Peroxydicarbonate modification of polypropylene and extensional flow properties
Introduction
Polypropylene (PP) is the fastest growing commodity resin in the polymer world market. It has many desirable properties when compared to other thermoplastics, such as high melting point, low density, high tensile modulus and low cost. Commercial PP is produced with Ziegler–Natta or metallocene catalysts resulting in highly linear chains and a relatively narrow molecular weight distribution. Conventional linear PP, however, has poor melt strength and cannot easily be used in processes where elongational flows are dominant, such as foaming, thermoforming, extrusion coating or blow molding. This is due to the extensional viscosity of the PP melt. Linear chains do not show strain hardening in extensional flow. Polymers with strain hardening elongational viscosity are known to exhibit high spinning viscosity and melt strength [1].
The influence of the molecular structure on the melt strength of polymers has been studied extensively. For several commercial polyethylenes (PE), Ghijssels et al. [2], [3], [4] found that the melt strength increases with decreasing Melt Flow Index (MFI) and decreases with increasing temperature in the same way as its viscosity. The melt strength of LDPE was a factor 2 higher than that of LLDPE and HDPE with the same MFI. This is probably caused by the presence of long chain branches (LCB) in the former polymer, which induce strain hardening in the elongational viscosity of the melt. The effect was stronger with the ‘tree-type’ than with the ‘comb-type’ long chain branching, while no effect could be seen by using different co-monomers (1-butene, 1-hexene and 1-octene) as short branches in LLDPE. In the case of PP, the samples that were used had linear chains. The melt strength of PP also increased strongly with decreasing MFI. A wider molecular mass distribution (MMD) led to higher melt strength.
The differences between several linear and branched PP and PE polymers were studied by Demaio and Dong [5]. The branched PP was obtained by electron beam (EB) irradiation. The melt strength of the branched PP was found to be ten times higher than the one of a linear PP with the same MFI. The melt strength of the LDPE was five times higher than HDPE. In that report, the melt strength was related to the loss tangent The melt tension rose as the loss modulus decreased, indicating that higher elasticity is coupled with higher melt strength.
Gotsis and Ke [6], among others, showed that both the spinning viscosity, as it is measured in a fiber spinning experiment (Rheotens®), and the uniaxial elongational viscosity depend strongly on the molecular structure of the melt. Three PE melts with equal MFI at 190°C were examined: A long chain branched polymer (LDPE), another with short branches (LLDPE) and a third without branches (HDPE). While their shear viscosities were the same in a large range of shear rates, the melts showed quite different elongational properties. The LDPE showed distinct strain hardening of the elongational viscosity and the highest melt strength of the three. On the other hand, the LLDPE showed very little strain hardening and low melt strength, even lower than that of HDPE. It seems that the broader MMD of the HDPE had a stronger contribution to strain hardening and melt strength than the shorter branches of LLDPE, at least at the strain rates, that were used. Münstedt et al. [7] have reported strain hardening for LLDPE, but only at low strain rates. This was probably due to a small fraction of very high molecular mass and its immiscibility with the rest of the material. It is doubtful, therefore, whether short branches can contribute effectively to higher entanglement density and thus to higher extensional properties. Long chain branching, on the other hand is the factor that contributes the most to the enhancement of the melt strength.
It is expected that the improvement of the melt strength behavior of PP will substantially contribute to the growth of this polymer in the plastics market. For this reason, many plastic manufacturers are developing HMS-PP grades. The melt strength of PP can be improved by increasing the molecular mass, broadening the distribution (e.g. creating a bimodal MMD) or by introducing branches. The latter seems to be the most efficient way and it may be achieved by radical reactions. Radicals can be introduced in the polymer chain by irradiation or by chemical free radical initiators, such as organic peroxides.
Patents exist for the manufacturing of HMS-PP by EB irradiation [8], [9]. The EB irradiation causes scission of the PP chains followed by crosslinking, leading to long chain branching. Patents for the modification of PP by chemical initiation also exist. The reactive extrusion of PP with peroxydicarbonates (PODIC) has been patented by Basell (formerly HIMONT) [10] and by Akzo-Nobel, [11], [12]. Other publications describe the modification of PP with peroxides in combination with multifunctional monomers [13] or in particular with butadiene gas in a post-reactor process [14], with the aim to introduce LCB on initially linear chains.
Similar to PE, the most successful way to improve the melt strength of PP seems to be the addition of long chain branching. In the present study, this was achieved by modification of (initially) linear PP in a reactive extrusion process in the presence of PODIC. Our aim is to evaluate the effectiveness of the several types of PODIC for this modification, study their effect on the melt strength, detect the presence of strain hardening in elongational flows and explain the mechanism which causes the improved melt strength.
Section snippets
Experimental
The PP used in the present study was a reactor grade powder from Borealis with a MFI of 3.2 g/10 min at 2.16 kg and at 230°C. The weight average molecular mass, Mw, was 450,000 (though this was reduced after extrusion to 410,000), Mw/Mn=6.4 and Mz/Mw=5.3. This polymer was stabilized with 0.1% antioxidant, Irganox 1010 (Ciba). The peroxides that were used are listed in Table 1. They are PODIC except the PND, which is a perester, are produced by Akzo Nobel and are commercially available, except for
Results and discussion
During most of the modifications, the torque on the motor of the extruder was higher than during the extrusion of pure PP and the polymer strand surface was rougher in appearance. Since the half time of the PODIC is relatively short at the processing temperatures used, the peroxides must have completely decomposed and reacted during the residence time of the polymer in the extruder.
Conclusions
Linear PP can be modified to obtain varied degrees of long chain branching using specific PODIC. The increase in branching number is reflected on the differences of the extensional rheology of the modified samples and their melt strength. All PODIC-modified samples show enhanced strain hardening and a higher stress at break. Further, the modifications lead to increased die swell and lower MFI. The amount of LCB can be controlled by the type and the amount of PODIC used for the modification. A
Acknowledgements
We thank Akzo Nobel Polymer Chemicals Research (Deventer) and R. Liebrand for supporting Rob Lagendijk during his M.Sc. project. We also thank P.J.C. van Haeren of Akzo Nobel Chemicals Research Arnhem for helping with the HTSEC measurements.
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