GB2219289A - Poly-vinylphosphonic acid and metal oxide or cermet or glass ionomer cement - Google Patents

Abstract

A glass ionomer cement composition comprises poly(vinyl phosphonic acid) dissolved in its own mass of water; and (in treble the total mass of the solution) a powder, being glass or MgO or a cermet. The powder has to be deactivated so that the setting reaction with the acid is not too fast, and this is achieved either by a raised Si :Al ratio or by heat-treating.

Description

1 n f-.. 'S a ") '1_---89 POLY-VINYLPHOSPHONIC ACID AND METAL OXIDE 130417

OR CERMET OR GLASS IONOMER CEMENT This Invention relates to poly(vinyl phosphonic acid), in particular to a method of synthesising it. In addition, the invention relates to a glass lonomer or metal oxide or cermet cement made using poly(vinyl phosphonic acid). This cement may find application for specialised surgical purposes, such as dental fillings and cements and as splint bandage materials.

Glass-ionomer cements were developed In the -early 1970's to meet requirements for a more acid-resistant filling material to replace dental silicate cement, formerly in widespread use.

Glass-ionomer cements set by an acid-base reaction between an aciddecomposable glass powder, which acts as a base, and a concentrated solution (typically 50%) of poly(acrylic acid) -(CH2 - CH)-, or its copolymers with methacrylic or itaconic 1 LUUM acids. On mixing the glass and the acid, cations (Ca2+ and A13+), released from the glass, cross-link the polyacid chains. A metal polysalt is formed which acts as a binding matrix for the partially reacted glass particles. Thus the glass-ionomer cement is a composite material, consisting of a glassy filler embedded in an insoluble metal polyacrylate matrix.

These cements are unusual in being adherent to tooth dentine and enamel, providing excellent seals when used as filling - materials. Thus, they prevent ingress of fluid and debris into the tooth at the restorative/cavity wall margin, and so prevent secondary tooth decay. They are however susceptible, before they are fully set, to contamination by moisture. They are a] so not sufficiently translucent to match tooth enamel optically.

Poly(vinyl phosphonic acid) -(CH 2 - CH)1 MOH) 2 is a stronger acid than poly(acrylic acid), and hence offers the possibility of producing stronger cements, with the adhesion characteristic of existing cements. However, it has not proved easy to synthesise, nor can it be simply used as a direct replacement for poly(acrylic acid).

ill According to the present invention, a cement composition comprises an intimately blended mixture of a water-containing liquid, a cationcatalysed 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 cation-leachable surgically acceptable glass powder containing Si and Al in the mole proportions 0.6 - 0.2:1 if previously heat-treated, otherwise exceeding 1. 6:1, in the proportions (I-x)g phosphonic acid e.g. PVPA: 1-5g glass: x g liquid, where x is from 0.3 0.7. The solvent is preferably aqueous, i.e. water, and preferably dissolves the acid, the glass being kept separate until use. However, the acid may be dried and mixed with the glass: the invention extends to this mixture, which is preferably packed in a sealed capsule (PVPA being hygroscopic) and which is made into cement by adding the solvent. In a modification, the glass may be replaced by a metal oxide such as 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.) 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 acid(s) plus water, and the second paste may be the glass powder suspended in aqueous thickening agent e.g. methylcellulose. If the two pastes have been formulated to appropriate concentrations, one could in use squeeze out equal lengths of paste from two tubes, or scoop out equal numbers of spoonfuls from two tubs, as an easy way to ensure that the mixture is 6f the correct composition.

Q, cl a The c.omposition may further contain acid-neutral i sing agents such as zinc oxide or aluminium phosphate or both, in a total amount of up to 10% by mass based on the acid solution, and/or complexing agents such as phosphonic-acid-based materials.

The glass powder preferably consists of particles substantially all of which are smaller than 100 microns, preferably smaller than 60 microns. Preferably the glass powder has been heat-treated at at least 4000C (preferably 450 - 60000 for at least 40 minutes (preferably at least 55 minutes). The Si:Al range of 0.6 - 2:1 yields an opaque product, which may be acceptable in appropriate cases. The glass may be washed in a dilute acid such as acetic acid, preferably in a y% concentration for z hours where yz < 4, preferably:-5 2. In place of glass, MgO deactivated at at least 9000C may be used.

The invention also extends to a pack comprising the components of the cement composition set forth above, so packed that when unpacked and mixed they form the cement. Poly (vinyl phosphonic acid).

Poly(vinyl phosphonic acid), PVPA, can be prepared by a free radical polymerisation in an inert solvent, using for example vinyl phosphonic acid monomer. The polymer is a light brown very hygroscopic solid, which dissolves readily in water to give solutions of up to 80-85% by weight. The solutions show good long term stability with no evidence of gelling. A 50% solution typically has a viscosity of 0.8 - 0.9 cP. mLn Pr-Qperties of PVPA Cements Poly(vinyl phosphonic acid) is a dibasic acid with pKa values (acidic strength) of 2 and 8 respectively, compared with the pKa of 5 - 6 for poly(acrylyc acid). It is expected therefore that the aforementioned cross-linking cations released from glass powder (Ca2+ and A 13+) would form stronger associations between the polyacid chains, leading to stronger cements.

In practice, however, cement formation with the glass types used in current glass-ionomer cements is difficult when PVPA is used in place of PAA. For example, a glass which has a setting c 1 - 4 time of 3-4 minutes with PAA solution will set in probably less than 45 seconds with PM solutions. It thus proved necessary to modify the cement system to allow a reasonable working and setting time for the cement paste, by:

i) using glasses with a different formulation which allows adequate working time, or by deactivating reactive glasses; i i) partially neutralising the PM solution, so reducing Its reactivity, and iii) use of additives, such as complexing agents, in the 10 liquid; these are believed to tie up cations as they are refeased from the glass and delay the setting of the cement.

Using a cement according to the invention as above, cements with setting time of about 5 minutes can be obtained, which is comparable with existing glass-ionomer cements G-20 minutes).

Dental Requirements 1. Adhesion.

Adhesion of a dental restorative material to the tooth substrate is of prime importance. The dental silicate cement (now no longer used since inadequately acid-resistant), typical of phosphoric acid based cements, did not adhere to the tooth enamel and as a consequence fluid and debris could leak Into the tooth at the edge of the filling, possibly resulting in secondary tooth decay. An adhesive filling material will (in contrast) effectively seal this edge. In addition, with non-adherent restoratives, the filling needs to be mechanically keyed into the tooth by undercutting, with the loss of excessive sound tooth material. This is not necessary with adhesive materials.

2. Biocompatibility and toxicity.

Glass-ionomer cements are known to be compatible with the oral tissues and do not irritate the dental pulp as phosphoric acid based cements do. This improvement isattributable to the very low difusion of bulky polymer molecules through the dental tubinals leading to the pulp. Cements based on PVPA are expected A 1 1 C' to be equally bland, and like PAA cements they also show very small temperature rises on setting.

Current glass-ionomer cements show resistance to acid attack and to staining and PVPA-based cements are expected to show similar properties.

The toxicity of the PVPA starting material vinyl phosphonyl dichloride is known to be high, but this is converted 100% to vinyl phosphonic.acid monomer and polymer. The toxicity of the VPA monomer is not known, but its complete removal from the polymer solution prior to cement formation is readily possible as described later. As a matter of interest, the monomer has been found to form insoluble cements with certain metal oxides, and would not therefore be expected to be leachable even if present in the initial polyacid solution.

3. Translucency.

If a restorative material is to be successfully used as an anterior filling material, then its translucency should match that of the surrounding tooth material, to give a "I i ve" appearance. Dentine has an opacity of 0.51 - 0.93 (CO.70 value, test disc thickness 1 mm, two discs compared for strength of reflected light when one sits on a white (reflectivity 0.70) and the other on a black background), while enamel has an opacity of 0.21 - 0.67. The opacity of glass-ionomer restorative material is affected by the refractive indices of the glass particles and of the gel matri x; a close match of the refractive indices results in a low-opacity cement.

The British Standard for glass-ionomer cements quotes limits for the opacity of cements as 0.35 - 0.90 (CO.70 value), and typical values are 0. 65 - 0.75 for filling cements, above the range for enamel. Dental silicate cements had a much lower opacity (B.S. opacity value 0.35 - 0.55, typically 0.45 - 0.55), but these are no longer in use. Measurement of the opacity of a cement prepared from PVPA and a modified dental silicate glass (7 gave a CO.70 value of 0.40 - 0.46 compared with 0.5 - 0.6 for a cement prepared from the same glass and PAA of a similar molecular weight as the PM. Such low opacity indicates that aesthetic anterior fillings should be possible using PM cements.

The invention will now be described by way of example.

The three accompanying drawings, which are described In more detail at the end, illustrate the setting properties of cements (both within and outwith the Invention) with reference to their composition as plotted on ternary composition diagrams, in which the counts are of mole per cent of Ca atoms, mole per cent of Si atoms and mole per cent of A] atoms unless otherwise stated. The same letters refer to the same glasses. Part 1 This part describes the preparation of a polymer of vinyl phosphonic acid from vinyl phosphonyl dichloride (VPDC). MC is dissolved in an equal volume of 1,1,1-trichloroethane and to this solution is added the initiator, azo-bis-isobutyronitrile, at a level of 3% by weight of the monomer. The mixture is heated under nitrogen, with stirring, in a flask equipped with a reflux condenser to a temperature of 700C for two hours. Heating is by means of a water bath, surrounding the flask, to maintain adequate temperature control. If the entire polymerisation (18 hours) were carried out at 700C, the yield would be 50-60%. Instead, after two hours at 700C, the temperature is lowered to 25 40 - 45c1C and maintained at this temperature for a further 16 18 hours, whereby the yield is increased to 85-90%. A viscous, orange-brown solution results, which is a solution of the crude polymer of vinyl phosphonyl dichloride. This is then hydrolysed by pouring slowly into a large volume of chilled water with stirring, removing the bulk of the HCl gas as it is formed by means of a vacuum. The hydrolysed product is then concentrated, and the excess organic solvent removed by vacuum distillation or by means of a rotary evaporator.

Ana-lysis of the product typically shows it to contain 85 - 90%. of the desired PM - poly(vinyl phosphonic acid), a solid, together with some 10 15% residual monomer.

C \ Part 2 This part describes the purification of the product prepared as described in Part 1. The PVPA must first be isolated by removing the water introduced by the hydrolysis, using a rotary evaporator or similar. The resulting solid is then dissolved in an equal or lesser weight of ethanol (other alcohols can be used), with gentle warming as necessary. The solution is then poured slowly into a large excess of a non-solvent, such as ethyl acetate, with stirring. A white precipitate forms, which is isolated from the solvent mixture, and contains some 5% of residual monomer. Repeating the dissolution in ethanol and the precipitation with ethyl acetate leaves no residual monomer detectable by P-31 NMR spectroscopy.

Pure PVPA is a hygroscopic, off-white solid. It is readily soluble in water up to concentrations greater than 807. by weight when prepared as described. The viscosity of an infinitely dilute solution (intrinsic viscosity), obtained by the extrapolation of the measured viscosities of solutions of concentrations 1, 2, 3 g/100ml (sodium salt of PVPA in IM NaCD is 0.08 ml/g, suggesting a polymer of low molecular weight, probably in the range 3000 - 5000.

Part 3 This part describes the cement-forming properties of aqueous solutions of poly(vinyl phosphonic acid) with various metal oxide powders, i.e. not with glass as such. In al I cases a 50% by weight aqueous solution of poly(vinyl phosphonic acid), prepared as described in Parts 1 and 2, is used as the cement-forming liquid. The liquid and a powdered metal oxide are mixed together on a glass block with a metal spatula at a 9 powder: ml liquid ratio which gives a homogeneous paste, and which is from 1:1 to 5:1 depending on the reactivity and bulk density of the metal oxide powder. Many metal oxides can be reacted with poly(vinyl phosphonic acid) in this way to yield fast setting, water-stable cements at room temperature.

c 8 - Metal oxides which are suitable for forming water-stable cements are:

Cu(l), Cu(II), COGI), COGID, Sn(II), BiGID, PbGI), PbGI,IV), Hg(II), Cd(II), Y(III), La(III), Mo(VI).

In addition, the following oxides reacted very vigorously with poly(vinyl phosphonic acid), giving cement-like products in less than 15 seconds: Zn(I1), Ca(I1), Mg(II). In an attempt to deactivate (reduce the reactivity of) these oxides, they were heat treated at 10000C for at least 24 hours. Only in the case of magnesium oxide could a practical cement be mixed in this way. The morphology of the MgO was also found to affect setting. "Light" MgO, a low density form, reacted very rapidly despite the heat treatment, whereas "heavy" MgO, a much denser powder, gave a suitable setting time.

Typical physical properties of some metal oxide-PVPA cements are given in the Table.

MgO CUO B'203 powder:liquid ratio (g/ml) 1:1 5:1 5:1 Setting time at 23 240C 340 sec 440 sec 120 sec Compressive strength: dry 49.1 MPa 54.5 MPa 18.2 MPa Compressive strength at 100% 56.6 MPa 44.6 MPa 55.2 MPa relative humidity Flexural strength 4.5 MPa 12 MPa 10 MPa Solubility (1 hour) 0.25%:-:50.01% 0.06 % The MgO setting time at 370C (as in the mouth) was under 200 seconds. That product (after 24 hours at 100% relative humidity) adhered not only to tooth enamel (5.0 MPa) and dentine (3.4 MPa) but also to base metals useful -in dental surgery such as stainless steel and cobalt-chrome alloys.

Part 4.

This part illustrates the properties of a typical cement prepared from an aqueous solution of poly(vinyl phosphonic acid) and an ion-leachable aluminosilicate glass. The polyacid solution used i s 50% m/m. The glass is prepared by mixing together 437 parts by weight silica, 230 parts by weight alumina, 129 parts by 1 c - 9 weight calcium fluoride, 175 parts by weight cryolite and 29 parts by weight aluminium phosphate and heating to 130011C for 75 minutes. The melt is cooled rapidly by pouring into water. The resulting glass is ground and sieved, and the fraction of particle size less than 45 microns used to prepare the cement. Before use, the glass powder is deactivated by heating in a furnace at 4500C for 90 minutes.

Part 4A The setting properties of the poly(vinyl phosphonic acid) cement of Part 4 are compared with commercial glass ionomer cements containing poly(acrylic acid) or similar. Setting times were determinied using a 453g Gillmore needle. Working times and setting rate were determined from Wilson rheograms.

Cement PVPA Fuji DeTrey Type 11 ASPA 9 powder/m] liquid 3.0 2.75 3.0 Working time 0.6 2.7 2.4 /mins, 230C Setting time 2.0 - - lmins, 230C Setting time 1.33. 7.5 4.25 lmins, 370C Setting rate 71.6 lmins, 230C The PM coment could be made with glass acetic-acid washed for 1 or 2 hours, acid concentration being 1% or 2%. All combinations extended the working time and slowed the setting rate, which could be useful, but 2 hours/2% did so rather strongly.

c Part 4B. - The mechanical properties of the poly(vinyl phosphonic acid) cement were compared with commercial glass ionomer cements. The cements were mixed at the powder/liquid ratios given in Part 4A.

Compressive strengths were determined on cylindrical specimens (12mm length, 6mm diameter), after storage for 24 hours.

Flexural strength was determined using specimens of dimensions 25x3x3 mm.

Cement PVPA Fuji DeTrey f Type II ASPA Compressive 75 174 140 strength/ MPa Flexural 10 8.9 9.8 strength/MPa Part 4C.

This Part Illustrates the translucency of the poly(vinyl phosphonic acid) cement. The cement paste is placed in a brass mould, such as will give a cement disc of diameter 2.0 cm and thickness 0.1 cm. The mould is closed with steel plates and the cement allowed to set. The cement disc is removed from the mould t c - 11 after one. hour, and subsequently stored In water for a further 24 hours. The opacity of this cement disc, when measured on a Hunterlab D2SA- 9 Tristimulus Colorimeter is superior to that of a similar cement prepared using poly(acrylic acid) as the cement forming liquid, and compares favourably with the opacity of a similarly prepared commercial dental silicate cements (DSC) and glass ionomer cements (G1). The C 0.70 opacities obtained for the cements are given in the Table.

cement 9 powder:ml liquid opacity % PM.2.2:1 44 PAA 2. 2:1 55 3. 5:1 60 D5C0) 3.5:1 47 DSC(2) 3.5:1 45 GIO) 2.75:1 69 GI(2) 3.0:1 85 PVPA Poly(vinyl phosphonic acid) PAA Poly(acrylic acid) DSCM "BioTrey" dental silicate cement DSCM "Achatit" GIM "Fuji Ionomer Type Il" GIM "DeTrey ASPA" P-air-t -kQ-.

This Part illustrates the rapid development of water stability of cements prepared from poly(vinyl phosphonic acid), and compares this feature with similar cements prepared from poly(acrylic acid) and with commercial glass ionomer cements. Thus, a specimen is prepared by placing the cement paste (powderlliquid ratio 39: 1 m]) in a brass mould such as to give 30 set cement disc of diameter 2 cm and thickness 0.2 cm. A length c - 12 of unwaxed dental floss is placed in the cement paste, by which to suspend the set cement disc. After being allowed to set for 7 minutes or 1 hour, as required, the disc is removed from the mou 1 d, and immediately suspended in a tared weighing bottle containing a fixed volume of water. After 24 hours in water, the disc is removed, and the water evaporated, leaving the residue leached from the cement disc. The water stability of the cement is thus determined from the difference in weight of the bottle and the weight of the cement disc.

The water stabilities of cements thus determined are given in the Table.

Cement PVPA Fuji DeTrey Type II ASPA Water-leachable 0.50 1.90 2.10 material, 7 min M Water-leachable (0.05 0.70 0.30 material, I hr M _ Part 5.

This example describes the cement forming properties of a range of glass powders based on that described in Part 4, and prepared by altering the pre-fired composition of the glass. Thus the effect of altering the amount of aluminium phosphate (SA-5D), the Si/Al ratio (SE-5H), and the amount of fluoride (calcium fluoride) (5J-SM) in the glass Is described. The glass compositions are listed below as parts by weight.

The setting times of cements prepared from these glass powders are given, measured at 23-24c1C and ambient humidity.

Deactivation of the glass powders is achieved by heating at 4500C for 90 minutes.

Z i c 1 glass 5102 A1203 no.

CaF2 Na3A1F6 A1P04 setting time (secs) unheated heated SA 416 219 123 166 144 <lo so 5B 416 219 123 166 171 <lo 40 SC 416 219 123 166 29 <20 70 SD 416 219 123 166 0 <10 130 SE 499 219 123 166 57 <lS 225 SF 582 219 123 166 57 <20 450 5G 332 219 123 166 57 <lo <15 5H 249 219 123 166 57 <lo <15 si 416 219 148 166 57 <lS 80 SK 416 219 172 166 57 <15 60 SL 416 219 98 166 57 <15 50 5M 416 219 74 166 57 20 70 c Part 6.

This Part describes the preparation of a cement from an ion-leachable glass and an aqueous solution of PVPA. A calcium fluoroaluminosilicate glass is prepared by mixing together 67 parts by weight of silica, 100 parts by weight of alumina and 100 parts by weight of calcium fluoride, and heating to a temperature of 1300 degC for 90 mins. The melt is rapidly cooled by pouring into cold water, and the resulting glass-ceramic crushed and sieved so as to collect the fraction of particle size less than 45 microns. The cement is prepared using a 50% by weight solution of poly(vinyl phosphonic acid) at a p/I ratio of 3:1. The resulting cementforming reaction is very vigorous, and a hard, dense cement-like mass results in less than 10 seconds.

The glass powder can be deactivated by heating to 6000C for at least 1 hour. When mixed with the poly(vinyl phosphonic acid) solution, this deactivated glass powder gives a setting time (at 23-24 degC) of 105 seconds. The compressive and flexural strengths of this cement are given below and are compared with the compressive strengths of similar cements prepared using poly(acrylic acid) as the cement forming liquid, and the same (unheated) glass powder.

Cement liquid PM PAM 1) PAM2) Compressive strength /MPa dry 100% RH 67 73 90 90 40 i Flexural strength /MPa 100% RH 11.6 c \ - is - (1) Allied Colloids I'Versicol E5" 50% w/w (2) Aldrich Co. PAA MW 2000 50% w/w PaC 7.

This Part illustrates the effect of altering the Si/Al ratio on the cement properties of the glass type described in Part 6.

Thus a series of glass powders was prepared as described, with Si/Al mole ratios from 2.5:1 to 0.23:1, by altering the amount of silica in the pre-fired mixture. This Is the ratio by number of the silicon atoms to the aluminium atoms present. Cements were mixed at 2.5g powder: 1 ml liquid and setting times measured at 23-240C. The Table also shows the effect of heating to 60011C on the reactivity of the glasses.

Glass Si/Al Relative masses in Settinq time (secs) no. mole prefired Mixture ratio S102 A1203 CaF2 unheated heated to 6000C is for 6 hours 7A 2.0:1 300 100 100 975 - 7B 2.2:1 266 100 100 340 1500 7C 1.94:1 233 100 100 190 - 7D 1.67:1 200 100 100 120 320 7E 1.11:1 133 100 100 <s <15 7F 0.78:1 94 100 100 <5 <15 7G 0.56:1 67 100 100 <15 105 7H 0.42:1 so 100 100 70 130 71 0.23:1 27 100 100 205 320 Glasses 7G, 7H and 71 after heating were partly crystallised, so that the "real" (amorphous) glass apart from the crystals would have had a composition deviating from the overall nominal composition. That actual composition was not determined. It may account for the reversal in the trend of setting times.

Similarly the Si/Al ratio has an effect on the physical properties of the set cements, as shown in the Table below. The glass powders for making the last three cements were heat-treated at 600c1C for 6 hours.

Si/Al Compressive Flexural g powder/ ratio strength/MPa strength/MPa ml liquid dry 1001. RH 100% RH 2.50 31.6 16.4 3.3 2.5 2.22 27.6 22.9 - 2.5 1.67 24.6 23.5 - 2.5 1.67 31.6 32.2 4.2 3.0 0.56 67.0 73.0 11.6 3.0 0.42 69.0 65.0 7.7 3.0 Part 8.

This Part illustrates the effect of altering the fluoride content of the type of glass described in Part 4. Thus cements were prepared as described, using glass powders of constant Si/Al mole ratio (viz. 1.48:1), but with varying amounts of fluoride.

The effect on the setting time of such cements (mixed at 2.5g powder: 1 m] liquid) is given in the Table.

Relative massei-jrefiring glass no. S'02 A1203 CaF2 mol% F setting time 8A 175 100 553 40.6 465 sec 8B 175 100 240 26.9 95 sec 8C 175 100 110 15.8 <10 sec Fluoride acts as a fluxing agent, making the glass easier to work (as may be imagined - in glass SA, fluorine atoms are 40.6% by number of the whole, all other species of atom added together being only 59.4%) Fluoride makes the glass more susceptible to acid attack, and it moderates the setting reaction.

Increasing the Ca:Al ratio generally decreases the setting time if no other factors are varied.

1 1 4? Cl.

Part 9.

This Part describes the effect of the incorporation of aluminium phosphate, zinc oxide and mixtures thereof In the aqueous poly(vinylphosphonic acid) solution, on the working time and the setting rate of the glass described in Example 4. A Wilson rheometer is used to determine these data on cements (3g powder: 1 M] liquid) mixed at 23-240C The glass powder was deactivated by heat- treating at 4500C for 90 minutes before use.

Additive Working time Setting speed (mins) (arbitrary units) PVPA 0.6 78.3 2%) 0.6 110.4 5%) ZnO 0.5 116.8 10%) 0.85 153.7 is 2%) 0.45 128.9 5%) A1P04 0.5 157.6 10%) 1+1% Z110+ 0.5 144.2 2.5+2.5% A1P04 0.7 118.0 Percentages are by mass on the liquid component.

Part 10.

This Part describes the effect of adding chelating compounds on the working properties of the cements described above. The additives were incorporated into the polymer solution so as to give a final solution containing 45% m/m PVPA and 5% m/m (unless otherwise stated) of the additive. A number of additives were tried: all were phosphonic acids manufactured by Monsanto Co. under the trade mame "Dequest". The Dequests tried and found to be useful were:

c Dequest 2000 aminotri-s(methyl phosphonic acid) Dequest 2010 hydroxyetliyl-diphosphonic acid Dequest 2060 diethylenetriaminepenta(methylenephosphonic acid) The modified PVPA solution was mixed with two types of glass (Parts 4 and 6 above, 3g powder: 1 ml liquid. The setting time was determined with a 453g Gillmore needle at 23-240C and ambient humidity (55%). The effect of Dequests on the setting of cements prepared from untreated and heat treated glasses was studied, and the results shown below. (The effect of Dequests on the compressive and flexural strengths of the cements of Part 7 was.studied and found to be insignificant).

An estimate of the working time (manipulability) of each of the cements was made by examination of the rheograms obtained using a Wilson rheometer for each of the cements.

Table 1 - liquid glass setting working timelsecs time/secs PM Part 4 80 20 PVPA/D2000 Part 4 110 20 PVPA/D2010 Part 4 95 40 PVPA/D2060 Part 4 80 20 PM Part 4 140 35 PVPA/D2000 Part 4 140 70 PVPA/D2010 Part 4 190 90 PVPA/D2060 Part 4 150 so but not deactivated by heating before use.

1 c Table 2 glass setting working compressive time/secs time/sec's strength (MPa) PM Part 6t <20 <10 PVPA/D2000 Part 6t so 30 PVPA/D2010 Part 6t 40 20 PVPA/D2060 Part 6t <30 <20 PM Part 6 90 so PM/D200OM) Part 6 120 70 75 PVPA/D2000 Part 6 130 85-90 81 PVPA/D2000(10l.)Part 6 190 100 109 PM/D2010(2%) Part 6 100 so 53 PVPA/D2010 Part 6 140 70 53 PVPA/D2010(10%)Part 6 180 70 76 PVPA/D2060 Part 6 120 80 t But not deactivated.

Deactivated by heat-treatment at 6000C for 90 minutes.

-CQ-m.pad son of PVPA and PAA QC-ments.

cement PVPA Fuji DeTrey Part 4 Type II ASPA 9 powder per m] liquid 3.0 2.75 3.0 WT (mins, 230C) 0.6 2.7 2.4 ST (mins, 370C) 1.3 3.75 4.25 ST (mins, 230C) 2.0 CS (MPa, 24 HO 75 174 140 FS (MPa, 24 Hr) 10 8.9 9.8 Sol - 7 mins 0.50 1.90 2.10 - 1 hour >0.05 0.70 0.30 Opacity (C0.70 value) 0.44 0.69 0.85 WT - ST = Working Time (measured by 289 needle leaving no impression) Setting Time (measured by 453g needle leaving no impression) CS - Compi; essive Strength FS - Flexural Strength Sol = % dissolved when immersed at stated time after mixing, for 24 hours in 370C water.

v i s i b 1 e visible c An ideal dental filling material will have a 3711Csetting time of 3 - 6 minutes from starting the mixing or 2 - 5 minutes from completing the mixing (itself done at 23110, following a relatively leisurely working time.

Part 11 This part describes the cement formed between an aqueous solution of poly(viny] phosphonic acid) and a tin cermet. The tin cermet was prepared by mixing glass powder (as described in Part 6, but ground to 18 Vm) with tin powder (Goodfellow Metals, sieved to <45 pm) in equal volumes (1:4 by weight). The mixture was compressed to form a 2.5 cm diameter disc under a pressure of tonnes. The discs were then sintered in a vacuum furnace at 2100C for 30 minutes before being ground to a powder and passed through a 15 pm sieve. - A cement was prepared by mixing a 50% by mass solution of PVPA with the cermet material prepared as described above, in the ratio 5g powder: 1 ml liquid. The cement was water stable on setting. It had the following properties:

setting time/secs (2300 600 (approx) compressive strength/MPa (1007. RH, 4 days) 36.2 Turning to the drawings, Figure 1 shows the cement setting properties of S'02 -A] 2 0.-CaF 2 powdered glass used with PWA, the glass having not been heat-treated. Foregoing Examples 7A - 7H are Glasses A - H respectively in this Figure; 71 i s J. The other Examples and the other Glasses do not correspond. The most suitable compositions describe an arc across the diagram. Figure 2 shows a selection of the glasses examined in Figure 1, with the difference that they Lave been heat treated (600"C 30 for 90 minutes), the glasses containing equal CaF 2 and Al 203 P1US varying amounts of SiO 2 Figure 3 compares some of the foregoing compositions and some with CaO Instead of CaF 2, and 2.5g glass used per ml PVPA.

S 3 k c 1 - 21 EXAMPLES OF PASTES Cements prepared by mixing two pastes, as opposed to a powder and liquid, have the advantage of ease of measuring and give an easier mix. Solutions of PVPA at concentrations of 80% m/m (by mass) can be prepared, these being viscous liquids. The possibility of using such concentrated solutions as part of a two paste glass lonomer type cement was investigated, the other paste being a glass powder suspended In water and some dispersing aid.

Materials.

80% m/m PVPA solution was used as one component of the cement. The glass paste was prepared by mixing G5 (i.e. the glass of Part 4 above) with sufficient water such that when this paste was mixed with the PVPA solution, a 507. m/m solution of PVPA in water resulted. In addition, various dispersing aids Is- were added to the glass paste to prevent drying out. Sufficient glass to give an approximately 3:1, or more usually 5:1, p/] ratio with the 50% PVPA solution was used. A density of 1.0 9 cm-1 was assumed for a 50% PVPA solution.

The dispersing aids used in this study were 11Cellosolve" (i.e. ethoxyethanol), Tween 80 (a non-ionic surfactant) and Dequest 2010 (explained In Part 10). The composition of the glass pastes are given in Table 3.

The G5 glass was used both unheated and heat -treated (4500C. 90 mins) where marked with asterisk (GS).

Results and Discussion.

Firstly, a cement was prepared from 80% PVPA and glass powder. A maximum p/1 ratio of 1:1 was possible, giving a stiff but workable paste (Paste 0). This cement had a very long setting time, and remained soft and pliable at >lhr. The poor setting characteristics of this cement confirmed that there is a minimum quantity of water required in the cement paste for the reaction to occur. Generally cements are prepared from 50-60% m/m PVPA solutions.

c The setting behaviour of the cements prepared from the glass pastes of Table I are shown in Table 5. The compressive strengths of the cements are shown in Table 2. All of the cements when mixed as two pastes gave a homogenous paste within 10 seconds, except paste no. 6. In this case the unheated glass reacted with D2010 in the glass paste, forming a friable solid. When mixed with the PVPA solution the resulting cement set very rapidly (< 20 seconds). With the heat deactivated glass (e.g. in Pastes 4 and 5) there was no reaction noticed between preparing the glass paste and forming the cement from it, generally about 5-10 minutes, although storage of such a paste may have shown similar problems to that observed for Paste 6. By adding the D2010 to the PVPA solution however, (Paste 7), such problems were overcome. This cement (Paste 7) used non-deactivated G5, and consequently had a working time similar to that obtained for a standard powder-liquid mix. However, the p/l ratio of this cement was almost double that which could be achieved with a normal mix.

In general, mixing cements from two pastes gave a homogeneous mix of higher p/l ratio than could be achieved by mixing a powder and liquid. The working times of the cement pastes were also increased, especially when using cellosolve in the glass paste.

However, the disadvantage with all of the cements was their low strength when compared with the normal powder-liquid cements, Table 2. Although the incorporation of D2010 did increase the strength of the cements, it was not able to fully compensate for the weakening effect of the other additives (i.e. cellosolve or Tween 80). Typically, a cement prepared from 50% PVPA, containing 5% D2010, and G5, at a p/l ratio of 3:1 has a compressive strength of 85-90 MPa.

The two paste cements may therefore be limited in their applications to temporary restoratives, for example. Further work is required to identify other dispersing aid, which might have less effect on the strength of the resulting cements.

J -1 Table 3. Composition of Glass Pastes.

Paste glass water dispersing nominal no. aid P/] 1 0.36g G5 0.099 3 drops 3:1 cellosolve 2 0.60g GS 0.099 6 drops 5:1 cellosolve 3 0.60g G5 0.09g 2 drops 5 Tween 80 4 0.609 G5 0.099 3 drops 5:1 D2010 0.609 G5 0.079 1 drop D2010 5:1 1 drop Tween 80 6 0.609 G5 0.07g 1 drop D2010 5:1 1 drop Tween 80 7 0.60g G5 0.079 2 drops Tween 80 5:1 (1 drop D2010 added to PM solution) All cements were prepared by mixing the glass pastes with 0.159 of 80% m/m PM solution.

Table 4. Compressive strength of Two-Paste Cements.

Glass paste No.

1 2 3 4 7 Compressive strength MPa 10.5 11.2 9.8 43.8 30.6 37.1 Compressive strengths were measured in cements 24 hours old, stored at 370C, 100% RH.

c Table 5. Paste N.

0 1 2 3 4 6 is 7 Steady but No setting minutes, No setting minutes, No setting minutes, No setting minutes, Setting characteristics very slow (>> 1 hour) for 5 minutes, rapid setting 6-11 substantially f ully set by 15 minutes.

for 6 minutes, rapid setting 7-15 substantially fully set by 20 minutes.

for 2 minutes, rapid setting 3-6 substantially fully set by 9 minutes.

for 1 minute, steady setting 2-18 substantially fully set by 20 minutes.

No setting for 3 minutes, steady setting 4-20 minutes, substantially fully set by 30 minutes. Friable solid + PM + set within 20 seconds.

No setting for 1 minute, steady setting 1-4 minutes, substantially fully set by 6 minutes.

h a -h c - 1

Claims (15)

- 25 CLAIMS

1. A cement composition, comprising an intimately blended mixture of a water-containing liquid, a cation-crosslinkable polymeric acid containing on average one phosphonic acid group per one to three backbone carbon atoms, and a cation-leachable surgically acceptable glass powder containing Si and Al In the mole proportions 0.6 - 0.2A if previously heat-treated, otherwise exceeding 1. 6A, in the mass proportions (1 -)() polymeric acid: (1 - 5) glass: x liquid, where x is from 0.3 - 0.7.

2. A cement composition according to Claim 1, wherein the glass has been metallised to form a cermet.

3. A cement composition according to Claim 1, wherein the glass has been washed In a dilute acid.

4. A cement composition according to Claim 3, wherein the dilute acid is acetic acid in a concentration of y% and the washing has been for z hours where yz < 4.

5. A cement composition according to any preceding claim, wherein the glass powder consists of particles all of which are smaller than 100 microns.

6. A cement composition according to any preceding claim, wherein the glass powder has been heat-treated at at least 4000C for at least 40 minutes.

7. A cement composition, comprising an Intimately blended mixture of a water-containing liquid, a cation-crosslinkable polymeric acid containing on average one phosphonic acid group per one to three backbone carbon atoms, and a metal oxide powder, in the mass proportions (1 - x) polymeric acid: (1 - 5) oxide: x liquid, where x is from 0.3 - 0.7.

8. A cement composition according to Claim 7, wherein the 30 metal oxide is CaO, ZnO or MgO.

9. A cement composition according to Claim 8, wherein the metal oxide Is MgO which has been heat-treated at at least 90011C.

10. A composition according to any preceding claim, wherein the polymeric acid Is or comprises poly(vinyl phosphonic acid).

C

11. A composition according to Claim 10, wherein the polymer acid further comprises poly(acrylic acid).

12. A composition according to any preceding claim, wherein the liquid is water.

13. A pack comprising two pastes which when mixed form a cement composition according to any preceding claim.

14. A pack according to Claim 13, wherein the first paste contains the acid(s) plus water and the second contains the powder suspended in aqueous thickening agent.

15. A pack comprising the acid(s) in dry form and the powder in the proportions and the compositions of a cement composition according to any preceding claim.

Published 1989 atThe Patent Offtce, State House, 66'71 High Holborn, London WClR 4TP. Further copies maybe obtainedfPom The Patent OMCe. Sales Branch, St Mary Cray. Orpington, Rent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con- 1/87 4-1