Polymer coatings and effect paints are extremely important for improvement of surfaces and the esthetic appearance of objects. A wide variety of color impressions and special color effects can be created in many different ways. Current polymer coatings use particles or pigments in a carrier polymer to provide color or achieve special effects such as a metallic sheen or the like. To achieve specific reflection effects, metal flakes, coated mica particles, or interference pigments based on liquid crystalline compounds for example are worked into a clear vehicle as a carrier polymer. Other pigments can additionally be added for free creation of the color impression.
Another possibility for coloration-producing effects consists in the use of liquid crystalline polymers or copolymers, oligomers (macromonomers), or monomers. Some of these liquid crystalline materials are appropriate for forming cohesive polymer films. They polymerize in the liquid crystalline phase and thus produce a paint layer with a special color effect. It is not necessary to work them into a carrier material such as a clear paint.
Known substances that exhibit a liquid crystalline state are generally elongated organic molecules which are able to assume a particular molecular arrangement. Depending on the arrangement of the liquid crystalline phase, only the wavelengths of the incident light that interfere with the equidistant lattice spacing of the liquid crystalline materials are reflected, so that particular color and reflection effects are generated. To make paints and other polymer coatings that exhibit certain wavelength reflections and light effects, it is necessary to fix the liquid crystalline phase or stabilize it mechanically. Particular liquid crystalline phases are formed in certain temperature ranges, whose position and size depends in turn on the chemical structure of the materials. Moreover, the color appearance of the liquid crystalline phases within the phase often depends on temperature, namely as the liquid crystalline phase is heated or cooled, different wavelengths are reflected. To preserve certain color or reflection effects, it is possible to fix a liquid crystalline phase by polymerization or chemical crosslinking of the initial molecules into a dense network. For this purpose, the starting materials must contain crosslinkable reactive groups.
The literature contains liquid crystalline monomer compounds with two identical terminal reactive groups such as diacrylates (J. Lub, D. J. Broer, R. A. M. Hikmet, K. G. J. Nierop, Liquid Crystals 18, 319 (1995)), diepoxides (D. J. Broer, J. Lub, G. N. Mol, Macromolecules 26, 1244 (1993)), and divinyl ethers (R. A. M. Hikmet, J. Lub, J. A. Higgins, Polymer 34, 1736 (1993); S. Jahromi, J. Lub, G. N. Mol, Polymer 35, 622 (1994)). Such monomers are usually crosslinked photochemically by photocycloaddition or by addition of a photoinitiator to the monomer mixture. Likewise, thermally initiated radical crosslinking or thermally initiated addition or condensation reactions are known. With these known monomer compounds, both terminal groups are always polymerized at the same time. To form a polymer film, the known liquid crystalline monomers are applied to the corresponding substrates and the polymerization reaction is initiated, producing the finished end product.
In this process, the application and adhesion of the initial monomers to the coated substrate create considerable difficulties. As a rule, liquid crystalline monomers are crystalline or powdered, so that they adhere poorly to the substrate and are difficult to apply in an even layer. Moreover, the known liquid crystalline materials are relatively invariable in terms of hardness, elasticity, and adhesion of the end product, namely the polymer film, and do not adjust readily to the requirements of specific applications.
The goal of the present invention is to provide compounds and a method of producing liquid crystalline polymers with better handling, processing, and end product properties than the prior art.
This goal is achieved by compounds with general formula Y.sup.1 --A.sup.1 --M.sup.1 --A.sup.2 --Y.sup.2 wherein
Y.sup.1 and Y.sup.2 are different from each other and Y.sup.1 is an acrylate or methacrylate residue and Y.sup.2 is a vinyl ether, epoxy, or azide residue, PA1 A.sup.1 and A.sup.2 are identical or different residues with the general formula C.sub.n H.sub.2n, in which n is a whole number from 0 to 20 and one or more methylene groups can be replaced by oxygen atoms, and PA1 M.sup.1 has the general formula --R.sup.1 --X.sup.1 --R.sup.2 --X.sup.2 --R.sup.3 --X.sup.3 --R.sup.4 -- wherein PA1 R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are identical or different covalently residues from the group --O--, --COO--, --CONH--, --CO--, --S--, --C.ident.C--, --CH.dbd.CH--, --CH.dbd.N--, --CH.sub.2 --, --N.dbd.N--, and --N.dbd.N(O)--, and R.sup.2 --X.sup.2 --R.sup.3 or R.sup.2 --X.sup.2 --R.sup.3 --X.sup.3 or X.sup.2 --R.sup.3 can also be a C--C bond, and PA1 X.sup.1, X.sup.2, and X.sup.3 are identical or different residues from the group 1,4-phenylene, 1,4-cyclohexylene; arylalkane or heteroarylalkane with 1 to 10 carbon atoms which contains one to three heteroatoms from the group O, N, and S, substituted with B.sup.1, B.sup.2, and/or B.sup.3 ; and cycloalkylene with 1 to 10 carbon atoms and substituted with B.sup.1, B.sup.2, and/or B.sup.3, wherein B.sup.1, B.sup.2, and B.sup.3 can be identical or different substituents from the group --H, C.sub.1 -C.sub.20 -alkyl, C.sub.1 -C.sub.20 -alkoxy, C.sub.1 -C.sub.20 -alkylthio, C.sub.1 -C.sub.20 -alkylcarbonyl, C.sub.1 -C.sub.20 -alkoxycarbonyl, C.sub.1 -C.sub.20 -alkylthiocarbonyl, --OH, halogen (fluorine, chlorine, bromine, iodine), --CN, --NO.sub.2, cycloalkyl, formyl, acetyl, and alkyl, alkoxy, or alkylthio residues with 1-20 carbon atoms interrupted by ether oxygen, thioether sulfur, or ester groups. PA1 a) prepolymers are made by polymerization, reacting the acrylate or methacrylate groups Y.sup.1 of a compound or a mixture of compounds according to claim 1, and thereafter PA1 b) they are crosslinked by polymerization, reacting vinylethyl, epoxy, or azide groups Y.sup.2. PA1 Y.sup.3 can be an acrylate or methacrylate group and Y.sup.4 can be a polymerizable residue from the group of vinyl ether, epoxy, and azide residues or a nonpolymerizable residue from the group --H, --CN, and cholesteryl, PA1 A.sup.3 and A.sup.4 are identical or different residues with the general formula C.sub.n H.sub.2n in which n is a whole number from 0 to 20 and one or more methylene groups can be replaced by oxygen atoms, and PA1 M.sup.2 has the general formula --R.sup.5 --X.sup.4 --R.sup.6 --X.sup.5 --R.sup.7 --X.sup.6 --R.sup.8 -- wherein PA1 R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are identical or different doubly bonded residues from the group --O--, --COO--, --CONH--, --S--, --C.ident.C--, --CH.dbd.CH--, --CH.dbd.N--, --CH.sub.2 --, --N.dbd.N--, and --N.dbd.N(O)--, and R.sup.6 --X.sup.5 --R.sup.7 or R.sup.6 --X.sup.5 --R.sup.7 --X.sup.6 can also be a C--C bond and PA1 X.sup.4, X.sup.5, and X.sup.6 are identical or different residues from the group 1,4-phenylene, 1,4-cyclohexylene; arylalkane or heteroarylalkane with 1 to 10 carbon atoms which contains one to three heteroatoms from the group O, N, and S, substituted with B.sup.1, B.sup.2, and/or B.sup.3 ; and cycloalkylene with 1 to 10 carbon atoms and substituted with B.sup.1, B.sup.2, and/or B.sup.3, wherein B.sup.1, B.sup.2, and B.sup.3 can be identical or different substituents from the group --H, C.sub.1 -C.sub.20 -alkyl, C.sub.1 -C.sub.20 -alkoxy, C.sub.1 -C.sub.20 -alkylthio, C.sub.1 -C.sub.20 -alkylcarbonyl, C.sub.1 -C.sub.20 -alkoxycarbonyl, C.sub.1 -C.sub.20 -alkylthiocarbonyl, --OH, halogen (fluorine, chlorine, bromine, iodine), --CN, --NO.sub.2 cycloalkyl, formyl, acetyl, and alkyl, alkoxy, or alkylthio residues with 1-20 carbon atoms interrupted by ether oxygen, thioether sulfur, or ester groups.
At the same time, a method of producing liquid crystalline polymers is provided wherein
The particular advantage of the compounds according to the invention is that, during polymerization, particularly the creation of liquid crystalline polymers, they make a two-step process possible. The compounds proposed have a molecular structure that confers liquid crystalline properties on them. In addition, they have polymerizable residues Y.sup.1 and Y.sup.2 which make it possible to fix the polycrystalline phase by polymerization. By contrast to known liquid crystalline monomers, which can be used to make liquid crystalline polymer coatings, the various reactive residues Y.sup.1 and Y.sup.2 of the compounds according to the invention are however crosslinkable by polymerization reactions with various initiation and reaction mechanisms. By polymerization of acrylate or methacrylate groups Y.sup.1, prepolymers can be made with degrees of polymerization that are not too high (oligomers). These oligomers form glasses, but still do not have sufficiently low viscosity to ensure good orientation. Moreover, by the usual methods of polymer chemistry, the molecular weight and other material parameters such as viscosity, film-forming properties, leveling, flow properties, solubility, color, sheen, swelling, workability, adhesion, elasticity, hardness, etc. can be affected.
A particularly advantageous property of the reactive groups Y.sup.1 and Y.sup.2 of the compounds according to the invention is that the initiation and reaction of prepolymerization of reactive groups Y.sup.1 can take place without the reactive groups Y.sup.2 being reacted. The resultant oligomers can be readily further processed following prepolymerization and have outstanding application properties by comparison to the known polycrystal monomers. The viscosity of the prepolymers makes it possible for example to apply them before final crosslinking like a paint with good leveling properties, good flow properties, high sprayability, and outstanding adhesion to the substrate to be coated. In a further polymerization step, the vinyl ether, epoxy, or azide groups Y.sup.2 that are still free are crosslinked. Adhesion of the end product to the substrate can be adjusted, for example by concentrating the reactive groups Y.sup.2, to the substrate in question.
The compounds according to the invention have still further advantages. Polymerization of the liquid crystalline materials is preferably done in two steps with prepolymerization. They can also be polymerized directly on a substrate with both functional groups Y.sup.1 and Y.sup.2 being polymerized. The user is thus able to choose between a one-step and a two-step polymerization process or combine them according to the application.
Particularly advantageous liquid crystalline monomers are compounds in which Y.sup.1 is an acrylate group and Y.sup.2 is a vinyl ether group. It is also advantageous for the number n of carbon atoms of residues A.sup.1 and A.sup.2 to be 1 to 10, and particularly preferably 2 to 6. It is advantageous for the liquid crystalline properties for the residues R.sup.1, R.sup.2, R.sup.3, and/or R.sup.4 to be --O--, --COO--. It is also advantageous for the X.sup.1 and X.sup.3 residues to be 1,4-phenylene and/or for R.sup.2 --X.sup.2 --R.sup.3 to be a C--C bond.
In a preferred embodiment of the method for producing liquid crystalline polymers, polymerization (a) is carried out by reacting the acrylate or methacrylate groups Y.sup.1 radically and polymerization (b) by reacting the vinyl ether, epoxy, or azide groups Y.sup.2 preferably photochemically, and particularly preferably cationically. Since polymerization reaction (a) is usually inhibited by oxygen, it is advantageous to conduct it in an organic solvent, preferably in degassed, absolute tetrahydrofuran. It is appropriate to initiate this reaction and to continue it, in the presence of a radical polymerization initiator, preferably 2,2'-azobis-(2-methylpropionitrile), dibenzoyl peroxide, or di-t-butyl peroxide, particularly preferably with an initiator concentration of 1 to 5 mol. %. For polymerization reaction (a), it is advantageous for there to be a reaction regulator, preferably 1-decanethiol, particularly preferably with a reaction regulator concentration of 1 to 10 mol %.
The various initiation and reaction mechanisms of radical polymerization of acrylate or methacrylate groups Y.sup.1 to the prepolymer on the one hand and the photochemical cationic polymerization (b) of vinyl ether, epoxy, or azide groups Y.sup.2 to the end product on the other hand, ensure that in the first step, when residue Y.sup.1 is being crosslinked, no reaction takes place with residues Y.sup.2 or of residues Y.sup.2 with one another. Since cationic polymerization by contrast with radical polymerization is not inhibited by oxygen, this makes the process simpler and simplifies handling of the prepolymer during crosslinking on the substrate. No steps need be taken to exclude oxygen by expensive inert gas technologies.
When prepolymerization (a) is conducted in an organic solvent, the oligomers, which are easy to handle, can be made in large quantities regardless of where they are further converted to the end product. The prepolymers usually have a very long shelf life.
By using a radical polymerization initiator and possibly adding a reaction regulator, the reaction conditions can be optimized in terms of yield and product properties.
For many applications, it is particularly advantageous to isolate the prepolymer after polymerization reaction (a). For this purpose, it is advantageous to precipitate it from hexane after polymerization reaction (a) and then advantageously reprecipitate and/or dry it.
For the application, to be able to apply the prepolymer like a paint to the substrate to be coated, it is particularly advantageous to dissolve it before polymerization (b) of the vinyl ether, epoxy, or azide groups in an organic solvent, preferably in chloroform or tetrahydrofuran, and to evaporate the solvent before the reaction itself.
The form of the prepolymer to be applied can be adapted to a great variety of application requirements. For example, it can also be sold in the solvent instead of the monomeric compounds, so that the end user need not make the prepolymer, saving work steps, manufacturing facilities, and cost. Moreover, solutions of the prepolymer can be made in any concentration and with specific viscosity, flow, wetting, and adhesion properties. Application to the substrate to be coated can be done in any suitable manner. Thus, for example, surfaces can be sprayed or painted or dipped in the solution of prepolymer. Depending on the requirement and the solvent used, it can be evaporated at room temperature or at an elevated temperature, at a negative pressure, or in an air stream. It must be borne in mind in this connection that polymerization reaction (b) with certain compositions already begins at an elevated temperature, which may be advantageous for certain applications.
In a preferred embodiment of the invention, polymerization (b) is conducted by reacting the vinyl ether, epoxy, or azide group in the presence of at least one photoinitiator, preferably a cationic photoinitiator. Advantageously the photoinitiator is present in a quantity of 0.5 to 10 wt. %, preferably 1 to 5 wt. %, and/or contains a diarylsulfonium salt, a diaryliodonium salt, or a mixture thereof. Examples of such photoinitiators are the commercial products Degacure KI 85 (Degussa), bis(4-tert-butylphenyl)iodonium hexafluorophosphate (Midori Chemical), 2,4,6-trimethylbenzoylethoxyphenylphosphine oxide (BASF) mixed with diphenyliodonium hexafluorophosphate, and 2,2-dimethoxy-2-phenylacetophenone mixed with diphenyliodonium tetrafluoroborate. Normally, polymerization reaction (b) is induced by ultraviolet radiation and/or the reaction product is post-cured by heat treatment.
The use of a photoinitiator has particular advantages since the course of the reaction up to the end product can be more easily initiated, regulated, and accelerated. By comparison to purely thermal curing of the prepolymers, induction of the polymerization reaction by ultraviolet radiation saves considerable energy cost, and is faster and more readily initiated, and easier to quantify. Heat treatment has advantages if the surface to be coated cannot be reached by the initiation radiation because it is in the shadow or inaccessible.
In a particularly preferred embodiment of the method of the present invention, polymerization (a) is conducted in the presence of additional compounds with the general formula Y.sup.3 --A.sup.3 --M.sup.2 --A.sup.4 --Y.sup.4 or (Y.sup.3 --A.sup.3).sub.2 --M.sup.2 --A.sup.4 Y.sup.4 wherein
These additional compounds, hereinafter known as comonomers, are particularly suitable for matching the properties of the polymerization end product to the application requirements in question. The adjustable mechanical properties of the end product are in particular adhesion, elasticity, and hardness of the polymer film on the substrate in question.
The use according to the invention of the comonomers to control the optical effect of the polymer film is particularly advantageous. Here it is particularly advantageous for residues A.sup.3 and/or A.sup.4 to be chiral. By using one or more chiral or chiral-nematic comonomers in different ratios mixed with the monomers of the present invention, any reflection wavelengths of the polymer films, from the ultraviolet to the infrared range, can be established. By copolymerization with the chiral compounds, highly crosslinked polymer films result, in which the reflection wavelength does not depend on temperature. Because of the chirality of the compounds, cholesteric side group prepolymers are formed in polymerization (a) which are then fixed as a cholesteric network in polymerization reaction (b).
Examples of comonomers suitable according to the invention are the following compounds:
4-[(S)-(2-acryloyloxy-2-methyl)ethoxy]-4'-cyanobiphenyl ##STR1## PA0 4-[(S) -2-acryloyloxy-2-methyl)ethoxy]-4'-[(S)-2-methylbutoxyl]-biphenyl ##STR2## PA0 {4'-[(S)-(2-acryloyloxy-2-methyl)ethoxy]phenyl}4-(2-vinyloxyethoxy)benzoate ##STR3##
Preferably, the copolymers have only one or two polymerizable residues Y.sup.3, which are exclusively involved in first polymerization reaction (a). Copolymers with only one acrylate or methacrylate group affect not only the optical effect but also the mechanical properties of the prepolymer and the end product. Such copolymers lead to chain terminations in the polymerization reaction and thus regulate the degree of crosslinking. The chiral centers A.sup.3 and/or A.sup.4 of the comonomers favor formation of a cholesteric phase in the liquid crystalline polymer. Such a cholesteric structure, which can only be accomplished with optically active molecules and represents a type of superstructure on the liquid crystalline textures of simple structure, contains liquid crystalline unit regions in a screw-shaped helical arrangement. The height of the individual helical unit regions, also known as pitch, determines the wavelength of the reflected light and hence the colors of the metallically iridescent reflections of the polymer film.
Examples of chiral vinyl ethers and divinyl ethers, suitable according to the invention as comonomers with free vinyl ether groups, and favoring a cholesteric phase, are the following compounds: ##STR4##