Polymer coatings with improved UV and heat stability

The present invention relates to coatings comprising electrically conductive polymers and fla- vones, their production and use, and dispersions for the production of such coatings.

Electrically conductive polymers from the class of polypyrroles, polyanilines and polythio- phenes are known from the literature. Poly(3,4-alkylenedioxythiophene) dispersions in particular have recently acquired technical importance, since they can be used, for example, for the production of conductive or antistatic coatings (see e.g. EP-A 440 957). In practice, however, it has been found that the electrical conductivity of the coatings from such dispersions is not always sufficiently stable for practical uses under a higher temperature and/or ultra-violet (UV) irradiation.

EP 1798259 Al and WO 2008/055834 Al disclose that e.g. the heat or UV stability of the electrical conductivity of coatings containing polythiophene dispersions can be increased if aromatic compounds containing at least two hydroxyl groups are added. Nevertheless, this stabilizing action, in particular the action on the heat stability, is not sufficient for many long- term uses.

There thus continued to be a need for conductive or antistatic coatings with improved heat or UV stability compared with the known coatings and for suitable dispersions for the production of such coatings.

The object of the present invention was therefore to provide such coatings with improved heat or UV stability and suitable dispersions for the production thereof. It has now been found, surprisingly, that dispersions containing at least one conductive polymer and at least one flavone are suitable for the production of coatings which have, for example, significantly better heat and UV stabilities.

The present invention therefore provides a dispersion comprising at least one electrically con- ductive polymer, at least one counter-ion and at least one dispersing agent, characterized in that it contains at least one flavone.

In the context of the invention, flavones are preferably understood as meaning compounds which fall under the following general formula (I)

(I), wherein the compounds carry the substituents shown in Table 1 at positions 2', 3, 5, 7, 8, 3', 4' and 5': Table 1:

Quercetin is particularly preferably employed as the flavone:

The flavones can be employed as the pure substance or as a mixture of various flavones. FIa- vones which can be used for such dispersions are commercially obtainable.

The flavones can be added to the dispersions according to the invention in an amount of from 1 to 100 per cent by weight (wt.%), preferably from 5 to 50 wt.%, particularly preferably from 10 to 40 wt.%, based on the solids content of electrically conductive polymer, such as, for ex- ample, the polythiophene of the general formula (II), in the dispersion.

In the context of the invention, electrically conductive polymers are understood as meaning in particular the compound class of π-conjugated polymers which have an electrical conductivity after oxidation or reduction. Preferably, conductive polymers are understood as meaning those π-conjugated polymers which, after oxidation, have a specific conductivity in the dried state of the order of at least 0.01 S cm ^"1 .

Preferred dispersions are those wherein at least one electrically conductive polymer is an optionally substituted polythiophene, an optionally substituted polyaniline or an optionally substituted polypyrrole.

The conductive polymer is or the conductive polymers are particularly preferably chosen from polyalkylenedioxythiophenes containing recurring units of the general formula (II)

(H)

wherein

A represents an optionally substituted C^Cs-alkylene radical, preferably an optionally substituted C ₂ -C ₃ -alkylene radical,

R independently of each other represents H, a linear or branched, optionally substituted CrQs-alkyl radical, an optionally substituted Cs-C ₁₂ -cycloalkyl radical, an optionally substituted C ₆ -C ₁₄ -aryl radical, an optionally substituted C ₇ -C ₁₈ -aralkyl radical, an optionally substituted Q-C-i-hydroxyalkyl radical or a hydroxyl radical, preferably a linear or branched, optionally substituted Ci-C ₄ -alkyl radical, an optionally substituted C ₁ - C ₄ -hydroxyalkyl radical or a hydroxyl radical, particularly preferably a linear or branched optionally substituted d-C ₄ -alkyl radical or a hydroxyl radical,

x represents an integer from 0 to 8, preferably an integer from 0 to 2, particularly preferably 0 or 1 and

in the case where several radicals R are bonded to A, these can be identical or different.

The general formula (II) is to be understood as meaning that x substituents R can be bonded to the alkylene radical A.

Polythiophenes with recurring units of the general formula (II) wherein A represents an optionally substituted C ₂ -C ₃ -alkylene radical and x represents 0 or 1 are particularly preferred.

Very particularly preferably, at least one electrically conductive polymer is poly(3,4- ethylenedioxythiophene), which is optionally substituted. In the context of the invention, the prefix poly- is to be understood as meaning that the polymer or polythiophene contains more than one identical or different recurring unit. The polythiophenes contain n recurring units of the general formula (II) in total, wherein n is an integer from 2 to 2,000, preferably from 2 to 100. The recurring units of the general formula (II) can in each case be identical or different within one polythiophene. Polythiophenes with in each case identical recurring units of the general formula (II) are preferred.

The polythiophenes preferably carry H on each of the end groups.

In preferred embodiments of the present invention, the dispersions contain at least one polyal- kylenedioxythiophene containing recurring units of the general formula (II) which have a spe- cific conductivity in the dried state of the order of at least 0.05 S cm ^"1 , preferably of at least 0.5 S cm ^"1 .

The solids content of electrically conductive polymer, in particular of a polyalkylenedi- oxythiophene containing recurring units of the general formula (I), in the dispersion is between 0.05 and 3.0 wt.%, preferably between 0.1 and 1.5 wt.%, particularly preferably between 0.3 and 1.0 wt.%.

In the context of the invention, Q-Cs-alkylene radicals A are preferably methylene, ethylene, n-propylene, n-butylene or n-pentylene. Q-C^-alkyl R preferably represent linear or branched Ci-Ci ₈ -alkyl radicals, such as methyl, ethyl, n- or iso-propyl, n-, iso-, sec- or tert-butyl, n- pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1-ethylpropyl, 1,1-dimethylpropyl, 1,2- dimethylpropyl, 2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl or n-octadecyl; preferably represents linear or branched Ci-C- ₄ -alkyl radicals, such as methyl, ethyl, n- or iso- propyl, n-, iso-, sec- or tert-butyl, Q-Q-alkyl moreover represents, for example, n-pentyl, 1- methylbutyl, 2-methylbutyl, 3-methylbutyl, neo-pentyl, 1-ethylpropyl, cyclohexyl, cyclopen- tyl, n-hexyl, 1,1-dimethylpropyl, 1 ,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3- methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1 ,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2- dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2- trimethylpropyl, 1,2,2-trimethylpropyl, 1 -ethyl- 1-methylpropyl, l-ethyl-2-methylpropyl or 1- ethyl-2-methylpropyl; in the context of the invention, Ci-C-rhydroxyalkyl R preferably repre- sents a straight-chain, cyclic, branched or unbranched Q-Gj-alkyl radical, which is substituted by one or more, but preferably one hydroxyl group; C ₅ -Ci ₂ -cycloalkyl radicals R represent, for example, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl or cyclodecyl; C ₆ -Ci ₄ - aryl radicals R represent, for example, phenyl or naphthyl, and C ₇ -Ci ₈ -aralkyl radicals R repre- sent, for example, benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, 3,5-xylyl or mesityl. The preceding lists serve to illustrate the invention by way of example and are not to be considered conclusive.

In the context of the invention, numerous organic groups are possible optional further substitu- ents of the radicals A and/or of the radicals R, for example alkyl, cycloalkyl, aryl, aralkyl, alkoxy, halogen, ether, thioether, disulphide, sulphoxide, sulphone, sulphonate, amino, aldehyde, keto, carboxylic acid ester, carboxylic acid, carbonate, carboxylate, cyano, alkylsilane and alkoxysilane groups and carboxamide groups.

Possible substituents for polyaniline or polypyrrole are, for example, the radicals A and R listed above and/or the further substituents of the radicals A and R. Unsubstituted polyanilines and polypyrroles are preferably used.

The context of the invention includes all the radical definitions, parameters and explanations above and given in the following, which are general or mentioned in preferred ranges, with one another, that is to say also in any desired combination between the particular ranges and preferred ranges.

The polythiophenes employed as conductive polymers in the dispersions can be neutral or cationic. In preferred embodiments they are cationic, "cationic" relating only to the charges on the polythiophene main chain. The polythiophenes can carry positive and negative charges in the structural unit, depending on the substituent on the radicals R, the positive charges being on the polythiophene main chain and the negative charges optionally being on the radicals R substituted by sulphonate or carboxylate groups. In this context, the positive charges of the polythiophene main chain can be partly or completely satisfied by the anionic groups optionally present on the radicals R. Overall, in these cases the polythiophenes can be cationic, neutral or even anionic. Nevertheless, in the context of the invention they are all regarded as cationic polythiophenes, since the positive charges on the polythiophene main chain are decisive. The positive charges are not shown in the formulae, since their precise number and position cannot be determined absolutely. However, the number of positive charges is at least 1 and at most n, wherein n is the total number of all recurring units (identical or different) within the polythiophene.

To compensate the positive charge, if this is not already done by the optionally sulphonate- or carboxylate-substituted and therefore negatively charged radicals R, the cationic polythio- phenes require anions as counter-ions.

Counter-ions can be monomelic or polymeric anions, the latter also being called polyanions in the following.

Polymeric anions are preferable to monomelic anions, since they contribute towards film formation and because of their size lead to electrically conductive films which are more stable to heat. However, in addition to the polymeric anions, the dispersions can also contain monomelic anions.

Polymeric anions here can be, for example, anions of polymeric carboxylic acids, such as polyacrylic acids, polymethacrylic acid or polymaleic acids, or polymeric sulphonic acids, such as polystyrenesulphonic acids and polyvinylsulphonic acids. These polycarboxylic and - sulphonic acids can also be copolymers of vinylcarboxylic and vinylsulphonic acids with other polymerizable monomers, such as acrylic acid esters and styrene.

Preferably, the dispersions according to the invention contain at least one anion of a polymeric carboxylic or sulphonic acid as a counter-ion.

The anion of polystyrenesulphonic acid (PSS) is particularly preferred as the polymeric anion.

The molecular weight of the polyacids which supply the polyanions is preferably 1 ,000 to 2,000,000, particularly preferably 2,000 to 500,000. The polyacids or their alkali metal salts are commercially obtainable, e.g. polystyrenesulphonic acids and polyacrylic acids, or can be prepared by known processes (see e.g. Houben Weyl, Methoden der organischen Chemie, vol. E 20 Makromolekulare Stoffe, part 2, (1987), p. 1141 et seq.). The dispersion can contain polymeric anion(s) and electrically conductive polymers in particular in a weight ratio of from 0.5:1 to 50:1, preferably from 1:1 to 30:1, particularly preferably 2:1 to 20:1. The weight of the electrically conductive polymers here corresponds to the weight of the monomers employed, assuming that complete conversion takes place during the polym- erization.

Monomelic anions which are used are, for example, those of Ci-Cao-alkanesulphonic acids, such as methane-, ethane-, propane-, butanesulphonic acid or higher sulphonic acids, such as dodecanesulphonic acid, of aliphatic CrC^-perfluorosulphonic acids, such as trifluoro- methanesulphonic acid, perfluorobutanesulphonic acid or perfluorooctanesulphonic, of ali- phatic C ₁ -C _2O -CaTbOXyHc acids, such as 2-ethylhexylcarboxylic acid, of aliphatic C ₁ -C _2O - perfluorocarboxylic acids, such as trifluoroacetic acid or perfluorooctanoic acid, and of aromatic sulphonic acids optionally substituted by Q-C^-alkyl groups, such as benzenesulphonic acid, o-toluenesulphonic acid, p-toluenesulphonic acid or dodecylbenzenesulphonic acid, and of cycloalkanesulphonic acids, such as camphorsulphonic acid, or tetrafluoroborates, hexafluorophosphates, perchlorates, hexafluoroantimonates, hexafluoroarsenates of hexa- chloroantimonates.

The anions of p-toluenesulphonic acid, methanesulphonic acid or camphorsulphonic acid are preferred as the monomelic anions.

Cationic polythiophenes which contain anions as counter-ions for charge compensation are also often called polythiophene/(poly)anion complexes in the technical field.

The total content of the electrically conductive polymer and counter-ion, for example in the form of such polymer/counter-ion complexes, in the dispersion according to the invention is, for example, between 0.05 and 10 wt.%, preferably between 0.1 and 2 wt.%, based on the total weight of the dispersion.

The dispersions according to the invention can contain one or more dispersing agents. Dispersing agents which may be mentioned are, for example, the following solvents: aliphatic alcohols, such as methanol, ethanol, i-propanol and butanol; aliphatic ketones, such as acetone and methyl ethyl ketone; aliphatic carboxylic acid esters, such as ethyl acetate and butyl acetate; aromatic hydrocarbons, such as toluene and xylene; aliphatic hydrocarbons, such as hexane, heptane and cyclohexane; chlorohydrocarbons, such as methylene chloride and dichloroethane; aliphatic nitriles, such as acetonitrile; aliphatic sulphoxides and sulphones, such as dimethyl- sulphoxide and sulpholane; aliphatic carboxylic acid amides, such as methylacetamide, di- methylacetamide and dimethylformamide; and aliphatic and araliphatic ethers, such as diethyl ether and anisole. Water or a mixture of water with the abovementioned organic solvents can furthermore also be used as a dispersing agent.

Preferred dispersing agents are water or other protic solvents, such as alcohols, e.g. methanol, ethanol, i-propanol and butanol, and mixtures of water with these alcohols; water is the particularly preferred solvent.

The dispersion can moreover contain further components, such as surface-active substances, e.g. ionic and nonionic surfactants, or adhesion promoters, such as e.g. organofunctional si- lanes or hydrolysates thereof, e.g. 3-glycidoxypropyltrialkoxysilane, 3- aminopropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, 3- methacryloxypropyltrimethoxysilane, vinyltrimethoxysilane or octyltriethoxysilane.

The dispersions according to the invention can contain further additives which increase the conductivity, such as e.g. compounds containing ether groups, such as e.g. tetrahydrofuran, compounds containing lactone groups, such as γ-butyrolactone, γ-valerolactone, compounds containing amide or lactam groups, such as caprolactam, N-methylcaprolactam, N ₅ N- dimethylacetamide, N-methylacetamide, N,N-dimethylformamide (DMF), N- methylformamide, N-methylformanilide, N-methylpyrrolidone (NMP), N-octylpyrrolidone, pyrrolidone, sulphones and sulphoxides, such as e.g. sulpholane (tetramethylene sulphone), dimethylsulphoxide (DMSO), sugars or sugar derivatives, such as e.g. sucrose, glucose, fructose, lactose, sugar alcohols, such as e.g. sorbitol, mannitol, furan derivatives, such as e.g. 2- furancarboxylic acid, 3-furancarboxylic acid and/or di- or polyalcohols, such as e.g. ethylene glycol, glycerol, di- and triethylene glycol. Tetrahydrofuran, N-methylformamide, N- methylpyrrolidone, ethylene glycol, dimethylsulphoxide or sorbitol are particularly preferably employed as conductivity-increasing additives.

The dispersions according to the invention can moreover contain one or more organic binders which are soluble in organic solvents or water-soluble, such as polyvinyl acetate, polycarbon- ate, polyvinylbutyral, polyacrylic acid esters, polyacrylamides, polymethacrylic acid esters, polymethacrylamides, polystyrene, polyacrylonitrile, polyvinyl chloride, polyvinylpyrrolidone, polybutadiene, polyisoprene, polyethers, polyesters, polyurethanes, polyamides, polyimides, polysulphones, silicones, epoxy resins, styrene/acrylic acid ester, vinyl acetate/acrylic acid 5 ester and ethylene/vinyl acetate copolymers, polyvinyl alcohols or celluloses.

The content of the polymeric binder in the dispersion according to the invention is 0.1 - 90 wt.%, preferably 0.5 - 30 wt.% and very particularly preferably 0.5 - 10 wt.%, based on the total weight of the dispersion.

Such an organic binder optionally contained in the dispersion can also optionally function as LO the dispersing agent if it is liquid at the given temperature.

The dispersions according to the invention can have a pH of from 1 to 14; a pH of from 1 to 8 is preferred.

Bases or acids, for example, can be added to the dispersions to adjust the pH. Those additions which do not impair the film formation of the dispersions and are not volatile at higher tem- L 5 peratures, e.g. soldering temperatures, such as e.g. the bases 2-(dimethylamino)-ethanol, 2,2'- iminodiethanol or 2,2',2"-nitrilotriethanol and the acid polystyrenesulphonic acid, are preferred.

The viscosity of the dispersion according to the invention can be between 0.1 and 100,000 mPa-s (measured at 20 ⁰ C at a shear rate of 100 s ^"1 ), depending on the method of application. 10 Preferably, the viscosity is 1 to 10,000 mPa-s, particularly preferably between 10 to 1,000 mPa-s.

The preparation of the dispersions according to the invention is carried out by first preparing, from the corresponding precursors for the preparation of conductive polymers, dispersions of electrically conductive polymers in the presence of counter-ions, for example analogously to >5 the conditions mentioned in EP-A 440 957. An improved variant for the preparation of these dispersions is the use of ion exchangers for removal of the inorganic salt content or of a part thereof. Such a variant is described, for example, in DE-A 196 27 071. The ion exchanger can be stirred with the product, for example, or the product is conveyed over a column filled with an ion exchanger column. Low metal contents, for example, can be achieved by using the ion exchanger.

The particle size of the particles in the dispersion can be reduced after the desalination, for example by means of a high pressure homogenizer. This operation can also be repeated in or- der to increase the effect. Particularly high pressures of between 100 and 2,000 bar have proved to be particularly advantageous here for greatly reducing the particle size.

Preparation of the polyaniline/polyanion, polypyrrole/polyanion or polythiophene/polyanion complex and subsequent dispersion or redispersion in one or more dispersing agent(s) is also possible.

For preparation of the dispersions according to the invention, the further components, such as, for example, the flavone, optionally further dispersing agent and optionally further additives, organic binders etc., are then added to these dispersions and the components are mixed, for example while stirring.

Corresponding monomers, for example, are understood as precursors for the preparation of conductive polymers, also called precursors in the following. Mixtures of various precursors can also be used. Suitable monomelic precursors are, for example, optionally substituted thio- phenes, pyrroles or anilines, preferably optionally substituted thiophenes, particularly preferably optionally substituted 3,4-alkylenedioxythiophenes.

As substituted 3,4-alkylenedioxythiophenes there may be mentioned by way of example the compounds of the general formula (III)

A Λ o ⁷ O

)ri _> Co

O

wherein

represents an optionally substituted Cj-Cs-alkylene radical, preferably an optionally substituted C ₂ -C ₃ -alkylene radical, R independently of each other represents H, a linear or branched, optionally substituted d-Cis-alkyl radical, an optionally substituted C ₅ -Ci ₂ -cycloalkyl radical, an optionally substituted C ₆ -Ci ₄ -aryl radical, an optionally substituted C ₇ -Ci ₈ -aralkyl radical, an optionally substituted Ci-C4-hydroxyalkyl radical or a hydroxyl radical, preferably a lin- ear or branched, optionally substituted Ci-C ₄ -alkyl radical, an optionally substituted C ₁ -

C ₄ -hydroxyalkyl radical or a hydroxyl radical, particularly preferably a linear or branched optionally substituted Ci-C ₄ -alkyl radical or a hydroxyl radical,

x represents an integer from 0 to 8, preferably an integer from 0 to 2, particularly preferably 0 or 1 and

in the case where several radicals R are bonded to A, these can be identical or different.

Very particularly preferred monomeric precursors are optionally substituted 3,4- ethylenedioxythiophenes, in a preferred embodiment unsubstituted 3,4- ethylenedioxythiophene.

Possible substituents for the abovementioned precursors, in particular for the thiophenes, pref- erably for the 3,4-alkylenedioxythiophenes, are the radicals mentioned for R for the general formula (III).

Possible substituents for pyrroles and anilines are, for example, the radicals A and R listed above and/or the further substituents of the radicals A and R.

Possible optional further substituents of the radicals A and/or the radicals R are the organic groups mentioned in connection with the general formula (II).

Processes for the preparation of the monomeric precursors for the preparation of conductive polymers are known to the person skilled in the art and are described, for example, in L. Gro- enendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 12 (2000) 481 - 494 and literature cited therein.

The dispersions according to the invention are outstandingly suitable for the production of electrically conductive or antistatic coatings having a heat stability and/or UV stability of the electrical properties. The present invention therefore furthermore provides electrically conductive or antistatic coatings obtainable from the dispersions according to the invention.

For production of the coatings according to the invention, the dispersions according to the invention are applied, for example, by known processes, e.g. by spin coating, impregnation, pouring, dripping on, spraying, misting, knife coating, brushing or printing, for example ink- jet, screen, gravure, offset or tampon printing, to a suitable substrate in a wet film thickness of from 0.5 μm to 250 μm, preferably in a wet film thickness of from 2 μm to 50 μm, and then dried at a temperature of at least from 20 ⁰ C to 200 ⁰ C.

The dispersions according to the invention show a significantly higher stability above room temperature, in particular at temperatures above 80 ⁰ C, of the electrical properties of the coatings produced therefrom.

The dispersions according to the invention also show a significantly higher stability to UV light of the electrical properties of the coatings produced therefrom.

The following examples serve to illustrate the invention by way of example and are not to be interpreted as a limitation.

Examples:

Comparison Example:

Test prints were produced with the commercial screen printing paste Clevios S V3 (manufacturer H.C.Starck GmbH, Goslar) using a screen of polyester fabric with a mesh number of 140/cm. The printed area had the dimensions of 10 x 2 cm ² . The prints were dried at 130 ⁰ C in a circulating air oven for 15 minutes (min). 2 conductive silver electrodes were then applied in the middle of the film at a distance of 2 cm at right angles to the longitudinal direction and the system was dried at room temperature for 24 hours (h). The conductive silver electrodes were then connected to a multimeter by means of clamps and the surface resistance was measured.

Surface resistance: 400 ohm/sq.

Example 1 according to the invention:

1.0 g of quercetin (Aldrich) was dissolved in 200 g of the screen printing paste from the comparison example, while stirring, and test prints were produced as described for the comparison example and the surface resistance determined.

Surface resistance 410 ohm/sq.

The test prints were then stored in air at 150 ⁰ C and the surface resistance was determined after 316 h:

Surface resistance [ohm/sq.]

Before storage After 316 h 150 ⁰ C

Comparison example 400 9700

Example 1 410 1340 Coatings produced from the dispersions according to the invention have a better heat stability than coatings produced from known dispersions, i.e. dispersions to which no flavone has been added.

Comparative Example 2:

Test prints were produced with the commercial screen printing paste Clevios S V3 (manufacturer H.C.Starck GmbH, Goslar) using a screen of polyester fabric with a mesh number of 140/cm. The printed area had the dimensions of 10 x 2 cm ² . The prints were dried at 130 ⁰ C in a circulating air oven for 15 minutes (min). Two gold electrodes were then applied by vapour deposition in the middle of the film with lengths of 2 cm at a distance of 1 cm at right angles to the longitudinal direction

The gold electrodes were then contacted and the electrical resistance determined by means of a multimeter and the surface resistance (doubled electrical resistance) calculated therefrom.

The prints were then exposed for 100 and 200 hours with 500 W/m ² with an Atlas Suntest CPS+ and subsequently the surface resistance measured again.

The average surface resistance of two prints before and after exposure is given in Table 2

Example 2 according to the invention:

1.0 g of quercetin (Aldrich) was dissolved in 200 g of the screen printing paste from comparative example 2 with stirring, and test prints were produced as described for comparative exam- pie 2 and the surface resistance determined before and after exposure.

The average surface resistance of two prints before and after exposure is given in Table 2 Table 2:

Surface resistance [ohm/sq.]

Coatings prepared from the inventive dispersions have better UV stability than coatings prepared from known dispersions, i.e. dispersions to which no flavone has been added.