Effect of Rheological Strain Hardening on Extrusion Blown Film of Polyvinylidene Fluoride

Abstract

In this work the effect of strain hardening on poly-vinylidene fluoride (PVDF) extrusion blown film is investigated. Controlled long chain branching is introduced via a multi-functional initiator to produce two PVDF samples with different molecular weight and chain architecture. The branched samples are compared to two reference resins having identical molecular weight and no chain branching. All samples are characterized by size exclusion chromatography (SEC) coupled with triple detection system comprising a Differential Refractive Index (DRI) detector, Intrinsic Viscosity (IV) detector, and Multi-Angle Laser Light Scattering (MALLS) detector, to determine their molecular weights and their distribution as well as to detect chain branching via measuring the coil size in dilute solution. The rheological properties are determined using oscillatory measurement and melt strength at 230°C while extensional viscosity measurements are conducted at 180°C to determine strain hardening at different extension rates. The resins are evaluated using a small scale extrusion blown film set-up to determine the blow-up ratio and the minimum thickness achievable. The characterization results show that the control samples are different in molecular weight and almost identical in the polydispersity index (Mw/Mn). The branched samples, however, have higher molecular weight and a slightly broader molecular weight distribution. Light scattering data together with inherent viscosity data show that the branched samples have a lower radius of gyration (RG) and inherent viscosity (IV) over the entire molecular weight distribution confirming chain branching. The rheological properties in oscillatory measurements show that the branched samples exhibit almost identical viscosities as the control samples. However, a broader transition from the Newtonian to the non-Newtonian region for the branched sample is observed confirming the SEC—MALLS results. This is corroborated using extensional viscosity and melt strength measurements, which show a significant strain hardening and an increase in melt strength, respectively. Blown film experiments show that the samples containing chain branching could be processed under similar process conditions as the control samples, and with a higher blow up ratio thereby achieving 5 μm film thickness with high clarity.

Effect of molecular structure on film blowing ability of thermoplastic zein

“…They found LDPEs as the most stable resins in the film blowing process for their distinct strain hardening behavior, whereas the HDPEs and LLDPEs were unstable for their narrower molecular weight distribution. Later, Mekhilef et al22 studied the effect of rheological strain hardening on extrusion blown film of polyvinylidene fluoride (PVDF). He observed that the strain hardening of the branched PVDF samples increases the processing window with respect to reference samples (not branched) and allows for the formation of a large and stable bubble, thus producing films with reduced thickness and higher clarity.…”

“…In these cases, the different processability of the polymers may be attributed to differences in molecular architecture due to factors such as branching and/or the presence of a high‐molecular‐weight tails18–20 that are not readily detected in shear but are more visible in extensional deformations. In fact, one of the key properties in extrusion blown film is extensional viscosity 21, 22. Indeed, it is well recognized that bubble stability, biaxial stretching, and thickness uniformity are very sensitive to extensional viscosity.…”

“…Another important rheological property in the blown film process is the haul‐off force 22. It is defined as the force necessary during processing to sustain the polymer weight when the bubble is formed.…”

Critical Role of Interfacial Diffusion and Diffuse Interphases Formed in Multi-Micro-/Nanolayered Polymer Films Based on Poly(vinylidene fluoride) and Poly(methyl methacrylate)

“…56 Also, a small amount of high-molecular-weight fraction containing chain branching might exist resulting from the synthesis of PVDF resins, which could also give a strain hardening typical of branched polymers. 58 A clear mechanism for the strain hardening of PVDF is still unclear for this moment and also not the aim of this study. A i (t) is the area of cross-section perpendicular to film coextrusion direction with A i (t) = W i (t)×H i (t), where W i (t) and H i (t) are the time-dependent width and thickness of a layer i, respectively.…”

Biopolymer Blends Based on Poly (lactic acid): Shear and Elongation Rheology/Structure/Blowing Process Relationships

“…Thus, the modified polymer resisted up to the elongational deformation without failure for the complete test durations. Considering the wide polydispersity for the modified PLA, this gives rise to the assumption that the effect of the branching structure on the strain hardening dominates [28,29]. Depending on the number of branches and molecular weight, entanglement density may be increased or decreased by branching.…”