Blocked polyisocyanates with built-in hals stabilizer

Details Blocked polyisocyanates with built-in hals stabilizer Blocked polyisocyanates with built-in hals stabilizer

1999-04-30

2001-03-20

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...

C528S073000, C252S182200, C252S182220, C546S244000

Reexamination Certificate

active

06204351

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to blocked polyisocyanates having a built-in HALS stabilizer, a process for their preparation, and a process for producing polyurethane (PUR) powder coatings.
2. Discussion of the Background
Blocked polyisocyanates are used for producing heat-curable one pack PUR baking systems which are storage stable. The masking or blocking of polyisocyanates is a well known way of affording temporary protection for isocyanate groups. The most common blocking agent used is &egr;-caprolactam, which forms with isocyantes a stable compound up to about 130-14 0° C. and which unblocks the blocked NCO groups at baking temperatures of 180° C. or higher.
The isocyantes preferred for heat-curable pulverulent compositions are (cyclo)aliphatic diisocyantes because of their excellent aging characteristics compared with aromatic isocyantes, which have the disadvantage of yellowing on baking, and aging in particular.
&egr;-Caprolactam-blocked isocyanate-polyol adducts based on isphorone diisocyanate (IPDI) in particular have become established as PUR powder curing agents. The reason for this is very probably the difference in reactivity between the two NCO groups in the IPDI, which permits controlled adduct formation from IPDI and the polyol (NCO:)H=2:1) with a narrow molecular weight distribution. A narrow molecular weight distribution of the curing agent is a prerequisite for good flow of the cured powder.
&egr;-Caprolactam-blocked IPDI melts at 53-55° C. Owing to the low melting temperature, the powders produced from this blocked IPDI cake together in storage. To increase the melting point, IPDI is subjected to a chain-lengthening reaction with polyol (NCO:OH=2:1) before &egr;-caprolactam blocking. DE-A-21 05 777 mentions polyols such as trimethylolpropane, trimethyl-1,6-hexanediol and diethylene glycol as chain lengtheners for IPDI and DE-A-25 42 191 mentions mixtures of di- and trifunctional polyols. Both are incorporated herein by reference.
The PUR powder coatings prepared with these curing agents have to be stabilizer against degradation by radiation. The stabilizers used are the known UV stabilizers based on benzotriazole (e.g. TINUVIN® 326) or based on strongly sterically hindered amines (e.g. TINUVIN® 770). The disadvantage with PUR powder coatings stabilized with these stabilizers is the limited lifetime of the stabilizers, which migrate to the surface over time and are destroyed there.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide partially or totally blocked polyisocyanates with which it is possible to produce stabilized PUR powder coatings without addition of a UV stabilizer.
The present invention accordingly provides partially or completely blocked polyisocyanates having a built-in HALS stabilizer, comprising blocking agents and adducts of diisocyantes and polyols of the general formula:
where R is H, alkyl, cycloalkyl of 1-20 carbon atoms and X is H,
where R is as defined above, the adducts preferably containing 2-20 NCO equivalents per OH equivalent.
DETAILED DESCRIPTION OF THE INVENTION
Diisocyantes particularly useful for the purposes of this invention are especially diisocyantes having an aliphatic, cycloaliphatic or (cyclo)aliphatic structure. Examples include those recited in Houben-Weyl, Methoden der organischen Chemie, volume 14/2, p. 61 ff. and J. Liebigs Annalen der Chemie, volume 563, p. 75-136, both incorporated by reference herein. Preference is generally given to the industrially readily obtainable aliphatic, cycloaliphatic and (cyclo)aliphatic diisocyantes of 6-14 carbon atoms, especially hexamethylene diisocyanate (HDI), 3-isocyanatomethyl-3,5,5-trimethylcoclohexyl isocyanate (IPDI) and dicyclohexylmethane 4,4′-diisocyanante (HMDI). Adducts of the diisocyantes and polyols may be made by any art-recognized process and is within the skill of the ordinary artisan.
Blocking agents particularly useful for the purposes of this invention are especially the compounds customary in PUR chemistry, such as oximes, acetone oxime, methyl ethyl ketoxime, methyl isobutyl detoximes, diisobutyl ketoxime and also acetophenone oxime, lactams, e.g., &egr;- caprolactam, seconday monoamines, e.g., diisobutylamine, dicyclohexylamine, tirazoles, e.g. 1,2,4-triazole, and also irreversible blocking agents such as alcohols, for example, methanol, ethanol, isporpanol, 2-ethylhexanol, which can be used in a mixture with the above mentioned reversible blocking agents. Preference is given to using &egr;-caprolactam, methyl ethyl ketoxime, and/or 1,2,4-triazole.
The present invention further provides a process for preparing partially or completely blocked polyisocyanates having a built-in HALS stabilizer, which comprises reacting the blocking agent with diisocyanate adducts preferably containing 2-8 NCO equivalents per OH equivalent.
The blocked polyisocyanates of the invention are preferably prepared in two stages, the first stage being a reaction of the diisocyanate with the TAD-OH. In a preferred reaction, an initial charge of diisocyanate at 80-120° C. is admixed with the TAD-OH in the course of a 2-3 h under nitrogen and in the absence of moisture with stirring in such a way that at least 2 and not more than 20, preferably 4-10, equivalents of NCO of the diisocyanate react per OH equivalent of the TAD-OH. In the second stage, the remaining NCO groups are then partially or totally blocked: the blocking agent mixture is added a little at a time to the diisocyanates-TAD-OH adduct at about 100-140° C. in such a way that the temperature does not rise above 150° C. On completion of the blocking agent or blocking agent mixture addition, the reaction mixture is heated at 120-140° C. for a further 2 h to complete the reaction. To speed up the reaction, a conventional urethanziation catalyst can be added, for example organotin compounds or triethylenediamine (Dabco), in an amount of 0.01 to 0.1% by weight as catalyst, based on the total mixture.
The following process has proved particularly advantageous for preparing the blocked compounds of the invention: in the first stage, the diisocyantes are reacted with blocking agent or blocking agent mixture at 80-150° C., and the second stage features the reaction of the remaining NCO groups with the polyols.
Examples of suitable TAD-OH compounds are 4-bis(2-hydroxyethyl)amino-2,2,6,6-tetramethylpiperidine, 4-bis(2-hydroxypropyl)amino-2,2,6,6-tetramethylpiperidine, 4-bis(2-hydroxybutyl)amino-2,2,6,6-tetramethylpiperdine, 4-bis(2-hydroxyethyl)amino-1-(2-hydroxyethyl)-2,2,6,6-tetramethylpiperidine (“TAD-triol”). In principle, the process of the invention can be carried out with any diols and/or triols which can be prepared in any known manner, or preferably according to the following reaction equation:
R:H, alkyl (e.g.; C
1
-C
20
), cycloalkyl of 1-20 carbon atoms;
The following polyols are used with particular preference:
The TAD-OH may be prepared in a 1-stage reaction TAD+monoepoxide at 100-130° C. 4-Amino-2,2,6,6-tetramethylpiperidine (TAD
0
may be prepared in the process described in DE-A-28 01 172, incorporated herein by reference, by reduction animation of the triacetoneamine prepared from acetone and NH
3
.
The compounds of the invention are generally compounds of the molecular weight range 500-1000, preferably 600-800. the products of the process have melting points (softening temperature) of 65-140° C., preferably 70-120° C., and a glass transition temperature of 25-100° C., preferably 35-85° C. Preferred compounds of the invention are additionally characterized by their containing the strongly sterically hindered amino group (calculated as NH) in an amount of 0.1-2% by weight, preferably 0.8-1.4% by weight, a free NCO group content of not more than 5% by weight, and a terminal blocked isocyanate group content (calculated as NCO) of 2-18% by weight, preferably 8-16% by weight.
The products of the process are useful as curing agents for higher functional thermoplastic compounds having Zerevitinov-active hydrogen ato

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