Material PropertiesControlled-peroxide degradation of polypropylene: Rheological properties and prediction of MWD from rheological data
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 () [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.
Section snippets
Materials
The PP was a homopolymer from Iran Petrochemical Co. with trade name S30S (MFI=1.1 g/10 min, density=0.9 g/cm3). The peroxide as initiator was dicumyl peroxide (DCP) provided by Hercules in powder form (Di-cup90, density=1.02 g/cm3).
Reactive extrusion process
The experiments were carried out in a laboratory, fully intermeshing, co-rotating, twin-screw extruder.
The diameter of each screw was 50 mm and L/D=15/1 with six heating/cooling zones. Before feeding to the extruder, PP and DCP were premixed in a turbo mixer at 70 °C for 7
Conclusions
From results reported in this study on the free-radical-induced degradation poly propylene (PP) in the melt phase using peroxide, the following conclusions are obtained:
- 1
Addition of peroxide to PP influences the melt rheological properties of PP.
- 2
MFI increases with increasing DCP concentration, whilst complex viscosity and storage modulus decrease.
- 3
Degraded resins show more Newtonian behavior and less elasticity compared to virgin PP.
- 4
From Cole–Cole plots it can be concluded that mean relaxation
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