WO2011134674A1

WO2011134674A1 - Improved light diffusing composition

Info

Publication number: WO2011134674A1
Application number: PCT/EP2011/002158
Authority: WO
Inventors: Weijun Zhou; William B. Marshall; Timothy J. Pope; William C. Sumner; Xuming Chen
Original assignee: Styron Europe Gmbh
Priority date: 2010-04-30
Filing date: 2011-04-29
Publication date: 2011-11-03
Also published as: TW201139532A

Abstract

The present invention relates to a light diffusing composition and its use for imparting light diffusing properties to a thermoplastic polymer. The present invention further relates to a polymer composition containing said novel light diffusing composition and to a compounded composition or formed article produced therefrom such as a cover. Said thermoplastic light diffusing composition comprises a) an inorganic particle, b) a polymeric particle differs in refractive index n at 589 nm by at least 0.05 from that of the thermoplastic resin polymer, and one or more of c), d), or mixtures thereof wherein c) is a wavelength downshifting material, and d) is an interference pigment. Further, the present invention relates to a luminous device comprising a light source, preferably one or more LED and a formed cover comprising said light diffusing composition.

Description

IMPROVED LIGHT DIFFUSING COMPOSITION

FIELD OF THE INVENTION

The present invention relates to a light diffusing composition and its use for imparting light diffusing properties to a thermoplastic polymer. The present invention further relates to a polymer composition containing said novel light diffusing composition and to a compounded composition or formed article produced therefrom such as a cover. Further, the present invention relates to a luminous device comprising a light source, preferably one or more LED and a formed cover comprising said light diffusing composition.

BACKGROUND OF THE INVENTION

The function of an illuminated sign is to capture the attention of customers, users, consumers, passers-by, etc., and therefore it is to be as visible as possible, both during the day and at night. It is therefore necessary for the elements that the sign contains (these may be letters, numbers, symbols, figures, etc.) to be visible in the daytime but also at night, or in half- darkness, when the sign is illuminated. It is therefore necessary for the sign to have a high contrast both in the daytime and at night.

An illuminated sign consists of a light source and a cover. Conventional light sources are incandescent lamps and/or neon tubes. More energy/heat efficient and longer life light emitting diodes (LEDs) are increasingly replacing traditional light sources. However, replacing a conventional light source with an LED light source results in a modification of the illumination and appearance of an illuminated sign. This is, in part, because an LED exhibits highly directional illumination whereas for example the illumination of a neon tube is from 0 to 360°. In addition, the emission spectrum of an LED is completely different from that of a conventional light source. The development of luminous devices incorporating one or more white LEDs with a luminous flux of greater than 3 lumen have changed the requirements of the thermoplastic suitable for use as the cover.

The function of the thermoplastic sign cover is to mask and protect the light source, while still ensuring good transmission of the light emitted by the light source. The cover also has the function of scattering the emitted light so that the illumination is uniform and not dazzling. The scattering of the light emitted by the light source is achieved by dispersing scattering particles of organic and/or mineral nature in the thermoplastic. The thermoplastic of the cover may be colored or may have decorative elements or patterns. The cover also often takes the form of a letter or any other sign.

It is well known to incorporate light diffusers into thermoplastic, transparent polymers. The light diffusers are generally in the form of a white powder. When light diffusers are incorporated into a thermoplastic polymer, the polymer composition and articles produced therefrom have light diffusing properties. Usually the resulting colors of these composites are called opalescent colors. They are semi-transparent or translucent to visible light, that is, they scatter transmitted light so that the light source is not visible. Light diffusing polymer compositions may be formed into sheets or films of various thicknesses, or into more complex shapes, such as lamp covers like globes etc. Light diffusing polymer compositions are widely used by the lighting industry for producing indicating panels, luminous displays, spotlights, street lighting, illuminated signs, etc. Accordingly, one important property of light diffusing polymer compositions and articles produced therefrom is a high resistance to thermal aging, particularly a high color stability when exposed to elevated temperatures over an extended time period.

It is known to incorporate certain inorganic additives into polymer compositions in order to impart to the compositions light-diffusing properties.

USP 6723772 discloses a scattering composition comprising a transparent plastic, based on methyl methacrylate or styrene, and organic or mineral scattering particles. The particle content is between 0.1 and 20 percent by weight.

US Publication 2004/0191550 discloses a sheet comprising a layer (A) composed of a resin based on methyl methacrylate and one or two layers (B) composed of a blend of a resin based on methyl methacrylate and a resin based on vinylidene fluoride. The sheet may contain organic or mineral scattering particles in the layer (A) and be illuminated by a neon lamp or by an LED.

US Publication 2004/0255497 discloses an illuminated sign in which the letters of the sign are made of a transparent plastic that scatters light. It does not matter what the light source is-LED, neon tube, incandescent lamp, etc.

EP 1369224 discloses a coextruded sheet comprising at least two layers, a layer of a thermoplastic filled with organic scattering particles and a layer of a transparent thermoplastic. The sheet may be illuminated by an LED. WO 2003/0523 1 5 discloses a luminous device that combines a cover made of a transparent plastic and a color LED. The transparent plastic contains 1 .5 to 2.5 percent by weight of BaSC>4 or polystyrene particles. In the case of crossl inked plastic particles, the content varies from 0.1 to 10 percent by weight. White LEDs are not described. In addition, the object of the invention is to ensure that an illuminated sign has the same color when it is off and when it is lit.

WO 2006/ 100127 discloses a luminous device comprising at least one LED wherein the cover is a single layer comprising organic particles, such as polyamide of PTFE particles, methyl-methacrylate-based cross-linked particles, cross-linked styrene-based particles or silicone particles for the purpose of scattering light. Alternatively, the scattering particles may be mineral of nature, such as BaSO^, Ti0₂, ZnO, CaC0₃, MgO, or AI2O3, or hollow glass microspheres.

EP 634445 discloses a light diffusing composition comprising a mixture of inorganic particles such as T1O2, silica gel, ZnS, ZnO, or MgTi0₃ and polymeric particles having a core/shell morphology wherein the core is a rubbery alkyl acrylate polymer. Said light diffusing composition demonstrates a similar spectrum of reflected light to transmitted light.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a light diffusing thermoplastic composition comprising a thermoplastic polymer and a) from 0.001 to 2 weight percent of an inorganic particle having an average particle diameter of from 0.1 to 1 microns and a refractive index of from 1 .9 to 3.2, b) from 0.01 to 10 weight percent of a polymeric particle having an average particle size in the range of from 0.2 to 20 microns that differs in refractive index n at 589 nm by at least 0.05 from that of the thermoplastic resin polymer, and one or more of c), d), or mixtures thereof wherein c) is from 0.1 to 1 ,000 ppm of a wavelength downshifting material, and d) is from 0.005 to 2 weight percent of an interference pigment.

In one embodiment of the present invention, the inorganic particles a) of the

thermoplastic light diffusing composition disclosed herein above are preferably titanium dioxide, silica gel, zinc sulfide, zinc oxide, MgTi0₃, or mixture thereof.

In another embodiment of the present invention, the polymeric particle b) of the thermoplastic light diffusing composition disclosed herein above are preferably a crosslinked acrylic polymer, a crosslinked styrene polymer, a crosslinked silicone polymer, or mixtures thereof. In another embodiment of the present invention, the polymeric particles b) of the thermoplastic light diffusing composition disclosed herein above preferably have a core/shell morphology comprising one or more shells wherein the core is a rubbery vinyl polymer comprising at least 15 weight percent of a polymerized alkyl acrylate or alkyl methacrylate based on the weight of the core.

In another embodiment of the present invention, the wavelength downshifting material c) of the thermoplastic light diffusing composition disclosed herein above preferably is an inorganic wavelength downshifting material selected from Sm³⁺, Cr³⁺, ZnSe, Eu, or Tb containing wavelength downshifting material, a quantum dot material, an organic downshifting material selected from Rhodamine, Coumarin, Rubrene, a laser dye, Alq3, TPD, Gaq₂Cl; a perylene carbonic acid or a derivative thereof, a naphthalene carbonic acid or a derivative thereof, a violanthrone or an iso-violanthrone or a derivative thereof, or mixtures thereof.

In another embodiment of the present invention, the interference pigment d) of the thermoplastic light diffusing composition disclosed herein above is preferably selected from mica flake + Ti0₂; mica flake + Zr0₂; Si0₂ flake + Ti0₂; Si0₂ flake + Zr0₂; Al₂0₃ flake + Ti0₂; Al₂0₃ flake + Zr0₂; glass flake + Ti0₂; glass flake + Zr0₂; glass flake + Si0₂ + Ti0₂; or combinations thereof.

In another embodiment of the present invention, the thermoplastic polymer of the thermoplastic light diffusing composition disclosed herein above is preferably selected from polycarbonate, polymethyl methacrylate, polystyrene, polystyrene and acrylonitrile copolymer, or mixtures thereof.

Another embodiment of the present invention is a single layer or multilayer thermoplastic light diffusion sheet with at least one layer comprising the thermoplastic light diffusing composition disclosed herein above.

Another embodiment of the present invention is a luminous devise comprising at least one light source and a cover made of the thermoplastic light diffusing composition disclosed herein above, preferably the at least one light source is a LED having a luminous flux greater than 3 Lm.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the diagram of a lamp type LED.

FIG. 2 shows an example of a letter of an illuminated sign illuminated by LEDs. FIG. 3 shows an illuminated sign comprising the letter of FIG. 2 at the top of a building during the daytime when it is not lit.

FIG. 4 shows the illuminated sign of FIG. 3 at the top of a building during night when it is lit.

DETAILED DESCRIPTION OF THE INVENTION

Surprisingly, it has been found that an article produced from a polymer composition comprising a combination of inorganic particles a), polymeric particles b), and one or both of a wavelength downshifting material c) and/or an interference pigment in the indicated amounts demonstrates good resistant to thermal aging, light diffusion properties, improved appearance in regard to brightness, and a difference in appearance depending on whether the light source is transmitted through or reflected off of the article.

The inorganic particles a) suitable for use in the present invention have an average particle diameter of from 0. 1 to I micrometer, preferably from 0. 1 to 0.8 micrometer, most preferably from 0.2 to 0.5 micrometer. Such inorganic particles and methods of producing them are well known in the art. By the term "average particle diameter" the number average is meant. The inorganic particles have a refractive index of from 1 .9 to 3.2, preferably from 2.0 to 2.9, most preferably from 2.0 to 2.7. Preferred inorganic particles a) are titanium dioxide (ΤΪΟΣ), silica gel, zinc sulfide (ZnS), zinc oxide (ZnO), MgTi0₃ particles, or mixtures thereof provided that their particle size distribution is within the indicated range. The most preferred inorganic particles are titanium dioxide particles. The inorganic particles can be used in the various modifications, for example the anatase, brookite or rutile configuration of titanium dioxide is useful. Titanium dioxide may be coated with a layer of siloxane. Coated titanium dioxide and a method of preparing it are known.

The inorganic particles a) are present in the thermoplastic light diffusing composition of the present invention in an amount equal to or greater than about 0.001 weight percent, preferably in an amount equal to or greater than about 0.005 weight percent, more preferably in an amount equal to or greater than about 0.01 weight percent, more preferably in an amount equal to or greater than about 0.05 weight percent, and more preferably in an amount equal to or greater than about 0.075 weight percent, wherein weight percent is based on the total weight of the thermoplastic light diffusing composition. The inorganic particles a) are present in the thermoplastic light diffusing composition of the present invention in an amount equal to or less than about 2 weight percent, preferably in an amount equal to or less than about 1 .5 weight percent, more preferably in an amount equal to or less than about 1 weight percent, more preferably in an amount equal to or less than about 0.5 weight percent, more preferably in an amount equal to or less than about 0.25 weight percent, and more preferably in an amount equal to or less than about 0.15 weight percent, wherein weight percent is based on the total weight of the thermoplastic light diffusing composition.

The polymeric particles b) suitable for use in the present invention may be a crosslinked acrylic polymer, a crosslinked styrene polymer, a crosslinked silicone polymer, or mixtures thereof. A preferred polymeric particle has a core/shell morphology having a core of a rubbery vinyl polymer. The rubbery vinyl polymer can be a homo-or copolymer of any of the monomers having at least one ethylenically unsaturated group which are well known to those skilled in the art to undergo addition polymerization under the conditions of emulsion polymerization in aqueous medium. Such monomers are listed in USP 4,226,752, column 3, lines 40-62, the teaching of which is included herein by reference. These monomers include 1 ,3-butadiene, 2- methyl- l ,3-butadiene, 2, 3-dimethyl- 1 ,3-butadiene, allylbenzene, diacetone acrylamide, vinylnaphthalene, chlorostyrene, 4-vinyl benzyl alcohol, vinyl benzoate, vinyl propionate, vinyl caproate, vinyl chloride, vinyl oleate, dimethyl maleate, maleic anhydride, dimethyl fumarate, vinyl sulfonic acid, vinyl sulfonamide, and methyl vinyl sulfonate. Particularly preferred monomers include for example, N-vinyl pyrolidone, vinyl pyridine, styrene, alpha-methyl styrene, tertiary butyl styrene, vinyl toluene, divinyl benzene, vinyl acetate, vinyl versatate, alkyl acrylates and methacrylates such as ethyl acrylate, butyl acrylate, 1 ,6-hexanediol diacrylate, ethylthioethyl methacrylate, methyl acrylate, isobornyl acrylate, 2-hydroxyethyl acrylate, 2- phenoxyethyl acrylate, glycidyl acrylate, 2-ethylhexyl acrylate, neopentyl glycol diacrylate, 2- ethoxyethyl acrylate, t-butylaminoethyl methacrylate, 2-methoxyethyl acrylate, methyl methacrylate, glycidyl methacrylate, benzyl methacrylate, ethyl methacrylate, acrylic acid, methacrylic acid, N-methyl methacrylamide, acrylonitrile, methacrylonitrile, acrylamide, N- (isobutoxymethyl)acrylamide, and the like. The rubbery vinyl polymer preferably contains at least 15 percent, more preferably at least 25 percent, most preferably at least 40 percent of a polymerized acrylate, methacrylate, monovinyl arene or optionally substituted butadiene and from 0 to 85 percent, more preferably from 0 to 75 percent, most preferably from 0 to 60 percent of one or more copolymerized vinyl monomers, based on the total weight of the rubbery vinyl polymer.

Preferred acrylates and methacrylates are alkyl acrylates or alkyl methacrylates which preferably contain I to 1 8, more preferably 1 to 8, most preferably 2 to 8, carbon atoms in the alkyl group, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, s-butyl or t-butyl or the hexyl, heptyl or octyl groups. The alkyl group may be branched or linear. The preferred alkyl acrylates are ethyl acrylate, n-butyl acrylate, isobutyl acrylate or 2-ethylhexyl acrylate. The most preferred alkyl acrylate is butyl acrylate.

Other useful acrylates are for example 1 , 6-hexanediol diacrylate, ethylthioethyl methacrylate, isobornyl acrylate, 2-hydroxyethyl acrylate, 2-phenoxyethyl acrylate, glycidyl acrylate, neopentyl glycol diacrylate, 2-ethoxyethyl acrylate t-butylaminoethyl methacrylate, 2- methoxyethyl acrylate, glycidyl methacrylate or benzyl methacrylate.

Preferred monovinyl arenes are styrene or alpha-methyl styrene, optionally substituted at the aromatic ring with an alkyl group, such as methyl, ethyl or tertiary butyl or with a halogen, such as chlorostyrene.

If substituted, the butadiene preferably is substituted with one or more alkyl groups containing I to 6 carbon atoms or with one or more halogens, most preferably with one or more methyl groups and/or one or more chlorines. Preferred butadienes are 1 , 3-butadiene, isoprene, chlorobutadiene, or 2, 3-dimethyl- l , 3-butadiene.

The rubbery vinyl polymer may contain one or more (co)polymerized acrylates, methacrylates, monovinyl arenes and/or optionally substituted butadienes. These monomers may be copolymerized with one or more other copolymerizable vinyl polymers, such as diacetone acrylamide, vinylnaphthalene, 4-vinyl benzyl alcohol, vinyl benzoate, vinyl propionate, vinyl caproate, vinyl chloride, vinyl oleate, dimethyl maleate, maleic anhydride, dimethyl fumarate, vinyl sulfonic acid, vinyl sulfonamide, methyl vinyl sulfonate, N-vinyl pyrrolidone, vinyl pyridine, divinyl benzene, vinyl acetate, vinyl versatate, acrylic acid, methacrylic acid, N-methyl methacrylamide, acrylonitrile, methacrylonitrile, acrylamide or N- (isobutoxymethyl)acrylamide.

One or more of the above-mentioned monomers are optionally reacted with 0 to 10 percent, preferably with 0 to 5 percent, of a copolymerizable polyfunctional cross-linker and/or with 0 to lO percent, preferably with 0 to 5 percent, of a copolymerizable polyfunctional graftlinker, based on the total weight of the core, If a cross-linking monomer is employed, it is preferably used at a level of from 0.05 to 5 percent, more preferably from 0.1 to 1 percent, based on the total weight of the core monomers. Crosslinking monomers are wel l known in the art and generally have a polyethylenic unsaturation in which the ethylenically unsaturated groups have approximately equal reactivity, such as divinylbenzene, trivinylbenzene, 1 , 3-or 1 , 4-triol acrylates or methacrylates, glycol di-or trimethacrylates or-acrylates, such as ethylene glycol dimethacrylate or diacrylate, propylene glycol dimethacrylate or diacrylate, 1 , 3-or 1 , 4-butylene glycol dimethacrylate or, most preferably, 1 , 3-or 1 , 4-butylene glycol diacrylate. If a graftlinking monomer is employed, it is preferably used at a level of from 0.1 to 5 percent, more preferably from 0.5 to 2.5 percent, based on the total weight of the core monomers. Graftlinking monomers are well known in the art and generally are polyethylenically unsaturated monomers having sufficiently low reactivity of the unsaturated groups to allow significant residual unsaturation to remain in the core subsequent to its polymerization. Preferred graftlinkers are copolymerizable allyl, methallyl or crotyl esters of a, β-ethylenically unsaturated carboxylic acids or dicarboxylic acids, such as allyl methacrylate, allyl acrylate, diallyl maleate, and allyl acryloxypropionate, most preferably allyl methacrylate.

Most preferably, the polymeric particles b) contain a core of a rubbery alkyl acrylate polymer, the alkyl group having from 2 to 8 carbon atoms, optionally copolymerized with from 0 to 5 percent cross-linker and from 0 to 5 percent graft-linker, based on the total weight of the core. The rubbery alkyl acrylate is preferably copolymerized with up to 50 percent of one or more copolymerizable vinyl monomers, for example those mentioned above. Suitable cross- linking and graft-linking monomers are well known to those skilled in the art and are preferably those disclosed in published EP 269324. For example, cross-linking monomers are generally monomers copolymerizable with the other core monomers and have polyethylenic unsaturation in which the ethylenically unsaturated groups have approximately equal reactivity, as for example divinylbenzene, glycol di- and trimethacrylates and acrylates and triol triacrylates and methacrylates. Graftlinking monomers generally are polyethylenically unsaturated monomers copolymerizable with the other core monomers and have sufficiently low reactivity of the unsaturated groups to allow significant residual unsaturation to remain in the core polymer subsequent to its polymerization, as for example allyl methacrylate, diallyl maleate and allyl acryloxypropionate. The core of the polymeric particles b) may contain residual oligomeric material used in the polymerization process to swell the polymer particles, but such oligomeric material has a high enough molecular weight to prevent its diffusion or being extracted during processing or use.

The polymeric particles b) contain one or more shells. Said one or more shells are preferably made of a vinyl homo-or copolymer. Suitable monomers for producing the shell(s) are listed in USP 4,226,752, column 4, lines 20-46, the teaching of which is incorporated herein by reference. These monomers can include ethylenically unsaturated vinyl monomers such as 1 ,3-butadiene, 2-methyl- l ,3-butadiene, 2,3-dimethyl- l ,3-butadiene, allylbenzene, diacetone acrylamide, vinylnapthalene, chlorostyrene, 4-vinyl benzyl alcohol, vinyl benzoate, vinyl propionate, vinyl caproate, vinyl chloride, vinyl oleate, dimethyl maleate, maleic anhydride, dimethyl fumarate, vinyl sulfonic acid, vinyl sulfonamide, methyl vinyl sulfonate, and preferably N-vinyl pyrolidone, vinyl pyridine, styrene, alpha-methyl styrene, tertiary butyl styrene, vinyl toluene, divinyl benzene, vinyl acetate, vinyl versatate, alkyl acrylates and methacrylates such as ethyl acrylate, butyl acrylate, 1 ,6-hexanediol diacrylate, ethylthioethyl methacrylate, methyl acrylate, isobomyl acrylate, 2-hydroxyethyl acrylate, 2-phenoxyethyl acrylate, glycidyl acrylate, 2-ethylhexyl acrylate, neopentyl glycol diacrylate, 2-ethoxyethyl acrylate, t-butylaminoethyl methacrylate, 2-methoxyethyl acrylate, methyl methacrylate, glycidyl methacrylate, benzyl methacrylate, ethyl methacrylate, acrylic acid, methacrylic acid, N-methyl methacrylamide, acrylonitrile, methalcrylonitrile, acrylamide, N-(isobutoxymethyl)acrylamide, and the like. One or more shells are preferably a polymer of a methacrylate, acrylate, vinyl arene, vinyl carboxylate, acrylic acid and/or methacrylic acid.

Preferred acrylates and methyacrylates are alkyl acrylates or alkyl methacrylates which preferably contain 1 to 18, more preferably 1 to 8, most preferably 2 to 8, carbon atoms in the alkyl group, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl or t-butyl, 2-ethylhexyl or the hexyl, heptyl or octyl groups. The alkyl group may be branched or linear. The preferred alkyl acrylate is ethyl acrylate. Other useful acrylates and methacrylates are those listed above for the core, preferably the 3-hydroxypropyl methacrylate. The most preferred alkyl methacrylate is methyl methacrylate. Preferred vinyl arenes are styrene or a-methyl styrene, optionally substituted at the aromatic ring with an alkyl group, such as methyl, ethyl or tertiary butyl or with a halogen, such as chlorostyrene.

A preferred vinyl carboxylate is vinyl acetate.

The shell(s) preferably contain(s) at least 15 percent, more preferably at least 25 percent, most preferably at least 40 percent of a polymerized methacrylate, acrylate or monovinyl arene and 0 to 85 percent, more preferably 0 to 75 percent, most preferably 0 to 60 percent of one or more vinyl comonomers, such as other alkyl methacrylates, aryl methacrylates, alkyl acrylates, aryl acrylates, alkyl and aryl acryl amides, acrylonitrile, methacrylonitrile, maleimide and/or alkyl and aryl acrylates and methacrylates being substituted with one or more substituents, such as halogen, alkoxy, alkylthio, cyanoalkyl or amino. Examples of suitable vinyl comonomers are listed above. Two or more monomers can be copolymerized.

The shell polymer may contain a cross-linker and/or a graft-linker which are the type indicated above with respect to the core polymer.

The shell polymers preferably comprise from 5 to 40 percent, more preferably from 15 to

35 percent, of the total particle weight.

The polymeric particles b) contain at least 15 percent, preferably from 20 to 80 percent, more preferably from 25 to 60 percent, most preferably from 30 to 50 percent, of a polymerized alkyl acrylate or methacrylate, based on the total weight of the polymer. Preferred alkyl acrylates and methacrylates are listed above. The alkyl acrylate or alkyl methacrylate constituent can be comprised in the core and/or in the shell(s) of the polymeric particles b). Homopolymers of an alkyl acrylate or methacrylate in the core and/or the shell(s) are useful; however, an alkyl (meth) acrylate is preferably copolymerized with one or more other types of alkyl (meth) acrylates and/or one or more other vinyl polymers, preferably those listed above. Most preferably, the polymeric particles b) contain a core of a poly(butyl acrylate) and one or more shells of poly(methyl methacrylate).

The polymeric particles b) are useful for imparting light diffusing properties to thermoplastic polymers. The refractive index n of core and of the shell(s) of the polymeric particles b) preferably is within +/- 0.25 units of, more preferably within +/-0.18 units of, most preferably within +/- 0.12 units of the refractive index of the thermoplastic polymer. The refractive index n of the core and of the shell(s) preferably is not closer than +/-0.003 units to, more preferably not closer than +/-0.0 I units to, most preferably not closer than +/-0.05 units to the refractive index of the thermoplastic polymer. The refractive index is measured according to ASTM D 542-50 and/or DIN 53400. A particularly usefully wavelength for comparison of the refractive index n of the core/shell rubber to the refractive index n of the thermoplastic polymer is 589 nm.

The polymeric particles b) generally have an average particle diameter of equal to or greater than about 0.2 micrometer, preferably an average particle diameter equal to or greater than about 0.5 micrometer, more preferably an average particle diameter equal to or greater than about 1 micrometer, and more preferably an average particle diameter equal to or greater than about 1.5 micrometer.

The polymeric particles b) generally have an average particle diameter of equal to or less than about 50 micrometer, preferably an average particle diameter equal to or less than about 20 micrometer, more preferably an average particle diameter equal to or less than about 10 micrometer, more preferably an average particle diameter equal to or less than about 7.5 micrometer, and more preferably an average particle diameter equal to or less than about 5 micrometer.

By "average particle diameter" the number average is meant. The polymeric particles b) are preferably a free-flowing powder.

The polymer particles b) can be produced in a known manner. Generally, at least one monomer component of the core polymer is subjected to emulsion polymerization to form emulsion polymer particles. The emulsion polymer particles are swollen with the same or one or more other monomer components of the core polymer and the monomer(s) are polymerized within the emulsion polymer particles. The swelling and polymerizing steps may be repeated until the particles have grown to the desired core size. The core polymer particles are suspended in a second aqueous monomer emulsion and a polymer shell is polymerized from the monomer(s) onto the polymer particles in the second emulsion. One or more shells can be polymerized on the core polymer. The preparation of core/shell polymer particles is disclosed in published EP 269324 and in USP 3,793,402 and USP 3,808, 180, both of which are incorporated herein by reference.

In general, the process may include: A) selecting as a rubbery core polymer composition a copolymer of an alkyl acrylate, the alkyl group having from 2 to 8 carbon atoms and the copolymer having a refractive index within +/-0.05 units of, but no closer than about +/-0.003 units to, the refractive index of the thermoplastic matrix polymer,

B) polymerizing particles of core polymer, from a first aqueous emulsion of one or more of the monomers which, when polymerized, produce the selected core polymer,

C) performing one or more steps of

1 ) swelling the paricles of core polymer with one or more of the monomers, which, when polymerized, produce the selected core polymer, and

2) polymerizing the swelling monomer within the particles of core polymer, until all of the monomers which lead to the selected core polymer have been polymerized, in these particles and the particles have reached a desired size within the range from about 2 to about 15 mu m, and

D) performing one or more steps of

1 ) suspending the core polymer particles in a second aqueous monomer emulsion, the monomers of which being polymerisable to form a polymer compatible with the matrix polymer, and

2) polymerizing onto the polymer particles a polymer shell from the monomer in the second emulsion.

The polymeric particles b) are present in the thermoplastic light diffusing composition of the present invention in an amount equal to or greater than about 0.01 weight percent, preferably in an amount equal to or greater than about 0.05 weight percent, more preferably in an amount equal to or greater than about 0.1 weight percent, more preferably in an amount equal to or greater than about 0.25 weight percent, and more preferably in an amount equal to or greater than about 0.5 weight percent, wherein weight percent is based on the total weight of the

thermoplastic light diffusing composition.

The polymeric particles b) are present in the thermoplastic light diffusing composition of the present invention in an amount equal to or less than about 20 weight percent, preferably in an amount equal to or less than about 10 weight percent, more preferably in an amount equal to or less than about 7.5 weight percent, more preferably in an amount equal to or less than about 5 weight percent, more preferably in an amount equal to or less than about 2.5 weight percent, and more preferably in an amount equal to or less than about 1 weight percent, wherein weight percent is based on the total weight of the thermoplastic light diffusing composition.

The wavelength downshifting material c) suitable for use in the present invention may comprise one or more organic and/or inorganic luminescent materials. Thus, a single wavelength downshifting material may be used, or, alternatively or additionally, a "chain" of wavelength downshifting materials may be used, e.g., in order to provide a wavelength downshifting "cascade".

Preferably, one or more of the following materials are used (solely or as mixtures): an organic luminescent material, preferably Rhodamine, Coumarin, Rubrene, a laser dye, Alq3, TPD, Gaq2CI; a perylene carbonic acid or a derivative thereof; a naphthalene carbonic acid or a derivative thereof; a violanthrone or an iso-violanthrone or a derivative thereof; an inorganic luminescent material, preferably Sm³⁺, Cr³⁺, ZnSe, Eu²⁺, Tb³⁺ downshifting luminescent materials, a semiconducting quantum dot material, or Ag nanoparticles. Therein, Alq3 is aluminium tris-(8-hydroxyquinoline), TPD is N, N'-diphenyl-N, N'-bis-(3-methylphenyl)- l , - biphenyl- 4-4'-diamine, and GaqjCl denotes bis-(8-hydroxy-quinoline)-chlorogallium.

The presence of a wavelength downshifting material in an amount of from l ppm to 2000 ppm, more preferably of from 200 ppm to 1000 ppm has proven to exhibit advantageous effects with regard to improved appearance, specifically brightness of the polymer compositions comprising the improved light diffusing composition of the present invention. Nevertheless, the precise optimum concentration may depend on the nature of the wavelength downshifting material(s) and/or the host material.

Further, in order to improve the spectral response in the short-wavelength region, it has proven to be advantageous if the wavelength downshifting material exhibits a maximum in absorption of electromagnetic radiation within a spectral range from 300 to 500 nanometers, preferably at approximately 400 nanometers. Further, preferably, the wavelength downshifting material may exhibit a maximum in emission of electromagnetic radiation within a spectral range from 400 to 700 nanometers, preferably within a range of 400 to 500 nanometers or within a range of 500 to 600 nanometers, and most preferably at approximately 500 nanometers.

As outlined above, the downshifting materials may preferably comprise a dye comprising one or more of the group consisting of a perylene carbonic acid or a derivative thereof, a naphthalene carbonic acid or a derivative thereof, a violanthrone or an iso- violanthrone or a derivative thereof.

As a fluorescent dye based on a perylene carbonic acid or a derivative thereof (herein after: "perylene dye"), preferably one or more of the following dyes may be used: a perylene tetracarbonic acid diimide, a perylene tetracarbonic acid monoanhydride monoimide, a perylene tetracarbonic acid dianhydride, a perylene dicarbonic acid imide, a perylene-3, 4-dicarbonic acid anhydride, a perylene dicarbonic acid ester, a perylene dicarbonic acid amide. Thereof, the most preferred are perylene dicarbonic acids, perylene dicarbonic acid imides or perylene tetracarbonic acid diimides or combinations thereof.

Therein, perylene dicarbonic acid imides are derived from perylene-3, 4-dicarbonic acid, and perylene dicarbonic acid esters and amides are derived from isomeric perylene- 3, 9- and -3, 10-dicarbonic acids.

Of the perylene carbonic acid imides, hydrogen or Ci-Cis -alk I are especially suited as substituents on the imide nitrogen atom.

The perylene dyes may be unsubstituted. However, preferably, they are substituted by 1 to 5 (in case of perylene tetracarbondiimide preferably 2 to 4) (het)aryloxy- or (het)arylthio substituent R.

The substituent R is defined as follows:

R is aryloxy, arylthio, hetaryloxy or hetarylthio, annulated with saturated or unsaturated 5- to 7- membered ring systems, which carbon atom backbone can be interrupted with one or more functional groups of -0-, -S-, -NR¹-, -N=CR'-, -CO-, -SO- and/or -SO²-, whereas the whole ring system can be substituted by one or more substituent (i), (ii), (iii), (iv) and/or (v):

(i) is C|-C3o-alkyl, which carbon atom chain can be interrupted by one or more groups of

-0-, -S-, -NR¹-, -N= CR¹-, -C≡ C-, -CR'= CR¹ -, -CO-, -SO- and/or -SO²- and can be substituted by one or more substituent of: Ci- C|₂-alkoxy, Ci-Ce-alkylthio, -C≡ CR¹,

-CR'= CR¹:, hydroxy, mercapto, halogen, cyano, nitro, -NR R³, -NR²COR³, -CONR²R³, -S0²NR²R³, -COOR², -S0₃R², aryl and/or saturated or unsaturated C₄-C₇-cycloalkyl, which carbon atom backbone can be interrupted by one or more groups of -0-, -S-,

-NR¹-, -N= CR¹-, -CR'= CR'-, -CO-, -SO- and/or -SO²-, whereas each of the aryl- and cycloalkyl substituent can be substituted by one or more substituent of Ci-Qs-alkyl and/or the previously as substituents for alkyl mentioned substituents. (ii) is Cs-Ce-cycloalkyl, which carbon atom backbone can be interrupted by one or more groups of -0-, -S-, -NR¹-, -N= CR¹-, -CR'= CR ¹-, -CO-, -SO- and/or -SO²- annulated with saturated or unsaturated 5- to 7-membered ring systems, which carbon atom backbone can be interrupted with one or more functional groups of -0-, -S-, -NR¹-,

-N= CR¹-, -CR'= CR¹-, -CO-, -SO- and/or -S0₂-, whereas the whole ring system can be substituted by one or more substituents of: Ci-C]₈-alkyl, C|-C|2-alkoxy, Ci-Ce-alkylthio, -C≡ CR¹, CR¹- = CR'₂, hydroxy, mercapto, halogen, cyano, nitro, -NR²R³, -NR²COR³, -CONR²R³, -S0₂NR²R³, -COOR² and/or -S0₃R²;

(iii) aryl or hetaryl, which can be annulated with further saturated or unsaturated 5- to 7- membered rings, which carbon atom backbone can be interrupted by one or more groups of -0-, -S-, -NR¹-, -N= CR¹-, -CR'= CR¹-, -CO-, -SO- and/or -S0₂-, whereas the whole ring system can be substituted by one or more substituents of: C| -C i8-alkyl, C | -Ci₂-alkoxy, C| -C6-alkylthio, -C≡ CR¹, CR'= CR'₂, hydroxy, mercapto, halogen, cyano, nitro, -NR²R³, -NR²COR³, -CONR R³, -

-COOR², -SO3R², aryl and/or hetaryl, which by itself can be substituted by C,-C₁₈- alkyl, Ci-C₁₂-alkoxy, hydroxy, mercapto, halogen, cyano, nitro, -NR²R³, -NR²COR³, -CONR²R³, -S0₂NR²R³, -COOR² and/or - SO3R²;

(iv) is one substituent -U-aryl, which can be substituted by one or more of the above mentioned substituents of the aryl substituent (iii), whereas U can be a group of -0-, - S-,

-NR¹-, -CO-, -SO- or -S0₂-;

(v) is Ci-C|₂-alkoxy, C)-C₆-alkylthio, -C≡ CR¹, Rl , CR'= CR'2, hydroxy, mercapto, halogen, cyano, nitro, -NR R³, -NR²COR³, -CONR²R³, -S0₂NR²R³, -COOR² or

-SO3R², whereas the substituent R for n > 1 can be identical or different from each other; R¹ hydrogen or C]-C₈-alkyl, whereas the residual R¹ can be identical or different, if they appear multiple times; R², R³ independently hydrogen; C)-C|₈-alkyl, which carbon atom chain can be interrupted with one or more groups of -0-, -S-, -CO-, -SO- and/or -S0₂ - and can be substituted one or more times by C|-C|₂-aIkoxy, C_rC6-alkylthio, hydroxy, mercapto, halogen, cyano, nitro and/or -COOR¹ ; aryl or hetaryl, which can be annulated by additional saturated or unsaturated 5- to 7- membered rings, which carbon atom backbone can be interrupted by one or more groups of -0-, -S-, -CO- and/or -S0₂-, whereas the complete ring system can be substituted one or more times by C|-C|₂-alkyl and/or the previously mentioned substituents of alkyl. In addition, perylene dyes can be substituted by cyanogroups. This substitution has great importance for perylene dicarbonic acid imide and perylene dicarbonic acid esters.

The following examples of especially suited perylene dyes shall be mentioned: Ν,Ν'- bis(2, 6-diisopropylphenyl)perylene-3, 4:9, 10-tetracarbonic acid diimide, N,N'-bis(2, 6- dimethylphenyl)perylene-3, 4:9, 10-tetracarbonic acid diimide, N,N'-bis(7- tridecyl)perylene-3, 4:9, 10-tetracarbonic acid diimide, N,N'-bis(2, 6-diisopropylphenyl)- 1 , 6, 7, 12-tetra(4-t- octylphenoxy)perylene-3, 4:9, 10-tetracarbonic acid diimide, Ν,Ν'- bis(2, 6-diisopropylphenyl)- 1 , 6, 7, 12-tetraphenoxyperylene-3, 4:9, 10-tetracarbonic acid diimide, N,N'-bis(2, 6- diisopropylphenyl)-l , 6- and -1 , 7-bis(4-tert- octylphenoxy)-perylene-3, 4:9, 10-tetracarbonic acid diimide, N,N'-bis(2, 6- diisopropylphenyl)-l , 6- and -1 , 7-bis(2,6-diisopropylphenoxy)- perylene-3, 4:9, 10- tetracarbonic acid diimide, N-(2, 6-diisopropylphenyl)perylene-3, 4- dicarbonic acid imide, N-(2, 6-diisopropylphenyl)-9-phenoxyperylene-3, 4-dicarbon acid imide, N-(2, 6- diisopropylphenyl)-9-(2, 6-diisopropylphenoxy)perylene-3, 4-dicarbonic acid imide, N- (2, 6-diisopropylphenyl)-9-cyanoperylene-3, 4-dicarbonic acid imide, N-(7-tridecyl)-9- phenoxyperylene-3, 4-dicarbonic acid imide, perylene-3, 9- and -3, 10-dicarbonic acid diisobutyl-ester, 4, lO-dicyanoperylene-3, 9- and 4, 9-dicyanoperylene-3, 10-dicarbonic acid diisobutyl-ester and perylene-3, 9- and -3, 10-dicarbonic acid di(2, 6- diisopropylphenyl)amide.

Perylene dyes are well known respectively described in USP 2008/0167467 (substitution with 0-, o'-disubstituted (thio)phenoxy substituent R), which is incorporated by reference herein in its entirety. They usually absorb in the wavelength region of 360 to 630 nanometer (nm) and emit between 470 to 750 nm.

Besides perylene dyes, other fluorescent dyes having similar structures may be employed, such as dyes on the basis of violanthrones and/or iso-violanthrones, such as the structures disclosed in US 4,486,587, which is incorporated by reference herein in its entirety. As a preferred example of well suited materials, alkoxylated violanthrones and/or iso-violanthrones, may be employed, such as 6, 15-didodecyloxyisoviolanthronedion-(9, 18).

Finally, as a further example of suitable fluorescent dyes, dyes on the basis of naphtha- lencarbonic acid derivatives may be named. Fluorescent dyes on the basis of naphthalene typically exhibit an absorption within the UV range at wavelengths of approximately 300 to 420 nm and exhibit an emission range at approximately 380 to 520 nm. Thus, as a further advantage, these dyes not only effect an efficient conversion of UV light into longer wavelength light, but also may form an efficient protection of the polymer composition against UV radiation.

Within the naphthalene carbonic acid derivatives, the most preferred are imides (e.g., naphthalene- 1 , 8:4, 5-teti acarbonic acid diimides, and especially naphthalene- 1 , 8-dicarbonic acid imides, most preferably 4, 5-dialkoxynaphthalene-l , 8-dicarbonic acid monoimides and 4- phenoxynaphthalene-1 , 8-dicarbonic acid monoimides, which are, in the following, abbreviated by "naphthalic imides"). Naphthalic imides, especially naphthalene-1 , 8:4, 5-tetracarbonic acid diimides, may also be unsubstituted in the naphthalene frame. Nevertheless, preferably, especially the naphthalene dicarbonic acid imides have one or preferably two alkoxy-, aryloxy- or cyano groups as substituents.

The alkoxy groups preferably comprise 1 to 24 C-atoms. Within the aryloxy groups, most preferred are phenoxy moieties, which may be unsubstituted or substituted.

As examples of naphthalic imides which are especially well suited, the following may be named: N-(2-ethylhexyl)-4, 5-dimethoxynaphthalene-l , 8-dicarbonic acid imide, N-(2, 6- diisopropyl-phenyl)-4, 5-dimethoxynaphthalene-l , 8-dicarbonic acid imide, N-(7-tridecyl)-4, 5- dimethoxy-naphthalene- 1 , 8-dicarbonic acid imide, N-(2, 6- diisopropylphenyl)-4, 5- diphenoxynaphthalene- 1 , 8-dicarbonic acid imide and Ν,Ν'- bis(2, 6-diisopropylphenyl)-l , 8:4, 5-naphthalene tetracarbonic acid diimide.

The wavelength downshifting material c) is present in the thermoplastic light diffusing composition of the present invention in an amount equal to or greater than about 0.01 parts per million (ppm), preferably in an amount equal to or greater than about 0.05 ppm, more preferably in an amount equal to or greater than about 0.1 ppm, more preferably in an amount equal to or greater than about 0.5 ppm, and more preferably in an amount equal to or greater than about 1 ppm, wherein ppm is based on the total weight of the thermoplastic light diffusing composition.

The wavelength downshifting material c) is present in the thermoplastic light diffusing composition of the present invention in an amount equal to or less than about 1000 ppm, preferably in an amount equal to or less than about 500 ppm, more preferably in an amount equal to or less than about 100 ppm, more preferably in an amount equal to or less than about 50 ppm, more preferably in an amount equal to or less than about 25 ppm, and more preferably in an amount equal to or less than about 10 ppm, wherein ppm is based on the total weight of the thermoplastic light diffusing composition. If the concentration of downshifting materials is too high, the re-absorption effect will decrease conversion efficiency.

We have found that an interference pigment d) alone or in combination with the wavelength downshifting material c) can further provide desirable light diffusing and color properties to the thermoplastic light diffusing composition of the present invention. Interference pigments are employed in many automotive and industrial paint formulations. Their effectiveness is due, in part, to the interplay of colors and to effects due to the various materials employed in their layered structures. Refractions and reflections of light at, and within, those layers cause interferences yielding selected colors. Interference pigments manipulate incident light by means of refractions and reflections such that the resultant refracted and reflected light generates color perceptions in the human eye and brain.

Interference pigments may be classified by either the method employed for their manufacture or by their structure. Substances, such as titanium dioxide or iron oxide, that have high indices of refraction, may, for example, be deposited on a transparent substrate, such as mica, as in the case of IRIODrN™, silicon dioxide, as in the case of

COLORSTREAM™, or aluminum oxide, as in the case of XIRALLIC™. Such pigments are produced using wet-chemical processes, while those having an aluminum layer as an internal reflector (VARIOCHROM™, CHROMAFLAIR™, and SPECTRAFLAIR™) are manufactured in high vacuum. Liquid crystals are also classed as interference pigments.

The interference pigment d) suitable for use in the present invention is based on one or more flake-form transparent substrates. Preferred substrates are phyllosilicates. Particularly suitable are natural and/or synthetic mica, flake-form aluminum oxides, glass flakes, Si0₂ flakes, Ti0₂ flakes, synthetic support-free flakes, BiOCl or other comparable materials. The glass flakes, A I 2O3 flakes and Si0₂ flakes may also be doped.

It is also possible to employ mixtures of different substrates or mixtures of identical substrates having different particle sizes. The substrates can be mixed with one another in any weight ratio. Preference is given to the use of 10: 1 to 1 : 10 mixtures, in particular 1 : 1 mixtures. Particular preference is given to substrate mixtures consisting of mica flakes having different particle sizes, in particular mixtures of N mica (less than 60 micrometer) and F mica (less than 25 micrometer). The size of the base substrates is not crucial per se and can be matched to the particular application.

In general, the flake-form substrates have a thickness between 0.05 and 5 micrometer, in particular between 0. 1 and 1 micrometer. The size in the two other dimensions is usually between 1 and 250 micrometer, preferably between 2 and 200 micrometer and in particular between 5 and 60 micrometer.

The thickness of at least one individual layer on the base substrate is essential for the optical properties of the pigment. The pigment must have at least one optically active layer, preferably a high-refractive-index layer, high-refractive-index layers here are taken to mean all layers which have a refractive index of n greater than 1.8, preferably n equal to or greater than 2.0.

The optical layer preferably consists of Ti0₂, Zr0₂, Sn0₂, ZnO, or mixtures or combinations thereof. The layer may be undoped or doped. Suitable dopants are, for example, alkaline-earth metals or compounds thereof, in particular calcium and magnesium. The doping proportion is generally at most 5 percent by weight, based on the respective layer.

The optical layer is particularly preferably a colorless layer, in particular a Ti0₂ layer. The Ti0₂ here can be in the rutile and in the anatase modification, preferably rutile.

The thickness of the optically active layer is preferably 30 to 350nm, in particular 50 to

250nm.

Particularly preferred interference pigments are: mica flake + Ti0₂; mica flake + Zr0₂;

S1O2 flake + Ti0₂; Si0₂ flake + Zr0₂; Al₂0₃ flake + Ti0₂; Al₂0₃ flake + Zr0₂; glass flake + Ti0₂; glass flake + Zr0₂; glass flake + Si0₂ + Ti0₂; or combinations thereof.

Suitable interference pigments are likewise multilayered pigments so long as they have at least one and at most two identical, optically active layers. Particular preference is given to multilayered pigments which have a Ti0₂-Si0₂ (optically inactive Iayer)-Ti0₂ layer sequence. Pigments of this type are known, for example, from EP 0882099. The optically inactive layers are generally Si0₂ and/or Al₂Cb layers having layer thicknesses of less than l Onm, preferably less than 5nm. In addition, the pigments may also comprise further auxiliary layers above or below the interference layer, for example for control of the rutilisation, the particle growth or for inhibiting the photoactivity. The preparation of interference pigments has been described many times in the literature and is known to the person skilled in the art, for example see US Publication 2008/0210133, incorporated by reference herein in its entirety.

The interference pigment d) are present in the thermoplastic light diffusing composition of the present invention in an amount equal to or greater than about 0.001 weight percent, preferably in an amount equal to or greater than about 0.005 weight percent, more preferably in an amount equal to or greater than about 0.01 weight percent, more preferably in an amount equal to or greater than about 0.05 weight percent, and more preferably in an amount equal to or greater than about 0.075 weight percent, wherein weight percent is based on the total weight of the thermoplastic light diffusing composition.

The interference pigment d) are present in the thermoplastic light diffusing composition of the present invention in an amount equal to or less than about 2 weight percent, preferably in an amount equal to or less than about 1.5 weight percent, more preferably in an amount equal to or less than about 1 weight percent, more preferably in an amount equal to or less than about 0.5 weight percent, more preferably in an amount equal to or less than about 0.25 weight percent, and more preferably in an amount equal to or less than about 0.15 weight percent, wherein weight percent is based on the total weight of the thermoplastic light diffusing composition.

The light diffuser composition of the present invention may contain other inorganic pigments, such as barium sulfate; however, the inclusion of such other inorganic pigments is less preferred. The light diffuser composition of the present invention may contain other organic light diffusers, such as crosslinked poly(methyl methacrylates), polyolefins, MBS-rubbers or another light diffuser described herein below. However, the light diffuser composition of the present invention is also very useful in the absence of any significant amounts of other pigments or light diffusers.

The light diffuser composition of the present invention can be prepared by blending the inorganic particles a), the polymeric particles b), and one or more of the wavelength

downshifting material c) and/or interference pigment d) in the above-indicated weight ratios. The blending can be conducted in an organic diluent. Preferably the dry components a), b), and one or more of c) and/or d), and any additional components are blended. Alternatively, one or more of the inorganic particles a) and/or the wavelength downshifting material c) and/or the interference pigment d) can be added prior to or during the production of the polymeric particles b).

The blending temperature is not critical. Room temperature is the most convenient one; however, decreased or elevated temperatures are also useful.

The light diffuser composition of the present invention is very useful for imparting light diffusing properties to any suitable thermoplastic polymer, preferably transparent.

The thermoplastic polymer generally is transparent. It may be clear or colored.

Preferably, it is a polyacrylate such as polymethyl methacrylate (P MA), a polystyrene (PS), a styrene/acrylonitrile copolymer (SAN), a polycarbonate (PC) or blends thereof. The light diffuser composition of the present invention is particularly useful for polycarbonates.

In the following paragraphs mainly polymer compositions are described which contain a polycarbonate as a thermoplastic polymer, although the present invention is not limited thereto.

Suitable polycarbonates are described in USP 4,722, 955, column 2, lines 6-42 and the references cited therein, which is incorporated by reference herein by reference. Exemplary carbonate polymers may include trityl diol carbonates; polycarbonates of bis(ar- hydroxyphenyl)alkylidenes (often called bisphenol-A type diols) including their aromatically and aliphatically substituted derivatives and carbonate polymers derived from other aromatic diols. The polycarbonates may also be derived from (1) two or more different dihydric phenols or (2) a dihydric phenol and a glycol or a hydroxy- or acid-terminated polyester or a dibasic acid in the event a carbonate copolymer or interpolymer rather than a hornopolymer is desired. Blends of the above carbonate polymers may also be used. The carbonate polymers may include ester/carbonate copolymers.

A polycarbonate generally is a polycondensate which is obtainable by reacting a diphenol, such as bisphenol-A and/or bishydroxyphenylfluorene, with phosgene or a diester of a carbonic acid, a dihydroxydiarylalkane, the aryl radicals of which carry one or more methyl groups or halogen atoms in the o-and/or m-position relative to the hydroxyl groups also being suitable, in addition to a unsubstituted dihydroxydiarylalkane. Examples of suitable diphenols which are useful as starting materials for a polycarbonate are listed in USP 4, 627,949, column 2, line 68-column 3, lines 1 -22, which is incorporated herein by reference. Exemplary diphenols are hydroquinone, resorcinol, 4,4'-dihydroxydiphenyl, bis-(hydroxy-phenyl)-alkanes, such as for example Ci -C₈ -alkylene- or C2 -C₈ -alkylidene-bis phenols, bis-(hydroxyphenyl)-cycloalkanes, such as, for example, C5 -C 15 -cycloalkylene- or C5 -C 15 -cycloalkylidene-bisphenols, bis- (hydroxy-phenyl) sulphides, ethers, ketones, sulphoxides or sulphones, and furthermore α,α '-bis- (hydroxy-phenyl)-diisopropylbenzene and the corresponding nuclear-alkylated or nuclear- halogenated compounds. Polycarbonates based on 2,2-bis-(4-hydroxyphenyl)-propane

(bisphenol A), 2,2-bis-(4-hydroxy-3,5-dichloro-phenyl)-propane (tetrachlorobisphenol A), 2,2- bis-(4-hydroxy-3,5-dibromophenyl)-propane (tetrabromobisphenol A), 2,2-bis-(4-hydroxy-3,5- dimethyl-phenyl)-propane (tetramethylbisphenol A) and l , l -bis-(4-hydroxyphenyl)-cyclohexane (bisphenol Z), and those based on trinuclear bisphenols, such as a, a'-bis-(4-hydroxyphenyl)-p- diisopropylbenzene may also be used. Most preferably, the polycarbonate is prepared from bisphenol-A and phosgene.

Polycarbonates and methods of producing them are well known in the art. For example the polycarbonate can be prepared by a known interfacial two phase process, a homogeneous organic solution process and/or a melt process. USP 4,092,288, which is incorporated herein by reference, discloses aromatic polycarbonates and methods of preparing them in column 4, lines 4-68 and in Example 1 . As described in that patent, aromatic polycarbonates may include dihydric phenols, for example, bisphenols such as bis(4-hydroxyphenyl) methane, 2,2-bis(4- hydroxyphenyl) propane, 2,2-bis(4-hydroxy-3-methylpheny!) propane, 4,4-bis(4-hydroxyphenyl) heptane, etc., dihydric phenol ethers such as bis(4-hydroxyphenyl) ether, bis(3,5-dichloro-4- hydroxyphenyl) ether, etc., dihydroxydiphenyls such as ρ,ρ'-dihydroxydiphenyl, 3,3'-dichloro- 4,4'-dihydroxydiphenyl, etc., dihydroxyaryl sulfones such as bis(4-hydroxyphenyl) sulfone, bis(3,5-dimethyl-4-hydroxyphenyl) sulfone, bis(3-methyl-5-ethyl-4-hydroxyphenyl) sulfone, etc., dihydroxy benzenes, resorcinol, hydroquinone, halo- and alkyl-substituted dihydroxy benzenes such as l ,4-dihydroxy-2-chlorobenzene, l ,4-dihydroxy-2,3-dichlorobenzene, 1 ,4- dihydroxy-3-methylbenzene, etc., and dihydroxy diphenyl sulfoxides such as bis(4- hydroxyphenyl) sulfoxide, bis(3,5-dibromo-4-hydroxyphenyl) sulfoxide, etc. Two or more different dihydric phenols or a copolymer of a dihydric phenol with glycol, a hydroxy or an acid terminated polyester, or a dibasic acid in the event a carbonate copolymer or interpolymer rather than a homopolymer can be used.

Generally, the polycarbonate may be prepared by reacting a dihydric phenol with a carbonate precursor in the presence of a molecular weight regulator and an acid acceptor. The carbonate precursors can be a carbonyl halide, a carbonate ester or a haloformate. Suitable, carbonyl halides are carbonyl bromide, carbonyl chloride, carbonyl fluoride, etc., or mixtures thereof. Typical of the carbonate esters are diphenyl carbonate, di-(halophenyl) carbonates such as di-(chlorophenyl) carbonate, di(bromophenyl) carbonate, di(trichlorophenyl) carbonate, di- (tribromophenyl) carbonate, etc., di-(alkylphenyl) carbonates such as di-(tolyl) carbonate, etc., di-(naphthyl) carbonate, di-(chloronaphthyl) carbonate, phenyl tolyl carbonate, chlorophenyl chloronaphthyl carbonate, etc., or mixtures thereof. Haloformates may include bishaloformates of dihydric phenols (bischloroformates of hydroquinone, etc.) or glycols (bishaloformates of ethylene glycol, neopentyl glycol, polyethylene glycol, etc.).

The reaction may be carried out in the presence of an acid acceptor, which could be either an organic or an inorganic compound. For example, an organic acid acceptor may be a tertiary amine such as pyridine, triethylamine, dimethylaniline, tributylamine, etc. Inorganic acid acceptor may be a hydroxide, a carbonate, a bicarbonate or a phosphate of an alkali or alkaline earth metal. Alternatively, polycarbonates can be prepared from diphenylcarbonate or dimethyl carbonate by transesterification.

Branched polycarbonates are also suitable. If the polycarbonate is branched, it preferably contains from 0.01 to 3 mole percent, more preferably from 0.05 to 2 mole percent of a branching agent, by the weight of the polycarbonate. Branched polycarbonates, methods of preparing them and suitable branching agents are for example described in USP 3,544,514, which is incorporated herein by reference, the published European patent application EP 41 1433 and in the references cited in EP 41 1433. A preferred branching agent is 1 , 1 , l -tris(4- hydroxyphenyl)ethane.

The polycarbonates preferably have a weight average molecular weight of from 10,000 grams per mole (g/mole) to 200, 000 g/mole, more preferably from 15,000 g/mole to 100,000 g/mole and most preferably from 17, 000 g/mole to 45,000 g/mole.

The end groups of the polycarbonate may be the same or different. The most preferred end groups are p-tert-butyl phenyl, p-octyl phenyl, or phenyl. End groups which can lead to a crosslinking of polycarbonate such as arylcyclobutene terminated carbonate polymers are particularly useful. The invention is not restricted to these examples.

The polymer composition of the present invention optionally contains an organic light diffuser in addition to the light-scattering polymeric particles b). If present, the amount of an additional organic light diffuser preferably is from 0.01 to 10 percent, more preferably from 0.02 to 5 percent, most preferably from 0.5 to 3 percent, by the weight of the thermoplastic polymer. Exemplary of useful known light diffusers are spherical cross-linked copolymers of I to 90 weight percent of cyclohexylmaleimide and 99 to 10 weight percent of styrene having an average diameter of 4 to 100 micrometer. Another useful known light diffuser is a poly(methyl methacrylate) resin having an average diameter of 0.5 to 100 micrometer, preferably I to 20 micrometer. The most preferred additional light diffusers are cross-linked homo-or copolymers which contain at least 15 weight percent, preferably from 20 to 80 weight percent, more preferably from 25 to 60 weight percent of a polymerized, optionally alkylated acrylate. If the optionally alkylated acrylate is copolymerized, one or more of the following monomers are preferred for copolymerization: vinyl arenes, such as styrene or an alkyl styrene like

methylstyrene or ethyl styrene; olefins, such as butadiene; acrylonitrile or maleimide. Two or more optionally alkylated acrylates can be copolymerized. If alkylated, the acrylate preferably contains a C|.₈ alkyl group, more preferably a C2.₈-alkyl group. The preferred alkyl acrylates are methyl acrylate, ethyl acrylate and butyl acrylate. These optional organic light diffusers do not have a core-shell morphology. Such organic light diffusers and methods of preparing them are generally known.

The polymer composition of the present invention may contain optional additives, such as a pigment or colorant, tackifier, mold release agent, impact modifier, filler, etc., provided that these optional additives do not have a negative influence on the optical properties of the polymer composition. Such optional additives are generally known in the art. If present, the polymer composition contains an impact modifier preferably in an amount of from 0.01 to 3 weight percent. The amount of a pigment or colorant preferably is from 0.0001 to 5 weight percent, if present at all. Preferred mold release agents are known esters of long fatty acids; their preferred amount is from 0.01 to 2 weight percent. A preferred filler is glass fibers, the preferred amount is from 1 to 20 weight percent. All percentages are based on the weight of the thermoplastic polymer.

The polymer composition of the present invention may also contain a stabilizer, such as an anti-oxidant and/or a UV stabilizer, such as a sulfur containing molecule, a phosphite, hindered phenol, hypophosphite, phosphonite and/or diphosphonite, such as tetrakis-(2, 4-di-tert butylphenyl) biphenylene diphosphonite, etc., which may have been added during the production of the polycarbonate and/or during the production of the polymeric particles b). One or more stabilizers are preferably comprised in the polycarbonate composition in an amount of from 0.01 to 5 percent, preferably from 0.05 to 2 percent, by the weight of the polycarbonate.

A preferred stabilizer is for example an organo-phosphite, preferably a phosphite of formula (I)

wherein R⁴ and R⁵ each independently are a C| .6-alkyl group, a Ci -3-hydroxyalkyl group or a C|. 3-alkoxy group such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, tert-butyl, a pentyl or a hexyl group, a hydroxymethyl, hydroxyethyl or hydroxypropyl group or a methoxy, ethoxy or propoxyl group. Preferred thereof is tri(2, 4-di-tert-butylphenyl)-phosphite (IRGAPHOS™ 168).

Another preferred stabilizer is a hindered phenol. Hindered phenols and their use as antioxidants are described in Ullmann's Encyclopedia of Industrial Chemistry, Volume 3, "Antioxidants", pages 95-98, 5th ed., 1985, VCH Verlagsgesellschaft mbH and in Encyclopedia of Polymer Science and Engineering, Volume 2, "Antioxidants", pages 75-91 , 1985 by John Wiley & Sons, Inc. Methods of preparing the hindered phenols are also well known in the art.

Preferred hindered phenols are those of formula (II)

wherein R⁴, R⁵ and R⁶ each independently are a Ci-6-alkyl group, a Ci -hydroxyalkyl group, or a Ci.3-aIkoxy group such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, tert-butyl, a pentyl or a hexyl group, a hydroxymethyl, hydroxyethyl or hydroxypropyl group or a methoxy, ethoxy or propoxyl group. Generally at least one, preferably at least two groups should provide steric hindrance to the molecule of formula (11). Preferably at least one, more preferably at least two of the groups R⁴, R⁵ and R⁶ are i-butyl or tert-butyl. Preferred examples of hindered phenols of formula III are 2, 6-di-tert-butyl-4-methylphenol, 2, 6-di-tert-butyl-4-sec-butylphenol, 4-(hydroxymethyl)-2, 6-di-tert-butylphenol or 2, 6-di-tert-butyI-4-methoxy-phenol.

Other preferred hindered phenols are those of formula (111)

wherein R⁷, R⁸ and R⁹ are arranged in the ortho and para positions to the hydroxyl group, R⁷ is a C].₆-alkyl group, R⁸ is a C|^-alkyl group or a group containing one or more ester, ether, amide, amine, phosphonite, phosphonate, thioester and/or thioether functionalities and containing up to 24, preferably up to 12 carbon atoms, such as the -CH₂-CH₂-C(0)-0-Ci₈ H₃₇ group or the -CH₂- S-C₈H|₇ group and R⁹ is a group containing one or more ester, ether, amide, amine, phosphonite, phosphonate, thioester and/or thioether functionalities and containing up to 24, preferably up to 12 carbon atoms, such as the -CH₂-CH2-C(0)-0-Ci₈H3₇ group or the -CH₂-S-C₈H |₇ group.

When R⁷ or R⁸ or both are a C|.6-alkyl group, they preferably are methyl, ethyl, n-propyl, i- propyl, n-butyl, i-butyl, tert-butyl, a pentyl or a hexyl group.

Preferred examples of hindered phenols of formula (III) are octadecyl 3, 5-di-tert-butyl-4- hydroxyhydrocinnamate, commercially available as IRGANOX™ 1076, 2-methyl-4, 6- bis((octylthio)-methyl)-phenol, commercially available as IRGANOX 1520, 2, 6-di-tert-butyl-4- (dimethylaminomethyl)phenol or 3, 5-di-tert-butyl-4-hydroxybenzyl di-O-ethyl phosphonate, commercially available as IRGANOX 1222.

Other preferred hindered phenols contain 2 phenolic groups, for example N,N'- 1 , 6- hexamethylene-bis-3-(3, 5-di-tert-butyl-4-hydroxyphenyl)propionamide, commercially available as IRGANOX 1098, 1 , 6-hexamethylene bis(3, 5-di-tert-butyl-4-hydroxyhydrocinnamate, commercially available as IRGANOX 259, triethylene glycol bis[3-(3-tert-butyl-4-hydroxy-5- methylphenyl)] propionate, commercially available as IRGANOX 24, N,N'-bis(3, 5-di-tert-butyl- 4-hydroxyhydrocinnamoyl)hydrazine, commercially available as IRGANOX MD 1024 and nickel or calcium bis[0-ethyl(3, 5-di-tert-butyl-4-hydroxybenzyl)]phosphonate, the latter being commercially available as IRGANOX 1425.

The most preferred hindered phenols are tetrakis[methylene(3, 5-di-tert-butyl-4- hydroxyhydrocinnamate)] methane, commercially available as IRGANOX 1010, 3, 5-di-tert- butyl-4-hydroxyhydrocinnamate, commercially available as IRGANOX 1076 or a 1 : 1 blend of IRGANOX 1010 and tri(2, 4-di-tert-butyl-phenyl)phosphite (IRGAPHOS 168, trademark), which blend is commercially available as IRGANOX B 225.

The hindered phenol is advantageously utilized in combination with a phosphine of the general formula PR' R^ZR³ (IV), wherein R¹, R² and R³ independently from each other represent an alkyl, cycloalkyl, aryl or aryl-alkyl group or an aryl group which is substituted at the aromatic ring with one or more halogens and/or one or more alkyl, hydroxy or alkoxy groups.

The radicals R¹ , R² and R³ can be identical or different. Of the alkyl groups those are preferred that have 1 to 18, preferably I to 12 carbon atoms, such as methyl, ethyl, n-propyl, i- propyl, n-butyl, s-butyl or t-butyl or the pentyl, hexyl, octyl, nonyl, decyl or octadecyl groups. The alkyl groups can be straight-chain or branched. Of the cycloalkyl groups those having 5 or 6 carbon atoms, such as cyclopentyl or cyclohexyl are preferred. Of the aryl groups those having from 6 to 14 carbon atoms, such as phenyl or naphthyl, are preferred. The aryl groups may be substituted with one or more of the above-mentioned alkyl groups and/or with one or more halogens, such as fluoride, chloride or bromide, and/or one or more hydroxy groups and/or one or more alkoxy groups. Alkoxy groups, if present, preferably contain 1 to 6 carbon atoms, such as the methoxy, ethoxy, n-propoxyl, i-propoxyl, n-butoxyl, s-butoxyl or t-butoxyl groups. If substituted, the aryl groups preferably are substituted with 1 , 2 or 3 substituent groups. In the aryl-alkyl groups the above-mentioned alkyl groups are preferred and the aryl group preferably is phenyl. Preferred aryl-alkyl groups are benzyl, butyl phenyl or tolyl. Triphenyl phosphine is the most preferred compound of formula (IV).

For preparing the polymer composition of the present invention 0.001 to 2 weight percent of the above-described inorganic particles a), 0.1 to 10 weight percent of the above-described polymeric particles b), optionally 0.1 to 1 ,000 ppm of the above-described wavelength downshifting material c), optionally 0.005 to 2 weight percent of the above-described interference pigment d) and, if desired, one or more of the above-mentioned optional additives are mixed with the thermoplastic polymer. These compounds may be premixed before blending the mixture with the thermoplastic polymer. Alternatively, these compounds may be mixed separately with the thermoplastic polymer. The inorganic particles a), the polymeric particles b), and one or more of the wavelength downshifting material c) and/or interference pigment d), and the optional additives may be added simultaneously or in sequence to the thermoplastic polymer. The sequence of addition and the temperature during the addition are not critical. The compounds may be mixed in their undiluted form or one or more of the compounds may be diluted with an aqueous or organic diluent instead of preparing a polymer composition containing the above-mentioned concentration of the particles a), b), and one or more of c) and/or d) master batches can be prepared containing higher concentrations of the inorganic particles a), polymeric particles b), and one or more of the wavelength downshifting material c) and/or interference pigment d). The master-batches can be blended later with a thermoplastic polymer to prepare polymer compositions containing the claimed concentration of the particles a) , b), and ne or more of c) and/or d). It is advisable to mix the inorganic particles a), the polymeric particles b), and one or more of the wavelength downshifting material c) and/or interference pigment d), and the optional additives with the thermoplastic polymer, which may contain optional additives, before the thermoplastic polymer is compounded to granules or pellets. The manner of dispersing or mixing the inorganic particles a), the polymeric particles b) , and one or more of the wavelength downshifting material c) and/or interference pigment d), and any optional additives with the thermoplastic polymer(s) is not critical. However, the process chosen should be one which results in a great degree of dispersion of all the additives throughout the thermoplastic polymer. Preferred mixing equipment are mixing rolls, ribbon blenders, dough mixers, Banbury mixers, etc.

The polymer composition can be compounded to granules or pellets by known extrusion techniques. If the polymer composition contains polycarbonate, the extrusion is preferably conducted at a temperature of from 200°C to 390°C, more preferably from 250°C to 390°C, most preferably from 260°C to 340°C. The mixture may be fed into an extruder and extruded into strands which are then comminuted into pellets or granules. Useful extruders are generally known in the art. The extruders generally have a single screw or a twin screw. When using a twin screw extruder, the screw speed is preferably from 50 to 100 rotations per minute (RPM), more preferably from 100 to 350 RPM. When using a single screw, one screw speed preferably is from 5 to 250 RPM, more preferably from 10 to 150 RPM. The pellets or granules may be formed into molded articles in a generally known manner, for example by injection-molding, injection blow molding, roll mill processing, rotational molding, extrusion, etc. If the polymer composition contains polycarbonate, the molding is preferably conducted at a temperature of from 200°C to 380°C, more preferably from 250°C to 380°C, most preferably from 260°C to 380°C.

Examples of molded articles are sheets, films, coextruded sheet, coextruded films, coextruded composites comprising at least one sheet and at least one film, lamp covers, luminaires, lamps, etc. The molded articles have an excellent surface appearance. The surface may be smooth or textured according to the requirements of the specific application.

An LED 1, FIG. 1, is a light source that exploits the physical principle of

electroluminescence, that is to say the emission of light by a semiconductor 5 when a potential difference is applied to a junction of the p-n type. The LED therefore comprises the

semiconductor itself and also other elements for the operation and handling of the LED, such as a transparent protective shell 7, supply contacts (anode 2/cathode 3), conducting wire 4, optionally a reflector cup 6 that reflects the emitted light, etc. According to the invention, the scattering particles are not dispersed in the transparent protective shell but in the cover.

Among LEDs that emit in the visible spectrum, distinction may be made between color LEDs and white LEDs. The emission spectrum of a color LED is very narrow and has a mid- height width of between 30 and nm, preferably between 30 and 50 nm (as may be seen in Figure 2). The emission spectrum depends on the nature of the p-n junction, and therefore on the chemical nature of material used for the semiconductor. Unlike another light source, such as an incandescent lamp or a neon lamp, the following equation connects the wavelength λ (in nm) of the emitted light to the bandgap Eband (in eV), which depends on the semiconductor material used:

where c denotes the velocity of light (3 x 108 m/s) and h denotes Planck's constant (4.136 x 10' eV/s).

For example, if the semiconductor material is gallium arsenide GaAs, the bandgap is 1 .35 eV and the LED emits in the near infrared at around 920 nm. Gallium phosphide arsenide GaAsP, gallium phosphide GaP, gallium nitride GaN and gallium indium nitride GalnN are other examples among other intermetallic compounds that can be used for manufacturing an LED.

The emission of white light by an LED is more problematic as it is necessary to combine the three essential components of white light, namely blue, red and green. It is also necessary to mix these three components so as to obtain a white light having good color rendition or CR l and a good color temperature (in Kelvin). At the present time, there are at least three technologies for obtaining emission of white light:

The first consists in combining an LED emitting in the blue with one or more phosphorescent compounds that reemit in the yellow. This is a method that is widely used at the present time, but the CRl (color rendition index) is poor, less than 75, and there is often a halo problem due to the fact that the blue light and the re-emitted yellow light do not mix everywhere uniformly;

The second consists in combining an LED emitting in the ultraviolet with one or more phosphorescent compounds, each re-emitting light in a given color. An example of a white LED using this technology is given in document US 6,084,250. For example, an LED emitting between 370 and 410 nm is combined with a mixture of phosphorescent compounds, namely Eu: BaMgAl | |0|₇, Cu: ZnS and YV0₄. This type of LED gives a white light having a good CRl, close to that of fluorescent lamps; and

The third consists in mixing the colors emitted by several color LEDs so as to obtain a white light. This technique has the advantage of being more flexible as it does not require the use of phosphorescent compounds. It is possible, by acting separately on the intensity of the LEDs, for the color temperature to be finely adjusted, thereby making it possible to obtain a whole range of white lights (ranging from "hot" white to cold blue-white). In practice, to obtain white light, it is necessary for the LEDs to emit monochromatic light corresponding to the sensitivity maxima of each type of cone present in the human eye (450 nm in the case of blue, 550 nm in the case of green and 620 nm in the case of red). If LEDs emitting at this precise frequency are not available, compensation is necessary by adjusting the emission intensity.

The emission spectra of white LEDs are broader than those of color LEDs. In the case of white LEDs, the scattering plastic according to the invention makes it possible to eradicate the drawbacks associated with the three technologies, for example the halo phenomenon associated with the first technology or else the imperfect color mixing of the third technology. The scattering plastic according to the invention is therefore well suited to the production of luminous devices incorporating one or more white LEDs.

White LEDs, which have a high luminous flux, of greater than 3 lumen (Lm), may be profitably employed as a light source in illuminated signs. Thanks to their long lifetime, the LEDs do not have to be frequently replaced, this being a major advantage, most particularly in the case of illuminated signs that are located high up, for example at the top of towers or buildings. Another advantage is that the LEDs are small light sources, thereby making it possible to manufacture illuminated signs that are more compact, and therefore easier to mount and to dismantle.

In one embodiment, the present invention is a luminous device 8, FIG. 2, comprising a light source, preferably at least one LED 1 , preferably a white LED wherein the light flux of which is equal to or greater than 3 Lm, advantageously equal to or greater than 5 Lm, preferably equal to or greater than 10 Lm, more preferably equal to or greater than 50 Lm, and at least one cover 9 a cover comprising the light diffusing composition disclosed herein above. The cover, among other things, makes it possible to ensure transmission of the light emitted by the LED(s); mask and protect the LED(s); provide a uniform and nondazzling illumination; reduce, or even eliminate, the drawbacks of LEDs, especially the halo effects in white LEDs; and ensure good visibility both in the daytime and at night.

Advantageously, the luminous flux is between 3 and 1 ,000 Lm, better still between 3 and 500 Lm, better still between 3 and 200 Lm, better still between 3 and 100 Lm, preferably between 5 and 100 Lm, even more preferably between 10 and 100 Lm and very preferentially between 50 and Lm. Thanks to white LEDs having a high luminous flux, it is possible to reduce the number of LEDs in order to obtain a given illumination. Compared with an incandescent lamp or a neon tube, it is possible to obtain luminous devices that are more compact and consume less electrical power. In addition, when the LED is lit, the cover appears to be substantially white and has, as L, a*, b* values, a luminosity of greater than 55, an a* value of between -7.5 and +7.5 and a b* value of between -7.5 and +7. 5.

In one embodiment, the cover comprises a single layer sheet or film.

In another embodiment, the cover comprises a multilayer sheet, multilayer film, or multilayer composite comprising one or more sheet and one or more film. Multilayer sheet having one or more layers, multilayer film, and multilayer composites may be made by any suitable means with coextrusion being preferred. A multi layer sheet may comprise one or more sheet substrate layer and/or one or more cap layer.

Typically, a sheet substrate layer may have a thickness equal to or greater than about 0.5 millimeter (mm), preferably equal to or greater than about I mm, more preferably equal to or greater than about 2mm, and more preferably equal to or greater than about 3mm. Typically, a sheet substrate layer may have a thickness equal to or less than about 5mm, preferably equal to or less than about 4.5mm, and more preferably equal to or greater than about 4mm. Typically, a sheet cap layer is from 1 micrometer to 500 micrometer, preferably of from 10 micrometer to 200 micrometer.

When the cover comprises more than one sheet, or more than one film, each film and/or each sheet may comprise one or all of components a), b), c), and d), such that between all the layers of the multi layer sheet, multi layer film, or multi layer composite there is the presence of components a) and b) and one or more of components c) and d). As an example, if the cover comprises a sheet substrate coextruded with cap layer, the sheet substrate may comprise components a) + b) + d) while the cap layer comprises component c).

There are several methods known to those skilled in the art for producing the cover of the luminous device of the invention. The cover may have all kinds of geometries, depending on the nature of the intended application. For example, it may be in the form of a flat, curved or domed sheet, whether rectangular or circular, in the form of a disc, etc. It may also take the form of a letter of the alphabet or of any other similar element in the case of an illuminated sign, as illustrated for example in FIG. 2. Examples are also given in the following documents: US Publication 2004/0255497, which is incorporated by reference herein in its entirety.

The cover has a thickness of between 0.05 and 15 cm, preferably between 0.1 cm and 10 cm, more preferably between 0.1 cm and 7 cm, more preferably between 0.1 cm and 5 cm, and even more preferably between 0.2 cm and 4 cm.

The cover is separated from the LED(s) by a distance of between 1 cm and 50 cm, better still between 2 cm and 50 cm, preferably between 2 cm and 20 cm and even more preferably between 3 cm and 20 cm. The luminous device is distinguished from edge-emitting devices, as described for example in EP 893481. This is because, in the luminous device according to the invention, it is not the edge of the cover that is illuminated but one of the faces of the cover. The luminous device according to the present invention has a variety of applications such as, for example, general illumination, interior lighting (living room lamps, office lamps, etc.); advertising displays; direction lighting or escape route marking; illuminated signs (in this case, the cover may especially have the form of a letter 8, FIG. 2, a number, a symbol or any other sign); traffic signaling; automobile lighting such as a headlamp, a daytime light, a direction indicator, a stop light, a fog lamp, reversing light, a mobile device, back light unit for an LED- TV, recessed down-lights, task lights, office under-shelf lights, kitchen under-cabinet lights, globes, lenses, optics; and the like.

FIG. 3 and FIG. 4 show an illuminated sign 11 at the top of a building 12. Among the letters making up the sign is again the letter 8 of FIG. 2, described above. FIG. 3 shows the sign during the daytime when it is not lit. FIG. 4 shows the sign at night, when it is lit.

In one embodiment of the present invention, articles made from the light diffusing composition of the present invention appear a different color and/or brighter when viewed by reflected light, preferably sun light (during the day) than they do when viewed from a back lit light source (during the night), whether the light source be an LED and/or a fluorescent light, and/or incandescent light. The degree and level of color difference is related to the specific wavelength downshifting material used. Not to be held to any particular theory, it is believed that ultra violet light in the 300 nm to 400 nm range is absorbed by the wavelength downshifting material and reflected in the visible range, typically blue fluorescent light between 400 nm to 500 nm range.

The difference in appearance is visible or detectable to the naked eye, but can be quantified through a change of yellow index of at least 50 when measured against a back-lit LED that is on then off. Alternatively, it may be detected by a change of chromaticity in at least one color coordinate (x, y) by an amount of 0.01 when a back-lit light source is on versus off.

The invention is further illustrated in further detail by the following, non-limiting examples.

EXAMPLES

The compositions for Examples 1 to 3 and Comparative Examples A to F are listed in Table I , and, unless otherwise noted, all amounts are in weight percent based on the total weight of the polycarbonate composition. The blend compositions are premixed in a glass bottle and the mixed samples are compounded using Haake PPLYLAB™ system with 60 min- 1 speed at 250°C for 5 minutes followed by a quick cooling process using a compressor, which produces a slab about 3mm thick.

The slabs of the compounded samples are cut to about 1 by 1 inch strips using a New

Hermes Shearer. The strips are placed in a compression mold faced with a KAPTON™ film to provide a smooth surface. The mold is placed in between platens of a Carver Compression Molder, which is pre-heated at 250°C. The platens are compressed under a pressure of 3000 pounds for 3 minutes, followed by a pressure of 10,000 pound for 3 minutes, followed by a pressure of 20,000 pounds for 2 minutes and finally cooled to ambient temperature under a pressure of 3,000 pounds for 3 minutes. The smooth side of the resulting 2.7mm plaque is used for optical property testing.

The following components are used in Examples I to 3 and Comparative Examples A to

F:

"PC" is a bisphenol-A polycarbonate homopolymer with a melt flow of 6 grams per

10 minutes as determined at 300°C under a load of 1 .2 kilograms;

"Ti0₂" is titanium dioxide and is available as TIONA™ RCL-4 from Cristal Global;

"C/S-l rubber" is a core/shell rubber having an average rubber particle size of about 6 microns wherein the shell is a MMA/EA copolymer comprising about 20 weight percent of the rubber with a refractive index (RI) of 1 .46 and the core is a BA/ALMA copolymer comprising the remaining 80 weight percent of the rubber having a RI of 1 .49, available as PARALOID™ EXL 5136 from The Dow Chemical Company;

"C/S-2 rubber" is a core/shell rubber having an average rubber particle size of about 0.1 to 0.2 microns wherein the shell is a MMA/EA copolymer comprising about 20 weight percent of the rubber with a refractive index (RJ) of 1.46 and the core is a BA/ALMA copolymer comprising the remaining 80 weight percent of the rubber having a RI of 1 .49, available as PARALOID PRD 137 from The Dow Chemical Company;

"LDSM" is a wavelength downshifting material avai lable as LUMOGEN™ Violet 570 from BASF; and

"1FP" is an interference pigment comprising Ti0₂ coated on clay available as

IRIODIN™ 97225 Ultra Rutile Blue Pearl SW from Merck Chemicals. The compression molded plaques comprising the compositions of Examples I to 3 and Comparative Examples A to F are subjected to the following optical property

evaluations and the results are listed in Table 1 :

"% T" is percent light transmittance as measured by using a BYK Gardner (Haze-Gard Plus) according to ASTM D- 1003 and the measured transmittance is total transmittance. The test specimens are measured using a Hunterlab Coiorquest in transmission mode, utilizing a light source C (daylight simulation), an observer angle of 2° and wave lengths of 400-700 nm;

"Dso" is determined from an optical goniphotometer (Model: GP-200) and refers to the angle Θ that can transmit light intensity which is 50 percent of the light intensity at a 0 degree viewing angle in a transmittance mode and is a measure of the light diffusion ability in a sample. The optical goniphotometer has a halogen light source and the sample holder could be rotated from -90 to 90 degree. The light intensity at a different sample rotation angle could be drawn in a polar axis graph and the light intensity profile could be also transferred into an X-Y axis graph. The half gain angle or D₅o was used to evaluate the light diffusion properties.

"L*, a*, and b*" values are used to characterize the principal color in the Commission Internationale d'Eclairage system (CIE-system) in reflectance (black) according to ASTM E 308. L* denotes the luminosity and extends from 0 (black) to 100 (white). The value a* measures the red and green of the color: the colors tending toward green have a negative a* value while those tending toward the red have a positive a* value. The b* value measures the blue and the yellow of the color: colors tending toward the yellow have a positive b* value while those tending toward the blue have a negative b* value. The L*, a*, b* values are measured using the reflection mode according to the ASTM E 313;

"YI" is yellowness index and is determined according to ASTM E 313; and

"Color Change" is a visual determination of whether a color difference can be perceived.

A plaque is back lit with an LED light source (MWL-4-12-W4 from USLED) and is viewed under two conditions: ( 1 ) when the LED is on and (2) when the LED is off. If any color change is perceivable, at any angle, between the two conditions, the sample is considered to demonstrate color change. If no color change is perceivable, at any angle, the sample is considered not to demonstrate color change. Examples of the present invention, Example I , 2 and 3 show color changes when viewed by reflected and transmitted light, while Comparable Examples A, B, D, E, and F, which are not examples of the present invention, do not show color changes. While Comparable Example C shows a color change, it has an unacceptable light transmittance because of the high level of IFP makinR it unacceptable for LED light diffusion applications.

SE (Scanning Electron Microscopy) was used to determine average particle diameter. In SEM, 100 particles were analyzed for the measurement of the average particle diameter of the particles.

The Becke Line microscopic method was used to determine the refractive index of the transparent particles.

The weight average molecular weight of the polymers was determined by a liquid chromatographic method (gel permeation chromatography). The fluid phase is tetrahydrofuran (THF) of a flow rate of I mL/min. 5μιη mixed D columns were used for separation. The detector was a UV DAD device (diode array detector). Calibration was performed by a set of polystyrene standards.

Table 1

Claims

What is claimed is:

1. A light diffusing thermoplastic composition comprising a thermoplastic polymer and a) from 0.001 to 2 weight percent of an inorganic particle having an average particle diameter of from 0.1 to 1 microns and a refractive index of from 1 .9 to 3.2,

b) from 0.01 to 10 weight percent of a polymeric particle having an average particle size in the range of from 0.2 to 20 microns that differs in refractive index n at 589 nm by at least 0.05 from that of the thermoplastic resin polymer,

and one or more of c), d), or mixtures thereof wherein

c) is from 0.1 to 1 ,000 ppm of a wavelength downshifting material,

and

d) is from 0.005 to 2 weight percent of an interference pigment.

2. The thermoplastic light diffusing composition of Claim 1 wherein the inorganic particles a) are titanium dioxide, silica gel, zinc sulfide, zinc oxide, MgTi03, or mixture thereof.

3. The thermoplastic light diffusing composition of Claim 1 wherein the polymeric particle b) is a crosslinked acrylic polymer, a crosslinked styrene polymer, a crosslinked silicone polymer, or mixtures thereof.

4. The thermoplastic light diffusing composition of Claim 1 wherein the polymeric particles b) have a core/shell morphology comprising one or more shells wherein the core is a rubbery vinyl polymer comprising at least 15 weight percent of a polymerized alkyl acrylate or alkyl methacrylate based on the weight of the core.

5. The thermoplastic light diffusing composition of Claim 1 wherein the wavelength downshifting material c) is an inorganic downshifting material selected from Sm³⁺, Cr³⁺, ZnSe, Eu, or Tb containing wavelength downshifting material, a quantum dot material, an organic wavelength downshifting material selected from Rhodamine, Coumarin, Rubrene, a laser dye, Alq₃, TPD, Gaq₂CI; a perylene carbonic acid or a derivative thereof, a naphthalene carbonic acid or a derivative thereof, a violanthrone or an iso-violanthrone or a derivative thereof, or mixtures thereof.

6. The thermoplastic light diffusing composition of Claim 1 wherein the interference pigment d) is selected from mica flake + Ti0₂; mica flake + Zr0₂; Si0₂ flake + Ti0₂; Si0₂ flake + Zr0₂; A1₂0₃ flake + Ti0₂; A1₂0₃ flake + Zr0₂; glass flake + Ti0₂; glass flake + Zr0₂; glass flake + Si0₂ + Ti0₂; or combinations thereof.

7. The l ight diffusing composition of Claim 1 wherein the thermoplastic polymer is selected from polycarbonate, polymethyl methacry!ate, polystyrene, polystyrene and acrylonitrile copolymer, or mixtures thereof.

8. A single layer or multi layer thermoplastic light diffusion sheet with at least one layer comprising the thermoplastic l ight diffusing composition of Claim 1 .

9. A luminous devise comprising at least one l ight source and a cover made of the composition of Claim 1 .

10. The luminous device of Claim 9 wherein the at least one l ight source is a LED having a luminous flux greater than 3 Lm.