Note: Descriptions are shown in the official language in which they were submitted.
CA 02213~47 1997-08-21
Acetophenone oxime-blocked PU powder coatinqs and their use
The present invention relates to a transparent or
pigmented, heat-curable, weather-stable polyurethane (PU)
powder coating of high reactivity.
The blocking of a polyisocyanate for the temporary
protection of the NCO groups is a long-established procedure
and is described, for example, in Houben-Weyl, Methoden der
organischen Chemie XIV 2, page 61 ff.
For the preparation of PU powder, c-caprolactam has
become established, from among the large number of blocking
agents, for blocking the NCO component.
In DE-B 21 05 777 a range of advantages are cited
as a reason for emphasizing ~-caprolactam-blocked adducts of
isophorone diisocyanate (IPDI) as hardeners for powder
coatings.
DE-A 25 42 191 relates to a blocked polyisocyanate
of IPDI, ~-caprolactam and a polyol which is a mixture of a
diol and a triol, and its use for preparing a heat-curable
pulverulent coating composition.
DE-A documents 27 35 497 and 28 42 641 describe a
pulverulent PU coating material based on a hydroxyl-
containing polyester, a polyacrylate or an epoxy resin, whose
hardener component is an ~-caprolactam-blocked mixture of
IPDI isocyanurate and monomeric IPDI.
DE-A 30 04 876 claims an ~-caprolactam-blocked
polyisocyanate which consists of an adduct of IPDI and a
polyol in a fixed OH/NCO ratio.
The disadvantage of the known ~-caprolactam-blocked
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CA 02213~47 1997-08-21
polyisocyanates is the relatively high unblocking temperature
of around 180-200~C. For a long time there has been great
interest, on economic grounds, in reducing this temperature
and/or the stoving time of such coating material. Attempts
have been made to use, instead of ~-caprolactam, an oxime,
whose adduct with an isocyanate is thermally cleaved at a
much lower temperature than 170~C. In the cured coating to
which an oxime has been added, however, there is the unwanted
formation of blisters, as described in DE-A 33 12 028 (p.3,
line 3) and Powder Coatings Bulletin, Dec. 1994, p.2.
Moreover, with such a reactive system, a slight yellowing is
found, which increases greatly on overbaking, as our own
experiments have shown.
A main object of the present invention is to
overcome the disadvantages of the prior art and, in
particular, to provide a PU powder coating which may be
transparent or pigmented, which is notable in particular for
high reactivity and which therefore permits curing at a low
temperature.
The present invention accordingly provides a
storage-stable, heat-curable polyurethane powder coating
composition, essentially comprising:
a) at least one polyisocyanate component blocked
totally or partially with acetophenone oxime,
b) a polymer having at least two hydroxyl groups,
and
c) if required, appropriate auxiliaries or
additives.
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The polyisocyanates in the context of this
application are preferably a diisocyanate of aliphatic or
(cyclo)aliphatic structure. Such a diisocyanate is very well
known. Rather than listing individual representatives here,
reference is made to the literature: Houben-Weyl, Methoden
der organischen Chemie, Volume 14/2, p.61 ff. and J. Liebigs
Annalen der Chemie, Volume 562, pp. 75-136. Preference is
generally given to the readily industrially available
aliphatic or (cyclo)aliphatic diisocyanates with 6-14 carbon
atoms, especially 3-isocyanatomethyl-3,5-5-
trimethylcyclohexyl isocyanate (isophorone diisocyanate) and
dicyclohexylmethane 4,4'-diisocyanate. In addition to the
monomeric diisocyanates, it is also possible to employ
isocyanurates (i.e., a trimer) prepared therefrom, which can
be prepared by known methods (cf. e.g DE-C documents
26 44 684, 28 21 109 and 29 16 201).
Finally, the polyisocyanates in the context of the
present invention may also be understood as including those
adducts which are obtained by reacting the abovementioned
monomeric, predominantly bifunctional polyisocyanates with
chain extenders which are common in isocyanate chemistry.
Compounds of this kind are listed, for example, in
DE-A 27 07 660. Preference is given to polyols whose
molecular weight is below 350, especially ethylene glycol and
trimethylolpropane. The chain extenders should be reacted
with the monomeric diisocyanates in such amounts that the
resulting adducts have on average at least two isocyanate
groups.
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- 4 -
The polyisocyanates employed as hardeners in the
novel coating composition preferably have a content of
blocked NCO groups of about 9-20%, more preferably 11-15% by
weight. Their melting point is generally within a wide range
and is preferably from about 60 to about 150~C, preferably
from about 70 to about 150~C. They are suitable for
preparing solvent-containing one-component PU stoving
enamels. The blocking of the polyisocyanates can be
undertaken either in inert solvents or else in bulk in the
polyisocyanate initial charge.
The inert solvents are those which do not react
with the polyisocyanates. Examples of them include ketones
such as acetone, methyl ethyl ketone, methyl isobutyl ketone,
cyclopentanone and cyclohexanone; aromatic compounds, such as
toluene, xylene, chlorobenzene and nitrobenzene; cyclic
ethers, such as tetrahydrofuran; esters, such as methyl
acetate and n-butyl acetate; aliphatic chlorinated
hydrocarbons, such as chloroform and carbon tetrachloride,
and polar aprotic solvents, such as dimethyl-formamide,
dimethylacetamide and dimethyl sulfoxide.
The blocking agent, i.e., acetophenone oxime, is
employed relative to the polyisocyanate preferably in a ratio
such that there is 0.5 - 1 mol of acetophenone oxime per NCO
group.
The reaction of the polyisocyanate with
acetophenone oxime is usually carried out in the temperature
range between 0 and 150~C. To carry out blocking rapidly and
completely a relatively high reaction temperature is
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CA 02213~47 1997-08-21
preferred. On the other hand, the reaction temperature must
be at least 10~C below the unblocking temperature of the
blocked polyisocyanate. Preference is given to the
temperature range which is about 15 to 35~C below the
unblocking temperature, i.e., at about 120~C.
In the blocking reaction, a catalyst may be used to
promote the isocyanate polyaddition reaction, for example
tin(II) octoate, dibutyltin dilaurate (DBTL), tertiary
amines, etc.
The polymers having at least two hydroxyl groups
are preferably polyesters, epoxy resins, and hydroxyl-
containing acrylates, each having a molecular weight of from
800 to 40,000.
For the powder coatings, the polyesters i.e.,
polyester polyols, are preferably those prepared from
terephthalic acid with a polyol such as 1,6-hexanediol,
neopentylglycol, 1,4-dimethanolcyclohexane and 2,2,2-
trimethylolpropane.
The epoxy resins are preferably those listed in
DE-A 29 45 113, page 12, line 1 to page 13, line 26.
The hydroxyl-containing polyacrylates are
preferably those described in DE-A- 30 30 359, page 14, line
21 to page 15, line 26.
The polymers having at least two hydroxyl groups
are employed generally in such an amount that the OH/NCO
molar ratio is l:n where n is a number from about 1.2 to
about 0.8, preferably from about 1.05 to about 0.95.
The auxiliaries and additives are very well known
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- 5a -
in the name and examples of them include leveling agents,
pigments, dyes, fillers, catalysts, thixotropic agents, W
stabilizers and antioxidants. The amount of these
substances, relative to the amount of the solid binder, may
vary within a wide range.
The PU powder coatings are prepared, for example,
by mixing the polymer having at least two hydroxyl groups and
the blocked polyisocyanate, possibly following addition of
the desired coating additives, in the stated ratios and
extruding the mixture at an elevated temperature. This
temperature must be above the melting points of the polymer
having at least two hydroxyl groups and blocked
polyisocyanate but below the unblocking temperature of the
blocked polyisocyanate.
Extrusion is followed by cooling and by milling to
a particle size of less than 250 ~m, preferably less than 100
~m. Subsequently, when required coarser fractions may be
removed by sieving and are returned to the mill.
The application of the powder coating to bodies
that are to be coated may take place in accordance with known
methods, for example by electrostatic powder spraying, by
fluidized-bed sintering or by electrostatic fluidized-bed
sintering. The coated articles are then cured in the
temperature range between 140 and 250~C for a sufficient
time, e.g., from 60 minutes to 1 minute, preferably at a
temperature between 150 and 200~C for from 20 to 5 minutes,
more preferably at a temperature between 150 and 180~C.
Substrates suitable for coating with the novel PU
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- 5b -
powder coatings are all those which withstand the curing
temperatures indicated without suffering a deterioration in
their mechanical properties. Examples are metals, glass,
ceramic and plastic.
The stoving enamels and powder coatings that are
obtained are notable relative to their conventional
counterparts for a more favorable relationship between
stoving temperature, curing time and defined quality
requirements.
Experimental section
I. Preparation of the hardener component
Example 1
270 parts by weight of acetophenone oxime were
added at 100~C to 222 parts by weight of IPDI at a rate such
that the temperature did not rise
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above 120~C. After the end of the addition of acetophenone oxime, the
reaction mixture was held at 120~C for 0 5 h more in order to complete the
reaction.
free NC0 content: 0.2 %
5 total NC0 content: 16 7 %
melting point: 55 - 58~C
Example 2
106 parts by weight of diethylene glycol were added dropwise over the
course of about 1 h at 80~C to 444 parts by weight of IPC?I. The reaction
10 mixture was then heated further at 80~C until the NC0 content had reached
15 3 %. Then 270 parts by weight of acetophenone oxime were added in
portions at 110CC and after the end of the addition of acetopnenone oxime!
the reaction mixture was heated further at 120~C until the NC0 content had
reached 0 2 %
15 free NC0 content: 0 2 %
total NC0 content: 10.1 %
melting point: 77 - 83~C
The Examples listed in Table 1 below were prepared in analogy to the
process described in Examples 1 and 2.
- 7 - o. z . 5094
Table 1:
Example Composition of the blocked isocyanate FreeTotal m.p.
No. NCO NCO
Mol of Diisocyanate Mol of chain extender Mol of acetophe- [~C
none oxlme
3 41PDI 1 ethyleneglycol 6 0.1 14.170-75
4 41P[)I 1 1,4-di(hydroxymethy!)cyciohexane 6 0.2 13.576-81
3 IPDI 1 trimethylolpropane (TMP) 3 0.1 13.7133 -138 D
6 6 IPDI 1 TMP 9 0.1 13.984 - 90
7 5 IPDI 1 triethanolamine 7 0.3 13.286 - 91 r
8 3 IPDI 1 tris(hydroxyethyl) isocyanurate 3 0.2 9.3148 -154
9 4 HMDI 1 TMP 5 0.1 11.1106 -111 ~1
1 HXDI - 2 0.1 17.766-71
HMDI: 4,4'-Methylenebiscyclohexyl isocyanate
HXDI: 1,4-bis(isocyanatomethyl)cyclohexane
CA 02213547 1997-08-21
- 8 - O.Z. 5094
Example 1 1
a) 3 parts by weight of DABC0 TMR were added with intense stirring at
80~C to 1000 parts by weight of iPI~I. After an induction period of
about 1 minute the terrlper2ture of the reaction mixture rose to 136~C
s over the course of about 3 minutes. When the temperature maximum
has been reached the reaction is at an end. Further heating at 1 30~C
brings no change in the NC0 content. After it has cooled to room
temperature the reaction mixture had an NC0 content of 26 %.
b) 836 parts by weight of acetophenone oxime were added in portions
at a rate such that the temperature of the reaction mixture did not rise
above 120~C to 1000 parts by weight of the partially trimerized IPDI
with an NC0 c~ntent of 26 % described under a). After the end of the
addition of acetophenone oxime the reaction mixture was heated
further at 120~C for about 0 5 h to complete the reaction.
free NCO content: 0.2 %
total NC0 content: 13.8 %
melting point: 88 - 91 ~C
Example 12 (Comparison example)
1000 parts by weight of the partially trimerized IPDI described under 11 a
were reaG.ed with 538 parts by weight of MEK oxime in analogy to 11 b.
free NC0 content: 0.1 %
total NC0 content: 16.7 %
melting point: 53 - 61 ~C
Example 13 (ComParison example)
2 5 700 parts by weight of c~aprolactam were added in portions at a rate such
that the temperature of the reaction mixture did not rise above 130~C and at
CA 02213547 1997-08-21
_ 9 _ o. z . 5094
120~C to 1000 parts by weight of the partially trimerized IPDI with an NCO
content of 26 % described under 11 a. After the end of the addition of
c-caprolactam, the reaction mixture w, s heated further at 120~C for about
2 h to complete the reaction.
free NCO content: 0.5 %
total NCO content: 15 0 %
rnelting point: 86 - 89~C
CA 02213547 1997-08-21
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CA 02213547 1997-08-21
- 11 - O.Z. 5094
C Pol~furethane powd~r cQatEnqs
General preparation procedure
The comminuted products - blocked polyisocyanates ~crosslinkers),
polyesters, leveling-agent masterbatch and, if appropriate, catalyst
5 rnasterbatch - are intimately mixed, together if appropriate with the white
pigment, in an edge runner mill and the mixture is then homogenized in an
extruder at up to 13û~C. After it has cooled, the extrudate is fractionated and
milled using a pin mill to a particle size < 100 ,um. The powder thus prepared
is applied using an electrostatic powder spraying unit at 60 kV to degreased,
10 optionally pretreated iron panels, which are stoved in a convection oven at
temperatures between 160 and 180CC.
Le~elin~-a~ent masterbatch
10 % of ~he leveling agent - a commercial copolymer of butyl acrylate and
2-ethylhexyl acrylate - is homogenized in the melt in the corresponding
15 polyester, and the solidified melt is cornminuted.
C~talyst masterbatch
5 % by weight of the catalyst - DBTL - is homogenized in the melt in the
corresponding polyester, and the solidified melt is comminuted.
The abbreviations in the tables below have the foilowing meanings:
LT = Layer thiekness in ~Im
El = Erichsen indentation in mm ~DIN 53156)
CH = Crosshatch test (DIN 53151)
GG 60~ ~ = Gardner gloss (ASTM-D 5233)
Irnp. rev. = Impact reverse in g m
HK = Konig hardness in sec (DIN 53157)
CA 02213547 1997-08-21
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- 13 - o. z . 5094
C 2 Transparent powder coatinqs
The transparent powder coatings as well were prepared, applied and stoved at between 150 and 1 70~C in accordance with
the process described.
Table 4:
Example C 1 2 3 4 5 6 r~ 8~') g 10~ D
Formulation ~
Crosslinker~romAExample() 23.5523.8014.3117.29 16.362394 23.94 20.11 13.59 13.59 ~"
(3) (6) (6) (9) (9) (1~ 11 ) r
Polyester as per B 1 76.4576.20 - - - 76.0676.0679.89 - -
Polyesteras per B 2 - - 85.6982.71 - - - - - -
Polyester as per B 3 - - - - 83.64 - - - 86.41 86.41
Notes All ~ormulations containec 0.5 % by weight leveling agent and 0.3 "~ by weight benzoin;
xj 0.1 % by weight DBTL; OH/NCO ratio is 1 : l, o) 1:0.8
Coatings data
LT 44-57 61-69 50-6054-63 57-62 46-5743-55 58-67 56-61 51-60
HK 186 199 198 187 194 189 196 191 188 191
CH 0 0 0 0 0 0 0 0 0
El > 10 > 10 > 10~ 10 ~ 10 9/10> 10 > 10 > 10 > 10
Notes Curing conditions: 170''C/10 - 12', 160~C/15 - 17', 150~CI25 - 30'
- CA 022l3547 l997-08-2l
- 14 - O.Z. 5094
C 3 PU powder coatin~s - Comparison examples
Example 1
Owing to the low melting range (53 - 61 ~C) the crosslirlker of Example A 12
is in principle unsuitable for preparing transparent and pigmented powder
s coatings in accordance with conventional powder-coating technology. The
cross!inker itself bakes during storage, and does therefore not have
adequate storage stability at room temperature.
Ready-to-spray powder coatings prepared therefrom do not comply with DIN
Standard 5599~1 part 7 - "Determination of blocking resistance". Moreover,
10 the coatings have a tendency varying from foaming to pinholing; in other
words, the surface of the coating is not flawless. The yellowing of the
transp~rent and pigmented coating films was at a relatively high level.
Example 2
c-caprolactam-blocked polyisocyanates, i.e. including the crosslinker of
15 Example A 13, are less reactive; in other words, the curing conditions
required for such coatin~s involve either higher temperatures or longer
baking times, eg. 200CC/12 - 15', 180~CJ15 - 20', 170~C/25 - 30' for
pigmented coating and 200~C/8 - 10', 180~C/12 - 15', 170~C/18 - 22',
160CC/25 - 30' for transparent coating.
