Composite sheet material

Abstract

A method of forming a composite sheet material, the method comprises energising a pair of electrodes to apply an electrostatic charge to a bed of fibres located therebetween thereby orienting at least some of the fibres to be substantially orthogonal to the electrodes and sandwiching at least some of the oriented fibres between a first sheet and a second sheet. The first sheet may be subsequently removed. A third sheet may be used to sandwich the fibres between the second sheet and the third sheet. Apparatus (100) is disclosed for carrying out the method.

Claims

1. A method of forming a composite sheet material, the method comprising locating a first sheet adjacent to a second of a pair of electrodes, energising the pair of electrodes to apply an electrostatic charge to a bed of fibres located therebetween thereby orienting at least some of the fibres of the bed of fibres to be substantially orthogonal to the pair of electrodes and adhered to the first sheet, and sandwiching at least some of the oriented fibres between the first sheet and a second sheet, and subsequently removing the first sheet, comprising subsequent to removing the first sheet, sandwiching at least sonic of the oriented fibres between the second sheet and a third sheet.

2. The method according to claim 1, wherein t bed of fibres is located in a first position adjacent a first of the pair of electrodes, the method further comprising positioning the first sheet between the bed of fibres and the second of the pair of electrodes, and energising the pair of electrodes causes at least some of the fibres to move from the bed of fibres toward the second electrode of the pair of electrodes such that at least some of the fibres attach or adhere to the first sheet in the orthogonal orientation.

3. The method according to claim 1, comprising positioning the second sheet with an adhesive surface thereof facing oriented fibres attached or adhered to the first sheet such that sandwiching the fibres causes at least some of the fibres to adhere to the adhesive surface of the second sheet.

4. The method according to claim 1, comprising positioning a third sheet with an adhesive surface thereof facing oriented fibres attached or adhered to the second sheet such that sandwiching them causes at least some of the oriented fibres to adhere to the adhesive surface of the third sheet.

5. The method according to claim 1, wherein the first sheet comprises a pressure sensitive or non-curable adhesive and the second sheet, comprises a structural or curable adhesive.

6. The method according to claim 1, comprising introducing resin between at least some of the oriented fibres.

7. The method according to claim 1, further comprising alternately interrupting and reapplying an electrostatic charge to the bed of fibres to orient at least some of the fibres to be substantially orthogonal to the electrodes prior to sandwiching at least some of the oriented fibres between a pair of sheets.

8. The method according to claim 1, wherein at least one of the electrodes is patterned to provide a pattern of oriented fibres in the composite sheet material.

9. The method according to claim 1, comprising moving at least one of the electrodes to orient fibres in one or more predetermined regions and/or according to a predetermined pattern to provide a pattern of oriented fibres in the composite sheet material.

Description

(1) Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

(2) FIG. 1 shows a series of images that illustrate the orientation and loading of fibres onto a first sheet in a method according to an embodiment of the invention;

(3) FIG. 2 is a schematic representation of the movement of one of the fibres during the orientation stage shown in FIG. 1;

(4) FIG. 3 illustrates the application of a second sheet to the first, oriented fibre carrying release sheet;

(5) FIG. 4 illustrates the removal of the first sheet from the second, oriented fibre carrying sheet;

(6) FIG. 5 illustrates the application of a third sheet to the second, oriented fibre carrying sheet to form a composite structure according to an embodiment of the invention;

(7) FIG. 6 is an image of the second, oriented fibre carrying sheet, which is an intermediate product of the method;

(8) FIG. 7 is a schematic representation of an apparatus according to an embodiment of the invention for forming a composite sheet material;

(9) FIG. 8 is a schematic representation of an apparatus according to another embodiment of the invention for forming a composite sheet material;

(10) FIG. 9 is a schematic representation of an apparatus according to yet another embodiment of the invention for forming a composite sheet material;

(11) FIG. 10 is a schematic representation of an apparatus according to a yet further embodiment of the invention for forming a composite sheet material; and

(12) FIGS. 11 and 12 are graphs illustrating the performance of a composite sheet formed in accordance with the present invention as compared with alternative composite structures.

(13) Referring to FIGS. 1 to 6, the invention provides a method of forming a composite sheet material using an apparatus 1 having a pair of spaced horizontal electrode or capacitor plates 10, 11. As shown in FIG. 1a, a bed 2 of fibres 20 is placed on a first, lowermost electrode plate 10 and a first, release sheet 3 is secured to a second, uppermost electrode plate 11. The release sheet 3 includes a pressure sensitive adhesive 30 on the major surface thereof that faces the first electrode plate 10. The release sheet 3 can be formed from any suitable material, such as a paper or plastic film.

(14) As shown in FIG. 1b, the electrode plates 10, 11 are then energised to apply an electrostatic charge to the bed 2, thereby causing at least some of the fibres 20 to be oriented substantially orthogonal to the electrode plates 10, 11. Continued application of the electrostatic charge causes at least some of the oriented fibres 20 to move from the bed 2 toward the second electrode plate 11. These fibres 20 adhere to the adhesive 30 of the first sheet 3 and their orientation is maintained through gravitational forces by virtue of their own weight. As shown in FIG. 1c, continued application of the electrostatic charge also causes fibres 20 to form chains depending from the first sheet 3. However, when the electrode plates 10 are de-energised the fibres 20 not adhered to the adhesive 30 of the first sheet 3 fall back into the bed 2 as shown in FIG. 1d. Optionally, power to the electrode plates 10, 11 may be cycled to alternately energise, de-energise and re-energise them in order to create a more dense array of oriented fibres 20 on the first sheet 3.

(15) FIG. 2 illustrates the electrostatic orientation of one of the fibres 20. In this embodiment, the first, lowermost electrode plate 10 is positively charged and the second, uppermost electrode plate 11 is negatively charged. Accumulation of charge is non-uniform due to the position of the fibre 20 in the bundle 2, which results in the force F.sub.1 at a first, lower end of fibre 20 being less than the force F.sub.2 at a second, higher end thereof. An initial orienting torque is therefore provided about a central point A. The force F.sub.2 at the second, higher end therefore lifts the fibre 20 toward the second electrode plate 11, reducing the distance between them and therefore increasing F.sub.2 further. The process continues until the fibre 20 is aligned with the vertical.

(16) The motion of the fibre 20 under the influence of the electrostatic field may be broken down into two components: rotational motion about the central point A, and vertical acceleration toward the second, uppermost electrode plate 11 and the positive x direction can be considered to be upward in this case.

(17) Because the initial positioning of the fibre 20 is not totally flat against the positive plate, the charge distribution will be non-symmetric, with the ‘higher’ end having a slightly greater net charge δQ.sub.2 than the net charge δQ.sub.1 at the lower end.

(18) By use of the infinite plate approximation, which is valid under the regime that the plate area is large compared to the distance between the plates, Gauss' Law can be used to find the electric field between the electrode plates 10, 11. This yields a uniform electric field E given by:

(19) $E=\frac{\sigma }{{ϵ}_0{ϵ}_r}$
where ε.sub.0 is the permittivity of free space and ε.sub.r is the relative permittivity of the dielectric. For air, ε.sub.r is approximately equal to 1.

(20) Both the charges δQ.sub.2 and δQ.sub.1 exert a torque on the fibre 20 that acts through its centre, central point A. The magnitude of the torque T produced by each end is given by:
τ=Fd
where F is the force on each end exerted by the electric field and d is the perpendicular distance from the action point of the force to the central point A. If the radius of the fibre 20 is considered to be r and its length 2L and the forces are assumed to act from the ends of the fibre 20, these forces act at a distance L from the centre. δQ.sub.2 produces an anticlockwise moment, whereas δQ.sub.1 produces a clockwise moment. As δQ.sub.2>δQ.sub.1, the anticlockwise moment is greater. The net torque τ.sub.N is given by:
τ.sub.N=(δQ.sub.2−δQ.sub.1)EL.Math.sin(θ),
where E is the electric field between the electrode plates 10, 11 as given above and θ is the angle between the axis of the fibre 20 and the vertical.

(21) This moment will cause a net rotation of the fibre 20 in an anticlockwise direction about the central point A. However, the angular momentum of the fibre 20 will carry it past the equilibrium position of vertical alignment. The resulting oscillation will be damped by friction with the fluid in between the electrode plates 10, 11 (air in this case), and may be described by the equation of motion:

(22) $I\frac{d^2\theta }{{\mathrm{dt}}^2}=-\beta \frac{d\theta }{\mathrm{dt}}-\left(\delta Q_2-\delta Q_1\right)\mathrm{EL}.\sin \left(\theta \right),$

(23) Where β is a damping constant and I is the moment of inertia of the fibre 20 about its midpoint diameter (as opposed to its major axis):
I=¼ mr.sup.2+⅓mL.sup.2.

(24) The second-order ordinary differential equation above has no analytical solution, however if it is assumed that the angular amplitude of the oscillations is ‘small’, then sin(θ)≈θ, and the differential equation may be re-written as:

(25) $I\frac{d^2\theta }{{\mathrm{dt}}^2}+\beta \frac{d\theta }{\mathrm{dt}}+\mathrm{\zeta \theta }=0.$

(26) Note that for convenience we have defined ζ as:
ζ=(δQ.sub.2−δQ.sub.1)EL.

(27) The solutions to this equation have the form:
θ=e.sup.λt,
where λ is a parameter given by:

(28) $\lambda =\frac{-\beta \pm \sqrt{{\beta }^2-4I\zeta }}{2I}$

(29) The discriminant value (β.sub.2−4Iζ) determines the type of damping that occurs. For β.sup.2>4Iζ the oscillations of the fibre 20 are overdamped, for β.sup.2<4Iζ they are underdamped and for β.sup.2=4Iζ they are critically damped.

(30) Expressed fully, we may say:

(31) $\theta \left(t\right)=e^{\frac{-\beta \pm \sqrt{{\beta }^2-4I\zeta }}{2I}t}$

(32) The net result of the rotational component of the motion will be that the fibre 20 is oriented with its length parallel to the electric field, and the end with the greater charge concentration (δQ.sub.2) will be closer to the second, negatively charged plate 11.

(33) The vertical motion of the fibre 20 is caused by the interaction of its net charge +Q with the electric field between the electrode plates 10, 11. The net force F on the fibre 20 due to this electric field is given by:

(34) $F=Q\frac{\sigma }{{ϵ}_0{ϵ}_r}$

(35) Account should be given to the viscous drag on the fibre 20 due to its passage through the dielectric, which is assumed to be linear and acts antiparallel to the motion, in the negative x direction.

(36) Account should also be given to the weight of the fibre 20, which will also act in the negative x direction whilst the buoyancy force W acts in the positive x direction.

(37) This yields the following second-order linear inhomogeneous ordinary differential equation of motion:

(38) $F=m\frac{d^{2}x}{{\mathrm{dt}}^2}=Q\frac{\sigma }{{ϵ}_0{ϵ}_r}+W-\alpha \frac{\mathrm{dx}}{\mathrm{dt}}-\mathrm{mg}$
where α is a numerical drag coefficient determined by the shape of the fibre 20, m is its mass and g is the gravitational field strength. Note that a will not have the same value as β in the rotational motion section as the cross-sections presented to the fluid by the fibre 20 will be different.

(39) Re-writing this in a form that is more straightforward to solve:

(40) $\frac{d^{2}x}{{\mathrm{dt}}^2}+\frac{\alpha }{m}\frac{\mathrm{dx}}{\mathrm{dt}}=Q\frac{\sigma }{m{ϵ}_0{ϵ}_r}+\frac{W}{m}-g$

(41) This equation has the general solution:

(42) $x\left(t\right)=\frac{\mathrm{Cm}}{\alpha }t+k_1\frac{{\mathrm{me}}^{-\frac{\alpha }{m}t}}{\alpha }+k_2$
where k.sub.1 and k.sub.2 are unknown constants, and C is the constant term, equal to:

(43) 0$Q\frac{\sigma }{m{ϵ}_0{ϵ}_r}+\frac{W}{m}-g$

(44) By use of the initial conditions that the fibre 20 is stationary to begin with

(45) $\left(\frac{\mathrm{dx}}{\mathrm{dt}}=0\right)$
and that it starts from a position adjacent the first, lowermost electrode plate 10 (x=0), we can solve for the unknown constants, and find that:

(46) $k_1=\frac{m}{\alpha }C\mathrm{and}{k}_2=-\frac{{\mathrm{Cm}}^2}{{\alpha }^2}$

(47) Thus, the linear equation of motion for the fibre 20 is:

(48) $x\left(t\right)=\frac{\mathrm{Cm}}{\alpha }\left(t+\frac{m}{\alpha }{e}^{-\frac{\alpha }{m}t}-\frac{m}{\alpha }\right)$

(49) Referring now to FIG. 3, the first sheet 3 carrying the oriented fibres 20, which can be referred to as a first preform or intermediate product IP.sub.1, is removed from the second electrode plate 11, placed on a substrate with the fibres 20 uppermost. A second sheet 4 having a structural adhesive 40 on a major surface thereof is placed on top of the fibres 20 with the adhesive 40 facing the fibres 20. In this embodiment, the second sheet 4 is formed of a carbon fibre reinforced polymer material. Pressure P is then applied to the second sheet 4 to sandwich the fibres 20 between the first and second sheets 3, 4, thereby providing a second preform or intermediate product IP.sub.2. At this point, the adhesive 40 of the second sheet 4 is preferably allowed or caused to dry or cure for a period sufficient to ensure that the fibres 20 are adhered to the second sheet 4 with a stronger bond than the pressure sensitive adhesive 30 of the first sheet 3, thereby providing a third preform or intermediate product IP.sub.3.

(50) Referring now to FIG. 4, the second or third intermediate product IP.sub.2, IP.sub.3 is overturned so that the second sheet 4 is lowermost and the first, release sheet 3 is uppermost. When the adhesive 40 of the second sheet 4 is sufficiently dried or cured, the first, release sheet 3 is then peeled off the fibres 20, leaving a fourth preform or intermediate product IP.sub.4 which is shown in FIG. 6.

(51) As illustrated in FIG. 5, a third sheet 5 having a structural adhesive 50 on a major surface thereof is placed on top of the fibres 20 with the adhesive 50 facing the fibres 20. In this embodiment, the third sheet 5 is also formed of a carbon fibre reinforced polymer material. Pressure P is then applied to the third sheet 5 to sandwich the fibres 20 between the second and third sheets 4, 5, thereby providing a fifth preform or intermediate product IP.sub.6.

(52) In one embodiment, the adhesives 40, 50 of the second and third sheets 4, 5 are allowed or caused to dry or cure completely to provide a finished composite sheet. In another embodiment, this arrangement is a sixth preform or intermediate product IP.sub.6 and fluid resin is then introduced, for example from one or more sides thereof, between the fibres 20 and the sheet is then rotated or spun S about its centre to expel excess resin. The resin is then dried or cured, for example using a heater (not shown) to provide a finished composite sheet.

(53) Referring now to FIG. 7, there is shown an apparatus 100 according to a second embodiment of the invention similar to the first embodiment, wherein like references depict like features that will not be described further herein. The apparatus 100 according to this embodiment includes a conveyor 101 with an endless belt 102 that is driven between a pair of rollers 103 and which runs between the electrode plates 10, 11. A hopper 104 supplies fibres 20 onto the conveyor to form the bed 2 upstream of the electrode plates 10, 11.

(54) The apparatus includes a supply means including first, second and third feed rollers 130, 140, 150 each carrying a respective first, second and third sheet material 131, 141, 151. The first sheet material 131 includes a pressure sensitive adhesive 132 on one major surface thereof, which is outermost on the first feed roller 130. The second sheet material 141 includes a structural adhesive 142 on one major surface thereof, which is innermost on the second feed roller 140. The third sheet material 151 also includes a structural adhesive 152 on one major surface thereof, which is outermost on the third feed roller 150.

(55) The first feed roller 130 is above the conveyor 101 and feeds the first sheet material 131 between the bed 2 of fibres 20 and the second, uppermost electrode plate 10 via a first alignment roller 133 with the adhesive 132 facing the fibres 20. The electrode plates 10, 11 are energised to orient and move the fibres 20 from the bed 2 to adhere to the adhesive 132 in an orthogonal orientation as described above in relation to the first embodiment. The first sheet material 131 then passes out of the space between the electrodes 10, 11 and any of fibres 20 which are not adhered to the first sheet material 131 fall back into the bed 2 as described above in relation to the first embodiment. The first sheet material 131 is then fed between a first pair of sandwiching rollers 134, 144. The second feed roller 140 is below the conveyor 101 and also feeds the second sheet 141 into the sandwiching rollers 134, 144 below the first sheet material 131 with its adhesive 142 facing the fibres 20. The sandwiching rollers 134, 144 apply pressure to force the fibres 20 into the adhesive 142 of the second sheet material 141. The first sheet material 131 is then fed from the uppermost sandwiching roller 134 to an exhaust roller 135 to remove it from the fibres 20. In this embodiment, the sandwiching rollers 134, 144 are heated to encourage the adhesive 142 to dry or cure sufficiently to retain the fibres as the first sheet material 131 is removed.

(56) The apparatus 100 also includes an optional resin introduction means in the form of a spray station 160 downstream of the first pair of sandwiching rollers 134, 144, which sprays resin 161 onto the layer of fibres 20 upstanding on the second sheet 141. The second sheet 141 with resin impregnated upstanding fibres 20 is then fed between a second pair of sandwiching rollers 145, 155. The third feed roller 150 feeds the third sheet 151 into the sandwiching rollers 145, 155 from above, such that the third sheet 151 is above the second sheet material with its adhesive 152 facing the fibres 20. The sandwiching rollers 145, 155 apply pressure to force the fibres 20 into the adhesive 152 of the third sheet material 151. In this embodiment, the second pair of sandwiching rollers 145, 155 are also heated to encourage the adhesive 142, 152 to dry or cure into the finished composite sheet material 160. The apparatus may also include downstream heaters for this purpose.

(57) Referring now to FIG. 8, there is shown an apparatus 200 according to a third embodiment of the invention similar to the apparatus 100 according to the second embodiment, wherein like references depict like features that will not be described further herein. The apparatus 200 according to this embodiment differs from that of the second embodiment in that two further pairs of electrodes 10′, 11′ and 10″, 11″ are included, which are spaced from one another to alternately apply, interrupt and reapply an electrostatic charge to the bed 2 of fibres 20 in order to create a more dense array of oriented fibres 20 on the first sheet material 131.

(58) Referring now to FIG. 9, there is shown an apparatus 300 according to a fourth embodiment of the invention similar to the apparatus 100 according to the second embodiment, wherein like references depict like features that will not be described further herein. The apparatus 300 according to this embodiment differs from that of the second embodiment in that the first feed roller 130 carries the third sheet material 151 and the first, release sheet material 131 is omitted. Thus, the third sheet material 151 is used to capture the fibres 20 as they are oriented by the electrode plates 10, 11 and is then fed between the first pair of sandwiching rollers 134, 144, where the fibres 20 are sandwiched between the second and third sheet materials 141, 151. The spray station 160 is located beneath the third sheet material 151 from which the oriented fibres 20 depend and sprays resin 161 thereon as it enters the sandwiching rollers 134, 144. This is a simplified process, but it may limit the selection of sheet material 151 to ensure that the electrostatic field permeates therethrough.

(59) Referring now to FIG. 10, there is shown an apparatus 400 according to a fifth embodiment of the invention similar to the apparatus 300 according to the fourth embodiment, wherein like references depict like features that will not be described further herein. The apparatus 400 according to this embodiment differs from that of the fourth embodiment in that the pair of electrodes 10, 11 is replaced with a series of three movable electrode pairs 410, 411, 410′, 411′ and 410″, 411″. Each of the electrode pairs 410, 411, 410′, 411′ and 410″, 411″ is movable relative to the bed 2 of fibres 20 in order to orient fibres 20 in one or more predetermined regions and/or according to a predetermined pattern to provide a pattern of oriented fibres 20 in the composite sheet material. It will be appreciated that the electrode pairs 410, 411, 410′, 411′ and 410″, 411″ may be used to alternately apply, interrupt and reapply an electrostatic charge to the regions of the bed 2 of fibres 20 in order to create the desired array density of oriented fibres 20.

(60) In each of the above-specified apparatus a resin spray station 160 (or other resin application means) may be provided. The resin application means may provide resin across the entire composite sheet or across portions thereof. For example, where the fibres have been patterned (for example using a patterned or localised electrode) the resin may be applied to areas where there are fibres and not to areas which are intended to provide free channels.

(61) Once the third sheet material 141 has been applied the composite product may be formed into a non-rectilinear shape and then the adhesives (and/or resin if applied) cured to retain the non-rectilinear shape. Alternatively, the composite product may be cut to a desired shape and then formed into a desired profile shape.

(62) FIG. 11 illustrates the results of a lateral compression test in which a composite sheet material A made in accordance with the method invention illustrated in FIG. 1-6 was compared with those of an aluminium honeycomb composite sheet material B and a Nomex honeycomb sheet material C. The composite sheet material A was constructed using E-glass fibres having an average length of 3 mm sandwiched between a pair of sheets formed of a unidirectional carbon fibre prepreg in an epoxy resin. The honeycomb composite sheets B, C were constructed using 3 mm cell structures also sandwiched between a pair of sheets formed of a unidirectional carbon fibre prepreg in an epoxy resin. FIG. 12 illustrates the maximum compressive strength achieved by each of the composite sheet materials A, B, C in a series of compression tests.

(63) It will be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention. It will also be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.