Solid polyurethane hardeners having triazine groups,...

Details Solid polyurethane hardeners having triazine groups,... Solid polyurethane hardeners having triazine groups,...

1999-11-01

2001-07-10

Gorr, Rachel (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From reactant having at least one -n=c=x group as well as...

C528S060000, C528S065000, C528S073000, C525S124000, C525S453000, C544S223000, C252S182200, C252S182220

Reexamination Certificate

active

06258915

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to solid polyurethane hardeners (curing agents) having triazine groups, to processes for preparing such compounds and to their use for preparing plastics, especially powder coating materials which crosslink to give high-gloss or matt, light- and weather-stable coating films.
2. Discussion of the Background
Externally or internally blocked polyisocyanates which are solid at room temperature constitute useful crosslinkers for thermally crosslinkable polyurethane (PU) powder coating materials. For example, DE-C 27 35 497 describes PU powder coating materials having outstanding weathering and thermal stability. The crosslinkers whose preparation is described in DE-C 27 12 931 consist of E-caprolactam-blocked, isocyanurate-functional isophorone diisocyanate. Urethane-, biuret- or urea-functional polyisocyanates whose isocyanate groups are likewise blocked are also known.
The disadvantage of these purely externally blocked systems lies in the stoichiometric elimination of the blocking agent during the thermal crosslinking reaction. Since, the blocking agent is able to escape into the environment, it is necessary on ecological and workplace safety grounds to take special measures to clean the waste air and/or to recover the blocking agent. Moreover, the crosslinkers are of low reactivity, and curing temperatures of more than 170° C. are required.
DE-A 3030539 and DE-A 3030572 describe processes for preparing uretdione-functional polyaddition compounds whose terminal isocyanate groups are blocked irreversibly with monoalcohols or monoamines. The chain-terminating constituents of the crosslinkers are particularly disadvantageous, leading to low network densities of the PU powder coatings and thus to moderate solvent resistances.
Hydroxyl-terminated, uretdione-functional polyaddition compounds are the subject of EP 0669353. Because of their di-functionality they have improved solvent resistance. A common feature of the powder coating materials based on these uretdione-functional polyisocyanates is that they do not emit any volatile compounds in the course of the curing reaction. However, the stoving temperatures are high, at temperatures of 180° C. or more.
The use of amidines as catalysts in PU coating materials is described in EP 803 524. However, there is no chemical reaction of the PU hardener with the catalyst.
It is therefore an object of the present invention to provide low-emission PU hardeners of high reactivity which are particularly suitable for producing plastics and ecologically valuable powder coating materials.
SUMMARY OF THE INVENTION
The present invention provides solid PU hardeners having triazine groups, a melting point of from 40° to 130° C. and a free NCO content of less than 2% by weight, consisting of the reaction product of:
A) a polyaddition compound having at least one uretdione group and prepared by reaction of uretdione-functional polyisocyanates and hydroxyl-functional or amine-functional chain extenders, and
B) a compound having at least one >C═N— group, the ratio of equivalents of the uretdione groups employed to the >C═N— group being between 1.0:0.05 and 1.0:1.0.
DETAILED DESCRIPTION OF THE INVENTION
The principle of this invention lies in the reaction of conventional uretdione-functional powder coating hardeners, or uretdione-functional polyaddition compounds prepared specifically for the purpose, with suitable compounds having at least one >C═N— group, such as imines or amidines, for example, to give new PU hardeners, with in some cases drastic changes in both physical and chemical properties (for example, melting point, glass transition temperature, NMR spectrum, reactivity). The ratio of equivalents of the uretdione groups employed to the >C═N— group varies between 1.0:0.05 and 1.0:1.0, inclusive of all values and subranges therebetween. Consequently, these novel hardeners belong to a group of mixed internally and externally blocked PU hardeners.
The compounds of the invention have an advantage over purely externally blocked powder coating hardeners, in that they release much less blocking agent and are more reactive. In addition, compounds of this invention have a much lower curing temperature, relative to the purely internally blocked powder coating hardeners.
Uretdione-functional polyisocyanates are well known and are described, for example, in U.S. Pat. Nos. 4,476,054, 4,912,210, 4,929,724 and EP 417 603. A comprehensive overview of industrially relevant processes for dimerizing isocyanates to uretdiones is given in J. Prakt. Chem. 336 (1994) 185-200. In general, the conversion of isocyanates to uretdiones takes place in the presence of soluble dimerization catalysts, such as dialkylaminopyridines, trialkylphosphines, phosphoramides or imidazoles. The reaction—optionally conducted in solvents but preferably in the absence of solvents—is stopped by adding catalyst poisons when a desired level of conversion is reached. Excess monomeric isocyanate is separated off subsequently by flash evaporation. If the catalyst is volatile enough, the reaction mixture can be freed from the catalyst at the same time as the monomer is separated off. In this case, the addition of catalyst poisons can be omitted. In principle, a broad range of isocyanates are suitable for preparing uretdione-functional polyisocyanates. For example, isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), 2-methylpentane diisocyanate (MPDI), 2,2,4-trimethylhexamethylene diisocyanate/2,4,4-trimethylhexamethylene diisocyanate (TMDI), norbornane diisocyanate (NBDI), methylenediphenyl diisocyanate (MDI) and tetramethylxylylene diisocyanate (TMXDI) are preferred.
The reaction of these uretdione-bearing polyisocyanates to give uretdione-functional polyaddition compounds involves the reaction of the free NCO groups with hydroxyl-containing monomers or polymers. For example, the hydroxyl-containing polymers may be polyesters, polythioethers, polyethers, polycaprolactams, polyepoxides, polyesteramides, or polyurethanes. Low molecular weight di-, tri- and/or tetra-alcohols, or alternatively, polyamines may be used as chain extenders, with or without monoamines and/or monoalcohols as chain terminators, as has been described, for example, in the patents EP 669 353, EP 669354, DE 3030572, EP 639 598 or EP 803524. By low molecular weight di-, tri- and/or tetra-alcohols is meant alcohols with a molecular weight that is less than approximately 1000 g/mole. Preferred uretdione-functional polyaddition compounds have a free NCO content of less than 2% by weight and a uretdione group content of from 3 to 17% by weight. In addition to the uretdione groups, the polyaddition compounds may also have isocyanurate, biuret, allophanate, urethane and/or urea structures.
Compounds suitable as component B are all those having at least one >C═N— group which are capable of reaction with component A. Examples thereof are imines, amidines, oxazolines, oximes or imidazoles.
Examples of preferred amidines suitable for preparing the PU hardeners of the invention are N,N-dimethyl-N′-phenylformamidine, 2-methyltetrahydropyrimidine, 1-acetyl-2-phenylimidazoline and 1-acetyl-2,4-dimethylimidazoline. Particular preference is given to N,N,N′-trisubstituted amidines, such as 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The preparation of such bicyclic amidines is described, for example, in EP 662 476.
The invention also provides a process for preparing compounds having triazine groups, which comprises reacting
A) a starting compound having at least one uretdione group and
B) a compound having at least one >C═N— group at temperatures of 20°-120° C.
The invention additionally provides a process for preparing solid PU hardeners having triazine groups, a melting point of 40°-130° C. and a free NCO content of less than 2% by weight by reacting:
A) a polyaddition compound having at least one uretdione group and prepared by reaction

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