Progress in Organic Coatings

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1 Progress in Organic Coatings 69 (2010) Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: Synthesis and characterization of diethylene glycol monobutyl ether Blocked diisocyanate crosslinkers Z. Ranjbar, Sh. Montazeri, M. Mohammad Raei Nayini, A. Jannesari Institute for Colorants, Paint and Coatings, Tehran, Iran article info abstract Article history: Received 7 June 2010 Accepted 10 August 2010 Keywords: Blocked isocyanate Deblocking temperature Polyurethane coatings Diethylene glycol monobutyl ether Typically blocked isocyanate systems are used to obtain the performance of two component polyurethane (PU) system in a one-component mixture. In this study four types of isocyanates namely, hexamethylene diisocyanate (HDI), diphenylmethane diisocyanate (MDI), isophorone diisocyanate (IPDI) and toluene diisocyanate (TDI) were blocked with diethylene glycol monobutyl ether (DEGMBE). Elimination of the isocyanate groups and the formation of urethane bonds were studied by FTIR spectroscopy and titration methods. Thermal dissociation of blocked diisocyanates was analyzed by DSC and TGA techniques. Deblocking temperature obtained by DSC and TGA techniques was compared. Based on DSC data, it was found that deblocking of blocked MDI and TDI starts at lower temperatures compared to that of the aliphatic one (HDI). Reactivity of the blocked IPDI is between blocked MDI and blocked TDI. In general, TGA results show the same trend as DSC except for IPDI which shows the lowest deblocking temperature. Deblocking temperature values obtained by TGA technique were lower than DSC values Elsevier B.V. All rights reserved. 1. Introduction Urethane chemistry brings the coatings industry a large variety of chemical, mechanical, physical benefits. One of the limitations of this chemistry is the 2K nature of this formulation [1]. One approach to the 1K PUs is using a thermally activating blocked isocyanate. A blocked isocyanate is an adduct containing a comparatively weak bond, formed by reaction between an isocyanate and a compound containing an active hydrogen atom. Blocked isocyanates have several advantages like marked reduction of moisture and water sensitivity, elimination of free isocyanate toxicity and possibility of formulating single pack heatcurable systems. Blocked isocyanates may be used in a wide range of coating applications such as coil coating, electro-coating, and powder coating. These coatings exhibit good adhesion, high weather resistance and good mechanical properties. Deblocking temperature of a blocked isocyanate depends on the structure of the isocyanate and the blocking agent, the chemical nature and the amount of curing catalyst and the deblocking reaction media [2,3]. Both aliphatic and aromatic isocyanates can be blocked by various blocking agents. The most widely commercially used blocking agents are alcohols, phenols, oximes, -caprolactam, dibutyl malonates, amides and imides [3 5]. Two different mechanisms have been proposed to explain the reaction of blocked isocyanates Corresponding author. Tel.: ; fax: address: ranjbar@icrc.ac.ir (Z. Ranjbar). and nucleophilic compounds namely elimination addition and addition elimination. According to the first mechanism, the reaction is believed to proceed as dissociation of a blocked isocyanate to yield the free blocking agent, who may evaporate, and free isocyanate group which then reacts with nucleophile compound. According to the second reaction mechanism, the nucleophile is added to blocked isocyanate to yield intermediate with a tetrahedral structure and then the blocking agent is eliminated in the curing film [2,3]. Ether alcohols and specially diethylene glycol monobutyl ether (DEGMBE) is extensively used to block isocyanate in coatings [3] since the residual blocking agent may introduce good performance and leveling properties in the curing film [1,6]. In this study, four types of isocyanate (1 aliphatic, 2 aromatics and 1cycloaliphatic) were blocked with diethylene glycol mono butyl ether. During the blocking reaction, the elimination of isocyanate group was studied by FTIR spectroscopy and titration methods. Thermal dissociation of the blocked diisocyanates was analyzed by DSC, TG/DTA techniques to clarify the impact of the isocyanate structure on deblocking temperature of these compounds. 2. Experimental 2.1. Materials Table 1 shows the materials used to prepare blocked isocyanates. All the materials were laboratory grade chemicals and were used as received without any further purification /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.porgcoat

2 Z. Ranjbar et al. / Progress in Organic Coatings 69 (2010) Table 1 Material properties. Material Chemical structure Supplier Purity MDI Diphenylmethane diisocyanate Aldrich Co., Germany 99.5% TDI Toluene diisocyanate Merck Co., Germany 20/80 mixture of 2,4/2,6 isomers HDI Hexamethylene Diisocyanate Merck Co., Germany 99% IPDI Isophorone Diisocyanate Merck Co., Germany 99% DEGMBE Diethylene glycol Monobutyl ether Merck Co., Germany 99% MIBK Methyl isobutyl ketone Merck Co., Germany 99% n-hexane Normal hexane Merck Co., Germany 99% DBTDL Dibutyl tin dilaurate Merck Co., Germany 2.2. Preparation of DEGMBE blocked diisocyanates Hexamethylene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate and toluene diisocyanate were blocked with diethylene glycol monobutyl ether (DEGMBE) by a typical procedure [1] with slight modifications. DEGMBE (0.1 M) was dissolved in 100 ml n-hexane in a three-necked glass reactor equipped with a condenser and dropping funnel. The mixture was agitated with a magnetic stirrer. Dry nitrogen was purged through the reactor and the reaction mixture was cooled to 0 C in an ice bath ml DBTDL (dibuthyl tin dilaurate) was then added as catalyst. Then 25 ml of a 0.05 M solution of diisocyanate in n-hexane was added to the mixture drop wise during 1 h and agitation was continued for another 1 h at 0 C. Then temperature was increased to 30 C under reflux condition for several hours depending on the isocyanate type. In case of HDI and MDI, the blocked isocyanates loose their solubility as they are formed. In case of TDI and IPDI, the reaction product is obtained by vacuum drying of the final reaction mixture Characterization methods for the blocked diisocyanates Real-time monitoring of the isocyanate conversion was performed using a Perkin-Elmer Spectrum One FTIR Spectroscope Equipped with a HATR accessory. A few drops of the reaction mixture were spread over the ATR crystal and the IR absorbance data were collected. To characterize the final blocked isocyanate, it was dry ground together with KBr powder and the mixture was pressed to a tablet. The tablet was then placed in the FTIR cell and the measurements were done. Completion of reaction was also monitored by taking out a sample of reaction mixture and determining the percentage of free Fig. 1. FTIR spectrum of (a) blocked MDI, (b) blocked TDI, (c) blocked HDI, (d) blocked IPDI.

3 428 Z. Ranjbar et al. / Progress in Organic Coatings 69 (2010) Fig. 2. Real-time FTIR spectrum of blocking MDI with DEGMBE. isocyanate content using dibutylamine titration as mentioned in ASTM D2572. Differential Scanning Calorimetry (DSC) thermograms from 0 to 300 C were obtained using a Perkin-Elmer DSC Pyris 6 instrument at heating rate of 10 K/min in a nitrogen atmosphere with a gas flow rate of 20 ml/min. Thermogravimetric analyses (TGA) were carried out using a Perkin-Elmer Diamond SII analyzer. The samples were heated from 30 to 500 C at a heating rate of 10 K/min in a nitrogen atmosphere with a gas flow rate of 20 ml/min. The effect of blocked isocyanate structure on curing condition was studied by addition of a hydroxyl-functional acrylic resin (Tacryl 392 H Tak Resin Co.) to blocked isocyanates in stoichiometric amount and were studied via DSC. 3. Results and discussion Fig. 3. Departing of NCO group from real-time FTIR spectrum of blocking MDI with DEGMBE FTIR and titration The reaction kinetics for each of diisocyanates with DEGMBE was studied through titrating of the free-nco content of the reaction mixture at different intervals during the blocking process. The FTIR spectra (Fig. 1a d) support the absence of free-nco groups in the all diisocyanates at the end of the blocking reaction. As FTIR spectra show the isocyanate characteristic band ( cm 1 ) is eliminated at the end of the reaction. The formation of urethane bonds is easily identified by the appearance of four characteristic bonds (1223, 1533, 1699, 3317 cm 1 ) [7,8]. The absorption frequencies of the blocked isocyanates are shown in Table 2. Figs. 2 and 3 show the disappearance of NCO peak (at cm 1 ) and the appearance of urethane peaks in the blocked MDI. The rate of consumption of NCO groups during the reaction is shown in Fig. 3. As could be seen, the blocking reaction goes to completion after four hours. These results indicate that the NCO groups Fig. 4. NCO content during the blocking reaction for TDI. of the MDI, TDI, HDI and IPDI molecules are completely blocked with blocking agent. Fig. 4 shows the consumed NCO content in the course of the blocking reaction for TDI. Table 3 shows the completion time of the blocking reaction for different isocyanates. So, one may conclude that the rate of reaction Table 2 Urethane bands resulting from blocking of diisocyanates [1,7,8]. DEGMBE blocked isocyanate (S) Amide ( NH CO ) (cm 1 ) (S) Carbamate ( NH COO ) cm 1 ) (VS) Carbonyle (C O) (cm 1 ) (S) Urethan (N H) (cm 1 ) MDI HDI TDI IPDI

4 Z. Ranjbar et al. / Progress in Organic Coatings 69 (2010) Scheme 1. Chemical structure of diethylene glycol monobutyl ether blocked (a) MDI, (b) TDI, (c) IPDI and (d) HDI. is in the following order: MDI = HDI > TDI > IPDI The blocking reaction is completed in 4 h for MDI and HDI, and in 8 h for TDI, while it takes 20 h to block all the free-nco groups of IPDI. This order shows that the more steric effect in the chemical structure. The more time to complete the reaction (Scheme 1). In other word, the more the steric hindrance, the longer will be the completion time of the blocking reaction Thermal analysis The onset of deblocking reaction has been determined by different analytical techniques [1,3,4]. It should be noted that the reported deblocking temperature depends on the characterization technique, heating rate, and other variables. Differential Scanning Calorimetry (DSC) is one of the most common techniques for the determination of the deblocking temperature. The presence of an endothermic peak in DSC thermographs of the blocked isocyanate may be assigned to the deblocking reaction and the evaporation of the blocking agent. DSC thermograms can appear an endothermic peak that is related to the deblocking reaction [1,4,9]. The results of the temperature sweep experiments for different blocked diisocyanates are shown in Fig. 5. Table 3 Reaction time and physical properties of blocked diisocyanates. DEGMBE blocked isocyanate Physical appearance Reaction time Solvent MDI Solid (mp = 30 C) 4 h in 30 degree n-hexane TDI Viscose liquid 8 h in 30 degree n-hexane HDI Solid (mp = 79 C) 4 h in 30 degree n-hexane IPDI Viscose liquid 20 h in 30 degree n-hexane Table 4 Deblocking temperatures of various blocked diisocyanates by DSC technique. DEGMBE blocked isocyanate Deblocking temperature (maximum) ( C) MDI TDI HDI IPDI Deblocking temperature (onset) ( C) Table 4 shows onset and maximum temperatures and also consumed heat during the deblocking process of the blocked diisocyanates which are obtained by using the DSC measurements. DSC thermograms of blocked HDI and MDI show another endothermic peaks at about 30 and 79 C, respectively, which correspond to the melting of these blocked diisocyanates. Dissociation temperature of the blocked diisocyanates determined by DSC is in the following order: blocked TDI < blocked IPDI < blocked MDI < blocked HDI The onset temperature of deblocking reaction depends on the chemical structure of diisocyanate. The high reactivity of isocyanate groups toward hydroxyl group of DEGMBE is essentially because of pronounced positive character of the C atom in cumulative double bond sequence consisting of nitrogen, carbon and oxygen, especially in aromatic systems [1,2,4]. In general, blocked aromatic isocyanates should be deblocked at lower temperatures than aliphatic isocyanates resulting from the greater electron-withdrawing potential of an aromatic ring as compared to an aliphatic group, in accordance with the results of the other researchers [2,10]. TDI exhibits the lowest deblocking temperature among the diisocyanates. As the benzene ring in TDI structure withdraws the electron pair on nitrogen atoms and increases the repulsion between nitrogen and hydrogen atoms in blocked isocyanate groups compared to MDI, HDI and IPDI. In addition it makes a higher steric effect than MDI [11,12]. Blocked aliphatic isocyanates with the isocyanate group on a tertiary carbon (IPDI), deblock at significantly lower temperatures which reported before about the other blocking agents [10,11,2]. This is a result of the steric influence of the cyclohexyl ring [13]. The appearance of the DSC endothermic peaks may be due to either the release of blocking agent or the evaporation of the dissociated components. The boiling points of the diisocyanates and DEGMBE are listed in Table 5. On the other hand rates of deblocking reactions and also their extent can be very dependent on volatiliza- Table 5 Thermal properties of reagents. Reagents Boiling point ( C) Melting point ( C) MDI TDI HDI IPDI DEGMBE

5 430 Z. Ranjbar et al. / Progress in Organic Coatings 69 (2010) Fig. 5. The DSC thermogram of (a) blocked HDI, (b) blocked MDI, (c) blocked IPDI, (d) blocked TDI. tion of the blocking agent. The peak temperature of the deblocking reaction shows the same trend as that of the boiling points of the individual isocyanates: blocked IPDI < blocked TDI < blocked HDI < blocked MDI TG/DTA graphs of the deblocking reaction of the blocked diisocyanates are shown in Fig. 6. TGA technique measures the changes in sample weight over a specified temperature range [14,9]. Dissociation temperatures of the blocked HDI, MDI, IPDI and TDI determined by TGA are listed in Table 6. As could be seen the onset temperature of weight loss is in the following order, as same the results of DSC measurements: blocked IPDI < blocked TDI < blocked MDI < blocked HDI Table 6 Dissociation temperatures of blocked isocyanates from TGA compared with DSC data. DEGMBE blocked isocyanate DSC maximum deblocking temperature ( C) DSC deblocking temperature (onset) ( C) TG/DTA (onset) ( C) MDI TDI HDI IPDI About blocked IPDI the temperature corresponding to the onset of weight loss is the lowest ( 131 C). It could be reasonable owing to the low boiling point of IPDI. As can be seen weight loss of blocked HDI starts at higher temperature compared to aromatic isocyanates (MDI and TDI) which Fig. 6. The TGA thermogram of (a) blocked IPDI, (b) blocked MDI, (c) blocked TDI and (d) blocked HDI.

6 Z. Ranjbar et al. / Progress in Organic Coatings 69 (2010) Fig. 7. The DSC thermograms of curing reaction of (a) blocked IPDI, (b) blocked TDI, (c) blocked HDI, (d) blocked MDI. are related to higher electron density on nitrogen atoms in aromatic compounds. This has been confirmed by DSC measurements too. It has been reported that the deblocking temperature determined by DSC differs from the deblocking temperature measured by TGA. This is caused by differences in sample pans and thermal effects in these two instruments [1,3]. Based on our observations, the deblocking temperature measured by DSC is greater than that obtained from TGA Curing Fig. 7 shows the curing thermograms of the synthesized blocked isocyanates mixed with a hydroxyl-functional acrylic resin in stoichiometric ratio of OH:NCO Results of thermal analysis of the mixture are presented in Table 7. As can be seen, the curing temperature is obviously lower than the deblocking temperatures reported in previous section, therefore it can be concluded that for the curing reaction of blocked isocyanates in this case the addition elimination mechanism is dominant. On the other hand curing temperatures are lower in case of aromatic blocked isocyanates compared to the aliphatic diisocyanate (HDI). This may be assigned to the fact that the C O single bond of the urethane linkage is highly polarized because of the electron-withdrawing character of the aromatic ring. In case of IPDI, the low deblocking temperature may be resulted from the steric stress on the C O single bond due to presence of cyclohexyl ring. The lower curing temperatures compared to deblocking temperature may also be related to the presence of hydroxyl groups which can accelerate the deblocking reactions [2,3,5]. Table 7 Comparison of dissociation temperatures of blocked isocyanates and curing reactions. Isocyanate blocked with DEGMBE H of curing (j/g) MDI TDI HDI IPDI Onset of curing temperature ( C) The heat of curing reactions of the blocked isocyanates shows a similar trend. blocked IPDI < blocked TDI < blocked MDI < blocked HDI 4. Conclusion DEGMBE blocked isocyanates were prepared. The blocking reaction progress was monitored by FTIR and titration methods. It was proved that after completion of the reaction all free isocyanates are consumed. None of the FTIR spectrum of blocked isocyanates shows any absorption at cm 1 range which indicates absence of isocyanate group. According to FTIR spectrums it was shown that blocked isocyanates were successfully produced. Differential scanning Calorimetry was used to determine deblocking temperature of the individual blocked isocyanates. It was shown that aromatic diisocyanates are deblocked at lower temperatures compared to aliphatic ones, in agreement to the other researchers report. TG/DTA was also used to investigate the dissociation temperature of blocked isocyanates. Deblocking temperatures determined by DSC and TGA techniques show the same trend but the deblocking temperatures determined by DSC are greater than those determined by TGA. The DSC thermograms of the curing reaction of the blocked isocyanates with an acrylic resin also showed that deblocking temperature is reduced upon mixing of blocked isocyanate and the acrylic resin. References [1] I. Ahmad, J.H. Zaidi, R. Hussain, A. Munir, Polym. Int. 56 (2007) [2] P. Thomas, Polyurethanes, Vol III, second ed., Wiley & Sons, [3] D.A. Wicks, Z.W. Wicks, Prog. Org. Coat. 41 (2001) [4] Z.W. Wicks, Prog. Org. Coat. 3 (1975) [5] Z.W. Wicks, Prog. Org. Coat. 9 (1981) [6] U. Poth, Automotive Coatings Formulation, Vincentz Pub., [7] G. Socrates, Infrared Characteristic Group Frequencies, Wiley, Chichester, [8] S.K. Wang, C.S.P. Sung, Macromolecules 35 (2002) 877. [9] M.G. Smolka, L. Haubler, D. Fischer, Thermochim. Acta 351 (2000) [10] D.A. Wicks, Z.W. Wicks, Prog. Org. Coat. 36 (1999) [11] P.L. Jansse, J. Oil Colour Chem., Assoc. 89 (1989) 478. [12] H. Kothandaraman, A. SultanNaser, J. Polym. 34 (3.) (1993). [13] I. Muramatsu, Y. Tanimoto, M. Kase, N. Okoshi, Prog. Org. Coat. 22 (1993) 279. [14] D.A. Wicks, Z.W. Wicks, Prog. Org. Coat. 43 (2001)