In experiments conducted as part of a continuing effort to incorporate multifunctionality into advanced composite materials, blends of multi-walled carbon nanotubes and a resin denoted "PETI-330" (wherein "PETI" is an abbreviation for "phenylethynyl- terminated imide") were prepared, characterized, and fabricated into moldings. PETI-330 was selected as the matrix resin in these experiments because of its low melt viscosity (<10 poise at a temperature of 280 °C), excellent melt stability (lifetime >2 hours at 280 °C), and high temperature performance (>1,000 hours at 288 °C). The multi-walled carbon nanotubes (MWCNTs), obtained from the University of Kentucky, were selected because of their electrical and thermal conductivity and their small diameters. The purpose of these experiments was to determine the combination of thermal, electrical, and mechanical properties achievable while still maintaining melt processability.
The PETI-330/MWCNT mixtures were prepared at concentrations ranging from 3 to 25 weight-percent of MWCNTs by dry mixing of the constituents in a ball mill using zirconia beads. The resulting powders were characterized for degree of mixing and thermal and rheological properties. The neat resin was found to have melt viscosity between 5 and 10 poise. At 280 °C and a fixed strain rate, the viscosity was found to increase with time. At this temperature, the phenylethynyl groups do not readily react and so no significant curing of the resin occurred. For MWCNT-filled samples, melt viscosity was reasonably steady at 280 °C and was greater in samples containing greater proportions of MWCNTs. The melt viscosity for 20 weight-percent of MWCNTs was found to be ≈28,000 poise, which is lower than the initial estimated allowable maximum value of 60,000 poise for injection molding. Hence, MWCNT loadings of as much as 20 percent were deemed to be suitable compositions for scale-up.
High-resolution scanning electron microscopy (HRSEM) showed the MWCNTs to be well dispersed in the polymer matrices, while high-resolution transmission electron microscopy shows splits in the walls of the MWCNTs but no catastrophic breakage of tubes. To further assess processing characteristics prior to scale-up, samples containing 10, 15, and 20 weight-percent of MWCNTs were processed through a laboratory melting extruder. HRSEM of the extruded fibers shows significant alignment of MWCNTs in the flow direction (see figure). For the samples containing 20 weight-percent of MWCNTs, difficulties were encountered during feeding, and the temperature of a rotor in the extruder rose to 245 °C because of buildup of frictional heat; this indicates that materials of this type having MWCNT concentrations ≥20 weight-percent may not be melt-processable.
On the basis of the results from the foregoing characterizations, samples containing 10, 15, and 20 weight-percent of MWCNTs were scaled up to masses of ≈300 g and used to make specimens having dimensions of 10.2 by 15.2 by 0.32 cm. These specimens were molded by (1) injecting the mixtures, at temperatures between 260 and 280 °C, into a tool made of the low thermal-expansion alloy Invar® and then (2) curing for 1 hour at 371°C. The tool was designed to impart shear during the injection process in an attempt to achieve some alignment of the MWCNTs in the flow direction.
Qualitatively, the moldings from the 10 and 15 weight-percent samples appeared to be good. The moldings were subsequently characterized with respect to thermal, mechanical, and electrical properties. However, as expected from the results of the extrusion experiments, the 20 weight-percent sample could not be injected because of its higher viscosity.
The hardness value of each molded PETI-330/MWCNT specimen was found to be lower than that of the neat resin in the sense that an indenter was found to penetrate to a greater depth or an enhanced plastic deformation of the material was observed. The neat resin specimen was found to be electrically insulating. For the other specimens, the electrical resistivity was found to decrease with increasing concentration of MWCNTs, ranging from 8.86 × 103 Ω /cm for the 10 weight-percent sample to 5.13 × 103 Ω/cm for the 15 weight-percent sample. The thermal conductivities were found to increase with the proportion of MWCNTs, ranging from 0.219 W/(m·K) for the neat resin specimen to 0.577 W/(m·K) for the 10 weight-percent specimen and 0.777 W/(m·K) for the 15 weight-percent specimen. This trend in thermal conductivity suggests that nanotubes form networks in the polymer matrices that conduct heat, but not to the extent expected based on the high thermal conductivity of the MWCNTs.
This work was done by John W. Connell, Joseph G. Smith, Emilie J. Siochi, and Dennis C. Working of Langley Research Center; Jim M. Criss of M&P Technologies; Kent A. Watson and Donavon M. Delozier of the National Institute of Aerospace; and Sayata Ghose of the National Research Council. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Materials category. LAR-17082-1