GB1595426A - Plastics which appear white in reflected and transmitted light - Google Patents

Description

(54) PLASTICS WHICH APPEAR WHITE IN REFLECTED AND TRANSMITTED LIGHT (71) We BAYER AKTIENGESELLSCHAFT a body corporate organised under the laws of the Federal Republic of Germany of 509 Leverkusen, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to plastics which appear white in reflected and transmitted light.

More particularly, the invention relates to plastics having optical properties (such as remission, transmission, hazing and colour tinge) which determine their practical application. The plastics according to the present invention are based on translucent polymers, such as polyacrylate, polymethacrylate, polycarbonate, polystyrene, polyesters or cellulose esters which are coloured with white pigments, for example, titanium phosphate lead hydrogen phosphate, zinc oxide, zinc sulphide, magnesium titanates, calcium titanate, anatase or rutile.

Conventional processes for producing white translucent plastics have initiated from the premise that the particle size of the pigments should lie in a narrow distribution range around a value do which was defined by Mie as long ago as 1905 d - 0.122 (m2 + 2) in Fm nO (m - 1) cf. also van de Hulst "Light Scattering by Small Particles", John Wiley and Sons, New York/London 1957, on the assumption that the maximum of the scattering coefficient lies at the wavelength normally used for white pigments, namely BmnX t 420 mm, no being the refractive index of the polymer, n being the refractive index of the pigment and m = n .

no German Offenlegungsschrift No. 2,314,253 claims particularly finely divided titanium dioxides for improving opalescence. In order not to impair the purity and gloss of the opalescent materials, the titanium dioxide pigment used should not contain any particles with a diameter of more than 0.15 ,um. It is also known to use rutile crystals with an average particle diameter of from about 0.05 to 0.10 llm and with as narrow a particle size distribution as possible, optionally with addition of further transparent phthalocyanine blue pigments or quinacridone red pigments.

This literature reference is consistent with the commercially available TiO2-types for colouring plastics which are optimised towards the finely divided side.

Disadvantages of these finely divided titanium dioxides, particularly when they have a particle diameter of less than 0.15 Rm, are their increased photoactivity attributable to their high specific surface, their frequently inadequate dispersibility and their inadequate colour neutrality in reflected and transmitted light.

It has surprisingly been found that, in addition to a high haze level, plastics containing coarsely divided white pigments also have an appearance which, in reflected and transmitted light, comes extremely close to the neutral white required.

Accordingly, the present invention provides a plastics material which appears white in reflected and transmitted light, comprising at least one translucent organic polymer and from 0.001 to 20% by weight, based on the polymer, of at least one white pigment, wherein the refractive index no of the polymer is in the range of from 1.4 to 1.65, wherein the refractive index n of the pigment is in the range of from 1.7 to 2.9, and wherein the average particle diameter d of the pigment is between 2 do and 5 d,, where 0.122 (m2 + 2) .

do = 0.122 (mZ + 2) (mZ 1) m, with n m = - nO Preferably, the white pigment is present in a quantity of from 0.005 to 5% by weight based on the polymer.

One advantage of the plastics material according to the invention is its white neutral colour in reflected and transmitted light for the required scattering of light and transparency to light.

It has also been found that, contrary to what has generally been assumed in the past, as narrow a particle size distribution as possible is not the critical factor, because pigments with a wide particle size distribution also produce the required properties of the plastics provided that the refractive index and average particle diameter lie within the range specified according to the invention. If there is a mixture of pigments, the values of d and n for each component should lie between the above mentioned relation.

According to DIN 53206.1 the value do may be termed the light scattering equivalent diameter. In this Application the average diameter d of the pigment particles is determined either by electron microscope or by the BET-method, the BET-method preferably being applied in the case of particle diameters of more than 1 llm. Accordingly, the diameter in question is the projection or surface equivalent diameter. These two values differ only slightly from one another, particularly in the case of calcined pigments, so that one of the two methods is applied in accordance with the prior art, depending upon the average particle diameter and the surface texture of the pigment. These methods measure the primary particle size even if the particles are present in agglomerates and the term "average particle diameter" used herein refers to the primary particle size.According to the present invention, the projection or surface equivalent diameter should be between 50% and 600% above the light scattering equivalent diameter.

Pigments suitable for the purposes of the invention have a refractive index in the range of 1.7 to 2.9. Suitable pigments are, for example, magnesium aluminium oxide, aluminium oxide, lead phosphate, lead hydrogen phosphate, titanium phosphate, aluminium titanate, tin oxide, zinc oxide antimony oxide, zirconium oxide, zinc sulphide, zinc titanate, magnesium titanate as MgTiO3, Mg2TiO4, MgTi2O5, calcium titanate and titanium dioxide in anatase or rutile form, either individually or in combination. The critical factor is the average particle size which should be at least 20% above the value do. The pigments are produced by conventional precipitation and/or calcination processes. By applying relatively high calcination temperatures, the particles become coarser and the particle size distribution range is widened to an extent, although according to the invention this does not have any adverse effect upon the colouring of plastics.

The necessary amount of pigment in the plastics material is determined by various parameters. According to the invention, the pigment and polymer are not independent of one another. As can be theoretically established, the scattering power of the coarsely divided pigments to be used in accordance with the invention decreases with their particle size so that, with relatively coarse particles, the quantity of pigment has to be greater if the haze of the coloured polymer is to remain constant. The refractive indices of the polymer and pigment enter into the particle size, as had already been established by Mie.For example, in the case of a polymer having a refractive index no of about 1.59, approximately the following quantities of pigment are necessary for obtaining the same haze as achieved by the addition of 0.01% of rutile: anatase 0.012%, zinc sulphide 0.015 to 0.02%, zinc oxide 0.04 to 0.06%, lead hydrogen phosphate 0.1 to 0.2%.

In practice, rutile pigments, for example, are added to the plastics material in quantities of from 0.001 to 1% by weight, depending upon the light transmittance required. The pigments according to the invention have to be added in larger quantities than comparable finely divided pigments if scattering, haze and light transmittance are to remain at their optimum levels. The pigmenting level is also dependent upon the wall thickness of the shaped plastics article. Shaped articles with a wall thickness of several millimetres have to be pigmented to a lower level than films with a thickness of a few micrometers if remission and transparency are to be comparable. However, the optical effect, particularly the neutral white colour of the pigmented plastics, is solely determined by the particle diameter and not by the concentration of the pigments.

If the context of this invention plastics are translucent and transparent thermoplastic and duroplastic polymers having a refractive index no in the range from 1.4 to 1.65, for example, polycarbonates, polyethylene, polystyrene, polyacrylate, polymethacrylate and their copolymers, also cellulose esters and polyesters.

In the context of this invention polycarbonates are polycondensates of the type which may be obtained by reacting diphenols, especially dihydroxy diaryl alkanes, with phosgene or diesters of carbonic acid, unsubstituted dihydroxy diaryl alkanes of which the aryl radicals carry methyl groups or halogen atoms in the o- and/or m-position to the hydroxyl group also being suitable. Branched-chain polycarbonates are also suitable.

Polycarbonates have average molecular weights (weight average) Mw of from 10,000 to 100,000, preferably from 20,000 to 40,000, as determined by measuring relative viscosity in CH2Cl2 at 25"C and at a concentration of 0.5% by weight.

Suitable diphenols are, for example, hydroquinone, resorcinol, 4,4'-dihydroxy diphenyl, bis-(hydroxyphenyl)-alkanes, for example, C1-C5-alkylene and C2-C8-alkylidene-bis- phenols, bis-(hydroxyphenyl)-cycloalkanes for example, C5-C15-cycloalkylene and C5-C1 -cycloalkylidene-bisphenols, bis-(hydroxyphenyl)-sulphides, ethers, ketones, sulphoxides or sulphones, also a,a'-bis-(hydroxyphenyl)-diisopropyl benzene and the corres ponding compounds which are alkylated or halogenated in the nucleus.

Preferred polycarbonates are those based on bis-(4-hydroxyphenyl)-2,2-propane (bisphe nol A), bis-(4-hydroxy-3 ,5-dichlorophenyl)-2,2-propane (tetrachlorobisphenol A), bis-(4 hydroxy-3 ,5-dibromophenyl)-2,2-propane (tetrabromobisphenol A), bis-(4-hydroxy-3,5 dimethylphenyl)2,2-propane (tetramethyl bisphenol A), bis-(4-hydroxyphenyl)-1,1 cyclohexane (bisphenol Z), and also those based on trinuclear bisphenols such as a,a'-bis-(4-hydroxyphenyl)-p-diisopropyl benzene.

Other diphenols suitable for the production of polycarbonates are described in US Patents Nos. 2,970,131; 2,991,273; 2,999,835; 2,999,846; 3,014,891; 3,028,365; 3,062,781; 3,148,172; 3,271,367; 3,271,368 and 3,280,078.

Suitable polystyrenes are homopolymers of styrene or copolymers of styrene with, preferably acrylinitrile and/or butadiene and/or maleic acid esters which are obtained, for example, by suspension polymerisation in the presence of catalysts from the monomers or from a mixture of the monomers with Mw of from 10,000 to 600,000 ( Mw is measured in DMF at c = 5 g/l and 20"C).

(For literature, see: Beilsteins Handbuch der Organischen Chemie, fourth Edition, Third Supplement, Vol. 5, pages 1163 - 1169, Springer Verlag 1964; H. Ohlinger, Polystyrene, Part 1, Herstellungsverfahren und Eigenschaften der Produkte [Manufacturing Processes and Properties of the Products] Springer Verlag, 1955).

Cellulose esters suitable for the purposes of the present invention are obtained by the usual methods, i.e. by esterifying cellulose with aliphatic monocarboxylic acid anhydrides, preferably acetic acid and butyric acid or acetic acid and propionic acid anhydride. The hydrolysis reaction which is carried out in the crude solution is controlled by a slight excess of water so that a low hydroxyl content (4 to 25) is obtained. The oxidising bleaching of the cellulose ester isolated from the solution has to be carried out in such a way that no more oxidising agent can be detected in the end product. This may require an aftertreatment with reducing agents.

In order to determine the OH-number, the free hydroxyl groups of the cellulose ester are esterified with acetic anhydride in pyridine, the excess anhydride is reacted with water and batch-titrated [Procedure: C.J. Mah, L.B. Genung and R.F. Williams, Analysis of Cellulose Derivatives, Industrial and Engineering Chemistry, Vol. 14, No. 12, 935 - 940 (1942)].

The viscosity of the cellulose esters should amount to between 0.3 and 0.5 poises, as measured on a 20% solution in acetone. Preferred cellulose esters have an acetic acid content of from 17% to 23% by weight and a butyric acid content of from 45% to 50% by weight in the case of the acetobutyrates, and a propionic acid content of from 61% to 69% by weight and an acetic acid content of from 2% to 7% by weight in the case of the acetopropionates. The OH-numbers are normally between 4 and 25. The mean weight averages of the molecular weights Mw are in the range from 10,000 to 1,000,000 and preferably in the range from 100,000 to 500,000.

Polyalkylene glycol terephthalates in the context of the invention are, for example, those based on ethylene glycol, 1,3-propane diol, 1,4-butane diol 1,6-hexane diol and 1,4-bis-hydroxymethyl cyclohexane. The molecular weights ( Mw) of these polyalkylene glycol terephthalates are between 10,000 and 80,000. The polyalkylene glycol terephthalates may be obtained by known methods, for example from terephthalic acid dialkyl ester and the corresponding diol by transesterification (see for example US Patents Nos.

2,647,885; 2,643,989; 2,534,028; 2,578,660; 2,742,494 and 2,901,466).

Starting, for example, with a lower alkyl ester of terephthalic acid, preferably the dimethyl ester, this ester is transesterified with an excess of diol in the presence of suitable catalysts to form the bis-hydroxyalkyl ester of terephthalic acid. Starting from 140"C, the temperature is increased to 210 C to 220"C. The alcohol liberated is distilled off. The condensation reaction is then carried out at temperatures of from 210 C to 280"C and, during the reaction, the pressure is reduced in stages to less than 1 Torr, the excess diol being distilled off.

Polyacrylates and polymethacrylates which can be coloured with the pigments according to the invention are homopolymers and copolymers of acrylic acid ester and methacrylic acid ester with molecular weights of from 104 to 107 and with 4 to 18 carbon atoms in the monomer unit for example, polyacrylic acid isobutyl ester, polymethacrylic acid methyl ester, polymethacrylic acid ethyl hexyl ester, polyacrilic acid ethyl ester, copolymers of different acrylic acid esters and/or methacrylic acid esters, for example, methacrylic acid methyl ester/acrylic acid cyclohexyl ester copolymers, also copolymers of acrylic acid esters and/or methacrylic acid esters with crosslinking agents, for example, 1,4-butane diol dimethacrylate, glycol dimethyacrylate, triglycol dimethacrylate, trimethylol propane trimethacrylate, allyl methacrylate, triallyl cyanurate, also copolymers of acrylic acid esters and/or methacrylic acid esters with styrene and/or a-methyl styrene and the graft polymers and copolymers and polymer mixtures consisting of acrylic esters, methacrylic esters, methacrylic acid esters, styrene and butadiene.

Olefin homopolymers and copolymers in the context of the present invention are, in particular, the poly-a-olefins containing from 2 to 10 carbon atoms in the monomer unit such as high-pressure and low-pressure polyethylene, polypropylene, poly-1-butene, polyisobutylene, poly-1-pentene or poly-4-methyl-1-pentene, copolymers of various aolefins, for example, ethylene/propylene copolymers or terpolymers, also homopolymers and copolymers of conjugated aliphatic dienes, for example, polybutadiene, polyisoprene, polychloroprene, styrene/butadiene and acrylonitrile/butadiene copolymers and the copolymers and graft polymers and polymer mixtures consisting of acrylonitrile, butadiene and styrene and copolymers of a-olefins with other copolymerisable, olefinically unsaturated compounds, for example, ethylene/vinyl chloride, ethylene/vinylacetate or ethylene/acrylic acid copolymers.

The polyester resin compositions are based on a, -ethylenically unsaturated polyesters and vinyl or vinylidene compounds copolymerisable therewith.

a, -Ethylenically unsaturated polyesters such as these are the usual polycondensation products of at least one, a, -ethylenically unsaturated dicarboxylic acid generally containing 4 or 5 carbon atoms or ester-forming derivatives thereof, optionally in admixture with up to 90 mole %, based on the unsaturated acid components, of at least one aliphatic saturated dicarboxylic acid containing from 4 to 10 carbon atoms or cycloaliphatic dicarboxylic acid containing from 8 to 10 carbon atoms or ester-forming derivatives thereof with at least one polyhydroxy compound, more especially a dihydroxy compound, containing from 2 to 8 carbon atoms - i.e. polyesters of the type described, for example, in J.R. Lawrence "Polyester Resins" Reinhold Publ. Corp., New York, 1960, pages 18 etseq, and Goerden-Vieweg Kunststoff-Handbuch, Vol. VIII ("Polyesters"), Carl Hanser Verlag, Munich 1973, pages 247 to 312.

Examples of preferred unsaturated dicarboxylic acids or their derivatives are maleic acid or maleic acid anhydride and fumaric acid. However, it is also possible to use, for example, mesaconic acid, citraconic acid, itaconic acid or chloromaleic acid. Examples of the aliphatic saturated and cycloaliphatic dicarboxylic acids or their derivatives which may be used in accordance with the invention are phthalic acid or phthalic acid anhydride, isophthalic acid, terephthalic acid, hexahydrophthalic or tetrahydrophthalic acid or their anhydrides, endomethylene tetrahydrophthalic acid or its anhydride, succinic acid or succinic acid anhydride and succinic acid esters and chlorides, adipic acid, sebacic acid.In order to produce substantially non-inflammable resins, it is possible to use, for example, hexachloroendomethylene tetrahydrophthalic acid (HET-acid), tetrachlorophthalic acid or tetrabromophthalic acid. Preferred polyesters contain maleic acid residues of which from 25 mole % to 75 mole % may be replaced by phthalic acid or isophthalic acid esters. Suitable dihydric alcohols are thylene glycol, 1,2-propane diol, 1,3-propane diol diethylene glycol, dipropylene glycol, thiodiglycol, 1,3-butane diol, 1,4-butane diol, neopentyl glycol, 1,6-hexane diol, 2,2-bis-(4-hydroxy-cyclohexyl)-propane, bis-alkoxylated bisphenol A, perhydrobisphenol and others. It is preferred to use ethylene glycol, 1,2-propane diol, diethylene glycol and dipropylene glycol.

Further modifications are possible by the incorporation of up to 10 mole %, based on the alcohol or acid component, of monohydric, trihydric and tetrahydric alcohols with 1 to 6 carbon atoms such as methanol, ethanol, butanol, allyl alcohol, benzyl alcohol, cyclohexanol and tetrahydrofurfuryl alcohol, trimethylol propane, glycerol and pentaerythritol and also mono-, di- and tri-allyl ethers and benzyl ethers of trihydric and polyhydric alcohols with 3 to 6 carbon atoms according to German Auslegeschrift No.

1,024,654, and also by the incorporation of mono-basic acids, such as benzoic acid, or long-chain unsaturated fatty acids, such as oleic acid, linseed oil fatty acid and ricinene fatty acid.

The acid numbers of the polyesters should be between 1 and 100 and preferably between 20 and 70, their OH-numbers between 10 and 150 and preferably between 20 and 100 and their molecular weights M,, measured as numerical averages, between about 500 and 5000 and preferably between about 1000 and 3000 (as measured by vapour pressure osmometry in dloxene and acetone; in the case of differing values, the lower value is taken as the correct value).

Suitable copolymerisable vinyl and vinylidene compounds are the unsaturated compounds normally encountered in polyester technology which preferably contain asubstituted vinyl groups or ss-substituted allyl groups, preferably styrene, but also for example nucleus-chlorinated and -alkylated or -alkenylated styrenes, in whose case the allyl groups may contain from 1 to 4 carbon atoms, for example vinyl toluene, divinyl benzene, a-methyl styrene, tert.-butyl styrene, chlorostyrenes; vinyl esters of carboxylic acids containing from 2 to 6 carbon atoms, preferably vinyl acetate; vinyl pyridine, vinyl naphthalene, vinyl cyclohexane, acrylic acid and methacrylic acid and/or their esters (preferably vinyl, allyl and methallyl esters) containing from 1 to 4 carbon atoms in the alcohol component, their amides and nitriles, maleic acid anhydride, semiesters and diesters with 1 to 4 carbon atoms in the alcohol component, semiamines and diamines or cyclic imides such as N-methyl maleic imide or N-cyclohexyl maleic imide; allyl compounds, such as allyl benzene, and allyl esters such as allyl acetate, phthalic acid diallyl ester, isophthalic acid diallyl ester, fumaric acid diallyl ester, allyl carbonates, diallyl carbonates, triallyl phosphate and triallyl cyanurate.

The cast polyester resins generally contain from 30 to 75 parts by weight of polyester and from 70 to 25 parts by weight of copolymerisable vinyl or vinylidene compounds.

In the case of polymer material having tinges of colour, corrections may have to be made by the addition of known dyes and/or optical lighteners.

The processing of the pigmented plastics and the use of auxiliaries is not impaired by the relatively coarsely divided pigments according to the invention. Auxiliaries which may be used as fillers for example, calcium carbonate, quartz powder, barium sulphate, kaolines heat stabilisers, UV-absorbers, optical lighteners, lubricants waxes, plasticisers, flameproofing agents or light stabilisers. The plastics material may be extruded, injection-moulded or rolled in the usual machines.

One criterion for the objective optical assessment of the plastics according to the invention is their yellowness index (ASTM D 1925-70). Test specimens with a haze of approximately 80% (ASTM 1003) were used for each of the following Examples. The degree of transmission for straight transmitted light (light type C DIN 5033) was determined with a Zeiss (Trade Mark) PM Q II spectral photometer. The yellowness index of plastics coloured in accordance with the invention should amount to less than 20.

Polymers containing coarsely divided pigments and having a yellowness index of less than 10 are preferred, values of from +5 to -5 being particularly desirable.

The invention is illustrated by the following Examples.

Example I A test specimen was produced by mixing a polycarbonate (nO = 2.76) having a-low dye number with 0.03% of a coarsely divided rutile pigment (n = 1.587) produced by calcination at an elevated temperature and having a central value of the volume distribution DZVL Of 0.78 llm and a particle size distribution range g-value of 1.47 and the resulting mixture was extended. The specific surface amounted to 2.7 m2/g and the yellowness index of the test specimen was measured at -16. The calculated value do was 0.19. The test specimen appeared substantially neutral in colour in both reflected and transmitted light.

Example 2 (Comparative test) A standard rutile pigment produced bathe sulphate process for the pigmenting of plastics with a specific surface of 7.4 m2/g and DZVL value of 0.23 m for a 6 g-value of 1.44 was worked into polycarbonate in accordance with Example 1. In this example n = 1.587, nO = 2.76 and do = 0.19. For a pigmenting level of 0.011% by weight, the test specimen appeared tinged with yellow in transmitted light and was found to have a yellowness index of 99.

Example 3 A TiO2 mixed pigment of rutile and anatase with a DZVL value of 0.43 clam, a 6 g-value of 1.49 and a specific surface of 4.9 m2/g was incorporated in polycarbonate in a quantity of 0.03% in accordance with Example 1 and measured. In this example n = 1.587, no =2.64 and do = 0.21. The test specimen was neutral in colour both in reflected and in transmitted light and had a yellowness index of -3.

Example 4 A pigment mixture of 10 parts by weight of the titanium dioxide used in Example 1 with one part by weight of a TiO2 pigment having a value DZVL of 0.37 and a 6 g-value of 1.35 was incorporated in accordance wtih Example 1. The refractive indices were n = 1.587 and no = 2.76 and the calculated value do was 0.19. For a pigmenting level of 0.025%, the test specimen was found to have a yellowness index of -3. The test specimen appeared neutral in colour both in reflected and in transmitted light. The pigment mixture had a DZVL value of 0.73 Rm and a 6 g-value of 1.68. The specific surface amounted to 3.6 m2/g.

Example 5 A polycarbonate granulate having a low dye number was mixed with 0.1% by weight of a coarsely divided zinc oxide white pigment and the resulting mixture was extruded. The refractive indices were n = 1.587 and nO = 2.02 and the calculated value do was 0.45. The ZnO-pigment had a specific surface of 0.8 m2/g. According to literature references (F.

Kindervater DVS. 13, 312 (1959)), an average particle diameter for spherical particles can be calculated therefrom in accordance with the following formula: 6 d = BET 6 Accordingly, the average particle diameter d amounted to 1.34 llm as calculated from the BET-surface in m2/g and the density 6 of ZnO of 5.6 g/cc.

The test speicmen with a wall thickness of 4 mm was investigated in the same way as described in the preceding Example. It appeared neutral in colour in reflected light and transmitted light and had a yellowness index of 12.

Example 6 (Comparative test) For comparison, a polycarbonate (nO = 2.02) was pigmented with 0.3% by weight of a finely divided ZnO pigment (n = 1.587) which was not in accordance with the invention and which had a specific surface according to BET of 3.6 m2/g corresponding to an average particle diameter of 0.30 llm. The calculated value do was 0.45. For a wall thickness of 1 mm and, hence, substantially the same haze, the test speicmen had a yellowness index of 110 and showed distinct yellowing of the transmitted light.

Example 7 0.06% by weight of the coarsely divided titanium dioxide pigment used in Example 1 but with a refractive index n = 1.562 was added to a polyester cast resin (nO = 2.76) which was then cast into 4 mm thick test plates and hardened for 15 hours at 90"C. The calculated value do was 0.19. For somewhat higher haze values, measurement produced a yellowness index of -8.

Example 8 (Comparative test) The pigmenting of a polyester cast resin (n = 2.76 corresponding to Example 7 was repeated for comparison with the standard TiO2-pigment of Example 2 but with a refractive index n=1.562, 0.03% by weight of the pigment being added. The calculated value do was 0.19. The 4mm thick test specimen was found by measurement to have a yellowness index of 74. The polyester plate with the relatively coarse pigment used in accordance with the invention added to it was considerably more neutral in colour and the transmitted light appeared distinctly less tinged with yellow than in the case of pigmenting with the TiO2-pigments commercially available for plastics. The haze values of the test specimens according to Examples 7 and 8 were of substantially the same order of magnitude.

Example 9 A granulate of a cellulose ester with a low colour value was mixed with 0.027 % by weight of the coarsely divided pigment mixture used in Example 4. The resulting mixture was extruded and then processed into 3mm thick test specimens. The refractive indices were n=1.48 and nO=2.76. The calculated value do was 0.18. The 3mm thick test specimen appeared neutral in colour in both reflected light and transmitted light. The test specimen had a yellowness index of 1.

WHAT WE CLAIM IS: 1. A plastics material which appears white in reflected and transmitted light, comprising at least one translucent organic polymer and from 0.001 to 20% by weight, based on the polymer, of at least one white pigment, wherein the refractive index no of the polymer is in the range of from 1.4 to 1.65, wherein the refractive index n of the pigment is in the range of from 1.7 to 2.9, and wherein the average particle diameter d of the pigment is between 2 do and 5 d,, where 0.122 (m2 + 2) ym, with d 0.122 (m2 - 1) n m = no 2. A plastics material as claimed in claim 1, which comprises one or more fillers, optical lighteners, stabilisers, plasticisers and/or other auxiliaries.

3. A plastics material as claimed in claim 1 or 2, which comprises the white pigment in a quantity of from 0.005 to 5% by weight based on the polymer component.

4. A plastics material as claimed in any of claims 1 to 3, wherein the white pigment is titanium phosphate, lead hydrogen phosphate, zinc oxide, zinc sulphide, magnesium titanate, calcium titanate, anatase or rutile, either individually or in admixture.

5. A plastics material as claimed in any of claims 1 to 4, wherein the polymer is polyacrylate, polymethacrylate, a polycarbonate, polystyrene, polyethylene, polyester or a cellulose ester.

6. A plastics material as claimed in claim 1, which comprises a polycarbonate and from 0.001 to 1% of titanium dioxide with an average particle diameter of from 0.40 to 0.80 um.

7. A plastics material as claimed in claim 6, wherein the titanium dioxide pigment has the anatase modification and has a specific surface according to BET of less than 5.5 m2/g.

8. A plastics material as claimed in claim 7, wherein the titanium dioxide pigment has specific surface according to BET of from 0.5 to 4.5 m2/g.

9. A plastics material as claimed in claim 8, which comprises a rutile pigment with a specific surface of less than 5.5 m2/g.

10. A plastics material as claimed in claim 9, wherein the rutile pigment has a specific surface of from 0.5 to 4.5 m2/g.

11. A plastics material which appears white in reflected and transmitted light substantially as herein described with reference to any of the specific Examples 1, 3, 4, 5, 7 and 9.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. Example 8 (Comparative test) The pigmenting of a polyester cast resin (n = 2.76 corresponding to Example 7 was repeated for comparison with the standard TiO2-pigment of Example 2 but with a refractive index n=1.562, 0.03% by weight of the pigment being added. The calculated value do was 0.19. The 4mm thick test specimen was found by measurement to have a yellowness index of 74. The polyester plate with the relatively coarse pigment used in accordance with the invention added to it was considerably more neutral in colour and the transmitted light appeared distinctly less tinged with yellow than in the case of pigmenting with the TiO2-pigments commercially available for plastics. The haze values of the test specimens according to Examples 7 and 8 were of substantially the same order of magnitude. Example 9 A granulate of a cellulose ester with a low colour value was mixed with 0.027 % by weight of the coarsely divided pigment mixture used in Example 4. The resulting mixture was extruded and then processed into 3mm thick test specimens. The refractive indices were n=1.48 and nO=2.76. The calculated value do was 0.18. The 3mm thick test specimen appeared neutral in colour in both reflected light and transmitted light. The test specimen had a yellowness index of 1. WHAT WE CLAIM IS:

1. A plastics material which appears white in reflected and transmitted light, comprising at least one translucent organic polymer and from 0.001 to 20% by weight, based on the polymer, of at least one white pigment, wherein the refractive index no of the polymer is in the range of from 1.4 to 1.65, wherein the refractive index n of the pigment is in the range of from 1.7 to 2.9, and wherein the average particle diameter d of the pigment is between 2 do and 5 d,, where 0.122 (m2 + 2) ym, with d 0.122 (m2 - 1) n m = no

2. A plastics material as claimed in claim 1, which comprises one or more fillers, optical lighteners, stabilisers, plasticisers and/or other auxiliaries.

3. A plastics material as claimed in claim 1 or 2, which comprises the white pigment in a quantity of from 0.005 to 5% by weight based on the polymer component.

4. A plastics material as claimed in any of claims 1 to 3, wherein the white pigment is titanium phosphate, lead hydrogen phosphate, zinc oxide, zinc sulphide, magnesium titanate, calcium titanate, anatase or rutile, either individually or in admixture.

5. A plastics material as claimed in any of claims 1 to 4, wherein the polymer is polyacrylate, polymethacrylate, a polycarbonate, polystyrene, polyethylene, polyester or a cellulose ester.

6. A plastics material as claimed in claim 1, which comprises a polycarbonate and from 0.001 to 1% of titanium dioxide with an average particle diameter of from 0.40 to 0.80 um.

7. A plastics material as claimed in claim 6, wherein the titanium dioxide pigment has the anatase modification and has a specific surface according to BET of less than 5.5 m2/g.

8. A plastics material as claimed in claim 7, wherein the titanium dioxide pigment has specific surface according to BET of from 0.5 to 4.5 m2/g.

9. A plastics material as claimed in claim 8, which comprises a rutile pigment with a specific surface of less than 5.5 m2/g.

10. A plastics material as claimed in claim 9, wherein the rutile pigment has a specific surface of from 0.5 to 4.5 m2/g.

11. A plastics material which appears white in reflected and transmitted light substantially as herein described with reference to any of the specific Examples 1, 3, 4, 5, 7 and 9.

12. A method of preparing a plastics material which appears white in reflected and

transmitted light wherein at least one translucent organic polymer and from 0.001 to 20% by weight, based on the polymer, of at least one white pigment are mixed together, the refractive index nO of the polymer being in the range of from 1.4 to 1.65, the refractive index n of the pigment being in the range of from 1.7 to 2.9 and the average particle diameter of the pigment being between 2 do and 5 d0 where 0.122 (m2 + 2) 0.122 (m2 + 2) in ltm with n0 -m-1) n m nO

13. A method of preparing a plastics material which appears white in reflected and transmitted light substantially as herein described with reference to any of the specific Examples 1, 3, 4, 5, 7 and 9.