1. Field of the Invention
The present invention relates to catalysts and a process for preparing polyisocyanates containing isocyanurate groups, to polyisocyanates thus prepared, and to their use.
2. Description of the Background
For high-grade one- and two-component polyurethane coating materials featuring high light stability and weathering stability, the isocyanate component used comprises, in particular, polyisocyanate mixtures containing isocyanurate and uretdione groups. The oligomerization or polymerization of isocyanates to give such polyisocyanates has been known for a long time. A range of preparation processes have been developed, differing from one another in catalyst selection, in the organic isocyanates used, or in technical parameters of the process (cf. e.g. GB Patent 1391066, EP 82 987, DE 39 02 078, EP 339 396, EP 224 165; see also H. J. Laas et al. in J. Prakt. Chem. 336 (1994), 185 ff.).
Isocyanates suitable for trimerization, examples being aromatic, cycloaliphatic, and aliphatic diisocyanates and higher polyisocyanates, can be prepared by a variety of methods (Annalen der Chemie 562 (1949), 75 ff.). Processes established in the industry include in particular preparation by phosgenating organic polyamines to the corresponding polycarbamoyl chlorides and the thermal cleavage of these chlorides into organic polyisocyanates and hydrogen chloride. Alternatively, organic polyisocyanates can be prepared without the use of phosgene, i.e., by phosgene-free processes. According to EP 126 299, EP 126 300 and EP 355 443, for example, (cyclo)aliphatic diisocyanates—such as 1,6-hexamethylene diisocyanate (HDI) and/or isomeric aliphatic diisocyanates having 6 carbon atoms in the alkylene radical, and 1-isocyanato-3-iso-cyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate or IPDI)—can be prepared by reaction of the parent (cyclo)aliphatic diamines with urea and alcohols to give (cyclo)aliphatic biscarbamic esters and the thermal cleavage thereof into the corresponding diisocyanates and alcohols.
For oligomerization, the (cyclo)aliphatic diisocyanates are reacted in the presence of the catalyst, with or without the use of solvents and/or auxiliaries, until the desired degree of conversion has been achieved. One of the terms used in this context is partial trimerization, since the target conversion is generally well below 100%. Afterward, the reaction is terminated by deactivating the catalyst and the excess monomeric diisocyanate is usually separated off, generally by flash or thin-film distillation. Deactivation is carried out thermally or by adding a catalyst inhibitor. Suitable examples include acids such as p-toluenesulfonic acid or bis(2-ethylhexyl)phosphate, alkylating agents or else acylating reagents.
As catalysts for the trimerization of isocyanates to the target polyisocyanates containing isocyanurate and possibly uretdione groups it is possible, for example, to use tertiary amines, phosphines, alkali metal phenoxides, amino silanes, quaternary ammonium hydroxides or quaternary ammonium carbonates. Highly suitable oligomerization catalysts also include hydroxides, halides or carboxylates of hydroxyalkylammonium ions (cf., e.g., EP 351 873, EP 798 299, U.S. Pat. No. 5,290,902), alkali metal salts, and tin salts, zinc salts, and lead salts of alkylcarboxylic acids. Depending on the catalysts, the use of various cocatalysts such as OH-functionalized compounds or Mannich bases composed of secondary amines and aldehydes or ketones, for example, is also possible.
Depending on the type of catalyst used and the reaction temperature, polyisocyanates are obtained with different proportions of isocyanurate and/or uretdione groups. The products are usually clear but may also have a greater or lesser yellow coloration depending on catalyst type, diisocyanate quality, reaction temperature and reaction regime. For the preparation of high-grade polyurethane coating materials, however, products whose color number is as low as possible are desired.
An appropriate catalyst may be selected according to different criteria. Of particular advantage with a view to the trimerization of isocyanates on an industrial scale, for example, is the use of quaternary hydroxylalkylammonium carboxylates as oligomerization catalysts. These choline-type catalysts are thermally labile. It is unnecessary to stop the trimerization on reaching the desired conversion by adding potentially quality-lowering catalyst inhibitors. Instead, targeted thermal deactivation allows optimum process control. The thermal lability also affords advantages from the standpoint of process safety. There is no possibility of uncontrolled reaction “runaway” provided the amount of catalyst added does not exceed the customary level by a multiple.
Aminosilyl compounds have proven advantageous for preparing high color quality polyisocyanates containing isocyanurate groups (U.S. Pat. Nos. 4,412,073, 4,537,961, 4,675,401, 4,697,014). In addition, they permit safe reaction control and can easily be deactivated using water or alcohols.
The class of the aminosilyl catalysts is hampered, however, by the disadvantage of low catalytic activity, so that economic space-time yields can be realized only when relatively large quantities of catalyst are used. This, however, is associated with further disadvantages. On the one hand, it constitutes a serious cost factor, since deactivation irreversibly destroys the catalyst which thus cannot be positively recycled into the process. On the other hand, relatively large amounts of the deactivated catalysts inevitably get into the product, with possibly adverse consequences for its profile of properties.
There is therefore a need for Si-based catalysts for preparing isocyanurate-containing polyisocyanates that do not have the disadvantages of the aminosilyl compounds known in the art.