Correlation of low density polyethylene rheological measurements, with optical and processing properties

Abstract

The processing properties of' low density polyethylene melts, such as drawdown and neck-in, and the final product properties, such as film haze and gloss, have been successfully correlated with rheological functions and the level of longchain branching. The rheological functions employed are the entrance pressure drop AP,, and the swell ratio So, determined at a specified shear stress using an orifice with a length/ diameter (LID) ratio of zero. The calculation of shear stress requires additional measurements using a die with a finite LID e.g. 20. The rheological functions AP,, and S,, are governed by the level of high molecular weight species and/or the level of long-chain branching(LCB). If determined at a constant shear stress, in order to eliminate the effect of viscosity, they are a relative measure of elasticity. Higher AP, and S,, indicate a higher level of LCB and correlate with poorer optical properties and drawdown in films.

A comprehensive investigation of the origins of surface roughness and haze in polyethylene blown films

“…1 and 2 is that it often serves as a good measure of the elasticity of the melt,42 and furthermore has been useful in correlating with other processing phenomena, such as die swell and melt fracture 43, 44. The choice of the low frequency at which γ ∞ was estimated rests in the fact that differences in the elastic character of various melts are magnified at low frequencies (or shear rates) because they probe the longest relaxation time behavior at these conditions 8. Furthermore, the approximation in eq.…”

“…A review of some of the earlier work in this area reveals that both low and high polymer melt elasticity have been shown, either directly or as we interpret their data now, to be responsible for high haze 2, 5–8, 10, 19. For example, it has been shown that shear modification (disentanglement) of highly branched, high‐pressure (also referred to as conventional), low‐density PE (HP‐LDPE) resins resulted in an improvement (decrease) in blown film haze due to a decrease in the melt elasticity 5, 19.…”

“…Optical properties such as haze and clarity are of considerable importance in the development of polyethylene (PE) blown and cast films for packaging applications. As a result, factors affecting these optical properties have been the focus of investigations for many years 1–36. It was recognized early on that in PE blown films, haze was primarily caused by the scattering of light due to surface irregularities 1–3, 10.…”

Melt flow index: More than just a quality control rheological parameter. Part I

“…Since the values of temperature and shear rate employed in an MFI test differ substantially from those encountered in actual large scale processes, MFI does not correlate directly with processing behavior, as has been shown by Smith3' as well as Shida et al 38 As far as processing is concerned, both high and low shear viscosities of the material are critical. Low shear properties are important in applications where melt strength, sagging are critical such as in blow molding and coating.…”

Prediction of processability at extrusion coating for low‐density polyethylene

“…Therefore, converters have to employ an appropriate grade that have a good balance between the maximum drawdown speed and the neck-in level. For the purpose, extensive efforts have been carried out to correlate the processability with the polymer structure [2][3][4][5] or the rheological properties [5][6][7][8][9][10][11], which can be evaluated in a laboratory scale, in order to avoid the processing trial by a manufacturing machine.…”