Low density composite materials, their production and use

Abstract

Laminar structures comprising a fiber reinforced layer bonded to an expandable filler containing layer to provide improved flexural stiffness to weight ratio at lower fiber loading; a process for the manufacture of the laminar structures in which the material are selected so that migration of the expandable filler into the fiber structure of the fiber reinforced layer as they expand is minimized or prevented.

Claims

1. A laminar structure comprising a fibre reinforced layer bonded to a layer comprising an expanded filler within a cured matrix wherein the fibre reinforced layer has a thickness of from 5 mm to 50 mm and the layer comprising the expanded filler within a cured matrix has a thickness of from 4 mm to 49 mm and the laminar structure has a reduced density in comparison to the uncured laminate of from 10% to 65%.

2. A laminar structure according to claim 1 in which the fibre reinforced layer has a weight ranging from 5 to 800 gsm.

3. A laminar structure according to claim 1 in which the expanded filler particles have an increased diameter by a factor of 2 to 20 in comparison to their diameter prior to expansion.

4. A laminar structure according to claim 1 wherein said fibre reinforced layer contains from 10 to 40 wt % by weight of the structure of fibres and wherein said layer contains cured matrix in an amount which is from 0.1% to 10% by weight in relation to said expanded filler.

5. A process for the production of laminar structures according to claim 4 comprising providing a prepreg comprising a resin containing from 10 to 40 wt % fibre, coating the prepreg with a mixture comprising 0.1 wt to 5 wt % of an expandable filler in a curable matrix to provide a multi layer structure and heating the structure to a temperature in the range 80° C. to 120° C. to cure the resin and the matrix material of both the prepreg and the mixture containing the filler and to cause expansion of the expandable filler.

6. A process according to claim 5 in which the expandable filler comprises expandable microspheres and the prepreg is curable at a temperature in the range 80° C. to 120° C. and the microspheres expand at a temperature in the range 80° C. to 100° C. and the matrix containing the microspheres cures at a temperature in the range of 100° C. to 120° C.

7. A process according to claim 5 comprising providing prepreg and providing a mixture comprising an expandable filler within a curable matrix wherein the expandable microsphere starts to expand at a temperature Tstart and completes expansion at a temperature Tmax and the curable matrix cures at a temperature Tg wherein Tstart and Tmax are both lower than Tg, said process further comprising applying the mixture to the prepreg at a temperature below Tstart to provide a laminar structure and heating the laminar structure to at least Tg to cause the microspheres to expand and the matrix to cure.

8. A process according to claim 7 in which the mixture of expandable microspheres and a curable matrix has a viscosity in the range 100 Pa.Math.s to 1000 Pa.Math.s over the temperature range of 60° C. to 85° C.

9. A process according to claim 5 wherein the resin of the prepreg cures at a temperature in the range Tstart to Tg.

10. A process according to claim 5 comprising heating the laminar structure up to Tg at a rate of from 1° C. to 5° C. per second, holding the temperature at Tg or higher for from 30 to 180 seconds and then cooling the laminar structure.

11. A process for making a moulded part or article in a single moulding step, said process comprising the steps of providing a laminar structure according to claim 4 and moulding said laminar structure in a single step by compressing said laminar structure whilst heating the laminar structure to expand the expandable filler, followed by subsequent cure of the matrix, whereby the volume of the laminar structure prior to cure is smaller than the volume of the cured structure.

Description

EXAMPLE 1

(1) A standard laminate was prepared from 4 layers of standard monolithic 0/90 carbon fibrous reinforcement comprising 4 plies and a resin matrix comprising the M77 resin as supplied by Hexcel. The standard laminate has a fibre volume content of 50% by volume as determined as outlined herein before. The laminate was cured at a temperature of 120° C.

(2) A low density laminate was prepared from a resin matrix which contained a mixture of M77 resin and an expandable filler of the type EXPANCEL 920 DET 40 d25 as supplied by Akzo Nobel. This matrix was sandwiched between carbon fibrous layers 0/90 to form a core. On either side of the core, additional carbon fibrous layers were provided which were each impregnated with M77 matrix resin. The low density laminate was again cured at a temperature of 120° C.

(3) The below Table 1 shows the comparative flexural results between a standard monolithic 0°/90° carbon laminate with 50% fibre volume content and the low density laminate.

(4) TABLE-US-00001 TABLE 1 Property Standard Laminate Low Density Laminate Density 1.50 0.63 Thickness (mm) 2 4 Mass areal weight (kg/m.sup.2) 3.1 2.52 Flexural stiffness (N/mm) 11.11 31.85 Failure load (N) 205 194 Failure strength (MPa) 855 195 Failure deflexion (mm) 20 6 Flexural modulus (GPa) 56.7 17.4 Fibre volume content (%) 50 10 Fibre areal weight (kg/m.sup.2) 1.8 0.72

(5) It is particularly noticeable that with a thickness twice the thickness of the standard laminate, the product of the invention provides a 19% weight reduction while the flexural stiffness is increased by a factor of 2.87 without any reduction in maximum load and there is a 60% mass reduction of fibre weight.

EXAMPLE 2

(6) This Example demonstrates the preparation of the expanded microsphere containing resin film material.

(7) 100 parts of HexPly M10, a proprietary Dgeba-DDM epoxy resin of Hexcel Composites, was mixed with 4.5 parts of Expancel microspheres from Akzo Nobel, grade 920 DET 40 d25. This grade of Expancel is an expanded grade with a 35-55 micrometers particle size and a 25 kg/m.sup.3 true density. The mixture was prepared at room temperature (21° C.) until complete wetting of Expancel particles by M10 resin occurred. The resulting density of the mixture was 0.36 g/cc. The resulting low density M10 mixing was B-staged at 23° C. for 7 days, the B-staged low density M10 batch was then divided in blocks and heated to 65° C. in an oven.

(8) The heated blocks were provided with a resin film by a nip roll process at 65° C. The resulting film had a 75 g/m.sup.2 areal weight and was rolled with onto a silicone paper foil. The resin may then be applied to any suitable medium such as fibrous reinforcement.

EXAMPLE 3

(9) This Example illustrates the production of low density prepregs.

(10) Carbon fabric/epoxy prepregs from Hexcel Composites, HexPly M49/42%/200T2X2/CHS-3K contains M49 resin, a 120° C. cure epoxy resin from Hexcel Composites and a fabric in the form of 200T2X2/CHS-3K, a 200 g/m.sup.2 Twill 22 carbon fabric with 3K high strength standard modulus carbon fibres from Hexcel Composites. The prepregs were used to produce four grades of low density carbon prepregs by applying low density resin films containing different concentrations of expandable filler Expancel 920 DET 40 d25 onto the prepregs as follows: Grade 1—HexPly M49/42%/200T2X2/CHS-3K with a layer of 75 g/m.sup.2 low density resin on one side, the low density resin containing 100 parts by weight of M49 resin in combination with 4.5 parts by weight of expandable filler particles; Grade 2—HexPly M49/42%/200T2X2/CHS-3K with two layers of 75 g/m.sup.2 low density resin, the low density resin containing 100 parts by weight of M49 resin in combination with 4.5 parts by weight of expandable filler particles as used in Grade 1 (with 2 plies of film); Grade 3—HexPly M49/42%/200T2X2/CHS-3K with three layers of the low density resin of Grade 1 to produce a 225 g/m.sup.2 resin on one side (with 3 plies of film); Finally, Grade 4—HexPly M49/42%/200T2X2/CHS-3K with four layers of the low density resin of Grade 1 to produce a 300 g/m.sup.2 resin on one side (with 4 plies of film).

(11) Five laminates have been moulded for mechanical comparison tests. Laminate 0: composed of 9 plies of the above M49 carbon fabric hexPly prepreg Laminate 1: composed of 4 plies of Grade 1 low density carbon prepregs whereby the centre is formed by two layers of the prepreg whereby the low density resin layers are in contact with one another. The centre layer thus formed is contacted with two further Grade 1 low density carbon prepregs whereby the low density resin layers of these prepregs contact the centre layer; Laminate 2: composed of 4 plies of Grade 2 density carbon prepreg with the same lay-up sequence as described with respect to Laminate 1; Laminate 3: composed of 4 plies of Grade 3 low density carbon prepreg with the same lay-up sequence as described with respect to Laminate 2; and Laminate 4: composed of 4 plies of Grade 4 low density carbon prepreg with the same lay-up sequence as described with respect to Laminate 3

(12) Test results for the five laminates are presented in Table 2.

(13) TABLE-US-00002 TABLE 2 Lami- Lami- Lami- Lami- Lami- — nate 0 nate 1 nate 2 nate 3 nate 4 Density 1.50 0.89 0.79 0.71 0.63 Thickness mm 2.13 1.81 2.49 3.21 4.08 Fibre % 46.9 24.5 17.8 13.8 10.9 Volume Content Traction Strength 861.3 427 213.2 215 142 (MPa) Modulus 58.9 31.3 19.8 18.7 13.2 (GPa) Flexion Strength 993.5 326.5 328.5 311.7 177.3 (MPa) Modulus 58.9 33.2 27.0 23.5 17.9 (GPa) ILSS Strength 61.2 13.4 10.5 11.3 8.5 (MPa)

(14) From the data in the Table we have calculated Modulus.sup.1/3/Density to compare the flexural stiffness of the various materials. In the below Table 3 the values are presented for the five laminates in relation to the standard Laminate 0.

(15) TABLE-US-00003 TABLE 3 Laminate Laminate Laminate Laminate Laminate 0 1 2 3 4 Modulus.sup.1/3/ 2.59 3.61 3.80 4.03 4.15 Density Gain (%) Ref 39 47 55 60

(16) The product of the invention has the important economic advantage that the carbon fibre content (which is costly) may be reduced whilst still reducing the weight and improving the mechanical properties of structural material.

(17) Table 4 compares a product of this invention with products of comparable rigidity based on traditional monolithic laminar structures and glass reinforced sandwich compositions which have the same stiffness.

(18) TABLE-US-00004 TABLE 4 Monolithic (single layer of Multi-Sandwich (as impregnated carbon fiber) Laminate 4 in Table 2) Thickness (h) 2.9 mm 4 mm Weight (m.sup.2) 4.5 kg/m.sup.2 2.8 kg/m.sup.2 Flexural Modulus 50 Gpa 19 Gpa Stiffness (E .Math. h.sup.3) 1216 1216 Volume of Fibre 50% (carbon) 11% (carbon) Volume of Resin 50% 42% Volume of Air — 47% Weight of Fibre 2.61 kg/m.sup.2 (58%) 0.80 kg/m.sup.2 (29%) Weight of Resin 1.74 kg/m.sup.2 (42%) 1.99 kg/m.sup.2 (71%)