GB2409682A

GB2409682A - Polycarboxylic acids with cenospheres

Info

Publication number: GB2409682A
Application number: GB0427515A
Authority: GB
Inventors: Huda Morgan; Peter Jonathan Samuel Foot
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-12-16
Filing date: 2004-12-16
Publication date: 2005-07-06
Also published as: GB0329056D0; GB0427515D0

Abstract

A low density, cross-linked glassy composite is formed by reacting polycarboxylic acids with cenospheres. The polycarboxylic acid is preferably polyacrylic acid or an alkylene dicarboxylic acid of the formula, HOOC(CH2)2-25COOH. The ratio of cenosphere to the polymer acid is preferably 5.0-0.5. The curing time may be accelerated by adding particles of basic oxides or hydroxides selected from calcium, magnesium, aluminium, zinc, or iron. Alternatively, dioic ore trioic aliphatic acids may be used as accelerators. In order to make the cenosphere surfaces more reactive, they may be etched using a hot aqueous alkali, such as sodium hydroxide. The cenospheres are typically sized in the range of 10-400 microns. The ceramic-like products may be suitable for use in bone repair or replacement or as structural composites.

Description

1 - 2409682 LIGHT-WEIGHT ORGANOCERAMICS FOR PROSTHETIC,

SURGICAL AND STRUCTURAL APPLICATIONS

Specification:

The present generation of materials for bone repair and replacement relies heavily on metal alloys and hydroxyapatite composites. The former are hardwearing, but inconveniently heavy, and the latter may lack durability.

This invention concerns the use of cenosphere (a cheap, inorganic industrial waste material) to produce novel ultra-light, strong glassionomer cement l0 materials suitable for applications such as bone replacements or repairs.

Conventional glass polyalkanoate (ionomer) cements are formed from ion leachable glasses and aqueous poly(acrylic acid). The acid degrades the glass, releasing metal cations (e.g. Al3+, Ca2+, Zn2+), which are chelated by the carboxylate groups of the polymer.

Cenosphere is a hard, inert and glassy material, which consists of hollow aluminosilicate spheres. It is a by-product of burning coal in electric power plants and has been used as an inert filler in materials such as cements, plastic composites and gypsum jointing compounds [R. M. CLAYTON and L. H. BACK, Journal of Engineering for Gas Turbines and Power 111 (1989) 679.]. Advantages in these existing roles include light weight, low thermal expansion, mouldability and big-compatibility. No use has previously been made of the potential for strong chemical bonding between basic sites at the cenosphere surfaces and suitable polymer host matrices; nor has cenosphere

-

previously been used as a constituent of big-materials such as bonerepair or replacement materials.

Size-selected cenosphere particles (preferably 10 to 400,um) are mixed and reacted with polymers with pendant acid functionalities (e.g. polycarboxylic acids such as polyacrylic acid), in order to produce crosslinked glassy particulate composites of very low density (400-900 kg ma). Although it is not an essential part of the present invention, the addition of suitable solid basic oxides or hydroxides to the mixture accelerates the curing time, by reactions l0 largely familiar to those versed in the art. Preferably, in order to retain good biocompatibility, these "accelerators" consist of particles of the oxides or hydroxides of calcium, magnesium, aluminium, zinc or iron, with average diameters in the range 20nm to 100 micrometres. Alternatively (or additionally), dioic or trioic aliphatic acids may be used as accelerators, for applications in which the concentration of the metallic components must be minimised.

in order to make the cenosphere surfaces more reactive, they may be treated chemically. The most effective treatment is that of surface etching with a hot (for example 50 to 100 C) dilute aqueous alkali (for example 0.1-2 M NaOH) , in order to roughen the surfaces, to deplete the concentration of acidic oxides such as SiO2 and Al2O3, and so to render the surface basic. (N.B.: It is important not to over-treat the surfaces, since this will lead to weakening or perforation of the cenospheres. Preferably a period between 1 and 20 min is found to be suitable for this purpose).

The cross-linkable acids required for use in this invention are difunctional or polyfunctional carboxylic acids defined by the following structural formulae: R1 R' * I, 1, R3 (CH2),n-COOH m=Oto10 < n < 3000 R1 = H - (CH2) x R2 = H - (CH2) y R3 = H - (CH2) z x=O, 1,2......

y=O, 1,2......

z =0, 1, 2......

HOOC -( CH2) -COOH Where n = 2 to 25 n.b no specific regio- or stereoisomers are implied by the structural formula above.

The ratio of cenosphere to polymer acid by mass may be in the range 5.0 to 0.5, but it has been found preferable to use a ratio between 1.5 and 0.75.

Where used, the mass fraction of metal oxide relative to the cenosphere may preferably be between 0.05 to 0.50, most advantageously between 0.06 and 0.3 for ZnO.

It is advantageous to mix the powder components with warm water to form a thick paste by stirring with the chosen accelerator (cross-linking agent), and the mixture is then cured for a period between 10 to 4000 seconds at a temperature of 25 to 80 C, depending on the composition. The composites produced according to the principle of this invention are durable, lightweight materials, containing constituents of low toxicity; they are suitable for use as bone substitutes/implants, for bone repair, or as general-purpose light-weight structural composites.

The principle of the present invention is illustrated (though not restrictively defined) by the following practical examples; these may readily be put into practice by those skilled in the art.

Example 1

Commercial cenosphere powder having a large range of particle sizes was sieved to produce a narrow distribution of particle sizes, approximately 44 - 53 m. This material was mixed with 35% poly(acrylic acid) solution in water at a ratio of 5:1 by mass. A solution of 30% m/v tartaric acid was prepared in 100ml of distilled water and added to the mixture at a ratio of 4:1 cenosphere powder to tartaric acid. The cement was allowed to set in cylindrical moulds for 30-60 min at a temperature of 47 - 60 C in a vacuum oven. Once they had dried, they were removed from their moulds, weighed and ready for compressive strength testing. Keeping the tartaric acid: cenosphere ratio l0 constant throughout the trials, the ratio of the cenosphere powder to poly (acrylic acid) solution was changed to 3:1 and 2:1. Three different samples for each different ratio were prepared.

Table 1: Compositions of cements prepared.* Sample Cenosphere PM Tartaric acid (Mass/g) (Mass/g) (Mass/g) 0.280 0.300 0.585 0.375 0.875 0.375 * = Masses of anhydrous constituents.

Table 2: Some physical properties of cements prepared.

Example 2

This example illustrates the use of zinc oxide as an accelerator. Cement was S made in an analogous manner to example 1 except that the tartaric acid was replaced by 0.77 9 of ZnO.

Table-3: Properties of typical cement.

Masses of ratios Ceno:PM:ZnO 6.16:1.617:0.77 Compressive strength = 11.5 MPa Young's Modulus = 3800 MPa Drying Time 85 min (Vacuum oven) at 80 C Density 835 kg me

Claims

Claims 1. Light-weight materials comprising substantially cenosphere

particles and one or more polyfunctional carboxylic acids, such that reaction at the surface of the cenospheres leads to the formation of a continuous organo-ceramic solid.
2. Materials as described in Claim 1, in which the polyfunctional carboxylic acid(s) is/are defined by the following structural formulae: R R7 * * Y R3 (CH2)m-COOH where: m = 0 to 1 0 < n < 3000 R. = H - (CH2) x R2 = H - (CH2) y R3 = H - (CH2) z x=O, 1,2 y=O, 1,2 z=O, 1,2
3. Materials as described in Claim 1, in which the polyfunctional carboxylic acid is poly(acrylic acid) or poly(methacrylic acid).
4. Materials as described in Claims 1, 2 or 3, in which at least a part of the carboxylic acid is a difunctional acid defined by the formula: HOOC -( CH2) -COOH Wheren=2to25
5. Materials as described in any of the above Claims, in which the process of curing (hardening) is accelerated by the presence of tartaric acid or a trioic acid.
6. Materials as described in any of the above Claims, in which the process of curing (hardening) is accelerated by the presence of an oxide or hydroxide of calcium, magnesium, aluminium, iron or zinc.
7. Materials as described in any of the above Claims, in which the curing process is carried out at a temperature between 25 and 80 C.
8. Materials as described in any of the above claims, in which the cenosphere particles have diameters in the range 10 to 400 micrometres.
9. Materials as described in any of the above claims, in which the cenosphere particles have been surface-treated by etching with an aqueous alkali solution.
10.The use of materials as described in any of the foregoing Claims, for the purpose of repair or replacement of natural bone, or as generalpurpose light-weight structural materials.