Compositions – Liquid crystal compositions
Reexamination Certificate
2000-04-10
2002-07-23
Wu, Shean C. (Department: 1756)
Compositions
Liquid crystal compositions
C106S493000, C106S494000, C106S499000, C106S506000
Reexamination Certificate
active
06423246
ABSTRACT:
TECHNICAL FIELD
The invention relates to thermostable pigments, films and effect coatings, and to mixtures for their production.
BACKGROUND ART
Materials having a liquid-crystalline structure with a chiral phase (LC materials) are known. The production of such materials from LC organosiloxanes is described, for example, in U.S. Pat. No. 5,211,877. Pigments containing aligned, three-dimensionally crosslinked substances having a liquid-crystalline structure with a chiral phase (LC pigments), their production and use are also known, for example from DE 42 407 43, which corresponds to U.S. Pat. No. 5,362,315.
One problem in the processing of LC materials, for example in surface finishes, is the varying color of the material depending on the application temperature. In paint systems employing conventional LC materials, for example those produced from LC organosiloxanes mentioned in U.S. Pat. No. 5,211,877 or the LC pigments disclosed in DE 42 407 43, different colors arise at processing temperatures of 130° C., the usual processing temperature in the OEM painting of vehicle bodies, than on processing at 80° C., the usual processing temperature for repair finishes.
This varying behavior of the LC pigment is generally caused by swelling of the pigment during production of paints and the de-swelling process during drying of these paints, i.e. by the action of solvents on the pigment. The swelling process, and the dissolving-out of uncrosslinked constituents from the pigment which may occur at the same time, modifies the structure of the pigment. Warming of the dried paint film results in pronounced relaxation of the helix of the cholesteric liquid crystal, i.e. the pitch of the helix is shortened. This results in a shortening of the central reflection wavelength, which is directly proportional to the pitch of the helix. The color of the pigment thus shifts toward shorter wavelengths. A clearly visually perceptible blue shift occurs. Depending on the selected processing temperature, i.e. the baking temperature of a paint containing LC pigments of this type, various degrees of relaxation of the helix in the pigments occur. This results in the undesired variation in hue with processing temperature.
EP 0 887 399 solves this problem by selecting crosslinkable LC organosiloxanes as pigment material which, after crosslinking, give LC pigments having maximum glass transition temperatures (T
g
) of 80° C. and low uncrosslinked, extractable contents. Thus, complete and uniform relaxation of the helix is achieved at all temperatures above T
g
, which means that the same color is always obtained at these temperatures. The property profile of these organosiloxanes is achieved by appropriate choice of flexible structural units, so-called spacers, in the LC monomers on which the pigments are based. Although these organosiloxanes modified with flexible structural units have quite good thermostability as LC pigments in the crosslinked state in the processing range 80° C.-130° C., they have, independently thereof, strong swelling especially processing window, of 80° C.-130° C., since incorporation of the spacers lowers the crosslinking density. LC pigments comprising mixtures in accordance with EP 0 887 399 exhibit strong interactions at room temperature with solvents usually used in paints, which results in strong swelling of the pigments and consequently a clearly visually perceptible red shift of the pigment color. In addition, the organosiloxanes modifications disclosed in EP 0 887 399 cause a significant increase in costs due to additional complex synthesis steps.
EP 0 887 398 discloses preparation of a system containing known LC pigments in which visually perceptible color differences at different processing temperatures are not evident, through selection of a suitable matrix. This system has the disadvantage that the user does not have a free choice of matrix for the LC pigments, but is subject to restrictions in selecting the paint components (vehicles and solvents).
EP 0 760 836, which corresponds to U.S. Pat. No. 5,807,497, discloses interference pigments comprising liquid-crystalline side-chain polymers made from low-molecular-weight, monofunctional or difunctional acrylates which are distinguished by higher color brightness, higher crosslinking density and better chemical resistance in conventional paints as well as simple production compared with the LC pigments disclosed hitherto. These LC pigments likewise have inadequate thermostability in the processing temperature range which is relevant for paint finishes of from about 80° C. to about 130° C. This inadequate thermostability is evident from shifts in the central reflection wavelengths of greater than 2 nm.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a mixture of crosslinkable, liquid-crystalline substances having a chiral phase (LC mixture) which is suitable for the production of thermostable pigments, films and effect coatings.
BEST MODE FOR CARRYING OUT THE INVENTION
For the purposes of the present invention, an article, for example a pigment, a film, or an effect coating, is thermostable if it does not exhibit any visually perceptible color differences in a paint or plastic matrix over a temperature range of from 80° C. to 130° C.
A color difference is generally accepted by paint users as being visually imperceptible if the color difference &Dgr;E* in accordance with DIN 5033 and DIN 6174 between two paint samples produced at different temperatures is ≦2 (L. Dulog, H. Schweiger, Farbe & Lack, 1997, 30-44). In order to achieve this aim in bright pigments (reflection >25%) in the green color region, in which the eye is particularly sensitive to color differences, experience has shown that it is necessary for the central reflection wavelengths to differ by ≦2 nm in the paint samples processed at 80° C. and 130° C., respectively.
This object is achieved by an LC mixture containing polymerizable groups, where at least 90% of the polymerizable groups are part of molecules containing at least two polymerizable groups (crosslinker molecules), wherein from 3.2 to 15 mmol of polymerizable groups are present per g of LC mixture.
The LC mixture according to the invention preferably comprises, as substances to which the polymerizable groups are bonded, low-molecular-weight, oligomeric or polymeric substances or mixtures of these substances having a chiral phase. These chiral phases can also be thermotropic twisted nematic, smectic or discotic phases. They can also be substances having a thermotropic or lyotropic phase.
The LC mixture according to the invention particularly preferably consists of mixtures of
compounds of the general formula Y
1
—A
1
—M
1
—A
2
—Y
2
, in which Y
1
and Y
2
are identical or different and are polymerizable groups, such as, for example, acrylate, methacrylate, epoxide, isocyanate, hydroxyl, vinyl ether or vinyl ester radicals, and
A
1
and A
2
are identical or different radicals of the general formula C
n
H
2n
, in which n is an integer from 0 to 20, and one or more methylene groups may be replaced by oxygen atoms, and
M
1
has the general formula —R
1
—X
1
—R
2
—X
2
—R
3
—X
3
—R
4
—, where R
1
, R
2
, R
3
and R
4
are identical or different divalent radicals selected from the group consisting of —O—, —COO—, —CONH—, —CO—, —S—, —C≡—C, —CH═CH—, —N═N— and —N═N(O)—, and R
2
—X
2
—R
3
or R
2
—X
2
or R
2
—X
2
—R
3
—X
3
can also be a C—C bond, and X
1
, X
2
and X
3
are identical or different radicals selected from the group consisting of 1,4-phenylenes, 1,4-cyclohexylenes, arylenes, or heteroarylenes having 6 to 10 atoms in the heteroaryl ring which contain 1 to 3 heteroatoms from the group consisting of O, N and S and which are substituted by B
1
and/or B
2
and/or B
3
, cycloalkylenes having 3 to 10 carbon atoms which are substituted by B
1
and/or B
2
and/or B
3
, where B
1
, B
2
and B
3
can be identical or different substituents selected from the group consisting of —H, C
1
-C
20
-alkyl, C
1
-C
20
-alkoxy, C
1
-C
20
-alkylthio, C
1
-C
20
-alkylcarbo
Kasch Michael
Kreuzer Franz-Heinrich
Küpfer Jürgen
Brooks & Kushman P.C.
Consortium fur Elektrochemische Industrie GmbH
Wu Shean C.
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