Microinjection molding of polypropylene/multi‐walled carbon nanotube nanocomposites: The influence of process parameters

Abstract

Polypropylene (PP) filled with 10 wt% multi‐walled carbon nanotube (CNT) nanocomposites were prepared via masterbatch dilution and subjected to microinjection molding (μIM) under various processing conditions. The molding conditions were altered by systematically changing the machine variables, such as: melt temperature, mold temperature, backpressure and injection velocity. A mold insert with a three‐step decrease in thickness along the flow direction was adopted. The effect of molding parameters on the electrical conductivity and dimensional stability of as‐molded microparts was evaluated using the design of experiments (DOE) method. The distribution of maximum shear rates along the flow direction was simulated via Moldflow, and the state of dispersion of CNT within the microparts was examined by scanning electron microscopy (SEM). In addition, the thermal behavior of the microparts molded from unfilled PP and PP/CNT 10 wt% nanocomposites at different sampling positions along the flow direction was studied by differential scanning calorimetry. Results showed that the crystallization process of unfilled PP taken from different regions of the microparts is temperature dependent, which was ascribed to the variations of shearing effects undergone by the polymer melt during μIM, while this effect is not significant for CNT loaded systems. POLYM. ENG. SCI., 58:E226–E234, 2018. © 2017 Society of Plastics Engineers

Properties of microinjection‐molded polypropylene/graphite composites

“…The FD σ for both the thick and middle sections of microparts is higher than the TD σ of corresponding counterparts, which indicates a role for the preferential orientation of conductive fillers along the melt FD. A similar trend has been observed for their CNT‐containing PP counterparts .…”

“…Pan et al reported that the cooling time drops dramatically with a reduction of mold cavity thickness. Therefore, it is anticipated that the generated structure in the thin section (0.2 mm) can instantly “freeze” and have little chance returning to a random orientation . In this scenario, the TD σ for the thin section of corresponding microparts can be substantially reduced due to a lack of sufficient conductive pathways.…”

“…Typical products from μIM include microgears, microneedles, microfilters, and microfluidic devices . The common feature for these microparts is the very high surface‐area‐to‐volume ratio (up to 10 3 –10 6 /m) , which poses certain challenges for the mold filling process due to the very high cooling rate effects in μIM . To avoid premature solidification of polymer melts, certain processing parameters in μIM (such as melt and mold temperatures as well as injection velocities) are typically higher than those in CIM .…”

Microinjection molding of multiwalled carbon nanotubes (CNT)–filled polycarbonate nanocomposites and comparison with electrical and morphological properties of various other CNT‐filled thermoplastic micromoldings

“…The orientation of CNT is enhanced with the increasing shearing effect in the melt filling direction, arising from the combined effects of higher injection speed and convergence of mold cavities in flow direction . In a recent study, Zhou et al found that an increase of melt temperature and mold temperature is advantageous to the enhancement of electrical conductivity ( σ ) in subsequent polypropylene (PP)/CNT microparts. Moreover, the morphology observations indicated that CNT achieved a relatively uniform dispersion in PS, whereas the appearance of CNT aggregates was detected in PP .…”

“…In a recent study, Zhou et al found that an increase of melt temperature and mold temperature is advantageous to the enhancement of electrical conductivity ( σ ) in subsequent polypropylene (PP)/CNT microparts. Moreover, the morphology observations indicated that CNT achieved a relatively uniform dispersion in PS, whereas the appearance of CNT aggregates was detected in PP . Thus, the melt and mold temperatures are important process parameters, which determine the microstructure of polymer nanocomposites in μIM.…”