Note: Descriptions are shown in the official language in which they were submitted.
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An aqueous two-component coating composition and the use
thereof
This invention relates to aqueous coating compositions
5 based on polyols, in particular for coating plastic
substrates, and to the use thereof.
Ever increasing quantities of mouldings made from plastic
substrates, in particular from polyolefins, for example
10 polypropylene in pure form or in modified form (PP blend),
are being used in the automotive industry due to the
excellent overall properties thereof in comparison with
other plastics. The elevated content of organic solvents is
increasingly being reduced by the development of water-
15 based coating systems.
Aqueous coating compositions based on epoxy resins oracrylate resins as two-component systems are thus known
from DE-A 41 23 860 or EP-A 0 358 979. However, adequate
2G adhesion to the substrate is not achievable when coating
plastic substrates, in particular polyolefin substrates
which have not been pretreated. Moreover, defects such as
pinholing occur in relatively thick films.
DE-A-39 10 901 describes aqueous coating compositions for
plastic components, the compositions containing aqueous
emulsions of acrylate resins or polyurethane resins,
chlorinated polyolefins optionally together with pigments
or additives. Production of these coating compositions is
highly complex due to a special melting process for the
chlorinated polyolefins and because of an azeotropic
distillation stage for the organic solvent.
According to EP-A-0 539 710, aqueous keying primer
compositions based on chlorinated polyolefins, organic
solvent, emulsifiers and pigments and optionally a film
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forming binder may be used for lacquer coating plastics.
Application of lacquers containing solvent onto such
priming layers, may cause the primer to dissolve, so
resulting in lacquer faults.
EP-A-0 587 061 (H 32 088) describes an aqueous coating
composition based on polyols, which composition moreover
contains chlorinated polyolefins, organic solvent and
optionally further binders, together with polyisocyanates.
The composition may be used on plastic substrates as a
primer, for single-layer lacquer coatings or as a base
lacquer without prior priming. Usable pigments are titanium
dioxide, talcum and carbon black. The coating composition
provides good adhesion, in particular to polyolefin plastic
substrates, together with a smooth, faultless surface.
Subsequent layers may also be applied in the conventional
manner, such as for example by spraying, roller
application, dipping. The coating composition is, however,
not suitable for electrostatic application of subsequent
layers.
The surface conductivity required for subsequent
electrostatic coating layers may be achieved by using
electrically conductive primers.
These contain, for example, electrically conductive carbon
black or graphite (c.f. Glasurit-Handbuch from BASF Lacke +
Farben AG, 1984, pages 606 to 607). The use of such
conductive lacquers is, however, disadvantageous, due to
the elevated organic solvent content thereof.
The object of the present invention is to provide a coating
composition for polyolefin substrates, in particular
polypropylene blends, in order to overcome the stated
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technical disadvantages, which composition is aqueous and,
in addition to providing excellent adhesion, allows
electrostatic application of subsequent layers. A further
object is to reduce the use of chlorinated polyolefins.
This object is achieved by a coating composition based on
crosslinking, water-borne two-component resin systems which
contain chlorinated and/or non-chlorinated polyolefins and
small proportions of organic solvent.
The coating composition according to the invention is
characterised in that it contains
10 to 40 wt.% of one or more polyols having a number
average molecular weight of 300 to 2000000,
an OH value of 10 to 400 mg of KOH/g and a
content of ionic groups and/or groups
convertible into ionic groups of up to 400
milliequivalents/100 g of solid resin,~0 0.5 to 10 wt.% of one or more chlorinated and/or non-
chlorinated polyolefins,
1 to 35 wt.% of electrically conductive pigments and/or
fillers,
5 to 30 wt.% of one or more organic solvents~5 25 to 75 wt.% of water and
1 to 30 wt.% of polyisocyanates having on average at
least two free isocyanate groups per
molecule, wherein the sum of the stated
constituents is 100 wt.% and the quantity
ratio between the polyisocyanate component
and the polyol component is selected such
that the number of reactive NCO groups to
the number of OH groups is in the ratio 5:1
to 0.5:1.
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Conventional lacquer additives, further pigments and/or
fillers, optionally together with further binders, may also
be present. In relation to the resins, the above-stated
weight percentages relate to resin solids. Water is the
principal solvent.
The coating composition should take the form of a two-
component coating composition. One component contains the
polyols and the other component the polyisocyanates. The
polyol component is in water-borne form and the
polyisocyanate component is dispersible in water together
with the polyol component. The polyolefins, solvents,
optional further binders, pigments, fillers and other
lacquer additives may be present in either one or in both
of the two components. Care must be taken to ensure the
storage stability of the individual components.
The viscosity of the ready-to-apply coating composition may
be adjusted to within the desired range by dilution with
water.
The quantity ratios of the conductive pigments and/or
fillers should be selected such that an adequate subsequent
coat may be applied electrostatically. The proportion of
these pigments is within a range from 1 to 35 wt.%. The
conductive pigments are preferably usable in a quantity of
2 to 30 wt.%
The coating composition according to the invention may
crosslink at temperatures as low as room temperature.
Crosslinking may be accelerated by adding catalysts or
raising the drying temperature. Preferred temperatures are
from 40 to 120~C.
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The polyol components usable according to the invention are
known binders, for example binders based on free-radically
polymerisable monomers containing OH groups, polyesters
containing OH groups and/or polyurethanes containing OH
groups. The number average molecular weight is 300 to 2x106,
the hydroxyl value 10 to 400. Ionic, nonionic groups or
groups convertible into ionic groups may also be present in
addition to the OH groups, ester groups, urethane groups.
Such groups may be substituents which are cationic or
convertible into cationic groups, for example amino groups.
Particularly suitable, however, are those substituents
which are anionic or convertible into anionic groups, for
example carboxyl groups, phosphoric acid groups together
with sulphonic acid groups. The content of ionic groups is
400 milliequivalents/100 g of solid resin. The polyol
component is in the form of an aqueous dispersion.
Examples of polymers based on free-rad cally polymerlsable
monomers are described in EP-A-0 358 979. These may be, for
example, styrenes, (meth)acrylic acid alkyl esters,
hydroxyalkyl esters of (meth)acrylic acid, monomers
containing epoxy or amide groups, such as for example
glycidyl (meth)acrylate or N-methoxymethyl
(meth)acrylamide. The ionic groups may be introduced by
means of olefinically unsaturated monomers having
functional groups convertible into ionic groups, for
example olefinically unsaturated mono- or dicarboxylic
acids, such as for example acrylic acid, maleic acid,
itaconic acid and optionally semi-esters thereof or by
means of compounds such as 2-acrylamido-2-methylpropane-
sulphonic acid. The polymers are produced using known
processes, for example by solution polymerisation.
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Water-dispersible polyurethane resins are described, for
example, in DE-A 41 24 453 and DE-A-40 00 889. The
polyurethane resins are produced, for example, by reacting
di- or polyhydric saturated linear or branched aliphatic or
cycloaliphatic polyalcohols with linear or branched
aliphatic, cycloaliphatic or aromatic polyisocyanates and
optionally linear or branched aliphatic or cycloaliphatic
monoalcohols.
Ionic groups are introduced, for example, by reacting
proportions of low molecular weight dialcohols which
contain a group which is anionic or capable of forming an
anion, preferably a carboxyl group.
The polyurethane polyols preferably have an OH value of 20
to 250. The acid value is 50 to 150, preferably 10 to 60.
The number average molecular weight is preferably 2000 to
50000.
Polyester polyols usable according to the invention are,
for example, the polyesters described in DE-A-32 13 160,
DE-A-28 24 418 and US-A-3 053 783. These may be linear or
branched oil-free polyesters based on di- or polyhydric
linear or branched aliphatic or cycloaliphatic saturated
polyalcohols, together with linear or branched aliphatic,
cycloaliphatic or aromatic di- or polybasic carboxylic
acids, which may optionally be polycondensed with linear or
branched aliphatic monoalcohols.
Usable alcohols preferably contain 2 to 21 C atoms, for
example hexanediol, neopentyl glycol, 2,2,4-trimethyl-
1,3-pentanediol. The di- or polybasic carboxylic acids
preferably contain 5 to 10 carbon atoms, for example
isophthalic acid, terephthalic acid, 1,3-cyclohexane-
dicarboxylic acid or butylisophthalic acid. Anionic groups
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may additionally be incorporated by reaction with low
molecular weight dialcohols which additionally contain acid
groups capable of forming an anion.
The molecular weight of the polyesters may also be
increased by reaction with diisocyanates. Further
modifications may optionally be introduced by means of
reactive groups, for example OH groups.
The polyester polyols usable according to the invention may
advantageously have a molecular weight of 500 to 50000,
preferably of 2000 to 50000, particularly preferably of
1000 to 15000 and very particularly preferably of 3000 to
15000.
The OH value is 20 to 200, the acid value 10 to 150.
The polyester polyols are produced using known processes,
for example azeotropically or in a melt by a stepwise
reaction. Once the desired final values have been achieved,
the polyester may optionally be diluted with solvents to
achieve a good application viscosity.
The properties of the polyesters may be influenced by means
of the dicarboxylic acids or polyalcohols which may be
used, for example flexibility may be increased with longer
chain aliphatic alcohols or elasticity reduced by
introducing aromatic dicarboxylic acids.
Once a proportion of the ionic groups present have been
neutralised, the polyols usable according to the invention
may be converted into the aqueous phase without further
emulsifiers. As a result of the production process, they
may still contain small residues of organic solvent. It is,
however, also possible to add emulsifiers, wherein the
added quantity should be as small as possible.
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The polyisocyanates usable according to the invention are
conventional, optionally hydrophilised and/or partially
blocked lacquer polyisocyanates. They have on average at
least two free isocyanate groups per molecule. Such
polyisocyanates are described, for example, in EP-A-
0 358 979. These are organic polyisocyanates, for example
diisocyanates having aliphatically, cycloaliphatically
and/or aromatically attached free isocyanate groups. Under
certain circumstances, various polyisocyanates may be mixed
together. A content of inert solvents may furthermore be
added to adjust viscosity. Examples of usable
polyisocyanates are known, conventional lacquer
polyisocyanates, such as for example hexamethylene
diisocyanate, isophorone diisocyanate, tetramethylxylylene
diisocyanate.
These may be converted by known processes into oligomers
having, for example, biuret, urethane, uretidione or
isocyanurate groups.
The composition according to the invention may contain
chlorinated and/or non-chlorinated polyolefins.
Chlorinated polyolefins which may be used are conventional
commercial materials either individually or as a mixture.
These materials may in particular be chlorinated
polyethylene, chlorinated polypropylene or chlorinated
copolymers thereof having a degree of chlorination of
preferably 10 to 45%. The number average molecular weight
of the chlorinated polyolefins is preferably 700 to 70000.
The chlorinated polyolefins may be used in modified form,
for example by incorporation of polar groups, such as for
example maleic anhydride. They may be used as resin powder
dissolved in an organic solvent or as an aqueous suspension
or emulsion.
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Alternatively, the content of chlorinated polyolefins may
be reduced by using non-chlorinated polyolefins, for
example according to DE 43 08 349 and JP 03 122 125 or
polyolefin/acrylate copolymers according to DE 44 32 985
having an average molecular weight of 1000 to 50000.
These polymers may optionally also be modified, for example
by incorporating polar groups, such as for example maleic
anhydride and acrylic acid.
The non-chlorinated polyolefins may be used alone or as a
mixture with the chlorinated polyolefins.
The chlorinated and non-chlorinated polyolefins may be used
as an aqueous dispersion. As a result of the production
process, these may still contain small proportions of
organic solvents, together with proportions of ionic or
nonionic emulsifiers. The polyolefin fraction may, for
example, be added to the coating composition in the form of
a dispersion in a similar manner to WO 93/01 244 or WO
90/12 056.
Suitable organic solvents are, for example, conventional
lacquer solvents such as ketones, hydrocarbons, alcohols,
glycol ethers, such as for example xylene, toluene,
mesitylene, benzyl alcohol. The quantity of organic
solvents should be kept as small as possible.
The coating compositions according to the invention contain
electrically conductive pigments and/or fillers. These
pigments or fillers may be inorganic or organic.
Commercially available opaque and transparent electrically
conductive pigments and/or fillers, as are known, for
example, for imparting anti-static properties to polymeric
coating materials for appliances, surfaces and components,
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may be used. These may be conductively coated TiO2, barium
sulphate, doped tin dioxide, doped zinc oxide (doped, for
example, with aluminium, gallium, antimony and bismuth),
conductively coated potassium titanate as well as
conductive grades of carbon black.
Once the coating composition according to the invention has
been applied to the substrate, these substances may produce
black to white colours, for example if grades of carbon
black or titanium dioxide are used.
Further colours may optionally be achieved by incorporating
coloured pigments. Light colours are preferred.
The pigments are preferably ground directly into the polyol
component. It is optionally possible, in particular in the
case of poorly dispersible pigments, such as for example
grades of carbon black, to use a pigment paste using
additional binders (grinding resins), for example those
based on polyurethane, polyacrylate, polyether and
polyester resins. The pigments are finely dispersed
therein. The apparatus and methods necessary for this
purpose are familiar to the person skilled in the art.
Conductive pigments may be present in the composition
according to the invention in a quantity of 1 to 35 wt.%.
~referably, 1 to 5 wt.% of dark to black pigments and 5 to
25 wt.~ of grey to light pigments are used.
The coating compositions may optionally contain further
pigments and additives. Usable additives are conventional
lacquer additives, such as for example wetting agents,
dispersion auxiliaries, anti-foaming agents, flow
promoters, catalysts, rheological additives together with
anti-cratering agents.
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The coating compositions according to the invention may
optionally additionally contain proportions of further
binders which preferably react with the crosslinking
components. These may be, for example, aqueous dispersions
of polyurethane, polyester, corresponding copolymers or
aqueous dispersions prepared from mixtures of such resins,
which are preferably mixed into the polyol component.
The coating compositions according to the invention are
produced by converting the film-forming resins into the
aqueous phase. The materials may be entirely or partially
neutralised in order to obtain a stable aqueous emulsion.
The polyolefin preparation is preferably present in the
binder component, optionally together with the further non-
reactive, water-dilutable binders. Additives and/or
pigments or fillers may additionally be present in this
component.
The second component contains the polyisocyanates. Under
certain circumstances, further additives (for example anti-
foaming agents) may be present in this constituent. A
proportion of the polyolefin preparation may also be
incorporated.
The polyisocyanate component is dispersed in the aqueous
dispersion of the binder component. The coating composition
according to the invention may be adjusted to a suitable
application viscosity with water.
Suitable substrates are preferably plastic substrates, in
particular modified or unmodified polyolefins. Examples of
these are in particular polyethylene or polypropylene
substrates together with substrates made from copolymers or
mixtures thereof, for example PP/EPDM blends.
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The coating composition according to the invention is
applied onto the substrate which has preferably not been
pretreated and has optionally been cleaned. The coating is
then chemically crosslinked, a process which may optionally
be supported by an elevated temperature. The crosslinking
temperature of the coating composition may be selected
depending upon the temperature sensitivity of the substrate
as the coating compositions according to the invention are
capable of crosslinking over a wide temperature range. A
temperature range of 40 to 120~C is preferred.
After crosslinking, homogeneously coated plastic substrates
having a faultless surface are obtained, wherein the
resultant coatings are distinguished by excellent adhesion
to the substrate despite the reduction in CPO content or
the use of non-chlorinated polyolefins and by good low
temperature impact resistance.
The coating compositions according to the invention are in
particular used as electrically conductive keying primers.
Further coatings, for example, base lacquer, clear lacquer
coatings or single-tone topcoat lacquer coatings may then
be applied thereon using electrostatic application methods.
These methods permit a distinct reduction in overspray in
comparison with conventional application methods.
The resultant multi-layer structures exhibit very good
adhesion to the substrate, good low temperature elasticity
and no changes in surface appearance on exposure to
moisture.
