The addition of nano-scale and micro-scale fillers has been proven to increase tensile and thermal properties in polymer composites. Orientation of high aspect fillers, however, has not been studied before despite being crucial to altering physical properties. When fibers are included during extrusion, they tend to align in the direction of the flow. This phenomena leads to longitudinal improvements in mechanical properties, and thus provides great benefits in some applications; however, it is beneficial to have improved properties in the transverse direction as well. Therefore, it is crucial to study reorientation phenomena in composites.
The purpose of this experiment is to study property enhancement resulting from fiber structure. The material properties are compared for the range of weight percentages of fillers. This is done for the purpose of finding an ideal fill concentration. Two dies are used to study different orientation distributions: straight and divergent. Thermal and tensile properties and optical micrographs are analyzed and compared.
Composites were processed on a Coperion ZDSK-28mm co-rotating, fully-intermeshing, twin-screw extruder. Polybutylene terephthalate (PBT) was used as the polymer matrix. 0 W% to 2 W% multi-walled carbon nanotubes (CNTs) and 0 W% to 30 W% carbon microfibers (CMFs) were used as fillers.
Preliminary results showed a clear trend in increased tensile strength of the composite with the increase of concentration of CMFs and CNTs in the slit die up to 25 W% CMF. After 25 W% CMF, however, there was a depreciation in properties. Similarly, thermal conductivity results have shown a clear peak at 25 W% CMF with 30 W% showing a decrease in thermal properties.
Preliminary results for the divergent die showed that, with addition of carbon microfibers to the polymer matrix, thermal properties of the composite increased up to 15 W%, then dropped and increased again as more CMFs were added. In addition, on average, material extruded through the divergent die showed better results of thermal conductivity than that extruded through the slit die. This indicates that when using a diverging die, fiber become oriented perpendicular in relation to the direction of the flow, thus improving heat flow in the transverse direction.