Multilayer element comprising a reinforcing material combined with a support layer by means of an electrostatic link

Abstract

The present invention concerns a multilayer element comprising a reinforcing material suitable for producing composite parts combined on at least one of the faces of same with a support layer characterized in that the reinforcing material and the support layer are combined by means of electrostatic forces, and a method for preparing such a material and a method for producing a composite part produced from at least one reinforcing material obtained from such an element, after having removed the support layer.

Claims

1. A multilayer element comprising a reinforcing material adapted to making composite parts associated on at least one of its faces with a support layer, the reinforcing material and the support layer being associated with each other by electrostatic forces, wherein, the reinforcing material does not present an adhesive nature at temperatures in the range 18° to 25° C.; and wherein, the reinforcing material consisting essentially of reinforcing fibers and by thermoplastic material, the weight of the thermoplastic material representing not more than 10% of the total weight of the reinforcing, and wherein, the electrostatic forces associating the reinforcing material with the support layer corresponds to a peeling force lying in the range 50 to 1000 mN; and wherein, the electrostatic forces associating the reinforcing material with the support layer correspond to a residual charge of voltage lying in the range 0.1 to 3 kV.

2. A multilayer element according to claim 1 wherein, the reinforcing material comprises one or more woven, non-woven, or unidirectional material fabrics comprising fibers selected from the group consisting of glass fibers, carbon fibers, aramid fibers and ceramic fibers.

3. A multilayer element according to claim 1 wherein the support layer has a thickness lying in the range 10 to 500 μm.

4. A multilayer element according to claim 1, wherein the support layer is made of an electrically insulating material.

5. A multilayer element according to claim 4, wherein the support layer presents resistivity lying in the range 10.sup.8 to 10.sup.17 Ω.Math.m.

6. A multilayer element according to claim 1 wherein the reinforcing material includes a thermoplastic binder.

7. A multilayer element according to claim 6, wherein the support layer comprises polyethylene terephthalate.

Description

(1) The examples given below with reference to the accompanying figures, serve to illustrate the invention, but they have no limiting character.

(2) FIG. 1 shows the results of voltage measurements and

(3) FIG. 2 shows the results of peeling force measurements.

EXAMPLES

(4) Multilayer elements of the invention were fabricated using: a reinforcing material constituted by a sheet of unidirectional carbon fibers (sold by the supplier Hexcel Corporation under the reference HT40 and presenting a weight of 150 grams per square meter (g/m.sup.2)) bonded on each of its faces to a web of copolyamide fibers having a thickness of 118 μm and weighing 6 g/m.sup.2 (sold by the supplier Protechnic, 41, avenue Montaigne, 68700 Cernay, France, under the reference 1R8D06, at 3 g/m.sup.2). Bonding was performed by heat using the adhesive nature of the thermoplastic web when hot, and was performed in compliance with the method described on pages 27 to 30 of application WO 2010/046609; a support layer constituted by a film of polyethylene terephthalate (sold under the reference PEPOLIT 150.8 by the supplier Effegidi International S.p.A, Via Provinciale per Sacca, 55, 43052 Colorno (Parma) Italy) having a thickness of 75 micrometers.

(5) Charges generation, and thus obtaining association with electrostatic force, were performed on samples of 150 mm×150 mm constituted by such a reinforcing material superposed with such a support layer.

(6) For this purpose, two unwinders were used: one supporting the plastics film; and another supporting the reinforcing material.

(7) The two sheets were guided and positioned one onto the other. It is important to establish contact between the two sheets as well as possible prior to entering the zone in which charge is created and thus in which electrostatic bonding occurs.

Description of Procedures

(8) Association by Creating Electrostatic Charge

(9) Use was made of a 0-30000V Fraser 7300P positive voltage generator (suitable for supplying a voltage that is adjustable over the range 0 to 30 kV at a current of 1 milliamps (mA)) and having a 7080 static electricity generator bar with a length of 300 mm (supplier Boussey Control). That bar gives off electricity from the generator in the form of a cloud of ions. The bar was positioned 25 mm above the sample. Beneath the sample, a conductive plate (aluminum angle bar) having a length of 140 mm was positioned and connected to ground, which bar extended parallel to the electricity generator bar. The conductive plate was also situated at 25 mm from the sample, which was thus at equal distances from the electricity generator bar and the conductive plate. The length of the conductive plate was selected so as to avoid projecting beyond the width of the sample, in order to avoid creating a preferred flow of ions between the bar and the plate.

(10) The sample was supported on two very fine nylon yarns, tensioned using a weight of 700 grams (g), so as to be positioned parallel to the bar and to the conductive plate. The conductive material could equally well face the generator bar or the conductive plate.

(11) The voltage selected for the generator was applied continuously for 10 seconds (s). The bar created a cloud of ions that was picked up by the outside face of the plastic film (beside the generator bar). On the opposite face (beside the reinforcement sample), a mirror image of the charges was formed. The plastics film constituted a barrier that retained the positive charge and that was thus attracted by the negative charge of the mirror image. The film was thus “stuck” against the reinforcing material by attraction between the positive and negative charges. Such attraction occurs once the applied voltage is greater than or equal to 15 kV.

(12) Measuring the Residual Charge Voltage

(13) The residual charge voltage on the sample was measured using a Fraser 715 static voltage measuring appliance. The measurements were performed in compliance with the manufacturer's recommendations, with calibration remote from a charged source, grounding, and then pointing orthogonally relative to the sample at a distance of 100 mm.

(14) Measuring the Peeling Force

(15) The sample was fastened on a plane support by means of double-sided adhesive tape in contact with the reinforcing material. A small rigid bar of width equal to the width of the film was fastened to one end of the plastics film in such a manner that the bar was perpendicular to the direction of the unidirectional fibers. A beaker was secured to the bar; water was poured progressively into the beaker using a pipette until the film separated from the reinforcing material. The unit comprising the bar, the beaker, and the water was then weighed.

(16) Results of Measuring Surface Voltage

(17) In order to perform the test, two series of six samples were produced, one at 15 kV and the other at 30 kV.

(18) The samples were all produced at the same time and manipulated only once, so as to be positioned on two tensioned nylon yarns providing support.

(19) The residual voltages were measured at defined time intervals.

(20) Regularly, a sample was taken in order to subject it to a peel strength test. Since that test is destructive, the number of samples diminished over time.

(21) FIG. 1 shows the results of voltage measurements on the samples averaged for each reading. It can thus be seen that the size of the population diminished regularly over time (from six individuals to one individual).

(22) It can thus be seen that the surface voltage drops considerably in the first few minutes after charging, regardless of whether the applied voltage was 15 kV or 30 kV. Thereafter, the voltage stabilized asymptotically around a value close to 0.3 kV, with this applying for both initial charge values.

(23) Results of Surface Voltage Measurements

(24) Certain measurements were performed on samples that had aged for several hours, whereas others were taken a few minutes after the charge generation step.

(25) FIG. 2 shows the various measurements taken: all of the unfilled-in marks were measured immediately after the charge generation step.

(26) The initial charge voltage appears to have no influence on peeling performance. A given residual surface voltage may correspond to various initial charge voltages, given that surface voltage decreases and then stabilizes over time. It is therefore possible to question the pertinence of measuring residual voltage in the first few minutes after charging. The result is subjected to variations that are too great in that time interval.

(27) In conclusion, it can be seen that: the residual surface voltage decreases quickly in the first few minutes after charging and stabilizes at a low level, with this applying regardless of the initial charge; the voltage of the initial charge, providing it is at least 15 kV, turns out to have no influence on the peeling performance, providing the peeling test is performed several tens of minutes after charging; and in the tests performed, the measured peeling force was on average equal to 11 g (i.e. 107.9 mN)±30%.

(28) Comparable results have been obtained with other types of plastics film as the support layer, and in particular with a polyester film of trademark Airtech® (reference: WL3800) having a thickness of 50 μm.