Process for manufacturing composite materials

Abstract

The invention relates to a prepreg comprising a structural layer of conductive fibres comprising thermosetting resin in the interstices, and a first outer layer of resin comprising thermosetting resin, and comprising a population of conductive free filaments located at the interface between the structural layer and the outer resin layer which, when cured under elevated temperature, produces a cured composite material comprising a cured structural layer of packed conductive fibres and a first outer layer of cured resin, the outer layer of cured resin, comprising a proportion of the population of conductive free filaments dispersed therein, and to a process for manufacturing prepregs wherein the electrically conductive fibres pass a fibre disrupting means to cause a proportion of the fibres on an external face of the sheet to become free filaments.

Claims

1. A disrupted sheet of unidirectional carbon fibers tor use in making a prepreg, said disrupted sheet being made by disrupting the surface of a sheet of unidirectional fibers that comprises a total amount of unidirectional fibers which have cross-sectional diameters in the range of 3 microns to 20 microns to form said disrupted sheet, said disrupted sheet comprising: a centrally located body of unbroken unidirectional carbon fibers having a first side and a second wherein said unbroken unidirectional carbon fibers extend in a lengthwise direction; a first external face located on the first side of said body of unbroken unidirectional carbon fibers, said first external face comprising said broken unidirectional comprising fibers extending in said lengthwise direction, said broken unidirectional carbon fibers being present in an amount that is equal to from 0.5 to 5.0 wt % of the total amount of said sheet of unidirectional carbon fibers; and a second external face located on the second side of said body of unbroken unidirectional carbon fibers, said second external face comprising broken unidirectional carbon fibers in an amount that is equal to from 0.5 to 5.0 wt % of the total amount of said sheet of unidirectional carbon fibers.

2. A disrupted sheet of unidirectional carbon fibers for use in making a prepreg according to claim 1 wherein said first external face comprises free filaments that are formed during formation of said broken unidirectional fibers.

3. A disrupted sheet of unidirectional carbon fibers for use in making a prepreg according to claim 2 wherein the free filaments in said first external face have a distribution of lengths with a mean length of less than 2.0 cm.

4. A disrupted sheet of unidirectional carbon fibers for use in making a prepreg according to claim 1 wherein said second external face comprises free filaments that are fanned during formation of said broken unidirectional carbon fibers.

5. A disrupted sheet of unidirectional carbon fibers for use in making a prepreg according to claim 4 wherein the free filaments in said second external face have a distribution of lengths with a mean length of less than 2.0 cm.

6. A prepreg comprising a disrupted sheet of unidirectional carbon fibers according to claim 1 which has been combined with a thermosetting resin.

7. A prepreg according to claim 6 wherein said thermosetting resin comprises an epoxy resin.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of a spreader bar arrangement.

(2) FIG. 2 is a schematic is a schematic representation of another spreader bar arrangement.

(3) FIG. 3 is an image of a cross-section through a cured laminate made up of plies of prepreg according to the present invention.

(4) FIGS. 4a to 4d are images of cross-sections through a cured laminate made of plies of prepreg according to the present invention.

(5) FIGS. 5a to 5d are images of cross-sections through a cured laminate made of plies of prepreg outside the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) In a preferred embodiment, the conductive fibres are unidirectional fibres and the disruption means involves passing the fibres over an abrasion surface, thereby causing breakage of a proportion of the fibres on the external face passing in contact with the abrasion surface, whilst the fibres not in contact with the abrasion surface remain unbroken.

(7) It has been found that breaking from 0.5 to 5.0 wt % of the fibres in at least one location provides good results.

(8) As discussed above, unidirectional fibre sheets are typically formed from a plurality of tows of fibres, which are spread out to merge together, prior to impregnation with resin. A common method of achieving this is to pass the fibres over a plurality of sequential spreader bars or rollers.

(9) It is therefore convenient for the abrasion surface to be incorporated in an existing spreader bar arrangement. Thus, in a preferred embodiment, the abrasion surface is the surface of a spreader bar.

(10) Furthermore, it has been found that if the abrasion surface spreader bar is positioned late in the sequence of spreader bars, then further improvements in conductivity can be obtained. Thus, preferably the abrasion surface spreader bar is in the last three, preferably in the last two, and most preferably is the last spreader bar in the sequence.

(11) The abrasion surface may be made from any suitable material, such as metal or ceramic, however tungsten carbide is preferred.

(12) In a preferred embodiment, the process of the invention involves passing the sheet of electrically conductive fibres to a second fibre disrupting means to cause a proportion of the fibres on the other external face of the sheet to become free fibres.

(13) Thus, at least two spreader bars may comprise abrasion surfaces, each one in contact with each of the external faces of the sheet of conductive fibres.

(14) A number of factors determine the breakage rate of fibres passing over the abrasive surface. For example the relative speed of movement over the surface, the roughness of the surface, the tension in the fibres, the area and the time spent in contact with the surface. Also the material properties of the fibres will be a factor, particularly their sizing type and percentage.

(15) However, it has been found that the roughness of the abrasive surface is a key parameter and thus preferably the abrasive surface has an R.sub.a roughness of at least 1.5 micrometres, more preferably at least 2.5 micrometres.

(16) Another important factor is the relative speed of movement over the surface. Preferably the relative speed of movement is from 2 to 20 m/min.

(17) Once the sheet of electrically conductive fibres comprising free fibres on one or both external faces is prepared, the next stage is resin impregnation.

(18) Resin impregnation may be carried out in a wide variety of ways which will be known to the person skilled in the art. Typically it involves bringing into contact with a face of the fibres a first layer of resin, comprising thermosetting resin. This is generally followed by compressing the resin and fibres to cause impregnation to occur.

(19) In a particularly preferred embodiment the resin is applied to a roller, the fibre sheet passing over a surface of the roller and the resin detaching from the roller to the fibre sheet. Compression can conveniently be carried out also by means of passing over rollers, which can be arranged as desired.

(20) Traditionally there are two main ways of introducing resin to the fibre sheet for impregnation. The first involves introducing all the resin to the fibres in a single stage. The second involves introducing part of the resin in a first stage and the remainder in a second stage. Such one-stage and two-stage processes have been widely employed. One advantage of the two-stage process is the opportunity to introduce different materials in each of the two resin compositions, in order to achieve desired effects.

(21) For example, a widely-used two-stage process involves a first stage of impregnating the fibres with resin followed by a second stage of bringing into contact with the impregnated resin another resin composition comprising thermoplastic toughener particles. This process produces two distinct layers in the prepreg, one of impregnated fibres and one of resin comprising the thermoplastic particles. Once a plurality of such prepregs are laid-up then an alternating layered structure is formed, comprising alternate layers of impregnated structural fibres with resin interlayers comprising toughener particles. Such an arrangement is known to give good mechanical properties following curing.

(22) The good mechanical properties are generally attributed to the presence of these so-called interlayers which are free of structural fibres. However, as discussed, these interlayers also contribute to the poor electrical conductivity through the thickness of the laminate, essentially because they provide a spacing between adjacent layers of conductive fibres.

(23) In the present invention, this problem of the interlayer causing low electrical conductivity is overcome, without affecting the good mechanical performance provided by the interlayer. Thus the impregnation process can be either a one-stage or two-stage process as desired.

(24) It is highly desirable that particulate material be dispersed within the outer resin layer or interlayer.

(25) The particulate material can be made from a wide variety of materials, however preferably they provide an additional useful function such as improved toughness or conductivity. Materials which are suitable include polyamide 6, polyamide 6/12, polyamide 12, conductive coatings on particulates formed from resins such as phenolic resins or from glass beads, coatings such as silver, carbon particles and/or microparticles and others.

(26) Once prepared, the prepregs according to the invention are typically laid-up to produce a curable laminate or prepreg stack. Due to the flexible nature of the prepregs they are able to take the form of structural bodies having a wide range of shapes and contours.

(27) Thus, the prepreg according to the invention may include additional layers of electrically conductive structural fibres, typically separated by interleaf resin layers. Such a stack may comprise from 4 to 200 layers of electrically conductive structural fibres with most or all of the layers separated by a curable thermosetting resin interleaf layer. Suitable interleaf arrangements are disclosed in EP0274899.

(28) In such a stack, typically a plurality of the interleaf layers comprise a population of conductive free filaments. In a preferred embodiment at least half of the interleaf layers comprise a population of conductive free filaments. It may even be desirable for at least 75% of the interleaf layers to comprise a population of conductive free filaments or even substantially all of the interleaf layers.

(29) Once formed, the interleaf layers are typically much thinner than the structural fibre layers. Thus, the ratio of the total thickness of the structural layers to the total thickness of the interleaf layers is from 10:1 to 3:1.

(30) The structural fibres may comprise cracked (i.e. stretch-broken), selectively discontinuous or continuous fibres.

(31) When unidirectional, typically the orientation of the fibres will vary throughout the stack, for example by arranging for unidirectional fibres in neighbouring layers to be orthogonal to each other in a so-called 0/90 arrangement, signifying the angles between neighbouring fibre layers. Other arrangements such as 0+45/?45/90 are of course possible, among many other arrangements.

(32) The structural fibres may be made from a wide variety of materials, provided they are electrically conductive, such as carbon graphite, metallized polymers aramid and mixtures thereof. Carbon fibres are preferred.

(33) Likewise the filaments may be made from the same selection of materials. In a preferred embodiment the free filaments are the same material as the structural fibres.

(34) Typically the fibres in the structural layer and the free fibres will generally have a circular or almost circular cross-section with a diameter in the range of from 3 to 20 ?m, preferably from 5 to 12 ?m. The free filaments will generally also have a circular or almost circular cross-section with a diameter in the range of from 3 to 20 ?m, preferably from 5 to 12 ?m.

(35) The curable resin may be selected from epoxy, urethane, isocyanate and acid anhydride, for example. Preferably the curable resin comprises an epoxy resin.

(36) Suitable epoxy resins may comprise monofunctional, difunctional, trifunctional and/or tetrafunctional epoxy resins.

(37) Suitable difunctional epoxy resins, by way of example, include those based on; diglycidyl ether of Bisphenol F, Bisphenol A (optionally brominated), phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldelyde adducts, glycidyl ethers of aliphatic diols, diglycidyl ether, diethylene glycol diglycidyl ether, aromatic epoxy resins, aliphatic polyglycidyl ethers, epoxidized olefins, brominated resins, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or any combination thereof.

(38) Difunctional epoxy resins may be preferably selected from diglycidyl ether of Bisphenol F, diglycidyl ether of Bisphenol A, diglycidyl dihydroxy naphthalene, or any combination thereof.

(39) Suitable trifunctional epoxy resins, by way of example, may include those based upon phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, aromatic epoxy resins, aliphatic triglycidyl ethers, dialiphatic triglycidyl ethers, aliphatic polyglycidyl ethers, epoxidized olefins, brominated resins, triglycidyl aminophenyls, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or any combination thereof.

(40) Suitable tetrafunctional epoxy resins include N,N,N,N-tetraglycidyl-m-xylenediamine (available commercially from Mitsubishi Gas Chemical Company under the name Tetrad-X, and as Erisys GA-240 from CVC Chemicals), and N,N,N,N-tetraglycidylmethylenedianiline (e.g. MY721 from Huntsman Advanced Materials).

(41) Once prepared the laminates are cured by exposure to elevated temperature, and optionally elevated pressure, to produce a cured laminate.

(42) As discussed above, a proportion of the free filaments migrate from the region sandwiched between the structured fibre layer and the adjacent resin layer to become dispersed within the resin layer itself. This occurs as the laminate is heated up but before curing occurs, as the viscosity of the resin drops dramatically.

(43) Once in the resin layer, or interlayer, curing begins as the temperature rises still higher. The curing process prevents further migration of the free filaments which become locked in place in the interlayer.

(44) Thus the interlayer becomes electrically conductive due to the network of contacting free fibres. Additionally the good mechanical performance provided by the interlayer is not negatively affected.

(45) It has been found that excellent electrical conductivity can be achieved when the interlayer comprising from 1 to 15 wt % of free filaments, preferably from 1 to 10 wt %.

(46) The cured laminates produced according to the invention have remarkably low electrical resistance, with a 3 mm thick laminate of twelve plies of prepreg having an electrical resistance of less than 3?, preferably less than 2?, more preferably less than 1 ? being possible, as measured in the Z-direction according to the test method described below.

(47) Thus, in a third aspect, the present invention relates to a process of raising the temperature of a prepreg according to the present invention below that where curing occurs but sufficient to reduce the viscosity of the resin in the prepreg, and for a sufficient duration to allow a proportion of the free fibres to migrate into the outer layer of resin.

(48) The invention will now be illustrated, by way of example.

EXAMPLES

Resistance of Composite Laminates Test Method

(49) A panel is prepared by autoclave cure that is 300 mm?300 mm?3 mm in size. The layup of the panel is 0/90. Specimens (typically three to four) for test are then cut from the panel that are 36 mm?36 mm. The square faces of the specimens should be sanded (for example on a on a Linisher machine) to expose the carbon fibres. This is not necessary if peel ply is used during the cure. Excess sanding should be avoided as this will penetrate past the first ply. The square faces are then coated with an electrically conductive metal, typically a thin layer of gold via a sputterer. Any gold or metal on the sides of the specimens should be removed by sanding prior to testing. The metal coating is required to ensure low contact resistance.

(50) A power source (TTi EL302P programmable 30V/2A power supply unit, Thurlby Thandar Instruments, Cambridge, UK) that is capable of varying both voltage and current is used to determine the resistance. The specimen is contacted with the electrodes (tinned copper braids) of the power source and held in place using a clamp (ensure electrodes do not touch each other or contact other metallic surfaces as this will give a false result). Ensure the clamp has a non-conductive coating or layer to prevent an electrical path from one braid to the other. A current of one ampere is applied and the voltage noted. Using Ohm's Law resistance can then be calculated (VA). The test is carried out on each of the cut specimens to give range of values. To ensure confidence in the test each specimen is tested two times.

(51) A continuous sheet of unidirectional carbon fibres was passed through the roller arrangement shown in FIG. 1. The rollers have a chrome surface with a very low surface roughness R.sub.a of less than 1.0 micrometres. The rollers were fixed in a non-rotating manner.

(52) The carbon fibres were then impregnated with an epoxy resin formulation comprising polyamide particles in a single stage, producing a prepreg having a layer of resin impregnated carbon fibres and an outer layer of resin comprising the polyamide particles.

(53) The prepregs were then cut to size and stacked together in a 0/90 arrangement symmetric from the center, with 12 plies. They were then cured by heating until fully cured.

(54) The resulting cured laminate was then tested for its electrical conductivity according to the above method.

(55) Subsequently, the rollers marked A and B were exchanged for tungsten carbide rollers having roughened surfaces of 3.0 micrometres and 6.0 micrometres in a variety of combinations and composite laminates made and tested in the same manner.

(56) The results are shown in table 1 below.

(57) TABLE-US-00001 TABLE 1 Spreader bar A Spreader bar B Electrical resistance, ohms Smooth Smooth 1.47 Rough, 6 ?m Smooth 0.87 Rough, 3 ?m Smooth 0.96 Rough, 6 ?m Rough, 3 ?m 0.47

(58) The dramatic effect of providing roughened spreader bars on the electrical resistance of the eventual cured laminate can be clearly seen.

Example 2

(59) A continuous sheet of unidirectional carbon fibres was passed through the roller arrangement shown in FIG. 2. The rollers have a chrome surface with a very low surface roughness R.sub.a of less than 1.0 micrometres. The rollers were allowed to freely rotate.

(60) The carbon fibres were then impregnated with an epoxy resin formulation comprising polyamide particles in a single stage, producing a prepreg having a layer of resin impregnated carbon fibres and an outer layer of resin comprising the polyamide particles.

(61) The prepregs were then cut to size and stacked together in a 0/90 arrangement, symmetric from the center with 12 plies. They were then cured by heating until fully cured.

(62) The resulting cured laminate was then tested for its electrical conductivity according to the above method.

(63) Subsequently, the rollers marked A and B and C were exchanged for tungsten carbide rollers having roughened surfaces of 3.0 micrometres and 6.0 micrometres in a variety of combinations and composite laminates made and tested in the same manner. Some of the laminates were also tested for their mechanical performance.

(64) The results are shown in table 2 below.

(65) TABLE-US-00002 TABLE 2 Electrical resistance, ILSS Multiangle A B C ohms (88 MPa) tensile (1240 MPa) UTS (2980 MPa) Smooth Smooth Smooth 3.8 93 1154 2676 6 ?m Smooth Smooth 1.1 106 1159 2518 6 ?m 8 ?m Smooth 1.0 6 ?m 8 ?m 9 ?m 1.05 98 1173 2704 9 ?m Smooth Smooth 1.2 9 ?m 8 ?m Smooth 0.8

(66) The dramatic effect of providing roughened spreader bars on the electrical resistance of the eventual cured laminate can be clearly seen. Additionally breakage of a proportion of the structural fibres has no measurable effect on mechanical performance.

(67) The laminate produced with spreader bar A at 6 micrometres was sectioned and an image of its cross-section taken, as shown in FIG. 3. FIG. 3 clearly shows the presence of free filaments in the interlayer of the laminate.

(68) FIGS. 4a to 4d show further images of a sample of cross-sections, at a variety of scales, of laminates produced with rough spreader bars as shown in table 2. The presence of free filaments in the interlayer can be clearly seen.

(69) FIGS. 5a to 5d show images of a sample of cross-sections, at a variety of scales, of laminates produced with smooth spreader bars as shown in table 2. No filaments can be seen in the interlayer region.

(70) Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited by the above-described embodiments, but is only limited by the following claims.