Patent Application
20150005432
Exemplary embodiments of the invention are related to composite materials and, more specifically, to fiber-reinforced composite materials.
Composite materials are well-known. One class of composite materials that is widely used in a variety of applications is the class of fiber-reinforced composites. Fiber-reinforced composites typically include a continuous phase, also called a matrix or binder, and also a discontinuous phase of fibers embedded in the binder. The fibers can be distributed into a mixture of the binder in a powder or fluid form prior to solidification, or the fibers can be in the form of a mat or fiber preform that is impregnated with binder to form the composite material. Although fiber-reinforced materials often utilize a polymer binder, other types of binders such as cement or metals can also be reinforced with fibers. Fiber-reinforced composite materials can provide significant benefits compared to homogeneous materials, including but not limited to strength, stiffness, impact resistant, strength to weight ratio, etc. However, new alternatives, which may offer performance benefits, are always welcome.
In an exemplary embodiment of the invention, a composite material comprises a matrix binder material and a plurality of elongated fiber strands, the strands comprising a longitudinally extending central portion having two or more lobes extending radially from the central portion and disposed longitudinally along the strand. The lobes can have any shape, and in some embodiments the lobes comprise a radially-extending portion and a cap portion. In some embodiments the fiber has four lobes extending radially from the central portion of the strand. Longitudinal channels are thus formed between the lobes, which can be filled with matrix binder material in the composite material.
The above features and advantages and other features and advantages of the invention are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings.
Other objects, features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which:
Referring now to
Referring now to
The fibers used in the invention, including those discussed above shown in
The dimensions of the lobes can be described as having a height, with the term ‘height’ being defined, with respect to the lobed fibers, as the distance between the outer edge of the fiber central or core portion, and the outer (radially from the center of the fiber) edge of the lobe. The height of one of the lobes 20, 40 in
Lobed fibers for fiber-reinforced composite materials can be formed from a variety of materials. In some embodiments, inorganic fibers such as glass or ceramic fibers are used, and can provide beneficial properties such as high stiffness and strength, as well as durability and ability to withstand sever processing conditions. Examples of specific inorganic materials include glass fibers such as E-glass, S-glass, etc, or ceramics such as silicon carbide. Polymeric fibers such as aramid fibers, or other known reinforcing fibers such as carbon fibers can also be used. In addition, precursor fibers such as polyacrylonitrile (“PAN”) can be formed in a lobed shape before conversion to carbon fibers. Lobed fibers can be formed by extruding the fiber material in a fluid state. Ceramic or glass materials can be heated to a fluid state (e.g., from 900 to 1100° C., depending on the softening point of the material, and then extruded through a high temperature die (e.g., formed from a high temperature ceramic or mineral). Polymer fibers can be heated to a fluid state, typically at lower temperatures than ceramics or glass (e.g., 250 to 350° C.), and can typically be extruded through metal dies. The extruded fiber, still hot and deformable, is typically routed into a cooling zone such as a cold water bath to cool and solidify the binder material in the shape of the lobed fiber imparted by the die. Some polymers can be dissolved in solvent to form a thick dope-like material that can be extruded, followed by evaporation of the solvent, although shrinkage during solvent evaporation could result in deformation of the lobed fiber.
The matrix binder material can be any of a number of materials known to be used for this purpose, including polymers such as thermoset resins such as epoxy resins, polyurethanes, etc., and also thermoplastic polymers such as acrylic polymers, polycarbonates, nylons, polyesters, etc. Other types of matrix binder materials can also be used with the selection of an appropriate fiber material. For example, Portland cement, metals (e.g., aluminum), and rubber can also be used as matrix binder materials. In the case of metals, the fiber should have a higher melting point than the metal, so the choice of fiber may be limited to certain materials like ceramics, and the fibers may be in the form of a mat that is impregnated with molten metal.
Fiber-reinforced composite materials can be prepared using a variety of techniques, as is known in the art. With some techniques, the fibers are dispersed in the binder that is in powder or fluid form and the binder is molded and cured. For example, with a thermoplastic polymer binder, the fibers can be dispersed in polymer that has been heated to its fluid state (often called a “melt”), or they can be dispersed with polymer powder that is then heated to its fluid state. The fluid polymer with fibers dispersed therein can then be formed into a fiber-reinforced composite material by conventional techniques such as extrusion, injection molding, or blow molding. With thermoset polymers, the fibers can be dispersed among the reactive components, which are then cured to form the fiber-reinforced composite material. In some embodiments, a pre-formed fiber mat can be impregnated with a fluid matrix binder material that is then cured or otherwise solidified to form the fiber-reinforced composite material. Another common technique is to impregnate a pre-formed fiber mat with a curable resin such as an epoxy resin. This article, also called a pre-preg or pre-form, can then be incorporated into a layup on a mold, optionally along with other pre-forms or pre-pregs, and subjected to heat and/or pressure to cure the resin, thereby forming the fiber-reinforced composite. Similar and/or analogous techniques can be used with other matrix binder materials such as aluminum, where, for example, molten aluminum can be cast into a mold where a pre-formed mat of high-temperature ceramic fibers is disposed.
As is known in the art, the matrix binder material and the reinforcing fibers can be selected to provide individual physical properties that cooperate to provide a desired set of properties to the fiber-reinforced composite material as a whole. In some embodiments, reinforcing fibers are used to increase the stiffness of an article formed from the composite material, and the fiber material has a Young's modulus, Ef, that is higher than the Young's modulus of the matrix binder material, Em. In some embodiments, the ratio of the modulus of the reinforcement to the matrix can be approximately 10. However, other fiber and matrix combinations that are much closer in properties can also be used such as a ratio of 8 to 12, for example. With respect to the above, Young's modulus values are at nominal ambient temperature (25° C.) or other specified operating temperature or temperature range for an article formed from the composite material. Such other operating temperatures could include temperatures from −40° C. to 200° C. or from −20° C. to 130° C., for example.
The invention is further described in the following non-limiting example.
Lobed fibers configured as shown in
The results in Table 1 show that the lobed fibers have a stronger stiffening effect than circular fibers of the same aspect ratio and volume fraction, and that the lobed fibers of lower aspect ratio and same volume fraction produce a significantly more isotropic material while maintaining stronger stiffness than typical molded circular fibers of higher aspect ratio and same volume fraction.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the present application.
