Dry process for manufacturing hybridized boron fiber/carbon...

Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor

Reexamination Certificate

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C156S166000, C156S180000, C156S307300, C156S307700, C427S374400, C427S398100, C427S385500

Reexamination Certificate

active

06514370

ABSTRACT:

ORIGIN OF INVENTION
This invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government or government purposes without payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the preparation and fabrication of prepreg tapes and ribbons and relates specifically to the preparation of polyimide resin/carbon fiber/boron fiber unidirectional solvent-free tape and ribbon having well wet-out and encapsulated boron fiber evenly distributed throughout the tape or ribbon, controlled dimensions and resin content, and composite structures having high mechanical properties.
2. Description of the Related Art
Reinforcing fibers comprising filaments combined with a matrix resin are known in the art and typically are called “towpregs.” A conventional towpreg consists of thousands of filaments impregnated with a continuous mass of matrix. The type of advanced reinforcing fibers typically used are available commercially in bundles of filaments known as “tows.” The number of filaments vary widely per tow and is denoted by the tow count. Many matrix resins are available that generally fall into one of two resin types within the related art: thermoplastic and thermoset polymers.
Thermoplastic polymers have been used widely as matrices for composites, and are potentially useful as matrices for advanced composites in aerospace applications. Thermoplastics have advantages over thermosetting materials in fracture toughness, impact strength, and environmental resistance. Thermoplastics also provide towpregs with indefinite shelf life, give the fabricator better quality assurance, and avoid the storage and refrigeration problems associated with thermosetting towpreg. Thermoplastic molecules are tougher than the rigid crosslinked network of the thermosets; few of the toughened thermosets have met the combined requirements of damage tolerance and hot/wet compression strength necessary for use in aerospace composites. The disadvantage of thermoplastic polymers as a composite matrix material is the difficulty of uniformly coating the fibers due to the high viscosity of the molten polymer.
Thermoset polymers also are used as matrices for towpreg. Typically, towpreg containing thermosetting prepolymer, although relatively flexible, is tacky, thus requiring a protective release coating, which must be removed before use. While thermoset towpreg is acceptable for filament winding, its tackiness and the requirement of a protective release coating make thermoset towpreg unfeasible for weaving, braiding, or the production of any chopped fiber feed stock for bulk or sheet molding compounds.
Continuous fiber towpregs can be produced by a number of impregnation methods including hot melt, solution, emulsion, slurry, surface polymerization, fiber commingling, film interleaving, electroplating, and dry powder techniques. A powder impregnation method and apparatus are disclosed in U.S. patent application Ser. No. 09/185142, filed Nov. 3, 1998, entitled Method and Apparatus to Fabricate a Fully-consolidated Fiber-Reinforced Tape from Polymer Powder Preimpregnated Fiber Tow Bundles for Automated Tow Placement (Belvin et al.), now abandoned the disclosure of which is herein incorporated by reference. U.S. patent application Ser. No. 09/185142 discloses the manufacture of a 3-inch wide product from powder pre-impregnated fiber-tow bundles that employ a number of techniques that are very specific to the fabrication of a placeable-grade 3-inch wide product using powder prepreg fiber-tow bundles as the precursor.
Precursor fabrication is completed in various techniques, such as the Powder Curtain Process (PCP) and other slurry operations. In the PCP, the resin powder is mechanically deposited onto the fiber tow bundles while being pulled through a series of process components. The use of a powder for the impregnation of fiber tow bundles creates a number of obstacles the tape manufacturer has to overcome for the fabrication of a placement grade product.
The PCP can use a powder having a particle size ranging from 3 microns to 15 microns. The particle size is important in the tape fabrication process due to the melt viscosity of the resin. The melt resembles a droplet on the tow bundle and may not completely wet-out (encapsulate) the filaments in the tows. During periods when the resin does not wet-out (encapsulate) the filaments of the tows, the shape of the tape/ribbon becomes irregular and jagged which facilitates the generation of voids. The lack of a smooth uniform surface and a large void content inhibits the placement process during the fabrication of a component.
In any of the mentioned techniques (PCP, Slurry) of powder impregnation, the main concern of the tape manufacturer is the resin content along the length of the tows. If the resin content varies to a large extent, dry areas will exist through-out the tape, and/or an over abundance of resin will be localized in one area. With dry areas, voids manifest themselves during the fabrication of the tape and are magnified during the placement process, creating a product that does not perform as predicted in operation. If the resin content is high in a localized area, the mechanical properties become more dependent on the resin than is typical for that area. A highly consistent resin application and distribution along the length of the fiber tow bundles generates a well consolidated tape product, allowing the automated tape placement machine to fabricate a low void, well consolidated part.
The significant advantages of the solution-coating method include ensuring a virtually even distribution of a coating on the towpregs and the elimination of voids during the tape fabrication and tow placement processes. The ultimate goal for almost all solution-coating applications is the ability to deposit a thin, even thickness, high quality coating as efficiently as possible. The polymeric matrix or resin also must be soluble at ambient and refrigeration storage temperatures.
Typically, towpregs made from solution-coated fiber bundles are not universally well-characterized geometrically, leading to difficulties in using such towpregs for processes when an accurate geometry is vital for the production of high-quality parts. Examples of processes which require an accurate geometry include filament winding, pultrusion, and automated tow placement, or ATP.
ATP is a process where composite ribbons or tapes are robotically managed and continually fed onto a tool or part surface and adhered by application of heat and pressure. ATP is particularly sensitive to the quality of the ribbon when considering low-flow matrix materials. The simultaneous assembly of adjacent ribbons (typically 4 to 34) or wide tape offers significant advances in the lay-up of composite materials. However, ribbons or tapes made from low-flow matrix materials typically lack a cross-sectional dimensional integrity, and more importantly, a standard rectangular cross-section. These structural defects complicate the ATP process and frequently render poor results. Although ribbons are bonded to their vertical neighbor (directly below) satisfactorily, the failure to make quality parts is generally attributed to the poor bonding of adjacent ribbons to each other. Low-flow thermoplastic parts made by using slit prepreg tapes are typically unconsolidated and exhibit excessive porosity and void content.
Ideally, tapes used in the ATP process are fully consolidated. Consolidation can be defined as the elimination of voids in a composite material during melt-processing. One method of accomplishing consolidation is pultrusion. This technique requires full ingestion of the unconsolidated composite material within an enclosed die with an exit area less than the inlet area. Within the heated closed die, processing of the polymeric matrix forces the polymer melt to flow axially to the filament array, whereas flow transverse to the filament array is generally 1/10 to 1/100 of the axia

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