Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02191251 2005-03-17
~ ~~ ~ 2191251
Daim 23 342/FC'~
Daimler-Benz-Aktiengesallschaft
Stuttgart
Effect coating material aa~ effect oat~nQ system.
gs,~eciallv for vcehiele bodices.
y~si nc liauid crystalline ir~ter~ene~ Qiame~.ts
ThQ invention relates to interference pigments
based on liquid-crystalline polymers for coloured
coating materials, useful for painting commodity
articles, especially motor vehicle bodies. Liquid-
crystalline polymers are already known, for example,
from DE 40 0 8 076 A, EP 66 137 A and US-A 5,188,760.
The auctomary automotivt solid-colour paints
generally involvo, incorporated irxto a altar vehicle
comprising synthetic rtsiu, colour pigments which deter
mice the desired coloration of the paint. Tht colour
is
effect of these pigments is based oa a spectrally selec-
five absorption effect, so that a apeetrally broad
traction of the incident - white - light is absorbed by
the pigmeata gad only a spectrally narrow fraction is
reflected. ~-
In the effort for brighter perceived colours,
especially for vehicle bodies, the so-called effect
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finishes have been developed. In the case of one group
thereof, the metallic paints, small metal flakes, inter
alia, as pigments are incorporated by mixing into the
paint vehicle. In the case of another group, coated mica
particles are used as pigments. With these effect fin-
fishes it is possible to achieve a better brightness of
colour than with the solid-colour paints; moreover,
depending on the direction of incidence of light and/or
direction of viewing, there is a slightly altered
perceived colour, which is desirable. EP 383 376 Al
describes such an effect finish, in which small mica
flakes are coated uniformly on all sides with a cross-
linked, liquid-crystalline polymer (LCP) in chiral
nematic arrangement. The coloured appearance of such
pigments comes about by means of an interference pheno-
menon. The only light waves of the incident light which
are reflec=ted are those whose wavelength interferes with
the equidistant interplanar spacings of the liquid-
crystalline polymers, whereas the light fractions of
other wavelengths pass through the transparent body of
the paint and are absorbed by t:he - preferably - dark
substrate. Plateletlike interference pigments of this
kind, aligned parallel to the painted surface, have a
defined first colour - base colour - when viewed
orthogonally and a second, shorter-wave colour when
viewed from an inclined direction. This viewing angle-
dependent coloured appearance of the painted surface
makes the coating system highly effective, and, for
specific applications in which such colour effects are
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perceived positively by the customer, highly desirable.
A disadvantage is the complex preparation of the inter-
ference pigments, making the effect coating material and,
correspondingly, the effect finish on the commodity
article very costly.
The preparation of interference pigments based on
polysiloxanes has become known, confidentially, to the
Applicant, these pigments each consisting wholly of small
fragments of a thin crosslinked film of liquid-crystal-
line polymer. These interference pigments are colourless
and transparently see-through. The colour effect which
can be achieved with them is based on the regular struc-
ture and on the uniform arrangement of the molecules in
the form of a liquid crystal and on the interference,
which can be traced back thereto, of a certain spectral
light fraction for which the pigment has a reflective
action. The other light fractions pass through the
pigment. By this means it is possible to achieve stunning
colour effects of different kinds, depending on the
configuration of the coating system and/or on the inter-
ference pigments and mixing thereof. Aside from the novel
aesthetic colour effect of the coating system and of the
coating material used therefor, this coating system also
offers a series of technical advantages. Owing to their
chemical composition, the interference pigments have a
specific weight which is approximately equal to that of
the vehicle of the pigments and/or the paint base.
Therefore, there is no mass-related separation of pig-
ments and paint base during the storage of the coating
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material or during its application by spraying using a
high-speed rotating bell, as is observed in the case of
conventional painting materials containing absorption
pigments - most based on metal - having high specific
weights. Since, moreover, the entire colour spectrum can
be produced by mixing just a few types of interference
pigments in different ratios at the painting plant,
stockkeeping on site can be limited to a few basic types
of paints, thereby simplifying very considerably the
logistics relative to the various paint colours. The
interference pigments of the type discussed are obtained
by knife-coating the liquid-crystalline polymers from the
liquid state onto a smooth substrate, for example on to
a polished roller, thereby forming a thin film. As a
result of the knife-coating procedure there is an align-
went of the molecules within the film, with the alignment
being better the thinner the film; only after this
orientation does the film exhibit an interference colour.
Owing to the monomer architecture of the molecules of the
liquid-crystalline polymers, equal interplanar spacings
are automatically established as, consequently, are
diffraction structures having a colour-selective effect,
in the shearing process in the course of knife-coating.
The effective interference colour of the pig-
ments, which are transparent and colourless per se, can
be formed by adapting, in the smectic or cholesteric
phases of the interference pigments, the equidistantly
spaced planes, in terms of their interplanar spacings, to
the wavelength of a particular coloration of the colour-
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defining coating layer, which can be achieved by suitable
chemical structuring measures in respect of the molecular
architecture. The Applicant has been able to prepare
interference pigments having the basic colour red and
others with the basic colour green. Advantageously it is
possible to produce intermediate colours by mixing
different interference pigments, together, into a base
coating material, the colouring depending on the mixing
ratio of the different interference pigments. Since this
mixing ratio is infinitely variable, it is also possible
to set, infinitely, all colour graduations for the basic
shade
The basic shade of the interference pigments is
determined by the perceived colour, or the colour which
is established when the painted surface is viewed perpen-
dicularly under perpendicular illumination. Since, with
a beam path directed diagonally to the surface, the
interplanar spacings - as a result of geometry - appear
altered relative to the orthogonal beam direction, the
perceived colour shifts towards a different colour, which
in the colour spectrum is displaced in the direction of
shorter wavelengths, depending on the relative direction
of viewing of the surface. In other words, depending on
the position of a certain part of the surface relative to
the beam path of the viewer, that part of the surface
appears in the basic colour or in the different, shorter-
wave colour. Interference pigments of basic colour red,
for example, can "flip" into the colour green.; with other
interference pigments, a flip in colour ("colour flop")
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between green and blue can be produced.
The intensity of the colours which can be per-
ceived in this context is all the greater the darker the
colour of the substrate which carries the colour-defining
coating layer, although the shade of this substrate must
be determined by colour pigments having an absorptive
effect. The basis for this, in fact, is that the light
fractions passing through the interference pigments are
absorbed more or less completely by the dark substrate,
and are absorbed all the more the darker the substrate.
The lighter (in colour) the substrate, the greater the
light fraction of the non-interfering light which is
likewise reflected back from the substrate and is super-
imposed on the fraction of light thrown back by the
interference pigments, so that the colour intensity of
the latter appears paler. The lighter, therefore, the
substrate;=-the lower the colour intensities of the basic
colour and of the flop colour. This goes so far that, on
chromium-plated bright components, for example, it is
absolutely impossible to achieve any colour effect, since
on a mirror-like substrate there is no spectral shift in
intensity in the light.
One condition for the occurrence of liquid-
crystalline phases is a rigid mesogenic molecular struc-
ture. The repeating units of liquid-crystalline polymers
comprise the mesogenic units. The two most frequently
realized molecular structures are the side-chain LC
polymers, in which the mesogenic units are chemically
fixed as side chains on the polymer backbone, and the
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main-chain LC polymers, in which the mesogenic units form
the polymer backbone, or part of the polymer backbone. In
addition to the homopolymers, it is possible to prepare
a large number of copolymers which may contain different
mesogenic units, or mesogenic and nonmesogenic units.
EP 358 208 A2 describes liquid-crystalline
organosiloxanes and their preparation, which contain
(meth)acryloxy groups and which can be employed, inter
alia, as pigments in paints. According to the reported
example, the liquid-crystalline polymers are formed from
a low molar mass nematic component, containing
methacrylic groups, and of a low molar mass chiral
compound, which are crosslinked. From this document,
however, it is not possible to obtain any indications of
how inteference pigments can be produced by selecting an
optimised composition of materials whir_h has improved
colour brightness.
WO 91/13125 describes pigments for paints, where
plateletlike substrates are coated with one or more metal
oxides, the coating comprising a chiral, irridescent
liquid-crystalline medium. In accordance with the
examples cited therein, the liquid-crystalline composi-
tions consist of low molar mass chiral and nematic,
P-pentyl-based and/or cholesteryl-based compounds. These
classes of compounds are unsuitable f:or the present
application case of weather-resistant and mechanically
resistant coating systems.
GB 2 132 623 A discloses liquid-crystalline
polymers which are obtained from low molar mass chiral
~
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cholestero-~ derivatives. The polymer films exhibit
irridescent properties. The addition of low molar mass
nematic compounds, which also include photopolymerizable
groups (e. g. 4-methoxyphenyl 4-(6-metha~ryloyloxyhexyl-
oxy] benzoate) is proposed. This document too, however,
does not reveal any indications of interference pigments
of the composition which is relevant or of interest in
the present case.
The object of the invention, is relation to the
various categories taken as the basis for the generic
types, namely interference pigments, proeecs for their
preparation, coating material and coating system applied
oa a commodity article, is to find a material base which
is independent of the base of the polysiloxanes and, in
effect coating materials and effect coatings, gives rise
to an improved brightness in the perceived colour; it
being intended that the interference pigment used. should
possess an improved chemical and physical compatibility
with the customary paint vehicles and should also be able
to be prepared in a technically simple manner at favour-
able cost.
One aspect of the present invention provides
interference pigments comprising liquid-crystal-
line side-chain polymers (LCPs), for coloured coating
materials, in which the side-group mesogeas have an at
least approximately nematic and/or smectic and/or
cholesteric order,
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- 7b -
c h a r a c t a r i z a d b y the conjunction of the
following features:
P the liquid-crystalline side-chain polymers (LCPs) of
the interference pigments are formed from a nematic
component and a chiral component which is crosslinked
with it,
o the osmotic component consisting of one of the mono- or
difunctional, low molar mass compounds listed below, or
a mixture thereof. namely:
- 1,4-diphenyl dil4-(acryloyloxy)alkyloxy]benzoates,
- 1,4-di(2-methyl)phenyl di[4-(acryloyloxy)alkyloxy]-
benzoates,
- I, 4-di (2-methoxy) phenyl di [4- (acryloyloxy) alkyloxy] -
beazoates,
a the chiral component consisting of one of the optically
isomeric low molar mass compounds having a centre of
chirality, which are listed below, or of a mixture
thereof, namely:
- 3,5-cholesteryl (acryloyloxyalkoxy)benzoate,
- 3,4-cholesteryl (acryloyloxyalkoxy)benzoate,
- 2,5-cholesteryl (acryloyloxyalkoxy)beazoate,
- 2,4-cholesteryl (acryloyloxyalkoxy)benzoate,
- /S-oestra-3,17-diyl (acryloylalkyloxy)benzoate,
- ~B-oestra-3,17-diyl acrylate,.
- R- or S-1,1'-bi-2-naphthoyl [4-alkoxybeazoyloxy].-
(4,4-bipheayloyloxyalkoxyacrylate)-,
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-7c-
- R- or S-1,1~-bi-2-naphthoyl [haxylterephthaloyl]-
(4.4-biphenyloyloxyalkoxyacrylats),
- 1,4-dipheayl R- or S-di[4-(3-aoryloyloxy-a-methyl-
propoxy) benzoate] ,
- 2 -octyl 4-(4-hexyloxybeazoyloxy]benzoate,
- 1.4-dipheayl di[4-(3-acryloyloxy-2-methylpropoxy]-
benzoate,
- 1,4-di(2-methyl)phenyl di[4-(3-acryloyloxy-2-methyl-
propoxy]benzoate,
- 1,4-~(2-methoxy)pheayl di[4-(3-acryloyloxy-2-methyl-
propoxy]benzoate,
- 1,4-diphenyl~ di[4-(1-aCryloyloxy 1-methylethoxy]-
benzoate,
- 1,4-di(2-methyl)phenyl di[4-(1-acryloyloxy-1-methyl-
ethoxy]benzoate,
- 1,4-di(2-methoxy)phenyl di[4-(1-acryloyloxy-1-methyl-
ethoxy]benzoate,
- 1,4-diphenyl di[4-(6-acryloyloxy-3-methylhexoxy]-
benzoate,
- 1.4-di(2-methyl)phenyl di[4-(6-acryloyloxy-3-methyl-
hexoxy]benzoate,
- 1.4-di(2-methoxy)phenyl di[4-(6-acryloyloxy-3-methyl-
hexoxy] benzoate.
The invention also provides a process for
the preparation of platelet-like interference
pigments, a coating material for painting
commodity articles such as vehicle bodies, and a
corresponding coating system.
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-7d-
It is known that liquid-crystalline polymers.
owing to their high molecular weight and the resulting
high viscosity, possess a long orientation time. This
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disadvantage is avoided in accordance with the claims by
the use of low molar mass mono- or difunctional
acrylates, methacrylates, epoxides or vinyl ethers, which
have a considerably shorter orientation time and can be
crosslinked in order to "freeze in" the mesophase formed.
The polymeric networks obtained in this way no longer
exhibit any liquid-crystalline transitions, and until
thermal decomposition they remain in a highly ordered
state which is conserved. The coloured films and/or the
interference pigments are obtained by admixing a low
molar mass chiral compound to the nematic component,
thereby inducing the formation of the chiral-nematic
reflecting phase.
In this context it has been found in practice, in
the course of the preparation of the colour-imparting
films and/or of the pigments, that it is preferable to
employ chzral compounds possessing good liquid-crystal
line properties and good crosslinking properties. If a
cholesteric network is prepared wa.th nonmesogenic
diacrylates, a crosslinking agent is added, for example
a 1,4-diacryloyloxybenzene.
Some of the nematic and chiral components
employed are available as commercial products, and/or
their preparation is known from the literature. The
preparation of the nematic component 1,4-diphenyl
di[4-(6-acryloyloxyhexyloxy)benzoate] is described by
D.J. Broer; J. Boven; G.N. Mol; G. Chaila; ?rlakromol.
Chem. 190, 2255 (1989) and the preparation of the nematic
component 4'-methoxybiphenyl 4-(8-acryloyloxy-3,6-dioxy-
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octoyloxy)benzoate has been carried out by M. Engel;
B. Hisgen; R. Keller; W. Kreuder; B. Reck; H. Ringsdorf;
H.-W. Schmidt; P. Tschirner; Pure and Appl. Chem. 57
(1985) 1009.
2-Octyl 4-(4-hexyloxybenzoyloxy)benzoate is
marketed as a commercial product by the company Merck
under the designation S 811. The chiral component choles-
teryl butyrate is marketed by the company Fluka under
order number 26 770. The preparation of the chiral
component cholesteryl acrylate employed is described by
A.C. De Visser; K. De Groot; J. Feyen; A. Brantjes;
J. Polym. Sci., A-1, 9, 1893 (1971). The preparation of
the chiral component cholesteryl 4-(6-acryloyloxyhexyl-
oxy) benzoate was carried out by esterification with chol-
esterol via the corresponding acid chloride. The
1,4-diacryloyloxybenzene employed as crosslinking agent
was prepared in accordance with L. Liebert;
L. Strzelecki; D. Vagogne; Bull. Soc. Chim. Fr. 9-10,
2073 (1975) .
The film produced in connection with the prepara-
tion of the interference pigments should at least after
curing have a thickness of from 3 to 15 arm. In the case
of thinner films, the desired interference-effective
colour phenomenon is lost, and, in the case of thicker
films, the alignment of the molecules is not as good, so
that the colour phenomenon is impaired by a milky appear-
once. By breaking down the cured film in small particles,
the interference pigments required in the present case
are produced in platelet form, in which likewise, or as
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before, the side-group mesogens are in at least approxi-
mately nematic and/or smectic and/or, preferably, chol-
esteric order. The plateletlike interference pigments
which can be used have a platelet. diameter in the order
of from 5 to 100 ~.m, or approximately from one to ten
times the film thickness; larger or smaller particles
produced during comminution of the fully cured film must
be systematically removed, for example by sieving oper-
ations. When the basecoat pigmented with them is applied
to a surface, the plateletlike pigments automatically
arrange themselves parallel to the surface as a result of
shear forces in the basecoat. The small particles, which
tend to approximate to the cubic form, do not, however,
align themselves in the applied coating material parallel
to the surface of the component, which is why they should
be systematically removed. The very large pigment plate-
lets maylgive rise to problems during application of the
coating material, which is why pigments of such a size
should also be removed by sieving.
Another expedient embodiment of the coating
system can be seen in keeping the substrate of the
colour-defining coating layer in a shade, brought about
by means of absorption pigments, which is such that it
approximately coincides with the basic colour or with the
"flop" colour of the interference pigments. This gives
the colour coincident with the substrate a particularly
intense and brilliant appearance. Although in tile other
viewing direction or illumination direction the colour of
the coated surface also appears in the respective sub-
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strate shade, it is tinted in the other colour of the
interference pigments and is strewn with finely dispersed
points of sparkle. In addition, it is of course also
possible, using absorption pigments, to give the
substrate carrying the colour-defining coating layer
neither a dark shade nor a shade in one of the colours of
the interference pigments but in a third shade, which for
example is in the colour spectrum '.between both colours or
has a particularly large spectral distance from at least
one of the two colours . In this way it is possible to
give the coated article a three-way perceived colour.
As an alternative or else a supplement giving the
substrate which carries the colour-defining coating layer
a shading appropriate to its use, it is also conceivable
to mix absorption pigments into the effect paint contain-
ing - in some instances possibly different - interference
pigments. 'By admixing dark absorption pigments to the
novel effect paint it is possible to obtain an impression
wholly comparable with that given by a dark substrate.
The situation is similar when light-coloured absorption
pigments are mixed in; they bring about an attenuation of
the colour flop and of the individual colour intensities.
By admixing absorption pigments in one of the effect
colours it is possible to achieve an intensification of
this perceived colour at the expense of the colour effect
of the other effect colour. The admixing of absorption
pigments to the effect paint is advisable if the inten-
tion is - for whatever reason - to paint a light-coloured
substrate, or even a substrate having a metallic gleam,
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with effect paint. A task of this kind may be required,
for example, when repainting old vehicles or in the
course of refinish operations.
In the text below the invention is illustrated in
more detail with reference to examples.
Production of the colour-imparting film:
The polymer films are produced in an in situ
photopolymerization in the cholesteric phase of the
corresponding monomer mixture. A monomer mixture consist-
ing of 50 mol o of a nematic component and 50 mol o of a
chiral component, and also approximately l~ by weight of
2,2-dimethoxy-2-phenylacetophenone, were dissolved in
1 ml of chloroform. The nematic component used was a
1,4-diphenyl di[4-(6-acryloyloxyhexyloxy)benzoate] and
the chiral component used was a cholesteryl aerylate. The
reaction, solution was applied to a substrate, for example
to a film, to a metal plate or to a slide, with a coating
thickness of about 5 ~.m. After evaporating the solvent,
the coated substrate was heated to 115°C, at which point
the mesophase was formed in the monomer mixture. Good
orientation was obtained by knife-coating the reaction
solution onto the substrate, treating it with electrical
or magnetic fields, or applying an alignment layer to the
substrate beforehand. The applied film was subsequently
cured by means of W light. The cured film, when viewed
straight on under perpendicular illumination, had a
violet colour.
By varying the content of the two components it
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is possible to establish reflection colours in the entire
visible region of the spectrum and in the IR range within
the film, as is evident from the table below.
Proportion of chiral monomer Reflection wavelength
(mold) ~R (nm)
1668
998.0
674.5
572.0
10 40 512.5
474.0
435.5
378.0
362.5
15 Preparation of the interference pigments:
The colour-imparting film was scraped off from
the substrate using a blade-like tool to give flake-like
structures. The flakes were subsequently comminuted with
an air jet mill (from Alpine); it is also possible to
20 employ other mills, incorporating gentle heating, for
plastics parts. The resulting milled material was then
sieved, and a sieve fraction having a mean size of about
30 ~m was used for further processing.
Preparation of the paint:
25 The interference pigments obtained were mixed
with a clearcoat in a ratio of from 1:7 to 1:20 parts by
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Daa.m 23 342/PCT - lg - ' , ~ ~ 91251
weight. The clearcoat used wars a two-compoaet~t topcoat
based on polyurethane: a.g. 2k-PU-decklack 0111 from BASF
Lacks and Farben or a "Standox~''" coating material from
Herberts.
Effect finishing of a bodywork components
The effect paint was applied by a nemxtic spray-
ing technique (equipment_ sATA) to a vehicle sunroof
cover whfeh had been primed uair~g a black filler coating,
Applicant's designation Yii. 0404. An electrostatic appli-
cation technique for the novel affect paint using high-
speed rotating spraying bells appears to be another
possibility. Following application, the effect paint was
first dried and then subjected to force thermal curing.
The result obtained on the bodywork .coutponant was
a coloured coating which, under diffuse white light -
with a cloudy sky - appeared irl an intense violet colora-
tion or in a green shade depending oa tha di.rectiort o!
viewing.
Another colour-iatparting film was prepared from
1,4-di(2-methyl)phenyl dil4-(6-acryloyloxy)hexyloxyl-
benzoate as n~atic campoaeat and 2,3-cholesteryl
(2-acryloyloxyethoxy)benzoate as cltixal component. The
film was produced by the procedure indicated in the
previous example, with the difference that the coated
substraCe was heated to 90°C.
For this film, the following reflection wavelengths
were established:
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Daim 23 342/PCT - 15 -
Proportion of chirai monomer Reflection wavelength
(mol$) ~R (am)
1668
998
5 30 674
35 572
40 512
45 474
50 435
10 60 378
70 362
The advantages achieved with the subject-matter
of the invention consist, in particular, in that with the
effect coating material and, respectively, with the
15 effect coating system an even higher brightness of
colour, with a high degree of light stability, is
achieved than it was possible to obtain with the inter-
ference pigments known to date. This can be explained by
the fact that the novel interference pigments develop
20 very narrow-band absorption curves for the various
colours. Moreover, the interference pigments possess
improved chemical and physical compatibility with the
customary coating materials with which they are mixed.
Relative to those known to date, the novel interference
pigments possess a substantially higher degree of cross-
linking, which is achieved in particular when the difunc~-
tional nematic diacrylates and difunctional chiral-
nematic diacrylates are used. The interference pigments
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prepared possess high mechanical stability and great
colour stability, and exhibit outstanding adhesiveness
and wettability. The starting components for the liquid-
crystalline side-chain polymers moreover, are simple and
inexpensive to prepare.
