Composite materials combine two or more distinct materials with complementary qualities, such as for instance lightness and strength. Various composite materials are known to the skilled person. For instance, honeycomb sandwiches, combining a honeycomb core and two facing panels, in metal, polymer and/or other materials, have long been used in a number of different applications, and in particular for structural elements in the aerospace and shipbuilding fields. Other composite materials combine a solid matrix of a first material with reinforcing elements, usually fibers, of a second material embedded in the matrix. Such composite materials include ceramic matrix composites (CMC), metal matrix composites (PMC) and polymer matrix composites (PMC). Advances in various fields, such as nanotechnology, have expanded the use of these materials to many technical fields, such as power generation, construction, medical implants and prostheses, transportation, etc. This has led to further competition to increase the performances and reduce the drawbacks of these materials.
Among composite materials, polymer matrix composites (PMC) and in particular fiber-reinforced polymers (FRP), such as, among others, carbon-, glass- and/or aramid-fiber reinforced polymers are particularly widespread. Fiber-reinforced polymers offer an advantageous combination of the properties, in particular the mechanical properties, of a polymer matrix and reinforcing fibers embedded in said polymer matrix. Both thermosetting and thermoplastic polymers are commonly used as matrices in such fiber-reinforced polymers. To produce a fiber-reinforced thermosetting polymer article, the fibers are first impregnated with a resin, i.e. a prepolymer in a soft solid or viscous state, shaped into a given form, usually by molding, and the resin is then irreversibly hardened by curing. During curing, the prepolymer molecules crosslink with each other to form a three-dimensional network. To initiate or at least accelerate this crosslinking reaction, the resin is usually energized using thermal heat transfer mechanisms and/or electromagnetic excitation. On the other hand, fiber-reinforced thermoplastic polymer composites can be produced by heating a thermoplastic so that it melts and impregnates reinforcing fibers. The production of fiber-reinforced thermoplastic polymer articles normally involves a heating stage in which the material is heated in order to soften the thermoplastic and enable processes such as forming or handling. Some preforms for fiber-reinforced thermoplastic polymer articles include a so-called comingled fabric in which the reinforcing fibers are mixed with thermoplastics. In this case the impregnation step takes place during forming.
Microwave heating technology is the most promising candidate for curing, drying, thermal treatment, inspection, post-consolidation, repair and a number of other processes for composite materials. A method for heating a fiber-reinforced polymer article using microwaves was disclosed in Japanese patent publication JP H5-79208 B2. According to this first prior art method, the fiber-reinforced polymer article is held in a mold made of a similar material with substantially the same dielectric properties. The mold containing the fiber-reinforced polymer is irradiated with microwaves, whose energy is converted into heat by both the mold and the fiber-reinforced polymer inside it. However, in this method, since the mold absorbs part of the microwave radiation, the dielectric heating of the fiber-reinforced polymer article may not be sufficiently homogeneous. In particular, in a thick-walled hollow article such as a pressure tank, the inner layers of the article could be insufficiently heated as a result.
Another method for heating up a fiber-reinforced polymer article using microwaves was disclosed in Japanese patent application Laid-Open JP H11-300766 A. According to this second prior art method, the fiber-reinforced polymer article is held in a mold made of a material that is substantially transparent to microwaves. In this method, the dielectric heating by the microwave radiation is substantially limited to the fiber-reinforced polymer, rather than the mold. However, this method also has the potential drawback of insufficiently homogeneous heating, in particular in thick-walled hollow articles.
To facilitate the penetration of the microwave radiation into the thick-walled article, it may be considered to dispense with the mold. However, when the fiber-reinforced polymer article has a high fiber-to-matrix ratio, a number of the embedded fibers, and in particular loose fiber ends, may be exposed. If the fibers are electrically conductive, as for instance carbon fibers are, the currents induced in the fibers by the microwave radiation during microwave curing will generate electric sparks and arcs, which can lead to local charring or even fire.