FIELD OF THE INVENTION

The present invention relates to dental adhesive compositions.

BACKGROUND

Dental adhesive compositions typically contain a range of components such as adhesion-promoting monomers, hydrophilic monomers, hydrophobic monomers, stabilisers, photo-initiators and co-initiators. These components and other suitable additives are known to those skilled in the art. These components are dissolved in a solvent.

Adhesion-promoting monomers typically possess a structure containing an acidic functionality (e.g. carboxylic, phosphoric, phosphonic, or sulfonic) and a polymerisable functional group, such as acrylate or methacrylate. Common examples of these monomers used in dental adhesives are 10-methacryloxydecyl dihydrogen phosphate (MDP) and 4-methacryloxyethyl trimellitic acid (4-MET).

Hydrophilic monomers possess mono- or multi-polymerisable functionality and are substantially miscible with water. They are generally incorporated in adhesive formulations to help compatibilise the more hydrophobic components with water. Common examples of these hydrophilic monomers are HEMA, glycerol dimethacrylate (GDMA) and hydroxypropyl methacrylate (HPMA).

Hydrophobic monomers typically possess mono- or multi-polymerisable functionality, but are substantially immiscible with water. They are generally added to provide strength and water-resistance to a cured adhesive. Common examples of these hydrophobic monomers are bisphenol A glycidylmethacrylate (BisGMA), urethane dimethacrylate (UDMA) and ethoxylated bisphenol A dimethacrylate (BisEMA).

Photo-initiators, optionally combined with co-initiators, may be added to provide adhesive formulations with the ability to cure upon exposure to an appropriate light source. Common photo-initiators used in dental adhesives are camphorquinone (CQ) and diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO). Co-initiators, such as ethyl 4-dimethylaminobenzoate (EDMAB) and dimethylaminoethyl methacrylate (DMAEMA), may be used to improve the reaction speed of some photo-initiator systems.

Stabilisers may be added to adhesive formulations to prevent premature polymerisation. Typically, the stabilisers are free-radical scavengers such as butylated hydroxytoluene (GHT), monomethyl ether of hydroquinone (MEHQ) or hydroquinone (HQ).

Organic solvents such as ethanol and acetone are typically incorporated in dental adhesive compositions to (1) reduce viscosity, (2) compatibilise resin ingredients with water, (3) improve surface wetting and (4) increase depth of penetration into dental tissues. Typically, the solvents also contain water. A substantial percentage of water is required in the final adhesive formulation to achieve a thorough wetting of the moist dentin.

Ideally, after serving these purposes, all solvents (including water) should be removed since residual solvents will only weaken a final cured adhesive layer, which is the foundation layer of good bonding between a tooth and a subsequent restorative material such as a composite, a glass ionomer cement or an amalgam. Poor bonding can result in marginal deterioration, leading to marginal staining and development of caries.

In general, solvent-containing adhesives have a solvent-removal step after application which involves air-blowing the applied layer for a period of time.

Solvents removed during this step typically include both organic solvents and water. Ethanol-solvated compositions will typically dry as an azeotrope of 96% ethanol and 4% water which, depending on the initial ethanol-water ratio, often leads to a high level of residual water, thus weakening the adhesive layer. On the other hand, ethanol is good for storage stability.

Acetone-containing compositions lose acetone quite readily upon air-blowing. However, due to absence of an acetone-water azeotrope, the water removal is again problematic, thus resulting in high levels of residual water.

Acetone-containing compositions are also difficult to store due to the high volatility of acetone. Bottles and containers readily lose acetone solvents causing the adhesive to become viscous and, in some cases, phase-separated.

SUMMARY OF THE INVENTION

It would be advantageous to have a solvent system that exhibits a good evaporation rate of organic solvent and water during air-blowing, whilst possessing reduced loss of volatiles upon storage.

In accordance with one aspect of the present invention there is provided a dental adhesive composition comprising an organic solvent component which contains methyl ethyl ketone. Preferably, the solvent also contains water.

It has been found that methyl ethyl ketone is able to form an azeotrope with water. The azeotrope has a lower boiling point and higher percentage water removal compared to a water/ethanol azeotrope. Preferably the methyl ethyl ketone is dissolved in water either alone or in conjunction with an alcohol such as ethanol or isopropanol.

Preferably the solvent has a boiling point in the range from 60° to 100° C. More preferably the solvent has a boiling point of less than 75° C. Still more preferably the solvent may have a boiling point in a range from 70° to 74° C.

