GB2310663A - Organic polyacid/base reaction cement - Google Patents

Abstract

For making a cement by reacting an organic polyacid (such as polyacrylic acid or polyvinylphosphonic acid or a copolymer of these) with an ion-leachable glass as the base, the glass contains the cations Al, Si, Ca and Zn wherein Si:Al >1.5. Zn is preferably 30-70 percent of those cations. The glass is made by heat treating an aqueous paste of Al, Si and Ca oxides, such as bentonite, with ZnO and a precursor of ZnO such as zinc acetate, whereby the zinc forms a layer on the glass and is also incorporated as a glass network former. The setting of the cement is speeded and sharpened by using around 2% MgCl 2 . The cement may be used as a dental or bone cement or in splints or the building industry.

Description

ORGANIC POLYACIDIBASE REACTION This invention relates to an organic polyacid/base reaction cement. Polyalkenoate cements, which are examples of such a cement, contain both carbon-carbon and metaloxygen bonds. They are employed as dental restorative materials.

The existing classes of polyalkenoate cements are glass-ionomer (e.g. UK Patent GB 1422337) and zinc polycarboxylate (UK Patent GB 1139430). Both of these cements are formed by the neutralisation reaction of polyacids such as poly(acrylic acid), PAA, with calcium alumino silicate and with zinc oxide respectively. Therefore, the cations responsible for the neutralisation reactions are Zn in the case of the former cement and Ca and Al in the case of the glass-ionomer cement. The concept of creating a combination of these two cements is not itself new. An ideal combined polyalkenoate cement would:i) retain the generic properties of polyalkenoate cements - adhesion and fluoride release; ii) possess the individual advantages of both the glass-ionomer and zinc polycarboxylate cements; iii) possess the disadvantages of neither of the cements, viz, for glass-ionomers, poor flexural strength and wear and early susceptibility to water dissolution; for zinc polycarboxylates, poor wetting and low compressive strengths.

Acids other than polyalkenoic acids can be used, instead or alongside, such as poly(vinyl phosphonic acid).

According to the present invention, a glass comprises, apart from 0 and F, atoms of Al, Si, Ca and Zn wherein SINAI > 1.5:1. Preferably Zn = 30-70 e.g. 45-55% of all those atoms apart from 0 and F. Preferably A1203 < 15% by weight of the glass. The glass is preferably in powder form, e.g. finer than 200, more preferably finer than 45 Um).

The glass is preferably ion-leachable and forms a cement with polyalkenoic acid.

The invention thus extends to a pack comprising (i) a water soluble organic polyacid (or mixture) or a hydrolysable precursor thereof, and (ii) a powdered glass as set forth above. The invention further extends to a glass reaction cement made from a glass as set forth above and a polyacid.

Thus, according to the invention, a process for making an organic polyacid/base reaction cement comprises mixing an oxide powder with an aqueous solution comprising polyalkenoic acid, wherein the oxide comprises aluminium, silicon and zinc.

Also according to the invention, there is provided a two-part pack for making an organic polyacid/base reaction cement, comprising an oxide powder containing aluminium, silicon and zinc; organic polyacid (or mixture) or a precursor which forms such an acid upon hydration; and water, wherein the one part contains one of the foregoing constituents and the other part contains the remainder, such that the two parts upon being mixed form a cement. The oxides of Al, Si and Ca can thus be present as an aqueous powdered paste.

The oxide preferably further comprises an alkaline earth such as wholly or mainly calcium, preferably in a quantity which would be (at least locally) insufficient by itself to form an acid-degradable glass with the aluminium and silicon, such as 4-15 g (as calcium carbonate) per 100 g of the total oxide.

The oxide powder may be made by heat-treating at e.g. 900-1300"C (typically 1200"C) for 1/4 to 2 hours a mixture of two or more oxide constituents or precursors thereof, such as zinc oxide, which may accordingly comprise a proportion (e.g. 1-3% preferably 2%) of zinc acetate as a precursor of zinc oxide. The zinc (oxide equivalent) may be present in a proportion of 15 to 70% of the total oxide powder. One of the oxide constituents may be an aluminosilicate clay. The ZnO reacts with A1203 to form ZnA1204 and with SiO2 to form ZnSi2O5, any ZnO in excess of those stoichiometries remaining as ZnO.

In the preferred heat-treatment range of 1000 to 12000C, compressive strength (and workable powder:liquid ratios) increase with increasing temperature, and working time and setting time both fall, but at the cost of a worsened (higher) setting time:working time ratio.

Preferably there is also present a chloride salt of a multivalent cation (e.g. MgC12, ZnC12, CaC12, AIC13), which can speed up the set and sharpen the snap-set effect, preferably in a weight proportion of 1/2 to 10% (e.g. 1 to 5% such as 11/2 to 3%) to the total oxide.

The acid may be a polyalkenoic e.g. poly(carboxylic acid) of molecular weight of the order of 104-106, such as poly(acrylic acid), e.g. of chain length 75000. In the aqueous solution, the water:polyalkenoic acid weight ratio may be (0.50 to 0.65):1, preferably (0.58 to 0.62): 1. In the process and in the two-part pack, the oxide:polyalkenoic acid weight ratio may be (1.10 to 1,.50):1, preferably (1.15 to 125):l,with the polyalkenoic acid preferably accounting for 32-40%, more preferably 34-38%, of the pack or of the starting mixture in the process. Polyalkenoic acid may be mixed with, copolymerised with, or even replaced by, poly(vinyl phosphonic acid).

The oxide usable in the invention will now be described by way of example.

The oxide powder may be regarded as a chemical mixture of polyalkenoate cement formers starting from an ion- (network and modifier) deficient alumino silicate. Zinc ions from organic (e.g. acetate, stearate or oxalate) and inorganic sources are then incorporated into the alumino silicate at elevated temperatures. The suitability of the ion-deficient aluminosilicate is judged primarily by its silica:alumina ratio. For the complete dissociation of the glass network necessary to form practical polyacid reaction cement this ratio must be less than or equal to 3:1. Therefore, for the ion-deficiency criterion to apply to the present cement, the silica:alumina ratio must be greater than 3:1.

In this way, the zinc ions are incorporated in the silicate network as network formers, in the place of the alumina tetrahedra of the glass component of the glass-ionomer cements. Also, excess zinc ions take on the role of network modifiers otherwise assumed by calcium ions in the glass-ionomer cements. The resulting material is acid degradable.

It is very versatile in that it possesses variable handling properties (working and setting time and sharpness of set) and mechanical properties. These are dependent on: i) the wide matrix of viable chemical composition; ii) thermal history; and iii) the choice of zinc ions precursor material.

The origin of the aluminosilicate is preferably (but not essentially) the naturally-occurring aluminosilicate, bentonite. Other natural or synthetic silicates with the prerequisite composition will suffice. Bentonite may be partly or wholly replaced by any other montmorillonite or smectite clay mineral, and clay materials based on beidellite, hectorite, sauconite and saponite, and more preferably nontronite, may be used.

Admixtures of these with quartz, muscovite, biotite, limonite, hydrous micas, cristobalite, feldspar and vermiculite or volcanic glass may be used. The particle sizes should be mostly in the colloidal range. Attapulgite may be present, as may sepiolite, corrensite, allophane or imogolite. Texas, Arkansas, Mississippi, Kentucky and Tennessee bentonites are preferred to those from Wyoming, South Dakota, Montana, Utah, Nevada and California, which swell in water. The deposition of a layer of zinc oxide, probably from an organic source, on suitable aluminosilicates is a preferred way of putting the invention into effect.

Preferably zinc is added as 1 zinc acetate:50-100 zinc oxide by weight. The zinc acetate acts as a precursor of zinc oxide. The zinc compound(s), calcium compound if present and bentonite (or equivalent) are, as powders, mixed in water and the aqueous mixture is fired and if necessary reground. If on the other hand depositing such a zinc oxide layer on glass, there will exist a distinct (if diffuse) bimaterial interface in the acid-degradable particles.

In other words, there exists a three dimensional interconnected and interwoven structure of zinc- and calcium-rich aluminosilicates. It is also possible that, added to this, the organic zinc chemicals employed in the cement compositions (e.g. the zinc acetate just mentioned) deposit a thin layer of zinc oxide on the calcium-rich aluminosilicates, separate from and additional to the three-dimensional zinc aluminosilicate.

The invention will now be described by way of example. The compositions of 62 oxides used and of bentonite, glass and "zinc oxide" are as follows: TABLE 1. The compositions of tvpical Bentonite. Glass-ionomer Cement Glass and zinc polvcarboxvlates "zinc oxide" (by weight)

Components Bentonite G338 "zinc oxide" Silica 64.8 24.93 Alumina 13.5 14.25 Calcium oxide 4.77 Calcium fluoride 12.82 Aluminium phosphate 24.22 Cryolite AlF3.3NaF 19.23 Aluminium fluoride - 4.56 Magnesium oxide 3.63 17.0 Zinc oxide 83.0 Ferric oxide 1.25 Sodium oxide 2.22 The bentonite consisted of granular particles smaller than }/4 mm.

G338 is the designation of a popular commercial glass for glass ionomer cement.

In the following oxide compositions: 1. Calcium carbonate, CaCO3 is used as a cheap and convenient source of calcium oxide (CaO) on firing (note that if CaO itself is used directly, it gives different results, working too fast); 2. ZnAc refers to zinc acetate; 3. The surfactant ("Nansa", trade mark) is needed to dissolve the xanthan gum ("Keltrol", trade mark) in water; 4. Various bentonite grades were used. All samples from Z46 onwards used EXM 622 ADP. The primary distinction between the bentonites is their calcium content bentonite 0408, EXM 585, EXM 58519 ADP and EXM 622 ADP contain 2, 3, 9 and 5 weight percent calcium respectively. The other oxides can also vary by a few percent, as would be expected from a naturally occurring mineral, as follows:

Bentonite grade EX 0408 | EXM 585 | EXM 58519 ADP | EXM 622 ADP X-ray fluorescence ANALYSIS (%) SiO2 68.6 66.8 61.7 64.8 TiO2 0.16 0.15 0.14 0.16 A1203 15.4 15.0 13.8 13.5 Fe2O3 1.11 1.06 1.00 1.25 CaO 2.33 3.73 9.73 4.77 MgO 3.03 2.97 2.85 3.63 K2O 1.26 1.18 1.10 0.32 Na2O 2.01 1.96 1.77 2.22 Mn3O4 0.06 0.05 0.05 0.03 ZrO2 0.02 0.02 0.02 0.02 SrO 0.02 0.02 0.02 0.11 Loss on ignition at 5.85 6.60 8.00 7.61 1025 C TOTAL 99.85 99.54 100.18 98.49 Sulphate -0.05% -0.05% -0.05% -1.75% The phases present in bentonite of grade EXM 622 ADP are as follows:montmorillonite- major cristobalite - 19% (by volume as determined by X-ray diffraction) calcite CaCO3 - 2.1% bassanite CaSO4.H2O - 5.0%.

OXIDE COMPONENTS BY WEIGHT

ZnO Bentonite CaCO3 Xanthan Gum Surfactant Water ZnAc Firing Regime Storage Time days Z1 50.0 30.5 16.5 0.67 0.05 150.0 - 1200C, 120 mins, O Z2 50.0 30.5 16.5 0.67 0.05 150.0 1.82 1200C, 120 mins, O 8 Z3 30.0 30.5 16.5 0.67 0.05 150.0 - 1200C, 120 mins, O 7 Z4 30.0 30.5 16.5 0.67 0.05 150.0 1.37 1200C, 120 mins, O 7 Z5 70.0 30.5 16.5 0.67 0.05 150.0 - 1200C, 120 mins, O 7 Z6 70.0 30.5 16.5 0.67 0.05 150.0 1.37 1200C, 120 mins, O 7 Z7 50.0 30.5 16.5 0.67 0.05 150.0 1.82 1200C, 120 mins, RT 8 Z8 30.0 30.5 16.5 0.67 0.05 150.0 1.37 1200C, 120 mins, RT 7 Z9 70.0 30.5 16.5 0.67 0.05 150.0 1.37 1200C, 120 mins, RT 7 Z10 70.0 30.5 16.5 0.67 0.05 150.0 1.37 1200C, 60 mins, RT 7 Z11 50.0 30.5 16.5 0.67 0.05 150.0 - 1200C, 60 mins, RT 8 Z12 50.0 30.5 16.5 0.67 0.05 150.0 1.82 1200C, 60 mins, RT 8 Z13 30.0 30.5 16.5 0.67 0.05 150.0 - 1200C, 60 mins, RT 7 Z14 30.0 30.5 16.5 0.67 0.05 150.0 1.37 1200C, 60 mins, RT 7 Z15 70.0 30.5 16.5 0.67 0.05 150.0 - 1200C, 60 minns, RT 7 Z20 50.0 30.5 16.5 0.67 0.05 150.0 1.82 1200C, 15 mins, RT 8 OXIDE COMPONENTS BY WEIGHT

Zno Bentonite CaCO3 Xanthan Gum Surfactant Water ZnAc Firing Regime Storage Time (days) Z21 50.0 30.5 16.5 0.67 0.05 150.0 1.82 1200C, 30 mins, RT 8 Z24 50.0 30.5 16.5 0.67 0.05 150.0 - 1000C, 60 mins, RT 8 Z25 50.0 30.5 16.5 0.67 0.05 150.0 - 1100C, 60 mins, RT 5 Z26 50.0 30.5 16.5 0.67 0.05 150.0 1.21 1200C, 60 mins RT 5 Z27 50.0 30.5 16.5 0.67 0.05 150.0 0.60 1200C, 60 mins, RT 5 Z28 50.0 30.5 16.5 0.67 0.05 150.0 0.60 1000C, 60 mins, RT 5 Z29 50.0 30.5 16.5 0.67 0.05 150.0 1.21 1000C, 60 mins, RT 5 Z30 50.0 30.5 16.5 0.67 0.05 150.0 1.82 1000C, 60 mins, RT 5 Z31 50.0 30.5 16.5 0.67 0.05 120.0 1.40 Z32 50.0 30.5 11.0 0.67 0.05 120.0 1.40 7 Z33 50.0 30.5 7.0 0.67 0.05 120.0 1.40 8 Z34 50.0 30.5 3.0 0.67 0.05 120.0 1.40 8 Z35 50.0 30.5 16.5 0.67 0.05 120.0 1.40 7 Z36 50.0 30.5 16.5 0.67 0.05 120.0 1.40 7 Z37 50.0 30.5 13.5 0.67 0.05 120.0 1.40 7 Z38 50.0 30.5 25.5 0.67 0.05 120.0 1.40 8 Z39 50.0 30.5 31.5 0.67 0.05 120.0 1.40 Z40 50.0 30.5 37.5 0.67 0.05 120.0 1.40 Thus, in oxides Z31-34 and Z37-40, the following respective masses of calcium carbonate were added to 67 g in each case of prefired ZnO + bentonite mix: 5.5 g, 3.7 g, 2.3 g, 1.0 g, 6.5 g, 8.5 g, 10.5 g and 12.5 g.

In the oxides Z1-Z30, bentonite EXM585 was used.

In the oxides Z31-Z44 the bentonite employed is EXO408, while a different batch of EXM585 was used in the oxide 245.

The bentonite EXM622 was employed for the production of the oxides Z46-Z62.

OXIDE COMPONENTS BY WEIGHT

ZnO Bentonite CaCO3 Xanthan Gum Surfactant Alumina Water ZnAc Z41 50.0 30.5 16.5 0.67 0.05 4.5 120.0 1.40 Z42 50.0 30.5 16.5 0.67 0.05 9.0 120.0 1.40 Z43 50.0 30.5 16.5 0.67 0.05 13.5 120.0 1.40 Z60 150.0 61.0 33.0 1.34 0.10 27.0 240.0 2.80 Z44 50.0 30.5 16.5 0.67 0.05 18.0 120.0 1.40 OXIDE COMPONENTS BY WEIGHT

ZnO Bentonite CaCO3 Xanthan Gum Surfactant MgO Water ZnAc Z45 50.0 30.5 16.5 0.67 0.05 - 120.0 1.40 Z46 16.7 10.17 5.5 0.22 0.017 - 40.0 0.47 Z47 16.7 10.17 - 0.22 0.017 2.2 40.0 0.47 Z48 10.0 10.17 5.5 0.22 0.017 - 40.0 0.47 Z49 3.3 10.17 5.5 0.22 0.017 - 40.0 0.47 Z50 16.7 10.17 5.5 0.22 0.017 2.2 40.0 0.47 Z51 2.0 10.17 5.5 0.22 0.017 - 40.0 0.47 Z52 1.0 10.17 5.5 0.22 0.017 - 40.0 0.47 Z53 0.5 10.17 5.5 0.22 0.017 - 40.0 0.47 Z54 - 10.17 5.5 0.22 0.017 - 40.0 0.47 Z58 16.7 10.17 1.5 0.22 0.017 - 40.0 0.47 Z59 16.7 10.17 3.5 0.22 0.017 - 40.0 0.47 Z62 100.0 61.0 14.0 1.34 0.10 - 240.0 2.80 OXIDE COMPONENTS BY WEIGHT

ZnO ZnF2 Bentonite CaF2 Xanthan Gum Surfactant Water ZnAc Z55 16.7 - 10.17 3.25 0.22 0.017 30.0 0.47 Z56 8.3 10.58 10.17 3.25 0.22 0.017 40.4 0.47 Z57 - 21.17 10.17 4.29 0.22 0.017 40.0 0.47 Z61 - 127.0 61.0 25.74 1.34 0.10 240.0 2.80 Oxide 257 was found by X-ray diffraction to contain 15 vol% fluorite CaF2, major amounts of willemite ZnSi2O4 and gahnite ZnAl2O4 and a possible trace of grossular Ca3Al2(SiO4)3, and containing 12.2% fluorine.

OXIDE COMPONENTS BY WEIGHT

ZnO G338(Glass Xanthan Gum Surfactant Water ZnAc Z16 70.0 40.0 0.67 0.05 150.0 Z17 35.0 28.0 0.33 0.025 50.0 Z18 35.0 47.0 0.33 0.025 50.0 Z19 35.0 20.0 0.33 0.025 75.0 1.5 The above oxides were used to form cements.

Composition of cement-forming glasses Z68 and Z69:

Cement Components (by wt) Z68 Z69 zinc oxide 15.69 12.27 zinc fluoride 7.49 11.65 Bentonite EXM 585 13.17 13.20 calcium carbonate 5.18 4.11 calcium fluoride 1.52 2.36 alumina 4.23 3.37 zinc acetate 0.60 0.60 Additional components present: water 51.81 51.93 xanthan gum 0.29 0.29 surfactant 0.002 0.002 Z68 is the chemical equivalent of physically blending 73 parts by weight of Z60 with 27 parts of 261 (from which the composition of Z60 can be readily deduced). Z69 is the chemical equivalent of physically blending 58 parts of Z60 with 42 parts ofZ6l. Physical blends of Z60 and Z61 (in the proportions 73:27 and 58:42 as above) were mixed with water and accelerated with calcium chloride and also with magnesium chloride. Other multivalent water soluble chlorides are also effective. Z68 and Z69 are relatively deactivated by the partial replacement of zinc oxide with zinc fluoride.

Two anhydrous cement powders were prepared: i) 5.0 g of dry poly(acrylic acid) PAA + 0.5 g tartaric acid + 10.0 g oft60; ii) 5.0 g of dry PAA + 0.5 g of tartaric acid + 10.0 g of 261 (fluoride-containing glass). The anhydrous cement powders were mixed with water at p:w of 3.4:1. This is equivalent to mixing the glasses with a 50% aqueous solution of PAA mediated with 5% tartaric acid.

Various physical blends of the two cement powders were mixed with water at this p:w. In some instances, minute quantities of magnesium chloride hexahydrate were included in the cement powders. The working and setting times of these cements were determined with the aid of Wilson rheometer at 22"C:

Description of Anhydrous Cement Powder Working Setting Time Time 0.34g of Z60 cement powder 0.5 3.0 0.30g Z60 cement + 0.04g Z61 cement 2.7 6.5 0.30g Z60 cement + 0.04g Z61 cement + 0.006g MgCl2 2.1 4.4 0.25g Z60 cement + 0.09g Z61 cement 2.4 7.6 0.25g Z60 cement + 0.09g Z61 cement + 0.01g MgCl2 2.0 4.2 0.20g Z60 cement + 0.14g Z61 cement + 0.01g MgC12 2.1 5.6 Note: The anhydrous cement powders were mixed with 0.10 g of water.

The cements were formed by mixing the requisite amounts of the oxide powder (P) with liquid (L) being 13% tartaric acid plus 35% E9 poly(acrylic acid) of chain length 75000 PAA aqueous solution - referred to as L1. Cement pastes were introduced into standard stainless steel (6 x 12) mm moulds, stored for 1 hour dry and then wet for the remainder of the storage period. Their properties are listed in Table 2. A different liquid was used in Table 2A, see later.

In Tables 1, and 3 to 6, 0 and RT refer to the fired oxide contained in the crucible being left in the furnace overnight, 0 (after being switched off), and RT (being allowed to equilibrate at room temperature) respectively.

WT(90) refers to the working time, i.e. the time taken for the amplitude of the oscillating rheometer to fall to 90% of its original amplitude. The setting time was the time taken for the amplitude of the rheometer to fall to 5%, So(5). ZnAc, ZnO and CaCO; refer to zinc acetate, zinc oxide and calcium carbonate respectively. Bnt refers to the aluminosilicate, bentonite.

TABLE 2. THE PROPERTIES OF THE CEMENTS AT 20 C

MATERIAL COMPRESSIVE FLEXURAL P:L HANDLING PROPERTIES STRENGTH (Mpa) STRENGTH by weight WT(90) ST(10) ST(5) ST(5)/WT(90) (mins) (mins) (mins) Z1 ()=Std 1.0 5.4 5.8 5.8 Deviation 11.79(3.13) 1.5:1 Z2 12.57(3.20) 1.5:1 Z3 16.36(1.50) 7.04(1.49) 1.5:1 Z4 16.94(0.54) 22.14(14.09) 1.5:1 Z5 17.79(0.30) 6.41(1.19) 1.5:1 Z6 23.91(1.02) 11.69(3.38) 2.0:1 1.8 2.8 3.0 1.7 Z7 19.64(1.53) 26.76(3.46) 2.0:1 3.5 5.8 6.0 1.7 Z8 22.23(0.28) 20.67(3.46) 2.0:1 2.3 5.8 6.5 2.8 Z9 25.05(1.02) 23.57(2.43) 2.5:1 1.9 4.0 4.4 3.8 Z10 24.37(0.91) 12.74(2.23) 2.0:1 0.8 2.8 3.0 2.3 Z11 22.40(0.84) 10.74(1.06) 2.5:1 2.9 6.1 6.7 2.1 Z12 20.54(0.39) 11.39(0.04) 2.5:1 2.5 4.8 5.2 1.7 Z13 17.74(1.04) 9.00(2.75) 2.5:1 3.6 5.7 6.2 2.1 Z14 24.73(1.33) 23.22(2.74) 2.5:1 2.5 4.8 5.2 4.2 Z15 22.30(1.85) 32.73(6.26) 1.5:1 0.7 2.7 3.0 Z20 TABLE 2. (continued) THE PROPERTIES OF THE CEMENTS AT 20 C

MATERIAL COMPRESSIVE FLEXURAL P:L HANDLING PROPERTIES STRENGTH (Mpa) STRENGTH by weight WT(90) ST(10) ST(5) ST(5)/WT(90) (mins) (mins) (mins) Z21 26.09(0.66) 29.54(4.63) -1.5:1 4.3 6.6 7.1 1.7 Z24 18.76(2.27) 24.78(4.34) -0.7:1 3.7 5.6 5.9 1.6 Z25 20.87(1.30) 27.55(3.50) -1.0:1 3.5 5.4 5.8 1.7 Z26 24.65(1.69) 30.67(2.95) -1.8:1 3.4 6.0 6.5 1.9 Z27 21.11(4.91) 18.59(1.31) 2.5:1 1.9 4.8 5.4 2.8 Z28 23.18(1.40) 21.51(3.54) -1.5:1 0.8 2.8 3.1 3.9 Z29 24.45(1.25) 26.48(1.47) -1.5:1 1.5 3.3 3.6 3.4 Z30 22.40(1.99) 24.15(8.17) -1.8:1 1.4 2.4 2.5 1.8 Z11B 22.66(0.91) 2.0:1 0.8 2.8 3.0 3.8 Z12B 22.58(1.26) 2.5:1 2.9 6.1 6.7 2.3 TABLE 2A.

MATERIAL COMPRESSIVE FLEXURAL HANDLING PROPERTIES STRENGTH (Mpa) STRENGTH WT(90) ST(5) ST/WT (mins) (mins) Z21 114.56 (8.38) 29.54(4.63) 4.3 7.1 1.7 Z31 141.3 (11.05) 27.39(2.77) 2.9 6.1 2.1 Z32 106.59(15.30) 27.57(3.04) 1.6 2.7 1.7 Z33 92.84 (9.16) 33.05(6.55) 1.3 2.1 1.6 Z34 80.39 (2.64) 24.22(2.35) 0.8 1.5 1.9 Z35 89.57 (8.20) - 3.8 8.2 2.2 Z36 82.58(20.13) - 3.7 5.2 1.54 Z37 120.00(10.96) 20.68(1.82) 3.2 6.0 1.9 Z38 117.32(16.69) 22.95(2.29) 3.0 6.1 2.0 Z39 102.33 (7.02) - 2.1 4.0 1.9 Z40 68.45(10.20) - 1.7 3.9 2.3 Z41 122.534(9.60) 24.01(3.28) 3.5 7.3 2.1 Z42 101.33(15.91) 23.87(5.65) 3.4 6.4 1.9 Z43 115.83(13.15) 20.94(2.56) 5.5 9.0 1.6 Z44 121.93 (7.49) 27.70(4.61) 2.7 6.3 2.3 TABLE 2A. (continued)

MATERIAL COMPRESSIVE FLEXURAL HANDLING PROPERTIES STRENGTH (Mpa) STRENGTH WT(90) ST(5) ST/WT (mins) (mins) Z46 135.43 (9.73) 27.70(2.51) 3.8 6.1 1.6 Z47 94.27 (2.35) 25.56(4.92) 1.3 4.5 3.5 Z48 122.49 (4.03) 28.31(3.27) 2.8 7.0 2.5 Z49 108.99 (9.31) 27.98(3.03) 2.4 6.2 2.6 Z50 105.74 (1.72) 28.10(3.08) 2.8 9.7 3.5 Z51 98.17 (1.93) 8.55(0.71) 2.5 6.5 2.6 Z52 74.58 (3.12) 7.39(0.14) 2.3 8.0 3.5 Z53

TABLE 3. THE EFFECT OF FIRING TIME ON CEMENT PROPERTIES (Oxides were fired at 1200C)

MATERIAL FIRING COMPRESSIVE P:L HANDLING PROPERTIES TIEM STRENGTH (Mpa) WT(90) ST(10) ST(5) ST(5)/WT (Z4) Z4 120 mins, O 16.36(1.50) 1.5:1 Z8 120 mins, RT 19.64(1.53) 2.0:1 3.5 5.8 6.0 1.7 Z14 60 mins, RT 17.74(1.04) 2.5:1 3.6 5.7 6.2 1.7 (Z6) Z6 120 mins, O 17.79(0.30) 1.5:1 Z9 120 mins, RT 22.23(0.28) 2.0:1 2.3 5.8 6.5 2.8 Z10 60 mins, RT 25.05(1.02) 2.5:1 1.9 4.0 4.4 2.3 (Z2) Z2 120 mins, O 11.79(3.13) 1.5:1 Z7 120 mins, RT 23.91(1.02) 2.0:1 1.8 2.8 3.0 1.7 Z12 60 mins, RT 22.40(0.84) 2.5:1 2.9 6.1 6.7 2.3 Z21 30 mins, RT 26.09(0.66) -1.5:1 4.3 6.6 7.1 1.7 Z20 15 mins, RT 22.30(1.85) 1.5:1 0.7 2.7 3.0 4.2 TABLE 4. THE EFFECT OF ZnO LEVEL ON CEMENT PROPERTIES (Oxides were fired at 1200C for 60 minutes then left at RT)

OXIDE ZnO COMPRESSIVE P:L HANDLING PROPERTIES LEVEL STRENGTH (Mpa WT(90) ST(5) ST(5)/WT(90) Z11 50g 24.37(0.91) 2.0:1 0.8 3.0 3.8 Z12 50g 22.40(0.84) 2.5:1 2.9 6.7 2.3 Z13 30g 20.54(0.39) 2.5:1 2.5 5.2 2.1 Z14 30g 17.74(1.04) 2.5:1 3.6 6.2 1.7 Z15 70g 24.73(1.33) 2.5:1 2.5 5.2 2.1 Z10 70g 25.05(1.02) 2.5:1 1.9 4.4 2.3 TABLE 5. THE EFFECT OF FIRING TEMPERATURE ON CEMENT PROPERTIES (Oxides were fired for 60 minutes then left at RT)

OXIDE FIRING COMPRESSIVE P:L HANDLING PROPERTIES LEVEL STRENGTH (Mpa) WT(90) ST(5) ST(5)/WT(90) (Z1) Z24 1000C 18.76(2.27) -0.7:1 3.7 5.9 1.6 Z25 1100C 20.87(1.30) -1.0:1 3.5 5.8 1.7 Z11B 1200C 22.66(0.91) 2.0:1 0.8 3.0 3.8 TABLE 6. THE EFFECT OF ZnaC LEVEL OF CEMENT PROPERTIES (Oxides were fired for 60 minutes then left at RT)

OXIDE ZnAc COMPRESSIVE P:L HANDLING PROPERTIES LEVEL STRENGTH (Mpa) WT(90) ST(5) ST(5)/WT(90) 1200C Z27 1g 21.11(4.91) 2.5:1 1.9 5.4 2.8 Z26 2g 24.65(1.69) -1.8:1 3.4 6.5 1.9 Z12B 3g 22.58(1.26) 2.5:1 2.9 6.7 2.3 1000C Z28 1g 23.18(1.40) -1.5:1 0.8 3.1 3.9 Z29 2g 24.45(1.25) -1.5:1 1.5 3.6 2.4 Z30 3g 22.40(1.99) -1.8:1 1.4 2.5 1.8 The liquids used in this specification had the following compositions by weight L I L2 L3 L4 (Table 2? ~~~~~ ~~~~~~ (Table 2A) Poly(acrylic acid) 35 40 45 50 Tartaric acid 13 13 13 13 Water 52 47 42 37 These influenced the compressive strength of cement made using Z21 as follows: Ll 26 MPa, L2 73 MPa, L3 90 MPa and L4 1141/2 MPa.

An alternative liquid was used in the following test, intended to demonstrate how, as mentioned earlier, Z68 and Z69 are deactivated due to the substitution of zinc oxide by zinc fluoride. The deactivation can be observed by their reaction with poly(acrylic acid) PAA, and with the altemative liquid, a copolymer of acrylic and vinyl phosphonic acid, PVPA/PAA.

Because the fluoride-containing minerals synthesized in this invention, typified by Z61, are more acidic than the traditional basic ion-leachable component of polyalkenoate cements, these fluoride-rich types are not fully decomposed by poly(acrylic acid) to form cements. The use of a stronger polyacid, such as poly(vinyl phosphonic acid), PVPA or the copolymer of vinyl phosphonic acid and acrylic acid as suitable cement formers has been investigated.

The copolymer of vinyl phosphonic acid and acrylic acid (PVPA/PAA) was supplied by A.H. Marks Ltd., Bradford, UK. Two solutions were prepared from the polymers, PVPA/PAA I and PVPA/PAA II:

Materials PVPA/PAA I PVPA/PAA II Weight (g) PVPA/PAA 5.00 5.00 Dequest D2010 0.60 1.20 (+) Tartaric acid 0.60 Water 3.80 3.80

Glass Reactivity with polyacid PAA PVPA/PAA Z60 Fast-setting, but practical Too reactive Z68 Slow-setting, but practical Too reactive especially at high p:l Z69 Slow-setting, but practical Fast-setting, practical especially at high p:l at low p:l Z72 Incomplete reaction, Fast-setting, practical non-practical cements 1 at wide p:l (p:l = powder:liquidratio) Properties of the Z68 and Z69 Cements

Cement Description | Compressive Biaxial Flexural Liquid p:1 Powder Strength(MPa) Strength(MPa) 50% PAA, 15% tartaric acid 2.5:1 | Z69 | 70.52 (2.98) 21.99(1.33) 40% PAA, 15% tartaric acid 2.5:1 Z68 80.38 (0.62) 28.87 (1.02) 50% PAA, 15% tartaric acid | 2.0:1 Z68 1 55.87 (1.52) 25.44 (2.41) Unlike in the case ofthe reaction ofZ61 with poly(acrylic acid) where the cements formed did not set, the cements formed between Z61 and PVPA/PAA set hard within 5 minutes.

The compressive and biaxial flexural strengths of cements stored for 24 hours in water at 37 C were determined: Properties of the Z61 Cements

Cement Description | Mechanical Properties Compressive Biaxial Flexural Powder Liquid p:l Strength (MPa) | Strength (MPa) Z61 PVPAIPAAI 3:1 133.07 (6.92) 44.54(5.17) Z61 PVPAIPAA II 3:1 1 135.89 (10.25) 39.84 (2.55) Turning to the effect of fluoride on materials according to the invention, it will be seen that the oxides Zl-Z15, Z20-Z54 and Z58, Z59 do not contain fluoride ions.

Z16-Z19 contained fluoride ions introduced in the form of cryolite. Z55 contained calcium fluoride, while Z56 and Z57 contained calcium fluoride and zinc fluoride, at different levels. All the cements formed from fluoride-containing oxides did not set when stored in anaerobic conditions, e.g. in compressive and flexural strength clamps. However, when these cements were exposed to air they set rapidly. They also set when they were heated at elevated temperatures > 60C. There are potential applications for these heat- and/or command-curable cements in dentistry, other biomedical fields and the general industrial sector, such as Dentistrv: 1. endodontic (root canal) fillers, where physical mixtures of the non-setting cements can be mixed with standard setting-cements to form flexible fluoride-releasing cement replacement for Gutta Percha; 2. one-component temporary filling materials; 3. core build-up/posterior filling materials; 4. specialist adhesive for the bonding of orthodontic bands and brackets and also as a general dental adhesive for bonding crowns, split teeth and endodontic posts; Biomedical: 1. replacement for the existing commercial Plaster of Paris and Fibre-glass splint bandages; 2. replacement for PMMA as a bone cement; and General Industrv: 1. hard and durable prototype material; 2. household one-component filler for plugging holes and supporting furniture onto walls; 3. roofing material for mending leaks; 4. general repair material for roads and bridges; and 5. component of building blocks and cement material for the construction industry.

As to Biomedical 1 above, commercial splint bandages are made from Plaster of Paris (POP) and resin impregnated with fibre glass. The POP splints are the traditional materials. They are bulky and have high setting exotherms.

The requirements for a splint bandage are: 1) working time, when the cement is manipulable, of4 minutes at 230C; 2) setting time of 5-6 minutes; 3) minimal, preferably negligible setting exotherm; 4) high strength:weight ratio.

A suitable polyalkenoate cement for a splint bandage will comprise three components: acid degradable mineral, tartaric acid and poly(acrylic acid). The tartaric acid may be blended with the mineral and dispersed in acetone, and the dry poly(acrylic acid) may be dissolved in a 9:1 or 19:1 acetone:water mixture.

Blending and dispersion of all the three polyalkenoate cement components, as practised previously, has been found to be deleterious - it leads to premature cement formation during the dispersion process. The obstacle to the use of polyalkenoate cements as a splint bandage material is the inefficient mixing of the cement components when introduced into water, unlike the thorough spatulation of the cements possible on the dental scale to obtain fast-setting dental materials.

The porous foam padding of conventional Plaster of Paris splint bandages was found not to be suitable for the polyalkenoate splint material. It allowed too much polyalkenoate material to leach out upon dipping the splint in water. In the case of the multi-component polyalkenoate cement there is greater need to keep as much of the splint material out of the water as possible. If the foam material is desired to be retained for its comfort, it is preferably covered by a tightly woven fabric, e.g. similar to the fabric covering the other face of a POP splint bandage. This could still be done to leave the foam covering outermost. The polyalkenoate cement can be wrapped in two layers of the conventional light outer cotton fabric. The result has been a reduction in the material leached out and an increase in strength of the resulting splint.

In industrial applications, cheap additives such as specialist sands or furnace slag can be added to the cements to make them more economical. In this way, the cement is only providing the setting reaction, while the added fillers improve the strength of the resulting material.

Cheap additives of this type include salts such as chlorides, nitrates, carbonates are capable of being introduced into the cement system. Chlorides give the benefit of a sharp quick set. Fluorides make the glass more wettable, which allows a higher practical powder:liquid ratio, which gives a stronger product. This is illustrated as follows: The present fluoride-containing glasses do not form acid/base cements with poly(acrylic acid). However, physical mixtures of these cements set hard when mixed with the standard, non-fluoride-containing glasses. As mentioned earlier, the setting of the resulting cements is speeded up and sharpened by the addition of minute quantities of di- and trivalent chlorides. Magnesium, calcium, zinc and aluminium chlorides can be employed in this way.

Z60 has been tested for its suitability for the bonding of orthodontic brackets.

It was tested in stainless steel lap-joints cemented with the following compositions:

Time of Age of Thickness Cement Composition Joint Bonded of cement Bond Strength Z60 plus liquid: Formation Joint layer (mm) (Mpa) 1. 50% PAA, 5% TA, 45 seconds 1 hr 0.68-0.80 4.59 (0.42) p:l 0.8:1 2. 50% PAA, 5% TA, 45 seconds 15 mins 0.17-0.22 5.92 (0.61) p:l 0.8:1 3. 50% PAA, 5% TA, 80 second 15 mins 0.16-0.65 5.97 (0.53) p:1 0.8:1 4. 50% PAA, 5% TA, 45 seconds 24 hours 0.14-0.51 7.87(0.36) p:1 0.8:1 PAA and TA refer to poly(acrylic acid) and tartaric acid respectively. "Time of joint formation" refers to the passage of time before the two halves of the lap joint are brought together to sandwich the cement and form a bonded joint. The measurement of this variable is a recognition of the fact that the cementing power of an adhesive diminishes with time (increase in viscosity) as it sets.

A cement with a bond strength of 5.5 MPa is considered likely to be sufficiently robust to maintain the integrity of the bonded brackets under the current clinical procedures.

Thus glasses according to the invention can offer the following properties compared with prior polyalkenoate cements:

COMPRESSIVE FLEXURAL SETTING/WORKING MATERIAL I STRENGTH STRENGTH TIMES (MPa) (MPa) Glass-ionomer 100-200 10-15 2.0-3.0 Zinc polycarboxylate 90-100 20-30 3.04.0 Invention 70-150 20-60 1.5-2.5 In addition they can be:1. radio-opaque - a property of the zinc polycarboxylate cements. The glass-ionomer cements are radiolucent; 2. fast-setting. The typical glass-ionomer cement has a ratio of setting:working time of 2-3:1, while the zinc polycarboxylate cement has a ratio of 3-5:1. Some fast-setting cement compositions according to the invention possess setting:working times of 1.5-1-8:1. This is due to the opposing effect of heat on zinc oxide and calcium aluminosilicate - heat deactivates zinc oxide while producing highly strained and reactive glasses; 3. very strong in flexure. This is a reflection of the zinc polyacrylate nature of the matrix of the resulting cements; 4. of high compressive strengths. This is a property normally associated with glass-ionomer cements.

5. less susceptible to early moisture attack. This is due to their relatively modest levels of calcium ions - calcium polyacrylate is soluble in water; 6. less translucent than the glass-ionomer cement but, also, less opaque than the zinc polycarboxylate cements. As in (5) above, this is a reflection of the modest levels of calcium ions in the cements; 7. with fracture properties (fracture toughness and toughness) greater than the existing polyalkenoate cements. This is reflected in their high flexural strengths.

8. a match for the light-activated glass-ionomer cements in mechanical properties, though not in aesthetics. However, they are stronger adhesives (due to the absence of the hydrophobic resins necessary for light activation) and they do not need to be applied and cured in successive thin layers.

Claims (21)

1. A process for making a polyalkenoate cement, comprising mixing an oxide powder with an aqueous solution comprising polyalkenoic acid, wherein the oxide comprises aluminium, silicon and zinc.

2. A process according to Claim 1, wherein the oxide further comprises an alkaline earth.

3. A process according to Claim 2, wherein the alkaline earth is present in a quantity which would be, at least locally, insufficient by itself to form an acid-degradable glass with the aluminium and silicon.

4. A process according to Claim 2 or 3, wherein the alkaline earth is wholly or mainly calcium.

5. A process according to any preceding claim, wherein the oxide powder is made by heat-treating a mixture of two or more oxide constituents or precursors thereof.

6. A process according to Claim 5, wherein one of said constituents is zinc oxide, which may comprise a proportion of zinc acetate as a precursor of zinc oxide.

7. A process according to Claim 5 or 6, wherein one of said constituents is an aluminosilicate clay.

8. A process according to any preceding claim, wherein the polyalkenoic acid is a poly(carboxylic acid) of molecular weight of the order of 1 04-1 06.

9. A process according to any preceding claim, wherein the setting of the cement is speeded by a chloride salt of a multivalent cation.

10. A two-part pack for making a polyalkenoate cement, comprising an oxide powder containing aluminium, silicon and zinc; a polyalkenoic acid or a precursor which forms a polyalkenoic acid upon hydration; and water, wherein the one part contains one of the foregoing constituents and the other part contains the remainder, such that the two parts upon being mixed form a cement.

11. A pack according to Claim 10, wherein the oxide further comprises an alkaline earth.

12. A pack according to Claim 11, wherein the alkaline earth is present in a quantity which would be, at least locally, insufficient by itself to form an acid-degradable glass with the aluminium and silicon.

13. A pack according to Claim 11 or 12, wherein the alkaline earth is wholly or mainly calcium.

14. A pack according to any of Claims 10 to 13, wherein the oxide powder is made by heat-treating a mixture of two or more oxide constituents or precursors thereof.

15. A pack according to Claim 14, wherein one of said constituents is zinc oxide, which may comprise a proportion of zinc acetate as a precursor of zinc oxide.

16. A pack according to Claim 14 or 15, wherein one of said constituents is an aluminosilicate clay.

17. A pack according to any of Claims 10 to 16, further comprising a chloride salt of a multivalent cation.

18. A pack according to any of Claims 10 to 17, wherein the polyalkenoic acid is a poly(carboxylic acid) of molecular weight of the order of 104-106.

19. A glass comprising the atoms Al; Si; Ca and/or Mg; and Zn, wherein Si:Al > 1.5:1.

20. A cement made by a process according to any of Claims 1 to 9 or by mixing the two parts of a pack according to any of Claims 10 to 19.

21. A cement according to Claim 20, wherein, in the process, the oxide is a glass according to Claim 19.