Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
1999-11-03
2002-12-17
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S480000, C428S483000, C428S500000, C428S520000, C428S522000, C428S542800, C156S278000, C156S280000, C156S313000, C156S326000
Reexamination Certificate
active
06495259
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the bonding of rubber to epoxy resin materials and composite matrices thereof. The present invention is particularly suitable for the bonding of rubber, whether vulcanized or unvulcanized, to curable epoxy resin materials or composite matrices including curable epoxy resins therein. The present invention is especially suitable for, but not limited to, the bonding of unvulcanized synthetic rubber material to an epoxy resin based composite matrix including fibrous material to form certain components of rocket motors constructed by the aerospace industry.
2. State of the Art
Certain components of rocket motors are often constructed by laying down a composite material, or matrix of materials, usually having strength-enhancing fibers embedded in a curable epoxy resin, onto a flame-resistant and, to a certain extents thermally insulative rubber substrate. The rubber substrate and the composite matrix substrate being co-cured at an elevated temperature at a preselected pressure ideally provide a final component consisting of the rubber substrate being securely bonded to the composite matrix substrate.
An exemplary component of a rocket motor constructed in such manner is the motor case assembly which forms the main portion of the rocket motor. Typically, motor cases are constructed to have an epoxy composite exterior and a flame-resistant rubber material lining the interior of the case. In many rocket motor designs, the motor case itself is the primary portion of the rocket motor wherein the epoxy composite exterior forms a rigid skin and propellant fuel is bonded or cast onto the rubber material lining installed within the interior of the rigid skin. In addition to having suitable flame-resistant properties, motor cases must be made as lightweight and as structurally rigid as is feasible because the cases can be quite large in larger motors. Thus, constructing rocket motor cases from epoxy composite materials lined with a flame-resistant rubber material is a common practice within the art due to composite materials being able to provide all of the requisite characteristics needed for rocket motor cases while the rubber material lining provides the needed flame resistance as well as provides a suitable surface to which propellant fuel can be bonded.
In constructing thermally insulated rocket motor cases, an unvulcanized synthetic rubber material such as ethylene propylene diene monomer (EPDM) rubber compound, available from a number of commercial rubber mixing facilities, including the Burke Rubber Company for example, islinitially formed about a generally cylindrically-shaped mandrel, or tooling form, in order to provide the inner size and configuration which the motor case is to have. The use of EPDM rubber material is desired for its flame resistance and thermally insulative properties as well as its low specific gravity. In other words, EPDM rubber material is very lightweight when compared to other commercially available rubber materials. Usually EPDM rubber material is first formed about the mandrel to a desired thickness. Then, typically a composite epoxy resin matrix containing carbon, graphite, boron, aramids, or other high-strength fibrous material, in the form of fibers, tows, strands, or tapes for example, immersed in, or prepregnated with, epoxy resin is hand-laid on or wound about the outer surface of the EPDM rubber by specially designed machines to provide a motor case assembly having an inner layer or lining of rubber material and a composite outer shell. Epoxy resins particularly suitable for forming the base material of the motor case's composite outer shell are typically aromatic diglycidyl ethers of bisphenol A or F variety and include exemplary widely used epoxy resins such as: EPON 828 and 826 from Shell Chemical Company; DER 332, 383, and 661 from Dow Chemical Company; and LY 9703 from Ciba-Geigy Corporation.
European Patent Application 0 486 044 A2 filed Nov. 15, 1991, entitled Damage Tolerant Composites Containing Infusible Particles, listing Hercules Incorporated as the Assignee thereof and which reference is incorporated herein, discloses such a damage-tolerant epoxy resin matrix having high-strength filaments therein.
The epoxy resin matrix, usually containing strength-enhancing fibers therein, is laid upon the outer surface of the EPDM rubber material to form an outer shell of a desired thickness, thereby creating a motor case assembly. The motor case assembly, usually including the tooling mandrel to provide support to the motor assembly, which comprises the uncured EPDM rubber material layer and the uncured fiber reinforced epoxy resin matrix layer, is typically placed in an oven or an autoclave and co-cured at an elevated temperature and at a preselected pressure which is usually the ambient, or atmospheric, pressure. As a result of co-curing the rubber material layer and the epoxy resin layer together or, more accurately, vulcanizing the rubber layer and curing the epoxy resin layer simultaneously while the rubber substrate and the epoxy substrate are in contact with each other, a generally suitable high-strength bonding of the two substrates occurs. Upon the assembly being co-cured and thus becoming bonded together, the assembly is removed from the oven, autoclave or other curing apparatus, and the removable mandrel is removed from within the now-bonded rocket motor case assembly.
The previously described rubber materials and aromatic cured epoxy resin based composite materials generally performed adequately together in forming a suitably strong and uniform bond between the rubber substrate and the epoxy resin composite substrate. However, it was discovered when bonding EPDM rubber materials, or compounds, to aliphatic type epoxy resins that the rubber-to-aliphatic epoxy bonds did not have the same bond qualities as did the rubber-to-aromatic epoxy bonds. Aliphatic epoxies are often designed to have enhanced, or at least differing, room temperature handling characteristics when compared with aromatic epoxies. Furthermore, aliphatic epoxies are generally designed to have particularly modified viscosity-related characteristics affecting the ability of fibers to be impregnated with the aliphatic epoxy resin and to have longer shelf lives in which the prepreg material could be stored prior to being used. In order to provide such desirable qualities, aliphatic epoxies have a different chemical composition which appears to significantly negatively affect their ability to be bonded to certain rubber substrates, at least when compared to the bonding of aromatic epoxy based substrates to EPDM rubber substrates. A prepreg fiber composition including such an aliphatic epoxy that is anhydride cured is discussed in U.S. Pat. No. 5,593,770, issued to Mumford et al. and entitled Chemorheologically Tailored Matrix Resin Formulations Containing Anhydride Curing Agents. The '770 patent is a continuation-in-part of U.S. Pat. No. 5,356,499, issued to Decker et al. and entitled Method For Increasing Fiber Strength Translation In Composite Pressure Vessels Using Matrix Resin Formulations Containing Surface Acting Agents, directed to a method for improving the strength of composite pressure vessels incorporating chemorheologically viscosity tailored epoxy resin formulations having certain viscosity characteristics. Both of these patents are incorporated by reference herein.
U.S. Pat. No. 5,656,703, issued to Costin et al. and entitled Coating Composition of Epoxy Resin, Metal Di(Meth)Acrylate and Poly(Meth)Acrylate, discloses a curable coating composition containing an epoxy resin, a polymethacrylate, and a metal salt to improve adhesive characteristics: of a coating on a substrate, but does not offer any teachings, or suggestions, directed to bonding substrates of differing materials together, such as rubber being bonded to an epoxy resin based substrate.
U.S. Pat. 5,763,629, issued to Fan et al. and entitled Alkoxylated Glycidyl (Meth)Acrylates and Method of Prepa
Alliant Techsystems Inc.
Dawson Robert
Robertson Jeffrey B.
TraskBritt
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