Compositions: coating or plastic – Coating or plastic compositions – Dental
Patent
1994-07-01
1995-05-02
Klemanski, Helene
Compositions: coating or plastic
Coating or plastic compositions
Dental
523116, C09K 300
Patent
active
054115841
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a dental cement made from a polymeric phosphonic acid and a glass, other metal oxide or cermet, to a method of making the cement and to materials for use in making the cement.
Such cements are described in GB-A-2219289 and it would be desirable to increase their compressive strength and to lengthen their working time.
According to the present invention, a cement composition comprises an intimately blended mixture of a water-containing liquid, at least 30% by weight of the composition being a cation-catalysed cross-linkable polymeric acid containing on average one phosphonic acid group per one to three backbone carbon atoms, such as poly(vinyl phosphonic acid) PVPA optionally containing poly(acrylic acid), and a metal oxide (e.g. ZnO) or cation-leachable surgically acceptable aluminosilicate (preferably fluoroaluminosilicate) glass powder preferably containing SiO.sub.2 and Al.sub.2 O.sub.3 in the mass proportions (0.6 to 3.0):1 such as (1 to 2):1, preferably (11/2 to 2):1, in the proportions (1 minus x)g phosphonic acid: 1 to 5 g glass: x g liquid, where x is from 0.3 to 0.7, characterised in that the acid has been pre-reacted in aqueous solution at elevated temperature with a fluoride and/or phosphate of preferably a divalent or trivalent metal e.g. of Zn, Sn, Mg, Ca, Al or B, or a mixture, e.g. ZnF.sub.2 +BPO.sub.4, preferably amounting to 21/2 to 121/2% (e.g. 4 to 8%) by weight of said acid, preferably in the presence of water, preferably present as solvent of the acid.
The glass powder is preferably two-phase and preferably consists of particles substantially all of which are smaller than 100 microns, preferably smaller than 60 microns. If it is desired to reduce its reactivity with the acid, the glass powder may be heat-treated, for example at at least 400.degree. C. (preferably 450.degree.-600.degree. C.) for at least 40 minutes (preferably at least 55 minutes).
While a glass rapidly quenched from a high temperature will usually be translucent, certain heat treatments yield an opaque glass, and hence an opaque cement, which may be acceptable in appropriate cases. The glass may alternatively be de-activated by washing in a dilute acid such as acetic acid, typically in a 21/2% weight concentration for 1 hour. Advantageously, the glass is admixed (e.g. by grinding together) with boron phosphate, in a preferred amount based on the glass of at least 10 wt %, more preferably 12-20 wt %. In place of glass, MgO deactivated at at least 900.degree. C. may be used.
The solvent is preferably water, and preferably dissolves the acid, the glass being kept separate until use. However, the acid may be pre-reacted and dried (optionally freeze-dried but more preferably spray-dried) and mixed with the glass; the invention extends to this mixture, and to a pack of dried acid plus glass, preferably packed in a sealed capsule (PVPA being hygroscopic) and which is made into cement by adding the said liquid. The acid may be accompanied by a desiccant such as hydrophobic silica. In a modification, the glass may be replaced by a metal oxide such as ZnO or MgO, preferably heat-treated, preferably of dense powder morphology, or by a cermet. (A cermet is a metallised glass powder, typically formed by fusing (by sintering) a metal powder (e.g. silver or tin) onto the surface of a glass powder, and may offer lower friction when burnished and greater wear resistance than glass in these cements.) A chelating agent e.g. a chelating phosphonic acid is preferably present, in a preferred amount of 10 to 20% by mass.
The invention extends to a pack comprising two pastes which when mixed form a cement composition as set forth above; the first paste may be the pre-reacted acid(s) plus water, and the second paste may be the glass or oxide powder suspended in aqueous thickening agent e.g. methylcellulose. Ordinary aqueous PVPA with optional complexant (described later) is a solution and therefore not entirely satisfactory as a paste, but pre-reacted PVPA as set forth above forms a gel and thus a true paste. If the t
REFERENCES:
patent: 4758612 (1988-07-01), Wilson et al.
patent: 5079277 (1992-01-01), Wilson et al.
patent: 5179135 (1993-01-01), Ellis et al.
Akinmade Ademola O.
Braybrook Julian H.
British Technology Group Limited
Klemanski Helene
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