Hardenable multi-part acrylic composition

Abstract

A hardenable multi-part acrylic composition comprises a solid first part and a storage stable liquid second part and optionally, further solid and/or liquid parts, the parts being operable to form a cement which hardens to a solid mass upon mixing. The composition further comprises an acrylic monomer component in the second part, an initiator component, a first sub-population of acrylic polymer particles in the first and/or further parts, a second sub-population of acrylic polymer particles in the first and/or further parts and optionally, one or more further sub-population(s) of acrylic polymer particles, and a radiopacifying filler. The initiator component amount is effective to polymerize the acrylic monomer component upon mixing. At least some of the radiopacifying filler is encapsulated within and/or adsorbed on the first sub-population of acrylic polymer particles and the second sub-population of acrylic polymer particles has a lower average particle size than the first.

Claims

1. A hardenable multi-part acrylic composition comprising a solid first part and a storage stable liquid second part and optionally, further solid and/or liquid parts, the parts adapted to form a cement which hardens to a solid mass upon mixing of the parts together, the composition further including: an acrylic monomer component in the second part, an initiator component, a first sub-population of acrylic polymer particles in the first and/or further parts, a second sub-population of acrylic polymer particles in the first and/or further parts and optionally, one or more further sub-population(s) of acrylic polymer particles, and a radiopacifying filler, wherein: the initiator component is present in an amount effective to polymerize the acrylic monomer component upon mixing and/or activating the parts together such that at least some of the radiopacifying filler is encapsulated within and/or adsorbed on the first sub-population of acrylic polymer particles; the second sub-population of acrylic polymer particles has a lower average particle size than the first sub-population; and the first sub-population is acrylic bead polymer particles produced by suspension polymerization.

2. The hardenable multi-part acrylic composition according to claim 1, wherein the first sub-population of acrylic polymer particles and/or the second sub-population of acrylic polymer particles are in the first part.

3. The hardenable multi-part acrylic composition according to claim 1, wherein between 20 and 100% w/w of the radiopacifying filler in the composition is encapsulated within and/or adsorbed on acrylic polymer particles of the first and/or second sub-population(s) of acrylic polymer particles.

4. The hardenable multi-part acrylic composition according to claim 1, wherein the level of radiopacifying filler in the hardenable multi-part composition is between 1 and 50% w/w.

5. The hardenable multi-part acrylic composition according to claim 1, wherein lower average particle size for the second sub-population of acrylic polymer particles ranges between 0.01-30 m.

6. The hardenable multi-part acrylic composition according to claim 1, wherein the second sub-population of acrylic polymer particles comprises less than 5% w/w of radiopacifying filler.

7. The hardenable multi-part acrylic composition according to claim 1, wherein average particle size of the first and/or second sub-populations of acrylic polymer particles having encapsulated and/or adsorbed radiopacifying filler ranges from 10 m to 1000 m.

8. The hardenable multi-part acrylic composition according to claim 1, wherein the radiopacifying filler comprises zirconium dioxide, strontium carbonate, powdered tantalum, powdered tungsten, barium sulphate, and mixtures thereof.

9. The hardenable multi-part acrylic composition according to claim 1, wherein the hardenable multi-part acrylic composition is adapted for use in the treatment of humans, animals, or bone.

10. A method of producing an acrylic cement from a multi-part acrylic composition comprising: (i) mixing the multi-part acrylic composition, the multi-part acrylic composition having a solid first part and a storage stable liquid second part and optionally, further solid and/or liquid parts, and further including: an acrylic monomer component in the second part, an initiator component, a first sub-population of acrylic polymer particles in the first and/or further parts, a second sub-population of acrylic polymer particles in the first and/or further parts and optionally, one or more further sub-population(s) of acrylic polymer particles, and a radiopacifying filler; and (ii) forming an acrylic cement from the mixed multi-part acrylic composition of step (i); wherein: the initiator component is present in an amount effective to polymerize the acrylic monomer component upon being mixed such that at least some of the radiopacifying filler is encapsulated within and/or adsorbed on the first sub-population of acrylic polymer particles; the second sub-population of acrylic polymer particles has a lower average particle size than the first sub-population; and the first sub-population is acrylic bead polymer particles produced by suspension polymerization.

11. A method of producing a hardenable multi-part acrylic composition according to claim 1 comprising: a) producing an acrylic polymer composition first part and a storage stable second part according to claim 1; b) wherein step a) comprises the step of polymerizing an acrylic monomer composition to form a sub-population of acrylic polymer particles wherein the polymerization is carried out in the presence of a radiopacifying filler to encapsulate the radiopacifying filler in acrylic polymer particles.

12. A hardenable multi-part acrylic composition comprising a solid first part and a storage stable liquid second part and optionally, a third or further solid or liquid parts, the parts being operable to form a cement which hardens to a solid mass upon mixing of the parts together, the composition further including: an acrylic monomer component in the second part, an initiator component, a first sub-population of acrylic polymer beads in the first and/or a further part, and a radiopacifying filler, a second sub-population of acrylic emulsion polymerized microparticles in the first and/or a further part, wherein: the initiator component is present in an amount effective to polymerize the acrylic monomer component upon being mixed and/or activated such that at least some of the radiopacifying filler is encapsulated within and/or adsorbed on the first sub-population of acrylic polymer beads produced by suspension polymerization.

13. The hardenable multi-part acrylic composition according to claim 12, wherein the emulsion polymerized microparticles are in the form of a network of coalesced emulsion polymerized microparticles.

14. The hardenable multi-part acrylic composition according to claim 13, wherein the network of coalesced emulsion polymerized particles is a porous larger coalesced particle having a large surface area resulting from voids in said particles, and wherein the larger coalesced particles have an average surface area of between 1 and 100 m.sup.2/g.

15. The hardenable multi-part acrylic composition according to claim 14, wherein the larger coalesced particles have an average total pore volume of between 0.005 and 0.5 cm.sup.3/g.

16. The hardenable multi-part acrylic composition according to claim 12, wherein a total amount of acrylic monomer in the hardenable composition is 10-70% w/w.

17. The hardenable multi-part acrylic composition according to claim 12, wherein at least 90% w/w of the total radiopacifying filler in the composition is present in the acrylic polymer composition first part.

18. The hardenable multi-part acrylic composition according to claim 12, wherein all or substantially all of the acrylic monomer component and the radiopacifying filler are located in separate parts of the composition so that the radiopacifying filler is not substantially present in the polymer matrix of the final hardened material.

19. The hardenable multi-part acrylic composition according to claim 12, wherein at least 90% w/w of the total first or further sub-population acrylic polymer particles with encapsulated and/or adsorbed radiopacifying filler in the composition are present in the acrylic polymer composition first part.

20. The hardenable multi-part acrylic composition according to claim 12, wherein at least 90% w/w of the total second or further sub-population acrylic polymer particles with lower average particle size in the composition are present in the acrylic polymer composition first part.

21. The hardenable multi-part acrylic composition according to claim 12, wherein at least 90% w/w of the total emulsion polymerized microparticles present in the composition whether in the second or further sub-populations are present in the acrylic polymer composition first part.

22. The hardenable multi-part acrylic composition according to claim 12, wherein the multi-part composition is a bone cement composition or dental composition.

23. The hardenable multi-part acrylic composition according to claim 1, wherein the lower average particle size subpopulation(s) are emulsion polymerized microparticles.

24. The hardenable multi-part acrylic composition according to claim 23, wherein when emulsion polymerized microparticles, the Z-average particle size of the lower average particle size sub-population(s) whether the second or further sub-population(s) is preferably in the range 0.01 to 2 m or when bead particles, the mean particle size of the lower average particle size sub-population(s) whether the second or further sub-population(s), is preferably, in the range 1-30 m.

25. The hardenable multi-part acrylic composition according to claim 12, wherein at least 90% w/w of the total acrylic polymer bead with encapsulated and/or adsorbed radiopacifying filler in the composition whether in the first or further sub-populations is present in the acrylic polymer composition first part.

26. The hardenable multi-part acrylic composition according to claim 1, wherein at least 90% w/w of the total acrylic polymer bead with encapsulated and/or adsorbed radiopacifying filler in the composition whether in the first or further sub-populations is present in the acrylic polymer composition first part.

27. The hardenable multi-part acrylic composition according to claim 1, wherein a total amount of acrylic monomer in the hardenable composition is 10-70% w/w.

28. The hardenable multi-part acrylic composition according to claim 1, wherein at least 90% w/w of the total radiopacifying filler in the composition is present in the acrylic polymer composition first part.

29. The hardenable multi-part acrylic composition according to claim 1, wherein at least some of the radiopacifying filler remains encapsulated within and/or adsorbed on the first sub-population of acrylic bead polymer particles after polymerization with said monomer and initiator.

30. The hardenable multi-part acrylic composition according to claim 1, wherein at least 90% w/w of the total first or further sub-population acrylic polymer particles with encapsulated and/or adsorbed radiopacifying filler in the composition are present in the acrylic polymer composition first part.

31. The hardenable multi-part acrylic composition according to claim 1, wherein at least 90% w/w of the total second or further sub-population acrylic polymer particles with lower average particle size in the composition are present in the acrylic polymer composition first part.

32. The hardenable multi-part acrylic composition according to claim 1, wherein the multi-part composition is a bone cement composition or dental composition.

Description

BRIEF DESCIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described with reference to the accompanying figures and examples in which:

(2) FIG. 1 shows the SEM image of the spray dried powder according to the invention; and

(3) FIG. 2 shows the SEM image of a bead polymer.

EXAMPLES

(4) Characterisation Techniques

(5) The Z-average particle size of the emulsion polymerized microparticles is determined using a Malvern Zetasizer nano series S particle size analyzer.

(6) The particle size (d10, d50, d90) of the powder produced from spray drying of the emulsion polymerized microparticles is determined by a Malvern Mastersizer 2000 particle size analyser. d10, d50, d90 are standard percentile readings from the particle size analysis. d50 is the size in microns at which 50% of the sample is smaller and 50% is larger. d10 is the size of particle below which 10% of the sample lies. d90 is the size of particle below which 90% of the sample lies.

(7) Reduced viscosity (RV, dl/g) is measured in chloroform (1 wt % solution) using an Ubbelohde viscometer type OB at 25 C.

(8) w/w % residual dibenzoyl peroxide content is determined by a titration method.

(9) The mean particle size of acrylic polymer beads is determined using a Coulter LS230 laser diffraction instrument.

(10) Dough time is measured according to BS ISO 5833:2002.

(11) Flexural strength of the hardenable compositions was determined by a three-point bend test according to ISO 1567:1997.

(12) Determination of surface area is by the method of Brunauer-Emmett-Teller (BET) according to ISO 9277:2010 using a Micromeritics Tristar II 3020 instrument operating at room temperature and using nitrogen as the absorptive gas.

(13) Determination of pore volume is by the method of Barrett-Joyner-Halenda (BJH) according to DIN 66134 using a Micromeritics Tristar II 3020 instrument operating at room temperature and using nitrogen as the absorptive gas.

(14) Pore size is determined by scanning electron microscopy (SEM) according to the following test method: Sprinkle the sample of acrylic polymer particles onto a conducting self-adhesive carbon tab on a standard aluminium SEM stub. Coat the sample with a thin layer of metal (Pt) by vacuum metallization to avoid charging in the SEM instrument. SEM images are taken using a Hitachi S4500 Field Emission SEM using accelerating voltage of 3 kV and working distance of 20 mm. Imaging is carried out on several particles and representative images obtained at different magnifications

Preparative Example 1

(15) Use of emulsion polymerization and spray drying to produce coalesced emulsion polymerized microparticles of poly(methyl methacrylate) (PMMA).

(16) Emulsion Polymerization

(17) 1.0 liter of deionised water is added to a five-liter round bottomed glass flask fitted with a nitrogen inlet, condenser and electrically operated stainless steel paddle stirrer. The water is heated to 82 C. by means of an electric heating mantle whilst stirring at 392 min.sup.1. A flow of nitrogen is passed through the vapour space of the flask above the surface of the liquid.

(18) A monomer mixture is prepared consisting of 500 grams of methyl methacrylate, 1.85 grams of 1-dodecanethiol content and 5.0 grams of 75% active sodium dioctylsulphosuccinate emulsifier (trade name: Aerosol OT). These components are mixed before use.

(19) With the temperature of the water at 82 C., a polymer seed (Stage 1) is prepared by adding 50 grams of the monomer mixture to the flask followed by 10 milliliters of a 2 wt % solution of potassium persulphate in deionised water. Following a slight exotherm, the reaction proceeds for thirty minutes until the temperature returns to 82 C.

(20) The core is then grown over the polymer seed particles (Stage 2) by firstly adding 20 milliliters of a 2 wt % solution of potassium persulphate in deionised water to the flask followed by continuous addition of 350 grams of the monomer mixture over approximately 35 minutes using a peristaltic pump. The reaction proceeds for a further fifteen minutes after the completion of the monomer mixture addition until the temperature returns to 82 C.

(21) 30.0 grams of 70% active benzoyl peroxide are dissolved in the remaining 100 grams of monomer mixture. This produces a residual benzoyl peroxide (BPO) content of approximately 2 wt % in the polymer product.

(22) The BPO-containing shell is then grown over the core (Stage 3) by firstly adding five milliliters of a 2 wt % solution of potassium persulphate in deionised water to the flask followed by continuous addition of the monomer mixture containing added BPO over approximately 10 minutes using a peristaltic pump. The reaction proceeds for a further fifteen minutes after all the monomer mixture has been added until the temperature has returned to 82 C.

(23) The reactor contents are then cooled to below 40 C. and filtered through a 150 micron screen. The resultant acrylic polymer emulsion has a reduced viscosity of 2.09 dl/g and a Z-average particle size of 215 nm.

(24) Spray Drying

(25) The emulsion is isolated as a powder by spray drying using a LabPlant SD05 laboratory spray dryer. The inlet temperature is 135 C., the outlet temperature is 80 C., the latex feed rate is set at 15, a 1.0 mm jet size is employed and the maximum settings for airflow rate and air compressor pressure are used.

(26) This produces a powder with particle size d10=8.6 microns, d50=25.9 microns, d90=62.9 microns and residual benzoyl peroxide of 2.02% w/w.

Preparative Example 2

(27) Preparation of acrylic polymer beads containing approximately 40% w/w of encapsulated and/or adsorbed X-ray opacifying filler for use in preparing a hardenable composition.

(28) The preparation of acrylic beads containing encapsulated and/or adsorbed barium sulphate is carried out in a two-step process. Firstly, the barium sulphate (from Sachtleben Chemie GmbH) is dispersed in a syrup prepared from dissolving polymer in monomer, followed by the transformation of the dispersion into barium sulphate-filled acrylic polymer beads by suspension polymerization.

(29) A 20% wt solution of poly(methyl methacrylate-co-N,N-dimethylamino ethyl methacrylate) (poly(MMA-co-DMAEMA) (RV=0.5 dl/g) in MMA is prepared by dissolving 100 grams of the poly(MMA-co-DMAEMA) in 400 g of MMA at room temperature. 300 grams of this syrup is transferred to a 2 liter glass flask equipped with stainless steel anchor-type stirrer and 400 grams of barium sulphate (medical grade) is added. The flask and contents are weighed and the weight recorded. The mixture is then stirred at room temperature for 5 hours at a stirrer speed of 1500-1900 rpm. 300 grams of MMA monomer is then added and stirring is continued at 1500 rpm for a further 40 minutes. The flask is reweighed and the reduction in weight due to evaporation of MMA calculated. The calculated amount of evaporated MMA is then added to the flask along with 10 grams of benzoyl peroxide (75% active) initiator and the mixture is stirred at 1500 rpm for 15 minutes at room temperature. This forms the organic phase of the suspension polymerization.

(30) Separately, the aqueous phase of the suspension polymerization is prepared by adding 2000 ml of deionized water and 8 grams of hydroxyethyl cellulose powder (Natrosol HEC 250HR from Aqualon Ltd) to a 5 liter glass flask containing a stainless steel anchor-type stirrer. The flask contents are stirred at 400 rpm and heated to 40 C. to dissolve the hydroxyethyl cellulose. The organic phase containing the barium sulphate dispersed in a monomer/polymer syrup is then added and the contents of the reactor flask heated to 82 C. using a water bath. The polymerization is continued at 82 C. until the reactor contents experience an exotherm, typically to approximately 90-92 C. The reactor flask is then cooled and the resultant acrylic polymer beads containing encapsulated and/or adsorbed barium sulphate are filtered, washed with deionized water, dried in an air circulating oven overnight at 50 C. and sieved through a 300 micron mesh. The resultant product has an ash content of 40.2% w/w, residual benzoyl peroxide content of 1.1% w/w, mean particle size of 75 microns. The ash content represents the amount of encapsulated and/or adsorbed barium sulphate in the acrylic polymer beads.

Preparative Example 3

(31) Preparative example 2 was repeated except that the amount of encapsulated and/or adsorbed barium sulphate in the acrylic polymer beads was approximately 30% w/w.

(32) A 20% wt solution of poly(methyl methacrylate-co-N,N-dimethylamino ethyl methacrylate) (poly(MMA-co-DMAEMA) (RV=0.5 dl/g) in MMA is prepared by dissolving 100 grams of the poly(MMA-co-DMAEMA) in 400 grams of MMA at room temperature. 300 grams of this syrup is transferred to a 2 liter glass flask equipped with stainless steel anchor-type stirrer and 300 grams of barium sulphate (medical grade) is added. The flask and contents are weighed and the weight recorded. The mixture is then stirred at room temperature for 5 hours at a stirrer speed of 1500-1900 rpm. 400 grams of MMA monomer is then added and stirring is continued at 1500 rpm for a further 40 minutes. The flask is reweighed and the reduction in weight due to evaporation of MMA calculated. The calculated amount of evaporated MMA is then added to the flask along with 10 grams of benzoyl peroxide (75% active) initiator and the mixture is stirred at 1500 rpm for 15 minutes at room temperature. This forms the organic phase of the suspension polymerization, which was then carried out in the same way as example 2. The resultant product has an ash content of 29.2% w/w, residual benzoyl peroxide content of 1.18% w/w, mean particle size of 78 microns. The ash content represents the amount of encapsulated and/or adsorbed barium sulphate in the acrylic polymer beads.

Example 1

(33) This example describes the blending of spray dried emulsion polymer of preparative example 1 with acrylic polymer beads containing encapsulated and/or adsorbed X-ray opacifying filler of preparative example 2 and a portion of unfilled acrylic polymer beads to firstly prepare a solid component and then a hardenable composition.

(34) A general lab scale method of blending spray dried emulsion powder with acrylic polymer beads is to use a tumble blending approach in a suitable container. The container is typically filled to three quarters of the total volume and the blending time is typically 15 to 30 minutes. 3.6 grams of the spray dried emulsion powder of preparative example 1, 15.0 grams of the acrylic polymer beads containing encapsulated and adsorbed X-ray opacifying filler of preparative example 2 and 1.4 grams of unfilled poly(methyl methacrylate) (PMMA) beads of mean diameter 75 microns are blended together according to the above method to form a solid component.

(35) The preparation of a hardenable composition is described as follows: Before mixing, the solid and liquid components are equilibrated for at least 10 hours in an incubator at 23 C. 20.0 g of the solid component is placed into a polypropylene beaker followed by 10.0 ml (9.40 grams) of a liquid component comprising methyl methacrylate (MMA) monomer containing 60 ppm of hydroquinone (HQ) inhibitor and 1% w/w with respect to MMA of N,N-dimethyl-para-toluidine (DMPT) accelerator. Hand mixing is then carried out at 23 C. for 30 seconds using a metal spatula, whereupon the material is covered and left to stand. Periodically, the material is assessed for initial mix consistency and dough time determined. For preparing specimens for mechanical testing, doughed material is packed into moulds preconditioned at 23 C. and allowed to harden. The amount of barium sulphate in the total mixture is 20.4% w/w. The flexural strength of the resultant material is 75.0 MPa.

Example 2

(36) This example describes the blending of spray dried emulsion polymer of preparative example 1 with a mixture of the acrylic polymer beads containing encapsulated and/or adsorbed X-ray opacifying filler of preparative examples 2 and 3 to firstly prepare a solid component and then a hardenable composition.

(37) Thus, 3.6 grams of the spray dried emulsion powder of preparative example 1, 10.82 grams of the acrylic polymer beads containing encapsulated and/or adsorbed X-ray opacifying filler of preparative example 2 and 5.58 grams of the acrylic polymer beads containing encapsulated and/or adsorbed X-ray opacifying filler of preparative example 3 are blended together according to the method of example 1 to form a solid component.

(38) The preparation of a hardenable composition is described as follows: Before mixing, the solid and liquid components are equilibrated for at least 10 hours in an incubator at 23 C. 20.0 g of the solid component is placed into a polypropylene beaker followed by 10.0 ml (9.40 grams) of a liquid component comprising methyl methacrylate (MMA) monomer containing 60 ppm of hydroquinone (HQ) inhibitor and 1% w/w with respect to MMA of N,N-dimethyl-para-toluidine (DMPT) accelerator. Hand mixing is then carried out at 23 C. for 30 seconds using a metal spatula, whereupon the material is covered and left to stand. Periodically, the material is assessed for initial mix consistency and dough time determined. For preparing specimens for mechanical testing, doughed material is packed into moulds preconditioned at 23 C. and allowed to harden. The amount of barium sulphate in the total mixture is 20.4% w/w. The flexural strength of the resultant material is 77.3 MPa.

Comparative Example 1

(39) Example 1 is repeated except that the acrylic polymer beads containing encapsulated and/or adsorbed barium sulphate are totally replaced with unfilled poly(methyl methacrylate) (PMMA) beads and the barium sulphate is added as a separate powder ingredient. Additionally, no spray dried emulsion polymer powder is added.

(40) Thus, 14.0 grams of PMMA bead polymer with residual BPO 2.94% w/w and mean particle size of 39 microns were blended with 6.0 grams of barium sulphate (from Sachtleben Chemie GmbH) according to the powder blending method of example 1 to form a solid component. The preparation of a hardenable composition is described as follows: Before mixing, the solid and liquid components are equilibrated for at least 10 hours in an incubator at 23 C. 20.0 g of the solid component is placed into a polypropylene beaker followed by 10.0 ml (9.40 grams) of a liquid component comprising methyl methacrylate (MMA) monomer containing 60 ppm of hydroquinone (HQ) inhibitor and 1% w/w with respect to MMA of N,N-dimethyl-para-toluidine (DMPT) accelerator. Hand mixing is then carried out at 23 C. for 30 seconds using a metal spatula, whereupon the material is covered and left to stand. Periodically, the material is assessed for initial mix consistency and dough time determined. For preparing specimens for mechanical testing, doughed material is packed into moulds preconditioned at 23 C. and allowed to harden. The amount of barium sulphate in the total mixture is 20.4% w/w. The flexural strength of the resultant material is 50.5 MPa.

Comparative Example 2

(41) The acrylic polymer beads containing encapsulated and/or adsorbed X-ray opacifying filler of preparative example 3 were used as the only powder ingredient of the solid component (no spray dried emulsion polymer powder or unfilled acrylic polymer bead is present).

(42) The preparation of a hardenable composition is described as follows: Before mixing, the solid and liquid components are equilibrated for at least 10 hours in an incubator at 23 C. 20.0 g of the solid component is placed into a polypropylene beaker followed by 10.0 ml (9.40 grams) of a liquid component comprising methyl methacrylate (MMA) monomer containing 60 ppm of hydroquinone (HQ) inhibitor and 1% w/w with respect to MMA of N,N-dimethyl-para-toluidine (DMPT) accelerator. Hand mixing is then carried out at 23 C. for 30 seconds using a metal spatula, whereupon the material is covered and left to stand. Periodically, the material is assessed for initial mix consistency and dough time determined. For preparing specimens for mechanical testing, doughed material is packed into moulds preconditioned at 23 C. and allowed to harden. The amount of barium sulphate in the total mixture is 20.4% w/w. The flexural strength of the resultant material is 71.6 MPa.

(43) Table 1 compares the flexural strength results for the final materials prepared in the examples with the comparative examples.

(44) TABLE-US-00001 TABLE 1 Amount of barium sulphate in final Flexural cement strength Characteristics of solid component.sup.1 (% w/w) (MPa) Example 1 Mixture consisting of beads containing 20.4 75.0 encapsulated barium sulphate, unfilled beads and spray dried emulsion powder Example 2 Mixture consisting of beads containing 20.4 77.3 encapsulated barium sulphate and spray dried emulsion powder Com- Mixture consisting of unfilled beads and 20.4 50.6 parative barium sulphate powder example 1 Com- Beads containing encapsulated barium 20.4 71.6 parative sulphate example 2 Notes: .sup.1Beads containing encapsulated barium sulphate means acrylic polymer beads containing encapsulated and/or adsorbed barium sulphate. 2. Unfilled beads means acrylic polymer beads that do not contain any radiopacifying filler. 3. Spray dried emulsion powder means coalesced emulsion polymerized microparticles of PMMA.

(45) Comparison of examples 1 and 2 with comparative example 1 shows how the mechanical properties (as measured by flexural strength) of the final hardenable compositions are enhanced by the use of acrylic polymer beads containing encapsulated and/or adsorbed barium sulphate to make the final hardenable composition instead of using barium sulphate as a separate powder ingredient.

(46) Further, comparison of example 2 with comparative example 2 (which both involve use of acrylic polymer beads containing encapsulated and/or adsorbed barium sulphate) demonstrates how the inclusion of spray dried emulsion powder in example 2 leads to an enhancement of flexural strength compared to comparative example 2, and a surprising improvement in the mechanical properties of the overall material.

Example 3

(47) This example demonstrates that the spray dried powder of preparative example 1 consists of a network of coalesced emulsion polymerized microparticles which is microporous. A sample of the spray dried powder of preparative example 1 was examined by scanning electron microscopy (SEM) to show the morphology of the material. The method involves sprinkling a sample of acrylic polymer particles onto a conducting self-adhesive carbon tab on a standard aluminium SEM stub. The sample is coated with a thin layer of metal (Pt) by vacuum metallization to avoid charging in the SEM instrument. SEM images were taken using a Hitachi S4500 Field Emission SEM using accelerating voltage of 3 kV and working distance of 20 mm. Imaging was carried out on several particles and representative images obtained at different magnifications.

(48) Brunauer-Emmett-Teller (BET) surface area analysis and Barrett-Joyner-Halenda (BJH) pore volume analysis was also carried out on the powder using a Micromeritics Tristar II 3020 instrument operating at room temperature and using nitrogen as the absorptive gas.

(49) A poly(methyl methacrylate) PMMA bead polymer prepared by suspension polymerization was studied as an example of a material that is not considered to be microporous. This was Colacryl B866, obtained from Lucite International Speciality Polymers & Resins Limited (mean particle size 39 microns and reduced viscosity 2.4 dl/g).

(50) FIG. 1 shows the SEM image of the spray dried powder of preparative example 1 and demonstrates that it consists of a network of coalesced emulsion polymerized microparticles. It also shows that the material is microporous in nature, consisting of pores of 0.01-0.2 microns (10-200 nm) in diameter.

(51) FIG. 2 shows the SEM image of Colacryl B866 PMMA bead polymer at the same magnification. In contrast to FIG. 1, this image shows that the PMMA bead polymer is not microporous.

(52) Table 2 shows the results of BET surface area and BJH pore volume analysis of the two materials. It can be seen that the spray dried powder of preparative example 1 has much higher surface area and pore volume than the PMMA bead polymer, again reinforcing that fact that the network of coalesced emulsion polymerized microparticles is microporous.

(53) TABLE-US-00002 TABLE 2 Results of BET and BJH analysis of the spray dried powder of preparative example 1 and a typical PMMA bead polymer Spray Spray dried Colacryl dried powder of Colacryl B866 bead powder of preparative B866 bead polymer preparative example 1 polymer (room example 1 (room (no temperature (no temperature degassing degassing degassing degassing of of sample) of sample) of sample) sample) Surface BET Surface 0.9968 1.4586 22.2453 22.1307 Area Area: m.sup.2/g BJH Adsorption 0.420 0.863 16.776 17.406 cumulative surface area of pores between 2.0000 nm and 500.0000 nm diameter: m.sup.2/g Pore Single point 0.915 1.642 39.091 39.023 Volume adsorption total pore volume of pores (cm.sup.3/g 10.sup.3) BJH Adsorption 0.729 1.496 133.932 135.282 cumulative volume of pores (cm.sup.3/g 10.sup.3) between 2.0000 nm and 500.0000 nm diameter: BJH Desorption 0.893 1.919 138.564 139.677 cumulative volume of pores (cm.sup.3/g 10.sup.3) between 2.0000 nm and 500.0000 nm diameter:

(54) Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

(55) All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

(56) Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(57) The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.