Differential scanning calorimetry in dynamic and isothermal modes was used to study the cure kinetics of the commercial epoxy system Narmco 5208, whose main components are bis[4-(diglycidylamino)phenyljmethane and bis(4-aminophenyl) sulfone. The data were analyzed in terms of a new mechanistic approach described in the preceding paper. The treatment explicitly takes into account both the epoxideamine reactions and the subsequent etherification reaction. The kinetics can be completely described in terms of three rate constants, which obey the Arrhenius relationship. Excellent agreement with the experimental data is obtained if the etherification reaction is assumed to be first order with respect to the concentrations of epoxide groups, hydroxyl groups, and the tertiary amine groups formed in the epoxide-amine reaction. This model applies over the whole range of conversion up to the point where the resin vitrifies and the reaction becomes diffusion-controlled. The effect of the diffusion control is described very well by an approach based on simple equations proposed in the literature. Altogether, the model allows accurate prediction of the degree of conversion over the whole range of cure and over the temperature range 160-200°C , which covers the usual curing conditions. Although the rate constants derived are specific to Narmco 5208, the model itself is generally applicable to other epoxy amine systems.
Samples of Narmco Rigidite 5208/WC3000 carbon-epoxy prepreg were exposed to ambient temperature (22°C) and 50% relative humidity for different periods up to 66 days. They were analyzed in depth using various techniques to determine the extent of the chemical changes occurring. The physicochemical techniques used were Fourier transform infrared spectroscopy (transmission, attenuated total reflection, diffuse reflection), liquid chromatography (reverse-phase, highspeed reverse-phase, and high-performance size-exclusion), thermal analysis (differential scanning calorimetry and thermogravimetric analysis), and pyrolysis-gas chromatography. All showed evidence of significant changes, the most sensitive being Fourier transform infrared spectroscopy (FT-IR) and reversephase liquid chromatography (RPLC). FT-IR showed that the number of unreacted epoxy groups decreases steadily at a rate of 0.34% per day, based on the initial amount. At the same time, the number of free amine-hardener molecules, as monitored by RPLC, drops at a rate of 1.05% per day. RPLC also showed that the amount of initial epoxy-amine reaction product increases significantly over the first 30 days, but then declines as it undergoes further reaction to give highermolecular-weight products. The heat of polymerization of the resin, measured by thermal analysis, decreased by 0.26% per day from its initial value of 561 J/g.
Samples of Narmco Rigidite 5208/WC3000 carbon‐epoxy composite prepreg were exposed to ambient temperature (22°C) and 50 percent relative humidity for different periods up to 66 days. The aging has a significant effect on prepreg physical properties such as tack, volatiles content, and gel time. A set of 4‐ply laminates made from aged prepreg was subjected to tensile testing, ultrasonic inspection, and optothermal inspection. No relationship could be discerned between laminate properties and prepreg aging time. However, variations in panel homogeneity were observed, and these correlated with thermal diffusivity and tensile modulus measurements, but not with ultimate tensile strength or elongation. A set of 6‐ply laminates was used to measure compressive properties, interlaminar shear strength, and physical properties. These panels also showed variations in porosity, again unrelated to aging, but in addition, the fiber/resin ratio was observed to decrease with aging time. Both factors were found to affect mechanical properties. The implications concerning the importance of monitoring the aging by physicochemical methods are discussed.
SynopsisA method based on diffuse reflection Fourier transform infrared (FT-IR) spectroscopy has been developed for determining the state of crystallinity in composite materials made from poly(phenylene sulfide) (PPS) reinforced with carbon fibers. Using this technique, good-quality spectra can be obtained directly from the surface of prepreg or molded composite; thus the method is rapid and nondestructive. Several peaks in the spectrum are sensitive t o the crystallinity and can be used for quantitative characterization purposes. The recommended indicator is the ratio of the heights of the peaks a t 1075 and 1093 cm-'. Using a range of samples of varying crystallinity prepared by annealing amorphous prepreg, it has been shown that there is a very good correlation between this ratio and the enthalpy of crystallization as determined by differential scanning calorimetry. The effects of such annealing, as well as heating in air a t high temperatures, have been investigated. INTRODUCTIONIn the rapidly expanding field of high-performance composites, there is increasing interest in replacing thermoset polymer matrices by engineering thermoplastics such as polyether ether ketone (PEEK) and poly(pheny1ene sulfide) (PPS). One important advantage of these materials compared to thermosets such as epoxies and polyimides is that they are nonreactive systems. Furthermore, they generally provide better impact strength (toughness), better high-temperature performance, better resistance to solvents and other chemicals, lower moisture absorption, and better flame resistance. Disadvantages associated with thermoplastics include their rigidity, the high temperatures required for processing, and the importance of controlling their degree of crystallinity.The properties, processing, and applications of PPS have been described in recent articles.'-3 The physical properties of a part made from PPS are affected by the degree of crystallinity, which may vary from less than 10% to greater than 60%, depending on the thermal hist01-y.~ In view of the importance of the degree of crystallinity and its dependence upon processing conditions, it would be desirable to have a fast and reliable method for Journal of Applied Polymer Science, Vol. 39, 1887-1902 (1990 determining the crystallinity in a finished part. Information concerning crystallinity can be obtained through a number of instrumental methods, including X-ray diffraction, differential scanning calorimetry, and transmission infrared spectroscopy. Recently, we have demonstrated the possibilities of using diffuse reflection (DR) Fourier transform infrared spectroscopy for examining the surface of prepregs and composites containing carbon fiber reinf~rcement.~ The main advantages of this method are rapidity, ease of sample preparation, and the fact that it can be nondestructive. Prepreg or molded parts can be examined "as is," without undergoing preparation requiring destruction of the sample. The object of this paper is to demonstrate the usefulness of diffuse reflection FT-IR for providi...
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