Synthesis of ADI/HDI hybrid isocyanurate and its application in polyurethane coating.
Keywords Bis(isocyanatomethyl)cyclohexane, Hexamethylene diisocyanate, Hybrid isocyanurate, Polyurethane, Structure, Coating
For high-quality two-component polyurethane coating materials, the polyisocyanate component is frequently an isocyanurate. Isocyanurates prepared by cyclic trimerization of isocyanates are well known in the literature. (1-8) Compared to the aromatic analogs, aliphatic isocyanate-based coatings exhibit excellent light and weather stability, and are principally used for outdoor applications. (9-14)
Bis(isocyanatomethyl)cyclohexane (ADI), a developmental cycloaliphatic diisocyanate, (15,16) shows good performance in a number of applications. Compared to conventional diisocyanates (HDI, IPDI, [H.sub.12]MDI, and TDI), ADI displays better properties in the polyurethane elastomers (16-18) dispersions, (19) and coatings etc. (20)
As shown in Fig. 1, ADI is composed of a mixture of 1,3-cis, 1,3-trans, 1,4-cis, and 1,4-trans bis(isocyanatomethyl)cyclohexane, where the 1,3-ADI content of the diisocyanate is in the range of 50-55%.
Coatings based on ADI isocyanurate have the advantage of fast drying, high hardness, and excellent solvent resistance. Low impact resistance and high viscosity are addressed by blending. Introducing HDI isocyanurate to ADI isocyanurate not only decreased the viscosity but also offered coatings with better impact resistance. (20) However, the conventional blending of the two isocyanurates can lead to coatings with lower distinctness of image (DOI) which is believed to arise from coating heterogeneity due to differential reactivity. To overcome these deficiencies, hybrid isocyanurate compositions based on ADI and HDI were prepared and reported on in this article.
The following reagents and materials were used as received: HDI (industrial grade, Bayer AG), 2-hydroxypropyl-trimethyl-azanium (DABCO TMR, Air Products and Chemicals, Inc.), dibutyl tin dilaurate (DABCO T-12, Air Products and Chemicals, Inc.), benzoyl chloride (chemically pure, Shanghai LingFeng Chemical Reagent Co., Ltd.), n-tetradecane (chromatographic pure, from Acros Organics.), butyl acetate (BA analytically pure, Shanghai LingFeng Chemical Reagent Co., Ltd.), Desmophen A870 = (hydroxyl-bearing polyacrylate, 70% in BA, [T.sub.g] = - 15[degrees]C, equivalent weight = 575, OH content = 2.95 wt%, Bayer AG), Desmodur N3300 (HDI isocyanurate, NCO content = 21.8 wt%, solid content = 100%, Bayer AG), ADI isocyanurate (NCO content = 13.1%, 70% in BA, developmental product from The Dow Chemical Company).
Synthesis of ADI/HDI hybrid isocyanurate
All the reactions were carried out in a 100-mL three-neck flask in an oil bath, which was equipped with a mechanical stirrer, an argon inlet, and a thermometer. In the first step, a calculated amount of ADI, HDI, and TMR was introduced into the reactor, and the mixture was stirred well to obtain a homogeneous liquid. Then the mixture was heated to the desired temperature. During the reaction, the mass fraction of the NCO group, residual content of ADI and HDI monomer, and the trimer content were measured by titration using the di-n-butyl amine method, GC, and GPC, respectively. As soon as the mass fraction of the NCO group dropped to the desired value, a calculated quantity of benzoyl chloride was added to terminate the reaction. (21) The mixture was continually stirred for 30 min and then the unreacted monomer was evaporated under vacuum to yield a colorless or pale yellow liquid.
Preparation of two-component polyurethane coatings
Coatings were formulated by first adding 0.1 wt% DABCO T12 (based on total resin solids) to Desmophen A870. The appropriate amount of hardener (ADI/HDI hybrid isocyanurate or ADI/HDI blend isocyanurate) was added to give a 1:1 ([NCO]:[OH]) ratio. Meanwhile, the solvent (BA) was also added with hand mixing to bring the solids to 56 wt%.
Clear films were prepared on black steel panels using a wet film applicator (100 [micro]m). Films were allowed to cure at 25[degrees]C and 50% relative humidity (RH). The film appearance properties were determined after drying for 7 days.
Characterization and evaluation
By measuring the mass fraction of the NCO group, the reaction degree was determined. The measurement of the mass fraction of the NCO group was in accordance with ASTM D 2572-97, and the value of the NCO conversion in the reaction was calculated according to equation (1):
NCO conversion = [[[NCO].sub.0] - [[NCO].sub.t]]/[[NCO].sub.0] x 100%, (1)
where [[NCO].sub.0] is the initial mass fraction of the NCO group and [[[NCO].sub.t] is the mass fraction of the NCO group at the interval of reaction time.
Gel permeation chromatography (GPC)
GPC was performed on a Waters 1515 equipped with a series of PS gel columns with a PS calibration. The tetrahydrofuran (THF) acted as the eluent at a flow rate of 1 mL/min at 40[degrees]C. The GPC samples were prepared from the reaction mixture every half an hour or an hour, which were first dissolved in THF and then deactivated by diethylamine; finally, the THF and excess diethylamine were distilled under vacuum at 130[degrees]C. The deactivated samples were dissolved in THF for GPC testing.
Gas chromatography (GC) analysis
GC was performed on a GC-3010A, equipped with capillary gas chromatographic column and flame ionization detector from Shanghai Institute of Computing Technology. The carrier gas was nitrogen. Injector and detector temperatures were set at 300[degrees]C. The oven temperature program was as follows: 80[degrees]C for 1 min, 80 to 280[degrees]C at 10[degrees]C/min, and 280[degrees]C for 10 min. The test sample was prepared by dissolving reactant and n tetradecane (internal standard) in ethyl acetate, with a concentration of about 3%.
Nuclear magnetic resonance (NMR) analysis
The [sup.1]H NMR (400 MHz) spectra were recorded in chloroform-d (CD[Cl.sub.3]) on a Bruker AVANCE 400 FT-NMR spectrometer.
Electrospray ionization mass spectrometry (ESI-MS) analysis
ESI-MS spectra were recorded on a Micromass LCT mass instrument operating in the positive-ion mode. The test samples were prepared by terminating ADI/HDI hybrid isocyanurate by methanol to eliminate the possible side reactions of NCO groups.
DOI analysis of coatings
DOI tests were performed on a Wave-Scan Plus5 Instrument (BYK-Gardner Company). The test samples were prepared on black steel panels after drying at 25[degrees]C and 50% RH for 7 days.
Gloss analysis of coatings
Gloss tests were performed on a Micro-TRI-Gloss Instrument (BYK-Gardner Company). The test samples were prepared on black steel panels after drying at 25[degrees]C and 50% RH for 7 days.
Results and discussion
For mild reaction conditions, i.e., low catalyst concentration and temperature, the reaction gave selectively the cyclic trimer. (22) Similar results were observed in our previous research of diisocyanate homotrimer. Higher trimer content is obtained at the lower NCO conversion. When the NCO conversion reaches 20-25%, the preferred balance between monomer consumption and trimer content is achieved. The various reaction conditions for synthesizing ADI/HDI hybrid isocyanu rate are listed in Table 1.
GPC analysis and results
As both ADI and HDI have two isocyanate groups, in the process of synthesizing ADI/HDI hybrid isocyanurate, a mixture that mainly consists of mono-, di-, and tri-isocyanurates is formed. Figure 2 shows the main chemical structures formed during the process of synthesizing ADI/HDI hybrid isocyanurate.
As shown in Fig. 3, the peaks at 9.8 and 9.2 min correspond to monomer and trimer, respectively. All other peaks from 7.8 to 9.5 min were attributed to oligomers. Quantitation of trimer content in the mixture was of primary interest. Deconvolution of the peaks in the GPC trace was processed with Peakfit v 4.12 software. In Fig. 3, the blue lines were the original GPC trace, and the other lines were the fitting curves by Peakfit v 4.12 software. We assumed that the mass fraction of each component was in accordance with the area fraction of each component (23) and thus the trimer content could be obtained.
The experimental results for trimer content are shown in Table 2. The trimer content for all the four samples after distillation was more than 35%. The trimer content of Sample 4 was significantly higher than that of the other three samples, which can be attributed to the milder reaction conditions. The trimer content in ADI/HDI hybrid isocyanurate can be controlled by adjusting the reaction conditions and the target NCO conversion.
GC analysis and results
In order to reduce the level of unreacted diisocyanate present in the product, monomer was removed by distillation at 150-170[degrees]C under high vacuum (P < 100 Pa). (24) The residual monomer content can be detected by GC analysis with n-tetradecane as the internal standard. (25,26)
Figure 4 reveals that n-tetradecane (internal standard) appeared at 8.87 min, ADI appeared as multiplet from 11.2 to 11.65 min, and HDI appeared at 8.53 min. The content of ADI (HDI) can be calculated by the peak areas of ADI (HDI) and n-tetradecane through the following function relation:
C = [f.sub.x] x [W.sub.i] x [A.sub.s]/[W.sub.s] x [A.sub.i], (2)
where C is the mass fraction of monomer, [f.sub.x] is the correction factor, [W.sub.i] is the mass of the internal standard sample, and [W.sub.s] is the mass of the sample. [A.sub.i] and As are the peak areas of the standard sample and ADI (HDI), respectively.
With equation (2), we can figure out the amount of unreacted ADI and HDI in the isocyanurate mixture as a function of reaction time. The amount of each individual isocyanate incorporated into the oligomer (reaction ratio) can be calculated by equation (3).
ADI/HDI = [([C.sub.A0] - [C.sub.At])/194] / [([C.sub.H0] - [C.sub.H0])/168], (3)
where [C.sub.A0] and [C.sub.H0] are initial mass fractions of ADI and HDI, and [C.sub.At] and [C.sub.Ht] are terminal mass fractions of ADI and HDI.
The residual monomer content and the reaction molar ratio of the two monomers are shown in Table 3. First, the residual monomer content decreased to an extremely low level after distillation, and the total residual HDI and ADI monomer contents were less than 3%. Besides, the residual isocyanate monomer content of HDI was significantly lower than that of ADI, which could be attributed to the lower boiling point of HDI. Finally, the reaction molar ratios of ADI/HDI increased with the feed ratio of the monomers, which will be discussed further in the following section (NMR analysis).
[sup.1]H NMR analysis and results
The [sup.1]H NMR spectra of ADI isocyanurate and HDI isocyanurate are compared in Fig. 5. In the [sup.1]H NMR spectrum of ADI isocyanurate, there are signals below 1.25 ppm, while in the [sup.1]H NMR spectrum of HDI isocyanurate, there is no signal in this region. So, the peaks in the region from 0.6 to 1.25 ppm are completely attributed to ADI and set as the internal standard. According to the enlarged view of the ADI isocyanurate spectrum (Fig. 5), the integration area from 0.6 to 2.3 ppm was 3.72 times as much as that from 0.6 to 1.25 ppm. Furthermore, according to our experimental results, we have found that the area ratio (3.72:1) is constant and independent of reaction condition and the feed ratio.
As shown in the supporting information, the chemical shifts of proton in hybrid isocyanurate at 0.6-2.3 ppm correspond to 10 hydrogens (A2-A7 and A2'-A9') of ADI and 8 hydrogens (H2-H5) of HDI. We defined the integration area from 0.6 to 1.25 ppm as [S.sub.1] and that from 0.6 to 2.3 ppm as [S.sub.2]; thus, the reaction molar ratio of ADI and HDI can be calculated by the following formulas:
[S.sub.ADI] = 3.72 x [S.sub.1]
[S.sub.HDI] = [S.sub.2] - 3.72 x [S.sub.1]
Reaction molar ratio: ADI:HDI(mol/mol) = [[S.sub.ADI]/10]/[[S.sub.HDI]8], (4)
where [S.sub.ADI] and [S.sub.HDI] are the integrating areas of the 10 protons belonging to ADI and the 8 protons belonging to HDI.
The [sup.1]H NMR spectra of all four samples are shown in Fig. 6. The results of the reaction ratio calculated by [sup.1]H NMR and GC are listed in Table 4. The quantitative analysis results of the reaction ratio calculated by NMR analysis are close to those obtained by GC analysis, which increases confidence in the accuracy of our data.
By plotting feed molar ratio with reaction molar ratio (ratio of ADI and HDI incorporated into product), a linear relationship is obtained (Fig. 7).
First, when the feed molar ratio is 1.0, the predicted reaction molar ratio is about 0.7, as shown with the blue dotted lines in Fig. 7. This means that HDI monomer has a higher reactivity than ADI monomer in the trimerization reaction. In addition, the favorable linear relationship indicates that the reaction ratio depends mainly on the feed ratio and is almost not affected by the reaction time or conversion. Furthermore, the linear fitting equation (equation (5)) can be used to prepare hybrid isocyanurates with the desired ratio of ADI to HDI in the product (reaction ratio).
y = 0.05097 + 0.63114x, (5)
where y is the reaction molar ratio of ADI/HDI and x is the feed molar ratio of ADI/HDI.
ESI-MS analysis and results
The hybrid isocyanurates are complex mixtures. ESIMS had been shown to be a very efficient characterization tool for polymers under 10,000 Da. (27-29) It is a soft ionization technique in which molecular ions are formed exclusively with no fragmentation. (30) Most components in the hybrid isocyanurate can be uniquely identified by molecular weight and quantified by the abundance on the basis of the assumption that the signal strength of abundance is independent of molecular weight. (31) The isocyanurate samples were terminated with methanol to eliminate the possible side reactions inherent to the NCO groups.
The ESI-MS spectrum of methanol-terminated hybrid isocyanurate (Sample 3) is shown in Fig. 8. It is clearly seen that the hybrid isocyanurate is a mixture of several components. The observed peaks are sodium cation adducts of the species originally present in the solution. For example, the peak set at 623 Da are sodium (23 Da) adducts of the methanol-terminated HDI homotrimer. The molecular weight (MW) of HDI monomer is 168 Da; therefore, the MW of homotrimer is 168 x 3 = 504 Da. The three methanol capping groups contribute 32 x 3 = 96 Da, and sodium adds 23 Da. Hence, the expected ion should appear at 504 + 96 + 23 = 623 Da. The ions at 624 and 625 Da are attributed to the isotopes of the molecular formula of [[C.sub.21][H.sub.48][N.sub.6][O.sub.9]][Na.sup.+], the methanol-terminated HDI homotrimer. The ions at 649,675, and 701 Da are assigned to the trimers of ADI + 2HDI(2),2ADI + HDI (3), and 3ADI (4), respectively (as shown in Fig. 9). The ESI-MS spectra of hybrid isocyanurates based on other two formulas are shown in Fig. 10. To sum up, the ESI-MS results demonstrate that the hybrid isocyanurates were synthesized successfully.
The quantitative analysis for the four isocyanurate samples was calculated by the related abundance on the basis of the assumption that the signal strength of abundance is independent of molecular weight. The results are listed in Table 5.
As shown in Fig. 11, with the ADI content increasing in the formula, the HDI homotrimer content decreased sharply, while the content of ADI homotrimer increased slowly. In our experiments, the higher hybrid level was obtained when the feed ratio of ADI/HDI was elevated. This was ascribed to the higher activity of HDI to generate homotrimer than ADI. It was also predicted that the hybrid level may reach up to 75% when the ADI content in formula was higher than 67 wt%.
Appearance of polyurethane coatings based on ADI/HDI hybrid isocyanurate
The appearance of coatings is an important factor in the overall product quality of engineered surfaces, especially in the automotive industry. (32,33) Information on coating components-hardeners are listed in Table 6. Two-component polyurethane clear coatings were prepared according to the formulations in Table 7.
The DOI and gloss results of coatings are listed in Table 8. All of the six samples displayed outstanding appearance property in DOI and gloss, especially in gloss (above 90[degrees]). As shown in Table 8, we found that there was no obvious difference in gloss between the two types of hardeners.
Table 8 shows that compared to the coating based on blended ADI&HDI isocyanurate, the coatings based on hybrid isocyanurate possess higher DOI value. Since the reactivity of ADI and HDI is different, the heterogeneity of 2K polyurethane coating will be directly influenced when blending ADI and HDI isocyanurate together as hardener. Hybrid isocyanurate is a good way to improve the DOI and gloss properties, which is extremely important in the applications requiring high-performance topcoats, such as in the aircraft coatings and automotive coatings etc.
The ADI/HDI hybrid isocyanurate with low residual monomer content was synthesized successfully with TMR as catalyst. The trimer content in isocyanurate composition was more than 35% after distillation. Both of the GC and [sup.1]H NMR analyses identified the favorable linear relationship between the amount of each isocyanate incorporated into the isocyanurate and feed ratio of the isocyanates. The reaction molar ratios of ADI/HDI in the hybrid trimer increased with the feed ratio of the monomers, and higher hybrid level was obtained when the feed ratio of ADI/HDI was elevated. This was ascribed to the higher activity of HDI to generate homo trimer than ADI. The analysis result has direct reference value to both further theoretical research and industrial synthesis of hybrid isocyanurate. Compared to ADI/HDI-blended isocyanurate, ADI/ HDI hybrid isocyanurate yields coatings with better DOI, and therefore shows tremendous potential in applications requiring high-performance topcoats.
The analysis of [sup.1]H NMR spectra of pure ADI isocyanurate and HDI isocyanurate.
Acknowledgments This research was supported by Dow Chemical Company Limited. The authors wish to thank Prof. Zhiping Zhang for his assistance.
Electronic supplementary material The online version of this article (doi:10.1007/s11998-014-9650-3) contains supplementary material, which is available to authorized users.
G. Wang ([mail]), K. Li, W. Zou, A. Hu, C. Hu
Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
Y. Zhu, C. Chen, G. Guo, A. Yang
Dow Chemical Company Limited, No. 936, Zhang Heng Road, Zhangjiang Hi-Tech Park, Shanghai 201203, China
R. Drumright, J. Argyropoulos
The Dow Chemical Company, Midland, MI 48674, USA
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Table 1: The various reaction conditions of synthesizing ADI/HDI hybrid isocyanurate Sample Formula TMR Temperature Time NCO ADI/HDI (wt) (wt%) ([degrees]C) (h) conversion (%) 1 1/2 0.1 65 2 22.30 2 1/1 0.1 65 2.25 22.43 3 2/1 0.1 65 3 22.07 4 5/1 0.08 65 7 20.44 Table 2: Trimer content of ADI/HDI hybrid isocyanurate obtained by GPC analysis Sample Reaction NCO Content of trimer in time (h) conversion (%) oligomer (%) Before After distillation distillation 1 2 22.30 53.03 36.71 2 2.25 22.43 54.77 38.97 3 3 22.07 54.40 39.82 4 7 20.44 67.94 53.27 Table 3: GC results of ADI/HDI hybrid isocyanurate Sample Monomer content Reaction Monomer content after reaction ratio ADI:HDI after distillation (mol/mol) ADI (wt%) HDI (wt%) ADI (wt%) HDI (wt%) 1 21.91 34.75 0.31# 1.61 1.31 2 29.45 22.93 0.66# 2.29 0.69 3 43.00 16.51 1.22# 1.31 0.50 4 57.55 8.77 2.83# 2.41 0.44 Bold values indicate that the reaction molar ratios of ADI/HDI in the hybrid isocyanurate increased from 0.31/1 to 2.83/1, when the feed weight ratio of ADI/HDI increased from 1/2 to 5/1 Note: The reaction molar ratios of ADI/HDI in the hybrid isocyanurate increased from 0.31/1 to 2.83/1 are indicated with #. Table 4: Comparison of the reaction ratio of ADI/HDI calculated by [sup.1]H NMR and GC Sample Formula ADI:HDI Reaction molar ratio ADNHDI wt/wt molar/molar GC [sup.1]H NMR 1 1:2 0.43 0.31 0.31 2 1:1 0.87 0.66 0.65 3 2:1 1.73 1.22 1.24 4 5:1 4.33 2.83 2.74 Table 5: Relative content of different components of the trimer by ESI-MS Peak (Da) Structural units Mole percentage (%) Sample 1 Sample 2 Sample 3 701/717 3ADI# + 3C[H.sub.3]OH + 2.64 2.15 11.61 [Na.sup.+]/[K.sup.+] 675/691 2ADI# + HDI# + 5.89 17.20 22.30 3C[H.sub.3]OH + [Na.sup.+]/[K.sup.+] 649/665 ADI# + 2HDI# + 33.52 45.22 46.72 3C[H.sub.3]OH + [Na.sup.+]/[K.sup.+] 623/639 3HDI# + 3C[H.sub.3]OH + 57.95 35.43 19.37 [Na.sup.+]/[K.sup.+] Hybrid 39.41 62.42 69.02 level (a) Bold entries indicate that with the ADI content increasing from 33.3 wt% to 66.7 wt% in the formula, the HDI homotrimer molar content in the hybrid isocyanurate decreased sharply from 57.95% to 19.37%, while the mol content of hybrid trimer HDI in the hybrid isocyanurate increased from 39.41% to 69.02% (a) Hybrid level: the percentage of hybrid trimer in all trimers Note: The ADI content increasing from 33.3 wt% to 66.7 wt% in the formula are indicated with #. Table 6: The detailed data for hardeners Property ADI/HDI blend isocyanurate (a) B-50 B-40 B-20 ADI (mol%) 50 [+ or -] 1 40 [+ or -] 1 20 [+ or -] 1 HDI (mol%) 50 [+ or -] 1 60 [+ or -] 1 80 [+ or -] 1 NCO content 14.70 15.02 15.68 (wt%) Non-volatile 75% in BA 75% in BA 75% in BA content Trimer -- (c) -- (c) -- (c) content (wt%) HDI/ADI <2 <2 <2 content (wt%) Property ADI/HDI hybrid isocyanurate (b) H-50 H-40 H-20 ADI (mol%) 50 [+ or -] 1 (d) 40 [+ or -] 1 (d) 20 [+ or -] 1 (d) HDI (mol%) 50 [+ or -] 1 (d) 60 [+ or -] 1 (d) 80 [+ or -] 1 (d) NCO content 15.35 14.94 16.16 (wt%) Non-volatile 75% in BA 75% in BA 75% in BA content Trimer 48.25 41.87 45.55 content (wt%) HDI/ADI <2 <2 <2 content (wt%) (a) ADI/HDI blend isocyanurate was prepared by mixing ADI-TRI and Desmodur N3300, in quantities corresponding to the desired HDI and ADI contents in the hardener (b) ADI/HDI hybrid isocyanurates were synthesized in the laboratory (c) The information of trimer content in blend isocyanurate is unpublished (d) The contents of ADI and HDI in hybrid isocyanurate sample are determined by NMR and GC analysis Table 7: The formulas for the polyurethane coatings Sample Polyol Hardener A Desmophen A870 H-50 B Desmophen A870 B-50 C Desmophen A870 H-40 D Desmophen A870 B-40 E Desmophen A870 H-20 F Desmophen A870 B-20 Table 8: Appearance properties of 2K polyurethane coatings with different ADI and HDI contents Sample DOI Gloss 20[degrees] 60[degrees] 85[degrees] A 84.6 90.7 95.7 98.4 B 80.0 90.8 95.0 98.1 C 95.5 91.5 94.5 94.8 D 85.2 90.1 93.8 94.0 E 95.9 91.5 94.0 94.6 F 87.1 91.3 94.0 94.8
Please note: Some tables or figures were omitted from this article.
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|Title Annotation:||hexamethylene diisocyanate; bis(isocyanatomethyl) cyclohexane|
|Author:||Wang, Guiyou; Li, Kang; Zou, Wei; Hu, Aiguo; Hu, Chunpu; Zhu, Yangping; Chen, Chen; Guo, Glynn; Yang|
|Publication:||Journal of Coatings Technology and Research|
|Date:||May 1, 2015|
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