Flow Instabilities in Linear and Branched Syndiotactic Poly(propylene)s

Abstract

Extrusion flow experiments of linear and branched syndiotactic poly(propylene)s were carried out. The work was focused on flow instabilities. Ionized radiation was employed to induce long chain branching in linear samples. Sharkskin and melt fracture were postponed in the case of slightly long branched samples, which possess an enhanced melt elasticity compared to linear samples. For the most elastic samples the nature of the flow instability changed: sharkskin disappeared and melt fracture was observed instead. The correlation between sharkskin and melt strength results is discussed.magnified image

Effect of stereoregularity of polypropylene on flow instability in capillary extrusion

“…On the contrary, gross melt fracture is detected for iPP-2 at the stress level of 0.178 MPa. These experimental results do not correspond with the data by earlier researchers 6,10 for unknown reasons. However, the theoretical approach to flow instability in capillary extrusion, proposed by Allal et al 22,23 and Yamaguchi et al, 24 supports the present experimental results.…”

“…24 It is apparent from Eqs. (1)- (3) that the experimental results reported by Tapadia et al 6 and Rojo et al 10 cannot be explained by the theoretical approaches. In this study, the flow instability in capillary extrusion for sPP produced by metallocene catalyst is compared with that for iPP employing commercially available materials including high viscous polymers.…”

“…In particular, there have been only two reports on the flow instability for sPP at the best of our knowledge. 6,10 Tapadia et al found that shark-skin failure is more apt to be the cause of the flow instability for sPP. They also showed the onset stress of shark-skin failure is lower than that of iPP.…”

Rheological Features and Flow‐Induced Crystallization of Branched Poly[ethylene‐co‐(1,4‐cyclohexanedimethylene terephthalate)] Copolyesters

“…In contrast, a reactive extrusion process of PET with polyanhydride and polyepoxide modifiers gave rise to increased viscosities, [18][19][20][21][22] attributed to a combination of long chain branching, molecular weight increase and broadening of the molecular weight distribution. In the previous research carried out by some of the authors on rheological effects of long chain branching in polyethylenes [29][30][31][32] and poly(propylenes), [33][34] a very clear positive deviation from the general approximate scaling law of linear polymers h 0M 3:4 w was observed for slightly branched samples. We recall that although moderate or high levels of LCB in polyethylene produce a decrease in viscosity, because of a reduction of the hydrodynamic volume, [35] low levels of LCB slow the diffusion of the chains resulting in a viscosity increase.…”

In situ Pressure Fluctuations of Polymer Melt Flow Instabilities: Experimental Evidence about their Origin and Dynamics

“…At higher shear rates, regular surface instabilities can appear called a ‘sharkskin’, which is characterized by both small amplitude and high frequency distortions 3. Metallocene short‐chain branching polyethylenes,4 ethylene/propylene copolymers,5 polymers with high molecular weights and low polydispersities,6 and syndiotactic poly(propylenes),7, 8 are examples of materials that develop this instability. Upon increasing the shear rate, some polymers show spurt instability (also named stick‐slip, and is different to the sharkskin phenomenon) characterized by large pressure oscillations where the extrudate alternates between rough and smooth sections.…”