Date of Award

5-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Ogale, Amod A

Committee Member

Lickfield , Gary C

Committee Member

Bruce , David A

Committee Member

Hirt , Douglas E

Abstract

Properties exhibited by blown films are controlled by the microstructure developed during their processing. Therefore, real-time measurement of microstructure during the blown film extrusion can help in better control and optimization of the process needed to obtain desired properties. The objectives of this research were (i) to conduct real-time microstructural measurements during single-layer and multi-layer blown film extrusion of polypropylene (PP) and low-density polyethylene (LDPE) using real-time Raman spectroscopy; (ii) to conduct real-time wide-angle X-ray diffraction (WAXD) during single-layer blown film extrusion of LDPE to obtain crystallinity and orientation values during the process; and (iii) to investigate the effect of blown film coextrusion on the microstructure of PP/LDPE bilayer films. The potential of real-time Raman spectroscopy as a rapid microstructure monitoring tool for better process control during blown film extrusion is demonstrated.

Real-time polarized Raman spectroscopy was conducted to measure the orientation development during blown film extrusion of low-density polyethylene (LDPE). Polarized Raman spectra were obtained at different locations along the blown film line, starting from the molten state near the die and extending up to the solidified state near nip-rolls. The trans C-C symmetrical stretching vibration of PE at 1130 cm-1 was analyzed for films possessing uniaxial symmetry. For the given peak, the principal axis of the Raman tensor is coincident with the c-axis of the orthorhombic crystal, and was used to obtain second (2(cos θ)>)( ) and fourth ( ) moments of the orientation distribution function. The orientation parameters (P2, P4) were found to increase along the axial distance in the film line even past the frost-line height (FLH). The P2 values also showed an increasing trend with crystalline evolution during extrusion consistent with past observations that molecular orientation takes place even after the blown film diameter is locked into place. It was also found that the integral ratio (I1132/I1064), obtained from a single, ZZ-backscattered mode, can provide a reasonable estimate of molecular orientation.

Although Raman spectroscopy is a convenient technique, it is not a primary measurement technique to obtain crystallinity and orientation in fibers or films. So for the first time, a real-time wide-angle X-ray diffraction (WAXD) technique was attempted during blown film extrusion. WAXD patterns were obtained at different axial positions in the film line starting from a location near the die up to the nip-roller. The composite X-ray diffraction patterns from the bubble were analyzed and quantified for crystallinity values. From the evolution of (110) and (200) peaks in the WAXD pattern, it was inferred that the crystallization process started near the frost-line height (FLH) and showed a steep increase at lower axial distance near the freeze line and then a gradual increase at higher axial distance in the film line. The differences in the profiles for crystallinity were also evident with changing processing conditions. The crystallinity results obtained using WAXD were found to be consistent with those from simultaneous real-time Raman spectroscopic measurements. Thus, for the first time, real-time WAXD technique was successfully used for measurement of microstructure during the single-layer blown film extrusion of low density polyethylene (LDPE).

Multi-layer blown films are of significant industrial importance to make packaging films with desirable properties through combination of two or more polymers. Therefore, real-time Raman measurements were extended from single-layer blown film extrusion to multi-layer blown film extrusion. Online spectroscopic measurements were carried out to estimate crystalline growth of the individual components of a bicomponent low-density polyethylene (LDPE) and polypropylene (PP) film (LDPE/PP). The 1296-1305 cm-1 band, observed predominantly for PE, was only slightly masked by the contribution from the PP layer. In contrast, the 809-841 cm-1 band was unique for PP and unaffected by the presence of the PE layer and 1418 cm-1 band was unique for PE. These distinct peaks enabled successful deconvolution of the superimposed spectra to enable crystallinity measurements during coextrusion of LDPE/PP films. Such real-time results have not been reported earlier in the literature for multi-layer films.

Finally, real-time Raman spectroscopy results were used to develop an understanding of processing-microstructure relationship for the blown film process. For bilayer films (PP/LDPE), the onset crystallization-time difference for PP and LDPE components was found to be an important parameter, which controls the orientation and morphology of the coextruded films. Although overall molecular orientation within PP and LDPE multiple layers was not affected, single-layer LDPE films displayed some row-nucleation, but not the LDPE layer in coextruded films. Also, there was a slight decrease of crystalline a-axis orientation for coextruded LDPE layer as compared to that for single-layer LDPE films. Thus, as one component of experimental research being conducted at the Center for Advanced Engineering Fibers and Films (CAEFF), this study was successful in generating real-time experimental results during the film formation that are critical for validating modeling results being generated in companion studies.

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