UV-stabilizers for siloxane systems

A UV-stabilizer comprising a mixture of hydroxybenzotriazole and a hydrolyzable silane containing at least one epoxy group is disclosed. The stabilizer is especially suitable as a component of a siloxane system for coating substrates, especially thermoplastic substrates and most especially polycarbonate substrates.

The present invention relates to non-volatile UV-stabilising mixtures for
 siloxane lacquer systems, which mixtures have certain
 hydroxybenzotriazoles as the UV-stabilising active structure and which are
 thus particularly suitable for the UV-stabilisation of thermoplastics, in
 particular of aromatic polycarbonates.
 Materials are frequently protected from the harmful influences of the
 environment by providing them with a protective surface. Siloxane-based
 lacquers have proved particularly suitable for this purpose, inter alia
 providing the materials with a scratch-resistant surface.
 These lacquers may contain so-called UV-stabilising substances in order to
 protect the lacquer itself and the underlying material, the so-called
 substrate, from harmful UV radiation. Apart from providing long-term UV
 protection, one requirement placed upon these substances is, inter alia,
 that they are not volatile so that they remain homogeneously distributed
 within the lacquer layer and do not escape from the lacquer layer either
 during curing or during subsequent use of the laquer. The UV-stabilising
 substances must furthermore not decompose rapidly, must be durably
 homogeneously miscible with the lacquers and the lacquer containing the
 UV-stabilising substances should be transparent
 U.S. Pat. Nos. 4,278,804 and 4,051,161 relate to UV-stabilising active
 substances and lacquers containing them. The substances disclosed therein,
 however, exhibit the disadvantage that they provide inadequate UV
 protection, they decompose too rapidly and/or the siloxane system
 containing the stabilisers has a yellow tinge.
 U.S. Pat. No. 5,438,142 furthermore discloses the UV-stabilising active
 substance, 1-(3'-(benzotriazol-2"-yl)-4'-hydroxyphenyl)ethane. This active
 substance, however, exhibits the disadvantage that it is not durably
 miscible with siloxane-based lacquers.
 This object thus arises of providing a UV-stabiliser system which does not
 exhibit the above-stated disadvantages.
 This object is achieved according to the invention by the provision of
 UV-stabilising mixtures containing hydroxybenzotriazoles of the general
 formula (I) below and hydrolysable silanes containing epoxy groups.
 ##STR1##
 R.sup.1 : H, C.sub.1- C.sub.18 alkyl, C.sub.5- C.sub.6 cycloalkyl, C.sub.6-
 C.sub.12 aryl,
 R.sup.2 : H, halogen, preferably C1, or C.sub.1- C.sub.12 alkyl,
 R.sup.3 : a single bond, C.sub.1- C.sub.12 alkylene, C.sub.5- C.sub.6
 cycloalkylene or phenylene,
 R.sup.4 : H, alkali metal, ammonium, alkaline earth metal, C.sub.1-
 C.sub.12 alkyl,
 ##STR2##
 The present invention furthermore provides UV-stabilising mixtures having a
 molar ratio of epoxy groups of the silane to the hydroxybenzotriazole of
 the general formula (1) which is greater than 1.4, preferably greater than
 2, particularly preferably greater than 8. The molar ratio of epoxy units
 of the silane to the hydroxybenzotriazole of the general formula (1)
 should not, however, exceed 1:100.
 The mixtures according to the invention are suitable for the
 UV-stabilisation of siloxane systems, in particular of scratch- and
 abrasion-resistant siloxane coating materials. Such UV-stabilised coating
 materials, preferably lacquers, may be used for coating materials of all
 kinds, such as for example wood, textiles, paper, stone articles, but
 preferably for coating plastics, metals, glass and ceramics, particularly
 preferably for coating thermoplastics and very particularly preferably for
 coating polycarbonates.
 The hydroxybenzotriazoles used for the non-volatile, UV-stabilising
 mixtures according to the invention are compounds of the general formula
 (1).
 Preferred compounds of the formula (1) are:
 ##STR3##
 The compounds of the formula (1) are either known from the literature or
 obtainable using processes known from the literature, for example in
 accordance with the reaction scheme disclosed on page 7 of EP-0 057 160.
 This patent application is introduced as a reference and is accordingly
 part of the disclosure of the present invention.
 Silanes containing epoxy groups are generally taken to mean compounds
 which, on the one hand, possess at least one epoxy ring and simultaneously
 have groups which form silanol structures under hydrolysing conditions.
 Epoxysilanes as are preferably used according to the invention are
 described, for example, in U.S. Pat. No. 2,946,701. They are compounds of
 the formulae (2) or (3):
 ##STR4##
 R.sup.5 is a divalent hydrocarbon residue having at most 9 carbon atoms or
 a divalent residue having at most 9 carbon atoms consisting of C, H and O
 atoms, wherein the O atom is present as an ether bond residue. R.sup.5 is
 preferably --CH.sub.2 OCH.sub.2 CH.sub.2 CH.sub.2 --.
 R.sup.6 is an aliphatic hydrocarbon residue having at most 4 carbon atoms,
 an acyl residue having at most 4 carbon atoms or a residue of the formula
 (CH.sub.2 CH.sub.2 O).sub.n Z, in which n is at least 1 and Z means an
 aliphatic hydrocarbon residue having at most 4 carbon atoms; m is 0 or 1.
 Production of these epoxysilanes is also described in U.S. Pat. No.
 2,946,701. This patent is accordingly introduced as a reference.
 Particularly preferred epoxysilanes are those compounds in which R.sub.6
 is methyl. They are commercially available, inter alia from the companies
 Union Carbide and Huls AG as:

A-187 or Dynasilan 3-glycidyloxypropyltrimethoxysilane
 Glymo
 A-186 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.
 Production of the UV-stabilising Mixtures
 The UV-stabilising components are produced by homogeneously mixing
 compounds of the general formula (1) with the hydrolysable silanes
 containing epoxy groups and heating this mixture. Heating should be
 performed for at least 30 minutes at at least 90.degree. C. The
 temperature should preferably be above 120.degree. C. during heating. It
 has proved particularly favourable to use a mixing ratio at which
 stoichiometrically more epoxy groups are present than the --R.sub.3
 --CO--OR.sub.4 --groups of the hydroxybenzotriazole of the general formula
 1. The molar ratio of epoxy units of the silane to the
 hydroxybenzotriazole of the general formula (1) should thus be greater
 than 1.4, preferably greater than 2, particularly preferably greater than
 8.
 The UV-stabilising components need not necessarily be produced separately
 so that they may subsequently be added to the siloxane system to be
 stabilised, but may also be synthesised in situ as a sub-stage during
 synthesis of the siloxane systems/siloxane coating materials.
 Siloxane systems/siloxane Coating Materials
 The siloxane systems are substantially thermally curing systems which
 preferably crosslink by a condensation reaction to yield
 --Si--O--Si--linkages. Other crosslinking mechanisms may proceed in
 parallel. Such systems are described, for example, in U.S. Pat. Nos.
 3,790,527, 3,865,755, 3,887,514, 4,243,720, 4,278,804, 4,680,232,
 4,006,271, 4,476,281, in DE-A 4 011 045, 4 122 743, 4 020 316, 3 917 535,
 3 706 714, 3 407 087, 3 836 815, 2 914 427, 3 135 241, 3 134 777, 3 100
 532, 3 151 350, in DE-A 3 005 541, 3 014 411, 2 834 606, 2 947 879, 3 016
 021, 2 914 427 and 4 338 361 and should be considered part of the present
 disclosure.
 The present invention accordingly also provides siloxane systems
 UV-stabilised according to the invention.
 Preferably used siloxane systems are those containing particulate material
 selected from among oxides, oxide hydrates, nitrides and carbides of Si,
 Al, Sb and B and of transition metals, preferably Ti, Ce, Fe and Zr, and
 having a particle size in the range from 1 to 100 nm, preferably from 2 to
 50 nm.
 The UV-stabilising mixture according to the invention should be added to
 the siloxane system in such a quantity, relative to the solids content of
 the siloxane system, that the proportion of hydroxybenzotriazole, relative
 to the solids content of the siloxane system, is 0.3 to 20, preferably 3
 to 15, particularly preferably 5 to 10 wt %.
 Reference is made to DE-A 2 914 427 and DE-A 4 338 361 with regard to the
 production of siloxane-based scratch-resistant coating systems and
 components thereof and these documents are thus part of the present
 description.
 Substrates, materials
 The siloxane systems provided with the UV-stabilising mixture according to
 the invention may be used as bulk materials and as coating materials.
 There are no restrictions as to the substrate materials which may be
 selected for coating. These UV-stabilised coating materials are preferably
 suitable for coating wood, textiles, paper, stone articles, metals, glass,
 ceramics and plastics and in particular for coating thermoplastics, as are
 for example described in Becker/Braun Kunststoffhandbuch, Carl Hanser
 Verlag, Munich, Vienna, 1972. They are very particularly suitable for
 coating transparent thermoplastics, preferably polycarbonates
 Conventional coating processes are used for coating purposes, for example
 dipping, flooding, pouring, spinning, spraying or brushing.
 The coating is applied to film thicknesses of, for example, 2 to 200 .mu.m,
 preferably of 2 to 30 .mu.m and particularly preferably of 5 to 15 .mu.m.
 The substrate may optionally be primed with a coupling agent or primer
 coat before application of the coating.
 The lacquers are preferably cured at temperatures of &gt;90.degree. C.
 For the purposes of the present invention, thermoplastic, aromatic
 polycarbonates include both homopolycarbonates and copolycarbonates; the
 polycarbonates may, in a known manner, be linear or branched.
 A proportion, up to 80 mol %, preferably of 20 mol % to 50 mol % of the
 carbonate groups in the suitable polycarbonates may be replaced by
 aromatic dicarboxylic acid ester groups. Such polycarbonates, which
 contain both acid residues of carbonic acid and acid residues of aromatic
 dicarboxylic acids incorporated in the molecular chain, are more
 accurately termed aromatic polyester carbonates. They are to be subsumed
 within the superordinate term of thermoplastic, aromatic polycarbonates.
 Details of the production of polycarbonates have been described in hundreds
 of patents over the past approx. 40 years. Reference is made, merely by
 way of example, to "Schnell, Chemistry & Physics of Polycarbonates",
 Polymer Reviews, volume 9, Interscience Publishers, New York, London,
 Sydney 1964, to D. C. Prevorsek, B. T. Debona & Y. Kesten, Corporate
 Research Center, Allied Chemical Corporation, Morristown, N.J. 07960,
 "Synthesis of poly(ester carbonate) copolymers" in Journal of Polymer
 Science, Polymer Chemistry edition, volume 19, 75-90 (1980), to D.
 Freitag, U. Grigo, P. R. Muller, N. Nouvertne', Bayer A G,
 "Polycarbonates" in Encyclopedia of Polymer Science & Engineering, volume
 11, second edition, 1988, pages 648-718 and finally to Dr. U. Grigo, Dr.
 K. Kircher & Dr. P. R. Muller "Polycarbonate" in Becker/Braun,
 Kunststoff-Handbuch, volume 3/1, Polycarbonate, Polyacetale, Polyester,
 Celluloseester, Carl Hanser Verlag, Munich, Vienna, 1992, pages 117-299.
 The thermoplastic polycarbonates have average molecular weights M.sub.w
 (determined by measuring relative viscosity at 25.degree. C. in CH.sub.2
 Cl.sub.2 at a concentration of 0.5 g per 100 ml of CH.sub.2 Cl.sub.2) of
 12000 to 400000, preferably of 18000 to 80000 and in particular of 22000
 to 60000.
 The present invention accordingly also provides coated materials,
 preferably polycarbonate and particularly preferably polycarbonate
 provided with a scratch-resistant coating.

EXAMPLES
 Example I
 a) UV-absorbing Starting Materials
 a1) 2-(2-hydroxy-3-tert.-butyl-5-(2-carboxyethyl)phenyl)benzotriazole, m.p.
 195.degree. C. (produced as in EP 0 057 160, Example 1)
 a2) 2-(2-hydroxy-3-cyclohexyl-5(3-carboxypropyl)phenyl)benzotriazole
 .alpha.) 132 g (0.75 mol) of 2-cyclohexylphenol are dissolved in 800 ml of
 dry chlorobenzene. 200 g (1.5 mol) of AlCl.sub.3 are then added at 5 to
 10.degree. C. followed at 0 to 10.degree. C. by a solution of 73.5 g (0.75
 mol) of maleic anhydride in 400 ml of chlorobenzene. After 12 hours at
 room temperature, the mixture is poured into iced water and acidified with
 concentrated HCl. 85 g of a powder having an m.p. of 187 to 190.degree. C.
 are obtained.
 .beta.) 34.5 g (0.25 mol) of o-nitroaniline are stirred in 300 ml of water
 and 69 ml of concentrated HCl. A solution of 17.3 g (0.25 mol) of sodium
 nitrite in 155 ml of water is then added dropwise at 5.degree. C. This
 solution is then added dropwise at 5.degree. C. to a solution of 68.5 g
 (0.25 mol) of compound a and 79.5 g (0.75 mol) of sodium carbonate in 1
 liter of water. 117 g of a solid having an m.p. of 155.degree. C. are
 obtained.
 .gamma.) 42.5 g (0.1 mol) of the azo dye .beta. are combined with 200 ml of
 2 n NaOH. 50 g of zinc powder are then added and 80 ml of 10 n NaOH are
 run in within 1 hour such that the temperature remains below 45.degree. C.
 The mixture is then heated to 90.degree. C. for 4 hours, filtered and the
 filtrate acidified with HCl.
 After recrystallisation from cyclohexane, 31 g of
 2-(2-hydroxy-3-cyclohexyl-5(3-carboxypropyl)phenyl)benzotriazole are
 obtained as colourless crystals of an m.p. of 165.degree. C.
 b) UV-stabilising component prepared from al and
 3-glycidyloxypropyltrimethoxysilane (Glymo)
 50 g of al and 450 g of 3-glycidyloxypropyltrimethoxysilane are introduced
 into a vessel and heated to 140 to 150.degree. C. under a nitrogen
 atmosphere while being stirred and are maintained at this temperature for
 one hour.
 This mixture (mixture 1) was varied as follows:
 Mixture:

2 100 g al 400 g Glymo
 3 150 g al 350 g Glymo
 4 200 g al 300 g Glymo
 5 100 g al 400 g .alpha.-(3,4-epoxycyclohexyl)ethyltrimeth-
 oxysilane
 c) Production of the Siloxane Coating Material According to DE-A 2 914 427
 (Coating Sol I)
 .alpha.) 19.8 g of glacial acetic acid, 210 g of distilled water and 227 g
 of isopropanol are added to 300 g of colloidal silicic acid having an
 SiO.sub.2 content of 30 wt. %. After thorough mixing, 900 g of
 methyltriethoxysilane are added and the mixture heated to 60.degree. C.
 while being stirred. The mixture is left at this temperature for 4 hours
 and then a further 1200 g of isopropanol are added to the mixture. Once
 the product has cooled to room temperature, the slightly opaque solution
 is filtered.
 .beta.) 340 g of isopropanol, 190 g of tetraethoxysilane and 360 g of
 methyltriethoxysilane are introduced into a vessel fitted with a stirrer
 and reflux condenser. This mixture is combined with 180 g of 0.05 n
 hydrochloric acid and co-hydrolysed by refluxing for five hours. The
 mixture is cooled to room temperature after the reaction. A solution is
 obtained which is a partial hydrolysate of tetraethoxysilane (5.1%,
 calculated as SiO.sub.2) and a partial hydrolysate of
 methyltriethoxysilane (12.6%, calculated as CH.sub.3 SiO.sub.1.5)
 Before use as a coating material, the two components .alpha.) and .beta.)
 are mixed together in a 1:1 ratio and dissolved in a mixture prepared from
 60 parts by weight of n-butanol, 40 parts by weight of acetic acid and 20
 parts by weight of toluene.
 d) Production of a Siloxane Coating Material According to DE-A 4 338 361
 (Coating Sol II)
 A boehmite sol was produced by combining 12.82 g of acetic acid-stabilised
 (6.4 wt. % acetic acid) boehmite powder with 104.62 g of 0.1 n HCl.
 Subsequent ultrasonication (20 minutes) produced a transparent, colourless
 solution, 24.3 g of which were combined with a mixture prepared from
 118.17 g of GPTS (3-glycidyloxypropyltrimethoxysilane) and 62.50 g of TEOS
 (tetraethyl orthosilicate). The reaction mixture was stirred for 2 hours
 at room temperature and then, while being cooled with ice, combined with
 18.93 g of aluminium tributoxyethanolate. The resultant clear sol was
 stirred for 2 hours at room temperature and then, while being cooled with
 ice, combined with 93.14 g of the above boehmite sol and 79.30 g of
 butoxyethanol.
 e) UV-stabilised Coating Sols I and II
 A 60 g portion of the UV-stabilising mixture 2 according to the invention
 was added to a 1000 g portion of each of coating sols I and II. Silica
 glass was coated with these compositions and UV light transmission
 measured with a Beckmann DU 70 photometer in the wavelength range from 250
 to 600 nm. The coating film was 5 .mu.m thick and absorbed &gt;98% of the
 radiation of a wavelength of &lt;350 nm critical for polycarbonate.
 Coating of Substrates and Testing of Coating Properties
 Bisphenol A polycarbonate sheets (T.sub.g =147.degree. C., M.sub.w 27500)
 of dimensions 105.times.150.times.4 mm were cleaned with isopropanol and
 primed by dipping in a mixture prepared from 3 wt. % of
 aminopropyltrimethoxysilane and 97 wt. % of ethylene glycol monobutyl
 ether followed by 30 minutes' heat treatment at 130.degree. C. The sheets
 were then provided with a 20 .mu.m film of one of coating sols I or II at
 a dipping speed V=100 cm/min. After flashing off for 10 minutes at room
 temperature, the coated sheets were dried for 1 hour at 130.degree. C. The
 film thickness of the scratch-resistant lacquers was approx. 5 .mu.m after
 drying. Once curing was complete, the coated sheets were stored for 2 days
 at room temperature and then exposed to a defined quantity of UV
 radiation.
 UV Exposure Testing
 The polycarbonate sheets were exposed to filtered xenon arc radiation with
 a water spray cycle to DIN 53387-1-A-X under the following test
 conditions:

Weathering apparatus: Xenon-WOM
 Radiation intensity at 340 nm: 0.35 W/m.sup.2 (preferably)
 Filter combination: inner: Pyrex, outer: Pyrex
 Blackboard temperature: 60.degree. C. .+-. 5.degree. C.
 Black standard temperature: 65.degree. C. .+-. 3.degree. C.
 Mode of operation: constant
 Water spray cycle: 102:18
 Relative atmospheric humidity: 60-80%
 Yellowing as a function of exposure time was used as the evaluation
 criterion for the weathering resistance of the lacquer-coated sheets. The
 corresponding yellowness of the sheets was determined as the Yellowness
 Index (Y.I.) to ASTM D 1925-70.
 Y.I. Values After Xenon-WOM 102:18 Weathering