Molecular structure and rheology of polypropylene in blown film processing
Creators: name: Ivanov, Ivan
Publisher: name: RMIT University
Year: 2024

description: Polypropylene (PP) is one of the most important polymers in use. It is a cheap commodity plastic, yet useful for manufacturing a range of versatile products. One of the prominent applications of PP is the production of biaxially oriented PP film (BOPP), which became an irreplaceable material for food and tobacco packaging and a substrate for printing the Australian banknotes. The ultimate properties of polymer products are the result of a combination of factors, such as molecular structure of the resin and processing conditions. Understanding the influence of these factors on the final polymer properties would allow cost-effective development of more efficient technologies for manufacturing high quality products. <br><br>Rheology was found to be the most useful tool for assessing the interrelations between several aspects of molecular structure (more precisely, molecular weight and molecular weight distribution, MW and MWD) and the processability of polypropylene. As a science of flow and deformation, rheology studies the various aspects of deformation that occur during the BOPP processing. Shear flow dominates during the extrusion of the PP melt, while the extensional deformation dominates when it's being blown into a bubble and stretched. <br><br>In the work presented here, several PP grades, including homo-polymers, co-polymers and blends, were characterised in terms of MW, MWD, shear and extensional rheology. Additionally, differential scanning calorimetry was used to determine the thermal properties of the grades and to provide information on the morphology of the materials at different temperatures. Gel-permeation chromatography was a method of choice to study the polymers' MWD; moreover, by coupling the viscometer to the refractive index detector, it was possible to study the long-chain branching (LCB). None of the PP grades showed presence of LCB. <br><br>The shear rheological characterisation was performed in both steady and dynamic mode. The steady shear measurements provided the most relevant data for prediction of the polymer melt behaviour under processing conditions. However, dynamic or oscillatory shear measurements produced a large set of data useful for estimation of the linear visco-elastic properties that can be related to their MWDs. Extensional rheology of PP was also carefully studied by measuring the transient extensional viscosity, also known as the stress growth coefficient, η<sup>+</sup><sub>E</sub>(t).<br><br>All measurements were conducted in such a way that would provide the most possible precision and accuracy. The error analysis was performed for all the rheological measurements. All tests were repeated several times in order to verify the repeatability. Wherever possible, the reproducibility of measurements was determined as well, by comparing the data obtained from different instruments. This was especially important in case of GPC analysis, which is a calibration-based technique and also for extensional rheometry, which is not a well-established method yet. <br><br>Furthermore, it was attempted to estimate the MWD of several PP homo-polymer grades from their steady shear viscosity curves, by inversion of blending laws. The resulting curves were much narrower than the GPC curves and did not retain the information on the broadness of distribution. The dynamic shear data were also utilised to estimate the MWD. First, the relaxation time spectra for polymer grades were calculated from their dynamic moduli, G' and G". A recently developed analytical relationship between relaxation spectrum and MWD was then applied to the commercial PP grades for the first time. The obtained distribution curves were narrower than the GPC ones, but they preserved their relative position and breadth, i.e. they could be used for comparison between grades. This provides a basis for the future work in utilisation of rheological measurement for MWD determinations. <br><br>Many correlations between rheological parameters and MW averages and polydispersities have been formerly known. Most common of them were tested and discussed for their reliability and practicality for polymer grades used in BOPP processing. The use of such correlations, e.g. between Mw and zero shear viscosity or between the degree of shear thinning and polydispersity, can provide an assistance for engineers involved in polymer processing. <br><br>Blending PP with an amorphous hydrocarbon resin - hydrogenated oligo-cydopentadiene (HOCP), has been recently shown to improve final BOPP properties, such as gas permeability. The use of a commercial modifier, EPI that consists of 50wt% PP and 50wt% HOCP, for blending with different PP grades, was studied here for the first time. Rheological and morphological characterisation of the blends was performed and the correlation between miscibility and the rheological behaviour was drawn. It was found that two factors were highly significant: -the concentration of the oligomeric resin, HOCP, which lowered the viscosity and dictated the onset of phase separation and -the MWD compatibility of the PP grade with the polypropylene pre-blended in EPI. <br><br>Extensional rheological measurements were conducted with PP grades in different morphological states, controlled by temperature. Strain hardening was found for PP stretched from the semi-molten state, which exists at temperatures slightly lower than its melting point, Tm. The extensional viscosity index, EVI was introduced in this study as a measure of strain hardening for polypropylene and a practical tool for relating the results of extensional rheological measurements to processability of PP grades at different conditions. The results were supported by shear rheological, GPC, DSC and optical microscopy testings, where any of the listed methods by itself was not sufficient to predict the behaviour of semi-molten PP under extensional deformation. <br><br>It was shown that the factors that dominate the processability of polymers in industrial processing and their interrelationships are very complex. By identifying the key parameters of interest for a specific process (e.g. extensional viscosity, strain, strain rates, temperature, shear viscosity etc. in case of film blowing) it may become possible to develop models that predict the processing behaviour of the polymer grades. This work provides a basis for developing such models.