Characterisation of fischer-tropsch wax/LLDPE blends

Abstract

Wax is often used as a processing additive in polymer compounding, particularly in thermoplastic processing, due to its ability to improve processability. Wax acts as an external lubricant in the polymer melt, reducing the melt viscosity and increasing the melt flow rate. It’s addition, in masterbatching operations, facilitates improved dispersion of additives and fillers, as well as easier mixing and extrusion. Furthermore, the addition of wax to the polymer can reduce the processing temperature, leading to energy savings and reduced wear on processing equipment. However, the effectiveness of wax as a processing additive is strongly dependent on the type and amount of wax used, as well as the specific polymer being processed and its processing conditions. This study investigated the flow behaviour and compatibility characteristics of Fischer-Tropsch (F-T) wax blended with linear low-density polyethylene (LLDPE), for its possible application as a processing aid package for highly filled pigment masterbatches. The samples were prepared by melt blending using extrusion. The blends were prepared in predetermined quantities of the F-T wax and LLDPE in increments of 10 wt-%. This study provides a survey of characterisation methods and principles using rheology, differential scanning calorimetry (DSC), and hot stage polarised optical microscopy (POM). Both sample preparation and characterisation work were conducted in a temperature range of 120 – 180 °C. Rheological behaviour of the F-T wax/LLDPE blends were measured using the cone-and-plate configuration. The results showed that small additions of LLDPE to F-T wax increased the viscosity of the blend significantly. The composition dependence of the zero-shear melt-viscosity of the blends was adequately represented by the Friedman and Porter mixing rule with alpa = 3.4. This is equivalent to the expression in which the viscosity is calculated via the weight-average molecular mass of the mixture, i.e., ηo = KM^3.4. This implies that the zero-shear melt viscosity was dominated by polymer chain entanglement. The activation energy for viscous flow was found to be insensitive to blend composition. A linear relationship in all the Han plots, i.e., the plots of the logarithm of the storage modulus (G') against the logarithm of the loss modulus (G") was observed. Within the experimental uncertainty, they were essentially unaffected by variations in blend composition, temperature and the applied angular frequency. Additionally, the Cole-Cole plots supported the notion that the wax/LLDPE blends were miscible in the molten state. These results suggest full miscibility of the F-T wax/LLDPE blend system down to temperatures as low as 120 °C. The melting and crystallisation behaviour were studied using hot-stage optical microscopy and differential scanning calorimetry (DSC) in isothermal and dynamic modes. DSC results revealed significant LLDPE melting point depression increasing with increasing wax content. Optical microscopic monitoring of isothermal crystallisation, of the LLDPE phase, showed that adding wax decreased the size of the polymer spherulites. Beyond 50 wt-% wax, it was not possible to distinguish the spherulites at the magnification applied (25). Overall, it was found that increasing the wax content delayed the onset of crystallisation, decreased the overall crystallinity, and reduced the size of the crystallites of the LLDPE-rich phase. The results from both techniques were consistent with partial co-crystallisation of the two components. In summary, all the results indicate full miscibility of the wax and the LLDPE in the melt and partial co-crystallisation in the solid state. Furthermore, in the dynamic DSC scans, the near complete absence of a wax-like melting peak for the blend containing 10 wt-% wax suggests complete miscibility at that concentration.