found by formulating a cross-linker containing a good balance of hydrophobic low molecular weight isocyanates, special hydrophilic modified isocyanates and special solvents, whereas the hydrophilic part helps to get the hardener into the dispersion the hydrophobic part has a higher affinity to the polyol and equalizes the viscosity influence of the hydrophylic component. Solvent composition supports both mechanisms. The polyol dispersion needs a mixing viscosity and spray viscosity to enhance the miscibility with hardener. The adjustment to spray viscosity is done after mixing of the polymers. 3. Potlife of at least 1 h is necessary. Potlife in waterborne clearcoat does not mean the same as in solventborne systems. In both potlife is the maximum time you can use the ready to spray mixture resulting in constant properties in application, wet and dry film properties. Potlife depends strongly on spray viscosity in solventborne systems. In waterborne viscosity remains more or less constant far beyond the potlife. Nevertheless side reactions with water become predominant not only on the surface of the isocyanate particles formed by the mixing process but also by penetrating inside (Fig. 6). Urea polymers are created by this reaction as shown before. Crosslinking density is reduced when isocyanatefunctionality of hardener decreases. Chemical resistance is lowered. Lower gloss and haze influence appearance of clearcoat after drying in a negative manner. This behavior of waterborne 2K clearcoat does not impair the spray application by increasing viscosity. Therefore, we need a stable potlife to be sure the resulting film properties are unchanged even with small deviations from recommended application time. This means we have to develop a formulation providing a mixture of OH- and NCO-functional polymers that prevents
Fig. 6. Isocyanate in water during potlife.
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W. Collong et al. / Progress in Organic Coatings 45 (2002) 205–209
Fig. 7. Emulsification of polyisocyanates in OH polymer (mixed clearcoat dispersion.
the water from reacting with NCO-groups of the hardener by sterical hindrance and hydrophobic behavior. This happens if the NCO-hardener is sufficiently dispersible in water to form particles. Additionally high affinity between hardener and more hydrophylic OH polymer is needed to end up with a dispersion of NCO-hardener covered by OH polymer (Fig. 7). This process should be finished as fast as possible to minimize the time of direct contact between isocyanate and water. This is not only done by proper polymer choice but also to a large extent by the paint formulation. Emulsifying components like solvents and polymeric additives either in the base material or in the hardener influence the potlife strongly.
One example has been evaluated using polymer thickener based on PUR chemistry. Potlife as well as spray viscosity stabilization are influenced by this additive. (Figs. 8–10). These results show the sophisticated task of the formulator. For whichever reason the best film properties in gloss and NCO stability were reached with quantities of thickener that lead to increasing viscosity during potlife. In this example, potlife only reaches 30–45 min, whereas the target is 60–90 min. On the basis of these results we decided to focus our development for car refinishes on a clearcoat that provides good application and dry film properties. Good application properties encompasses mixing process, spray application and potlife. We used a modified urethane polyester as OH polymer in combination with a mixture of low molecular weight hydrophobic and hydrophylic polyisocyanate as cross-linker. Formulation was adjusted to get a mixture of three parts base material with one part hardener resulting in a OH/NCO balance of 1/1.5. Viscosity was adjusted by a mixture of water with special solvents. Paint material can be mixed by hand as usual in paint shops of car refinishers. Reactivity is balanced to a potlife of 90 min. As we have seen above the most critical factor is water evaporation after spraying. Best film properties are achieved if most of water has evaporated before film forming process and crosslinking
Fig. 8. Viscosity as a function of thickener content.
Fig. 9. NCO content as a function of thickener content.
W. Collong et al. / Progress in Organic Coatings 45 (2002) 205–209
209
Fig. 10. Gloss as a function of thickener content.
heavy duty trucks and busses. Thus, main customers today are found in this area.
Table 2 Properties of waterborne refinish clearcoat Solid content (ready for use) VOC Potlife Overspray absorption Vertical stability Blister free up to Drying time Adhesion on basecoat Sandability/polishability Appearance Direct gloss Hardness after aging Chemical resistance Weather resistance
35% 2.0 90 min Excellent 120 m 120 m 60 min at 60 ◦ C or 16 h at room temperature Excellent Good the next day Excellent 90 Good Good Excellent
3. Conclusion Waterborne clearcoat has a reasonable chance of being accepted in the car refinishing market. Referring to the present profile first application were successfully performed on busses and heavy duty trucks, whereas solventborne systems may be corrected to customer needs waterborne clearcoat properties are much more influenced by the choice of polymers. Acknowledgements
have started. Table 2 shows some properties of the present refinish clearcoat. This formulation is the best compromise of latitude in application, film quality and drying time. Acceleration in drying either by faster curing or raising physical drying reduces latitude in application by reduced threshold for blistering and overspray absorption. Blistering is dominated by water release after film forming, whereas overspray absorption is effected by poor resolving of clearcoat particles in the low amount of organic solvents in the clearcoat. Both lack of overspray absorption and blistering at low film thickness are unacceptable for customers, whereas slow drying is a minor problem especially in the after-market of
This work was strongly supported by a lot of partners. Special thanks to Dr. Joachim Probst (Bayer) who supported us by doing a lot of analytical work. Further Reading [1] J. Probst, M. Melchiors, E. Juerens, M. Sonntag, Aqueous two-component polyurethane systems for coatings, in: Proceedings of the IUPAC Symposium, Warsaw, July 2000. [2] M. Sonntag, E. Juergens, Aqueous 2-Pack Polyurethane Coatings for Automotive Refinish and Transportation, Bayer Publication. [3] M. Melchiors, et al., Recent developments in aqueous two-component polyurethane coatings, Prog. Org. Coat. 40 (2000) 99. [4] DE 19 607 672, EP 654 051, EP 654 053, EP 654 055, EP 358 979, EP 542 105, EP 578 940, EP 669 352, EP 751 179, EP 469 389, EP 456 210, etc.