Generation of high performance polyphenylene sulfide-thermotropic liquid crystalline polymer composite filaments for use in fused filament fabrication

Abstract

In this work, a novel dual extrusion process was used to fabricate high performance continuously-reinforced composite filaments for use in Fused Filament Fabrication (FFF). The processing conditions required to reinforce polyphenylene sulfide (PPS) using a commercially available thermotropic liquid crystalline polymer (TLCP), composed of terephthalic acid (TA), 4-hydroxybenzoic acid (HBA), hydroquinone (HQ) and hydroquinone derivatives (HQ-derivatives) were established. The TLCPs possess high mechanical properties arising from the stearic hindrance of their rodlike monomers. The TLCP of interest melts at 320 ℃ but can stay molten up to around 35 ℃ below its melting temperature when cooled from 360 ℃, exhibiting a phenomenon known as supercooling behavior. However, the melt stability was found to vary drastically with the temperature. At 290 and 295 °C, the sample begins to solidify in 43 and 317 seconds, respectively. However, at 300 °C and above, the melt is stable for more than 3600 seconds. Isothermal time sweeps performed on PPS indicate an unstable melt complex viscosity above 320 ℃. Shear step strain tests performed on the TLCP indicate that the TLCP should be heated to at least 360 °C to melt all the residual crystallites. Due to the difference in processing temperatures, the filaments are generated using a unique process in which the higher melting point TLCP is spun within the matrix to form an in situ reinforced filament. The resins are plasticated in two different extruders at different temperatures and then the stream of the TLCP is injected into the stream of PPS. The composite stream is passed through a series of static mixers which subdivide the streams into finer streams, and then the composite stream is drawn to impart molecular orientation to the TLCP phase. The filaments so produced are processed in FFF by melting the PPS without melting the TLCP. The properties of the plaques generated from these strands will be reported.