Development of Kevlar®/polypropylene towpregs through optimized commingling nozzle and processing parameters for thermoplastic composites

The current study aims at optimizing nozzle diameter and processing conditions for producing quality Kevlar®/polypropylene commingled towpregs meant for thermoplastic composite applications. ANSYS 3D CFD simulation and various experimental techniques are used for the same purpose. The study also investigates the effect of optimized commingled towpregs on the consolidation quality, void % and flexural properties of the developed UD composites. It is concluded that the nozzle diameter of 7 mm has proven to be ideal for producing quality towpregs as it leads to the formation of high nip frequency at 4.5 bar air pressure. The other nozzle diameters, 3 mm, 5 mm, and 9 mm, performed poorly in forming nips. Among these diameters, the 7 mm nozzle has optimum free space between the internal wall of the nozzle and the feed fibre bundle. This optimum space keeps the filaments within the potential air interaction zone and thus leading to the individual filament displacement and entrapment to form stable nips. The Box-Behnken design expert is used for obtaining ideal commingling conditions. The optimized Kevlar®/PP towpregs are produced using a custom fabricated commingling machine with a 7 mm diameter nozzle at the processing conditions - 4.5 bar air pressure, 2 m/min delivery speed, 3% and 1% overfeed, respectively for Kevlar® and polypropylene (PP). These commingled towpregs are flexible, compact, and regular in structure. These towpregs have the highest nip frequency, nip stability, weavability and homogeneous fibre-matrix mixing quality in yarn cross-section along the length. Using such towpregs makes the textile pre-forming effortless and yields composites with better resin impregnation quality and low void content. The UD Kevlar®/PP composites made through these towpregs offered good consolidation quality with a low void (4.3%). These composites showed flexural strength and modulus of 130 MPa and 28 GPa, respectively.