WO2022083996A1

WO2022083996A1 - An oral care composition for whitening benefits

Info

Publication number: WO2022083996A1
Application number: PCT/EP2021/076905
Authority: WO
Inventors: Somnath Das; Satyajit Samadder
Original assignee: Unilever Ip Holdings B.V.; Unilever Global Ip Limited; Conopco, Inc., D/B/A Unilever
Priority date: 2020-10-22
Filing date: 2021-09-30
Publication date: 2022-04-28

Abstract

The invention relates to an oral care composition for whitening teeth. This is achieved through a composition that comprises a specific zinc layered hydroxide with a plate like morphology and an orally acceptable base. Disclosed is a tooth whitening composition comprising: (a) a zinc layered hydroxide having the chemical formula Zn2+(OH)2-x(Am-)x/mnH2O, where x has a value between 0 and 2; "A" is chosen from acetate, nitrate, chloride, sulphate, oxalate, citrate, phosphate; m has a value from 1 to 3; and n has a value from 0 to 10; and (b) an orally acceptable base wherein the zinc layered hydroxide has a plate like morphology, where plate-like means that the length and breadth are orders of magnitude larger than the thickness.

Description

AN ORAL CARE COMPOSITION FOR WHITENING BENEFITS

Field of the Invention

The present invention relates to an oral care composition for providing whitening benefits to teeth.

Background of the Invention

Long-lasting whitening of teeth is of considerable interest to consumers. Foods and drinks such as tea, coffee and wine may form dental stains by directly depositing chromogens on the tooth surface. Attraction of materials to the tooth surface plays a critical role in the deposition of extrinsic dental stain. The chromogens in these beverages that are responsible for causing dental stain are known as tannins and are composed of polyphenols such as catechins. These materials generate color due to the presence of conjugated double bonds and are thought to interact with the tooth surface via an ion exchange mechanism.

Traditional tooth whitening methods involve either peroxide bleaching from kit formats or abrasive stain removal from toothpaste formats. Additionally highly white particles like titanium dioxide are included in oral care compositions, that are expected to deposit on the surface of the tooth thereby ensuring a more white appearance of the tooth. Recently there has been a move to get away from traditional whitening agents like those based on titanium due to expected regulatory pressures in certain countries I regions. In light of this, the present inventors with their extensive knowledge and research in the area of tailored crystal structures of layered metal hydroxides, have hit upon a specific zinc layered hydroxide (ZLH) that has a plate like structure of a specific morphology/ aspect ratio that provides enhanced whitening as compared to titanium hydroxide.

W02007/063508 (P&G) relates to a dentifrice compositions free of abrasive material comprising a hydrophilic clay material, a phytic acid compound, an effective amount of an oral care active and a polar solvent carrier that provides similar cleaning benefits as compared to one with an abrasive. One example of the oral care active is a zinc containing layered material having a zinc lability of greater than about 25%. GB2243775 (Unilever) relates to compositions comprising therapeutically active metal ion delivering compounds that slowly dissolve in the acid environment of the oral fluid. These compounds are layered inorganic materials with three-dimensional structures containing zinc, copper, tin or silver ions.

KR102120992 B1 (Webiotree, 2020) discloses metal layered hydroxide composite comprising an active ingredient and a surface modifier. It exhibits sustained-release effects.

WO2018230325 A1 (Lion Corp) discloses oral care compositions containing hexagonal plate-like zinc oxide that provides tooth whitening effect.

US2018256468 A1 (Colgate) discloses oral care composition comprising a zinc amino acid halide complex that can retain in the oral cavity for a time sufficient to form a precipitate.

US2007/122358 A1 (P&G) discloses a dentifrice comprising hydrophilic clay material, a phytic acid compound, an effective amount of an oral care active, and a polar solvent carrier. The composition is substantially free of abrasive material.

The above-mentioned art known to the present inventors indicate incorporation of layered zinc compounds in oral care compositions but do not disclose the specific morphology of plate like structure developed by the present inventors to ensure whitening of teeth.

It is thus an object of the present invention to provide for a composition for whitening teeth.

It is another object of the present invention to provide for such a composition that does not include titanium based whitening particles.

Summary of the Invention

The present invention relates a tooth whitening composition comprising

(a) a zinc layered hydroxide having the chemical formula Zn²⁺ (OH)2-_X (A^m )x/m . nl-LO; where x has a value between 0 and 2; A is chosen from acetate, nitrate, chloride, sulphate, oxalate, citrate, phosphate; m has a value from 1 to 3; and n has a value from 0 to 10; and

(b) an orally acceptable base. wherein the zinc layered hydroxide has a plate like morphology, where plate-like means that the length and breadth are orders of magnitude larger than the thickness.

Detailed Description of the Invention

Except in the examples, or where otherwise explicitly indicated, all numbers in this description indicating amounts of material or conditions of reaction, physical properties of materials and/or use may optionally be understood as modified by the word “about”.

All amounts are by weight of the final composition, unless otherwise specified.

It should be noted that in specifying any ranges of values, any particular upper value can be associated with any particular lower value.

For the avoidance of doubt, the word “comprising” is intended to mean “including” but not necessarily “consisting of” or “composed of”. In other words, the listed steps or options need not be exhaustive.

The disclosure of the invention as found herein is to be considered to cover all embodiments as found in the claims as being multiply dependent upon each other irrespective of the fact that claims may be found without multiple dependency or redundancy.

Where a feature is disclosed with respect to a particular aspect of the invention (for example a composition of the invention), such disclosure is also to be considered to apply to any other aspect of the invention (for example a method of the invention) mutatis mutandis.

“Oral care composition” for the purposes of the present invention means a paste, powder, liquid, gum, serum or other preparation for cleaning the teeth or other surfaces in the oral cavity. “Tooth paste” for the purpose of the present invention means a paste or gel dentifrice for use with a toothbrush. Especially preferred are tooth pastes suitable for cleaning teeth by brushing for about one minutes, preferably two minutes.

“Mouth wash” for the purpose of the present invention means liquid dentifrice for use in rinsing the mouth. Especially preferred are mouth washes suitable for rinsing the mouth by swishing and/or gargling for about half a minute before expectorating.

The present invention relates to a tooth whitening composition comprising a zinc layered hydroxide have a specific formula and having a specific plate like morphology; and an orally acceptable base.

The zinc layered hydroxide as per this invention has a structure having the chemical formula:

Zn²⁺ (OH)₂-X (A^m-)x/_m . nH₂O

Where x has a value between 0 and 2;

A is chosen from acetate, nitrate, chloride, sulphate, oxalate, citrate, phosphate; m has a value from 1 to 3; and n has a value from 0 to 10, where plate-like means that the length and breadth are orders of magnitude larger than the thickness.

The preferred anions from “A” are chloride, nitrate, acetate and sulphate and most preferably acetate, nitrate and citrate.

As per the highly preferred aspect of the present invention the zinc layered hydroxide has the possible chemical formula:

Zn²⁺ (OH)₂.X (A^m-)x/_m . nH₂O where x has a value between 0.1 and 1.9;

“A” is chosen from acetate, nitrate, chloride, sulphate, oxalate, citrate, phosphate; m has a value from 1 to 3; and n has a value from 0 to 10.

Layered metal hydroxide compounds can be classified into two main types, namely layered double hydroxide and layered hydroxide salt. One example is zinc layered hydroxide (ZLH). ZLH have a structure like brucite, but instead of metal replacement, hydroxide ions are removed from the structure and replaced by a water molecule, or other type of oxo-anions. The layered hydroxide is formed when part of the hydroxyl groups from the brucite-like structure are substituted with water molecules. Oxo-anions are then positioned in the second coordination sphere of the metal to stabilize the electrostatic charge, or by the direct substitution of other single charged anions.

The zinc layered hydroxide has a plate like morphology. By plate like is meant that the length and breadth are orders of magnitude larger than the thickness, preferably several fold larger than the thickness. The length of the plates of zinc layered hydroxide is preferably in the range of 0.1 to 5 micron, more preferably in the range of 1 to 2.5 micron. The breath of the plates of zinc layered hydroxides is preferably in the range of 0.1 to 2.0 micron, more preferably in the range of 0.1 to 0.5 micron, most preferably in the range of 0.2 to 0.5 micron. The thickness of the plates of zinc layered hydroxides is preferably in the range of 20 to 50 nm, more preferably in the range of 30 to 50 micron. The zinc layered hydroxide for use in the present invention may be monophasic or biphasic, preferably it is biphasic.

Thus a more preferred aspect of the invention relates to a composition of the invention wherein the zinc layered hydroxide has a length in the range of 2 to 5 micron, breadth in the range of 0.1 micron to 0.5 micron; and thickness in the range of 20 to 50 nm.

The ZLH is preferably included in 0.5 to 20%, more preferably 1 to 15%, most preferably 2 to 10% by weight of the composition.

Without wishing to be bound by theory, the present inventors believe that the plate like morphology of the zinc layered hydroxide provides for the right aspect ratio which delivered the desired high whitening while minimizing abrasion.

The composition of the invention comprises an orally acceptable base. The orally acceptable base preferably comprises an abrasive, a surfactant, a volatile alcohol, a humectant or combinations thereof. More preferred orally acceptable bases include an abrasive, a humectant or mixtures thereof. The orally acceptable base preferably is included in as much as 99.8%, more preferably at as much as 96% by weight of the composition. The orally acceptable base is included in at least 80% more preferably at least 90%, by weight of the composition. The oral care composition of the present invention may be delivered in the form of an ointment, a gel, a dentifrice or a mouthwash. Dentifrices include forms like toothpaste and toothpowder. A composition is most preferably presented in the form of a toothpaste, a toothpowder or a mouthwash.

Oral care composition of the present invention preferably comprises an abrasive. The abrasive may preferably be calcium carbonate or silica. Gels usually contain silica, whereas opaque creams generally contain calcium based abrasives, especially chalk (calcium carbonate). In opaque toothpastes, the compositions have 5 to 60 wt% calcium based abrasive. In more preferred compositions it is 30 to 60 wt% and further more preferably from 35 to 55 wt%. Optimal compositions have 40 to 55 wt% calcium based abrasive.

A preferred abrasive is fine ground natural chalk (FGNC), which is a form of chalk. It is obtained from limestone or marble. FGNC may also be modified chemically or physically by coating during milling or after milling by heat treatment. Typical coating materials include magnesium stearate or oleate. The morphology of FGNC may also be modified during the milling process by using different milling techniques, for example, ball milling, air-classifier milling or spiral jet milling.

FGNC may be used as the sole calcium based abrasive. However, FGNC may also be used with the other calcium based abrasives for some balance of abrasion. Usually the particle size of chalk is from 1 to 60 pm, and preferred sizes range from 1 to 15 pm.

Other preferred calcium based abrasives include dicalcium phosphate (DCP), calcium pyrophosphate and precipitated calcium carbonate (PCC), which preferably are included at 25 to 55 wt%, more preferably 35 to 50 wt%.

Other abrasives may also be used depending upon the intended degree of abrasion. These include synthetic abrasive polishing agents such as amorphous precipitated silica and silica gels. It is preferred that gel compositions of the invention comprises silica as the abrasive. Abrasive silica for incorporation in the composition of the invention is preferably one with a low refractive index. The low refractive index silicas, used as abrasives in the present invention are preferably silicas with an apparent refractive index (Rl) in the range of 1.41 to 1.47, preferably 1.435 to 1.445 , preferably having a weight mean particle size of between 5 and 15 mm, a BET (nitrogen) surface area of between 10 and 100 m²/g and an oil absorption of about 70 - 150 cm³/100 g, but abrasive silicas with a lower apparent refractive index may also be used . Typical examples of suitable low refractive index abrasive silicas (e.g. having an Rl of between 1.435 and 1.445. The amount of these silicas in the composition of the present invention generally ranges from 5-60% by weight , usually 5-20% by weight of the composition.

Other abrasive agents include magnesium carbonate, sodium metaphosphate, potassium metaphosphate, zirconium silicate, potassium metaphosphate, magnesium orthophosphate, tricalcium phosphate, magnesium orthophosphate, trimagnesium phosphate, aluminum silicate, zirconium silicate and perlite.

Suitable humectants are preferably used in the oral care composition of the present invention and they include, for example, glycerin, sorbitol, propylene glycol, dipropylene glycol, diglycerol, triacetin, mineral oil, polyethylene glycol (preferably, PEG-400), alkane diols like butane diol and hexanediol, ethanol, pentylene glycol, or a mixture thereof. Glycerin, polyethylene glycol, sorbitol or mixtures thereof, are the preferred humectants, most preferred ones being glycerol (also known as glycerine) and sorbitol.

The humectant may be present in the range of from 10 to 90% by weight of oral care compositions. More preferably, the humectant makes up from 25 to 80%, and most preferably, from 45 to 70% by weight of the composition, based on total weight of the composition and including all ranges subsumed therein.

Preferably, an oral care composition comprises a surfactant. Preferably the composition comprises at least 0.01% surfactant by weight of the composition, more preferably at least 0.1% and most preferably from 0.5 to 7%. Suitable surfactants include anionic surfactants, such as the sodium, magnesium, ammonium or ethanolamine salts of C8 to C18 alkyl sulphates (for example sodium lauryl sulphate), C8 to C18 alkyl sulphosuccinates (for example dioctyl sodium sulphosuccinate), C8 to C18 alkyl sulphoacetates (such as sodium lauryl sulphoacetate), C8 to C18 alkyl sarcosinates (such as sodium lauryl sarcosinate), C8 to C18 alkyl phosphates (which can optionally comprise up to 10 ethylene oxide and/or propylene oxide units) and sulphated monoglycerides. More preferably the surfactant comprises or is an anionic surfactant. The preferred surfactants are sodium lauryl sulphate and/or sodium dodecylbenzene sulfonate. Most preferably the surfactant is sodium lauryl sulphate. Other suitable surfactants include nonionic surfactants, such as optionally polyethoxylated fatty acid sorbitan esters, ethoxylated fatty acids, esters of polyethylene glycol, ethoxylates of fatty acid monoglycerides and diglycerides, and ethylene oxide/propylene oxide block polymers. Other suitable surfactants include amphoteric surfactants, such as betaines or sulphobetaines. Mixtures of any of the above described materials may also be used. Most preferred surfactants are an alkali metal alkyl sulphate or a betaine.

In a preferred embodiment, the composition comprises a thickener. Thickeners that may be used in this invention include, sodium carboxymethyl cellulose (SCMC), hydroxyl ethyl cellulose, methyl cellulose, ethyl cellulose, gum tragacanth, gum arabic, gum karaya, xanthan gum, sodium alginate, carrageenan gum, guar gum, Irish moss, starch, modified starch, silica based thickeners including silica aerogels, magnesium aluminum silicate (e.g., Veegum), Carbomers (cross-linked acrylates) and mixtures thereof.

Typically, thickening silica, sodium carboxymethyl cellulose and/or a Carbomer is/are preferred thickeners for use in the composition of the invention.

Thickening silica is especially preferred to be used in gel toothpastes.

Gel toothpastes generally contain upto 8.5 wt% thickening silica whereas opaque toothpastes typically contain 3 to 4 wt% thickening silica.

Thickener, when present, preferably makes up from 0.01 to about 10%, more preferably from 0.1 to 9%, and most preferably, from 1 .5 to 8% by weight of the composition.

Water may preferably be included in 5 to 95%, in particular 10 to 75%, and especially at from 10 to 60%, further more preferably 10 to 45% by total weight of the composition. When the oral care composition of this invention is a toothpaste or gel, the same typically has a viscosity from about 30,000 to 180,000 centipoise, and preferably, from 60,000 to 170,000 centipoise, and most preferably, from 65,000 to 165,000 centipoise.

The oral care composition of the present invention may contain a variety of other ingredients which are common in the art to enhance physical properties and performance. These ingredients include antimicrobial, anti-caries agents, plaque buffers, fluoride sources, vitamins, plant extracts, desensitizing agents, anti-calculus agents, biomolecules, flavors, proteinaceous materials, preservatives, opacifying agents, coloring agents, pH-adjusting agents, sweetening agents, particulate abrasive materials, polymeric compounds, buffers and salts to buffer the pH and ionic strength of the compositions, and mixtures thereof. Such ingredients typically and collectively make-up less than 20% by weight of the composition, and preferably, from 0.0 to 15% by weight, and most preferably, from 0.01 to 12% by weight of the composition, including all ranges subsumed therein.

The zinc layered hydroxide for use in the present invention is preferably prepared using a process comprising the steps of a) mixing a zinc salt in an basic aqueous solution for extended period of time to generate a precipitate;

(b) filtering the precipitate and drying it to prepare the zinc layered hydroxide.

The reaction to prepare the zinc layered hydroxide may be biphasic or monophasic. By biphasic is meant that two sources of zinc are used, a soluble zinc salt and an insoluble zinc oxide. In the monophasic route only a soluble zinc salt is used. Of the two processes biphasic is more preferred. In the biphasic process, the soluble salt of zinc is preferably a zinc salt of carboxylic acid. Preferred zinc carboxylates are acetate, oxalate, phthalate, malonate, citrate, or zinc salts of EDTA, NTA, or DTPA. The process may be carried out in water or an aqueous solution of a crystal habit modifier e.g. a polyhydric alcohol in water. The reaction in a mixture of polyhydric alcohol and water is preferred. Preferred polyhydric alcohols have 2 to 4 carbon atoms with the most preferred polyhydric alcohols being ethylene glycol, propylene glycol, glycerol or polyethylene glycol. When polyhydric alcohol in water is used, it is preferably included in amount of 5 to 20 wt% polyhydric alcohol in water. The zinc layered hydroxide which is used in the present invention is therefore most preferably prepared using a process comprising the steps of a) mixing a zinc salt with zinc oxide in an aqueous solution of polyhydric alcohol for extended period of time to generate a precipitate; (b) filtering the precipitate and drying it to prepare the zinc layered hydroxide.

The reaction in the above process is preferably carried out for 24 to 48 hours.

According to another aspect of the present invention there is provided a non-therapeutic method of whitening teeth comprising the steps of (i) applying the composition of the present invention on to a tooth surface for at least one minute; and (ii) rinsing the surface to be substantially free of the composition. The method is preferably for achieving cosmetic benefits.

The invention will now be illustrated with the help of the following non-limiting example.

EXAMPLES

Examples A, B, 1-4: Whitening provided by various samples within and out side the invention:

The following samples were analyzed for whitening benefits using various samples.

Example - A: Tea stained hydroxyapatite (HAP) disk.

The following procedure was used to prepare the tea stained disk:

HAP Tea Staining Protocol

Step 1 : Take 2 gm of tea leaves in a beaker.

Step 2: Add 100 ml boiling distilled water to the beaker and wait for two minutes.

Step 3: Filter the solution using a filter paper after two minutes, into a petri dish. Step 4: Immerse pristine HAP disks into the solution (-100 ml volume, while the tea was still warm) for 5 minutes and then reverse the sides and let it sit for another 5 minutes.

Step 5: Dip rinse each disk in Milli Q water twice.

Step 6: Leave it in air for drying for 20-30 minutes, with the circular disk standing on its edge and both sides exposed to air. Examples B, 1-4:

The disks so obtained as mentioned above were now ready for further experiments where whitening samples were coated on to the disks using the following drop casting protocol.

HAP drop casting protocol

Step 1 : Drop 10OpI of test sample on the stained full disk.

Step 2: Spread the sample as evenly as possible across the disk.

Step 3: Let it dry in the air for 20 minutes.

Step 4: Keep it in the oven at 40 °C for 10 minutes to dry and measure the L*-a*-b* values.

The whitening samples dropped on to the disks above were as follows:

Example B: Titania STV 455 at 1 wt%

Example 1 : Zinc layered hydroxide (monophasic) at 1 wt%

Example 2: Zinc layered hydroxide (monophasic) at 2 wt%

Example 3: Zinc layered hydroxide (biphasic) at 1 wt%

Example 4: Zinc layered hydroxide (biphasic) at 2 wt%

The monophasic zinc layered hydroxide was prepared using the following procedure: a) Prepare a stock aqueous solution of 400 ml zinc acetate dihydrate [Zn(Ac)₂.2H₂O], 0.18M and 400 ml of Sodium Hydroxide [NaOH], 0.18 M. b) In a pool of water (400 ml), mix both under constant stirring (400 rpm) for 5 hours. c) Let the solution stand overnight (~ 16 hours). d) Filter using vacuum filtration. e) Dry in an oven at 60 °C, followed by vacuum drying, in a desiccator.

The biphasic zinc layered hydroxide was prepared using the following procedure: a) Prepare a stock aqueous solution of 50 ml zinc acetate dihydrate [Zn(Ac)₂.2H₂O], 0.9M. b) Prepare a suspension of zinc oxide [ZnO] (~ 3.7 gm) in 50 ml water (or combination of water and solvent) by stirring. c) Add zinc acetate solution to this suspension and stir for 7 hours. d) Let it stand overnight (~ 14 hours). e) Stir for another ~ 9 hours. f) Let it age for ~ 68 hours. g) Vacuum filter. h) Dry in an oven at 60 °C, followed by vacuum drying in a desiccator.

The whiteness of the samples of HAP disks prepared above (Examples A, B, 1-4) were then measured using the following procedure:

Procedure used for L*-a*-b* measurement

Stain measurement on surface was measured using a Konica Minolta

Spectrophotometer (CM2600D) with the following attributes:

Wavelengths 360 nm -740 nm, L*, a* and b* values, instrument calibrated using standard black and white surfaces as provided by instrument maker.

L* presented here is the average of at least five measurements taken on the circular surface of 0.5 cm diameter in the SCI and SCE modes with Small aperture.

The data on the L* along with the morphology of the samples of zinc layered hydroxide is summarised in Table 1 below:

TABLE 1

* Highly inhomogeneous distribution of rectangular plates: length of 0.5 to 2 microns; breadth of 100-300 nm; thickness of 40-50 nm

** Homogeneous Sheet-like plates: length of 2 to 5 microns; breadth of 100 to 200 nm; thickness of 20 to 50 nm

The data in the above table -1 indicates that the use of zinc layer hydroxide meeting the plate like structure requirement performs superior to titanium dioxide in whitening stained HAP disks (which is a surrogate for stained teeth). Biphasic ZLH is seen to be superior to monophasic ZLH.

Example C, D, 5-8: Use of ZLH prepared using a process comprising presence of a crystal habit modifier

Experiments were carried out similar to examples A,B, 1-4 except that during the process of preparing the ZLH, a crystal habit modifier, propylene glycol, at various concentrations (1, 5 or 10 wt%) was used as shown in Table 2 below. The L* value is also summarized.

TABLE 2

* Homogeneous Sheet-like plates: length of 2 to 5 microns; breadth of 200 to 500 nm; thickness of 30 to 50 nm

The data in Table- 2 indicates that ZLH of plate like morphology with slightly different dimensions can be prepared with certain crystal modifiers and such ZLH also gives whiteness superior to conventional agents like titanium di oxide.

Claims

1. A tooth whitening composition comprising

(a) A zinc layered hydroxide having the chemical formula Zn²⁺ (OH)2-_X (A^m )x/m nFW where x has a value between 0 and 2;

“A” is chosen from acetate, nitrate, chloride, sulphate, oxalate, citrate, phosphate; m has a value from 1 to 3; and n has a value from 0 to 10; and

(b) an orally acceptable base wherein the zinc layered hydroxide has a plate like morphology, where platelike means that the length and breadth are orders of magnitude larger than the thickness.

2. A composition as claimed in claim 1 wherein the zinc layered hydroxide has length in the range of 0.1 micron to 5 micron, breadth in the range of 0.1 micron to

2 micron; and thickness in the range of 20 to 50 nm.

3. A composition as claimed in claim 2 wherein the zinc layered hydroxide has a length in the range of 2 to 5 micron, breadth in the range of 0.1 micron to 0.5 micron; and thickness in the range of 20 to 50 nm.

4. A composition as claimed in any one of the preceding claims wherein the orally acceptable base is selected from an abrasive, a surfactant, a volatile alcohol, a humectant or combinations thereof.

5. A composition as claimed in any one of the preceding claims wherein the abrasive is selected from calcium carbonate or silica.

6. A composition as claimed in any one of preceding claims wherein the humectant is a polyhydric alcohol preferably sorbitol or glycerol.

7. A composition as claimed in any one of the preceding claims wherein the surfactant is sodium lauryl sulphate, sodium dodecyl benzene sulfonate or mixtures thereof, preferably sodium lauryl sulphate. A composition as claimed in any one of the preceding claims wherein the composition is a toothpaste, a toothpowder or a mouthwash. A composition as claimed in any one of the preceding claims wherein the zinc layered hydroxide is prepared using a process comprising the steps of

(a) mixing a zinc salt with zinc oxide in an aqueous solution of polyhydric alcohol for extended period of time to generate a precipitate;

(b) filtering the precipitate and drying it to prepare the zinc layered hydroxide. A composition as claimed in claim 9 wherein the zinc salt is a zinc carboxylate chosen from zinc acetate, oxalate, phthalate, malonate, citrate, or a zinc salt of EDTA, NTA, or DTPA. A composition as claimed in claim 9 or 10 wherein the polyhydric alcohol is chosen from ethylene glycol, propylene glycol, glycerol or polypropylene glycol. A composition as claimed in any one of the preceding claims 9 to 11 wherein the reaction is carried out for a period of time from 24 to 48 hours. A non-therapeutic method of whitening teeth comprising the steps of:

(i) applying the composition as claimed in any one of the preceding claims on to a tooth surface for at least one minute; and

(ii) rinsing said surface to be substantially free of the composition.