Controlled-peroxide degradation of polypropylene: Rheological properties and prediction of MWD from rheological data

Abstract

In this study, some experimental results of the peroxide-degradation process of polypropylene (PP) in a co-rotating twin-screw extruder to produce controlled-rheology polypropylene (CRPP) are presented. The peroxide was dicumyl peroxide (DCP) and the concentration of DCP was in the range 0–0.6 wt%. It was found that the rheological properties of PP change significantly during reactive extrusion. Melt flow index (MFI) increased with DCP concentration. Complex viscosity decreases with increasing DCP concentration. Degraded resins showed more Newtonian behavior than the origin resin. Cole–Cole plots indicated that mean relaxation times of CRPP resins are shorter than for virgin PP, and the relaxation mechanism is more uniform for modified samples. From dynamic rheological data, molecular weight (MW) and molecular weight distribution (MWD) were calculated. Results indicated that MW decreases and MWD becomes narrower with increasing peroxide concentration.

Introduction

Modification of the molecular structure of polypropylene (PP) during reactive extrusion after polymerization by the action of peroxide-generated radicals is known as controlled rheology or vis-breaking. The product of this process is so called the controlled-rheology polypropylene (CRPP) [1], [2], [3]. The process involves the blending of appropriate peroxides [4] with PP. During melt extrusion of this mixture the peroxide thermally decomposes to produce peroxy radicals. These radicals attack the polymeric chains [5], [6]. This process converts the low melt flow index (MFI) commodity resins to polymers with higher MFI (up to 100 times the original value) that have superior processing properties because of the reduced viscosity and elasticity. The advantages of CRPP are less elasticity, less shear sensitivity, less part warpage and better physical properties such as clarity and gloss [7], [8]. There is a large body of publications that describe peroxide degradation of PP [9], [10], [11], [12], [13]. The effect of processing parameters and the kinetics of controlled degradation process were studied by Ryu et al. [12], while the rheological behavior of CRPP was studied by Berzin et al. [13].

It is well recognized that the rheological properties of polymer melts depend strongly on the molecular structure, especially molecular weight (MW) and molecular weight distribution (MWD) [14]. In recent years, there has been considerable interest in this area regarding the prediction of MW and MWD from rheological data. Since there are many viable methods of determining the MWD of flexible polymers, such as gel permeation chromatography, light scattering, intrinsic viscosity, etc., it is important to note what specific advantages having a rheological MWD determination method provides.

It should be noted that many polymers of commercial importance, such as Teflon, polyethylene and PP, are only slightly soluble in common solvents at ambient temperatures. Therefore, rheological methods are best for obtaining the MWD of these polymers. Several methods have been used for prediction of MWD from rheological data [15], [16], [17] and some software has also been developed. William H. Tuminello and N. Cudre-Mauroux showed that MWD can be determined from linear viscoelastic melt properties, such as the shear storage modulus, G′(ω) or the stress–relaxation modulus G(t) and from viscosity vs. shear rate η ($\dot{\gamma }$) [18].

In our two previous papers, mechanical, thermal and some rheological properties of controlled-degraded PP were investigated [19], [20]. In this study, a Cole–Cole plot was used to evaluate the relaxation mechanism of CRPP. In addition, MWD and MW of different samples containing different concentrations of peroxide were calculated from dynamic rheological data.