THREE ROUTES TO STRUCTURAL NANOCOMPOSITES THROUGH NANOPARTICLE INFUSION INTO MATRIX, FIBER AND CORE MATERIALS
Dr. Hassan Mahfuz
Center for Advanced Materials (T-CAM)
Thursday, June 17th, 2004, 2:00 pm
Ben Bandy Conference Room
UK Center for Applied Energy Research
It has been established in recent years that polymer-based composites reinforced with a small percentage of strong fillers can significantly improve the mechanical, thermal and barrier properties of the pure polymer matrix. Moreover, these improvements are achieved through conventional processing techniques without any detrimental effects on processability, appearance, density and aging performance of the matrix. The benefits of nanoparticle infusion comes from the fact that the large amount of interphase zones in nanocomposites may serve as catalysts for prolific crack growth creating a greater amount of new surfaces. The creation of new surfaces can serve as efficient mechanisms to dissipate kinetic energy, for example, in the event of an impact. These interphase zones can also be visualized as defects, the density of which will be very high in nanocomposites such that the spacing between them will approach interatomic distances and a large fraction of atoms will sit very adjacent to a defect. Any brittle crack developed in the material will therefore, get deflected and branch out into these defects, attributing a crack-blunting feature to the composites. If these unique features can be imparted to the two main constituents, namely, the fiber and the matrix of structural composites, it may bring about significant improvement in the performance of the resulting composites.
In order to develop structural composites based on the above concept of nanotechnology a systematic study has been carried out at Tuskegee during the last two years. The study began with the infusion of both micron and nano-sized SiC particles into an epoxy matrix. The amount of particle loading varied from 1.5 - 3.0 % by weight of the resin. Nanoparticle infusion was carried out through high intensity Vibrasound liquid processor. Ultrasonic mixing utilized high energy sonic waves to force an intrinsic mixing of particles with the matrix via sonic cavitations. It has been observed that with equal amounts of loading; nanoparticle infusion brings about superior thermal and mechanical properties to the matrix than what is usually obtained by the micro fillers infusion. It was also found that nanoparticle infusion increases the decomposition temperature of the matrix significantly, adds crystallinity to the matrix, and also reduces the void content. These improvements were eventually reflected on the mechanical properties by increasing the strength and stiffness by about 20-25% under tension and flexure.
Once the matrix was modified, it was reinforced with commercial fibers (satin weave carbon) through vacuum assisted resin transfer molding (VARTM) process to fabricate laminated composites. It is to be noted that the resin retained sufficient viscosity with 1.5 - 3.0%wt loading to be processed through VARTM. These laminates were then tested under quasi-static, high strain rate, and cyclic loading. In each case, similar enhancement in mechanical properties was found as it was observed with the nano-modified matrix. Flexural fatigue behavior was however, somewhat different; it was seen that only below a threshold stress level, nanophased composites were outperforming their neat counterparts.
In another route, nanoparticles having aspect ratios such as carbon nanowhiskers and carbon nanotubes were doped with textile precursors. Two types of polymer precursors were used; linear low density polyethylene and nylon-6. The fillers in each case were carbon nanowhiskers and multi-walled carbon nanotubes (MWCNT). Amount of loading varied from 1-2% by weight of the base polymer. Nanoparticles were first mixed with the base polymer in powdered form. The mixture was then dried in a hopper and fed through a single screw extrusion machine. It passed through three stages of heating where the temperature was set slightly above the melting temperature of the polymer. After heating and at the end of the screw, the molten mass was forced through two stages of mixing. In the final stage, filaments were extruded through a micron size orifice. The extruded filaments sequentially went through a cooling trough, a tension device, a heater, and eventually wound into a filament winder to form spools. In one attempt, these filaments were cut into strands, laid out in 00 fashions in several layers, and consolidated in a compression molding machine. Test coupons were extracted from these laminates and were tested under tension and flexure. It was found that the improvement with carbon nanowhiskers was around 17% while it increased to 34% with MWCNT. Out of the four systems (two fillers and two polymers) investigated, the system with Nylon-6 infused with MWCNT yielded the most promising results. Tension tests on individual filament of this system showed about 150-300% improvement in strength and stiffness with 1% MWCNT loading. TEM studies revealed that extrusion technique caused sufficient alignment of MWCNT along the length of the filament which may have caused the gain in mechanical properties.
In the third route, an innovative technique to develop polyurethane foams containing nano particles has been introduced. Polymethylene polyphenylisocyanate (part A) is mixed with nanoparticles such as SiC and TiO2, and irradiated with a high power ultrasound liquid processor. In the next step, the modified foams containing nanoparticles are mixed with part B (containing polyol resin systems, surfactant, and an amine catalyst) through a high-speed mechanical stirrer. The mixture is then cast into rectangular molds to make nanophased foam panels. Test coupons were then extracted from the panels to carry out morphological and mechanical characterizations. The as-prepared foams were characterized by scanning electron microscopy (SEM), X-ray diffraction, and thermo gravimetric analysis (TGA). The SEM studies have shown that the particles are non-agglomerated and well dispersed in the entire volume of the foam. The foam cells are well ordered and uniform in size and shape. The TGA analyses indicate that the modified foams are thermally more stable than the parallel neat system. Quasi-static flexure tests under three-point bend configuration have also been conducted with both modified and neat foams. Test results show a significant increase (approximately in the range of 50-70%) in the flexural strength and stiffness of the nanophased foams over the neat system. This enhancement in flexural properties have been demonstrated repeatedly with multiple batches and with at least three specimens tested from each batch.
In summary, it has been observed that in all cases of nanoparticle infusion namely in matrix, in textile precursors and in liquid foam there are significant improvements in chemical and mechanical properties of the resulting nano-composites. This improvement takes place with a very low loading of nanoparticle, and there seems to be an optimal loading corresponding to each system of nanoparticle and the polymer. The study also ensures that the traditional composite fabrication routes can be successfully utilized to make structural nanocomposites.