Note: Descriptions are shown in the official language in which they were submitted.
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O.Z. 0062/02112
Heat curable cathodic electrocoatina composition
The present invention relates to a heat curable
coating composition for cathodic electrocoating ~hich is
water thinnable on protonation with an acid and contains
a chain or step growth polymerization product as binder
and a mixture of blocked polymethylenepolyphenyl
isocyanates as crosslinking agent.
Polymeric binders with free OH and/or free NH
groups can be crosslinked with isocyanato-containing
components at above 100C. Since isocyanate groups react
even at low temperatures, they are usually blocked with
reactive, low molecular weight compounds. The blocking
agent used can be any kind of compound containing OH, NH
or acidic CH (see Progres~ Org. Coatings 9 (1981), 3-28).
At higher temperatures the reaction products re-
eliminate the isocyanate groups; the isocyanate groups
can then react with OH- and NH containing binders with
crosslinking. The equilibxium shifts toward crosslinking
(transurethanization) as a result of the evaporation of
the low molecular weight blocking component.
To make it possible for polyisocyanates to be
handled safely, a small increase in the molecular weight
is produced in a preliminary stage. For instance, a
diisocyanate can be reacted in a first stage with a
triol; this produces under suitable reaction conditions
a trimerized i~ocyanate havinq a distinctly lower vapor
pressure and reduced skin absorption and also a favorable
crosslinking trifunctionality. However, this prelLminary
reaction of the diisocyanate makes industrial production
of electrocoatings costly.
US-A-4 296 010 describes coating compositions
comprising a binder and a crosslinking agent based on
diphenylmethane 4,4'-diisocyanate (MDI) or a mixture of
about 50 % each of diphenylmethane 4,4~-diisocyanate and
polymethylenepolyphenyl isocyanate (crude MDI~. It is
true that such coating compositions produce nonyPllowing
coatings, but their storage life is not satisfactory in
that the dispersion of the coating composition will
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gradually form a sediment. According to EP-A-256 050,
this problem disappears on using as crosslinker a blocked
polymethylenepolyphenyl isocyanate containing more than
5 % of diphenylmethane 2,4-diisocyanate. ~he isocyanate
5mixture can contain up to 75 %, preferably 10-50 %, of
polyisocyanates with three or more rings. It has now been
found that such electrocoating compositions likewise give
rise to sediments in the bath.
It is an object of the present invention to
provide crosslinkers for cathodic electrocoating which
- have a high content of aromatic structures in order
to ensllre good corrosion protection,
- despite the aromatic structures do not cause white
topcoats to yellow,
15 - deblock at a low baking temperature and then exhibit
high reactivity,
- despite the low deblocking temperature form stable
formulations or electrocoating baths,
- have no crystallization tendency in order that
gelling and sedimenta~ion may be effectively sup-
pressed, and
- are simple and inexpensive ts produce from in-
expen~ive, commercially available raw materials.
~e have found that this object is achieved when
the crosslinker is a mixture of blocked polymethylene-
polyphenylisocyanates which contains less than 25 % by
weight of diphenylmethane diisocyanates.
The present invention accordingly provides a heat
curable coating composition for cathodic electrocoating,
water thinnable on protonation with an acid, comprising
(A) from 50 to 95 % by weight of a chain or step growth
polymerization product having functional groups and
(B) from 50 to 5 % by weight of a blocked polymethylene-
polyphenyl isocyanate as crosslinker, wherein the
35crosslinker comprises a mixture of blocked
polyisocyanates of the formula
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OC~-ECH~ ~--NCO (I)
OCN n
where n is from 0 to 10, but contains less than 25 % of
diphenylmethane diisocyanates where n=0.
Preferably, the polyisocyanate mixture according
to the present invention contains from 5 to 20 %, in
particular from 12 to 18 ~, of diphenylmethane di-
isocyanates, the diphenylmethane 2,4'-diisocyanate
content prPferably being less than 5 ~, in particular
less than 2 %. The viscosity of the polyisocyanate
mixture at 35C is preferably from 2,500 to 8,000, in
particular from 3,500 to 6,000 [mPa.s].
The polyisocyanate mixture with less than 25 % of
diphenylmethane diisocyanates can be prepared by a
distillation of crude MDI in which preferably the di-
phenylmethane 4,4'-diisocyanate i9 distilled off and the
higher molecular weight portions axe concentrated. The
diphenylmethane diisocyanate content of the isocyanate
mixture can be determined in a conventional manner, for
example by column chromatography or by HPLC.
The polyisocyanate mixture according to the
present invention is poIyfunctional even without tri-
marization; that is, correctly put together, the mix~ure
will have a functionality of greater than 3Ø Conse-
quently, the preliminary txLmerization can be dispensed
wi~h; this reduces production costs.
The polyisocyanates of the structure II) react
with any blocking componen~ known to those skilled in the
art, so that a wide range of different crosslinkers can
be ~yn~hesized. For instance, using alcohols it is
possible to prepare crosslinXers having deblocking
temperatures of around 160C; amines will give blocked
crosslinkers for a baking temperature of around 130C.
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The polyisocyanates are liquid even at low
tPmper~tures and have no tendency to crystallize. Simi-
larly, the reaction products with the blocking components
are stable and make it possible to formulate nongelling
coatings and stable electrocoating baths.
Even at high baking temperatures the crosslinkers
do not cause white topcoats to yellow.
The crosslinkers require distinctly less solvent
in their synthesis than the prior art. Consequently the
solvent content of the coating compositions formulated
therewith can be reduced.
By partial blocking with tertiary amines and
subsequent quaternization thereof it is possible ~o
produce cros~linker dispersions of high stability.
In the electrocoating bath, the polyisocyanates
are present in the blocked state. They are blocked by
reacting the polyisocyante mixture with a stoichiometric
amount of blocking agent corresponding to the isocyanate
content. Depending on the blocking component the reaction
temperatures range from 20 to 60C. The reaction times
range from two to three hours until a residual isocyanate
value of 0 is reached.
The reaction can be carried out using a certain
amount of solvent depending on the type of blocking
agent; it is an advan~aye to use no sol~ent at all. It is
advisable to add small amounts of a low molecular weight
alcohol toward the end of the reaction.
Suitable blocking agents are-
- alcohols: primary, secondary or tertiary;
preferably alkylene glycol monoethers or
polyalkylene glycol monoethers;
- amines: primary and secondary; ~ diamines with
disubstitution on one of the nitrogens;
alkoxyamines; trishydroxyalkylamines;
preferably aliphatic amines;
- oximes;
- hydroxyimides;
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- heterocyclic compounds such as triazoles, Lmida-
zoles, imidazolines, and so on;
- lactams;
- phenols;
- active methylene derivatives such as ethyl acetate,
malonic acid, enamines;
Particular preference is given to:
ethylene glycol monopropyl ether (PG),
diethylene glycol monobutyl ether (BDG),
diethylene glycol monohexyl ether,
triethylene glycol monomethyl ether,
dipropylene glycol monoisopropyl ether,
dibutylamine,
diallylamine,
N-ethyl-N-phenylamine,
di[2-methoxyethyl]amine,
N,N-dimethyl-1,3-propylenediamine,
methylethanolamine, diethanolamine,
methyl ethyl ketoxime,
2a 3,4-dimethylphenol.
As component tA) it is possible to use synthetic
resin binders known in the art with primary and/or
secondary hydroxyl groups and/or primary, secondary
: and/or tertiary amino groups, preferably with an average
molecular weight M~ of from 500 to 20,000, such as amino-
epoxy resins, amino-poly(meth)acrylate resins and/or
amino-polyurethane resins with an amin~ number of from
30 to 150. The use of amino-epoxy resins is preferred or
basecoats which are to give a high corrosion protection
level. The synthetic resin binder contains at least one
amino group per molecule. The lower limit for the amina
number should be 45, preferably 70, and the upper limit
should be 120, preferably 100. Examples of amino-epoxy
resins are reac~ion products of epoxy-containiny resins
with preferably terminal epoxy groups with saturated
and/or unsaturated secondary and/or primary amines or
amino alcohols. These reaction products may be modified
at the alkyl moiety by at leas~ one primary and/or
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secondary hydroxyl group, by the mono- or dial~ylamino
group and/or by a primary amino group temporarily protec-
ted by ketiminization.
As epoxy resins it is possible to use any desired
material, provided it has an average molecular weight of
from 300 to 60,000 and contains on average from 1.0 to
3.0 epoxy groups per molecule, preferably compounds
having two epoxy groups per molecule. Preference is given
to epoxy resins having average molecular weights of from
350 to 5,000, in particular from 350 to 2,000. Particu-
larly preferred epoxy resin~ are for example glycidyl
ethers of polyphenols containing on average at leaqt two
phenolic hydroxyl groups in the molecule and preparable
in a conventional manner by etherification with an
epihalohydrin in the preqence of alkali. Aromatic poly-
epoxides having a high epoxy equivalent weight can be
prepared from those having lower epoxy equivalent weight
and polyphenols.
The introduction of amino groups can be effected
in one of the usual reactions as known to the person
skillad in the art and as described for example in
EP 134 983, EP 165 556 or EP 166 314.
As well as the abovementioned components it is
possible for further substances to be added such as
pigments, coating assistants, solvents and hardener
catalysts. The coating compositions thus prepared can be
applied to substrates such as wood, plastic or m~tal in
a conventional manner. For cathodic electrocoating, the
synthetic resin is converted into a water-soluble form,
together with the additives mentioned, by protonation
with an acid. Preferred acids ~re carboxylic acids such
as formic acid, acetic acid or lactic acid, but it is
also possible to use organic acids, for example
phosphoric acid. Subsequently this dispersion is admixed
with a dispersion of the crosslinker in the desired
ratio. It is of course also possibl~ to add the additives
mentioned to ~he crosslinker and then to disperse the
mixture.
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A cathodic electrocoating bath is in general
adjusted to a solids content of from 5 to 30 % by weight.
Deposition customarily takes place at from 15 ~o
40C in the course of from 0.5 to 5 min and at a pH
within the range of from 4.0 to 8.5, preferably at a
neutral pH, and at a voltage of from 50 to 500 V. The
electrically conducting object to be coated is connected
as the cathode at the time. After a rinse, the deposited
film is cured at above 100C (object temperature) for
20 min.
EXAMPLES
Preparation of binder dispersion A
1805 g of a liquid epoxy resin having an epoxy
equivalent weight of 188 are mixed in a 5-l stirred flask
with 450 g of p-nonylphenol, 63 g of xylene and 7 g of
dimethylbenzylamine and heated to 130C. ~hen an epoxy
equivalent weight o 460 has been reached, 440 g of
xylene are added; then the mixture is cooled down to
80C. A mixture of 126 g of diethanolamine and 90 g of
N-methylethanolamine are added dropwise. After stirring
for one hour at 80C a further 73 g of ethanolamine are
added dropwise. After two hours' stirring at ~0C the
mixture is diluted with 127 g o$ hexylglycol. The solids
content is ~0 ~ and the molecular weight M~ is 3025
(measured by gel permeation chromatography) coupled with
a polydisper3ity of 1.65.
Prepara~ion o~ crosslinker dispersion B
A polyisocyanate mixture is used comprising
14.9 ~ of diphenylmethane 4,4' diisocyanate, 1.0 ~ of
diphenylmethane 2,4'-diisocyanate, and 84.1 ~ of tri-
cyclic or higher polyisocyanates having a viscosity at
25C of 7070 mPa.s and an isocyanate value of 30.6 %.
343 g of this polyisocyanate mixture are dissolved in
169 g of methyl ethyl ketone. 332.5 g of di(2-methoxy-
ethyl)amine are added at room temperature in the courss
of half an hour. The temperature rises all the while to
56~C. After 20 minutes' stlrring 195 g of isobutanol are
metered in; the mixture is then cooled down. The solids
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content is 63.1 %. The 55% solution in 1-metho~y-2-
propanol (Solvenon PM) has a viscosity of 102 mPa.s at
25C.
Preparation of pigment paste
660.8 g of epoxy resin EPON 828 (from SHELL),
260.6 g of bisphenol A and 61.5 g of dodecylphenol are
mixed and heated to 110C until a clear solution has
formed. Then 0.98 g of ethyltriphenylphosphonium iodide
is added, whereupon the temperature rises to 150C. After
the exothermic reaction has died down, the mixture is
maintained at 130C for 90 min. It is then diluted with
513.5 g of 2-butoxyethanol and cooled to 80C, at which
point 244.2 g of thiodiethanol (50 % active) are added
dropwise in the course of 30 min. Then 134.1 g of di-
methylolpropionic acid and 30.6 g of water are added.
98 g of this resin are made into a paste with 175 g of
titanium dioxide, 8 g of lead silicate, 35 g of aluminum
silicate, 11 g of dibutyltin oxide, 3.5 g of pearl black
and 169.5 g of water.
Preparation of electrocoating bath:
508.2 g of binder A are mixed with 68 g of
polypropylene glycol phenyl ether and 273.6 g of cross-
linker B, and then 15 g of glacial acetic acid and 593 g
of water are added. 590 g of this water-solvent mixture
are distilled off under reduced pressure at 45~C while at
the same time a further 975 g of water are added. The
result is an aqueous secondary dispersion having a solids
content of 30 %. The bath has a storage life of more than
6 weeks; no separation or crystallization occurs.
Electrocoating
The bath described i5 used for coating a phospha-
tized steel panel. ~he d~position voltage is 360 volts
and the tear-off voltage is 380 volts. The result is a
22.5 ~m thick film which can be baked at 140C to form a
coating which is free of any yellowing.
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