Real-time x-ray diffraction: crystalline orientation during blown film extrusion
Blown film extrusion is a commercial process used to produce high-volume polymeric films. The properties the polymeric films exhibit are controlled by the microstructure (crystallinity and orientation) developed during their processing. Conventional manufacturing techniques use off-line measurements on the processed polymer to empirically modify the process to obtain the desired product properties, but this trial-and-error procedure is time-consuming and expensive.1 Therefore, real-time measurement of microstructure during the blown film extrusion can help in better process control and optimization.
The potential of real-time Raman spectroscopy as a rapid microstructure monitoring tool during blown film extrusion has been demonstrated previously.2 However, Raman spectra must be calibrated using primary techniques such as differential scanning calorimetry, density measurements, or x-ray diffraction. We recently reported the first real-time wide-angle x-ray diffraction (WAXD) measurements during blown film extrusion of a low-density polyethylene (LDPE).3
Here, we summarize the crystalline orientation development during film blowing of LDPE that we observed using WAXD.4 The composite x-ray diffraction patterns were interpreted to explain the microstructural transformation during the blown-film process. A custom-built x-ray diffraction system (Rigaku/MSC, shown in Figure 1) was used throughout this study. We chose a constant blow-up ratio (BUR) of 0.6 and take-up ratios (TURs) of 2.5 and 5.5 for the real-time measurements based on the stability of the bubble. A BUR of less than one led to a uniaxial extension in the axial direction with about equal shrinkage in the other two orthogonal directions (normal and transverse). The crystalline reflections (110) and (200) from a polyethylene bubble are separated or conjoined depending on the bubble diameter. However, reflections that are unaltered after scattering from one of the cylindrical faces can be used to obtain the azimuthal profiles from the WAXD scans.
We analyzed the real-time x-ray patterns for changes in the diffracted intensities with respect to the azimuthal angle along the axial distance of the film line. The normalized intensities of the (110) and (200) reflections are shown plotted as a function of azimuthal angle for various axial distances in Figure 2(a) and (b), respectively. Two-dimensional WAXD images for the bubble at two axial distances along the line are also displayed in Figure 2. The patterns show the evolution of preferred orientation of the crystallites in the bubble. Significant reduction in the amorphous region (yellow) in the pattern can also be observed, indicating that the orientation and crystallization occur simultaneously. The peaks tend to become narrower at higher axial distances, indicating the evolution of anisotropy of the crystallites with an increase in stretching of the bubble. The (110) reflection appears as a broad azimuthal peak near the frost-line height (FLH) (~76cm). But at higher distances, the reflection tends to show a maximum that is about 25° from the equator. Similarly, the a-axis reflection of the (200) peak gradually transforms from a broad profile at a lower axial distance into a narrower profile aligned near the meridian at 0° at a higher axial distance. The azimuthal profiles confirm the evolution of orientation and crystallization in the polyethylene bubble (subjected to low–intermediate stresses during processing) starting from an isotropic state at lower distances, to a Keller-Machin type I microstructure at higher distances.
The calculated Hermans orientation profiles (fa, fb, fc) for the two TURs are displayed in Figure 3. Trends are shown with solid lines for low TURs and dotted lines for high TURs. The orientation along all three axes does not significantly change as a function of process time for the case of low take-up speed. However, at higher TURs we observed a significant change in the fa, fb, and fc values. For a twisted row-nucleated lamellar morphology, both the a-axis and c-axis preferentially orient in the machine direction, while the b-axis aligns perpendicular to the a- and c-axis.
In summary, we successfully used x-ray diffraction to determine orientation evolution during the blown-film extrusion process.4 The results indicated that significant orientation occurs past the FLH, even after the bubble has solidified. This result is consistent with our earlier study2 using real-time Raman spectroscopy, which confirmed that uniaxial orientation parameters (P2, P4) increase along the axial distance in the film line even past the FLH. Thus, refinement of chain orientation continues after the blown film diameter is locked into place. These results provide a fundamental understanding of the microstructural transformation that occurs during the process. In future, the work will explore other processing conditions to formulate processing-structure-property correlations.
- T. Kanai and G. A. Campbell, Film Processing, Hanser, 1999.
- G. Gururajan and A. A. Ogale, Molecular orientation evolution during low-density polyethylene blown film extrusion using real-time Raman spectroscopy, J. Raman Spectrosc. 40, pp. 212, 2009.
- G. Gururajan, H. Shan, G. Lickfield and A. A. Ogale, Real-time wide-angle x-ray diffraction during polyethylene blown film extrusion, Polym. Eng. Sci. 48, pp. 1487, 2008.
- G. Gururajan and A. A. Ogale, Real-time crystalline orientation measurements during low-density polyethylene blown film extrusion using wide-angle x-ray difraction, Polym. Eng. Sci., 2012.