Influence of long chain branching on fiber diameter distribution for polypropylene nonwovens produced by melt blown process

摘要

In this work, linear isotactic polypropylene (L-PP) and long-chain branched polypropylene (LCB-PP) miscible blend, both having comparable weight average molecular weight, zero-shear viscosity, and polydispersity index, were used to produce nonwovens via melt blown technology in order to understand the role of long chain branching in the fiber diameter distribution. Basic morphological characteristics of produced nonwoven samples have been determined using digital image analysis of scanning electron microscope images considering different magnifications to capture nanofibers as well as microfibers. At the same air flow rate, polymer flow rate, and temperature, the average fiber diameters were the same, 1.6 mu m, but the coefficient of variation, CV, was greater for the linear PP than for the blend. Material elasticity was assessed by reptation-mode relaxation time, lambda, determined by fitting of deformation rate dependent shear viscosity by Cross and Carreau-Yasuda models as well as via fitting of frequency dependent loss and storage moduli master curve by a two-mode Maxwell model. It was found that lambda is higher for LCB-PP in comparison with L-PP and the Cross model gives a meaningful relaxation time while the Carreau-Yasuda model does not despite giving a better numerical fit. Extensional rheology was assessed by the strain rate dependent uniaxial extensional viscosity (estimated from the entrance pressure drop using the Gibson method). The infinite shear to zero-shear shear viscosity ratio eta(infinity)/eta(0) (obtained directly from the shear viscosity data measured in a very wide shear rate range) was shown to be proportional to the maximum normalized extensional viscosity at very high extensional strain rates, eta(E),(infinity)/(3 eta(0)). eta(infinity)/eta(0) was related to temperature and basic molecular characteristics of given polymers via simple equation. It was observed that extensional viscosity for both samples first decreases with increased extens