1 HIGH PURITY MONOALKYLTIN COMPOUNDS AND USES THEREOF Field of the invention [0001] The present invention relates to high purity monoalkyltin compounds, more specifically to alkyltin compound compositions containing monoalkyltin as major compound, and minor quantities of di- and/or trialkyltin compounds. [0002] The present invention also relates to the preparation processes of such high purity monoalkyltin compounds, as well as to the uses of said monoalkyltin compounds, such as chlorine-containing polymer-stabilisers, glass coating chemicals and catalysts, and the like. Prior art and Technical problem [0003] Tin-based stabilisers are widely known and used in chlorine-containing polymers and co-polymers, such as for example poly(vinyl chloride) (PVC) polymers and PVC-based copolymers. Such uses of tin-based stabilisers are for example described in the Kirk Othmer "Encyclopedia of Chemical Technology", 4'^ Edition, (1994), vol. 12, Heat Stabilisers, pp. 1071-1091. [0004] These known tin-based stabilisers are used as mixtures of mono-alkyltin with dialkyltin compounds. For example widely used tin-based compounds in PVC polymers and copolymers are mixtures of mono- and di-methyltin compounds, mono- and di-butyltin compounds or mono- and di-octyltin compounds, such as those sold under the name Thermolite® by Arkema. [0005] However, tri-alkyltin compounds are known to be toxic compounds, and di-alkyltin compounds have recently been classified as toxic compounds. Toxicity of tin compounds is known to be linked to mono-, di- and tri-alkyi tin compound contents, particularly toxicity is increasing from mono-, to di- and to tri-alkyi tin compound contents. Therefore it is nowadays highly relevant to develop mono-alkyltin compounds, with low levels of di- and tri-alkyi tin compounds, in order to avoid toxicity issues. [0006] Attempts to the production of high purity mono-alkyltin chlorides have been conducted, for example through redistribution of tetra-alkyltin and tri-alkyltin compounds with tin tetrachloride followed by fractional distillation: mono-alkyltin chlorides were obtained in relatively pure form but di-alkyltin chlorides were always co-produced in significant quantities rendering these routes less attractive from an industrial perspective. [0007] Recently developed technology as disclosed in EP-A-01 225177 and EP-A-1 743 898 allows the more selective production of mono-alkyltin chlorides without producing significant amount of di- and tri-alkyltin species as by-products. These technologies provide access to high purity monoalkyltin compounds through relatively short purification procedures. 2 [0008] Therefore an objective of the present invention is to provide non-toxic or less toxic tin-based compounds or compositions for stabilising polymers, more specifically chlorinecontaining polymers, e.g. PVC polymers and copolymers. [0009] Another objective of the present invention is to provide high purity monoalkyltin compounds, i.e. compositions with a major content of monoalkyltin compound and minor contents, and preferably only traces of, and even more preferably no di- and tri-alkyltin compounds, for use as stabilisers for chlorine-containing polymers, especially PVC polymers, post-chlorinated PVC and copolymers thereof, as well as for uses as a catalyst, or in glass coating compositions. Brief description of tfie invention [0010] The inventors have now discovered that high purity monoalkyltin trihalides can be converted to other mono-alkyltin derivatives that are of interest for several existing applications. The technology developed thus provides high purity mono-aikyltin compounds on an industrial scale as more benign alternatives to currently applied mixtures of monoand di-alkyltin compounds. [0011] The inventors have also discovered that high purity mono-alkyltin compounds, when used for example as stabilisers for chlorine-containing polymers and copolymers, provide to said polymers and copolymers remarkable and unexpected stability properties, more specifically thermal properties, long term colorhold retention properties, transparency properties (specifically for polymer films of variable thickness), among others. Detailed description of tiie invention [0012] In a first aspect, the present invention relates to a composition comprising: • from 85 wt% to 99.999 wt%, preferably from 90 wt% to 99.999 wt%, more preferably from 95 wt% to 99.99 wt%, still more preferably from 97 wt% to 99.99 wt% (limits included) of at least one monoalkyltin compound of formula RSn(T)3, in which R is linear, branched or cyclic C1-C20 alkyl, preferably C1-C10 alkyl, and T is a ligand; • from 0.001 wt% to 10 wt%, preferably from 0.001 wt% to 1 wt%, more preferably from 0.01 wt% to 0.5 wt%, most preferably from 0.01 wt% to 0.1 wt%, of at least one dialkyltin compound of formula R2Sn(T)2, in which R and T are as defined above; • from 0.001 wt% to 5wt%, preferably from 0.005 wt% to 1 wt%, more preferably from 0.01 wt% to 0.5 wt%, most preferably from 0.01 wt% to 0.1 wt%, of at least one trialkyltin compound of formula R3Sn(T), in which R and T are as defined above; and • from 0 wt% to 5 wt%, more preferably from 0 wt% to 1 wt%, still more preferably from 0 wt% to 0.2 wt% of one or more impurities. 3 [0013] According to another aspect, the composition of the present invention comprises a weight ratio of (mono-allPVC Technology", W. V. Titow, 4th ed., Elsevier Publishers, 1984, section 6.10.2, pages 171-173, (group G), section 6.10.5 page 174, (group H), section 6.10.3, page 173, (group I) and section 6.10.4, pages 173-174 (group J). It is also possible to use mixtures of different plasticizers. The plasticizers can be employed in an amount of, for example, from 5 to 20 parts by weight, judiciously from 10 to 20 parts by weight, per 100 parts by weight of chlorine-containing polymers and resins, such as PVC. Rigid or semi rigid PVC contains preferably up to 10%, with particular preference up to 5% of plasticiser, or no plasticiser. [0117] As other additives, pigments may be used, suitable substrates of which are known to the person skilled in the art. Examples of inorganic pigments are Ti02, zirconium oxidebased pigments, BaS04, zinc oxide (zinc white) and lithopones (zinc sulphide/barium sulphate), carbon black, carbon black/titanium dioxide mixtures, iron oxide pigments, Sb203, (Ti, Ba, Sb)02, Cr203, spinels, such as cobalt blue and cobalt green, Cd(S, Se), ultramarine blue. Organic pigments are, for example, azo pigments, phthalocyanine pigments, quinacridone pigments, perylene pigments, diketopyrrolopyrrole pigments and anthraquinone pigments. Preference is also given to Ti02 in micronised form. [0118] Epoxidized fatty acid esters and other epoxy compounds may also be used as additives to the chlorine-containing polymers together with the stabilising composition of the invention. Particularly suitable such esters are those of fatty acids from natural sources (fatty acid glycerides), such as soybean oil or rapeseed oil. It is, however, also possible to employ synthetic products such as epoxidized butyl oleate. Epoxidized polybutadiene and polyisoprene can also be used, as they are or in partially hydroxylated form, or else homoor copolymeric glycidyl acrylate and glycidyl methacrylate can be used. These epoxy compounds can also have been applied to an alumo salt compound; in this regard see also DE031 818. [0119] Customary antioxidants may also be used as additives, alone or in combination. Examples of suitable antioxidants are alkylated monophenols, for example, 2,6-di-teAtbutyl- 4-methylphenol, 2-tert-butyl-4,6-dimethylphenol, 2,6-di-te/t-butyl-4-ethylphenol, 2,6-dite/ t-butyl-4-n-butylphenol, 2,6-di-tert-butyl-4-/so-butylphenol, 2,6-di-cyclopentyl-4- 26 methylphenol, 2-(a-methylcyclohexyl)-4,6-dimethylphenol, 2,6-di-octadecyl-4-methylphenol, 2,4,6-tricyclohexylphenol, 2,6-di-fert-butyl-4-methoxymethylphenol, 2,6-dinonyl-4- methylphenol, 2,4-dimethyl-6-(1 '-methylundec-1 '-yl)phenol, 2,4-dimethyl-6-(1 '- methylheptadex-1'-yl)phenol, 2,4-dimethyl-6-(1'-methyltridec-1'-yl)phenol, octylphenol, nonylphenol, dodecylphenol and mixtures thereof. [0120] Other examples are alkylthiomethylphenols, such as 2,4-dioctylthiomethyl-6-fertbutylphenol, 2,4-dioctylthiomethyl-6-methylphenol, 2,4-dioctylthiomethyl-6-ethylphenol, and 2,6-didodecylthiomethyl-4-nonylphenol; alkylated hydroquinones, such as 2,6-di-tert-butyl- 4-methoxyphenol, 2,5-di-tert-butylhydroquinone, 2,5-di-terf-amylhydroquinone, 2,6- diphenyl-4-octadecyloxyphenol, 2,6-di-terf-butylhydroquinone, 2,5-di-fe/t-butyl-4-hydroxyanisole, 3,5-di-teAf-butyl-4-hydroxyanisole, 3,5-di-tert-butyl-4-hydroxyphenyl stearate, bis(3,5-di-tert-butyl-4-hydroxyphenyl) adipate; hydroxylated thiodiphenyl ethers, such as 2,2'-thiobis(6-fert-butyl-4-methylphenol), 2,2'-thiobis(4-octylphenol), 4,4'-thiobis(6-tert-butyl- 3-methylphenol), 4,4'-thiobis(6-tert-butyl-2-methylphenol), 4,4'-thiobis-(3,6-di-secamylphenol)- 4,4'-bis-(2,6-dimethyl-4-hydroxyphenyl)disulphide; alkylidenebisphenois, such as 2,2'-methylenebis(6-tert-butyl-4-methylphenol), 2,2'-methy!enebis(6-fert-butyl-4- ethylphenol), 2,2'-methylene bis-[4-methyl-6-(a-methylcyclohexyl)phenol], 2,2'- methylenebis(4-methyl-6-cyclohexylphenol), 2,2'-methylenebis(6-nonyl-4-methylphenol), 2,2'-methylenebis-(4,6-di-te/t-butylphenol), 2,2'-ethylidenebis(4,6-di-teAt-butylphenol), 2,2'-ethylidenebis(6-terf-butyl-4-isobutylphenol), 2,2'-methylenebis[6-(a-methylbenzyl)-4- nonylphenol], 2,2'-methylenebis[6-(a,a-dimethylbenzyl)-4-ncnylphenol], 4,4'-methylene bis(2,6-di-ferf-butylphenol), 4,4'-methylene bis-(6-tert-butyl-2-methylphenol), 1,1-bis-(5-fertbutyl- 4-hydroxy-2-methylphenyl)butane, 2,6-bis-(3-fert-butyl-5-methyl-2-hydroxybenzyl)-4- methylphenol, 1,1,3-tris-(5-ferf-butyl-4-hydroxy-2-methyl-phenyl)butane, 1,1-bis-(5-terfbutyl- 4-hydrcxy-2-methylphenyl)-3-n-dodecylmercaptobutane, ethylene glycol bis[3,3-bis- (3'-tert-butyl-4'-hydroxyphenyl)butyrate], bis(3-tert-butyl-4-hydroxy-5-methyl-phenyl)dicyclopentadiene, bis[2-(3'-teft-butyl-2'-hydroxy-5'-methylbenzyl)-6-fert-butyl-4-methylphenyl] terephthalate, 1,1-bis-(3,5-dimethyl-2-hydroxyphenyl)butane, 2,2-bis-(3,5-di-fertbutyl- 4-hydroxyphenyl)propane, 2,2-bis(4-hydroxyphenyl)propane, 2,2-bis-(5-tert-butyl-4- hydroxy-2-methylphenyl)-4-n-dodecylmercaptobutane, 1,1,5,5-tetra-(5-fert-butyl-4-hydroxy- 2-methylphenyl)pentane; benzyl compounds, such as 3,5,3',5'-tetra-tert-butyl-4,4'- dihydroxy-dibenzyl ether, octadecyl 4-hydroxy-3,5-dimethylbenzyl-mercaptoacetate, tris(3,5-di-teAf-butyl-4-hydroxybenzyl)amine, bis(4-fer/-butyl-3-hydroxy-2,6-dimethylbenzyl)- dithioterephthalate, bis(3,5-di-tert-butyl-4-hydroxybenzyl) sulphide, isooctyl 3,5-di-terf-butyl- 4-hydroxybenzyl-mercaptoacetate; hydroxybenzylated malonates, for example, dioctadecyl 2,2-bis(3,5-di-tert-butyl-2-hydroxybenzyl)malonate, di-oactadecyl 2-(3-tert-butyl- 4-hydroxy-5-methylbenzyl)malonate, di-dodecyl mercapto-ethyl-2,2-bis(3,5-di-tert-butyl-4- 27 hyciroxybenzyl)malonate, di-[4-(1,1,3,3-tetramethylbutyl)-phenyl]-2,2-bis(3,5-di-tert-butyl-4- hydroxybenzyl)malonate; aromatic hydroxybenzyl compounds, for example, 1,3,5-tris(3,5- di-ferf-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene, 1,4-bis(3,5-di-teAf-butyl-4- hydroxybenzyl)-2,3,5,6-tetramethylbenzene, 2,4,6-tris(3,5-di-ferf-butyl-4-hydroxybenzyl) phenol; triazine compounds, such as 2,4-bisoctylmercapto-6-(3,5-di-feAt-butyl-4- hydroxyanilino)-1,3,5-triazine, 2-octylmercapto-4,6-bis-(3,5-di-tert-butyl-4-hydroxyanilino)- 1,3,5-triazine, 2-octylmercapto-4,6-bis-(3,5-di-fert-butyl-4-hydroxyphenoxy)-1,3,5-triazine, 2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenoxy)-1,2,3-triazine, 1,3,5-tris(3,5-di-fe/t-butyl-4- hydroxybenzyl)isocyanurate, 1,3,5-tris-(4-te/t-butyl-3-hydroxy-2,6-dimethylbenzyl) isocyanurate, 2,4,6-tris-(3,5-di-fe/t-butyl-4-hydroxyphenylethyl)-1,3,5-triazine, 1,3,5-tris(3,5- di-fe/t-butyl-4-liydroxyphenylpropionyl)hexaliydro-1,3,5-triazine, 1,3,5-tris-(3,5-dicycloiiexyl- 4-hydroxybenzyl) isocyanurate; phospinates and phosphonites, for example, dimethyl-2,5- di-ferf-butyl-4-hydroxybenzylphosphonate, diethyl-3,5-di-ferf-butyl-4-hydroxybenzylphosphonate, dioctadecyl-3,5-di-terf-butyl-4-hydroxybenzylphosphonate, dioctadecyl-5-tert-butyl-4-hydroxy-3-methylbenzylphosphonate, calcium salt of monoethyl 3,5-di-fert-butyl-4-hydroxybenzylphosphonate, tetrakis-(2,4-di-fe/t-butylphenyl)-4,4'- biphenylenediphosphonite, 6-isooctyloxy-2,4,8,10-tetra-tert-butyl-12/-/-dibenzo-[d,g]-1,3,2- dioxaphosphocine, 6-fluoro-2,4,8,10-tetra-teAf-butyl-12-methyl-dibenzo-[d,g]-1,3,2- dioxaphosphocine; acylaminophenols, such as 4-hydroxylauranilide, 4-hydroxystearanilide, octyl N-(3,5-di-tert-butyl-4-hydroxyphenyl)carbamate; esters of 3-(3,5-di-te/t-butyl-4- hydroxyphenyl)propionic acid with mono- or polyhydric alcohols, e.g. with methanol, ethanol, octanol, octadecanol, 1,6-hexanediol, 1,9-nonanediol, ethylene glycol, 1,2-propane diol, neo-pentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, dipentaerythritol, tris(hydroxyethyl) isocyanurate, N,N'- bis(hydroxyethyl)oxalamide, 3-thia-undecanol, 3-thia-pentadecanol, trimethylhexanediol, trimethylolpropane, ditrimethylolpropane, 4-hydroxymethyl-1 -phospha-2,6,7- trioxabicyclo[2.2.2]octane; esters of 3-(5-teAt-butyl-4-hydroxy-3-methylphenyl)propionic acid with mono- or polyhydric alcohols, for example, with methanol, ethanol, octanol, octadecanol, 1,6-hexanediol, 1,9-nonanediol, ethylene glycol, 1,2-propanediol neo-pentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, tris(hydroxy)ethyl isocyanurate, N,N'-bis(hydroxyethyl)oxaiamide, 3-thiaundecanol, 3- thiapentadecanol, trimethylhexanediol, trimethylolpropane, 4-hydroxymethyl-1-phospha- 2,6,7-trioxabicyclo[2.2.2]octane; esters of 3-(3,5-dicyclohexyl-4-hydroxyphenyl)propionic acid with mono- or polyhydric alcohols, for example, with methanol, ethanol, octanol, octadecanol, 1,6-hexanediol, 1,9-nonanediol, ethylene glycol, 1,2-propanediol, neopentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pantaerythritol, tris(hydroxy)ethyl isocyanurate, N,N'-bis(hydroxyethyl)oxalamide, 3-thia-undecanol, 3- 28 thiapentadecanol, trimethylhexanediol, trimethylolpropane, 4-hydroxymethyl-1-phospha- 2,6,7-trioxabicyclo[2.2.2]octane; esters of 3,5-di-te/t-butyl-4-hydroxyphenylacetic acid with mono- or polyhydric alcohols, for example, with methanol, ethanol, octanol, octadecanol, 1,6-hexanediol, 1,9-nonanediol, ethylene glycol, 1,2-propanediol, neopentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, tris(hydroxy)ethyl isocyanurate, N,N'-bis(hydroxyethyl)oxalamide, 3-thia-undecanol, 3-thiapentadecanol, trimethylhexanediol, trimethylpropane, 4-hydroxymethyl-1 -phospha-2,6,7- trioxabicyclo[2.2.2]octane; amides of 3-(3,5-di-fert-butyl-4-hydroxyphenyl)propionic acid, such as, for example, N,N'-bis(3,5-di-feAt-butyl-4- hydroxyphenylpropionyl)hexamethylenediamine, N,N'-bis(3,5-di-fert-butyl-4- hydroxyphenylpropionyl)trimethylenediamine, N,N'-bis(3,5-di-fert-butyl-4- hydroxyphenylpropionyl)hydra2ine; vitamin E (tocopherol) and derivatives. [0121] As antioxidants, preference is given to 2,2-bis(4-hydroxyphenyl)propane, esters of 3,5-di-te/t-butyl-4-hydroxyphenylpropionic acid with octanol, octadecanol or pentaerythritol or tris(2,4-di-feAf-butylphenyl) phosphite. It is also possible, if desired, to employ a mixture of antioxidants of different structures. [0122] The antioxidants can be employed in an amount of, for example, from 0.01 to 10 parts by weight judiciously from 0.1 to 10 parts by weight and in particular, from 0.1 to 5 parts by weight per 100 parts by weight of PVC. [0123] Other additives may be UV absorbers and light stabilisers, examples of which are 2-(2'-hydroxyphenyl)benzotriazoles, such as, for example 2-(2'-hydroxy-5'- methylphenyl)benzotriazole, 2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)benzotriazole, 2-{5'-tertbutyl- 2-2'-hydroxyphenyl)benzotriazole, 2-(2'-hydroxy-5'-(1,1,3,3- tetramethylbutyl)phenyl)benzotriazole, 2-(3',5'-di-fert-butyl-2'-hydroxyphenyl)-5- chlorobenzotriazole, 2-(3'-fe/t-butyl-2'-hydroxy-5'-methylphenyl)-5-chlorobenzotriazole, 2- (3'-sec-butyl-5'-terf-butyl-2'-hydroxyphenyl)benzotriazole, 2-(2'-hydroxy-4'- octoxyphenyl)benzotriazole, 2-(3',5'-di-te/t-amyl-2'-hydroxyphenyl)benzothiazole, 2-(3',5'- bis(a,a-dimethylbenzyl)-2'-hydroxyphenyl)benzothiazole, mixtures of 2-(3'-tert-butyl-2'- hydroxy-5'-(2-octyloxycarbonylethyl)phenyl, 0-chlorobenzotriazole, 2-(3'-fert-butyl-5'-[2-(2- ethylhexyloxy)carbonylethyl]-2'-hydroxyphenyl)-5-chlorobenzotri-azole, 2-(3'-tert-butyl-2'- hydroxy-5'-(2-methoxycarbonylethyl)phenyl)-5-chlorobenzotriazole, 2-(3'-fert-butyl-2'- hydroxy-5'-(2-methoxycarbonylethyl)phenyl)benzotriazole, 2-{3'-tert-butyl-2'-hydroxy-5'-(2- octyloxycarbonylethyl)phenyl)benzotriazole, 2-(3'-te/t-butyl-5'-[2-(2- ethylhexyloxy)carbonylethyl]-2'-hydroxyphenyl)benzotriazole, 2-(3'-dodecyl-2'-hydroxy-5'- methylphenyl)benzotriazole and 2-(3'-teAf-butyl-2'-hydroxy-5'-(2- isooctyloxycarbonylethyl)phenylbenzotriazole, 2,2'-methylenebis[4-(1,1,3,3- tetramethylbutyl)-6-benzotriazol-2-ylphenol]; the transesterification product of 2-[3'-tert29 butyl-5'-(2-methoxycarbonylethyl)-2'-hydroxyphenyl]benzotriazole with polyethylene glycol 300; where R = 3'-tert-butyl-4'-hydroxy-5'-2/-/-benzotriazol-2-yl-phenyl; 2- hydroxybenzophenones, for example the 4-hydroxy-, 4-methoxy-, 4-octyloxy-, 4-decyloxy, 4-dodecyloxy-, 4-benzyloxy, 4,2',4'-trihydroxy-, 2'-4,4'-dimethoxy-derivatives; esters of substituted or unsubstituted benzoic acids, for example 4-tert-butylphenyl salicylate, phenyl salicylate, octylphenyl salicylate, dibenzoylresorcinol, bis(4-tert-butylbenzoyl)resorcinol, benzoylresorcinol, 2,4-di-fe/t-butylphenyl 3,5-di-ferf-butyl-4-hydroxybenzoate, hexadecyl- 3,5-di-tert-butyl-4-hydroxybenzoate, octadecyl 3,5-di-tert-butyl-4-hydroxybenzoate, 2- methyl-4,6-di-fert-butylphenyl 3,5-di-ferf-butyl-4-hydroxy-benzoate; acrylates, for example ethyl-a-cyano-3,|3-diphenylacrylate or isooctyl-ethyl-a-cyano-p,p-diphenylacrylate, methyl- G-carbo-methoxycinnamate, methyl-a-cyano-p-methyl-para-methoxycinnamate or butyl-acyano- 3-methyl-para-methoxycinnamate, methyl-a-carbomethoxy-para-methoxycinnamate, N-(3-carbomethoxy-cyanovinyl)-2-methylindoline; nickel compounds, for example nickel complexes of 2,2'-thiobis[4-(1,1,3,3-tetramethylbutyl)phenol], such as the 1:1 or 1:2 complex, with or without additional ligands such as n-butylamine, triethanolamine or N-cyclohexyldiethanolamine, nickel dibutyldithiocarbamate, nickel salts of monoalkyi esters such as the methyl or ethyl ester, of 4-hydroxy-3,5-di-tert-butylbenzylphosphonic acid, nickel complexes of ketoximes, such as of 2-hydroxy-4-methylphenylundecylketoxime, nickel complexes of 1-phenyl-4-lauroyl-5-hydroxypyrazole, with or without additional ligands; oxalamides, for example 4,4'-dioctyloxyoxanilide, 2,2'-dioctyloxy-5,5'-di-te/t-butyloxanilide, 2,2'-didodecyloxy-5,5'-di-tert-butyloxanilide, 2-ethoxy-2'-ethyl-oxanilide, N,N'-bis(3-dimethylaminopropyl)oxalamide, 2-ethoxy-5-tert-butyl-2'-ethyloxyanilide and its mixture with 2-ethoxy-2'-ethyl-5,4'-di-tert-butyl-oxanilide, mixtures of ortho- and paramethoxy and of ortho- and para-ethoxy-di-substituted oxanilides; 2-(2-hydroxyphenyl)- 1,3,5-triazines, for example 2,4,6-tris(2-hydroxy-4-octyloxyphenyl)-1,3,5-triazine, 2-(2- hydroxy-4-octyloxyphenyl)-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, 2-(2,4- dihydroxyphenyl)-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, 2,4-bis(2-hydroxy-4- propyloxyphenyl)-6-(2,4-dimethylphenyl)-1,3,5-triazine, 2-(2-hydroxy-4-octyloxyphenyl)-4,6- bis(4-methylphenyl)-1,3,5-triazine, 2-(2-hydroxy-4-dodecyloxyphenyl)-4,6-bis(2,4-dimethylphenyl)- 1,3,5-triazine, 2-[2-hydroxy-4-(2-hydroxy-3-butyloxypropyloxy)phenyl]-4,6-bis(2,4- dimethylphenyl)-1,3,5-triazine, 2-[2-hydroxy-4-(2-hydroxy-3-octyloxypropoxy)phenyl]-4,6- bis(2,4-dimethylphenyl)-1,3,5-triazine; sterically hindered amines, for example bis(2,2,6,6- tetramethyl-piperidin-4-yl) sebaceate, bis(2,2,6,6-tetramethylpiperidin-4-yl)succinate, bis(1,2,2,6,6-pentamethylpiperidin-4-yl)sebaceate, bis(1-octyloxy-2,2,6,6- tetramethylpiperidin-4-yl)sebaceate, bis(1,2,2,6,6-pentamethylpiperidyl) n-butyl-3,5-di-te/tbutyl- 4-hydroxybenzylmalonate; the condensate of 1-hydroxyethyl-2,2,6,6-tetramethyl-4- hydroxypiperidine and succinic acid, linear or cyclic condensates of N,N'-bis(2,2,6,6- 30 tetramethyl-4-piperidyl)hexamethylenecliamine and 4-fert-octylamino-2,6-dichloro-1,3,5-striazine, tris(2,2,6,6-tetramethyl-4-piperidyl)nitrilotriacetate, tetrakis(2,2,6,6-tetramethyl-4- piperidyl)-1,2,3,4-butanetetraoate, 1,1 '-(1,2-ethanediyl)-bis(3,3,5,5- tetramethylpiperazinone), 4-benzoyl-2,2,6,6-tetramethylpiperidine, 4-stea ryloxy-2,2,6,6- tetramethylpiperidine, bis(1,2,2,6,6-pentamethylpiperidyl), 2-n-butyl-2(2-hydroxy-3,5-di-ferfbutylbenzyl) malonate, 3-n-octyl-7,7,9,9-tetramethyl-1,3,8-triazaspiro[4.5]decane-2,4-dione, bis(1-octyloxy-2,2,6,6-tetramethylpiperidyl)sebaceate, bis(1-octyloxy-2,2,6,6-tetramethylpiperidyl) succinate, linear or cyclic condensates of N,N'-bis(2,2,6,6-tetramethyl-4- piperidyl)hexamethylenediamine and 4-morpholino-2,6-dichloro-1,3,5-triazine; the condensate of 2-chloro-4,6-di-(4-n-butylamino-2,2,6,6-tetramethylpiperidyl)-1,3,5-triazine and 1,2-bis(3-aminopropylamino)ethane, the condensate of 2-chloro-4,6-di(4-n-butylamino- 1,2,2,6,6-pentamethylpiperidyl)-1,3,5-triazine and 1,2-bis(3-aminopropylamino)ethane, 8- acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triazaspiro[4.5]decane-2,4-dione, 3-dodecyl-1- (2,2,6,6-tetramethyl-4-piperidyl)pyrrolidone-2,5-dione, 3-dodecyl-1 -(1,2,2,6,6-pentamethyl- 4-piperidyl)pyrrolidine-2,5-dione, mixtures of 4-hexadecyloxy- and 4-stea ryloxy-2,2,6,6- tetramethylpiperidine; the condensate of N,N'-bis(2,2,6,6-tetramethyl-4- piperidyl)hexamethylendiamine and A-cyclohexylamino-2,6-dichloro-1,3,5-triazine, the condensate of 1,2-bis(3-aminopropylamino)ethane and 2,4,6-trichloro-1,3,5-triazine, and also 4-butylamino-2,2,6,6-tetramethylpiperidine (CAS Reg. No. [136504-96-6]); N-(2,2,6,6- tetramethyl-4-piperidyl)-n-dodecylsuccinimide, N-(1,2,2,6,6-pentamethyl-4-piperidyl)-ndodecylsuccinimide, 2-undecyl-7,7,9,9-tetramethyl-1-oxa-3,8-diaza-4-oxo-spiro[4.5]decane; 1,1-bis(1,2,2,6,6-pentamethyl-4-piperidyloxycarbonyl)-2-(4-methoxyphenyl)ethene, N,N'- bisformyl-N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)hexamethylenediamine, the diester of Amethoxymethylenemalonic acid with 1,2,2,6,6-pentamethyl-4-hydroxypiperidine, poly[methylpropyl-3-oxy-4-(2,2,6,6-tetramethyl-4-piperidyl)]siloxane; the reaction product of maleic anhydride-a-olefin copolymer and 2,2,6,6-tetramethyl-4-aminopiperidine or 1,2,2,6,6-pentamethyl-4-aminopiperidine. [0124] Still as additives, mention may be made of blowing agents, examples thereof being organic azo and hydrazo compounds, tetrazoles, oxazines, isatoic anhydride, and also sodium carbonate and sodium bicarbonate. Preference is given to azodicarboxamide and sodium bicarbonate and mixtures thereof. [0125] Still other additives may be used in the context of the present invention, and among them, mention may be made of impact modifiers, thermal modifiers, processing aids, gelling agents, antistatic agents, biocides, fungicides, metal passivators, optical brighteners, flame retardants, antifogging agents and compatibilisers, which are described in "Kunststoffadditive", R. Gachter; H. Muller, Carl Hanser Verlag, 3rd ed., 1989, and in the "Handbook of Polyvinyl Chloride Formulating" E. J. Wickson, J. Wiley & Sons, 1993, and in 31 "Plastics Additives" G. Pritchard, Ciiapman & Hall, London, 1st ed., 1998. Impact modifiers are also described in detail in "Impact Modifiers for PVC", J. T. Lutz/D. L. Dunkelberger, John Wiley & Sons, 1992. [0126] The compositions of the present invention can also contain other stabilisers such as amino-uracils and particularly 6-aminouracils disclosed for instance in US 6,174,941 B1 and/or thiouracils and particularly 4-amino-6-hydroxy-2-mercapto-pyrimidine. [0127] Hydrazides and hydrazide systems may also be used as additives. Preferred hydrazides are selected among one or more hydrazides compounds of formula R^'-CO-NH-R'', in which * R^'' is chosen from among: C1-C30 linear or branched alkyl and C2-C30 mono- or poly-unsaturated hydrocarbon, each of these groups possibly containing one or more heteroatoms, and possibly being substituted with one or more phenyl groups (substituted or not), epoxy groups, cycloaliphatic or heterocyclic group, halogen atom(s), hydroxy and/or alkoxy, phenyl, benzyl, naphthyl, toluyi group, optionally substituted by -OH, -CI, -alkoxy, -alkyl, - cycloalkyi, -COOR" or OCOR" (where R" is C1-C12 alkyl), thienyl, -CH=CH-CO-NH-NH-R^-X-CO-NH-NH2, -CH=CH-CO-R'^ -(CH2)n-C6H5 (with n varying from 1 to 5), -NH-NH2; * R^^ is chosen from hydrogen and the group -CO-R^^; * R^^ and R^'*, which may be the same or different are chosen from among C1-C30 linear or branched alkyl and C2-C30 mono- or poly-unsaturated hydrocarbon, each of these groups possibly containing one or more heteroatoms, and possibly being substituted with one or more phenyl groups (substituted or not), epoxy groups, cycloaliphatic or heterocyclic group, halogen atom(s), hydroxy and/or alkoxy; phenyl, benzyl, naphthyl, toluyi group, optionally substituted by -OH, -CI, -alkoxy, -alkyl, - cycloalkyi, -COOR" or OCOR" (where R" is C1-C12 alkyl); * X is chosen from among C1-C30 linear or branched alkylene and C2-C30 mono- or polyunsaturated bivalent hydrocarbon, each of these groups possibly containing one or more heteroatoms, and possibly being substituted with one or more phenyl groups (substituted or not), epoxy groups, cycloaliphatic or heterocyclic group, halogen atom(s), hydroxy and/or alkoxy, phenylene, benzylene, naphthylene, toluylene group, optionally substituted by -OH, -CI, - alkoxy, -alkyl, - cycloalkyi, -COOR" or OCOR" (where R" is C1-C12 alkyl); * R^^ and R^^ also possibly be linked by a covalent bond when R^^ = -CH=CH- and R^^ = - CO- (ketonic function). [0128] Preferred hydrazides are those for which at least one hydrazide is such as R" is C^-C^^ alkyl group (e.g. methyl, butyl, octyl, ethyl-2-hexyl, stearyl, lauryl); -X-CO-NHNH2 where X is C1-C17 alkyl (e.g. methyl, butyl, octyl, ethyl-2-hexyl, stearyl, lauryl), or 1,3-phenyl 32 group substituted or not; ort/70-substituted phenol, benzenic cycle, isophthalic group, naphthol, cyclo-S-pentadiene-2,4, C6H5-CH2- and R^^ is H, COR^^ with R^^ being preferably chosen from among C1-C17 alkyl (e.g. methyl, butyl, octyl, ethyle-2-hexyle, stearyl, lauryl) and a benzenic ring). [0129] Particularly preferred co-stabilisers and additives are chosen from among epoxydised soy bean oil, hydrotalcite, zeolite, dihydropyridine-based compounds as described hereinbefore, a-phenylindole, 3-diketones, phosphates, polyols, THEIC (isocyanurate), metallic salts (especially Zn, Mg, or Ca stearate(s)), acetylacetonate (acac) salts such as Ca(acac)2, perchlorate-based compounds (such as sodium or potassium perchlorates), and the like, as well as mixtures thereof. [0130] According to another embodiment of the present invention, preferred co-stabilisers are chosen from among dihydropyridine compounds. Particularly valuable compositions of the invention comprise dihydro-1,4-dimethyl-2,6-dicarbododecyloxy-3,5-pyridine (Stavinor® D507, Arkema, or "DHP") as co-stabiliser. Particularly advantageous mixtures of co-stabiliser and additive may be for example mixtures of DHP with sodium perchlorate. [0131] Therefore, according to another aspect, the present invention relates to a formulation comprising at least one composition of the invention as disclosed above, i.e. a composition mainly comprising at least one monoaikyltin-based compound as a major compound, and at least one dihydropyridine-based compound, preferably dihydro-1,4- dimethyl-2,6-dicarbododecyloxy-3,5-pyridine, and optionally at least one perchlorate compound, preferably sodium perchlorate. [0132] Still according to another embodiment of the present invention, preferred costabilisers are chosen from among metal soaps or metals salts of carboxylic acids. Particularly valuable compositions of the invention comprise disodium diadipate as costabiliser. [0133] Still other conventional additives may be added to the composition according to the invention, and to the optional co-stabiliser(s). This additive may be all and any additives used in the domain of application and the nature of which will be evident for the skilled artisan. [0134] The present invention also relates to chlorine-containing polymers comprising at least one composition as defined above, and optionally at least one co-stabiliser, such as defined above, preferably a dihydropyridine-based compound, such as DHP, and/or one or more additive(s), such as previously described, preferably a perchlorate compound, such as sodium perchlorate. [0135] Because of the high content of mono-alkyltin species, and the low content of the di-alkyltin species, and the very low (traces) of tri-alkyltin species within the compositions of the present invention, said compositions present a low to very low toxicity versus di- and/or 33 tri-alkyltin compound compositions. More particularly the compositions of the present invention are less toxic than (and even non toxic according to the regulations when compared to) the known and used compositions containing both mono- and di-alkyltin species, wherein the weight ratio of mono- to di-alkyltin species is less than 90/10. [0136] According to another embodiment, the composition of the present invention may be used to stabilise chlorine-containing polymers as defined above (matrix), said chlorinecontaining polymers being processed by any method known in the art, such as, for example, extrusion, calendaring, injection, injection-moulding, to prepare articles based on the said matrix. [0137] According to a preferred embodiment, the amount of composition within the chlorine-containing polymers ranges from 0.2 to 5 phr and preferably 0.5 to 3 phr (the term "phr" means per hundred of resin). Still according to another preferred embodiment, the thermal stability may even be increased by adding a friction or shear rate controlling agent, a viscosity controlling agent, or mixtures thereof, and particularly a viscosity reducing agent and/or a friction or shear rate decreasing agent..Friction or shear rate controlling, and particularly reducing, agents are well known in the art, as are the viscosity controlling, and particularly reducing agents. Friction or shear rate can also be decreased by processing conditions, according to known techniques of the art. Similarly viscosity can also be reduced by modifying the structure of the halogenated resin, for example when the chlorine-containing polymers is a copolymer, again according to techniques well known in the art [0138] The stabilised polymers in accordance with the invention can be prepared in a manner known perse using devices known perse such as the abovementioned processing apparatus to mix the stabilising composition of the invention and any further additives with the polymers. In this case, the stabilisers can be added individually or as a mixture or else in the form of so-called master batches. [0139] The stabilised polymers in accordance with the present invention can be brought into the desired form by known methods. Examples of such methods are milling, calendering, extruding, injection moulding or spinning, and also extrusion blow moulding. The stabilised polymers can also be processed to foam materials. [0140] Stabilised polymers in accordance with the invention are suitable, for example, for the manufacture of hollow articles (bottles), packaging films (thermoform sheets), blown films, pipes, foam materials, heavy profiles (window frames), transparent-wall profiles, construction profiles, sidings, fittings, office films, and apparatus enclosures (computers, domestic appliances). Preference is given to rigid or semi-rigid PVC films (opaque or transparent), PVC rigid foam articles and PVC pipes for drinking water or wastewater, pressure pipes, gas pipes, cable-duct and cable protection pipes, pipes for industrial 34 pipelines, seepage pipes, flow-off pipes, guttering pipes and drainage pipes. For further details on this subject see "Kunststoffhandbuch PVC", Vol. 2/2, W. Becker / H. Braun, 2"" ed., (1985), Carl Hanser Verlag, pages 1236-1277. [0141] The present invention also relates to articles comprising at least one polymer matrix and at least one composition according to the present invention, and optionaiiy one or more additive(s) and/or co-stabiliser(s), as hereinbefore described. [0142] According to a preferred embodiment of the invention, the article is a rigid or semirigid, opaque or transparent film, especially those chosen from among rigid or semi-rigid, opaque or transparent films, shrink-films, adhesive films, sheets, fittings, profiles (window, in-door), edge-bands. [0143] Preferably, the article is an opaque or transparent rigid or semi-rigid film, which may be advantageously used for thermoforming (or not) and preparing food-packaging, pharmaceutical blisters, plastic cards (such as credit cards), furniture films and technical packaging films, and generally all kinds of opaque and transparent films. [0144] The present invention is now illustrated with the following examples, which do not aim at limiting the sought scope of protection, which is defined by the annexed claims. Examples: PART 1: Preparation of monoalkyltin compounds Example 1: Distillation of mono-octvltin trichloride from a thermal redistribution mixture of octyltin chlorides. [0145] A mixture (928 g) consisting of mono-octyltin trichloride (59.0%), di-octyltin dichloride (39.5%), tin tetrachloride (1.2%) and tri-octyltin chloride (0.3%) that resulted from a thermal redistribution of tetra-octyltin and SnCU (1:2 molar ratio), was fractionally distilled at 8 mbar. The distillate (520 g) collected at a vapour temperature of 145°C, consisted of mono-octyltin trichloride (99.3%), di-octyltin dichloride (0.5%) and tin tetrachloride (0.2%) according to GC analysis after ethylation with excess EtMgCI. Example 2: Mono-octvltin tns((2-ethvlhexvl Imercaptoacetafe) [0146] 2-Ethylhexylmercaptoacetate (371 g; 1.82 mo!) and water (102 g) were added under stirring to 200 g (0.589 mol) of mono-octyltin trichloride obtained from Example 1. Then 290 g (1.81 mol) of a 25% solution of sodium hydroxide (NaOH) in water was added in ca. 1.5 hours at a reaction temperature of 50°C. [0147] After the reaction the pH of the water layer was adjusted to pH = 5-6 and the two phases were allowed to separate. The water layer was drained and volatiles were removed 35 by distillation (10 mbar) up to a pot temperature of 110°C. The product was filtered through 8 |jm filter paper using Dicacel as filter aid to afford 462 g (93%) of the final product. [0148] GC analysis after ethylation with excess EtMgCI indicated that the purity of this product was > 99.5% with less than 0.5% di-octyltin bis(2-ethylhexylmercaptoacetate) being present. Tri-organotin species were not detectable (< 0.1%). Example 3: Re-distillation of mono-octyltin trichloride obtained from thermal redistribution [0149] Re-distillation of mono-octyltin trichloride (at 8 mbar, 145°C) that had been obtained from the procedure described in Example 1, afforded mono-octyltin trichloride in 99.7 % purity (by GC analysis after ethylation with excess EtMgCI) with only a small quantity of di-octyltin dichloride (0.03 %). Example 4: Mono-octyltin tris((2-ethvlhexvlmercaptoacetate) [0150] 2-Ethylhexylmercaptoacetate (744 g; 3.64 mol) and 204 g of water were added under stirring to 400 g (1.18 mol) of mono-octyltin trichloride from Example 3. Then 567 gram (3.54 mol) of a 25% solution of NaOH in water was added in ca. 2 hours at a reaction temperature of 50°C. [0151] After the reaction the pH of the water layer was adjusted to pH = 5-6 and the two phases were allowed to separate. The water layer was drained and volatiles were removed by distillation (10 mbar) up to a pot temperature of 110°C. The product was filtered through 8 |jm filter paper using Dicacel as filter aid to afford 985 gram (98%) of the final product. [0152] GC analysis after ethylation with excess EtMgCI, indicated that the purity of this product was 97.9 % with minor amounts of other species being present: tin tetrakis(2-ethyl hexyl mercaptoacetate) (0.26%), di-octyltin bis(2-ethylhexylmercaptoacetate) (<0,1%), trioctyltin (2-ethylhexylmercaptoacetate) (< 0,01%). Example 5: Mono-octyltin trichloride from 1-octene, HCI and SnCb. [0153] A reaction vessel equipped with magnetic stirring, was brought under inert atmosphere and charged with 0.55 g of Pd(PPh3)4 (0.46 mmol). Next, 110 mL of a 0.80 M solution of anhydrous SnCb in THF (88 mmol) were added, followed by 220 mL of 1-octene (1.40 mol). The vessel was placed in an oil bath and heated to 50°C. Subsequently, 332 mL of a 0.25 M HCI solution in THF (83 mmol) were added and the resulting pale yellow solution was stirred for 1.5 hours at 50°C. 36 [0154] Volatiles were removed in vacuum (80°C, 70 mbar) and hexane (100 mL) was added to the resulting suspension. Filtration followed by concentration in vacuum afforded 24.5 g of a yellow-orange liquid that, according to GC-analysis (after alkylation with excess EtIVIgCI), consisted of mono-octyltin trichloride (89.7%), di-octyltin dichloride (0.09%), trioctyltin chloride (< 0,01%) and residual solvent. Example 6: Mono-octvltin tris((2-ethvlhexvlmercaptoacetate) [0155] 2-ethylhexylmercaptoacetate (37.0 g; 0.181 mol) and 8.90 g of water were added under stirring to 20.64 g (0.058 mol) of mono-octyltin trichloride obtained from Example 5. Then 26.5 g (0.17 mol) of a 25% solution of NaOH in water was added in ca. 2 hours at a reaction temperature of 50°C. [0156] After the reaction the pH of the water layer was adjusted to pH = 5-6 and the two phases were allowed to separate. The water layer was drained and volatiles were removed by distillation (9 mbar) up to a pot temperature of 110°C. The product was filtered through 8 |jm filter paper using Dicacel as filter aid to afford 41.6 g of the final product. [0157] GC analysis after ethylation with excess EtMgCI, indicated that this product consisted of mono-octyltin tris((2-ethylhexylmercaptoacetate) (95.4%) and di-octyltin bis(2-ethylhexylmercaptoacetate) (<0,1%). Residual solvent as measured by measuring the weight loss on drying at 50°C (3.0%) and residual 2-ethylhexylmercaptoacetate (not quantified) accounted for the remaining weight of the sample. A sample of this compound was tested for PVC stabilisation. PART 2: Application examples [0158] In the following examples, the term "phr" means per hundred of poly(vinyl chloride) (PVC) resin (e.g.: 0.2 phr means 0.2 g per 100 g of PVC). All parts are given by weight. Example A Comparative study of monoalkyltin- vs. dialkyltin compounds on long term colorhold retention in PVC transparent rigid films [0159] A PVC formulation typical for transparent rigid film (amounts of each component in phr are given below) is evaluated using a Collin two-roll mill, the rolls of which are heated to 200°C. The rotational speeds of the two cylinders are respectively adjusted to 20 rpm and 24 rpm, providing sufficient friction to gelate PVC and sufficient heat to thoroughly study the thermal stabilisation efficiency of stabilisers. The gap between the cylinders is adjusted to 0.5 mm. Samples are withdrawn from the cylinders at regular time intervals, their coloration being recorded. 37 [0160] The components of the various PVC formulations and their amounts are the following: • PVC (Lacovyl RB8010, Arkema, kW = 57): 100 • Epoxydised soybean oil (Ecepox PBS, Arkema) 1 • Hydrogenated castor oil (Loxiol G15, Henkel) 0.6 • Oxidized PE wax (A-C 316A, Honeywell) 0.12 • Process aid (Plastistrength 551, Arkema) 0.6 • Antisticking process aid (Plastistrength 770, Arkema) 1 • MBS impact modifier (Clearstrength 320, Arkema) 8 • Stabiliser (cf. list below) 1.3 [0161] The various stabilisers are the following: • Stab 1 = Composition of Example 6 comprising high purity mono-octyltin-(2- ethylhexylmercaptoacetate); • Stab 2 = 30% mono octyl tin (2-ethylhexylmercaptoacetate) and 70% di-octyltin-(2- ethylhexylmercaptoacetate); • Stab 3 = 60% mono octyl tin (2-ethylhexylmercaptoacetate) and 40% di-octyltin-(2- ethylhexylmercaptoacetate). [0162] The b* value in Hunter L*a*b* scale (b*, ASTM Standard E 313) is measured on each sample withdrawn and the results are reported in Table 1 below: - Table 1 - ^TTTl Stab 1 I Stab 2 ' '^ (according to the invention) (comparative) 2 7.1 ~ 7 3 7.8 " 7.8 4 8.4 9.2 5 94 10.5 6 9.9 13.3 7 ^Q^ 14.6 8 1Z1 16.9 9 13^0 18.0 10 15J 21.1 12 22.2 26.5 15 " 43.6 53.7 [0163] The results listed in Table 1 above show the efficiency of the stabiliser containing a high content of mono-alkyltin compound, i.e. high purity mono-alkyltin compound as described in the present invention, not only on the initial colour of the PVC formulation but 38 also on the long term colorhold retention compared to formulations containing both monoand di-alkyltin compounds. Example B Comparative study of mono-alkyltin vs. di-alkyltin compounds on long term colorhold retention in PVC used in film calendaring production [0164] A PVC formulation containing a copolymer widely used in film calandering production (amounts of each component in phr are given below) is evaluated using a Collin two-roll mill, the rolls of which are brought to 190°C. The rotational speeds of the two cylinders are respectively adjusted to 20 rpm and 24 rpm, providing sufficient friction to gelate PVC and sufficient heat to thoroughly study the thermal stabilisation efficiency of stabilisers. The gap between the cylinders is adjusted to 0.5 mm. Samples are withdrawn from the cylinders at regular time intervals, their coloration being recorded. [0165] The components of the PVC formulation and their amounts are the following: • PVC (Lacovyl S071, Arkema, kW = 57): 60 • Copolymer vinyl chloride - vinyl acetate (Lacovyl GA 7701H, Arkema KW = 57) 40 • Epoxydised soybean oil (Ecepox PBS, Arkema) 1 • Glycerol mono-oleate (Loxiol G10, Henkel) 0.7 • Pentaerythritol adipate stearate (Loxiol G70S, Henkel) 0.4 • Process aid (Plastistrength 550, Arkema) 1 • Antisticking process aid (Plastistrength 770, Arkema) 0.7 • MBS impact modifier (Clearstrength 303H, Arkema) 8 • Stabiliser (see list below) 1.5 or 1.7 [0166] The same stabilisers Stab 1, Stab 2 and Stab 3 as defined above are tested. Stab 2 and 3 are used at 1.5 phr and Stab 1 at 1.7 phr to compare formulations at equal tin content. [0167] The b* value in Hunter L*a*b* scale (b*, ASTM Standard E 313) is measured on each sample withdrawn and the results are reported in Table 2 below: 39 -- Table 2 - ^'^^.^^ Stab 2 Stab 3 ^ ' I . ? (comparative) (comparative) 2 7.5 8 7.7 3 8J 9^4 9J 4 8.8 11 1^ 5 8^6 12 r6 9.3 13 U 7 8^9 14 r8 10 14 12 9 10 15 12 10 r\ 17 14 12 13 20 1^ 15 17 41 30 20 40 65 61 [0168] As in previous Example A above, the results listed in Table 2 show the efficiency of the stabiliser containing mono-alkyltin compound of high purity as described in the present invention not only on the initial colour of the PVC formulation but also on the long term colorhold retention compared to formulations with a much higher di-alkyltin compound content, i.e. mixtures of mono- and di-alkyltin compounds as known in the art. Example C Comparative study on long term colorhold retention of thin- and thick- PVC films used in film calendaring production, each containing mono-alkyltin- and/or di-alkvitin compounds [0169] Each PVC formulation used in example B containing a copolymer widely used in film calandering production is milled during 5 minutes at 180°C (Collin two-roll mill with rolls brought to 180°C, rotational speeds of the two cylinders are respectively adjusted to 20 rpm and 24 rpm) to obtain a thin film of 0.5 mm thickness. [0170] Measurement of colour is made, as well as on a thicker film obtained from the thin film pressed under a plate press at a temperature of 190°C, during 5 minutes. This thicker film is 4 mm thick and colour (b*) is recorded as described above. 40 [0171] The results are reported in Table 3 below: - Table 3 - B* value (according to , ^^^^^ , , ^*^^ ^ , the invention) (go^P^raf/ye; (comparative) m^?ml^^ 7.8 9.6 8.3 (U.5 mm) ™ckfi'"^ V5 ^^ 7? (4 mm) [0172] Early colour provided by very high purity mono octyl tin stabiliser is excellent compared to current technology also in thick films. Example D Comparative Haze study of thin- and thick- PVC films used in film calanderinq production. each containing monoalkyltin- ordialkyltin compounds [0173] The PVC formulation used in Example A widely used in film calandering production is milled during 5 minutes at 180°C (Collin two-roll mill with rolls heated to 180°C, rotational speeds of the two cylinders respectively adjusted to 20 rpm and 24 rpm) to obtain a thin film of 0.5 mm thickness. [0174] Measurement of optical properties such as haze in a PVC sample is made, on both thin and thicker film obtained from the thin film pressed under a plate press at a temperature of 190°C, during 5 minutes. This thicker film is 4 mm thick and haze is recorded. [0175] The haze value is measured on a Hazemeter according to ASTM D1003 on each sample and the results are reported in Table 4 below: - Table 4 - \ Stabl \ TZiTo Haze value (^% )' (' acc.o rdin5g to, the (c, om^p^a^r°a t\i.v e) invention) ' ^ ' Thin film -. ^_ ^ .,. m ^ mm^ 0-76 0.74 (u.o mm) Thick film ., _ .„ lA \ 7.9 10 (4 mm) [0176] Very high purity mono-alkyltin stabiliser is able to decrease the haze and then gives to the finished film an excellent level of transparency. 41 Example E Comparative colour study of PVC films containing a monoalkyltin-compound and an organic co-stabiliser [0177] The advantageous use of a co-stabiliser together with a high purity mono-alkyltin stabiliser is illustrated: the formulation described below is evaluated using a Collin two-roll mill, the rolls of which are heated to 190°C. The rotational speeds of the two cylinders are respectively adjusted to 20 rpm and 24 rpm, providing sufficient friction to gelate PVC and sufficient heat to thoroughly study the thermal stabilisation efficiency of stabilisers. [0178] The gap between the cylinders is adjusted to 0.5 mm. Samples are withdrawn from the cylinders at regular time intervals, their coloration being recorded. [0179] The tested formulations are the following: • PVC (Lacovyl RB8010, Arkema, kW = 57) 100 • Epoxydised soybean oil (Ecepox PBS, Arkema) 1 • Hydrogenated castor oil (Loxiol G15, Henkel) 0.6 • Oxidized PE wax (A-C 316A, Honeywell) 0.12 • Process aid (Plastistrength 551, Arkema) 0.6 • Antisticking process aid (Plastistrength 770, Arkema) 1 • MBS impact modifier (Clearstrength 320, Arkema) 8 • Stab 1 1.3 • Dihydro-1,4-dimethyl-2,6 dicarbododecyloxy-3,5-pyridine ('DHP'; CAS 36265-41-5; Stavinor® D507, Arkema) 0 to 0.2 [0180] The b* value in Hunter L*a*b* scale (b*, ASTM Standard E 313) is measured on each sample withdrawn and the results are reported in Table 5 below: - Tables - ^, . , 1 o* t -. I Steb 1 + I Stah 1 + T(mm.) Stabi Q^QS pf,r PHP O.lphrDHP 2 7.4 7.1 7.1 4 8^3 7^8 7^8 6 9J 8^4 7^9 ~ 8 10 9.1 8.2 10 11 10.1 9.1 12 12^9 12 10^6 14 14.7 13.7 11.7 16 17.3 16.5 ~ 14.1 18 20.3 23^8 17.5 20 26.9 28 17.6 42 [0181] It is well known that dihydropyridine-based compounds alone, such as DHP alone, give poor results in this test. However, when combined with a highly pure mono-alkyltin stabiliser according to the invention, the co-stabilising effect is drastically improved in terms of initial colour and colour hold during a dynamic thermal stability test. [0182] This is the evidence of a synergistic effect resulting from the use of dihydropyridine-based compounds together with high purity mono-alkyltin compounds. [0183] A good effect of initial colour may also be evidenced by preparing first a film on two roll mill (190°C, 5 minutes) and then pressing several sheets until a thickness of 4 mm (plate press at 190°C during 10 minutes with pressure: 30 bars, during 60 sec, 200 bars during 40 sec, 300 bars during 180 sec). Yellow index (Yl) and L* value in Hunter L*a*b* scale (b*, ASTM Standard E 313) is given in table 6 below: - Table 6 - T c T T T l Steb 1 + \ Stab 1 + I Stab 1 + ^ 0.05 phr DHP 0.1 phr DHP 0.2 phr DHP Yl 35.5 27J 25^5 24 L* ~79!3~ 81.3 8Z1 81.6 [0184] Yellow index decreases with adding a dihydropyridine-based compound with the composition of the invention (highly pure mono-alklytin compound) and L* value increases attesting a higher whiteness. Example F Comparative colour study of PVC films containing a monoalkyltin-compound and another organic co-stabiliser based on dissodium adipate [0185] The advantageous use of another co-stabiliser together with a high purity monoalkyltin stabiliser is illustrated: the formulation described below is evaluated using a Collin two-roll mill, the rolls of which are heated to 195°C. The rotational speeds of the two cylinders are respectively adjusted to 20 rpm and 24 rpm, providing sufficient friction to gelate PVC and sufficient heat to thoroughly study the thermal stabilisation efficiency of stabilisers. [0186] The gap between the cylinders is adjusted to 0.5 mm. Samples are withdrawn from the cylinders at regular time intervals, their coloration being recorded. [0187] The tested formulations are the following: • PVC(LacovylS071, Arkema, kW = 57) 100 • Epoxydised soybean oil (Ecepox PB3, Arkema) 1 • Oxidized PE wax (A-C 316A, Honeywell) 0.12 • Process aid (Plastistrength 551, Arkema) 1.2 • Antisticking process aid ^lastistrengtti 770„ Arkema) t 43 • MBS impact modifier (Clearstrength 303H, Arkema) 8 • Glycerol monostearate (Loxiol G12, Henl