Effect of shear modification on the rheological behavior of two low‐density polyethylene (LDPE) grades

Abstract

Shear modification treatment represents a special shearing history affecting mainly the elastic behavior of polymer melts. This process has been attributed to reversible physical changes in the entanglement structure of the polymer chain network. Shear modification studies were performed for two well-characterized low-density polyethylene (LDPE) grades differing in molecular weight distribution and degree of long chain branching. The shear working of the material was carried out using a specially designed shearing unit producing definable amounts of pure shear in a continuous process. Measurements of the dynamic storage modulus, G ', steady-state shear compliance; J,, extrudate swell, melt flow index, and the extensional behavior (Rheotens test) indicate that primarily properties associated with the elasticity of the melt are reduced in value. The observed reduction is found to correlate with the mean specific energy dissipated during sample preparation. Comparing the two LDPE grades showed that higher degrees of modification can be obtained at lower energy input levels for the more highly branched grade. Reversibility tests were performed and complete recovery of the initial material behavior was observed. Comparison of measurement results for samples prepared using the shearing unit presented here and a Brabender Plasticorder indicates that the degree of modification depends not only on the molecular structure of the polymer but also on the manner in which the shearing history is imposed upon the material.

Grafting of polyethylenes by reactive extrusion. II. Influence on rheological and thermal properties

“…Although the MFI value is supposed to represent the reciprocal of the particular viscosity measured at the MFI condition of temperature and shear stress, it is strongly influenced by the elasticity of the polymer because of the short L / D ratio and the flat entry of the capillary. The highest MFI values found for the resins extruded once (Table VI) are due probably to the effect of shear modification 5. The influence of the grafting degree, screw configurations, grafting temperature, and type of peroxide on the MFI value is consistent with the mean residence time in the extruder and the kinetics of the peroxide decomposition as was found in previous work 13.…”

“…The rheology of polyethylene (PE) melts has been extensively studied 1–5. The correlations between rheological functions and molecular parameters such as weight‐average molecular weight ( M w ), molecular weight distribution (MWD), short‐ and long‐chain branching (SCB and LCB), and comonomer content have been reported 1–4.…”

Gas atomization of polymers

“…This prior deforming of the melt is said to cause partial disentanglement of the polymer network structure, thus changing the density of the material [24].…”

“…Therefore, not only does the atomization process lower the percent crystallinity of a material, it lowers the density as well. The decrease in density due to atomization can be compared to the prior working of a polymer melt which causes partial disentanglement of the polymer network structure, thus changing its density [24].…”

“…The results from the dynamic testing which was completed using the DSR, will be useful to assess the ability of the current GAP system to atomize higher viscosity materials. The dynamic testing gives material information on molecular relaxation times which will be important when atomizing these materials [24].…”