Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-05-02
2004-05-11
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S482000, C427S485000, C427S475000
Reexamination Certificate
active
06733845
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the impregnation of a fibrous or a filamentary network with powder, especially for producing a composite comprising a continuous, rigid or flexible, matrix with which said network is in intimate contact. This invention relates not only to the composite obtained by this process but also to a preform for the composite obtained according to this process.
2. Description of the Prior Art
Composites reinforced by fibers embedded in thermoplastic matrices are a category of extremely useful materials, making it possible in particular to produce materials having excellent mechanical properties for masses substantially less than those of metals. Furthermore, these materials are obtained by simple molding, after having coated the reinforcing fibers or filaments with the thermoplastic resin intended to form the matrix of the composite. Of course, the mechanical properties of the composite thus obtained depend on the quality of the interface between the reinforcing fibers or filaments and the matrix.
This therefore assumes that there is good cohesion between the fibers or filaments and the matrix. Two factors essentially determine this cohesion; these are, on the one hand, the adhesion properties between the resin and the reinforcing fibers or filaments, that is to say the choice of the material intended to form the matrix, and, on the other hand, the void content within the composite. This second factor obviously results from the ability of the resin to infiltrate between the fibers or filaments of the fibrous mass. This is because each fiber or filament, or each portion of fiber or filament, which is not embedded in the matrix does not contribute, or contributes only partially, to the mechanical properties of the composite. Consequently, the void content correspondingly reduces the mechanical properties of the composite.
In conventional processes proposed for incorporating the thermoplastic resin into the fibrous or filamentary mass, the resin is melted in order make it penetrate in the liquid state into the fibrous or filamentary mass to be impregnated, after which this fibrous mass thus impregnated may be molded in order to give it the shape of the composite component that it is desired to obtain. The drawback of this approach stems from the difficulty in making the molten resin penetrate perfectly between the fibers or filaments on account of the viscosity of these resins.
Among the processes used for trying to remedy this drawback, the post-impregnation of fabric has already been proposed, by passing it through a bath containing the thermoplastic matrix in a suitable solvent. The drawbacks of this process are well known and are those associated with the use of solvents, namely the to need to recover the solvent, the risk of not completely recovering it and the hygiene problems. Furthermore, and something which is perhaps the most troublesome, is that it is the highest-performance resins that are inert with respect to the main solvents.
It has also been proposed to incorporate the thermoplastic matrix using a weaving process known by the name “Comingle” in which a mixture of reinforcing fibers and matrix fibers are woven. After weaving, the woven is heated, in order to make the matrix present in the form of fibers melt, and is then compressed, in order to obtain the desired component. During the heating, and during the compression, it is necessary for the matrix in the form of fibers to melt and then migrate, in order to penetrate between the reinforcing fibers. It is not obvious how to obtain a homogeneous distribution of the matrix in this way.
Various solutions have been proposed in order to try to remedy this drawback and to allow the void content to be reduced. Thus, in EP-B1-0,226,420, EP-B1-0,354,139 and EP-B1-0,466,618 it has been proposed to form yarns comprising a mixture of reinforcing fibers or filaments, such as glass, aramid or carbon reinforcing fibers or filaments, with thermoplastic fibers. Once these yarns have been placed in a mold so as to form the desired reinforcing structure, the thermoplastic of the yarn is melted to form the matrix, so that the reinforcing fibers or filaments mixed with the thermoplastic fibers are embedded in the thermoplastic matrix. Given that the thermoplastic fibers are mixed intimately in the composite yarn with the reinforcing fibers or filaments, the void content of the composite obtained is low.
The drawback of this solution is that the cost of producing such a composite yarn is high, so that this is a solution at the very most reserved for rare applications for a few top-of-the-range products or products requiring advanced technologies. On the other hand, its cost does not make this solution competitive for most applications in which the abovementioned conventional coating technique continues to be used.
It has already been proposed to introduce the matrix in powder form into a yarn or into a fibrous mass consisting of a woven or a nonwoven. Thus, a process has been proposed in which a yarn is impregnated with powder of small particle size in a fluidized bed in which the powder is held in suspension. The powder-coated yarn is then coated with a sheath of thermoplastic compatible in nature with that of the powder. This process, known by the name FIT, is used for the manufacture of a flexible prepreg capable of being woven. However, it should be noted that the flexibility of the prepreg depends on the thickness of the extruded sheath. If this sheath is very thin, the prepreg is actually flexible but the sheath is brittle; if the sheath if thick, it is less brittle but the prepreg is then less flexible.
In order for this prepreg to retain its flexibility, the powder must not melt inside the sheath. Consequently, it can migrate within the yarn during handling operations. The polymer of which the sheath is composed and which, in the end product, will contribute to the formation of the matrix, will be unable to contribute to the cohesion of the end product unless it migrates sufficiently between the reinforcing fibers and unless it is subjected to sufficiently high compression.
Another category of technique uses, in particular, fiberglass mats. These mats are impregnated with molten polymer by the calendering of thermoplastic films, by the compression molding of resin films and of mats, by the casting of molten polymer between two mats sandwiched by two calendered polymer films or else by the electrostatic spraying of powdered resin onto the mat followed by the melting of the matrix and compression of the assembly.
FR 2,258,254 has proposed a DC electrostatic spraying process for introducing powder into a fibrous material. Such a process is similar to that of electrostatic painting. The powder bonds to the first fibers that it encounters, so that it rapidly blocks the pores in the network and prevents its penetration. This is confirmed by the low fiber content of the specimens tested by the authors of that document.
O. R. Yurkevitch has described, in an article entitled “On the role of electric forces in the processing of composites prepared with polymeric matrices” in Polymer Engineering & Science, Vol. 36, No.8, Apr. 1 1996, pages 1087-1091, an impregnation process in which a fluidized bed is formed from the powder to be impregnated and this powder is at the same time charged in an electrostatic field. The powder is kept moving by the fluidized bed and the charges that are induced on the moving particles are attracted by the filaments to be coated, so that better penetration of the powder should be obtained. However, it should be noted that the tested specimens according to that document are formed from ten superimposed pre-impregnated layers which are hot pressed in order to produce the composite, a situation which does not make it possible to know whether the process does actually allow the powder to p
Caramaro Laurence
Lamure Gerard
Materials Technics Societe Anonyme Holding
Parker Fred J.
Sturm & Fix LLP
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