Method for manufacturing a composite part from a preimpregnated material with a semi-crystalline matrix having an amorphous surface layer

Abstract

A method for manufacturing a composite part includes preparing a stack of plies made of a starting material, applying a vacuum bag to the stack of plies, and subjecting the stack of plies to a temperature and pressure cycle in an autoclave. The starting material is a laminate material of resin matrix reinforced with a fiber material. The matrix has a core layer of semi-crystalline thermoplastic resin and a pair of outer layers of amorphous thermoplastic resin arranged on opposite sides of the core layer. The glass transition temperature of the amorphous thermoplastic resin is below the melting point of the semi-crystalline thermoplastic resin. The autoclave temperature cycle heating rapidly the stack of plies to a working temperature above the transition temperature, but below the melting point, keeping the stack of plies at the working temperature during a time period for compaction alone; and cooling the stack of plies.

Claims

1. A method for manufacturing a composite part, comprising the following steps: preparing a stack of plies, said stack comprising a plurality of plies, each of the plurality of plies being made of a starting material; applying a vacuum bag to the stack of plies, and applying vacuum within the vacuum bag; and subjecting the stack of plies to a temperature and pressure cycle in an autoclave; wherein said starting material is a laminate material comprising a resin matrix reinforced with a fiber material, wherein the matrix comprises a core layer of semi-crystalline thermoplastic resin having a melting point, and a pair of outer layers arranged on opposite sides of the core layer, each outer layer consisting of amorphous thermoplastic resin having a glass transition temperature, wherein the glass transition temperature of the amorphous thermoplastic resin is below the melting point of the semi-crystalline thermoplastic resin; wherein adjacent plies of the plurality of plies make contact with each other by the respective outer layers of amorphous thermoplastic resin; and wherein the temperature and pressure cycle in an autoclave comprises: applying pressure in an increasing manner until a working pressure is reached, and rapidly heating the stack of plies until a working temperature which is above the glass transition temperature of the amorphous thermoplastic resin but below the melting point of the semi-crystalline thermoplastic resin is reached; keeping the stack of plies at the working temperature during a time period for compaction alone; and cooling the stack of plies and releasing the pressure after the temperature has dropped below the glass transition temperature of the amorphous thermoplastic resin.

2. The method according to claim 1, wherein the rate of increase of the temperature during heating of the temperature and pressure cycle is greater than 3 C./min.

3. The method according to claim 1, wherein the time period for compaction alone during the temperature and pressure cycle is shorter than 30 minutes.

4. The method according to claim 1, wherein preparing the stack of plies comprises heat-welding at least some of the plies to one another at local points of the plies.

5. The method according to claim 1, wherein said semi-crystalline thermoplastic resin is polyether ether ketone.

6. The method according to claim 1, wherein said amorphous thermoplastic resin is poly(ether imide).

7. The method according to claim 1, wherein said fiber material comprises carbon fibers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred but non-limiting embodiments of the invention will now be described, with reference to the accompanying drawings in which:

(2) FIG. 1 shows a schematic cross-sectional view of a prepreg used as starting material for the method according to the invention; and

(3) FIGS. 2 and 3 are schematic representations which show different operative steps of the method according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) With reference to FIG. 1, a prepreg 1 to be used as starting material for the method according to the invention is shown in schematic form. This prepreg 1 consists of a laminate material comprising a matrix based on resin reinforced with a fiber material. The fiber material may be composed of fiber of any type known in the sector, for example glass fiber, carbon fiber, or a combination of these. Moreover, the fibers may be arranged in different configurations, for example in a unidirectional layer, in several layers having orientations different from each other, or as fabric. In any case, the composition and the arrangement of the fibers are not essential for the purposes of the invention.

(5) The matrix of the prepreg 1 according to the invention comprises a core layer 11 of semi-crystalline thermoplastic resin having a melting point T.sub.f. This semi-crystalline thermoplastic resin is for example polyether ether ketone, or PEEK, which has a melting point T.sub.f of about 350 C. A respective outer layer 12, 13 of amorphous thermoplastic resin having a glass transition temperature T.sub.g is applied on each of the opposite sides of the core layer 11, the glass transition temperature T.sub.g of the amorphous thermoplastic resin being below the melting point T.sub.f of the semi-crystalline thermoplastic resin. This amorphous thermoplastic resin is for example poly(ether imide), or PEI, which has a glass transition temperature T.sub.g of about 200 C.

(6) A preferred example of a prepreg of the type described above is a prepreg in which the fiber is a carbon fiber of the Intermediate Module or Intermediate Strength type (for example AS4 or IM7), and the matrix is of the thermoplastic resin type obtained by placing a PEEK film between two PEI films. Even more preferably, the prepreg has a thickness of about 0.250 mm with a percentage by weight of thermoplastic resin equal to about 37% of the overall weight of the prepreg, and a weight per unit area of the carbon fiber of about 290 g/m.sup.2, the matrix being obtained from a PEEK film having a thickness of about 0.080 mm and from two PET films each having a thickness of about 0.020 mm. The selection of these film thicknesses is particularly preferred since it is able to achieve a semi-crystalline/amorphous distribution which represents a compromise between the need to reduce the amorphous part to the minimum for preserving the structural properties, and the need to have a sufficient quantity for good adhesion between the prepregs in the lamination process.

(7) The aforementioned material may be produced for example using one of the methods described in the publication EP 2 109 532. The first of these methods described envisages that a strip of semi-crystalline thermoplastic resin film and two strips of amorphous thermoplastic resin film are respectively supplied from respective rolls. These strips are passed inside a heating chamber where they are heated to a temperature above the melting point T.sub.f of the semi-crystalline thermoplastic resin and where they are compacted using rolls into a single film. Immediately after compaction the strip of high-temperature film is transported into a constant-temperature zone where it encounters reinforcing fibers which are supplied in the form for example of fabric yarns or a strip of fabric. The multi-layer resin film is made to penetrate between the fibers by means of hot rolling Immediately after interpenetration between fiber and resin (impregnation) the article (strip of reinforced film) is cooled in a controlled manner to avoid amorphization of the semi-crystalline thermoplastic resin.

(8) The second of the methods described in EP 2 109 532 envisages that a strip of semi-crystalline thermoplastic resin film reinforced with fiber material and two strips of amorphous thermoplastic resin film are respectively supplied. This production method is similar to that described above, except for the fact that it is not required to supply fiber material, this being already present in the strip of semi-crystalline thermoplastic resin film.

(9) With reference to FIGS. 2 and 3, a method for manufacturing a composite part according to the invention is now described.

(10) A stack of plies 30 is formed in a manner known per se on a tool 20 configured according to the production requirements. Each ply 30 is formed by a part made of the preimpregnated material 1 described above. Consequently, each ply 30 makes contact with an adjacent ply 30 by means of respective outer layers 12, 13 of amorphous thermoplastic resin. Optionally, with regard to the preparation of the stack of plies 30, it is possible to heat-weld at least some of the plies 30 to each other at local points of the latter. This may be convenient in order to obtain greater stability of the stack during processing. The heat-welding process envisages applying heat to local points of the plies so that at these points a temperature above the glass transition temperature T.sub.g of the amorphous thermoplastic resin, but below the melting point T.sub.f of the semi-crystalline thermoplastic resin, is reached.

(11) A vacuum bag 40 is then applied, in a manner conventional per se, to the stack of plies 30 within which a vacuum is created. The pressure inside the vacuum bag may be for example about 0.2 atm.

(12) The stack of plies 30 with the vacuum bag 30 is then subjected to a temperature and pressure cycle in an autoclave. With regard to this cycle, the pressure inside the autoclave may be set to a value of a few atmospheres, in a similar manner to that which occurs for processes which use conventional thermosetting materials. In particular, the pressure is raised to a working pressure p.sub.w of a few atmospheres, which is reached before or during heating of the stack of plies, and then maintained until the end of the process.

(13) As regards the temperature inside the autoclave, it is envisaged that initially the stack of plies 30 is heated rapidly to a working temperature T.sub.w above the glass transition temperature T.sub.g of the amorphous thermoplastic resin, but below the melting point T.sub.f of the semi-crystalline thermoplastic resin. The term rapidly is understood as meaning that the rate of increase of the temperature during heating of the temperature and pressure cycle is greater than that which is generally applied in the case of conventional thermosetting materials, in particular higher than 3 C./min, or even much higher than 3 C./min. More specifically, it is possible to preset the process so that heating occurs with the maximum rate of increase of the temperature permitted by the plant, with a consequent substantial reduction in the process times.

(14) Following heating, the stack of plies 30 is kept at the working temperature T.sub.w for a period of time sufficient for compaction of the material. In fact, unlike in the case of thermosetting materials, the autoclave treatment is not required to produce a chemical transformation of the material, but only a compaction of the plies, as required for conventional thermoplastics. In particular, the time interval for compaction alone during the temperature and pressure cycle is shorter than 30 minutes, for example about 10 minutes.

(15) The stack of plies 30 is therefore compacted at a temperature which is relatively low, but in any case sufficient to cause melting solely of the amorphous resin layers and thus obtain tackiness of the preimpregnated laminae, while there is no melting of the semi-crystalline resin layer and therefore no problems which could be associated with this melting process and consequent control of the cooling speed occur.

(16) Finally, the stack of plies 30 is cooled. As explained above, the cooling step does not require any particular control, since the only part to have undergone transformation is the amorphous part of the layers 12 and 13 of each ply 30.

(17) With cooling of the stack the pressure is also naturally released; in particular, the pressure is released after the temperature has dropped below the glass transition temperature T.sub.g of the amorphous thermoplastic resin. Finally, removal of the vacuum bag is performed, in order to allow any further processing of the composite part thus obtained.

(18) Comparing a composite obtained from a prepreg produced as described in the above-mentioned example with a composite obtained from a conventional prepreg with PEEK and carbon fiber, for a given fiber weight per unit area and resin content, the mechanical properties of the composite obtained from the prepreg according to the invention (tension, compression, in-plane shear, open-hole tension and compression, compression after impact) are substantially equivalent to those of the composite obtained from the conventional prepreg, with the exception of the interlaminar shear properties which, for the composite obtained from the prepreg according to the invention, are no less than 90% of the interlaminar shear properties of the composite obtained from the conventional prepreg.

(19) Moreover, from the production point of view, the process is greatly simplified since it is necessary to reach, for compaction, a relatively low temperature (temperature higher than the T.sub.g of the amorphous thermoplastic, by a sufficient amount to ensure its transition and permit tackiness), in any case lower than the temperature to be reached for the conventional semi-crystalline thermoplastic prepreg. Moreover, for the conventional semi-crystalline thermoplastic prepreg, it is necessary to control the cooling temperature, whereas this is not necessary for the prepreg according to the invention.