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Isophorone diisocyanate in blocking agent free polyurethane powder coating hardeners: analysis, selectivity, quantumchemical calculations
Progress in Organic Coatings 48 (2003) 201–206
Isophorone diisocyanate in blocking agent free polyurethane powder coating hardeners: analysis, selectivity, quantumchemical calculations E. Spyrou a,∗ , H.J. Metternich b , R. Franke b a
Coatings and Colorants, Degussa AG, D-45764 Marl, Germany b Infracor GmbH, Degussa Group, D-45764 Marl, Germany
Abstract Blocking agent free polyurethane powder coating formulations are emission free and thus comply with the highest ecological requirements. In addition the resulting coatings exhibit an excellent performance in non-yellowing light-stable applications for exterior use. Essential component of the formulations is isophorone diisocyanate (IPDI)-uretdione, an internal blocked derivative of IPDI. For the first time it is now possible to identify each of the 10 isomers by 13 C-NMR-spectroscopy. This opportunity leads to fundamental conclusions concerning the selectivity of IPDI in the uretdione generating step and the reactivity of the different uretdione isomers in the crosslinking reaction. Quantumchemical calculations confirm the experimental results. © 2003 Elsevier B.V. All rights reserved. Keywords: Polyurethanes; Isophorone diisocyanate; Blocking agent free; Powder coatings; Uretdiones
1. Introduction
2. Experimental
Polyurethane (PU) coatings are currently taking the lead thanks to their excellent coating properties, good weather stability and versatility of formulation. For over 30 years, powder coating technology has made it possible to apply PU formulations in a solvent-free and hence environmentally friendly manner. However, conventional PU-powder coatings release blocking agents during the hardening process, that are undesired both ecologically and economically. Internally blocked and hence emission free PU-powder coatings do not have this drawback [1–3]. The topic of this lecture is the chemical basis of such innovative formulations, that is, isophorone diisocyanate (IPDI)-“uretdione”. Due to the complexity of the isomer mixture little is known up to now about this interesting raw material. In the following, the analysis of this compound, the selectivity of the various NCO-groups and the reactivity of individual isomer compositions are investigated. Finally, quantumchemical calculation methods are discussed.
2.1. NMR-spectroscopy [4–7]
∗ Corresponding author. E-mail address: [email protected] (E. Spyrou).
0300-9440/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0300-9440(03)00104-8
13 C: Bruker AMX 500 (500 MHz), Internal Standard TMS
(optionally, relaxant chromacetyl acetonate); 13 C-NMR spectra were 1 H-broad band-decoupled [8]. A positive sign denotes a low-field shift relative to the standard; a negative sign indicates a high-field shift. Starting compounds. The IPDI-uretdione mixtures used in the experiments come from Degussa AG. For structural assignment in the NMR spectra, isomer-pure uretdione mixtures were made from pure cis-IPDI and pure trans-IPDI. The degree of purity of these reference substances was >95% in all cases. The evaluation was done via the carbonyl carbon atoms of the uretdione ring, which show resonance between 159 and 156 ppm. The ring formation purely from aliphatic isocyanate groups appears at a low-field, while the uretdione ring formed purely from a cycloaliphatic isocyanate shows a high-field shift. Available for an unambiguous assignment of the individual resonance signals in the uretdione ring were uretdione synthesized from isomer-pure trans- and cis-IPDI, and mixtures of different isomer composition. A prediction of the shift values supported the calculated assignment [8–11].
202
E. Spyrou et al. / Progress in Organic Coatings 48 (2003) 201–206
O
O R
C N + N C O
C
catal.
R
R
N
N
R
∆T
OH-Polymer
2 RNCO
PU-Network
C
O
Isocyanates
Isocyanates
Uretdione
Fig. 1. Formation, cleavage and film building reaction of uretdione.
O
C N
H
= Symbol for cycloaliphatic NCO-group H3C H3C
CH3
O
= Symbol for aliphatic NCO-group
C N
Fig. 2. Structure of IPDI.
The NMR experiments were conducted on 30% solutions at a field intensity of 500 MHz. Uretdiones are temperature-sensitive and should not be heated over 30–40 ◦ C. Carbonyl carbon atoms undergo different NOE enhancement, and the use of an inverse gated decoupling measuring program [8–12] is needed for quantification. Alternatively, a chrom acetyl acetonate additive can be used to obtain shorter measuring times. In all experiments conducted at a higher field, only a differentiation of the three ring combinations of aliphatic/ aliphatic, aliphatic/cycloaliphatic and cycloaliphatic/cycloaliphatic could be achieved.
3. Uretdione
2.2. Quantumchemical calculations
4. NMR-spectroscopic investigations
All ab initio and density functional theory (DFT) calculations have been performed with the TURBOMOLE [13] suite of programs. The reported reaction energies result from DFT calculations employing the B3LYP hybrid functional [14–16] and using on all atoms a split-valence basis set with polarization functions (SVP) [17]. The initial structures of reactants and products have been generated with the semiempirical PM3 method [18] using MOPAC2000 [19]. These structures have been fully optimized on DFT level using the B3LYP functional and employing SVP basis sets. Comparison with experimental geometry parameters from X-ray diffraction for analogous molecules revealed a maximum error of 2 pm (0.002 nm) for bond lengths and of 3◦ for bond angles. Extensive test calculations in the framework of ab initio quantum-mechanical methods yield an estimated relative error of ±0.5 kcal/mol for the reaction enthalpies reported in our study.
The starting compound IPDI contains two different isocyanate groups, the CH-bound (cycloaliphatic) isocyanate group and the CH2 -bound (aliphatic) group (Fig. 2) [20]. During dimerization, three possible combinations occur for the isocyanate groups in the uretdione ring system: a uretdione system comprised of aliphatic or cycloaliphatic isocyanate groups and the mixed aliphatic/cycloaliphatic
Uretdione denotes a four-membered heterocyclic compound obtained by suitable catalysis from dimerization of two isocyanate groups (Fig. 1). Such compounds are thermally labile, that is, they split into their original components under the influence of temperature. The isocyanate groups released can then react with a hydroxyl group-containing resin to harden the coating film [1–3]. IPDI is now used almost exclusively as a starting material for such uretdiones (Fig. 2).
CH3 H3C
CH3
CH3 NCO
H3C
NCO NCO
H3C
OCN
cis-IPDI (75%)
trans-IPDI (25%)
Fig. 3. Different isomers of IPDI.
E. Spyrou et al. / Progress in Organic Coatings 48 (2003) 201–206
the different structures can be calculated using the integral stages of the corresponding signal lines.
Table 1 The 10 different isomers of IPDI-uretdione Symbol
Term
Structure
A1 A2 A3
Trans-CH2 –trans-CH2 Trans-CH2 –cis-CH2 Cis-CH2 –cis-CH2
B1 B2 B3 B4
Trans-CH2 –cis-CH Cis-CH2 –trans-CH Cis-CH2 –cis-CH Trans-CH2 –trans-CH
C1 C2 C3
Cis-CH–cis-CH Trans-CH–cis-CH Trans-CH–trans-CH
heterocyclus. Additionally, IPDI occurs in a mixture of two different isomers (Fig. 3), the cis form (75%) (equatorial aliphatic NCO-group) and the trans form (25%) (axial aliphatic NCO-group). This results in additional permutations, and therefore, a total of 10 isomer combinations are detectable in the uretdione mixture (Table 1). In the past, this complexity made it extremely difficult to classify precisely the individual isomers. Now for the first time, all 10 isomers can be distinguished and the isomer distribution quantified with a sophisticated NMR-spectroscopic analysis method. The individual analysis techniques are described in detail in the experimental part. Figs. 4 and 5 show the assignment of the signal lines and the associated integrals. As regards the chemical shift values of the individual isomers, the molar percentage of
Fig. 4.
203
13 C-NMR
5. Selectivity of IPDI The assignment of the signals of the individual uretdione isomers now makes it possible for the first time to infer to what extent they form, that is, what selectivity IPDI has in the dimerization reaction. IPDI is known to have considerable selectivity in urethane reactions [21–23]. This means that the two different NCO-groups react at different rates with alcohols. In the case of NCO-containing pre-polymers (the same holds true of trimeric isocyanurates), this fact plays a crucial role in terms of monomer content, viscosity, NCO content and storage stability. Selectivity during dimerization causes certain uretdione isomers to be formed preferentially. Far-reaching consequences can be expected. Based on the isomer distribution of IPDI (Fig. 3), a certain distribution of the 10 possible isomers is to be expected for statistical reasons alone. This statistical distribution is compared in Table 2 with the actual isomer contents, measured by 13 C-NMR. We see from this that aliphatic NCO-groups are reacted more rapidly than cycloaliphatic isocyanate groups (by a factor of 2–3) in the dimerization reaction. Further, it is evident that, in the composition of IPDI-uretdione, essentially four isomers dominate: A3, B3, A2 and B1 (in sum 78%). Based on the percentage of deviation of the actual content from the statistically anticipated value, a qualitative classification of the formation rates of the individual isomers can be observed (Fig. 6), in this case defined as selectivity. Without any selectivity all deviations would be zero.
spectrum of the uretdione isomer mixture, section carbonyl signals.
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Fig. 5.
13 C-NMR
spectrum of the uretdione isomer mixture, section uretdione signals.
Table 2 Content of the different uretdione isomers in theory and practice Symbol
Term
Structure
A1 A2 A3
Trans-CH2 –trans-CH2 Trans-CH2 –cis-CH2 Cis-CH2 –cis-CH2
1.6 9.4 14.0
3.4 14.3 30.2
B1 B2 B3 B4
Trans-CH2 –cis-CH Cis-CH2 –trans-CH Cis-CH2 –cis-CH Trans-CH2 –trans-CH
9.4 9.4 28.1 3.1
11.0 7.1 21.7 5.9
C1 C2 C3
Cis-CH–cis-CH Trans-CH–cis-CH Trans-CH–trans-CH
14.0 9.4 1.6
4.4 1.9 0.1
100.0
100.0
Statistical (mol%)
Sum
[%]
uretdione isomers content: statistic and NMR-results
150 123
100
86 72
selectivity =
[NMR-results] - [statistic] [statistic]
* 100%
48
50
13
theoretical value 0
-20
-50
-27 -67 -80
-100
-94
-150 A3
A1
B4
A2
B1
B3
B2
C1
C2
C3
Fig. 6. Deviation of the spectroscopically found isomer contents from statistical anticipation.
NMR (mol%)
E. Spyrou et al. / Progress in Organic Coatings 48 (2003) 201–206 H [kcal/mol]
205
reaction enthalpy - selectivity of uretdione isomers
[%]
140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100
13 enthalpy ( H)
12
selectivity
11 10 9 8 7
A1
A2
A3
B1
B4
B3
B2
C2
C1
C3
Fig. 8. Comparison of the reaction enthalpy (calculated) with the selectivity (Fig. 6).
Fig. 7. Uretdione isomer “trans-CH–trans-CH” (C3).
6. Quantumchemical calculations The reaction enthalpy of molecules can be determined by suitable quantumchemical calculation procedures. The calculation effort increases with increasing molar weight and the precision of the results decreases accordingly. Due to the dramatic development of computer hardware, however, considerably more complex structures can be calculated than just a few years ago. The application of the quantumchemical methods to the uretdione molecule results in the optimized structure of a selected isomer represented three-dimensionally in Fig. 7. A comparison of the reaction enthalpies (H) determined quantumchemically with the selectivity (NMR-spectroscopic results) shows a surprisingly similar pattern (Fig. 8). This consistency suggests that the dimerization reaction of IPDI proceeds largely under thermodynamic control.
Remarkable in this comparison is the deviation of the two isomers A3 and B4 from the calculated values. A3 is known to crystallize during the formation reaction. Thus, the high experimentally determined figure can be explained by the shift in the thermodynamic equilibrium. A similar reason for B4 is suspected.
7. Reactivity of uretdione The selectivity of IPDI and the resulting isomer distribution in the uretdione are not only of theoretical interest. Efforts in the powder coating field are increasingly aimed at low-temperature hardening. The motives for this are not only centered around energy savings but also relate to reducing cycle times and the opportunity to coat temperature-sensitive substrates as well. This raises the question as to the reactivity of the different uretdione isomer compositions as a function of the catalysts used. An experimental model system was used for this purpose. Uretdione
Fig. 9. Experimental model for evaluation of the reactivity of different uretdione compositions.
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E. Spyrou et al. / Progress in Organic Coatings 48 (2003) 201–206
8. Conclusions
Reactivity of uretdiones with ethyl hexanol catalysis 1% DBTL (150 ˚C)
conversion [%] 100
80 60 40 20 0 0
5
10
standard (30% A3)
15
experimental product (66% A3)
time 20 [min]
Fig. 10. Reactivity of different uretdione compositions with DBTL catalysis.
Reactivity of uretdiones with ethyl hexanol catalysis 1% DBN (150 ˚C)
conversion [%] 100
IPDI-uretdione is the basic component in blocking agent free PU-powder coating hardeners. Due to the complexity of the uretdione mixture little is known up to now about this compound. For the first time it is possible to assign the signals in a 13 C-NMR spectrum to each of the 10 different uretdione isomers in the mixture. Thus the content of every isomer can be measured and compared with the statistically anticipated value. The deviation of those two figures reveals a substantial selectivity of IPDI in the generation step of uretdione. This analytically determined selectivity corresponds quite well with the quantumchemical calculated reaction enthalpies (H) of the uretdione isomers. The reactivity of IPDI-uretdione depends significantly on the composition of the isomers and on the used catalyst. These results are a promising starting point in order to increase the reactivity of blocking agent free PU-powder coating hardeners. References
80 60 40 20 0 0
5 standard (30% A3)
10
15
experimental product (66% A3)
time [min]
20
Fig. 11. Reactivity of different uretdione compositions with DBN catalysis.
was blocked irreversibly with 2-ethyl hexanol at the outer NCO-groups, leaving the uretdione group intact (Fig. 9). In a second step the reaction of the excess 2-ethyl hexanol with the isocyanate units of the uretdione ring at 150 ◦ C in the time curve then enabled the reactivity of selected compositions to be determined. The content of the remaining uretdione groups (reaction of uretdione with butyl amine followed by back titration of excess amine with hydrochloric acid) was used as a measure of conversion. Dibutyl tin dilaurate (DBTL) and 1,5-diazabicyclo[4.3.0]non-5-en (DBN) were used as catalysts. In Figs. 10 and 11, an experimental product (66 mol% A3) is compared with the commercial isomer mixture (30 mol% A3) by way of example. Depending on the catalyst, one or the other isomer composition is more reactive. Consequently, the isomer distribution has a major influence on reactivity and, furthermore, the catalyst selection must be carefully coordinated with it. This is a significant step, in the search for highly reactive emission free PU-powder coating hardeners.
[1] W. Grenda, J.-V. Weiß, Uretdione based crosslinkers for powder coatings, in: Coating Symposium Powder Coatings Technology, Mulhouse, 2001. [2] W. Grenda, J.-V. Weiß, Farbe and Lack 6 (2000) 97. [3] W. Grenda, J.-V. Weiß, Polyurethane hardeners for powder coating specialities, ger., VILF-Tagung, Kaarst, 1999. [4] C.H. Sotak, C.L. Dumoulin, G.C. Levy, Top. Carbon-13 NMR Spectros. 4 (1984) 91–121. [5] M.L. Martin, G.J. Martin, J.J. Delpuech, Practical NMR Spectroscopy, Heyden, London, 1984. [6] H.-O. Kalinowski, S. Berger, S. Braun, 13 C-NMR-Spektroskopie, Georg Thieme Verlag, Stuttgart/New York, 1984. [7] L.D. Field, S. Sternhell, Analytical NMR, Wiley, Chichester, 1989. [8] E.G. Paul, D.M. Grant, J. Am. Chem. Soc. 86 (1964) 2984. [9] E.G. Paul, D.M. Grant, J. Am. Chem. Soc. 85 (1963) 1701. [10] G.C. Levy, G.L. Nelson, Carbon-13 Nuclear Magnetic Resonance for Organic Chemists, Wiley, New York, 1972. [11] E. Breitmaier, W. Voelter, 13 C NMR Spectroscopy, Monographs in Modern Chemistry, vol. 5, Verlag Chemie, Weinheim, 1974. [12] E.D. Becker, High Resolution NMR, Academic Press, New York, 1980. [13] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett. 162 (1989) 165. [14] A.D. Becke, Phys. Rev. A 36 (1988) 3098. [15] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [16] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [17] A. Schäfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 97 (1992) 2571. [18] J.J.P. Stewart, J. Comp. Chem. 10 (1989) 221. [19] J.J.P. Stewart, MOPAC2000, Fujitsu Limited, 1999. [20] A. der Heyde, W. Hübel, R. Boese, Die Angewandte Makromolekulare Chemie 153 (1987) 1–13. [21] E. Spyrou, Farbe Lack 106 (2000) 126–130. [22] R. Lomölder, F. Plogmann, P. Speier, JCT 69 (1997) 868. [23] E. Spyrou, R. Lomölder, XXIV, FATIPEC-Kongress, vol. B, Interlaken, 1998, pp. 159–174.
Isophorone diisocyanate in blocking agent free polyurethane powder coating hardeners: analysis, selectivity, quantumchemical calculations E. Spyrou a,∗ , H.J. Metternich b , R. Franke b a
Coatings and Colorants, Degussa AG, D-45764 Marl, Germany b Infracor GmbH, Degussa Group, D-45764 Marl, Germany
Abstract Blocking agent free polyurethane powder coating formulations are emission free and thus comply with the highest ecological requirements. In addition the resulting coatings exhibit an excellent performance in non-yellowing light-stable applications for exterior use. Essential component of the formulations is isophorone diisocyanate (IPDI)-uretdione, an internal blocked derivative of IPDI. For the first time it is now possible to identify each of the 10 isomers by 13 C-NMR-spectroscopy. This opportunity leads to fundamental conclusions concerning the selectivity of IPDI in the uretdione generating step and the reactivity of the different uretdione isomers in the crosslinking reaction. Quantumchemical calculations confirm the experimental results. © 2003 Elsevier B.V. All rights reserved. Keywords: Polyurethanes; Isophorone diisocyanate; Blocking agent free; Powder coatings; Uretdiones
1. Introduction
2. Experimental
Polyurethane (PU) coatings are currently taking the lead thanks to their excellent coating properties, good weather stability and versatility of formulation. For over 30 years, powder coating technology has made it possible to apply PU formulations in a solvent-free and hence environmentally friendly manner. However, conventional PU-powder coatings release blocking agents during the hardening process, that are undesired both ecologically and economically. Internally blocked and hence emission free PU-powder coatings do not have this drawback [1–3]. The topic of this lecture is the chemical basis of such innovative formulations, that is, isophorone diisocyanate (IPDI)-“uretdione”. Due to the complexity of the isomer mixture little is known up to now about this interesting raw material. In the following, the analysis of this compound, the selectivity of the various NCO-groups and the reactivity of individual isomer compositions are investigated. Finally, quantumchemical calculation methods are discussed.
2.1. NMR-spectroscopy [4–7]
∗ Corresponding author. E-mail address: [email protected] (E. Spyrou).
0300-9440/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0300-9440(03)00104-8
13 C: Bruker AMX 500 (500 MHz), Internal Standard TMS
(optionally, relaxant chromacetyl acetonate); 13 C-NMR spectra were 1 H-broad band-decoupled [8]. A positive sign denotes a low-field shift relative to the standard; a negative sign indicates a high-field shift. Starting compounds. The IPDI-uretdione mixtures used in the experiments come from Degussa AG. For structural assignment in the NMR spectra, isomer-pure uretdione mixtures were made from pure cis-IPDI and pure trans-IPDI. The degree of purity of these reference substances was >95% in all cases. The evaluation was done via the carbonyl carbon atoms of the uretdione ring, which show resonance between 159 and 156 ppm. The ring formation purely from aliphatic isocyanate groups appears at a low-field, while the uretdione ring formed purely from a cycloaliphatic isocyanate shows a high-field shift. Available for an unambiguous assignment of the individual resonance signals in the uretdione ring were uretdione synthesized from isomer-pure trans- and cis-IPDI, and mixtures of different isomer composition. A prediction of the shift values supported the calculated assignment [8–11].
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E. Spyrou et al. / Progress in Organic Coatings 48 (2003) 201–206
O
O R
C N + N C O
C
catal.
R
R
N
N
R
∆T
OH-Polymer
2 RNCO
PU-Network
C
O
Isocyanates
Isocyanates
Uretdione
Fig. 1. Formation, cleavage and film building reaction of uretdione.
O
C N
H
= Symbol for cycloaliphatic NCO-group H3C H3C
CH3
O
= Symbol for aliphatic NCO-group
C N
Fig. 2. Structure of IPDI.
The NMR experiments were conducted on 30% solutions at a field intensity of 500 MHz. Uretdiones are temperature-sensitive and should not be heated over 30–40 ◦ C. Carbonyl carbon atoms undergo different NOE enhancement, and the use of an inverse gated decoupling measuring program [8–12] is needed for quantification. Alternatively, a chrom acetyl acetonate additive can be used to obtain shorter measuring times. In all experiments conducted at a higher field, only a differentiation of the three ring combinations of aliphatic/ aliphatic, aliphatic/cycloaliphatic and cycloaliphatic/cycloaliphatic could be achieved.
3. Uretdione
2.2. Quantumchemical calculations
4. NMR-spectroscopic investigations
All ab initio and density functional theory (DFT) calculations have been performed with the TURBOMOLE [13] suite of programs. The reported reaction energies result from DFT calculations employing the B3LYP hybrid functional [14–16] and using on all atoms a split-valence basis set with polarization functions (SVP) [17]. The initial structures of reactants and products have been generated with the semiempirical PM3 method [18] using MOPAC2000 [19]. These structures have been fully optimized on DFT level using the B3LYP functional and employing SVP basis sets. Comparison with experimental geometry parameters from X-ray diffraction for analogous molecules revealed a maximum error of 2 pm (0.002 nm) for bond lengths and of 3◦ for bond angles. Extensive test calculations in the framework of ab initio quantum-mechanical methods yield an estimated relative error of ±0.5 kcal/mol for the reaction enthalpies reported in our study.
The starting compound IPDI contains two different isocyanate groups, the CH-bound (cycloaliphatic) isocyanate group and the CH2 -bound (aliphatic) group (Fig. 2) [20]. During dimerization, three possible combinations occur for the isocyanate groups in the uretdione ring system: a uretdione system comprised of aliphatic or cycloaliphatic isocyanate groups and the mixed aliphatic/cycloaliphatic
Uretdione denotes a four-membered heterocyclic compound obtained by suitable catalysis from dimerization of two isocyanate groups (Fig. 1). Such compounds are thermally labile, that is, they split into their original components under the influence of temperature. The isocyanate groups released can then react with a hydroxyl group-containing resin to harden the coating film [1–3]. IPDI is now used almost exclusively as a starting material for such uretdiones (Fig. 2).
CH3 H3C
CH3
CH3 NCO
H3C
NCO NCO
H3C
OCN
cis-IPDI (75%)
trans-IPDI (25%)
Fig. 3. Different isomers of IPDI.
E. Spyrou et al. / Progress in Organic Coatings 48 (2003) 201–206
the different structures can be calculated using the integral stages of the corresponding signal lines.
Table 1 The 10 different isomers of IPDI-uretdione Symbol
Term
Structure
A1 A2 A3
Trans-CH2 –trans-CH2 Trans-CH2 –cis-CH2 Cis-CH2 –cis-CH2
B1 B2 B3 B4
Trans-CH2 –cis-CH Cis-CH2 –trans-CH Cis-CH2 –cis-CH Trans-CH2 –trans-CH
C1 C2 C3
Cis-CH–cis-CH Trans-CH–cis-CH Trans-CH–trans-CH
heterocyclus. Additionally, IPDI occurs in a mixture of two different isomers (Fig. 3), the cis form (75%) (equatorial aliphatic NCO-group) and the trans form (25%) (axial aliphatic NCO-group). This results in additional permutations, and therefore, a total of 10 isomer combinations are detectable in the uretdione mixture (Table 1). In the past, this complexity made it extremely difficult to classify precisely the individual isomers. Now for the first time, all 10 isomers can be distinguished and the isomer distribution quantified with a sophisticated NMR-spectroscopic analysis method. The individual analysis techniques are described in detail in the experimental part. Figs. 4 and 5 show the assignment of the signal lines and the associated integrals. As regards the chemical shift values of the individual isomers, the molar percentage of
Fig. 4.
203
13 C-NMR
5. Selectivity of IPDI The assignment of the signals of the individual uretdione isomers now makes it possible for the first time to infer to what extent they form, that is, what selectivity IPDI has in the dimerization reaction. IPDI is known to have considerable selectivity in urethane reactions [21–23]. This means that the two different NCO-groups react at different rates with alcohols. In the case of NCO-containing pre-polymers (the same holds true of trimeric isocyanurates), this fact plays a crucial role in terms of monomer content, viscosity, NCO content and storage stability. Selectivity during dimerization causes certain uretdione isomers to be formed preferentially. Far-reaching consequences can be expected. Based on the isomer distribution of IPDI (Fig. 3), a certain distribution of the 10 possible isomers is to be expected for statistical reasons alone. This statistical distribution is compared in Table 2 with the actual isomer contents, measured by 13 C-NMR. We see from this that aliphatic NCO-groups are reacted more rapidly than cycloaliphatic isocyanate groups (by a factor of 2–3) in the dimerization reaction. Further, it is evident that, in the composition of IPDI-uretdione, essentially four isomers dominate: A3, B3, A2 and B1 (in sum 78%). Based on the percentage of deviation of the actual content from the statistically anticipated value, a qualitative classification of the formation rates of the individual isomers can be observed (Fig. 6), in this case defined as selectivity. Without any selectivity all deviations would be zero.
spectrum of the uretdione isomer mixture, section carbonyl signals.
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E. Spyrou et al. / Progress in Organic Coatings 48 (2003) 201–206
Fig. 5.
13 C-NMR
spectrum of the uretdione isomer mixture, section uretdione signals.
Table 2 Content of the different uretdione isomers in theory and practice Symbol
Term
Structure
A1 A2 A3
Trans-CH2 –trans-CH2 Trans-CH2 –cis-CH2 Cis-CH2 –cis-CH2
1.6 9.4 14.0
3.4 14.3 30.2
B1 B2 B3 B4
Trans-CH2 –cis-CH Cis-CH2 –trans-CH Cis-CH2 –cis-CH Trans-CH2 –trans-CH
9.4 9.4 28.1 3.1
11.0 7.1 21.7 5.9
C1 C2 C3
Cis-CH–cis-CH Trans-CH–cis-CH Trans-CH–trans-CH
14.0 9.4 1.6
4.4 1.9 0.1
100.0
100.0
Statistical (mol%)
Sum
[%]
uretdione isomers content: statistic and NMR-results
150 123
100
86 72
selectivity =
[NMR-results] - [statistic] [statistic]
* 100%
48
50
13
theoretical value 0
-20
-50
-27 -67 -80
-100
-94
-150 A3
A1
B4
A2
B1
B3
B2
C1
C2
C3
Fig. 6. Deviation of the spectroscopically found isomer contents from statistical anticipation.
NMR (mol%)
E. Spyrou et al. / Progress in Organic Coatings 48 (2003) 201–206 H [kcal/mol]
205
reaction enthalpy - selectivity of uretdione isomers
[%]
140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100
13 enthalpy ( H)
12
selectivity
11 10 9 8 7
A1
A2
A3
B1
B4
B3
B2
C2
C1
C3
Fig. 8. Comparison of the reaction enthalpy (calculated) with the selectivity (Fig. 6).
Fig. 7. Uretdione isomer “trans-CH–trans-CH” (C3).
6. Quantumchemical calculations The reaction enthalpy of molecules can be determined by suitable quantumchemical calculation procedures. The calculation effort increases with increasing molar weight and the precision of the results decreases accordingly. Due to the dramatic development of computer hardware, however, considerably more complex structures can be calculated than just a few years ago. The application of the quantumchemical methods to the uretdione molecule results in the optimized structure of a selected isomer represented three-dimensionally in Fig. 7. A comparison of the reaction enthalpies (H) determined quantumchemically with the selectivity (NMR-spectroscopic results) shows a surprisingly similar pattern (Fig. 8). This consistency suggests that the dimerization reaction of IPDI proceeds largely under thermodynamic control.
Remarkable in this comparison is the deviation of the two isomers A3 and B4 from the calculated values. A3 is known to crystallize during the formation reaction. Thus, the high experimentally determined figure can be explained by the shift in the thermodynamic equilibrium. A similar reason for B4 is suspected.
7. Reactivity of uretdione The selectivity of IPDI and the resulting isomer distribution in the uretdione are not only of theoretical interest. Efforts in the powder coating field are increasingly aimed at low-temperature hardening. The motives for this are not only centered around energy savings but also relate to reducing cycle times and the opportunity to coat temperature-sensitive substrates as well. This raises the question as to the reactivity of the different uretdione isomer compositions as a function of the catalysts used. An experimental model system was used for this purpose. Uretdione
Fig. 9. Experimental model for evaluation of the reactivity of different uretdione compositions.
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8. Conclusions
Reactivity of uretdiones with ethyl hexanol catalysis 1% DBTL (150 ˚C)
conversion [%] 100
80 60 40 20 0 0
5
10
standard (30% A3)
15
experimental product (66% A3)
time 20 [min]
Fig. 10. Reactivity of different uretdione compositions with DBTL catalysis.
Reactivity of uretdiones with ethyl hexanol catalysis 1% DBN (150 ˚C)
conversion [%] 100
IPDI-uretdione is the basic component in blocking agent free PU-powder coating hardeners. Due to the complexity of the uretdione mixture little is known up to now about this compound. For the first time it is possible to assign the signals in a 13 C-NMR spectrum to each of the 10 different uretdione isomers in the mixture. Thus the content of every isomer can be measured and compared with the statistically anticipated value. The deviation of those two figures reveals a substantial selectivity of IPDI in the generation step of uretdione. This analytically determined selectivity corresponds quite well with the quantumchemical calculated reaction enthalpies (H) of the uretdione isomers. The reactivity of IPDI-uretdione depends significantly on the composition of the isomers and on the used catalyst. These results are a promising starting point in order to increase the reactivity of blocking agent free PU-powder coating hardeners. References
80 60 40 20 0 0
5 standard (30% A3)
10
15
experimental product (66% A3)
time [min]
20
Fig. 11. Reactivity of different uretdione compositions with DBN catalysis.
was blocked irreversibly with 2-ethyl hexanol at the outer NCO-groups, leaving the uretdione group intact (Fig. 9). In a second step the reaction of the excess 2-ethyl hexanol with the isocyanate units of the uretdione ring at 150 ◦ C in the time curve then enabled the reactivity of selected compositions to be determined. The content of the remaining uretdione groups (reaction of uretdione with butyl amine followed by back titration of excess amine with hydrochloric acid) was used as a measure of conversion. Dibutyl tin dilaurate (DBTL) and 1,5-diazabicyclo[4.3.0]non-5-en (DBN) were used as catalysts. In Figs. 10 and 11, an experimental product (66 mol% A3) is compared with the commercial isomer mixture (30 mol% A3) by way of example. Depending on the catalyst, one or the other isomer composition is more reactive. Consequently, the isomer distribution has a major influence on reactivity and, furthermore, the catalyst selection must be carefully coordinated with it. This is a significant step, in the search for highly reactive emission free PU-powder coating hardeners.
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