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At least 181 records · Page 10

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NASA NTRS1982

Detection of trace quantities of dimethylacetamide during the curing process of a polyimide film

An experimental study of positive ion clustering of dimethylacetamide (DMAC) with water molecules using high pressure mass spectrometry techniques is reported. The possibilities of using these techniques to determine the concentration levels of DMAC evolved from a polyimide film during the curing process, are examined. The test sample of gas contained DMAC of the order of a few micrograms, water, nitrogen, oxygen, and other compounds that might evolve during the curing cycle of a polyimide film. It is found that PPM concentrations such as DMAC lost from polyimide during curing stages can be detected with this technique. Improvement of the detection of lower concentrations (50 picograms) can be made by using electron multipliers with a gain in the 10 to the 8th power range and pulse counting techniques.

Smith, A. C.↗

NASA NTRS1984

Evaluation of long-duration exposure to the natural space environment on graphite-polyimide and graphite-epoxy mechanical properties (A0175)

The primary objective of this experiment is to accumulate the needed operational data associated with the exposure of graphite-polyimide and graphite-epoxy material to the environments of space. The experiment will be mounted in two 3-in.-deep peripheral trays. Graphite-polyimide specimens will occupy 1 1/3 trays and the graphite-epoxy specimens will occupy two-thirds of a tray. The experiment approach requires two matched sets of specimens with traceable records that are maintained for materials processing and specimen quality. After fabrication, one set of each test specimen will be sectioned and structurally tested to serve as a data baseline. After the flight, the other set of specimens will undergo extensive measurements of mechanical properties for comparison with the original data baseline. Structural testing of the graphite-polyimide specimens will provide strength and elastic data in tension, compression, and shear. Transverse tension microcracking and crack propagation will be evaluated by photomicroscopy. Structural testing of the graphite-epoxy specimens will include verification of laminate, core, adhesive, and fatigue properties as applied to the design and analysis of the payload bay door. Microcracking and crack propagation will also be analyzed by photomicroscopy.

Powell, J. H.↗

NASA NTRS1984

Evaluation of two polyimides and of an improved liner retention design for self-lubricating bushings

Two different polyimide polymers were studied and the effectiveness of a design feature to improve retention of the self lubricating composite liners under high load was evaluated. The basic bearing design consisted of a molded layer of chopped graphite-fiber-reinforced-polyimide (GFRP) composite bonded to the bore of a steel bushing. The friction, wear, and load carrying ability of the bushings were determined in oscillating tests at 25, 260 and 315 C at radial unit loads up to 260 MPa. Friction coefficients were typically 0.15 to 0.25. Bushings with liners containing a new partially fluorinated polymer were functional, but had a lower load capacity and higher wear rate than those containing a more conventional, high temperature polyimide. The liner retention design feature reduced the tendency of the liners to crack and work out of the contact zone under high oscillating loads.

Sliney, H. E.↗

NASA NTRS1983

Acetylene-terminated polyimide adhesives

The nadic-encapped LARC-43 addition polyimide exhibits excellent flow, is easy to process, and can be utilized for short terms at temperatures up to 593 C. It retains good lap shear strength as an adhesive for titanium after aging in air up to 125 hours at 316 C; but lap shear strength degrades with longer exposures at that temperature. Thermid 600, an addition polyimide that is acetylene encapped, exhibits thermomechanical properties even after long term exposure in at air at 316 C. An inherent drawback of this system is that it has a narrow processing window. An acetylene encapped, addition polyimide which is a hybrid of these two systems was developed. It has good retention of strength after long term aging and is easily processed. The synthesis and characterization of various molecular weight oligomers of this system are discussed as well as the bonding, aging, and testing of lap shear adhesive samples. Previously announced in STAR as N83-18910

Hanky, A. O.↗

NASA NTRS1984

Thermoplastic/Melt-Processable Polyimides

Several polyimides were prepared which show promise for aircraft composite applications. This was achieved through a systematic polymer synthesis program where the glass transition temperatures were greatly lowered when compared to the older polyimide systems. Several of the materials were shown to be hot-melt processable and are attractive matrix resin candidates especially in light of their high g(sub Ic) values. At least two of these polyimides are available for evaluation and others are on the research horizon.

St.clair, T. L.↗

NASA NTRS1985

Fire-resistant phosphorus containing polyimides and copolyimides

Phosphorus-containing polyimides and copolyimides are synthesized in a two-step polycondensation reaction from 1- (diorganooxyphosphonl)methly 2,4- and 2,6-diaminobenzenes and tetracarboxylic anhydride. The diorgano position of the diorganooxyphosphonyl group includes alkyl, such as ethyl, substituted alkyl, such as 2-chloroethyl, and aryl such as phenyl. The tetracarboxylic anhydries include compounds such as pyrometallitic dianhydride and benzophenone tetracarboxylic dianhydride. The glass transition temperature (Tg) of the polyimides is reduced by incorporation of the (dialkoxyphosphonyl)methyl groups. The phosphorus-containing copolyimides show a considerably higher degree of fire-resistance as compared to that of the corresponding common polyimides.

Mikroyannidis, J. A.↗

NASA NTRS1984

Rheological characterization of addition polyimide matrix resins and prepregs

Although graphite-reinforced polyimide matrix composites offer outstanding specific strength and stiffness, together with high thermal oxidative stability, processing problems connected with their rheological behavior remain to be addressed. The present rheological studies on neat polyimide resin systems encountered outgassing during cure. A staging technique has been developed which can successfully handle polyimide samples, and novel methods were applied to generate rheological curves for graphite-reinforced prepregs. The commercial graphite/polyimide systems studied were PRM 15, LARC 160, and V378A.

Maximovich, M. G.↗

NASA NTRS1984

The development of aerospace polyimide adhesives

Few materials are available which can be used as aerospace adhesives at temperatures in the range of 300 C. The Materials Division at NASA-Langley Research Center developed several high temperature polyimide adhesives to fulfill the stringent needs of current aerospace programs. These adhesives are the result of a decade of basic research studies on the structure property relationships of both linear and addition aromatic polyimides. The development of both in house and commercially available polyimides is reviewed with regards to their potential for use as aerospace adhesives.

St. Clair, A. K.↗

NASA NTRS1984

Polyimides - Tribological properties and their use as lubricants

Friction, wear, and wear mechanisms of several different polyimide films, solid bodies, composites, and bonded solid lubricant films are compared and discussed. In addition, the effect of such parameters as temperatures, type of atmosphere, contact stress, and specimen configuration are investigated. A friction and wear transition occurs in some polyimides at elevated temperatures and this transition is related to molecular relaxations that occur in polyimides. Friction and wear data from an accelerated test (pin-on-disk) are compared to similar data from an end use test device (plain spherical bearing), and to other polymers investigated in a similar geometry.

Fusaro, R. L.↗

NASA NTRS1985

PMR polyimides from solutions containing mixed endcaps

Previous studies have shown that partial substitution of p-aminostyrene (PAS) for the monomethylester of endo-5-norbornene-2, 3-dicarboxylic acid (NE) lowered the cure temperature of PMR polyimides from 316 to 260 C, but the modified PMR polyimides required higher compression-molding pressures than state-of-the-art PMR-15. In this study PMR polyimides are prepared employing three encaps: NE, PAS, and endo-N-phenyl-5-norbornene-2,3-dicarboximide (PN). The effect of PN addition on the processing characteristics and glass transition temperatures of graphite fiber-reinforced PMR composites is studied. The room temperature and short-time 316 C mechanical properties of the composites are determined. The weight loss and mechanical property retention characteristics of the composites after exposure in air at 316 C are also determined.

Delvigs, P.↗

NASA NTRS1985

Recent developments in polyimide and bismaleimide adhesives

Research on high temperature resin systems has intensified. In the Aerospace Industry, the motivation for this increased activity has been to replace heat resistant alloys of aluminum, stainless steel and titanium by lighter weight glass and carbon fiber reinforced composites. Applications for these structures include: (1) engine nacelles involving long time exposure (thousands of hours) to temperatures in the 150 to 300 C range, (2) supersonic military aircraft involving moderately long exposure (hundreds of hours) to temperatures of 150 to 200 C, and (3) missile applications involving only brief exposure (seconds or minutes) to temperatures up to 500 C and above. Because of fatigue considerations, whenever possible, it is preferable to bond rather than mechanically fasten composite structures. For this reason, the increased usage of high temperature resin matrix systems for composites has necessitated the devlopment of compatible and equally heat stable adhesive systems. The performance of high temperature epoxy, epoxy phenolic and condensation polyimide adhesives is reviewed. This is followed by a discussion of three recently developed types of adhesives: (1) condensation reaction polyimides having improved processing characteristics; (2) addition reaction polyimides; and (3) bismaleimides.

Politi, R. E.↗

NASA NTRS1985

Transfer molding of PMR-15 polyimide resin

Transfer molding is an economically viable method of producing small shapes of PMR-15 polyimide. It is shown that with regard to flexural, compressive, and tribological properties transfer-molded PMR-15 polyimide is essentially equivalent to PMR-15 polyimide produced by the more common method of compression molding. Minor variations in anisotropy are predictable effects of molding design and secondary finishing operations.

Reardon, J. P.↗

NASA NTRS1985

High temperature resistant polyimide from tetra ester, diamine, diester and N-arylnadimide

The invention described relates to improved polyimide resins which are noted for their high thermal and oxidative stability, high strength at elevated temperatures and which exhibit many other outstanding physical and chemical properties, especially useful in high temperature applications. The polyimides are prepared by the reaction, with application of heat of a mixture of monomers comprising: (1) a dialkyl or tetraalkyl ester of an aromatic tetracarboxylic acid, (2) and aromatic diamine, (3) a monoalkyl or dialky ester of a dicarboxylic acid, and (4) a N-arylnadimide such as N-phenylnadimide. Polyimides of monomers (1), (2) and (3) are known.

Pater, R. H.↗

NASA NTRS1985

Polyimide analysis using diffuse reflectance-FTIR

The thermal imidization of a number of polyimide precursors in the form of powders, films, and prepregs was examined by an in situ diffuse reflectance-FTIR technique where infrared spectra were determined while the material was being heated. An analysis of these spectra revealed that, with the exception of one water soluble adhesive, each precursor developed an anhydride band around 1850 cm/cu during imidization. This band diminished in intensity during final stages of cure. Efforts were made to quantify the amount of anhydride in several samples. Evidence obtained could be interpreted to mean that poly(amic acid) resins undergo an initial reduction in molecular weight during imidization before recombining to achieve their ultimate molecular weights as polyimides. Several reports in the literature are cited to support this interpretation. This report serves both to document anhydride formation during imidization and to increase our fundamental understanding of how polyimides cure.

Young, P. R.↗

NASA NTRS1985

Aerospace applications of PMR polyimide composites

The current status of the novel class of processable, addition-type polyimides known as PMR (for in situ polymerization of monomer reactants) polyimides, developed by NASA at the Lewis Research Center, is reviewed. Highlights of PMR technology studies conducted at NASA Lewis are presented. Several examples of industrial applications of PMR-15 polyimide composites to aerospace structural components are examined.

Serafini, T. T.↗

NASA NTRS1987

Acetylene-Terminated Polyimide Siloxanes

Siloxane-containing addition polyimides yield toughened high-temperature adhesives and matrix resins. Addition polyimide made by reaction of aromatic tetracarboxylic acid dianhydride with aromatic diamine in presence of ethynyl-substituted aromatic monoamine. Acetylene-terminated siloxane imide cured by heating to yield acetylene-terminated polyimide siloxane.

St. Clair, Terry L.↗

NASA NTRS1986

A new readily processable polyimide

As part of an effort to develop tough solvent resistance thermoplastics for potential use as structural resins on aerospace vehicles, a new processable polyimide was evaluated. The synthesis involved the reaction of a new diamine, 1,3-bis 2-(3-aminophenoxy)ethyl ether, with 3,3',4,4'-benzophenonetetracarboxylic dianhydride to form the polyamic acid and subsequent conversion of it to the polyimide. Various physical properties such as thermal stability, solvent resistance, glass transition temperature, crystalline melt temperature, melt viscosity and mechanical properties such as fracture toughness, adhesive, film and composite properties are reported. Of particular interest is the extremely high titanium to titanium tensile shear strength obtained for this polyimide.

Harris, F. W.↗

NASA NTRS1987

A new readily processable polyimide

Harris, F. W.↗