GB2359499A

GB2359499A - Sphere for use in composite material

Info

Publication number: GB2359499A
Application number: GB9930677A
Authority: GB
Inventors: Robert Kenneth Oram
Original assignee: Balmoral Group Ltd
Current assignee: Balmoral Group Ltd
Priority date: 1999-12-24
Filing date: 1999-12-24
Publication date: 2001-08-29
Also published as: GB9930677D0

Abstract

A sphere for use in a composite material comprises (i) a core (1), (ii) a first layer (2) comprising polymeric material containing glass, mineral or carbon fibres, (iii) a second layer (3) comprising polymeric material comprising natural fibres and (iv) a third layer (4) comprising polymeric material containing glass flakes. The core may comprise expanded polystyrene. The polymeric material may be epoxy resin. The natural fibre, which may be acetylated, may be coir, jute, hemp, wood fibre, sisal, tow, kenaf or straw. The spheres may be dispersed in a matrix of syntactic foam to form a composite material for sub-sea use for example for buoyancy or insulation.

Description

2359499 1 2 3 4 This invention relates to composite materials and methods

of 5 their manufacture. More especially but not exclusively the 6 invention relates to composites for sub-sea use for for example 7 buoyancy or insulation.

8 COMPOSITE MATERIAL 9 Known composites can comprise a matrix of for example epoxy 10 resin. Bubbles may be dispersed in the matrix. In some 11 composites the bubbles are simply air or gas bubbles. In some 12 composites the bubbles are encased for example using 13 microspheres of glass or polymeric material. Where the bubbles 14 are encased the composite may be known as a syntactic material. 15 Typically microspheres have a diameter in the range of 15 to 16 20Ogm.

17 18 19 20 21 22 23 24 25 To decrease still further the density of the composite it is known to include other larger buoyant objects in the matrix. In view of the high forces which may be imposed on the objects they are generally spherical. They are known as macrospheres or minispheres depending upon their size. Macrospheres may have a diameter of the order of 20cm while minispheres have a diameter of the order of 1cm. Macrospheres may be made of 1 metal (as for example described in WO 99/44 881), glass, ceramic 2 or plastics material as described in GB 2 167 017.

3 A common material from which spheres are made is fibre reinforced plastics material for example glass fibre re-inforced 6 epoxy resin. A problem with this material is that the spheres may 7 fail by buckling. To reduce the likelihood of this kind of failure hitherto it has been necessary to provide a heavy wall to the sphere. The heavy wall increases the density of the sphere which is not required. The invention seeks to reduce this problem.

8 9 10 11 12 According to the invention there is provided a sphere for use in a 13 composite material the sphere comprising 14 i) a core 15 ii) a first layer comprising polymeric material containing glass, mineral or carbon fibres, iii) a second layer comprising polymeric material containing natural fibres, and iv) a third layer comprising polymeric material containing glass flakes.

21 22 According to the invention there is provided a method of making 23 a composite material comprising the steps of 24 a) dispersing spheres of the invention in a syntactic foam precursor matrix, and 1 2 b) curing the precursor.

3 7 11 12 13 4 According to the invention there is provided a method of making 5 a sphere for use in a composite material the method comprising i) providing a core, ii) providing a first layer comprising polymeric material containing glass, mineral or carbon fibres around the core, iii) providing a second layer comprising polymeric material containing natural fibres around the first layer, and iv) providing a third layer comprising polymeric material containing glass flakes around the second layer.

14 According to the invention there is provided a syntactic foam 15 having spheres of the invention dispersed therein.

16 17 It has been unexpectedly found that by replacing at least a 18 proportion of the fibres with fibres derived from natural products 19 good results can be achieved. Examples of suitable fibres include 20 coir, jute, hemp, wood fibre, sisal, tow and straw especially when 21 acetylated as for example as described in WO 98 01 497 and WO 22 97 19 979 and kenaf.

1 An example of the invention will now be described be way of non2 limiting example by reference to the accompanying figure which 3 is a cross section of a sphere.

4 As an initial step polystyrene beads are expanded in known way 6 to give a core 1. The invention is not restricted to polystyrene 7 beads. The beads form a base on which a coating (to be described 8 hereinafter in more detail is formed). The coating provides 9 strength and water resistance to the sphere. Other materials preferable of low density can be used.

11 12 A mixture of a polymer or polymer precursor such as epoxy resin 13 in which is incorporated one or more of glass, mineral and carbon fibres is applied to the beads for example by mixing in a drum to give a first layer 2.

16 17 A second coating is then applied to give a second layer 3. This 18 coating can comprise a polymer or polymer precursor such as 19 epoxy resin in which is incorporated a natural fibre.

21 A third coating is next applied to give a third layer 4. This can comprise glass flakes dispersed in a polymer or polymer 23 precursor such as epoxy. Those skilled in the art will be able to 24 devise other suitable dispersed solids which are generally in plate 25 form. Examples could include micas and other dispersants.

1 2 A coating may be at least partially cured before application of the 3 next. During process the core may collapse if processing is 4 conducted at a sufficiently high temperature. Collapse is not 5 important since it is the coatings which provide strength and 6 water resistance to the sphere.

7 8 Beads or spheres formed in accordance with the invention are 9 both strong and lightweight. Accordingly they are superior to 10 conventional beads prepared by a single coating step. While we 11 do not wish to be bound by this theory we believe that in use 12 spheres may fail by buckling. Resistance to buckling is a function 13 of shell thickness to a greater extent than it is a function of 14 material elastic modulus. It is generally accepted that resistance 15 to buckling is proportional to the elastic modulus in tension, E, 16 of the material but to the square of the material thickness. Glass, 17 carbon and mineral fibres are dense and hence a thick, buckling 18 resistant, coating is heavy which reduces the buoyancy of the 19 composite. Natural fibres such as acetylated coir or jute are of 20 lower density and hence a thick layer is not as heavy as if 21 conventional fibres were used.

22 23 Desirably the natural fibre containing material is not applied 24 directly to the initial bead. The reason for this is that the fibres 25 then tend to be positioned radially. This makes it difficult to 1 make a non porous bead. It is believed, although we do not wish 2 to be bound by this theory, that this is due to the very low 3 density of the bead. Once the bead has a density of about 4 200kgm-3this tends not to be a problem natural fibre containing 5 material can then be applied.

6 7 The natural fibre tends to comprise a bundle of smaller fibres. If 8 the outer surface of the bead has exposed fibre bundles water 9 can, under conditions of prolonged exposure to water pressure, 10 wick down the fibre and penetrate the sphere.

12 Applying an outer coating comprising glass fibres reduces this 13 problem. While however glass fibres can be monofilaments they 14 may not bond well to the polymer matrix. Water can then 16 penetrate the bead. Better performance can be achieved by using 17 an outer coating containing glass flakes since the flakes are 18 relatively two-dimensional and tend to lay flat to the bead 19 surface. A composite coating comprising multiple layers of 20 21 overlapping glass flakes, all fully encapsulated in an epoxy resin 22 23 24 25 matrix has a very low water permeation rate.

Claims

1 Claims

2 3 1. A sphere for use in a composite material the sphere 4 comprising i) a core 6 ii) a first layer comprising polymeric material containing 7 glass, mineral or carbon fibres, 8 iii) a second layer comprising polymeric material containing 9 natural fibres, and iv) a third layer comprising polymeric material containing 11 glass flakes.

12 13 2. A sphere as claimed in claim 1 wherein the natural fibre is 14 selected from coir, jute, hemp, wood fibre, sisal, tow, kenaf and straw.

16 17 18 19 20 21 22 23 5. A composite material comprising spheres as claimed in any 24 one of the preceding claims dispersed in a matrix of syntactic foaM.

3. A sphere as claimed in claim 1 or claim 2 wherein the natural fibre is acetylated.

4. A sphere as claimed in any one of the preceding claims wherein the polymeric material comprises epoxy resin.

1 4 6. A method of making a composite material comprising the steps of a) dispersing spheres as claimed in any on 5 in a syntactic foam precursor matrix, and 6 7 8 11 12 13 14 is 16 17 e b) curing the precursor.

of claims 1 to A 9 7. A method of making a sphere for use in a composite 10 material the method comprising i) providing a core, ii) providing a first layer comprising polymeric material containing glass, mineral or carbon fibres around the core, iii) providing a second layer comprising polymeric material containing natural fibres around the first layer, and iv) providing a third layer comprising polymeric material containing glass flakes around the second layer.

18 21 22 8. A method as claimed in claim 7 wherein the natural fibre is selected from coir, jute, hemp, wood fibre, sisal, tow, kenaf and straw.

23 9. A method as claimed in claim 7 or claim 8 wherein the 24 natural fibre is acetylated.

1 9 11 12 13 14 16 17 18 19 1 10. A method as claimed in any one of claims 7 to 9 wherein the 2 polymeric material comprises epoxy resin.

3 4 11. A method as claimed in any one of claims 7 to 10 wherein the core comprises expanded polystyrene.

6 7 12. A syntactic foam having spheres as claimed in any one of 8 claims 1 to 4 dispersed therein.