Effect of stereoregularity of polypropylene on flow instability in capillary extrusion

Syndiotactic polypropylenes (sPPs) with several microstructures (i.e., syndiotacticities and molecular weights) and synthesized by means of two metallocenic catalysts were melt-blended with 1 and 3 wt % organophilic layered silicates in the presence of a compatibilizer. X-ray diffraction and transmission electron microscopy analysis showed that the clay was well dispersed in the composites, although the filler morphology depended on the polymer microstructure. Polypropylenes with low syndiotacticities and molecular weights presented the best clay dispersion. Nonisothermal differential scanning calorimetry analysis showed that the polymer microstructure and the clay content modified the thermal behavior of the composites. The compatibilizer and the clay acted as nucleant agents to increase the crystallization temperature of the matrix. Moreover, the double endothermic peak observed during heating scan and associated with the melt/recrystallization/remelt processes of the pure polymer matrix was reduced in the composites. With regard to the mechanical properties under tensile conditions, a synergic effect of the compatibilizer and the clay was observed. In particular, the addition of the compatibilizer alone was able to increase by about 20% the elastic modulus relative to the neat samples, whereas increases between 35 and 50% were measured when the clay was also added, depending on the polymer microstructure. Our results show that the microstructure of sPPs had strong effects on the behavior of its composites with clay in the presence of a compatibilizer.

Section: Resultsmentioning

confidence: 96%

Effect of the polymer microstructure on the behavior of syndiotactic polypropylene/organophilic layered silicate composites

Palza

Zurita

2011

“…This equation provides the information on processing failures due to the pronounced melt elasticity (high Deborah number) including the shark‐skin failure . When a polymer melt has narrow molecular weight distribution, that is, small f , Deborah number becomes large at a constant shear stress.…”

Section: Resultsmentioning

confidence: 99%

Viscoelastic properties and extrusion processability of poly(vinyl butyral)

Arayachukiat

Siriprumpoonthum

Nobukawa

et al. 2014

Self Cite

Rheological properties and flow instability at capillary extrusion of a random terpolymer composed of vinyl butyral, vinyl alcohol, and vinyl acetate, that is denoted as PVB in this article, are studied. It is found that the rubbery plateau modulus G 0 N is 1.3 MPa at 100 C from the oscillatory shear modulus. Furthermore, the average molecular weight between entanglement couplings M e is found to be 2670. Because of the relatively high value of G 0 N , it shows rubbery region in the wide temperature range (90 C-180 C). At the capillary extrusion, the surface instability (shark-skin failure) appears prior to volumetric melt fracture. The onset stress of the shark-skin failure, ca. 0.18 MPa, is similar to that of polyethylene, although PVB used in this study has narrow molecular weight distribution. Moreover, the apparent slippage is not detected, presumably due to good adhesion to the die wall. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40337.

“…The mechanism of flow instability at capillary extrusion has been studied for a long time 29–45. The gross melt fracture is the flow instability occurred at die entrance, which is prominent for a polymer melt with high melt elasticity, such as LDPE and poly(vinyl chloride) 45, 46. Meller found that elongational stress generated by contraction flow at die entrance decides the occurrence of gross melt fracture 36.…”

Section: Introductionmentioning

confidence: 99%

“…Since LDPE exhibit marked strain‐hardening in elongational viscosity, leading to high elongational stress, gross melt fracture is always detected. Recently, Mieda and Yamaguchi revealed that binary blends of LDPE and LLDPE with high molecular weight exhibit severe gross melt fracture owing to enhanced elongational stress 45. On the contrary, LLDPE and HDPE show shark‐skin failure before gross melt fracture when shear stress is beyond the critical value.…”

Section: Introductionmentioning

confidence: 99%

“…Roughly speaking, two types of mechanism have been proposed for shark‐skin failure; one is owing to cohesive rupture at surface of extrudates by sudden large deformation at die exit, originally proposed by Cogswell30; and the other is due to the detachment of a melt from die surface accompanied with cracks by adhesive failure, which was summarized in recent works by Kulikov et al39, 43 Moreover, the critical stresses of shark‐skin failure of both mechanisms were theoretically derived by Allal et al,42 which agree with the experimental data obtained by Yamaguchi et al37 using model polyolefins. Yamaguchi et al37, 44, 45 also explained the onset of shark‐skin failure as well as gross melt fracture using the concept of Deborah number. Although their explanation does not predict the mechanism, it provides the direction to solve the problem.…”

Section: Introductionmentioning

confidence: 99%

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Effect of shear history on flow instability at capillary extrusion for long‐chain branched polyethylene

Siriprumpoonthum

Mieda

Doan

et al. 2011

Self Cite

Effect of applied processing history on flow instability at capillary extrusion is studied using a commercially available low-density polyethylene (LDPE) having long-chain branches. It is found that processing history in an internal mixer in a molten state depresses long-time relaxation mechanism associated with long-chain branches, which is known as ''shear modification.'' Consequently, the onset of output rate for melt fracture increases greatly. Furthermore, it should be noted that the sample having intense shear history shows shark-skin failure without volumetric distortion, although it has been believed that LDPE exhibits gross melt fracture at capillary extrusion. The reduction of elongational viscosity by the alignment of long-chain branches along to the main chain is responsible for the anomalous rheological response. As a result, the sample shows shark-skin failure like a linear polyethylene at a lower output rate than the critical one for gross melt fracture.