Engineering topics

Meador, Michael A.

Publications and source records attributed to Meador, Michael A..

71 records · Page 4

NASA NTRS1999

An Approach to Processable Polyimides

The use of polymer matrix composites (PMC's) in aircraft engines can lead to substantial weight savings over metals. This weight reduction correlates into better fuel economy, increased speed, and increased passenger load. Typically, high performance PMC's possess high thermal-oxidative stabilities (TOS) and high glass transition temperatures (Tg's) to withstand temperatures up to 316 C (600 F). One of the leading high temperature resins system available today is PMR-15 (Polymerization of Monomeric Reactants, MW=1500). This thermosetting polyimide utilizes addition curing through polymer endcaps which enables hand lay-up processing of carbon fiber composite parts with low void contents. However, the large amount of hand labor raises manufacturing costs and prohibits the use of PMR-15 in many aerospace applications. Resin Transfer Molding (RTM) provides an economical alternative, but it requires a melt Viscosity of less than 10(exp 3) centipoise (cP). This is much lower than the minimum melt viscosity of PMR-15 (about 10(exp 6) cP). To improve the processability of polyimides, the polymer backbone can be modified by incorporating flexible linkages, such as branching. bulky pendant groups, kinked structures, and twisted or non-coplanar moietes . The focus of this paper will be the introduction of non-coplanar biaryls into the PMR polyimide backbone to increase processability while maintaining high temperature performance.

Gariepy, Christopher A.↗

NASA NTRS1998

"Green" High-Temperature Polymers

PMR-15 is a processable, high-temperature polymer developed at the NASA Lewis Research Center in the 1970's principally for aeropropulsion applications. Use of fiber-reinforced polymer matrix composites in these applications can lead to substantial weight savings, thereby leading to improved fuel economy, increased passenger and payload capacity, and better maneuverability. PMR-15 is used fairly extensively in military and commercial aircraft engines components seeing service temperatures as high as 500 F (260 C), such as the outer bypass duct for the F-404 engine. The current world-wide market for PMR-15 materials (resins, adhesives, and composites) is on the order of $6 to 10 million annually.

Meador, Michael A.↗

NASA NTRS1997

Electron Beam Cured Epoxy Resin Composites for High Temperature Applications

Electron beam curing of Polymer Matrix Composites (PMC's) is a nonthermal, nonautoclave curing process that has been demonstrated to be a cost effective and advantageous alternative to conventional thermal curing. Advantages of electron beam curing include: reduced manufacturing costs; significantly reduced curing times; improvements in part quality and performance; reduced environmental and health concerns; and improvement in material handling. In 1994 a Cooperative Research and Development Agreement (CRADA), sponsored by the Department of Energy Defense Programs and 10 industrial partners, was established to advance the electron beam curing of PMC technology. Over the last several years a significant amount of effort within the CRADA has been devoted to the development and optimization of resin systems and PMCs that match the performance of thermal cured composites. This highly successful materials development effort has resulted in a board family of high performance, electron beam curable cationic epoxy resin systems possessing a wide range of excellent processing and property profiles. Hundreds of resin systems, both toughened and untoughened, offering unlimited formulation and processing flexibility have been developed and evaluated in the CRADA program.

Janke, Christopher J.↗

NASA NTRS1995

Synthesis of Substituted 2,3,5,6-tetraarylbenzo(1,2-b:5,4-b')difurans

A series of substituted 2,3,5,6-tetraarylbenzo(l,2-b:5,4-b')difurans 1 was synthesized. This synthesis is based upon the photocyclization of 2,5-dibenzoylresorcinol dibenzyl ethers to the corresponding tetrahydrobenzo(1,2-b:5,4-b')difurans. Treatment of the photoproducts with methanesulfonyl chloride in pyridine afforded 1 in overall yields ranging from 30-72%. A number of these compounds have high fluorescence quantum yields (of phi(sub f) = 0.76-0.90), and their fluorescence spectra exhibit large solvatochromic shifts. These compounds may be suitable for use as fluorescent probes.

Abdul-Aziz, Mahmoud↗

NASA NTRS1994

Easily Processable High-Temperature Polyimide

Polymerization without volatile gases leads to high-quality resins. N-CYCAP (amiNe substituted CYClophane Addition Polyimide) is thermoset polyimide developed containing 4-amino{2.2}paracyclophane as end cap. Current improvements in synthesis of end cap include increasing yield to 60 percent overall and decreasing manufacturing time by simplifying synthetic procedure. Enables large-scale production and increases viability of cyclophane end caps for polymers for high-temperature applications. In comparison with neat resin moldings and composite matrix resins made of PMR-II-50, those made of N-CYCAP polymers found to be processable with lower void content and higher thermo-oxidative stability.

Sutter, James K.↗

NASA NTRS1994

Composites research at NASA Lewis Research Center

Composites research at NASA Lewis is focused on their applications in aircraft propulsion, space propulsion, and space power, with the first being predominant. Research on polymer-, metal-, and ceramic-matrix composites is being carried out from an integrated materials and structures viewpoint. This paper outlines some of the topics being pursued from the standpoint of key technical issues, current status, and future directions.

Levine, Stanley R.↗

NASA NTRS1991

Electrically conducting polymers for aerospace applications

Current research on electrically conducting polymers from 1974 to the present is reviewed focusing on the development of materials for aeronautic and space applications. Problems discussed include extended pi-systems, pyrolytic polymers, charge-transfer systems, conductive matrix resins for composite materials, and prospects for the use of conducting polymers in space photovoltaics.

Meador, Mary Ann B.↗

NASA NTRS1991

Addition Curing Thermosets Endcapped with 4-Amino (2.2) Paracyclophane

A new family of addition curing polyimides were prepared that contained 4-amino (2.2)-paracyclophane as the endcap. An improved synthesis of the endcap 4-amino-(2.2) cyclophane was accomplished increasing the yield to 60 percent and simplifying the procedure. DSC and rheological analysis of endcapped polyimide oligomers confirmed that the onset for polymerization of the ethylene bridge was 250 C. C-13 CP/MAS NMR was used to determine the structural changes of the oligomers after thermal treatment. The cyclophane capped polyimides were successfully compression molded to form void free neat resin specimens. Tg's as high as 353 C were obtained by thermomechanical analysis for postcured samples. Preliminary thermal stability studies suggest that these resins have a high onset of decomposition ranging from 549 to 567 C.

Waters, John F.↗

NASA NTRS1990

A review of properties and potential aerospace applications of electrically conducting polymers

An overview of current research in conducting polymers is presented. Emphasis is placed on development of materials useful for aeronautic and space applications. Research on organic conducting polymers began in the early 1970s with the discovery of polyacetylene. Since then, many polymers which share structural characteristics with polyacetylene have been prepared which conduct electricity, especially when they are doped with suitable agents. Problems with environmental instability, difficult processing, poor mechanical properties and high cost have slowed the development of conducting polymers. However, practical use of these materials is imminent, based on recent refinements in understanding how polymers conduct, more systematic approaches to the development of new materials, and significant improvements in both the processing and properties.

Meador, Mary Ann B.↗

NASA NTRS1990

High-temperature polymer matrix composites

Polymers research at the NASA Lewis Research Center has produced high-temperature, easily processable resin systems, such as PMR-15. In addition, the Polymers Branch has investigated ways to improve the mechanical properties of polymers and the microcracking resistance of polymer matrix composites in response to industry need for new and improved aeropropulsion materials. Current and future research in the Polymers Branch is aimed at advancing the upper use temperature of polymer matrix composites to 700 F and beyond by developing new resins, by examining the use of fiber reinforcements other than graphite, and by developing coatings for polymer matrix composites to increase their oxidation resistance.

Meador, Michael A.↗

NASA NTRS1990

Photochemical approaches to ordered polymers

The photocyclization of o-benzyloxyphenyl ketone chromophores provides an efficient, high yield route to the synthesis of 2,3-diphenylbenzofurans. The synthesis and solution of photochemistry of a series of polymers containing this chromophore is described. The photocuring of these polymers is a potential new approach to the synthesis of highly conjugated polymers based upon a p-phenylene bisbenzofuran repeat unit.

Meador, Michael A.↗

NASA NTRS1989

Photochemistry Of 2,5-Diacyl-1, 4-Dimethylbenzenes

Experiments described in report revealed potentially useful aspects of photochemistry of 2,5-dibenzoyl-1, 4-dimethylbenzene (DBX) and 2,5-diacetyl-1, 4-dimethylbenzene (DAX). Behavior of these compounds reminiscent of orthoalkylphenyl ketones, studied from similar perspective for more than two decades.

Meador, Michael A.↗

NASA NTRS1988

Addition polymers from 1,4,5,8-tetrahydro-1,4;5,8-diepoxyanthracene and Bis-dienes. 2: Evidence for thermal dehydration occurring in the cure process

Diels-Alder cycloaddition copolymers from 1,4,5,8-tetrahydro-1,4;5,8-diepoxyanthracene and anthracene end-capped polyimide oligomers appear, by thermogravimetric analysis (TGA), to undergo dehydration at elevated temperatures. This would produce thermally stable pentiptycene units along the polymer backbone, and render the polymers incapable of unzipping through a retro-Diels-Alder pathway. High resolution solid 13C nuclear magnetic resonance (NMR) of one formulation of the polymer system before and after heating at elevated temperatures, shows this to indeed be the case. NMR spectra of solid samples of the polymer before and after heating correlated well with those of the parent pentiptycene model compound before and after acid-catalyzed dehydration. Isothermal gravimetric analyses and viscosities of the polymer before and after heat treatment support dehydration as a mechanism for the cure reaction.

Meador, Mary Ann B.↗

NASA NTRS1987

High temperature polymer matrix composites

With the increased emphasis on high performance aircraft the need for lightweight, thermal/oxidatively stable materials is growing. Because of their ease of fabrication, high specific strength, and ability to be tailored chemically to produce a variety of mechanical and physical properties, polymers and polymer matrix composites present themselves as attractive materials for a number of aeropropulsion applications. In the early 1970s researchers at the NASA Lewis Research Center developed a highly processable, thermally stable (600 F) polyimide, PMR-15. Since that time, PMR-15 has become commercially available and has found use in military aircraft, in particular, the F-404 engine for the Navy's F/A-18 strike fighter. The NASA Lewis'contributions to high temperature polymer matrix composite research will be discussed as well as current and future directions.

Meador, Michael A.↗

NASA NTRS1987

A preliminary study of ester oxidation on an aluminum surface using chemiluminescence

The oxidation characteristics of a pure ester (trimethyolpropane triheptanoate) were studied by using a chemiluminescence technique. Tests were run in a thin-film micro-oxidation apparatus with an aluminum alloy catalyst. Conditions included a pure oxygen atmosphere and a temperature range of 176 to 206 C. Results indicated that oxidation of the ester (containing 10 to the minus 3rd power M diphenylanthracene as an intensifier) was accompanied by emission of light. The maximum intensity of light emission (I sub max) was a function of the amount of ester, the concentration of intensifier, and the test temperature. The induction period or the time to reach one-half of maximum intensity (t sub 1/2) was an inverse function of test temperature. Decreases in light emission at the later stages of a test were caused by depletion of the intensifier.

Jones, William R., Jr.↗

NASA NTRS1987

The 2.5-diacyl-1,4-dimethylbenzenes: Examples of bisphotoenol equivalents

The photochemistry of 2,5-dibenzoyl(DBX)-and 2,5-diacetyl-1,4-dimethylbenzene (DAX) has been investigated. Both compounds readily undergo photoenolization similar to 0-alkylphenyl ketones. However, unlike 0-alkylphenyl ketones DAX and DBX are each capable of undergoing two tandem photoenolizations. Photoenols derived from o-alkylphenyl ketones have been successfully trapped with Diels-Alder dienophiles to provide a convenient synthesis of substituted tetralins. Similarly, Diels-Alder trapping of DBX photoenils afforded substituted tetra- and octahydro anthracenes. Further mainpulation of these photadducts provided the corresponding anthracenes in good yield. The photochemistry of DAX and DBX will be discussed, in particular their use in the synthesis of substituted anthracenes.

Meador, Michael A.↗

NASA NTRS1986

Ester oxidation on an aluminum surface using chemiluminescence

The oxidation characteristics of a pure ester (trimethyolpropane triheptanoate) were studied by using a chemiluminescence technique. Tests were run in a thin film microoxidation apparatus with an aluminum alloy catalyst. Conditions included a pure oxygen atmosphere and a temperature range of 176 to 206 C. Results indicated that oxidation of the ester (containing .001 M diphenylanthracene as an intensifier) was accompanied by emission of light. The maximum intensity of light emission was a function of the amount of ester, the concentration of intensifier, and the test temperature. The induction period, or the time to reach one-half of maximum intensity was inversely proportional to test temperature. Decreases in light emission at the later stages of a test were caused by depletion of the intensifier.

Jones, William R., Jr.↗