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At least 127 records · Page 7

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NASA NTRS1979

Thermochemical characterization of some thermally stable thermoplastic and thermoset polymers

The thermochemical and flammability properties of some thermally stable polymers considered for use in aircraft interiors are described. The properties studied include: (1) thermomechanical properties such as glass transition and melt temperature; (2) dynamic thermogravimetric analysis in anaerobic environment; (3) flammability properties such as oxygen index, flame spread, and smoke evolution; and (4) selected physical properties. The thermoplastic polymers evaluated include polyphenylene sulfide, polyaryl sulfone, 9,9-bis(4-hydroxyphenyl)-fluorene polycarbonate-poly(dimethylsiloxane) and polyether sulfone. The thermoset polymers evaluated include epoxy, bismaleimide, a modified phenolic, and polyaromatic melamine resin. These resins were primarily used in the fabrication of glass-reinforced prepregs for the construction of experimental panels. Test results and relative rankings of some of the flammability parameters are presented, and the relationship of the molecular structure, char yield, and flammability properties of these polymers are discussed.

Kourtides, D. A.↗

NASA NTRS1978

Thermochemical and flammability properties of some thermoplastic and thermoset polymers - A review

The thermochemical and flammability properties of thermoplastic and thermoset polymers are discussed. The results of a thermogravimetric analysis of the polymers conducted on a DuPont 950 thermogravimetric analyzer using both nitrogen and air atmospheres are presented. Experimental data on smoke evolution are given, and the methodology for assessing the relative toxicity of the pyrolysis effluents is described. The values obtained from the flammability tests are compared with the stoichiometric char yield, and it is shown that the ignition tendency of the polymers is a linear function of the resin char yield

Kourtides, D. A.↗

NASA NTRS1979

Toughening of thermosetting polyimides

Work directed toward increasing the resistance to crack propagation of thermoset polyimides is described. Rubber modification and Teflon microfiber impregnation techniques for increasing fracture toughness are investigated. Unmodified Kerimid 601 has a fracture surface work value of 0.20 in-lbs/sq in. Dispersed particles of amine terminated butadiene acrylonitrile liquid rubber or of silicone rubber do not raise this value much. By contrast, 5 percent of well fibrillated Teflon produces an eight-fold increase in fracture toughness. Further process improvements should increase this factor to 20-30.

Gollob, D. S.↗

NASA NTRS1980

Oxygen index tests of thermosetting resins

The flammability characteristics of nine thermosetting resins under evaluation for use in aircraft interiors are described. These resins were evaluated using the Oxygen Index (ASTM 2863) testing procedure. The test specimens consisted of both neat resin and glass reinforced resin. When testing glass-reinforced samples it was observed that Oxygen Index values varied inversely with resin content. Oxygen values were also obtained on specimens exposed to temperatures up to 300 C. All specimens experienced a decline in Oxygen Index when tested at an elevated temperature.

Gilwee, W. J., Jr.↗

NASA NTRS1979

Advanced thermoset resins for fire-resistant composites

The thermal and flammability properties of some thermoset polymers and composites are described. The processing and evaluation of composites fabricated from currently used resins and advanced fire-resistant resins are also described. Laboratory test methodology used to qualify candidate composite materials includes thermochemical characterization of the polymeric compounds and evaluation of the glass reinforced composites for flammability and smoke evolution. The use of these test methods will be discussed in comparing advanced laminating resins and composites consisting of modified epoxies, phenolics and bismaleimide, with conventional baseline materials consisting of epoxy.

Kourtides, D. A.↗

NASA NTRS1982

Fabricating Thin-Film High-Temperature Thermoset Resins

To prepare an epoxy thin film, quantity of uncured epoxy to be cast placed in vacuum oven and heated to melting temperature. Vacuum of about 30 mm Hg is applied to deaerate epoxy charge. Pressure is cycled with each foaming until all air and excess volatiles are revoved. thermoset (cross-linked) resin is cast between thin, flexible, releasing substrate films. Films less than 0.025 mm in thickness are made routinely with this facility.

Dickerson, G. E.↗

NASA NTRS1984

Thermoset-thermoplastic aromatic polyamide containing N-propargyl groups

The compounds of the class of aromatic polyamides useful as matrix resins in the manufacture of composites or laminate fabrication were developed. The process for preparing this thermoplastic-thermoset polyamide system involves incorporating a latent crosslinking moiety along the backbone of the polyamide to improve the temperature range of fabrication thereof wherein the resin softens at a relatively low temperature (approx. 154 C) and subsequently sets-up or undergoes crosslinking when subjected to higher temperature (approx. 280 C).

St.clair, T. L.↗

NASA NTRS1987

Thermosetting Fluoropolymer Foams

New process makes fluoropolymer foams with controllable amounts of inert-gas fillings in foam cells. Thermosetting fluoropolymers do not require foaming additives leaving undesirable residues and do not have to be molded and sintered at temperatures of about 240 to 400 degree C. Consequently, better for use with electronic or other parts sensitive to high temperatures or residues. Uses include coatings, electrical insulation, and structural parts.

Lee, Sheng Yen↗

NASA NTRS1986

Modeling the curing process of thermosetting resin matrix composites

A model is presented for simulating the curing process of a thermosetting resin matrix composite. The model relates the cure temperature, the cure pressure, and the properties of the prepreg to the thermal, chemical, and rheological processes occurring in the composite during cure. The results calculated with the computer code developed on the basis of the model were compared with the experimental data obtained from autoclave-curved composite laminates. Good agreement between the two sets of results was obtained.

Loos, A. C.↗

NASA NTRS1988

Chemoviscosity modeling for thermosetting resin systems, part 3

A new analytical model for simulating chemoviscosity resin has been formulated. The model is developed by modifying the well established Williams-Landel-Ferry (WLF) theory in polymer rheology for thermoplastic materials. By introducing a relationship between the glass transition temperature (T sub g (t)) and the degree of cure alpha(t) of the resin system under cure, the WLF theory can be modified to account for the factor of reaction time. Temperature-dependent functions of the modified WLF theory parameters C sub 1 (T) and C sub 2 (T) were determined from the isothermal cure data. Theoretical predictions of the model for the resin under dynamic heating cure cycles were shown to compare favorably with the experimental data. This work represents a progress toward establishing a chemoviscosity model which is capable of not only describing viscosity profiles accurately under various cure cycles, but also correlating viscosity data to the changes of physical properties associated with the structural transformations of the thermosetting resin systems during cure.

Hou, T. H.↗

NASA NTRS1988

Instability-related delamination growth in thermoset and thermoplastic composites

Mixed-mode crack propagation in compressively loaded thermoset and thermoplastic composite columns with an imbedded through-width delamination is investigated. Beam theory is used to analyze the geometrically nonlinear load-deformation relationship of the delaminated subregion. The elastic restraint model (ERM), combined with existing FSM modeling of the crack-tip region, yields expressions for the Mode I and Mode II components of the strain energy release rate G(I) and G(II) to predict the critical load at the onset of delamination growth. Experimental data were generated for geometries yielding a wide range of G(I)/G(II) ratios at the onset of crack growth. A linear mixed-mode crack growth criterion in conjunctuion with the ERM provides good agreement between predicted and measured critical loads for both materials studied.

Gillespie, John W., Jr.↗

NASA NTRS1990

Acetylene-chromene terminated resins as high temperature thermosets

A novel phase transfer catalyzed process for the preparation of propargyl ethers has been developed. The propargyl ethers serve as precursors to a new class of thermosetting resins called acetylene-chromene terminated (ACT) resins. Heat treatment of a solution of propargyl ethers with various catalysts, followed by removal of solvent leads to the ACT resins via partial conversion of the propargyl ether groups to chromenes. This process reduces the energy content of the resin systems and reduces the amount of shrinkage found during cure. Due to the presence of the solvent the process is safe and gives rise to low viscosity products suitable for resin transfer molding and filament winding type applications. Due to the high glass transition temperature, high modulus, and low moisture uptake the cured resins display better than 232 C/wet performance. The thermal stability of the ACT resins in air at 204 C is superior to that of conventional bismaleimide resins. The resins also display excellent electrical properties.

Godschalx, J. P.↗

NASA NTRS1991

Flexible thermoset towpregs by electrostatic powder fusion coating

Thermoset prepregs of expoxy and polyimide have been produced by electrostatic deposition of charged fluidized polymer powders on spread continuous fiber tows. The powders are melted onto the fibers by radiant heating to adhere the polymer to the fiber. This process produces towpreg uniformly and rapidly without imposing severe stresses on the fibers. The towpregs produced by this novel process were consolidated to make unidirectional laminates for mechanical testing. Low void content samples have been made and demonstrated by C-scan and scanning electron microscopy. The mechanical properties of unidirectional laminates are equivalent to composites fabricated by conventional techniques.

Yang, Pei-Hua↗

NASA NTRS1993

Predicting Failure Behaviors Of Thermoset Polymers

Useful predictions made from measurements on test-tube quantities. Mathematical model for viscoelastic failure behavior of epoxy or other thermoset polymer reduces considerably time and expense necessary to evaluate new polymer. One quickly obtains first-order estimates of viscoelastic properties, and decides more easily whether material rejected or good candidate for more thorough evaluation. Only properties required by model are equilibrium pressure vs. volume vs. temperature (PVT) response and linear viscoelastic shear and bulk moduli.

Bowles, Kenneth J.↗

NASA NTRS1992

Lower temperature curing thermoset polyimides utilizing a substituted norbornene endcap

Methoxycarbonyl bridgehead substituted nadic diacid monomethyl ester, when used as an endcapping monomer, lowered the cure temperature of thermoset PMR polyimides without seriously affecting other desirable properties, such as glass transition temperature and thermal oxidative stability. The C-13 CP/MAS NMR of model compounds was used to follow the cure of resin systems using both the unmodified nadic endcap and the methoxycarbonyl-substituted endcap. Rheological analysis and differential scanning calorimetry DSC also provided evidence for the lower curing nature of the substituted endcap. Two regioisomers of the bridgehead-substituted endcap were isolated, and their chemical structures were elucidated by X-ray crystallography. The model compound and molecular modeling studies conducted ruled out the possibility of regioisomeric imide formation in the substituted endcaps.

Waters, John F.↗

NASA NTRS1996

A Method for Characterizing Thermoset Polyimides

Thermoset polyimides have great potential for successfully meeting tough stress and temperature challenges in the advanced aircraft development program. However, studies of structure-property relationships in these materials have not been very successful so far. Positron lifetime spectroscopy (PLS) has been used to investigate free volumes and associated parameters in a series of variable, segmental molecular weight samples. The free volume correlates well with the molecular weight M(sub c), the cross-link density v, and the coefficient of thermal expansion (CTE) of these materials. Currently, no other techniques are available for direct measurement of these parameters, particularly for polymers in solid phase. Experimental results and their interpretations are presented.

Ranganathaiah, C.↗

NASA NTRS1994

Morphology of Thermoset Polyimides by Positron Annihilation Spectroscopy

Thermoset polyimides have great potential for successfully meeting tough stress and temperature challenges in the advanced aircraft development program. However, studies of structure/property relationships in these materials have not been very successful so far. Positron annihilation spectroscopy has been used to investigate free volumes and associated parameters. It has been noted that the free volume correlates well with the molecular weight, cross-link density and thermal coefficient of expansion of these materials. Currently no other techniques are available for direct measurement of these parameters. Experimental results and their interpretations will be discussed.

Ranganathaiah, C.↗

NASA NTRS1996

Performance of Integrated Fiber Optic, Piezoelectric, and Shape Memory Alloy Actuators/Sensors in Thermoset Composites

Recently, scientists and engineers have investigated the advantages of smart materials and structures by including actuators in material systems for controlling and altering the response of structural environments. Applications of these materials systems include vibration suppression/isolation, precision positioning, damage detection and tunable devices. Some of the embedded materials being investigated for accomplishing these tasks include piezoelectric ceramics, shape memory alloys, and fiber optics. These materials have some benefits and some shortcomings; each is being studied for use in active material design in the SPICES (Synthesis and Processing of Intelligent Cost Effective Structures) Consortium. The focus of this paper concerns the manufacturing aspects of smart structures by incorporating piezoelectric ceramics, shape memory alloys and fiber optics in a reinforced thermoset matrix via resin transfer molding (RTM).

Trottier, C. Michael↗