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At least 145 records · Page 8

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NASA NTRS1983

Synthesis and characterization of bisimide amines and bisimide amine-cured epoxy resins

An attempt is made to develop tough, moisture resistant, high char yield epoxy resins by means of novel bisimide amine (BIA) hardener curing agents and a state-of-the-art epoxy resin system. The BIAs are isolated as mixtures containing monomer, oligomer, and polymeric species, and then characterized by elemental analysis and high pressure liquid chromatography. The bisimide amine-cured epoxies (IMEs) were characterized with respect to moisture absorption, thermal properties, and physical and mechanical properties, as well as in the role of matrices in Celion 6000/IME composites. The relative toughness characteristics of each IME formulation was measured by the 10 deg off-axis tensile test, measuring the uniaxial tensile strength, shear strength, and shear-strain-to-failure of the composite systems.

Scola, D. A.↗

NASA NTRS1983

Mechanism and kinetics of the curing process in a resin system

The present investigation is concerned with the development of a cure reactivity model for a resin system consisting of tetraglycidyl diaminodiphenyl methane (TGDDM) and diaminodiphenyl sulfone (DDS), taking into account the temperature range from 153 to 177 C. Differential scanning calorimetry and FT-IR spectroscopy have been used to investigate the cure kinetics of the resin with the objective to obtain a correlation between the total conversion and the rates of formation and disappearance of specific functionalities. It was found that the overall process can be satisfactorily described in terms of two first order processes.

Moacanin, J.↗

NASA NTRS1986

Phthalocyanine Tetraamine Epoxy-Curing Agents

Tough fire- and chemical-resistant epoxies produced by using metalphthalocyanine tetraamines (MPT's) of copper, cobalt, or nickel as curing agents. Synthesis of MPT's commercially realizable and gives pure compounds with almost 90-percent yield. Synthesis applicable for metals with atomic radii of about 1.35 angstroms, including Cu, Co, Ni, Zn, Fe, Pt, Al, and V. Possible to use metal phthalocyanines to cure epoxy resins in homogeneous reaction.

Fohlen, G. M.↗

NASA NTRS1984

Prepreg cure monitoring using diffuse reflectance-FTIR

An in situ diffuse reflectance-Fourier transform infrared technique was developed to determine infrared spectra of graphite fiber prepregs as they were being cured. A bismaleimide, an epoxy, and addition polyimide matrix resin prepregs were studied. An experimental polyimide adhesive was also examined. Samples were positioned on a small heater at the focal point of diffuse reflectance optics and programmed at 15 F/min while FTIR spectra were being scanned, averaged, and stored. An analysis of the resulting spectra provided basic insights into changes in matrix resin molecular structure which accompanied reactions such as imidization and crosslinking. An endo-exothermal isomerization involving reactive end-caps was confirmed for the addition polyimide prepregs. The results of this study contribute to a fundamental understanding of the processing of composites and adhesives. Such understanding will promote the development of more efficient cure cycles.

Young, P. R.↗

NASA NTRS1986

Monitoring Prepregs As They Cure

Quality IR spectra obtained in dynamic heating environment. New technique obtains quality infrared spectra on graphite-fiber-reinforced, polymeric-matrix-resin prepregs as they cure. Technique resulted from modification of diffuse reflectance/Fourier transform infrared (DR/FTIR) technique previously used to analyze environmentally exposed cured graphite composites. Technique contribute to better understanding of prepreg chemistry/temperature relationships and development of more efficient processing cycles for advanced materials.

Young, P. R.↗

NASA NTRS1986

Tetraglycidyl epoxy resins and graphite fiber composites cured with flexibilized aromatic diamines

Studies were performed to synthesize new ether modified, flexibilized aromatic diamine hardeners for curing epoxy resins. The effect of moisture absorption on the glass transition temperatures of a tetraglycidyl epoxy, MY 720, cured with flexibilized hardeners and a conventional aromatic diamine was studied. Unidirectional composites, using epoxy-sized Celion 6000 graphite fiber as the reinforcement, were fabricated. The room temperature and 300 F mechanical properties of the composites, before and after moisture exposure, were determined. The Mode I interlaminar fracture toughness of the composites was characterized using a double cantilever beam technique to calculate the critical strain energy release rate.

Delvigs, P.↗

NASA NTRS1985

Ultrasonic characterization of changes in viscoelastic properties of epoxy during cure

A technique for using the longitudinal velocity (LV) of an ultrasonic wave to monitor the extent of cross linking (CL) during the cure of thermosetting resins is described. The method was developed by monitoring the rate of change in LV during the cure of a bisphenol-A epoxy resin with an amine adduct. The experiment included variations in the temperature and stoichiometry in order to express the rate of change in terms of the reaction kinetics. The pulse-echo method was used with a single transducer operating at 20 MHz. Numerical models were defined to account for the acoustic response of a single layer, the attenuation and the reflection coefficient. A linear relationship was observed between the inverse of the temperature and the log of the rate of change in the velocity, supporting the theory that the velocity could be used to monitor the extent of the cross-linking reaction. An activation energy of 11.9 kcal/mole was calculated for the mixture being investigated.

Winfree, W. P.↗

NASA NTRS1987

Dielectric Monitoring of Curing Composites

Report describes preliminary attempts at dielectric monitoring of curing of graphite/epoxy and carbon/phenolic composites. Objective is to develop dielectric monitoring for optimizing curing process and reduce incidence of failures of produced composite structures.

Goldberg, Benjamin E.↗

NASA NTRS1986

Dynamic dielectric analysis - A nondestructive cure process monitoring method

The cure processes of TGDDM-based epoxy thermosets and polysulfone thermoplastic resins were monitored by means of computer-controlled dynamic dielectric measurements made at frequencies 5 Hz to 5 million Hz. The values of capacitance and conductance were recorded, and the data were used to calculate the complex permittivity, an intensive property which is characteristic of the material's state of cure and independent of the amount of material measured. The frequency dependence of the complex permittivity value was used to determine the conductivity and mean relaxation time. The conductivity which is also an intensive variable and is independent of sample size, was correlated with the viscosity in both the epoxy and polysulfone systems.

Kranbuehl, D. E.↗

NASA NTRS1987

A cure-rate model for the Shuttle filament-wound case

An epoxy and carbon fiber composite has been used to produce a light-weight rocket case for the Space Shuttle. A kinetic model is developed which can predict the extent of epoxy conversion during the winding and curing of the case. The model accounts for both chemical and physical kinetics. In the model, chemical kinetics occur exclusively up to the time the transition temperature equals the reaction temperature. At this point the resin begins to solidify and the rate of this process limits the rate of epoxy conversion. A comparison of predicted and actual epoxy conversion is presented for isothermal and temperature programmed cure schedules.

Cagliostro, D. E.↗

NASA NTRS1988

Investigation Of Epoxy Curing

Kinetic model describes major features of curing process. Report describes investigation of curing of epoxy resin used in carbon-fiber/epoxy version of cases of Space Shuttle booster rockets.

Cagliostro, D. E.↗

NASA NTRS1987

Degree of cure and viscosity of Hercules HBRF-55 resin

The rate of cure and viscosity were measured for Hercules HBRF-55 resin. The rate of cure was measured by differential scanning calorimetry, while the viscosity was measured by a parallel disk and plate type apparatus. The data were fitted to analytic expressions to make them suitable for use in numerical calculations.

Bhi, S. T.↗

NASA NTRS1986

Dynamic dielectric analysis for nondestructive cure monitoring and process control

Dynamic dielectric analysis (DDA) is an effective in situ NDE method that can monitor the reaction status in thermosets and the phase changes in thermoplastics, including slow reactions occuring late in the cure cycle and recrystallization during annealing. The effects of moisture and resin history on reaction rate can also be determined, as can ionic and dipolar contributions. The ionic mobility parameter is noted to be an excellent monitor of viscosity above the glass transition temperature. The ability of DDA to monitor cure rate variations in a thick section during autoclaving has been demonstrated.

Kranbuehl, D. E.↗

NASA NTRS1989

Method of controlling a resin curing process

An analytical technique for controlling the curing process of fiber-reinforced composite materials that are formed using thermosetting resins. The technique is the percent gel method and involves development of a time-to-gel equation as s function of temperature. From this equation a rate-of-gel equation is then determined, and a percent gel is calculated which is the product of the rate-of-gel times time. Percent gel accounting is used to control the proper pressure application point in an autoclave cure process to achieve desired properties in a production composite part.

Charles Neal Webster↗

NASA NTRS1988

In situ composite cure monitoring using infrared transmitting optical fibers

The development of infrared-transmitting optical fibers as sensors for monitoring the cure of advanced composite materials is reported. Fourier transform infrared spectra are presented which were remotely sensed during the cure of a high performance polyimide resin and a graphite/polyimide matrix prepreg using an 0.1 mm O.D. x 3 m chalcogenide optical fiber. A discussion of the fiber and sensor element, absorption mechanism and potential applications is presented.

Young, Philip R.↗

NASA NTRS1990

FTIR Monitoring Of Curing Of Composites

Infrared-sensing optical fiber system developed to monitor principal infrared absorption bands resulting from vibrations of atoms and molecules as chemical bonds form when resin cured. System monitors resin chemistry more directly. Used to obtain Fourier transform infrared (FTIR) spectrum from graphite fiber/polyimide matrix resin prepreg. Embedded fiber optic FTIR sensor used to indicate state of cure of thermosetting composite material. Developed primarily to improve quality of advanced composites, many additional potential applications exist because principal of operation applicable to all organic materials and most inorganic gases. Includes monitoring integrities of composite materials in service, remote sensing of hazardous materials, and examination of processes in industrial reactors and furnaces.

Druy, Mark A.↗

NASA NTRS1990

Fourier transform infrared (FTIR) fiber optic monitoring of composites during cure in an autoclave

Real-time in situ monitoring of the chemical states of epoxy resins was investigated during cure in an autoclave using infrared evanescent spectroscopy. Fiber evanescent sensors were developed which may be sandwiched between the plies of the prepreg sample. A short length of sapphire fiber was used as the sensor cell portion of the fiber probe. Heavy metal fluoride glass optical fiber cables were designed for connecting the FTIR spectrometer to the sensor fiber within the autoclave. The sapphire fibers have outstanding mechanical thermal properties which should permit their use as an embedded link in all thermoset composites. The system is capable of operation at a temperature of 250 C for periods up to 8 hours without major changes to the fiber transmission. A discussion of the selection of suitable sensor fibers, the construction of a fiber-optic interface, and the interpretation of in situ infrared spectra of the curing process is presented.

Druy, Mark A.↗

NASA NTRS1991

New addition curing polyimides

In an attempt to improve the thermal-oxidative stability (TOS) of PMR-type polymers, the use of 1,4-phenylenebis (phenylmaleic anhydride) PPMA, was evaluated. Two series of nadic end-capped addition curing polyimides were prepared by imidizing PPMA with either 4,4'-methylene dianiline or p-phenylenediamine. The first resulted in improved solubility and increased resin flow while the latter yielded a compression molded neat resin sample with a T(sub g) of 408 C, close to 70 C higher than PME-15. The performance of these materials in long term weight loss studies was below that of PMR-15, independent of post-cure conditions. These results can be rationalized in terms of the thermal lability of the pendant phenyl groups and the incomplete imidization of the sterically congested PPMA. The preparation of model compounds as well as future research directions are discussed.

Frimer, Aryeh A.↗