Synthesis of isophorone diisocyanate (IPDI) based waterborne polyurethanes: Comparison between zirconium and tin catalysts in the polymerization process
Introduction
Aqueous polyurethane (PU) dispersions have been gaining importance in a wide number of applications due to their versatility and their environmental friendly character [1], [2]. Their excellent properties such as good chemical resistance, high flexibility, adhesion to many polymers, and glass surfaces and film-forming ability at room temperature make them suitable for myriad of applications.
Although recently PUs have been successfully prepared by miniemulsion polymerization [3], [4], PU dispersions are almost exclusively obtained using a two-step procedure [5]. Several processes have been developed, having in common the first step, where ionic groups containing polyurethane is prepared in a non-aqueous system, and then dispersed in water in a second step. Depending on the selected route, the chain extension can take place either in the first or in the second step.
Regardless of the preparation method, tin compounds are widespread used as catalysts in PU systems. However, due to their high toxicity several attempts have been made in order to replace them by different friendly catalysts, such as bismuth, aluminium or zirconium chelates [6]. Zirconium compounds seem to be interesting as catalysts because of their low toxicity [7].
Different literature reports indicate the ability of zirconium compounds to selectively catalyze the isocyanate–hydroxyl reaction over the isocyanate–water reaction [8], which makes them ideal candidates in order to obtain waterborne polyurethanes. Nevertheless, due to the complexity of obtaining PU dispersions, more investigations about the activity of zirconium based compounds are needed if the final objective is the replacement of tin catalysts.
In this work different waterborne polyurethanes (WBPU) were synthesized using a two-step procedure. The article is centred in the first step where ionic groups containing polyurethanes were synthesized in acetone (using the so called “acetone process”). Two different catalysts were investigated, dibutyltin diacetate (DBTDA) and zirconium acetyl acetonate Zr(acac). The main goal of the work is to compare the effectiveness of both catalysts.
Section snippets
Materials
Isophorone diisocyanate (IPDI), trans-1,4-cyclohexylene diisocyanate, 1,3-bis(isocyanate methyl)-cyclohexane, 2,2-bis(hydroxymethyl) propionic acid (DMPA), 1,4-butanediol (BD), poly(1,4-butylene adipate) end-capped diol (Mn ≈ 1000) (PBAD), triethylamine (TEA), acetone d-6, dibutyltin diacetate (DBTDA) and zirconium acetyl acetonate (Zr(acac)) were supplied by Aldrich Chemical Corporation. Cyclohexane was purchased from Lab-Scan. Dry acetone was supplied by Panreac. All materials were used as
Catalytic activity
Due to the low reactivity of isocyanate groups present in isophorone diisocyanate (IPDI) [9] towards hydroxyl groups, the use of a catalyst is needed to carry out the polymerization process. Different polymerizations were performed using 350 ppm of both types of catalysts in presence and absence of triethylamine. Infrared spectra were performed at different reaction times for a reaction mixture catalyzed with 350 ppm of DBTDA (Fig. 1).
As expected, when the reaction advances, the intensity of the
Conclusions
Isophorone diisocyanate based waterborne polyurethanes can be synthesized using zirconium acetyl acetonate. As this catalyst has a relatively low toxicity (it is on average from 10 to 20 times less toxic than comparable tin compounds) it seems to be a good candidate in this application to replace the general use of tin catalysts. Nevertheless, in order to avoid hydrolysis of the zirconium complex, acid groups of the functionalized diol must be previously neutralized.
IPDI primary and secondary
Acknowledgements
The authors would like to thank the financial support of the Ministerio de Ciencia e Innovación (Interhybrid Project no. MAT 2005-08033-C02-01) and the Fondo Social Europeo (FSE) for the development of this work. H. Sardon also thanks the Ministerio de Ciencia e Innovación for a PhD grant. The authors also thank the NMR service of the University of the Basque Country.
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